Mechanistic aspects of the selective methanation of CO over Ru/TiO2 catalyst

The mechanism of selective methanation of CO/CO2 over 5%Ru/TiO2 catalyst is investigated employing in situ FTIR spectroscopy (DRIFTS) and transient mass spectrometry techniques. It is shown that interaction of the prereduced catalyst with the reaction mixtures results in the development of various Ru-bonded carbonyl species on reduced and partially oxidized sites as well as on sites located at the metal-support interface. The nature and population of these species depend strongly on feed composition and reaction temperature. Results of the present study confirm our previous suggestion that the mechanism of CO methanation includes both dissociative and associative reaction pathways. The former, which dominates at lower reaction temperatures, involves hydrogenation of surface carbon produced by dissociative adsorption of CO, whereas the latter involves hydrogenation of CO species adsorbed at the metal-support interface. At low temperatures, typically <250 °C, dissociation of CO results in accumulation of adsorbed oxygen species which cannot be removed from the catalyst surface, rendering it inactive. Catalytic activity is restored at higher temperatures, where partially oxidized sites are reduced efficiently by adsorbed hydrogen atoms. The associative reaction pathway is the only one which is operable under conditions of CO2 methanation and proceeds via intermediate formation of carbonyl species at the metal-support interface, produced by the RWGS reaction. Selective methanation of CO in CO/CO2 mixtures occurs efficiently under conditions where the dissociative reaction pathway is operable and the associative reaction pathway is completely suppressed.