Kinetics and mechanism of CCU photoreductive degradation on TiOi: The role of trichloromethyl radical and dichlorocarbene

Article abstract of DOI:10.1021/jp951431k

The mechanism of photoreduction of CCU on illuminated TiO₂ surfaces was investigated by selectively trapping transient free radical intermediates. Dichlorocarbene and trichloromethyl radical were trapped with 2,3dimethyl-2-butene during the photocatalytic degradation of CCLt. The rate of formation of trapped :CCl2 and 'CCla was found to be a function of [H₂₂O], pH, [CCLt], the nature of the dissolved gas, and light intensity. Dissolved oxygen was not essential for the degradation of CCLt. The production rate of trapped dichlorocarbene showed light intensity dependencies of second, first, and half order with progressively increasing light intensity. A two-electron photoreductive pathway (via dichlorocarbene formation) was found to be the dominant mechanism leading to the full degradation of CCLt. Since dichlorocarbene is hydrolyzed under basic conditions, the pH and water concentration were found to be integral parameters controlling the complete degradation of CCU to CO, CU2, and HC1. Kinetic equations describing the formation of trapped dichlorocarbene were derived from a proposed mechanism. The comparison of the predicted rate expression to the observed data suggested that the observed two-electron transfer occurred consecutively.

Full text of DOI:10.1021/jp951431k

J. Phys. Chem. 1996, 100, 2161-2169
2161
Kinetics and Mechanism of CCl4 Photoreductive Degradation on TiO2: The Role of
Trichloromethyl Radical and Dichlorocarbene
Wonyong Choi and Michael R. Hoffmann*
W. M. Keck Laboratories, California Institute of Technology, Pasadena, California 91125
X
ReceiVed: May 23, 1995; In Final Form: October 30, 1995
The mechanism of photoreduction of CCl
transient free radical intermediates. Dichlorocarbene and trichloromethyl radical were trapped with 2,3-
dimethyl-2-butene during the photocatalytic degradation of CCl . The rate of formation of trapped :CCl and
], the nature of the dissolved gas, and light intensity.
Dissolved oxygen was not essential for the degradation of CCl . The production rate of trapped dichlorocarbene
4 2
on illuminated TiO surfaces was investigated by selectively trapping
4
2
•
3 2 4
CCl was found to be a function of [H O], pH, [CCl
4
showed light intensity dependencies of second, first, and half order with progressively increasing light intensity.
A two-electron photoreductive pathway (via dichlorocarbene formation) was found to be the dominant
mechanism leading to the full degradation of CCl
the pH and water concentration were found to be integral parameters controlling the complete degradation of
CCl to CO, CO , and HCl. Kinetic equations describing the formation of trapped dichlorocarbene were
4
. Since dichlorocarbene is hydrolyzed under basic conditions,
4
2
derived from a proposed mechanism. The comparison of the predicted rate expression to the observed data
suggested that the observed two-electron transfer occurred consecutively.
-
•
-
Introduction
CCl + e f CCl + Cl
(1)
(2)
4
cb
3
Semiconductor photocatalysis has been intensively studied
and investigated for a wide variety of applications such as water
-
-
CCl + 2e f :CCl + 2Cl
4
cb
2
1
2
3
splitting, organic chemical synthesis, metal recovery, and
water and air treatment.4 Among the wide-band-gap semicon-
ductors, TiO2 is the most widely used due to its outstanding
In this study, we present direct evidence for competing one-
and two-electron-transfer steps to CCl by trapping the short-
lived intermediates. The transient intermediates were trapped
using 2,3-dimethyl-2-butene as follows:
4
5
photostability. In particular, its application to hazardous waste
remediation has emerged as a viable technology.
In colloidal TiO2, both the photogenerated electrons and holes
are likely transferred to substrates at the solid/liquid interface.
6
While hole-transfer reactions (i.e., photooxidation) have been
widely studied due to the strong oxidation potential (E° ) +2.7
V vs NHE at pH 7) of the valence-band edge of TiO2, electron-
transfer reactions have been investigated primarily as an
auxiliary step to minimize charge-pair recombination. Not many
studies on chemistry induced by photoelectron transfer on TiO2
6
-10
are found.
Carbon tetrachloride degradation is initiated by a one-electron
reduction step that may be followed by a series of oxidation or
reduction steps. The degradation of CCl4 has been studied using
1
1
7b
12
γ-ray radiolysis, TiO2 photocatalysis, sonolysis, electroly-
sis,
1
3,14
15
and photolysis on MgO. In addition, CCl4 reduction
is known to occur in aquatic environments in the presence of
1
6
17
18
mineral surfaces and on iron powders. In a recent study,
we demonstrated that CCl4 can be photochemically degraded
through the mechanism involving photoreduction by conduction-
band electrons in the presence of TiO2 and suitable electron
donors. On the basis of the fact that CCl4 was found to be
fully degraded in the absence of dissolved oxygen,18 we also
suggested that the initial electron-transfer process may involve
both one- and two-electron transfer reactions. The one- and
two-electron steps result in the formation of trichloromethyl
radical and dichlorocarbene as intermediates, respectively. Both
pathways result in the release of chloride ions (dissociative
electron capture).
The product of reaction 3, C7H11Cl3 (4,4,4-trichloro-2,3,3-
trimethyl-1-butene) will be referred to as TCCl3, and that of
reaction 4, C7H12Cl2 (1,1-dichloro-2,2,3,3-tetramethylcyclopro-
pane), as TCCl2. In this paper, we examine the kinetics of the
formation of TCCl3 and TCCl2 and propose a mechanism for
CCl4 photodegradation consistent with our kinetic observations.
Experimental Section
Titanium dioxide (Degussa P25), which is a known mixture
of 80% anatase and 20% rutile with an average particle size of
2
3
0 nm and a reactive surface area of ∼50 m /g, was used as a
photocatalyst without further treatment. TiO2 concentrations
were maintained at 0.5 g/L in all experiments. To explore the
*
To whom correspondence should be addressed.
Abstract published in AdVance ACS Abstracts, January 1, 1996.
X
0022-3654/96/20100-2161$12.00/0 © 1996 American Chemical Society

2
162 J. Phys. Chem., Vol. 100, No. 6, 1996
Choi and Hoffmann
effects of dissolved gas (Figure 4), the particle suspensions were
bubbled with O2, air, and N2, respectively, for 30-40 min before
addition of the trapping agent and CCl4. The pH of the
suspension was adjusted with 1 N HClO4 or 1 N NaOH. CCl4
(
Baker) and 2,3-dimethyl-2-butene (Aldrich) were used as
received.
Steady-state photolyses were performed with a 1000-W Xe
arc lamp (Spindler and Hoyer). Light was filtered through a
0-cm IR water filter and a UV band-pass filter (310-400 nm,
1
Corning). The filtered light was focused onto a 35-mL quartz
reactor cell loaded with TiO2 suspension. The head space above
the suspension was minimized. Light intensity measurements
were performed by chemical actinometry using (E)-R-((2,5-
dimethyl-3-furyl)ethylidene)isopropylidenesuccinic anhydride
1
9
(
1
Aberchrome 540). The typical light intensity was ∼1.4 ×
-3
-1
-1
0
einstein L min . Two types of photolysis experiments
were carried out. Detection of TCCl3 and TCCl2 was carried
out in a water/acetonitrile mixture in order to increase the
solubility of 2,3-dimethyl-2-butene (trap) and CCl4 up to 100
and 10 mM, respectively. Water concentrations were varied
from 0 to 10 M. In order to investigate the degradation of CCl4,
photolyses were carried out in aqueous suspensions in the
presence of methanol and 2-propanol (0.1 M) as electron donors.
The desired concentrations of CCl4 in water were prepared by
dilution of a saturated CCl4 stock solution (5.1 mM).
Sample aliquots were obtained with a 1-mL syringe, filtered
through a 0.45-µm nylon filter, and injected into a 2.5-mL glass
vial with a PTFE/silicone septum-lined threaded cap. CCl4 and
intermediates were extracted with 0.5 mL of pentane im-
mediately after sampling. Sample vials were stored at 4 °C in
the dark up to 48 h before analysis. CCl4 degradation, the
formation of intermediates, and the formation of products were
followed chromatographically with a Hewlett-Packard (HP)
Figure 1. Mass spectra (extracted from a GC total ion chromatogram)
of (a, top) TCCl2 and (b, bottom) TCCl3 which were generated during
the CCl
4 2
photolysis in the presence of TiO and traps. The mass signal
5
880A gas chromatograph (GC) equipped with a 63Ni electron
intensities were normalized.
capture detector and a HP-5 column (cross-linked 50% PhMe
silicone, 25 m × 0.32 mm × 1.05 µm). Nitrogen was used as
the carrier gas. For the detection and identification of TCCl3
and TCCl2, a HP 5890 II gas chromatograph with a HP-5
column serially connected to a HP 5965B infrared detector
2
-butene (trap), are shown in Figure 1. The molecular ion peak
of TCCl2 (m/e ) 166) was too weak to be detected at its in-
situ concentration (<5 µM). However, the GC retention time
and mass spectrum of TCCl2 exactly matched those of the
authentic compound. In the case of TCCl3, the relative intensity
ratios of the chlorine isotope peaks at masses M, M + 2, and M
(IRD) and a HP 5972A mass selective detector (MSD) was used.
The carrier gas, in this case, was helium. The GCs were
calibrated daily with external standards, and duplicate measure-
ments were made for each sample. The bottom aqueous phase
+
4 were 3:3:1, which indicated that three chlorine atoms were
2
1
contained in the molecular ion. The presence of the terminal
CdC double bond was confirmed by the FTIR spectrum, which
showed a CdC stretching vibration at 1630 cm and an olefinic
CsH stretching vibration at 3109 cm .
(
or water/acetonitrile phase) in the sampling vial was analyzed
-1
-
by ion-exchange chromatography (IC) for Cl ion. The IC
system was a Dionex Bio-LC system equipped with a conduc-
tivity detector and a Dionex OmniPac PAX-500 column (8 µm
-1
The production of TCCl2 during photolysis was linear up to
3
0 min, as shown in Figure 2. TCCl3 production showed a
×
5 mm × 250 mm).
similar trend. Most of data shown in this paper were reproduc-
ible within (10%. The production of trapped intermediates
appeared to depend on [H2O]. This effect is shown in Figure
1
,1-Dichloro-2,2,3,3-tetramethylcyclopropane (TCCl2) was
20
synthesized using the method of Doering and Henderson. The
product was a white powder having a melting range of 51.0-
3
. Production of both TCCl2 and TCCl3 increased with
51.5 °C. No significant impurity was detected by GC/MS. Since
increasing water concentration up to 1 M. However, further
increases in the water concentration resulted in a reduced
production of TCCl2, while TCCl3 production continued to
increase slowly. On the other hand, chloride production was
found to be significant only in an excess of water (10 M). The
production of Cl- at lower water concentrations (e1 M) in
Figure 3 was negligible although the production of TCCl2 and
TCCl3 was a clear indication of chloride production. The
chloride production corresponding to the formation of TCCl2
at [H2O] ) 1 M was ∼7 µM. As for TCCl3, a similar amount
of chloride was expected to be generated even though the
concentration of TCCl3 was not determined. Therefore, the total
chloride production at [H2O] ) 1 M may not exceed 20 µM,
which is similar to the in-situ chloride impurity concentration
authentic samples of 4,4,4-trichloro-2,3,3-trimethyl-1-butene
TCCl3) were not available, its quantification was prevented.
(
However, analyses of mass and FTIR spectra provided sufficient
information to identify the formation of TCCl3. The formation
of TCCl2 and TCCl3 was followed by monitoring mass signals
of m/e ) 131 and m/e ) 165, respectively. These peaks
represented fragment ions of the molecular ion less chlorine
atom (m/e ) M - 35).
Results
Formation of TCCl2 and TCCl3 and Degradation of CCl4.
The mass spectra of TCCl2 and TCCl3, which were generated
during the photolysis of CCl4 in the presence of 2,3-dimethyl-

CCl4 Photoreductive Degradation on TiO2
J. Phys. Chem., Vol. 100, No. 6, 1996 2163
Figure 2. Time-dependent production of TCCl2 during the photolysis
with varying amounts of water (0, 0.2, 1, and 10 M). The experimental
-
1
conditions were: [CCl
4
-1
]
0
) 5 mM, [TiO
2
] ) 0.5 g L , I ) 1.43 ×
-3
-1
1
0
einstein L min (310 < λ < 400 nm), [trap] ) 0.1 M, air
equilibration, and no pH adjustment.
Figure 4. Production of (a, top) TCCl2 and (b, bottom) TCCl3 as a
function of irradiation time in TiO
and O gases before irradiation. Water concentration was 10 M, and
other experimental contions were the same as those of Figure 2.
2 2
suspensions saturated with N , air,
2
Figure 3. Comparison of the production of TCCl2, TCCl3, and
chloride after a 30 min irradiation as a function of water concentration
in TiO suspension. Other experimental conditions were the same as
2
those of Figure 2. The concentration of TCCl3 was not determined.
production in an air-saturated suspension was significantly
delayed compared with its production in a N2-saturated suspen-
sion.
The influence of [CCl4]0 on the production rates of TCCl2
and TCCl3 is compared for two different light intensities in
Figure 5. The production of TCCl2 showed similar saturation
-
in the TiO2 (Degussa P25) suspension. At such low [Cl ], the
titania surface can serve as a buffer site, retaining the additional
chloride produced. Such a retention can be significant when
the dielectric constant of the solution (mostly acetonitrile) is
low.
-
5
-1
-1
behavior at both low (6.1 × 10 einstein L min ) and high
-3
-1
-1
(1.4 × 10 einstein L min ) light intensities. However,
the production rate of TCCl3 at the lower light intensity reached
a saturation level sooner than the system at high light intensity.
The solid lines are fits to a Langmuir-Hinshelwood equation
(ν ) kK[CCl4]/(1 + K[CCl4])). The corresponding constants
Since dissolved O2 is an alternative electron acceptor compet-
•
ing with CCl4 and since it is known to react fast with CCl3,
the effect of dissolved oxygen on the formation of TCCl2 and
TCCl3 was investigated. In Figure 4, the production of TCCl2
and TCCl3 in illuminated TiO2 suspensions, which were sparged
with nitrogen, air, and oxygen prior to photoirradiation, is
compared. In the presence of O2, the apparent induction period
increased with increasing [O2]. This trend is consistent with
the effect of scavenging CB electrons by O2. The appearance
of an induction period in the N2-saturated system can be
attributed to surface adsorbed oxygen, which was not readily
removed by purging alone. After the induction period, during
which oxygen was consumed, the production rate of TCCl2 and
TCCl3 in the O2-saturated system increased to the level of the
oxygen-free system. In the case of TCCl2 production, there
was little difference between N2- and air-saturated solutions
except for the initial induction period. However, TCCl3
for TCCl2 are k )155.7 nM min- and K ) 330 M for high
1
-1
light intensity and k ) 11.2 nM min- and K ) 200 M for
1
-1
low light intensity.
The production rates of TCCl2 are plotted as a function of
light intensity in Figure 6. Three distinct regions are apparent.
Over a broad range, the light intensity dependence changes from
2
.0
-4
-1
-1
νTCCl2 ∝ I (I < 1.6 × 10 einstein L min ) to νTCCl2 ∝
1
.0
-4
-4
-1
-1
I
(1.6 × 10 < I < 3.5 × 10 einstein L min ) and
0.5 -4 -1
finally to νTCCl2 ∝ I (I > 3.5 × 10 einstein L-1 min ).
The measured rates in the third region appear to deviate from
the square-root dependence. This may be due to light-scattering
losses in the turbid suspension at higher light intensities. The
average photon fluxes per particle at I ) 1.6 × 10- and 3.5 ×
4

2
164 J. Phys. Chem., Vol. 100, No. 6, 1996
Choi and Hoffmann
Figure 7. pH (relative scale) dependence of the production rate of
TCCl2, TCCl3, and chloride in the TiO suspension having the same
2
conditions as those of Figure 2. Water concentration was 10 M. The
production rate of TCCl3 is relative.
transition at 5.5 × 10- einstein L-1 min . In a 2-propanol
4
-1
photooxidation study using pure rutile powder (10 g/L), Egerton
and King reported the light intensity order changed from first
23a
-5
-1
-1
order to half order at 1.2 × 10 einstein L min . On the
2
3b
other hand, Okamoto et al. showed that the light intensity
dependence of phenol photooxidation using anatase (2 g/L)
-5
changed from first order to half order around (1.7-3.4) × 10
einstein L- min .
1
-1
The light intensity dependencies of TCCl3 production were
also measured (data not shown due to the undetermined TCCl3
concentration) and showed the similar behavior of second-, first-,
and half-order transition with increasing light intensity. Even
though the production of TCCl3 requires only one electron
Figure 5. Production of (a, top) TCCl2 and (b, bottom) TCCl3 as a
function of initial CCl concentration. The concentration dependences
are compared at two different light intensities (1400 vs 61 µE L
min ). Water concentration was 10 M, and other experimental contions
were the same as those of Figure 2.
4
-
1
-1
(unlike TCCl2 which needs two electrons), it needs a hydroxyl
radical (a hole-equivalent) instead (reaction 3). Therefore, the
overall process of TCCl3 production is equivalent to the two-
electron transfer in terms of the total number of charge transfer,
which makes its light intensity dependence indistinguishable
from that of TCCl2.
The formation of products and intermediates in TiO2/UV-
catalyzed reactions often shows a strong pH dependence. In
order to investigate the effects of pH, the production rates of
TCCl2, TCCl3, and Cl- were measured as a function of pH
(Figure 7). The absolute pH values of the suspensions were
not given on the abscissa of Figure 7, since the reaction medium
was a mixture of water and acetonitrile in which the proton
2
4
-
activity was not measured. However, the log [OH ]add can
be used as a surrogate for pH. The production rate of TCCl2
-
showed a maximum at log [OH ]add ∼ -4 while that of TCCl3
monotonously decreased with increasing base concentration.
-
Both production rates dropped quickly above log [OH ]add >
3.5. On the other hand, the chloride production rate, which
-
Figure 6. Light intensity dependence of TCCl2 production rate under
provides a measure of total CCl4 degradation, showed an
anticorrelation with the production rate of TCCl2. The pH-
dependent behavior was also investigated in an aqueous solution
the same experimental conditions of Figure 2. Water concentration was
2
.0
1
I
0 M. The solid lines represent the light-intensity dependence of I ,
, and I , respectively.
1
.0
0.5
of CCl4 (Figure 8), which were measured using a pH-stat
4
-1
-6
-6
18,25
1
0- einstein L min-1 (or 1.8 × 10 and 3.9 × 10 einstein
titration technique.
This technique measured the consump-
-2
-1
cm min ) are 176 and 385 photons/(particle‚s), respectively
tion rate of the standard base (NaOH 0.10 N) titrant, which was
added to the reactor in order to maintain the pH of the system
constant. The acid generation resulting from CCl4 degradation
to HCl, which is equivalent to the base consumption during
titration, can be considered as an indicator of CCl4 degradation.
The acid generation rate showed a drastic increase above pH
(
assuming an average particle size of 30 nm).
Such transition light intensities seem to vary with the
22
experimental conditions. In our previous study using the same
photocatalyst (Degussa P25, 0.5 g/L), the photodegradation of
chloroform showed the light intensity order transition at 6.87
10 einstein L min . As for Fe -doped (0.5 atom %)
quantum-sized TiO2, the same photoreaction showed the
-5
-1
-1
3+
-
×
10. This behavior is also seen for Cl production, as shown in
6b
Figure 7.

CCl4 Photoreductive Degradation on TiO2
J. Phys. Chem., Vol. 100, No. 6, 1996 2165
TABLE 1: Elementary Reaction Steps of CCl
4
Photolysis in
UV-Irradiated TiO
2
Photoexcitation
hν
-
+
R1
TiO2
CCl
9
8 e + hvb
cb
Adsorption
+ >S (surface site) T CCl4,ad
4
R2
R3
R4
O + >S T O2,ad
2
T + >S T Tad
Electron Transfer
-
-
O
>
2,ad + ecb T O
2
R5
R6
R7
R8
TiIV + ecb T >Ti (tetr
-
III
-
)
-
•
-
CCl4,ad + ecb f CCl
3
+ Cl
III
•
-
CCl4,ad + >Ti f CCl
3
+ Cl
•
•
-
-
3
CCl
CCl
3
+ ecb T CCl
R9
III
-
3
+ >Ti T CCl
3
R10
R11
R12
-
-
+ Cl-
CCl4,ad + 2ecb f CCl
3
-
3 2
T :CCl + Cl
-
CCl
Hole Transfer
O (or >TiOH) f OH (or >TiOH ) + H
Figure 8. pH dependence of CCl
by monitoring the acid generation rate, in an aqueous TiO
4
degradation, which was measured
suspension
+
•
•
+
h
vb + H
2
R13
R14
2
•
•
-
1
-4
2
OH + DH f D + H O
where [CCl
4
]
0
) 5.1 mM, [TiO
2
] ) 0.5 g L , I ) 2.1 × 10 einstein
-
1
-1
L
min (λ ) 320 ( 10 nm), [CH
3
OH] ) 0.1 M, and there is air-
Recombination
Intermediate and Product Formation
-
+
equilibration.
e
cb + hvb f TiO
2
R15
2
•CCl
CCl
CCl
CCl
3
f C
2
Cl
6
R16
R17
R18
R19
R20
R21
R22
R23
R24
R25
R26
R27
•
+ D•
3
3
3
+ DH f CHCl
3
•
•
•
•
+ O
2
f OOCCl
3
•
+ T f TCCl3
•
2
TCCl3 + OH f TCCl3 + H O
-
+
CCl
3
+ H f CHCl
f C Cl
+ T f TCCl2
OOCCl ff CO
+ 3Cl-
CCl + H
CO + OH f HCOO
3
2
:CCl
CCl
2
2
4
:
2
•
3
2
-
:
2
2
O/OH f CO + 2HCl
- -
-
•
HCOO + OH ff CO
2
while no intermediates were detected at pH 12.4. The degrada-
tion of CCl4 was found to be much faster at pH 12.4 than at pH
2
.7.
Photochemical Mechanism of CCl4 Degradation on TiO2.
We propose a detailed mechanism of CCl4 photolysis in TiO2
suspension that is listed in Table 1 in order to interpret our
results. The intermediate trap (2,3-dimethyl-2-butene) is sym-
bolized as T; DH indicates a suitable electron donor (i.e., trap,
acetonitrile, alcohols) with abstractable hydrogen atoms. In this
proposed mechanism, several assumptions are made: (i) adsorp-
tion equilibria for various intermediate species are omitted; (ii)
only indirect hole transfers are considered; (iii) only recombina-
tion between free charge carriers is considered; (iv) the actual
2
6
surficial reaction sites are not specified (For example, the
bimolecular reaction of reaction R23 can take place between
two adsorbates (Hinshelwood-type) or between one adsorbate
and the other in the solution (Rideal-type) or between both in
the solution after desorption.); (v) the degradation reaction of
TCCl2 is not shown because it is negligible in the presence of
excess hole scavengers such as the trap during the initial portions
of the reaction.
In addition to the above assumptions, we have to keep in
mind that the following reactions are in competition with
reactions 3 and 4:
Figure 9. Degradation of CCl
4
and the production of intermediates
suspension at (a, top) pH 2.7 and (b,
and chloride in an aqueous TiO
2
•
•
bottom) pH 12.4. Other experimental conditions were [TiO
2
] ) 0.5 g
, I ) 1.46 × 10 einstein L min (310 < λ < 400 nm), [(CH
CHOH] ) 0.1 M, and N saturation prior to light illumination.
CCl + O f OOCCl
(5)
(6)
3
2
3
-1
-3
-1
-1
L
3 2
) -
•
2
2 CCl f C Cl
3 2 6
•
•
CCl + CH CN f CHCl + CH CN
(7)
(8)
(9)
The product and intermediate analyses were also determined
for photocatalytic CCl4 degradation in aqueous TiO2 suspensions
at pH 2.7 (Figure 9a) and pH 12.4 (Figure 9b). At pH 2.7,
intermediates such as CHCl3, C2Cl4, and C2Cl6 were formed
3
3
3
2
:
CCl + H O f CO + 2HCl
2 2
2
:CCl f C Cl
2 2 4

2
166 J. Phys. Chem., Vol. 100, No. 6, 1996
Choi and Hoffmann
TABLE 2: Kinetic Rate Equations of TCCl2 Production for Three Different Cases of Electron Transfers to CCl
4
d[TCCl2]
dt
case
low light intensities
high light intensities
1H
k I ]
abs1/2[CCl4,ad
I
k
I
abs2[CCl4,ad
]
1L
-
CCl4,ad + ecb
where
where
+
a
+
k k k f ([H ])
k k k f ([H ])
a R23 R7 1
a
R23 R7 1
•CCl
+ ecb-
k1L
)
[T]
k1H )
[T]
3
2
1/2
k′R13 ∑k [S ]
k k
i
i
R9 R15
II
k
2L
I
abs2[CCl4,ad
]
2H
k Iabs[CCl4,ad]
CCl4,ad + 2ecb-
where
where
+
b
+
k k f ([H ])
k k f ([H ])
a
R23 2
a
R23 2
k2L
)
[T]
k2H
)
[T]
2
k′R13
kR8
kR15
III
kR8
[
CCl4,ad
]
[CCl4,ad]
k-R6
k-R6
2
1/2
k I
k I
3H abs
3
L abs
2
kR8
kR8
1
+
[CCl4,ad
]
1 +
[CCl4,ad
]
(
)
(
)
k-R6
k-R6
CCl4,ad + >TiIII
where
where
2
+
c
+
k kR23k f ([H ])
k k k f ([H ])
a
R6
3
IV 2
a
R23 R6 3
IV
•CCl
+ >TiIII
k3L
)
[Ti ] [T]
k3H
)
[Ti ][T]
3
2
1/2
k′R13 k-R6∑k [S ]
kR10kR15
i
i
kR9
kR11
kR10
a
+
b
+
c
+
f ([H ]) )
.
f ([H ]) )
.
f ([H ]) )
.
1
+
2
+
3
+
k-R9 + kR12 + kR21[H ]
k-R9 + kR12 + kR21[H ]
k-R10 + kR12 + kR21[H ]
•
As a result of this competition, only a small fraction of CCl3
and :CCl2 generated during the photolysis was trapped ef-
fectively. The relative trapping efficiency was optimized by
using a high concentration of 2,3-dimethyl-2-butene (e.g., 0.1
M).
where
kR12
ka )
kR23[T] + kR25[H O]
2
Kinetic Analysis of TCCl2 Formation. From the mecha-
nism of Table 1, the observed TCCl2 production rate (Figures
-
Similar steady-state analysis for [CCl3 ] yields
-
5
and 6) can be expressed as (reaction R23)
d[CCl ]
3
kR10[•CCl ][>Ti ] - k
III
-
3
)
[CCl ] -
3
-R10
dt
d[TCCl2]
-
3
-
3
+
)
kR23[:CCl ][T]
(10)
2
kR12[CCl ] - k [CCl ][H ] ≈ 0
dt
R21
kR10[•CCl ][>Ti ]
III
Since the initial rates of TCCl2 formation were measured after
the induction period, reaction steps involving dissolved oxygen
-
3
[
CCl ] )
)
+
3
k-R10 + kR12 + kR21[H ]
(reactions R5 and R18) can be ignored. The reverse reaction
+
•
III
of reaction R12 is not considered because the chloride produc-
tion is small during the initial period. Three different cases of
electron transfers to CCl4 are treated separately: (i) sequential
f ([H ])[ CCl ][>Ti ] (13)
3
3
where
-
one-electron (free ecb ) transfers (reactions R7 and R9); (ii) a
-
kR10
simultaneous two-electron (free ecb ) transfer (reaction R11);
+
f ([H ]) )
III
3
(
(
iii) sequential one-electron (trapped electron, >Ti ) transfers
+
k-R10 + kR12 + kR21[H ]
reactions R8 and R10). The derived kinetic rate equations for
each case under the two extreme conditions of low and high
light intensities are summarized in Table 2. We present the
derivation of the kinetic expression for TCCl2 production for
only case iii, since it represents the most complex mechanism.
The concentration of :CCl2 during the photolysis can be
assumed to be in steady-state according to eq 11:
•
The corresponding steady-state treatment for CCl3 yields
•
d[ CCl ]
3
III
•
III
)
kR8[CCl4,ad][>Ti ] - k [ CCl ][>Ti ] +
R10 3
dt
-
3
•
2
•
k-R10[CCl ] - 2k [ CCl ] - [ CCl ]∑k [S ] ≈ 0 (14)
R16
3
3
i
i
-
3
•
kR12[CCl ] ) k [:CCl ][T] + k [:CCl ][H O] +
where ki and Si refer to the reactions that deplete CCl3, such
R23
2
R25
2
2
•
2
as reactions R17-19. Since the [ CCl3] term is negligible, we
2
2
kR22[:CCl ] (11)
2
can rewrite eq 14 as
The term reflecting the bimolecular recombination of :CCl2 is
negligible relative to the other terms such that
III
-
kR8[CCl ][>Ti ] + k
[CCl ]
3
•
4,ad
-R10
[ CCl ] )
(15)
3
III
kR10[>Ti ] + ∑k [S ]
i
i
-
kR12[CCl ]
3
-
[
:CCl ] )
) k [CCl ]
(12)
When the back-reaction rate of reaction R10 is slow compared
to the rate of reaction R8, eq 15 can be simplified to
2
a
3
kR23[T] + kR25[H O]
2

CCl4 Photoreductive Degradation on TiO2
J. Phys. Chem., Vol. 100, No. 6, 1996 2167
III
kR8[CCl4,ad][>Ti ]
kR8
[•
CCl ] )
(16)
CCl4,ad
[
]
3
k-R6
III
d[TCCl2]
kR10[>Ti ] + ∑k [S ]
1/2
H abs
i
i
) k I
(25)
3
dt
kR8
1
+
[CCl4,ad
]
III
The trapped electron, >Ti , concentration is given by
(
)
k-R6
III
d[>Ti ]
where
IV
-
III
)
k [Ti ][e ] - k-R6[>Ti ] -
R6 cb
+
dt
k k k f ([H ])
a
R23 R6 3
IV
III
•
III
k3H
)
[Ti ][T]
kR8[CCl4,ad][>Ti ] - k [ CCl ][>Ti ] +
1/2
R10
3
kR10kR15
-
k-R10[CCl ] ≈ 0 (17)
3
Discussion
•
k [Ti ][e -] + k-R10[CCl ]
IV
-
From our experimental results, we conclude that both CCl3
and :CCl2 are formed via one- and two-electron reductions of
CCl4 on illuminated TiO2 surfaces. Similar observations of
distinct one- and two-electron pathways were made by Bahne-
III
R6
cb
3
[
>Ti ] )
(18)
•
k-R6 + k [CCl4,ad] + kR10[ CCl ]
R8
3
7
a
The second term in the numerator of eq 18 can be neglected,
because the electron trapping reaction (reaction R6) is very fast
compared to the reverse reaction of reaction R10.2 The third
term in the denominator of eq 18 is much smaller than the
second term, since the steady-state concentration of trichlorom-
ethyl radical should be much lower than the adsorbed CCl4
concentration. Therefore, eq 18 is reduced to
mann et al. during the photoreduction of halothane (2-bromo-
2-chloro-1,1,1-trifluoroethane). However, they noted that the
presence of surficial Pt deposits on the TiO2 was essential for
the two-electron reduction pathway to occur.
The effect of water on the production rate of TCCl2 and
TCCl3 (Figure 3) can be interpreted in terms of the role of the
hydroxyl radical in the hole-scavenging mechanism. Efficient
hole scavenging is essential for maximizing conduction-band
electron transfer, since most of the photogenerated electrons in
the colloidal TiO2 system recombine with holes. Two hole-
scavenging mechanisms are currently being discussed among
7
k [Ti ][e -]
IV
III
R6
cb
[
>Ti ] )
(19)
k-R6 + k [CCl4,ad]
R8
2
8,29
researchers: direct and indirect hole transfers
The steady-state concentration of free charge-carriers under
illumination can be related to the absorbed light intensity, Iabs
as follows:
hvb+ + D f D•+
direct hole transfer
(26)
2
6
hvb+ + H O (or >Ti OH ) f OH (or >Ti OH )
IV
-
•
IV
•
2
d[hvb+]
Iabs - kR13[hvb+][H O] - kR15[hvb+][e ] ≈ 0
-
DH
•
)
8 D + H O
indirect hole transfer (27)
2
cb
2
dt
(20)
where D and DH represent electron donors. The trap (2,3-
dimethyl-2-butene) can serve as an electron donor for both direct
and indirect hole transfer. However, the data in Figure 3
indicate that indirect hole transfer, which involves water
molecules and surficial hydroxyl radicals, is a more efficient
hole-scavenging pathway. Adding 2-methyl-2-propanol or
Iabs ) kR13[ecb-][H O] + kR15[ecb
- 2
]
2
by assuming [hvb+] ≈ [ecb-]) (21)
(
for low light intensities
2
-propanol as additional hole scavengers had little effect on the
Iabs
Iabs
[
ecb-] )
)
(where k′R13 ) kR13[H O])
production rate of TCCl2 and TCCl3. Similar conclusions,
which were based on H-D kinetic isotope effects, were drawn
previously.
2
kR13[H O]
k′R13
2
(22)
18,28
When [H2O] g 1 M, the hole-scavenging
efficiency leveled off.
and for high light intensities
ecb-] )
-
Since Cl production was negligible over the range 0.0 M <
1
/2
[H2O] < 1.0 M, in which the production of TCCl2 and TCCl3
Iabs
•
[
(23)
was rapid (Figure 3), most :CCl2 and CCl3 radicals were trapped
(
)
kR15
with no further degradation. However, when [H2O] ) 10 M,
-
the [Cl ] yield was 200 µM which corresponded to ∼50 µM
Combining eqs 10, 12, 13, 16, 19, 22, and 23 yields for low
light intensities
CCl4 degraded. This implies that CCl4 degradation is significant
only with the presence of an excess amount of water. The
principal role of water in CCl4 degradation does not seem to be
as a source of hydroxyl radicals, since the hole-scavenging
efficiency leveled off at [H2O] g 1 M. This indicates that the
hydrolysis of the intermediate :CCl2 may play a primary role
in the complete degradation of CCl4.
kR8
[
CCl4,ad
]
d[TCCl2]
k
-R6
2
)
k I
(24)
3
L abs
2
dt
kR8
1
+
[CCl4,ad]
(
)
k-R6
Dichlorocarbene is known to undergo hydrolysis (reaction
8
) while trichloromethyl radical does not. The hydrolysis
where
reaction of dichlorocarbene has been studied in detail by
3
0
Robinson, who proposed the following mechanism:
2
R6
+
k kR23k f ([H ])
a
3
IV 2
k )
[Ti ] [T]
2OH-, H O
3
L
2
2
+
-
k′R13 k-R6∑k [S ]
:CCl2
8 CO + 2H + 2Cl
(28)
(29)
i
i
OH-(slow)
-
and for high light intensities
CO
8 HCOO

2
168 J. Phys. Chem., Vol. 100, No. 6, 1996
Choi and Hoffmann
As [H2O] increases, reaction 28 competes effectively with
trapping reaction 4. As a result, the [TCCl2] decreased as [H2O]
increased from 1 to 10 M while [TCCl3] continued to increase
to the formation of dichlorocarbene, which is then rapidly
hydrolyzed with little chance of recombination.
Oxygen also plays a dual role. It competes as an electron
acceptor with CCl4, and it serves as a reactant (reaction 5), which
(Figure 3). The rapid increase in the CCl4 degradation rate at
-
high pH, as shown in Figures 7 and 8, can be attributed primarily
to the base-catalyzed reaction of eq 28.
is essential for the release of three Cl ions from trichloromethyl
radical. The proposed mechanism for this pathway based on
In our previous study,18 we attributed the enhanced rate of
CCl4 reduction at high pH solely to the larger thermodynamic
driving force of conduction band (CB) electron transfer to CCl4,
which is a result of normal Nernstian behavior. However, this
study indicates a more complex process. Increasing TCCl2
pulse radiolysis of CCl4 is as follows:
11
-
•
-
CCl + e f CCl + Cl
(35)
(36)
(37)
(38)
(39)
(40)
4
aq
3
O + e f O2•-
-
2
aq
production coincides with decreasing TCCl3 production at log
-
+
O2•- + H T HO2•
[
OH ]add < -4.0 (Figure 7). In this region, total chloride
production from reductive dechlorination and hydrolysis remains
constant. The fate of CCl3 should be independent of pH. This
•
•
•
CCl + O f OOCCl
3
2
3
implies that a larger thermodynamic driving force for CB
electron transfer increasingly favors a two-electron transfer over
•
•
-
2 OOCCl f O + 2 OCCl
3 2 3
a one-electron transfer to CCl4. For the range of log [OH ]add
>
-3.5, however, both the production of TCCl2 and TCCl-3
•
2
•
HO + OCCl f O + HOCCl
3
rapidly decrease with a concurrent increase in total Cl
production.
3
2
•
•
The two-electron transfer represented by the reaction of eq 2
can be broken into several steps as follows.
OCCl + (CH ) COH f HOCCl + CH (CH ) COH
3
3 3
3
2
3 2
(41)
-
•
-
+
-
CCl + e f CCl + Cl
(30)
(31)
(32)
HOCCl f COCl + H + Cl
3
2
(42)
(43)
4
cb
3
•
CCl + e T CCl -
-
cb
+
-
COCl + H O f CO + 2H + 2Cl
2 2 2
3
3
CCl3- T :CCl + Cl-
Since the production of TCCl3 in the presence of air is retarded
relative to that of TCCl2 (Figure 4), we conclude that either
CCl3 or the intermediate carbon-centered radical in reaction 3
2
•
Equation 32 has been shown to be the rate-determining step in
the basic hydrolysis of chloroform.31 If the photogenerated
is scavenged by oxygen addition to some extent while :CCl2 is
not. Therefore, some of the CCl4 could be degraded completely
by following reactions 35-43. However, reactions 35-43 do
not seem to be the dominant pathway for CCl4 degradation in
the UV/TiO2 system for the following reasons: (i) carbon
tetrachloride is fully degraded in the absence of dissolved
oxygen; (ii) the total chloride production is significant only in
the presence of excess water and at high pH; (iii) the relative
distribution of the intermediates and their pH dependence cannot
be explained by a mechanism involving only the trichloromethyl
radical. The two-electron transfer to CCl4 and the subsequent
hydrolysis of dichlorocarbene seem to be a main path for CCl4
degradation in the UV/TiO2 system.
dichlorocarbene is rapidly scavenged by reaction 28 at high pH,
the equilibrium of reaction 32 shifts to the right side of the
equation. Sequentially, so does that of reaction 31. As a result,
we can expect more trichloromethyl radicals to lead to dichlo-
rocarbene with increasing pH. However, we cannot expect
TCCl2 production to increase continuously with pH, as is shown
in Figure 7, because the hydrolysis reaction (reaction 28) starts
to compete with reaction 4. From the above argument, we can
say that increasing pH has a dual effect on CCl4 degradation.
One part of this effect is the shift in the CB electron potential
to a more negative value according to Nernstian behavior, which
results in an increase in the electron-transfer rate, and the other
part is an increase in the rate of hydrolysis of dichlorocarbene.
This latter effect leads to the full degradation of CCl4 and favors
dichlorocarbene formation over trichloromethyl radical forma-
tion by shifting the equilibria of reactions 31 and 32.
The resulting kinetic expressions for cases i and iii (from
Table 2) predict the observed light intensity dependence of
TCCl2 production: the production rate is proportional to Iabs
2
1
/2
at low light intensities and Iabs at high light intensities.
However, cases ii, which involves the simultaneous two-electron
transfer, does not explain the square-root dependence of TCCl2
production at high light intensities. On the basis of this analysis,
CCl4 appears to be reduced through sequential one-electron
transfers instead of a simultaneous two-electron transfer.
The initial TCCl2 production rates (Figure 5a) show apparent
Langmuir-Hinshelwood-type kinetics with respect to the initial
CCl4 concentration. Even though both cases i and iii predict
the observed light intensity dependence, they have different
The relative effect of pH on intermediate formation (Figure
) is consistent with the data shown in Figure 7. The
9
intermediates are formed at pH 2.7, where both TCCl2 and
TCCl3 are detected, but they are absent at pH 12.4, where
neither TCCl2 nor TCCl3 are seen. Hexachloroethane (C2Cl6)
and perchloroethylene (C2Cl4) are formed through the recom-
bination of two trichloromethyl radicals (reaction 6) and two
dichlorocarbenes (reaction 9), respectively. Chloroform (CHCl3)
could be formed through two pathways.
[
CCl4] dependencies. Case i shows that the reaction rate will
•
be proportional to [CCl4,ad] under both low and high light
intensities. On the other hand, case iii predicts that the
concentration dependence should be different between the low
and high light intensity conditions especially at higher CCl4
concentrations. The production rates under the low-intensity
irradiation should start to decrease at higher CCl4 concentration
according to case iii. This behavior can be rationalized in terms
CCl + (CH ) CHOH f CHCl + (CH ) C˙ OH (33)
3
3 2
3
3 2
CCl3- + H f CHCl
+
(34)
3
Since all of the chlorinated intermediates were derived from
either :CCl2 or CCl3, their formation should be directly
correlated with the formation of TCCl2 and TCCl3. At pH 12.4,
trapped conduction-band electron transfer to CCl4 seems to lead
•
•
of the competition for limited trapped electrons between CCl3

CCl4 Photoreductive Degradation on TiO2
J. Phys. Chem., Vol. 100, No. 6, 1996 2169
References and Notes
and CCl4. However, the low-intensity limit of case iii is not
consistent with the experimental observations (Figure 5a)
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concentration dependence between the low and high light
intensity cases. However, we cannot rule out the possibility
that case iii is operative at high light intensities. We note that
the high-intensity limit of case iii takes the typical form of
Langmuir-Hishelwood kinetics.
On the basis of the above argument, we conclude that the
reduction of CCl4 proceeds through consecutive free electron
transfers at low light intensities (case i) and through the
consecutive trapped or free electron transfers at high light
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<
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7,32
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6
Acknowledgment. Financial support from the Advanced
Research Projects Agency (ARPA) and the Office of Naval
Research (ONR) (Grant N0014-92-J-1901) under the auspices
of the Department of DefensesUniversity Research Initiative
Program (DOD-URI) is gratefully acknowledged.
(
JP951431K