The structure and catalytic properties of supported platinun catalysts

+166hlm.;24c

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@c-Al{2}O{3} catalyst was determined by EXAFS after low temperature reduction (LTR: 300}o{ C) and high temperature reduction (HTR: 450}o{ C). The clusters have excellent thermal stability as the particle size remains 12 atoms per cluster upon increasing the reduction temperature from 300}o{ C to 450}o{ 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 a 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 a. 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

A New Method for Parameterization of Phase Shift and Backscattering Amplitude

Parameterization of phase and backscattering amplitude with cubic splines is described. Using these cubic spline, the analytical partial derivatives of the plane wave EXAFS function can be calculated. The use of analytical partial derivatives decreases the CPU time needed for a refinement by over 60% for a three shell system compared to a refinement with partial derivatives calculated with the finite difference method

The Influence of Metal-Support Interactions on the Whiteline Intensity

The whiteline intensity of Pt/K-LTL catalysts reduced at 300, 450, and 600 Deg decreases with increasing redn. temp. This change in whiteline intensity was ascribed to the removal of hydrogen from the metal-support interface by redn. at higher temp

Influence of Hydrogen Pretreatment on the Structure of the Metal-Support Interface in Pt/Zeolite Catalysts.

Platinum supported on H-LTL, K-LTL, and H-MAZ was reduced at temperatures from 573 to 873 K and the structure of the metal-support interface was determined by EXAFS. In all samples, the platinum was highly dispersed, with metal particle sizes from 5-1l atoms. After low temperature reduction (LTR, 573 K), the distance between the platinum atoms and the oxide atoms of the support (Pt-O distance) is 2.7 Å, which is significantly longer than the Pt-O distance of 2.2 Å observed after high temperature reduction (HTR, 873 K). At intermediate reduction temperatures both the long and the short Pt-O distances are observed. The shortening of the Pt-O distance, combined with the decrease in both the Debye-Waller factor and the inner potential shift, implies a stronger interaction between the platinum atoms and the oxide support. A structural model is proposed wherein the longer Pt-O distance results from the presence of hydrogen in the interfacial layer between the metal particle and the support. During high temperature reduction, the hydrogen is released from the interface, leaving platinum in direct contact with the support. The shortening of the Pt-O interfacial distance is accompanied by a decrease in the hydrogen chemisorption capacity, and may additionally be related to changes in the catalytic properties