HYDROGEN AS A MODIFIER OF THE STRUCTURE AND ELECTRONIC-PROPERTIES OF PLATINUM IN ACIDIC ZEOLITE LTL - A COMBINED INFRARED AND X-RAY-ABSORPTION SPECTROSCOPY STUDY

The structure and electronic properties of platinum in Pt/H-LTL after reduction at 300 degrees C and heating in helium to 500 or 690 degrees C were determined using X-ray absorption and infrared spectroscopy. After reduction at 300 degrees C, the platinum particles were metallic, consisted of 4 or 5 atoms, and were located at 2.64 Angstrom from the oxygen atoms in the zeolite framework. The particles remained metallic but increased in size to approximate to 13 atoms during hydrogen desorption by heating in a helium flow up to 690 degrees C. Simultaneously, the distance between metal particle and oxygen atoms of the zeolite framework was shortened to 2.05 Angstrom. After reduction at 300 degrees 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 Pt/H-LTL catalyst to levels lower than bulk metal values. The linear CO band shifted from 2071 to 2084 cm(-1) upon hydrogen desorption, due to the increased particle size and/or 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.