M Vaarkamp

A New Method for Parameterization of Phase Shift and Backscattering Amplitude

Abstract Parameterization of phase and backscattering amplitude with cubic splines is described. Using the cubic spline, the analytical partial derivatives of the plane wave EXAFS function can be calcalated. 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 derivaties calculated with the finite difference method.

Sulfur Poisoning of a Pt/BaK-LTL Catalyst: A Catalytic and Structural Study Using Hydrogen Chemisorption and X-ray Absorption Spectroscopy

The sulfur poisoning of a Pt/BaK-LTL catalyst has been studied with X-ray absorption spectroscopy and hydrogen chemisorption. The fresh catalyst contained highly dispersed platinum inside the zeolite pores. EXAFS analysis determined a Pt-Pt coordination number of 3.7, suggesting an average platinum cluster size of 5 or 6 atoms, consistent with the TEM and chemisorption data (H/Pt = 1.4). The catalyst was poisoned with H2S until the dehydrocyclization activity of n-hexane decreased to 30% of fresh activity. The first-shell PtPt coordination number increased to 5.5, indicating a growth of the average platinum cluster size to 13 atoms. Hydrogen chemisorption measurements of the poisoned catalyst show a decrease in the H/Pt value to 1.0. The EXAFS data also provide evidence for the presence of sulfur adsorbed on the surface of the platinum particles with a PtS bond distance of 2.27 A. The high sensitivity of the Pt/LTL catalyst to poisoning by very low levels of sulfur is attributed to the loss of active platinum surface by adsorption of sulfur and the growth of the platinum clusters. Much of the available platinum surface was found to be capable of chemisorbing hydrogen, but with no activity for dehydrocyclization. Growth of the platinum particle was sufficient to block the pore. In the sulfur-poisoned catalyst, only the sulfur-free platinum atoms exposed through the pore windows remain active. The evidence suggests the location of the sulfur was at or near the metal-zeolite interface. Since both high activity and selectivity require extremely small platinum particles, regeneration of sulfur-poisoned catalysts will require removal of the adsorpted sulfur and restoration of the original particle size.

Pt clusters in BaKL zeolite : characterization by transmission electron microscopy, hydrogen chemisorption, and X-ray absorption spectroscopy

Platinum supported on BaKL zeolite was characterized by Transmission Electron Microscopy (TEM), hydrogen chemisorption, and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. The results of all three techniques indicate the presence of highly dispersed platinum in the zeolite pores. There is no evidence of platinum outside the zeolite pores. The EXAFS data determine a Pt-Pt coordination number of 3.7, suggesting that the average platinum cluster in the zeolite consists of 5 or 6 atoms, consistent with the TEM and chemisorption data. The EXAFS data also provide evidence of the platinum-zeolite interface, indicated by Pt-O contributions at 2.14 and 2.70 Å, and a Pt-Ba contribution at 3.8 Å. The Pt/BaKL zeolite is one of the most highly dispersed supported platinum samples and one of the most structurally uniform supported metal catalysts.

The Relation between Catalytic and Electronic Properties of Supported Platinum Catalysts: The Local Density of States as Determined by X-Ray Absorption Spectroscopy.

Abstract The intensity of the white line of the LII and LIII X-ray absorption edge spectra of small platinum particles increases with decreasing particle size. The combined white line intensity of the LII and LIII X-ray absorption spectra for platinum catalysts with comparable average particle size is higher when dispersed on acidic than on neutral supports. This indicates that platinum particles are more electron deficient on acidic than on neutral supports. The propane hydrogenolysis TOF for platinum supported on γ-Al2O3 or H-LTL is found to be more than an order of magnitude higher than for platinum supported on a non-acidic K-LTL zeolite. The differences in catalytic behavior are related to differences in the d-band density of states.

The Influence of Metal-Support Interactions on the Whiteline Intensity

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

Influence of the reduction temperature on the structure of the metal particles and the metal-support interface of Pt/γ-Al2O3 catalysts

The structure of the metal particles and the metal-support interface of a Pt/γ-Al2O3 catalyst was determined by EXAFS after low temperature reduction (LTR: 300°C) and high temperature reduction (HTR:450°C). The clusters have excellent thermal stability as the particle size remains 12 atoms per cluster upon increasing the reduction temperature from 300°C to 450°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.

The influence of hydrogen pretreatment on the structural and catalytic properties of a Pt/K-LTL catalyst

ABSTRACT A platinum, non–acidic K–LTL catalyst, reduced at 300, 450, and 600°C, was characterized by extended X–ray absorption spectroscopy (EXAFS), hydrogen chemisorption, hydrogen temperature programmed desorption (H2–TPD) and methylcyclopentane hydrogenolysis. Reduction at 300°C produces small platinum crystallites with an interfacial layer of hydrogen. Reduction at 450°C increases the particle size, releasing interfacial hydrogen. This irreversible hydrogen desorption is observed in the TPD around 300°C. Reduction at 600°C results in further growth of the platinum cluster with the loss of the remaining interfacial hydrogen. In the TPD, a second high temperature H2 desorption is observed at around 610°C. Because of the confined space within the zeolite pore, as the platinum particle size approaches the pore size, there is a reduction in hydrogen chemisorption capacity and catalytic activity compared to a particle of equivalent size on an amorphous support.