OXIDATION OF CARBON-MONOXIDE CATALYZED BY RHODIUM (110)

The oxidation of carbon monoxide on Rh(110) has been studied using a thermal molecular beam apparatus, with a flux of reactant at the crystal surface of approximately 0.01 ML s-1, in the temperature range 300-700 K. Results are reported for three types of experiments, (1) where the crystal is exposed to a mixed beam of CO and oxygen and reaction is allowed to reach steady state, (2) where the crystal is predosed with oxygen and then exposed to a beam of CO, and (3) where reaction is allowed to reach steady state at one temperature (usually 370 K), and the crystal temperature is then raised at approximately 10 K min - 1. A kinetic model has been developed for the reaction that assumes Langmuir-Hinshelwood kinetics with coverage dependent sticking probabilities and activation energies, and that describes the experimental results well over all of the conditions studied. The reactivity of the rhodium crystal may be divided into two regimes, depending on oxygen coverage. At theta(O) > 0.6 ML, reaction becomes much slower than at lower oxygen coverages, especially at T > 400 K. At low temperatures reaction is inhibited by the presence of CO, but above T almost-equal-to 400 K the steady state coverage of CO becomes small and reaction is then inhibited by adsorbed oxygen.