DEPENDENCE OF TEMPERATURE-PROGRAMMED METHANATION OF H-2 PRESSURE

The effect of H2 partial pressure (0.1-2 atm) on the temperature-programmed reaction (TPR) of adsorbed CO in flowing H2/He was studied for Ni catalysts supported on SiO2, A1203, and TiO2. On Ni/SiO2, as H2 partial pressure decreased, the methanation rate decreased. The methanation rate of CO adsorbed on Ni/SiO2 was approximately first order in H2 pressure below 0.8 atm and zero order between 0.8 to 2 atm. On Ni/TiO2 and Ni/Al2O3 catalysts, two CH4 peaks form; one is due to hydrogenation of CO that is adsorbed on Ni and the other is due to hydrogenation of a H-CO complex (most likely CH3O) that is adsorbed on the TiO2 and A1203 Supports. Formation of CH4 from CO on Ni has the same dependence on H2 pressure as that seen on Ni/SiO2. However, hydrogenation of the H-CO complex is negative order in H2 partial pressure; the peak temperature of CH4 decreases with a decrease in the H2 partial pressure. Apparently, H2 inhibits hydrogenation of the complex by occupying surface sites that are needed for other species. A possible explanation is that the H-CO complex diffuses to vacant sites on the Ni surface to decompose and be hydrogenated, and at higher H2 pressures, more of these sites are occupied by hydrogen. © 1990.