Nickel nanoparticle/carbon catalysts derived from a novel aqueous-synthesized metal-organic framework for nitroarene reduction

Article abstract of DOI:10.1016/j.jallcom.2020.157348

Carbon-supported, non-noble metal-based catalysts derived from metal-organic frameworks (MOFs) are attractive alternatives to noble metal-based systems, but typical syntheses of the starting MOFs are not desirable from an environmental and practical perspective (e.g., they rely on non-innocuous organic solvents and long reaction times). Here, we report the preparation of a Ni-based MOF in aqueous medium, at moderate temperature (95 °C) and in a short reaction time (<30 min), which was used as a sacrificial template to access a family of Ni nanoparticle/carbon hybrids that were then tested as catalysts in the reduction of 4-nitrophenol (4-NP). The MOF-derived hybrids exhibited a mesoporous texture, with specific surface areas between ～250 and 450 m² g^?1 depending on the carbonization temperature applied to the MOF, as well as high Ni contents (between ～36 and 57 wt%). Notwithstanding the latter, the metal was homogeneously distributed throughout the carbon matrix in the hybrid and was quite resistant to extensive agglomeration and sintering, even at temperatures as high as 1000 °C. With increasing carbonization temperature, the Ni component was seen to go through different crystal phases, i.e., Ni₃C phase → Ni hexagonal close-packed phase → Ni face-centered cubic phase. The results of the catalytic tests suggested the former and latter phases to be the most active towards the reduction of 4-NP, with catalytic activity values as high as 0.039 mol_4-NP mol_Ni^?1 min^?1.

Full text of DOI:10.1016/j.jallcom.2020.157348

Journal of Alloys and Compounds 853 (2021) 157348
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Nickel nanoparticle/carbon catalysts derived from a novel aqueous-
synthesized metal-organic framework for nitroarene reduction
*
ꢀ
ꢀ
F. Julian Martín-Jimeno , Fabian Suarez-García , Juan I. Paredes , Amelia Martínez-Alonso ,
Juan M.D. Tascon
ꢀ
Instituto de Ciencia y Tecnología del Carbono, INCAR-CSIC, C/ Francisco Pintado Fe 26, 33011, Oviedo, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Carbon-supported, non-noble metal-based catalysts derived from metal-organic frameworks (MOFs) are
attractive alternatives to noble metal-based systems, but typical syntheses of the starting MOFs are not
desirable from an environmental and practical perspective (e.g., they rely on non-innocuous organic
solvents and long reaction times). Here, we report the preparation of a Ni-based MOF in aqueous me-
dium, at moderate temperature (95 ꢀC) and in a short reaction time (<30 min), which was used as a
sacrificial template to access a family of Ni nanoparticle/carbon hybrids that were then tested as catalysts
in the reduction of 4-nitrophenol (4-NP). The MOF-derived hybrids exhibited a mesoporous texture, with
specific surface areas between ꢁ250 and 450 m2 gꢂ1 depending on the carbonization temperature
applied to the MOF, as well as high Ni contents (between ꢁ36 and 57 wt%). Notwithstanding the latter,
the metal was homogeneously distributed throughout the carbon matrix in the hybrid and was quite
resistant to extensive agglomeration and sintering, even at temperatures as high as 1000 ꢀC. With
increasing carbonization temperature, the Ni component was seen to go through different crystal phases,
i.e., Ni3C phase / Ni hexagonal close-packed phase / Ni face-centered cubic phase. The results of the
catalytic tests suggested the former and latter phases to be the most active towards the reduction of 4-
Received 17 June 2020
Received in revised form
15 September 2020
Accepted 25 September 2020
Available online 28 September 2020
Keywords:
Ni/C hybrids
Nickel nanoparticles
MOF carbonization
Nitroarene reduction
ꢂ1
NP, with catalytic activity values as high as 0.039 mol4-NP mol minꢂ1
.
Ni
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
nitroarenes into their corresponding anilines is an important re-
action that can be catalysed by different metal nanoparticles.
Nowadays, the highest catalytic activity in this reaction is achieved
by noble metal-based catalysts, such as Au, Ag, Pd and Pt [12e17].
One promising solution is to replace these noble metals by more
abundant and cost-effective transition metals, such as Cu, Ni, or Co,
which have good catalytic activities, especially when they are in the
form of clusters or nanoparticles [18]. However, metallic nano-
particles have a strong tendency to form large aggregates due to
their high surface energy, resulting in a remarkable reduction of
their catalytic activity and limiting their industrial application
[19e21]. One way to solve these problems is preparing supported
catalysts, where the catalyst nanoparticles are dispersed in
different materials. In the case of Ni, heterogeneous catalysts have
been prepared by supporting Ni nanoparticles on carbon natotubes,
graphene, porous carbons, silica, zeolites, etc. [5,7,10,20e27].
Among others, carbon materials are especially appropriate as a
support due to their unique properties, such as low density and
weight, high chemical and thermal stability, and the possibility of
modifying their surface chemistry and porous texture.
The development of highly active and selective catalysts is of
great importance in many industrial chemical processes. For many
chemical reactions, the best catalysts are expensive and scarce
noble metals (Pt, Pd, Au, etc), making the development of new
catalysts based on cheaper metals (e.g., Ni or Cu) the subject of
intense research [1e5]. Nitroarene compounds have an important
role in the chemical industry, but in most of the cases they are
generated as by-products of the pharmaceutical, agrochemical, dye
and other industries [6e8]. Furthermore, nitroarenes are an
important source of pollution from such industries and they also
pose health risks due to their carcinogenic effects [9,10]. On the
other hand, anilines are very important reagents in the preparation
of several drugs as well as in the polymer, dye and many other
industries [11]. Thus, the reduction (i.e. hydrogenation) of
* Corresponding author.
E-mail address: fabian@incar.csic.es (F. Suarez-García).
ꢀ
https://doi.org/10.1016/j.jallcom.2020.157348
0925-8388/© 2020 Elsevier B.V. All rights reserved.

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F.J. Martín-Jimeno, F. Suarez-García, J.I. Paredes et al.
Journal of Alloys and Compounds 853 (2021) 157348
Different methods have been studied for the dispersion of
metallic nanoparticles in carbon materials, such as impregnation
with metal salts followed by reduction or by means of addition of
metal compounds during the synthesis of soft-templated carbons
[22,24]. A novel method that allows the preparation of materials
with a very high metal content and at the same time with a high
dispersion of the metallic nanoparticles is the use of metal-organic
frameworks (MOFs) as sacrificial materials [28]. Thus, different
MOFs containing the desired metal ions or metallic clusters were
synthesized and pyrolyzed in inert atmosphere at high tempera-
ture to trigger the thermal decomposition of the MOF and give
metal or metal oxide/carbon nanohybrids. The large variety of
MOFs with different metal nodes and organic linkers, the highly
periodic arrangement of the metal ion or metallic cluster in the
MOF structure and the large variety of possible pyrolysis conditions
are conducive to obtaining hybrid metal/carbon materials with
tunable properties.
There are only a few examples in the literature of Ni-containing
MOFs that have been pyrolyzed to produce Ni/carbon catalysts
[23,25,29e32]. Most of these works report the carbonization of
MOFs prepared using aromatic acids (e.g., terephthalic acid or
benzene-tricarboxylic acid) as organic linkers. These MOFs are
generally prepared in non-innocuous organic solvents and require
long reaction times [23,25,30e32], making this route unattractive
from an environmental and practical perspective. To address this
issue, we report here the synthesis of a new nickel-based metal
organic framework (NiOF), where Ni acts as the coordinating metal
and imidazole as the ligand, thus being an analogue to zeolitic
imidazolate frameworks (ZIFs). To this end, an environmentally
friendly water-based synthesis procedure was developed using
moderate temperatures (95 ꢀC) and short reaction times (<30 min).
This novel Ni-based MOF was carbonized at different temperatures
and the resulting Ni/carbon hybrids were tested as catalysts in the
reduction of 4-nitrophenol, affording a competitive activity when
compared to previous Ni-based catalysts.
a number between 4 and 10 to denote the carbonization temper-
ature used (400e1000 ꢀC).
2.2. Characterization techniques
Thermogravimetric analysis (TGA) were performed in a SDT
Q600 (TA Instrumets) thermogravimetric analyser. The thermal
stability of NiOF was studied under an argon flow (50 cm3/min) at a
heating rate of 5 ꢀC/min up to 950 ꢀC. The nickel content in the
nickel/carbon catalysts (NiCx samples) was determined in the same
thermobalance, but under air flow (50 cm3/min) with a heating
program of 10 ꢀC/min up to 950 ꢀC and maintaining this temper-
ature for 30 min.
Field-emission scanning electron microscopy (FE-SEM) images
were obtained on a Quanta FEG 650 microscope (FEI Company).
Transmission electron microcopy (TEM) studies were carried out
with a JEOL 2000 EX-II instrument operated at 160 kV. Specimens
for TEM were prepared by dispersing the sample in ethanol via a
short sonication treatment and then drop-casting ~40
mL of the
resulting dispersion onto a copper grid (200 square mesh) covered
with a lacey carbon film. Raman spectroscopy was performed with
a LabRam instrument (Horiba JobineYvon) using a 532 nm wave-
length laser at a low incident power (0.5 mW) to avoid altering the
samples. X-ray diffractograms were obtained in a D5000 diffrac-
tomer (Siemens) using Cu Ka radiation, with a step size of 0.015ꢀ
and a step time of 1 s. N2 adsorptionedesorption isotherms were
measured at ꢂ196 ꢀC in an ASAP 2010 (Micromeritics) volumetric
apparatus. The samples were previously outgassed at 150 ꢀC
overnight. From the adsorption brand of the isotherms the
apparent surface area (SBET) was obtained by applying the
Brunauerꢂ EmmettꢂTeller (BET) method in the 0.01 to 0.25 P/P0
range, the micropore volume (VDR,N2) was calculated using the
DubininꢂRadushkevich (DR) equation in the 0.001 to 0.15 relative
pressure range, and the total pore volume (VT) was estimated as the
volume of liquid nitrogen adsorbed at 0.975 relative pressure. Pore
size distributions and microprore (Vmp), mesopore (Vmp) and total
pore volumes (Vtotal) were obtained by applying the QSDFT method
developed by Quantachrome [35].
2. Material and methods
2.1. Materials preparation
2.3. Catalytic activity studies
The nickel/carbon catalysts were obtained by a simple two-step
procedure: (1) synthesis of a nickel-imidazole complex (i.e. NiOF)
and (2) thermal treatment to obtain a hybrid material formed by Ni
nanoparticles embedded in a carbon matrix. In the first step, a
novel nickel-imidazole complex was prepared following a simple
method in aqueous medium. A typical synthesis procedure was the
following: 20 g of nickel acetate tetrahydrate [NiOAc, Alfa Aesar, >
98% assay] were dissolved in 50 mL of deionized water and 10.102 g
of 2-methyl-imidazole [2-mIm, Acros Organics, > 99% assay] were
dissolved in another 50 mL of water. A molar ratio between re-
actants [2-mIm]:[NiOAc] ¼ 6:1 was used. Both solutions were
separately pre-heated at 95 ꢀC, and when this temperature was
reached the NiOAc solution was poured into the 2-mIm solution.
The reaction started immediately and the colour of the solution was
seen to change to orange, which indicated the formation of the
nickel-imidazole complex [33,34]. The mixture was kept at 95 ꢀC in
argon atmosphere and under vigorous stirring for 30 min, after
which it was cooled in an ice-bath and the formed solid was
recovered by centrifugation after 4 washing cycles with water. After
that, the obtained metal-organic framework (NiOF) was dried in
vacuum at 200 ꢀC to remove the non-reacted 2-mIm molecules that
occluded the pores of the material. Finally, the NiOF was heat-
treated in a horizontal tubular furnace in a N2 atmosphere
(99,999% pure) at different temperatures, from 400 to 1000 ꢀC, for
30 min. The resulting materials were referred as NiCx, where “x” is
The catalytic activity was evaluated towards the reduction of 4-
nitrophenol (4-NP, Sigma-Aldrich, 100% assay) to 4-aminophenol
(4-AP) with NaBH4 (Sigma-Aldrich, > 99% assay) in aqueous me-
dium. Nickel/carbon catalysts were ground and mesh at a particle
size < 63 mm before the catalytic experiments. The experiments
were carried out in a catalytic reactor (500 mL) using a total volume
of 300 mL of solution. To this end, 4.5 mg of catalyst were dispersed
in 292.85 mL of ultrapure water and 3.6 mL of a solution (5$10ꢂ3 M)
of 4-NP in water were added. The temperature of the reactor was
then set to a given target value (5,15 or 25 ꢀC) and kept for 1 h. After
this time, 3.55 mL of a solution (3 M) of NaBH4 in water were added
to the dispersion to start the reduction of 4-NP. Previous work in
our group [36] has demonstrated that the activity of catalysts for
nitroarene reduction increases with increasing amount of NaBH4
until a plateau is reached at a certain amount of reductant. Tests
using different concentrations of NaBH4 were carried out to
determine the smallest amount needed to yield the highest cata-
lytic activity (35.5 mM in this case) to ensure that the catalysts were
working at their full potential. Additionally, the corresponding high
NaBH4 to 4-NP molar ratio ensured a large excess of the former, and
thus that its concentration remained almost constant during the
reaction, thus leading to simpler kinetic equations that are only
dependent on the 4-NP concentration and not on NaBH4 concen-
tration (i.e. pseudo-first order reactions, see below).
2

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F.J. Martín-Jimeno, F. Suarez-García, J.I. Paredes et al.
Journal of Alloys and Compounds 853 (2021) 157348
The reaction progress was followed by UVevis absorption
spectroscopy measuring the absorption peak located at 400 nm
that is characteristic of deprotonated 4-NP (i.e., 4-nirphenolate),
which is generated in the basic medium induced by the presence of
NaBH4 (the corresponding absorption peak of non-deprotonated 4-
NP is downshifted to 316 nm), but is absent form 4-AP [37]. 2.5 mL
aliquots were taken from the reactor every 5 min until the complete
transformation of 4-NP to 4-AP, which typically took place between
60 and 90 min after the start of the reaction.
The formation of 4-AP (indeed 4-amimophenolate) was quan-
titatively determined from the absorbance of its characteristic band
at 299 nm. Calibration curves relating 4-NP and 4-AP (Sigma-
Aldrich, > 99% assay) concentration and their absorbance at 400
and 299 nm, respectively, were determined using aqueous solu-
tions of different concentrations (from 0 to 0.1 mM) in the presence
of 35.5 mM of NaBH4 (Figure S1). The reaction yields were obtained
by determining the concentration of 4-AP at the end of the
reduction reaction relative to the initial concentration of 4-NP. As
the prepared nickel/carbon catalysts had some porosity (see below)
it was necessary to rule out any potential artefacts in the catalytic
tests that could be caused just by the adsorption of 4-NP on the
catalysts. To this end, tests were run under the same experimental
conditions as described above but in the absence of NaBH4 and
adjusting the pH to 10 by adding NaOH. In this case, the absorbance
at 400 nm remained unchanged after 60 min (Figure S2) indicating
that removal of 4-NP from the reaction solution by adsorption on
the catalyst was negligible.
Fig. 1. XRD patterns of NiOF carbonized between 400 (NiC4) and 1000 ꢀC (NiC10) as
well as the starting (non-carbonized) NiOF. The different crystalline structures
involving nickel are noted: Ni3C (△) (PDF 01-072-1467); Ni-FCC (,) (PDF 03-065-
2865); Ni-HCP (ꢃ).
volatization of residual organic ligands (i.e. 2-mIm) that were not
eliminated during the drying step at 200 ꢀC. The thermal degra-
dation of NiOF occurred in a narrow temperature interval between
407 and 482 ꢀC, with a mass loss of 36 wt%. Above the later tem-
perature, a much slower and steadily mass loss is observed up to
950 ꢀC. The final yield in inert atmosphere was ~53 wt% at 950 ꢀC.
The catalyst cyclability and stability were studied for the two
most active catalysts (see below). Eight consecutive reduction re-
actions were carried out in the cuvette of the UVevis spectropho-
tometer by adding fresh 4-NP at the beginning of each cycle and
measuring the absorbance decay at 400 nm. Molar ratios and
concentrations similar to the ones used in the catalytic reactor were
used in the cuvette experiments.
3.2. NiOF-derived Ni/carbon hybrids
The obtained NiOF was carbonized in inert atmosphere at
temperatures between 400 and 1000 ꢀC to obtain nickel/carbon
hybrids. N2 adsorption-desorption isotherms and pore size distri-
butions obtained by the QSDFT method for NiOF and its carbonized
counterparts are plotted in Fig. 2 and the corresponding porous
textural parameters are summarized in Table 1.
All the catalytic tests were repeated 3 times to obtain a mean
value for each sample.
3. Results and discussion
All samples, including the NiOF, exhibited type IV isotherms,
which are characteristic of mesoporous materials [39]. The amount
of N2 adsorbed, especially at high relative pressures, increased with
carbonization temperature up to 600 ꢀC, decreasing at higher
temperatures. This fact was reflected in the different porous
textural parameters, Table 1, and in their variation as a function of
the carbonization temperature (Figure S4). Sample NiC6 had the
maximum total pore volume (0.73 cm3 gꢂ1) and mesopore volume
3.1. Nickel-imidazole MOF (NiOF)
The method developed in this work allowed the preparation of a
novel nickel-imidazole MOF (denoted as NiOF) with high yields
(about 98 wt% with respect to Ni) following a simple process and
using water as the solvent. The resulting solid had an ordered
structure, as can be seen in the X-ray diffractogram shown in Fig. 1,
(0.64 cm3 ꢂ1). The largest BET surface area was obtained for the
g
with the highest intensity peak centered at 2
q
q
¼ 13.64ꢀ and a
¼ 23.8, 26.8, 30.7,
sample prepared at 500 ꢀC (NiC5), with a value of 449 m2 gꢂ1. These
values could be considered moderate if they are compared with
typical values for other porous materials such as MOFs or activated
carbons, but it should be noted that present materials incorporated
a large amount of non-porous nickel with values ranging between
35.8 and 57.5 wt% (see Table 1). Hence, the porous textural pa-
rameters were recalculated with respect to the carbon weight, and
BET surface areas around 800 m2 gꢂ1 and total pore volumes up to
1.37 cm3 gꢂ1 were obtained (see Table S1 and Figure S5b), indi-
cating that the carbonaceous fraction of the samples possessed a
highly developed porous texture. With respect to the carbon con-
tent, the pore texture development was seen to be maximum for
the sample prepared at 700 ꢀC (i.e., NiC7).
The carbonization temperature not only had a strong effect on
the amount of Ni (Table 1) but also on the nickel particle size dis-
tribution. Figs. 3 and 4 show representative SEM and TEM images,
respectively, for samples carbonized at different temperatures. The
morphology of the materials (Fig. 3) evolved with temperature
from a continuous, rough morphology with nanometre-sized holes
shoulder at 2
q
¼ 15.38ꢀ. Low intensity peaks at 2
33.9, 46.8 and 49.1ꢀ could be also noticed. According with these
peaks, the solid possessed a tetragonal structure with a D4h point
group [38]. Using the two principal peaks (13.64 and 15.38ꢀ, which
correspond to the (200) and (202) crystallographic planes,
respectively), it was possible to calculate the unitary cell parame-
ters: a ¼ b ¼ 12.973 Å and c ¼ 29.978 Å.
The nickel content in the solid was determined by TGA in air,
yielding a residual weight of 26.9 wt% (d.b.) at 950 ꢀC which cor-
responds to a nickel oxide (NiO) product, as demonstrated by X-ray
diffraction (XRD) (see Figure S3). This amount of NiO corresponded
to a nickel content of 21.1 wt%, in reasonable agreement with a
theoretical value of 18.9 wt% for a nickel di-imidazole compound,
where Ni2þ is coordinated with four 2-mIm molecules in a square-
plane configuration and two acetate anions as counter-ion [33,38].
The synthesized materials exhibited a high thermal stability in
inert atmosphere, see Figure S4, showing a first small mass loss,
about 3 wt%, between 180 and 381 ꢀC, which could be due to the
3

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F.J. Martín-Jimeno, F. Suarez-García, J.I. Paredes et al.
Journal of Alloys and Compounds 853 (2021) 157348
Fig. 2. N2 adsorption-desorption isotherms (a, b) and QSDFT pore size distribution (c, d) for pristine NiOF and NiOF carbonized at different temperatures (NiCx samples).
Table 1
Yield and porous textural parameters deduced from N2 adsorption at e 196 ꢀC on pristine NiOF and NiOF samples carbonized at different temperatures.
Sample
T (ꢀC)
Ni (wt.%)
VT (cm3$gꢂ1
)
SBET (m2$gꢂ1
)
VDR,N2
QSDFT (cm3$gꢂ1
Vmp (dp < 2 nm)
)
(cm3$gꢂ1
)
Vmp (2 < dp < 50 nm)
Vtotal
NiOF
NiC4
NiC5
NiC6
NiC7
NiC8
NiC9
NiC10
e
21.4
35.8
43.0
44.2
50.5
49.5
53.4
57.5
0.23
0.28
0.64
0.73
0.68
0.64
0.56
0.50
81
0.03
0.08
0.15
0.15
0.14
0.11
0.11
0.09
0.01
0.06
0.08
0.07
0.06
0.05
0.05
0.04
0.21
0.21
0.54
0.64
0.60
0.58
0.49
0.44
0.22
0.27
0.62
0.71
0.66
0.62
0.54
0.49
400
500
600
700
800
900
1000
251
449
429
393
316
302
259
(NiC4) to an arrangement of close-packed but clearly distinct
nanoparticles (NiC8). The nickel particle size tended to increase
temperature (Fig. 4). It is important note that the nickel nano-
particles were homogeneously distributed throughout the carbon
matrix in all the samples, as can be seen from the TEM images.
While their number and size increased with treatment tempera-
ture, the nanoparticles were retained as individual, non-sintered
entities, even for the sample prepared at the highest temperature
(NiC10).
The nickel particle size distribution was determined from the
TEM images. The particle diameter of a minimum of 1000 different
nanoparticles for each sample was measured using the image
treatment software ImageJ [40], and the particle size distribution
histograms are given in Fig. 5. At temperatures up to 500 ꢀC, the
particle size distributions were narrow, with most sizes below
10 nm and centered on sizes around 4 nm. As carbonization tem-
perature increased, the distributions widened and the typical par-
ticle sizes becomes larger, although for samples prepared up to
700 ꢀC the particles were well below 20 nm. At 800 ꢀC and above,
the particle size distributions became clearly wider with a sub-
stantial percentage of the nanoparticles being several tens of
nanometres in size. However, even for these samples the most
frequent sized remained below 20 nm.
The average nickel nanoparticle size, Lb parameter, was also
estimated by applying the Scherrer’s equation (Equation (1)) to the
X-ray diffractograms shown in Fig. 1 [41,42]. This parameter eval-
uated the crystal dimension using the integral breadth, bb, which is
defined as the total area under the diffraction maximum divided by
the corresponding peak intensity. The bb parameter includes an
instrumental contribution, ceq, that needs to be removed to
4

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F.J. Martín-Jimeno, F. Suarez-García, J.I. Paredes et al.
Journal of Alloys and Compounds 853 (2021) 157348
Fig. 3. Representative FE-SEM images for NiOF carbonized at: (a, b) 400 ꢀC; (c, d) 600 ꢀC and (e, f) 800 ꢀC.
determine the actual integral breadth, b0b, as indicated in Equation
other. Both parameters increased with carbonization temperature,
with a significant increase being observed between 700 and 800 ꢀC.
From 400 to 700 ꢀC the average size and Lb increase from 3.8 to 7.5
and 5.8e10.7 nm, respectively, and at 800 ꢀC these parameters
change to 19.8 and 22.5 nm, respectively. On the other hand, the
modal particle size did not exhibit a significant increase between
700 and 800 ꢀC and only underwent a limited increase in the whole
carbonization temperature range (from 3 to 10 nm between 400
and 1000 ꢀC).
(2). In our case: c2eq ¼ 7.7$10ꢂ6. Finally, Kb is the Scherrer’s constant,
which depends on the definition of ‘breadth’, the crystallite shape
and the crystallite-size distribution [42].
Kb
b0b cos
l
Lb ¼
(1)
q
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
À
Á
An interesting effect of the treatment temperature on the evo-
lution of the crystalline phases of nickel was observed by X-ray
diffraction (Fig. 1). As it is well known, a face-centered cubic (FCC)
structure is the stable crystalline phase of nickel (PDF 03-065-
2
b0b ¼ ðbbÞ2 ꢂ ceq
(2)
Table 2 collects the modal and average particle size obtained
from TEM images (Fig. 5) as well as the Scherrer’s crystal dimension
parameter for the different NiCx samples. The average values ob-
tained from TEM and Lb are in reasonable agreement with each
2865), with typical peaks at 2
peaks were observed in all carbonized NiOF samples except in the
q
¼ 44.51, 51.85 and 76.37ꢀ. These
5

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F.J. Martín-Jimeno, F. Suarez-García, J.I. Paredes et al.
Journal of Alloys and Compounds 853 (2021) 157348
Fig. 4. Representative TEM images for NiOF carbonized at: (a) 400, (b) 600, (c) 800 and (d) 1000 ꢀC.
case of NiC4, where only an intense peak at 44.74ꢀ was noticed,
which could be assigned to the metastable form of Ni3C in rhom-
bohedral structure [43,44] (PDF 01-072-1467). The sample treated
at 400 ꢀC also displayed two peaks at 13.64 and 15.38ꢀ that cor-
responded to non-decomposed NiOF, i.e., this temperature was not
high enough to fully decompose the NiOF as suggested by the TGA
results (Figure S4). The thermal decomposition of Ni3C is known
start at around 450e475 ꢀC [44,45], but in our case it could not
observed by TGA because the thermal decomposition of the NiOF
occurred in the same temperature interval (Figure S4). Samples
prepared at temperatures between 500 and 700 ꢀC exhibited
defined and narrow peaks, corresponding with high crystalline
FCC nickel nanoparticles with a calculated cell dimension of
a ¼ 0.3525 nm, indicating that the stable Ni phase was attained
only at a temperature of 800 ꢀC.
Going back to the nanoparticle size histograms (Fig. 5), the
discrepancies between modal values and average size and Lb can be
understood. As can be seen, up to 700 ꢀC the particle size distri-
butions are narrow, but above this temperature they become wide,
increasing the particle size dispersion. This is reflected in the
average values (average size and Lb), but not in modal size. The
presence of Ni3C and HCP phases insures a strong interaction be-
tween the carbonaceous component and Ni nanoparticles, pre-
venting their movement and therefore their sintering. Above
700 ꢀC Ni-FCC is the only structure present in the sample and due to
the evolution of the Ni crystalline phases explained above, most
nickel nanoparticles are expected to continue to maintain a strong
interaction with the carbonaceous support after transformation
from Ni3C to Ni-FCC, which would cause that the modal size does
not increase much. However, as temperature increases a greater
number of Ni nanoparticles will have less interaction with the
carbon support, being able to spill over the surface and therefore
sintering, increasing the particle size dispersion and therefore the
average nanoparticle size.
additional peaks at 2
q
¼ 41.85, 44.5 (this peak cannot be distin-
guished from the highest intensity peak of FCC Ni phase), 47.55 and
62.53ꢀ. As reported previously [45e48], these peaks can be
ascribed to an intermediate phase formed during the conversion of
rhombohedral Ni3C (where carbon adopts inner atom-positions in
the lattice) [44] into the stable Ni-FCC structure. In this interme-
diate phase, nickel atoms could be forced to adopt a compressed
crystalline form of hexagonal close-packed (HCP) phase due to the
loss of interstitial carbon atoms and to the effect of the graphite-like
shell formed over the nickel particles, which blocks the reorgani-
zation of the Ni atoms into the FCC structure [45]. In our case, the
peak position and the calculated hexagonal cell dimensions
(a ¼ 0.2492 nm and c ¼ 0.4015 nm) were the same as those re-
ported previously by other authors [45] and matched with PDF 00-
001-1277 corresponding to the HCP structure of cobalt. Finally,
samples NiC8, NiC9 and NiC10 only exhibited three intense, well-
The evolution of the carbonaceous component with heat treat-
ment temperature was studied by Raman spectroscopy (Fig. 6).
First, the Raman spectrum of non-carbonized NiOF reflected the
vibrational modes of 2-mIm [49]. The most intense bands were
6

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F.J. Martín-Jimeno, F. Suarez-García, J.I. Paredes et al.
Journal of Alloys and Compounds 853 (2021) 157348
Fig. 6. Raman spectra of pristine NiOF and NiOF carbonized at different temperatures
(between 400 and 1000 ꢀC).
were observed together with the emergence of the 2D band, which
is known to be related to the development of a significant graphitic
character in carbon materials with decreasing concentration of
defects [51,52]. The latter was confirmed by determining the inte-
grated intensity ratios of the D and G bands (ID/IG ratio) as a well-
known proxy for defect content in graphitic materials. Samples
NiC8, NiC9 and NiC10 had an ID/IG ratio of ~0.92, ~0.63 and ~0.58,
respectively, indicating decreasing defect content with increasing
temperature. Furthermore, Raman spectra in samples NiC9 and
NiC10 showed a shoulder on the high wavenumber side of the G
band, which could be assigned to the so-called D0 band
(~1612 cmꢂ1), which is also a defect-related band. The significant
increase in the graphitic character of the samples treated at 800 ꢀC
and above could be explained by taking into account that Ni ca-
talyses the graphitization of amorphous carbon close to the metal
Fig. 5. Nickel particle size histogram distributions obtained by manually counted and
measured diameters of nickel grains in TEM photographs (~1000 particles for each
sample were measured).
Table 2
Nickel particle sizes as a function of the carbonization temperature.
T (ꢀC)
Particle size (nm)
Modal(a)
Average(a)
Scherrer
L(bb)
400
500
600
700
800
900
1000
3
4
6
7
3.8
4.4
7.3
7.5
19.8
20.2
36.9
5.8
6.9
8.6
10.7
22.5
25.1
26.7
particles by
a
solution-diffusion-precipitation mechanism
[48,53,54]. Indeed, TEM images for samples prepared above 800 ꢀC
showed the presence of carbon nanofilaments (see Figure S6)
produced during the heat treatment.
10
8
6e10
a
Values obtained by manually counting-measuring nickel particles (~1000 par-
ticles/sample) fromTEM images.
3.3. Catalytic activity of the NiOF-derived Ni/carbon hybrids in the
reduction of 4-nitrophenol
b
Calculated by Scherrer equation using the integral breath approximation. J.I.
Langfor and A.J.C. Wilson [42].
The reduction of 4-NP to 4-AP by NaBH4 is usually employed as a
model reaction to assess the catalytic activity of metallic nano-
particles [55] but has also relevance from a practical perspective
(e.g., in the synthesis of certain analgesic and antipyretic drugs)
[11]. The evolution of the reaction was followed with UVevis ab-
sorption spectroscopy measuring the evolution (decrease) of the
absorption band located at ~400 nm that is characteristics of 4-
nitrophenolate, i.e., the deprotonated form of 4-NP that is gener-
ated in the basic medium of the reaction by the NaBH4. The for-
mation of 4-AP was confirmed by the emergence of the 4-
aminophenolate characteristic band at ~299 nm (see Fig. 7 and
S7). The concentration of 4-NP at the beginning of the reaction and
that of 4-AP at the end were determined by means of their corre-
sponding UVevis calibration curves (Figure S1). Specifically, the
absorbance values at 299 nm for 4-AP at the end of the reduction
correlated with vibrational stress of the imidazole ring
(1467 cmꢂ1), symmetric tensional vibration of methyl group
(2928 cmꢂ1) and symmetric tensional vibration of the HC]CH
bond (3150 cmꢂ1). After heat treatment, the Raman spectra of the
NiCx samples were dominated by three main features that are
characteristic of graphitic (or, at least, sp2-based) carbons [50],
namely, the G band (~1582 cmꢂ1), the defect-related D band
(~1348 cmꢂ1), and the second-order 2D band (~2692 cmꢂ1). Sam-
ples prepared at temperatures up to 700 ꢀC exhibited a strong
overlap of very broad D and G bands as well as an absence of any 2D
band, indicating a highly disordered or even amorphous carbon
structure. Above this temperature, well defined D and G bands
7

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F.J. Martín-Jimeno, F. Suarez-García, J.I. Paredes et al.
Journal of Alloys and Compounds 853 (2021) 157348
reaction varied between 0.113 and 0.126 (see Fig. 7 and S7 and
Table S2), which corresponded to 4-AP concentrations between
0.056 and 0.062 mM. Taking into account the stoichiometry of the
reaction and the initial concentration of 4-NP, the corresponding
reaction yields were close to 100% (Table S2) for all the catalysts,
except for NiC9 and NiC10, indicating that 4-NP was efficiently and
selectively reduced to 4-AP by the Ni/carbon hybrids. The some-
what lower reaction yield for samples NiC9 (93%) and NiC10 (92%)
could be simply due to the fact that reduction was not yet
completed at the time of the measurement, as can be noticed in
Figure S7-f1 and -g1, where some absorbance at 400 nm was
observed in the latter spectra.
be calculated by a logarithmic fit of absorbance (Ln A/A0) vs time
(Equation (4)) as can be seen in Fig. 7b:
dC4NP
dt
ꢂ
¼ kap C4NP
(3)
(4)
C4NP
C4NP; t¼0
A4NP
A4NP; t¼0
ꢂLn
¼ kap t ≡ ꢂ Ln
¼ kap
t
, where, C4NP and C4NP, t¼0 are concentration of 4-NP at the time t
and at the beginning of the reaction, respectively. In order to make a
better comparison of the catalyst performance, two activity pa-
rameters were defined by normalizing kap to the amount of catalyst
used per unit of volume, k’ (Equation 5), and to the total amount of
nickel in each sample, k’’ (Equation 6):
NaBH4 was used in a very large excess so that the reaction
approximately became pseudo-first-order with respect to 4-NP
(Equations (3) and (4)). Thus, the apparent rate constant (kap) can
kap
k0 ¼
k00 ¼
(5)
(6)
mcat
=
Vreact
k0
~
xNi
, where mcat is the total mass of Ni/carbon hybrid used (about
~
4.5 mg), Vreact is the total volume of reaction (300 mL) and xNi is the
nickel weight percentage in each sample (Table 1). Likewise, to
facilitate comparisons with other Ni-based catalysts reported in the
literature, the overall catalytic activity defined as the number of
moles of 4-NP reduced per mole of nickel per unit time, ActNi, was
also calculated.
The different measured parameters are collected in Table 3 and
for the different NiCx samples and plotted in Fig. 8 as a function of
teat treatment temperature. Sample NiC4 (material prepared at
400 ꢀC) boasted the highest catalytic activity of all samples
ꢂ1
(Kap ¼ 0.172 minꢂ1, k’ ¼ 0.191 L gꢂ1
s
ꢂ1, k’’ ¼ 0.534 L g sꢂ1
;
Ni
ꢂ1
ActNi ¼ 0.039 mol4NP mol
s
ꢂ1). While an overall trend of
decreasing activity parameters with increasing heat treatment
temperature between 400 and 1000 ꢀC was apparent in Fig. 8, a
local maximum could be noticed at 700 ꢀC.
Ni
To rationalize this behaviour, several points have to be taken
into account. First, the monotonous increase in the size of the Ni
nanoparticles in the NiCx samples with increasing the heat treat-
ment temperature (see Table 2 and Fig. 5) entails a decrease in the
specific surface area of the Ni catalyst, so a diminution in the cat-
alytic activity of the samples when normalized to their Ni content is
expected. However, although this can roughly explain the overall
trend (decrease in catalytic activity with heat treatment tempera-
ture), it cannot explain the substantial increase in activity between
600 and 700 ꢀC. Second, because the Ni nanoparticle were
embedded in a porous carbon matrix (see Fig. 4), another factor
that could critically impact their catalytic performance would be
the accessibility of the reagent to the nanoparticle surface through
the pore network of the carbon matrix. In this case, reagent access
would be expected to be facilitated for carbon matrixes with a well-
developed porous texture, particularly in terms of mesopore vol-
ume. Nevertheless, the most active catalyst (NiC4) was also the
poorest regarding pore texture development, suggesting that such
an effect was not a determinant of the observed catalytic trends.
Third, the different crystalline phases of nickel observed by X-ray
diffraction in the samples prepared at different temperatures could
bear a significant influence on the reported catalytic behaviour. We
note that the sharp decrease in catalytic activity between 400 and
600 ꢀC (see Fig. 8), where the increase in Ni nanoparticle size was
relatively modest, was coincident with the emergence of the HCP
Ni phase (Fig. 1), and that the almost total disappearance of this
Fig. 7. (a) Representative evolution of the absorption spectra during the reduction of
4-NP to 4-AP with NaBH4 catalysed by NiC7 at 25 ꢀC; (b) corresponding plot of Ln (A/
A0) vs time. Data for other NiCx samples are compiled in Figure S7. A denoted the
absorbance measured at 400 nm at a time t, and A0 is the absorbance at the beginning
of the reaction.
8

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F.J. Martín-Jimeno, F. Suarez-García, J.I. Paredes et al.
Journal of Alloys and Compounds 853 (2021) 157348
Table 3
Kinetic constants and catalytic activities of the Ni/Carbon hybrids catalysts at 25 ꢀC.
Sample
kap (minꢂ1
)
k’
k’’
ActNi (mol4NP $ molꢂNi1$ minꢂ1
)
(L$gꢂ1$sꢂ1
)
(L$gꢂNi1$sꢂ1
)
NiC4
NiC5
NiC6
NiC7
NiC8
NiC9
NiC10
0.172
0.104
0.061
0.129
0.088
0.055
0.046
0.191
0.115
0.067
0.143
0.097
0.061
0.051
0.534
0.268
0.153
0.283
0.197
0.114
0.088
0.039
0.020
0.012
0.020
0.015
0.008
0.007
(volume < 3 mL) or in low volume reactors, where the amount of
catalyst to volume ratio was very high, which should favour the
kinetics of the process, as we also noted in the higher values of Kap
obtained in the cyclabilty experiments carried out in the spectro-
photometer cuvettes under similar conditions than in the reactor
experiments. Thus, the concentration of catalyst (i.e., Ni þ carbon)
and Ni used in those works were also included in Table S3. In our
case, we used a reaction volume of 300 mL, which should more
closely resemble a real practical situation. Taking this into account,
the best catalyst prepared in this work (NiC4) displayed one the
best performance for the investigated reaction and only catalysts
reported by Chen et al. [58] and Xia et al. [10] (measured at 30 ꢀC)
possessed a higher activity than ours using similar catalyst con-
centrations. As in the case of other Ni-based catalysts, our Ni/car-
bon hybrids also have a ferromagnetic character (Figure S8), which
allows their easy recovery comparing to non-ferromagnetic
catalysts.
The cyclability of the most active catalysts (i.e. NiC4 and NiC7)
was studied by means of eight consecutive reduction reaction
carried out in the cuvette of the UVevis spectrophotometer by
adding fresh 4-NP at the beginning of each cycle and measuring the
absorbance decay at 400 nm (Figure S9). As can be seen, after 8
consecutive cycles the reaction rate constant, Kap, is maintained for
both samples, thus demonstrating their stability and recyclability.
Leaching of Ni into the reaction solution was not thought to be an
issue. After the cyclability tests, the solid catalyst was allowed to
sediment from the reaction solution, and then the latter was
collected and used as a possible catalyst in a subsequent 4-NP
reduction reaction. No significant catalytic activity was detected,
which indicated that Ni molecular species or clusters did not leach
from the solid catalyst to any substantial extent.
Finally, we studied the effect of temperature on the catalyst
activity. Experiments at 15 and 5 ꢀC were carried out with sample
NiC4 and shown in Figure S10. Applying the Arrhenius equation
and plotting LnKap vs. 1/T (Equation (7) and Fig. 9), the apparent
activation energy (Eact) and the pre-exponential factor (A) could be
calculated (Table 4):
Fig. 8. Plot of the kinetic constants and ActNi at 25 ꢀC as a function of NiOF carbon-
ization temperature. Note that k’ is referred to the total mass (carbon þ nickel) and k’’
is calculated with respect to the nickel amount in the catalyst.
phase at 700 ꢀC, was concurrent with a substantial increase in
catalytic activity. Such observations strongly suggest that HCP Ni is
considerably less active than both FCC Ni and Ni3C which would
account for some of the peculiar catalytic trend observed in Fig. 8.
Indeed, recent theoretical studies [56] on the dissociation of CO in
FCC and HCP Ni catalysts have shown that the latter phase is less
active than the former. In our case, this lower activity of the hex-
agonal phase can also be explained taking into account the mech-
anism discussed above for the formation of the FCC Ni phase from
Ni3C: the transformation of Ni3C to FCC Ni occurs through the
formation of the intermediate HCP phase by the diffusion of carbon
atoms from the bulk to the surface of the Ni particles, where they
precipitate and form a graphitic carbon shell [45]. This carbon shell
can partially cover the catalyst surface, making it less accessible to
the reagents, decreasing the catalytic activity of HCP Ni
nanoparticles.
It is not straightforward to compare the behaviour of different
catalysts reported in the literature because the conditions under
which they are tested differ in the amount of catalyst used, the
concentration of the reagents, the volume of the reactor and the
temperature, which all have an effect on the catalyst activity.
Table S3 lists different kinetic parameters and ActNi for a series of Ni
catalysts supported onto carbon materials reported in the literature
[7,10,22,25,29,31,32,57e61]. In several of these works, the kinetic
studies were carried out in UVeVis spectrophotometer cuvettes
E
RT
Eact
RT
act
Kap ¼ Aeꢂ / Ln k ¼ Ln A ꢂ
(7)
Eact was determined to be 93.9 kJ molꢂ1 and A was 4.34$1015 sꢂ1
.
This activation energy was similar to value reported by Xia et al.
[10] for Ni nanoparticles supported on carbon black (80.7 kJ molꢂ1
and lower than that for unsupported Ni catalyst (129.3 kJ molꢂ1
)
)
reported by same authors, but higher than those reported by other
authors [7,19]. The activation energy depends on the reaction
mechanism, being that of the highest energy stage. In the case of
our catalysts, it seems that supporting the nanoparticles on the
carbon material produces a decrease in the activation energy with
respect to that presented by the not supported nickel in the
reduction of 4-NP to 4-AP with NaBH4 [10]. On the other hand, a
higher activation energy is also indicative of a higher dependence
9

ꢀ
F.J. Martín-Jimeno, F. Suarez-García, J.I. Paredes et al.
Journal of Alloys and Compounds 853 (2021) 157348
reduction of 4-nitrophenol to 4-aminophenol. Indeed, the catalytic
activity was strongly dependent on the heat treatment tempera-
ture, generally decreasing with increasing temperature, but pre-
senting a local maximum at 700 ꢀC. This behaviour was related to
the different catalytic activity of the various nickel phases, with Ni-
HCP being a relatively low activity phase. The catalyst prepared at
400 ꢀC was the most active, boasting one of the highest kinetic
parameters reported in the literature for the investigated reduction
reaction using non-noble metals (Kap ¼ 0.172 minꢂ1, k’’ ¼ 0.534 L
gꢂNi1 sꢂ1 and ActNi ¼ 0.039 mol4NP molNꢂi1 minꢂ1).
CRediT authorship contribution statement
F. Julian Martín-Jimeno: Investigation, Methodology, Data
ꢀ
ꢀ
curation. Fabian Suarez-García: Conceptualization, Methodology,
Data curation, Writing - original draft, Funding acquisition. Juan I.
Paredes: Methodology, Data curation, Writing - review & editing,
Funding acquisition. Amelia Martínez-Alonso: Writing - review &
ꢀ
editing, Funding acquisition. Juan M.D. Tascon: Writing - review &
editing, Funding acquisition.
Fig. 9. Plot of Lnkap against 1/T for the reduction of 4-NP at different temperatures: 5,
15 y 25 ꢀC.
Declaration of competing interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Table 4
Kinetic parameters obtained for experiments carried out at 25, 15 and 5 ꢀC in
presence of sample NiC4. A and Eact were calculated using the Arrhenius equation.
T (ꢀC) kap [minꢂ1
kap,NiC4 A,NiC4
]
k’ [L$gꢂca1t$sꢂ1
]
k’’ [L$gꢂNi1$sꢂ1
k’’NiC4 A00
]
Eact [kJ$molꢂ1
]
Acknowledgments
k’NiC4 A’NiC4
NiC4
Funding by the Spanish Ministerio de Economía y Com-
petitividad (MINECO) and the European Regional Development
Fund (ERDF) through project MAT2015-69844-R and by the Span-
25
15
5
0.172
0.033
0.011
4.34E15 0.191 4.82E15 0.534 1.35E16 93,9
0.036
0.012
0.102
0.035
ꢀ
ish Ministerio de Ciencia, Innovacion y Universidades, the Spanish
ꢀ
Agencia Estatal de Investigacion (AEI) and ERDF through project
RTI2018-100832-B-I00 is gratefully acknowledged. Partial funding
of the reaction rate with the temperature. Therefore, a small in-
crease in temperature will produce a significant increase in the
kinetic constants allowing to achieve very high reaction rates at
moderate temperatures.
ꢀ
by Plan de Ciencia, Tecnología e Innovacion (PCTI) 2013e2017 del
Principado de Asturias and the ERDF through project IDI/2018/
000233 is also acknowledged.
Appendix A. Supplementary data
4. Conclusions
Supplementary data related to this article can be found at
https://doi.org/10.1016/j.jallcom.2020.157348.
In this work, we have synthesized a new nickel-based metal-
organic framework (NiOF) by reaction between nickel acetate and
2-methyl-imidazole in water at a moderate temperature and in a
short time. This NiOF was used as a precursor for the synthesis of
mesoporous Ni nanoparticle/carbon hybrids by heat treatment at
temperatures between 400 and 1000 ꢀC. The NiOF solid was a
mesoporous crystalline material with a tetragonal structure (D4h
point group symmetry), a specific surface area of 81 m2g-1 and a
mesopore volume of 0.21 cm3g-1. By controlling the heat treatment
temperature, both the Ni nanoparticle size and the amount of
nickel increased from 3.8 to 36.9 nm and from 35.8 to 57.5 wt%,
respectively. Despite the large fraction of metal content, the ma-
terials exhibited a homogeneous dispersion of Ni nanoparticles
with limited agglomeration or sintering, probably due to their
stabilization in the porous carbon matrix. The Ni nanoparticles
went through different crystalline phases during the carbonization
process. At 400 ꢀC, Ni3C was the dominant phase, while at tem-
peratures above 700 ꢀC the thermodynamically stable Ni face-
centered cubic phase (FCC) remained as the only crystalline form
present in the samples. The nanoparticles transitioned from Ni3C to
Ni-FCC through a metastable hexagonal close-packed phase (HCP).
The presence of such crystalline forms appeared to have a notice-
able effect on the performance of the materials as a catalyst for the
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