CO + O2 and CO + NO Reactions over Pd/Al2O3 Catalysts

The kinetics of the CO + NO and CO + O2 reactions have been studied over several different Pd/Al2O3 powder catalysts covering a wide range of average Pd particle sizes. The structure-insensitive nature of the CO + O2 reaction over Pd has been exploited to determine the relative dispersions in several Pd/Al2O3 powder catalysts by measuring the rate of that reaction and normalizing against surface area. This method, assuming particles of hemispherical shape, yields average particle sizes that are consistent with observations using TEM. For the CO + NO reaction over the same catalysts, a pronounced structure sensitivity is evident that results in higher reaction activities over larger Pd particles. Differences between CO oxidation rates for the CO + O2 reaction prior and subsequent to exposure to the CO + NO reaction are suggestive of the formation of an inhibiting, site-blocking species during the latter reaction. These results are discussed with reference to surface science and kinetics studies over single crystal and model planar Al2O3 supported Pd catalysts which have indicated that the structure-sensitive formation and stabilization of an inactive atomic N species plays a significant role in determining the reaction activity over a particular crystal plane or particle size.