Pore structure, catalytic reaction performance

The CVD catalyst exhibits good catalytic performance for the selective oxidation/ammoxida-tion of propene as shown in Table 8.5. Propene is converted selectively to acrolein (major) and acrylonitrile (minor) in the presence of NH3, whereas cracking to CxHy and complete oxidation to C02 proceeds under the propene+02 reaction conditions without NH3. The difference is obvious. HZ has no catalytic activity for the selective oxidation. A conventional impregnation Re/HZ catalyst and a physically mixed Re/HZ catalyst are not selective for the reaction (Table 8.5). Note that NH3 opened a reaction path to convert propene to acrolein. Catalysts prepared by impregnation and physical mixing methods also catalyzed the reaction but the selectivity was much lower than that for the CVD catalyst. Other zeolites are much less effective as supports for ReOx species in the selective oxidation because active Re clusters cannot be produced effectively in the pores of those zeolites, probably owing to its inappropriate pore structure and acidity. 

When modeling phenomena within porous catalyst particles, one has to describe a number of simultaneous processes (i) multicomponent diffusion of reactants into and out of the pores of the catalyst support, (ii) adsorption of reactants on and desorption of products from catalytic/support surfaces, and (iii) catalytic reaction. A fundamental understanding of catalytic reactions, i.e., cleavage and formation of chemical bonds, can only be achieved with the aid of quantum mechanics and statistical physics. An important subproblem is the description of the porous structure of the support and its optimization with respect to minimum diffusion resistances leading to a higher catalyst performance. Another important subproblem is the nanoscale description of the nature of surfaces, surface phase transitions, and change of the bonds of adsorbed species. 

The catalytic reaction rate is limited by the intraparticle mass transfer rate. If the rate is relatively slow, both activity and selectivity are lowered. As a result, the support must have a low pore diffusional resistance (high effectiveness factor). For a given pore volume, the surface area and the strength of the support increase as the pore diameter decreases, and the pore diffusional resistance decreases as the pore diameter increases. Thus, an appropriate pore structure must be determined for the support to achieve optimal catalytic performance. 

Catalyst characterization tests include measurement of surface areas, chemisorption, pore-size distributions, crystal structure as determined by X-ray crystallography, reaction mechanisms as revealed by kinetics, and isotopic tracers and diagnostic catalytic reactions to test functional capabilities. These have been interpreted in terms of variation of catalyst preparation-structure-performance relationships. 

Different active components and the catalytic reactions have different requirements on the pore structures of activated carbon. Therefore, it should balance the proportion of the macroporous, mesopores and microporous by the concrete conditions in order to optimize the performances of catalyst. 

The specific surface area and pore structure are the most basic macroscopic physical properties of solid catalysts. Pore and surface are the reactive rooms of heterogeneous catalytic reactions, and the amount of surface area directly influences the level of catalytic activity. If the surface properties of catalyst are uniform, then their activity is directly proportional to their surface area. Catalytic reactions are generally influenced by the diffusion under industrial conditions, and the activity, selectivity and lifetime and almost all properties of catalyst are related to these two macroscopic physical properties. Although the activity for most catalysts is not proportional to their surface area, the surface area is still a visual physical quantity to evaluate catalyst performance, and sometimes acts as a control index of preparation. 

SEM and TEM images give detailed information about the porous structure of a supported heterogeneous catalyst (pore size distribution, typical sizes of the particles, etc.). The information from SEM and TEM images can be used in the reconstruction of porous catalytic medium. In the digitally reconstructed catalyst, transport (diffusion, permeation), adsorption, reaction, and combined reaction-diffusion processes can be simulated (Stepanek et al., 2001a). Parametric studies can be performed, and the resulting dependencies can serve as a feedback for the catalyst development. 

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