Shape-dependent reactivity and chemoselectivity of nanogold towards nitrophenol reduction in water

Article abstract of DOI:10.1016/j.jorganchem.2020.121361

Although the catalytic activity of nano-gold surfaces for the reduction of nitro compounds has been known, the effect of their shape has been rarely evaluated. Here, the synthesis, characterization, and application of both gold nanoworms (GNW) and gold nanospheres (GNS) are described. Both GNW and GNS were characterized using SEM, TEM, UV–Vis, FTIR, and XPS spectroscopy. The catalytic efficiency of GNW with an average dimensions of 2 × 250 nm (D × L) towards the hydrogenation of nitrophenol, a pollutant present in industrial wastewater, is higher (TOF 3675 h^?1) than that of spherical GNS (10 ± 1 nm), for which TOF is 1838 h^?1 in water using NaBH₄ as the reductant. The selectivity of 4-aminophenol is 100% for both GNS and GNW.

Full text of DOI:10.1016/j.jorganchem.2020.121361

Journal Pre-proof
Shape-dependent reactivity and chemoselectivity of nanogold towards nitrophenol
reduction in water
Sodiq Adeyeye Nafiu, Md Abdul Aziz, M. Nasiruzzaman Shaikh
PII:
S0022-328X(20)30263-1
DOI:
https://doi.org/10.1016/j.jorganchem.2020.121361
JOM 121361
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 20 March 2020
Revised Date: 17 May 2020
Accepted Date: 28 May 2020
Please cite this article as: S.A. Nafiu, M.A. Aziz, M.N. Shaikh, Shape-dependent reactivity and
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Shape-dependent reactivity and chemoselectivity of nanogold towards
nitrophenol reduction in water
Sodiq Adeyeye Nafiu,a Md. Abdul Aziz,b M. Nasiruzzaman Shaikh*, b
aDepartment of Chemistry, King Fahd University of Petroleum & Minerals (KFUPM),
Dhahran, Saudi Arabia
bCenter of Research Excellence in Nanotechnology (CENT), King Fahd University of
Petroleum & Minerals (KFUPM), Dhahran 31261, Saudi Arabia
Abstract
Although the catalytic activity of nano-gold surfaces for the reduction of nitro compounds
has been known, the effect of their shape has been rarely evaluated. Here, the synthesis,
characterization, and application of both gold nanoworms (GNW) and gold nanospheres
(GNS) are described. Both GNW and GNS were characterized using SEM, TEM, UV-Vis,
FTIR, and XPS spectroscopy. The catalytic efficiency of GNW with an average dimensions
of 2×250 nm (D×L) towards the hydrogenation of nitrophenol, a pollutant present in
industrial wastewater, is higher (TOF 3675 h-1) than that of spherical GNS (10±1 nm), for
which TOF is 1838 h-1 in water using NaBH4 as the reductant. The selectivity of 4-
aminophenol is 100% for both GNS and GNW.
Keywords: Gold nanoparticles; nitrophenol; chemoselectivity; turn over frequency;
azoxyarene
1

Introduction
Catalytic reduction of nitroarenes has gained increased attention due to its high selectivity
and generation of less waste, when compared with conventional reduction reactions using
metals such as iron [1], tin [2], or zinc [3]. Conventional reduction employs a toxic organic
solvent and also requires hydrogen gas, which normally involves the use of a reactor capable
of withstanding very high pressures. Catalytic reduction of nitrophenol is an example of
catalytic transfer hydrogenation (CTH), which can be catalyzed using homogeneous catalysts
including the complexes of noble metals such as Pd [4], Rh [5], Ru [6], and Ir [7], and a non-
noble metal Ni [8].
Until the 1990s, gold was considered to be catalytically inert for hydrogenation reactions due
to its poor adsorption of molecules from the gaseous phase [9]. However, a major
breakthrough in gold catalyzed hydrogenation was achieved when gold nanoparticles
supported on a suitable oxide were employed for the selective hydrogenation of even
complex organic molecules containing more than one functional group [10]. Catalytic
hydrogenation of nitroarenes to their corresponding amines is among the most important
hydrogenation reactions catalyzed by gold nanoparticles [11-15]. This reaction is important
because functionalized anilines are intermediates in the production of pharmaceuticals [16],
herbicides, polymers, and fine chemicals [17]. Aminophenols produced by the reduction of
nitrophenols are also vital starting materials and intermediates in the manufacture of many
indispensable chemicals, including polymers, dyes, and agricultural and pharmaceutical
products [18]. For example, 4-aminophenol is employed as a developer in processing black
and white photographs and as intermediate in the synthesis of paracetamol. Furthermore, 4-
nitrophenol has been identified as a highly toxic environmental pollutant. Hence, catalytic
hydrogenation can also be used in environmental remediation [19].
In this study, colloidal gold nanospheres (GNS) and gold nanoworms (GNW) were developed
and used as heterogeneous catalysts to selectively reduce a series of nitroarenes using sodium
borohydride as the hydrogen source in aqueous medium at a temperature of 50 °C. Both GNS
and GNW catalysts are highly selective for the reduction of nitroarenes to aminoarenes and
azoxyarenes. These AuNPs have exceptional properties and will enhance the engineering of
new catalyst technology that is environmentally benign and cost effective.
2

Experimental
Materials and methods: All chemicals were purchased from Sigma-Aldrich and were used
as-received unless otherwise stated. Standard procedures were followed for the production of
dry and deoxygenated solvents. Deionized (DI) water (specific conductivity: 18.2 MΩ) was
used in all the experiments. UV-Vis absorption spectroscopic data were obtained on a
UV-Vis-NIR system, model Cary 500 from Agilent, USA within the wavelength
window of 200-800 nm. TEM images were acquired on a JEOL JEM2100F transmission
electron microscope. The TEM samples were prepared by dropping an ethanolic
suspension on a copper grid and allowing it to dry at room temperature. The amount of
nanogold in the catalyst was determined by inductively coupled plasma optical
emission spectrometry (ICP-OES; PlasmaQuant PO 9000 - Analytik Jena). The
samples were first digested in a dilute mixture of HNO3 and HCl. Calibration curves
were prepared for gold using standard solutions (ICP Element Standard solutions,
Merk). Samples for SEM (Lyra 3, Tescan) imaging were prepared from a chunk of
decorated jute stem on alumina stubs and coated with gold in an automatic gold coater
(Quorum, Q150T E). The surface chemistry was determined using an X-ray Photoelectron
Spectroscope (XPS) equipped with an Al-Kα micro-focusing X-ray monochromator
(ESCALAB 250Xi XPS Microprobe, Thermo Scientific, USA). The binding energy scale
was calibrated, and base pressure used. The chamber pressure was 2 x 10-9 torr. Catalytic
products were identified by a Shimadzu 2010 Plus gas chromatograph attached with a
mass spectrometer (GC-MS, Japan). The disappearance of the reactant and sequential
appearance of the product was recorded in real time, identifying the species in terms of
their molecular ion (M+) by comparing and matching them with the available Wiley
library of the mass spectral database, in addition to the identification of mass
fragmentation. Catalytic reactions were performed in a 10-place parallel reactor, from Bibby
Scientific, UK (model# Electrothermal Onmi Series 1025).
Synthesis of gold nanospheres (GNS)
Gold nanospheres (GNS) were synthesized using a slightly modified version of a method
reported in the literature [20, 21]. Briefly, 19.42 mg of pamoic acid (PA) was dissolved in 15
mL of distilled water in an Eppendorf tube and sonicated for 15 min. Following sonication,
100 µL of 1.0 M NaOH (aq.) was added to the solution, followed by distilled water to make
3

the volume 25 mL and sonicated for further 15 min to obtain a clear solution of 2.0 mM PA.
An aliquot of 250 µL of 1.0 M NaOH (aq.) was added to the resulting solution and further
sonicated for 1 min. Subsequently, 25 mL of 1.413 mM of HAuCl4 solution was added to the
resulting PA solution and sonicated for 15 min. Finally, the solution was allowed to stand
undisturbed for 60 min for completion of the formation of GNS.
Synthesis of gold nanoworms (GNW)
Gold nanoworms (GNW) were also synthesized using a slightly modified version of a
method reported in the literature [20, 21]. Initially, 80 µL of 1 M NaOH solution (aq.) was
added to 18 mL of water containing 11.53 mg of 2-naphthol (2-NP) and the solution was
sonicated for 10 min. Further 200 µL of 1 M NaOH was added to the resulting solution and
the sonication was continued until 2-NP dissolved. The solution volume was then adjusted to
20 mL with water to make the final concentration of 2-NP 4.00 mM. Finally, 20 mL of 1.34
mM solution of HAuCl4 (prepared by diluting 2 mL of 13.4 mM HAuCl4 (aq.)) was added to
the solution of 2-NP under sonication. The solution becomes a dark-grey color immediately,
which confirms the formation of GNW, and the sonication was continued for another 15 min.
The final solution was then allowed to stand for 60 min for completion of the formation of
GNW.
Procedure for catalytic reduction of nitroarenes using GNS and GNW
0.5 mmol of nitroarene were dissolved in a solution of 25 mmol of NaBH4 in 5 mL of
distilled water. The resulting solution was stirred for 5 min. at 50 °C and 2.72 x 10-4 mmol of
GNS or GNW were added with continued stirring. The progress of the reactions was
monitored using UV-visible spectrophotometry with continued stirring till the completion of
the reaction. The crude product was extracted with ethyl acetate (2 x 5 mL) and the organic
layer was dried with anhydrous Na2SO4. The residue obtained by evaporating the solvent of
the dried organic layer under vacuum was dissolved in ethyl acetate and analyzed using GC-
MS to determine the percentage conversion and selectivity towards the product (azoxylarenes
or aminoarenes).
4

Results and discussion
Synthesis and characterization
Both the gold nanospheres and the gold nanoworms were prepared using sonochemical
methods. As described earlier, pamoic acid coated gold nanoshperes were synthesized using
sodium hydroxide as the deprotonating agent. The guided two-dimensional growth of
nanoworms was steered using 2-napthol as the coating agent by sonication under the same
conditions.
The morphological structure of the prepared material was evaluated using transmission
electron microscopy (TEM). TEM images show the uniform distribution of surface coated
gold nanospheres with an average diameter of 10±1 nm (Fig 1a), which was confirmed by
measuring the diameter of the particles using the dynamic light scattering (DLS)
technique(Fig 1b). Moreover, the low standard deviation indicates the monodispersity of
GNS. A uniform and homogeneous distribution of 2-naphthol coated nanoworms, which
appear like worms, with dimensions of 2 nm × 255 nm (D x L) were confirmed by TEM
micrographs, supplemented by DLS (Figs. 1c and d). HRTEM image revealed the atomic
structure of GNW (Fig 1e). UV-Vis spectra show absorption peaks at 520 nm (GNS) and
flattened absorption spectra (GNW), indicating the formation of nanogold (Fig 1f). The size
of both GNS and GNW is reproducible and they are uniformly dispersed under the described
conditions.
Fig. 1 GNS: (a) TEM image (b) DLS for particle distribution; GNW: c) TEM image (d) DLS for particle
distribution (e) HRTEM of GNW and (f) UV-Vis spectra of GNS and GNW.
5

X-ray photoelectron spectroscopy (XPS) was performed on the as-synthesized nanomaterials
to determine the electronic states of gold (Fig 2). The XPS spectra indicate the presence of C
and Au in both GNS and GNW. The formation of metallic gold (Auo) in GNS is confirmed
by the appearance of peaks at binding energies of 84.33 and 87.97 eV corresponding to the
4f7/2 and 4f5/2 gold orbitals respectively [20, 22] (Fig 2a). The peaks at binding energies at
284.48, 285.48, and 289.78 eV clearly indicate the presence of -C-C- bond, -C-O- and –C=O
group, respectively [20, 23]. The presence of this functional group indicates that the GNS is
carboxylic acid functionalized. A similar trend was observed for GNW, with the presence of
the peaks
Fig 2. XPS curve fitting of GNS a) gold b) carbon and GNW of c) gold d) carbon
at 84.3 eV and 88.0 eV which reveal the formation of metallic gold (Fig 2c). Besides, the
presence of -C-C- bond (peak at 284.6 eV) (Fig. 2d) is appeared in GNW. Another broad
peak between binding energies at 284.7 eV and 292 eV (Fig. 2d) is appeared which might be
presence of –C-O and C=O i.e. presence of -COO- along with other –C-O on GNW. The
6

lower intensity of this broad peak indicates the presence lower amount of -COO- on AuNW
compared to that of GNS. Perhaps this -COO- was generated by the degradation of 2-NP
during the reduction of Au3+ ion. The generated -COO- and –C-O on GNW helped to make
stable GNW aqueous solution.
Catalytic performance of GNS and GNW
The catalytic activity and selectivity of GNS and GNW were evaluated using 4-nitrophenol
(4-NP) as the benchmark substrate for the catalytic hydrogenation in water, and the results
are summarized in Table 1. The efficacy of the catalyst was investigated by varying the
amount of the hydrogen source, i.e., NaBH4, temperature, and the duration of the reaction.
The progress of the reaction was monitored using GC-MS and solution-state UV-Vis
spectroscopy. 4-aminophenol (4-AP) is not produced in the absence of NaBH4 or the catalyst,
i.e., GNS or GNW. Hence, the catalytic activity of nanogold plays an important role in the
reduction of the nitroarenes to the corresponding aminoarenes in water using NaBH4.
Fig 3. Variation of the UV-Vis absorption spectra with time of the 4-nitrophenol reduction with NaBH4 in water
in the presence of a) GNS b) GNW at 50 °C.
The reduction of 4-NP to 4-AP does not occur at room temperature and at 40 °C with GNS
after 30 minwhen 0.5 mmol of NaBH4, an equimolar amount of 4-NP, is used. Only the
conversion of 4-NP to the 4-nitrophenolate intermediate takes place. Hence, the amount of
NaBH4 was increased by 10-fold to 5 mmol, and also the reaction temperature was increased
from 40 °C to 50 °C. After 40 min, 4-nitrophenolate intermediate still accounted for almost
7

60% of the conversion. However, with the increase of the amount of NaBH4 to 25 mmol and
prolonging the reaction time to 60 min, the reaction mixture contained only 4-aminophenol
and not even traces of the 4-nitrophenolate intermediate was detected, as shown in Fig 3a.
Hence, the combination of 25 mmol of NaBH4 and 50 °C are the most suitable conditions for
the complete conversion of 0.5 mmol 4-NP to 4-AP in 1 h using 2.72 x 10-4 mmol of GNS as
the catalyst. Optimized conditions determined for the GNS catalyst were also found to be
most suitable for the reduction with the GNW catalyst using the same substrate in 30 min.
The progress of the reaction was further monitored by UV-Vis absorption spectroscopic
o
studies at 50 C and the results are shown in Fig 3. UV-Vis spectra of the reaction mixture
were obtained at 10, 20, 30, 40, 50, and 60 min (Fig. 3a), and more than 60% of 4-
aminophenol was formed after 40 min. The peak due to 4-nitrophenolate is negligible after 50
min and after 60 min only the 4-aminophenol peak at 300 nm is present. As indicated in Fig
3b, the 4-AP peak at 300 nm begins to appear at t = 3 min and the amount of nitrophenolate
gradually decreases after the addition of the GNW catalyst, and in 30 min all nitrophenolate
is reduced to 4- aminophenol at 50 oC.
Table 1. Conversion, selectivity, and turnover frequency (TOF) of the reduction of nitroarenes by sodium
borohydride catalyzed by GNS and GNW.
Entry Catalysts
Substrate
Conv. Selectivity (%)b
(%)a
TOF (h-1)
Amine Azoxy
1
2
GNS
GNW
R1 = R2 = R4 = H; R3 = OH
100
100
100
100
---
---
1838
3675
3
4
5
6
7
8
9
10
11
12
GNS
GNW
GNS
GNW
GNS
GNW
GNS
GNW
GNS
GNW
R1 = R2 = R3 = R4 = H
91
100
86
100
93
100
96
100
96
100
11
---
23
100
30
100
9
91
28
36
89
100
77
---
70
---
91
9
1672
3675
1580
3675
1709
3675
1764
3675
R1 = Me; R2 = R3 = R4 = H
R1 = R3 = R4 = H; R2 = Me
R1 = R2 = R4 = H; R3 = Me
R1 = R2 = R4 = H; R3 = Br
72
64
1764
3675
8

13
14
15
16
GNS
GNW
GNS
R1 = R2 = R4 = H; R3 = Cl
R1 = R2 = R4 = H; R3 = NH2
92
66
100
100
26
74
---
---
---
1691
2426
1838
3675
100
100
100
GNW
17
18
GNS
GNW
R1 = R4 = Me; R2 = R3 = H
100
100
100
100
---
---
1838
3675
aFor the reaction of 0.5 mmol of nitrophenol and 25 mmol of NaBH4 in 5 mL water at 50 C with GNS (for 60
mins) or GNW (30 mins) as the catalyst, determined by gas chromatography (GC); bConfirmed by gas
chromatography-mass spectrometry (GC-MS).
ͦ
Having established the catalytic activity of GNS and GNW catalysts for the reduction of 4-
NP, the catalytic reduction was applied to a series of nitroarenes with structurally divergent
functional groups to evaluate their comparative conversion and selectivity, and the results are
presented in Table 1. For the GNS-catalyzed reaction, 86%, 93%, and 96% of 2-nitrotoluene
(2-NT), 3-nitrotoluene (3-NT), and 4-nitrotoluene (4-NT), respectively, were converted to
their corresponding reduced products (azoxytoluene and aminotoluene) in 1 h with a
selectivity of 77%, 70%, and 91% for azoxyarene, respectively. The increase of the
conversion is most likely due to the decrease of the steric hindrance in going from 2-NT,
through 3-NT to 4-NT, which in turn increases the reactivity (Table 1, entries# 5, 7, and 9).
Meanwhile, quantitative conversion (100%) of 2-NT, 3-NT, and 4-NT with a selectivity of
100%, 100%, and 91% for the amine, respectively, were recorded in the presence of the
GNW catalyst (Table 1, entries# 6, 8, and 10). The reactivity of the GNS-catalyzed reduction
of halogen-substituted nitrobenzenes was also evaluated and 96% and 92% of 1-bromo-4-
nitrobenzene and 1-chloro-4-nitrobenzene, respectively (Table 1, entries# 11 and 13), is
reacted in 1 h. In the case of the GNW-catalyzed reduction, 100% and 66% conversion of 1-
bromo-4-nitrobenzene and 1-chloro-4-nitrobenzene, respectively, were achieved in 30 min
(Table 1, entries# 12 and 14) without the formation of even traces of the dehalogenation
product. The lower conversion of 1-chloro-4-nitrobenzene is most likely due to the higher
electron withdrawing ability of chlorine compared to bromine, which implies that electron
withdrawing substituents lower the reactivity of nitroarenes [24, 25]. GNS and GNW
catalyzed reduction of 4-nitroaniline selectively produced the amines in 1 h and 30 min,
respectively, (Table 1, entries# 15 and 16).
The turnover frequency (TOF) is calculated on the basis of the number of nitroarene
molecules reacted per mole of GNS or GNW catalyst per hour. A very high TOF of 3675h-1
9

was achieved with the GNW catalyst compared to spherical nanogold, which results in a
turnover frequency of 1838 h-1 for the reduction of 4-NP to 4-AP at 50° C. Thus, GNW
demonstrates better catalytic efficiency over its spherical counterpart in terms of both
conversion and selectivity. This may be due to the higher surface area of GNW allowing a
higher number gold atoms on the surface to interact with sodium borohydride to form the
gold-hydride intermediate, which is mainly responsible for the reduction of the nitro group to
its amine.
Kinetics study of GNS and GNW
The performance of the GNS catalyst was evaluated at the optimized reaction conditions
using 4-NP as the model substrate and sodium borohydride as the reductant in water. The
reduction of 4-NP was then monitored using UV-Vis absorption spectroscopy at 40, 50, and
60 °C, and the results are shown in Fig. 4a. An absorption peak at 300 nm corresponding to
the 4-aminiphenol is present after 1 and 10 min at 50 °C for GNW and GNS catalyzed
reduction of 4-NP respectively. Almost all the 4-NP is reduced to 4-AP after 20 and 40 min
using GNW and GNS catalyst respectively at 50 °C (Fig. 3).
The optimized conditions were determined using 25 mmol of NaBH4 and 0.5 mmol of 4-NP,
and thus, the reduction is expected to follow a pseudo-first order reaction with respect to 4-
nitrophenol [26]. The rate of the GNS catalyzed reaction was calculated at 40, 50, and 60 °C.
The plot of ln(Ct/Co) vs. reduction time reveals that the rate of the reduction increases as the
temperature is varied from 40 °C to 60 °C and to 50 °C in that order. The rate constants (k)
obtained from the slope of the respective plots are 2.44 x 10-3 mol L-1 min-1, 4.52 x 10-3 mol
L-1 min-1, and 2.64 x 10-3 mol L-1 min-1 at 40, 50, and 60 °C, respectively (Fig. 4a). A similar
kinetic study of the GNW catalyzed 4-NP reduction indicates that the rate constants (k) are
7.79 x 10-4 mol L-1 min-1, 5.20 x 10-3 mol L-1 min-1, and 1.57 x 10-3 mol L-1 min-1 at 40, 50,
10

and 60 °C, respectively (Fig. 4b).
Fig. 4 Plot of ln(Ct/C0) versus time for a) GNS and b) GNW catalysts
General mechanisms for the reduction of nitroarenes with GNS or GNW
The reduction of nitroarenes to the corresponding aminoarenes involves a direct reduction
pathway (Scheme 1). In this pathway, nitroarenes are converted into nitrosoarenes and then to
N-phenylhydroxylamines, which are further reduced to produce aminoarenes. However, the
condensation of nitrosoarene and N-phenylhydroxylamines results in the formation of
azoxyarenes. This is referred to as the condensation pathway (Scheme 1).
Scheme 1. General mechanisms for the reduction of nitroarenes using GNS and GNW.
Comparison of the GNS and GNW catalysts with literature data
The efficacy of the GNS and GNW catalysts were established by comparing the data obtained
from the GNS and GNW catalyzed nitrophenol reduction into aminophenol with the results
11

of other gold nanoparticles in the literature under similar reaction conditions as shown in
Table 2. TOF values shown in Table 2, revealed that the GNS (TOF 1838 h-1) and GNW
(TOF 3675 h-1) catalysts employed in this study display much higher catalytic activity and far
superior to other AuNPs used for this reaction.
Table 2. Comparison of the results obtained with the GNS and GNW catalysts for the nitrophenol
reduction with previously reported results in the literature
Serial
NO.
1
Catalytic system
Reaction condition
Conv.
(%)
100
90
100
100
100
100
100
100
100
100
TOF
(h-1)
845
Ref.
NAP-Mg-Au(0)
Starch-crt@Au
Au(0)TpPa-1
NaBH4, H2O
NaBH4, H2O:EtOH (1:1)
NaBH4, H2O
25
27
28
29
30
31
32
33
This
work
2
3
4
5
6
7
8
9
1.53 X10-3
3
1.75 X10-4
1
Au-CeO
Au@BSA
NaBH4, H2O
NaBH4, H2O
NaBH4, H2O
NaBH4, H2O
NaBH4, H2O
NaBH4, H2O
NaBH4, H2O
Fe3O4@PDA@Au
Colloidal AuNPs
Colloidal GNPs
Colloidal GNS
Colloidal GNW
302
15
941
1838
3675
10
Acknowledgements
MNS gratefully acknowledges the National Plan for Science, Technology and Innovation
(MAARIFAH)-King Abdulaziz City for Science and Technology through the Science and
Technology Unit at King Fahd University of Petroleum and Minerals (KFUPM), the
Kingdom of Saudi Arabia, award number 15-NAN4650-04.
12

Conclusions
Colloidal GNS and GNW catalysts were developed, characterized, and applied for
heterogeneous catalytic hydrogen transfer reactions. The catalysts developed displayed
excellent reactivity towards the reduction of a range of nitroarene compounds in the presence
of NaBH4 as the reducing agent in water at 50° C, resulting in up to 100% conversion. The
results show that the reduction of nitroarenes produces aminoarenes and/or azoxyarenes.
GNW was found to be more selective towards the amine than with the GNS. The formation
of only aminoarene product when both 4-nitrophenol and 4-nitroaniline are reduced is an
indication that the mechanism for the reduction of these two compounds involves only the
direct pathway (Scheme 1). However, the mechanism for the reduction of nitroarenes, which
leads to the formation of both aminoarene and azoxyarene products, involves a combination
of both the direct and condensation pathways (Scheme 1).
13

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15

Highlights
1. Spherical (10±1 nm) and wire (250 nm long) of nanogold are prepared using pamoic
acid and naphthol as reducing agents, respectively.
2. The prepared catalyst was thoroughly characterized using SEM, TEM, XPS and other
analytical and spectroscopic techniques.
3. Reported for the first time, nitroarene has been selectively reduced to aromatic amine in
water in 30 min (conversion 100%) using GNW as the catalyst.
4. Turnover frequency was calculated and it is very high (3675 and 1838 h-1)
5. Rate constant at different temperatures were determined for both GNS and GNW.
6. Results were compared with the literature data.

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1. __Dr. M. Nasiruzzaman Shaikh
2. _Dr. Md. Abdul Aziz
__18-03-2020_
____18-03-2020___
3. _Mr. Sodiq Adeyeye Nafiu
18-03-2020