Enhanced catalytic benzene oxidation over a novel waste-derived Ag/eggshell catalyst

Article abstract of DOI:10.1039/c8ta10822f

Catalytic oxidation plays a key role in transforming gaseous benzene into harmless matter, in which it is essential to look for a low-cost, environmentally friendly and efficient green catalyst/support. Herein, Ag nanoparticle (Ag NP) loaded-eggshell catalysts were successfully synthesized by a simple impregnation method, in which waste eggshells were used as an efficient template and catalyst support. SEM, XRD and ICP-OES results show that a different number of Ag NPs are uniformly distributed on the surface of eggshells, which are defined as Ag₁/eggshell (10.8%), Ag₂/eggshell (19.9%) and Ag₃/eggshell (34.3%) catalysts according to the amounts of loaded Ag NPs. Then, the activity between the three catalysts and the pure Ag NPs was assessed based on the catalytic oxidation of benzene. Our results show that the Ag NP loaded-eggshell catalysts exhibit superior catalytic activity compared with the pure Ag NPs. Considering the balance between the cost and catalytic performance, the Ag₂/eggshell catalyst was selected as the optimum research object in the subsequent work. The Ag₂/eggshell catalyst calcined at 500 °C exhibited excellent catalytic activity, which is assigned to the unique channel structure of eggshell, good low-temperature reducibility, and high dispersion of particles (sizes effects) on the surface of eggshell as well as the synergetic interaction between the Ag NPs and eggshell. Furthermore, the Ag₂/eggshell catalyst exhibited predominant stability even after 200 h of on-stream reaction. The reaction mechanism is put forward based on the in situ FTIR experimental results, in which some carboxylate intermediate species are confirmed. Accordingly, the waste eggshell is considered as a promising supporting material for the large-scale synthesis of noble metal catalyst with high catalytic performance and stability for VOC oxidation.

Full text of DOI:10.1039/c8ta10822f

Journal of
Materials Chemistry A
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Enhanced catalytic benzene oxidation over a novel
waste-derived Ag/eggshell catalyst†
Cite this: DOI: 10.1039/c8ta10822f
ab
b
b
b
b
Yunlong Guo, Da-Peng Yang, * Minghuan Liu, Xiaoyan Zhang, Yisong Chen,
a
a
cd
Jiale Huang, * Qingbiao Li and Rafael Luque
*
Catalytic oxidation plays a key role in transforming gaseous benzene into harmless matter, in which it is
essential to look for a low-cost, environmentally friendly and efficient green catalyst/support. Herein, Ag
nanoparticle (Ag NP) loaded-eggshell catalysts were successfully synthesized by a simple impregnation
method, in which waste eggshells were used as an efficient template and catalyst support. SEM, XRD and
ICP-OES results show that a different number of Ag NPs are uniformly distributed on the surface of
1 2 3
eggshells, which are defined as Ag /eggshell (10.8%), Ag /eggshell (19.9%) and Ag /eggshell (34.3%)
catalysts according to the amounts of loaded Ag NPs. Then, the activity between the three catalysts and
the pure Ag NPs was assessed based on the catalytic oxidation of benzene. Our results show that the Ag
NP loaded-eggshell catalysts exhibit superior catalytic activity compared with the pure Ag NPs.
Considering the balance between the cost and catalytic performance, the Ag
2
/eggshell catalyst was
selected as the optimum research object in the subsequent work. The Ag
2
/eggshell catalyst calcined at
ꢀ
5
00 C exhibited excellent catalytic activity, which is assigned to the unique channel structure of
eggshell, good low-temperature reducibility, and high dispersion of particles (sizes effects) on the
surface of eggshell as well as the synergetic interaction between the Ag NPs and eggshell. Furthermore,
2
the Ag /eggshell catalyst exhibited predominant stability even after 200 h of on-stream reaction. The
Received 9th November 2018
Accepted 21st December 2018
reaction mechanism is put forward based on the in situ FTIR experimental results, in which some
carboxylate intermediate species are confirmed. Accordingly, the waste eggshell is considered as
a promising supporting material for the large-scale synthesis of noble metal catalyst with high catalytic
performance and stability for VOC oxidation.
DOI: 10.1039/c8ta10822f
rsc.li/materials-a
to be the most promising method and it can be performed at
1
. Introduction
lower temperatures with low cost and high oxidation effi-
6,8–10
Volatile organic compounds (VOCs) generated from ciency.
The key issue of catalytic oxidation is the prepara-
petrochemical-related production processes,
product tion of inexpensive catalysts with high activity. Compared to
consumption behavior and motor vehicle exhaust are identied transition-metal oxides, supported noble metals are widely
as dominant factors to air pollution and are harmful to human used for the oxidation of VOCs because of their high catalytic
1–4
health. Hence, it is urgent to eliminate the pollution of VOCs. performance and excellent stability. Generally, the activity of
In recent years, plenty of treatment ways such as physical, supported noble metals has a great relationship with the
6
,11
chemical and biological methods are used for the abatement of species of noble metals, the nature of support, etc.
VOCs. Among them, catalytic oxidation of VOCs is considered
5–7
In the past few decades, supported Ag catalysts have attrac-
ted much attention because of their low cost and relatively high
1
2,13
a
catalytic performance.
noble metal (Pd, Pt, Au, and Rh) catalysts,
catalyst shows a lot of merits. On one hand, Ag is one of the
Compared with other supported
Department of Chemical and Biochemical Engineering, College of Chemistry and
14–18
the supported Ag
Chemical Engineering, Xiamen University, Xiamen 361005, PR China. E-mail: cola@
xmu.edu.cn; Fax: +86-05922184822; Tel: +86-05922183088
b
College of Chemical Engineering and Materials Science, Quanzhou Normal University, cheapest noble metals. On the other hand, Ag can also provide
Quanzhou 362000, Fujian Province, PR China. E-mail: yangdp@qztc.edu.cn; Fax: +86-
sufficient active sites for almost all kinds of catalytic oxidation
0
59522199816; Tel: +86-059522199816
2,19–21
reactions.
Therefore, the selectivity of Ag as the active site
rather than other noble metals is essentially investigated.
Recently, some literature studies reported the oxidation
c
Departamento de Quimica Organica, Universidad de Cordoba, Campus de Rabanales,
Edicio Marie Curie (C-3), Ctra Nnal IV-A, Km. 396 E-14014, Cordoba, Spain. E-mail:
rafael.luque@uco.es; Fax: +34-957212066; Tel: +34-957211050
d
Peoples Friendship University of Russia (RUDN University), 6 Miklukho-Maklaya str., performance of VOCs on supported Ag catalysts, like Ag/
2
22
11
23
1
†
1
17198, Moscow, Russia
Mn
O
, Ag/MnO
,
Ag/CeO
,
and Ag/Al
2
O
,
which demon-
2
3
2
2
3
Electronic supplementary information (ESI) available. See DOI:
0.1039/c8ta10822f
strate that Ag catalysts are promising for the reaction.
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ꢁ
1
However, most of the used catalyst supports are transition concentrations of 0.05, 0.1, and 0.2 mol L was added to
metal oxides, which have some obvious disadvantages with a 100 mL conical ask, respectively. Aerwards, 2.0 g of eggshell
a high-cost, non-environmentally friendly and complex prepa- powder was added to the above Ag precursor solution under
ration process. Therefore, looking for an available supporting magnetic stirring for 12 h. Then, the suspension was centri-
ꢀ
material with low-cost, green and simple preparation tech- fuged and the solid in the retentate was dried at 60 C in air for
ꢀ
nology is urgently needed. Nature demonstrates a diverse 6 h followed by calcination at 300, 400, 500 and 600 C for 3 h
ꢀ
ꢁ1
species through evolution, affording us important inspiration with a ramp rate of 2 C min in a muffle furnace, respectively.
12
for materials design and synthesis. Eggshell is a kind of Finally, the different Ag loaded-eggshell catalysts were obtained
natural biomaterial produced by poultry and composed of (dened as Ag /eggshell (10.8%), Ag /eggshell (19.9%) and Ag
about 96% CaCO , 1% MgCO , 1% Ca (PO ) , and organic eggshell (34.3%)).
1
2
3
/
3
3
3
4 2
24,25
materials (mainly proteins), as well as water.
The eggshell
In addition, another type of Ag catalyst was also synthesized
support is composed of a foamy layer of cuticle, a calcite or for comparison. The Ag /com-CaCO (19.9% and calcined at
2
3
ꢀ
calcium carbonate layer and two shell membranes, with 7000– 500 C for 3 h) catalyst was prepared by using the same method
7 000 funnel-shaped pore canals distributed unevenly on the stated above except that com-CaCO substituted for eggshell as
shell surface. Every day, a mass of eggshells is treated as waste a support. Ag NP solids were prepared according to the method
1
3
26
27
29
in landll, which becomes a source of environment pollution.
developed by us with some slight modications. For a typical
In fact, eggshell has intrinsic pore and hierarchical structures synthesis, 50 mg of ascorbic acid was added to the AgNO
and can be used as a calcium catalyst, additive and soil condi- solution (100 mL) with a concentration of 0.05 mol L ; the
3
ꢁ1
2
4,28
tioner.
Natural eggshell not only provides a cost-effective and mixture was stirred at room temperature for approximately
ꢀ
environmentally sound feedstock to produce heterogeneous 5 min followed by centrifuging and dried at 60 C in air for 8 h.
catalysts, but also acts as an excellent platform for the reduction, Finally, the Ag NPs were obtained.
dispersion and stabilization of nanoparticles. More importantly,
the hierarchical and porous structure is very much in favor of
mass and energy transfer. To the best of our knowledge, few
2.3 Catalyst characterization
reports are available in the literature on the use of waste eggshell The physicochemical properties of the eggshell support and Ag
as a support to synthesize catalysts for VOC oxidation (most work catalysts were characterized by using different techniques.
focused on calcium sources for the synthesis of additives, bio- Power X-ray diffraction (XRD) analysis was performed on an X-
diesel, tissue scaffolds and adsorbents).
ray diffractometer (Rigaku Ultima IV, Japan) with Cu Ka radia-
Herein, we report a facile one-step route for preparing the tion at 40 kV and 30 mA to identify lattice parameters and to
Ag/eggshell catalysts, where the eggshell from kitchen waste is analyze the crystal phases. The surface morphology, pore
used as an efficient support and template. The Ag loaded- structure, and composition of the samples as well as Ag NP size
eggshell samples were utilized for the oxidation of benzene, and dispersion on the supports were obtained by scanning
and it is found that these catalysts show outstanding activity for electron microscopy (SEM) (ZEISS SIGMA, Germany) which was
the catalytic oxidation of benzene. It is believed that the facile equipped with an EDS elemental composition analyzer. The Ag
synthesis, available large-scale production, environmental NP size of the supported Ag catalysts was estimated by using
friendliness, low cost, non-toxic and excellent properties will Sigma Scan Pro soware (SPSS Inc., Version 4.01.003). High-
make the eggshell support widely used in the practical appli- angle annular dark-eld-scanning transmission electron
cation of VOC oxidation.
microscopy (HAADF-STEM) and energy dispersive X-ray spec-
troscopy (EDX) mapping were carried out by using using a Tec-
nai F30 microscope (TECNAI F30, USA) to identify the chemical
compositions of the crystal phases. Textural properties were
measured using a Micromeritics Tristar system (Tristar II3020,
USA). Before the surface area measurement, all samples were
2. Materials and methods
2.1 Materials and agents
The waste eggshells were obtained from the canteen located in
Quanzhou Normal University, China. The raw eggshells were
carefully rinsed with deionized (DI) water several times and
then dried overnight at room temperature. The dry eggshells
were ground and sieved through a 100-mesh screen to obtain
the eggshell powder. Silver nitrate (AgNO ) was purchased from
Aladdin (Shanghai, China). Commercial CaCO3 (com-CaCO3)
was purchased from Sinopharm Chemical Reagent Co. Ltd.
ꢀ
degassed at 300 C for 3 h. The actual loadings of Ag on the
eggshell support were obtained by inductively coupled plasma
optical emission spectrometry (ICP-OES) (5100 SVDV, USA). The
surface elements and chemical states of Ag/eggshell were pro-
bed by X-ray photoelectron spectroscopy (XPS) using mono-
chromated Al Ka (1486.7 eV) as an X-ray source in which the
binding energies (BEs) were calibrated with C 1s 284.8 eV
3
(Physical Electronics, USA). UV-vis diffuse reectance spec-
(Guangzhou, China). All chemicals were of analytical grade and
trometry (UV-vis DRS) characterization was obtained in the
wavelength range of 200–800 nm using a Cary 5000 spectro-
used as received. DI water was used throughout this work.
photometer (Varian Cary 5000, USA) and dehydrated BaSO was
4
2
.2 Preparation of Ag catalysts
used as the reference. Thermogravimetric (TG) characterization
ꢀ
The typical impregnation method was used to prepare the was performed from 30 to 800 C in the temperature range at
ꢀ
ꢁ1
ꢁ1
Ag/eggshell catalysts. Firstly, an AgNO solution with different a heating rate of 10 C min with an air ow of 50 mL min
3
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over a thermobalance (SDT Q600, USA). FTIR spectra were 2.5 In situ FTIR
performed on a Nicolet Avatar 660 (Nicolet 6700, USA), where
the KBr powder was mixed with the samples and then pressed
into an appropriate disk. Hydrogen temperature-programmed
In situ Fourier transform infrared spectroscopy (abbreviated to
in situ FTIR) was performed on a Nicolet Avatar 660 (Nicolet
ꢁ1
6
700, USA) at a resolution of 4 cm with 32 scans, which is
reduction (H
ical adsorption analyzer (MFTP-3060, China) by using a H
5/95, v/v) mixture as a reducing gas. Prior to the H -TPR oper-
2
-TPR) experiments were performed on a chem-
equipped with an MCT detector (cooled with liquid nitrogen)
and a stainless steel IR cell (CaF windows). Prior to each
2
–Ar
2
(
2
experiment, the IR cell was ushed with owing N
2
at a rate of
ꢁ1
ation, the samples were pretreated in an Ar ow of 30 mL min
at 300 C for 3 h, and the pretreated samples were heated from
RT to 900 C at a ramp rate of 5 C min in the H -Ar mixture of
5
ꢁ1
90 mL min for 2 h. At rst, the catalyst (approximately 15 mg)
ꢀ
was pressed into a supported disk. Then, the sample was under
ꢀ
ꢀ
ꢁ1
2
ꢀ
ꢀ
vacuum at 300 C for 0.5 h and subsequently activated at 300 C
ꢁ
1
0 mL min
.
ꢁ1
for 1 h in a 40% O
60 C. The background spectrum was collected under 40%
2
/N
2
(50 mL min ) stream and then cooled to
ꢀ
2
2 2
O /N and automatically subtracted from the next sample
2
.4 Catalytic activity measurements
spectra. Aerward, 1000 ppm benzene/40% O
2
/N
2
was intro-
ꢁ
1
ꢀ
The catalytic activity of benzene was performed in a continuous duced to the IR cell at a ow rate of 50 mL min at 260 C, and

ow xed-bed quartz microreactor (inner diameter ¼ 5 mm) subsequently the in situ FTIR spectra were measured at different
using a catalyst amount of 50 mg. The reactant gas contains times.
000 ppm benzene, where the benzene/O molar ratio was 1/400
1
2
and the remaining N was the balance gas, passing through the
2
reactor bed with different ow rates, resulting in the different 3. Results and discussion
ꢁ
1
ꢁ1
SV values of 20 000, 60 000, and 100 000 mL g
h , respec-
3
.1 Characterization of the pure eggshell and Ag/eggshell
tively. The temperatures of the column oven, gasication
chamber and detector were 130, 240 and 250 C, respectively for
CO and benzene detection. To probe the effect of water vapor
2
on the catalyst activity of benzene oxidation, 5.0 and 10.0 vol%
water vapor were introduced into the reaction system by passing
the reactant gas through a water saturator at 33 and 46 C. In the
case of analyzing the CO effect, 10.0 vol% CO was also intro-
2
duced into the reaction system using a high-purity CO cylinder.
catalysts
ꢀ
3.1.1 XRD analysis. The crystalline structure of pure
eggshell and the varying amounts of Ag NP loaded-eggshell
catalysts was characterized by XRD. As shown in Fig. 1A, the
ꢀ
main diffraction peaks of pure eggshell at 2q ¼ 23.2, 29.5, 31.6,
ꢀ
36.1, 39.5, 43.3, 47.7, 48.7, 57.6 and 60.8 were assigned to
2
2
3
CaCO (calcite, JCPDS PDF #29-0305). Compared with the pure
eggshell, the additional peaks of varying amounts of Ag NP
loaded-eggshell catalysts centered at 38.2, 44.4, 64.4, 77.4 and
The gas concentrations from the microreactor were analyzed
online by using a gas chromatograph (GC-9160, Ouhua China)
equipped with double FID detectors. A TDX-01 packed column
ꢀ
30
81.5 were ascribed to metallic Ag (JCPDS PDF #04-0783). It
can be seen that the peak intensities increased with the Ag
loading amount, conrming the excellent incorporation of Ag
(
1 m ꢂ 2 mm) and a SE-54 capillary column (30 m ꢂ 0.25 mm ꢂ
0
.5 mm) were employed to analyze CO and benzene concen-
2
NPs and the CaCO
no extra diffraction peaks were observed from the XRD patterns,
which indicated the pure phase of metallic Ag and CaCO . Other
components in the eggshell are too low in content to be
observed from XRD patterns, which could also manifest the
relatively pure Ag loaded-eggshell catalysts. Furthermore, the
magnication patterns of Ag peaks are analyzed in Fig. 1B in
detail. The diffraction peaks of Ag (111) and (200) become much
3
support. In addition, it is worth noting that
trations with Ar and N as protective gases, respectively. In the
2
catalytic activity experiments, the benzene conversion (Xbenzene
and CO
equations:
)
3
2
selectivity (YCO2) were calculated according to the
C
in ꢁ Cout
X
benzene
¼
ꢂ 100%
(1)
C
in
CðCO
CðCO
2
Þout
complete
Y
CO2
¼
ꢂ 100%
(2) stronger and sharper at increased Ag content, suggesting the
formation of larger and more abundant concentration of Ag
NPs. Comparably, almost no peak shis were observed in the
2
Þ
For the kinetic studies, the turnover frequencies (TOFAg
were calculated according to the equation:
)
diffraction peaks of Ag (111) and (200), demonstrating that no
related Ag compounds are formed, consistent with results in
Fig. 1A.
3.1.2 STEM observations and corresponding elemental
mapping. The morphologies of Ag NP loaded-eggshell catalysts
C
benzene
X
benzene
V
gas
À
Á
ꢁ1
TOFAg
¼
s
(3)
n
Ag
where Cin and Cout are the benzene concentration correspond- can be observed from the STEM images. Fig. 2A–E show the
ing to the inlet and outlet, respectively, C(CO out is the outlet HAADF-STEM and mapping images of the Ag
2
)
2
/eggshell catalyst.
CO concentration and C(CO )complete is the CO concentration The EDX elemental mapping images were applied to elucidate
2
2 2
of complete conversion of benzene to carbon dioxide, and Vgas is the existence and the dispersion of Ag, Ca, C and O elements on
the total molar ow rate, Cbenzene is the benzene concentration the catalyst. The microstructure of Ag /eggshell was also
2
of the gas mixture, and nAg is the molar amount of Ag in the determined by HRTEM. As shown in Fig. 2F, the HRTEM image
catalyst.
of the Ag
2
/eggshell catalyst demonstrates the lattice spacings of
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Fig. 1 XRD patterns of pure eggshell and xAg/eggshell catalysts: (A) overall profiles and (B) magnification of Ag peaks.
Fig. 2 (A) HAADF-STEM image of Ag
HRTEM image of Ag /eggshell.
2
/eggshell and the corresponding EDX elemental mapping images of (B) Ag, (C) Ca, (D) C and (E) O and (F)
2
0.241 and 0.307 nm, which were ascribed to the (111) plane of created in between particles). Loading Ag onto the eggshell
Ag and (112) plane of CaCO , respectively according to the support did not obviously change the textural properties of the
3
standard PDF cards of Ag (JCPDS PDF #04-0783) and CaCO3 supported Ag catalyst, indicating that the majority of Ag NPs
(JCPDS PDF #29-0305). It can also be observed (from Fig. S1†) were highly dispersed on the eggshell (Table 1).
that a number of good-quality spherical Ag NPs were highly 3.1.4 XPS analysis. XPS analysis was performed to charac-
dispersed on the surface of the eggshell support. It is assured terize the electronic states of the Ag/eggshell catalysts with
that plentiful Ag NPs were well generated on the surface of the different Ag NP loadings. As shown in Fig. 4A, two binding
eggshell. In addition, it can be seen from Fig. S2† that the pure energies with one doublet of Ag 3d are detected. The peaks at
Ag NPs are all spherical and have a uniform particle size, which about 368.3 and 374.3 eV are assigned to the surface metallic
0
is consistent with the Ag NPs dispersed on the eggshell support. Ag species. A shi to lower binding energy by 0.2 eV was
3
.1.3 BET analysis. The textural properties of the eggshell observed for Ag of Ag /eggshell (368.2 eV), compared to Ag /
2
1
3
support and Ag catalysts have an important effect on the cata- eggshell and Ag /eggshell (368.4 eV). It has been reported that
lytic performance. As shown in Fig. 3A and B, N
2
adsorption– the Fermi energy levels would compromise to a new value in the
desorption isotherm curves of eggshell and supported Ag cata- case of a strong synergistic effect between Ag NPs and the
19
lysts show a characteristic isotherm pattern of non-porous support. As a result, electron transfer from Ag NPs to the
2
ꢁ1
materials (surface area 1–2 m g ). The existing porosity in eggshell support occurred, decreasing the core-level binding
the materials is due to interparticle meso/macroporosity (voids energy of Ag. Hence, the lower shi for Ag of Ag /eggshell was
2
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Fig. 3 Nitrogen adsorption–desorption isotherm and the corresponding pore size distribution curves (inlet): (A) xAg/eggshell catalysts with
different Ag loadings and (B) the optimum Ag /eggshell calcined at different temperatures.
2
Table 1 Textural properties of various samples
531.2 eV and 532.9 eV, which can be derived from the asym-
metrical element O 1s of the catalysts. The binding energy peak
a
BET surface area
Pore volume
34
at 531.2 eV pertains to the Olatt (lattice oxygen) in CaCO
3
, and
2
ꢁ1
3
ꢁ1
Sample
(m g
)
(cm g
)
the other at 532.9 eV can be ascribed to the adsorbed oxygen
ꢁ
ꢁ
2ꢁ 35,36
Eggshell
<5
<5
<5
<5
<5
<5
<5
0.008
0.005
0.004
0.003
0.010
0.007
0.008
species Oads (adsorbed oxygen, e.g., O2 , O or O2 ).
Ag
1
Ag
2
Ag
3
Ag
2
Ag
2
Ag
2
/eggshell
/eggshell
/eggshell
/eggshell (300 C)
/eggshell (400 C)
3.1.5 TG and FTIR analysis. TG characterization was
carried out to investigate the decomposition of the pure
eggshell and xAg/eggshell catalysts. The TG curves of all the
samples are shown in Fig. 5A. The TG curve of the pure eggshell
ꢀ
ꢀ
ꢀ
ꢀ
/eggshell (600 C)
support demonstrated a slight decline at 217–373 C and
ꢀ
a
a sharp decrease at 560–769 C, which is attributed to the
Calculated from the volume adsorbed at P/P
0
¼ 0.99.
decomposition of the eggshell membrane and CaCO , respec-
3
tively. And the weight loss of the eggshell membrane was
accounted for 2.1 wt%. Compared with the pure eggshell, all the
a proof of the stronger synergistic effect between Ag nano-
xAg/eggshell catalysts showed an extra decrease in the range of
particles and the eggshell support than that of Ag /eggshell and
1
ꢀ
1
30 to 250 C, which is attributed to the decomposition of
Ag /eggshell, resulting in improved catalytic properties.
3
AgNO
3
. Furthermore, the weight loss of the different Ag/
Furthermore, the signals of the metallic Ag become stronger
and stronger (Ag /eggshell > Ag /eggshell > Ag /eggshell) with
3 2 1
ꢀ
eggshell catalysts between 130–250 C is obviously different
3 2 1
(ranked as Ag /eggshell > Ag /eggshell > Ag /eggshell), which is
the increased Ag loadings. Besides, no other peaks were
observed, which elucidated the complete conversion of oxida-
consistent with the data obtained by ICP-OES. In addition, the
FTIR spectra of the eggshell and xAg/eggshell catalysts were
obtained to validate the existence of different functional groups.
As shown in Fig. 5B, nine characteristic absorption bands were
observed in every spectrum. This indicates that they have
similar FTIR absorption. Moreover, the positions and intensi-
ties of the absorption bands were not affected by the loading of
Ag NPs on the eggshell. The characteristic absorption band
+
tion valence state Ag to metallic Ag during the preparation
process. It was also in accordance with the results of XRD
(Fig. 1). Fig. 4B shows the Ca 2p spectra of the three catalysts,
and one can see that a set of peaks occurred corresponding to
the Ca 2p3/2 and 2p1/2 electronic states. The peaks at 347.2 eV
reect the Ca 2p3/2 features while the peaks with a binding
energy of about 350.6 eV belong to Ca 2p1/2. As a result, the peak
position spacing between Ca 2p3/2 and Ca 2p1/2 from all the
catalysts was around 3.5 eV. Then, the C 1s peaks (Fig. 4C) of the
silver supported catalysts have two binding energies with one
double, which is located at around 284.7 and 289.7 eV. The peak
corresponding to C–C/C–H bonds was observed in all the
samples at approximately 284.7 eV assigned to the carbon from
inadequate combustion of organics, and this peak was always
ꢁ
1
centering at 3434 cm was assigned to –OH stretching vibra-
37
tion from adsorbed water. The weak band centering at 2976,
ꢁ
1
2
875 and 1800 cm can be ascribed to C]O bands derived
ꢁ1
from carbonate. The moderate absorption peak at 2517 cm
was the characteristic peak of CaCO
characteristic absorption peaks at 1422 and 878 cm corre-
spond to the C–O stretching and bending modes of CaCO
3
. Another two well-dened
ꢁ
1
3
,
3
8
ꢁ1
respectively. The peak at 1639 cm can be attributed to N–H
31,32
elucidated as a reference calibration.
Besides, the peak ob-
ꢁ1
bending mode. The absorption band at 709 cm
can be
tained at 289.7 eV demonstrated the presence of the carbonate
39
assigned to Ca–O bonds. In summary, these peaks are char-
33
group in the Ag/eggshell catalysts. Furthermore, it can be seen
from Fig. 4D that there are two binding energy peaks at around
acteristic functional groups of eggshell.
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Fig. 4 XPS spectra of (A) Ag 3d, (B) Ca 2p, (C) C 1s and (D) O 1s for the xAg/eggshell catalysts.
3
.1.6
formed to demonstrate the reducibility of the samples. reduction peak of all the catalysts shied to a lower temperature
According to the H
-TPR proles in Fig. 6, there is only one than the one of pure eggshell which indicated that Ag NP
2 2
H -TPR analysis. H
-TPR experiments were per- that when Ag NPs were loaded onto the eggshell support, the
2
reduction peak in all the samples. The prole of pure eggshell loading could obviously enhance the reducibility of the
ꢀ
(
Fig. 6A) showed a single peak at 759 C, which is ascribed to eggshell. As the Ag loading increases, the reduction peak
CO generation from the decomposition of eggshell. No peaks gradually shied to a lower temperature. Based on the Mars–
2
6,40
assigned to the reduction of AgO to Ag were observed, which van Krevelen (redox) mechanism,
such shis demonstrate
indicated that all the Ag precursors were completely converted a strong interaction between Ag NPs and the eggshell support,
0
into Ag in this preparation method. It can be seen from Fig. 6A which could benet the low-temperature reducibility of the
Fig. 5 (A) Simultaneous TG profiles and (B) FTIR spectra of pure eggshell and xAg/eggshell materials.
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Fig. 6
2 2
H -TPR profiles of (A) xAg/eggshell catalysts with different Ag loadings and (B) the optimum Ag /eggshell calcined at different
temperatures.
0
43
supported Ag catalysts and enhance the catalytic performance. were assigned to Ag species. And no absorption bands at
+
As shown in Fig. 6B, despite all the catalysts containing the about 200 nm associated with Ag were observed in the three
same Ag contents, the reduction peaks shied to higher samples, which demonstrated that all the Ag precursors were
temperature with respect to the increased calcination temper- reduced completely. As for Fig. 7B, all the Ag
2
/eggshell catalysts
ature of the Ag /eggshell, affirming that higher calcination calcined at different temperatures have similar peak positions
2
conditions could decrease the reducibility of the catalysts. In and intensities, which indicated that they have the same Ag
conclusion, when Ag NPs were loaded on the eggshell, the species and content.
reducibility of the eggshell was enhanced, demonstrating
a synergistic effect between the Ag and eggshell in Ag /eggshell. 3.2 Catalytic performance in benzene oxidation
2
3
.1.7 UV-vis DRS analysis. UV-vis DRS was further applied
The pure eggshell did not have any catalytic activity for the
to identify the Ag oxidation state of the xAg/eggshell catalysts. It
can be seen from Fig. 7A that no peaks were observed above
00 nm on pure eggshell. In contrast, when Ag NPs were loaded
onto the eggshell support, the absorption bands of xAg/eggshell
at about 314 and 348 nm were ascribed to the oxidized silver
clusters (Agn ), which were formed by the interaction between
the eggshell and metallic silver. And the peak intensities at
14 nm became stronger as the Ag NP loading increased, which
was helpful for the catalytic oxidation of benzene. As is well
known, Ag and oxidized silver clusters (Ag
with respect to ethylene epoxidation, as opposed to Ag .
absorption bands of xAg/eggshell (Fig. 7A) at 389 and 405 nm
oxidation of benzene until the reaction temperature reached
ꢀ
3
80 C (Fig. S3†). The effect of Ag loading on the catalytic
3
performance of benzene was further investigated. As can be
seen in Fig. 8A, the catalytic performance of Ag/eggshell cata-
ꢀ
lysts (all calcined at 500 C) was signicantly improved by
d+
immobilizing the Ag NPs onto the eggshell, compared with the
pure eggshell. What is more, the higher Ag loading resulted in
higher benzene conversion at a certain temperature. However,
the benzene conversion increase became not obvious when the
41
3
0
d+
n
) are active species
2
Ag NP loading exceeded 19.9%. Hence, Ag /eggshell was deter-
+
41,42
The
mined to be an optimal Ag loading, in which the T90% (the
ꢀ
temperature for achieving 90% benzene conversion) was 225 C,
Fig. 7 UV-vis DRS profiles of (A) xAg/eggshell catalysts with different Ag loadings and (B) the optimum Ag
2
/eggshell calcined at different
temperatures.
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Fig. 8 Benzene conversion as a function of reaction temperature over (A) pure Ag NPs and. xAg/eggshell catalysts with different Ag loadings and
ꢁ1
ꢁ1
(
B) effect of calcination temperature of Ag
2
/eggshell at SV ¼ 20 000 mL g
h .
ꢀ
ꢀ
ꢀ
which is lower than that of reported other kinds of catalysts sequence of Ag
Table S2†). Besides, the catalytic oxidation ability of pure Ag (0.112) > Ag
2
/eggshell-500 C (0.153) > Ag
2
/eggshell-400 C
ꢀ
(
2
/eggshell-300 C (0.080) > Ag
2
/eggshell-600
C
NPs was also tested and the conversion was lower than that of (0.026), which is consistent with the catalytic curves observed in
Ag
2
/eggshell and Ag
3
/eggshell catalysts at a certain temperature. Fig. 8B. Furthermore, it can be seen from the XRD patterns
ꢀ
The TOFAg values were calculated at low conversions at 190 C (Fig. S5†) that with the increase of calcination temperature, the
under a kinetically controlled regime for the oxidation of peaks belonging Ag NPs tended to become narrow, which
ꢁ
4
ꢁ1
benzene. It is demonstrated that the TOFAg (10
s ) values of demonstrated that the sizes of the generated Ag NPs were
the samples decreased in the sequence of Ag /eggshell (0.153) > increasing. This observation was further conrmed by the SEM
2
Ag
with the lowest TOFAg value being observed over pure Ag NPs
and highest TOFAg value of Ag /eggshell. This calculation result eggshell calcined at different temperatures are described in
explains why Ag /eggshell was chosen as the catalyst support. Fig. S6.† It can be seen that the spherical Ag NPs were highly
3
/eggshell (0.129) > Ag
1
/eggshell (0.073) > pure Ag NPs (0.005), images, as shown below.
SEM images and size distributions (insets) of the Ag
2
/
2
2
Considering the characterization analysis of Fig. 6A and 7A, the distributed on the surfaces of the eggshell supports. The
observed enhanced catalytic activity for benzene oxidation average particle size increased slowly as the calcination
ꢀ
results from a synergistic interaction between the eggshell and temperature increased from 300 to 500 C, which corresponded
Ag NPs.
to 197.7, 246.5, and 276.3 nm, respectively. Nevertheless, the
Besides, SEM images and size histograms of Ag NPs with average particle size of Ag NPs dramatically increased to
ꢀ
different Ag contents are shown in Fig. S4.† It is obvious that the 402.0 nm when the calcination temperature was 600 C, which
spherical Ag NPs were uniformly distributed on the eggshell suggested the obvious agglomeration of the Ag NPs at this
supports. The mean diameter of Ag NPs increased from 237.5 to temperature. In general, the smaller the noble nanoparticles of
308.8 nm, with the increase of Ag content from 10.8 to 34.3%, supported noble metal catalysts, the greater the number of
which is consistent with the phenomenon observed in Fig. 1. active sites and interfaces, and it will be benecial to the cata-
The superior performance observed for the highest Ag loading lytic activity enhancement. However, according to the H -TPR
2
might be ascribed to the larger Ag content and presence of analysis in Fig. 6B, as the calcined temperature of the Ag2/
active Ag sites which results in a higher catalytic activity for eggshell increased, the reduction peaks shied to higher
benzene oxidation.
temperature, which leads to the decrease of catalytic activity.
Calcination is necessary to decompose the AgNO
3
for the Therefore, the catalytic performance of Ag NP loaded-eggshell
formation of Ag loaded-eggshell catalysts. Therefore, different catalysts is a comprehensive effect of Ag NP dispersion/size
calcination temperatures might inuence the size, morphology and synergistic effect. Therefore, choosing an available calci-
and polymorphism of the resulting products, and nally, the nation temperature for preparing the catalyst is extremely
44
catalytic activity. Considering the Ag /eggshell catalyst as the important to the catalytic activity of the reaction system. As
2
optimum option in the system, we tested its catalytic activity mentioned, the optimal catalytic activity of Ag /eggshell was
2
ꢀ
ꢀ
calcined at 300, 400, 500 and 600 C. As shown in Fig. 8B, the obtained at 500 C.
benzene conversion increased as the temperature increased
ꢀ
from 300 to 500 C, but declined signicantly as the tempera-
ture reached 600 C. Hence, 500 C was regarded as the optimal 3.3 Stability of the Ag /eggshell catalyst
2
ꢀ
ꢀ
calcination temperature. The TOFAg values were calculated at
It is obvious that both the catalytic performance and stability
are equally important parameters in various industrial appli-
cations. Thus, the durability of the as-prepared Ag /eggshell
2
ꢀ
90 C corresponding to the low conversions. It is conducted
1
ꢁ
4
ꢁ1
that the TOFAg (10
s ) values of the samples decreased in the
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35,47
catalyst was determined. As shown in Fig. 9, the benzene activity of the catalysts.
In order to examine the inuence of
conversion remains constant (ꢃ95%) within 200 h of testing, water vapor on the catalytic performance of the Ag /eggshell
2
which demonstrated the excellent durability of the catalyst. In catalyst, the on-stream benzene oxidation experiments in the
order to explore what happened to the catalyst before and aer presence of 5.0 and 10.0 vol% water vapor were carried out. As
reaction, a series of experiments including SEM, TG/DTG, XRD, shown in Fig. 10A, when the 5.0 vol% water vapor was intro-
XPS and UV-vis DRS are carried out. As indicated in Fig. S7,† all duced into the feed steam, the benzene conversion and CO
2
these characterization results of the catalyst aer 200 h at yield were reduced from 98.6 and 98.9% to 83.2 and 77.4%,
ꢀ
2
30 C reaction are much the same as the unreacted one. No respectively. When the content of water vapor was further
2
obvious changes in terms of textural/surface properties were increased to 10.0 vol%, the benzene conversion and CO yield
observed for Ag catalysts aer 200 h reaction. Compared to the decreased to 80.3 and 73.4%, respectively. Thus, with the
catalyst before reaction, slight binding energy shis of Ag 3d increase of water vapor content, the conversion is increasingly
and O 1s aer reaction were found. The peaks slightly shied to reduced. However, aer water vapor is cut off, the benzene
lower binding energy, assigned to the differential charging conversion would be restored to the original values again. These
effect by the interaction between Ag and the support, as dis- results show that the structural integrity of the Ag/eggshell
19
cussed above. These results also indicated an optimum catalyst is kept even for long term water vapor running tests.
stability of the catalyst under long-term reaction conditions.
Obviously, water vapor plays a negative role in catalyzing
benzene oxidation, which is attributed to the competitive
adsorption between water vapor and benzene.
14,35
3.4 Effects of space velocity, water vapor and CO
2
As another product of benzene oxidation-CO , its effect on
2
To meet the requirements of industrial applications, the treat-
ment of VOCs is usually carried out at a high space velocity.
Furthermore, the high SV will have an essential inuence on the
catalytic performances of the Ag/eggshell catalysts. Therefore,
the inuence of various SV values ranging from 20 000 to
benzene conversion was also examined. As can be seen in
Fig. 10B, the introduction of 10.0 vol% CO
2
in the feed steam
ꢀ
caused a decrease (by ca. 14.4% for 280 C and ca. 6.5% for
ꢀ
3
00 C, respectively) in the catalytic performance of the Ag
2
/
eggshell catalyst. However, the benzene conversion will be
almost restored to the original values when the CO was cut off.
ꢁ1
ꢁ1
1
00 000 mL g
catalyst was investigated. As shown in Fig. S8,† it is a general
fact that benzene conversion and CO yield decreased with the
increase of SV at the same reaction temperature. In the case of
h
on the catalytic activity of the Ag /eggshell
2
2
The negative effect might be due to the surface active site
2
deactivation of the Ag /eggshell, which was caused by the
2
formed carbonate species coating. It is noticeable that the
benzene conversion decreased about 14.4% for a reaction
ꢁ
1
ꢁ1
the biggest SV (100 000 mL (g
of benzene and total CO
a temperature of about 280 C, which is lower than that of re-
ported other kinds of catalysts,
mances of the Ag/eggshell based catalyst. Moreover, one can see
that the benzene conversion is equated with the CO yield on
the whole, indicating that benzene is completely oxidized into
CO over the Ag /eggshell catalyst, and no byproducts are
h
)), the complete conversion
/eggshell were obtained at
yield of Ag
2
ꢀ
ꢀ
2
temperature of 280 C, compared to 6.5% for 300 C. The reason
is that higher temperature would promote the decomposition
rate of carbonate species. The resulting benzene conversion of
ꢀ
45,46
revealing the good perfor-
2
the CO introduction is a balance of carbonate species accu-
2
mulation and degradation. These carbonate species would be
almost decomposed aer CO2 was cut off, which led to the
recovery of the surface active sites of the catalyst. The conver-
2
2
produced through the catalytic reaction.
It has been generally accepted that water vapor introduced
into the feed steam has an important effect on the catalytic
sion aer the cut off of CO
inal values, suggesting that fewer carbonate species are still on
the active sites of the Ag /eggshell catalyst.
2
is about 1.9% lower than the orig-
2
2 2 3
3.5 Comparison between Ag /eggshell and Ag /com-CaCO
catalysts
As shown in Fig. 11, the benzene oxidation conversion was done
using Ag /eggshell and Ag /com-CaCO catalysts. It is worth
2
2
3
noting that both catalysts had the same preparation method
and corresponded with the same Ag loading amount (19.9%).
From it, one can see that the activity of the Ag
signicantly outperformed that of the Ag /com-CaCO
To nd out the possible reason, SEM observation and BET
analysis were also performed for the Ag /com-CaCO catalyst.
As shown in Fig. S9,† the Ag NPs were distributed inhomo-
geneously on the surface of com-CaCO , and their sizes are
2
/eggshell catalyst
2
3
catalyst.
2
3
3
either too large or too small. By calculation, the average Ag NP
size of the Ag /com-CaCO (627.1 ꢄ 284.3 nm) was much larger
2
3
Fig. 9 Benzene conversion as a function of on-stream reaction time
ꢁ1
ꢁ1
h and
than that of the Ag
2
/eggshell (276.3 ꢄ 90.7 nm shown in
over Ag
2
/eggshell. Reaction conditions: SV ¼ 20 000 mL g
ꢀ
reaction temperature ¼ 230 C.
Fig. S6c†). Therefore, it is concluded that the Ag NPs in the
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ꢀ
Fig. 10 Benzene conversion over Ag
and 300 C at SV ¼ 100 000 mL g
2
/eggshell (A) in the presence or absence of water vapor at 280 C and (B) effects of 10.0 vol% CO
h .
2
at 280
ꢀ
ꢁ1 ꢁ1
Ag/com-CaCO catalyst tended to agglomerate to form oversized 3.6 In situ FTIR studies
3
particles during the preparation process. The reason for this
agglomeration may be due to the difference in the supports. As
mentioned above, eggshell is a kind of natural biomaterial
3
containing CaCO and a layer of membrane (proteins and
In situ FTIR characterization was performed to explore the
intermediate species along with the oxidation of benzene, and
the result of the Ag
be observed that the Ag
2
/eggshell catalyst is shown in Fig. 12. It can
/eggshell catalyst demonstrated similar
2
polysaccharides). Thus, the eggshell membrane plays a crucial
role in regulating the size and distribution of Ag NPs during the
preparation process because of the strong metal–protein
spectra at different reaction time intervals. The band at
ꢁ1
3
055 cm can be assigned to the phenylic C–H vibration of the
49
ꢁ1
aromatic ring. The band located at 2938 cm is associated
with the C–H stretching vibrations. The band at 1619 cm can
24,48
bonding interactions.
In addition, it can be seen from
50
ꢁ1
Fig. S10† that the eggshell is composed of porous structure
characters, which increased the contact area and was benecial
for the reaction gas and energy transmission. In order to explore
be ascribed to the vibration of the aromatic ring which was
51
adsorbed parallel to the catalyst surface. It is commonly
ꢁ1
accepted that the band at 1570 cm should be attributed to the
the textural properties in detail, BET analysis of com-CaCO
Ag /com-CaCO was also carried out in comparison. As shown in
Fig. S11 and Table S1,† the BET surface areas of eggshell and
Ag /eggshell are very similar as compared to com-CaCO and
Ag /com-CaCO . The difference in the morphology and chemical
3
and
52
ꢁ1
phenolate species. In addition, the band at 1710 cm can be
2
3
53
specially assigned to the C]O from quinone species. Besides,
ꢁ1
the bands at 1306, 1438, and 1519–1542 cm are characteristic
2
3
53
ꢁ1
of maleate species. The bands at 1349–1355 cm
(CH3
2
3
54
stretching vibration) should be assigned to the acetate species.
Besides, the band at 1400 cm was attributed to o-benzoqui-
none. Furthermore, the band at 1595 cm can be assigned to
carboxylate species (ring vibrations), derived from the oxidation
of the breaking ring. These results above are in accordance
composition may be the reason for different catalytic activity. On
one hand, the oversized Ag NPs will reduce the surface area of
the active sites. On the other hand, the interaction between the
ꢁ1
55
ꢁ1
oversized-Ag NPs and the CaCO
3
support will be signicantly
50
weakened, thereby decreasing the catalytic performance.
with previous research on the oxidation of benzene.
ꢁ1
ꢁ1
h , (A)
Fig. 11 Benzene oxidation catalyzed by the Ag catalysts with different supports as a function of temperature at SV ¼ 100 000 mL g
2
benzene conversion and (B) CO yield.
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ꢀ
2 2 2
Fig. 12 In situ FTIR spectra in a 1000 ppm benzene/40% O /N stream at 260 C over Ag /eggshell at different times.
In order to determine the source of oxygen required for the Oads species play essential roles during the VOC catalytic
initiation step of catalytic benzene oxidation, the in situ FTIR oxidation. On the basis of the O 1s spectra and in situ FTIR
experiments of 1000 ppm benzene/N
eggshell catalyst at 260 C were also performed. It can be seen reacted with the active oxygen species. The surface active oxygen
2
adsorption over the Ag
2
/
results (Fig. 12 and S12†), it was demonstrated that benzene
ꢀ
from Fig. S12† that four broad bands centered at 3058, 1700, species could be supplemented by gas phase oxygen along with
ꢁ1
1
495 and 1388 cm can be observed, respectively. It demon- the catalytic benzene oxidation process. Benzene rst reacted
strated similar band regions, compared to Fig. 12. It is calculated with the active oxygen species at the Ag-eggshell interface and
that the band intensities were weaker than the spectra charac- produced phenolate species (A); the phenolate species possess
terized by the 1000 ppm benzene/O /N , and the reason may be an oxygen-containing functional group which is reserved as an
2
2
53
that the oxygen supply is insufficient in the oxygen-consuming ortho–para position director and an electron provider. There-
process. Comparing Fig. 12 with Fig. S12,† it is generally fore, benzoquinones are further generated through the trans-
accepted that benzene rst reacted with the active oxygen species formation of the phenolate species (B); as the reaction proceeds,
of the catalyst, other than the oxygen gas. When the active oxygen the active oxygen species would give rise to the cleavage of
was consumed, the oxygen gas was transformed into active benzene rings resulting in the formation of carboxylate species
oxygen on the catalysts, thereby achieving the oxygen circulation (C), which are the main intermediate species during the cata-
process of the catalytic performance on the catalyst.
lytic process; nally, the carboxylate species can further react
with active oxygen species into the nal products (i.e., CO and
2
H O) (D).
2
3.7 Reaction mechanism
Combining the characterization discussed above, the reaction
mechanisms can be demonstrated in Fig. 13. As is well known,
4. Conclusions
Waste eggshells were rstly used as a template and support to
prepare Ag/eggshell catalysts, and their application to VOC
catalytic oxidation is investigated. The strategy is facile, envi-
ronmentally friendly and economical. Due to the involvement of
biomolecules, the limited-sized Ag NPs were well dispersed on
the surface of the eggshell. Compared with the Ag/com-CaCO
3
catalyst, the Ag/eggshell showed an outstanding catalytic
oxidation performance towards benzene, which is attributed to
the even particle size distribution and channel structures. The
ꢀ
optimal parameter of T90% with the Ag
2
/eggshell is 257 C at SV
ꢁ
1
ꢁ1
¼
100 000 mL g
h . In addition, it keeps excellent stability
even aer 200 h of on-stream reaction. The introduction of
water vapor into the feed steam induced a decrease in the
catalytic activity, and the introduction of CO
2
also led to
a decrease in the catalytic activity. However, they did not affect
the structure of the catalysts. In situ FTIR results show that
carboxylate species are the main intermediate species in the
catalytic benzene oxidation, which would be nally oxidized to
Fig. 13 Proposed mechanism for the catalytic benzene oxidation over
the Ag /eggshell catalyst.
2
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2 2
the products–CO and H O. Based on these results, the reaction 16 C. Chen, F. Chen, L. Zhang, S. Pan, C. Bian, X. Zheng,
mechanism for the catalytic oxidation of benzene over the Ag/
eggshell catalyst was proposed. The present work clearly
X. Meng and F. S. Xiao, Chem. Commun., 2015, 51, 5936–
5938.
showed the potential use of waste eggshell-derived Ag catalysts 17 Z. Wu, J. Deng, Y. Liu, S. Xie, Y. Jiang, X. Zhao, J. Yang,
for VOC oxidation and gave us a valuable insight into designing
eggshell-based catalysts/supports for industrial applications.
H. Arandiyan, G. Guo and H. Dai, J. Catal., 2015, 332, 13–24.
18 M. Manzoli, F. Vindigni, T. Tabakova, C. Lamberti,
D. Dimitrov, K. Ivanov and G. Agostini, J. Mater. Chem. A,
2017, 5, 2083–2094.
Conflicts of interest
19 F. Yang, J. Huang, T. Odoom-Wubah, Y. Hong, M. Du,
D. Sun, L. Jia and Q. Li, Chem. Eng. J., 2015, 269, 105–112.
There are no conicts to declare.
2
2
2
0 F. Yang, X. Jing, J. Huang, D. Sun and Q. Li, Ind. Eng. Chem.
Res., 2015, 54, 5373–5380.
1 X. Jing, J. Huang, H. Wang, M. Du, D. Sun and Q. Li, Chem.
Eng. J., 2016, 284, 149–157.
2 B. Bai, Q. Qiao, H. Arandiyan, J. Li and J. Hao, Environ. Sci.
Technol., 2016, 50, 2635–2640.
Acknowledgements
This work was sponsored by the National Natural Science
Foundation of China (21536010, 41673088 and 81472001), the
Minjiang Scholars Program of Fujian Province, the Tongjiang
Scholars Program of Quanzhou City, the Natural Science
Foundation of Fujian Province (2015J05030) and the Quanzhou
City Science & Technology Program of China (2017G023). The
publication was also prepared with support from the RUDN
University Program 5-100.
2
2
3 H. Einaga and A. Ogata, Environ. Sci. Technol., 2010, 44, 2612.
4 M. Nasrollahzadeh, S. M. Sajadi and A. Hatamifard, Appl.
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7 S. J. Abbas, P. V. R. K. Ramacharyulu, H. H. Lo, S. I. Ali and
S. C. Ke, Appl. Catal., B, 2017, 210, 276–289.
8 R. Mallampati and S. Valiyaveettil, ACS Sustainable Chem.
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