Characteristics and mechanism of toluene removal by double dielectric barrier discharge combined with an Fe2O3/TiO2/γ-Al2O3catalyst

Article abstract of DOI:10.1039/d0ra07938c

Removal of volatile organic compounds (VOCs) by non-thermal plasma technology produced by dielectric barrier discharge has become a hotspot due to its rapid reaction at room temperature, convenience without preheating and high removal efficiency of VOCs. Ways to improve the removal efficiency of mixed VOCs and the control of by-products in the discharge process are urgently needed. In view of the above shortcomings, the research progress of catalysts to improve the removal efficiency of VOCs by dielectric barrier discharge in recent years was reviewed; in this research, we conducted experiments on the removal efficiency of toluene under high flow rate experimental conditions (input voltage 10-70 V, mixed gas flow rate 1 m3 h-1, inlet concentration 1000 mg m-3). The research used γ-Al2O3, TiO2/γ-Al2O3 and Fe2O3/TiO2/γ-Al2O3 as catalysts in a reactor with a discharge gap of 6 mm, and a 99.7% removal efficiency of toluene was achieved when the SIE was 183.4 J L-1. The added catalyst significantly reduced the concentration of by-products. At the same time, experiments with multiple catalysts to improve the removal efficiency and the performance of the discharge effect under different humidity levels were analyzed. The removal efficiency of toluene was optimized and recyclable, the formation of by-products was effectively suppressed when the catalysts in the reactor were Fe2O3/TiO2/γ-Al2O3, TiO2/γ-Al2O3 and γ-Al2O3, and the discharge length was 1 : 2 : 2.

Full text of DOI:10.1039/d0ra07938c

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Characteristics and mechanism of toluene removal
by double dielectric barrier discharge combined
with an Fe2O3/TiO2/g-Al2O3 catalyst
Cite this: RSC Adv., 2020, 10, 41511
abc
Jiaze Ren,abc Jiangyou Wuabc and Lanlan Wuabc
*
Rui Wang,
Removal of volatile organic compounds (VOCs) by non-thermal plasma technology produced by dielectric
barrier discharge has become a hotspot due to its rapid reaction at room temperature, convenience without
preheating and high removal efficiency of VOCs. Ways to improve the removal efficiency of mixed VOCs
and the control of by-products in the discharge process are urgently needed. In view of the above
shortcomings, the research progress of catalysts to improve the removal efficiency of VOCs by dielectric
barrier discharge in recent years was reviewed; in this research, we conducted experiments on the
removal efficiency of toluene under high flow rate experimental conditions (input voltage 10–70 V,
mixed gas flow rate 1 m3 ꢀ1, inlet concentration 1000 mg mꢀ3). The research used g-Al2O3, TiO2/g-
h
Al2O3 and Fe2O3/TiO2/g-Al2O3 as catalysts in a reactor with a discharge gap of 6 mm, and a 99.7%
removal efficiency of toluene was achieved when the SIE was 183.4 J Lꢀ1. The added catalyst
significantly reduced the concentration of by-products. At the same time, experiments with multiple
catalysts to improve the removal efficiency and the performance of the discharge effect under different
humidity levels were analyzed. The removal efficiency of toluene was optimized and recyclable, the
formation of by-products was effectively suppressed when the catalysts in the reactor were Fe2O3/TiO2/
g-Al2O3, TiO2/g-Al2O3 and g-Al2O3, and the discharge length was 1 : 2 : 2.
Received 17th September 2020
Accepted 30th October 2020
DOI: 10.1039/d0ra07938c
rsc.li/rsc-advances
The VOCs in the reactor are mainly degraded in two ways: in the
rst, the molecular structures of the VOCs themselves are
Introduction
destroyed by high-energy electrons and split into smaller particles,
while in the second, high-energy electrons collide with gas mole-
cules such as N2 and O2. Free radicals and active ions with different
degrees of reactivity are generated, and these products react with
VOCs to achieve the effect of purifying the VOCs.12
In order to reduce the yield of by-products in the process of
removing low-temperature plasma generated by dielectric barrier
discharge to remove VOCs and improve the reaction efficiency,
a catalyst is usually used for the cooperative reaction. Studies13–20
have shown that catalysts can effectively increase the reaction
area, improve the removal efficiency of VOCs along with the
selectivity of CO2, and reduce the formation of by-products.21
Volatile organic compounds (VOCs), as the main components of
air pollution, are produced in large quantities in the production
processes of leather goods, inks, and decoration materials. Due
to the health hazards of VOCs and their negative impact on the
environment, the issue of effective removal of VOCs is urgent.
Traditional methods such as physical adsorption, chemical
absorption, condensation or biological methods are difficult to
put into practical industrial application due to their cost and
concentration limitations.1–5 Low-temperature plasma has
become a research hotspot because of its high response rate to
VOCs, the low cost of the processing technology and the
convenience of opening and closing the experimental device.6–8
Dielectric barrier discharge, a method for efficiently generating
low-temperature plasma, involves applying an alternating voltage
to two electrodes covered by an insulating medium to break down
the working gas between the electrodes, accelerate free electrons,
endow the gas with high energy and cause electron collapse.9–11
Materials and methods
Experimental setup
The experimental system consisted of a gas inlet system, DBD
reactor (discharge length 200 mm, discharge distance 6 mm; the
high-voltage electrode was stainless steel mesh, and the low-
voltage electrode was quartz), pulse power supply (0–20 kV,
50% duty cycle, f ¼ 10 kHz, fm ¼ 200 Hz, CTP-2000 K, pulse width
2.5 ꢁ 10ꢀ3 s, China Jiangsu Suman Plasma Technology Co., Ltd.)
and analysis system. Compressed air generated by an air
compressor passes through the rotor owmeter and enters the
aState Key Laboratory of Safety and Health for Metal Mines, Maanshan 243000, China.
E-mail: 564044540@qq.com
bSinosteel Maanshan General Institute of Mining Research Co., Ltd., Maanshan
243000, China
cHuawei National Engineering Research Center of efficient recycling for Metal Mineral
Resources Co., Ltd., Maanshan 243000, China
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mixing box in three ways. In the rst way, air enters the cylinder
containing toluene (l); in the second way, air enters the empty
cylinder to control the air ow rate (l); in the third way, air enters
the water-mixed gas bottle to control the humidity. Experimental
gas enters the mixing box aer the three-way gas merges and then
enters the dielectric barrier discharge reactor aer mixing evenly.
The experimental system adjusts the gas ow rate, humidity and
initial concentration of toluene through the ow meter and the
manual valve of the air compressor to control the concentration
of toluene at about 1000 mg mꢀ3 and the relative humidity at
about 30%. The inlet and outlet concentrations of toluene and
CO2 were measured by activated carbon adsorption/carbon
analysis and gas chromatography, referring to the National
Environmental Protection Standard of the People's Republic of
China (HJ584-2010). The NOx concentration was analyzed by
a ue gas analyzer (KM950, Kane International Co. Ltd.). The O3
concentration was measured by the iodometric method (Fig. 1).
The toluene removal efficiency was dened as
Preparation of Fe2O3/TiO2/g-Al2O3
10 g of titanium dioxide powder was mixed with a certain
amount of Fe(NO3)3 solution, stirred well and then ultrasoni-
cally dispersed for 10 min; aer being kept open at 120 C for
ꢂ
4 h, the solution was evaporated and the powder was removed.
100 mL deionized water containing 0.1% polyethylene glycol
was added; the mixture was stirred uniformly and ultrasonically
dispersed for 30 minutes. The activated g-Al2O3 was soaked in
this mixture for 30 minutes, then calcinated in a muffle furnace
ꢂ
at 150 C for 1 hour and calcinated under increasing tempera-
ꢂ
ture at a rate of 10 C every 15 minutes for 2 hours.
Measuring method of energy density
An error occurred in the power measurement caused by the
action of the alternating electric eld because the phase
difference between the discharge voltage and current is difficult
to measure. Therefore, in this study, we used the Lissajous
method to determine the discharge power. The principle is to
connect a measuring capacitor in series with the internal load of
the power supply (its capacitance value CM is known and is
much larger than the equivalent capacitance of the reactor
load); the circuit diagram is shown in Fig. 2.
h ¼ (C0 ꢀ C)/C0 ꢁ 100%.
The selectivity of CO2 was dened as
S
¼ C /7(C0 ꢀ C).
CO2
CO2
At the same time, the voltage U(t) applied to the load and the
voltage Uc(t) on the measuring capacitor were connected to the
oscilloscope, and the relationship of the U(t)–t curves, Uc(t)–t
curves and closed response U (the Lissajous curve of t)–Uc(t) is
shown in Fig. 3. By integrating the Lissajous gure area, the
discharge power of the reactor can be calculated by a formula.
From this, the specic input energy of the DBD reactor can be
calculated. The calculation formula is as follows:22,23
Preparation of catalyst
Activation of g-Al2O3: deionized water was used to clean the
alumina pellets, the pellets were dried at 100 ꢂC for 3 h, and the
temperature was raised to 550 ꢂC at 5 ꢂC per minute in a muffle
furnace. Aer roasting for 3 h, the pellets were set aside.
P (W) ¼ fm ꢁ capacitance ꢁ Slissajous
Preparation of TiO2/g-Al2O3
TiO2 powder was dispersed in 100 mL of deionized water con-
taining 0.1% polyethylene glycol and ultrasonically dispersed at
30 ꢂC for 30 min; then, the activated g-Al2O3 pellets were
immersed in the catalyst dispersion for 10 min. The powder was
À
Á
60 ꢁ P ðWÞ
Q ðL minꢀ1Þ
SIE J Lꢀ1
¼
ꢂ
removed and roasted in a muffle furnace at 150 C for 2 h.
Fig. 1 Experimental system.
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Fig. 4 XRD patterns of bare g-Al2O3, TiO2/g-Al2O3 and Fe2O3/TiO2/g-
Al2O3.
Fig. 2 Circuit diagram.
The SEM results (Fig. 6) also support this point of view: as
shown in Fig. 6(a), the surface of the bare g-Al2O3 is smooth; as
shown in Fig. 6(b), TiO2(anatase) is loaded on the surface of g-
Al2O3 as small unevenly dispersed particles, showing an irreg-
ular morphology; in Fig. 6(c), the Fe2O3 hematite in the form of
irregular akes and blocks is loaded on the surface of TiO2/g-
Al2O3. It can be seen that compared with small TiO2 particles,
larger akes and massive particles appear on the surface of the
catalyst. However, the surface of the catalyst still maintains an
overall dense morphology, indicating that the loading of Fe2O3/
TiO2 does not change the basic structure of the catalyst carrier
g-Al2O3.
It can be seen from Fig. 7(a) that the xps spectra of O 1S of
hematite, anatase and g-Al2O3 can be tted to three forms of
peaks. The diffraction peak with a binding energy of 529.76 eV
corresponds to the lattice oxygen (Oa) on the catalyst surface.
The diffraction peak with a binding energy of 531.58 eV corre-
sponds to the adsorbed oxygen (Oc) on the catalyst surface, and
the diffraction peak with binding energy at 532.96 eV corre-
sponds to the hydroxyl oxygen or oxygen in water (Od) due to
adsorption on the catalyst surface. Among them, adsorbed
oxygen (Oc) accounts for a higher proportion, which can
improve the catalytic activity of the catalyst and is benecial to
the oxidation process of VOCs. The diffraction peak binding
energy in the xps spectrum of g-Al2O3 is 74.41 eV, indicating
that Al exists in the form of Al3+. The XPS spectrum of Ti in the
catalyst is shown in Fig. 7(c). The diffraction peaks of anatase at
458.35 eV and 463.98 eV correspond to Ti 2p3/2 and Ti 2p1/2.
This shows that Ti4+ exists on the catalyst surface. The XPS
spectrum of Fe 2p in the catalyst is shown in Fig. 7(d). The
diffraction peaks of hematite at 710.59 eV and 723.46 eV
correspond to Fe 2p1/2 and Fe 2p3/2, respectively, and the
Results and discussion
Characterization of the catalysts Fe2O3, TiO2, and g-Al2O3
Fig. 4 shows the XRD patterns of g-Al2O3 and other catalysts
based on g-Al2O3. As shown in Fig. 4, the diffraction peaks of
Al2O3 appear at 2q ¼ 19.45ꢂ, 31.93ꢂ, 37.63ꢂ, 39.49ꢂ, 45.86ꢂ, and
67.03ꢂ, which correspond well to PDF no 10-0425; this proves
that the crystal form of alumina is g-Al2O3.
Aer loading TiO2, the diffraction peaks appearing at 2q ¼
25.28ꢂ, 37.8ꢂ, 48.05ꢂ, 62.69ꢂ, and 75.02ꢂ correspond well to the
standard pd of anatase (PDF no 21-1272). Aer loading Fe2O3,
the diffraction peaks appearing at 2q ¼ 23.85ꢂ, 32.85ꢂ, 35.08ꢂ,
40.32ꢂ, 48.82ꢂ, 53.5ꢂ, and 61.53ꢂ correspond to hematite (PDF
no 88-2359). However, the peak intensity corresponding to
Fe2O3 (hematite, PDF number 88-2359) is low and has a slight
deviation. This is because loading Fe on Al2O3 will shi the peak
to a larger angle.24 Due to the low loading and good spreading,
the peak of Fe2O3 is not obvious in the XRD test results;
however, the EDS test results (Fig. 5) show that sample 2 and
sample 3 are successfully loaded with Ti and Fe.25
accompanying peaks are at 718.16 eV, indicating that hematite
on the surface of the catalyst exists in the form of Fe3+
.
The
26
diffraction peaks of the three elements of Al, Ti and Fe in the
catalyst are all shied to a certain extent compared to the
standard XPS spectrum. Combined with XRD, it can be seen
Fig. 3 The relationship of U–Uc(t).
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Fig. 5 EDS patterns of bare g-Al2O3 (a), TiO2/g-Al2O3 (b) and Fe2O3/TiO2/g-Al2O3 (c).
that the interaction between g-Al2O3, anatase and hematite in
the catalyst changes the electron cloud density of the surface
elements, which is benecial to the catalytic oxidation of
VOCs.27
Inuence of input voltage on output power and SIE
As shown in Fig. 8, the increase of the specic input energy (SIE,
which is also called specic energy density) showed a trend of
slow and then fast with increasing input voltage. Aer the input
Fig. 6 SEM patterns of bare g-Al2O3, TiO2/g-Al2O3 and Fe2O3/TiO2/g-Al2O3.
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Fig. 7 XPS patterns of O (a), Al (b), Ti (c), and Fe (d) on the surface of Fe2O3/TiO2/g-Al2O3.
voltage was greater than 60 V, the growth rate gradually tended
to be at. The SIE values of the three curves represented by the
reactor with catalyst added were signicantly higher than the
SIE value of the curve represented by a single DBD reactor
because the addition of catalyst reduced the discharge gap,
increased the peak voltage and ionized more high-energy
particles. Therefore, the collision probability of high-energy
particles and gas molecules in the dielectric barrier discharge
process was improved, and the value of SIE increased.
Moreover, the power increased slowly when the input voltage
was 10–30 V, and the trend grew faster when the input voltage
was 30–60 V. The curve with the added catalyst was similar to
the value of the power compared to the single DBD due to the
addition of the catalyst, which increased the peak voltage. At the
same time, the smaller discharge interval caused the current to
decrease; therefore, there is not much difference in the power
value.
Inuence of SIE on the toluene removal efficiency and
selectivity of CO2
In the single DBD process, the removal efficiency of toluene and
the selectivity of CO2 were both low because the residence time
of toluene gas in the DBD process was relatively short and it
could not be completely degraded. As a result, it was difficult to
generate CO2. The addition of the catalyst signicantly
improved the removal rate of toluene at various energy densi-
ties. The g-Al2O3 catalysts loaded with TiO2 and TiO2/Fe2O3
increased the removal efficiency of toluene by 20.48% and
28.94% compared with a single DBD and the selectivity of CO2
was increased by 32.55% and 35.85%, respectively. It can be
seen from Fig. 9 that the removal rate of toluene by the reactor
loaded with different catalysts improved with increasing SIE,
which showed an increasing trend of fast and then slow. This is
because with the increase of the applied electric eld, more free
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radicals were produced during the reaction, and the electrons at high temperature, and could not be used as a reactant to
produced had higher energy. These high-energy particles would promote the oxidation of toluene; therefore, the concentration
accelerate the decomposition of toluene. However, the high- of NOx gradually increased.28,29
energy electrons and free radicals generated were sufficient to
The DBD reactor had the worst mineralization efficiency but
destroy the lower-energy chemical bonds in the pollutants as the highest NOx output. It was found that the excessive energy
the energy density gradually increased, and the removal rate in the DBD reactor was consumed in the process of forming
increased slowly as the energy density continued to increase NOx, which was an important reason for its low mineralization
because the generated electrons had higher energy and more rate. Under the same value of SIE value, NOx was generated due
radical groups decomposed chemical bonds and grouped with to the short reaction time between the active group and the
higher energy moieties, such as benzene rings. At the same toluene molecule, in which the toluene molecule could not be
time, the pollutants completely decomposed and oxidized the puried while the active group was consumed. Aer adding the
intermediate products generated during the reaction into CO2 catalyst, on the one hand, the pollutants and background gas
and H2O. This change improved the selectivity of CO2; however, molecules were adsorbed on the surface of the carrier, and the
it slowed the increase in the removal rate as well.
probability that the active groups on the carrier surface would
Under the same SIE, the removal rate of toluene by the react with the pollutant gas molecules and oxygen molecules
reactor loaded with catalyst was higher than that of the ordinary was improved compared with N2 molecules. On the other hand,
DBD reactor, indicating that the presence of g-Al2O3 pellets the precursors of NOx-active oxygen atoms and excited nitrogen
loaded with TiO2 and Fe2O3 could effectively promote the molecules or nitrogen atoms were consumed on the surface of
degradation of organic matter. When the SIE was higher than the catalyst, resulting in a decrease in the concentration of NOx.
150 J Lꢀ1, the difference in the removal rate was reduced Due to the above reasons, the reactor loaded with the catalyst
because the energy was enough to destroy the structure of most had a higher energy utilization rate.
toluene molecules without the help of an external catalyst.
As shown in Fig. 10, the reactor loaded with g-Al2O3 catalyst
Therefore, the addition of the catalyst could improve the energy could control the production of NOx well. When the surface of g-
utilization rate to a certain extent and reduce the energy Al2O3 was loaded with TiO2 and Fe2O3, the NOx concentration of
consumption during the DBD reaction process.
the two reactors was higher than that of the g-Al2O3 reactor, and
the total NOx concentration of the two reactors was not much
different.
Inuence of SIE on NOx concentration
In the DBD process, electrons collided with background gas
molecules such as O2 and N2, generating ground and excited
oxygen and nitrogen atoms. These oxygen atoms and nitrogen
atoms would further react with O2 and O3 to produce NO and
NO2. This study investigated the amount of NOx produced by
the DBD reactor and the catalyst-loaded reactor under different
values of SIE. As shown in Fig. 10, the NOx concentration
gradually increased as the SIE increased because the higher SIE
reected the higher discharge energy in the reaction. The extra
NOx generated could not be reduced like O3, which decomposes
Inuence of SIE on O3 concentration
The principle of the synergy between the DBD process and
a heterogeneous catalyst is that the ultraviolet light and high-
energy particles generated during the DBD discharge process
can excite TiO2 and generate hole–electron pairs. At the same
time, g-Al2O3 as the catalyst carrier plays a role in adsorbing
pollutant molecules, while the addition of Fe2O3 also improves
the catalytic activity. Under the action of the DBD electric eld,
the active material produced will react with the pollutant
molecules adsorbed on the surface of the catalyst, and the holes
Fig. 8 The relationships between SIE and the change in output power Fig. 9 The relationship between the removal efficiency and selectivity
with input voltage in different types of DBD reactors.
of CO2 with SIE in different types of DBD reactors.
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Fig. 12 Removal efficiencies of different TiO2 catalytic reactors.
Fig. 10 The relationships between SIE and NOx concentration in
different types of DBD reactors.
energy electrons and free radical groups were generated, and
these high-energy electrons and free radical groups would
decompose O3 into O2 when the energy density increased at the
same time. O3 was not only a byproduct but also an oxidant
which could oxidize toluene and the intermediate products. The
above reasons together led to an increase in ozone content.
When toluene was introduced, the O3 concentration
decreased. On the one hand, toluene and O3 would directly react
on the surface of the catalyst so that O3 could be consumed; on
the other hand, toluene would react with the precursor that
generated ozone on the surface of the catalyst, which indirectly
led to a decrease in ozone content. However, the output of ozone
decreased slightly aer adding toluene to the reactor because
the by-products generated during the reaction covered the
active sites on the surface of the catalyst carrier, reducing the
catalytic activity of g-Al2O3. At the same time, these byproducts
occupied the active sites on the surface of the catalyst, pre-
venting toluene from reacting with the active oxygen groups on
the surface.
will also directly oxidize the pollutant molecules or oxidize H2O
molecules to form $OH. During the reaction process, ozone
went through the following process: (1) the hole–electron pair
generated aer TiO2 was excited had strong oxidizing proper-
ties, and a large amount of O3 was generated; (2) during the
DBD process, a large amount of O3 was generated; (3) direct
reactions with pollutant molecules on the catalyst surface
occurred; (4) the pollutant molecules reacted with the precur-
sors, which produced O3 on the surface of the catalyst.
In the DBD reaction process, O3 is an inevitable by-product.
This study investigated the changes in the concentration of O3
produced in the DBD process in different reactors under
different values of SIE. As shown in Fig. 11, the amount of O3
produced in all the DBD reactors showed an initial rapidly
increasing trend and then slowly increased with increasing SIE
because the increase of SIE would lead to an increase in the
energy and in the number of higher-energy electrons inside the
reactor. These higher-energy electrons could easily destroy the
O]O bond, converting O2 into O3; meanwhile, more high-
The O3 concentration in the reactor loaded with TiO2/g-Al2O3
was higher than that in the reactor loaded with blank g-Al2O3.
TiO2 could be excited to generate hole–electron pairs under the
action of DBD, and the holes had a strong oxidizing ability;
Fig. 11 The relationships between SIE and O3 concentration in
different types of DBD reactors.
Fig. 13 Ozone concentrations of different TiO2 catalytic reactors.
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This showed that the addition of Fe2O3 not only consumed the
oxygen used to generate ozone but also promoted the decom-
position of ozone or make ozone form active groups to partici-
pate in the degradation reaction and further reduce the
concentration of ozone. The concentrations of ozone produced
by the two catalytic reactors were not much different. The O3
concentration produced by the reactor loaded with Fe2O3/TiO2/
g-Al2O3 was lower than that in the reactor loaded with TiO2/g-
Al2O3. This result was in agreement with the previous removal
rate comparison, which further showed that the Fe2O3/TiO2/g-
Al2O3 catalyst has higher catalytic activity.
Four catalytic reactors were prepared by loading TiO2 on g-
Al2O3 and directly coating TiO2 on the electrode. The removal
efficiency of toluene and the output of O3 were evaluated under
different SIE values. It can be seen from Fig. 12 and 13 that with
the help of the g-Al2O3 carrier, the TiO2 carried on the g-Al2O3
carrier showed a higher toluene removal efficiency and a lower
O3 output than pure TiO2. This is because more pollutant
molecules will be adsorbed onto the surface of the catalyst,
which increases the probability of reacting with active
substances on the catalyst surface. Aer TiO2 is excited, the
carrier facilitates the exposure of pollutant molecules to the
generated holes. As shown in Fig. 12, reactors with different
TiO2 wt% show similar treatment efficiencies and ozone
Fig. 14 SIE under different ozone production levels.
meanwhile, a large amount of O3 was generated, and the O3
would also be decomposed on the surface of the catalyst during
the reaction process.30
In addition, it can be seen from Fig. 11 that the concentra-
tion of ozone was lower in the reactors loaded with Fe2O3/TiO2/
g-Al2O3 and TiO2/g-Al2O3 than in the DBD reactor. The
concentration of ozone in the former reactor was the lowest.
Fig. 15 Byproducts generated in DBD (a), DBD + g-Al2O3 (b), DBD + TiO2/g-Al2O3 (c), DBD + Fe2O3/TiO2/g-Al2O3 (d).
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Fig. 16 Reaction mechanism.
production. Among these, the catalytic reactor with about
15 wt% TiO2 has the best effect. This is because a suitable
weight percentage of TiO2 can not only generate enough elec-
tron–hole pairs to oxidize pollutant molecules, but will also not
cover too many catalyst sites, thereby affecting the adsorption
effect of the pollutant molecules. Pure TiO2 shows lower pro-
cessing efficiency and higher ozone production. This is because
the lack of catalyst carrier adsorption of pollutant gas molecules
reduces the residence time of gas molecules in the reactor and
is not conducive to the consumption of ozone on the surface of
the catalyst. Through control experiments on the amount of
ozone produced by different reactors under different SIE values,
the research showed that under different O3 concentration
levels, the reactor loaded with Al2O3 has the highest SIE value;
the SIE values of the reactors loaded with TiO2/g-Al2O3 and
Fe2O3/TiO2/g-Al2O3 are slightly lower, indicating that the Al2O3
catalyst has the best controlling effect on ozone, and the results
are also consistent with the research of ozone production under
the same SIE (Fig. 14).
Byproduct analysis
Active groups containing oxygen and nitrogen were formed
during the discharge process aer toluene and background gas
molecules (N2 and O2) entered the reactor. These active groups
and the high-energy electrons generated by the discharge would
react with the toluene molecules to form a large amount of
intermediate product. The chemical bonds with low bond
energy in the toluene molecule would be destroyed by the
particles, active groups with relatively low energy and some
chemical bonds with higher bond energy, such as benzene
rings, in this process. The energy would also be reduced during
the collision with the particles, resulting in preliminary puri-
cation of the toluene molecules.
As shown in Fig. 15(b), g-Al2O3 had the function of adsorbing
organic gas molecules, prolonging their residence time in the
DBD process, and increasing the probability of collisions
between high-energy electrons generated in the DBD process
and gas molecules; this facilitates the production of high-energy
Fig. 17 Mechanism of co-processing.
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Table 1 Effects of different catalyst discharge length ratios on toluene treatment
Discharge length ratio
Value of SIE when the
Selectivity of CO2 when
O3 concentration when the NOx concentration when the
(Fe2O3/TiO2/g-Al2O3 : TiO2/ highest removal efficiency is the highest removal
highest removal efficiency is highest removal efficiency is
g-Al2O3 : g-Al2O3)
reached (J Lꢀ1
)
efficiency is reached (%)
reached (mg mꢀ3
)
reached (mg mꢀ3
)
1 : 1 : 1
1 : 1 : 2
1 : 2 : 1
1 : 2 : 2
2 : 1 : 1
2 : 1 : 2
2 : 2 : 1
207.8
216.9
189.7
183.4
205.8
218.6
210.1
58.87
59.52
69.32
67.74
42.31
49.58
54.56
2260
2010
2450
2180
3130
2790
3740
131.58
128.56
130.42
117.66
141.87
127.41
147.69
particles or strong oxidizing properties, promoting the decom- oxidize small molecules and continue to degrade toluene.31 The
position of the organic gas. combination of DBD and g-Al2O3 achieved the lowest concen-
Aer TiO2 was loaded on g-Al2O3, the concentrations of tration of ozone and NOx in the experiment; therefore, g-Al2O3
benzyl formate, benzaldehyde and benzyl alcohol were signi- was considered as the third-stage catalyst to decompose ozone
cantly reduced. At the same time, the concentrations of benzoic and NOx in order to increase the selectivity of CO2 and reduce
acid, formic acid, and acetic acid increased, which can be found the by-product concentration. Aer determining the types of
in Fig. 15(c). The results showed that the combination of pho- catalysts in the different discharge regions, the discharge
tocatalysis and DBD strengthened the degradation ability of lengths occupied by different catalysts were compared. The
toluene molecules, which was benecial to further improve- research results showed that the removal efficiency could reach
ment of the selectivity of CO2.
more than 99.7% under a certain value of SIE (Table 1).
The results showed that the reactor with a discharge length
When Fe2O3/TiO2/g-Al2O3 was added to the DBD reactor,
a large number of active groups and high-energy particles ratio of 1 : 2 : 2 only needed 183.4 J Lꢀ1 of SIE when the removal
generated by the DBD process and catalytic reaction were efficiency reached the highest level, and the selectivity of CO2
adsorbed on the surface of the catalyst carrier, and a series of was slightly lower than that of 1 : 2 : 1 because the catalytic
reactions occurred on the active sites of the catalyst which activity of g-Al2O3 was lower than those of the other two, resulting
further improved the decomposition rate of the organic in a decrease in the selectivity of CO2. At the same time, the O3
mixture. It can be seen from Fig. 15(d) that the addition of Fe2O3 and NOx concentrations in this type of reactor were lower than in
further reduced the concentration of methyl benzoate. More the other kinds of reactors when it reached the highest treatment
small molecules appeared under the synergistic effect of the efficiency. Compared with the Fe2O3/TiO2/g-Al2O3-DBD reactor,
photocatalysis and DBD reaction, which showed that the addi- the reactor with a discharge length ratio of 1 : 2 : 2 achieved
tion of Fe2O3 promoted the decomposition of toluene and 21.16% increased selectivity of CO2, the O3 output decreased by
intermediate products, pushing the reaction forward in the 26.9%, and the NOx content decreased by 9.1%.
direction of producing smaller molecules. The inferred reaction
mechanism is shown in Fig. 16 and 17.
Effects of humidity on the processing efficiency
By comparing the toluene removal efficiencies of different
reactors under different humidity levels, it could be found that
Synergistic effects of different catalysts
From the research above, it can be found that different catalysts
have their own advantages and disadvantages when cooperating
with DBD to treat toluene. Therefore, this study considered the
location of different catalysts in the DBD reactor according to
the processing characteristics to maximize the removal effi-
ciency and selectivity of CO2 and reduce the generation of by-
products. Compared with other catalysts, Fe2O3/TiO2/g-Al2O3
had higher treatment efficiency and selectivity of CO2. There-
fore, Fe2O3/TiO2/g-Al2O3 was used as the catalyst in the rst
section to degrade part of the toluene and promote the reaction
in the direction of generating more small molecules. However,
it can be seen from Fig. 15 that the DBD process using Fe2O3/
TiO2/g-Al2O3 as a catalyst would produce more kinds of by-
products. The synergy of g-Al2O3 loaded with TiO2 and DBD
could further reduce the output of by-products, especially for
the removal of small molecule products. Therefore, this study
the reactor loaded with g-Al2O3 and the single DBD reactor
Fig. 18 The relationships between humidity and removal efficiency in
considered using TiO2 as the catalyst in the second section to different types of DBD reactors.
41520 | RSC Adv., 2020, 10, 41511–41522
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Fig. 21 Photograph of Fe2O3/TiO2/g-Al2O3 before use (1), after use
(2), and after regeneration (Re).
Fig. 19 Long term experiment of removal efficiency.
carbon deposits produced gradually reduced the efficiency of the
catalyst, and the treatment efficiency eventually dropped to about
88.6%. Aer 48 hours of use, there were carbon deposits on the
surface; then, the different types of catalysts were placed in a DBD
environment for surface discharge treatment for 2 hours. Aer
surface discharge, it can be seen from Fig. 21 that the carbon
deposits on the surface basically disappeared. From the SEM
image (Fig. 20), it can be found that the small particles on the
surface of the mixed catalyst still exist, and there is no obvious
change in particle size; however, it can be seen from Fig. 20 that
the surface of the catalyst became slightly rough and uneven,
possibly due to corrosion by carbon deposits. The mixed catalyst
was placed into the DBD reactor, and the test of the removal rate
of toluene was continued. As shown in Fig. 19, the removal effi-
ciency was basically restored.
showed a small increase when the humidity was 10–30%
because H2O was decomposed into hydroxyl radicals in the DBD
process to promote the decomposition of toluene molecules
under an SIE of 200 J Lꢀ1. When the humidity was 30–90%,
there was a sharp drop because too many H2O molecules could
not undergo timely decomposition, which reduced the electron
density and accelerated the demise of free radicals. At the same
time, the presence of H2O molecules would compete with other
groups to adsorb on the catalyst surface, reducing the number
of active sites for toluene and O3 adsorption.32 The reactor
loaded with TiO2 and Fe2O3 catalysts was less affected by
humidity than the rst two reactors, which proved that these
two catalysts could break down more water molecules and had
stronger catalytic activity (Fig. 18).
Conclusions
This study compared the removal rates, mineralization rates, SIE
values and by-product outputs of different catalysts when syner-
gizing DBD to treat toluene. Based on the value of SIE, the advan-
tages and disadvantages of different catalysts to remove toluene
were considered, and a mixed-catalyst synergistic DBD reaction was
designed. Compared with the single catalyst and DBD treatment of
toluene, a removal rate of more than 99.7% and CO2 selectivity of
67.74% were achieved under the conditions of an Fe2O3/TiO2/g-
Al2O3 : TiO2/g-Al2O3 : g-Al2O3 discharge length ratio of 1 : 2 : 2 and
an SIE value of 183.4 J Lꢀ1, while the concentrations of O3 and NOx
were reduced as well. In addition, the reactor in which the multiple
catalysts worked together was stable under different humidity
conditions, which indicates that this kind of reactor has the
potential to remove toluene in a complex ambience.
Regeneration of catalyst
The research showed that aer 48 hours of reaction, due to the
accumulation of by-products on the surface of the catalyst, the
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was nancially supported by the Anhui Provincial
Science and Technology Major Special Project (17030801034).
Fig. 20 SEM patterns of regenerated g-Al2O3, TiO2/g-Al2O3 and
Fe2O3/TiO2/g-Al2O3.
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