Homogeneous palladium nanoparticles surface hosts catalyzed reduction of the chromophoric azo (-N=N-) group of dye, acid orange 7 by borohydride in alkaline media

Article abstract of DOI:10.1002/kin.20883

In alkaline media, well-characterized gelatin-stabilized palladium (GPd) nanoparticles catalyze the reduction of the azo group containing pollutant dye, Acid Orange 7 (AO7) by sodium borohydride (NaBH4) to 1-amino-2-napthol and sulfanilic acid. Kinetic observations and detailed FTIR studies suggests that the reaction follows Langmuir-Hinshelwood kinetic model, where during the reaction both AO7 and borohydride are adsorbed on the GPd surface. Plots of lnk_o versus ln[AO7] or ln[NaBH₄] show that the order of reaction with respect to AO7 and NaBH₄ remains almost same over different molar ratios of [NaBH₄]/[AO7]. The catalyzed reaction shows an initial induction period (t0) due to a surface-restructuring process of GPd nanoparticles, and (1/t₀) can be defined as the rate of surface restructuring. The activation energy of the catalyzed reaction and energy of the surface-restructuring process of GPd are estimated as 22 ± 3 and 25 ± 7 kJ M^-1, respectively.

Full text of DOI:10.1002/kin.20883

Homogeneous Palladium
Nanoparticles Surface
Hosts Catalyzed Reduction
of the Chromophoric Azo
(–N=N–) Group of Dye, Acid
Orange 7 by Borohydride in
Alkaline Media
RANENDU SEKHAR DAS,1 BULA SINGH,2 ARABINDA MANDAL,3 RUPENDRANATH BANERJEE,1
SUBRATA MUKHOPADHYAY1
1Department of Chemistry, Jadavpur University, Kolkata 700 032, India
2Department of Chemistry, Visva-Bharati, Santiniketan 731 235, India
3Department of Chemistry, Haldia Government College, Purba Medinipur 721 657, India
Received 24 April 2014; revised 10 September 2014; accepted 10 September 2014
DOI 10.1002/kin.20883
Published online 20 October 2014 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: In alkaline media, well-characterized gelatin-stabilized palladium (GPd) nanopar-
ticles catalyze the reduction of the azo group containing pollutant dye, Acid Orange 7 (AO7) by
sodium borohydride (NaBH4) to 1-amino-2-napthol and sulfanilic acid. Kinetic observations
and detailed FTIR studies suggests that the reaction follows Langmuir–Hinshelwood kinetic
model, where during the reaction both AO7 and borohydride are adsorbed on the GPd surface.
Plots of lnko versus ln[AO7] or ln[NaBH4] show that the order of reaction with respect to AO7
and NaBH4 remains almost same over different molar ratios of [NaBH4]/[AO7]. The catalyzed
reaction shows an initial induction period (t0) due to a surface-restructuring process of GPd
nanoparticles, and (1/t0) can be defined as the rate of surface restructuring. The activation
energy of the catalyzed reaction and energy of the sꢀuC rface-restructuring process of GPd are
estimated as 22 3 and 25 7 kJ M−1, respectively. 2014 Wiley Periodicals, Inc. Int J Chem
Kinet 46: 746–758, 2014
INTRODUCTION
Correspondence to: R. S. Das; e-mail: ranendu147@gmail.com
Supporting Information is available in the online issue at
Since the accidental discovery of the first synthetic dye,
Mauveine, in the middle of 19th century [1], numerous
www.wileyonlinelibrary.com.
C
ꢀ
2014 Wiley Periodicals, Inc.

REDUCTION OF CHROMOPHORIC AZO GROUP OF DYE, ACID ORANGE 7 BY BOROHYDRIDE
747
other dyes of different classes had been synthesized
and commercialized in accord with the huge usage in
different industries such as textiles, leather, and paper
[2–6]. Among different class of dyes, azo dyes, which
consists of one or more azo groups (–N=N– bond),
alone accounts for almost half of the synthetic dyestuffs
used in industries [7–9].
capped palladium nanoparticles (GPd) and studied the
detailed kinetics of the catalyzed reduction of AO7
by sodium borohydride in aqueous alkaline media
([NaOH] = 0.1–0.5 M) keeping in view the facile use
of the nanoparticles-catalyzed reaction to remove the
harmful azo dye remnants from the effluents.
The escalating use of dyestuffs became alarming
when it was found that a large amount of colorants
used for dyeing is released through the sewage wa-
ter from the industries [7–9]. Since except one single
compound (4,4ꢁ-dihydroxyazo-benzene), naturally oc-
curring products do not contain the azo group [10],
it is expected that the released azo dye contaminants
will be harmful both for human and for all other living
beings [11–14]. It has been found that azo dyes are rela-
tively persistent pollutants because under aerobic con-
ditions they are hard to biodegrade [9,15–18] compared
to their reduced amine products [13]. Consequently, in
many countries, the release of dye-contaminated efflu-
ents in environment is highly prohibited and the subject
has long been a topic of debate from the standpoints of
socio-political views.
To confront this major challenge to degrade these
wastewater pollutants, different biochemical and phys-
iochemical techniques, mainly fungal and bacterial
decomposition [19–21], enzymatic degradation [22],
coagulation [23,24], adsorption [25–27], photocat-
alytic oxidation [28–30], Fenton oxidation [30–33],
advanced oxidation processes [34,35], and chemical
reduction [36–42], have been frequently employed to
decolorize and scavenge these colorants from waste
water.
Since most of the biochemical and biophysical pro-
cesses require a large amount of adsorbents along with
other means, such as intricate reactors and intensive
power supply [9,38], researchers have put much em-
phasis on the chemistry of different metals or metal
composites (containing mostly iron, nickel, copper,
etc.) mediated reduction of azo dyes [36–42]. Although
these reduction processes are efficient but one serious
concern lies in the usual practice of large usage of metal
dosages, which generate metal sludge as secondary
wastes [31]. In search of a more suitable process, there-
fore, we have turned our focus on the use of noble metal
nano-like palladium, which under aerobic condition,
even in the presence of astoundingly small concen-
tration (10−10 M, vide infra), catalyzes the reduction
of an azo dye, Acid Orange 7 (AO7, 4-(2-hydroxy-
1-naphthylazo)benzenesulfonic acid) by sodium boro-
hydride (NaBH4) to 1-amino-2-napthol and sulfanilic
acid at ambient conditions [13,43,44].
EXPERIMENTAL
Materials
Palladium(II) chloride (Aldrich, USA), sodium citrate
dihydrate (Rankem, India), gelatin (Merck, Germany),
Acid Orange 7 (Aldrich, USA), sodium borohydride
(Merck, Germany), sodium nitrate (Merck, Germany),
and sodium hydroxide (SRL, India) were used as re-
ceived. Milli-Q water and doubly distilled water was
used for the preparation of nanoparticles and kinetic
experiments, respectively. All the other chemicals used
were of analytical grade.
Preparation of Gelatin-Stabilized
Palladium Nanoparticles (GPd)
Palladium nanoparticles had a substantial history of
synthesis and usage [45]. Here, an easy and one-pot
synthetic procedure was developed for the preparation
of gelatin-stabilized palladium nanoparticles (GPd).
Palladium nanoparticles were prepared from the pre-
cursor Pd(II) ions by the reduction with trisodium
citrate. Citrate ions reduce Pd(II) to Pd(0), and then
through the nucleation process Pd(0) forms palladium
nanoparticles. Citrate ions also primarily stabilize the
palladium nanoparticles electrostaticlly, forming an
electrical double layer at the particle interfaces [46].
Since the interaction between the nanoparticles and
the citrate ions is relatively weak, aqueous gelatin was
added to the preformed palladium nanoparticles for
better and prolonged stabilization. Gelatin is a com-
mon, naturally occurring denatured product of colla-
gen, which is well known as a stabilizing agent. More-
over, it does not react with the reactants or passivate
the nanoparticle surfaces [47–49].
GPd was prepared by following the procedure:
50 mL of an aqueous PdCl2 solution (2.5 × 10−4 M)
was heated to boiling with vigorous stirring. Then 2 mL
of 1% (w/v) aqueous trisodium citrate solution was
added to the solution, and the solution was allowed to
boil for 1 h with vigorous stirring. The formation of
the Pd nanoparticles can be followed by the appear-
ance of yellow color in the solution. Then 5 mL of 1%
(w/v) aqueous gelatin solution was added to the pre-
formed Pd nanoparticles solution and the boiling was
In this study, we have developed a quick and easy
method of synthesis of stable homogeneous gelatin-
International Journal of Chemical Kinetics DOI 10.1002/kin.20883

748
DAS ET AL.
continued for another 30 min with stirring. Finally,
the yellow GPd solution was gradually cooled to room
temperature and was stored in a cold, dry, and dark
place. This stock GPd solution was always used after
sonication.
0.5
0.0
Uncatalyze
t0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
k0
Instrumentation
Absorbance and UV–vis spectra were recorded using a
Shimadzu(Japan) spectrophotometer (UV-1700) with
1.000-cm quartz cuvettes. Transmission electron mi-
croscopic (TEM) analyses were performed in a Hi-
tachi(Japan) (H-9000 NAR) instrument on samples
prepared by placing a drop of GPd solution on a Cu
grid, precoated with carbon films, followed by solvent
evaporation under vacuum. The GPd solution was cen-
trifuged in a Heraeus(USA), Biofuge primo R machine
and Fourier transform infrared spectroscopy (FTIR)
data were collected using a Shimadzu(Japan) FT-IR
8400S spectrometer. The GPd solution was regularly
sonicated in a Rivotek(India) ultrasonic bath (50 Hz)
before any experimentation.
catalyze
0
500
1000
time (s)
1500
2000
Figure 1 Plot of ln(A/A0) versus time for uncatalyzed and
GPd-catalyzed reduction of AO7, where A0 and A are, respec-
tively, the corresponding absorbance values at time zero and
time t, measured at 485 nm. The catalyzed reduction follows
a first-order kinetics (represented by the broken black line)
with an induction period (t0). Conditions: [AO7] = 0.05 mM,
[NaBH4] = 2.5 mM, [GPd] = 3.33 × 10−9 M, μ = 1.0 M,
[NaOH] = 0.5 M, and T = 25.0 0.1°C.
The buffer solutions were prepared following the
literature method [50] and were measured with a Tosh-
niwal(India) pH-meter (CL-54, India), calibrated as
usual using standard buffers.
1.0
1
Kinetics
At ambient temperature and fixed ionic strength (μ
= 1.0 M), in alkaline media (0.1–0.5 M NaOH) AO7
is reduced mainly to the corresponding amines, viz.,
1-amino-2-napthol and sulfanilic acid [13,43,44] by
sodium borohydride in the presence of GPd, whereas
the uncatalyzed reduction of AO7 by sodium borohy-
dride contributes nothing to the observed rate under
similar reaction conditions (Fig. 1). Moreover, neither
the GPd solution and/or aqueous gelatin solution (con-
centration ࣘ 0.0075%) can reduce AO7, nor the ad-
dition of mere aqueous gelatin solution catalyzes the
reduction of AO7 by borohydride. In both cases, the
spectrum of AO7 does not change at least for 2 h. Thus
it ensures that under the specified kinetic conditions,
only in the presence of GPd nanoparticles, AO7 is re-
duced by borohydride and the gelatin molecules only
acts as a stabilizing agent of the nanoparticles.
0.5
0.0
20
400
500
600
wavelength (nm)
Figure 2 Spectral changes for the GPd nanoparticles-
catalyzed reduction of AO7 by sodium borohydride at an
interval of 2 min. Condition: [AO7] = 0.05 mM, [NaBH4]
= 10 mM, [GPd] = 3.33 × 10−9 M, μ = 1.0 M, [NaOH] =
0.5 M, and T = 25.0 0.1°C.
The reduction reaction was monitored spectropho-
tometrically following the change in the absorbance of
the reaction mixture in situ in an electrically controlled
thermostat. Kinetic data were measured at 485 nm
where only AO7 shows absorption due to the pres-
ence of the color-generating chromophore (–N=N–
group) [32]. The complete reduction of AO7 can thus
be confirmed when the absorbance at 485 nm reaches
to a near-zero value (Fig. 2). Moreover, the reduction
reactions of AO7 were performed in the presence of a
large excess of borohydride over AO7. Usually, a reac-
tion mixture was prepared by the successive addition
of a measured amount of the GPd solution from the
stock solution, sodium nitrate solution (to maintain the
ionic strength, μ), and an alkaline solution of AO7 fol-
lowed by the final quick addition of the borohydride
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REDUCTION OF CHROMOPHORIC AZO GROUP OF DYE, ACID ORANGE 7 BY BOROHYDRIDE
749
solution. The final volume of the reaction mixture was
3.0 mL. Linear, standard least-square fit to the derived
first-order equation (ln(A/A0) = −kot (where A0 and A
are the respective absorbance values of AO7 at time
zero and time t) of the decay plot of absorbance with
time yielded the observed first-order rate constants, k-o.
Here, it should be mentioned that an initial time delay,
i.e., an induction period (t0) has been observed in the
catalyzed reaction (Fig. 1), which was not considered in
the determination of k-o values. The first point of time
where the linear plots of ln(A/A0) over the initial and
subsequent experimental points intersects each other
was taken as the induction time, t0. The presence of
the induction period does not arise due to the presence
of aerial oxygen [51] or the impurities in the reaction
medium since on passing argon or on addition of the
chelating agent, like dipicolinic acid [52], the induction
period remains as usual. This induction period origi-
nates from the inherent and adsorbate-induced surface
restructuring of nanoparticles [53], which has been ad-
dressed later in detail. We assume that during this short
period of time the adsorption of the reactants and the
rate of the reduction reaction was yet to achieve full
pace.
RESULTS AND DISCUSSION
Characterization of GPd
The formation of GPd was characterized by
TEM imaging and UV–vis spectroscopy. The
high-resoluttion transmission electron microscopy
(HRTEM) images of the GPd are shown at two differ-
ent magnifications in Fig. 3. High-resolution images
show that most of the GPd nanoparticles are hexag-
onal, indicating a shell structure [54], and the diam-
eter of a single nanoparticle was found to be about
4.5 nm.
Figure 4 presents the UV–vis spectra of aqueous so-
lution of PdCl2 and aqueous solution of GPd nanopar-
ticles. The spectrum of GPd nanoparticles shows the
absence of characteristic absorbance peaks of PdCl2 at
235 and 413 nm, whereas the former one arises due
to ligand-to-metal charge transfer transition [54–56].
Thus it can be logically assumed that Pd(II) ions have
been mostly reduced to Pd(0) during the formation of
nanoparticles.
Figure
3
High-resolution transmission electron mi-
croscopy images of GPd nanoparticles (a) having an average
diameter of nearly 4.5 nm (b).
2.0
1.5
1.0
0.5
A
B
0.0
200
The presence of gelatin molecules as the stabiliz-
ing agent on the nanoparticles’ surface has been con-
firmed from the FTIR spectrum of GPd (Fig. 5). The
FTIR spectrum of GPd was recorded after the cen-
trifugation of the stock GPd solution (at 12,000 rpm
for 30 min), followed by washing with Milli-Q water
300
400
500
600
wavelength (nm)
Figure 4 UV–vis spectra of aqueous solution of PdCl2
(6.25 × 10−4 M) (A) and the aqueous solution of GPd
nanoparticles (B).
International Journal of Chemical Kinetics DOI 10.1002/kin.20883

750
DAS ET AL.
100
20
16
12
8
80
60
40
1595 cm-1
1631 cm-1
4
3600 - 3200 cm-1
3000
0
4000
2000
cm-1
1000
0
10
20
30
40
1010[GPd] (M)
Figure 5 FTIR spectrum of GPd.
Figure 6 Observed rate, ko of the catalyzed reduction of
AO7 linearly increases with increasing concentration of GPd.
Conditions: [AO7] = 0.05 mM, [NaBH4] = 10 mM, μ =
1.0 M, [NaOH] = 0.5 M, and T = 25.0 0.1°C.
to remove excess gelatin. The main absorption bands
at 1620 cm−1 (C=O stretching of amide) and 1595
cm−1 (N–H bending) arise due to the amide groups of
gelatin molecules [57–59]. The spectrum confirms the
presence of the gelatin molecule as the capping agent
on the surface of Pd nanoparticles. Gelatin molecules
generally have been found to attach itself electro-
statically to the metal surface by their amine group
[49,58,59].
Effect of [GPd] on ko
The linear rise of the observed rate constant, ko, with
increasing [GPd] represents the general observation
of the pseudo–first-order reaction (Fig. 6; Table S1 in
the Supporting Information). The plot passes through
the origin, which signifies that the reduction reaction
between AO7 and borohydride happens only in the
presence of the catalyst, and the contribution of the un-
catalyzed reaction is negligible. Moreover, the plot en-
sures that the increasing gelatin concentration (0.0015–
0.0075%) along with increasing [GPd] does not inter-
fere with the reaction or passivate the catalyst in any
way [53].
Concentration of GPd
The HRTEM images show that the shapes of the GPd
nanoparticles are mostly hexagonal with an estimated
diameter of 4.5 nm (vide supra). Here, on the as-
sumption that all the Pd(II) was reduced to Pd(0) and
the nanoparticles are near spherical in shape with the
same diameter as already mentioned, Eq. (1) gives
an approximate concentration of GPd nanoparticles
[60]:
Effect of [AO7] and [NaBH4] on ko Values
Figures 7 and 8 describe the variation of ko val-
ues with increasing [AO7] and [NaBH4], respectively
(Tables S2 and S3, respectively, in the Supporting
Information). It can be seen that in both cases val-
ues of the ko at first increase with the increase in
[AO7] and [NaBH4] reaching to an optimum value,
then ko decreases on a further increase in [AO7] and
[NaBH4].
The kinetic results firmly conclude that the reduc-
tion of AO7 by borohydride is a surface-catalyzed re-
action where one or more reactants are adsorbed on
the nanoparticle surface. Two kinetic models, viz.,
Eley–Rideal or by Langmuir–Hinshelwood mecha-
nism, have been successfully employed to explain such
surface reactions [62]. Here, since in both cases (Figs. 7
and 8) ko shows similar behavior, it can be suggested
[GPd] = [Pd2+ions]/(V GPd/V Pd atom
)
(1)
where V refers to the corresponding volumes where the
atomic radius of the Pd atom was taken as 0.137 nm
[61]. The approximate concentration of stock GPd
solution was calculated as 5.0 × 10−8 M, and the
concentration of the GPd catalyst used in the reac-
tion media is in the order of 10−10 M. Although
the concentration of GPd is an rough estimate of
the desired parameter but it serves the indispensable
need in understanding the dependence of observed
reaction rates on the GPd concentration and other
factors.
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REDUCTION OF CHROMOPHORIC AZO GROUP OF DYE, ACID ORANGE 7 BY BOROHYDRIDE
751
20
16
12
8
100
80
B
60
A
40
4000
4
3000
2000
cm-1
1000
0
0.00
Figure 9 FTIR spectrum of pure AO7.
0.05
0.10
0.15
[AO7] (mM)
Figure 7 Variation of ko values with [AO7]. Conditions:
[NaBH4] = 2.5 mM (A), 10 mM (B); [GPd] = 3.33 ×
10−9 M; μ = 1.0 M; [NaOH] = 0.5 M; T = 25.0 0.1°C.
as displayed in Figs. 7 and 8, also needs an expla-
nation. It can be seen that while the optimum con-
centration for AO7 lies around 0.025 mM, for NaBH4
it is around 5 mM. A suitable account for this dif-
ference can be found in the disparity of the size of
the reactants to be absorbed and the possible mode of
adsorption.
The interaction of NaBH4 with the metal nanoparti-
cle surface had long been a subject of study. The inter-
action proceeds through several complex steps [68,69],
and now it has been firmly established that ultimately
the borohydride ion transfers a hydrogen species to the
nanoparticle surface [63,64]. On the other hand, the
interaction of AO7 with GPd can be followed by the
change in the FTIR spectrum of pure AO7 and GPd–
AO7 composite (Figs. 9 and 10, respectively).
24
C
D
20
16
12
8
4
0
0
4
8
12
16
The FTIR spectrum of pure AO7 shows several
absorption bands, which are assigned as follows:
3060–3030 cm−1 (aromatic =C–H stretching), 1600–
1450 cm−1 (aromatic C=C stretching), 1618 cm−1
(C=N stretching), 1556 cm−1 (related to NH–N=C
grouping), 1504 cm−1 (N–H bending), 1446 cm−1
(N=N stretching), 1204 and 1120 cm−1 (coupling of
[NaBH4] (mM)
Figure 8 Variation of ko values with [NaBH4]. Conditions:
[AO7] = 0.025 mM (C), 0.05 mM (D); [GPd] = 3.33 ×
10−9 M; μ = 1.0 M; [NaOH] = 0.5 M; T = 25.0 0.1°C.
−
SO3 stretching and aromatic =C–H bending), and
that both the reactants AO7 and borohydride are
adsorbed on the GPd surface and both the reactants
compete with the same reaction sites [63–67]. Thus, in
the presence of a certain amount of [GPd], when [AO7]
or [NaBH4] is increased, the reaction sites on the GPd
surface are more occupied by the corresponding reac-
tants and the rate also increases. But after reaching an
optimum when the concentration of each reactant is
further increased, that results in the occupation of re-
action sites by only that reactant and the observed rate
constants decreases due to the lack of other reactant
([NaBH4] or [AO7], respectively).
1037 cm−1 (C–OH stretching) [70–73].
Here, it is important to note that AO7 exists as an
equilibrium between two tautomeric forms, viz., the
azo form (A) and the hydrazone form (B) (Scheme 1)
due to a rapid intramolecular proton exchange [73].
The presence of FTIR bands of AO7 at 1620 and 1446
cm−1 is in good agreement with the UV–vis spectrum
of AO7 (Fig. 2). Figure 2 also shows two peaks at 430
and 485 nm, which supports the coexistence of both azo
and hydrazone tautomeric forms of AO7, respectively
[72,73].
The coexistence of both tautomeric forms is further
supported by the absence of the FTIR band for carbonyl
stretching of the pure azo form of AO7, which usually
The relative difference in the optimal concentra-
tion of AO7 and NaBH4 after which ko decreases,
International Journal of Chemical Kinetics DOI 10.1002/kin.20883

752
DAS ET AL.
100
80
60
40
4000
3000
2000
cm-1
1000
Figure 10 FTIR spectrum of the GPd–AO7 composite.
SO3Na
SO3Na
H
GPd
O
O
N
H
O
N
N
N
N
S
N
H
O
O
O
GPd-AO7
azo form (A)
hydrazone form (B)
AO7
Scheme 1
appears at 1570 cm−1. Owing to the delocalization of
the H atom between oxygen and nitrogen atoms of
AO7, a hydrazone tautomer is formed; and owing to
the resulting intramolecular hydrogen bond (N–H–O),
the double bond character of the carbonyl group is
substantially reduced [72]. When AO7 is adsorbed on
GPd, it can be seen that the FTIR bands at 1204 and
Effect of Media Alkalinity on ko
The observed ko values have been found to be de-
creasing with the increasing alkalinity of the reaction
media (Fig. 11; Table S4 in the Supporting Informa-
tion). Such a trend can be explained if the behavior
of adsorbent GPd and AO7 and sodium borohydride
in alkaline media is considered. With increasing alka-
linity, borohydride ions are more stabilized and thus
it is expected that a less number of hydride ions will
be available on the catalyst surface, which in turn can
have an effect on the decreasing rate. Moreover, AO7 in
solution undergoes protolytic equilibria [74] as shown
below:
−
1120 cm−1 (SO3 stretching) and 1307 cm−1 (C–O–
H stretching) have almost collapsed. On the basis of
similar observation by Stylidi et al. and others [70,72],
it can be assumed that AO7 is adsorbed on the GPd
surface through the oxygen atom of the carbonyl group
of the hydrazone form and two oxygen atoms of the
sulfonate group (Scheme 1).
Since, an AO7 molecule with two phenyl groups is
bulkier than the adsorbed hydrogen species, on adsorp-
tion AO7 molecules will cover up larger surface area
than the other adsorbates. Thus on a small increase in
[AO7], the surface tends to be more covered and the
rate starts to decrease, whereas for a similar effect to
take place for [NaBH4] a relatively high concentration
is required.
K
1
L
2− + H+ ꢀ LH−, K1 = [LH−]/[L2−][H+] (2)
K
2
LH− + H+ ꢀ LH2, K2 = [LH2]/[LH−][H+] (3)
The relative concentration of the three forms of
AO7, viz., LH2, LH−, and L2− depend on the pH of
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REDUCTION OF CHROMOPHORIC AZO GROUP OF DYE, ACID ORANGE 7 BY BOROHYDRIDE
753
The kinetic observations are best modeled by the
Langmuir–Hinshelwood surface mechanism, and the
rate of the reaction can simply be expressed as follows
[63]:
40
30
20
10
0
Rate = −d[AO7]/dt = ko[AO7]
= (kKA[AO7]αKB[NaBH4]β)/(1+KA[AO7]α
+ KB[NaBH4]β)2
(4)
where k is the true rate constant and KA and KB
are the adsorption coefficients of AO7 and sodium
borohydride, respectively. The exponents α and β
are the corresponding reaction order in [AO7] and in
[NaBH4] [62], which has also been shown to be re-
lated to the heterogeneity of the nanoparticles surface
[78].
If both α = β = 1, then the above equation reduces to
the classical Langmuir–Hinshelwood equation, which
can be rearranged as [79]
0
5
10
1/[OH-] (M-1)
Figure 11 Variation of ko versus 1/[OH−]. Conditions:
[AO7] = 0.05 mM, [NaBH4] = 10 mM, [GPd] = 3.33 ×
10−9 M, μ = 1.0 M, and T = 25.0 0.1°C.
([AO7]/ko[AO7])1/2
the medium. The values of logarithm of the protona-
tion constants K1 and K2 are 10.6 and 1.0 [75] which
accounts for the deprotonation of naphthalene OH and
SO3H groups, respectively [76]. Thus, in 0.1–0.5 M
NaOH media AO7 is mostly present as a doubly de-
protonated L2− form. Now, as the media alkalinity is in-
creased, the palladium surface becomes more negative
due to the enhanced interaction of the nanoparticles’
surface with the OH− ions [77]. Overall, an increase
in media alkalinity results in the poor ele−ctrostatic at-
traction between the AO7 anion, the BH4 anion, and
the negatively charged GPd surface. Thus, due to the
lower extent of adsorption of AO7 and borohydride
on GPd surface, the reaction rate is also diminished
gradually [74–76].
= (1 + KB[NaBH4])/(kKAKB[NaBH4])1/2
+ [AO7](KA/kKB[NaBH4])1/2
(5)
Or, ([NaBH4]/ko[AO7])1/2
= (1+KA[AO7])/(kKAKB[AO7])1/2
+ [NaBH4](KB/kKA[AO7])1/2
(6)
Thus plots of ([AO7]/ko[AO7])1/2versus [AO7] and
([NaBH4]/ko[AO7])1/2 versus [NaBH4] should be a
straight line and indeed the plots have been found to
be acceptably good straight lines (Figs. 12 and 13, re-
spectively) [79].
Since the plots are linear and continuous, we can
argue that the interaction between the adsorbate AO7
and the surface hydrogen species are relatively homo-
geneous. If the interaction had been heterogeneous,
there would have been break in plots [53]. The ob-
served continuous nature is further supported by the
near constancy of the exponent values over the total
reactant concentrations or in general at different mo-
lar ratios of [NaBH4]/[AO7]. On the assumption that
the adsorption of [AO7] and [NaBH4] on the GPd sur-
face is not so strong (since they are supposed to be
reversible), Eq. (4) is reduced to
Proposed Reaction Mechanism
Following the kinetic observations and the discussion
described above, the catalyzed reaction steps are sim-
ply described in Scheme 2.
In step 1, the borohydride anions are adsorbed on
the GPd surface to provide surface-adsorbed hydrogen
species [63,64] and in step 2 AO7 molecules are ad-
sorbed on the GPd surface. In step 3, AO7 is reduced
to the corresponding amines, viz., 1-amino-2-napthol
and sulfanilic acid, and then the products desorb from
the GPd surface and the bare surface again takes part
in the catalytic cycle. Steps 1 and 2 both are supposed
to be reversible as they describe the fast adsorption or
desorption processes, and step 3 is the rate-determining
step [64].
Rate = ko[AO7] = kKA[AO7]αKB[NaBH4]β (7)
or, ko = kKA[AO7]α−1KB[NaBH4]β
(8)
International Journal of Chemical Kinetics DOI 10.1002/kin.20883

754
DAS ET AL.
NH2
H
N N
NH2
O
O
O
B
-
-
H4
-
SO3
H
-
H 4
BH4
B
AO7
Step 2
S
H
H
H
H
H
H
Step 1
O
O
H
Step 3
H
H
H
H
H
H
GPd
GPd
GPd
GPd
Scheme 2
0.028
34
32
30
28
26
24
22
0.024
0.020
0.016
0.012
0.008
0.004
H
G
E
F
0.000
0.004
0.008
0.012
0.016
0.00004
0.00008
[AO7] (M)
0.00012
[NaBH4] (M)
Figure 13 Plot of ([NaBH4]/ko[AO7])1/2 versus [NaBH4].
Conditions: [AO7] = 0.025 mM (G), 0.05 mM (H); [GPd]
= 3.33 × 10−9 M; μ = 1.0 M; [NaOH] = 0.5 M; and T =
25.0 0.1°C.
Figure 12 Plot of ([AO7]/ko[AO7])1/2 versus [AO7]. Con-
ditions: [NaBH4] = 2.5 mM (E), 10 mM (F); [GPd] = 3.33
× 10−9 M; μ = 1.0 M; [NaOH] = 0.5 M; and T = 25.0
0.1°C.
The slopes of the plots of lnko versus ln[AO7] or
lnko versus ln[NaBH4] (Figs. 14 and 15, respectively)
at the fixed concentration of other adsorbate (NaBH4
or AO7, respectively) provide the corresponding expo-
nent values with respect to the reactants [80]; numerical
values of which are given in Table I. Both in Figs. 14
and 15, there is an increasing and decreasing linear
segment, which signifies the mutual competition of the
reactants toward the surface sites and consequently the
variation in the reduction rate, ko (vide supra). The
mathematical signs of the exponent values indicate
this variation in the rate (positive and negative signs
correspond to the increasing and decreasing trends,
respectively) and thus only the numerical values are
considered.
It can be seen that the numerical values of α and β
corresponding to the increasing and decreasing parts
over different molar ratios are almost constant if we
consider the error range. The variation in exponent val-
ues usually indicates the irregularity in the activation
energy of the adsorption process [80]. Thus we can con-
clude that the interaction of the adsorbate molecules
with the GPd surface is homogeneous.
-6.4
B
-6.6
-6.8
A
-7.0
-11.5
-11.0
-10.5
-10.0
-9.5
-9.0
ln[AO7]
Figure 14 Plot of lnko versus ln[AO7]. Conditions:
[NaBH4] = 2.5 mM (A), 10 mM (B); [GPd] = 3.33 ×
10−9 M; μ = 1.0 M; [NaOH] = 0.5 M; and T = 25.0
0.1°C.
Moreover, if we relate the exponent values α and
β to the reaction order, it can be said that the nature
of mutual dependence of AO7 and adsorbed hydrogen
species remains invariant on the change in the molar ra-
tio. Since, for the reduction of the azo bond, each AO7
International Journal of Chemical Kinetics DOI 10.1002/kin.20883

REDUCTION OF CHROMOPHORIC AZO GROUP OF DYE, ACID ORANGE 7 BY BOROHYDRIDE
755
40
-6.1
-6.2
-6.3
-6.4
-6.5
-6.6
-6.7
30
C
20
D
10
intercept = 9.8
-7.0
-6.5
-6.0
-5.5
-5.0
-4.5
-4.0
0
0
5
10
15
ln[NaBH4]
[NaBH4] (mM)
Figure 15 Plot of lnko versus ln[NaBH4]. Conditions:
[AO7] = 0.025 mM (C), 0.05 mM (D); [GPd] = 3.33 ×
10−9 M; μ = 1.0 M; [NaOH] = 0.5 M; and T = 25.0
0.1°C.
Figure 16 Plot of 1/t0 versus [NaBH4]. Conditions: [AO7]
= 0.05 mM, [GPd] = 3.33 × 10−9 M, μ = 1.0 M, [NaOH]
= 0.5 M, and T = 25.0 0.1°C.
Table I Representative Values of Exponent α and β at
Different Molar Ratios
results both from the spontaneous restructuring process
and the adsorbate-induced process [82–84]. Nanopar-
ticles, especially which are made up of the platinum
group metals, due to relativistic effects, are more likely
to go through this restructuring process [83]. The initial
time period, t0, is thus a measure of extent of surface
restructuring, and 1/t0 has been conceived as the rate
of such a restructuring process [63,65]. The plot of 1/t0
versus [NaBH4] is shown in Fig. 16, which shows that
the overall restructuring process increases with an in-
crease in [NaBH4]. At zero [NaBH4], the presence of
an intercept value is particularly noteworthy, which ac-
tually represents the rate of occurrence of spontaneous
restructuring (1/t0,sp) [63,65]. Thus, the plot (1/t0 –
1/t0,sp) versus [NaBH4] (Fig. 17) describes the sole ef-
fect of adsorbate-induced restructuring of the surface,
which passes through the origin and increases linearly
with the increasing adsorbate concentration. If we plot
1/t0 against [GPd] (Fig. 18), naturally the plot passes
through the origin, confirming that the nanoparticle
surface is responsible for the presence of the induction
period.
Exponent
Value
Molar
Ratioa
Exponent
Value
Molar
Ratioa
αb
αc
βd
βe
1.65
1.36
0.36 0.08
0.23 0.02
167–100
667–400
50–200
25–100
1.44 0.03
1.48 0.03
0.22 0.05 200–600
0.31 0.08 100–300
50–22
200–80
aMolar ratio = [NaBH4]/[AO7].
b[NaBH4] = 2.50 mM, [GPd] = 3.33 × 10−9 M, μ = 1.0 M,
[NaOH] = 0.5 M, and T = 25.0 0.1°C.
c[NaBH4] = 10.0 mM, [GPd] = 3.33 × 10−9 M, μ = 1.0 M,
[NaOH] = 0.5 M, and T = 25.0 0.1°C.
d[AO7] = 0.025 mM, [GPd] = 3.33 × 10−9 M, μ = 1.0 M,
[NaOH] = 0.5 M, and T = 25.0 0.1°C.
e[AO7] = 0.05 mM, [GPd] = 3.33 × 10−9 M, μ = 1.0 M, [NaOH]
= 0.5 M, and T = 25.0 0.1°C.
molecule needs four neighboring hydrogen species and
the adsorbed hydrogen species are distributed over the
nanoparticle surface, the reduction of the azo group
successfully proceeds through a isotropic interaction
and a single mechanistic pathway [81].
Role of Induction Period, t0
Effect of Temperature on Catalysis and
Surface Restructuring
The presence of an induction period, t0, had been pro-
posed to be a manifestation of the effect of surface re-
structuring of the GPd nanoparticles [63,65]. Since the
atoms resting on the nanoparticle surface possess more
potential energy than the atoms comprising the bulk of
the particle, the surface always tends to reshuffle the
atomic arrangement to lower the excess surface energy
[82]. The rearrangement of the surface atoms usually
Both the rates of catalysis and the overall surface re-
structuring process increase with the increase in reac-
tion temperature. The surface-restructuring energy and
the activation energy of the GPd-catalyzed reduction
of AO7 by NaBH4 were evaluated from the plot of
ln(1/t0) versus 103/T and lnko versus 103/T (Figs. 19
and 20. respectively; Tables S5 and S6, respectively,
International Journal of Chemical Kinetics DOI 10.1002/kin.20883

756
DAS ET AL.
25
20
15
10
5
-5.4
-5.6
-5.8
-6.0
-6.2
-6.4
0
3.15 3.20 3.25 3.30 3.35 3.40 3.45 3.50
0
5
10
15
103/T (K-1)
[NaBH4] (mM)
Figure 19 Plot of ln(1/t0) versus 103/T. Conditions: [AO7]
= 0.05 mM, [NaBH4] = 10 mM, [GPd] = 3.33 × 10−9 M,
μ = 1.0 M, and [NaOH] = 0.5 M.
Figure 17 Plot of (1/t0 – 1/t0,sp) versus [NaBH4]. Condi-
tions: [AO7] = 0.05 mM, [GPd] = 3.33 × 10−9 M, μ =
1.0 M, [NaOH] = 0.5 M, and T = 25.0 0.1°C.
-5.8
-6.0
-6.2
-6.4
-6.6
12
8
4
-6.8
0
3.15 3.20 3.25 3.30 3.35 3.40 3.45 3.50
0
10
20
30
40
50
103/T (K-1)
1010 [GPd] (M)
Figure 20 Plot of lnko versus 103/T. Conditions: [AO7] =
0.05 mM, [NaBH4] = 10 mM, [GPd] = 3.33 × 10−9 M, μ
= 1.0 M, and [NaOH] = 0.5 M.
Figure 18 Plot of 1/t0 versus [GPd]. Conditions: [AO7] =
0.05 mM, [NaBH4] = 10.0 mM, μ = 1.0 M, [NaOH] =
0.5 M, and T = 25.0 0.1°C.
in the Supporting Information) following the simple
Arrhenius equation, lnko = ln A – Ea/RT, where ko
and A are the rate constant and the preexponential
constant, respectively. From the slope of the corre-
sponding plots, the calculated value of the surface-
restructuring energy was found to be 25 7 kJ M−1
this GPd catalyzes the reduction of the harmful azo
dye, AO7 by sodium borohydride in alkaline media.
The reduction takes place on the GPd surface, and
the kinetics follows the Langmuir–Hinshelwood ki-
netic model, where both AO7 and borohydride com-
pete for the same surface site for adsorption on
the GPd surface. The adsorption of AO7 is evident
from the changes in the corresponding FTIR spec-
tra. Detailed analysis also suggests that the reduc-
tion of AO7 by borohydride is isotropic over dif-
ferent molar ratios of [AO7]/[NaBH4]. The presence
of an induction period (t0), which accounts for the
surface-restructuring processes, emphasizes the cru-
cial role of the nanoparticles surface in the catalyzed
reaction.
and that of the activation energy, Ea was 22 3 kJ M−1
,
which is very close to that value as found earlier
[63].
CONCLUSIONS
Well-characterized GPd nanoparticles were prepared
by an one-pot synthesis method. A small amount of
International Journal of Chemical Kinetics DOI 10.1002/kin.20883

REDUCTION OF CHROMOPHORIC AZO GROUP OF DYE, ACID ORANGE 7 BY BOROHYDRIDE
757
The award of a senior research fellowship (Council for Sci-
entific & Industrial Research, New Delhi, India) to R. S. Das
is gratefully acknowledged.
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