Conversion of neopentane (hydrogenolysis and isomerizationwere performed using the FEFF7 code. The electron charge on the three "support" oxygens was changed from +0.05 to −0.01 electron to mimic changes in the support Madelung potential, which for the cluster is dominated by the nearest neighbor oxygen charge. The trends found in these theoretical AXAFS results are in excellent agreement with the experimental Pt AXAFS data and suggest that a metal cluster-support potential model is adequate for describing the changes seen in the experimental AXAFS. The experimental AXAFS results can also be understood using a molecular orbital scheme. This molecular orbital scheme further indicates that the metal-support interaction not only changes the ionization potential of the Pt valence orbitals but also induces a charge rearrangement from the Pt 6s orbitals within the particle to the oxygens of the Ptsupport interface and vice versa. This charge rearrangement is also indicated by the AXAFS through the shift, R, in AXAFS peak position. Both effects influence the electronic structure of the Pt particles. The changes in the electronic structure alter the catalytic properties of the Pt surface atoms by varying the bond strength and bond order (single or bridged) to the catalytic intermediates. The consequences of the metal-support interaction for tailor-made supported metal catalysts will be discussed.

The sulfidation of disk-shaped MOO, particles with a thickness of 5-10 nm supported on a 5-nm-thick layer of Si02 on Si(100) in a mixture of 10% H2S in H2 at atmospheric pressure has been studied as a function of temperature. XPS and SIMS indicate the formation of Mo4+OS, at the surface and Mo(1V) oxides (probably H1.6Mo03 or MOO?) in the interior of the particles at temperatures between 20 and 100 OC, whereas MoS2 forms at temperatures of 125 OC and higher. Sulfur is present in two forms, as S2-and in a second form which is most probably Sz2-or SH-, but not elemental sulfur. The additional sulfur species disappear at temperatures between 150 and 200 OC. Rutherford backscattering analysis indicates S:Moatomicratios of 1-1.5 at sulfidation temperatures below 100 OC and of 2-2.5 above 100 OC. It is concluded that the sulfidation of Moo3 to MoS2 proceeds through a Mo(1V) oxysulfide, formed initially at the outside of the particle, and Mo(1V) oxide in the interior of the particle. Sulfidic species are believed to facilitate the reduction of Moo3 to Mo(1V) species at low temperatures.

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