Hydrogen photochemical evolution

It is interesting to note that in the photochemical evolution of hydrogen from aqueous solutions of Fe+2 ions (17) H atoms appear as intermediate, but preceding formation of e aq could not be proved. 

Mallouk and coworkers explored the electrochemistry and photochemical evolution of hydrogen from water using a zeolite modified electrode. Methylviologen (MV ) was encapsulated in platinized zeolite L powder onto which ZifiTMPyP" ) was externally adsorbed (Figure 106). Taking advantage of the zeolite microstructure in an attempt... 

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

Fretwell R, Douglas P (2002) Nanocrystalline Ti02-Pt photoelectrochemical cells-UV induced hydrogen evolution from aqueous solution of ethanol. Photochem Photobiol Sci 1 793-798... 

Abe R, Hara K, Sayama K, Domen K, Arakawa H (2000) Steady hydrogen evolution from water on Eosin-Y-fixed Ti02 photocatalyst using a silane coupling reagent under visible light irradiation. J Photochem Photobiol A Chem 137 63-69... 

Tsuji I, Kudo A (2003) Hydrogen evolution from aqueous sulfite solution under visible light irradiation over Pb and halogen-codoped ZnS. J Photochem Photobiol A 156 249-252... 

Under photochemical etching, special demands should be made on the oxidizing agent in the solution. Above all, it must be reduced with the participation of conduction-band electrons alone (see above) and at potentials less negative than the potential of reducing water or H+ ions to molecular hydrogen, in order to avoid gas evolution. 

UV irradiation of low-valent cations (e.g. V2+, Cr2+, Fe2+) in acidic aqueous solution leads to photooxidation of the metal and evolution of H2.110 This reaction, described by equation (47), is thought to involve the formation of the oxidized metal and an electron in the primary photochemical act. Immediate scavenging of the photoelectrons by protons yields hydrogen atoms which then combine to produce H2. Some interest has been shown in this type of system as a component of a photochemical water-splitting cycle.110 111... 

The primary photochemical step, resulting from the absorption of the photon, must produce transition to a spectroscopically recognizable upper state—excitation or ionization, in which the Franck-Condon principle is observed. Which of these is involved in the process of hydrogen evolution could be clearly ascertained for one of the simple systems in which Reaction 1 occurs 02-free aqueous solutions of iodide ions. The absorption spectrum of such solutions is well known, showing two bands with maxima at 226 and 194 m/z. Photolytic light at 254 m/z is absorbed near the onset of the 226 m/z max. band. 

At the end of 1960 s, Honda and Fujishima found that in a photochemical cell employing titanium dioxide and platinum electrodes, irradiation of the titanium dioxide electrode resulted in splitting of water into hydrogen and oxygen (18,19). This work has had an extraordinarily strong impact for the research on the photochemical action of various semiconductors inducing evolution of hydrogen from water as well as new catalytic reactions (20). 

Photochemical H2-Evolution and Hydrogenation Processes Using Heterogeneous Catalysts.180... 

Kim, Y. I. Atherton, S. J. Brigham, E. S. Mallouk, T. E. Sensitized layered metal oxide semiconductor particles for photochemical hydrogen evolution from non-sacrificial electron donors, J. Phys. Chem. 1993, 97, 11802. 

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. 

Randt, C. and Senger, H., Participation of the two photosystems in light dependent hydrogen evolution in Scenedesmus obliquus, Photochem. Photobiol., 42, 553, 1985. 

The fast bimolecular reaction HgBr + Brj HgBr2 + Br has been shown to be responsible for the observed rapid and efficient regeneration of HgBr2 in cyclic operation of the repetitively pulsed HgBr photodissociation laser. Hydrogen evolution has been observed from photochemically reduced tin(iv) porphyrins in the presence of colloidal platinum. ... 

Silicon is highly unstable in aqueous electrolytes due to the formation of an insulating oxide film which prevents the use of n-Si as photoanode. On the other hand, the silicon electrode has poor kinetics for hydrogen evolution which is not desirable for its use as a photocathode. Many methods have been explored to stabilize Si electrodes in aqueous solutions for possible applications as photochemical cells. They include coating the surface with noble metals, metal oxides, metal silicides, or organic materials as shown in Table 6.6. Also, some redox species, the reduction of which can favorably compete with the oxidation of silicon, can be used to stabilize silicon anodes... 

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