CO/NO and CO/NO/O2 reactions over a Au–Pd single crystal catalyst

NO reduction by CO was investigated over a AuPd(1 0 0) model catalyst at near atmospheric pressures. The alloy catalyst exhibits higher CO2 formation rates below ∼550 K than does pure Pd, although the binding energy of NO on the alloy surface is substantially less, i.e., its dissociation tendency is much less, compared with pure Pd. This behavior is rationalized by the fact that the low CO/NO-binding energies with the alloy surface provide a substantial population of empty ensembles for NO dissociation at relatively low temperatures. Moreover, AuPd(1 0 0) catalyzes the CO + NO reaction with much higher N2 selectivity than does pure Pd. Reaction kinetics data reveal that contiguous Pd sites are essential for NO dissociation. The reaction orders in CO and NO pressures, vastly different from Pd and Rh, are also a consequence of the low-binding energies of CO and NO on the alloy surface. It is also found that low-pressure NO promotes the CO + O2 reaction via gas-phase NO2 formation; the latter dissociates to form O(ads) more efficiently than does O2 below ∼600 K. However, when the NO pressure exceeds a critical value, gas-phase NO2 causes surface oxidation and thus inhibits CO2 formation. The current study suggests that AuPd alloys should be superior catalysts compared to traditional catalysts with respect to the “cold start” problem in catalytic automobile pollutant removal.