Dual Purpose Photocatalysis for Water Treatment and Hydrogen Production

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Comment on "Photocatalytic Oxidation of Arsenite over TiO(2): Is Superoxide the Main Oxidant in Normal Air-Saturated Aqueous Solutions?

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Photoelectrochemical Conversion of the As(III)/As(V) Redox Couple on Illuminated Titanium Dioxide Electrodes in a Wide Concentration Range (M – mM)

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Concentration-Dependent Photoredox Conversion of As(III)/As(V) on Illuminated Titanium Dioxide Electrodes

The photoconversion of As(III) (arsenite) and As(V) (arsenate) over a mesoporous TiO2 electrode was investigated in a photoelectrochemical (PEC) cell for a wide range of concentrations (mu M-mM), under nonbiased (open-circuit potential measurements) and biased (short-circuit current measurements) conditions. Not only As(III) can be oxidized, but also As(V) can be reduced in the anoxic condition under UV irradiation. However, the reversible nature of As(III)/As(V) photoconversion was not observed in the normal air-equilibrated condition because the dissolved O-2 is far more efficient as an electron acceptor than As(V). Although As(III) should be oxidized by holes, its presence did not increase the photooxidation current in a monotonous way: the photocurrent was reduced by the presence of As(III) in the micromolar range but enhanced in the millimolar range. This abnormal concentration-dependent behavior is related with the fate of the intermediate As(IV) species which can be either oxidized or reduced depending on the experimental conditions, combined with surface deactivation for the water photooxidation process. The lowering of the photooxidation current in the presence of micromolar As(III) is ascribed to the role of As(IV) as a charge recombination center. Being an electron acceptor, the addition of As(V) consistently lowers the photocurrent in the entire concentration range. A global concentration-dependent mechanism is proposed accounting for all the PEG results and its relation with the photocatalytic oxidation mechanism is discussed.X112221sciescopu

Simultaneous production of hydrogen with the degradation of organic pollutants using TiO2 photocatalyst modified with dual surface components

The simultaneous production of hydrogen and degradation of organic pollutants (4-chlorophenol, urea, and urine) was successfully achieved using titania photocatalysts which were modified with both anion adsorbates (fluoride or phosphate) and (noble) metals (Pt, Pd, Au, Ag, Cu, or Ni). The dual-function photocatalysis worked only when both components coexisted on the surface of TiO2, whereas TiO2 modified with a single surface component (F-TiO2, P-TiO2, or Pt/TiO2) was inactive under the same experimental condition. Two main surface-modified photocatalysts, F-TiO2/Pt (surface fluorinated and platinized) and P-TiO2/Pt (surface phosphated and platinized), were similarly active for dual-function photocatalysis in the anoxic suspension under UV irradiation. With these catalysts employed, the degradation of 4-chlorophenol (or urea) was accompanied by the concurrent production of H-2. The synergistic effect greatly depended on the kind of metal and pH. The activity of F-TiO2/Pt gradually decreased with increasing pH, which makes the application of F-TiO2/Pt limited to the acidic pH region. On the other hand, P-TiO2/Pt exhibited a consistent activity over a wide range of pH, which makes P-TiO2/Pt a more practical dual-function photocatalyst. The synergistic effect of anions and metal deposits on the surface of TiO2 enhanced the interfacial electron transfer and reduced the charge recombination which resulted in a maximum of 20-fold increase of H-2 production compared to metal deposited TiO2 in the presence of 4-chlorophenol.open11109115sciescopu

Band Energy Levels and Compositions of CdS-based Solid Solution and Their Relation with Photocatalytic Activities

CdS-based solid solution (AgIn) xCd(2(1-x))S(2) (x = 0, 0.02, 0.05, 0.1, 0.2, 1.0) was prepared through hydrothermal reaction. The physicochemical and optical properties of the as-prepared solid solution samples were characterized by X-ray diffraction (XRD), diffuse reflectance UV-visible absorption spectroscopy (DRS) and N2 adsorption-desorption isotherms. Through controlling the composition of CdS solid solution, its band-gap energy (E-g) can be easily tuned from ca. 2.4 to 1.8 eV. The band edge energy levels of all samples were determined through the measurement of the open circuit potential (OCP) and onset potential of photocurrent (E-onset) for their respective electrodes. The decrease in Eg of the solid solutions is attributed to the downward and upward shift of conduction and valence band potentials, respectively. It was found that In 5s5p and Cd 5s5p states mainly contribute to the bottom of conduction band, and Ag 4d and S 3p states mainly contribute to the top of valence band as confirmed by theoretical density of states (DOS) calculation. To test the photocatalytic activity of the prepared solid solution materials, the evolution of H-2 and the reduction of polyoxometalate (POM: PMo12O403-) under visible light irradiation were used as probe reactions. The photocatalysis test results were well correlated with the band structures determined from the photoelectrochemical measurements and DOS calculation. The relative positioning between the band-edge levels and the substrate conversion potentials was the most critical and this can be controlled with the solid solution-based photocatalysts.open1199sciescopu

Response to Comment on ""Photocatalytic Oxidation Mechanism of As(III) on TiO2: Unique Role of As(III) as a Charge Recombinant Species

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