Surface phenomena catalyst

The physical meaning of the parameter 2FNG/I is obvious It expresses the time required to form a monolayer of oxide ions on a surface with NG adsorption sites when the oxide ions are supplied at a rate I/2F. This proves that NEMCA is a surface phenomenon (not a bulk phenomenon and not a phenomenon at the tpb) taking place over the entire gas-exposed catalyst electrode surface. 

The importance of catalyst stability is often underestimated not only in academia but also in many sectors of industry, notably in the fine chemicals industry, where high selectivities are the main objective (1). Catalyst deactivation is inevitable, but it can be retarded and some of its consequences avoided (2). Deactivation itself is a complex phenomenon. For instance, active sites might be poisoned by feed impurities, reactants, intermediates and products (3). Other causes of catalyst deactivation are particle sintering, metal and support leaching, attrition and deposition of inactive materials on the catalyst surface (4). Catalyst poisons are usually substances, whose interaction with the active surface sites is very strong and irreversible, whereas inhibitors generally weakly and reversibly adsorb on the catalyst surface. Selective poisons are sometimes used intentionally to adjust the selectivity of a particular reaction (2). 

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. 

Catalysts may be porous pellets, usually cylindrical or spherical in shape, ranging from 0.16 to 1.27 cm (Vm to Vi in) in diameter. Small sizes are recommended, but the pressure drop through the reactor increases. Among other shapes are honeycombs, ribbons, and wire mesh. Since catalysis is a surface phenomenon, a physical property of these particles is that the internal pore surface is nearly infinitely greater than the outside surface. 

This phenomenon is the same for ail types of geometric shapes of catalysts, monoliths, pellets or nets, In a monolith the mass transfer, between gas bulk and the outer surface of catalyst, is not particular good since the flow, at least in the boundary layers, tends to become laminar, but the pressure drop is low. In a packed bed with pellets the mass transfer is very good in general, but the pressure drop is high. A stack of nets, however, combine good mass transfer and low pressure drop, it is in between a monolith and a packed bed. 

More generally, if this structural modification is a common phenomenon, it is liable to occur during all catalytic reactions, even at moderate temperatures, at the surface of catalysts which have not been sintered at high temperatures. A study of the mechanism of decomposition of nitrous oxide at 250° on a divided nickel oxide [NiO(250°)J, during successive runs, is particularly interesting in this respect. 

Catalytic activity is essentially a surface phenomenon -201) hence particularly sensitive to the presence of impurities on the surface, which are often responsible for poisoning or for modifications in the catalytic selectivity The presence of sulfur on a metallic catalyst may induce rearrangement in the surface structure resulting in modification of the electronic distiibution mostly by dative bond formation, and, for higher coverages, in strong adatom-adatom interaction. 

The first stage is the formation of the active center, illustrated here using a-TiClj as the catalyst. The suggestion is that alkylation of the 5-coordinated TP+ ion takes place by an exchange mechanism after chemisorption of the aluminum alkyl on the surface of the TiClj crystal. The four chloride ions remaining are the ones firmly embedded on the lattice, and the vacant site is now ready to accommodate the incoming monomer unit. The reaction is confined to the crystal surface, and the active complex is purely a surface phenomenon in heterogeneous systems. 

Adsorption equilibria are normally considered in connection with processes occurring in porous media filters, catalysts, adsorbents, chromatographs, and rock of petroleum reservoirs. In macroporous and mesoporous media, adsorption is normally accompanied by another surface phenomenon, the capillary condensation. These two types of surface phenomena are closely connected because they are both produced by surface forces. On the other hand, these phenomena are relatively independent and may, to some extent, be discussed separately [3]. Moreover, the description of the coexistence of the adsorbed films and capillary condensate in the same capillary is a nontrivial problem. We present capillary condensation and adsorption separately, although their eommon roots are discussed in Section II. The (more or less) comprehensive description of the thermodynamics of multicomponent capillary condensation... 

In previous papers we demonstrated that the same nitridation approach can be also successfully applied for the incorporation of nitrogen into the framework of different Y zeolites, this making possible the preparation of porous basic catalysts active in the Knoevenagel condensation reaction [13-14]. The present work was undertaken in order to understand the modifications induced by nitridation and to provide a picture of the chemical rearrangements that occur upon nitrogen incorporation into ultrastable Y zeolite (Si/Al ratio of 13). Since catalysis is a surface phenomenon we choose to characterize the local environments of nitrogen, silicon and aluminum by X-ray photoelectron spectroscopy (XPS). A clear identification of these modifications is essential to allow a control of the preparation parameters for more efficient basic catalysts. 

Catalysts commonly lose activity in operation as a result of accumulation of materials from the reactant stream. Catalyst poisoning is a chemical phenomenon, A catalyst poison is a component such as a feed impurity that as a result of chemisorption, even in smaH amounts, causes the catalyst to lose a substantial fraction of its activity. For example, sulfur compounds in trace amounts poison metal catalysts. Arsenic and phosphoms compounds are also poisons for a number of catalysts. Sometimes the catalyst surface has such a strong affinity for a poison that it scavenges it with a high efficiency. The... 

Preferential deposition of weaMy adsorbed dissolved components on the outer surface of the catalyst body is a similar phenomenon but can occur after impregnation during removal of the solvent. When the impregnated particle is heated, the Hquid phase expands and coats the exterior surfaces with dissolved species as the solvent evaporates. In particularly severe cases, the entire outer surface of the catalyst can become completely coated with a soHd that blocks access to the underlying pore stmcture. 

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