This study concerned the preparation, characterisation and evaluation of different proton conductors for the Reverse Water Gas Shift Reaction (RWGS) and their evaluation as electrolytes for Solid Oxide Fuel Cells (SOFC) under H₂ and O₂. Materials with both catalytic and conductive properties are of a great interest, as their use in electrocatalytical systems may be very important. Sr₃CaZr₀.₅Ta₁.₅O₈.₇₅ (SCZT), BaCe₀.₉Y₀.₁O₂.₉₅ (BCY10) and Ba₃Ca₁.₁₈Nb₁.₈₂O₈.₇₃ (BCN18), were the initial materials in this study. Thermogravimetric analysis under different atmospheres (5%H₂/Ar, Ar, 5%CO₂, etc), were performed on SCZT and BCN18; with both materials being shown to be stable under reducing and oxidising conditions. Conductivity measurements by two terminal a.c. impedance were also conducted on SCZT and BCN18 under oxidising and reducing atmospheres. As found in literature, BCN18 showed mixed conductivity; with electronic conductivity at high temperatures and pure ionic conductivity below 550ºC, This behaviour was shown in chapter 3, where the change on conduction process was observed upon different gas feeds. Its maximum conductivity values for the different atmospheres were: 8.50x10⁻⁵ S/cm (Dry 5%H₂/Ar at 200ºC), 4.24x10⁻⁴ S/cm (Wet 5%H₂/Ar at 500ºC) and 4.48x10⁻³ S/cm (Air at 900ºC). SCZT proton conducting behaviour was also measured (wet and dry 5%H₂/Ar). Exhibiting an order of magnitude higher in total conductivity upon hydration of the gas feed (σdry=1.01x10⁻⁶ and σwet=1.07x10⁻⁵ at 450ºC). The doping of barium cerate with Zr and Zn by Tao and Irvine, lead to a more stable and easily sinterable BaCe₀.₅Zr₀.₃Y₀.₁₆Zn₀.₀₄O₃ (BCZYZ). Following this work, the introduction of ZnO as a sintering aid to SCZT and BCN18 resulted in Sr₃CaZr₀.₄Ta₁.₅Zn₀.₁O₈.₇₅ (SCZTZ), and Ba₃(Ca₁.₁₈Nb₁.₇₀Zn₀.₁₂)O₈.₅₅ (BCNZ); with higher final densities (above 90% dense). As with BCN18, BCNZ also exhibited mixed conductivity; with higher total conductivity values than BCN18. A maximum of total conductivity of 1.85x10⁻³ S/cm at 900ºC for BCNZ was measured against 6.99x10⁻⁴ S/cm at 900ºC for BCN18. A change in conductivity process was observed when using air or wet 5%H₂/Ar, achieving a maximum of 3.85x10⁻⁴ S/cm at 400ºC when under wet hydrogen. All materials (as powders) have shown catalytic activity for the reverse water gas shift (RWGS) reaction, with the lowest conversion temperature onset at 400ºC for SCZT and a maximum conversion of CO₂ to CO of 42%, with circa 0.52 and 0.59 mmol/s.m² of CO produced at 900ºC by BCN18 and BCZYZ, respectively. No relation between mechanisms for the RWGS and σ for these materials were expected below 10% conversion, as no correlation was found between their activation energies. BCY10 as shown a partial decomposition when exposed to the RWGS reaction, for what BCZYZ After fuel cell testing under H₂ and O₂ both SCZTZ and BCNZ showed mixed conductivity. SCZTZ under different hydrogen partial pressures, exhibited a behaviour close to a pure proton conductor, however, when exposed to both reducing and oxidising conditions, its behaviour did not follow the theoretical values. On the other hand, BCNZ behaves as a pure ionic conductor below 500ºC; with increasing influence of the electronic conductivity on temperature increase. However, as seen for BCNZ conductivity data from 2 terminal a.c. impedance, below 650ºC wet 5%H₂ exhibited the highest conductivity values. This, in additions to the pure ionic conductive behaviour below 400ºC (from the effective ionic transport number), suggests that BCNZ becomes closer to a pure proton conductor with temperature decrease.

Ce travail s’inscrit dans le contexte d’un fort accroissement des émissions de gaz à effetde serre au niveau mondial. Une idée est de réutiliser ce CO2 comme matrice de stockageénergétique pour fabriquer un carburant de synthèse en le combinant avec de l’hydrogèneproduit à partir de décomposition de l’eau par apport d’énergie nucléaire ou renouvelable,évitant ainsi le recours au pétrole ou au charbon. Cette idée prend tout son sens dans lecontexte spécifique français où l’électricité, majoritairement produite par énergie nucléaire etrenouvelable a une faible empreinte carbone. Dans ce cadre nous nous proposonsd’hydrogéner le CO2 en gaz de synthèse par la réaction Reverse Water-Gas-Shift (RWGS),lequel gaz de synthèse est alors transformé en carburant. Ce projet de recherche est composéde deux parties principales :La première partie se focalise sur le développement d’un catalyseur sélectif et stable pourla réaction de RWGS à température modérée (723-773 K). A cet égard nous avons procédé àune modélisation conjointe de la micro-cinétique de la réaction de RWGS et des principalesréactions parasites pour déterminer un composé multi-métallique innovant ; celui-ci a pu êtreconfronté avec succès aux catalyseurs industriels utilisés, dans les conditions optimales de laréaction de RWGS. Dans une deuxième partie, nous avons effectué un remontagethermodynamique de l’ensemble d’une conversion du CO2 issu de fumées industrielles encarburant de synthèse (dimethyl ether, DME) sur un cas concret à grande échelle en France.La simulation du procédé CO2 to DME montre une efficacité énergétique du procédé de 52%et une réduction des émissions du CO2 de la cimenterie de 88%. / This work is in the context of large-scale efforts to enhance greenhouse gas emissionsmitigation. A potential way to recycle CO2 as a carbon feedstock to produce a synthetic fuelby the conversion of CO2 and hydrogen, produced from water electrolysis using nuclear orrenewable energy. This process may be sustainable in some specific context like in Frenchcontext; French electricity is mainly generated by nuclear and renewable energies that havelow carbon footprints. In this work, a synthetic fuel is produced by CO2 hydrogenation intosynthesis gas via the Reverse Water-Gas Shift (RWGS) reaction, then synthesis gas isconverted into a synthetic fuel. This research project consists of two main parts:The first part focuses on the development of a selective and stable catalyst for the RWGSreaction at moderate temperature (723-773 K). We have applied the micro-kinetic approach ofthe RWGS reaction and its side reactions in order to determine a multi-metallic catalyst,which has shown to perform better selectivity and stability than a conventional, commercialcatalyst under the optimal operating conditions of the RWGS reaction. In the second part, weconducted the simulations of a large-scale dimethyl ether (DME) production process by theconversion of CO2 from industrial flue gases in the French context. The simulation of the CO2to DME process showed the process energy efficiency of 52% and the emissions reductionpotential of 88% of total CO2 emissions.

CO2 global emissions exceed 30 Giga tonnes (Gt) per year, and the high atmospheric concentrations are detrimental to the environment. In spite of efforts to decrease emissions by sequestration (carbon capture and storage) and repurposing (use in fine chemicals synthesis and oil extraction), more than 98% of CO2 generated is released to the atmosphere. With emissions expected to increase, transforming CO2 to chemicals of high demand could be an alternative to decrease its atmospheric concentration. Transportation fuels represent 26% of the global energy consumption, making it an ideal end product that could match the scale of CO2 generation. The long-term goal of the study is to transform CO2 to liquid fuels closing a synthetic carbon cycle. Synthetic fuels, such as diesel and gasoline, can be produced from syngas (a combination of CO and H2) by Fischer Tropsch synthesis or methanol synthesis, respectively. Methanol can be turned into gasoline by MTO technologies. Technologies to make renewable hydrogen are already in existence, but CO is almost exclusively generated from methane. Due to the high stability of the CO2 molecule, its transformation is very energy intensive. Therefore, the current challenge is developing technologies for the conversion of CO2 to CO with a low energy requirement. The work in this dissertation describes the development of a recyclable, isothermal, low-temperature process for the conversion of CO2 to CO with high selectivity, called Reverse Water Gas Shift Chemical Looping (RWGS-CL). In this process, H2 is used to generate oxygen vacancies in a metal oxide bed. These vacancies then can be re-filled by one O atom from CO2, producing CO. Perovskites (ABO3) were used as the oxide material due to their high oxygen mobility and stability. They were synthesized by the Pechini sol-gel synthesis, and characterized with X-ray diffraction and surface area measurements. Mass spectrometry was used to evaluate the reducibility and re-oxidation abilities of the materials with temperature-programmed reduction and oxidation experiments. Cycles of RWGS-CL were performed in a packed bed reactor to study CO production rates. Different metal compositions on the A and B site of the oxide were tested. In all the studies, La and Sr were used on the A site because their combination is known to enhance oxygen vacancies formation and CO2 adsorption on the perovskites. The RWGS-CL was first demonstrated in a non-isothermal process at 500 °C for the H2-reduction and 850 °C for the CO2 conversion on a Co-based perovskite. This perovskite was too unstable for the H2 treatment. Addition of Fe to the perovskite enhanced its stability, and allowed for an isothermal and recyclable process at 550 °C with high selectivity towards CO. In an effort to decrease the operating temperature, Cu was incorporated to the structure. It was found that Cu addition inhibited CO formation and formed very unstable oxide materials. Preliminary studies show that application of this technology has the potential to significantly reduce CO2 emissions from captured flue gases (i.e. from power plants) or from concentrated CO2 (adsorbed from the atmosphere), while generating a high value chemical. This technology also has possible applications in space explorations, especially in environments like Mars atmosphere, which has high concentrations of atmospheric carbon dioxide.

[ES] Teniendo en cuenta el agotamiento de los combustibles fósiles y la creciente concentración de CO2 en la atmósfera, la hidrogenación de CO2 es una forma prometedora de convertir el CO2 en productos químicos y combustibles de carbono de alto valor añadido. Considerando la gran influencia del tamaño de partícula, la composición química, la naturaleza del soporte y las condiciones de operación sobre el comportamiento catalítico de los catalizadores, se han desarrollado una serie de catalizadores para la hidrogenación de CO2 basados en metales abundantes no nobles y polisacáridos naturales como precursores del grafeno. En la presente tesis doctoral, las especies metálicas soportadas sobre una matriz de carbono grafítico defectuosa, con diferentes tamaños de partículas, muestran diferente actividad catalítica y selectividad para la hidrogenación de CO2. Se prepararon, de forma controlada, nanopartículas de aleaciones de Co y Co-Fe soportadas en grafenos dopados con N defectuosos, con una amplia distribución de tamaño de nanopartículas, para la reacción de Sabatier, presentando una selectividad a metano superior al 90% con valores de conversión de CO2 superiores al 85%. En el caso de un solo metal, Co o Fe, y sus aleaciones en forma de "clusters" y pequeñas nanopartículas soportadas en el mismo material, la selectividad de la hidrogenación de CO2 cambia a CO, en lugar de metano, obteniéndose un valor del 98 % y alcanzando una conversión de CO2 del 56%. Conviene resaltar que, los catalizadores basados en "clusters" de aleaciones de metal con una carga de metal incluso por debajo del 0.2 % en peso, exhiben una mayor selectividad y rendimiento que los que tienen nanopartículas de aleaciones de Co-Fe más grandes que varían de 1 a 4 nm y una carga de metal más alta en una composición similar. Siguiendo la línea de investigación de hidrogenación de CO2, se desarrollaron una serie de nanopartículas de aleaciones de Co-Fe soportadas sobre grafenos dopados con N defectuosos con distribución de tamaño de nanopartículas controlada en el rango de 7-17 nm, obteniendo una selectividad hacia hidrocarburos C2+ alrededor del 45% y una conversión del CO2 cercana al 60%. Además, se realizó un estudio comparativo de la actividad catalítica de catalizadores similares basados en Co-Fe con promotores e inhibidores para la hidrogenación de CO2, observando su influencia en la conversión y selectividad de CO2. Finalmente, además de los catalizadores basados en Co-Fe, también se han preparado catalizadores basados en Cu-ZnO mediante un método de dos pasos. Estas nanopartículas de Cu-ZnO soportadas sobre grafeno defectuoso dopado con N exhiben una alta selectividad hacia la conversión de CO2 a metanol. / [CA] Tenint en compte l'esgotament dels combustibles fòssils i la creixent concentració de CO2 en l'atmosfera, la hidrogenació de CO2 és una forma prometedora de convertir el CO2 en productes químics i combustibles de carboni d'alt valor afegit. Considerant la gran influència de la grandària de partícula, la composició química, la naturalesa del suport i les condicions d'operació sobre el comportament catalític dels catalitzadors, s'han desenvolupat una sèrie de catalitzadors per a la hidrogenació de CO2 basats en metalls abundants no nobles i polisacàrids naturals com a precursors del grafé. En la present tesi doctoral, les espècies metàl·liques suportades sobre una matriu de carboni grafític defectuosa, amb diferents grandàries de partícules, mostren diferent activitat catalítica i selectivitat per a la hidrogenació de CO2. Es van preparar, de manera controlada, nanopartícules d'aliatges de Co i Co-Fe suportades en grafens dopats amb N defectuosos, amb una àmplia distribució de grandària de nanopartícules, per a la reacció de Sabatier, presentant una selectivitat a metà superior al 90% amb valors de conversió de CO2 superiors al 85%. En el cas d'un sol metall, Co o Fe, i els seus aliatges en forma de "clústers" i xicotetes nanopartícules suportades en el mateix material, la selectivitat de la hidrogenació de CO2 canvia a CO, en lloc de metà, obtenint-se un valor del 98% i aconseguint una conversió de CO2 del 56%. Convé ressaltar que, els catalitzadors basats en "clústers" d'aliatges de metall amb una càrrega de metall fins i tot per davall del 0.2% en pes, exhibeixen una major selectivitat i rendiment que els que tenen nanopartícules d'aliatges de Co-Fe més grans que varien d'1 a 4 nm i una càrrega de metall més alta en una composició similar. Seguint la línia d'investigació d'hidrogenació de CO2, es van desenvolupar una sèrie de nanopartícules d'aliatges de Co-Fe suportades sobre grafens dopats amb N defectuosos amb distribució de grandària de nanopartícules controlada en el rang de 7-17 nm, obtenint una selectivitat cap a hidrocarburs C2+ al voltant del 45% i una conversió del CO2 pròxima al 60%. A més, es va realitzar un estudi comparatiu de l'activitat catalítica de catalitzadors similars basats en Co-Fe amb promotors i inhibidors per a la hidrogenació de CO2, observant la seua influència en la conversió i selectivitat de CO2. Finalment, a més dels catalitzadors basats en Co-Fe, també s'han preparat catalitzadors basats en Cu-ZnO mitjançant un mètode de dos passos. Aquestes nanopartícules de Cu-ZnO suportades sobre grafé defectuós dopat amb N exhibeixen una alta selectivitat cap a la conversió de CO2 a metanol. / [EN] Considering the depletion of fossil fuels and the increasing atmospheric CO2 concentration, CO2 hydrogenation is a promising way to convert CO2 into value-added carbon-containing chemicals and fuels. Taking into account the significant influences of the particle size, chemical composition, nature of the support, and operation conditions on the catalytic performance of catalysts, a series of catalysts for CO2 hydrogenation have been developed based on the use of abundant non-noble metals and natural polysaccharides as graphene precursors. In the present PhD Thesis, metal species supported on defective graphitic carbon matrix with different particle sizes show different catalytic activity and selectivity for CO2 hydrogenation. Under effective control, Co and Co-Fe alloy nanoparticles wrapped on defective N-doped graphenes with a broad nanoparticle size distribution were prepared and performed for the Sabatier reaction, exhibiting a selectivity to methane over 90 % at CO2 conversion values over 85 %. In the case of single Co or Fe metal and their alloys in the form of clusters and small nanoparticles wrapped on the same support, the selectivity for CO2 hydrogenation shifts to CO, rather than methane, reaching a conversion of 56 % with 98 % CO selectivity. It is worth noting that the metal alloy clusters-based catalysts with the metal loading even below 0.2 wt.% exhibit a higher selectivity and better performance than the ones with larger Co-Fe alloy nanoparticles ranging from 1-4 nm and higher metal loading in a similar composition. Following the research line for CO2 hydrogenation, a series of Co-Fe alloy nanoparticles supported on defective N-doped graphenes with controlled nanoparticle size distribution in the range of 7-17 nm are developed, obtaining a selectivity towards C2+ hydrocarbons about 45% with a CO2 conversion close to 60%. In addition, a comparative catalytic activity of similar Co-Fe-based catalysts with promoters and poison has been studied for CO2 hydrogenation to observe their influence on CO2 conversion and selectivity. Finally, besides Co-Fe-based catalysts, Cu-ZnO-based catalysts have also been prepared by a two-step method. These Cu-ZnO nanoparticles supported on N-doped defective graphene exhibit a high selectivity for CO2 conversion to methanol. / Peng, L. (2021). Metal Nanoparticles Wrapped on Defective Nitrogen-doped Graphitic Carbons as Highly Selective Catalysts for C02 Hydrogenation [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/172329 / TESIS