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Ind. Eng. Chem. Res. 1997, 36, 4712-4718
Photocatalytic Degradation of 2-Chlorophenol in TiO2 Aqueous Suspension: Modeling of Reaction Rate L. Rideh, A. Wehrer, D. Ronze, and A. Zoulalian* LERMAB Equipe: Ge´ nie des Proce´ de´ s, Universite´ Henri Poincare´ Nancy 1, BP 239, 54506 Vandoeuvre les Nancy Ce´ dex, France
The photoassisted oxidation of a dilute aqueous solution of 2-chlorophenol (2-CP) was investigated, over a heterogeneous catalyst of TiO2 (anatase), in an annular photocatalytic reactor. The effects of some physical and chemical parameters such as 2-CP concentration, catalyst concentration, dissolved oxygen concentration, pH, temperature, and absorbed light intensity were studied in order to optimize the process. The experiment, carried out in the presence of electron scavengers such as metallic ions, shows that the reaction rate is significantly higher than that obtained when oxygen is used alone. The results obtained in this study have led us, on the basis of experimentally determined adsorption, to propose a kinetic approach in which the rate-determining step is the reaction of OH• radicals, identified by a spin trapping technique (EPR), with adsorbed 2-CP. A kinetic model proposed was based on a Langmuir type adsorption involving a competition between solvent and substrate with a supplementary assumption that the further oxygen adsorption sites were different from those of 2-CP and the oxygen adsorption obeys the Freundlich isotherm. This model is able to justify the observed dependence of the pollutant disappearance rate on dissolved 2-CP concentration, oxygen partial pressure, and absorbed light intensity. in an illuminated suspension of TiO2 according to the following stoichiometry:
Introduction The photocatalytic degradation of toxic organic compounds in a semiconducting TiO2 surface has recently received an increased attention as an attractive method to destroy a variety of organic pollutants in waste water (1, 2). Particular attention has been directed toward aliphatic and aromatic recalcitrant pollutants dissolved in water. In recent literature these degradation methods are generally referred to as advanced oxidation processes (3). This oxidative process makes use of oxygen as a catalyst, a semiconductor irradiated by a light whose energy must be higher than or at least equal to their band-gap. Titanium dioxide, in anatase phase, has been utilised in most cases of the investigation performed so far, the reason of this choice being the activity shown by this compound together with its chemical inertia and its non-photocorrosivity. Photocatalytic reactions occur when charge separations are induced in a large band-gap semiconductor by excitation with ultra-band-gap radiations. The electron/ hole pairs so-formed either recombine in the bulk of the catalyst or migrate to the catalyst-solution interface where they may be trapped at defect sites. The trapped charge carriers react either by electron/hole recombination or by participation in a interface redox process with reduction occurring by the trapped holes. Photocatalytic oxidation of organic compounds is frequently supposed to occur via hydroxyl radicals formed by oxidation of surface bound hydroxide ions or water at the sites of a positive hole while the predominant reductive step in aerated systems is the transfer of the trapped electron to adsorbed molecular oxygen to create a superoxide radical anion (4). Hydroxyl radicals are very reactive neutral species; they react rapidly and nonselectively with organic compounds and are the common oxidizing agent. 2-CP was employed as an environmentally relevant model pollutant. It was shown that 2-CP was degraded S0888-5885(97)00100-0 CCC: $14.00
TiO2/hν
2C6H4OHCl + 13O2 98 2HCl + 12CO2 + 4H2O (1) The aim of the present work is to examine this photochemical reaction. The effects on the reaction rate of 2-CP concentration, catalyst concentration, dissolved oxygen concentration, pH, temperature, and absorbed light intensity are studied. Several kinetic models describing the mechanism of the photooxidation reaction have been proposed; in particular, the Langmuir-Hinshelwood law has been qualitatively proved in numerous cases of photocatalytic degradation (5). However, a detailed description of the relation between the phenomenon of adsorption and kinetics of photodegradation could be ambiguous. The kinetic model proposed describes satisfactorily the observed dependence of the pollutant disappearance rate on dissolved 2-CP concentration, on oxygen partial pressure, and on absorbed light intensity. Experimental Section Photoreactor. Experiments were realized in a differential conversion photoreactor. The experimental apparatus is constituted by a 2 L cylindrical Pyrex batch tank where the gas flowing through the suspension is introduced by a Teflon distributor. In addition to the agitation due to gas bubbles, the suspension was stirred mechanically and pumped through a recirculator external loop which includes the photoreactorsthis is an annular Pyrex tube inside which the irradiation source is placed. The illumination was carried out with a HPK 125 medium pressure mercury lamp; the power emitted by the radiation source was measured by uranyloxalate actinometry, obtaining a value of 1.98 × 10-5 Einstein‚L-1‚s-1. For most experiments, a filter solution of CuSO4 was circulated through the Pyrex reactor © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4713
Figure 1. Effect of initial concentration on degradation of 2-CP. Table 1. Operating Conditions of Experiments parameters
values
2-CP initial concentration TiO2 concentration optical thickness light intensity
10-4-(6 × 10-3) kmol m-3 0.012-0.2 kg m-3 (3.7 × 10-3)-(5.05 × 10-3) m (2.36 × 10-6)-(11.8 × 10-6) Einstein‚L-1‚s-1 (0.15 × 105)-105 Pa 15-65 °C 19.4 × 10-6 m3 s-1 14.2 × 10-6 m3 s-1 990 rpm
oxygen partial pressure temperature gas flow rate suspension flow rate stirring rate
jacket to eliminate radiation with λ < 320 nm and to provide cooling. The purpose of blocking radiation below 320 nm was to eliminate as much direct photolysis of 2-CP as possible. Reagents. Titanium dioxide (TiO2) was used, provided by Degussa P25. It is in the anatase form and has a BET surface area of 50 ( 15 m2‚g-1 and an average particle diameter of 30 nm. The standard grade 2-CP was purchased from Merck. All other chemicals were of reagent-grade quality and used without any further purification. The water used in the preparation of all the solutions was obtained from a Millipore Waters Milli-Q water purification system. Analysis. All samples were taken using a syringe and then immediately filtered using a 0.45 µm cellulose acetate membrane (type HA Millipore). The 2-CP concentration was measured in a Water HPLC (510) equipped with a Nova-Pak C18 column. UV detection was performed at 254 nm and the mobile phase was 50% acetonitrile/50% water at 0.75 mL‚min-1. The operative conditions of experiments are summarized in Table 1. Results and Discussion Effect of Initial Concentration of 2-CP. A study of the effect of initial 2-CP concentration allowed us to define the optimal concentration field for photocatalytic degradation practice. Figure 1 shows the decrease of normalized 2-CP concentration as a function of time for the experiments conducted at different initial concentrations of 2-CP. From the plot, it can be seen that the photodegradation conversion of 2-CP decreases with the increasing initial concentration of 2-CP. A plausible explanation of this behavior can be the following: as the initial concentration increases, more and more organic substances are adsorbed on the surface of TiO2,
Figure 2. Variation of reaction rate with catalyst concentration.
but the intensity of light and illumination time are constant; consequently, the OH• formed on the surface of TiO2 is constant, the relative number of OH• attacking 2-CP decreases, and thus the photodegradation efficiency decreases too (6). Effect of Catalyst Concentration. Many authors have investigated under different experimental conditions the evolution of the reaction rate as a function of catalyst concentration (6, 7). Our results are in good agreement with those found in literature. A series of experiments were carried out to find an optimum catalyst concentration by varying the concentration of TiO2 on the reaction from 12 × 10-3 to 0.3 kg‚m-3. Results illustrated in Figure 2 show that the initial photodegradation rate increases linearly with catalyst concentration up to 0.2 kg‚m-3. Above this amount, increased turbitity of the solution reduced the light transmission through the solution, while below this level, it is assumed that the catalyst surface and the absorption of light by TiO2 were limiting. Effect of Oxygen Partial Pressure. The limitation of the rate of photocatalytic degradation is attributed by most researchers to the recombination of photogenerated hole-electron pairs. Oxygen adsorbed on the surface of titanium dioxide prevents the recombination process by trapping electrons according to the following reaction:
O2 + e- f O2•-
(2)
Barbeni et al. (8) have reported that the partial pressure of oxygen is a crucial factor in the photocatalytic reaction. Experiments with a controlled amount of oxygen were monitored. Results reported in Figure 3 show that a 2-CP degradation rate increases nonlinearly with oxygen partial pressure. We can assume that the reaction rate is a function of the fraction of adsorption sites occupied by oxygen; hence oxygen adsorption becomes a limiting factor at very low dissolved oxygen concentrations. Effect of Light Intensity. The observed results indicate that saturation of the catalyst by the incident photons was not reached and that the rate of formation of the electron-hole pairs is directly proportional to the incident flux of efficient photons. Results reported in Figure 4 show that the initial reaction rate is first order with respect to the light intensity. This finding means that our experiments were carried out in the region of
4714 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997
Figure 3. Effect of oxygen partial pressure on degradation of 2-CP.
Figure 4. Variation of reaction rate with absorbed light intensity.
Figure 5. Effect of temperature on degradation of 2-CP.
lower intensity. Thus, higher intensities will produce higher rates without losing efficiency. In addition, this linearity is an indication that mass transfer is not limiting the reaction rate. Consequently, the conversion rate of adsorbed species represents the limiting step in the process. Effect of Temperature. Figure 5 shows the normalized concentration curves of 2-CP versus time for the experiments conducted at different temperatures. The mentioned positive influence can clearly be ob-
Figure 6. Arrhenius plot for degradation of 2-CP on irradiated TiO2.
Figure 7. Dependence of rate constant on pH.
served; the apparent energy of activation obtained from the Arrhenius plot (Figure 6) was 6.23 kJ‚mole-1. This value agrees well with that found for OH• radical reactions (9). On the basis of an energy of activation of 6.23 kJ‚mol-1, a temperature increase of 103 °C would be required to double the rate constant. This fact showed that the oxidation rate of 2-CP did not change significantly in the range of 15-65 °C. An expected increase of the reaction rate constant with increasing temperature is possibly compensated by a decrease of the adsorption equilibrium constant (10). Effect of pH. Results obtained from experiments with varying pH from 3 to 12 are illustrated in Figure 7. From the plot we can see that the rate constant is lower in the acid medium than in the basic medium and it is more or less constant in the neutral pH range. pH has a higher direct effect on the conversion rate; it can affect either the surface properties of the photocatalyst or the chemical form of the substrate. TiO2 has an amphoteric character with a point of zero charge around pH equal to 6 (5), and the substrate can undergo acid-base equilibria. Consequently, the adsorption of the substrate may be affected, strongly influencing the degradation rate (11). Effect of Dissolved Metallic Ions. The influence of metallic ions such as Ag+, Cr2O72-, Cu2+, and Fe2+ on the photocatalytic degradation process was studied. Figure 8 shows the effect of the presence of some metallic ions on the photocatalytic degradation of 2-CP.
Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4715
Figure 8. Effect of various metals on degradation of 2-CP.
They enhance the rate of degradation. This finding is in agreement with the works of Fujihira et al. (12) and Pavlik and Tantayanon (13). Both groups have reported the improvement of photocatalytic oxidation of organic compounds by inclusion of metallic ions in the test solutions. The enhancement was proposed to occur via the trapping of photogenerated electrons in TiO2 by the metallic ions, this preventing the recombination of electron-hole pairs and thus increasing the opportunity of formation of OH• on the surface of TiO2. The presence of metallic ions and oxygen, which act both as efficient electrons scavengers onto the semiconductor surface, modifies the photoactivity of TiO2. Results show that the 2-CP photodegradation rate increases remarkably when they are saturated by oxygen and metallic ions. The increase in photoactivity when oxygen is present together with metallic ions can be justified by taking into account that the presence of a photoreduced metal on TiO2 would improve charge separation and oxygen reduction efficiency, thus beneficially influencing the oxidation process too. Kinetic Analysis Kinetic modeling of the primary degradation steps is essential for any practical application of the process. In order to obtain a greater precision on the kinetic parameters of photodegradation, we have measured independently kinetic and isotherm adsorption of 2-CP and oxygen on TiO2. 2-CP adsorption isotherm can be represented by a Langmuir adsorption type (Figure 9). At 25 °C, the adsorption constant and maximal adsorbed concentration are respectively
Figure 9. Isotherm adsorption of 2-CP over TiO2.
Figure 10. Isotherm adsorption of oxygen over TiO2.
as we know. For this reason we do not have a comparison point in the literature. In this work, the Freundlich isotherm parameter (n) agrees well with that given, in general, for a Freundlich isotherm between 0 and 1. Some kinetic models describing photocatalytic oxidation on an illuminated semiconductor have been proposed (14-16). Our paper reports a simple kinetic model according to the partial sequence described by the following equations: K2-CP
2-CP + Sx 798 2-CPads
[2-CP]ads ) K2-CPC2-CP
K2-CP ) 3600 m3‚kmol-1
1 + K2-CPC2-CP
Qmax ) 0.107 kmol‚kg-1
KO
2
On the other hand, the oxygen adsorption is carried out using a dynamic method. Results show, in the studied range, that oxygen adsorption obeys the Freundlich isotherm (Figure 10). The Freundlich parameters are respectively
k′ ) 1.35 × 10
-5
mol
0.62
O2 + Sy 798 O2,ads k1
TiO2 + hν 98 e- + h+ k2
e- + h+ 98 heat -1
[O2]ads ) k′COn2,ads
(3)
(4)
r1 ) k1Iabs
(5)
r2 ) k2[e-][h+]
(6)
-0.38
‚g ‚L
n ) 0.38 A direct measurement of oxygen adsorption on TiO2 in an aqueous suspension has not been realized as far
If we suppose that the crystal electric charge is determined by a carrier charge
[e-] ) [h+]
r2 ) k2[h+]2
(7)
4716 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997
After they were transfered to catalyst surface, electrons and holes could react with adsorbed species to generate OH• radicals. The holes (h+) react specifically with adsorbed reducing species according to the following reactions: k3
h+ + H2Oads 98 OH•ads + H+
tant than other processes, then [OH•]ads can be written as
[OH•]ads )
k5
[h+]
k7[Cinert]
[OH•]ads ) kOH[h+]
(8)
according to eq 11:
k4
h+ + OH-ads 98 OH•ads
(9) [OH•]ads ) kOHkIIabs ) k′IIabs
r3 ) k3[h+][H2O]ads + k4[h+][OH-]ads (10)
Adsorbed oxygen (O2)ads reacts with the (2-CP)*ads radical according to the reaction
r3 ) k5[h+]
kr
O2,ads + (2-CP)*ads 98 product where k5 ) k3[H2O]ads + k4[OH-]ads. At stationary state, the rate of formation of holes is equal to the rate of their disappearance:
r1 ) r2 + r3 + 2
(15)
(16)
If step 16 is rate determining, then
r ) kr[O2]ads[2-CP]*ads
(17)
From the quasi-equilibrium 12, [2-CP]*ads can be written as
k1Iabs ) k2[h ] + k5[h ] +
Results obtained in this work show that the reaction rate is proportional to the intensity light; this is proof that the range of the photonic flux is low. Under these conditions we can suppose that the photogenerated electron-holes concentration is low:
[2-CP]*ads ) k6[OH•]ads[2-CP]ads [2-CP]*ads ) k6k′IIabs[2-CP]ads [2-CP]*ads ) k′′IIabs[2-CP]ads
(18)
Finally, the reaction rate can be rewritten as
k5[h+] . k2[h+]2
r ) krk′′IIabs[O2]ads[2-CP]ads and
r ) k′rIabs[O2]ads[2-CP]ads k1 [h ] ) Iabs ) kIIabs k5 +
(11)
OH• radicals react with adsorbed 2-CP according to the following reaction: k6
OH•ads + 2-CPads 798 (2-CP)*ads
(12)
Taking into account the isotherm nature of 2-CP and oxygen adsorption, we can state that oxygen and 2-CP adsorption occurs at different sites. This hypothesis was already used by several authors (17-19). The reaction rate becomes
r)
[2-CP]*ads represents an active form of adsorbed 2-CP. OH• radicals could be consumed on the TiO2 surface by the inactive adsorbed species (Cinert), k7
OH•ads + Cinert 98 inactive species
(13)
or they recombine: k8
2OH•ads 98 H2O2
(14)
At stationary state, we admit that [OH•]ads is constant:
d[OH•]ads ) k5[h+] - k6[OH•]ads[2-CP]ads dt k7[OH•]ads[Cinert] - k8[OH•]2ads ) 0 In addition, if we suppose that the process of OH• radical consummation and recombination is less impor-
(19)
k′rIabsk′CO2nK2-CPC2-CP 1 + K2-CPC2-CP r)
k′′rIabsCO2nC2-CP 1 + K2-CPC2-CP
(20)
where kr′′ is equal to k′rk′K2-CP. Al-Akabi and Serpone (1988) and Pelizzetti et al. (1993) have admitted that the rate must include competitive adsorption by solvent and pollutant (2, 5). Under these reasonable conditions, the reaction rate can be expressed as follows:
r)
k′′rIabsCO2nC2-CP 1 + KsCs + K2-CPC2-CP
(21)
where Ks and K2-CP are the equilibrium adsorption constants of solvent and 2-CP, respectively, C2-CP is the concentration of 2-CP, and Cs is the concentration of the solvent. Moreover, to the extent that Cs > C2-CP and Cs remains essentially constant, the part of surface covered by the solvent is approximately unchanged for
Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4717
Figure 11. Measured and predicted 2-CP reduced concentration versus time for diffrent 2-CP initial concentrations.
Figure 12. Measured and predicted 2-CP reduced concentration versus time for different oxygen partial pressures.
all the reactant concentrations used. Under these conditions, eq 21 can be further simplified to
r)
k′′rIabsCO2nC2-CP K + K2-CPC2-CP
(22)
where K is equal to 1 + KsCs Inserting into the Langmuir-Hinselwood equation the value of K2-CP and n determined from dark adsorption measurements, the parameters K and kr′′ were evaluated by fitting the results obtained at different initial concentrations of 2-CP, at different oxygen concentrations and at different absorbed light intensities. They were found to be kr′′ ) 1197.6 (L/mol)1.38 and K ) 2.25. With these optimized values the representation of our results is less satisfactory than that obtained if we do not fix the order n of oxygen concentration. The optimal value of n is 0.20 and the corresponding values of K and kr′′ are
K ) 2.25 kr′′ ) 329.60 (L/mol)1.2 We note however that the exponent given by kinetic law (n ) 0.20) is less than the one measured in the absence of chemical transformation (n ) 0.38); it is probable that oxygen adsorption sites are partially covered by intermediate products of the degradation reaction. With these last values, the obtained relationship (eq 22) is able to represent very satisfactorily the observed decays of C2-CP versus time and the observed dependence of the measured kinetic constant on initial 2-CP concentration (Figure 11), on the dissolved oxygen concentration (Figure 12), and on the absorbed light intensity (Figure 13). Results obtained with this model were compared with those deduced from the other hypothesis of oxygen adsorption (oxygen adsorption occurs on the same site as 2-CP or oxygen adsorption is Langmuir type). With this hypothesis, representation of our results is not as satisfactory as the one given by the detailed kinetic model. Conclusion This study has permitted us to show that organic compounds, which are refractory to classical methods,
Figure 13. Measured and predicted 2-CP reduced concentration versus time for different absorbed light intensity.
can be degraded using a photocatalytic process. The results obtained have allowed us to specify the influence of some parameters on the efficiency of the TiO2 process to degrade 2-CP. It was shown that above a maximum pollutant concentration, the reaction rate became independent of concentration. For a fixed geometry and consequently for a fixed optical thickness, our results show that there is an optimal catalyst concentration for which all the light is absorbed. It was proved that it is useful to operate at higher temperatures and in basic medium, to enhance the reaction rate. The kinetics of 2-CP photocatalytic degradation obeys the Langmuir-Hinshelwood model. 2-CP adsorption occurs in competition with that of the solvent one on the same sites. Therefore, dissolved oxygen adsorption occurs on different sites. According to our experimental results, solvent and 2-CP adsorption are of Langmuir type and the dissolved oxygen adsorption is probably a Freundlich type. Literature Cited (1) Minero, C.; Aliberti, C.; Pelizzetti, E.; Terzian, R.; Serpone, N. Kinetic studies in heterogeneous photocatalysis. 6. AM1 simulated sunlight photodegradation over titania in aqueous media: A first case of fluorinated aromatics and identification of intermediates. Langmuir 1991, 7, 928-936. (2) Al-Akabi, H.; Serpone, N. Kinetic studies in heterogeneous photocatalysis. 1. Photocatalytic degradation of chlorinated phenols in aerated aqueous solutions over TiO2 supported on a glass matrix. J. Phys. Chem. 1988, 92, 5726-5731.
4718 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 (3) Haarstrick, A.; Kut, O.; Heinzle, E. TiO2-assisted degradation of environmentally relevant organic compounds in wastewater using a novel fluidized bed photoreactor. Environ. Sci. Technol. 1996, 30, 817-824. (4) Schmelling, D. C.; Gray, K. A. Photocatalytic transformation and mineralization of 2,4,6-trinitrotoluene (TNT) in TiO2 slurries. Water Res. 1995, 29, 2651-2662. (5) Pelizzetti, E.; Minero, C.; Peramauro, E. Photocatalytic processes of organic water contaminants. In Chemical Reactor Technology for Environmentally Save Reactors and Products; de Lasa, H. I et al., Eds.; Kluwer: The Netherlands, 1993; pp 577607. (6) Mengyne, Z.; Shifn, C.; Yaown, T. Photocatalytic degradation of organophosphorus pesticides using thin films of TiO2. J. Chem. Tech. Biotechnol. 1995, 64, 339-344. (7) Kawaguchi, H. Dependence of photocatalytic reaction rate on photocatalyst concentration in aqueous suspensions. In Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-Akabi, H., Eds.; Elsevier: Amsterdam, The Netherlands, 1993; pp 665-673. (8) Pelizzetti, E. Concluding remarks on heterogeneous solar photocatalysis. Sol. Energy Mater. Sol. Cells 1995, 38, 453-457. (9) Matthews, R. W. Photooxidation of organic impurities in water using thin films of titanium dioxide. J. Phys. Chem. 1987, 91, 3328-3333. (10) Koster, T. P. M.; Assink, J. W.; Van der Veen, C. Photocatalytic oxidation of multi-component organochlorine mixtures in water. In Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-Akabi, H., Eds.; Elsevier: Amsterdam, The Netherlands, 1993; pp 613-618. (11) Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. Photolysis of chloroform and other organic molecules in aqueous TiO2 suspensions. Environ. Sci. Technol. 1991, 25, 494-500. (12) Fujihira, M.; Satoh, Y.; Osa, T. Heterogeneous photocatalytic oxidation of aromatic compounds on TiO2. Nature 1981, 293, 206.
(13) Pavlik, J. W.; Tantayanon, S. Photocatalytic oxidations of lactams and N-acylamines. J. Am. Chem. Soc. 1981, 103, 67556761. (14) Ollis, D.; Yung, H. C.; Budiman, L.; Li Lee, C,G. Heterogeneous photoassisted catalysis conversion of perchloroethylene, dichloroethane, chloroacetic acids and chlorobenzenes. J. Catal. 1984, 88, 89-96. (15) Ollis, D. Heterogeneous photocatalysis for water purification. Prospects and problems. In Homogeneous and heterogeneous photocatalysis; Pelizzetti, E., Serpone, N., Eds.; Reidel Publishing Co.: Boston, 1986, pp 651-656. (16) Minero, C. A rigorous kinetic approach to model primary oxidative steps of photocatalytic degradations. Sol. Energy Mater. Sol. Cells 1995, 38, 421-430. (17) Terzian, R.; Serpone, N.; Minero, C.; Pelizzetti, E. Photocatalyzed mineralization of cresols in aqueous media with irradiated titania. J. Catal. 1991, 128, 352-365. (18) Yung, H, C.; Li Lee, C. G.; Ollis, D. Heterogeneous photocatalysis: degradation of dilute solutions of dichloromethane (CH2Cl2), chloroform (CHCl2), and carbon tetrachloride (CCl4) with illuminated TiO2 photocatalyst. J. Catal. 1983, 82, 418-423. (19) Matthews, R. W. Kinetics of photocatalytic oxidation of organic solutes over titanium dioxide. J. Catal. 1988, 111, 264272.
Received for review February 3, 1997 Revised manuscript received June 27, 1997 Accepted July 11, 1997X IE970100M
X Abstract published in Advance ACS Abstracts, October 1, 1997.
