The Photochemical Decomposition of Hydrogen Peroxide

It is well known that hydrogen peroxide which absorbs fairly strongly in the region below 3100 A. undergoes photochemical decomposition. 

Dawsey, and Rice (25) suggested in 1929 that the photochemical primary process in the quartz ultraviolet was represented by  

If this process is combined with the chain reactions (2) and (3), one should obtain, in conjunction with a process for the disappearance of the free radicals (chain breaking), an adequate representation of the photochemical reaction. In fact, it has been shown that the experimental material, although incomplete in many respects, is in good agreement with such a mechanism. Kornfeld (26) found that the quantum yield for the decomposition is far greater than unity (values up to 50 were recorded) which clearly indicates the operation of a chain mechanism. The interaction of two radicals such as  

A number of photochemical experiments using the sector method have been carried out mainly by Allmand and Style (32) and more recently by Lea (31). These experiments give lifetimes of 0.5 sec. to 1 sec. for the active radicals. However, there are some unexplained features in this work and experiments under well-defined conditions are still required for a detailed discussion of the lifetimes. 

In neutral or acid solutions the hydrogen peroxide is present in solution almost entirely as H2O2, while in alkaline solutions (pH 12) appreciable amounts of the anion H02 must be present. This is also shown clearly by the change in the absorption spectrum (33). In the latter case the photochemical primary process is presumably represented by the electron transfer process  

---

J.W. Moffett, O.C. Zafiriou (1993). The photochemical decomposition of hydrogen peroxide in surface waters of the Eastern Caribbean and Orinoco River. J. Geophys. Res., 98,2307-2313. 

J.P. Hunt, H. Taube (1952). The photochemical decomposition of hydrogen peroxide. Quantum yields, tracer and fractionation effects. Nature, 74, 5999-6002. 

However, although the Haber-Willstatter chain reactions have been assumed to occur in certain catalytic systems, notably the ferrous-ferric ion system (4), more recent evidence to be described subsequently, does not support this assumption. On the other hand, such reactions appear to offer the most plausible explanation for the photochemical decomposition of hydrogen peroxide, although even here a satisfactory analysis of the kinetics has yet to be made. 

The acrylamide post-polymerization initiated by photochemical (k = 313 nm) decomposition of hydrogen peroxide in aqueous media can take place in the course of several days 78) after cutting off the UV source. Three structures of the terminal radicals have been suggested... 

In addition to the photoelectrochemical (catalysed by Ti02) mechanism discussed above, also the chemical and photochemical mechanisms of H2O2 decomposition are to be considered. In strongly irradiated, alkaline solutions the direct photolysis of hydrogen peroxide (strictly speaking-of HO2 ions) should seriously be taken into account for the... 

Several pathways can account for the decomposition of hydrogen peroxide In natural waters. Some of these decay processes not only remove H2O2, but also result In the oxidation of chemicals, possibly Including various pollutants, that are present In natural waters. These processes Include direct oxidation (5), peroxidase-catalyzed oxidation (6), and free radical oxidation Initiated by photochemical or metal-catalyzed decomposition (7). Little Is known about the significance and rates of these various processes under environmental conditions, but they have all been shown to occur rapidly with certain organic substrates In the laboratory. 

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 first step in the peroxide-induced reaction is the decomposition of the peroxide to form a free radical. The oxygen-induced reaction may involve the intermediate formation of a peroxide or a free radical olefin-oxygen addition product. (In the case of thermal and photochemical reactions, the free radical may be formed by the opening up of the double bond or, more probably, by dissociation of a carbon-hydrogen bond in metal alkyl-induced reactions, decomposition of the metal alkyl yields alkyl radicals.)... 

Egerton [197, 198J found that hydrogen peroxide vapour and its radical decomposition products can degrade polymers. Also Egerton et al. [198, 199, 201] reported that many different dyes, such as acriflavin and eosin, sensitize the photochemical oxidation of textile fibres. 

A wide variety of peroxides have been used to produce alkyl radicals, either directly as fragments of the decomposition of peroxides, or indirectly by hydrogen abstraction from suitable solvents. The production of alkyl radicals used in homolytic alkylation has been accomplished by thermal or photochemical homolysis and recently also by redox reactions due to the possibilities offered by alkylation in acidic aqueous solution. 

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. 

Temperature will affect the degradation rate of different organic pollutants. Weir et al. (1987) reported that benzene and hydrogen peroxide are insensitive to temperature because photochemically induced reactions often have low activation energies. Koubek (1975) stated that temperature has little effect on the oxidation of refractory organics however, Sundstrom et al. (1986) observed that the decomposition rates of some halogenated aliphatics increased with temperature. 

R CHORj. Such radicals have been formed by hydrogen atom abstraction from the ether by radicals produced from thermal decomposition of peroxides (67, 75, 76). Similar radicals may be produced in photochemical processes, either by direct irradiation (29, 54), or by the use of a photosensitizer or a photoinitiator, such as acetone or benzophenone (21, 64, 66). The ether radicals once produced, participate in a variety of chemical reactions. It might be noted that resonance forms as illustrated... 

---