STRUCTURAL AND CHEMISORPTIVE PROPERTIES OF MODEL CATALYSTS - COPPER SUPPORTED ON SIO2 THIN-FILMS

The structural and chemical properties of model silica-supported copper catalysts have been investigated with infrared reflection-absorption spectroscopy and temperature-programmed desorption using CO as a probe molecule. The isosteric heats of adsorption of carbon monoxide on the model catalysts were measured at pressures between 10(-8) and 10(-3) Torr and temperatures between 180 and 300 K. The model catalysts were prepared by evaporating copper onto a planar silica thin film (approximately 100 angstrom) which, in turn, was supported on a Mo(110) surface. When deposited at 100 K, copper initially forms a two-dimensional structure on silica; however, annealing induces the ultrathin copper films (< 3 ML) to form small clusters. The unannealed copper films have a significant density of low-coordinated copper sites, whereas the annealed films consist of copper clusters with structures similar to low-index [(111), (110)] and high-index copper planes [(211) or (311)]. The distribution of the facets depends upon the initial copper coverage. The dispersion of the copper on the planar silica films is high, approximately 0.5, for the annealed films with theta(Cu) < 2 ML. Carbon monoxide desorbs in a single peak centered at approximately 210 K from the unannealed copper film, but in several peaks centered between 150 and 220 K from the annealed film. The CO adsorption energy strongly depends on the CO coverage, varying from approximately 17 kcal/mol at the low coverage limit to approximately 1 0 kcal/mol near CO saturation. The copper structure, the dispersion, the particle size distribution, and the heats of CO adsorption for the model systems are remarkably similar to the corresponding values for silica-supported copper catalysts prepared by the ion-exchange method. These similarities demonstrate that the method of preparation described here using a dispersed metal on an oxide film produces an excellent model for the analogous supported metal catalyst.