Carbonization of Co-BDC MOF results in magnetic C@Co nanoparticles that catalyze the reduction of methyl orange and 4-nitrophenol in water

Article abstract of DOI:10.1016/j.molliq.2019.111059

Herein we report a simple, facile and green technique for the preparation of magnetic cobalt nanoparticles (NPs) embedded on porous carbon (C@Co) nanocatalyst using MOFs template and explored for the catalytic reduction of Methyl orange (MO) and 4-Nitroph

Full text of DOI:10.1016/j.molliq.2019.111059

Accepted Manuscript
Carbonization of Co-BDC MOF results in magnetic C@Co
nanoparticles that catalyze the reduction of methyl orange and
4-nitrophenol in water
Md. Ariful Ahsan, Olivia Fernandez-Delgado, Eva Deemer,
Huiyao Wang, Ahmed A. El-Gendy, Michael L. Curry, Juan C.
Noveron
PII:
DOI:
Article Number:
Reference:
S0167-7322(19)30627-0
https://doi.org/10.1016/j.molliq.2019.111059
111059
MOLLIQ 111059
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Accepted date:
1 February 2019
22 May 2019
26 May 2019
Please cite this article as: M.A. Ahsan, O. Fernandez-Delgado, E. Deemer, et al.,
Carbonization of Co-BDC MOF results in magnetic C@Co nanoparticles that catalyze
the reduction of methyl orange and 4-nitrophenol in water, Journal of Molecular Liquids,
https://doi.org/10.1016/j.molliq.2019.111059
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ACCEPTED MANUSCRIPT
Carbonization of Co-BDC MOF results in magnetic C@Co nanoparticles that catalyze the
reduction of methyl orange and 4-nitrophenol in water
Md. Ariful Ahsan,a* Olivia Fernandez-Delgado,a Eva Deemer,b Huiyao Wang,c Ahmed A. El-
Gendy,d Michael L. Curry,e Juan C. Noverona*
aDepartment of Chemistry, University of Texas at El Paso, El Paso, TX-79968, USA.
bDepartment of Materials Science, University of Texas at El Paso, El Paso, TX-79968, USA.
cCURRL, New Mexico State University, Las Cruces, NM-88003, USA.
dDepartment of Physics, University of Texas at El Paso, El Paso, TX-79968, USA.
eDepartment of Chemistry, Tuskegee University, Tuskegee, AL-36088, USA.
Corresponding Authors: jcnoveron@utep.edu; mahsan2@miners.utep.edu
Abstract
Herein we report a simple, facile and green technique for the preparation of magnetic
cobalt nanoparticles (NPs) embedded on porous carbon (C@Co) nanocatalyst using MOFs
template and explored for the catalytic reduction of Methyl orange (MO) and 4-Nitrophenol (4-
NP). The morphology, structure and composition of the nanocatalyst were characterized by using
Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS),
Transmission Electron Microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy,
thermogravimetric analysis (TGA) and Vibrating Sample Magnetometer (VSM) techniques.
XRD analysis confirmed the presence of metallic Co NPs as well as their high crystalline

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structure. TEM analysis revealed that all the Co NPs are wrapped and protected by the porous
carbon matrix. Catalytic studies revealed that the prepared magnetic C@Co nanocatalyst could
catalyze the reduction of MO and 4-NP in 2 min effectively. Furthermore, the nanocatalyst
showed an outstanding reusability and recyclability characteristics that remained unchanged even
after several cycles. Consequently, this research offers a highly active, recyclable and
magnetically isolable nanocatalyst, which can be utilized for many catalytic industrial processes.
In this regard, the magnetic C@Co nanocatalyst could be a potential candidate for the
wastewater treatment.
Key Words: Catalytic reduction; Co NPs; Methyl orange; MOFs; 4-Nitrophenol; Porous carbon.
1. Introduction
Transition metal nanoparticles (NPs) have been widely used in catalysis and many other
fields in recent times due to their excellent physico-chemical properties [1-3]. But, the
coalescence and aggregation of the metal NPs during their usage possess a huge limitation in
their practical application due to their high activity and high surface energy. Therefore, it is very
important to find a suitable or functionalized support on which the metal NPs can lie while
retaining their physical and chemical properties [4-5]. Metal-organic frameworks (MOFs) have
drawn increasing attention in different fields due to their tunable pore structures and designable
frameworks [6-7]. In recent times, MOFs have been using as ideal precursors to prepare
nanoporous carbon materials directly by carbonization in controllable atmosphere [8-9]. This
carbon-based nanomaterials synthesized by this new MOF templated method possess many
benefits such as high crystallinity, large internal surface area, tuned in modification and
tailorable porous structure, as a result of this, many researchers focused on preparing suitable
and desired nanocatalysts derived from MOFs precursors [10-12]. During the process of

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pyrolysis, the carbon produced from the organic linkers of MOFs coated the metal NPs as well as
prevented them from aggregating. Besides, the in situ preparation method makes the metal NPs
more stably and uniformly dispersed in porous carbon than other procedures of loading metal
NPs into carbon matrix [11].
Furthermore, magnetic nanoparticles have been drawing much attention recently as an
essential class of nanocatalysts since they show exciting properties that are essentially different
from the bulk materials. Also, the separation of these tiny particles from solution or reaction
mixture is hard by traditional methods [13]. Therefore, utilization of magnetic nanoparticles
which can be easily recoverable by an external magnet is one of the key strategies to overcome
this separation problem [14-16]. Then these system allows quantitative recovery of the magnetic
catalyst by simply utilizing an external permanent magnet which makes the system more cost-
effective and can be used potentially for different industrial applications.
Recently, the water pollution is becoming a major concern all over the world. There are
several techniques such as adsorption [17-20], photocatalysis [21], catalysis [22-23] etc. are
utilizing for treating the wastewater. The compound 4-nitrophenol (4-NP) is a known common
organic pollutant in industrial and agricultural wastewater, whereas 4-aminophenol (4-AP), the
reduced form of 4-NP is an important intermediate for preparing many antipyretic/analgesic
drugs and can be utilized as a corrosion inhibitor, anti-corrosion lubricant and photographic
developer [22-25]. Furthermore, many industries such as cosmetic, plastic and textile produce
numerous dyes and use them to color their products. The dyes having N N groups for instance,
methyl orange (MO) are toxic and even carcinogenic that lead to a serious threat for aquatic
living organisms [26-28]. Thus, their catalytic reduction is a key topic in the field of contaminant
elimination. Presently, various noble metal catalysts such as Cu, Ni, Pt, Pd, Au or Ag have been

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used extensively due to their high catalytic performance [29-31]. Though, as the noble metals are
expensive and scarce, the high cost restricts their large-scale application in environmental
pollutants removal. Recently, Co nanoparticles and composites with Co nanoparticles have
become more exciting than noble metal catalysts due to their noticeably lower cost as well as
magnetic separability [31].
The current study reports a comparatively simple, cheap and facile method for the
synthesis of magnetic C@Co nanocatalyst by direct carbonization of Co-BDC MOF for the
catalytic reduction of 4-NP and MO in presence of NaBH4. The nanocatalyst was characterized
thoroughly by using SEM, EDS, TEM, TGA, XRD, FTIR, Raman and VSM instruments. This
method allows high-content magnetic Co nanoparticles to locate uniformly over the porous
carbon matrix. The obtained Co nanoparticles of several nanometers supported on the
nanoporous carbon matrix are highly dispersible in water and capable of effective catalysis for
MO and 4-NP reduction. The as-synthesized C@Co nanocatalyst showed excellent catalytic
performance towards the reduction of 4-NP and MO and also, can be recycled for several cycles
with higher catalytic activity in case of both pollutants.
2. Experimental
2.1 Materials
All chemicals were reagent grade and used as received. Co(NO3)2.6H2O, benzene-1,4-
dicarboxylic acid or terephthalic acid (BDC), dimethylformamide (DMF), Sodium borohydride
(NaBH4), 4-nitrophenol and methyl orange were purchased from Sigma-Aldrich. Deionized (DI)
water was obtained from Milli-Q® instrument, Millipore Corporation.

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2.2 Synthesis of the nanocatalyst
2.2.1. Synthesis of Co-BDC MOF
Co-BDC MOF was synthesized as follows: 132 mg of terephthalic acid was dissolved
into 10 mL of DMF. In the same time, a solution of Co(NO3)2.6H2O (720 mg in 10 mL water) in
water was also prepared. Then the salt solution was poured into the terephthalic acid solution and
stirred for 10 minutes. The resulting solution was then transferred into a glass reactor and put it
0
in an over for 24 h at 100 C. Then the Co-BDC MOF crystals were collected by vacuum
filtration and washed for three times with water and DMF. The obtained crystals were dried in a
vacuum oven at 110 oC for 12 h. The structure of the Co-BDC MOF is given in Fig. S1.
2.2.2. Synthesis of magnetic C@Co nanocatalyst
The C@Co nanocatalyst was prepared by direct carbonization of Co-BDC MOF as
synthesized under continuous argon gas flow without any pretreatment. The carbonization was
o
carried out at 600 oC for 1 h with a heating rate of 5 C/min. The C@Co nanocatalyst was black
in color and put it into a glass vial for further use.
2.3 Sample Characterization
The X-ray diffraction (XRD) analysis was carried out using a Bruker D8 Discover X-ray
Diffractometer to analyze the crystal phases of the prepared C@Co nanocatalyst. Scanning
electron microscopy (SEM) equipped with an Energy-dispersive X-ray spectroscopy (EDS) was
utilized to study the morphology as well as the elemental compositions of the prepared
nanocatalyst (Model: Hitachi S-4800). Transmission electron microscopy (TEM) images of the
C@Co nanocatalyst were taken by TEM, Model: Hitachi H-7650. A tiny amount of C@Co
nanocatalyst was dispersed in methanol with an ultrasonic treatment before transferring to a
copper grid supported carbon film. Raman spectrum of the nanocatalyst was obtained using a
DXR Smart Raman. Thermal gravimetric analysis (TGA) of the nanocatalyst was conducted on a

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Mettler Toledo thermogravimetric instrument. UV–Vis spectra of the MO and 4-NP solutions
were recorded on a UV-Vis spectroscopy (Model: Agilent Cary 50 Conc) using the absorbance
mode. The magnetic properties of the nanocatalyst at room temperature was measured by means
of Versalab. Vibrating sample magnetometer (VSM) from Quantum Design in the magnetic field
range of -3 T to 3T. An inductively coupled plasma-optical emission spectrometer (ICP-OES)
was used to investigate the leaching of the cobalt nanoparticles (manufacturer: Perkin Elmer and
model: Optima 4300DV).
2.4 Catalytic reduction of MO and 4-NP
The catalytic activity of the prepared magnetic C@Co nanocatalyst was investigated
through the reduction of MO and 4-NP in presence of NaBH4 as reducing agent. In a typical
experiment, 4 mL of 20 ppm MO and 1 mg of C@Co nanocatalyst were mixed together in a
quartz cuvette and sonicated for 2 min to make a homogeneous mixture. Afterward, 0.5 mL of
0.1 M freshly prepared NaBH4 solution was added directly to the cuvette and then measured the
absorbance using a UV-vis spectrophotometer at a range of 200-600 nm. The similar procedure
was followed for 4-NP reduction reaction.
To evaluate the efficacy of the catalytic reduction reaction, the remaining fraction of MO or 4-
NP is calculated by the following equation [32].
Remaining fraction = Ct/Ci ×100%
(1)
Where Ci and Ct are concentrations of MO or 4-NP at time 0 and t, respectively.

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3. Results and discussion
3.1 Characterization of the C@Co nanocatalyst
3.1.1 XRD analysis
The Co-BDC MOF was prepared by a green solvothermal method. The MOF precursor
can be easily synthesized by mixing cobalt nitrate salt solution with BDC solution and then
heated in an oven for 24 h. Then the obtained MOF powder was analyzed by XRD measurement
and the result is shown in Fig. S2. XRD analysis of the Co-BDC MOF precursor reveals that the
MOF is crystalline in nature. The peak positions and relative intensities of the synthesized MOF
have good matching with the reported values of the Co-BDC MOF [33], confirming the high
crystallinity of the MOF precursor.
The obtained Co-BDC MOF powder was then directly carbonized under a constant argon
gas flow at 600 °C. After the carbonization process, the MOF precursor was fully converted to
magnetic C@Co nanocatalyst. To confirm the constituents of the prepared nanocatalyst as well
as the crystallinity, XRD analysis was further carried out and the result is shown in Fig. 1a. The
nanocatalyst shows a weak and broad peak at ~25°, which may be ascribed to the (002)
diffraction peak of the graphitic carbon with a low degree of graphitization. Besides this
diffraction peak, three prominent peaks are also found from the figure at approximately 44.2o,
51.5o and 75.9° that are ascribed to the (111), (200) and (220) crystal planes of the metallic Co
of the face centered cubic (fcc) structure, respectively [34]. Furthermore, there are no diffraction
peaks corresponding to the CO-BDC MOF and thus, it confirms that the Co-BDC MOF was
completely decomposed. Moreover, there are no additional peaks of impurity found in the XRD
analysis of C@Co nanocatalyst, confirming the presence of purely Co phase in nanocatalyst

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sample without the existence of any trace of oxides (e.g. CoO, Co3O4) and the result was further
supported the EDS analysis. Therefore, the XRD analysis clearly demonstrates the successful
synthesis of pure magnetic C@Co nanocatalyst.
3.1.2 Raman analysis
Raman spectroscopic analysis of the magnetic C@Co nanocatalyst is presented in Fig.
1b. The Raman spectra of the nanocatalyst showed two characteristic peaks at about 1580 cm-1
and 1347 cm− 1 corresponding to the G (graphite) and D (defects and disorder) band, respectively
and these peaks are known as characteristic Raman peaks for carbon-based composites [35]. It
has been stated the intensity ratio (IG/ID) of the G to D band in the Raman spectra is a key
parameter for understanding the degree of crystallization of the graphitic carbon in C@Co
nanoctalyst and the degree of graphitization is related to the intensity ratio (IG/ID) inversely
whereas, the amount of defect sites is proportional to the IG/ID ratio. The intensity ratio (IG/ID)
value of the C@Co nanocomposite is 1.17 and this value is higher than that of the crystalline
graphitic carbon phases (< 1) [35].
3.1.3 SEM and TEM analysis
The morphology of the Co-BDC MOF and magnetic C@Co nanocatalyst was first
investigated by the scanning electron microscopy (SEM) as shown in Fig. S3 and Fig. 2(a-b),
respectively. Fig. S3 shows that the Co-BDC MOF has a shale-shaped structure and the other
researchers found the similar result [33]. Fig. 2(a-b) shows the SEM micrographs of the C@Co
nanocatalyst at different magnifications (e.g. 5 and 10 µm). It can be seen from the figure that
the surface of the MOF is covered with many nanoscaled particles (as marked by the red and
green arrow, respectively in the figure) and these NPs are also highly dispersed. It can be

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concluded from the figure that the MOF displays high stability as a support for the preparation of
metal nanoparticles as there is not much change observed after carbonization though the surface
is little bit distorted.
TEM analysis was further conducted to get the in-depth knowledge about the morphology
of the prepared magnetic C@Co nanocatalyst and the results are presented in Fig. 3(a-b). The
figure shows the TEM images of the C@Co nanocomposite at different magnification (viz. 50
and 100 nm). From the figure, it is clear that the Co NPs are almost uniformly dispersed in the
porous carbon matrix and most importantly, all the Co NPs are wrapped by the thin porous
carbon. Therefore, there is no reunion of the Co NPs as well. The distribution range of the Co
NPs was mostly distributed from 20 nm to 30 nm measured by TEM analysis.
3.1.4 EDS analysis
Afterward, the chemical composition of as prepared magnetic C@Co nanocatalyst was
investigated quantitatively and qualitatively by EDS and element color mapping measurements
as shown in Fig. 4(a-d). The EDS spectrum of the C@Co nanocayalyst reveals that the
nanocatalyst is primarily composed of Co and C and their mass contents are 93.75% and 6.25%,
respectively in the sample. Further, the elemental color mapping also demonstrates and verifies
the result of EDS showing the presence of Co and C in the nanocatalyst sample.
3.1.5 TGA analysis
Co-BDC MOF is one of the most readily available and well-studied cobalt based MOFs.
The framework of this MOF is mainly comprised of infinite chains of CoO6 octahedra as corner-
sharing and four parallel running chains are linked to each chain from infinite chains via the
organic BDC linker [36]. Thermogravimetric analysis (TGA) was performed to investigate the

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thermal behavior of the Co-BDC MOF and the result is shown in Fig. 5. From the figure, a
o
o
drastic weight loss was observed between 400 C and 490 C for the Co-BDC MOF. After this
point, there is no further weight loss with the increasing temperature, reflecting that the organic
ligands of the MOF are completely decomposed [36].
The TGA analysis was further carried out to calculate the loading amount of Co NPs in
the porous carbon at a temperature range of 25 to 600 oC. From the TGA curve, it was found that
the loading content of the Co NPs was about 91.52 % and has a good agreement with the EDS
result. Obviously, the Co content is increased in the nanocomposite during the carbonization
process as the carbon content is usually declined with increasing the temperature.
3.1.6 Magnetic properties
The magnetic properties of the synthesized C@Co nanocatalyst have been characterized
using studies of the field dependence of the magnetization at room temperature (300 K) and field
range of -3T to 3T Fig. 6. Fig. 6 shows hysteresis loops of the magnetization revealing
ferromagnetic behavior of the C@Co sample. The sample yields saturation magnetization (MS)
of 109 emu/g and coercivity (HC) of 650 Oe. In order to estimate the mass ratio of the magnetic
materials in the carbon-impeded particles, the comparison with the MS of the related bulk Co
was done yielding 67 wt% of the magnetic Co in the C@Co system [37]. The magnetization
studies provide another alternative to deduce information on the size of the magnetic domains by
evaluating the initial slopes of the hysteresis loop [38]. The analysis of the experimental data in
Fig. 6 yield magnetic domain size of 4 nm.

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3.2 Catalytic reduction of MO and 4-NP
The catalytic activity of the prepared magnetic C@Co nanocatalyst was studied using the
reduction of MO and 4-NP on aqueous medium. In that purpose, pollutant solution was taken in
a quartz cuvette and NaBH4 solution was added to the pollutant solution subsequently. Finally,
the C@Co nanocatalyst was utilized to catalyze the degradation of MO and 4-NP. The color of
the MO and 4-NP solution was disappeared quickly, indicating the successful
degradation/reduction of the pollutants. Fig.7 and Fig. S4 shows the reduced products of MO
and 4-NP as hydrazine products and 4-aminophenol.
Organic dyes are one kind of chemical substances that are broadly used in plastic, paint,
paper, textile, pharmaceuticals, cosmetic, food and leather industries [39-40]. Their presence in
water are posing serious threat to the humankind all over the world. Thus, their complete
removal from wastewater is very important as well as a matter of widespread research. Methyl
orange was used in this study as a model organic pollutant/dye and the catalytic performance of
the prepared magnetic C@Co nanocatalyst was examined in the reduction of MO. The time
dependent UV-vis spectroscopic measurement of the catalytic reduction of MO is given in
Fig.8a. It is obvious from the figure that the reduction of MO is occurred very fast after the
addition of C@Co nanocatalyst into the MO solution and the reaction was completed in about 2
min. These advise that the C@Co nanocatalyst is responsible for the probable cleavage of the –
N=N- bonds that are present in the structure of MO and thereby decolorizing the MO solution
[41]. The change in adsorption band was monitored for MO at 465 nm over time. Furthermore,
the appearance of new adsorption bands over reaction time indicates the formation of aromatic
products from the catalytic reduction of MO as seen in the figure. The catalytic reduction of MO
was also carried out with only NaBH4 as seen in Fig. 8b. The results demonstrate that the

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reducing agent NaBH4 didn’t have significant effect on the reduction reaction. The percentage
yield was 98.58% for the MO reduction reaction as shown in Fig. 8c.
Furthermore, the magnetic C@Co nanocatalyst was also applied towards the catalytic
reduction of 4-NP. Fig. S4 displays the picture of the whole 4-NP catalytic reduction reaction.
Generally, the 4-NP solution shows the adsorption band at around 317 nm, but the peak shifted
to 400 nm after the addition of NaBH4. This is due to the transformation of 4-NP to 4
nitrophenolate ion followed by a color change from light yellow to bright yellow [24]. When the
C@Co nanocatalyst was introduced into the 4-NP solution, the adsorption band at 400 nm
decreased gradually and a new adsorption band at 300 nm appeared which corresponded to 4-AP
as seen in Fig. 9a. The catalytic reduction reaction was finished in about 2 min, reflecting the
excellent catalytic performance of the prepared magnetic C@Co nanocatalyst. This excellent
catalytic performance may be due to the (1) higher loading amount of Co NPs with more
catalytic active sites is favorable to increase the catalytic performance; (2) porous carbon
surrounded the Co NPs has a 3D structure as well as thinner, letting the 4-nitrophenolate ion
quickly pass through and reach and (3) easier hydrophilic characteristic because of the little
contents of porous carbon, which makes it suitable to absorb BH4- ion on the surface of Co NPs.
Furthermore, to find out the significance of the C@Co catalyst, we have demonstrated the same
reaction under the similar experimental conditions in the presence of NaBH4, it has been noticed
that there was no formation of new product such as 4-AP as seen in Fig. 9b. It is well established
that the NaBH4 works as a source of H2 production for the catalytic reduction reaction but it
requires the addition of an appropriate catalyst to progress the reaction. Fig.9c shows the
remaining fraction of 4-NP after the reduction reaction as well as it shows the percentage yield
(99.34%). Moreover, the catalytic reduction reaction was also carried for MO and 4-NP in

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presence of only the C@Co nanocatalyst to verify the adsorption properties of the nanocatalyst
and the results are shown in Fig. S5(a-b). It is clear from the figure that the nanocatalyst does not
adsorb MO or 4-NP by itself.
A pseudo-first order kinetic model can be utilized to investigate the catalytic rate. The
kinetic model can be expressed by the equation 2 [24].
ln(Ct/C0) = − k t
(2)
Where Co is the initial concentration of MO and 4-NP, Ct is the concentration of MO and 4-NP at
time t, and k is the apparent rate constant. In this investigation, it is presumed that the mass
transport effect is not significant, therefore the measured value of Ct in solution is close to the
MO/4-NP concentration in solution near the Co surface. Fig. 8d and Fig. 9d illustrates a linear
relationship between the reaction time and ln(Ct/C0) for the catalytic reduction of MO and 4-NP
by C@Co nanocatalyst in presence of NaBH4. The apparent rate constant values are found to be
0.042 s-1 and 0.033 s-1 for MO and 4-NP reduction reaction, respectively. Furthermore, the ratio
of k to m is defined as an activity parameter (κ = k/m), where m is the mass of the catalyst (g)
and k is the apparent rate constant (s-1). The calculated activity parameter values are 42 s−1g−1
and 33 s−1g−1 for MO and 4-NP reduction, respectively and are presented in Table 1. The
catalytic activity of the magnetic C@Co nanocatalyst in terms of κ is much higher for the
catalytic reduction of MO and 4-NP in presence of NaBH4 than that of many other catalysts
reported previously as shown in Table S1 [42-49]. The reduced solutions were further analyzed
using ICP-OES instrument to investigate the leaching of cobalt from the C@Co nanocatalyst
during the reduction process and the results revealed that there was undetectable level of cobalt
in each of the 4-NP and MO reduced solutions.

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3.3 Mechanism of MO reduction
Electron relay system is usually used to explain the dye reduction mechanism by a
catalyst. At first, the Co nanoparticles start the catalytic reduction reaction by conveying
-
electrons (e- s) from the electron donor BH4 to the MO dye molecule, where the nanocatalyst
-
accepted the e- s from donor BH4 and relayed them to the MO molecule. Actually, when the
reducing agent NaBH4 is added to the MO solution, then the Co NPs may trap the hydride from
−
BH4 . Afterward, the hydride might be adsorbed on the surface of the Co NPs and subsequently,
-
transfers its e- to the Co NPs. The hydrogen atom is formed from the BH4 due to the electron
-
transfer from BH4 to Co NPs and then the formed hydrogen atom attacks a nearby MO molecule.
Finally, the electrons on the Co NPs are released to MO molecule (an electron acceptor), making
their reduced form (hydrazine derivative) as seen in Fig. 10. This reduction mechanism is similar
to previously reported models including the Eley-Rideal and Langmuir–Hinshelwood
mechanisms [50-51].
3.4 Reusability
In both environmental and economical aspects, reusability is very important for any kind
of catalyst. Therefore, the reusability of magnetic C@Co nanocatalyst was investigated by
running repetitive MO and 4-NP reduction cycles. C@Co nanocatalyst was separated after each
run by unsing an external magnet and washed with water for next cycle. The percentage
reduction of MO and 4-NP decrease from 98.58% to 96.82% and 99.34% to 97.83%,
respectively after the fifth cycle. The results of MO and 4-NP reduction in five consecutive
reactions are given in Fig. 11(a-b) and the nanocatalyst displayed an excellent reusability in both
cases. The higher conversion efficiency of the C@Co nanocatalyst suggested that the Co

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nanoparticles were stable as well as protected by the porous carbon matrix, completely
representing the good stability of the C@Co nanocatalyst.
4. Conclusion
In conclusion, we have successfully synthesized magnetic C@Co nanocatalyst by using
Co-BDC MOF template, via a green and facile carbonization method under argon atmosphere.
The obtained results from the catalytic reduction reactions demonstrated the excellent catalytic
performance of the prepared C@Co nanocatalyst towards the catalytic reduction of MO and 4-
NP in presence of NaBH4. Additionally, this unique magnetic nanocatalyst could be simply
recovered from the reaction mixture using an external magnet. Therefore, the synthesis route
reported in this study could be an ideal platform to make highly effective magnetic nanocatalysts
for many heterogeneous catalytic reduction reactions.
Acknowledgments
This work was supported by NSF grants ERC Nanotechnology-Enabled Water Treatment
Center 1449500, CHE-0748913 and USDA 2014-38422-22078. We thank Dr. Echegoyen’s
research group for their kind help with the TGA and Raman analysis. We also would like to
thank Dr. Jorge Gardea-Torresdey and Dr. Jose Angel Hernandez-Viezcas for the help with the
ICP-OES analysis.

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Table 1. Reaction time and rate constants of the catalytic reduction of MO and 4-NP by C@Co
nanctalyst.
Catalyst Pollutants Amount of
Catalyst
Reaction
Time
120 Sec
120 Sec
Rate
Yield
(%)
98.58
99.34
Correlation
Coefficient
0.946
Constant
0.033 s-1
0.042 s-1
C@Co
MO
1 mg
1 mg
4-NP
0.999

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Figures:
Fig 1. (a) XRD spectra and (b) Raman spectra of the magnetic C@Co nanocatalyst.
13012
Fig 2. (a-b) SEM images of the magnetic C@Co nanocatalyst at different magnifications.
Fig 3. (a-b) TEM images of the C@Co nanocatalyst at different magnifications.

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Fig 4. (a-d) EDS color mapping and elemental analysis of the C@Co nanocatalyst.
Fig. 5. TGA curve of the Co-BDC MOF and as synthesized magnetic C@Co nanocatalyst.

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120
C@Co
Ms= 109 emu/g
80
40
Dmag= 4 nm
0
-40
-80
-120
T= 300 K
-30000 -20000 -10000
0
10000 20000 30000
H (Oe)
Fig. 6. Magnetization dependence on external magnetic field at room temperature for C@Co
nanocatalyst.
Fig. 7. Schematic view of the catalytic reduction of MO by magnetic C@Co nanocatalyst.

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Fig. 8. UV-vis spectra for the catalytic reduction of MO: (a) in presence of C@Co and NaBH4,
(b) in presence of NaBH4 only; (c-d) time vs Ct/Ci and time vs –ln(Ct/Ci) plot.

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Fig. 9. UV-vis spectra for the catalytic reduction of 4-NP: (a) in presence of C@Co and NaBH4,
(b) in presence of NaBH4 only; (c-d) time vs Ct/Ci and time vs –ln(Ct/Ci) plot.

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Fig. 10. Schematic view representing the synthesis of C@Co nanocatlyst along with the
mechanism of catalytic reduction of MO and 4-NP.
Fig. 11. Reusability test of the magnetic C@Co nanocatalyst for the reduction (a) 4-NP and (b)
MO in presence of NaBH4.