Cocatalysts, deposition

Tertiary stibines have been widely employed as ligands in a variety of transition metal complexes (99), and they appear to have numerous uses in synthetic organic chemistry (66), eg, for the olefination of carbonyl compounds (100). They have also been used for the formation of semiconductors by the metal—organic chemical vapor deposition process (101), as catalysts or cocatalysts for a number of polymerization reactions (102), as ingredients of light-sensitive substances (103), and for many other industrial purposes. 

Both heterogeneous and homogeneous disproportionation catalysts are known. All contain a transition metal component with derivatives of Mo, W, and Re being the most important. Heterogeneous catalysts are generally metal oxides deposited on a support such as silica or alumina (1, 4). Homogeneous catalysts in general require a non-transition metal derivative as cocatalyst (2, 3). 

The gas-phase process, successfully commercialized independently by Bayer and USI,417 involves passing a mixture of ethylene, acetic acid and oxygen over a supported palladium catalyst contained in a multitubular reactor at 150 °C and about 5-10 atm pressure. The overall yield in vinyl acetate is about 92%, and the major by-product is C02. The catalyst consists of a palladium salt (e.g. Na2PdCl4) deposited on silica (or alumina) in the presence of a cocatalyst (e.g. HAuC14), reduced and impregnated with potassium acetate before use.384,418 The lifetime of the catalyst is about 2... 

Acetoxylation of propene to allyl acetate can be performed in the liquid phase with high selectivity (98%) in acetic acid in the presence of catalytic amounts of palladium trifluoroacetate. The stability and activity of this catalyst can be considerably increased by adding copper (II) trifluoroacetate and sodium acetate as cocatalysts (100 °C, 15 bar, reaction time = 4 h, conversion = 70%, selectivity = 97%). Gas-phase procedures for the manufacture of allyl acetate are described in several patents and use conventional palladium catalysts deposited on alumina or silica, together with cocatalysts (Au, Fe, Bi, etc.) and sodium acetate. The activity and selectivity reported for these catalysts are very high (100-1000 g l-1 h-1, selectivity = 90-95% ).427 A similar procedure has been used for the synthesis of methallyl acetate from 2-methylpropene.428... 

Several research approaches are pursued in the quest for more efficient and active photocatalysts for water splitting (i) to find new single-phase materials, (ii) to tune the band-gap energy of TJV-active photocatalysts (band-gap engineering), and (iii) to modify the surface of photocatalysts by deposition of cocatalysts to reduce the activation energy for gas evolution. Obviously, the previous strategies must be combined with the control of the s)mthesis of materials to customize the crystallinity, electronic structure, and morphology of materials at nanometric scale, as these properties have a major impact on photoactivity. 

The deposition of noble metals (e.g.. Ft, Rh) or metal oxides (e.g., NiO, RUO2) onto photocatalyst surfaces is an effective way of enhancing photocatalyst activity (Sato and White, 1980 Subramanian et al., 2001). The cocatalyst improves the efficiency of photocatalysts, as shovm in Figure 15, as a result of (i) the capture of CB electrons or VB holes from the photocatalysts (Maruthamuthu and Ashokkumar, 1988), thereby reducing the possibility of electron-hole recombination and (ii) the transference... 

The standard electrode potentials are far more anodic than that of one-electron transfer process, -0.284 V (SHE) and the visible-light photocatalytic activity of platinum-loaded tungsten(VI) oxide could be interpreted by enhanced multiple-electron transfer process by deposited platimun (45), since it is well known that platinum and the other noble metals catalyze such multiple-electron transfer processes. Similar phenomena, cocatalyst promoted visible-light photocatalytic activity, have been reported with palladium 46) and copper oxide (47). Thus, change of reaction process seems beneficial to realize visible-light photocatalytic activity. 

At that time, we thought that the reverse reaction would be suppressed if nanoparticulate noble metal cocatalysts could be coated by a certain shell, forming a core/shell-like configuration, as shown in Fig. 9. More specifically, it was expected that the access of H2 and O2 molecules to the noble metal core would be suppressed upon such a shell coating. As such a shell component, we paid attention to Cr203 that has been reported to function as catalyst for some hydrogen-related reactions (e.g., (de)hydration) [61, 62] and to be deposited through photochemical reduction process from Cr(VI) species as the precursor. PhotoreductiOTi of Cr(VI) ions over... 

Afterward, the notion of unspecified carbon deposition with an olefin-like composition (CH ) has been gradually transformed to Polymethylbenzenes (PMBs) by many research groups [87,97]. Those PMBs serve as scaffolds/cocatalysts, where methanol is added and olefins are eliminated in a closed catalytic cycle [87,98]. It is therefore indicated that the interplay between the inorganic framework and the organic reaction centers dictates the activity and selectivity. However, according to Ref. [97], the role of PMBs as the major hydrocarbon pool species appears to be independent of the zeotype catalyst chosen. Haw et al. [87] provided both experimental and theoretical evidence in favor of PMBs as the driving force for the hydrocarbon pool mechanism. In 1998, by means of pulse-quench reactions on an H-ZSM-5 catalyst and GC-MS and MAS NMR analysis, it was reported... 

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