Structure Sensitive Selectivity of the NO-CO Reaction over Rh(110) and Rh(111)

Abstract

We have studied the effects of temperature, NO conversion, and NO-CO ratio 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 measured 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 activation energy (Ea), 27.2 vs 34.8 kcal/mol, than does the (111) surface. We attribute this behavior to a slightly more facile NO dissociation process on the more open (110) surface. Although the turnover numbers for NO reaction can be quite similar on the two different surfaces, we find large differences between Rh(110) and Rh(111) with regard to their selectivities for the two competitive nitrogen-containing products, N2O vs N2. The more open Rh(110) surface tends to make significantly less N2O than Rh(111) under virtually all conditions that we probed with these experiments. This can be understood in terms of the relative surface coverages of adsorbed NO and N atoms on the two surfaces. Notably, more facile NO dissociation on Rh(110) appears to lead to greater steady-state concentrations of adsorbed N atoms than is present on the (111) surface. Higher N atom coverages on the (110) surface favor N atom recombination (N2 formation) more than the NO + N reaction (N2O formation) on Rh(110) relative to Rh(111). Indeed, Rh(110) surfaces 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 to adsorbed NO.