SUPPORT AND TEMPERATURE EFFECTS IN PLATINUM CLUSTERS .2. ELECTRONIC-PROPERTIES

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 an 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 from the EXAFS data exhibits no dependence on the measu rement temperature and the acidity of the zeolite support. The implica tions of these results for current XANES-based methods for estimation of valence-band charge count are discussed.