Geometric influences, catalytic activity

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,... 

The high catalytic activity of enzymes has a number of sources. Every enzyme has a particular active site configured so as to secure intimate contact with the substrate molecule (a strictly defined mutual orientation in space, a coordination of the electronic states, etc.). This results in the formation of highly reactive substrate-enzyme complexes. The influence of tfie individual enzymes also rests on the fact that they act as electron shuttles between adjacent redox systems. In biological systems one often sees multienzyme systems for chains of consecutive steps. These systems are usually built into the membranes, which secures geometric proximity of any two neighboring active sites and transfer of the product of one step to the enzyme catalyzing the next step. 

In many catalytic systems, nanoscopic metallic particles are dispersed on ceramic supports and exhibit different stmctures and properties from bulk due to size effect and metal support interaction etc. For very small metal particles, particle size may influence both geometric and electronic structures. For example, gold particles may undergo a metal-semiconductor transition at the size of about 3.5 nm and become active in CO oxidation [10]. Lattice contractions have been observed in metals such as Pt and Pd, when the particle size is smaller than 2-3 nm [11, 12]. Metal support interaction may have drastic effects on the chemisorptive properties of the metal phase [13-15]. Therefore the stmctural features such as particles size and shape, surface stmcture and configuration of metal-substrate interface are of great importance since these features influence the electronic stmctures and hence the catalytic activities. Particle shapes and size distributions of supported metal catalysts were extensively studied by TEM [16-19]. Surface stmctures such as facets and steps were observed by high-resolution surface profile imaging [20-23]. Metal support interaction and other behaviours under various environments were discussed at atomic scale based on the relevant stmctural information accessible by means of TEM [24-29]. 

Small metal particles as model systems for electrocatalysts are crucial to unravel the influence of electronic or geometrical structure on the catalytic activity. The effect of metal particle size on electrochemical reactivity has been proposed to exist for the electro-oxidation of alcohols as well as for the reduction of oxygen [7,75,76], both vital processes that require much deeper understanding for the development and... 

After the altered catalytic activity has been attributed to identified defects, so far as possible, there may remain a final ambiguity as to the nature of the interaction between defect and substrate that brings about the alteration. This ambiguity arises from the dual natmre of many defects and from the possible influence of geometric as well as of electronic parameters upon catalysis. 

For small clusters with up to 10-20 atoms, the interaction with the support can play a direct role on the geometrical or electronic structure of the cluster, thus notably affecting its reactivity [212]. In this respect, point defects or morphological irregularities on the oxide surface can influence the chemical properties of the particle, hence its catalytic activity [16,213,214]. 

The selectivity of the metal sulfate catalyst is influenced by many factors besides its acidic property, such as geometric structure involving a pore structure, arrangement of basic sites, polarity of the surface, etc. For example, the relative values of the first-order rate constants (per imit acidity at pK — 3) of the depolymerization catalyzed by nickel sulfate, cupric sulfate, and silica-alumina were found to be 1100 300 1. The difference may be attributed to the differences in acid-base bi-functional catalysis of these catalysts. This view may be said to have originated in 1948 when Turkevich and Smith (45) showed that the isomerization of 1-butene to 2-butene is catalyzed by metal sulfates, sulfuric acid, phosphoric acid, etc., but little by acetic acid, hydrogen chloride, etc. The high catalytic activity of the catalysts of the former group is considered as due to acid-base bifunctional catalysis as illustrated by Fig. 14. Independently, Horiuti (45a) advanced the same idea... 

The catalytic activity and selectivity of platinum, palladium, rhodium and ruthenium salts supported on various polyamides was studied, and the correlation between their reduction capability to the metallic state and the formation of coordination complexes with polyamide was established [115]. Concerning supported metals, the influence of the geometrical factor and the feasibility of electronic interaction of the supported metal with polyamide was discussed. It was found that the activity of Rh complexes decreases with an increase in the distance between amide groups of polyamide and correlates with the change in polymer crystallinity [107]. 

In earlier studies using ideally flat metal surfaces, it has been established that geometric factors, such as interatomic distance and particular arrangements of atoms, exposed at the surfaces with specific Miller indices and the presence of coordinatively unsaturated atoms due to various defects, along with electronic factors, such as percentage of d-character, exert significant influence on the catalytic activity of metals [33, 34]. In a similar manner, size effects in catalytic activity of... 

In the case of 2,2 -DBDI, geometric effects were clearly evident. Reactivity was influenced by the steric effects wich lead to significant intramolecular catalytic activity. These effects were responsible for the whole reaction pattern. 

Little attentions are paid on the reaction transport in the catalysts unless the pore size of catalyst is too small to geometrically confine the diffusion of reactant molecules in the pore. However, if the catalytic support consisted of active species, rather than traditional support materials, the new catalytic support could also have chemical effect with reactants. The influence of chemical effect of such active support on the transport mechanism of reactants is unknown. In recent years, some investigators proposed the unsupported catalyst for designing the new catalyst. Instead of being loaded on the catalytic support, active species are made into the mesoporous material to increase the highly catalytic activity. Though it obtains some progress, the unit catalytic performance of unsupported catalyst cannot show a proportional improvement compared with that of supported catalysts (Eijsbouts et al, 2007). 

In fact, geometric and electronic properties influence on the catalytic activity. These tables show that the n-type solids are the most active and that the interatomic distance, metal-oxygen 2 A, and the electronegativity 2.0 eV adjust better the activity results ... 

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