New photocatalysts: Dispersed semiconductor materials with microheterojunctions

We have obtained new semiconductor materials with microheterojunctions by substitution of surface cadmium ions in CdS by Cu(I), Cu(II), and Bi(III) ions. We have shown that the photocatalytic activity of their dispersions in the reaction of evolution of H₂ from alcohol-water mixtures is considerably higher than in the original cadmium sulfide.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 29, No. 3, pp. 270–274, May–June, 1993.We would like to acknowledge Academician V. D. Pokhodenko of the Academy of Sciences of Ukraine for drawing our attention to the prospects for investigation of complex photocatalytic systems based on semiconductors with different types of conductivity.     

Photochemical and photocatalytic properties of zeolites containing titanium(iv) complexes and metallic palladium

An attempt has been made to create organized, highly efficient photocatalytic systems in the example of composite materials consisting of blocks of a metallocomplex photocatalyst and a dark-stage catalyst, assembled on a zeolite matrix. The spectral and photochemical properties of these systems have been investigated, and also their use in catalysis of hydrogen evolution from alcohol-water media. L. V. Pisarzhevskii Institute of Physical Chemistry, National Academy of Sciences of the Ukraine, 31 Nauka Prospect, Kiev 252039, Ukraine. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 34, No. 2, pp. 103–108, March–April, 1998.     

Mechanism of photolysis of peroxy complexes of titanium(IV)

Conditions were found under which radical pairs become stabilized during the photolysis of peroxy complexes of titanium(IV) in alcoholic solutions. It was shown that precursors are dimeric complexes of titanium(IV) comprising peripheral peroxy ligands as well as a peroxy bridge. Irradiation by light corresponding to the peroxy ligand — the central ion charge transfer bands, leads to the initiation of an intraspheric reaction, as a result of which two peripheral peroxy ligands undergo a one-electron oxidation, while the bridge undergoes a two-electron reduction. The radical particles (HO₂ ^. and O₂ ) formed from the peripheral peroxy ligands become stabilized at strictly delineated distances relative to one another, which are determined by the structure of the initial dimeric peroxy complex of titanium(IV).Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 6, pp. 678–685, November–December, 1988.     

Photocatalysis of reactions involving reduction of nitrogen-containing compounds by ethanol in an organized 18-tungsten diphosphate-cadmium sulfide system

Cadmium sulfide and heteropolytungstate were found to form an organized photocatalytic system providing separation of photogenerated charges. Reduction of nitrobenzene, nitrosobenzene, and phenylhydroxylamine to aniline has been investigated in this system; kinetic patterns have been established and quantum yields of the reactions have been determined.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 30, No. 4, pp. 206–210, July–August, 1994.The authors are grateful to the Fund for Fundamental Research of the Ukrainian State Committee on Science and Technology for supporting this work.     

Sensitization of cadmium sulfide by cyanine dyes in the photocatalytic production of hydrogen

The possibility of photocatalysis of hydrogen production from aqueous alcohol mixtures by heterostructures of cadmium sulfide with dyes under the action of light with >600 nm was shown. A probable mechanism of the sensitization is proposed on the basis of an examination of the spectral, photochemical, and electrochemical data obtained.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 31, No. 6, pp. 370–374, November–December, 1995.The authors are grateful to S. V. Shinkorenko for providing the dyes that they synthesized for the first time.     

Catalysis by titanium (IV) ions of reactions forming chlorohydrins from olefins

A study was made of reactions forming chlorohydrins from cyclohexane and other olefins acted upon by H₂O₂ and HCl or H₂O₂ and TiCl₄ in alcoholic solutions. The catalyzing action of titanium (1V) was established, and possible reaction mechanisms are discussed.L. V. Pisarzhevskii Institute of Physical Chemistry, National Academy of Sciences of Ukraine, 31 Prosp. Nauki, 252039, Kiev, Ukraine. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 32, No. 5, pp. 295—299, September–October, 1996. Original article submitted February 9, 1996.     

Electronic and ESR spectroscopic study of thiocyanate-containing coordination compounds of titanium(III) in alcohol solutions

In an investigation of electronic and ESR spectra, it has been established that titanium(III) with the thiocyanate ion (in ethanol) forms distorted tetrahedral and octahedral complexes. The tetrahedral compounds, which are hydroxo complexes of titanium(III) that contain one hydroxyl group and three thiocyanate ions in the coordination sphere, give signals in the ESR spectra with g_o=1.960, A_o=15.0·10^–4 cm^–1 g=1.958, A ^Ti =26.4·10^–4cm^–1, g= 1.691, A^Ti=9.0·10^–4 cm^–1. In the ESR spectra of liquid solutions, additional splitting on the nitrogen atoms is manifested(A _o ^N =2.2·10^–4 cm^–1), which indicates bonding of thiocyanate ions through the nitrogen atom. The pseudo-octahedral complexes of titanium(III), containing from two to six thiocyanate ions in the coordination sphere, are characterized by d-d absorption bands in the 520–590 nm region; and at 77°K, they give anisotropic ESR signals. On the basis of the temperature dependences of the equilibrium constants for the reaction of conversion of the tetrahedral complexes to the octahedral complexes, and also the lifetimes of the tetrahedral complexes, values have been estimated for the heat of reaction (H=26.8 kJ/mole) and the entropy change (S=–1.66 kJ/mole·°K) of the equilibrium process, and also the activation energy for the reaction of titanium(III) thiocyanate formation (E=37 kJ/mole).Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 5, pp. 560–567, September–October, 1985.     

Influence of added water on pathways and efficiency of photocatalytic reactions in ethanol solutions of titanium tetrachloride

Electronic spectroscopy, ESR, and gas-liquid chromatography have been used in studying the action of light with =254 nm on ethanol-water solutions of titanium tetrachloride. It has been established that, in the course of irradiation, a photocatalytic process develops with the participation of coordination compounds of titanium(IV) and (III), resulting in oxidation and breakdown of the ethanol to form acetaldehyde, methane, ethane, ethyl chloride, and molecular hydrogen. In the presence of small amounts of added water (up to 0.5 M), the efficiency of CH₄ and C₂H₇ formation is much lower but the yield of H₂ is higher, in comparison with the yields of these products in absolute ethanol. With further increases in the water concentration, the rates of formation of methane, ethane, and acetaldehyde increase. The maximum quantum yields () are as follows: (CH₄)=1.8 · 10^–4 and (C₂H₆)=1 · 10^–3 in absolute ethanol; (CH₃CHO)= 1.6·10^–2 in ethanol with the addition of 6.0 M H₂O; (H₂)=6.7·10^–3 in solutions containing 0.5 M H₂O. The observed differences in the product yields are explained by a change in the composition and structure of the titanium compounds that act as the photocatalyst, a change that takes place when the water concentration is varied. A mechanism is proposed for these reactions.Translated from Teoreticheskaya i Éksperimentai'naya Khimiya, Vol. 22, No. 1, pp. 51–58, January–February, 1986.     

Photocatalytic liberation of hydrogen from C1-C4 alcohols with the participation of titanium complexes

It was established that in UV irradiation of solutions of TiCl₄ in methanol, ethanol, isopropanol, and butanol, alcohol-chloride complexes of titanium(III) are formed. The quantum yields of the formation of coordination compounds of titanium(III) depend on the nature of the alcohol: 0.08 (methanol); 0.13 (ethanol); 0.20 (butanol); 0.22 (isopropanol). As complexes of titanium(III) accumulate in solution, there is a liberation of molecular hydrogen. The quantum yields of the formation of hydrogen, determined in a steady-state process, are correlated with the values of the C-H bond energy at the -carbon atom of the alcohol and are equal to 2·10^–3, 3.4·10^–3, 4.3·10^–3 and 1·10^–2 for solutions in methanol, butanol, ethanol, and isopropanol, respectively. A substantial increase in the quantum yield of the formation of molecular hydrogen was detected when a heterogeneous catalyst (palladium on silica gel) was used, and the possible mechanism of the process of photocatalytic liberation of hydrogen from alcohols with the participation of titanium complexes is discussed.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 3, No. 2, pp. 181–186, March–April, 1987.The authors would like to thank V. M. Granchak for his participation in the discussion of the results.