STRUCTURE SENSITIVE SELECTIVITY OF THE NO-CO REACTION OVER RH(110) AND RH(111)

We have studied the effects of temperature, NO conversion, and NO-CO r atio on the activity and selectivity of the NO-CO reaction at high (1 Torr < P < 100 Torr) pressures over the Rh(110) and Rh(111) surfaces. Under the conditions used in this study, the NO-CO activity, as measur ed by the rate of NO loss, is between 1.3 and 6.3 times faster over Rh (110) than over Rh(111), The (110) surface exhibits a lower apparent a ctivation energy (E(a)), 27.2 vs 34.8 kcal/mol, than does the (111) su rface. We attribute this behavior to a slightly more facile NO dissoci ation process on the more open (110) surface. Although the turnover nu mbers for NO reaction can be quite similar on the two different surfac es, we find large differences between Rh(110) and Rh(111) with regard to their selectivities for the two competitive nitrogen-containing pro ducts, N2O vs N-2. The more open Rh(110) surface tends to make signifi cantly less N2O than Rh(111) under virtually all conditions that we pr obed with these experiments, This can be understood in terms of the re lative surface coverages of adsorbed NO and N atoms on the two surface s, Notably, more facile NO dissociation on Rh(110) appears to lead to greater steady-state concentrations of adsorbed N atoms than is presen t on the (111) surface. Higher N atom coverages on the (110) surface f avor N atom recombination (N-2 formation) more than the NO + N reactio n (N2O formation) on Rh(110) relative to Rh(111). Indeed, Rh(110) surf aces were found to be largely composed of adsorbed N atoms and lesser quantities of NO in postreaction XPS measurements. In contrast, Rh(111 ) surfaces showed only X-ray photoelectron spectroscopy features due t o adsorbed NO. (C) 1995 Academic Press, Inc.