Preparation of highly active heterogeneous Au@Pd bimetallic catalyst using plant tannin grafted collagen fiber as the matrix

Article abstract of DOI:10.1016/j.molcata.2012.07.028

Au@Pd bimetallic nanoparticles (NPs) catalysts were synthesized by a seeding growth method using bayberry tannin grafted collagen fiber (BT-CF) as the matrix. In this method, Au³⁺ was first reductively adsorbed onto BT-CF to form Au NPs, and then they serve as the seeds for the over growth of Pd shell. The morphology of BT-CF-Au@Pd catalyst was observed by TEM and SEM, and the core-shell structure of the Au@Pd was confirmed by EDS and XRD. It was found that the as-prepared BT-CF-Au₉@Pd₃ catalyst showed excellent synergy effect in liquid-phase hydrogenation of cyclohexene, whose reaction time was three times faster than that catalyzed by BT-CF-Pd catalyst under the same conditions. Meanwhile, the BT-CF-Au₉@Pd₃ catalyst could be re-used four times without significant loss of activity. In the fourth run, the substrate conversion was still as high as 92.70%, much better than that by using commercial Pd/C catalyst (42.60%). Additionally, BT-CF-Au₉@Pd₃ catalyst exhibited high hydrogenation activity to various alkenes and nitro-compounds. For example, the TOF of allyl alcohol, styrene and nitrobenzene hydrogenations reached 10,980, 14,732 and 1379 mol mol^-1 h^-1, respectively.

Full text of DOI:10.1016/j.molcata.2012.07.028

Journal of Molecular Catalysis A: Chemical 366 (2013) 8–16
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Preparation of highly active heterogeneous Au@Pd bimetallic catalyst using plant
tannin grafted collagen fiber as the matrix
Jun Maa,b, Xin Huanga,b, Xuepin Liaoa,b, Bi Shia,∗
a Department of Biomass Chemistry and Engineering, Sichuan University, Chengdu 610065, PR China
b National Engineering Laboratory for Clean Technology of Leather Manufacture, Sichuan University, Chengdu 610065, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Au@Pd bimetallic nanoparticles (NPs) catalysts were synthesized by a seeding growth method using
bayberry tannin grafted collagen fiber (BT-CF) as the matrix. In this method, Au3+ was first reductively
adsorbed onto BT-CF to form Au NPs, and then they serve as the seeds for the over growth of Pd shell.
The morphology of BT-CF-Au@Pd catalyst was observed by TEM and SEM, and the core–shell structure
of the Au@Pd was confirmed by EDS and XRD. It was found that the as-prepared BT-CF-Au9@Pd3 catalyst
showed excellent synergy effect in liquid-phase hydrogenation of cyclohexene, whose reaction time was
three times faster than that catalyzed by BT-CF-Pd catalyst under the same conditions. Meanwhile, the
BT-CF-Au9@Pd3 catalyst could be re-used four times without significant loss of activity. In the fourth
run, the substrate conversion was still as high as 92.70%, much better than that by using commercial
Pd/C catalyst (42.60%). Additionally, BT-CF-Au9@Pd3 catalyst exhibited high hydrogenation activity to
various alkenes and nitro-compounds. For example, the TOF of allyl alcohol, styrene and nitrobenzene
hydrogenations reached 10,980, 14,732 and 1379 mol mol−1 h−1, respectively.
Received 3 March 2012
Received in revised form 18 July 2012
Accepted 22 July 2012
Available online 31 July 2012
Keywords:
Core–shell structure
Metallic catalyst
Bimetallic nanoparticles
Tannin grafted collagen fiber
Liquid-phase hydrogenation
© 2012 Published by Elsevier B.V.
1. Introduction
reusability of the catalysts. To fulfill this purpose, it is essentially
important to find a suitable matrix that metal NPs can be tightly
More recently, core–shell bimetallic NPs have attracted growing
interest in the field of catalysis because the catalytic performances
of these NPs, including activity and selectivity, can be adjusted by
varying the composition, core size and shell thickness [1–3]. In
general, the core–shell bimetallic NPs are prepared by a two-step
seeding-growth method, where one kind of metallic nanoparticles
are first synthesized and then serve as seeds for the overgrowth
of secondary metal. According to this method, some kinds of
core–shell bimetallic NPs have been successfully synthesized, such
as Ru@Pt [4], Rh@Pd [5], Au@Pd [6] and Au@Ag [7].
The core–shell bimetallic NPs are commonly supported by solid
matrices through impregnation method so that the heterogeneous
catalysts can be obtained. However, although the heterogeniza-
tion of core–shell bimetallic NPs can be achieved by impregnation
method, the supported NPs are somewhat easily detached from the
solid matrices owing to the weak interactions between the NPs and
the matrices. Moreover, aggregation of NPs may also occur during
the reactions. As a result, the catalytic activity of the catalysts is
substantially decreased during recycles. Ideally, the bimetallic NPs
should be stably anchored onto the solid matrix, which prevents
the leakage and aggregation of the metal NPs, thus providing a good
anchored on.
Plant tannins are soluble polyphenols extracted from plants and
have high affinity toward various metal species. In our previous
work [8–10], plant tannins were chemically grafted onto collagen
fibers, and then, the resultant materials were used as supporting
matrices to prepare heterogeneous Pd NPs catalyst. Due to the
anchoring effect of plant tannin to the Pd NPs, the leaching of metal
species was considerably suppressed during recycles, and accord-
ingly, the catalyst exhibited excellent reusability. Moreover, the
as-prepared Pd NPs showed extremely highly catalytic activity due
to the fibrous morphology of the supporting matrix. Consequently,
it is feasible in principle to prepare highly active and reusable het-
erogeneous bimetallic NPs catalyst by using plant tannin-grafted
collagen fiber as the supporting matrix.
Our recent studies showed that, due to the strong reduction
ability of the plant tannins, they were able to directly reduce Au3+
to Au NPs in solution, and the formed Au NPs were still well dis-
persed and stabilized by the plant tannins [11,12]. Furthermore,
quantum chemistry calculations suggested that the surface of the
Au NPs anchored by plant tannins was negatively charged because
of the electron donating–accepting interactions between the plant
tannins and the Au NPs [13]. Based on these results, we assumed
that these negatively charged Au NPs anchored by the plant tan-
nin grafted-collagen fiber could serve as the in situ seeds for the
∗
Corresponding author. Tel.: +86 28 85400356; fax: +86 28 85400356.
E-mail address: shibitannin@vip.163.com (B. Shi).
overgrowth of positively charged second metal species, like Pd2+
.
1381-1169/$ – see front matter © 2012 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.molcata.2012.07.028

J. Ma et al. / Journal of Molecular Catalysis A: Chemical 366 (2013) 8–16
9
Table 1
Therefore, it can be expected that a highly stable heterogeneous
core–shell bimetallic NPs catalyst would be conveniently prepared
by a two-step seeding-growth method using plant tannin grafted-
collagen fiber as the supporting matrix. In this way, the synthesis of
heterogeneous bimetallic Au@Pd NPs would become quite simple
since the Au@Pd NPs are in situ formed, anchored and dispersed on
the plant tannin-grafted collagen fiber, which integrates the proce-
dures of preparation and heterogenization of Au@Pd NPs. However,
the implementation of this strategy requires the rational control-
ling of molar ratio of Au3+/Pd2+ because Pd2+ would be mainly
chelated by the phenolic hydroxyls of tannin rather than adsorbed
on the surface of Au NPs if the Au3+ reductively adsorbed onto the
BT-CF does not consume all the phenolic hydroxyls of BT.
To confirm our assumption, bayberry tannin (BT), a kind of
condensed tannin, was first grafted onto collagen fiber (CF) using
glutaraldehyde as the cross-linking agent. Then, the obtained BT-
grafted CF (BT-CF) was used as supporting matrix to reductively
adsorb Au3+, followed by adsorption and chemical reduction of
different amounts of Pd2+. All the as-prepared heterogeneous BT-
CF anchored Au/Pd NPs (BT-CF-Au/Pd) catalysts were analyzed by
TEM and EDS in order to identify the size and structure of the
metal NPs. Indeed, heterogeneous BT-CF-Au/Pd catalysts with or
without Au@Pd core–shell structure were both prepared by vary-
ing the molar ratio of Au3+/Pd2+. The BT-CF-Au/Pd catalyst with
Au core/Pd shell structure (denoted as BT-CF-Au@Pd) was further
characterized by XRD and XPS analyses. Subsequently, the catalytic
hydrogenations of unsaturated organic compounds were used to
evaluate the catalytic activity of the as-prepared catalysts.
Metal compositions of BT-CF-Aux/Pdy, BT-CF-Au and BT-CF-Pd analyzed by ICP-AES.
Samples
Metal composition
Au (mmol)
Pd (mmol)
Au/Pd molar ratio
BT-CF-Au9/Pd1
BT-CF-Au9/Pd3
BT-CF-Au1/Pd1
BT-CF-Au
2.47
2.50
1.52
2.50
0
0.28
0.84
1.47
0
8.82:1
8.93:3
1:1.03
–
BT-CF-Pd
2.8
–
was obtained by filtration, followed by fully washed with deionized
water and dried in vacuum at 308 K for 24 h. Additionally, the BT-
CF-Au9/Pd3, and BT-CF-Au1/Pd1 catalysts with different molar ratio
were prepared, in which the volume of HAuCl4 solution (1.0 g/L)
was 27.0 mL, 10.5 mL, respectively. BT-CF-Au and BT-CF-Pd cat-
alysts were also prepared by the similar procedure as described
above. According to the ICP-AES analysis, the actual contents of Pd2+
and/or Au3+ of all the catalysts were determined, and summarized
in Table 1.
2.4. Characterization of catalysts
The surface morphology of the catalysts was observed by Scan-
ning Electron Microscopy (SEM, JEOL LTD JSM-5900LV). X-ray
Photoelectron Spectroscopy (XPS, Kratos XSAM-800, UK) analyses
were conducted by employing Mg Ka X-radiation (hv = 1253.6 eV)
and a pass energy of 31.5 eV. All of the binding energy peaks of
XPS spectra were calibrated by placing the principal C1s bind-
ing energy peak at 284.7 eV. The peaks from all high resolution
core spectra were fitted by XPSPEAK 4.1 software, using mixed
Gaussian–Lorentzian functions. X-ray Diffraction (XRD, Philips
X’Pert Pro-MPD) studies were performed to identify the formation
of Au/Pd crystal phase by using Cu Ka X-radiation (ꢀ = 0.154 nm).
The size and distribution of Au/Pd NPs on BT-CF were deter-
mined using Transmission Electron Microscopy (TEM, FEI-Tecnai
G2 microscope) operated at an acceleration voltage of 200 kV. The
core–shell structure of the BT-CF-Au@Pd catalysts was confirmed
using Energy Dispersive Spectroscopy (EDS). Ultra Violet Diffuse
Reflectance (UV-DR) Spectroscopy was measured using Varian Cary
5E UV-vis NIR spectrophotometer.
2. Experimental
2.1. Reagents
Collagen fiber (CF) was prepared from cattle skin according
to our previous work [10]. PdCl2, HAuCl4 (99%), borohydride
(NaBH4), glutaraldehyde (50%), and other chemicals were all ana-
lytic reagents. Bayberry tannin (BT) was purchased from a plant of
forest product in Guangxi Province (China).
2.2. Preparation of BT-grafted CF (BT-CF)
3.0 g of BT was dissolved in 100 mL of distilled water, and then
5.0 g of CF was added. The mixture was stirred at 298 K for 2 h and
then filtrated. The collected filter cake was added into 50.0 mL of
glutaraldehyde solution (2.0 wt.%) at pH 6.5, and the mixture was
stirred at 318 K for 6 h, which allows the chemical grafting of BT
onto CF. When the reaction was completed, BT-CF was collected by
filtration, fully washed with distilled water and dried in vacuum
at 308 K for 12 h. According to the ultraviolet–visible spectra mea-
surement of residual BT in solution, the grafting degree of BT on the
BT-CF was determined to be 42.39% in weight.
2.5. Catalytic hydrogenation of unsaturated organic compounds
Pd catalysts are commonly used in catalytic hydrogenation of
unsaturated organic compounds. Therefore, the catalytic hydro-
genation of cyclohexene was employed as a probe reaction to
evaluate the catalytic activity and reusability of the BT-CF-Au/Pd
catalysts. Briefly, a certain amount of BT-CF-Au/Pd catalyst (con-
taining 2.5 ␮mol metal Pd), cyclohexene (10.0 mmol) and methanol
(10.0 mL) were introduced into a 50.0 mL stainless steel autoclave
equipped with a stirring bar. Then, the catalytic hydrogenation was
conducted under 1.0 MPa of H2 at 298 K, and the obtained products
were analyzed by gas chromatography. After reaction, the catalyst
was recovered by filtration, thoroughly washed with methanol,
and then reused. For comparison, the catalytic hydrogenation of
cyclohexene was also carried out under the same experimental
conditions using BT-CF-Au, BT-CF-Pd and commercial 5% Pd/C as
catalyst, respectively.
2.3. Preparation of BT-CF-Au/Pd
1.0 g of BT-CF was suspended in 27.0 mL of HAuCl4 solution
(Au3+ = 1.0 g/L, pH = 2.0). The mixture was stirred at 303 K for 4 h,
which allows the adsorption and reduction of Au3+ onto the BT-CF,
resulting in the formation of BT-CF anchored Au NPs (BT-CF-Au).
Subsequently, 2.0 mL of PdCl2 solution (Pd2+ = 1.0 g/L) was added
into the above mixture, and the pH of the mixture was adjusted
to 4.0 using 0.1 M NaOH. The mixture was stirred at 303 K for 4 h
in order to adsorb Pd2+ onto the BT-CF-Au. After this, the mixture
was filtrated and fully washed with deionized water. The filter cake
was re-dispersed in 10.0 mL of distilled water, and then 100 mL of
0.1 M NaBH4 solution was drop-wise added into the solution in
order to reduce the Pd2+ to Pd(0). Finally, BT-CF-Au9/Pd1 catalyst
In addition, hydrogenations of 2-methyl-3-buten-2ol, acrylic
acid, a-methacrylic acid, styrene, nitrobenzene, o-nitrotoluene,
and m-nitrotoluene catalyzed by BT-CF-Au@Pd catalyst were also
carried out to evaluate the universal application of

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J. Ma et al. / Journal of Molecular Catalysis A: Chemical 366 (2013) 8–16
BT-CF-Au@Pd. The turnover frequency (TOF) of the catalysts was
calculated as:
Au NPs if the phenolic hydroxyls are consumed by the reductive
adsorption of Au3+, as shown in Scheme 1d. After reduction by
NaBH4, the Pd2+ would be reduced to Pd shell over the Au NPs, thus
forming the Au@Pd NPs (Scheme 1e). However, Pd2+ will be mainly
chelated by the phenolic hydroyxls of BT if the amount of Au3+ is
not sufficient enough to consume the major positions of phenolic
hydroxyls of BT (Scheme 1g). Under such conditions, the reduction
of Pd2+ would lead to the formation of individual Pd NPs and/or the
alloy of Au and Pd, instead of Au@Pd core–shell NPs (Scheme 1h).
hydrogenated substrate (mol)
TOF (mol mol−1 h−1) =
Pd (mol) × reaction time (h)
3. Results and discussion
3.1. Preparation of BT-CF-Au/Pd catalysts
The molecular structure of BT is shown in Scheme 1a. It mainly
consists of flavan-3-ols, in which the C6 and C8 of the A-rings
have high nucleophilic reactivity toward electrophilic reagents, like
aldehydes. Additionally, collagen fiber is also able to react with
aldehydes via the NH2 on its side chains. Therefore, we used glu-
taraldehyde as the bifunctional cross-linking agent to form bridges
between BT and CF, as shown in Scheme 1b. On the other hand,
BT has a large number of adjacent phenolic hydroxyls on its B-
rings. Our previous study has proved that the phenolic hydroxyls
of BT were able to reduce Au3+ into Au NPs [13]. Thus, it can be
assumed that the BT grafted onto CF can still reduce Au3+ to form
Au NPs, and the formed Au NPs should also be anchored onto the
BT-CF (Scheme 1c). Actually, the UV-DR spectrum of the BT-CF-Au
indicated the presence of characteristic absorption peak of Au NPs
around 520 nm (supporting information 1), which confirmed the
formation of Au NPs on BT-CF.
3.2. Characterization of the catalysts
The morphology and size distribution of the catalysts were
determined by Transmission electron micrographs (TEM). TEM
images of monometallic BT-CF-Au, BT-CF-Pd NPs and bimetallic
BT-CF-Au1/Pd1 NPs catalysts are shown in Fig. 1a–c, and the cor-
responding histograms of particle size distribution are presented
in Fig. 1d–f. As shown in Fig. 1a, the NPs in monometallic BT-
CF-Au are roughly spherical, and the corresponding particle size
are around 10.65 1.2 nm. In Fig. 1b, the monometallic BT-CF-Pd
exhibited much smaller Pd particle size with average diameter of
3.72 0.7 nm. However, the spherical NPs in BT-CF-Au1/Pd1 appear
in uneven distribution, with average sizes located around 9.75 nm
and 23.25 nm, respectively. EDS point analyses of distinct NPs in
BT-CF-Au1/Pd1 were adopted to further identify the composite dis-
tribution.
Based on our previous quantum chemistry calculations [13], the
Au NPs anchored on BT-CF is negatively charged. Therefore the pos-
itively charged Pd2+ should be mainly adsorbed on the surface of
As shown in Fig. 2, the NPs with large diameter in BT-CF-Au1/Pd1
are found to be mono-Au NPs. The small NPs are too small to be
analyzed by EDS method. However, the above ICP-AES analysis has
Scheme 1. Proposed preparation mechanism of BT-CF-Au/Pd.

J. Ma et al. / Journal of Molecular Catalysis A: Chemical 366 (2013) 8–16
11
Fig. 1. TEM images of BT-CF-Au (a), BT-CF-Pd (b) and BT-CF-Au1/Pd1 (c), and the histograms of size distribution of BT-CF-Au (d), BT-CF-Pd (e) and BT-CF-Au1/Pd1 (f).
BT-CF-Au1/Pd1, low amount of Au3+ (1.0 g/L, 10.5 mL) was
employed to synthesize Au NPs. Under such conditions, a small
amount of Au0 seeds was initially formed, which further grown
to form large Au nanoparticle along with the successive reduction
of Au3+. However, when high amount of Au3+ (1.0 g/L, 27.0 mL) was
used for the preparation of BT-CF-Au, the Au0 seeds initially formed
onto BT-CF were much more than those onto BT-CF-Au1/Pd1, and
accordingly, the further growth of Au0 seeds should be considerably
suppressed due to their large numbers. As a result, the use of high
amount of Au3+ leads to the formation of smaller Au nanoparticles.
TEM image of BT-CF-Au9/Pd1 NPs is illustrated in Fig. 3. It is
observed that the NPs in BT-CF-Au9/Pd1 have an average particle
confirmed that the molar ratio of Pd/Au is close to 1:1. Hence, it is
reasoned that the small NPs observed in Fig. 1b should be mono-Pd
NPs. That is, when the molar ratio of Au3+/Pd2+ is 1:1, the reduc-
tively adsorbed Au3+ on the BT-CF is not enough to consume all
the active phenolic hydroxyls, and accordingly, the subsequently
adsorbed Pd2+ would be mainly chelated by the residual phenolic
hydroxyls of BT, and then in situ reduced by the addition of NaBH4
to form individual Pd NPs. Interesting, the Au NPs in BT-CF-Au1/Pd1
are relatively larger than those in monometallic BT-CF-Au. Consid-
ering the preparation mechanism proposed in Scheme 1, the varied
size of Au NPs should be attributed to the differences in reduc-
tive growth of Au3+ in those catalysts. During the preparation of

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J. Ma et al. / Journal of Molecular Catalysis A: Chemical 366 (2013) 8–16
Fig. 2. EDS point analyses of a distinct NP in BT-CF-Au1/Pd1.
Fig. 3. TEM image and the histogram of size distribution of BT-CF-Au9/Pd1.
Fig. 4. EDS point analyses of a distinct NP in BT-CF-Au9/Pd1.

J. Ma et al. / Journal of Molecular Catalysis A: Chemical 366 (2013) 8–16
13
Fig. 5. TEM image and the histogram of size distribution of BT-CF-Au9/Pd3.
Table 2
Content of Au and Pd at different points of a distinct NP in BT-CF-Au9/Pd1.
Location
Content of Au (%)
Content of Pd (%)
1
2
3
95.29
6.98
48.62
4.70
93.02
51.37
size of 10.63 1.9 nm. EDS point analyses of a distinct NP in BT-
CF-Au9/Pd1 is shown in Fig. 4. The contents of Au and Pd at each
point are summarized in Table 2. Clearly, Au is rich in the interior of
the particle, while Pd is mainly located at the edge of the particle.
These results confirm a typical Au core/Pd shell structure of the NPs
in the BT-CF-Au9/Pd1 catalyst. As proposed in Scheme 1c–e, when
most of the phenolic hydroxyls of BT are consumed by the reductive
adsorption of Au3+, the subsequently adsorbed Pd2+ will be mainly
located on the negatively charged surface of Au NPs via electro-
static attraction, and then form Pd shell by the reduction of NaBH4.
Considering that the NPs in BT-CF-Au9/Pd1 have core–shell struc-
ture, the catalyst was denoted as BT-CF-Au9@Pd1 in the following
text.
To further investigate the preparation mechanism of Au@Pd
bimetallic catalyst, BT-CF-Au9@Pd3 was prepared according to the
same procedures used for preparation of BT-CF-Au9@Pd1. The mor-
phology and particle size distribution of Au9@Pd3 NPs are shown in
Fig. 5. Roughly spherical NPs are still formed in BT-CF-Au9@Pd3, and
the corresponding average particle size is 11.20 2.7 nm, which
is relatively larger than that of BT-CF-Au9@Pd1. This should be
attributed to the formation of thicker Pd shell when a higher
amount of Pd2+ is employed. To confirm this, high-resolution TEM
(HRTEM) observation of BT-CF-Au9@Pd3 was carried out. As shown
in Fig. 6, a distinct variation in contrast between the dark gold core
and the lighter palladium shell was seen, while the lattice fringes
of Pd cannot be observed, which suggests that the crystallinity of
the Pd shell is quite low.
Fig. 6. Transmission electron micrographs of BT-CF-Au9/Pd3.
The XRD patterns of BT-CF-Pd, BT-CF-Au, BT-CF-Au9@Pd1 and
BT-CF-Au9@Pd3 are shown in Fig. 7. As for BT-CF-Au, it exhibits
an amphorous peak at 22◦ that belongs to the BT-CF supporting
matrix. In addition, the characteristic peak of face centered cubic
Au is clearly observed at 39.34◦, which confirms the anchoring
of highly crystallized Au NPs on the BT-CF. With regard to the
bimetallic NPs with Au core/Pd shell structure (BT-CF-Au9@Pd1 and
BT-CF-Au9@Pd3), the characteristic peaks of face centered cubic
Au is still evident but its intensity is gradually decreased along
with the increase of Pd content in the catalysts, which presents
the core–shell nature of BT-CF-Au9@Pdx (x = 1, 3, respectively). In
addition, the characteristic peak of Pd cannot be observed in the
XRD patterns of all BT-CF-Au@Pd samples, which is possibly due to
Fig. 7. XRD patterns of BT-CF-Pd (a), BT-CF-Au (b), BT-CF-Au9@Pd1(c) and BT-CF-
Au9@Pd3 (d).

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J. Ma et al. / Journal of Molecular Catalysis A: Chemical 366 (2013) 8–16
binding energies of 84.2 and 87.8 eV, showing the presence of Au0
[14]. Another pair of doublets located at 85.0 and 89.0 eV should
belong to Au oxides [15]. In Pd 3d XPS spectrum of BT-CF-Au9@Pd3
(Fig. 9b), two peaks located at 335.8 eV and 340.9 eV, which are
attributed to elemental Pd. In Fig. 9d, the O 1s spectrum of BT-CF
shows two peaks at 531.5 and 532.8 eV, which belong to the car-
bonyl oxygen (
C O) and the phenolic hydroxyl oxygen ( C OH),
respectively. After anchoring Au/Pd NPs, a new peak appears at
533.6 eV, as shown in Fig. 9c, which confirms the electron donat-
ing/accepting interactions between BT and the NPs.
The surface morphology of the BT-CF-Au9@Pdx (x = 1, 3, respec-
tively) catalyst is shown in supporting information 2. It can be
observed that all the catalysts have well defined fibrous mor-
phology, which suggests that the fibrous structure of the natural
collagen fiber is well preserved in the BT-CF-Au9@Pdx catalysts.
Compared with monolithic catalysts, fibrous catalysts have bet-
ter geometric flexibility and lower mass transfer resistance [16,17],
and thus often exhibit high catalytic activity. Actually, many efforts
in recent years have been paid on the development of fibrous matrix
supported catalysts because of the unique mass transfer property of
this kind of catalysts [18–21]. From this view point, the high activ-
ity of the BT-CF-Au9@Pdx catalyst can be expected in the following
catalytic experiments.
Fig. 8. Survey scan of BT-CF-Au9@Pd3 in the range of 0–1100 eV.
the fact that the crystallization of Pd shell is disturbed by the BT
shell around the surface of the Au core.
3.3. Activity evaluation of the BT-CF-Au/Pd catalyst
The XPS survey scan spectrum of BT-CF-Au9@Pd3 is shown in
Fig. 8. The main components of BT-CF-Au9@Pd3, such as O and C,
exhibit strong absorption intensity, while Pd and Au show relatively
weak absorption intensity because of their low content (0.43% and
0.59%, respectively). Subsequently, the XPS analyses of individual
Au, Pd and O element in the BT-CF-Au9@Pd3 catalyst were carried
out. As shown in Fig. 9a, the Au 4f XPS spectrum of BT-CF-Au9@Pd3
consists of two pairs of doublets from spin–orbital splitting of 4f7/2
and 4f5/2. The spin–orbital splitting photoelectron was located at
We chose cyclohexene as model molecule to investigate the cat-
alytic activity of the BT-CF-Au/Pd catalysts. For comparison, the
catalytic reaction was also carried out under the same conditions
using BT-CF-Au and BT-CF-Pd as catalysts. The dependence of the
conversion of cyclohexene on reaction time is shown in Fig. 10.
BT-CF-Au had no catalytic activity for the hydrogenation of
cyclohexene under the experimental conditions. BT-CF-Pd was very
active for the hydrogenation of cyclohexene, by which the substrate
Fig. 9. Au 4f (a), Pd 3d (b) and O 1s XPS (c) spectra of BT-CF-Au9@Pd3, and O 1s XPS spectrum of BT-CF (d).

J. Ma et al. / Journal of Molecular Catalysis A: Chemical 366 (2013) 8–16
15
Table 3
Hydrogenation activity of BT-CF-Au9@Pd3 for other substrates.
Entry
1a
Substrate
Product
Molar ratio (substrate/Pd)
4000
TOF (mol mol−1 h−1
)
Allyl alcohol
10,980
14,732
2a
3a
styrene
4000
4000
Acrylic acid
10,262
6266
4a
2-Methyl-3-buten-2ol
4000
5a
6b
a-Methacrylic acid
Nitrobenzene
2000
2000
3624
1379
7c
m-Nitrotoluene
o-Nitrotoluene
2000
2000
856
355
8c
a
Solvent: methanol; pressure: 1 MPa H2; temperature: 298 K.
Solvent: methanol; pressure: 1 MPa H2; temperature: 313 K.
Solvent: methanol; pressure: 2 MPa H2; temperature: 313 K.
b
c
conversion reached nearly 100% when the reaction was conducted
for 92 min. As expected, BT-CF-Au1/Pd1 showed similar activity
and the substrate was completely converted in 90 min. However,
the catalytic activity of BT-CF-Au9@Pd1 and BT-CF-Au9@Pd3 were
much higher than that of the BT-CF-Pd. When the BT-CF-Au9@Pd1
was used, the time needed for completing conversion of the sub-
strate was reduced to 58 min, indicating an excellent synergy
effect between Pd and Au for the reaction. Moreover, when BT-
CF-Au9@Pd3 was employed as the catalyst, the time required for
completing conversion of the substrate was further decreased to
40 min, which only counts for 1/3 of the reaction time required
by using the BT-CF-Pd catalyst. Compared with BT-CF-Au9@Pd1,
the higher activity of BT-CF-Au9@Pd3 is due to the well structured
core–shell of Au@Pd.
The operating stability of the as-prepared catalyst was further
investigated, in comparison with that of commercial Pd/C catalyst.
As showed in Fig. 11, the activity of the as-prepared BT-CF-Au9@Pd3
catalyst shows no significant loss of activity even re-used for 4
times in consideration of inevitable loss of catalyst during filtra-
tion, while the activity of commercial Pd/C catalyst was rapidly
lost when re-used in the second time, which is probably due to the
leaching of Pd species from the support. The satisfactory reusability
Fig. 10. Dependence of conversion of cyclohexene on reaction time over the BT-CF-
Au/Pd catalysts. Reaction condition: metal Pd (2.5 ␮mol), cyclohexene (10 mmol),
methanol (10 mL), temperature (298 2 K) and pressure (1 MPa H2).
Fig. 11. Operating stability of BT-CF-Au9@Pd3 catalyst in catalytic hydrogenation
of cyclohexene.Reaction condition: metal Pd (2.5 ␮mol), cyclohexene (10 mmol),
methanol (10 mL), temperature (298 2 K) and pressure (1 MPa H2).

16
J. Ma et al. / Journal of Molecular Catalysis A: Chemical 366 (2013) 8–16
of the BT-CF-Au9@Pd3 catalyst should be attributed to the anchor-
ing effect of phenolic hydroxyl of BT toward the metal NPs, which
suppresses the agglomeration and leakage of NPs during catalytic
reaction process.
China (2011AA08A108), Science and Technology Department of
Sichuan Province, China (2010JY0056), and Education Department
of Sichuan Province, China (09ZB065).
To evaluate the universal application of BT-CF-Au9@Pd3 in
liquid-phase hydrogenation, its catalytic activity for hydrogena-
tion of some typical alkenes and nitro-compounds were tested.
As shown in Table 3, BT-CF-Au9@Pd3 shows high catalytic hydro-
genation activity for all the substrates containing C C bond. The
TOF of allyl alcohol, styrene and acrylic acid are 10,980, 14,732
and 10,262 mol mol−1 h−1, respectively (Entries 1–3). As expected,
the presence of methyl-substitute causes a significant steric hin-
drance in the substrate, which leads to a decreased catalyst activity,
as observed in hydrogenation of 2-methyl-3-buten-2ol and a-
methacrylic acid (Entries 4 and 5). In addition, BT-CF-Au9@Pd3 is
also effective for liquid-phase hydrogenation of nitro-compounds
(Entries 6–8). For example, the TOF of nitrobenzene hydrogenation
reaches 1379 mol mol−1 h−1 (Entry 6) when the reaction was con-
ducted under 1 MPa H2 at temperature 313 K with a substrate/Pd
molar ratio of 2000.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.molcata.2012.07.028.
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Au/Pd bimetallic catalysts were successfully prepared by a
seeding growth technique using tannin-grafted collagen fiber as
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We acknowledge the financial support provided by National
High Technology Research and Development Program 863 of