Composition-dependent catalytic activity of bimetallic nanocrystals: AgPd-catalyzed hydrodechlorination of 4-chlorophenol

Article abstract of DOI:10.1021/cs400282a

Ag-Pd bimetallic nanocrystals (NCs) with tunable compositions and narrow size distributions were produced by a one-pot synthesis. The NC growth process was investigated by time-dependent TEM, XRD, and UV-vis studies. In the hydrodechlorination of 4-chlorophenol, the AgPd_x (x = 2, 4, 6, 9, 19) showed pronounced composition-dependent catalytic activities, leading to the AgPd₉ catalyst with excellent activity.

Full text of DOI:10.1021/cs400282a

Letter
pubs.acs.org/acscatalysis
Composition-Dependent Catalytic Activity of Bimetallic
Nanocrystals: AgPd-Catalyzed Hydrodechlorination of
4‑Chlorophenol
Hongpan Rong,† Shuangfei Cai,† Zhiqiang Niu,*,† and Yadong Li*,†,‡
†Department of Chemistry, Tsinghua University, Beijing, 100084, People’s Republic of China
‡Department of Chemistry and the State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University, Beijing,
100084, People’s Republic of China
S
* Supporting Information
ABSTRACT: Ag−Pd bimetallic nanocrystals (NCs) with
tunable compositions and narrow size distributions were
produced by a one-pot synthesis. The NC growth process was
investigated by time-dependent TEM, XRD, and UV−vis
studies. In the hydrodechlorination of 4-chlorophenol, the
AgPdx (x = 2, 4, 6, 9, 19) showed pronounced composition-
dependent catalytic activities, leading to the AgPd9 catalyst with
excellent activity.
KEYWORDS: Ag, Pd, nanocrystals, hydrodechlorination
Herein, we prepared narrowly distributed AgPdx NCs with
tunable compositions and investigated their growth mechanism
by time-dependent TEM, XRD, and UV−vis studies. The as-
prepared monodispersed AgPdx NCs were deposited on carbon
black and performed high catalytic activity for the HDC
reaction at room temperature and atmospheric pressure. A
pronounced composition-dependent activity trend was estab-
lished in the HDC reaction, and the AgPd9/C catalysts showed
the highest catalytic activity among all AgPdx NCs/C.
recious metals are widely used in chemical engineering.
1−6
PRecently, bimetallic NCs, such as Pt-based
and Pd-
based7−12 nanostructures, have attracted tremendous research
interest due to their better catalytic properties than
monometallic NCs. Previous studies have shown that the
catalytic performance of bimetallic NCs has a close relationship
with their composition because the relative amount of each
constituent metal could tune the electronic state of the primary
catalytically active component.13,14
The AgPdx NCs were synthesized by coreduction of AgNO3
and Pd(acac)2 in octadecylamine (see details in the Supporting
Information). Figure 1 depicts the TEM images of as-prepared
AgPdx NCs. By varying the ratios of AgNO3 and Pd(acac)2, the
compositions of AgPdx (x = 2, 4, 6, 9, 19) NCs could be finely
tuned while their sizes remained in the range of 3−5 nm. When
AgNO3 was the only precursor, Ag NCs of (11.9 1.4) nm
(Supporting Information Figure S1a) were obtained; when
Pd(acac)2 was the only precursor, Pd NCs of (7.5 1.7) nm
were produced (Supporting Information Figure S1b). A
representative high-resolution (HR) TEM image of the
AgPd4 NCs (Supporting Information Figure S2) shows that
the products exhibit good crystallinity and the interplanar
spacings of lattice fringes are 0.23 nm. The XRD patterns of the
as-obtained AgPdx NCs are shown in Figure 1f. The diffraction
peaks positioned between the standard peaks of Ag (JCPD-65-
Chlorophenols (CPs) are widely present in waste waters of
paper, dyes, pesticides, textiles, and petrochemicals.15,16
Because of the stability of the chlorine−carbon bonds, CPs
are difficult to biodegrade. Thus, to develop mild methods for
dechlorination of CPs is of great importance. Compared with
other precious metal catalysts from group VIII (Ru, Rh, and
Pt), Pd has a better performance in the hydrodechlorination
(HDC) reaction and lower cost.17−22 For instance, Pd−Ag
sol−gel catalyzed 1,2-dichloroethane to ethylene was carefully
studied by Heinrichs and co-workers.23−26 Similar catalysts
(Pd−Ag/SiO2 and Pd−Cu/SiO2 cogelled xerogel catalysts)
used for the same reaction have been reported by Lambert and
co-workers.22 Although systematic investigations of HDC
process have been performed in these studies, the synthesis
method of the xerogel Pd−Ag/SiO2 was somewhat compli-
cated, which included drying under vacuum, calcination, and
reduction. Therefore, the facile synthesis of monodispersed
Ag−Pd NCs and investigation of their properties toward HDC
reactions under mild conditions are of significance.
Received: January 28, 2013
Revised: June 5, 2013
© XXXX American Chemical Society
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Letter
Figure 2. XPS spectra of the (a) Ag 3d and (b) Pd 3d regions for the
AgPdx NCs.
Figure 1. TEM images of as-prepared (a) AgPd2, (b) AgPd4, (c)
AgPd6, (d) AgPd9, and (e) AgPd19 NCs and (f) their corresponding
XRD patterns.
6174) and Pd (JCPD-65-2871), demonstrating the formation
of the bimetallic phase of Ag and Pd. As expected, with an
increase in the Pd content, the peaks continuously shifted from
Ag standard peaks to Pd. The exact atomic ratios of the AgPdx
NCs were determined by energy dispersive X-ray (EDX)
spectroscopy (Supporting Information Figure S3) and
inductively coupled plasma−atomic emission spectroscopy
(ICP−AES; Supporting Information Table S1), which is almost
identical to that of the precursors. This observation indicates
the atomic ratio of the products could be effectively controlled
by the addition content of the precursors.
The nature of the particle surface was studied by Fourier
transform infrared (FT-IR) spectra and X-ray photoelectron
spectroscopy (XPS). Supporting Information Figure S4 shows
the FT-IR spectra of as-prepared Pd, AgPd4, and Ag NCs. The
peaks around 3440 and 1630 cm−1 of Pd, AgPd4, and Ag NCs
were assigned to the N−H stretch and N−H scissor,
respectively. These two peaks shifted to higher wavenumbers
compared with pure octadecylamine, indicating the interaction
of NCs with the nitrogen atoms of residual capping ligands.
The AgPdx NCs were further characterized by XPS. A slight
enrichment of silver on the surface of AgPdx NCs was observed
(Supporting Information Table S1). The surface compositions
of AgPd2, AgPd4, AgPd6, AgPd9, and AgPd19 were determined
to be Ag1.7Pd2, Ag1.3Pd4, Ag1.3Pd6, Ag1.2Pd9, and Ag1.7Pd19,
respectively. As shown in Figure 2, the binding energies of the
Ag 3d5/2 lie in the range of 367.6−367.7 eV, which are lower
(about 0.6 eV) than the bulk Ag (368.2 eV). The Pd 3d5/2
binding energies obtained from AgPdx (x = 19, 9, 6) are very
near the value of bulk Pd (335.4 eV); however, when the
atomic ratio of Ag/Pd further was increased to 1/4 and 1/2, the
Pd 3d5/2 binding energies exhibited negative shifts of 0.2 and
0.4 eV, respectively. The negative core level shifts relative to
bulk Ag and Pd could be ascribed to the increase in the charge
density in the d band with a loss in the sp band.27
Figure 3. TEM images of AgPd4 NCs obtained at different reaction
times: (a) 1, (b) 3, (c) 10, and (d) 30 min. (e) XRD patterns and (f)
UV−vis spectra of the products obtained from different reaction times.
typical TEM images of products obtained at different reaction
times in the preparation of AgPd4.28 In a 1 min reaction, most
of the products were 10 nm NCs (Figure 3a). As the reaction
proceeded, the 10 nm NCs gradually disappeared while more
and more 4 nm NCs appeared (3−10 min, Figure 3c−d). In a
30 min reaction, all the products evolved to 4 nm NCs. Figure
3e and f shows the corresponding XRD patterns and absorption
spectra of the samples shown in Figure 3a−d, respectively. The
products collected in a 1 min reaction are mainly Ag NCs, as
indicated by the XRD and characteristic SPR (surface plasma
resonance) band at 409 nm observed in the absorption
spectrum. Thereafter (3−30 min), the characteristic SPR peak
of Ag disappeared, and the XRD patterns shifted to higher 2θ
The formation process of AgPdx NCs was investigated
through time-dependent TEM, XRD, and UV−vis studies by
using AgPd4 as a model system. Figure 3a−d displays a series of
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Letter
degree, indicating the Ag−Pd alloy formed at the consumption
of Ag NCs. The chemical process observed in the growth of
AgPd4 could be attributed to galvanic replacement of Ag with
Pd.29
The as-prepared AgPdx NCs were then supported on carbon
black and submitted to the HDC reaction under mild
conditions (room temperature and atmosphere pressure).
First, AgPd4/C (Supporting Information Figure S5) was used
as a probe catalyst to screen the best reaction conditions in the
HDC of 4-chlorophenol. Previous studies revealed that the HCl
formed during the reaction could poison the Pd and decrease
the reaction rate.30 Therefore, neutralizer is necessary, and we
used NaOH as the base to test different solvents (Table 1,
Figure 4. (a) The yield of phenol versus time during the HDC
reaction catalyzed by different AgPdx NCs/C and (b) TOFs in the first
hour for the HDC reaction of 4-chlorophenol. Conditions: 4-
chlorophenol (1 mmol, 128.6 mg), base (1.5 mmol, 60.0 mg), catalyst
loading (0.6 mol % based on initial 4-chlorophenol), THF (3 mL), n-
tridecane (0.3 mmol), H2 balloon, 25 °C, 3 h.
a
Table 1. Effect of Different Solvents and Different Bases
the C−Cl bond.31 In the dechlorination of 1,2-dichloroethane
on Ag−Pd/SiO2, Heinrichs and co-workers found that
chlorides first formed on the Ag surface from 1,2-dichloro-
ethane, followed by a HDC process triggered by hydrogen
adsorbed on Pd.25,26 Considering the addition of Ag promoted
the HDC reaction of 4-chlorophenol, as observed in this work,
a sound mechanism was proposed as follows: 4-chlorophenol
first absorbs on the surface of the AgPdx NCs, and the breaking
of C−Cl bond occurs on the Ag sites. H atoms supplied via the
dissociation of H2 on Pd sites facilitate the HDC reaction and
render the regeneration of metallic Ag from chlorinated silver
(Supporting Information Figure S6).
In conclusion, narrowly distributed AgPdx NCs with
controllable compositions have been prepared through a facile
method. The as-prepared NCs performed well in a HDC
reaction of 4-chlorophenol into phenol under mild conditions
(room temperature and atmospheric pressure), and a
pronounced composition-dependent catalytic activity of
AgPdx NCs/C in this reaction was observed, which further
prompts us to develop optimal bimetallic nanocatalysts with
improved activities. We envision such a rational approach could
be extended to other novel heterogeneous catalysts.
b
entry
base
solvent
DMSO
yield (%)
1
2
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
KOH
0
CH3OH
1,4-dioxane
H2O
14
47
51
60
66
99
98
3
4
5
DMF
6
toluene
THF
7
8
THF
c
9
Na2CO3
THF
10
c
10
CH3CH2COONa
THF
27
a
Conditions: 4-chlorophenol (1 mmol, 128.6 mg), base (1.5 mmol),
catalyst loading (0.6 mol % based on initial 4-chlorophenol), solvent
(3 mL), n-tridecane (0.3 mmol), H2 balloon, 25 °C, 4 h. Determined
b
c
by GC. Reaction time was prolonged to 24 h.
entries 1−7). The yields of phenol in dimethyl sulfoxide
(DMSO), methanol, 1,4-dioxane, H2O, N,N-dimethylforma-
mide (DMF), toluene, and tetrahydrofuran (THF) were 0,
14%, 47%, 51%, 60%, 66%, and 99%, respectively. Different
bases were also tested in the THF system (Table 1, entries 7−
10). It shows that the yield of the HDC reaction is greatly
affected by the basicity of neutralizer for HCl. When the
neutralizer is a strong base (NaOH, KOH), the reaction
equilibrium moves to the right, and the reaction rate is high. In
contrast, the reaction rate is sluggish when the base is weak
(NaCO3, CH3CH2COONa).
Using THF and NaOH as the optimal solvent and base, we
explored the role of Pd and Ag in the HDC reaction. Figure 4a
shows the yield of phenol versus time during the HDC reaction
catalyzed by different catalysts. Not surprisingly, Ag nano-
particle (11.9 nm, Supporting Information Figure S2a) was
inactive, indicating Pd is the intrinsic active sites for the HDC
reaction.25,26 Meanwhile, as the benchmark catalysts, commer-
cial Pd/C (Figure 4a) was found to be inferior to the AgPdx
NCs/C catalysts, implying the promoting role of Ag in the
HDC reaction. Moreover, the atomic ratio of Ag/Pd was found
to have a pronounced influence over the activity of the AgPdx
NCs/C catalysts. As displayed in Figure 4b, the turnover
frequencies (TOFs) of AgPdx NCs/C catalysts showed a typical
volcano curve: it initially improved along with the increase in
Pd (x < 9) until it reached a peak value at 322 h−1, then it
dropped when more Pd was added. According to Sinfelt’s
report, Ag is able to absorb the chlorinated molecules and break
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details, HRTEM image of AgPd4 NCs, TEM
images of as-prepared Ag and Pd NCs, ICP and XPS results,
effects of different solvents, EDX spectroscopy of as-prepared
AgPdx NCs, TEM image of AgPd4 supported on carbon black,
TOF calculation details. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: (Z.N.) zqniu@mail.tsinghua.edu.cn, (Y.L.) ydli@mail.
tsinghua.edu.cn.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the State Key Project of
Fundamental Research for Nanoscience and Nanotechnology
(2011CB932401 and 2011CBA00500), National Key Basic
Research Program of China (2012CB224802), and the
National Natural Science Foundation of China (Grants Nos.
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Letter
(29) Chen, J.; Wiley, B.; McLellan, J.; Xiong, Y.; Li, Z.-Y.; Xia, Y.
Nano Lett. 2005, 5, 2058−2062.
20921001 and 21131004). We are grateful for helpful
discussions and suggestions from Prof. Wei He (Tsinghua
University).
(30) Aramendia, M. A.; Borau, V.; Garcia, I. M.; Jimenez, C.; Lafont,
F.; Marinas, A.; Marinas, J. M.; Urbano, F. J. J. Mol. Catal. A: Chem.
2002, 184, 237−245.
ABBREVIATIONS
(31) Fung, S. C.; Sinfelt, J. H. J. Catal. 1987, 103, 220−223.
■
NCs, nanocrystals; CPs, chlorophenols; HDC, hydrodechlori-
nation; XRD, X-ray diffraction; EDX, energy dispersive X-ray;
ICP−AES, inductively coupled plasma−atomic emission spec-
troscopy; XPS, X-ray photoelectron spectroscopy; SPR, surface
plasma resonance; THF, tetrahydrofuran; TOF, turnover
frequency
REFERENCES
■
(1) Niu, Z. Q.; Wang, D. S.; Yu, R.; Peng, Q.; Li, Y. D. Chem. Sci.
2012, 3, 1925−1929.
(2) Hong, J. W.; Kang, S. W.; Choi, B.-S.; Kim, D.; Lee, S. B.; Han, S.
W. ACS Nano 2012, 6, 2410−2419.
(3) Wang, D. S.; Zhao, P.; Li, Y. D. Sci. Rep. 2011, 1, 37.
(4) Wu, J. B.; Yang, H. Nano Res. 2011, 4, 72−82.
(5) Lim, B.; Jiang, M.; Yu, T.; Camargo, P. H. C.; Xia, Y. N. Nano Res.
2010, 3, 69−80.
(6) Yu, X. F.; Wang, D. S.; Peng, Q.; Li, Y. D. Chem. Commun. 2011,
8094−8096.
(7) Niu, Z. Q.; Peng, Q.; Zhuang, Z. B.; He, W.; Li, Y. D. Chem.
Eur. J. 2012, 18, 9813−9817.
(8) Wu, Y. E.; Wang, D. S.; Zhao, P.; Niu, Z. Q.; Peng, Q.; Li, Y. D.
Inorg. Chem. 2011, 50, 2046−2048.
(9) Yin, Z.; Zhou, W.; Gao, Y. J.; Ma, D.; Kiely, C. J.; Bao, X. H.
Chem.Eur. J. 2012, 18, 4887−4893.
(10) Yang, C. W.; Chanda, K.; Lin, P. H.; Wang, Y. N.; Liao, C. W.;
Huang, M. H. J. Am. Chem. Soc. 2011, 133, 19993−20000.
(11) Xu, J.; Wilson, A. R.; Rathmell, A. R.; Howe, J.; Chi, M.; Wiley,
B. J. ACS Nano 2011, 5, 6119−6127.
(12) Chang, S. H.; Su, W. N.; Yeh, M. H.; Pan, C. J.; Yu, K. L.; Liu,
D. G.; Lee, J. F.; Hwang, B. J. Chem.Eur. J. 2010, 16, 11064−11071.
(13) Zhang, J.; Yang, H.; Fang, J.; Zou, S. Nano Lett. 2010, 10, 638−
644.
(14) Mazumder, V.; Chi, M.; More, K. L.; Sun, S. J. Am. Chem. Soc.
2010, 132, 7848−7849.
(15) Nair, A.; Pillai, M. K. K. Sci. Total Environ. 1992, 121, 145−157.
(16) Graham, L. J.; Atwater, J. E.; Jovanovic, G. N. AIChE J. 2006, 52,
1083−1093.
(17) Arcadi, A.; Cerichelli, G.; Chiarini, M.; Vico, R.; Zorzan, D. Eur.
J. Org. Chem. 2004, 2004, 3404−3407.
(18) Babu, N. S.; Lingaiah, N.; Prasad, P. S. S. Appl. Catal., B 2012,
111, 309−316.
(19) Diaz, E.; Casas, J. A.; Mohedano, A. F.; Calvo, L.; Gilarranz, M.
A.; Rodriguez, J. J. Ind. Eng. Chem. Res. 2008, 47, 3840−3846.
(20) Molina, C. B.; Pizarro, A. H.; Casas, J. A.; Rodriguez, J. J. Water
Sci. Technol. 2012, 65, 653−660.
(21) Sun, C.; Wu, Z.; Mao, Y.; Yin, X.; Ma, L.; Wang, D.; Zhang, M.
Catal. Lett. 2011, 141, 792−798.
(22) Lambert, S.; Ferauche, F.; Brasseur, A.; Pirard, J. P.; Heinrichs,
B. Catal. Today 2005, 100, 283−289.
(23) Heinrichs, B.; Delhez, P.; Schoebrechts, J. P.; Pirard, J. P. J.
Catal. 1997, 172, 322−335.
(24) Heinrichs, B.; Noville, F.; Schoebrechts, J. P.; Pirard, J. P. J.
Catal. 2000, 192, 108−118.
(25) Heinrichs, B.; Schoebrechts, J. P.; Pirard, J. P. J. Catal. 2001,
200, 309−320.
(26) Heinrichs, B.; Noville, F.; Schoebrechts, J. P.; Pirard, J. P. J.
Catal. 2003, 220, 215−225.
(27) Slanac, D. A.; Hardin, W. G.; Johnston, K. P.; Stevenson, K. J. J.
Am. Chem. Soc. 2012, 134, 9812−9819.
(28) The reaction temperature was lowered to 170 °C, at which
temperature the reaction speed was slower.
1563
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