Crystalline mesoporous γ-Al2O3 supported palladium: Novel and efficient catalyst for Suzuki-Miyaura reaction under controlled microwave heating

Article abstract of DOI:10.1016/j.catcom.2013.03.006

Crystalline mesoporous γ-Al₂O₃ has been successfully synthesized under high temperature hydrothermal conditions and used as anexcellent support for Pd(OAc)₂. The resulting γ-Al ₂O₃-Pd showed excellent catalytic activity for Suzuki-Miyaura cross-coupling reaction. As low as 0.2 mol% loading of Pd was needed to obtain cross-coupling products in good to excellent yields in aqueous DMF under microwave irradiation. Moreover, the novel catalyst can be easily recovered by filtration and reused without any diminishing observable activity after five runs.

Full text of DOI:10.1016/j.catcom.2013.03.006

Catalysis Communications 37 (2013) 27–31
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Crystalline mesoporous γ-Al O supported palladium: Novel and efficient
2
3
catalyst for Suzuki–Miyaura reaction under controlled microwave heating
Gaofeng Feng, Fujian Liu, Chen Lin, Weiting Li, Shengnan Wang, Chenze Qi ⁎
Zhejiang Key Laboratory of Alternative Technologies for Fine Chemicals Process, Shaoxing University, Shaoxing 312000, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Crystalline mesoporous γ-Al O has been successfully synthesized under high temperature hydrothermal condi-
tions and used as an excellent support for Pd(OAc) . The resulting γ-Al O –Pd showed excellent catalytic activity
2 2 3
for Suzuki–Miyaura cross-coupling reaction. As low as 0.2 mol% loading of Pd was needed to obtain cross-
coupling products in good to excellent yields in aqueous DMF under microwave irradiation. Moreover, the
novel catalyst can be easily recovered by filtration and reused without any diminishing observable activity
after five runs.
2
3
Received 12 December 2012
Received in revised form 5 February 2013
Accepted 6 March 2013
Available online 15 March 2013
Keywords:
©
2013 Elsevier B.V. All rights reserved.
Mesoporous material
Heterogeneous catalysis
Suzuki–Miyaura reaction
2 3
γ-Al O
1
. Introduction
which form complex with dissolved palladium salt [19–23]. Additionally,
a variety of inert support materials are now available to fix Pd species,
including activated carbon [24], zeolites [25], silica gel [26], porous glass
[27], polymers [28] and mesoporous materials [29]. It should be noted
that mesoporous materials used as effective catalysts supporters have
received considerable attention because of their unique characters
including large BET surface areas, uniform pore sizes and adjustable com-
positions [30–32]. However, mesoporous materials with amorphous
frameworks generally exhibited relative low stabilities, which severely
constrain their wide applications. On the other hand, mesoporous mate-
rials with crystalline frameworks usually possess high hydrothermal
stability, which is crucial for their application in environmentally friendly
catalysis, particularly when water or aqueous organic mixture was
employed as the reaction media. Therefore, the development of novel
mesoporous material with good hydrothermal stability and finding its
application in heterogeneous Suzuki–Miyaura cross-coupling reaction
are quiet desirable.
The palladium catalyzed Suzuki–Miyaura cross-coupling reaction [1]
of aryl (pseudo) halides with arylboronic acids has been proved as a
powerful tool for synthesis of biaryl compounds, which are of consider-
able importance in the fields of pharmaceuticals, agrochemicals, and
functional materials [2–4]. The main advantages of this methodology
lie in the low toxicity of boron compounds, broad tolerance of functional
groups, and wide application scope of substrates. Numerous progresses
have been made in this field [5–9]. Nowadays, through developing
novel ligands [10–13], Suzuki–Miyaura reaction could be carried out
with high reaction rate, excellent selectivity and yields under ambient
conditions. However, the ligand-promoted, or even ligand-free [14–16],
homogeneous catalysis undoubtedly suffers from reusing palladium
species and separating it from the reaction system, resulting in both con-
taminating the final products and polluting the environment. Moreover,
most ligands developed so far are usually water- and/or air-sensitive,
expensive, and difficult to make. Both disadvantages in homogenous
catalysis may result in serious negative impact on their possible industrial
applications.
Heterogeneous catalysis [17,18], featuring easy separation and
recycling, is a promising alternative. The development of heterogeneous
catalyst, particularly the ligand-free alternatives, with excellent activity
for Suzuki–Miyaura reaction is highly appealing and it represents one of
the most widely concerned directions in recent years. In this regard, Pd
immobilized onto a solid support as a complex are commonly employed;
that is the surface of the support was covalently functionalized by ligands,
We report here the high temperature hydrothermal synthesis of
crystalline mesoporous γ-Al
solid support for palladium through simple adsorption. The resulting
γ-Al –Pd is a new kind of efficient and reusable heterogeneous cat-
2 3
O , which could be used as an excellent
2 3
O
alyst in Suzuki–Miyaura cross-coupling reaction under controlled
microwave heating.
2
. Experimental
2.1. Reagents and characterization methods
All reagents and organic solvents were obtained from commer-
⁎
Corresponding author. Tel./fax: +86 575 8834 5682.
E-mail address: qichenze@usx.edu.cn (C. Qi).
2
cial resource and used without further purification. Pd(OAc) was
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.03.006

28
G. Feng et al. / Catalysis Communications 37 (2013) 27–31
purchased from TCI and ultrapure water was used in Suzuki–Miyaura
reaction.
cavity and heated at the final temperature 150 °C for 45 min. After
cooling to room temperature, the reaction mixture was filtrated
through Büshner Funnel with sintered disc. After washing with H O
2
and EtOAc several times, the catalyst was recovered and dried under
vacuum for 2 h. The recovered catalyst was used directly in the follow-
ing reactions. The filtrate was taken GC analysis or carried out column
chromatography following the procedure indicated in Section 2.2.4.
The results were listed in Table 2.
1H NMR spectra were recorded with Bruker AV-400 spectrometer.
GC analysis was performed on Agilent 7890 instrument. X-ray powder
diffraction (XRD) of samples was recorded on a Rigaku D/max2550 PC
powder diffractometer using nickel-filtered CuKα radiation; The specific
surface areas (SBET), total pore volumes, micropore volumes (Vmic) and
2
micropore area (Amic) of the samples was obtained from N adsorption
isotherms at −195.8 °C using a TristarII3020 apparatus of Micromeritics
Company. Before adsorption measurements, the samples were out-
gassed at 250 °C under vacuum for 4 h. XPS spectra were performed
on a Thermo ESCALAB 250 with Al Kα radiation, and binding energies
were calibrated using the C1s peak at 284.9 eV.
3. Results and discussion
2 3
Instead of traditional method, the present mesoporous γ-Al O was
synthesized by a new method, namely through a sequence of self assem-
bly of aluminum isopropoxide with P4VP template at room temperature
in ethanol, hydrothermal treatment at 180 °C for 24 h, and calcinations
2
.2. Preparation of the samples
at 550 °C for 5 h. Obtaining the mesoporous γ-Al
into a solution of Pd(OAc) in THF at room temperature until the solution
became colorless. After evaporating THF, washing with CH Cl , and drying
under vacuum, γ-Al supported Pd(OAc) (denoted as γ-Al –Pd)
was obtained as a pale brown powder.
X-ray diffraction (XRD) analysis was used to determine the crystal-
line structures of γ-Al and γ-Al –Pd. According to six broad diffrac-
tion peaks associated with 220, 331, 222, 400, 511, and 440 reflections in
Fig. 1, we concluded that highly crystalline γ-Al phase was formed in
our new method. Notably, after incorporated with Pd(OAc) , the
resulting γ-Al –Pd also showed the crystalline characters of γ-Al
indicating its good crystalline degree even after introduction of Pd spe-
cies. Interestingly, Pd signals in γ-Al –Pd were not found, which dem-
2 3
O , it was then put
2
.2.1. Synthesis of poly4-vinylpyridine (P4VP) template
4
2
-Vinylpyridine monomer (2.0 g) was added to a solution of AIBN
0.05 g) in ethanol (10 mL) and the mixture was rapidly heated to
0 °C. After vigorous stirring at the same temperature for 6 h, the P4VP
template (1.9 g) was obtained with molecular weight about 26,000.
2
2
(
8
2
O
3
2
2 3
O
2
O
3
2 3
O
2
.2.2. Synthesis of mesoporous γ-Al
2 3
O
P4VP (2.0 g) template was dissolved in ethanol (50 mL), and then
2 3
O
aluminum isopropoxide (5.1 g) was added into the mixture under vig-
orous stirring. After stirring the mixture at room temperature for 24 h,
ethanol was slowly evaporated by stirring at the same temperature
for 48 h to afford a brown solid. The solid was then rapidly transferred
into an autoclave and hydrothermally treated at 180 °C for 48 h. The
resulted brown solid was calcined at 550 °C for 5 h in the air to provide
2
O
2 3
2 3
O ,
2 3
O
onstrating that the Pd species showed smaller particle sizes and highly
dispersion characters. Both characters are beneficial for its application
in catalysis.
2 3
the mesoporous γ-Al O (1.2 g).
2 3 2 3
The porosity and surface area of γ-Al O and γ-Al O –Pd were mea-
2
.2.3. Synthesis of mesoporous γ-Al
Mesoporous γ-Al (1.0 g) was added to a solution of Pd(OAc)
0.04 g) in THF (20 mL) at room temperature and the resulting mixture
2
O
3
supported Pd(OAc)
2
sured by nitrogen adsorption–desorption analysis. As shown in Fig. 2,
both samples showed type IV isotherms and gave a sharp capillary con-
O
2 3
2
(
densation step at p/p
ence of mesostructure in the sample [33]. The obtained γ-Al
γ-Al –Pd have relative high BET surface areas at 202 and 183 m /g,
and uniform pore diameters at 24.0 and 22.6 nm, respectively. Com-
pared with γ-Al , the decreased surface area and small pore diameter
of γ-Al –Pd are attributed to the introduction of Pd(OAc) in γ-Al
which could increase the density of the framework and block the
mesopores of γ-Al . The high BET surface areas and abundant
would also be helpful for improving its catalytic
0
of 0.80–0.95, obviously demonstrating the pres-
was stirred at the same temperature for 5 h (the color of the solution
became colorless). THF was then slowly evaporated at the same tem-
perature for 12 h. The resulting sample was then washed with large
2 3
O
and
2
2 3
O
amount of CH
Cl
2 2
and dried under vacuum. The theoretical Pd loading
2 3
O
(
4 wt.%) was considered as the Pd content in the catalyst and the
O
2 3
2
2 3
O ,
2 3
resulting γ-Al O –Pd was then used for Suzuki–Miyaura reaction.
2 3
O
2
.2.4. General procedure for Suzuki–Miyaura cross-coupling reaction
Suzuki–Miyaura cross-coupling reactions were carried out on Biotage
2 3
mesoporosity of γ-Al O
activity.
Initiator EXP instrument with temperature measured by an IR sensor.
The microwave-assisted reaction time is the hold time at the final tem-
perature (see supporting information). A 10 mL pressure-safe vial was
charged with aryl halide (0.5 mmol), aryl boronic acid (0.75 mmol),
We then performed X-ray photoelectron spectroscopy (XPS) for
γ-Al –Pd. As depicted in Fig. 3, γ-Al –Pd showed the signals of
Al2p, C1s, Pd3d, and O1s, confirming that Pd(OAc) had been success-
fully grafted onto the framework of γ-Al . Notably, the binding ener-
gies around 338.1 and 343.1 eV were attributed to the signals of Pd
suggesting that Pd(OAc) was intact after being fixed into the γ-Al
To evaluate the catalytic activity of the novel mesoporous γ-Al
O
2 3
2 3
O
2
2 3
O
2
+
K CO
2 3
2 3
(1.0 mmol), γ-Al O –Pd (5.6 mg, 0.2 mol%), DMF (3.0 mL), and
,
2
H O (1.0 mL) sequentially. The vial was then tightly sealed with a cap
2
2 3
O .
O –Pd
2 3
containing a silicon septum. The loaded vial then placed into the micro-
wave reactor cavity and heated at the final temperature 150 °C for
catalyst, Suzuki–Miyaura cross-coupling reaction was then carried out
under microwave irradiation in aqueous DMF at 150 °C. The Pd
loading is as low as 0.2 mol% and the results were illustrated in
Table 1. Excellent isolated yields (93–95%) could be achieved with
aryl iodide and aryl bromide possessing an alkyl group (entry 1, 3
and 4). Also, the reaction was compatible with substrates containing
both electron-donating and electron-withdrawing groups. For example,
reaction of 4-iodoacetophenone with phenylboronic acid provided the
corresponding product in 81% yield (entry 2). While the reaction of
phenylboronic acid with 3-bromoanisole afforded the corresponding
biaryl product in 91% isolated yield (entry 5). Four kinds of substituted
aryl boronic acids were also tested and they all underwent smoothly,
affording the desired products in good to excellent yields (entry
7–10). However, relative lower yield (85%) for 4-chlorophenyl boronic
acid was observed, probably owing to the further reaction of product
45 min. After cooling to room temperature, the reaction mixture was
diluted with water and the resultant mixture was extracted with
EtOAc. The combined organic layer was washed with brine, dried over
anhydrous Na SO , and evaporated under reduced pressure. The residue
2 4
was purified by column chromatography over silica gel to provide the
corresponding products. The yields were listed in Table 1.
2
.2.5. General procedure for recycling test in Suzuki–Miyaura
cross-coupling reaction
A 30 mL pressure-safe vial was charged with 4-iodotoluene
(
5 mmol), phenylboronic acid (7.5 mmol), K
2
CO
3
(10.0 mmol),
γ-Al –Pd (56 mg, 0.2 mol%), DMF (12 mL), and H
2
O
3
2
O (4 mL) sequen-
tially. The vial was then tightly sealed with a cap containing a silicon
septum. The loaded vial was then placed into the microwave reactor

G. Feng et al. / Catalysis Communications 37 (2013) 27–31
29
Table 1
2 3
Catalytic activity test of γ-Al O
–Pd in Suzuki–Miyaura cross-coupling reaction.a
Entry
1
R1
H
X
I
R2
Product
Yield (%)b
4-Me
94
2
H
I
4-COMe
81
3
4
H
H
Br
Br
4-Me
95
93
4-t-Bu
5
6
7
H
Br
Cl
Br
3-OMe
3-OMe
4-Me
91
21
92
H
OMe
8
9
Cl
Br
Br
Br
4-Me
4-Me
4-Me
85
90
95
t-Bu
F
1
0
a
Reaction conditions: Boronic acid (0.75 mmol), aryl halide (0.5 mmol), K
2
CO
3
2 3 2
(1 mmol), γ-Al O –Pd (5.6 mg, containing Pd 0.001 mmol, 0.2 mol%), DMF (3 mL), H O (1 mL),
MW, 150 °C, 45 min. According to GC analysis, full conversion was observed in all entries except for entry 6.
b
Isolated yield.
with 4-chlorophenyl boronic acid, as insoluble polymer was observed
during extraction (entry 8). Inspired by the phenomenon, the reaction
of 3-chloroanisole with phenyl boronic acid was then tried. However,
the desired product was isolated in only 21% yield under the same
conditions due to the incompletion of the reaction (entry 7). It should
be noted that Suzuki–Miyaura reaction could be carried out under milder
conditions. For example, heating the mixture of 4-bromotoluene and
phenylboronic acid in aqueous DMF at 100 °C in oil bath for 15 min in
Table 2
2 3
Recycling γ-Al O
–Pd heterogeneous catalyst for Suzuki–Miyaura reaction of 4-bromotoluene and phenylboronic acid.a
Run
1
2
3
4
5
Conv. (%)
GC Yield (%)
Isolated yield (%)
100
>99
94
100
100
100
100
>99
95
>99
>99
>99
b
b
b
nd
nd
nd
a
Reaction conditions: Boronic acid (7.5 mmol), aryl halide (5.0 mmol), K
nd: Not detected.
2 3 2 3 2
CO (10 mmol), γ-Al O –Pd (56 mg, 0.2 mol%), DMF (12 mL), H O (4 mL), MW, 150 °C, 45 min.
b

30
G. Feng et al. / Catalysis Communications 37 (2013) 27–31
any diminishing observable activity after five runs. 100% conversion, 99%
Al
2
O
3
GC-yield and 95% isolated yield could be achieved, respectively, in the
fifth run. We reason that the high reactivity of the catalyst maintained
in the recycling test is probably due to the high dispersion character of
3
11
2
20
2
22
400
4
40
b
2 2 3 2
Pd(OAc) in mesoporous γ-Al O and high affinity of Pd(OAc) to
γ-Al , which may avoid aggregation and leaching of Pd species to a
certain extent during the reaction.
2 3
O
511
2 3
The high catalytic activity of γ-Al O –Pd catalyst in Suzuki–Miyaura
Al
2 3
O -Pd
reaction encouraged us to investigate the catalyst after reaction. XPS
analysis was carried out after the first Suzuki–Miyaura reaction cycle.
According to the binding energies around 335.4 and 340.6 eV instead
of original 338.1 and 343.1 eV, we could conclude that the original
Pd was basically transformed to Pd0 after reaction (see supporting
information). The result is good for the following Suzuki–Miyaura reac-
tion, because it is not necessary to transform Pd to Pd0 before the
catalytic circle. Also, ICP analysis was perform to determine the leaching
of Pd and we found that 8 wt.% of Pd was lost after five reaction cycles.
a
II
II
2
0
30
40
50
60
70
80
2
Theta (degree)
2 3 2 3
Fig. 1. XRD patterns of mesoporous γ-Al O (b) and γ-Al O –Pd (a).
4
. Conclusions
the presence of the catalyst, the reaction was quite clean and 79% GC
yield could be achieved. Extending the reaction time to 25 min, GC
yield was improved to 87%. Also, the same reaction was tried under
microwave heating at 100 °C for 10 min, 95% GC yield could be
achieved. The results indicated that microwave irradiation is beneficial
for the present transformations.
2 3
In conclusion, crystalline mesoporous γ-Al O was successfully
prepared under high temperature hydrothermal conditions and used
as a support for Pd(OAc) through simple adsorption. The resulting
γ-Al –Pd acted as a new and efficient heterogeneous catalyst in
Suzuki–Miyaura reaction under controlled microwave heating in aque-
ous DMF, even use 0.2 mol% loadings of Pd(OAc) . Moreover, it was
2
2 3
O
One of the advantages of heterogeneous catalyst is reusability,
which is of great importance in their application in industry. The
2
demonstrated that the catalyst could be recovered easily through filtra-
tion and reused five runs without loss of activity. The results imply
that γ-Al O –Pd may be a potential alternative for other palladium-
2 3
γ-Al
using phenylboronic acid and 4-bromotoluene under the standard con-
ditions (DMF-H O, 150 °C, 45 min). Although Suzuki–Miyaura reaction
2 3
O –Pd was therefore tested for recoverability and reusability
2
catalyzed cross-coupling reactions.
could be carried out under milder conditions, the hash conditions were
employed in recycling test in order to evaluate the high stability of the
catalyst. After the reaction, the catalyst was recovered through a
Acknowledgment
2
sequence of filtration, washing with H O and EtOAc, and drying under
vacuum. The recovered catalyst was used directly in the following reac-
tions without any other manipulation. According to the results illustrated
in Table 2, the γ-Al O –Pd catalyst could be recovered and reused without
2 3
Financial support by the University of Shaoxing is acknowledged
with thanks. We also thank Dr. Fujian Liu in Zhejiang University for
discussions and help.
1
1
1
400
200
000
3
offset 4cm /g
5
4
4
3
2
1
0
0
.6
.8
.0
.2
.4
.6
.8
.0
3
offset 600 (cm /g)
c
8
6
4
2
00
00
00
00
0
2
b Al O
3
Al
2
O
3
-Pd
a
0
.0
0.2
0.4
0.6
0.8
1.0
0
20 40 60 80 100 120 140 160
Pore diameter (nm)
Relative pressure (p/p0)
2
Fig. 2. Nitrogen-adsorption–desorption isotherms of γ-Al
sponding pore size distribution curve for (c) is offset by 4.0 cm /g along the vertical axis for clarity.
2
O
3
(b) and γ-Al
2
O
3
–Pd (a). The isotherm for (b) is offset by 600 m /g along the vertical axis for clarity and the corre-
3

G. Feng et al. / Catalysis Communications 37 (2013) 27–31
31
References
A
[
1] N. Miyaura, A. Suzuki, Chemical Reviews 95 (1995) 2457–2483.
O1s
[
2] M. Liu, S.-J. Su, M.-C. Jung, Y. Qi, W.-M. Zhao, J. Kido, Chemistry of Materials 24
(
24) (2012) 3817–3827.
3] P.J. Hajduk, M. Bures, J. Praestgaard, S.W. Fesik, Journal of Medicinal Chemistry 43
2000) 3443–3447.
[
(
[4] G. Bringmann, D. Menche, Accounts of Chemical Research 34 (2001) 615–624.
[5] F. Alonso, I.P. Beletskaya, M. Yus, Tetrahedron 64 (2008) 3047–3101.
[6] M. Tobisu, T. Xu, T. Shimasaki, N. Chatani, Journal of the American Chemical Society
1
33 (2011) 19505–19511.
7] Q.-L. Luo, J.-P. Tan, Z.-F. Li, W.-H. Nan, D.-R. Xiao, Journal of Organic Chemistry 77
2012) 8332–8337.
8] G.A. Molander, S.L.J. Trice, S.M. Kennedy, Journal of Organic Chemistry 77 (2012)
678–8688.
9] K. Inamoto, C. Hasegawa, K. Hiroya, Y. Kondo, T. Osako, Y. Uozumi, T. Koi, Chemical
Communications 48 (2012) 2912–2914.
10] R. Martin, S.L. Buchwald, Accounts of Chemical Research 41 (2008) 1461–1473.
11] W.-M. Dai, Y. Zhang, Tetrahedron Letters 46 (2005) 1377–1381.
[
[
[
(
C1s
8
Al2p
Pd3d
[
[
[
12] S.-S. Zhang, Z.-Q. Wang, M.-H. Xu, G.-Q. Lin, Organic Letters 12 (2012) 5546–5549.
7
00
600
500
400
300
200
100
0
[13] A. Ros, B. Estepa, A. Bermejo, E. Alvarez, R. Fernandez, J.M. Lassaletta, Journal of
Organic Chemistry 77 (2012) 4740–4750.
Binding energy (eV)
[
[
[
[
[
[
[
[
14] J.-P. Wan, C. Wang, R. Zhou, Y. Liu, RSC Advances 2 (2012) 8789–8792.
15] C. Liu, Y. Zhang, N. Liu, J. Qiu, Green Chemistry 14 (2012) 2999–3003.
16] C. Liu, Q. Ni, P. Hu, J. Qiu, Organic and Biomolecular Chemistry 9 (2011) 1054–1060.
17] Á. Molnár, Chemical Reviews 111 (2011) 2251–2320.
18] L. Yin, J. Liebscher, Chemical Reviews 107 (2007) 133–173.
19] J. Yang, P. Li, L. Wang, Synthesis (2011) 1295–1301.
20] G. Lv, W. Mai, R. Jin, L. Gao, Synlett (2008) 1418–1422.
21] K. Mandai, T. Korenaga, T. Ema, T. Sakai, M. Furutani, H. Hashimoto, J. Takada,
Tetrahedron Letters 53 (2012) 329–332.
B
Al2p
[
22] X. Jin, K. Zhang, J. Sun, J. Wang, Z. Dong, R. Li, Catalysis Communications 26 (2012)
1
99–203.
23] Z.-C. Wu, Y. Huang, Y.-N. Lu, T.-X. Tao, Z. Zhang, Catalysis Communications 29 (2012)
58–161.
[
1
[
24] G. Lu, R. Franzen, Q. Zhang, Y. Xu, Tetrahedron Letters 46 (2005) 4255–4259.
[
25] L. Djakovitch, K. Koehler, Journal of the American Chemical Society 123 (2001)
5990–5999.
[
[
26] S. Paul, H.J. Clark, Green Chemistry 5 (2003) 635–638.
27] C. Schmöger, T. Szuppa, A. Tied, F. Schneider, A. Stolle, B. Ondruschka, ChemSusChem
1
(2008) 339–347.
[
[
[
28] P. Zhang, Z. Weng, J. Guo, C. Wang, Chemistry of Materials 23 (2011) 5243–5249.
29] H. Yang, G. Li, Z. Ma, Journal of Catalysis 276 (2010) 123–133.
30] Y. Wan, D.Y. Zhao, Chemical Reviews 107 (2007) 2821–2860.
8
2
80
78
76
74
72
70
68
66
[31] F.J. Liu, L. Wang, Q. Sun, L.F. Zhu, X.J. Meng, F.-S. Xiao, Journal of the American
Binding energy (eV)
Chemical Society 134 (2012) 16948–16950.
[
32] F.J. Liu, T. Willhammar, L. Wang, L.F. Zhu, Q. Sun, X.J. Meng, W. Carrillo-Cabrera,
X.D. Zou, F.-S. Xiao, Journal of the American Chemical Society 134 (2012) 4557–4560.
33] D.Y. Zhao, Q.S. Huo, J.L. Feng, B.F. Chmelka, G.D. Stucky, Journal of the American
Chemical Society 120 (1998) 6024–6036.
[
Pd3d
5
/2
C
Pd3d
3/2
3
50 348
346 344 342
340 338 336 334
332 330
Binding energy (eV)
2 3
Fig. 3. XPS spectra of γ-Al O –Pd.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.catcom.2013.03.006.