Ever since the discovery of heterogeneous gold catalysts for low-temperature oxidation reactions, the mechanism of O2 dissociation on these materials has been controversial. We report Au L3-edge X-ray absorption near-edge structure (XANES) data for an active Au/TiO2 catalyst which indicate that fully reduced metallic gold particles on a reducible support (TiO2) form activated gold-oxygen complexes in the absence of CO. It is possible that these play a role in the mechanism of CO oxidation. These results were obtained with a highly active powder catalyst and the conclusions agree with those derived from model catalysts, for which it was shown that reduced gold is the active component in the oxidation of CO.

No abstract

The origin of CO oxidation performance variations between three different supported Au catalysts (Au/CeO 2 , Au/Al 2 O 3 , Au/ TiO 2 ) was examined by in situ XAFS and DRIFTS measurements. All samples were prepared identically, by depositionprecipitation of an aqueous Au(III) complex with urea, and contained the same gold loading ($1 wt %). The as-prepared supported Au(III) precursors exhibited different reduction behaviour during exposure to the CO/O 2 /He reaction mixture at 298 K. The reducibility of the Au(III) precursor was found to decrease as a function of the support material in the order: titania > ceria > alumina. The as-prepared samples were inactive catalysts, but Au/TiO 2 and Au/CeO 2 developed catalytic activity as the reduction of Au(III) to metallic Au proceeded. Au/Al 2 O 3 remained inactive. The developed catalytic CO oxidation activity at 298 K varied as a function of the support as follows: titania > ceria > alumina $ 0. The EXAFS of samples pretreated in air at 773 K and in H 2 at 573 K reveals the presence of only metallic particles for Au/TiO 2 and Au/Al 2 O 3 . Au(III) supported on CeO 2 remains unreduced after calcination, but reduces during the treatment with H 2 . CO oxidation experiments performed at 298 K with the activated samples show that the presence of metallic gold is necessary to obtain active catalysts (Au/CeO 2 is not active after calcination) and that the reducible supports facilitate the genesis of active catalysts, while metallic gold particles on alumina are not active.

High surface area (HS) A1F3 samples have been examined by X-ray photoelectron spectroscopy (XPS). The experimentally observed binding energy (BE) shifts were analysed by reference to core level BEs obtained from ab initio total energy calculations on a range of different, clean and hydroxylated alpha- and beta-A1F3 surfaces. Examination of the two components visible in the A1 2p emission indicates that surface A13+ sites can, depending on the local geometric structure, contribute to both a high BE peak at 77.0 eV and a low BE peak at 76.1 eV. Consequently, the areas under the peaks do not quantitatively correlate with surface area or Lewis acidity. However, a significant correlation between the number of surface A1 centres with dangling F or OH groups and the appearance of an A1 2p emission component at a BE lower than in the alpha-A1F3 bulk is predicted. The experimental F 1s emission data indicate that dangling F species are essentially absent. Examination of the O 1s emission suggests that HS A1F3 handled at room temperature under any practical laboratory conditions, including glovebox environments, probably contains intrinsically a significant amount of OH groups and adsorbed water, which results in the covering of A1F3 surfaces by dangling or bridging OH groups. These Bronsted acid species must be removed by treatment at higher temperature before HS A1F3 reagents can fully develop their Lewis acidity.

No abstract

The pretreatment conditions (temperature, medium) have a strong influence on the N2O decomposition activity of Fe−ZSM-5 catalysts (J. Jia et al., J. Catal. 2002, 210, 453). We used X-ray absorption spectroscopy to follow in situ the structural changes of the iron oxide clusters in an Fe−ZSM-5 sample, which was prepared by chemical vapor deposition of FeCl3, during different pretreatments and during N2O decomposition. Reductive pretreatment in H2 always decreased the Fe−Fe coordination number; i.e., it broke the iron oxide clusters into smaller units by removing bridging oxygen atoms. When reduction was carried out at low temperatures (673 K), this process was reversed during reoxidation by N2O. When reduction was carried out at high temperatures (873 K), the smaller fragments were stabilized by forming strong bonds to the zeolite matrix and did not agglomerate during reoxidation. An analysis of the Fe−O coordination numbers showed that temperatures above 673 K are required to dehydroxylate the iron oxide clusters. Reduction at 673 K did not significantly change the Fe−O coordination number because the oxygen remained adsorbed on the iron cluster in the form of H2O (or as a hydroxy group). This is an important result because the degree of (de)hydroxylation of the iron oxide clusters has implications on their catalytic activity. During N2O decomposition the catalyst was in a fully oxidized state. The EXAFS spectra of the samples before and after reaction with N2O were identical, indicating that only a very small concentration of sites, which is not detectable by EXAFS, is responsible for the N2O decomposition activity.

No abstract

A new spectroscopic cell has been designed for studying model catalysts using in situ or operando X-ray absorption spectroscopy. The setup allows gas treatment and can be used between 100 and 870 K. Pressures from 10(-3) Pa up to 300 kPa can be applied. Measurements on model systems in this particular pressure range are a valuable extension of the commonly used UHV characterization techniques. Using this setup, we were able to analyze the Au L3 EXAFS of a silica wafer covered with sub-monolayer concentrations of gold (0.05 ML). By modifying the sample holder, powder catalysts can also be analyzed under plug-flow conditions. As an example, the reduction of a Au/SiO2 powder catalyst prepared from HAuCl4 was followed.