Environ. Sci. Technol. 1994, 28, 1661-1668
Products, Intermediates, Mass Balances, and Reaction Pathways for the Oxidation of Trichloroethylene in Air via Heterogeneous Photocatalysis William A. Jacoby,' Mark R. Nimlos, and Daniel M. Blake National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401
Rlchard D. Noble and Carl A. Koval University of Colorado, Boulder, Colorado 80302 ~~~
~~
Studies of the photocatalytic reaction of a solution of trichloroethylene in the air and in contact with UVirradiated titanium dioxide have produced conflicting reports in regard to the compositionof the product mixture. This paper resolves these discrepancies by reporting the results of experiments designed to identify and quantify intermediates, products, and reaction pathways. Mass balances are closed in differential and integral modes to ascertain the effects of factors such as the extent of conversion, feed composition, and photon energy on the composition of the product stream. Dichloroacetyl chloride, phosgene, carbon dioxide, carbon monoxide, and hydrogen chloride were observed in the effluent of photocatalytic reactors featuring thin films of titanium dioxide catalyst. These observations were made with a gas-phase Fourier transform infrared spectrometer. The instrument directly samples the effluent from the reactor without splitting or dilution. A direct sampling molecular beam mass spectrometer used in a parallel study has also identified molecular chlorine as a component of the effluent.
Introduction Traditional approaches to pollution remediation often involve the concentration, separation, or immobilization of the contaminants, usually accomplished through phase change. The primary drawback of these techniques is that the absolute toxicity of the pollutant is not diminished. Trichloroethylene (TCE) in air may be decomposed via gas-solid heterogeneous photocatalysis. This technique has many of the characteristics of the ideal remediation process, which would efficiently achieve the complete decomposition of dilute environmental toxins into less mephitic constituents at near-ambient temperatures. The objective of this paper is to report the results of an investigation into the catalytic selectivity of a UVirradiated titanium dioxide (TiO2) photocatalyst for the reaction of TCE in air. Raupp and his co-workers (1-6) performed the initial research in this area. They observed the destruction of TCE in the air in contact with irradiated Ti02 at ambient pressure and temperature with the efficient use of photons. They reported the gas-phase products carbon dioxide (Cod and hydrogen chloride (HC1) and concluded that the reaction goes to completion (i.e., complete mineralization without products of incomplete oxidation) according to the stoichiometry proposed by Pruden and Ollis for the aqueous-phase system (7). Other researchers (8-10) used a sol-gel process to make highly porous Ti02 pellets. This material was placed in
* Author to whom correspondence should be addressed: INTERNET: jacoby @ tcplink.nrel.gov. 0013-936X/94/0928-1661$04.50/0
0 1994 American Chemical Society
an externally illuminated packed bed, and no gas-phase products were reported other than C02 and HC1. Aspecies postulated to be chloroacetate was also observed on the catalyst surface via diffuse reflectance Fourier transform infrared spectrometry (FTIR). In our studies using direct sampling analytical methods (11-14), we have observed a number of compounds in the effluent of a photocatalytic reactor fed with a mixture of TCE in air. These include dichloroacetyl chloride (DCAC), phosgene (COC12),carbon monoxide (CO),and molecular chlorine (CW,in addition to C02 and HC1. These products have been confirmed by other researchers (15, 16). This paper extends our earlier efforts by resolving the discrepancies pertaining to products and intermediates reported in the above-referenced work. We have performed a series of experiments designed to quantify the intermediate and products, to provide carbon and chlorine mass balances, to directly study the reaction of the intermediate, and to identify factors that can affect the composition of the product mix. All of these results are interpreted within the context of a set of proposed reaction pathways.
Experimental Section The experimental apparatus has been discussed in previous publications (11, 13, 14). The photocatalytic reactors used during the course of this investigation had an annular geometry. An annular reactor consists of a glass tube coated on the inner surface with a Ti02 photocatalyst. Inside the glass tube is a cylindrical UV light source, which also serves as the inner surface of the annulus. The gas flows through the annular region. Hydraulic diameter is the crucial factor effecting the mass transport of reactants to the catalyst surface. Hydraulic diameter is defined as the inside diameter of the glass tube minus the outside diameter of the lamp, or twice the distance between the light source and the catalyst film. The application of a thin, uniform Ti02 coating on the inside of the glass reactor tubes was essential. The Pyrex surface to be coated was first etched with a 5 M sodium hydroxide solution at 100 OC. An aqueous suspension containing 5% Degussa P-25 (anatase) Ti02 was used to introduce the catalyst to the glass support. The coated reactor was then baked for 1 h at 150 "C. After poorly adhered catalyst particles were rinsed off with distilled water, the sequence was repeated until a smooth and opaque (to UV) coating was achieved (17). Two values for reaction rates are reported in Tables 1 and 2. The areabased rate, denoted by the subscript A, is computed using the geometricarea of the opaque catalyst film. The weightbased rate, denoted by the subscript W, is computed using the weight of the catalyst film. A third basis for the rate Environ. Sci. Technol., Vol. 26, No. 9, 1994
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Table 1. Representative TCE Data chlofeed feed prod. prod. prod. prod. prod. prod. quan- carbon rine data hydr. flow res. (mTorr (mTorr (mTorr (mTorr (mTorr (mTorr (mTorr (mTorr conv. rateA ratew tum bal- balin- dia. (mLi time of of of of of of of of TCE (pmol/ (pmoli yield ance ance CO) HCl) ( % ) gs-l) gs-l) ( % ) (%) (%) dex (mm) min) (ms) TCE) HzO) TCE) DCAC) COC12) COz) 1 2 3 4 5 6 7 8 9 10 11 12 13 14
7.3 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9
887 3126 156 231 1225 29 4484 8.0 5.8 6253 8700 4.1 8817 4.1 8789 4.0 4.1 8907 8898 4.1 8825 4.1 4.1 8825 8867 4.1 8742 3.0
55 52 52 55 55 56 56 55 54 54 54 54 55 197"
0 0 12 34 40 44 45 45 45 45 46 47 49 186
0 604 604 599 600 602 604 1540 1522 3797 3827 3827 7629 748
3 0 13 9 8 6 6 5 4 4 3 3 3 6
42 40 22 8 6 4 4 4 4 4 3 3 3 4
38 39 15 6 5 4 4 4 4 4 4 4 4 3
20 21 11 5 3 2 2 2 2 2 2 2 2 2
29 23 8 3 2 1 1 0 0 0 0 0 0 1
100 100 75 36 27 20 19 18 17 16 14 13 12 5
2.9 5.0 29.3 55.5 60.2 62.1 61.0 55.3 53.4 49.5 44.2 40.4 39.8 82.4
0.2 1.7 9.9 18.8 20.4 21.0 20.7 18.7 18.1 16.7 15.0 13.7 13.5 12.3
3 3 19 35 38 39 39 35 34 31 28 26 25 52
97 96 96 98 100 99 100 101 100 100 100 100 102 100"
74 66 83 92 95 96 96 96 96 96 96 97 98 99
TCE feed partial pressure calculated from product partial pressures constrains carbon balance to 100%. Table 2. Representative DCAC Data
data hydr. flow india. (mL/ dex (mm) min)
res. time (ms)
7.3 7.3 7.3 0.9 7.3 0.9 7.3 7.3 0.9 7.3 0.9 0.9 0.9
1445 1445 1551 53 1445 52 1439 1454 13.3 1436 13.3 13.5 7.3
1 2 3 4 5 6
7 8 9 10 11 12 13
1797 1797 1675 1159 1797 1167 1805 1786 4569 1809 4571 4528 8546
prod. feed" feed prod. prod. prod. prod. (mTorr (mTorr (mTorr (mTorr (mTorr (mTorr (mTorr conv. rateA DCAC bmol/ of of of of of of of m2 9-1) DCAC) HzO) DCAC) COC12) c o d HCl) (%) CO) 59 50 48 34 46 32 47 50 19 53 32 53 50
55 51 53 275 38 200 25 3 1288 0 54 963 157
13 15 16 14 21 16 26 30 13 36 26 46 45
24 20 18 10 14 8 12 11 2 9 3 3 3
49 34 32 21 24 16 20 19 8 17 7 8 6
19 15 15 10 12 8 10 10 3 9 3 3 2
58 54 53 37 47 32 41 40 7 35 11 9 10
78 70 67 59 55 50 45 40 33 33 20 14 11
5.5 4.1 3.8 9.0 3.0 7.0 2.5 2.4 11.2 2.1 11.1 12.4 17.3
chloquan- rine ratew tum bal(pmoli yield ance g 9-1)
(%)
(%)
0.24 0.18 0.17 1.9 0.13 1.5 0.11 0.11 2.3 0.09 2.3 2.6 7.2
5 4 3 6 3 4
83 93 95 96 100 100 101 101 85 100 97 96 100
2 2 7 2 7 8 11
DCAC feed partial pressure calculated from product partial pressures constrains carbon balance to 100%.
of the reaction may be obtained by multiplying by the surface area of the Degussa P-25 TiO2, which is 50 m2/g. The Ti02 catalyst can be excited by photons with wavelengths shorter than 385 nm. The most frequently used photon source in this investigation was an 8-W fluorescent black light (Sylvania FBT5/BLB) with a spectral maximum at 356 nm. The intensity of the UV radiation on the catalyst surface in the annular photocatalytic reactors varied inversely with hydraulic diameter between 3.5 and 5.3 mW/cm2. This was determined with a Blak-Ray ultraviolet radiometer (Model 5-221), which is specifically designed to measure the near-UV output from fluorescent black lights. A low pressure mercury arc germicidal lamp (Sylvania G8T5) with a spectral maximum at 254 nm was also used to ascertain the effect of higher energy photons. A Nicolet 8220 FTIR is plumbed downstream of the reactors, although it can also be fed directly from the injector assembly for feed gas analysis. It collects spectra with a 2 wave number resolution and has a multipass sample cell with a 9.85-m path length. Beer's law works well in dilute gas mixtures, although some deviations from linearity were observed for CO and HC1 leading to a higher measurement uncertainty for these compounds (14).An optimum system for quantitative analysis of these diatomic 1662 Environ. Sci. Technol., Vol. 28, No. 9, 1994
moleculeswould have higher resolution (19). Additionally, during experimentation and calibration of the FTIR, adsorption of HC1 vapor throughout the flow path and analytical chamber caused difficulties in achieving a steady-state reading for this compound. This error was exacerbated by the presence of water in the feed gas. Despite these challenges, appropriate calibration and classical least squares analysis of overlapping peaks provided excellent mass balances; typically 100 ?d f 4 5% of all carbon atoms are accounted for a t all extents of conversion (see Table 1). The FTIR was calibrated at a standard operating pressure of 67 kPa (500 Torr), and experiments were carried out a t room temperature, unless otherwise noted. NBS traceable gas standards (Matheson) were used to calibrate the FTIR for dilute mixtures of TCE, C02, CO, HC1, and COC12. Standards for other organic and chlorinated organic liquids, such as DCAC, were created by injecting the neat liquid directly into the evacuated analytical chamber. The pressure increase upon injection was measured and then compared with the pressure increase predicted by the ideal gas law to verify complete vaporization and accurate quantification. The FTIR's analytical chamber was then back-filled to appropriate analysis pressure with high-purity nitrogen.
Carbon Dioxide
Air and Moisture UV/Ti02
Air (Direct Oxidation Pathway)
UVITi02
Carbon Monoxide
-
1
Molecular Chlorine
Figure 1. Proposed reaction pathways for heterogeneous photocatalytic oxidation of TCE.
All analytical results qualitatively agree with mass spectrometry results, which also identified Clz during the photocatalytic oxidation of TCE (13). 50
Results and Discussion Two heterogenous photocatalytic reaction pathways for the oxidation of TCE in air are proposed. One pathway proceeds through the DCAC intermediate while the second pathway features direct oxidation of the TCE to (photocatalytically) stable products. We have observed that the proportions of observed products in the effluent from a photocatalytic reactor change with water vapor partial pressure. We propose that this change is associated with the reaction of the DCAC intermediate. Figure 1illustrates these hypotheses, and supporting evidence is presented below. All of the reactions shown in Figure 1are represented as requiring UV light and TiOz. Initial experiments established that both UV light and Ti02 must be present for the oxidation of TCE to occur. Similarly, oxidation of a solution of DCAC in air required both photons of appropriate energy and catalyst. The working hypothesis for the investigation was that the entire process occurs on the catalyst surface. A series of experiments was performed to test this hypothesis. Chlorine atoms propagated through a chain reaction (13)and hydroxyl radicals (19)have been proposed as the oxidative species in heterogeneousphotocatalysis mediated by TiOz. If this process is propagated homogeneously, one would expect the effective scavenging of gas-phase chlorine atoms and/or hydroxyl radicals to dramatically decrease the reaction rate. Comparison of gas-phase rate constants for the reactions of TCE, ethane (CZHe), and carbon tetrachloride (CCl4) with chlorine atom and hydroxyl radical reveals that C2H6, if present in sufficient excess, would effectively suppress gas-phase reactions involving both chlorine atom and hydroxyl radical while CC14 would not (20). Neither CZH6 nor CCL were reactive as single-componentmixtures with air in the photocatalytic reactor, implying that dissociative adsorption is not an operative mechanism. Since molecular adsorption is unlikely, one may conclude that these species are not strongly interacting with the Ti02 surface (21). A photocatalytic reactor with a hydraulic diameter of 0.9 mm and a residence time of about 3 ms was used to compare the rate of TCE destruction for the following feed solutions (with air as the solvent): a single-component mixture of TCE, a dual-component mixture of CzH6 and TCE at a molar ratio of about l O : l , and a dual-component mixture of C c 4 and TCE at a molar ratio of about 1O:l.
45
40
Rote p m o 1/ m 2 / s
30
25
5
0
5
0
61 mtarr TCE
131 m t a r r C 2 i 6 F e e d Conccsltizn
Flgure 2. TCE destruction rate as a function of feed composition: single-component feed, dual-component feed with CPHB,and dualcomponent feed with CCI4.
Figure 2 shows that neither CzH6 nor CC14significantly inhibited the reaction rate of TCE. These data indicate that a gas-phase chlorine radical chain reaction is not the operative mechanism. Further, it is unlikely that gasphase hydroxyl radicals are responsible for homogeneous reactions. A chain reaction propagated on the catalyst surface remains a possibility since quantum yields in excess of unity for the TCE reaction have been reported (6,22). Thus, it appears that the reaction is purely heterogeneous. The pathways proposed in Figure 1 dictate that, in addition to TCE and DCAC, the following compounds are found in the effluent of the photocatalytic reactor fed with a mixture of TCE and air: COC12, COZ,CO, Clz, and HC1. An FTIR spectra of a photocatalytic effluent showing TCE, DCAC, and COClB has been previously published (12-14). Figure 3 documents the presence of COz, CO, and HC1 by comparing the spectra of single-componentstandards with a representative product spectra. Molecular beam mass spectrometry (MBMS), also operated in the direct sampling mode, has been used in a parallel study to investigate the photocatalytic oxidation of TCE and other compounds. Nimlos et al. (12,13) used thin-film annular photocatalytic reactors identical to those used in this investigation as well as reactors featuring Ti02 catalyst supported on foamed alumina frits and on Environ. Sci. Technol., Voi. 28, No. 9, 1994
11383
feed partial pressure is increased, up to 85% of the carbon atoms from the reacted TCE molecules have been measured as DCAC molecules in the product mixture (see entry 14 in Table 1).This establishes that the reaction pathway through the DCAC intermediate is significant during the photocatalytic oxidation of TCE. The other products of the TCE reaction (COC12, COZ, CO, Clz, and HC1) are stable in the photocatalytic reactor on the millisecond time scale. Data at residence times of up to 1min are presented in Figure 5. The abscissa is the carbon atom product fraction, the fraction of the carbon atoms from reacted TCE molecules that become incorporated into a particular carbon-containing product molecule. CoClz is converted to COz (and HC1) at long residence times in the photocatalytic reactor, and the CO fraction remains essentially constant. This is due to the non-photo-heterogeneous hydrolysis reaction that is discussed below. Representative product mixtures from the photocatalytic oxidation of TCE in air a t various extents of conversion are shown in Table 1. Differential conditions were achieved in a photocatalytic reactor with a hydraulic diameter of 0.9 mm operating with a residence time of about 4 ms. The discrepancy between carbon and chlorine balances shown in Table 1 supports the theory that the formation of molecular chlorine gas is occurring (13), although adsorbed chlorine containing intermediates (8) and/or products (e.g., HC1) would also contribute to an inferior chlorine balance. Integral data in Table 1were taken a t lower flow rates using a photocatalytic reactor with a 7.3-mm hydraulic diameter, providing a residence time of up to 3 s. Complete conversion of the TCE was observed, although a trace of the DCAC intermediate appeared in the effluent gas. Using a photocatalytic reactor with a 0.9-mm hydraulic diameter, complete conversion of TCE was achieved during a residence time of 75 ms, and the DCAC intermediate had been destroyed in 230 ms (see Figure 4 and Table 1).This
7 ,L ~ $ 0 0
so00
asoo
PBOO
8700 "2
-
moo o=
iaoo
e4.x
Em0
=zoo
*loo
Loo
II
Figure 3. Standard spectra compared with product spectra.
fiberglass mesh. Fluorescent black lights, an argon ion laser, and natural sunlight provided photons. In all permutations, the MBMS confirmed and complemented the observations made with the FTIR. Additionally, Clz, which does not absorb infrared radiation, was observed. Figure 1 shows a reaction pathway through a DCAC intermediate. The relationship between residence time in the photocatalytic reactor and the partial pressures of the reactant, intermediate, and products in the effluent is shown in Figure 4. During the experiment, a nominal feed of 55 mTorr of TCE (1mTorr = 0.133 Pa) and 600 mTorr of water in solution with 500 Torr of air was fed at various flow rates into a photocatalytic reactor with a hydraulic diameter of 0.9 mm. Complete conversion of TCE is achieved in 75 ms. DCAC is a major product at short residence times (e100 ms), but its signal disappears at longer residence times (>240 ms). This behavior is characteristic of an intermediate; DCAC forms during the destruction of TCE but is not stable in the photocatalytic reactor. When TCE BO-
coc 1, I
_-.-
-
_,_.-.-
_.__.----
-_./
-.-
OCAC
0
sc
150
100
zoo
250
Residence Time (715)
Figure 4. Product partial pressures from the photocatalytic destruction of TCE as a function of residence time: short residence times. 1664
Environ. Sci. Technol., Vol. 28, No. 9,1994
c.=l-= c o t 12
0. 4
Carbon Atom Product F r a c t 1 on
O . 1
......................... & " " "
co ....................................
........................... A
"'t Residence T i m e (S)
Figure 5. Product carbon atom fractions from the photocatalytic destruction of TCE as a function of residence time: long residence tlme.
highlights the importance of hydraulic diameter and its effect on mass transport. Again, CO2, COC12, CO, and HC1are formed in significant quantities, and the formation of C12is proposed as a possible explanation for an inferior chlorine balance. DCAC is formed from TCE and is subsequently destroyed in a photocatalytic reactor. Therefore, knowledge of the photocatalytic oxidation of DCAC is necessary to understand the TCE reaction. The product solutions from a DCAC feed to photocatalytic reactors operating under differential and integral conditions are shown in Table 2. Although the same products were identified by FTIR from the reaction of DCAC as were identified from the reaction of TCE, a comparison of Tables 1and 2 reveals the difference in product proportions under integral conditions. During the reaction of TCE, about two C02 molecules and two COClz molecules are formed for each CO molecule, while COClz and CO are formed in approximately equimolar amounts from DCAC. This provides evidence of an additional photocatalytic reaction pathway for TCE other than the pathway through the DCAC intermediate, Le., the direct oxidation pathway shown in Figure 1. Another striking difference exists in the proportion of HC1in the product mixtures. More HC1 is detected during the reaction of DCAC than during the reaction of TCE. The change in the composition of the feed gas (from TCE to DCAC) effects the adsorption equilibria among the reactants and products, and previously adsorbed HC1 desorbs in the presence of a DCAC feed. An analysis of data taken when feeding photocatalytic reactors with 0.9- and 7.3-mm hydraulic diameters with mixtures of DCAC in the air at various DCAC partial pressures, water vapor partial pressures, residence times, and extents of conversionrevealed that water vapor partial pressure was the principal factor effecting the composition of the product mix. Figure 6 shows ratios of products as
a function of the water vapor partial pressure. Two observations are apparent in this figure. First, as noted above, CO and COCl2 are formed in approximately equimolar amounts regardless of reaction conditions. Second, the ratio of CO2 to COCl2 is dependent on water vapor partial pressure. Under dry conditions, two COZmolecules are formed for each COClz molecule. As humidity increases, this ratio climbs to as high as 4. This change in product proportions as a function of water vapor partial pressure suggests the parallel reaction pathways for the DCAC intermediate shown in Figure 1. Experiments were run to determine the effect of water vapor partial pressure on the product mixture from the photocatalytic oxidation of TCE in air. Differential data were taken with feed mixtures containing a 55-mTorr partial pressure of TCE and between 0 and 3.5 Torr of water vapor. A photocatalytic reactor with a hydraulic diameter of 0.9 mm was operated at a residence time of about 3 ms. Figure 7 shows the results of these experiments. The primary effect is the reduction in DCAC carbon atom product fraction with increasing water vapor partial pressure. The destruction rate of TCE decreases with humidity since water vapor inhibits TCE adsorption, while adsorption of DCAC appears unaffected by the presence of moisture (14). Thus, surface-generatedDCAC competes more effectively with adsorbing TCE for available surface oxidative species, and the carbon atom fraction of DCAC in the reactor effluent decreases relative to the other products (14). The slopes of the lines fit to the data indicate that the COCl2 and CO fractions increase in equal proportions, while the C02 fraction increases at a greater rate. This is consistent with the discussion of the effect of water vapor partial pressure on the DCAC reaction presented above. The evolution of COClz and the other products shown in Figure 1 has been observed consistently during the oxidation of TCE via gas-solid heterogeneous photocaEnviron. Sci. Technol., Vol. 28, No. 9, 1994 1665
U
O iI
1GO
IO
Water
Vapor P o r t i a ! (rntorr)
1000 ~
-000
Pressare
Flgure 6. Product ratios from the photocatalytic destruction of DCAC. 0.6~...
DCAC
0.5-
Carbon Atom
Fract 1 o n
o.o! G
2000
1000
3000
WATER VAPOR P A R T I A L PRESSURE (rntorr)
Flgure 7. Carbon atom product fractions of the photocatalyticdestruction of TCE under differential conditions as a function of water vapor partial pressure. The uncertainty in the curve fits is represented by the 95% confidence intervals (dashed curves).
talysis in our laboratory. Other investigators, notably those in Anderson’s group at the University of Wisconsin (8IO), have reported conditions under which COClz was not observed. This discrepancy is resolved by the series of experiments described here. An annular photocatalytic reactor was packed with Ti02 pellets prepared via a sol-gel technique by Professor Anderson and operated a t 68 “C, according to Anderson’s procedure (9, IO). Figure 8 shows TCE conversion and the carbon atom product fractions of COClz and DCAC 1666
Environ. Sci. Technoi.. Vol. 28, No. 9,1994
as a function of residence time. While both COClz and DCAC are formed a t short residence times, they are destroyed at longer residence times. This is consistent with Anderson’s results, which were taken a t a residence time on the order of 500 ms (9, IO). COZ,CO, and HC1 were also observed in the reactor effluent. The experiment was repeated at ambient temperature with similar results. Reactor design is the key to the discrepancy between our observations and Anderson’s findings. The essential difference is that our annular photocatalytic reactors
0
I
/TCE
0
Conversion
j
Carbon
+.*
P A troo md u c t
Residence T i m e (ms)
Figure 8. Heterogeneous photocatalysis of TCE in a packed bed of TiOn pellets: TCE conversion (left abscissa) and COClp and DCAC product atom fractions (right abscissa) as a function of residence time.
feature a thin film of Ti02 catalyst coated on the glass tube that forms the outer surface of the annulus. Consequently, almost the entire flow volume of the reactor is void volume. A packed bed, by contrast, has a relatively low void volume. Complete and quantitative hydrolysis of COC12 to C02 and HC1 in a “dark”reactor, packed with activated carbon pellets downstream of a thin-film annular photoreactor, has been achieved in our laboratory (11). y-Alumina pellets also successfully catalyzed this dark hydrolysis. This suggested the series of experiments that generated the data presented in Figure 9. The first group of bars illustrates the carbon atom fractions of COC12, C02, and CO in the effluent of a thinfilm annular reactor. Despite long residence time (-5 s), COC12 is not destroyed via heterogeneous photocatalysis, though all of the DCAC has reacted. The second group of bars represents the effluent of a series of two reactors: the thintfilm annular photoreactor and a photoreactor packed with Anderson’s Ti02 crystals. The packed bed was not illuminated. The COCl2 in the effluent of the thin-film photoreactor underwent hydrolysis in the dark packed-bed reactor. (An increase in the HCl signal was also observed.) The third group of bars illustrates the effluent from the reactor packed with Anderson’s crystals under near-UV illumination. COCl2 is being produced photocatalytically, but is being simultaneously destroyed via a non-photoheterogeneous process. Anderson’s crystals have greater surface area (190 m2/g)and porosity (55 % ) relative to the nonporous Degussa P-25 crystals used to make the thinfilm reactors (8). The pellets loaded into the packed-bed reactor were about 1 mm in diameter. A reasonable hypothesis, therefore, is that the illuminated outer layer operated as a photocatalyst, while the dark interior of the pellet provided the active sites for the hydrolysis reaction.
The fourth group of bars represents the effluent from the thin-film photocatalytic reactor in series with a dark reactor packed with Ti02 pellets manufactured by Norton Chemicals. The Norton material also catalyzed the hydrolysis of COCl2 in the dark, though not as efficiently as the Anderson material. Figures 8 and 9 illustrate that the destruction of TCE and DCAC occurs via heterogeneous photocatalysis,while COCl2 hydrolysis proceeds in the dark in contact with a packed Ti02 bed (as well as a variety of other surfaces). The thin-film photocatalytic reactors, which do not have sufficient surface area to hydrolyze the phosgene during a 5-s residence time, effectively deconvolute the light and dark reactions, allowing the products of the photocatalytic reaction to be explicitly identified. Additionally, the constant level of CO implies that it is formed during the photocatalytic oxidation of TCE, not during the subsequent dark hydrolysis of COC12,and that the photocatalytic chemistry is similar in both the thin-film and packed-bed reactors. Another factor that might have an impact on the stoichiometry is the wavelength of the UV light. The band gap of Ti02 is 3.2 eV, and photons with wavelengthsshorter than 385 nm are required to excite the catalyst. The light sources for the photocatalytic reactors used in collecting the data reported thus far were fluorescent black lights that provide photons in the region of 320-400 nm with a primary peak at 356 nm. A germicidallamp with a primary spectral output at 254 nm was employed to ascertain the effect of higher energy photons. Changing the UV wavelength did not significantly effect the product mixture in differential or integral modes of operation (14). This is consistent with a purely heterogeneous mechanism; any photon exceeding the band gap excites the catalyst and generates an electron-hole pair. These species either participate in a chemical reaction or recombine. Environ. Sci. Technol., Vol. 28, No. 9, 1994 1667
;orton TJTCX
.:e
Flgure 9. Carbon atom product fractions from thin-film and packedbed reactors.
Acknowledgments The authors would like to thank Tom Milne, Kim Magrini Bair, John Falconer, Ron West, Mark Anderson, and Maura Jacoby for their contributions to this work. L i t e r a t u r e Cited (1) Dibble, L. A.; Raupp, G. B. J . Mol. Catal. 1992, 77, 297. (2) Dibble, L. A.; Raupp, G. B. Environ. Sei. Technol. 1992,26, 492. (3) Dibble, L. A.; Raupp, G. B. Catal. Lett. 1990, 4 , 345. (4) Dibble, L. A.; Raupp, G. B. Proceedings of the Arizona Hydrological Society 1st Annual Symposium; Sep 16-17, 1988, Phoenix; 1988; pp 221-229. (5) Dibble, L. A. Ph.D. Thesis, Arizona State University, 1989. (6) Raupp, G. B. Presentedat the First International Conference on Ti02 Photocatalytic Purification and Treatment of
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Water and Air. London, Ontario, Canada, Nov, 1992. (7) Pruden, A.; Ollis, D. F. J . Catal. 1983, 60, 404. (8) Anderson, M. A.; Yamazaki-Nishida, S.;Cervera-March, S. Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-Ekabi, H., Ed.; Elsevier Science Publishers: New York, 1993; pp 405-420. (9) Zeltner, W. A. Hill, C. G.; Anderson, M. A. Chemtech 1993, 27 (3), 21. (10) Yamazaki-Nishida,S.; Nagano,K. J.; Phillips, L. A.; CerveraMarch, S.;Anderson, M. A. J. Photochem. Photobiol. 1993, A70 (l),95. (11) Blake, D. M.; Jacoby, W. A.; Nimlos, M. R.; Noble, R. D. Proceedings of the Sixthlnternational Symposium on Solar Thermal Concentrating Technologies; Sep 28-Oct 2,1992, Mojacar, Spain; Ciemat: Madrid, 1993; pp 1223-1231. (12) Nimlos, M. R.; Jacoby, W. A.; Blake, D. M.; Milne, T. A. Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-Ekabi, H., Eds.; Elsevier Science Publishers: New York, 1993; pp 387-392. (13) Nimlos, M. R.; Jacoby, W. A,; Blake, D. M.; Milne, T. A. Environ. Sei. Technol. 1993, 27 (4), 732. (14) Jacoby, W. A. Ph.D. Thesis, University of Colorado, 1993. (15) Holden, W.; Marcellino, A.; Valic, D.; Weedon, A. C. Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-Ekabi, H., Eds.; Elsevier Science Publishers: New York, 1993; pp 393-404. (16) Kutsuna, S.; Ebihara, Y.; Nakamura, K.; Ibuski, T. Atmos. Environ. 1993, 27 (4), 599. (17) This method was developed based upon discussions with Dr. Kim Magrini Bair of the National Renewable Energy Laboratory, Golden, CO, 1991. (18) Hanst, P.; Hanst, S. Gas Analysis Manual for Analytical Chemists: V o l u m e I Infrared Measurements of Gases and Vapors; Infrared Analysis Incorporated: Anaheim, CA, 1990; p 26. (19) Teichner, S. J.; Formenti, M. Photoelectrochemistry, Photocatalysis and Photoreactors; Schiavello, M., Ed.; D. Reidel: Dordrecht, Holland, 1985; pp 457-489. (20) The National Institute of Standards and Technology Kinetics Data Bases, 1992. (21) Campbell, I. M. Catalysis at Surfaces; Chapman and Hall: New York, 1988; p 145. (22) Berman, E,; Dong, J. Chemical Oxidation Technologies for the Nineties: Eckenfelder, W., et al., Eds.; Technomic: Lancaster, PA, 1994; Vol. 3, pp 183-189.
Received for review January 13, 1994. Revised manuscript received M a y 10, 1994. Accepted M a y 20, 1994.@ @Abstractpublished in Advance ACS Abstracts, July 1, 1994.
