One-Pot Synthesis of Heterobimetallic Metal–Organic Frameworks (MOFs) for Multifunctional Catalysis

Article abstract of DOI:10.1002/chem.201901939

A one-pot synthesis of bimetallic metal–organic frameworks (Co/Fe-MOFs) was achieved by treating stoichiometric amounts of Fe and Co salts with 2-aminoterephthalic acid (NH₂-BDC). Monometallic Fe (catalyst A) and Co (catalyst F) were also prepared along with mixed-metal Fe/Co catalysts (B–E) by changing the Fe/Co ratio. For mixed-metal catalysts (B–E) SEM energy-dispersive X-ray (EDX) analysis confirmed the incorporation of both Fe and Co in the catalysts. However, a spindle-shaped morphology, typically known for the Fe-MIL-88B structure and confirmed by PXRD analysis, was only observed for catalysts A–D. To test the catalytic potential of mixed-metal MOFs, reduction of nitroarenes was selected as a benchmark reaction. Incorporation of Co enhanced the activity of the catalysts compared with the parent NH₂-BDC-Fe catalyst. These MOFs were also tested as electrocatalysts for the oxygen evolution reaction (OER) and the best activity was exhibited by mixed-metal Fe/Co-MOF (Fe/Co batch ratio=1). The catalyst provided a current density of 10 mA cm^?2 at 410 mV overpotential, which is comparable to the benchmark OER catalyst (i.e., RuO₂). Moreover, it showed long-term stability in 1 m KOH. In a third catalytic test, dehydrogenation of sodium borohydride showed high activity (turnover frequency=87 min^?1) and hydrogen generation rate (67 L min^?1 g^?1 catalyst). This is the first example of the synthesis of bimetallic MOFs as multifunctional catalysts particularly for catalytic reduction of nitroarenes and dehydrogenation reactions.

Full text of DOI:10.1002/chem.201901939

A Journal of
Accepted Article
Title: One-pot Synthesis of Heterobimetallic Metal-Organic
Frameworks (MOF) for Multifunctional Catalysis
Authors: Bushra Iqbal, Murtaza Saleem, Salman Noshear Arshad,
Jamshaid Rashid, Naveed Hussain, and Muhammad Zaheer
This manuscript has been accepted after peer review and appears as an
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201901939
Link to VoR: http://dx.doi.org/10.1002/chem.201901939
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Chemistry - A European Journal
10.1002/chem.201901939
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One-pot Synthesis of Heterobimetallic Metal-Organic Frameworks (MOF) for
Multifunctional Catalysis
a
b
a
c
d
Bushra Iqbal , Murtaza Saleem , Salman Noshear Arshad , Jamshaid Rashid , Naveed Hussain ,
a
Muhammad Zaheer *
a
Department of Chemistry and Chemical Engineering, Lahore University of Management
Sciences (LUMS), Lahore 54792 Pakistan
Department of Physics, Lahore University of Management Sciences (LUMS), Lahore 54792
b
Pakistan
c
Department of Environmental Science, Faculty of Biological Sciences, Quaid-i-Azam
University, Islamabad 45320, Pakistan
d
State Key Laboratory of New Ceramics and Fine Processing, School of Material Science and
Engineering, Tsinghua University, Beijing, P.R. China.
*
Corresponding author: muhammad.zaheer@lums.edu.pk
Abstract
A one-pot synthesis of bimetallic Metal-Organic Frameworks (Co/Fe-MOFs) was achieved by
2
reacting the stoichiometric amounts of Fe and Co salts with 2-aminoterephthalic acid (NH -
BDC). Monometallic Fe (catalyst A) and Co (catalyst F) were also prepared along with mixed-
metal Fe/Co catalysts (B-E) by changing Fe/Co ratio. For mixed-metal catalysts (B-E) SEM-
EDX analysis confirmed the incorporation of both Fe and Co in the catalysts. However, a
spindle-shaped morphology typically known for a Fe-MIL-88B structure and confirmed by
PXRD analysis was only observed for catalysts A-D. To test the catalytic potential of mixed-
metal MOFs, reduction of nitroarenes was selected as a benchmark reaction. Incorporation of Co
2
enhanced the activity of the catalysts as compared to the parent NH -BDC-Fe catalyst. These
MOFs were also tested as electro-catalysts for Oxygen Evolution Reaction (OER) with best
activity exhibited by mixed-metal Fe/Co-MOF (Fe/Co batch ratio =1) with a current density of
-
2
1
0 mA.cm at 410 mV overpotential, which is comparable to the benchmark OER catalyst (i.e.
RuO ). Moreover, it showed long-run stability in 1M KOH. In a third catalytic test,
2
dehydrogenation of sodium borohydride showed high activity (TOF: 87/min) and hydrogen
generation rate (67 L/min/g catalyst). This is the very first example of the synthesis of bimetallic
MOFs as multifunctional catalysts particularly for catalytic reduction of nitroarenes and
dehydrogenation reactions.
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Introduction
Metal-Organic Frameworks (MOFs), the crystalline nanoporous materials constructed from
metallic nodes and multipodal organic ligands, have become attractive materials in many fields
[
1]
[2]
[3]
[4]
[5]
such as gas storage , separation , sensing , drug delivery and energy storage. Compared to
other crystalline nanoporous materials like zeolites and zeotypes, MOFs offer higher surface
area, larger cavities (that would allow effective mass transport) and fine-tuning of the structure at
[
6]
the molecular level. Despite all these benefits, pristine MOFs have not gained attention in
industrial catalysis to an extent as the zeolites do, for instance, in the petroleum industry.
[
7]
However, MOFs can be engineered for potential applications in heterogeneous catalysis ,
[
8]
[9]
[10]
[11]
photocatalysis , asymmetric catalysis , acidic and basic catalysis , biocatalysis
,
[
12]
[13]
[14]
supramolecular catalysis,
biomass conversion
and electrocatalysis.
These catalytic
[
15]
applications of MOFs can be attributed to various strategies such as pore surface engineering,
[
16]
[17]
[18]
defect engineering, and encapsulation of the active species such as nanoparticles (NPs)
[
19]
and metal complexes . Although, MOFs embedded with transition metal NPs via Metal-
[
20]
[21]
Organic Chemical Vapor Deposition (MOCVD),
double solvent approach , solution
[
22]
[23]
infiltration , and Atomic Layer Deposition (ALD)
have been widely applied as catalysts,
development of catalysts with well-defined active sites is highly desirable for the activity and
selectivity of the catalysts. Recently, various single-site MOF catalysts have been developed by
[
24]
creating open metal sites or through the functionalization of the organic linkers. Post-synthetic
[
25]
cation exchange
development of multifunctional MOFs, for instance for gas uptake
Computational studies have also shown the potential of such bimetallic MOFs for various
at secondary building units of MOFs is another potential strategy for the
[
26]
[27]
and catalysis.
[
28]
applications, such as C-H bond activation.
Generation of defects and synergistic effect
between the two metals in bimetallic MOFs could lead to enhanced catalytic activity. Here we
report a one-pot method, eliminating the post synthetic process, for the partial substitution of Fe
[
29]
2
with Co in an amine-functionalized MOF [NH -MIL-88B(Fe)] . The effects of metal
substitution on crystallinity, crystal structure, stability and catalytic activity were investigated.
Catalytic reduction of nitroarenes, catalytic dehydrogenation of sodium borohydride and
electrocatalytic water oxidation were used as model test reactions. Very recently this strategy has
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[
30]
been adopted to develop highly active electrocatalysts for water oxidation and energy storage.
[
31]
However the use of bimetallic MOFs for thermochemical catalysis are scarce. To the best of
our knowledge this is the first example of a bimetallic MOFs for multifunctional single-site
[
32]
catalysis, thereby, extends the application domains of iron-based MOFs.
Experimental
2
-Aminoterephthalic acid (NH
2
-BDC, 99%), Iron(III) chloride hexahydrate (FeCl
3 2
.6H O, 99%),
Cobalt(II) chloride hexahydrate (CoCl
2
.6H O, 99%) and N,N-dimethylformamide (DMF, 99.9%)
2
were purchased from Sigma-Aldrich and used without any further purification. All the organic
solvents were of analytical grade.
Synthesis of the catalysts
A total of six catalysts were synthesized by gradual replacement of Fe in an NH
Co (Table S1). For the synthesis of catalyst B (NH -BDC-Fe0.5Co0.5), equimolar amounts of
FeCl .6H O (1 mmol) and CoCl .6H O (1 mmol) were dissolved in 7.5 ml of DMF. 1 mmol of
-aminoterephthalic acid was separately dissolved in 7.5 ml DMF. Both solutions were then
mixed by stirring for 15 min and transferred into a 100 mL Teflon-lined autoclave and heated at
20°C for 24 h. The brown product was separated by centrifugation, washed (with DMF and
2
-BDC-Fe with
2
3
2
2
2
2
1
ethanol) and vacuum dried at 110°C overnight. The remaining catalysts were synthesized by a
similar method by varying the Fe and Co concentration. The details are provided in the
supporting information (Table S1).
Characterization
X-ray diffraction (XRD) patterns were obtained on BRUKER (D2 Phaser) diffractometer
operated at 30 kV and 10 mA using Ni-filtered Cu-Kα irradiation (Wavelength 1.5406 Å).
Surface morphology and compositional analysis were characterized using scanning electron
microscopy (FEI NOVA Nano SEM 450) equipped with energy dispersive X-ray spectroscopy
(EDX). The N
2
adsorption-desorption measurements were performed by using Quantachrome
Nova 2200e. Prior to the measurement, samples were degassed under vacuum at 363 K
-
1
overnight. IR spectra were recorded on Bruker Alpha Platinum ATR in the 400–4000 cm
region. NMR analysis was conducted by Bruker AVANCE III 400-600 MHz.
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Thermogravimetric analysis was performed over a TA instrument (SDT Q600) from room
temperature to 700°C (10°/min ramp) under an air flow (100mL/min).
Catalytic studies
Catalytic reduction of nitroarenes
In a 100 mL round bottom flask, 1 mmol of the substrate was dissolved in 10 mL of methanol-
4
ethanol mixture (1:1) and 20 equivalents of NaBH were added. The solution was subjected to
constant stirring and 20 mg of MOF catalyst was added at room temperature. The conversion of
nitroarenes to amines was monitored with Thin Layer Chromatography (TLC) and finally by
GC-MS and NMR.
Electrochemical water oxidation over mixed metal MOFs
Electrochemical experiments were performed in 1M KOH at scan rate of 50 mV/sec using
GAMRY Reference 600 potentiostat and employing fluorinated tin oxide (FTO, ≤ 15Ω, working
electrode), Hg/HgO (reference electrode) and Pt wire (counter electrode) in a standard three-
2
electrode cell. 1 cm area of FTO was coated with the catalyst (0.2 mg) by drop casting followed
by drying in air. The recorded potential in all the experiments is presented vs reversible hydrogen
electrode (RHE).
Catalytic Dehydrogenation of sodium borohydride
In a typical experiment, NaBH
4
(2 mmol) and catalyst (10 mg) were charged in a sealed flask
) solution
fitted with a gas evolution measuring apparatus. 10 mL of NaOH (2 wt % w.r.t NaBH
4
was injected into the reaction flask under continuous stirring. The volume of the evolved gas was
noted from the gas measuring apparatus. All the catalytic tests were carried out at a given
temperature under an inert atmosphere.
RESULTS AND DISCUSSION
A series of MOFs, labeled A through F, with varying Fe:Co batch ratio (A= 1:0,B= 1:1,C=
1
2
:3,D= 1:9, E= 1:19 and F= 0:1) respectively with 2-aminoterephthalic acid (NH -BDC) were
prepared
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Scheme 1. (Top) Schematic illustration for the synthesis of mixed metal (Fe/Co) MOFs with
NH -BDC linker. Various possibilities for the existence of SBUs are shown at the bottom. (a)
Individual trinuclear Fe O(μ -O) cluster (b) individual Co O(μ -O) cluster. (c) Co coordination
with the –NH function of the linker. Color codes: Co (Purple), Fe (Orange), O (red), C (grey).
2
3
3
3
3
2
according to the Scheme 1. A MIL-88 structure consists of trimeric secondary building units
[
33]
(
SBUs) which are basically M
Soft Acid Base (HSAB) concept, Fe(III) can preferable bind with carboxylate while Co(II) can
either coordinate with N-H (c in Scheme 1) or replace one of the iron from Fe O cluster (a) to
3 3
(μ -O) clusters as shown in Scheme 1. According to Hard and
3
[
34]
III
make Fe
has not been reported.
independent Co O cluster (b). In forthcoming sections, we will try to experimentally confirm the
2
Co(μ
3
-O) cluster. Existence of more than one bivalent atom in a M 3ꢀ(μ
3
-O) cluster
[
35]
Lastly, Co(II) can also incorporate into MOF structure as an
3
existence of possibilities (a-c).
-
FT-IR spectra of the materials are shown in Figure 1a. Characteristic bands at1572 and 1428 cm
1
[36]
,
present in all materials, were assigned to symmetric and anti-symmetric stretching of C–O.
-
1
Similarly peak at ~1250 cm was present in as prepared MOFs and was attributed to C-N
-
1[37]
stretch. Coordinated DMF appeared at~1655 cm
for all the catalysts, although signal was
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Figure 1. a) A comparison of FT-IR spectra of MOFs (A-F) in spectral range 4000-500cm-1. b)
-
1
A magnified view of far-IR range 800-500cm .
masked in the case of catalysts A and B due to peak broadening. The absence of a band around
-
1
1
678 cm indicated the absence of any unreacted NH
A planar Fe (µ -O) cluster with a D3h symmetry (Scheme 1a) shows an asymmetric in-plane
vibration for central oxygen at ~600 cm .
Fe CoO) above mentioned band is discontinued and splits into two components due to the
reduction of symmetry from D3h to C2v
should have been observed with no band at ~600 cm for the formation of Fe
2
-BDC.
3
3
-
1 [38]
However in the case of a mixed-metal cluster
(
2
[
39]
-1
.
Therefore two bands at 720-710 and 565-545 cm
-
1
2
CoO cluster in the
[
35]
-1
MOFs. However due to low resolution of FT-IR spectrum in 800-400 cm range, we could not
confirm the presence of a mixed-metal Fe CoO cluster in our catalysts.
All MOFs showed N-H symmetric and antisymmetric bands at 3358 and 3466 cm respectively.
As Co(II) is a soft acid, it would have bonded with –NH soft base and resulting in N-H
stretching vibrations to lower frequency. However, as no change in the position of these bands
2
-
1
2
[
40]
was observed, we conclude that Co(II) is not bonded with NH
therefore must be a part of the nodes.
2
functions (c in Scheme 1) and
The morphology, crystallite size and elemental composition of mono and bimetallic MOFs were
investigated with SEM-EDX. The catalysts A-D showed a spindle-shaped morphology, which
was retained upon the incorporation of cobalt (Figure S1 in supporting information SI).ꢀ
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Increasing cobalt content (c-e) resulted in phase separation and both octahedrons (distorted) and
spindles were observed in catalysts C and D. Spindle morphology was almost lost in MOF-E
while monometallic Co-MOF (f) was rod-like in appearance.
In order to confirm the incorporation of Co within the framework, EDX analysis was done and
elemental maps are presented in the columns 2-6 (left to right) in Figure 2. In all mixed-metal
MOFs (B-E), presence of both Fe and Co was confirmed as evident from the EDX elemental
maps. EDX analysis shows that rather low amount of Co was actually incorporated in mixed-
metal MOFs than that was attempted which could be due to strong affinity of carboxylate for
Fe(III) and strength of Fe-O bonds. ICP-OES analysis of the materials depicts that Fe/Co ratio in
products were lower than the stoichiometric amounts used in the synthesis (see Table S2).
st
Figure 2. (a-f) SEM micrographs of as prepared MOFs (1 column) along with EDX elemental
nd
rd
th
th
th
maps for C (2 column), O (3 column), N (4 column), Fe (5 column) and Co (6 column).
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X-ray Powder Diffraction (PXRD) analysis of MOFs was carried out to confirm their
crystallinity. Comparison of X-ray diffraction patterns with simulated PXRD are presented in
Figure 3a. At least (002) and (101) reflections of Fe-MIL-88B structure were seen for A-D at
[
41]
9
.2 and 10.2 2theta respectively. Similarly a weak signal at 113° was assigned to reflection
from (102) plane while the one at 20.6° was attributed to diffraction from (202) plane. This
confirms the structural integrity of MOF structure upon the incorporation of cobalt within the
framework in the catalysts A-D. Catalysts E was however X-ray amorphous showing only a
broad and weak reflection at 20.6°. Any reflections from Co(0) or Co
x y
O were absent in the
diffraction patterns which further confirmed that Co has become a part of the node. PXRD
[
42]
pattern of Co-MOF (F) could not be matched with any known Co-MOF.
Thermogravimetric analysis (TGA) was carried out for the structural elucidation and
investigating the stability of the catalysts. Thermal behavior of all MOFs could be divided into
three regions of mass loss (Figure 3b). First step at 69-75°C in thermograms, corresponded to
the loss (7%) of the solvated molecules (DMF) and is in agreement with the empirical formula
[
41]
M
3
O(DMF)
3
(NH
2
-BDC)
3
.DMF.
A second step at 272-276°C was assigned to the loss of
coordinated DMF which
Figure 3. a) PXRD pattern of various MOFs in comparison with the simulated XRD patter of
NH
2
-MIL-88B. b) Thermograms of MOFs (A-F).
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again agrees with theoretical calculations. Above 290°, the structure started to decompose and
weight loss at this temperature was attributed to the departure of two linker molecules. The loss
of third linker moves to a lower temperature with an increase in cobalt content which could be an
indirect evidence for the existence cobalt secondary building units (SBUs) with weaker Co-O
bonds. The structural collapse was completed by 500°C with a ~72% weight loss.
Surface and pore size distribution of the MOFs was analyzed by nitrogen physisorption and
adsorption-desorption isotherms along with the pore size distribution is presented in Figure 4. A
[
43]
MIL-88B structure is known to have low surface area due to shrinkage upon solvent removal.
2
-1
As expected, as-synthesized catalysts showed low surface area (12-83 m .g ) except catalyst C
2
-1
(
Figure 4c) which showed a high surface area of 239 m .g and highly ordered bimodal porosity
averaging at 2.4 and 3.0 nm. Although the isotherm and pore size distribution of this catalyst was
[
44]
very similar to a MIL-101 structure, PXRD pattern match reasonable with MIL-88B (Figure
b). However SEM images (see Figure S1) clearly show the existence of both spindle-shaped
Fe-MIL-88B) and distorted octahedral (Fe-MIL-101) crystals therefore we conclude that upon
increasing cobalt content, formation of MIL-101-NH structure is favored as observed by others
too. The molar ratio of Fe and Co in this catalyst, as analyzed by ICP-OES was found to be
.7 (see Table S2).
Catalysts A, B and D showed a H3 type hysteresis previously reported for MIL-88B structure
1
(
2
[
45]
2
[
43]
whereas H2 hysteresis shown by E could be due to percolation phenomenon. Pore size
distribution was rather broad in the case of catalysts B and D while for the rest of the catalysts
(excluding C) an average pore diameter of 4.8-5.4 nm was observed.
Catalyst B was also analyzed with XPS to check the oxidation state of the elements contained by
the MOF and their coordination environment (Figure 5). XPS can also be used for
characterization of mixed-metal MOFs as the coordination environment of each metal will be
[
46]
influenced by that of another metal in vicinity. Survey scan showed the presence of C, N, O,
Fe and Co as identified by their typical binding energies (Figure 5a). Carbon (1s) spectrum
(
Figure 5b), at high resolution was deconvoluted into three peaks at 284.6, 286 and 288.6 eV
corresponding to C=C, C-O and C=O bonds. Likewise the nitrogen 1s peak was deconvoluted
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into two peaks centering at 399.5 and 400.3 eV and were assigned to amine and pyrollic nitrogen
respectively.
Figure 4. a-e) Absorption-desorption isotherm of the catalysts (A-E) with corresponding pore
size distribution in the inset. f) Pore size distribution of catalyst E. Catalysts F did not show any
porosity.
Figure 5. XPS analysis of catalyst B. a) Survey scan b) C; c) N ;d) Co; e) Fe and f) O.
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Co 2p high resolution spectrum (Figure 5d) consisted of two peaks at 782.0 and 786.7 and
[
47]
absence of shakeup satellite peaks suggested the presence of Co(III).
presence of a mixed-metal Fe Co(μ -O) cluster with Co in 3+ oxidation state.
Figure 5e) at 711.2 ad 725.2 correspond to 2p3/2 and 2p1/2 states of iron(III) respectively and a
This rules out the
[
48]
2
3
Fe XPS peaks
(
[
49]
difference of 14eV between the two peaks is comparable to Fe(III) in Fe
2
O
3
.
No unusual
features were found in high resolution oxygen 1s spectrum (Figure 5f).
Catalytic studies
Reduction of nitroarenes to corresponding amines has served as a benchmark reaction for testing
[
50]
the activity of nanocatalysts (NCs). Also the reaction is industrially important as functionalized
[
51]
amines are important feedstock for the synthesis of pharmaceuticals, dyes and agrochemicals.
NaBH is a strong reducing agent and more stable and does not require high-pressure reactors as
4
required by the catalytic studies under molecular hydrogen. Catalytic hydrogenation of
nitroarenes with earth-abundant metals is of particular importance due to scarce availability of
[
52]
noble metals.
Catalytic reduction of nitroarenes, therefore, was taken as a benchmark reaction to test catalytic
activity of mono and bimetallic MOFs. Firstly we studied the solvent effect (Table S4) using 4-
nitrotoluene as the substrate. The yield of the aniline product was very low in water (Entry 1) and
ethanol (Entry 2) most likely due to insolubility of the nitro substrates. The maximum yield of 4-
methylaniline was obtained in ethanol-methanol (1:1) mixture (Entry 3).
Next, catalytic activity of mono and bimetallic MOFs was tested for the reduction of 4-
nitrotoluene and results are presented in Table 1.
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Table 1. Reduction of 4-nitrotoluene to various possible products
Entry
Catalyst
Conv. (%)
Yield (%)
AT
0
AZ
0
AOB
1
2
3
4
5
6
7
8
No catalyst
15
0
5
0
No NaBH
4
0
0
A
10
4
20
---
4.3
10
5
15
---
30
9
B
>99
81
>99
52
>80
>70
65
FeCo-MIL-101
C
D
E
>90
>85
>89
10
8.1
3.2
Reaction conditions: Substrate: 1 mmol; Catalyst: 20 mg (0.226mg of Co); NaBH4: 20 mole equivalents with
respect to substrate; Solvent (ethanol/methanol): 10 mL; Temperature: Room temperature.
Formation of the aniline product did not occur in the absence of any catalyst (entry 1) or with
reducing agent only (entry 2). Pristine Fe-MOF (A) provided 4% yield of 4-methylaniline at 10%
conversion of nitro substrate (Entry 3). The side products were azobenzene and azoxybenzene.
Almost quantitative yield to 4-methylaniline was by catalyst B without the formation of any
noticeable side products (Entry 4). Interestingly the closely related catalyst BDC-Fe0.5Co0.5
provided 81% conversion but only 52% selectivity to the aniline product (entry 5). The catalysts
C, D and E provided 80%, 70% and 65% of the product respectively with some azo and azoxy
side compounds (Entries 6-8).
To explore the wide-range applicability of the catalyst B, a variety of nitro substrates bearing
electron withdrawing and electron-donating groups at different positions were investigated under
optimized condition.
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Table 2. Catalytic reduction of various monostubstitutednitroarenes.ꢀ
Reduction of various nitroarenes using catalyst B. Conditions: room temperature, 1mmol of nitro substrate, 20 mg
catalyst, 20 eq. of NaBH and 10ml of ethanol: methanol mixture (1:1). *Hydrogenationꢀtookꢀplaceꢀhere.ꢀIsolatedꢀ
4
yieldsꢀareꢀgivenꢀinꢀparenthesis.
The catalyst showed outstanding chemoselective reduction of the most-exigent industrially
relevant nitroarenes substrates (Table 2) into corresponding aniline products. For instance,
bromo and chloro groups were tolerated well (entries 4-8) with any dehalogenation side reaction
[
53]
observed for Pd-based catalyst.
Substrates containing amino (entry 9) or cyano (entry 10)
groups are notoriously known to coordinate with Pd catalysts leading to their deactivation.
However, for the catalyst B, all targeted products were achieved with appreciable selectivity. It is
important to mention here that most of the reported non-noble metals catalysts for nitroarenes
reduction are active at temperatures >80°C.
The reuse of the catalyst was studies up to four runs. A slight drop-in the activity was observed
after first and the successive runs (see Figure S2 in SI) which could be due to the filling of pores
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with sodium metaborate.
SEM micrographs of the catalysts, before and after the catalysis
(Figure S3) show structural integrity during catalysis.
Electrochemical Water Oxidation
Electrochemical water splitting is a sustainable method for the production of hydrogen as a clean
[
55]
fuel. The anode half reaction, known as the oxygen evolution reaction (OER) is kinetically
sluggish and requires large overpotential(η). Benchmark catalysts for OER are based on the
[
56]
scarcely available noble metals (Ir, Pt and Ru) and need to be replaced with earth abundant
[
57]
metals (Fe, Co, Ni) with OER activity at low overpotential.
Electrochemical activity of monometallic catalysts A, F and mixed-metal catalyst B was tested
for OER in 1M KOH using three-electrode system. Cyclic voltammogram (CV) revealed
2
efficient OER activity requiring overpotential of 410 mV to achieve 10mA/cm current density
(
2
Figure 6a) comparable to benchmark catalyst (RuO ; 400 mV). Tafel slope was calculated from
the steady state polarization curves close to the onset potential to evaluate the comparative
kinetics of the modified electrodes. Small Tafel slope means fast kinetics and easy transport of
[
58]
the charge species across the electrode-electrolyte interfaces. Tafel slope value of 101mV/dec
for catalyst B (Figure 5b) as compared to 173 and 293 mV/dec for A and F respectively,
indicated the synergy between the two metals facilitating the sluggish OER. The slope value
-
1
suggested that formation of MO specie form the adsorbed OH on the exposed metal site was the
[
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rate-determining step.
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Figure 6. (a) OER polarization curve (b) Tafel plots and (c) Chronopotentiometry plot for B in
comparison with RuO and monometallic A and F catalysts.ꢀ
2
We have further calculated the electrochemically active surface (EASA) and double layer
capacitance for each catalyst (Figure S3). From the cyclic voltammograms of A, B and F we see
increase in the current density by increasing the scan rate in the non-Faradaic region from where
2
double layer capacitance is calculated. The highest EASA was found for B (15.9m /g) as
compared to monometallic A and F. Further, stability of the catalyst B was tested by
Chronopotentiometry (Figure 6c). No notable increase in the applied potential required to
-
2
maintain 10mA.cm current density was observed up to six hours. A minor positive shift in
potential might be due to bubble formation on the surface of FTO. The catalysts was collected
after the reaction and analyzed by PXRD and SEM. Absence of peaks at 15-25° (2θ), suggested
minor damage to the structure probably due to the dissolution of linker under highly basic
conditions. (Figure S5). SEM micrographs of the used catalyst showed structurally intact
crystals along with deformed ones (Figure S4).
Catalytic Dehydrogenation
Dehydrogenation of sodium borohydride (SBH) serves as another benchmark reaction to check
[
54]
the catalytic activity of materials. We tested the potential of our catalysts for dehydrogenation
o
of base-stabilized SBH at 80 C and found that as compared to monometallic Fe (A) and Co (F),
bimetallic MOF (B) showed higher activity as shown in Figure 7a. The initial hydrogen
-
1 -1
generation rate was calculated to be 5.4, 16.7, and 67 L min g for A, F and B respectively
(
Table S5 in SI).
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Figure 7. Catalytic hydrolysis of NaBH .a) effect of monometallic and bimetallic catalysts on
HGR; b) effect of temperature; c) Arrhenius plot (see table S6).
Further catalytic studies, for instance, the effect of temperature on hydrogen generation rate were
performed with catalyst B. As shown in Figure 7b volume of generated hydrogen increased with
-
1 -1
increase in temperature and the highest value (67 Lmin g Co) was obtained at 80°C. In
comparison to other Co-based catalysts (see Table S7 in SI), our catalyst provided high turnover
-
1
−1
frequency (TOF: 5263 h ) and low activation energy (27 kJ mol ) for the dehydrogenation
reaction which could be attributed to the presence of Co at the nodes (Figure 7c).
Conclusions
To summarize, a series of bimetallic MOFs with modulated molar ratios of Fe/Co have been
successfully synthesized in a one-pot approach. FT-IR and XPS studies suggested the absence of
2 3
mixed-metal Fe Co(μ -O) cluster and rather the existence of secondary building units (SBUs)
with one kind of metal only. Mixed-metal MOFcatalyst with a Fe/Co batch ratio of one showed
excellent activity and selectivity for reduction of nitroarenes to corresponding amines at room
temperature. The same catalyst also showed excellent activity and stability for OER. Lastly, the
activity of mixed-metal MOF catalyst for catalytic dehydrogenation of sodium borohydride was
-
1
tested and the lowest activation energy (27 kJ/mol) and the highest TOFs 5263 h ) were
achieved as compared to the literature reports. The high catalytic performance can be credited to
high porosity, as well as the synergic effect between the two metals in MOF frameworks.
Acknowledgements
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Authors are thankful to Higher Education Commission (HEC) of Pakistan for the financial
support under grant NRPU-5910.
Conflict of interest
Authors declare no conflict of interest.
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