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The kinetics of NO reduction by CO have been investigated over a Rh SiO 2 catalyst. These studies have been complemented by in situ infrared studies. The specific activity is found to be sensitive to the nature of the catalyst pretreatment. Preoxidation in NO increases the specific activity for NO reduction by 50% over that observed when the catalyst is prereduced. Pretreatment has little effect, though, on the selectivity for forming N 2 versus N 2 O. For NO conversions below 50%, the kinetics for NO reduction, and for N 2 and N 2 O formation, are positive order in CO but inverse order in NO. The activation energies for all three processes are equivalent. Infrared spectra taken under reaction conditions show that the surface is nearly saturated by adsorbed NO. Smaller coverages by CO and NCO species are also observed. The results of both the spectroscopic and rate studies can be interpreted on the basis of a …