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Abstract:
A new experimental method for obtaining real-time in situ infrared reflection-absorption spectra of any reproducibly reversible process has been applied to reveal the time-dependent coverages of adsorbates during the low-temperature oscillatory regime of the NO + CO reaction on Pt{100}. Simultaneous acquisition of real-time mass spectrometry data is used to relate the coverages to the rates of reaction, providing new insight into the mechanisms involved in the oscillation. NO and CO are shown to be present at estimated coverages varying between 0.06 and 0.20 monolayers (ML) for CO and between 0.27 and 0.36 ML for NO at different points in the cycle. The relative phases of these two coverage oscillations differ very significantly. CO is present in both atop and bridging geometries, but the two species exhibit identical adsorption and reaction behavior, attributed to the high rate of equilibration between the two configurations. Overall, the results show strong similarities to the predictions of the well-established model of Fink et al. (J. Chem. Phys. 1991, 95, 2109), but there are also notable differences. The results indicate a significant role for the nonreactive displacement of NO by CO not included in this model. Some of the other discrepancies may relate to unavoidable surface inhomogeneity associated with reaction front propagation at key points in the oscillatory cycle such as the "surface explosion" involving coadsorbed NO and CO. The coexistence of nonreacting CO poisoned islands are also found to occur at the lowest temperatures.
