Photocatalytic transfer hydrogenation

Subsequendy, Khan and Rao (1990) studied the photocatalytic transfer hydrogenation of cyclohexane, in the absence of hydrogen gas flow, using water as the hydrogen source and ethanol as the electron donor. The authors reported that a very low yield of cyclohexane (7—8%) was obtained in aqueous ethanol solutions (80 20 ethanol-water) with K[Ru(H-EDTA)Cl] 2H2O and Pt/CdS/Ru02 as the semiconductor. 

Hanaoka et al. (1999) reported the effect of additive and of rhodium colloid catalyst (Rh4(CO)i2) for selective photocatalytic transfer hydrogenation of 1,5-COD to COE. The solution of rhodium colloid particles was prepared with an irradiation method that consists of stirring a solution with Rh4(CO)n in 2-propanol and acetone for 32 min... 

Figure 20.4 Reaction path of photocatalytic transfer hydrogenation of 1,5-COD.

Figure 20.5 Additive effect by salts of transition light metals on the photocatalytic transfer hydrogenation of 1,5-COD (reaction condition Rli4(CO)i2 3 mg, 2-propanol 32 ml, acetone 8 ml, 20 °C, under N2).

The adsorption of ketones on Ti02 surface and their solubdity in the solvent is an important parameter for the efficiency of the photocatalytic transfer hydrogenation. 

In this last decade, a green method as the photocatalytic transfer hydrogenation has been developed for the photocatalytic reduction of a nitro group to an amino group by several steps. 

Aniline, acetaldehyde, and water were the main products of the photocatalytic transfer hydrogenation of nitrobenzene after 24 h of UV light irradiation. 

Hanaoka, T., Matsuzaki, T., Sugi, Y. (1999). Selective photocatalytic transfer hydrogenation to 1,5-cyclooctadiene with fight transition metal modified rhodium colloid catalyst. Journal... 

Ohtani B, Goto Y, Nishimoto S-I, Inui T (1996) Photocatalytic transfer hydrogenation of Schiff bases with propan-2-ol by suspended semiconductor particles loaded with platinum deposits. J Chem Soc, Faraday Trans 92(21) 4291 295... 

Ti02 is the most widely used catalyst for photocatalytic reactions (and for transfer hydrogenation) under UV light. Kohtani, Yoshioka, Saito, Kudo, and Miyabe (2012)... 

The introduction of an impurity into a specimen (accompanied by a change in tv and es ) will transfer us from one point to another in Fig. 9. Suppose that when a donor impurity is introduced into the specimen (decrease in v and e8 ), we are transferred from the point A to the point B. This involves a decrease in K, as can be seen from Fig. 9. Such a decrease in the photocatalytic effect caused by the addition of donor impurities has been observed by Kohn and Taylor (40) who studied the photoreaction of hydrogen-deuterium exchange on zinc oxide exposed to y radiation. 

If the positive effect is observed on a specimen deposited in the hydrogen atmosphere, then after the specimen is calcined in vacuo, this being accompanied by an increase of es, it is replaced by the negative effect (transfer from the point A to the point F in Fig. 9). Such an inversion (change of sign) of the photocatalytic effect due to the calcination of the specimen in vacuo (after it is annealed in hydrogen) was observed by Kohn and Taylor (40) who worked with hydrides of various metals. 

At present, several stable photocatalytic systems for production of hydrogen from water and organic compounds are made of semiconducting oxides and suitable proton reducing catalyzer. An efficient electron transfer between inorganic semiconductor and bacterial hydrogenase was shown to result in hydrogen photoproduction. 

The selective oxidation and, more generally, the activation of the C-H bond in alkanes is a topic of continuous interest. Most methods are based on the use of strong electrophiles, but photocatalytic methods offer an interesting alternative in view of the mild conditions, which may increase selectivity. These include electron or hydrogen transfer to excited organic sensitizers, such as aryl nitriles or ketones, to metal complexes or POMs. The use of a solid photocatalyst, such as the suspension of a metal oxide, is an attractive possibility in view of the simplified work up. Oxidation of the... 

CQDs can be efficient chromophores for photon harvesting and photoconversion [55] and it was shown that photogenerated electrons could be transferred to gold or platinum nanoparticles for the photocatalytic conversion of carbon dioxide and splitting of water for hydrogen generation [55,56], In both cases, CQDs similar to titania... 

Wu et al. (1999b) reported that the intercalated Fe Oj showed photocatalytic activity of evolution from a CH OH-HjO solution by transfer of the photoelectron from Fe Oj to the host layered compound, which decreased the recombination of electrons and holes under visible light irradiation. However, it was found in this work that hydrogen evolution under visible light was not due to photocatalytic decomposition of water (Wu et al., 1999b). 

Photocatalytic oxidation of 2,4-dichlorophenoxyacetic acid (2,4-D) was investigated (Sun and Pignatello, 1995). In addition to the dominant hydroxyl radical mechanism, Sun and Pignatello found evidence that direct hole oxidation may be the mechanism for the photocatalytic degradation of some organic compounds. The assumed mechanism for this oxidation is H+ acting as an electron-transfer oxidant, while O behaves like a free OH and abstracts H or adds to C=C multiple bonds. Hole oxidation has been used to explain the oxidation of oxalate and trichloroacetate ions, which lack abstractable hydrogens or unsaturated C-C bonds. Whether the reaction... 

Among the practical consequences of micellar-contained electron transfer systems are the photocatalytic evolution of hydrogen [56], the containment of photoactive semiconductors within the protective micellar core [57], and the use of such systems as kinetic models for mechanistic characterization of light-responsive redox herbicides [58]. 

Recently, it was reported that loading small amount of platinum onto tungsten(VI) oxide enhances the visible-light photocatalytic activity significantly and this is caused by the catalytic action of platinum to induce multiple-electron transfer to oxygen 44). Reactions of two and four-electron transfer processes are as follows (potential in parentheses is standard electrode potential versus standard hydrogen electrode at pH 0). 

An exciting aspect of atom-transfer reactivity is the possibility of photocatalytic processes. The metal dihydrides formed in the initial photochemical hydrogen atom-transfer reaction may be turned over to... 

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