PdAu alloy nanoparticles supported on nitrogen-doped carbon black as highly active catalysts for Ullmann coupling and nitrophenol hydrogenation reactions

Article abstract of DOI:10.1039/c9ra01685f

Noble metal-based catalysts have been proven to be active for catalytic organic reactions. The selectivity and conversion can be improved by integration with proper carrier materials, and further modulated by tuning the composition as well as the electronic structure of the active noble metals. Compared with unsupported monometallic catalysts, the synergistic interactions between neighboring metals and the combined effects between the carrier materials and the active components often give rise to positive influences on the enhancement of the catalytic efficiency and selectivity. In this work, we report a facile process for the fabrication of nitrogen-doped carbon black (NCB) supported PdAu bimetallic nanoparticles (NPs) with a uniform dispersion and narrow size distribution. The PdAu/NCB catalyst with a Pd/Au mole ratio of 1/1 shows the highest activity towards both Ullmann coupling reactions of aryl halides and the hydrogenation reaction of nitrophenols. Moreover, this bimetallic catalyst also exhibits a superior recycling durability to that of monometallic Pd/NCB and Au/NCB catalysts. The enhanced catalytic performance of the bimetallic catalyst is mainly due to the large BET specific surface area (125.45 m² g^-1) and the synergy between the individual components of the catalyst.

Full text of DOI:10.1039/c9ra01685f

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PdAu alloy nanoparticles supported on nitrogen-
doped carbon black as highly active catalysts for
Ullmann coupling and nitrophenol hydrogenation
reactions†
Cite this: RSC Adv., 2019, 9, 17812
ab
a
Jiawei Xia,‡a Xinglong Zhangb and Yongsheng Fu
*
Fengyan Han,
‡
Noble metal-based catalysts have been proven to be active for catalytic organic reactions. The selectivity
and conversion can be improved by integration with proper carrier materials, and further modulated by
tuning the composition as well as the electronic structure of the active noble metals. Compared with
unsupported monometallic catalysts, the synergistic interactions between neighboring metals and the
combined effects between the carrier materials and the active components often give rise to positive
influences on the enhancement of the catalytic efficiency and selectivity. In this work, we report a facile
process for the fabrication of nitrogen-doped carbon black (NCB) supported PdAu bimetallic
nanoparticles (NPs) with a uniform dispersion and narrow size distribution. The PdAu/NCB catalyst with
a Pd/Au mole ratio of 1/1 shows the highest activity towards both Ullmann coupling reactions of aryl
halides and the hydrogenation reaction of nitrophenols. Moreover, this bimetallic catalyst also exhibits
a superior recycling durability to that of monometallic Pd/NCB and Au/NCB catalysts. The enhanced
catalytic performance of the bimetallic catalyst is mainly due to the large BET specific surface area
Received 6th March 2019
Accepted 16th May 2019
DOI: 10.1039/c9ra01685f
rsc.li/rsc-advances
(125.45 m2 ꢀ1) and the synergy between the individual components of the catalyst.
g
Ullmann coupling reactions,8–11 the exorbitant price of the
complex catalysts, non-recyclable property and complicated
post-processing are still the main blocks for such a homoge-
neous catalysis.
Introduction
The synthesis of symmetrical di- or poly-aromatic compounds is
one of the important processes in industry, and shows a wide
application in the production of pharmaceuticals, natural
products and organic conductors.1–3 The Pd-catalyzed Suzuki,
Stille, Negishi and Ullmann coupling reactions are commonly
used for the fabrication of carbon–carbon bonds to generate
biaromatic compounds.4–7 Among the reactions above, the
homogeneous Ullmann coupling of aryl halides is one of the
most versatile strategies for the synthesis of symmetrical biar-
omatic compounds. However, the traditional homogeneous
Ullmann coupling reaction is always carried out at a relatively
high temperature in an organic medium with a high boiling
point, leading to extensive energy-consumption and compli-
cated post-processing. Hence, the development of highly effi-
cient catalysts and the exploration of mild reaction conditions
are in great demand. Even though a variety of homogeneous
catalysts have been explored recently for homogeneous
To date, with the rapid development of nanotechnology,
more and more attention has been paid to the design and
synthesis of nanomaterials with unique structures as catalysts
for organic reactions. Metallic NPs were widely studied in the
areas of the hydrogenation reaction, coupling reaction, etc.12,13
Compared with monometallic NPs, multimetallic NPs oen
reveal enhanced optical, electronic and catalytic activity on
account of the synergistic effect between different metal
atoms.14–18 As a result, remarkable progress was achieved in the
controllable synthesis and potential applications of bimetallic
catalysts on the basis of a well-understood design strategy. The
most studied bimetallic catalysts, such as Pd-based bimetallic
catalysts, including Pd–Au,19,20 Pd–Ni,21 Pd–Ag22 and Pd–Cu,23
have been widely investigated for varied catalytic applications in
oxidation, reduction and coupling reactions. Among these
bimetallic catalysts, bimetallic PdAu materials have attracted
extensive attention in the eld of heterogeneous Ullmann
coupling reactions due to their excellent catalytic activity and
durability. For example, Sakurai et al. synthesized bimetallic Au/
Pd nanoclusters for the Ullmann coupling of chloroarenes
under an argon atmosphere at a low temperature with a high
yield.24 Wang and co-workers prepared Au–Pd alloyed
aKey Laboratory for So Chemistry and Functional Materials of Ministry Education,
Nanjing University of Science and Technology, Nanjing 210094, China
bCollege of Science, Institute of Materials Physics and Chemistry, Nanjing Forestry
University, Nanjing 210037, China. E-mail: fengyanhan@njfu.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra01685f
‡ These authors contributed equally to this work.
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nanochain networks by using 4-aminopyridine as the structure-
directing agent for the Ullmann intermolecular and intra-
molecular homocoupling and the catalyst exhibited a high
activity and good stability.25
Synthesis of PdAu/NCB
Nitrogen-doped carbon black (NCB) was rst synthesized
through a hydrothermal process according to our previous
work.42 PdxAuy/NCB nanocatalysts with a metal loading of
10 wt% were fabricated through a co-reduction process in the
liquid system, where x and y are the relative molar ratios of Pd
and Au, respectively, and x + y ¼ 10. Typically, 20 mg of NCB was
dispersed in 20 mL of ethylene glycol (EG) with sonication for
10 min to obtain a uniform suspension. Then 5 mL of L-ascorbic
acid (1.0 mg mLꢀ1) aqueous solution was dropped in with
continuous stirring. Aer that, 5 mL of a fresh aqueous solution
of metal precursors which contains the required amounts of
K2PdCl4 and HAuCl4$9H2O was added into the above mixture
with vigorous stirring at room temperature. Aer 2 h, the solid
residue was ltered, washed with water and freeze-dried for
further use, labelled as PdxAuy/NCB. The schematic fabrication
process is shown in Scheme 1.
It is reported that the combination of active components
with some certain carrier materials can largely improve the
activity and stability of the catalysts. The carrier materials can
help disperse the active components and prevent their aggre-
gation, and meanwhile reduce the dosage of active components,
especially for the precious metals, which is in favor of
a controlled catalysis cost. Among various carrier materials,
such as hydrotalcite,26 graphitic carbon nitride,27 metal–organic
frameworks,28 graphene29,30 and carbon materials,31 XC-72
carbon black is one of the most promising in the near future
because of many admirable properties, such as permeable
pores, large surface area, excellent conductivity, good stability
and chemical inertness.32–34 As reported, nitrogen doping is
considered one of the most effective ways to further modify the
properties of carbon materials, where the more negatively-
charged nitrogen atoms can help to gain interactions between
the active components and the substrates.35–39 The lone pair
electrons in the nitrogen atoms can also act as Lewis base sites,
which can partly modulate the electronic properties, basicity
and hydrophilicity of carbon black.40,41 To the best of our
knowledge, few works have been reported about the well-
dispersed nitrogen-doped carbon black (NCB) supported
bimetallic PdAu NPs in organic catalysis.
According to the amounts of Pd and Au precursor in the
solution, catalysts with different molar ratios of Pd : Au were
obtained, and were set as 1 : 9, 3 : 7, 5 : 5, 7 : 3 and 9 : 1,
respectively. Moreover, Pd/NCB and Au/NCB were also prepared
in the same way but with only the Pd or Au precursor in the
solution.
Characterization
The morphology and microstructure of the catalyst were char-
acterized by transmission electron microscopy (TEM, JEOL JEM-
2100). To further identify the state of Pd and Au in the nano-
hybrid, PdAu/NCB was observed by high-angle annular dark-
eld scanning TEM (HAADF-STEM) and energy dispersive X-
ray spectroscopy (EDX) mapping analysis (Tecnai G2 F30 S-
Twin TEM). The structure of the catalyst was characterized by
X-ray diffraction (XRD, Bruker D8 Advance diffractometer)
analysis using Cu Ka radiation (l ¼ 0.15406 nm). The chemical
state of each element was obtained by X-ray photoelectron
spectroscopy (XPS) spectra on a PHI-5000C ESCA system
(Thermo ESCALAB 250) with an Al Ka X-ray (E ¼ 1486.6 eV) as
the excitation light source. The nitrogen adsorption–desorption
isotherm and BJH pore size distribution were measured by
using a TriStar II3020 automated gas adsorption analyzer at 77
K. Inductively Coupled Plasma Optical Emission Spectroscopy
(ICP-OES) was performed on an ICP-OES analyzer (Agilent 730).
The yield of biphenyl was measured by high-performance liquid
chromatography (HPLC, Agilent 1260 Innity, Eclipse XDB-C18
column) analysis according to an external standard method.
The absorbance was measured by a UV-vis spectrophotometer
(PERSEE TU-1900).
In this work, we report a facile, mild and surfactant-free
process for the synthesis of a well-dispersed NCB supported
bimetallic PdAu catalyst with an average particle size of 3.4 nm.
The Pd/Au mole ratios can be easily tuned through controlling
the amounts of the metal precursors. The PdAu/NCB catalyst
with a Pd/Au ratio of 1/1 exhibits the most activity for both the
Ullmann coupling reaction and the hydrogenation reaction of
nitrophenols. Compared with the monometallic Pd/NCB or Au/
NCB catalysts, the synergistic effect between two different
metals and the combined effect between the nanoparticles and
NCB lead to an enhanced catalytic performance, achieving
a cost-effective Ullmann coupling reaction and nitrophenol
hydrogenation reaction.
Experimental details
Materials
XC-72 carbon black (CB) was supplied by the Cabot Corpora-
tion. Bromobenzene (AR., 99%) and magnesium sulfate anhy-
drous (MgSO4, AR.) were purchased from the Aladdin Industrial
Cooperation. Ethylene glycol (EG, 99.0%), nitric acid (HNO3, 65–
68%), isopropanol (IPA) and potassium carbonate (K2CO3) were
of analytical grade and purchased from Sinopharm Chemical
Reagent Co., Ltd. L-Ascorbic acid (AR.) was purchased from
Ullmann coupling reaction
ꢁ
Xilong Scientic Co., Ltd. Potassium tetrachloropalladate The Ullmann coupling reaction was carried out at 40 C in air
(K2PdCl4, 99.99%) and chloroauric acid (HAuCl4$9H2O, 99.99%) for 3 hours. Typically, 1.0 mmol of aryl halide (1 equiv.),
were obtained from Alfa Aesar. All the chemicals in this work 3.0 mmol of base (3 equiv.) and 4 mg of the as-obtained catalyst
were used without further purication, and deionized water was were added into a mixed solution of 4 mL of IPA and 4 mL of
used for the preparation of various solutions throughout the water. Parts of mixture were taken out every 30 minutes for
entire study.
determination with HPLC. For the optimization of the reaction
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Scheme 1 Synthetic scheme of PdAu/NCB NPs for the Ullmann coupling of bromobenzene and reduction of 4-nitrophenol.
conditions, 200 mL of the above mixture was added into 4 mL of resolution TEM image of the Pd5Au5/NCB catalyst. The lattice
methanol followed by syringe ltration. The yield of biphenyl plane distance of the Pd5Au5 particle is estimated as 0.230 nm,
was measured by HPLC analysis according to an external stan- which is between the lattice plane distance of Pd (111) and Au
dard method. While for the catalyst applicability study, the (111), 0.223 nm and 0.234 nm, respectively, indicating the
catalyst was removed by simple ltration, and then the formation of an alloy structure.43,44 Furthermore, the elemental
remaining solution was extracted with ethyl acetate three times. composition of the Pd5Au5/NCB was further investigated by the
The collected organic layers were combined, dehydrated with HAADF-STEM technique (Fig. 1C and D). The STEM-EDX
MgSO4 and nally rotary evaporated. The resulting crude mapping results conrm that C, N, O, Pd and Au elements are
product was separated by column separation to give an isolated apparently observed in Pd5Au5/NCB. A homogeneous distribu-
yield.
tion of both Pd and Au is found (Fig. 1D), indicating the
formation of a PdAu alloy on the surface of NCB.
XRD analysis was used to further study the structure of the
catalysts. Fig. 2A shows the XRD patterns of NCB, Pd/NCB, Au/
NCB and PdAu/NCB catalysts with different Pd–Au molar ratios.
For all the PdAu/NCB catalysts with different Pd/Au molar
ratios, a broad peak centered at 2q ¼ 25ꢁ can be observed, which
is ascribed to C (002) in the NCB material. As for Pd/NCB, the
diffraction peaks centered at 2q ¼ 40.09ꢁ, 46.60ꢁ, 68.00ꢁ and
81.90ꢁ are assigned to the (111), (200), (220), and (311) lattice
planes of face-centered cubic Pd, respectively. Similarly, for Au/
NCB, the peaks located at 38.18ꢁ, 44.44ꢁ, 64.65ꢁ, 77.56ꢁ and
81.77 correspond to (111), (200), (220), (311) and (222). For the
PdAu/NCB samples, an obvious shi (Fig. 2B) can be observed
for the main peaks with an increase in the Pd content, sug-
gesting the formation of a PdAu alloy rather than a core–shell
structure.45–47 Meanwhile, the decreased intensity of the peaks
implies a lower crystallization degree.
Nitrophenol hydrogenation reaction
Nitrophenol hydrogenation in the presence of an excess amount
of NaBH4 was carried out in an aqueous solution. Typically,
100 mg of NaBH4 was mixed with the 20 mL of an aqueous
solution of nitrophenol (100 mg Lꢀ1), and then 85 mL of the
catalyst suspension (1 mg catalyst/1 mL H2O) was added to the
above solution to start the reaction. In order to monitor the
hydrogenation process, 3.0 mL of the solution was taken out of
the reaction mixture at specic time intervals (2 minutes) fol-
lowed by syringe ltration and nally measured by a UV-vis
spectrophotometer to give time-dependent UV-vis spectra.
Results and discussion
Morphology and structure
The typical IUPAC type IV pattern with a hysteresis loop in
Fig. 2C shows that the Pd5Au5/NCB has a Brunauer–Emmett–
Teller (BET) surface area (SBET) of 125.45 m2 gꢀ1 and a pore
As shown in Fig. 1A, the Pd5Au5 alloy nanoparticles are
uniformly dispersed on the surface of the NCB, with an average
particle size of 3.4 nm and a narrow size distribution ranging
from 2 to 5 nm (Fig. 1B). The inset of 1A exhibits the high-
volume of 0.24 cm3 gꢀ1
. The corresponding pore size
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Fig. 1 (A) TEM image, the inset is a HRTEM image; (B) particle size distribution; (C) HAADF-STEM images of Pd5Au5/NCB; (D) EDS mapping of
C, N, O, and Pd and Au element.
distribution (Fig. 2D) shows that the NCB material itself has the aggregated particles. According to these results, the large
a pore size of around 3.60 nm, while the mesopore with a size of specic surface area and abundant pores are expected to
34.96 nm simply reects the average size of the voids between enhance the catalytic performance.
Fig. 2 (A) XRD patterns of NCB and PdAu/NCB with different Pd/Au molar ratios; (B) the enlarged XRD patterns between 2q ¼ 30–50ꢁ; (C)
nitrogen adsorption–desorption isotherm; (D) BHJ pore size distribution of the Pd5Au5/NCB.
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XPS characterization was employed to obtain the supercial major nitrogen types in our support material are pyridinic N
element information of the as-prepared Pd5Au5/NCB catalyst. and pyrrolic N. As for the Au 4f spectrum of the Pd5Au5/NCB
The XPS survey scans of Pd/NCB, Au/NCB and Pd5Au5/NCB are catalyst, it is well tted with two pairs of peaks (Fig. 3D). One
shown in Fig. 3A. The peaks of C, O, Pd and Au can be seen doublet located at 83.8 and 87.5 eV (ref. 52 and 53) is assigned to
clearly on the Pd5Au5/NCB composite. Additionally, a weak peak Au0, while the other at BE ¼ 84.9 and 88.6 eV corresponds to
of N 1s can also be observed, which indicates that nitrogen is Au+. Similarly, Pd0 (BE ¼ 340.3 and 335.1 eV), Pd2+ (BE ¼ 336.5
successfully doped into carbon black via the facile hydro- and 341.7 eV) and Pd4+ (BE ¼ 337.8 and 343.0 eV) are three
thermal treatment. The high-resolution XPS tting spectrum of typical Pd species that appear in the Pd5Au5/NCB composite54,55
C 1s concludes that C exists in the form of a sp2 C]C, a defect (Fig. 3E). It is worth noting that, compared with Au/NCB and Pd/
peak or sp2 C]N, C–OH, C]O or C–N and O–C]O with the NCB (Fig. 3D and E), both XPS spectra of Au 4f and Pd 3d in
binding energy (BE) centered at 284.6, 285.2, 286.1, 287.1 and Pd5Au5/NCB have shied to a lower binding energy, suggesting
289.1 eV, respectively (Fig. 3B).48–50 The convoluted result of the electron transfer to Pd and Au metals. Due to the conduc-
the N 1s spectra shows three types of N species, including tivity of NCB, we can deduce that these electrons came from
pyridinic N (398.8 eV, 39.9%), pyrrolic nitrogen (399.8 eV, NCB, conrming the interaction between PdAu NPs and NCB.56
48.7%) and amino functional groups (401.3 eV, 11.4%), Compared with the PdAu/CB sample (Fig. S1†), the Pd and Au
respectively (Fig. 3C).48,51 Also, the corresponding data of the peak positions of PdAu/NCB shi to higher binding energies,
atomic percentage of each nitrogen type indicated that the which is attributed to the metal-support interaction (MSI)
Fig. 3 (A) Global XPS spectrum of Pd5Au5/NCB; XPS spectra of C 1s (B), N 1s (C), Au 4f (D) and Pd 3d (E).
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between PdAu nanoparticles and NCB,57 suggesting an
enhanced attraction force between the PdAu atoms and the NCB
support.48
Table 2 Optimization of the experimental conditions for the Ullmann
coupling reaction of bromobenzenea
Ullmann coupling reaction
In our work, the Ullmann coupling of bromobenzene was
employed as a model reaction to conduct the optimization of
the reaction conditions with PdAu/CB and PdAu/NCB compos-
ites as the catalysts. The activities of PdAu/NCB with different
Pd/Au molar ratios as well as their monometallic counterparts
are summarized in Table 1. Monometallic Au/NCB was totally
inactive for the Ullmann coupling of bromobenzene, and the
monometallic Pd/NCB could only give a yield of 6%. Herein, it
can be inferred that Pd is the only active site for the coupling
reaction. By alloying Au with Pd, a great enhancement of the
catalytic activity can be obtained. At an Au/Pd ratio of 5/5, the
biphenyl yield reaches the highest value of 95% within 3 h. The
yield of biphenyl over Pd5Au5/CB was only 16% under the same
conditions. Hence, Pd5Au5/NCB was considered the best cata-
lyst for further study.
Entry
Base
Solvent (mL mLꢀ1
)
Yield (%)
1
2
3
4
5
6
7
8
K2CO3
K2CO3
K2CO3
K2CO3
KOH
NaHCO3
NH4HCO3
Et3N
NaOH
C5H5N
HCOONa
K2CO3
K2CO3
K2CO3
IPA
EtOH
C4H4O
H2O
52
70
60
52
55
30
10
8
60
20
17
94
82
75
IPA/H2O ¼ 4/4
IPA/H2O ¼ 4/4
IPA/H2O ¼ 4/4
IPA/H2O ¼ 4/4
IPA/H2O ¼ 4/4
IPA/H2O ¼ 4/4
IPA/H2O ¼ 4/4
IPA/H2O ¼ 4/4
EtOH/H2O ¼ 4/4
C4H4O/H2O ¼ 4/4
9
10
11
12
13
14
As the bases and solvents play an important role in the Ull-
a
mann coupling reaction,
a control variate method was
Reaction conditions: bromobenzene (1 mmol), base (3 mmol), solvent
(8 mL), PdAu/NCB(4 mg, 10 wt% Pd5Au5 loading, the calculated moles of
Pd5Au5 is 0.0013 mmol), 40 ꢁC, 3 h. Yields were determined by HPLC
according to the standard curve based on biphenyl.
employed to study the inuence of each factor. As shown in
Table 2, various bases, such as K2CO3, KOH, NaHCO3,
NH4HCO3, NaOH, C5H5N, HCOONa and Et3N (entries 5–12),
were employed in the Ullmann coupling reaction. Among these
bases, K2CO3 was the most efficient for the Ullmann reaction in
the IPA/H2O system with a 94.6% yield of biphenyl (entry 12).
So, we used K2CO3 as the base in the subsequent experiments
because of its high efficiency and low cost. Organic solvents
such as IPA, EtOH, C4H4O and H2O gave moderate yields
(entries 1–4). Interestingly, a mixed water-organic solvent can
tremendously increase the yield of biphenyl (entries 12–14) in
the presence of K2CO3 as a base, especially, the highest yield of
95% can be obtained by using the IPA/H2O system (entry 12).
Because the base and organic reactants are more easily dis-
solved in the mixed solutions, organopalladium intermediates
are easily formed, thus facilitating the reaction. Additionally,
the desired product of biphenyl will separate out because of its
limited solubility in the IPA/H2O medium, driving the reaction
equilibrium in the forward direction. Hence, the IPA/H2O
system is considered to be the best solvent because it is safe,
easily available and environmentally friendly. So, the optimized
reaction conditions are as follows: bromobenzene (1 mmol),
K2CO3 (3 mmol), Pd5Au5/NCB (4 mg), IPA (4 mL), and water (4
withdrawing functional groups under the optimized condi-
tions. As shown in Table 3, the activity of 4-OCH3-substituted
aryl benzenes follows the sequence of I > Br > Cl because of the
difference between the electronegativity of halogen atoms (Cl >
Br > I), where higher electronegativity leads to a stronger
carbon–halogen bond and thus a lower activity (entries 1–3).58
This result is consistent with the work reported by Zhang et al.,59
in which ion exchange resin supported Au alloyed with Pd single
atoms was explored to serve as an effective and robust catalyst
for the Ullmann reaction of aryl halides. For other OCH3-
substituted bromobenzenes (entries 4–5), the 2-OCH3
substituted bromobenzene gives a lower yield of 85.1% due to
steric hindrance.60 Moreover, for para-position substituted aryl
bromides with electron-donating (–OH and –CH3) or electron-
withdrawing (–CN, –CHO, –NO2) functional groups, desirable
yields can be obtained under required conditions (entries 6–10).
In addition, 4-NO2 iodobenzene can give the corresponding
symmetrical products with a yield of 88.6% in 2 h (entry 11).
However, Dhital et al.24,61 reported that Au–Pd alloy nanoclusters
stabilized by PVP could efficiently catalyze the Ullmann
coupling of chloroarenes but not for bromoarenes and iodide
arenes, and aryl iodide was a stronger inhibitor by forming
a stable complex with a gold component. Obviously, the as-
prepared PdAu/NCB catalysts are more stable and adaptable
to different substrates.
ꢁ
mL) at 40 C for 3 h.
The generality of the catalyst for the Ullmann coupling
reaction was examined by extending different bromobenzene
derivatives containing electron-donating and electron-
Table 1 Ullmann coupling of bromobenzene over PdAu/NCB cata-
A possible pathway for the Ullmann coupling of aryl halides
catalyzed by the PdAu/NCB is proposed in Scheme 2. First, the
aryl halides are adsorbed onto the surface of PdAu NPs followed
by the activation (step A) and cleavage of the carbon–halogen
(C–X) bond to generate metal-aryl and metal-halogen interme-
diates (step B). Later, a carbon–carbon bond is formed by
lysts with various Au/Pd ratiosa
Catalyst
Yield (%)
Au Pd1Au9 Pd3Au7 Pd5Au5 Pd7Au3 Pd9Au1 Pd
49 74 95 51 18
0
6
a
Reaction conditions: bromobenzene (1 mmol), K2CO3 (3 mmol), IPA (4
mL), H2O (4 mL), PdAu/NCB (4 mg, 10 wt% PdAu loading), 40 ꢁC, 3 h.
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Table 3 Generality of the Pd5Au5/NCB catalyst in the Ullmann coupling reaction involving different aryl halidesa
Entry
1
Aryl halide
Product
Time
3 h
Yield (%)
95
2
3
5 h
1 h
85
97
4
5
3 h
3 h
92
85
6
3 h
3 h
3 h
3 h
3 h
81
90
92
88
87
7
8
9
10
11
2 h
88
a
Reaction conditions: aryl halide(1 mmol), K2CO3 (3 mmol), IPA (4 mL), H2O (4 mL), Pd5Au5/NCB (4 mg, 10 wt% Pd5Au5 loading), 40 ꢁC, 3 h, isolated
yield.
reductive elimination (step C) and the bi-aryl product is des- effect between individual components: (1) the abundant oxygen
orbed from the active site and into the solution, accompanying containing functional groups and large surface area on of NCB
the recovery of alloy nanoparticles by isopropanol (step D).62–64 could anchor the PdAu NPs, which is in favor of the dispersion
As demonstrated by many reports, alcohols (such as ethanol or and stability of the catalyst. Moreover, nitrogen-doping can
isopropanol) can be used as reducing agents for Ullmann increase the interaction between the PdAu and NCB, thus
homocoupling.65–67
enhancing the catalytic durability.68–70 (2) In the case of the
Based on the above investigation, the excellent activity of Ullmann coupling reaction, Pd appears to be more catalytically
PdAu/NCB in Ullmann coupling can probably be attributed to active than Au. The synergistic effect creates highly active sites,
the unique nanostructure of the catalyst and the synergistic which is benecial to the oxidation addition of aryl halides.52 (3)
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the excessive amount of NaBH4, the pseudo-rst-order kinetics
related to 4-NP can be used to evaluate the kinetic rate constant,
and the corresponding kinetic equation can be written as eqn
(1):
A
c
ln
¼ ln
¼ ꢀkt
(1)
A0
c0
where c is the concentration of 4-NP during the reaction, c0 is
the initial concentration of 4-NP, k is the rate constant and t is
the reaction time.
Catalysts with different molar ratios of Pd and Au were
synthesized in order to further investigate the catalytic activity
for the hydrogenation of 4-NP to 4-AP. The ln c/c0 vs. reaction
time over PdAu/NCB with various Pd/Au molar ratios is shown
in Fig. 4B. Combining the results of Fig. 4B with Table 4, we can
clearly conclude that all of the PdAu catalysts showed a higher
catalytic activity compared with the corresponding Pd/NCB and
Au/NCB, indicating the synergistic effect between Pd and Au
nanoparticles in the PdAu alloy. Notably, the Pd5Au5/NCB
catalyst exhibited the highest catalytic activity within 12 min to
nish the reaction. However, only a 20% conversion of 4-NP
could be obtained over Pd5Au5/CB within 12 min [Fig. S2†].
Obviously, N-doping can improve the catalytic performance due
to an enhanced attraction force between PdAu atoms and the
NCB support.48,57
Scheme 2 Possible reaction pathway for the Ullmann coupling of aryl
halides catalyzed by PdAu/NCB.
For the Ullmann coupling reaction itself, the almost biphenyl
with a noncoplanar conjugated structure, poor polarity and
limited solubility will quickly escape from the surface of the
catalyst, driving the reaction in the forward direction.71
We also investigate the effect of temperature on the hydro-
genation reaction. Four different temperatures ranging from 20
to 35 ꢁC were chosen for the reduction of 2-NP, 3-NP and 4-NP.
The activation energy (Ea) can be calculated according to the
Arrhenius equation (eqn (2)):
Hydrogenation of nitrophenols
The PdAu/NCB alloy catalyst is not only active for the Ullmann
coupling reaction, but also behaves well in the hydrogenation
reaction of nitro compounds. Here, the catalytic reduction of p-
nitrophenol (4-NP) to p-aminophenol (4-AP) was set as a model
reaction to estimate the activity of the catalysts. The reaction
was carried out in an aqueous solution using an excess amount
of NaBH4 as the reductant. As shown in Fig. 4A, the maximum
UV-vis adsorption wavelength of 4-NP is at 400 nm under
alkaline conditions, while the peak maximum of 4-AP is around
303 nm.53 In the presence of NaBH4 and catalyst, the peak
assigned to 4-NP decreases as the reaction proceeds, and
meanwhile the peak of 4-AP increases concomitantly, which can
be used to monitor the reaction process. Taking into account
ꢀEa
ln k ¼
þ ln A
(2)
RT
where k is the rate constant, Ea is the activation energy, R is the
ideal gas constant (8.314 J molꢀ1 Kꢀ1), T is the temperature, and
A is the Arrhenius factor. As shown in Fig. 5 and Table 4, a larger
k was achieved at a higher temperature. The k values of the 2-NP,
3-NP and 4-NP reduction were calculated as 0.2071, 0.1971 and
0.2114 minꢀ1, respectively, at 298.15 K. The activation energies
(Ea) of the catalytic reduction for 2-NP, 3-NP and 4-NP over
Pd5Au5 are calculated to be 55.12, 65.13 and 20.88 kJ molꢀ1
,
Fig. 4 (A) UV-vis adsorption spectra of 4-NP catalyzed by Pd5Au5/NCB at certain time intervals; (B) pseudo-first-order plot of ln(c/c0) against
reaction time for the hydrogenation of 4-NP over different catalysts.
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ꢀ ꢁ
ꢀ
ꢁ
ꢀ ꢁ
DS# DH#
1
Table 4 Pseudo-first-order kinetics constants for the 4-NP reduction
of different catalystsa
k
kB
h
ln
¼ ln
þ
ꢀ
(3)
(4)
T
R
R
T
Entry
Catalyst
k (minꢀ1
)
R2
DG# ¼ DH# ꢀ TDS#
1
2
3
4
5
6
7
Pd5Au5/NCB
Pd9Au1/NCB
Pd7Au3/NCB
Pd3Au7/NCB
Pd1Au9/NCB
Pd/NCB
0.2895
0.1930
0.1543
0.1137
0.0872
0.0575
0.0372
0.9961
0.9724
0.9813
0.9902
0.9893
0.9785
0.9812
Based on the experimental results and the literature,78,79 the
high activity of Pd5Au5/NCB can be attributed to a synergistic
effect and the unique nanostructure. The nitro group as an
electron-withdrawing group enhances the p–p stacking inter-
actions by reducing the electron density of p electrons, dimin-
ishing the repulsive electrostatic interactions between the
aromatic rings.80 It provides a higher concentration of nitro-
phenol molecules near to the PdAu NPs on N-doping carbon
black, leading to a highly efficient contact between the reactant
molecules and the active sites, thus enhancing activity.81
Au/NCB
a
Reaction conditions: 4-NP (20 mL, 100 mg Lꢀ1), catalyst suspension
(85 mL, 1 mg mLꢀ1), NaBH4 (50 mg), 25 ꢁC.
respectively. Obviously, 4-NP exhibits the lowest activation
energy and the highest rate constant k.
Thermodynamic parameters, including the activation of
entropy (DS#), enthalpy (DH#) and Gibbs energy change (DG#),
are calculated following eqn (3) and (4), where kB is the Boltz-
mann constant (1.38 ꢂ 10ꢀ23 J Kꢀ1) and h is the Planck constant
(6.63 ꢂ 10ꢀ34 J).72 For Pd5Au5/NCB, the values of DS#, DH# and
DG# are listed in Table 5. The positive values of activation of
enthalpy show that the reduction of 2-NP, 3-NP and 4-NP is an
endothermic reaction and the positive DG# values indicate that
the hydrogenation needs the catalyst.73 The negative entropy
(DS#) values indicate that randomness on the catalyst-solution
interface decreased during the catalytic reduction process.74 In
addition, the DG# values increase as the temperature rises,
which indicates that the reduction process of 2-NP, 3-NP and 4-
NP needs energy.75–77
Cycling performance
The recoverability and reusability of the catalyst are of great
importance. The Pd5Au5/NCB was used in the Ullmann coupling
ꢁ
ꢁ
of bromobenzene at 40 C and the reduction of 4-NP at 25 C
over 10 cycles. As shown in Fig. 6, both the conversion efficiency
of 4-NP and yield of the Ullmann coupling over Pd5Au5/NCB
maintained at over 80%, suggesting a good reusability of
Pd5Au5/NCB in the reaction.
The recovered catalyst was re-characterized using XPS, TEM
HAADF and ICP. XPS analysis (Fig. S3†) reveals that PdAu
nanoparticles maintain their state aer the tenth use. TEM
HAADF images of the tenth cycle of catalysis (Fig. S4†) show the
Fig. 5 Pseudo-first-order plot of ln c/c0 against reaction time for the reduction of 2-NP (A), 3-NP (B) and 4-NP (C) catalyzed by Pd5Au5; plot of
ln k against 1/T for the reduction of 2-NP, 3-NP and 4-NP (D).
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Table 5 Kinetic and thermodynamics parameters of catalytic reaction for 2-NP, 3-NP and 4-NP over Pd5Au5/NCB
Substrate
2-NP
T (K)
K (minꢀ1
)
Ea (kJ molꢀ1
)
DH (kJ molꢀ1
)
DS (J molꢀ1 Kꢀ1
ꢀ122.34
)
DG (kJ molꢀ1
)
293
298
303
308
293
298
303
308
293
298
303
308
0.1643
0.2071
0.3233
0.4829
0.1467
0.1971
0.3385
0.5217
0.1912
0.2114
0.2540
0.2896
55.12
44.8
80.64
81.25
81.86
82.48
80.31
80.71
81.11
81.52
82.58
84.70
86.83
88.96
3-NP
4-NP
65.13
20.88
57.1
ꢀ80.6
19.13
ꢀ212.9
aryl halides and for nitrophenol hydrogenation. According to
the results, the PdAu/NCB nanoparticles are more active
compared to the corresponding monometallic counterparts;
moreover, Pd5Au5/NCB exhibits the highest activity. The price of
the catalyst is low due to the low price of NCB. The enhanced
catalytic performance can be ascribed to the unique nano-
structure of the catalyst including the large BET surface, small
size of PdAu NPs, excellent PdAu dispersion, strong adsorption
of organic molecules on the surface of NCB, and the synergistic
effect between two different metals and the combined effect
between the nanoparticles and NCB.
Conflicts of interest
There are no conicts to declare.
Fig. 6 Cycling performance of Pd5Au5/NCB for the reduction of 4-NP
and the Ullmann coupling of bromobenzene.
Acknowledgements
This work was supported by the Fundamental Research Funds
for the Central Universities (No. 30916014103), NSF of China
(No. 51572125), the Natural Science Foundation of Jiangsu
Province (No. BK20171423) and PAPD of Jiangsu.
PdAu particles had slightly grown to 4–6 nm, indicating the
mild agglomeration of the particles, which may have caused the
reduction in the catalytic activity aer the tenth use. Metal
leaching of the catalyst before and aer the reaction was studied
using ICP. The Au and Pd contents of the Pd5Au5/NCB are
shown in Table S1† before reaction and aer ten reaction cycles.
These results demonstrate that the amount of metal loss can be
5%, which may be cause the decrease of the activity of Pd5Au5/
NCB NPs.82
Furthermore, we performed a ltration test in the Ullmann
coupling of PhBr at 40 ꢁC. The Pd5Au5/NCB was ltrated out
using a syringe lter aer 1 h. As indicated in Fig. S5,† the
catalytic reaction did not occur aer the catalyst was ltered,
which conrmed the heterogeneous nature of the catalysis.
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