Effect of operational key parameters on photocatalytic degradation of phenol using nano nickel oxide synthesized by sol-gel method

Article abstract of DOI:10.1016/j.molcata.2010.12.011

Photocatalytic oxidation of phenol was studied using nickel oxide (NiO) nanoparticles synthesized by sol-gel method. The morphology of synthesized catalyst was studied by using field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDX) and high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). The average particle size was 6.5 nm as estimated by Scherrer formula and was confirmed by HRTEM and FESEM analysis. The photocatalytic activity of NiO was evaluated by degradation of phenol under irradiation of unique source like UV laser. The influence of various key parameters such as amount of photocatalyst, initial solution pH and the initial concentration of phenol was investigated. The operational parameters show the expected influence regarding the efficiency of the photocatalytic degradation process. The photocatalytic degradation efficiency of NiO was found to decrease with the increase in the initial solution pH. The degradation of the phenol followed a pseudo first-order rate kinetics with k = 5.7 × 10^-2 min^-1.

Full text of DOI:10.1016/j.molcata.2010.12.011

Journal of Molecular Catalysis A: Chemical 336 (2011) 64–71
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Effect of operational key parameters on photocatalytic degradation of phenol
using nano nickel oxide synthesized by sol–gel method
Khizar Hayata, M.A. Gondalb,∗, Mazen M. Khaleda, Shakeel Ahmedc
a Chemistry Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
b Laser Research Group, Physics Department and Center of Excellence in Nano Technology (CENT), King Fahd University of Petroleum & Minerals, Box 372, Dhahran 31261,
Saudi Arabia
c Center for Refining and Petrochemical, Research Institute, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history:
Photocatalytic oxidation of phenol was studied using nickel oxide (NiO) nanoparticles synthesized by
sol–gel method. The morphology of synthesized catalyst was studied by using field emission scan-
ning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDX) and high resolution
transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). The average particle size was
6.5 nm as estimated by Scherrer formula and was confirmed by HRTEM and FESEM analysis. The
photocatalytic activity of NiO was evaluated by degradation of phenol under irradiation of unique
source like UV laser. The influence of various key parameters such as amount of photocatalyst, ini-
tial solution pH and the initial concentration of phenol was investigated. The operational parameters
show the expected influence regarding the efficiency of the photocatalytic degradation process. The
photocatalytic degradation efficiency of NiO was found to decrease with the increase in the initial
solution pH. The degradation of the phenol followed a pseudo first-order rate kinetics with k = 5.7 ×
Received 24 August 2010
Received in revised form
14 December 2010
Accepted 16 December 2010
Available online 24 December 2010
Keywords:
Nano nickel oxide
Heterogeneous photocatalysis
Phenol
Waste water treatment
Nanoparticles
10−2 min−1
.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Nanopowders could be prepared with narrow size distribu-
tion, higher purity, well dispersed and spherical shape by this
method. Moreover, the physical and chemical properties of the
nanopowders might be easily controlled by varying the processing
parameters and no need of expensive agents or special equip-
ment.
The uses of toxic compounds are creating a multiple environ-
mental problems. There is an urgent demand to develop a method
for solving these problems. Among these, removal of phenol and
chlorinated phenols became more essential because of their high
toxicity, extensive use and their weak retention in soil sediments.
Sources of phenol include waste solutions in various industries,
such as petroleum refining and plastics.
Traditional wastewater treatment techniques include activated
carbon adsorption, chemical oxidation and biological digestion, in
which the photoassisted catalytic decomposition of organic pol-
lutants in water and wastewater employing semiconductors as
photocatalysts has been a promising technique during the past two
decades [24–29].
The photocatalytic process starts with the irradiation of a
semiconductor material by light with sufficient energy to excite
the electrons form the valence to the conduction band generat-
ing extremely reactive electron/hole (e−/h+) pairs which migrate
to the surface of the semiconductor where they can react with
Nanomaterials have recently attracted extensive interest for
their unique properties in various fields as compared to their
bulk counterparts [1–6]. Semiconductor metal oxide nanopar-
ticles have been studied due to their novel optical, electronic,
magnetic, thermal and mechanical properties and potential appli-
cation in catalyst, gas sensors and photo-electronic devices [7–11].
More research has been focused on the types and modifica-
tion of the catalyst, the photocatalytic principles, the factors
influencing photocatalytic rate and the reaction kinetics [12–17].
Development of new photocatalysts for waste water treatment
is of current interest [18]. These days, much attention has been
focused to synthesize high-quality and ultra-fine powders with
the required characteristics in terms of their size, morphol-
ogy and microstructure. There are different methods reported
for the synthesis of NiO nanoparticles, such as hydrothermal
synthesis, carbonyl, and pulsed laser ablation, microemulsion,
precipitation–calcination and sol–gel methods [19–23]. Compared
with the conventional methods, sol–gel has many advantages.
∗
Corresponding author. Tel.: +966 38602351; fax: +966 38602293.
E-mail address: magondal@kfupm.edu.sa (M.A. Gondal).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.12.011

K. Hayat et al. / Journal of Molecular Catalysis A: Chemical 336 (2011) 64–71
65
adsorbed species.
6000
NiO/SC + hꢀ → e− + h+
(1)
5000
(O2)ads + e− → O2
(2)
−
•
H2O → OH− + H+
(3)
4000
•
O2 − + H+ → HOO•
(4)
•
3000
The hydroperoxyl radical formed in (4) also has scavenging prop-
erty as O2 thus doubly prolonging the lifetime of photohole:
2000
1000
0
HOO• + e− → HO2
HOO− + H+ → H2O2
(5)
(6)
−
The ability of the photocatalytic technique to completely degrade
organics into water and CO2, without generating any harmful
byproducts, has popularized its role as a wastewater purifier [30].
In the present work, a novel laser-based photocatalytic process has
been tested for the degradation of phenol using NiO. In case of laser,
there is high absorption of photons at a particular laser emission
wavelength by photocatalysts as compared with broad spectral
sources like lamps where most of the emission wavelengths lie out-
side the band gap energy of the catalyst. In addition, the drawback
associated with the photo-degradation process with conventional
setups using lamps is the longer irradiation time (hours or days) and
thermal stability of UV lamp over longer periods of time [31–33].
Our purpose of use of laser as source of excitation in this study is
to optimize the photo-degradation process and to identify the key
parameters for optimization process and once these parameters
are optimized, laser could be replaced with solar radiation for pilot
plant studies.
The band gap of nano NiO is in the range of 3.6–3.8 eV as
reported in the literature [34,35] therefore 266 nm wavelength
(UVC) was applied for the optimization of the process for the first
time in this work to enhance the degradation process. TiO2 is
well studied catalyst for photocatalytic applications [36]. However,
for improvement of efficiency and to enhance the reaction rate,
work on other catalysts like ZnO, NiO, Fe2O3 and MnO2 is being
investigated worldwide [37]. Under our experimental conditions,
nano-NiO catalysts demonstrated better photocatalytic efficiency
for degradation of phenol at 266 nm as compared with TiO2. The
reason could be the suitability of band gap for NiO at 266 nm and
the nano structured nature of the self synthesized NiO applied in
this work.
20
40
60
80
2 Theta (deg)
Fig. 1. X-ray diffractrogram of nano NiO calcined at 500 ◦C.
monohydrate were accurately weighed and dissolved in deionized
water. Subsequently, the mixed solution was heated at a temper-
ature of 65 ◦C with continuous stirring for three days. The green
gel so obtained was dried at 100 ◦C for 24 h. The precursors were
heated in a programmable furnace at the rate of 1 ◦C/min to achieve
the temperature of 500 ◦C and allowed to stay for 6 h at this tem-
perature.
The crystalline structure of the product was characterized by
a rotating-target X-ray diffractometer (Japan Rigaku D/Max-2400)
equipped with a monochromatic high-intensity Cu K␣ radiation
˚
(=1.5418 A, 40 kV, 100 mA). The average crystalline size of the NiO
nanoparticles was estimated from the full width half maximum
(FWHM) and the peak position of an XRD line broadened according
the Scherrer formula [38]:
0.9ꢁ
B cos ꢂ
d =
(7)
where d is the average crystallite size, ꢁ is the wavelength of the
X-ray (0.15418 nm), B is the full width at half maximum (FWHM,
radian) and ꢂ is the Bragg angle (degree). The value of FWHM
was obtained by performing profile fitting using an XRD pattern
processing software. The mean particle size of the as-synthesized
products is calculated according to this at about 6.5 nm and further
confirmed with the help of SEM and TEM results.
In this paper, NiO nanoparticles were synthesized by sol–gel
method and characterized by the use of X-ray diffraction (XRD),
field emission electron microscopy (FESEM) and high resolution
transmission electron microscopy (TEM). The photocatalytic activ-
ity of nano NiO was investigated for the oxidation of phenol in
aqueous solution. The effect of key operational parameters on phe-
nol degradation process such as amount of catalyst, initial pH and
phenol concentration was also studied.
The peaks positions appear at 2ꢂ = 37.20 ◦, 43.20 ◦, 62.87 ◦,
75.20 ◦ and 79.38 ◦ as depicted in Fig. 1 and can be indexed as (1 1 1),
(2 0 0), (2 2 0), (3 1 1), and (2 2 2) crystal planes of the bulk NiO,
respectively. All these diffraction peaks can be perfectly indexed
to the face centered cubic (FCC) crystalline structure of NiO which
is in accordance with that of the standard spectrum (JCPDS No. 04-
0835). The XRD pattern shows that the samples are single-phase
and no any other impurities distinct diffraction peak except the
characteristic peaks of FCC phase NiO was detected.
2. Experimental
Fig. 2(a) and (b) shows the FESEM image and EDX of NiO
nanoparticles. It is evident from Fig. 2(a) that the NiO nanoparti-
cles had spherical shapes and were well dispersed with smooth
surface and uniform size. Fig. 3 shows a HRTEM micrograph of
the NiO nanoparticles. The results of FESEM and HRTEM observa-
tions further confirmed and verified the relevant results obtained
by XRD.
2.1. Materials
Chemicals such as Ni (NO3)2·6H2O and citric acid monohydrate
used in the preparation of nano-NiO particles, were of analytical
grade and were used without further purification. Deionized water
was used throughout the study. Phenol used in this study was pur-
chased from Fluka. Methanol of HPLC grade was used as solvent.
2.3. Photocatalytic degradation of phenol
2.2. Catalyst synthesis and its characterization
Photocatalytic activity of the synthesized photocatalyst was
evaluated using aqueous solution of phenol under ultraviolet laser
(266 nm) irradiation. The experiments were carried out in a locally
Nickel oxide nanoparticles were prepared using the sol–gel
process. Appropriate amount of Ni (NO3)2·6H2O and citric acid

66
K. Hayat et al. / Journal of Molecular Catalysis A: Chemical 336 (2011) 64–71
Fig. 2. (a) Field emission scanning electron micrograph of nano NiO. (b) Energy dispersive X-ray spectroscopy of nano NiO.
designed photocatalytic reactor with quartz window for illumina-
tion of UV laser irradiation. The stock solution of phenol containing
100 mg L−1 was prepared in deionized water. The photoreactor was
loaded with 100 mL of aqueous suspension of nano NiO in phe-
nol. The aqueous suspension was magnetically stirred in the dark
for 15 min to ensure the establishment of adsorption equilibrium.
Then, the suspension was irradiated with UV light at constant stir-
ring speed and laser energy. Nd: YAG laser (Brilliant B Quantel,
ꢁ = 266 nm, pulse duration = 6 ns) was used to irradiate the solution.
After regular intervals, 5.0 mL of the sample was withdrawn from
the reactor and filtered using nano syringe filters. High performance
liquid chromatograph (Waters 2996 Photodiode Array Detector,
Waters 1525 Binary HPLC pump and Waters 717 plus Autosampler)
was used to analyse the laser irradiated samples. The station-
ary phase consisted in a Purospher Star RP-18 endcapped column
(250 mm × 4.6 mm, 5 ␮m particles) working at room temperature.
A mixture of water and methanol with a gradient concentration
was used as the mobile phase at a flow rate of 1.0 mL min−1. The
removal of phenol after time t was calculated by peak area method.
To study the effect of pH on photodegradation rate, the initial pH
of the suspension was adjusted by the addition of either NaOH or
HNO3 solutions.
3. Results and discussion
3.1. Effect of laser UV irradiation and photocatalyst containing
phenol using nano NiO
Photocatalytic degradation of phenol was investigated without
NiO catalyst as well as in the absence of laser UV light. Only nano
NiO catalyst without UV laser irradiation could not catalyze the
degradation of the phenol. It was observed that phenol degradation
was not significant in the absence of laser or nano NiO. The results
indicate that both the catalyst and light are essential for efficient
phenol degradation. The photodegradation of phenol in aqueous
solution as a function of laser irradiation time in the presence of
nano-structured NiO is shown in Fig. 4.
The rate constant obtained from the plot shown in Fig. 5 was
5.7 × 10−2 min−1. The percentage degradation of phenol was cal-

K. Hayat et al. / Journal of Molecular Catalysis A: Chemical 336 (2011) 64–71
67
Fig. 3. High resolution transmission electron micrograph of nano NiO.
culated using the following relation:
1.2
1
1 − Ct
Phenol degradation (%) =
× 100
(8)
C0
where C0 = initial concentration of phenol and Ct = concentration of
phenol at any time ‘t’.
The results demonstrate that almost 97% of phenol was
degraded within 60 min using laser irradiation by nano NiO cat-
alyst. The results obtained in this study demonstrate that the
percentage of phenol degradation increases with the increase in
laser irradiation exposure time. This increase in phenol degradation
might be due to increase in the amount of produced OH• radicals
with time.
0.8
0.6
0.4
0.2
0
Without Catalyst
nano NiO
3.2. Effect of catalyst weight on photocatalytic degradation of
phenol using nano NiO
The optimum weight of catalyst is an important parameter to
be investigated to avoid ineffective excess of catalyst and to ensure
the total absorption of efficient photons. For this study, the weight
of nano NiO in the suspension was varied from 1.0 to 6.0 g L−1
.
0
20
40
60
An increase in the catalyst loading up to an amount of 6.0 g L−1
resulted in a decrease in the rate constant as depicted in Fig. 6
which can be attributed to a screening effect due to the redun-
dant dispersion of UV radiation caused by the ample amount of
suspended photocatalyst. In addition, particles tend to agglomer-
ate, making a significant fraction of the catalyst to be inaccessible
to either adsorbing the molecules or absorbing the laser radiations,
with consequent decrease in the active sites available to the cat-
alytic reaction [39,40]. The results presented in Table 1 indicate
Time (min)
Fig. 4. A plot showing the degradation of phenol as a function of laser exposure
time for an aqueous solution of phenol in presence of NiO. Experimental conditions:
phenol concentration = 1.064 mM, V = 100 mL, NiO = 1.0 g L−1, and laser exposure
time = 60 min.
Table 1
Effect of key parameters on photo-degradation rate of phenol using nano-NiO.
Parameters
Rate constant k (min−1
)
R2
Photodegradation rate (␮M min−1
)
1.0
2.0
3.0
4.0
6.0
0.0570
0.0540
0.0481
0.0433
0.0321
0.9641
0.9809
0.9732
0.9643
0.8879
60.65
57.46
51.18
46.07
34.15
Catalyst loading (g L−1
)
5.0
6.5
9.0
0.0663
0.0582
0.0254
0.0115
0.9885
0.9727
0.9475
0.9315
70.54
61.93
27.03
12.24
Initial solution pH
11.0

68
K. Hayat et al. / Journal of Molecular Catalysis A: Chemical 336 (2011) 64–71
4
3.5
3
2.5
2
1.5
1
0.5
0
0
20
40
60
Time (min)
Fig. 5. A plot of ln C0/C as a function of laser irradiation time for an aqueous solution of phenol in presence of NiO.
that the photodegradation rate decreases as the amount of cata-
lyst increases from 1.0 to 6.0 g L−1. Following these observations,
it was decided to keep the amount of nano NiO at the opti-
mum value of 1.0 g L−1 in subsequent photocatalytic degradation
experiments.
the results presented in Table 1 that the photoefficiency of the
process decreases and phenol removal becomes slow with a k
of 1.15 × 10−3 min−1 at pH 11. Similar behavior of photocata-
lysts was also reported by some researchers during their studies
[41–43].
Nano NiO shows amphoteric behavior in aqueous media. Elec-
tric charge properties of both, catalyst and substrate, are found to
play an important role on adsorption process. The point of zero
charge (pzc) of nano NiO, i.e. the point at which the surface charge
density is zero, was found to be of 10.5 [44]. At pH higher than
pHpzc nano NiO surface is negatively charged and NiO•− appears
to be the predominant form of NiO. For lower pH values, nano
NiO surface is in the protonated form (Ni OH2+). Phenol shows
a pKa of 9.9 (at 25 ◦C) under aqueous media. If pH < pKa, phenol
3.3. Effect of pH on photocatalytic degradation of phenol using
nano NiO
The pH of the suspension was varied in the range (5.0–11.0) to
study its effect on the photocatalytic degradation of phenol. The
plot of pH versus ln C0/C is depicted in Fig. 7. It is evident from
the results that the photocatalytic degradation rate was increased
when pH was decreased from natural to pH = 5. It is clear from
4
1.0 g L-1
3.5
2.0 gL-1
3
3.0 gL-1
4.0 gL-1
2.5
6.0 gL-1
2
1.5
1
0.5
0
0
20
40
60
Time(min)
Fig. 6. The change of rate constant as a function of catalyst loading for photocatalytic degradation of phenol using NiO.

K. Hayat et al. / Journal of Molecular Catalysis A: Chemical 336 (2011) 64–71
69
4.5
4
5
0.532 mM Phenol
4.5
4
1.064 mM Phenol
1.596 mM Phenol
2.128 mM Phenol
2.66 mM Phenol
pH =5.0
pH =6.5
pH =9.0
pH =11.0
3.5
3
3.5
3
2.5
2
2.5
2
1.5
1
1.5
1
0.5
0
0.5
0
0
20
40
60
0
20
40
60
Time (min)
Time (min)
Fig. 8. The diagram showing the effect of initial dye concentration on photocatalytic
degradation rate of phenol by nano NiO.
Fig. 7. Effect of initial pH on photodegradation of aqueous solution of phenol in
presence of NiO.
3.5. Kinetics of photocatalytic degradation phenol
is in the molecular form (C6H5OH) and at pH > pKa the molecule
undergoes deprotonation becoming negatively charged (C6H5O−).
Under these conditions, both catalyst and substrate are negatively
charged, developing repulsive forces between them. Consequently,
the phenol adsorption is decreased, which decreases its degrada-
tion.
The kinetic of photocatalytic degradation of phenol obeys
pseudo first order behavior and can be expressed as equations. i.e.:
−dC
dt
kKC
rate =
=
+
= kapp
C
(9)
1 + KC
1
kapp
1
kcK
C0
kc
=
(10)
3.4. Effect of concentration on photocatalytic degradation of
phenol using nano NiO
where C0 = initial concentration of phenol; C = concentration of phe-
nol at any time ‘t’; k, kapp denote rate constant, and the apparent
pseudo-first order rate constant, respectively. K and kc are phe-
nomenological constants that depend on some operational factors
including the radiation intensity. The reaction mechanism consid-
ered for heterogeneous photocatalyzed reactions consist of two
main steps: fast adsorption of the reactants on the catalyst sur-
face and a slow step of reaction in the adsorbed phase of the
organic compound and a photogenerated hydroxyl radical. The
results reported in the literature [47,48] demonstrate that kc and K
are light intensity-dependent.
A plot of 1/kapp versus initial dye concentration is presented
in Fig. 9. In photo-induced reactions, the number of active cata-
lyst sites is less than the total surface adsorption sites. Both kc and
K depend on the intensity of absorbed light. Results showed that
kc is proportional to the incident photon flux while K decreased
for higher light intensity [49–52]. Experimental data reported in
Table 2 demonstrate that photocatalytic oxidation of phenol fol-
lows a pseudo-first order kinetic model. As photon flow increases,
the number of photogenerated HO• radicals rise up, accelerating the
oxidation of the substrate molecules. Simultaneously, the increase
of surface active species such as electrons, holes and HO• radicals
lead to a decrease in the available sites for adsorption of the primary
molecule and therefore a decrease in K.
The effect of initial phenol concentration was investigated vary-
ing the concentration from 0.532 mM to 2.66 mM which is depicted
in Fig. 8. The photo degradation rates are presented in Table 2.
The degradation rate for decomposition of the phenol under inves-
tigation was found to increase with the increase in substrate
concentration up to a certain level and reaches to saturation limit
which is in agreement with a number of studies reported earlier
[45]. This saturation trend of degradation rate could be due to the
scavenging of the majority of photogenerated OH radicals by phe-
nol at its higher concentration.
The photocatalysed degradation of phenol in solution is initi-
ated by the photoexcitation of the semiconductor, followed by the
formation of electron–hole pair on the surface of catalyst. Highly
oxidative valence band holes and various oxygen-containing radi-
cal species (e.g., •OH, O2•−, HO2•) are produced [46]. Among these
species, holes and •OH radicals play the most important roles in the
photodegradation of organic pollutants. The high oxidative poten-
tial of the hole (hVB+) in the catalyst permits the direct oxidation of
phenol to reactive intermediates. The overall quantum efficiency
depends from the competition between charge-carrier recombina-
tion, trapping, and interfacial charge transfer.
Table 2
Effect of substrate concentration on rate constant of phenol degradation using nano-NiO.
S. no.
Phenol conc. (mM)
(kapp)/min−1
Photodegradation rate (10−5 M min−1
)
R2
1/kapp
1
2
3
4
5
0.532
1.064
1.596
2.128
2.66
0.0876
0.0613
0.0430
0.0317
0.0246
4.66
6.52
6.86
6.74
6.54
0.8833
0.986
0.9977
0.9415
0.9691
11.42
16.31
23.25
31.55
40.65

70
K. Hayat et al. / Journal of Molecular Catalysis A: Chemical 336 (2011) 64–71
45
40
35
30
25
20
15
10
5
0
0
0.5
1
1.5
2
2.5
3
Initial concentration of Phenol(mM)
Fig. 9. A plot of 1/kapp against initial phenol concentration.
4. Conclusions
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tocatalytic degradation of phenol (97%) using nano NiO and UV
laser irradiation was achieved within short span of time as com-
pared with conventional setups like lamps. The degradation rate
was increased significantly by increasing the initial pH of the solu-
tion while it decreases with increasing substrate concentration. The
results obtained obey pseudo-first order rate kinetics.
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The support by King Fahd University of Petroleum and Minerals
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