Transition metal photocatalysts

M.V. Yakunichev, Photostimulated Emission of Singlet Oxygen from the Surface of Deposited Oxides of Transit Metals (Photocatalysts), PhD Thesis, Moscow, 1986 (in Russian)... 

The photoassisted activity is especially useful for degradation of strongly hazardous substances or recalcitrant pollutants, which are resistant to removal in chemical and/or biochemical processes. Transition metal photocatalysts are able to stimulate the indirect photodegradation of molecules that resist direct photolysis. Thus, they can activate species not absorbing sunlight, or those whose excited states populated by simlight absorption are chemically inefficient. 

Electron transfer processes involving excited transition metal photocatalysts are examined in Section 6.8.2. Reactive radical ions are usually generated from neutral... 

As an example, Yoon [34] has studied the inter- and intramolecular 2+2 cycloaddition of unsaturated ketones (an example in Scheme 8.19). A chiral version of the reaction cannot be obtained by simply adding a chiral catalyst, because electronic excitation promotes the addition of the aromatic ketone also when not complexed, as indeed it is often the case with this kind of reactions. However, he was successful when he used two catalysts, viz., a visible-absorbing transition metal photocatalyst and a stereocontrolling Lewis acid cocatalyst. Under these conditions, the excited molecule could react only with a molecule complexed with a chiral ligand (Scheme 8.20). 

While the majority of transition metal photocatalysts are based upon ruthenium and iridium complexes, similar intermolecular cyanation reactions can also be accomplished using gold-based transition metal photocatalysts. 

The benefit of using transition metal photocatalysts is often selective activation over organic molecules, which typically do not absorb light in the visible range. However, a broad range of organic dyes and porphyrins with... 

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

Thus to conclude, it can be inferred that the ultrasound alone is less effective in degrading phenol compared to the photocatalyst, TiC>2. However, when the two techniques are combined, the result is most effective. Nevertheless, the role of transition and inner transition metal ions is still not clear although their effect could be seen - may be the lower concentrations of these ions could not play any effective role as of now until a further study is carried out. 

Surface doping of oxide colloids and nanostructured electrodes with transition metal ions and complexes is of great interest for improving efficiency and selectivity of photocatalysts and photoelectrodes. Such surface ions as electron donors or acceptors play an important role as catalytic active centers, in charge transfer and in adsorption. There were many publications on this subject and we will try to bring forward the most... 

Another approach is the introduction of some transition metals, such as Cr, V, Fe, Cu, Mn, Co, Ni, Mo, and La into the synthesis mixture of the MCM-41, MMS with a Si/Me ratio of 80 [85], It was then demonstrated that the presence of the transition metal salts in the gel during the hydrothermal synthesis process hinders the action of the template, which results in MCM-41 pores that are not well formed. These materials were then loaded with TiOz, via the solgel method, and the activity of the Ti02/TM-MCM-41 catalysts in the degradation of the 4-chlorophenol reaction was tested in the presence of UV and/or visible light. It was shown that although some metals are deleterious, others can improve the performance of photocatalysts and even enable them to utilize visible light [85],... 

Dyes such as erythrosin B [172], eosin [173-177], rose bengal [178,179], rhodamines [180-185], cresyl violet [186-191], thionine [192], chlorophyll a and b [193-198], chlorophyllin [197,199], anthracene-9-carboxylate [200,201], perylene [202,203] 8-hydroxyquinoline [204], porphyrins [205], phthalocyanines [206,207], transition metal cyanides [208,209], Ru(bpy)32+ and its analogs [83,170,210-218], cyanines [169,219-226], squaraines [55,227-230], and phe-nylfluorone [231] which have high extinction coefficients in the visible, are often employed to extend the photoresponse of the semiconductor in photoelectro-chemical systems. Visible light sensitization of platinized Ti02 photocatalyst by surface-coated polymers derivatized with ruthenium tris(bipyridyl) complex has also been attempted [232,233]. Because the singlet excited state of these dyes is short lived it becomes essential to adsorb them on the semiconductor surface with... 

The first popular photocatalysts in homogeneous media were transition metal carbonyls (eg M(CO)6, where M=Cr, Mo, W Fe(CO)5, Fe3(CO)i2, Ru3(CO)i2, Mn2(CO)10) and their derivatives [28], Recently, other systems have been intensively studied, eg iron(III) complexes, especially in the photo-Fenton reactions [63-78, 139-142],polyoxometallates,porphyrins, metalloporphyrins,and other porphyrinoid sensitizers. 

The effects of other transition metal ions are qualitatively similar, although their extent is decidedly smaller. Complexes of copper, chromium, and manganese with central atoms in their higher oxidation states are fairly good candidates to play the role of an environmental photocatalyst [20],... 

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