Nature of the Metal–Support Interaction in Supported Pt Catalysts: Shift in Pt Valence Orbital Energy and Charge Rearrangement

Abstract

Conversion of neopentane (hydrogenolysis and isomerization on Pt/LTL, Pt/SiO2–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 interaction for Pt-supported catalysts. The turnover frequency (TOF) of both the 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 the presence of extra-framework Al. The intensity of the experimental Pt atomic XAFS correlates with the TOF. Ab initio scattered wave cluster calculations on a Pt4O3 cluster were 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 Pt-support interface and vice versa. This charge rearrangement is also indicated by the AXAFS through the shift, ΔR, in AXAFS peak position. Both effects influence the electronic s tructure 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.