Design of advanced Catalysts and new Strategies for biogas Reforming

TESIS DOCTORAL Diseño de catalizadores avanzados y nuevas estrategias para el reformado de biogás Design of advanced catalysts and new strategies for biogas reforming Juan Luis Martín Espejo Instituto de Ciencia de Materiales de Sevilla Universidad de Sevilla -2025- Diseño de catalizadores avanzados y nuevas estrategias para el reformado de biogás Juan Luis Martín Espejo Memoria presentada por Juan Luis Martín Espejo ante la Universidad de Sevilla para optar al título de Doctor en el Programa de Doctorado en Ciencia y Tecnología de Nuevos Materiales Abril de 2025 Fdo. Juan Luis Martín Espejo DIRECTORES Fdo. Laura Pastor Pérez Fdo. Tomás Ramírez Reina TUTOR Fdo. José Antonio Odriozola Gordón Table of Contents Chapter 1. Introduction and objectives ................................................................ 1 1. General introduction ....................................................................................... 3 2. Aim and objectives ......................................................................................... 8 3. Thesis outline ............................................................................................... 10 Chapter 2. Experimental ...................................................................................... 13 1. Experimental techniques .............................................................................. 15 1.1. X-Ray Diffraction (XRD) .................................................................... 15 1.2. Inductively Coupled Plasma Spectrometry (ICP-OES) ....................... 18 1.3. Nitrogen Physisorption and Textural Properties .................................. 18 1.4. Scanning Electron Microscopy (SEM) ................................................ 20 1.5. Transmission Electron Microscopy (TEM) ......................................... 21 1.6. RAMAN Spectroscopy ........................................................................ 22 1.7. Temperature-Programmed Reduction with Hydrogen (H2-TPR) ........ 23 1.8. X-ray Photoelectron Spectroscopy (XPS) ........................................... 24 1.9. UV-Vis Spectroscopy .......................................................................... 26 1.10. Thermogravimetric Analysis (TGA) ................................................... 26 1.11. Temperature-Programmed Oxidation followed by Mass Spectrometry (TPO-MS) .......................................................................................................... 27 2. Catalytic performance .................................................................................. 28 2.1. Thermo-catalytic equipment ................................................................ 28 2.2. Plasma-catalytic equipment ................................................................. 32 Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures .................................................................................................. 37 1. Introduction .................................................................................................. 39 1.1. Thermodynamic considerations ........................................................... 40 1.2. Reaction mechanism ............................................................................ 44 1.3. Catalytic considerations for DRM ....................................................... 48 1.4. Concluding remarks ............................................................................. 69 2. Catalyst synthesis ......................................................................................... 71 3. Effect of nickel loading on Ni-based cerium zirconate ................................ 74 3.1. Catalysts characterisation .................................................................... 75 3.2. Catalytic activity ................................................................................ 108 3.3. Post-reaction characterisation ............................................................ 121 4. Effect of calcination temperature on Ni-based cerium zirconate ............... 127 4.1. Catalysts characterisation .................................................................. 128 4.2. Catalytic activity ................................................................................ 141 4.3. Post-reaction characterisation ............................................................ 148 5. Conclusion .................................................................................................. 153 Chapter 4. Thermo-catalytic biogas reforming using other formulation structures 155 1. Bimetallic formulation Ni:Co for low-temperature DRM.......................... 158 1.1. Introduction ....................................................................................... 158 1.2. Catalyst synthesis .............................................................................. 163 1.3. Catalysts characterisation .................................................................. 165 1.4. Catalytic activity ................................................................................ 177 1.5. Conclusion ......................................................................................... 181 2. Sacrificial template approach using a Ni:Ce MOF-based catalyst ............. 183 2.1. Introduction ....................................................................................... 183 2.2. Catalyst synthesis .............................................................................. 186 2.3. Catalysts characterisation .................................................................. 188 2.4. Catalytic activity ................................................................................ 205 2.5. Catalytic activity role of other support structures .............................. 210 2.6. Stability tests and post-reaction characterisation ............................... 214 2.7. Conclusion ......................................................................................... 221 3. Conventional wet impregnation vs Modified pechini method ................... 223 3.1. Introduction ....................................................................................... 223 3.2. Catalyst synthesis .............................................................................. 225 3.3. Catalysts characterisation .................................................................. 226 3.4. Catalytic activity ................................................................................ 237 3.5. Post-reaction characterisation ............................................................ 247 3.6. Conclusion ......................................................................................... 250 4. Summary .................................................................................................... 251 Chapter 5. Plasma-catalytic biogas reforming ................................................. 253 1. Introduction ................................................................................................ 256 1.1. Generics ............................................................................................. 256 1.2. Geometric factors .............................................................................. 261 1.3. Operating factors ............................................................................... 266 1.4. C2+ and oxygenates ............................................................................ 271 1.5. Concluding remarks ........................................................................... 279 2. Catalytic activity ........................................................................................ 280 2.1. Ni-based cerium zirconate ................................................................. 282 2.2. Ni:Ce MOF-based catalyst ................................................................ 289 2.3. Comparison ........................................................................................ 293 3. Conclusion .................................................................................................. 297 Chapter 6. Overall conclusion and future directions ...................................... 299 Thesis outputs ........................................................................................................ 307 Bibliography .......................................................................................................... 311 Chapter 1. Introduction and objectives 6 the development of a circular economy by utilising carbon-rich gases as feedstocks for chemical production [12,14–17]. Transforming biogas into valuable chemicals reduces dependency on fossil-based feedstocks, thereby addressing multiple environmental and economic challenges simultaneously. Acetic acid can be an interesting intermediate product generated from biogas which can be later transformed into other valuable products. Traditionally, acetic acid is industrially produced via an indirect route, carbonylation of methanol, using syngas (foremost formed by CO and H2) [18]. Globally, three main environmental drawbacks are found in this synthesis pathway: a) it is an energy-intensive process, b) syngas is mainly produced from fossil fuels (e.g., carbon and natural gas) and c) environmental impacts coming from many chemical waste discharges [19,20]. Research efforts are now focused on promoting alternatives. Furthermore, current trends in manufacturing are shifting towards more sustainable chemical routes to lessen the environmental impact on the atmosphere and hydrosphere caused by industrial processes. Therefore, the alternative BIO-routes proposed, illustrated in Figure 1, using biogas as a source, could be considered an appealing alternative compared to the traditional route. Figure 1. Traditional acetic acid production (right side) versus alternative BIOroutes to acetic acid production from biogas (left side). Chapter 1. Introduction and objectives 7 Dry reforming of methane (DRM) can be used to produce syngas from biogas. This process, however, faces a series of disadvantages: it is an energy-intensive process and suffers from deactivation of the catalyst [21]. Despite the appreciable positive environmental potential, DRM cannot be considered an industrially mature process yet since the development of a robust catalyst remains a challenge [21–23]. The incorporation of biogas-DRM into the acetic acid route represents an innovative lowcarbon path taking the edge over the traditional process in terms of sustainability. On the other hand, chemically and stoichiometrically, the direct, one-pot transformation of CH4 and CO2 into acetic acid (Equation 1) is a very appealing atom-efficient process (BIO-direct route). Nonetheless, this one-step process proposed is quite unfavourable thermodynamically [24,25]. CH4 + CO2 → CH3COOH Equation 1 A smart way to overcome these thermodynamic limitations is the use of a nonconventional technology, i.e., non-thermal plasma (NTP). Currently, NTP is considered an enabling green technology for the near future of energy sector as it may allow chemical processes to be based on renewable energy sources instead of fossil fuels [22]. Indeed, the combination of NTP and a robust catalyst, known as plasmacatalysis or hybrid plasma catalysis, offers a great potential to optimise reaction conditions for the production of acetic acid [22,23,26]. Chapter 1. Introduction and objectives 8 2. Aim and objectives The aim of this thesis is to develop and explore innovative catalytic processes and design cost-effective catalysts for the efficient reforming of biogas into value-added chemicals. This research focuses on addressing the limitations of conventional biogas utilisation methods, which are typically energy-intensive, environmentally unsustainable and economically constrained. The study builds upon the bio-routes, emphasising both direct and indirect catalytic pathways to transform biogas into highvalue products. The initial phase involves the systematic investigation of a series of catalysts under thermo-catalytic DRM conditions. These conditions serve as a benchmark for evaluating catalytic performance, stability and potential deactivation mechanisms such as coking and sintering. The study further extends the framework by incorporating non-thermal plasma (NTP) technology, an emerging green energy solution, to overcome thermodynamic and kinetic limitations inherent to DRM. The combination of NTP with catalytic systems is expected to enhance process efficiency, reduce energy requirements and enable reaction conditions that are not feasible under conventional thermal setups. To achieve this, the following specific objectives were set: Objective 1: Development and synthesis of advanced, tailored catalysts for biogas transformation, with a particular focus on combining structural, surface and textural properties to maximise catalytic performance. Chapter 1. Introduction and objectives 9 Objective 2: Comprehensive characterisation of catalysts preand post-reaction using advanced analytical techniques to establish structure-performance relationships and understand deactivation mechanisms such as coking and sintering. Objective 3: Optimisation of catalytic performance for syngas and C2+ and/or oxygenates hydrocarbons (acetic acid) production using hybrid plasma-catalysis to enhance efficiency and overcome thermodynamic limitations. Chapter 1. Introduction and objectives 10 3. Thesis outline This thesis is structured as follows: Chapter 2 describes the experimental techniques and methodologies used throughout the study, including catalyst synthesis protocols, advanced material characterisation techniques and catalytic testing setups. It provides a comprehensive overview of the specific conditions and equipment employed for evaluating the performance and stability of the developed catalysts under various reaction environments. Chapter 3 focuses on the thermo-catalytic reforming of biogas using nickel-based cerium zirconate structures. It investigates the effects of metal loading and calcination conditions on the catalytic performance and stability during DRM reactions. This chapter presents a thorough analysis of the reaction mechanisms, thermodynamic considerations and the impact of the properties of the catalyst on activity and deactivation resistance. Chapter 4 explores alternative catalytic formulations and synthesis methods. It compares bimetallic systems, such as Ni:Co formulations, sacrificial template-based catalysts using a MOF and a comparation between two synthesis approaches, modified version of Pechini method and wetness impregnation. This chapter aims to identify innovative strategies to enhance catalytic efficiency, reduce reaction temperatures and improve catalyst durability for biogas reforming. Chapter 5 delves into the application of non-thermal plasma (NTP) technology to biogas valorisation. This chapter discusses the fundamentals of plasma-catalysis, reactor design considerations and the integration of NTP with catalytic processes. It Chapter 1. Introduction and objectives 11 presents experimental results for biogas dry reforming reactions and evaluates the potential for producing value-added chemicals, such as syngas, C2+ and/or oxygenates hydrocarbons, under plasma conditions. Chapter 6 concludes the findings of the thesis, summarising the key achievements and contributions to the field of biogas valorisation. It also outlines the future perspectives and research directions for the development of sustainable catalytic processes and advanced materials to address the challenges of greenhouse gas emissions and promote the transition to a circular economy. Chapter 1. Introduction and objectives 12 Chapter 2. Experimental Chapter 2. Experimental 14 Chapter 2. Experimental 15 1. Experimental techniques The main aspects of the techniques employed are briefly described in this section. 1.1. X-Ray Diffraction (XRD) X-Ray crystallography is a non-destructive characterisation technique used in the field of materials science to determine the crystalline domain of a solid sample. It is an essential tool for the crystalline determination of a wide variety of materials and is based on the phenomenon of X-ray diffraction in a crystalline solid. X-Ray diffraction (XRD) is based on the principle of constructive interference. Xrays, with wavelengths on the scale of angstroms, comparable to the interatomic distances in a crystal, can penetrate solids and probe their internal structure. When these rays encounter the regular and ordered atomic planes of a crystal, they reflect at specific angles. The arrangement and properties of the atoms in the crystal lattice influence this scattering, causing the solid to diffract in a way characteristic of its composition and structure. The resulting angular reflections produce a distinctive diffraction pattern that can be observed and recorded by a detector. XRD analysis enables the identification of different phases in synthesised solid samples, along with determining the average crystallite size and providing a semiquantitative estimation of the crystalline phase content. It also offers insights into the unit cell type, interatomic distances, crystal plane orientation, symmetry and the presence of defects or impurities. The theoretical foundations of the technique lie in Bragg’s Law and the Debye-Scherrer Equation. Chapter 2. Experimental 22 image is formed from these transmitted electrons. The image is then magnified and captured on a CCD camera or a fluorescent screen, providing detailed structural and morphological insights. Equipment and measurements The TEM micrographs were obtained using a JEOL 2100Plus electron microscope (200 kV) with a LaB6 filament, featuring a structural resolution of 0.14 nm between lines and 0.23 nm between points, and singleand double-tilt sample holders. The microscope is equipped with an Oxford Instruments X-Max 80T EDX analyser and a Gatan CCD camera for image recording. The samples were deposited without solvents onto a copper sample grid. 1.6. RAMAN Spectroscopy Raman spectroscopy is a non-destructive spectroscopic technique used to study excitation modes in molecules, including vibrational and rotational transitions. It relies on the inelastic scattering of light when a monochromatic laser interacts with a sample, resulting in scattered light with energy different from the incident light. This energy difference arises from molecular vibrations, electron movements or molecular rotations, with vibrations occurring on a much faster timescale than rotations. In Raman spectroscopy, vibrational transitions provide structural information about the material, including molecular symmetry and potential adsorbed species or sites. Chapter 2. Experimental 23 The radiation used to excite molecules typically has a wavelength between approximately 200 and 1100 nm, although not all transitions are allowed. This technique is particularly useful for characterising molecular structures and interactions within a material. Equipment and measurements The Raman analysis of calcined catalytic structures was performed using a Horiba Jobin Yvon® dispersive microscope with a green/red laser (λ = 532.14/784.56 nm) and a maximum power of 20 mW, featuring a 1000 µm confocal aperture. The samples were examined with a 50x magnification objective and intensity filters D06 and D1. 1.7. Temperature-Programmed Reduction with Hydrogen (H2-TPR) Temperature-programmed reduction with hydrogen (H2-TPR) is a technique used to study the redox behaviour of materials. In this method, the solid sample is exposed to a reductive gas mixture, typically containing hydrogen, while the temperature is ramped according to a programmed schedule. The hydrogen concentration is monitored over time to determine when the sample consumes it. This information helps determine the oxidation states of reducible species, their size and their interactions with the support or other species. Chapter 2. Experimental 24 The H2-TPR technique provides valuable insights into the reducibility of species present in the solid. It allows the analysis of the complete reduction of catalysts, interactions between the components, and changes that may occur under external factors, such as catalytic reactions. Equipment and measurements H2-TPR was conducted on the calcined catalysts in a standard U-shaped quartz reactor linked to a thermal conductivity detector (TCD). A flow of 5% H2 (v/v) diluted in Ar at a rate of 20 mL min-1 was employed. TPR measurements were performed using 100 mg of catalyst and a heating rate of 10 ºC min-1 from ambient temperature to 900 ºC. Additionally, a CO2 (s)/acetone cold trap was used to condense the water formed during the process. 1.8. X-ray Photoelectron Spectroscopy (XPS) X-ray Photoelectron Spectroscopy (XPS) is a surface analysis technique used to determine the elemental composition, chemical states and electronic environments of atoms within the outermost layers of a material. The method is based on the photoelectric effect, where a material is irradiated with monochromatic X-rays, typically using an Al Kα or Mg Kα radiation source. This interaction causes core electrons in the atoms to be ejected, provided that their binding energy is lower than the photon energy. The energy of the emitted electrons is measured by an electron energy analyser and the binding energy is calculated as the difference between the Chapter 2. Experimental 25 photon energy and the kinetic energy of the emitted electrons. This binding energy is unique for each element and its chemical state, enabling the identification of the elements present on the surface and their oxidation states or bonding environments. XPS is highly surface-sensitive, probing a depth of approximately 1 to 10 nm, as the photoelectrons originate only from the outermost layers due to inelastic scattering within the sample. The technique provides quantitative data on the atomic percentages of elements and can also detect shifts in binding energies that reveal information about chemical bonding, electronic structure and surface modifications. Equipment and measurements X-ray photoelectron spectroscopy (XPS) analyses were conducted using a VGMicrotech Multilab 3000 spectrometer equipped with a hemispherical electron analyser and a Mg Kα X-ray source (h = 1253.6 eV) operating at 300 W. Powder samples were pressed into small stainless-steel cylinders and maintained in the analysis chamber until a residual pressure of approximately 5 × 10-7 N m-2 was achieved prior to recording the spectra. Data were collected at a pass energy of 50 eV. Peak intensities were determined by integrating each peak after subtracting a Shirleytype background and fitting the experimental curves using a Lorentzian (30%) and Gaussian (70%) line combination. The binding energy (BE) of the C 1s peak at 284.6 eV was used as an internal standard, with an accuracy of ± 0.2 eV. Samples were reduced ex situ under flowing hydrogen at 800 ºC for 1 h and stored in octane before analysis. Chapter 2. Experimental 26 1.9. UV-Vis Spectroscopy UV-Vis spectroscopy measures the absorption or reflectance of light in the ultraviolet, visible or near-infrared regions of the spectrum, corresponding to electronic transitions in molecules. When light is absorbed, electrons are excited to higher energy levels, and the resulting spectrum provides qualitative information about the electronic structure and bonding through the position of absorption maxima. Quantitative analysis is possible using the Lambert-Beer law, which relates absorbance to concentration, but the technique is often primarily applied to qualitative analysis due to limitations like overlapping peaks and scattering effects. Equipment and measurements The experiments on calcined samples were performed on a UV-Vis Avantes AvaLight-DH-S-BAL spectrometer, equipped with an optical fiber sensor, covering a wavelength range from 150 to 1100 nm. During the measurements, barium sulphate (BaSO4) was used as a reference material to establish the baseline for diffuse reflectance spectra. 1.10. Thermogravimetric Analysis (TGA) Thermogravimetric Analysis (TGA) monitors changes in the mass of the catalyst as a function of temperature or time under a controlled atmosphere. Chapter 2. Experimental 27 Equipment and measurements The thermogravimetric analysis was performed using a simultaneous TG/DTA/DSC instrument, STA449 F5 Jupiter (Netzsch), coupled with a quadrupole mass spectrometer. The experiments were conducted at a gas flow rate of 50 mL min-1 of air or N2, from room temperature up to 900 ºC and a heating rate of 20 ºC min-1. 1.11. Temperature-Programmed Oxidation followed by Mass Spectrometry (TPO-MS) Temperature-Programmed Oxidation (TPO) is a technique used to study the oxidation behaviour of materials by exposing a sample to an oxidising gas while gradually increasing the temperature. As the sample oxidises, gases like CO2, CO or H2O are released and monitored using detectors. Equipment and measurements The TPO experiment was conducted on post-reaction samples using a diluted oxygen flow (10% O2 (v/v) in He) at a rate of 50 mL min-1. The temperature program ranged from room temperature to 900 ºC, with a heating ramp of 10 ºC min-1. Before the analysis, the samples were pretreated in a helium atmosphere for 30 min at room temperature. The composition of the effluent gases was monitored using a Pfeiffer Vacuum Prisma Plus mass spectrometer, controlled by the Quadera® software, monitoring mass-to-charge (m/z) ratios from 2 to 100. Chapter 2. Experimental 28 2. Catalytic performance 2.1. Thermo-catalytic equipment The performance of the catalytic materials was studied in the DRM reaction regime using a tubular Hastelloy reactor with an internal diameter of 9 mm and a length of 400 mm in an automated Microactivity Reference apparatus from PID Eng&Tech presented in Figure 2. Figure 2. Microactivity Reference setup for catalytic performance evaluation. The setup consisted of a down-flow fixed-bed tubular reactor housed in an electric furnace and enclosed within a temperature-controlled hot box. Gaseous reactants were supplied to the reactor through mass flow controllers. A 6-port valve enabled the reactant gases to either flow through the reactor or bypass it directly to the outlet. To Chapter 2. Experimental 29 protect the system and the valve from potential catalyst particle contamination, 10 μm sintered filters were installed at both the reactor inlet and outlet. Downstream, the product gas exited the hot box and was cooled down using a Peltier cooler. The liquid part (if present) was recovered and the remaining gas flow was pressure-regulated within the hot box via a servo-controlled micrometric regulating valve before being directed to a gas analyser. The catalytic samples were sieved in the range of 100-200 µm fraction and 200 mg was then placed inside the reactor over a quartz wool bed, together with a thermocouple to control the temperature in the same spot. The catalysts were positioned at the centre of the reactor. Prior to the activity tests, the catalysts were in situ reduced in a reducing atmosphere using a flow of 50 mL min-1 of 40% H2 (v/v) in N2, at 800 ºC for 1 h and a heating rate of 7.5 ºC min-1 from ambient temperature. The reaction was carried out with a reactant feed flow of 100 mL min-1 and a volume ratio of N2:CH4:CO2 2:1:1 and an absolute pressure of 1 atm. To study the influence of the temperature, temperature-programmed experiments were performed on each catalyst, every 50 ºC, from 500 to 800 ºC (or 400 to 700 ºC), until steady-state conditions were achieved at each step, which occurred after 1.5 h. A heating rate of 5 ºC min-1 was used. An example of the conditions is presented in Figure 3. The weight hourly space velocity (WHSV) was fixed at 30 L gcat-1 h-1. In addition, stability tests were performed using the same reduction conditions and maintaining the selected temperature constant throughout the experiment. Chapter 2. Experimental 30 Figure 3. Example of a temperature programme of an experiment to study the effect of temperature on DRM. The composition of the product gas stream was analysed using an on-line gas chromatography instrument (Agilent Technologies) equipped with a HayeSep Q and Mol sieve 5A column. The reactant conversions (Xi), Equation 4 and Equation 5, and the H2/CO molar ratio, Equation 6, were calculated as follows. XCH4 (%)=FCH4,in − FCH4,out FCH4,in x 100 Equation 4 XCO2 (%)=FCO2,in − FCO2,out FCO2,in x 100 Equation 5 H2/CO =FH2,out FCO,out Equation 6 Chapter 2. Experimental 31 where F is the volumetric flow rate of CH4, CO2, H2 and CO, respectively, at standard temperature and pressure and the subscripts in or out correspond to either the inlet or the outlet reactor flow. The carbon balance was closed ± 5%, and the experimental error of the measurements was estimated to be within ± 3%, which should be considered when analysing conversion and product selectivity values. The reactor was evaluated in an empty state (blank experiment) to assess its intrinsic reactivity, yielding conversions below 3% across all tested temperatures. The flow rate was cross-checked using a custom-made bubble-timer flowmeter and additionally confirmed with a portable flowmeter. All bottled gases used in the experiments, supplied by Air Liquide or Linde, had a minimum reactant purity of 99.9% (v/v), with impurities present at ppm(v) levels. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 38 Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 39 1. Introduction Dry reforming of methane (DRM) offers a viable solution for reducing environmental impact while generating valuable syngas, a crucial intermediate for various industrial applications. This process aligns with global initiatives aimed at lowering carbon footprints and advancing sustainable practices in the chemical sector. A particularly noteworthy advantage of DRM is its synergy with biogas upgrading, presenting opportunities to utilise underexploited resources such as methane and carbon dioxide from organic waste and agricultural activities [18,30]. By leveraging two of the most prevalent greenhouse gases as reactants, DRM facilitates their transformation into hydrogen and carbon monoxide, the primary components of syngas. As shown in Equation 9, this reaction not only mitigates emissions but also establishes a renewable route for producing essential raw materials in the chemical industry [31–35]. This dual function underscores the strategic importance of DRM in fostering a circular and low-carbon economy. CH4 + CO2 → 2CO + 2H2 ΔG = 247.3 - 0.25 x T kJ mol-1 Equation 9 Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 40 1.1. Thermodynamic considerations Dry reforming of methane is a highly endothermic reaction, requiring substantial energy input to drive the conversion of CH4 and CO2. With a standard enthalpy of ΔH298K 0 of 247.3 kJ mol-1, this reaction is significantly influenced by temperature and pressure [36,37]. The high stability of CH4 and CO2, demonstrated by their dissociation energies of 435 kJ mol-1 for CH3-H and 526 kJ mol-1 for CO-O, respectively, presents a challenge for efficient catalytic conversion [38]. As temperature increases, the Gibbs free energy of the reaction (ΔG298K 0 of 170.3 kJ mol1 [39,40]) becomes more negative, indicating greater thermodynamic favourability. At room temperature, DRM is not spontaneous due to a positive ΔG; however, at temperatures above 800 ºC, ΔG becomes sufficiently low or negative, allowing the reaction to proceed and approach its thermodynamic equilibrium conversion [37,41,42]. The entropic contribution to the Gibbs equation (ΔG = ΔH - T x ΔS) further enhances the feasibility of DRM at high temperatures. Lower pressures are generally favourable for the reaction, as they align with Le Chatelier’s principle by favouring the formation of more gas-phase products. Under typical operating conditions, DRM is accompanied by several side reactions that significantly impact product distribution and catalyst performance. These side reactions exhibit distinct thermodynamic properties and temperature-dependent equilibrium constants, which play a critical role in the overall process dynamics [37,43,44]. One of the most relevant side reactions is the reverse water-gas shift (RWGS) reaction (Equation 10). This reaction competes directly with DRM by consuming hydrogen and producing water, which alters the desired H2/CO ratio in Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 41 syngas. Methane decomposition (Equation 11) contributes significantly to carbon deposition on the catalyst surface, which can lead to deactivation through coking. It is thermodynamically favoured at high temperatures. The Boudouard reaction (Equation 12) involves the disproportionation of carbon monoxide to produce carbon and carbon dioxide. This exothermic reaction becomes more prominent at lower temperatures and is a major contributor to catalyst deactivation through coking. The methanation reaction (Equation 13) further consumes syngas components, reducing overall yield, while carbon dioxide hydrogenation (Equation 14) and carbon monoxide hydrogenation (Equation 15) can also lead to carbon deposition and water formation, introducing additional challenges in controlling product distribution [37,45]. CO2 + H2 ↔ CO + H2O ΔG = 41.17 - 0.04 x T kJ mol-1 Equation 10 CH4 ↔ C + 2H2 ΔG = 74.87 - 0.08 x T kJ mol-1 Equation 11 2CO ↔ C + CO2 ΔG = -172.44 + 0.18 x T kJ mol-1 Equation 12 CO + 3H2 ↔ CH4 + H2O ΔG = -206.20 + 0.21 x T kJ mol-1 Equation 13 CO2 + 2H2 ↔ C + 2H2O ΔG = -90.10 + 0.09 x T kJ mol-1 Equation 14 H2 + CO ↔ H2O + C ΔG = -131.27 - 0.13 x T kJ mol-1 Equation 15 At temperatures below 700 ºC, DRM conversion is limited and side reactions such as methane decomposition and the Boudouard reaction become more prominent due to their lower activation energies, exacerbating catalyst deactivation due to carbon formation, as observed in equilibrium constants of reactions of Figure 5 [41,46]. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 42 Nevertheless, at higher temperatures, the equilibrium constant for DRM improves significantly, promoting CH4 and CO2 conversion while reducing carbon deposition owing to the Boudouard reaction and methane decomposition, which are less thermodynamically favourable. However, methane decomposition remains kinetically relevant and must be addressed through catalyst design to ensure high coking resistance. The RWGS reaction displays distinct behaviour in this thermal regime. Although thermodynamically less favourable at high temperatures due to its exothermic nature, increased hydrogen availability can enhance its kinetics, potentially altering the syngas composition. Consequently, precise control over reaction parameters and catalyst formulation is essential to achieve an optimal H2/CO ratio for downstream applications [41]. Figure 5. Equilibrium constants of reactions as a function of the temperature. Reprinted from [47], with permission from Elsevier. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 43 With a stoichiometric H2/CO ratio close to 1.0, syngas produced via DRM is particularly suitable for synthesising some oxygenated chemicals such as acetic acid, dimethyl ether and oxo-alcohols [38,48,49]. Compared to alternative reforming methods, including methane partial oxidation and steam methane reforming, DRM provides a more balanced H2/CO ratio in these chemical synthesis processes [32,40]. Nonetheless, due to the influence of RWGS and other competing side reactions, the H2/CO ratio of DRM, which is ideally unity, is often slightly lower in practice. This deviation arises from hydrogen consumption in RWGS and hydrogenation reactions, necessitating precise control [41]. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 44 1.2. Reaction mechanism As shown in Figure 5, the DRM equilibrium constant increases significantly at temperatures above 700 ºC, enabling nearly complete conversions beyond this threshold. Consequently, at temperatures exceeding this value, thermodynamic limitations no longer restrict high CH4 and CO2 conversions, provided an appropriate catalyst is employed. An effective catalyst for the DRM reaction must exhibit both high activity and stability under reaction conditions. The selection of an appropriate catalyst must prioritise its ability to kinetically minimise carbon formation and suppress unwanted side reactions which are favoured at high temperature [41,50]. While variations exist depending on the catalytic system 1 , it is generally agreed that the process follows four fundamental steps: adsorption and activation of reactants, surface reactions forming intermediates and desorption of products, represented in Figure 6 [40,41,51,52]. The first step is the dissociative adsorption of methane on the active metal sites of the catalyst. This process involves breaking the C-H bonds in methane, resulting in the formation of surface-bound carbon (C*) and adsorbed hydrogen (H*). Methane activation is considered the rate-determining step of DRM [53–55] due to the high bond dissociation energy of methane. However, the total dissociation energy of the bond CHx-H depends on the hosting surface and the entire catalytic system which may be controlling the surface metal work function. Consequently, lower CHx-H bond dissociation energies are required in catalysed 1 At elevated temperatures, the reaction pathways may shift due to changes in the interaction of the surface with intermediates. This underscores a limitation of the widely used DRIFTS (Diffuse Reflectance Infrared Fourier Transform Spectroscopy) technique, as its sample chambers are typically restricted to a maximum of 600 ºC, preventing a complete characterisation of the reaction mechanism under industrially relevant conditions. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 45 decomposition. Some studies suggest that methane activation primarily occurs on the metal surface, while others indicate that, for certain catalysts, activation may take place at interfacial sites where metal-support interactions play a critical role [41,56– 58]. Simultaneously, CO2 adsorption occurs, typically on oxygen vacancy sites or basic supports. Once adsorbed, CO2* dissociates into surface oxygen (O*) and carbon monoxide (CO*). The oxygen species generated in this step play a crucial role in the oxidation of carbon deposits formed from methane decomposition, thereby mitigating coke formation, a major challenge in DRM. The dissociation and reduction of CO2* are known to be structure-sensitive, being promoted at defect sites such as corner atoms. CO2 adsorption on metal oxides, commonly used as supports in DRM catalysts, is an essential step in the process, as it allows the interaction with active sites and facilitates the redox cycle [50,59,60]. Figure 6. Reaction steps for DRM. (a) Adsorption of CH4 and CO2, (b) Desorption of CO and H2, (c) Formation of surface hydroxyls from hydrogen and oxygen spillover and (d) Surface hydroxyls and oxygen species oxidising CHx species to form CHxO species. Reprinted from [50], with permission from De Gruyter Brill. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 46 After the reactants are adsorbed, surface reactions occur to form intermediates and products. Hydrogen spillover from the metal surface to the support is predicted by most kinetic models [41], allowing hydrogen to react with oxygen species and form hydroxyl groups. Oxygen spillover from the support to the metal may also be expected. Nevertheless, at temperatures above 800 ºC, the presence of hydroxyl groups on the support is unlikely [61]. The adsorbed carbon atoms (C*) react with surface oxygen (O*) to form adsorbed carbon monoxide (C* + O* → CO*), while adsorbed hydrogen atoms (H*) recombine to produce molecular adsorbed hydrogen (2H* → H2*). The interplay between these reactions determines the overall efficiency of DRM and the H2/CO ratio in the produced syngas. A well-balanced reaction pathway ensures optimal syngas yield while minimising unwanted side reactions and excessive carbon deposition. The final step of the mechanism involves the desorption of hydrogen and carbon monoxide from the catalyst surface. Desorption is relatively fast and depends on the ability of the catalyst to efficiently release the products. Effective desorption prevents product accumulation on active sites, ensuring sustained catalytic activity and high turnover rates [50]. A study by Aldana et al. [62] found that hydrogen dissociates on Ni0 sites while carbon dioxide is activated on the ceria-zirconia support, forming carbonates that can be hydrogenated into formate and further into methoxy species. The superior performance of Ni/CeZr catalyst compared to Ni-silica catalyst was attributed to the presence of weak basic sites on the support, which enhanced CO2 adsorption, and to the stable metal-support interactions that contribute to catalyst stability. The study further highlighted that competitive CO-H2 adsorption on Ni0 sites, which is more Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 47 prevalent in Ni-silica, could lead to deactivation due to carbonyl accumulation and metal sintering. In contrast, Ni/CeZr maintained higher activity and stability due to its metal-support interactions and the distinct reaction pathway involving carbonate and formate species. Other study by Pan et al. [63] also supported this theory. Their findings revealed that the superior activity of Ni/Ce0.5Zr0.5O2 was attributed to the presence of medium basic sites, which facilitated the formation and rapid hydrogenation of monodentate carbonate species. In contrast, Ni/γ-Al2O3 predominantly exhibits strong basic sites, leading to the formation of less reactive bidentate formate intermediates. These results highlighted the crucial role of medium basic sites in promoting CO2 activation and accelerating methanation, providing further insights into the influence of catalyst support properties on reaction pathways and overall performance. Another study by Wang et al. [52] also confirmed that Ni0 is the active phase in DRM, proposing the schematic diagram of the whole reaction network shown in Figure 7 with all the elementary steps herein mentioned [52]. Figure 7. Schematic diagram of the whole reaction network of dry reforming. Reprinted from [52], with permission from Elsevier. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 54 structural separation, where different components fail to form a uniform material, reducing their effectiveness. Some combinations help balance acidity and basicity and improve stability but can weaken metal-support interactions, limiting metal dispersion and increasing sintering risk. Additionally, excessive basicity can cause overadsorption of CO2, leading to unwanted carbon accumulation instead of preventing it. While the intrinsic properties of the support are crucial in determining catalytic behaviour, additional modifications can further enhance performance. The incorporation of promoters plays a key role in optimising catalysts for DRM. Surfaceinduced promoters modify the physicochemical properties of the catalyst by influencing acidity, basicity, metal-support interactions and redox properties. Promoters are used in small amounts, usually from 0.01 to 10 wt.% [51,85]. In DRM, promoters can be classified into three main groups: alkali and alkaline-earth metals (e.g., Li, K, Mg, Na, Ba, Ca), rare-earth metals (e.g., Ce, Pr, La, Sm) and other elements (e.g., Zr, Au, Ag, Bi, As, Tb, Gd, Cu, Sn, V, B, Mn, Mo) [74]. Alkali and alkaline earth metals are widely used as promoters due to their ability to increase the basicity of the support [86]. The presence of alkali metals also strengthens electron transfer mechanisms, stabilising CO2-derived intermediates and enhancing overall catalytic efficiency [51,86,87]. Alkali adatoms enhance CO2 adsorption by increasing its binding energy, promoting the formation of CO2radicals, CO3* and O* species, thereby improving catalyst reactivity and stability [41]. The literature contains numerous reports on alkali or alkaline earth metal-promoted catalysts, as summarised by Hu and Ruckenstein [55]. For instance, Alipour et al. [88] has reported that adding small amounts of Ba and Ca over Ni/γ-Al2O3 enhances coke resistance due to Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 55 interaction between the active phase and the support, leading to changes of the basic properties of the catalyst and the adsorption of CO2. A study by Dias and Assaf [89] found that CaO as a promoter in Ni/γ-Al2O3 catalysts influences sintering resistance, metal-support interactions and reducibility. Low Ca concentrations enhanced CO2 adsorption and CH4 conversion, while higher Ca levels increased Ni electron density, reducing both conversions. Excessive Ca also led to pore blocking and surface area loss, negatively affecting performance. Another study by Owgi et al. [90] examined the effect of Ce, Sr, Cs and Sm as promoters on Ni/FSA catalysts. Cs-Ni/FSA showed the highest stability over 50 h, with minimal coke formation due to enhanced Ni reducibility and carbon oxidation. Promoters modified the catalyst structure, reducing surface area but improving performance. Cs was identified as the most effective promoter for improving catalyst stability and coke resistance, highlighting its potential for optimising Ni-based DRM catalysts. Nisa et al. [91] investigated the effect of base promoters (Mg, Ca, Na and K) on Ni/MCM-41. The results showed that Mg and Ca were the best promoters, reducing carbon deposition. Mg-Ni/MCM-41 exhibited the highest CH4 and CO2 conversions due to its stronger metal-support interactions and higher basicity. Na and K promoted severe sintering, leading to structural collapse and lower catalytic performance. In an interesting study by Azancot et al [92], the role of K as a promoter in a aluminium magnesium spinel (MgAl2O4) was studied. The findings revealed the formation of a Ni-O-K phase, where K modified the electronic and structural properties of Ni, stabilising it in a +3 oxidation state. This phase inhibited CO dissociation and suppressed carbon formation. Potassium also improved Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 56 metal-support interactions, reducing sintering and maintaining high activity over extended operation. As mentioned, rare-earth metal oxides have been extensively studied as effective promoters due to their high oxygen storage capacity and redox properties. Noble metals, such as Ru, Pt, Pd and Rh, have also been explored limitedly as promoters due to their high cost [41,93]. The study by Ferrandon et al. [94] applied high-throughput experimentation to systematically evaluate 95 Ni-based catalysts modified with various promoters (B, Mg, Cu, Fe, Mn, Co, Sn, Zn and V) on different oxide supports (Al2O3, SiO2, TiO2, CeO2 and TiO2-MgO) for DRM. The promoters were introduced via an organometallic grafting technique, optimising Ni dispersion, reducibility and interaction with the support. The results demonstrated that the type of oxide support had a greater influence on catalytic activity and stability than the choice of promoter. Fe was identified as the most effective promoter for Ni catalysts supported on Al2O3 and MgO, while Cu improved performance on SiO2 at higher temperatures, and Mn enhanced conversion on TiO2 at 800 ºC. B and Ce were effective in suppressing carbon deposition, while Sn promoted Ni dispersion and prevented sintering. The best-performing catalysts, including NiFe/Al2O3 and NiFe/MgO, achieved methane conversions above 95% at 800 ºC and remained stable for over 25 h. Coke analysis revealed that amorphous carbon was primarily responsible for deactivation, while filamentous carbon did not significantly impact catalyst stability. To further mitigate deactivation and enhance catalyst performance, the role of oxygen vacancies (OVs) and oxygen storage capacity (OSC) becomes critical. OVs are intrinsic anionic defects in metal oxides that can be manipulated through synthesis Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 57 conditions. These vacancies act as active sites for CO2 adsorption and activation, facilitating C-O bond cleavage and enhancing the overall reactivity of the catalyst [40,70]. The ability of the support to store and release oxygen under varying conditions is fundamental to its role in DRM. Under oxidising conditions, metal oxides retain oxygen, while under reducing conditions, they release it, promoting the formation of OVs and enabling dynamic oxygen exchange [51,70]. The presence of mobile oxygen species on the catalyst surface is essential for in situ carbon oxidation, effectively preventing coke accumulation. These oxygen species react with surface carbon to form CO and intermediate carbonate species, thereby gasifying deposited carbon. A high concentration of redox-active oxygen intensifies surface carbon gasification, improving catalyst stability and longevity [74]. Wang and Ruckenstein [95] studied the effect of reducible and irreducible oxide supports on Rh-based. Irreducible supports (γ-Al2O3, MgO, La2O3) showed higher stability due to strong metal-support interactions, while SiO2 and Y2O3 deactivated due to Rh sintering. Reducible supports (CeO2, ZrO2) exhibited prolonged activation periods due to dynamic oxygen exchange but did not ensure long-term stability. MgO and La2O3 formed stable Rh-based compounds, improving durability. Supports like CeO2 and ZrO2, despite their high OSC, exhibited prolonged activation periods and inconsistent long-term stability, while Ta2O5 and TiO2 suffered from significant deactivation. In contrast, irreducible oxides such as MgO and La2O3 provided superior stability by forming stable Rh-based compounds (MgRh2O4 and LaRhO3), which helped anchor Rh particles, reducing sintering and maintaining catalytic activity over extended operation. Differently, Ibrahim et al. [96] investigated the effect of different zirconia Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 58 sources. The findings showed that variations in ZrO2 sources affected catalyst properties, including surface area, reducibility and resistance to carbon deposition. The study highlighted that oxygen vacancies in ZrO2 play a crucial role in CO2 activation by providing active sites for CO₂ adsorption and dissociation. These vacancies facilitate the cleavage of the C-O bond, promoting the formation of reactive oxygen species that contribute to carbon oxidation. The ability of ZrO2 to create and replenish oxygen vacancies under reaction conditions enhances its effectiveness in carbon removal, reducing catalyst deactivation. On the other hand, ceria and CeO2-based catalysts have been widely studied due to their exceptional OSC, strong reducibility and ability to generate mobile oxygen pools. CeO2 facilitates CO2 activation by promoting oxygen vacancy formation, allowing for efficient oxygen migration from the bulk lattice to the surface, where it participates in carbon oxidation. The redox cycling between Ce4+ and Ce3+ enhances oxygen mobility, enabling continuous removal of deposited carbon and reducing coke formation [40]. Interestingly, Gao et al [97] found that hierarchically structured CeO2 significantly enhanced catalyst performance by increasing OV concentration and metal-support interaction. XPS analysis confirmed that Ce3+ and Ce4+ species coexisted, forming a reversible redox cycle where lattice oxygen dissipated, generating intrinsic OVs. Among the tested structures, Ni/CeO2-D (dumbbell-like morphology) displayed the highest OV concentration and best DRM performance, indicating that morphology tuning can enhance redox properties and CO2 conversion. Da Fonseca et al. [98] studied the impact of Ni crystallite size on carbon deposition and OV availability. Increasing calcination temperature led to larger Ni crystallite Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 59 sizes and lower OV concentrations, negatively affecting carbon removal. When Ni particles were large, poor metal-support contact reduced the influence of CeO2 OVs in carbon oxidation, increasing deactivation risk. This suggests that optimising both Ni dispersion and OV density is crucial to mitigating coke formation and enhancing catalyst durability. As mentioned in previous studies, optimising OSC enhances carbon oxidation, but its effectiveness depends on metal dispersion and particle size, which influence active site availability and stability. Higher dispersion results in more active sites being available for the reaction, thereby improving catalytic efficiency [32,74]. The impact of support materials and preparation methods is crucial in controlling metal dispersion and particle size. Supports with high surface areas, such as mesoporous alumina, silica or ceria-zirconia composites, provide better anchoring for Ni particles, preventing excessive growth and sintering [70]. The size of active metal particles also influences catalytic activity, carbon deposition and resistance to deactivation. Smaller particles exhibit higher metal dispersion, providing more reactive sites for CH4 and CO2 activation. However, their stability at high temperatures is a challenge due to sintering, which leads to particle growth, loss of active sites and reduced catalyst efficiency [74]. Ni-based catalysts, when Ni particles are too large, usually promote methane decomposition, leading to excessive carbon deposition [32]. Conversely, extremely small Ni particles can lead to rapid sintering, reducing long-term stability [64]. It has been observed that Ni particle sizes below a certain threshold (typically < 6 nm) demonstrate superior resistance to carbon formation. This is attributed to their lower tendency to catalyse methane decomposition, a key source of carbon deposition Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 60 [64,70]. One study by Zhang et al. [99] investigated Ni-β-zeolites, demonstrating that strengthening Ni-Si interactions via solid-state grinding and controlled calcination significantly suppresses sintering and coking. By tuning metal-support interactions, catalysts exhibited improved stability over 100 h of operation. The study found that well-dispersed Ni nanoparticles (NPs), especially those anchored within the zeolite framework, maintained smaller sizes and better resistance to deactivation. These findings underscore the importance of precise synthesis control in enhancing DRM efficiency. Another study by Osojnik Crnivec et al. [100] explored the synthesis of Ni-Co bimetallic catalysts supported on ceria-zirconia using two solvothermal approaches: hydrothermal and glycothermal methods. It was found that glycothermal synthesis resulted in smaller active metal particles (~ 6 nm), contributing to greater resistance against carbon accumulation and improved dispersion. In contrast, hydrothermally prepared supports produced larger crystallites (30-40 nm), which were more prone to carbon deposition. A separate investigation by Rahemi et al. [101] focused on Ni/Al2O3-ZrO2 nanocatalysts synthesised via impregnation and treated with non-thermal plasma. Plasma treatment improved the dispersion of Ni particles, leading to a narrower particle size distribution with smaller Ni NPs compared to conventionally prepared catalysts. The strong metal-support interactions observed in plasma-treated catalysts reduced sintering, lowered activation temperatures and significantly enhanced long-term catalytic stability. The research concluded that smaller Ni particles (~ 5-10 nm) exhibited superior activity, while larger particles were prone to deactivation by coke formation. Another study by Shen et al. [102] examined the use of mesoporous SBA-15 as a support for Ni-based catalysts, focusing Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 61 on controlling Ni nanoparticle size and dispersion. A solid-state impregnation (SSI) method was employed to confine ultra-small Ni NPs (~ 3-4 nm) within the mesopores of SBA-15, preventing particle aggregation and enhancing resistance to sintering and coking. Compared to conventional wetness impregnation, which resulted in larger NiO particles, the SSI-prepared catalysts exhibited smaller, more uniformly dispersed Ni particles, leading to higher catalytic activity and improved stability over extended DRM operation. Overall, the metal-support interaction (MSI) and the catalyst preparation method are crucial. The strength of this interaction can significantly influence the dispersion of active metal particles, their sintering resistance and the extent of carbon deposition. Metal-support interactions are generally categorised into weak metal-support interactions (WMSI) and strong metal-support interactions (SMSI) [103]. The latter is particularly beneficial in DRM, as it enhances metal dispersion, stabilises smaller metal particles and minimises carbon deposition [64,93]. SMSI can manifest through electronic effects, where charge transfer modifies metal reactivity, geometric effects, where the support partially or fully encapsulates the metal particles to prevent sintering and bifunctional effects, where dual sites facilitate CH4 and CO2 activation while preventing coke formation [64]. The synthesis method significantly influences MSI strength and catalyst performance. Techniques such as impregnation, coprecipitation, sol-gel and atomic layer deposition (ALD) affect metal dispersion, particle size and MSI strength [32,38,70,73] Interestingly, one study by Phichairatanaphong and Donphai [104] investigated the impact of the Ce/Zr ratio and surface chemistry of Ni/CeO2-ZrO2. Using template-assisted co-precipitation under Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 62 controlled pH conditions (5-10), the synthesised catalysts were evaluated at 700 ºC. The findings indicated that an optimal Ce/Zr ratio (Ni/CeZr(8.5)) enhances oxygen vacancy density, improving conversions. The strong MSI, high Ni dispersion and enhanced oxygen mobility contributed to superior catalytic stability. These results underscore the role of Ce-Zr tuning in optimising redox and structural properties. Structured approaches, including confinement strategies, sandwiched catalysts, coreshell and core-yolk particles, tubular, hydrotalcite and perovskites, fluorites, pyrochlore-based materials, also enhance SMSI by controlling metal placement within supports, leading to improved sintering resistance and catalyst durability. Some literature reviews are referred about the most recent advances [51,67,105–107]. An emerging area of interest in DRM catalysis involves the use of perovskite and pyrochlore-type structures as advanced materials. Perovskites and perovskite-type materials are a class of crystalline oxides named for their resemblance to the mineral perovskite, exhibiting the general formula ABO3 or A2BO4 [70,108,109]. In these structures, the A-site is typically occupied by a larger cation, such as rare-earth elements or alkaline earth metals, which fit into the dodecahedral coordination sites, while the B-site is occupied by a smaller transition metal cation (3d, 4d or 5d), which is located in octahedral sites [110]. The stability of the perovskite structure is determined by the relationship between the ionic radii of the A, B and oxygen ions, expressed through the tolerance factor (t) equation (Equation 16) [111]: t = rA+ rO √2 (rB+ rO) Equation 16 Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 63 where rA, rB and rO represent the ionic radii of the A-site cation, B-site cation and oxygen anion, respectively. The perovskite structure is ideal when t is close to 1, leading to a cubic symmetry, though distortions to tetragonal, orthorhombic or rhombohedral structures occur when t deviates significantly [112]. Perovskites are highly versatile due to their tunable composition and structure. The substitution of different cations at Aand B-sites allows the modulation of physicochemical properties such as oxygen mobility, redox capacity and thermal stability. These characteristics are particularly valuable in catalytic applications, where perovskites act as precursors to highly dispersed active metal nanoparticles upon reduction. In these cases, the B-site cation, which governs catalytic activity, is reduced to form small, well-dispersed metallic particles, while the A-site cation serves as a structural stabiliser. Ni-containing perovskites are particularly useful as they can release Ni metal upon reduction, forming active sites for methane reforming reactions [112,113]. Perovskite oxides typically require high calcination temperatures during synthesis, leading to low textural properties and specific surface areas generally below 10 m2 g-1 [114]. However, their inherent thermal stability makes them attractive for high-temperature catalytic processes. Activation via reduction or exposure to reactive atmospheres induces structural changes, leading to improved dispersion of metal species and the generation of oxygen vacancies, enhancing catalytic performance [112,113]. Shiozaki et al. [115] and Hayakawa et al.[116] described the formation of small, well-dispersed metallic Ni particles from a well-defined crystalline perovskite structure. The solid-phase crystallisation method was used to synthesise Ni/BaTiO3 and Ni/Ca0.8Sr0.2TiO3 catalysts, ensuring high Ni dispersion and SMSI. These catalysts Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 70 In light of these challenges, this chapter aims to address the limitations of existing catalysts by developing an optimised system incorporating nickel-substituted ceriumzirconium mixed oxides. The selection of cerium and zirconium as the backbone for the catalyst structure was motivated by their ability to enhance oxygen mobility and thermal resilience. Cerium, through its Ce4+/Ce3+ redox cycle, provides exceptional oxygen storage and release capabilities, mitigating carbon deposition, while zirconium improves structural integrity, resists sintering and strengthens metalsupport interactions. However, even Ce-Zr-based materials have shown limitations in maintaining long-term redox performance and stability. Trying to overcome these issues, nickel was incorporated into the lattice through the synthesis method, leveraging the close ionic radii of Ni2+ (0.69 Å) and Zr4+ (0.72 Å) to achieve effective substitution while preserving structural integrity. This approach is intended to minimise free Ni particles that are prone to sintering and carbon accumulation. To further enhance catalyst performance, the modified Pechini method was selected as the preferred synthesis route. This sol-gel approach enables controlled metal dispersion at the molecular level, ensuring homogeneous active site distribution and improving stability. However, acknowledging the challenges in industrial scalability, this study also considers how synthesis parameters influence large-scale applicability. Additionally, this chapter evaluates the impact of nickel loading and calcination temperature, incorporating considerations to optimise performance. By addressing both material limitations and practical challenges in DRM implementation, this research contributes to the development of more robust and industrially viable catalysts for sustainable syngas production. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 71 2. Catalyst synthesis The mixed oxide structure of cerium and zirconium with nickel was prepared through a modified version of the Pechini method, a type of sol-gel method [134], described elsewhere [135–137], as represented in Figure 9. Figure 9. Schematics of the synthesis method of the Ni-based cerium zirconate structure using a modified citrate synthesis method. As precursors, zirconyl nitrate (ZrO(NO3)2·6H2O, 99 wt.% purity), cerium nitrate (Ce(NO3)3·6H2O, 99 wt.% purity), provided by Sigma-Aldrich, and nickel nitrate (Ni(NO3)2·6H2O, 98 wt.% purity), provided by Alfa Aesar, were used. Each precursor was dissolved separately in sufficient deionised water and then mixed together. For this synthesis method, nickel is substituting stoichiometrically zirconium atoms in the formula, modifying the mixed oxide formula to Ce2Zr2-xNixO7-δ. This substitution is more likely to occur because the radius of Ni2+ (0.69 Å) is similar to that of Zr4+ (0.72 Å), enabling a seamless integration without causing significant structural distortion [138]. Then, citric acid (CA) (≥ 99 wt.% purity), provided by Sigma-Aldrich, was dissolved in deionised water and added to the mixture of precursors in order to help chelate the metal ions, forming a stable, homogeneous solution. The quantity of CA added was calculated to obtain a molar ratio of CA:metal of 1.2:1, referring to metal Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 72 as the sum of cerium, zirconium and nickel [139,140]. The solution was then heated while stirring to 90-100 ºC to ensure metal complexation, and ethylene glycol (EG) (99 wt.% purity), provided by PanReac AppliChem, was incorporated to the solution drop-wise, using a molar ratio of EG:CA 1:1. EG acts as a cross-linking agent. The solution was stirred and concentrated, evaporating the water, until a viscous, resinlike mixture was formed. The stirring was then stopped and the dense gel was left at 100 ºC to produce the polyesterification reaction between CA and EG. The decomposition of the nitrate precursors led to plumes of NOx. Once the toxic gas released, the resulting solid was dried at 120 ºC overnight. The resulting compound was powdered in an agate mortar and then calcined in a crucible for 8 h at 1000 ºC, using a heating rate of 7.5 ºC min-1. The material was calcined in a furnace with air and a gas outlet, ensuring the removal of combustion by-products during the heat treatment process. The combustion process removed the organic components, leaving behind a fine oxide powder. The catalysts were finally sieved using the 100-200 µm fraction. Ni-incorporated cerium zirconate mixed oxide catalysts with stoichiometric formula Ce2Zr2-xNixO7-δ were engineered using different nickel loadings, where x = {0, 0.5, 1, 1.25, 1.5} represents the degree of nickel substitution at zirconium sites in the structure. These values correspond to nominal nickel contents of 0, 5, 10, 12.5 and 15 wt.%, respectively, ensuring a direct stoichiometric substitution in the B-site of the Ce2Zr2-xNixO7-δ lattice. This dual notation allows for a clear correlation between the molar substitution parameter (x) and the actual Ni weight percentage used in the formulations. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 73 For instance, for the substitution of zirconium with a molar ratio of x = 0.5, to create 5 g of theoretical calcined catalysts Ce2Zr1.5Ni0.5O7-δ, the precise amounts of precursors required were: • ZrO(NO3)2·6H2O = 4.60 g. • Ce(NO3)3·6H2O = 7.86 g. • Ni(NO3)2·6H2O = 1.33 g. In this example, a total of 9.12 g of CA and 2.70 g of EG were used. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 74 3. Effect of nickel loading on Ni-based cerium zirconate The selection of catalyst loading, particularly in terms of metal content, is critical for optimising catalytic performance, cost-effectiveness and stability in DRM. In catalytic systems, especially those involving supported nickel catalysts, choosing the appropriate metal loading significantly influences activity, selectivity, resistance to deactivation and overall economic feasibility [141,142]. Typically, metal loadings from 1 to 15 wt.% are chosen in research due to their optimal balance between high dispersion, good activity, resistance to deactivation and cost-effectiveness. Many commercial catalysts present nickel contents ranging from 5 up to 50-60 wt.% [143– 149]. Increasing the nickel content may lead to practical and economic drawbacks, including poor dispersion and higher deactivation rates. Following previous studies [130,131], this section focuses on the synthesis and characterisation of Ni-substituted cerium zirconate mixed oxide catalysts. Specifically, 0, 5, 10, 12.5 and 15 wt.% nickel content are chosen. Catalysts were henceforth referred to as CZ, CZN5, CZN10, CZN12 and CZN15, respectively. These variations allowed the investigation of how different amounts of nickel affected the catalytic properties and performance. The series of catalysts thus prepared were systematically characterised to study the impact of nickel substitution on factors such as reducibility, surface area, crystallinity and catalytic activity. These evaluations provided insights into the effects of varying nickel loadings on the overall performance and properties of the cerium zirconate mixed oxide catalysts. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 75 3.1. Catalysts characterisation The thermogravimetric analysis (TGA) results for the CZN10 powder precursor provided valuable insights into the decomposition and calcination process of the catalyst. This powder precursor was obtained after polyesterification reaction and subsequent drying at 120 ºC. Conducted both in air and in an inert atmosphere, the TGA experiments highlighted the importance of oxygen in achieving complete combustion and obtaining the final mixed oxide catalyst structure. In Figure 10.a, the weight loss against temperature is shown, whereas Figure 10.b displays the mass spectrometric signals corresponding to mass-to-charge (m/z) ratios of 18 (H2O), 30 (NOx), 32 (O2) and 44 (CO2), under an oxidising atmosphere. The displayed weight loss profiles revealed multiple stages of decomposition. The first significant weight loss observed below 170 ºC, accounting for approximately 13-16% of the total weight, was attributed to the desorption of moisture and physically adsorbed water from the catalyst precursor. Following the dehydration phase, a gradual weight loss was observed starting at 170 ºC. This is linked to the onset of the combustion process, indicated by the mass spectrometric data (Figure 10.b), which showed small signals for CO2 (m/z = 44) and NOx (m/z = 30), along with a corresponding release of H2O (m/z = 18). This early combustion stage reflected the decomposition of organic components, such as citric acid and ethylene glycol, that were used during the synthesis process to chelate the metal ions and form a homogeneous precursor gel. This stage continued until about 300 ºC, where the most significant combustion event took place. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 76 Figure 10. TGA of the pre-calcined CZN10 powder. a) Weight against temperature and b) Mass signal of the decomposition under oxidising environment for m/z: 18, 30, 32 and 44. At approximately 300 ºC, the main combustion event occurred, characterised by a sharp drop in the TGA curves under air. The mass spectrometer signal showed a clear increase in CO2 and H2O production, while the O2 signal decreased, confirming a Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 77 strong oxidative combustion reaction. This was the key step where the majority of the organic components were burned off, leaving behind the oxide structure of the catalyst. Interestingly, the NOx signal did not significantly increase during this stage, suggesting that the bulk of nitrogen-containing compounds were decomposed in the earlier and later combustion steps. This major combustion event under air was crucial to the formation of the desired metal oxide phases, as indicated by the rapid weight loss and the pronounced gas evolution. After that, no relevant weight loss was observed. The comparison of the TGA data under air and inert conditions revealed that the combustion process was more efficient and complete in the presence of oxygen. Under inert conditions, the decomposition occurred more slowly, and the sharp combustion step observed at 300 ºC in air was much less pronounced. This suggested that oxygen facilitated the breakdown of the organic matrix and accelerated the removal of volatiles. Additionally, under inert conditions, a noticeable weight loss was observed above 600 ºC, which was not present in the air calcination. This weight loss was likely associated with structural changes and reduction processes in the catalyst, potentially indicating a transition from higher to lower oxidation states. The presence of oxygen was found to be necessary in order to obtain the final calcined catalyst; therefore, the catalysts were calcined in a furnace chamber with a sufficient oxygen supply. This ensured complete combustion of the organic template and the formation of the desired mixed oxide structure. The chemical composition of nickel and the textural properties of the prepared samples are listed in Table 1. The metal loading of the catalysts closely matched the Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 78 nominal values of 5, 10, 12.5, and 15 wt.% of Ni, demonstrating the effectiveness of the preparation method in precisely achieving the desired active phase loading. However, for the high-nickel-containing samples (CZN12 and CZN15), the actual nickel contents were slightly lower than the intended values. This discrepancy may suggest that there is a threshold for optimal nickel uptake in the synthesis process. Table 1. Chemical composition and textural properties of the calcined catalysts. Sample NiNOM (wt.%) NiICP-OES (wt.%) SBET (m2 g-1) Pore volume (cm3 g-1) x 10-3 CZ 0 0 4 9.2 CZN5 5.0 5.0 4 8.8 CZN10 10.0 9.5 3 7.7 CZN12 12.5 11.6 2 3.0 CZN15 15.0 13.7 2 1.6 Nitrogen adsorption-desorption isotherms were first obtained and analysed to determine the textural properties of the prepared catalysts. Regarding the textural properties of the samples, we observed the noteworthy low surface area of all of the synthesised materials in contrast to benchmark supported catalysts, which typically exhibit much higher surface areas [32,38,51,76]. This finding is essential to consider, as the engineered materials presented here are still regarded as active for the DRM, despite their low surface area. It is important to note that the DRM reaction does not necessarily require high-surface area catalysts to achieve optimal performance. This may suggest that structural factors, rather than extensive surface area, play a key role in catalytic performance. Moreover, a slight tendency of reduction in surface area was Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 79 observed as nickel loading increased, indicating a potential trade-off between nickel content and textural characteristics. This decrease in surface area was accompanied by a more pronounced reduction in pore volume, which suggests that nickel particles may be blocking some of the catalyst pores. This blockage can impede reactant access to active sites, potentially affecting catalytic activity. In addition, nitrogen adsorptiondesorption isotherms are illustrated in Figure 11. All the synthesised catalysts exhibited typical type IV isotherms, which are characteristic of mesoporous materials, with an H1 hysteresis loop. The presence of a H1 hysteresis loop within the isotherm indicated the existence of mesopores, with diameters ranging between 2 and 50 nm. This structural feature is particularly significant, as it suggests that the catalysts possess a well-defined porosity which can effectively facilitate the diffusion of reactants during catalytic reactions due to the relatively large pore diameter. Such a mesoporous structure is advantageous in catalytic applications, as it allows for improved accessibility of the reactants to the active sites within the catalyst, despite the low surface area [150]. However, it was noteworthy that the relatively small hysteresis loop observed for the CZN15 sample may indicate a less-developed mesoporous structure. This observation aligned with the previously noted threshold for optimal nickel uptake measured by ICP-OES, where the actual nickel content in the CZN15 sample was lower than expected. This correlation suggests that higher nickel loading can adversely affect the development of the mesoporous characteristics within the catalyst. The variations in nickel loading across the different catalyst samples appear to play a critical role in influencing their mesoporous structures. Notably, the CZN10 sample demonstrated the most favourable balance between Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 86 in crystallite size, as larger crystals produce narrower diffraction peaks according to Scherrer’s equation. On the other hand, it was observed that zirconium was not completely incorporated into the inorganic complex structure in CZN5 and CZN10, appearing a diffraction line around 30º 2θ which may correspond to ZrO2 tetragonal structure with space group P42/nmc (ICDD Card No. 00-024-1164). This is possibly due to the competitive interaction between Ni and Zr during the synthesis. This phase could arise from excess zirconium segregating out of the Ce-Zr lattice as nickel was incorporated into the structure. From CZ to CZN10 sample, an interesting, progressive phenomenon of peak splitting was observed, where the diffraction lines began to separate into two distinct peaks, as observed in the inset of Figure 14. This indicates the formation of two notable crystalline phases: the tetragonal Ce0.5Zr0.5O2 and the cubic Ce2Zr2O7 pyrochlore. The bifurcation of these peaks pointed to a phase separation induced by nickel incorporation (Ce0.5Zr0.5-xNixO2-δ / Ce2Zr2-xNixO7-δ). The splitting of these diffraction peaks is an indicator of a mixed oxide system in which both tetragonal and cubic structures coexist. For Ce2Zr2O7 pyrochlore cubic structure, the diffraction lines at 2θ values of 28.9, 33.6, 48.1 57.1, 59.9 and 70.4º were attributed to the (222), (400), (440), (622), (444) and (800) crystal planes. However, as the nickel content increased beyond 10 wt.%, the diffraction peaks returned to a single dominant pattern, corresponding primarily to the cubic Ce2Zr2O7 pyrochlore phase. This suggests that higher nickel loading stabilised this cubic phase, suppressing the tetragonal structure observed at lower nickel content. Additionally, the increased crystallinity seen with Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 87 higher nickel loading likely contributed to the disappearance of the phase splitting phenomenon seen at lower nickel contents. Considering the radius ratio rA/rB, which is relevant factor for A2B2O7-δ inorganic complex structures [120,129], the substitution of Ni on B-site within the structure produced a decrease in this parameter since Ni radius is smaller than that of Zr, which was associated with a rearrangement of the structure. The reduction in the lattice parameter further supports the partial incorporation of nickel into the crystal lattice, as the smaller ionic radius of Ni2+, compared to Zr4+, likely caused this lattice contraction. When the loading of Ni was further increased, the peak splitting phenomenon vanished, appearing mainly one diffraction pattern corresponding again to Ce2Zr2O7 cubic structure. Therefore, it was proved that the incorporation of Ni produced variations in the inorganic oxides formed in this material. XRD also indicated that part of the nickel was incorporated into additional crystalline phases. Peaks at 33.5, 48.1 and 59.8º were attributed to the formation of a cubic perovskite CeNiO3 phase with space group Pm3 m (Material Project Card No mp866095). The presence of this phase further supports the idea that nickel is integrated into the crystal lattice, modifying the overall structure. Moreover, small amounts of NiO were also detected in all Ni-containing samples, as evidenced by diffraction peaks at 37.3, 43.3, 62.9 and 75.4º, corresponding to rhombohedral NiO (ICDD Card No. 00-022-1189). In general, the intensity of the diffraction line attributed to NiO species remained the same as the Ni content increased. The appearance of the secondary phase CeNiO3 may be indicative of the saturation point of nickel solubility within the cerium-zirconium lattice. As nickel content increased, it became more Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 88 difficult to maintain a single solid solution and, as a result, nickel tended to segregate, forming nickel oxide species and CeNiO3. This segregation of phases can be associated with a loss of structural homogeneity. Interestingly, the most intense NiO diffraction peak (Figure 15) at 43.3º shifted slightly towards higher angles as the nickel content was increased, reaching 43.5º in the CZN15 sample. This shift is indicative of the formation of non-stoichiometric NiO1-y species, where y increases as nickel content rises in the catalysts. Indeed, the ratio Ni/O was closer to orthorhombic Ni4O3 (mp-656887) than NiO in CZN15. Figure 15. Stacked XRD pattern of the calcined catalysts zooming most intense peak of NiO. In conclusion, the XRD results highlighted several important trends. The introduction of nickel into the cerium-zirconium oxide structure not only caused a shift in the Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 89 diffraction peaks but also significantly enhanced the crystallinity of the material. The partial incorporation of nickel into the lattice, coupled with the emergence of additional phases such as CeNiO3 and NiO, further supports the notion that nickel plays a critical role in reshaping the crystalline framework of the catalyst. These structural changes are expected to influence the catalytic performance of these materials. Raman spectroscopy was performed to gain further structural insights of the calcined catalysts, as depicted in Figure 16. The analysis focused on the spectral region ranging from 150 to 700 cm-1, as this range contains crucial vibrational information relevant to the cerium zirconate mixed oxide structure. Figure 16. Raman pattern of calcined catalysts from 150 to 700 cm-1. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 90 The catalysts exhibited three or four distinct peaks, corresponding to the cerium zirconate mixed oxide structure observed. The most prominent peak, located at 477 cm-1, was attributed to the symmetric stretching vibrational mode of Ce-O8, which is characteristic of the cubic fluorite structure of Ce4+. This peak corresponds specifically to the F2g mode, associated with the symmetric vibrations of oxygen atoms surrounding cerium cations in the fluorite lattice [154,155]. The persistence of this peak in all the samples, regardless of the nickel content, indicated that the fluorite structure was largely preserved across the entire range of nickel-incorporated catalysts. However, a gradual decrease in the intensity of this band was observed as the nickel content increased from CZN5 to CZN15, implying that nickel incorporation into the cerium zirconate structure introduced some degree of distortion. This decrease in intensity likely resulted from oxygen vacancies or defects created by the partial substitution of nickel for zirconium in the crystal lattice, leading to changes in the vibrational properties of the oxygen atoms. In addition to the primary fluorite-related band, a peak around 305 cm-1 was observed, corresponding to the B1g O-Zr-O bending vibrations, and another band around 600-620 cm-1 was attributed to the B3g stretching vibration mode of Zr-O in the cerium zirconate structure. The intensities of these bands decreased progressively as the nickel loading increased and the Zr loading diminished. This reduction in intensity suggests that the partial substitution of nickel at the B-site, where Zr4+ is located, disrupted the vibrational modes associated with the Zr-O bonds. Upon increasing nickel content, the presence of stoichiometric zirconium decreased in the lattice, which weakened these vibrational signals. This observation aligned with the XRD analysis, which indicated that nickel doping affects Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 91 the crystallographic environment around the zirconium atoms, leading to changes in the local bonding environment. Moreover, a peak near 250 cm-1 was detected, which is associated with the vibrational modes of tetragonal ZrO2 [155,156]. This signal was particularly intense in the CZN5 sample, providing further confirmation of the XRD results, which suggested the presence of a tetragonal ZrO2 phase in the nickelincorporated catalysts. The emergence of this peak pointed to the formation of small amounts of the tetragonal ZrO2 phase in certain catalysts, especially at lower nickel loadings. As the nickel content increased, this peak diminished in intensity, suggesting that higher nickel loading suppressed the formation of the tetragonal ZrO2 phase, shifting the structure more toward the cubic phase and reinforcing the formation of Ce2Zr2O7. The Raman analysis complements the calcined XRD findings, reinforcing the notion that nickel doping significantly alters the cerium-zirconium oxide structure by introducing defects. These changes became more pronounced with higher nickel loadings, reflecting the complex interplay between nickel incorporation, crystallinity and defect formation in these mixed oxide systems. Diffuse Reflectance (DR) UV-Visible spectroscopy provided relevant information regarding electronic properties of the prepared samples, as shown in Figure 17. By examining the absorbance curves within the range of 150 to 1100 nm, a detailed analysis of how nickel incorporation affected the electronic properties of the ceriumzirconium-based mixed oxides was conducted. The analysis was performed using barium sulphate (BaSO4) as a reference for the baseline measurements (Figure 17.a) and later using undoped cerium zirconium oxide (CZ) as a reference (Figure 17.b), allowing for a comparative study of the effect of Ni loading on the catalytic structure. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 92 In the ultraviolet (UV) region, spanning from 200 to 400 nm in Figure 17.a, a high absorbance was detected across all the calcined materials, which is characteristic of CeO2and ZrO2-type materials [157,158]. These transitions, particularly the O 2p → Ce 4f charge transfer, are typical of cerium oxide and reflect the electronic structure of the Ce-O bond. The CZ reference (undoped with nickel) displayed a sharp and intense absorbance peak around 250-300 nm [159,160]. This high absorbance is characteristic of the fluorite-like environment, where cerium ions are surrounded by oxygen atoms, facilitating these electronic transitions. Nonetheless, as nickel was incorporated into the material, there was a noticeable reduction in the intensity of this UV peak. This suggested that the incorporation of Ni2+ into the lattice influences the matrix, possibly by introducing electronic distortions. The partial substitution of nickel for zirconium in the structure altered the local coordination environment, which disrupted the regular Ce-O bonding and electronic transitions. The reduction in UV absorbance may also imply that nickel is modifying the electronic band structure, likely by creating additional defects or oxygen vacancies [161,162]. In the visible region (400-700 nm), as shown in Figure 17.b, a progressive increase in absorbance was observed as the nickel content increased from CZN5 to CZN15. This increase in visible light absorption was likely due to the presence of Ni2+ species, which typically absorb light through d-d transitions in the visible range. These transitions are characteristic of transition metal oxides like NiO or other Ni2+ structures, where the Ni²⁺ ions absorb visible light due to electronic excitations between the d-orbitals. This trend indicated that increasing the nickel content led to a more substantial contribution from NiO species to the overall absorbance. More Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 93 precisely, weak d-d bands of octahedrally coordinated Ni2+ were observed at 450 and 650 nm [163,164]. Figure 17. Absorbance spectra of calcined catalysts from 150 to 1100 nm using a reference BaSO4 (upper) and CZ (lower). Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 94 At higher nickel concentrations (Figure 17.b), especially in the CZN15 sample, there was a marked increase in absorbance at longer wavelengths in the near-infrared (NIR) region (700-1100 nm). This significant rise in NIR absorbance was likely due to the formation of NiO aggregates, which corresponds to the development of more extensive active nickel sites. The aggregation of NiO particles or their interaction with matrix support could lead to the formation of larger NiO domains, contributing to higher light absorption in the NIR region. Again, weak d-d bands of octahedrally coordinated Ni2+ were observed at 724 nm [164,165]. XRD analysis was also performed on all the samples reduced at 800 ºC for 1 h to investigate the redox behaviour of the catalysts (Figure 18). This analysis provided insight into the structural changes occurring in the catalyst series with varying nickel content (CZN5, CZN10, CZN12, CZN15) compared to the nickel-free mixed oxide (CZ). In the first place, it was observed that the cerium zirconate oxide systems did not show an apparent change in the XRD patterns after the reduction treatment. The fluorite Ce0.5Zr0.5O2 (ICDD Card No. 00-038-1436) and the Ce2Zr2O7 pyrochlore (ICDD Card No. 00-008-0221) phase remained predominant across the catalyst series, both in the calcined and reduced samples. The peaks corresponding to this phase (indicated by stars in the graph) remained in similar positions, indicating that the main structure did not undergo significant changes after reduction. This suggests that the cerium-zirconium matrix was quite stable, even under reducing conditions. Nevertheless, a very slight shift to higher angles upon reduction was noticed, as seen by comparing the inset images of Figure 14 and Figure 18. For CZ, the displacement was occurring from 29.4 to 29.6º 2θ. For CZN5 and CZN10, a similar behaviour was Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 95 observed, whereas CZN12 and CZN15 catalysts did not show this behaviour clearly. This phenomenon suggests a slight lattice contraction, possibly related to the partial reduction of Ce4+ to Ce3+ in the cerium-zirconium structure. The reduction of Ce4+ can cause a slight distortion in the lattice, contributing to the shift of the peaks towards higher angles. Figure 18. Normalised XRD diffractogram of the reduced catalysts loading series. After the reduction treatment, nickel oxide species disappeared from the diffractogram. Characteristic peaks of metallic Ni0 at 44.6 and 51.8º 2θ were observed, corresponding to the characteristic planes of (111) and (200) of cubic Fm3 m metallic Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 102 a statistical average rather than a direct measure of the real dispersion of nickel species. Consequently, while higher nickel content enhances reducibility and promotes lattice oxygen mobility, it might simultaneously lead to reduced dispersion, impacting catalytic performance. Lastly, XPS analysis of the reduced catalysts (Figure 21) provided in-depth information about the oxidation states and electronic environments of the key elements (Ni, Ce, Zr and O) within the catalyst structure. The Ni 2p3/2 region (Figure 21.a), the spectra revealed the coexistence of Ni0 and Ni2+ species in all samples. Ni0/Nitotal ratio was calculated. For instance, in CZN5 and CZN10, these values were 0.37 and 0.49, respectively. A mixture of metallic Ni0 (851-853 eV) and Ni2+ (854857 eV) was observed on the surface of the catalysts [174,175]. As the nickel content increased, the intensity of the Ni0 peak became more pronounced until CZN10, indicating a higher proportion of reduced nickel in the catalysts with higher nickel concentrations. The presence of the Ni2+ peak on the surface even on the reduced samples suggested that not all Ni2+-containing species had been fully reduced, possibly due to strong interactions [176–178] and the cerium-zirconium matrix, which inhibited its reduction. In the Ce 3d region (Figure 21.b), the spectra showed distinct peaks corresponding to Ce4+ and Ce3+. There are six peaks corresponding to the three pairs of spin-orbit doublets of related to Ce4+ and four peaks corresponding to the two doublets related to Ce3+ [179–181]. The Ce4+ species were predominant across all the surface of the samples, indicating that most cerium remained in its oxidised state. However, the presence of Ce3+ indicated a partial reduction of cerium (Ce4+ to Ce3+), which is Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 103 common in cerium-zirconium catalysts under reducing conditions [176,179]. The proportion of Ce3+ slightly increased with the nickel content, suggesting that nickel promoted the reducibility of cerium. This might be in alignment with the presence of CeNiO3 species. Therefore, nickel may be contributing to the formation of oxygen vacancies within the lattice, improving the redox capacity of cerium by facilitating oxygen mobility. This redox capacity is crucial for catalytic performance, as the Ce4+/Ce3+ cycle allows for efficient oxygen storage and release [176,182,183]. Table 3. Binding energies (eV) of Ni 2p3/2 and Ni0/Nitot ratio of XPS analysis. eV Ni0 Ni2+ Ni0/Nitot CZN5 851.7 852.9 0.37 854.8 CZN10 852.2 855.3 0.49 CZN12 852.5 854.0 0.47 856.0 CZN15 852.2 854.0 0.46 856.1 The Zr 3d spectra (Figure 21.c) showed zirconium oxide peaks across all samples, typically displayed at 182.2 and 184.5 eV [104], indicating that zirconium remained stable in its oxidised form and did not undergo reduction, despite the decrease in concentration as the nickel content increased. This stability may note that zirconium serves as a structural stabiliser within the cerium-zirconium mixed oxide framework, without directly participating in the redox processes observed for nickel and cerium. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 104 Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 105 Figure 21. XPS spectra of the a) Ni 2p3/2, b) Ce 3d, c) Zr 3d regions and d) O 1s for the series of catalysts with nickel CZN5, CZN10, CZN12 and CZN15. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 106 In the O 1s region (Figure 21.d), the spectra provided valuable information about the nature of oxygen in the catalysts. Three key types of oxygen are observed: lattice oxygen O1 (529.5-530.5 eV), oxygen vacancies O2 (531.0-532.0 eV) and adsorbed oxygen O3 (532.0-533.0 eV) [104]. Lattice oxygen, which is part of the stable crystalline structure, remained relatively constant but slightly decreased in intensity as the nickel content increased. This slight decrease indicates that the incorporation of nickel promoted the formation of oxygen vacancies by facilitating the reduction of cerium from Ce4+ to Ce3+, leading to a gradual release of oxygen from the lattice. Meanwhile, oxygen vacancies generally increased with rising nickel content, although the trend was not entirely linear across all samples. This increase highlighted the role of nickel in enhancing the defect sites within the structure, improving the oxygen mobility and redox capacity of the catalyst. Oxygen vacancies are critical for storing and releasing oxygen, which is essential for redox reactions, making these materials highly active for catalytic processes. The presence of adsorbed oxygen suggests greater surface reactivity, making them more effective for processes involving oxygen as a reactant [129,184,185]. Overall, the rise in adsorbed oxygen, coupled with a slight reduction in lattice oxygen, may underscore a positive impact of nickel in enhancing the redox properties and surface reactivity of the catalyst. These changes may make the catalysts more effective in facilitating redox reactions. The characterisation of Ni-based cerium-zirconium mixed oxide catalysts provided a cohesive understanding of how structural, electronic and redox properties evolve with nickel content. Across various techniques, a consistent picture emerged, emphasising the balance between nickel dispersion, structural organisation and metal-support Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 107 interactions as critical factors defining the system. XRD analysis revealed the formation of two stable Ce-Zr phases, with nickel influencing the lattice structure. These interactions were particularly evident at intermediate nickel loadings, where nickel species were closely integrated into the oxide matrix, facilitating strong metalsupport interactions. This finding aligned with H2-TPR results, which indicated enhanced reducibility at these loadings, suggesting favourable integration of nickel into the oxide matrix. Moreover, the dual-phase structure observed in low Ni-content catalysts, particularly CZN5, likely played a role in oxygen vacancy formation, enhancing both oxygen mobility and carbon resistance. Having established a comprehensive picture of the physicochemical properties of the catalysts, the next step is to assess how these characteristics translate into catalytic performance. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 108 3.2. Catalytic activity The reduced catalysts were tested under DRM reaction conditions explained in Chapter 2, Section 2. Catalytic performance. The effect of the temperature in DRM was studied at a temperature range from 500 to 800 ºC and atmospheric pressure, using a reactant gases molar ratio of CO2:CH4 1:1. As it can be observed in Figure 22, the DRM reaction tests on the reduced catalysts revealed a consistent trend where CO2 conversion surpassed CH4 conversion across all systems and temperature ranges. This may have been due to the higher activation energy of CH4 than CO2 (i.e., the energy barrier to active C-H cleave bond is higher than CO2 dissociation), requiring higher temperatures in agreement with DFT results reported elsewhere [186,187]. Besides, the possible occurrence of the reverse water-gas shift (RWGS) reaction (CO2 + H2 → CO + H2O), which competes with DRM, may have contributed to the higher CO2 conversion due this parallel route consuming CO2 simultaneously while producing CO and H2O. As the temperature rose, both CO2 and CH4 conversions increased due to the endothermic nature of the reaction [188], reaching their peak at 800 ºC, where the conversion of CH4 reached 45% (Figure 22.a) and CO2 conversion approached 60% (Figure 22.b) for the CZN10 catalyst. These values were closer to the equilibrium conditions [37], which highlighted the temperature dependence of the reaction, particularly for CH4, which requires high temperatures to overcome its activation energy. Focusing on the two best results, CZN5 and CZN10, the conversion gap between them narrowed as the temperature was increased for both CH4 and CO2, even surpassing CO2 conversion offered by CZN10 catalysts if compared to CZN5 at 800 ºC. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 109 Figure 22. Effect of the temperature on (a) CH4 conversion and (b) CO2 conversion with H2/CO ratio, for all the catalysts from 500-800 ºC. Reaction conditions: P = 1 atm, N2:CH4:CO2 2:1:1, 100 mL min-1, WHSV = 30 L g-1 h-1. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 110 Regarding the loading of nickel, interesting findings were noted. The CZ catalyst, which contains no nickel, served as a useful reference in the analysis. Its very low activity in both CH4 and CO2 conversion further underscored the essential role of Ni0 in driving the DRM reaction. CH4 is acknowledged to be activated by Ni active centres whereas CO2 by the support [67,189]. While the ceria-zirconia support had little inherent redox activity, it is clear that nickel is necessary for efficiently catalysing both methane dissociation and syngas production. The poor performance of CZ catalyst highlighted that the synergistic interaction between nickel and the support is crucial for successful DRM catalysis. As the metal loading was increased from 0 to 10 wt.%, the conversion of both CH4 and CO2 rose due to the higher Ni concentration presented in the sample. While CZN10 showed the highest performance, CZN5 also exhibited commendable results, with a slight decrease in conversion compared to CZN10, despite having half the nickel content. This suggests that nickel dispersion and interaction with the Ce-Zr support played a critical role in determining catalytic activity, linked to the accessibility of Ni active sites to the reactant gases. The slightly lower conversion observed in CZN5 compared to CZN10 could be attributed to fewer available Ni0 active sites, as observed in XPS, but the presence of the Ce0.5Zr0.5O2 phase in CZN5, interacting with nickel, might have compensated for this to a significant extent. This mixed-phase support may have enhanced the interaction between Ni particles while maintaining the accessibility of the Ni active centres. The shift of reduction peaks in H2-TPR to higher temperatures suggested a strong metalsupport interaction with the increasing nickel loading, which may interfere in the activity of the catalysts. This is consistent with the trend observed in BE in Ni0 on Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 111 XPS. Anyhow, CZN10 showed the highest catalytic performance among the studied series. When the metal loading was further increased to 15 wt.% Ni, a decrement of the conversion could be observed despite having more Ni in the samples. The accessibility of Ni active centres and its interaction with the support structure influenced this. Despite the similar mean Ni crystallite size observed among all the reduced samples, dispersion probably decreased at higher nickel loadings. Therefore, an excessively strong interaction and incorporation of nickel into the bulk structure of the catalyst could have limited the number of available active sites. Conversely, weak interactions may promote the sintering of metal particles in stability tests. Achieving a balanced interaction strength is essential to maximise catalytic performance. Indeed, there appears to be an optimum amount of Ni which maximised the conversion in DRM, being close to 10 wt.%. From 10 wt.% up, despite the more stoichiometric Ni, substituting stoichiometrically Zr, the activity did not improve. At these moderate nickel loadings (CZN5 and CZN10), the nickel particles were more efficiently active across the surface of the support for DRM conditions, likely ensuring a higher proportion of exposed active sites. These optimal conditions enhanced the efficiency of methane dissociation and CO2 activation. The textural properties of the catalysts were moderate, which might initially have suggested a limitation in catalytic performance. Despite the relatively low surface area, the catalysts demonstrate excellent performance, indicating that the interaction of the phases play a more significant role than textural properties alone. In terms of H2/CO molar ratio, the tendency was to increase the ratio as the temperature increases, as observed in the inset of Figure 22. This increment might be Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 118 catalytic performance, as it may have restricted oxygen mobility and redox cycling efficiency. Figure 24. Stability test on CZN5 over 100 h at (a) 600 and (b) 800 ºC. Reaction conditions: P = 1 atm, N2:CH4:CO2 2:1:1, WHSV = 30 L g-1 h-1. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 119 Figure 25. Stability test on CZN10 over 100 h at (a) 600 and (b) 800 ºC. Reaction conditions: P = 1 atm, N2:CH4:CO2 2:1:1, WHSV = 30 L g-1 h-1. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 120 The stability results at 600 and 800 ºC highlighted the different challenges faced by the CZN10 catalyst at two different temperatures. At 600 ºC, the main issue was carbon formation, which led to a gradual decline in CH4 and CO2 conversion over time. However, the decrease was relatively slow, suggesting that the ceria-zirconia mixed oxide structure provided some resistance to coke formation, although it could not entirely prevent it. The ability of the support to store and mobilise oxygen helped to mitigate carbon buildup, but activity loss was inevitable in this temperature range due to the nature of the reactions occurring. At 800 ºC, the catalysts showed better stability. Nickel particle agglomeration could contribute to the loss of activity, but it seemed to have a less severe impact than carbon formation at lower temperatures. Additionally, the higher H2/CO ratio at 800 ºC indicated that the catalyst was more efficient in producing syngas at higher temperatures, making it more suitable for industrial applications where a higher hydrogen content in syngas is desired [192]. In conclusion, CZN5 and CZN10 demonstrated reasonable stability at both 600 and 800 ºC, though each temperature presented unique challenges. At 600 ºC, carbon formation was expected to be the primary obstacle, while at 800 ºC, nickel particle agglomeration may have reduced the availability of active sites. Nevertheless, the high stability observed at 800 ºC, along with the improved H₂/CO ratio, suggested that this catalyst was more efficient and durable when operated at higher temperatures, where the DRM reaction proceeds more efficiently and coke formation is less of an issue. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 121 3.3. Post-reaction characterisation Deactivation of the catalyst is mainly caused by carbon deposition and/or sintering of the active phase. In order to elucidate these phenomena discussed in Section 3.2. Catalytic activity, the best performing catalysts were analysed after long-term activity test by XRD (Figure 26) in order to detect any structural changes after the different treatments it underwent. The post-reaction XRD analysis revealed several important observations regarding the behaviour and stability of both CZN5 and CZN10 catalyst after 100 h of testing at 600 and 800 ºC. The inorganic crystalline structure of the sample, mainly formed by Ce2Zr2O7, Ce0.5Zr0.5O2 and CeNiO3, remained practically intact since no significant differences in the diffraction characteristics between the reduced and the post-reaction catalysts were found. This is a crucial point because it confirms that the catalyst did not undergo major phase changes or degradation that could lead to a severe loss of activity. The retention of these active phases suggests that the engineered catalyst formulation was robust and thermally stable, which is essential for sustained catalytic performance under harsh reaction conditions. Additionally, a key observation from the XRD data is the increase in the mean crystallite size of metallic nickel Ni0. The mean crystallite size of Ni for CZN10 was calculated using Scherrer equation, increasing from 33.7 nm in the fresh-reduced catalyst to 39.8 nm after the reaction at 600 ºC and to 37.3 nm after the reaction at 800 ºC. CZN5 underwent a similar trend, from 32.8 to 35.0 after the reaction at 600 ºC and 32.2 nm after the reaction at 800 ºC. This general growth in crystallite size is indicative of sintering, a phenomenon where metal particles agglomerate and grow in size during the reaction, leading to a reduction in the dispersion of the active Ni sites. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 122 Figure 26. Normalised XRD pattern series calcined/reduced/post-reaction of (a) CZN5 and (b) CZN10. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 123 The increase in particle size was more pronounced at 600 ºC, which was somewhat unexpected because sintering is typically more severe at higher temperatures. The variation of CZN5 Ni mean crystallite size was lower, which may confirm the advantageous role of the coexistence of Ce0.5Zr0.5O2 and Ce2Zr2O7 crystalline phases. Metallic nickel, as mentioned before, remained present in the post-reaction samples, despite the increase in crystallite size. In CZN10 (Figure 26.b), a slight displacement of the characteristic peaks 44.4 and 51.7º 2θ, corresponding to the characteristic planes of (111) and (200) of metallic Ni0 to lower angles, was observed, indicating that part of the surface Ni0 was oxidising again to form NiO1-y species. On the other hand, the XRD pattern after 100 h at 800 ºC showed the presence of NiO. In contrast, CZN5 (Figure 26.a) did not show that behaviour, further confirming the positive nickelsupport interaction reached. In general, the presence of Ni0 was confirmed to be in its reduced, active form after the reaction, which is essential for the catalytic performance in DRM. The difference in crystallite growth between reaction temperature 600 and 800 ºC also suggests that there were different stabilisation mechanisms at play at these temperatures. At 600 ºC, the larger increase in crystallite size indicated that sintering was more severe. At 800 ºC, the crystallite growth seemed lower. The exsolution mechanism, previously reported in a similar structure [130], is an important factor to consider in the behaviour of the catalysts and its role in maintaining catalytic activity. Exsolution is the process where transition metals such as nickel migrate from the bulk of the support material to the surface, typically driven by reduction at elevated temperatures. This phenomenon can lead to the formation of highly dispersed and Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 124 stabilised metal nanoparticles on the surface of the support, enhancing the activity and stability of the catalysts [193–196]. In the case of CZN5 and CZN10 catalysts, the exsolution mechanism might be occurring under reaction conditions, likely played a key role in the performance and stability, particularly at 800 ºC. Nickel might be partially exsolving from the bulk structure and forming stable, well-dispersed nanoparticles on the surface [129,130]. This process helped prevent excessive sintering by anchoring the nickel particles to defect sites in the support, such as oxygen vacancies. At reaction temperature 800 ºC, the exsolved nickel was better dispersed and stabilised, showing a trend towards mitigating particle growth due to the interplay between formation of highly-dispersed Ni cluster by exsolution and particle growth due to sintering, maintaining catalytic activity. Exsolution also contributed to a reduction in carbon deposition, as the exsolved particles are more resistant to carbon formation and facilitate its removal through gasification. The fact that crystallite growth was more significant at 600 ºC could be linked to a less efficient exsolution process at this lower temperature [197–200]. Carbon deposition was also studied as a cause of deactivation. The formation of carbon was observed in XRD pattern of CZN10 after 100 h at 600 ºC (Figure 26.b), where a peak attributed to graphitic carbon was detected. This peak corresponds to the graphite lattice plane (002) of carbon nanotubes at 26º 2θ. At 800 ºC, no carbon structures were observed. In CZN5, no graphite was noticed. Carbon formation is hard to avoid due to the intricate reaction, since C-H activation of CH4 involves the formation of carbon species, widely studied elsewhere [201]. Besides, the reaction temperature has an important influence on carbon deposition. It must be emphasised Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 125 that carbon deposition is thermodynamically more favoured at moderate temperatures, between 600 and 750 ºC [67]. The presence of graphitic carbon (Cc) at 600 ºC also indicated that the conditions at this temperature were not sufficient to facilitate the gasification of carbon (C + CO2 → 2 CO), a process that helps remove carbon deposits from the catalyst surface. As a result, carbon built up, leading to the formation of more stable and structured graphitic carbon, which is harder to remove [37]. Besides, carbon deposition is closely related to Ni particle size, since the larger the clusters the more favoured the carbon deposition. Due to the increment of Ni crystallite size, carbon deposition may be enhancing. Carbon can cover the active nickel sites, blocking their ability to dissociate CH4 and activate CO2, resulting in the gradual loss of activity observed. However, the observed decline of activity was relatively slow, indicating that the ceria-zirconia support played a role in mitigating carbon accumulation, likely through its ability to generate and mobilise oxygen (due to oxygen vacancies). These vacancies could have interacted with the carbon deposits, oxidising them to form CO and thus reducing the impact of coke formation, though not completely eliminating it. TEM images of CZN10 of the (a) reduced, (b) after 48 h reaction and (c) after 100 h at 600 ºC reaction were shown in Figure 27 to untangle the presence of carbon. Figure 27.a shows a particle of the reduced catalyst. After 48 h at 600 ºC, some carbon nanotubes (CNT) or “whiskers” (Cγ) are formed, as observed in Figure 27.b. Nevertheless, the amount is negligible. After 100 h at 600 ºC, significant amount of CNTs appeared. These CNTs started growing from the interface of the active metal phase and the support structure, “pulling out” part of the Ni particle from the surface. Nevertheless, despite the formation of carbon deposits, the catalyst remained Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 126 acceptable since this carbon was partially covering the active sites of Ni, being the rest of the Ni atoms accessible for the reaction. A similar behaviour was reported for a Ni-substituted La pyrochlore [130]. Overall, the main causes of catalyst deactivation at 600 ºC were the carbon deposits around nickel particles and the increase of Ni clusters. This situation was overcome when the reaction was run at 800 ºC where our post-stability XRD pattern showed a carbon-free sample which explained the excellent conversion levels under these reaction conditions for a continuous 100 h test. Figure 27. TEM images of CZN10. (a) Reduced, (b) after 48 h reaction and (c) after 100 h reaction at 600 ºC. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 127 4. Effect of calcination temperature on Ni-based cerium zirconate Once the effect of nickel loading on the catalyst properties had been studied, the effect of the calcination temperature on the best catalytic structure was scrutinised. In this section, the effect of calcination conditions on 10 wt.% Ni-based cerium zirconate oxide (CZN10) is examined, using the modified citrate synthesis method. Calcination temperature was selected as a key variable in our synthesis process [32] and its effect on the properties of the catalysts was thus studied. The resulting fresh catalyst, with 10 wt.% nickel incorporated, was calcined in a crucible for 8 h at 800, 900, 1000, 1100 and 1200 ºC, using a heating rate of 7.5 ºC min-1. The correlation between the calcination temperature in catalyst synthesis and the properties of the catalysts was also discussed. These complex catalytic structures were evaluated to understand the differences in the performance of the catalysts to gain further insight into the optimum calcination temperature. To simplify, a special notation was chosen. The catalysts were referred to as CZN800, CZN900, CZN1000, CZN1100 and CZN1200 corresponding to calcination temperatures of 800, 900, 1000, 1100 and 1200 ºC, respectively. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 134 metallic Ni0 can be attributed to the high dispersion, which may result in crystallites too small to be detected by XRD, or in pyrophoric clusters prone to rapid reoxidation upon air exposure during sample handling [203]. The presence of reduction events associated with nickel species in H2-TPR, discussed later, further supports this interpretation. Figure 30. Normalised XRD diffractogram of the reduced catalysts. Therefore, at lower calcination temperatures, nickel remained highly dispersed, preventing its detection in XRD after reduction. Nonetheless, at higher calcination temperatures, the interaction between nickel and the support increased, stabilising the Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 135 Ni phase within the mixed oxide structure, favouring the formation of more stable phases such as ZrO2 and CeNiO3. Meanwhile, nickel that was not fully incorporated into the lattice likely sintered into larger particles due to enhanced thermal mobility, as previously observed in Table 4. While previous studies on similar structures suggested that a proportion of Ni inserted within the lattice may undergo exsolution upon reduction [129,130,140], the observed effects in this study are more consistent with the influence of calcination temperature on phase segregation rather than exsolution during reduction. In order to corroborate these results, H2-TPR was performed. The reducibility of the calcined systems was followed by H2-TPR measurements to further investigate the redox behaviour of our mixed-oxide systems. H2-TPR results were also relevant in order to understand the activation of metallic nickel of the catalyst prior the assessment of the catalytic performance of the catalysts for DRM. The TCD signal, which indicates hydrogen consumption, was collected and represented in Figure 31 for all the samples. As a reminder, in the H2-TPR showed in Figure 20, the catalyst without any nickel was found to have little reducibility between 400 and 600 ºC, ascribed to reduction events of Ce4+ to Ce3+ because of the Ce species in the catalyst. Therefore, the catalyst without nickel was considered not to have any relevant reducibility events owing to the strong interaction of the solid lattice formed which inhibited the removal of surface oxygen up to 900 ºC. Besides, it is generally accepted that Ni2+ is reduced to Ni0 without any intermediate oxide (NiO + H2 → Ni0 + H2O) [204–206]. Nonetheless, hydrogen consumption may be attributed to the reduction of different Ni2+-containing species. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 136 Figure 31. H2-TPR profile of the calcined samples. At first glance, three main events occur at three different temperature regions, as remarked in Figure 31. The first region started at a temperature of 230 ºC. Interestingly, this signal appeared in CZN800, becoming more intense in CZN900 and CZN1000 at 270 ºC. Nevertheless, the signal rapidly disappeared in CZN1100, being negligible at CZN1200. This signal was ascribed to the reduction of highly dispersed Ni2+ species on the surface of the catalytic samples. In addition, hydrogen consumption signal between 300 to 350 ºC was attributed to the reduction of Ni2+ species allocated on the sub-surface of the catalyst, weakly interacting with the crystalline phases and easily reducible, as reported elsewhere [185]. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 137 A second reduction region was observed as the major event in the range of 380 to 500 ºC, corresponding primarily to the reduction of bulk Ni2+ species, although other contributions cannot be excluded [118,130,173]. The relatively high reduction temperatures indicate a strong NiO-support interaction, which intensified with increasing calcination temperature. The gradual peak shift from 450 to 470 ºC in the reduction profiles with increasing calcination temperature further supports this hypothesis. This behaviour correlates with XRD findings, which indicate that as calcination temperature increased, nickel segregated from the lattice, forming bulk NiO phase. These structural modifications likely contributed to the stabilisation of Ni2+ within the mixed oxide matrix, making its reduction more energy demanding. Finally, region 3 presented a low signal in CZN900 and CZN1000, but as the calcination temperature increased beyond 1000 ºC, hydrogen consumption became more pronounced, suggesting that at higher temperatures, Ni species with stronger interactions and CeNiO3, previously identified in XRD, may have undergone partial reduction, as reported in similar studies [207]. The significant increase in H2 consumption in CZN1100 and CZN1200 in region 3, compared to regions 1 and 2, indicates that higher calcination temperatures reinforced NiO-support interactions, making nickel species more resistant to reduction. The presence of CeNiO3 in these high-temperature samples further supports that this phase was only partially reduced, leaving bulk crystalline domains intact. The intensified hydrogen consumption in CZN1100 and CZN1200 at high temperatures suggests that Ni species became increasingly embedded within the oxide matrix, strengthening their interaction with the support and making their reduction more difficult. This aligns with the theory of Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 138 phase segregation, where NiO species that were not fully integrated into the lattice migrated to the surface or formed highly stable Ni-O-Ce/Zr structures, further complicating their reducibility. As a result, the stronger metal-support interaction at extreme calcination temperatures led to a higher energy requirement for Ni reduction, explaining the pronounced hydrogen consumption in this region. Notwithstanding, H2 consumption remained similar, as seen in Table 5, except of the CZN800 sample. Region 3 could not be assigned exclusively to the reduction of Ni2+ species. This increment might be attributed to the partial reduction of Ce4+ species to Ce3+, as reported before [118,208,209]. This effect may have provoked the greater formation of CeNiO3. In addition, the presence of ZrO2 observed in calcined XRD at these calcination temperatures may also have been responsible for the peak-shifting toward higher reduction temperature, due to the presence of stable oxide species. The appearance of Zr4+ may have increased oxygen vacancies, enhancing the bulk oxygen mobility and helping to improve the oxygen storage capacity (OSC) and redox properties [210]. Ultimately, the step observed in CZN800 at 800 ºC was due to the maximum calcination temperature reached during the synthesis process. After 800 ºC, structural changes of mixed oxides structure in CZN800 continued to occur, as previously reported on XRD results on Figure 28 and literature [130], distorting the signal. By quantifying the hydrogen uptake using CuO as a calibration standard, a quantitative analysis of the hydrogen consumption for the samples was performed. The results, noticed in Table 5, showed that hydrogen consumption was 1.05 times (for CZN800) and 1.55-1.62 times (for the rest of the catalysts) larger than the one Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 139 required for a theoretical single reduction event of Ni2+ to Ni0. This confirmed that Ce species were suffering a reduction event. Therefore, as stated before, the consumption in the second but more prominently in the third region could not be completely ascribed to the reduction of Ni2+ species. Table 5. Hydrogen uptake of samples during H2-TPR. Catalyst H2 consumption / Catalyst mass (mmol /gcat) H2 consumption / NiO ratio (mol/mol) CZN800 1.89 1.05 CZN900 2.87 1.60 CZN1000 2.78 1.55 CZN1100 2.70 1.51 CZN1200 2.89 1.62 In conclusion, calcination temperature significantly influenced the redox behaviour and structural properties of the catalytic system. XRD confirmed that higher calcination temperatures led to phase segregation, favouring the formation of tetragonal ZrO2 and promoting the growth of larger NiO clusters rather than their integration into the lattice. The increased thermal energy facilitated Ni particle coalescence, forming larger crystallites, which was reflected in the evolution of reduction profiles in H2-TPR. As calcination temperature increased, Ni species exhibited stronger interactions with the support, making their reduction more challenging. This was evidenced by the shift in H2-TPR profiles, where Ni species required higher temperatures for reduction, suggesting their stabilisation within the mixed oxide structure. At lower calcination temperatures, nickel remained highly Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 140 dispersed, making its reduction less pronounced in XRD. However, beyond a certain threshold, segregation of NiO became more prominent. Additionally, Ce4+ species were partially reduced to Ce3+ at higher temperatures, as confirmed by quantification of hydrogen uptake. Overall, the results indicate a tradeoff between calcination temperature, Ni dispersion and metal-support interaction. Optimising the calcination temperature is crucial for balancing Ni dispersion, interaction strength and redox flexibility, all of which are essential for achieving high catalytic performance and long-term stability. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 141 4.2. Catalytic activity The catalytic activity was tested under DRM conditions explained in Chapter 2, Section 2. Catalytic performance. Firstly, a screening of the temperature was conducted from 500 to 800 ºC at 1 atm using a molar ratio of N2:CO2:CH4 2:1:1. Due to the endothermicity of the reaction, the conversion of both CO2 and CH4 increased with increasing temperature for all the catalysts, noticed in Figure 32. At low temperatures, the conversion was far from the equilibrium [37,175] for both CO2 and CH4 but, when the temperature was increased, this value became higher, reaching acceptable values for all the catalytic systems. Conversions reached values of 40% for CH4 and 55% for CO2 at 800 ºC despite the low SBET reported. On the other hand, the CO2 conversion was greater than the CH4 conversion for all the catalytic systems at all temperatures, ascribed to the higher activation energy of CH4 molecule [186,211]. In addition, the potential incidence of the reverse water-gas shift (RWGS) reaction, which competes with DRM, could have contributed to the higher CO2 conversion by consuming CO2 through a parallel reaction pathway [174]. CZN800 exhibited the lowest CO2 and CH4 conversions among the catalysts. XRD and H2-TPR results suggest that at this lower calcination temperature, Ni was more strongly integrated into the Ce2Zr2O7 or in Ce2Zr2-xNixO7-δ phase, limiting its reducibility and leading to a lower availability of Ni0 active sites, which are crucial for DRM. In fact, this was confirmed by the quantitative H2-TPR analysis. Qualitative H2-TPR analysis showed that Ni species in CZN800 required medium and higher reduction temperatures, indicating a stronger interaction with the oxide lattice. Reduced XRD data further confirmed that CZN800 had the lowest Ni0 crystallite size, Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 142 demonstrating that sintering was not a dominant factor in its lower activity. Instead, its limited catalytic performance was attributed to the difficulty in reducing Ni species confined within the mixed oxide structure, reducing the number of accessible active sites. Additionally, the lower calcination temperature resulted in a less defined crystalline structure, which may have impacted the redox flexibility required for CO2 activation, further contributing to the reduced conversions observed. In contrast, CZN900 showed improved performance. At this calcination temperature, the segregation of Ni from Ce2Zr2-xNixO7-δ likely started to occur, leading to the appearance of highly dispersed Ni0 species detected in H2-TPR. The increased availability of Ni0 sites contributed to the higher catalytic activity observed. The balance between dispersion and accessibility allowed CZN900 to achieve higher conversions than CZN800 while maintaining a sufficiently high dispersion to prevent significant aggregation. As calcination temperature increased, nickel-support interactions strengthened, further stabilising Ni species within the oxide matrix. While this stabilisation enhanced dispersion at moderate calcination temperatures (900-1000 ºC), higher calcination temperatures (≥ 1000 ºC) led to increased phase segregation, marked by the formation of ZrO2 and the appearance of CeNiO3. The segregation process, evidenced by XRD and H2-TPR, correlated with the progressive increase in NiO crystallite size detected in reduced XRD. The H2-TPR analysis indicated that while highly dispersed Ni species were still present at 1000 ºC, phase-separated nickel species were beginning to dominate, requiring higher temperatures for reduction. The presence of CeNiO3 in particular, contributed to the observed higher-temperature reduction events, as its reduction is more complex than that of NiO. Chapter 3. Thermo-catalytic biogas reforming using Ni-based inorganic complex structures 143 Figure 32. Effect of the temperature on (a) CO2 conversion and (b) CH4 conversion with H2/CO ratio, for all the catalysts from 500-800 ºC. Reaction conditions: P = 1 atm, N2:CH4:CO2 2:1:1, 100 mL min-1, WHSV = 30 L g-1 h-1. Chapter 4. Thermo-catalytic biogas reforming using other formulation structures 246 (5Ni/CZ and 10Ni/CZ) exhibited higher CH4 and CO2 conversions across all temperatures, with 10Ni/CZ achieving the best overall performance due to the enhanced availability of Ni0 active sites and the presence of the Ce0.5Zr0.5O2 phase, which improved oxygen mobility and CO2 activation. In contrast, Pechini-derived catalysts (CZN5 and CZN10) showed slightly lower conversions, attributed to stronger metal-support interactions that reduced Ni reducibility and the predominance of the Ce2Zr2O7 phase, which limited redox activity. Long-term stability tests revealed that while all catalysts experienced an initial deactivation phase, CZN5 maintained stable performance due to the partial reduction of Ce4+ to Ce3+, which enhanced oxygen mobility and mitigated carbon accumulation. However, CZN10 exhibited the most pronounced deactivation, likely due to limited Ni accessibility and lower oxygen mobility. The H2/CO ratio trends confirmed that wet-impregnated catalysts favoured methane decomposition, whereas Pechini-derived catalysts exhibited higher RWGS activity, influencing syngas composition. Chapter 4. Thermo-catalytic biogas reforming using other formulation structures 247 3.5. Post-reaction characterisation The post-characterization of catalysts after the 100-hour stability test provides insights into structural changes, metal particle sintering and carbon deposition, which directly influence the long-term performance of the catalysts. The XRD patterns of 5Ni/CZ and 10Ni/CZ, calcined and after the reaction, which can be observed in Figure 64, show interesting findings. Figure 64. XRD pattern of 5Ni/CZ and 10Ni/CZ calcined and after 100 h at 800 ºC. The post-characterisation of catalysts after the 100-hour stability test confirms that the support structure remained intact, demonstrating thermal stability and resistance at Chapter 4. Thermo-catalytic biogas reforming using other formulation structures 248 elevated temperatures. The Ce0.5Zr0.5O2 and Ce2Zr2O7 phases were preserved, indicating that the ceria-zirconia framework remained structurally stable under prolonged DRM conditions. The persistence of these phases suggests that the redox and oxygen mobility properties of the support were maintained, which is critical for CO2 activation and carbon oxidation. XRD analysis confirmed that Ni0 was preserved in all catalysts after 100 h of reaction, demonstrating the stability of metallic Ni under DRM conditions. The absence of NiO peaks post-reaction suggests that reoxidation did not occur, likely due to the reducing environment of the reaction. However, Ni sintering might be happening in all samples, with 5Ni/CZ exhibiting a size of 44 nm and 10Ni/CZ reaching 51.3 nm, while CZN5 and CZN10 showed lower sintering effects, with crystallite sizes of 32.2 nm and 37.3 nm, respectively (see Figure 26). This suggests that Pechini-derived catalysts maintained better Ni dispersion over time due to stronger metal-support interactions, potentially contributing to their greater long-term stability despite lower initial activity. Graphitic carbon deposition was detected in both 5Ni/CZ and 10Ni/CZ, as indicated by the appearance of graphite peaks in the post-reaction XRD patterns. This suggests that methane decomposition was more pronounced in these catalysts, which correlates with their higher H2/CO ratios. However, the preserved catalytic activity of 5Ni/CZ and 10Ni/CZ indicates that carbon formation was not severe enough to cause significant deactivation, likely due to the oxygen mobility provided by Ce0.5Zr0.5O2, which allowed for continuous oxidation of carbon deposits. In contrast, CZN5 and CZN10 exhibited lower carbon deposition, as discussed in Chapter 3, possibly due to Chapter 4. Thermo-catalytic biogas reforming using other formulation structures 249 higher RWGS activity and lower methane decomposition, which contributed to their lower H2/CO ratios. The results confirm that wet-impregnated catalysts exhibited higher Ni sintering but maintained strong activity due to better CH4 activation, while Pechini-derived catalysts experienced less Ni sintering due to stronger Ni-support interactions which in turn limits long-term CH4 conversion. The preservation of Ni0 in all catalysts confirms their structural stability, while the Ce0.5Zr0.5O2 phase in 5Ni/CZ and 10Ni/CZ likely contributed to superior catalytic performance. Meanwhile, the retention of Ce2Zr2O7 in Pechini-derived catalysts provided thermal resistance at the cost of lower overall performance. The support structures demonstrated high stability, ensuring that deactivation was primarily driven by Ni sintering and carbon formation rather than structural degradation. Among all catalysts, 10Ni/CZ exhibited the best balance of high initial activity and stability. Chapter 4. Thermo-catalytic biogas reforming using other formulation structures 250 3.6. Conclusion The evaluation of Ni-based cerium-zirconium oxide catalysts for DRM confirmed that the synthesis method significantly influences Ni accessibility, reducibility and overall catalytic behaviour. Wet impregnation proved effective in producing catalysts with readily available Ni0 active sites, resulting in higher initial CH4 and CO2 conversions, while the Pechini method led to stronger metal-support interactions, promoting redox stability and limiting immediate Ni availability. The presence of Ce0.5Zr0.5O2 in wetimpregnated catalysts enhanced oxygen mobility, facilitating CO2 activation. Despite the differences in initial activity, both synthesis methods yielded catalysts with good long-term stability, as confirmed by the 100-hour DRM test. Ni0 was preserved postreaction, indicating structural durability, though wet-impregnated catalysts exhibited greater Ni sintering. Wet impregnation remains an effective strategy for maximising Ni availability, while the modified Pechini method provides a highly stable support, ensuring Ni remains active and well-distributed across the surface. Chapter 4. Thermo-catalytic biogas reforming using other formulation structures 251 4. Summary Chapter 4 examined innovative catalytic formulations for thermo-catalytic biogas reforming. Three main approaches were explored: bimetallic Ni:Co formulations for low-temperature DRM, a sacrificial template strategy using a Ni:Ce MOF-based catalyst and a comparative study of conventional wet impregnation versus the Pechini synthesis method. The first section evaluated Ni:Co bimetallic catalysts, which introduced cobalt to modify the redox properties of nickel-based catalysts. The goal was to enhance methane activation, increase resistance to carbon deposition and improve durability. Structural characterisations reveal strong metal-support interactions; however, Ni:Co catalysts failed to surpass the activity of monometallic Ni catalysts, limiting their practical advantage. The second approach investigated a sacrificial template strategy using a Ni:Ce MOFderived catalyst. MOF pyrolysis generated highly dispersed active sites and tunable porosity, leading to improved nickel dispersion and metal-support interactions. While this strategy enhanced initial catalytic activity, stability was compromised due to sintering and weak interactions between the active phase and the support, which accelerated deactivation. The role of other support structures in catalytic activity was also explored although the initial system NMC1 resulted the best performing material, a result which is highly commendable in view of its very low Ni loading. The final section compared wet impregnation and the Pechini method for catalyst synthesis. While wet impregnation ensured high metal loading, the Pechini method Chapter 4. Thermo-catalytic biogas reforming using other formulation structures 252 enhanced catalyst homogeneity and stability by forming a more robust metal-oxide matrix, reducing sintering and coke formation. However, wet impregnation ultimately proved more effective, yielding better performance. Among the tested catalysts, 10Ni/CZ emerged as the most promising, demonstrating superior stability and activity. Chapter 5. Plasma-catalytic biogas reforming Chapter 5. Plasma-catalytic biogas reforming 254 Chapter 5. Plasma-catalytic biogas reforming 255 In this last chapter, plasma was investigated as a non-conventional biogas upgrading route. Specifically, non-thermal plasma (NTP) technology is applied in combination with solid catalysts in a hybrid configuration, commonly referred to as plasmacatalysis. The use of plasma allows the generation of highly energetic states, offering a promising alternative to overcome the thermodynamic and kinetic limitations inherent to conventional thermo-catalytic reforming, where high activation energies and carbon deposition pose significant challenges. To evaluate the synergy between plasma activation and the catalysts, selected materials previously studied under thermal conditions are employed. From Chapter 3, three mixed oxide cerium zirconate, CZ, CZN5 and CZN10, are chosen due to their representative behaviour and distinct nickel loadings. Additionally, NCM1, a MOFderived Ni:Ce catalyst developed and characterised in Chapter 4 is included due to its unique structural properties and dispersion. The selection of these materials allows for a comprehensive assessment of how catalyst composition, nickel dispersion and support structure influence plasma-catalytic performance. This chapter explores the behaviour of these catalysts under varying plasma power inputs, focusing on their role in enhancing biogas conversion and selectivity towards syngas and light hydrocarbons. The potential benefits of plasma-catalysis in mitigating catalyst deactivation, improving energy efficiency and broadening operational flexibility are addressed, supporting the development of sustainable pathways for biogas valorisation. Chapter 5. Plasma-catalytic biogas reforming 262 voltage and ground electrodes, determines the electric field strength and ionisation efficiency. A smaller gap intensifies the electric field, accelerating electrons and increasing CH4 and CO2 dissociation in plasma-assisted DRM. Nonetheless, excessively small gaps promote energy losses through surface recombination, while larger gaps require higher breakdown voltages, increasing power demands. Discharge volume, which defines the total plasma-occupied space, plays a key role in reactor scalability, allowing higher reactant throughput. However, ensuring plasma uniformity across large volumes is challenging, as energy density fluctuations can cause incomplete conversion and undesired byproducts [22,264]. Figure 66. Typical geometry of a DBD reactor. For example, Tao et al. [265] investigated the effect of discharge gap variation on the performance of a DBD plasma reactor for DRM using NiMgAlCe hydrotalcitederived catalysts. With a constant gas flow rate, they observed that at a 1.5 mm gap, conversion was primarily governed by residence time rather than plasma properties. At an optimised 2.0 mm gap, conversion was no longer limited by residence time but rather by plasma characteristics, with further increases reducing ionisation efficiency Chapter 5. Plasma-catalytic biogas reforming 263 and reactant conversion. Similarly, Khoja et al. [266] explored discharge gap effects in reactors with quartz and alumina dielectrics, finding that CO2 and CH4 conversion in DRM increased with gap width up to 3 mm for quartz and 4 mm for alumina, beyond which efficiency declined due to reduced electron density. The enhancement at moderate gaps was attributed to improved electron-molecule collisions and sufficient residence time for reactants to undergo dissociation. Another study by Khoja et al. [267] demonstrated a non-linear relationship between discharge volume and reactant conversion over DRM conditions, with performance initially improving due to enhanced electric field distribution before declining as plasma uniformity decreased. On the other hand, Duan et al. [268] investigated CO2 degradation in a DBD microplasma reactor, reporting that increasing the discharge gap from 0.5 to 0.9 mm slightly improved CO2 conversion by extending gas residence time. However, the effect was minimal, suggesting that specific energy input (SEI) played a more dominant role. They also found that an external electrode length of 80 mm yielded maximum CO2 conversion. The dielectric barrier material is another key component, governing charge accumulation, discharge characteristics and overall efficiency. It prevents continuous arcs by limiting charge buildup, leading to controlled microdischarges that sustain non-equilibrium plasma conditions, where high-energy electrons selectively activate chemical bonds without excessive gas heating. High-permittivity materials enhance charge storage and local electric fields, increasing microdischarge intensity. However, excessively high permittivity can lead to energy losses if stored charge is not efficiently released. Common dielectric materials include quartz, alumina, Chapter 5. Plasma-catalytic biogas reforming 264 borosilicate glass and ceramics, each offering a balance of dielectric strength, thermal resistance and plasma stability. The thickness of the dielectric layer is also critical; while thicker layers enhance charge storage and prevent excessive current flow, they can reduce energy coupling efficiency, necessitating careful material selection to optimize plasma performance [264]. Electrode configuration plays a crucial role in plasma formation. Electrodes can be designed in wire, mesh, cylindrical or planar geometries, each affecting plasma properties differently. Wire electrodes generate localised electric fields, creating intense microdischarges that enhance molecular dissociation but can lead to nonuniform plasma distribution. Mesh electrodes improve electric field uniformity, enabling better reaction control, while cylindrical electrodes, commonly used in coaxial designs, enhance discharge stability and catalyst interaction. The placement of the high-voltage electrode, either inside or outside the dielectric barrier, further influences plasma intensity and energy distribution. Internal electrodes generate stronger electric fields and higher discharge densities, improving conversion levels but are more susceptible to degradation. External electrodes improve thermal management and longevity but may reduce discharge intensity [22]. Various DBD reactor configurations have been developed to enhance plasma-catalyst interactions and improve reaction efficiency. The coaxial configuration, with the high-voltage electrode inside a cylindrical dielectric tube and the ground electrode external, ensures a well-defined plasma region with uniform discharge, making it suitable for plasmaassisted DRM. The annular configuration, where plasma forms in the gap between two concentric cylinders, optimises gas flow dynamics and catalyst exposure, Chapter 5. Plasma-catalytic biogas reforming 265 improving reactant activation. Planar configurations, where parallel electrodes are separated by flat dielectric plates, are commonly used for surface-based plasma treatments and thin-film catalysis [22,261]. By carefully adjusting these parameters, DBD reactors can be fine-tuned to achieve higher efficiency, better energy utilisation and greater selectivity in plasma-assisted reactions. Chapter 5. Plasma-catalytic biogas reforming 266 1.3. Operating factors The performance of a DBD plasma reactor is governed by multiple process parameters that directly impact conversion efficiency, selectivity and energy consumption. Controlling these variables is essential for optimising reactor stability, enhancing reaction kinetics and ensuring scalability. Key parameters include gas flow rate, feed composition, operating pressure and temperature, discharge power, input power, specific energy input, catalyst loading, carrier gas and frequency. The gas flow rate significantly influences reactant conversion and product selectivity by dictating residence time within the plasma zone. This parameter is directly related to the gas hourly space velocity (GHSV), which expresses the volumetric gas flow rate relative to reactor volume. High flow rates reduce residence time, limiting interactions between reactants and reactive plasma species, thereby lowering conversion efficiency. Conversely, lower flow rates extend residence time, enhancing reactant activation but potentially favouring undesirable side reactions. The study of Khoja et al. [266] also investigated the effect of GHSV on the DRM in DBD plasma reactors using quartz and alumina as dielectric materials. The study revealed that at lower GHSV values, both CH4 and CO2 conversions were significantly higher, with alumina demonstrating superior performance due to its higher dielectric constant and surface properties. As GHSV increased from 45 to 230 h-1, a gradual decline in conversion was observed for both materials, with quartz experiencing a more pronounced reduction. The authors attributed this decline to shorter residence time and reduced dissociation probability of reactant molecules, which limited the interaction between high-energy electrons and gas species. While selectivity towards Chapter 5. Plasma-catalytic biogas reforming 267 CO and H2 remained relatively stable across different GHSV values, higher flow rates favoured the formation of hydrocarbons such as C2H6, suggesting that increased GHSV promotes chain growth reactions. The CH4/CO2 molar ratio plays a pivotal role in determining syngas composition and reaction pathways. A CO2-rich feed enhances oxidation reactions, mitigating carbon deposition and improving catalyst stability, while a CH4-rich feed favours hydrocarbon formation and may accelerate carbon accumulation. The addition of diluent gases such as Ar, He, or N2 modifies electron energy distribution, influencing plasma stability and reactivity. Pu et al. [269] explored the influence of noble gas mixing on electron energy distribution function (EEDF) in an inductively coupled plasma (ICP) discharge, focusing on Ar, Xe, Ne and He in nitrogen N2 plasmas. Their results demonstrated that different noble gases significantly alter electron energy distribution due to variations in ionisation cross-sections and excitation potentials. On the other hand, Rahmani and Nikravech [270] studied Ar dilution in a surface DBD reactor and found that increasing Ar content up to 66% enhanced plasma stability, lowered breakdown voltage, and increased electron density by 60%, leading to improved CH4 and CO2 conversion. However, excessive Ar dilution reduced energy efficiency and carbon balance due to increased soot formation and diminished selectivity for higher hydrocarbons and liquid products. Park and Cha [271] examined the influence of N2 and Ar dilution on microdischarge behaviour and discharge power. Their findings indicated that N2 dilution maintained stable discharge power regardless of concentration, whereas decreasing Ar dilution significantly reduced discharge power due to differences in ionisation efficiency. This effect was attributed to weaker Chapter 5. Plasma-catalytic biogas reforming 268 Penning ionisation interactions with Ar compared to N2. Additionally, higher N2 or Ar dilution ratios facilitated stable microdischarge patterns, whereas lower ratios led to sporadic discharges, highlighting the role of metastable states in sustaining plasma formation. Pressure and temperature profoundly impact plasma characteristics. DBD reactors typically operate at atmospheric or near-atmospheric pressure, simplifying reactor design and eliminating the need for vacuum systems. At reduced pressures, plasmagenerated radicals and ions exhibit extended lifetimes due to lower collision frequencies, prolonging reaction pathways. In contrast, elevated pressures enhance molecular dissociation and reaction rates, although excessive pressure may suppress plasma formation and compromise discharge uniformity [256]. Regarding operating temperature, Ahasan et al. [272] investigated the effect of temperature using a NiO/CeO2 nanorod-supported catalyst in a DBD reactor. Their study examined temperature ranges from 150 to 500 ºC, demonstrating that plasma enables methane and carbon dioxide conversion at much lower temperatures than thermal DRM. While no significant conversion was detected below 300 ºC in purely thermal DRM, the introduction of plasma facilitated detectable syngas production (CO and H2) even at 150 ºC, with concentrations increasing progressively with temperature. This study showed significantly enhanced conversion levels compared to thermal DRM, particularly at higher temperatures, where the synergetic effect of plasma and catalyst interaction led to increased production of CO and H2. At 500 ºC, plasma-assisted DRM achieved CO and H2 molar fractions of 33 and 25%, respectively, while thermal-only DRM produced lower yields. What is clear is that reactor designs must Chapter 5. Plasma-catalytic biogas reforming 269 ensure efficient heat dissipation to maintain stable plasma conditions and prevent excessive catalyst deactivation. Specific energy input (SEI) and energy efficiency are crucial parameters in plasmaassisted DRM, as they directly influence reactant conversion and process sustainability. SEI, expressed in kJ L-1 or eV per molecule, represents the energy supplied per unit volume of processed gas and is a key determinant of reaction efficiency. While increasing SEI enhances molecular activation and conversion, excessive input power can lead to diminishing returns due to energy losses from gas heating, non-productive reactions and radical recombination. Optimising SEI requires precise adjustment of power input, feed gas composition and reactor parameters to maximise conversion while minimising energy consumption. Additionally, the frequency of applied voltage significantly affects plasma behaviour by influencing microdischarge formation and energy transfer efficiency. Higher frequencies generally improve reactant activation, but beyond a certain threshold, plasma saturation effects limit additional gains in conversion efficiency [23]. Duan et al. [268] observed that increasing input power enhanced CO2 conversion up to 25.5 W, after which energy losses reduced efficiency. SEI correlated positively with conversion but negatively with energy efficiency, indicating the necessity of balance. Frequencies exceeding 17.5 kHz weakened the electric field, reducing discharge power and molecular activation, confirming that excessive frequency increases do not necessarily enhance conversion. Discharge power, another critical factor, governs plasma species generation, impacting reaction rates and selectivity. Excessive power input may decrease energy Chapter 5. Plasma-catalytic biogas reforming 270 efficiency if not managed properly. In a study by Mei et al. [273], a maximum CO2 and CH4 conversion of 29.8 and 49.1%, respectively, was achieved at a discharge power of 60 W, demonstrating the strong influence of power input on reaction performance. Nonetheless, the highest fuel production efficiency (FPE) occurred at 30 W, suggesting that beyond a certain threshold, additional power input did not proportionally improve process efficiency. Similarly, Suttikul et al. [274] investigated Ni/Mg-Al2O3 catalysts in a parallel plate DBD. Regarding discharge power, the study found that increasing the power led to higher CH4 and CO2 conversions, as well as improved selectivities toward H2 and CO. However, excessive power input resulted in diminished energy efficiency due to increased energy consumption without proportional improvements in reaction performance. Similar findings were found in other studies [256,275]. Optimising operating parameters in DBD plasma reactors is crucial for achieving high conversion levels, enhanced selectivity and sustainable energy efficiency. Factors such as feed flow rate, feed composition, operating pressure, temperature and discharge power must be carefully adjusted to balance conversion and energy utilisation. SEI and frequency modulation directly impact energy efficiency, while carrier gas selection influences plasma behaviour. Understanding the interplay of these parameters allows for better reactor design, improved process stability and increased viability for large-scale applications. Chapter 5. Plasma-catalytic biogas reforming 271 1.4. C2+ and oxygenates Plasma-assisted catalysis presents a promising route for the direct conversion of CH4 and CO2 into value-added oxygenates and C2+ molecules, overcoming the thermodynamic limitations of conventional catalytic processes. The formation of oxygenates, particularly acetic acid, methanol and ethanol, has been explored under various plasma conditions, demonstrating that the selective activation of CH4 and CO2 can facilitate pathways otherwise restricted under purely thermal processes. NTP, particularly DBD reactors, enables efficient molecular activation by generating reactive species, including radicals and vibrationally excited states, which drive oxygenate formation at significantly lower temperatures compared to traditional catalytic methods. As introduced in Chapter 1, one of the objectives of this work is to explore alternative strategies for the direct utilisation of CO2 and CH4 beyond syngas production. In this context, the BIO-Direct route aims at the direct transformation of CO2 and CH4 into more advanced C2+ and/or oxygenated products, such as acetic acid, offering a more atom-economical and potentially valuable pathway for C1 resource valorisation. This reaction (Equation 1), however, faces substantial thermodynamic limitations (ΔGo298K=71.0 kJ mol-1). As it can be observed in Figure 67, the equilibrium amount of direct acetic acid formation increases with temperature. Equilibrium studies indicate that at 1 bar pressure, the acetic acid yield is negligible due to these constraints. Increasing the pressure to 30 bar enhances the yield slightly, reaching an optimum at approximately 300 ºC, yet remaining relatively low [25]. This was confirmed by Huang et al. [276], who studied the viability of the direct thermal Chapter 5. Plasma-catalytic biogas reforming 278 discharge behaviour and the presence of a water film favoured the production of oxygenates since they condense and dissolve, thus (i) enhancing their formation rate and (ii) preventing them from decomposition. The presence of water was also studied by Rahmani et al. [289], in this case in the form of steam. The presence of water in the system improved the conversion and energy efficiency. At the lowest amount of steam, the selectivity of acetic acid was maximum, a noteworthy 24.5%. When the steam amount is increased in the system, two phenomena occurred: (i) the excess of water lower the energy available to create excited species, thus reducing the effect of the plasma and (ii) -CH3 radicals tend to react with -OH radicals to form methanol, instead of undergoing to further reaction pathways. On the other hand, the presence of argon has been studied by Rahmani et al. [270]. Its presence enhanced the density and electron temperature in the system, improving the conversion of CH4 and CO2 towards liquid oxygenates. At 34% argon addition, a maximum of liquid oxygenates was observed, being acetic acid a main product. Chapter 5. Plasma-catalytic biogas reforming 279 1.5. Concluding remarks To date, no studies have explored the use of cerium zirconate mixed oxides or the Nibased catalysts synthesised in this work under plasma-assisted conditions for the production of oxygenated compounds, revealing a significant research gap. Most of the available literature focuses on Cuand Co-based systems, which have shown the highest selectivity toward acetic acid, particularly in the presence of Cu+ species, known to stabilise key intermediates. In contrast, the use of Ni-based catalysts in plasma-assisted processes targeting oxygenates has been scarcely reported, and no conclusive evidence has been found regarding their selectivity toward C-C coupled products. This absence of data highlights the relevance of investigating Ni-based formulations, especially considering that Ni is the active phase in this document. Other factors such as discharge gap, dielectric, CH4:CO2 ratio, specific energy input (SEI) and the use of Ar, O2 and H2O have been shown in the literature to influence the plasma environment and improve selectivity. However, further optimisation is needed to better control product distribution and maximise oxygenate yields. Overall, these findings reinforce the importance of catalyst design, active phase selection and reactor parameter optimisation in plasma-assisted DRM conversion. In particular, exploring the performance of Ni-based catalysts under plasma conditions offers a promising direction. Chapter 5. Plasma-catalytic biogas reforming 280 2. Catalytic activity The mixed oxide structure of cerium and zirconium with nickel prepared through a modified version of Pechini method (CZ, CZN5 and CZN10) and the MOF-derived catalyst NCM1 were tested under plasma conditions to assess their activity and efficiency. A power input screening was performed in the range of 20 to 50 W, following the reaction conditions detailed in Chapter 2. This evaluation aimed to compare their performance in the plasma-assisted DRM reaction and identify trends related to plasma power input. The selection of key parameters in this study was based on insights from the literature on plasma-assisted catalytic reactions, ensuring an optimal balance between energy efficiency, reaction performance and practical implementation. The discharge gap of 4 mm was chosen to provide a sufficiently strong electric field while avoiding excessive energy losses and recombination effects that could limit the generation of active species. A discharge length of 10 mm was selected to ensure sufficient plasma exposure time for reactant activation while maintaining stable plasma operation and minimising unwanted thermal effects. The gas flow rate was set at 50 mL min-1 with a CH4:CO2 molar ratio of 1:1, a composition previously used in thermal DRM to produce syngas in a balanced ratio, minimising carbon deposition and optimising conversion levels. Quartz was selected as the dielectric material due to its high breakdown voltage, excellent thermal resistance and stability under plasma conditions. Compared to other dielectric materials, quartz ensures consistent discharge characteristics, allowing for stable plasma generation and efficient catalyst interaction [290]. The input power was varied between 20 and 50 W to assess the influence of energy input on reaction performance taking into account Chapter 5. Plasma-catalytic biogas reforming 281 that excessive power input can lead to energy losses and promote undesirable side reactions. The discharge frequency was set at 54 kHz. The system operates at ambient temperature, a key advantage of plasma-assisted catalysis, enabling the activation of stable molecules such as methane and carbon dioxide without requiring external thermal energy input. The choice of AC rather than DC was made based on its superior ability to sustain stable discharge while preventing electrode degradation and excessive heat accumulation. Stainless steel was used for both internal and external electrodes due to its electrical conductivity, durability and resistance to corrosion in plasma environments. The electrodes, measuring approximately 10 mm in length, were designed to ensure a uniform discharge field, promoting homogeneous plasma formation and improving overall reaction efficiency. A catalyst loading of 0.3 g for CZ, CZN5 and CZN10 was selected based on previous studies [27,291], ensuring an adequate balance between active site availability and optimal plasma-catalyst interactions. In the case of NCM1, 0.1 g of catalysts was used. A higher catalyst loading could lead to excessive surface recombination of reactive species, reducing plasma efficiency, while a lower loading may not provide sufficient active sites for catalytic enhancement. The absence of argon or other noble gases in the system was a deliberate choice to focus on direct plasma activation of reactants without the influence of additional ionisation effects. While noble gases are often used to enhance plasma stability and electron density, their presence could introduce additional complexity in determining the intrinsic effects of plasma-assisted catalysis on methane and carbon dioxide conversion. Chapter 5. Plasma-catalytic biogas reforming 282 2.1. Ni-based cerium zirconate Methane and carbon dioxide conversion results can be observed in Figure 69 and Figure 70, respectively. The experimental results revealed a direct correlation between plasma power and the conversion of CH4 and CO2, with higher power levels leading to increased conversion in all cases. This trend is attributed to the greater availability of energetic electrons and reactive species, which enhanced molecular activation. However, CH4 consistently exhibited higher conversion levels than CO2, reflecting the differences in bond strength and activation requirements. Plasma alone (NC) achieved a notable CH4 conversion, indicating that direct electron impact plays a dominant role in methane dissociation, whereas CO2 conversion was more dependent on catalytic interactions. Figure 69. CH4 conversion at different input power. Chapter 5. Plasma-catalytic biogas reforming 283 Among the catalysts tested, CZN10 showed the highest performance for both CH4 and CO2 conversion, surpassing CZ and CZN5. The improved activity of CZN10 suggests that the combination of nickel and cerium zirconate optimised the interaction between plasma-generated species and the catalyst surface. Nickel facilitated CH4 activation by providing active sites for dissociation, while cerium zirconate enhanced CO2 conversion by creating oxygen vacancies that promote CO2 adsorption and reduction. The lower performance of CZ in CH4 conversion indicated that cerium zirconate alone did not significantly contribute to methane activation. Indeed, there is a clear trend in CH4 conversion which confirms the role of Ni: CZN10 > CZN5 > CZ. Figure 70. CO2 conversion at different input power. CO2 conversion remained lower than CH4 conversion across all power levels, consistent with its stronger C=O bond. The trend observed, CZN10 > CZ > NC > CZN5, suggests that cerium zirconate played an impotant role in CO2 activation. The Chapter 5. Plasma-catalytic biogas reforming 284 improved performance of CZN10 indicated that an optimal balance between nickel and cerium enhanced both CH4 and CO2 conversion by providing complementary catalytic functions. The higher metallic surface area and metal dispersion observed in CZN10, together with the presence of the highest amount of nickel, could explain this behaviour. Figure 71. Product molar fraction distribution across different input power. The selectivity trends observed in the plasma-assisted DRM revealed clear distinctions between non-catalytic (NC) system and the catalysts, particularly in the formation of C2 hydrocarbons. In the NC case, ethane (C2H6) was the dominant C2 Chapter 5. Plasma-catalytic biogas reforming 285 product, reaching a selectivity of approximately 26%, while ethylene (C2H4) remained relatively low at 1-2%. This suggests that in the absence of a catalyst, the plasma environment favoured the recombination of CH3 radicals, leading to the formation of saturated hydrocarbons. The dominance of C2H6 in NC aligns with literature findings on plasma-driven hydrocarbon formation, where radical recombination pathways prevail when no catalytic surface is present to mediate secondary reactions [292], as observed in Figure 72. Figure 72. Schematic overview of the dominant reaction pathways for the conversion of CH4 and CO2. The thickness of the arrow lines is correlated to the importance of the reaction path. Reprinted from [292], with permission from John Wiley and Sons. However, upon introducing catalysts, a significant shift occurs in the C2 product distribution. Ethane selectivity is drastically suppressed, dropping to around 3%, while ethylene selectivity increases to 23%. This shift indicates that the catalytic Chapter 5. Plasma-catalytic biogas reforming 286 surface facilitated dehydrogenation reactions or more complex recombinations which inhibited partially the pure recombination of CH3 radicals. Catalysts with Ni and/or only Ce-Zr enhanced C-H bond scission and favour unsaturated hydrocarbon formation [293–295]. The suppression of C2H6 also suggests that catalytic surfaces effectively prevent radical recombination, directing the reaction pathway towards the formation of more reactive intermediates. The selectivity towards hydrogen and carbon monoxide remained stable across all tested catalysts and power levels, with no significant variations observed between NC, CZ, CZN5 and CZN10. This consistency suggests that the plasma environment primarily dictated syngas formation, with the catalyst exerting a more pronounced effect on conversion and selectivity toward other species. The H2/CO ratio was maintained within the range of 0.4-0.5 across all conditions, indicating that the balance between CH4 and CO2 activation remained steady despite variations in plasma power and catalyst composition. This stability highlights the robustness of the reaction pathways governing syngas production, where plasma-generated reactive species efficiently drive methane and carbon dioxide dissociation, independent of specific catalytic effects. In conclusion, the near-complete suppression of C2H6 in favour of C2H4 across all catalytic systems underscores the role of plasma-catalyst interactions in steering selectivity toward added value unsaturated hydrocarbons. This shift suggests that catalysts facilitated dehydrogenation pathways, likely by stabilising intermediate species that favoured C-H bond scission over full hydrogenation. The interplay between plasma-generated radicals and catalyst surface interactions appears to be key Chapter 5. Plasma-catalytic biogas reforming 287 in determining the composition of hydrocarbon products, further reinforcing the selective control achievable in plasma-assisted reforming. The presence of C3 hydrocarbons across all conditions indicates that higher-order radical coupling reactions occured within the discharge zone, involving CH3 and C2 species. This phenomenon aligns with known plasma-driven mechanisms, where radical recombination leads to chain growth [295]. The fact that the H2/CO ratio remains stable across power levels and catalysts further supports the idea that the primary reaction equilibrium was controlled by plasma conditions rather than catalyst-specific effects. The observed trends suggest that while catalysts enhanced conversion efficiency, the plasma environment established the dominant reaction pathways, leading to a predictable and stable product distribution. Regarding energy efficiency (EE), the results show a clear trend as a function of plasma power for the different catalyst systems tested, observed in Figure 73. As the input power increases from 20 W to 50 W, EE improves for all configurations, reflecting the enhanced conversion of reactants due to the increased availability of energetic species in the plasma. The non-catalytic system (NC) exhibits relatively high energy efficiency, confirming that plasma alone can drive DRM effectively. However, the introduction of catalysts modifies this trend, particularly for CZN10, which consistently achieved the highest EE values across all power levels. The superior performance of CZN10 could be attributed to the synergistic interaction between the Ni active sites and the cerium-zirconium mixed oxide support, which enhances electron transfer and promotes CO2 activation through oxygen vacancies. This improved electron mobility likely increased the efficiency of CH4 activation, Chapter 5. Plasma-catalytic biogas reforming 294 suggesting that the increased nickel content in CZN10 may have enhanced activity, although to a limited extent. Figure 77. CO productivity for all the catalysts across the different power input. Similar trends were observed in CO productivity. NCM1 again led with values ranging from approximately 0.85 × 10-3 to 2.0 × 10-3 mol min-1·𝑔𝑐𝑎𝑡 −1 . The results imply that NCM1 effectively promotes both CH4 and CO2 activation in plasma-hybrid environment, likely due to enhanced surface properties, such as higher dispersion of active sites or increased oxygen vacancy concentration derived from its unique structural features. Chapter 5. Plasma-catalytic biogas reforming 295 Figure 78. C2H4 productivity for all the catalysts across the different power input. In the case of C2H4 productivity, although the values were an order of magnitude lower than those for H2 and CO, NCM1 again exhibited significantly higher productivity than the other catalysts, increasing from approximately 2.5 × 10-4 to 4.7 × 10-4 mol min-1·𝑔𝑐𝑎𝑡 −1 . The formation of ethylene under plasma-catalytic conditions suggests that in addition to dry reforming, plasma-induced C-C coupling pathways are favoured, particularly over NCM1. This behaviour indicates the multifunctional capability of NCM1 to facilitate both syngas production and hydrocarbon coupling to ultimately deliver advanced chemicals beyond syngas. What is clear from these results is that metal dispersion is a key factor in the observed catalytic performance. While CZN10 contains 10 wt.% nickel and NCM1 only 1 wt.%, the latter outperformed the former under plasma conditions. Notably, in conventional thermocatalytic systems, NCM1 lacked stability due to sintering at high Chapter 5. Plasma-catalytic biogas reforming 296 temperatures. However, in plasma-catalysis, the absence of thermal sintering effects allowed NCM1 to exhibit significantly enhanced activity, attributed to a higher Ni0/Nitotal availability ratio, larger surface area and superior active metal dispersion. These attributes are critical under plasma activation, where maximising the accessibility and reactivity of active sites is more impactful than sheer metal loading. Thus, the enhanced performance of NCM1 under plasma conditions is directly linked to its structural advantages and improved nickel dispersion, which collectively drove its superior catalytic behaviour. In addition, all the selectivity combined can be consulted in Figure 79. Figure 79. Summary of the product distribution obtained in all the study cases. Chapter 5. Plasma-catalytic biogas reforming 297 3. Conclusion The plasma-catalytic study conducted in Chapter 5 demonstrated that metal dispersion plays a critical role in determining catalytic performance under non-thermal plasma conditions. Among the catalysts tested, NCM1, despite its low nickel loading (1 wt.%), outperformed CZ, CZN5 and CZN10 in terms of productivity and conversion, owing to its superior Ni0 availability, higher surface area and enhanced dispersion. Other features like dielectric properties are very likely relevant and play a role in plasma catalysis. Presumably NMC1 presents very different dielectric features that those of the mixed oxide CZ family but dielectric behaviour of the prepared solids is beyond the scope of the present thesis and remains as research quest to be addressed in future works. The CH4 and CO2 conversions achieved were significantly enhanced with plasma power, confirming the effective activation of biogas components in the hybrid system. The catalytic systems notably increased selectivity towards C2H4 over C2H6, particularly when compared to the non-catalysed plasma condition (NC), which favoured higher C2H6 production. Furthermore, C2 and C3 hydrocarbons were consistently detected, highlighting the presence of plasma-induced coupling reactions. While no oxygenates were obtained under the tested conditions, the formation of light hydrocarbons such as ethylene and propane indicate a potential basis for downstream oxygenate synthesis, provided that conditions favouring oxygen insertion can be developed in future studies. These findings underline the promise of plasma-catalysis for syngas and hydrocarbon production from biogas, while pointing to the importance of catalyst design and dispersion control to maximise efficiency and expand product scope. Chapter 5. Plasma-catalytic biogas reforming 298 Chapter 6. Overall conclusion and future directions Chapter 6. Overall conclusion and future directions 300 Chapter 6. Overall conclusion and future directions 301 This thesis was conceived within the specific framework of biogas valorisation, with the initial ambition of opening low-carbon (direct and indirect) routes for added value chemicals production such as syngas, acetic acid and C2/C2+ chemicals. In particular, acetic acid was initially targeted. While this overarching objective has guided the research from its inception, the complexity of the dry reforming of methane reaction, especially under plasma-assisted conditions, combined with the inherent challenges of catalyst design, meant that this goal could not be fully achieved within the current experimental scope. Nonetheless, the work led to significant scientific progress, especially through the indirect route, yielding promising results that lay a strong foundation for further development. Although the direct route toward acetic acid via plasma-catalysis has not yet delivered the desired product, the progress made indicates a clear direction forward. In this regard, the thesis not only delivers technical insights but also closes with a reflection that connects directly with the initial research vision, highlighting both the achievements and the road ahead. The early chapters contextualised the relevance of biogas valorisation as a strategy to mitigate greenhouse gas emissions and promote circular carbon use. Biogas was redefined not merely as a by-product of organic waste, but as a renewable carbon feedstock suitable for chemical synthesis. This conceptual shift laid the groundwork for exploring both dry reforming of methane and plasma-assisted catalysis as viable, low-carbon pathways for sustainable energy and chemical production. In this sense, this work demonstrates the sustainable production of syngas, ethylene, propane, ethane among other chemicals from biogas model mixtures using custom-made catalysts under thermal and/or plasma catalysis. Chapter 6. Overall conclusion and future directions 302 In the thermo-catalytic section of the study, particular emphasis was placed on the synthesis and evaluation of nickel-based cerium zirconate catalysts, selected for their redox versatility and strong metal-support interactions. A detailed investigation into the effects of nickel loading and calcination temperature demonstrated that these parameters are crucial for fine-tuning the structural and chemical properties of the catalysts. Specifically, variations in the nickel content significantly influenced metal dispersion, electronic properties and surface availability, which in turn affected methane activation and carbon deposition resistance. Likewise, calcination temperature played a vital role in determining phase composition, surface area and the development of oxygen vacancies, which are essential for CO2 activation and in situ carbon oxidation. The catalysts exhibiting intermediate nickel loading and optimised calcination conditions showed a favourable balance between dispersion and thermal stability, resulting in higher reforming activity and prolonged resistance to deactivation mechanisms such as sintering and coking. Subsequent chapters broadened the investigation by exploring alternative catalytic formulations and synthesis methodologies beyond the cerium zirconate-based catalytic systems. This included the design of bimetallic Ni:Co catalysts, the development of MOF-derived structures through sacrificial template strategies and a direct comparison between conventional wet impregnation and the modified Pechini synthesis route. Although the incorporation of cobalt alongside nickel introduced structural and electronic interactions between the two metals, these synergistic effects did not translate into improved catalytic performance. The bimetallic systems exhibited no enhancement in activity under the conditions studied, suggesting that the Chapter 6. Overall conclusion and future directions 303 specific metal ratio, dispersion or interaction with the support may not have been optimal. These findings highlight the complexity of bimetallic formulations in DRM, where synergistic effects do not inherently guarantee superior performance, and careful tuning of synthesis parameters is essential. In contrast, the use of metalorganic frameworks as sacrificial templates allowed for the generation of catalysts with well-dispersed active phases. These MOF-derived materials exhibited increased surface area and accessible active sites, which initially translated into superior catalytic performance under dry reforming conditions. The specific activity in terms of TOF is highly remarkable and takes the edge over many of the DRM catalysts reported so far resulting in very active MOF-derived catalysts with very low Ni loading (1 wt.%). However, despite their promising activity at early stages, this strategy proved inadequate in terms of long-term stability, as the catalysts underwent rapid deactivation due to metal sintering. The lack of structural robustness under continuous operation underscores the need for improved thermal stability in MOFderived architectures, particularly when applied to high-temperature reforming environments. Furthermore, the synthesis method was shown to significantly influence the physicochemical properties and catalytic behaviour of the final materials. 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