In situ synthesis of Au-Pd bimetallic nanoparticles on amine-functionalized SiO2 for the aqueous-phase hydrodechlorination of chlorobenzene

Article abstract of DOI:10.1039/c4ra07122k

Highly dispersed Au-Pd nanoparticles (NPs) with an average size of ～3.0 nm were synthesized by in situ reduction of HAuCl₄ and PdCl₂ on an amine-functionalized SiO₂ support. The structural and electronic properties of the

Full text of DOI:10.1039/c4ra07122k

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PAPER
In situ synthesis of Au–Pd bimetallic nanoparticles
on amine-functionalized SiO2 for the aqueous-
phase hydrodechlorination of chlorobenzene
Cite this: RSC Adv., 2014, 4, 48254
Xinkui Wang, ab Qinggang Liu,b Zihui Xiao,b Xiao Chen,b Chuan Shi,b Shengyang Tao,b
*
Yanqiang Huangc and Changhai Liangb
Highly dispersed Au–Pd nanoparticles (NPs) with an average size of ꢀ3.0 nm were synthesized by in situ
reduction of HAuCl4 and PdCl2 on an amine-functionalized SiO2 support. The structural and electronic
properties of the Au–Pd NPs were investigated systematically using STEM-EDX, XRD, UV-vis and XPS
spectroscopy. It is demonstrated that the composition of the Au–Pd NPs can be tuned by varying the
ratio of Au- to Pd-containing salt in solution. In the aqueous-phase hydrodechlorination of
chlorobenzene, the Au–Pd NPs showed a much higher activity and chlorine resistance than the
monometallic Au and Pd counterparts. The effects of base and solvent on the activity over the supported
Au–Pd NPs were also investigated.
Received 15th July 2014
Accepted 23rd September 2014
DOI: 10.1039/c4ra07122k
www.rsc.org/advances
mentioned a benecial effect of Au on Pd for a number of
hydrogenation reactions, such as the hydrogenation of aromatic
compounds,18 the hydrogenation of dienes,19 and the HDC of
trichloroethene.20 However, to the best of our knowledge, the
utilization of SiO2 supported Au–Pd bimetallic catalysts for the
HDC of aryl chlorides has not been reported so far.9,21
1. Introduction
Aryl chlorides have been widely used as solvents, odorizers,
insect repellents, fungicides, and intermediates in the synthesis
of organic chemicals.1 Unfortunately, these aryl chlorides have
acute toxicity and high bioaccumulation potential in the envi-
ronment. Therefore, there is growing interest to develop
methods to eliminate the toxic chlorine groups in aryl chlorides.
Among the various detoxication techniques, the catalytic
hydrodechlorination (HDC) is a preferred choice because it does
not generate toxic compounds and the remaining hydrocarbons
can be recycled.2 To date, precious metals such as Pt,3 Rh4 and
Pd,5 and transition metals such as Ni,6 Fe7 and Co8 are the most
frequently used catalysts for HDC in either liquid- or gas-phase
environments. Pd has exhibited superior performance than
other metal catalysts in terms of activity and selectivity.9
However, it suffers from deactivation due to coke deposition,10
formation of surface metal halides,11 or leaching of the active
species.12 In order to enhance the durability of Pd, a second
metal is incorporated to modify its surface geometric arrange-
ment and electronic properties. Several bimetallic catalysts
such as Pd–Cu,13 Pd–Ag,14 Pd–Pt,15 Pd–Ni,16 and Pd–Fe17 have
been reported, with improved catalytic performance over their
corresponding monometallic counterparts. Many reports have
Several methods have been proposed to prepare uniform
bimetallic nanoparticles. Wet impregnation is a conventional
method that can prepare a variety of supported Au–Pd cata-
lysts,22,23 but usually with large size and severe agglomeration of
particles. Recently, Hutchings et al. reported a novel method to
support Au–Pd nanocrystals on activated carbon via a sol-
immobilization technique.24 Nevertheless, the protecting
agents (such as PVP) must be removed by high-temperature
calcination before catalytic performance tests, which inevi-
tably leads to agglomeration of the Au–Pd nanoparticles.25 Our
earlier work reported an in situ method to anchor Au nano-
particles on the organic–inorganic hybrid silica with reducible
functional groups (–SiCH2CH2CH2NHCH2OH).26 In the present
work, we explore this method to synthesize supported Au–Pd
bimetallic catalysts. The Au–Pd bimetallic nanoparticles
exhibited high mixture homogeneity of the two components
and high dispersion on the support. The addition of Au
signicantly improved the dispersion of Pd, and the catalytic
performance for HDC of chlorobenzene was greatly improved.
aState Key Laboratory of Fine Chemicals, School of Chemistry, Dalian University of
Technology, Dalian 116024, China. E-mail: wangxinkui@dlut.edu.cn; Fax: +86-411-
84986329
2. Experimental
2.1. Catalyst synthesis
bLaboratory of Advanced Materials and Catalytic Engineering, School of Chemistry,
The organic–inorganic hybrid silica supported Au–Pd nano-
particles were prepared according to our previous work.26 First,
4.7 mL of 3-aminopropyl-triethoxy-silane (APTES) was added
Dalian University of Technology, Dalian 116024, China
cState Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, Dalian, 116023, China
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into 150 mL of deionized water. Then, 2.0 mL of formaldehyde
solution (37%) was added into the mixture and induced a rapid
hydrolysis in APTES, indicated by a white precipitate that
formed in less than 30 s. Aer stirring at room temperature for 2
h, the obtained solid was ltered and washed with an excess
amount of deionized water. The solid was then dried in vacuum
2.3. Catalyst evaluation
In a typical reaction procedure, Au–Pd/SiO2 (0.10 g) was added
to a stirred ethanol solution of chlorobenzene (CB) (0.338 g),
triethylamine (Et3N) (0.455 g, 1.5 equivalence vs. the number of
chlorine atoms) and 40 mL solvent in a 50 mL reactor equipped
with a mechanical stirring system under mild conditions (25 ꢁC,
1 atm H2). The product yield was quantied by GC-FID
(Shanghai Tianmei, GC7890II equipped with an HP-5 column
(30 m ꢃ 0.32 mm ꢃ 0.25 mm)) analysis using mesitylene as
ꢁ
at 120 C for 12 h, which yielded a solid white powder.
Au–Pd bimetallic catalysts were prepared as follows. First,
1.0 g of SiO2-org was impregnated with an aqueous solution of
PdCl2 and HAuCl4, at various Pd–Au molar ratios, at 100 ꢁC for 2
h under stirring. The mixture was ltered and washed with an
excess amount of deionized water until no chlorine ions were
detected in the ltrate. Following drying in vacuum at 120 ꢁC for
12 h, Au–Pd/SiO2 catalyst samples were obtained, with Pd–Au
molar ratios of 0.2, 1.0, and 2.0. Supported monometallic Au
and Pd NPs, employed as controls, were prepared in the same
manner. The metal loadings of all ve catalysts are shown in
Table 1.
ꢁ
internal standard. The injection-port temperature was 250 C,
the detector temperature was 280 ꢁC, and the oven temperature
ꢁ
was held at 120 C. Conversion of the CB was calculated based
on the following equation:
C0 ꢂ Ct
Conversionð%Þ ¼
ꢃ 100
C0
where C0 is the concentration of the CB before reaction (mg
Lꢂ1); Ct is the concentration of the CB aer a certain reaction
time (mg Lꢂ1).
For the recyclability test, the Au–Pd/SiO2 catalyst was sepa-
rated aer reaction from the reaction mixture by centrifugation.
The ltrate was then thoroughly washed with water and
ethanol, dried at 120 ꢁC in vacuum, and then reused as catalyst
for the next run.
2.2. Catalyst characterization
The gold and palladium contents in the Au–Pd/SiO2 catalysts
were analyzed by means of inductively coupled plasma-atomic
emission spectroscopy (ICP-AES) aer the sample was dis-
solved in aqua regia. X-ray diffraction (XRD) analysis was carried
out on a PW3040/60 X'Pert PRO (PANalytical) diffractometer (Cu
Ka X-ray source, operated at 40 kV and 50 mA). Transmission
electron microscopy (TEM) images were recorded using a FEI
Tecnai G2 Spirit microscope operated at 120 kV. Scanning
transmission electron microscopy (STEM) images were recor-
3. Results and discussion
3.1. Catalyst characterization
3.1.1. TEM/STEM. TEM images of the Au/SiO2, Pd/SiO2,
ded on a FEI Tecnai G2 F30 S-Twin microscope that was oper- and bimetallic AuPd1.0/SiO2 catalysts are presented in Fig. 1. For
ated at 300 kV. An energy-dispersive X-ray (EDX) spectroscopic all samples, the particles were uniformly dispersed on the SiO2
detecting unit was used to collect the EDX spectra for elemental support with sizes below 4 nm. Unfortunately, Au–Pd bimetallic
analysis. The specimen was prepared by ultrasonically nanoparticles cannot be determined or distinguished from the
dispersing the solid powder into ethanol and drops of the corresponding monometallic counterparts by TEM or HRTEM,
suspension were deposited onto a clean carbon-enhanced since Au and Pd have the same crystal structure and similar
copper grid and then dried in air. X-ray photoelectron spec- lattice parameters.21 In order to obtain the composition of the
troscopy (XPS) spectra were obtained with a VG ESCALAB 250 Au–Pd bimetallic nanoparticles, high-angle annular dark-eld
equipped with a monochromated Al Ka radiation source (1486.6 STEM (HAADF-STEM) and EDX were used to analyze the
eV) under a residual pressure of 10ꢂ9 Torr. The binding energy randomly selected NPs from the representative AuPd1.0 bime-
scale was referenced to C1s at 284.6 eV.
tallic catalyst. As shown by the HAADF-STEM image, the Au–Pd
Table 1 XPS Pd 3d and Au 4f binding energies (eV) and surface atomic ratios of the catalysts
Peak position [eV]
Sample
Metal loadinga (wt%)
Au0 4f7/2
Pd0 3d5/2
PdII [%]
Atomic ratio Pd–Aub
Au/SiO2
AuPd0.2/SiO2
Au 1.0 (1.1)
Au 1.0 (1.2)
84.4
84.2
—
334.1
—
6.8
—
0.14 (0.17)
Pd 0.11 (0.11)
Au 1.0 (1.2)
Pd 0.54 (0.56)
Au 1.0 (0.94)
Pd 1.08 (0.862)
Pd 0.5 (0.54)
AuPd1.0/SiO2
AuPd2.0/SiO2
83.7
84.3
—
334.3
334.6
335.0
20.9
32.1
72.3
1.0 (0.86)
1.85 (1.89)
—
Pd/SiO2
a
The theoretical metal loading of Au–Pd, and the datum in parentheses is the actual metal loading measured by ICP-AES. b The atomic ratio of Pd–
Au measured by XPS, and the datum in parentheses is the ratio value measured by ICP-AES.
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Fig. 3 XRD patterns for the catalysts (i) Au/SiO2, (ii) Pd/SiO2, (iii)
AuPd0.2/SiO2, (iv) AuPd1.0/SiO2, and (v) AuPd2.0/SiO2.
except a broad diffraction peak that is attributed to the SiO2
support. This can be explained by the fact that the lower loading
of Au and Pd NPs were highly dispersed on the support and in
small size below the detection limit of the X-ray beam.28 As the
Pd loading increased, a weak and broad diffraction peak
ranging from 38ꢁ to 40ꢁ emerged, indicating the Pd atoms
entered into the crystal lattice of Au, forming Pd–Au bimetallic
NPs rather than a mixture of Pd and Au NPs.25,29
3.1.3. UV-vis spectra. From the DR UV-vis spectrum (Fig. 4),
the Au/SiO2 sample exhibited a peak maximum at 530 nm,
which was attributed to the surface plasmon resonance (SPR) of
the Au NPs.30 However, this peak disappeared upon the addition
of Pd. It has been shown that the incorporation of Pd to the
crystal lattice of Au can change the band structure of Au,
resulting in the absence of the SPR peak.24,31,32
Fig. 1 Set of TEM images and corresponding size distribution from
particles at the (a) Au/SiO2, (b) Pd/SiO2, and (c) AuPd1.0/SiO2 catalysts.
bimetallic NPs immobilized by SiO2 were in size of 3–4 nm
(Fig. 2a), almost identical to that obtained from TEM. Both the
signal of Au and Pd were detected in all the bright spots
randomly chosen (Fig. 2b1–3), indicative of a highly intimately
mixed phase of Au–Pd had been formed in bimetallic nano-
particles rather than physical mixtures of pure Au and Pd
particles.24,27 It should be noted that the electron beam for EDX
measurements was <0.8 nm in diameter, which is much smaller
than the size of the Au–Pd NPs.
3.1.4. XPS. To obtain further information about the struc-
ture and electronic properties of the bimetallic NPs, XPS char-
acterization was performed for the three AuPdx/SiO2 catalysts.
Fig. 5 and Table 1 show the tted XPS spectra and the resulting
data, respectively.
The XPS data showed that both the peaks ascribed to Au0 4f
and Pd0 3d shied to lower binding energy with respect to the
monometallic Au/SiO2 and Pd/SiO2. Such an observation was
reported previously and attributed to the modication of the
electronic structure of Au by the incorporation of Pd atoms.33 In
the Pd 3d spectra, the peaks with the binding energies of 335.0
and 340.2 eV were attributed to the Pd0 3d5/2 and 3d3/2 in
monometallic Pd/SiO2. However, a negative shi of 0.9 eV was
observed for the Pd0 3d5/2 and 3d3/2 peaks in the AuPd0.2/SiO2.
3.1.2. XRD. Fig. 3 shows the XRD patterns of the AuPdx/
SiO2 samples. There were no obvious diffraction peaks resolved
in the XRD patterns of the Au/SiO2, Pd/SiO2, and AuPd0.2/SiO2
Fig.
2 HAADF-STEM images of as-synthesized (a) AuPd1.0/SiO2
sample and EDX point analysis spectra of the selected area 1 (b1), 2 (b2) Fig. 4 UV-vis spectra for the catalysts (i) Au/SiO2, (ii) AuPd0.2/SiO2, (iii)
and 3 (b3).
AuPd1.0/SiO2, (iv) AuPd2.0/SiO2, and (v) Pd/SiO2.
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Fig. 6 Conversion for the HDC of CB with various catalysts (a) and
proposed reaction mechanism of HDC reaction over AuPdx/SiO2 (b).
Fig. 5 XPS spectra of (a) Pd 3d and (b) Au 4f in the different catalysts.
also contribute to the enhancement of the Pd activity. Except for
the active sites on Pd, the bimetallic Pd–Au NPs could also
provide Pd–Au interfacial sites as new active sites for the HDC
reaction.9 Furthermore, the inert Au atoms can adsorb the
chlorine atoms39 and weaken the C–Cl bond, facilitating the
elimination of Cl and preventing Pd from poisoning.
Two mechanisms of the HDC reaction of aromatic chlorides
have been proposed by Likholobov and co-workers.40 If the HDC
proceeds by a radical mechanism, the formed phenyl radicals
either interact with the adsorbed hydrogen atoms to form
benzene or recombine to form diphenyl.38 Since the latter was
not detected in the current work, it is presumed that the reac-
tion proceeded as follows (Fig. 6b). Au induced the adsorption
of CB and Pd served to activate H2. The adsorbed H formed on
the Pd surface reacted with the adsorbed CB to generate
benzene and HCl. The base was used to neutralize the HCl and
inhibit the poisoning of Pd.
Further increasing the Pd loading, the negative binding energy
shi in the Pd0 peaks showed a gradual decrease to 0.7 eV for
AuPd1.0/SiO2 and 0.4 eV for AuPd2.0/SiO2. The large span of the
binding energy (BE) shis correspond to a strong interaction
between Au and Pd.34 The homogeneity of the Au–Pd bimetallic
NPs appears to be dependent on the Pd content. While Pd was
uniformly dispersed within Au at low Pd content, the incorpo-
rated Pd would be isolated from Au and exist as islands when Pd
was in excess.
In addition to Pd0, the deconvolution of the Pd 3d XPS
spectra of all the samples gave another two peaks ascribed to
the Pd2+ 3d5/2 and 3d3/2. The formation of the Au–Pd bimetallic
NPs signicantly reduced the degree of Pd oxidation, as indi-
cated by the decreased fraction of Pd2+ species. This phenom-
enon implies that the interaction between Au and Pd might
result in the modication of the electronic structure of Pd,
which facilitated the protection of Pd from oxidation.35,36 The
ratios of Pd–Au calculated from the XPS data demonstrate that
there is no obvious surface enrichment of either Pd or Au,
excluding the possibility for a core–shell structure.
3.3. Effect of base
The effect of the base in the HDC of CB was investigated over the
most active AuPd1.0/SiO2 catalyst (Fig. 7a). The results indicated
that the conversion of CB was signicantly affected by the
basicity of neutralizer. When no base was added, the conversion
of CB reached 10% within 30 min but a rapid activity loss of the
catalyst was observed in the following period. Once the
neutralizer was added, the reaction equilibrium moved to the
right and the reaction rate was accelerated. In this case, the
HDC reaction rate decreased in the order of KOH z NaOH >
Et3N > Na2CO3, consistent with the decreasing basicity of
neutralizer. This conrms that a strong base was more effective
to neutralize the HCl.14 Although Na2CO3 and Et3N have almost
3.2. HDC reaction
It has been reported that the HCl generated during the HDC
reaction could poison Pd and severely reduce the reaction rate.37
Hence, in the present case, a base (Et3N) was used to neutralize
HCl. Since the aromatic ring resonance stability enough,
benzene was produced selectively and none of the ring hydro-
genated compounds were detected under the mildness condi-
tion. The Pd/SiO2 showed a relatively low activity with the
conversion of only 78.2% CB in 5 h, whereas the Au/SiO2 was
almost not active in the HDC reaction (Fig. 6a). However, the
Au–Pd bimetallic NPs showed greatly enhanced activity
compared with the Pd/SiO2, suggesting a key role of Au in
promoting HDC activity. The optimal AuPd1.0/SiO2 catalyst
could complete the conversion of CB within 2 h. The promotion
of the activity of Pd upon the addition of the inert Au could be
explained by the dilution and electronic effects. In this case, the
Pd was diluted and formed islands on the surface defects of Au,
providing more active sites than pure Pd surface.9,38 In addition,
Fig. 7 The effect of different bases (a) and solvents (b) on the HDC of
the modication of the electronic properties of Pd by Au could CB over AuPd1.0/SiO2.
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identical basicity, the reaction rate in Et3N system was much
higher than that in Na2CO3. This might be due to the fact that
Et3N possessed a better solubility in ethanol than Na2CO3.41
4. Conclusions
Highly dispersed Au–Pd NPs with small size have been prepared
through a facile in situ reduction method. The results reveal that
high homogeneity of the two components have been formed in
all the bimetallic catalysts, and a pronounced composition-
dependent catalytic activity of AuPdx/SiO2 in the HDC reaction
was observed. Furthermore, the catalyst is easily recoverable
and can be reused several times without obvious leaching or
loss of activity. The signicantly improved catalytic perfor-
mance of the bimetallic catalyst can be ascribed to the high
dispersion and modied electronic properties of Pd. However,
the intrinsic mechanism underlying these effects still requires
further investigation.
3.4. Effect of solvent
Solvent effects on the HDC of CB were investigated over
AuPd1.0/SiO2 using KOH as the base (Fig. 7b). The results
showed that ethanol was the most preferable solvent for this
reaction, followed by methanol, i-propanol, dimethylforma-
mide (DMF), and dimethylsulfoxide (DMSO). Wang and co-
workers reported that the increase in the polarity of solvent
favored the transfer of chloride species from the catalyst
surface to the reaction liquor.42 The highest activity for the
HDC of CB was obtained in methanol rather than n-hexane
with weak polarity. However, applying strong polarity solvents
(i.e., DMF, DMSO) in the present case greatly reduced the
Acknowledgements
catalytic activity. Especially when DMSO was used as the This work was supported by the National Natural Science
solvent, no desired products were produced. This is likely due Foundation of China (20906008, 21176037, 21373037 and
to the high coordination ability of these solvents, which might 51273030) and the Fundamental Research Funds for the Central
deactivate the active sites by interacting strongly with the Universities (DUT09RC (3) 158 and DUT13RC (3) 41).
metal center.43 A desired result was obtained with the less
polar protic solvents, such as ethanol, methanol, and i-
propanol. Protic solvents have been reported to be hydrogen
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