IR study of CO adsorption on Cu–Rh/SiO 2 catalysts, coked by reaction with methane

Abstract

Infrared spectra are reported of CO adsorbed on silica-supported Rh, Cu, and Cu–Rh catalysts before and after coking by heat treatment in methane at 673 K, and after subsequent exposure to hydrogen at 293, 373 or 473 K. The surface of Cu–Rh particles was enriched with copper, but all surfaces contained alloy phases. Addition of copper to rhodium blocked sites for the formation of Rh(CO) 2, decreased the proportion of sites which linearly adsorbed CO, but favoured bridge-bonded CO on Rh sites. Cu sites in Cu/SiO 2 were not poisoned by methane treatment, but Cu sites and all types of Rh site in Cu–Rh/SiO 2 underwent poisoning in the sequence Cu–CO>Rh–CO>Rh 2CO≈Rh 2(CO) 3. CO band shifts caused by coking of rhodium sites are attributed to a decrease in dipolar coupling. Dipole coupling changes were more significant when the Rh surface was modified by Cu than by coking. Coke deposits, at least in the presence of CO, aggregated into patches on exposed Rh surfaces. Similar effects of coking occurred for linear CO on copper sites. Rh/SiO 2 gave a high yield of methane, but only a low yield of ethane on coke hydrogenation. Copper largely poisoned the catalytic activity, which it is tentatively proposed primarily involved sites on rhodium which adsorb CO in a bridging configuration.