A molecular beam study of the kinetics of the catalytic reduction of NO by CO on Rh(111) single-crystal surfaces

Abstract

Steady-state rates for the catalytic reaction of NO with CO on Rh(111) surfaces have been measured under isothermal conditions by using a molecular beam approach with mass spectrometry detection. Systematic studies were carried out as a function of surface temperature, NO+CO beam composition, and total beam flux. A maximum in reaction rate was observed between 450 and 900 K, the exact temperature depending on the NO:CO beam ratio. Indeed, a synergistic behavior was seen where the loss in reactivity induced by increasing the CO concentration in the beam is partly compensated by a higher surface temperature. The data presented here are consistent with the rate-limiting step of the overall NO reduction process being the surface recombination of atomic nitrogen atoms resulting from fast dissociation of NO adsorbed molecules. Temperature-programmed desorption and CO titration experiments were also performed after the isothermal kinetic runs in order to estimate the surface coverages of the reactants during the steady-state reactions. The NO+CO conversion rate was found to be directly proportional to the coverage of atomic oxygen on the surface. The relation between reaction rates and nitrogen coverages, however, proved to be much more complex; an inverse correlation was in fact seen in most cases between those two parameters. The build-up of a critical coverage of atomic nitrogen was found to be necessary to trigger the nitrogen recombination step to N₂. This critical coverage of strongly held nitrogen was determined to not depend in any significant way on the composition of the beam, but to decrease with reaction temperature in all cases.