Highly Efficient and Selective Co@ZIF-8 Nanocatalyst for Hydrogen Release from Sodium Borohydride Hydrolysis

Article abstract of DOI:10.1002/cctc.201900051

Sodium borohydride hydrolysis catalyzed by composite nanomaterials has long been proposed as a viable possibility for hydrogen generation, but it required either expensive noble metals or sophisticated composites. Herein, the catalysis of NaBH₄ hydrolysis is studied with single earth-abundant Fe, Co, Ni, and Cu nanoparticles supported by a metal organic framework (MOF), ZIF-8. It appears that CoNPs@ZIF-8 is the most efficient and selective catalyst in this series. The turnover frequency (TOF) related to the total atoms number within the cobalt nanoparticles reaches 14000 mL_H2 min^?1 g_Co^?1 at 30 °C, and the TOF(s) related to the number of surface atoms of the cobalt nanoparticles 19400 mL_H2 min^?1 g_Co^?1 at 30 °C. Kinetic studies including a high kinetic isotope effect disclosed using D₂O (k_H/k_D=6.85) and the favorable effect of NaOH lead to the suggestion of an oxidative addition of a O?H bond of water in the rate-determining step.

Full text of DOI:10.1002/cctc.201900051

Accepted Article
Title: Highly Efficient and Selective Co@ZIF-8 Nanocatalyst for
Hydrogen Release from Sodium Borohydride Hydrolysis
Authors: Chun Luo, Fangyu Fu, Xiaojiao Yang, Jianyu Wei, Changlong
Wang, Jing Zhu, Deschun Huang, Didier Astruc, and
Pengxiang Zhao
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Highly Efficient and Selective Co@ZIF-8 Nanocatalyst for
Hydrogen Release from Sodium Borohydride Hydrolysis
Chun Luo,[a] Fangyu Fu,[b] Xiaojiao Yang,[a] Jianyu Wei,[a] Changlong Wang,[b] Jing Zhu,[a] Deshun
Huang,[a] Didier Astruc* [b] and Pengxiang Zhao* [a]
Abstract: Sodium borohydride hydrolysis catalyzed by
composite nanomaterials has long been proposed as a viable
possibility for hydrogen generation, but it required either
expensive noble metals or sophisticated composites. Herein, the
catalysis of NaBH4 hydrolysis is studied with single earth-
abundant Fe, Co, Ni, and Cu nanoparticles supported by a
metal organic framework (MOF), ZIF-8. It appears that
CoNPs@ZIF-8 is the most efficient and selective catalyst in this
series. The turnover frequency (TOF) related to the total atoms
number within the cobalt nanoparticles reaches 14000 mLH2
transition metals in place of noble metal NPs or other
sophisticated composite catalysts.[9] It is well known that
supported transition metal NPs display remarkable catalytic
efficiencies
improvement,
because
NP
supports
synergistically boost the catalytic activities of NP catalysts
for this reaction as well as others.[10-11]
Metal Organic Frameworks (MOFs) are among the most
widely used supports for transition metal NPs.[12] Compared
to other supports, they possess high porosity, low density,
large specific surface area, regularity of pores, adjustable
pore size, diversity of topologies, and curability.[13] Among
the many types of MOF, the ZIF series reported by Yaghi’s
group includes a large number of robust molecular sieve
materials.[14] In 2006, Yaghi et al. reported the synthesis and
structure of ZIF-1 to ZIF-12 including ZIF-8 that has
permanent porosity, high surface area, hydrophobicity,
thermal stability and chemical stability, making it stand out
from the other catalytic carriers.[14a]
-1
min-1 gCo at 30°C, and the TOF(s) related to the number of
-1
surface atoms of the cobalt nanoparticles 19400 mLH2 min-1 gCo
at 30°C. Kinetic studies including a high kinetic isotope effect
disclosed using D2O (kH/kD = 6.85) and the favorable effect of
NaOH lead to the suggestion of an oxidative addition of a O-H
bond of water in the rate-determining step.
Previous reports have shown that ZIF-8 is an excellent
support of transition metal NP-catalyzed reactions.[14e,15] In
order to extend the application of NP@ZIF-8 catalysts for
the hydrolysis of NaBH4, the Fe, Co, Ni, and Cu NP
catalysts supported by ZIF-8 are prepared in this work using
the one-pot reduction method.[14e,15] The nanostructures are
characterized by Transmission Electron Microscopy (TEM),
X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-
Teller (BET) and Inductively Coupled Plasma Optical
Emission Spectroscopy (ICP-OES). Kinetic isotope effects
(KIEs) are used for the mechanistic study of the reaction.
The activities of these first-raw metal@ZIF-8 catalysts are
compared, and in particular focus is applied here to the new
remarkable nanocatalyst CoNPs@ZIF-8 whose activity for
NaBH4 hydrolysis is excellent.
Introduction
Energy problems have become the key of sustainable
development in the last two decades.[1] Hydrogen is clean
and renewable, and it is regarded as the most promising
energy in the future, particularly in the automotive field.[2] For
instance, the Toyota, Hyunai and Honda companies have
recently exhibited their hydrogen-powered vehicles
(https://www.wikiwand.com/en/Hydrogen
vehicle).[3]
However, further improvement of safety and efficient
utilization of hydrogen are still demanded, and hydrogen
storage plays a critical role to achieve this goal.[4] Among all
hydrogen storage materials, NaBH4 hydrogen storage has
attracted a great attention[5-7]
since its discovery by
Schlesinger in the 1950’s.[6] due to i) light weight and small
volume; ii) high purity of hydrogen; iii) controllable hydrogen
production rate; iv) satisfactory hydrogen storage capacity
(10.8 wt.%); v) high stability in alkaline solution; vi) safe and
pollution-free properties. Hydrogen is produced upon NaBH4
hydrolysis according to equation 1:
Results and Discussion
1. Synthesis and Characterizations of the ZIF-8 NPs.
The ZIF-8 NPs are rapidly synthesized in water following
previous reports (see the experimental section).[14e,15] The
ZIF-8 nanomaterial prepared in this way is stable even upon
reflux in methanol or water during more than a day. A TEM
image and the size distribution are represented in Figure S1,
showing their hexagonal shape. In more details, the ZIF-8
NPs are around 79 nm in average size.
NaBH4 + 4H2O  NaB(OH)4 + 4H2
(1)
From the stoichiometry of this reaction, 1 mol NaBH4
produces 4 mol H2 during the hydrolysis process under
ambient conditions, which is much more efficient than other
reported hydrogen storage materials. Hence, a lot of efforts
have been devoted to accelerate the hydrolytic
dehydrogenation of NaBH4 by using efficient, durable and
recyclable catalysts.[8] Although these noble metal
nanoparticles are ideal catalysts for hydrogen generation
from NaBH4 hydrolysis, their high prices and rarity on earth
limits the interest in their practical investigation. Therefore,
developing noble-metal-free catalysts is timely and has
already attracted interest. One possible solution is to use
supported nanoparticles (NPs) of cheap earth-abundant
In order to gain more insight into the ZIF-8 cavities,
Brunauer−Emmett−Telle
(BET)
gas
sorptometry
[15]
measurements
were conducted to examine its porous
nature. Figure S2 displays the N2 adsorption/desorption
isotherm and the pore-size distribution of the synthesized
ZIF-8 NPs. The isotherms are identified as type I (Figure
S2a), which is characteristic of microporous materials. From
the adsorption curve, the N2 adsorption volume increases
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The metal content was measured by Inductively Coupled
Plasma Optical Emission Spectroscopy (ICP-OES).
significantly at very low relative pressures and high relative
pressures, indicating the presence of micropores and
textural mesoporosity. Identical conclusions are obtained in
Figure S2b: the pore-size distribution obtained from the
isotherm presents a number of pores less than 2.5 nm as
well as some mesoporosity between 15 to 37 nm in the
sample. The BET specific surface area of the sample is
calculated from the N2 isotherms at 77.3 K and is found to
be around 1501.9 m2 g-1. The single-point total volume of the
pores at P/P0 = 0.989 is 1.317 cm-3 g-1.
Interestingly, the edges of ZIF-8 become more rounded after
the deposition of the metal NPs such as CoNPs@ZIF-8
(Figure 1a). In order to deeply investigate the sizes of the
MNPs by TEM, the ZIF-8 framework is then digested using a
solution of ethylenediaminetetraacetic acid (EDTA), and the
MNPs are stabilized by poly(vinylpyrrolidone) (PVP, MW =
10000).[15] The TEM image of the Co NPs reveals that the
NPs are quite monodispersed (Fig. S3a). The average size
of the Fe NPs, Co NPs, Ni NPs, Cu NPs are respectively 1.4
nm, 1.5 nm, 2.1 nm, and 3.5 nm. Figure S3 shows these Fe,
Co, Ni and Cu NPs, and Figures S4-S7 show their size
distributions.
2. Synthesis and Characterizations of the Nanocatalysts.
A series of MNPs@ZIF-8 of the late first-raw transition
metals (M = Fe, Co, Ni and Cu) nanocatalysts are prepared
[15]
by the deposition-precipitation (DP) method
with in-situ
reduction by NaBH4. The mixture of metal cations and ZIF-8
is stirred continuously under N2 at ambient temperature for
two hours. Then 1 mL aqueous solution of freshly prepared
NaBH4 is quickly added, and the nanocatalysts are
successfully prepared by further stirring for another 30 min.
(Scheme 1) .
The XPS spectrum of Zn 2p3/2 in ZIF-8 nanocatalysts and
the XPS spectra of Co 2p3/2 in CoNPs@ZIF-8
nanocatalysts have been recorded and are presented in
Figures S8 and S9, respectively. Figure S9 shows the
presence of both Co(0) at 778.9 eV and Co(III) at 780.6 eV
in the CoNPs@ZIF-8 recorded by XPS. It is assumed that
some of the catalyst has been oxidized before or during the
XPS experiments due to extensive handling. The catalysis
experiments are carried out immediately after synthesis with
full protection from air, however, and these samples used for
catalysis are thus believed to be air-free. Consequently in
these samples, the presence of Co(III) is expected to be
negligible.
Scheme 1. Synthesis of the nanomaterials MNPs@ZIF-8.
3. Hydrolysis of sodium borohydride.
3.1. Comparison between various metal NPs.
Here the turnover frequency (TOF) related to the total atom
number within the MNPs and the TOF(s) related to the
number of surface atoms of the MNPs are calculated to
evaluate the hydrolysis of sodium borohydride (the
calculations of the average TOF and TOF(s) values are
given in the SI). The hydrolysis of NaBH4 is conducted in
water at 30 ± 1 °C by adding 6 mol% (measured by ICP-
OES) of the nanocatalyst (Figure 2a). This comparison
demonstrates that the nanocatalyst CoNP@ZIF-8 has the
best TOF of 2935 mLH2
Figure 1. TEM images: (a) CoNPs@ZIF-8 at 20 nm scale; (b) CoNPs
after digestion of the ZIF-8 framework with EDTA, and NP capping with
PVP. See the size distribution histogram in Figure S4
Figure 2. Comparison between various metal NPs in hydrogen release
efficiencies from NaBH4 hydrolysis for the ZIF-8 supported late first-raw
transition metal catalysts: (a) time plots of the hydrolysis of NaBH4
catalyzed by various metal NPs; (b) comparison of TOF values related to
various metal NPs. NaBH4 (0.75mmol) is in water at 30 ± 1 °C with 6
mol% MNPs@ZIF-8 nanocatalyst.
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min-1 gMetal-1 and TOF(s) = 4076 mLH2 min-1 gMetal-1 (See SI)
among the four earth-abundant nanocatalysts. Here ZIF-8
shows a high selectivity for Co NPs among the four metal
NPs, because the steric constraints inside the MOF cavity
bring about dramatic stereoelectronic factors controlling the
selectivity among the distinct MNPs. The TEM images of
MNPs@ZIF-8 (Figure 2) indicate that (i) most of Fe NPs are
outside the ZIF-8 framework, (ii) most of Co, Ni and Cu NPs
are inside the ZIF-8 framework, whereas the dispersibility is
getting worse in turn. It clearly appears that the location of
the FeNPs outside ZIF-8 is responsible for its weak
efficiency. In other terms, localization of MNPs inside ZIF-8
brings efficiency. Selectivity is best for CoNPs, however,
compared to the NiNPs and CoNPs. The observed
polydispersity also is in accord with observed optimized
efficiency and selectivity of CoNPs. Hence, the
Figure 5a shows the plots of the hydrogen volume vs. time
for various amounts of CoNPs. The logarithmic plot of the H2
generation rate vs. the concentration of CoNPs is
represented in Figure S10a. The slope obtained is 1.645,
Figure 4. Hydrolysis of NaBH4 (0.75mmol) catalyzed by CoNPs@ZIF-8 in
the presence of 0.2M-0.5M NaOH (final concentration) in water, at 30 ±
1 °C with 6 mol% nanocatalyst: (a) time plots of the hydrolysis of NaBH4
catalyzed by CoNPs@ZIF-8 in the presence of different NaOH
concentration; (b) comparison between TOF values related to NaOH
concentration.
CoNPs@ZIF-8 nanocatalysts are chosen for further studies.
3.2. Comparison between various supports of the
CoNPs@ZIF-8 catalyst.
The ZIF-8 support has been compared to other supports
including multi-layer MXenes, single-walled carbon
nanotubes and MoS2 (Figure 3a) in order to optimize the
catalyst support for NaBH4 hydrolysis. Figure 3b
demonstrates that ZIF-8 is indeed the best support with a
TOF of 2935 mLH2 min-1 gCo-1 and a TOF(s) of 4076 mLH2
min-1 gMetal-1; thus ZIF-8 is finally selected as the carrier for
further studies of this reaction.
Figure 3. Comparison between various supports for hydrogen release
from NaBH4 (0.75mmol) hydrolysis at 30 ± 1 °C with 6 mol% supported
Co NPs: (a) time plots of the hydrolysis of NaBH4 catalyzed by Co NPs
loading on different supports; (b) comparison of TOF values related to
various supports.
3.3. Influence of NaOH.
Previous reports have shown that the presence of NaOH
enhances the catalytic activity of NaBH4 hydrolysis (see the
references in Table 1). The effect of NaOH concentration on
the hydrolysis reaction of NaBH4 is studied here in order to
eventually further optimize the catalytic efficiency. The effect
of the NaOH concentration is compared between 0.2M and
0.5M (Figure 4a). Figure 4b shows that the optimum
concentration of NaOH for catalytic activity enhancement is
0.4M. In this situation, the TOF is enhanced to 14023 mLH2
,
-1
min-1 gCo-1 and the TOF(s) to 19476 mLH2 min-1 gCo (see
Figure 5. Kinetic studies: (a) time plots of the hydrolysis of NaBH4
catalyzed by Co@ZIF-8 with various catalyst amounts; (b) time plots of
the hydrolysis of NaBH4 catalyzed by Co@ZIF-8 with various NaBH4
concentration; (c) time plots of the hydrolysis of NaBH4 catalyzed by
Co@ZIF-8 at various temperatures.
Table 1). Besides, the best TOF in the presence of 0.4M
NaOH is almost 5 times higher than that found in the
absence of NaOH. Consequently, the following kinetic
studies are tested in the presence of 0.4M NaOH.
3.4. Reaction kinetics, kinetic isotope effect, mechanism
and comparison with literature.
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showing that the H2 generation rate for NaBH4 hydrolysis
catalyzed by CoNPs@ZIF-8 is second-order in catalyst
concentration. In the same way, the slope of the logarithmic
plot of H2 generation vs. concentration of NaBH4 is 1.052
(Fig. 5b and Fig. S10b), showing that the H2 generation rate
is first order in NaBH4 concentration. From Fig. 5c and Fig.
S10c, the activation energy (Ea) is obtained in accordance
with the Arrhenius equation (Equation 2). The calculations
indicate that the activation energy (Ea) is approximately 62.9
kJ/mol of NaBH4 hydrolysis with the nanocatalyst
CoNPs@ZIF-8 (Figure 5e and 5f and Equation 2). This
In the presence of the catalyst, both the hydridic and
protonic hydrogen atoms are transferred to the zero-valent
transition-metal nanoparticle surface by oxidative additions
of the B-H and O-H bonds respectively to form a CoNP-
dihydride before reductive elimination of both H2 and
BH3OH- The most difficult oxidative addition of the water O-
H bond is facilitated by hydrogen bonding with the hydridic
hydrogen of borohydride that decreases the electronic
density of the water O-H bond (Scheme 2). The OH- anions
coordinate to the NP surface thus enhancing its electron
density, which activates the oxidative addition step up to a
value also is lower than most Co-based catalysts (vide infra). certain extent. We suspect that when the OH- concentration
becomes too high, too many surface-OH ligands inhibit the
activation of the substrates by the NPs.
Ea
(2)
lnk  lnA 
RT
Where k（mLH2 min-1 gCo-1）is the reaction rate, A is the pre-
exponential factor, R (8.314 J mol-1 K-1) is gas constant, and
T (K) is the reaction temperature.
Previously, it has been demonstrated that half of the
hydrogen atoms in H2 comes from water for the metal-
catalyzed borohydride hydrolysis,[7a] and the O-H bond
cleavage of H2O is the rate-determining step as measured
by the kinetic isotope effect (KIE).[16a] Here, the KIE is further
investigated to investigate the rate-determining step (RDS)
of the NaBH4 hydrolysis reaction catalyzed by CoNPs@ZIF-
8. Indeed, the KIE value found for the present reaction
(Figure 6) including the NaBH4−D2O system, kH/kD = 6.85, is
very high and represents a primary KIE, showing that the O-
H bond cleavage of H2O proceeds in the RDS.
Scheme 2. Suggested initial phase of the mechanism for the
CoNPs@ZIF-8-catalyzed NaBH4 hydrolysis. BH3OH- produced undergoes
a
similar mechanism, and in such sequences the H atoms are
successively substituted by OH groups on boron.
Conclusion
A straightforward, gentle, and environmentally friendly way
has been disclosed to synthesize MNPs@ZIF-8 (M = Fe, Co,
Ni and Cu) nanocatalysts. In the process of synthesizing
nanocatalysts by in-situ reduction, ZIF-8 is used as a
template for the syntheses of metal NPs to make
monodispersed metal NPs. Among these four cheap earth-
abundant metals, simple Co NPs have the best TOF of 2935
mLH2 min-1 gCo-1 and TOF(s) = 4076 mLH2 min-1 gCo-1 at 30 °C
without NaOH. Practically, the catalytic activity of the
CoNPs@ZIF-8 nanocatalyst is enhanced from TOF = 2935
-1
mLH2 min-1 gCo to 14023 mLH2 min-1 gCo-1 and to TOF(s) =
Figure 6. Hydrogen release in the NaBH4-H2O system and in the NaBH4-
D2O system catalyzed at 30±0.1°C by 6 mol% CoNPs@ZIF-8 (KIE).
19476 mLH2 min-1 gCo in the presence of 0.4 M NaOH at
-1
30 °C, whereas a TOF value of 25717 mLH2 min-1 gCo-1 and a
TOF(s) value of 35718 mLH2 min-1 gCo are obtained at
-1
It appears that this reaction obeys the Langmuir-
Hinshelwood (LH) model:[16b] i) two reactants are adsorbed
on the solid catalyst, then the adsorbed molecules undergo
a bimolecular reaction; ii) the adsorption and desorption rate
is much greater than the surface reaction rate. This means
that the chemical reaction of the two adsorbed molecules is
the RDS. The LH model (Scheme 2) according which the
reaction kinetics is controlled by surface restructuration is
shown by the fact that the value of the second Damköhler
number (DaII),[16b] equal to 0.015, is <<1 (SI).
40 °C.This exceeds the performances of most non-noble
metal catalysts, and even some noble metal catalysts.
As shown in Table 1, the catalytic performance of
CoNPs@ZIF-8 for NaBH4 hydrolysis surpasses that of most
Co-based catalysts owing to the regular pore size and large
specific surface area of ZIF-8. This cobalt catalyst is also
about as efficient as the sophisticated Co composites, but it
presents the great advantage of simplicity.
The high selectivity for this Co catalyst is a specific property
of the ZIF-8 supporting template. The very high kinetic
isotope effect disclosed here shows that the O-H bond
cleavage of water clearly occurs in the rate-determining
state. In the ground state, the Na+BH3H-…HOH ensemble
involves hydrogen bonding between a hydridic borohydride
hydrogen atom and an acidic hydrogen atom of water.
Extraction of these two hydrogen atoms to form H2 is not
possible under ambient conditions in the absence of catalyst.
The high deuterium kinetic isotope effect indicates activation
of the hydridic borohydride hydrogen and protic water
hydrogen by the metal nanoparticle surface to form the
dihydride that further reductively eliminate to form H2. The
hydrolysis of another hydrogen precursor, ammonia-borane
was best catalyzed by NiNPs@ZIF-8 or a Ni-containing alloy,
at variance with the present results indicating that Co is the
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Experimental Section
Table 1. Comparison of recent literature results on Co-based catalysts for
the hydrolysis of NaBH4.
1.
General data:
Catalyst
n(catalyst
)/n(NaBH4
)
TOF
Ea
T
Ref.
(mLH2·min-
1·g-1)
(kJ·m
ol-1)
(°C)
All the flasks were washed with a solution of aqua regia (VHCl/VHNO3
= 3:1) to remove any metal residue. Milli-Q water was used in all
reactions. Milli-Q water was bubbled with nitrogen for 1 hour to
remove oxygen before catalyst preparations and catalytic reactions.
All the catalytic reactions were conducted under nitrogen in order to
avoid any oxidation of the NPs.
Co-BC
11%
-
31000[a]
58.8
-
80
30
[17]
[18]
Co-B NPs
26000[b]
-
-
-
Fe-Co-B/Ni foam
Co-Ni-B/Cu sheet
Co-Sn-B/GP
Co-B/Ni foam
Co NPs@C2N
Co2B
22000[b]
14778[b]
11272[a]
11000[b]
8903[b]
27
30
25
30
30
30
25
25
50
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
2. Characterizations. Transmission electron microscope (TEM).
The TEM images were recorded by Tecnai G2 F20 S-TWIN (200 kV).
Brunauer−Emmett−Teller (BET) measurements were made at 77.3
42.8
39.6
33
K
under N2 on an automatic volumetric adsorption apparatus
(QUADRASORB 2QDS-30). The sample was dried at 60℃ in vacuo
overnight before the test. Around 100 mg samples were filled into
glass ampoules and outgassed in high vacuum at 573 K for 3 h
before the start of the sorption measurements. X-ray photoelectron
spectra (XPS) measurements were performed on a PHI5000 XPS
system with an Al Ka X-ray source (1486.6 eV photons). The core-
level signals were obtained at a photo electron take off angle of
45°with respect to the sample surface. The X-ray source was
operated at a reduced power of 150 W. The pressure in the analysis
chamber was maintained at 10-8 Torr or lower during each
measurement. The sample was stuck to the conductive paste, and
the surface charge of the sample was neutralized using a low-
energy electron beam. Therefore, the C1s hydrocarbon peak at
283.8 eV was not subjected to charge correction. The metal content
of all the samples was measured by Inductively Coupled Plasma
Optical Emission Spectroscopy (ICP-OES) using an Agilent 5100
ICP-OES spectrometer.
-
6%-11%
66.2
77.9
42.1
35.4
-
8500[b]
CoPNAs/Carbon
Cloth
Co@g-C3N4-rGO
composite
Intrazeolite
cobalt(0)
nanoclusters
-
5960(CoP)
5865[a]
25.90%
5575[a]
34
25
[27]
13%
5360[a]
5233[b]
37.1
16.5
25
30
[28]
[29]
Co-Co3O4@C-II
Co0.9Cu0.1
-
-
Hydroxyapatite-
supported
cobalt(0)
5000[a]
53
25
[30]
13%
nanoclusters
3. Preparation of the ZIF-8 NPs: According to a previous report, 2-
methylimidazole (22.99g, 272 mmol) was dissolved in 80 mL water,
and stirring was conducted at 35 °C in order to form a homogeneous
solution. Then, an aqueous solution of Zn(NO3)2•6H2O (1.19g, 4
mmol) dissolved in 9 mL water was rapidly injected into the aqueous
solution. The mixture was stirred for 30 min, and the product was
collected by centrifugation and washed with water (repeated for
three times). The ZIF-8 NPs were then dried at 60 °C in vacuo
overnight. After fine grinding, these NPs were dried for further use.
-
4028[b]
4000[a]
-
30
25
[31]
[32]
Co2P nanowires
Co(CO3)0.5(OH)·0.1
1H2O nanowire on
Ti
19%
39.8
Co-ZIF-9
-
-
-
-
3642[a]
-
40
25
50
35
[33]
[34]
[35]
[36]
Co3O4 NA/Ti
1940(CoO4)
563[a]
59.8
40.2
57.7
Co-F Cubes
CoB/ZIF-8
4. Preparation of the nanocatalysts: Typically, a Schlenk flask
was charged with 50 mg ZIF-8/MXenes/CNTs/MoS2 under nitrogen.
Then 3 mL water was injected, and the mixture was stirred for 30
min. The metal precursor dissolved in 1 mL water was injected. This
mixture was stirred continuously at room temperature for 2 h. Then,
1 mL of a freshly prepared NaBH4 aqueous solution (10 equiv. per
metal) was quickly added, and the nanocatalysts were successfully
prepared by further stirring for another 30 min.
506[b]
14023[a]
/19476[c]
This
work
CoNPs@ZIF-8
6%
62.9
30
40
25717[a]
/35718[c]
[a] TOF is related to the total atoms number within the cobalt NPs and the
unit of the TOF is mLH2·min-1·gCo
.
[b] The unit of the TOF is mLH2·min-
-1
1·gcat-1. [c] The TOF(s) is related to the surface atom number of the cobalt
NPs (see detailed calculations in SI). [d] These literature results were
essentially obtained using various NaOH concentrations between 0.25 M
and 2 M.
5. Digestion of the ZIF-8 framework with EDTA, and NP capping
with PVP for TEM characterization: PVP (MW = 10000, 10 equiv.
per NPs) solubilized in 13 mL water was directly added to the above
nanocatalyst solution, and the solution was stirred for 30 min. EDTA-
2Na·2H2O (10 equiv.) in 2 mL water was then added. The mixture
was continuously stirred for 1 h. Several drops of this solution are
used for TEM characterization.
best late first-raw metal in the case of NaBH4 hydrolysis.
[15,16a]This difference confirms that the ZIF-8 support is very
selective
to
the
substrate
structure.
However,
it remains challenging to develop even more highly active,
stabilizing, earth-abundant non-noble metal catalysts for
NaBH4 hydrolysis and other kinds of catalytic reactions.
6. Hydrogen release from sodium borohydride hydrolysis:
Following the preparation of the nanocatalyst, 2 mL of aqueous
solution of freshly prepared NaBH4 was directly injected into the
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10.1002/cctc.201900051
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above nanocatalyst solution. Hydrogen release immediately started
and was collected by the drainage gas collection method. The
exhaust vent at one end of the Schlenk flask was connected to the
inverted graduated cylinder through a rubber tube. The conversion
of NaBH4 was quantitative and generated 4.0 equivalents of H2.
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Acknowledgement
Financial support from the Science and Technology
Department of Sichuan Province (218JY0287), the
Discipline Development Foundation of Science and
Technology on Surface Physics and Chemistry Laboratory
(XKF201505), the Natural Science Foundation of China
(21806151,21601166), the University of Bordeaux and the
Centre National de la Recherche Scientifique (CNRS) is
gratefully acknowledged.
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Keywords: metal organic frameworks (MOFs) • ZIF-8 •
sodium borohydride • hydrogen release • cobalt
nanoparticles • primary kinetic isotope effect
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Text for Table of Contents
Chun Luo,[a] Fangyu Fu,[b] Xiaojiao
Yang,[a] Jianyu Wei,[a] Changlong
Wang,[b] Jing Zhu,[a] Deshun Huang,[a]
Didier Astruc* [b] and Pengxiang Zhao* [a]
Page No. – Page No.
Efficient and Selective Co@ZIF-8
Nanocatalyst for Hydrogen Release
from Sodium Borohydride Hydrolysis
((Insert TOC Graphic here: max. width:
5.5 cm; max. height: 5.0 cm))
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