NO+CO+O₂ reaction kinetics on Rh(111): a molecular beam study

Abstract

Steady-state rates for the chemical conversion of NO+CO+O₂ mixtures on Rh(111) surfaces have been measured by using a molecular beam setup with mass spectrometry detection. The changes in the partial pressure of reactants (NO, CO, O₂) and products (N₂ and CO₂) have been used as a measure of the reaction rates for temperatures between 435 and 766 K and varying beam compositions around those observed in automobile exhausts. The addition of oxygen was found to inhibit the activity of the rhodium catalyst toward NO reduction in most cases, as expected. The reason for this behavior, however, was determined not to be the consumption of some of the CO in the mixture by the added O₂, but rather a poisoning of the adsorption of CO by adsorbed atomic oxygen. In fact, oxygen addition to the NO+CO mixtures reduces not only the rates of N₂ production, but also those of CO₂ formation. Moreover, NO was proven to always compete favorably against O₂ for the consumption of CO. Optimum reaction rates for the production of both N₂ and CO₂ are reached at temperatures around 500-600 K and under conditions leading to stoichiometric coverages of all reactants. An interesting consequence of this is the fact that with CO-rich mixtures the addition of oxygen sometimes actually facilitates, not poisons, NO reduction, presumably because it helps in the removal of the excess CO from the surface. A synergy was observed in terms of reaction rate maxima between temperature and beam composition, CO-richer mixtures requiring higher temperatures to reach comparable reaction rates. This is explained by a decrease in CO surface coverage because of the increase in desorption rate with temperature, a trend that also explains the gradual increase in poisoning of the NO reduction activity of Rh by molecular oxygen with increasing temperature. Studies on the reaction between CO and O₂ were also carried out in order to isolate and identify the contributions of the surface oxygen deposited by dissociation of molecular oxygen and by NO to the production of CO₂, and alternations between oxygen-rich and oxygen-lean beams were used to test cyclic processes as a way to better manage NO reduction under net oxidizing atmospheres.