The photocatalytic decomposition of phenol in oxygenated aqueous suspensions of lightly-reduced anatase TiO2, being the most satisfactory among the semiconductors investigated from the standpoint of the photocatalytic activity and stability, has been investigated at the optimum pH 3.5. The products at the initial stage of the reaction were hydroquinone, pyrocatechol, 1,2,4-benzenetriol, pyrogallol, and 2-hydroxy-1,4-benzoquinone. These intermediates underwent further photocatalytic oxidation via acids and/or aldehydes finally into CO2 and H2O. A reaction scheme involving hydroxyl radicals as real reactive species has been proposed. Although H2O2 was formed via O2\ewdot produced by electron trapping of adsorbed oxygen, its concentration remained constant at a low value during the reaction. About 0.7 mole of O2 was consumed for the consumption of one mole of phenol at the initial stage of the reaction. These results indicated that hydroxyl radicals were formed not only via holes but also via H2O2 from O2\ew...

Heterogeneous Photocatalytic Decomposition of Phenol over TiO2 Powder

Bioinorganic photochemistry: frontiers and mechanisms.

Heterogeneous photocatalysis has become a comprehensively studied area of research during the past three decades due to its practical interest in applications including water–air depollution, cancer therapy, sterilization, artificial photosynthesis (CO2 photoreduction), anti-fogging surfaces, heat transfer and heat dissipation, anticorrosion, lithography, photochromism, solar chemicals production and many others. The utilization of solar irradiation to supply energy or to initiate chemical reactions is already an established idea. Excited electron–hole pairs are generated upon light irradiation of a wide-band gap semiconductor which can be applied to solar cells to generate electricity or in chemical processes to create/degrade specific compounds. While the field of heterogeneous photocatalysis for pollutant abatement and mineralisation of contaminants has been extensively investigated, a new research avenue related to the selective valorisation of residues has recently emerged as a promising alternative to utilise solar light for the production of valuable chemicals and fuels. This tutorial review will focus on the potential and applications of solid photonanocatalysts for the selective transformation of biomass-derived substrates.

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Heterogeneous photocatalytic nanomaterials: prospects and challenges in selective transformations of biomass-derived compounds

The photocatalytic decomposition efficiency of gaseous 2-propanol was studied using a titanium dioxide thin film under very weak UV light; the incident UV light intensity was 36 nW−45 μW·cm-2. Under such low-intensity UV illumination, the value of the quantum yield (QY) increased gradually with decreasing number of absorbed photons and finally saturated (28%) for a number of absorbed photons less than 4 × 1011 quanta·cm-2·s-1 for an initial 2-propanol concentration of 1000 ppmv. Thus, purely light-limited conditions were reached. For lower initial concentrations, the QY values decreased, but the same maximum QY value as that for 1000 ppmv was also approached with decreasing light intensity. We discuss these results in terms of the normalized absorbed photon number (Inorm/s-1), a parameter that we have defined as the ratio of the number of absorbed photons ([photon]ab) to the number of adsorbed 2-propanol molecules ([M]ad). When all of the experimental QY values were plotted as a function of Inorm, all of ...

Kinetics of photocatalytic reactions under extremely low-intensity uv illumination on titanium dioxide thin films

The generation of hydroxyl (•OH) radicals plays a key role in the heterogeneous photocatalytic degradation of organic pollutants in aqueous suspensions of TiO2. The quantum yield of this process is thus an important parameter; however, it is not easy to measure in a particulate system arising from problems caused by light scattering from the particles. In this work, a reliable method for the determination of the quantum yield of hydroxyl radical production in heterogeneous systems has been developed, based on measurements of •OH radical generation rates and the photon flux absorbed by TiO2 suspensions. In this procedure, a modified integrating sphere method was used to determine the true fraction of light absorbed by TiO2 suspensions. A ferrioxalate chemical actinometer was used to measure the incident photon flux. As a check on the quantum yield method, good agreement with known literature values was obtained for quantum yield measurements of the photochemical generation of the p-benzosemiquinone (BQ•-) ...

Determination of the Quantum Yield for the Photochemical Generation of Hydroxyl Radicals in TiO2 Suspensions

The photocatalytic oxidation of liquid propan-2-ol to propanone has been investigated using suspensions of TiO2 irradiated with filtered u.v. radiation at temperatures in the range 277–313 K. The dependence of reaction rate on the square root of the intensity of the incident radiation, together with low quantum yields, reflect the dominance of photoelectron and photohole recombination within the TiO2. The activation energy for reaction on pure rutile using 366 nm radiation, 27 kJ mol–1, was independent of reaction conditions. For pure anatase, five doped rutiles and coated anatase and rutile pigments, the activation energy under standardized conditions ranged from 31 to 91 kJ mol–1. These energies are considered to reflect the solid-state properties of the catalysts rather than the chemistry of propan-2-ol oxidation, which proceeds by a radical mechanism.

Photocatalytic oxidation of liquid propan-2-ol by titanium dioxide

The photocatalytic oxidation of liquid ethanol, propan-1-ol, propan-2-ol and butan-2-ol, together with the binary mixtures ethanol + propan-2-ol and propan-1-ol + propan-2-ol, has been investigated using rutile suspensions irradiated with 366 nm u.v. light at temperatures in the range 275–313 K. Activities and activation energies for product formation from the individual alcohols were effectively identical. The single activation energy is considered to be the energy required to promote photoelectrons from traps into the rutile conduction band. With the binary mixtures both the activity and activation energy for total carbonyl compound formation were composition independent and approached those of the individual alcohols. Selectivities for specific products were a function of both composition and temperature, and are considered to reflect the availability of the parent alcohols within a saturated physisorbed layer on the rutile surface. The results are discussed in terms of an established radical mechanism.

Photocatalytic oxidation of liquid alcohols and binary alcohol mixtures by rutile

The photocatalytic oxidation of aqueous I–, usually as neutral 0.1 mol kg–1 KI, has been investigated using suspensions of pure anatase and rutile, five doped rutiles and two commercial anatase pigments. Reaction only proceeded in the presence of both O2 and u.v. radiation to yield I2, which reacted with excess I– to form I3–; there was no evidence for IO– or IO3– formation. The dependence of initial reaction rate on the square root of the intensity of the incident 366 nm radiation, together with low quantum yields, reflect the recombinative loss of photoelectrons and photoholes greatly exceeding their consumption in productive reaction steps. No activation energy for I2 formation was observed with any of the catalysts, and in this respect the photo-oxidation of I– differs from that of liquid alcohols on the identical catalysts. A mechanism is proposed in which I2 and OH– result from electron transfer between charged and neutral species adsorbed on the TiO2 surface, leading to an overall reaction: 4I–+ O2+ 2H2O +hvâ†� 2I2+ 4OH–.

Paul R. Harvey

Papers

Photocatalytic oxidation of liquid propan-2-ol by titanium dioxide

Photocatalytic oxidation of liquid alcohols and binary alcohol mixtures by rutile

Photocatalytic oxidation of iodide ions by titanium dioxide