Quantum yields reduction, photo

The oxidation of oxalic acid by mercuric chloride to give CO2 and mercurous chloride is a classic example of an induced reaction. This reaction is extremely slow unless small quantities of chromic acid and manganous ions are added, whereon facile reduction takes place Addition of permanganate or persulphate and some reducing agents is also effective and the oxidation proceeds readily under photo- or X-irradiation (Eder s reaction). The large quantum yield points to a chain mechanism , which could also function with an inducing oxidant, viz. 

Photoreduction of benzophenone by primary and secondary amines leads to the formation of benzpinacol and imines [145]. Quantum yields greater than unity for reduction of benzophenone indicated that the a-aminoalkyl radical could further reduce the ground state of benzophenone. Bhattacharyya and Das confirmed this in a laser-flash photolysis study of the benzophenone-triethylamine system, which showed that ketyl radical anion formation occurs by a fast and a slow process wherein the slow process corresponds to the reaction of a-aminoalkyl radical in the ground state of benzophenone [148]. Direct evidence for similar secondary reduction of benzil [149] and naphthalimides [150] by the a-aminoalkyl radical have also been reported. The secondary dark reaction of a-aminoalkyl radicals in photo-induced electron-transfer reactions with a variety of quinones, dyes, and metal complexes has been studied by Whitten and coworkers [151]. 

A relatively small amount of energy is required to do the job in a typical reprocessing plant. We estimate, under the most ideal circumstances,that 2000 Watts of absorbed light for a typical 5 ton/day plant would be necessary to achieve the reductive. stripping of plutonium. (This is based on the assumption that the dominant photo-reaction will be UO reduction with a Quantum Yield. 

Spectral variations of the ratio 4>o2I4>ei between the quantum yields for the photo-stimulated adsorption of oxygen and hydrogen, which also scales with y, was obtained for the reduced and oxidised surfaces of Zr02 (eq. 5.143 and Fig. 5.54). Reduction or oxidation of the Zr02 surface leads to changes in the surface potentials of this metal oxide, and thus to changes in the subsurface electric field. 

The photodegradation reactions involve not only the photooxidation, but also the photoreduction. For example, Chu W. and Javert C.T. (1994) reported a photoreduction reaction for aromatic compounds in the presence of hydrogen sources in which high reaction quantum yields were observed. Many dyes are also noted for their ease of deco lour ization in the absence of oxygen when a suitable electron donor or hydrogen source is present. For example, in the scheme IV, formation of colourless leuco-form of anthraquinone dye is observed after photo-reduction that the main structural integrity of the dye molecule retains (Rys P. and Zollinger H., 1972). 

In addition there is a report of a water soluble polymeric (copolymer) photoinitiators. These initiators were synthesized by copolymerizing 2-acryloxy and 2-acrylamido anthraquinone monomers with three water-soluble comonomers acrylamide, 2-acrylamido-2-methylpropane sulfonic acid and 2-acryloxyethyl trimethylammonium iodide. The polymerization activity correlates well with their measured photo reduction quantum yields in water... 

The reaction rate and the product yields are low compared to the natural photosynthesis. The catalyst is deactivated in the operation stage itself. Hence the quantum efficiencies are often very low. For the real life implementation of the photocatalytic reduction of CO2 or so called artificial photo-... 

---