Support and temperature effects in platinum clusters .2. Electronic properties

The effects of Bronsted acidity on the spatial structure and electronic properties of platinum clusters supported on zeolite Y have been examined with X-ray absorption (XANES and EXAFS) and X-ray photoelectron spectroscopy. The clusters contain 10-25 Pt atoms on average, with a nearest-neighbor distance of 2.70 +/- 0.01 Angstrom, static disorder in the atomic distribution-while certainly present in the supported metal-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% enhancement of near-edge features in the L(2.3) X-ray absorption spectra, as well as 0.2-0.3 eV shifts in Pt 4f and 4d core-level binding energies and valence-band thresholds. Evidence is presented that the interaction between the electronic levels of the cluster and the support is not dominated by charge-transfer effects but results in the creation of unoccupied antibonding states above the Fermi level. Structure of atomic origin isolated from the EXAFS data exhibits no dependence on the measurement temperature and the acidity of the zeolite support. The implications of these results for current XANES-based methods for estimation of valence-band charge count are discussed.