CO methanation on highly active Ru/TiO₂ catalysts at low CO: H₂ ratios: Mechanism and rate-determining step derived from transient measurements

We have systematically investigated mechanistic details of the CO methanation reaction over a supported Ru catalyst in an idealized H-2-rich reformate gas mixture with low CO:H-2 ratio, employing isotope labelling techniques. From a comprehensive set of transient FTIR spectroscopy measurements, following the buildup / disappearance of different adsorbed species upon exchange of isotope marked reactants during reaction or during desorption, we derive that under these conditions the reaction is dominated by an associative reaction pathway, involving first the formation of adsorbed formyl intermediates, followed by hydrogenation of these intermediates and C-O bond breaking. Formate formation can be excluded. Measurements of kinetic isotope effects (KIEs) revealed a weak secondary inverse KIE with values of r(CH4)/r(CD4) between 0.6 (175 degrees C) and 1.0 (230 degrees C), indicating that at lower temperatures C-H bond formation, most likely HCOad hydrogenation, is rate-determining, while at higher temperatures the reaction is increasingly controlled by C-O bond breaking.