ELECTRONIC METAL SUPPORT INTERACTION IN PT CATALYSTS UNDER DEUTERIUM ETHENE REACTION CONDITIONS AND THE MICROSCOPIC NATURE OF THE ACTIVE-SITES

The electronic states and the nature of active sites of Pt/Y2O3, Pt/V2O3, and Pt/TiO2 in the working state were investigated by X-ray absorption near-edge structure (XANES) spectroscopy, kinetics, and isotope-tracing techniques. The density of the unoccupied 5d state of Pt decreased as the reduction temperature of catalysts increased, but its degree strongly depended on the nature of support. High-temperature reduction of Pt/ZrO2 and Pt/Y2O3 decreased the unoccupied d density of Pt more than TiO2 and Nb2O5, probably because the reduced region of support was restricted in the vicinity of the Pt particles. The unoccupied d density of Pt on V2O3 was independent of the reduction temperature due to the metallic nature of the support. For this catalyst, the decoration by the support oxide occurred by the reduction at 773 K. The d states of Pt were largely modified under the D2-ethene reaction conditions as proved by XANES. The electrons of Pt on ZrO2 were removed by adsorbed ethene, being negatively charged, while Pt on Y2O3 behaved as an electron acceptor, leaving a positive charge on ethene. The nature of the active sites was also characterized by the D profile of ethane in the D2-ethene reaction. Pt/TiO2 reduced at 773 K had two kinds of sites with different surface H/D ratios under working conditions-it was suggested that one was the bare metal site of Pt mainly producing ethane-d2 and the other was the peripheral site of TiOx islands producing ethane-d0. The active site of Pt/V2O3 reduced at 773 K had oxide nature, suggesting that Pt particles may be completely covered with VOx.