In situ solid-state NMR studies of trichloroethylene photocatalysis: Formation and characterization of surface-bound intermediates

Article abstract of DOI:10.1021/ja974192i

In situ solid-state NMR methodologies have been employed to investigate the photocatalytic oxidation of trichloroethylene (TCE) over two TiO₂-based catalysts, Degussa P-25 powder and a monolayer TiO₂ catalyst dispersed on porous Vycor glass. ¹³C magic angle spinning (MAS) experiments reveal that similar reaction intermediates form on the surfaces of both catalysts. Long- lived intermediates, including dichloroacetyl chloride (Cl₂HCCOC1, DCAC), carbon monoxide, and pentachloroethane and final products CO₂, phosgene (Cl₂CO), and HCl were observed under dry conditions. The presence of molecular oxygen was found to be essential for TCE photooxidation to proceed. Adsorbed water was found to greatly reduce the formation of phosgene. The formation of surface-bound dichloroacetate and trichloroacetate species was observed and identified via ¹³C cross polarization MAS experiments. Dichloroacetate, which forms from mobile DCAC, appears to be bound to the nonirradiated surfaces of the powdered TiO₂ catalysts and further degradation was not possible. Formation of di- and trichloroacetate also takes place on the TiO₂/PVG catalyst in the absence of light; however, their concentrations are low. Degradation studies of these surface-bound species indicate that the photooxidation of dichloroacetate, is slow and results in the formation of phosgene and CO₂, while trichloroacetate remains resistive to degradation on the TiO₂/PVG catalyst. Our results also indicate that the formation of DCAC and phosgene seems to be a general result of TCE degradation which is not limited to TiO₂ photocatalysis but instead may be more characteristic of the types of initiating species which are formed by UV irradiation. However, the TiO₂ surface is the most effective in terms of the observed initial rates of degradation.

Full text of DOI:10.1021/ja974192i

4388
J. Am. Chem. Soc. 1998, 120, 4388-4397
In Situ Solid-State NMR Studies of Trichloroethylene Photocatalysis:
Formation and Characterization of Surface-Bound Intermediates
Son-Jong Hwang, Chris Petucci, and Daniel Raftery*
Contribution from the Department of Chemistry, H. C. Brown Laboratory, Purdue UniVersity,
West Lafayette, Indiana 47907-1393
ReceiVed December 10, 1997
Abstract: In situ solid-state NMR methodologies have been employed to investigate the photocatalytic oxidation
of trichloroethylene (TCE) over two TiO2-based catalysts, Degussa P-25 powder and a monolayer TiO2 catalyst
13
dispersed on porous Vycor glass. C magic angle spinning (MAS) experiments reveal that similar reaction
intermediates form on the surfaces of both catalysts. Long-lived intermediates, including dichloroacetyl chloride
(Cl2HCCOCl, DCAC), carbon monoxide, and pentachloroethane and final products CO2, phosgene (Cl2CO),
and HCl were observed under dry conditions. The presence of molecular oxygen was found to be essential
for TCE photooxidation to proceed. Adsorbed water was found to greatly reduce the formation of phosgene.
The formation of surface-bound dichloroacetate and trichloroacetate species was observed and identified via
1
3
C cross polarization MAS experiments. Dichloroacetate, which forms from mobile DCAC, appears to be
bound to the nonirradiated surfaces of the powdered TiO2 catalyst and further degradation was not possible.
Formation of di- and trichloroacetate also takes place on the TiO2/PVG catalyst in the absence of light; however,
their concentrations are low. Degradation studies of these surface-bound species indicate that the photooxidation
of dichloroacetate is slow and results in the formation of phosgene and CO2, while trichloroacetate remains
resistive to degradation on the TiO2/PVG catalyst. Our results also indicate that the formation of DCAC and
phosgene seems to be a general result of TCE degradation which is not limited to TiO2 photocatalysis but
instead may be more characteristic of the types of initiating species which are formed by UV irradiation.
However, the TiO2 surface is the most effective in terms of the observed initial rates of degradation.
I. Introduction
environmental contaminant. Chlorine photosensitized oxygen
3
inhibition and O( P) initiated oxidation of TCE with molecular
Heterogeneous semiconductor photocatalysis represents an
emerging area of environmental catalysis with the broadly
defined goal of efficiently detoxifying hazardous organic
pollutants. Photocatalytic processes using a variety of semi-
conductor materials has been intensively investigated after an
initial report that indicated the potential of TiO2 materials for
photochemical conversion and light harvesting.1 Presently, a
growing interest has developed for efficient and inexpensive
methodologies to reduce environmental problems caused by
toxic chemicals.2 Due to its stability and nontoxicity, TiO2
has been most frequently investigated for the degradation of a
variety of environmentally harmful organic molecules including
halogenated and nonhalogenated compounds. A number of
researchers have reviewed the important features of TiO2
surface chemistry at both the liquid-surface and gas-surface
8-10
oxygen were established two decades ago.
More recently,
studies of the photocatalytic oxidation of TCE using semicon-
ductor photocatalysts have been reported at both the liquid-
11-17
16-36
solid
and gas-solid
interfaces. Currently, the variety
(8) Bertrand, L.; Franklin, J. A.; Goldfinger, P.; Huybrechts, G. J. Phys.
Chem. 1968, 72, 396.
(
01.
9) Sanhueza, E.; Hisatsune, I. C.; Heicklen, J. Chem. ReV. 1976, 76,
8
(
10) Sanhueza, E.; Heicklen, J. Int. J. Chem. Kinetics 1974, 6, 553.
,3
(11) Pruden, A. L.; Ollis, D. F. J. Catal. 1983, 82, 404.
(
(
12) Ahmed, S.; Ollis, D. F. Solar Energy 1984, 32, 597.
13) Zhang, Y.; Crittenden, J. C.; Hand, D. W.; Perram, D. L. EnViron.
Sci. Technol. 1994, 28, 435.
(14) Glaze, W. H.; Kenneke, J. F.; Ferry, J. L. EnViron. Sci. Technol.
993, 27, 177.
1
(
15) Matthews, R. W. J. Catal. 1988, 111, 264.
(
16) Lichtin, N, N.; Avudaithai, M. EnViron. Sci. Technol. 1996, 30, 2014.
4
-7
interfaces.
(17) Larson, S. A.; Falconer, J. L. In Photocatalytic Purification and
Treatment of Water and Air; Ollis, D. F., Al-Ekabi, H., Eds.; Elsevier:
Amsterdam, 1993; p 473.
There has been a great deal of interest in the photooxidation
of trichloroethylene (TCE) due to its role as a significant
(18) Hwang, S. -J.; Petucci, C.; Raftery, D. J. Am. Chem. Soc. 1997,
1
19, 7877.
(
(
1) Fujishima, A.; Honda, K. Nature 1972, 37, 238.
2) Schiavello, M. In Photocatalysis and EnVironment: Trends and
(19) Fan, J.; Yates, Jr. J. T. J. Am. Chem. Soc. 1996, 118, 4686.
(20) Hung, C.-H.; Marinas, B. J. EnViron. Sci. Technol. 1997, 31, 562.
(21) Hung, C.-H.; Marinas, B. J. EnViron. Sci. Technol. 1997, 31, 1440.
(22) Nimlos, M. R.; Jacoby, W. A.; Blake, D. M.; Milne, T. A. EnViron.
Applications; Schiavello, M., Ed.; NATO ASI Series C; Boston, 1987; Vol.
2
37, p 351.
3) Mills, A.; Davies, R. H.; Worsley, D. Chem. Soc. ReV. 1993, 22,
17.
(
Sci. Technol. 1993, 27, 732.
4
(23) Jacoby, W. A.; Nimlos, M. R.; Blake, D. M.; Noble, R. D.; Koval,
C. A. EnViron. Sci. Technol. 1994, 28, 1661.
(24) Larson, S. A.; Falconer, J. L. Appl. Catal. B: EnViron. 1994, 4,
325.
(25) Berman, E.; Dong, J. In Chemical Oxidation: Technologies for the
Ninties; Eckenfelder, W. W., Bowers, A. R., Roth, J. A., Eds.; Technomic
Pub. Co., Inc.: Lancaster, 1993; p 183.
(
4) Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 341.
(5) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem.
ReV. 1995, 95, 69.
6) Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. Chem. ReV. 1995, 95,
35.
7) Hagfeldt, A.; Gratzel, M. Chem. ReV. 1995, 95, 49.
(
7
(
S0002-7863(97)04192-9 CCC: $15.00 © 1998 American Chemical Society
Published on Web 04/23/1998

Solid-State NMR Studies of Trichloroethylene Photocatalysis
J. Am. Chem. Soc., Vol. 120, No. 18, 1998 4389
of different approaches to TCE detoxification is growing and
reaction pathway to produce DCAC when low surface area TiO2
37
19
includes the oxidation by nanoscale iron particles and biotrans-
catalysts were employed. Recently, Fan et al. reevaluated the
formation methods.3
8-41
In the present work, the photocatalytic
role of O2 radical and concluded that it served as the major
-
oxidation of TCE on TiO2 catalysts was investigated with an
aim of examining surface reaction mechanisms.
oxidizing agent of TCE over a powdered TiO2 catalyst. DCAC
was identified as a major reaction intermediate in their study.
18
18
The photocatalytic activity of TiO2 is known to result from
the generation of electron-hole pairs in the bulk semiconductor
which can migrate to the surface and form OH and O2- radicals.
Both of these radical species have the potential to oxidize
organic molecules at the TiO2 surface. However, despite the
contributions from a number of research groups, detailed
mechanisms of the photocatalytic oxidation processes at the TiO2
surface remain elusive, particularly regarding the initial steps
involved in the radical reactions, which may involve one or
When H2 O was used as a coreactant, O was not incorporated
into any of the products, which prompted the authors to rule
19
out the possibility that a OH radical mechanism was dominant.
As reviewed above, precise identification and characterization
of the reaction intermediates formed during the photocatalytic
reaction of TCE is critical for the determination of reaction
mechanisms. For the detection of chemical species resulting
from TCE oxidation, most of the previous studies utilized
analytical methods such as GC, GC/MS, FT-IR, MS, or trapping
agents for the detection of chlorinated compounds and CO2.
Inhibition of further reactions during sampling was unavoidable
in many cases. In some cases, in situ detection of intermediates
-
more of the following radical species: O2 , OH, and Cl.
Although complete mineralization may occur, the reaction
mechanisms are complex as demonstrated by the large number
of intermediates that have been observed. This situation is
particularly evident in reaction conditions where the oxygen
without interruption of the reaction was employed using FT-IR
19,30,36
spectroscopy.
While sensitive, FT-IR methods exhibit
2
0,21
concentration is low.
Regarding mechanisms, the relative
difficulties in direct quantitation, and although this can be
-
roles of O2 , OH and Cl as initiating species are still under
consideration. Identification of dichloroacetaldehyde (Cl2-
22,23
avoided by the complimentary use of GC/MS,
careful and
separate calibration runs for each intermediate are necessary.
CHCHO) as an intermediate led to the postulation that OH
Recently, with an attempt to resolve some of the issues
discussed above and to provide complementary information for
understanding the complex surface chemistry, we introduced a
new approach to the study of photocatalytic reactions, namely
in situ solid-state nuclear magnetic resonance (SSNMR) spec-
radicals acted as the initiating oxidizing agent.1
1,42
In this
scenario, the role of molecular oxygen on the TiO2 surface was
limited to electron-trapping. Based on the same intermediate,
a reductive pathway14 involving either the direct capture of a
photoexited electron by TCE and/or possible initiation by
hydroperoxyl radical (HOO)30 was also suggested. On the other
hand, identification of dichloroacetyl chloride (Cl2CHCOCl,
DCAC) as a major intermediate in studies by Nimlos and co-
workers at the gas-solid interface22 as well as the observed
high quantum yield (molecules degraded/incident photon)
invoked the reconsideration of previous work on chlorine
photosensitized oxygen inhibition8 and resulted in the adop-
tion of a Cl radical initiated chain reaction mechanism. They
proposed a mechanism in which OH radical attacked TCE in
the process of Cl radical formation.22 In studies of gas-phase
18
43,44
troscopy. SSNMR techniques, particularly in situ methods,
have played an invaluable role in the study of a broad range of
issues involved in heterogeneous catalysis due to the wealth of
structural and dynamical information available via NMR. As
described in our previous work, in situ SSNMR is useful in
exploring the complex reaction chemistry of TCE both in the
gas-phase and on the catalyst surface during the course of the
reaction due to its atomic specificity, high resolution, and
quantitative capabilities. Several new reaction intermediates
were identified, and a number of the important intermediates
that had previously been observed in other studies were evident
-10
33
TCE photooxidation using porous TiO2 pellets, Anderson et al.
concluded that the Cl radical chain reaction constituted a minor
18
in the spectra and could be quantified. In this paper, we report
our more detailed studies on the photocatalytic oxidation of TCE
under several different reaction conditions. In particular, we
have focused on the formation and evolution of surface-bound
species on TiO2 catalysts. By elucidating the structures of all
of our observed reaction intermediates involved during the TCE
photodegradation we have additional information on the reaction
mechanisms. Finally, a comparison of the photocatalytic
reactions over several catalysts allows us to draw some
conclusions regarding the surface chemistry of TCE.
(
(
(
(
(
(
26) Lichtin, N. N.; Avudaithai, M. Res. Chem. Intermed. 1994, 20, 755.
27) Luo, Y.; Ollis, D. F. J. Catal. 1996, 163, 1.
28) Dibble, L. A.; Raupp, G. B. Catal. Lett. 1990, 4, 345.
29) Dibble, L. A.; Raupp, G. B. EnViron. Sci. Technol. 1992, 26, 492.
30) Phillips, L. A.; Raupp, G. B. J. Mol. Catal. 1992, 77, 297.
31) Yamazaki-Nishida, S.; Cervera-March, S.; Nagano, K. J.; Anderson,
M. A.; Hori, K. J. Phys. Chem. 1995, 99, 15814.
32) Yamazaki-Nishida, S.; Nagano, K. J.; Phillips, L. A.; Cervera-March,
S.; Anderson, M. A. J. Photochem. Photobiol. A:Chem. 1993, 70, 95.
33) Yamazaki-Nishia. S.; Fu, X.; Anderson, M. A.; Hori, K. J.
Photochem. Photobiol. A: Chem. 1996, 97, 175.
34) Annapragada, R.; Leet, R.; Changrani, R.; Raupp, G. B. EnViron.
Sci. Technol. 1997, 31, 1898.
35) In Photocatalytic Purification and Treatment of Water and Air; Ollis,
(
(
(
II. Materials and Methods
(
D. F., Al-Ekabi, H., Eds.; Elsevier: Amsterdam, 1993. (a) Holden, W.;
Marcellino, A.; Valic, D.; Weedon, A. C. p 393. (b) Nimlos, M. R.; Jacoby,
W. A.; Blake, D. M.; Milne, T. A. p 387. (c) Anderson, M. A.; Yamazaki-
Nishida, S.; Cervera-March, S. p 405. (d) Al-Ekabi, H.; Butters, B.; Holden,
W.; Powell, T.; Story, J. p 719.
A. Catalyst Preparation. Photocatalytic oxidation reactions were
carried out in sealed glass NMR tubes at room temperature. Catalyst
45
samples consisting of approximately 180 mg of TiO
2
powder were
packed into 5 mm glass NMR tubes (Norell), which were then attached
to a gas manifold. The powder was first evacuated at 773 K for 4 h to
remove both weakly and strongly adsorbed water molecules as well as
the majority of surface hydroxyl groups and then calcined at 773 K in
(36) Driessen, M. D.; Goodman, A. L.; Miller, T. M.; Zaharias, G. A.;
Grassian J. Phys. Chem. B 1998, 102, 549.
(
(
42.
37) Wang, C.-B.; Zhang, W.-X. EnViron. Sci. Technol. 1997, 31, 2154.
38) Wilson, J. T.; Wilson, B. H. Appl. EnViron. Microbiol. 1985, 49,
a ceramic heater under 1 atm of O
2
gas for another 4 h. Typically, the
twice during calcination.
2
1
O
2
was pumped out and replaced with fresh O
2
(
39) Hopkins, G. D.; McCarty, P. L. EnViron. Sci. Technol. 1995, 29,
-
5
Evacuation down to 2 × 10 Torr was followed by cooling the
628.
(
(
40) McCarty, P. L. Science 1997, 276, 1521.
41) Newman, L. A.; Strand, S. E.; Choe, N.; Duffy, J.; Ekuan, G.;
(43) Haw, J. F. In NMR techniques in catalysis; Bell, A. T., Pines, A.,
Eds.; Marcel Dekker, Inc.: New York, 1994; p 139.
(44) Haw, J. F.; Nicholas, J. B.; Xu, T.; Beck, L. W.; Ferguson, D. B.
Acc. Chem. Res. 1996, 29, 259.
Ruszaj, M.; Shurtleff, B. B.; Wilmoth, J.; Heilman, P.; Gordon, M. P.
EnViron. Sci. Technol. 1997, 31, 1062.
(42) Turchi, C. S.; Ollis, D. F. J. Catal. 1990, 122, 178.

4390 J. Am. Chem. Soc., Vol. 120, No. 18, 1998
Hwang et al.
2
Figure 1. UV transmittance spectra of PVG (dotted curve) and a TiO /
PVG catalyst (solid curve). A spectrum (solid circles) of the light from
the Xe arc lamp acquired at the output of the quartz rod is also shown.
Figure 2. Schematic diagram of the in situ optical/MAS NMR probe
head used in the present study.
sample through a 10 mm gap in the radio frequency coil. Figure 2
shows a schematic diagram of the probe head. The optical/MAS NMR
powdered sample to room temperature in the heater. 13C labeled TCE
(
Cambridge Isotope Laboratories) was then introduced into the gas
manifold, and frozen onto the calcined TiO powder using a liquid
nitrogen trap. The amount of TCE used was routinely 48 µmol. O
1
13
probe is doubly tuned for H and C observation at frequencies of
00 and 75.4 MHz, respectively. A 300 W Xe arc lamp (ILC
2
3
2
Technology) was used as the UV light source. Near infrared light
produced by the lamp was filtered by a dichroic mirror (Oriel Corp.),
and the remaining near UV light was delivered to the sample region
inside the magnet using a liquid-filled optical light guide (Oriel Corp.)
that had a polished 70 mm long suprasil quartz rod attached to its end.
The quartz rod reduced the effects of radio frequency pickup by the
probe coil from the aluminum-encased liquid-light guide. The light
quickly diverges from the quartz rod to cover the entire sample region.
The light spectrum obtained at the output of the quartz rod is displayed
in Figure 1. The spectrum indicates that light used in this study ranges
in wavelength from 350 to 550 nm and its maximum matches the onset
(60 µmol) was subsequently loaded onto the sample, and the NMR
tube was sealed off 10-12 mm above the catalyst sample.
A second TiO catalyst was prepared by dispersing TiO
2
2
(with
coverage of a monolayer) on the surface of transparent porous Vycor
glass (PVG). The preparation method of supported monolayer TiO
catalysts has been established by Anpo et al.46-48 As depicted in eq 1,
2
2
of the absorption region for the TiO /PVG catalyst. The near UV light
power that reaches the sample was measured to be 5 mW by standard
ferrioxalate actinometry.50
the catalyst was prepared from the gas phase hydrolysis of TiCl
4
with
OH groups at the PVG surface. Typically, 88 µmol of TiCl (Aldrich)
4
was reacted at room temperature using a PVG rod (Corning 7930, BET
Results and Discussion
2
surface area ∼150 m /g, pore diameter 40 Å), that was 3.6 mm in
diameter and 12 mm long, weighed 180 mg, and had been previously
degassed and calcined at 773 K for 4 h each. Since the saturated vapor
A. Photocatalytic Oxidation of TCE on the TiO Powder
Catalyst. Figure 3a shows proton-decoupled C MAS NMR
spectra obtained during the in situ photooxidation of TCE in
the presence of O2 and the powdered TiO2 catalyst. Similar
results were observed and reported earlier. The narrow line
widths of the peaks indicate that these species are very mobile
and most likely exchange rapidly between the surface and the
gas phase. Typically each NMR spectrum consists of the
average of 48 transients that were obtained with a delay time
of 4 s between acquisitions, such that the time evolution could
2
pressure of TiCl
typically four reaction cycles were used to insure the complete reaction
of all the surface OH groups with TiCl . HCl gas produced from the
reaction was completely evacuated and another loading of 88 µmol of
TiCl followed. The anchoring process was modified by controlling
the amount of TiCl introduced for different coverages, and the whole
process could be repeated to create multilayer coverages. After
anchoring, the catalyst was hydrated, degassed, and calcined under O
at 773 K. The UV-visible transmission spectrum (see Figure 1) of
the TiO /PVG catalyst exhibited a strong absorption starting around
70 nm, which is characteristic of TiO
dispersed on PVG.46 PVG
4
(11 Torr) corresponds to 88 µmol in our gas rack,
13
4
1
8
4
4
2
2
13
3
2
be monitored on the time scale of a few minutes. The C spin-
lattice relaxation times were measured to be between 0.7 and
displays a similarly strong absorption at much shorter wavelengths.
Nevertheless, it is possible to irradiate this catalyst in a homogeneous
fashion, such that the near UV light can penetrate the entire catalyst.
For the MAS NMR experiments, both ends of the PVG rod were fitted
with plastic end caps whose outer diameter was adjusted to fit snugly
against the inner wall of the NMR tube. In this way, the sealed NMR
tubes could be spun at speeds as high as 2.7 kHz.
1
.2 s for all of the mobile species, with the exception of
phosgene, which has a T1 of 2.3 s. NMR spectra collected as
a function of UV irradiation time show the degradation of TCE
and creation of a number of long-lived intermediates, including
carbon monoxide (CO), DCAC, phosgene, and pentachloroet-
hane (C2HCl5), and their conversion to the final products CO2
and phosgene. Assignment of these intermediates is confirmed
B. NMR Methods. For the in situ NMR experiments, a home-
4
9
built NMR probe based on a design by Gay was constructed. A light
pipe was incorporated into the probe to bring the near UV light to the
1
3
by comparing C NMR shifts for liquid samples reported in
5
1
the literature or prepared in our lab and by taking proton
coupled spectra during the photoreactions. The fact that TCE
(
45) TiO2 P-25 powdered catalyst is a gift from Degussa Corp. The P-25
2
has roughly 70% anatase and a surface area of ∼50 m /g.
46) Anpo, M. Aikawa, N.; Kubokawa, Y.; Che, M.; Louis, C.; Giamello,
E. J. Phys. Chem. 1985, 89, 5017.
47) Yamashita, H.; Ichihashi, Y.; Harada, M.;Stewart, G.; Fox, M. A.;
Anpo, M. J. Catal. 1996, 158, 97.
48) Osipenkova, O. V.; Malkov, A. A.; Malygin, A. A. Russ. J. General
Chem. 1994, 64, 498.
(
(49) Gay, I. D. J. Magn. Reson. 1984, 58, 413.
(50) Hatchard, C. G.; Parker, C. A. Proc. Royal Soc. London 1956, A235,
518.
(51) Stadtler Standard Carbon-13 NMR spectra Bio-Rad Laboratories:
Philadelphia, PA, 1994.
(
(

Solid-State NMR Studies of Trichloroethylene Photocatalysis
J. Am. Chem. Soc., Vol. 120, No. 18, 1998 4391
broad resonances at 64.0 and 177.3 ppm and a number of
spinning sidebands indicated by asterisks. This result provides
definitive evidence for the presence of surface-bound species.
The integrated signal intensities of the two isotropic resonances
and their associated side bands are very nearly the same,
indicating the formation of a single surface species containing
two carbon atoms. We have identified this species as dichlo-
-
roacetate (Cl2CHCOO ), which presumably forms from the
reaction of DCAC with surface hydroxyl groups. The assign-
ment is based on the fact that the spinning sidebands show
chemical shift anisotropy patterns characteristic of a methyl
carbon and a carbonyl carbon. The proton-coupled 13C CP/
MAS (contact time 50 µs) spectrum (not shown) displayed a
number of spinning sidebands, which outline a Pake doublet
1
3
powder pattern that is characteristic of an isolated C-H spin
pair. This Pake pattern is centered at 64.0 ppm, indicating that
the methyl carbon has one attached proton. The chemical shift
of the carbonyl carbon (177.3 ppm) is somewhat high compared
to the chemical shift of dichloroacetic acid in liquid (63.8 and
5
1
1
70.0 ppm). Such a shift to down field might be associated
with acetate bond formation of the carbonyl carbon to titanium
sites on the catalyst surface. Continued UV irradiation did not
break up dichloroacetate, while the other, more mobile species
were converted into phosgene and CO2. These results as well
as the results of detailed experiments on the fate of dichloro-
acetate on the TiO2/PVG catalyst (see section III.B.2. below)
clearly indicate that some reaction intermediates such as DCAC
move to dark regions of the catalyst and react to form stable,
strongly surface-bound species.
In addition to the formation of dichloroacetate, the photo-
oxidation of TCE on the powdered TiO2 catalyst did not lead
to complete mineralization since phosgene did not oxidize
further under dry conditions even after continued UV irradiation.
In order to investigate whether phosgene could be converted
thermally to CO2, the sample was heated after the UV
Figure 3. (a) Proton-decoupled 13C MAS NMR spectra obtained during
the photocatalytic oxidation of 48 µmol of TCE and 60 µmol of O
2
on
the Degussa P-25 TiO powdered catalyst. The UV irradiation time is
2
1
13
indicated in min. (b) A spectrum obtained with H- C cross polariza-
tion (contact time 3 ms) recorded after the UV light was turned off.
The vertical scale of the spectrum (b) is expanded 10× compared to
the spectra in (a). The asterisks indicate spinning sidebands of surface-
bound dichloroacetate.
13
irradiation. C MAS NMR measurements were followed after
the sample was heated at 373 K and later to 473 K for 1 h.
There were no remarkable changes observed in the NMR
spectrum upon heating the sample to 373 K. However, all the
phosgene decomposed to produce CO, C2Cl4, C2Cl6, and CO2
after heating at 473 K for 1 h. In similar experiments, Fan et
oxidation leads to the creation of intermediates/products con-
taining at most one hydrogen atom aided in the peak assign-
ments. The chemical shifts and J couplings of the observed
compounds are compiled in Table 1. Most of the carbon
containing intermediates identified by SSNMR in the reaction
with the powdered catalyst are in good agreement with those
observed in previous gas phase photooxidation studies of TCE
using several different types of TiO2 photoreactors.1
19
al. reported the conversion of phosgene to tetrachloroethylene
(C2Cl4) on TiO2 at 473 K. The NMR spectrum obtained after
heating at 473 K also indicates that all of the pentachloroethane
13
was converted. However, the C CP/MAS spectrum obtained
9-23,35a,b,36
after the thermal reaction indicated that the surface-bound
dichloroacetate species was not significantly affected.
However, there was no indication of the formation of mono- or
1
1,14
dichloroacetaldehyde observed in previous liquid phase
or
Separate experiments were conducted on samples where a
small quantity of water was deliberately left on the TiO2 catalyst
by carefully controlling the conditions of the evacuation step.
In situ studies revealed that the production of dichloroacetate
was somewhat diminished, and a concomitant formation of
dichloroacetic acid was observed. This latter species is mobile
other gas phase reaction studies in a similar packed catalyst
3
1,32,35c
bed reactor.
The surface water content in these experi-
ments was extremely low since the powder was evacuated at
1
7
73 K. The H NMR MAS spectrum (not shown) obtained for
this sample showed no indication of H2O although a very small
and broad peak was observed that is due to surface OH groups.
A carbon balance obtained by integrating the peak areas of
the spectra shown in Figure 3a indicates a significant loss of
signal (up to 50%) from the original TCE concentration after
(
see section III.B.3. below), and its slow photooxidation lead
31,52
to the formation of CO2, in agreement with previous studies.
We also note that TCE photocatalysis was not observed in
samples prepared without added O2.
50 min of UV irradiation. Therefore, an extended accumulation
B. Photocatalytic Oxidation of TCE on TiO2/PVG Cata-
lyst. In a second series of experiments, we examined TCE
photooxidation using the supported monolayer TiO2 catalyst,
which was synthesized in a manner described in section II-A
above. The catalyst used in the experiments described below
had a TiO2 coverage of one monolayer. The photocatalytic
with a longer delay time (20 s) was obtained immediately after
ceasing UV irradiation. Broad peaks with low intensities were
observed at the base of the narrow lines, and these broad
1
8
resonances are responsible for the apparent loss of signal. In
order to characterize the broad components, a separate spectrum
1
13
was obtained using H- C cross polarization (CP), and this
spectrum is displayed in Figure 3b. The spectrum shows two
(52) Sauer, M. L.; Ollis, D. F. J. Catal. 1996, 163, 215.

4392 J. Am. Chem. Soc., Vol. 120, No. 18, 1998
Hwang et al.
Table 1. Chemical Shifts of Species Observed over TiO
2
Powder
species
chemical formula
chemical shifts (ppm) and J couplingsa
trichloroethylene (TCE)
dichloroacetyl chloride (DCAC)
pentachloroethane
carbon dioxide
phosgene
carbon monoxide
dichloroacetate
C
C
C
CO
CCl
CO
2
2
2
HCl
HCl
HCl
3
3
5
116.7, 124.0 (JC-C ) 103 Hz, JH-C ) 201 Hz)b
O
70.0, 167.5 (JC-C ) 65 Hz, JH-C ) 186Hz)
79.5, 100.0 (JC-C ) 50 Hz, JH-C ) 185 Hz)
124.0
143.0
183.0
2
2
O
Cl
2
HCCOO-Ti(O-)
3
64.0, 177.3
a
The error bars for the chemical shifts are (0.5 ppm for the mobile species, (1.0 ppm for the surface bound species, and the error bars for the
b
J couplings are (5 Hz.
JH-C2 ) 12.5 Hz estimated from TCE in CDCl
3
but not observable in this work.
properties of TiO2/PVG have been studied and reviewed by
methyl and carbonyl resonances, respectively, which are very
close to those observed on the TiO2 powder surface. The other
pair of chemical shifts found in this experiment (63.7 and 162.1
ppm) is assigned to dichloroacetate bound to Si sites on the
surface (see below for a description of dichloroacetate formation
on pure PVG). The chemical shift of the carbonyl carbon of
trichloroacetate shows the similar upfield shift in going from
Ti (171.0 ppm) to Si (159.1 ppm) sites as does dichloroacetate.
Although the TiO2/PVG was prepared using an excess amount
of TiCl4 in order to produce a full monolayer coverage of
titanium dioxide, this result indicates that there are some SiO2
surface sites that remain. Nevertheless, it is possible that DCAC
reacts preferentially with (-O)3SiOH, so that quantitation of
the relative numbers of Ti and Si sites is not possible at this
time. The signal from surface adsorbed CO2 is also enhanced
via cross polarization (from H atoms in surface OH groups) in
the CP/MAS spectrum.
5
3,54
Anpo et al.
It has been observed that by dispersing TiO2
over the PVG surface both the physical and chemical nature of
TiO2 are significantly modified with the result that the catalytic
activity and selectivity can be enhanced as compared to bulk
5
3,54
TiO2.
1
. TCE + O2. A number of studies of TCE photooxidation
were performed under dry conditions. The TiO2/PVG catalyst
was evacuated at 773 K for 4 h and subsequently loaded with
48 µmol of TCE and 96 µmol of O2, as before for the powdered
TiO2 catalyst.
As can be seen in the 13C MAS NMR spectra shown in Figure
4
a, TCE degrades quickly to form a number of intermediates
including DCAC, oxalyl chloride (Cl2C2O2), pentachloroethane,
trichloroacetaldehyle (Cl3CCHO), and trichloroacetyl chloride
(Cl3CCOCl) in a trace amount. Ultimately these species are
converted to the final products phosgene and CO2. The NMR
shifts and J couplings of these intermediates are listed in Table
1H MAS NMR spectra were also collected and show the
presence of TCE and surface OH groups (see Figure 4c). The
intensity ratio of [TCE]/[OH] was calculated to be ∼0.5 in the
2
. In contrast to our previous experiments involving the TiO2/
18
PVG catalyst with a coverage of roughly a quarter monolayer,
1
dry sample. H MAS NMR spectra show the destruction of
the formation of CO is not seen and the formation of Cl3CCHO
and Cl2C2O2 is suppressed. Direct integration of the NMR peaks
allows us to estimate the gradual evolution of individual species
TCE (6.4 ppm) and formation of DCAC (6.2 ppm). Although
not evident in Figure 4c Cl3CCHO was found to resonate at
1
∼
9.0 ppm in a separate experiment. The H NMR spectra taken
(see below).
during the reaction also indicate no significant loss of surface
hydroxyl groups, suggesting that OH groups do not participate
in TCE oxidation process to a significant extent. The consump-
tion of OH groups in photocatalytic oxidation reactions has been
a subject of some controversy. From the in situ FT-IR
After 230 min of UV irradiation, the extended Bloch decay
spectrum (top spectrum in Figure 4b) exhibits the presence of
phosgene and CO2. CO2 spinning sidebands are evident and
indicate the presence of a small quantity of strongly adsorbed
CO2. Integration of the peaks results in CO2:phosgene ratio of
30
measurements of TCE photooxidation, Raupp et al. observed
2
:1, and the total carbon balance of the mobile species was
the consumption of water or/and surface hydroxyls on untreated
calculated to be 85% of the initial TCE concentration. There
is no clear indication of the presence of surface-bound species
in the Bloch decay spectrum. However, the 13C CP/MAS
spectrum (bottom spectrum in Figure 4b) does show the
existence of at least two surface-bound species which are
responsible for the minor loss in the carbon balance. We
19
TiO2 surface, while Fan et al. reported no OH consumption
on a TiO2 surface. Production of HCl is anticipated but not
1
resolved by H NMR (see Figure 4c). However, slight
downfield shifts (∼1 ppm) of the OH peak are observed,
indicating an increased acidity of the surface protons, which is
55
therefore suggestive of the adsorption of some HCl. Formation
identify these species as dichloroacetate and trichloroacetate
of gas-phase HCl in the reaction was confirmed using FT-IR.
For this measurement, we placed the NMR sample which had
been previously irradiated with UV light into a test tube with
-
(
Cl3CCOO ) which are bound to the surface of TiO2/PVG via
chemical bonds to both Si and Ti sites. As with the powdered
catalyst, we interpret the formation of these surface species as
due to the reaction of DCAC and trichloroacetyl chloride with
surface OH groups. As seen in Figure 4b, the spectrum is very
complex due to overlapping resonances of the carbonyl groups
on Ti sites and Si sites as well as their spinning sidebands.
Assignments (see Table 2) were made by comparing the
chemical shifts of surface-bound species observed on the
powdered TiO2 surface with those on the SiO2 (pure PVG), as
will be discussed below. The chemical shifts of dichloroacetate
bound to Ti sites are found to be 63.7 and 175.0 ppm for the
-5
an iron ball. The tube was evacuated down to 2 × 10 Torr,
after which the NMR sample was opened using the iron ball.
All the products in gas phase were transferred into a pre-
evacuated cell for the FT-IR measurement. The FT-IR spectrum
(
not shown) clearly reveals the presence of CO2 and phosgene
as well as HCl.
. Reaction of Surface Bound Species on TiO2/PVG.
From the photodegradation study of TCE on the TiO2 powder
see Part III.A., above), further degradation of dichloroacetate
2
(
formed in the dark regions of the powder was impossible.
(
53) Anpo. M. Res. Chem. Intermed. 1989, 11, 67.
(55) Pfeifer, H. In Solid-State NMR II: Inorganic Matter; Diehl, F.,
Gunther, H., Kosfeld, R., Seeling, J., Eds.; Springer-Verlag: Berlin, 1994;
p 33.
(54) Anpo, M.; Yamashita, H. In Surface Photochemistry; Anpo. M.,
Ed.; John Wiley & Sons: New York, 1996; p 117.

Solid-State NMR Studies of Trichloroethylene Photocatalysis
J. Am. Chem. Soc., Vol. 120, No. 18, 1998 4393
(1) A NMR sample was prepared by loading 48 µmol of TCE
and 24 µmol of O2 on the surface of completely dried TiO2/
PVG.
(2) UV irradiation was continued until there was no more
degradation of TCE observed. The O2 deficiency prevented
further degradation, in accordance with the stoichiometry of the
following TCE oxidation reaction
3
Cl CdCHCl + 5O f 4CO + 3HCl + 2COCl + Cl
2 2 2 2 2
which is in accordance with our observed 2:1 ratio of CO2:
phosgene. Note that it is possible to write reactions which do
not result in Cl2 production, but these change the relative ratio
of CO2 and phosgene or involve H2O as a reactant. A 13C MAS
spectrum acquired after 92 min of UV irradiation is displayed
in Figure 5a and reveals the formation of DCAC, phosgene,
pentachloroethane, Cl3CCHO, and CO as well as unreacted
TCE.
(3) The sample was stored in the dark for 28 days to allow
the reaction of DCAC with surface OH groups to proceed. A
spectrum shown in Figure 5b clearly indicates the growth of
broad resonances during the dark period while the DCAC peaks
decreased markedly. The concentration of phosgene also
decreased most likely due to its reaction with surface OH groups
to form CO2 and HCl.
(4) The sample was then heated at 423 K for 30 min to
complete the hydrolysis of the remaining DCAC. Figure 5c
shows the resulting NMR spectrum, which reveals the further
increase in intensity of broad peaks that result from the
consumption of DCAC. Additional phosgene was also con-
sumed. It is also possible to associate at least some of the
decreased phosgene concentration with the formation of another
-
surface bound species, which may be chloroformate (ClCOO )
that is produced via reaction of phosgene with surface OH
groups (see below).
(5) After heating, the NMR sample was broken and the
volatile species were released. The catalyst was then transferred
-
5
to a fresh NMR tube and evacuated to a pressure of 2 × 10
Torr. O2 (96 µmol) was loaded, and the sample was again
1
3
sealed. The C Bloch decay (Figure 5d) and CP/MAS (Figure
f) spectra acquired at this stage of the experiment are shown.
5
The Bloch decay spectrum clearly shows the removal of all gas
phase species after evacuation, while the CP/MAS spectrum
indicates the presence of the surface-bound dichloroacetate both
on Ti sites (63.7 and 175.0 ppm) and on Si sites (63.7 and 162.1
ppm). Significant differences in chemical shift for the carbonyl
carbons in dichloroacetate adsorbed on the two different metal
sites are evident, while the chemical shifts of the methyl carbons
adsorbed on both sites are found to be essentially identical.
These dichloroacetate resonances also appear to be narrower
than the corresponding signals on the TiO2 powder (see Figure
Figure 4. MAS NMR spectra obtained during the in situ photooxi-
2
dation of TCE on the TiO /PVG catalyst. The spinning rate was 2.0
1
3
kHz. (a) Proton-decoupled C spectra acquired during UV irradiation.
3
c), suggesting that the surface sites on the TiO2/PVG sample
1
3
(
b) A C spectrum obtained with extended accumulation (top) and a
53,54
are more homogeneous.
6) As the final step, in situ NMR observation was made
1
3
C CP/MAS (contact time 3 ms, bottom) after the lamp was turned
(
2
off. The asterisks indicate spinning sidebands of strongly adsorbed CO .
1
1
during UV irradiation of the sample. The degradation of
dichloroacetate did occur, although the reaction rate was much
slower than that observed for TCE. NMR measurements made
after a UV irradiation time of 310 min are shown in Figure 5
parts e and g for the Bloch decay and CP/MAS experiments,
respectively. From both spectra, it is evident that upon UV
irradiation most of the dichloroacetate is decomposed to form
phosgene and CO2. After 310 min of irradiation, destruction
of dichloroacetate was complete as indicated by the disappear-
ance of the methyl carbon peak at 63.7 ppm. A very small
signal corresponding to the formation of trichloroacetate (92.1
(
c) H MAS NMR spectra acquired during photooxidation (probe H
background subtracted). Abbreviations: dichloroacetate (DCAc); trichlo-
roacetate (TCAc).
However, dichloroacetate formation on the TiO2/PVG surface
was suppressed in the presence of UV light. Since it was not
clear whether dichloroacetate, once formed on the catalyst
surface, could be photodegraded, we examined the reactivity
of dichloroacetate by using an experimental setup as described
below.

4394 J. Am. Chem. Soc., Vol. 120, No. 18, 1998
Hwang et al.
Table 2. Additional Observed Species from TiO
2
/PVG Catalysts
species
chemical formula
chemical shifts (ppm)
dichloroactic acid
chloroform
oxalyl chloride
trichloroacetaldehyde
trichloroacetyl chloride
trichloroacetic acid
dichloroacetate
dichloroacetate
trichloroacetate
trichloroacetate
C
CHCl
2
H
2
Cl
3
2
O
2
63.0, 167.5 (JC-C ) 66 Hz)
77.2
C
C
C
C
Cl
Cl
Cl
Cl
2
2
2
2
Cl
HCl
Cl
HCl
2
O
2
159.0
3
O
93.0, 177.0 (JC-C ) 44 Hz, JH-C ) 195 Hz)
94.0, 164.0 (JC-C ) 75 Hz)
88.0, 164.0 (JC-C ) 80 Hz)
63.7, 162.1
63.7, 175.0
89.2, 159.1
4
O
3
O
2
2
2
3
3
HCCOO-Si(O-)
HCCOO-Ti(O-)
CCOO-Si(O-)
CCOO-Ti(O-)
3
3
3
3
92.1, 171.0
to form surface-bound dichloroacetate in a dark reaction, while
continuous irradiation breaks down the dichloroacetate to form
phosgene and CO2 at the surface of the TiO2/PVG catalyst. The
results also imply that the formation of phosgene during the
photooxidation of TCE primarily originates from photooxidation
of DCAC, supporting previous findings by Jacoby et al.2
According to their report, phosgene and CO are formed in
approximately equimolar amounts from DCAC. The production
of CO was not observed in our in situ NMR study of
dichloroacetate photodegradation.
2,23
3
. TCE + O2 + H2O. The influence of water on the
photooxidation of TCE was investigated by introducing H2O
0.1 mmol) before TCE (48 µmol) and O2 (96 µmol) were
(
1
loaded onto the TiO2/PVG catalyst. H NMR spin counting
was used to quantify the amount of H2O adsorbed on the catalyst
surface before the reaction was initiated. The surface water
content was determined to be roughly equivalent to one
1
monolayer coverage. The observed H line width of 700 Hz
indicates that this water is strongly associated with surface
hydroxyl groups and very likely evenly distributed over the
surface of the 40 Å pores of the catalyst. Figure 6a shows the
13
resulting C MAS NMR spectra as a function of UV irradiation
time, while Figure 6b shows a kinetic plot from direct integration
of peak areas shown in Figure 6a. As shown in Figure 6,
degradation of TCE upon UV illumination occurs in a very
similar fashion to that observed under dry conditions. However,
the formation of dichloroacetic acid (63.0 and 167.5 ppm) as a
major intermediate is observed. Narrow NMR line widths for
dichloroacetic acid indicate that it is very mobile over the
hydrated surface or may be in fast exchange with the gas phase.
The carbonyl carbons in both DCAC and dichloroacetic acid
were found to resonate at the same chemical shift, 167.5 ppm
Figure 5. Proton decoupled 13C MAS and CP/MAS spectra acquired
during the photocatalytic and dark reaction steps as described in the
text. Bloch decay spectra: (a) after UV irradiation for 92 min; (b) after
dark reaction for 28 days; (c) after heating at 423 K for 30 min; (d)
after evacuation at room temperature; (e) after UV irradiation for 310
min; CP/MAS spectra (contact time 3 ms): (f) same conditions as (d);
(
see Figure 6a). Once formed, dichloroacetic acid could be
further photooxidized, but its degradation rate was slow.
Formation of phosgene is almost suppressed in the presence of
surface water, in good agreement with a previous report.19 CO,
Cl3CCHO, and Cl2C2O2 are not found under these conditions.
However, small quantities of DCAC are observed at an early
stage during the UV irradiation. A second experiment was run
with roughly half the amount of adsorbed water. Formation of
DCAC was predominant at the early stage of UV illumination,
while dichloroacetic acid was produced with a concomitant
decrease in the amount of DCAC during the photoreaction. From
these results, it is evident that the destruction of TCE leads to
formation of DCAC and immediate hydrolysis of DCAC follows
(g) same conditions as (e). Abbreviations: dichloroacetate (DCAc);
Trichloroacetate (TCAc).
and 171.0 ppm for trichloroacetate bound to Ti sites, 89.2 and
159.1 ppm for trichloroacetate bound to Si sites) was observed,
and their intensities were found to increase slowly as the
irradiation time increased. Further irradiation for 90 more min
did not affect the signal intensities of trichloroacetate on either
site. This result indicates that trichloroacetate, once formed
from DCAc, is Very resistiVe to further destruction, both when
bound to Ti and to Si sites on the catalyst. Finally, a peak
appearing at 152 ppm (underneath a sharp CO2 spinning
sideband) was observed and is indicated by an open arrow in
Figure 5g. The chemical shift of this peak could correspond to
1
9,33,35a
to form dichloroacetic acid.
It is not clear at this point
whether phosgene formation is suppressed due to the presence
of water or whether phosgene is efficiently decomposed as soon
as it forms under hydrated conditions. However, phosgene is
-
-
either CO3 or ClCOO on the surface.
56
The experimental results described in this section demonstrate
that once formed, DCAC reacts with surface hydroxyl groups
known to react with water to yield CO2 and HCl, and therefore
the latter case is more plausible.

Solid-State NMR Studies of Trichloroethylene Photocatalysis
J. Am. Chem. Soc., Vol. 120, No. 18, 1998 4395
Figure 7. 13C MAS NMR spectra obtained for the reaction of TCE
on PVG. (a) Bloch decay spectrum acquired after UV photooxidation
reaction for 20 min. (b) CP/MAS spectrum (contact time 3 ms) acquired
after forming DCAc in the dark. (c) Bloch decay spectrum after UV
reaction for 340 min.
those experiments, irradiation was performed with shorter
wavelengths to compensate for the fact that PVG has an
6
2
absorption maximum near 260 nm. We also examined the
reactions of TCE on PVG alone and used the same procedure
as described above for the photooxidation of TCE on TiO2/
PVG, so that direct comparison between TiO2/PVG and PVG
systems should be possible. Figure 7 shows three 13C NMR
spectra obtained from these examinations, and the findings are
summarized below.
Our data indicate that photooxidation reactions are possible
at room temperature and result in the degradation of TCE and
the formation of DCAC, Cl2C2O2, CO, Cl3CCHO, phosgene,
Figure 6. (a) Proton-decoupled 13C MAS spectra obtained during TCE
1
3
and CO2. The C MAS NMR spectrum in Figure 7a shows
the presence of these reaction intermediates and products after
UV exposure for 20 min. PVG also provides surface hydroxyl
groups to form surface bound dichloroacetate when DCAC is
present. Figure 7b exhibits the dichloroacetate resonances (64
and 162 ppm) which are identical to the peaks assigned to
dichloroacetate adsorbed on the surface of the TiO2/PVG
catalyst. Dichloroacetate on PVG is presumably bound to Si
sites. When the photooxidation of dichloroacetate was examined
in the presence of O2 using the procedure described above for
investigating its degradation on the TiO2/PVG catalyst, photo-
degradation was observed as well as the production of phosgene
and CO2. The formation of trichloroacetate also was observed
toward the end of the dichloroacetate destruction. Upon
continued UV irradiation, reaction intermediates were converted
to phosgene and CO2, indicating that PVG behaves in a similar
manner to the TiO2/PVG catalyst, although the reaction rate is
found to be slower than that found for TiO2/PVG. In particular,
the initial degradation rate of TCE appears to about an order of
magnitude slower than for the TiO2 catalysts. Figure 7c shows
photocatalytic degradation with O
TiO /PVG catalyst. (b) The resulting kinetic plot obtained by integration
of the spectra in (a).
2 2
and H O as coreactants over the
2
4
. TCE + H2O. Hydrated samples were prepared by fully
saturating the catalyst with water followed by evacuation for
0 min at room temperature. TCE (48 µmol) was used as
3
13
before. C MAS NMR spectra (not shown) were acquired
during UV irradiation for a period of 2 h. There was no
indication of any TCE degradation. In addition, the thermal
reaction of TCE in the presence of the catalyst and adsorbed
water was examined by heating the same sample at 423 K for
13
an hour. We did not observe any change in C MAS NMR
spectrum. Anpo et al.4
6,53,54,57
have reported the photocatalytic
hydrogenolysis of unsaturated hydrocarbons (alkenes and
alkynes) and the photocatalytic isomerization of alkenes on both
TiO2 powder catalysts and TiO2/PVG catalysts. In their studies
of photocatalytic hydrogenolysis, reactions were run in the
presence of H2O but without O2. However, our findings indicate
that the presence of O2 is essential to initiate oxidation of TCE.
It is also possible to conclude that if any OH radical is involved
in the photocatalytic oxidation of TCE, the radical does not form
or does not attack TCE in the absence of oxygen.
1
3
a
C MAS NMR spectrum obtained after photooxidation of
TCE for 340 min. The spectrum indicates the presence of
phosgene and CO2 as final products as well as a minor
concentration of trichloroacetic acid (Cl3CCO2H).
As can be seen in Figure 1, the spectrum of UV light we
have used in these experiments has a maximum at ∼450 nm
with small contributions from light with wavelengths shorter
than 350 nm. At the same time, the UV transmittance spectrum
of PVG (degassed at 773 K) indicates that PVG starts absorbing
UV light around 350 nm. A speculation that photooxidation
of TCE on PVG is driven primarily by the UV photons available
C. TCE Oxidation on PVG. The photocatalytic activity
5
8-61
of pure Vycor glass has been reported by Anpo et al.
In
(
56) Morrison, R. T.; Boyd, R. N. Organic Chemistry; Allyn and
Bacon: Boston, 1987; 5th ed., p 886.
57) Anpo, M.; Aikawa, N.; Kodama, S.; Kubokawa, Y. J. Phys. Chem.
984, 88, 2569.
58) Anpo, M.; Kubokawa, Y.; Fujii, T.; Suzuki, S. J. Phys. Chem. 1984,
8, 2572.
(
1
(
8
(
(
(
59) Anpo, M.; Yun, C.; Kubokawa, Y. J. Catal. 1980, 61, 267.
60) Anpo, M. Chem. Lett. 1987, 1221.
61) Yun, C.; Anpo, M.; Kubokawa, Y. Chem. Lett. 1979, 631.
(62) Anpo, M.; Yun, C.; Kubokawa, Y. J. Chem. Soc., Faraday I 1980,
76, 1014.

4396 J. Am. Chem. Soc., Vol. 120, No. 18, 1998
Hwang et al.
with wavelengths well below 350 nm led us to run another
control experiment. TCE photooxidation was carried out on
PVG using a 350 nm UV cutoff filter as well as an IR filter.
The results obtained were very similar in terms of the observed
reaction intermediates; however, the reaction rate becomes
somewhat slower.
bare PVG and TiO2/PVG indicate that the same or at least
similar initiating species are involved. In fact, the titanium oxide
species dispersed on PVG might just assist the photooxidation
of TCE on the surface. The assistance becomes possible by
providing surface sites which can form [Ti -O ] species.
These active sites will be created by absorbing UV light with
longer wavelength (∼50 nm) in comparison with the wavelength
needed in PVG (SiO2). Note that the TiO2 surface was the most
effective in terms of the observed initial rates of degradation.
3
+
-
D. Discussion of the Influence of Radical Species. TCE
is a very reactive species that degrades quickly in the presence
of UV light and the catalytic surfaces featured in these studies.
Currently, complete elucidation of the reaction mechanisms is
not possible in part due to the large number of reactive radical
species that are most likely present in low abundance. In
addition, direct comparisons of the results from different
research groups are challenging due to the large number of
unique reaction conditions, different photoreactors, and analyti-
cal methods that have been used to study TCE photodegradation
reactions. The NMR experiments described above shed light
on a number of questions related to the photocatalytic surface
chemistry of TCE. In particular, these experimental results yield
information that will be useful to help resolve the following
issues.
2. Involvement of Cl Radical. It is also evident that Cl
radicals play an important role in the TCE photodegradation.
Most of the reaction intermediates need to involve either Cl
addition or loss of Cl radicals. For example, the formation of
pentachloroethane is postulated to take place by the addition of
two Cl radicals to TCE. Formation of trichloroacetyl chloride
and trichloroacetate most likely takes place as a result of H atom
abstraction and an addition of Cl radical to DCAC and
dichloroacetate, respectively. The photosensitizing effect re-
cently observed in mixtures of TCE with other organic pollutants
2
4,26,27
also supports a mechanism involving Cl radicals.
How-
ever, a reaction pathway proposed by Nimlos et al.22 which
associates the direct attack of Cl radical on the TCE double
bond still needs to be proved. While a Cl radical chain reaction
mechanism can explain the formation of DCAC and phosgene,
1
. Involvement of O2 versus OH in TCE Photooxidation.
As discussed in the Introduction, the identity of the initiating
species in photocatalytic oxidation processes has been under
considerable debate. In the liquid phase, a strong case has been
made for the involvement of hydroxyl radical in initiating the
photodegradation process. In the gas-solid interface, formation
of monochloroacetyl chloride or dichloracetaldehyde is antici-
pated when OH radical inserts directly into the TCE double
8
-10
in early studies
the complete mineralization of TCE was
never observed nor were the yields of CO production very high
2
18
even after long UV irradiation times. In addition, we and
1
9
others have observed the necessity of O as a reagent in order
2
to initiate the TCE photocatalysis. As noted in section III.B.2,
31
bond. However, from our observation of DCAC as the major
insufficient O caused the degradation of TCE to be incomplete.
2
intermediate as well as other reports1
8,19,22,23,35a
it is evident that
The Cl radical mechanism is therefore not sufficient to explain
OH radical does not attack TCE directly, even though OH
radical might form as a result of the interaction between the
photogenerated hole and OH /H2O species at the surface. If
the complete mechanisms of TCE degradation reactions as
observed by ourselves and others, although the addition of Cl-
containing reagents as a source of Cl radicals has been observed
to increase the reaction rate in the degradation of nonhalogenated
-
OH radicals are involved in TCE photooxidation, as proposed
by Howe et al.,63 their role should be defined as assisting the
formation of other reactive radicals which readily attack TCE.
A common denominator in our experiments is the observed
reactivity of TCE over the surfaces of TiO2 powder, TiO2/PVG,
66
species. Our observed quantum yields (as measured from the
degradation of TCE within 20 min using 5 mW of UV light)
corresponds to roughly 100% which represents a high yield,
but it is not as high as the value of Φ ∼ 100 reported by
5
,6,53,54,63
8
and PVG employed. According to previous studies,
one possible process which can occur at all of these surfaces is
Bertrand et al. The propagation of Cl radical chain reactions
may be dependent on a number of factors related to experimental
conditions.
3
+
-
3+
-
the creation of [Ti -O ] or [Si -O ] species due to charge
transfer upon UV illumination. Observed radical species
common to the three different surfaces
and O3 . We anticipate that O2 or oxygen radicals forming
from electronic activation on the surface act as the TCE-
attacking species. O2 is known to be the most stable thermally
3. Reaction Pathways for Degradation of DCAc. The
photodegradation of the surface-bound dichloroacetate species
is considered to proceed via an attack of oxygen radicals such
5,6,53,54,64
-
-
are O2 , O ,
-
-
-
as O2 or O . In our examination of the dichloroacetate
degradation using in situ NMR, we did not observe the formation
of any gas phase species other than phosgene and CO2. It is
also evident that dichloroacetate breaks down while it is bound
to the surface rather than desorbing from the surface prior to
degradation.
-
among these radicals.65 Insertion of one of these radicals into
the TCE double bond can be suggested as the radical initiation
step. As a result of the insertion, formation of DCAC is
expected to occur although the detailed reaction steps in this
process are not evident in our present study. Since O2 is
necessary to break down dichloroacetate, further destruction of
DCAC which leads to the formation of phosgene and CO2 is
also expected to involve oxygen radical species. However, in
this case, the question of the role of TiO2 arises. Anpo et al.
have associated the photocatalytic activity of PVG with forma-
Our finding that trichloroacetyl chloride and trichloroacetate
are both very resistive to further oxidation indicates that the
abstraction of a chlorine atom from these reaction intermediates
as well as other species is either not possible or extremely slow.
We expect that trichloroacetyl chloride, trichloroacetate, and,
27,52
to a somewhat lesser extent, dichloroacetate
play a primary
3+
-
61
tion of [Si -O ] surface site as a result of charge transfer.
Our observations of the same reaction intermediates on both
role in catalyst deactivation processes, although the formation
of these species may be inhibited by hydration.
Finally, we reported18 that dichloroacetate accumulated on
the TiO2 powder surface is very persistent for further degradation
both photocatalytically and thermally. It is likely that the active
oxygen radicals important for initiating the photodegradation
reactions appear to be unable to attack dichloroacetate residing
(
(
(
(
63) Howe, R. F.; Gratzel, M. J. Phys. Chem. 1987, 91, 3906.
64) Tanaka, K.-I. J. Phys. Chem. 1974, 78, 555.
65) Shiotani, M.; Moro, G.; Freed, J. H. J. Chem. Phys. 1981, 74, 2616.
66) d’Hennezel, O.; Ollis, D. F. In 11th International Congress on
Catalysis. -40th AnniVersary; Hightower, J. W., Delgass, W. N., Iglesia,
E., Bell, A. T., Eds. Stud. Surf. Sci. and Catal. 1996, 101, 435.

Solid-State NMR Studies of Trichloroethylene Photocatalysis
J. Am. Chem. Soc., Vol. 120, No. 18, 1998 4397
on the dark portions of the TiO2 surface. Our experimental
findings indicate that the diffusion of these active radical species
is not great. Trichloracetate formation was not observed on
the TiO2 particle surface because the UV light was able to reach
only a small minority of the TiO2 surface, even though the
evidence for trichloroacetate formation on Ti sites is strong (see
section III.B.1). Hydration of the catalytic surface can serve
to promote formation of acetic acids which are more mobile
than the acetates and which can be photooxidized, although at
a slow rate. Similar results have been observed by others using
catalysts that were not as completely dehydrated as those in
photodegradation, whereas their role in the direct attack of the
TCE double bond needs further investigation. DCAC and
phosgene were major reaction intermediates identified in the
TCE photooxidation reactions, and they were observed in all
cases, both in TiO2-based catalysts and in reactions over PVG.
The formation of surface-bound dichloroacetate as a result of a
surface reaction between mobile DCAC and surface hydoxyl
groups was observed via C CP/MAS experiments. Further
destruction of dichloroacetate to phosgene and CO2 was found
to be slow but possible only in the presence of UV light and
O2, which indicates the advantage of employing TiO2/PVG
catalysts in reactors. However, once trichloroacetate was
formed, it could not be effectively degraded, indicating that at
least for surface adsorbates, hydrogen abstraction may play a
key role in the degradation process.
1
3
3
1,52
this study.
IV. Conclusions
In situ SSNMR has been shown to be useful in investigating
the photoinitiated surface reactions because of its ability to
precisely identify reaction intermediates both in the gas phase
and on the catalyst surface and to directly quantitate these
species. Using these methods, the photocatalytic oxidation of
TCE over titania-based catalysts has been studied. TCE
degrades efficiently at the gas-solid interface in the presence
of molecular oxygen to produce a number of long-lived
intermediates. Oxygen radical species created via UV activation
of added O2 are likely candidates to be primarily involved in
attacking the TCE double bond in the initiation step. The
involvement of Cl radicals is an important feature in TCE
Acknowledgment. Partial support for this research from the
National Science Foundation (CHE 94-22235 and CHE 97-
33188 CAREER Award), the Donors of the Petroleum Research
Foundation administered by the American Chemical Society,
and from the AT&T/Lucent Technologies Industrial Ecology
Program is gratefully acknowledged. C.P. thanks the Purdue
Research Foundation for a fellowship. D.R. is a Cottrell Scholar
of the Research Corporation.
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