Environ. Scl. Technol. 1093, 27, 732-740
Direct Mass Spectrometric Studies of the Destruction of Hazardous Wastes. 2. Gas-Phase Photocatalytic Oxidation of Trichloroethylene over TiOs: Products and Mechanisms Mark R. Nlmlos,’ Wllllam A. Jacoby, Danlel M. Blake, and Thomas A. Mllne National Renewable Energy Laboratory, Golden, Colorado 80401
The gas-phase photocatalytic oxidation of trichloroethylene (TCE) over titanium dioxide was investigated as a potential method for destroying this common pollutant. The results from this study agree with earlier studies in that high levels of destruction of TCE were achieved. Accompanying these high rates of destruction were high quantum yields (approaching unity). However, directsampling mass spectrometry and gas-phase Fourier transform infrared (FTIR) spectroscopy revealed that there are significant quantities of byproducts produced [phosgene, dichloroacetyl chloride (DCAC), carbon monoxide, molecular chlorine]. The DCAC has been rationalized on the basis of a chemical reaction mechanism in which the TCE molecules are oxidized in a chain reaction involving C1 atoms. This mechanism appears to be validated by tests with other chlorinated ethylenes (perchloroethylene, dichloroethylenes). Phosgene may arise at least partially from the photocatalytic oxidation of DCAC, and molecular chlorine may result from the recombination of chlorine atoms. The results of this study are discussed relative to aqueous-phase photocatalytic oxidation of TCE where chlorinated intermediates have been observed. Introduction
slurries and identified a partially oxidized intermediate, dichloroacetaldehyde (DCAAD). Recent aqueous-phase studies of the reaction of TCE and perchloroethylene (PCE) have reported partial oxidation products (8) in a Ti02-photocatalytic system and a UV-peroxide system. Several interesting results were discovered in this work. In both the photocatalytic and the UV-peroxide systems, dichloroacetic acid (DCAA), DCAAD, and trichloroacetaldehyde (TCAAD) were detected from the reaction of TCE. Higher yields of intermediates were measured from PCE under reaction conditions similar to those used for TCE. The major byproduct was DCAA and a small amount of trichloroacetic acid (TCAA), but no acetaldehydes were seen. We report a study of the gas-phase, heterogeneous photocatalytic oxidation of TCE in reactor systems using a molecular beam mass spectrometer (MBMS) or a Fourier transform infrared (FTIR) spectrometer as the analytical tools. Using these “direct-sampling” instruments, we have measured the rates of oxidative destruction of TCE and have followed the formation of intermediates. Products formed from other chlorinated ethylenes (perchloroethylene, dichloroethylenes) are also reported. The data have been used to formulate a chemical mechanism for the photocatalytic oxidation.
Trichloroethylene (TCE) is the most common and abundant pollutant in groundwater in the United States (I), and there is currently a great deal of interest in developing processes which can destroy this compound. Many of the commonly used treatment technologies for TCE simply change the phase of the pollutant (such as adsorption on carbon) and do not destroy the TCE. A process being studied at the laboratory scale for the removal of TCE involves gas-phase photocatalysis over the semiconductor Ti02 in the presence of sunlight. This semiconductor is stable and inexpensive, and it has an absorption spectrum (2)which overlaps the solar spectrum (3). Though the gas-phase photocatalytic oxidation of TCE is interesting in its own right, it also provides an opportunity to help elucidate chemical mechanisms that may be important in aqueous-phase photocatalytic oxidation. In the gas phase, chemical reactions will be free of the effects of solvent molecules and studies in the gas phase may allow a determination of the fundamental reaction mechanisms. In addition, the detection of reactive intermediates may be easier in the gas phase. The gas-phase photocatalytic oxidation of TCE was first studied by Dibble and Raupp (4-6). These authors reported high rates of reaction for the photocatalytic oxidation of TCE and no byproducts. The photocatalytic oxidation of TCE in aqueous Ti02 slurries has also been studied because this process may have the potential to decontaminate polluted groundwater. Pruden and Ollis (7) measured the rate of removal of TCE from aqueous
The experimental equipment used in a large portion of this study consists of a single-pass, catalytic reactor, an argon ion laser (Coherent, Inova 200-20/4, Palo Alto, CA) €or a light source (emission lines at roughly 330 and 360 nm), and a molecular beam mass spectrometer sampling system. Figure 1shows a schematic representation of the reactor, where the catalyst was supported upon a foamed alumina frit (Hi-Tech Ceramics, Alfred, NY) and the laser beam was expanded so as to illuminate the entire frit. The entire reactor was made from pyrex glass to prevent corrosion and surface reaction of intermediates. The MBMS (Figure 2) is an instrument that can be used to sample products from ambient pressure reactors. A more detailed discussion of the MBMS can be found in earlier publications (9, IO), and only a brief description will be provided here. Ambient pressure gases pass through a small sampling orifice (shown on the left-hand side of the figure) into a vacuum chamber held at roughly 50 mTorr by a molecular drag pump (OsakaVacuum, Osaka, Japan). The gas plume from this expansion is skimmed downstream, and the resulting molecular beam is sent into a collinear, triplequadrupole mass spectrometer (Extrel, Pittsburgh, PA). At the front of the mass spectrometer, the molecular beam passes through an electron impact ionization source (The electron energy was 25 eV.) where positive ions are formed, extracted, and focused into the first quadrupole. During normal operation, this quadrupole serves as the
732
0013-936X/93/0927-0732$04.00/0
Environ. Sci. Technol., Vol. 27, No. 4, 1993
Experimental Section
@ 1993 American Chemical Society
:I
Frit with TiOz
Tubing Clamp
\ He Flow
Window Sweep)+
-
1
1
t
He Flow (Make-up)
ExpandedLaserBeam
Schematic representation of the laser-frit reactor. The beam from the argon ion laser is expanded to fill ihe entire frit with Flgure 1. UV light.
mass filter and the second and third quadrupoles are set to pass all ions to a conversion dynode detector. A PCbase data system (Teknivent, St. Louis, MO) is used to control the mass spectrometer and collect the ion signals. The triple-quadrupole was used to help identify products through the use of collisionally induced dissociation (CID) mass spectrometry. In these experiments acollision gas was introduced into the second chamber, and ions selected by the first quadrupole (Ql) were fragmented. The fragments were analyzed by the third quadrupole (Q3). The experiments were conducted in the “parent ion” mode in which the third quadrupole is fixed and the first quadrupole scans the nascent ions that produce the fragment seen by the third quadrupole. In experiments with the laser-frit reactor, gases containing the compound of interest are passed through the frit and products are sampled at the orifice of the MBMS. Technical grade helium flows through an injector where TCE is introduced using a calibrated syringe pump. The He-TCE mixture is mixed with technical grade oxygen that has been saturated with water vapor. The concentration of water is -6000 ppmv. The flow rates of the oxygen and helium are set so that the oxygen is -20% of the total. The mixture of helium, oxygen,water, and TCE then passes through theTi0z-coatedfrit, and the products are sampled by the MBMS orifice. In these experiments, the gas flow runs countercurrent to the photon flux. As Figure 1 shows a small flow of technical grade helium is added downstream from the catalyst to ensure that the window is kept clean. Further downstream a make-up flow of technical grade helium is added to satisfy the pumping requirements of the MBMS sampling orifice. The product gases are heated to -200 “C before the sampling orifice, which also increases the sensitivity of the MBMS. A small flow of argon gas is added to the make-up helium which serves as a tracer speciesto monitor fluctuations in the MBMS sensitivity. During typical experiments, the ion signals for TCE are directly compared with and without ultraviolet light. As
a baseline, the instrument collects mass spectra without any ‘WE. Next, the TCE is introduced and t heTCE signal is allowed to come to a steady state as determined by observing the rn z 130 ion signal. The laser is turned on, and the product signals are allowed to come to a steadystate value. By making a direct comparison between the steady-state levels with and without the light, a number of systematicerrors can he reduced. However,quantifying product concentrations with the MBMS is moredifficult. l h e photocatalyst used in experiments with the laser was prepared by Hi-Tech Ceramics and consisted of a reticulated alumina foam (927 a-alumina-10 pores in.) coated with Degussa P-25 titanium dioxide. The open structure ofthis monolithicsupport (averagepore diameter is 1.5 mm) is such that light easily penetrates deep into the catalyst support. This, in effect, increased the illuminated surfacearea relative toa flat surfaceofcatalyst and allowed longer contact time. The thickness of the catalyst (1.2cmJ waschosen toensure that the fullvolume of the monolith was illuminated. The diameter of the frit was 6.3 cm, the gas volume in the frit was 32 mL, and the total weight of the Ti02 was 490 mg. To obtain quantitative results for the formation of products, we have used a gas-phase FTIR spectrometer (Nicolet 8220, Madison, WI) to directly analyze the gases Since thelRahsorptionsignal isdeterminedonly by theconcentrationofthegasesunder the range of conditions used, the instrument needs to be calibrated only once at a given pressure. Furthermore, library spectra available from Nicolet can be used to determine concentrations of products for which there are no ready standards. Figure 3 shows a schematic representation of the experimentalapparatus for the FTIR measurements. The TCE and water were introduced into theevacuated injector (a high-pressure, stainless cylinder), which was then backfilled with air to 100 psig. The TCE-containing gas (at 500 Torr pressure) was then passed through a pyrex reactor, which had been partially coated with ‘I‘iO?. The TiOI coated 13 cm’ of the reactor tube. An 8-W black light hulh was centered in the reactor. The product gases were then exhausted through pyrex and Teflon tubing into the gas cell (9.85, path length) of the FTIR. The catalyst was supported on the inside of the pyrex reactor using a simple coating method. After the pyrex wasetchedwith2MNaOH,theTiOlwascoatedontothe inside of the reactor tube from an aqueous slurry and the reactor w a s thendriedat 150°C. Thecoating was repeated several times until the reactor was opaque to ultraviolet light. A weighingofthereactor heforeandafter thecoating allowed a determination of the catalyst weight (13 mgJ. No effort was made to determine the surface area or the thicknessof thecoating. Experiments were alsoconducted on the MBMS using these annular reactors. Results
Reaction Rates. High levels of destruction of TCE were measured using both the MBMS and the FTIR. No reaction wasobserved in theabsenreof UV light orcatalyst. Furthermore, high levels of destruction were found using a variety of reactors, catalysts, and supports. These results are consistent with those reported by Dibble and Haupp (4-6).
The reaction rates for the 1,l-dichloroethylene (1.1DCE), cis-1.2-dichloroethylene (C-1,2-DCEJ, trans-1,2Envlron Sci TeChnOl..
VoI. 27. ho. 4.
1993
7SS
Water-cooled Plate Skimmer
Figure 2. Schematic representation of the molecular beam mass spectrometer (MBMS).
= , r SYRINGE PORT
140.00
~
1 12
INJECTOR
I
AIR A b N2 BLACK LIGHT
-
X
1
Fit r V = 0 99
w F.T.I.R.
Figure 3. Schematic drawing of the FTIR apparatus used to study the gas-phase photocatalyticoxidationof TCE. The TCE and air are mlxed in the pressurized InJector. The mixed gas passes over the illuminated reactor and Into the FTIR analyzer.
dichloroethylene (t-1,2-DCE),and perchloroethylene were measured and found to be lower than those for TCE. However,the rates for these other compounds were higher than the rates for the nonchlorinated compounds. The experimental measurements were fit to a pseudo-firstorder decay as a function of time and rate constants for PCE and 1,2-DCE are about 60% and 20% of the rate for TCE. The difference between the values obtained for c-1,2-DCE and t-1,2-DCE was negligible. Experiments were also conducted using dichloroacetyl chloride (DCAC) as a starting material and the same frit reactor used for the TCE reactions. As will be shown later, DCAC was found to be an intermediate in the photocatalytic oxidation of TCE. DCAC gave a slower photocatalytic oxidation rate than TCE. We have also measured the photocatalytic oxidation rate of TCE as a function of the laser intensity impinging upon the frit reactor. Figure 4 shows a plot of TCE destruction as a function of light intensity at a relatively low concentration (57 ppmv) and a high concentration (1030ppmv). The plot shows the experimentally measured points and curves that were generated by linear regression fits to the points. At the low concentration, a square-root dependence gave the best match to the data, while at the 734
[TCE] = 1030 ppm
Envlron. Sci. Technol., Vol. 27, No. 4, 1993
[TCE1=57ppm
zooo Fit rA2= 0 95
1-
1 2
0 00
'
000
050
100
150
L
'
'
200
250
' 0
300
Llght Intensity (1OA.S Eis)
Figure 4. Plot of the rate of photocatalytic oxidation of TCE as a function of the light intensity. At a high concentration (1030 ppmv), the data fit a straight line (correlation coefficient, 6 = 0.99). At a low concentration(57 ppmv), the data are better fit by a square-root functlon (P = 0.95).
higher concentration, a linear dependence gave a better fit. Products. The photocatalytic oxidation of TCE in all of these experiments resulted in the formation of six prominent products: hydrogen chloride, carbon monoxide, carbon dioxide,molecular chlorine,dichloroacetylchloride, and phosgene. The molecular chlorine, DCAC, CO, and phosgene have not been previously reported in gas-phase studies with TiOz. These species may be more difficult to detect with gas chromatography than with the directsampling techniques employed in this study. Figure 5 presents a typical mass spectrum of the products from the photocatalytic oxidation of TCE, and Table I presents the peak positions and identities. The peaks in these spectra were identified first by comparison to the National Institute of Standards (NIST) mass spectral library. For DCAC and phosgene, mass spectra were obtained from standards. Identification was simplified in many cases because of the characteristic peak intensity patterns which arise as a result of the natural abundance of the chlorine isotopes.
5000
-
I
111+
146+
95+
76+ 0 50
70
90
110
130
150
170
190
Mass (arnu) Figure 5. Mass spectrum of the products from the photocatalytic oxldatlon of TCE.
Figure 6 shows the parent ion CID spectra for the rnlz 83+ and 63+ fragments obtained from the photocatalytic oxidation of TCE while Figure 7 shows the parent ion CID spectrum for rnlz 83+ and 63+ obtained from a standard DCAC. As the spectra show, the parent ions for the rnlz 83" ion from TCE photocatalytic oxidation and from a DCAC standard were identical. (The parent ion spectra for mlz 85+ were also identical.) In Figure 6, the parent ion spectrum for m f z 63+ shows that both phosgene (ml z 98+) and DCAC (mlz 146+)give the rnlz 63+ fragment. The parent ion spectrum for rnlz 63+ shown in Figure 7 shows that although DCAC can form the rnlz 63+fragment, DCAC will not account for the rnlz 98+ parent. The formation of molecular chlorine from TCE was established by the peaks in the mass spectrum at rnlz 70+, 72+,and 74+. In the library spectrum (at 50-eV ionization energy), these peaks also arise from phosgene. However, we have measured the mass spectrum of phosgene on our MBMS and found that, under our operating conditions (25-eV ionization energy, free-jet internal-state cooling), these peaks are insignificant. The identity of these peaks was also confirmed with CID mass spectrometry. The bottom of Figure 8 shows a typical FTIR spectrum obtained from the photocatalytic oxidation of TCE. The FTIR spectra of standards of TCE, DCAC, and phosgene are shown on the top of this figure, which confirms that these species are present in the product stream. The FTIR analysis also identified CO, COZ,and HC1as products from the oxidation of TCE. As a check of its analytic capabilities, we measured the carbon and chlorine balances from the photocatalytic oxidation of TCE using concentration data from the FTIR spectrometer and the annular-black-light reactor. The concentrations of the products and the mass balances are collected in Table 11. To help unravel the mechanism for the photocatalytic oxidation of TCE, the mass spectra of products from PCE, c-1,2-DCE, t-1,2-DCE, and 1,l-DCE were compared to TCE. Table I collects the peaks from these spectra and gives their probable assignments. These peaks were also identified by comparison to library spectra, All four precursors produced phosgene and molecular chlorine. PCE also produced trichloroacetyl chloride, 1,l-DCE produced chloroacetyl chloride, and 1,2-DCE produced a small amount of DCAC. The products from t-1,2-DCE and c-1,2-DCE were identical. The products from c-1,2DCE and PCE were confirmed with FTIR. The mass spectra of products from the photocatalytic oxidation of DCAC were also measured, and a typical
product spectrum is shown in Figure 9. The top of this figure shows a mass spectrum of unreacted DCAC (200 ppmv) while the bottom spectrum is a plot of the mass spectrum when the DCAC is passed over the illuminated catalyst. As can be seen from this figure, the major products from the oxidation are phosgene (mlz 63+) and molecular chlorine (mlz 70+). The formation of phosgene from DCAC was also confirmed by FTIR spectroscopy. The time profiles of the products from the photocatalytic oxidation of TCE can help elucidate the chemical reaction mechanism. Figure 10 shows a plot of the relative ion intensities of phosgene and DCAC formed as a function of the residence time in the illuminated catalyst frit. The residence time was estimated as the volume of the frit divided by the total volumetric flow rate. This is only an estimate since the light profile in the frit could not be measured. As the plot shows, at short residence times, the DCAC and the phosgene concentrations increase quickly, while the TCE concentration decreases quickly. The DCAC signal reaches a maximum at -0.6 s and then diminishes, while the phosgene concentration continues togrow. In other experiments which used longer residence times (so higher light intensity), the concentration of the phosgene also maximizes and then slowly decreases. MBMS experiments with the reactors shown in Figures 1and 3 gave the same products from TCE, suggesting that the reaction mechanism was unchanged. However, with the frit reactor the yield of DCAC appeared to be higher. In addition, the yield of DCAC increased as the radius of the annular reactor tube decreased.
Discussion Global Quantum Yields. A comparison of the global quantum yields measured for the photocatalytic oxidation of TCE with other compounds reveals an interesting trend. Global quantum yields can be determined by dividing the number of molecules oxidized per second by the photon count rate (photonsls) impinging upon the reactor. This is often a lower limit for the true quantum yield (number of molecules reacted divided by the number of photons absorbed) because the UV light may be reflected by Ti02 or may pass through the catalyst support. As an example, estimated global quantum yields can be calculated for the high concentration curve in Figure 4. For these data, the global quantum yield varies from 0.5 to 0.8. Similar calculations with Dibble and Raupp's (4-6) data also give high global quantum yields. These high global quantum yields are in contrast to the global quantum yields determined for nonchlorinated species. For instance, the rates (and thus the global quantum yields) for ethylene, propylene, tetramethylethylene, propanol, and ethanol were about 1order of magnitude lower than the rates for TCE under identical conditions. These results for the gas phase are an interesting contrast to results from aqueous-phase photocatalysis. In the aqueous phase, the global quantum yields for photocatalysis of TCE are usually lower than 0.01 and differ little from the global quantum yields for other compounds (11). Relevant ChemicalReaction Mechanisms. The high global quantum yields measured for the photocatalytic oxidation of TCE and the observed products can be rationalized by a chemical reaction mechanism in which C1 atoms attack the TCE, thus initiating oxidation. This chemical mechanism was proposed by Sanhueza et al. (12) Eflvlron. Scl. Technol., Vol. 27, No. 4, 1993
795
Table I. Products from Chlorinated Ethylenes: Assignments of the Ion Peaks in the Figures ion mass, m/z
ion
60,62 61,63 61,63 63.65 70,72, 74
TCE
CHClCC12" CHzCClz" CHCICHCP COClZ CHClzCCl(0) CC13CC1(0) Clz CHClzCCl(0) CH2ClCC1(0) cclzcclz" CHClzCCl(0) CC12CC12" CHClCClz" CHzCClz" CHClCHCP COClZ CHClzCCl(0) CHzClCCl(0) CC1~CCl(O) CHClCC12" CC13CCl(O) CHCl&Cl(O) CClZCClZ~
X
Clzt CZHC10+ CH2ClC(O)+ cc12+ CHClz' CZC12+ C2HC1zt CH2CC1Zt CHClCHCl+ coc12+ CHClzC(O)+ CHZClCCl(0)' Cc13' CHClCClZ+ CC13C(O)+ CHClZCCl(O)+ cc12cc12+
76,78
77,79 82,84,86 83,85,87 94, 96,98 95,97,99 96,98,100 96,98,100 98,100,102 111,113,115 112,114,116 117,119,121 130,132,134,136 145,147,149,151 146,148,150,152 164,166,168,170,172 (I
molecular precursor
CZHCl+ CH 2 CC1' CHCHCP COCP
PCE
1,l-DCE
1,P-DCE
X X X
X X
X
X
X X X
X X
X
X
X X X
X X
X X X X
X X X
X
X
X X
X X X
X X
Starting material.
(Parents of 63+(
981
/Parents of 63+1
146t
146+
m,
70
80
90
100
110
120
130
140
150
160
70
,,,, ,,, ,
, ,
,,,,,,,,,
100
90
80
Mass (arnu)
, , , , , , , , , ,, I ,
110
A
, ,,,,,!-
120
130
140
150
160
Mass (amu)
il lt !
I
111t
lParentof1
Parentsof83+]
I
146t
146+ 90
100
I,
1181.
110
120
1 130
140
150
II 160
Mass (amu) Flguro 7. CID parent spectra for the peaks m/z 63+ and 83+ which arise from neat DCAC. Notice that in the parent spectrum for mlz 63+,the signal for mlz 98+ is very weak.
to explain products identified from the homogeneous reaction of C1 atom with chlorinated ethylenes. In their experiments, C11 was photolyzed in the presence of chlorinated ethylenes. A comparison of the products from the work by Sanhueza et al. with the products in this study leads us to believe that C1 atom initiated oxidation plays 736
Environ. Sci. Tachnol.. Vol. 27, No. 4, 1993
a significant role in the photocatalytic oxidation of TCE and the other chlorinated ethylenes studied here. In the mechanism proposed by Sanhueza et al. (12),the C1 atom adds to the carbon atom bonded to the hydrogen atom CCl,=CHCl+ C1- CHCl2CCl2' The resulting alkyl radical reacts with 02 (reaction 2) to form a peroxy radical, which reacts with another peroxy
ethylenes. The basis for suggestinga mechanism involving C1 atom attack relies upon the fact that for PCE, TCE, and 1,l-DCE we have found that acetyl chlorides were significant products and that they are identical to those found in homogeneous reactions with C1 atom. In Table I11we have shown the major products measured from this work. It should be noted that the MBMS did not detect CHCl(O), the main product from the homogeneous, C1 atom initiated oxidation of 1,2-DCE. Instead, DCAC was detected. This compound is not consistent with the mechanism outlined above, but it may arise from an alternative reaction of the alkoxy radical. For instance, the alkoxy radical formed from 1,2-DCEwould be CHCl2CHC10'. This species is susceptible to H atom abstraction by 02 (or by a radical) to form DCAC CHC1,CHClO'
+ 0, ('OH)
-
CHC1,CClO Photocatalysis Products
700
900
1100
1300
1500
1700
1900
Frequency (ern*-I) Flgurr 8. FTIR spectrum of products from the photocatalytic oxldatlon of TCE compared to the spectra of DCAC, phosgene, and TCE.
radical (reaction 3) to give an alkoxy radical CHCl,CCl,'
+ 0,
-
CHCl,CC1,00'
2[CHCl~CC1~00']+2[CHCl,CC120'1
(2)
+ 02
(3)
The alkoxy radical can then lose a C1 atom to form the DCAC
-
CHC12CC120' CHCl,CCl(O)
+ C1
(4)
or the C-C bond can rupture to form phosgene and a dichloromethyl radical
-
CHC12CC120' CHC1,'
+ CC1,O
(6)
This reaction is exothermic but may be kinetics limited in homogeneous, gas-phase systems. This limitation may be removed on the catalyst surface. Whatever the mechanism, the fact that there are more chlorine atoms in the product than in the starting material again suggests C1 atom reactions. Molecular chlorine which was formed during photocatalysis is also an indication of the presence of C1 atoms. The recombination of C1atoms to form molecular chlorine will be a chain-termination step for reactions 1-5 and will be possible because reactions 1-5 produce no net loss in C1 atom concentration. As can be seen from Table 11,the C1 atom balances measured with the FTIR were short of 100% in spite of the good carbon atom closures. This is consistent with the presence of molecular chlorine measured by the MBMS because this molecule cannot be detected with the FTIR. C1Atom Formation. A possible source for the C1atoms in our work is the reaction of TCE with photocatalytically produced radicals. Consider the 'OH radical, which is known to be produced on wet Ti02 under UV illumination (16). Homogeneous, gas-phase studies have shown that the reaction of 'OH with TCE will produce C1 atoms (17) through the following series of reactions:
(5)
The branching ratio for (4) and (5) depends upon the relative strength of the C-C bond and the alkoxy carbon C-C1 bond. In homogeneous experiments with TCE (12) C1atom initiated oxidation yielded 90 % DCAC, indicating that C-C1 bond was weaker. The same result has been found when TCE was directly photolyzed in air (13). Sanhueza et al. (12)also reported on the C1atom initiated oxidation of the other chlorinated ethylenes, and again, the final product distribution depended upon the relative strengths of the C-C bond and the C-C1 bond in the corresponding alkoxy radical. The products formed in their studies are presented in Table 111. These results appear consistent with group additivity (14)determination of the heats of formation of the relevant radicals (15). They are also consistent with results obtained from homogeneous, gas-phase studies of chlorinated ethanes (15). Photocatalytic Mechanism. C1Atoms. We propose that the mechanism outlined in reactions 1-5 is applicable to the gas-phasephotocatalytic oxidation of the chlorinated
+ 'OOH (H,O)
CCl,=CClH 'CC1,CHClOH
-
+ *OH
+ 0,
-
'CC1,CHClOH
(7)
'00CC1,CHClOH
(8)
2(*00CCl2CHC10H) 2('OCCl,CHClOH) 'OCC1,CHClOH
-
+ 0,
CHClOHCCl(0) + C1
(9) (10)
It was demonstrated that roughly 50% of the reactions of 'OH with TCE produced C1 atoms (17). It is plausible that reactions 7-10 could occur on the Ti02 surface during the photocatalytic oxidation of TCE as an initiation step to the chain reaction 1-4. It is also possible that 0 atoms may be formed on the Ti02 surface and may react with TCE. The production of 0 atoms on Ti02 has often been invoked in the gasphase photocatalytic oxidation (18,191of organics on TiO2. Homogeneous, gas-phase reactions of 0 atom (20) with TCE have been shown to give C1 atoms as a product. Environ. Scl. Technol., Vol. 27, No. 4, 1993 737
Table 11. Results from FTIR Measurements8 resid time, s
feed TCE
TCE
coz
co
1.9 0.97 0.45 0.25 0.20 0.17 0.17 0.17
103 110 106 106 106
2 15 25 43 50 57 60 61
89 67 61 45 39 40 38 38
37 32 30 23 21
118
114 114
21
20 20
products HC1
COClZ
DCAC
73 64 59 44 39 39 32 33
3 3 3 4 4 4 3
4 2
4 4 4 10 11
12
atom balance, % C Clb 98 90 97 97 98 94 96 96
1
51 56 65 73 77 77 78 79
The concentrations are given in ppmv as measured using an FTIR spectrometer and were determined after background subtraction. Molecular chlorine is a likely candidate for the missing C1 atoms. (I
,
9000
I
Table 111. Comparison of C1 Atom Intiated Oxidation to Photocatalytic Oxidation
compounda
productsb ( % )
CClZCClZ
70
5
90
110
130
150
170
190
Mass (arnu)
20000
-m
;63t
15000
a
m
;
10000
0
5000 0
50
70
90
110
130
150
170
190
Mass (arnu)
Figure Q. Mass spectra of DCAC neat (top) and the products of photocatalytic oxidatlon of DCAC (bottom). 140
CC13CCl(O) (75), COCli (25) CClzCHCl CHClzCCl(0) (go), coc12 CCl?CH? CHyClCCl(0) (98), . . COCl? trans-CHClCHCl CHClO (71), COClZ (3) cis-CClzCClz CHClO (71), COClZ (3)
,
---
100000
i
20000
*
L
O 020
040
060
080
100
120
io000
140
Residence Time (s)
Flgute 10. Plot showlng the formatlon of byproducts from the photocatalytic oxidation of TCE as a function of residence time in the frit.
Afurther possibility for the formation of C1atoms could be the direct oxidation of C1-. Chloride ion will result from the complete mineralization of TCE and may be present on the surface of the TiO2. It has been suggested 738
Environ. Scl. Technol., Vol. 27, No. 4, 1993
300 200 172 21.5 21.5
products this work CCl&Cl(O), coc12 CHClzCCl(O), COClZ CH2ClCCl(O), coc12 COClz, CHClzCCl(0) COC12, CHC12CCl(0)
Chlorinated ethylenes. Reference 12.
earlier that chloride ion may be oxidized (21)by illuminated TiO2. Phosgene Formation. The mechanism outlined in reactions 1-4, while accounting for the formation of the DCAC based upon the reaction of TCE with C1 atoms, does not account for the formation of phosgene. As Table I11 shows, phosgene is only a minor product in the homogeneous reaction of C1 atom with TCE. It might be that the branching ratio for reactions 4 and 5 changes on the catalyst surface. The time profiles in Figure 10 also suggest that phosgene is formed from a reaction of the DCAC intermediate, and Figure 9 supports this hypothesis. Another possible source of phosgene is the reaction of chlorine atoms with carbon monoxide (22). A likely route for the reaction of DCAC to give phosgene is through an H atom abstraction by a photogenerated radical (such as 'OH)
'OH + CHCl,CCl(O)
.
global quantum yieldb
-
'CCl2CC1(0)+ H20 (11) It is unlikely that the 'OH radical would add to the C=O bond (23),and abstraction of a C1 atom is thermodynamically unfavorable. Reaction 12 could be followed by addition of 02 to form a peroxide and subsequent reaction of the peroxide radical to form an alkoxy radical
'oocc1,cc1(o) +
0, + *cc1,cc1(0)
(12)
2'O0CCl2CC1(O) 2'0CCl2CC1(O) 0, (13) The alkoxy radical should decompose to phosgene and CC10'
-
'0cc1,cc1(0) cc1,o + CC10' (14) The further decomposition of CClO' is a potential source
of the observed carbon monoxide
-
CClO' c1+ co (15) The experiments in which the yield of the DCAC increased with decreasing reactor radius are a result of the competition between TCE and DCAC for reactive species on or near the surface. With the larger reactors, bulk diffusion limits the access of the TCE to the area near the surface and the DCAC can more effectively compete for reactive species. This also explains the higher apparent yield of DCAC for the frit reactor. The mechanisms outlined above for the photocatalytic oxidation of TCE are by no means complete. We have not attempted to account for the reactions of electrons and holes produced in the solid upon absorption of light. These reactions are intrinsic in the formation of the radicals discussed above. The *OH radical is thought (16) to be formed by the oxidation of OH- by holes, while 0 atom (18,19) and C1 atom (21) are thought to be formed by a series of reactions involving electrons and holes. Undoubtedly, another important reaction is the reduction of molecular oxygen O,+e--O; (16) which has been discussed elsewhere (18). Table I11 lists the global quantum yields for the homogeneousC1atom initiated oxidation of the chlorinated ethylenes (12). Since the C1 atoms were formed by photodissociation of Clz, this quantity was determined by dividing the number of chlorinated ethylene molecules reacted by the number of photons absorbed by the Clz molecules. As can be seen, the global quantum yield is often very high because the reaction series 1-4 is a chain reaction with respect to the C1 atom. Combining these reactions produces the global reaction
-
CCl,=CHCl+ C1+ 1/20,CHCl,CC1(0) + C1 (17) The chain reaction shown in (17) may explain the high reaction rates seen for the gas-phase photocatalytic oxidation of TCE relative to the gas-phase photocatalytic oxidation of nonchlorinated organic compounds, where chain reactions would be less likely. The dependence of the reaction rate upon light intensity shown in Figure 5 is typical of photocatalytic systems (24). The square-root dependence at low concentrations is a result of hole-electron recombination. For TCE this may also be due to the recombination of C1 atoms. An important consideration in the chain reaction shown in (17) is whether or not these reactions occur in the gas phase above the catalyst or on the catalyst surface. The similarity between these photocatalysis results and the homogeneous experiments would seem to suggest that the C1 atom reactions occur in the gas phase. However, it could also be argued that similar reaction channels may be available to species adsorbed to the catalyst surface. For instance, another possible formation route for the alkoxy radical would be a reaction of the peroxy radical formed in (2) with surface peroxy radicals. The reaction scheme outlined above for the formation of DCAC from TCE (1-4) may also be of importance in aqueous-phase photocatalysis. Consider, for instance, the formation of dichloroacetic acid in the aqueous-phase photocatalysis of TCE (8). It was suggested that dichloroacetaldehyde (DCAD)could be oxidized toDCAA, which
was identified as an intermediate in the photocatalytic oxidation of TCE. The reaction of chlorinated aldehydes to give chlorinated acids has been demonstrated (8). However, there is the possibility that the DCAA could be formed as a result of the C1 atom reactions shown in (1-4). The DCAC formed from these reactions would be quickly hydrolyzed to give DCAA in aqueous solutions. By the same reasoning, the formation of trichloroacetic acid in aqueous photocatalysis may also result from the reaction of PCE with C1 atoms. The formation of byproducts from photocatalytic oxidation of gaseous TCE is of concern but should not limit the use of this technologyfor waste removal. As was shown, DCAC can be removed by longer residence times. Phosgene and molecular chlorine can be removed by a caustic scrubber, which would be required to remove HC1.
Conclusions The gas-phase photocatalytic oxidation of trichloroethylene has been studied using the direct-sampling capabilities of an MBMS and of gas-phase FTIR spectroscopy. As in earlier studies, we found that the oxidation of TCE was a rapid reaction with fairly high global quantum yields. However, unlike earlier reports, we found high yields for the following byproducts: dichloroacetyl chloride, phosgene, CO, and molecular chlorine. Tests with other chlorinated ethylenes also give byproducts which are consistent with a mechanism involving C1 atom attack of the chlorinated ethylene. Reactions involving the attack of TCE by C1 atom are likely to involve a chain mechanism, which could explain the high global quantum yieldsfor the oxidation of TCE. Studies with DCAC show that phosgene can be produced by the photocatalytic oxidation of this compound. This may be the source of much of the phosgene in this system and shows that the DCAC can be removed from the system with long residence times. Acknowledgments The programmatic and technical support (under DOES Solar Industrial Program) of Dr. Frank Wilkins, Dr. John Anderson, and Dr. Hal Link is gratefully acknowledged. Help with the operation of the MBMS was provided by David A. Gratson, Carolyn C. Elam, and Dr. Dingneng Wang. We thank Professors Gregory B. Raupp at Arizona State University, Richard D. Noble at the University of Colorado, and William H. Glaze at the University of North Carolina at Chapel Hill for useful discussions. Literature Cited Dyksen, J. E.; Hess, A. F. J.-Am. Water Works Assoc. 1983, 74, 394-403. Herrmann, J.; Pichat, P. J. Chem. SOC., Faraday Trans. 1 1980, 76, 1138-1146. Hulstrom, R.; Bird, R.; Riordan, C. Solar Cells 1985, 15, 365-391. Dibble, L. A.; Raupp, G. B. Catal. Lett. 1990,4, 345-354. Dibble, L. A,; Raupp, G . B. Proceedings of the Arizona Hydrological Society, First Annual Symposium; Sept 1617, Phoenix AZ, 1988; pp 221-229. Dibble, L. A.; Raupp, G. B. Environ. Sci. Technol. 1992,26, 492-495. Pruden, A. L.; Ollis, D. F. J. Catal. 1983,82, 404-417. Kenneke, J. F.;Ferry, J. L.; Glaze,W. H. Extended Abstracts of Papers, 203rd National Meeting of the American Chemical Society, San Francisco, CA; American Chemical Society: Washington, DC, 1992; ENVR 31. Envlron. Scl. Technol., Vol. 27. No. 4, 1003
739
(9) Milne, T. A,; Soltys, M. N. J. Anal. Appl. Pyrol. 1983, 5, 93-110. Evans, R. J.; Milne, T. A. Energy Fuels 1987, 1, 123-137. Evans, R. J.; Milne, T. A. Energy Fuels 1987,1, 311-319. (10) Nimlos, M. R.; Milne, T. A. Environ. Sci. Technol. 1992,26 545-552. (11) Mathews, R. W. Water Res. 1986,20, 569-578. (12) Sanhueza, E.; Hisatsune, I. C.; Heiklen, J. Chem.Rev. 1976, 76, 801-826. (13) Blystone, P. C.; Johnson, M. D.; Haag, W. R. Proceedings of Air and Waste Management Association, 85th Annual Meeting, June 21-26, Kansas City, KS, 1992. (14) Benson, S. W., Thermochemical Kinetics, 2nd ed.; John Wiley and Sons: New York, 1976. (15) Bertrand, L.; Exsteen-Meyers, L.; Franklin, J. A.; Huybrechts, G.; Olbregts, J. Int. J . Chem. Kinet. 1971, 3, 89-96. (16) Anpo,M.; Shima,T.; Kubokawa, Y. Chem.Lett. 1985,17991802.
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Environ. Sci. Technol., Vol. 27, No. 4, 1993
(17) Kleindienst, T. E.; Shepson, P. B.; Nero, C. M.; Bufalini, J. J. Int. J. Chem. Kinet. 1989,21, 863-884. (18) Teichner, S. J.; Formenti, M. In Photoelectrochemistry, Photocatalysis and Photoreactors;Schiavello, M., Ed.; D. Reidel Publishing Co.: Boston, 1985; pp 457-489. (19) Formenti, M.; Teichner, S. J. Catalysis 1977, 2, 87-106. (20) Sanhueza, E.; Heicklen, J. Int. J. Chem. Kinet. 1974, 6, 553-565. (21) Munuera, G.; Navio, A. Proceedings of the 7thZnternational Congresson Catalysis;Elsevier: Amsterdam, 1981;pp 11951197. (22) Ohta, T. Bull. Chem. SOC.Jpn. 1983, 56, 869-872. (23) Atkinson, R. J. Phys. Chem. Ref. Data 1989, Monogram 1. (24) Egerton, T. A.; King, C. J. J. Oil. Colour Chem. Assoc. 1979, 62, 386-391.
Received for review August 31, 1992. Revised manuscript received December 28, 1992. Accepted December 28, 1992.
