Organic surface reactions, catalytic mechanisms

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). 

Kinetic measurement of the interfacial reaction is still very important in relation to the biological reactions taking place at the cell membrane and the surface of biological organism. The catalytic mechanism of enzyme at the liquid-liquid interface is not well understood yet. The interaction between the enzyme and the substrate at the oil-water interface has to be investigated urgently. 

Many enzymes use redox centers to store and transfer electrons during catalysis. These redox centers can be composed of metals such as iron or cobalt, or organic cofactors such as quinones, amino acid radicals, or flavins. In order to fully appreciate the catalytic mechanisms of these enzymes, it is often necessary to determine the free energy required to reduce or oxidize their protein redox centers. This is called the redox potential. The measurement of enzyme redox potentials can be performed by either direct or indirect electrochemical methods. The type of electrochemistry suitable for a particular protein system is simply dictated by the accessibility of its redox center to the electrode surface. Because most reactions catalyzed by enzymes occur within hydrophobic pockets of the protein, the redox sites are often far from the surface of the protein. Unless an electron transfer path exists from the protein surface to the redox center, it is not feasible to use direct electrochemistry to measure the redox potential. Since only a few enzymes (most notably certain heme-containing enzymes) have such electron transferring paths and... 

To summarize these results, it becomes now clear that EMIR Spectroscopy is particularly well suited to follow the fate of the different small adsorbed organic residues, resulting from the chemisorption of small organic molecules, such as CH3OH. The nature and the quantity of adsorbed species depend strongly on the structure of the catalytic surface, on the concentration of methanol in solution, on the adsorption time, on the applied electrode potential,... All these various experimental conditions lead to a great variety of adsorbed species, and control their surface distribution. According to these spectroscopic data, the reaction oxidation mechanisms of methanol adsorption and oxidation at platinum electrodes... 

Partial oxidations over complex mixed metal oxides are far from ideal for singlecrystal like studies of catalyst structure and reaction mechanisms, although several detailed (and by no means unreasonable) catalytic cycles have been postulated. Successful catalysts are believed to have surfaces that react selectively vith adsorbed organic reactants at positions where oxygen of only limited reactivity is present. This results in the desired partially oxidized products and a reduced catalytic site, exposing oxygen deficiencies. Such sites are reoxidized by oxygen from the bulk that is supplied by gas-phase O2 activated at remote sites. 

Without substantial artistic talent, depicting organic reaction mechanisms on surfaces is difficult. Over the years, a variety of methods have been invented and used with differing successes. Frequently used is an asterisk, an M, or sometimes the symbol of the metal catalyst to designate a surface catalytic... 

Two examples of the application of SERS and potential-difference IRRAS methods to the identification of adsorbed intermediates and reaction mechanism elucidation are also described, involving the catalytic electrooxidation of carbon monoxide and small organic molecules on transition-metal surfaces. 

M. W. Roberts reviews the contribution of photoelectron spectroscopy to provide chemical information at the molecular level to the catalytic reactions on surfaces. The use of organic probes to study the rate-determining steps and mechanisms of catalytic reactions is reviewed by R. W. Maatman and M. Kraus, respectively. 

However, the proponents of the Radical Mechanism would argue that, whatever the actual agent, the fact remains that the process provides a means of generating fluorine at a rate, and in a form which is conducive to controlled reaction with organic compounds, probably constrained on a favourable, possibly catalytic [189] heat-sink surface, in such a way as to overcome the two principal problems inherent in all elemental fluorinations [190], namely... 

Titanium containing materials have been investigated for various reactions, but selective oxidations with H202 as the oxidant have attracted the most interest. For these reactions, the formation of surface titanium peroxo compounds with H202 and the subsequent transfer of the peroxidic oxygen to the organic reactants have been proposed to explain the mechanism by which titanium participates in the catalytic cycle (Notari, 1988). 

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