Photocatalysts complexes

To this category belong, e.g., homogeneous photocatalytic systems based on soluble metal complexes or organic dyes as photocatalysts. Instructive examples are photoreactions assisted by heteropolyacids (HPAs), transition meal complexes with carbonyl, phosphine or some other ligands, and metal porphyrins. 

Scheme 7 illustrates the transformations of a bicycloheptadiene in the presence of transition metal complexes as photocatalysts or photogenerated catalysts [11] ... 

Procedure 10% aqueous solution of potassium iodide, KI, when exposed to sunlight, liberated I2 due to the photolytic decomposition and gave blue colour with freshly prepared starch solution. The intensity of blue coloured complex with the starch increased many fold when the same solution was kept in the ultrasonic cleaning bath. As an extension of the experiment, the photochemical decomposition of KI could be seen to be increasing in the presence of a photocatalyst, Ti02, showing an additive effect of sonication and photocatalysis (sono-photocatalysis) However, the addition of different rare earth ions affect the process differently due to the different number of electrons in their valence shells. 

Catalysis is known as the science of accelerating chemical transformations. In general, various starting materials are converted to more complex molecules with versatile applications. Traditionally, catalysts are divided into homogeneous and heterogeneous catalysts, biocatalysts (enzymes), photocatalysts, and electrocatalysts, which are mainly used... 

In classical kinetic theory the activity of a catalyst is explained by the reduction in the energy barrier of the intermediate, formed on the surface of the catalyst. The rate constant of the formation of that complex is written as k = k0 cxp(-AG/RT). Photocatalysts can also be used in order to selectively promote one of many possible parallel reactions. One example of photocatalysis is the photochemical synthesis in which a semiconductor surface mediates the photoinduced electron transfer. The surface of the semiconductor is restored to the initial state, provided it resists decomposition. Nanoparticles have been successfully used as photocatalysts, and the selectivity of these reactions can be further influenced by the applied electrical potential. Absorption chemistry and the current flow play an important role as well. The kinetics of photocatalysis are dominated by the Langmuir-Hinshelwood adsorption curve [4], where the surface coverage PHY = KC/( 1 + PC) (K is the adsorption coefficient and C the initial reactant concentration). Diffusion and mass transfer to and from the photocatalyst are important and are influenced by the substrate surface preparation. 

The selective oxidation and, more generally, the activation of the C-H bond in alkanes is a topic of continuous interest. Most methods are based on the use of strong electrophiles, but photocatalytic methods offer an interesting alternative in view of the mild conditions, which may increase selectivity. These include electron or hydrogen transfer to excited organic sensitizers, such as aryl nitriles or ketones, to metal complexes or POMs. The use of a solid photocatalyst, such as the suspension of a metal oxide, is an attractive possibility in view of the simplified work up. Oxidation of the... 

Krissanasaeranee, M., et al., Complex carbon nanotube-inorganic hybrid materials as next-generation photocatalysts. Chemical Physics Letters, 2010. 496(1-3) p. 133-138. 

In this section, Ru(bpy)j+ and its polymer analogoues are mainly described, since the Ru complex is best known as the photocatalyst that can photolyze water theoretically. 

Bis(2,2 -bipyridyl)ruthenium(II) was anchored onto poly(4-vinyIpyridine) (PVP) (6) 28>29), but the polymer complex is not suitable as photocatalyst, because it is susceptible to photoaquation. A polymer complex containing Ru bpy) + pendant groups was first prepared by reaction of polystyrene (PSt) as shown in Eq. (15)30). 

Platinized and sensitized (by ruthenium polypyridyl complexes) layered alkali-metal titanates, niobates, and titaniobates were used as photocatalysts for H2 and IJ production [91]. The use of reversed micelles as microreactors was reviewed in a feature article [92]. 

Recently, the interest in Re (I) complexes has been increased due to their potential utility for the activation and reduction of C02 into CO and C032 in a purpose of construction of artificial photosynthetic systems [11-13]. Rhenium Complexes such as ReX(CO)3(bpy) (X=C1, Br) and Re(CO)2(bpy)[P(OEt)3]2 have been used as photocatalysts for C02 reduction to CO in solvent mixture of triethanolamine/dimethylformamide [12,13]. Most of the research on photochemical activation and reduction of C02 using Re(I) complexes have focused on the homogeneous solution systems. There are few reports concerned about the encapsulation of rhenium complexes into molecular sieves and their photochemical application to the photochemical reduction of C02. 

Poly(pyridyl)ruthenium complexes, typically, [Ru(bpy)3]2+ have frequently been used as photocatalysts in the redox reactions between electron donors (Dred) and acceptors (Aox) to yield the oxidized (Dox) and reduced (Ared) forms (Eq. 20) [34-37] ... 

Alcohols such as methanol, 2-propanol, and benzhydrol are cleanly oxidized to the corresponding carbonyl compounds upon photoexcitation with Na3PWi2O40 in water or with (n-P NfoPW C in CH3CN (406). The quantum yields appear to be governed by the oxidation potential of the alcohol, the availability of a-hydrogens, and the tightness of complexation with the photocatalyst The reactivity order is primary alcohol > secondary alcohol > tertiary alcohol. 

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