Catalytic properties, porosity effects

High r factors are, however, not without some other complications since they imply porosity of materials. Porosity can lead to the following difficulties (a) impediment to disengagement of evolved gases or of diffusion of elec-trochemically consumable gases (as in fuel-cell electrodes 7i2) (b) expulsion of electrolyte from pores on gas evolution and (c) internal current distribution effects associated with pore resistance or interparticle resistance effects that can lead to anomalously high Tafel slopes (132, 477) and (d) difficulties in the use of impedance measurements for characterizing adsorption and the double-layer capacitance behavior of such materials. On the other hand, it is possible that finely porous materials, such as Raney nickels, can develop special catalytic properties associated with small atomic metal cluster structures, as known from the unusual catalytic activities of such synthetically produced polyatomic metal clusters (133). 

A short review has been published by Tsitsishvili in which the structures of porous zeolites, their porosities, surfaces, types of lattice, and the effect of chemical modification on their adsorption, c omatographic, spectroscopic, and catalytic properties have been examined. The effect of the nature of the cations on the properties of the zeolites has also been discussed. 

Porosity Effects on Catalytic Properties Naturai, Periodic, and imprinted Porosities 967... 

The effective diffusivity Dn decreases rapidly as carbon number increases. The readsorption rate constant kr n depends on the intrinsic chemistry of the catalytic site and on experimental conditions but not on chain size. The rest of the equation contains only structural catalyst properties pellet size (L), porosity (e), active site density (0), and pore radius (Rp). High values of the Damkohler number lead to transport-enhanced a-olefin readsorption and chain initiation. The structural parameters in the Damkohler number account for two phenomena that control the extent of an intrapellet secondary reaction the intrapellet residence time of a-olefins and the number of readsorption sites (0) that they encounter as they diffuse through a catalyst particle. For example, high site densities can compensate for low catalyst surface areas, small pellets, and large pores by increasing the probability of readsorption even at short residence times. This is the case, for example, for unsupported Ru, Co, and Fe powders. 

The dealumination process is associated with a change in the porosity within the crystals and may sometimes cause a drastic loss of crystallinity. The microporous adsorbents of the faujasite type are so arranged that the Si/Al ratio increases as the munber of cations and the average electrostatic field within the framework decrease. To assess the effect of the Si/Al ratio on the activity and acidity of Y zeolites, it is desirable to compare samples with similar extents of exchange, since the degree of exchange has a significant influence on the catalytic and acidic properties of faujasites. 

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