TRANSIENT KINETIC ANALYSIS OF METHANE SYNTHESIS OVER UNSUPPORTED Co and Rh/SiO2

Concentration jump and isotopic transient experiments show that at H₂/CO = 3, one atmosphere pressure, and 523 K both unsupported cobalt and Rh/SiO₂, have near monolayer coverage of molecular CO but coverage of the primary reactive intermediate below 5% of a monolayer. Modeling the data for unsupported Co requires inclusion of a second reactive surface carbon pool about four times less active than the primary pool. Coverages of both reactive intermediates increase with temperature and with H₂/CO ratio, the latter suggesting that CO dissociation is assisted by hydrogen. On Rh/SiO₂, deactivation appears to be caused by growth of an inactive surface carbon species that blocks sites for the reactive intermediate. Initially, CO dissociation is rate determining over the Rh catalyst. Over the Co catalyst and over Rh after long reaction times or at lower temperature, hydrogcnation of the reactive intermediate produced by CO dissociation has a low enough rate constant to allow the coverage of the intermediate, relative to the rate of reaction, to attain a value which is small but sufficient to slow the dynamic response of ¹³CH₄ to a CO to ₁₃CO switch. Thus, CO dissociation is not rate determining in these systems.