Low-temperature methanol steam reforming

Low-Temperature Methanol Steam Reforming Hugo José Lopes Silva Dissertation presented for the degree of Doctor of Philosophy in Chemical and Biological Engineering by the University of Porto – Faculty of Engineering LEPABE – Department of Chemical Engineering University of Porto – Faculty of Engineering Porto, 2015 Dissertation supervised by: Adélio Miguel Magalhães Mendes Full Professor Department of Chemical Engineering University of Porto – Faculty of Engineering Cecília Mateos Pedrero Postdoctoral Researcher Department of Chemical Engineering University of Porto – Faculty of Engineering Financial support: Acknowledgements iii Acknowledgements I would like to acknowledge the Portuguese National Founding Agency for Science, Research and Technology (FCT), for the attributed grant SFRH/BD/45890/2008. I am grateful to the European Union’s Seventh Framework Programme (FP7/2007-2013) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement No [303476] due to the funding received for part of this work. I also acknowledge financing from FCT through the project PTDC/EQUEQU/104217/2008. To LEPABE and DEQ that offered me the conditions to conduct the research activity. I express a sincere feeling of gratitude to my supervisors. To Professor Adélio Mendes for giving me the opportunity of being part of an outstanding research group that combines several areas of knowledge. Thank you also, for all the passionate and enthusiastic scientific discussions that revealed to be a truly learning experience. To Dr. Cecilia Mateos Pedrero, I would like to extend my gratitude; thank you for all the support, knowledge, advises and guidance that lead to the definition of this thesis. In days of struggle, you were strength. Your friendship is a gift that I really hope to save and respect through its different timings. I am also grateful to Dr. Sandra Sá, colleague that I had the opportunity to work at LEPABE within the methanol steam reforming (MSR) research field, and that had the valuable contribution concerning the project/assembly of an in-house set-up for evaluating the MSR catalysts performance. The opportunity to be integrated in the same project and work together was fundamental for the results presented in this dissertation. Also, I would like to acknowledge Professor Sousa for the learning experience during the assembly of the control and acquisition box of the set-up, which included LabView programming. I cannot forget to express my gratitude to Dr. Alfredo Tanaka for sharing its vast knowledge and experience in the field of materials science, which was crucial for finding a path in this work. Thank you. Acknowledgements iv I would like to acknowledge Dr. Katarzyna Eblagon for her relevant contribution in chapter 4 of this thesis, concerning the performance of Pd/ZnO catalysts for MSR. The Instituto de Tecnología Química (ITQ), in the Polytechnic University of Valencia (UPV), partner of the BeingEnergy Project, I would like to acknowledge the research period in their facilities and to Abdessamad Grirrane and Professor Hermenegildo Garcia for the reception and guidance. Finnally, to Dr J. L. Jorda Moret and to Dr. P. H. Concepción also from ITQ-UPV for the discussion and analysis of the Pd/ZnO samples through in-situ XRD and CO DRIFT studies, respectively. Paulo Ribeirinha is acknowledged for the simulated results of the kinetic models presented in Chapter 5. The work in this thesis, presented as a compilation of scientific publications, is the outcome of the endeavor of several co-autors, and I would like to acknowledge each and every one for their valuable contributions. To my coleagues in LEPABE I want to say thank you for all the companionship and good moments shared. If you find reasons to smile there is no space for concerns. I would like to say to my Parents and to my sister Raquel that there is not enough time to reattribute all the love and sacrifices that were made during my period at FEUP. Thank you. Finally, to Susana I cannot say that I do not ask myself: where were you all this time? What makes me happy at the present moment is that we have a long future in front of us. I want to live the largest chapter of my life with you by my side. Abstract v Abstract Climate changes due to greenhouse-gas emissions and the continuous growth of energy demand are triggering the search for cleaner and sustainable energy sources for the near future, where hydrogen combined to fuel cell technology is expected to have a key role. Despite being the most attractive fuel for polymer electrolyte membrane fuel cells (PEMFCs), hydrogen storage is still a limiting factor with the currently available technologies, which entails safety risks, non-competitive overall efficiency and lower volumetric density when compared to other fuels. The current options are pressurized hydrogen (700 bar, 39 kgH2·m-3), liquefied hydrogen at cryogenic temperatures (21 K, 70.8 kgH2·m-3) and storage in solids (e.g. Mg2FeH6, 150 kgH2·m-3). Therefore, the on-site hydrogen production in an integrated and compact energy system with fuel cells has been pointed out as an alternative solution for stationary and transportation applications. In this scenario, the methanol steam reforming reaction (MSR) is one very attracting alternative that takes into account all the advantages inherent to the use of the simplest of all alcohols as a hydrogen carrier. This thesis focuses on the development and study of highly efficient catalysts for MSR. The state of the art of these catalysts can be divided in two main groups: copper-based and the group 8-10 metal-based catalysts, largely represented by the Pd/ZnO formulation. Both groups of catalysts are addressed in this thesis and different strategies for improving their performance are presented. As a first approach in this work, a urea-assisted hydrothermal synthesis method was deeply studied with the purpose of tailoring the physicochemical properties of a metal oxide that is ubiquitous in both groups of MSR catalysts - zinc oxide (ZnO). The inclusion of Pluronic P123 block copolymer in the preparation method revealed to be crucial for obtaining highly dispersed ZnO microflowers with enhanced surface area and higher proportion of polar crystal planes (higher polarity). Additionally, the type of metal salt precursor influenced the morphology and polarity properties of ZnO. In this way, when zinc acetate was used it occurred the formation of highly faceted microflowers, whereas zinc nitrate led to urchin-like structures with lower Abstract vi polarity. This simple and easily scalable synthesis method was crucial for evaluating the influence of ZnO properties in a catalytic system for MSR. In a following study, the developed ZnO supports were used for the preparation of Cu/ZnO catalysts. The activity and selectivity of these catalysts was confirmed to be strongly related with the surface area and polarity properties of ZnO. While increasing the surface area, higher dispersion of active copper particles was attained and consequently the activity was enhanced. A noteworthy result was the lower carbon monoxide production of the catalysts with higher proportion of polar planes. Inclusively, the selectivity at high conversion levels was significantly better than a reference commercial catalyst (CuO/ZnO/Al2O3 from Süd-Chemie) under the same kinetic conditions. As an alternative to the copper-based catalysts, the more recent Pd/ZnO formulation shows a surprising shift of selectivity towards MSR when a PdZn alloy is formed. In another study presented within the framework of this thesis, a series of ZnO supports prepared by the hydrothermal route, were calcined under different gas atmospheres (i.e. H2, N2, O2 and air). The support calcined in a H2 atmosphere presented an enhanced performance for MSR, which was associated to the higher concentration of oxygen vacancies on ZnO surface. Again, in this group of catalysts, the support properties had an impact on the catalyst performance. As a final study, a novel CuZrDyAl catalyst formulation was prepared by the coprecipitation method and a kinetic study within the low-temperature MSR range was performed. The developed catalyst was then compared with the CuO/ZnO/Al2O3 commercial catalyst and showed better performance in terms of selectivity (namely, yielding lower CO concentration) and activity. This behavior was attributed to the improved reducibility of the copper particles in the CuZrDyAl catalyst. The parameters of a simple power-law equation and two mechanistic kinetic models were determined. The best fitting with the experimental data was obtained when using mechanistic Model 3, based on the reported work from Peppley et al. for the commercial CuO/ZnO/Al2O3. Noteworthy, is the small number of MSR kinetic studies within the temperature range of 170 ºC-200 ºC. Resumo ix Resumo As mudanças climáticas devido às emissões de gases estufa e o crescimento contínuo do consumo energético fomentam a procura de novas fontes de energia ambientalmente limpas e sustentáveis para um futuro próximo, onde se espera que hidrogénio combinado com a tecnologia das celúlas de combustível venha a ter um papel crucial. Apesar de ser o melhor combustível para as células de combustível de membrana de permuta iónica, o armazenamento de hidrogénio ainda é um fator limitante com as tecnologias actualmente disponíveis, envolvendo riscos para a segurança, uma eficiência global pouco competitiva e uma menor densidade volumétrica quando comparado a outros combustíveis. As opções atuais são o armazenamento de elevada pressão (700 bar, 39 kg H2·m-3), a liquefação do hidrogénio a temperaturas criogênicas (21 K, 70,8 kg H2·m-3) e o armazenamento em sólidos (por exemplo Mg2FeH6, 150 kg H2·m-3). A produção in situ de hidrogénio num sistema integrado de energia e compacto com células de combustível, tem sido apontada como uma solução alternativa para aplicações estacionárias e no sector dos transportes. Neste cenário, a reação de reformação com vapor de metanol é uma alternativa muito atractiva que tem em conta todas as vantagens inerentes à utilização do mais simples de todos os álcoois para o transporte de hidrogénio. O principal foco desta tese foi o desenvolvimento e estudo de catalisadores altamente eficientes para a reformação com vapor de metanol. Com base no estado da arte destes catalisadores, estes podem ser divididos em dois grupos principais:os catalisadores à base de cobre e os catalisadores do grupo 8-10, representados principalmente pela formulação de Pd/ZnO. Ambos os grupos de catalisadores são abordados nesta tese e diferentes estratégias para a melhoria do seu desempenho são apresentadas. Como uma primeira abordagem neste trabalho, foi desenvolvido um método de síntese hidrotérmica com a finalidade de controlar as propriedades físico-químicas de um óxido de metal que é ubíquo em ambos os grupos de catalisadores para a reformação com vapor de metanol - óxido de zinco (ZnO). A inclusão de um copolímero (Pluronic P123) no método de preparação revelou-se crucial para a obtenção de microestruturas de ZnO altamente dispersas, com uma elevada área Resumo viii superficial e uma maior proporção de planos cristalinos polares (maior polaridade). Além disso, o tipo de precursor usado também influenciou a polaridade e a morfologia do ZnO. Desta forma, quando acetato de zinco foi utilizado ocorreu a formação de microestruturas altamente facetadas, enquanto que o nitrato de zinco conduziu à formação de estruturas com menor polaridade. Este método de síntese simples e que possibilita um fácil aumento de escala, foi de importância crucial para avaliar a influência das propriedades do ZnO num sistema catalítico para a reformação com vapor de metanol. Num segundo estudo, os suportes catalíticos de ZnO foram usados para a preparação de catalisadores de Cu/ZnO. A actividade e selectividade destes catalisadores revelou-se estar relacionada com a área superficial e polaridade do ZnO. Com o aumento da área superfical, maior dispersão de partículas de cobre activas foi alcançada e, consequentemente, a actividade aumentou. Um resultado interessante foi a menor produção de monóxido de carbono dos catalisadores preparados usando suportes de maior polaridade. Inclusive, a seletividade para níveis elevados de conversão foi significativamente melhor do que a selectividade de um catalisador comercial de referência (CuO/ZnO/Al2O3 - Süd-Chemie). Como uma alternativa aos catalisadores à base de cobre, a formulação de Pd/ZnO mostra uma surpreendente mudança de selectividade no sentido da reação de reformação com vapor de metanol, quando uma liga metálica de PdZn é formada. Em outro estudo apresentado no âmbito desta tese, uma série de suportes de ZnO preparados pelo método de síntese hidrotérmica, foram calcinados sob diferentes atmosferas de gasosas (H2, N2, O2 e ar). O suporte de ZnO calcinado numa atmosfera de H2 apresentou um melhor desempenho, o que foi associado à maior concentração de lacunas de oxigénio na superfície deste suporte. Mais uma vez, neste grupo de catalisadores, as propriedades do suporte tiveram um impacto sobre o desempenho do catalisador. Num último estudo, uma nova formulação catalítica para a reformação com vapor de metanol é apresentada: CuZrDyAl. Este catalisador foi ulizado para realizar um estudo cinético numa gama de baixas temperaturas. O desempenho do catalisador desenvolvido foi comparado com o catalisador comercial de referência Resumo ix CuO/ZnO/Al2O3 e apresentou melhores resultados em termos de seletividade (ou seja, produzindo menor concentração CO) e atividade. Este resultado encontrou-se estar relacionado com a melhoria da reducibilidade das partículas de cobre. Foram determinados os parâmetros de ajuste para um modelo empírico e para dois modelos mecanísticos. O melhor ajuste dos dados experimentais foi obtido para o modelo 3, com base no trabalho publicado por Peppley et al. para o catalisador comercial de CuO/ZnO/Al2O3. É de realçar o número reduzido de estudos cinéticos na gama de temperaturas entre 170 ºC-200 ºC. Figure Captions xviii Figure 2.6 - SEM images of the ZnO products prepared with different nP123/nAc molar ratios: A) 0.03 (ZnAcP3T90), B) 0.07 (ZnAcP7T90), C) 0.10 (ZnAcP10T90) and D) 0.20 (ZnAcP20T90). _______________________________________________________ 53 Figure 2.7 - Evolution of the specific surface area of ZnO samples as a function of the Pluronic P123 content (ZnAcPyT90 series: Zn-acetate and hydrothermal temperature 90 ˚C). Lines were added for readability. _________________________________ 54 Figure 2.8 - XRD pattern and I(002)/ I(100) ratio of the ZnO samples synthesized from Zn-acetate, at 90 ˚C and with different Pluronic P123 content. _______________ 55 Figure 2.9 – O 1s XPS spectrum for (A) ZnAcP0T90 and (B) ZnAcP10T90 samples prepared in the absence and presence of Pluronic P123, respectively. _________ 56 Figure 2.10 - SEM images of the ZnO samples synthesized at different hydrothermal temperatures: A) 70 °C (ZnAcP10T70); B) 90 °C (ZnAcP10T90) and C) 110 °C (ZnAcP10T110). ______________________________________________________ 60 Figure 2.11 - XRD pattern and I(002)/I(100) ratio of the ZnO samples prepared at different hydrothermal temperatures. ___________________________________ 61 Figure 2.12 - Photocatalytic degradation of MB over: ZnAcP10T90 (SBET= 80 m2·g-1; I(002)/I(100) = 1.10), ZnNP10T90 (SBET=76 m2·g-1;I(002)/I(100)= 0.62) and ZnAcP7T90 (SBET=53 m2·g-1; I(002)/I(100)= 0.92). Lines are added for readability. _____________________ 62 Figure 2.13 - pH and temperature of the hydrothermal solution as a function of the reaction time for ZnAcP10T90 sample. Lines are for readability. _______________ 63 Figure 2.14 – SEM images showing the morphology evolution during the hydrothermal synthesis of the HZC in the presence of Pluronic P123 (same synthesis conditions as for ZnAcP10T90 sample: Zn-acetate, 0.10 (nP123/nAc) molar ratio and 90 °C), after: (A) 30 min, (B) 75 min, (C) 150 min, (D) 195 min and (E) 24 h. ________ 64 Figure 2.15 - SEM images showing the morphology evolution during the hydrothermal synthesis of the HZC in the absence of Pluronic P123 (same synthesis conditions of ZnAcP0T90 sample: Zn-acetate, no P123 and 90 °C), after: (A) 215 min, (B) 230 min, (C) 250 min, (D) 270 min and (E) 24 h. _________________________ 64 Figure 3.1 - Catalytic activity at 180 °C as a function of the specific surface area of ZnO carriers and copper dispersion. _____________________________________ 94 Figure Captions xvii Figure 3.2 - Evolution of CO concentration (at 300 °C) as a function of the polarity ratio of ZnO carriers. _________________________________________________ 95 Figure 3.3 - Overall product molar fraction vs time on stream for MSR reaction in the Pd/Al2O3 MR at T = 220 °C, transmembrane pressure = 2.0 bar, WHSV = 0.95 h-1, H2O/CH3OH = 2.5/1. __________________________________________________ 98 Figure 4.1 - XRD pattern of Zn4CO3(OH)6•H2O (precursor), ZnO calcined in N2 (ZnO_N2), ZnO calcined in O2 (ZnO_O2) and ZnO commercial (ZnO_COM). _____ 118 Figure 4.2 - HRTEM micrographs of A) ZnO_H2 and B) ZnO_COM. ____________ 120 Figure 4.3 - SEM images of the ZnO particles prepared by calcination in different atmospheres. A) ZnO_O2; B) close-up of ZnO_O2; C) ZnO_H2; D) close-up of ZnO_H2; E) particle size distribution of ZnO_O2 and F) particle size distribution of ZnO_H2. 121 Figure 4.4 - XRD pattern obtained during reduction of PdZn_ZnO_COM at various temperatures (°C). Large peak at 2θ = 47.7°is ascribed to ZnO (102) plane [29]. _ 124 Figure 4.5 - Left: HRTEM image of a single polycrystalline PdZn particle supported on ZnO_H2. The lattice fringes of PdZn (101) and ZnO (100) are marked. Right: HRTEM image of single PdZn and Pd particles supported on ZnO_COM. _____________ 124 Figure 4.6 - (A) HRTEM image of PdZn/ZnO_H2. (B) Particle size distribution of the same sample. ______________________________________________________ 126 Figure 4.7 - Comparison of the O 1s spectra of PdZn_ZnO_H2 (top) and PdZn_ZnO_O2 (bottom) with the assigned peaks from various oxygen species. _ 130 Figure 4.8 - Representative highly resolved core level Zn 2p3/2 spectrum of PdZn_ZnO_COM. ___________________________________________________ 132 Figure 4.9 - The Zn LMM line of PdZn/ZnO catalysts as compared to pure ZnO_H2_support. ___________________________________________________ 132 Figure 4.10 - Representative XPS spectra of the elemental peaks of Pd 3d in PdZn/ZnO_H2 ______________________________________________________ 134 Figure 4.11 - H2-TPR profile of ZnO_H2 and PdZn/ZnO catalysts. ______________ 136 Figure 4.12 - CO adsorption at room temperature on A) PdZn/ZnO_H2; B) PdZn/ZnO_N2; C) PdZn/ZnO_O2 and D) PdZn/ZnO_COM. Spectra 1–4 were recorded with increasing time of CO exposure. ___________________________________ 139 Figure Captions xviii Figure 5.1 - Nitrogen adsorption–desorption isotherm of synthesized CuZrDyAl after calcination at 360 °C (A) and as received G66 MR (B) samples. SEM images of both samples with a magnification of x40,000 (inset). __________________________ 155 Figure 5.2 - XRD patterns of synthesized CuZrDyAl after calcination at 360 °C and as received G66 MR samples. ___________________________________________ 156 Figure 5.3 - H2–TPR profiles of the CuZrDyAl and G66-MR catalysts. The H2-TPR profile of a bulk CuO is also shown for comparison. _______________________ 158 Figure 5.4 - Parity plots of the experimental and predicted reaction rates using different models. Reaction conditions: steam/methanol = 1.5, P = 1 bar, catalyst weight (W) = 0.4 g, methanol flow rate = 0.02-0.06 mL/min. ________________ 165 Figure 5.5 - Experimental (symbols) and simulated (solid lines; Model 1 and Model 3) results for methanol conversion versus the space-time ratio atdifferent temperatures. Reaction conditions: steam/methanol = 1.5, P = 1 bar, catalyst weight (W) = 0.4 g, methanol flow rate = 0.02 – 0.06 mL/min. _____________________ 167 Figure 5.6 – Methanol conversion as a function of 3 0  cat CH OH WF ration of synthesized CuZrDyAl and commercial G66-MR (Süd-Chemie) catalysts; reaction conditions : steam/methanol = 1.5, T = 180 °C, P = 1 bar, catalyst weight (W) = 0.4 g, methanol flow rate = 0.02-0.06 mL/min. ________________________________________ 168 Table Captions xix Table Captions Table 1.1 - Physicochemical properties of copper catalysts found in the literature. 13 Table 1.2 - Influence of the preparation method on the performance of different Cubased catalysts for MSR. ______________________________________________ 14 Table 1.3 - Influence of the type of promoter on the performance of different Cubased catalysts for MSR. ______________________________________________ 16 Table 1.4 - Summary table for MSR over various supported group 8–10 catalysts. 18 Table 1.5 - Energy consumption for heating the water/methanol mixture (molar ratio of 1.5) and for performing the MSR reaction. _________________________ 23 Table 2.1 - Experimental parameters studied for the preparation of ZnO samples (ZnxPyTz: x Zn-precursor; Py Pluronic P123 amount; Tz synthesis temperature). __ 51 Table 2.2 - XPS relative composition of the O1s peak for the series of ZnO samples prepared with increasing Pluronic P123 concentration. _____________________ 57 Table 3.1 - Table 1. Experimental parameters studied for the preparation of ZnO samples, calcination temperature series (ZnAc-CT: Ac Zn-acetate as precursor; CT: calcination temperature); Zn-precursor series (ZnOx-375: x stands for Zn-acetate (Ac) or Zn-nitrate (N); both samples were calcined at 375 °C). ________________ 81 Table 3.2 - H2-TPR data, CuO mean crystallite size and dispersion for CuO/ZnO samples. ___________________________________________________________ 88 Table 3.3 - Methanol conversion, CO2 and H2 yields, and CO amount measured at 220 °C and 300 °C in the conventional fixed bed reactor;   3 01 cat CH OH cat W F 83kg mol s     . ________ 93 Table 3.4 - Permeation characteristics of the fresh composite Pd/Al2O3 membrane at 300 °C and ΔP = 1.0 bar. ______________________________________________ 97 Table 3.5 - Methanol conversion (into gas) and output molar fractions (H2, CO and CO2) at different temperatures, WHSV = 0.95 h-1 and transmembrane pressure = 1.0 bar _______________________________________________________________ 98 Table 3.6 – Hydrogen permeate purity and hydrogen recovery vs reaction pressure at 330 °C H2O/CH3OH = 1.5/1 and WHSV = 2.73 h-1 during MSR reaction in the Pd/AL2O3 MR. _______________________________________________________ 99 Table Captions xx Table 3.7 – Flow rates of the gases present in the permeate stream at different reaction pressure during MSR reaction at 330 °C, WHSV = 2.73 h-1. ____________ 99 Table 3.8 – Hydrogen permeate purity and hydrogen recovery vs WHSV at 330 °C, H2O/CH3OH = 1.5/1 and reaction pressure = 2.5 bar during MSR in a Pd/Al2O3 MR. _________________________________________________________________ 100 Table 3.9 - Permeation characteristics of the composite Pd/Al2O3 membrane at the end of the whole experimental campaign. _______________________________ 100 Table 4.1 - The physicochemical characterization of the ZnO supports studied. _ 118 Table 4.2 - Catalytic performance of a series of PdZn/ZnO catalyst in low temperature MSR together with corresponding particle size measured from HRTEM images. ___________________________________________________________ 126 Table 4.3 - BE and relative intensities of components of O 1s XPS spectra. _____ 131 Table 4.4 - BE and composition of Pd 3d XPS spectra of studied PdZn/ZnO catalysts. _________________________________________________________________ 134 Table 5.1 - Average crystallite size of CuO determined from the XRD data using the Scherrer equation and chemical composition (ICP) of CuZrDyAl and G66 MR samples. __________________________________________________________ 157 Table 5.2 - H2-TPR data of bare CuO, CuZrDyAl and G66 MR catalysts. ________ 159 Table 5.3 - Parameters determined for the different models. The adsorption equilibrium enthalpies (∆Hi) are in kJ·mol-1 and the entropies of adsorption (∆Si) are in J·mol·k-1 and values labeled with “p” refers to the data from Peppley et al. [13]. _________________________________________________________________ 165 Table 5.4 - Comparison between the parameters obtained for the CuO/ZnO/Al2O3 (Süd-Chemie, G66 MR) and the CuZrDyAl catalysts in this work when considering Model 3 __________________________________________________________ 168 Chapter 1 Chapter 1 3 Chapter 1. Introduction The global energy system has currently two major concerns that call an immediate intervention: the continuous growth of energy demand and climate changes. According to the international energy agency (IEA) the global primary energy demand is projected to have an increase of 1.5 % per year between 2007 and 2030 [1]. The predicted scenario indicates a 40 % increase in tonnes of oil equivalent (toe), assuming that no changes will occur in the energy sector – Figure 1.1. Figure 1.1 - World primary energy demand by fuel relative to the IEA projections in 2008 (image adapted from IEA 2009 [1]). This indicates an unsustainable trend of the present energy system. The climbing of energy demand is combined with a dependence in hydrocarbons resources (oil, gas and coal) that are finite. As a reminder, the recent rising in oil demand placed the price per barrel in a new record at July 2008, exceeding the 140 US dollars per barrel [2]. The energy sector has also been held responsible for the alarming climate changes of our time, mainly due to greenhouse-gas emissions (see Figure 1.2). This awareness is causing political intervention to settle targets of CO2 concentration levels. As a belief, the barrier of the 450 ppm in CO2 equivalent is the upper limit for maintaining the temperature changes below the 2 ˚C, having as reference the pre-industrial period [3]. Introduction 4 Figure 1.2 - Atmospheric concentration of CO2 in connection with the temperature changes (image adapted from IEA 2013 [3]). Looking at these projections is easy to understand that is imperative to develop cleaner and sustainable energy sources. 1.1. Fuel Cells As an environmentally friendly technology, fuel cells are expected to have a key role in the near future for the clean production of electric power. By definition, a fuel cell is an energy conversion device that electrochemically converts chemical energy within two reactants, a fuel and an oxidant, into electrical energy, heat and water as by-product [4]. The fuel cells working principle is very simple and four main components are responsible for the electrochemical reactions: electrically negative electrode (anode) where the fuel is supplied, electrically positive electrode (cathode) where an oxidant is supplied, an electrolyte that separates the two electrodes and an external electrical circuit that connects the electron migration from the anode to the cathode side [5]. Generally, fuel cells are classified according to the nature of electrolyte used. Based on this criterion, the following types of fuel cells were developed: alkaline fuel cells (AFC) that use alkaline solution electrolytes; phosphoric acid fuel cells (PAFCs) using phosphoric acid electrolyte; polymer electrolyte membrane fuel cells (PEMFC) using proton exchange membranes; molten carbonate fuel cells (MCFC) using molten carbonate salt electrolytes; solid oxide fuel cells (SOFC) using ceramic ion conducting electrolyte in solid oxide form. Chapter 1 5 When compared to other fuel cells, PEMFCs are regarded as the most promising and versatile reaching the portable (5 W-20 kW), stationary (0.5 kW-400 kW) and transport (1 kW-100 kW) areas of application [5]. This technology is discussed in the following section. 1.1.1. PEMFCs The polymer electrolyte membrane fuel cells (PEMFCs) are considered the best suitable fuel cells for commercial applications, especially as mentioned before for the transport and small scale stationary applications. Their popularity is mainly due to the simple operation process, moderate operating temperatures (60-80 ˚C), fast starting up time, high power densities (ca. 300-1000 mW·cm-2) [6] and long-term stability (ca. more than 20,000 h [7]). Typically, hydrogen is the elected fuel, leading to the higher efficiencies. However, a common subtype of PEMFCs is the direct methanol fuel cells (DMFCs) that use methanol as fuel instead of hydrogen. Nevertheless, there are several challenges faced by DMFC technology that include high catalyst usage, which results in higher costs, lower power density, lower efficiency as result of methanol crossover and shorter operable life compared with direct hydrogen systems [8-10]. Therefore, the former technology has lower competiveness than the hydrogen operated PEMFCs. However, the PEMFCs face some drawbacks of its own to overcome. One common disadvantage presented by PEMFC is the high cost of the anode catalyst, typically made of a Pt alloy; the catalyst loading is of 0.2-0.3 mg·cm-2 for both anode and cathode [11]. Another crucial matter is the sensitivity of the electrocatalyst to fuel impurities. For instance, the presence of CO even in trace amounts (10 ppm) causes a serious loss of effectiveness of the anode electrocatalyst [5]. Presently, the Pt-Ru alloy catalysts are responsible for improving the tolerance levels for 200 ppm [5]. Nevertheless, the hydrogen supplied to the anode should be highly pure. Recently high temperature polymer electrolyte fuel cells (HT-PEMFCs) technology shifted the tolerable limits of CO to 20,000-30,000 ppm [12]. These fuel Introduction 12 selectivity. However, concerning the catalytic activity, the later catalysts have in most studies the disadvantage of producing less hydrogen than the copper-based ones. 1.4.1. Copper-based catalysts The most common catalysts for MSR are the copper-based ones. In order to improve their catalytic activity, several approaches are reported in the literature. Some studies are based on the addition of promoters [40-45], while others focus on the effect of the preparation method [46-50]. The later effect has been investigated by several authors, who found that each step can affect the properties of the catalyst. In other words, similar catalysts prepared by different methods can present distinct catalytic properties. 1.4.1.1. Status of copper catalysts: metal dispersion, surface area and particle size The performance of copper-based catalysts is affected by the status of copper. More specifically, high values of copper dispersion and metal surface area, along with small particle sizes, are the targets to attain for the production of highly active catalysts. A summary of the physicochemical properties of various catalysts found in the literature is presented in Table 1.1. In its turn, the influence of the preparation method and promoter on the performance of the steam reforming reaction catalysts in presented in Table 1.2 and 1.3. To facilitate the comparison between different catalysts, some authors use the commercially available CuO/ZnO/Al2O3 catalyst as a reference [51-53]. An extensive variety of preparation methods can be found in the literature for the copper-based catalysts, namely the conventional co-precipitation [40, 54-56] and wet impregnation [53,57-59] methods. To enhance the catalytic activity and selectivity of the synthesised catalysts, some authors propose new preparation methods or a variation of the conventional ones. It is worth noticing that, in this study, selectivity is always towards the formation of CO unless said otherwise: Chapter 1 13 2 100  CO CO CO CO F SFF (1.7) where, FCO and FCO2 are CO and CO2 flow rates, respectively, and SCO is the selectivity towards the formation of CO. Table 1.1 - Physicochemical properties of copper catalysts found in the literature. Catalyst Preparation method SBET (m2·g-1) SCu (m2·g-1) Cu dispersion (%) Cu/ZrO2 [56] IMP 13.1 1.0 - Cu/ZrO2 [56] CP 64.2 3.5 - Cu/ZrO2 [56] OGCP 71.5 18.4 - Cu/ZnO [59] CP 47.5 20.8 - Cu/ZnO/Al2O3 [59] CP 90.8 23.7 - Cu/ZnO/ZrO2/Al2O3 [59] CP 129.7 25.9 - Cu/Zn/Al [60] WT 63.9 - - Cu/Zn/Al [60] HS 84.5 - - Cu/Zn/Al [60] CP 93.7 - - Cu/ZnO [40] CP 40.7 16.0 - Cu/ZnO [40] HP 76.4 41.6 - Cu/ZnO/Al2O3 [40] HP 97.5 47.0 - Cu/ZnO [61] CP 48.6 20.8 9.6 Cu/ZnO/Al2O3 [61] CP 91.9 22.2 11.3 Cu/ZnO/ZrO2 [61] CP 81.8 15.5 13.2 Cu/ZnO/ZrO2/Al2O3 [61] CP 116.2 23.3 23.2 Cu/Zn/Al [62] WT 152 - - Cu/Zn/Ce/Al [62] WT 162 - - Cu-Mn [63] CP 9.6 - - Cu-Mn [63] CP 55.2 - - Cu-Mn spinel [63] SRG 118.1 - - CuO/CeO2 [64] CT 153 - - CuMn2O4 [64] ST 144 - - CuO/ZnO/Zr2O2 [64] CP 64 - - CP: Co-precipitation CT: Carbon template HP: Homogeneous precipitation HS: Hydrothermal synthesis IMP: Impregnation OGCP: Oxalate gel co-precipitation SRG: Soft reactive grinding technique ST: Silica template WT: Wet impregnation Introduction 14 Table 1.2 - Influence of the preparation method on the performance of different Cu-based catalysts for MSR. Catalyst Prep. method T (°C) XMeOH (%) Activity (μmolH2·gcat-1·s-1) yCO SCO (%) W/F0 kgcat·mol-1s Cu/ZrO2 [56] IMP 260 10 3 0 - - Cu/ZrO2 [56] CP 260 62 56 0.009 - - Cu/ZrO2 [56] OGCP 260 100 90 0.005 - - Cu/ZnO [40] CP 250 46.4 51 - 0.4 - Cu/ZnO [40] HP 250 94.2 105 - 0.4 - Cu/ZnO/Al2O3 [40] HP 250 97.3 109 - 1 - CuZn [65] IMP 230 0 - - - - CuZn [65] IMP 300 36.8 - - 7.1 - CuZn [65] IMP-BD 230 59.5 - - 0 - CuZnZrAl [66] SQ 260 61 - - 4.5 150 CuZnZrAl [66] CI-NP 260 63 - - 3.0 150 CuZnZrAl [66] CI 260 65 - - 7.0 150 Cu/Zn/Al2O3 [66] (Commercial) - 260 60 - - 3.2 150 Cu-Mn [41] OGCP 260 59.7 51 - 1.4 - Cu-Mn [41] CP 260 65.7 56 - 1.1 - Cu-Mn spinel [41] SRG 260 92.9 79 - 0.7 - CuO/ZrO2 [42] PTSG 250 92 - 0.002 - - CuO/ZnO/Al2O3 [42] (Commercial) - 250 92 - 0.1 - - Cu/Zn/Zr/Al [48] WT 260 60 159 0.016 - 11.0 Cu/Zn/Zr/Al [48] CP 260 97 261 0.008 - 11.0 Cu/ZnO [49] CP 240 43.0 36 0.0018 - Cu/ZnO [49] CP 240 48.6 41 0.0022 - Cu/ZnO [49] SRG-C 240 52 44 0.001 6 - Cu/ZnO [49] SRG-N 240 70.9 59 0.002 6 - CP: Co-precipitation CI: Co-impregnation CI-NP: Co-impregnation with nanoparticle precursor HP: Homogeneous precipitation IMP: Impregnation IMP-BD: Modified impregnation with 1,3butanediol OGCP: Oxalate gel co-precipitation PTSG: Polymer template sol-gel method SQ: Sequential impregnation SRG: Soft reactive grinding technique SRG-N: Soft reactive grinding of oxalic acid with Cu/Zn nitrate precursors SRG-C: Soft reactive grinding of oxalic acid with Cu/Zn carbonate precursors WT: Wet impregnation Chapter 1 15 Promoters have been used to influence the status of copper and enhance the performance of the catalyst. Studies on the promotional effects of zirconia [59, 61] have revealed that this structural promoter decreases the CO selectivity – Table 1.3. For instances, Lindström et al. [50] studied the effect of zirconia in aluminasupported monolithic Cu-Zn catalysts. Although the Zr doped catalysts were less active than the ones without Zr, the selectivity towards CO2 was higher. Jeong et al. [59] compared the performance of Cu/ZnO/Al2O3 to the one of Cu/ZnO/ZrO2/Al2O3, and reported an increase of approximately 16 % in methanol conversion and a CO molar fraction 7.3 times lower, due to the presence of ZrO2. Additionally, zirconia can promote copper dispersion as presented in Table 1.4. Agrell et al. [61] reported an increase of ca. 37.5 % in copper dispersion after adding ZrO2 to Cu/ZnO catalyst. Finally, ZrO2, as well as ZnO, can prevent copper particles from aggregation and help stabilize the crystal size of copper [68]. Another structural promoter is Al2O3, which provides a larger surface on which copper can be dispersed [59, 61]. Agrell et al. [61] observed an increase of both total surface area (48.6 to 91.9 m2·g-1) and copper dispersion (9.6 to 11.3 %) due to Al2O3 – Table 1.3. A similar effect can be attained by adding Cr2O3 [69, 70], which acts as stabilizer of the copper structure reducing sintering. The promotional effects of CeO2 have been described in the literature [47, 52, 64, 70], in particular, Liu et al. [53] reported high activity of the Cu/CeO2 catalysts compared to Cu/ZnO, Cu/Zn(Al)O and Cu/Al2O3 with the same Cu loading and under the same reaction conditions. It was suggested that the high activity of the Cu/CeO2 catalysts was due to the highly dispersed Cu metal particles and the strong metal-support interaction between the Cu metal and CeO2 support. The catalytic activity has been reported to improve with the addition of yttria [71] which appears to stabilize a high copper surface area [72]. Finally, Houteit et al. [54] reports that cesium oxide can prevent copper oxide crystallites from sintering and its reduction into metallic Cu. Introduction 16 Table 1.3 - Influence of the type of promoter on the performance of different Cu-based catalysts for MSR. Catalyst T (°C) XMeOH (%) Activity (μmolH2·gcat-1·s-1) yCO SCO (%) W/F0 kgcat·mol-1s Cu/ZnO [59] 260 75 - 0.0073 - Cu/ZnO/Al2O3 [59] 260 79 - 0.0073 - Cu/ZnO/ZrO2/Al2O3 [59] 260 92 - 0.0010 - Cu/Zn/Al [73] 250 39 - - 0.6 24.3 Cu/Zn/Zr/Al [73] 250 61 - - 0.4 24.3 Cu/SiO2 [74] 300 50 - - 0.15 ZnO/Cu/SiO2 [74] 300 75 - - 0.6 Cu/Zn/Al2O3 [74] (Commercial) 300 57 - - 1.1 Cu/ZnO/Al2O3 [75] (Commercial) 270 59.0 - - - 8.0 Cu/ZnO/Al2O3 [75] 270 89.2 - - 0.92 8.0 Cu/ZnO/ZrO2/Al2O3 [75] 270 92.4 - - 0.97 8.0 Cu/ZnO/CeO2/ZrO2/Al2O3 [75] 270 89.4 - - 0.85 8.0 Cu/ZnO/CeO2/Al2O3 [75] 270 79.3 - - 0.66 8.0 Cu/ZnO [61] 308 90 - 0.0014 - Cu/ZnO/Al2O3 [61] 305 90 - 0.0011 - Cu/ZnO/ZrO2 [61] 295 90 - 0.0005 - Cu/ZnO/ZrO2/Al2O3 [61] 278 90 - 0.00045 - Cu/Zn/Al [48] 260 76 203 0.011 - Cu/Zn/Ce/Al [48] 260 90 244 0.0006 - Cu/Zn/Al2O3 [76] 300 100 185 0.012 - Cu/Cr/Al2O3 [76] 300 63 117 0.011 - Cu/Zr/Al2O3 [76] 300 44 81 0.0075 - Cu-Cr2O3 [69] 240 - 28 - 5 Cu-ZnO [69] 240 - 24 - 7 Cu-CoO [69] 240 - 17 - 14 CuO/CeO2 [77] 250 80 75 - - CuO/CeO2/ZrO2 [77] 250 - 97 - - Cu/CeO2 [53] 260 91 135 - 2.3 17.6 Cu/ZnO [53] 260 67 99 - 0.9 17.6 Cu/Zn(Al)O [53] 260 58 86 - 0.8 17.6 Cu/Al2O3 [53] 260 22 32 - 0.4 17.6 Cu-Mn-O [72] 240 99 - - 3.1 Cu-Ce-O [72] 240 37 - - 0.8 CuMn-spinel [78] 213 77 - - 0.2 non-spinel CuMn [78] 214 61 - - 0.1 CuZn [79] 250 39 - 0.0011 - Pd/CuZn [79] 250 47 - 0.003 - CuYPrAl [80] 320 99.1 120 - 0.2 25.0 CuCeAl [81] 350 99.8 16.7 0.0 - Chapter 1 17 1.4.2. Group 8 – 10 Catalysts The group 8 - 10 metal-based catalysts were firstly studied by Iwasa et al. [8288]. Based on their preliminary findings, other authors have later studied the influence of several factors on the catalytic performance for MSR [36, 37, 89-98]. The most commonly used catalysts in this group are the palladium supported in zinc oxide (Pd/ZnO), due to its anomalous high performance after the formation of a PdZn alloy [83]. Accordingly, the preparation method and the pretreatment conditions of the Pd/ZnO catalysts have been thoroughly studied in the literature [8994]. A different approach focused on the search of new types of supports, which presented higher surface areas than the commercially available ZnO support [38, 9597]. Finally, other authors have synthesized catalysts based on different alloy species and studied the effect of Zn addition on bimetallic catalysts [39, 87, 99]. Table 1.4, presents a summary of the results obtained in the literature for the group 8-10 catalysts. From the former review of results reported in the literature is easy to understand that ZnO is a common material in both groups of catalysts: copper-based and group 8-10. Despite being a controversy matter, the role of ZnO is proved to benefic for copper-based catalysts for stabilizing and increasing the dispersion of copper crystallites and in group 8-10 revelead to be essential for the formation of a selective alloy. Introduction 18 Table 1.4 - Summary table for MSR over various supported group 8–10 catalysts. Catalyst T (°C) R.T. (°C) Activity SCO (%) XMeOH (%) Smetal (m2 g−1 ) Dispersion (%) PdP1, L2 [87] 220 500 – 100 10.9 9.9 2.1 Pd P2, L1 [83] 200 – 2.0 99.9 – – 2.1 Pd/SiO2 P2, L1 [83] 200 – 0.13 100 – – 7.2 Pd/SiO2 P2, L2 [100] 220 – 0.37 100 – – 1.4 Pd/SiO2 P2, L2 [85] 220 500 100 0.09 – – Pd/SiO2 P1, L2 [87] 220 500 100 15.7 42.4 9 Pd/Al2O3 P2, L1 [83] 200 – 1.9 98.6 – – 13.1 Pd/La2O3 P2, L1 [87] 200 – 3.1 92 – – 5 Pd/Nd2O3 P2, L1 [87] 200 – 3.7 93 – – 25.3 Pd/Nb2 O5 P2, L1 [87] 200 – 1.5 95.8 – – 19.6 Pd/MgOP1,L2 [87] 220 500 – 93.4 41 49 10.4 Pd/In2O3 P1, L2 [87] 220 250 – 4.5 28.3 9.9 2.1 Pd/Ga2O3 P1, L2 [87] 220 500 – 5.4 21.2 12.3 2.6 Pd/CeO2 P1, L2 [87] 220 500 – 77.3 62.4 170.6 36.2 Pd/A.C.P1, L2 [88] 220 500 – 100 2.3 – – Pd/HfO2 P1, L2 [88] 220 500 – 100 13.6 – – Pd/Ta2O5 P1, L2 [88] 220 500 – 100 6.0 – – Pd/ZrO2 P2, L1 [83] 200 – 4.0 80 – – 28.9 Pd/ZrO2 P1, L2 [87] 220 500 – 81.6 64.3 31.1 6.6 Pd/ZnOP2, L1 [83] 200 – 8.3 3 – – 10.7 Pd/ZnOP1, L2 [87] 220 500 – 0.8 54.2 10.4 2.2 Pd/ZnOP1, L2 [85] 220 500 – 1.9 56.3 – – Pd/ZnO P2, L2 [85] 220 500 – 0.5 20.5 – – PtP1, L2 [87] 220 500 – 43.3 3 1.65 0.6 Pt/Ga2O3 P1, L2 [87] 220 500 – 24.5 5.4 7.2 2.63 Pt/In2O3 P1, L2 [87] 220 500 – 1.7 30.6 7.7 2.81 Pt/ZnOP1, L2 [87] 220 500 – 4.6 27.6 7.1 2.58 Pt/ZnOP1, L2 [85] 220 500 – 4.4 27.9 – – Pt/SiO2 P2, L2 [85] 220 500 – 74.4 0.3 – – Pt/SiO2 P2, L2 [100] 220 – 0.42 74.4 – – 2.1 Pt/SiO2 P1, L2 [87] 220 500 – 81.2 10.3 92.3 33.5 Ni/ZnOP1, L2 [85] 220 500 – 95.3 15.7 – – Ni/SiO2 P2, L2 [85] 220 500 – 98.9 7.3 – – Ni/SiO2 P2, L2 [100] 220 - 2.0 98.9 – – 10.6 Ni/ZnOP1, L2 [87] 220 500 – 97 19.1 2.7 0.4 Co/ZnOP1, L2 [85] 220 500 – 91.1 20.3 – – Co/ZnOP1, L2 [87] 220 500 – 86.7 13.1 6.2 0.92 Ru/ZnOP1, L2 [88] 220 500 – 96.2 9.5 – – Ir/ZnOP1, L2 [88] 220 500 – 79.6 2.6 – – P1: inlet partial pressure of water and methanol equal to 10.1 kPa. P2: inlet partial pressure of water and methanol equal to 24.3 kPa. L1: metal loading of 1 wt%. L2: metal loading of 10 wt%.R.T.: reduction temperature. Chapter 1 19 1.4.3. Zinc Oxide Zinc oxide (ZnO) is a material that can be used for a wide range of applications and has a high added industrial value [101-103]. One of its major applications is in heterogeneous catalysis, for instance in the methanol reactions, as a main component in the catalysts for methanol synthesis or MSR [104-107]. Typically, ZnO has a wurtzite crystal structure, which is constituted by polar surfaces and non-polar surfaces. Surface studies indicate that the non-polar surfaces,   ZnO 1010 and   ZnO 1010 , have a fairly low density of atomic defects such as vacancies and are electrostatic stable [108]. On the other hand, polar surfaces have unbalanced charges, one is terminated in Zn2+ and the other in O-2, corresponding respectively to   Zn ZnO 0001 and   O ZnO 0001 planes of the crystal [109, 110]. The role of the specific crystallographic orientation of the exposed catalytic surface of ZnO and its morphology has been addressed in literature regarding methanol dissociation, more particularly for catalysts which are prepared using ZnO powder as a support. Karim et al. reported that faceted Pd/ZnO catalysts are more active for MSR [36]. Among the exposed surfaces, the polar   Zn ZnO 0001 is considered to have an active pair of ions that readily dissociate methanol [112], whereas non-polar surfaces are not active for methanol dissociation [113]. The effect of different nanoshapes (short rods, long rods and polyhedral) of ZnO in MSR reaction were studied by Flytzani-Stephanopoulos et al.; these authors concluded that polyhedral shape was the most active for MSR, followed by the short rods and then for the long rods, due to the higher number of polar facets exposed [114]. Plane-density functional theory calculations performed by Guo et al. also indicate the contribution of polar facets for a low-temperature pathway reaction of MSR [115]; according to their calculations the dissociation of both water and methanol have low or null barriers on the polar   Zn ZnO 0001 . Despite the attributed importance of the polar ZnO surfaces for catalysis, the nonpolar surfaces are dominant in the commercially available ZnO powders [116]. These powders have a prismatic morphology where the non-polar surfaces correspond to six Introduction 20 of the exposed surfaces for chemical reactions (see Figure 1.5) [110]. Therefore, in the catalysis point of view, by increasing the polar surface ratio in comparison with the non-polar, the catalyst activity could be improved. Additionally, commercial ZnO has a low surface area, around 10 m2·g-1 [96]. Figure 1.5 - Schematic representation of a commercially available ZnO crystal (A) and a SEM image of the ZnO material acquired to Sigma Aldrich under the framework of this thesis (B). There are several methods for the synthesis of ZnO nanostructures being the hydrothermal method one of the most applied for its simplicity and mild conditions [117]. It is known that the growth velocities of the crystal planes in the hydrothermal method follow this preferential order: V(0001) > V(1011) > V(1010) [118, 119]. Due to the faster growth in the V(0001) plane direction the most exposed surfaces are the non-polar. This is easier to imagine in Figure 1.6, where a hexagonal unit cell of ZnO is represented and the growth along the c-axis would lead to higher proportion of non-polar facets. AB Chapter 1 21 Figure 1.6 - - Schematic representation of the ZnO hexagonal unit cell. In order to expose the polar ZnO facets, capping agents could be added to the synthesis method, since they can inhibit the growth in the V(0001) direction by chemical binding [101, 119, 120]. Block copolymers (i.e Pluronic P123) are wellknown for their unique properties as template agents and structural polymorphism. The polyethylene oxide-polypropylene oxide polyethylene oxide (PEO-PPO-PEO) based triblock copolymers are an example of nonionic surfactants where the temperature, concentration and type of solvent medium extremely affect the molecular arrangement. Below the critical micellar temperature (CMT), molecules are present in solution in a non-aggregated state as unimers and they suffer a reorganization forming micelles above the CMT – Figure 1.7. These micelles are constituted by a hydrophobic core of PPO and a hydrophilic shell of PEO blocks. Introduction 28 [14] http://www.fuelcelltoday.com, in 8-1-2014. 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Jensen, Integration of high temperature PEM fuel cells with a methanol reformer, Journal of Power Sources, 145 (2005) 392-398. [125] http://serenergy.com/products/systems/h3-350/, in 22-10-2014. Simple Urea-assisted Hydrothermal Synthesis Method 44 2.2. Experimental 2.2.1. Synthesis of ZnO samples Zinc acetate dihydrate (Zn(CH3COO)2·2H2O; 99.0%), zinc nitrate hexahydrate (Zn(NO3)2·6H2O; 98.0%), urea (CO(NH2)2; 99.5%), glacial acetic acid (CH3COOH; 99.7%), block copolymer poly(ethylene glycol)-block-poly(propylene glycol)-blockpoly(ethylene glycol) (Pluronic P123, PEG20-PPG70-PEG20), all analytical grade reactants were obtained from Sigma-Aldrich and used without further purification. Distilled water was used throughout the experiments. In a typical synthesis the ZnO support precipitate was obtained as following: 5 mmol of zinc salt precursor (acetate or nitrate, Table 2.1), 100 mmol of urea and given amounts of Pluronic P123 (Table 2.1) were mixed in 100 ml of distilled water. The pH of the solution was adjusted to 5.0 with glacial acetic acid and stirred for 2 h under ambient conditions. Then, the mixture was poured into a 150 ml Teflon-lined autoclave and maintained at a given temperature (Table 2.1) for 24 h and cooled down to room temperature naturally. The white precipitate was thoroughly washed with distillated water and dried overnight at 110 °C. The final ZnO powder was obtained after calcination in a muffle furnace at a given temperature (Table 2.1) for 30 minutes. 2.2.2. Characterization The specific surface area (SBET) of the ZnO materials was determined by standard N2 gas adsorption method using a Quantachrome Autosorb-1 apparatus. XRD measurements were taken on a Rigaku Miniflex 2. SEM analysis was performed using a FEI Quanta 400 scanning electron microscope equipped with an energy dispersive X-ray high vaccum detector (EDX). HRTEM/SAED images were obtained using a FEI Titan High Base microscope. DSC-TGA analyses were performed using a Netzsch TG 209 F1 Iris instrument. The photocatalytic activity of selected ZnO samples was assessed based on the decomposition of methylene blue (MB solution: 0.01 g·L-1). During the experiments, 100 ml of solution was poured Chapter 2 45 into a glass flask with 50 mg of ZnO. A UV lamp (Vilber Lourmat, BLB 365 nm, 2 x 6 W) was placed over the solution and maintained at the same distance during the experimental tests (irradiance of 10 W·m-2). The extent of MB degradation was determined measuring the absorbance of the solution at 645 nm. XPS analyses were performed using a Kratos Axis Ultra HSA equipment, with Vision software for data acquisition and CASA XPS software for data analysis. The analysis was carried out with a monochromatic Al Kα X-ray source (1486.7 eV), operating at 15 kV. For the quantification of the elements, sensibility factors provided by the manufacturers were used. For these experiments, Zn 2p, O 1s and C 1s bands were recorded. The binding energies were calibrated by fixing the C-(C, H) contribution of the C 1s adventitious carbon at 285.0 eV. 2.3. Results and discussion 2.3.1. Characterization of the ZnO precursor: the role of Pluronic P123 The thermal decomposition of the ZnO precursor (before calcination) for the samples prepared in the absence (ZnAcP0T90) and presence of Pluronic P123 (ZnAcP10T90, Table 2.1) was examined by TG-DSC. The following nomenclature for the prepared ZnO samples was used (Table 2.1), ZnxPyTz, where: x denotes the zinc precursor, zinc-acetate (Ac) or zinc-nitrate (N); Py stands for the amount of Pluronic P123 expressed as (nP123/nAc) molar ratio percentage (Table 2.1) and Tz represents the synthesis temperature in ˚C (Table 2.1). Accordingly, the ZnAcP10T90 sample was prepared from zinc-acetate as precursor, with a (nP123/nAc) molar ratio percentage of 10 and at 90 ˚C. Simple Urea-assisted Hydrothermal Synthesis Method 46 Figure 2.1 - (A) TGA-DSC analyses of the ZnO-precursor (before calcination at heating rate of 10°·min-1 in static air for ZnAcP10T90sample; (B) thermal decomposition of the ZnO-precursor (HZC) followed by mass spectroscopy The TG-DSC results obtained for sample ZnAcP10T90 are shown in Figure 2.1-A. As depicted in this figure, this sample was decomposed in only one fast step, denoted by a sharp peak centered at 250 °C. This was accompanied by a weight loss of 26 % in the temperature interval of 175-300 °C. The sample prepared without P123 behave similarly (results not shown here). In fact both samples exhibit the same TG profile, indicating that Pluronic P123 was removed during the washing step. The evolution of the CO2 during the thermal decomposition of ZnAcP10T90 sample was followed by mass spectroscopy. As shown in Figure 2.1-B, a large CO2 peak is observed centered at 250 °C, in agreement with TG-DSC data. The decomposition value for the zinc hydroxycarbonate (hydrozincite) according to equation (2.1) is 26.3 wt.%, which is in good agreement with the TG results (26 %).     5 3 2 2 26 Zn CO OH 5ZnO 3H O 2CO   (2.1) On the other hand, according to the XRD pattern of both samples (not shown), all the diffraction peaks could be indexed as hydrozincite (hereafter referred to as HZC, Fig. 2.2), Zn5(CO3)2(OH)6, (JCPDS Card No. 19-1458). Chapter 2 47 Figure 2.2 - XRD pattern of Zn5(CO3)2(OH)6 precursor (HZC) and ZnO samples prepared in the absence (ZnAcP0T90) and presence of Pluronic P123 (ZnAcP10T90) after calcination at 375 °C for 30 minutes. Inset (A): SAED pattern of ZnAcP0T90 sample. Inset (B): SAED pattern of ZnACP10T90 sample. The effect of Pluronic P123 on the morphology of the as-prepared HZC samples (before calcination) is nicely illustrated in Figure 2.3. In the absence of additive, spherical clusters of HZC appear to be agglomerated forming larger clusters of about 200 m (Figure 2.3-A). The addition of Pluronic P123 resulted in well-dispersed microspheres (Figure 2.3-B), which are in fact microflower-like in morphology (inset Figure 2.3-B). Comparing images in Figure 2.3, it is clear that spherical HZC architectures are obtained, regardless of Pluronic P123 content, being the main difference the higher dispersion of these microspheres in the presence of Pluronic P123. The calcination effect on the structure and morphology of the resulting ZnO solids (from HZC prepared in the absence and presence of Pluronic P123) was analyzed by XRD and SEM. Simple Urea-assisted Hydrothermal Synthesis Method 48 Figure 2.3 - SEM images of the as-prepared HZC samples obtained in the absence (A) and presence of Pluronic P123 (B) (nP123/nZnAc molar ratio = 0.10). It is important to mention that repetition of the synthesis under identical conditions led to similar results. SEM micrographs in Figure 2.4 reveal very different morphologies for both samples. In the absence of Pluronic P123 (Figure 2.4-A) non homogeneous ZnO structures (ZnAcP0T90), consisting of a combination of plates and spheres, were formed. The enlarged SEM image of one single sphere in Figure 2.4B-C, shows that it is composed of randomly close-packed porous nanosheets. On the contrary, in the presence of Pluronic P123 (sample ZnAcP10T90) welldispersed ZnO microspheres were obtained (Figure 2.4-D). Moreover, SEM images in Figure 2.4-D and E clearly show that the ZnO product calcined at 375 °C inherited the flower-like morphology of the HZC precursor (Figure 2.3-B), evidencing its thermal stability. Chapter 2 49 Figure 2.4 - Low and high magnification SEM images of ZnO samples ZnAcP0T90 (A-C) and ZnAcP10T90 (D-F). According to Figure 2.4-D-F, calcination of HZC resulted in ZnO samples (sample ZnAcP10T90) made up of micro-flowers uniformly distributed and of smaller diameter (21 m) than those seen in the HZC precursor (35 m) (Figure 2.3-B). Detailed observation of a single micro-flower in Figure 2.4-E shows that they are in fact an assembly of nanosheets that grow radially from the center outward, and the entire structure resembles a carnation. The micro-flowers in ZnAcP10T90 sample show a lot of voids between the various nanosheets (Figure 2.4-E and F), in opposition to the compact structure shown by the ZnAcP0T90 sample (prepared without Pluronic P123; Figure 2.4-B and C). This is also reflected in the higher specific surface area that is 43% larger for sample ZnAcP10T90 (80 m2·g-1) than that of the sample ZnAcP0T90 (46 m2·g-1). Thus, the presence of Pluronic P123 helps to enhance the formation of spherical ZHC structures during the hydrothermal synthesis. As a result the ZnO product obtained after calcination, inherits the flowerlike morphology of the ZHC precursor maintaining also the high dispersion. The XRD patterns of ZnAcP0T90 and ZnAcP10T90 samples are shown in Figure 2.2 with an SAED image of each nanostructure in the inset. It is found that both samples are highly crystalline, and the diffraction peaks in every pattern can be indexed to hexagonal würtzite-type ZnO (JCPDS No. 36–1451). Simple Urea-assisted Hydrothermal Synthesis Method 50 No other phases were detected indicating pure ZnO samples. Further EDX results confirmed that the ZnO samples were composed of solely Zn and O. However, the diffraction intensity ratios of (002) polar plane to (100) nonpolar plane are clearly different from one sample to another. In the present work, the term “polarity” is used to refer the relative intensities ratio of the polar and the nonpolar planes, (I(002)/I(100)). In this way, intensity ratio values higher than the würtzite reference (I(002)/I(100)= 0.73) denote a higher polarity, and consequently a higher ratio of exposed polar facets, and vice versa. The polarity values (I(002)/I(100)) for samples ZnAcP0T90 and ZnAcP10T90 are 0.74 and 1.10, respectively. Clearly, the former sample shows the same polarity as the reference würtzite whereas the later has a higher polarity. This indicates that the use of Pluronic P123 enhances the polarity of the ZnO material. SAED patterns were also obtained of samples ZnAcP0T90 and ZnAcP10T90 (insets A and B in Figure 2.2). The ZnAcP0T90 sample shows a diffraction pattern characteristic of a fully polycrystalline material with preferential growth along the caxis direction (inset A in Figure 2.2). On the other hand, the SAED pattern of the ZnAcP10T90 sample, confirms that they are single crystals (inset B in Figure 2.2). These results demonstrate that the addition of Pluronic P123 strongly influences the morphology and the growth preference of ZnO materials prepared by the hydrothermal route. In particular, in the presence of Pluronic P123 hierarchical ZnO microflowers whose surface is dominated by their (002) polar planes were formed. 2.3.2. The influence of the ZnO precursor Since the most promising ZnO material was obtained in the presence of Pluronic P123 (ZnAcP10T90), we focused on the optimization of its preparation. Accordingly, a series of ZnO samples were prepared following the same procedure as for the ZnAcP10T90 sample by only changing one parameter while keeping the rest to be constant. The following parameters were studied: ZnO precursor, Pluronic P123 concentration and hydrothermal synthesis temperature (Table 2.1). The influence of Chapter 2 51 these parameters on the specific surface area, morphology, structure and growth habit (polarity) of ZnO products is discussed in next sections. Table - 2.1 - Experimental parameters studied for the preparation of ZnO samples (ZnxPyTz: x Znprecursor; Py Pluronic P123 amount; Tz synthesis temperature). Parameter studied Range Sample name ZnO - precursor Zn acetate ZnAcP10T90 Zn nitrate ZnNP10T90 P123 concentration (nP123/nAc) molar ratio% 0 ZnAcP0T90 3 ZnAcP3T90 7 ZnAcP7T90 10 ZnAcP10T90 20 ZnAcP20T90 Synthesis temperature (°C) 70 ZnAcP10T70 90 ZnAcP10T90 110 ZnAcP10T110 The influence of the Zn-precursor on the morphology of the prepared ZnOs is presented in Figure 2.5. The use of Zn-nitrate (ZnNP10T90) resulted in the formation of sea urchin-like ZnO structures (Figure 2.5-A) made of very thin ZnO nanowires (ca. 25 nm thick; Figure 2.5-B). As previously indicated, the sample synthesized from Zn-acetate (ZnAcP10T90) presents a lamellar flower-like architecture (Figure 2.5-C and D), made up of assembled porous ZnO nanosheets (ca. 23 nm thick). Considering the size of these ZnO materials, it is found that the Zn-nitrate precursor leads to smaller ZnO structures (mean size 12 µm; Figure 2.5-A) in comparison with its Zn-acetate counterpart (mean size 20 µm; Figure 2.5-B). Simple Urea-assisted Hydrothermal Synthesis Method 52 Figure 2.5 - SEM images of ZnO samples prepared with different types of metal salts: zinc nitrate, ZnNP10T90, (A and B) and zinc acetate, ZnAcP10T90, (C and D). Both samples have a quite similar specific surface area (80 and 76 m2·g-1 for samples ZnAcP10T90 and ZnNP10T90 samples, respectively), and also show the same würtzite structure although the relative intensities of the polar (002) and nonpolar (100) XRD planes is very different. The latter suggests that the growth habit of ZnO crystals is different in both samples. As already mentioned, the ZnAcP10T90 sample has a higher polarity (I(002)/I(100)= 1.10) than the reference würtzite. In contrast, the ZnNP10T90 sample (prepared from Zn nitrate) shows lower polarity (I(002)/I(100)= 0.60). This indicates that the Zn salt precursor influences the growth habit of the resulting ZnO products.This might be related to the different morphologies of both ZnO products as reported in [26-28]. In the case of ZnAcP10T90 sample the flowers are composed of plates (Figure 2.5-D); these kind of structures are likely originated from lateral growth along the nonpolar facets of ZnO then explaining their increased polarity in a similar fashion as described in [20, 22, 29, 30]. On the other hand, the urchin structures formed in ZnNP10T90 are made up of needles (Figure 2.5-B). According to the literature [31-33] the usual growth along c-axis is expected to occur in such case, explaining the lower polarity of the sample ZnNP10T90 sample. Chapter 2 53 2.3.3. The influence and role of the Pluronic P123 concentration The SEM images of the samples prepared with increasing Pluronic P123 concentration (Table 2.1) are depicted in Figure 2.6. The sample prepared with the lowest Pluronic P123 amount (ZnAcP3T90; nP123/nAc molar ratio = 0.03) consists of ZnO particles of irregular shapes (Figure 2.6-A). The morphology of ZnAcP7T90 and ZnAcP10T90 samples (nP123/nAc molar ratio = 0.07 and 0.10, respectively; Table 2.1) is similar. In fact, both samples have a flower-like morphology (mean size of 20 µm) assembled from ZnO plates (Figure 2.6-B and C), although sample ZnAcP10T90 has a higher number of voids between the sheets (compare inset in Figure 2.6-B and C). The structure became more compact as the P123 concentration increases (Figure 2.6D), resulting in the formation of non-uniform agglomerated particles, where no ZnO nanoplates are apparent (inset Figure 2.6-D) as for the ZnAcP20T90 sample (nP123/nZnAc molar ratio= 0.20). Figure 2.6 - SEM images of the ZnO products prepared with different nP123/nAc molar ratios: A) 0.03 (ZnAcP3T90), B) 0.07 (ZnAcP7T90), C) 0.10 (ZnAcP10T90) and D) 0.20 (ZnAcP20T90). Simple Urea-assisted Hydrothermal Synthesis Method 60 the urea hydrolysis is slow, so that the concentration of ammonium and bicarbonate ions is low and consequently less ZnO particles are formed leading to the quasiflower structures of Figure 2.10-A. However, at 90 °C the urea decomposition is faster and more ZnO particles are formed, giving rise to the micro-flowers in Figure 2.10-B. Finally, at 110 °C the urea hydrolysis is likely too fast which might cause the agglomeration and formation of the large cauliflower-like architectures shown in Figure 2.10-C. Figure 2.10 - SEM images of the ZnO samples synthesized at different hydrothermal temperatures: A) 70 °C (ZnAcP10T70); B) 90 °C (ZnAcP10T90) and C) 110 °C (ZnAcP10T110). The specific surface area of this group of samples increases with hydrothermal temperature in the 70 °C - 90 °C interval (SBET = 64 and 80 m2·g-1, for ZnAcP10T70 and ZnAcP10T90). Further increase of reaction temperature causes a significant decrease of the specific surface area (ZnAcP10T110 SBET = 50 m2·g-1). The evolution of the SBET with temperature is in good agreement with the formation of larger ZnO agglomerates at higher reaction temperature, as evidenced by SEM. It was found that the peak intensity of XRD for the analyzed samples tends to increase with the temperature – Figure 2.11. Since the sample weight used in XRD analysis is nearly the same, this is attributed to a higher degree of crystallinity of the resulting ZnOs. Regarding the polarity of this series of samples, it is observed that sample ZnAcP10T70 shows nearly the same value (I(002)/I(100)= 0.75) as the würtzite (I(002)/I(100)= 0.73). This suggests that both samples follow the typical growth habit of würtzite with preferential orientation along c-axis, resulting in ZnO materials whose Chapter 2 61 surface is dominated by non-polar planes. The sample ZnAcP10T90 exhibits the highest polarity (I(002)/I(100)= 1.10), which represents a higher ratio of exposed polar facets, as already indicated. The ZnAcP10T110 sample has the lowest polarity among this series of samples, which is even lower than the würtzite. This indicates that this sample has a higher ratio of non-polar planes. The variation in polarity observed here is probably related to the different morphology of the various samples. By comparison, the ZnO particles obtained at 90 °C show the optimal morphology, SBET and also polarity. Figure 2.11 - XRD pattern and I(002)/I(100) ratio of the ZnO samples prepared at different hydrothermal temperatures. 2.3.6. Evaluation of the photocatalytic activity The photocatalytic activity of a semiconductor such as ZnO dependents on the optical properties, specific surface area, particle size and morphology of the particles [13, 42-44]. In particular a high polarity (higher number of defects) and high specific surface area are expected to increase the photocatalytic activity of ZnO materials [13, 45]. Simple Urea-assisted Hydrothermal Synthesis Method 62 The results presented in preceding sections evidence that both the polarity and specific surface area of ZnO materials prepared by the hydrothermal route depend on the synthesis conditions. To discriminate if the photoactivity of the prepared ZnO samples is influenced by the SBET and/or the polarity, samples with similar SBET but greatly differing in polarity (ZnAcP10T90 and ZnNP10T90) and samples with similar polarity but with very different specific surface areas (ZnAcP10T90 and ZnAcP7T90) were selected. The idea is to use the photocatalytic test as an indirect confirmation of polarity trends. Accordingly, the degradation of methylene blue (MB) was assessed using three ZnO samples already mentioned: ZnAcP10T90, ZnNP10T90 and ZnAcP7T90. Figure 2.12 shows the results obtained. Sample ZnNP10T90 (lowest polarity) exhibited a lower degradation rate, taking approximately 200 min to achieve total photocatalytic degradation. On the other hand, samples with higher polarity (ZnAcP10T90 and ZnAcP7T90) exhibited a prominent photocatalytic activity degrading the MB in 100-120 min, despite having different SBET areas. These results show that the photoactivity of the prepared ZnO materials, under the present operation conditions, dependes on the polarity rather then the SBET surface area. On the other hand our “more polar” ZnO materials (ZnAcP10T90 and ZnAcP7T90) show comparable [46] or better photocatalytic performance [47, 48] as compared to other ZnOs tested under similar operating conditions. Figure 2.12 - Photocatalytic degradation of MB over: ZnAcP10T90 (SBET= 80 m2·g-1; I(002)/I(100) = 1.10), ZnNP10T90 (SBET=76 m2·g-1;I(002)/I(100)= 0.62) and ZnAcP7T90 (SBET=53 m2·g-1; I(002)/I(100)= 0.92). Lines are added for readability. Chapter 2 63 2.3.7. Role of Pluronic P123 on the ZnO formation mechanism To gain some understanding about the role of the Pluronic P123 on the formation, growth process and morphology of HZC precursor, two samples, with and without P123, were prepared following the same experimental protocol as that used for the synthesis of samples ZnAcP10T90 and ZnAcP0T90. During both syntheses a small amount of sample was taken out at different intervals and characterized by SEMEDX. It should be noted that no product was collected if the hydrothermal time was inferior to 30 minutes (in the presence of P123) and 215 minutes (without P123). The evolution of the pH and temperature of hydrothermal solution during sampling was also recorded. The results are displayed in Figure 2.13. Figure 2.13 - pH and temperature of the hydrothermal solution as a function of the reaction time for ZnAcP10T90 sample. Lines are for readability. The temperature of the solution steadily increased during the first 3 h of reaction and stabilizing at ca. 85 °C (Figure 2.13) until the end of the synthesis (24 h). The variation of pH with reaction time follows a similar trend (Figure 2.13); it slightly increases with temperature during the first 50 minutes of reaction, then it increases sharply between 60 °C to 82 °C, and finally reaches a constant value of around 8.4. The pH history is essentially related to the homogeneous urea decomposition. Simple Urea-assisted Hydrothermal Synthesis Method 64 Consequently, the formation and growth of HZC precipitate particles should be governed by the gradual urea hydrolysis, as nicely illustrated in Figure 2.13. Figure 2.14 – SEM images showing the morphology evolution during the hydrothermal synthesis of the HZC in the presence of Pluronic P123 (same synthesis conditions as for ZnAcP10T90 sample: Zn-acetate, 0.10 (nP123/nAc) molar ratio and 90 °C), after: (A) 30 min, (B) 75 min, (C) 150 min, (D) 195 min and (E) 24 h. Figure 2.15 - SEM images showing the morphology evolution during the hydrothermal synthesis of the HZC in the absence of Pluronic P123 (same synthesis conditions of ZnAcP0T90 sample: Zn-acetate, no P123 and 90 °C), after: (A) 215 min, (B) 230 min, (C) 250 min, (D) 270 min and (E) 24 h. Chapter 2 65 The different contrast in the low voltage SEM images in Figure 2.14-A and B suggests some degree of heterogeneity in these samples. Accordingly, the EDX images of the materials show that in the bright areas Zn is the major element. The dark shades have a different composition, where N and C appear as the main components along with O and traces of Zn; urea should be the main component of these regions. As mentioned, some Zn-rich particles (bright areas in Figure 2.14-A) are already formed during the first 30 minutes of the hydrothermal reaction. At this stage and according to Figure 2.13, the urea hydrolysis is likely very slow due to the low temperature of hydrothermal solution, so the equilibrium of urea hydrolysis is shifted towards the left side, in good agreement with EDX results that indicate high N contents. This is also consistent with the low pH (Figure 2.13), which evidences that the equilibrium of urea hydrolysis (Equation 2.2) lies to the left side; there is a much greater concentration of urea than bicarbonate and ammonium ions, and no significant H+ consumption take place, so pH remains essentially unchanged at 5. This also agrees with reported works on urea hydrolysis [40]. After 75 minutes of reaction, HZC species in the form of thin sheets are formed, as shown in the inset of Figure 2.14-B. As the reaction proceeds, the number of HZC particles increases (Figure 2.14-C), and the sheets appear now assembled in larger structures (inset in Figure 2.14-C). The spherical particles observed after 3 h of reaction (Figure 2.14-D) strongly resemble the micro-flowers observed after 24 h of hydrothermal reaction (inset in Figure 2.3-B). Similar experiments were conducted in the absence of Pluronic P123. The obtained SEM images are shown in Figure 2.15. In this case the precipitated particles are formed at much higher reaction times (215 min vs. 30 min in the synthesis without P123). As apparent in this figure, the morphology evolution of the various materials with reaction time follows a similar tendency to that found in the presence of P123, although the solids show quite different morphology. In general, larger clusters are formed in the absence of Pluronic P123, as observed in Figures 2.3-A and 2.15-E (final product). Interestingly, the SEM images of the resulting HZC material in Figure 2.14-E (after 24 h of reaction), shows that spherical clusters are Simple Urea-assisted Hydrothermal Synthesis Method 66 formed regardless of Pluronic P123, but they appear better dispersed and have a more uniform aggregate size when P123 is used. Thus, it is clear that under identical synthesis conditions, the use of Pluronic P123 enhances the dispersion of the formed HZC precipitates with the spherical morphology being characteristic of this kind of zinc basic hydroxy carbonates (ZHC). 2.3.8. ZnO Formation Mechanism In order to understand the role played by the Pluronic P123 on the formation process of ZnO in our experimental conditions, the aqueous chemistry of zinc and the urea hydrolysis will be analyzed henceforth. Upon dissolution of a zinc salt (Zn-Acetate herein) in water, Zn2+ cations become octahedrally coordinated forming [Zn(OH2)6]2+. The hexaqua zinc species are quite stable in very acidic medium but undergo hydrolysis at higher pHs. According to the literature [49, 50] around pH 5 cations [Zn(OH2)5(OH)]+ are the predominant species in solution. Thus, initial conditions of pH and temperature (at instant t = 0, in Fig. 2.13), urea and Pluronic P123 are stable in solution, since the solution temperature is very low (room temperature) for the urea decomposition and Pluronic micellization, and zinc is mostly as [Zn(OH2)5(OH)]+. The mixture is then heated and the temperature of the solution gradually increases and around 50 °C micellization of Pluronic P123 is expected to occur [36]. The Pluronic micelles would interact with the polar species existing in solution, creating in this way the active sites for further nucleation and growth. At this point, the temperature is still too low for urea hydrolysis, and since the pH remains unchanged around 5, [Zn(OH2)5(OH)]+ species are stable in solution. However, the solution temperature continues increasing and around 70 °C fast urea decomposition occurs. The optimum temperature for the urea hydrolysis is at about 85 °C [40], which is accompanied by a sharp increase of pH (herein, after 75 minutes, Figure 2.13). The decomposition of urea in aqueous solutions takes place in two stages [40]. The first one involves the formation of ammonium cyanate, which is followed by the irreversible hydrolysis of cyanate ions. Thus, the hydrolysis of urea can be written as follows: Chapter 2 67   2 2 3 4 2 CO NH 3H O HCO 2NH OH        (2.2) Obviously, the extent of the reaction defines the release of bicarbonate ions and OH-, resulting in a gradual increase of pH, as observed in Fig. 2.13. According to [50], [Zn(OH2)5(OH)]+ species are complexed by HCO3anions giving rise to [Zn(OH)(HCO3)(OH2)3]. On the other hand, as pH raises the former species become unstable and undergo ligand exchange between the OHand HCO3leading to neutral [Zn(OH2)4(OH)2] species. Then, in the experimental conditions of the present study, the HZC nuclei are likely generated from condensation reactions between both [Zn(OH2)4(OH)2] and [Zn(OH)(HCO3)(OH2)3] according to equation 2.3 [50]:             2 3 2 5 3 2 4 2 3 6 2 3 Zn OH OH 2 Zn OH HCO OH Zn OH CO 20H O        (2.3) The formed nuclei then grew up to produce primary ZHC nanoparticles, which aggregated to form sheets (Figure 2.14-B). Subsequent growth leads to larger HZC structures (Figure 2.14-C and D), which are thermodynamically more stable, and finally to the hierarchical micro-flowers in Figure 2.14-E. In the synthesis carried out with Pluronic P123, since the nucleation process occurs in preferential sites, highly dispersed ZHC microflowers are generated from an oriented growth mechanism. However, in the absence of Pluronic, homogeneous nucleation takes place randomly, which leads to the formation of larger HZC particles during the growth step (Figure 2.14) that undergo agglomeration giving rise to the large structures displayed in Figure 2.14-E. Finally, the HZC solid will decompose into ZnO upon heating according to equation 2.1, releasing CO2 and H2O (as evidenced by TPD and TG) to generate the porous ZnO microstructures illustrated in Figure 2.4-E. Moreover the ZnO solid synthesized in the presence of P123, ZnAcP10T90 sample, after calcination maintains the original flower-like architectures as well as the high dispersion of the HZC precursor (Figure 2.4-E). As a result of the low dispersion, the sample obtained without P123 (ZnAcP0T90), leads to a more compact and shapeless ZnO material after calcination, as illustrated in Figure 2.4-C. Simple Urea-assisted Hydrothermal Synthesis Method 68 Comparing Figures 2.14 and 2.15 it can be seen that the nucleation-growth process depends on the presence of P123. In this former case, P123 acts as a template, providing the sites for nucleation and growth, contributing to the formation of highly dispersed ZHC particles during growth. As a result homogeneous and dispersed ZnO solids in the form of microflowers are obtained upon calcination. Our results suggest that Pluronic P123 acts as growth director and dispersing agent. 2.4. Conclusions Pluronic P123 block copolymer strongly influenced the morphology, polarity and specific surface area of the ZnO materials prepared by the urea-assisted hydrothermal method. In the presence of Pluronic P123, hierarchical ZnO microflowers whose surface is dominated by (002) polar planes were formed. Contrarily to this, large clusters agglomerates are formed in its absence and surface is dominated by (100) non-polar planes. Moreover the use of Pluronic P123 leads to ZnO materials with enhanced SBET. The morphology and polarity is also affected by changing the metal salt from zinc acetate to zinc nitrate precursor. In this latter case, an urchin-like structure is obtained and proportion of polar planes is only of 0.60 (ZnNP10T90). The synthesis temperature seems to affect the decomposition rate of urea, being obtained at 70 °C and 110 °C quasi-flower and large cauliflower-like architectures, respectively. The materials with higher polarity showed higher photocatalytic activity for the decomposition of methylene blue, evidencing that even with similar surfaces areas the polar surfaces are more reactive, mainly due to the higher number of defects such as oxygen vacancies. The role of Pluronic 123 in the initial steps of the hydrothermal synthesis was clearly visible through backscattering images on SEM, acting as a template and providing the sites for nucleation and growth. This simple and easily scalable method allows the synthesis of ZnO with a highly faceted morphology, combined with high surface area and polarity, making these materials very promising for several applications, such as catalysis. Chapter 2 69 2.5. Acknowledgments The research leading to these results has received funding from the European Union’s Seventh Framework Programme (FP/20072013) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement nº. [303476] 10. The work of Hugo Silva was supported by FCT, grant SFRH/BD/45890/2008. The authors also acknowledge financing from FCT through the project PTDC/EQUEQU-EQU/104217/2008. Ms. Joana Angelo is acknowledged for her help in photocatalytic tests. CEMUP is also thanked. Simple Urea-assisted Hydrothermal Synthesis Method 56 Chapter 3 77 Chapter 3. CuO/ZnO catalysts for methanol steam reforming: the role of the support polarity and surface area 2 Abstract The effect of surface area and polarity ratio of ZnO support on the catalytic properties of CuO/ZnO catalyst for methanol steam reforming (MSR) are studied. The surface area of ZnO was varied changing the calcination temperature and its polarity ratio was modified using different Zn precursors, zinc acetate and zinc nitrate. It was found that the copper dispersion and copper surface area increase with the surface area of the ZnO support, and the polarity ratio of ZnO strongly influences the reducibility of copper species; a higher polarity ratio promotes the reducibility, which is attributed to a strong interaction between copper and the more polar ZnO support. Interestingly, it was observed that the selectivity of CuO/ZnO catalysts (lower CO yield) increases with the polarity ratio of ZnO carriers. As another key result, CuO/ZnOAc375 catalyst has proven to be more selective (up to 90 %) than a reference CuO/ZnO/Al2O3 sample (G66-MR, Süd Chemie). The activity of the best performing catalyst, CuO/ZnOAc375, was assessed in a Pdcomposite membrane reactor and in a conventional packed-bed reactor. A hydrogen recovery of ca. 75 % and a hydrogen permeate purity of more than 90 % was obtained. The Pd-based membrane reactor allowed to improve the methanol conversion, by partially supressing the methanol steam reforming backward reaction, besides upgrading the reformate hydrogen purity for use in HT-PEMFC. 2C. Mateos-Pedrero, H. Silva, D.A. Pacheco Tanaka, S. Liguori, A. Iulianelli, A. Basile, Adélio Mendes, CuO/ZnO catalysts for methanol steam reforming: The role of the support polarity ratio and surface area, Appl. Catal. B Environ. 174-175 (2015) 67–76. CuO/ZnO catalysts for methanol steam reforming 78 3.1. Introduction The methanol steam reforming (MSR) reaction has received much attention in the past few decades as an attractive route of producing hydrogen for small-scale polymer electrolyte membrane fuel cells (PEMFC): 3 2 2 2 CH OH H O 3H CO 1 049.7 H KJ mol    (3.1) MSR catalysts are usually divided in two main groups: Cu-based and the more recent Pd-based ones [1]. Regardless the catalyst type ZnO support has a ubiquitous presence. Although CuO/Zn-based catalysts are used in industry since the 1960s, the role of ZnO in these catalysts system remains unclear despite the efforts made to elucidate its role [2-6]. For instance, Karim et al. investigated the effect of ZnO morphology on the reactivity of PdZnO catalysts for MSR [7] and concluded that the activity was higher for faceted ZnO materials [7]. In line with the former work, the theoretical studies by Smith et al. demonstrated that the polar crystalline surfaces of ZnO has null energetic barrier for both methanol and water dissociation [8]. On this basis, one could assume that ZnO with higher ratio of polar surfaces, namely higher polarity ratio, would lead to MSR catalysts with enhanced activity. This concept has in fact gained more attention as evident from the studies by Boucher et al. [9, 10], who investigated the influence of the properties of various carriers (mainly shape and defects) on the reactivity of Au-based catalysts for WGS and MSR reactions. These authors concluded that for different ZnO nanoshapes the activity increased when the binary catalysts were prepared with more polar supports (higher polarity ratio). Nevertheless, to our knowledge there is no study over CuO/ZnO catalysts that establishes a relation between the support polarity and the selectivity towards MSR. However, this aspect is of crucial importance for fuel cell applications where the presence of CO should be minimized as much as possible since even ppm levels of CO irreversibly poison Pt electrodes. Recently, a simple urea-assisted hydrothermal method for tailoring the physicochemical properties of ZnO materials was reported by the research team [11]. It was found that the specific surface area, morphology and polarity ratio of the resulting ZnO solids were strongly affected by the synthesis conditions employed Chapter 3 79 [11], in particular, the presence and concentration of surfactant (Pluronic P123) and type of metal salt precursor (Zn-acetate vs. Zn-nitrate). The main conclusions of this study were: (i) the addition of Pluronic P123 results in better dispersion of ZnO particles (hierarchical ZnO microflowers are formed), higher polarity ratio (higher ratio of (002) polar planes), and ZnO materials with enhanced surface area; (ii) the morphology, polarity ratio and reactivity are also affected by the Zn salt used as precursor. The use of Zn-nitrate led to urchin-like ZnO structures (ZnO microflowers were formed when using Zn-acetate) with lower polarity ratio (higher proportion of (100) non-polar planes) than their acetate derived counterparts . The ZnO sample with the highest polarity ratio (the acetate derived ZnO) also exibithed the highest photoactivity, which is ca. 2 times higher than that of the “less polar” (lower polarity ratio) nitrate derived ZnO. These results suggest that both samples have different reactivity, being higher for the ZnO with higher polarity ratio [11]. MSR reaction should be carried out at low temperature to exploit the favorable thermodynamics to yield low CO, but it is equilibrium limited and then, for high conversions, the back reaction penalizes the overall reaction rate. The use of a Pdbased membrane reactor allows hydrogen product to be continuously removed from the reaction medium and then enhances the overall reaction kinetics resulting in enhanced conversions and in the production of a high purity hydrogen stream. Low temperature PEMFCs require hydrogen with very low concentrations of CO; the automotive standard imposes a maximum CO concentration of 0.2 ppm (ISO 146872). This high purity hydrogen can be obtained using a Pd-based purification process or, with advantages, using a Pd-based membrane reactor. However, Pd-membranes are poisoned by CO, which adsorbs on the membrane surfaces inhibiting the hydrogen permeation [12]. Pd-based composite membranes are characterized by a thin Pd layer deposited onto porous substrates and show high permeability and selectivity to hydrogen [13-16]. A growing attention is then been devoted to Pdcomposite membranes that have - among others - the advantage of lower cost and higher permeability because of the reduced palladium content utilized in these membranes [13-18]. Numerous studies deal with MSR reaction carried out in both dense and composite Pd-based MRs [17-25]. In most of them, it has been demonstrated that these MRs made possible higher performances than conventional CuO/ZnO catalysts for methanol steam reforming 80 packed bed reactors (CR) in terms of methanol conversion and hydrogen yield with the further benefit of producing high-grade hydrogen. Dense self-supported Pd-Ag membranes with a thickness of 50 μm and composite Pd-based membranes with Pdlayers thicker than 10 μm were used in previous in previous studies of steam reforming of methanol [18, 21-23]. This work used a thin composite membrane of ca. 8 μm deposited onto a ceramic support and the direct content of the catalyst with the composite membrane is accessed in terms of methanol conversion, hydrogen recovery and hydrogen permeate purity as well as permeation characteristics stability. In this context, the first part of this work investigates the role of ZnO surface area and polarity ratio on the activity-selectivity of CuO/ZnO catalysts at low temperature. Two types of ZnO samples were prepared as detailed in [11] and used as supports of CuO/ZnO catalysts: a series of ZnO samples with different specific surface area and similar polarity and a group of ZnO samples with similar specific surface area but different polarity ratio. It should be noted, however, that in the present work the term “polarity ratio” is used to refer the relative intensities of the polar and nonpolar planes of ZnO, ( 002 100( ) ( ) /II ). A commercial isotropic würtzite ZnO from Sigma-Alldrich was taken as a reference and studied by XRD. The reference ZnO sample gave a value 0.73 for the (002)/(100) intensity ratio, thus, intensity ratio values higher than the würtzite reference ( 002 100 0 73 ( ) ( ) /.II ) denote a higher polarity ratio, and consequently, a higher ratio of exposed polar facets, and vice-versa. The second part of this work evaluates the performances of the best CuO/ZnOAc375 catalyst, among the ones reported in this work, in a Pd-membrane reactor. 3.2. Experimental 3.2.1. Preparation of ZnO supports ZnO samples were prepared by a modified hydrothermal method as detailed elsewhere [11]. In a typical preparation, 1.1 g of Zn salt precursor (zinc acetate or Chapter 3 81 zinc nitrate), 6 g of urea and 3 g of P123 Pluronic block copolymer were mixed 100 mL of water. The pH was adjusted to 5 and the solution was stirred for 2 hours under ambient conditions. Then, the mixture was poured into a teflon lined autoclave and kept at 90 °C for 24 h. The precipitate was thoroughly washed with distillated water and dried at 110 °C overnight. The resulting solid was calcined in a muffle furnace at given temperature for 30 min. Table 3.1 - Table 1. Experimental parameters studied for the preparation of ZnO samples, calcination temperature series (ZnAc-CT: Ac Zn-acetate as precursor; CT: calcination temperature); Zn-precursor series (ZnOx-375: x stands for Zn-acetate (Ac) or Zn-nitrate (N); both samples were calcined at 375 °C). Parameter studied Range Sample name SBET (m2·g-1) Polarity*     002 100 II Calcination temperature (CT) (°C) 300 ZnAC-300 64 0.76 350 ZnAC-350 71 0.78 375 ZnAC-375 80 1.10 400 ZnAC-400 54 0.80 ZN-precursor Zn-acetate ZnAC-375 80 1.10 Zn-nitrate ZnAC-375 77 0.60 *Ratio between XRD plane (002) and plane (100) – indicates the polarity degree of the ZnO carriers. The polarity ratio of a isotropic würtzite ZnO from Sigma-Alldrich was 0.73. Table 3.1 shows the ZnO samples prepared. The following nomenclature for ZnO samples was used (Table 3.1), ZnOx-CT, where: x denotes the zinc precursor, zincacetate (Ac) or zinc-nitrate (N) and CT represents the calcination temperature in °C (Table 3.1). Accordingly, the ZnAc-375 sample was prepared from zinc-acetate as precursor and calcined at 375 °C for 30 minutes. SEM images of the series ZnO materials (ZnOAC-CT) obtained at different calcination temperatures are shown in Fig. 3.1. CuO/ZnO catalysts for methanol steam reforming 82 Figure 3.1 - Influence of the calcination temperature on the morphology of ZnOAc-CT samples: (A) before calcination and calcined in air at 300 °C (B), 375 °C (C) and 400 °C (D). 3.2.2. Preparation of the CuO/ZnO catalysts CuO/ZnO catalysts were prepared by impregnation of the ZnO supports with an aqueous solution of copper nitrate (the amount of copper calculated to achieve a nominal metal loading of 15 wt. %). The pH was adjusted to 6 by dropwise addition of ammonium hydroxide. The resulting slurry was dried at 110 °C overnight and calcined at 360 °C during 8.5 h. As for ZnO carriers, CuO/ZnO catalysts will be denoted in terms of the ZnO supports calcination temperature (CT) and the type of zinc precursor used; thus when zinc nitrate was used the catalyst was named CuO/ZnON-CT and when prepared from zinc acetate it was named CuO/ZnOAc-CT, where CT denotes the calcination temperature in °C. 3.2.3. Materials characterisations The specific surface area was measured by N2 physisorption at -196 °C in a Quantachrome Autosorb-1 Instruments apparatus. The surface area (SBET) was calculated using the Brunauer-Emmett-Teller (BET) equation. X-ray powder diffraction (XRD) analyses were carried using a Cu-Kα radiation (30 KV/15 mA and λ=0.154 nm) in a Rigaku Miniflex 2 equipment. CuO crystallite size was assessed by the Debye-Scherrer equation,   cosDK     , where D is the average size of the CuO crystallites, K is the Scherrer’s constant 0.94,  is the wavelength of X-ray, and  is the full width at half maximum. The XRD pattern was measured at ambient temperature and for the calcined samples of CuO/ZnO at a 2  range of 10-80° with a Chapter 3 83 step width of 0.06°·s-1. Temperature-programmed reduction (TPR) experiments were performed using a ChemBET Pulsar TPR/TPD equipped with a thermal conductivity detector (TCD). In a typical TPR experiment approximately 50 mg of sample was held by quartz wool and placed in a Ushaped quartz reactor. The sample was heated from 50 °C to 400 °C at a heating rate of 5 °C·min-1 under a flow of 5 % H2/Ar. Hydrogen consumption was measured by TCD. The copper dispersion was determined by temperature programmed desorption of H2 (H2-TPD), following a similar procedure as reported by Amorim de Carvalho et al. [26]. Accordingly, the sample was reduced under a flow of 5 % H2/Ar. Then, the sample was cooled to 0 °C with an ice bath and pure H2 was passed during 1 h. Then, the temperature was lowered to -196 °C using liquid nitrogen under a pure H2 flow (30 cm3·min-1). After 1 h, H2 was switched to He flow (50 cm3·min-1) for 30 min. The temperature was then raised up to the 400 °C and desorption of H2 was monitored by using a TCD detector. Copper dispersion is defined as the ratio of the surface copper atoms to the total copper atoms present in the catalyst. 3.2.4. MSR with a conventional reactor The activity and selectivity of the catalysts were determined for MSR reaction using an in house built set-up. Steam reforming of methanol was performed at atmospheric pressure in a tubular reactor (7.25 mm i.d.) placed inside an oven. The reaction temperature was recorded inside the packed bed reactor using a thermocouple. The reactor was loaded with 200 mg of catalyst (180-350 µm) diluted with 200 mg of glass spheres. Plug flow conditions were ensured keeping catalyst bed length to catalyst size ratio above 50 ( 50 reactor particle Ld ) and the reactor diameter to size ratio above 30 ( 30 reactor particle dd ) [27]. Activity measurements were performed in the temperature range of 180 °C to 300 °C and space-time ratio of 3 01 cat. CH OH cat. W F 83kg mol s     . Prior to the catalytic activity measurements, the catalyst was reduced in situ using a diluted hydrogen stream (40 vol. % of H2 balanced with N2), at 240 °C for 2 h. The gas feed flow rate was controlled by mass flow controllers from Bronkhorst (model F-201C, ± 0.1 FS). Required flow rate of CuO/ZnO catalysts for methanol steam reforming 84 methanol aqueous solution was controlled using a Controlled Evaporation and Mixing (CEM) system (Bronkhorst). The condensable reactants were separated from the gas mixture in a condenser at ca. 0 °C, placed outside the oven. Hydrogen and carbon dioxide were analysed in a quadruple mass spectrometer (Pfeiffer Vacuum OmniStar GSD 320). Trace amounts of carbon monoxide were measured using a CO infra-red analyser (Signal Instruments, 7100 FM, accuracy: ± 0.2 ppm). The methanol conversion ( 3 CH OH X ) and CO output molar fraction ( CO  ) were calculated by applying equations (3.2) and (3.3). Methanol Conversion : 2 3 ,, , CO out CO out MeOH CH OH in QQ X Q   (3.2) CO output molar fraction : , , CO out CO TOT out Q Y Q  (3.3) 3.2.5. MSR with a composite Pd-Al3O3 membrane reactor A sketch of the Pd/Al2O3 membrane reactor (MR) used is shown in Figure 3.2. The composite Pd-based membrane is made of a thin Pd layer (~ 7 μm) deposited via electroless plating onto a porous Al2O3 support. The membrane has been produced at Nanjing University of Technology (the porous Al2O3 support is from Gao Q Funct. Mat. Co.), and used at ITM-CNR, with 7.5 cm of total length and 5.0 cm of active length, 1.3 cm of O.D. It was housed in a stainless steel module, having 12 cm of length, 1.5 cm of O.D., equipped with two gaskets at both membrane ends for preventing permeate and retentate streams to mix. The MR annulus was packed with the CuO/ZnO catalyst. Prior to the reaction tests, the permeability of the composite Pd-membrane to hydrogen has been obtained at T = 300 °C and for a transmembrane pressure (ΔP) of 1.0 bar. Chapter 3 85 Figure 3.2 – Conceptual scheme of the composite Pd-Al2O3 MR with the catalyst (in powder form) packed in two the MR annulus. The performance of the MR has been first analysed, in terms of methanol conversion and gas selectivity. The effect of temperature in the range 220 – 300 °C was assessed at 2.0 bar, ~ 0.95 h-1 weight hourly space velocity (WHSV) and H2O/CH3OH feed molar ratio equal to 2.5/1. The permeate pressure has been kept constant at 1.0 bar in the whole experimental campaign. Afterwards, the investigation has been focused on MR performance in terms of hydrogen recovery and hydrogen permeate purity by varying both reaction pressure and WHSV. The reaction pressure was varied from 1.5 bar to 2.5 bar, WHSV from 1.37 h-1 to 2.73 h-1. The temperature was kept constant at 330 °C and H2O/CH3OH feed molar ratio equal to 1.5/1. The MR has been heated up under helium and a P680 HPLC pump (Dionex) has been used for supplying liquid methanol and water. The mixture was vaporized with nitrogen supplied at a constant flow rate of 22.0 mL/min and fed to the MR. The retentate stream was directed to a cold trap in order to condensate the unreacted water and methanol. Both permeate and retentate stream compositions were analysed using a temperature programmed HP 6890 GC with two thermal conductivity detectors, heated at 250 °C and using Ar as carrier gas. The GC was equipped with three packed columns: Porapack R 50/80 (8 ft 1/8 inch) and CarboxenTM 1000 (15 ft 1/8 inch) connected in series, and a Molecular Sieve 5 Ǻ (6 ft 1/8 inch). The permeability of the membrane was obtained for monocomponent streams of H2, N2 and He using a bubble-flow meter; at least 10 experimental values were obtained. CuO/ZnO catalysts for methanol steam reforming 92 the more polar sample. Thus, it could be concluded that ZnO polar surfaces interacts strongly, at least to a higher extent than the less polar ZnO surfaces (ZnON-375), with highly dispersed copper particles modifying their electronic properties, and resulting in samples with enhanced reducibility. A recent study of Pd/ZnO catalysts pointed out in the same direction [32]. In fact, authors reported preferential formation of a PdZnβ alloy on polar ZnO facets [32], suggesting a different interaction between palladium particles and polar ZnO surfaces, as in the present case. 3.3.2. Catalytic activity of CuO/ZnO samples in conventional reactor The evolution of methanol conversion with the reaction for some reprensentative CuO/ZnO catalysts is presented in Figure 3.6. Overall the methanol conversion, H2 and CO2 yields increase with temperature, as expected (Figure 3.6, Table 3.3). Regardless the catalyst and the temperature, H2 and CO2 are the main products with CO formation initiating at 300 °C when methanol approaches complete conversion (Table 3.3). No CO was observed below 300 °C, expecting for CuO/ZnON-375 (Table 3.3). This is easily understood since CO is a by-product produced by endothermic reactions methanol decomposition (MD) and reverse water gas shift (RWGS) [1]. Figure 3.6 – Evolution of methanol conversion with reaction temperature for CuO/ZnOAc-300, CuO/ZnOAc-375 and CuO/ZnON-375 catalysts   3 01 cat CH OH cat W F 83kg mol s     . Chapter 3 93 Table 3.3 - Methanol conversion, CO2 and H2 yields, and CO amount measured at 220 °C and 300 °C in the conventional fixed bed reactor;   3 01 cat CH OH cat W F 83kg mol s     . Sample Activity (μmol/gmet s) 3 CH OH X (%) 2 CO y (%) 2 H y (%) COb (μmol) Temperature (°C) 180 220 300 220 300 220 300 300 CuO/ZnOAc-300 20.8 11.7 70.6 11.6 70.3 11.7 73.1 1.32 CuO/ZnOAc-350 22.6 13.8 74.8 13.7 76.6 13.8 76.5 1.29 CuO/ZnOAc-375 39.6 15.9 (<1) 82.2 (1.8) 15.7 80.9 16.1 84.1 1.06 (0.015) CuO/ZnOAc-400 17.4 7.9 64.2 7.8 60.1 7.9 62.5 1.25 CuO/ZnON-375 24.1 14.7 (<1) 80.1 (2.4) 14.1 72.7 14.3 75.6 7.42 (0.010) Cu/ZnO/Al2O3a 7.65 63.8 90.1 60.3 82.1 62.1 83.5 9.61 a: Commercial CuO/ZnO/Al2O3 (66/24/10 wt. %) catalyst (G66-MR) supplied by Süd Chemie. Values in brackets correspond to conversion and μmol of CO obtained when MSR tests were performed only with ZnO support; b: corresponds to the CO reformate concentration at 300 °C, no CO was detected below this temperature (< 0.5 μmol/mL) except for CuO/ZnON-375 sample that produces 1.67 μmol/mL and 3.1 μmol/mL of CO at 220 °C and 260 °C, respectively. The catalytic activity at 180 °C as a function of the specific surface of ZnO support and CuO dispersion is illustrated in Figure 3.7. Overall, the activity of the CuO/ZnO catalyst increases with the copper dispersion, with the later increasing as the surface area of ZnO support does. ZnO supports with larger surface areas are able to better disperse Cu particles, leading to a higher number of exposed active sites (Cu sites) and consequently to a higher activity. It is also interesting to note that catalysts prepared from ZnO supports with similar surface areas (CuO/ZnOAc-375 and CuO/ZnON-375) have comparable copper dispersion and behave similarly in terms of activity (Figure 3.7), in good agreement with our previous assumption. CuO/ZnO catalysts for methanol steam reforming 94 Figure 3.1 - Catalytic activity at 180 °C as a function of the specific surface area of ZnO carriers and copper dispersion. Under the conditions of the present study, the dispersion of copper (or copper surface area) is the predominant factor governing the activity of CuO/ZnO catalysts in MSR. This agrees with other published results [33-35] reporting a linear correlation between the activity of Cu-based catalysts and the copper surface area. On the contrary, there is no clear correlation between the CO production and the surface area of ZnO carriers or copper dispersion. In fact, all the catalysts obtained from ZnO with different SBET produce similar amounts of CO (Table 3.1 and 3.3, series of CT catalysts). However, the two catalysts with similar surface areas but very different polarity ratio (CuO/ZnOAc-375 and CuO/ZnON-375) show the largest difference in selectivity. Clearly, the different selectivity of CuO/ZnOAc-375 and CuO/ZnON-375 samples cannot be ascribed to the ZnO surface area or copper dispersion. The CO produced at 300 °C as a function of the ZnO polarity ratio is illustrated in Figure 3.8. Data presented in this figure suggests that the selectivity (regarded as CO produced) is related to the polarity ratio of the ZnO supports, or in other words to the preferential exposure of polar or nonpolar facets of ZnO. In fact, copper catalysts supported on ZnOs with similar polarity ratio (Table 3.1), which in turn are very close to that of the würtzite reference (no anisotropic), produced nearly the same amount of CO (Table 3.3, Figure 3.7). Conversely, copper catalyst supported on the ZnOs showing the highest difference in polarity ratio ZnON-375 ZnOAc-375 (Table 3.1), Chapter 3 95 which in turn are significantly different from that of the würtzite reference (anisotropic), show also the largest differences in selectivity (Table 3.3, Figure 3.7): the lower the polarity ratio, the higher the CO production (Figure 3.9). Thus, a more polar ZnO support gives more selective samples (CuO/ZnOAc-375), namely, producing lower CO amounts and vice versa. Figure 3.2 - Evolution of CO concentration (at 300 °C) as a function of the polarity ratio of ZnO carriers. The polarity of ZnO is linkely related to the presence of defects. Typical ZnO materials exhibit a würtzite structure with the polar planes corresponding to the basal planes of the hexagonal würtzite unit cell [36-38]. The würtzite configuration has preferential exposure of non-polar facets (lower polarity ratio). It is well-accepted that higher proportion of polar facets means also a higher number of defects, such as oxygen vacancies, that may play a crucial role in methanol and water activation [39, 40]. It might be thought that the polar ZnO support itself is responsible for the enhanced MSR selectivity. In order to verify this hypothesis, MSR activity measurements were conducted over both polar (ZnOAc-375) and nonpolar (ZnON-375) ZnO samples under the same operating conditions as those used for CuO/ZnO samples. It was observed that both supports give very low methanol conversions (< 3 %) with almost complete selectivity towards CO2 (so, negligible CO production - Table 3.3). Therefore, the ZnO support alone does not explain the enhanced selectivity. CuO/ZnO catalysts for methanol steam reforming 96 It is then reasonable to assume that in the present case the nature of copper ZnO support interaction (evidenced by TPR) could account for the differences in selectivity. ZnO was also found to affect the activity and selectivity of PdZnO catalysts in MSR reaction [32]. In line with this finding, a recent study about the influence of ZnO facets on the performances of Pd/ZnO catalysts for MSR also reached the same conclusion [41, 42]. Authors reported that at comparable Pd/ZnO catalyst composition, the polar sample was more selective than the nonpolar due to the preferential formation of the PdZnβ phase, which is selective towards CO2, on the polar ZnO [41, 42]. From the results compiled in Table 3, it can be inferred that the polarity ratio of ZnO support does not exert any promoting effect on activity but clearly affects the selectivity (Figure 3.7). TPR results evidenced strong interactions between copper and the more polar ZnOAc-375 support, which facilitates the reducibility of copper oxide leading to enhanced selectivity (decreases CO formation). This suggests that sites of particular reactivity may exist at the Cu–ZnO polar interfaces that are responsible for the higher selectivity of the more polar catalyst, CuO/ZnOAc-375. Despite our results do not allow identifying the exact role of the ZnO polarity ratio on the selectivity of CuO/ZnO catalysts, they clearly point out to its relevant role on the selectivity of the catalyst and suggest that the CuO-ZnO interface is involved in the MSR selectivity. Another interesting finding of the present study is that the activity (per mass of metal) at 180 °C of the best in-house catalyst, CuO/ZnOAc-375, is up to 5-fold higher (Table 3.3) than that of a commercial CuO/ZnO/Al2O3 catalyst (66/24/10 wt. %; G66-MR, from Süd Chemie). Moreover, at comparable methanol conversion (300 °C, Table 3.3) the in-house sample produces considerably less CO (up to 90% lower, Table 3.3), further evidencing the high selectivity of CuO/ZnOAc-375 catalyst. The first part of this study identified catalyst CuO/ZnOAc-375 to have the highest catalytic activity among the prepared catalysts and the highest selectivity of all Chapter 3 97 catalysts. This catalyst was then selected to pack a Pd-based membrane reactor. The results obtained are presented and discussed in the next section. 3.3.3. Catalytic activity of CuO/ZnOAC-375 in the Pd/Al2O3 composite membrane reactor Before the reaction tests, the permeation characteristics of the fresh Pd/Al2O3 membrane were investigated at T = 300 °C and ΔP = 1.0 bar. Table 3.4 shows the ideal selectivities obtained during the pure gas permeation tests. Table 3.4 - Permeation characteristics of the fresh composite Pd/Al2O3 membrane at 300 °C and ΔP = 1.0 bar. Pure gas (i) Ji (mol·-2·s-1) Permeancei (mol·-2·s-1·Pa-1) αH2/i H2 1.42x10-1 1.42x10-6 1 N2 2.36x10-5 2.36x10-10 >6000 He 4.29x10-5 4.29x10-10 ~ 3300 The MSR on the composite Pd/Al2O3 MR were carried out by varying the temperature in the range 220 - 300 °C, at 2.0 bar, H2O/CH3OH feed molar ratio of 2.5/1 and WHSV = 0.95 h−1. The objective of this first experimental campaign was evaluating the CuO/ZnOAc-375 catalyst performance in terms of activity and stability. Based on both permeate and retentate streams, Table 3.5 illustrates both methanol conversion and output molar fractions for different reaction temperatures. Though the composite Pd-based membrane has defects, besides hydrogen only CO2 was found in the permeate stream. In particular, it is worth noting that a temperature increase allows two positive effects on the MR system: the first effect is related to the increase of the reaction rate with the temperature; the second one is due to the H2 permeation through the membrane. In the latter case, at higher temperature the hydrogen permeation through the membrane is enhanced and, consequently, this induces a higher H2 removal from the reaction to the permeate side, favouring the shift of the CuO/ZnO catalysts for methanol steam reforming 98 MSR reaction towards further products formation as well as higher methanol consume. Table 3.5 - Methanol conversion (into gas) and output molar fractions (H2, CO and CO2) at different temperatures, WHSV = 0.95 h-1 and transmembrane pressure = 1.0 bar Temperature (°C) Overall product molar fraction (%) 220 260 300 H2 74.56 74.46 74.18 CO 0.75 0.88 1.25 CO2 24.69 24.66 24.57 CH3OH conversion (%) 12.4 47.1 97.4 Figure 3.10 highlights the stability of the catalyst as confirmed by the constant trend of H2, CO, CO2 selectivities with respect to time on stream up to 3 h of operation at steady state conditions. A similar trend was confirmed in all the MR experimental tests of this work, suggesting that the catalyst is stable under long time operation. Figure 3.3 - Overall product molar fraction vs time on stream for MSR reaction in the Pd/Al2O3 MR at T = 220 °C, transmembrane pressure = 2.0 bar, WHSV = 0.95 h-1, H2O/CH3OH = 2.5/1. Chapter 3 99 Pressure Effect The second campaign of experiments aimed to obtain high grade and high yields of hydrogen in permeate side. The reaction tests were carried out at 330 °C, feed molar ratio equal to 1.5/1, WHSV = 2.73 h−1 and by varying the reaction pressure between 1.5 - 2.5 bar. Table 3.6 shows the permeated hydrogen purity and the hydrogen recovery at 330 °C and at various reaction pressures. Table 3.6 – Hydrogen permeate purity and hydrogen recovery vs reaction pressure at 330 °C H2O/CH3OH = 1.5/1 and WHSV = 2.73 h-1 during MSR reaction in the Pd/AL2O3 MR. Pressure (bar) H2 permeate purity (%) H2 recovery (%) 1.5 90 57 2.0 90 62 2.5 88 64 The hydrogen recovery increases with the reaction pressure, overcoming 65 % at 2.5 bar. Indeed, the highest reaction pressure maximizes the permeation driving force to hydrogen favouring the hydrogen permeation and then the hydrogen recovery. Nevertheless, the permeated hydrogen purity decreases with the reaction pressure. The composite palladium membrane should have pinholes that allow the diffusion/convection of other gas species. As the reaction pressure increases, more hydrogen is removed from the reaction medium making its permeation driving force to decrease. At the same time, the driving force to the other gas species increases, causing the purity of hydrogen at the permeate side to decrease. On the other hand, no CO was detected in the permeate side, as shown in Table 3.7. Table 3.7 – Flow rates of the gases present in the permeate stream at different reaction pressure during MSR reaction at 330 °C, WHSV = 2.73 h-1. Reaction Pressure (bar) Flow rate gas in the permeate (mL·min-1) 1.5 2.0 2.5 H2 1.27 1.90 1.97 CO2 0.11 0.16 0.2 CuO/ZnO catalysts for methanol steam reforming 100 WHSV Effect Hydrogen recovery and hydrogen permeate purity have been determined as a function of the WHSV at T = 330 °C, 2.5 bar reaction pressure and H2O/CH3OH feed molar ratio equal to 1.5/1. Table 3.8 shows that, as expected, the hydrogen recovery increases when WHSV decreases (higher residence times); the hydrogen recovery increased to around 75 % at 1.37 h-1. Table 3.8 – Hydrogen permeate purity and hydrogen recovery vs WHSV at 330 °C, H2O/CH3OH = 1.5/1 and reaction pressure = 2.5 bar during MSR in a Pd/Al2O3 MR. WHSV (h-1) H2 permeate purity (%) H2 recovery (%) 1.37 88 72 2.05 88 66 2.73 87 63 Furthermore, the permeated hydrogen purity did not vary significantly with WHSV being ca. 91 %. Also in this case, no CO presence was noticed in the permeate side. At the end of the whole experimental reaction campaigns, the permeation characteristics of the Pd/Al2O3 membrane have been checked again at T = 300 °C and ΔP = 1.0 bar. Table 3.9 reports the new ideal selectivities values, which are different from the fresh membrane; the permeability increased and the selectivity decreased by a factor of ca. 2. This probably occurred because of the effect of thermal cycles that greatly affect the gas permeation characteristics of the composite Pd-membrane. Table 3.9 - Permeation characteristics of the composite Pd/Al2O3 membrane at the end of the whole experimental campaign. Pure gas (i) Ji (mol·-2·s-1) Permeancei (mol·-2·s-1·Pa-1) αH2/i H2 9.87·10-2 1.95·10-6 1 N2 2.60·10-5 5.14·10-10 ~ 3800 He 6.70·10-5 1.32·10-9 ~ 1500 Chapter 3 101 3.4. Conclusions The effect of the surface area and polarity ratio (related to the facet defects) of ZnO supports on the activity-selectivity of CuO/ZnO catalysts for MSR was studied. The ZnO surface area was varied by changing the calcination temperature of the prepared ZnO materials, whereas, its polarity was modified using different Zn precursors, acetate or nitrate. Both the copper surface area and the dispersion increased with the surface area of ZnO support. The polarity ratio of ZnO carrier carrier did not significantly affectcopper dispersion dispersion but it influences notably the reducibility of copper species. A higher polarity ratio of ZnO promotes the reducibility of copper oxide, which is attributed to a strong interaction between copper species and the more polar facets of ZnO, which in higher proportion in the ZnO support with higher polarity ratio. The activity of CuO/ZnO catalysts shows a marked dependence on the surface area of the ZnO supports, which in turn affect the dispersion of copper. In fact, those catalysts with higher surface area (and also Cu dispersion) also exhibit higher activity. Interestingly, the experimental results evidence that the selectivity of CuO/ZnO catalysts is closely related to the polarity ratio of ZnO carriers: the selectivity increases with the polarity ratio, probably due to the presence of more selective Cu-ZnO sites at the Cu-ZnO polar interface. The CuO/ZnOAc-375 catalyst also shows a drastic improvement of selectivity as compared to commercial CuO/ZnO/Al2O3 sample. In fact, at similar methanol conversion the in-house catalyst has proven to be up to 90 % more selective than the commercial sample. Best performing catalyst CuO/ZnOAc-375 was further tested in a Pd-membrane reactor, made of a thin Pd-layer deposited on Al2O3 support. Two studies were performed, one concerning the catalyst performance in terms of methanol conversion and gas selectivity stability and the other concerning the MR performance in terms of hydrogen recovery and hydrogen permeate purity for various operating conditions. During the first set experiments, the best result has been reached for 300 °C, 2.0 bar and WHSV = 2.73 h−1 with 97 % of methanol conversion. Furthermore, a CuO/ZnO catalysts for methanol steam reforming 90 . Chapter 4 109 Chapter 4. Ultraselective low temperature steam reforming of methanol over PdZn/ZnO catalyst – Influence of induced support defects on catalytic performance 3 Abstract The influence of the calcination atmosphere of ZnO precursor (Zn4(CO)3(OH)6·H2O) on the catalytic performance of a series of PdZn/ZnO catalysts was studied for production of H2 via low temperature (180 °C) direct methanol steam reforming (low temperature-MSR). The catalytic activity and selectivity of PdZn/ZnO were found to be strongly influenced by the calcination atmosphere of ZnO precursor and increased from oxidizing to reducing atmosphere, following the order (O2< air< N2< H2). As a result, a very active catalyst was obtained by simply supporting Pd on ZnO calcined in H2. Further evidence from XPS and TPR analysis indicated that calcination in reducing atmosphere gave rise to a significant increase in the concentration of oxygen vacancies on the surface of ZnO support. Thus, the superb performance of the best catalyst was attributed to the defect chemistry of ZnO support; mainly to the amount of oxygen vacancies present in the interface region, which act as additional active sites for water adsorption and subsequent activation. In addition, the formation of CO was drastically suppressed by replenishment of oxygen vacancies on ZnO support. Thus, it is clear that the abundance of specific active sites on PdZn/ZnO catalyst is strongly influenced by the preparation route of the ZnO support. Additionally, the PdZn alloy was discovered to be unstable under prolonged exposure to CO atmosphere and the stability test under methanol steam reforming conditions showed a 24% drop in conversion over 48 h testing period. This phenomena can have detrimental effect on the performance of this type of catalytic systems in continuous prolonged duty cycle time on-stream. 3 K. M. Eblagon, P. H. Concepción, H. Silva, and A. Mendes, Ultraselective low temperature steam reforming of methanol over PdZn/ZnO catalysts—Influence of induced support defects on catalytic performance. Appl. Catal. B Environ. 154–155 (2014) 316–328 Ultraselective low temperature Steam Reforming of Methanol over PdZn/ZnO catalysts 110 4.1. Introduction Hydrogen will surely contribute to the world energy market in the mid-term to long-term future [1, 2]. However, one of the major obstacles of “Hydrogen Economy” is finding a feasible method to store and distribute sufficient amount of hydrogen mainly for transportation sector [3]. Recently, reformers that can extract hydrogen from fuels gained increased research interest due to the fact that they can be integrated directly with polymer electrolyte fuel cell (PEMFC). As a result, high purity H2 can be produced in situ on board of the vehicle via well-established steam reforming of alcohols from biomass, or hydrocarbons [4]. In this regard, methanol is a promising hydrogen carrier candidate for the future [5] mainly because it has high H to C ratio, low chemical energy bond [6] and can be produced either by reduction CO2 or from diverse bio-based resources [7]. Methanol steam reforming (MSR) is accompanied with side-reactions such as: methanol decomposition (MD), water gas shift (WGS) and reversed water gas shift (RWGS) [8, 9]. The extent of these processes depends on the feed composition, reaction conditions and the catalyst used. Due to MD and RWGS reactions being always present, the reformate gas inevitably contains CO contaminant which even in trace levels can poison the PEMFC anode [2, 10]. For example, high temperature PEMFC (HT-PEMFC) using polybenzimidazole membranes working at 180 °C can only tolerate a maximum of 1–2% CO in the anode feed [11]. One approach to decrease the amount of produced CO is to lower the temperature of MSR to the point at which MD is suppressed and WGS reaction is favoured due to the thermodynamics of these processes [9]. A catalyst able to work efficiently at lower temperatures is also cost-effective and desirable taking into consideration the integration between the endothermic MSR and exothermic PEMFC operation. Furthermore, low operating temperatures are beneficial for applications as power supplies for small portable devices, where heat and space management are of primary concern [12]. However, the development of a suitable catalytic system promoting both endothermic MSR and exothermic WGS at reasonably low temperatures (below 200 °C) remains a challenge. Chapter 4 111 Cu based catalysts are typically used for MSR due to their relatively low cost, very high activity and low CO production at temperatures below 300 °C [13]. Thus, several catalyst formulations were studied, such as binary compositions; Cu/ZnO, Cu/SiO2, Cu/CeO2 [4], commercially based Cu/ZnO/γ-Al2O3 and its variations with added promoters; Cu/ZrO2/γ-Al2O3, Cu/Cr/γ-Al2O3 [2] or CuTiP/γ-Al2O3 [6]. More recently CuZnGaOx [12] and CuZn catalyst promoted by rare earth metals such as Tb and Pr [14] were reported to have high activity at low temperature MSR. Nevertheless, Cu based catalysts have some considerable drawbacks which include pyrophoricity and easy deactivation due to thermal instability [7, 15] or coke formation [13]. Additionally, these catalysts are sensitive towards condensing steam [6], which results in declining activity and mechanical integrity of the catalytic system under duty cycle conditions [11].The search for a more suitable catalyst for MSR has led to a discovery that the inherent high selectivity of Pd catalyst towards MD can be drastically switched to high selectivity in MSR by simply changing the material of the support [16]. Apparently, Pd supported on hard-to-reduce oxides MxOy (M= Si, Al, Mg, Zr, Pr, Ce, La) is highly selective to MD [17, 18]. On the other hand, Pd supported on easily reducible oxides such as ZnO, Ga2O3 or In2O3 becomes active and selective in MSR. Unusual behaviour of these Pd based catalysts was attributed to a formation of alloys PdMe (Me = Zn, Ga, In) upon proper reductive pre-treatment, leading to a bifunctional synergism between intermetallic and oxide species which is necessary for a good catalytic performance in MSR [19, 20]. In addition, the in situ partial oxidation of PdZn nanoparticles was observed recently to result in the formation of a large interface between the intermetallic PdZn and small ZnO patches, which improved the selectivity to CO2 in MSR [20]. It is thus evident that the performance of Pd/ZnO catalyst is not only dependent on the chemical composition or size of the active species, but it is also governed by the type of active sites present on its surface [21]. Moreover, there was an indication that addition of Zn to Pt or Pd altered the barriers for C–H cleavage and thus changed the stability of various intermediates in MSR [22]. The PdZn/ZnO interface was found to increase water activation at lower temperatures, therefore promoting CO2 selectivity [23]. However, the role of ZnO in the performance of PdZn/ZnO is not yet fully understood. Ultraselective low temperature Steam Reforming of Methanol over PdZn/ZnO catalysts 112 ZnO is an anisotropic oxide which contains intrinsically unstable polar facets with point defects that compensate for a surface dipole moment and non-polar facets which exhibit densely packed Zn and O atoms that are electrically neutral [24]. In addition, stronger electronic interaction and facile PdZn alloy formation was reported on Pd supported on ZnO with majority of polar facets exposed on the surface. Furthermore, it was shown previously that the exposure of ZnO to atmospheric pressure of H2, can change the concentration of the Schottky defects mainly that of oxygen vacancies [25]. In addition, the creation of defects through reduction modifies the d-band states of a reducible metal oxide (such as ZnO) which changes the interaction with the adsorbates [26]. As a result, it can be expected that the increased concentration of oxygen vacancies in ZnO support might improve the performance of the PdZn/ZnO catalyst at low temperature MSR. With this regard, the present work will be focused on the performance of PdZn alloys supported on the ZnO calcined in N2, O2, air and H2 in low temperature MSR. The catalytic results will be compared to the performance of PdZn supported on commercial ZnO (Sigma–Aldrich). The aim is to provide an insight into the influence of the active sites present on the surface of ZnO support on the activity and selectivity of the PdZn/ZnO catalytic system. Moreover, thorough catalyst characterization was carried out to gain a better understanding of the key parameters controlling the performance of PdZn/ZnO catalysts for low temperature MSR. Thus, the extent of synergetic interaction between PdZn and various ZnO was studied using analytical techniques such as X-ray Diffraction (XRD), Temperature Programmed Reduction (TPR) and X-ray Photoelectron Spectroscopy (XPS). Selected catalysts and ZnO supports were additionally characterised by high resolution transmission electron microscopy (HRTEM) and scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM-EDX). The active sites and the stability of PdZn/ZnO catalysts was characterised by CO absorption IR DRIFT spectroscopy. Finally, the stability test of the most active catalyst was carried out to access its long term performance under prolonged exposure to methanol steam reforming conditions. Chapter 4 113 4.2. Experimental 4.2.1. Hydrothermal method of ZnO preparation All the reagents were purchased from Sigma–Aldrich and were used without any pretreatment. The ZnO supports were obtained following a conventional hydrothermal synthesis [27]. Zn(CH3COO)2 was used as a Zn2+ precursor, CO(NH2)2 as precipitant and P123 Pluronic block copolymer as a surfactant. In a typical hydrothermal synthesis; 1.1 g of Zn(CH3COO)2, 6 g of urea and 3 g of P123 were dissolved in 100 mL of distilled water. Subsequently, the pH of the solution was fixed at 5.0 using CH3COOH and the mixture was left to homogenize at ambient conditions for 2 h. After a homogenous mixture was obtained, the solution was sealed in a hydrothermal teflon lined reactor and heated in a laboratory oven at 90 °C for 24 h to ensure the complete precipitation of zinc carbonate. Subsequently, the obtained solid product was filtered and washed several times with distilled water and dried overnight at 90 °C in air in an oven. The final step of the preparation of ZnO supports was the calcination of the prepared precursor at 370°C, with a heating rate of 4.2 °C/min in a flow (200 mL/min) of a selected gas (H2, O2, air, N2) and with a dwell time of 0.5 h. The prepared ZnO supports are hereafter referred as: ZnO_H2 – ZnO calcined in H2; ZnO_N2 – ZnO calcined in N2. ZnO_COM is a ZnO commercial support (Sigma–Aldrich), etc. 4.2.2. Synthesis of PdZn/ZnO catalysts The 4.7 wt.% PdZn/ZnO catalysts were prepared following a standard wet impregnation technique, using hydrothermally prepared ZnO supports (Section 4.2.1) and ZnO commercial support (Sigma–Aldrich, ZnO nanopowder) for comparison. In a typical procedure, the appropriate amount of ZnO support was wetted by 10 mL of chloroform to prepare a slurry. Subsequently, a solution of a calculated amount of palladium acetate (Sigma–Aldrich, reagent grade 98%) in 5 mL of chloroform was added dropwise to the slurry of the ZnO support under vigorous ultrasonic treatment. Ultraselective low temperature Steam Reforming of Methanol over PdZn/ZnO catalysts 114 The solvent was then dried out at ambient conditions under continuous magnetic stirring while placed in a fume hood. PdZn alloy formation is necessary to obtain good catalytic activities in MSR, thus the catalyst reduction temperature was optimized by monitoring various reduction temperatures using an on-line XRD technique. The optimum temperature of PdZn alloy formation was chosen at 400 °C in H2 flow (200 mL/min), with a heating rate of 4.2 °C/min and a dwell time of 2 h. 4.2.3. Physicochemical characterization of supports and catalyst The physicochemical properties of ZnO supports were characterised using a variety of methods. The morphology and qualitative composition was obtained from SEM images and EDX, respectively. The SEM micrographs were recorded using a high resolution (Schottky) environmental scanning electron microscope with X-ray microanalysis and backscattered electron diffraction pattern analysis working at 20 kV. Shape and sizes of the particles were obtained using image analysis software (ImageJ). The average diameter of the ZnO assemblies was obtained measuring at least 100 particles from three different regions of each of the samples. The composition of the supports was studied at a magnification of about 600k. Four different areas of each of the samples were examined by EDX analysis with working distance set at 25 mm, dead time 50% and a process time of 6 min. The crystallographic characterisation of ZnO supports was obtained using the XRD technique. The XRD pattern of the selected samples was collected using a Philips PW1729 diffractometer operating in Bragg-Brentano focusing geometry and using Cu Kα radiation at wavelengths Cu Kα1 = 154.06 pm and Cu Kα2 = 154.439 pm. The data was collected at 2θ angles (20–70°), with a step size of 0.02°, step speed of 0.5°/min and at 1.25 s per step. The obtained X-ray scans were compared to those of standard database and the phases were assigned comparing the data available in literature. Chapter 4 115 BET surface area of ZnO supports was measured following a standard procedure, obtaining N2 11 point adsorption isotherms at 77 K. Prior to the measurements, the samples were outgassed in vacuum at 300 °C for 2 h. The neat ZnO supports and as-prepared PdZn/ZnO catalysts were characterised using a TEM CM 20 (accelerating voltage 310 and 208 kV at nominal magnification) and a HRTEM JEOL 2010 (accelerating voltage 600 kV, at nominal magnification 590 kV). For determination of a particle size distribution, not less than 100 particles from different areas of the sample were measured, using the Scandium software from Olympus Soft Imaging Solutions. The determination of the lattice d-spacing was carried out using high resolution micrographs of the lattice, taking at least five measurements from different areas of the sample. The estimated error of the measurements was ±0.01 nm. The interaction between metal and support in PdZn/ZnO calcined at various atmospheres was probed using X-ray photoelectron spectroscopy (XPS). The XPS analysis was performed using a Kratos AXIS Ultra HSA, with VISION software for data acquisition and CASAXPS and XPS Peak 41 software for data analysis. The analysis was carried out with a monochromatic Al Kα X-ray source (1486.7 eV), operating at 15 kV (90 W), in FAT mode (Fixed Analyser Transmission), with a pass energy of 40 eV for regions ROI and 80 eV for survey. Data acquisition was performed with a pressure lower than 6 1.0 10  Pa using a charge neutralisation system. The modelling of the spectra was performed using peak fitting with Gaussian–Lorentzian (80%) peak shape and Shirley type background subtraction. The TPR-curves were measured using a quartz flow-through apparatus. Consumption of H2 was obtained using a Balzers GAM-415 quadrupole mass spectrometer. Hydrogen response was calibrated by pulsing 1 mL of 3% H2 in argon and measuring the corresponding response. TPR-conditions: H2/Ar (3 vol%),flow 20 mL/min, heating rate 3 °C/min in the temperature range 30–600 °C. The nature of the Pd species and the type of active sites present in the prepared PdZn/ZnO catalysts was further characterised by CO-chemisorption IR-DRIFT analysis. The IR-DRIFT analysis was carried out using powder catalyst. The spectra Ultraselective low temperature Steam Reforming of Methanol over PdZn/ZnO catalysts 116 of adsorbed CO were recorded at room temperature (25 °C) with a Nexus 8700 FTIR spectrometer using a DTGS detector with 4 cm−1 resolution. An IR cell allowing in situ treatments under controlled gas atmosphere and temperature was connected to a vacuum system with gas dosing facility. For IR studies the samples were pressed into self-supported wafers and diluted in ZnO (Sigma–Aldrich) using previously optimised weight ratios. The samples were treated at 300 °C in a 5% H2 in N2 flow (15 mL/min) for 2 h followed by evacuation at 10-4 mbar at 350 °C for 1 h. After the activation procedure, the samples were cooled down to 25 °C under dynamic vacuum conditions followed by CO dosing at increasing pressure (0.4–15 mbar). Several IRDRIFT spectra were recorded with increasing exposure time, until no further changes were observed in the spectra indicating full CO saturation of the surface. The amount of Pd in the samples was determined with an ICP-OES (Vista RL, Varian) after matrix-matched calibration. Not more than 5 mg of the sample was dissolved in 2 mL of aqua regia. Finally, the solutions were diluted in 50 mL volumetric flasks and the analysis was carried out. The number of surface Pd sites was quantified by CO pulse chemisorption experiments which were conducted using a Micromeritics AutoChem II 2920 analyzer. Catalysts (typically 0.3–0.5 g) were loaded in a quartz tube that was placed in the built-in furnace. The temperature in the catalyst layer was controlled using a type-K thermocouple and the flow of gasses was regulated by a built-in mass flow controllers. The catalysts were typically purged with dry helium at 100 °C for 1 h, followed by reduction in 5% H2/Ar (flow rate 50 mL/min) at 400 °C for 2 h and then it was cooled down to 50 °C. Subsequently, the cleaned sample was exposed to pulses of 0.5 mL CO until five consecutive pulses yielded identical signal areas. The dispersion of Pd was estimated from the ratio of the number of moles of surface Pd atoms to the total number of moles of Pd atoms present in the catalyst. Chapter 4 117 4.2.4. Catalytic tests The catalytic activity tests of MSR were performed in a fixed bed reactor at pressures between 1 and 1.6 bar, with the following experimental conditions: steam/methanol molar ratio of 1.5 and a contact time of W/F0 = 83 kg mol−1s (where W is the mass of the catalyst, 200 mg and F0 is the flow rate of methanol). The stainless steel microreactor was placed in an electrically heated furnace and the temperature was controlled inside the furnace as well as in the catalytic bed using a K-type thermocouple inserted into the catalytic bed. Argon was used as carrier gas and the flow of reactants was controlled by mass flow meters and the gas products were analysed by an online mass spectrometer. The concentration of CO (ppm) in the products was measured directly using an on-line CO analyser (reading error ± 2 ppm). An average of at least 10 measurements was taken to reduce the reading error. Prior to the catalytic test, the catalyst was pre-treated in situ for 2 h by heating up to 200 °C in a stream of Ar (30 mL/min). Subsequently, the hydrogen gas was switched on and the final stream consisted of 70% H2/Ar. Finally, the catalyst was cooled down to 180 °C and the reaction mixture was introduced. An average of four measurements spaced by 15 min was taken to assure that a stable state of the catalyst performance was reached. 4.3. Results and discussion 4.3.1. Structural and Surface Characterisation of ZnO supports The hydrothermally prepared ZnO supports showed significantly higher values of BET surface area in comparison to ZnO_COM. The BET surface area results are gathered in Table 4.1. Within in-house made samples, higher surface areas were obtained for ZnO supports calcined in oxidative atmospheres. Larger BET surface area after calcination in air/O2 than in N2 were previously observed in case of other oxides [28]. Ultraselective low temperature Steam Reforming of Methanol over PdZn/ZnO catalysts 124 Figure 4.4 - XRD pattern obtained during reduction of PdZn_ZnO_COM at various temperatures (°C). Large peak at 2θ = 47.7°is ascribed to ZnO (102) plane [29]. Lattice fringes of 0.28 nm were measured from HRTEM micrographs for ZnO_O2, ZnO_H2 and ZnO_COM which can be attributed to the exposure of a ZnO non-polar (100) plane [29] which is in agreement with the XRD findings. The formation of PdZn alloy under reduction atmosphere in all of the samples was also further supported by the lattice spacing measurements from HRTEM micrographs. A representative HRTEM image of PdZn_ZnO_H2 is shown in Figure 4.5. Figure 4.5 - Left: HRTEM image of a single polycrystalline PdZn particle supported on ZnO_H2. The lattice fringes of PdZn (101) and ZnO (100) are marked. Right: HRTEM image of single PdZn and Pd particles supported on ZnO_COM. Chapter 4 125 As it can be seen in Figure 4.5 left, a lattice spacing of 0.22 nm was obtained for the nanoparticle, which matches the value reported for PdZn alloy (111) [35]. It should be noted, that the lattice fringes of the particles in the range of 3-4 nm were easily obtained, however measuring d-spacings of smaller particles also present in the samples was difficult due to the contrast from the support. Therefore, we have examined more closely selected area of sample PdZn_ZnO_COM (see Figure 4.5, left) that contained the highest average particle size. The image revealed the lattice spacing of the smaller particle (on the left side of Figure 4.5) to be 0.23 nm. This result can suggest the presence of Pd (111), which possesses lattice spacing slightly higher as compared to that of PdZn alloy in accordance with literature findings [35]. It should be underlined that due to the difference in lattice spacing between PdZn and Pd being not more than 3% [35], the phase of these small particles in our case could only be conclusively identified as Pd by joined results from HRTEM, TPR, XPS and CO adsorption-DRIFT analysis described later in this work. A representative lower magnification HRTEM image of the same sample together with the corresponding particle size distribution is shown in Figure 4.6. Similarly to these results, the HRTEM analysis of the remaining PdZn/ZnO catalysts, showed finely dispersed particles with no visible agglomeration, regardless of the calcination atmosphere of ZnO precursor. The mean diameters obtained from HRTEM images of PdZn supported on hydrothermally synthesized ZnO were in the range of 2.1 nm to 3.4 nm, which is significantly lower than the mean diameter of the PdZn supported on ZnO_COM (8.8 nm). The sizes of the particles are gathered in Table 4.2. A clear influence of the morphology of ZnO support was observed on the crystallization of PdZn alloy, leading to changes in size of the nanoparticles. In general, smaller particles would be expected on the higher surface area supports, mainly due to higher Pd dispersion and thus longer diffusion distances between neighboring PdZn and decreased sintering of these particles. Nevertheless, no clear trend was observed between the particle size and the surface area of ZnO support in the studied catalysts. Ultraselective low temperature Steam Reforming of Methanol over PdZn/ZnO catalysts 126 Figure 4.6 - (A) HRTEM image of PdZn/ZnO_H2. (B) Particle size distribution of the same sample. Table 4.2 - Catalytic performance of a series of PdZn/ZnO catalyst in low temperature MSR together with corresponding particle size measured from HRTEM images. Sample Activity (μmol/gmet·s) CO concentration (ppm) HRTEM particle size (nm) PdZn_FEUP_O2 42.6 1450 3.3 PdZn_FEUP_air 46.8 1400 2.4 PdZn_FEUP_N2 63.8 700 2.1 PdZn_FEUP_H2 87.9 146 3.3 PdZn_ZnO_COM 38.3 309 8.1 4.3.3. Methanol Steam Reforming A good catalyst for low temperature MSR should provide high water and methanol conversions to hydrogen and carbon dioxide while minimising occurrence of any side-reactions. The prepared PdZn alloyed catalysts immobilized on ZnO calcined under various atmospheres were tested for activity and selectivity in MSR at 180 °C using an in-house built MSR set-up as described in the Experimental section. The results of the catalytic activity and selectivity expressed by CO concentration in ppm, are gathered in Table 4.2 and compared to PdZn alloy supported on ZnO_COM. The BET surface area of ZnO supports and their polarity can be found in Table 4.1.It was rather surprising to find that the catalytic activity was independent from the BET surface area of ZnO support (compare Table 4.1 with Table 4.2), Chapter 4 127 which is in contrast with the literature reports [27]. On the other hand, selectivity to CO was higher for PdZn catalysts supported on higher surface area ZnO, which were calcined in oxidizing atmosphere. This result can be associated with the apparently lower reducibility of high surface area ZnO supports. Moreover, the selectivity to CO was found to be inversely proportional to the activity for the PdZn supported on hydrothermally obtained ZnO supports (excluding PdZn/ZnO_COM). Lower selectivity to CO was achieved by PdZn supported on ZnO with higher exposure of polar facets which is in agreement with the literature [25]. However, no direct trend was established between increased polarity of ZnO and activity of the PdZn/ZnO catalysts, which can be explained by the fact that the range of the polarities studied was very narrow. Additionally, smaller PdZn alloy particles obtained on hydrothermally produced ZnO displayed better activity per gram of metal than their bigger counterparts supported on ZnO_COM. However, in order to account for the particle size differences between PdZn_ZnO_H2 and PdZn_ZnO_COM, the TOF of these samples was compared and the results are gathered in the supplementary material. The amount of active sites was obtained by CO pulse chemisorption taking into consideration the similar amount of PdZn alloy present on the surface of these catalysts, as showed later in the paragraph describing XPS results of the present work. Similar extent of alloy present is essential in case of this comparison, because the presence of Zn in Pd decreases the amount of chemisorbed CO and therefore alters the Pd:CO stoichiometry, leading to false number of measured active sites on the catalyst [46]. The TOF of PdZn_ZnO_H2 was 1.01 s−1 and that of PdZn_ZnO_COM was 0.71 s−1, which is in the range of the values reported in the literature for similar systems [16]. These results show that independently of the particle size, the most active catalyst was obtained by supporting PdZn on ZnO calcined in reductive atmosphere. Nevertheless, the greatest differences in performance could be assigned to differences in calcination atmosphere of ZnO precursor (H2, O2, N2, air). As it can be clearly seen in Table 4.2, the activity of PdZn/ZnO catalysts increased with calcination atmosphere from oxidative (O2) to reducing (H2). It should be underlined that over twofold increase in activity was achieved by supporting PdZn on ZnO_H2 as compared to ZnO_O2. Moreover, the selectivity to CO dropped significantly on Ultraselective low temperature Steam Reforming of Methanol over PdZn/ZnO catalysts 128 the PdZn supported on ZnO calcined in N2 and H2 as compared to PdZn supported on ZnO calcined in oxidative atmospheres. The decrease in CO production can be a result of more efficient water activation in these supports, prerequisite for CO2 selective catalyst in MSR [24]. Overall, the anticipated exceptional increase in activity and selectivity of PdZn supported on ZnO calcined in H2 atmosphere can be associated with the presence of different active sites, mainly oxygen defects on the surface of the support of this catalyst, which alter the adsorption process of substrates and intermediates during low temperature MSR. It is clear that the difference in catalytic performance as a result of the calcination atmosphere of ZnO showed cannot be attributed to a difference in particle size of the PdZn alloy, as virtually the same particle sizes were obtained in this family of catalysts, regardless of the calcination atmosphere of ZnO. As it was mentioned before, significantly higher size of PdZn particles was noted on the ZnO_COM, which can be attributed to the difference in morphology of the ZnO support. Oxygen vacancies present on the surface of an oxide catalyst are important for several catalytic reactions [31]. However, generally the amount of oxygen vacancies is expected to be higher in ZnO with higher exposure of polar facets, because they are formed to counterbalance the large surface dipole moment [24]. The vacancies can also be created on the non-polar facets of the crystal by treatment in reductive atmosphere, given the right reaction conditions. Much higher conductivity has been previously reported for ZnO materials calcined in oxygen poor atmospheres, which was attributed to a reversible removal of the oxygen atoms from the ZnO lattice and releasing free carriers [37]. Thus, it is likely that the outstanding performance of the PdZn/ZnO_H2 catalyst in the present work is a result of a higher concentration of oxygen vacancies that are produced according to Eq. (4.1) [37], even though the exposure of polar facets on this support is not the highest in this work (see Table 4.1) 22 24   i O O e Eq. (4.1) These oxygen defects in ZnO create additional active sites to the ones existing on PdZn and on PdZn/ZnO interface, which are responsible for water adsorption. Such a synergy drastically increases the activity of the catalysts at low temperature MSR. Additionally, the improved water activation increases the selectivity towards CO2 by Chapter 4 129 decreasing the activity of PdZn/ZnO in MD. On the other hand, when the ZnO precursor is calcined in oxidative atmospheres, the surplus oxygen is able to enter the interstitial sites of ZnO and push the interstitial oxygen atoms back into the lattice, or fill in the existent oxygen vacancies. Thus the amount of VO–Oi pairs formed decreases. As a result, less available active sites are present on the surface of ZnO leading to poorer catalytic performance. Noteworthy, further evidence had to be obtained to support this speculation. 4.3.4. XPS study of Oxigen vacancies in ZnO supports XPS is a very useful technique to investigate the chemical and electronic properties of the heterogeneous catalysts. Particularly, the presence of defects in the structure of ZnO can cause changes in chemical bonding resulting in the shifts of Binding Energy (BE) of the elements in XPS [36]. Thus, the XPS spectra of Pd 3d, Zn 2p, O 1s and C 1s were recorded for selected catalysts and a representative ZnO_H2 support. A general XPS survey spectra of all of the catalyst samples showed Pd, Zn, C and O throughout the experiments. With the goal to investigate the abundance of the oxygen vacancies on the PdZn supported on various ZnO supports, the electronic state of the O 1s XPS peak was firstly analysed in higher resolution. A resulting comparison of XPS spectra of oxygen 1sregion of the PdZn/ZnO_H2 and PdZn/ZnO_O2 is shown in Figure 4.7. In all cases studied, the obtained O1s asymmetric peak was coherently fitted with three components in agreement with previously reported studies [38]. Accordingly, the oxygen species on the lowest side of the O 1s spectrum labelled as Oa can be attributed to Zn-O bonds of highly crystalline ZnO. The component with the medium BE (labelled Ob) can be assigned to O2ions in oxygen deficient areas of the ZnO lattice. The changes in intensity of this component mirror the variation in the concentration of oxygen vacancies in ZnO support. In addition, the highest BE peak labelled Oc is associated with adsorbed OH groups on the surface of ZnO. These groups most likely originate from dissociation of adsorbed water on ZnO, which was produced during reduction of PdO [39]. Ultraselective low temperature Steam Reforming of Methanol over PdZn/ZnO catalysts 130 Figure 4.7 - Comparison of the O 1s spectra of PdZn_ZnO_H2 (top) and PdZn_ZnO_O2 (bottom) with the assigned peaks from various oxygen species. The BE and composition of O 1s spectra of the representative catalysts and ZnO_H2 support are gathered in Table 4.3. As indicated by the results, the highest amount of vacancies (Ob) was found in PdZn_ZnO_H2. Over 52% of the total oxygen species present on the surface of this catalyst was associated with oxygen defects. On the other hand, the lowest amount of oxygen vacancies was found in PdZn_ZnO_COM. Moreover, the PdZn supported on ZnO_COM had the highest relative concentration of stoichiometric oxygen (Oa) which agrees well with the XRD result (see Figure 4.1), showing that ZnO_COM had a more crystalline nature than the hydrothermally produced counterparts. In addition, a small shift towards lower values of the BE of oxygen can be noted in case of the most active catalyst as compared to other samples (see Table 4.3). This can suggest that the electrons are easier to be excited in ZnO_H2 due to the significantly higher mobility of interstitial charge associated with oxygen vacancies. A similar shift of BE of O 1s was previously reported for Cu supported on polar ZnO and was linked to the presence of defects in the ZnO support [21]. Further, as shown in Table 4.3, the composition of O Chapter 4 131 1s peak in ZnO_H2 did not change significantly upon addition of Pd and subsequent PdZn alloy formation via reduction in H2. However, a shift of BE towards higher values was observed in PdZn_ZnO_H2 as compared to pure support ZnO_H2 (Table 4.3), which can be attributed to the electronic inter-action between PdZn and support. The electronic synergy leads to a lower charge concentration and thus a higher oxidation state of oxygen. Table 4.3 - BE and relative intensities of components of O 1s XPS spectra. Sample Oa BE (eV) % Ob BE (eV) % Oc BE (eV) % PdZn_ZnO_H2 530.2 4.6 530.7 52 531.9 43.4 ZnO_H2 529.8 5.9 530.2 53 531.7 40.7 PdZn_ZnO_O2 530.2 40.5 530.8 40.2 532.0 19.2 PdZn_ZnO_COM 530.4 66 531.6 11 532.3 23 PdZn_ZnO_air 530.5 58 531.1 19 532.1 22 With regards to the chemical state of Zn in PdZn/ZnO catalysts, the XPS spectra of Zn 2p and Auger Zn LMM peaks were also analysed. The representative XPS spectra of Zn 2p3/2 of PdZn_ZnO_COM is shown in Figure 4.8. The obtained asymmetric Zn peak could be fit-ted with two components. Thus, two Zn species coexisted in all of the analyzed catalysts. The lower binding energy was assigned to metallic Zn and the higher BE was assigned to oxidized Zn [40, 41]. Similar XPS spectra were obtained for the other studied catalysts. Ultraselective low temperature Steam Reforming of Methanol over PdZn/ZnO catalysts 132 Figure 4.8 - Representative highly resolved core level Zn 2p3/2 spectrum of PdZn_ZnO_COM. Different amount of oxygen vacancies in ZnO supports was also confirmed by studying the Auger line of Zn LMM. The BE of Auger Zn LMM is generally more sensitive to the chemical environment [38]. As shown in Figure 4.9, there is a positive shift of BE in the presence of an alloy, which is slightly higher in case of the most active PdZn/ZnO_H2 sample. The shift of Zn LMM peaks from low BE to higher values in the presence of the PdZn alloy was caused by the decreased negative charge on Zn. This can be attributed to a synergy between support and PdZn particles in which the electrons from Zn interact with the positively charged oxygen vacancies unquestionably present on the interface in the PdZn/ZnO_H2 sample. Figure 4.9 - The Zn LMM line of PdZn/ZnO catalysts as compared to pure ZnO_H2_support. Chapter 4 133 The influence of the calcination atmosphere of ZnO on the chemical and electronic state of Pd species in PdZn/ZnO catalyst was analysed in detail. It was expected that the small local variation of the electronic charge concentration in the ZnO supports would influence the BE of Pd in the resulting PdZn/ZnO catalysts. In our study, careful fitting of the obtained XPS signal of Pd 3d in case of all of the catalysts studied here showed similarly the coexistence of three different Pd species. For example, a representative Pd 3d XPS spectrum of PdZn/ZnO_H2 catalyst is shown in Figure 4.10. The Pd 3d region presented a doublet of Pd 3d5/2 and Pd 3d3/2 at 335.04 and 340.35 eV, which was assigned to Pd (0) species. Another doublet positioned at 335.93 and 341.29 eV can be assigned to Pd in PdZn alloy. With accordance to the literature, the bimetallic bonding with Zn produces positive BE shift in the core levels and valence d band of the group 10 metals [42]. The positive shift is connected with the reduction of electron population and subsequent shift of the valence d orbital. The remaining third doublet at 336.6 and 342.4 eV would be attributed to oxidized Pd. The presence of oxidized Pd could be expected due to the fact that Pd easily reacts with oxygen from air at ambient conditions and the samples were not pre-reduced in situ before the XPS-experiment. The XPS assignment agrees well with the values reported in the literature [17, 43]. The existence of Pd in the metallic state can be the result of not complete alloy formation, or the decomposition of PdZn alloy upon air exposure to Pd and Zn [43]. Additionally, the presence of separately existing metallic Pd particles on XPS spectra agrees well with our results from HRTEM image analysis of lattice spacing of single particles.