Immobilizing highly catalytically active noble metal nanoparticles on reduced graphene oxide: A non-noble metal sacrificial approach

Article abstract of DOI:10.1021/ja511511q

In this work, we have developed a non-noble metal sacrificial approach for the first time to successfully immobilize highly dispersed AgPd nanoparticles on reduced graphene oxide (RGO). The Co₃(BO₃)₂ co-precipitated with AgPd nanoparticles and subsequently sacrificed by acid etching effectively prevents the primary AgPd particles from aggregation. The resulted ultrafine AgPd nanoparticles exhibit the highest activity (turnover frequency, 2739 h^-1 at 323 K) among all the heterogeneous catalysts for the dehydrogenation of formic acid to generate hydrogen without CO impurity. The sacrificial approach opens up a new avenue for the development of high-performance metal nanocatalysts.

Full text of DOI:10.1021/ja511511q

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Communication
Immobilizing highly catalytically active noble metal nanoparticles
on reduced graphene oxide: a non-noble metal sacrificial approach
Qiang Xu
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 28 Dec 2014
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Immobilizing highly catalytically active noble metal
nanoparticles on reduced graphene oxide: a non-noble
metal sacrificial approach
Journal: Journal of the American Chemical Society
Manuscript ID: ja-2014-11511q.R2
Manuscript Type: Communication
Date Submitted by the Author: 20-Dec-2014
Complete List of Authors: Chen, Yao; National Institute of Advanced Industrial Science and
Technology (AIST),
Zhu, Qi-Long; National Institute of Advanced Industrial Science and
Technology (AIST),
Tsumori, Nobuko; Toyama National College of Technology,
Xu, Qiang; National Institute of Advanced Industrial Science and
Technology (AIST),
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Immobilizing highly catalytically active noble metal nanoparticles on
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†,‡
,†
Yao Chen, Qi-Long Zhu, Nobuko Tsumori, and Qiang Xu∗
†
National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka, 563-8577, Japan
Toyama National College of Technology, 13, Hongo-machi, Toyama, 939-8630, Japan
‡
Supporting Information Placeholder
activity for the dehydrogenation of formic acid in the
heterogeneous catalytic system.
ABSTRACT: In this work, we have developed a non-noble
metal sacrificial approach for the first time to successfully
immobilize highly dispersed AgPd nanoparticles on reduced
graphene oxide (RGO). The Co (BO ) co-precipitated with AgPd
3
3 2
nanoparticles and subsequently sacrificed by acid etching
effectively prevents the primary AgPd particles from aggregation.
The resulted ultrafine AgPd NPs exhibits the highest activity
–1
(
turnover frequency, 2739 h ) among all the heterogeneous
catalysts for the dehydrogenation of formic acid to generate
hydrogen without CO impurity. The sacrificial approach opens up
a new avenue for the development of high-performance metal
nanocatalysts.
Supported noble metal nanoparticles (NPs) have attracted great
attention in catalysis due to their high activity in chemical
reactions. Surface atoms often function as active sites, and thus
the control of NP size can tailor the catalytic activity. Unfortu-
nately, primary particles have high surface energies, rendering
1
severe Oswald ripening or grain growth during the synthesis. An
2
efficient approach is to immobilize the NPs on special supports.
Among all the available support materials, reduced graphene ox-
ide (RGO) consisting of chemically converted monolayer carbon
atoms has emerged as one of the most promising supports for
catalysts because the unique advantage of hydrophilicity and large
specific surface area of its precursor, graphene oxide (GO), makes
it possible to anchor well-dispersed metal NPs in the solution-
Figure 1. XRD patterns of the precipitations of the reaction be-
tween Co(CH
nealing (823 K, 3 h, Ar), (c) JCPDS: 25-0102 (Co
Co Ag0.1Pd0.9/RGO and (g)
(Co
COOH) and NaBH
(a) before and (b) after an-
3
2 4
(BO ), (d)
)
3 2
3
3
based controlled reduction. However, it is still a major obstacle
6
Ag0.1Pd0.9, (e) (Co
)Ag0.1Pd0.9/RGO.
6
)Ag0.1Pd0.9, (f) Co
6 6
to obtain monodispersed metal particles with very small sizes on
unfunctionalized RGO, where the aggregation of NPs cannot be
4
Cobalt was chosen as the sacrificial agent for the NNMSA. The
reaction between Co(CH COOH) and NaBH in a molar ratio of
perfectly overcome. Hence, immobilizing highly dispersed noble
3
2
4
metal NPs with high catalytic activity on RGO remains a critical
challenge in the heterogeneous catalysis.
1: 3.7 under an ambient condition for 2 h results in the formation
of a black precipitation, which exhibits very broad X-ray diffrac-
tions (XRD) (Figure 1a), characteristic of an amorphous phase.
The sample crystallizes by annealing in Ar atmosphere at 823 K
for 3 h, displaying the XRD peaks (Figure 1b) corresponding to
On the other hand, formic acid (FA) has been proposed as a
5
suitable liquid carrier for hydrogen, which can be formed by a
6
7
biomass processing and reduction of carbon dioxide. Hydrogen
generation from FA holds great potential for application to
10
Co (BO ) (JCPDS: 25-0102, Figure 1c). The precipitation
3
3 2
8
hydrogen powered fuel cells (FCs), and hence downsized metal
Co (BO ) can be readily dissolved in H PO (Figure S1). The
3
3 2
3
4
NPs as high-performance nanocatalysts are highly desirable for
reaction between NaBH4 and the mixed solution of
Co(CH COOH) , AgNO and K PdCl in a molar ratio of 6: 0.1:
9
this reaction. Herein, for the first time, we report the
3
2
3
2
4
immobilization of ultrafine AgPd NPs on RGO by a new non-
noble metal sacrificial approach (NNMSA), in which a cobalt
compound co-precipitated during the reduction of precursors and
subsequently sacrificed by acid etching prevents the noble metal
NPs from aggregation. The resulted catalyst exhibits the highest
0.9 also produces a black precipitation Co Ag Pd0.9, giving an
6
0.1
additional XRD peak at 39.9° for Pd (Figure 1d). Etching
Co Ag Pd with H PO (2 h, room temperature) gives the sam-
6
0.1
0.9
3
4
ple (Co )Ag0.1Pd0.9, for which the broad XRD band due to
6
Co (BO ) disappears while the characteristic Pd peaks are clearly
3
3 2
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observed (Figure 1e). In the presence of GO, the reaction between
NaBH and the mixed solution of Co(CH COOH) , AgNO and
of primary AgPd particles on RGO (Figure 2a). More severe
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aggregation is observed for Co Ag0.1Pd0.9/RGO in Figure 2b and
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3
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K PdCl in the same molar ratio (6: 0.1: 0.9) yields
S3a. In contrast, (Co )Ag Pd /RGO shows highly dispersed
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6
0.1
0.9
Co Ag Pd /RGO, which is followed by the H PO etching to
NPs (2–4.5 nm) on RGO in Figure 2c, d and S3b. Combining with
the XRD and ICP results, it can be concluded that the well-
dispersed AgPd NPs are immobilized on RGO by NNMSA,
illustrating that the sacrificial Co (BO ) prevents the primary
6
0.1
0.9
3
4
form (Co )Ag0.1Pd0.9/RGO. Both of Co Ag Pd0.9/RGO and
6
6
0.1
(
Co )Ag0.1Pd0.9/RGO exhibit another broad XRD shoulder at 24°
6
1
1
arising from RGO (Figure 1 f and g).
3
3 2
AgPd particles from aggregation as shown in Scheme 1.
Figure shows the nitrogen sorption isotherms of
Ag0.1Pd0.9/RGO, Co Ag Pd /RGO and (Co )Ag0.1Pd0.9/RGO.
The inductively coupled plasma optical emission spectroscopic
ICP-OES) analysis reveals that the molor ratio of Co: Ag: Pd in
(
(
3
Co )Ag0.1Pd0.9/RGO is 0.08: 0.1: 0.9, indicating that almost all
6
6
0.1
0.9
6
the co-precipitated Co (BO ) can be removed by H PO . In the
When AgPd NPs are loaded on RGO by direct deposition, the
restacking of RGO is partially avoided. As result,
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X-ray photoelectron spectroscopic (XPS) measurements, no Co
2p signals are observed for (Co )Ag Pd0.9/RGO before and after
a
Ag0.1Pd0.9/RGO yields a type-IV curve with a hysteresis loop and
6
0.1
2
Ar sputtering (Figure S2a). The Ag 3d and Pd 3d spectra reveal
a Brunauer–Emmett–Teller (BET) specific surface area of 183 m
–1
that (Co )Ag0.1Pd0.9/RGO is composed of metallic Ag and Pd
g , indicative of mesopores between the RGO layers.
Co Ag Pd /RGO shows a very low N uptake with a specific
surface area as low as 6 m g , implying that the surface of sepa-
rated RGO are completely occupied by a large amount of
Co (BO ) with AgPd, which is consistent with the TEM observa-
6
1
2
covered by a thin oxidized layer which can be easily removed by
Ar sputtering (Figure S2b and c).
6
0.1
0.9
2
2
–1
3
3 2
tions. It is worth noting that a drastically increased N uptake is
2
2
–1
observed for (Co )Ag0.1Pd0.9/RGO (310 m g ), illustrating that
6
the elimination of Co (BO ) by the H PO4 etching endows
3
3 2
3
(Co )Ag0.1Pd0.9/RGO with high porosity, which would facilitate
6
the diffusion of reactants to the metal NPs in catalysis.
Figure 2. TEM images of (a) Ag0.1Pd0.9/RGO, (b)
Co Ag Pd /RGO, and (c, d) (Co )Ag0.1Pd0.9/RGO.
6
0.1
0.9
6
Scheme 1. Schematic illustration of immobilizaiton of AgPd NPs
on RGO by (a) the direct deposition and (b) the non-noble metal
sacrificial approach (NNMSA).
Figure 3. Nitrogen adsorption–desorption isotherms of
Ag0.1Pd0.9/RGO, Co Ag Pd /RGO and (Co )Ag0.1Pd0.9/RGO at
6
0.1
0.9
6
7
7 K.
Dehydrogenation of FA was selected as a probe reaction to in-
vestigate the catalytic performance of highly dispersed AgPd NPs
on RGO. A series of (Co )Ag1-yPdy/RGO catalysts (x represents
x
the molar ratio of Co/(Ag + Pd) and y is the molar percentage of
Ag in AgPd) are tested for hydrogen generation from FA in a FA–
sodium formate (SF) system. At y = 0.9, the catalytic activity for
the dehydrogenation of FA increases with increasing the amount
of sacrificial Co (BO ) until x reaches 6 (Figure 4 and S4), illu-
3
3 2
minating that the non-noble metal sacrifical agent plays a decisive
role in giving rise to high catalytic activity by decreasing the size
of AgPd NPs. It is found that the molar ratio of SF to FA has an
obvious effect on the performance of the (Co )Ag0.1Pd0.9/RGO
6
catalyst. The activity of the catalyst for decomposition of FA in-
creases with the molar ratio of SF to FA until the value reaches
2
.5, after which further increase in the ratio of SF to FA does not
give more positive effect on the decomposition of FA (Figure S5).
The decompositon of FA catalyzed by (Co )Ag0.1Pd0.9/RGO
6
The morphologies of Ag0.1Pd0.9/RGO, Co Ag0.1Pd0.9/RGO and
(nAgPd/nFA = 0.02, n /n = 2.5) is completed within 1.1 min with
6
SF FA
(
Co )Ag0.1Pd0.9/RGO are investigated by transmission electron
the generation of 146 mL gas (H + CO ). Gas chromatography
6
2 2
microscopy (TEM). Ag0.1Pd0.9/RGO exhibits severe aggregation
measurements demonstrate that no CO can be detected from the
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evolved gas (H + CO ) at the level of 1 ppm (Figure S6). It is
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noted that the activity of (Co )Ag0.1Pd0.9/RGO is much higher
6
than that of (Co )Pd/RGO though monometallic Ag NPs on RGO
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is inactive for this reaction (Figure S7), indicating that Ag is an
13
excellent promoter for Pd-catalyzed FA decomposition. Addi-
tionally, both (Co )Ag0.1Pd0.9/RGO and Ag0.1Pd0.9/RGO show
6
much better catalytic properties than the support-free
(
Co )Ag0.1Pd0.9 and Ag Pd catalysts (Figure S8), suggesting
6 0.1 0.9
that RGO as a support to immobilize the metal NPs is essential
and effective for avoiding their aggregation. (Co )Ag Pd0.9/RGO
6
0.1
-
1
gives an average TOF of 2756 ± 180 h at 323 K based on a full
conversion of FA catalyzed by AgPd (Figure S4a), indicating that
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(
Co )Ag0.1Pd0.9/RGO is the most active catalyst as the highest
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-
1
TOF record (2623 h ) was reported with the Pd/MSC-30 catalyst
for the heterogeneous dehydrogenation of formic acid at 323 K.
It is worth noting that TOF of (Co )Ag0.1Pd0.9/RGO at 298 K (453
h ) is lower than that of Pd/MSC-30 (750 h ), opposite to the
results at 323 K, for which the higher apparent activation energy
of (Co )Ag Pd /RGO (43.1 kJ mol ) than that of Pd/MSC-30
14
6
-1
-1
–
1
6
0.1
0.9
1 14
–
(38.6 kJ mol ) accounts (Figure 5). For durability test, the
(Co )Ag0.1Pd0.9/RGO catalyst was recycled by washing with water
6
after each cycle, of which the activity kept almost unchanged
under the same reaction condition over 5 cycles (Figure S9), in
agreement with the retained particle size of AgPd on RGO (Figure
S10). The strong anchoring effect between the AgPd NPs and the
RGO surface in (Co )Ag0.1Pd0.9/RGO makes NNMSA an efficient
6
approach towards the development of high-performance hetero-
geneous metal nanocatalysts.
In summary, we have developed a non-noble metal sacrificial
approach (NNMSA), in which Co (BO ) co-precipitated with
3
3 2
AgPd NPs and subsequently sacrificed by acid etching plays an
important role in immobilizing highly dispersed AgPd NPs on
Figure 5. (a) Volume of the generated gas (CO + H ) versus time
2
2
RGO. The particle size of AgPd in (Co )Ag0.1Pd0.9/RGO prepared
6
for the dehydrogenation of FA over (Co )Ag0.1Pd0.9/RGO at dif-
6
by NNMSA is much smaller than that in Ag0.1Pd0.9/RGO prepared
ferent temperatures, (b) Arrhenius plot and TOF values of the
by the direct deposition method. The (Co )Ag0.1Pd0.9/RGO cata-
6
dehydrogenation of FA over (Co )Ag0.1Pd0.9/RGO (nAgPd/nFA
=
6
lyst shows much higher activity than Ag0.1Pd0.9/RGO for the FA
dehydrogenation. The present strategy opens up a new avenue for
the development of high-performance metal nanoparticle cata-
lysts.
0
.02, n /n = 2.5).
SF FA
ASSOCIATED CONTENT
Supporting Information
Experimental details, additional material characterization and
catalytic performance. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
E-mail: q.xu@aist.go.jp
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
The authors thank the reviewers for valuable suggestions, Ms. Jun
Li for XPS and Dr. Takeyuki Uchida for TEM measurements and
AIST and METI for financial support.
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