Photoelectrochemical behavior of nanostructural composites based on polycrystalline cadmium sulfide

Nanostructural composites can be used as suspension electrodes; they are made by substitution cadmium(II) at the surfaces of CdS microcrystals for Cu(I, II), Fe(III), Bi(III), and Sb(III). Features are considered in the photoelectrochemical action mechanism of the various nanocomposites. L. V. Pisarzhevskii Institute of Physical Chemistry, National Academy of Sciences of Ukraine, 31 Prospekt Nauki, Kiev 252039, Ukraine. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 35, No. 2, pp. 79–82, March–April, 1999.     

Formation and conversions of titanium(iii) complexes in the photocatalytic production of hydrogen from alcohols

EPR and spectrophotometry were used to study the products of the photochemical reduction of titanium(IV) ions in alcohols which act as photocatalysts for the production of molecular hydrogen. Tetrahedral complexes of the type [TiOHL₃] and [TiORL₃] were detected besides the octahedral complexes [TiCl₂(ROH)₄]⁺ and [TiCl₃(ROH)₃]. The EPR spectra of the compounds at 77 and 295°K have been obtained and their parameters determined. The influence of temperature, the addition of water, and alkali on the concentration ratio of the compounds studied has been established. It has been found that the addition of small amounts of water increases the content of the hydroxocomplex, while at the same time the alkoxocomplex decomposes irreversibly. Analogous changes take place when the photolysis time is increased. One of the conditions for effective hydrogen evolution is maintaining the temperature close to room temperature or slightly higher. The efficiency of the process increases somewhat after prolonged operation of the system.Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 23, No. 3, pp. 295–302, May–June 1987.     

CdS nanoparticles in porous silicate glasses: energy characteristics, photocatalytic activity, and the effect of silver ion doping

Energy diagrams were constructed for CdS nanoparticles doped with silver ions and incorporated into silicate matrices, which are new nanostructural materials. The effect of the amount of silver introduced on the efficiency of the photocatalytic evolution of hydrogen from aqueous sulfide-sulfite solutions was demonstrated, and the interrelationship between the photocatalytic activity of these materials and their energy characteristics was established. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 34, No. 6, pp. 360–365, November–December, 1998.     

Photoreduction dehalogenation of polychlorobenzenes by ethanol

It has been demonstrated that in ethanolic solutions in the presence of oxygen the photochemical and photocatalytic consecutive dehalogenation of hexachlorobenzene to benzene can take place. The kinetic rules governing the processes have been established and the possible mechanisms discussed.Translated from Teoreticheskaya i Éxperimental'naya Khimiya, Vol. 30, No. 1, pp. 34–38, March–April, 1994.     

Peculiarities of the Behavior of Porous Titanium Dioxide in the Photocatalytic Evolution of Molecular Hydrogen from Aqueous Ethanolic Solutions

Peculiarities of the behavior of porous titanium dioxide in the photocatalytic evolution of molecular hydrogen from aqueous ethanolic solutions are shown to arise from the accumulation of long lived "free" electrons and titanium(III) ions in the irradiated samples, and the slow transport of electrons to the surface of the TiO₂ particles. This explains the dependence of the effectiveness of the process on both the intensity of the irradiation and on the length of time the irradiated samples are kept in the dark.     

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.     

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.     

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.