Comparative kinetic studies of CO$z.sbnd;O2 and CO$z.sbnd;NO reactions over single crystal and supported rhodium catalysts

The kinetics of the COO2 and CONO reactions over single crystal Rh(111) and over alumina-supported Rh catalysts have been compared at realistic reactant pressures. For the COO2 reaction, there is excellent agreement between both the specific rates and activation energies measured for the two types of Rh catalysts. The CONO reaction, on the other hand, exhibits substantially different activation energies and specific reaction rates between the single crystal and supported catalysts. This indicates that the kinetics of the CONO reaction, unlike the COO2 reaction kinetics, are sensitive to changes in catalyst surface characteristics. The kinetic data for the COO2 and CONO reactions over Rh(111) and RhAl2O3 were analyzed using mathematical models which account for the individual elementary reaction steps established from surface chemistry studies of the interactions of CO, NO, and O2 with Rh surfaces. The model, when used with the parameter values similar to those reported in the surface chemistry literature, can quantitatively fit the CO oxidation rate data over both the single crystal and supported Rh catalysts. The kinetics of the CO NO reaction over Rh(111) can also be well described by a reaction model using parameter values taken from surface chemistry studies. However, the rate data for the CONO reaction over supported Rh can be rationalized by assuming that the dissociation of molecularly adsorbed NO occurs much more slowly on supported Rh than on Rh(111).

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