Hydrogen and temperature effects on the coverages and activities of surface intermediates during methanation on Ru/SiO2

Methanation on Ru can be considered to be a representative example of many hydrogenation reactions. Steady-state isotopic transient kinetic analysis (SSITKA), one of the most powerful surface kinetic techniques capable of in situ assessing reaction parameters such as abundance of surface intermediates and intrinsic activity, was used to study the effects of hydrogen partial pressure and temperature on the fundamental surface reaction parameters for methanation (for a fixed P-CO of 0.036 bar) on Ru/SiO2. Although absolute hydrogen coverage under reaction conditions is not able to be measured due to the hydrogen isotope effect, relative hydrogen surface concentration as a function of P-H2 was able to be estimated at constant temperature from SSITKA parameters. Increasing the hydrogen partial pressure at constant temperature caused an expected increase in the relative surface concentration of hydrogen and a concomitant increase in the abundance of CHx species on the surface, N-M, possibly due to increased hydrogenation. At higher partial pressures of H-2, two active pools of methane intermediates (alpha and beta) were able to be observed in the activity distribution analysis. However at low P-H2, only the most active species (alpha) was detected. It was found that N-M was additionally dependent on temperature and deactivation. At low H-2/CO ratios (H-2/CO = 5), no increase in N-M with increasing temperature was detected, which is suggested to be an effect of site blockage by the formation of greater amounts of inactive surface carbon at higher temperatures. At low temperatures, both alpha- and beta-carbon were observed in the activity distribution analysis. However, at higher temperatures the less active beta-carbon could not be detected. This can probably be explained by a transformation of beta carbon into inactive gamma carbon under these conditions. At high H-2/CO ratios (H-2/CO = 20), an increase in N-M was observed with increasing temperature. This was attributed mainly to a more efficient hydrogenation of the surface active carbon. (C) 1997 Academic Press.