Metal nanoparticles with high catalytic activity in degradation of methyl orange: An electron relay effect

Article abstract of DOI:10.1016/j.molcata.2010.12.001

Gold, silver and platinum nanoparticles have been synthesized following a green approach by reducing the corresponding salt using tannic acid as reducing agent at room temperature in aqueous medium. The reaction is instantaneous and the average diameter of the particles formed is around 10 nm in all the three cases as measured by TEM. These nanoparticles have been used as a catalyst for the degradation of methyl orange in the presence of sodium borohydride (NaBH₄). Silver nanoparticles have a drastic catalytic effect as compared to gold or platinum nanoparticles on the degradation of methyl orange in the presence of sodium borohydride. From the kinetic data it is concluded that the rate constant follows the order: k_{Ag nanoparticles} k_{Au nanoparticles} > k _{Pt nanoparticles} k_{uncatalyzed reaction}. The high catalytic effect of silver nanoparticles has been attributed to its low value of work function as compared to Au and Pt. The uncatalyzed reaction does not show any decrease in the absorbance value within the given experimental time due to the large kinetic barrier, i.e. high activation energy. Decrease in absorbance value for uncatalyzed reaction is observed after nearly 48 h that too at a very high concentration of reducing agent, thereby indicating that reaction is extremely slow and reduction of methyl orange is thermodynamically feasible.

Full text of DOI:10.1016/j.molcata.2010.12.001

Journal of Molecular Catalysis A: Chemical 335 (2011) 248–252
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Metal nanoparticles with high catalytic activity in degradation of methyl orange:
An electron relay effect
Nikesh Gupta, Henam Premananda Singh, Rakesh Kumar Sharma∗
Nanotechnology Research Group, Department of Chemistry, University of Delhi, Delhi 110 007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Gold, silver and platinum nanoparticles have been synthesized following a green approach by reducing
the corresponding salt using tannic acid as reducing agent at room temperature in aqueous medium. The
reaction is instantaneous and the average diameter of the particles formed is around 10 nm in all the
three cases as measured by TEM. These nanoparticles have been used as a catalyst for the degradation
of methyl orange in the presence of sodium borohydride (NaBH4). Silver nanoparticles have a drastic
catalytic effect as compared to gold or platinum nanoparticles on the degradation of methyl orange
in the presence of sodium borohydride. From the kinetic data it is concluded that the rate constant
follows the order: kAg nanoparticles ꢀ kAu nanoparticles > kPt nanoparticles ꢀ kuncatalyzed reaction. The high catalytic
effect of silver nanoparticles has been attributed to its low value of work function as compared to Au
and Pt. The uncatalyzed reaction does not show any decrease in the absorbance value within the given
experimental time due to the large kinetic barrier, i.e. high activation energy. Decrease in absorbance
value for uncatalyzed reaction is observed after nearly 48 h that too at a very high concentration of
reducing agent, thereby indicating that reaction is extremely slow and reduction of methyl orange is
thermodynamically feasible.
Received 26 August 2010
Received in revised form
19 November 2010
Accepted 5 December 2010
Available online 14 December 2010
Keywords:
Methyl orange (MO)
NaBH4
Metal nanoparticles
Environmentally benign
Green approach
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
catalysis. Generally, sphere-shaped inorganic nanoparticles, both
semiconducting and metallic, have been of particular interest for
There has been growing interest for the creation of nanoscale
materials with a complex shape and controlled size under ambient
conditions in aqueous solutions. For metallic nanoparticles, inter-
esting optical and electronic effects [1–4] are expected in the size
range of 10–100 nm since the mean free path of an electron in a
metal is 10–100 nm [5,6]. There are various routes of synthesis of
metal nanoparticles [7–14] in the size range of 10–100 nm but the
subsequent use of these particles for various applications are either
hampered by the drastic method of synthesis or removal of haz-
ardous chemicals which are used for synthesis. Recently attention
has been given to the green route of synthesis of metal nanoparti-
cles because of its soft nature as well as safety and environmental
concerns [15–18]. Some fundamental studies have revealed that
biomolecules can selectively recognize inorganic surfaces [19,20].
Tannic acid is typically hydrolyzed to tannin, a mixed gallotannin
composed of gallic acid esters of glucose, which has been used for
the biomimetic growth of inorganic nanoparticles [21]. Controlling
the size and shape of nanocrystalline materials is a key issue in
such type of catalytic reactions. It was observed that the optical
properties of silver and gold nanoparticles are tunable throughout
the visible and near-IR regions of the spectrum as a function of the
nanoparticle size, shape and local environment. Nanoparticles of
gold and silver with nanometer scale dimensions are of particular
interests as catalysts in organic and inorganic reactions [22,23].
The electron transfer step plays a vital role in the degradation of
dyes. This step depends on the potential difference, as there can be
large redox potential difference between the donor and acceptor,
which can hinder the passage of electrons between them [24,25].
An effective catalyst, such as silver, gold or platinum nanoparticles
with an intermediate redox potential value between the donor and
acceptor helps in the electron transfer and thus acts as an elec-
tron relay system [26]. Gold, silver and platinum nanoparticles are
well-known examples of this type of redox catalyst and it is known
that when metal particles become nanoparticles, their electrode
potential differs from each other. The turnover frequency (TOF) of
metal nanoparticles increases as the size decreases [27]. However,
the probability of aggregation also increases as the size decreases
[28]. Therefore, the presence of stabilizers and various surfactants
is desirable to have a control over this problem. Tannic acid an
important component of green tea leaves can be used as reduc-
∗
Corresponding author. Tel.: +91 9310050453.
E-mail address: sharmark101@yahoo.com (R.K. Sharma).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.12.001

N. Gupta et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 248–252
249
Fig. 1. (a) High resolution transmission electron microscopic pictures of Ag, Au and Pt nanoparticles and (b) SAED pattern of Ag, Au and Pt nanoparticles.
ing agent to reduce noble metal salts into metallic nanoparticles as
well as it works as a capping agent to stabilize the particles so that
no surfactant is necessary [29,30].
In this study we have synthesized Au, Ag and Pt metal
nanoparticles using tannic acid and shown their applications
as a catalyst in the degradation of methyl orange. The reduc-
tion of methyl orange by NaBH4 in absence of catalyst is
thermodynamically favourable, but it is kinetically difficult.
Metal nanoparticles provide an alternative path for the reac-
tion as they reduce the activation energy, hence reducing kinetic
barrier, thereby making it thermodynamically and kinetically
favourable.

250
N. Gupta et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 248–252
2. Experimental
2.1. Materials
600
2 min
Au Nanoparticles
Ag Nanoparticles
550
500
All the chemicals used were of AR grade. The aqueous solution
was prepared in double-distilled water. Chloroauric acid (HAuCl4),
silver nitrate (AgNO3) and chloroplatinic acid (H2PtCl6) were pur-
chased from SRL, Merck and Spectrochem respectively. Tannic
acid and sodium borohydride (NaBH4) was obtained from Merck
and spectrochem respectively. Methyl orange was procured from
Thomas baker. All chemicals were used as such without any further
purification.
1 min
450
400
350
300
250
200
5 min
Pt Nanoparticles
2.2. Equipment
0
5
10
15
20
25
All spectra were recorded in UV–Vis Spectrophotometer-1601,
Shimadzu and constant temperature water circulator from Haake
instruments was used to maintain the temperature throughout the
reaction.
Time of stirring (in hours)
Fig. 2. The plot of ꢀmax of all the three metal nanoparticles (Au, Ag and Pt) versus
the time of stirring.
2.3. Experimental procedure
2.3.1. Synthesis of gold, silver and platinum nanoparticles using
tannic acid
stabilizer for metal nanoparticles. The ꢀmax of the prepared metal
nanoparticles was observed at 528 nm for Au, 416 nm for Ag and
238 nm for Pt. The average particle size of all the three metals is
around 10 nm as measured by transmission electron microscopy
[Fig. 1(a)]. The particles obtained are spherical and reasonably
monodispersed as seen from TEM images. The electron diffrac-
tion pattern (SAED) as shown in Fig. 1(b) clearly concludes that the
particles are crystalline in nature. Due to capping effect of tannic
acid, no aggregation of these nanoparticles occurs and size remains
constant irrespective of the time of stirring of reaction mixture.
The prepared nanoparticles are stable as their ꢀmax value does
not change with the time of stirring (Fig. 2) due to the presence
of capping agent in tannic acid. We noticed that the dispersion
of these nanoparticles in water is clear and quite stable for more
than 150 days. The metal nanoparticles thus prepared by the above-
mentioned procedure were used as catalyst for the reduction of
methyl orange by NaBH4. The rate of degradation of colour of MO in
the presence of NaBH4 was monitored in the presence and absence
of metal nanoparticles spectrophotometrically at a wavelength of
464 nm in water as a medium as shown in Fig. 3. The figure shows
that the reduction of MO with NaBH4 in absence of catalyst at 20 ◦C
occurs at an extremely slow rate which is not visible from Fig. 3(d),
indicating that the reaction is very slow under the given time con-
straint. But on the addition of metal nanoparticles degradation of
the dye MO is greatly enhanced, especially in case of silver nanopar-
ticles as compared to gold and platinum nanoparticles [Fig. 3(a–c)].
The catalytic efficiency of different metal nanoparticles are dif-
ferent as shown in Fig. 3 and from the graph we observed that
there is a rapid decay in concentration of MO in the presence
of Ag nanoparticles in just 9 min (ꢁOD9 min = 1.63), as compared
to Au (ꢁOD40 min = 0.70) and Pt (ꢁOD40 min = 0.52) nanoparticles
in 40 min whereas there is no degradation up to 100 min for the
uncatalyzed reaction (ꢁOD100 min = 0). The reduction of MO in the
presence of NaBH4 using tannic acid instead of metal nanoparti-
cles shows exactly the same pattern as in case of absence of metal
nanoparticles [similar to Fig. 3(d)] which confirmed that metal
nanoparticles are acting as a catalyst and not tannic acid. The rate
of reduction of MO has been found to be more in case of silver
nanoparticles than platinum or gold nanoparticles, i.e. it follows the
order—RateAg nps > RateAu nps > RatePt nps as shown in Fig. 4. The dif-
ference in the rate of reduction in the presence of three metals can
be attributed to difference in their work function values (eV). From
Fig. 5, it is clearly indicative that as the value of the work function
decreases, total change in the absorbance of MO increases by suffi-
To 5 mL of tannic acid (1.4 × 10−4 M), 100 ␮L of 3% w/v HAuCl4 or
1 mL of 0.05% w/v AgNO3 or 100 ␮L of 2% w/v H2PtCl6 was mixed
to synthesize gold, silver or platinum nanoparticles respectively.
The mixture was stirred for 2 min at room temperature for gold
nanoparticles and for 1 min for silver nanoparticles while it was
stirred for a longer time, i.e. for about 5 min at room temperature
for platinum nanoparticles. The solution changes to violet, yellow-
ish brown and yellow for gold, silver and platinum nanoparticles
respectively. The synthesized metal nanoparticles was as such used
as catalyst for the degradation of methyl orange by NaBH4 and the
rate constants (k) were determined.
2.4. Characterization of the metal nanoparticles
2.4.1. High resolution transmission electron microscopy (HRTEM)
TEM pictures were taken with TECNAI G2-30 U TWIN instru-
ment. After preparation the metal nanoparticles were centrifuged
and re-dispersed in double distilled water by sonication for
2–3 min. A drop of diluted solution was put on the copper grid and
the grid was dried under ambient conditions. After complete drying
of the grid, a TEM picture of the particles was taken.
2.4.2. Kinetic study for the degradation of methyl orange
To a mixture containing 50 ␮L of methyl orange (5 × 10−3 M) and
100 ␮L of sodium borohydride (0.5% w/v) solution, 50 ␮L of 0.01%
aqueous dispersion of metal nanoparticles was added in a 3.5 mL
capacity quartz cuvette. Total volume of the mixture was made up
to 3.2 mL by adding the required amount of double distilled water.
The reaction was studied in a spectrophotometer cuvette kept at
a desired temperature by circulating water from a thermostated
water bath. For the uncatalyzed reaction 50 ␮L of metal nanoparti-
cles was replaced by an equal amount of double distilled water. The
reaction was followed by studying the time-dependent absorbance
(A) in a UV–visible spectrophotometer at 464 nm for 10 min and the
fall in A with time at desired temperature was noted.
3. Results and discussion
We have adopted the greener route for the synthesis of metal
nanoparticles in aqueous phase using pure tannic acid instead of
strong and harmful reducing agents. In our approach, tannic acid
is serving a dual role, i.e. it is acting as a reducing agent as well as

N. Gupta et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 248–252
251
2.0
1.5
1.0
0.5
0.0
2.0
1 min
(b)
(a)
1 min
2 min
.
.
.
With Au nps
With Ag nps
1.5
Temp . = 20 C
9 min
Temp . = 20 C
40 min
1.0
Δ OD
Δ OD
= 0.067
Δ OD
= 1.63
0.5
= 0.70
0.0
300
300
400
500
600
400
500
600
Wavelength (nm)
Wavelength (nm)
2.0
1.5
1.0
0.5
0.0
2.0
1.5
1.0
0.5
0.0
1 min
(c)
(d)
Uncatalyzed
1 min
100 min
With Pt nps
Temp . = 20 C
or
With Tannic acid
40 min
Temp .= 20 C
Δ OD
Δ OD
= 0.023
= 0.52
Δ OD
= 0
300
400
500
600
300
400
500
600
Wavelength (nm)
Wavelength (nm)
Fig. 3. Comparison of plot of absorbance versus wavelength for the reduction of methyl orange in the presence of NaBH4 at 20 ◦C using (a) Ag nps, (b) Au nps, (c) Pt nps and
(d) only tannic acid (uncatalyzed).
cient amount, i.e. work function and change in OD are exponentially
related to each other.
Which on integration gives,
I = Io e−ACl
From the expression,
Here I is the energy associated with the radiation, A is the integral
absorption coefficient, C is the molar concentration and L is the path
length [31]; we concluded that the energy is exponentially related
to absorbance and this is in accordance with our observation of
work function (or energy) with OD (or absorbance).
2
−dI = (8˘3/3h2)|ꢂlm| (I/c) hꢃlm (NC/1000) dl and
A = {8˘3N/3hc (1000)} ꢃlm|ꢂlm
|
2
It is concluded that, −dI = AIC dl
The degradation of methyl orange follows first order kinet-
ics, and from Table 1, it can be seen that the rate constant, k,
5.8
Pt
5.6
5.4
5.2
Au
5.0
4.8
4.6
4.4
Ag
4.2
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
Δ OD
Fig. 4. Comparison of plot of decrease in absorbance versus time for catalyzed and
uncatalyzed reaction at 20 ◦C; inset: digital photograph of the prepared nanoparti-
cles (after 150 days).
Fig. 5. Plot of work function (Wo) versus ꢁOD for reaction catalyzed by different
metal nanoparticles after 9 min at 20 ◦C.

252
N. Gupta et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 248–252
Table 1
Comparison of rate constant (k) for the catalyzed reduction using different catalysts (Au, Pt and Ag nanoparticles) with uncatalyzed reduction.
S. no.
Concentration of catalyst
Concentration of MO
Concentration of NaBH4
Temperature (◦C)
Rate constant (k) (min−1
)
1
2
3
4
100 ␮L of 0.01% Pt nps solution
100 ␮L of 0.01% Au nps solution
100 ␮L of 0.01% Ag nps solution
Without catalyst
50 ␮L of 5 × 10−3
50 ␮L of 5 × 10−3
50 ␮L of 5 × 10−3
50 ␮L of 5 × 10−3
M
M
M
M
100 ␮L of 0.5%
100 ␮L of 0.5%
100 ␮L of 0.5%
100 ␮L of 0.5%
20
20
20
20
0.0029
0.0049
0.5853
Very small of order 10−17 (negligible)
calculated for Pt, Au, Ag nanoparticles as catalysts and uncat-
alyzed reactions are 0.0029 min−1, 0.0049 min−1, 0.5853 min−1
and 10−17 min−1 respectively. Therefore, it is concluded that
Ag nanoparticles reduces the coloured methyl orange to its
colourless form faster as compared to Au and Pt nanopar-
ticles. These metal nanoparticles can be used in the indus-
tries for the degradation of various dyes, as the dyes are
harmful when decanted without reducing them to colourless
form.
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The authors are thankful to Prof. Amarnath Maitra, Department
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useful comments. We are highly thankful to University Grant Com-
mission, Government of India, for providing the financial assistance
in the form of a research project. We are also thankful to Mr. Arun
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images.