Photocatalysis reactions

Keywords Photocatalysis Reaction mechanisms Novel photocatalysis ... 

The formation of halohydrins can be promoted by peroxidase catalysts.465 Recently 466 it has been shown that photocatalysis reactions of hydrogen peroxide decomposition in the presence of titanium tetrachloride can produce halohydrins. The workers believe that titanium(IV) peroxide complexes are formed in situ, which act as the photocatalysts for hydrogen peroxide degradation and for the synthesis of the chlorohydrins from the olefins. The kinetics of chlorohydrin formation were studied, along with oxygen formation. The quantum yield was found to be dependent upon the olefin concentration. The mechanism is believed to involve short-lived di- or poly-meric titanium(IV) complexes. 

In all examples discussed so far, the transient and the persistent species were continuously generated. Now, we consider the behavior when both radicals are initially present and then react without further generation by the cross-reaction of R" with Y" and by the self-terminations. This initial situation may be created by a pulse photolysis of suitable precursor molecules. The first theoretical treatment was given by O Shaughnessy et al.,144 and the resulting equations were later used to determine cross-reaction constants in the photocatalysis reaction of polyoxo-tungstates (Scheme 29).94... 

Homogeneous transition metal photocatalysis reactions generating HO radicals, often referred to as advanced oxidation processes (AOP),1518 are powerful methods to remediate (i.e. to remove or to destroy) organic pollutants from aqueous solutions.1519... 

Figure 3.13 TI02 modified micro-channel chip for photocatalysis reaction,and cross-sectional images of the channel (Takei et al., 2005).

There are various interactions between photos and materials. Photocatalysis reaction is one of them. The catalytic reaction and photochemical reaction can be... 

Garcfa, J., Oliveira, J., Silva, A., et al. (2007). Comparative Study of the Degradation of Real Textile Effluents by Photocatalysis Reactions Involving UV/T1O2/H2O2 and UV/Fe " /H202 Systems, J. Hazardous Materials, 147, pp. 105-110. 

Heterogeneous photochemical reactions fall in the general category of photochemistry—often specific adsorbate excited states are involved (see, e.g.. Ref. 318.) Photodissociation processes may lead to reactive radical or other species electronic excited states may be produced that have their own chemistry so that there is specificity of reaction. The term photocatalysis has been used but can be stigmatized as an oxymoron light cannot be a catalyst—it is not recovered unchanged. 

A large variety of organic oxidations, reductions, and rearrangements show photocatalysis at interfaces, usually of a semiconductor. The subject has been reviewed [326,327] some specific examples are the photo-Kolbe reaction (decarboxylation of acetic acid) using Pt supported on anatase [328], the pho-... 

The radicals are then involved in oxidations such as formation of ketones (qv) from alcohols. Similar reactions are finding value in treatment of waste streams to reduce total oxidizable carbon and thus its chemical oxygen demand. These reactions normally are conducted in aqueous acid medium at pH 1—4 to minimize the catalytic decomposition of the hydrogen peroxide. More information on metal and metal oxide-catalyzed oxidation reactions (Milas oxidations) is available (4-7) (see also Photochemical technology, 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). 

In the last two decades we have witnessed in photocatalysis, as a science, a continuous shift from phenomenological approaches to studies at the molecular level. With accumulation of information obtained in such studies, the accents in the work aimed at development of new photocatalysts and new photocatalytic reactions and technologies, are expected to more and more shift from empirical search to intentional design. 

Most of the so far designed photocatalytic systems can be divided into three categories simple molecular ones, organized molecular assemblies and semiconductor systems. In this section typical photocatalytic behaviour and reaction mechanisms will be discussed for photocatalysis with systems of all these types. 

Thus, photocatalysis and photogenerated catalysis indeed open up rather reach opportunities in fine organic synthesis, including some new reactions and nontraditional pathways for some known reactions. More efforts should be made in engineering of appropriate photocatalytic reactors for such synthesis. 

Chemical models of photosynthesis have been used to investigate two types of reactions photosynthesis and photocatalysis. In photosynthetic processes the standard Gibbs free energy of the reaction is positive, and solar energy is utilized to perform work. In photocatalytic processes the free energy is negative and solar energy is used to overcome the activation barrier. 

Photoinduced ET at liquid-liquid interfaces has been widely recognized as a model system for natural photosynthesis and heterogeneous photocatalysis [114-119]. One of the key aspects of photochemical reactions in these systems is that the efficiency of product separation can be enhanced by differences in solvation energy, diminishing the probability of a back electron-transfer process (see Fig. 11). For instance, Brugger and Gratzel reported that the efficiency of the photoreduction of the amphiphilic methyl viologen by Ru(bpy)3+ is effectively enhanced in the presence of cationic micelles formed by cetyltrimethylammonium chloride [120]. Flash photolysis studies indicated that while the kinetics of the photoinduced reaction,... 

Like water, the interaction of 02 with Ti02 also has implications for photocatalysis. As with water again, reactions of 02 with Ti02 are also important because 02 will form part of the environment in many Ti02 applications. 

Photocatalysis involve the production of reactions because of the incidence of light on a semiconductor material. Unlike metals, these materials have a forbidden energy band, which extends from the top of the so-called valence band to the bottom of the conduction band (Figure 18). 

Oxidation reactions are the most studied processes owing to the well-known ability of illuminated Ti02 in water to produce reactive oxygen species. In this context, heterogeneous photocatalysis could contribute to the replacement of hazardous compounds such as KMn04 and K2Cr20 [13]. 

The presence of two types of catalytic centers (e.g., oxidative and reductive) in the same material can give rise to the possibility of multi-step photocatalysis in a one-pot procedure. C-C coupling, for example, is a field of great interest and a recent very good review was published [221]. C-N coupling reactions are also of interest. 

However, the pathways for these reactions, particularly in the gas phase, have been only -.rtially characterized. In a wide variety of these reactions, coordinatively unsaturated, highly reactive metal carbonyls are produced [1-18]. The products of many of these photochemical reactions act as efficient catalysts. For example, Fe(C0)5 can be used to generate an efficient photocatalyst for alkene isomerization, hydrogenation, and hydrosilation reactions [19-23]. Turnover numbers as high as 3000 have been observed for Fe(C0)5 induced photocatalysis [22]. However, in many catalytically active systems, the active intermediate has not been definitively determined. Indeed, it is only recently that significant progress has been made in this area [20-23]. 

Nosaka, Y., Photoelectrochemical reactions at semiconductor microparticles, in Photocatalysis, Science and Technology, Kaneko, M. and Okura, I. (Eds), Kodansha/Springer, Berlin, 2002, Chap. 5. 

The ambient temperature and the possible use of solar UV are the advantages of photocatalysis moreover, Ti02 is not toxic. The reaction mechanisms of Ti02 photocatalytic oxidation of azo dyes was similar to the biodegradation process of oxidation of azo dyes with OH radical. 

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