Titania-photocatalyzed transfer hydrogenation reactions with methanol as a hydrogen source: Enhanced catalytic performance by Pd-Pt alloy at ambient temperature

Article abstract of DOI:10.1002/cctc.201300905

Hydrogenation reactions are of great importance in scientific research and in industry productions. Herein, we designed a novel system to realize photocatalytic transfer hydrogenation by using solar light as the energy input and methanol as the hydrogen source. In this reaction, titania loaded with Pd-Pt bimetallic alloy nanocrystals as a cocatalyst exhibited photocatalytic performance that was remarkably superior to that exhibited by titania with Pd or Pt alone as the cocatalyst. This work has shed light on the rational design of multifunctional catalysts through selecting appropriate bimetallic alloys as efficient cocatalysts. Light up, as if you have a catalyst: Photocatalytic transfer hydrogenation is efficiently realized on Pd-Pt/TiO₂ under mild reaction conditions with the use of light irradiation as the energy input and methanol as the hydrogen source at ambient temperature. The Pd-Pt alloy cocatalyst exhibits enhanced catalytic performance relative to that of the monometallic Pd or Pt component. Copyright

Full text of DOI:10.1002/cctc.201300905

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DOI: 10.1002/cctc.201300905
Titania-Photocatalyzed Transfer Hydrogenation Reactions
with Methanol as a Hydrogen Source: Enhanced Catalytic
Performance by Pd–Pt Alloy at Ambient Temperature
[a]
[a]
[a]
[a, c]
[a, b, c]
Yubao Zhao, Feng Pan, Hui Li, Guo Qin Xu,
and Wei Chen*
Hydrogenation reactions are of great importance in scientific
research and in industry productions. Herein, we designed
a novel system to realize photocatalytic transfer hydrogenation
by using solar light as the energy input and methanol as the
hydrogen source. In this reaction, titania loaded with Pd–Pt bi-
metallic alloy nanocrystals as a cocatalyst exhibited photocata-
lytic performance that was remarkably superior to that exhibit-
ed by titania with Pd or Pt alone as the cocatalyst. This work
has shed light on the rational design of multifunctional cata-
lysts through selecting appropriate bimetallic alloys as efficient
cocatalysts.
However, as a result of the low reduction potential of the
electrons in the photoinduced conduction band [À0.5 V vs.
normal hydrogen electrode (NHE)] and the strong oxidizing
power of the valence band holes (+2.7 V vs. NHE), titania-pho-
tocatalyzed reduction reactions are not as popular as oxidation
[5,12,16]
reactions.
To realize efficient photocatalytic reduction re-
actions, it is crucial to enhance the reducing power of the cata-
lyst. It has been found that loading of noble metals as cocata-
lysts on titania can significantly improve the reducibility of the
[17,18]
titania photocatalyst.
By choosing appropriate noble
metals as cocatalysts, some unsaturated organic functional
groups are expected to be effectively reduced under light illu-
mination. In addition, photocatalytic reductions are always ac-
[19]
The selective hydrogenation of organic functional groups is of
great significance for the industrial production of fine chemi-
cals, pharmaceutical intermediates, and many other prod-
companied by oxidation reactions. Methanol, which may be
derived from biomass, can play a dual role and act as both the
sacrificial agent and the effective hydrogen donor. This route,
if realized, will possess desirable advantages, such as the use
of a green and sustainable energy source, mild reaction condi-
tions, and a transfer hydrogenation mechanism.
[
1,2]
ucts.
Traditional heterogeneous hydrogenation reactions
always employ high temperatures and pressurized molecular
hydrogen, which clearly cause mass energy consumption and
explosion hazards in operation. A mild heterogeneous hydro-
genation process is highly favorable from the perspective of
As to the noble metal cocatalyst, platinum (Pt) has been
widely applied in titania-photocatalyzed water splitting for its
high activity towards the reduction of protons to produce hy-
drogen, whereas palladium (Pd) is an efficient catalyst in hy-
drogenation reactions with the use of molecular hydrogen
[
3,4]
academic research and practical applications.
Titania photo-
catalysis, owing to its mild reaction conditions and the sustain-
ability of the energy source (solar light) employed, has attract-
ed intensive research interest and has been widely applied in
the elimination of environmental pollutants, in organic synthe-
[17]
under high temperatures. By combining titania with the Pt
and Pd cocatalysts, photocatalytic transfer hydrogenation reac-
tions could be realized under mild reaction conditions. More-
over, as a result of the different work functions of Pt and Pd,
electron migration can happen between closely contacted Pt
and Pd clusters, which thereby leads to the formation of rela-
tively positively charged Pt sites (electron deficient) and nega-
tively charged Pd sites (electron redundant) on the surface of
[
5–10]
sis, and in hydrogen evolution.
Among various applica-
tions, titania-photocatalyzed organic synthesis routes are espe-
cially appealing because of novel reaction mechanisms. Various
types of these reactions have been realized through photoca-
talysis, such as oxidation, reduction, isomerization, and
[
11–15]
polymerization.
[20–22]
the Pd–Pt alloy particles.
Such heterogeneity of the sur-
face electron distribution can positively contribute to en-
[23,24]
hanced catalytic activity.
Therefore, the Pd–Pt alloy is ap-
[a] Dr. Y. Zhao, Dr. F. Pan, Dr. H. Li, Prof. G. Q. Xu, Prof. W. Chen
Department of Chemistry
National University of Singapore
parently a suitable cocatalyst for titania to realize photocatalyt-
ic transfer hydrogenation reactions efficiently.
3
Science Drive 3, 117543 (Singapore)
Herein, for the first time, we report studies on photocatalytic
transfer hydrogenation reactions catalyzed by titania with
a Pd–Pt alloy cocatalyst. A series of organic chemicals with dif-
ferent types of unsaturated functional groups were selectively
reduced with hydrogen derived from methanol under mild re-
action conditions, that is, light irradiation and ambient temper-
ature. In these reactions, the cocatalyst, titania loaded with the
Pd–Pt alloy in a weight ratio of 7:3, exhibited remarkably en-
hanced photocatalytic performance relative to that exhibited
by pure Pd- or Pt-loaded titania. The hydrogen-transfer reac-
E-mail: phywchen@nus.edu.sg
[
b] Prof. W. Chen
Department of Physics
NUS Environmental Research Institute
National University of Singapore
117542 (Singapore)
[
c] Prof. G. Q. Xu, Prof. W. Chen
National University of Singapore (Suzhou) Research Institute
3
77 Lin Quan Street, Suzhou Industrial Park, Jiang Su, 215123 (China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300905.
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tion mechanism was also confirmed by hydrogen isotope
tracer studies.
The titania used in this work was P25, which is commercially
available and composed of an anatase–rutile mixed phase. The
photocatalyst (Pd–Pt/TiO ) was prepared by the photoreduc-
2
tion method (see the Experimental Section for details). The
amount of alloy loaded was 1.2 wt%, and the photocatalysts
with different Pd/Pt weight ratios are denoted by Pd Pt
/
x
100Àx
TiO , such as Pd Pt /TiO , which indicates that the titania was
2
70 30
2
loaded with 1.2 wt% of the alloy with a Pd/Pt weight ratio of
:3.
The average titania crystal size was 25.7 nm [calculated from
the anatase (101) diffraction peak by using the Scherren equa-
7
Figure 2. Diffuse reflectance UV/Vis spectra of the photocatalysts.
2
À1
tion], and the BET surface area of titania was 44.8 m g (Fig-
ure S1, Supporting Information). As shown in Figure 1b, the
lattice distance of 0.35 nm corresponds to the anatase (101)
and (011) planes with a dihedral angle of 828. Rutile crystals
with (110) lattice spacing of 0.33 nm are also clearly observed.
Because Pd and Pt share the same facet-centered cubic (fcc)
structure and because their lattice constant is almost identical
with only 0.77% mismatch, they can easily form bimetallic
nanocrystal alloys if their ions exist in the reducing environ-
[
25]
ment.
In the high-resolution TEM images of Pd/TiO2,
Pd Pt /TiO , and Pt/TiO , the Pd, Pd–Pt alloy, and Pt particles
70
30
2
2
on titania are all approximately the same in size, that is, 5 nm.
Figure S2 show the X-ray photoelectron spectroscopy (XPS)
patterns of the photocatalysts. The peaks located at binding
energies of 334.8 and 70.8 eV correspond to Pd3d5/2 and
Pt5f , respectively.
Figure 3. Catalytic performance of Pd
weight ratios in the transfer hydrogenation of acetophenone.
x
2
Pt100Àx/TiO with different Pd–Pt
7
/2
As shown in the UV/Vis spectra (Figure 2), the absorbance of
Pd/TiO and Pt/TiO in the UV region is lower than that of pure
2
2
titania. However, with the bimetallic alloy as the cocatalyst, the
absorbance of Pd Pt /TiO in the UV region increased dramat-
Pd Pt /TiO with a conversion of 94.3% and a selectivity for
70
30
2
70 30
2
ically relative to that of pure Pt- or Pd-loaded titania and pure
titania. These changes in absorbance result from the formation
1-phenylethanol of 92.3% within 2.5 h of light irradiation. Fur-
ther increases in the percentage of Pt in the alloy resulted in
decreased catalytic activity of the photocatalysts. On Pt/TiO2,
the transfer hydrogenation reaction proceeded with a very low
yield of the product. The above investigations clearly con-
firmed the feasibility of the photocatalytic transfer hydrogena-
tion reaction and the enhancement resulting from the alloy co-
catalyst in this process.
[
23,26]
of the alloy structure.
Moreover, the enhanced UV absorb-
ance of the sample, in photocatalysis, is of great significance
[
27]
for improved photocatalytic performance.
Reduction of acetophenone was chosen as the model reac-
tion to test the feasibility of the photocatalytic transfer hydro-
genation reaction [Eq. (1)]. With methanol as the hydrogen
donor, acetophenone was selectively reduced to 1-phenyletha-
nol on Pd/TiO upon irradiation at room temperature (Figure 3,
2
Table S1). As the percentage of Pt in the Pd–Pt alloy was in-
creased, the photocatalytic activity of Pd–Pt/TiO increased dra-
2
matically, and the optimum performance was obtained on
To further confirm the univer-
sality of this photocatalytic
transfer hydrogenation reaction
with methanol as the hydrogen
source and the beneficial effect
of the alloy, Pd/TiO , Pd Pt /
2
70 30
TiO , and Pt/TiO were used in
2
2
the reduction of a series of sub-
strates with unsaturated car-
bonyl and alkene functional
groups. As shown in Table 1,
2 2 2
Figure 1. High-resolution TEM images of the photocatalysts: a) Pd/TiO , b) Pd70Pt30/TiO , and c) Pt/TiO .
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with a high selectivity of 99%
on Pd/TiO and Pd Pt /TiO . Al-
Table 1. Photocatalytic transfer hydrogenation reactions of
photocatalysts.
a
series of organic substrates on the
Conversion [mol%] (Selectivity [%])
[
a]
2
70 30
2
though Pt-loaded titania was
not active in this hydrogenation
reaction, with the addition of Pt
in Pd, the conversion on
Pd Pt /TiO increased almost
Entry Reaction Substrate
time [h]
Product
Pd/TiO
2
Pd70Pt30/TiO
2
Pt/TiO
2
39.3
94.3
5.0
70
30
2
1
2
2.5
6.5
(
(
97.9)
(92.3)
(31.1)
10-fold relative to the conver-
sion on Pd/TiO . This transfer
2
hydrogenation reduction reac-
tion was also applied to some
alkenes, such as styrene and
12.1
63.6
(92.3)
5.2
(51.3)
99.1)
1,3-cycloheptadiene
(Table 1,
entries 6 and 7). As to styrene
oxide, selectivity to the direct 2-
phenylethanol hydrogenation
product was low, and the main
product resulted from conden-
sation reactions of the reduc-
tion intermediates with metha-
nol (Table 1, entry 8). An acetyl
group connected to a hetero-
aromatic ring was also selective-
4
81.8)
0.5
98.3
(90.6)
46.1
(99.0)
3
4
2
(
(
23.8
99.8
(99.0)
4.2
(16.9)
1.5
96.5)
6.9
63.0
(99.0)
0
(–)
5
6
7
6
(
(
(
99.0)
ly reduced to
a
hydroxy
17.2
99.7
(95.2)
7.7
(88.5)
2
group with the formation of
minor condensation byproducts
88.5)
(
Table 1, entry 9). Most impor-
69.1
99.8
(96.2)
6.3
(88.9)
tantly, in all of the transfer hy-
drogenation reactions, Pd Pt /
2.25
96.0)
7
0
30
TiO2 clearly exhibited photoca-
talytic performance that was su-
perior to that of pure Pt- or Pd-
loaded titania, which firmly vali-
dates the enhanced effect of
the alloy cocatalyst in the trans-
fer hydrogenation reaction. This
enhancement effect may result
from the synergistic effect of
the Pt and Pd sites in the pro-
duction of the active hydrogen
species and in the addition of
hydrogen to the substrate, re-
spectively. In addition, the
aforementioned surface elec-
tron heterogeneity and the im-
proved UV absorbance also con-
tribute to the enhanced photo-
catalytic activity of titania
loaded with a Pd–Pt bimetallic
alloy cocatalyst.
9
5.9
99.8
(A: 30.9,
B: 54.2)
87.2
(A: 11.6,
B: 77.7)
8
4
(A: 15.0,
B: 63.6)
72.9
95.6
(A: 89.0,
B: 5.4)
71.8
(A: 51.1,
B: 19.4)
9
4.5
(A: 68.9,
B: 11.6)
[
(
a] Reaction conditions: The reaction mixture was prepared by adding photocatalyst (9 mg), substrate
0.1 mmol) and toluene (0.1 mmol, internal standard) in methanol (5 mL). The mixture, in a 10 mL Pyrex glass
vial, was deoxygenated through argon bubbling. The mixture was magnetically stirred and irradiated with
a 300 W xenon lamp (PE300BF).
several aromatic ketones were hydrogenated to produce the
corresponding aromatic alcohols with high selectivity (Table 1,
entries 1–4). The methyl-substituted aromatic ketone (Table 1,
entry 2) was less reactive, but a longer reaction time led to
a higher conversion without severely decreasing the selectivity
of the product. In terms of aliphatic ketones (Table 1, entry 5),
With hydrogen isotope tracing studies, we were able to un-
derstand the reaction mechanism of this photocatalytic hydro-
genation reaction more clearly. Reduction of acetophenone
was employed as a model reaction. With CH OH as the hydro-
3
gen donor, the mass spectrum of 1-phenylethanol presents
+
a molecular ion peak at m/z=122 [M ]. With CH OD as the hy-
3
+
2-methylcyclohexanone was reduced to 2-methylcyclohexanol
drogen donor, the molecular ion peaks shift to [M +1] and
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+
+
[
M +2]. The [M +1] peak corresponds to 1-phenylethanol,
Experimental Section
which results from the reduction of the carbonyl group of ace-
tophenone by the addition of one deuterium atom and one
General
+
hydrogen atom; the [M +2] peak is the product obtained
All chemicals were from Sigma–Aldrich except P25, which was
from Thermal Fisher Scientific (Acros Organics). The catalysts were
prepared by a photoreduction reaction. In a typical synthesis, P25
after hydrogenation of acetophenone by two deuterium atoms
(
Figure S3).
(0.5 g) was dispersed in water (70 mL) in a 100 mL Pyrex glass reac-
According to all of the above investigations, we tentatively
tor. Methanol (10 mL) was added to the dispersion as a sacrificial
agent. The reactor was sealed and deoxygenated through argon
bubbling for 40 min with a flow rate of 100 mLmin . The reactor
proposed a reaction mechanism for the photocatalytic transfer
hydrogenation reaction (Figure 4). Upon irradiation, photoin-
À1
was then illuminated with a 300 W xenon lamp (PE300BF) under
À1
magnetic stirring. A solution of H PtCl (1.36 mg PtmL , 1.296 mL)
2
6
À1
and a solution of PdCl2 (1.36 mg PdmL , 3.024 mL) was mixed
and deoxygenated through argon bubbling in a sealed glass vial,
and this solution was injected into the Pyrex reactor by using a sy-
À1
ringe pump with a flow rate of 0.1 mLmin . The reactor was illu-
minated for 5 h. After the reaction, the catalyst sample was centri-
fuged and washed with water until the filtration was neutral in pH.
The sample was then dried in a vacuum at 808C for 6 h.
Photocatalytic reactions and product analysis
The reaction mixture was prepared by adding photocatalyst
(9 mg), substrate (0.1 mmol) and toluene (0.1 mmol, internal stan-
dard) in methanol (5 mL). The mixture, in a 10 mL Pyrex glass vial,
was sealed and deoxygenated through argon bubbling for 10 min
À1
with an argon flow rate of 50 mLmin . The mixture was magneti-
cally stirred and irradiated with a 300 W xenon lamp (PE300BF) for
2.5 h at room temperature. The product was analyzed by using an
Agilent GC7890A and Shimadzu GCMS-TQ8030 triple quadrupole
gas chromatograph mass spectrometer.
Figure 4. Proposed reaction mechanism for the photocatalytic transfer hy-
drogenation reduction of acetophenone.
À
duced conduction band electrons (e ) and valence band holes Acknowledgements
+
(
h ) are generated in the titania crystals. The photoinduced
electrons are transferred to the noble metal cocatalyst, owing
to the lower Fermi energy level of Pt and Pd relative to the
The authors acknowledge the support by Singapore NRF CREATE-
SPURc project, MOE grants R143-000-542-112 R143-000-505-112,
and the NUS SPORE program.
[
17,28]
conduction band of titania.
The valence band holes with
strong oxidizing power diffuse to the surface of the titania par-
ticles to initiate the oxidation of the adsorbed methanol into
formaldehyde or other deep-oxidation products, and this is ac-
companied by the release of hydrogen ions. The photoinduced
electrons on the surface of the noble metal reduce the protons
into hydrogen atoms, which results in the formation of surface
hydride species that are ready to react with the unsaturated
C=O or C=C groups of the surface-adsorbed substrate mole-
cules. The energy of the photons (hn) powers the catalytic
cycle, which constantly transfers hydrogen from methanol to
the target substrate.
Keywords: hydrogenation
reduction · titania
· methanol · photocatalysis ·
[
[
1] R. A. Sheldon, H. Bekkum, Fine Chemicals through Heterogeneous Cataly-
sis, Wiley-VCH, Weinheim, 2007, pp. 351–471.
2] a) T. Mallat, A. Baiker, P. Ratnasamy, A. P. Singh, S. Sharma, H. W. Kou-
wenhoven, H. van Bekkum, M. H. W. Burgers, M. Kraus, C. T. O’Connor,
M. P. McDaniel, A. Baker, H. U. Blaser, U. Kragl, C. Wandrey, Handbook of
Heterogeneous Catalysis, Wiley-VCH, Weinheim, 2008, pp. 2334–2447;
b) C. Descorme, P. Gallezot, C. Geantet, C. George, ChemCatChem 2012,
4, 1897–1906.
[
[
3] G. Brieger, T. J. Nestrick, Chem. Rev. 1974, 74, 567–580.
4] R. A. W. Johnstone, A. H. Wilby, I. D. Entwistle, Chem. Rev. 1985, 85, 129–
In conclusion, heterogeneous photocatalytic transfer reduc-
tions were realized on Pd–Pt/TiO with methanol as the hydro-
2
170.
gen donor; this initiated a novel strategy of combining light
energy and methanol (possibly from biomass) to realize a sig-
nificant hydrogenation process under mild reaction conditions.
Moreover, TiO2 loaded with Pd–Pt alloy nanocrystals at
a weight ratio of 7:3 exhibited enhanced photocatalytic activity
[5] C. Chen, W. Ma, J. Zhao, Chem. Soc. Rev. 2010, 39, 4206–4219.
[
6] Y. Zhao, W. Ma, Y. Li, H. Ji, C. Chen, H. Zhu, J. Zhao, Angew. Chem. 2012,
24, 3242–3246; Angew. Chem. Int. Ed. 2012, 51, 3188–3192.
1
[
7] a) J. Lyu, L. Zhu, C. Burda, ChemCatChem 2013, 5, 3114–3123; b) W.
Zhao, W. Ma, C. Chen, J. Zhao, Z. Shuai, J. Am. Chem. Soc. 2004, 126,
4782–4783.
[
[
8] S. Yurdakal, G. Palmisano, V. Loddo, V. Augugliaro, L. Palmisano, J. Am.
Chem. Soc. 2008, 130, 1568–1569.
9] X. Chen, L. Liu, P. Y. Yu, S. S. Mao, Science 2011, 331, 746–750.
relative to that of Pd/TiO . This enhancement effect of the Pd–
2
Pt alloy on the photocatalytic performance of the catalyst also
shed light on the design of other bimetallic alloy catalysts with
enhanced catalytic performance.
[
10] a) Y. Zhao, F. Pan, H. Li, D. Zhao, L. Liu, G. Q. Xu, W. Chen, J. Phys. Chem.
C 2013, 117, 21718–21723; b) Z. Zheng, J. Zhao, H. Liu, J. Liu, A. Bo, H.
ꢀ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 454 – 458 457

CHEMCATCHEM
COMMUNICATIONS
www.chemcatchem.org
Zhu, ChemCatChem 2013, 5, 2382–2388; c) J. W. Shi, X. Zong, X. Wu,
H. J. Cui, B. Xu, L. Wang, M. L. Fu, ChemCatChem 2012, 4, 488–491.
11] G. Palmisano, V. Augugliaro, M. Pagliaro, L. Palmisano, Chem. Commun.
[21] I. Moysan, V. Paul-Boncour, S. Thiebaut, E. Sciora, J. M. Fournier, R.
Cortes, S. Bourgeois, A. Percheron-Guegan, J. Alloys Compd. 2001, 322,
14–20.
[
2
007, 3425–3437.
[22] J. L. Rousset, J. C. Bertolini, P. Miegge, Phys. Rev. B 1996, 53, 4947–4957.
[23] S. Sarina, H. Zhu, E. Jaatinen, Q. Xiao, H. Liu, J. Jia, C. Chen, J. Zhao, J.
Am. Chem. Soc. 2013, 135, 5793–5801.
[
[
12] M. A. Fox, M. T. Dulay, Chem. Rev. 1993, 93, 341–357.
13] X. Lang, H. Ji, C. Chen, W. Ma, J. Zhao, Angew. Chem. 2011, 123, 4020–
4
023; Angew. Chem. Int. Ed. 2011, 50, 3934–3937.
14] M. Zhang, Q. Wang, C. Chen, L. Zang, W. Ma, J. Zhao, Angew. Chem.
009, 121, 6197–6200; Angew. Chem. Int. Ed. 2009, 48, 6081–6084.
15] J. Zhao, Z. Zheng, S. Bottle, A. Chou, S. Sarina, H. Zhu, Chem. Commun.
013, 49, 2676–2678.
16] Q. Chen, W. Ma, C. Chen, H. Ji, J. Zhao, Chem. Eur. J. 2012, 18, 12584–
2589.
[24] M. Chen, D. Kumar, C. W. Yi, D. W. Goodman, Science 2005, 310, 291–
293.
[25] H. Zhang, M. Jin, Y. Xia, Chem. Soc. Rev. 2012, 41, 8035–8049.
[26] Y. Sugano, Y. Shiraishi, D. Tsukamoto, S. Ichikawa, S. Tanaka, T. Hirai,
Angew. Chem. 2013, 125, 5403–5407; Angew. Chem. Int. Ed. 2013, 52,
5295–5299.
[27] A. L. Linsebigler, G. Lu, J. T. Yates, Chem. Rev. 1995, 95, 735–758.
[28] J. Disdier, J. M. Herrmann, P. Pichat, J. Chem. Soc. Faraday Trans. 1 1983,
79, 651–660.
[
[
[
[
2
2
1
17] W. N. Wang, W. J. An, B. Ramalingam, S. Mukherjee, D. M. Niedzwiedzki,
S. Gangopadhyay, P. Biswas, J. Am. Chem. Soc. 2012, 134, 11276–11281.
18] X. Chen, S. Shen, L. Guo, S. S. Mao, Chem. Rev. 2010, 110, 6503–6570.
19] A. Fujishima, T. N. Rao, D. A. Tryk, J. Photochem. Photobiol. C 2000, 1, 1–
[
[
21.
[20] Y. Jiang, R. Bꢁchel, J. Huang, F. Krumeich, S. E. Pratsinis, A. Baiker, Chem-
Received: October 23, 2013
SusChem 2012, 5, 1190–1194.
Published online on January 2, 2014
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