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Abstract:
At a partial pressure of 10-7 mbar, the reaction between NO and CO on Pt{100} exhibits oscillatory behavior in two distinct temperature regimes. Oscillations in the high-temperature regime (380-411 K) are accompanied by a phase transition from the (1x1) surface to the hex surface. Using infrared reflection-absorption spectroscopy (IRAS) and a novel method of data acquisition, we show that during the oscillation cycle, the only molecular species present on the surface is atop CO, adsorbed on the (1x1) phase at very low coverage
(~0.03-0.007 ML). Furthermore, the minimum in the CO coverage coincides with the maximum reaction rate, as measured by the partial pressure of CO2. From a comparison of these data with previously published LEED and PEEM studies of the same system, it can be seen that the high-reaction-rate branch of the oscillatory cycle coincides with the maximum area of the surface in the hex phase. This is in contrast to previously proposed mechanisms, which assume that the (1x1) surface is the active phase. Since the hex surface is inactive for NO dissociation, we conclude that defects on the hex surface, created during the (1x1)-hex phase transition and known to be active for NO dissociation, are responsible for the high-reaction-rate branch.
Removal of these defects by annealing provides the means by which the reaction returns to the low-rate branch of the cycle. This annealing process also accounts for the observation that the period of oscillation decreases with temperature.
