Stephen R. McEvoy

Role of Nanoparticles in Photocatalysis

The aim of this review paper is to give an overview of the development and implications of nanotechnology in photocatalysis. The topics covered include a detailed look at the unique properties of nanoparticles and their relation to photocatalytic properties. Current applications of and research into the use of nanoparticles as photocatalysts has also been reviewed. Also covered is the utilization of nanoparticles in doped, coupled, capped, sensitized and organic–inorganic nanocomposite semiconductor systems, with an effort to enhance photocatalytic and/or optical properties of commonly used semiconductor materials. The use of nanocrystalline thin films in electrochemically assisted photocatalytic processes has been included. Finally, the use of nanoparticles has made a significant contribution in providing definitive mechanistic information regarding the photocatalytic process.

Photocatalytic oxidation of organics in water using pure and silver-modified titanium dioxide particles

The simultaneous photocatalytic degradation of organic compounds and reduction of silver ions in titanium dioxide suspensions at pH 3.0-3.5 has been studied. The organic compounds of interest were sucrose and salicylic acid. The presence of silver ions in TiO 2 suspensions was found to enhance the photooxidation of high loadings (2001) μgC) of sucrose. However, for low sucrose loadings (100 μg C), pure TiO 2 particles performed as well as modified TiO 2 particles. An optimum silver ion loading of 2.0 at.% Ag + was observed for the mineralisation of 2000 μg C sucrose. At this silver ion loading, the mineralisation of 2000 μg C sucrose was enhanced by a factor of approximately 4.0 (based on 90% overall oxidation rates). In contrast, the addition of silver ions to TiO 2 suspensions did not have any significant effect on the photocatalytic mineralisation of salicylic acid to carbon dioxide, for both low and high loadings of salicylic acid in the suspension. It was also observed that pure TiO 2 particles performed as well as silver-modified TiO 2 particles for the degradation of 2000 μg C salicylic acid. The higher activity of silver ion-modified titanium dioxide suspensions for sucrose mineralisation is predominantly due to the presence of small silver particles on the titania surface, rather than due to the trapping of electrons during the reduction of silver ions. Approximately 50% of the initial mass of silver ions added to TiO 2 suspensions were reduced to metallic silver deposits in the presence of sucrose and salicylic acid mineralisation at the 2.0 at.% Ag + loading. Nanosize silver deposits on TiO 2 particles act as sites of electron accumulation where the reduction of adsorbed species such as oxygen occur. The enhanced reduction of oxygen through better electron-hole separation in Ag/TiO 2 particles compared to pure TiO 2 particles increases the rate of sucrose mineralisation. Therefore, it is proposed that the rate-limiting step in the sucrose photooxidation reaction is the transfer of electrons to dissolved oxygen molecules, whereas in the case of salicylic acid degradation and mineralisation, the rate-limiting step is the attack of salicylic acid molecules and its degradation intermediate products by holes and hydroxyl radicals. Hence silver deposits on TiO 2 particles are not beneficial for the photocatalytic degradation and mineralisation of salicylic acid but are advantageous for the mineralisation of sucrose.

Photocatalytic degradation of phenol in the presence of near-UV illuminated titanium dioxide

Abstract The photocatalytic oxidation of aqueous phenol solutions in the presence of near-UV illuminated titanium dioxide has been studied at concentrations of 1–100 μM. The titanium dioxide was used as a free suspension and attached to sand. Under identical illumination conditions the degradation rate with the free suspension was approximately 2.6 times faster than with the immobilized catalyst. The quantum yield for the disappearance of phenol from a 100 μM solution in 0.1% TiO2 suspension illuminated with ‘Blacklight Blue’ 360–370 nm maximum emission radiation was approximately 0.6%. Direct solar illumination gave degradation rates comparable with that achieved with a 100 W medium pressure mercury lamp. In less than 1 h the midday summer sun degraded 90% of a 100 μM phenol solution, of volume 0.5 l in a 145 mm diameter open dish. The disappearance of phenol and the formation of carbon dioxide with illumination time was quantitatively accounted for by numerical integration of simultaneous differential equations based on Langmuir-Hinshelwood kinetics. An unexpected observation emerging from this analysis was a dependence of the adsorption equilibrium constant on the reciprocal of the initial phenol concentration. This dependence occurred at each solution pH studied. A possible explanation for the observation is given in terms of phenol reformation reactions competing with irreversible phenol degrading reactions and free diffusion of OH radicals from the catalyst surface at low solute concentrations.

Occurrence and prevention of photodissolution at the phase junction of magnetite and titanium dioxide

A stable magnetic photocatalyst was prepared by coating a magnetic core with a layer of photoactive titanium dioxide. A direct deposition of titanium dioxide onto the surface of magnetic iron oxide particles proved ineffective in producing a stable magnetic photocatalyst, with high levels of photodissolution being observed with these samples. This observed photodissolution is believed to be due to the dissolution of the iron oxide phase, induced by the photoactive the titanium dioxide layer due to electronic interactions at the phase junction in these magnetic photocatalysts. The introduction of an intermediate passive SiO2 layer between the titanium dioxide phase and the iron oxide phase inhibited the direct electrical contact and hence prevented the photodissolution of the iron oxide phase. The magnetic photocatalyst is for use in slurry-type reactors from which the catalyst can be easily recovered by the application of an external magnetic field.

A comparison of 254 nm and 350 nm excitation of TiO2 in simple photocatalytic reactors

Photocatalytic reactors of simple annular design were manufactured from blacklight fluorescent tubes and germicidal tubes of identical dimensions and electrical power requirements. Salicylic acid and phenol selected as model water contaminants were analysed at various illumination times and TiO2 loading conditions to determine degradation rates. The shorter wavelength 254 nm radiation is considerably more effective in promoting degradation than radiation centred at 350 nm and the optimum rate occurred with a lower catalyst loading than that required at 350 nm. However, even with 350 nm radiation, in oxygenated solutions containing 94 ppm of phenol and greater, the phenol was degraded at a rate of approximately 2400 ppm l−1 kWh−1 and was mineralized at the equivalent of 820 ppm total organic carbon destroyed l−1 kWh−1. Phenol and Degussa P25 TiO2 catalyst provide a basis for future comparative photocatalytic reactor performance evaluations.

Studies on the Mineralization and Separation Efficiencies of a Magnetic Photocatalyst

A bench-scale system was used to demonstrate the application of a recently developed magnetic titanium dioxide photocatalyst for the degradation of organics in aqueous systems. The integrated water treatment system included a photoreactor and magnetic separator for the recovery of the magnetic photocatalyst particles. Methylene blue (MB) was used as a test pollutant. The mineralization and bleaching efficiencies of methylene blue using the magnetic photocatalyst were found to be comparable to those of Degussa P25. The magnetic photocatalyst also demonstrated high magnetic separation efficiency. The recycled photocatalyst particles were found to have the same separation efficiency as the fresh catalyst, although the mineralization and bleaching efficiencies were lower.

Formation of ammonium and nitrate ions from photocatalytic oxidation of ring nitrogenous compounds over titanium dioxide

Abstract The relative concentrations of ammonium and nitrate ions were measured during the photocatalytic oxidation of theophylline, proline, pyridine, and piperidine. It was found that the formation of inorganic nitrogeneous species occurred almost instantaneously upon illumination and at a significantly greater rate than carbon dioxide production, irrespective of whether the nitrogen heteroatom was contained in a saturated or an aromatic ring. The opening of the ring systems was postulated not to be the rate determining step in the formation of these species. The concentration versus time profiles for the formation of carbon dioxide, ammonium and nitrate for each compound indicated that a number of reaction intermediates, referred to here as organic and inorganic nitrogenous intermediates, have to be involved for the photocatalytic conversion of initial organic nitrogen to the inorganic species. A framework is thereby proposed for the formation pathway of these species. Indirect evidence was used to support the existence of such intermediates.

Analytical monitoring systems based on photocatalytic oxidation principles

Abstract Under photocatalytic oxidation, an organic compound in water can be quantitatively converted to ions corresponding to the elemental composition of the compound. Based on this principle, a range of analytical monitoring systems can be constructed for selective monitoring of organic pollutants containing these elements in aqueous systems.

Content of chloride, nitrate, and sulphate in snow samples collected from the Snowy Mountains region of Australia

Time-sequential and precipitation (depth profiling) sampling procedures were used to collect snow samples at nine sites (spread across 100 km) in the Snowy Mountains region to determine their chloride, nitrate, and sulphate content. Analysis of the data indicated that the concentrations of anions varied significantly during an individual storm event. Large variation was also found for snow samples collected between storms at a given site. The overall mean concentrations of chloride, nitrate and sulphate were 0.25, 0.13 and 0.09 mg kg−1, which compare favourably with those reported elsewhere and may be considered as typical values for chloride, nitrate, and sulphate in snow samples collected from a pristine area.