INFLUENCE OF THE REDUCTION TEMPERATURE ON THE STRUCTURE OF THE METAL PARTICLES AND THE METAL-SUPPORT INTERFACE OF PT GAMMA-AL2O3 CATALYSTS/

The structure of the metal particles and the metal-support interface o f a Pt/gamma-Al2O3 catalyst was determined by EXAFS after low temperat ure reduction (LTR: 300 degrees C) and high temperature reduction (HTR : 450 degrees C). The clusters have excellent thermal stability as the particle size remains 12 atoms per cluster upon increasing the reduct ion temperature from 300 degrees C to 450 degrees C. However, the stru cture of the metal-support interface is a strong function of the reduc tion temperature. After LTR and in the presence of chemisorbed hydroge n the metal particles are at a distance of 2.68 Angstrom from the supp ort oxygen ions. This long Pt-O distance is due to the presence of hyd rogen between the platinum atoms and the support. Increasing the reduc tion temperature results in the removal of hydrogen from the metal-sup port interface, simultaneously placing the metal particles in direct c ontact with the support oxygen atoms at a distance of 2.28 Angstrom. T he release of interfacial hydrogen during high temperature reduction i s changing the metal-support interaction, which in turn changes the el ectron density distribution in the metal particles. The electronic and thereby the catalytic properties of the platinum metal particles are therefore a function of the reduction temperature. The results show th at the influence of the reduction temperature on the structure of the metal-support interface of platinum particles on a amorphous support i s similar to platinum particles which reside in cavities of zeolites.