Mechanism of photooxidation of trichloroethylene on TiO2: Detection of intermediates by infrared spectroscopy

Article abstract of DOI:10.1021/ja952155q

The photooxidation of trichloroethylene (TCE) on TiO₂ has been investigated using infrared spectroscopy for kinetic studies of the production of intermediate species and for investigation of the reaction mechanism. Trichloroethylene is oxidized by chemisorbed molecular O₂ when TiO₂ band gap radiation (hv > 3.1 ± 0.1 eV) is incident. An intermediate species, dichloroacetyl chloride, HCl₂CCOCl, was identified. At 300 K, Cl₂CO, CO, CO₂, HCl, and H₂O are final photooxidation products. At 473 K, Cl₂CO undergoes a thermal side reaction on TiO₂ to produce Cl₂C=CCl₂. Studies in which oxygen-labeled H₂O was present indicated no incorporation of the oxygen label into any of the intermediates, showing that the OH· driven oxidation mechanism proposed by others is not operative. At 150 K, TCE blocks TiO₂ sites, significantly retarding O₂ adsorption and slowing the photooxidation reaction.

Full text of DOI:10.1021/ja952155q

4
686
J. Am. Chem. Soc. 1996, 118, 4686-4692
Mechanism of Photooxidation of Trichloroethylene on TiO2:
Detection of Intermediates by Infrared Spectroscopy
Jingfu Fan and John T. Yates, Jr.*
Contribution from the Surface Science Center, Department of Chemistry, UniVersity of Pittsburgh,
Pittsburgh, PennsylVania 15260
X
ReceiVed June 30, 1995
Abstract: The photooxidation of trichloroethylene (TCE) on TiO2 has been investigated using infrared spectroscopy
for kinetic studies of the production of intermediate species and for investigation of the reaction mechanism.
Trichloroethylene is oxidized by chemisorbed molecular O2 when TiO2 band gap radiation (hν > 3.1 ( 0.1 eV) is
incident. An intermediate species, dichloroacetyl chloride, HCl2CCOCl, was identified. At 300 K, Cl2CO, CO,
CO2, HCl, and H2O are final photooxidation products. At 473 K, Cl2CO undergoes a thermal side reaction on TiO2
to produce Cl2CdCCl2. Studies in which oxygen-labeled H2O was present indicated no incorporation of the oxygen
•
label into any of the intermediates, showing that the OH driven oxidation mechanism proposed by others is not
operative. At 150 K, TCE blocks TiO2 sites, significantly retarding O2 adsorption and slowing the photooxidation
reaction.
I. Introduction
examine the reaction mechanism of the photooxidation of TCE
which is currently under discussion.
1
2-17
The use of TiO2 as a photocatalyst for environmental cleanup
is of great current interest since TiO2 is stable and inexpensive
and has a UV absorption partially overlapping the solar
There has been active interest in the photooxidation of TCE
1
2-17
using TiO2.
As a widely used solvent in industrial
1,2
spectrum. Electron-hole pairs are photolytically generated by
exciting the TiO2 semiconductor with UV light:
processes, TCE is the most common and abundant pollutant in
1
8
ground water in the United States. Since oxidation rates are
typically orders of magnitude higher in the gas phase than in
aqueous solution, an approach that strips the TCE from water
and then carries out the UV photooxidation in the resulting air
-
+
TiO + hν f e + h
(1)
2
1
3,19
stream is being considered.
Thus, the study of the TiO2
It has been postulated that the holes which diffuse to the TiO2
surface react with surface OH groups. The subsequently formed
OH radicals (OH ) are proposed to be oxidizing agents. In
this OH radical model, in order to prevent recombination of
the holes with electrons, it was postulated that acceptor
molecules are needed to trap the electrons to prevent electron-
photocatalyzed oxidation of TCE is practically useful. Previous
1
4-16
•
3
studies
have shown that a significant amount of toxic
byproducts (chlorinated molecules) were formed in the photo-
oxidation of TCE using TiO2 in both aqueous solution and the
gas phase. In studies of TCE photooxidation on TiO2 in aqueous
20
4
suspension, Pruden and Ollis suggested that hydroxyl radicals
initiate the reaction, and that dichloroacetaldehyde (DCAAD)
hole pair recombination in the bulk and on the TiO2 surface.
Oxygen is an electron-trapping molecule and it was found to
1
6
be indispensable for photooxidation reactions.5
-8
is an intermediate. Glaze, Kenneke, and Ferry argued that in
addition to the hydroxyl radical initiated oxidative reaction
channel, a parallel reductive pathway, involving the conduction
band electrons of TiO2, also plays an important role. This
reductive pathway yields dichloroacetaldehyde and dichloro-
acetic acid (DCAA) as initial products from TCE. In studies
2
However, in
a study of the photooxidation of methyl chloride on single crystal
TiO2(110), Lu, Linsebigler, and Yates found that while both
surface defect sites and oxygen adsorption are necessary for
the photooxidation reaction to occur, the adsorbed oxygen is
the actual oxidizing agent. Other studies also suggest that the
5
8
-11
role of oxygen may not just be limited to electron-trapping.
of the TiO photocatalyzed gaseous TCE oxidation, Anderson,
In this paper, the photooxidation of trichloroethylene (TCE) on
powdered TiO2 was studied. The purpose of this study is to
(12) Phillips, L. A.; Raupp, G. B. J. Mol. Catal. 1992, 77, 297.
(
13) Dibble, L. A.; Raupp, G. B. EnViron. Sci. Technol. 1992, 26, 492.
X
Abstract published in AdVance ACS Abstracts, May 1, 1996.
(14) Nimlos, M. R.; Jacoby, W. A.; Blake, D. M.; Milne, T. A. EnViron.
Sci. Technol. 1993, 27, 732.
(
(
(
(
(
1) Linsebiger, A. L.; Lu, G.; Yates, J. T., Jr. Chem. ReV. 1995, 95, 735.
2) Wold, A. Chem. Mater. 1993, 5, 280.
(15) Holden, W.; Marcellino, A.; Valic, D.; Weedon, A. C. In Photo-
catalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-
Ekabi, H., Eds.; Elsevier Science Publisher: New York, 1993; p 393.
(16) Glaze, W. H.; Kenneke, J. F.; Ferry, J. L. EnViron. Sci. Technol.
1993, 27, 177.
(17) Zhang, Y.; Crittenden, J. C.; Hand, D. W.; Perram, D. C. EnViron.
Sci. Technol. 1994, 28, 435.
(18) Dyksen, J. E.; Hess, A. F., III. J. AWWA 1982, August, 394.
(19) (a) Blystone, P. G.; Johnson, M. D.; Haag, W. R. Air & Waste
Management Association, 85th Annual Meeting, June 21, 1992, Kansas
City, MO. (b) Blystone, P. G.; Johnson, M. D.; Haag, W. R.; Daley, P. F.
Advanced Ultraviolet Flash Lamps for the Destruction of Organic Con-
taminants in Air. In Emerging Technologies In Hazardous Waste Manage-
ment III; Tedder, D. W., Pohland, F. G., Eds.; ACS Symposium Series,
518; American Chemical Society: Washington, DC, 1993; p 380.
(20) Pruden, A. L.; Ollis, D. F. J. Catal. 1983, 82, 404.
3) Jaeger, C. D.; Bard, A. J. J. Phys. Chem. 1979, 83, 3146.
4) Gerischer, H.; Heller, A. J. Phys. Chem. 1991, 95, 5261.
5) Lu, G.; Linsebigler, A.; Yates, J. T., Jr. J. Phys. Chem. 1995, 99,
7
5
626.
(
6) Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 341.
(7) Wang, C.; Heller, A.; Gerischer, H. J. Am. Chem. Soc. 1992, 114,
230.
8) Wong, J. C. S.; Linsebigler, A.; Lu, G.; Fan, J.; Yates, J. T., Jr. J.
Phys. Chem. 1995, 99, 335.
9) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem.
ReV. 1995, 95, 69.
10) Hoffmann, A. J.; Carraway, E. R.; Hoffmann, M. R. EnViron. Sci.
Technol. 1994, 28, 776.
11) Stafford, U.; Gray, K. A.; Kamat, P. V.; Varma, A. Chem. Phys.
Lett. 1993, 55.
(
(
(
(
S0002-7863(95)02155-X CCC: $12.00 © 1996 American Chemical Society

Mechanism of Photooxidation of Trichloroethylene on TiO2
J. Am. Chem. Soc., Vol. 118, No. 19, 1996 4687
Nishida, and March21 proposed a mechanism involving hydroxyl
radicals and monochloroacetate as intermediates. Phillips and
12
Raupp suggested that hydroxyl radical or hydroperoxide radical
initialized the reaction and involved dichloroacetaldehyde as
an intermediate. In contrast, Nimlos, Jacoby, Blake, and
Milne1
4,22
suggested that TCE is oxidized in chain reactions
initiated by chlorine atoms. Thus, various mechanisms are
currently proposed for TCE photooxidation on the TiO2 surface.
In this paper, the photooxidation of gaseous TCE was
performed on a dehydroxylated TiO2 sample in a controlled
fashion. The use of dehydroxylated TiO2 established a condition
that the amount of surface hydroxyl groups on the TiO2 surface
2
3
is small, and the possibility of both photooxidation reaction
and gas-solid isotopic exchange via hydroxyl groups was
2
4,25
minimized.
We then compared these results with experi-
ments where significant amounts of water were present. We
studied photooxidation reactions at 300 K where surface sites
are accessible to gaseous TCE, water, and oxygen molecules.
We also examined the photooxidation of TCE on TiO2 surfaces
at 150 K. Since TCE was adsorbed to high coverage on the
TiO2 surface at 150 K, the active surface sites were less
accessible to oxygen under these conditions. We also investi-
gated the thermal and photooxidation reactions at 473 K, where
2
Figure 1. Gas-phase infrared spectra for Cl CdCHCl (TCE) photo-
oxidation at 300 K: (a) before irradiation; (b) after 105 min of UV
irradiation; and (c) after 435 min of UV irradiation. λ ) 325 ( 50
nm.
obtained from Matheson Gas Products with a quoted purity of 99.999%.
The oxygen-18 enriched oxygen gas was obtained from ICON Services
Inc. with a quoted isotopic purity of 96 atom %. The TiO
2
powder
(
P25) was purchased from Degussa. Ultrapure H O was prepared by
2
a thermal side reaction which does not take place at 300 K was
observed.26 Fourier Transform Transmission Infrared (FTIR)
ion-exchange and distillation. The following procedure was followed
to ensure a homogeneous mixture of TCE and oxygen inside the infrared
reaction cell: TCE of the desired quantity was first introduced to the
reaction cell and condensed on the TiO sample holder using liquid
2
nitrogen. Oxygen was then introduced and the TCE was allowed to
evaporate back into the gas phase by warming the sample holder and
analysis and isotopic labeling methods were used to determine
the reactions products and kinetics of the photooxidation by
spectroscopic analysis in the gas phase and on the TiO2 surfaces.
II. Experimental Section
TiO
The P25 TiO
structures, and a surface area of ∼50 m /g at room temperature.
Before any experiments, the TiO was pretreated at 473 K in vacuum
2
to room temperature.
2
is reported to have 70% anatase and 30% rutile
Experiments were carried out in an infrared cell capable of operation
at sample temperatures from 150 to 1500 K. The internal volume of
2
28
3
2
the cell is 385 cm . A detailed description of this cell can be found
8,27
overnight to remove any adsorbed water, and then at ∼623 K for more
elsewhere.
grid using a spraying technique developed in this laboratory. The
temperature of the TiO deposit could be very easily adjusted between
50 and 1500 K using electrical heating of the grid and a programmable
2
In brief, the TiO particles were supported on a tungsten
than 2 h to remove the majority of surface hydroxyl groups and organic
contamination.23 The amount of TiO
used in an experiment was ∼30
2
2
2
mg sprayed over a grid of ∼5.2 cm area. The TiO
2
weight could be
1
visually estimated during the sample preparation27 and accurately
controller operating off of feedback from a thermocouple welded to
the grid. The reaction cell was set up so that infrared spectra of the
determined after performing the oxidation reactions.
TiO
2
surface and of the gaseous species inside the cell could be
The UV source is a 350-W high-pressure mercury arc lamp (Oriel
Corp.). A 325 ( 50 nm bandpass filter was used in all the experiments
except for the photothreshold measurements. The photon flux at 325
measured alternatively. KBr single crystals were used as infrared
windows and were sealed by concentric viton O-rings. The window
seals were differentially pumped by a mechanical pump. The system
was pumped by both turbomolecular and ion pumps and a base pressure
better than 1 × 10 Torr was achieved routinely. The pressure
measurements were carried out using a capacitance manometer (Bara-
tron, 116A, MKS) or using the ion current drawn by the ion pump
17
-2 -1
nm is 1 × 10 photons cm
s
with an absolute accuracy of (10%
29
in our measurement using a EG&G (HUV-4000B) photodiode.
A
-
8
series of interference filters (Oriel Corp., ∼10 nm width) in the range
of 200 to 500 nm was used in our laboratory for the UV photon-energy
dependent study and the photon fluxes at each wavelength were
determined with the photodiode. A manual shutter was also installed
(
Varian, 921-0062). The analytical system we used in this study
consists of a FTIR spectrometer (RS-1, Mattson), a gas chromatograph
5890 series II, Hewlett Packard, column DB-624, FID detector), and
a quadrupole mass spectrometer (M100M, Dycor Electronics Inc.).
The CHClCCl
(TCE) was purchased from Aldrich with a quoted
-1
to accurately control the UV exposure time. In all experiments, 4 cm
(
spectral resolution and signal averaging of 100 scans were used in the
infrared measurements.
2
minimum purity of 99+%. The compound was transferred under
nitrogen into a glass storage bulb after being passed through an alumina
column to remove the small amount of added stabilizer. TCE was
then further purified with several freeze-pump-thaw cycles using
liquid nitrogen and ethanol-liquid nitrogen baths. The oxygen was
III. Results
A. Room Temperature Gas-Phase Photooxidation. Figure
1
shows the gas-phase infrared spectra taken at three different
reaction stages of TCE oxidation on TiO2 at 300 K. Before
reaction, Figure 1a shows a typical gaseous TCE infrared
spectrum; the positions and assignments of the observed infrared
bands are summarized in Table 1. After mixing ∼4 Torr of
TCE and ∼10 Torr of oxygen in the cell and irradiating with
UV light for 105 min, we observed that the trichloroethylene
(
21) Anderson, M. A.; Yamazaki-Nishida, S.; Cervera-March, S. In
Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F.,
Al-Ekabi, H., Eds.; Elsevier Science Publisher: New York, 1993; p 405.
(
22) Nimlos, M. R.; Jacoby, W. A.; Blake, D. M.; Milne, T. A. In
Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F.,
Al-Ekabi, H., Eds.; Elsevier Science Publisher: New York, 1993; p 387.
(TCE) gas-phase spectral features disappeared completely.
(
(
(
(
23) Fan, J.; Yates, J. T., Jr. J. Phys. Chem. 1994, 98, 10621.
24) Sato, S. J. Phys. Chem. 1987, 91, 2895.
25) Sato, S. Langmuir 1988, 4, 1156.
26) Lu, G.; Linsebigler, A.; Yates, J. T., Jr. J. Phys. Chem. 1994, 98,
Dichloroacetyl chloride (DCAC, CHCl2COCl), CO, phosgene
(28) Highly Dispersed Metallic Oxides Produced by Aerosil Process;
Degussa Technical Bulletin Pigments No. 56, 1990, p 13.
(29) Hanley, L.; Guo, X.; Yates, J. T., Jr. J. Chem. Phys. 1989, 91, 7220.
(30) Tanaka, K.; Blyholder, G. J. Phys. Chem. 1972, 76, 1807.
1
1733 and references therein.
27) Basu, P.; Ballinger, T. H.; Yates, J. T., Jr. ReV. Sci. Instrum. 1988,
9, 1321.
(
5

4
688 J. Am. Chem. Soc., Vol. 118, No. 19, 1996
Fan and Yates
Table 1. Assignments of the Observed Infrared Frequencies in the
Photooxidation of TCE at 300 K
compds
infrared freq (cm-1)
Cl
2
CdCHCl (g)
3169 (C-H); 3099 (C-H); 1586, 1555 (CdC);
1251 (CH-Cl); 944, 783, 631 (C-Cl);
1
2
(TCE)
848 (C-H)
1
6
Cl
2
CH-COCl (g) 1820, 1788 (Cd O); 1225 (C-H); 1076,
4
5
(
DCAC)
(g)46
(g)33
2
989 (C-C); 800, 741 (C-Cl ); 1783,
1
8
1
751 (Cd O)
16
COCl
2
1833, 1820 (Cd O); 1682 [2(C-Cl
2
)]; 852,
1
8
6
62 (C-Cl
2
); 1793, 1783 (Cd O)
16
16
18
18
CO
CO (g)
CHCl
2
2349 ( OdCd O); 2331 ( OdCd O)
1
6
18
2143 (Ct O); 2091 (Ct O)
3
(g)
1219, 773
Figure 3. Energy dependence of the calibrated and normalized
photooxidation yield for TCE oxidation at 300 K. The CO yields are
2
obtained from infrared band intensities. The reaction time is 300 min;
initial oxygen pressure is ∼4 Torr; initial TCE pressure is ∼2 Torr.
most of the DCAC was already oxidized in Figure 1c, the
-
1
remaining peaks at 1219 and 773 cm must be due to an
additional species. These two peaks are assigned to CHCl3
based on standard spectra. Thus, the detected products from
TCE photooxidation on TiO2 are CO2, COCl2, CO, HCl, and a
trace of CHCl3. Although our experiment was performed in a
closed reaction cell, the reaction kinetics and reaction products
as detected by infrared measurements are in general agreement
2
Figure 2. Kinetics for Cl CdCHCl (TCE) oxidation at room temper-
ature. For easy comparison, only normalized gas-phase concentrations
are shown.
1
4,22
15
with that reported by Nimlos et al.
and by Holden et al.
in their studies of TCE photooxidation in a flowing air stream.
Closely related intermediates and final products were also
reported for photooxidation of TCE in aqueous solution.16 In
a separate experiment, we found that DCAC underwent pho-
tooxidation in oxygen on TiO2, forming CO2, CO, COCl2, and
HCl as products. This further suggested that DCAC was an
intermediate in the photooxidation of TCE. Our carbon balance
analysis based on infrared absorption calibrations using pure
CO2 and CO shows ∼67 mol % CO2 and ∼14 mol % CO in
the final product mixture after extended UV exposure. The
COCl2 and CHCl3 yield therefore corresponds to 19 mol % by
(Cl2CO), CO2, and HCl spectral features appeared in the infrared
spectrum as revealed in Figure 1b. The assignments of these
newly formed species are also summarized in Table 1. After
extensive UV irradiation (435 min), Figure 1c shows that most
of the DCAC had converted to CO2, phosgene, CO, and HCl.
The kinetics of the TCE oxidation reaction are plotted in Figure
2. The measurements in Figure 2 were obtained from gas-phase
infrared band intensities and were normalized for easy com-
parison. The concentrations for CO2, CO, TCE, DCAC, and
COCl2 were based on their infrared bands at 2349, 2143, 1555,
difference. Assuming as an order of magnitude that each surface
2
-1
site on TiO is of 10 Å area, the complete conversion of TCE
1
077, and 1682 cm , respectively. Figure 2 clearly shows that
2
to oxidation products in these experiments involves about 3-4
DCAC, as a gas-phase reaction intermediate, is further converted
to CO2, CO, and phosgene. Also, the maximum concentration
for DCAC occurred at the time when TCE was completely
consumed, an indication that DCAC was directly formed from
TCE. The constant slope for the TCE consumption curve before
primary molecular oxidation events per surface site. The
reaction rate is controlled by O2 activation at vacancy defect
5
sites (as was determined on single-crystal TiO2(110) ) and if
these sites occupy ∼10% of the surface, then 30-40 primary
molecular oxidation events occur per active defect site at
complete conversion.
1
00 min suggests that the rate of TCE oxidation remained
relatively unchanged during most of the reaction. This may
suggest that the photooxidation reaction follows zero-order
kinetics with TCE.30 The kinetics for HCl formation are not
plotted in Figure 2 due to difficulties in performing a quantitative
A separate experiment was done to look at the possibility of
thermal reactions inside the reaction cell which might occur in
the dark. In this experiment, the UV light was turned off after
30 min of total UV irradiation and an infrared spectrum was
obtained just before the light was turned off. After 1 h another
infrared spectrum was recorded. No change in the TCE
absorption was observed in these two infrared spectra. How-
ever, the photooxidation was observed to resume after the UV
light was turned back on.
Figure 3 shows the result of TCE oxidation under UV
irradiation with photon energies of 2.85 ( 0.03, 3.06 ( 0.04,
3.41 ( 0.05, and 3.71 ( 0.05 eV. All the experiments in Figure
3 were performed at room temperature for 300 min, using ∼2
Torr of TCE and ∼4 Torr of oxygen. The yields of CO2 are
-
1
analysis of the intensity of its rotational bands at ∼2900 cm .
The change in infrared absorption intensities for DCAC was
-
1
consistent for the bands at 1788, 1077, and 741 cm , which
follow the trend shown in Figure 2 for DCAC. However, this
-1
is not the case for bands at 1225 and 800 cm . Careful analysis
shows that each of the two bands actually was a superimposition
of two bands. In Figure 1b, we observed that the band centered
-
1
-1
at 1225 cm has a shoulder at 1210 cm , and the band centered
-
1
-1
at 800 cm has a shoulder at 780 cm . In Figure 1c, the
-
1
-1
band originally at 1225 cm was shifted to 1219 cm , and
that originally at 800 cm was shifted to 773 cm . Since
-
1
-1

Mechanism of Photooxidation of Trichloroethylene on TiO2
J. Am. Chem. Soc., Vol. 118, No. 19, 1996 4689
Figure 5. Development of infrared spectra of photolytically generated
species on the TiO surface during TCE oxidiation at 150 K: (a) before
2
UV irradiation; (b) after 30 min of UV irradiation; and (c) after 80
min of UV irradiation. The insert shows the change of adsorbed TCE
Figure 4. Isotopic shifts for the frequencies of carbonyl stretches of
DCAC, phosgene, CO, and CO
2
during TCE oxidation at 300 K in
16
18
16
O
2
,
O
2
with and without the presence of H
2
O. PH2O ) ∼10 Torr.
(See text for details.)
concentration on the TiO
2
surface.
based on the IR band intensities and are normalized to the
absolute photon-fluxes through the band pass filters measured
before and after each experiment. Figure 3 shows that the CO2
production is very low below a photon energy of 3.1 eV. The
threshold near 3.1 ( 0.1 eV corresponds to the band gap of
cm- are observed for C O2, while Figure 4b clearly shows
1
16
-1
-1
that missing branches at 3675, 3572 cm and 3637, 3524 cm
are observed for C O O and C O2, respectively. The
observation of both isotopic oxygen-16 and oxygen-18 in the
1
6
18
18
rutile and anatase TiO2.3
1,32
CO product in Figure 4b indicates that oxygen from both lattice
The rate of CO2 production
2
increases significantly for photon energies above the band gap.
This result is consistent with other studies reported for gas-
phase and aqueous-solution oxidation of TCE in which control
experiments showed no photooxidation of TCE in the absence
TiO2 and gas-phase O2 is involved in the final oxidation to CO2,
since TiO2 is the only possible source of oxygen-16 and 18O2
is the only possible source of oxygen-18. The extra valleys at
-
1
-1
3568 cm
in Figure 4a and 3500 cm in Figure 4b are due to
of TiO2 particles under UV irradiation (λ > 290 nm).1
4-16
DCAC and phosgene products as shown by a separate calibra-
tion experiment. In this experiment, when the DCAC was all
consumed and COCl2 was thermally reacted at 473 K (see
The increase in the reaction rate with increasing UV photon
energy above the threshold energy at ∼3.1 eV shown in Figure
3
(
was also observed in other studies on single-crystal TiO2-
section III.C for details), we observed that the valley features
110).5 Since O2 is the oxidation agent, the differences in
at 3568 and 3500 cm-1 disappear while the other features of
reaction rates may be attributed to differences in O2-TiO2
excitation. As the photon energy increases, more energetic
electrons will be produced and will tunnel into a broadened
acceptor band in adsorbed O2, giving an increase in reaction
rate over a wide range of photon energy above 3.1 eV, as
observed.
CO2 remain as shown in the spectrum plotted in the dashed
line in Figure 4b. Figure 4c shows the infrared spectra obtained
16
during TCE oxidation with oxygen-18 in the presence of H2 O.
Figure 4c demonstrates that only oxygen-18 is incorporated into
DCAC, COCl2, and CO based upon the positions of the carbonyl
stretching frequencies for those molecules using the same
arguments above when discussing Figures 4a and 4b. This
indicates that the molecular oxygen involvement in the oxidation
reaction is not changed by the presence of water vapor, and
Figure 4 shows the IR spectrum for TCE oxidation on TiO2
1
8
using O2 at 300 K. The results obtained using oxygen-16 are
also shown in Figure 4a for comparison. In Figure 4a, the three
-
1
16O is not incorporated in the photo-
infrared bands at ∼1800 cm correspond to the carbonyl
stretches of DCAC and phosgene. Upon using oxygen-18,
Figure 4b shows that all the carbonyl stretching frequencies were
that oxygen-16 from H2
oxidation products. Unfortunately, the overtone region for CO2
in Figure 4c cannot be used to determine the isotopic content
-
1
-1
of CO in the experiment done in the presence of water due to
shifted down by ∼39 cm . The CO stretch at ∼2100 cm
2
-1
the interference from the OH stretch modes of water. However,
for carbon monoxide was also shifted down ∼52 cm . The
fact that only oxygen-18 is incorporated into DCAC, phosgene,
and CO indicates that only the oxygen from the gas phase is
involved in the oxidation of TCE to DCAC, phosgene, and CO.
The isotopic content of CO2 is not easily discerned from the
CO2 stretching frequencies at ∼2300 cm due to fairly small
isotopic shifts and overlapping of multiple bands. However,
the overtone and combination bands for the CO2 molecule
by comparing the spectra in Figure 4a-c, the very similar
-
1
spectral profiles of the CO2 stretch mode at ∼2300 cm in
Figures 4b and 4c suggest that the isotopic content of carbon
18
16
dioxide in Figure 4c in the presence of water is mostly C O O
-
1
and C18O as was also found in the absence of H 16O. Figure
2
2
4c, compared to Figure 4b, shows that in the presence of H2O,
the yield of CO2 is increased and the yield of DCAC is
decreased. This is probably due to the hydrolysis of DCAC
-
1
containing different oxygen isotopes at ∼3700 cm are well
separated and well documented.3
3-35
and COCl to produce CO , HCl, and dichloroacetic acid.19,36
Observation of the CO2
2 2
missing spectral branches provides the clearest information.
Figure 4a shows that only two missing branches at 3713, 3610
B. Photooxidation on TiO2 at 150 K. Figure 5 shows the
IR spectra of TCE adsorbed on TiO2 surfaces before and after
UV irradiation at 150 K. In these experiments, TCE was first
condensed onto the TiO2 surface at 150 K, then gaseous O2
was introduced into the cell. After TCE was condensed on the
(
(
31) Kraeutler, B.; Bard, A. J. J. Am. Chem. Soc. 1978, 100, 5985.
32) Tang, H.; Berger, H.; Schmid, P. E.; Levy, F. Solid State Commun.
1
994, 92, 267.
33) Herzberg, G. Molecular Spectra and Molecular Structure II, Infrared
and Raman Spectra of Polyatomic Molecules; D. van Nostrand: Princeton,
945.
-
1
(
surface, we observed infrared bands at 3161 and 3077 cm
1
(36) The Merck Index, An Encyclopedia of Chemicals, Drugs, and
Biologicals; 11th ed.; Budavari, S., O’Neil, M. J., Smith, A., Heckelman,
P. E., Eds.; Merck & Co., Inc.: Rahway, 1989.
(
(
34) Berney, C. V.; Eggers, D. F., Jr. J. Chem. Phys. 1964, 40, 990.
35) Chackerian, C., Jr.; Eggers, D. F., Jr. J. Chem. Phys. 1965, 43, 757.

4
690 J. Am. Chem. Soc., Vol. 118, No. 19, 1996
Fan and Yates
Figure 7. Isotopic shifts of carbonyl stretching frequencies of
Figure 6. Infrared spectra of the OH stretching region for the
photooxidation of TCE at 150 K. The absorption spectra are shown at
the left, and the corresponding difference spectra are shown on the
right. The left-hand spectra have been ratioed against the spectrum
obtained prior to TCE adsorption, clearly showing the conversion of
isolated hydroxyl groups to associated hydroxyl groups by TCE
adsorption; this process is reversed on photooxidation.
photolytically generated adsorbed species from TCE oxidation on TiO
2
at 150 K in 16
18
2 2
O and O .
associated OH groups interacting with TCE on the surface. The
-
1
infrared peaks at ∼3700 cm are due to isolated OH groups.
The negative infrared absorption of the isolated OH groups
results from ratioing of sample transmittance containing ad-
sorbed TCE with that of the background without TCE, indicating
loss of isolated OH groups after adsorption of TCE onto the
TiO2 surface. The difference spectra are shown in the right
panel of Figure 6. Spectrum 6d is obtained by subtracting
spectrum 6a from 6b, and spectrum 6e by subtracting spectrum
for TCE (the symmetric and asymmetric C-H stretch modes),
-
1
-1
at 1589 cm for the CdC stretch, at 1246 cm for the CH-
-
1
Cl bend, and at 1090 cm for the C-Cl stretch, based on the
assignment for gas-phase TCE.12 After UV irradiation for ∼30
min, Figure 5b shows new multiple infrared features in the
-1
-1
1
600-1800-cm region with a peak at 1737 cm . There was
6
a from 6c. The difference spectra show that the intensity of
-
1
also an increase in intensity in the band at 1090 cm . Based
on a separate experiment using pure DCAC, the newly formed
bands in Figure 5b are assigned to DCAC. Figure 5c shows
the infrared spectrum after a total UV irradiation of ∼80 min;
the associated OH stretches decreased and the intensity of the
isolated OH stretches increased during UV irradiation, suggest-
ing that isolated OH groups on the surface were regenerated
from associated OH groups during the photooxidation due to
consumption of adsorbed TCE. This is not surprising since a
similar phenomenon was also observed on other oxide sur-
-
1
we observed further growth of the band at 1737 cm due to
continued DCAC production, and a relatively large increase in
-
1
the shoulder band at 1719 cm due to COCl2 production. At
the same time, we observed development of a shoulder peak at
570 cm and a new peak at 1225 cm , and further increase
in the intensity of the band at 1090 cm . We assign those
newly developed bands at 1719, 1225, and 1090 cm to COCl2
adsorbed on TiO2. The shoulder at 1570 cm is tentatively
assigned to surface carboxylate species and will be discussed
later.37
38
faces. However, Figure 6 illustrates a very important point
that the hydroxyl groups on the TiO2 surface were not consumed
in the photooxidation process.
-
1
-1
1
-
1
Figure 7 shows the infrared spectra of surface species
following partial TCE oxidation (as an adsorbed species) on
TiO2 using oxygen-18 at 150 K. The result using oxygen-16
is also shown for the purpose of comparison. In the oxidation
using oxygen-16, in Figure 7a, we observed multiple bands in
-1
-
1
The insert of Figure 5 shows the kinetics of TCE oxidation
on the TiO2 surface at 150 K. It shows that the rate of loss of
TCE decreases with time and the oxidation reaction essentially
stops after 40 min of UV irradiation. The insert also shows
that ∼30% of the TCE is still unreacted after 80 min of UV
irradiation. The termination of the photooxidation was not due
to lack of oxygen since we observed that the TCE was
completely oxidized after heating up the TiO2 to 300 K to
vaporize adsorbed species and performing the gas-phase pho-
tooxidation at 300 K as in previous experiments. The experi-
ments shown in Figure 5 which involve the presence of adsorbed
layers on TiO2 at 150 K yield very small levels of TCE oxidation
compared to the gas-phase experiments at 300 K.
-1
-1
the 1600-1800-cm region, and a shoulder at 1570 cm which
we assigned to the carbonyl stretches of DCAC, COCl2, and
surface carboxylate groups, respectively. The band at 1589
-1
cm is the CdC stretch of TCE which remains on the TiO2
surface after partial oxidation. When oxygen-18 is used, Figure
7
∼
b shows that the carbonyl stretches of DCAC and COCl2 at
-1
-1
1800 cm are shifted ∼35 cm to lower wavenumbers, and
-1
the shoulder at ∼1570 cm of the surface carboxylate is also
-1
shifted to 1551 cm . The isotopic shifting we observed in
Figure 7 is consistent with our assignments of those bands to
carbonyl stretches of DCAC, COCl2, and surface carboxylate
18
species. The shifts upon using oxygen-18 indicate that O2 in
the gas phase is involved in the oxidation of TCE to DCAC,
COCl , and carboxylate species on the TiO surface. This
Figure 6 shows the infrared spectral changes in the OH
stretching region during the photooxidation of TCE on the TiO2
surface at 150 K. In the absorption spectra shown in the left
panel, spectrum (a) was obtained before any UV irradiation,
spectrum (b) was obtained after 30 min of UV irradiation, and
spectrum (c) was obtained after a total of 80 min of UV
2
2
isotopic result is the same as we saw in Figure 4 for the gas-
phase photoreaction at room temperature. Figure 7 also shows
that the IR spectra of adsorbed CO and CO are also shifted
2
upon using oxygen-18, showing that oxygen from the gas phase
was also involved in the production of those small quantities
of CO and CO2.
-
1
irradiation. The broad peak at ∼3550 cm corresponds to the
(
37) Nakamoto, K. Infrared and Raman Spectra of Inorganic and
(38) Beebe, T. P., Jr.; Crowell, J. E.; Yates, J. T., Jr. J. Phys. Chem.
1988, 92, 1296.
Coordination Compounds, 4th ed.; John Wiley & Sons: New York, 1986.

Mechanism of Photooxidation of Trichloroethylene on TiO2
J. Am. Chem. Soc., Vol. 118, No. 19, 1996 4691
B. Sequence of Photooxidation Reactions. The overall
reactions observed here may be written as in eqs 2-7:
TiO2
CHCldCCl (g)
8 CHCldCCl (a)
(2)
(3)
(4)
2
2
TiO2
O (g)
8 O (a)
2
2
TiO2
O (a) + hν
8 O*(a)
2
2
TiO2
1
/2
O* (a) + CHCldCCl (a)
8 Cl CH-COCl(a) (5)
2
2
2
TiO2
Cl CH-COCl(a) T Cl CH-COCl(g)
(6)
2
2
TiO2
Cl CH-COCl(a) + O* (a)
8 ... Cl CO(g) + CO(g) +
2
2
2
Figure 8. Gas-phase infrared spectrum obtained after photooxidation
CO (g) + HCl(g) + CHCl (g)(trace) (7)
2 3
1
8
of TCE at 473 K in
2
O for 365 min. The insert shows the change of
TCE concentration with reaction time based on infrared measurements.
The stoichiometry of reaction 7 with respect to carbon and
oxygen could be determined from our experiments as ap-
proximately
C. Photooxidation and Thermal Oxidation at 473 K. The
degree of involvement of thermal processes in the sequence of
steps observed in the photooxidation of TCE at 300 K was
investigated by carrying out similar measurements at 473 K.
Figure 8 shows the infrared spectrum obtained after photooxi-
dation of TCE at 473 K using 18O2 for 365 min. It can be seen
that COCl2 is absent in the spectrum and only CO2, CO, HCl,
and a small quantity of tetrachloroethylene, C2Cl4, are observ-
able in the gas phase. The presence of C2Cl4 is indicated by
CHCl COCl + 1.17O (a) f
2
2
0.38Cl CO + 0.28CO + 1.34CO (8)
2
2
In addition, we have observed the thermal reaction at 473 K:26
4
73 K
2Cl CO(g) + TiO (reduced)
8
2
2
O2
-
1
C Cl (g) + TiO (oxidized) (9)
2 4 2
the small bands at 912 and 780 cm . Control experiments
indicate that no thermal reaction occurs between TCE and O2
at 473 K over TiO2. Thus TCE is only photooxidized at 473
K. In a control experiment, we found that COCl2 reacts rapidly
with TiO2 at 473 K by means of a thermal process, giving C2-
Cl4 in the gas phase in the presence of O2(g).
The constant slope observed in Figure 2 for TCE may be
attributed to the electron-transfer kinetics for electrons from the
TiO2 surface to a saturated layer of adsorbed O2, as discussed
4
30
by Gerischer for O2 on TiO2, and by Tanaka and Blyholder
for CO oxidation on ZnO. It is suggested that the electron-
transfer step (eq 4) is the rate-controlling step. This will give
apparent zero-order kinetics for TCE under conditions where a
saturated layer of TCE is present.
The insert of Figure 8 shows the kinetics of TCE consumption
in the photooxidation at 473 K. The gradual decrease in the
slope of the TCE consumption curve clearly suggests that the
rate of TCE consumption decreases with reaction time, which
is in contrast to the results observed in the oxidation carried
out at 300 K.
The observation of CHCl3 suggests that a side reaction was
also occurring along with the oxidation reaction. The formation
40
of CHCl3 was also reported by Sanhueza and Heicklen in the
3
41
reaction of O( P) with TCE, and by Huybrechts and Meyers
IV. Discussion
in the gas-phase chlorine-photosensitized oxidation of TCE.
However, in the chlorine-photosensitized oxidation, trichloro-
ethylene epoxide and CCl4 were also formed. The fact that we
only observed CHCl3 suggests that CHCl3 was formed through
A. Band Gap Excitation for TiO2-Source of Activation
for TCE Photooxidation. The band gap of rutile-TiO2 is near
31,32
3
.02 eV while for anatase-TiO2 the band gap is 3.2 eV.
powdered TiO2 used in this work consists of a mixture of anatase
70%) and rutile (30%), and exhibits a photooxidation threshold
The
3
3
O( P). The origin of O( P) may be due to the following
(
42
mechanism, as suggested by Teichner and Formenti.
for TCE near 3.1 eV, as shown in Figure 3. This is consistent
with the band gaps of anatase and rutile, and within the error
of the experimental measurements.
-
-
O (a) + e f O (a)
(10)
(11)
2
2
An alternative view, involving a photochemical radical chain
mechanism, is not supported by these results. The UV
-
+
O (a) + h f 2O*(a)
2
39
absorption threshold for TCE is at ∼4.5 eV, far above the
C. Comparison of the Photooxidation of TCE at Various
Temperatures. The two experiments at 150 and 300 K are
very informative for understanding the photooxidation of TCE.
At 300 K, the coverage of adsorbed TCE in equilibrium with
TCE(g) is expected to be small in comparison to experiment at
3
.1-eV experimental threshold observed here.
Both O2 and TiO2 are necessary for the photooxidation of
TCE, in accordance with previous studies.1
4-16,21
TiO2 lattice
oxygen is excluded from involvement in the photochemistry,
except in an isotopic exchange process with the final product,
CO2. This oxygen isotopic exchange process probably involves
a carboxylate species which was observed on the surface
following TCE photooxidation.
(40) Sanhueza, E.; Heicklen, J. Int. J. Chem. Kinet. 1974, 6, 553.
(41) Huybrechts, G.; Meyers, L. Trans. Faraday Soc. 1966, 62, 2191.
(42) Teichner, S. J.; Formenti, M. Heterogeneous Photocatalysis. In
Photoelectrochemistry, Photocatalysis and Photoreactors, Fundamentals
and DeVelopments, Series C; Schiavello, M., Ed.; D. Reidel Publishing
Company: Dordrecht, The Netherlands, 1984; Vol. 146, pp 457-489.
(39) Hubrich, C.; Stuhl, F. J. Photochem. 1980, 12, 93.

4
1
692 J. Am. Chem. Soc., Vol. 118, No. 19, 1996
Fan and Yates
50 K where a condensed layer of TCE is produced. Two large
exclude, at the sensitivity limits of our measurements, the
involvement of processes like
differences in behavior were noted under these two conditions:
1) At 300 K, complete consumption of TCE occurred to
produce COCl2, CO, CO2, HCl, and CHCl3.
2) At 150 K, incomplete reaction occurred for equivalent
(
+
•
+
H O + h f OH + H
(12)
2
(
quantities of TCE, O2, and TiO2 and a similar UV fluence.
This comparison suggests that site blocking occurs under
conditions where the condensed layer of TCE is present. It is
likely that TCE blocks access of O2(g) to the surface sites
necessary for addition of O2 by the photoprocess. In addition,
photoproducts such as COCl2 may also block sites needed by
O2, turning off the photooxidation process as seen in Figure 5.
The isotopic results are the same for the experiments at 150
and 300 K. In both cases, only the oxygen from the gas-phase
molecular O2 was observed to be incorporated in the reaction
intermediates.
as postulated by others.1,3,6,9,43,44
V. Summary of Results
The kinetics and product sequence for the photooxidation of
trichloroethylene (TCE) on powdered TiO2 have been studied
using infrared spectroscopy, in separate experiments where little
background water (or surface hydroxyls) is present or alterna-
tively where large amounts of background water are employed.
The following results have been obtained:
(1) TCE is photooxidized on TiO2 by electronically activated
D. Role of H2O in the Photooxidation of TCE. Practical
photooxidation reactions are often carried out in the presence
of water. Therefore in Figure 4 we carried out the photooxi-
dation of TCE with 18O2 in the presence of H2 O(g). The
infrared spectra of the gas proucts indicate that H2 O is
ineffective in incorporating oxygen-16 into the intermediate
chemisorbed O . All oxidation products originate from O rather
2
2
than lattice oxygen in TiO , with the exception of the CO2
2
product which probably exchanges oxygen atoms with lattice
oxygen via a surface carboxylate species.
16
1
6
(2) The photothreshold for photooxidation of TCE on TiO2
is near 3.1 eV, indicative of band gap excitation in TiO2 leading
to O2 excitation by electron-hole pair excitation in the TiO2 and
thereby to the photocatalytic oxidation process.
(DCAC) or into the final products (CO and Cl2CO). This and
the fact that surface hydroxyl groups were not consumed during
photooxidation, as shown in Figure 6, proVide compelling
eVidence that surface hydroxyl groups are not inVolVed in the
photooxidation of TCE, in contrast with mechanisms postulated
(3) A sequence of chemical steps are observed in the
photooxidation of TCE, which first photooxidizes to Cl2HC-
COCl (DCAC). Subsequent continued photooxidation produces
Cl2CO, CO, CO2, HCl, and CHCl3 at 300 K.
1
2,21
in other studies.
The results of Figure 4 also indicate that
H2O causes an enhancement of the CO2 yield and a decrease in
the level of DCAC and COCl2 observed as intermediate species.
This is probably due to reactions of water with DCAC and
(4) At 473 K, a thermal reaction converting Cl2CO to
Cl2CdCCl2 on TiO2 is observed. This molecule then undergoes
photooxidation itself.
1
9,36
COCl2.
(5) Evidence is presented showing that at 150 K, the
The lack of oxygen-16 incorporation from water into the
photooxidation products indicates that mechanisms involving
adsorption of TCE (and possibly the photooxidation products
of TCE) is able to retard the photooxidation process possibly
by blockage of sites for O2 adsorption.
•
3,20,21
OH species as oxidizing agents
are probably inoperative
in TCE photooxidation over TiO2, consistent with recent studies
(6) The isotopic labeling of products in the photooxidation
of CH3Cl photooxidation over TiO2(110).5 These observations
of TCE to DCAC, Cl2CO, CO, and CO2 is uninfluenced by the
presence of isotopically labeled H2O. This indicates that
(
43) Okamoto, K.; Yamamoto, Y.; Tanaka, H.; Tanaka, M. Bull Chem.
Soc. Jpn. 1985, 58, 2015.
44) Blake, D. M.; Webb, J.; Turchi, C.; Magrini, K. Solar Energy Mater.
991, 24, 584.
45) Miyake, A.; Nakagawa, I.; Miyazawa, T.; Ichishima, I.; Shiman-
ouchi, T.; Mizushima, S. Spectrochim. Acta 1958, 13, 161.
46) Hannus, I.; Kiricsi, I.; Tasi, Gy; Fejes, P. Appl. Catal. 1990, 66,
•
mechanisms involving OH as an oxidizing agent are unlikely.
(
1
Acknowledgment. We acknowledge with thanks the Army
Research Office for financial support.
(
(
L7.
JA952155Q