Synergistic effect of microwave rapid heating and weak mineralizer on silica-stabilized tetragonal zirconia nanoparticles for enhanced photoactivity of Bisphenol A

Article abstract of DOI:10.1016/j.molliq.2018.04.068

Silica-doped tetragonal mesoporous zirconia nanoparticles (SZ) were successfully synthesized with various concentrations of ammonia solution(NH₄OH) using microwave-assisted method for photodegradation of Bisphenol A (BPA). The phase of SZ catalyst was pure mesoporous tetragonal ZrO₂ with nano in size (5–10 nm). The increasing concentration of NH₄OH solution enhanced the hydrolysis and condensation process, as confirmed by formation of additional Zr–O–Zr, Si–O–Si and Si–O–Zr bonds. The formation of oxygen vacancy (V_o) and Zr³⁺ surface defects (ZSD) also increased proportionally with the concentration of NH₄OH solution. Both properties and the Si–O–Zr bond are supposed to play important roles in stabilizing the tetragonal phase of ZrO₂. The V_o and ZSD also lowered the band-gap energy (from 4.35 eV to 3.05 eV) and act as electron acceptors. Catalytic testing showed that 8-SZ gave the highest initial rate (0.372× 10^?3 mM min^?1) of 10 mg L^?1 BPA at pH 7 using 0.5 g L^?1 catalyst within 4 h while under similar conditions, 5-SZ, 3-SZ, 0-SZ, ZrO₂ and TiO₂ only gave 0.263 × 10^?3, 0.211 × 10^?3, 0.145× 10^?3, 0.026× 10^?3 and 0.305 × 10^?3 mM min^?1, respectively. These results demonstrate that higher numbers of Si–O–Zr, V_o, and ZSD seemed to be the main factors in the high photoactivity of SZ prepared in 8 M NH₄OH solution (8-SZ). A kinetic study demonstrated that the photodegradation followed the pseudo-first-order Langmuir-Hinshelwood model. The mechanism study using scavenging agents proved that photogenerated hole was the main active species, thus led more hydroxyl radicals formed to degrade the BPA. The 8-SZ also exhibited a great potential on degradation of various phenols and dyes in wastewater.

Full text of DOI:10.1016/j.molliq.2018.04.068

Accepted Manuscript
Synergistic effect of microwave rapid heating and weak
mineralizer on silica-stabilized tetragonal zirconia nanoparticles
for enhanced photoactivity of Bisphenol A
N.S. Hassan, A.A. Jalil, S. Triwahyono, N.F. Khusnun, S.M. Izan,
K. Kidam, A. Johari
PII:
DOI:
Reference:
S0167-7322(17)36050-6
doi:10.1016/j.molliq.2018.04.068
MOLLIQ 8971
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
19 December 2017
22 March 2018
11 April 2018
Please cite this article as: N.S. Hassan, A.A. Jalil, S. Triwahyono, N.F. Khusnun, S.M.
Izan, K. Kidam, A. Johari , Synergistic effect of microwave rapid heating and weak
mineralizer on silica-stabilized tetragonal zirconia nanoparticles for enhanced
photoactivity of Bisphenol A. The address for the corresponding author was captured
as affiliation for all authors. Please check if appropriate. Molliq(2017), doi:10.1016/
j.molliq.2018.04.068
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ACCEPTED MANUSCRIPT
Synergistic effect of microwave rapid heating and weak mineralizer on silica-stabilized
tetragonal zirconia nanoparticles for enhanced photoactivity of Bisphenol A
N.S. Hassana, A.A. Jalila,b*, S. Triwahyonoc, N.F. Khusnuna, S.M. Izanc, K. Kidama and A.
Joharia,b
aDepartment of Chemical Engineering, Faculty of Chemical and Energy Engineering,
Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia
bCentre of Hydrogen Energy, Institute of Future Energy, 81310 UTM Johor Bahru, Johor,
Malaysia
cDepartment of Chemistry, Faculty of Science,Universiti Teknologi Malaysia, 81310 UTM
Johor Bahru, Johor, Malaysia
*To whom correspondence should be addressed,
Aishah Abdul Jalil (Ph.D.)
Tel: 60-7-5535581 Fax: 60-7-5536165
Email: aishahaj@utm.my
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Abstract
Silica-doped tetragonal mesoporous zirconia nanoparticles (SZ) were successfully synthesized
with various concentrations of ammonia solution(NH4OH) using microwave-assisted method
for photodegradation of Bisphenol A (BPA). The phase of SZ catalyst was pure mesoporous
tetragonal ZrO2 with nano in size (5-10 nm). The increasing concentration of NH4OH solution
enhanced the hydrolysis and condensation process, as confirmed by formation of additional
Zr−O−Zr, Si−O−Si and Si−O−Zr bonds. The formation of oxygen vacancy (Vo) and Zr3+
surface defects (ZSD) also increased proportionally with the concentration of NH4OH
solution. Both properties and the Si-O-Zr bond are supposed to play important roles in
stabilizing the tetragonal phase of ZrO2. The Vo and ZSD also lowered the band-gap energy
(from 4.35 eV to 3.05 eV) and act as electron acceptors. Catalytic testing showed that 8-SZ
gave the highest initial rate (0.372× 10-3 mM min-1) of 10 mg L-1 BPA at pH 7 using 0.5 g L-1
catalyst within 4 h while under similar conditions, 5-SZ, 3-SZ, 0-SZ, ZrO2 and TiO2 only
gave 0.263 × 10-3, 0.211 × 10-3, 0.145× 10-3, 0.026× 10-3 and 0.305 × 10-3 mM min-1,
respectively. These results demonstrate that higher numbers of Si−O−Zr, Vo, and ZSD seemed
to be the main factors in the high photoactivity of SZ prepared in 8M NH4OH solution (8-SZ).
A kinetic study demonstrated that the photodegradation followed the pseudo-first-order
Langmuir-Hinshelwood model. The mechanism study using scavenging agents proved that
photogenerated hole was the main active species, thus led more hydroxyl radicals formed to
degrade the BPA. The 8-SZ also exhibited a great potential on degradation of various phenols
and dyes in wastewater.
Key Words : Silica-doped mesoporous zirconia nanoparticles; tetragonal phase; microwave;
ammonium hydroxide; photoactivity
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1.0
Introduction
Bisphenol A (BPA) is widely used to produce epoxy resins, polycarbonate plastics and
other plastics products, which considered as a recalcitrant water contaminant that cannot be
readily removed from the water due to complex aromatic molecular structure and low
biodegradability [1,2]. Therefore, the treatment of such pollutant is of great interest in order to
reduce the concentrations of that compounds [3]. Several methods have been explored such as
adsorption, filtration, and chemical oxidation [4-6]. Nevertheless, these methods have
weaknesses such as producing large amounts of sludge, time consuming, and generating
secondary products [7].
An advanced oxidation process (AOPs) can be an alternative to conventional methods
by heterogeneous semiconductor photocatalysts such as TiO2, Fe2O3, ZnO, CuO, ZrO2, CdS,
and SnO2 for the removal of such pollutants from wastewater [8]. Recently, ZrO2 also has
been recognized as a potential photocatalyst for the oxidation of chemicals, water splitting,
and carbon dioxide reduction regardless of its wide band gap energy (5.0 eV) [9]. In fact, pure
ZrO2 has three temperature dependent stable phases: monoclinic, cubic, and tetragonal, which
the latter has attracted more interest among researchers recently as a photocatalytic active
material for photocatalytic degradation of dyes and phenolic compounds [10,11].
However, due to its formation has to conduct at high temperature (1100-2370ºC), various
synthesis methods have been developed such as chemical precipitation, hydrothermal, and
microemulsion in order to stabilize its phase at room temperature by either reducing the
particles size to few nanometers or by doping which introduces defects in the lattice [12]. In
addition, adjusting the reaction conditions such as precursor, pH of solution during hydrolysis,
surfactant, mineralizers, processing conditions and calcination temperature also could control
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the crystal growth [13]. However, those methods facing several shortcomings such as long
reaction time, uneven temperature distribution, and/or, strong reduction conditions.
Over the last decade, microwave (MW)-assisted methods become an effective way to
fabricate nanomaterials such as TiO2, ZnO and Mn2O3 [14,15]. The uniformity and rapid
heating can produce small particles with narrow size distribution and high purity than the
conventional method [16,17]. Recently, we also reported a facile MW method for the
synthesis of mesoporous titania and mesoporous silica nanoparticles, and its use in
photocatalysis and drug delivery system, respectively [7,18]. Besides, MW also shorten the
synthesis time and enhanced the crystal growth that leads to a large pore volume, pore width
and surface area. Due to these advantages, an extension of this study is worth continuing.
Furthermore, the studies relating the synergistic influences of MW-heating and weak
mineralizer on phase stabilization of ZrO2 nanocrystals have not been reported. In this
perspective, the present work aimed to investigate the effect of concentrations of NH4OH
aqueous solution on the structure of silica-doped tetragonal mesoporous zirconia nanoparticles
(SZ) that prepared via MW irradiation. The physicochemical properties of the SZ catalysts
were detail characterized using XRD, TEM, surface area analysis, FTIR and ESR. The
proposed structure, performance, kinetic analysis, proposed reaction mechanism, regeneration
of the catalysts toward degradation of bisphenol A (BPA) as well as application on other
organic pollutants are also discussed in order to confirm their responsibility in the
photocatalytic reactions. The remarkable properties generated during the MW irradiation are
believed could diversify its use not only in the photocatalytic reaction but also for other
applications.
2.0
Experimental
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2.1
Materials
Bisphenol A (BPA, >98% purity) was used without further purification. Hydrochloric
acid (HCl), cetyltrimethylammonium bromide (CTAB), sodium hydroxide (NaOH), and 2-
propanol (Pr-OH), were obtained from Merck Sdn. Bhd., Malaysia. (3-
aminopropyl)triethoxysilane (APTES), ammonium hydroxide solution (NH4OH) and
Zirconium (IV) propoxide (ZrP) were obtained from QRec, Malaysia and Sigma Aldrich,
respectively. 0.1 M HCl and NaOH solution were used for adjustment of pH solution.
2.2
Preparation of catalyst
Silica doped tetragonal mesoporous ZrO2 was synthesized as follows; CTAB
surfactant (4.68 g) was added into the mixture of propanol (120 mL), distilled water (720
mL), and 8 M NH4OH solution (29 mL). The solution was stirred for 30 min at 323 K
followed by addition of ZrP (5.7 mL) and APTES (1.053 mL) and continued for 2 h in water
bath. Then, the solution was placed in the domestic microwave oven (Samsung ME711K) for
heating, which was intermittently for 2 h to form a sol-gel of the SZ. The obtained sample was
collected, dried and calcined at 1123 K for 3 h to give a white powder which denoted as 8-SZ
catalyst. Similar procedure was performed under various NH4OH solution concentration: 5 M,
3 M and 0 M and denoted as 5-SZ, 3-SZ and SZ, respectively. Due to the limitation, 8 M was
chosen as the highest NH4OH solution concentration that can be prepared in this study.
2.3
Characterization
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X-ray diffraction (XRD) was used to analyze the phases of the catalysts by D8
ADVANCE Bruker X-ray diffractometer at a 2θ angle ranging from 3◦ to 90◦. Transmission
electron microscope (TEM) (JEOL JEM-2100F) was used for determination of morphology
and size of the catalyst. Meanwhile, Perkin Elmer Spectrum GX Fourier-transform infrared
(FT-IR) Spectrometer was used to identify the functional groups of the catalysts by KBr
method. The surface defect and oxygen vacancy was confirmed by ESR analysis by JEOL
JESFA100 ESR spectrometer. The textural properties were determined using a Beckman
Coulter SA 3100 surface area analyzer. In addition, The Kubelka-Munk (K-M) plots was used
for determination of band gap and Perkin Elmer UV-Vis/DRS Spectrophotometer was used
for optical absorption analysis.
2.4
Photodegradation of BPA
The experiments regarding degradation of BPA were conducted in batch reactor with
four 400 W metal halide lamps (λ=400 nm) as a visible light source and a cooling system.
Before being exposed to light irradiation,a desired concentration of BPA was stirred in the
dark for 1 h to achieve adsorption-desorption equilibrium. Each set of experiment was
conducted three times to ensure the accuracy. The pH = 7 and 10 mg L−1 were initial pH and
concentration of BPA solution, respectively.
3.0
Results and Discussion
3.1
Catalyst characterization
3.1.1 Crystallinity, phase and structural studies
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Fig. 1A shows the wide-angle XRD patterns of silica and silica-doped tetragonal
zirconia nanoparticles (SZ) synthesized under various concentrations of NH4OH solution. A
broad diffraction peak centered at 2Ɵ= 23° was observed for SiO2, indicating its amorphous
phase (JCPDS No. 29-0085) (Fig. 1A/a) [18]. The absence of this peak in SZ catalysts may
be due to its presence are too diminutive to be detected by XRD as similar phenomenon was
reported when lower metal content was loaded onto ZrO2 [19, 20]. All the SZ shown in Fig.
1A could be indexed as tetragonal phase as according to the JCPDS card No. 79-1771, the
XRD diffractogram for the tetragonal phase of ZrO2 was at peaks 29.9° (101), 34.8° (110),
50.1°(112), 59.0°(200), 60.0° (211), and 63.2°(202) (Fig. 1A/b-e) [15, 21]. The additional
peaks at 59.0° and 63.2° for 5-SZ and 8-SZ demonstrated their high crystallinity compared to
others, which may be evidence that the higher concentration of base improved crystal growth.
A
B
Tetragonal
C
Tetragonal
Monoclinic
e
d
c
0.273 nm
b
5 nm
a
20 30 40 50 60 70
20 30 40 50 60 70
2θ ( )
2θ ( )
Fig. 1 XRD patterns of (A) a) SiO2, b) SZ, c) 3-SZ, d) 5-SZ, and e) 8-SZ; (B) commercial
ZrO2 (C) TEM and HRTEM images of 8-SZ (as inset)
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No reflections of monoclinic phases were observed as compared to the commercial ZrO2 (Fig.
1B), signifying the high purity of SZ without any impurities. The peak intensities seemed
increased with the increasing NH4OH solution concentration, signifying the enhancement in
the crystallinity of the SZ, which most probably due to the improvement in the structural
arrangement of the ZrO2 network [22].
Table 1 Textural properties of catalysts
Catalyst
Surface
area
Pore
Average Crystallite
Band
gap
volume
(cm3/g)
0.0332
0.2653
0.2572
0.2405
0.1845
pore size
(nm)
5.1
size
(nm)a
19.8
6.3
(m2/g)
7.1
(eV)b
4.35
3.75
3.45
3.25
3.05
commercial ZrO2
SZ
136.6
130.6
110.6
100.7
5.1
3-SZ
5-SZ
8-SZ
5.1
6.5
6.1
8.9
4.3
10.2
aCrystallite size calculated using Debye–Scherrer equation at 2θ =29.9˚
bBand gap calculated using Kubelka–Munk (K–M) plots
The crystallite size of the SZ was calculated based on the (101) diffraction peak using
the Debye-Scherer equation as follows,
k
 cos
 
(1)
where τ is particles size, k is shape factor (k = 0.9), β is the line width at half maximum height
(radian), λ is the wavelength of X-ray radiation (0.15418 nm), and θ is the angular position of
the peak maximum (radian). The average size of SZ crystallites ranged from 6-10 nm (Table
1). Size increased with increasing concentrations of NH4OH solution due to the agglomeration
of small crystals [23]. The broad diffraction peaks supported that the crystallites of SZ were
nano in size [24]. This result was confirmed by the TEM images shown in Fig. 1C, in which
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the diameter of black particles of ZrO2 were in the range of 5-10 nm (red square). While, its
interplanar spacing (yellow square) was also determined to be 0.273 nm (inset figure), which
implied the d spacing of (101) plane of tetragonal ZrO2 (JCPDS 42-1164) [13]. These results
revealed that the pure tetragonal phase of ZrO2 with ≤ 10 nm of crystallite size was simply
synthesized by the MW-assisted method, which are rather difficult to be produced by other
conventional methods such as sol-gel and co-precipitation that mainly resulted in mixed
phases [25].
3.1.2 Textural studies
The pore size distribution and nitrogen adsorption–desorption isotherms of the
commercial ZrO2 and SZ was shown in Fig. 2. For the commercial ZrO2, a H4 hysteresis loop
with type IV isotherm with was observed, which is the dominant of pore with uniform size
and shape (Fig. 2A). Meanwhile, the SZ catalysts exhibited H3 hysteresis loop with isotherm
type IV (Fig. 2B-E), confirming an adsorption profile for mesostructured material [26]. The
filling of intraparticle pores within the samples can be observed at P/Po=0.3-0.4, while
nitrogen condensation within interparticle voids was observed at a relative pressure of 0.5-
0.99. Based on the Fig. 2B-E, the intraparticle pores of SZ remain unchanged but the
interparticle pores obviously reduced by the increasing NH4OH solution concentration. Table
1 also demonstrates that the surface area and pore volume of the catalyst decreased upon
addition of NH4OH solution. The pore size distribution was shown in Fig. 2F clearly suggests
that the commercial ZrO2 possessed a very narrow monomodal pore dimension, while the SZ
synthesized using the MW-assisted method were distorted to a multi-modal pore distribution
by the increasing NH4OH solution concentration, particularly after the addition of ≥ 5M
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NH4OH solution, which is evidence of the pore-blockage effect. This may be due to the
consecutively collapsed pore walls that reduced the pore volume as well as the surface area of
200
150
100
50
12
9
8-SZ
5-SZ
3-SZ
SZ
B
F
E
D
A
C
ZrO2
6
3
0
0
2
3 4 5 6 7 8 9 10
0
0.2 0.4 0.6 0.8 1
P/P0
Pore width (nm)
Fig. 2 N2 adsorption-desorption isotherms plots of (A) commercial ZrO2, (B) SZ, (C) 3-SZ,
(D) 5-SZ, (E) 8-SZ, (F) Pore size distribution of catalysts
the catalysts. This result suggested the possible inclusion of silica from the APTES has
interfered the arrangement of ZrO2 matrix [27]. Similar reduction in surface area was
observed when zinc was introduced into mesostructured silica nanoparticles [28].
3.1.3 Vibrational spectroscopy
A FTIR spectra of the commercial ZrO2 and SZ in the region between 4000−400 cm-1
was shown in Fig 3A. The SZ catalysts displayed four main bands corresponded to −OH
stretching (3456 cm-1), bending vibrations of the −OH groups of the physisorbed moisture
(1620 cm-1), Si−O−Si asymmetric stretching (1070 cm-1), and ZrO2 skeletal (580 cm-1) [29,
30]. For clarity, a Gaussian curve-fitting was plotted (Fig. S1) and the intensities of those
important bands are summarized in Fig. 3B. It was seen that the Si−O−Si bonds at 1160 and
1070 cm-1 were increased in intensities by the increasing NH4OH solution concentration.
While, the band at 955 cm-1 decreased in parallel with the emergence of a new band at 950
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cm-1, implying the Zr ions was exchanged with the hydrogen of Si−OH groups to form new
Si−O−Zr bonds. Similar interactions were observed previously when metals were supported
onto mesostructured silica nanoparticles and alumina [31, 32]. The bands in range of 700−580
and 550−400 cm-1 also showed increasing trend which represented the stretching vibrations of
Zr−O−Zr chains and tetragonal phase of ZrO2, respectively [33, 34]. The absence of
monoclinic phase was apparently seemed in all SZ catalysts as compared to commercial ZrO2,
in which the band should appear at 740 cm-1. This result is in agreement with the XRD data,
confirming the purity of tetragonal phase of the SZ [35]. These results suggested that the
presence of Si−O−Zr bonds in the ZrO2 network might play important role to rigorously
suppress the transformation of tetragonal to monoclinic phase [36, 37]. It can be explained by
the constrainment effect of the ZrO2 particles with the surrounding amorphous silica matrix
which is enough to preserve the particle growth, thus stabilized the tetragonal phase of ZrO2
[9].
B
C
A
3740
e
464
464 (t-Zr-O)
580
580 (Zr-O-Zr)
e
d
c
3610-3400
740
740 (m-Zr-O)
1620
950
950 (Si-O-Zr)
1070
955
955 (Si-OH)
580
1070
1160
1160 (Si-O-Si)
1070 (Si-O-Si)
3456
d
b
a
c
b
740
3800
3600
Wavenumber(cm-1)
3400
3200
4000
2800
Wavenumber (cm-1)
1600
400
ZrO
ZrO2
SZ
3-SZ
5-SZ
8-SZ
2
Fig. 3 (A) FTIR spectra of a) commercial ZrO2, b) SZ, c) 3-SZ, d) 5-SZ and e) 8-SZ, (B)
Intensity of Gaussian peak area in the 1300-400 cm-1 region, (C) IR spectra for all activated
catalyst in the OH stretching region.
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Next, Fig. 3C shows IR measurements for the physisorbed water by evacuating the
catalysts at 623 K. The sharp band at 3740 cm-1, which typically attributed to the tetragonal
phase and its bibridged hydroxyl groups of the ZrO2 framework, seemed to decrease in
intensity with increasing NH4OH solution concentration [38]. Similar result was also observed
for a broad band at 3610−3450 cm-1, which predictably assigned to Zr(OH)4 [39]. These
decreases most probably due to the fast condensation rate at higher concentration of alkaline
solution that led to the formation of additional Zr−O−Zr, Si−O−Zr, and Si−O−Si bonds, as
confirmed by their increasing formation at band 580, 950, 1070, and 1160 cm-1 in Fig. 3B.
3.1.4 Chemical oxidation state determination
In general, ESR spectroscopy is used to study the unpaired electrons as well as the
occurrence of paramagnetic ions in materials. Fig. 4A shows that all SZ catalysts have similar
ESR signals at g =2.011 and g=1.978, 1.951 and 1.907, which are corresponded to oxygen
vacancy (Vo) and Zr3+ surface defect (ZSD), respectively [40, 41]. Due to similar trend, the
signal of ZSD at g= 1.907 was chosen as a representative for all ZSD. It could be seen from
the peaks intensities summarized in Fig. 4B that the commercial ZrO2 possessed higher
amount of ZSD but somewhat lower Vo. While, the intensities of both properties for SZ seem
increased proportionally with the increasing NH4OH solution concentration, with the 8-SZ
gave the highest amount of them. It is postulated that both properties appeared due to the
Zr−OH terminal bonds breaking on the SZ surface during condensation as a result of the
alkaline treatment, microwave heating as well as calcination [42, 43]. In addition, uniformity
of heat distribution enhanced the condensation of excess NH4OH solution to reduce the
number of OH- ions from filling the oxygen vacancies in the lattice, thereby the higher
amount of Vo could have stabilized the tetragonal phase of the ZrO2 [13, 44]. Due to the
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abundant numbers of both properties, it is presumed that most of the SZ have a good
prospective for the enhanced photocatalytic reaction.
1.907
1.978
A
B
3+
1.901
1.907 (Zr
)
1.951
2.001
2.011 (Vo)
2.011
e
d
c
b
a
1.85 1.9 1.95
g-value
2
2.05
ZrO
ZrO2
SZ
3-SZ 5-SZ 8-SZ
2
Fig. 4 (A) ESR spectra of a) commercial ZrO2, b) SZ, c) 3-SZ, d) 5-SZ and e) 8-SZ, (B)
Intensity of the related g-value
Fig S2A shows the UV-DRS spectra of all catalysts recorded at the range of 200-600
nm. Two main bands were observed at 230 and 305 nm correspond to ligand to metal charge
transfer band (LMCT) from O2- to an isolated Zr4+ ion in tetrahedral configuration [45].
Compare with commercial ZrO2, the band for SZ was blue-shifted from 230 nm to 225 nm,
signifying the decrease in its crystallite size via the addition of NH4OH solution into the
system. While, the band-gap energy of catalysts calculated by the Kubelka-Munk (K-M)
equation (Fig S2B) was found to decrease by the increasing concentration of NH4OH solution
(Table 1), supporting the ESR data regarding the presence of high amount of Zr3+ and Vo
between the valence and conduction band [46,47]. It was noticed that this result follows the
quantum size effect (< 10 nm), where the band-gap energy is inversely proportional to the
crystallite size of SZ catalysts [48].
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3.1.5 Proposed structure of SZ nanoparticles
The mechanism and structure for formation of SZ are proposed regarding on the
characterization results above, as in Fig 5. The CTAB created a micellar system in water-
propanol media and subsequent addition of zirconium source and APTES into the prepared
template formed the nanosized spheres [49]. The NH4OH solution acted as a precipitating
agent and also a basic catalyst for the hydrolysis to form Zr(OH)4. Afterwards, the formation
of hydroxyl and oxo-bridges between tetramer units were occurred due to polycondensation
processes in the microwave via olation (Zr−OH + H2O−Zr → Zr−OH−Zr + H2O) and
Zr (OBu)4
APTES
H2O/PrOH
NH4OH
MW
Calcine
(1123 K)
CTAB
Micelles
O
O
O
Si
O
Si
O
O
O
Vo
HO
O
O
O
Zr
O
Zr
O
O
Zr
O
Zr3+
Zr3+
O
O
Fig. 5 Schematic diagram of formation of SZ
oxolation (Zr−OH + HO−Zr → Zr−O−Zr + H2O) reaction [50]. The silica source underwent
the process as the ZrO2; the dramatic changes in pH mainly due to the gradual addition of
NH4OH solution and thus, accelerated both hydrolysis and condensation processes to form
additional Zr−O−Zr, Si−O−Zr, and Si−O−Si bonds [22, 39]. Then, a white powder of SZ was
obtained by removing the template via calcination process. On the other hand, well
condensation simultaneously removed additional surface oxygen and led to the formation of
Vo and ZSD. In fact, generated electrons were trapped on the Zr4+ sites to form Zr3+ [51].
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3.2
Photocatalytic performance and kinetic analysis
Next, the photoactivity of catalysts were examined on a degradation of BPA for 240
min and the results are illustrated in Fig. 6A. The photolysis of BPA under visible light (inset
figure) shows a meagre degradation of 5% (0.006 × 10-3 mM min-1), suggesting the higher
stability of the pollutant. From Fig 6A, it observed that the effect of NH4OH solution
concentration demonstrated that the 8-SZ gave the highest value of 0.372× 10-3 mM min-1,
followed by TiO2, 5-SZ, 3-SZ, SZ, ZrO2 and with 0.305× 10-3, 0.263 × 10-3, 0.211 × 10-3,
0.145 × 10-3 and 0.026 × 10-3 mM min-1, respectively. The pH 7 and 0.5 g L-1 was the
optimum condition for the highest degradation of BPA (Fig. S3A and S3B). The effectiveness
of 8-SZ compared to commercial ZrO2 most likely due to its high purity of tetragonal phase
and higher numbers of Vo that led to the lowest band-gap energy [46]. The defects in crystals
especially on the surface of catalysts (Vo and Zr3+) play major roles in the photocatalytic
0.45
0.3
0.15
0
2.5
2
A
0.01
0.005
0
B
10 mg L-1
10 ppm
Photolysis
30 mg L-1
30 ppm
50 mg L-1
50 ppm
70 mg L-1
70 ppm
-1
100 ppm
100 mg L
1.5
1
0.5
0
0
60 120 180 240
t (min)
Fig. 6 (A) Catalytic performance of the catalysts under visible light towards BPA and
photolysis reaction (as inset), (B) Kinetic studies of 8-SZ at different initial concentrations
15

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reaction as active sites for the adsorption of O2, which acts as a promoter in the oxidation of
organic pollutant [52]. Similar phenomenon was reported in the degradation of 2-CP over
mesoporous titania nanoparticles [7].
Table 2 The kinetics parameters of photodegradation of BPA in different initial
concentration using 8-SZ
Initial concentration
Reaction rate, kapp
Initial reaction rate, ro
(mg L-1)
10
(x10-3 min-1)
(x10-2 mg L-1 min-1)
8.5
4.8
4.1
3.6
2.6
8.5
30
50
70
100
14.4
20.5
25.2
26.0
The kinetics of photodegradation was studied based on the effect of initial
concentration of BPA in the range of 10-100 mg L-1 using 8-SZ catalyst (Table 2). Fig. 6C
shows the linear plot of ln (C0/Ct) vs. irradiation time, suggesting photodegradation follows
first-order which evaluated by the Langmuir–Hinshelwood model [53]. The increasing initial
concentration with decrease in kapp values indicated that the system was favorable at low
concentration. The calculated values of KLH (adsorption coefficient of reactant) and kr
(reaction rate constant) for 8-SZ were 0.04968 L mg-1 and 0.23657 mg L-1 min-1, respectively,
in which BPA adsorption on the surface of the SZ was the controlling step of the process [7].
3.3
Proposed mechanism for photodegradation of BPA
In order to further investigation of the mechanism of BPA photodegradation, the 8-SZ
catalyst was chosen and four scavenging agents were used in order to identify the key factor
in the reaction: potassium peroxydisulfate (PP), ammonium oxalate (AO), isopropanol (IP)
and potassium iodide (PI), which act as photogenerated electron (e-), photogenerated hole
(h+), hydroxyl radicals (•OH) in bulk solution, and hydroxyl radicals adsorbed on the catalyst
16

ACCEPTED MANUSCRIPT
surface (•OHads), respectively [54]. Fig. 7A shows that the initial rate of BPA degradation
decreased to 0.237× 10-3 mM min-1, 0.269× 10-3 mM min-1, 0.196× 10-3 mM min-1, and 0.253
× 10-3 mM min-1 by the addition of IP, PI, AO, and PP, respectively, signifying the
photogenerated hole was the main active species for degrading the BPA. Accordingly, the
mechanism was proposed for degradation of BPA over SZ as in Fig. 7B. Under UV light
irradiation, the holes were created at valence band (VB) caused by electrons excitation to
conduction band (CB) of SZ (Eq. 2). The photogenerated holes (h+) which have high
oxidation potential, play two important roles in this degradation, in which they directly
oxidized the BPA to reactive intermediate (Eq. 3), and in parallel reacted with the surface-
0.45
A
O2
B
e-
e
e- e-
e-
CB
-
•O-2
hv
e-
Zr3+ state
H2O2
Vo
0.3
0.15
0
•OH
BPA
VB
H2O
h+ h+ h+ h+
Degraded product
•OHads
BPA
Fig. 7 (A) Photodegradation efficiencies of BPA in the presence of scavengers, (B)
proposed mechanism for photodegradation of BPA over 8-SZ catalyst
adsorbed hydroxyl groups (OH-) or water to yield surface-adsorbed hydroxyl radicals (•OHads)
as shown by the decrease in degradation efficiency by PI, which then degraded the BPA (Eq.
4).
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SZ  hv  e  h
(2)
(3)
BPA  h  oxidation of BPA
OH  h  OHads
(4)
Electron-hole pair recombination was suppressed by the Vo and ZSD, which acted as electron
 (Eq. 5)
acceptors. The electrons then reduced by the oxygen to superoxide anion radicals
O2
before reacting with H+ in the water to produce HOO● and finally generate OH● for the partial
or complete mineralization of BPA (Eq. 6-8) [1].
O2  e  O2
(5)
O2  e  2H   H2O2
(6)
H2O2  e  OH OH
(7)
OH  SZ  degraded SZ  CO2  H2O
(8)
The degradation of BPA was further investigated by GC-MSD and the mechanism is
proposed in Fig. 8. From the identified m/z determined by GC-MS, as well as compared with
the results of previous studies, three compounds were identified as the main intermediates as
presented in Fig. S4 [55]. Firstly, OH may cause cleavage between one of the benzenes and
oxidize the molecules to form phenol. Then, glycolic acid and 2-butanol were formed by ring
opening reaction before being finally mineralized into carbon dioxide and water.
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2-butanol
OH
Bisphenol A
Phenol
94
76
CO2 + H2O
OH
HO
OH
44
18
HO
O
228
OH
76
Glycolic acid
Fig. 8 Proposed mechanism of degradation BPA by GC-MSD.
3.4
Stability
The recyclability of photocatalyst is very important to their practical use. Fig. 9A
shows the stability of catalyst that evaluated by repeating the experiments for five cycles
using the 8-SZ catalyst. It is obviously seen that the catalyst remained active and insignificant
decrease in degradation over time may be due the heat treatment which caused an aggregation
of the catalyst and decreased the surface area. This result indicated that the catalyst is
photochemically stable and highly reusable [56].
0.45
0.3
0.15
0
0.45
0.3
0.15
0
A
B
2-CP3-CP4-CP TCP MB
1
2
3
4
5
Cycles
Fig. 9 (A) Stability of 8-SZ catalyst on photodegradation of BPA, (B) Photocatalytic activity
of various organic pollutant.
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The 8-SZ was also tested for the degradation of various organic pollutants such as 2-
chlorophenol (2-CP), 3-chlorophenol (3-CP), 4-chlorophenol (4-CP), trichlorophenol (TCP),
and methylene blue (MB), and the results are shown in Fig. 9B with degradation percentage
of 90% (0.427×10-3 mM min-1), 70% (0.319×10-3 mM min-1), 75% (0.350×10-3 mM min-1),
50%(0.170×10-3 mM min-1), and 60% (0.252×10-3 mM min-1), respectively. This result
demonstrated that the 8-SZ has a great potential to be used for degradation of phenols and
dyes wastewater.
4.0
Conclusion
As conclusion, a simple MW-assisted method was applied for preparation of silica-
doped tetragonal mesoporous zirconia nanoparticles (SZ). The physicochemical properties of
the catalysts were studied by XRD, TEM, N2 adsorption-desorption, FTIR, ESR and UV-DRS
analyses. Different behaviors were observed when the SZ were prepared under various
concentration of NH4OH aqueous solution. Significantly, the FTIR verified the formation of
additional Zr−O−Zr, Si−O−Si, and Si−O−Zr bonds by the increasing concentration of NH4OH
solution, along with the formation of higher number of Vo and ZSD which were detected by
the ESR. Those properties were found to play important roles in stabilizing the tetragonal
phase of ZrO2. The 8-SZ gave the highest value of 0.372× 10-3 mM min-1, followed by 5-SZ,
3-SZ, SZ and commercial ZrO2 with 0.263 × 10-3, 0.211 × 10-3, 0.145 × 10-3 and 0.026 × 10-3
mM min-1, respectively. A study of the effects of scavengers showed that the photogenerated
hole was the key factor in the reaction. The Si-O-Zr, Vo and ZSD played an important role as
electron acceptors to hinder the electron-hole recombination. The kinetic study showed that
the degradation of BPA followed the pseudo–first-order as determined by the Langmuir–
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ACCEPTED MANUSCRIPT
Hinshelwood model. In addition, the regeneration test revealed that the photocatalyst
remained stable even after five cycling runs. Besides stabilized the tetragonal phase of ZrO2,
the MW-assisted method also is believed could be applied for synthesis of various catalysts
for various applications.
Acknowledgement
The authors are grateful for the financial support by the Fundamental Research Grant
Scheme (Grant No. 14H72), awards of MyPhD Scholarship (Nurul Sahida Hassan) from the
Ministry of Higher Education Malaysia and to the Hitachi Scholarship Foundation for their
support.
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Graphical abstract
0.45
0.3
0.15
0
-
OH
-OH
ZrO2
4.35 eV
Vo
-OH
-OH
8MSZ
3.05 eV
O
O
O
O
Zr
O
Si
O
Si
O
O
O
Vo
HO
O
Zr
O
Zr
O
Zr3+
Zr3+
O
O
O
O
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Research Highlights
 Oxygen vacancies (Vo) and silica stabilized the tetragonal structure of ZrO2
 Microwave irradiation generated Vo and Zr3+ surface defects (ZSD) in ZrO2
 Increased NH4OH solution concentration greatly enhanced hydrolysis
 Vo and ZSD lowered the band gap of SZ from 4.35 to 3.05 eV
 8M SZ showed the highest initial rate of degradation (0.372× 10-3 mM min-1) of
bisphenol A
27