Study of transition metal carbide catalysts of group 5 in the RWGS reaction

Abstract

[eng] Nowadays there is a growing interest in the capture and utilization of CO2 because of the increase of its concentration in the atmosphere. Captured CO2 can be recycled through different processes to obtain several value-added products, such as fuels. Current processes for the production of synthetic fuel and of some other products use syngas, a mixture of CO and H2 of varying composition, as feedstock. Syngas can be obtained by the catalytic reduction of CO2 to CO using an excess of H2 through the reverse water gas shift (RWGS) reaction. The RWGS reaction is endothermic and to reach appropriate conversion values, needs high temperature and the presence of a catalyst. Suitable catalysts should present not only high activity and selectivity, but also a good stability and low cost. In this context, transition metal carbides (TMCs) have been proposed as a good alternative to conventional catalysts to be used in the RWGS reaction. Theoretical and experimental investigations have shown that molybdenum carbide presents good catalytic behavior in the RWGS reaction. However, very few investigations about the use of TMCs of group 5 as catalysts in this reaction have been reported. The main objective of this work is the study of the CO2 reduction to CO under RWGS conditions over catalysts based on transition metal carbides of group 5 (G5TMC=VC, NbC and TaC). Additionally, some of these materials have been tested as catalysts in the methanol steam reforming (MSR). The catalysts have been characterized before and after reaction by means of different techniques, such as, X-ray diffraction, N2 adsorption-desorption isotherms, Raman spectroscopy, X-ray photoelectron spectroscopy, H2-temperature programmed reduction, scanning and transmission electron microscopy, and precession electron diffraction. In addition, studies of CO2 adsorption and reactivity have been carried out using thermogravimetry, calorimetry and in-situ diffuse reflectance infrared spectroscopy. The preparation of bulk and, ɣ-Al2O3-, TiO2-, SBA-15-, SiO2-, CeO2-, ZrO2- and activated carbon-supported catalysts, based on G5TMC has been performed avoiding the use of carburizing methods. A sol-gel method using 4,5- dicyanoimidazole as carbon source, metal chloride and alkoxide as metal precursors, and different thermal treatments has been developed. The use of metal alkoxides resulted in the preparation of bulk G5TMCs with smaller crystallite size compared to that obtained when metal chloride was used. Moreover, for metal alkoxide precursors, a decrease of the crystallite size of G5TMC was observed when the 4,5-dicyanoimidazole/metal precursor molar ratio was increased. An increase of the temperature of treatment led to a higher crystallite size of the G5TMCs. For supported vanadium carbide catalysts, VCx with different crystallite sizes were obtained as a function of the support and/or method of preparation used. The catalysts were tested under RWGS conditions, CO2/H2=1/3 in the 573- 873 K range. The catalysts based on vanadium carbide were active and highly CO selective, reaching values of CO selectivity near to 100% above 773 K. The samples based on niobium carbide and tantalum carbide were inactive under the experimental conditions used. For bulk vanadium carbide catalysts, the coexistence of two different phases, VC (stoichiometric), and V8C7 (C deficient) was determined. The presence of a higher amount of V8C7 phase was related with a smaller VCx crystallite size and a higher CO2 adsorption energy. The catalyst, which presented a higher presence of V8C7, showed a better catalytic behavior in all the temperature range studied. The experimental results obtained in this work have been interpreted in the light of theoretical studies performed in the group of Prof. Illas (University of Barcelona) in a frame of a collaborative research. The dissociative adsorption of CO2 to CO+O over G5TMC is proposed; a subsequent surface oxy-carbide species formation and CO release would take place. The presence of C vacancies in vanadium carbide catalysts allows the reactants to absorb more strongly, lowering the energy barrier for both H2 and CO2 dissociation steps. VC- based catalysts with smaller VCx crystallite size, showed lower activation energy in the RWGS reaction under the conditions used. Supported vanadium carbide catalysts produced a higher amount of CO per mol of V than the corresponding bulk catalyst. VC/Al2O3, VC/SiO2 and VC/AC, which had the smallest crystallite sizes of VCx phase showed the best catalytic behavior in the RWGS reaction. Some catalysts have been tested in the RWGS during 4 days, showing a high stability under the reaction conditions used. Finally, several bulk and supported G5TMC catalysts were tested under MSR conditions (CH3OH/H2O=1/1, 573-723 K). CH4 was the main product found when vanadium carbide catalysts were used; this is related with the methanol decomposition. On the other hand, for niobium carbide and tantalum carbide catalysts, HCHO was the main product, which could be formed via methanol dehydrogenation. After MSR catalytic tests, the characterization of used catalysts revealed a high SBET reduction and the presence of abundant carbon deposits, which is associated with the high deactivation of the catalysts observed under the reaction conditions used.

[spa] El principal objetivo de este trabajo es el estudio de catalizadores basados en carburos de metales de transición del grupo 5 (CMTG5=VC, NbC and TaC) en la reducción selectiva de CO2 a CO bajo condiciones de reacción inversa de desplazamiento de agua (RWGS por sus siglas en inglés). Adicionalmente, algunos de estos materiales se han probado como catalizadores en el reformado de metanol con vapor (MSR por sus siglas en inglés). Los catalizadores se han caracterizado adecuadamente antes y después de su uso en reacción mediante diversas técnicas físico-químicas. Asimismo, se han llevado a cabo estudios de adsorción y reactividad de CO2 mediante termogravimetría, calorimetría y espectroscopia infrarroja en modo reflectancia difusa in situ. Se han ensayado los diferentes materiales bajo condiciones de RWGS, CO2/H2=1/3 en el rango de 573-873 K. Los catalizadores basados en carburo de vanadio fueron activos y altamente selectivos a CO alcanzando valores cercanos al 100% a partir de 773 K. Los materiales basados en carburo de niobio y carburo de tántalo fueron inactivos bajo las condiciones experimentales usadas. En los catalizadores de carburo de vanadio másico se pudo determinar la coexistencia de dos fases, una estequiométrica VC, y otra deficiente en C, V8C7. La presencia de una mayor cantidad de la fase V8C7 se relaciona con el menor tamaño de cristalito y se refleja en una mayor energía de adsorción de CO2. El catalizador que presentó mayor presencia de la fase V8C7 mostró el mejor comportamiento catalítico en todo el rango de temperatura estudiado. Todos los catalizadores soportados de carburo de vanadio produjeron una mayor cantidad de CO por mol de V que el catalizador másico correspondiente. Los catalizadores soportados, VC/Al2O3, VC/SiO2 y VC/AC, que tuvieron los menores tamaños de cristalito de la fase VCx mostraron el mejor comportamiento catalítico. Algunos catalizadores se ensayaron en la RWGS durante 4 días, mostrando una elevada estabilidad bajo las condiciones de reacción utilizadas. Finalmente, diversos catalizadores másicos y soportados de CMTG5 se ensayaron bajo condiciones de MSR, CH3OH/H2O=1/1 en el rango de 573-723 K. Con los catalizadores de carburo de vanadio se obtuvo como producto mayoritario CH4. Por otra parte, con los catalizadores de carburo de niobio y carburo de tántalo, se obtuvo mayoritariamente HCHO. La caracterización de los catalizadores usados en el MSR reveló una gran reducción del área superficial SBET y la presencia de abundantes depósitos carbonosos, lo que se asocia con la elevada desactivación observada bajo las condiciones de reacción utilizadas.