Catalytic photooxidation of pentachlorophenol using semiconductor nanoclusters

Article abstract of DOI:10.1021/jp0012653

Pentachlorophenol (PCP) is a toxic chlorinated aromatic molecule widely used as a fungicide, a bactericide, and a wood preservation, and thus is ubiquitous in the environment. We report photooxidation of PCP using a variety of nanosize semiconductor metal oxides and sulfides in both aqueous and polar organic solvents and compare the photooxidation kinetics of these nanoclusters to widely studied bulk powders such as Degussa P25 TiO₂ and CdS. We study both the light- intensity dependence of PCP photooxidation for nanosize SnO₂ and the size dependence of PCP photooxidation for both nanosize SnO₂ and MoS₂. We find an extremely strong size dependence for the latter which we attribute to its size-dependent band gap and the associated change in redox potentials due to quantum confinement of the hole-electron pair. We show that nanosize MoS₂ with a diameter of d = 3.0 nm and an absorbance edge of approximately 450 nm is a very effective photooxidation catalyst for complete PCP mineralization, even when using only visible-light irradiation.

Full text of DOI:10.1021/jp0012653

7334
J. Phys. Chem. B 2000, 104, 7334-7343
Catalytic Photooxidation of Pentachlorophenol Using Semiconductor Nanoclusters
J. P. Wilcoxon†
Nanostructures and AdVanced Materials Chemistry, Department 1152, Sandia National Laboratories,
Albuquerque, New Mexico 87185-1421
ReceiVed: April 3, 2000; In Final Form: June 5, 2000
Pentachlorophenol (PCP) is a toxic chlorinated aromatic molecule widely used as a fungicide, a bactericide,
and a wood preservation, and thus is ubiquitous in the environment. We report photooxidation of PCP using
a variety of nanosize semiconductor metal oxides and sulfides in both aqueous and polar organic solvents
and compare the photooxidation kinetics of these nanoclusters to widely studied bulk powders such as Degussa
P25 TiO
2
and CdS. We study both the light- intensity dependence of PCP photooxidation for nanosize SnO
and MoS . We find an extremely
2
and the size dependence of PCP photooxidation for both nanosize SnO
2
2
strong size dependence for the latter which we attribute to its size-dependent band gap and the associated
change in redox potentials due to quantum confinement of the hole-electron pair. We show that nanosize
MoS
2
with a diameter of d ) 3.0 nm and an absorbance edge of ∼450 nm is a very effective photooxidation
catalyst for complete PCP mineralization, even when using only visible-light irradiation.
I. Introduction
It is clear that more effective methods of treatment of these
chlorinated aromatics must be sought. To this end, a few groups
have been investigating photocatalytic oxidation of these
compounds to form harmless CO2 and HCl, a process referred
Contamination of sediments and aqueous water systems by
halogenated organic compounds presents a serious environmen-
tal threat due to their toxicity and resistance to biodegradation.
These chemicals are widely employed as pesticides, insecticides,
and wood preservatives and thus are ubiquitous in the environ-
ment of both industrialized and agrarian nations. Even chemicals
which have been banned for years, such as DDT and its
analogues, still pose major environmental threats and, in fact,
constitute the basis of several EPA superfund sites. A subgroup
of these chemicals referred to as chlorinated aromatics includes
chlorinated benzenes and biphenyls (PCBs), pentachlorophenol
2
-4
to as total mineralization.
The semiconductor catalyst of
choice in these studies has been TiO2, a white, photostable,
nontoxic powder, whose principal deficiency is an absorbance
edge which starts at about 385 nm, allowing less than 3%
utilization of the solar spectrum.
It would be a major boon to have a visible-light-absorbing
semiconductor catalytic material available, which is also pho-
tostable and nontoxic. Such a photocatalyst would make it
possible to exploit sunlight as the sole energy source required
for detoxification. To this end we have employed our expertise
in nanocluster synthesis and processing to make and purify
nanoparticles of MoS2, whose band-gap and absorbance edges
can be adjusted by particle size on the basis of the quantum
confinement of the hole-electron pair. In a recent paper we
demonstrated the use of these new photocatalysts to destroy
phenol, and demonstrated a strong effect of size or band gap
(PCP), and insecticides such as DDT.
PCP, the topic of this paper, is widely found in the environ-
ment, and it belongs to a family of chlorinated phenols that are
among the most toxic chemicals known to man. In general, the
more chlorines on the aromatic phenol ring the greater the
1
biohazard. It has been postulated that the widespread detection
of PCP and its analogues in the environment may result from
combustion, water treatment with chlorine in the presence of
organic materials, and municipal sewage treatment plants and
incinerators. Once discharged into the environment, these rather
water- insoluble compounds, (10-20 ppm in water, typically),
seep into the sediment of rivers, lakes, and other bodies of water
and continually leach out into the water supply, eventually
affecting the entire mammalian food chain.
5
on the rate of photooxidation.
The question of the effect of catalyst size on the photocatalytic
activity has not been investigated very much for nanosize
materials, particularly those whose size is small enough to affect
their electronic energy levels via quantum confinement. Farin
5
and co-workers investigated the photoactivity of colloidal TiO2
Microbial degradation and naturally occurring hydrolysis of
these compounds is a very slow process (e.g., for 4-choloro-
phenol at 9 °C the half-life is nearly 500 days). Some direct
for propene photohydrogenation and found that the rate in-
creased ∼7-fold upon decreasing the colloid size from 16 to 8
nm. Since a 4-fold increase in surface area occurs over this size
regime, their observed rate increase is mostly accounted for by
this increase in surface area and is probably not due to changes
in either surface morphology or electronic levels. (The Bohr
diameter for which quantum confinement effects are expected
to be important is significantly smaller than 8 nm for TiO2).
2
photodegradation also occurs, although the limited optical
absorbance of chlorinated aromatics above 350 nm in wave-
length makes this process painfully slow. Sometimes this
direct photolysis can actually lead to more toxic products (e.g.,
direct photolysis of PCP has been reported to lead to octachlo-
rodibenzo-p-dioxin, an even more toxic species than its pre-
6
More recently, Korgel and Monbouquette investigated photo-
3
cursor).
catalyzed reduction of nitate ions in deaerated water using
dispersed 2 nm CdS nanocrystals stabilized with organic ligands.
They found a very strong size dependencesa 10% decrease of
†
E-mail: jpwilco@sandia.gov.
1
0.1021/jp0012653 CCC: $19.00 © 2000 American Chemical Society
Published on Web 07/19/2000

Catalytic Photooxidation of Pentachlorophenol
J. Phys. Chem. B, Vol. 104, No. 31, 2000 7335
TABLE 1: Synthetic Details for Catalysts Used in Photooxidation Studiesa
surfactant/
solvent
precursor/
concentration
sulfiding or
hydrolysis agent
sample name
rate of mixing
0.1 mL/min
pH (final)
2
2
SnO (d ) 25 nm)
2-propanol
Sn(isopropoxide)/
HCl@pH ) 1.5
5
0 mg/mL
Sn(isopropoxide)/
5 mg/mL
Ti(isopropoxide)/
0 mg/mL
Ti(isopropoxide)/
5 mg/mL
SnO
2
(d ) 58 nm)
2-propanol
2-propanol
2-propanol
HCl@pH ) 1.5
HCl@pH ) 1.5
HCl@pH ) 1.5
0.01 mL/min
0.1 mL/min
0.01 mL/min
NA
2
2
TiO
2
#28(d ) 20 nm)
#10(d ) 55 nm)
2
5
TiO
2
2
2
MoS
MoS
MoS
2
(d ) 3.0)
(d ) 4.5)
(d ) 8-10)
DTAC(8%)/
c6OH(11%)/c8
DTAB(8%)/
c6OH(11%)/c8
DTAB((1%)/
c6OH(1.5%)/c8
MoCl
MoCl
MoCl
4
4
4
/0.002 M
/0.002 M
/0.002 M
H
H
H
2
2
2
S(0.01 M)
S(0.01 M)
S(0.01 M)
NA
NA
NA
2
2
NA
NA
a
DTAC ) dodecyltrimethylammonium chloride (Fluka Chemicals, 99%); DTAB ) dodecyltrimethylammonium bromide (Fluka Chemicals,
9%).
9
the nanocrystal size resulting in a 5-fold increase in the rate of
reduction! However, a sacrificial electron donor, formate, had
to be utilized to prevent photocorrosion of the nanoparticles
and to allow the reduction to occur.
described extensively by others in the literature.11,12 We modified
this general, acid-catalyzed metal alkoxide hydrolysis procedure
by employing an aqueous solution of HCl at pH ∼ 1.5 which
was slowly injected using a programmable syringe pump (KD
Scientific Co., model 100 pump) into a rapidly stirred titanium
or tin isopropoxide (Aldrich Chemical) in 2-propanol solution.
It was discovered that both the pH and rate of addition of acid
catalyst could be used to control the final size. In general we
found that pH ∼ 2 or less was required to ensure long term
(i.e., weeks to months) colloidal stability against aggregation.
The final (post-mix) ratio of water to 2-propanol volumes was
10:1 in each case. In general, using higher pH values and slower
mixing of the two solutions gave rise to larger nanoclusters as
determined after 24 h by dynamic light scattering (DLS). We
were able to vary the size as measured by DLS from about 10
nm to 150-200 nm, but we were only interested in the 10-30
nm size regime for photocatalysis purposes. Table 1 gives further
details of the nanocluster synthesis, nanocluster size, and
physical properties.
In this paper we investigate the photooxidation kinetics and
products formed for a standard material, Degussa P25 TiO2, as
compared to nanosize TiO2, SnO2, and MoS2. We examine the
light-intensity dependence for nanosize, fully dispersed SnO2
compared to a slurry of TiO2 (Degussa) at the same concentra-
tion. We show that because of the intense multiple scattering
observed in slurries compared to dispersion of nanocrystals it
is necessary to study both systems in a light-intensity regime
where the reaction kinetics are both first-order in PCP.
We study photooxidation by nanosize SnO2 in aqueous
systems and, for the first time, a system consisting almost
entirely of a polar organic acetonitrile, and show that total PCP
mineralization occurs even with severely limited oxygen and
water concentrations. Furthermore, we show that while simple
salts such as NaCl poison the activity of P25 TiO2, certain
cationic surfactants can actually enhance its activity.
Degussa P25 TiO was purchased from Degussa Chemical
2
To understand the effect of catalyst size on PCP photooxi-
dation kinetics we investigated various sizes of nanosize SnO2
and MoS2. However, in the former case the two sizes studied
were larger than the size at which quantum confinement effects
would be expected to affect the electronic energy levels of the
material. We found a negligible size dependence for nanosize
SnO2, a surprising result in view of the nearly 4-fold difference
in the surface areas of the two samples investigated. For nanosize
MoS2, strong quantum confinement effects are present which
lead to a shift of as much as 2 eV in the band gap compared to
bulk MoS2. This large energy shift is invoked to explain the
enormous size-dependence of the observed PCP photooxidation
kinetics using this catalyst. We find that d ) 3.0 nm MoS2 which
has an absorbance edge near 450 nm can mineralize PCP at a
rate exceeding even that of P25 TiO2, even though only visible
irradiation was employed in our MoS2 studies while full Xe
lamp illumination was used in the our studies of P25 TiO2.
Company and used as received by making a stock solution of
10 mg/mL TiO in milli-Q (Millicore Corp.) water. This stock
2
solution was diluted as appropriate to achieve the desired catalyst
concentration. No buffers or salts were added unless otherwise
noted to any of our powder slurries or nanocluster solutions.
CdS powder, 99.9% pure, ALFA Chemicals, was similarly used
as received and dissolved in milli-Q water to make a standard
10 mg/mL solution which was then diluted for use in our
photoreactor. Other powders investigated were also metals grade,
purchased from ALFA and then made into stock slurries at 10
mg/mL. These stock solutions were used to prepared either 1
mg/mL or 0.1 mg/mL slurries in milli-Q water.
Purification/Processing of the Nanocatalysts. The as-
synthesized MoS2 nanoclusters were purified by extraction from
the nonpolar solution in which they were prepared, octane, into
a water-miscible but octane-immiscible solvent, acetonitrile
(ACN). The MoS2 clusters in ACN were dried to a very small
volume (e.g., ∼0.5 mL) by centrifugal evaporation (∼40×
reduction in volume) using a Centrivap (Labconco Corp.) with
a cold trap, and then these reduced volume samples were added
to water to form the catalyst solution.
Removal of ions and excess surfactants and 2-propanol from
the nanocluster solutions of MoS2, TiO2, and SnO2 using the
as-prepared solutions was achieved by dialysis using a 500 M.W.
cutoff Spectra-Por (Spectrum Medical Industries, Inc.) dialysis
membrane and dialysis against 1 L of milli-Q water with one
II. Materials Synthesis and Experimental Procedures
Synthesis. In a previous paper describing the photooxidation
of phenol, we gave details concerning the synthesis of nanosize
7
MoS2. The electronic and optical properties of this remarkable,
structurally anisotropic, indirect band-gap semiconductor are
discussed elsewhere.8 Nanosize TiO2 and SnO2 were prepared
by the controlled hydrolysis of the corresponding metal iso-
propoxide in 2-propanol, and this general procedure has been
-10

7336 J. Phys. Chem. B, Vol. 104, No. 31, 2000
Wilcoxon
2
Figure 1. TEM of a field of d ) 3.0 MoS nanoclusters.
change over a 12-24 h period. Some aggregation as determined
by DLS of the MoS2 and TiO2 nanoclusters occurred under these
conditions, but the catalytic activity improved substantially. The
nanocluster solutions were still visually transparent, temporally
stable, and completely dispersed without agitation. All nano-
cluster catalyst solutions exhibited negligible light scattering
in contrast to the control suspension of Degussa TiO2 at a
concentration of 0.1 mg/mL, which was milky-white and cloudy,
and which exhibited intense multiple scattering. We note that
the concentration of TiO2 used in the present studies was about
1
6-20 times less than that used in most previous work, to try
to minimize multiple scattering effects on the control catalyst.
Even so, the intense multiple scattering characteristic of slurries
of TiO2 or CdS makes direct comparison of the quantum effi-
ciency (Q.E.) of Degussa TiO2 to the corresponding nanocluster
solutions very difficult. Viable realistic approaches to address
1
1
this issue have been discussed extensively in the literature.
Figure 2. XRD from a d ) 4.5 dried, nanocrystalline powder (dotted
Catalyst Characterization. TEM and HRTEM were used
to determine the nanocrystallinity of the materials, average
cluster size, and polydispersity. The results were corroborated
by use of DLS to measure the hydrodynamic diameter of the
clusters in solution at the pH used for the photooxidation
experiments. Except for nanosize TiO2 this pH was ∼4.0 (the
typical pH of our deionized milli-Q water) and the solution was
not buffered, nor purged with any gases. It was necessary to
keep the pH of the nanosize TiO2 solutions at pH ∼ 2.0 to
prevent aggregation. In the case of MoS2, the clusters were so
small that TEM gives a better measure of cluster size than DLS,
but in the other cases the sizes reported were obtained from
DLS which gives a better ensemble averaged size than TEM.
Figure 1 shows a field of MoS2 nanoclusters with an average
size of d ) 3 nm.
2
line) compared to a bulk powder of MoS (solid line).
X-ray fluorescence (XRF) spectroscopy (Spectrace QuantX
system) was used to determine the inorganic composition of
the purified, processed nanocluster solutions as well as to obtain
the absolute metal concentrations indicated in later figures.
Using XRF analysis, it was also possible to determine ap-
proximately the metal-to-sulfur ratio despite the notoriously low
(∼40 ppm) XRF sensitivity to low Z elements such as S, and
to compare this to bulk powders of MoS . An example is shown
2
in Figure 3, where bulk MoS powder is compared to nano-
2
cluster solutions of purified MoS . In this figure we have
2
indicated lines which arise from the cell windows and from the
Ar present in the air environment. It was found that the ratio of
Mo:S in our nanoclusters was ∼1:2.5 to 1:3, showing some
excess sulfur on the nanocluster surface which could not be
removed by our purification procedures. Later work using anion
chromatography showed that this sulfur was in the form of
SAD has been used to determine that, for the case of nano-
clusters of MoS2, clusters large enough to give good electron
diffraction (i.e., d > 4.5 nm), a crystal structure identical to
bulk MoS2 was observed. Figure 2 shows XRD from a d )
.5 nm MoS2 solution dried to form a powder compared to data
8
-2
-2
adsorbed S not SO4 . This is fortunate since the latter
oxidized form acts as the catalyst poison.
4
from a commercial MoS2 (99.7%, Alfa) powder obtained on
the same instrument. Except for the broadening due to the small
domain size, the structures are identical. Further details concern-
ing the optical properties and physical characterization of MoS2
are given elsewhere.8
Catalyst Optical Properties. The optical absorbance proper-
ties of our nanocluster catalysts were determined using a Cary
2300 UV-visible-NIR spectrometer at the same concentrations
used in the photooxidation studies. Slurries of Degussa TiO2 in
water were shaken and the spectrum rapidly obtained before

Catalytic Photooxidation of Pentachlorophenol
J. Phys. Chem. B, Vol. 104, No. 31, 2000 7337
Figure 5. UV-visible absorbance spectra of PCP in three common
HPLC solvents, water, methanol (MeOH), and acetonitrile (ACN).
Figure 3. XRF lines from high purity bulk powder are co-plotted with
purified nanosize MoS . Both Mo ΚR and S ΚR lines are shown.
2
solvent gradient from 70:30 ACN:H2O to 100% ACN over 20
min time to give good elution peak shape of the nanoclusters
themselves. We find that the PCP absorbance under the mixed
organic:H2O conditions most closely resembles the H2O absor-
bance shown below so we chose to obtain PCP concentrations
on the basis of the absorbance at 325, 250, and 215 nm.
Photooxidation Reactor. Our photooxidation reactor consists
of a custom built (Ace Glassware Co.) cylindrical reactor with
a flat glass base and an O-ring sealed quartz, Ace threaded
window holder, with an aperture larger (∼3 cm) than our
collimated Xe lamp output beam, (∼1.5 cm). The reactor has a
total volume of about 60 mL, and we use 40 mL of liquid in all
our reactions. The reactor has an O-ring sealed, Ace threaded,
4
5° sidearm for liquid- or gas-phase sampling through a septa,
if desired, but in all the studies to be described we simply
removed the O-ring plug and sampled 0.6 mL of the sample at
various irradiation times for analysis. This aliquot was filtered
using a Hewlett-Packard HPLC filter (0.45 micron, cellulose)
to remove suspended catalysts into a standard 2 mL crimp-top
HP HPLC vial for either HPLC or GC/MS analysis. We tested
to make sure no PCP was being adsorbed onto the filter. This
filter allowed the nanosize catalysts to pass through and so we
could simultaneously do HPLC analysis of the MoS2 nanocluster
catalysts.
Figure 4. UV-visible absorbance spectra of nanocatalysts and control
bulk powder (Degussa P25, TiO ) are shown. All concentrations were
.1 mg/mL.
2
0
powder settling occurred. The results of these absorbance
measurements for nanosize TiO2, SnO2, MoS2, and the Degussa
TiO2 slurries are shown in Figure 4. The strong multiple
scattering of semi-opaque TiO2 slurries even at 0.1 mg/mL in
the 0.2 cm path length cell obscures the true band-edge
absorbance of this material, which is clearly exhibited by the
optically clear 20 nm diameter TiO2 nanocluster solution. As
can be observed in this figure, the TiO2 slurry multiple scattering
effectively enhances the absorbance probability of a photon in
the relevant range of light output from our Xe arc lamp (300-
We used a commercial 400 W Xe arc lamp from Oriel. The
output of this lamp is very close to that of the solar spectrum
when combined with the 700 nm short-pass filter used. Overhead
illumination of the cylindrical reactor is very advantageous since
the flat-bottom geometry of our cell allows rapid magnetic
stirring of the catalyst solutionsa vital aspect affecting the
photooxidation rate of the powder slurries used as control
catalysts. This stirring is unimportant for the nanosize catalysts,
but for consistency we maintained the same rate for these
catalysts too. To study only visible-light photooxidation a 400
nm long-pass filter was also used to limit the incident irradiation
wavelength, λ to 400 nm < λ < 700 nm. The lamp light output
is monitored continuously by a Newport research (Newport
Corp., model 835) power meter with an IEEE-488 interface to
a MacIntosh IIci computer, to track total irradiation time and
any power variations. The incident power was measured using
4
00 nm) which corresponds to the actual TiO2 absorbance (see
the TiO2 (d ) 20 nm) nanocluster absorbance curve). Light
scattering is negligible for SnO2, MoS2, and TiO2 nanocluster
solutions, and the absorbance curves of Figure 4 represent pure
absorbance for these solutions.
PCP Optical Properties. To determine the optimum HPLC
detector wavelengths to monitor the PCP concentration as a
function of irradiation time, we first obtained the PCP absor-
bance spectrum at 10 ppm in water, methanol, and acetonitrile
using our Cary 2300 spectrophotometer. These spectra are
shown in Figure 5. Note that the PCP absorbance is solvent
dependent. This fact is important because the chromatography
was run in an isocratic 70:30 MeOH:H2O ratio to optimize
separation and peak shape, while still having an acceptably short
run time for the metal oxide nanocatalysts. In the case of the
MoS2 nanoclusters the chromatography was performed in a
2
the 1 cm size calibrated photodiode probe from Newport
Research Corp. Neutral density filters on suprasil quartz
substrates (Oriel Corp.) were used to attenuate the incident light
by known amounts.
HPLC, GC/MS, and Cl Ion Analysis of PCP Photooxi-
dation Kinetics. To study the rate of photooxidation of PCP

7338 J. Phys. Chem. B, Vol. 104, No. 31, 2000
Wilcoxon
we used a HP 1050 high-pressure liquid chromatography
col, a logical intermediate to be formed by OH radical attack
on a Cl on the PCP ring. However, our inability to identify
most intermediates was not a major problem as all these
compounds were eventually completely mineralized themselves.
On the basis of the rate of appearance of these minor peaks
compared to the rate of disappearance of the PCP peak, it was
clear that these photooxidized compounds were not true
intermediates on the pathway to complete mineralization, but
rather, minor alternative photooxidation paths.
(
HPLC) system equipped with a photodiode array (PDA), a
refractive index (RI), and a fluorescence (FL) detector(s).
Unfortunately, the latter detector, though the ultimate in
sensitivity for non-chlorinated aromatics like phenol, is nearly
useless for PCP detection due to the quenching of the fluores-
cence by the chlorines on the aromatic ring of PCP. Neverthe-
less, we excited at 250 nm and detected fluorescence at 320
nm using this detector to detect any non-chlorinated aromatic
byproducts at the ppb level that might be formed during the
photooxidation process. The entire spectrum from 200 nm (the
solvent cutoff) to 600 nm is available at each time point in the
elution peak chromatogram, but for quantitation purposes we
chose to monitor at the three broad main features in the PCP
spectrum shown in Figure 5, 325, 250, and 215 nm. The latter
wavelength gives the most sensitivity, while the former two can
be used to avoid any detector saturation effects at high PCP
concentrations and to check for consistency. Stock solutions of
At the conclusion of several of the photooxidation reactions
that were run long enough to achieve >99% disappearance of
-
the PCP elution peak we used a Cl selective electrode to
determine [Cl]. We were able to verify that the expected amount
of free chloride was generated as calculated from the initial
concentration of 10 ppm PCP, so complete detoxification was
accomplished with the best nanosize photocatalysts using only
visible illumination as well as with nanosize SnO2 and Degussa
TiO2 using full (UV + visible) lamp illumination. Because of
the small reactor volume and large Cl electrode size, it was not
possible to follow the [Cl] in situ.
10, 1, and 0.1 ppm PCP in milli-Q water were prepared, and a
linear detector response over this concentration range was
verified. A minimum sensitivity of about 0.02 ppm was
determined for absorbance at 215 nm. This is significantly better
sensitivity than we could achieve using our HP 5890/5972 GC/
MS even using selective ion monitoring.
III. Experimental Results and Discussion
Direct Photolysis. It is interesting to compare the results of
direct photolysis of PCP with that observed with Degussa TiO2.
In Figure 6a we show the absorbance at 215 nm, A(215 nm) vs
elution time, t for different irradiation times using unfiltered
Xe lamp illumination of PCP at 10 ppm in water. The products
observed at the elution peaks of t ) 6.0 min and t ) 3.27 min
were not observed in the catalytic photooxidation experiments.
We found that measurements of the [Cl] using a Cl selective
electrode at t ) 480 min were consistent with nearly complete
mineralization of the PCP under direct photolysis. This contrasts
with some claims that only toxic intermediates such as oc-
tachlorodibenzo-p-dioxin are formed under direct photolysis.14
The UV-visible spectrum at t ) 6.0 is not consistent with such
a product formation.
Obtaining acceptable peak elution symmetry and retention
time stability from a highly chlorinated aromatic like PCP is a
very challenging chromatography problem. We found that using
an HP analytical ODS200 column (reverse phase c18 terminated
silica, 200 mm (length) by 4.6 mm (diameter) with 5 mm
particle packing and 120 Å pores), and running a 60:40 or 70:
30 methanol (MeOH):H2O mobile-phase mixture gave the best
elution profiles, with good retention time stability over many
runs. Various composition gradients of this mixture to 100%
MeOH were also used to improve the separation if multiple
organics were present. Using this solvent mixture it was also
possible to detect the nanoclusters of MoS2, but we found that
acetonitrile (ACN) gave more reproducibility for the nanocluster
peak shape and elution time so a 60:40 ACN:H2O to 100% ACN
was used in the MoS2 nanocatalyst studies. This condition
separates the nanoclusters, any surfactant, and PCP completely.
We tried to use an independent column of the same type for
the studies involving nanoclusters since some adsorption of the
nanoclusters onto the column over many injections was found
to alter the chromatographic elution behavior (i.e., peak shape,
retention time) of the PCP, though not its quantitation (i.e., peak
area). In all of our studies we inject 50 µL of the 600 µL sample
taken at a given time point in the irradiation, and analyze the
area under the elution peak at the three monitoring wavelengths,
taking the average as the concentration of PCP at that point in
time.
15
Pelizzetti et al. studied the direct photolysis of PCP starting
from an initial concentration of 12.5 ppm at pH ) 3.0, and
using a 1500 W Xe arc lamp with filters providing irradiation
from 310 to 800 (conditions very similar to the present work
except for the 3-fold increase in lamp intensity) and surprisingly
observed a much slower rate of direct photolysis than we show
in Figure 6a, even though their light intensity was more than
3-fold lamp greater than in the present work. This may be due
to differences in reactor geometry.
Catalytic Photooxidation. The generally accepted description
of the complete mineralization of PCP in water is
2HOC Cl (PCP) + 7O ) 4HCO H + 8CO + 10HCl (1)
6
5
2
2
2
In previous work,16 other intermediates in the complete reaction
Generally, because of the low concentrations and limited
solubility of PCP in water (∼20 ppm), it is not possible to use
gas chromatography/mass spectrocopy (GC/MS) to directly
identify unknown photooxidation byproducts observed in HPLC
using ion fragmentation patterns in MS, so we purchased several
putative photooxidation intermediate candidates such as tetra-
chlorocatechol from Aldrich Chemicals and ran HPLC on these
chemicals under identical chromatography conditions as used
in the PCP photooxidation experiments. This allowed us to
identify any possible intermediates via retention time alone. We
also had the luxury of the complete absorbance spectrum of
each elution peak to identify whether a compound observed was
aromatic or not. Nevertheless, we were not able to identify all
intermediates positively and we never observed tetrachlorocate-
1 included p ) cholinil (C Cl (dO) ) or its reduced form.
6
4
2
However, detection of this compound is difficult due to its low
optical extinction coefficient in water. Also, this compound is
reported to be unstable when stored in water and must be
extracted into ether to allow detection by HPLC, so we did not
expect to observe this compound. We also did not monitor the
CO or formic acid concentrations because of the limited reactor
2
volume we employed. However, our [Cl] measurement at
completion (i.e., >99.9% PCP removal) t ∼ 480 min was
consistent with the stoichiometry described in eq 1.
When Degussa TiO2 is added to the water at a concentration
of 0.1 mg/mL the absorbance at 215 nm (A(215 nm) chromato-
grams) shows different reaction byproducts than observed in

Catalytic Photooxidation of Pentachlorophenol
J. Phys. Chem. B, Vol. 104, No. 31, 2000 7339
Figure 7. Normalized PCP concentration vs irradiation time using a
4
00 W Xe arc lamp with 300 nm < λ < 700 nm irradiation. Initial
PCP concentration was 10 ppm in all cases, and the incident light
intensity was 250 mW/cm .
2
other studies is difficult. However, our conditions are most
1
6
similar to those employed by Mills et al. in which a 450 W
Xe arc lamp with IR cutoff filter was used and an initial
concentration of 1.2 ppm was employed with 0.2 mg/mL of
Degussa TiO2 catalyst. They reported that complete mineraliza-
tion was achieved under these conditions in about 3 h, consistent
with our results taking into account our 8-fold larger concentra-
tion of PCP and 2-fold lesser catalyst concentration.
Our photocatalytic oxidation results using nanosize SnO2 are
shown in Figure 6c. (Bulk SnO2 is nearly inactive for PCP
photocatalysis). Here we observe breakdown products more
similar to those of the direct photolysis results of Figure 6a,
but with accelerated kinetics. No peak at t ) 7.6 min is observed.
Nanosize SnO2 has not been studied previously so these
results cannot be compared directly to other work. However,
the rate of disappearance is comparable to that observed from
Degussa TiO2. Since SnO2 is cheaper than TiO2, but is
ineffective as a bulk photocatalyst, these results are encouraging
as they provide evidence that a robust metal oxide material in
nanosize form can be competitive with Degussa TiO2 slurries.
By integrating the areas observed in Figure 6a-c and similar
data collected for the other nanocluster catalysts studied, one
can obtain the kinetics behavior shown in Figure 7. There are
obviously dramatically different rates of photooxidation of PCP
for these various metal oxides. As has been found by many
others, Degussa P25 has remarkable catalytic activity compared
to nanosize TiO2 (d ) 20 nm). Though initially Degussa TiO2
is more active than the SnO2 nanoclusters, they all reach >99%
complete mineralization in ∼8 h.
Figure 6. Photooxidation of PCP in water using a 400 W Xe arc lamp
with the 300 nm < λ < 700 nm pass filters and measured incident
2
intensity of I
o
) 250 mW/cm is followed by the absorbance monitored
by a PDA at 215 nm, A(215 nm) vs elution time, t for three cases: (a)
no catalyst, (b) Degussa TiO (0.1 mg/mL) slurry, (c) nanosize (0.1
mg/mL) SnO . The chromatograms for different irradiation times are
2
2
offset for clarity.
1
6
direct photolysis, Figure 6a. In fact, we observe different
byproducts for each type of catalytic material we examined,
TiO2, SnO2, and MoS2.
In Figure 6b we show the results for catalytic photooxidation
of PCP using Degussa P25 TiO2 at a concentration of 0.1 mg/
mL in milli-Q water. There is less buildup of intermediates than
observed in the direct photolysis results of Figure 6a. The elution
peaks at t ) 7.6 min and t ) 5.8 min are also destroyed more
rapidly. Also, the photooxidation kinetics are significantly
accelerated with the photocatalyst present.
In previous work by Mills et al., the kinetics of photooxi-
dation using Degussa P25 TiO2 at 0.2 mg/mL was observed to
be 0th order in [PCP] for [PCP] > 0.6 ppm. In contrast, Barbeni
1
et al. showed first-order behavior and an exponential decrease
in [PCP] with irradiation time. Our results for both direct
photolysis and to a lesser extent photocatalysis using Degussa
TiO2 and nanosize TiO2 and SnO2 also show an exponential
decrease with time. However, the higher concentrations of TiO2
used by previous workers and the resulting stronger multiple
scattering may make previous determinations of the true kinetics
ambiguous, as we show in our light- intensity studies.
The apparently faster rate of photooxidation using Degussa
P25 TiO2 compared to the other materials is partly due to its
intense multiple scattering of light which serves to confine the
incident photons more effectively in the reactor. In other words,
Our observations of the increase in photooxidation rates with
TiO2 compared to direct photolysis are consistent with other
work using Degussa P25.1 Since our reactor geometry, initial
concentrations, and catalyst concentrations are significantly
different from previous work a direct comparison with these
,16

7340 J. Phys. Chem. B, Vol. 104, No. 31, 2000
Wilcoxon
Figure 9. Relative PCP concentration (all initial concentrations were
Figure 8. Relative concentration of PCP vs irradiation time for several
1
0 ppm in water) vs irradiation time using a 400 W Xe arc lamp with
nanocluster metal oxide catalysts using 10-fold attenuated incident light
the 300 nm < λ < 700 nm pass filters and measured incident intensity
2
intensity I ) 0.1 I
o
) 25 mW/cm .
2
of I
o
2
) 250 mW/cm , and using Degussa P25 TiO at 0.1 mg/mL.
its effectiVe absorbance cross-section is larger. The other
nanosize photocatalyst solutions, being transparent, are some-
what light-intensity limited at the 0.1 mg/mL or less nanocatalyst
concentrations employed in these studies. This effect is il-
lustrated most easily by comparison of the light-intensity
dependence of Degussa TiO2 vs nanosize SnO2 as we do in the
next section of this paper.
that described below for nanoclusters of MoS2, where a very
strong size dependence is observed due to quantum confinement
effects.
Effect of Incident Light Intensity on Photooxidation
Kinetics. Because of the strong light dependence demonstrated
in Figures 7 and 8, we undertook a systematic investigation of
the incident light-intensity dependence of nanosize SnO2
From Figure 7 we observe that the SnO2 nanoclusters are
less effective for photooxidation of the PCP than the P25 TiO2
and, surprisingly, the smaller d ) 25 nm SnO2 nanoclusters
are less efficient than the d ) 58 nm SnO2 nanoclusters despite
their 4-fold advantage in geometric surface area. However, they
are much more active than the nanosize TiO2. We have also
investigated the PCP photooxidation reaction in these systems
with the incident intensity reduced 10-fold to see if photons
are the “limiting reagent” in the nanocatalyst case but not in
the TiO2 slurry suspension case. This appears to be the case as
shown in Figure 8. In the case of a slurry catalyst, the intense
multiple scattering effectively increases the light path of the
incident photons and thus their absorbance probability and so
there are many more photons available than are needed for the
compared to Degussa TiO powder slurries. We did not attempt
to make a similar comparison in the case of nanosize MoS2
2
because of the significant difference in the absorbance edges
of TiO and nanosize MoS .
2
2
Figure 9 shows the change in photooxidation rate of PCP
using a slurry of 0.1 mg/mL Degussa TiO over a range of
2
2
incident intensities of I ) 250 mW/cm (no light attenuation)
o
to I ) 0.01I . We were initially surprised by the very weak
o
dependence of the kinetics on the incident intensity in the range
0.1I < I < I . Apparently, due to extreme confinement of the
o
o
light in the reactor due to multiple scattering from the slurry
suspension, there are more than enough photons available so
the reaction acts as if it is nearly zero-order in the light intensity.
Only when the light intensity is reduced by about a factor of
100 does the kinetics of PCP photooxidation become exponen-
tial, as is the case with all the nanosize photocatalysts
investigated. Therefore, one should really be comparing the PCP
photooxidation kinetics for nanosize TiO and SnO to slurries
2
reaction at the full lamp light intensity, 250 mW/cm . Thus,
even when the photon intensity is reduced by an order of
magnitude the slurry reaction does not slow proportionally, and
the reaction kinetics for the slurry are not exponential under
either of these irradiation conditions. However, under these
irradiation conditions the nanocluster solutions are “starved”
for photons and do slow in a proportional fashion and exhibit
an exponential decrease in [PCP] with time.
2
2
of TiO2 in this light-intensity regime. Unfortunately, this
intensity is at least 2 orders of magnitude lower than has been
previously investigated using Degussa TiO and any organic.
2
Contrast the behavior shown in Figure 9 by the slurry
From these studies one can conclude that it is necessary to
study the strongly multiply scattering TiO2 slurries at vastly
reduced (10-100 fold) light intensities, to compare them to
nanocluster dispersions at similar concentrations. For example,
comparing Figures 7 and 8 we see that >99% photooxidation
of PCP is reached in ∼120 min for Degussa TiO2 compared to
four times as long for nanosize SnO2, while, when the intensity
is reduced 10-fold, Degussa TiO2 takes ∼240 min for >99%
photooxidation while nanosize SnO2 reaches this value in only
twice the time.
suspension of TiO to that exhibited by a nanocluster solution
of SnO2 using the same reactor geometry and irradiation
2
conditions. The results of this experiment are shown in Figure
10. The same exponential PCP oxidation kinetics were observed
for the nanosize TiO (d ) 20 nm), SnO (d ) 25 nm), SnO (d
2
2
2
) 58 nm), and MoS (d ) 4.5 nm). For each of the nonscattering
2
nanocluster solutions we observe a rate that is roughly propor-
tional to the incident light intensity, and the PCP oxidation
kinetics appear exponential (i.e., first-order in [PCP]) over the
entire light-intensity range investigated.
We observe that there is little difference in photocatalytic
oxidation activity for the two sizes of SnO2 nanoclusters under
these lower light level conditions. From this observation we
can conclude that an increase in specific surface area alone is
not enough to accelerate the kinetics for nanoclusters in this
colloidal size range. This observation contrasts markedly from
Other Powder Slurries. It has been demonstrated that certain
materials, when prepared as semiconductor photoelectrodes, are
effective photoelectrochemical catalysts. The best-known
example of this is RuO2 which has been deposited on colloidal
TiO2 enhancing the rate of water photooxidation substantially.17
We were interested in seeing if bulk powders of this and other
1
7

Catalytic Photooxidation of Pentachlorophenol
J. Phys. Chem. B, Vol. 104, No. 31, 2000 7341
Figure 10. Relative PCP concentration (all initial concentrations were
Figure 11. Relative PCP concentration (10 ppm at t ) 0) vs irradiation
2
1
0 ppm in water) vs irradiation time using a 400 W Xe arc lamp with
time using a Xe arc lamp with I
Degussa TiO
o
) 250 mW/cm for no catalyst, and
the 300 nm < λ < 700 nm pass filters and a measured incident intensity
2
powder at 0.1 mg/mL in two solvents, milli-Q water
2
of I
mg/mL.
o
) 250 mW/cm , and using nanosize, d ) 26 nm SnO
2
at 0.1
and acetonitrile (ACN) containing 1 vol % water.
similar materials would provide good photooxidation catalysts
for organics such as PCP. Two of these materials, formulated
as slurry suspensions in milli-Q water were studied. The powders
were the highest grade purity available, obtained from ALFA
Chemicals. The results were rather surprising, showing that these
materials actually reduced the rate of normal photolysis and
essentially acted as “anti-oxidation agents”, preserving the PCP.
We found this also to be true of bulk powders of MoS2, WO3,
and SnO2. The mechanism of this effect is quite mysterious,
but does indicate that the presence of suspended, micron-size,
inorganic materials may substantially decrease the rate of
photooxidation of organic pollutants. This effect deserves more
extensive examination, but does emphasize the remarkably rapid
rates of photooxidation we observe in nanosize semiconductors
as illustrated in Figure 10 . It is interesting to note that PtS2
has the same layered hexagonal structure found in MoS2, but
in bulk form is very ineffective as a bulk photocatalyst as is
bulk MoS2 powder, probably due to having too narrow a band
gap. As we will see, nanosize MoS2 with its substantially wider
gap is very effective as a photocatalyst.
Figure 12. Relative PCP concentration (10 ppm initially in water) vs
irradiation time. The effect of inorganic ions and surfactants on the
photooxidation kinetics of PCP in water is shown.
however. This shows there are subtle differences in the
photooxidation mechanism for the two solvents. The important
conclusion from this experiment is that the presence of
significant amounts of H2O is not critical for the photooxidation
of PCP. It would be interesting to extend these experiments to
a wider range of organic chemicals, solvents, and photocatalysts
to test the generality of our results.
Effect of Surfactants and Salts on PCP Photooxidation.
In many real world situations, there are both multiple organic
pollutants, inorganic salts, and even surfactants present in the
water. Very little research is available in the literature comparing
the effects of these complexities on the photooxidation kinetics,
and nothing is known concerning their effects on PCP photo-
oxidation. Thus, we examined separately the effect of a simple,
common salt, NaCl, and a common type of organic, water-
soluble surfactant, a quaternary ammonium salt, dodecyltri-
methylammonium chloride (DTAC), on the photooxidation
kinetics. We selected this surfactant for investigation since we
use it to stabilize nanosize MoS2 against aggregation, and
because it has the common counterion, Cl, for comparison to
NaCl, so we can separate out the effect of Cl from that of the
organic dodecyltrimethylammonium entity.
Photooxidation of PCP in Polar, Nonaqueous Solvents. It
has been postulated that photooxidation of organic materials
using semiconductors in water requires the presence of OH
radicals generated from the water to proceed at any appreciable
1
5
rate. Furthermore, many researchers deliberately aerate their
photocatalyst suspensions in order to optimize the photooxida-
tion process, and claims have been made numerous times that
nearly total quenching of the photooxidation process occurs
1
when inert gas purging is employed. Thus, the use of aprotic
organic solvents should cause a severe quenching of the
photooxidation due both to the lack of OH radicals and reduced
oxygen levels because of reduced gas solubility.
Figure 11 shows a study of the effect of solvent on the
photooxidation of PCP both in direct photolysis (no catalyst)
and using Degussa TiO2 at 0.1 mg/mL. As observed in this
figure, though the rate of photooxidation is 2-5× slower in
ACN, photocatalysis does occur, indicating the mechanism for
PCP photooxidation in water and in ACN is similar. This is
verified by the presence of the same t ) 7.6 min elution peak
(see Figure 2) representing a photooxidation intermediate in the
HPLC chromatogram for both solvents. As stated earlier this
intermediate is not a catechol (i.e., the product of OH radical
attack on a Cl on the aromatic ring). In ACN, it is a larger peak
which is more slowly broken down when compared to water,
The results of our investigations are summarized in Figure
12, for a single catalyst, Degussa P25 TiO2 in water at 0.1 mg/
mL under 300 nm < λ < 700 nm irradiation from our 400 W

7342 J. Phys. Chem. B, Vol. 104, No. 31, 2000
Wilcoxon
Xe arc lamp. We first observe that even low levels of NaCl
have a poisoning effect on the catalyst, though it is mild in this
case. Similar observations have been made in field tests of TiO2
where diionization of the aqueous waste stream have been found
to be necessary.19 In the laboratory, Barbeni et al. reported no
1
-
3
inhibition by NaCl at 1 × 10 M, so it appears that it is
inhibitory only at the higher levels investigated here.
We expected the role of surfactants to be inhibitorysblocking
access to surface sites on the semiconductor. However, our
surprising observation is the significant acceleration of the
photooxidation of PCP in the presence of the surfactant DTAC.
We made similar observations of enhanced photooxidation rates
in the case of nanosize MoS2 and SnO2. Clearly, the surfactant
does not serve to block key surface sites as we expected. Instead
it somehow aids the electron and/or hole transfer process. It is
interesting to note that the use of a structurally similar surfactant,
dodecyltrimethylammonium bromide (DTAB), also accelerates
the photooxidation process, though not as much as DTAC, and
that these observed increases in photooxidation rate are also
seen using ACN as a solvent. So the counterion, Br or Cl, in
the polar headgroup of the surfactant is important. It is important
to note that the HPLC elution peak corresponding to the
surfactant DTAC does not change in position or area, showing
that these surfactants are robust compared to PCP during
photooxidation. These types of multicomponent experiments are
worth extending to other organic chemicals such as 4-chlo-
rophenol, or phenol, due to the unexpected positive effects of
the presence of these cationic surfactants.
Figure 13. Relative PCP concentration in water vs irradiation time
using a 400 W Xe arc with long- and short-pass filters allowing only
4
00 nm < λ < 700 nm to reach the stirred solutions.
methods. We confirmed that the MoS2 nanocluster elution peak
area and corresponding optical spectrum of the nanoclusters
under visible irradiation showed no changes with irradiation
time, demonstrating that the MoS2 nanoclusters were acting as
true photocatalysts. There seemed to be no substantial improve-
ment in the MoS2 nanocluster photooxidation kinetics when full
lamp (300 nm < λ < 700 nm) was used. This result is not
surprising given the relatively strong visible absorbance of both
sizes of MoS2 nanoclusters shown in Figure 3 coupled with the
small lamp output below 400 nm.
Figure 13 demonstrates the dramatic effect of nanocluster size
in the strong quantum confinement regime on PCP photooxi-
dation kinetics. The change in size in this regime, it must be
remembered, affects both the electronic valence and conduction
band energy levels shifting them to more favorable values with
decreasing size. Adding to the complexity of a direct comparison
of the size effect in MoS2 to that of control catalysts such as
Degussa P25 TiO2 is the larger surface area and change in
relative number of Mo edge sites with decreasing size. The key
observation, nevertheless, is that both d ) 4.5 nm and d ) 3.0
nm MoS2 are substantially more active than CdS, though this
slurry suspension effectively absorbs a substantially larger
amount of incident light than the nearly transparent (i.e.,
nonscattering) MoS2 solutions. It should also be noted, that
though we did not perform a detailed study of the effect of
nanocatalyst concentration on PCP photooxidation kinetics, this
effect is weak (almost linear) for nanoclusters in the 0.01 to
6
In the studies of Korgel and Monbouquette of the reduction
of nitrate ion by nanosize CdS which was surface stabilized by
charged organic thiols, they also found significant photocatalytic
activity despite the strong surface binding of these ligands.
Photooxidation of PCP Using Nanosize MoS2 and Visible
Irradiation. In addition to the difficulties noted above in
comparing transparent solutions of nanoclusters of TiO2 or SnO2
to strongly multiply scattering slurry suspensions of TiO2, in
the case of nanosize MoS2 we also have qualitatively different
absorbance characteristics between these oxide and sulfide
materials, which render direct comparison of the photooxidation
kinetics of our de facto standard, Degussa P25, to nanosize MoS2
impossible. Basically, the problem is that under illumination
using a 400 nm long-pass filter, TiO2 has no activity, as
expected, since its does not absorb light in the 400 nm < λ <
7
00 nm region. So, for purposes of comparison to a bulk material
suspension, we chose to use a metals grade CdS bulk powder
ALFA), whose absorbance onset of 525 nm is similar to that
0
.1 mg/mL range due to the low PCP concentrations used (10
(
ppm). Our opinion is that the dramatic differences observed in
the rates of photooxidation of PCP by d ) 3.0 MoS2 at 0.09
mg/mL and d ) 4.5 mg/mL d ) 4.5 MoS2 at 0.036 mg/mL, is
due primarily to the wider band gap of the former nanoclusters.
of nanosize MoS2, and which, unlike TiO2, does exhibit some
photooxidation activity using 400 nm < λ < 700 nm irradiation.
Figure 13 shows the relative PCP concentration vs time for
two sizes of MoS2 nanoclusters compared to CdS. The
concentration of each MoS2 solution as determined by XRF is
shown in the figure, and it should be noted that due to some
losses during the MoS2 purification/dialysis, the concentrations
are less than that of the CdS slurry suspensions. All other
reaction conditions were identical.
This is because d ) 4.5 nm MoS with an absorbance onset of
2
nearly 550 nm absorbs substantially more of the visible light
than the d ) 3.0 nm MoS solution, with an absorbance onset
2
of about 450 nm. It is worth noting that the PCP photooxidation
kinetics curve terminates at t ) 120 min for the case of d )
3.0 nm MoS simply because the next point measured at t )
2
Measurements of PCP concentration vs irradiation time by
HPLC using a 70:30 ACN:H2O to 100% ACN gradient elution
allowed us to also determine the concentration of the surfactant-
stabilized MoS2 nanoclusters while we observed the destruction
of the PCP. It was necessary to use ACN instead of MeOH as
the organic component in these experiments to get good,
reproducible HPLC of the nanoclusters. However, the HPLC
peak symmetry of the PCP was not as good under these
conditions, so we analyzed the irradiated samples using both
240 min showed no detectable PCP (i.e., less than 20 ppb)! By
way of Very indirect comparison, this is significantly better PCP
photooxidation than is achieved using Degussa P25 TiO2 and
full lamp irradiation (i.e., 300 nm < λ < 700 nm)!
Another strong argument favoring the effect of energy level
shifts compared to a simple increase in surface area with
decreasing size on the photooxidation kinetics is our observation
of nearly identical photooxidation rates for d ) 26 nm and d )
58 nm SnO2 nanoclusters. These SnO2 sizes are much too large

Catalytic Photooxidation of Pentachlorophenol
J. Phys. Chem. B, Vol. 104, No. 31, 2000 7343
to affect valance and conduction band levels due to quantum
confinement, yet the increase in specific surface area (total area/
gram of catalyst) is very significant (almost 4×, about the same
as that expected for the two sizes of much smaller MoS2
nanoclusters!
function adequately in aqueous systems containing a significant
amount of miscible organic solvents.
In the case of nanosize SnO2 we observed little or no size
dependence of the photooxidation rate on size when comparing
d ) 26 nm to d ) 58 nm SnO2 colloids at the same mass
concentration. Thus, it appears having larger available surface
area, by itself, does not make this material more active.
The very strong MoS2 size dependence we observe for this
complex PCP photooxidation experiment is consistent with that
observed for simpler photoredox reactions we have previously
performed using time-resolved fluorescence to follow the
We also examined our d ) 8-10 nm MoS2 nanoclusters
7
employed in previous studies of phenol photooxidation using
visible irradiation and found very little activity, showing how
important the quantum confinement and concomitant energy
levels shifts are on the positive results of Figure 13. MoS2 in
the 8-10 nm size range absorbs intensely throughout the visible
range 400 nm < λ < 700 nm, but has a very small shift in the
10
electron transfer (E.T.) rates. In these experiments we observed
a dramatic reduction of E.T. rates to bipyridine and substituted
bipyridine molecules as a function of increasing cluster size. It
is also consistent with the strong size dependence we previously
1
0
valence or conduction band levels relative to the bulk.
Although its specific surface area is admittedly considerably
lower, (∼10×, this area reduction is not sufficient to explain
its lack of activity.
7
observed in the visible-light-driven photocatalysis of phenol,
where d ) 8-10 nm MoS2 had negligible activity while d )
IV. Conclusions
4.5 MoS was active.
2
Using HPLC analysis we showed that the product intermedi-
ates in the photooxidation of PCP depend on whether a
photocatalyst is used and also on the material type of the
catalyst. Nanosize SnO2, for example, gave different products
than powders of Degussa TiO2 or nanosize MoS2.
Acknowledgment. This work was supported jointly by the
Division of Materials Sciences, Office of Basic Energy Sciences
and the Energy Research/Environmental Management Program
of the U.S. Department of Energy under contract DE-AC04-
9
4AL8500. Sandia is a multiprogram laboratory operated by
We demonstrated a poisoning effect upon addition of a simple
salt, NaCl, to slurries of TiO2. More surprisingly, we observed
that certain cationic surfactants actually enhance the activity of
TiO2 slurries and the degree of enhancement depended on the
counterion, Cl or Br. Furthermore, HPLC showed that these
cationic surfactants themselves were not photooxidized.
We also investigated the kinetics of PCP photooxidation as
a function of light intensity comparing nanosize SnO2 and TiO2
to slurries of TiO2. We showed that a fair comparison of the
activity of these materials requires an investigation of the
kinetics in light-intensity regimes where the reaction is first-
order in PCP. For slurry systems, the intense multiple light
scattering inherently increases the light collection efficiency of
the reactor giving rise to an almost negligible dependence of
the PCP photooxidation rate on the light intensity in the regime
Sandia Corporation, a Lockheed-Martin Company, for the U.S.
Department of Energy.
References and Notes
(
95.
1) Barbeni, M.; Pramauro, E.; Pelizzetti, E. Chemosphere 1985, 14,
1
(2) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W.
Chem. ReV. 1995, 95, 69.
(
3) Mills, A.; Davies, R. H.; Worsley, D. Chem. Soc. ReV. 1993, 417.
(4) Serpone, N.; Maruthamuthu, P.; Pickat, P.; Pelizzetti, E.; Hidaka,
H. J. Photochem. Photobiol., A: Chem. 1995, 85, 247.
(5) Farin, D.; Kiwi, J.; Avnir, D. J. Phys. Chem., 1989, 98, 5851.
(
6) Korgel, B. A.; Monbouquette, H. G. J. Phys. Chem. B 1997, 101,
5
010.
(7) Thurston, T. R.; Wilcoxon, J. P. J. Phys. Chem. 1998, 103, 11.
(8) Wilcoxon, J. P.; Newcomer, P.; Samara, G. A. J. Appl. Phys. 1997,
81, 7934.
(9) Tributsch, H. Z. Naturforsch. 1977, 32a, 972.
10) Parsapour, F.; Kelley, D. F.; Craft, S.; Wilcoxon, J. P. J. Chem.
2
2
25 mW/cm < Io < 250 mW/cm . In this same intensity regime
(
nanosize SnO2 shows a proportional reduction with decreasing
light intensity and first-order kinetics in [PCP], demonstrating
that photons are a limiting “reagent” for these nanosize materials
at the [PCP] typically found in the environment (1-10 ppm).
We made the first studies of PCP photooxidation using
slurries of TiO2 and nanosize SnO2 in a polar organic solvent,
ACN containing 1% H2O. Surprisingly, we found significant,
though reduced compared to pure H2O, photocatalytic activity
for both types of catalysts, despite the much lower O2 and H2O
levels. This may indicate the role of hydroxyl radicals is not as
critical in the total mineralization process as other researchers
have suggested. It also indicates these photocatalysts will
Phys. 1996, 104, 1.
(11) Kiwi, J.; Borgarello, E.; Duonghong, D.; Gratzel, M. Stud. Surf.
Sci. Catal. 1983, 16, 135.
(
12) Jean, H.; Ring, T. A. Langmuir 1986, 2, 251.
(13) Serpone, N.; Terzian, R.; Lawless, D.; Kennipohl, P.; Sauve, G. J.
Photochem. Photobiol., A: Chem. 1993, 73, 11.
14) Plimmer, J. R.; Knliengebiel, U. I.; Crosby, D. G.; Wong, A. S.
AdV. Chem. Ser. 1973, 120, 44.
15) Pelizzetti, E.; Barbeni, M.; Pramauro, E.; Serpone, N.; Borgarello,
(
(
E.; Jamieson, A.; Hidaka, H. Chim. Ind. 1985, 67, 623.
(16) Mills, G.; Hoffmann, M. R. EnViron. Sci. Technol. 1993, 27, 1681.
(
(
(
17) Hagfeldt, A.; Gratzel, M. Chem. ReV. 1995, 95, 49.
18) Serpone, N. Res. Chem. Intermed. 1994, 20, 953.
19) Crittenden, J. C.; Zhang, Y.; Hand, D. W.; Perram, D. L.; Marchard,
E. G. Water EnViron. Res. 1996, 68 (3), 270.