Electronics of supported catalysts

Electronics of Supported Catalysts Georg-Maria Schwab The Effect of a Magnetic Field on the Catalyzed Nondissocitive Parahydrogen Conversion Rate P. W. Selwood... 

Electronics of Supported Catalysts Georg-Maria Schwab... 

Atomic Number Imaging of Supported Catalyst Particles by Scanning Transmission Electron Microscope... 

Ideally, we would like to study the structure and composition of supported, dispersed catalyst particles in the same configuration used in the chemical technology. However, the determination of the atomic surface structure of the catalyst particle that is situated inside the pores of the high-surface-area support by LEED, for example, is not possible. This technique requires the presence of ordered domains 200 A or larger to obtain the sharp diffraction features necessary to define the surface structure. Even Auger electron emission, which is the property of individual atoms and can even be obtained from liquid surfaces, can only be employed for studies of supported catalyst surfaces with difficulty. Identification of the active sites does require the determination of the structure and composition of the catalyst surface, however. To avoid the difficulties of carrying out these experiments on supported... 

The term electronics normally refers to the theory and function of the electronic devices and circuits used so universally for measurement, control, and computation. Conventional electronics is an important tool also in the study of supported catalysts. 

The electronic structure of a metal particle may be affected by perturbations in its environment (gases, carrier, neighboring supported material) and is larger for smaller particles. In the case of supported catalysts the latter effect is termed the support effect. Edge spectroscopy is a sensitive tool for detecting charge transfer between metal and support or adsorbates. Pure metal-support effects are difficult to observe, since these are frequently perturbed by size and electronic effects. 

The use of a synthetic model system has provided valuable mechanistic insights into the molecular catalytic mechanism of P-450. Groves et al. [34]. were the first to report cytochrome P-450-type activity in a model system comprising iron meso-tetraphenylporphyrin chloride [(TPP)FeCl] and iodosylbenzene (PhIO) as an oxidant which can oxidize the Fe porphyrin directly to [(TPP)Fe =0] + in a shunt pathway. Thus, (TPP)FeCl and other metalloporphyrins can catalyze the monooxygenation of a variety of substrates by PhIO [35-40], hypochlorite salts [41, 42], p-cyano-A, A -dimethylanihne A -oxide [43-46], percarboxylic acids [47-50] and hydroperoxides [51, 52]. Catalytic activity was, however, rapidly reduced because of the destruction of the metalloporphyrin during the catalytic cycle [34-52]. When (TPP)FeCl was immobilized on the surface of silica or silica-alumina, catalytic reactivity and catalytic lifetime both increased significantly [53]. There have been several reports of supported catalysts based on such metalloporphyrins adsorbed or covalently bound to polymers [54-56]. Catalyst lifetime was also significantly improved by use of iron porphyrins such as mew-tetramesitylporphyrin chloride [(TMP)FeCl] and iron mcA o-tetrakis(2,3,4,5,6-pentafluorophenyl)por-phyrin chloride [(TPFPP)FeCl], which resist oxidative destruction, because of steric and electronic effects and thereby act as efficient catalysts of P-450 type reactions [57-65]. 

A wide variety of other techniques are available for the characterization of supported catalyst systems including X-ray absorption fine structure (EXAFS), Mossbauer, Auger electron. X-ray, and u.v. spectroscopies, magnetic susceptibilities, electron spin resonance spectroscopy, and transmission electron microscopy. However these techniques have not been employed to any significant effect. 

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