Wen Yuan Lin

Bactericidal activity of TiO2 photocatalyst in aqueous media : toward a solar-assisted water disinfection system

Irradiation of suspensions of Escherichia coli ([approximately] 10[sup 6] cells/mL) and TiO[sub 2] (anatase) with UV-visible light of wave-lengths longer than 380 nm resulted in the killing of the bacteria within minutes. Oxygen was found to be a prerequisite for the bactericidal properties of the photocatalyst. Bacterial killing was found to adhere to first-order kinetics. The rate constant was proportional to the square root of the concentration of TiO[sub 2] and proportional to the incident light intensity in the range [approximately] 180- [approximately] 1660 [mu]E s[sup [minus]1] m[sup [minus]2]. The trends in these simulated laboratory experiments were mimicked by outdoor tests conducted under the summer noonday sun in Texas. The implications of these results as well as those of previous investigations in terms of practical applicability to solar-assisted water treatment and disinfection at remote sites are discussed relative to water technologies currently considered as viable as alternatives to chlorination. 24 refs., 8 figs.

Photocatalytic Reduction and Immobilization of Hexavalent Chromium at Titanium Dioxide in Aqueous Basic Media

The thermodynamic aspects of the photocatalytic reduction of Cr(VI) at TiO[sub 2] are first discussed and contrasted for pH 0 and pH 10 aqueous media. The photocorrosion of TiO[sub 2] is shown to be thermodynamically less favorable in basic media. A scheme for the one-step removal of Cr(VI) at TiO[sub 2] via its reduction and subsequent immobilization [as Cr(OH)[sub 3]] is demonstrated for pH 10 chromate solutions. Dissolved oxygen is shown to inhibit the reduction of Cr(VI). A preliminary kinetics analysis reveals adherence of the initial rate of Cr(VI) reduction to the simple Langmuir model.

Homogeneous and heterogeneous photocatalytic reactions involving As(III) and As(V) species in aqueous media

Thermodynamic considerations show that photogenerated holes in UV-irradiated TiO2 can oxidize As(III) to As(V). Similarly, the photogenerated electrons can reduce As(V) to the elemental state. Experimental data are presented in verification of these expectations. An electron acceptor (e.g., O2) is needed for the hole transfer pathway whereas the heterogeneous photocatalytic reduction of As(V) is facilitated by a hole scavenger such as methanol. The oxidation of As(III) to As(V) also proceeds in the dark when a large excess of H2O2 is present. However, this process is very facile with simultaneous UV irradiation of the peroxide. These data point toward the feasibility of a two-step pollution abatement scheme for the initial oxidation and subsequent immobilization of the toxic As(III) species.

Photocatalytic Removal of Nickel from Aqueous Solutions Using Ultraviolet‐Irradiated TiO2

The photocatalytic removal of Ni(II) using ultraviolet (UV)-irradiated TiO{sub 2} suspensions is described. The photocatalytic reaction is shown to be electron-mediated, and occurs both via direct electron transfer from the photoexcited TiO{sub 2} particles to Ni(II), and an indirect route as described by previous authors. The latter requires the presence of an organic additive (acetate, formate, or methanol) that generates an intermediate radical reductant via an initial hole-mediated reaction. The effects of inorganic anions (chloride, sulfate) on the rate of Ni(II) removal are described as are those of other variables including TiO{sub 2} dose and solution pH. The direct electron-transfer route is shown to be kinetically sluggish, although it can be made fast by the addition of facile hole acceptors. The dark adsorption of Ni(II) on the TiO{sub 2} (anatase) surface is also quantified in a chloride medium. The present data are discussed within the framework of previous studies on the UV/TiO{sub 2} photocatalytic deposition of Ni(II) and other metal ions.

Sonolytic enhancement of the bactericidal activity of irradiated titanium dioxide suspensions in water

Irradiated TiO2 suspensions in water were used to inactivate Escherichia coli and Hansenula polymorpha. Two types of batch reactors employing static and recirculating solutions were used in this study. Sonolysis using a 20 kHz ultrasonic unit was found to enhance the microorganism inactivation in all instances, although the enhancement was more modest for the batch recirculation reactor. These data are interpreted within the framework of four possible mechanisms. The mechanism based on sonolytic creation of ·OH appears to provide the most satisfactory explanation of the data trends. The present data also implicate ·OH as the dominant bactericidal agent in irradiated TiO2 suspensions.

Photocatalytically Generated Bimetallic ( Pt – Au ∕ C – TiO2 ) Electrocatalysts for Polymer Electrolyte Fuel Cell Applications

Heterogeneous photocatalysis was used to prepare bimetallic Pt―Au modified carbon―TiO 2 matrices for use in polymer electrolyte fuel cells. These new generation electrocatalysts were characterized by transmission electron microscopy, energy-dispersive X-ray analyses, X-ray diffraction, and X-ray photoelectron microscopy. The electrocatalytic activity of these materials for the oxygen reduction reaction (ORR) was assessed by rotating disk hydrodynamic voltammetry. Of the three variant scenarios that can be envisioned for photocatalytic deposition of the two metals, i.e., sequential deposition (with Pt first and Au second or Au first and Pt second) or simultaneous deposition of Pt and Au on the C―TiO 2 nanocomposite surface from a single bath, electrocatalyst samples with Pt decorating the initially deposited Au nanoclusters (designated as Pt/Au/C―TiO 2 ) performed the best in terms of ORR kinetic facility, even relative to the monometallic case of Pt supported on C―TiO 2 . The durability of these electrocatalysts (in terms of corrosion) was assessed via galvanostatic polarization tests; once again Pt/Au/C―TiO 2 fared best relative to the other two samples as well as the Pt/C―TiO 2 control case. For all the electrochemical analyses, the total metal loading in the electrocatalysts was kept constant at 20% (by mass) for meaningful comparison.

Manifestation of photocatalysis process variables in a titanium dioxide-based slurry photoelectrochemical cell

This paper examines how variables that influence photocatalytic events at ultraviolet (UV)-irradiated TiO{sub 2} particles manifest in the transient response from a slurry photoelectrochemical (PEC) cell. These include the nature of the ambient gas, electrolyte pH and composition, and the presence of hole acceptors in the electrolyte. Unlike the composite photocurrent response observed in previous studies on TiO{sub 2}-based slurry PEC cells, the photoresponse was almost exclusively anodic under the conditions extant in this work. The highest photoresponse was obtained in neutral electrolytes. Both O{sub 2} and Cl{sup {minus}} exerted a complex effect on the photoresponse, depending on the magnitude of the collector-electrode potential. Pulsed photoexcitation of the TiO{sub 2} particles enables decoupling of processes with variant time constants in the slurry PEC cell, such as O{sub 2} adsorption on TiO{sub 2}, adsorption of TiO{sub 2} particles on the collector-electrode surface, and mass transport of TiO{sub 2} particles to the collector electrode. Finally, the influence of three types of environmental pollutants: viz., organic (formaldehyde), inorganic [As(III)], and biological (Escherichia coli), on the slurry cell photoresponse, is discussed.

INTERFACIAL CHEMISTRY AT METAL/CDTE CONTACTS AS PROBED BY DIFFERENTIAL SCANNING CALORIMETRY

All four possible chemical reactivity patterns, namely, outdiffusion of Te (metal‐Cd alloy formation), Cd outdiffusion (metal telluride compound formation), comparable chemical reactivity of the metal towards both Cd and Te (no Cd or Te outdiffusion), and chemical inertness of the metal towards CdTe, were differentiated via the differential scanning calorimetry (DSC) technique from a study of the interaction of nine different metals toward CdTe powder. The fusion signatures of free Cd or Te, exotherms due to compound or alloy formation, along with the thermal transitions of the metal telluride and/or the intermetallic were used for this purpose. These reactivity patterns are discussed within the framework of two different thermodynamic models. Both virgin and chemically etched CdTe surfaces were examined, and found to exhibit rather different reactivity trends towards the metal. The ramifications of these results in terms of the electronic properties of metal/CdTe contacts are discussed. Finally, DSC is s...