K. Vinodgopal

Enhanced Rates of Photocatalytic Degradation of an Azo Dye Using SnO2/TiO2 Coupled Semiconductor Thin Films.

We present here for the first time the results from electrochemically assisted photocatalytic experiments using coupled TiO{sub 2}/SnO{sub 2} semiconductor thin films in the degradation of textile dye effluent. We show that by using such a system the oxidative efficiency of photocatalytic semiconductor systems in degrading a commercial azo dye such as Acid Orange 7 (AO7) can be improved. The results presented here on the decolorization of the azo dye AO7 represents a major step forward in the development of new advanced oxidation processes for the treatment of such industrial waste. The improved charge separation as a result of coupling two semiconductor systems with different energy levels and the applied anodic bias is responsible for the enhancement in the rate of photocatalytic degradation. 20 refs., 6 figs.

Ultrasonic mineralization of a reactive textile azo dye remazol black B

The degradation of a reactive black dye in oxygen saturated aqueous solution has been investigated using a high frequency ultrasonic generator. The OH radical initiated oxidative degradation of the dye results in 65% mineralization as measured by the decrease in the total organic content. Ion chromatography indicates that the only remaining components are oxalate, sulfate and nitrate ions.

Photochemistry of textile azo dyes. Spectral characterization of excited state, reduced and oxidized forms of Acid Orange 7

The spectral characterization of the singlet and triplet excited states of an azo dye (Acid Orange 7 (AO)) was performed using picosecond and nanosecond laser flash photolysis. The excited singlet state lifetime, estimated from the bleaching recovery, is around 135 ps. The triplet excited state generated by triplet—triplet (TT) energy transfer shows a difference absorption maximum at 650 nm with a lifetime of μs. The oxidized and reduced forms of the dye were generated by reacting AO with pulse radiolytically generated azide (N3) and eaq radicals. These radicals were also detected as the primary intermediates in visible-laser-induced photoionization. The photoelectrochemical reduction of the dye in UV-irradiated TiO2 colloidal suspension results in the irreversible decolorization of the dye.

Electrochemically assisted photocatalysis using nanocrystalline semiconductor thin films

The principle and usefulness of electrochemically assisted photocatalysis has been illustrated with the examples of 4-chlorophenol and Acid Orange 7 degradation in aqueous solutions. Thin nanocrystaliine semiconductor films coated on a conducting glass surface when employed as a photoelectrode in an electrochemical cell are effective for degradation of organic contaminants. The degradation rate can be greatly improved even in the absence of oxygen by applying an anodic bias to the TiO2 film electrodes. A ten-fold enhancement in the degradation rate was observed when TiO2 particles were coupled with SnO2 nanocrystallites at an applied bias potential of 0.83 V versus SCE.

Photocatalytic reduction of azo dyes Naphthol Blue Black and Disperse Blue 79

Photocatalytic reduction of two textile azo dyes, Naphthol Blue Black (NBB) and Disperse Blue 79 (DB79) has been carried out in colloidal WO3 and TiO2 suspensions. Under bandgap excitation of the semiconductor colloids these dyes undergo irreversible reduction as they react with the trapped electrons. The quantum efficiency for the photocatalytic reduction of these dyes were 5.4% and 4.8% for NBB and DB79 respectively. The kinetics and mechanism of the interfacial charge transfer in these colloidal suspension has been elucidated with transient absorption spectroscopy. The reaction between the dye and trapped electrons is diffusion limited and occurs with rate constants of 1.1×108 M−1s−1 and 4.0×107 M−1s−1 for NBB and DB79 respectively.

Hydroxyl radical-mediated advanced oxidation processes for textile dyes: a comparison of the radiolytic and sonolytic degradation of the monoazo dye Acid Orange 7

Acid Orange 7, a textile azo dye, has been partially mineralized and degraded using γ-radiolysis and sonolysis. These two different advanced oxidation processes (AOP) are effective in producing ⋅OH radicals and cause complete destruction of the chromophore. The reaction mechanism of dye degradation has been probed by analyzing the reaction products with HPLC. In both cases, the final end products of degradation are oxalate and formate ions. The intermediates observed are all similar. While hydroxybenzenesulfonic acid is the major observed reaction intermediate in the oxidation processes, the pulse radiolysis studies indicate that the OH radical initiated pathway for attack occurs via the initial formation of 1,2-naphthaquinone and subsequent breakdown into oxalic acid.

Electrochromic and photoelectrochemical behavior of thin WO3 films prepared from quantized colloidal particles

The electrochromic, photochromic and photoelectrochemical properties of thin semiconductor WO3 films, prepared from quantum-size colloids, have been examined. These films have been found to exhibit reversible electrochromic and photochromic behavior (blue coloration). The trapped electrons in the particulate films are thought to be the major species responsible for the blue coloration. The photoelectrochemical measurements have shown a very low conversion efficiency of WO3 particulate films. This is most probably due to the efficient trapping of electrons by the lattice defects in the film.

Structural changes and catalytic activity of platinum nanoparticles supported on C60 and carbon nanotube films during the operation of direct methanol fuel cells

Comparison of the structure and activity of Pt nanoparticles anchored on two nanostructured carbon supports, C60 and carbon nanotubes (CNTs) provides insight into their electrocatalytic activity in direct methanol fuel cells. The local structure of platinum atoms during the initial stages of the catalytic oxidation of methanol was probed using x-ray absorption spectroscopy. A large fraction of the Pt atoms in the Pt–C60 nanocomposite continuously undergoes structural changes during the initial stages of methanol oxidation. The Pt-CNT system, however, proves to be more robust in maintaining its initial morphology and higher electrocatalytic activity. These observations reflect the importance of the carbon support in controlling the catalyst morphology and activity during methanol oxidation.

Environmental photochemistry: Electron transfer from excited humic acid to TiO2 colloids and semiconductor mediated reduction of oxazine dyes by humic acid

The ability of naturally occurring Suwanee River Humic acid to sensitize a large bandgap semiconductor such as colloidal TiO2 has been investigated by fluorescence emission. The charge injected from the humic acid sensitizer into the semiconductor was used to reduce a series of oxazine dyes viz:, N,N,N’,N’-tetraethyloxonine and Nile Blue A. The mechanism of such a sensitized reduction process was elucidated by laser flash photolysis methods. The quantum yield for such a reduction calculated from these transient absorption techniques was 0.005.

Reaction pathways and kinetic parameters of sonolytically induced oxidation of dimethyl methylphosphonate in air saturated aqueous solutions

The oxidation of dimethyl methylphosphonate (DMMP) was examined under ultrasonic conditions (640 kHz) in oxygen saturated aqueous solutions. Acetic acid, formic acid, methylphosphonic acid, phosphate, and oxalic acid have been identified as the major products produced during the sonolytic irradiation of DMMP. The initial rates of oxidation were determined as a function of initial DMMP concentration. The kinetic behavior of the system is consistent with the Langmuir-Hinshelwood model implying oxidative processes occur at or near the gas-liquid interface during cavitation. Mechanistic implications and conclusions are discussed based on the product distributions and kinetic parameters.