Catalytic decomposition of nitric oxide (NO) over Pd/Al2O3 and Ag−Pd/Al2O3 has been studied using the pulse transient response technique coupled with in situ infrared (IR) and mass spectrometry (MS) at 723−823 K. In the absence of H2, pulsing NO over the Pd/Al2O3 catalyst produces adsorbed NO species (i.e., Pd+−NO, Pd0−NO, and Pd−NO−) as well as gaseous N2, O2, and N2O products. Transient responses of the N2 and O2 profiles show that the addition of Ag onto Pd/Al2O3 catalyst shifts the O2 profile forward, increases oxygen formation and the oxidation resistance of Pd, but did not decrease the amount of retained oxygen (Oret) and did not improve the catalytic cycle for NO decomposition. Oret on the Pd surface is not able to desorb in the temperature range of this study; however, Oret on Ag−Pd/Al2O3 can be desorbed at higher temperatures than its formation and adsorption temperature. The presence of H2 during the NO pulse allowed NO reduction to occur, producing N2, N2O, O2, NH3, and H2O. Pd/Al2O3 is a more active catalyst for the formation of NH3 and H2O than Ag−Pd/Al2O3. Comparison of the transient gaseous product responses over Pd/Al2O3 and Ag−Pd/Al2O3 catalysts show that Ag (i) promotes the formation of N2, shifting its profile forward, and (ii) suppresses the formation of NH3 and H2O, delaying their formation. The lack of the initial activity of Ag−Pd/Al2O3 for NH3/H2O formation can be attributed to the alloy state of Ag−Pd on Al2O3. As the NO reduction proceeds in the presence of H2, adsorbed oxygen produced from N−O dissociation could cause the dealloying of Ag−Pd, producing Pd sites, which exhibited high selectivity for NH3/H2O formation.
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