Hydrogen peroxide photochemical formation

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). 

The transformation of isoquinoline has been studied both under photochemical conditions with hydrogen peroxide, and in the dark with hydroxyl radicals (Beitz et al. 1998). The former resulted in fission of the pyridine ring with the formation of phthalic dialdehyde and phthalimide, whereas the major product from the latter reaction involved oxidation of the benzene ring with formation of the isoquinoline-5,8-quinone and a hydroxylated quinone. 

Cooper WJ, Zika RG, Photochemical formation of hydrogen peroxide in surface and ground water exposed to sunlight, Science 220 711—712, 1983. 

Anastasio, C., B. C. Faust, and J. M. Allen, Aqueous Phase Photochemical Formation of Hydrogen Peroxide in Authentic Cloud Waters, J. Geophys. Res., 99, 8231-8248 (1994). 

The OH (X2n) can be generated from the photolysis of water, hydrogen peroxide, and nitric and nitrous acid. Reactions of OH(A2n) with various hydrocarbons arc important in understanding photochemical smog formation (see Section VIII—2). 

The decomposition of liquid water and the following reactions are the results of a typical chemical effect. In this case, however, overall water splitting does not occur because oxygen is not obtained but hydrogen and hydrogen peroxide are. On the other hand, it is impossible to decompose water by photochemical reaction under illumination with a xenon lamp. Although it is possible to decompose water by photocatalytic reaction using a desirable photocatalyst and photoirradiation, it is difficult to decompose in practice because of rapid backward reaction, the formation and accumulation of intermediates onto the surface of photocatalyst,10) and other reasons. 

The products of the photochemical reaction of oxygen and hydrogen in a flow system are ozone, hydrogen peroxide, and water. Mechanisms for the formation of these products are discussed below. 

Photochemical operations offer several routes of hydroxyl radical formation by UV irradiation. The formation of hydroxyl radicals by irradiation of samples doped with hydrogen peroxide or ozone is the state-of-the-art in water treatment. Two comprehensive reviews cover the historical development of the UV photo-oxidation technique as a pretreatment step in the inorganic analysis of natural waters, its principles and the equipment available, and its principal applications in the analytical field.3,4 They include tables summarizing the elements determined, the analytical techniques used, and the sample matrices studied. 

Laser flash photolysis has been used to study the primary photochemical reactions involving the excited state of the photosensitizers as well as the photochemically generated intermediate species. A sequence of reactions leading to oxygen photoreduction, with the concomitant formation of hydrogen peroxide, is proposed in every case. 

Fig. 4 Proposed mechanism of hydrogen peroxide formation by the photochemical systems. While lumiflavin appears to react via reductive quenching by semicarbazide, the ruthenium complex appears to react via oxidative quenching by molecular oxygen.

Siefert et al. [136] simulated the chemical conditions of cloudwater using ambient aerosol samples suspended in an aqueous solution. Electron donors that are known to exist in atmospheric cloudwater (oxalate, formate, and acetate) were then added to the simulated cloudwater, and the solution irradiated with UV fight at A, > 300 nm. In all cases, H2O2 and Fe(II)aq were produced as a function of irradiation time. In addition, H2O2 was also produced without added electron donors simply using ambient aerosols collected from four different sites around the US. In addition, the production of Fe(II)aq showed that Fe from the ambient aerosol was available for photochemical redox reactions. In addition, the simultaneous release of Fe(II) and hydrogen peroxide will result in the indirect photochemical production of hydroxyl radical as follows ... 

Pehkonen S, Pettersson M, Lundell J, Khriachtchev L, Rasanen M (1998) Photochemical Studies of Hydrogen Peroxide in Solid Rare Gases Formation of the HOH.O( P) Complex,/. Phys. Chem. 102 7643-7648. 

This scheme of interrelated primary photochemical and subsequent radical reactions is comphcated by the back reaction of hydrogen atoms and hydroxyl radicals with formation of water (Fig. 7-16, reaction 2) or the dimerization of the latter with formation of hydrogen peroxide (Fig. 7-16, reaction 3). Furthermore, hydroxyl radicals are scavenged by hydroperoxyl radicals with formation of oxygen and water (Fig. 7-16, reaction 5) or by hydrogen peroxide to yield hydroperoxyl radicals and water (Fig. 7-16, reaction 4). In addition, hydroxymethyl radicals (HOCH ) formed by reaction 1 (Fig. 7-16) are able to dimerize with formation of 1,2-ethane-diole (Fig. 7-16, reaction 7) or they disproportionate to yield methanol and formaldehyde (Fig. 7-16, reaction 8). 

The ambient atmosphere at the mobile instrument site in Hoboken, N.J. contained up to 4 pphm of H2O2 on a day with high solar radiation and apparent photochemical smog formation. Hydrogen peroxide was observed between 12 00 A.M. and 2 00 P.M. On days when solar radiation was low because of cloud cover, no H2O2 was observed. 

N.M. Scully, D.R.S. Lean, D.J. McQueen, W.J. Cooper (1995). Photochemical formation of hydrogen peroxide in lakes Effects of dissolved organic carbon and ultraviolet radiation. Can. J. Fish. Aquat. Sci., 52,2675-2681. 

W.J. Cooper, D.R.S. Lean (1989). Hydrogen peroxide concentration in a northern lake Photochemical formation and did variability. Environ. Sci. Technol, 23, 1425-1428. 

C.L. Wilson, N.W. Hinman, R.P. Sheridan (2000). Hydrogen peroxide formation and decay in iron-rich geothermal waters The relative roles of biotic and abiotic mechanisms. Photochem. Photobiol, 71,691-699. 

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