ON THE RELATION BETWEEN PARTICLE MORPHOLOGY, STRUCTURE OF THE METAL-SUPPORT INTERFACE, AND CATALYTIC PROPERTIES OF PT GAMMA-AL2O3/

The relation between the catalytic activity, electronic properties, mo rphology, and structure of the metal-support interface was studied for Pt/gamma-Al2O3 after low (300 degrees C, LTR) and high temperature re duction (450 degrees C, HTR). EXAFS revealed that after LTR the platin um particles were three-dimensional, contained 11 Pt atoms on average, and were at a distance of 2.7 Angstrom from the support oxygen. Durin g HTR, the morphology of the platinum particles changed from three-dim ensional to rafts with a structure similar to Pt(100), as indicated by a decrease in the Pt-Pt coordination number and the absence of the th ird coordination shell in the EXAFS spectrum, After HTR the Pt-O dista nce was shortened to 2.2 Angstrom due to the desorption of hydrogen fr om the metal-support interface, The shortening of the Pt-O distance up on HTR treatment agrees with previous studies on zeolite supported pla tinum, However, zeolite supported platinum particles retained their th ree-dimensional structure upon HTR. The changes in the structure of th e catalyst affected the catalytic, chemisorption, and electronic prope rties. After HTR the selectivity for hydrogenolysis of both neopentane and methylcyclopentane to methane decreased. At the same time the spe cific activity for neopentane isomerization and methylcyclopentane rin g opening increased. The hydrogen chemisorption capacity after HTR was lower than after LTR, HTR shifted the asymmetric linear CO infrared a bsorption from 2063 to 2066 cm(-1). Comparison of the Pt L(III) and L( II) X-ray absorption edge intensities after LTR and HTR revealed that the number of holes in the d-band of the platinum atoms increased by 9 .5% during HTR. It was suggested that the decrease in hydrogen chemiso rption capacity, hydrogenolysis selectivity, and number of holes in th e d-band are related to the change in the structure of the metal-suppo rt interface. The increase in specific activity for isomerization of n eopentane and the shift in the CO infrared absorption band with a rais e in reduction temperature agreed with reported results for single cry stals and was attributed to the higher concentration of atoms with Pt( 100) symmetry in the catalyst after HTR. (C) 1996 Academic Press, Inc.