Structure sensitive selectivity of the NO-CO reaction over rhodium single crystal catalysts

摘要

The control of automotive emissions of nitrogen oxides (NOx) in passenger cars is accomplished by a heavy reliance on after-treatment of the engine exhaust using catalytic converters that contain a mixture of platinum (Pt), rhodium (Rh), and sometimes palladium (Pd). In this paper we examine the effect of surface structure on the NO-CO activity and selectivity by comparing the reactivity of Rh-110 and Rh-111 single crystal catalysts. Selectivity for the two possible nitrogen containing products from NO reduction, N2O and N2, is particularly interesting. Here we report that the selectivity of the NO-CO reaction is quite sensitive to the structure of the Rh catalyst metal surface. 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. Furthermore, under the conditions used in this study, the NO-CO activity over Rh-110, as measured by the rate of NO loss, is somewhat faster than over Rh-111 with a lower apparent activation energy (Ea), 27.6 vs. 35.4 kcal/mol. We attribute these results to the greater tendency of the more open (110) surface to dissociate NO. Notably, more facile NO dissociation on Rh-110 would lead to greater steady-state concentrations of adsorbed N-atoms; thus, the (110) surface favors N-atom recombination over the surface reaction between adsorbed NO and N-atoms to make N2O. In support of this, post-reaction surface analysis shows only NO on the Rh-111 surface while the Rh-110 surface contains predominantly N-atoms and much lower concentrations of adsorbed NO. NO dissociation on Rh-110 is more favorable than on Rh-111, in part, because it is less-severely poisoned by high surface concentrations of NO. In addition, the more-open (110) surface may be intrinsically more active for the elementary process of dissociating adsorbed NO.