Pulse Transient Responses of NO Decomposition and Reduction with H₂on Ag−Pd/Al₂O₃

Catalytic decomposition of nitric oxide (NO) over Pd/Al 2 O 3 and Ag−Pd/Al 2 O 3 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 H 2, pulsing NO over the Pd/Al 2 O 3 catalyst produces adsorbed NO species (i.e., Pd + −NO, Pd 0 −NO, and Pd−NO − ) as well as gaseous N 2, O 2, and N 2 O products. Transient responses of the N 2 and O 2 profiles show that the addition of Ag onto Pd/Al 2 O 3 catalyst shifts the O 2 profile forward, increases oxygen formation and the oxidation resistance of Pd, but did not decrease the amount of retained oxygen (O ret ) and did not improve the catalytic cycle for NO decomposition. O ret on the Pd surface is not able to desorb in the temperature range of this study; however, O ret on Ag−Pd/Al 2 O 3 can be desorbed at higher temperatures than its formation and adsorption temperature. The presence of H 2 during the NO pulse allowed NO reduction to occur, producing N 2, N 2 O, O 2, NH 3, and H 2 O. Pd/Al 2 O 3 is a more active catalyst for the formation of NH 3 and H 2 O than Ag−Pd/Al 2 O 3 . Comparison of the transient gaseous product responses over Pd/Al 2 O 3 and Ag−Pd/Al 2 O 3 catalysts show that Ag (i) promotes the formation of N 2, shifting its profile forward, and (ii) suppresses the formation of NH 3 and H 2 O, delaying their formation. The lack of the initial activity of Ag−Pd/Al 2 O 3 for NH 3 /H 2 O formation can be attributed to the alloy state of Ag−Pd on Al 2 O 3 . As the NO reduction proceeds in the presence of H 2, adsorbed oxygen produced from N−O dissociation could cause the dealloying of Ag−Pd, producing Pd sites, which exhibited high selectivity for NH 3 /H 2 O formation.