Light-driven oxygen scavenging by titania/polymer nanocomposite films

We demonstrate that UV illumination of nanocrystalline TiO2 films in the presence of excess organic hole scavengers can result in the deoxygenation of a closed environment. The kinetics of deoxygenation are investigated under continuous UV illumination as a function of film preparation and hole scavenger employed. Optimum deoxygenation is observed using methanol as a hole scavenger, although efficient deoxygenation is also observed for a range of different polymer/TiO2 nanocomposite films deposited on glass and plastic substrates. Transient absorption spectroscopy is used to probe the kinetics of the deoxygenation reaction, focusing on the kinetics of the reduction of oxygen by photogenerated TiO2 electrons. Under aerobic conditions, this oxygen reduction reaction is observed to exhibit first order kinetics with a rate constant of 70 s(-1), more than one order of magnitude faster than alternative reaction pathways for the photogenerated electrons. These observations are discussed in terms of the Langmuir-Hinshelwood equation for photocatalytic action. (C) 2004 Elsevier B.V. All rights reserved.

Characterisation of the photocatalyst Pilkington Activ (TM): a reference film photocatalyst?

Pilkington Glass Activ(TM) represents a possible suitable successor to P25 TiO2, especially as a benchmark photocatalyst film for comparing other photocatalyst or PSH self-cleaning films. Activ(TM) is a glass product with a clear, colourless, effectively invisible, photocatalytic coating of titania that also exhibits PSH. Although not as active as a film of P25 TiO2, Activ(TM) vastly superior mechanical stability, very reproducible activity and widespread commercial availability makes it highly attractive as a reference photocatalytic film. The photocatalytic and photo-induced superhydrophilitic (PSH) properties of Activ(TM) are studied in some detail and the results reported. Thus, the kinetics of stearic acid destruction (a 104 electron process) are zero order over the stearic acid range 4-129 monolayers and exhibit formal quantum efficiencies (FQE) of 0.7 X 10(-5) and 10.2 x 10(-5) molecules per photon when irradiated with light of 365 +/- 20 and 254 nm, respectively; the latter appears also to be the quantum yield for Activ(TM) at 254 nm. The kinetics of stearic acid destruction exhibit Langmuir-Hinshelwood-like saturation type kinetics as a function of oxygen partial pressure, with no destruction occurring in the absence of oxygen and the rate of destruction appearing the same in air and oxygen atmospheres. Further kinetic work revealed a Langmuir adsorption type constant for oxygen of 0.45 +/- 0.16 kPa(-1) and an activation energy of 19 +/- 1 Kj mol(-1). A study of the PSH properties of Activ(TM) reveals a high water contact angle (67) before ultra-bandgap irradiation reduced to 0degrees after prolonged irradiation. The kinetics of PSH are similar to those reported by others for sol-gel films using a low level of UV light. The kinetics of contact angle recovery in the dark appear monophasic and different to the biphasic kinetics reported recently by others for sol-gel films [J. Phys. Chem. B 107 (2003) 1028]. Overall, Activ(TM) appears a very suitable reference material for semiconductor film photocatalysis. (C) 2003 Elsevier Science B.V All rights reserved.

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.

PHOTOMINERALIZATION OF SALICYLIC-ACID - A KINETIC-STUDY

The kinetics of the photomineralization of salicylic acid (SA) sensitized by Degussa P25 titanium dioxide (TiO2) dispersions in oxygenated aqueous solution are reported as a function of the following experimental parameters: [TiO2], percentage of O-2, [SA], temperature (T) and light intensity (I). The kinetics of SA photomineralization conform to a Langmuir-Hinshelwood kinetic scheme with SA and O-2 adsorbed at different sites with apparent Langmuir adsorption coefficients of (6.1 +/- 1.2) x 10(4) mol(-1) dm(3) and 0.061 +/- 0.007 kPa(-1) respectively. The overall activation energy for the system was determined as 4.6 +/- 0.2 kJ mol(-1). Two major stable reaction intermediates are identified (dihydroxybenzoic acids (DHBA) and catechol (C)) and the existence of a further pathway involving one or more very unstable and, as yet, unidentified reaction intermediates is proposed. A kinetic model is presented which describes the temporal behaviour of the concentrations of SA, CO2 and the major photogenerated intermediates (DHBA and C). This model is used to predict successfully the temporal behaviour of the major intermediates in the photomineralization of SA under non-standard conditions.

PHOTOMINERALIZATION OF 4-CHLOROPHENOL SENSITIZED BY TITANIUM-DIOXIDE - A STUDY OF THE INITIAL KINETICS OF CARBON-DIOXIDE PHOTOGENERATION

The kinetics of photomineralization of 4-chlorophenol (4-CP) sensitized by Degussa P25 TiO2 in O2-saturated solution is studied as a function of the following different experimental parameters: pH, [TiO2], percentage O2 [O2], [4-CP], T, I, lambda and [KNO3]. At pH 2 and T=30-degrees-C the initial relative rate of CO2 photogeneration R(CO2) conforms to a Langmuir-Hinshelwood-type kinetic scheme and the relationship between R(CO2) and the various experimental parameters may be summarized as follows: R(CO2) = gammaK(O2)[O2](I(a))(theta)K(4-CP]0/(1 + K(O2])(1 + K(4-CP)[4-CP]0) where gamma is a proportionality constant, K(O2) = 0.044 +/- 0.005[O2]-1, theta = 0.74 +/- 0.05 and K(4-CP) = (29 +/- 3) x 10(3) dm3 mol-1. The overall activation energy for this photosystem was determined as 16 +/- 2 kJ mol-1. This work forms part of an overall characterization study in which it is proposed that the 4-CP-TiO2-O2 photosystem is adopted as a standard test system for incorporation into all future semiconductor-sensitized photomineralization studies in order to facilitate comparisons between the results of the different studies.

PHOTOMINERALISATION OF 4-CHLOROPHENOL SENSITIZED BY TITANIUM-DIOXIDE - A STUDY OF THE INTERMEDIATES

The photomineralisation of 4-chlorophenol (4-CP) sensitised by Degussa P25 TiO2 in O2-saturated solution represents a possible standard test system in semiconductor-sensitised photomineralisation studies. As part of a detailed examination of this photosystem, the results of the temporal variations in the concentrations of 4-CP, CO2, Cl- and the major organic intermediates, namely, 4-chlorocatechol (4-CC), hydroquinone (HQ), benzoquinone and 4-chlororesorcinol, are reported. The observed variations in [4-CP], [4-CC], [HQ] and [CO2] fit those predicted by a kinetic model which utilises kinetic equations with a Langmuir-Hinshelwood form and assumes that there are three major possible routes in which the photogenerated hydroxyl radicals can react with 4-CP, ie. 4-CP --> 4-CC, 4-CP --> HQ and 4-CP --> (unstable intermediate) --> CO2 and that these routes have the following probabilities of occurring: 48%, 10% and 42%.

Optimisation of Kinetic Parameters for an After-treatment Catalyst

Mathematical modelling has become an essential tool in the design of modern catalytic systems. Emissions legislation is becoming increasingly stringent, and so mathematical models of aftertreatment systems must become more accurate in order to provide confidence that a catalyst will convert pollutants over the required range of conditions. 
Automotive catalytic converter models contain several sub-models that represent processes such as mass and heat transfer, and the rates at which the reactions proceed on the surface of the precious metal. Of these sub-models, the prediction of the surface reaction rates is by far the most challenging due to the complexity of the reaction system and the large number of gas species involved. The reaction rate sub-model uses global reaction kinetics to describe the surface reaction rate of the gas species and is based on the Langmuir Hinshelwood equation further developed by Voltz et al. [1] The reactions can be modelled using the pre-exponential and activation energies of the Arrhenius equations and the inhibition terms. 
The reaction kinetic parameters of aftertreatment models are found from experimental data, where a measured light-off curve is compared against a predicted curve produced by a mathematical model. The kinetic parameters are usually manually tuned to minimize the error between the measured and predicted data. This process is most commonly long, laborious and prone to misinterpretation due to the large number of parameters and the risk of multiple sets of parameters giving acceptable fits. Moreover, the number of coefficients increases greatly with the number of reactions. Therefore, with the growing number of reactions, the task of manually tuning the coefficients is becoming increasingly challenging. 
In the presented work, the authors have developed and implemented a multi-objective genetic algorithm to automatically optimize reaction parameters in AxiSuite®, [2] a commercial aftertreatment model. The genetic algorithm was developed and expanded from the code presented by Michalewicz et al. [3] and was linked to AxiSuite using the Simulink add-on for Matlab. 
The default kinetic values stored within the AxiSuite model were used to generate a series of light-off curves under rich conditions for a number of gas species, including CO, NO, C_{3H8 and C3H6. These light-off curves were used to generate an objective function.} 
This objective function was used to generate a measure of fit for the kinetic parameters. The multi-objective genetic algorithm was subsequently used to search between specified limits to attempt to match the objective function. In total the pre-exponential factors and activation energies of ten reactions were simultaneously optimized. 
The results reported here demonstrate that, given accurate experimental data, the optimization algorithm is successful and robust in defining the correct kinetic parameters of a global kinetic model describing aftertreatment processes.

Photocatalytic destruction of geosmin using novel pelleted titanium dioxide

Geosmin is produced by cyanobacteria and actinomycetes in surface waters. It causes undesirable earthy off-flavours in freshwater fish and is a major concern for the drinking water industry. This paper presents the first published study on the use of the novel pelleted Ti0₂ photocatalyst, Hombikat K01/C, for the removal of geosmin from water. Ti0₂ in pelleted form eliminates the requirement for the separation of the catalyst from the water following treatment which is normally the case with the widely used powdered catalysts. A laboratory reactor was designed to limit system loss since the compound adsorbs to a wide range of surfaces. Initial concentration, aeration rate and irradiation were evaluated. It was found that degradation of geosmin followed the Langmuir-Hinshelwood model. Elevated aeration had no effect on the photocatalytic removal of geosmin, but increasing irradiation was found to increase degradation rates. The catalyst proved effective within 10 min under optimum conditions. 

Mathematical modelling of quantum yield enhancements of methyl orange photooxidation in aqueous TiO2 suspensions under controlled periodic UV LED illumination

Quantum yields of the photocatalytic degradation of methyl orange under controlled periodic illumination (CPI) have been modelled using existing models. A modified Langmuir-Hinshelwood (L-H) rate equation was used to predict the degradation reaction rates of methyl orange at various duty cycles and a simple photocatalytic model was applied in modelling quantum yield enhancement of the photocatalytic process due to the CPI effect. A good agreement between the modelled and experimental data was observed for quantum yield modelling. The modified L-H model, however, did not accurately predict the photocatalytic decomposition of the dye under periodic illumination.

The Destruction of Cyanobacterial Toxins by Titanium Dioxide

Cyanobacterial (blue-green algal) toxins are extremely toxic naturally occurring substances which display hepato- and neurotoxic behaviour (1, 2). In this paper we report the application of titanium dioxide photocatalysis for the destruction of two of these compounds, microcystin-LR and anatoxin-a. The destruction of microcystin appears to follow Langmuir-Hinshelwood kinetics although a discrepancy was observed between adsorption constants determined for the photocatalytic process with those obtained from dark isotherms. A square root dependence between illumination intensity and rate of microcystin destruction was noted. When the destruction was performed in the presence of the naturally occurring pigment it appeared that the pigment also contributes to the destruction of the toxin. Toxicity studies on the photocatalysed toxin solutions indicates that the toxicity is substantially reduced within 30 min photolysis.

Controlled periodic illumination in semiconductor photocatalysis

Controlled periodic illumination is a hypothesis postulated in the early 1990s for enhancing the efficiency of semiconductor photocatalytic reactions. This technique has been proposed to improve photocatalytic efficiency by the nature of photon introduction alone. Before its application in semiconductor photocatalysis, controlled periodic illumination had been investigated in other fields including photosynthesis. This paper presents a detailed review of the state of the art research undertaken on the application of controlled periodic illumination in semiconductor photocatalysis. The review briefly introduces semiconductor photocatalysis, and then presents a detailed explanation of this technique, its importance to photocatalytic efficiency, an overview of previous results of its application in significant studies and present knowledge. Results from previous as well as some of the most recent studies indicate potential applications of controlled periodic illumination in areas other than just the improvement of the efficiency of the photocatalytic process.

Photocatalytic degradation of humic acid in saline waters. Part 1. Artificial seawater: influence of TiO2, temperature, pH, and air-flow

We report the first systematic study on the photocatalytic oxidation of humic acid (HA) in artificial seawater (ASW). TiO2 (Degussa P25) dispersions were used as the catalyst with irradiation from a medium-pressure mercury lamp. The optimum quantity of catalyst was found to be between 2 and 2.5 g l(-1); whiled the decomposition was fastest at low pH values (pH 4.5 in the range examined), and the optimum air-flow, using an immersion well reactor with a capacity of 400 ml, was 850 ml min(-1). Reactivity increased with air-flow up to this figure, above which foaming prevented operation of the reactor. Using pure. oxygen, an optimal flow rate was observed at 300 nil min(-1), above which reactivity remains essentially constant. Following treatment for 1 h, low-salinity water (2700 mg l(-1)) was completely mineralised, whereas ASW (46000 mg l(-1)) had traces of HA remaining. These effects are interpreted and kinetic data presented. To avoid problems of precipitation due to change of ionic strength humic substances were prepared directly in ASW, and the effects of ASW on catalyst suspension and precipitation have been taken into account. The Langmuir-Hinshelwood kinetic model has been shown to be followed only approximately for the catalytic oxidation of HA in ASW. The activation energy for the reaction derived from an Arrhenius treatment was 17 ( +/-0.6) kJ mol(-1). (C) 2003 Elsevier Science Ltd. All rights reserved.

The impact of water concentration on the catalytic oxidation of ethanol on platinum electrode in concentrated phosphoric acid

The electro-oxidation of ethanol on platinum in phosphoric acid opens the door to promote the oxidation reaction at higher temperatures. However, the effect of the presence of water is not well understood. In this work, the electro-oxidation of ethanol on platinum was studied in concentrated phosphoric acid containing different concentrations of water at room temperature. The results show that effect of bulk water on the rate electro-oxidation is highest at 0.60 V and decreases for increasing potentials. This was suggested as due to the increasing formation of oxygenated species on the electrode surface with potential, which in turn is more efficient than the increase of water content in the electrolyte. Altogether, these results were interpreted as an evidence of a Langmuir-Hinshelwood step involving oxygenated species as one of the adsorbed partners. (C) 2009 Elsevier B.V. All rights reserved.

TiO(2)-PHOTOCATALYZED DEGRADATION OF PHENOL IN SALINE MEDIA IN AN ANNULAR REACTOR: HYDRODYNAMICS, LUMPED KINETICS, INTERMEDIATES, AND ACUTE TOXICITY

The photocatalytic degradation of phenol in aqueous suspensions of TiO(2) under different salt concentrations in an annular reactor has been investigated. In all cases, complete removal of phenol and mineralization degrees above 90% were achieved. The reactor operational parameters were optimized and its hydrodynamics characterized in order to couple mass balance equations with kinetic ones. The photodegradation of the organics followed a Langmuir-Hinshelwood-Hougen- Watson lumped kinetics. From GC/MS analyses, several intermediates formed during oxidation have been identified. The main ones were catechol, hydroquinone, and 3-phenyl-2-propenal, in this order. The formation of negligible concentrations of 4-chlorophenol was observed only in high salinity medium. Acute toxicity was determined by using Artemia sp. as the test organism, which indicated that intermediate products were all less toxic than phenol and a significant abatement of the overall toxicity was accomplished, regardless of the salt concentration.

Avaliação de catalisadores a base de carbeto e nitreto de molibdênio na obtenção de lactitol via hidrogenação da lactose

Seeking a greater appreciation of cheese whey was developed to process the hydrogenation of lactose for the production of lactitol, a polyol with high added value, using the catalyst Ni / activated carbon (15% and 20% nickel), the nitride Mo2N, the bimetallic carbide Ni-Mo/ activated carbon and carbide Mo2C. After synthesis, the prepared catalysts were analyzed by MEV, XRD, laser granulometry and B.E.T. The reactor used in catalytic hydrogenation of lactose was the type of bed mud with a pressure (68 atm), temperature (120 oC) and stirring speed (500 rpm) remained constant during the experiments. The system operated in batch mode for the solid and liquid and semi-continuous to gas. Besides the nature of the catalyst, we studied the influence of pH of reaction medium for Mo2C carbide as well as evaluating the character of the protein inhibitor and chloride ions on the activity of catalysts Ni (20%)/Activated Carbon and bimetallic carbide Ni-Mo/Activated Carbon. The decrease in protein levels was performed by coagulation with chitosan and adsorption of chloride ions was performed by ion exchange resins. In the process of protein adsorption and chloride ions, the maximum percentage extracted was about 74% and 79% respectively. The micrographs of the powders of Mo2C and Mo2N presented in the form of homogeneous clusters, whereas for the catalysts supported on activated carbon, microporous structure proved impregnated with small particles indicating the presence of metal. The results showed high conversion of lactose to lactitol 90% for the catalyst Ni (20%)/Activated Carbon at pH 6 and 46% for the carbide Mo2C pH 8 (after addition of NH4OH) using the commercial lactose. Monitoring the evolution of the constituents present in the reaction medium was made by liquid chromatography. A kinetic model of heterogeneous Langmuir Hinshelwood type was developed which showed that the estimated constants based catalysts promoted carbide and nitride with a certain speed the adsorption, desorption and production of lactitol