Catalyst surface, accessibility heterogeneity

Heterogeneous catalysis is a surface phenomenon, therefore the overall kinetic parameters are dependent on the real exposed catalyst surface area. In the supported systems only a part of the photocatalyst is accessible to light and to substrate. Besides, the immobilized catalyst suffers from the surface deactivation since the support could enhance the recombination of photogenerated electron-hole pairs and a limitation of oxygen diffusion in the deeper layers is observed. 

Textural properties are important in the field of catalyst design for heterogeneous catalysis Surface area and pore size determine the accessibility to active sites and this is often related to catalytic activity and selectivity in catalysed reactions Therefore textural properties are often a target of catalyst design. 

The coordination environment around titanium in TiCla (in the solid-state) is six, i.e., each titanium is surrounded by six chlorines. The chlorines are involved as intermolecular bridging ligands between adjacent titanium centers. It is important to realize that this is the solid-state structure and the catalyst system is heterogeneous. Accordingly, only the surface sites will be active and bulk of the material will be not accessible to the reaction. It is reasonable to propose that at the surface several titanium cen-... 

Dispersed catalysts are defined as heterogeneous catalysts flowing alrnig with the reactants in a reactor system. The residence time of catalyst and reactants are thus equivalent and consequently, a continuous renewal of the catalyst-reactant interface is afforded. They demonstrate their usefulness in case of competitive fast deactivation or when accessibility of reactants to catalyst surface is hampered by feed characteristics. This kind of situation typically correi nds to heavy feeds processing, and effectively, the use of dispersed catalysts is practically limited to coal liquefaction and petroleum residues conversion. 

Through such chemisorption studies, the values of >, have been determined not only by geometric accessibility, but also by the chemical heterogeneity of the surface. This can result in abnormal values of D, and demonstrates the scale effect on the kinetics and selectivity of catalytic reactions. For such studies, Farin and Anvir [213] derived the equations that can be applied for characterization of supported catalysts ... 

The hydrogenation of unsaturated polymers like polyisoprene is based on the mobility of a soluble catalyst in the reaction medium. In the hydrogenation of such unsaturated polymers the soluble catalyst brings its active site to the C=C bonds in the polymer chain. In contrast, a heterogeneous catalyst requires that the polymer chain unfold to gain access to a catalytically active site on the surface of a metal particle. 

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