NO-CO ACTIVITY AND SELECTIVITY OVER A PT10RH90(111) ALLOY CATALYST IN THE 10-TORR PRESSURE RANGE

We have studied the effects of temperature, NO conversion, and NO-CO ratio on the activity and selectivity of the NO-CO reaction over a Pt10Rh90(111) surface. The NO-CO activity over the Pt10Rh90(111) is very similar to that over the Rh(111) surface from 573 to 648 K in that both surfaces have the same E(a), reaction orders, products, and selectivities. The turnover numbers for the Pt10Rh90(111) alloy are slightly lower than those for Rh(111), when compared on a per surface atom basis; however, the rates per surface Rh atom are virtually unchanged. This behavior suggests that the primary effect of Pt is to dilute the Rh surface atom concentration; however, it is equally consistent with electronic modification of all surface atoms. The surface composition remains essentially unchanged over the range of reaction conditions that we explored; however, we did not go extremely oxidizing, which is the condition known to have the largest effect on the surface composition. The Pt10Rh90(111) single crystal mimics the behavior of supported Pt-Rh catalysts in that both show high (approximately 75%) selectivity for N2O at low temperature, low conversion, or high NO-CO ratio and low or zero N2O production at high temperature and high NO conversion. Our conclusion is that the N2O selectivity and the overall reaction rate are controlled by the NO adsorption/desorption equilibrium. Adsorbed NO strongly inhibits the NO dissociation reaction, keeping surface N coverages low. However, once the temperature is raised or the NO pressure lowered, surface NO coverages fall, accelerating the NO dissociation reaction and likewise increasing the N + N reaction. We conclude that these surface kinetics explain the curious N2O selectivities observed during light-off of supported catalysts. (C) 1994 Academic Press, Inc.