Phenol photodegradation

Augugliaro, V. Davi, E. Palmisano, L. Schiavello, M. Sclafani, A. Influence of hydrogen peroxide on the kinetics of phenol photodegradation in aqueous titanium dioxide dispersion, Appl. Catal. 1990, 65, 101. 

The carbon balance between reacted phenol and produced CO2 and dihydroxylated compounds was satisfactory (> 98%) for phenol conversion less than 60%. On this basis it maybe assumed that at the used experimental conditions the stable intermediate products of phenol photodegradation do not compete with phenol for photoadsorption and oxidation, at least in the time needed for 60% conversion. 

Figure 5.17 (a) Three-dimensional dependence of the rate of phenol photodegradation in illuminated... 

Trillas, M., Pujol, M. andDomenech, X., 1992, Phenol photodegradation over titanium dioxide. 7. Chem. Tech. Biotech., 55(l) 85-90. 

It can be observed in Table 7.4 that for phenol photodegradation the PTEFeq and PTEFneq leads to essentially the same results and this is consistent with the frequently adopted assumption of chemical special species at equilibrium. As it can also be observed in Table 7.4, the MeB display however, for PTEFeq and PTEFneq quite different values. This emphasizes the critical importance of properly modeling photocatalytic degradation processes taking place under non-equilibrium conditions. Thus, adequate modeling of these processes as well as calculation of the related kinetic parameters leads to proper calculation of PTEF reactor energy efficiencies. 

TABLE 27.1 First-Order Rate Constant (k) for Phenol Photodegradation and Correlation Coefflcient (R ) for the First-Order Plot ... 

The exterior durabiHty of relatively stable coatings can be enhanced by use of additives. Ultraviolet absorbers reduce the absorption of uv by the resins and hence decrease the rate of photodegradation. Eurther improvements can be gained by also adding free-radical trap antioxidants (qv) such as hindered phenols and especially hindered amine light stabilizers (HALS). A discussion of various types of additives is available (113). 

Tetrahydrofuran has been reported to exhibit an absorption maximum at 280 nm (52,56), but several workers have shown that this band is not produced by the purified solvent (30,41,57). Oxidation products from THF have been invoked in order to account for the appearance of the 280-nm band in PVC films that are solvent-cast from THF in air (57. 581. However, in some reported cases (56,59), this band was undoubtedly produced, at least in part, by a phenolic antioxidant (2.6-di-tert-butyl-p-cresol)(59) in the solvent. Since certain -alkylphenols have now been shown to be powerful photosensitizers for the dehydrochlorination of PVC (60), it is clear that antioxidant photosensitization might well have been responsible for some of the effects attributed previously (56) to THF alone. On the other hand, enhanced rates of photodegradation under air have also been observed for PVC films cast from purified THF (57), a result which has been ascribed to radical formation during the photooxidation of residual solvent (57,61). Rabek et al. (61) have shown that this photooxidation produces a-HOO-THF, a-HO-THF, and y-butyro-lactone, and they have found that the hydroperoxide product is an effective sensitizer for the photodehydrochlorination of PVC at X = 254 nm (61). 

During photolysis, the double bond content of the polysilane(P-l)(15mol% in this experiment) decreased to 10mol%, as measured by 1H-NMR spectroscopy. However, the ratio, quantum yield of scission(Q(S))/quantum yield of crosslinking(Q(X)), was not affected by the reaction of the double bond. West and his coworkers have reported that poly((2-(3-cyclohexenyl)-ethyl)methylsilane-co-methylphenylsilane) crosslinked upon irradiation(55). The difference between our results and West s may lie in the amount of the double bond and inhibitation of the radical closslinking by the phenol moiety. Polysilane with a halogen moiety, P-8, photodecomposed rapidly, compared with P-1 or P-3. The introduction of a chloride moiety was effective for the sensitization of the photodegradation. Similar results has already been reported(55). 

Photolytic. The UV photolysis (7, = 300 nm) of bifenox in various solvents was studied by Ruzo et al. (1980). In water, 2,4-dichloro-3 -(carboxymethyl)-4 -hydroxydiphenyl ether and 2,4-di-chloro-3 -(carboxymethyl)-4 -aminodiphenyl ether were identifled. In cyclohexane, 2,4-dichloro-4 -nitrodiphenyl ether and methyl formate were the major products. In methanol, a dichloro-methoxy phenol was identified. Photodegradation occurred via reductive dechlorination, de-carboxymethylation, nitro group reduction, and cleavage of the ether linkage (Ruzo et al., 1980). 

Irradiation of isoprocarb and promecarb resulted in PFR to ortho- andpara-hydroxybenzamides. The photodegradation of analogous carbamate pesticides (bendiocarb, bendiocarb, ethiofencarb,furathiocarb,fenoxycarb, and pirimicarb) has also been examined, but in these cases, the most general result was formation of the corresponding phenols [298]. 

Photodegradation rates of ortho derivates present good correlation with the thermodynamic stability of sigma-complexes formed between the aromatic ring and the surface OH-radicals. Rates decrease in the order -OCH3 (guiacol) > -Cl (2-chlorophenol) -H (phenol) > -OH (catechol). ... 

As previously mentioned, a key point in the optimization of the catalysts photodegradation of phenol and its derivates, is the minimization of the electron-hole recombination and the intimate connection of this process with the anion vacancies present in the size-limited, nanometric oxide particles. Minimization of the overall amount of oxide defects has a significant impact on the reaction rate. Traditional methods for improving electron-hole charge separation beyond what can be obtained with bare titania, involves doping mainly with although surface... 

This is consistent with a study by Feilberg and Nielsen (1999b), who investigated the influence of other aerosol components on the photodegradation rates of representative particle-associated nitro-PAHs in a model system consisting of the nitro-PAH dissolved in cyclohexane along with various known constituents of diesel exhaust and wood smoke particles. These cosolutes included PAHs, substituted phenols, hydroxy-PAHs, oxy-PAHs, and substituted benzaldehydes. 

However, when H-atom-donating cosolutes, e.g., certain phenols, were added, the photodegradation rates of both 1-nitropyrene and 3-nitrofluoranthene increased. In this case, the reaction occurred via H-atom abstraction from the phenol by the electronically excited nitro-PAHs. Feilberg and Nielsen concluded that the photodegradation of nitro-PAHs on both diesel particles and wood smoke proceeds primarily by radical formation. However, H-atom abstraction by the excited triplet states of 1-nitropyrene and 2-nitrofluoranthene may also contribute. 

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