Evidence for a carbonate species during CO oxidation on Deactivated Rh(111)

Abstract

In earlier work from this laboratory on the oxidation of CO over a single-crystal Rh(111) model catalyst, it was found that the kinetics of the reaction change dramatically when the reaction conditions were varied from reducing to oxidizing. These earlier studies suggested the possibility that the observed changes in reaction kinetics arose from the formation of a surface carbonate species, i.e., RhO(CO 2), on oxidized Rh which alters the kinetics and decreases the reaction rate because of the tenacious bonding of this complex. In order to further investigate this suggestion, we have recently performed a high-resolution electron energy loss, HREELS (surface vibrational spectroscopy), study of the intermediate species present on the Rh surface under various reaction conditions in conjunction with temperature-programmed desorption (TPD) measurements. The results provide further evidence that such a carbonate complex is present on the surface after reaction under oxidizing conditions. However, we also observed that under these conditions the surface is hydroxylated, probably arising from the presence of impurity H 2O or H 2 in the reactor. TPD results confirm the importance of these -OH groups in stabilizing the carbonate complex. For example, CO 2 adsorbed on a “clean” oxide (prepared by high temperature oxygen exposures in ultrahigh vacuum) was found to desorb at temperatures more than 100°C lower than CO 2 desorption from a hydroxylated Rh-oxide surface. In the latter case, CO 2 desorption likely arises from the reaction-limited decomposition of the carbonate species. In fact, the earlier kinetic studies are most consistent with an overall reaction mechanism for CO oxidation under oxidizing conditions involving the rate-limited decomposition of such a surface-bound carbonate species.