Surface modification by loading alkaline hydroxides to enhance the photoactivity of WO3

Article abstract of DOI:10.1039/c3cy00781b

Surface modification on WO₃ was carried out by loading KOH, NaOH and CsOH to study the influence of surface interactions on photocatalytic activity of this material. Photodegradation of 2-propanol, methanol, ethanol, acetone and propane in gas phase under visible light irradiation shows loading of KOH, NaOH and CsOH on WO₃ enhanced the photodegradation rate about 4 and 3 times respectively. Loading of KOH on TiO₂ were carried out for comparison and the results showed enhancement of photoactivity by KOH loading for WO₃ which was much higher than that of TiO₂. Increasing the rate of the photocatalytic reaction is attributed to compensation of the high tendency of WO₃ for H⁺ donation and making multi-electron O₂ reduction by loading of KOH on the surface.

Full text of DOI:10.1039/c3cy00781b

Catalysis
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Surface modification by loading alkaline
hydroxides to enhance the photoactivity of WO3
Cite this: Catal. Sci. Technol., 2014,
4, 657
M. Khajeh Aminian* and M. Hakimi
3
Surface modification on WO was carried out by loading KOH, NaOH and CsOH to study the influence
of surface interactions on photocatalytic activity of this material. Photodegradation of 2-propanol,
methanol, ethanol, acetone and propane in gas phase under visible light irradiation shows loading of
KOH, NaOH and CsOH on WO3 enhanced the photodegradation rate about 4 and 3 times respectively.
Received 7th October 2013,
Accepted 4th November 2013
2
Loading of KOH on TiO were carried out for comparison and the results showed enhancement of
photoactivity by KOH loading for WO
3
which was much higher than that of TiO
2
. Increasing the rate of
DOI: 10.1039/c3cy00781b
+
the photocatalytic reaction is attributed to compensation of the high tendency of WO
and making multi-electron O reduction by loading of KOH on the surface.
3
for H donation
www.rsc.org/catalysis
2
indicated that there exists an optimum pH condition for
oxygen evolution. The optimum pH seems to be determined
by the competition between the influence of the concentra-
1
. Introduction
Photocatalyst materials have been extensively investigated for
decomposition of harmful organic substances under visible
light irradiation.
−
tion of OH on the photo-oxidation and photoreduction
1
–5
15,16
The photocatalytic reaction originates
processes.
It is mainly because the pH of a solution
from a strong potential of photogenerated electrons and
holes for a redox reaction. Several attempts have already been
performed to study the influence of crystallinity, electronic
band structure, surface area, particle size and morphology on
the activity of a photocatalyst material. Surface interactions
and band edges exert great influence on the photocatalytic
reaction rate. Investigation of the adsorption influence on
controls the valence and conduction band potentials as well
as the surface chemistry and surface interactions of a
photocatalyst material. There are three basic phenomena
influenced by the pH of the solution: (i) semiconductor
flat-band potential, (ii) adsorption of the electroactive species
6
–9
−
(OH ions), and (iii) photo-electrochemical oxidation of water
−
and OH ions competing with other reactants able to form
1
3
the photocatalytic reaction rate of TiO suspended in an acid-
powerful oxidants on irradiation. In fact, the pH of a solu-
tion can shift the valence band and the conduction band
potentials and it influences the photo-electrochemical
process rate. On the other hand, the pH of the solution
controls the photo-electrochemical process by controlling the
adsorption and desorption of the ions on the surface. It
means interactions with cationic electron donors and elec-
tron acceptors will be favored for the photocatalytic activity at
high pH under conditions in which the pH > pHzpc, while
2
ified aqueous solution of silver salts shows the amount of
+
silver ions (Ag ) adsorbed on the TiO
2
surface, predominantly
The initial rate of the
1
0,11
determining the reaction rate.
acetone formation, as well as that of the Ag deposition,
followed a formal Langmuir equation as a function of
2
-propanol concentration; the rate was proportional to the
1
2
amount of surface adsorbed 2-propanol as a hole scavenger.
Several researches reported the rate of the photocatalytic
reaction in aqueous media strongly depends upon the pH of
anionic electron donors and acceptors will be favored at low
1
1,13–15
17
the suspension.
photocatalytic oxidation of Naphthol Blue Black dye on WO
in alkaline aqueous solution is much slower than that in
It was found that the rate of the
pH under conditions in which pH < pHzpc.
3
It was proposed that the surface OH groups react with
photogenerated holes to form the surface-bound OH radical
1
3
17
acidic solution. There are some reports show increasing the
(radical OHads), which then oxidizes the surface adsorbates.
1
0,11
pH accelerates the photoreduction process
formation. Also, it was observed that the oxygen evolution
and acetone
Several researchers also reported that the OHads radical is an
important specie for the oxidation reaction because the
1
2
1
5
rate was enhanced by the pH up to a pH of 12. This
Physics Department, Yazd University, Yazd, Iran. E-mail: kh.aminian@yazd.ac.ir
This journal is © The Royal Society of Chemistry 2014
diffusion of OHads radical from the TiO surface into the
2
1
8,19
bulk solution is minimal.
Nakato et al. investigated
the photo-oxidation of water adsorbed on the TiO2 surface
by in situ FT-IR absorption and photoluminescence
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measurements and concluded that the oxygen photo evolu-
3
. Results and discussion
tion is initiated by a nucleophilic attack of a H O molecule
2
20,21
on a photogenerated hole at a surface lattice O site.
3
The results of the photocatalytic evaluation of WO loaded with
Although the photo reactions which take place in gaseous
media are similar to those in aqueous media in some charac-
teristics, however they may be different in some other charac-
teristics. In this paper the surface interactions and the
different amounts of KOH are depicted in Fig. 1. Fig. 1a shows
the concentration of CO generated during photodegradation
2
3
of 2-propanol on pure WO , 0.05-wt, 0.075-wt, 0.1-wt and 5-wt
which all samples were heated at 450 °C for 2 h. One observes
surface band potentials during a photoreaction on WO in a
loading of KOH on WO from zero up to 0.1 wt% of the mate-
3
3
gaseous media were studied. It was investigated that how
surface modification of WO3 by alkaline hydroxides such
as KOH, NaOH and CsOH can enhance the photoactivity of
this material.
3
rial can enhance the photoactivity of it rather than pure WO .
Since the reaction between 2-propanol molecules and KOH is
important and unknown in this experiment, it is useful to test
a sample which was loaded with a high amount of KOH for this
experiment. In this case, the sample 5-wt, which was loaded
with 5 wt% of KOH, was tested for photodegradation of
2
-propanol. Obviously the surface of 5-wt is mostly covered by
2
. Experimental methods
potassium hydroxide or potassium oxide. The results of
photodegradation of 2-propanol on the surface of 5-wt was
shown in Fig. 1a. It shows photoactivity of 5-wt is less than that
of pure WO . Fig. 1a concludes that the sample 0.075-wt owns
the highest activity which is more than 4 times higher than
that of a pure one.
Different photocatalyst systems of alkaline hydroxide on WO3
and TiO were prepared by impregnation method. 100 mg of
KOH (Wako corp.) were dissolved in 100 g distilled water
separately. Five samples of 1.0 g commercial WO -monoclinic
Wako corp., BET surface area 5.2 m g ) were mixed with
.50, 0.75, 1.0 and 1.5 ml KOH solution separately to load
WO samples with KOH for 0.05, 0.075, 0.10 and 0.15 wt% of
the material respectively. These samples were named 0.05 wt,
.075 wt, 0.1 wt and 0.15 wt respectively. Loading of KOH on
WO with 5 wt% of the material was carried out by mixing of
.0 g WO with 1.0 g aqueous solution containing 50 mg KOH.
2
3
3
2
−1
(
0
Fig. 1b shows the result of the repeatability test of the
3
3
photoactivity of 0.05-wt (450 °C) and pure WO (450 °C)
sample. One observes the photoactivity of 0.05-wt (450 °C)
and pure WO3 (450 °C) samples has not changed so much
after using it for 3 times. Fig. 1c shows the results of photore-
0
3
1
3
3
action on pure WO , 0.05-wt (450 °C) and unheated 0.05-wt
It was named 5-wt. The mixtures were dried on hot plate
together with CO generation from degradation of 2-propanol
2
and heated at 450 °C for 2 h. Loading of NaOH and CsOH on
in the dark at 60 °C on 0.05-wt (450 °C). It was shown that
the photoactivity of 0.05-wt (450 °C) and unheated 0.05-wt
are close to each other. Since this idea may come to mind
3 2
WO and loading of KOH on TiO (P25-Degussa Corp., BET sur-
2
−1
face area 50 m g ) were performed using the same routine.
The photocatalytic evaluation was carried out in the gas
phase in a cylindrical air-filled static Pyrex glass vessel (500 ml
of total volume) for degradation of methanol, ethanol,
that CO
2
generation is because of the heating effect of irradi-
generation in the
ation and not not photoexcitation, CO
2
dark at 60 °C was investigated in the presence of 2-propanol
2
-propanol, acetone and propane to CO
2
. The photocatalyst
vapour. The result in Fig. 1c shows the catalytic activity of
powders (~0.6 g) were evenly dispersed on the bottom of a
circular glass dish to have a uniform area of ~8 cm and the
dish was mounted in the vessel. The vessel was filled by artifi-
KOH loaded samples is negligible and it confirms CO
generation in the previous experiments is mainly because of
photoexcitation. Photoactivity of 0.05-wt and pure WO was
2
2
3
cial pure air (O
2
–N
2
= 1 : 9) to replace CO
2
-containing natural
investigated in presence of other organic materials such as
methanol, ethanol, acetone and propane. Fig. 1d shows CO2
generation rate from the photodegradation of methanol,
ethanol, 2-propanol, acetone and propane in the same condi-
tions. It can be observed in this figure that although the CO2
generation rates for different organic materials are not the
same; it is well demonstrated that in all cases the photoactivity
of 0.05-wt is much higher than that of pure WO3.
air. Some air enriched with 2-propanol or other mentioned
organic material vapor was injected into the vessel which the
final concentration of organic material in the vessel is about
1
30 ppm. After injection of organic material vapor and prior
to light irradiation, the vessel was kept in the dark for one
hour to get adsorption saturation of organic material vapor
on the sample surface. All photocatalytic reactions were
carried out at room temperature. Upon light irradiation, the
gaseous sample (0.5 ml) was periodically extracted from the
reaction vessel and measured on a gas chromatograph
To find the reason why loading of WO
enhanced the photoactivity of WO and check if it works for
other alkaline hydroxides, similar experiments were carried
out for WO samples loaded with NaOH and CsOH.
Fig. 2a and b shows the concentration of CO generated
3
with KOH
3
(GC-14B, Shimadzu) equipped with a flame ionization detec-
3
tor (FID) and a methanizer-FID for analysis of organic mate-
2
rials and CO . The light source was a 300 W xenon arc lamp
during the photodegradation of 2-propanol on WO samples
2
3
(
ILC Technology, CERMAX LX-300). A cutoff filter (L42, Hoya
loaded with NaOH and CsOH respectively. One can observe
the photoactivity of the samples loaded with NaOH and
Optics) was used to obtain visible light (λ > 420 nm). UV–visible
diffuse reflectance spectrum was measured on a UV–visible
spectrometer (UV-2500, Shimadzu) at room temperature.
CsOH enhanced up to 3 times than that of pure WO . By
3
comparing of Fig. 1a, 2a and b it is found loading of WO
3
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Fig. 1 (a) Photodegradation results of 2-propanol on WO
3
loaded with different amounts of KOH under visible light irradiation; (b) test of the
repeatability of photodegradation experiments, (c) photodegradation rate comparison between 0.05-wt before and after heating at 450 °C
together with CO generation in dark at 60 °C, (d) a comparison between CO generation rate in the photodegradation test of different organic
materials such as methanol, ethanol, 2-propanol, acetone and propane on the surface of both pure WO and 0.05-wt.
2
2
3
with KOH, NaOH and CsOH has a similar effect on the
photoactivity of WO . Fig. 2c shows the highest activity sam-
ples loaded with KOH, NaOH and CsOH. It shows the sam-
ples loaded with KOH are more reactive than those loaded
(Fig. 3b) seems to be almost the same, while some nanorods
have been formed on 5.0-wt 450 °C (Fig. 3c). It can be
concluded that the surface change after loading of 0.05 wt%
KOH is little; however a different phase of material has been
formed on the surface after loading of 5 wt% KOH.
3
with NaOH or CsOH. The photoactivity of KOH/WO
3
for
photodegradation of 2-propanol is about 50% related to that
The optical properties of 0.05-wt (450 °C) were investi-
gated by UV–visible diffuse reflectance spectroscopy and the
2
2
of Pt/WO
3
.
The apparent quantum efficiency of KOH/WO
3
at 440 nm for 2-propanol and propane was measured
as about 3 and 18% respectively. Also, the photoactivity of
3
result is shown in Fig. 4. The spectrum of pure WO is also
shown for comparison. One observes the absorption edges of
both samples are at the same wavelength. The shape of the
absorption spectrum of 0.05-wt (450 °C) is very similar to that
of pure WO ; except that reflectance of WO above absorption
2
3
KOH/WO
of acetaldehyde, the photodegradation rate for KOH/WO
little higher than that of CuBi O /WO , while CO2
3
was compared to CuBi
2
O
4
/WO
3
for degradation
3
is a
2
4
3
3
3
photogeneration is about 70% of the full amount obtained
using CuBi /WO . It seems that the enhancement of
edge was decreased by KOH loading which implies that some
defects were formed in WO by KOH loading. It is concluded
2
O
4
3
3
photoactivity is not only because of the chemical property,
but because of photo-oxidation and photo-reduction pro-
cesses on the surface.
that loading of WO3 with KOH does not create a new elec-
tronic energy level for photo-excitation of the electrons. X-ray
diffraction (XRD) spectra of pure WO and 0.05-wt sample
3
The SEM images of the samples are shown in Fig. 3. The
show WO was crystallized in monoclinic structure. It can be
3
surface of pure WO
3
450 °C (Fig. 3a) and 0.05-wt 450 °C
observed there is no noticeable change in the structure after
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Fig. 2 Results of the photodegradation of 2-propanol on WO
3
loaded with different amounts of (a) NaOH and (b) CsOH under visible light irradiation.
3
It can be observed the sample loaded with NaOH or CsOH enhanced the photodegradation rate up to 3 times compared to pure WO . (c) Comparison
of the highest activity of the samples loaded with KOH, NaOH and CsOH. Samples loaded with KOH are more reactive than those loaded with
NaOH or CsOH.
Fig. 3 SEM images of (a) pure WO
3
450 °C, (b) 0.05-wt 450 °C and (c) 5.0-wt 450 °C. The SEM images show there is no difference between pure
WO and 0.05-wt, while some nanorods have been formed on 5.0 wt.
3
loading of KOH on the surface, except the intensity of the
peaks has slightly decreased after KOH loading, which proves
a formation of some defects on the surface.
shows different peaks for oxygen and tungsten, but the ratios
of the peak intensity are the same for all samples and it can
be concluded the surface group has not changed significantly.
To study the changes of the surface groups after loading
of KOH, XPS experiment has been carried out on the sam-
ples. Fig. 5a and b show XPS spectra of oxygen and tungsten
for the samples loaded by different amounts of KOH in high
resolution. For both oxygen and tungsten, the intensity of the
peaks is different and the position has shifted a little. The
peaks of oxygen have been dissociated to two components
Surface area measurement of pure WO and 0.05-wt using
3
2
−1
BET method yields 5.2 and 5.6 m g respectively. It can be
found from surface area measurement that immersion of
WO3 in the solution of KOH makes some corrosion on the
3
surface of WO which increases the surface area of material
slightly. Also the gas adsorption experiment on the surface of
KOH loaded WO3 was carried out in dark conditions using
2-propanol vapor in the glass vessel of the photoreaction. The
results of the gas adsorption have been shown in Fig. 6.
2-Propanol molecules are adsorbed on the surface of the
photocatalyst material and the concentration of 2-propanol
has been reduced in time. The gas adsorption experiment
results in the adsorption rate on the surface are almost the
5
x
30.5 eV (belongs to WO ) and 531.3 eV (belongs to WOH). It
is found that the ratio for the intensity of two components
for all samples is approximately the same. The peaks of
tungsten have been dissociated and it is found that they are
+
6
+5
composed of W (35.8 and 37.9 eV) and W (34.7 and
3
6.8 eV) with the same ratio for all samples. XPS spectra
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Fig. 4 (a) UV–visible diffuse reflectance spectra of pure WO
3
and 0.05-wt sample. There is no noticeable change after loading except some
defects which reduced the reflectance above absorption edge after loading. (b) Absorbance spectra of pure WO
absorption edge has not changed after loading. (c) XRD spectra of pure WO and 0.05-wt sample show WO
structure and the crystal structure has not changed after loading of KOH on the surface.
3
and 0.05-wt sample show the
was crystallized in monoclinic
3
3
Fig. 5 XPS spectra of oxygen (a) and tungsten (b) on the surface of WO
different and the position has shifted a little. (a) For all samples, the peak of oxygen is composed of 530.5 eV (belongs to WO
(
3
loaded by different amounts of KOH. The intensity of the peaks is
) and 531.3 eV
belongs to WOH) with the same ratio. (b) The peaks of tungsten are composed of W (35.8 and 37.9 eV) and W (34.7 and 36.8 eV). Although
x
+6
+5
the intensity of the spectra is different, but the ratios of the peaks for all samples are the same and it seems the surface group has not changed
significantly.
same for pure WO and WO loaded with KOH and this con-
that of WO , KOH loading was carried out in different concen-
3
3
3
firms the results of surface area measurement.
trations. Fig. 7 shows the results of CO2 generation during
photodegradation of 2-propanol on P25 loaded with 0.1, 0.3
and 0.75 wt% of KOH without a cutoff filter. It can be
observed the activity of the sample loaded with 0.5 wt% of
To do more investigation on this phenomenon, the influ-
ence of loading of KOH on photoactivity of TiO2 (P25) was
investigated. Since the surface area of P25 is different from
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2
3
reactions. In fact, some states on the surface which are
formed by dangling bonds, interatomic spaces, surface
hydroxyl groups, etc., can be traps of both electrons and
2
5
holes and perform charge separation. To date, a great
deal of research has been conducted on the adsorption and
26–29
2
photocatalytic reactions of surface active ligands on TiO ,
but their relationship is not yet well understood. Chelation of
surface Ti atoms with electron-donating bidentate ligands,
such as catechols, facilitates the charge transfer across the
3
0,31
photocatalyst materials to the reactant molecules.
The
photogenerated holes in TiO2 are promptly scavenged by
chemisorbed molecules on the surface. It seems that this
charge transfer reaction is very rapid and competes with the
charge recombination and trapping of the photogenerated
3
2
holes by other surface defects. It has been reported that
aliphatic alcohols, such as methanol, ethanol, and 1-propanol,
adsorbed on TiO2 surface can be present in two forms: a
hydrogen-bonded physisorbed species and a chemisorbed
Fig. 6 2-Propanol molecules in the glass vessel are adsorbed on the
surface of photocatalyst material and the concentration of 2-propanol
has been reduced in time. The adsorption rate on the surface is almost
3 3
the same for pure WO and WO loaded with KOH.
3
3
alkoxide species. Kinetic experiments indicate chemisorbed
3
4
alkoxide species react much more rapidly. Also it was
material is slightly higher than that of pure P25, while the
other samples own the same activity as pure P25 approxi-
mately. It seems the influence of KOH loading on
reported that photodegradation of 2-propanol on TiO could
proceed through two parallel routes; the first route occurred
2
through the formation of acetone from the H-bonded
2-propanol species. The second route occurred through
relatively rapid oxidation of the 2-propoxide species to form
photoactivity of P25 is similar to that of WO ; except the
3
enhancement of photoactivity for loading on WO
higher than that of P25.
3
is much
3
5
CO directly.
2
Since the amount of KOH loaded on WO is a few percent,
It was demonstrated that surface modification of Cs is a
3
less than one monolayer, it can not change the morphology
of the material, and it is expected the loaded material forms
ions or clusters of K on the surface. The reaction between
method to improve activity for photocatalytic O evolution
2
+
3
36
and Fe reduction over WO in the solution. It was consid-
3
ered that the reason for the increase in photo-oxidation activity
+
2
+
WO
3
and KOH on the surface is the same as the one already
was the partial ion-exchange of Fe with Cs formed on the
2
4
reported on the surface of alumina:
surface of H–Cs–WO during the photocatalytic reaction. The
3
+
3
3 3
selective adsorption of Fe on WO and H–Cs–WO and
+
3
–WOH + KOH → –WOK + H
2
O
(1)
two-step photo excitation via Fe were considered as the main
causes of photoreduction improvement. However, the activity
of H–Cs–WO for photocatalytic O evolution and silver reduc-
Sayama et al. suggested that the defects on the surface of
the photocatalyst play important roles in the photocatalytic
3
2
+
tion in the presence of Ag was generally lower than that of
Also, it was reported that surface treatment using Cs
and Fe ions on WO
3
6
+
WO
3
.
+
2
+
3
can proceed VO
2
reduction and water
3
7
+3
oxidation. Since photoactivity was enhanced for Fe and
+
+
VO
2
but it was decreased for Ag , it can be understood that
and H–Cs–WO
the photo-oxidation–reduction process on WO
3
3
depends on a lot of factors and the reason for improvement is
not trivial.
One may think that photodegradation of organic materials
on KOH loaded WO3 in the gas phase proceeds through a
similar mechanism. Corresponding to two last reported
3
6,37
papers,
ion exchange and two-step photo excitation via
ions are the main causes for promotion of photoactivity.
Actually ion exchange and two-step photo excitation are
impossible in gas phase, this experiment, and they are not
promising to explain promotion of photoactivity. In addition
to the mentioned reasons, there are some more problems.
The first one is that KOH loading promotes photodegradation
rate of various kinds of organic materials including alcohols,
propane and acetone via different mechanisms. The other
2
Fig. 7 Photodegradation of 2-propanol TiO (P25) under UV light irra-
diation, loaded with different amounts of KOH. KOH loading on P25
enhanced the photodegradation rate slightly.
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one is why loading of KOH on WO3 enhanced the
photoactivity several times while the enhancement of the
photoactivity on TiO is slight.
2
slightly. The enhancement of photoactivity of WO was attrib-
3
uted to two phenomena. The first one is catalytic property of
OH groups on the surface and compensation of the high
+
Considering all these experiments and reports, it can be
tendency of WO3 for H donation. The second one is multi-
assumed that promotion of photoactivity of WO in gas phase
electron O2 reduction to which the high activity of KOH
3
after KOH loading is due to catalytic property of OH groups
on the surface and to multi-electron reduction. Actually pHzpc
of WO is 1 and it shows WO has a high tendency to release
3 2
loaded WO is attributed, with regard to single electron O
reduction which is considered for TiO2.
3
3
+
+
H
or donate H to other materials. Obviously loading of
Acknowledgements
KOH can recompense the acidic property and shift the surface
property from acidic to basic property. This phenomenon can
facilitate the photocatalytic reaction of organic molecules on
the surface.
The high valence band energy, EVB, in WO3 makes the
surface holes act as powerful oxidizing sites for generating
radical oxidants in reactions with organic molecules. In our
experiments the level of EVB is less important since the poten-
tial is high enough to oxidize the molecules, while the con-
We would like to express our sincere gratitude and thanks
to the National Institute of Materials Science (NIMS), and
especially to Prof. Jinhua Ye who provided the facilities for
the experiments and study of the photocatalytic materials.
References
1 R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga,
Science, 2001, 293, 269.
2 T. Kako, Z. Zou, M. Katagiri and J. Ye, Chem. Mater., 2007,
19, 198.
3 Y. Irokawa, T. Morikawa, K. Aoki, S. Kosaka, T. Ohwaki and
Y. Taga, Phys. Chem. Chem. Phys., 2006, 8, 1116.
4 D. Wang, T. Kako and J. Ye, J. Am. Chem. Soc., 2008,
130, 2724.
duction band energy level, ECB, of WO3 (+0.35 V) is not
+
negative enough for one-electron oxygen reduction (O
2
+ H +
−
e
→ HO
2
, −0.046 V). The other reason may consider for
enhancement of photodegradation is multi-electron reduc-
tion. Abe has reported that two-electron oxygen reduction
+
−
+
−
(O
2
+ 2H + 2e → H
2 2 2
O , +0.68 V and O + 4H + 4 e →
2
2
2
H O, +1.23 V vs. NHE pH 0) proceeded over Pt/WO . It
2
3
seems reasonable to consider that such multi-electron
reductions more readily proceed on the surface of
alkaline/WO , compared to the bare surface of WO . Unfortu-
nately, we do not have any apparatus to check which mecha-
nism is more probable for this experiment.
5 T. Arai, M. Yanagida, Y. Konishi, Y. Iwasaki, H. Sugihara and
K. Sayama, Catal. Commun., 2008, 9, 1254.
6 T. Tachikawa, M. Fujitsuka and T. Majima, J. Phys. Chem. C,
2007, 111, 5259.
7 T. Tachikawa, S. Tojo, M. Fujitsuka, T. Sekino and
T. Majima, J. Phys. Chem. B, 2006, 110, 14055.
8 H. Zhuang, C. Lin, Y. Lai, L. Sun and J. Li, Environ. Sci.
Technol., 2007, 41, 4735.
9 M. Khajeh Aminian and J. Ye, J. Mater. Res., 2010, 25, 141.
10 S. Nishimoto, B. Ohtani, H. Kajiwara and T. Kagiya, J. Chem.
Soc., Faraday Trans. 1, 1983, 79, 2685.
O
2
3
3
The next question is why loading of KOH on TiO cannot
2
promote the photocatalytic reaction as high as that on WO
It is well known that the CB bottom of TiO
3
.
2
lies at −0.16 V,
slightly more negative than one-electron oxygen reduction
potential, and single-electron reduction is generally considered
3
8,39
the main pathway for electron consumption over TiO
2
.
It is
reasonable to suppose that enhancement of the photoactivity
11 B. Ohtani, Y. Okugawa, S. Nishimoto and T. Kagiya, J. Phys.
Chem., 1987, 91, 3550.
12 B. Ohtani, M. Kakimoto, H. Miyadzu, S. Nishimoto and
T. Kagiya, J. Phys. Chem., 1988, 92, 5773.
for KOH loaded TiO
since it possessing sufficient CB level for the reduction of O
2
is considerably less than that for WO
3
2
by single electron. The other reason is that pHzpc of TiO , 5.5,
2
showing a low tendency for TiO
2
, in contrast to WO
release or donate H , and KOH loading on TiO is not as
highly effective as on WO for photocatalytic reactions.
3
, to
13 M. Hepel and J. Luo, Electrochim. Acta, 2001, 47, 729.
14 O. P. Panwar, A. Kumar, R. Ameta and S. C. Ameta, Maced. J.
Chem. Chem. Eng., 2008, 27, 133.
+
2
3
1
5 W. S. Nam, E. Y. Kim and G. Y. Han, Korean J. Chem. Eng.,
008, 25, 1355.
2
4
. Summary
1
6 K. Sayama, K. Mukasa, R. Abe, Y. Abe and H. Arakawa,
WO and TiO were loaded by different amounts of alkaline
J. Photochem. Photobiol., A, 2002, 148, 71.
3
2
hydroxide and the influences of loading KOH on the
photocatalytic activity of WO3 and TiO2 were investigated.
Photocatalytic evaluation was carried out using photodegradation
of 2-propanol, methanol, ethanol, acetone and propane in gas
phase under visible light irradiation. It was shown KOH load-
ing for 0.075 wt% of material enhanced the photodegradation
17 M. R. Hoffmann, S. T. Martin, W. Choi and
D. W. Bahnemann, Chem. Rev., 1995, 95, 69.
18 Y. Mao, C. Schöneich and K. D. Asumus, J. Phys. Chem.,
1991, 95, 10080.
19 T. Tachikawa, M. Fujitsuka and T. Majima, J. Phys. Chem. C,
2007, 111, 5259.
rate of 2-propanol on WO
and CsOH loading enhanced it up to 3 times; as well loading
of KOH on TiO2 enhanced the photoactivity of this material
3
more than 4 times, while NaOH
20 R. Nakamura and Y. Nakato, J. Am. Chem. Soc., 2004, 126, 1290.
21 R. Nakamura, T. Okamura, N. Ohashi, A. Imanishi and
Y. Nakato, J. Am. Chem. Soc., 2005, 127, 12975.
This journal is © The Royal Society of Chemistry 2014
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View Article Online
Paper
Catalysis Science & Technology
2
2
2
2
2
2
2 R. Abe, H. Takami, N. Murakami and B. Ohtani, J. Am.
30 T. Rajh, L. X. Chen, K. Lukas, T. Liu, M. C. Thurnauer and
D. M. Tiede, J. Phys. Chem. B, 2002, 106, 10543.
31 P. C. Redfern, P. Zapol, L. A. Curtiss, T. Rajh and
M. C. Thurnauer, J. Phys. Chem. B, 2003, 107, 11419.
32 T. Tachikawa, Y. Takai, S. Tojo, M. Fujitsuka and T. Majima,
Langmuir, 2006, 22, 893.
33 K. S. Kim, M. A. Barteau and W. E. Farneth, Langmuir, 1988,
4, 533.
34 S. Pilkenton, S. J. Hwang and D. Raftery, J. Phys. Chem. B,
1999, 103, 11152.
Chem. Soc., 2008, 130, 7780.
3 T. Arai, M. Yanagida, Y. Konishi, Y. Iwasaki, H. Sugihara and
K. Sayama, J. Phys. Chem. C, 2007, 111, 7574.
4 H. Ma, S. Li, B. Wang, R. Wang and S. Tian, J. Am. Oil Chem.
Soc., 2008, 85, 263.
5 Z. Liu, Z. G. Zhao and M. Miyauchi, J. Phys. Chem. C, 2009,
1
13, 17132.
6 L. G. C. Rego and V. S. Batista, J. Am. Chem. Soc., 2003,
25, 7989.
1
7 T. Paunesku, T. Rajh, G. Wiederrecht, J. Maser, S. Vogt,
N. Stojićević, M. Protić, B. Lai, J. Oryhon, M. Thurnauer and
G. Woloschak, Nat. Mater., 2003, 2, 343.
35 W. Xu and D. Raftery, J. Phys. Chem. B, 2001, 105, 4343.
36 Y. Miseki, H. Kusama, H. Sugihara and K. Sayama, J. Phys.
Chem. Lett., 2010, 1, 1196.
2
8 T. Rajh, Z. Saponjic, J. Liu, N. M. Dimitrijevic, N. F. Scherer,
M. Vega-Arroyo, P. Zapol, L. A. Curtiss and M. C. Thurnauer,
Nano Lett., 2004, 4, 1017.
37 Y. Miseki, H. Kusama and K. Sayama, Chem. Lett., 2012,
41, 1489.
38 M. Mrowetz, W. Balcerski, A. J. Colussi and M. R. Hoffmann,
J. Phys. Chem. B, 2004, 108, 17269.
2
9 N. M. Dimitrijevic, Z. V. Saponjic, B. M. Rabatic and T. Rajh,
J. Am. Chem. Soc., 2005, 127, 1344.
39 T. Hirakawa and Y. Nosaka, Langmuir, 2002, 18, 3247.
664 | Catal. Sci. Technol., 2014, 4, 657–664
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