Attila K. Horváth

Photocatalytic oxidation of oxalic acid enhanced by silver deposition on a TiO2 surface

Photooxidation of oxalic acid over TiO2 surface in the presence of silver ion was studied. Simultaneous deposition of silver and oxidation of oxalic acid were followed under irradiation of aqueous suspensions of pH 2.5–3.5. The rate of the reduction of Ag+ was increased by the progress of silver deposition until a considerable depletion of the silver ion in the liquid phase was achieved. Then it was slowed down when the transport of Ag+ from the bulk of the liquid phase to the surface of the semiconductor particle became the rate-determining process. It has been demonstrated that, after the completion of silver deposition, the small metal particles on TiO2 surface enhance the efficiency of the semiconductor by a factor of 5 for the photooxidation of oxalic acid.

Temperature dependence study of five-coordinate complex formation of zinc(II) octaethyl and tetraphenylporphyrin

Abstract Five coordinate complex formation of zinc(II)–octaethylporphyrin (Zn(OEP)) and zinc(II)–tetraphenylporphyrin (Zn(TPP)) have been studied in the presence of seven systematically selected electron donor molecules. Stability constants in toluene were determined at various temperature ranged between 20 and 60°C by absorption and steady-state fluorescent measurements from which thermodynamic parameters were determined. Depending on the porphyrin and the axial ligand the entropy is changing between −127 and −61 J mol−1 K−1 while the enthalpy is ranging from −49 to −24 kJ mol−1.

Transition metal complex exciplexes

Abstract The photophysical and photochemical properties of exciplexes are summarized considering the electronic structure and role of energetics and kinetics in the formation and stabilization of these molecular entities. Exciplexes involving transition metal complexes, are known as transition metal complex exciplexes (TMCEs) and are classified on the basis of the reactive site of the excited metal complex and the properties of the ground state reactant. There are two main groups; ligand-centered exciplexes and metal-centered exciplexes. A survey of the literature of TMCEs is presented. Common features of exciplexes classified into different groups are revealed. A strategy for searching for new TMCEs is suggested.

Temperature dependence study on photophysics of RuL(CN)4 2− complexes: effects of diimine ligand and solvent deuteration

Abstract Photophysics of [RuL(CN)4]2− complexes, where L=2,2′-bipyridine (bpy), 4,4′-dimethyl-2,2′-bipyridine (dmb), 4,4′-diphenyl-2,2′-bypiridine (dpb), 1,10-phenantrolin (phen), has been investigated by luminescence spectroscopy and flash photolysis at various temperatures. Luminescence quantum yield and lifetime of the 3MLCT excited state have been measured in aqueous solutions and in D2O. The rate of direct deactivations to the ground state via radiative and non-radiative decay and the parameters of the thermally activated deactivation pathway have been determined. The rate coefficients of the radiative decay are quite close to each other (4×104–7×104 s−1) and are not sensitive to the replacement of H to D in the solvent, while the rate of non-radiative decay strongly depends on the nature of the polypyridyl ligand and on solvent deuteration. The activation energy and the pre-exponential factor of the thermally activated deactivation also depend on the nature of the α,α′-diimine ligand. The value of the potential barrier and the pre-exponential coefficient vary between 1010–1400 cm−1 and 5.5×108–14×108 s−1, respectively. Analysis of the emission spectra and the decay kinetics has confirmed that the excited state distortion and the interaction between the excited species and the solvent molecule through H or D bond are the factors that count in determining the rate of non-radiative deactivation pathways.

Recent Developments in the Chemistry of Thiourea Oxides

Thiourea dioxide is one of the best known, important, and stable products of thiourea oxidation. This compound has long been considered as an effective reducing agent for many years. Traditional areas of its application include the textile and paper industries. In recent years, however, thiourea dioxides and trioxides have been widely used in new fields including organocatalytic, polymerization, and phase-transfer reactions; reduction of graphene and graphite oxides; bitumen modifications; synthesis of guanidines and their derivatives; and studying nonlinear dynamical phenomena in chemical kinetics. The review gives a detailed survey of the latest developments and main trends in the chemistry and application of thiourea mon-, di-, and trioxides.

Effect of Chloride Ion on the Kinetics and Mechanism of the Reaction between Chlorite Ion and Hypochlorous Acid

The effect of chloride ion on the chlorine dioxide formation in the ClO 2 (-)-HOCl reaction was studied by following .ClO 2 concentration spectrophotometrically at pH 5-6 in 0.5 M sodium acetate. On the basis of the earlier experimental data collected without initially added chloride and on new experiments, the earlier kinetic model was modified and extended to interpret the two series of experiments together. It was found that the chloride ion significantly increases the initial rate of .ClO 2 formation. At the same time, the .ClO 2 yield is increased in HOCl but decreased in ClO 2 (-) excess by the increase of the chloride ion concentration. The two-step hydrolysis of dissolved chlorine through Cl 2 + H 2O left harpoon over right harpoon Cl 2OH (-) + H (+) and Cl 2OH (-) left harpoon over right harpoon HOCl + Cl (-) and the increased reactivity of Cl 2OH (-) compared to HOCl are proposed to explain these phenomena. It is reinforced that the hydrolysis of the transient Cl 2O 2 takes place through a HOCl-catalyzed step instead of the spontaneous hydrolysis. A seven-step kinetic model with six rate parameters (constants and/or ratio of constants) is proposed on the basis of the rigorous least-squares fitting of the parameters simultaneously to 129 absorbance versus time curves measured up to approximately 90% conversion. The advantage of this method of evaluation is briefly outlined.

Kinetics and Mechanism of the Chlorine Dioxide-Tetrathionate Reaction

The trithionate-chlorine dioxide reaction has been studied spectrophotometrically in a slightly acidic medium at 25.0 ± 0.1 °C in acetate/acetic acid buffer monitoring the decay of chlorine dioxide at constant ionic strength (I = 0.5 M) adjusted by sodium perchlorate. We found that under our experimental conditions two limiting stoichiometries exist and the pH, the concentration of the reactants, and even the concentration of chloride ion affects the actual stoichiometry of the reaction that can be augmented by an appropriate linear combination of these limiting processes. It is also shown that although the formal kinetic order of trithionate is strictly one that of chlorine dioxide varies between 1 and 2, depending on the actual chlorine dioxide excess and the pH. Moreover, the otherwise sluggish chloride ion, which is also a product of the reaction, slightly accelerates the initial rate of chlorine dioxide consumption and may therefore act as an autocatalyst. In addition to that, overshoot-undershoot behavior is also observed in the [(·)ClO(2)]-time curves in the presence of chloride ion at chlorine dioxide excess. On the basis of the experiments, a 13-step kinetic model with 6 fitted kinetic parameter is proposed by nonlinear parameter estimation.

Photoinduced electron transfer and luminescence in aqueous bromocuprate(I) complexes

Abstract Luminescence, laser flash photolysis and continuous photolysis studies of equilibrated solutions of CuBr 2 − and CuBr 3 2− were carried out in the UV region. Excitation of the absorption band at 279 nm in CuBr 3 2− results in emission centered at 475 nm, with a lifetime of 710 ns in neutral solution, and quenched by hydronium ions with a rate constant of 6.2 × 10 8 M −1 s −1 . Neutral solutions of the complexes produce hydrated electrons when they absorb 15 ns pulses of laser light at 266 nm. The electrons are scavenged by the copper(I) species itself with a second-order rate constant of 7.5 × 10 9 M −1 s −1 , and by hydronium ions with a second-order rate constant of 1.3 × 10 10 M −1 s −1 at 0.5 M ionic strength. Individual quantum yields of electron production, determined at 1 M ionic strength, are 0.67 for CuBr 2 − and 0.34 for CuBr 3 2− . Continuous photolysis of acidic solutions of the complexes reveals a dependnece on hydronium ion concentration which is different from that for the scavenging of electrons, a dependence on Br − concentration and an action spectrum consistent with the 279 nm absorption band as the photoactive state. These plus other observations and arguments support a mechanism for dihydrogen evolution, involvin the formation of a steady state hydride intermediate which reacts with H + to form dihydrogen.

Mechanism of photoinduced redox reactions in aqueous solutions of [Fe(bpy)(CN)4]2-

Abstract The mechanism of redox reactions initiated by UV excitation of [Fe(bpy)(CN) 4 ] 2− in aqueous solutions has been investigated by continuous photolysis, laser kinetic spectroscopy and pulse radiolysis. Hydrated electrons formed in the light induced reaction are shown to react with ground state [Fe(bpy)(CN) 4 ] 2− to form [Fe(bpy)(CN) 4 ] 3− characterized by its absorption spectra and reactions. The back reaction between [Fe(bpy)(CN) 4 ] − and [Fe(bpy)(CN) 4 ] 3− takes place with k l.8±4x10 10 M −1 s −1 . The quantum yield of solvated electron formation determined by using nitrate ions as electron scavenger is 0.14±0.01 and 0.068±0.007 at μ∼ 0.01 and μ∼ 1.0, respectively. Addition of methylviologen (MV 2+ ) to the solution of iron(II) complex leads to production of MV+ via direct electron scavenging and an electron transfer reaction from [Fe(bpy)(CN) 4 ] 3− . The latter process takes place with k = 9.0±0.6 x 10 9 M −1 s −1 . The decay of MV + occurs in reactions with both iron(III) and iron(II) complexes.

General Pathway of Sulfur-Chain Breakage of Polythionates by Iodine Confirmed by the Kinetics and Mechanism of the Pentathionate−Iodine Reaction

The pentathionate-iodine reaction has been studied spectrophotometrically at T = 25.0 ± 0.1 °C and at an ionic strength of 0.5 M in both the absence and presence of an initially added iodide ion at the pH range of 3.95-5.15. It was found that the pH does not affect the rate of the reaction; however, the iodide ion produced by the reaction strongly inhibits the oxidation. Therefore, it acts as an autoinhibitor. The kinetic curves also support the fact that iodide inhibition cannot be explained by the formation of the unreactive triiodide ion, and S(5)O(6)I(-) along with the iodide ion has to be involved in the initiating rapid equilibrium being shifted far to the left. Further reactions of S(5)O(6)I(-), including its hydrolysis and reaction with the iodide ion, lead to the overall stoichiometry represented by the following equation: S(5)O(6)(2-) + 10I(2) + 14H(2)O → 5SO(4)(2-) + 20I(-) + 28H(+). A nine-step kinetic model with two fitted parameters is proposed and discussed, from which a rate equation has also been derived. A brief discussion about the general pathway of sulfur-chain breakage of polythionates supported by theoretical calculations has also been included.