On the Relation Between Particle Morphology, Structure of the Metal-Support Interface and Catalytic Properties of Pt/gamma-Al2O3.

Abstract

The relation between the catalytic activity, electronic properties, morphology, and structure of the metal-support interface was studied for Pt/@c-Al{2}O{3}after low (300}o{C, LTR) and high temperature reduction (450}o{C, HTR). EXAFS revealed that after LTR the platinum particles were three-dimensional, contained 11 Pt atoms on average, and were at a distance of 2.7 A from the support oxygen. During HTR, the morphology of the platinum particles changed from three-dimensional 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 third coordination shell in the EXAFS spectrum. After HTR the Pt-O distance was shortened to 2.2 A due to the desorption of hydrogen from the metal-support interface. The shortening of the Pt-O distance upon HTR treatment agrees with previous studies on zeolite supported platinum. However, zeolite supported platinum particles retained their three-dimensional structure upon HTR. The changes in the structure of the catalyst affected the catalytic, chemisorption, and electronic properties. After HTR the selectivity for hydrogenolysis of both neopentane and methylcyclopentane to methane decreased. At the same time the specific activity for neopentane isomerization and methylcyclopentane ring opening increased. The hydrogen chemisorption capacity after HTR was lower than after LTR. HTR shifted the asymmetric linear CO infrared absorption from 2063 to 2066 cm}-{}1{. Comparison of the Pt L{I}{I}{I}and L{I}{I}X-ray absorption edge intensities after LTR and HTR revealed that the number of holes in thed-band of the platinum atoms increased by 9.5% during HTR. It was suggested that the decrease in hydrogen chemisorption capacity, hydrogenolysis selectivity, and number of holes in thed-band are related to the change in the structure of the metal-support interface. The increase in specific activity for isomerization of neopentane and the shift in the CO infrared absorption band with a raise in reduction temperature agreed with reported results for single crystals and was attributed to the higher concentration of atoms with Pt(100) symmetry in the catalyst after HTR.