Catalytic Activity of Cobalt Nanoparticles for Dye and 4-Nitro Phenol Degradation: A Kinetic and Mechanistic Study

Article abstract of DOI:10.1002/kin.21089

Cetyltrimethylammonium bromide (CTAB) capped nanosized α-cobalt hydroxides (CoNPs) were prepared to the removal of Congo red at room temperature in the absence and presence of sunlight. UV/visible spectra reveal that the CoNPs and NaBH₄ have no catalytic effects toward Congo red and 4-nitrophenol (4-NP) degradation, whereas CoNPs act as an efficient catalyst with a small amount of NaBH₄. The catalytic reaction has an induction period followed by an autoacceleration, which depends on the [NaBH₄]. The reaction follows fractional-order kinetics with respect to [NaBH₄] for Congo red degradation. The activation energies were found to be 24 and 74 kJ mol^?1, respectively, for Congo red and 4-NP catalyzed reactions. The AgNPs has no catalytic activity toward Congo red. The degradation pathway mechanism with observed kinetics has been proposed and discussed, which follows the following rate law:. (Formula presented.).

Full text of DOI:10.1002/kin.21089

Catalytic Activity of Cobalt
Nanoparticles for Dye and
4-Nitro Phenol
Degradation: A Kinetic and
Mechanistic Study
MOHAMMAD NAVED KHAN,1 OMMER BASHIR,1 TABREZ ALAM KHAN,1
SHAEEL AHMED AL-THABAITI,2 ZAHEER KHAN2
1Department of Chemistry, Jamia Millia Islamia, New Delhi, 110 025, India
2Department of Chemistry, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah, 21589, Saudi Arabia
Received 4 December 2016; revised 17 February 2017; accepted 20 February 2017
DOI 10.1002/kin.21089
Published online in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Cetyltrimethylammonium bromide (CTAB) capped nanosized α-cobalt hydroxides
(CoNPs) were prepared to the removal of Congo red at room temperature in the absence and
presence of sunlight. UV/visible spectra reveal that the CoNPs and NaBH4 have no catalytic ef-
fects toward Congo red and 4-nitrophenol (4-NP) degradation, whereas CoNPs act as an efficient
catalyst with a small amount of NaBH4. The catalytic reaction has an induction period followed
by an autoacceleration, which depends on the [NaBH4]. The reaction follows fractional-order
kinetics with respect to [NaBH4] for Congo red degradation. The activation energies were
found to be 24 and 74 kJ mol−1, respectively, for Congo red and 4-NP catalyzed reactions. The
AgNPs has no catalytic activity toward Congo red. The degradation pathway mechanism with
observed kinetics has been proposed and discussed, which follows the following rate law:
k KC Ka[H+][BH4−][Congo red]T
d[Congo red]
−
=
1+Ka[H+]+KC Ka[H+][BH−4
]
d t
ꢀC
2017 Wiley Periodicals, Inc. Int J Chem Kinet 1–17, 2017
aimed toward understanding the role of experimen-
tal conditions, methods of preparations, and pres-
ence of stabilizers in the synthesis and characteriza-
tion of different shapes and sizes of cobalt nanopar-
ticles (nanocones, nanoplates, nanodisks, nanorods,
nanosheets, nanoneedles, nanowires, nanoflake, and
butterfly-like structures) [1–7]. Generally, researchers
used azo (─N═N─) dyes in industries as a col-
oring agent. The removal and/or degradation of
INTRODUCTION
Owing to their potential applications in chemistry,
biology, engineering, and industries as a catalyst,
the chemical literature contains abundant reports
Correspondence to: Zaheer Khan; e-mail: drkhanchem@yahoo.
co.in.
Supporting Information is available in the online issue at
www.wileyonlinelibrary.com.
ꢀC
2017 Wiley Periodicals, Inc.

2
KHAN ET AL.
excess dyes from the textile effluents by using suit-
able nanomaterials has been the subject of investi-
gations [8–11]. Therefore, Xu and Zeng used com-
plementary precipitation methods and reported the
synthesis of cobalt-hydroxide-carbonate ([Co(OH)x-
(CO3)0.5(2–x)·nH2O] nanorods with NaOH + Na2CO3
and urea [12]. It has been established that cobalt
hydroxides exist in α- and β- polymorphic forms
(hydrotalcite- and brucite-like morphologies, respec-
tively) and have a hexagonal layered structure [13].
Out of these two species, α-hydroxide hydroxy defi-
cient consists of stacks of positively charged layers of
composition Co(OH)2–x(H2O)x, which intercalate an-
ions and water molecules in the interlayer space to
restore charge neutrality [14]. Synthesis of needle-like
and sheet shapes α-cobalt(II) hydroxides have been re-
ported by Gedanken et al. [1,15]. The cobalt(II) nitrate
solution was also used in the preparation of α-cobalt
hydroxide in the presence of 1 wt% of fructose as ad-
ditive by the electrochemical reduction method [16].
Geng et al. synthesized the hierarchical dumbbell-like
CoO(OH) nanostructures, which have large active sur-
face areas, and exhibited a superior sensitivity to CO
at room temperature [17]. The synthesis of the metal-
hydroxide-carbonates usually involves coprecipitation
of metal salts with an alkaline carbonate at constant
pH [18].
exhibited the best oxygen evolution reaction
electrocatalyst [31].
Bakshi in his pioneering review suggested that
the fine-tuning of the desired morphologies could be
achieved by controlling the surfactant architecture as
well as its self-assembly behavior [32]. He also sug-
gested that the morphology (nanorods, nanoribbons,
and nanobelts) depended on the adsorption of ionic
surfactants on the crystal planes of the tiny metal nano-
materials [33]. Therefore, CTAB and PVA were used
as the stabilizers for the synthesis of CoNPs. A liter-
ature review showed that the systematic kinetics of
Congo red (azo dye) degradation by using CoNPs
as a catalyst was not studied in detail. In this pa-
per, first, we synthesized α-cobalt nanoparticles with
and without stabilizers. Second, the effect of the cat-
alytic and photocatalytic conditions on the efficiency
for the degradation of Congo red by CoNPs was in-
vestigated in detail in the absence and presence of
sunlight. The mode of investigation was mainly ki-
netic; other relevant data were also obtained regard-
ing the synthesis. This paper contributes experimental
evidence to the degradation of Congo red and 4-NP
by CTAB-capped CoNPs at room temperature for the
first time. In addition, the effect of sunlight irradiation
and AgNPs was also been studied on the Congo red
degradation.
Chen et al. reported the synthesis of uniform
cobalt nanoparticles (size in the range 1.8–4.4 nm)
by the reduction of CoCl2 with NaBH4 inside the
reverse micelles of didodecyldimethylammonium bro-
mide [19]. Pileni and his coworkers reported the syn-
thesis of trioctylphosphine-coated cobalt nanomateri-
als [20,21]. Generally, a Co2+–NaBH4 redox system
with and without stabilizers (organic solvents, mixture
of surfactants, silica) was used for the synthesis of sta-
ble cobalt nanoparticles [22–25]. The complexities of
this redox system, formation of ultrafine particles of
Co2B, metallic Co, or Co(BO)2 as the product(s) have
been discussed by Brown et al. [26] and Klabunde
et al. [27]. Jiang and his coworkers described a sim-
ple cost-effective method for the synthesis of cobalt–
alginate hydrogels and suggested that the CoAHs acted
as a effective and recyclable catalyst for hydrogen gen-
eration from the hydrolysis of NaBH4 [28]. Ai et al. re-
ported the composition and structure of CoNPs embed-
ded in porous N-rich carbon derived from cobalt con-
taining a metal organic frame work and used as an effi-
cient bifunctional electrocatalyst for water splitting and
degradation of toxic organic substances (rhodamine
B and 4-nitrophenol (4-NP)), respectively [29,30].
These investigators synthesized the CoWO4 nanos-
tructures (nanoflakes, nanorods, and nanocuboids) by
solvothermal method. Out of these, CoWO4 nanoflakes
EXPERIMENTAL
Materials
Cobalt nitrate (Co(NO3)2; formula weight
=
182.94), sodium borohydride (NaBH4; formula
weight = 37.83), cetyltrimethylammonium bromide
(C19H42BrN; CTAB; formula weight = 364.45),
Congo red (C32H22N6Na2O6S2; formula weight =
696), 4-nitrophenol (C6H5NO3; formula weight =
139.11), poly(vinyl)alcohol (PVA; 99–100% hy-
drolyzed), and 4-aminophenol (C6H7NO; formula
weight = : 109.12) were used with out further pu-
rification (all from Merck, Mumbai, India; purity
ࣙ99%). Aqueous solutions of all reagents were pre-
pared using double-distilled deionized water. NaBH4,
and Co(NO3)2 solutions (0.01 mol dm−3) were pre-
pared daily in boiled and cooled distilled water (to
arrest the aerial oxidation) by dissolving the required
amounts prior to use and later stored in amber-colored
bottles and standardized by the reported method. Stock
PVA solutions were prepared by slow stepwise addi-
tion of PVA to deionized water while rapidly stirring
to avoid the aggregation of PVA.
International Journal of Chemical Kinetics DOI 10.1002/kin.21089

CATALYTIC ACTIVITY OF COBALT NANOPARTICLES FOR DYE AND 4-NP DEGRADATION
3
analysis was done using the Fison (VG) ESCA 210
spectrometer equipped with an MgKa X-ray source
to examine the oxidation states of the CTAB-capped
CoNPs.
Preparation and Characterization of
Nanosized Cobalt Materials
In a typical synthesis of nanosized cobalt materials,
NaBH4 solutions (3.3 × 10−3 mol dm−3) were added
into the different separate reaction vessels containing
an aqueous solutions of Co2+ ions only, Co2+ ions
+ PVA (2 mL (4%), and Co2+ ions + CTAB (=0.8
× 10− 3 mol dm−3). Each reaction flask has constant
[Co2+] = 1.6 × 10−3 mol dm−3, held at room temper-
ature for 2 h. As the reaction proceeds, the light pink
color of Co2+ ions was disappeared and prefect trans-
parent blue color appeared, indicating the formation
of nanosized α-cobalt hydroxide [15]. To find out the
preliminary information about the morphology, spec-
tra of reaction mixtures were recorded at different time
intervals on a UV/visible spectrophotometer (UV-260
Shimadzu, with 1 cm quartz cuvettes) under different
experimental conditions (vide infra). For the measure-
ments of surface properties, CoNPs were separated by
centrifugation (10,000 rpm for 30 min), washed with
deionized water three times, and dried under vacuum
for 3–4 h.
Fourier transform infrared spectroscopy (FTIR) was
used to characterize the presence of chemical bond-
ing information on metal–oxygen, metal anions, and
hydroxyl-specific chemical groups in the materials.
Reductant, NaBH4, all stabilizers (CTAB, and PVA),
and synthesized stabilizers-capped cobalt nanomateri-
als were pulverized and mixed to KBr powder, pressed
into a pellet, and dried for 24 h at room temper-
ature. FTIR spectra were recorded in the range of
4000–500 cm−1 collected after 40 scans at a reso-
lution of 4 cm−1 using a FT-IR spectrophotometer
(IR Prestige-21, IRAffinity-1, FTIR-8400S, Shimadzu
Corporation Analytical and Measuring Instrument Di-
vision). Morphologies and structures of the samples
were investigated using a transmission electron mi-
croscope (TECHNAI-320 KV JAPAN), operating at
80 kV (TEM; together with selected area electron
diffraction (SAED) experiments). For specimen prepa-
ration, few drops of reaction mixtures were spread onto
a commercial copper grid coated with amorphous car-
bon layers, followed by a drying treatment at room
temperature. X-ray diffraction patterns were obtained
Kinetics of Congo Red and 4-Nitrophenol
Degradation
The catalytic activity was monitored by fading the
Congo red color in the absence and under solar irradi-
ation at different time intervals. For the measurement
of reaction rates, the two-necked reaction vessel con-
taining required [CoNPs] and [Congo red] equipped
with a double-surface condenser to prevent evaporation
was kept immersed in the water bath thermostated at
room temperature, and the solution was left to stand for
5 min to attain equilibrium. The degradation was initi-
ated with the addition of required volume of [NaBH4]
(whenever necessary). The progress of the reaction
was followed spectrophotometrically by recording the
spectra (absorbance decay at 495 nm; λmax of Congo
red) of the reaction mixture at definite time intervals
with and without sunlight. The pseudo–first-order rate
constants were calculated from the initial part of the
slopes of the plots of ln (absorbance) versus time with
a fixed time method. The pH of the reaction mixture
was also measured at the end of each kinetic exper-
iment, and it was observed that pH drift during the
course of the reaction is very small (with in 0.05 unit).
We did not observed any significant decay at 495 nm.
Then, NaBH4 (10.0 × 10−3 mol dm−3) was added into
the reaction solution under stirring. A UV-260 Shi-
madzu with a 1-cm quartz cuvettes UV/Vis spectropho-
tometer was used to monitor the Congo red degra-
dation rates under different experimental conditions.
The same experiments were also performed for the
CoNPs reduction of 4-NP to 4-Amino phenol (4-AP)
at 400 nm, λmax of 4-nitrophenolate, in the presence of
NaBH4.
Products Identification
The complete oxidation and/or degradation of Congo
red is indicated by the disappearance of its color. At the
end of the reaction, the colorless mixture was shaken
with few drops of ether and treated for the spot test
with 1.0% solution of sodium rhodizonate followed
by few drops of 2 N acetic acid. The appearance of
violet precipitate suggests the formation of benzidine
as one of the oxidation product of Congo red under our
experimental conditions [27].
˚
using Ni-filtered Cu Kα radiation (λ = 1.54056 A)
of a (Rigaku X-ray diffractometer, XRD) operating at
40 kV and 150 mA, the Bragg angle (2θ) in the range
from 10 to 80°). Elemental composition analyses were
carried out using energy dispersive X-ray spectroscopy
(EDX) on a TECHNAI-320 KV Japan, operating at
80 kV. A Fisher Scientific digital pH meter (910 fitted
with a combination electrode) was used for pH mea-
surements. X-ray photoelectron spectroscopy (XPS)
International Journal of Chemical Kinetics DOI 10.1002/kin.21089

4
KHAN ET AL.
3.0
2.5
2.0
1.5
1.0
0.5
0.0
RESULTS AND DISCUSSION
General Considerations
Time (min)
20.0
15.0
10.0
5.0
2.0
1.0
The cobalt–NaBH4 redox system has a complicated
reaction path, mechanisms, and different kinetics with
respect to [NaBH4], which have been attributed to the
various stoichiometry of the reaction (molar ratios of
the reactants) and formation of various products. Aque-
ous NaBH4 solutions were unstable and alkaline in
nature (a quick increase of pH of the solution) due
to the fast hydrolysis with water and formation of
NaOH [34]. Therefore, a 1:2 molar ratio of Co2+ ions
and NaBH4 and freshly prepared NaBH4 (after 10 min
of preparation time) solutions were used to the synthe-
sis of CoNPs in the absence and presence of stabilizers.
Upon addition of NaBH4 (3.3 × 10−3 mol dm−3), in a
Co2+ ions (1.6 × 10−3 mol dm−3) solutions with and
without required concentrations of stabilizers, the ap-
pearance of perfect transparent stable different colors
was observed and their optical images at different time
intervals are summarized in Fig. S1 (in the Supporting
Information). Initially, a light pink color of Co2+ ions
(optical image; Fig. S1 in the Supporting Information)
becomes dark blue (Fig. S1B in the Supporting Infor-
mation) with in a mixing reaction time. As the reac-
tion time increases, the intensity of the color increases
and/or decreases, and finally sky blue color appeared
within 90 min (Fig. S1 in the Supporting Information).
Surprisingly, no precipitate was observed for the long
reaction time, i.e., 2 h. Stability and intensity of the re-
sulting blue-colored cobalt sols depend on the capping
action of the CTAB. Cobalt hydroxide is known to ex-
ist in the blue (α-cobalt hydroxide) and pink (β–cobalt
hydroxide) forms, but the former is unstable, rapidly
ages to the latter, and only the pink β-phase has been
particularly characterized and reported in the literature.
Therefore, we may state confidently that the α-cobalt
hydroxide nanomaterials are blue in color, which con-
firms that the formed blue color might be due to the
nanoparticles of α-cobalt hydroxide under the present
experimental conditions [1,14]. Interestingly, the re-
duction of Co2+ into Co0 has occurred by NaBH4, but
the resulting blue sols are not stable, and finally, blue-
green precipitate deposited at the walls of the reaction
vessel (Fig. S1F in the Supporting Information) after
ca. 150 min, whereas such a type of deposition was not
appeared in the presence of CTAB for up to 5 h. We did
not observed the any color change from blue to pink
(conversion of α-cobalt hydroxide to β-cobalt hydrox-
ide) and pink to brown black (conversion of β-cobalt
hydroxide to cobalt oxyhydroxide) during the entire
studies, which ruled out the formation of β-cobalt hy-
droxide and cobalt oxyhydroxide.
pure dye
CoNPs
300
400
500
600
700
800
900
Wavelength(nm)
Figure
1
UV–vis spectra of pure CoNPs (1.6
×
10−3 mol dm−3), pure Congo red (1.4 × 10−4 mol dm−3),
and CoNPs + Congo red at different time intervals. [Color
figure can be viewed at wileyonlinelibrary.com]
Adsorption Ability for Congo Red
The degradation and/or adsorption of the Congo
red from aqueous solution using the as-prepared
CoNPs was investigated spectrophotometrically.
Figure 1 shows the spectra of pure CoNPs, pure Congo
red, and a mixture of CoNPs + Congo red. Interest-
ingly, Congo red spectrum has two peaks at 340 and
495 nm due to the π–π-* transition of –NH and azo
groups, respectively. On the other hand, CoNPs spec-
trum shows a steep rise in absorption at very short
wavelengths. In the visible range, the absorption was
flat and featureless [35,36]. The absorption spectrum of
mixed solution of Congo red–CoNPs shows a blueshift
(from 340 to 335 nm) and redshift (from 495 to
500 nm) intense bands at 465 nm as well as high-
energy blueshift bands at 329 nm (as indicated by ar-
rows; Fig. 1), indicating the adsorption of Congo red
on to the surface of nanoparticles. From the successive
UV–vis spectrum of Congo red adsorption (Fig. 1), we
can directly observe that the CoNPs are not in position
to remove and/or degrade the Congo red concentra-
tion under our experimental conditions. Therefore, in
the next experiment, a required [NaBH4] (10.0 × 10−3
mol dm−3) was used to monitor the degradation. The
degradation of the Congo red by CoNPs in the presence
of NaBH4 was confirmed by the gradual decrease of
its absorption peak intensity at 340 and 495 nm. The
Congo red color was completely discharged within ca.
7 min of the reaction time as indicated by an arrow
(typical example; Fig. 2), i.e., the completed reaction
was, the peak at 495 nm, due to the π–π-* transition
International Journal of Chemical Kinetics DOI 10.1002/kin.21089

CATALYTIC ACTIVITY OF COBALT NANOPARTICLES FOR DYE AND 4-NP DEGRADATION
5
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
100
495 nm
80
60
40
20
0
285 nm
340 nm
Pure CR
CR+ NaBH4 + CoNPs ; 1 min
103[CoNPs] (mol dm-3)
CR+ NaBH4 + CoNPs ; 2 min
CR+ NaBH4 +CONPs ; 4 min
CR+ NaBH4 +CONPs ; 6 min
Pure CoNPs
3.2
1.6
0
2
4
6
8
10
12
Time (min)
300
400
500
600
700
800
900
Wavelength(nm)
Figure 3 Variation of Congo red% degradation (2.1 ×
10−4 mol dm−3) with time. [Color figure can be viewed
at wileyonlinelibrary.com]
Figure 2 Degradation of Congo red (2.1 × 10−4 mol
dm−3) using CoNPs (3.2 × 10−3 mol dm−3) as a catalyst in
the presence of NaBH4 (10.0 × 10−3 mol dm−3) at differ-
ent time intervals. The peak ca. 495 nm shows the decrease
of absorbance intensity due to Congo red and 290–285 nm
shows the increase of absorbance intensity due to formation
of the ─NH2 group. [Color figure can be viewed at wileyon-
linelibrary.com]
[Congo red] [38]. Inspection of Fig. 3 indicates that ca.
86% and 97% Congo red can be absorbed after 8 and
4 min, respectively, at room temperature. Interestingly,
the degradation process is sigmoid in nature (the in-
duction period is followed by an autoacceleration) at
lower [CoNPs] (Fig. S2 in the Supporting Informa-
tion). At lower [CoNPs] (ࣘ1.6 × 10−3 mol dm−3),
absorbance remains constant (induction period) up to
ca. 4 min of the reaction time, which might be ascribed
to the time taken for the activation of catalyst surface
and/or transfer of electrons from NaBH4 to Congo red
through CoNPs. We observed that the induction pe-
riod has been terminated at higher [catalyst] as well as
[NaBH4] completely.
of azo groups that were no longer observed, whereas
control experiment (with or without NaBH4; Fig. 1)
showed no appreciable change in peak intensity for at
least 40 min to the observation period. Interestingly,
the appearance of a new peak (at 295 nm; indicated by
an arrow) and disappearance of azo groups peak at
495 nm, respectively, were noticed simultaneously
(Fig. 2). Finally, the reaction mixture shows a peak at
285 nm after of the completion of the reaction, which
might be due to the degradation and/or reduction of
the ─N═N─ group into the ─NH2 group. These re-
sults (appearance of peak at 285–290 nm) are in good
agreement to the observations of Pal et al. regarding
the use of AgNPs as a catalyst for the reduction of
aromatic nitro compounds into corresponding amino
compounds in the presence of NaBH4 [37].
Kinetics and Mechanism of Congo Red
Degradation
To determine the reduction rates of Congo red with
CoNPs + NaBH4, kinetics of these reactions was stud-
ied spectrophotometrically by monitoring the absorp-
tion peak decay at 495 nm with varying [CoNPs],
[NaBH4], and [Congo red] under pseudo–first-order
conditions with respect to [NaBH4] over [Congo red]
([NaBH4] ࣙ 10 times; Fig. 2). At lower [CoNPs] (ࣘ1.6
× 10−3 mol dm−3), degradation of Congo red has an
induction period, whereas at higher [CoNPs] (ࣙ3.2 ×
10−3 mol dm−3), the concentration of the Congo red
changed linearly with time. In the presence of sun-
light, degradation of Congo red has been completed
during the mixing time (ca 1 min; Fig. 4) under similar
reaction conditions. The representative plots of log (ab-
sorbance) versus time are shown in Fig. 5 for varying
[NaBH4] and [Congo red] in the absence and presence
The percentage of Congo red degradation was cal-
culated by the following formula, and its variation with
reaction time is shown in Fig. 3.
C0 − Ct
Congo red degradation(%) =
× 100 (1)
C0
where, C0 is the initial concentration of Congo red so-
lution and Ct is the concentration of the Congo red
solution at time t (min). All [Congo red] were mea-
sured by the absorbance value at 495 nm in UV–vis
spectra as the absorbance is directly proportional to the
International Journal of Chemical Kinetics DOI 10.1002/kin.21089

6
KHAN ET AL.
25
20
15
10
5
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Time(min)
103[Co0] (mol dm-3)
Pure congo red
Congo red + NaBH4
3.2
1.6
1 min ; NPs + congo red + NaBH4
0
0
5
10
15
-3
20
3
10 [NaBH ] (mol dm
)
4
Figure 6 Effects of [NaBH4] on the pseudo–first-order
rate constant to the degradation of Congo red (2.1 ×
10−4 mol dm−3) by CoNPs as a catalyst at room temperature.
[Color figure can be viewed at wileyonlinelibrary.com]
300
400
500
600
700
800
900
Wavelength(nm)
Figure 4 Effect of sunlight on the degradation of Congo
red (2.1 × 10−4 mol dm−3) using CoNPs (3.2 ×
10−3 mol dm−3) as a catalyst in the presence of NaBH4
(10.0 × 10−3 mol dm−3. [Color figure can be viewed at
wileyonlinelibrary.com]
200
150
100
0.8
0.4
1
3
-3
0
10 [Co ] (mol dm
)
50
0
3.2
1.6
0.0
2
-0.4
-0.8
-1.2
0
50
100
150
-1
200
250
5
1 CR + CoNPs
-
3
2 CR + CoNPs + BH4-
3 CR + CoNPs + BH
1[NaBH ] (mol dm )
4
4
4 CR + CoNPs + BH44-
-
Figure 7 Plots of 1/ kobs versus 1/[NaBH4] for Fig. 7 data.
[Color figure can be viewed at wileyonlinelibrary.com]
3
5 CR +CoNPs+ BH4 +sl
0
2
4
6
8
10
Time (min)
Figure 5 (A) Plot of log (absorbance) versus time for the
degradation of Congo red (2.1 × 10−4 mol dm−3) using
CoNPs (1.6 × 10−3 (◦) and 3.2 × 10−3 mol dm−3 (•), (ꢀ),
(ꢁ), and (ꢂ)) as a catalyst with and without NaBH4 = 0.0 (•),
10.0 × 10−3 mol dm−3 (◦), and 20.0 × 10−3 mol dm−3
(ꢀ, ꢂ ꢁ). [Color figure can be viewed at wileyonlineli-
brary.com]
order rates constant (kobs/s−1) were calculated from
the slopes of the plots of log(absorbance) versus time
as a function of [NaBH4] at two different [CoNPs].
These results are depicted graphically in Fig. 6 as a
kobs-[NaBH4] profiles, which shows that the Congo
red degradation follows fractional-order kinetics with
respect to [NaBH4] (plots of kobs against [NaBH4] de-
viate from the linearity). Furthermore, the plots of log
kobs versus log[NaBH4] are straight line (figures not
shown) with slopes = 0.70 and 0.83 indicating the or-
der with respect to [NaBH4] to be fractional. On the
other hand, the plot of 1/kobs versus 1/[NaBH4] is lin-
ear with a positive intercept and positive slope (Fig. 7).
Such a plot is indicative of Michaelis–Menten
of sunlight irradiation, respectively. Interestingly, the
induction period has also been terminated with higher
[NaBH4] = 20.0 × 10−3 mol dm−3 (Fig. 5A; ꢀ, ꢂ, and
ꢁ). This becomes obvious from Fig. 5, which shows the
smooth plots of log (absorbance) versus time obtained
for the degradation of Congo red. The pseudo–first-
International Journal of Chemical Kinetics DOI 10.1002/kin.21089

CATALYTIC ACTIVITY OF COBALT NANOPARTICLES FOR DYE AND 4-NP DEGRADATION
7
behaviors (kinetic proof for complex formation be-
tween Congo red-CoNPs and BH−4 .
Wenqi and his coworkers reported that the that the
peak position of Congo red aqueous solutions depends
solution, molar ratios of the reactants, as well as a mix-
ing order of NaBH4 have great influence on the mor-
phology of AgNPs and CoNPs [24,46]. To determine
the stoichiometry of as-prepared CoNPs sols, UV–
vis spectra were recorded at different Co2+/NaBH4
ratios as a function of time. These results indicate
that cobalt nanosols has no significant absorption peak
in the whole visible region (from 300 to 800 nm;
Fig. 1). Our spectra are in good agreement to the obser-
vations of Janata et al. regarding the formation of cobalt
sols (optical absorption increases smoothly in the UV
region without a maximum up to 200 nm) [35]. There-
fore, exact Co2+/NaBH4 ratios cannot be determined
under our exper−imental conditions. The Congo red
(–SO3−) and BH4 incorporated into the Stern layer of
of the pH of the working reaction mixture (λmax
=
576, 567, and 496 nm, respectively, at pH 2.18–3.16,
3.86, and ࣙ4.71 for the different structures of Congo
red) [39]. Stopa et al. reported the pKa of Congo red
by using used the acid–base titration method and sug-
gested that the ionization of Congo red amino groups
takes place at pH 4.5–5.5, which should be associated
with spectral transition [40]. At low pH, Congo red
becomes a cationic form and shows two tautomeric
forms of protonated Congo red, i.e., ammonium-rich
and azonium-rich species. The fresh solution shows the
predominance of the ammonium-rich variety whereas
after 1 h the azonium-rich variety predominated and
the isoelectric point was found to be at pH 3.0 [41].
Therefore, a series of experiments were performed to
determine the pH under different experimental condi-
tions (pH6.2, 6.2, 6.3, 6.2, 6.3, 6.3 and 8.1, 8.0, 8.1,
8.1, 8.1, 8.0 for [CTAB] = 0.7, 0.8, 1.0, 1.2, 1.4, 1.6 ×
10−3 mol dm−3 and [CoNPs] = 1.2, 1.3, 1.6, 2.0,
2.5, 3.2 × 10−3 mol dm−3 in the presence of fixed
[NaBH4] = 10.0 × 10−3 mol dm−3 and [Congo red]
= 1.4 × 10−4 mol dm−3). The pH values was found
to be nearly constant with increasing [CoNPs] in the
presence of [CTAB] = 0.8 × 10−3 mol dm−3. On
the other hand, addition of CoNPs and NaBH4 solu-
tions has no significant effect on the pH of Congo red
solutions under our experimental conditions. In aque-
ous solution, Congo red participates in the acid–base
equilibrium and ammonium- and azonium-rich species
exist in solution; concentrations of these species de-
pend on the pH. The azonium species is converted
into ammonium species due to the resonance delocal-
ization processes. Between about pH 5.0 and 6.0, the
azonium-rich species is the principal species.
CTAB micelles and formed ion pairs with the cationic
+
head group of CTAB micelles (CH3(CH2)15N(CH3)3
)
through strong electrostatic interactions [33]. The non-
polar naphthalene ring penetrates into the hydrophobic
core of CTAB. As a result, micelles concentrate the
all reactants and catalyst, CoNPs, into its small vol-
ume. Thus we may state confidently that the degra-
dation reactions involve the adsorption of Congo red
(acceptor) and NaBH4 (donor) on to the surface of
CoNPs, catalyst, and /or electron transfer mediator
(Scheme 1).
In Scheme 1, the first reaction represents the for-
mation of a complex between azonium species of
Congo red, BH−4 and CoNPs (Eq. (2)). In the next
step, the reductant (NaBH4; donor) releases an elec-
tron to the surface of the CoNPs from where the
─N═N─ linkage gains an electrons and is thus re-
duced. Florence reported the electrochemical reduction
of several azo compounds having different electron-
attracting or -releasing substituents [47]. They also
NH2
NH2
Pal and his coworkers suggested that a reaction be-
tween the dyes and NaBH4 has been considered to be an
electron transfer reaction in which NaBH4, dyes, and
metal nanoparticles act as a donor, acceptor, and elec-
tron transfer mediator, respectively [42]. Therefore, the
presence of catalyst certainly plays a vital role in these
degradation reactions as an electron relay effect [43].
The addition of nanocatalyst having a potential inter-
mediate between those of the donor and acceptor makes
the electron transfer between them very easy. Interest-
ingly, Congo red degradation occurs only in the pres-
ence of both CoNPs and NaBH4 in the present studies
(Figs. 1 and 2). It is well known that the morphologies
of the metal nanoparticles strongly depend on the na-
ture of the reducing agent as well as the presence on
the stabilizers [33,44,45]. The aging of NaBH4 aqueous
..
..
N
+
N
+
N
N
H
H
(PCR)
KC
nCo0
O
S
O-
O-
S
O
-
+ 2BH4
O
O
NH2
NH2
H
H
N
N
N
N
(2)
(3)
- - -
(Co0)n
- - -
(Co0)n
BH4
BH4
(Complex)
O
S
O-
O-
S
O
O
O
k, slow, rds
+6H2O
NH2
NH2
2
+ H2N-
-NH2 + 2H3BO3 + 4H2 + nCo0
O
S
O-
O
Scheme 1 Reductive degradation mechanism of Congo red
with BH−4 and CoHNPs.
International Journal of Chemical Kinetics DOI 10.1002/kin.21089

8
KHAN ET AL.
provided the evidence that reduction of aromatic azo
compounds is a four-electron process, which proceeds
through the formation of an unstable intermediate hy-
drazobenzene (most labile than the parent azocom-
pound). Its formation is generally not detected and
complete reductive cleavage of the azo linkage occurs,
resulting in the formation of aromatic amines, which
are considered a hazardous class of compounds be-
cause of their mutagenic and carcinogenic properties.
By analogy with previous results [47,48], we assume
that the Congo red–borohydride–CoNPs complex un-
dergoes the multielectron transfer oxidation–reduction
mechanism (Eq. (3)), which leads to the formation of
benzidiene [27], boric acid, and o-aminonaphthionic
acid [27,48]. The mechanism presented in Scheme 1
is in good agreement to the suggestions of Miller and
his coworkers regarding the involvement of colloidal
particles in the electron-transfer reactions via the elec-
tron relay effect [49]. Unfortunately, CoNPs’ spec-
tra do not have any well-defined noticeable surface
resonance plasmon absorption in the visible region
(Fig. 1). Therefore, we cannot draw any statement re-
garding the surface modification of nanoparticles at the
end of the degradation processes. The proposed mech-
anism is further supported by analysis of the products
(vide supra). A spot test for the benzidine was pos-
itive [27]. On the basis of the mechanism shown in
Scheme 1, the following rate law is derived:
Under pseudo–first-order conditions, the rate law
can be expressed by Eq. (6)
d[Congo red]
Rate = −
= kobs[CR]T
(6)
dt
Comparing Eqs. (5) and (6), the following relation-
ship is obtained:
k KC Ka[H+][BH−4 ]
1 + Ka[H+] + KC Ka [H+][BH−4 ]
kobs
=
(7)
Under our experimental conditions (ca. [H+] = 1.0
× 10−6 mol dm−3 and Ka = 1.0 × 10−5 mol dm−3), Ka
[H+] can be neglected in comparison to 1. Therefore,
Eq. (7) can be simplified to Eq. (8).
k KC Ka[H+][BH−4 ]
1 + KC Ka [H+][BH−4 ]
kobs
=
(8)
The derived rate law clearly accounts for the
fractional-order kinetics with respect to [BH−4 ] under
pseudo–first-order conditions, i.e., [BH−4 ] > [Congo
red]. Furthermore, Eq. (8) can be verified by rearrang-
ing it to the following equation:
1
1
1
=
+
(9)
k KC Ka [H+][BH−4 ]
kobs
k
d[Congo red]
Rate = −
= k KC Ka[H+][BH−4 ][CR]
dt
(4)
Equation (9) requires that plot of 1/kobs versus
1/[BH−4 ] at constant [H+] to be linear with positive
intercepts on the y-axis, and this was found to be so
(Fig. 7). From the intercepts and slopes of these plots,
the values of the rate constants of the slow step (k)
and KCKa [H+] could be calculated and were found
The total [Congo red] at any time t, is given as
[Congo red]T = [CR] + [PCR] + [complex]
to be 5.0 × 10−2
s
−1, 29.0 and 4.8 × 10−2
s
−1, 48.4,
respectively, for [CoNPs] = 1.6 × 10−3 and 3.2 ×
10−3 mol dm−3 at room temperature. Substituting the
values of KCKa [H+], k, and [BH−4 ] in Eq. (8), rate con-
stants, kcal have been calculated in various kinetic runs
(Table I). The close agreement between the kobs and kcal
provides the supportive evidence to the proposed mech-
anism (Scheme 1) and also conforms the validity of rate
law (Eq. (8)). Thus we may state confidently that the
Congo red oxidation followed the Michaelis–Menten
reciprocal relationship. Thus we may state confidently
that the derived rate law and Fig. 7 follow a general
Langmuir–Hinshelwood type mechanism:
[Congo red]T = [CR] + Ka[CR][H+]
+ KC Ka[CR][BH−4 ][H+]
[Congo red]T
[CR] =
1 + Ka[H+] + KC Ka[H+][BH−4 ]
Substituting the value of [CR] in Eq. (4), we get
d[Congo red]
k KC Ka[H+][BH4−][Congo red]T
1 + Ka[H+] + KC Ka[H+][BH−4 ]
−
=
d t
d[reactant]
k KC[reactant]
1 + KC[reactant]
Rate = −
=
(10)
(5)
dt
International Journal of Chemical Kinetics DOI 10.1002/kin.21089

CATALYTIC ACTIVITY OF COBALT NANOPARTICLES FOR DYE AND 4-NP DEGRADATION
9
Table I Comparison between the kobs and kcal for the Degradation of Congo Red (2.1 × 10−4 mol dm−3) by BH4− in
the Presence of CoNPs (3.2 × 10−3 mol dm−3) at 25° C.
103[CoNPs] (mol dm−3
)
1.6
3.2
103[BH−4 ]
(mol dm−3
)
103 kobs (s−1
)
103 kcal (s−1
)
(kobs kcal/kcal
)
103 kobs (s−1
)
103 kcal (s−1
)
(kobs – kcal/kcal)
0.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
18.0
20.0
0.0
5.5
8.2
10.2
12.5
15.0
15.7
17.5
18.7
19.2
0.0
5.4
8.2
10.4
12.3
15.8
15.7
17.3
18.8
19.4
0.0
8.1
0.0
8.1
+0.01
0.00
–0.01
+0.01
–0.05
0.00
0.00
0.00
11.2
13.8
15.5
17.8
20.0
21.1
22.2
23.1
11.2
14.0
16.3
18.3
19.8
20.7
23.2
24.2
–0.01
–0.05
–0.02
+0.01
+0.01
–0.04
–0.04
+0.01
0.00
–0.01
using the Eyring equation:
-1.64
-1.68
-1.72
-1.76
-1.80
-1.84
-1.88
-1.92
k = (kBT/h) exp(ꢀS#/R) exp(−ꢀH #/RT ) (11)
103[CoNPs] (mol dm-3)
slope = 1266.6
3.2
1.6
where the symbols have their usual meaning. The eval-
uation of Ea, ꢀH#, and ꢀS# of the CoNPs degradation
of Congo red provides useful information regarding
the catalytic role of CoNPs. The energy-profile dia-
gram (Fig. 9) clearly shows that the activation energy is
lower for the catalyzed reaction,whereas we did not ob-
served the degradation in the absence of borohydride.
A complete account for all the factors that influence
ꢀH# and ꢀS# is not possible because kobs does not
represent a single elementary step as it is a complex
function of association, ionization, and complexation
constants. Furthermore though a change in tempera-
ture, it is known to produce changes in the adsorption
of nanomaterials onto the surface of dye. Therefore,
adsorption phenomenon in an aqueous solution is ki-
netically important.
slope = 1333.3
3.24
3.26
3.28
3.30
-1
3.32
3.34
3.36
3
10 / T(K
)
Figure 8 Arrhenius plots for the degradation of Congo
red (2.1 × 10−4 mol dm−3) by CoNPs as a catalyst in the
presence of NaBH4 (10.0 × 10−3 mol dm−3). [Color figure
can be viewed at wileyonlinelibrary.com]
Overall observations to the degradation of Congo
red are also summarized in Table II. Surprisingly, in-
spection of Figs. 5 and 6 clearly suggests that the
sunlight irradiation has a significant impact on the
Congo red degradation and rate constant markedly in-
creases from 15.5 × 10−3 to 35.8 × 10−3 s−1 with sun-
light under similar reactant and catalyst concentrations
(Table II). It has been established that the radiation-
assisted mechanisms are quite different and/or have
different reaction paths in comparison to others [50].
The photocatalytic oxidization of H2O and the deox-
idization of O2 by photogenerated holes and photo-
generated electrons, respectively, occurred under UV
irradiation. In the present study, the presence of H2O
where k is a reaction rate constant and KC is an adsorp-
tion coefficient or complex formation constant.
Activation Parameters and Role of Sunlight
Irradiation
To determine the activation parameters, a series of re-
actions were carried out at different temperatures in
the range of 25–35°C. The Arrhenius plot of log(kobs
)
versus 1/T was linear, and the activation energy was
calculated from the slopes of Fig. 8. The enthalpy and
entropy of activation (ꢀH# and ꢀS#) were calculated
International Journal of Chemical Kinetics DOI 10.1002/kin.21089

10
KHAN ET AL.
Figure 9 Energy reaction coordinates profile to the degradation of Congo red (2.1 × 10−4 mol dm−3) by CoNPs as a catalyst
in the presence of NaBH4 (10.0 × 10−3 mol dm−3). [Color figure can be viewed at wileyonlinelibrary.com]
Table II Observations and Values of Various Parameters for the Degradation of Congo Red (2.1 × 10−4 mol dm−3
)
and 4-NP (1.0 × 10−4 mol dm−3) by NaBH4 (10.0 × 10−3 mol dm−3) in the Presence of CoNPs (3.2 × 10−3 mol dm−3).
The Average Linear Regression Coefficient, γ = 0.998, were Obtained for Each kinetic Run
Observations and Parameters
Reaction Conditions
103 kobs (s−1
)
Ea (kJ mol−1
)
ꢀH# (kJ mol−1
)
ꢀS# (J K−1 mol−1
)
Congo red + CoNPs
Congo red + BH−4
Stable; no degradation
Stable; no degradation
23
Congo red + CoNPs + BH4−
2.5
15.5
25
24
–106a
–107
21
Congo red + CoNPs + BH−4 + sunlight
Congo red + AgNPs
35.8; very fast
Stable; no degradation
Stable; no degradation
Stable; no degradation
Stable; no degradation
71
Congo red + AgNPs + BH4−
4-NP + BH−4
4-NP + CoNPs
4-NP + BH−4 + CoNPs
1.1
1.7
74
75
–110a
–110
72
a[CoNPs] = 1.6 × 10−3 mol dm−3
.
molecules in the reaction mixture is responsible for the
generation of the OH radical, which destabilize the azo
linkage (─N═N─) of the Congo red. The CoNPs might
have high surface-to-volume ratio, which can produce
a large amount of these active oxygenated species un-
der light illumination. As a result, the photocatalytic
efficiency of the used catalyst increases with sunlight
irradiation [51]. Finally, the photocatalytic degradation
rate of Congo red is higher in comparison to the nonra-
diation processes, whichmight beattributed to thepres-
ence of two reaction sites, i.e., azo groups (─N═N─)
in the Congo red. In respect that the •OH radical adds
onto the ─N═N─ double bond in a very fast reaction
and the conjugated system of Congo red molecule is
International Journal of Chemical Kinetics DOI 10.1002/kin.21089

CATALYTIC ACTIVITY OF COBALT NANOPARTICLES FOR DYE AND 4-NP DEGRADATION
broken easily. The •OH radical addition to the ─N═N─
11
bond produced the hydrazyl radical (N–N(OH) a sim-
ilar radical to –N–NH–. This reaction probably led to
the very fast destruction of the intensive color of the
Congo red (Eq. (12))
sun light
Congo red + CoNPs + NaBH4 → Products (12)
very fast
Reduction of 4-Nitrophenol to
4-Aminophenol
The catalytic activity of CoNPs was also evaluated
by using the reduction of 4-NP to 4-AP in the pres-
ence of NaBH4. The UV–vis spectra of 4-NP in water
were recorded in the absence and presence of NaBH4.
Figure 10 (A; red line) shows that 4-NP has an ab-
sorption peak at 315 nm in an aqueous solution. It is
well known that the aqueous NABH4 solution is un-
stable and alkaline in nature (pH ca. 8.2) due to the
hydrolysis of NaBH4 and sodium metaborate. Upon
addition of NaBH4, the peak was shifted from 315
to 400 nm (redshift, ca. 75 nm), which might be due
to the formation of 4-nitrophenolate in alkaline con-
ditions (Fig. 10A; black line). Time-resolved UV–
vis spectra of 4-NP + NaBH4 reaction mixture were
also recorded. Surprisingly, 4-nitrophenolate peak at
400 nm remained unchanged for ca. 24 h, indicating
that only NaBH4 is not in position to reduce the 4-NP
under our experimental conditioned. These observa-
tions are summarized in Scheme 2.
Scheme 3 Mechanism to the reduction of 4-nitophenolate
by BH−4 in the presence of CoNPs.
Equation (1) was used for the calculation of percent-
age reduction of 4-NP. Reaction–time profiles sug-
gested that the 60% 4-NP has been reduced within
10 min of the reaction time (Fig. 10B). The pseudo–
first-order rate constants were also calculated from the
slopes of the plots of log(absorbance) versus time by
using a fixed time method (Fig. 10C) for two [CoNPs]
at a constant [NaBH4]. In a typical experiment, the
used CoNPs are separated from the reaction system
(CoNPs + 4-AP) by using a magnetic bar. The solu-
tions were decanted, and a magnetic bar containing
the CoNPs was dispersed in the double distilled water.
The resulting black particle dispersion solution was
reused for the degradation of 4-NP under similar ex-
perimental conditions. These experiments suggest that
the reusability of the CoNPs catalyst exhibits a sim-
ilar catalytic performance for running eight cycles of
the reaction (Fig. 10D), which might be due to the
excellent stability and a long life of the as-prepared
CoNPs catalyst, which are in accordance with the re-
sults of Li et al. to the reduction of 4-NP by CoNPs [30].
The activation energy, enthalpy of activation, and en-
tropy of activation were also calculated by using Ar-
rhenius and Eyring equations. These values are given
in Table II along with the rate constants. Inspection
of Fig. 10A clearly indicates the formation of 4-AP
as a sole product to the reduction of 4-NP (disap-
pearance of the 400-nm peak with the appearance of
new peak at ca. 315 nm). From these observations,
the overall mechanism for the reaction is presented in
Scheme 3.
In Scheme 2, Eqs. (13) and (14) represent the
hydrolysis of NaBH4. 4-NP converted into the 4-
nitrophenolate in alkaline medium (Eq (15)).
Interestingly, the absorbance of 4-nitrophenolate
decreases with time in the presence of NaBH4
+
CoNPs, indicating that the NABH4 and CoNPs acted
as a source of hydrogen and catalyst, respectively,
and reduction of 4-NP to 4-AP takes place (Fig. 10).
NaBH4 + 2H2O
NaBO2 + 4H2
(13)
(14)
NaBO2 + H2O
OH
NaOH + H3BO3
O-
+ OH-
+ H2O
(15)
In Scheme 3, the redox reaction proceeds through
the formation of a complex between 4-nitrophenolate,
CoNPs, and BH−4 (Eq. (16)). In the next step,
the complex undergoes to the four-electron transfer
(rate-determining step) from the BH−4 (donor) to 4-
nitrophenolate (acceptor) via CoNPs (electron transfer
-
-
NO2
NO2
Scheme 2 Formation of 4-nitrophenolate in the presence
of NaBH4.
International Journal of Chemical Kinetics DOI 10.1002/kin.21089

12
KHAN ET AL.
80
70
60
50
40
30
20
10
0
3.0
2.5
2.0
1.5
1.0
0.5
0.0
B
A
-
4NP + BH4
4NP
4NP+BH4-+CoNPs; Time=2 min
4
6
8
10
12
14
16
18
20
22
24
26
3
-3
10 [CoNPs] (mol dm
)
1.6
3.2
0
5
10
15
20
25
30
200
300
400
500
600
700
Wavelength(nm)
Time (min)
C
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
-0.4
D
4-NP + BH4-
4-NP + BH4-+ CoNPs
4-NP + BH4- + coNPs
60
50
40
30
20
10
0
103[CoNPs] (mol dm-3)
0.0
1.6
3.2
0
1
2
3
4
5
6
7
8
0
4
8
12
16
20
24
28
Cycle number
Time (min)
Figure 10 Reduction of 4-NP by CoNPs in the presence of NaBH4 (A). Degradation percentage of 4-NP (B), pseudo–first-
order plots (C)) for the reduction of 4-NP to 4-AP, and reusability test of the CoNPs (D). Reaction conditions: [CoNPs] =
3.2 × 10−3 mol dm−3, [NaBH4] = 2.0 × 10−3 mol dm−3, [4-NP] = 1.0 × 10−4 mol dm−3. [Color figure can be viewed at
wileyonlinelibrary.com]
mediator) and results in the formation of 4-AP
(Eq. (17)) [30,37].
Comparison with AgNPs
To compare the catalytic efficiency of CoNPs with
other nanomaterials, for the degradation of Congo
red, a series of kinetic runs were performed by using
AgNPs as a catalyst. Under our experimental condi-
tions, [Congo red] = 2.1 × 10−4 mol dm−3, AgNPs
= 3.2 × 10−3 mol dm−3 (prepared by the reduction
of AgNO3 with NaBH4 in the presence of CTAB;
Fig. 11), NaBH4 = 10.0 × 10−3 mol dm−3, temper-
Figure 11 TEM image of AgNPs prepared by the reduction
ature = 25 °C, the degradation of Congo red is not
of Ag+ by NaBH4 in the presence of CTAB.
observed for a short traction time, i.e., 2 h, whereas the
International Journal of Chemical Kinetics DOI 10.1002/kin.21089

CATALYTIC ACTIVITY OF COBALT NANOPARTICLES FOR DYE AND 4-NP DEGRADATION
13
blueshift) from 1474 to 1376 cm−1 and peak inten-
sity also decreased for the CTAB–CoNPs. Compar-
ison of the all FTIR spectra clearly shows that the
nano α-cobalt hydroxide is stabilized and capped by
CTAB. This is not very surprising judgment from the
fact that cationic (–N(CH3)3+) plays an important role
in the complexation and ion pair formation with a re-
actant (BH−4 ) and resulting positive charge hydroxide
layers and/or sheet-like nanosized α-cobalt hydroxide
through electrostatic, hydrogen bonding, and van der
Walls interactions [14]. EDX analyses show the types
and nature of chelating agents that influence the mor-
phology of the synthesized nanomaterials. Thus, we
may state confidently that stabilizers have a significant
impact on the CoNPs size and shape [27].
Figure 12 shows the TEM images, and correspond-
ing SAED images of the as-prepared α-cobalt hy-
droxides having fibrillar turbostratic needles-like sheet,
multilayered aggregated sheet, and dumbbell shaped
morphology for the absence and presence of CTAB
and PVA, respectively. The turbostratic morphology
is a characteristic of the α-cohydroxides nanoparti-
cles [54]. Our TEM images do not consist of hexago-
nal platelet morphology, which is the characteristic of
β-cobalt hydroxides [15]. Cobalt hydroxide nanoparti-
cles were formed at the beginning. Then, the nanoparti-
cles were aggregated to form the nanoneedles. Finally,
the nanoneedles self-assembled and/or aggregated
forming beautiful cobalt hydroxide nanostructures to
minimize the surface energy of nanoneedles [22].
On the basis of the observed results (blue color,
optical images, and TEM images (Fig. 12), it is
concluded that the as-prepared materials are mainly
made up of α-cobalt hydroxides with the platelet
morphologies.
degradation is very fast in these experimental condi-
tions with CoNPs (Figs. 2 and 3 and Table II). There-
fore, the catalytic activity of CoNPs and AgNPs is in
the order CoNPs > AgNPs. This is not very surpris-
ing judging from the fact that Congo red has a strong
complex-forming tendency with AgNPs, which might
be due to electrostatic interactions between the nega-
tive charge (–OSO3−) of Congo red and positive surface
of AgNPs, i.e., Ag42+ [38].
Morphology of Nanocatalyst
Figure S2 (in the Supporting Information) shows the
FTIR spectra of α-cobalt hydroxide and compares it
with that of pure NaBH4 and CTAB. In both spectra,
a broad band centered at ca. 3441, and 3441 cm−1 for
BH−4 - and CTAB-capped materials, respectively, re-
veals that the hydroxyls is involved in hydrogen bond-
ing with the intercalated water molecules. Our spectra
do not consist of a sharp peak at 3630 cm−1 (charac-
teristic peak for free hydroxyl groups in brucite-like
structures), ruling out the possibility to the formation
of β-cobalt hydroxide. The band at ca. 1614, 1640,
1627, and 1627 cm−1 might be attributed to the bend-
ing vibration mode of the interlayer water molecules
in both samples. Inspection of all spectra clearly indi-
cates that the O–H peak positions in the region from
3500 to 3000 cm−1 and bending water molecules are
strongly dependent on the presence and nature of the
stabilizers. These spectral features are in good agree-
ment with the results of Gedanken et al. [1], Kamath
et al. [14], and Fernandez et al. [52] regarding the
synthesis of α-cobalt hydroxide nanomaterials under
different experimental conditions.
Figure S2A (in the Supporting Information) has no
vibration frequencies in the boron–hydrogen stretch-
ing region (from 2300 to 2100 cm−1) [53], conforming
that all BH−4 have been oxidized by Co2+ ions and/or
hydrolyzed by water simultaneously. The nitrate group
is FTIR active and shows various strong vibrations
at 1400–900 cm−1 region that can be seen in FTIR
spectra of the α-CoNPs intercalated with nitrate an-
ions (Fig. S2A in the Supporting Information). Since
FTIR analysis is a very sensitive technique, it can be
concluded that the NO3− group is the only anion (from
the salt reagents) incorporated in the cobalt hydroxide
nanomaterials. The infrared spectrum is characterized
by a broad band centered at 3450 cm−1, characteristic
of the OH stretching vibration. The peaks observed at
3021, 2931, 2848, and 1474 cm−1 are assigned to the
CH2-stretching vibrations, antisymmetric deformation
of alkyl chains, and C–N vibrations in pure CTAB, re-
spectively (Fig. S2B in the Supporting Information).
The C–N vibrations peak of pure CTAB shifted (little
It has been established that layered double α-cobalt
hydroxides (isostructural with hydrotalcite like) con-
sist of positively charged Co(OH)2–x layers and anions
(NO3−, CO32−, Cl−, SO42−, etc.) as charge balancing
in the interlayer gallery. On the other hand, the β-
cobalt hydroxides (brucite-like; stoichiometric phase
of the composition Co(OH)2) has hexagonal packing
of hydroxyl ions with Co(II) occupying alternate rows
of octahedral sites. The α-hydroxides have a larger
˚
interlayer spacing (>7 A, dependent on intercalated
˚
anions) than that of the β-form (4.6 A) [55]. Out of
these stabilizers, as-prepared α-cobalt hydroxides has
the same morphology (needles-shape aggregated and
trubostatic) in the absence and presence of CTAB. On
the other hand, morphology has been changed drasti-
cally in the presence of PVA, which might be due to
the electrostatic repulsion and/or interaction between
the polar positive (–N(CH3)3+) head group of CTAB
with Co2+ ions, BH−4 ions, and positive charge surfaces
International Journal of Chemical Kinetics DOI 10.1002/kin.21089

14
KHAN ET AL.
Figure 12 TEM and SAED images of CoNPs without stabilizer (A), with CTAB (B), and PVA (C).
of α-cobalt hydroxides. PVA–OH groups lone pair
electrons are also responsible for the such types of
interactions. The XPS spectrum of the Co2p (where
2 = n (principle quantum number), and p = (az-
imuthal quantum number) region for the CoNPs was
also recorded. It has been established that the XPS
spectra of cobalt has various peaks at 779 and 793
eV for Co0, 780 and 795 eV for Co2+ ions, and 778
and 794 eV for Co3+ ions [28,56]. The as-prepared
CoNPs have characteristic peaks of Auger parameter,
Co2p3/2 and Co2p1/2 (where 2 and p have their useful
significance), at 779 and 795 eV (Fig. 13), which can be
attributed to the metallic Co0. These results are in good
agreement to the observations of Li and his cowork-
ers regarding the presence of typical valence state of
Co0 [28,31].
Co 2P
3/2
Co 2P
1/2
760
770
780
790
800
Binding Energy(ev)
Figure 13 XPS of synthesized CoNPs. [Color figure can
be viewed at wileyonlinelibrary.com]
The XRD pattern of as-synthesized α-cobalt hy-
droxide nanoparticles are given in Fig. 13. From the
XRD pattern, an obvious main peak of Co can be
observed, but it is difficult to judge the Co phase
(Fig. 14). The absence of the characteristic diffraction
peaks for Co nanoparticles shows that the α-hydroxides
are poorly ordered phases and exhibit broad bands in
their XRD patterns. Therefore, resulting nanomateri-
als could be amorphous, since the pattern is feature-
less (Fig. 14), which might be due to the amorphous
in nature, and to the presence of interlamellar water
molecules bound to the –OH groups through hydro-
gen bonding. This is not unexpected, as blue-colored
α-cobalt hydroxides obtained in the presence of
organic additives (glucose, lactose, fructose, glycerol,
and citric acid) shows broad peaks and scaly particles in
the XRD patterns and TEM images, respectively [16].
These results are in good agreement to the observations
of Liz–Marzan regarding the synthesis of amorphous
nanosize cobalt materials [57]. However, the peak po-
sitions at 2θ = 44.8, 47.1, and 49.5° can be indexed as
α-cobalt [58]. No distinct peak corresponding to CoO
or CoB phases is detected, indicating that a single α-Co
phase is obtained. The synthesis of cobalt hydroxide in
International Journal of Chemical Kinetics DOI 10.1002/kin.21089

CATALYTIC ACTIVITY OF COBALT NANOPARTICLES FOR DYE AND 4-NP DEGRADATION
15
a simple sonication method that leads to autoconver-
sion of α-cobalt hydroxides to β-phase under a high
pH value [15]. In the present study, we did not used an
aqueous NaOH to maintain the high pH condition to
avoid the formation of β-hydroxides. It was observed
that the pH of the reaction mixture is ca. 8.0 under our
experimental conditions, which is attributed to the hy-
drolysis of used NaBH4 by water. On the basis of above
results, we may safely conclude that the morphologies
of CoNPs depend on the presence of a stabilizer under
identical experimental conditions (Scheme 4) [32].
900
800
700
600
500
400
A)
10
20
30
40
50
60
70
80
SUMMARY
°
2θ
We demonstrated a simple method to the synthesis
of α-cobalt blue-colored nanosols in the absence and
presence of CTAB and PVA. The characterization of
nanoparticles was conducted using UV–vis, FTIR, and
TEM techniques. These results confirmed that stabi-
lizers have significant impacts on the morphology of
as prepared cobalt nanoparticles. The crystalline na-
ture and purity of nanoparticles were also confirmed
by XRD spectra. We used resulting CoNPs as a cata-
lyst for the degradation of Congo red and 4-NP in an
aqueous solution. The complete degradation of Congo
red takes place within ca. 6 min at room temperature
in the presence of CoNPs and BH−4 , which may be at-
tributed to the adsorption of CoNPs -BH−4 and Congo
red through electrostatic as well as van der Waals in-
teraction. The degradation process has an induction
period followed by an autocatalyst path. The induction
period strongly depends on the [CoNPs] and [NaBH4]
B)
1000
900
800
700
600
500
400
10
20
30
40
50
60
70
80
°
2θ
Figure 14 XRD of CoNPs without a stabilizer (A) and with
CTAB (B).
Scheme 4 Formation of different morphologies by using CTAB and PVA as the stabilizing agents.
International Journal of Chemical Kinetics DOI 10.1002/kin.21089

16
KHAN ET AL.
under pseudo–first-order conditions. Sunlight irradia-
tion increases the catalytic efficiency of the CoNPs by
excitation of electrons from a conduction band to a va-
lence band. AgNPs are not a suitable catalyst for the
removal and/or degradation of Congo red under our
experimental conditions.
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