Abstract
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 e13 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.
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