J. Am. Chem. Soc. 1998, 120, 8265-8266
8265
excitation and in situ FTIR detection. A catalyst consisting of a
1-cm2 tungsten mesh coated with 25-nm TiO2 particles (Degussa
P25) was prepared to provide a clean, dry surface on which the
photocatalytic degradations could occur. Samples were prepared
following methods originally developed by Yates and co-workers
in their investigation of chloromethane and trichloroethylene
(TCE) photooxidation.18-20 Briefly, the catalyst was prepared by
first spraying a 90% acetone/10% water suspension of TiO2
particles (100 g/L) onto a tungsten mesh, which was heated
electrically to 340-370 K, allowing the liquid to evaporate. The
TiO2-coated sample was calcined in the presence of O2 at 773 K
for 5 h and then mounted into the reaction chamber where it was
purged with N2 gas for 1 h. Finally, the sample was heated
electrically at 473 K for 2 h under high vacuum. Pressures in
the reaction chamber were maintained at 10-5 Torr prior to
photodegradation experiments. CH2Cl2 (Fisher Scientific Co.)
was purified by filtration through an alumina column20 and by
subsequent vapor pressure distillation. Approximately 12 µmol
of the reactant and 35 µmol of O2 were introduced into the IR
cell. Either broad band UV, produced by a high-pressure Xe-
Hg lamp (Oriel) operating at 350 W with a water cell for IR
absorption, or 351-nm light from a Xe-F excimer laser (Lambda
Physik Compex) operating at 20 Hz with ca. 26- to 100-mJ, 10-
ns pulses, was used to initiate the reactions. FTIR scans were
typically obtained every 15 min during the lamp experiments and
every 30 min during the laser experiments. Reference FTIR
spectra were acquired immediately before introducing reagents
into the reaction cell. Additionally, GC/MS analysis of the gas-
phase species was performed using an online Varian Saturn 2
ion trap system.
In Figure 1, IR spectra are shown, indicating the progress of
the photocatalytic oxidation of CH2Cl2 in the presence of O2 using
lamp irradiation. Initial FTIR scans 1A and 1D show the
characteristic peaks for CH2Cl2; 2990, 1276, and twin peaks at
764 and 749 cm-1. Spectra 1B and 1E, collected after 60 min of
irradiation, indicate a decrease in the CH2Cl2 absorption at 2990
cm-1 and an intensity shift within the twin peak structure.
Characteristic chloroform (CHCl3) absorption bands were ob-
served at 1219 and 771 cm-1. Additionally, CO2 and CO bands
centered around 2345 and 2166 cm-1, respectively, were observed.
Spectra 1C and 1F, taken after 135 min, show the appearance of
three new species, CCl4 at 795 cm-1, CCl2O with two peaks at
1831 and 849 cm-1, and HCl by a series of peaks 3031 to 2679
cm-1. The observed absorption bands were assigned by com-
parison with IR standards21 and literature data for the case of
phosgene.22 Approximately 0.12 µmol of CCl2O was produced,
representing 1% of the total carbon species within the system.
GC/MS scans confirmed the presence of all of the above species
in runs taken at various times during the experiment.
Changing the Product State Distribution and
Kinetics in Photocatalytic Surface Reactions Using
Pulsed Laser Irradiation
Matthew L. Miller, John Borisch, Daniel Raftery,* and
Joseph S. Francisco*
H.C. Brown Laboratory, Department of Chemistry
Purdue UniVersity, West Lafayette, Indiana 47907-1393
ReceiVed February 10, 1998
Heterogeneous semiconductor photocatalysis has shown prom-
ise for the degradation of a number of volatile organic species
that pose significant environmental concerns.1-5 Photocatalytic
methods can provide significant advantages over existing hazard-
ous waste treatments such as incineration,6 physical processes such
as adsorption,7 and bioremediation.8,9 Studies have shown that
complete mineralization is often possible, even for chlorinated
species, while capital, fuel, and energy costs for photocatalytic
processes can be significantly lower than for comparable thermal
treatments.10 The initial steps involved in photocatalysis that
include the excitation of electrons to the conduction band and
migration of these electrons and holes to the surface are well-
known, although a number of issues pertaining to the complex
surface chemistry are not fully understood.
One question we raise is the effect of using different irradiation
methods to initiate the surface chemistry. Nearly all of the
heterogeneous photocatalytic experiments to date have employed
continuous UV lamps, with few exceptions.11-13 For example,
in the case of dichloromethane (CH2Cl2) photocatalytic degrada-
tion using UV lamps, complete mineralization to HCl, CO2, and
Cl- occurs in the liquid phase,14-16 while in the gas phase, the
undesirable intermediate phosgene (CCl2O) is also observed.17
In this paper, we report our initial results on the photodegradation
of CH2Cl2 over a TiO2 photocatalyst, in which we observe
dramatic changes in the product distribution that depend on the
UV irradiation source. In experiments using a continuous xenon
arc lamp, we observe the formation of CCl2O, HCl, and CO2 in
agreement with previous studies. However, when we use a pulsed
351-nm excimer laser, CCl2O and HCl are no longer produced,
and the kinetics of the CH2Cl2 degradation are altered.
Experiments were conducted using a glass reaction cell that
provided an orthogonal, crossed beam arrangement for UV
* To whom correspondence should be addressed.
(1) Schiavello, M., Ed. Photocatalysis and EnVironment; Kluwer Academic
Publishers: Dordrecht, The Netherlands, 1988.
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of Water and Air; Elsevier: Amsterdam, 1993.
(3) Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 341.
(4) Linsebigler, A. L.; Lu, G.; Yates, J. T. Chem. ReV. 1995, 95, 735.
(5) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem.
ReV. 1995, 95, 69.
Figure 2 contains spectra collected during a photocatalytic
experiment using the pulsed UV laser source. Spectra 2A and
2D again show the characteristic frequencies of CH2Cl2 prior to
irradiation. After 270 min of laser irradiation, spectra 2B and
2E follow the trends established by the lamp experiment, with
CH2Cl2 absorption diminishing as the chloroform, CO2, and CO
bands increase in intensity. However, spectra 2C and 2F, taken
after 540 min of irradiation, show marked differences compared
to the results obtained with lamp irradiation. Although CCl4 is
observed, no HCl or CCl2O is apparent in these two spectra. GC/
(6) Freeman, H. M. In Incinerating Hazardous Wastes; Technomic
Publishing Co.: Lancaster, PA, 1988; p 375.
(7) Patterson, J. W. Industrial Wastewater Treatment Technology, 2nd ed.;
Butterworth Publishers: Boston, 1985.
(8) De Renzo, D. J. Biodegradation Techniques for Industrial Organic
Wastes; Noyes Data Corporation: Park Ridge, NJ, 1980; p 358.
(9) National Research Council. In Situ Bioremediation: When does it work?
National Academy Press: Washington, D. C., 1993; p 33.
(10) Miller, R. In Proceedings of the 1st International EPRI/NSF Sympo-
sium on AdVanced Oxidation; EPRI TR-102927-V2, Electric Power Research
Institute: Palo Alto, 1993; pp 2-27 to 2-28.
(11) Nimlos, M. R.; Jacoby, W. A.; Blake, D. M.; Milne, T. A. EnViron.
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(12) Sczechowski, J. G.; Koval, C. A.; Noble, R. D. J. Photochem.
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(18) Basu, P.; Ballinger, T. H.; Yates, J. T., Jr. ReV. Sci. Instrum. 1988,
59, 1321.
(19) Wong, J. C. S.; Linsebigler, A.; Lu, G.; Fan, J.; Yates, J. T., Jr. J.
Phys. Chem. 1995, 99, 335.
(13) Bahnemann, D. W.; Hilgendorff, M.; Memming, R. J. Phys. Chem. B
1997, 101, 4265.
(14) Pelizzetti, E.; Minero, C.; Maurino, V.; Sclafani, A.; Hidaka, H.;
Serpone, N. EnViron. Sci. Technol. 1989, 23, 1380.
(20) Fan, J.; Yates, J. T., Jr. J. Am. Chem. Soc. 1996, 118, 4686.
(21) Sadtler Infrared Prism Spectra. Sadtler Research Laboratories, Inc:
Philadelphia, PA, 1992.
(15) Hsiao, C.-Y.; Lee, C.-L.; Ollis, D. F. J. Catal. 1983, 82, 418.
(16) Tanguay, J. F.; Suib, S. L.; Coughlin, R. W. J. Catal. 1989, 117, 335.
(17) Lichtin, N. N.; Avudaithai, M. EnViron. Sci. Technol. 1996, 30, 2014.
(22) Bailey, C. R.; Hale, J. B. Philos. Mag. 1938, 25, 98.
S0002-7863(98)00462-4 CCC: $15.00 © 1998 American Chemical Society
Published on Web 07/29/1998