Kinetics of the co/no surface reactions by the direct observation of the adsorbed species

Dynamic reflection infrared spectroscopy (DRIRS) was used to characterize the adsorbed state dynamics of CO and NO during adsorption/desorption and the CO/NO reaction on a platinum foil. The adsorbed states of CO and NO were characterized by stretching vibrations in the 1400-2100 cm^-1 range. A square wave modulated reactant source provided a flux of one species to the surface at a pressure equivalent of 10^-7 to 10^-5 mBar, while the background pressure of the other species was independently controlled. Sample temperatures were varied from 325 to 500 K. For CO adsorption, a single IR band was observed that shifted from 2040 to 2075 cm^-1 with increasing coverage. In the CO adsorption/desorption experiments, two distinct kinetic regimes were observed. At temperatures below 420 K, where the CO coverages were high (>50% saturation) desorption was characterized by a rate constant k = 10⁵ e^-7100/Ts^-1. When the CO coverage was low, corresponding to the temperatures >450 K, the desorption rate constant was k = 10¹⁵ e^-18,000/Ts^-1. The change in the effective rate constant for CO desorption is suggested to result from repulsive interactions between adsorbed CO molecules. Two NO absorption bands were observed at 1630 and 1770 cm^-1 which were attributed to NO absorbed on restructured and relaxed Pt surfaces. The combination of surface restructuring and NO decomposition precluded the accurate determination of NO desorption kinetics. The presence of NO on the surface increased the rate at which CO was removed from the surface. The CO and NO reaction on the surface produced a maximum in the CO turnover rate dependent on the reactant pressure ratio, coverage, and temperature. Modulating the CO pressure affected the coverage of NO in both types of adsorption sites, and modulation of the NO pressure produced modulations in the CO coverage. Assuming a uniform surface reaction, the data for the rate constant of CO removal during the CO/NO reaction due to a CO pressure modulation with constant NO pressure or an NO pressure modulation with constant CO pressure were consistent with the rate-limiting step in the CO/NO reaction being a bimolecular surface reaction between adsorbed CO and adsorbed NO. The data were not consistent with NO dissociation being the rate-limiting reaction step. Alternative possibilities resulting from reactions at surface defects or more complex reaction mechanisms could also account for the data.