Photodecomposition of ozone sensitised by a film of titanium dioxide on glass

High levels of ozone (typically 850 ppm) are readily decomposed by semiconductor photocatalysis, using a thin film of the semiconductor titanium dioxide (Degussa P25 TiO2) cast on a glass tube, and UVA light, i.e. light of energy greater than that of the bandgap of the semiconductor (ultra-bandgap light); in the absence of this light the thermal decomposition of ozone is relatively slow. The semiconductor films show no evidence of chemical or photochemical wear with repeated use. At high levels of ozone, i.e. 100 ppm less than or equal to [O-3] less than or equal to 1400 ppm, the initial rate of ozone decomposition by semiconductor photocatalysis is independent of [O-3], whereas, at lower ozone concentrations, i.e. 5 ppm less than or equal to [O-3] less than or equal to 100 ppm, the initial rate of ozone photodestruction decreases in a smooth, but non-linear, manner with decreasing [O-3]. The kinetics of ozone photodecomposition fit a Langmuir-Hinshelwood type kinetic equation and the possible mechanistic implications of these results are briefly discussed. (C) 2002 Elsevier Science B.V. All rights reserved.

Atmospheric pressure chemical vapour deposition of titanium dioxide coatings on glass

Atmospheric pressure chemical vapour deposition of titanium dioxide coatings on glass substrates was achieved by the reaction of TiCl4 and a co-oxygen source (MeOH, EtOH, (PrOH)-Pr-i or H2O) at 500-650degreesC. The coatings show excellent uniformity, surface coverage and adherence. Growth rates were of the order of 0.3 mum min(-1) at 500degreesC. All films are crystalline and single phase with XRD showing the anatase TiO2 diffraction pattern; a = 3.78(1), c = 9.51(1) Angstrom. Optically, the films show minimal reflectivity from 300-1600 nm and 50-80% total transmission from 300-800 nm. Contact angles are in the range 20-40degrees for as-prepared films and 1-10degrees after 30 min irradiation at 254 nm. All of the films show significant photocatalyic activity as regards the destruction of an overlayer of stearic acid.

Optical sensors for carbon dioxide: an overview of sensing strategies past and present

The evolution of the optical sensor for CO2 over the past two decades is outlined and illustrated through examples of luminescent-based sensors. The basic principles and design of the early 'wet covered' type sensor, in which a pH sensitive dye in an aqueous buffer is covered by a gas permeable, ion impermeable, membrane, are outlined. The gradual move from the 'wet covered' types of CO2 optical sensor to 'solid-water droplet' type sensors and then onto 'solid' sensors is charted. The basic design and principles of operation of the modern 'solid' optical sensor for P-CO2 is covered in some detail. Other sensing strategies outside the simple use of pH-sensitive dyes are also considered, most notably those based on luminescence lifetime measurements.

Thin plastic film colorimetric sensors for carbon dioxide: Effect of plasticizer on response

Different plasticizers, including phosphate-, phthalate-and adipate-based types were used in the creation of a range of colorimetric plastic film sensors for CO2, The different types of plasticizer used in the formulation of a colorimetric plastic film sensor for CO2 affect the response and recovery times of the sensor differently, An effective plasticizer was taken as one that decreased the response and recovery times of the final film sensor when exposed to an alternating atmosphere of 0-5% CO2. On this basis, the most efficient plasticizers appeared to be phosphate-based, followed by phthalate- and adipate-based plasticizers, This trend appears to reflect the degree of the polymer-plasticizer compatibility. Increasing the amount of plasticizer in the film formulation decreased the response and recovery times of the sensor dramatically, The sensitivity of the film sensor towards CO2 appears to decrease with increasing plasticizer effectiveness; thus, the general order of film CO2 sensitivity with respect to plasticizer type was found to be adipate > phthalate > phosphate. In general, the response of the optical films towards CO2 was found to be temperature sensitive [typically, Delta H = -(44-55) kJ mol(-1)], The phosphate-based plasticized films appear to be less temperature sensitive than the other plasticized films, and 2-ethylhexyl diphenylphosphate appears particularly effective in this respect (Delta H = -18.5 kJ mol(-1)).

EFFECT OF ULTRASOUND ON THE KINETICS OF REDUCTION OF HEXACYANOFERRATE(III) BY THIOSULFATE IONS MEDIATED BY RUTHENIUM DIOXIDE HYDRATE

Ultrasound promotes the reduction of hexacyanoferrate(III) by thiosulfate ions mediated by RuO2 . xH(2)O under diffusion-controlled conditions. There is a strong correlation between the measured first-order rate constant and the absorbance of the dispersion, which, in turn, is closely related to the specific surface area of the catalyst. The enhancement in rate with ultrasonic irradiation appears to be largely associated with the dispersive action of the ultrasound on the aggregated particles of RuO2 . xH(2)O. The rate of reaction increases with increasing %duty cycle and ultrasonic intensity. The measured overall activation energies for the reaction with and without ultrasound, i.e. 18 +/- 1 and 20 +/- 1 kJ mol(-1), respectively, are very similar to those expected for a diffusion-controlled reaction. The homogeneous reaction is not promoted by ultrasound.

COLORIMETRIC POLYMER FILM SENSORS FOR DISSOLVED CARBON-DIOXIDE

Plasticized and unplasticized polymer colorimetric film sensors for gaseous CO2, containing the dye m-cresol purple, are tested as sensors for dissolved CO2. The plasticized polymer m-cresol purple film sensor develops a measurable degree of opacity when exposed to aqueous solution, especially in neutral, compared with alkaline, solution. However, it is shown that a presoaked, fogged plasticized polymer m-cresol purple film does function as a quantitative sensor for dissolved CO2 over the range 0-4% CO2. An unplasticized polymer m-cresol purple film remains largely dear upon exposure to aqueous solution and also functions as a quantitative sensor for dissolved CO2 over the range 0-4% CO2. However, in both types of films the dye interacts with electrolytes present in solution; invariably the dye appears to be converted from its initial deprotonated form (blue) to its protonated form (yellow) and the rate of this process appears to increase with increasing ionic strength, anionic charge and decreasing pH. The 90% response and recovery times for an unplasticized film are determined as 19 s(CO2:0-->5%) and 21 s (CO2:5-->3.6%), respectively.

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.

A KINETIC-STUDY OF THE BLEACHING OF RHODAMINE-6G PHOTOSENSITIZED BY TITANIUM-DIOXIDE

The results of a kinetic study of the bleaching of the photostable dye rhodamine 6G by dissolved oxygen, photosensitized by TiO2, are reported. The observed variations in the initial rate of dye photobleaching as a function of the O2 percentage, temperature, incident light intensity and concentrations of rhodamine 6G and sacrificial electron donor are described and the results are rationalized using a proposed photochemical reaction scheme. The photosensitized bleaching of rhodamine 6G dye by TiO2 has a formal quantum yield of 2.65 X 10(-3), but the rate of complete photomineralization is about twofold slower. The overall activation energy for the semiconductor-sensitized dye photobleaching process is 15.0 +/- 1.5 kJ mol-1.

TUNING COLOURIMETRIC AND FLUOROMETRIC GAS SENSORS FOR CARBON-DIOXIDE

The basic theory behind conventional colourimetric and fluorimetric optical sensors for CO2 is examined and special attention is given to the effect on sensor response of the key parameters of initial base concentration and dye acid dissociation constant, K(D). Experimental results obtained in aqueous solution using a variety of different dyes and initial base concentrations are consistent with the predictions made by the theoretical model. A series of model-generated pK(D) versus %CO2 curves for different initial base concentrations allow those interested in constructing an optical CO2 sensor to readily identify the optimum dye/initial base combination for their sensor; the response of the sensor can be subsequently fine-tuned through a minor variation in the initial base concentration. The model and all its predictions appear also to apply to the new generation of plastic film CO2 sensors which have just been developed.

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.