Publications by authors named "Diek C Koningsberger"

The influence of changes in coordination number, interatomic distances, and oxidation state on the intensity and centroid position of the Fourier transform (FT) of the atomic X-ray absorption fine structure (AXAFS) peak of vanadium oxide bulk model compounds and alumina-supported vanadium oxide clusters has been investigated. Using Na3VO4 and V2O5 as model compounds, it has been shown that the nearest neighbour shells have a pronounced influence on the AXAFS intensity; specifically, a 40 % decrease in intensity was observed between these two compounds. Secondly, the influence of partial reduction of the vanadium oxide species has been determined; this led to a 50 % decrease in the AXAFS intensity and to an increase in the centroid position.

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The effect of the point of zero charge (PZC) of the support oxide (Al(2)O(3), Nb(2)O(5), SiO(2) and ZrO(2)) on the molecular structure of hydrated vanadium oxide species has been investigated with EXAFS spectroscopy for low-loaded vanadium oxide catalysts. It was found that the degree of clustering (i.e.

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The influence of the support oxide on the molecular structure of a VO(4) cluster and its interfacial geometry has been determined for SiO(2), Nb(2)O(5), and ZrO(2) as supports. Raman, IR, UV-vis-NIR diffuse reflectance, electron spin resonance, and extended X-ray absorption fine structure (EXAFS) spectroscopies were used to characterize the supported vanadium oxide clusters after dehydration. It has been found that for all supports under investigation the vanadium ion is tetrahedral coordinated and consists of one V=O and three V-O bonds.

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X-ray absorption spectroscopy at the Cr K- and L(2,3)-edges was used to study the assembling process of a heterogeneous Cr-based single-site catalyst. The starting point was a Phillips-type system with monochromate species anchored on a silica surface, which was first reduced to a variety of different surface Cr(II) species. The reduced sample was modified with a 1,3,5-tribenzylhexahydro 1,3,5-triazine (TAC) ligand in the presence of CH(2)Cl(2) as solvent to yield a heterogeneous single-site Cr-based catalyst active in the trimerization of ethylene.

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Atomic XAFS is a very attractive technique for probing electronic properties of supported metal nanoclusters. For platinum nanoparticles on different supports, the technique is found to be in good agreement with infrared CO adsorption measurements. The advantages of AXAFS, however, are that no probe molecule is required and that real-time measurements under reaction conditions are possible.

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Pd K-edge X-ray absorption near-edge spectroscopy (XANES) is used to probe the unoccupied molecular orbitals in bidentate diphosphine Pd complexes. Complexes containing a series of bidentate diphosphine ligands (PP) are examined to study the effect of the ligand bite angle on the charge redistribution in these complexes. Different coordinating moieties (XX) have been used to induce a range of Pd oxidation states.

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Atomic XAFS on [PtCl(NCN)-Z] pincer complexes shows it to be a sensitive probe for the determination of the electron density on the metal atom, similar yet complementary to NMR.

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Application of in situ X-ray absorption near-edge spectroscopy (XANES) at the Al K-edge provides unique insight into the flexibilty of the aluminum coordinations in zeolites as a function of treatment or during true reaction conditions. A unique, previously not observed, pre-edge feature is detected in zeolites H-Mordenite and steamed and unsteamed H-Beta at temperatures above 675 K. Spectra simulations using the full multiple scattering code Feff8 identify the unique pre-edge feature as three-coordinate aluminum.

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UV-Vis, combined with ED-XAFS shows, for the first time, the evolution of inactive Pd dimers and trimers, that are a possible first stage in the deactivation process of important palladium catalysed reactions, leading to larger palladium clusters and eventually palladium black.

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X-ray absorption fine structure (XAFS) spectroscopy is used to unravel the structure of homogeneous catalysts in their catalytically active phase (solution), for which other characterization techniques failed in providing detailed structural information. Application of in situ EXAFS shows that the conformation of the allylic fragment of (P-P)Pd(1,1-dimethylallyl) catalytic intermediate complexes in solution (i.e.

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