Redox catalysts bifunctional

Efficient Water Cleavage by Visible Light in Colloidal Solutions of Bifunctional Redox Catalysts... 

This problem has been overcome only recently throu the development of bifunctional redox catalysts (1 ). The latter are distinguished by the fact that Pt and RUO2 are loaded onto the same mineral carrier particle. Colloidal Ti02 was the first material to be used as a siipport. It fulfills four different functions in the water splitting system (Figure 8) 1) It serves as... 

Figure 8. Schematic illustration of the intervention of a colloidal TiOfbased bifunctional redox catalyst in the cleavage of water by visible light.

Figure 9. Cyclic water cleavage by visible light in the presence of Ru(bipy)i as a sensitizer and bifunctional redox catalyst as an electron relay.

Fig. 8.1a, b. Schemes for the visible light-induced decomposition of water a colloidal redox catalysts are present as separated entities. b Bifunctional redox catalyst Ru02 and Pt are codeposited on the colloidal Ti02 carrier particle... 

Although photoelectrochemical systems are able to offer respectable conversion efficiencies, the refinement of other solution-based processes continues. Gratzel has reviewed photoredox processes, paying particular attention to the use of organized assemblies such as micelles and vesicles. He emphasizes the central role of efficient colloidal metal catalysts in these schemes and also describes the recent development of bifunctional redox catalysts that allow the combination of cycles for the generation of hydrogen and oxygen. 

Figure 3 The reaction scheme for the bifunctional redox catalyst. RuOj/Pt on Ti02- The conduction and valence hand energies are compared with the standard hydrogen and oxygen potentials...

Kiwi J, Borgarello E, Pellizzetti E, Visca M, Gratzel M (1980) Cyclic water cleavage by visible light drastic improvement of yield of H2 and O2 with bifunctional redox catalysts. Angew Chem Int Ed 19 646-648... 

An interesting approach is to combine both the formation of HMF from fructose with the oxidation to FDCA in one pot. The aim is to convert the unstable HMF before it can react further to levulinates or degrade into humins. An FDCA yield of 25% was reported using a PtBi/C oxidation catalyst in combination with a solid acid in water/methyl iso-butyl ketone biphasic reaction medium [23]. Recently, Ribeiro and Schuchardt [14] reported a 72% fructose conversion and excellent greater than 99% selectivity to FDCA when using Co(acac) - Si02 as a bifunctional acid-redox catalyst at 160 °C and 20 bar air pressure. 

It is worth mentioning that both the carboxylation of epoxides and anilines are acid-base reactions, which do not entail redox processes. Therefore a catalyst active in these reactions must provide acid-base functionality. In this perspective, positively charged gold could be the real player, although a co-catalytic or promotion effect of ze-rovalent gold could also be important. Therefore the catalysts for the oxidative carbonylation of aniline, supported on Merck Ion-exchanger IV, could be actually bifunctional. On one side, Au could catalyze the oxidation of CO with O2 to CO2, a reaction for which it is... 

Metal oxides possess multiple functional properties, such as acid-base, redox, electron transfer and transport, chemisorption by a and 71-bonding of hydrocarbons, O-insertion and H-abstract, etc. which make them very suitable in heterogeneous catalysis, particularly in allowing multistep transformations of hydrocarbons1-8 and other catalytic applications (NO, conversion, for example9,10). They are also widely used as supports for other active components (metal particles or other metal oxides), but it is known that they do not act often as a simple supports. Rather, they participate as co-catalysts in the reaction mechanism (in bifunctional catalysts, for example).11,12... 

Once the multi-step reaction sequence is properly chosen, the bifunctional catalytic system has to be defined and prepared. The most widely diffused heterogeneous bifunctional catalysts are obtained by associating redox sites with acid-base sites. However, in some cases, a unique site may catalyse both redox and acid successive reaction steps. It is worth noting that the number of examples of bifunctional catalysis carried out on microporous or mesoporous molecular sieves is not so large in the open and patent literature. Indeed, whenever it is possible and mainly in industrial patents, amorphous porous inorganic oxides (e.g. j -AEOi, SiC>2 gels or mixed oxides) are preferred to zeolite or zeotype materials because of their better commercial availability, their lower cost (especially with respect to ordered mesoporous materials) and their better accessibility to bulky reactant fine chemicals (especially when zeolitic materials are used). Nevertheless, in some cases, as it will be shown, the use of ordered and well-structured molecular sieves leads to unique performances. 

Each zeolite type can be easily obtained over a wide range of compositions directly by synthesis and/or after various post synthesis treatments. Moreover, various compounds can be introduced or even synthesized within the zeolite pores (ship in a bottle synthesis). This explains why zeolites can be used as acid, base, acid-base, redox and bifunctional catalysts, most of the applications being however in acid and in bifunctional catalysis. 

POMs are promising catalysts for acid, redox and bifunctional catalysis. In many structures, the transition metal addenda atoms such as Mo or W exist in two oxidation states, which results in different redox properties as determined by polarog-raphy. The exceptional ability of heteropolyanions to act as electron reservoirs has been demonstrated by the preparation and characterization of numerous reduced derivatives [32]. They also exhibit high solubility in polar solvents, which means that they can be used in homogeneous catalysis. The wide range of applications of heteropoly compounds are based on their unique properties which include size, mass, electron and proton transfer (and hence storage) abilities, thermal stability. 

In addition, substitution of variable oxidahon state metal ions such as Co, Fe and Cu generates materials with redox catalytic properties, for example References [57, 64]. Hydrotalcites themselves have been used as basic supports for metals, leading to bifunctional catalysts. An example of this is provided in the study of de Jong and coworkers in which palladium-hydrotalcite combinations were reported to be active for single-stage synthesis of MIBK from acetone and hydrogen [65]. 

Figure 13. (a, b) Schematic representation of the oxidation pathways using redox molecular sieves (a) homolytic free radical autoxidation and (b) heterolytic oxygen transfer, (c) Oxidation of styrene to styrene oxide and transformation to 2-phenylacetaldehyde using a bifunctional Ti-silicalite catalyst. 

When talking about bifunctional catalysis, one thinks immediately of catalysts possessing metal and acid functions. It is well known that traces of olefins accelerate the acid-catalyzed conversion of hydrocarbons and that such a catalysis usually results in rapid deactivation. More stable catalytic activity for the isomerization of paraffins is achieved by bifunctional catalysis, i.e., the association of a hydrogenation function of a metal with an acidic function of a support. In this case, the amount of olefins is controlled by the hydrogenation-dehydrogenation equilibrium. This topic has received considerable attention and has been earlier reviewed by Weisz [130]. However, bifunctional catalysis cannot be restricted to catalysts composed of metal and support with acid sites, but also with supports possessing acid-base pairs, basic or redox sites [131]. This is illustrated by some upcoming short examples. 

They have outstanding properties which are of great value for catalysis, such as strong Bnansted acidity [8], ability to catalyze reversible redox reactions under mild conditions [9], and high solubility in water [10]. In most applications, they are used as acid, redox, and bifunctional catalysts in homogeneous and heterogeneous systems. 

We are investigating bifunctional catalysts in which one component of the catalyst adsorbs or oxidizes CO and the other component dissociates water. Our present research is focusing on metal-support combinations to promote this bifunctional mechanism. The metallic component is chosen to adsorb CO at intermediate adsorption strengths (platinum [Pt], Ru, palladium [Pd], PtRu, PtCu, cobalt [Co], ruthenium [Ru], silver [Ag], iron [Fe], copper [Cu], and molybdenum [Mo]). The support is chosen to adsorb and dissociate water, typically a mixed-valence oxide with redox properties or oxygen... 

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