Active photocatalysts

In situ metallization has been claimed to provide a convenient method for the preparation of metal-deposited and metal sulfide deposited CdS during photocatalytic decomposition of aqueous sulfide. As-prepared MS/CdS and M/CdS bifunctional photocatalysts (MS = Pt or Ir sulfide M = Pt or Ir) were reported to be more active photocatalysts than CdS and ex-situ metallized CdS [285]. 

Recently, we have shown that the combination of barium tetratitanate, BaTi40g and sodium hexatitanate, NagTigOis, with ruthenium oxides leads to active photocatalysts for water decomposition[1,2]. The unique feature of these photocatalysts is that no reduction of the titanates is required to be activated this is intrinsically different from conventional photocatalysts using TIO2 which are often heat-treated in a reducing atmosphere. Such different photocatalytic characteristics suggest that efficiency for the separation of photoexcited charges (a pair of electrons and holes) which is the most important step in photocatalysis is... 

Fluoride is also used as an anionic dopant. An early study demonstrated that F substitutes surface OH species leading to an increase in the degradation of phenol at least three times faster than an undoped sample [56]. Other anions are also effective as dopants in reducing the Ti02 bandgap. Chloride, for example, was shown to give active photocatalysts in the visible range thanks to the red shift in the absorption spectra (Eg = 3 eV) but also increased surface acidity [57]. 

Mixed Ti-W oxides, after deposition of Pt, are active in the mineralization of toluene with sunlight excitation due to the low bandgap (2.7 eV) caused by the presence of both W and Pt in the anatase structure [87]. Vanadium was also used to prepare visible light active photocatalysts in dye degradation [88]. 

Martinez-Ferrero, E., Sakatani, Y., Boissire, C., Grosso, D., Fuertes, A., Fraxedas, J., and Sanchez, C. (2007) Nanostructured titanium oxynitride porous thin films as efficient visible-active photocatalysts. Advanced Functional Materials, 17 (16), 3348—3354. 

All of these reports indicate that the deposition of titania upon available supports seems to be a good strategy to prepare active photocatalysts. 

Yu, B. Y. Kwak, S.-Y., Carbon quantum dots embedded with mesoporous hematite nanospheres as efficient visible light-active photocatalysts. J. Mat. Chem. 2012,22 8345-8353. 

Takata, T., Shinohara, K., Tanaka, A., Kara, M., Kondo, J.N., Domen, K. 1997a. A highly active photocatalyst for overall water splitting with a hydrated layered perovskite structure. J Pho-tochem PhotobiolA Chem 106 45 9. 

Liu H, Gao L (2004) (Sulfur, Nitrogen)-codoped rutile-titanium oxide as a visible-light-activated photocatalyst. J Am Ceram Soc 87 1582-1584... 

Chemoselecti vity could potentially be achieved if the oxidation potential of a desired donor adsorbate lies between the valence band edges of two possible semiconductor photocatalysts. Since TiOj has a more positive valence band edge than does CdS, it should be the more active photocatalyst. Consistent with this idea, decarboxylation of organic acids, Eq. (5), is much more efficient on irradiated suspensions of rutile than of CdS... 

The course of hydrocarbon photocatalyzed oxidations seems to depend significantly on the relative positions of the valence band edge of the active photocatalyst and the oxidation potential of the substrate. For example, in contrast to the clean oxidation of toluene described above, lower activity was observed in neat benzene, a substrate whose oxidation potential lies at or slightly below the valence band edge This observation implies the importance of radical cation formation (via photoinduced electron transfer across the irradiated interface) as a preliminary step to hydrocarbon radical formation. If beitzene-saturated aqueous semiconductor suspensions are... 

In Part I the fundamental aspects of photocatalysis are described. Photoelectrochemical processes at semiconductors are the most important basics for all photocatalytic reactions (Chapter 2). Design, preparation and characterization of active photocatalysts have been an important research subject,... 

Effective Screening of Active Photocatalysts for Water Splitting Using Na2C03 Addition Method... 

This paper concentrates on a detailed description of the primary events occurring immediately after the absorption of a photon within a single titanium dioxide particle in an aqueous environment. This restriction was made, because 1) titanium dioxide seems to be the most active photocatalyst, and 2) the photocatalytic treatment of polluted water seems to be a promising application for an interfacial electron transfer serving the environment. 

Of the water oxidation catalysts mentioned, all use either a powerful chemical oxidant (e.g., CeIV, OC1, Oxone, Ihi OOI I) or an electrode to drive the reaction. Efforts to couple the oxidation to a photoprocess have not yielded an active photocatalyst but nonetheless are beginning to yield some promising results as shown previously in Fig. 9 for the Ru-Mm dyad.254 256... 

Several research approaches are pursued in the quest for more efficient and active photocatalysts for water splitting (i) to find new single-phase materials, (ii) to tune the band-gap energy of TJV-active photocatalysts (band-gap engineering), and (iii) to modify the surface of photocatalysts by deposition of cocatalysts to reduce the activation energy for gas evolution. Obviously, the previous strategies must be combined with the control of the s)mthesis of materials to customize the crystallinity, electronic structure, and morphology of materials at nanometric scale, as these properties have a major impact on photoactivity. 

In the development of active photocatalysts imder visible light, it is essential to control their electronic energy structure. The strategies for controlling the energy structure of photocatalysts for water splitting may be classified in three ways (i) cation or anion doping, (ii) use of mixed semiconductor composites, and (iii) use of semiconductor alloys. 

These two complexes were also found to be active photocatalysts for the polymerization of ethylene (77, 47, 47a). Irradiation of solutions of either complex under 1 atm ethylene resulted in the rapid formation of high-molecular-weight polyethylene. The rate of ethylene polymerization was increased by the addition of various metal halides prior to photolysis. The mechanism of this reaction was not investigated but the authors postulated that the active species were photogenerated Ziegler-Natta-type catalysts (77). 

This review has been written in order to clarify fundamental aspects of photocatalysis, an important subject in inorganic and material chemistry, not to present a list of studies on photocatalysis reported so far, since it seems rather difficult to make a complete review by introducing all or a large part of the reported studies on photocatalysis of relatively long history. This review is based on the author s experience in studies on photocatalysis and topics are limited to so-called semiconductor photocatalysis definition and examples of photocatalysis, its principle and kinetics, visible light-induced photocatalysis, and design of active photocatalysts are discussed in detail. 

PHOTOCATALYSIS BY INORGANIC SOLID MATERIALS 421 VI. Design of Active Photocatalysts... 

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