PHOTOCATALYTIC DEGRADATION OF 4-CHLOROPHENOL MEDIATED BY TIO2 - A COMPARATIVE-STUDY OF THE ACTIVITY OF LABORATORY MADE AND COMMERCIAL TIO2 SAMPLES

The photocatalytic efficiencies of laboratory made and commercial TiO2 samples were compared using a standard test reaction: the photomineralization of 4-chlorophenol (4-CP) to CO2, H2O and HCl mediated by Degussa P25 TiO2 in a batch reactor. The results show that the rate of photodegradation of 4-CP, sensitized by a sample of TiO2, shows no clear simple dependence on physical characteristics such as the degree of crystallinity, the surface area and the percentage of H2O.

OXIDATION OF CHLORIDE TO CHLORINE BY CERIC IONS MEDIATED BY THE 3 FORMS OF CRYSTALLINE CARBON

C-60 is more effective than graphite or diamond as a redox catalyst for the oxidation of chloride to chlorine by cerie ions.

KINETICS OF REDUCTION OF HEXACYANOFERRATE(III) BY THIOSULFATE IONS MEDIATED BY RUTHENIUM DIOXIDE HYDRATE

The kinetics of reduction of hexacyanoferrate(III) by excess thiosulfate, mediated by RuO2.xH2O, are investigated. At high concentrations of S2O32- (0.1 mol dm-3) the kinetics of Fe(CN)63- reduction are first order with respect to [Fe(CN)63-] and [RuO2.xH2O] and independent of [Fe(CN)64-], [S2O32-] and [S4O62-]. At relatively low concentrations Of S2O32- (0.01 mol dm-3) and in the presence of appreciable concentrations of Fe(CN)64- and S4O62- (0.01 mol dm-3) the kinetics depend directly upon [Fe(CN)63-] and [RuO2.xH2O] and inversely upon [Fe(CN)64-]. Both sets of kinetics can be rationalised using an electrochemical model of redox catalysts in which a reversible reduction reaction [Fe(CN)63- + e- --> Fe(CN)64-] is coupled to an irreversible oxidation reaction (s2O32- - e- --> 1/2S4O62-), by a dispersion of RuO2.xH2O microelectrodes. At high concentrations Of S2O32- this model predicts that the kinetics of Fe(CN)63- reduction are controlled by the rate of diffusion of the Fe(CN)63- ions to the RuO2.xH2O particles. The kinetics observed at low concentrations of S2O32- are predicted by the electrochemical model, assuming that the Tafel slope for the oxidation Of S2O32- to S4O62- on the RuO2.xH2O particles is 56.4 mV decade-1.

OXIDATION OF RUTHENIUM(II) TRIS(2,2'-BIPYRIDINE) IONS BY THALLIC IONS IN NITRIC-ACID, MEDIATED BY RUTHENIUM DIOXIDE HYDRATE - AN EXAMPLE OF REVERSIBLE HETEROGENEOUS REDOX CATALYSIS

The kinetics of the oxidation of Ru(bpy)32+ to Ru(bpy)33+ by T13+ ions, catalyzed by a dispersion of RuO2-xH2O in 3 mol dm-3 HNO3, are reported as a function of [Ru(bpy)32+], [Tl3+], [Tl+], [RuO2.xH2O], and temperature. The kinetics of Ru(bpy)32+ oxidation fit an electrochemical model of redox catalysis involving electron transfer between the two electrochemically reversible redox couples, i.e. Ru(bpy)33+/Ru(bpy)32+ and Tl3+/Tl+, mediated by the dispersion of microelectrode particles of RuO2.xH2O. In this model, the rate of reaction is assumed to be controlled by the diffusion of Ru(bpy)32+ toward, and Ru(bpy)33+ away from, the catalyst particles. The Arrhenius activation energy for the catalyzed reaction is 25.9 +/- 0.7 kJ mol-1, and the changes in enthalpy and entropy for the reaction are 36 +/- 2 kJ mol-1 and 127 +/- 6 J mol-1 K-1, respectively. This work describes a rare example of reversible heterogeneous redox catalysis.

OXIDATION OF CHLORIDE TO CHLORINE BY CERIC IONS MEDIATED BY DIFFERENT CARBON-BLACKS

A number of different carbon blacks are tested for activity as chlorine catalysts in the oxidation of chloride (2 mol dm-3 in 0.5 mol dm-3 H2SO4) to chlorine by Ce(IV) ions, that is,

OXIDATION OF WATER BY MNO4- MEDIATED BY THERMALLY ACTIVATED RUTHENIUM DIOXIDE HYDRATE

The kinetics of oxidation of water to oxygen by MnO4-, mediated by thermally activated ruthenium dioxide hydrate, has been studied. The rate of catalysis is 0.8 order with respect to the surface concentration of MnO4- (which in turn appears to fit a Langmuir adsorption isotherm) and proportional to the catalyst concentration, but is independent of the concentration of manganese(II) ions. The catalysed reaction appears to have an activation energy of 50 +/- 1 kJ mol-1. These observed kinetics are readily rationalised using an electrochemical model in which the catalyst particles act as microelectrodes providing a medium for electron transfer between the highly irreversible oxidation of water to O2 and the highly irreversible reduction of MnO4- to Mn2+.

MECHANISM OF THE REDUCTION OF WATER BY REDUCED METHYL VIOLOGEN MEDIATED BY PLATINIZED ALUMINA - H/D ISOTOPE EFFECTS

The rate of oxidation of reduced methyl viologen (MV+4) by water, catalyzed by colloidal Pt/Al2O3, is reduced by a factor of congruent-to 5 when D2O is used as a solvent rather than H2O in the presence of a pH 4.40 acetate buffer. In contrast, the rate measured in the presence of a pH 3.05 buffer is reduced only slightly when D2O replaces H2O. H/D isotope separation factors for the methyl viologen mediated reduction of water to hydrogen catalyzed by Pt/Al2O3 are 4.22 (+/- 0.15) at pH 4.40 and 5.99 (+/- 0.11) at pH 3.05, at 25-degrees-C. These data are interpreted in terms of the electrochemical model for metal-catalyzed redox reactions with a pH-dependent mechanism for the hydrogen-evolving reaction. It is proposed that hydrogen atom combination on the catalyst surface is the rate-limiting step at pH 4.40, whereas at pH 3.05 diffusion of MV2+4 is rate limiting and hydrogen evolution proceeds via the electrochemical reaction between a surface-adsorbed hydrogen atom and a solution-phase proton.