Adsorbed species and reaction rates for NO-CO-O-2 over Rh(111)

We have studied the NO-CO-O-2 reaction over a Rh(111) catalyst by monitoring the reaction products (CO2, N2O, and N-2) and the infrared (IR) intensity of surface CO and NO at various partial pressures of NO, CO and O-2, and sample temperatures. The selectivity for N2O formation, apparent activation energy for product formation, and NO consumption rate during NO-CO-O-2 are identical to those measured during the NO-CO reaction. The IR measurements show that during NO-CO-O-2 the same two adsorbed species, NO at 1640 cm(-1) and linear CO at similar to 2040 cm(-1), are present in the same surface concentrations as during NO-CO. For this reason the NO-CO-O-2 kinetics are dominated by the NO-CO kinetics, the NO consumption is rate limited by dissociation of adsorbed NO, and the N2O selectivity is dominated by surface NO coverage. In contrast, O-2 consumption is adsorption rate limited with the NO-CO adsorption-desorption equilibrium controlling the vacant sites required to dissociatively adsorb O-2. These kinetic and IR data of the CO-NO-O-2 reaction and our interpretation of them agree with previous studies over supported Rh catalysts and thus confirm the previously proposed explanation. From RAIRS and kinetic data we estimate the rate constant for the CO + O --> CO2 elementary step. The pre-exponential factor for this rate is 2 x 10(10) s(-1), a factor of 50 smaller than previous estimates. This rate constant is important to the NO-CO-O-2 kinetics because it affects O coverage, which, under certain conditions, inhibits NO consumption.