Mechanistic Study of the Selective Methanation of CO over Ru/TiO2 Catalysts: Effect of Metal Crystallite Size on the Nature of Active Surface Species and Reaction Pathways

The effect of metal crystallite size on the reaction mechanism of competitive methanation of CO and CO2 is investigated over Ru/TiO2 catalysts of variable metal content (0.5 and 5.0 wt %) and crystallite size (2.1 and 4.5 nm) employing in situ FTIR and transient mass spectrometry techniques. Results show that although both catalysts follow the same mechanistic pathways, the relative population of reaction intermediates differs. Specifically, the nature of Ru-bonded carbonyl species detected on the catalyst surface upon interaction with the CO/CO2-containing gas mixtures is the same for both catalysts. However, their relative population depends significantly on ruthenium crystallite size and determines catalytic activity. The population of multicarbonyl species adsorbed on partially oxidized Ru sites (Run+-(CO)x) decreases significantly with increasing Ru crystallite size, whereas the opposite is observed for CO species linearly bonded on reduced Ru crystallites (Rux-CO). This is correlated with the higher catalytic activity found for the 5%Ru/TiO2 catalyst, since evidence has been provided that surface species adsorbed on reduced ruthenium sites are direct precursors of methane.