H. Dexpert

The influence of polydispersity and inhomogeneity on EXAFS of bimetallic catalysts

The effect of polydispersity and inhomogeneity of supported bimetallic catalysts on the EXAFS analysis is investigated with some simple model calculations. These show that EXAFS is very insensitive to polydispersity. Polydispersity and inhomogeneous distribution of the metals over the particles however have only limited influence on the ability to distinguish between core-shell particles and particles with random distribution of both metals.

Bimetallic Reforming Catalysts: EXAFS Investigation of the Particle-Growing Process during the Reduction Step

Abstract EXAFS has been used to study the reduction which leads to the active species of PtRh and PtRe reforming catalysts. The first moments of the formation of the metallic cluster have been followed kinematically at the platinum absorption edge for two catalysts, PtRh/γ-Al 2 O 3 and PtRe/γ-Al 2 O 3 , using two different modes for the EXAFS setup (different synchrotron stations). The experiments have been made in situ , the sample being under flowing hydrogen at high temperature. The main result is the appearance of long metal-metal distances, of the same range order as that of the bond lengths existing in the alumina network. They occur solely for a very low number of metal atoms and are probably due to an intergrowth process with the carrier. The phenomenon seems to be general and disappears with the growth of the particle.

EXAFS studies of bimetallic PtReAl2O3 and PtRhAl2O3 catalysts in the first stages of preparation, drying, and calcination

Abstract EXAFS (extended X-ray absorption fine structure) measurements have been applied to highly dispersed bimetallic Pt-(Re or Rh)/alumina catalysts in the first stages of their preparation, drying and calcination. In order to understand the role played by the second metal, different (Re or Rh) Pt ratios have been considered. In both systems, as early as the drying step, we observed a large substitution of chlorine by oxygen atoms in the first coordination sphere of platinum, ex H 2 PtCl 6 . These oxygens are attributed to the alumina carrier and the detailed analysis of the absorption signal, in particular for the PtRh case, leads to the conclusion that anchoring sites of different strength exist at the surface of the carrier. Their respective relations with the various metals loadings and atomic ratios are discussed.

An X-ray absorption study of the reconstitution process of bovine Cu, Zn superoxide dismutase by Cu(I)-glutathione complex

The CU(I)GSH complex has recently been shown to be a good candidate for delivering copper to the active site of Cu‐free Cu,Zn superoxide dismutase both in vivo and in vitro. In this work X‐ray absorption spectroscopy has been used to characterize the CU(I)GSH complex and to follow in vitro the reconstitution of Cu,Zn superoxide dismutase from the copper‐free protein and this complex. The results obtained indicate that the copper is directly transferred as Cu(I) from the GSH complex into the empty copper binding site. No evidence has been obtained for a ternary complex in which the metal is bound to both GSH and the protein.

First experimental evidence of a zinc binuclear cluster in AlcR protein, mutational and X-ray absorption studies.

AlcR is the transcriptional activator of the ethanol utilization pathway in Aspergillus nidulans. The zinc DNA-binding domain contains ligands of zinc, six cysteines (Zn2Cys6) or five cysteines and one histidine (Zn2Cys5His). The utilisation of complementary approaches such as X-ray absorption spectroscopy, mutational analysis, zinc content evaluation, determination of specific binding connecting structural and biological data, have allowed to determine zinc environment and to analyse the involvement of amino acids. The determination by EXAFS of zinc ligands (four sulphur atoms), the Zn content in the protein (2:1), the evaluation of the distance between two zinc atoms (3.16 +/- 0.02 angstroms), together with the total loss of specific DNA-binding activity when one cysteine ligand is mutated, are in favour of a zinc cluster model in which six cysteine sulphurs ligate two zinc atoms. XANES spectra of wild type and H10A AlcR protein are virtually identical indicating that Histidine 10 does not have a direct contribution in zinc ligation but electrophoretic mobility shift assays show that His10 is involved in DNA-binding. In contrast, proline 25 does not seem to play any direct role in the DNA-binding activity but XANES spectra of Pro25A AlcR protein are slightly modified comparing to the wild type protein spectra. This suggests a role of the proline in the stabilisation of the Zn cluster structure. AlcR DNA-binding domain belongs to the zinc binuclear class family (Zn2Cys6) with unique characteristics resulting from its primary and secondary structures and its binding specificity toward direct and inverted repeat target.

Experimental set-up for in-situ X-ray absorption spectroscopy analysis of photochemical reactions: the photocatalytic reduction of gold on titania

Abstract An experimental set-up is described for in-situ absorption spectroscopy analysis of photochemical and photocatalytic reactions. It consists of a photochemical reactor connected to a peristaltic pump and an X-ray absorption transmission cell specially designed for liquid samples. The criteria for selecting the experimental conditions are discussed and tested by the study of the photocatalytic reduction of Auṫ+ cations on colloidal TiO2. The results show that it is possible to follow in real time the two-step mechanism of the photoreduction of noble metals on titania, i.e. the nucleation step and then the growth of the metallic crystallites.

In situ EXAFS study of the effect of hydrocarbon deposition over Pt/Al2O3 and Pt–Re/Al2O3 catalysts

The structural evolution of Pt/Al2O3 and Pt–Re/Al2O3 catalysts has been studied in situ by extended X-ray absorption fine structure (EXAFS) spectroscopy during the catalytic reforming of n-heptane at high pressures and temperatures. The results obtained for the monometallic catalyst show the formation of a platinum–carbon bond during the catalytic reaction at 733 K, without marked changes in the size of platinum particles. In contrast, in the case of Pt–Re/Al2O3, this new Pt–C bond is formed only at lower temperatures, and is accompanied by important changes in the structure of the metallic particles. These results show very clearly that the properties of platinum are modified by the presence of rhenium, and explain the higher stability of the bimetallic reforming catalysts in terms of the formation of a bimetallic phase, more resistant to deposition of well structured carbonaceous residues than the monometallic one.In addition, the results obtained at the Re LIII edge show the presence of long Re–O distances (2.20 A) in the reduced Pt–Re/Al2O3 catalyst, compatible with the existence of a rhenium phase incorporated into the alumina support. This phase is partially segregated to the surface and reduced after contact of the catalyst with an H2–hydrocarbon mixture.

X-ray absorption spectroscopy of the continuous change from Ce(IV) to Ce(III)

Abstract The fast acquisition method for X-ray Absorption Spectroscopy was used to obtain the Extended X-ray Absorption Fine Structure (EXAFS) and the X-ray Absorption Near Edge Structure (XANES) of the dynamical redox process in the Ce(IV)/Ce(III) chemical solutions system. From the XANES data, the relative concentration of both species was determined as a function of time and from the EXAFS data two different distances and two coordination numbers were found for the initial and final states, respectively. The analysis of the consecutive steps of the reaction indicates the absence of intermediate valence states of the cerium ions.

Dispersive XAS studies of the reduction kinetics of a highly dispersed supported catalyst

Abstract The reduction kinetics of a highly dispersed supported platinum catalyst were studied using dispersive x-ray absorption spectroscopy. A significant difference in the rate of reduction could be seen between the two temperatures studied (470K and 486K). Preliminary analysis of the data showed that the data could be best fit by two exponentials, indicating a two step reduction process.

EXAFS study of transition metal chlorides intercalated in graphite

Abstract The X-ray absorption spectroscopy (EXAFS) technique has been used for studying the local environment of the metal in graphite intercalation compounds with transition metal dichlorides. For these compounds the octahedral environment of the cation is preserved after the intercalation process. A simulation of the first coordination shell with a two metal-ligand distance distribution enables the characterization of a strong distortion of the octahedra. The study of the radial distribution function around the metal atoms gives evidence of the existence of a second coordination shell.