Elementary steps involved in the hydrogenation of the linear CO species adsorbed on a Ru/Al 2O 3 catalyst

Abstract

The adsorption of CO on a reduced 3.5% Ru/Al 2O 3 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 CH 4 in the temperature range 443–478 K in a flow of pure hydrogen. The rate of appearance of CH 4 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 CH 4 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.