IR study of CO adsorption on Cu-Rh/SiO2 catalysts, coked by reaction with methane

Infrared spectra are reported of CO adsorbed on silica-supported Rh, Cu, an d Cu-Rh catalysts before and after coking by heat treatment in methane at 6 73 K, and after subsequent exposure to hydrogen at 293, 373 or 473 K. The s urface of Cu-Rh particles was enriched with copper, but all surfaces contai ned alloy phases. Addition of copper to rhodium blocked sites for the forma tion of Rh(CO)(2), decreased the proportion of sites which linearly adsorbe d CO, but favoured bridge-bonded CO on Rh sites. Cu sites in Cu/SiO2 were n ot poisoned by methane treatment, but Cu sites and all types of Rh site in Cu-Rh/SiO2 underwent poisoning in the sequence Cu-CO > Rh-CO > Rh2CO approx imate to 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. Cok e deposits, at least in the presence of CO, aggregated into patches on expo sed Rh surfaces. Similar effects of coking occurred for linear CO on copper sites. Rh/SiO2 gave a high yield of methane, but only a low yield of ethan e on coke hydrogenation. Copper largely poisoned the catalytic activity, wh ich it is tentatively proposed primarily involved sites on rhodium which ad sorb CO in a bridging configuration. (C) 1999 Elsevier Science B.V. All rig hts reserved.