Photochemical splitting of water into H2 and O2 using solar energy is a process of great economic and environmental interest. Since the discovery of the first water splitting system based on TiO2 and Pt in 1972 by Fujishima and Honda, over 130 inorganic materials have been discovered as catalysts for this reaction. This review discusses the known inorganic catalysts with a focus on structure–activity relationships.

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Inorganic Materials as Catalysts for Photochemical Splitting of Water

In view of the diminishing oil resources and the ongoing climate change, the use of efficient and environmentally benign technologies for the utilization of renewable resources has become indispensible. Therein, hydrogenolysis reactions offer a promising possibility for future biorefinery concepts. These reactions result in the cleavage of C-C and C-O bonds by hydrogen and allow direct access to valuable platform chemicals already integrated in today's value chains. Thus, hydrogenolysis bears the potential to bridge currently available technologies and future biomass-based refinery concepts. This Review highlights past and present developments in this field, with special emphasis on the direct utilization of cellulosic feedstocks.

Hydrogenolysis Goes Bio: From Carbohydrates and Sugar Alcohols to Platform Chemicals

In this review, for a variety of metals and semiconductors, an attempt is made to generalize observations in the literature on the effect of process conditions applied during photodeposition on (i) particle size distributions, (ii) oxidation states of the metals obtained, and (iii) consequences for photocatalytic activities. Process parameters include presence or absence of (organic) sacrificial agents, applied pH, presence or absence of an air/inert atmosphere, metal precursor type and concentration, and temperature. Most intensively reviewed are studies concerning (i) TiO2; (ii) ZnO, focusing on Ag deposition; (iii) WO3, with a strong emphasis on the photodeposition of Pt; and (iv) CdS, again with a focus on deposition of Pt. Furthermore, a detailed overview is given of achievements in structure-directed photodeposition, which could ultimately be employed to obtain highly effective photocatalytic materials. Finally, we provide suggestions for improvements in description of the photodeposition methods applied when included in scientific papers.

Methods, Mechanism, and Applications of Photodeposition in Photocatalysis: A Review.

Although the use of CO as a reductant had been in the past confined to few reactions, its use in organic synthesis, especially in the reductive carbonylation of nitro aromatics and the oxidative carbonylation of aromatic amines, has increased dramatically. Since the discovery of CO-induced reduction of nitro groups, there has been a wide spread increase of interest in the application and mechanistic understanding of this reaction. In a major review published in 1988 it was noted, that in practice no studies of the mechanism of N-carbonylation of aromatic nitro compounds with alcohols leading to carbamates have been carried out. This review clearly shows a major change since that publication. Indeed, metal-catalyzed reductive carbonylation of nitro aromatics using CO as reducing agent has been in the past 10 years the subject of intense investigation both in academia and in the chemical industry. Several articles and reviews have covered the subject up to the late 1980s. The authors will concentrate on more recent literature, but sometimes older data will be used to establish an understanding of these reactions. 127 refs.

A Review of the Selective Catalytic Reduction of Aromatic Nitro Compounds into Aromatic Amines, Isocyanates, Carbamates, and Ureas Using CO.

Nitrogen dioxide and carbonate radical anion: two emerging radicals in biology.

Catalytic amounts of RuCl2(PPh3)3 in the presence of excess of N-methylmorpholine N-oxide (NMO) in DMF, oxidizes secondary alcohols to ketones. Spectral and electrochemical studies reveal the formation of an oxocomplex of Ru(IV), on adding excess of NMO. In the proposed mechanism, the Ru(IV)-oxo complex formed from Ru(II) and NMO, attacks the substrate in the rate determining step.

RuCl2(PPh3)3 catalyzed oxidation of secondary alcohols with N-methylmorpholine N-oxide

The selective oxidation of thioethers to sulphoxides and of PPh3 to PPh3O can be effected by NMO in DMF as solvent in the presence of Ru(III) chloride as catalyst. Kinetic investigations indicate that the orders with respect to the catalyst and oxidant are one each. The order with respect to the substrate is variable being fractional order at low concentrations and zero at high concentrations. Spectrophotometric studies reveal the formation of a 1:1 complex between the substrate and the catalyst. A mechanism consistent with the above observations has been proposed and verified.

Kinetics and mechanism of Ru(III)-catalysed oxidation of organic sulphides and triphenylphosphine by N-methylmorpholine N-oxide

Rate constants for the reactions of the borate radical B(OH) 3 O⨪ with various para -substituted benzoic acids were measured by flash kinetic spectrophotometry. The product analyses and kinetic studies are consistent with an electron transfer mechanism followed by decarboxylation. In the case of the p -aminobenzoate anion the —NH 2 and —COO − groups react simultaneously with the borate radical providing two parallel pathways with different rate constants.

Reactions of the borate radical with para-substituted benzoic acids (photo-kolbe reaction)

The effect of substituents on the reactivity of aniline with phosphate radical

Ruthenium(III) catalysed oxidation of 1-phenylethanol and substituted 1-phenylethanols by phenyliodosoacetate (pia) in acetic acid is studied and a probable mechanism suggested. The proposed mechanism involves the formation of a complex of Ru(III) and (pia) which reacts with the substrate, S, in a slow step. Spectrophotometric evidence for this complex is presented. Studies on the effect of substituents suggest a transition state involving a π-complex.

J. C. Kuriacose

Papers

RuCl2(PPh3)3 catalyzed oxidation of secondary alcohols with N-methylmorpholine N-oxide

Kinetics and mechanism of Ru(III)-catalysed oxidation of organic sulphides and triphenylphosphine by N-methylmorpholine N-oxide

Reactions of the borate radical with para-substituted benzoic acids (photo-kolbe reaction)

The effect of substituents on the reactivity of aniline with phosphate radical

Ruthenium(III) catalyzed oxidation of 1-phenylethanol and substituted 1-phenylethanols by phenyliodosoacetate