Elementary steps involved in the hydrogenation of the linear CO species adsorbed on a Ru/Al2O3 catalyst

The adsorption of CO on a reduced 3.5% Ru/Al2O3 catalyst in the temperature range (300-478 K) leads mainly to the formation of a linear CO species (denoted by L) adsorbed on Ru sites characterized by an IR band at 2047 cm(-1) at 300 K. The Linear CO species is hydrogenated to CH4 in the temperature range 443-478 K in a flow of pure hydrogen. The rate of appearance of CH4 as a function of the time in hydrogen follows a decreasing exponential profile which indicates that a limiting elementary step at the surface controls the overall process. It is shown that this step is the hydrogen-assisted dissociation of CO, while the other elementary steps leading to CH4 are fast. In the course of the hydrogenation reaction, the hydrogen surface concentration does not change significantly with the removal of the L species (no competition between the L species and hydrogen). This is due to the fact that in the presence of adsorbed CO on the surface, hydrogen is adsorbed in an amount similar to that determined on a surface without any adsorbed CO but the sites involved are different. (C) 2000 Elsevier Science B.V. All rights reserved.