Metal Nanoparticles Covered with a Metal-Organic Framework: From One-Pot Synthetic Methods to Synergistic Energy Storage and Conversion Functions

Article abstract of DOI:10.1021/acs.inorgchem.6b00911

Hybrid materials composed of metal nanoparticles and metal-organic frameworks (MOFs) have attracted much attention in many applications, such as enhanced gas storage and catalytic, magnetic, and optical properties, because of the synergetic effects between the metal nanoparticles and MOFs. In this Forum Article, we describe our recent progress on novel synthetic methods to produce metal nanoparticles covered with a MOF (metal@MOF). We first present Pd@copper(II) 1,3,5-benzenetricarboxylate (HKUST-1) as a novel hydrogen-storage material. The HKUST-1 coating on Pd nanocrystals results in a remarkably enhanced hydrogen-storage capacity and speed in the Pd nanocrystals, originating from charge transfer from Pd nanocrystals to HKUST-1. Another material, Pd-Au@Zn(MeIM)₂ (ZIF-8, where HMeIM = 2-methylimidazole), exhibits much different catalytic activity for alcohol oxidation compared with Pd-Au nanoparticles, indicating a design guideline for the development of composite catalysts with high selectivity. A composite material composed of Cu nanoparticles and Cr₃F(H₂O)₂O{C₆H₃(CO₂)₃}₂ (MIL-100-Cr) demonstrates higher catalytic activity for CO₂ reduction into methanol than Cu/γ-Al₂O₃. We also present novel one-pot synthetic methods to produce composite materials including Pd/ZIF-8 and Ni@Ni₂(dhtp) (MOF-74, where H₄dhtp = 2,5-dihydroxyterephthalic acid).

Full text of DOI:10.1021/acs.inorgchem.6b00911

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pubs.acs.org/IC
Metal Nanoparticles Covered with a Metal−Organic Framework:
From One-Pot Synthetic Methods to Synergistic Energy Storage and
Conversion Functions
,
†,‡
§
,†,∥,⊥
Hirokazu Kobayashi,* Yuko Mitsuka, and Hiroshi Kitagawa*
†
Division of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa, Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
JST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
‡
§
∥
Shoei Chemical Inc., 5-3 Aza-wakazakura Fujinoki-machi, Tosu-shi, Saga 841-0048, Japan
Institute for Integrated Cell-Material Sciences, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan
⊥
INAMORI, Frontier Research Center, Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan
ABSTRACT: Hybrid materials composed of metal nanoparticles and metal−organic
frameworks (MOFs) have attracted much attention in many applications, such as
enhanced gas storage and catalytic, magnetic, and optical properties, because of the
synergetic effects between the metal nanoparticles and MOFs. In this Forum Article,
we describe our recent progress on novel synthetic methods to produce metal
nanoparticles covered with a MOF (metal@MOF). We first present Pd@copper(II)
1
,3,5-benzenetricarboxylate (HKUST-1) as a novel hydrogen-storage material. The
HKUST-1 coating on Pd nanocrystals results in a remarkably enhanced hydrogen-
storage capacity and speed in the Pd nanocrystals, originating from charge transfer
from Pd nanocrystals to HKUST-1. Another material, Pd−Au@Zn(MeIM) (ZIF-8,
2
where HMeIM = 2-methylimidazole), exhibits much different catalytic activity for
alcohol oxidation compared with Pd−Au nanoparticles, indicating a design guideline
for the development of composite catalysts with high selectivity. A composite material composed of Cu nanoparticles and
Cr F(H O) O{C H (CO ) } (MIL-100-Cr) demonstrates higher catalytic activity for CO reduction into methanol than Cu/γ-
3
2
2
6
3
2 3
2
2
Al O . We also present novel one-pot synthetic methods to produce composite materials including Pd/ZIF-8 and Ni@Ni (dhtp)
2
3
2
(
MOF-74, where H dhtp = 2,5-dihydroxyterephthalic acid).
4
1
1
12
1
. INTRODUCTION
HKUST-1, and ZIF-8, using a solid grinding method
followed by H reduction. The solution infiltration is reported
2
Metal−organic frameworks (MOFs) or porous coordination
polymers, consisting of metal ions connected by organic
bridging ligands, have received much attention in a wide range
of applications such as gas storage
optical, magnetic, and ion-conducting properties, owing to
as a convenient method to prepare nanoparticles immobilized
in MOFs (Figure 1b). When the MOFs are soaked in the metal
precursor solution, the precursors fill the MOF pores by
1
−3
4,5
and separations, and
6
,7
capillary force, which is followed by H or NaBH reduction to
2
4
8
−10
obtain nanoparticles immobilized within the MOFs. This
solution infiltration method has been employed for the
synthesis of various nanoparticles/MOF composites such as
Cu/MIL-101, Ag/HKUST-1,
(MOF = MOF-5, HKUST-1, MIL-100, and MIL-101 ),
their high porosity and designability.
Recently, multifunc-
tional MOF composites, which include metal nanoparticles,
have received increasing interest because of the possibility of
enhancing gas storage and catalytic, magnetic, and optical
properties, all of which have been attributed to the synergistic
effects between the MOFs and metal nanoparticles.
1
5,16
23,24
Au/MIL-101,
Pd/MOF
43
34
28
33
44−50
and Pt/MOF (MOF = MIL-101
), which were tested for
Various methods for synthesizing metal nanoparticle/MOF
composites have been developed, and accordingly several types
of composite structures have been reported that relate to the
method of synthesis (Figure 1). The first attempts at loading a
MOF with metal nanoparticles were performed by simply
their catalytic properties. Moreover, using this method,
31
25
26
bimetallic nanoparticles such as Ni/Pt, Au/Ni, Au/Pd,
29
and Pd/Cu have been successfully incorporated into MOFs.
A chemical vapor deposition or chemical vapor infiltration
(CVD or CVI) method has also been used to create
11−13
5
1−66
mixing the MOF and metal precursor via solid grinding
or
nanoparticles/MOF composites.
By this method, a
14−50
solution infiltration,
and the metal nanoparticles/MOFs
precursor is loaded into porous MOFs in the vapor phase
were fabricated by reducing the metal precursor. A solid
grinding method is reported as a classical method to obtain
metal nanoparticles deposited on a MOF (Figure 1a). Haruta
and co-workers reported Au nanoparticles deposited on several
11
Special Issue: Metal-Organic Frameworks for Energy Applications
Received: April 14, 2016
11
11
11
porous MOFs, CPL-1, CPL-2, MIL-53(Al), MOF-5,
©
XXXX American Chemical Society
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Figure 1. Representative structures of the composites consisted of metal nanoparticles and a MOF.
7
1
and is subsequently decomposed and/or reduced to obtain
nanoparticles inside the MOF pores (Figure 1b); this method is
advantageous because there is better control over the resultant
nanoparticle size. Fischer and co-workers first employed MOF-
MOF copper(II) 1,3,5-benzenetricarboxylate (HKUST-1).
Remarkably, the Pd nanocrystals covered by HKUST-1 have
twice the storage capacity of the bare Pd nanocrystals.
The composite material with a core/shell structure, where
HKUST-1 is coated onto the surface of the Pd nanocrystals,
was synthesized via a bottom-up approach. In a typical
synthetic method, Pd nanocrystals with a flat surface were
used as the core of the composite material. Pd nanocubes
were first prepared in an aqueous solution by the reduction of
5
as a host framework and obtained Cu, Pd, and Au
5
nanoparticles/MOF composites by using (η -C H )Cu(PMe ),
5
5
3
3
5
(
η -C H )Pd(η -C H ), and (CH )Au(PMe ) as organometal-
3 5 5 5 3 3
51
lic precursors, respectively. So far, various kinds of nano-
103
5
1,53,59,61,62
51,52
51,53
55
particles such as Pd-,
Ni-,
Au-,
Cu-,
Mg-,
5
6,57
63
64
Pt-, and Ru/MOF composites have been obtained
Na PdCl with ascorbic acid in the presence of bromide ions as
2
4
through the gas-phase method. Furthermore, bimetallic Ni/
5
8
66
the capping agents to form {100} facets, as reported previously.
Following this, HKUST-1 coating onto the Pd nanocubes was
performed by forming a suspension of Pd nanocubes in an
ethanolic solution of Cu(NO ) ·3H O and trimesic acid and
Pd and Ru/Pt nanoparticles/MOFs were also successfully
created by simultaneous gas-phase loading of the corresponding
alloy precursors.
3
2
2
Recently, nanoparticles/MOF materials have also been
prepared through encapsulation of presynthesized nanoparticles
stirring at room temperature for 48 h.
67−70
To examine the structure of the composite material, the
powder X-ray diffraction (PXRD) measurements with synchro-
tron radiation were performed at the BL02B2 beamline of
SPring-8 with a wavelength of 1.000 Å. As shown in Figure 2a,
by growing the MOF around them (Figure 1c).
The
functional cavities of the MOF remain unimpeded, and the
composite materials can provide a synergistic function that is
derived from both the nanoparticles and MOFs. The core/shell
structure composed of a nanoparticle core and a MOF shell has
received increasing attention for the development of gas
7
1
72−85
67,72,86−91
storage and magnetic,
lytic
optical,
and cata-
9
2−102
materials. Because the MOFs work as molecular
sieves, the core/shell composites are expected to be effective
catalysts with high selectivity and activity, as well as efficient
molecular sensing materials. In addition, if the MOF can act as
a partition between nanoparticles, it is also expected to prevent
the sintering of nanoparticles during the reaction, resulting in
high durability. The interfacial region between MOFs and metal
nanocrystals also plays an important role for combining the full
potential functionalities of each component. In this Forum
Article, we describe our recent progress in the development of
metal nanoparticles covered with a MOF (metal@MOF) for
hydrogen-storage or catalytic materials. Novel synthetic
methods for the composite materials are also presented.
2
. HYDROGEN STORAGE IN PD@HKUST-1
Hydrogen is an essential component in many industrial
processes and also a potential clean energy source. As a
hydrogen-storage material, the metal/MOF composite is a
promising candidate. In particular, Pd/MOF composites have
received much attention because Pd can absorb large amounts
of hydrogen at ambient temperature and pressure. For example,
1
4
28
Pd/SNU-3(Zn) and Pd/MIL-100(Al) have been reported
to exhibit an enhanced hydrogen-storage uptake because of the
spillover effect (the diffusion of H atoms dissociated on the Pd
surface into the MOF), although the total hydrogen-storage
capacity of the Pd did not change. In this section, we
demonstrate the first example of altering the hydrogen-storage
properties of Pd nanocrystals by coverage with the porous
Figure 2. (a) PXRD pattern and TEM images of (b) Pd nanocrystals
and (c) a Pd@HKUST-1 composite material. The radiation
wavelength was 1.000 Å. Reprinted with permission from ref 71.
Copyright 2014 Nature Publishing Group.
B
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the diffraction pattern of the composite material consists of
separate contributions from Pd and HKUST-1. From the
transmission electron microscopy (TEM) images shown in
Figure 2b,c, the mean diameters of the Pd nanocrystals before
and after coating were estimated to be 10.0 ± 0.9 and 10.0 ±
0
.9 nm, respectively, so the mean diameter and the shape of the
Pd nanocube were maintained. In addition, the MOF thin film
was observable around the surface of the Pd nanocubes after
hybridization with HKUST-1. We carried out high-angle
scattering annular dark-field scanning transmission electron
microscopy (HAADF-STEM) and energy-dispersive X-ray
(EDX) analysis to investigate the composite state of Pd and
HKUST-1 in detail.
Figure 3a shows a HAADF-STEM image of the Pd@
HKUST-1 composite. Parts b and c of Figure 3 correspond to
Figure 4. PC isotherms of Pd nanocrystals and Pd@HKUST-1 at 303
K. Reprinted with permission from ref 71. Copyright 2014 Nature
Publishing Group.
storage capability of Pd nanocrystals is doubly enhanced by the
HKUST-1 covering.
The hydrogen-storage capacity of Pd is known to correlate
104
with the electronic states of Pd.
For Pd@HKUST-1, the
change in the electronic structure due to the HKUST-1 coating
on the surface of Pd was examined by X-ray photoelectron
spectroscopy (XPS) measurements. As shown in Figure 5, the
Figure 3. (a) HAADF-STEM image and (b−d) EDX mappings of
Pd@HKUST-1: (b) Cu element; (c) Pd element; (d) an overlay of the
Pd and Cu elements. Reprinted with permission from ref 71.
Copyright 2014 Nature Publishing Group.
Pd and Cu STEM−EDX maps as the main constituent metals
of Pd nanocubes and HKUST-1, respectively. An overlay of the
Pd and Cu maps shown in Figure 3d revealed that the Cu
elements of HKUST-1 are distributed around the surface of the
Pd nanocubes. From these results, it is concluded that we
successfully synthesized the composite material of Pd nano-
crystals coated with HKUST-1.
We then investigated the hydrogen-storage capacity of Pd@
HKUST-1 by measurement of the hydrogen pressure−
composition (PC) isotherms at 303 K using an automatic PC
isotherm apparatus (BELSORP-max, MicrotracBEL Corp.).
Figure 4 shows the results of PC isotherms for Pd nanocrystals
and Pd@HKUST-1. The logarithm of the hydrogen pressure is
plotted against the hydrogen concentration (H/Pd). Both
materials absorbed hydrogen with increasing hydrogen
pressure, but the hydrogen concentration at 101.3 kPa was
Figure 5. XPS spectra of (a) Cu 2p and (b) Pd 3d for Pd nanocrystals,
HKUST-1, and Pd@HKUST-1. Reprinted with permission from ref
71. Copyright 2014 Nature Publishing Group.
binding energy of Cu 2p originating from HKUST-1 is shifted
to lower energy by coverage of the surface of the Pd
nanocrystals. On the other hand, the binding energy of Pd
3d is shifted to higher energy. The XPS spectra of Pd@
HKUST-1 suggest that the electronic states of Pd@HKUST-1
are different from those of bare Pd nanocubes and pure
HKUST-1 and that charge transfer occurs from the Pd
nanocubes to HKUST-1 in the composite.
In order to discuss the relationship between the significantly
enhanced hydrogen-storage capacity and the electronic state of
Pd@HKUST-1, we first examined the electronic states of Pd
and palladium hydride (PdH). Figure 6 shows the calculated
0
.5 H/Pd in bare Pd nanocubes, while the H/Pd in Pd@
104
HKUST-1 was 0.87. Pd@HKUST-1 absorbs nearly double the
amount of hydrogen of bare Pd nanocubes. Taking into
total densities of states (DOS) of pure Pd and PdH.
As
shown in Figure 6a, the DOS of Pd around the Fermi level E is
F
consideration that pure HKUST-1 adsorbs nearly zero H at
composed of the 5s band with a broad bandwidth and the 4d
band with a narrow bandwidth. The total number of valence-
2
303 K (Figure 4, inset), the results indicate that the hydrogen-
C
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Figure 6. Electronic DOSs of Pd and PdH around the Fermi level.
Reprinted with permission from ref 104. Copyright 1978 American
Physical Society.
band electrons around E is 10, the same as the number of
F
valence electrons of a Pd atom.
When hydrogen is absorbed inside the Pd lattice to form
chemical bonds, Pd−H bonding and Pd−H antibonding states
are formed between nonbonding states of Pd (Figure 6b).
Then, the 4d conduction bands of Pd are filled up by the
addition of a 1s electron of the H atom, and the energy level at
EF apparently rises. Further hydrogen absorption in Pd causes
destabilization of the electronic states of PdH because the 1s
electron of the H atom begins to occupy the 5s band, which has
a low DOS. Therefore, the hydrogen concentration of Pd
strongly depends on the number of holes in the 4d band. In
other words, it is possible to control the hydrogen
concentration by means of alloying, which is based on the
1
12
113
Figure 7. Electronic DOSs for (a) Pd−Ag
and (b) Pd−Rh
obtained by band-structure calculations. Reprinted with permission
from refs 112 and 113. Copyright 1991 and 2006 American Physical
Society.
9
2−102,110,111
catalysis,
and these materials often exhibit
enhanced catalytic properties because of synergistic effects,
especially when metal nanoparticles are encapsulated within the
MOFs. Furthermore, tuning the pore size of the MOF can
allow the MOF to act as a selective sieve for reactants,
potentially providing high catalytic selectivity. An MOF can be
used as a well-defined and tunable support for metal
nanoparticles, which distinguishes MOFs from other traditional
supports for heterogeneous catalysis. These catalytic studies can
be mainly divided into two categories according to the type of
reaction: liquid- and gas-phase reactions. In this section, we first
introduce Pd−Au@Zn(MeIM) (ZIF-8, where HMeIM = 2-
methylimidazole) as a liquid-phase reaction catalyst, which was
the first report of a bimetallic nanoparticles@MOF catalyst.
Cu nanoparticles/Cr F(H O) O{C H (CO ) } (MIL-100-
Cr) is subsequently presented as an example of a gas-phase
reaction catalyst.
105
change in the number of Pd 4d band holes.
1
06−108
106,109
For examples, Pd−Ag
and Pd−Rh
are classical
hydrogen absorption alloys. In the Pd−Ag−H system, by the
addition of Ag into Pd, the number of available 4d band holes
decreases (Figure 7a) because Ag has one more electron than
Pd. As a result, the hydrogen-storage capacity of Pd−Ag alloys
106−108
is decreased with increasing Ag content.
On the other
hand, in the Pd−Rh−H system, the number of available 4d
band holes increases because Rh has one less electron than Pd
(
Figure 7b). Therefore, the hydrogen-storage capacity of Pd−
2
106,109
Rh alloys increases with increasing Rh content.
Thus, the
92
hydrogen-storage capacity of Pd-based hydrogen-storage alloys
can be controlled on the basis of the band filling of Pd.
In our case, the enhanced hydrogen-storage capacity of Pd
nanocrystals in Pd@HKUST-1 originates from the increase in
the available 4d band holes of Pd due to charge transfer from
the Pd nanocubes to HKUST-1 in Pd@HKUST-1. The
development of hydrogen-storage alloys has typically been
based on alloying. These results demonstrate the first example
to control the hydrogen-storage properties of metal nanocryst-
als by coating a MOF on the metal surface. These findings
suggest that a MOF coating can alter not only the surface/bulk
reactivity of the nanocrystals but also the physical properties,
such as the magnetic or optical properties.
3
2
2
6
3
2 3 2
Pd−Au bimetallic nanoparticles with a mean diameter of 4.6
nm were synthesized in an aqueous solution by reducing
HAuCl under H gas in the presence of Pd nanoparticles, as
reported previously. The ZIF-8 coating on the surface of the
bimetallic nanoparticles was performed by adding Zn(NO ) ·
H O and 2-methylimidazole as ZIF-8 precursors in the
4
2
3
2
6
2
presence of the Pd−Au nanoparticles.
The diffraction pattern of the composite material consisted of
two kinds of Pd−Au and ZIF-8 diffractions. The positions of
the Pd−Au diffraction peaks were located between those of
pure Pd and Au. The lattice constant of the Pd−Au bimetallic
nanoparticles estimated from the Rietveld refinement followed
Vegard’s law. These results suggest that the obtained Pd−Au
nanoparticles form solid−solution alloys in which Pd and Au
are homogeneously mixed at the atomic level. Figure 8a shows
3
. PD−AU@ZIF-8 FOR AEROBIC ALCOHOL
OXIDATION AND CU/MIL-100 FOR CO
REDUCTION
2
The composite materials composed of metal nanoparticles and
MOFs have been particularly developed for the field of
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We also focused on Cu/MOF composites as gas-phase
reaction catalysts for methanol synthesis from CO2 and
hydrogen. MIL-100-Cr with superior thermal, water, and
chemical stability was selected as the MOF. MIL-100-Cr was
114
prepared by a previously reported hydrothermal synthesis.
The composite materials were synthesized by thermal
decomposition of copper acethylacetonate [Cu(acac) ] as a
2
Cu precursor in the presence of MIL-100-Cr. In a typical
synthesis, MIL-100-Cr and Cu(acac) were dispersed in an
2
acetone solution, and the mixture was stirred at room
temperature for 1 h. After impregnation, the solid was collected
by centrifugation. The solid including MIL-100-Cr and
Cu(acac) was then heated at 350 °C for 1 h under vacuum
2
to produce a composite of MIL-100-Cr and Cu nanoparticles
(
Cu/MIL-100-Cr). A γ-Al O -supported Cu catalyst (Cu/γ-
2 3
Al O ) was also prepared by the same method to compare its
2
3
catalytic activity with that of the Cu/MIL-100-Cr catalyst.
As shown in Figure 9a, the diffraction pattern of the
composite material is composed of overlapping patterns of
Figure 8. (a) HAADF-STEM image and (b−f) EDX mappings of Pd−
Au@ZIF-8: (b) Zn element; (c) Pd element; (d) Au element; (e) an
overlay of the Pd and Au elements, and (f) an overlay of the Pd, Au,
and Zn elements. Reprinted with permission from ref 92. Copyright
2
014 Wiley-VCH.
a HAADF-STEM image. Parts b−d of Figure 8 correspond to
Zn, Pd, and Au STEM−EDX maps as the main constituent
metals of ZIF-8, Pd, and Au, respectively. Figure 8b reveals that
ZIF-8 has a hexagonal shape with an approximate size of 250
nm. From an overlay of the Pd and Au maps shown in Figure
Figure 9. (a) PXRD pattern and (b) N sorption isotherms at 77 K for
MIL-100-Cr (black) and Cu/MIL-100-Cr (red). TEM images of (c)
2
8e, it was demonstrated that Pd and Au atoms randomly are
distributed over the whole particle to form a solid−solution
structure, which is consistent with the results of the X-ray
diffraction (XRD) pattern. In addition, no surface solid−
solution Pd−Au nanoparticles are observed, and the nano-
particles are perfectly covered with ZIF-8 (Figure 8f).
Cu/MIL-100-Cr and (d) Cu/γ-Al O .
2
3
diffraction from Cu and MIL-100-Cr. The N2 sorption
isotherms indicated a decrease in the total volume for Cu/
MIL-100-Cr associated with the formation of Cu nanoparticles
(Figure 9b). From the TEM images of Cu/MIL-100-Cr (Figure
9c), the mean diameter of Cu nanoparticles was estimated to be
ca. 45 nm. On the other hand, the mean diameter of Cu
nanoparticles in Cu/γ-Al O was ca. 30 nm (Figure 9d).
In order to investigate the synergistic catalytic activity created
by hybridization of the Pd−Au nanoparticles and ZIF-8, we
tested the activity of the Pd−Au nanoparticles and Pd−Au@
ZIF-8 for aqueous alcohol oxidation. The catalytic oxidations of
cyclopentanol to ketone for Pd−Au nanoparticles and Pd−
Au@ZIF-8 were 82.2 and 1.9%, respectively, and the ZIF-8
coating on the surface of Pd−Au nanoparticles resulted in lower
conversion. Considering that the ZIF-8 pore is a very
hydrophobic because of the existence of the methyl group,
poor conversion of Pd−Au@ZIF-8 may originate from the
inhibition of access hydrophilic reductant onto Pd−Au
catalysts. The decrease in the catalytic activity of Pd−Au
nanoparticles by the ZIF-8 coating indicates a design guideline
for the development of composite catalysts with high selectivity
by choosing an appropriate MOF according to the target
reaction.
2
3
To investigate the catalytic activity of the composite material
for CO reduction into methanol, we carried out activity tests
2
using a fixed-bed flow reactor. The catalytic test was performed
using a gas mixture of H (72%), CO (14%), and He (14%)
2
2
with 140 mL/min at 220 °C. The produced methanol was
analyzed by flame ionization gas chromatography. It is known
that Cu shows almost no activity for the conversion of CO to
2
methanol. In our case, Cu/γ-Al O produced an extremely
2
3
small amount of methanol (0.2 μmol/g ·h). Interestingly, the
cat
Cu/MIL-100-Cr catalyst produced 2.0 μmol/g ·h of methanol,
cat
and the catalytic activity of Cu/MIL-100-Cr was 10 times
higher than that of Cu/γ-Al O . Considering that the mean
2
3
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Figure 10. Representative synthesis methods for metal nanoparticles/MOF composites.
diameter of Cu nanoparticles in Cu/MIL-100-Cr is larger than
that in Cu/γ-Al O , the significantly enhanced catalytic activity
2
3
for the synthesis of methanol may originate from the synergistic
effect at the interface between Cu nanoparticles and MIL-100-
Cr. The detailed mechanism is currently investigated.
4
. ONE-STEP SYNTHETIC METHODS FOR
METAL/MOF: PARTIAL THERMAL DECOMPOSITION
OF MOF AND MICROWAVE (MW) SYNTHESIS
The synthetic methods for metal nanoparticle/MOF compo-
sites are mainly classified into two types; one is hybridization
with metal nanoparticles after MOFs were preprepared as
templates (Figure 10a) and the other is hybridization with a
MOF on preprepared metal nanoparticles (Figure 10b). The
preparation methods reported to date for metal/MOF
nanomaterials have required at least a two-step reaction, so
the further development of facile hybridization methods of
metal and MOFs is essential. In this section, we introduce two
examples of one-step synthetic methods for metal nano-
particles/MOF including a MW irradiation technique (Figure
10c).
As a first demonstration, we describe a one-pot synthesis of
composites composed of Pd nanoparticles and ZIF-8 by the
Figure 11. (a) XRD pattern, (b) IR spectra, and (c) N sorption
isotherms at 77 K for ZIF-8 (black), ZIF-8-MW (blue), and Pd/ZIF-
2
MW method. Zn(NO ) ·6H O, 2-methylimidazole, and
3
2
2
8
-MW (red).
palladium acethylacetonate [Pd(acac) ] were dissolved in
2
N,N-dimethylformamide. The reaction solution was heated at
1
80 °C for 1 h under MW irradiation using Biotage Initiator+.
sorption isotherms were measured at 77 K for ZIF-8, ZIF-8-
MW, and Pd/ZIF-8-MW, respectively (Figure 11c). Before the
As shown in Figure 11a, the XRD patterns reveal that the
sample synthesized by the MW method (ZIF-8-MW) has the
same structure as that of a commercially available ZIF-8,
Basolite Z1200. Furthermore, the sample (Pd/ZIF-8-MW)
synthesized by the same synthetic method as that of ZIF-8-
measurement of N gas sorption, each sample was activated at
2
150 °C for 12 h. For ZIF-8-MW and Pd/ZIF-8-MW, similar
typical type I sorption behaviors originating from the
microporosity were observed as expected for ZIF-8. The slight
decrease in the total volume for Pd/ZIF-8-MW indicates
hybridization with Pd nanoparticles.
We further investigated the composite state of Pd/ZIF-8-
MW using microscopy. Figure 12a shows a bright-field STEM
(BF-STEM) image of Pd/ZIF-8-MW. From Figure 12a, the
mean diameter of Pd nanoparticles was estimated to be 14.5 ±
MW, except for the addition of Pd(acac) , displays an XRD
2
pattern composed of the diffraction peaks of ZIF-8 and Pd. IR
spectra confirm that the absorption bands of ZIF-8-MW and
Pd/ZIF-8-MW are consistent with those of ZIF-8 (Figure
11b). The IR spectrum of Pd/ZIF-8-MW also suggests that
bare Pd without any protecting agent hybridizes with ZIF-8. N2
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Figure 12. (a) HAADF-STEM image and (b−d) EDX mappings of
Pd/ZIF-8-MW: (b) C element; (c) Zn element; (d) Pd element.
3.1 nm. Parts b−d of Figure 12 reveal that the Pd nanoparticles
are distributed around ZIF-8. The MW-assisted synthesis is a
low-cost, rapid, and scalable method. This MW technique is
expected to be a powerful tool for the one-pot synthesis of
various metal nanoparticles/MOF materials in the future.
Finally, we introduce a method to generate nanoparticles in
situ through partial thermal decomposition of a MOF as an
example of a novel one-pot hybridization method for metal@
Figure 14. (a) PXRD patterns of Ni-MOF-74 (black line), 300-12h
(green line), 350-12h (blue line), and 400-12h (red line). The
radiation wavelength was 9.98 Å. (b) Rietveld analysis for 350-12h.
Reprinted with permission from ref 115. Copyright 2015 Royal Society
of Chemistry.
1
15
The lattice constants estimated from Rietveld refinement of
the PXRD pattern for sample 350-12h (Figure 14b) were
consistent with those of each component, MOF-74 and Ni
MOF (Figure 13). We used Ni-MOF-74, Ni (dhtp) (H dhtp
2
4
115
nanoparticles. The crystal size of Ni was estimated to be ca. 5
nm. To investigate the composite state of Ni nanoparticles and
MOF-74 in sample 350-12h, we performed measurements of
HAADF-STEM and EDX analysis. As shown in Figure 15a, Ni
nanoparticles were highly dispersed all over the MOF. The
mean diameter was estimated to be ca. 4 nm, which is in
agreement with the crystal size from the diffraction pattern.
From the high-resolution TEM image, Ni-MOF-74 in sample
Figure 13. Scheme for the synthesis of Ni@MOF via partial thermal
decomposition of a MOF. Reprinted with permission from ref 115.
Copyright 2015 Royal Society of Chemistry.
350-12h exhibited well-defined crystalline lattice fringes, and
the lattice spacing was estimated to be 11 Å, which corresponds
to the Ni-MOF-74 (2−10) lattice plane (Figure 15b). The
mapping data shown in Figure 15d−f demonstrate that the Ni
nanoparticles were distributed within Ni-MOF-74.
=
2,5-dihydroxyterephthalic acid), with large, cylindrical, one-
dimensional pores (diameter of 11 Å) to create a hybrid
material containing Ni nanoparticles. Ni-MOF-74 was heated at
Here, we explain a possible mechanism of Ni@MOF-74 via
partial thermal decomposition of MOF-74 (Figure 16). For
generation of the Ni nanoparticles within Ni-MOF-74, it seems
that the redox activity of the 2,5-dihydroxyterephthalate (dhtp)
ligand is essential for reducing Ni2+ to generate Ni nano-
particles. This is supported by previous reports showing that for
quinine-based metal complexes there is significant electron
2
50−350 °C under vacuum to obtain composites, Ni
nanoparticles contained within MOF-74 (Ni@MOF-74).
Figure 14a shows the PXRD patterns of Ni-MOF-74 heated
at 300, 350, and 400 °C for 12 h under vacuum, respectively
0
(300-12h, 350-12h, and 400-12h). The sample 300-12h shows
a diffraction pattern identical with that of Ni-MOF-74. With
increasing temperature, broad diffraction peaks originating from
a face-centered-cubic (fcc) Ni lattice appeared in addition to
the Ni-MOF-74 pattern (350-12h). The higher heat treatment
temperature at 400 °C (400-12h) gave rise to complete
decomposition of Ni-MOF-74, and only a fcc Ni pattern was
observed.
116
transfer from the ligand to the metal ion. From this, it is
reasonable to consider that, for Ni-MOF-74 undergoing partial
2
+
thermal decomposition, the dhtp ligand reduces the Ni
centers, which is accompanied by cleavage of the Ni−O
bond. This electron transfer would then generate a semi-
quinone-like radical, which would promote further reduction of
G
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Inorganic Chemistry
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reduction, and electrochemical reduction have been reported,
but the synthesis of several nanometer-scale-sized Ni is still
challenging owing to the tendency of Ni nanoparticles of this
size to easily aggregate and oxidize. The new findings provide a
facile synthetic method not only for highly dispersed Ni@MOF
hybrid materials but also for several nanometer-scale-sized Ni.
Actually, the size of the Ni nanoparticles is precisely
controllable at the nanometer level by changing the conditions
115
of the heat treatments, such as temperature and/or time.
This approach is also expected as a useful method for the
creation of various kinds of metal nanoparticle composites
because a MOF-74 analogue, including Mg, Mn, Fe, Co, or Zn
as the metal cation, exists. The further development of
multifunctional materials is expected via this method of thermal
decomposition of MOFs.
5
. CONCLUSION
Over the years, significant progress has been made in the
combination of metal nanoparticles and MOF materials to form
composite materials. In this Forum Article, we introduced novel
synthetic methods for producing the composite materials as
well as some applications, including hydrogen storage and
catalysis. These types of hybrid materials are still in an early
phase of development, so the origin of the enhanced properties
associated with the synergistic effect between the nanoparticles
and MOF and the mechanism of these interactions still remains
unclear. In the future, by clarifying the origins or mechanisms
through systematic studies, we expect that the targeted design
of nanoparticles and MOF will be possible, leading to the
creation of a variety of useful hybrid materials.
Figure 15. (a) TEM image of 350-12h with electron diffraction (inset)
and (b) the high-magnification image. (c) HAADF-STEM image, (d)
C−K STEM−EDX map, and (e) Ni−K STEM-EDX map of 350-12h.
(
f) Reconstructed overlay image of the maps shown in parts d and e.
Reprinted with permission from ref 115. Copyright 2015 Royal Society
of Chemistry.
AUTHOR INFORMATION
Corresponding Authors
E-mail: hkobayashi@kuchem.kyoto-u.ac.jp.
E-mail: kitagawa@kuchem.kyoto-u.ac.jp.
■
*
*
Ni2+ and ultimately generate a more stable quinone (Figure
1
6). These multistep redox reactions are a reasonable
0
Notes
mechanism for the generation of Ni nanoparticles in Ni-
MOF-74 during partial thermal decomposition.
The authors declare no competing financial interest.
Ni is a 3d transition metal and has been studied in various
fields, especially for magnetic or catalytic applications, which
require nanosized Ni. With respect to the synthesis of Ni
nanoparticles, various methods such as sonochemical or
thermal decomposition of organometallic precursors, chemical
ACKNOWLEDGMENTS
■
This work was supported by Core Research for Evolutional
Science and Technology from JST and a Grant-in-Aid for
Scientific Research (B) (Grant 15H03784) from JSPS. The
Figure 16. (a) Structure of MOF-74. (b) Possible mechanism of Ni@MOF-74 via partial thermal decomposition of MOF-74.
H
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Inorganic Chemistry
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authors are grateful to our collaborators who have assisted with
this work, especially Dr. Guangqin Li, Dr. Jared M. Taylor,
Prof. Ryuichi Ikeda, Prof. Yoshiki Kubota, Dr. Kenichi Kato,
Prof. Masaki Takata, Tomokazu Yamamoto, Dr. Shoichi Toh,
Prof. Syo Matsumura, Megumi Mukoyoshi, Dr. Kohei Kusada,
Dr. Mikihiro Hayashi, Prof. Teppei Yamada, Prof. Mitsuhiko
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DOI: 10.1021/acs.inorgchem.6b00911
Inorg. Chem. XXXX, XXX, XXX−XXX