A COMPARATIVE-STUDY OF CO AND CO2 HYDROGENATION OVER RH SIO2/

The hydrogenation of CO and CO2 over Rh/SiO2 have been investigated fo r the purpose of identifying the similarities and differences between these two reaction systems. In situ infrared spectroscopy was used to characterize the surface of the catalyst. Exposure of the catalyst to CO or CO2 produced very similar infrared spectra in which the principa l features are those for linearly and bridge-bonded CO. In the case of CO2 adsorption, a band for weakly adsorbed CO2 could also be observed . For identical reaction conditions the rate of CO2 hydrogenation to m ethane is higher than that for CO hydrogenation. The activation energy for CO hydrogenation is 23.2 kcal/mol and that for CO2 hydrogenation is 16.6 kcal/mol. The partial pressure dependances on H-2 and COz (z = 1, 2) are 0.67 and -0.80, respectively, for CO hydrogenation, and 0.5 3 and -0.46, respectively, for CO2 hydrogenation. Infrared spectroscop y reveals that under reaction conditions the catalyst surface is nearl y saturated by adsorbed CO. The spectra observed during CO and CO2 hyd rogenation are similar, the principal difference being that the CO cov erage during CO hydrogenation is somewhat higher than that during CO2 hydrogenation. The CO coverage is insensitive to Hz partial pressure, but increases slightly with increasing COz partial pressure. Transient -response experiments demonstrate that the adsorbed CO is a critical i ntermediate in both reaction systems. It is proposed that the rate det ermining step in the formation of methane is the dissociation of H2CO, produced by the stepwise hydrogenation of adsorbed CO. A rate express ion derived from the proposed mechanism properly describes the experim entally observed reaction kinetics both under steady-state and transie nt-response conditions. (C) 1996 Academic Press, Inc.