Small metal particles EXAFS

The characteristics of reforming catalysts make them one of the most frequent cases where EXAFS is used. The very small metallic particles cannot be detected in transmission electron microscopy the long range order required for XRD analysis is absent, and the low metal contents make XPS analysis difficult. The observation of XANES structures at Pi and Re thresholds can be used to determine the electronic state. Some examples of reference compounds are shown in Figure 11.10. In particular a sharp peak (called a white line for historical reasons) is visible at the edge with an intensity related to the oxidisation state of the element. This peak in the absorption coefficient is a consequence of the existence of the empty electron states close to zero binding energy. 

Figure 6.20 shows an example in which QEXAFS has been used in combination with XRD to study the temperature programmed reduction of copper oxide in a Cu/ZnO/Al203 catalyst for the synthesis of methanol [43,44]. Reduction to copper metal takes place in a narrow temperature window of 430-440 K, and is clearly revealed by both the EXAFS pattern and the appearance of the (111) reflection of metallic copper in the XRD spectra. Note that the QEXAFS detects the metallic copper at a slightly lower temperature than the XRD does, indicating that the first copper metal particles that form are too small to be detected by XRD, which requires a certain extent of long range order [43,44],... 

A central question with respect to supported metal catalysts is that of the structure of the metal-support interface. Various possibilities have been proposed, varying from interfaces consisting of a mixed metal aluminate or silicate layer [17] or the presence of metal ions which serve as anchors between particle and support [18] to the attractive interaction between ions of the support and the dipoles that these ions induce in the metal particle [19]. EXAFS highlights the atomic surroundings of an atom in the catalyst, and if the supported metal particles are sufficiently small, oxygen atoms in the metal-support interface give a measurable contribution to the EXAFS spectrum. 

Catalysts prepared from iridium neutral binary carbonyl compounds and several supports have been studied extensively. Small Ir (x = 4, 6) clusters supported on several oxides and caged in zeolite, and their characterization by EXAFS, have been prepared [159, 179, 180, 194-196]. The nuclearity of the resulting metallic clusters has been related with their catalytic behavior in olefin hydrogenation reactions [197]. This reaction is structure insensitive, which means that the rate of the reac-hon does not depend on the size of the metallic particle. Usually, the metallic parhcles are larger than 1 nm and consequently they have bulk-like metallic behavior. However, if the size of the particles is small enough to lose their bulk-like metallic behavior, the rate of the catalytic reaction can depend on the size of the metal cluster frame used as catalyst. 

The characterization of zeolite-entrapped metal clusters by chemisorption of hydrogen or carbon monoxide provides valuable information, requiring, however, careful interpretation. When metal particles in zeolites approach monoatomic dispersion, the ratio of chemisorbed hydrogen to metal (H/M) fails to provide reliable information on metal dispersion. A first convincing example that the H/M ratio can actually decrease with increasing metal dispersion has been provided by determining the H/M ratio and the EXAFS profiles of reduced Pd/NaY and Pd/HY. It was found that the H/M ratio in Pd/HY is significantly smaller than that in Pd/NaY, particularly for small Pd particles when reduction was performed at temperatures below 500°C... 

Structure-sensitive reactions are extensively discussed in the catalytic literature, but careful examination of the published work reveals that on the atomic scale the catalytic materials used in these studies are in general poorly characterized with respect to particle size and structure. Extended X-ray absorption fine structure (EXAFS) has been successfully applied to the study of small particles on supportsand small metal molecules in matrices subject to the caveat that samples of these materials consist of a distribution of particle sizes. Information thus obtained is an average over the entire distribution. Supported, monosized clusters have not yet been used in catalytic studies. However, Woste and coworkers demonstrated in the first experiment where monosized clusters were deposited that Ag4 is the critical cluster... 

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