5726
J . Phys. Chem. 1988, 92, 5726-5731
rheological investigation for aqueous C,E7 solutions can confirm the results from light scatterings3'
Conclusion In this work, the measurement of laser light scattering was carried out for aqueous NaCl solutions of C,E7. Moreover, the size and mutual interaction of rodlike micelles in dilute regime were analyzed at a finite micelle concentration, and the properties of the entangled micelles in dilute and semidilute regimes were examined on the basis of the scaling laws. Several important conclusions were obtained: (1) The Debye plots and the diffusion coefficients exhibit minimum values around the threshold micelle concentration of overlap, above which micelles entangle together. (2) Whereas the threshold micelle concentrations of overlap decrease with an increase in NaCl concentration and the alkyl chain length, the aggregation number, radius of gyration, and hydrodynamic radius of rodlike micelles increase with them. (3) While the second virial coefficient of rodlike micelles is small and scarcely depends on micelle aggregation number, the friction coefficient remarkably increases with micelle size and strongly (37) Imae, T.; Sasaki, M.; Ikeda, S.J. Colloid Interface Sci., in press.
contributes to the hydrodynamic virial coefficient, resulting a negative large virial value. (4) When a value of 13/75 is applied instead of 2/15 for the coefficient A on angular dependence of diffusion coefficient, the numerical values of the second virial coefficient change from positive to negative. (5) The characteristic exponent v for several scaling laws associated with light scattering experiments in dilute and semidilute regimes ranges between 0.58 and 0.71, in general, independent of NaCl concentration and the alkyl chain length. Moreover, the v values from static and dynamic light scattering are consistent with each other. (6) The v values increase in the order of alkyltrimethylammonium halides C C g 7C dimethyloleylamine oxide, reflecting the difference of the flexibility of rodlike micelles. (7) The deviation from the straight line on the scaling laws was observed at micelle concentrations higher than 10-100 times the threshold value of overlap, suggesting the transition from the semidilute regime to the concentrated regime. Acknowledgment. I acknowledge Professor Shoichi Ikeda for valuable suggestions. Registry No. CloE7, 39840-09-0; CI2E7,3055-97-8; CI4E7,4003679-1; Ci6E7,4486-31-1; NaC1, 7647-14-5.
Klnetlc Studies In Heterogeneous Photocatalysis. 1. Photocatalytic Degradatlon of Chlorinated Phenols in Aerated Aqueous Solutions over TIO, Supported on a Glass Matrix Hussain AI-Ekabi and Nick Serpone* Department of Chemistry, Concordia University, 1455 de Maisonneuve Blvd. West, Montreal, Quebec, Canada H3G-lM8 (Received: October 22, 1987; In Final Form: April 25, 1988)
The photocatalytic degradation of phenol, 4-chlorophenol, 2,4-dichlorophenol, and 2,4,5-trichlorophenolover Ti02 (anatase) has been investigated by using three photochemical reactors. Ti02 was used as a thin film, coating the internal surface of a glass coil (reactors I and 11) or the external surface of glass beads (reactor 111). The degradation of the four phenolic compounds, in a continuous recirculation mode in all three reactors, approximatesfirst-order kinetics to near-complete degradation. The Langmuir-Hinshelwood kinetics have been modified slightly to rationalize the first-order behavior in solid-liquid reactions and to argue in favor of a surface reaction; the degradation reactions occur on the Ti02 particle surface. In the multipass mode experiments, both reactors I and I1 exhibit higher degradation rates for phenol at the higher flow rates. By contrast, the greater degradation is associated with the lower flow rates in the single-pass mode experiments.
Introduction The presence of chlorinated hydrocarbons and other organic contaminants in drinking waters in North Americai2 and Europe3 can have serious consequences on human health and on the quality of the environment." The widespread utilization of chlorinated aromatic compounds as pesticides and herbicides is attracting increased concern. The degradation of these compounds is possible via chemical, photochemical, and biological processes: but most (1) Syrnons, J. M.; Bellor, T. A.; Carswell, J. K.; De Marco, J.; Kropp, K. L.; Robeck, G. G.; Seeger, D. R.; Slocurn, C. J.; Smith, B. L.; Stevens, A. A. J.-Am. Water Works Assoc. 1975, 67, 634. (2) Foley, P. D.; Missingham, G. A. J.-Am. Water Works Assoc. 1976, 68, 105. (3) Rook, J. J. J.-Am. Water Works Assoc. 1976, 68, 168. (4) Hutzinger, O., Ed. The Handbook of Enuironmental Chemistry; Springer-Verlag: Berlin, 1982.
of these require long treatment periods and in practice are often difficult to apply in the disposal of wastes. The photocatalytic mineralization of chlorinated hydrocarbons and other organic contaminants in water mediated by illuminated T i 0 2 has been demonstrated.5* TiOz dispersions and conventional (5) (a) Hsiao, C.-Y.; Lee, C.-L.; Ollis, D. F.J. Catal. 1983, 82, 418. (b) Pruden, A. L.; Ollis, D. F. Enuiron. Sci. Technol. 1983, 17,628. (c) Pruden, A. L.; Ollis, D. F. J. Catal. 1983, 82, 404. (d) Ollis, D. F.; Hsiao, C.-Y.; Budirnan, L.; Lee, C.-L. J. Catal. 1984,88,89. (e) Nguyen, T.; Ollis, D. F. J. Phys. Chem. 1984.88, 3386. (f) Ollis, D. F. Enuiron. Sci. Technol. 1985, 19, 480. (6) (a) Matthews, R. W. J. Chem. SOC.,Faraday Trans. 1 1984,80,457. (b) Matthews, R. W. J. Catal. 1986, 97, 565. (c) Matthews, R. W. Water Res. 1986,20, 569. (d) Matthews, R. W. J. Phys. Chem. 1987, 91, 3328. (e) Matthews, R. W. Sol. Energy 1987, 38, 405. (f) Matthews, R. W. Aust. J. Chem. 1987. 40. 667. (7) Okamoto: K.; Yamarnoto, Y.; Tanaka, H.; Tanaka, M. Bull. Chem. SOC.Jpn. 1985, 58, 2015.
0022-3654/88/2092-5726$01 SO10 0 1988 American Chemical Society
Photocatalytic Degradation of Chlorinated Phenols near-UV radiation were used in most of these studies. Therefore, a technique utilizing sunlight that does not involve filtration and resuspension of the photocatalyst is desirable. Recently, Matthewsa' has employed Ti02in a stationary phase illuminated with UV light. In this regard, we earlier showed the photocleavage of HIS with CdS supported on a polycarbonate matrixgaand more recently the degradation of 2,4,5-trichlorophenol and the photoreduction and recovery of gold(II1) with illuminated TiOz supported on glass beads.gb We herein report the kinetics of the photodegradation of phenol (Ph), 4-chlorophenol (4-CP), 2,4-dichlorophenol (2,4-DCP), and 2,4,5-trichlorophenoI (2,4,5-TCP) in aerated aqueous media on T i 0 2 supported on a glass matrix in a flow system.'0 The photodegradation was carried out by using a thin film of TiOz coated either onto the internal surface of a glass coil or the external surface of glass beads. Simulated sunlight (AMI) and conventional fluorescent (Arnx N 366 nm) lamps were used as the radiation sources. The Langmuir-Hinshelwood kinetics, which have been applied successfully for describing solid-gas reactions, are adapted here with some modifications to meet the conditions inherent in solid-liquid reactions. We also show that the photocatalyzed degradation of phenol and its monochloro, dichloro, and trichloro derivatives can be done efficiently and completely to C 0 2 and HCl with TiOZin a stationary phase and with AM1 simulated sunlight.
Experimental Section Chemicals. Phenol (Fisher), 4-chlorophenol and 2,4-dichlorophenol (Anachemia), and 2,4,5-trichlorophenol (Eastman) were laboratory reagent grade and were used without further purification. Doubly distilled water was used throughout this study. Photochemical Reactors. A Pyrex glass tube of 1.5-m length and 0.8-cm i.d. was wound 13 times to form a coil. A slurry of 165 mg of TiOz (Degussa, P-25) in 5 mL of water was sonicated for 15 min with a 250-W sonicator (Sonics and Materials, Inc.), operated at 75 W; the milky suspension was subsequently introduced in the coil to cover uniformly the entire internal surface of the glass support. The suspension was evaporated to dryness under vacuum with warm air (heat gun) blown around the external surface of the coil. The surface area of the internal surface of the coil was 374 cm2; the coating of TiOz on the surface of the coil was ca. 0.44 mg.cm-2. The volume of the coil is 75 cm3. The Ti02/coil arrangement was illuminated with two radiation sources: (a) with a Solarbox lamp (CO.FO.MEGRA, Milano) equipped with a 340-nm cutoff filter (reactor I); the spectral distribution of the lamp above 340 nm simulates AMI solar radiation; (b) with six (ultraviolet, blacklight) fluorescent tubes (lifetime ca. 7500 h; wavelength range 300-400 nm; maximum emission at 366 m; intensity 0.9 mW cm-2 per lamp at 30 cm from the source; power rating 15 W; each lamp contains Hg-standard fluorescent fill; data were supplied by Ultraviolet Products, Los Angeles) surrounding (8) (a) Barbeni, M.; Minero, C.; Pelizzetti, E.; Borgarello, E.; Serpone, N.; Chemosphere 1988, 16, 2225. (b) Borello, R.; Minero, C.; Pramauro, E.; Pelizzetti, E.; Serpone, N.; Hidaka, H., submitted for publication in Chemosphere. (c) Pelizzetti, E.; Borgarello, M.; Minero, C.; Pramauro, E.; Borgarello, E.; Serpone, N. Chemosphere 1988, 17, 499. (d) Barbeni, M.; Pramauro, E.; Pelizzetti, E.; Vincenti, M.; Borgarello, E.; Serpone, N. Chemosphere 1987, 16, 47. (e) Hidaka, H.; Kubota, H.; Gratzel, M.; Pelizzetti, E.; Serpone, N. J . Photochem. 1986,35,219. (f) Borgarello, E.; Serpone, N.; Barbeni, M.; Minero, C.; Pelizzetti, E.; Pramauro, E. Chim. Ind. (Milan) 1986, 68, 53. (g) Barbeni, M.; Pramauro, E.; Pelizzetti, E.; Borgarello, E.; Serpone, N.; Jamieson, M. A. Chemosphere 1986, 15, 1913. (h) Pelizzetti, E.; Barbeni, M.; Pramauro, E.; Serpone, N.; Borgarello, E.; Jamieson, M. A.; Hidaka, H. Chim. Ind. (Milan) 1985, 67, 623. (i) Hidaka, H.; Kubota, H.; Gratzel, M.; Serpone, N.; Pelizzetti, E. N o w . J. Chim. 1985, 9, 67. 6 ) Barbeni, M.; Pelizzetti, E.; Borgarello, E.; Serpone, N. Chemosphere 1985, 14, 195. (k) Barbeni, M.; Pramauro, E.; Pelizzetti, E.; Borgarello, E.; Gratzel, M.; Serpone, N. N o w . J. Chim. 1984, 8, 547. (9) (a) Borgarello, E.; Serpone, N.; Liska, P.; Erbs, W.; Gratzel, M.; Pelizzetti, E. Gazz. Chim. Ifal. 1985, 115, 599. (b) Serpone, N.; Borgarello, E.; Harris, R.; Cahill, P.; Borgarello, M.; Pelizzetti, E. Sol. Energy Mafer. 1986, 14, 121. (10) While our work was in progress, we became aware of Matthews' work along similar lines as reported here (see ref 6d,e).
The Journal of Physical Chemistry, Vol. 92, No. 20, 1988 5727
0
120
240
360
TIME,min
Figure 1. Plot showing changes in the concentrations of Ph as a function of irradiation time in reactor I in a continuous recirculation mode at 10 mlmin-' flow rate. The insert is a plot of In (normalized [Ph]) versus time.
the glass coil and mounted on a setup with back aluminum reflectors (Sargent-Welch, S-44230) (reactor 11). Reactor I11 was constructed by surrounding a Pyrex glass tube of 3-cm i.d. and 38-cm long with a wider Pyrex tube of the same length so that a spacing of 0.6 cm separated the external and internal tubes. This spacing was filled with glass beads of 3-mm diameter coated with Ti02.9b The irradiation source was the same as for reactor 11. A short Tygon tubing was fitted to the inlet and outlet ports to allow circulation of the solution through the reactors. In the continuous recirculation mode experiments, 250 mL of solution was pumped at the desired flow rate. Aliquots (3 mL) were retrieved from the reservoir at certain time intervals during each run and analzyed; the solution was returned to the reservoir after analysis. In the single-pass-mode experiments, the solution was gravity fed to the reactor, and samples were withdrawn for analysis from the outlet. The reaction temperature varied between 25 and 35 O C , depending on the flow rate and the radiation source. Analyses. The degradation of Ph and 4-CP was followed by determining the concentration of unreacted phenol by fluorimetric methods on a Perkin-Elmer MPF-44B spectrofluorimeter. The excitation and emission wavelengths for Ph were set at 273 and 295 nm, respectively; for 4-CP, they were 280 and 310 nm, respectively. The fluorescence technique adequately monitored changes in concentration down to 10.2 hM. Unless otherwise noted, the initial concentrations of Ph and 4-CP were 17.8 and 13.0 pM, respectively. Both the 2,4-DCP and 2,4,5-TCP substrates emit very weakly. Inasmuch as HCl is one of the degradation products (reaction 1; for 2,4,5-TCP), the extent of Cl3C6HZ0H+ 5.50z
hu
io, 6COz + 3HC1
(1)
degradation of the higher chlorinated phenols was followed by measuring changes in pH as a function of irradiation time; further confirmation was obtained on determining (for complete degradation) the total C1- concentration by using a C1- ion selective electrode. The amount of C1- leached from the T i 0 2 was negligible. The initial concentrations of 2,4-DCP and 2,4,5-TCP were 153.4 and 126.6 gM, respectively, unless otherwise stated. Conversion for an arbitrary set of conditions was reproducible to within &lo% over the period required to complete a set of experiments.
Results and Discussion Kinetic Analysis. It is important to establish initially whether the degradation reaction takes place in the adsorbed state (phenol adsorbed to Ti02 film) or whether the Ti02 semiconductor surface merely provides an active species (-OH, for instance), which desorbs into solution and subsequently reacts. Below we discriminate between the two possibilities invoking the LangmuirHinshelwood (L-H) kinetic treatment. The L-H treatment is
Al-Ekabi and Serpone
5728 The Journal of Physical Chemistry, Vol. 92, No. 20, 1988 TABLE I: Apparent First-Order Rate Constants k' and t i l 2 for the Degradation of Ph, 4-CP, 2,4-DCP, and 2,4,5-TCP in Reactors I, 11, and 111 in a Continuous Recirculation Mode at 10 rnL-rnin-' Flow Rate reactor
I
io^, compound Ph" 4-CPb 2,4-DCF 2,4,5-TCpd
tl12, min 75 105 570 570
min-' 9.2 6.6 1.3 1.3
I1
111
1 0 3 ~ tl12, min-I min 55 12.5 8.9 77 1.6 427 1.6 427
1 0 3 ~ tl12, min-I min 9.7 71 9.5 73 1.7 408 1.7 408
"Initial concentration = 17.8 pM. *Initial concentration = 13.0 cInitial concentration = 153.4 pM. dInitial concentration = 126.6 pM.
pM.
known to be a good model for the description of solid-gas reactions. Extrapolation of this model to solid-liquid reactions requires some modifications, especially when the surface is that of TiOz particles in aqueous solutions; the TiOz is known to be covered with hydroxyl groups and molecular water. The degradation of the four phenols in aerated aqueous solutions as a function of irradiation time was carried out in reactor I in a continuous recirculation mode. The results for Ph are illustrated in Figure 1 . Plots of the natural logarithm of normalized concentrations of solutes versus irradiation time (insert) show good linearity. Under the conditions of the experiments, the reactions approximate first-order kinetics to a high degree of degradation (>85% for Ph). This behavior is rationalized in terms of modified forms of the L-H kinetic treatment, which has been used successfully as a qualitative model to describe solid-liquid reactions." We recognize two extreme situations in defining the surface coverage, 8, of the semiconductor particle: (i) both the reactant and the solvent compete f o r the same active sites; (ii) both the reactant and solvent are adsorbed on the surface without competing for the same active sites. According to the L-H model, the rate of a unimolecular surface reaction, R,Iz is proportional to 8 and will follow eq 2a or 2b, respectively, when the reactant is more strongly adsorbed on the surface than the products:
+ KCo + K,C,) = k,8 = k,KCo/(l + KCo)
R = -dC/dt = k,8 = k,KCo/(l R = dC/dt
(2a) (2b)
where k, is the reaction rate constant, 8 is the fraction of the surface covered by the reactant, K is the adsorption coefficient of the reactant, Cois the initial concentration of the reactant, K, is the adsorption coefficient of the solvent, and C, is the concentration of the solvent. Inasmuch as the hydrated TiOz surface is covered with hydroxyl groups and molecular water and since both phenol and water molecules could, in principle, adsorb on this surface via hydrogen bonds, their competition for the same active sites cannot be ignored. Moreover, to the extent that C, > Coand that C,remains essentially constant, the part of TiOz surface covered by the solvent is approximately unchanged at all the reactant concentrations used. Integration of eq 2a and 2b yields eq 3a and 3b, respectively:
co
ln-+C
K
1
+ K,C, (Co-C)
L O
In C
k*K =t 1 + K,C,
+ K(Co - C ) = k,Kt
(3a) (3b)
Obviously, both eq 3a and 3b are the sum of zero-order and (1 1) (a) Al-Ekabi, H.; de Mayo, P. J. Phys. Chem. 1986,90, 4075. (b) de Mayo, P. Tetrahedron 1986.42, 6277. (c) Hasegawa, T.; Al-Ekabi, H.; de Mayo, P. hngmuir 1986, 2, 362. (12)The applicability of the first-order kinetics requircs that the concentration of OH radicals, produced on the surface, be constant. This was, indeed, the case as the light intensity and the amounts of TiOz and water are kept constant. (13) Howe, R. F.; Gritzel, M. J. Phys. Chem. 1987, 91, 3906.
0
80
160
TIME, min Figure 2. Effect of the initial concentrations on the degradation of Ph in reactor I1 in a continuous recirculation mode. Flow rate = 10 mL. ' , initial concentrations are 3.5 (O), 8.8 ( O ) , 17.8 (a), and 34.6 pM (A). The insert is a plot of fluorescence intensity (au) versus [Ph] in pM. TABLE II: Apparent First-Order Rate Constants k' and till of the Photodegradation of Ph at Different Initial Concentrations in Reactor I1 in a Continuous Recirculation Mode at 55 mLmin-' Flow Rate IPhl, W M 3.5 8.8 17.8 34.6
i03k! min-I
tl 1 ) . min
33.9 28.9 23.6 15.1
20 25 31 46
first-order rate equations, and their contribution to the overall reaction depends essentially on the initial concentration C,. When Co is very small, eq 3a and 3b reduce to eq 4. Thus, a plot of In (Co/C) = k't (4) In (Co/C) versus irradiation time should give a straight line (as is the case in Figure 1) whose slope equals the apparent first-order rate constant, k'. The k'values for the degradation of the four phenols in reactor I are presented in Table I, along with the values of t l / z . The corresponding values for the photocatalyzed disappearance of the chlorophenols, carried out with reactors I1 and 111 in a continuous recirculation mode, are also collected in Table I. Since the initial concentrations of Ph and 4-CP (17.8 and 13.0 pM, respectively) were lower than those of 2,4-DCP and 2,4,5TCP (153.4 and 126.6 pM, respectively), any comparison of the results of Table I is restricted to each couple separately. The rates of degradation of 2,4-DCP and 2,4,5-TCP are identical (within experimental error) in each of the three reactors; the corresponding rates of Ph versus 4-CP are slightly different; Ph decomposes faster. The slow rates of degradation of 2,4-DCP and 2,4,5-TCP, in comparison with those of Ph and 4-CP are understandable as higher concentrations were used in the former couple and firstorder kinetics are involved. It should be noted that the performance of both reactors I and I1 did not change (within experimental error) over an 8-month period of continued use. Effect of Phenol Concentration. The results summarized in Table I show that the photocatalytic degradation of Ph proceeds faster in all three reactors. Ph was therefore chosen for further detailed studies. Initially, the concentration range in which the fluorescence intensity (ZJ of Ph increases linearly with concentration needed to be established, because of the potential for association. The insert in Figure 2 indicates that the fluorescence intensity of Ph increases linearly with concentration up to -34.6 pM; Zfreaches a maximum at -133.0 pM. At higher concentration, where association and/or self-quenching may occur, the fluorescence intensity decreases. The Ph concentration was maintained at 17.8 pM throughout. Figure 2 also illustrates the effect of initial concentrations (3.5-34.6 pM) on the photocatalytic degradation of Ph in reactor
The Journal of Physical Chemistry, Vol. 92, No. 20, 1988 5729
Photocatalytic Degradation of Chlorinated Phenols
c o l!-JM
0
0
20
10
200
400
TIME, min
30
c~(Io+~M-~) Figure 3. (a) Plot of the reciprocal of the degradation rate ( R ) versus the reciprocal of the initial concentration Coof Ph (0). Each sample was irradiated for 30 min. (b) Plot of t l l zversus Cofor Ph at different initial concentrations ( 0 ) .
Figure 4. Effect of flow rate on the concentrationof Ph in the reservoir from reactor 1 in a continuous recirculation mode. Flow rates are 5 (A), 10 (a), 55 ( O ) , and 125 ml-min-’ (0). The insert depicts plots of In
(normalized [Ph]) versus time.
TABLE 111: Apparent First-Order Rate Constants k’ and t l p of the Photodegradation of Ph in Reactors I and I1 in a Continuous Recirculation Mode at Different Flow Ratesu
reactor flow rate, mL.min-’ 5 10
55 125 250
I1
I iO3kk:min-l 7.2 9.2 12.2 11.5
t 1 p min
91 15 57 61
W k ’ , rnin-l
tip, min
12.5 24.5 30.5 39.2
56 28 23 18
0
“Initial [Ph] = 71.2 p M .
I1 in a continuous recirculation mode. The apparent first-order rate constants k’ and the corresponding tllt values are given in Table 11. The k’values decrease with increasing Ph concentration. versus the A plot of the reciprocal of the degradation rate (K1) reciprocal of the initial concentration of Ph (Cc’) is linear (Figure 3a) with a nonzero intercept; this together with the data of Figure 1 suggests that the Langmuir-Hinshelwd model is appropriate and, as required by this kinetic treatment, the reaction occurs on the surface. Additional kinetic support for this conclusion arises from application of eq 5, which obtains from eq 3 at C/Co= 0.5.
Clearly, both eq 5a and 5b will yield linear plots when tl12is plotted against Co (Figure 3b). We conclude that the Langmuir-Hinshelwood model cannot discriminate between the two extreme situations i and ii above. Accordingly, no attempts were made to determine the values of K and k, as they afford no clue as to which situation prevails in solid-liquid reactions without characterization of the adsorption isotherm of the system and without a knowledge of the value of K,; the concentration of water is known (-55.5 M). That the photocatalytic degradation of Ph occurs on the TiOz surface also finds support from recent EPR studies by Howe and GratzelI3 on hydrated T i 0 2 under UV irradiation. Effect of Flow Rate. The effect of flow rate on the concentration of Ph (initial [Ph] = 71.2 p M ) with irradiation time in a continuous recirculation mode (reactor I) is shown in Figure 4. Clearly, the rate of degradation increases with an increase in flow rate. The k’values at various flow rates and the corresponding tl12values are summarized in Table 111. Plots of k’ versus flow rate are depicted in Figure 5, which demonstrates that
0
30
60
90
120
150 180 210
240
FLOW RATE, mP/min Figure 5. Effect of flow rate on the apparent first-order rate constant k’in a continuous recirculation mode in (a) reactor I (0) and (b) reactor
I1 ( 0 ) .
“0
50
100
150
TIME,min
Figure 6. Effect of flow rates on the concentration of Ph in the reservoir from reactor I1 in a continuous recirculation mode. Flow rates are 10 ( O ) , 55 (0), 125 (A),and 250 mL-min-’ (0). The insert shows plots of In (normalized [Ph]}versus time.
the rate of degradation increases nonlinearly with flow rate, reaching a plateau at about 55 ml-min-’. Presumably, the higher flow rate maintains a higher concentration (in time) of Ph on the TiOz surface by simultaneous replacement of the degraded Ph
5730 The Journal of Physical Chemistry, Vol. 92, No. 20, 1988
TiO,
30
hu
Al-Ekabi and Serpone hVB+ + ecB-
H ~ O ( S+ ) hVB+ OH-(S)
-+
+ hVB+
-
.OH
(6)
+ H+
(7)
.OH
(8) Recently, Okamoto et aL7 suggested that these radicals form not only via the valence-band holes (reactions 7 and 8) but also via H 2 0 2from 02'-.It is generally accepted that oxygen plays an important role in semiconductor-mediated reactions by trapping the conduction-band electron as superoxide ion (027and thus delaying the electron-hole recombination process: 101
I
I
5
10
1 15
1
20
Flow Rate ,mP/mtn Figure 7. Plot showing the effect of flow rate on the percent of degraded Ph in the outlet stream of reactor I in a single-pass mode. Initial concentration of Ph = 17.8 rM.
molecules. However, when the same set of experiments was conducted with reactor I1 in a multipass mode, the plots of In (normalized [Ph]) versus time deviate (downward curvature) from first-order kinetics. Figure 6 shows that the deviations set in early in the degradation, especially at the higher flow rates. A similar observation was recently reported for the photodegradation of salicylic acid over irradiated Ti02.6d Application of least-squares methods to estimate k'values gave correlation coefficients L0.995 in all cases, indicating that these systems still approximate first-order kinetics. It is noteworthy that plots of k'versus flow rate (for reactor 11) show that k' increases almost linearly a t flow rates >55 mL-min-' (Figure 5b). This behavior is different from that of reactor I, where the rate of degradation of Ph reaches a limiting value a t a flow rate (FR) of 2 5 5 mL-min-' (Figure 5a). In this regard, Matthew@ has empirically developed an L-H type equation, k' = k*PFR/(l + PFR), to describe the dependence of k' on flow rate but failed to observe the linear correlation at higher flow rates. Work is in progress to assess the significance of this linear behavior. It must be pointed out that the degradation of Ph, under the conditions used here, is a light-induced reaction mediated by illuminated TiOz, as no change was observed under direct irradiation (no TiOJ or with TiOz in the dark. Moreover, the possible formation of a photointermediate(s), which could in principle thermally participate in the degradation of Ph in a secondary path, is also excluded as no postirradiation effects were observed. The foregoing observations prompt us to tentatively rationalize the deviations seen in Figure 6 (insert). We look at the crucial role of OH radicals in Ph degradation and the routes from which these radicals are formed as potentially responsible for the deviations. Hydroxyl radicals have been considered the reactive species involved in the photocatalytic degradation of many organic compounds.6*'"'6 These radicals are also implicated in the degradation of Ph inasmuch as hydroquinone, pyrocatechol, and 1,2,4-trihydroxybenzene have been detected as primary and secondary hydroxylation products in our1' and other lab~ratories.~ There are two routes through which OH radicals can be Matthews6a has suggested that reaction of the valence-band "holes" with either adsorbed H 2 0 or with the surface OH- groups on the T i 0 2 particle is the major route of formation of OH radicals: (14) Izumi, I.; Dunn, W. W.; Wilbourn, K. 0.;Fran, F. F.; Bard, A. J. J . Phys. Chem. 1980.84, 3207. ( I 5) Fujihira, M.; Satoh, Y . ;Osa, T. Bull. Chem. Soc. Jpn. 1982.55.666. (16) Izumi, I.; Fan, F. F.; Bard, A. J. J . Phys. Chem. 1981, 85, 218. (17) Al-Ekabi, H.; Serpone, N., unpublished results. (18) Cundall, R. B.; Rudham, R.; Salim, M. S. J . Chem. SOC.,Faraday Trans. 1 1976, 72, 1642. (19) Harvey, P. R.; Rudham, R.; Ward, S.J . Chem. SOC.,Faraday Trans. 1 1983, 79, 138 1 . (20) Herrmann, J.-M.;Pichat, P. J . Chem. SOC.,Faraday Trans. 1 1980, 76, 1 1 38. (21) Munuera, G.; Rives-Arnau, V.; Saucedo, A. J. Phys. Chem. 1979,75, 736.
+ + + + -
0 2
+ eCB-
02'-
H+
02*-
(9) H202could be formed from 02*via reactions 10-13:6a~16~18-20 HO2.
HOF
(10)
+0 2 HO2- + 0 2
HO2.
H202
HO2.
02'-
HOy
H+
(11) (12)
H202
(13) Cleavage of H202by any of the reactions 14-16 may yield O H radicals:7,15J8,19 H202
H202
+ eCB-
+ 02'-
-
.OH + OH*OH + OH- + 0
(14) 2
(15)
H202A 2.OH (16) The rate of reaction of the O H radical with phenol in homogeneous solutions is known to be very fast (Le., k = 1.4 X 1O1O M-' s-1).22*23On the assumption that the Ti02surface does not drastically change the rate of reaction, the rate-determining step may implicate the formation of OH radicals (see also ref 24-28). The source of irradiation is the only major difference between reactors I and 11; the major difference in their characteristics would be expected to show up in reaction 16. Since other wavelengths could reach the reaction mixture in reactor 11, formation of O H radicals via reaction 16 as an additional source of OH. is possible. We attempted to test this hypothesis by reducing the light intensity, thereby diminishing the contribution of reaction 16 to the reactions 6-15, and eliminating the source of the deviations in the In [Ph] versus time plots of Figure 6. Unfortunately, the light intensity in reactor I1 could be reduced by only 50% with our setup. No major changes were noted in the In [Ph] versus time plot a t 50% reduction. However, the apparent degradation rate constant k' decreased by 34% (from 0.0236 to 0.0156 m i d ) and the corresponding increased by 52% under these irradiation conditions. Clearly, an understanding of the factors that govern the behavior of reactors I and I1 is required. Future work will address this question. The effect of flow rate on the percent degradation of Ph in the outlet stream of reactor I in the single-pass mode (gravity feed) is shown in Figure 7 . It is evident that more degradation is observed at the lower flow rates. This observation is not, as it would first appear, in conflict with the results from the multipass (22) Adams, G. E.; Michael, B. D. Trans. Faraday SOC.1967.63, 1171. (23) Land, E. J.; Ebert, M. Trans. Faraday SOC.1967, 63, 1181.
(24) Our presently available results do not preclude direct reaction of a surface-adsorbed substrate with the positive valence-band holes of the semiconductor to yield, initially, some cation radical. This would certainly be the expectation if the redox chemistry in the presence of an illuminated semiconductor were carried out in a neutral (inert) solvent such as CHJCN.2S However, in a solvent like water, a totally different redox chemistry might reasonably be expectedz6as demonstrated by the complete mineralization of halocarbons in aqueous media.z7 In nonaqueous solutions, these halocarbons are not mineralized.28 (25) Fox, M. A,; Chen, C. C. J . Am. Chem. SOC.1981, 103, 6757. (26) Fox, M. A. In Photocatalysis;Serpone, N . , Pelizzetti, E., Eds.; Wiley: New York, to be submitted for publication. (27) Ollis, D. F. In Homogeneous and Heterogeneous Photocatalysis; Pelizzetti, E., Serpone, N . , Eds.; Reidel: Dordrecht, The Netherlands, 1986; p 651. (28) Barbeni, M.; Pramauro, E.; Pelizzetti, E.; Borgarello, E.; Gratzel, M.; Serpone, N . Nouu.J . Chim. 1984, 8, 547.
J. Phys. Chem. 1988, 92, 5731-5738 mode (Figures 4-6), where the higher flow rates led to more degradation, but is consistent with it. At the lower flow rates in a single-pass mode, the residence time of the Ph molecules at or near the Ti02 surface is longer. For a flow rate of 5 mL-min-I, the Ph residence time is ca. 15 min; 26% of the Ph is photodegraded. By contrast, irradiation of a stationary Ph solution for 15 min in the same reactor (nonflow) leads to 38% degradation.
Conclusions The photocatalytic degradation of Ph, 4-CP, 2,4-DCP, and 2,4,5-TCP over TiO, supported on a glass matrix and carried out in three different photochemical reactors has been demonstrated. The degradation follows approximate first-order kinetics for several half-lives. The results are understood in terms of modified (for solid-liquid reactions) Langmuir-Hinshelwood kinetics. The data
5731
are consistent with the degradation reaction occurrirg on the semiconductor surface. The future challenge will be to improve on the photoreactors to achieve more efficient degradation in shorter times with simulated sunlight. The factors and the optimum conditions under which a practical device can be tested in the disposal of waste components remain to be established. Acknowledgment. This work has benefited from support by the Natural Sciences and Engineering Research Council of Canada. We are grateful to the NATO Scientific Affairs Division for an exchange grant (No. 843/84) and to Prof. E. Pelizzetti for discussions. Registry No. Ph, 108-95-2; 4-CP, 106-48-9; 2,4-DCP, 120-83-2; 2,4,5-TCP, 95-95-4; TiOz, 13463-67-7; COz, 124-38-9; HC1,7647-01-0; HzO,7732-18-5.
Catalytic Etchlng of Platinum Folk and Thin Films in Hydrogen-Oxygen Mixtures Victor W. Dean: Michael Frenklach,' and Jonathan Departments of Chemical Engineering and Materials Science and Engineering, The Pennsylvania State University, 133 Fenske Laboratory, University Park, Pennsylvania 16802 (Received: October 30, 1987; In Final Form: March 16, 1988)
The catalytic etching of platinum thin films and foils during hydrogen oxidation was studied in an impinging flow reactor. It was found that when the surface temperature was greater than 725 K and the initial O:H ratio was greater than 2:1, metal was transported away from the reaction focus and concomitantly large platinum particles (1 pm) were found downstream on top of the original structure (catalytic etching). Under all other conditions (single gas, vacuum, O:H ratios less than 2:1, temperature less than 725 K) all changes could clearly be attributed to thermally accelerated movement toward equilibrium (thermal etching). Catalytic etching is attributed to the interaction of homogeneously formed radicals, most likely HO,, with the platinum surface, and the subsequent evaporation of metastable platinum-containing species. Furthermore, it is suggested that the metastable species interact in the gas phase via a homogeneous nucleation process to produce the large platinum single crystals which are observed downstream from the reaction focus.
Introduction Catalytic etching in ethylene-oxygen mixtures of platinum foils' and thin films2 and the sintering of silica-supported platinum particles under the same conditions were recently s t ~ d i e d . ~It was shown that the etching observed could not be attributed to the sum of the etching effects of the products and/or reactant gases. In particular it was found that treatment in oxygen alone over a wide range of temperatures did not result in morphological changes in foils or thin films that were different than those measured in any other individual gas. Thus, it seemed unlikely, despite prior that the observed changes could be attributed to the effect of oxygen. It was proposed that under reaction conditions radical species which form homogeneously react with the platinum surface. This results in the formation of volatile (gas-phase), metastable, intermediates. The intermediates decompose at some other location leading to massive amounts of metal transport and, consequently, gross surface reconstruction. Probably most difficult to determine with certainty is the identity of the active radical. However, computer modeling of the ethylene oxidation reaction, using the mechanism of Harris et al.,' indicates that the H 0 2 radical is the dominant radical present under those conditions of temperature, pressure, and initial gas phase mixture during which etching was found to take place in the previous studies.'*2 In the present work etching of platinum foils and thin films during hydrogen oxidation was carried out in order to test the above model. Hydrogen oxidation was chosen f Department of Chemical Engineering. $Materials Science and Engineering Department. *To whom correspondence should be addressed.
0022-3654/88/2092-5731$01.50/0
because it is generally understood that the H02radical is dominant at low temperatures for this reaction as ell.^-'^ Moreover, the number of radicals present in both reaction processes is very small. Thus, if etching occurs in a morphologically similar fashion in both cases, it would suggest that one of this small set of radicals is responsible. It was shown in this study that the model of catalytic etching developed previously is consistent with the etching observed in hydrogen-oxygen mixtures. Also, some additions have been made to the general radical-etching model presented in earlier studies. In particular, it is suggested that the platinum particles found (1) Wu, N. L.; Phillips, J. J . Phys. Chem. 1985, 89, 591. (2) Wu, N. L.; Phillips, J. J . Appl. Phys. 1986, 59, 769. (3) Wu, N. L.; Phillips, J., accepted for publication in J. C a r d . (4) McCabe, R. W.; Pignet, T.; Schmidt, L. D. J . Catal. 1974, 32, 114. (5) Flytzani-Stephanopoulos,M.; Schmidt, L. D. Chem. Eng. Sci. 1979, 34, 365. (6) Flytzani-Stephanopoulos,M.; Schmidt, L. D. Prog. Surf. Sci. 1979, 9, 83. (7) Harris, S.J.; Weiner, A. M.; Blint, R. J. Combusr. Flame, in press. ( 8 ) Dougherty, E.; Rabitz, H. J . Chem. P h p . 1980, 72, 6571. (9) Westbrook, C. K.; Dryer, F. L.; Schug, K. P. Symp. (Inr.) Combusr. [Proc.] 1982, 19rh, 153. (10) Warnatz, J. In Combustion Chemistry; Gardiner Jr., W. C., Ed.; 1984; Chapter 5. (1 1) Chinnick, K. J.; Gibson, C.; Griffiths, J. F. Proc. R . Soc. London, A 1986, 405, 129. (12) Chinnick, D. J.; Griffiths, J. F. J. Chem. Soc.,Faraday Trans. 2 1986, 82 --,881
(13) Chinnick, K. J.; Gibson, C.; Griffiths, J. F.; Kordylewski, W. Proc. R . Soc. London, A 1986, 405, 117. (14) Baulch, D. L.; Drysdale, D. D.; Horne, D. G.; Lloyd, A. C. Eualuared Kinetic Data for High Temperature Reactions; Buttenvorths: London, 1972; VOl. 1.
0 1988 American Chemical Society
