KINETICS OF CO OXIDATION BY N2O OVER RH(111)

We have studied, for the first time, the reaction of CO with N2O over a Rh(111) catalyst at pressures between 1 and 20 Torr in an apparatus that couples a moderate pressure reactor with an ultrahigh vacuum analysis chamber. Using 4 Torr of CO and 4 Torr of N20 between 570 and 670 K, we measured an apparent activation energy (Ea) of 40.0 kcal/mol. By varying the reactant pressures (T = 623 K), we determined that the reaction orders are + 1.1 in N2O pressure and -1.2 in CO pressure. Both Ea and the reaction order are in good agreement with previously reported measurements over alumina supported Rh particles. Kinetic modeling of the data suggests that CO and N2O compete for a limited number of vacant sites on a surface where OCO ≥ 0.9 ML. Under these conditions, the rate limiting step is N2O dissociation. We estimate, based on the model, that the barrier for N2O dissociation (EdissN is 17.5 kcal/mol. The model also predicts that the N2O sticking coefficient (SN) must be greater than 0.005. Our measurements show that there is excellent agreement between Rh(111) and supported Rh reaction kinetics with regards to both Ea and the CO pressure dependence. These experimental results add still another reaction to the growing list of cases where single crystals are excellent models for more practical supported catalysts. Further, our modeling of these results allows us to estimate two previously unknown quantities, SN and EdissN. © 1992.