Catalytic properties of transition metal carbides for CO2 reduction and hydrogen evolution reaction.

Abstract

Renewable energy conversion and storage plays an important role in global efforts to mitigate the effects of anthropogenic climate change. In particular, the catalytic reduction of carbon dioxide to fuels and other important chemicals is a very active area of modern chemistry, together with the search for energetically efficient routes for hydrogen production. All these catalytic processes occur mainly on the surface of the catalysts and, therefore, a deep and detailed understanding of the structure and molecular processes that take place on the surface constitutes an essential step for the efficient design and development of new catalytically active materials for these purposes. Therefore, in this thesis, the catalytic properties towards the reduction of carbon dioxide (CO2RR) and the hydrogen evolution reaction (HER) of tungsten and molybdenum carbides were studied. Today, the catalytic properties of this family of materials is a very promising research topic. They are of an extreme interest for development of the new economic and highly catalytic materials towards these reactions, in order to replace the traditional catalysts based on noble metals, in an effort to make economically viable many different technologies available for energy production and storage. Two different approaches were used in this thesis. Theoretical methods based on the functional density theory (DFT) were used to estimate adsorption and activation energy of different initial and intermediate species during HER and CO2RR, on hexagonal and cubic phases of tungsten carbide. The catalytic properties of these materials towards these reactions has been mainly studied experimentally, and detailed theoretical studies would allow a better understanding of the factors that control their catalytic activity. In contrast, experimental methods were used to study the catalytic activity of molybdenum carbide, mainly for two reasons. On the one hand, some works have reported a greater catalytic activity towards the HER for some phases of molybdenum carbides, compared to that of tungsten carbides. On the other hand, at the moment a significant number of theoretical studies on the catalytic activity of these materials for HER and CO2 reduction are available already. Initially, the interaction of the tungsten carbide model surfaces with CO, CO2, H and H2 was examined by DFT and the obtained results were analyzed, considering different chemical nature of the surface. Thus, surfaces of the hexagonal phase with exposed carbon atoms and tungsten atoms, or mixed atom cubic phase surfaces were studied. Subsequently, the study was extended to theoretical estimation of the catalytic activity towards the reduction of CO2 and the HER of systems composed of copper supported on hexagonal tungsten carbide. It is well known that the modification of the electronic and chemical properties of a given material can be achieved effectively by introducing another element into its network, either by the formation of heteroatomic bonds ("ligand effect") or by the change in the average length of the atom- V atom bond (“strain effect”). Therefore, an experimental investigation of the changes in activity for the hydrogen evolution reaction was carried out by doping molybdenum carbide with other transition metals in slightly acidic and alkaline media. The results show a strong adsorption of atomic hydrogen on the clean surface of the WC, while H2 is mainly adsorbed dissociatively. An important role of the C sites is revealed for atomic H adsorption and for the H-H bond cleavage. Similarly, the interaction of tungsten carbide with CO and CO2 demonstrates the importance of sites terminated in carbon atoms as one of the reasons for different reactivity of a C- and W-terminated (0001) hexagonal WC surfaces. The importance of exposed C sites can also be seen, analyzing the system in which copper atom is supported on tungsten carbide: the Cu particle supported on the surface of the WC with exposed carbon atoms yields charge, while on the metallic termination a reduction of copper has been observed, or, in other words, Cu atoms gained charge from the support. These differences in the charge of the adsorbed Cu on the carbide, result in the changes in the catalytic activity of the composite material. For example, CO adsorption on all Cu / WC surfaces is very strong for Cu coverage smaller than the monolayer, which could result in catalyst deactivation. However, this effect is smaller for systems in which a complete Cu monolayer is supported, and which do not suffer from the contribution of the strong CO adsorption on the support. In contrast, CO2 adsorption is weaker in systems with a copper monolayer supported on C-terminated WC (0001), compared to adsorption on Cu monolayer, supported on metal-terminated tungsten carbide, although the dissociation barriers are similar in case of the two terminations. Due to these differences in reactivity between systems with coverages smaller than monolayer and systems with monolayer, it is concluded that the CuML/C-WC surface would be a better catalyst for CO2 reduction, due to the lower degree of poisoning of the surface by the products of CO2 dissociation. This difference in reactivity could be used as an important criterion in the synthesis of new composite materials based on tungsten carbides. The results obtained in relation to the adsorption of atomic hydrogen, Hads, demonstrate a strong adsorption of atomic hydrogen on all Cu / WC surfaces, although the exact magnitude of this adsorption varies with the termination of the support. A detailed analysis of these data, within the conceptual framework currently accepted by the scientific community to describe the reactivity of the materials towards the HER, suggests that little catalytic activity is expected towards this reaction of the materials composed of copper monolayer and sub-monolayer, supported on carbon-terminated WC surface, while in the case of composite materials comprised of copper monolayer supported on the metal-terminated WC (0001) surface, the reactivity is likely to be much higher. Finally, the experimental study conducted on molybdenum carbide modified with other transition metals showed that pH has a significant influence on how the doping affects the reactivity VI of the final material. At pH = 5 doping has a slightly detrimental effect on the activity towards HER, while at pH = 9, a favorable effect was observed. On the other hand, a common result for all synthesized materials, in both alkaline and acidic media, was the noticeable drop in activity towards the HER of all materials after they were electrochemically oxidized using high positive potentials. This result is solid evidence that the measured catalytic activity must be attributed to the presence of molybdenum carbide in the synthetized compounds , and not to the existence of surface oxides of transition metals on the electrode. Finally, it is important to highlight that, in general, the results, reported in this thesis, show not only that transition metal carbides exhibit catalytic activity for essential reactions such as HER and carbon dioxide reduction, but also that their properties they can be modified, by introducing other transition metals in the carbide structure, for the development of materials, catalytically more active than the initial material. (Tomado de la fuente)

En esta tesis, las propiedades catalíticas hacia la reducción de dióxido de carbono y la reacción de evolución de hidrógeno (del inglés HER) de los carburos de tungsteno y molibdeno fueron estudiadas. Hoy en día, las propiedades catalíticas de esta familia de materiales es un destacado tópico de investigación, en la búsqueda de nuevos materiales económicos y altamente catalíticos hacia estas reacciones, con el objeto de sustituir los tradicionales catalizadores basados en metales nobles, en un afán de tornar económicamente viables muchas de las diferentes tecnologías disponibles para la producción y almacenamiento de energía. Dos aproximaciones diferentes fueron empleadas en esta tesis para cumplir con ese objetivo principal. Por un lado, métodos teóricos basados en el empleo de la teoría del funcional de la densidad (del inglés: Density functional theory DFT ) fueron empleados para estimar energía de adsorción y activación de diferentes especies participantes e intermedias durante las HER y reducción de dióxido de carbono, sobre diferentes fases de carburo de tungsteno. El comportamiento catalítico de estos materiales hacia esas reacciones ha sido principalmente estudiado experimentalmente, y estudios teóricos detallados permitirían una mejor comprensión de los factores controlantes de la actividad catalítica de estos materiales. Por el contrario, métodos experimentales fueron utilizados para el estudio de la actividad catalítica de los carburos de molibdeno. Se revela un papel importante de los sitios C para la adsorción de H atómico y para la ruptura del enlace H-H. De manera similar, la interacción del carburo de tungsteno con CO y CO2 evidencia la importancia de los sitios terminados en átomos de carbono en la diferente reactividad de las superficies (0001) de WC hexagonal, terminadas en C y W. La importancia de los sitios terminados en C también se ve el reflejada al analizar el sistema en el cual átomo de cobre son soportados sobre el carburo de tungsteno, para modificar su actividad y se llega a la conclusión de que la superficie Cu ML /C-WC sería un mejor catalizador para reducción de CO2 , debido al menor grado de envenenamiento de la superficie por los productos de disociación de CO 2 , como es el CO. Por otro lado, los resultados obtenidos con relación a la adsorción de hidrógeno atómico, Hads, demuestran una fuerte adsorción del H ads en todas las superficies de Cu/WC, aunque la magnitud exacta de esta adsorción varía con la terminación del soporte. Finalmente, el estudio experimental realizado con los carburos de molibdeno dopados con otros metales de transición mostró que el pH tiene una influencia significativa sobre el efecto del dopaje en la reactividad del material final. A pH = 5 el dopaje tiene un efecto ligeramente perjudicial sobre la actividad hacia la HER, mientras que a pH = 9, se observó un efecto favorable. (Tomado de la fuente)