Photochemical water splitting

A photochemical process could use the energy of sunlight to split water into H2 and 02, the former then being used as a fuel which can be stored indefinitely. The splitting of water requires in principle a photoactivated catalyst dissolved or dispersed in water. The energy requirement for the overall reaction H20— H2 + 02 is 1.23 eV (per electron). In conditions of electrolysis it would however proceed extremely slowly at this potential difference which pertains to a thermodynamic equilibrium, and it is well known that a substantial overvoltage of the order of 0.5 V is required in practice to drive this reaction. 

An improvement in the efficiency of photochemical splitting of water was the incentive for Morris [49]. A parallel optical screening method was developed to select photocatalytically active catalysts by their adsorption spectra (UV and visible light). 

From the point of view of organic synthesis, the overall process consists of the formation of carbohydrates (CH20) by the reduction of carbon dioxide. The essence of the process is the use of photochemical energy to split water and concomitantly to reduce C02. Many proteins and small molecules are involved in photosynthetic machinery. Inorganic species are in the centre of photosynthesis as pigments in light harvesting, substrates, products, catalysts, and electron transfer mediators. 

Osterloh FE. Inorganic materials as catalysts for photochemical splitting of water. Chem Mater 2008 20 35-54. 

Photochemical splitting of water achieved by combining two photocatalytic reactions on suspended Ti02 particles namely, the reduction of water to H2 using bromide ions and the oxidation of water using Fe(III) species. High efficiency also observed for the photoassisted OER on TiCte in the presence of Fe(III) ions. 326, 327... 

Closely related is the so-called photochemical diode,489 consisting of either a metal/ semiconductor Schottky barrier or a p n junction, which generates the voltage needed on illumination, to split water. Photochemical diodes are discussed along with other twin photosystem configurations in the next Section. 

A first successful attenpt to split water photochemically this way was made by us in 1979 (Ji2). A copolymer of maleic anhydride and styrene was used as a protective agent for the Pt sol. This is suitable to achieve selectivity since it provides functions with pronounced hydrophobicity. Of the redox products formed in the li t reaction MV is relatively hydrophobic and will therefore interact with the Pt. Ru(bipy)j on the other hand is prone to interact with the hydrophilic and negatively charged RUO2 surface. One disadvantage of this system is that the quant an yield of water splitting is small and that... 

Water photochemical splitting cyclic process, 353 Fe OFe moiety, 348 osmocene, 349-353... 

The essence of natural photosynthesis is the use of photochemical energy to split water and reduce CO2. Molecular oxygen is evolved in the reaction, although it appears at an earlier stage in the sequence of reactions than the reduction of carbon dioxide. Photochemical processes produce compounds of high chemical potential, which can drive a multistep synthetic sequence from CO2 to carbohydrate in a cyclic way. Reaction (16) is quite endoergic and thus thermodynamically very improbable in the dark (AG° = 522 kJ per mole of CO2 converted). Production of one molecule of oxygen and concomitant conversion of one molecule of carbon dioxide require the transfer of four electrons ... 

However, by 3.5 billion years ago photochemical splitting of water, or photosynthesis was happening. 

Osterloh, F. E., Inorganic Materials as Catalysts for Photochemical Splitting of Water, Chem. Mater. 2008, 20, 35 54. 

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