Effect of H 2 partial pressure on surface reaction parameters during CO hydrogenation on Ru-promoted silica-supported Co catalysts

Abstract

Steady-state isotopic transient kinetic analysis (SSITKA), one of the most powerful techniques for the investigation of surface reactions, was used to study the effect of hydrogen partial pressure on the fundamental surface reaction parameters for methanation on ordered mesoporous silica (MCM-41) and amorphous SiO 2-supported CoRu catalysts. The abundances, coverages, and lifetimes of surface intermediates of the reaction were measured under reaction conditions and their dependence upon hydrogen partial pressure was determined. Although absolute hydrogen coverage under reaction conditions is not measurable due to the hydrogen isotope effect, relative hydrogen surface concentration as a function of P H 2 could be estimated from SSITKA parameters. Increasing the hydrogen partial pressure at a constant reaction temperature of 220 °C not only caused the expected increase in the relative surface concentration of hydrogen but also increased the abundance of surface methane intermediates ( N M), possibly due to increased hydrogenation. The impact of P H 2 on N M for MCM-41-supported CoRu catalysts was similar to that for SiO 2-supported ones, showing an approximately twofold increase in N M as P H 2 increased from 0.23 to 1.71 bar. The relative concentration of surface hydrogen, however, increased fourfold. The abundance of surface methane intermediates and the surface coverages were significantly higher for the MCM-41-supported CoRu catalysts. The average surface reaction residence time of the methane intermediates ( τ M) consistently decreased with increasing hydrogen partial pressure due to the fact that the pseudo first order rate constant (1/ τ M) contains the hydrogen surface concentration term. There was no difference, however, in the intrinsic site activity since the average surface reaction residence times of methane intermediates ( τ M) for SiO 2- and MCM-41-supported CoRu catalysts were essentially identical for a given partial pressure of hydrogen, regardless of Co loading. This also indicates that the type of silica support used (amorphous SiO 2 or MCM-41) did not have an impact on surface hydrogen concentration, contrary to the case for H 2 chemisorption at 100 °C. The increase in rate with increasing hydrogen partial pressure resulted due to the increase in methane surface intermediates and, more importantly, the increase in hydrogen surface concentration.