Ralph W. Matthews

Kinetics of photocatalytic oxidation of organic solutes over titanium dioxide

The kinetics of photooxidation to CO2 of 22 organic solutes over a UV illuminated film of Degussa P25 titanium dioxide have been studied over a 100-fold concentration range for each solute, generally from about 1 to 100 mg liter−1. The dependence of the photooxidation rate on concentration obeyed a simple two-coefficient Langmuir expression for each solute. The coefficients reflecting the degree of adsorption on the TiO2 and the limiting rate at high concentrations enable the prediction of photocatalytic rates in any TiO2-based photoreactor of a similar type once a reference rate has been determined for one of the solutes. A reaction mechanism is proposed for the oxidation of aromatic compounds involving peroxyhydroxycyclohexadienyl- and mucondialde-hyde-type compounds as important intermediates. The solutes studied were benzoic acid, salicylic acid, phenol, biphthalate, 2-chlorophenol, 4-chlorophenol, monochlorobenzene, nitrobenzene, methanol, ethanol, n-propanol, 2-propanol, acetone, ethyl acetate, acetic acid, formic acid, sucrose, 2-naphthol, umbelliferone, chloroform, trichloroethylene, and dichloroethane.

Photo-oxidation of organic material in aqueous suspensions of titanium dioxide

Twenty-one organic compounds, known to be possible contaminants of water, have been treated with u.v.-illuminated titanium dioxide powder in aqueous suspension. In each case the organic material was converted to carbon dioxide. The rate of carbon dioxide formation did not vary greatly from one compound to another; even nitrobenzene was converted at 40% of the maximum rate observed with 2,4 dichlorophenol. Chlorobenzene, benzoic acid and 4-chlorophenol were selected for more detailed studies of the effect of solute concentration, pH and products other than CO2. Results were compared using illumination by a 100 W medium pressure mercury lamp, an 0.25 m2 parabolic trough solar collector and by solar illumination of a flat bed. The CO2 production rate from 300 ml of 10−3 M 4-chlorophenol in contact with 0.15 g TiO2 solar illuminated with the 0.25 m2 collector was 20 μM min−1 which corresponds to the oxidizing radicals generated in water by a 60Co gamma ray dose rate of not less than 8.3 krad min−1. The possible use of solar-illuminated TiO2 for the disinfection of water is suggested.

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.

Photooxidative degradation of coloured organics in water using supported catalysts. TiO2 on sand

Abstract The photocatalytic oxidation of methylene blue, Rhodamine B, methyl orange and salicylic acid have been studied in the presence of immobilized titanium dioxide coated sand in a flat bed configuration illuminated with a 100 W medium pressure mercury lamp and sunlight. The disappearance of solute in each case approximately obeys first order kinetics with the apparent first order decay constant increasing with decreasing solute concentration. Flow rate data, total organic carbon analyses and the effect of hydrogen peroxide are also given. The marked dependence of degradation rates on flow rates indicated a substantial mass transfer limitation but the rates were much greater in the photocatalytic system than for the natural u.v. degradation rates. The geometric arrangement lends itself to scaling up of the process in continuous operation without major engineering cost. Times for removal of 50% of solute for 151. of each solution of initial concentration 10 μM at a flow rate of 13.81/min per m 2 of sunlight at the equinox were 12–22 min. The corresponding times for 50% mineralization were 21–46 min.

Purification of water with near—u.v. illuminated suspensions of titanium dioxide

Abstract Benzoic acid, salicyclic acid, phenol, 2-chlorophenol, 3-chlorophenol, 4-chlorophenol, nitrobenzene, methanol, ethanol, acetic acid and formic acid in aerated, aqueous suspensions of TiO 2 were illuminated with near u.v. light. Each solute was present over a range of concentration; generally 1–50 mg 1 −1 . The rate of oxidation to carbon dioxide was measured at each concentration. The dependence of the rate on concentration obeyed simple Langmuir-Hinshelwood (L-H) kinetics for each solute. Empirical constants have been determined enabling the prediction of mineralization rates at any solute concentration in 0.1% (w/w) TiO 2 suspensions. The integrated L-H rate expression gives reasonable agreement with observed time-dependent CO 2 formation curves. A comparison was made between CO 2 formation rate using 4-chlorophenol illuminated with artificial and natural u.v. light. The results indicate that 90% mineralization of all the solutes except formic acid initially present as 50 mg solute 1 −1 suspension occurs within 2–3 h with 1 m 2 of sunlight. Formic acid was mineralized at approximately twice the rate of the other solutes.

An adsorption water purifier with in situ photocatalytic regeneration

Abstract Simple devices are described for the purification of water combining adsorption of organic impurities with in situ photocatalytic oxidation regeneration of the adsorbent/photocatalyst. The adsorbent was silica gel and the photocatalyst was Degussa P25 TiO 2 in one purifier and colloidal TiO 2 on silica gel in the other. The adsorbent/photocatalysts were arranged in the annuli of hollow cylinders, with tubular fluorescent lamps occupying the central cavity. Apart from the central illumination the purifiers were analogous to chromatographic columns. The arrangement permitted more uniform distribution of light to the photocatalyst, better access of water to the photoactivated catalyst, improved contact time between impurities in the water and the photoactivated surface, and continuous regeneration of the adsorbing surface, by destructive photocatalytic oxidation. A single pass at 20 ml min −1 under 20-W illumination reduced the phenol concentration from 940 μg liter −1 , in the influent, to 940 ng liter −1 in the effluent stream, i.e., a 99.9% removal of phenol.

Photocatalytic oxidation for total organic carbon analysis

Abstract A new method and apparatus for the determination of organic carbon in water are described. Photocatalytic oxidation of organic matter by near-UV illuminated titanium dioxide is used to convert organic carbon to carbon dioxide. Atmospheric oxygen is the oxidant. The light source is a 20-W black-light fluorescent tube. The carbon dioxide equilibrates between the reaction loop and a conductivity detection loop and the recirculated air forms the interface between the two loops. The method is suitable for analysis of water containing 0.1–30 μg ml−1 organic carbon and sample volumes of 1–40 ml. Higher concentrations can be determined after appropriate dilution. Times for 99% oxidation of organic solutes to carbon dioxide are of the order of 10 min or less. The main advantages of the apparatus are low cost and ease of operation.

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.

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.

Catalytic oxidation of formic acid in aqueous suspensions of platinized titanium dioxide

Platinized titanium dioxide particles produce hydrogen when illuminated with bandgap light in the presence of aqueous solutions of many organic compounds. Oxygen inhibits this reaction and, therefore, usually precludes the use of aerated suspensions. Solutions of formic acid, however, produce hydrogen unaffected by oxygen. This reaction has been studied before, but never in the dark. The catalytic oxidation of formic acid in aqueous suspensions of platinized titanium dioxide in the dark has been investigated, and the findings are presented here.