Complex model catalysts under UHV and high pressure conditions: CO adsorption and oxidation on alumina-supported Pd particles

The growth of metal particles on ordered oxide surfaces provides a strategy to prepare well-defined model systems for supported catalysts, which can b y easily studied by most surface-science techniques. Here, we focus on Pall adium particles grown on an ordered Al2O3 film on NiAl(110), a system which has previously been characterized in detail with respect to its structural , electronic and adsorption properties. In this contribution, we will provide several examples, showing how adsorpt ion and reactivity phenomena on these systems can be addressed over a press ure range from ultrahigh vacuum (UHV) to near atmospheric pressure. In the low pressure region, we apply a combination of molecular beam methods and i n-situ infrared reflection absorption spectroscopy (IRAS). For CO adsorptio n, angular resolved scattering and sticking coefficient measurements and st ructural information allow us to quantify different adsorption channels inc luding reverse spillover effects. The coverage dependent kinetics of CO oxi dation is derived and discussed in comparison with the single crystal kinet ics. The adsorption of CO on alumina supported Pd aggregates at low and high pre ssure, i.e. from 10(-7)-200 mbar, is examined by IR-VIS sum frequency gener ation (SFG) vibrational spectroscopy. At low pressure, the CO adsorption si te distribution (bridged vs. on-top) depends on the particle surface struct ure and temperature, but under reaction conditions, the site occupancy is m ainly governed by the CO pressure. The impact of these results on the extra polation of UHV data to high pressure catalysis is discussed. (C) 2000 Else vier Science B.V. All rights reserved.