No abstract

A novel analysis method of the platinum L 3 and L 2 X-ray absorption edges of Pt/LTL catalysts indicates that chemisorbed hydrogen induces an antibonding resonance state just above the Fermi level at the Pt L 3 edge. The difference in energy of this antibonding state (Eres) with respect to the Fermi level is strongly influenced by the acidity/alkalinity of the LTL support: Eres decreases with increasing alkalinity. The decrease in Eres can also be correlated with the decrease of the neopentane hydrogenolysis turnover frequency of Pt/LTL with increasing support alkalinity. These results provide the most direct experimental evidence that the support acidity/alkalinity alters the actual chemical bond between the surface platinum atoms and the reacting molecule.

The electronic and geometric e †ects induced by hydrogen chemisorption on small platinum particles supported on high surface-area saponite clay and zeolite LTL were studied by near edge X-ray absorption Ðne structure (XAFS) spectroscopy. A new subtraction procedure was developed to separate the electronic from geometric e †ects. A signiÐcant PtÈH extended X-ray absorption Ðne structure (EXAFS) scattering (structural e †ect) was found for energy values between 0 and 20 eV. In addition, the PtÈH antibonding state (electronic e †ect) was found to produce a shape-resonance and was isolated from the near edge of the X-ray L 3 absorption spectrum. Moreover, for Pt/LTL the shape and energy of the shape-resonance was found to strongly depend on the acidity/alkalinity of the support material, implying a direct inÑuence of the support on the electronic properties of the platinum particles. The results of the study of the resonance state and the PtÈH EXAFS scattering demonstrate the potential of these techniques for characterization of hydrogen chemisorption, metal-promoter, and metal-support e †ects in catalysis research.

The structure and electronic properties of platinum in WH-LTL after reduction at 300 "C and heating in helium to 500 or 690 "C were determined using X-ray absorption and infrared spectroscopy. After reduction at 300 "C, the platinum particles were metallic, consisted of 4 or 5 atoms, and were located at 2.64 A from the oxygen atoms in the zeolite framework. The particles remained metallic but increased in size to e 1 3 atoms during hydrogen desorption by heating in a helium flow up to 690 "C. Simultaneously, the distance between metal particle and oxygen atoms of the zeolite framework was shortened to 2.05 A. After reduction at 300 "C and in the presence of chemisorbed hydrogen, the platinum atoms in the PT/H-LTL catalyst had more holes in the d-band than bulk platinum. Hydrogen desorption decreased the number of holes of the platinum atoms in the WH-LTL catalyst to levels lower than bulk metal values. The linear CO band shifted from 2071 to 2084 cm-' upon hydrogen desorption, due to the increased particle size andor the change in the structure of the metal-support interface. The apparent contradiction between the shift to higher wavenumbers of the linear CO band and the decreased number of holes in the d-band was attributed to the interaction of CO with filled d-orbitals and the effect of chemisorbed hydrogen on the distribution of the local density of states.

The reactivity, structure, and sulfur tolerance is compared for platinum supported on acidic and alkaline LTL zeolite. In the absence of sulfur, EXAFS spectroscopy indicates that small metallic platinum particles of approximately 6 to 14 atoms/cluster are present. The TOF for neopentane hydrogenolysis and isomerization is ca 100 times higher on the acidic LTL due to the metalsupport interaction. Saturation of the platinum by H 2 S, results in the formation of surface Pt-S bonds with a bond length of 2.33Å. Comparison of the EXAFS results of the sulfur poisoned catalysts, indicates that the S to Pt ratio is lower for platinum on the acidic zeolite. Catalytically, the initial activity (per gram) of both catalysts is greatly reduced after sulfur poisoning due to the loss of exposed platinum, however, the initial neopentane TOFs are nearly unchanged. Because of its higher TOF, the catalytic activity (per gram) of sulfur poisoned, acidic Pt/LTL is comparable to that of nonsulfur poisoned alkaline Pt/LTL. In both sulfur poisoned catalysts the neopentane isomerization selectivity increases compared to the sulfur free catalyst. Although the initial TOFs of the nonsulfur poisoned and sulfur poisoned catalysts are the same, there is a rapid loss in activity due to coke formation in the sulfur poisoned, alkaline LTL, while rate of deactivation by coke in the sulfided, acidic LTL is much lower. The increased sulfur tolerance of acidic supported noble metal catalysts appears to result primarily from the higher intrinsic TOF. In addition, because of its resistance to coke deactivation, the acidic supported sulfur tolerant catalyst is able to maintain stable catalytic activity.