Interaction of NO with alumina supported palladium model catalysts

Abstract

The interaction of NO with a well-defined supported Pd model catalyst has been investigated, employing a combination of IR reflection absorption spectroscopy (IRAS) and molecular beam methods. The model catalyst is based on an ordered Al₂O₃ film, grown on NiAl(110) and has been characterized in detail with respect to its geometric and electronic structure previously. In a first step the interaction of NO with the pure Al₂O₃ model support is studied. It is observed that NO slowly decomposes at low surface temperature (100 K), resulting in the formation of a variety of N_xO_y surface species. The decomposition process involves strong structural transformations of the Al₂O₃ film and is initiated at oxide defect sites. The adsorption of NO on the Pd/Al₂O₃ model system is explored systematically as a function of surface coverage and temperature. For NO adsorption on the Pd particles, characteristic absorption bands are identified by comparison with single crystal data, which are indicative of particle-specific sites such as edges and other defects. At 300 K and above NO dissociation occurs on the Pd particles, whereas the low-temperature decomposition channel on the alumina support is found to be strongly suppressed in the presence of the Pd particles. Finally, co-adsorption of NO with oxygen- and CO-precovered Pd particles is investigated. Preadsorbed oxygen results in the formation of a mixed adsorbate layer with an enhanced NO population of on-top sites on the particles. NO adsorption on a CO layer, on the other hand, gives rise to a compression of the CO layer and an enhanced population of on-top sites by CO.