David F. Ollis

Photocatalytic degradation of organic water contaminants: Mechanisms involving hydroxyl radical attack

Hydroxyl and other oxygen-containing radicals are known to be present during the degradation of organic water pollutants in illuminated TiO2 photocatalyst slurries. It is proposed that the hydroxyl radical, OH., is the primary oxidant in the photocatalytic system. Four possible mechanisms are suggested, all based on OH. attack of the organic reactant. The cases of reaction on the surface, in the fluid, and via a Rideal mechanism are shown to yield expressions similar to Langmuir-Hinshelwood (L-H) rate forms. Compared with traditional L-H constants, the derived kinetic parameters represent fundamentally different reactions and properties. A rate parameter independent of organic reactant is predicted by the model and substantiated by experimental degradation data. On the basis of these model results, the kinetic parameters for the photocatalytic degradation may be estimated from data on the photocatalysts physical properties, the knowledge of electron-hole recombination and trapping rates, and the values of second-order reaction rate constants for hydroxyl radicals.

The chemistry and catalysis of the water gas shift reaction: 1. The kinetics over supported metal catalysts

The water gas shift (WGS) reaction (CO + H2O → CO2 + H2) is catalyzed by many metals and metal oxides as well as recently reported homogeneous catalysts. In this present paper the kinetics of the WGS reaction as catalyzed by alumina-supported Group VIIB, VIII, and IB metals are examined. For several metals a strong effect of support on metal activity is observed. For example, the turnover number (rate per surface metal atom) of Pt supported on Al2O3 is an order of magnitude higher than the turnover number of Pt on SiO2. The turnover numbers (at 300 °C) of the various alumina-supported metals studied for WGS decrease in the order Cu, Re, Co, Ru, Ni, Pt, Os, Au, Fe, Pd, Rh, and Ir. For these metals the range of activity varies by more than three orders of magnitude. It is shown that a volcano-shaped correlation exists between the activities of these metals and their respective CO heats of adsorption. The partial pressure dependencies of the reactants on these metals are reported for the first time. Over most metals the CO order of reaction is near zero and the H2O order of reaction is near 12. A reaction sequence including formic acid as an intermediate is proposed in order to account for the apparent bifunctionality of the supported catalyst systems. This approach leads to a power rate law, r = kPCOXPH2OZ(1 − X)2, an expression shown to be consistent with the experimental parameters obtained in these kinetic studies.

Heterogeneous photocatalytic oxidation of gas-phase organics for air purification: Acetone, 1-butanol, butyraldehyde, formaldehyde, and m-xylene oxidation

Photocatalyzed degradations of trace levels of various oxygenates and an aromatic in air were carried out using near-UV-illuminated titanium dioxide (anatase) powder. The initial rates of degradation for acetone, 1-butanol, formaldehyde, and m-xylene were well described by Langmuir-Hinshelwood rate forms. No reaction intermediates were detected for acetone oxidation at conversions of 5–20%. Butyraldehyde was the main product of 1-butanol oxidation for conversions of 20–30%. The influence of 5% water (simulating partial humidification) in the feedstream varied strongly: water vapor inhibited acetone oxidation, but had no influence on the 1-butanol conversion rate. m-Xylene conversion was enhanced by trace water addition, but inhibited at higher water levels. Some catalyst deactivation was detected between 1-butanol runs; the activity could be easily recovered by illuminating the catalyst in fresh air. Formaldehyde was also successfully oxidized. These results, taken together with earlier literature citations for photocatalyzed total oxidation of methane, ethane, trichloroethylene (but see (27)), toluene, and a very recent report for oxidation of odor compounds, indicate a favorable technical potential for photocatalyzed treatment of air in order to degrade and remove all major classes of oxidizable air contaminants.

Contaminant degradation in water

This paper describes the process by which heterogeneous photocatalysis is used for the study of the degradation of chloromethanes, bromomethanes, chloroethanes, chloroethylenes and bromoethylenes, chlorobenzene, and chloroacetic acids in dilute aqueous solutions. Rate equations are presented for each of the halocarbons. In addition, solar applications and the potential use for water purification are discussed. 32 references, 6 figures, 1 table.

Photoassisted heterogeneous catalysis: The degradation of trichloroethylene in water

The complete mineralization of trichloroethylene (Cl2CCClH), in dilute aqueous solutions, to HCl and CO2 is demonstrated with heterogeneous photoassisted catalysis using illuminated titanium dioxide (TiO2). An intermediate, dichloroacetaldehyde, is identified, and a photoassisted reaction sequence is proposed. A simple Langmuirian rate equation satisfactorily represents both the disappearance of initial reactant, trichloroethylene, and intermediate, dichloroacetaldehyde, as well as the inhibitory influence of product HCl. The present paper and two related reports (A. L. Pruden and D. F. Ollis, Environ. Sci. Technol., in press; C.-Y. Hsiao, C.-L. Lee, and D. F. Ollis, J. Catal. 82, 418 (1983)) establishing mineralization of chloromethanes, indicate some potential for removal of the two most common chlorocarbon contaminants from water via heterogeneous photocatalysis.

Heterogeneous Photocatalysis for Purification, Decontamination and Deodorization of Air

A research review of gas–solid heterogeneous photocatalysis is presented, ranging from details of pioneering works, which dealt with basic phenomena like oxygen and water vapor adsorption, to recent applications to pollutant removal in contaminated atmospheres. Special interest is taken in describing the different reactor configurations studied so far in this emerging and promising field.

Heterogeneous photoassisted catalysis: Conversions of perchloroethylene, dichloroethane, chloroacetic acids, and chlorobenzenes

Abstract The chlorinated hydrocarbons perchloroethylene, dichloroethane, monochloroacetic and dichloroacetic acids, in dilute aqueous solutions, are completely mineralized to HCl and CO2 by photoassisted heterogeneous catalysis with an aqueous slurry of near-uv illuminated TiO2. Trichloroacetic acid was dehalogenated at a negligible rate. Rate parameters from the present and our earlier studies indicate that the relative chlorocarbon mineralization rates at the 1 to 50-ppb levels of relevance to water supply contamination appear to be dichloroacetaldehyde ⪢ trichloroethylene > perchloroethylene > dichloroacetic acid ∼ dichloromethane > trichloromethane ∼ (1,2)-dichloroethane ∼ monochloroacetic acid > tetrachloromethane > trichloroacetic acid. The heterogeneously photocatalyzed conversion of monochlorobenzene in dilute (70–400 ppm) aqueous solutions yields (1) ortho- and para-chlorophenol, which may subsequently be dechlorinated to yield aromatic oxygenates (ortho- and para-benzoquinone, and the corresponding hydroquinones), or (2) condensation products such as 4,4′-dichloro-1,1′-biphenyl. No evidence of ring opening was noted. Similar but more complex behavior was noted with a dichlorobenzene.

Mixed reactant photocatalysis: Intermediates and mutual rate inhibition

Near UV-illuminated slurries of titanium dioxide were used to study the photocatalyzed degradation kinetics of benzene and perchloroethylene (PCE) as water contaminants. In all runs, the concentration(s) of reactant(s) (initially 15–500 μM) as well as the evolution of CO2 final product were monitored. The single component initial rate data were well described by Langmuir-Hinshelwood rate forms, and integration of these rate equations provided simulations of the entire time course of each reaction. Perchloroethylene degraded completely without any evidence of kinetically significant chemical intermediates; however, adequate simulation of the benzene time course data required the incorporation in the model of at least two reaction intermediates. Subsequent GCMS analysis of a benzene reaction slurry revealed several intermediate species, with positive identification of phenol and quinone. Using the kinetic rate parameters obtained from the single component initial rate data and including competitive intermediate terms, the Langmuir-Hinshelwood rate form was able to simulate the results of several binary reactant experiments. The data showed that the effect of PCE concentration on benzene reaction rate was negligible, while benzene and its intermediates significantly inhibited PCE degradation. Simulations with this simple model for diminution of both reactants as well as CO2 product evolution were quite good for low initial reactant concentrations, but overestimated the competitive inhibitory effect of high (≈ 300 μM) initial PCE concentrations.

Heterogeneous photocatalysis: degradation of dilute solutions of dichloromethane (CH2Cl2), chloroform (CHCl3), and carbon tetrachloride (CCl4) with illuminated TiO2 photocatalyst

In dilute (10–200 ppm) aqueous solutions, the chloromethanes CCl4, CHCl3, and CH2Cl2 are completely mineralized to CO2 and HCl by the heterogeneous photocatalyst TiO2. The reaction rates in the absence of accumulated product are described by a simple Langmuir form, rate = k · K[chloromethane]1 + K[chloromethane] The relative rate constants are in the approximate ratio of 29 (trichloro-): 9 (dichloro): 1 (tetrachloromethane). Chloride ion inhibits the rate of degradation, as do product protons, as shown by experiments with added initial CsCl or HCl.

Photocatalyzed oxidation of alcohols and organochlorides in the presence of native TiO2 and metallized TiO2 suspensions. Part(II): Photocatalytic mechanisms

A detailed description of various mechanisms for the photocatalytic oxidation of alcohols and organochlorides in an aerated or a deaerated system is given. This description includes mechanisms of surface reactions and radical reactions based on our experimental data presented. A surface mechanism of direct oxidation of substrates in photocatalyst surfaces is proposed as a favorable pathway for photocatalytic oxidations of methanol and ethanol, also a possible pathway for chloroform, trichloroethylene (TCE) and dichloropropionic acid (DCP) in M/TiO2 aqueous slurries.