Influence of the preparation method in the metal-support interaction and reducibility of Ni-Mg-Al based catalysts for methane steam reforming

Influence of the preparation method in the metal-support interaction and reducibility of Ni-Mg-Al based catalysts for methane steam reforming Lola Azancot*, Luis F. Bobadilla, José L. Santos, José M. Córdoba, Miguel A. Centeno and José A. Odriozola Instituto de Ciencia de Materiales de Sevilla – Departamento de Química Inorgánica, Centro mixto CSIC – Universidad de Sevilla, Av. Américo Vespucio 49, 41092 Sevilla (Spain) Corresponding author (*): lola.azan[email protected]c.es Abstract Ni-Mg-Al based catalysts were prepared using different preparation methods (impregnation, impregnation-coprecipitation and coprecipitation) and tested in steam reforming of methane. The differences observed in catalytic activity were directly correlated to the physicochemical properties and the different degree of Ni-Mg-Al interaction. The reducibility results showed that the catalyst prepared by the impregnation-coprecipitation method presented the most optimal metal-support interaction to reduce the NiO preserving the Ni0 particles highly dispersed on the support surface. These results demonstrate that the structure and catalytic performance of Ni- Mg-Al based catalysts can be tuned by controlling the metal-support interaction through of the preparation method. Keywords: Ni-Mg-Al catalysts; metal-support interaction; reducibility; methane steam reforming; H2 production. 1. Introduction The most important procedure for large scale manufacture of hydrogen for synthesis of ammonia, methanol, and other petrochemicals is the catalytic steam reforming of natural gas [1, 2]. Methane is the majority component of natural gas and the main reaction that takes place in the steam reforming process is: CH4 + H2O → CO + 3H2 ΔH0298K = 205.8 kJ mol-1 (1) This reaction is highly endothermic and requires high temperatures (700 – 900 ºC) to achieve high conversions because of the strong C-H bond of methane molecule [3]. An excess of steam is typically used to promote the methane conversion and favour the gasification of the carbonaceous deposits formed during the reaction [4]. Moreover, under these conditions the water gas shift (WGS) reaction also takes place producing further hydrogen and CO2 according to the following equation: CO + H2O ↔ CO2 + H2 ΔH0298K = - 41 kJ mol-1 (2) Ni-based catalysts have been extensively proved as efficient catalysts for steam reforming of methane due to its excellent C-H breaking ability. However, these kind of catalysts are prone to deactivation by coke deposition and metal sintering. Different strategies have been proposed to overcome these difficulties. For instance, it has been demonstrated that the addition of a basic metal oxides such as MgO facilitates the coke removal through the gasification of carbon deposits on the catalyst surface [5]. Conventionally, magnesium aluminate spinel (MgAl2O4) is used as an optimal catalyst support for steam reforming of methane due to its low acidity and excellent thermal stability [6, 7]. Guo et al. [8] reported that the use of MgAl2O4 as a support for Ni-based catalysts produce highly dispersed active Ni species very active and resistant to coke formation. Coleman et al. [9] reported that MgAl2O4 spinel compound exhibits moderate acidic-basic sites strength compared to the pure oxide supports and the moderation of the acid–base properties enhances the activity, selectivity, and stability of the catalyst. In this sense, the utilization of Ni-Mg-Al mixed oxides as catalysts for reforming reactions has been also widely investigated due to their particular properties [10-13]. Several parameters such as composition or calcination temperature can influence notably the catalytic properties of Ni–Mg–Al-mixed oxides existing a compromise between the reducibility of Ni and the acid-base properties of material. Lin et al. [14] studied the effect of Ni loading in a series of catalysts prepared from Ni–Mg–Al layered double hydroxides and obtained an efficient catalyst with 12 wt.% of Ni for dry reforming of methane. Pérez- López et al. [15] evaluated the influence of the catalyst composition and the influence of the thermal treatment on the properties of Ni–Mg–Al based catalysts for CO2 reforming of methane. They found an optimal Ni/Mg ratio of 1 < Ni/Mg < 5 and the results revealed that reduction temperature has a strong influence in the catalytic activity and the selectivity. In other study, Li et al. [16] studied the dependence of the composition and reduction pretreatment conditions on the catalytic performance of catalysts containing Ni, Mg and Al. The best catalytic results were obtained for an optimized molar composition of Ni/Mg/Al = 9/66/25. In addition, Katheria et al. [17] also investigated the effect of calcination temperature on the physico-chemical and catalytic properties of Ni/MgAl2O4 catalyst for steam reforming of methane. They stated that an increase in the calcination temperature resulted in a decrease in the Ni crystallite size and the degree of reduction which was attributed to formation of a solid solution of NiO with the support. It is well established in the literature that the metal-support interaction plays a crucial role in the catalytic performance of metal supported catalysts. An optimal metal-support interaction can increase the number of catalytically active sites and prevent the metal sinterization enhancing the catalytic performance [18, 19]. Bonding interactions between active metal and oxide support depend on the chemical nature and surface properties of support and the electronic structure of metal [18]. The role of the oxide support is not only limited to control the active metal dispersion but also often it has an important influence in the catalytic reaction via metal-support interaction [20]. The support can play a key role in controlling the size, shape, dispersion and structural stability of metal active sites during the reaction. The metal-support interaction can be optimized during the synthesis of the catalyst [21] and the preparation method that provides the opimal metalinteraction interaction is always pursued [22]. For instance, Guil-López et al. [23] demonstrated that Ni–Mg–Al interactions governs the reforming activity and they found that Ni-support interactions depend on the nature of the support and the method of preparation. Özdemir et al. [24] evaluated the effect of the calcination temperature on structural and catalytic properties of Ni/MgAl2O4 catalyst. They claimed that high calcination temperatures are beneficial to obtain an optimal catalyst which possess strong metal-support interaction, preserves the Ni dispersion high after reduction, increases the surface basicity and stabilizes the Ni particles against sintering. Within this perspective, a close attention must be played to the preparation method and the treatment stages of the catalyst to achieve an optimal metal support-interaction. Considering all these observations, we have performed a comprehensive study to gain more experimental evidences about how affect the preparation method in the metalsupport interaction of Ni-Mg-Al based catalysts for the generation of hydrogen towards methane steam reforming. The control of the oxide-metal interactions provides a unique opportunity to achieve the rational design of catalysts. 2. Experimental 2.1. Catalysts preparation The catalysts used in this study were prepared using three different procedures, namely (i) impregnation, (ii) coprecipitation-impregnation, and (iii) coprecipitation. The detailed preparation methods of the three catalysts are described below. In all three cases, Al2O3/MgO molar ratio of 1 was used to obtain the stoichiometric spinel MgAl2O4 with nickel loading of 10 wt. %. (i) For impregnation method, a commercial γ-alumina (Spheralite SCS505) was previously milled to obtain particles below the 100 µm. Then, the alumina was impregnated with a 0.7 M solution of Mg(NO3)2·6H2O in ethanol and rotavapored at 50 ºC for removing the solvent excess. Afterwards, the solid was dried overnight at 100 ºC and calcined at 900 ºC for 24 h. The sample was denoted as MgAl (IM). Subsequently, an aqueous 0.05 M solution of nickel precursor salt (Ni(NO3)2·6H2O) was impregnated on MgAl (IM) and the excess of solvent was removed through roto-evaporation at 50 ºC. Finally, the solid was dried overnight at 100 ºC and calcined at 550 ºC for 4 h. The catalyst obtained using this procedure was labelled Ni-MgAl (IM). (ii) In the coprecipitation-impregnation method, the support was firstly prepared by coprecipitation of Mg and Al precursor salts and subsequently the nickel was added by impregnation. For it, an aqueous 0.35 M solution of Mg(NO3)2·6H2O and 0.7 M of Al (NO3)3·9H2O was prepared using an Mg/Al molar ratio of 0.5. An aqueous 2 M solution of NaOH and 0.5 M of Na2CO3 was dropped into the above mixture stirring at 70 ºC until pH = 9. The solution obtained was aged in those conditions for 18 h. Once this stage was completed, the sample was filtered and washed with abundant deionized water to remove sodium and carbonate residues. Finally, it was dried overnight at 100 ºC and calcined at 900 ºC for 24 h. The resulting solid was further impregnated with nickel and thermally treated following an identical procedure as the one described above. This support and catalyst were designated as MgAl (CO-IM) and Ni-MgAl (CO-IM), respectively. (iii) In the other procedure, the catalyst was prepared in a single step by coprecipitation of Ni, Mg and Al. An aqueous solution of the three precursor salts in the form of nitrates Mg(NO3)2·6H2O (0.35 M), Al(NO3)3·9H2O (0.7 M) and Ni(NO3)2·6H2O) (0.05 M) was dropped with an aqueous 2 M solution of NaOH and 0.5 M of Na2CO3 stirring at 70 ºC until pH = 9 was obtained. The resulting mixture was further aged for 18 h at 70 ºC in agitation. Afterwards, the solid obtained was filtered and washed several times with deionized water to remove sodium and carbonate impurities. Finally, the solid was dried overnight at 100 ºC and calcined at 550 ºC for 4 h. This catalyst was assigned as Ni- MgAl (CO). 2.2. Characterization techniques The chemical composition of the solids was estimated by X-ray fluorescence (XRF) spectrometry using a Panalytical AXIOS PW4400 sequential spectrometer with Rh tube as source of radiation. For the measurements, the samples were pelletized onto pressed wafers containing 6 wt. % of wax. The textural properties of the samples were obtained from nitrogen adsorptiondesorption isotherms at liquid nitrogen temperature in a Micromeritics Tristar II apparatus. Before analysis, the samples were degassed at 250 °C for 4 h in vacuum. The crystalline structure and phases identification of the samples was determined by X- ray diffraction (XRD) on a Siemens D-500 diffractometer equipped with Ni-filtered Cu Kα radiation using a voltage of 40 kV and current of 40 mA. The diffraction patterns were recorded from 10° to 90° (2 theta) in step-scan mode at a step of 0.05° and a counting time of 300 s per step. The crystal structure refinements were performed by means of the Rietveld method using the FULLPROF program [25]. A Thompson–Cox–Hastings pseudo-Voigt function convoluted with an axial divergence asymmetry function was chosen to generate the peak shapes. The following parameters were refined: background points, zero point, scale factor, pseudo-Voigt parameters of the peak shape, atomic positions and cell parameters. Structural analysis by Raman spectroscopy was performed in a dispersive Horiva Jobin Yvon LabRam HR800 Confocal Raman microscope with a 20 mW He-Ne green laser (532.14 nm) operating at 5 mW and using a grating of 600 grooves mm-1. A 50x objective (Olympus) was used for both focusing the excitation beam on the sample and collecting the scattered light with a confocal pinhole of 1000 μm. The Raman spectrometer was calibrated using a silicon wafer. Temperature programmed reduction (H2-TPR) measurements were performed in a conventional U-shaped reactor charged with 50 mg of sample. The temperature was increased from RT to 900 °C with constant heating rate of 10 °C min−1 and passing through the catalyst a flow of 50 mL min−1 STP certified 5% H2 in Ar gas mixture. A cold trap of dry ice and acetone was used to condense the water and CO2 formed during reduction. The H2 consumption was followed by a TCD detector and quantified by using CuO powder (99.999%) as standard. Furthermore, the reducibility of the catalysts was also investigated by in situ XRD analysis using a high temperature cell Anton Paar HTK 1200 coupled with an X'Pert Pro Philips diffractometer. The system was equipped with X'Celerator detector with a step of 0.05° and a counting time of 300 s per step. The diffractograms were recorded in the 25 – 900 °C temperature range with acquisition per 100 ºC with heating rate of 10 ºC min-1 under a flow of 100 mL min−1 of 5% H2 in Ar. High resolution transmission electron microscopy (HR-TEM) micrographs of the reduced catalysts were obtained on a FEI Talos electron microscope operated at 200 kV and equipped with a field emission filament. For samples preparation, a few milligrams of each catalyst were deposited directly on 300 meshes holey carbon coated copper free TEM-grid. The particle size distribution and the mean particle sizes were estimated from HR-TEM micrographs by single particle measurement of at least 100 particles. The average nickel particle size was estimated using the following equation: 𝐷[3,2]=∑𝐷𝑖 3𝑣𝑖 ∑𝐷𝑖 2𝑣𝑖 (Eq. 1) where D is the diameter of the particle and v corresponds to the number of particles with this diameter [26]. Temperature-programmed surface reaction of methane (CH4-TPSR) was carried out in a fixed-bed quartz reactor. The calcined samples were heated from room temperature to 900 ºC under flow of 50 mL min-1 STP of 10% v/v CH4 in Ar. The effluent was monitored with a mass spectrometer PFEIFFER Vacuum PrismaPlus controlled by Quadera software. The experiments were performed without a previous reduction pretreatment of the sample to investigate the redox abilities of Ni as function of the metal-support interaction. The MS data selected to analyse the evolution of methane decomposition were CH4 (m/z = 15), H2 (m/z = 2), CO (m/z = 28) and H2O (m/z = 18) for the Ni catalysts. Temperature programmed oxidation (TPO) of spent catalysts was conducted in a fixedbed quartz reactor. The samples were heated from room temperature to 900 ºC with heating rate of 10 °C min−1 under a flow of 50 mL min-1 STP of 20% v/v O2 in helium. The CO2 signal (m/z = 44) was followed by mass spectrometry. 2.3. Catalytic activity measurements Methane steam reforming reaction was carried out in a homemade reaction system including a fixed-bed quartz reactor with internal diameter of 9 mm. The catalytic activity was measured using 100 mg of catalyst diluted in SiC both sieved in the 100-200 µm fractions and achieving a catalytic bed volume of 0.5 mL. Before the reaction, the catalysts were reduced in a flow of 50% H2/N2 (100 mL min-1 STP) at 800 ºC for 1 h. All reactions were performed at atmospheric pressure in the 750 - 850 ºC temperature range using a steam-to-carbon molar ratio of 1.24 (27.7 mL min-1 STP of H2O and 23.3 mL min- 1 STP of CH4), and a space velocity of 60 L g-1 h-1. Reactants and products were analysed and quantified by gas chromatography using an Agilent 7890A model gas chromatograph equipped with a Porapak® Q, a Molecular Sieve 5A, and two thermal conductivity detectors (TCD). The CH4 conversion and H2 yield was calculated according to Eqs. (2) and (3), respectively: CH4 conversion (% )=𝑛𝐶𝐻4𝑖𝑛− 𝑛𝐶𝐻4𝑜𝑢𝑡 𝑛𝐶𝐻4𝑖𝑛 x100 (Eq. 2) H2 yield (%) = 𝑛𝐻2𝑜𝑢𝑡 2 𝑛𝐶𝐻4𝑖𝑛+ 𝑛𝐻2𝑂𝑖𝑛 x100 (Eq. 3) Being n the molar flow of CH4, H2O and H2 respectively and the subscripts in or out correspond to the inlet or the outlet flow. 3. Results and discussion 3.1. Chemical composition and textural properties Chemical compositions obtained by XRF (Table 1) indicate that the loading of Ni, Mg and Al were almost identical to the nominal values for both Ni-MgAl (IM) and Ni-MgAl (CO-IM) catalysts. This indicates that these preparation methods were carried out effectively in both cases. Nevertheless, the Ni-MgAl (CO) catalyst presents a higher value of Ni content than the theoretical one. This fact is owed to the amphoteric nature of Mg (OH)2, which has a Ksp of 5.61 x 10-12 much higher than Ni(OH)2 (5.48 x 10-16) and Al(OH)3 (3 x 10-32). Consequently, the partial precipitation of Mg provokes a small loss of Mg2+ increasing the Ni content [27]. The Al content also was below the nominal value in this latter sample, which can be related with a small loss of Al ions during the coprecipitation and the washing steps. The textural properties of the supports and catalysts prepared are also listed in Table 1. It can be noticed that the support prepared by impregnation method leads to lower specific surface area (SBET) compared to the support prepared by coprecipitation method. This observation indicates that the preparation method affects directly to the textural properties of the MgAl2O4 spinel. On the other hand, a slight decrease in the specific surface area and pore volume was observed in both supports after Ni impregnation, indicating that nickel species were incorporated inside the pores of the support. Regarding the Ni-MgAl (CO) catalyst, this sample showed the smaller average pore diameter. However, the value of specific surface measured for this sample is similar to the value obtained for Ni-MgAl (CO-IM) sample. Figure 1 shows the nitrogen adsorption-desorption isotherms and pore size distributions for all the prepared samples after calcination. As can be observed, the isotherms of all reactions explains the presence of CO observed during the methane reaction decomposition in the three samples. 3.4. Activity tests for methane steam reforming Figure 8 includes the methane conversion and the hydrogen yield as function of the reaction temperature obtained for the three Ni catalysts prepared by the different methods. As can be observed, both conversion and yield increased with the temperature in all the samples and followed an order of Ni-MgAl (CO-IM) > Ni-MgAl (IM) > Ni-MgAl (CO). Interestingly, Ni-MgAl (CO-IM) catalyst exhibited the highest methane conversion and hydrogen yield. This may be due to the fact that in this catalyst there are a considerable amount of metallic Ni small particles highly actives and well dispersed on the MgAl2O4 support. By contrast, the results of TPR (Fig.4) showed that in the Ni-MgAl (IM) catalyst Ni species have strong interactions with the support (MgAl2O4) or is forming part of the structure NiMgAl2O4, being thus hardly reducible and consequently less active. On the other hand, the low activity observed in Ni-MgAl (CO) catalyst can be attributed to the presence of large Ni particles formed during the reduction pretreatment that decreases notably the effectivity of the catalyst. Numerous studies have demonstrated that the catalytic activity for methane steam reforming is affected by the nickel particle size, metal dispersion and the interaction of the nickel particles with the support [43, 44]. For better comparison, we have estimated the intrinsic site activity of the three catalysts in order to evaluate the relative efficiency of the Ni sites. The intrinsic activity provides direct information about the catalytic essence correlating the nature with the structure of the active sites. The parameter used for this purpose was the turnover frequency (TOF), which represents the number of molecules of methane transformed per active site and per second according to the following equation (Eq. 4): TOF = r DNi [s−1] (Eq. 4) where r is the reaction rate expressed as moles of CH4 transformed per mol of Ni and per second, and DNi corresponds to the nickel dispersion [45]. The nickel dispersion was determined on the basis of a cuboctahedron shape model assuming the average Ni particle size obtained by TEM (Figure 6) for the reduced catalysts [46]. The dispersion discloses the total number of nickel atoms accessible for the steam reforming of methane reaction. The estimated values of Ni dispersion after reduction at 800 ºC are included in Table 1. As can be observed, the three Ni-based catalysts presented a similar value of Ni dispersion. From Figure 9, it can be seen that the Ni-MgAl (CO) exhibits a poor catalytic activity in the methane steam reforming reaction while Ni-MgAl (CO-IM) sample presents the highest specific reaction rate. The obtained catalytic data correlate with the balance between the strength of the metal-support interaction and the reducibility of the catalyst. The catalytic performance can be increased by boosting the number of active nickel sites on the surface of reduced catalysts. Among the catalysts tested, Ni-MgAl (CO-IM) catalyst has the optimal metal-support interaction exhibiting the best catalytic performance in terms of methane conversion and hydrogen production. Therefore, it can be assumed that the catalytic activity can be improved by controlling the balance between the reduction degree and the strength of the metal-support interaction towards the adequate preparation method. These results are in agreement with the data reported by Nieva et al. [47] in which large Ni particles with none metal-support interaction led to a very unstable and inactive catalysts whereas small Ni particles in intimate contact with a zinc or magnesium aluminate-like matrix are catalysts highly active in steam reforming of methane. 3.5. Study of carbon deposits Temperature-programmed oxidation (TPO) analysis and Raman spectroscopy were used to characterize the coke species deposited on the three catalysts after methane steam reforming reaction. Figure 10 displays the evolution of the m/z = 44 signals assigned to CO2 during the TPO experiments for the three spent catalysts. From a quantitative point of view, as the area under the TPO curve is proportional to the amount of deposited carbon, it can be stated that the carbon deposited in the Ni-MgAl (CO) catalyst is relatively smaller in comparison with the other two catalysts. The temperature at which the CO2 maximum appeared provides information about the nature of the carbonaceous species deposited onto the catalyst surface. These carbonaceous deposits have different structure orders and morphologies depending on the reaction conditions and the structure of the catalyst. Zhang and Verykios [48] classified the coke deposits in three types, designed as Cα, Cβ and Cγ, respectively. The assigned to Cα usually corresponds to amorphous carbon and can be removed at low temperatures whereas Cβ and Cγ are associated to graphitic or filamentous carbon species very thermally stable. These latter carbon species are hardly gasified and consequently are causing of the catalyst deactivation. The TPO results displayed in Figure 10 show that the amount of Cβ and Cγ carbon species are considerably larger in the two impregnated catalysts. This fact can be understood taking into account that the specific activity for methane transformation is significantly minor in the Ni-MgAl (CO) catalyst in comparison with the other two Ni-impregnated catalysts. Another aspect to be considered is the acidbase properties of the samples. Ni-MgO (CO) sample is essentially composed by a NiO- MgO solid solution and it is well known that MgO is the responsible for the alkaline character. The presence of MgO favors the gasification of carbonaceous deposits [49]. In contrast, for Ni-impregnated catalysts, the nickel species are supported and dispersed on MgAl2O4 spinels. The absence or low content of MgO on surface could increase the acidity in the sample promoting an increase of the amount of coke deposited. The nature of carbon deposits was also evaluated by Raman spectroscopy. As can be observed in Fig. 11, the spectra obtained for all spent catalysts showed two characteristic bands, one at 1348 cm-1 (D-band) associated to the morphological disorder and defects corresponding to graphite with structural imperfections and/or amorphous carbon, and another one at 1589 cm-1 (G-band) that is ascribed to the in-plane C-C stretching vibrations characteristic of ordered structure carbon [50]. Moreover, it should be noticed the appearance of a weak shoulder at ca. 1620 cm-1 (D´-band) and although its connotation is not well understood it is believed that is related to the presence of defects in the graphite structure [51]. In order to identify the structural differences between the carbon species deposited on the samples, Beyssac et al. [52] proposed a simple classification according to the value of R2, which is estimated from the D-to-(G+D) bands intensity ratio. These authors suggested that values of R2 higher than 0.5 are typical of amorphous carbon species while that values lower than 0.5 correspond to structural carbon species. As shown Fig. 11, the Ni-MgAl (IM) and Ni-MgAl (CO-IM) catalysts presented values of R2 slightly lower than 0.5 whereas the Ni-MgAl (CO) sample shows a value of R2 superior to 0.5. This observation indicates that the carbon deposited on the latter catalyst is more amorphous in agreement with the TPO results. Considering findings mentioned above in combination with the economic feasibility of Nibased catalysts, it can be concluded that in view of designing a commercial catalyst for steam reforming, it is crucial the utilization of an adequate preparation method, to adjust the metal-support interaction and the reducibility of the active metal. 4. Conclusions The degree of nickel-support interaction plays a fundamental role in the performance of nickel-based catalysts. The goal of the present study was to investigate the influence of the preparation method on the degree of Ni-Mg-Al interaction and the structure-activity relationship in the methane steam reforming reaction. Differences observed in activity are related to the degree of Ni-Mg-Al interaction and according to the reducibility results the Ni-MgAl (CO-IM) catalyst presented the optimal metal-support interaction to reduce the NiO at lower temperatures and preserve the Ni0 particles highly dispersed on the support surface. This catalyst is basically formed of very small Ni particles strongly interacting with the MgAl2O4 spinel-like matrix and exhibits the higher catalytic activity and the major production of hydrogen. In conclusion, these results demonstrate that the structure and catalytic performance of Ni based catalysts can be tuned by controlling the Ni-support interaction towards of the preparation method. Acknowledgments Financial support for this work has been obtained from the Spanish Ministerio de Economía y Competitividad – MINECO (ENE2015-66975-C3-2-R) co-financed by FEDER funds from the European Union. Lola Azancot and José L. Santos acknowledge the MINECO for their PhD fellowship BES-2016-0077475 and BES-2014-068244, respectively. Moreover, Luis F. Bobadilla and José M. Córdoba thank the contracts of Spanish Government financed in part by the European Regional Development Fund through the “Juan de la Cierva incorporación” program (IJCI-2015-26348) and “Ramón y Cajal” program (RYC-2013-12437), respectively. References [1] S. Sengodan, R. Lan, J. Humphreys, D. Du, W. Xu, H. Wang, S. 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