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A mechanochemical synthesis method has been used to synthesize CoRu nanoparticles supported on CeO2 for methane dry reforming. In this work, we study the effect of Ru addition to Co/CeO2-based catalysts and of the synthesis method by screening their catalytic activity, using synchrotron X-ray diffraction (XRD), and operando near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). Ruthenium addition directly impacts the reducibility of cobalt species and results in smaller particle sizes, as demonstrated by H2-temperature programmed reduction and XRD. NAP-XPS shows that Ru modifies the metal-support interaction, as evidenced by the higher Ce3+/Ce ratios for the bimetallic samples and tuning the oxidation state of Ru. The synthesis method also influences the dispersion of Co and Ru on the surface. Mechanochemically-prepared samples (mono- and bimetallic) outperformed the conventionally-synthesized counterparts by reaching higher CH4 and CO2 conversions, resulting in a stable CoRu/CeO2 catalyst for 24 h at 700 °C and yielding an H2/CO ratio close to 1., Peer Reviewed, Postprint (published version)

(English) Heterogeneous catalysis is one of the most important chemical technologies available today to produce most chemicals at the basis of industrial production. The transition to a greener energy production demands new challenges and new chemicals to be used as energy sources and vectors. Established processes such as methane steam reforming (MSR) are used to produce hydrogen to be used in fuel cells, and syngas which is the base reactant mixture for the synthesis of other chemicals. Bio-methane generated from biomass or other renewable sources can be used to produce hydrogen and syngas in a sustainable way. One way to improve the performance of a heterogeneous catalyst is to prepare bimetallic catalysts exploiting the synergy between two different metals for the active phase. Catalyst synthesis is also a fundamental step to control the catalytic properties of the material and ultimately to increase the sustainability of the whole process. In this PhD thesis, two bimetallic systems based on Ni-Fe and Pt-Ni were studied for methane steam reforming. Together with the incipient wetness impregnation synthesis, one of the most common industrial methods to prepare catalysts mechanochemistry synthesis was used as a green chemistry alternative. In Chapter 4, Ni-Fe bimetallic catalysts have been studied on different supports comparing the two synthesis methods, the effect of the support and the amount of Fe in the alloy. Iron is a redox promoter which can enhance the coke resistance of Ni-based catalysts, and it should provide sulphur resistance too. Thorough investigation with synchrotron in situ techniques such as XRD and NAP-XPS under reaction conditions allowed to study the catalyst structure during the reaction. The rearrangement of the Ni-Fe particles and the enrichment of Fe on the surface was observed under MSR conditions. Other catalysts based on different support have shown different behaviours: on non-reducible oxide supports, the activity of the bimetallic s, (Català) La catàlisis heterogènia és una de les tecnologies més importants disponibles a dia d'avui per a la producció de la majoria de productes químics i de molts processos industrials. La transició cap a una producció d'energia més verda suposa nous reptes i la producció de nous compostos químics per a ser utilitzats com a fonts d'energia i vectors energètics. La reformació de metà amb aigua (MSR, methane steam reforming) és un procés ben establert per a la producció d’hidrogen per ser utilitzat en piles de combustible i gas de síntesi, el qual pot servir per a la fabricació d’altres compostos químics. El biometà generat a partir de biomassa o altres fonts renovables és una manera de produir hidrogen i gas de síntesi de manera sostenible. Una manera de millorar el rendiment d'un catalitzador heterogeni és preparar catalitzadors bimetàl·lics aprofitant la sinèrgia entre dos metalls diferents. La preparació del catalitzador és un pas fonamental per tal de controlar les propietats catalítiques del material i incrementar la sostenibilitat de tot el procés. En aquesta tesis s'han estudiat dos sistemes bimetàl·lics de Ni-Fe i Pt-Ni en la reformació de metà en presència d'aigua. S'ha estudiat, a més de la síntesis convencional d'impregnació, que és el mètode utilitzat a nivell industrial per a la preparació de catalitzadors, la utilització de la mecanoquímica com una alternativa més propera a la química verda. En el Capítol 4 s'han estudiat catalitzadors bimetàl·lics Ni-Fe sobre diferents suports comparant els dos mètodes de síntesi, l'efecte del suport i la quantitat de Fe. El ferro és un promotor redox que pot millorar la resistència als dipòsits de carboni dels catalitzadors de níquel i també pot proporcionar certa resistència respecte el sofre. S'ha estudiat l'estructura del catalitzador en condicions de reacció mitjançant tècniques in situ dutes a terme amb llum sincrotró, com ara XRD i NAP-XPS. En condicions de MSR, s'ha observat la reordenació de les particules N, Postprint (published version)

Ni-Fe nanocatalysts supported on CeO2 have been prepared for the catalysis of methane steam reforming (MSR) aiming for coke-resistant noble metal-free catalysts. The catalysts have been synthesized by traditional incipient wetness impregnation as well as dry ball milling, a green and more sustainable preparation method. The impact of the synthesis method on the catalytic performance and the catalysts’ nanostructure has been investigated. The influence of Fe addition has been addressed as well. The reducibility and the electronic and crystalline structure of Ni and Ni-Fe mono- and bimetallic catalysts have been characterized by temperature programmed reduction (H2-TPR), in situ synchrotron X-ray diffraction (SXRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Their catalytic activity was tested between 700 and 950 °C at 108 L gcat-1 h-1 and with the reactant flow varying between 54 and 415 L gcat-1 h-1 at 700 °C. Hydrogen production rates of 67 mol gmet-1 h-1 have been achieved. The performance of the ball-milled Fe0.1Ni0.9/CeO2 catalyst was similar to that of Ni/CeO2 at high temperatures, but Raman spectroscopy revealed a higher amount of highly defective carbon on the surface of Ni-Fe nanocatalysts. The reorganization of the surface under MSR of the ball-milled NiFe/CeO2 has been monitored by in situ near-ambient pressure XPS experiments, where a strong reorganization of the Ni-Fe nanoparticles with segregation of Fe toward the surface has been observed. Despite the catalytic activity being lower in the low-temperature regime, Fe addition for the milled nanocatalyst increased the coke resistance and could be an efficient alternative to industrial Ni/Al2O3 catalysts., Peer Reviewed, Postprint (published version)

Mechanochemical synthesis for the preparation of bimetallic catalysts is gaining increasing interest, and in recent years, some important milestones have been reached. However, the complexity of mechanochemically prepared bimetallic supported catalysts still leaves many open questions that need a systematic approach to be solved. In this work, we summarize our experience of mechanochemically milling bimetallic catalysts, introducing some key parameters that should be taken into account, particularly the thermal stability and hydrophilicity of precursor salts, and the effect of the milling order, highlighting the differences with wet synthesis methods. Finally, we will provide some suggestions on the application of the design of experiments approach to the rationalization of the milling procedure for the preparation of supported bimetallic catalysts., Peer Reviewed, Postprint (published version)