On the underlying mechanisms of the low observed nitrate selectivity in photocatalytic NOx abatement and the importance of the oxygen reduction reaction.

Semiconductor photocatalysis could be an effective means to combat air pollution, especially nitrogen oxides, which can be mineralized to nitrate. However, the reaction typically shows poor selectivity, releasing a number of unwanted and possibly toxic intermediates such as nitrogen dioxide. Up to now, the underlying principles that lead to this poor selectivity were not understood so a knowledge-based catalyst design for more selective materials was impossible. Herein, we present strong evidence for the slow oxygen reduction being one the causes, as the competing back-reduction of nitrate leads to the release of nitrogen dioxide. Consequently, engineering the photocatalyst for a better oxygen reduction efficiency should also increase the nitrate selectivity.

The Effect of Interfacial Chemical Bonding in TiO2-SiO2 Composites on Their Photocatalytic NOx Abatement Performance.

The chemical bonding of particulate photocatalysts to supporting material surfaces is of great importance in engineering more efficient and practical photocatalytic structures. However, the influence of such chemical bonding on the optical and surface properties of the photocatalyst and thus its photocatalytic activity/reaction selectivity behavior has not been systematically studied. In this investigation, TiO2 has been supported on the surface of SiO2 by means of two different methods: (i) by the in situ formation of TiO2 in the presence of sand quartz via a sol-gel method employing tetrabutyl orthotitanium (TBOT); and (ii) by binding the commercial TiO2 powder to quartz on a surface silica gel layer formed from the reaction of quartz with tetraethylorthosilicate (TEOS). For comparison, TiO2 nanoparticles were also deposited on the surfaces of a more reactive SiO2 prepared by a hydrolysis-controlled sol-gel technique as well as through a sol-gel route from TiO2 and SiO2 precursors. The combination of TiO2 and SiO2, through interfacial Ti-O-Si bonds, was confirmed by FTIR spectroscopy and the photocatalytic activities of the obtained composites were tested for photocatalytic degradation of NO according to the ISO standard method (ISO 22197-1). The electron microscope images of the obtained materials showed that variable photocatalyst coverage of the support surface can successfully be achieved but the photocatalytic activity towards NO removal was found to be affected by the preparation method and the nitrate selectivity is adversely affected by Ti-O-Si bonding.

Different Roles of Water in Photocatalytic DeNOx Mechanisms on TiO2: Basis for Engineering Nitrate Selectivity?

The nitrate selectivity of TiO2 has important consequences for its efficiency as a NOx depollution photocatalyst. Most emphasis is typically given to photocatalyst activity, a measure of the rate at which NOx concentrations are reduced, but a reduction in NOx concentration (mainly NO + NO2) is not necessarily a reduction in atmospheric NO2 concentration because the catalytic process itself generates NO2. With NO2 being considerably more toxic than NO, more emphasis on nitrate selectivity, a measure of the NOx conversion to nitrate, and how to maximize it, should be given in engineering photocatalytic systems for improved urban air quality. This study, on the importance of adsorbed water in the photocatalytic oxidation of NOx, has identified important correlations which differentiate the role that water plays in the oxidation of NO and NO2. This observation is significant and offers insights into controlling nitrate selectivity on TiO2 and the potential for increased effectiveness in environmental photocatalyst applications.

Properties and photochemistry of valence-induced-Ti(3+) enriched (Nb,N)-codoped anatase TiO2 semiconductors.

Nb and N codoped TiO2s are outstandingly versatile semiconductor oxides. Their high conductivity makes them valid alternatives to commercially available, but very expensive, conductive oxides. They show increased photonic efficiencies compared to the cases of solely Nb or N doped TiO2, when used as visible light sensitised photocatalysts. Furthermore, they are excellent materials for O2 sensors at very low temperature. Despite these remarkable properties, a clear picture of the electronic and optical mechanisms induced by the simultaneous presence of the dopants has just begun to be understood. Using a combination of electron paramagnetic resonance (EPR) spectroscopy, electrochemical impedance spectroscopy (EIS) and optical spectroscopy, we present here novel fundamental insights into the mechanisms responsible for the enhanced conductivity and visible light photochemistry.

Remediation of a chlorinated aromatic hydrocarbon in water by photoelectrocatalysis.

Photoelectrocatalysis driven by visible light offers a new and potentially powerful technology for the remediation of water contaminated by organo-xenobiotics. In this study, the performance of a visible light-driven photoelectrocatalytic (PEC) batch reactor, applying a tungsten trioxide (WO(3)) photoelectrode, to degrade the model pollutant 2,4-dichlorophenol (2,4-DCP) was monitored both by toxicological assessment (biosensing) and chemical analysis. The bacterial biosensor used to assess the presence of toxicity of the parent molecule and its breakdown products was a multicopy plasmid lux-marked E. coli HB101 pUCD607. The bacterial biosensor traced the removal of 2,4-DCP, and in some case, its toxicity response suggests the identification of transient toxic intermediates. The loss of the parent molecule, 2,4-DCP determined by HPLC, corresponded to the recorded photocurrents. Photoelectrocatalysis offers considerable potential for the remediation of chlorinated hydrocarbons, and that the biosensor based toxicity results identified likely compatibility of this technology with conventional, biological wastewater treatment.

The use of 'exotic' framework structures in waste management.

The use of porous framework materials in waste management applications has the potential to be a powerful tool in toxic metal remediation. The properties that these materials possess, including high surface area and ion-exchange capacity, are theoretically valuable. Furthermore, the flexibility of many of these frameworks allows the potential for immobilisation of waste materials with the framework of the material, in addition to the traditional capture in the pore structure. However, for either of these routes to be useful for waste management purposes, these structures must also be stable in any proposed storage media. This study examines the stability of a range of porous materials whose frameworks are made out of zinc and arsenic, both considered toxic minesite wastes, when exposed to aqueous media. The three frameworks examined (sodalite analogue Na(6)(H(2)O)(8)(ZnAsO(4))(6), open framework K(3)Zn(4)O(AsO(4))(3).3.5H(2)O, and an ABW type framework NH(4)ZnAsO(4)) all have similar hydrothermal synthetic routes and bulk framework compositions, but differ in counter ion used, pore size and complexity of structure. The phases were examined before and after storage in an aqueous environment, and their crystallinity and leaching were determined. All phases prepared were found to be extremely unstable outside their original synthetic environment, and very soluble when exposed to water, calling into question their practical use in any environment.

Synthesis and proposed crystal structure of a disordered cadmium arsenate apatite Cd5(AsO4)3Cl(1-2x-y)O(x)[symbol: see text](x)OH(y).

During a study into the synthesis of minerals composed of mining wastes aimed at improving their immobilisation, a cadmium arsenate apatite has been prepared by hydrothermal methods. The structure of this apatite was analysed by single crystal X-ray diffraction, and was found to consist of a standard apatite framework based on Cd(5)(AsO(4))(3)X, where X represents an anion resident on the (0,0,0.25) site. The framework is hexagonal with the space group P6(3)/m(no 176), a= 9.9709(8), c= 6.4916(4)[Angstrom]. The X ion site is predominantly occupied by Cl(-) ions; however due to significant shortening of the c axis exhibited by all cadmium containing apatite phases, a pure chlorapatite is not possible without a significant cation deficiency. No evidence of the necessary deficiency was found in the crystal structure. For larger bromo- and iodo-apatites significant modulations along the c-axis are required to accommodate the halide. This paper examines a number of compensation mechanisms and proposes that a minor disorder of chloride, oxide and hydroxide located on the X ion site provides the required charge compensation mechanism. This is contrary to previous complex modulations proposed in the literature. The proposed chemical formula is Cd(5)(AsO(4))(3)Cl(1-2x-y)O(x)[symbol:see text](x)OH(y) where [symbol: see text] represents a vacancy.

New low temperature synthetic route to an ammonium zinc arsenate zeolite analogue with an ABW-type structure.

The synthesis and stability of zeolite analogues are of potential interest to the waste management community as waste metals may be used in the framework of these materials. A new synthetic route to an ABW-type ammonium zinc arsenate is proposed. The stability of this material is then examined in an aqueous environment by quantitative X-ray diffraction and leach testing.