Photoreductive dehalogenation of halogenated benzene derivatives using ZnS or CdS nanocrystallites as photocatalysts

Article abstract of DOI:10.1021/es001114d

ZnS nanocrystallites (nc-ZnS) prepared in N,N-dimethylformamide (DMF) photocatalyze dehalogenation of halogenated benzenes to benzene as the final product from chlorinated benzenes and to difluorobenzenes from fluorinated benzenes in the presence of triethylamine (TEA) as an electron donor under UV light irradiation (λ > 300 nm). When CdS nanocrystallites (nc-CdS) are used as a photocatalyst (λ > 400 nm), halogenated benzenes are photoreductively dehalogenated, yielding trichlorobenzene from hexachlorobenzene and tetrafluorobenzene isomers from hexafluorobenzene as the final products. Photoformed electrons on nc-ZnS and nc-CdS have such negative reduction potentials that these electrons reduce polyhalogenated benzenes, leading to the successive dehalogenation. nc-ZnS exhibits higher photocatalytic activity than nc-CdS due to the more negative potential of the electrons on nc-ZnS than that on nc-CdS. The higher activities of nc-ZnS and nc-CdS compared to their bulk forms are explained as being due to their quantum size effects and the adsorptive interaction between the substrates and the nanosized photocatalysts. ZnS nanocrystallites (nc-ZnS) prepared in N,N-dimethylformamide (DMF) photocatalyze dehalogenation of halogenated benzenes to benzene as the final product from chlorinated benzenes and to difluorobenzenes from fluorinated benzenes in the presence of triethylamine (TEA) as an electron donor under UV light irradiation (λ>300 nm). When CdS nanocrystallites (nc-CdS) are used as a photocatalyst (λ>400 nm), halogenated benzenes are photoreductively dehalogenated, yielding trichlorobenzene from hexachlorobenzene and tetrafluorobenzene isomers from hexafluorobenzene as the final products. Photoformed electrons on nc-ZnS and nc-CdS have such negative reduction potentials that these electrons reduce polyhalogenated benzenes, leading to the successive dehalogenation. nc-ZnS exhibits higher photocatalytic activity than nc-CdS due to the more negative potential of the electrons on nc-ZnS than that on nc-CdS. The higher activities of nc-ZnS and nc-CdS compared to their bulk forms are explained as being due to their quantum size effects and the adsorptive interaction between the substrates and the nanosized photocatalysts.

Full text of DOI:10.1021/es001114d

Environ. Sci. Technol. 2001, 35, 227-231
nant role in the dehalogenation process, yielding intermedi-
ary aldehyde and carboxylic acids which still contain carbon-
chlorine bonds (3, 4, 7-10, 17).
On the other hand, the excited electrons on nc-ZnS and
nc-CdS exhibit highly negative reduction potentials, and the
nanocrystallites are effective photocatalysts for the pho-
Photoreductive Dehalogenation of
Halogenated Benzene Derivatives
Using ZnS or CdS Nanocrystallites
as Photocatalysts
2
toreduction of CO , aromatic ketones, amides, and electron-
deficient alkenes under UV or visible light irradiation (1a, h,
j) when triethylamine (TEA) is employed as an electron donor.
In our previous communication, the high activity of nc-ZnS
for dechlorination of chlorinated benzene was reported briefly
H E N G B O YI N , YU J I W A D A ,
T A K A YU K I K I T A M U R A , A N D
S H O Z O YA N A G I D A *
(
1i). Keeping these facts in mind, we thoroughly investigated
Material and Life Science, Graduate School of Engineering,
Osaka University, Suita, Osaka 565-0871, Japan
photocatalytic dehalogenation of halogenated benzenes
using nc-ZnS or nc-CdS under UV and visible light irradiation,
respectively, in the presence of TEA as an electron donor.
The present paper deals with photoreductive dehalogenation
using nc-ZnS or nc-CdS as a photocatalyst, by taking into
account the fact that the toxicity of organic halides can be
decreased by decreasing the halogen content in compounds
ZnS nanocrystallites (nc-ZnS) prepared in N,N-dimeth-
ylformamide (DMF) photocatalyze dehalogenation of
halogenated benzenes to benzene as the final product
from chlorinated benzenes and to difluorobenzenes from
fluorinated benzenes in the presence of triethylamine (TEA)
as an electron donor under UV light irradiation (λ > 300
nm). When CdS nanocrystallites (nc-CdS) are used as a
photocatalyst (λ > 400 nm), halogenated benzenes are
photoreductively dehalogenated, yielding trichlorobenzene
from hexachlorobenzene and tetrafluorobenzene isomers
from hexafluorobenzene as the final products. Photoformed
electrons on nc-ZnS and nc-CdS have such negative
reduction potentials that these electrons reduce polyhalo-
genated benzenes, leading to the successive dehaloge-
nation. nc-ZnS exhibits higher photocatalytic activity than nc-
CdS due to the more negative potential of the electrons
on nc-ZnS than that on nc-CdS. The higher activities of nc-
ZnS and nc-CdS compared to their bulk forms are
explained as being due to their quantum size effects and
the adsorptive interaction between the substrates and the
nanosized photocatalysts.
(
18).
Experimental Section
Materials. Halogenated benzenes were obtained from the
following sources: monochlorobenzene, dichlorobenzenes,
trichlorobenzenes, tetrachlorobenzenes, 1,4-difluoroben-
zene, and 1,2,3,4-tetrafluorobenzene from Wako Pure Chemi-
cals Industries; pentachlorobenzene, hexachlorobenzene,
1,2-difluorobenzene, 1,3-difluorobenzene, 1,2,4-trifluoroben-
zene, 1,3,5-trifluorobenzene, and hexafluorobenzene from
Tokyo Kasei Organic Chemicals; and 1,2,3-trifluorobenzene,
1
,2,3,5-tetrafluorobenzene, 1,2,4,5-tetrafluorobenzene, and
pentafluorobenzene from Aldrich Chem. Co. Zinc perchlorate
and cadmium perchlorate (from Mitsuwa) were used as
received. Triethylamine (TEA) (from Wako) was used after
fractional distillation. N,N-Dimethylformamide (DMF) (from
Wako) was used as delivered. All chemicals mentioned above
are guaranteed reagent grade. Highly pure CdS (99.99%,
particle size 2.5 µm) and ZnS (99.9%, particle size 3 µm) bulk
powders were commercially purchased from Furuuchi Pure
Chemicals and Nacalai Tesque, respectively, and in this paper,
they are called the bulk powders.
Preparations of nc-ZnS and nc-CdS in DMF. ZnS and
CdS nanocrystallites were prepared in DMF using the
methods similar to those described in previous studies (1f,
Introduction
Photocatalytic reactions have attracted the attention of
scientists in the fields of photosynthesis (1, 2) and pollutant
treatment (1, 3-15). In particular, with respect to the
environmental protection issues, halogenated chemicals,
such as pesticides, halogenated benzenes, and halogenated
aliphatic hydrocarbons, are difficult to decompose completely
k). A DMF solution 5 mL of Zn(ClO
(ClO ‚6H O (5 mM) was taken into a Pyrex tube, and then
the solution was flushed with nitrogen for 15 min. After oxygen
purging, H S was introduced into the DMF solution under
4
)
2
‚6H
2
O (5 mM) or Cd-
4
)
2
2
2
stirring and cooling in an ice-water bath for 30 or 8 s to
prepare nc-ZnS or nc-CdS, respectively. The particle size
ranges of nc-ZnS and nc-CdS in DMF were 1.6-2.7 and 2-5
nm, respectively (1f, k). The concentrations of the colloidal
solutions of the nanocrystallites are denoted as those of the
formulas of ZnS and CdS, which are called the diatomic
concentration.
Procedure for ZnS- or CdS-Photocatalyzed Dehaloge-
nation of Halogenated Benzene. To 1 mL of nc-ZnS or nc-
CdS (5 mM) DMF solution in a Pyrex tube (diameter 8 mm)
was added 1 mL of a DMF solution of halogenated benzene
containing TEA (2 M) and dodecane as an internal standard.
The photocatalytic reaction mixture was degassed again using
nitrogen for 15 min in order to purge dissolved oxygen. The
reaction tube was closed using a rubber plug and irradiated
under magnetic stirring using a high-pressure mercury lamp
2 2
to CO , H O, and HCl by incineration, and toxic chemicals
such as dioxins are detected in incinerator fields. Recently,
many researchers have devoted their efforts to performing
the photodehalogenation of polyhalogenated compounds
under mild conditions (1i, 3-11, 13, 14, 16). There exist two
pathways for photocatalytic dehalogenation in which semi-
conductor particles are used as photocatalysts; one is
2
photocatalytic oxidation using nanosized TiO as a catalyst
under atmospheric conditions (3, 4, 7-10), and the other is
photocatalytic reduction using semiconductors as catalysts
and some electron donors (1i, 13, 14). In general, the oxidation
potentials of the halogenated compounds are highly positive,
suggesting difficulty of oxidation in nature. Furthermore, in
photocatalytic oxidation, the hydroxyl radical plays a domi-
(
λ>300 nm, 500 W) when nc-ZnS was used as a photocatalyst,
*
Corresponding author phone: +81-66879-7924; fax: +81-66879-
7
875; e-mail: yanagida@chem.eng.osaka-u.ac.jp.
or a tungsten halogen lamp through a saturated aqueous
1
0.1021/es001114d CCC: $20.00

2001 Am erican Chem ical Society
VOL. 35, NO. 1, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2 2 7
Published on Web 11/23/2000

TABLE 1. Dechlorination of Chlorinated Benzenes Photocatalyzed by nc-ZnS under UV Lighta
composition of reaction mixtures (mol %)
react.
Ered. (V vs SCE)b
catalyst
react. time (h) conv. (%)
ben
32
m
1,2-di 1,3-di 1,4-di 1,2,3-tri 1,2,4-tri tetra
m
m
,2-di
,3-di
,4-di
,2,3-tri
,2,4-tri
tetra
tetra
tetra
-2.44
nc-ZnS
none
4.2
4
31
9
69
9
91
64
58
64
1
1
1
1
1
-2.22
-2.20
-2.20
-1.96
-2.00
-1.76
nc-ZnS
nc-ZnS
nc-ZnS
nc-ZnS
nc-ZnS
nc-ZnS
bulk ZnS
none
4
4
4
3.9
5
4
5
5
59
56
73
90
69
75
26
23
4.3
2.9
8
trace 36
trace 15
41
44
27
34
4
1.6
21
9
2
10
39
7.8
31
44
26
24
trace
1.6
6.9
25
74
77
trace trace
2.7 trace
a
Reaction conditions: reactant (25 m M), TEA (1 M), and nc-ZnS (diatom ic concentrated 2.5 m M) or bulk ZnS (5 m g) in 2 m L of DMF. Abbreviations:
tetra, 1,2,3,4-tetrachlorobenzene; 1,2,4-tri, 1,2,4-trichlorobenzene; 1,2,3-tri, 1,2,3-trichlorobenzene; 1,4-di, 1,4-dichlorobenzene; 1,3-di, 1,3-
b
dichlorobenzene; 1,2-di,1,2-dichlorobenzene; m , chlorobenzene; ben, benzene. From ref 19 in DMSO.
FIGURE 2. Photodechlorination of 1,2,3,4-tetrachlorobenzene with
FIGURE 1. nc-ZnS-catalyzed photodechlorination of 1,2,3,4-tetra-
chlorobenzene in DMF. Conditions are as listed in Table 1.
Symbols: b 1,2,3,4-tetrachlorobenzene, 9 1,2,4-trichlorobenzene,
TEA in DMF. Conditions are identical to those in Figure 1, except
for the absence of nc-ZnS. Symbols: b 1,2,3,4-tetrachlorobenzene,
9 1,2,4-trichlorobenzene, 2 1,4-dichlorobenzene, and 4 hydrogen.
[
1,2,3-trichlorobenzene, 2 1,4-dichlorobenzene, 1 1,2-dichlo-
robenzene, 0 1,3-dichlorobenzene, ] chlorobenzene, 4 hydrogen,
and O diethylamine.
Spectral analyses of ZnS, CdS, and their interaction with
the chlorine-containing compounds were performed using
Hitachi U-3300 spectrometer. The diffusion reflectance
spectra of nc-ZnS and nc-CdS were measured for the samples
obtained by depositing freshly prepared nanocrystallites on
filter papers.
sodium nitrite solution filter (λ > 400 nm, 300 W) when nc-
CdS was used as a photocatalyst. The lamps were cooled by
water, and the reaction tube was maintained in a water bath
(
298 K) during irradiation.
For the commercially available ZnS or CdS bulk powder-
Results and Discussion
photocatalyzed dehalogenation, all of the procedures were
the same as mentioned above, except that the nanocrystallites
were replaced by the bulk powder.
Determ ination of Apparent Quantum Yields. The quan-
tum yields for dechlorination of 1,2,3,4-tetrachlorobenzene
6.25 mM) photocatalyzed by nc-ZnS (2.5 mM) and nc-CdS
2.5 mM) were determined under irradiation with mono-
chromatic lights at 313 and 405 nm, respectively, by using
mL of a DMF solution containing TEA (1 M) in a 4 mL
quartz cell. The monochromatic lights (313 and 405 nm) were
obtained by using a Xe lamp (TRIAX 180, Instrument S. A.,
Inc. Edison NJ). The intensity of incident light was monitored
by the conventional actinometry method using tris(oxalate)
iron(III).
Photodechlorination Using ZnS. When nc-ZnS was used as
a photocatalyst, chlorinated benzenes were successively
dechlorinated to benzene at the final stage. The amounts of
consumed substrates were in balance with those of the
products, indicating that stoichiometric dechlorination oc-
curred in this system. No other byproducts other than the
partially dechlorinated benzenes were detected, indicating
that this catalytic reaction was very selective. The conversions
and the product distributions for dechlorination under
various conditions are listed in Table 1. Under UV light
irradiation without nc-ZnS, the photodechlorination rate was
much lower than that with nc-ZnS. Under visible light (λ >
(
(
2
4
00 nm) irradiation, no dechlorination took place even in
the presence of nc-ZnS. The commercially available ZnS bulk
powder exhibited little activity for the dechlorination of
tetrachlorobenzene, even compared to the blank experiment,
and the final dechlorinated product was 1,4-dichlorobenzene.
Analysis. The liquid-phase reaction mixtures were ana-
lyzed by gas chromatography using a fused silica capillary
column (HiCap-CBP20, 25 m × 0.2 mm, Shimadzu) and a
flame ionization detector. Dodecane was used as an internal
standard sample. The concentrations of the halogenated
benzenes were determined using the ratios of the peak areas
of halogenated benzene to those of dodecane. Hydrogen was
analyzed by gas chromatography using a Shimadzu GC-8A
apparatus equipped with a thermal conduction detector and
a column packed with active carbon.
Figures 1 and 2 show the time profiles of the dechlorination
of 1,2,3,4-tetrachlorobenzene in the presence and the absence
of nc-ZnS, respectively. Although the photocatalytic dechlo-
rination of 1,2,3,4-tetrachlorobenzene clearly showed higher
rate than that in the blank experiment, the product distribu-
tions were similar for the two systems, i.e., the predominant
2
2 8
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 1, 2001

SCHEME 1
FIGURE 3. nc-CdS-catalyzed photodechlorination of hexachlo-
robenzene in DMF. Conditions are as listed in Table 2. Symbols:
b hexachlorobenzene, 9 pentachlorobenzene, 2 1,2,4,5-tetrachlo-
robenzene, [ 1,2,3,4-tetrachlorobenzene, 1 1,2,4-trichlorobenzene,
and ] diethylamine.
products were 1,2,4-trichlorobenzene in the trichlorobenzene
derivatives and 1,4-dichlorobenzene in the dichlorobenzene
derivatives, suggesting the formation of common intermedi-
ates in the dechlorination processes for the photocatalytic
and photochemical systems. Tetrachlorobenzene was se-
lectively and stepwise dechlorinated to form less-chlorinated
homologues, through trichlorobenzene, dichlorobenzene,
and monochlorobenzene to benzene as the final product.
Diethylamine was formed as an oxidation product from TEA
as the reaction progressed. Hydrogen was also competitively
formed in the photocatalysis. In addition, the main products
were found to be 1,4-dichlorobenzene and 1,2-dichloroben-
zene for dechlorination of 1,2,4-trichlorobenzene and 1,2,3-
trichlorobenzene, respectively (see Table 1). The main
pathways in the photoreductive dechlorination catalyzed by
nc-ZnS are demonstrated in Scheme 1. Chlorinated benzenes
combine with electrons from the conduction bands of nc-
ZnS to form corresponding radical anions, and TEA should
transfer their electrons to the holes formed in the valence
bands of nc-ZnS under UV light irradiation (1k). The electron-
transfer process in the catalysis process should be different
from that in the photochemical process in which chlorinated
benzene radical anion is generated through the direct transfer
of an electron from TEA to an excited state of chlorinated
benzene as described by Freeman et al. (16a, c, d). However,
the common intermediates, i.e., the radical anion of the
substrate, are formed for the two systems. Hydrogen atoms
which replace chlorine atoms should come from TEA. The
chloride anions formed from chlorinated benzene derivatives
should combine with protons from TEA to form HCl as
confirmed by Freeman et al. (16c, d).
of hexachlorobenzene was pentachlorobenzene when bulk
CdS was used in place of nc-CdS, suggesting that the
photocatalytic activity of nc-CdS was much higher than that
of the bulk CdS. No photodechlorination took place without
CdS nanocrystallites.
The nc-CdS-catalyzed photodechlorination of hexachlo-
robenzene (Figure 3) demonstrated that hexachlorobenzene
was completely dechlorinated to form less-chlorinated
benzene derivatives in 1 h. Pentachlorobenzene, which was
first formed with the elapse of time, reached a maximum
after hexachlorobezene was completely dechlorinated, un-
dergoing further dechlorination. After a short induction time,
1
,2,4,5(1,2,3,4)-tetrachlorobenzene appeared, but the forma-
tion of 1,2,4-trichlorobenzene required a long induction time.
Among the photoformed tetrachlorobenzene isomers, the
main product was 1,2,4,5-tetrachlorobenzene, and further
photocatalytic dechlorination yielded only 1,2,4-trichlo-
robenzene as the final dechlorinated product. As shown in
Table 2, the amounts of consumed substrates were in balance
with those of the products, indicating that stoichiometric
dechlorination also occurred for nc-CdS. Diethylamine was
also formed as an oxidation product as the reaction pro-
gressed. The main photoreductive pathways are demon-
strated in Scheme 2.
The apparent quantum yield for dechlorination of 1,2,3,4-
tetrachlorobenzene to 1,2,4- and 1,2,3-trichlorobenzene by
nc-CdS irradiated by monochromatic light (405 nm, 1.897 ×
-
8
The apparent quantum yield for dechlorination of 1,2,3,4-
tetrachlorobenzene to 1,2,4- and 1,2,3-trichlorobenzene by
nc-ZnS irradiated by monochromatic light (313 nm, 5.430 ×
10 einstein/ min) was 1.4%.
Photodefluorination Using nc-ZnS and nc-CdS. The
photocatalysis using nc-ZnS was successfully extended to
defluorination of fluorinated benzene derivatives. (Table 3)
The defluorination proceeded even in the absence of nc-
ZnS, but the defluorination rate in the presence of nc-ZnS
was much faster than that in the absence of nc-ZnS. The
conversion of hexafluorobenzene or 1,2,4-trifluorobenzene
in the presence of nc-ZnS after 4-h irradiation was six and
-
9
1
0
einstein/ min) was 8.6%.
Photodechlorination Using CdS. When nc-CdS was used
as a photocatalyst under visible light irradiation (λ > 400
nm), chlorinated benzenes were photoreductively dechlo-
rinated, yielding trichlorobenzene without further dechlo-
rination (Table 2). The final product in the dechlorination
TABLE 2. Dechlorination of Chlorinated Benzenes Photocatalyzed by nc-CdS under Visible Lighta
composition of reaction mixtures (mol %)
react.
HCB
Ered. (V vs SCE)b
catalyst
react. time (h)
conv. (%)
1,2,4-tri
1,2,4,5-tetra
1,2,3,4-tetra
PCB
HCB
-1.32
nc-CdS
bulk CdS
nc-CdS
6
6
6
100
20
36
4
80
14
2
20
0
80
HCB
,2,3,4-tetra
1
36
64
a
Reaction conditions: reactant (6.25 m M), TEA (1 M), and nc-CdS (diatom ic concentrated 2.5 m M) or bulk CdS (5.5 m g) in 2 m L of DMF.
Abbreviations: HCB, hexachlorobenzene; PCB, pentachlorobenzene; 1,2,3,4-tetra, 1,2,3,4-tetrachlorobenzene; 1,2,4,5-tetra, 1,2,4,5-tetrachlorobenzene;
,2,4-tri, 1,2,4-trichlorobenzene. b From ref 19 in DMSO.
1
VOL. 35, NO. 1, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2 2 9

SCHEME 2
SCHEME 3
FIGURE 4. Diffusion reflectance spectra of ZnS and CdS: (1) nc-
ZnS deposited on filter paper, (2) bulk ZnS, (3) nc-CdS deposited
on filter paper, and (4) bulk CdS.
at 310 and 490 nm, respectively (Figure 4). The blue shift in
absorption of the freshly prepared sulfide nanocrystallites
compared to that of the bulk sulfides is explained as being
due to the quantum size effect. The higher photocatalytic
activities of nc-ZnS and nc-CdS than their bulk forms should
be ascribed to the quantum size effects of nanocrystallites.
four times higher, respectively, than that in the absence of
nc-ZnS. After 4-h irradiation, hexafluorobenzene, pentafluo-
robenzene, tetrafluorobenzene, and trifluorobenzene were
defluorinated to form less fluorinated derivatives, but dif-
luorobenzene was not defluorinated any further. The com-
mercially available ZnS bulk powder was of little activity for
the defluorination of hexafluorobenzene compared to the
blank experiment. The amounts of the consumed substrates
were in balance with those of products. The photocatalytic
defluorination was a successive process similar to that of
chlorinated benzene, and the final product was the diflu-
orinated derivatives (Scheme 3(1)).
The quantum size effect gives rise to electrons having
highly negative reduction potential. The conduction band
energy of nc-ZnS was estimated to be -2.3 V vs SCE, whereas
bulk ZnS was estimated to be -2.0 vs SCE (1g). The
conduction band energy of bulk CdS was reported to be -1.90
V vs SCE (1l). However, the photoexcited nc-CdS was
estimated to produce electrons having a potential of -2.21
V vs SCE in DMF (1f). The more negative conduction band
energy of nc-ZnS than that of nc-CdS and the more negative
conduction band energies of nc-ZnS and nc-CdS than those
of their corresponding bulk forms should explain the reason
nc-ZnS is of higher catalytic activity than nc-CdS and why
the nanosized sulfides are of higher activities than their bulk
forms.
When nc-CdS was used as a photocatalyst, hexafluo-
robenzene was photodefluorinated to form tetrafluoroben-
zene isomers as the final products via pentafluorobenzene
(
Table 3). No defluorination of hexafluorobenzene took place
under dark or in the absence of nc-CdS. The commercially
available CdS bulk powder did not show any photocatalytic
activity for the defluorination of hexafluorobenzene. The
amount of defluorinated hexafluorobenzene was in balance
with that of the produced pentafluorobenzene and tet-
rafluorobenzene, supporting successive and selective de-
fluorination, as shown in Scheme 3(2).
Mechanism of Photocatalytic Dechlorination. The onsets
of the diffusion reflectance UV-visible absorption of com-
mercially available micron-size ZnS and CdS were observed
at 375 and 550 nm, respectively. However, the freshly
prepared nc-ZnS and nc-CdS exhibited the absorption onsets
Figure 5 shows that the absorption onset of the mixture
of tetrachlorobenzene and nc-ZnS (spectrum 3) exhibited
red-shift, compared to that of only nc-ZnS (spectrum 1) or
tetrachlorobenzene (spectrum 2) in DMF. The absorption
onsets of the mixtures of hexafluorobenzene and nc-CdS
(Figure 6, spectrum 2) and tetrachlorobenzene and nc-CdS
(Figure 6, spectrum 3) also exhibited red-shift, compared to
that of nc-CdS alone in DMF (Figure 6, spectrum 1). These
a
b
TABLE 3. Defluorination of Fluorinated Benzenes Photocatalyzed by nc-ZnS and nc-CdS
composition of reaction mixtures (mol %)
react.
HFB
HFB
HFB
PFB
Ered. (V vs SCE)c
catalyst
react. time (h)
conv. (%)
DFB
TrFB
TeFB
PFB
HFB
-2.11
nc-ZnS
4
4
4
4
4
4
4
4
6
6
6
64
11
14
57
37
44
36
9
trace
60
11
14
48
40
89
86
d
none
bulk ZnS
nc-ZnS
nc-ZnS
nc-ZnS
nc-ZnS
-2.35
-2.40
8
33
35
63
91
44
63
60
1
1
1
1
,2,4,5-TeFB
4
5
37
9
,2,3,4-TeFB
,2,4-TrFB
,2,4-TrFB
-2.40 to-2.64
d
none
HFB
HFB
HFB
nc-CdS
38
0
0
trace
38
62
100
100
e
none
bulk CdS
a
Reaction conditions: reactant (25 m M), TEA (1 M), and nc-ZnS (diatom ic concentrated 2.5 m M) or bulk ZnS (5.2 m g) in 2 m L of DMF. Abbreviations:
HFB, hexafluorobenzene; PFB, pentafluorobenzene, TeFB, tetrafluorobenzene isom ers; TrFB, trifluorobenzene isom ers; DFB, difluorobenzene
b
isom ers. Reaction conditions: reactant (6.51 m M), TEA (1 M), and nc-CdS (diatom ic concentrated 2.5 m M) or bulk CdS (6 m g) in 2 m L of DMF.
c
From ref 20 in DMF. d Blank experim ents, other conditions are the sam e as that with nc-ZnS as a catalyst. e Blank experim ent, other conditions
are the sam e as that with nc-CdS as a catalyst.
2
3 0
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 1, 2001

and selective reduction process. Since no byproducts are
formed in the present dehalogenation processes, the exten-
sion of these processes will provide a new concept for the
detoxification of highly toxic polyhalogenated aromatics such
as dioxin at ambient temperature.
Acknowledgments
This work was partly supported by Grant-in-Aid for Scientific
Research from the Ministry of Education, Science, Sports,
and Culture of Japan (Nos. 09490023, 11358006) and “Re-
search for the Future” Program, JSPS 96P00305. We gratefully
acknowledge useful discussion with Drs. Hiroaki Fujiwara
and Kei Murakoshi, and H. Y. gratefully thanks Mr. Sadao
Murasawa for his financial support in Japan.
FIGURE 5. UV-vis spectra of nc-ZnS-catalyzed system in DMF: (1)
nc-ZnS, (2) 1,2,3,4-tetrachlorobenzene (TeCB), and (3) mixture of
TeCB and nc-ZnS. The concentrations are the same as that in nc-
ZnS-catalyzed system.
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FIGURE 6. UV-vis spectra of nc-CdS-catalyzed system in DMF: (1)
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Taking into account the strong adsorptive interaction
between sulfide nanocrystallites and substrates, the mech-
anism of photocatalytic dechlorination by nc-ZnS or nc-CdS
is depicted in Scheme 4. The valence band electrons of nc-
ZnS and nc-CdS are excited by light irradiation to their
conduction bands, giving rise to the formation of electron
and hole pairs. The conduction band electrons are transferred
to chlorinated benzenes which are adsorbed on the surfaces
of the nanocrystallites, resulting in the selective dehaloge-
nation. TEA, as a sacrificial electron donor, is effectively
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Received for review March 20, 2000. Revised manuscript
received October 3, 2000. Accepted October 9, 2000.
The photocatalytic dehalogenation that occurs under the
conditions examined in the present study is a consecutive
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