SUPPORT AND TEMPERATURE EFFECTS IN PLATINUM CLUSTERS .1. SPATIAL STRUCTURE

The effects of Bronsted acidity on the spatial structure and electroni c properties of platinum clusters supported on zeolite Y have been exa mined with X-ray absorption (XANES and EXAFS) and X-ray photoelectron spectroscopy. The clusters contain 10-25 Pt atoms on average, with a n earest-neighbor distance of 2.70 +/- 0.01 Angstrom. Static disorder in the atomic distribution-while certainly present in the supported meta l-is shown to be symmetric on average. Increasing Bronsted acidity of the zeolite support has no measurable systematic effect on the spatial structure of the clusters but results in reproducible 5-10% enhanceme nt of near-edge features in the L(2.3) X-ray absorption spectra, as we ll as 0.2-0.3 eV shifts in Pt 4f and 4d core level binding energies an d valence-band thresholds. Evidence is presented that the interaction between the electronic levels of the cluster and the support is not do minated by charge-transfer effects but results in the creation of unoc cupied antibonding states above the Fermi level. Structure of atomic o rigin isolated Fr om the EXAFS data exhibits no dependence on the meas urement temperature and the acidity of the zeolite support. The implic ations of these results for current XANES-based methods for estimation of valence-band charge count are discussed.