Room-Temperature Catalytic Reduction of Aqueous Nitrate to Ammonia with Ni Nanoparticles Immobilized on an Fe3O4@n-SiO2@h-SiO2–NH2 Support

Article abstract of DOI:10.1002/ejic.201700082

Efficient and selective catalytic reduction of aqueous nitrate to ammonia was achieved over Ni nanoparticles immobilized on Fe₃O₄@n-SiO₂@h-SiO₂–NH₂ [a magnetic hierarchical mesoporous amine-functionalized (M-HMAF) silica] by using hydrazine hydrate as a reducing agent. The high hierarchical mesoporous structure (surface area of 416 m² g^–1 and pore size of 3.7 nm) and Fe₃O₄ core (ca. 7 nm) of the M-HMAF silica support resulted in a high dispersion of Ni nanoparticles over the support and easy recovery of the catalyst, respectively. Interestingly, the Ni/M-HMAF silica catalyst exhibited an excellent catalytic turnover (275 mmol g_metal^–1 h^–1) compared with most of the existing catalysts for the conversion of nitrate ions at room temperature. The mechanistic study using UV/Vis spectroscopy revealed that the catalytic conversion of nitrate ions to ammonia proceeded through in situ generated nitrite ions. Subsequently, the ammonia produced from nitrate ions was isolated and analyzed by ¹H and ¹⁵N NMR spectroscopy, whereas the N₂ gas released as a byproduct of hydrazine was characterized by GC–MS.

Full text of DOI:10.1002/ejic.201700082

A Journal of
Accepted Article
Title: Room temperature catalytic reduction of aqueous nitrate to
ammonia over Ni nanoparticles immobilized on Fe3O4@n-
SiO2@H-SiO2-NH2 support
Authors: Rohit Kumar Rai, Deepika Tyagi, and Sanjay Kumar Singh
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201700082
Link to VoR: http://dx.doi.org/10.1002/ejic.201700082

10.1002/ejic.201700082
European Journal of Inorganic Chemistry
Room temperature catalytic reduction of aqueous nitrate to
ammonia over Ni nanoparticles immobilized on Fe3O4@n-SiO2@H-
SiO2-NH2 support
Rohit Kumar Rai,[a] Deepika Tyagi,[a] and Sanjay Kumar Singh*[a,b]
[a]Discipline of Chemistry and [b]Discipline of Metallurgy Engineering and Materials
Science, Indian Institute of Technology Indore, Simrol, Indore, 453552, India
E-mail: sksingh@iiti.ac.in
Homepage URL: http://iiti.ac.in/people/~sksingh/
Abstract
Efficient and selective catalytic reduction of aqueous nitrate to ammonia was
achieved over Ni nanoparticles immobilized on Fe3O4@n-SiO2@H-SiO2-NH2 (magnetic
hierarchical mesoporous amine-functionalized: M-HMAF) silica using hydrazine hydrate as a
reducing agent. High hierarchical mesoporous structure (surface area of 416 m2·g-1 and pore
size of 3.7 nm) and Fe3O4 core (ca. 7 nm) of the M-HMAF silica support resulted in, a high
dispersion of Ni nanoparticles over the support and an easy recovery of the catalyst,
respectively. Interestingly, Ni/M-HMAF silica catalyst exhibited excellent catalytic turnover
(275 mmol·gmetal-1·h-1), as compare to most of the existing catalysts for the conversion of
nitrate ions at room temperature. Mechanistic study using UV-vis spectroscopy revealed that
the catalytic conversion of nitrate ions to ammonia proceeded through in-situ generated nitrite
ions. Subsequently, the ammonia produced from nitrate ions was isolated and analyzed by 1H
15
and N NMR, whereas N2 gas released as a by-product of hydrazine was characterized by
GC-MS.
Introduction
High level of nitrate contamination of ground water has gained major environmental
concern during past decades.[1,2] In human beings, nitrate ions can be easily converted into
1
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nitrite or N-nitroso compounds which are known to be hazardous chemicals and causing
various health issues such as blue baby syndrome, hypertension, and cancer.[1,2] Therefore,
removal of nitrate from contaminated ground water is one of the most important
environmental challenges. From economical point of view, catalytic reduction of nitrate ions
is one of the most promising approach for removal of nitrate ions from ground water as
compared to several others such as ion exchange, electrodialysis, reverse osmosis and
microbiological denitrification.[3] Therefore, a variety of noble metal based monometallic and
bimetallic catalysts have been extensively investigated for the catalytic reduction of nitrate
ions.[4-10] From economical point of view, non-noble metal based catalysts could represent a
class of high-performing low-cost catalysts for nitrate ions reduction.[11-15] In this direction,
heterogeneous catalysts, such as TiO2 based catalysts, were also explored for photocatalytic
nitrate reduction in the aqueous solution.[16] Despite the fact that Ni based catalysts have
shown excellent catalytic activity for hydrogenation of nitro compounds to corresponding
amines,[17,18] literature revealed that non-noble metal (for e.g. Ni) exhibits only poor activity
for nitrate/nitrite reduction at room temperature.[8] Hence, for most of the non-noble metal
based catalysts, higher temperature/pressure or presence of noble metals as a promoter or
activator were found to be essential to enhance the activity of non-noble metal catalysts for
nitrate reduction.[11-13] Okuhara et al. reported Zr-modified porous Ni catalysts for nitrate
reduction to ammonia, where small amounts of Zr greatly enhance the activity of Ni by
suppressing the oxidation of Ni0 atoms (0.74 mmol·gmetal-1·h-1).[11] Later they also reported a
similar Pt-promoted enhancement in the catalytic activity of Ni catalyst for the hydrogenation
nitrate and nitrite ions to ammonia at 60 °C (7.0 mmol·gmetal-1·h-1).[12] Recently, Lefferts et al.
reported a carbon doped nickel hairy foam for nitrite reduction (~16% conv.) with ca. 70%
selectivity for ammonia at 20 °C. Though Pd decorated nickel hairy foam showed enhanced
catalytic activity (47% conv.), but the presence of C-doped nickel particles was found to be
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responsible for the activity of nickel hairy foam similar to those of noble metals.[13] In the
direction of development of non-noble based active catalysts, our main aim of this research is
to develop cost-effective/ inexpensive yet highly active catalysts for the complete removal of
nitrate ions from the ground water. Moreover, catalysts can be easily recovered from the
reaction medium by applying external magnet, and thus can be reused.
In the last few decades, magnetic materials with highly porous architectures, for
example, mesoporous silica shell with magnetic core have received particular attention, as
these materials displayed high stability, can be synthesized easily, their functionalized
architecture can be fine tuned, and thus they find wide application in various fields including
as catalyst carrier for several catalytic reactions.[19,20] For example, Yin et al. embedded Au
nanoparticle between the non-porous and porous SiO2 layer and used it for the catalytic
reduction of nitrophenol, where catalyst durability upon recycling the catalyst has been
enhanced.[21] Jin and Wang et al. reported similar system, where the magnetic core of 200 nm
was layered with a n-SiO2 of 30-40 nm and the top layered of hierarchical mesoporous SiO2
(10-20 nm). The synthesized materials showed high surface area (447 m2·g-1) and thus used
as a carrier for the loading of Pt nanoparticles and observed enhanced catalytic hydrogenation
of nitro compound with good recyclability.[22] More recently, Sun et al. synthesised the
Fe3O4@SiO2/Pd nanocatalyst for catalytic reduction of nitrate ions (51 mmol·gmetal-1·h-1).[9]
Herein, we report a mild synthetic methodology for the fabrication of magnetic
hierarchical mesoporous amine-functionalized (M-HMAF) silica and employed it as an ideal
catalyst carrier by immobilizing Ni nanoparticles on M-HMAF silica support (Ni/M-HMAF
silica catalyst). The as-synthesised Ni/M-HMAF silica catalyst was first employed for the
room temperature catalytic reduction of nitro compounds to corresponding amines in water,
and subsequently high catalytic activity was achieved for the catalytic reduction of aqueous
nitrate to ammonia at room temperature. Reaction products/by-products were isolated and
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1
analyzed by H and 15N NMR, and GC-MS. Further, mechanistic studies using UV-vis
spectroscopy were performed to identify possible intermediates during the catalytic reduction
of nitrate ions to ammonia.
Result and discussion
Synthesis and characterisation of magnetic hierarchical mesoporous amine-
functionalized silica (Fe3O4@n-SiO2@H-SiO2-NH2)
Synthesis of magnetic hierarchical mesoporous amine-functionalized (M-HMAF)
silica was performed following a simple seed-mediated procedure, as illustrated in Figure 1.
Pre-synthesized nanoparticulate Fe3O4 magnetic seeds (~7 nm), dispersed in water-ethanol
(1:4 v/v) solution were coated with non-porous SiO2 (n-SiO2) using TEOS solutions at pH ~
9 to obtain Fe3O4@n-SiO2 nanoparticles. The as-synthesized Fe3O4@n-SiO2 nanoparticles
were treated with acid to obtain purified Fe3O4@n-SiO2 nanoparticles, free from bare Fe3O4.
Further hierarchical mesoporous amine-functionalized silica shell (H-SiO2-NH2) was
fabricated over Fe3O4@SiO2 by a template mediated sol-gel procedure using TEOS and
APMS in the presence CTAB.[23a] Subsequently, the CTAB template was removed to obtain
the purified magnetic hierarchical mesoporous amine-functionalized (M-HMAF) silica.
Figure 1. Schematic presentation for the synthesis of magnetic hierarchical
mesoporous amine-functionalized (M-HMAF) silica.
Powder X-ray diffraction (P-XRD) patterns (Figure 2 and Figure S1) of the as-
synthesized M-HMAF silica showed a very broad signal peak at 2θ = 22.6° suggesting an
amorphous silica structure. In the P-XRD pattern of M-HMAF silica, a very weak peak for
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Fe3O4 core was also observed at 2θ = 35.6°, presumably due to the very high Si/Fe ratio,
suggesting the presence deeply embedded Fe3O4 core (Figure S1). Particle size of Fe3O4 was
found to be ~7 nm, as estimated by Scherrer equation from the P-XRD data, which is in good
agreement with the expected particle size of ~6 nm.[24] Scanning electron microscopic (SEM)
images displayed the spherical shape (with an average particle size of ca. 400 nm) and the
porous nature of the synthesized M-HMAF silica (Figure 2 and Figure S2-S3). FTIR analysis
of the as-synthesized M-HMAF silica showed vibration bands at 1066 cm-1, 796 cm-1 and 952
cm-1 respectively for asymmetric Si–O–Si stretching, symmetric Si–O–Si stretching, and Si–
OH stretching vibrations. The broad band observed at ca. 3010-3500 cm-1 was attributed to
N–H (–NH2 group) and O–H (Si–OH) stretching. Moreover, a weak band at 1510 cm-1 for N-
H bending was also appeared, further confirming the presence of –NH2 groups (Figure
S4).[23]
a
b
200 nm
1 µm
0.16
Adsorption
Desorption
400
300
200
100
0.12
0.08
0.04
0.00
d
c
0
10 20 30 40 50 60
Pore size (nm)
10 20 30 40 50 60 70 80
0.0
0.2
0.4
0.6
0.8
1.0
2 (degree)
Relative pressure (p/p0)
Figure 2. a, b) SEM images, c) P-XRD spectra and d) N2 adsorption-desorption
isotherm and pore size distribution (inset) of as-synthesized M-HMAF silica.
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European Journal of Inorganic Chemistry
Figure 2d displayed that a typical type-II nitrogen-sorption isotherm for the as-
synthesized M-HMAF silica. The observation of type H3 hysteresis loop in the P/P0 range of
0.4-0.9 suggesting the presence of bigger and slit-shaped pores, characteristics of hierarchical
porous materials.[23] Barrett-Joyner-Halenda (BJH) theory suggested an average pore size of
3.7 nm, which can be attributed to the mesoporous nature of the synthesized M-HMAF silica
(Figure 2d in the inset). The Brunauer-Emmett-Teller (BET) surface area and pore volume of
the as-synthesized M-HMAF silica are 416 m2·g-1 and 0.64 cm3g-1 respectively, which are
considerably higher compared to other analogous porous silica.[23a,21,22] In general, the iron
oxides are readily dissolved under acidic conditions,[25] which may limit their application.
However, in the presented methodology M-HMAF silica was synthesized in acidic condition
at elevated temperature, hence, M-HMAF silica displayed high stability in acidic medium.
Encouraged with the unique structural advantages of the as-synthesized M-HMAF
silica, such as highly porous, amine functionalized, high stability in acidic condition, high
surface area, we further attempted to the use M-HMAF silica as a carrier for loading active
metal nanoparticles. To this point, Ni loaded M-HMAF silica (Ni/M-HMAF silica catalyst)
was synthesized using an impregnation-reduction procedure. Ni precursor (NiCl2·6H2O) was
supposed to undergo complexation with amine groups of M-HMAF silica, which was
subsequently reduce to Ni nanoparticles upon addition of NaBH4 to obtain Ni/M-HMAF
silica catalyst. The estimated loading of Ni via NiCl2·6H2O precursor on M-HMAF silica
support was 11.7 wt%. Analogously, Pd and Pt nanoparticles loaded M-HMAF silica were
also synthesized. P-XRD patterns of Ni/M-HMAF silica showed peak at 2θ = 46.2°
corresponding to the face-centered cubic (111) plane of the metallic Ni (JCPDS 65-0380)
(Figure S1).[17] SEM images of Ni/M-HMAF silica showed no visible structural damages
suggesting that pores and the spherical structure of Ni/M-HMAF silica catalyst remain intact
even after Ni nanoparticles loading (Figure 3). The corresponding energy dispersive X-ray
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spectroscopy (EDS) patterns and elemental mapping also confirmed the presence of Ni, Fe
and Si elements in the synthesized Ni/M-HMAF silica catalyst (Figure 3 and Figures S2 and
S5). The transmission electron microscopy (TEM) images also showed a uniform spherical
morphology of the Ni/M-HMAF silica catalyst (Figure 3 and Figure S5). However, the
loaded Ni nanoparticles are not clearly visible, presumably due to the very high dispersion of
Ni nanoparticles (Ni/Si = 0.07) over M-HMAF silica support (Figure 3 and Figure S2).
Thermo gravimetric analysis (TGA) curves suggested that the Ni/M-HMAF silica catalyst is
thermally stable up to 200 °C (Figure S6). The BET surface area of Ni/M-HMAF silica
catalyst was reduced to 274 m2·g-1 as compared to the bare M-HMAF silica, inferred that
presumably Ni nanoparticles located inside the channels of hierarchical pores of M-HMAF
silica (Figure S7).[26]
a
c
d
b
Si
Ni
Ni
Fe Ni
0
2
4
6
8
10 12 14
200 nm
200 nm
100 nm
keV
e
g
h
f
Ni
Fe
Si
1 µm
f
1 µm
1 µm
1 µm
1 µm
e
Figure 3. a, b) TEM images, c) SEM image, d) EDS spectra, e) SEM image and f-h)
corresponding Si, Ni and Fe elemental mapping of the Ni/M-HMAF silica catalyst.
Catalytic reactions using metal nanoparticle loaded M-HMAF silica
At first, catalytic activity of Ni/M-HMAF silica catalyst was investigated for the
reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP), an important intermediate in the
preparation of several analgesics and antipyretic drugs such as paracetamol, acetanilide and
so on. Progress of the catalytic reaction was monitored by UV-vis spectroscopy (Figure 4).
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As illustrated in Figure 4, aqueous solution of 4-NP exhibited a strong absorption peak at 317
nm, whereas upon addition of hydrazine monohydrate to the solution peak at 317 nm
disappeared and a new peak at 400 nm appeared corresponding to the formation of phenolate
species of 4-NP (Figure 4a). In the presence of a catalytic amount of Ni/M-HMAF silica
catalyst, intensity of the peak at 400 nm gradually decreases with the appearance of a new
peak at 297 nm corresponding to the formation of the product, 4-AP. Absorption spectra after
3h, showed that the absorption peak at 400 nm completely eliminated, suggesting the
complete conversion of 4-NP to 4-AP (Table S1, entry 1, and Figure 4b). Progress of the
catalytic reaction can also be monitored apparently by visible color change from initial
yellow (4-NP) to final colorless (4-AP) (Figure 4c). After completion of the catalytic
reaction, Ni/M-HMAF silica catalyst was separated out by an external magnet (Figure 4d)
and reused for subsequent catalytic runs (Table S1, entry 4). Along with the catalytic
reduction of 4-NP, the reduction of other substituted aromatic nitro compounds to other
corresponding amines was also successfully accomplished over Ni/M-HMAF silica catalyst
(Table S1, entries 2-5).[17]
0 min
30 min
1 h
1.5 h
2.0 h
3.0 h
a
b
400 nm
317 nm
200 250 300 350 400 450 500
Wavelength (nm)
200 250 300 350 400 450 500
Wavelength (nm)
d
c
Magnetic
Separation
4-NP
4-AP
Figure 4. a) UV-vis spectra of 4-nitrophenol (4-NP) before and after addition of
hydrazine monohydrate, b) time-scaled UV-vis plot for the reduction of 4-NP in
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aqueous solution using Ni/M-HMAF silica catalyst, c) visible color change form initial
yellow of 4-NP to colorless 4-AP and d) magnetic separation of Ni/M-HMAF silica
catalyst after the catalytic reaction.
Encouraged by the high catalytic activity for reduction of nitro compounds achieved
with Ni/M-HMAF silica catalyst, we were curious to explore if inorganic nitrate ions can also
be reduced in a similar manner. Nitrate ions are considered as one of the major
contaminations in water, and therefore its facile reduction would be interesting to explore.[1,2]
Interestingly, our investigations showed that aqueous nitrate can also be converted to
ammonia with the aid of hydrazine monohydrate over Ni/M-HMAF silica catalyst at room
temperature. Complete conversion of nitrate ions to ammonia in the presence of hydrazine
monohydrate, as a reducing agent, is expected to follow the below equation (1):
-
NO3 + 2N2H4
2N2 + NH4OH + H2O + OH-
(1)
Therefore, progress of the catalytic reduction of nitrate ions could be easily monitored
by measuring the volume of N2 gas released per equivalent of nitrate ions. Catalytic reactions
were performed with varying amount of hydrazine (0.25 to 6.0 equiv. to nitrate ions) inferred
that complete conversion of nitrate ions can be achieved using 2 equivalents of hydrazine
(Figure 5a, Table S2), which is well in accordance to the equation (1). Increasing the amount
of hydrazine (> 2 equivalent), resulted in no further increase in the amount of N2 release, but
only the reaction completion time can be reduced. Thus, these results clearly demonstrated
that N2H4/NO3- ratio of 2:1 is an optimum ratio to achieve complete conversion of NO3- to
+
NH4 over Ni/M-HMAF silica catalyst. It is important to mention that, in the absence of
either nitrate ions or hydrazine, there is no evolution of N2 gas (Figure S8a and Table S2). It
is well established that the complete decomposition of hydrazine will release 3 equivalents of
gases (N2 and H2 gas).[27,28] Therefore, identity of gases evolved during the catalytic reduction
of nitrate ions, was confirmed by GC-MS (Figure 5b). Results indicated that only N2 gas was
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released during the catalytic reduction of nitrate ions in the presence of hydrazine, and
hydrogen was completely consumed for the reduction of nitrate ions to ammonia. This is in
agreement with the volumetric measurement, by water displacement, of the amount of gas
released during the catalytic reaction. Formation of ammonia as a sole product during the
catalytic reduction of nitrate ions were also characterized in the form of ammonium ions by
1H ( = 7.42, t) and 15N NMR ( = 406.9) spectroscopy (Figure 5c and Figure S9-S10). After
analysis of nitrogen products formed during the catalytic reaction, we propose that the total
nitrogen mass (nitrate, N2 and ammonia) is reasonably balanced during the course of catalytic
reduction of nitrate ions, by identifying ammonium as a final product from nitrate ion
reduction and N2 gas from hydrazine (as a hydrogen source), and is in accordance with the
above equation (1). Moreover, in contrast to high activity observed with Ni/M-HMAF silica
catalyst, analogously synthesized catalysts with other metals Pt and Pd loaded on M-HMAF
silica were found to be inactive for the reduction of nitrate ions to ammonia under analogous
condition (Figure S8b). Along with Ni/M-HMAF silica catalyst, Ni/SiO2 nonporous silica[29]
catalyst was also investigated for the nitrate ion reduction under optimised reaction condition.
In contrary to high performance of Ni/M-HMAF silica catalyst, Ni/SiO2 catalyst showed
lower catalytic activity for the nitrate ions reduction (Figure S11), suggesting that the
hierarchical mesoporous architecture of the M-HMAF silica support is advantageous to
achieve higher catalytic activity. The synthesised Ni/SiO2 catalysts was also characterised by
SEM, EDX elemental mapping and PXRD (Figure S12-S13). After completion of the
catalytic reactions, Ni/M-HMAF silica catalysts was recovered by an external magnet and
reused for subsequent catalytic runs (Figure S14).
To further investigate the reaction pathway for the reduction of aqueous nitrate to
ammonia, progress of the catalytic reaction was monitored by UV-vis spectroscopy (Figure
5d). As inferred from the Figure 5d, initial absorption band at 300 nm corresponding to
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nitrate ions decrease with the progress of the reaction, and a new shoulder peak at 354 nm
appeared. This new shoulder peak, which gradually became prominent with progress of the
reaction, was assigned to the in-situ generated nitrite ions. Nitrite ions have been considered
as a most favourable intermediate during the reduction of nitrate ions.[4-15,30] Further the
complete reduction of nitrate ions to ammonia via in-situ nitrite was established by the
disappearance of all absorption peaks.
d
a
c
b
Retention time (min)
Figure 5. a) Catalytic reduction of nitrate ions to ammonia in aqueous solution with
different equivalents of hydrazine, b) GC-MS of N2 gas released during the catalytic
1
reaction, c) characterization of ammonia (in form of ammonium chloride) by H and
15N NMR spectroscopy, and d) time-scaled UV-vis spectral plot for the reduction of
nitrate ions, showing the in-situ generation of nitrite ions.
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Scheme 1. Two-step process for room temperature catalytic reduction of aqueous
nitrate ions to ammonia, via in-situ generated nitrite ions, over Ni/M-HMAF silica
catalysts.
Based on the experimental results, reduction of nitrate ions to ammonia by hydrazine over
Ni/M-HMAF silica catalyst is considered to be a two-step process, where the in-situ
formation of nitrite from nitrate ions occurs at the first step and, subsequently, in-situ
generated nitrite ions are converted to ammonia at the second step (Scheme 1).[4-15,30] It was
suggested that selectivity of products for the nitrate ions reduction depend on the pH of the
reaction medium. In basic medium, ammonia formation occurred because of the formation of
adsorbed NH2 species.[31] During catalytic reduction of nitrate ions, hydroxide ions generated,
which could increase the pH of the solution and thus increase the selectivity for the formation
of ammonia over N2. Therefore, using hydrazine monohydrate as a reducing agent was found
to be advantageous, because of the basic in nature of hydrazine, pH of the solution increases
which led to the formation ammonia, as a sole product. To study the effect of various
reducing agents, experiments were performed for the nitrate reduction using other reducing
agent such as NaBH4 and HCOOH. It is worthy to mention that, both the reducing agents
NaBH4 and HCOOH were unable to reduce nitrate ions under optimised reaction condition,
whereas hydrazine efficiently reduce the nitrate ions to ammonia over Ni/M-HMAF silica
catalyst under analogous reaction conditions.
In contrary to earlier reports that non-noble metal catalysts are mostly inactive or
poorly active for nitrate reduction to nitrogen compounds,[8] the as-synthesised Ni/M-HMAF
silica catalyst exhibit excellent catalytic activity (275 mmol·gmetal-1·h-1) which is higher than
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the most of the reported catalysts (Table S3). Higher activity for complete reduction of
aqueous nitrate ions to ammonia at room temperature over Ni/M-HMAF silica catalyst is
significant, as most of the earlier Ni-based catalyst were either poorly active (hairy nickel
[13]
foam: 0.096 mmol·gmetal-1·h-1
)
or need a promoter/activator to enhance the catalytic
activity (Pt promoted Ni: 7.0 mmol·gmetal-1·h-1 and Zr modified porous Ni: 0.7 mmol·gmetal-1·h-
1 [11,12]
)
(Table S3). Hence, through this work, we believe that a precisely designed and tuned
non-noble metal based catalyst has all the potential to exhibit high catalytic performance for
the reduction of nitrate ions in water, which is so for remains elusive.
Conclusions
In summary, double silica coated (inner non-porous silica and the outer hierarchical
mesoporous amine-functionalized silica layer) magnetic core (M-HMAF silica) was
fabricated through a simple synthetic seed mediated CTAB template approach. The as-
synthesized M-HMAF silica, displayed high surface area (416 m2·g-1) and pore size (3.7 nm),
and thus employed as a catalyst carrier for the loading of Ni nanoparticles. The synthesised
Ni/M-HMAF silica catalyst exhibited excellent catalytic performance for the reduction of
aromatic nitro compounds to corresponding amines with aid of hydrazine monohydrate at
room temperature in water. Employing similar methodology, catalytic reduction of aqueous
nitrate to ammonia over Ni/M-HMAF silica catalysts was achieved at room temperature with
significantly high catalytic turnover rate (275 mmol·gmetal-1·h-1), as compared to the most of
the existing catalysts based on noble metals. Ammonia produced was analyzed by 1H and 15N
NMR analyses. Mechanistic study revealed the formation of nitrite ions (in-situ) during the
catalytic reduction of aqueous nitrate to ammonia. Hence, to the best of our believe, Ni/M-
HMAF silica catalyst represents the first report on the highly active Ni based catalyst, free
from noble metals, for room temperature complete conversion of aqueous nitrate to ammonia.
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Experimental section:
Materials. Cetyltrimethylammonium bromide (CTAB, Sigma-Aldrich, 99%), aqueous
ammonia (NH4OH, 30%), hydrochloric acid (HCl, 36%), potassium tetrachloroplatinate
(K2PtCl4, Sigma-Aldrich 99.99%), potassium tetrachloropalladate (K2PdCl4, Sigma-Aldrich,
98%), nickel chloridehexahydrate (NiCl6·6H2O, Alfa Aesar, 98%) tetraethoxysilane (TEOS,
Sigma-Aldrich, 98%) and 3-aminopropyltrimethoxysilane (APMS, Sigma-Aldrich, 97%),
sodium borohydride, ethanol and diethyl ether were purchased from Alfa Aesar. All
chemicals were of analytical grade, and used without further purification.
Characterization. High purity metal salts, chemicals were used for the experiments. NMR
spectra were recorded on a Bruker Avance 400 (400 MHz) spectrometer. Chemical shifts are
reported in ppm relative to the centre of the singlet at 7.26 ppm for CDCl3, 2.49 ppm in
DMSO-d6, 13C NMR in D2O. 15N NMR was recorded in DMSO-d6 using urea as a reference.
The powder X-ray diffraction (P-XRD) patterns of the samples were recorded using Rigaku
smart lab automated multipurpose X-ray diffractometer system with CuKα1 radiation (λ =
1.5408 Å). Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy
(EDS) were taken using a Carl Zeiss Supra-55, scanning electron microscope and EDS
Oxford instruments model X-Max, energy dispersive X-ray spectroscopy (Au-sputtered
samples were used). To check the thermal stability, thermo gravimetric analysis (TGA) was
done using Mettler Toledo TGA/DSC 1 analyser. Fourier transformed infrared spectroscopy
(FTIR) was done using instruments of Perkin Elmer Spectrum Version 10.5.1 in the wave
number range of 500-4000 cm-1. Transmission electron microscopy (TEM) images were
obtained using a JEM-1400 microscope (JEOL) operated at 100-120 kV. The N2 gas
adsorption isotherms were performed at 77 K on Autosorb IQ, Quantachrome sorptometer.
Prior to analysis the samples were outgassed at 120 °C for 8 h (increasing rate 10 °C/min).
Brunauer–Emmett–Teller (BET) specific surface areas were calculated by using adsorption
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data in the P/P0 range of 0.05–0.20. Pore size distributions were estimated from adsorption
branch of the isotherm using the Barrett, Joyner, and Halenda (BJH) method. Pore volumes
were determined from the amounts of N2 adsorbed at the single point of P/P0 = 0.99. GC−MS
analysis was performed on a Shimadzu GCMS-QP2010 Ultra and GC-2010 Plus system in EI
(electron impact) mode using RT- Msieve 5A column.
Synthesis of magnetic hierarchical mesoporous amine-functionalized silica
(Fe3O4@n-SiO2@H-SiO2-NH2). Step 1: Synthesis of Fe3O4 nanoparticles coated
by silica (Fe3O4@n-SiO2): Fe3O4@n-SiO2 nanoparticles were prepared by previously
reported method with little modification.[24] In a round bottom flask containing 100 mg of
Fe3O4 magnetic nanoparticle was ultrasonically dispersed in 240 ml ethanol and 60 ml water,
then the pH of the solution was adjusted to 9 with an addition of aqueous ammonia solution.
To this, 4 mL of TEOS was added under vigorous stirring at room temperature. After 10 h,
solution was again stirred at 50 °C for 12 h to further hydrolyze the TEOS. The synthesized
particles were magnetically separated and washed with the deoxygenated distilled water and
anhydrous ethanol until neutral pH. Further, Fe3O4@n-SiO2 nanoparticles were treated with
ethanolic HCl solution (10 mL conc. HCl in 100 mL ethanol) at 70 °C for 5 h to purify the
Fe3O4@n-SiO2 by eliminating bare Fe3O4 nanoparticles. The Fe3O4@n-SiO2 nanoparticles
were again magnetically separated out and washed with the deoxygenated distilled water and
anhydrous ethanol till neutral pH and vacuum dried at 50 °C for 12 h. Step 2: Synthesis
of hierarchical mesoporous amine-functionalized silica (Fe3O4@n-SiO2@H-
SiO2-NH2): To the synthesized and purified Fe3O4@n-SiO2 nanoparticles, 0.5 g of CTAB
was added and dissolved in 70 mL of H2O, 0.8 mL of aqueous ammonia, 20 mL of diethyl
ether and 10 mL of ethanol.[23a] The mixture was vigorously stirred for 0.5 h at room
temperature; then 2.5 mL of TEOS and 0.10 mL of APMS were quickly and sequentially
added to the mixture. The resulting mixture was again vigorously stirred at room temperature
15
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10.1002/ejic.201700082
European Journal of Inorganic Chemistry
for 4 h. A brownish white precipitate was obtained, magnetically separated, washed with pure
deionised water and ethanol. Then, template extraction was performed by adding the
precipitate in ethanolic HCl (10 mL of conc. HCl in 100 mL of ethanol) followed by stirring
at 70 °C for 7 h. The extracted precipitate was magnetically separated and washed with pure
deionised water and ethanol, and dried in air at 60 °C, to obtain purified Fe3O4@n-SiO2@H-
SiO2-NH2 (M-HMAF silica).
Loading of metal nanoparticles on metal nanoparticles loaded M-HMAF silica
(Metal nanoparticles = Ni, Pd and Pt). To the round bottom flask, 100 mg of M-HMAF
silica were suspended in 95 mL of deionised water by sonication for 10 min.[23a] To this,
NiCl2·6H2O (0.048 g, 0.20 mmol) was added and again sonicated for 10 min. A freshly
prepared NaBH4 solution (0.050 g in 5 mL) was added dropwise and sonicated for 10 min to
a get black solution of Ni nanoparticles supported on Fe3O4@n-SiO2@H-SiO2-NH2. The
supported Ni nanoparticles were magnetically separated out from the solution and used for
the catalytic reactions. Accordingly, Pd and Pt nanoparticles were also synthesized and used
for the catalytic reactions.
General procedure for the catalytic reduction of nitro compounds to
corresponding amines at room temperature. To a reaction flask containing an
aqueous suspension of freshly prepared Ni/M-HMAF silica catalyst (25 mg) was added 0.25
mmol of nitro compounds and 1.0 mmol of hydrazine monohydrate were then added to the
mixture, and the reaction contents were stirred at room temperature. The progress of the
reaction was monitored by thin layer chromatography. After completion of the reaction, the
reaction mixture magnetically separates out the catalysts. The reaction mixture was extracted
with dichloromethane (3 × 5 mL), the organic layer was dried over anhydrous Na2SO4 and
filtered off, and the solvent was evaporated under reduced pressure. All of the products were
characterized by 1H NMR.
16
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10.1002/ejic.201700082
European Journal of Inorganic Chemistry
General procedure for catalytic reduction of nitrate ions at room temperature.
To the reaction flask containing freshly prepared Ni/M-HMAF silica catalyst (25 mg)
suspended in 5 mL of deionised water was added nitrate ions (1.0 mmol) and hydrazine
monohydrate (2.0 mmol). After completion of the reaction, ammonia produced was converted
1
to ammonium salt (NH4Cl) by addition of aqueous HCl solution. H and 15N NMR was
employed to analyze ammonium salt. Volume of N2 gas released was measured by water
displacement using a burette and the identity of the gases released was confirmed by GC-MS.
Acknowledgements
This work is financially supported by IIT Indore and SERB (DST), New Delhi. R. K. R. and
D. T. thank CSIR New Delhi and UGC New Delhi, respectively for their SRF grants. Authors
also thank SIC, IIT Indore, for extended instrumentation facilities.
Keywords: catalysis; supported nanoparticles; nickel; nitrate reduction; room temperature.
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European Journal of Inorganic Chemistry
(Graphical abstract)
Room temperature catalytic reduction of aqueous nitrate to
ammonia over Ni nanoparticles immobilized on Fe3O4@n-SiO2@H-
SiO2-NH2 support
Rohit Kumar Rai,[a] Deepika Tyagi,[a] and Sanjay Kumar Singh*[a,b]
[a]Discipline of Chemistry and [b]Discipline of Metallurgy Engineering and Materials
Science, Indian Institute of Technology Indore, Simrol, Indore, 453552, India
E-mail: sksingh@iiti.ac.in
Magnetic hierarchical mesoporous amine-functionalized (M-HMAF) silica with higher total
surface area (416 m2.g-1) and pore size 3.7 nm were designed and employed as an ideal
carrier to obtain Ni nanoparticle loaded M-HMAF silica catalyst for the catalytic reduction of
aqueous nitrate to ammonia. High catalytic turnover (275 mmol·gmetal-1·h-1) was achieved
using the Ni/M-HMAF silica catalyst for the catalytic reduction of aqueous nitrate to
ammonia at room temperature, where the reaction proceeds via in situ generated nitrite ions.
Ni based catalyst, H2O, r.t.
High catalytic turnover
275 mmol·gmetal-1·h-1
15N NMR
1H NMR
23
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