A kinetic study on the adsorption and reaction of hydrogen over silica-supported ruthenium and silver-ruthenium catalysts during the hydrogenation ofcarbon monoxide

The simultaneous adsorption and reaction of hydrogen with preadsorbed carbo n monoxide was investigated over a series of silica-supported ruthenium and silver-ruthenium bimetallic catalysts to elucidate the role of site sensit ivity on hydrogen adsorption and methane formation. The specific rate of me thane synthesis, which is a direct measure of the catalyst activity toward the Fischer-Tropsch synthesis, measured at 460 Torr and temperatures from 4 00 to 500 K, varied from 0.01 x 10(-3) to 3 x 10(-3) s(-1). As little as 3% Ag reduced the methanation rate of Ru by 80%. The temperature dependence o f the turnover frequency showed that the apparent activation energy for met hanation dropped from 24 kcal mol(-1) for the monometallic ruthenium case t o 18 kcal mol(-1) for the bimetallic case. Since silver does not adsorb or react with either hydrogen or carbon monoxide, these results show that silv er does not act merely as a dilutant in the bimetallic system. These observ ations are elucidated in terms of a new model for surface-sensitive hydroge n adsorption, termed "portal site mediated adsorption," where low-coordinat ion edge and corner sites on the catalyst surface act as sinks for rapid, d issociative adsorption of weakly bound, highly mobile surface hydrogen. In situ H-1-NMR measurements of surface hydrogen coverages during reaction wer e used to confirm the kinetic model treatment of the reaction data. (C) 199 9 Academic Press.