Supported metal catalysts diffraction

COHEN ET AL. Diffraction from Supported Metal Catalysts... 

HREM methods are powerful in the study of nanometre-sized metal particles dispersed on ceramic oxides or any other suitable substrate. In many catalytic processes employing supported metallic catalysts, it has been established that the catalytic properties of some structure-sensitive catalysts are enhanced with a decrease in particle size. For example, the rate of CO decomposition on Pd/mica is shown to increase five-fold when the Pd particle sizes are reduced from 5 to 2 nm. A similar size dependence has been observed for Ni/mica. It is, therefore, necessary to observe the particles at very high resolution, coupled with a small-probe high-precision micro- or nanocomposition analysis and micro- or nanodiffraction where possible. Advanced FE-(S)TEM instruments are particularly effective for composition analysis and diffraction on the nanoscale. ED patterns from particles of diameter of 1 nm or less are now possible. 

Figure 2. Use of diffraction peak widths and areas to determine average Pt crystallite size and weight percent Pt in a supported metal catalyst.

For supported metal catalysts, no simple calculation is possible. A direct measurement of the metal crystallite size or a titration of surface metal atoms is required (see Example 1.3.1). TWo common methods to estimate the size of supported crystallites are transmission electron microscopy and X-ray diffraction line broadening analysis. Transmission electron microscopy is excellent for imaging the crystallites, as illustrated in Figure 5.1.5. However, depending on the contrast difference with the support, very small crystallites may not be detected. X-ray diffraction is usually ineffective for estimating the size of very small particles, smaller than about 2 nm. Perhaps the most common method for measuring the number density of exposed metal atoms is selective chemisorption of a probe molecule like H2, CO, or O2. 

The metal in a supported metal catalyst may be characterized by electron microscopy (providing a distribution of crystallite sizes), X-ray diffraction line broadening (providing an average size for crystallites larger than about 4 nm), and specific chemisorption (titration) with compounds such as H2 or CO [providing an estimate of the exposed metal area (v5. the total surface area of metal plus support)]. 

Various approaches have been described in the literature for the characterization of carbon-supported metal catalysts. The catalysts are usually analyzed before and after (postmortem) their electrochemical operation with conventional ex situ techniques such as x-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive x-ray analysis (EDX), x-ray photoelectron spectroscopy (XPS), and x-ray absorption spectroscopy (XAS). Although ex situ analysis provides an important starting point in catalyst characterization, one must keep in mind that significant morphological changes may occur under the operational conditions. It is thus vitally important... 

The present work is concerned with the influence of the pretreatment of an alumina-aerogel supported Pd catalyst on its activity in propane oxidation. The state of Pd and the crystalline state of alumina aerogel were investigated by grazing-incidence X-ray diffraction (GIXD). To our knowledge, this method is applied for the first time to supported metal catalysts. In addition, a comparison of the structural and catalytic oxidation properties of Pd, Pt and Pt-Rh supported on alumina carriers was made. 

The intensity of the diffracted radiation is related to the square of the atomic number and hence for the case of supported metal catalysts, the lower limit of sensitivity is likely to fall around 0.3 to 0.8 wt% loading. Additionally, once the crystallite size falls below ca. 2-4 nm, the diffraction line becomes so broad and... 

The performance of a supported metal or metal sulfide catalyst depends on the details of its preparation and pretreatraent. For petroleum refining applications, these catalysts are activated by reduction and/or sulfidation of an oxide precursor. The amount of the catalytic component converted to the active ase cind the dispersion of the active component are important factors in determining the catalytic performance of these materials. This investigation examines the process of reduction and sulfidation on unsupported 00 04 and silica-supported CO3O4 catalysts with different C03O4 dispersions. The C03O4 particle sizes were determined with electron microscopy. X-ray diffraction (XRD), emd... 

The title Spectroscopy in Catalysis is attractively compact but not quite precise. The book also introduces microscopy, diffraction and temperature programmed reaction methods, as these are important tools in the characterization of catalysts. As to applications, I have limited myself to supported metals, oxides, sulfides and metal single crystals. Zeolites, as well as techniques such as nuclear magnetic resonance and electron spin resonance have been left out, mainly because the author has little personal experience with these subjects. Catalysis in the year 2000 would not be what it is without surface science. Hence, techniques that are applicable to study the surfaces of single crystals or metal foils used to model catalytic surfaces, have been included. 

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