Hydrogen from photochemical reduction

Two different types of zinc-porphyrins coordinated diiron complex act as catalysts for the photochemical reduction hydrogen evolution from water. In this system... 

Alkanes are formed when the radical intermediate abstracts hydrogen from solvent faster than it is oxidized to the carbocation. This reductive step is promoted by good hydrogen donor solvents. It is also more prevalent for primary alkyl radicals because of the higher activation energy associated with formation of primary carbocations. The most favorable conditions for alkane formation involve photochemical decomposition of the carboxylic acid in chloroform, which is a relatively good hydrogen donor. 

The reaction pathways by which the net transfer of a hydrogen atom from an amine to a photoexcited ketone has been extensively examined in the nanosecond [23, 25-30], picosecond [20, 22, 31-33], and femtosecond [24] time domains. The following mechanism, as it pertains to the photochemical reduction of benzophenone (Bp) by N, A-dimethylaniline (DMA), is derived from these numerous studies. Only an overview of the mechanism will be presented. The details of the studies leading to the mechanism will not be given for specifics, the reader is referred to the original literature. 

The photochemical reduction of 1-methylquinolinium ions by (TMS)3SiH proceeds regioselectively to afford the corresponding 1,4-dihydroquinones in a water-acetonitrile solvent system (Reaction 4.47) [83]. Mechanistic studies demonstrated that the reactions are initiated by photoinduced electron transfer from the silane to the singlet excited states of 1-methylquinolinium ions to give the silane radical cation-quinolinyl radical pairs, followed by hydrogen transfer in the cage to yield 1,4-dihydroquinones and silicenium ion. Silyl cations are quenched by water. 

Here we report a technique of direct synthesis of the photocatalytically active mesoporous composites Ti02/M (M = Cu, Ni, Co, Fe, Zn, Ag etc.) via photochemical reduction of the metal cations adsorbed on highly developed surface of a mesoporous titanium dioxide. The Ti02/M composites were found to be efficient photocatalysts of hydrogen evolution from water-alcohol mixtures. 

From a photochemical reduction of the water-soluble porphyrin 32, hydrogen was obtained in a reaction sequence (Scheme 2) simulating the biosynthesis of chlorophyll as well as the hydrogen formation in photosynthesis (82AG132). 

Hydrogen peroxide, formed from hydroxyl radicals, also can be involved. Photochemical reduction is negligible in sulfuric acid solution, because of the low concentration of Ce ions. Furman stated that 0.1 Af solutions of Ce(TV) in sulfuric acid solutions are stable for at least 6 years, but was of the opinion that nitrate and ammonium ions are undesirable. Stored at room temperature, 0.1 M Ce(IV) in nitric or perchloric acid solutions showed a 0.01 to 0.03% decrease in concentration per day. ° Protection from light is necessary for nitric and perchloric but hot for sulfuric acid solutions. 

Fig. (13). Photochemical reduction of carbon dioxide into formic acid using tricyclic tertiary amine as a sacrificial donor (a) Plant photosynthesis (b) amine radical cation formed from tricyclic amine (c) regeneration of amine by hydrogenation using Pd catalyst. Reproduced with permission from Ref. [58, 59]. 2011 McMillan Publishers Ltd.

Several studies in recent years have focussed on the preparation and characterization of metal particles in zeolites (1). Various ways have been developed to introduce metals and metal precursors including sublimation (2), adsorption (3), and ion-exchange (4). If it is desirable to remove the ligands from the precursor metal complexes many approaches can be used including high temperature reduction in hydrogen (5), photochemical degradation (6), sodium (7) or cadmium (8) reduction, H atom reduction (9), low temperature vacuum treatments (10) and plasma methods (11). 

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... 

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