Metal-support interactions medium

Metal-support interactions can be defined as being weak, medium or strong.28 Non-reducible metal oxides such as silica, alumina, and magnesia as well as carbon or graphite are considered to exert only a weak influence on the metal and, thus, exhibit only a weak metal-support interaction (WMSI). Zeolites (Chapters 10 and 13) exert a medium metal-support interaction (MMS1)28 while metals supported on reducible oxides when reduced at high temperatures exhibit a strong metal-support interaction (SMSI).27-32... 

Lopez et al. [27] prepared Pd/SiC>2 catalysts under both acidic (pH = 3) and basic (pH = 9) conditions in the sol-gel step and reported that an acid medium promotes the formation of small metal crystallites. This finding is consistent with the formation of a micro-porous silica gel network at a low pH. By comparing samples prepared by the sol-gel method and impregnation, these authors found in the former a stronger metal-support interaction which they ascribed to the square planar palladium complex used as a precursor. Finally, their results showed that the method of preparation as well as the conditions used in each method impact on how these catalysts deactivate in the hydrogenation of phenylacetylene. 

The concept of mechanical fixation of metal on carbon makes catalytic applications at high temperatures possible. These applications require medium-sized active particles because particles below 2nm in size are not sufficiently stabilised by mechanical fixation and do not survive the high temperature treatment required by the selective etching. Typical reactions which have been studied in detail are ammonia synthesis [195, 201-203] and CO hydrogenation [204-207]. The idea that the inert carbon support could remove all problems associated with the reactivity of products with acid sites on oxides was tested, with the hope that a thermally wellconducting catalyst lacking strong-metal support interactions, as on oxide supports, would result. 

This type of co-catalyhc influence is well known in heterogeneous catalysis, in which for some reachons an acidic support will achvate a metal catalyst more efficiently than a neutral support In this respect, the acidic ionic liquid can be considered as a liquid acidic support for the transihon metal catalysts dissolved in it As one would expect, in those cases in which the ionic liquid acts as a co-catalyst, the nature of the ionic liquid becomes very important for the reactivity of the transihon metal complex. The opportunity to opdmize the ionic medium used, by vari-ahon of the halide salt, the Lewis acid, and the ratio of the two components forming the ionic liquid, opens up enormous potential for ophmizahon. However, the choice of these parameters may be restricted by some possible incompahbilihes with the feedstock used. Undesired side reachons caused by the Lewis acidity of the ionic Hquid or by strong interaction between the Lewis acidic ionic Hquid and, for example, some oxygen funchonalihes in the substrate have to be considered. 

For the detailed study of reaction-transport interactions in the porous catalytic layer, the spatially 3D model computer-reconstructed washcoat section can be employed (Koci et al., 2006, 2007a). The structure of porous catalyst support is controlled in the course of washcoat preparation on two levels (i) the level of macropores, influenced by mixing of wet supporting material particles with different sizes followed by specific thermal treatment and (ii) the level of meso-/ micropores, determined by the internal nanostructure of the used materials (e.g. alumina, zeolites) and sizes of noble metal crystallites. Information about the porous structure (pore size distribution, typical sizes of particles, etc.) on the micro- and nanoscale levels can be obtained from scanning electron microscopy (SEM), transmission electron microscopy ( ), or other high-resolution imaging techniques in combination with mercury porosimetry and BET adsorption isotherm data. This information can be used in computer reconstruction of porous catalytic medium. In the reconstructed catalyst, transport (diffusion, permeation, heat conduction) and combined reaction-transport processes can be simulated on detailed level (Kosek et al., 2005). 

The current results provide no support for either mechanism by which the metal ion impact on inhibition could arise from direct interactions between the metal ion and the inhibitors. The results reported here used medium-sized basis... 

The agreement between the experimental results and the calculated spectra demonstrates that Maxwell-Garnett effective medium theory effectively accounts for such dipole-dipole interactions. Similarly, the reflectance properties of the films are also affected by the interparticle spacing. This is supported by the data shown in Fig. 3, where the specular reflectance for films with different interparticle separation are compared to the spectra calculated by means of Eqs. (11), (18) and (19) [13]. Again, as the particles approach each other, the reflectance red-shifts and broadens. The effect is not as dramatic as seen for absorption, but the film changes from a green reflected hue to a metallic gold as (j) increases. 

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