The photoassisted monooxygenation of C6H12 to C6H10O and C6H11OH by molecular oxygen has been studied on TiO2 powder catalyst dispersed in neat C6H12 and in C6H12/CH2Cl2 mixtures. The composition of the mixed solvent has a strong influence on the selectivity of the process: an increase in the content of CH2Cl2 brings about both an enhancement in the rate of formation of mono-oxygenated products and a decrease in the production of CO2. At the same time, the alcohol to ketone ratio increases in the mixed solvent. An explanation of this behavior is proposed which is based on the observed decrease in the adsorption strength of intermediates (C6H11OH and radicals) as the solvent composition is varied from pure C6H12 to mixtures of it with increasing amounts of CH2Cl2. The results of experiments with different O2 partial pressures are reported. The process is unaffected for O2 partial pressures > 200 Torr. For lower values the formation of dicyclohexyl becomes significant and reaches a maximum at a pO2 of 60 Torr. In O2-free media containing C(NO2)4 as the electron scavenger, the formation of C6H10O decreases markedly while that of C6H11OH is essentially the same as that in oxygenated media. In the mechanism proposed, the reaction of cyclohexyl radicals with O2 and/or activated oxygen species is the main route leading to the ketone.

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The photochemical reactions of cis-Pt(gly)z, PtC12-, and PtBrs2-are investigated and their mechanisms are discussed. By using radioisotopic techniques, it is demonstrated that the cis + trans photoisomerization of Pt(g1y)z takes place by an intramolecular twisting mechanism. An attempt is made to correlate this mechanism with available information on the structure of excited states of square-planar complexes. PtC12-and PtBr62-undergo photoaquation reactions irradiating either in the charge transfer or in the ligand field bands. The quantum yield values of the photoaquation reactions are reported and discussed on the basis of the energy level diagrams of the two complexes. A comparison is also made between photoaquation and photoexchange reactions of PtBre2-, and it is shown that no relation exists between their mechanisms.

The photoreduction of uranyl solutions on illuminated TiO, suspensions and electrodes has been investigated. The photoreduction leads to the formation, on TiO, , of an uranium oxide having a stoichiometry close to U, O, .Adsorption of uranyl species in the dark was examined. It was found that the degree of adsorption depended on pH and on the formation of uranyl complexes in solution.Initial rates of photoreduction are given for the cases when propan-2-01, sodium acetate or sodium formate are used as scavengers of the photogenerated holes.With the use of microelectrode theory, it was possible to calculate the photoreduction rates from photoelectrochemical data obtained with TiO, electrodes. The reduction rates calculated in this way were from 3.5 to 4 times lower with the electrode than with the suspension, in agreement with the larger area available in the latter case.

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The rates of the reactions of hydroxyl radicals (OH) with styrene, a-methylstyrene, and P-methylstyrene have been measured by irradiating mixtures of these aromatic olefins and NO in an environmental chamber a t 298 K. Experimental conditions were used whereby the competition of ozone with OH in oxidizing the hydrocarbons could be considered negligible.The rate constant values, obtained by a relative method using isooctane as reference hydrocarbon, are: styrene (5.3 f 0.5) X lo-" cm3/molec.s, a-methylstyrene (5.3 f 0.6) X lo-" cm~/molec.s, and P-methylstyrene (6.0 f 0.6) X lo-" cm3/molec-s. A simplified kinetic treatment of the experimental data shows that styrene and 0-methylstyrene are stoichiometrically converted to benzaldehyde, suggesting that OH attack occurs only on the aliphatic moiety of the aromatic olefins. Benzaldehyde was observed to undergo consecutive oxidation by OH, and its maximum formation yield was about 60%. A reaction mechanism is proposed where the primary rate-determining OH attack leads to the formation of 1-hydroxy-2-phenyl-2-ethenyl radicals, from which benzaldehyde is formed through fast intermediate reactions.

The photooxidation of the 1,3-butadiene-NO-air system at 298 f 2 K was investigated in an environmental chamber under simulated atmospheric conditions. The irradiation gave rise to the formation of acrolein in a 55% yield, based on 1,3-butadiene initial concentration for all the experimental runs.The rate of formation of acrolein was the same as that of 1,3-butadiene consumption, indicating that acrolein is the major product of the 1,3-butadiene oxidation in air.The dependence of acrolein concentration on irradiation time showed that a secondary process, identified as an oxidation of acrolein by .OH radicals, was occurring during the photochemical runs. The rate constant of this secondary process was determined by measuring the relative rates of disappearance of acrolein and n-butane during the irradiation of acrolein-n-butane-NO-air mixtures. The so obtained relative rate constant value was placed on an absolute basis using a reported rate constant for the n-butane + .OH reaction; a value of (1.6 f 0.2) X 1O'O M-l sec-l was obtained.