Nature of the metal-support interaction in supported Pt catalysts: Shift in Pt valence orbital energy and charge rearrangement

Conversion of neopentane (hydrogenolysis and isomerization on Pt/LTL, Pt/Si O2-Al2O3, Pt/MgO-Al2O3) and tetralin (hydrogenation on Pt/Y) catalytic data , combined with spectroscopic Pt atomic XAFS (AXAFS) data, and theoretical calculations are utilized to elucidate the nature of the metal-support inte raction for Pt-supported catalysts. The turnover frequency (TOF) of both th e neopentane and tetralin conversion strongly depends on the composition of the support. The TOF increases with increasing acidity, polarization power of the charge-compensating cations (Na+, H+, K+, La3+), Si/Al ratio, and t he presence of extra-framework Al. The intensity of the experimental Pt ato mic XAFS correlates with the TOR Ab initio scattered wave cluster calculati ons on a Pt4O3 cluster were performed using the FEFF7 code. The electron ch arge on the three "support" oxygens was changed from +0.05 to -0.01 electro n to mimic changes in the support Madelung potential, which for the cluster is dominated by the nearest neighbor oxygen charge. The trends found in th ese theoretical AXAFS results are in excellent agreement with the experimen tal 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 metal-support interaction not only changes the ionization potential of the Pt valence or bitals but also induces a charge rearrangement from the Pt 6s orbitals with in the particle to the oxygens of the Pt-support interface and vice versa. This charge rearrangement is also indicated by the AXAFS through the shift, DeltaR, in AXAFS peak position. Both effects influence the electronic stru cture of the Pt particles. The changes in the electronic structure alter th e catalytic properties of the Pt surface atoms by varying the bond strength and bond order (single or bridged) to the catalytic intermediates. The con sequences of the metal-support interaction for tailor-made supported metal catalysts will be discussed. (C) 2001 Academic Press.