Supported catalysts, electronic state

Tin incorporated mesoporous Sn-MFI catalysts with different Si/Sn ratio using microwave were synthesized with carbon as hard template. These tin MFI catalysts were characterized using various physicochemical techniques XRD reviled the formation of more crystalline MFI structures which was further supported by the SEM and TEM imaging which clearly showed well ordered zeolite single crystals with mesoporosity. The N2 sorption isothers reviled the formation of bimodal mesoporous zeolites and the presence of tin in tetrahedral site was confirmed by FTIR (970 cm 1) and XPS (3ds/2 and 3 dj 2 electronic states). The thus synthesized mesoporous Sn-MFI catalysts with different Si/Sn ratios were used in studying the catalytic Baeyer-Villiger Oxidation (BVO) of cyclic ketones... 

As described above, XAS measurements can provide a wealth of information regarding the local structure and electronic state of the dispersed metal particles that form the active sites in low temperature fuel cell catalysts. The catalysts most widely studied using XAS have been Pt nanoparticles supported on high surface area carbon powders,2 -27,29,so,32,33,38-52 represented as Pt/C. The XAS literature related to Pt/C has been reviewed previ-ously. In this section of the review presented here, the Pt/C system will be used to illustrate the use of XAS in characterizing fuel cell catalysts. 

The catalysts used in low temperature fuel cells are usually based on small Pt particles dispersed on a carbon support with typical particle sizes in the range 1 — 10 nm in diameter. The XAS provides a measure of the average electronic state and local coordination of the absorbing atom, for example, Pt, on a per-atom basis, as described above. Thus, the XAS, for both the XANES and EXAFS regions, of such Pt/C catalysts reflects the size of the particles. 

In this paper the term electronics is used with a different connotation it stands for the description of electron transitions to and from, as well as within supported catalysts. It will be shown that in many cases such transitions are responsible for the enhanced or reduced catalytic activities of supported catalysis. In this concept, supported catalysts are described as solid state systems in which a catalytically active component (the... 

The electronic structure of a solid metal or semiconductor is described by the band theory that considers the possible energy states of delocalized electrons in the crystal lattice. An apparent difficulty for the application of band theory to solid state catalysis is that the theory describes the situation in an infinitely extended lattice whereas the catalytic process is located on an external crystal surface where the lattice ends. In attempting to develop a correlation between catalytic surface processes and the bulk electronic properties of catalysts as described by the band theory, the approach taken in the following pages will be to assume a correlation between bulk and surface electronic properties. For example, it is assumed that lack of electrons in the bulk results in empty orbitals in the surface conversely, excess electrons in the bulk should result in occupied orbitals in the surface (7). This principle gains strong support from the consistency of the description thus achieved. In the following, the principle will be applied to supported catalysts. 

A very clear-cut example for the influence of the electronic factor in supported catalysts, again involving a thin-layer metal type, is represented in Fig. 2. Here the carriers are commercially available samples of doped carborundum (SiC) which by itself is catalytically entirely inactive. In the abscissa of Fig. 2 we have arranged these samples in the order of their conductivity as stated by their manufacturers. The concentration of positive holes increases towards the right and that of the quasi-free electrons towards the left. Grains of these supports approximately 1 mm in size were covered with a thin layer of silver by the usual mirror produc-... 

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