On the performance and mechanisms of toluene removal by FeOx/SBA-15-assisted non-thermal plasma at atmospheric pressure and room temperature

Article abstract of DOI:10.1016/j.cattod.2014.07.026

FeO_x/SBA-15 catalysts were prepared via impregnation and utilized for toluene removal in dielectric barrier discharge (DBD) plasma at atmospheric pressure and room temperature. Toluene removal was investigated in the environment of various mixed N₂/O₂ plasmas, showing that toluene removal efficiency and CO_x selectivity were greatly increased by FeO_x/SBA-15 and that the organic intermediates were greatly reduced by catalysts. In pure N₂ plasma, the bulk oxygen in the catalyst was involved in the toluene oxidation, and the 3%FeO_x/SBA-15 catalyst showed the optimal toluene oxidation activity. The catalysts were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), N₂ adsorption-desorption, X-ray photoelectron spectroscopy (XPS), H₂ temperature-programmed reduction (H₂-TPR) and O₂ temperature-programmed desorption (O₂-TPD), showing that toluene oxidation was closely related to the highly dispersed nature of iron on the SBA-15 surface, the reduction temperature of Fe²⁺ and the oxygen adsorption ability of the catalyst. The pathways of toluene decomposition in the combination of FeO_x/SBA-15 with a non-thermal plasma (NTP) system were proposed based on the identified intermediates.

Full text of DOI:10.1016/j.cattod.2014.07.026

Catalysis Today 242 (2015) 274–286
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
On the performance and mechanisms of toluene removal by
FeOx/SBA-15-assisted non-thermal plasma at atmospheric pressure
and room temperature
Meijuan Lua, Rong Huanga, Junliang Wua,b, Mingli Fua,b, Limin Chena,b, Daiqi Yea,b,∗
a College of Environment and Energy, South China University of Technology, Guangzhou 510006, China
b Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control (SCUT), Guangzhou 510006, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 May 2014
Received in revised form 6 July 2014
Accepted 10 July 2014
Available online 10 August 2014
FeOx/SBA-15 catalysts were prepared via impregnation and utilized for toluene removal in dielectric
barrier discharge (DBD) plasma at atmospheric pressure and room temperature. Toluene removal was
investigated in the environment of various mixed N2/O2 plasmas, showing that toluene removal effi-
ciency and COx selectivity were greatly increased by FeOx/SBA-15 and that the organic intermediates
were greatly reduced by catalysts. In pure N2 plasma, the bulk oxygen in the catalyst was involved in
the toluene oxidation, and the 3%FeOx/SBA-15 catalyst showed the optimal toluene oxidation activity.
The catalysts were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), N2
adsorption–desorption, X-ray photoelectron spectroscopy (XPS), H2 temperature-programmed reduc-
tion (H2-TPR) and O2 temperature-programmed desorption (O2-TPD), showing that toluene oxidation
was closely related to the highly dispersed nature of iron on the SBA-15 surface, the reduction tempera-
ture of Fe2+ and the oxygen adsorption ability of the catalyst. The pathways of toluene decomposition in
the combination of FeOx/SBA-15 with a non-thermal plasma (NTP) system were proposed based on the
identified intermediates.
Keywords:
Dielectric barrier discharge
Toluene removal
Catalyst of FeOx/SBA-15
Oxygen species
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
its special micropore structure [20]. Thus, the residence time of
toluene can be further increased if SBA-15 is added into the plasma
Volatile organic compounds (VOCs), as chief atmospheric pollu-
tion precursors, are harmful to humans and the environment [1].
The toluene is a typical VOC in that it is emitted in high levels
and difficult to abate [2]. Consequently, considerable attention has
been paid to the development of methods for its removal. Recently,
NTP technology has emerged as a promising method [3–11]. How-
ever, it also suffers from high energy consumption and incomplete
oxidization, and the process is difficult to control [12,13]. To over-
come these problems, the used of a catalyst has been coupled
with NTP [14–16], wherein the VOCs removal rate increases with
the assistance of adsorption [17]. Considering the importance of
adsorption, various porous materials, such as ␥-Al2O3 and molecu-
lar sieves, have been studied [18–20]. The mesoporous material
SBA-15 shows relatively high adsorption for toluene because of
system. However, SBA-15 is chemically inert for various catalytic
applications because of its lack of acid and redox sites [21]. The
incorporation of metal is an efficient method to improve the catal-
ysis properties of this material. Among the various metal oxides,
iron oxide has received extensive attention due to its relatively low
cost, high activity, and environmental friendliness [22,23] as well as
the O2 transportation via facile interconversion between Fe2+ and
Fe3+ states [24]. Our previous work has also disclosed the outstand-
ing performance of FeOx for toluene degradation in DBD plasma
[25]. Therefore, it is vital to incorporate iron into SBA-15 mate-
rial for VOCs plasma-catalysis oxidation because of the contrasting
characteristics of these two materials. However, at present, the
application of NTP coupled with FeOx/SBA-15 catalysis in toluene
degradation is scarce in the literature.
Oxygen species serve as an oxidizing agent to scavenge the
VOCs and form carbon oxides. Their two main sources are gas-
phase oxygen in the feed gas stream and bulk-phase oxygen in
the catalyst. The gas-phase oxygen can be dissociated into atomic
oxygen by discharge [26], and can promote the decomposition of
CF4 into COx in air thermal plasmas [27]. Previous research shows
∗
Corresponding author at: College of Environment and Energy, South China Uni-
versity of Technology, Guangzhou Higher Education Mega Centre, Panyu District,
Guangzhou 510006, China. Tel.: +86 20 39380516; fax: +86 20 39380518.
E-mail address: cedqye@scut.edu.cn (D. Ye).
http://dx.doi.org/10.1016/j.cattod.2014.07.026
0920-5861/© 2014 Elsevier B.V. All rights reserved.

M. Lu et al. / Catalysis Today 242 (2015) 274–286
275
that the decomposition efficiency of benzene increases with the
CCO
O2 partial pressures [28]. However, the catalysts, especially transi-
tion metal oxides, generally contain lattice oxygen, surface oxygen
and adsorbed oxygen [29,30]. Regardless of type, all oxygen in the
catalyst can be described as the bulk-phase oxygen of the catalyst,
which can also oxidize VOCs into COx in the plasma [31,32]. Herein,
gas-phase oxygen in the feed gas stream and bulk-phase oxygen in
the catalyst are both expected to play important roles during VOC
removal.
In this work, iron loaded on SBA-15 was selected as a catalyst
to oxidize toluene (100 ppm) in various mixed N2/O2 plasmas at
atmospheric pressure and room temperature. To better understand
the structure and possible metal–support interaction of the iron
oxide species and SBA-15 support, the catalysts were characterized
by SEM, XRD, N2 adsorption–desorption, XPS, H2-TPR and O2-TPD.
The organic by-products of toluene oxidation were identified to
provide a better understanding of the VOC oxidation mechanism
over FeOx/SBA-15 catalysts.
2
CO selectivity % =
( )
× 100%
× 100%
2
7 C − C
(
)
)
0
CCO
x
CO selectivity % =
( )
x
7 C − C
(
0
where C0 and C are the initial and final concentrations of toluene,
CCO is the outlet concentration of CO2 and CCO is the sum of the
outlet concentrations of CO and CO2. All concentrations are units of
ppm and the data were recorded by an online gas chromatograph
after the plasma starting for 30 min, which was a stabilized state
for toluene removal.
x
2
2.2. Plasma system
The cylinder reactor was made of quartz glass, with a length
of 180 mm, inner diameter of 10 mm and wall thickness of 1 mm.
The quartz cylinder was wrapped by copper wire as the ground
electrode, delimiting the length of the discharge zone to 30 mm. A
stainless-steel rod with a diameter of 2 mm was used as the inner
electrode and was connected to an AC high-voltage power supply
that varied from 0 to 15 kV at a frequency of 50 Hz, resulting in a dis-
charge gap of 4 mm. The applied voltage and current of the reactor
were measured by a high-voltage probe and a digital power meter.
The discharge power was the product of the applied voltage and
the current [14,33–35]. The SED was calculated from the following
expression:
2. Experimental
2.1. Experimental setup
Fig. 1 shows a schematic diagram of the experimental arrange-
ment designed for toluene removal via the combination of a catalyst
with plasma. The toluene used was obtained by bubbling toluene
with a N2 gas stream in a bubbler. High-purity N2 (99.999%) was
divided into two streams. The first stream was allowed to pass
through the pure toluene liquid (99.5%) (Guangzhou Chemical
Reagent, China) kept in an ice and water bath (0 ◦C) to produce
the toluene vapor. The second stream was mixed with high-purity
O2 (99.99%) in a mixing chamber. The combined stream was then
mixed with vaporized toluene in another mixing chamber. The gas
flow rates were adjusted and controlled using mass flow controllers
(MFCs) (Seven Star Co., China). The mixture was introduced into the
reactor at a flow rate of 300 mL/min, with an initial toluene concen-
tration of 100 ppm and a gas residence time of 0.45 s. All of the gases
used in this study were purchased from Guangzhou Jun Qi Gas Co.,
Ltd.
Toluene oxidation was carried out in a fixed-bed flow reactor
that containing 0.2 g of the catalyst (40–60 mesh) and 1.6 g of silica
sand (40–60 mesh). All of the experiments in the discharge stages
were conducted at atmospheric pressure and room temperature.
The gas leaving the reactor was analyzed using an online gas chro-
matograph (GC2014C, Shimadzu) equipped with two FID detectors:
one for organic compounds, featuring a TG-BOND Q column (30 m,
0.32 mm) (60 ◦C), and one, equipped with a methanizer, for carbon
monoxide and carbon dioxide analysis using a 5 A molecular sieve
(2 m, 2 mm) and Poraplot Q column (4 m, 2 mm) (60 ◦C).
The organic by-products (gas phase) were preconcentrated by
active carbon placed downstream of the NTP reactor, and then
the active carbon was dispersed into the CS2 solution (Chro-
matographic Grade) under ultrasonic vibration for 120 min. The
supernatant was filtered with a 0.22-␮L filter membrane and ana-
lyzed by a GC–MS (QP2010, Shimadzu) using an Rtx-5MS capillary
column. The column temperature was 40 ◦C for the first 2 min
and was then increased to 220 ◦C at 6 ◦C/min, where it was main-
tained for 5 min. The MS detector was run in scan mode with a
mass range of 45–450 amu. The electron ionization (EI) method was
used. MS identification was conducted using the NIST 08 databank
(NIST/EPA/NIH Mass Spectral Library). The organic by-products on
the catalyst surface were analyzed using the same procedure.
The toluene removal was expressed as follows:
ꢀ
ꢁ
discharge power
gas flow rate L/s
W
)
)
(
SED J/L
=
(
All of the experimental results were compared based on specific
energy density (SED) in this investigation.
2.3. Catalyst preparation
Catalysts were prepared via impregnation using SBA-15 (Nan-
jing XFNANO Materials Tech Co., Ltd.) as the support and iron nitrate
ethanol solution of the desired concentration as the precursor. The
support was stirred in the iron solution for 24 h at room temper-
ature, and the solvent was then removed by evaporation at 60 ◦C.
The residue was dried at 120 ◦C for 12 h, and calcined at 500 ◦C
for 4 h. The resulting catalysts contained 1%, 3% and 5% (wt.%) iron
loading and were denoted as 1%FeOx/SBA-15, 3%FeOx/SBA-15 and
5%FeOx/SBA-15, respectively.
2.4. Catalyst characterization
SEM microphotographs were obtained with an S-3700N elec-
tron microscope (Hitachi, Japan) operating at 5.0 kV.
XRD patterns were collected using a D8 ADVANCE X-ray
diffractometer (Bruker, Germany) with Ni-filtered Cu K␣ radiation
(k = 0.15418 nm) in the 2ꢀ range of 0.6 to 5◦ and 5 to 90◦ at a scan-
ning rate of 4◦ min−1. The X-ray tube was operated at 40 kV and
40 mA.
N2 adsorption–desorption isotherms were measured at −196 ◦C
using a Micromeritics ASAP 2020 system. The sample (0.1 g)
was pretreated for 6 h at 300 ◦C in
a vacuum system, and
the Brunauer–Emmett–Teller (BET) equation (relative pressure
between 0.06 and 0.20) was employed to calculate the specific sur-
face areas. The pore size and pore volume were calculated from
the adsorption branches of N2 physisorption isotherms and the
Barrett–Joyner–Halenda (BJH) model.
The composition of elements and their valences on the catalyst
surface were analyzed by XPS using an ESCALAB 250 spectrome-
ter (Thermo Fisher Scientific, USA) equipped with a hemispherical
C0 − C
toluene removal efficiency % =
( )
× 100%
C0

276
M. Lu et al. / Catalysis Today 242 (2015) 274–286
Reactor
GC
MFC
MFC
MFC
Mixing chamber
Ozone monitor
R
Digital power meter
High voltage probe
Mixing chamber
Mixture of ice
and water
Bubbler for
toluene vapor
production
Oxygen
Nitrogen
AC high voltage power supply
Fig. 1. Schematic of the experimental setup.
electron analyzer, employing a Mg K␣ radiation source (1253.6 eV)
of 25 W. All binding energies were referenced to the C 1s line at
284.6 eV, which provided an accuracy of 0.48 eV within the full
scanning range of 0 to 1100 eV. XPS peak 4.1 software was used for
curve fitting.
H2-TPR analysis was performed on a Micromeritics AutoChem
II 2920 instrument. First, 40 mg of the catalyst was pretreated at
300 ◦C using high-purity Ar (30 mL/min) for 30 min and then cooled
to 60 ◦C. Subsequently, the flow of a H2–Ar mixture (10% H2 by
volume) was switched on, and the catalyst was heated to 700 ◦C
at a rate of 10 ◦C/min. The H2 consumption was monitored using a
thermal conductivity detector (TCD).
with the increase of oxygen content, reaching a maximum (51%)
at 3% O2, and then continuously decreased at higher O2 contents.
When various catalysts were packed into the NTP system, the
effect of oxygen’s electronegative character disappeared, and the
toluene removal efficiency was notably increased by SBA-15, SiO2
and FeOx/SBA-15. The extent of improvement was dependent on
the amount of iron loading, where that in the packed FeOx/SBA-15
reactor was higher than that in the packed SBA-15 or SiO2 reactor
at the various O2 concentrations.
3.1.2. Selectivity for CO2 and COx
Fig. 3 showed the selectivity of CO2 and COx as a function of
O2 concentration with or without the catalyst. For the plasma-
alone and plasma-catalysis systems, the selectivity for CO2 and COx
increased with the increase of O2 concentration. Under the various
O2 concentrations, the catalysts loaded with 1% and 3%Fe assisted
the formation of CO2 and COx and increased selectivity remark-
ably. In contrast, the SBA-15, SiO2 and 5%FeOx/SBA-15 catalysts
did not improve the selectivity of CO2 and COx at any of the O2
concentrations. In addition, when the catalysts loaded with 1% or
3%Fe were introduced into the NTP reactor, the proportion of CO2
in the COx significantly increased, which indicated that complete
oxidation of toluene was enhanced. According to the results for
toluene removal efficiency and COx selectivity, the 3%Fe loading
catalyst showed excellent toluene degradation performance under
the various plasma conditions.
O2-TPD analysis was performed using the same apparatus as
described for H2-TPR. First, 100 mg was pretreated at 300 ◦C using
high-purity He (30 mL/min) for 30 min and then cooled to 60 ◦C.
Subsequently, the flow of an O2–He mixture (5% O2 by volume)
was switched for 60 min. High-purity He was then switched on,
and the system was allowed to stabilize. The sample was heated
up to 800 ◦C at a rate of 10 ◦C/min. The experiment without O2 pre-
adsorption was performed as above except that the procedure for
O2–He adsorption was omitted.
3. Results
3.1. Toluene decomposition performance
3.1.1. Toluene removal characteristics
Fig. 2 showed the dependence of the toluene removal efficiency
on O2 concentrations ranging from 0 to 20% by volume. For the
toluene removal by plasma alone, removal efficiency increased
3.1.3. Analysis of the toluene oxidation residues
As observed in Fig. 3, a COx selectivity of less than 100% indi-
cated that some unidentified by-products formed in the reactor.
Their species was indicated using GC–MS, as presented in Fig. 4.
Nine gaseous organic by-products from toluene decomposition
by NTP alone were identified, and their abundance decreased
with increasing O2 concentration, suggesting that the COx selec-
tivity increased with increasing O2 content, consistent with the
results of Fig. 3. When SBA-15 was used in the NTP process,
the content of some organic by-products, such as benzyl alco-
hol; formic acid, phenylmethyl ester; acetic acid, phenylmethyl
ester; benzenemethanol, 2-nitro-; tetradecane; hexadecane and
heptadecane; decreased in the gas phase, even disappearing at
O2 concentration above 5%. At the same time, the abundance
of benzaldehyde also decreased under the various plasma con-
ditions. Meanwhile, using the 3%FeOx/SBA-15 packing material,
the by-product contents further decreased, even disappearing. The
toluene concentration in the various toluene oxidation experiment
followed the order of NTP > NTP + SBA-15 > NTP + 3%FeOx/SBA-15,
which agreed with toluene removal efficiency results for the dif-
ferent processes (Fig. 2).
NTP
NTP+SBA-15
NTP+1%FeO /SBA-15
x
120
80
40
0
NTP+3%FeO /SBA-15
x
NTP+5%FeO /SBA-15
x
NTP+SiO
2
N
2
1%O 3%O 5%O 10%O 20%O
2 2 2 2 2
Oxygen Content
Fig. 2. Effect of O2 concentration on toluene removal efficiency. (Initial concentra-
tion of toluene: 100 ppm, SED: 192 J/L).

M. Lu et al. / Catalysis Today 242 (2015) 274–286
277
80
60
40
20
0
NTP
NTP+SBA-15
NTP+1%FeO
NTP+3%FeO
NTP+5%FeO
NTP+SiO
NTP
NTP+SBA-15
NTP+1%FeO
NTP+3%FeO
NTP+5%FeO
A
B
120
80
40
0
/SBA-15
/SBA-15
/SBA-15
/SBA-15
/SBA-15
/SBA-15
x
x
x
x
x
x
NTP+SiO
2
2
N
2
1%O 3%O 5%O 10%O 20%O
N
1%O 3%O 5%O 10%O 20%O
2 2 2 2 2
2
2
2
2
2
Oxyg2en Content
Oxygen Content
Fig. 3. Effect of O2 concentration on (A) CO2 selectivity and (B) COx selectivity. (Initial concentration of toluene: 100 ppm, SED: 192 J/L).
TIC
A
99%N2+1%O2
(x1,000,000)
(x10,000,000)
5.0
1.5
1.0
0.5
0.0
NTP+3%FeOx/SBA-15
Toluene
NTP+3%FeOx/SBA-15
NTP+SBA-15
NTP+SBA-15
2.5
NTP
NTP
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
Time (min)
TIC
B
97%N2+3%O2
(x1,000,000)
(x10,000,000)
NTP+3%FeOx/SBA-15
NTP+SBA-15
4.0
3.0
2.0
1.0
0.0
Toluene
1.0
0.5
0.0
NTP+3%FeOx/SBA-15
NTP+SBA-15
NTP
NTP
5.0
10.0
15.0
20.0
25.0
30.0
35.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
Time (min)
TIC
C
95%N2+5%O2
(x1,000,000)
(x100,000)
7.5
5.0
2.5
0.0
Toluene
NTP+3%FeOx/SBA-15
NTP+SBA-15
7.5
5.0
2.5
0.0
NTP+3%FeOx/SBA-15
NTP+SBA-15
NTP
NTP
5.0
10.0
15.0
20.0
25.0
30.0
35.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
Time (min)
Fig. 4. Detailed GC–MS spectrum and identified peaks of organic by-products from toluene decomposition in the gas phase. (Reaction conditions: toluene, 100 ppm; SED,
192 J/L).

278
M. Lu et al. / Catalysis Today 242 (2015) 274–286
TIC
D
90%N2+10%O2
(x100,000)
(x1,000,000)
4.0
3.0
2.0
1.0
0.0
Toluene
5.0
NTP+3%FeOx/SBA-15
NTP+3%FeOx/SBA-15
NTP+SBA-15
2.5
0.0
NTP+SBA-15
NTP
NTP
5.0
10.0
15.0
20.0
25.0
30.0
35.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
Time (min)
TIC
E
80%N2+20%O2
(x100,000)
(x1,000,000)
5.0
2.5
0.0
7.5
Toluene
NTP+3%FeOx/SBA-15
NTP+3%FeOx/SBA-15
5.0
2.5
0.0
NTP+SBA-15
NTP
NTP+SBA-15
NTP
5.0
10.0
15.0
20.0
25.0
30.0
35.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
Time (min)
Peak #
Time(min)
Compound name
1
2
3
4
5
6
7
8
9
4.67
9.69
Toluene
Benzaldehyde
Benzyl alcohol
Formic acid, phenylmethyl ester
Acetic acid, phenylmethyl ester
Benzenemethanol, 2-nitro-
Tetradecane
11.78
12.98
15.245
15.495
21.11
23.34
25.44
Hexadecane
Heptadecane
Fig. 4. (Continued.)
The organic by-products deposited on the 3%FeOx/SBA-15
catalyst surface were also identified (Fig. 5) to obtain a better
understanding of the toluene decomposition on the catalyst
surface. The number of different by-products decreased as O2
concentration increased, with only toluene and benzaldehyde
being observed in the 80%N2 + 20%O2 plasma process. In con-
trast, numerous nitrogen-containing species, such as benzene,
1-methyl-2-nitro-; benzenemethanol,4-nitro-; benzene, 1-
methyl-4-nitro-; and phenol,4-methyl-2-nitro-; were identified
under the condition of low O2 concentration. Those species
decreased and even vanished with increasing O2 concentration.
During toluene degradation with NTP + 3%FeOx/SBA-15, the spec-
trum of toluene emerged on the 3%FeOx/SBA-15 surface, and
the abundance was higher than that of all of the by-products,

M. Lu et al. / Catalysis Today 242 (2015) 274–286
279
(x1,000,000)
TIC
2.0
99%N2+1%O2
97%N2+3%O2
95%N2+5%O2
1.0
0.0
90%N2+10%O2
80%N2+20%O2
5.0
10.0
15.0
20.0
25.0
30.0
35.0
Time (min)
Peak #
Time(min)
Compound name
1
2
3
4
5
6
4.67
9.69
15.32
15.495
16.725
Toluene
Benzaldehyde
Benzene, 1-methyl-2-nitro-
Benzenemethanol, 4-nitro-
Benzene, 1-methyl-4-nitro-
Phenol, 4-methyl-2-nitro-
17.205, 17.555
Fig. 5. Detailed GC–MS spectrum and identified peaks of organic by-products from toluene decomposition on the 3%FeOx/SBA-15 catalyst surface. (Reaction conditions:
toluene, 100 ppm; SED, 192 J/L).
indicating that the toluene could be adsorbed by the catalyst under
discharge conditions. Moreover, the benzaldehyde was detected
in all of the samples, implying that it was the key intermediate
by-product during toluene degradation.
catalytic test in toluene degradation, the amount of oxygen
decreased for the 3%FeOx/SBA-15 catalyst but increased for
1%FeOx/SBA-15 or 5%FeOx/SBA-15 catalyst.
3.3. Catalyst characterization
3.2. Toluene destruction in the N2 plasma
3.3.1. SEM
The catalyst morphology was observed by SEM (Fig. 9). The
surface of SBA-15 was very smooth, while some small granules
appeared on the iron-loaded SBA-15 surface, and the particle size
increased with increased iron loading. The iron oxide particles were
evenly dispersed on the SBA-15 surface for the 1%FeOx/SBA-15
and 3%FeOx/SBA-15 samples but aggregated when the iron loading
exceeded 3% (Fig. 9D).
Fig. 2 had showed that the toluene removal efficiency was
markedly improved by FeOx/SBA-15 catalysts in N2 plasma. To
study the influence of the bulk-phase oxygen of the catalyst on
the toluene plasma-catalysis oxidation mechanism, the yields of
CO2 and CO in toluene removal with catalyst-assisted N2 plasma
were measured (Fig. 6). As seen, the quantities of CO2 and CO
were related to the iron loading, with the 3%-Fe-loading catalyst
exhibiting the highest oxidative activity. The amounts of CO2 and
CO decreased along with the increasing of reaction time, but could
still be detected after reaction for 200 min.
Fig. 7 presented the toluene removal efficiency, CO2 yield and
CO yield as a function of SED in the 3%FeOx/SBA-15-assisted N2
plasma process. The results showed that the toluene removal effi-
ciency, CO2 yield and CO yield increased with the increase of SED,
which indicated that the toluene oxidation by the surface oxy-
gen of catalyst was related to the SED. To assess the role of the
bulk oxygen of the catalyst in the toluene oxidation, surface oxy-
gen content of the used and fresh catalysts were characterized
by XPS (Fig. 8), the data were summarized in Table 1. After the
3.3.2. Pore texture and XRD
The pore textures of the FeOx/SBA-15 catalysts were presented
in Table 2. As seen, the surface area, pore volume and pore diameter
were decreased by the iron loading (except the 1% iron loading),
and the extent of the decrease increased with increasing iron
loading. However, three well-resolved peaks (1 0 0), (1 1 0) and
(2 0 0) typical of hexagonal SBA-15, mesoporous silica [21,36], were
clearly identified in all of the catalysts (Fig. 10A), which indicated
that the highly ordered structure of SBA-15 was retained in the
iron loading process. No obvious iron oxide diffraction peaks were
observed in the wide-angle range for the 1% iron loading sample
(Fig. 10B), while Fe2O3 and Fe3O4 diffraction peaks appeared for the
Table 1
Oxygen content on the surfaces of the fresh catalysts and those used in N2 plasma
process as obtained via XPS.
Table 2
Surface area, pore diameter and pore volume of different FeOx/SBA-15 catalysts.
Catalyst
SED (J/L)
Fresh (at.%)
Used (at.%)
Catalyst
Surface
Pore volume
(cm3/g)
Pore diameters
(nm)
SBA-15
1%FeOx/SBA-15
3%FeOx/SBA-15
192
192
122
192
284
192
74.94%
73.98%
73.56%
74.43%
74.29%
73.35%
72.66%
73.33%
74.10%
area (m2/g)
SBA-15
508
518
496
486
0.85
0.84
0.81
0.78
6.7
6.5
6.5
6.4
1%FeOx/SBA-15
3%FeOx/SBA-15
5%FeOx/SBA-15
5%FeOx/SBA-15
73.24%

280
M. Lu et al. / Catalysis Today 242 (2015) 274–286
40
30
20
10
0
SBA -15
SBA -15
A
B
1%FeO /SBA -15
1%FeO /SBA -15
x
x
40
3%FeO /SBA -15
x
3%FeO /SBA -15
x
5%FeO /SBA -15
5%FeO /SBA -15
x
x
30
20
10
0
0
50 100 150 200 250 300
0
50 100 150 200 250 300
Time (min)
Time (min)
Fig. 6. (A) CO2 and (B) CO yields in toluene removal using the catalysts-assisted N2 plasma process. (Initial concentration of toluene: 100 ppm, SED: 192 J/L).
100
80
60
40
20
0
60
45
30
15
0
A
60
45
30
15
0
B
C
122 J/L
192 J/L
284 J/L
122 J/L
192 J/L
284 J/L
0
50 100 150 200 250 300
Time (min)
0
50 100 150 200 250 300
Time (min)
122
192
SED (J/L)
284
Fig. 7. The effect of SED on (A) toluene removal efficiency, (B) CO2 yield and (C) CO yield in the 3%FeOx/SBA-15-assisted N2 plasma process. (Initial concentration of toluene:
100 ppm).
5%FeO /SBA-15-192J/L
A
x
B
5%FeO /SBA-15
x
3%FeO /SBA-15-284J/L
x
3%FeO /SBA-15
x
3%FeO /SBA-15-192J/L
x
3%FeO /SBA-15-122J/L
x
1%FeO /SBA-15
x
1%FeO /SBA-15-192J/L
x
SBA-15
SBA-15-192J/L
530
531
532
533
534
535
536
537
530
531
532
533
534
535
536
537
B.E.(eV)
B.E.(eV)
Fig. 8. O 1s core level XPS spectra of catalysts: (A) fresh catalyst, (B) used catalyst in the N2 plasma.
3%- and 5%-Fe-loading catalysts, demonstrating the formation of
crystalline Fe2O3 and Fe3O4 on SBA-15 surface with increasing iron
loading.
was found for the 3%-Fe-loading catalyst, which also had the best
performance for toluene oxidation under the plasma conditions,
suggesting that the oxidation states of iron played a crucial role in
the toluene degradation.
3.3.3. XPS
Table 3
The oxidation states and iron content in the near-surface region
were measured using XPS (Fig. 11, Table 3). According to the XPS
spectra for Fe 2p (Fig. 11), the binding energies of 710.9–711.6 eV
and 712.5–714.4 eV could be ascribed to the presence of Fe2+ and
Fe3+ species, respectively. The relative percentages of each species
were summarized in Table 3. Quantitatively, it was found that the
Fe2+ and Fe3+ content on the catalyst surface changed as the iron
loading increased. The highest Fe2+ content and lowest Fe3+ content
Atomic surface compositions of the FeOx/SBA-15 catalysts obtained via XPS.
Atomic composition (%)
Fe (%)
Fe3+
Catalyst
Si
O
Fe
Fe2+
SBA-15
25.06
25.42
25.74
25.44
74.94
73.98
73.56
73.24
1%FeOx/SBA-15
3%FeOx/SBA-15
5%FeOx/SBA-15
0.60
0.70
1.32
44.26
37.02
70.08
55.74
62.98
29.92

M. Lu et al. / Catalysis Today 242 (2015) 274–286
281
Fig. 9. SEM images of various catalysts: (A) SBA-15, (B) 1%FeOx/SBA-15, (C) 3%FeOx/SBA-15, (D) 5%FeOx/SBA-15.
Fe O
Fe O
3
2
3
4
A
B
(100)
5%FeO /SBA-15
x
(110)
(200)
5%FeO /SBA-15
x
3%FeO /SBA-15
x
3%FeO /SBA-15
x
1%FeO /SBA-15
x
1%FeO /SBA-15
x
SBA-15
SBA-15
1
2
3
4
5
0
15 30 45 60 75 90
2 Theta (degree)
2 Theta (degree)
Fig. 10. The low-angle (A) and wide-angle (B) XRD patterns of different FeOx/SBA-15 catalysts.
3.3.4. H2-TPR
The catalytic properties of FeOx/SBA-15 could be determined by
the reducibility, which was measured by H2-TPR. In the TPR curves
of the Fe-loaded catalysts (Fig. 12), the low-temperature reduction
peak was centered at 342 ◦C with a shoulder peak at 492 ◦C, which
could be attributed to the reduction of Fe2O3 → Fe3O4 → Fe [21].
Another shoulder peak at 585 ◦C or 602 ◦C appeared with increas-
ing Fe loading and was ascribed to the reduction of Fe2+ to Fe0 [37].
Moreover, the temperature of Fe2+ reduction decreased slightly
with increasing iron loading, being lowest for the 3%-Fe-loading
catalyst, and then increased as the Fe loading increased further.
Quantitative evaluation of the reduction peaks (Table 4), which
could be ascribed to the catalyst active center, revealed that the
hydrogen consumption increased with increasing Fe loading. How-
ever, the best performance of toluene oxidation was achieved using
the 3%-Fe-loading catalyst. These findings indicated that the cat-
alyst performance was not exclusively determined by the active
5%FeO /SBA-15
x
3%FeO /SBA-15
x
1%FeO /SBA-15
x
706
708
710
712
B.E.(eV)
714
716
718
720
Fig. 11. Fe 2p core level XPS spectra of FeOx/SBA-15 catalysts.

282
M. Lu et al. / Catalysis Today 242 (2015) 274–286
3.4. Evaluation of catalyst stability
320
325
The stability of 3%FeOx/SBA-15 catalyst under condition of
toluene plasma-catalytic oxidation was assessed with experi-
ments at room temperature and atmospheric pressure. It could
be observed that toluene removal proceeded stably during the
first 12 h, after which the toluene removal efficiency decreased
(Fig. 14A). This observation indicated that the catalyst suffered
slight deactivation during prolonged toluene oxidation. However,
after the catalytic test (39 h) in toluene plasma-catalytic oxida-
tion, no disruption of 2D-hexagonal structure or phase change was
observed in the XRD analysis of the catalysts, because the three typ-
ical peaks {(1 0 0), (1 1 0), (2 0 0)} were clearly identified (Fig. 14B),
and no new iron diffraction peak was appeared (Fig. 14C).
478
602
438
5%FeO /SBA-15
x
342
585
3%FeO /SBA-15
x
492
1%FeO /SBA-15
x
SBA-15
100 200 300 400 500 600 700 800 900
Temperature (
)
4. Discussion
Fig. 12. H2-TPR profiles of various FeOx/SBA-15 catalysts.
The dispersion of metal particles on the support was
usually correlated with the catalytic activity of a metal cat-
alyst. From the SEM and XRD results, it was known that
the FeOx could be highly dispersed on the SBA-15 sur-
face at lower loading. The particle size increased with the
iron loading, and aggregation was observed at a loading of 5%Fe,
which might depress the adsorption of toluene on the catalyst.
From the adsorption breakthrough curves of toluene over different
catalysts (Fig. 15), the toluene adsorption ability decreased with
increasing of Fe loading, and the SBA-15 showed the best perfor-
mance. Consequently, the residence time of toluene in the DBD
reactor could increase because of the sorption of SBA-15. According
to previous literature [17], the increasing of residence time could
improve the VOCs removal efficiency. Thus, the toluene adsorp-
tion over the SBA-15 could be explained one reason of toluene
removal. In order to further study the effect of catalyst adsorption
on the toluene removal, experiment of toluene removal by coupling
SiO2 with NTP system was carried out (Figs. 2 and 3). Although
the toluene removal efficiency was greatly increased in compar-
ison with NTP alone, the CO2 selectivity and COx selectivity had
no significant increase. Compared with SBA-15-assisted NTP pro-
cess, the toluene removal efficiency in SiO2-assisted NTP process
was not improved in the environment of 100%N2 plasma and even
decreased under the condition of 80%N2 + 20%O2 plasma. And from
the adsorption breakthrough curves of toluene over SiO2, we could
know that it had not the capability of toluene adsorption (Fig. 15).
These results further suggested that the toluene adsorption over
catalyst played an important role in the plasma catalysis system.
Additionally, it was interesting to observe that the COx selec-
tivity using FeOx/SBA-15 in DBD reactor was much higher than
that using pure SBA-15. This phenomenon was probable due to
the excellent oxygen-supplying ability of FeOx, which had been
suggested in the process of toluene N2 plasma-catalytic oxidation
(Fig. 6). Previous studies revealed that VOC removal was better cat-
alyzed using mainly catalysts (TiO2, ␥-Al2O3, zeolites) rather than
supported metal catalysts [28,40]. However, in our study, the inter-
action of FeOx and SBA-15 was found to play an important role
in toluene removal using the combination of FeOx/SBA-15 with
plasma. The stability of catalyst 3%FeOx/SBA-15 was also studied
(Fig. 14), despite slight deactivation during prolonged toluene oxi-
dation reaction, the structure of the catalyst was perfectly stable
over the period of 39 h for 100 ppm of toluene plasma-catalysis
oxidation, and no active component run off was detected. So the
activity of the catalyst could be recovered through the appropriate
method.
Table 4
H2 consumption of various catalysts.
Catalyst
T1 (◦C)
T2 (◦C)
T3 (◦C)
Total H2
(mmol/g)
SBA-15
–
–
492
438
478
–
–
585
602
–
1%FeOx/SBA-15
3%FeOx/SBA-15
5%FeOx/SBA-15
342
325
320
0.66
1.30
2.18
center, as the more active centers were present, the stronger the
interaction between the active ingredient and carrier.
3.3.5. O2-TPD
O2-TPD is often used to identify the oxygen species in a cat-
alyst. Various oxygen species could be identified in the catalysts
(Fig. 13). The peaks at 80 ◦C, 370 ◦C and >500 ◦C were ascribed to
the desorption of weak adsorption molecular oxygen (O2−), chem-
ically adsorbed oxygen (O−) and lattice oxygen (O2−), respectively,
from the catalyst surface [38,39]. To study the role of iron in the
oxygen adsorption, O2-TPD was measured in another way (with-
out the O2 adsorption procedure). It was found that the extent of
oxygen desorption, especially that of O2−, significantly increased
after iron loading, which revealed that the oxygen adsorption on
the catalyst was promoted by iron loading. Meanwhile, the desorp-
tion temperature of O2− was lower for the case of O2 pre-adsorption
than for the case without it, implying the improvement of lattice
oxygen activity by adsorbed oxygen, which was beneficial for the
complete oxidation of toluene.
After O pre-adsoption
5%FeO /SBA-15
x
2
Without O pre-adsoption
2
3%FeO /SBA-15
x
After O pre-adsoption
2
Without O pre-adsoption
2
1%FeO /SBA-15
x
After O pre-adsoption
2
Without O pre-adsoption
2
After O pre-adsoption
2
SBA-15
Without O pre-adsoption
2
100
200
300
400
500
600
700
800
The influence of oxygen on VOC removal has been extensively
studied, revealing that the maximum VOC decomposition efficiency
occurred at a given oxygen content, for example, 5% for toluene
Temperature (ºC)
Fig. 13. O2-TPD profiles of various FeOx/SBA-15 catalysts.

M. Lu et al. / Catalysis Today 242 (2015) 274–286
283
A
B
C
(100)
100
80
60
40
20
0
Fe O
2
3
Fe O
3
4
(110)
(200)
2
1
3
4
5
25
50
2 Theta (degree)
75
0
10
20
Time (h)
30
40
2 Theta (degree)
Fig. 14. (A) Toluene removal over catalyst as a function of time, (B) low-angle and (C) wide-angle XRD patterns of catalyst after 39 h catalytic test. (Catalyst: 3%FeOx/SBA-15;
initial concentration of toluene: 100 ppm, feed gas: 80%N2 + 20%O2, SED: 192 J/L).
nitrogen excited species, especially the metastable states of nitro-
ꢂ
1.0
0.8
0.6
0.4
gen molecules (N2 (A3
+)), which were considered an energy
u
reservoir [43–45]. Correspondingly, some nitrogen-containing
species were identified in the gas phase and on the catalyst surface
(Figs. 4 and 5). On the other hand, oxygen excited species could
also be formed in the case of a gas stream containing oxygen. Such
species demonstrated higher oxidizability for VOCs, resulting in
a decrease in organic by-products and an increase in the toluene
removal efficiency with increasing oxygen content. However, a
lower toluene destruction efficiency was found in higher-oxygen-
content gas. This finding could be attributed to the electronegativity
of the oxygen molecules and the decrease in the oxygen excited
species due to the quenching of the high-energy ions (R1) [43–46].
SBA-15
1%FeO /SBA-15
x
3%FeO /SBA-15
x
5%FeO /SBA-15
x
SiO
2
0.2
0.0
0
50
100
150
200
NO2∗ + O2 → O(3P) + O(3P) + N2
(R1)
Time (min)
Fig. 15. Adsorption breakthrough curves of toluene over different catalysts.
It had been widely accepted that the loading amount of the
active component and the oxidation states were important factors
that determine VOC oxidation. Iron usually exists as Fe2+ and Fe3+
in oxide, and could transport O2 via interconversion between Fe2+
and Fe3+ [24]. As seen from the XPS results (Fig. 11 and Table 3),
the Fe2+ content was the highest for the 3%-Fe-loading catalyst,
[41], 20% for CF4 [42] and 10% for C2F6 [8]. We found the maximum
toluene removal efficiency at 3% (Fig. 2). The differences between
these results were attributed to the difference in the experimen-
tal conditions. The VOCs could be destroyed by collision with the
(x100,000)
5.0
SBA-15
2.5
0.0
3%FeOx/SBA-15
5.0
Peak #
10.0
15.0
20.0
25.0
30.0
35.0
Time(min)
Compound name
1
2
3
4
5
6
4.67
6.96
7.17, 7.81
9.69
11.78
15.32
Toluene
Ethybenzene
o-xylene, p-xylene
Benzaldehyde
Benzyl alcohol
Benzene, 1-methyl-2-nitro-
Phenol,4-methyl-2-nitro-
7
17.205
Fig. 16. GC–MS chromatogram of the organic by-products of toluene decomposition on the catalyst surface. (Reaction condition: toluene, 100 ppm; feed gas, 90%N2 + 10%O2;
SED, 192 J/L).

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M. Lu et al. / Catalysis Today 242 (2015) 274–286
Fig. 17. Pathways of toluene degradation in the gas phase and on the FeOx/SBA-15 surface.
which had the best catalytic performance. Thus, the Fe2+ content
might play a vital role in the destruction of toluene molecules in the
FeOx/SBA-15-catalyst-assisted plasma system. As is known, Fe3+
has a better deoxidization capability because of its lower reduc-
tion temperature (Fig. 12). The catalyst with 5%Fe loading had the
highest content of Fe3+, but its catalytic activity toward toluene
was poor due to the stronger interaction of FeOx with the support
(Table 4). This further demonstrated that FeOx and SBA-15 both
played important roles in the toluene oxidation. In terms of the O2-
TPD results, it was found that iron loading increased the oxygen
adsorption, especially that of the molecular oxygen on the catalyst
surface. Because of this characteristic, the oxygen could transfer
from the gas phase to the catalyst surface, consequently decreased
the O2 electronegative characteristics in the gas phase. Moreover,
after oxygen adsorption over the catalyst surface, the oxygen con-
sumption (as follows (R2)–(R4) [43]), could also decrease.
Therefore, Fe loading improved the utilization efficiency of active
oxygen. Meanwhile, we also found that the adsorbed oxygen
improved the activity of lattice oxygen (Fig. 13), which was favor-
able for the toluene complete oxidation [47].
It had been reported that bulk-phase oxygen species were key
factors for the oxidation activity of metal oxide catalysts [48].
Reactions without oxygen in the feed were performed for toluene
oxidation at room temperature. As expected, the oxidation of
toluene over the various FeOx catalysts occurred in the absence of
any gaseous oxygen (Figs. 6 and 7), which was in agreement with
the results reported by Ogata [31] and Futamura [49]. It is known
that electrical discharge is an effective mechanism for activating
gas-phase oxygen. The higher the applied voltage, the more high-
energy electrons, atoms, ions, free radicals and excited species are
created [43], which were beneficial for the catalyst activation [50].
Thus, the yields of CO2 and CO in the N2 plasma process increased
with increasing SED (Fig. 7B and C). It should also be noted that
a large amount of sub-surface oxygen was present in the catalyst.
During toluene oxidation, this oxygen could move to the catalyst
surface [51], resulting in the observation of COx after 3 h of the
reaction had passed. In addition, the bulk-oxygen in the 1% or 5%Fe-
loading catalyst could not quickly react with the VOCs adsorbed on
ꢃ
+
N2(A3
u) + O → NO + N(2D)
(R2)
N(2D) + O2 → O + NO
(R3)
(R4)
NO + O + M → NO2 + M

M. Lu et al. / Catalysis Today 242 (2015) 274–286
285
the catalyst surface, resulting in an increase in the oxygen content
of the sample after reaction (Table 1). These observations suggested
that the bulk-phase oxygen of the 3%FeOx catalyst might be more
easily activated by plasma than that of other catalysts.
the collision of electrons or oxidation and (2) the reaction on the
catalyst surface caused by the oxidation of active species (such as
O• and OH•). Among the multiple oxidation processes in the cou-
pled FeOx/SBA-15 and NTP system, the Fe2+ was considered to be
an important contributor because of its outstanding O2 adsorption
ability. SBA-15alsoplayedanimportantrolebecauseof itsexcellent
toluene adsorption ability.
Different pathways for VOC decomposition by NTP had been
proposed according to the identified intermediates in previous
studies [13,43,52]. However, most studies focused on the path-
ways in the gas phase. In this study, possible pathways at the
interface between the gas phase and catalyst were relevant, which
were scarce in the literature. As can be seen in Fig. 4, the Fe
loading markedly decreased the number of species and the con-
centration of organic by-products. To study the influence of Fe
loading on the toluene oxidation over the catalyst, an additional
experiment was carried out with 3%FeOx/SBA-15 and pure SBA-
15 in a 90%N2 + 10%O2 feed gas (Fig. 16). Compared to SBA-15,
the number of species and the concentration of the by-products
were remarkably decreased for the catalyst with 3% FeOx load-
ing, and no nitrogen-containing intermediates, ethybenzene and
xylene were observed. This finding indicated that Fe loading pro-
moted the deep oxidation of toluene, forming more CO2 and CO
and thereby increasing the CO2 and COx selectivity (Fig. 3). Another
hand, the content of some organic by-products in the gas phase
was also decreased by the SBA-15 (Fig. 4), while the CO and CO2
selectivity were not improved by it (Fig. 3). Thus, the benefits of
utilization of SBA-15 in this system might be considered to adsorb
or incompletely oxidize for the toluene.
Thus, based on the organic by-products of toluene decompo-
sition in the gas phase and on the catalyst surface, the following
toluene oxidation mechanism was proposed, as shown in Fig. 17.
The principle processes of toluene destruction were induced by
energetic electrons, active oxygen species as well as hydroxyl
and nitrogen excited species in the reactor. The tentative reaction
scheme included two parts: a reaction in the gas phase, namely,
the direct removal caused by the collision of electrons or oxidation
caused by the gas-phase radicals (O•, OH•, N2•, NO•, NO2•), and
a reaction on the catalyst surface, namely, the reaction between
adsorbed toluene or other intermediate by-products (such as ben-
zaldehyde, benzyl alcohol and nitrogen-containing intermediates)
and the active species (such as O• and OH•). Moreover, the O2 in the
gas stream might be fixed on the catalyst surface via facile inter-
conversion between Fe2+ and Fe3+ states and then be transported
to the toluene or intermediate organic by-products over the cat-
alyst, leading to the formation of CO2. As known, an opening of
the aromatic ring happened both in gas-phase and on catalyst sur-
face [13,14], and then a series of oxidations of the aromatic ring
steps could take place. While no opening of the aromatic ring inter-
mediates were identified in this study. Thus, we discussed only a
conceivable pathway that was operative in the gas-phase and on
the catalyst surface based on the results from the GC–MS, the path-
ways of COx formation from toluene derivatives was not discussed
in this study.
Acknowledgments
We would like to thank Hangao Liu and Peitao Wang for help-
ing us to build experimental system. Thanks Mr. Don Barnes, Prof.
Shiming Luo, Dr. Shafiq UR Rehman, and Dr. Yanxia Su for valuable
suggestions and English check. This work was carried out with the
supported of “the National Natural Science Foundation of China (no.
50978103, no. U1201231, no. 51108187 and no. 51378218)”.
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