Hollow Nickel–Cobalt Layered Double Hydroxide Supported Palladium Catalysts with Superior Hydrogen Evolution Activity for Hydrolysis of Ammonia Borane

Article abstract of DOI:10.1002/cctc.201800459

Although ammonia borane (NH₃BH₃, AB) has been identified as an excellent hydrogen-storage medium, the development of a highly active catalyst that can function under mild conditions for controllable hydrogen release is still a great challenge. The synergistic effect induced by interactions between metal nanoparticles and a support has been widely applied in thermocatalytic conversion processes. In this work, Pd nanoparticles (NPs) highly dispersed on hollow NiCo layered double hydroxide (LDH) were designed for efficient hydrogen generation from AB at room temperature. During the hydrolytic dehydrogenation of AB, Pd/α-LDH and Pd/β-LDH exhibited catalytic activities with total turnover frequency (TOF) values of 49.5 and 28.1 min^?1 with activation energy (E_a) values of 20.56 and 37.56 kJ mol^?1, respectively, at 298 K; thus, these catalysts outperform most Pd-based catalysts. The improved catalytic effect was attributed to the controllable size and fine distribution of the Pd NPs and the collaborative effect provided by the hydroxide of α-LDH and the intercalated anions (HO^?). This catalysts design principle can be easily transferred to other catalyst research fields for energy-conversion and -storage purposes.

Full text of DOI:10.1002/cctc.201800459

Accepted Article
Title: Hollow NiCo Layered Double Hydroxide Supported Pd Catalysts
for Superior Hydrogen Evolution Activity for Hydrolysis of
Ammonia Borane
Authors: Yinghua Zhou, Suqin Wang, Zhiyan Zhang, Nicholas
Williams, Yong Cheng, and Jing Gu
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the VoR from the journal website shown below when it is published
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To be cited as: ChemCatChem 10.1002/cctc.201800459
Link to VoR: http://dx.doi.org/10.1002/cctc.201800459
A Journal of
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10.1002/cctc.201800459
ChemCatChem
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Hollow NiCo Layered Double Hydroxide Supported Pd Catalysts
for Superior Hydrogen Evolution Activity for Hydrolysis of
Ammonia Borane
b
Ying-Hua Zhou,*[a, b] Suqin Wang,[a] Zhiyan Zhang,[a] Nicholas Williams,[ ] Yong Cheng,[a] and Jing Gu*[b]
Abstract: Although ammonia borane (NH3BH3, AB) has been
identified as an excellent hydrogen storage medium, there is still a
great challenge towards developing a highly active catalyst that can
function at mild conditions for controllable hydrogen release. The
synergistic effect induced by the interactions between metal
nanoparticles and a support has been widely applied in the thermo-
catalytic conversion process. In this work, Pd nanoparticles (NPs)
highly dispersed onto hollow NiCo layered double hydroxide (LDH),
were designed for efficient hydrogen generation from AB at room
temperature. During the hydrolytic dehydrogenation of AB, Pd/α-LDH
and Pd/β-LDH had been demonstrated to exhibit catalytic activities
with a total turnover frequency (TOF) of 49.5 and 28.1 min-1 and an
activation energy (Ea) of 20.56 and 37.56 kJ mol-1, respectively at 298
K, outperforming most Pd-based catalysts. The improved catalytic
effect can be contributed to the controllable size, fine distribution of
Pd NPs, and the collaborative effect provided by the hydroxide of α-
LDH with intercalated anions(OH-). This catalysts design principle can
be easily transferred into other catalyst research fields for energy
conversion and storage purposes.
towards dehydrogenation.[6,7] Among them, ammonia borane
(NH3BH3, AB) is considered to be an efficient carrier, with superb
stability in aqueous and methanol solutions, a high volumetric
density of 146 g·L-1, and a hydrogen capacity of 196 g·kg-1.[8,9]
Hydrogen stored in the AB can be released by either thermal
decomposition or solvolysis.[10,11] Considering the high energy
input of thermal decomposition (> 100 °C), numerous research
has been focused on the hydrolytic dehydrogenation of AB (eq 1)
in the presenence of metal catalysts under ambient conditions.[12]
Hundreds of metal catalysts have been exploited to promote
the efficiency of AB dehydrogenation.[13] Among them, Pd NPs
based catalytic systems served as a favorable catalyst for AB
hydrolysis, due to their lower costs relative to other precious
metals and higher activity compared to the non-noble metal
catalysts.[14,15] It is well known that although the small-sized metal
NPs can provide more catalytic active sites, such NPs usually
suffer from deactivation and instability issues induced by the
migration and agglomeration of metal NPs in the reaction. This
problem can usually be solved by immobilizing NPs on the
appropriate catalyst supports.[16]
Hydrogen is the simplest and cleanest fuel carrier, usually
utilized in a fuel cell setup to generate power, producing only
water and heat as byproducts.[1,2] Hydrogen produces the highest
energy per mass among any fuel. Unfortunately, due to its low
volumetric density under ambient conditions, the major obstacle
that prevents the building of the hydrogen economy is the
development of advanced storage methodology that have
potential for higher energy density per volume.[3-5] Subsequently,
it is also important for the development of technologies that could
release of hydrogen in a controllable fashion under mild
conditions from said medium. The chemical hydrides are
promising candidates due to their high gravimetric hydrogen
densities, favorable thermodynamic, and kinetic properties
Layered double hydroxides (LDH) are a class of synthetic
anionic clays, which frequently act as a support for catalysts. LDH
can be represented by a formula of [M2+ M3+x(OH)2]x+ (An-)x/n
1-x
•mH2O, which features the positive-charged brucite Mg(OH)2-like
host layers (M2+ and M3+ represent metallic cations) and
exchangeable interlayer galleries, consisting of anions (An-) and
water (H2O).[17] Metal catalysts tethered onto LDH experience
improved catalytic performance by enhancing catalytic efficiency,
selectivity and stability. The metal NPs can be placed in a
controllable and highly dispersed manner on the plane of LDH,
while the preferred orientation of anions will be located at the
interlayer.[18] The LDH selected here is a NiCo compound, and
was chosen for three reasons: 1) this LDH had been proven to act
as an excellent electroactive material, which may facilitate the
charge transfer in between the metal and the support.[19] 2) A
metal organic framework (MOF) template can be used to as a
simple precursor to construct these LDH nanocomposites. After
conversion, it forms a hollow cubic structure, which will increase
the surface-to-volume ratios enhancing the support of the
catalysts.[20] 3) Among NiCo LDH compounds studied, α-phase
has already received extensive attentions,[21] whereas the β-
phase, which might be another active phase, had been rarely
investigated. The α-structure of LDH consists of Ni(OH)2 layers
with intercalated anions or water. However, the β-form presents a
hexagonal close-packed structure of Ni2+ and OH- ions.[22] In
addition to its support functional role, we suspected the OH- anion
on LDH could further improve the catalytic activity of the metal
[a]
Associate Prof. Y.-H. Zhou, S. Wang, Z. Zhang
The Key Laboratory of Functional Molecular Solids, Ministry of
Education, Anhui Laboratory of Molecule-Based Materials (State
Key Laboratory Cultivation Base),
College of Chemistry and Materials Science
Anhui Normal University
Wuhu, Anhui 241002, China
E-mail: yhzhou@ahnu.edu.cn
[b]
Assistant Prof. J. Gu
Department of Chemistry and Biochemistry
San Diego State University
San Diego, CA 92182-1030, USA
E-mail: jgu@sdsu.edu
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201800459
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decorated LDH based on the following discoveries. Recently, the
catalytic mechanism of AB originated hydrogen releasing has
been studied, in which it demonstrated that the formation of
hydroxyl group from the deprotonation of water molecule is the
rate-determining step.[23] Furthermore, it is shown that the OH-
anion could significantly promote the catalytic activity during the
controlled-release of hydrogen from AB.[24] Therefore, it is highly
desirable to develop a catalyst with the superior efficiency of AB
hydrolysis by the synergistic effects induced by both the
interactions between metal NPs and the natural hydroxyl anions
on LDHs.
the edge and center. At the same time, NiCo LDH TEM imaging
(Figure S3) taken from a direction in between two faces
demonstrated that all faces of the nanobox are closed. The
thickness of the shell is about 50 nm, while the depth of the hollow
box is ca. 200 nm. The elemental mapping of a LDH (Figure S2)
nanobox confirms the uniform distribution of the Ni, Co and O
elements throughout the surface of the nanobox. The formation
process of hollow LDH nanoboxes might be explained as the
following process: firstly, during the reaction between Ni(NO3)2
and ZIF-67, an extra proton will be produced. The generated
proton will etch away the center of the ZIF-67 nanocubes while
the Ni2+ ions hydrolyze. The Co2+ ions released from the
decomposed ZIF-67 may be partially oxidized by NO3- ions or O2
in the solution to form Co3+.[27] As the reaction progresses, the
Co2+/Co3+ ions will co-precipitate with Ni2+ ions to form a layer of
NiCo LDH, which eventually leads to formation of hollow NiCo
LDH nanoboxes.
Scheme 1. Schematic diagram for the preparation of Pd/LDH.
Herein, we report a rational design and fabrication strategy of a
nanocatalyst with highly dispersed Pd NPs anchored to the NiCo
LDH hollow nanoboxes. The NiCo LDH nanoboxes were
synthesized by the acidic etching of ZIF-67 nanocubes. The
loading of Pd NPs onto NiCo LDH nanocubes was achieved by in
situ reduction of Pd NPs via NiCo LDH, occuring after the
2-
electrostatic attraction between PdCl4 and positively-charged
LDH in solution. NiCo LDH would serve as both the reductant and
as a size-controlling support (Scheme 1). In comparison with
Pd/β-LDH, Pd/α-LDH catalyst exhibited the superior catalytic
activity and durability towards the AB hydrolysis with a TOF value
-1
of 49.5 molH2 molPd min-1 and Ea value of 20.56 kJ mol-1 under
ambient conditions.
ZIF-67 nanocubes were obtained by a simple precipitation
reaction, in which Co(NO3)2 was added into an aqueous solution
of 2-methylimidazole at room temperature.[25] The as-synthesized
ZIF-67 structure was confirmed by powder X-ray diffraction (XRD)
(Figure S1), in which the diffraction peaks of as-synthesized ZIF-
67 matches well with the simulated results.[26] When ZIF-67
nanocubes, a sacrificial template, were reacted with Ni(NO3)2 in
an ethanol solution, the diffraction peaks of ZIF-67 disappeared,
indicating the complete conversion of ZIF-67 into the formation of
NiCo LDH during the acidic etching process. The FESEM and
TEM images of ZIF-67 and the converted NiCo LDH are shown in
Figure 1, indicating that the original ZIF-67 nanocubes assemble
with a highly uniform size of ~300 nm (Figure 1a and 1b). Since
the inside of LDH nanocubes are empty, the faces of nanoboxes
might collapse and form a fake hole as demonstrated in the SEM
image (Figure 1c). However, TEM (Figure 1d) imaging
unambiguously reveals that the surface of the nanoboxes were
completely enclosed by LDH nanosheets. More LDH nanosheets
are present on the edges of the walls than on the center of the
nanoboxes faces (Figure S2a). Furthermore, TEM (Figure 1d)
image reveals a hollow interior presented at the center of the
nanocubes by the obvious image contrast difference in between
Figure 1. SEM(left) and TEM(right) images of ZIF-67 nanocubes (a, b) and NiCo
LDH nanoboxes (c, d).
TEM images (Figure 2a, b) of the 0.0025 mol/g Pd/α-LDH
indicate that the Pd NPs uniformly dispersed on the surface of the
LDH nanoboxes, showing hollow nanobox morphology with a size
of ~200 nm × 200 nm. The size distribution of Pd NPs was
determined to be in ranges from 3.5-7.5 nm with an average
particle size of ca. 5.6 nm (Figure 2c). HRTEM imaging of the
0.0025mol/g Pd/α-LDH indicates that the lattice space of the Pd
NPs is 0.227 nm (Figure 2d), corresponding well with the lattice
spacing of Pd(111) (JCPDS no. 46-1043). The selected area
diffraction (SAED) pattern inset in Figure 2d further proves the
crystalline structure of Pd nanoparticle. X-ray energy dispersive
spectroscopy (EDS) (Figure S4) demonstrates the co-existence
of cobalt, nickel and palladium elements in the catalyst. The
palladium atomic ratio in Pd/α-LDH and Pd/β-LDH were found to
be 23.78 wt.% and 21.38 wt.%, respectively by ICP-OES (Table
S1). Furthermore, TEM were utilized here to characterize the
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10.1002/cctc.201800459
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metal, as well as the peaks of NiCo LDH. Three peaks located at
40.1, 46.7 and 68.1˚, corresponding to the crystal structure of Pd
(JCPDS no. 46-1043) at (111), (200), (220) orientations were
identified. Another three characteristic peaks, corresponding to
(006), (009), and (110) phases of a typical NiCo LDH structure,
can be observed at 22.5, 33.7, and 60.2˚ (Figure 3a),
respectively.[24] In respect to the XRD patterns of Pd/β-LDH
(Figure 3b), exclusive peaks can be identified at 19.2, 33.0, 38.5,
52.1, 59.0, and 62.7˚, which correspond to the (001), (110), (101),
(102), (110), (111) facets of β-phase Ni(OH)2 (JCPDS no. 14-
0117)[28] and Co(OH)2 (JCPDS no. 51-1731).
XPS was employed here to further uncover the chemical nature
of the catalysts. Figure 4 shows the XPS survey spectra of the
Pd/α-LDH, indicating the presence of Pd, Ni, Co, and O. The
binding energies (BEs) of the 335.3 and 340.5 eV located in the
Pd 3d5/2 and 3d3/2 range can be ascribed to presence of the
metallic Pd(0) state (Figure 4a). Unexpectedly, the Pd2+ peaks
with BE of 337.0 and 342.4 eV are also observed (Figure 4b). The
Pd2+ might have originated from two sources: 1. It might be
generated from the precursor Na2PdCl4. However, the 3d5/2 peak
of Pd2+ in the Na2PdCl4 should be located at 338.0 eV. Thus, the
possibility
Figure 2. Representative TEM images (a, b), the corresponding distribution size
histograms (c) and HRTEM image (d) of 0.0025mol/g Pd/α-LDH. Inset in (d) is
the SAED pattern recorded from the whole particle.
Figure 3. The XRD patterns of Pd/α-LDH (a) and Pd/β-LDH (b) with various Pd
NPs loading quantities.
structures and compositions of NiCo LDH immobilized different
loading amounts of Pd NPs (0.0005, 0.0015, 0.0025, 0.005 mol/g
Pd/α-LDH and Pd/β-LDH) (Figure S5 and S6). From the TEM
images, an intensive degree of particle aggregation and
degradation of the original hollow LDH nanoboxes was noticed
with a Pd high loading amount of 0.005 mol/g Pd/α-LDH and Pd/β-
LDH. One possible reason is that hollow LDH nanoboxes lack
sufficient strength to withstand a large number of nanoparticles
resting on its surfaces, resulting in the collapse of the structure.
Otherwise, the size and shape of the metal NPs does not show
obvious change with the tuning of loading amounts. However, the
density of NPs on the LDHs layer increases with the increasing
loading concentration. Due to the structure confinement effect
induced by the nanoporous LDHs, the Pd NPs are highly
dispersed and have a well-proportioned size distribution. As
compared with α-phase NiCo LDH (Figure 2), a similar particle
size and dispersity were observed for the β phase Pd/β-LDH
(Figure S6). Also, the chemical structure information of pure Pd,
NiCo LDH and Pd/LDH were studied by the XRD (Figure 3). XRD
patterns of the samples exhibit characteristic reflections of Pd
Figure 4. The XPS Survey spectra of Pd/α-LDH sample (a), the Pd 3d peak (b),
Ni 2p peak (c), and Co 2p peak (d) of the Pd/α-LDH sample.
2-
of the PdCl4 absorbed on LDH sheet is excluded. 2. The Pd2+
peaks might be assigned to PdO at the surface of Pd NPs due to
the exposure of oxygen, which has been observed from previous
research.[29] The Ni 2p spectrum reveals two satellite peaks (BE
= 861.7 and 880 eV) and two main peaks (BE= 855.7 and 873.7
eV) (Figure 4c), which are separately identified as Ni 2p3/2 and Ni
2p1/2 verifying the existence of Ni2+. For the Co 2p spectrum, the
Co2+ peaks were located at 781.3 and 796.6 eV with two
accompanied satellite peaks (BE=785.8 and 802.4 eV) (Figure
4d). Moreover, the peaks presents at the 780.1 eV are attributed
to the generation of Co3+ from Co2+.[30] The formation of Co3+ is an
indication that the well-controlled size of Pd NPs was established
by the redox reactions happening in between Co2+ in the NiCo
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2−
LDH support and PdCl4 in the solution,[31] considering that the
(pH 14), due to concentration of OH- exceeding its optimum
level.[40] Pure LDH without loading Pd nanoparticles exhibits
potential value of φ(PdCl42-/Pd)θ and φ(Co(OH)3/Co(OH)2)θ is
0.591 and 0.17 V vs SHE, respectively.
negligible catalytic activity, regardless of
acidic or basic
conditions. Therefore, it could be speculated that AB should be
activated by metal NPs to produce the activated complex
intermediates, which could be attacked by OH- anions and/or
water molecules in the following previously studied mechanistic
,
steps.[13,40 41]
Table 1. Catalytic activity of pallium-related catalysts in hydrogen generation
from the hydrolysis of ammonia borane at 298K.
Catalyst
nmetal/nAB
TOF
(min−1
Ea
(kJ mol−1
Ref.
)
)
Pd/α-LDH
0.022
0.020
0.0189
0.011
0.06
49.5
28.1
45
20.56
37.56
-
This work
This work
[14]
Pd/β-LDH
Pd@MIL-101
Pd/CeO2
Figure 5. Time course for hydrogen production from AB aqueous solution (5 mL,
0.2 M) over pvp-Pd NPs, NiCo LDH, Pd/α-LDH and Pd/β-LDH at 298 K.
29
68
[15]
RGO@Pd
26.3
21.1
15.5
9.7
40
[32]
Pd/PPy
0.002
0.005
0.056
0.027
-
33.5
-
[33]
The catalysts activity was investigated by hydrolyzing of AB, for
hydrogen generation, at room temperature. Pure NiCo LDH is
nearly inactive, whereas PVP-protected Pd NPs release 2.3 equiv
H2 per AB in about 46 min (Figure 5). The catlytic acitivities for
0.0025 mol/g Pd/α-LDH and Pd/β-LDH enhanced, with ~3 equiv
H2 per AB generated within only 2.80 min and 5.20 min,
respectively. Moreover, based on Pd atom amounts determined
by ICP-OES, the turnover frequency (TOF) of Pd/α-LDH has been
found to be 49.5 min−1, which is considerably better than that of
Pd/β-LDH (28.1 min−1). Table 1 displays the activities of 0.0025
mol/g Pd/α-LDH and Pd/β-LDH, in comparison with various Pd
related catalysts documented in the literatures.[14,15,32-39]
Comparing with the other Pd catalysts, we conclude the Pd/α-
LDH presented in this study is one of the state-of-art Pd catalysts
for AB hydrolysis. Mechanistically, the outstanding performance
of this Pd catalyst can be explained by the synergic effect of the
controlled size and distribution loading of the Pd catalyst with the
presence of interlayer anion (OH-), all of which improved the
kinetics of the hydrolysis process. As demonstrated by He et. al.
using kinetic isotope effect measurements, breaking the O-H
bond in the metal-activated H2O is the rate determining step in the
reaction of AB hydrolysis catalyzed by metal NPs.[23] The
introduction of OH- during AB hydrolysis should promote the
kinetics of hydrolysis intermediate and facilitate the hydrolytic H2
evolution process in the confined nanostructure medium. Both
phases contain OH- anions, which is expected to improve the
catalytic activity. Catalytically, the α-phases had superior
performance compared to the β-phases. Better OH- mobility in the
α-phases than the β-phases could explain this phenomena.[24] To
verify this hypothesis, AB dehydrogenation over 0.0025 mol/g
Pd/α-LDH had been performed with the different pH values. It was
found that the catalytic rate of hydrogen production decreased
(Figure S7) when the solution was adjusted to a pH value of 6
compared with pH 8. At the same time the rate of H2 generation
slightly increased when the pH value was raised from 8 to 12.
However, the reaction rate dropped in a more alkaline medium
Pd-CDG
[34]
Pd/graphene
Pd74Co26/MCN
Ag3Pd2/PDA-HNT
Pd@Co/graphene
Pd/PDA-Fe3O4
30.82
58.8
22.8
-
[35]
208.1
90
[36]
[37]
0.09
37.5
14.5
[38]
0.00895
65
[39]
In optimizing the catalytic activity, the loading amount of Pd NPs
immobilized to NiCo LDH is an influential parameter. As shown in
figure S8 the catalytic activity is proportional to the amount of Pd
NPs loaded (<0.0025 mol/g) if the structure remains intact. If the
loading amount passes 0.0025 mol/g Pd/LDH, its activity will
decrease. The possible reason is that the hollow structure of LDH
nanoboxes cannot support catalysts on its surface after a certain
loading amount, without collapsing, which is correlated well with
the material characterization results (Figure S5 and S6).
Concurrently, temperature plays an important role in the
process of hydrogen release from AB. The catalytic
dehydrogenation rates of Pd/α-LDH and Pd/β-LDH towards AB
hydrolysis at different temperatures were investigated (Figure 6).
The results showed that the rate of hydrogen generation rate
increased sharply when the temperature was increased from 298
to 318 K, indicating that a high reaction temperature will enhance
dehydrogenation kinetics and release hydrogen at a slightly
elevated temperature. According to the Arrhenius plots (Figures
6b, d),[41,42] the obtained apparent activation energy (Ea) of the AB
dehydrogenation involving Pd/α-LDH and Pd/β-LDH was
calculated to be 20.56 and 37.56 kJ mol-1, respectively. The Ea of
Pd/α-LDH was lower than most other reported Pd metal catalysts.
The ameliorable activity can be explained by the collaborative
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effects from the confined Pd nanoparticle and OH- anion
enhancing the hydrolysis kinetics.
temperature chemical compounds decomposition.
A
more
efficient strategy for releasing hydrogen under ambient conditions
facilitated by a suitable catalyst is critical for the economically
feasible application of hydrogen storage. Herein, we designed a
novel strategy to construct hollow NiCo LDH nanoboxes, which
were employed as supports to immobilize highly active Pd NPs
for AB hydrolysis. The NiCo LDH was directly converted from ZIF-
67 nanocubes and changed into a hollow structure in the reaction
process. The Pd/α-LDH exhibited superior catalytic activity with a
TOF value of 49.5 min-1 and an extremely low activation energy
of 20.56 kJ mol-1. The presence of interlayer anions (OH-) in the
NiCo LDH played an important role of changing the binding
kinetics and facilitating the activation process in the AB
hydrolyses reaction. In addition, the as-synthesized catalysts
exhibited high recycle stability. This work highlights that the
rational design of materials with controllable morphologies as the
catalysts supports can finely tune the interactions between
supports and active metal NPs. This result demonstrated here
could provide
a
great foundation in understanding the
collaborative effects happening in between catalysts and the
support materials for AB hydrolysis or other catalysts-support
interaction in sustainable energy production and chemical
synthesis applications.
Figure 6. Time course for hydrogen production from AB aqueous solution (5 mL,
0.2 M) and Arrhenius plots of AB hydrolytic dehydrogenation catalyzed by Pd/α-
LDH (a, b) and Pd/β-LDH (c, d) at different temperatures.
Experimental Section
Moreover, the recyclable stability is another crucial parameter
in the practical application of catalysts. The durability tests of
Pd/α-LDH and Pd/β-LDH were carried out at 298 K. As shown in
Figure 7, even after the 5 cycles, the productivity of H2 remained
almost unchanged. The reaction completion time of the
dehydrogenation over Pd/α-LDH is 2.7, 3.5, 3.7, 3.9 and 4.5 min
from first to fifth run, respectively. A slight decrease of reaction
Materials. Cetrimonium bromide (CTAB, Aladdin), Cobalt nitrate
(Co(NO3)2·6H2O, Sinopharm Chemical Reagent), Nickel nitrate
(Ni(NO3)2·6H2O, Sinopharm Chemical Reagent), Palladium chloride
(PdCl2, Aldrich), Sodium chloride (NaCl, Sinopharm Chemical Reagent),
Sodium hydroxide (NaOH, Sinopharm Chemical Reagent), ethanol
(CH3CH2OH, Sinopharm Chemical Reagent, 99%) and ammonia borane
(NH3BH3, Aldrich, 97%) were used as obtained. Deionized water was
utilized in all experiments.
-
rate could be attributed to the increased concentration of BO2
product and the slight increase in the solution viscosity.[43] After 5
cycles the integrity of Pd/α-LDH and Pd/β-LDH structures were
almost entirely preserved and no aggregation of Pd was found to
occur on the surface of LDH (Figure S9), indicating that the two
supported NP catalysts had long durability, which can be
attributed to the formation of NPs immobilized by hollow LDH
supports.
Synthesis of NiCo LDH nanoboxes: The ZIF-67 nanocubes were
prepared via a simple surfactant-mediated method in an aqueous solution
at the room temperature according to the previous literature.[24] 30 mg of
ZIF-67 nanocubes was dispersed into a round bottomed flask containing
20 mL ethanol and ultra-sonicated for 10 min. Further, the collected ZIF-
67 nanocubes were reacted with Ni(NO3)2 to form NiCo LDH in an ethanol
solution with minor modification compared to the literature.[20] 5 mL of
ethanol solution containing 100 mg of Ni(NO3)2·6H2O was slowly added
into the ethanol solution of ZIF-67. The mixture was refluxed for 1.5 h
under stirring at 358 K. Finally, the product NiCo LDH, was collected by
centrifugation, and subsequently washed with ethanol for three times and
vacuum dried at 313 K for overnight.
Synthesis of Pd/α-LDH catalyst: Firstly, Na2PdCl4 stock solution was
prepared by dissolving 44 mg of PdCl2 and 29 mg of NaCl (2 equiv with
respect to PdCl2) in 25 mL of deionized water and stirred at 30 min.
Secondly, 5 mL aqueous solution of 20 mg NiCo LDH was slowly added
into 5 mL Na2PdCl4 solution under vigorous stirring. Thirdly, the pH value
of solution was adjusted to ~10 by 0.1 M NaOH solution. After that, the
mixture was hydrothermally treated at 373 K for 8 h to obtain Pd/α-LDH.
The products were collected by filtration, washing with ethanol for three
times and dried in vacuum at 313 K for 12 h.
Figure 7. Durability test for hydrogen generated from AB aqueous solution over
the as-synthesized catalysts at 298 K. Pd/α-LDH (a) and Pd/β-LDH (b).
Synthesis of Pd/β-LDH catalyst: The procedure was similar to the above
except that condition of hydrothermal treatment is at an elevated
temperature of 393 K for 6 h. Finally, the products were collected by
In summary, hydrogen storage for subsequent use ranges from
approaches utilizing high pressure, cryogenics, and high
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10.1002/cctc.201800459
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washing with ethanol for three times and dried in vacuum at 313 K for 12
h.
Normal University (No. 2015cxsj142), and Undergraduate
Innovative training program of Anhui Normal University
(201610370486, 201610370490). Jing Gu want to acknowledge
the SDSU startup funds, the SDSU University Grants Program,
and NSF award CEBT-1704992.
Characterizations. The crystal phase properties of the synthesized
nanocatalysts were analyzed with a Shimadzu X-ray diffractometer-6000
(XRD-6000) using Cu Ka radiation at 40 kV and 40 mA (λ = 0.1542 nm).
The morphologies and sizes of the samples were determined by using a
transmission electron microscope (TEM, HT-7700), and energy dispersive
X-ray (EDS) spectroscopy on Field Emission Scanning Electron
Microscope (FESEM, S-4800) was used for elemental analysis. The lattice
fringe was captured by a high-resolution transmission electron microscope
(HRTEM, JEOL) at an accelerating voltage of 200 kV. X-ray photoelectron
spectroscopy (XPS) measurements were performed with a Thermo Fisher
Scientific ESCALAB 250X imaging electron spectrometer. Pd contents of
the samples were determined by using Agilent 7700ce inductively coupled
plasma-optical emission spectroscopy (ICP-OES) after the sample was
completely dissolved in a mixture of aqua regia (HNO3/HCl =1/3 volume
ratio).
Keywords: ammonia borane • dehydrogenation • hydrolysis •
heterogeneous catalysis • layered double hydroxide
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This work was supported by the National Natural Science
Foundation of China (NO. 21771004), Natural Science
Foundation of Anhui Province (No. 1408085MKL21), Special and
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Postgraduate Scientific Research and Innovation Project of Anhui
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Entry for the Table of Contents
COMMUNICATION
Pd nanoparticles (NPs) were highly
dispersed onto the hollow NiCo layered
double hydroxide (LDH) derived from
ZIF-67, through the in situ reduction
method. The nanocomposite exhibited
the superior catalytic performance for
hydrogen generation from ammonia
borane in aqueous solution at room
temperature.
Author(s), Corresponding Author(s)*
Ying-Hua Zhou,* Suqin Wang, Zhiyan
Zhang, Nicholas Williams, Yong Cheng
and Jing Gu*
Page No. – Page No.
Hollow
NiCo
Layered
Double
Hydroxide Supported Pd Catalysts for
Superior Hydrogen Evolution Activity
for Hydrolysis of Ammonia Borane
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