Investigation of the photocatalytic activity of TiO2-polyoxometalate systems

Article abstract of DOI:10.1021/es0106568

The present study reports the investigation of polyoxometalate catalyzed electron transfer from the conduction band of photoexcited TiO₂ to molecular oxygen. The oxidation of 1,2-dichlorobenzene (DCB) was used as an index reaction for evaluating the photocatalyst systems TiO₂-PW₁₂O₄₀^3-, TiO₂-SiW₁₂O₄₀,^4- and TiO₂-W₁₀O₃₂^4- in oxygenated aqueous solution. Addition of these polyoxometalate (POM) anions to TiO₂ suspensions resulted in significant rate enhancement for DCB oxidation. Photodegradation kinetics exhibited [POM] dependence, experiencing different maximum (k = 0.0318 min^-1, 0.0108 min^-1, and 0.0066 min^-1) for each POM at different [POM] (0.1 mM PW₁₂O₄₀^3-, 0.07 mM SiW₁₂O₄₀,^4- and 1 mM W₁₀O₃₂,^4- respectively). The probability that the difference in the adsorption affinity of POMs on TiO₂ surface could account for the observed ranking of photodegradation rates was ruled out by adsorption isotherm experiments that revealed similar binding constants for each POM (467 M^-1, 459 M^-1, and 417 M^-1 for PW₁₂O₄₀^3-, SiW₁₂O₄₀^4-, and W₁₀O₃₂^4-, respectively). DCB degradation over TiO₂ with O₂ or POM+O₂ systems can be modeled by the Langmuir-Hinshelwood (saturation kinetics) model. The concentration-independent rate constants (k_L-H) for TiO₂-O₂, TiO₂-W₁₀O₃₂^4-, TiO₂-SiW₁₂O₄₀^4-, and TiO₂-PW₁₂O₄₀^3- were 0.0818, 0.152, 0.421, and 0.638 min^-1, respectively. An analysis of ΔG for electron transfer from the conduction band of TiO₂ to POMs in this study shows that the electron transfer takes place even when it is endothermic.

Full text of DOI:10.1021/es0106568

Environ. Sci. Technol. 2001, 35, 3242-3246
However, there are strategies to reduce hole-electron
Investigation of the Photocatalytic
recombination and increase photocatalyst efficiency. Typi-
cally, these involve the addition of sacrificial oxidants to
scavenge conduction band electrons and enable holes to
oxidize a targeted organic substrate. The least expensive of
these is dioxygen; however, oxidants such as ozone, per-
oxydisulfate, periodate, bromate, chlorate, hydrogen per-
oxide, and others have all been successfully applied (14-
Activity of TiO -Polyoxometalate
2
Systems
R U YA R . O Z E R A N D J O H N L . F E R R Y*
Department of Chemistry and Biochemistry, University of
South Carolina, Columbia, South Carolina 29208
1
9). It is also possible to remove conduction band electrons
electrochemically, by depositing the photocatalyst on a
conductive substrate and placing it at a sufficiently positive
potential to harvest the photogenerated electrons (20-22).
All of these techniques can boost photooxidation efficiency
and have been thoroughly reviewed by other authors (4, 6,
The present study reports the investigation of polyoxo-
metalate catalyzed electron transfer from the conduction
band of photoexcited TiO2 to molecular oxygen. The oxidation
of 1,2-dichlorobenzene (DCB) was used as an index
reaction for evaluating the photocatalyst systems TiO2-
2
3, 24).
Although added oxidants can have dramatic effects on
photooxidation efficiency, it is difficult for any of them to
compete on a cost-benefit basis with the potential of atmo-
spheric dioxygen. Therefore, several investigators are taking
the approach of developing surface modified photocatalysts
that possess new, synthesized sites for catalyzing electron
transfer from the conduction band to dioxygen (25-33).
Typically, this is accomplished by the addition of transition
metal islands to the photocatalyst particle. This addition can
occur by dispersing metals in the semiconductor lattice by
coprecipitation techniques or on the outside of the photo-
catalyst particle by photodeposition as metal islands (27).
The effect of coprecipitation is difficult to predict; ions such
3
-
4-
4-
PW12O40 , TiO2-SiW12O40, and TiO2-W10O32 in
oxygenated aqueous solution. Addition of these polyoxo-
metalate (POM) anions to TiO2 suspensions resulted in
significant rate enhancement for DCB oxidation. Photodeg-
radation kinetics exhibited [POM] dependence, experiencing
-
1
-1
different maximum (k ) 0.0318 min , 0.0108 min , and
.0066 min ) for each POM at different [POM] (0.1
mM PW12O40 , 0.07 mM SiW12O40, and 1 mM W10O32,
respectively). The probability that the difference in the
-
1
0
3-
4-
4-
as Fe3 and Cu at very low concentrations inhibit electron-
+
2+
adsorption affinity of POMs on TiO2 surface could account
for the observed ranking of photodegradation rates was
ruled out by adsorption isotherm experiments that revealed
2
+
hole recombination (34-37), while others including V (38),
Mo3 (39), and Cr (38) have been reported to facilitate the
recombination of charge-carriers. Photodeposition of metal
islands can be especially problematic because the island size
can potentially grow large enough to “shade” the particle or
block active sites.
+
3+
-1
similar binding constants for each POM (467 M , 459
M , and 417 M for PW12O40 , SiW12O40 , and W10O324-,
-
1
-1
3-
4-
respectively). DCB degradation over TiO2 with O2 or
POM+O2 systems can be modeled by the Langmuir-
Hinshelwood (saturation kinetics) model. The concentration-
independent rate constants (kL-H) for TiO2-O2, TiO2-
Here, we report the investigation of TiO
2
-polyoxometalate
(
POM) photocatalytic system s that utilize the anions
3-
4-
4-
PW12O40 , SiW12O40 , and W10O32 as electron scavengers
4
-
4-
3-
W10O32 , TiO2-SiW12O40 , and TiO2-PW12O40 were
for photogenerated conduction band electrons. Yoon and
co-workers recently reported enhanced photocatalytic re-
-
1
0
.0818, 0.152, 0.421, and 0.638 min , respectively. An analysis
of ∆G for electron transfer from the conduction band of
TiO2 to POMs in this study shows that the electron transfer
takes place even when it is endothermic.
duction rates for methyl orange in a composite TiO -
2
3
-
PW12
O
40 system. In this paper, the authors interpreted their
and
finding to be a result of joint photoexcitation of the TiO
2
POM, with photoexcited POM serving as an electron-transfer
catalyst to methyl orange (40). In contrast, our system utilizes
POMs as ground-state oxidants to facilitate hole-electron
separation.
Introduction
Semiconductor photocatalysts have received a great deal of
attention for their potential to utilize solar photons in
chemical processes, including such diverse areas as pho-
The photocatalytic activity of all the systems in this study
were measured by the photooxidation of the index chemical
1,2-dichlorobenzene (DCB) in aqueous solution. These POMs
are known to be moderate ground-state oxidants with
reduction potentials of H NaPW O (-0.023 V vs SCE) (41),
2
toelectrochemical electricity generation, CO fixation, and
the remediation of hazardous waste streams (1-10). The
photoexcitation of these materials results in the promotion
of a valence band electron to the conduction band of the
semiconductor, creating strongly oxidizing sites (holes) and
reducing sites (electrons) in the photocatalyst (11). These
sites may react with materials adsorbed to the photocatalyst
surface, or they may recombine without effecting a change
in the immediate environment (9, 12, 13). Hole-electron
recombination is a serious problem for the development of
photocatalytically based technologies, and quantum yields
for photocatalytic processes tend to be quite low (often less
than 0.05).
2
12 40
H SiW O (-0.187 V vs SCE) (41), and Na W O (-1.270
4
12 40
4
10 32
V vs SCE) (42). It has been demonstrated that the POMs in
this study are structurally stable in their reduced state and
are readily reoxidized by dioxygen or hydrogen peroxide (41,
43-46). In our work, we observe that these oxidants are more
effective than dioxygen alone at removing conduction band
electrons from TiO , even when POM reduction is energeti-
2
cally disfavored. The suggested sequence of reactions portrays
the POM as an electron carrier between the photocatalyst
particle and dioxygen (Scheme 1). This is analogous to the
use of methyl viologen as an electron carrier, as reported by
Willner and co-workers (47). The use of POMs in this study
as electron carriers led to an apparent increase in DCB
*
Corresponding author phone: (803)777-2646; fax: (803)777-9521;
e-mail: ferry@psc.sc.edu.
2
degradation rates of a factor of 8 relative to TiO alone.
3
2 4 2
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 15, 2001
10.1021/es0106568 CCC: $20.00

2001 Am erican Chem ical Society
Published on Web 06/29/2001

SCHEME 1. POM-Mediated Electron Transfer from Conduction
Band of TiO to the Molecular Oxygen
mL EPA vials. The samples were centrifuged, and UV-vis
absorbance was recorded by a Perkin-Elmer 283 spectro-
photometer.
2
Adsorption isotherms were obtained by mixing 0.1 wt %
Degussa TiO
of POMs at pH 1. The resulting suspensions were shaken for
4 h, centrifuged, and filtered through Micropore filter (2.5
2
in 20 mL EPA vials with varying concentrations
2
mm). UV-vis absorbance spectra of the filtrate were recorded
and used to quantify the extent of adsorption.
GC-ECD and GC-ITMS Conditions. GC-ECD Operating
Conditions. GC-ECD analyses were done on a Hewlett-
Packard 5890 GC equipped with a 7672A autoinjector,
Chemstation integration package, and electron capture
Experimental Section
Materials. TiO (P-25; ca. 80% anatase, 20% rutile; average
2
particle diameter is 30 nm with surface area 50 m / g) was
2
detector. Carrier gas was He, and N was used as makeup.
The carrier gas flow rate was 1.3 mL/ min, and the method
detection limit for DCB was 0.018 µM. The injector port was
set for splitless operation at 250 °C. The autoinjector volume
was set at 1 µL. The analytical column was a 30 m DB-5 with
a 0.25-µm film thickness. The temperature program was as
follows: 1 min at 80 °C, increase at a rate of 3 °C/ min to 180
2
kindly supplied by Degussa Co. Methyl tert-butyl ether
2
(
MTBE, GC quality) was obtained from Burdrick and Jackson.
HNa
from Fluka. Na
Duncan et al. (48). 1,2-Dichlorobenzene (DCB, 99%), 2,3-
dichlorophenol (99+%), 3,4-dichlorophenol (99+%), HClO
99+%), and 4-bromoanisole (BA, 99%) were obtained from
2
PW12
O
40 and H
4
SiW12
O40 (reagent grade) were obtained
4
10
W O32 was prepared by the method of
°
C, isothermal at 180 °C for 1 min, increase at 20 °C / min to
4
2
50 °C, and isothermal for 7 min.
(
GC-ITMS Operating Conditions. GC-ITMS analyses were
Aldrich Chemical Co. ASTM grade water (18 M Ω cm
resistivity) was used for all experiments. Aqueous DCB
solutions were prepared from a stock DCB-saturated aqueous
solution. Analytical DCB stocks were prepared in GC grade
methanol. The internal standard for all quantitation was BA
performed on a Varian 3800 GC equipped with a Varian 8200
autoinjector and Varian Saturn 2000 ITMS. The injector port
was set for splitless operation at 250 °C. The autoinjector
volume was set at 1 µL. The analytical column was a 30 m
DB-5 with a 0.25-µm film thickness. Carrier gas (He) flow
rate was 1.3 mL/ min, and the method detection limit for
DCB in the sample matrix was 0.41 µM. The temperature
program was as follows: 1 min at 80 °C, increase at a rate
of 3 °C/ min to 180 °C, isothermal at 180 °C for 1 min, increase
at 20 °C / min to 250 °C, and isothermal for 7 min. This method
was applied to byproduct analysis also.
2
4
-
spiked into the extracting solvent. W10O32 was confirmed
by a comparison of the UV-vis absorption spectrum of
product to that reported in the literature (49, 50). The UV-
vis spectrum is provided in the Supporting Information.
Experimental Procedure
The reactor used in all experiments was an Ace Glass Schlenk
reactor. The light source was a 100 W Hg arc lamp (Osram)
filtered through a monochromator (Photon Technologies
Incorporated) to select for output at 350 nm. The slits were
set at 0.6 mm with a (2.4 nm band-pass. Lamp output (12.6
Result and Discussion
In this paper, we report a controlled exploration of the
catalyzed electron-transfer process from the conduction band
-
6
×
10 einstein/ min) was constant over the time frame of
the experiments, as determined by ferrioxalate actinometry
51).
Aqueous TiO
adjusted to pH 1 with HClO
of TiO
metalate anions, PW12
2
to dioxygen in water. We employed three polyoxo-
3
-
4-
4-
32
O
40 , SiW12
O
40 , and W10
O
as
(
electron-transfer promoters. These anions were chosen
because (a) their documented ability to undergo many
successive electron transfers without chemical degradation
(45, 52-55); (b) their small size and low absorbance at the
wavelengths encountered in this study, which guaranteed
that they would not affect light absorbing/ scattering of the
2
suspensions were prepared (0.1 wt %) and
following POM addition.
4
Experimental conditions were restricted to pHs < 3, because
of the hydrolytic instability of the POMs in this study (45).
The suspension was stirred for 15 min to ensure complete
mixing. Aqueous DCB was added to achieve the desired
substrate concentration. The reactor was allowed to equili-
brate in the dark with stirring for 30 min prior to the beginning
of illumination. The Hg arc lamp was always allowed to
stabilize at least 15 min before use. Illuminations were carried
out at 23 °C with constant stirring.
baseline TiO
which suggested they would tend to sorb to TiO
2
suspension; (c) their similar structure (56),
surfaces
2
with comparable binding constants; and (d) their widely
differing reduction potentials (41, 42), which allowed us to
probe their activity as a function of the ∆G for electron
transfer.
Samples (2 mL) were removed from the sample port with
an adjustable spring-loaded syringe (Manostat). Samples
were immediately placed in 20-mL EPA vials that had been
precharged with 5 mL of chilled extraction solvent (MTBE
with BA as an internal standard). To inhibit any further
possible reaction, the samples were buffered to pH 7.0 with
The photooxidation of 1,2-dichlorobenzene (DCB) was
used as the index reaction for evaluating the efficiency of the
various photocatalytic systems. A baseline data set was
2 4
established with a 0.1 wt % TiO suspension at pH 1 (HClO ),
air-saturated, no added POMs. Under these conditions, the
photocatalyzed oxidation of DCB proceeded rapidly (Figure
1), with the concurrent production of chlorophenolic byprod-
ucts in agreement with the observations of Ollis et al. (57)
DCB was not observed to photodegrade directly in the
absence of photocatalyst under our experimental conditions.
Likewise, DCB alone with any of the polyoxometalates did
not undergo any oxidation. Oxygen was required for rapid
DCB oxidation in all experiments, even in the presence of
2 3
the addition of 0.2 mL of 1.0 M Na CO and stored in the
dark. DCB was immediately extracted from the aqueous phase
by rapidly mixing samples on a vortex mixer for 30 s. The
organic layer was removed and stored at 4 °C until analysis
by GC-ITMS (gas chromatography with ion trap mass
spectrometer) or GC-ECD (gas chromatography with electron
capture detector).
For reduced POM measurements, the TiO
2
-POM system
o 2
POMs. Varying [DCB] over TiO revealed that DCB photo-
was degassed under high-purity nitrogen for 2 h prior to
illumination. Samples were taken by cannula (24 in., 14 gauge,
Aldrich Chemical Co) and placed in sealed and degassed 10
degradation could be modeled using the Langmuir-Hin-
shelwood approach (Figure 2) (57-59), with a correlation
-
1
-1
factor of 0.97, KL-H of 84.6 M , and kL-H of 0.0818 min
VOL. 35, NO. 15, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3 2 4 3

FIGURE 1. Index reaction of DCB degradation with 0.1 wt % TiO
2
;
[
, DCB; ×3,4-dichlorophenol; O, 2,3-dichlorophenol, all experiments
-
6
at 350 nm, 23 °C, pH ) 1, I ) 12.6 × 10 eins/min.
FIGURE 3. Pseudo-first-order degradation of DCB in the presence
3
-
4-
4-
O
32
of TiO
systems. ), TiO
TiO
(
)
2
, TiO
2
-PW12
2
O
40 , TiO
2
-SiW12
(wt 0.1%)-W10
40 (0.5 mM); 0, TiO (wt 0.1%)-PW12
) 45.0 µM, 23 °C, pH
O
40 , and TiO
2
-W10
4-
(wt 0.1%); *, TiO
2
O32 (0.5 mM); O,
4-
3-
O
40
2
(wt 0.1%)-SiW12
O
2
0.5 mM), all experiments at 350 nm, [DCB]
1, I ) 12.6 × 10 eins/min.
o
-
6
-
1
FIGURE 2. A plot of the experimental (rate constants) for DCB
-
1
2
degradation vs ([DCB]
o
)
with TiO (wt 0.1%) (R ) 0.97), all
2
-
6
experiments at 350 nm, 23 °C, pH ) 1, I ) 12.6 × 10 eins/min.
rate 0- ) (kL-H KL-H[DCB]0)-1 + k-1
1
L-H
where KL-H is the association constant and kL-H is the
concentration-independent rate constant. Langmuir-
Hinsheelwod type behavior has been regarded as an indica-
tion of a saturation kinetics (57, 58).
In the presence of oxygen, the addition of polyoxometalate
2
anions to TiO suspensions resulted in significant rate
enhancement for DCB oxidation (Figure 3). The magnitude
of the rate enhancement was “mapped out” over the range
of [POM] from 0.04 to 2 mM at a constant [DCB] of 40.0 µM.
o
Over this range, we observed that k for DCB degradation
experienced a different maximum value for each of the POMs
studied (Figure 4a-c). The maximum k for DCB degradation
3
-
-1
occurred at 0.1 mM PW12O40 (k ) 0.0318 min ), at 0.07
4
-
-1
4-
32
mM SiW12
O
40 (k ) 0.0108 min ), and at 1 mM W10
O
-
1
(
k ) 0.0066 min ). This can be contrasted against k (0.0041
-
1
min ) of TiO
2
2
-O at this DCB concentration. We also
observed that [POM] concentrations above 2 mM resulted
in significant decreases in k in our system. Speculatively, it
can be assumed that this is due to (a) competition for surface
sites between POMs and DCB and (b) rapid “short circuiting”
of the photocatalyst by direct reoxidation of reduced POM
by holes or some combination of the two.
The dependence of k on [POM] is in profile similar to
what is observed in other studies where metal-island
deposition was used as a strategy for enhancing oxidation
rates (25). In those studies, the decrease in rate was often
attributed to the occlusion of active sites on the photocatalyst
surface. In this study, adsorption isotherms for POMs on
FIGURE 4. (a) Pseudo-first-order degradation rate constants of DCB
3-
with TiO -PW12O40 , all experiments at 350 nm, 23 °C, pH ) 1,
2
-6
I ) 12.6 × 10 eins/min. (b) Pseudo-first-order degradation rate
4-
constants of DCB with TiO -SiW12O40 , all experiments at 350 nm,
2
TiO
that excess POM addition may result in similarly “capped”
surface sites. At pH 1, the TiO surface is positively charged,
2
were determined (Figure 5), to evaluate the possibility
-6
2
3 °C, pH ) 1, I ) 12.6 × 10 eins/min. (c) Pseudo-first-order
degradation rate constants of DCB with TiO -W O 4-, all experi-
2 10 32
-
6
2
ments at 350 nm, 23 °C, pH ) 1, I ) 12.6 × 10 eins/min.
while all the POMs in this study are negatively charged, which
suggests that ion-exchange may facilitate surface-POM
binding. In these experiments, the association constants (K)
centrations of electron scavengers are more effective than
3
-
4-
4-
higher even at relatively high oxygen concentration (2 × 10-3
for PW12
4
O
40 , SiW12
O
40 , and W10O32 were determined as
-
1
-1
-1
-3
67 M , 459 M , and 417 M , respectively (Table 1) (60).
mol dm ) (61). This has clear importance for the eventual
These association constants are high compared to DCB’s K
of 84.6 on TiO , and the difference supports the hypothesis
that the inhibitory effect of high [POM] is an artifact of
competition for surface sites. It also explains the somewhat
counterintuitive observation that in this case lower con-
application of POMs for enhancing photocatalytic systems.
For instance, the maximum k for DCB in the presence of
2
3
-
PW12O40 is obtained at a POM concentration of 0.1 mM.
3
-
-1
Given that K for PW12
O40 is 467 M , this corresponds to
only 30% surface coverage.
3
2 4 4
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 15, 2001

3-
FIGURE 5. Adsorption isotherms of TiO
2
-PW12
2
O40 (0), TiO -
4-
4-
SiW12O40 (O), and TiO -W10O32 (*) system.
2
TABLE 1. Reduction Potentials, Calculated Free Energy Change
for the Reduction of POMs by Conduction Band Electrons of
2 2
TiO , Binding Constants of POM on TiO Surface (Derived from
Adsorption Isotherms), and Concentration Independent
Degradation Rate Constants of DCB (Derived from
Langmuir-Hinshelwood Plots)
E
∆G
K (M-1)
kL-H
(kJ/mol) (TiO2-POM) (min 1)
-
(
V vs NHE)
O2
-0.125 (64)
13.1
81
84.6
417
459
467
0.0818
0.152
0.421
0.638
Na4W10O32
H4SiW12O40
H2NaPW12O40
-1.0288 (41)
0.0542 (41) -23.5
0.2182 (42) -39.3
It should also be noted that the binding constants of
contrasted POMs on TiO are similar. This finding excludes
2
the possibility that the observed difference on the photo-
degradation rate of DCB with three contrasting POMs over
-
1
TiO
2
is not simply caused by the adsorption.
FIGURE 6. (a) A plot of the (observed degradation rate) of DCB
-
o 2
) with TiO -PW12O40 (0.1 mM) (R )0.99), all experiments
6
1
3-
2
DCB oxidation continued to follow Langmuir-Hinshel-
wood kinetics even in the presence of POMs (Figure 6a-c).
It is notable that kL-H for DCB is found to be much higher
vs ([DCB]
at 350 nm, 23 °C, pH ) 1, I ) 12.6 × 10 eins/min. (b) A plot of the
-
-
1
-1
o
(observed degradation rate) of DCB vs ([DCB]
) with TiO
2
-
4-
2
SiW12
O40 (0.1 mM) (R ) 0.97), all experiments at 350 nm, 23 °C,
in these experiments than in TiO
indicate a fundamental change in the TiO
2
-O
2
(Table 1). This could
surface with POM
-
6
pH ) 1, I ) 12.6 × 10 eins/min. (c) A plot of the (observed
2
-
1
-1
o
degradation rate) of DCB vs ([DCB]
)
2
with TiO -Na
4 10
W O32 (1.0
adsorption, which would be consistent with Serpone and
co-worker’s finding that anionic surfactants increase the
oxidation rate of rhodamine B (62). In that work, it was
proposed that surfactant adsorption facilitated rhodamine
B’s oxidation by neutralizing surface charges. It is also possible
that DCB adsorption is exactly the same in the presence of
POMs but that the more efficient scavenging of conduction
band electrons by POMs results in a faster turnover of
occupied sites. The concomitant reduction of polyoxometa-
lates was qualitatively supported by the formation of a blue
color in the suspensions during irradiation in the absence
of oxygen (as verified by UV-vis absorption spectra, see
Supporting Information).
2
mM) (R ) 0.98), all experiments at 350 nm, 23 °C, pH ) 1, I ) 12.6
-
6
×
10 eins/min.
is a more useful predictor of the activity of added electron
scavengers (59, 67). Using the one-electron E° for dioxygen
64) and the POMs in this study, we determined the free
energy change for the reduction of these species by TiO
according to the following
(
2
o
∆
G ) -F(E (acceptor) - E°(TiO ))
(1)
2
o
where F is Faraday’s constant, E (acceptor) is the standard
o
2
reduction potential of the acceptor vs NHE, and E (TiO ) is
The TiO -POM association constants are also useful for
2
the standard reduction potential of the conduction band
electrons. The reduction potential for conduction band
electrons was determined to have a value of -0.189 V vs
NHE under our experimental conditions (pH 1) based on
evaluating the role of adsorption as the dominant variable
in predicting the effectiveness of POMs as electron scaven-
gers. A comparison of the measured values shows that the
3
-
4-
4-
32
POMs can be ranked as PW12
O
40 = SiW12
O
40 > W10
O
,
which does not agree with the significant differences we
observed in ranking the effectiveness of each POM by
measuring their effect on kL-H (Table 1).
E°(TiO2) ) -0.13 - 0.059 (pH)
(2)
Under our experimental conditions (pH 1), superoxide
produced from the one electron reduction of molecular
The resulting ∆G values (Table 1) show that the reduction
4
-
of W10
O
32 by TiO
2
is actually more endothermic than the
.
-
+
.
oxygen is readily protonated (O
2
+ H f HOO pK
a
) 4.88)
reduction of O
2
4-
under our experimental conditions, even
is a more effective electron scavenger as
.
(
63, 64). It is possible that HOO radicals produced from the
though W10O
32
reoxidation of reduced POMs could interfere with our
observable directly reacting with 1,2-dichlorobenzene. How-
ever, the published bimolecular rate constants for reactions
indexed by kL-H. The implication is that even POMs that are
not spontaneously reduced may still serve as effective electron
transfer promoters, opening the field to the consideration of
materials that may be more resistant to hydrolytic decom-
position at more environmentally relevant pHs. The reduction
of other polyoxometalates in this study is more favored than
.
-
.
between O
the order of 10 -10 M
when compared to the rate of HOO dismutation.
Previous investigations of electron transfer from TiO
2
/ HOO and aromatic compounds are only on
3
5
-1 -1
s
(65, 66), which is insignificant
.
2
have
the reduction of O
2
and is apparently kinetically favored also,
3
-
shown that ∆G for the one-electron reduction of the acceptor
with PW12O40 the most effective electron scavenger. The
VOL. 35, NO. 15, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3 2 4 5

coexistence of O
2
and POMs in the TiO
2
solution induces a
(30) Vinodgopal, K.; Kamat, P. Environ. Sci. Technol. 1995, 29, 841.
(
31) Kalyanasundaram, K.; Gratzel, M. Coord. Chem. Rev. 1998, 77,
47.
competition between these two for the photogenerated
conduction band electrons. The increased photodegradation
rate of DCB in the presence of relatively POMs indicates that
3
(
32) Nazeeruddin, K.; Kay, A.; Rodicio, I.; Baker, R.; Muller, E.; Liska,
P.; Vlachopoulos, N.; Gratzel, M. J. Am. Chem. Soc. 1993, 115,
2
electron transfer to POMs is favored over that to O .
6
382.
(
(
(
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Acknowledgments
We would like to express our gratitude to Dr. Dean Duncan
of Los Alamos National Laboratory for helpful discussions
and to the South Carolina Commission for Higher Education
for support of this work.
(
(
36) Butler, E. C.; Davis, A. P. J. Photochem. Photobiol. A: Chem.
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Supporting Information Available
(
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Chem. 1991, 56, 113.
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UV-vis absorbance of synthesized Na
4
W
10
O
32 and filtered
3
-
TiO -PW12O32 (0.1 mM) in deoxygenated illuminated aque-
2
(
ous solution. This material is available free of charge via the
Internet at http:/ / pubs.acs.org.
(
(
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Received for review February 16, 2001. Revised manuscript
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