Catalyst/catalytic activity/center/properties

Altliough activated adsorjition does not necessarily precede catalysis, the investigation of the activated adsorption as well as the investigation of adsorption by means of IR spectra and of magnetic properties of chemisorbed molecules, must be important for catalysis. Of no less importance is the study of bulk chemical compounds which are similar to the supposed surface compounds, for example, the alcoholates in the dehydration of alcohols. From this point of view, one should study the properties of nitrides, carbides, hydrides, and other similar compounds. These studies are necessary because they permit one to make a judgment of the chemical forces which are displayed under conditions similar to catalysis. It should be borne in mind, however, that this evidence is indirect, as it refers not to the catalytically active centers themselves but to the surface, which is much larger they refer not to the activated complex, but to more stable compounds formed with the help of a catalyst. 

As various irregularities of crystals lattice are discussed above, it is now necessary to analyze their effect and importance in heterogeneous catalysis. There are at least two facts confirming the relation of crystal irregularity with the catalytic active center on catalyst surface. One of them is on those sites where the dislocations and surface point defects occur, and the atomic arrangements would differ from the others sites in catalyst surface, while surface atomic space and the properties of stereochemistry would remain the important factors to decide the catalytic activity. 

Unfortunately, at present the information characterizing the properties of the active bond in polymerization catalysts is very scant. The analogy between the features of the active bonds in the propagation centers and those of the transition metal-carbon bond in individual organometallic compounds is sure to exist, but as in the initial form the latter do not show catalytic activity in olefin polymerization this analogy is restricted to its limits. 

The different classes of Ru-based catalysts, including crystalline Chevrel-phase chalcogenides, nanostructured Ru, and Ru-Se clusters, and also Ru-N chelate compounds (RuNj), have been reviewed recently by Lee and Popov [29] in terms of the activity and selectivity toward the four-electron oxygen reduction to water. The conclusion was drawn that selenium is a critical element controlling the catalytic properties of Ru clusters as it directly modifies the electronic structure of the catalytic reaction center and increases the resistance to electrochemical oxidation of interfacial Ru atoms in acidic environments. 

The model shown in Scheme 2 indicates that a change in the formal oxidation state of the metal is not necessarily required during the catalytic reaction. This raises a fundamental question. Does the metal ion have to possess specific redox properties in order to be an efficient catalyst A definite answer to this question cannot be given. Nevertheless, catalytic autoxidation reactions have been reported almost exclusively with metal ions which are susceptible to redox reactions under ambient conditions. This is a strong indication that intramolecular electron transfer occurs within the MS"+ and/or MS-O2 precursor complexes. Partial oxidation or reduction of the metal center obviously alters the electronic structure of the substrate and/or dioxygen. In a few cases, direct spectroscopic or other evidence was reported to prove such an internal charge transfer process. This electronic distortion is most likely necessary to activate the substrate and/or dioxygen before the actual electron transfer takes place. For a few systems where deviations from this pattern were found, the presence of trace amounts of catalytically active impurities are suspected to be the cause. In other words, the catalytic effect is due to the impurity and not to the bulk metal ion in these cases. 

On the other hand, a catalyst in which the CrV04 was one of major constituents had little catalytic activity for the ammoxidation of xylene. These observations indicate that the nature and the distribution of metal ions and oxygen ion on the catalyst surface affect the catalytic activity and selectivity. It is difficult to predict the relationship between the adsorptivity of reactants and the physical properties of catalyst, but it may be assumed that adding more electronegative metal ions affects the electronic properties of the vanadium ion, which functions as an adsorption center. Further details on the physical properties of catalysts for the ammoxidation of xylenes will be reported later. 

Recent work by Rabo et al. (57) opens new possibilities for controlling the activity and selectivity of zeolite catalysts. Occlusion of various guest molecules into the sodalite cavities of Y zeolites can significantly change the catalytic properties of the zeolites for carbonium-type reactions. Anions of occluded salts are located close to the center of the sodalite cavity and strongly influence the arrangement of cations in the faujasite lattice and hence the catalytic activity. 

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