Catalyst geometric factors

Despite various attempts, no single universal correlation between bulk properties and catalytic activity of solids has been found. It is now recognized that the geometric factor and the electronic factor cannot be separated from one another and that catalytic activity should be considered along with catalyst selectivity to arrive at an understanding of heterogeneous catalysis (Sachtler, 1981). 

In contrast to the aforementioned binary oxides, V2Os has a stronger oxidation power and is able to attack hydrogen attached to the aromatic nucleus. Sometimes attention is drawn to the importance of a layer structure in the catalyst or to geometric factors (e.g. Sachtler [270]). Unexpectedly, however, very effective vanadium-based catalysts exist which operate in the molten state, indicating that a fixed structure is not important. The catalytic activity of molten oxide phases seems to occur exclusively in the oxidation of aromatic hydrocarbons over V2Os-based catalysts, such systems have not been reported for the selective oxidation of olefins. 

SBY, yet the HDN activities of the catalysts are almost the same, especially for Mo-Ni / Zr-Si-Al catalyst. It is well known that not only surface chemistry of the support but also geometrical factors, like the surface area and pore-size distribution, are of major importance for performance of HDN catalyst. The pores are not only paths for reactants and products but also influence the deposition of the active metals during preparation. Mo-Ni/Zr-Si-Al catalyst has bigger surface area (over 600 M2/g) than SBY(240 M2/g), from the point of effective diffusivity, Zr-Si-Al is better than SBY. If the acidity of Zr-Si-Al support was increased properly by some modification methods, the synthesis samples would be a good HDN catalytic materials. 

In principle pentadienyls can bond to transition elements in at least three basic ways, tj3, and tjs (Fig. 1). These can be further subdivided when geometrical factors are considered. If r 5 coordination could be converted to rj3 orr/1, one or two coordination sites could become available at the metal center, and perhaps coordinate substrate molecules in catalytic processes. Little is known about the ability of pentadienyl complexes to act as catalysts. Bis(pentadienyl)iron derivatives apparently show naked iron activity in the oligomerization of olefins (144), resembling that exhibited by naked nickel (13). The pentadienyl groups are displaced from acyclic ferrocenes by PF3 to give Fe(PF3)5 in a way reminiscent of the formation of Ni(PF3)4 from bis(allyl)nickel (144). 

Two factors may be responsible for specific routes of adsorption and transformation of a hydrocarbon molecule on the catalyst surface a geometrical factor and an electronic factor. Certainly no ultimate conclusions with respect to adsorption geometry may be drawn on the basis of very small models of the site. We would rather show interrelations between the electronic structure of the host cluster or molecule and the preferable interaction geometry with the guest hydrocarbon molecule and point out the consequences regarding further reaction routes. The results discussed in this paragraph will be based on DFT geometry optimization within the LDA approximation... 

In catalysis, one of the key concepts to understand catalytic action is the so-called active sites [28], The essential concept behind this term is the fact that catalytic activity in solids is restricted to specific sites in the catalyst surface. Another factor influencing catalytic activity is the geometric factor, that is, a properly spaced array of atoms on the solid surface, named Balandin multiplets,... 

Besides, the geometric factor is important in the catalytic activity of iron, that is, the catalyst surface has irregularities such as steps and kinks that expose atoms with low coordination numbers which appears to be particularly reactive [22],... 

In the same group, the oxide of tungsten was observed to be a selective dehydration catalyst, irrespective of hydrogen or air pretreatment.Nevertheless, the octene distribution from octan-2-ol was affected by the pretreatment and as O " ions were removed, fra s-oct-2-ene became favoured and geometric factors are probably of importance. It was suggested that high cisitrans ratios may occur either when basic oxygen sites crowd closely around the catalytic site or when the metal ion has a lower co-ordination number. 

Thus, to obtain definitive results on the influence of parameters such as lattice defects, electronic and geometric factors, and contaminants, it is essential to develop and apply techniques which permit the study of simple systems under conditions such that the several contributing factors can be separated and investigated individually. Since chemisorbed gases are included in the class of objectionable contaminants, it is necessary to employ the most effective means available of cleaning the catalyst in high vacuum. This requirement places severe limitations on the construction of the reaction chamber, the size of the catalyst, the pressures of the reactants, and the means of detecting the reaction constant. 

Among the many reactions in which Cr20a acts as a catalyst, particular interest attaches to that which converts open-chain to aromatic hydrocarbons, for two reasons. These are (a) that both electronic and geometrical factors are involved and (b) that highly specific effects are produced by the action of AI2O3 as a catalyst support or promoter. 

Although the original form of this theory has been seriously criticized for several reasons (3), it provided a stimulus to further interest in problems of this type. Subsequent study of the relation between the dimensions of organic molecules, and of the catalysts which are efToctive in their conversions, has proved most interesting and has established the fact that two-point adsorption frequently occurs. It is with reactions of this type that this article is concerned in particular, the importance of the geometrical factor in defining the properties of a catalytic surface is considered. 

Other Syntheses Related to the Fischer-Tropsch Process Comparatively little is yet known of some synthetic reactions which obviously resemble the Fischer-Tropsch process very closely, but they are worth brief mention because they are also likely to be controlled by geometrical factors. The Oxo synthesis (15) of aldehydes by the interaction of ethylene or other olefins with carbon monoxide and hydrogen is carried out in contact with cobalt catalysts at temperatures in the range 110-150°, and under a pressure of 100-200 atmospheres. Cyclic olefins react similarly for example, cyclohexene gives hexahydrobenzaldehyde. There can be little doubt that a two-point adsorption of the hydrocarbon must take place and that the adsorbed molecule then reacts with carbon monoxide and hydrogen the difference between this process and that responsible for the normal hydrocarbon synthesis is that adsorbed carbon monoxide survives as such under the less drastic temperature conditions which are employed. Owing to the fact that a variety of isomeric aldehydes are produced, this system deserves further detailed study on geometrical lines. 

In comparison with the information about geometrical factors for metallic catalysts, little parallel progress has been made with oxides or sulfides. This is largely due to the uncertainty which exists concerning the precise chemical composition of these catalysts in their active state, and partly to the lack gf reliable data for the crystal structures of some of the lower oxides and sulfides. 

These few suggestions will indicate that a better knowledge of the geometrical factors in catalysis can only be obtained by the combined results of several distinct methods of theoretical and practical treatment. One of the most important of these is to increase the information available about the nature of the bond between the adsorbed molecule and the catalyst, and about its dependence on the chemical properties of the catalyst elements. 

In this article, no account has been included of the evidence provided by work on promoter actions or on catalyst poisons. Both these have made contributions to the study of the geometrical factor, and are likely to do so more extensively in the future, but these subjects are themselves of such importance in any report on catalysis that they should be the subject of a separate treatment. 

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