TiO2-Photocatalyzed Oxidative Degradation of CH3CN, CH3OH

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TiO2-Photocatalyzed Oxidative Degradation of CH3CN, CH3OH, C2HCl3, and CH2Cl2 Supplied as Vapors and in Aqueous Solution under Similar Conditions NORMAN N. LICHTIN* AND MUTHUSAMI AVUDAITHAI Department of Chemistry, Boston University, 590 Commonwealth Avenue, Boston, Massachusetts 02215-2507

Acetonitrile (CH3CN), methanol (CH3OH), trichloroethylene (C2HCl3, TCE), and methylene chloride (CH2Cl2) were degraded in stirred batch reactors in the presence of O2 at room temperature over films of pristine P-25 TiO2 under irradiation centered at 360 nm. Molarities of organics, activities of O2, irradiation flux, deployment of catalyst, and reactor configuration were matched for vaporized and aqueous organics. The dependence of initial rates and the photoefficiencies of removal on concentrations of reactants and photon flux were measured. Identities and rates of formation and decay of some intermediates were determined. Langmuir coefficients were determined for dark adsorption of the organic vapors on films of P-25 in the presence of dry and water-saturated air. Surface concentrations in equilibrium with aqueous solutions in the dark were calculated with the aid of Henry’s law constants from the Langmuir coefficients of the water-saturated vapors. Initial rates of removal of all four compounds from the vapor phase were much higher than from aqueous solution. Initial photoefficiencies of g1 molecule removed per incident photon were observed with vaporized CH3OH and TCE in dry air. Water vapor strongly inhibited the conversion of CH3OH and CH2Cl2. CH3CN, TCE, and CH2Cl2 vapors appeared to react with both electrons and holes, and homolytic dissociation of the C-C and C-Cl bonds of CH3CN and CH2Cl2 appeared to occur.

Introduction Much of the rapidly growing literature on heterogeneous photocatalytic oxidative degradation of organic compounds in aqueous media is summarized in several recently published chapters (1-6) and is cited in a recent bibliography (7). A major motivation of this work is its potential * Corresponding author telephone: (617) 353-2493; fax: (617) 3536466; e-mail address: [email protected].

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for use in purification of water. A smaller but steadily growing recent literature is devoted to heterogeneous photocatalytic autoxidation of vaporized organic compounds (6-27). For the past half-dozen years, a major motivation of the latter work has been its potential for use in purification of air and, together with air stripping, water (28). The literature suggests major differences between the chemistries of heterogeneous photocatalytic reactions with O2 of organic compounds supplied in water and in the vapor phase. Among the observed differences are the much larger range of reactivities found with vaporized organic substrates and the observation of photoefficiencies greater than one molecule of substrate converted per incident photon with vaporized but not with aqueous organics (see, e.g., refs 23 and 29). The higher rates of molecular diffusion in the vapor phase can facilitate mass transport to the catalyst and chain reactions (23, 24, 26). However, the use of different reactor configurations and modes of deployment of catalyst for the vapor and aqueous reactions has complicated the explanation of observed differences. Except for a recent cursory report (29), two symposium papers (30, 31), and an abstract of a symposium presentation (32), there do not appear to be reports in the literature comparing the photocatalyzed reactions with O2 of vaporized and aqueous organic compounds under similar conditions. Results of such a comparison of the reactions of CH3CN, CH3OH, C2HCl3 (TCE), and CH2Cl2 are reported here.

Experimental Section Reagents. TiO2 was Degussa P-25 (finely divided, mostly anatase, partly amorphous, some rutile, surface area 50 m2 g-1). High purity air, O2, N2, mixtures of O2 and N2, and Ar were from WESCO (grade 2.0) and contained no more than 10 ppm of water vapor. Vapor-phase reaction mixtures made up with these dry gasses, designated in this paper by dry, were found by GC to typically contain 0.2-0.3% of water vapor, presumably introduced with organic components. TCE was Baker Analyzed or Fisher ACS Certified. CH3CN, CH3OH, and CH2Cl2 were Fisher ACS Certified. Laboratory deionized water was used for preparation of solutions, suspensions, and water vapor. Deployment of Catalyst. TiO2 was deployed as a thin [approximately 10 µm thick (23)] film of particles deposited on the interior surface of the reactor. The catalyst coating was prepared by placing 50 mL of a 20 w/v % aqueous slurry in the reaction vessel and mechanically rotating the vessel horizontally in air for 5 min. The reaction vessel was then drained and dried by heating in an oven under air at 150-200 °C for at least 1 h. Used catalyst was removed from the reactor, and a fresh coating of pristine catalyst was applied before each experiment. Reactor. Batch reactors adapted from the Ace Glass 7840 closed reactor were used for both vapor- and aqueousphase reactions. The reactor comprised a 450-mL magnetically stirred cylindrical Pyrex vessel equipped with a port capped with a silicone rubber septum for syringe transfers, a second port for circulating vapors, and an axially aligned 6-W (electrical) water-cooled Vilbur-Lourmat T-6L 360-nm fluorescent lamp. The reaction vessel was con-

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to the vapor phase over aqueous TCE in the closed reactor by continuously circulating the vapor at 50 mL/min through a 1-L flask containing air. Because of the relatively low consumption of O2 by CH2Cl2, it was not necessary to add O2 to the closed reactor during the course of its reactions. Detachment of catalyst from the reactor wall was negligible in both vapor- and liquid-phase experiments. For aqueous TCE, both the liquid phase and the vapor phase over the liquid were sampled. Lamp Output. The output of the lamp was determined periodically by ferrioxalate actinometry (33). Its full output, initially 1.5 × 10-6 Einstein s-1 or 3.2 × 10-9 Einstein s-1 cm-2 of irradiated catalyst, declined 6% in 140 h of use. The time required for radiation output to become steady after turning on the lamp was negligible. Outputs were reduced by covering the lamp with sleeves of copper screening. Analyses of Reactants and Products. The four organic reactants as well as the products NO2, HNO3, CO2, and C2N2 were determined, and the products C2N2, CHCl3, and CH2O were identified by gas chromatography on a PerkinElmer Sigma 300 instrument using a Supelco 80.100 Porapak Q column and thermal conductivity detection (TCD). NO2, Cl2, and HNO3 were also identified with Matheson-Kitagawa gas analyzing tubes, and HNO3 was additionally identified colorimetrically as the brucine derivative (34). COD (chemical oxygen demand) was determined by chromic acid oxidation (35). Mass spectra of minor or intermediate products were measured with a Finnegan MAT 90 mass spectrometer coupled to a Varian 3400 gas chromatograph. The concentration of chloride ion was determined by ionselective potentiometry. Dark Adsorption Isotherms on TiO2. CH2Cl2 and Water-Saturated TCE. Dark adsorption of CH2Cl2 from dry

TABLE 1

Langmuir Coefficients for Dark Adsorption onto P-25 TiO2 from Air at 25 °C compd

atmosphere

a (L g-1)

b (L µmol-1)

CH3CN CH3CN CH3OH CH3OH TCE

Adsorption on Thin Film of P-25 dry air 2.4 ( 0.3 H2O-satd air 0.74 ( 0.004 dry air 8.8 ( 0.6 H2O-satd air 1.3 ( 0.015 dry air 0.33 ( 0.04

0.0015 0.00015 0.0036 0.0022 0.0079

CH2Cl2 CH2Cl2 TCE TCE

Adsorption on P-25 Powder dry air 0.31 ( 0.04 H2O-satd air 0.14 ( 0.02 5.5 dry air (23) 0.28 H2O-satd air

0.0033 0.0037 0.37 0.00022

TABLE 2

Henry’s Law Constants in Water, H, at 25 °C (36) compd

H (10-5 atm L mmol-1)

compd

H (10-5 atm L mmol-1)

CH3CN CH3OH

2.007 0.6945

TCE CH2Cl2

1167.0 247.6

nected to a tank of air, O2, or O2/N2 for system purging. Known amounts of organic vapors or aqueous solutions were introduced through the transfer port. Stirring was continued in the dark until adsorption, as measured by GC, reached equilibrium, and the reaction was then started by turning on the lamp. For reactions of aqueous solutions of CH3OH or CH3CN, 35 mL/min O2 was bubbled continuously through the solution. GC analysis of the solutions during dark runs showed no detectable change in the concentrations of these two compounds. O2 was supplied TABLE 3

Initial Kinetic Constants for Removal of CH3CN, CH3OH, TCE, and CH2Cl2 in Reactions with O2; Apparent Orders in Organic Reactant, Oorg,i,rem, Rate Constants, kr,i,rem, and photoefficiencies, Ep,i,rem, at Initial Equilibrated Concentrations of Reactants (Org.Cpd.)i; Irradiation by 3.2 × 10-9 Einstein s-1 cm-2 at 360 nm (Org.Cpd.)ih compd

phase

CH3CN CH3CN CH3CN CH3CN CH3CN CH3OH CH3OH CH3OH CH3OH CH3OH CH3OH CH3OH TCE TCE TCE TCE CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2

solna-c

aq aq solna-c dry vapd,e dry vapd,e dry vapd,e aq solnb,c,f aq solnb,c,f aq solnb,c,f dry vapd,e dry vapd,e dry vapd,e dry vapd,e aq solne-g aq solne-g aq solne-g dry vapc,d aq solne,g aq solne,g aq solne,g aq solne,g 3% H2O, vapd,e dry vapd,e dry vapd,e dry vapd,e dry vapd,e

Oorg,i,rem 0.55 0.55 0.90 0.90 0.90 0.45 0.45 0.45 0.9 0.9 0.9 0.9 1 1 1 0 0.75 0.75 0.75 0.75 1 1 1 1 0.7

kr,i,rem µM0.45

h-1

0.56 0.56 µM0.45 h-1 0.069 µM0.1 min-1 0.069 µM0.1 min-1 0.069 µM0.1 min-1 14.1 µM0.55 h-1 14.1 µM0.55 h-1 14.1 µM0.55 h-1 0.16 µM0.1 min-1 0.16 µM0.1 min-1 0.16 µM0.1 min-1 0.16 µM0.1 min-1 0.38 h-1 0.38 h-1 0.38 h-1 15 mM h-1 0.36 µM0.25 h-1 0.36 µM0.25 h-1 0.36 µM0.25 h-1 0.36 µM0.25 h-1 0.72 h-1 2.7 h-1 2.7 h-1 2.7 h-1 13.8 µM0.3 h-1

Ep,i,rem molec/photon

mM

mol %

0.005 0.009 0.065 0.12 0.8 0.036 0.064 0.11 0.13 0.35 0.60 1.1 0.0049 0.013 0.075 1.6 0.0008 0.0018 0.0083 0.016 0.007 0.022 0.044 0.14 0.34

0.95 3.8 0.22 0.44 3.8 1.2 4.9 24.7 0.23 0.44 1.1 2.25 0.38 1.5 7.6 0.045-2.0 0.044 0.11 1.1 11.7 0.11 0.063 0.11 0.38 2.2

0.0017 0.0068 0.5 1.0 8.5 0.0022 0.0088 0.045 0.50 1.0 2.5 5.0 0.00068 0.0027 0.014 0.11-5.0 0.00008 0.0002 0.002 0.02 0.25 0.14 0.25 0.86 5.0

a Ref 31. b Loss of organic reactant determined by COD. c 1 atm O . d Loss of organic reactant determined by GC. e 1 atm air. f Ref 30. 2 organic reactant determined by analysis for Cl-. h Supplied to reactor.

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and water-saturated air and of TCE from water-saturated air at 23-25 °C on catalyst powder, which had been pretreated by purging over night at 250-300 °C with 100 mL/min flowing He and then cooled to room temperature in the He atmosphere, was measured by a previously described (23) differential gas chromatographic method. Adsorption of CH2Cl2 from dry or water-saturated air and of TCE from water-saturated air onto thin films of TiO2 was too weak to be measured by the procedure described below. CH3CN, CH3OH, and Dry TCE. Isotherms were measured for the dark adsorption of these compounds on thin films of P-25 TiO2 on 75 × 25 × 1 mm microscope slides (Fisher’s Finest). The films were prepared by immersing the slides in 20 w/v % aqueous slurries of P-25 and draining and drying overnight at room temperature with flowing air. Slides coated with weighed amounts of catalyst were transferred to a heavy-walled, three-neck 1150-mL flask that was then purged with 150 mL/min dry air for 30 min before introducing a VOC. The rest of the procedure paralelled that employed with powdered VOCs (23). Supplied concentration ranges were 0.010-0.20 mol % for CH3CN, 0.020-1.0 mol % for CH3OH, and 0.010-1.0 mol % for TCE. FTIR Spectrometry. See ref 23 for apparatus and procedures. Dark Adsorption Isotherms and Henry’s Law Constants. Adsorption data were analyzed by means of the Langmuir expression:

Csurf ) aC0/(1 + bC0)

(1)

where Csurf is the surface concentration of organic reactant in µmol/g, C0 is its concentration in the vapor phase in µmol/L, and a and b are constants. Values of a and b are presented in Table 1, and Henry’s law constants are presented in Table 2. Kinetics and Photoefficiencies. Apparent initial kinetic orders of removal of organic reactants in concentration of organic reactants were evaluated from plots of log(initial rate of removal of organic) vs log(initial concentration of organic). Initial rates were based on consumption of 30% of concentrations present after equilibration of dark adsorption. Apparent initial kinetic orders in concentration of organic reactant, Oorg,i,rem, initial rate constants, kr,i,rem, and values of initial photoefficiencies of removal of organic reactant, Ep,i,rem, defined by eq 2, are presented in Table 3 for reactions of TCE vapor and aqueous solutions of CH3CN, CH3OH, and TCE with 1 atm of O2 and for vapors of CH3CN, CH3OH, and CH2Cl2 as well as aqueous CH2Cl2 with 1 atm of air.

Ep,rem ) no. of molecules of organic reactant removed (2) no. of photons incident on catalyst The dependence of initial rates of removal of the four vaporized organic reactants by reaction with air on concentration of water vapor is plotted in Figure 1, along with initial rates in aqueous solution under otherwise identical conditions, all based on GC analyses. The dependence of the initial kinetic order in O2 of the reaction of vaporized and dissolved CH2Cl2 on concentration of CH2Cl2 along with the initial kinetic orders in O2 of the reactions of the other three vaporized organics at 1 mol % concentrations are presented in Figure 2. Data on the dependence

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FIGURE 1. Photodecomposition of organic vapors at different concentrations of water vapor and in aqueous solution. Initial rates in aqueous solution in µM/min: CH3CN, 0.2; CH3OH, 2.7; TCE, 0.7; CH2Cl2, 0.24. Reaction conditions: 450 mL batch reactor; 6-W 360nm lamp; fresh P-25 film; air atmosphere, 1% CH3CN, CH3OH, and TCE, 0.25% CH2Cl2.

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 6, 1996

FIGURE 2. Initial order in O2 in the removal of the four compounds from dry vapor and of CH2Cl2 from aqueous solution, Oi,O2. Reaction conditions: 450 mL batch reactor; 6-W 360-nm lamp; fresh P-25 film. (a) 1% TCE vapor, (b) 5% CH2Cl2 vapor, (c) 1% CH3CN vapor, (d) 1% CH3OH vapor, (e) 0.25% CH2Cl2 vapor, (f) 0.11 mM aqueous CH2Cl2. Underlined points were not used in calculating reaction order.

of the initial rates of reaction on photon flux for all four vaporized organics and for aqueous CH2Cl2 are presented in Figure 3. The data of Figures 2 and 3 on consumption of vaporized organics is based on GC analyses while data for aqueous CH2Cl2 is based on chloride ion analyses. Final Products and Intermediates. From CH3CN. Equation 3 represents a plausible stoichiometry of complete oxidation.

CH3CN + 4O2 f 2CO2 + H2O + HNO3

(3)

We have previously (30) compared apparent removal of CH3CN from aqueous solution by reaction with 1 atm of O2 as measured by COD calibrated vs CH3CN with its actual removal under the same conditions as measured by GC. In the solution reaction, conversion as measured by COD was only moderately slower than the initial very slow removal of CH3CN as measured by COD; e.g., when GC analysis showed 60-85% of dissolved CH3CN had been removed,

FIGURE 3. Initial order in photon flux in the removal of dry vaporized organics and aqueous CH2Cl2, Oi,pf. Reaction conditions: 450 mL batch reactor; 6-W 360-nm lamp; fresh P-25 film; dry air. (a) 1% TCE vapor, (b) 1% CH3OH vapor, (c) 0.11 mM aqueous CH2Cl2, (d) 1% CH3CN vapor, (e) 5% CH2Cl2 vapor, (f) 0.25% ) 0.11 mM CH2Cl2 vapor.

FIGURE 4. Removal of vaporized CH3CN, formation and decay of intermediates, and formation of CO2. Reaction conditions: 450 mL batch reactor; 6-W 360-nm lamp; fresh P-25 film; dry air atmosphere, 1% CH3CN.

50-70% of the corresponding reducing equivalents had been consumed. In the fast reaction of dry CH3CN vapor with air, production of CO2 appears to lag somewhat farther behind the removal of CH3CN; e.g., when 40% or 15% of initially dark-equilibrated 0.95 mM CH3CN remained in the vapor, about 17% or 33%, respectively, of stoichiometric CO2 had formed, corresponding to 38% and 39%, respectively, of the difference between initial and remaining CH3CN. The fraction of CH3CN removed by chemical reaction that had been converted to CO2 cannot however be evaluated exactly because the amount of CH3CN that desorbed from the catalyst is not known. In the reaction of CH3CN vapor in ambient air (31) as well as in our more recent work on the reaction of CH3CN vapor with dry air, NO2 was identified as a product and, as shown in Figure 4, built up almost to its maximum yield while CH3CN decayed completely. Almost complete decay of NO2 took about 3 times as long as removel of vaporized CH3CN. An intermediate characterized by a strong GC/MS peak at mass 52, corresponding to C2N2, reached its maximum concentration when about 95% of the CH3CN had been removed

FIGURE 5. Removal of vaporized CH3OH, formation, decay of methyl formate, and formation of CO2. Reaction conditions: 450 mL batch reactor; 6-W 360-nm lamp; fresh P-25 film; dry air atmosphere, 1% CH3OH.

and then decayed while CO2 continued to increase. Nitric acid was identified as a product of the reaction with dry air as it had been with ambient air (31). From CH3OH. Consumption of aqueous CH3OH was followed only by measuring COD. Thus, information relating the disappearance of aqueous methanol to the formation of CO2 is lacking. The degree to which the formation of CO2 from dry CH3OH vapor lagged behind its removal decreased with increasing concentration of O2; e.g., in 20 mol % of O2, generation of the stoichiometric yield of CO2 took twice as long as complete removal of CH3OH, while in 100 mol % O2 the times required for complete removal of CH3OH vapor and for formation of CO2 were similar. The molar ratio of CO2 produced to vaporized CH3OH removed (not corrected for desorption of CH3OH from the catalyst) after 15 min of irradiation in respectively 2.3%, 6.2%, 20%, and 100% dry O2 was 0.25, 0.40, 0.55, and 0.85. It can be inferrred that this O2dependent lag results from dependence on partial pressure of O2 of their rates of oxidation, which is greater for one or more intermediates than for CH3OH. Two intermediates were detected by GC analysis and shown to be formaldehyde and methyl formate. Figure 5 shows the buildup and decay of methyl formate during a typical run with vaporized CH3OH. We have previously found by FTIR spectrometry that formic acid is produced from vaporized methanol and is strongly adsorbed on TiO2 (23). From TCE. Consumption of aqueous TCE was followed only by the determination of ionic chloride. Thus, no information on stoichiometry was obtained. Chloroform was identified by GC as a minor product of the reaction of vaporized and 0.49 mM aqueous TCE. Data were obtained that demonstrate that the generation of CO2 lagged far behind the removal of vaporized TCE. It was also found that substantially less than 100% of the carbon of vaporized TCE was converted into CO2 and/or that long-lived intermediates are generated. Extensive data on intermediates and final products produced in the oxidative TiO2photocatalyzed decomposition of vaporized (19, 20, 23, 37) and aqueous (38) TCE are reported in recent publications. From CH2Cl2. Consumption of CH2Cl2 in aqueous solution was followed only by monitoring chloride ion. Thus, information relating disappearance of aqueous CH2Cl2 to formation of CO2 is lacking. Figure 6 shows that the generation of CO2 in the reaction of CH2Cl2 vapor with

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3, initial dark Csurf values vary by factors of as much as 3.4 for CH3CN, 2.0 for CH3OH, and 3.6 for TCE. Furthermore, the ratio of calculated initial surface concentration to initial vapor concentration varies by factors of as much as 5 for CH3CN and CH3OH and 12 for TCE. Table 3 shows that, over these ranges of vapor concentration, the order in concentration of organic reactant and rate constant are constant for each of these compounds. Dark Surface Concentrations of Aqueous Organic Reactants. Fugacities (i.e., thermodynamic activities), faq (in atm), of reactant vapors in equilibrium with their dilute aqueous solutions of known millimolar concentration, (solute), can be calculated with FIGURE 6. Removal of vaporized CH2Cl2 and formation of CO2 in the presence and absence of water vapor. Reaction conditions: 450 mL batch reactor; 6-W 360-nm lamp; fresh P-25 film; dry air atmosphere, 1% CH2Cl2.

both dry and water-saturated air lagged well behind the removal of CH2Cl2 and that the rates of both processes were much lower with water-saturated than with dry air. FTIR absorption spectra of vapor-phase reaction mixtures transferred from the reactor after consumption of 80% of CH2Cl2 revealed the presence of phosgene (COCl2), HCl, and CO as well as CO2. Generation of Cl2 was demonstrated with Matheson-Kitagawa tubes.

Discussion Some General Considerations. Concentration of O2. Vapor pressures and, therefore, activities of O2 were matched in experiments with vaporized and aqueous CH2Cl2 and TCE, which are summarized in Table 3. However, molar concentration of O2 in the vapor phase at 1 atm (or lower pressure) and 25 °C is approximately 33 times its concentration in an aqueous solution equilibrated with the vapor (39). In the reactions of CH3CN and CH3OH, the kinetics of which is summarized in Table 3, the concentration of O2 in the vapor experiments was 6.6 times its concentration in the solution experiments. How much this difference contributed to the higher rates observed in the vapor phase is not known at present. Molar Concentrations vs Mole Fractions. The mole fraction of a component of a gas at 1 atm and 25 °C is approximately 1350 times its mole fraction in aqueous solution when its molarity is the same in the two phases. Thus, at equimolar concentrations of reactants in the two phases, competition with reactants for adsorption on the catalyst by molecules of the major components of the bulk phase, i.e., N2 or H2O, is much greater in aqueous solutions because of both the much greater preponderance of water molecules over reactant molecules and the stronger adsorption of water as compared to O2 or N2. Dark Surface Concentrations of Vaporized Organic Reactants. Substitution of the a and b Langmuir coefficients for films of TiO2, which are assembled in Table 1, and initial concentrations of organic reactants from Table 3 into eq 1 provides values of the initial dark surface concentrations of CH3CN, CH3OH, and TCE in equilibrium with their dry vapors in the reactor. It is assumed here that the values of the Langmuir coefficients of the organic reactants are not significantly affected by the composition of O2-N2 mixtures. These calculations show that, over the ranges of equilibrated dry vapor concentrations assembled in Table

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faq ) H × (solute)

(4)

using values of H (Henry’s law constants) assembled in Table 2. The fugacities, expressed in µmol L-1, assuming ideal behavior of reactant vapors can then be used in conjunction with the Langmuir equation, eq 1, and the Langmuir constants for water-saturated vapors, which are assembled in Table 1 to calculate dark surface concentrations in equilibrium with aqueous solutions. These calculations show that, for CH3CN and CH3OH, Csurf was indistinguishable from aC0 where a is the a coefficient of eq 1. Reactions of Individual Compounds. CH3CN. As previously reported, vaporized CH3CN is much more reactive to TiO2-photocatalyzed oxidation than is aqueous CH3CN (23, 30, 31). Its initial rate of removal is approximately half order in CH3CN in aqueous solution and approximately first order for the reaction of the dry vapor. As a consequence of the difference in order, the factor by which Ep,i,rem of the vapor reaction exceeds that of the aqueous reaction increases with increasing concentration and is of the order of 102 at the highest concentrations measured. It does not appear to be possible to draw any simple mechanistic inference from the fact that the kinetic order and specific rate of removal of CH3CN from its dry vapor are constant over a concentration range in which the ratio of its surface concentration to its vapor concentration in the dark varies by a factor of 5. Perhaps, dark adsorption equilibria differ widely from adsorption equilibria under irradiation. The fact (see Figure 2) that the rate of removal of CH3CN from its 1 mol % dry vapor is independent of the concentration of O2 can be interpreted as indicating that, at this concentration, adsorbed CH3CN replaces O2 as an electron trap. The fact (Figure 3) that, under the conditions for which Oi,O2 ∼ 0, Oi,pf ) 0.7, i.e., is less than 1, indicates significant competition of nonreactive charge recombination with reactive capture of charge. This inference is consistent with Ep,i,rem ) 0.12 at the highest flux used in determination of Oi,pf. (See Table 3). The formation of cyanogen, C2N2, as an intermediate product of the reaction of CH3CN vapor indicates the generation of CN• radicals, which dimerize to form C2N2. The formation of this radical suggests the possibility that the interaction of a hole, other oxidizing moiety, or CH3CN•+ radical cation with an adsorbed CH3CN•- radical anion can cause dissociation of the CH3CN. Presumably, the resulting (CH3•)vapor radical reacts rapidly with O2 while at least some of the (CN•)vapor radical undergoes dimerization to C2N2.

Calculated dark surface concentrations of CH3CN in equilibrium with its aqueous solutions vary linearly with the concentration of CH3CN over the range of concentrations assembled in Table 3. This linearity is not in accord with the possibility that the apparent approximately squareroot dependence on CH3CN concentration of the initial rate of removal of CH3CN from solution is an artifact of nonlinear variation of its surface concentration with its solution concentration. This discord may be an artifact of using dark adsorption data in place of unavailable adsorption data under irradiation. As shown in Table 3 and Figure 1, aqueous CH3CN is much less reactive than its vapor, yet water vapor reduces the reactivity of vaporized CH3CN only slightly. Perhaps, the low reactivity of aqueous CH3CN is due to the strong interaction of adsorbed CH3CN with liquid water. Such an interaction could help dissipate the heat of charge recombination and inhibit the dissociation of CH3CN. CH3OH. As seen in Table 3, the initial kinetic order in CH3OH of removal of CH3OH from the dry vapor phase is the same as for CH3CN, approximately first. In the dark, Csurf,i for adsorption of CH3OH varies by a factor of 2 and Csurf,i/C0 by a factor of 5 over the range of measured initial dry vapor concentrations. As with CH3CN, the kinetic order in CH3OH and dark-adsorption data do not suggest a mechanistic inference, and adsorption data under irradiation are needed. The initial rate of removal of CH3OH vapor, unlike that of CH3CN vapor, increases with increasing partial pressure of O2 (Figure 2). The fact that the order in concentration of O2 is fractional and diminishes with increasing concentration of O2 may reflect the saturation of adsorption of O2. These data are consistent with O2 functioning as an electron trap. The initial photoefficiency of removal of CH3OH from the dry vapor at its highest measured concentration, 1.1 (Table 3), corresponds to 1.4 molecule removed per absorbed photon (23). The fact (Figure 3) that, under the conditions of the reaction of dry CH3OH vapor for which Oi,O2 ) 0.25-0.5, Oi,pf ) 0.55 indicates significant competition of unreactive charge recombination with reactive capture of charge. This inference is consistent with Ep,i,rem ) 0.35 at the highest flux used in determination of Oi,pf. (See Table 3.) Calculated dark surface concentrations of CH3OH in equilibrium with its aqueous solutions vary linearly with the concentration of CH3OH over the range of concentrations assembled in Table 3. As is the case for CH3CN, this linearity is not in accord with the possibility that the apparent approximately square-root dependence of initial rate of removal of CH3OH (or CH3CN) from solution is an artifact of nonlinear variation of its surface concentration with its solution concentration. This discord may be an artifact of using dark-adsorption data as is the case for CH3CN. As shown in Figure 1, the initial rate of removal of CH3OH from the vapor phase, unlike that of CH3CN, is strongly inhibited by water vapor. TCE. As shown in Table 3, the initial rate of removal of TCE from the dry vapor phase by reaction with 1 atm of air is zero order in TCE and Ep,i,rem ) 1.6 over a factor of 45 in (TCE)i. The fact that Oi,O2 for removal of 1% TCE is 0 from 2 to 100 mol % of O2 (Figure 2) suggests that TCE, not O2, is the principal electron trap. Data recorded in Figure 3 establish that the initial rate of removal of 1 mol % of vaporized dry TCE in the presence of air varies linearly with radiation flux over a 10-fold range of the latter. These

data indicate that the rate of removal of TCE from the dry vapor was determined by the rate of absorption of photons by TiO2 and that the reaction of adsorbed TCE with each photogenerated oxidizing or reducing species (e.g., hole or electron) results in the removal of 1 molecule of TCE. The fact that Ep,i,rem is 1.6 rather than 2 is consistent with our prior observation that a similar film of P-25 TiO2 absorbed 78% of incident radiation at and near 360 nm (23). As shown in Figure 1, the initial rate of removal of 1 mol % TCE vapor by its reaction with air does not vary significantly with variation of the water content of the vapor phase from undetectable (achieved by predrying the TCE) to 3 mol % even though the data of Table 3 show that liquid water strongly inhibits the removal of TCE. Perhaps, as suggested for CH3CN, a strong interaction between adsorbed TCE and liquid water causes the low reactivity of aqueous TCE. CH2Cl2. Data recorded in Figure 1 show that the initial rate of removal of vaporized CH2Cl2, like that of CH3OH, diminished steeply with increasing partial pressure of water vapor. CH2Cl2 is the reactant for which the initial rates of the reactions in water-saturated air and in aqueous solution at matched molarities as well as the kinetic order in organic reactant in dry air and in aqueous solution were most similar. The large difference in mole fractions does not have a major effect. Caution must, however, be exercised in interpreting the kinetic data for aqueous CH2Cl2 because its removal was equated to one-half the yield of chloride, and the stoichiometric and temporal relationships of chloride formed to CH2Cl2 consumed were not measured. Since adsorption on thin films of CH2Cl2 was not measurable, its Csurf cannot be compared with the kinetics of its removal. The first-order rate constant for reaction of the dry vapor was constant over a 10-fold increase in its concentration. However, a further 5-fold increase in the vapor concentration was associated with a decrease in apparent order in CH2Cl2 or in apparent first-order rate constant. This decrease may be a consequence of reduced accessibility of the surface to O2 at higher concentrations of CH2Cl2 and resulting enhancement of electron capture by methylene chloride, if the latter process is a less efficient route to decomposition than is the initial attack on CH2Cl2 by an oxidizing moiety. Such a supposed operation of two mechanisms, one of which is initiated by oxidative attack and the other by reduction, is analogous to the dual reaction pathways for TiO2-photocatalyzed degradation of aqueous TCE and perchloroethylene in the presence of O2 that has been recently postulated (38). The fact that the kinetic order in O2 of removal of 0.11 mM CH2Cl2 from aqueous solution and from dry vapor was respectively 0.3 and 1 suggests that, in spite of the 33-fold lower concentration of O2 in water than in the vapor, reactive adsorption sites of O2 were closer to saturation in the presence of liquid water. Evidence for the formation of COCl2, CO, and Cl2 in the reaction of CH2Cl2 vapor taken together with evidence for electron capture by CH2Cl2 suggests a reaction sequence involving homolytic fission of a C-Cl bond analogous to that postulated to rationalize the formation of C2N2 from CH3CN. Discussion of Overall Results. The kinetic orders in the organic reactants of initial rates of photocatalytic removal of dry vaporized CH3CN, CH3OH, TCE, and CH2Cl2 are different from the corresponding orders for the aqueous organic reactants. Consequently, ratios of Ep,i,rem

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of conversion at similar initial molar concentrations of organic reactant in the gas phase and in solution vary with concentration in directions that depend on which order was higher. The highest observed ratios of Ep,i,rem of the dry vapor reaction to those of the aqueous reaction at equimolar initial concentrations varied from 25 for CH3OH and CH2Cl2 to 300 for TCE. Water vapor strongly inhibited the reactions of vaporized CH3OH and CH2Cl2 but had little effect on the rates of reaction of vaporized CH3CN and TCE. No single explanation appears capable of rationalizing these reactant-specific effects of liquid and gaseous water.

Acknowledgments The authors wish to express their appreciation for support of this work by EPA Grant R817831-01-0 to Boston University.

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Received for review October 31, 1995. Revised manuscript received February 7, 1996. Accepted February 19, 1996.X ES950816D X

Abstract published in Advance ACS Abstracts, April 15, 1996.