Zinc oxide active sites

To summarize the qualitative findings, the methanol synthesis activity in the binary Cu/ZnO catalysts appears to be linked to sites that also irreversibly chemisorb CO and not to sites that adsorb CO reversibly. Since irreversible adsorption of CO follows linearly the concentration of amorphous copper in zinc oxide, these sites are likely to be that part of the copper solute that is present on the zinc oxide surface. No correlation of the catalyst activity and the copper metal surface area, titrated by reversible form of CO or by oxygen, could be found in the binary Cu/ZnO catalysts (43). In contrast with this result, it has been claimed that the synthesis activity is proportional to copper metal area in copper-chromia (47), copper-zinc aluminate (27), and copper-zinc oxide-alumina (46) catalysts. In these latter communications (27,46,47), the amount of amorphous copper has not been determined, and obviously there is much room for further research to confirm one or another set of results and interpretations. However, in view of the lack of activity of pure copper metal quoted earlier, it is unlikely that the synthesis activity is simply proportional to the copper metal surface area in any of the low-temperature methanol-synthesis catalysts. 

The zinc oxide component of the catalyst serves to maintain the activity and surface area of the copper sites, and additionally helps to reduce light ends by-product formation. Selectivity is better than 99%, with typical impurities being ethers, esters, aldehydes, ketones, higher alcohols, and waxes. The alumina portion of the catalyst primarily serves as a support. 

Catalytic Oxidation. Catalytic oxidation is used only for gaseous streams because combustion reactions take place on the surface of the catalyst which otherwise would be covered by soHd material. Common catalysts are palladium [7440-05-3] and platinum [7440-06-4]. Because of the catalytic boost, operating temperatures and residence times are much lower which reduce operating costs. Catalysts in any treatment system are susceptible to poisoning (masking of or interference with the active sites). Catalysts can be poisoned or deactivated by sulfur, bismuth [7440-69-9] phosphoms [7723-14-0] arsenic, antimony, mercury, lead, zinc, tin [7440-31-5] or halogens (notably chlorine) platinum catalysts can tolerate sulfur compounds, but can be poisoned by chlorine. 

While PDF was originally proposed to be a zinc-metalloprotease [51], it is now generally accepted that Fe is the physiologically relevant metal ion occupying the active site in vivo [52], The native forms of most PDF enzymes are highly unstable due to propensity to oxidation, rendering them difficult to purify [53, 54], However, the Fe can be suitably replaced by either or Co, both of which provide a stable enzyme and main-... 

The introduction of redox activity through a Co11 center in place of redox-inactive Zn11 can be revealing. Carboxypeptidase B (another Zn enzyme) and its Co-substituted derivative were oxidized by the active-site-selective m-chloroperbenzoic acid.1209 In the Co-substituted oxidized (Co111) enzyme there was a decrease in both the peptidase and the esterase activities, whereas in the zinc enzyme only the peptidase activity decreased. Oxidation of the native enzyme resulted in modification of a methionine residue instead. These studies indicate that the two metal ions impose different structural and functional properties on the active site, leading to differing reactivities of specific amino acid residues. Replacement of zinc(II) in the methyltransferase enzyme MT2-A by cobalt(II) yields an enzyme with enhanced activity, where spectroscopy also indicates coordination by two thiolates and two histidines, supported by EXAFS analysis of the zinc coordination sphere.1210... 

Over zinc oxide it is clear that only a limited number of sites are capable of type I hydrogen adsorption. This adsorption on a Zn—O pair site is rapid with a half-time of less than 1 min hence, it is fast enough so that H2-D2 equilibration (half-time 8 min) can readily occur via type I adsorption. If the active sites were clustered, one might expect the reaction of ethylene with H2-D2 mixtures to yield results similar to those obtained for the corresponding reaction with butyne-2 over palladium That is, despite the clean dideutero addition of deuterium to ethylene, the eth-... 

Adsorption of ethylene as an olefinic species would not be likely to occur on the zinc half of the active site. A rigid ethylene molecule could not approach the sequestered zinc ions because of steric restrictions hence, ethylene would be confined primarily to the oxide part of this layer. In... 

Let us now look at the chemistry of the reaction of water and hydrogen with the active sites. When water reacts with the active site, it seems quite clear that this should be viewed as heterolytic fission of an OH bond with the proton adding to the oxide ion and the hydroxide ion adding to the zinc ion. This is shown schematically below ... 

This review has been concerned largely with interactions and reactions of unsaturated hydrocarbons with zinc oxide. The picture of the active site as a metal oxide pair capable of heterolytic fission of an acidic C—H bond provides a consistent framework for discussion of these results. We believe this view may be generally applicable. In its application, however, we must keep in mind that zinc oxide may be much more effective for heterolytic cleavage (i.e., more basic) than oxides such as, say, alumina.4... 

B. Better tools available, but no consensus on mechanism or active site—1980 to 2006. Rhodes et al.291 published a comprehensive review on the heterogeneously catalyzed water-gas shift mechanism in 1995. Included in that discussion was the copper/zinc oxide/alumina system. The conclusion was that this system appears to be constructed of small metallic islands of copper resting on a zinc oxide alumina phase. Zinc oxide may exert some impact on catalytic activity, but it was suggested in the review that the contribution is small. It was indicated that strong evidence exists to support either a formate or a redox mechanism, and the authors even suggest the possibility that both mechanisms might occur, though insufficient data exist to determine which mechanism predominates. 

Klier and coworkers—Role of ZnO in stabilizing Cu in Cu+ oxidation state, proposed to be the active site. Klier and coworkers235 241 provided a different explanation for the role of zinc in promoting the activity of Cu/ZnO catalysts. They suggested that zinc stabilizes the Cu in the Cu1 + oxidation state, and that it is the Cu ions in the 1 + oxidation state that serve as the active sites. 

An alternative method for removing Zn from the active site of zinc proteins is to use aromatic nitroso compounds such as 3-nitrosoben-zamide and 6-nitroso-l,2-benzopyrone (382). These agents can oxidize Zn-bound cysteine S and can inhibit HIV-1 infection in human lymphocytes. They also eject zinc from isolated HIV-1 nucleocapsid zinc-fingers and from intact HIV-1 virions. 

Guanine is the most easily oxidizable natural nucleic acid base [8] and many oxidants can selectively oxidize guanine in DNA [95]. Here, we focus on the site-selective oxidation of guanine by the carbonate radical anion, COs , one of the important emerging free radicals in biological systems [96]. The mechanism of COs generation in vivo can involve one-electron oxidation of HCOs at the active site of copper-zinc superoxide dismutase [97, 98], and homolysis of the nitrosoperoxycarbonate anion (0N00C02 ) formed by the reaction of peroxynitrite with carbon dioxide [99-102]. 

The latter reaction will occur (at temperatures sufficiently high to overcome the activation energy in Fig. 1) because adsorption of hydrogen (Type A) will upset the electronic equilibrium between the number of electrons leaving the O sites and those returning from the zinc oxide. There will be fewer empty O sites after the Type A adsorption, hence more electrons will leave the O sites than return. As more 0 sites are formed, more hydrogen will be adsorbed. 

During catalysis of superoxide disproportionation, the copper centre is reversibly oxidized and reduced by successive encounters with superoxide, giving 02 and H202 (equations 67 and 68). The zinc(II) almost certainly has a structural role in the formation and stabilization of the active site,1352 and indirectly in enhancing the reactivity of the copper. 

Cu—Zn superoxide dismutases (SODs) [87,88] are abundant in eukaryotic cells and may serve to protect cells against the toxic effects of superoxide or deleterious oxy-products derived from 02 . The active site copper and zinc ions are 6.3 A apart and are bridged by a histidine imidazolate. In the oxidized form Cu(II) is roughly pentacoordinate, with four His N s and a water molecule. A highly conserved Arg residue is thought to stabilize Cu(II)-bound anions (e.g., Cu(II)—02 ) a redox reaction releases 02, generating Cu(I), which can reduce more 02 substrate to give peroxide and Cu(II). 

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