Bimetallic alloy nanocrystals encapsulated in ZIF-8 for synergistic catalysis of ethylene oxidative degradation

Article abstract of DOI:10.1039/c4cc04479g

Highly dispersed PtPd alloy nanocrystals (NCs) were firstly successfully encapsulated in microporous zeolitic imidazolate framework ZIF-8 (PtPd@ZIF-8) with tunable compositions by a bottle around ship approach. The PtPd@ZIF-8 catalyst showed excellent synergistic photocatalytic activity in the transformation of the adsorbed ethylene into CO₂ and H₂O at room temperature. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc04479g

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Bimetallic alloy nanocrystals encapsulated in
ZIF-8 for synergistic catalysis of ethylene
oxidative degradation†
Cite this: Chem. Commun., 2014,
50, 10115
Received 12th June 2014,
Accepted 14th July 2014
Yuanbiao Huang, Yaohong Zhang, Xuxing Chen, Dongshuang Wu, Zhiguo Yi and
Rong Cao*
DOI: 10.1039/c4cc04479g
www.rsc.org/chemcomm
Highly dispersed PtPd alloy nanocrystals (NCs) were firstly success- dialkyl ketones.9a AuPd alloy NPs dispersed on the MIL-101 have
fully encapsulated in microporous zeolitic imidazolate framework been prepared via a colloidal method followed by reduction with
ZIF-8 (PtPd@ZIF-8) with tunable compositions by a ‘‘bottle around NaBH4.10a However, the incorporation of bimetallic alloy NPs into
ship’’ approach. The PtPd@ZIF-8 catalyst showed excellent synergistic MOFs host matrices still remains a great challenge because a
photocatalytic activity in the transformation of the adsorbed ethylene mixture of monometal NPs is often obtained by using traditional
into CO2 and H2O at room temperature.
approaches.3a Moreover, the NPs usually do not occupy the MOF
cavities, but are supported on the external surfaces of MOFs.10
Ethylene is a very important industrial material that produces
Recently, bimetallic alloy nanoparticles (NPs) have received much
attention because they usually show a great enhancement in polyethylene. However, ethylene released from flowers, fruits, and
synergistic catalytic properties due to their interplay of electronic vegetables can accelerate aging and spoilage of the plants, which is
and lattice effects.1 However, the metallic NPs could be dispersed not conducive to long-term storage. Consequently, in order to main-
on supports to prevent agglomeration to the bulk materials, which tain freshness in some special environments, such as fruit storage
generally leads to the loss of the catalytic properties.2 Therefore, areas, the removal of trace amounts of ethylene is necessary.11 The
suitable supports for NP catalysts are imperative.
catalytic oxidation degradation of ethylene into CO2 and H2O is an
Metal–organic frameworks (MOFs), also known as porous effective method to remove low concentrations of ethylene. Ultrafine
coordination polymers or porous coordination networks, have been powdered TiO2 photocatalysts have been used to degrade ethylene.12
emerging as supports for metal NPs due to their high surface areas, Hao have made attempts to develop gold catalysts supported on
tunable pore structures and the presence of organic functional mesoporous Co3O4 for the removal of ethylene.13 Recently, Fukuoka
groups.3,4 Especially, zeolitic imidazolate framework (ZIF) ZIF-8, a found that Pt NPs supported on mesoporous silica MCM-41 show
subclass of MOFs, not only has high thermal and chemical stability,5 high activity for ethylene oxidation under low-temperature condi-
but also shows great potential in gas separation6 and catalysis as a tions.14 However, Au and Pt are very expensive metals. Therefore,
host matrix.7,8c However, to date, most of reported studies have the bimetallic alloys incorporating less expensive metals such as
mainly focused on MOF-immobilized monometal NPs.3 There have Pd could increase the economic value and accelerate the practical
been only a limited number of studies on the use of MOFs as host applications of these catalysts.
matrices to support bimetallic NPs8–10 and even fewer on bimetallic
Herein, we report a general encapsulation strategy for the
alloy NPs embedded into MOFs for heterogeneous catalysis.8a–c–10 full incorporation of PtPd alloy nanocrystals (NCs) into ZIF-8.
Recently, Xu successfully prepared AuNi alloy NPs immobilized on The presynthesized PtPd alloy NCs were completely encapsu-
MIL-101 by using the double solvent method (DSM).8b By using lated in ZIF-8 crystals by the crystallization process of ZIF-8 in
the MOCVD (metal–organic chemical vapor deposition) method, methanol at room temperature. To the best of our knowledge,
Kempe introduced PdNi alloy NPs into the cavities of MIL-101, up to now, there are no reports of using a ‘‘bottle around ship’’
which exhibited synergistic catalytic effects in the hydrogenation of approach to embed bimetallic alloy NPs into MOFs.15
The cubic PtmPdn alloy NCs (with an average edge length of
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, 155, Yangqiao Road West,
Fuzhou, 350002, China. E-mail: rcao@fjirsm.ac.cn; Fax: +86 0591 87648168;
Tel: +86 0591 83796710
6.5 nm, Fig. S1, ESI†) with different compositions, were prepared
using a solvothermal method.16 The obtained alloy NCs can be easily
encapsulated in the crystals of ZIF-8 through the assembly process.
The samples were denoted as PtmPdn@ZIF-8, and m/n is the atomic
ratio of Pt and Pd. For comparison purposes, the monometallic NC
catalysts, Pt@ZIF-8 and Pd@ZIF-8, were also easily prepared using
† Electronic supplementary information (ESI) available: Experimental proce-
dures; TEM images, EDX spectra, nitrogen sorption results, XPS spectra for
catalysts; and results of catalytic recycle reactions. See DOI: 10.1039/c4cc04479g
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 10115--10117 | 10115

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Fig. 2 PXRD patterns of ZIF-8, Pt5Pd5@ZIF-8, Pt@ZIF-8, Pd@ZIF-8, and
Pt5Pd5@ZIF-8 after ten runs of catalysis.
Fig. 1 (a) Representative TEM and HRTEM (b) images of Pt5Pd5@ZIF-8;
(c) edge length distribution of Pt5Pd5@ZIF-8; (d) line-scanning profile
across a cubic Pt5Pd5 NCs encapsulated in ZIF-8, which is indicated in
the inset of (d). Inset in (b) is the FFT pattern of an individual Pt5Pd5 NC.
stabilized by imidazole may play an important role in the assembly
process.7b Compared to the standard binding energy peak values of
Pt(0) and Pd(0), the values are shifted to lower binding energies,
suggesting a modification of the electronic structure of the surface Pt
the same approach. Transmission electron microscopy (TEM) images and Pd atoms, which could further be indicative of the formation of
show that the well dispersed PtmPdn alloy NCs were fully encapsulated bimetallic alloys.10a
in ZIF-8 nanocrystals (about 140 nm) with a rhombic dodecahedral
shape (Fig. 1a, Fig. S4, ESI†). Interestingly, no particles were observed (712.9 cm2
Compared with pure ZIF-8 (1023.5 cm2 gÀ1), Pt5Pd5@ZIF-8
À1) shows decreased Brunauer–Emmett–Teller (BET)
g
on the outside surfaces of each ZIF-8 crystal. The mean size of the surface areas (Fig. S9, ESI†), which may be attributed to the non-
alloys is about 6.2 nm (Fig. 1c), which is similar to that of PtmPdn alloy porous bimetallic NCs and traces of PVP located at the surfaces.7a,b
NCs before encapsulation (Fig. S1, ESI†). This proves that the PtPd However, the incorporation of bimetallic NCs does not alter the
alloy NCs were stable after the process of encapsulation, and did not pore-size distribution of the matrix of ZIF-8 (Fig. S10, ESI†), because
agglomerate to the bulk. A high-resolution TEM (HRTEM) image the larger size of PtPd alloy NCs (6.5 nm) cannot occupy the cavities
shows that it is a single crystal with well-defined fringes (Fig. 1b). It (1.16 nm) of the framework. Therefore, the PtPd alloy NCs are not
has a typical face-centered cubic (fcc) Pd structure, which corresponds encapsulated in the cages of ZIF-8 but located in the intercrystals.7b
to a Pd(111) interplanar spacing of 0.22 nm. However, the lattice
The microporous material ZIF-8 shows good ethylene adsorption
spacing of the nanocube before encapsulation is 0.19 nm, consistent capacity (42 mg gÀ1) at room temperature under atmospheric pres-
with the {200} lattice spacing of the face-centered cubic (fcc) Pt or Pd sure.18 Encapsulation of PtmPdn alloy NCs did not affect the adsorp-
(Fig. S1, ESI†).16 It is obvious that there is a homogeneous distribution tion uptake (Fig. S11, ESI†). PtmPdn@ZIF-8 catalyst systems were then
of Pt/Pd elements in the compositional scanning line profile of investigated in the ethylene oxidation degradation reaction (Table 1).
Pd and Pt across an single nanocube encapsulated in the crystals of Although the parent material ZIF-8 showed high adsorption capture
ZIF-8, suggesting the formation of an alloy structure.16
of ethylene, it could not effectively degrade the captured ethylene
The powder X-ray diffraction (PXRD) patterns of the Pt5Pd5@ZIF-8 under the visible light (Table 1, entry 1). This can be ascribed to the
samples show two sets of peaks (Fig. 2). One set of peaks in the 2y fact that ZIF-8 shows ultraviolet absorptive behavior due to a wide
range of 5–38 degree is identical with that of ZIF-8,5–7 which indicates band gap of 4.9 eV,19 although the solar light consists of B5% UV
that the frameworks of ZIF-8 are stable after the encapsulation (wavelength 200–400 nm).20 It is no surprise that the Pt5Pd5 NCs
process. The other four strong characteristic peaks at 2y = 40.01, showed almost no activity (Table 1, entries 2 and 3), which might be
46.51, 67.81, and 81.61 can be assigned to the (111), (200), (220) and attributed to the agglomeration of the NPs without the protection of
(311) crystalline planes of PtPd alloy NPs with a face-centered-cubic the support.14 Interestingly, the noble metal NPs remarkably promote
(fcc) phase structure, respectively. The four diffraction peaks are the photocatalysis.21 More than 50% ethylene was converted into CO2
located between the corresponding peak positions of Pt@ZIF-8 and and H2O by using Pt@ZIF-8 assisted by solar light from a 300 W
Pd@ZIF-8 (Fig. 2), which strongly proves the formation of a PtPd xenon lamp at room temperature (entry 4), which was higher
alloy.16,17 The X-ray photoelectron spectroscopy (XPS) spectra (Fig. S8, than the conversion achieved using Pd@ZIF-8 almost under the
ESI†) of the Pt5Pd5@ZIF-8 show the binding energies of Pt(0) (73.7 eV same conditions (entry 5). Pt5Pd5@ZIF-8 showed only about 5%
and 70.5 eV, 4f5/2 and 4f7/2, respectively) and Pd(0) (340.6 eV and conversion of ethylene at room temperature or at 80 1C under
335.3 eV, 3d3/2 and 3d5/2, respectively). No obvious peaks of Pt2+ and dark conditions (entries 6 and 7). Consequently, it has been
Pd2+ are observed, which indicates that the PtPd NCs are stable after excluded that heat led to the transformation of ethylene. Expect-
the encapsulation procedure. The PVP protected PtmPdn NCs edly, the bimetallic PtmPdn@ZIF-8 catalyst system showed higher
10116 | Chem. Commun., 2014, 50, 10115--10117
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Table 1 The activity of PtmPdn@ZIF-8 for ethylene oxidation degradationa
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Entry Catalyst
Pt (Pd) loading (wt%) C2H4 conversion (%)
´
41, 5262; (c) A. Corma, H. Garcıa and F. X. L. i. Xamena, Chem. Rev.,
1
ZIF-8b
Pt5Pd5
Pt5Pd5
Pt@ZIF-8
0 (0)
o5
o5
o5
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2c
3d
4
4.95 (2.70)
4.95 (2.70)
5.03 (0)
53.2
5
6
7
8
Pd@ZIF-8
Pt5Pd5@ZIF-8e
Pt5Pd5@ZIF-8 f
Pt3Pd7@ZIF-8
Pt5Pd5@ZIF-8
Pt7Pd3@ZIF-8
0 (2.75)
29.5
4.8
5.0
4.95 (2.70)
4.95 (2.70)
4.95 (6.31)
4.95 (2.70)
4.95 (1.16)
68.2
93.1
82.7
56.1
90.1
83.2
84.3
94.2
9
10
11
12
13
14i
Pt@ZIF-8, Pd@ZIF-8 5.03 (0), 0 (2.75)
Pt5Pd5@ZIF-8g
Pt5Pd5@ZIF-8h
Pt5Pd5@ZIF-8
4.95 (2.70)
4.95 (2.70)
4.95 (2.70)
4.95 (2.70)
15 j Pt5Pd5@ZIF-8
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a
Conditions: 200 mg catalyst; C2H4: 100 ppm; O2: 1.11 Â 104 ppm; N2:
b
balance; temperature, 25 1C; time: 2 h; Xenon lamp, 300 W. 9 h.
c
f
d
e
35 mg catalyst. 35 mg catalyst, dark conditions. Dark conditions.
g
h
i
80 1C, dark conditions. 50 mg catalyst. mg catalyst. 200 ppm
j
C2H4. 50 ppm C2H4.
conversions at 25 1C (entries 8–10). The use of a mixture of pure
Pt@ZIF-8 and pure Pd@ZIF-8 in a 1 : 1 ratio (entry 11) showed a
clearly lower catalytic activity than the corresponding bimetallic
alloy catalyst Pt5Pd5@ZIF-8 (entry 9), which further indicated a
synergistic catalytic effect.9a Notably, over 83.2% ethylene conversion
was accomplished using a very low amount of the catalyst (25 mg,
entry 13) under the similar conditions. Increasing (200 ppm) or
decreasing (50 ppm) the ethylene concentration also gave high
conversion (entries 14 and 15). More importantly, reusability studies
at ambient temperature exhibited no significant decrease in the
conversion after 10 catalytic cycles (Fig. S13, ESI†), showing great
potential applicability to warehouse storage.
In conclusion, we have demonstrated a facile synthetic strategy
via a ‘‘bottle around ship’’ approach to prepare MOF-encapsulated
PtPd alloy NCs with tunable compositions for the first time. The
MOF-immobilized alloy NCs were firstly used in the synergistic
catalysis of ethylene oxidation degradation. The catalytic proper-
ties are highly composition dependent with B50% Pt showing the
optimum activity. It not only reduces the amount of noble Pt used,
but also enhances the catalytic activity. Our studies reveal that
ZIF-8 not only serves as a support to prevent the aggregation of the
alloys, but also adsorbs ethylene and promotes photodegradation
of ethylene into CO2 and H2O.
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The authors acknowledge the financial support from the 973
Program (2011CB932504 and 2013CB933200), NSFC (21273238 and
21221001, and 21331006), the ‘‘Strategic Priority Research Program’’
of CAS (XDA09030102), the NSF of Fujian Province (2014J05022) and
the Chunmiao Project of Haixi Institute of CAS (CMZX-2014-004).
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Chem. Commun., 2014, 50, 10115--10117 | 10117