Organic surface reactions, catalytic

Gomez-Sainero et al. (11) reported X-ray photoelectron spectroscopy results on their Pd/C catalysts prepared by an incipient wetness method. XPS showed that Pd° (metallic) and Pdn+ (electron-deficient) species are present on the catalyst surface and the properties depend on the reduction temperature and nature of the palladium precursor. With this understanding of the dual sites nature of Pd, it is believed that organic species S and A are chemisorbed on to Pdn+ (SI) and H2 is chemisorbed dissociatively on to Pd°(S2) in a noncompetitive manner. In the catalytic cycle, quasi-equilibrium ( ) was assumed for adsorption of reactants, SM and hydrogen in liquid phase and the product A (12). Applying Horiuti s concept of rate determining step (13,14), the surface reaction between the adsorbed SM on site SI and adsorbed hydrogen on S2 is the key step in the rate equation. 

The mechanism of action, and organization of the catalytic sites, in hydrogenases are different from a solid catalyst such as platinum. For a start, the reaction of H2 with hydrogenase involves heterolytic cleavage into a hydron and a hydride. This contrasts with the reaction of H2 at the surface of a metal such as platinum, which is usually considered to involve the homolytic cleavage into two hydrogen atoms. Moreover in the enzyme, the catalyst is a cluster of metal ions (with oxidation states +2 or -h3) rather than the metal (oxidation state 0). 

A key aspect of metal oxides is that they possess multiple functional properties acid-base, electron transfer and transport, chemisorption by a and 7i-bonding of hydrocarbons, O-insertion and H-abstraction, etc. This multi-functionality allows them to catalyze complex selective multistep transformations of hydrocarbons, as well as other catalytic reactions (NO,c conversion, for example). The control of the catalyst multi-functionality requires the ability to control not only the nanostructure, e.g. the nano-scale environment around the active site, " but also the nano-architecture, e.g. the 3D spatial organization of nano-entities. The active site is not the only relevant aspect for catalysis. The local area around the active site orients or assists the coordination of the reactants, and may induce sterical constrains on the transition state, and influences short-range transport (nano-scale level). Therefore, it plays a critical role in determining the reactivity and selectivity in multiple pathways of transformation. In addition, there are indications pointing out that the dynamics of adsorbed species, e.g. their mobility during the catalytic processes which is also an important factor determining the catalytic performances in complex surface reaction, " is influenced by the nanoarchitecture. 

It appears that the incorporation of metal adatoms into adsorbate structures stabilizes the reaction intermediates, and therefore, can be expected to be a general phenomenon on catalytic metal surfaces, at least for metal particles large enough to be considered as metallic. The dynamic processes of incorporation, release, and mass transport of metal adatoms may occur on the time scale of surface reactions and affect the reactive behavior of the intermediates, that is to say, the reaction kinetics. Indeed, STM studies have shown that the kinetic oscillation in some surface reactions can be partially attributed to the spatial organization of reactive species on the surfaces and the structural change in such complex surfaces on the time scale of reaction [69]. The structural complexity of the active surfaces and the origin of unusual surface reaction kinetics are of interest, and may be connected. Recently, such a relationship was established in the autocatalytic decomposition of formate and acetate on the Ni(llO) surface [21]. 

In addition to protection against corrosion, the microparticles can also be highly catalytic for the oxidation of organics. The reaction time needed for complete oxidation can be reduced several fold. The significant catalytic activity is due to the newly generated irregular surface on the sodium carbonate microparticles. The organic molecules attach to the surface and become more susceptible to oxidation. 

In order to improve the efficiency of activated carbon in organic compound removal, catalytic reactions are favored by modifications of the activated carbon surface. The functional groups are increased by oxidation treatment (see section on impregnation with metals or metal oxides). The metal coating the porous surface plays the role of catalyst or reacts with soluble organic compounds. 

Nickel and Platinum—These two metals (in the form of Raney nickel and Pt-black) are used for electrocatalytic organic hydrogenation reactions (i.e., the electrochemical generation of hydrogen on the catalytically active, high surface area cathode followed by the chemical reaction of adsorbed hydrogen with the organic substrate). 

In some instances, there still exist conflicting reports about the surface acidity-catalytic activity correlation. These differences may arise not only from the use of different reaction conditions and different approaches to preparing or modifying the catalysts but also from a poor characterization of the materials employed. Indeed, the detailed physicochemical characterization of the catalytic materials, as well as the study of their interaction with reagents and products, still represents well-recognized problems in the use of heterogeneous catalysis for organic syntheses. 

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