The interest in heterogeneous photocatalysis is intense and increasing, as shown by the number of publications on this theme which regularly appear in this journal, and the fact that over 2000 papers have been published on this topic since 1981. This article is an overview of the field of semiconductor photocatalysis : a brief examination of its roots, achievements and possible future. The semiconductor titanium dioxide (TiO 2 ) features predominantly in past and present work on semiconductor photocatalysis; as a result, in the most of the examples selected in this overview to illustrate various points the semiconductor is TiO 2 .

An overview of semiconductor photocatalysis

In this paper, visible light assisted degradation of various pollutants using different methods has been briefly reviewed. These methods have been broadly divided into two main categories. In the first category, the use of TiO2 semiconductor facilitating the photooxidative degradation of organic pollutants has been presented. This semiconductor has been treated in several ways. A major aim of these treatments is to maximize the range of wavelength in the visible light region for wastewater treatment. In the second category, various ways of degrading organic pollutants without the use of TiO2 semiconductor have been briefly outlined. The role of Fe(II)/Fe(III), etc., species in the sensitization process of various substrates helping it in the process to photocatalytic degradation reactions of organic pollutants has been highlighted. Also, the usage of semiconductors other than TiO2 has been critically analyzed.

Visible light induced photocatalytic degradation of organic pollutants

Oxidation into CO2 is a major solution to CO abatement in air depollution treatments. The development of catalytic converters led to an extraordinary high number of publications on metal catalysts during the last fifty years. Due to the increasing price of noble metals and to remarkable progresses in oxide syntheses, catalytic oxidation of carbon monoxide over oxide catalysts has recently gained in interest, even if some oxides are known to present remarkable activity since the beginning of the 20th century. In this Review, the kinetics and mechanism of CO oxidation on single and mixed oxides are examined, alongside the catalyst structures

Catalytic Oxidation of Carbon Monoxide over Transition Metal Oxides

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.

The photocatalytic oxidation of liquid propan-2-ol to acetone has been investigated using irradiated suspensions of pure rutile. It is shown that ultraviolet radiation, rutile and a source of oxygen are all necessary for measurable reaction to occur at 310 K. The parameters controlling the photoactivity of pure rutile, together with effects of adding hydrogen peroxide, water or dissolved salts to the reaction mixture, have been investigated. Supporting measurements with doped rutile and less pure anatase have also been made.The experimental results, which include values of the quantum yield, do not support a chain mechanism for acetone formation. Mechanisms are proposed, in which reactive species are formed by the surface trapping of both photoelectrons and photoholes.

Photocatalytic oxidation of propan-2-ol in the liquid phase by rutile

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

Infrared spectra of NO and NO + CO mixtures on Rh/Al2O3 are reported. At ambient temperatures NO alone was adsorbed non-dissociatively and gave no detectable reaction products. At T 413 K NO underwent partial decomposition to N2O(g) at surface sites which were rapidly poisoned as the reaction proceeded. The addition of NO to preadsorbed CO eliminated spectral bands characteristic of linear and bridged carbonyl species, but maxima due to gem-dicarbonyl species remained. The main products of the CO+NO reaction detectable by infrared spectroscopy were N2O(g), adsorbed isocyanate and CO2, either gaseous or chemisorbed. The extent of isocyanate formation depended on the relative amounts of NO and CO present, the gas-phase pressures of NO and CO and the order of addition of NO and CO to the Rh/Al2O3 catalyst. The order of addition did not affect the generation of N2O. The results are discussed in terms of reaction mechanism and types of catalytically active adsorption site. Dissociative adsorption of CO2 on Rh dispersed on alumina led to an adsorbed CO species giving an infrared band at 2015 cm–1.

Infrared study of the interactions between NO and CO on Rh/Al2O3 catalysts

The photocatalytic oxidation of liquid propan-2-ol to propanone has been investigated using semiconductor–zeolite composites consisting of either CdS or TiO2 in Y zeolite. CdS-based composites were prepared by cation exchange with Cd2+ followed by sulfidation with either Na2S or H2S, whereas TiO2-based composite was prepared by Ti(OEt)4 impregnation followed by hydrolysis and calcination. Rate measurements at 303 K were made to assess how the activity of CdS-based material depended on reaction conditions; differences from CdS supported on γ-Al2O3 are ascribed to smaller CdS particles and the higher adsorption potential of the zeolite matrix for O2 and H2O. However, rate measurements under standard conditions over the temperature range 283–308 K yielded the same activation energies as for CdS and TiO2 supported on γ-Al2O3, 53 ± 2 kJ mol–1 for CdS-based catalysts and 21 ± 1 kJ mol–1 for TiO2-based catalysts. It follows that the physico-chemical properties of the semiconductor control the activation energy, whereas the zeolite matrix is responsible for controlling semiconductor particle size during preparation and modifying the effective reaction conditions through adsorption.

Photocatalytic oxidation of propan-2-ol by semiconductor–zeolite composites

The photocatalytic dehydrogenation of liquid propan-2-ol by suspensions of platinum and other metals supported on anatase has been investigated by following propanone formation under an N2 atmosphere. Measurements were made over the range 278–303 K using filtered 366 nm u.v. radiation.With Pt/TiO2 prepared by H2 reduction there was an appreciable dark reaction, which was absent with catalysts prepared by photodeposition. Reaction on photodeposited catalysts was consistently associated with an activation energy of 20 kJ mol–1, although the activity fell in the sequence Pt/TiO2 > Pd/TiO2 > Rh/TiO2 > Au/TiO2= 0 for catalysts with a metal content of 0.5 wt%. The activation energy is identical to that for photoreaction on the anatase support in the presence of O2 and is believed to be associated with the transport of photoelectrons through the anatase to either metal particles or adsorbed O2.

Robert Rudham

Papers

Photocatalytic oxidation of propan-2-ol in the liquid phase by rutile

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

Infrared study of the interactions between NO and CO on Rh/Al2O3 catalysts

Photocatalytic oxidation of propan-2-ol by semiconductor–zeolite composites

Photocatalytic dehydrogenation of liquid propan-2-ol by platinized anatase and other catalysts