Synthesis of high surface area transition metal sponges and their catalytic properties

Article abstract of DOI:10.1039/c9nj02096a

We have reported sucrose-mediated facile route for the preparation of high surface area cobalt (Co), nickel (Ni), and copper (Cu) sponges. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed the sponge-like and nanoparticle-like morphologies of the metal sponges. The catalytic activity of the metal sponges was studied via the reduction of organic pollutants, including 4-nitrophenol (4-NP), methyl orange (MO) and methylene blue (MB) in water. It was found that the Cu sponge exhibited the fastest rate for the reduction of 4-NP, MO, and MB, followed by the Co and Ni sponges, respectively. The metal sponges exhibited excellent catalytic stability for the reduction of 4-NP for multiple cycles. In addition, due to their magnetic properties, the Co and Ni sponges could be easily recovered and reused by applying an external magnetic field.

Full text of DOI:10.1039/c9nj02096a

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Rosales, S. A. Ricardo, R. Arrieta, H. Kim, K. A. Sultana, Y. Lin, D. Villagran and J. Noveron, New J. Chem.,
2019, DOI: 10.1039/C9NJ02096A.
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The role of atropisomers on the photo-reactivity and fatigue of
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Synthesis of high-surface area transition metal sponges and their catalytic properties
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Md. Tariqul Islam,ab* Jose Rosales,a Ricardo Saenz-Arana,a Roy Arrieta,a Hoejin Kim,c Kazi Afroza Sultana,d Yirong
Lin,c Dino Villagran,ab Juan C. Noveron.ab*
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a Department of Chemistry, University of Texas at El Paso, 500 West University Avenue, El
Paso, Texas 79968, USA.
b Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment,
6100 Main St, Houston, TX 77005, USA.
c Department of Mechanical Engineering, University of Texas at El Paso, 500 West University
Avenue, El Paso, Texas 79968, USA.
d Department of Environmental Science and Engineering, University of Texas at El Paso, 500
West University Avenue, El Paso, Texas 79968, USA.
Keywords: Metal sponges, Catalysis, Reduction, 4-nitrophenol, Methyl orange, Methylene blue.
Abstract
Sucrose mediated facile route for the preparation of high-surface area cobalt (Co), nickel
(Ni), and copper (Cu) sponges are reported. The scanning electron microscopy (SEM),
transmission electron microscopy (TEM) images revealed the sponge-like and nanoparticles-like
morphology of the metal sponges. The catalytic activity of the metal sponges was studied by the
reduction of organic pollutants viz. 4-nitrophenol (4-NP), methyl orange (MO) and methylene
blue (MB) in water. It was found that the Cu sponge was much faster for the reduction of 4-NP,
MO, and MB, which was followed by the Co and Ni sponges, respectively. The metal sponges
exhibited excellent catalytic stability for the reduction of 4-NP for multiple cycles. Additionally,
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due to the magnetic properties, the Co and Ni sponges could be easily recovered and reused upon
applying an external magnetic field.
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1. Introduction
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Catalyst plays a key role in chemical process industries by the transformation of raw
materials into valuable products. A large portion of world economy is directly or indirectly
associated with the performance of catalysts. It is considered that about 30% of the gross
domestic product in European economies and the processing of over 80% of all manufactured
products somehow is associated with catalysis.1 Thus the development of catalyst is immensely
important to make sure a global economic success and a sustainable society with fundamental
aspects such as energy, food and water.
In this regard, transition and noble metal-based catalysts are some of the most widely
used ones due to the availability, cheaper price, stability, and the extraordinary catalytic
properties of these metals.2,3,4,5 These catalysts are applied in a variety of chemical
transformations6 such as hydrogenations,7 dehydrogenations,8 alkylation,9 amination,10 and
coupling reactions.11 Recently, the application of catalysts for sustainable environmental
remediation has garnered much attention.12,13 For example, the catalytic degradation of organic
pollutants,14 catalytic reduction of nitrates15 and hexavalent chromium,16 catalytic
dehydrohalogenation of halogenated compounds,17 and advanced oxidation processes (e.g.
fenton process)18,19,20 are some of the important types of catalytic reactions that are of special
interests. Catalytic processes for environmental remediation have advantages over other
methods.21 For example, the tunability in the reaction kinetics by tuning the morphology and the
composition of the catalysts, the reusability, and cyclic stability. Therefore, the development of a
self-supported and high-surface area (e.g. porous metals) catalyst based on facile method and
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cheap and readily available metals is highly desirable. In terms of the metal, the transition metals
are of prime choice due to their comparatively low price, availability, and intriguing catalytic
properties. For example, porous Ni for the selective hydrogenation of carbonates into formic
acid22 and the use of Raney® Ni and Raney® Co for the hydrogenation of the organic
compounds are some of the catalysts to mention. The porosity increases the surface area and
opens the active sites of a metal, which are extremely important for the better performance of the
catalysts. The porosity further facilitate elevated mass transport due to their higher surface
areas.23 Besides catalysis, porous metals have a great deal of applications including but not
limited to electro/catalysis,24 sorption,25 fuel cell electrode materials,26, biofiltration,27 hydrogen
and energy storage,28 supercapacitors and batteries29,30, heat dissipation31, and so on.
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The commonly employed method for the preparation of self-supported porous metal
include the templated synthesis, where solid template such as alumina, pumice, sugar crystal,
NaCl crystal, polymer nanoparticles, etc. are used as the sacrificial materials.32,33,34 Morphology-
wise, the template can be porous or solid in nature and the morphology of the final porous metal
is dependent on the morphology of the template. If the template is porous (such as porous
alumina or pumice), the metal salt precursor is impregnated into the pores of the template.
Thermal or chemical reduction of metal ion into the porous supports followed by the removal of
the porous template make the porous metals. The porous metals prepared by this method usually
have extraordinary higher specific surface area than the pure metal powders or foils.35 The
synthesis of the metal sponges or porous metals by the templated method has some drawbacks
such as the requirement of multiple steps of synthesis and the necessity of the washing off the
templates after the synthesis. In addition to the templated synthesis, there are template free
synthesis methods where the porous metals could be synthesized by the direct chemical and
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electrochemical reduction of the metal slats.36,32 For example, Katla and colleagues reported the
synthesis of porous gold, silver, platinum, and palladium nanosponges simply by the chemical
reduction of the metal salts by sodium borohydride in aqueous solution.37 However, these
methods are limited to the synthesis of small amount of materials and may not be adapted to the
large scale synthesis for the practical applications. Therefore, the development of a method for
the synthesis of porous metals that does not require a template and can prepare a large amount of
catalyst (porous metals) by a simple procedure with the adaptability for industrial scale
production can be very lucrative in this area of research.
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Herein, we report a facile method for the preparation of self-supporting and high-surface
area transition metal (e.g. cobalt, nickel, and copper) sponges by heating a paste of metal salt and
sucrose. The synthesis requires heating in two steps. In the first step, a mixture of metal nitrate
salt and sucrose was heated at ~180 °C in the presence of air for about 30 min to obtain a foam-
like substance. In the second step, foam-like substance was heated at 600 °C under argon to
obtain the metal sponges. The utilization of sucrose as a soft sacrificial template for the
preparation of metal sponges provides multiple advantages. For example, sucrose has reductive
alcohol groups capable of reducing metal ions, which facilitates the nucleation and the growth of
metallic clusters. Additionally, sucrose is a bio-derived organic compound having cheaper price,
sucrose is readily available, and it is not associated with any kind of chemical hazards. To the
best of our knowledge, the method presented here is novel and extremely simple in the sense of
not requiring any harmful solvents and chemicals while at the same time avoiding multiple steps.
Therefore, it could be readily extended to the fabrication of macroporous frameworks of various
other transition metals, metal oxides, and their hybrid nanocomposites. Additionally, the method
could be adapted for the industrial-scale production of the high-surface area metal, metal oxide
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porous structures and their nanocomposites for numerous applications. The as-prepared metal
sponges have been applied for the catalytic reduction of anthropogenic organic pollutants viz. 4-
NP, MO, and MB in water. The reduction of 4-NP to 4-aminophenol (4-AP) is important with
respect to the environmental remediation because of the toxic and carcinogenic nature of the 4-
NP. Additionally, the 4-AP has usefulness as a photographic developer, corrosion inhibitor,
precursor for the synthesis of paracetamol, etc.38 On the other hand, the catalytic reduction of
MO and MB can be considered as an intriguing method towards their reductive degradation for
the purpose of water treatment.39,40
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2. Materials and Methods
2.1. Materials
All the chemicals were used as received. Sucrose (>99.5%), Co(NO3)2.6H2O (99.999%
trace metals basis), Ni(NO3)2.6H2O (99.999% trace metals basis), 4-nitrophenol (O2NC6H4OH ≥
99%), and sodium borohydride (NaBH4 = 99%) were purchased from Sigma Aldrich. Methyl
Orange (C14H15N3O3S >98.0%) and methylene blue were obtained from TCI AMERICA and
EMD Millipore Corporation, respectively. Milli-Q water (> 18.20 MI cm resistivity) was
obtained from Milli-Q (Advantage A-10) water filter system.
2.2. Characterization Techniques
Agilent Cary 50 Conc UV-visible Spectrophotometer was used for the UV-visible
studies. Quartz cuvette (10 mm path length) was used as the sample holder. Transmission
electron microscopy (TEM) images were taken using a Hitachi H-7650 electron microscope. An
acceleration voltage of 80 kV was used. Carbon filmed copper grids with 200 mesh (Electron
microscopy sciences) were used for TEM imaging. A suspension of the metal sponge in ethanol
was prepared by bath sonication and a drop of this suspension was deposited onto the grid and
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air dried before imaging. Scanning Electron Microscopy (SEM) image and the Energy
Dispersive X-ray (EDX) experiments were carried out using a Hitachi S-3400N Type II scanning
electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer. Carbon tape
was used as the substrate for the SEM imaging. Bruker D8 Discover X-ray diffractometer with
Cu Kα radiation (λ = 0.15418 nm) was used for the powder X-ray diffraction (XRPD) spectrum.
Micromeritics ASAP 2020 Surface Area and Porosity Analyzer was used for nitrogen adsorption
isotherms. Samples were weighed, heated at 150 °ꢀC, and evacuated at 10ꢀmmHg for 12ꢀh before
the adsorption measurements. After evacuation, nitrogen isotherms were obtained volumetrically
at 77ꢀK. The obtained data was fitted into the Brunauer-Emmett-Teller (BET) equation to
determine the surface area.
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2.3. Preparation of the Co, Ni, and Cu Sponges
All of the three metal sponges were prepared following the same experimental
procedures. In detail, 400 mg sucrose and 4 mmol corresponding metal nitrate hydrate salt was
mixed in a 100 ml glass beaker. The mixture was slowly melted on a hotplate heated at 120 °C.
After heating and mixing on the hot plate for a while, a uniform viscous mixture was obtained.
Then, the viscous mixture was immediately put in an oven, that was preheated to ∼180 °C, for
about 20 min. After this heating period a foam-like substance consisting of decomposed metal
nitrate salts and sucrose was obtained. Since the decomposition of metal nitrate salts formed NO2
and NOx gases, the furnace was placed in a fume hood with proper ventilation. Afterwards, the
foam was annealed at 600 °C for 1 h in a tube furnace, under the flow of Argon, at a heating rate
of 10 °C/min. For cooling, normal cooling rate of the furnace was utilized. After the furnace was
completely cooled to room temperature the metal sponges were taken out from the furnace,
which was stored under ambient condition for the characterization and catalytic applications.
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For the synthesis of copper sponge, a minor modification was conducted. The mixture of sucrose
and copper nitrate salt was first ground by a mortar and pestle and then the mixture was put into
the oven at ~180 °C for the formation of foams.
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2.4. Catalytic activity of the Co, Ni, and Cu Sponges
The catalytic performance of metal sponges was evaluated by the reduction of 4-
nitrophenol (4-NP), methyl orange (MO), and methylene blue (MB) in water, Scheme 1. Excess
sodium borohydride (NaBH4) was used as the reducing agent. As NaBH4 spontaneously
decomposes in water, it is usually used in excess. The reduction of 4-NP, MB, and MO was
monitored by the gradual decrease of their characteristic absorbance at 400, 464, and 665 nm,
respectively with aid of UV-visible spectrophotometer.41,42,43
Scheme 1. Scheme for the metal sponges catalyzed reduction of 4-NP, MO, and MB in water by
the NaBH4.
For the reduction of 4-NP, 10 mL of 4-NP aqueous solution (0.2 mM) was mixed with 10
mg of the metal sponges by 10 min of bath sonication in a glass scintillation vial. Afterwards, 1
mL of freshly prepared NaBH4 aqueous solution (50 mg NaBH4 in 1mL water) was added into
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the reaction mixture and the reaction course was monitored by using the UV-visible
spectrophotometer.
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For the catalytic reduction of MB and MO, same experimental procedure was followed
except the concentration of MB and MO solution used was 15 ppm and 25 mg NaBH4 in 1 mL
water used as the reducing agent. The time dependent decolorization/reduction of MB and MO
was monitored by their characteristic absorbance at 665 and 464 nm, respectively.
3. Results and Discussion
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3.1. Synthesis of the Metal Sponges
The synthesis of Co, Ni, and Cu metal sponges was carried out by heating a paste
consisting of metal nitrate salt and sucrose (commonly known as table sugar), Scheme 2. The
heating was performed in three steps. First, a mixture of the sucrose and metal nitrate salt was
slowly melted on a hot plate at 120 °C to obtain a uniform viscous paste. Then, the paste was
immediately heated at ∼180 °C in a box furnace in the presence of air to obtain a foam-like
composite. The foam-like composite consisted of partially carbonized sucrose and partially
decomposed metal salts, Equation 1. When heated at this temperature, the nitrate salts started to
decompose to generate different gases (Equation 1), which caused the sticky mixture to blow up
in the form of foam-like composite during the heating process.44 The generation of the gases
induced the porosity of the foam-like composite and eventually in the metal sponges.
C12H22O11 (s) + Co(NO3)2.6H2O(s) ➔Cfoam + CoO (s) + N2(g) + H2O (g)+ NOx (g)
Cfoam + CoO (s) ➔ Co (s) + CO2(g) (2)
(1)
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Scheme 2. Scheme showing different stages for the preparation of metal sponges. a) mixture of
metal nitrate salt and sucrose; b) metal nitrate decomposition into a foam-like composite; c)
further heating of the foam in argon to obtain the metal sponges.
Finally, the foam-like composite was further heated at 600 °C in presence of argon to
obtain the porous metal with the presence of a trace amount of carbon, Equation 2. When heated
at 600 °C in argon atmosphere, the partially decomposed metal salt (metal oxide mostly)
undergone carbothermal reduction forming porous metallic frameworks. In the carbothermal
reduction, the carbon gets oxidized to form CO2 gas while reducing metal oxides to zero valent
metals. The generation of CO2 gas may have caused additional porosity of the metallic sponges.
Incomplete oxidation may have left behind trace of carbon in the porous metal catalysts.
The advantage of the method is that the experimental conditions is simple, fast, and
adaptable to industrial scale. Moreover, the method does not require any expensive or hazardous
reagents, experimental conditions, and post-synthesis modifications or treatments. The method
could be adapted for the industrial-scale production of the high-surface area metal, metal oxide
porous structures and their nanocomposites for numerous applications. We also assume that the
method can be employed to prepare metals of different pore size and surface area by varying the
experimental conditions and parameters such as metal precursor to the sucrose molar ratio,
heating temperature and rate, etc.
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3.2. Characterization of the Metal Sponges
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The metal sponges were thoroughly characterized in terms of the morphology,
crystallinity, specific surface area, and the thermal stability analyses. The. SEM and TEM
images were taken to study the nanoscale morphology of the metal sponges. The elemental
compositions of the metal sponges were qualitatively and quantitatively determined by the SEM-
EDS analysis. The crystallinity and specific surface area and thermal stability of the metal
sponges were investigated by the XRPD, BET surface area analysis, and the thermal gravimetric
analysis.
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3.2.1. SEM Images of the Metal Sponges
The SEM image analysis revealed the porous morphology of the as-synthesized cobalt,
nickel, and copper sponges, Figure 1. For instance, the SEM image of the Co sponge (Fig. 1a,1b)
demonstrated a self-supported three-dimensional interconnected network consisting of thin
filaments. The filaments were seen to be nanoassembled and entangled together to form a 3
dimensional-interconnected structure. Further magnification revealed that the filaments were not
uniform in terms of size and shape, figure 1b. The filaments were often seen to have many
branches indicating their formation during the foam formation and/or carbothermal reduction
step of synthesis. A porous morphology with a high volume of void space was also observed
from the SEM images. This void space and the porous structure induced the high specific surface
area of the metal sponges, which is very important for the purpose of catalysis.
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Figure 1. Typical SEM images of the cobalt, nickel, and copper sponges with different
magnifications showing the morphology.
Similarly, the SEM images of the Ni and Cu sponges revealed the porous morphology,
Figure 1c-1f. The porosity and void spaces of the Ni and Cu sponges can be clearly seen in the
SEM images. However, the Ni and Cu sponges were found to be less porous and filamentous as
compared to the Co sponge. This indicate the less surface area of the Ni and Cu sponges
compared to the Co sponge, which is further supported by the BET surface area analysis.
Morphology-wise, the Ni and Cu sponges were seen to be more like two-dimensional sheet like
structure with perforations.
3.2.2. EDX Spectrum of the Metal Sponges
The EDX spectrum was carried out to determine the qualitative and quantitative
elemental analysis of the metal sponges, Figure 2. For example, Figure 1a demonstrates EDX
peaks that are corresponding to cobalt, oxygen, and carbon. A high intensity of the cobalt peak
indicates that the sponge is consisting of mostly cobalt metal, whereas; less intense peaks of
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carbon and oxygen indicates that there is some amount of carbon and oxygen in the cobalt
sponge. Oxygen peak may have arisen from the protective oxide layer on the metal sponges.
Likewise, the EDS spectrum of the nickel, and copper sponges showed a high abundance of
nickel and copper with comparatively low abundance of carbon and oxygen. The strong cobalt,
nickel, and copper peaks indicate that the sponges are mostly metallic in nature. However, the
carbon and oxygen peaks indicate that the sponges have some percentage of carbon too. The
crystalline metallic nature of the metal sponges was further analyzed by the XRPD analysis
(discussed below).
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Figure 2. EDX spectrum of the a) cobalt, b) nickel, and c) copper sponges showing the
qualitative and quantitative elemental composition.
The percent elemental composition of the metal sponges, obtained from the EDS
analysis, is also shown in Figure 2. It was found that the Co sponge had about 3.5 % C, 0.6 %
oxygen, and 95.8 % metallic cobalt. The Ni sponge was composed of about 4.8 % C, 0.1 %
oxygen, and 95.1 % metallic nickel, whereas; the Cu sponge was composed of about 12.4 % C,
1.0 % oxygen, and 86.6 % metallic copper.
3.2.3. TEM Images of the Metal Sponges
The TEM image analysis was further performed to characterize the morphology of the
metal sponges. Typical TEM images of the metal sponges with different magnifications are
shown in Figure 3. From the TEM images it was found that the metal sponges had two different
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morphologies. For example, the Co sponge was seen have two different morphologies viz. the
sponge-like and the particle-like morphologies that are shown in Figure 3a and 3b, respectively.
Further analysis revealed that the sponge like morphology (Figure 3a) consisted of filamentous
metal nanowire of varying dimensions and it was mostly observed where there was no carbon
film associated. We assume that the sponge-like morphology may have formed due to the fusion
and aggregation of the smaller nanoparticles during the high-temperature heating steps of the
synthesis. However, the particles like morphology was observed where there was carbon film,
Figure 3b. The carbon film was originated from carbonization of sucrose and the metallic
nanoparticles were seen to have varying particle sizes. We assume that the carbon film may have
coated the metallic nanoparticles and prevented their further growth and aggregation leading to
the sponge like morphology.
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Figure 3. TEM images of cobalt, nickel, and copper sponges showing the sponge-like and
nanoparticles-like morphologies. Scale bars: 500, 100, 1000, 100, 1000, and 500 nm for figures a
through f, respectively.
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Like the Co sponge, the Ni and Cu sponges demonstrated two types of morphologies. The
average thickness of the Co, Ni, and Cu filaments in the sponges were calculated to be 56, 230,
and 120 nm, respectively. The average size of the Co, Ni, and Cu nanoparticles in the sponges
were calculated to be 9.5, 21, and 41 nm, respectively.
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3.2.4. XRPD Pattern of the Metal Sponges
The X-Ray powder diffraction patterns were obtained to determine the crystalline
properties of the metal sponges. The XRPD pattern of the Co sponge is shown in Figure 4a. The
diffraction peaks located at 2θ = 44.1◦, 51.5◦ and 75.8◦ can be attributed to the 111, 200, and 220
crystalline faces of the metallic cobalt.45 Moreover, this type of diffraction peaks corresponds to
the face-centered-cube (fcc) crystalline phase of the metallic cobalt.46 The absence of any other
undesired peaks indicate that the cobalt sponge is highly pure with crystalline metallic property
and it has low or undetectable amount of cobalt oxides on it. This result is compatible with the
EDS studies that were discussed above.
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Figure 4. XRPD pattern of the a) cobalt, b) nickel, and c) copper sponges showing the
crystalline structure.
Similarly, the XRD pattern of the Ni and Cu sponges are shown in Figure 4b and 4c. The
XRPD patterns of the Ni (2θ = 44.3◦, 51.7◦ and 76.1◦) and Cu (2θ = 43.9◦, 50.1◦ and 73.8◦)
sponges correspond to the face-centered cube (fcc) phase of the crystal lattice.47,48 Additionally,
the highly intense peaks of the Ni and Cu sponges without having additional peaks indicate their
high purity, crystallinity, and the presence of no or undetectable amount of oxide layer on them.
The Debye-Scherrer’s Formula (Equation 3) was employed to determine the average crystallite
size of the metal sponges. The XRPD peak corresponding to the (111) crystalline face was
utilized in this regard.49
d = kλ /β Cos(θ)
(3)
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where d is the crystallite size, k is the shape factor (0.9), λ is the wavelength of Cu Kα X-ray
radiation (1.54 Å), θ is the Bragg diffraction angle, and β is the full width at half maximum of
the respective diffraction peak. Employing the Debye-Scherrer formula, the average crystallite
size of the Co, Ni, and Cu sponges where calculated to be 38.9, 25.6, and 27.3 nm, respectively.
It could be observed that the crystallite sizes of the metal sponges, obtained from the XRPD
analysis, were different from the average particle size measured by the TEM analysis. This
because of the fact that the crystallite size is the size of a coherently diffracting domain of a
crystal in a particle and it is not necessarily be same as the particle size determined from the
TEM images.
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3.2.5. Surface Area analysis of the Metal Sponges
The specific surface area, pore size, and pore volume of the metal sponges were
measured using nitrogen gas adsorption analysis. Table 1 shows the BET surface area, pore size,
and pore volume of the metal sponges obtained from the N2 adsorption experiments. As can be
seen from the BET analysis that the cobalt sponge had the greatest surface area ca. 17.3935 m2/g,
while copper has the least surface area of the three at 2.7280 m2/g. The Ni sponge had the BET
surface area of about 8.85 m2/g.
Table 1. BET surface area, total pore volume, and average pore diameter of the Co, Ni, and Cu
sponges.
Sample
BET surface area
(m2/g)
Total pore volume
(cm3/g)
Average pore
diameter (nm)
Co Sponge
Ni Sponge
Cu Sponge
17.3935 m²/g
8.8472 m²/g
2.7280 m²/g
0.006750 cm³/g
0.003236 cm³/g
0.004188 cm³/g
67.731 Å
74.118 Å
66.102 Å
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This difference in surface can be correlated to the porosity and void spaces of the metal
sponges, shown in the SEM images, Figure 1. correlates to the pore volume and pore diameter.
Observing the SEM image, the difference in porosity is observable. The SEM images revealed
that the Co sponge had much higher porosity and void space compared to the Ni and Cu sponges
and thereby the Co sponge demonstrated the highest BET surface area. The comparative pore
volume and average pore diameter of the metal sponges are also given in Table 1.
3.2.6. Thermogravimetric Analysis
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In order to further quantify the percent amount of carbon in the metal sponges the thermal
gravimetric analysis was carried out. As shown in Figure 5, the metal sponges lost less than 5 %
weight what heated to about 800 °C.
Figure 5. Thermal decomposition profile of the metal sponges showing the percent carbon in the
sponges.
This indicate that percent weight of carbon in the metal sponges. From the results of this
study it was found that the Co, Ni, and Cu sponges had only about 4.2, 1.7, and 1.8 weight
percent carbon in the sponges.
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3.3. Catalytic Activity of the Co, Ni, and Cu Sponges
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The catalytic activity of the Co, Ni, and Cu sponges was studied by their ability to
reduce 4-NP, MO, and MB in water with NaBH4 at the unadjusted pH. The reduction of 4-NP,
MO, and MB with NaBH4 was monitored by the UV-visible spectroscopy by the lowering of
their characteristic absorption bands at 400, 464, and 665 nm, respectively. The reaction could be
observed by the naked-eye too by the decolorization of the inherent color of the substrates. The
experimental results of the catalytic reactions were analyzed by the pseudo-first-order rate
equation, which is expressed as follows:
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– ln(Ct/C0) = kt.
(4)
where C0 and Ct represent the initial and time-dependent concentrations of the substrates,
respectively. The time-dependent percent reduction of 4-NP, MO, and MB was calculated by
using the following equation:
푃푒푟푐푒푛푡 푟푒푑푢푐푡푖표푛 = 퐶표 ― 퐶푡 × 100% = 퐴표 ― 퐴푡 × 100%
(5)
퐶표
퐴표
where Ao and At represent the the initial and time-dependent absorbance of 4-NP, MO and MB at
400, 464, and 665 nm, respectively.
To study the mechanism of the reaction, the Langmuir−Hinshelwood model was utilized.
Langmuir−Hinshelwood model suggests that the reaction proceeds through the concomitant
adsorption of the reducing agent and the substrate on the catalyst surfaces to complete the
reaction.50,51
3.3.1. Catalytic Reduction of 4-NP
The catalytic reduction of 4-NP by the metal sponges is shown in Figure 6. The reaction
was studied up to 10 min period of time and within which the reaction proceeded to more than
completion. The reaction was monitored by the time-dependent UV-visible absorption spectrum
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of 4-nitrophenolate. A representative UV-visible absorption spectrum of 4-nitrophenolate is
shown in Figure 6a. A gradual decrease in the absorbance at 400 nm is characteristic to the
reduction of 4-NP; whereas the increase in the absorbance at 300 nm is characteristic to the
formation of 4-AP, which could be clearly observed from Figure 6a.
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The time-dependent percent reduction of 4-NP by the metal sponges is shown in figure
6b. It was found that the Cu sponge was the most active in doing the reduction, which was
followed by the Co, and Ni sponges, respectively. Cu sponge took about 1 min to completely
reduce the 4-NP into 4-AP, whereas Co and Ni sponges took about 6 min and 10 min to reduce
more than 95 % of the 4-NP, respectively. In contrast, the reduction of 4-NP in the absence of
catalyst showed negligible amount of reduction, which indicated the robustness of 4-NP to
undergo reduction without the catalyst.
Figure 6. Time-dependent a) UV-visible spectrum of 4-NP during its reduction on the Cu
sponge, b) percent reduction of 4-NP catalyzed by different metal sponges, and c) Pseudo-first
order reaction kinetics of the catalytic reduction of 4-NP.
The kinetics of the catalytic reduction of 4-NP by the Co, Ni, and Cu sponges is shown in
Figure 6c. The experimental results followed a linear relationship when –ln(Ct/C0) was plotted
against reaction time (t) (Equation 4). This suggested that the reduction followed the pseudo-first
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order kinetics, which makes sense as NaBH4 was used in a large excess in the reaction. The
apparent rate constant (kapp) of the reduction of 4-NP was obtained directly from the slope of the
straight line of the –ln(Ct/C0) vs. time (t) graph. For the Co, Ni, and Cu sponge the kapp was
calculated to be 1.31 ✕10-2 s-1, 9.10 ✕10-3 s-1 and 1.04 ✕10-1 s-1, respectively. The kapp of the
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uncatalyzed reaction was calculated to be 6.0 ✕10-5 s-1, which is much slower than the Co, Ni,
and Cu sponges catalyzed reactions.
The linear behavior of the –ln(Ct/C0) vs. time (t) graph indicated that all the reaction
followed the Langmuir–Hinshelwood (LH) model. This further suggested that the reduction of 4-
NP happened through the simultaneous adsorption of the sodium borohydride and the 4-
nitrophenolate on the surface of the metal sponges.
3.3.2. Catalytic Reduction of Methyl Orange (MO)
The reduction of MO with NaBH4 using the metal sponges as catalysts is shown in Figure
7. The gradual decrease in absorbance at 464 nm and 275 nm corresponds to the reduction of
MO and the gradual increase in absorbance at 245 nm corresponds to the formation of 4-
aminobenzenesulfonate and 4-N,N-dimenthylaminobenzene, Figure 7a. The time-dependent
percent reduction of MO to 4-aminobenzenesulfonate and 4-N,N-dimenthylaminobenzene by the
metal sponges is shown in figure 7b. The Cu sponge showed higher activity for the reduction of
MO than the Co and Ni sponges. In the absence of catalyst, a minor reduction of MO was
observed, however it is insignificant in comparison to the metal sponges catalyzed ones.
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Figure 7. Time-dependent a) UV-visible spectrum of MO during its reduction on the Co sponge,
b) percent reduction of MO catalyzed by different metal sponges, and c) Pseudo-first order
reaction kinetics of the catalytic reduction of MO.
The kinetics of MO reduction by the metal sponges is shown in Figure 7c demonstrating
a linear relationship following the LH model, which also suggests a pseudo-first order kinetics as
NaBH4 was used in excess. The kapp for the Co, Ni, and Cu sponge was calculated to be 1.67
✕10-2 s-1, 3.8 ✕10-3 s-1 and 1.58 ✕10-1 s-1, respectively. The kapp of the uncatalyzed reaction was
calculated to be 2.0 ✕10-4 s-1, which is many times slower than the metal sponge catalyzed
reactions.
3.3.3. Catalytic Reduction of MB
The catalytic reduction of MB on the metal sponges is shown in Figure 8. Like 4-NP and
MO, the reaction of MB was monitored by its reductive decolorization. During the catalysis, a
gradual decrease in the characteristic absorbance of MB at 300 and 665 nm was observed, Figure
8a.52 The deep blue colored MB turned into a colorless reduced product viz. leucomethylene
blue. The time-dependent percent reduction of MB by different metal sponges is shown in figure
8b. Out of the three sponges, it was found that the Cu sponge had the most catalytic activity,
which was followed by the Co, and Ni sponges, respectively. The Cu sponge took about 1 min to
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completely reduce the MB, whereas Co and Ni sponges took about 6 min and 10 min to reduce
more than 95 % of the MB, respectively. When doing a control experiment without the catalyst,
about 10 % reduction of MB was observed, which indicated that the metals sponges were
responsible for its catalytic reduction.
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Figure 8. Time-dependent a) UV-visible spectrum of MB during its reduction on the Ni sponge,
b) percent reduction of MB catalyzed by different metal sponges, and c) Pseudo-first order
reaction kinetics of the catalytic reduction of MB.
The kinetics of the catalytic reduction of MB is shown in Figure 8c. The experimental
results followed a linear relationship when –ln(Ct/C0) was plotted against reaction time (t)
(Equation 4). This suggested that the reduction followed the pseudo-first order kinetics and
Langmuir–Hinshelwood (LH) model. For the Co, Ni, and Cu sponges the kapp was calculated to
be 1.13 ✕10-2 s-1, 1.08 ✕10-2 s-1, and 4.24 ✕10-2 s-1, respectively. The kapp of the uncatalyzed
reaction was calculated to be 1.0 ✕10-4 s-1, which indicates the robustness of MB to undergo
reduction without the catalysts.
To avoid any confusion that the 4-NP, MO, and MB may have adsorbed on the metal
sponges during the catalysis, several control experiments were carried out. For this, mixture of
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substrates (4-NP, MO, and MB solutions) and metal sponges were bath sonicated for 10 min.
Afterwards, the percent adsorption was calculated. For all three metal sponges, less than 5
percent adsorption was obtained. This eventually confirms that the decolorization of the 4-NP,
MO, and MB solutions happened due to the catalytic reduction, not by physical adsorption.
The catalytic performance of the metal sponges for the reduction of 4-NP, MO, and MB
is compared with other transition metal catalysts in Table 2. All of the catalytic processes
summarized in the table below employed NaBH4 as the reduction agent. By comparison, the Co,
Ni, and Cu sponges demonstrated higher/similar catalytic activities compared to various other
transition metal catalyst systems reported in the literature.
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Table 2. Comparisons of catalytic performance of the Co, Ni, and Cu sponges for reduction of 4-
NP, MO, and MB Over various transition metal catalyst systems
Catalysts
Substrate Rate constant (s-1) References
Co sponge
Co sponge
Co sponge
Ni sponge
Ni sponge
Ni sponge
Cu sponge
4-NP
MO
1.31 ✕10-2
1.67 ✕10-2
1.13 ✕10-2
9.10 ✕10-3
3.8 ✕10-3
This work
This work
This work
This work
This work
This work
MB
4-NP
MO
MB
1.08 ✕10-2
1.04 ✕10-1
4-NP
This work
This work
Cu sponge
Cu sponge
MO
MB
4-NP
4-NP
4-NP
MO
1.58 ✕10-1
4.24 ✕10-2
~2.7 ✕10-2
~1.4 ✕10-2
~1.25 ✕10-2
~2.5 ✕10-2
~4 ✕10-3
This work
53
p(MAc)–Cu
p(MAc)–Co
p(MAc)–Ni
p(MAc)–Cu
p(MAc)–Co
p(MAc)–Ni
53
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53
MO
MO
~2 ✕10-3
-2
54
p(TA)-Cu
MB
~1.13 ✕
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-3
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Cu microsphere
Ni nanotube arrays
MB
MB
~6.55 ✕
~3.7 ✕
-2
-3
57
Ni/CPM-1
MB
~9.71 ✕
-3
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Ni/CPM-2
p(TA)-Co
Amid-p(MAc-co-AN)–Cu 4-NP
Amid-p(MAc-co-AN)–Co 4-NP
MB
4-NP
~9.51 ✕
~7.16 ✕
~7.08 ✕
~1.24 ✕
-3
-3
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-3
Moreover, the Co, Ni, and Cu sponges have the advantages of easy separation
(centrifugation or magnetic decantation) and being highly stable allowing recycling and
reusability. Most importantly, the facility of the synthesis methodology of the catalysts could be
employed or adapted for the large-scale production in the industrial setup.
3.3.4. Stability of the Metal Sponges for the Reduction of 4-nitrophenol
Cyclic stability and reusability are two important and desired properties of the catalysts.
The cyclic stability of the metal sponges was demonstrated by the catalytic reduction of 4-NP for
five cycles. For the cyclic stability of the Co, Ni, and Cu sponges, the percent reduction of 4-NP
was calculated after 6, 10, and 1 min of reaction, respectively. The percent reduction of 4-NP to
4-AP was calculated by the use of UV-visible spectroscopy and Equation 5. For example, when
Cu sponge was used, percent reduction of 4-NP was calculated after 1 min of reaction in each
cycle of catalysis. Afterwards, the catalyst was separated from reaction mixture by
centrifugation, washed with deionized water and reused in the same manner. For the Co and Ni
sponges, percent reduction of 4-NP was calculated after 6 and 10 min of reaction, respectively in
every cycle. The catalysts were separated by magnetic decantation, washed with deionized water
and was reused for the following cycles.
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Figure 9. Cyclic stability of the a) Cobalt b) Nickel, and c) Copper sponges for the reduction of
4-NP.
Figure 9 demonstrates the cyclic stability of the metal sponges for the 4-NP reduction. It
was found that all of the three metal sponges were highly stable for the reduction of 4-NP
throughout the cycles. It showed that the Co and Cu sponged could effectively reduce more than
98 % of the 4-NP in every cycle of the reaction. Whereas, the Ni sponge reduced up to 96 % 4-
NP in every cycle. No or negligible drop of catalytic activity was observed throughout the
cycles, which indicated the robustness of the catalysts. Therefore, from this study it could be
concluded that the Co, Ni, and Cu sponges could potentially be used a highly stable and efficient
catalyst for the reduction of 4-NP in water.
4. Conclusions and Summary
A simple method for the preparation of porous cobalt, nickel, and copper sponges by
heating a mixture of sucrose and metal nitrate salts is reported. The metal sponges were found to
have high porosity with particles and filamentous morphology. All of the three metal sponges
demonstrated excellent catalytic activity for the reduction of 4-nitrophenol, methyl orange, and
methylene blue in water in the presence of NaBH4. Excellent cyclic stability of the sponges
indicated their robustness and potential application in a variety of other catalytic reactions. Also,
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from the synthesis methodology perspective, the procedure could be adapted for the preparation
of a wide variety of transition and mixed metal sponges for numerous applications.
Author Information
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Corresponding Authors:
Md Tariqul Islam, Email: mtislam@miners.utep.edu / tariqul.lab@gmail.com
Juan C. Noveron, Email: jcnoveron@utep.edu
Notes
The authors declare no conflicting financial interest.
Acknowledgement
Financial support from NSF grants CHE-0748913, the NEWT-1449500, and USDA 2014-
38422-22078 are gratefully acknowledged. Authors would like to thank Dr. Huiyao Wang for the
kind assistance with TEM and SEM imaging techniques.
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