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supported catalyst showed appreciable styrene conversion activity
(styrene conversion of 35%) but with poor selectivity for styrene oxide
(28%); in this case, benzoic acid (36%) was found to be the major
product. The low selectivity for the benzaldehyde indicates that the
oxidation of the aldehydes to corresponding acids by TBHP is faster
than the aldehyde formation reactions. The Pd–Cu resulted in an
increase in both substrate conversion and product selectivity of the
catalyst, depending on the possible synergetic effects of alloys. This
indicates the important role played by the alloy of the catalyst in
styrene epoxidation. The Pd0.3Cu0.7 catalyst showed the best perfor-
mance with a significant improvement in both styrene conversion
(
75%) and styrene oxide selectivity (70%) after a reaction for 8 h,
19,20
which is higher than that reported in previous work.
Fig. S5 (ESI†)
shows the strong influence of the reaction time on the epoxidation of
styrene. The conversion of styrene increased continuously as the
reaction time and could reach as high as 93% for 16 h. The selectivity
to styrene oxide slightly increased from 51 to 70%. Further prolonging
the time led to a decrease in selectivity, which is mainly caused by the
isomerization of styrene oxide or over-oxidation at high temperature
Fig. 4 Cyclic voltammetric curves for (A) Pd0.5Cu0.5, (B) Pd0.7Cu0.3, (C) Pd
black catalyst on a GC electrode, in 0.5 M KOH + 1 M ethanol solution at a
À1
scan rate of 50 mV s . Current densities were normalized with reference to
2
the geometric area of a working electrode (0.07 cm ). Insets: CV curve in the
À1
absence of ethanol, scan rate: 100 mV s , reference electrode: Ag/AgCl reaction.
with saturated KCl. (D) Mass activities of Pd0.5Cu0.5, Pd0.7Cu0.3 and Pd black,
In summary, we have successfully synthesized a series of Pd–Cu
nanoalloys with tunable compositions (Pd0.2Cu0.8
Pd0.5Cu0.5, Pd0.7Cu0.3, Pd0.8Cu0.2) and controlled sizes (5.2 nm,
the mass activity of Pd based catalysts is evaluated by forward peak potential.
,
Pd0.3Cu0.7,
For electro-oxidation of ethanol, in particular, bimetallic Pd–Cu 6.8 nm, 8.1 nm, 16.4 nm, 19.9 nm). The as-prepared Pd–Cu catalysts
catalysts with a composition of Pd0.5Cu0.5 exhibited the highest mass showed much better performances toward both ethanol electro-
activity toward electrochemical oxidation of ethanol almost 4 times oxidation and styrene epoxidation. For ethanol electro-oxidation,
higher than that of Pd black and 1.7 times that of Pd Cu , thus Pd Cu exhibited the highest mass activity which is about 4 times
0.7 0.3
0.5 0.5
suggesting that bimetallic Pd–Cu catalysts with appropriate copper higher than that of Pd black and 1.7 times higher than that of
content have considerable potential as non-Pt electrocatalysts for Pd Cu . For styrene epoxidation, the as-prepared Pd Cu catalyst
0.7 0.3
0.3 0.7
DEFCs. The mechanism still needs a detailed analysis, which is part displayed much better catalytic efficiency than Pd–Cu NCs with other
of our ongoing research. Cu/Pd molar ratios. These preliminary results indicate that high-
Styrene oxide is an industrially important organic intermediate performance Pd–Cu alloyed nanocatalysts can be developed by tuning
widely used in the synthesis of fine chemicals and pharmaceuticals. the microstructures of the NCs, which are expected to have promising
Over the past few decades, many homogeneous and heterogeneous applications in fuel cells and organic reactions.
catalysts have been developed and used to catalyze the epoxidation of
Financial support of this work by the State Key Project of
18
styrene. Here, the epoxidation of styrene was used to evaluate the Fundamental Research for Nanoscience and Nanotechnology
catalytic performance of the as-prepared catalysts. Generally, benz- (2011CB932401, 2011CBA00500, 2012CB224802), the Founda-
aldehyde is a major by-product which is produced from the breaking tion for the Author of National Excellent Doctoral Dissertation
of a CQC bond followed by direct oxidation or from further oxidation of P. R. China (201321), Specialized Research Fund for the
of styrene oxide. Therefore, a good catalyst with both fast conversion Doctoral Program of Higher Education (20130002120013), and
and high selectivity is always desired. The results showing the the National Natural Science Foundation of China (21221062,
influence of composition of the as-prepared catalysts on styrene 21131004, 21390393, 21322107) is gratefully acknowledged.
conversion and product selectivity are presented in Table 1. The Pd
Notes and references
1 C. G. Hu, H. H. Cheng, Y. Zhao, Y. Hu, Y. Liu, L. M. Dai and L. T. Qu,
Adv. Mater., 2012, 24, 5493.
Table 1 Catalytic performance of the as-prepared catalysts for styrene
epoxidation
a
Selectivity (%)
Styrene
2 C. X. Xu, Y. Zhang, L. Q. Wang, L. Q. Xu, X. F. Bian, H. Y. Ma and
Y. Ding, Chem. Mater., 2009, 21, 3110.
Styrene
Benzoic
3
4
5
6
Z. Yin, W. Zhou, Y. J. Gao, D. Ma, C. J. Kiely and X. H. Bao,
Chem. – Eur. J., 2012, 18, 4887.
Q. Gao, Y. M. Ju, D. An, M. R. Gao, C. H. Cui, J. W. Liu, H. P. Cong
and S. H. Yu, ChemSusChem, 2013, 6, 1878.
K. W. Wang, W. D. Kang, Y. C. Wei, C. W. Liu, P. C. Su, H. S. Chen
and S. R. Chung, ChemCatChem, 2012, 4, 1154.
K. H. Park, Y. W. Lee, Y. Kim, S. W. Kang and S. W. Han, Chem. – Eur. J.,
Catalyst
Pd
conversion (%) oxide
Benzaldehyde acid
Others
35
28
40
70
38
48
30
31
17
44
29
36
22
9
14
18
6
7
4
4
5
Pd0.5Cu0.5 83
Pd0.3Cu0.7 75
Pd0.2Cu0.8 52
Pd0.7Cu0.3 39
2013, 19, 8053.
a
Reagents and conditions: 0.1 g catalyst, 10 mmol styrene, 20 mL
acetonitrile, 15 mmol TBHP, 80 1C, 8 h.
7 M. Yamauchi, R. Abe, T. Tsukuda, K. Kato and M. Takata, J. Am.
Chem. Soc., 2011, 133, 1150.
4590 | Chem. Commun., 2014, 50, 4588--4591
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