Evidence for light-induced oxygen exchange in the oxidation of liquid hydrocarbons on oxygen 18-labelled titanium dioxide

Article abstract of DOI:10.1039/c3ra40460a

Titanium dioxide (TiO₂) nanoparticles labelled with ¹⁸O isotopes were prepared via a sol-gel method with titanium alkoxide and ¹⁸O water precursors. The presence of ¹⁸O in the labelled TiO₂ sample was confirmed using Raman spectroscopy and secondary-ion mass spectrometry. The photocatalysis of cyclohexane under UV light was investigated using ¹⁸O-labelled TiO₂ as the catalyst; this reaction yielded ¹⁸O-containing cyclohexanone as the major product. The ¹⁸O content in the cyclohexanone product dramatically decreased with increasing reaction time. This isotopic experiment demonstrates that the oxygen atom in cyclohexanone originates from surface oxygen in ¹⁸O-labelled TiO₂. Similar photoreactions performed with toluene and para-xylene as the hydrocarbon substrates afforded oxidation products containing ¹⁸O isotopes. The migration of oxygen atoms in ¹⁸O-labelled TiO₂ into liquid hydrocarbons provides direct evidence for the involvement of the solid oxide in the interfacial reaction between a liquid substrate and surface oxygen in a solid catalyst.

Full text of DOI:10.1039/c3ra40460a

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Evidence for light-induced oxygen exchange in the
oxidation of liquid hydrocarbons on oxygen 18-labelled
titanium dioxide3
Cite this: RSC Advances, 2013, 3,
9402
Jongmyung Choi,a Donghyeon Kang,a Kyu Hyung Lee,a Byeongno Lee,a Kyung
Joong Kimb and Nam Hwi Hur*a
Titanium dioxide (TiO2) nanoparticles labelled with 18O isotopes were prepared via a sol–gel method with
titanium alkoxide and 18O water precursors. The presence of 18O in the labelled TiO2 sample was
confirmed using Raman spectroscopy and secondary-ion mass spectrometry. The photocatalysis of
cyclohexane under UV light was investigated using 18O-labelled TiO2 as the catalyst; this reaction yielded
18O-containing cyclohexanone as the major product. The 18O content in the cyclohexanone product
dramatically decreased with increasing reaction time. This isotopic experiment demonstrates that the
oxygen atom in cyclohexanone originates from surface oxygen in 18O-labelled TiO2. Similar photoreactions
performed with toluene and para-xylene as the hydrocarbon substrates afforded oxidation products
containing 18O isotopes. The migration of oxygen atoms in 18O-labelled TiO2 into liquid hydrocarbons
provides direct evidence for the involvement of the solid oxide in the interfacial reaction between a liquid
substrate and surface oxygen in a solid catalyst.
Received 25th January 2013,
Accepted 9th April 2013
DOI: 10.1039/c3ra40460a
www.rsc.org/advances
ide (TiO2) can be explained via the Mars–van Krevelen
mechanism. Almeida and co-workers investigated the photo-
Introduction
oxidation of cyclohexane over anatase TiO2 under an 18O-
labelled oxygen (O2) atmosphere using attenuated total-
reflectance Fourier-transform infrared (FT-IR) spectroscopy;
their results suggest that the oxygen in the cyclohexanone
Selective oxidation catalysed by solid oxides is an important
industrial process for converting hydrocarbons into valuable
chemicals such as alcohols, aldehydes, and acids at high
temperatures.1–3 It is known that oxidation proceeds through
two independent steps that are known as the Mars–van
Krevelen mechanism.4 In the first step surface oxygen atoms
are incorporated into the substrate, yielding oxygenated
products and simultaneously producing oxygen vacancies on
the oxide surface. In the second step, the vacant sites are
filled by molecular oxygen.5 Accordingly, the gas-phase
oxidation occurs at the surface of the solid oxide. The
Mars–van Krevelen mechanism has been widely used to
explain heterogeneous catalytic reactions that typically occurs
at high temperature.4,6–8
It has been reported that the Mars–van Krevelen mechan-
ism can be applied to homogeneous catalytic reactions
performed in solution under mild conditions, specifically
oxidation promoted by soluble polyoxometallate compounds
under an oxygen atmosphere.9,10 It has also been claimed that
photocatalytic oxidation of hydrocarbons over titanium diox-
product originates as surface oxygen in TiO2.8 Civis and co-
ˇ
workers reported that surface oxygen in TiO2 and oxygen in
CO2 can be exchanged during photo-reactions;11 they studied
light-induced oxygen exchange between 18O-labelled TiO2 and
CO2 molecules using a high-resolution IR spectrometer.
Although their labelling studies did not include any mass
spectral data, it is conceivable that the surface oxygen in TiO2
plays
a crucial role in the photo-oxidation of gaseous
hydrocarbon molecules. Light-induced gas-to-solid reactions
over TiO2 surfaces are quite analogous to the oxidation of
hydrocarbons catalysed by solid oxides at high temperature.
We thus reasoned that, under ambient conditions, the same
photo-oxidation process might occur at the liquid–solid
interface, and oxygen transfer from TiO2 to the liquid
substrate would provide direct evidence of the Mars–van
Krevelen mechanism.
Herein we demonstrate that the photo-oxidation of cyclo-
hexane over 18O-labelled TiO2 under UV irradiation afforded
18O-labelled cyclohexanone as the major product. The 18O-
isotope labelling experiments along with the mass spectro-
metric product analyses clearly demonstrate that oxygen atoms
in the TiO2 surface are transferred to the oxidation products.
aDepartment of Chemistry, Sogang University, Seoul 121-742, Korea.
E-mail: nhhur@sogang.ac.kr
bDivision of Industrial Metrology, Korea Research Institute of Standards and Science,
Daejeon 305-600, Korea
Electronic supplementary information (ESI) available. See DOI: 10.1039/
3
c3ra40460a
9402 | RSC Adv., 2013, 3, 9402–9407
This journal is ß The Royal Society of Chemistry 2013

View Article Online
RSC Advances
Paper
(l . 300 nm) using a 300 W xenon Lamp (Newport). The
incoming light was passed through a water filter to block
infrared radiation. The measurements were conducted over 12
h. The products that evolved from the liquid substrate were
initially analysed using a gas chromatograph (GC; Younglin,
Acme-6000) equipped with a flame ionization detector. To
enable quantitative analysis, the peak areas were calibrated
with authentic samples. In addition to the GC measurements,
To further confirm this result, the same photocatalytic
experiments were performed using toluene and para-xylene
as the hydrocarbon substrates; the corresponding 18O-labelled
products were obtained, thereby confirming that the oxygen
atoms in the oxidation products originate from the surface
oxygen atoms in 18O-labelled TiO2. Our results provide direct
evidence that photocatalytic reactions that occur at liquid–
solid interfaces can also be explained by the Mars–van
Krevelen mechanism.
1
the H NMR spectra were also recorded on a Varian 400-MHz
Gemini operating at 400 MHz. All chemical shifts were
referenced to CDCl3 at 7.26 ppm.
The 18O contents of the products were quantitatively
determined from mass spectra that were obtained using a
GC combined with a mass spectrometer (Agilent 5973N) at the
Organic Chemistry Research Centre in Sogang University.
Similar experiments were performed with toluene (99.5%,
Aldrich) and para-xylene (.99%, Aldrich) in place of cyclohex-
ane.
Experimental
Synthesis and characterization of 18O-labelled TiO2
18O-labelled TiO2 was prepared in a stainless steel autoclave
(Parr 4714, 45 mL) according to the following procedure: the
glass liner of the autoclave was charged with 4 mL of
tetraisopropyl orthotitanate (TCI) and
1
mL of H218O
(Cambridge Isotope Laboratories, 97%). After the gauge and
gauge-block assembly were attached to the autoclave, the
autoclave was placed in an oil bath on a hot plate. After
heating at 200 uC for 3 h, the autoclave was removed from the
oil bath. The white crystalline solid that formed on the bottom
of the glass liner was filtered and washed with ethanol several
times to remove the isopropyl alcohol and water. The solid
product was then completely dried in an oven at 80 uC. The
yield of 18O-labelled TiO2 based on the amount of tetraisopro-
pyl orthotitanate used was typically greater than 92%. The 18O-
labelled TiO2 powder was characterized using powder X-ray
diffraction (XRD), transmission electron microscopy (TEM),
Raman spectroscopy, and secondary-ion mass spectrometry
(SIMS) depth profiling. The powder XRD patterns were
obtained using a Rigaku diffractometer (Cu-Ka) operating at
40 kV and 150 mA. TEM analyses were carried out on a JEOL
JEM-2100F. Raman spectra were obtained at room temperature
using a LabRam HR Raman spectrometer (Horiba Jobin-Yvon)
equipped with a liquid-nitrogen-cooled CCD multichannel
detector. A 514 nm Ar-ion laser was used as the excitation
source. The relative ratio of 16O and 18O in the 18O-labelled
TiO2 was determined using SIMS depth profiling with a
magnetic sector SIMS (IMS-7f, Cameca). A cesium-ion beam
was used as the primary ion source to enhance the negative
ion intensity. For comparison, unlabelled TiO2 was prepared
following the same procedure with deionized water (H2O) in
place of H218O; the product was then characterized in a similar
manner.
Results and discussion
The 18O-labelled TiO2 sample prepared from tetraisopropyl
orthotitanate and H218O in an autoclave was first characterized
using powder XRD. Fig. 1 shows an XRD pattern of the 18O-
labelled TiO2. All peaks are well-indexed to theoretical
diffraction peaks of anatase TiO2 and no peaks corresponding
to rutile TiO2 phase were evident. The diffraction peaks are
broad as a result of the nanometre-scale sizes of the
particles.12 The average crystallite size, which was calculated
using the Scherrer equation, is 7.1 nm. Fig. 2 shows a high-
resolution TEM image of the 18O-labelled TiO2 sample. It
appears that particles are evenly dispersed without severe
agglomeration. Their average size, estimated from the TEM
image, is approximately 6–9 nm, which agrees well with the
XRD result.
Raman spectroscopy was employed to verify the presence of
18O in the 18O-labelled TiO2 sample; the 18O isotope induces
significant shifts and splitting of active Raman vibrational
modes. It is known that anatase TiO2 has six Raman-active
vibrations including one A1g, two B1g, and three Eg modes.13,14
Photocatalytic reaction
A 0.16 g sample of 18O-labelled TiO2 was placed in a 2 mL vial
that was sealed with a screw cap and then dried at 150 uC for 4
h in vacuo to remove adsorbed water. Cyclohexane (99.5%,
Aldrich) was dried using molecular sieves (Aldrich) and then
degassed three times via the freeze–pump–thaw method to
afford anhydrous cyclohexane. A 0.8 mL aliquot of anhydrous
cyclohexane was added to the sealed vial using a syringe.
Oxygen gas (99.995%, Aldrich) was then flowed into the vial to
saturate the atmosphere with O2. The 18O-labelled TiO2
powder dispersed in cyclohexane was irradiated with UV light
Fig. 1 XRD pattern of 18O-labelled TiO2. All the XRD peaks are indexed on the
basis of theoretical patterns of anatase TiO2.
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 9402–9407 | 9403

View Article Online
Paper
RSC Advances
Fig. 2 Typical TEM image of as-synthesized 18O-labelled TiO2 particles.
Fig. 4 SIMS depth profile of 18O-labelled TiO2 as a function of time. The red,
blue, and black curves correspond to the signals from 18O, 16O, and 48Ti,
respectively. The inset is the relative percentages of 16O and 18O.
Only the A1g mode is ascribed to purely oxygen vibrations
while the B1g mode is associated with titanium vibrations. The
other four vibrational modes are attributed to combined
oxygen and titanium vibrations.15 As shown in Fig. 3, the A1g
mode at 513 cm21 in the Raman spectrum of 18O-labelled TiO2
was red-shifted relative to that in the spectrum of unlabelled
in the labelled sample are 38.56% and 61.44%, respectively. It
is worth mentioning that a Ti18O2 sample that contains mainly
18O isotopes has been prepared from TiCl4 and H218O;14
however, this process results in chlorine-contaminated pro-
duct. Our method, which is based on titanium alkoxide, avoids
chlorine contamination to afford the chlorine-free 18O-labelled
TiO2 product, albeit with an isotopic mixture of 18O and 16O.17
However, this product is better suited to studying the
oxidation of various organic substrates over titanium oxide
because of the absence of chlorine impurities.
TiO2, but the B1g(1) peak was virtually unchanged at 399 cm21
.
An isotopic shift was also observed in the Eg(3) mode at 639
cm21. However, the other three modes were not discernible in
the Raman spectrum because the B1g(2) peak overlaps with the
A1g mode and the two other Eg modes appear far below 160
cm21. The distinctive shift of the Raman peaks in the 18O-
labelled TiO2 sample provides direct evidence for the 18O
labelling of TiO2.
To determine the relative 18O and 16O contents of the 18O-
labelled TiO2 sample, compositional SIMS depth profiling was
employed with a caesium-ion beam as the primary ion source
to enhance the negative ion intensity.16 The relative ratio of
16O and 18O isotopes is directly determined from the
intensities of 16O and 18O since their ionization yields remain
nearly constant due to their same chemical properties. As
shown in Fig. 4, 16O and 18O are evenly distributed in the 18O-
labelled TiO2 sample: the relative percentages of 16O and 18O
On the basis of these initial characterization results, we
investigated the photo-oxidation of cyclohexane with 18O-
labelled TiO2 to elucidate the involvement of the surface of the
titanium oxide in the oxidation process. The photocatalytic
reaction was performed under UV irradiation in an O2
atmosphere, yielding cyclohexanone as the major product
along with a small amount of cyclohexanol. Over 90%
selectivity towards cyclohexanone was achieved, which is
similar to that obtained in previous studies.18–20 High
selectivity towards cyclohexanone was also confirmed by the
1H NMR spectra, which are shown in Fig. S1, ESI. We carefully
3
monitored the reaction over time by analysing the 18O/16
O
ratio via mass spectrometry and gas chromatography. A
control experiment was also performed using unlabelled
TiO2 as the photocatalyst.
The top and bottom panels of Fig. 5(a) show two mass
spectra of cyclohexanone obtained from reactions over 18O-
labelled and unlabelled TiO2 catalysts, respectively. The mass
spectra of both cyclohexanones show molecular ion (M+)
peaks at m/z = 98. In addition, a small peak appears at M+1 in
each spectrum, which represents small amounts of isotopically
substituted cyclohexanones containing natural 13C and 2H
isotopes.21 However, the spectra differ significantly in their
isotopic peaks because of the 18O isotopes in the cyclohex-
anone generated over 18O-labelled and unlabelled TiO2. A
distinctive M+2 peak at m/z = 100 is evident only in the
cyclohexanone generated from the 18O-labelled TiO2 catalyst,
which clearly indicates that 18O isotopes are present in the
product. This 18O isotope experiment provides direct evidence
Fig. 3 Raman spectra of 18O-labelled TiO2 (red curve) and unlabelled TiO2
(dashed curve) excited by a 514.5 nm laser. The inset is an enlarged view of the
same Raman spectra.
9404 | RSC Adv., 2013, 3, 9402–9407
This journal is ß The Royal Society of Chemistry 2013

View Article Online
RSC Advances
Paper
Fig. 6 Cyclohexanone formation via the photocatalytic oxidation of cyclohexane
over 18O-labelled TiO2 as a function of time. The left vertical axis represents the
concentration of cyclohexanone(&) and the right axis is the percentage of 18O
isotope included in cyclohexanone(.). The horizontal axis is the UV-irradiation
time.
Fig. 5 Selected mass spectral data of (a) cyclohexanone, (b) benzaldehyde, and
(c) 4-methylbenzaldehyde produced on 18O-labelled TiO2 (red line) and
unlabelled TiO2 (black line). The asterisks mark the molecular ion peaks that
contain 18O.
irradiation time, which is shown in Fig. 6. With increasing
irradiation time, the amount of cyclohexanone produced from
the reaction over 18O-labelled TiO2 increases almost linearly
while the 18O content of the cyclohexanone drastically
decreases. These time-dependent 18O data suggest that only
surface oxygen in TiO2 interacts with cyclohexane over the
entire reaction period and bulk oxygen does not participate in
the oxidation reaction. In detail, cyclohexane initially reacts
with surface oxygen atoms to form cyclohexanone, leaving
behind oxygen vacancies on the TiO2 surface.23 Subsequently,
the oxygen vacancies that are associated with reduced titanium
cations, presumably Ti3+, are filled by 16O atoms from
atmospheric O2.24–26 As the reaction proceeds, a large number
of 16O atoms are integrated into the TiO2 surface, producing a
16O-rich TiO2 surface and eventually resulting in the produc-
tion of 16O-rich cyclohexanone. Labelling experiments also
indicate that exchange between the 18O atoms in TiO2 and the
16O atoms in molecular oxygen (O2) is less likely to occur.27–30
It is also important to note that the bulk oxygen atoms in TiO2
did not readily diffuse into the oxygen vacancies on the TiO2
surface.31–33 To show that bulk oxygen atoms did not diffuse
into the vacant sites in surface, we carefully examined the
Raman spectra of fresh and used 18O-labelled TiO2 samples. As
for the incorporation of oxygen atoms from TiO2 into
cyclohexane.
To confirm the migration of surface oxygen from TiO2 into
the product, the scope of the photo-reactions was expanded to
include toluene and para-xylene as hydrocarbon substrates.2,22
The oxidation of toluene and para-xylene yielded mainly
benzaldehyde and 4-methylbenzaldehyde, respectively, which
were evidenced by 1H NMR spectra (Fig. S2 and S3, ESI ). Other
3
products such as benzoic acid and 4-methylbenzoic acid were
not observed in the 1H NMR spectra. Their signals usually
appear at about 12.5–13.0 ppm. Accordingly, we could exclude
the possibility of formation of carboxylic acid derivatives from
the UV irradiation of toluene and para-xylene with TiO2.
As anticipated, a distinctive M+2 peak at m/z = 108 was
observed in the mass spectrum of benzaldehyde obtained from
the photo-oxidation of toluene over 18O-labelled TiO2 (top
panel of Fig. 5(b)). Fig. 5(c) shows the mass spectra of
4-methylbenzaldehyde generated from the photo-oxidation of
para-xylene over unlabelled (bottom panel) and 18O-labelled
(top panel) TiO2 catalysts. As anticipated, the presence of an
additional peak at m/z = 122 that corresponds to the M+2 peak
provides clear evidence of the participation of oxygen from the
TiO2 surface in the oxidation process. The additional 18O-
labelled experiments confirmed the migration of surface
oxygen into the oxidation products; this result suggests that
the Mars–van Krevelen mechanism is also valid for photo-
catalytic reactions at liquid–solid interfaces.
shown in Fig. S4, ESI, both Raman spectra are virtually
3
identical. This indicates that the 18O and 16O atoms are not
exchanged within the TiO2 lattice. It is thus conceivable that
oxygen vacancies are filled solely by O2 at the liquid–solid
interface. It has been shown that O2 molecules diffuse freely
across the oxide surface and dissociate to fill oxygen vacancies
on oxide surfaces.25,26,34–36
Based on the observed isotopic distribution of the 18O
atoms in cyclohexanone, we envision that the oxidation of
cyclohexane and subsequent filling of oxygen vacancies in the
TiO2 surface may occur via the following steps, which are
schematically illustrated in Fig. 7: the first step is the
formation of cyclohexanone by the interaction of cyclohexane
with surface TiO2 oxygen (18O/16O). In the subsequent step, the
To elucidate the role of the surface TiO2 oxygen in
hydrocarbon oxidation and the mechanism by which the
oxygen vacancies in TiO2 are filled, we carefully determined
the 18O content of the cyclohexanone product as a function of
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 9402–9407 | 9405

View Article Online
Paper
RSC Advances
2 H. Rezala, H. Khalaf, J. L. Valverde, A. Romero, A. Molinari
and A. Maldotti, Appl. Catal., A, 2009, 352, 234–242.
3 Q. Wang, M. Zhang, C. Chen, W. Ma and J. Zhao, Angew.
Chem., Int. Ed., 2010, 49, 7976–7979.
4 P. Mars and D. W. van Krevelen, Chem. Eng. Sci. Spec.
Suppl., 1954, 3, 41–59.
5 V. I. Avdeev and V. N. Parmon, J. Phys. Chem. C, 2009, 113,
2873–2880.
6 V. Shapovalov and H. Metiu, J. Catal., 2007, 245, 205–214.
7 S. Specchia, F. Conti and V. Specchia, Ind. Eng. Chem. Res.,
2010, 49, 11101–11111.
8 A. R. Almeida, J. A. Moulijn and G. Mul, J. Phys. Chem. C,
2011, 115, 1330–1338.
9 A. M. Khenkin, L. Weiner, Y. Wang and R. Neumann, J. Am.
Chem. Soc., 2001, 123, 8531–8542.
10 L. Nakka, J. E. Molinari and I. E. Wachs, J. Am. Chem. Soc.,
2009, 131, 15544–15554.
ˇ
´
´
11 S. Civis, M. Ferus, P. Kubat, M. Zukalova and L. Kavan, J.
Fig. 7 A schematic diagram of the reaction mechanism of the formation of
cyclohexanone on 18O-labelled TiO2.
Phys. Chem. C, 2011, 115, 11156–11162.
12 M. Niederberger, M. H. Bartl and G. D. Stucky, Chem.
Mater., 2002, 14, 4364–4370.
resultant oxygen vacancy on the TiO2 surface is filled by (16O)2
that is dissolved in the cyclohexane; the (16O)2 molecule
adsorbs on the TiO2 surface, dissociates, and fills two vacant
sites presumably via diffusion of an oxygen atom produced
from the dissociated (16O)2 molecule to a neighbouring vacant
site. Continuous reactions of cyclohexane on the TiO2 surface
eventually yield 16O-rich cyclohexanone.
13 X. Chen and S. S. Mao, Chem. Rev., 2007, 107, 2891–2959.
14 L. Kavan, M. Zukalova, M. Ferus, J. Ku¨rti, J. Koltai and
ˇ
S. Civis, Phys. Chem. Chem. Phys., 2011, 13, 11583–11586.
15 O. Frank, M. Zukalova, B. Laskova, J. Ku¨rti, J. Koltai and
L. Kavan, Phys. Chem. Chem. Phys., 2012, 14, 14567–14572.
16 K. J. Kim, J. S. Jang, D. W. Moon and H. J. Kang, Metrologia,
2010, 47, 253–261.
17 T. Yan, J. Gong and C. B. Mullins, J. Am. Chem. Soc., 2009,
131, 16189–16194.
18 C. Giannotti, S. Le Greneur and O. Watts, Tetrahedron Lett.,
1983, 24, 5071–5072.
19 G. Lu, H. Gao, J. Suo and S. Li, J. Chem. Soc., Chem.
Commun., 1994, 2423–2424.
20 P. Du, J. A. Moulijn and G. Mul, J. Catal., 2006, 238,
342–352.
21 I.-H. Nam, K. Murugesan, Y.-M. Kim, I.-H. Yang and Y.-
S. Chang, Int. J. Mass Spectrom., 2006, 252, 256–260.
22 Y. Li, B. Wen, C. Yu, C. Chen, H. Ji, W. Ma and J. Zhao,
Chem.–Eur. J., 2012, 18, 2030–2039.
23 J. F. Montoya, J. Peral and P. Salvador, ChemPhysChem,
2011, 12, 901–907.
24 H. Over, Y. D. Kim, A. P. Seitsonen, S. Wendt, E. Lundgren,
M. Schmid, P. Varga, A. Morgante and G. Ertl, Science,
2000, 287, 1474–1476.
Conclusions
In summary, we successfully prepared chlorine-free, anatase-
phase 18O-labelled TiO2 in an autoclave. By using the 18O-
labelled TiO2 as a photocatalyst, we demonstrated that the
oxygen atoms in the products generated from the photocata-
lytic oxidation of hydrocarbons originate from surface oxygen
in TiO2. It was also determined that O2 molecules dissolved in
the hydrocarbon substrate fill the oxygen vacancies in TiO2.
Our isotopic labelling results clearly demonstrate that the
Mars–van Krevelen mechanism accounts for the photo-oxida-
tion that occurs at the liquid–solid interface.
25 C. T. Campbell, Science, 2003, 299, 357.
¨
26 R. Schaub, E. Wahlstrom, A. Rønnau, E. Lægsgaard,
Acknowledgements
I. Stensgaard and F. Besenbacher, Science, 2003, 299,
377–379.
27 H. Courbon, M. Formenti and P. Pichat, J. Phys. Chem.,
1977, 81, 550–554.
28 P. Pichat, J. M. Herrmann, H. Courbon, J. Disdier and M.
N. Mozzanega, Can. J. Chem. Eng., 1982, 60, 27–32.
29 S. Sato, T. Kadowaki and K. Yamaguti, J. Phys. Chem., 1984,
88, 2930–2931.
NHH thanks the Korea Centre for Artificial Photosynthesis
(2012M1A2A2671782) and the Converging Research Centre
Program (2012K001486) funded by the Ministry of Education,
Science, and Technology through the National Research
Foundation of Korea.
30 R. Nakamura and Y. Nakato, J. Am. Chem. Soc., 2004, 126,
1290–1298.
Notes and references
1 U. Schuchardt, D. Cardoso, R. Sercheli, R. Pereira, R. S. da
31 S. Sato, J. Phys. Chem., 1987, 91, 2895–2897.
32 A. Galdikas, C. Descorme and D. Duprez, Solid State Ionics,
2004, 166, 147–155.
´
Cruz, M. C. Guerreiro, D. Mandelli, E. V. Spinace and E.
L. Pires, Appl. Catal., A, 2001, 211, 1–17.
9406 | RSC Adv., 2013, 3, 9402–9407
This journal is ß The Royal Society of Chemistry 2013

View Article Online
RSC Advances
Paper
33 P. Pichat, H. Courbon, R. Enriquez, T. T. Y. Tan and
R. Amal, Res. Chem. Intermed., 2007, 33, 239–250.
34 W. S. Epling, C. H. F. Peden, M. A. Henderson and
U. Diebold, Surf. Sci., 1998, 412–413, 333–343.
35 M. A. Henderson, W. S. Epling, C. L. Perkins, C. H. F. Peden
and U. Diebold, J. Phys. Chem. B, 1999, 103, 5328–5337.
36 M. A. Henderson, Surf. Sci. Rep., 2002, 46, 1–308.
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 9402–9407 | 9407