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 A. Static disorder in the atomic distributionwhile certainly present in the supported metalis 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% enhancement of near-edge features in the L2,3 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. Stru...