In situ solid-state NMR observations of photocatalytic surface chemistry: Degradation of trichloroethylene

Full text of DOI:10.1021/ja9703990

J. Am. Chem. Soc. 1997, 119, 7877-7878
In Situ Solid-State NMR Observations of
7877
Photocatalytic Surface Chemistry: Degradation of
Trichloroethylene
Son-Jong Hwang, Chris Petucci, and Daniel Raftery*
Department of Chemistry, Purdue UniVersity
1393 Brown Laboratory of Chemistry
West Lafayette, Indiana 47907
ReceiVed February 6, 1997
ReVised Manuscript ReceiVed May 27, 1997
Environmental catalysts that can efficiently degrade hazardous
chemicals are of growing importance. Among these, TiO2
photocatalysts show high potential for effective degradation of
harmful species, particularly chlorine containing compounds,
which are chemically persistent.1-3 TiO2 photocatalysts can
display efficiencies (degradation rate/unit energy) that are 1-2
orders of magnitude higher than thermal catalysts, particularly
for the low chlorocarbon concentrations often encountered.4 Gas-
surface experiments5-10 show that reactions occur both in the
gas phase and/or on the surface of TiO2 catalysts and can involve
the following radical initiators: O2-, OH, and Cl. Although a
number of studies have been carried out by means of GC,7-9
MS,6,8 and FT-IR,5-8,10 a detailed understanding of the reaction
mechanisms remains elusive. In the present work, we report a
new approach to the study of photocatalysis, namely in situ
solid-state nuclear magnetic resonance (SSNMR) spectros-
copy.11-13 SSNMR methods are advantageous because they
allow a quantitative examination of the reactions on the catalyst
surface as well as in the gas phase. We have characterized the
reaction of trichloroethylene (TCE) over two types of TiO2
photocatalysts and have identified new intermediates in the
complex surface chemistry.
Figure 1. Proton-decoupled 13C MAS NMR spectra obtained during
the photocatalytic oxidation of TCE on 170 mg of Degussa P-25 TiO2.
Closed reaction cells were prepared by sealing off 5 mm NMR tubes
after doubly 13C labeled TCE and O2 were introduced onto previously
calcined and evacuated (both at 773 K for 5 hours) TiO2 catalysts.
Approximately 5 mW of near UV light (350-450 nm) is delivered
evenly over the surface of the spinning sample via a liquid light guide
terminated by a 50 mm quartz rod. Spectra were obtained in a Varian
Unity Plus 300 spectrometer operating at 75.4 MHz for 13C and using
a homebuilt in situ MAS probe. (a) Reaction of 48 µmol of TCE with
60 µmol of O2 (48 scans each, delay 4 s). The UV irradiation time is
indicated in minutes. Assignments: TCE (116.7 and 124 ppm with
13C-13C coupling, J ) 103 Hz), DCAC (70 and 167 ppm), C2HCl5
(79.5 and 100 ppm), CO2 (124 ppm), phosgene (144.5 ppm), and CO
(184 ppm). (b) Spectrum (2000 scans, 20 s recycle time) recorded after
Figure 1 shows proton-decoupled 13C magic angle spinning
(MAS) NMR spectra obtained from photooxidation of TCE in
the presence of O2 and Degussa P-25 TiO2 powder.14 The NMR
spectra show the degradation of TCE, the formation of dichlo-
roacetyl chloride (Cl2CHCOCl, DCAC), CO, phosgene (CCl2O),
and pentachloroethane (C2HCl5), and their conversion to the
final product CO2. 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. Assignment of
these intermediates are confirmed by comparing 13C NMR shifts
for liquid samples reported in the literature15 or prepared in our
lab and from proton coupled spectra during the photoreactions.
Most of the intermediates identified above are in good agreement
1
UV light was turned off. (c) A spectrum obtained with H-13C cross
polarization. The asterisks indicate spinning sidebands of surface bound
dichloroacetate (center bands: 64 and 177.3 ppm).
(1) Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 341.
(2) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem.
ReV. 1995, 95, 69.
(3) Linsebigler, A. L.; Lu, G.; Yates, Jr. J. T. Chem. ReV. 1995, 95, 735.
(4) Miller, R. In Proceedings of the 1st International EPRI/NSF
Symposium on AdVanced Oxidation; EPRI TR-102927-V2, Electric Power
Research Institute: Palo Alto, 1993; p 2.
with previous reports.8,10 There was no indication of the
formation of mono- or dichloroacetaldehyde observed in previ-
ous liquid-phase16 or gas-phase7 reaction studies.
(5) Phillips, L. A.; Raupp, G. B. J. Mol. Catal. 1992, 77, 297.
(6) Nimlos, M. R.; Jacoby, W. A.; Blake, D. M.; Milne, T. A. EnViron.
Sci. Technol. 1993, 27, 732.
A carbon balance obtained from the peak areas of the
aforementioned six species, however, indicates a significant loss
of signal (up to 50%) from the original TCE concentration. A
spectrum acquired using an extended accumulation time (Figure
1b) exhibits additional broad peaks which account for the
apparent loss in signal. The observed line widths result from
adsorption site heterogeneity. A separate spectrum obtained
with cross polarization (CP) is displayed in Figure 1(c) and gives
further proof of the strong adsorption of this species, which we
identify as dichloroacetate (DCAc). DCAc presumably forms
(7) 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: Amsterdam, 1993; p 405.
(8) 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: Amsterdam, 1993; p 393.
(9) Luo, Y.; Ollis, D. F. J. Catal. 1996, 163, 1.
(10) Fan, J.; Yates, Jr. J. T. J. Am. Chem. Soc. 1996, 118, 4686.
(11) Haw, J. F.; Nicholas, J. B.; Xu, T.; Beck, L. W.; Ferguson, D. B.
Acc. Chem. Res. 1996, 29, 259.
(12) Haddix, G. W.; Reimer, J. A.; Bell, A. T. J. Catal. 1987, 106, 111.
(13) Stepanov, A. G. Catal. Today, 1995, 24, 341.
(14) TiO2 P-25 powdered catalyst is a gift from Degussa Corp.
(15) Stadtler, Standard Carbon-13 NMR Spectra; Bio-Rad Laborato-
ries: Philadelphia, PA, 1994.
(16) Pruden, A. L.; Ollis, D. F. J. Catal. 1983, 82, 404.
S0002-7863(97)00399-5 CCC: $14.00 © 1997 American Chemical Society

7878 J. Am. Chem. Soc., Vol. 119, No. 33, 1997
Communications to the Editor
Figure 3. Reaction kinetic plot from the TCE reaction on the TiO2/
PVG photocatalyst.
H2O to form dichloroacetic acid (Cl2CHCO2H). No surface-
bound species were observed from the photocatalytic degrada-
tion of TCE on the TiO2-anchored PVG catalyst in contrast to
the powdered catalyst.
TCAC and Cl3CCO2H species are more evident in Figure
2(b), in which a separate UV irradiation was carried out with
the addition of a small amount of water and with an extended
signal accumulation after the UV lamp was turned off. The
spectrum shows DCAC, Cl2CHCO2H, TCAC, and Cl3CCO2H
as well as phosgene and CO2. A dark reaction takes place which
transforms DCAC and TCAC to their corresponding acids on a
time scale of several hours. It is clear from these experiments
that the unstable intermediates DCAC and TCAC are first
formed from TCE photodegradation and later form acids via
reaction with surface water molecules. However, they remain
as mobile species and do not form acetates in the presence of
light. Upon further UV irradiation they convert to CO2.
Quantitative information on the fate of each species can be
acquired from the direct integration of peak areas in the 13C
MAS NMR spectra. Figure 3 shows such a kinetic plot from
the experimental results of Figure 2(a). Kinetic studies showed
that the fastest rates of catalysis were observed under conditions
of excess oxygen and low concentrations of water. However,
the addition of water was useful in decreasing the lifetime of
potentially harmful intermediates such as phosgene and OC.
There was no observed reaction of TCE without added O2.
Detailed kinetic analysis of the photooxidation is still under
investigation and will be published separately.
In light of the intermediates and kinetics observed in these
experiments, our NMR studies strongly support the position that
molecular oxygen is the primary initiating species in TiO2
photocatalysis. Additional 1H NMR investigations indicated that
there was no significant loss of surface hydroxyl groups, further
suggesting that a mechanism involving surface-bound H2O or
OH groups5,7 as the initiating species in TCE degradation is
not suitable. However, the formation of chlorine radicals would
explain the formation of C2HCl5 and TCAC. The formation of
surface-bound chloroacetates may play a role in the observed
deactivation of titania catalysis.9,19 The use of supported
catalysts also had a significant effect on the observed reactions,
and such systems may be useful in tuning the chemistry to favor
certain species. In situ SSNMR methods will be advantageous
to investigate these and other issues relevant to photocatalytic
surface chemistry.
Figure 2. 13C MAS NMR spectra obtained during TCE photooxidation
on the PVG supported TiO2 catalyst. The catalyst was prepared from
a gas phase reaction of TiCl4 (48 µmol) on a degassed and calcined
PVG rod followed by hydration and recalcination. The NMR samples
were prepared with the same procedure used for the powdered catalyst.
(a) Spectra acquired during UV irradiation. Assignment: Cl2CHCO2H
(63 and 167.5 ppm), Cl3CCO2H (88 and 164 ppm), TCAA (93 and
177 ppm), TCAC (94 and 164 ppm), and OC (159 ppm). (b) A spectrum
obtained with extended accumulation after the lamp was turned off.
The asterisks indicate spinning sidebands of strongly surface adsorbed
CO2.
from the reaction of DCAC with surface hydroxyl groups.
Continued UV irradiation did not break up DCAc, while other,
more mobile species were converted into CO2. These results
indicate that some reaction intermediates such as DCAC migrate
to the nonirradiated, central portion of the catalyst and react to
form stable, strongly surface-bound species.
A second TiO2 photocatalyst consisting of highly dispersed
TiO2 (roughly 25% of a monolayer) on the surface of transparent
porous Vycor17 glass (PVG) was employed18 in order to
minimize the UV light scattering problem of the powdered
catalyst. 13C MAS NMR spectra (Figure 2a) show that TCE
quickly breaks down to form a number of reaction intermediates
and final products phosgene and CO2 in samples prepared under
dry conditions. Among these species, trichloroacetaldehyde
(Cl3CCHO, TCAA), DCAC, CO, and phosgene were observed
as for the powdered catalyst and/or other previous reports.6,8,10
Three new intermediates, oxalyl chloride (ClCOCOCl, OC),
trichloroacetyl chloride (Cl3CCOCl, TCAC), and trichloroacetic
acid (Cl3CCO2H) were identified. The small size of the spinning
sidebands evident in the spectra indicate that only a small
fraction of the CO2 is bound to the surface. Photocatalytic
reactions in the presence of H2O as a coreactant resulted in the
observation of similar intermediates; however, TCAA and OC
were not observed, and phosgene was rapidly and almost
completely converted to CO2. DCAC was found to react with
Acknowledgment. Support from the National Science Foundation
(CHE 94-22235), the donors of the Petroleum Research Fund, and the
AT&T/Lucent Technologies Industrial Ecology Faculty Fellowship
program is gratefully acknowledged.
(17) Corning Inc., Corning, NY.
(18) Anpo, M.; Aikawa, N.; Kubokawa, Y.; Che, M.; Louis, C.; Giamello,
E. J. Phys. Chem. 1985, 89, 5017.
JA9703990
(19) Sauer, M. L.; Ollis, D. F. J. Catal. 1996, 163, 215.