The simultaneous adsorption and reaction of hydrogen with preadsorbed carbon monoxide was investigated over a series of silica-supported ruthenium and silver-ruthenium bimetallic catalysts to elucidate the role of site sensitivity on hydrogen adsorption and methane formation. The specific rate of methane synthesis, which is a direct measure of the catalyst activity toward the Fischer-Tropsch synthesis, measured at 460 Torr and temperatures from 400 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 of the turnover frequency showed that the apparent activation energy for methanation dropped from 24 kcal mol(-1) for the monometallic ruthenium case to 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 silver does not act merely as a dilutant in the bimetallic system. These observations are elucidated in terms of a new model for surface-sensitive hydrogen adsorption, termed "portal site mediated adsorption," where low-coordination edge and corner sites on the catalyst surface act as sinks for rapid, dissociative adsorption of weakly bound, highly mobile surface hydrogen. In situ H-1-NMR measurements of surface hydrogen coverages during reaction were used to confirm the kinetic model treatment of the reaction data.