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 of a Pt/gamma-Al2O3 catalyst was determined by EXAFS after low temperature 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 reduction temperature from 300 degrees C to 450 degrees C. However, the structure of the metal-support interface is a strong function of the reduction temperature. After LTR and in the presence of chemisorbed hydrogen the metal particles are at a distance of 2.68 Angstrom from the support oxygen ions. This long Pt-O distance is due to the presence of hydrogen between the platinum atoms and the support. Increasing the reduction temperature results in the removal of hydrogen from the metal-support interface, simultaneously placing the metal particles in direct contact with the support oxygen atoms at a distance of 2.28 Angstrom. The release of interfacial hydrogen during high temperature reduction is changing the metal-support interaction, which in turn changes the electron 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 that the influence of the reduction temperature on the structure of the metal-support interface of platinum particles on a amorphous support is similar to platinum particles which reside in cavities of zeolites.