Mechanistic Study of the Selective Methanation of CO over Ru/TiO2 Catalyst: Identification of Active Surface Species and Reaction Pathways

Selective methanation of CO can be used as the final purification step of re-formate gas for the generation of hydrogen-rich gas streams suitable for PEM fuel cell applications. In the present study, the adsorption/desorption characteristics of reactants, the nature of active sites, and the mechanism of CO/CO2 hydrogenation reactions have been investigated over 5% Ru/TiO2 catalyst with the use of in situ FTIR and temperature-programmed techniques. Results show that adsorption of hydrogen and CO are activated processes. The amount of adsorbed hydrogen increases by a factor of 2 with the increase of adsorption temperature from 25 to 200 degrees C, mainly due to migration of hydrogen atoms from the metal to the support. Adsorbed CO species evolve during TPD in the form of CO2, which is produced via the Boudouard and water-gas shift (WGS) reactions. Adsorption of CO2 occurs via formation of formate and carbonate species associated with the support whereas in the presence of H-2 part of these species are converted to Ru-bonded carbonyls via the RWGS reaction at the metal-support interface. Depending on the experimental conditions employed, interaction of CO-containing gas mixtures with the catalyst surface results in the development of a variety of bands in the v(CO) region attributed to mono- and multicarbonyl species adsorbed on reduced ruthenium crystallites (Ru-x sites), partially oxidized ruthenium (Run+ sites), and the metal-support interface. Evidence is provided that methanation of CO occurs via two distinct reaction pathways. The first one, which dominates at lower reaction temperatures, involves hydrogenation of surface carbon produced by dissociative adsorption of CO, whereas the second involves hydrogenation of Ru-x-CO species. The latter pathway is the only one that is operable under conditions of CO2 methanation and proceeds with intermediate formation of Ru-bonded carbonyls at the metal-support interface via the RWGS reaction. Selective methanation of CO in CO/CO2 mixture occurs under conditions where conversion of CO2 is almost completely suppressed because of the kinetically faster hydrogenation of surface carbon, which is produced only in the presence of gas-phase CO.