Titanium photocatalysis

Catalysis (qv) refers to a process by which a substance (the catalyst) accelerates an otherwise thermodynamically favored but kiaeticahy slow reaction and the catalyst is fully regenerated at the end of each catalytic cycle (1). When photons are also impHcated in the process, photocatalysis is defined without the implication of some special or specific mechanism as the acceleration of the prate of a photoreaction by the presence of a catalyst. The catalyst may accelerate the photoreaction by interaction with a substrate either in its ground state or in its excited state and/or with the primary photoproduct, depending on the mechanism of the photoreaction (2). Therefore, the nondescriptive term photocatalysis is a general label to indicate that light and some substance, the catalyst or the initiator, are necessary entities to influence a reaction (3,4). The process must be shown to be truly catalytic by some acceptable and attainable parameter. Reaction 1, in which the titanium dioxide serves as a catalyst, may be taken as both a photocatalytic oxidation and a photocatalytic dehydrogenation (5). 

Heterogeneous Photocatalysis. Heterogeneous photocatalysis is a technology based on the irradiation of a semiconductor (SC) photocatalyst, for example, titanium dioxide [13463-67-7] Ti02, zinc oxide [1314-13-2] ZnO, or cadmium sulfide [1306-23-6] CdS. Semiconductor materials have electrical conductivity properties between those of metals and insulators, and have narrow energy gaps (band gap) between the filled valence band and the conduction band (see Electronic materials Semiconductors). 

Asahi, R., Morikawa, T., Ohwaki, T., Aoki, K., and Taga, Y. (2001) Visible-light photocatalysis in nitrogen-doped titanium oxides. Science, 293 (5528), 269-271. 

Sakthivel, S. and Kisch, H. (2003) Daylight photocatalysis by carbon-modified titanium dioxide. Angewandte Chemie International Edition, 42 (40), 4908-4911. 

Kowalska, E., Remita, H., Colbeau-Justm, C., Hupka, J., and BelloniJ. (2008) Modification of titanium dioxide with platinum ions and dusters application in photocatalysis. Journal of Physical Chemistry C, 112 (4), 1124-1131. 

Nanomaterials can also be tuned for specific purposes through doping. Specifically, the effect of the presence of manganese oxides on photocatalysis involving primarily titanium dioxide will be considered in this section. Titanium dioxide is a well-known photocatalyst and will be considered separately. K-OMS-2, which has a cryptomelane structure, is illustrated in Figure 8.4. Not all the literature discussed in this section, however, involves OMS tunnel structure materials. For example, amorphous manganese oxide (AMO) is also discussed as a photocatalyst. Manganite (MnOOH) is also included in battery applications. 

Fujishima, A. and Zhang, X. (2006) Titanium dioxide photocatalysis present situation and future approaches. Comptes Rendus Chlmle, 9, 750-760. 

To be used as photocatalysts, especially in the so-called clean technologies, active materials must fulfill the following requirements (1) very low toxicity, (2) resistance to photo-corrosion, (3) high availability, (4) high catalytic efficiency, and (5) low cost. From all the materials cited above, titanium dioxide and its derivatives seem to offer the best answer to these requirements, being by far the most commonly utilized photocatalysts. To be used in gas-phase photocatalysis, in addition to the above requirements, two other conditions are still necessary, that is, a very small pressure drop and an easy recovery. 

Titanium dioxide has also been involved in the photocatalysis of toxic inorganic substances to yield harmless or less-toxic species. Sterilisation of drinking water by chlorine yields potentially carcinogenic compounds so that ozone has been used as an alternative sterilising agent. Bromate... 

Since the discovery of photoelectrochemical splitting of water on titanium dioxide (TiOj) electrodes (Fujishima and Honda, 1972), semiconductor-based photocatalysis has received much attention. Although TiO is superior to other semiconductors for many practical uses, two types of defects limit its photoeatalytic activity. Firstly, TiO has a high band-gap (E =3.2 eV), and it can be excited only by UV light (k < 387 nm), which is about 4-5% of the overall solar spectmm. Thus, this restricts the use of sunlight or visible light (Kormann et al., 1988). Secondly, the... 

Fujishima, A., Rao, T.N., Tryk, D.A. 2000. Titanium dioxide photocatalysis. J Photochem Photo-biol C Photochem Rev 1 1-21. 

Kato H, Hori M, Konta R, Shimodaira Y, Kudo A (2004) Construction of Z-scheme type heterogeneous photocatalysis systems for water splitting into H2 and O2 under visible light irradiation Chem Lett 33 1348-1349 Asahi R, Morikawa T, Ohwaki T, Aoki K, Taga Y (2001) Visible light photocatalysis in nitrogen-doped titanium oxide. Science 293 269-271... 

Rutile and anatase are used in photocatalysis and as catalyst supports and we describe some important structural features later. The titanium-oxygen system plays a key role in understanding stoichiometric variations. The highest oxide, Ti02, is polymorphic and the four known crystal structures are rutile, anatase, brookite and a high-pressure form similar to a-Pb02. 

Abstract A colloidal solution of titanium dioxide (TiO ) nanoparticles was prepared by the solvothermal method and dip-coated onto a polypropylene fabric with TMOS binder. The prepared TiO particles, colloidal solution and the coated fabrics were characterized by X-ray diffraction, SEM and TEM. The results showed that the TiO particles prepared by the solvothermal method were composed of anatase which uniformly coated the snbstrate. Photocatalysis induced bactericidal properties of coated fabrics were tested by measuring the viability of Escherichia coli. It was fonnd that solvothermally prepared TiO coatings have the ability to kill E. coli. This nniqne property of TiO makes it an ideal candidate in producing self-sterilizing protective masks and in providing bactericidal and self-cleaning properties to a variety of snrfaces. 

Ti02. - Semiconductor photocatalysis based on titanium dioxide (Ti02)... 

When titanium oxides are irradiated with UV light that is greater than the band-gap energy of the catalyst (about X < 380 nm), electrons (e ) and holes (h+) are produced in the conduction and valence bands, respectively. These electrons and holes have a high reductive potential and oxidative potential, respectively, which, together, cause catalytic reactions on the surfaces namely photocatalytic reactions are induced. Because of its similarity with the mechanism observed with photosynthesis in green plants, photocatalysis may also be referred to as artificial photosynthesis [1-4]. As will be introduced in a later section, there are no limits to the possibilities and applications of titanium oxide photocatalysts as environmentally harmonious catalysts and/or sustainable green chemical systems. ... 

S. Sakthivel and H. Kisch, Daylight Photocatalysis by Carbon-Modified Titanium Dioxide, Angew. Chem. Int. Ed. 2003,42, 4908. 

In the process of photocatalysis, the electrons and holes produced on photoirradiated Ti02 powders are trapped at the particle surface to form unpaired-electron species (step (4) in Fig.D.3). Photocatalytic reactions are actually the reactions of these radicals with reactant molecules at the Ti02 surface. Electron spin resonance (ESR) spectroscopy has been used for the detection of the photoproduced radicals on Ti02 at low temperatures such as 77 K. It has been reported that photoproduced electrons are trapped at various different sites titanium atoms on the surface or inside the particles, or oxygen molecules adsorbed on the surface. On the other hand, photoproduced holes are trapped at lattice OAygen atoms near the particle surface or at surface hydroxyl groups. We analyzed these radical species for several Ti02 photocatalysts that are commercially available, and found that the differences in the photoproduced radicals resulted from different heat-treatment conditions and the reactivity with several molecules.17)... 

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