Hisao Hidaka

Exploiting the interparticle electron transfer process in the photocatalysed oxidation of phenol, 2-chlorophenol and pentachlorophenol : chemical evidence for electron and hole transfer between coupled semiconductors

The photocatalysed oxidation of phenol, 2-chlorophenol and pentachlorophenol was re-examined under conditions in which TiO2 anatase was sensitized by CdS in air-equilibrated aqueous media; this was to assess whether or not the interparticle electron transfer pathway, first discovered a decade ago (N. Serpone, E. Borgarello and M. Gratzel, J. Chem. Soc., Chem. Commun., (1984) 342) and subsequently applied to enhance reductive processes on titania, could also be applied to photo-oxidative processes. The results indicate that combinations such as CdS/TiO2 lead to an enhanced rate of disappearance of the initial substrate by a factor greater than two, consistent with the notion that (irradiated) CdS electrons are vectorially displaced onto the non-illuminated TiO2 particulates. Cadmium sulphide is a poorer photo-oxidation catalyst than titania. Other semiconductor materials have also been examined under a variety of conditions of pH and irradiation wavelength. The data also show that when both semiconductors in a coupled system are illuminated simultaneously and their valence and conduction bands are suitably disposed, both electron and hole transfer occur (as in the CdS/TiO2, ZnO/TiO2, TiO2/Fe2O3 and ZnO/Fe2O3 couples), which will influence the efficiency of photo-oxidation. N2O-saturated aqueous dispersions of TiO2 have no effect on the photo-oxidation of phenol, although it was expected that nitrous oxide would scavenge the photogenerated electrons in a manner similar to chemisorbed molecular oxygen, and enhance the efficiency. It is suggested that the role of oxygen in photo- oxidations may be more than just a simple electron scavenger.

Chemical oxidation and DNA damage catalysed by inorganic sunscreen ingredients

Titanium dioxide (TiO2) has been noted (US Federal Register, 43FR38206, 25 August 1978) to be a safe physical sunscreen because it reflects and scatters UVB and UVA in sunlight. However, TiO2 absorbs about 70% of incident UV, and in aqueous environments this leads to the generation of hydroxyl radicals which can initiate oxidations. Using chemical methods, we show that all sunscreen TiO2 samples tested catalyse the photo‐oxidation of a representative organic substrate (phenol). We also show that sunlight‐illuminated TiO2 catalyses DNA damage both in vitro and in human cells. These results may be relevant to the overall effects of sunscreens.

Photooxidative N-demethylation of methylene blue in aqueous TiO2 dispersions under UV irradiation

Methylene blue (MB) is a representative of a class of dyestuffs resistant to biodegradation. Its decomposition was examined in aqueous TiO2 dispersions under UV illumination to assess the influence of temperature, pH, concentration of dissolved oxygen (DOC), initial concentration of MB, and light intensity on the kinetics of decomposition. Hypsochromic effects (i.e. blue shifts of spectral bands) resulting from N-demethylation of the dimethylamino group in MB occurs concomitantly with oxidative degradation. The maximum quantity of MB adsorbed on TiO2, and the kinetics of degradation of MB and of total organic carbon (TOC) removal were also measured at constant pH 4. Photobleaching of MB solutions takes place at low DOCs and is caused by a reversible reductive process involving photogenerated electrons on TiO2. The rate of degradation of MB remains fairly constant regardless of whether the dispersion was purged with oxygen prior to irradiation or with air during the light irradiation period. The photocatalytic process depends on light intensity, but not on the total light energy absorbed. The photoreaction followed pseudo-first-order kinetics even at high MB concentrations (0.3 mM). The temperature dependence of the photodegradation kinetics was assessed (Ea=8.9kJ/mol), as well as the relative photonic efficiency, ξr, relative to phenol (0.48).

TiO2-assisted photodegradation of dye pollutants II. Adsorption and degradation kinetics of eosin in TiO2 dispersions under visible light irradiation

Abstract The TiO2-assisted photodegradation of anionic eosin has been examined in TiO2 aqueous dispersions under illumination by visible light. Eosin is easily decomposed photochemically by visible light in the presence of TiO2 particles. The degradation kinetics followed a Langmuir-Hinshelwood type equation. The rate was greater in acid media than in neutral and alkaline media, which correlates with the adsorption behavior of eosin on the TiO2 surface. Adsorption of eosin is a prerequisite for the TiO2-assisted photodegradation. The evolution of CO2 occurred concomitantly with the photodegradation of eosin. A plausible mechanism of degradation is discussed.

TiO2-assisted photodegradation of dyes: A study of two competitive primary processes in the degradation of RB in an aqueous TiO2 colloidal solution

The photocatalytic degradation of rhodamine B (N,N,N′,N′-tetraethylrhodamine, RB) had been studied in a colloidal TiO2 solution under both UV and visible light irradiation. Two different photodegradation processes of RB, photobleaching and N-deethylation, were examined in detail. The N-deethylation of RB was attributed to the oxidation by HOO· or OH· radicals generated from the dye-photosensitized and/or TiO2 catalyzed reactions; while the photobleaching of RB was found to occur via dye cationic radical. The aromatic rings of RB cannot be destroyed in the absence of oxygen under visible light irradiation. Various additives, Fe3+, EDTA, MV2+, different atmosphere (O2 and N2), time-resolved laser flash photolysis and spin trapping electron paramagnetic resonance (ESR) were used to determine the possible mechanisms involved.

Photo-Fenton degradation of a dye under visible light irradiation

Abstract The photodegradation of Malachite Green (MG) under visible light irradiation (λ>470 nm) in the presence of Fe3+/H2O2 or Fe2+/H2O2 was studied in comparison with the dark reaction. It was found that visible light irradiation can accelerate significantly the rate of MG degradation, comparing to that in the dark. This provides possibly another approach to the treatment of dye-polluted waters using visible light or sunlight. Evidence for enhancement of ⋅OH radical generation by visible light irradiation was obtained using DMPO spin-trapping EPR spectroscopy. A probable degradation mechanism of MG by Fe3+/H2O2 under visible irradiation is discussed, which involves an electron transfer from the excited dye by visible light into Fe3+ in the initial photo-Fenton degradation. Conversion of Fe3+ to Fe2+ was detected in the degradation process of MG. The results of the total organic carbon (TOC) reduction in the photodegradation of MG under visible light irradiation supported the proposed mechanism by comparing to those in the degradation of sodium dodecylbenzenesulfonate (DBS). The photodegradation of MG in a Fe3+/C2O42−/H2O2 system irradiated by UV or visible light and reacted in the dark provided another proof of the proposed mechanism.

Photooxidation mechanism of dye alizarin red in TiO2 dispersions under visible illumination: an experimental and theoretical examination

Visible light-induced photocatalytic oxidation of the dye alizarin red (AR) has been examined in TiO2 aqueous dispersions. The ESR spin-trapping technique was used to detect active oxygen radicals formed during in situ visible irradiation of AR/TiO2 dispersions. Evidence for the production of superoxide (O2⋅− or HOO⋅) (formed in the reduction of O2) and hydroxyl radicals (⋅OH) (formed by a multistep reduction) in the initial photoexcitation stage is presented. Meanwhile, the pathway of the photooxidation of AR is theoretically predicted on the basis of molecular orbital (MO) calculations by frontier electron densities and point charges on all the individual atoms of the dye. The relatively high negative point charges on the sulfonic oxygens lead to a strong adsorption of the dye onto the TiO2 particle surface through the sulfonate function. The position of the dye molecule attacked by the active oxygen species (e.g., O2⋅− or HOO⋅ radicals) and/or O2 is correlated with frontier electron densities, there is a perfect agreement between MO calculations and the results of experiments. A plausible mechanism of photooxidation under visible irradiation is discussed.

Highly selective deethylation of rhodamine B: Adsorption and photooxidation pathways of the dye on the TiO2/SiO2 composite photocatalyst

The photocatalytic degradation of Rhodamine B (RhB) with TiO2 and TiO2/SiO2 in the aqueous dispersion was investigated under both the visible light (λg 480 nm) and UV irradiation. The detailed photocatalytic oxidative process of RhB under these different conditions was revealed by measurement of the isoelectric points of the catalysts, UV-VIS spectra, HPLC and LC-MS. RhB adsorbs on the surface of TiO2/SiO2 particles by the positively-charged diethylamino group while, in the case of net TiO2, it adsorbs through the negatively-charged carboxyl group under the experimental conditions (pH ∼ 4.3). In the RhB-TiO2/SiO2 system, RhB firstly underwent a highly selective stepwise deethylation process before the destruction of the chromophore structure under visible light irradiation. The average yield of the every deethylation step was higher than 86%. It is confirmed that visible light-induced photocatalytic degradation of dye proceeds on the surface of catalysts rather than in the bulk solution and active oxygen species preferentially attack the molecular portion that connects directly to the surface of catalysts. This work provides a possibility for the modification of the surface characteristics of TiO2 to adsorb effectively the special colored organic molecules in selective mode for selective modification or deeply extent photooxidation.

Standardization protocol of process efficiencies and activation parameters in heterogeneous photocatalysis: relative photonic efficiencies ζr

The quantum yield Φ is crucial in homogeneous photochemistry; in heterogeneous photocatalysis this term remains elusive since the number of absorbed photons remains experimentally difficult to asses. A comprehensive method to standardize and compare process efficiencies in heterogeneous photocatalysis was proposed earlier by describing the relative photonic efficiency ζr (J. Photochem. Photobiol., A: Chem., 73 (1993) 11). The method of determining ζr is herein tested for the photocatalyzed degradation of phenol as the standard process and Degussa P25 TiO2 as the standard photocatalyst. The effects of light intensity, reactor geometry, pH, temperature, concentration of reactant and concentration of TiO2 on ζr were examined for the photodegradation of three substituted phenols: 2-methylphenol, 2,4-dimethylphenol and 4-chlorophenol. Relative photonic efficiencies of other phenolic substrates are reported for a given set of conditions. Efficiencies on varying the nature and the source of TiO2 for the photodegradation of phenol were also explored. These ζr are useful to assess process quantum yields once the actual quantum yield for a standard process (Φstand, for a given photocatalyst and a standard organic substrate) has been rigorously determined; thus Φ = ζrΦstand.

Photocatalyzed N-demethylation and degradation of methylene blue in titania dispersions exposed to concentrated sunlight

Abstract S imulated wastewaters that con tain methylene blue (MB) were bleached in a photocatalytic aqueous TiO 2 dispersion illuminated by concentrated sunlight using a parabolic round concentrator reactor (PRCR). The kinetic analysis was carried out well when the temporal concentration variation was a function of the concentrated light energy irradiated. The p hotocatalyzed N-demethylation of MB takes place concurrently with photocatalytic decomposition of MB by pseudo- first-order kinetics. The dependence of the photo- decomposition kinetics on the initial concentration of MB is consistent with the Langmuir– Hinshelwood model. Elimination of TOC (total organic carbon) also occurs by pseudo- first-order kinetics prior to full bleaching of the aqueous TiO 2 dispersion, after which the TOC level decreases only slightly. Also, compared with the open to air, the photodegradation of MB is not influenced by molecular oxygen bubbl ing continuously through the reactant suspensions during illumination . TiO 2 loadings and flow rates markedly a ffect the degradation o f MB. Under concentrated sunlight, t he relative photonic efficiency of MB photodegradation i s ζ rel =0.49 (relative to phenol) . T he efficiency ζ rel for the degradation of MB is independent of photoreactor geometry (cylindrical bottle reactor versus round-bot tomed flask), of light sources (solar light concentrator versus a Hg lamp) and of the operating mode used (flow versus batch operation).