Enhanced plasma-catalytic decomposition of toluene over Co-Ce binary metal oxide catalysts with high energy efficiency

Article abstract of DOI:10.1039/c9ra00794f

In-plasma catalysis has been considered as a promising technology to degrade volatile organic compounds. Heterogeneous catalysts, especially binary metal oxide catalysts, play an important role in further advancing the catalytic performance of in-plasma catalysis. This work investigates the toluene decomposition performance over Co-Ce binary metal oxide catalysts within the in-plasma catalysis. Co-Ce catalysts with different Co/Ce molar ratios are synthesized by a citric acid method. Results show that the catalytic activity of Co-Ce catalysts is obviously superior to those of monometallic counterparts. Especially, Co_0.75Ce_0.25O_x catalyst simultaneously realizes highly efficient toluene conversion (with a decomposition efficiency of 98.5% and a carbon balance of 97.8%) and a large energy efficiency of 7.12 g kW h^-1, among the best performance in the state-of-art literature (0.42 to 6.11 g kW h^-1). The superior catalytic performance is further interpreted by the synergistic effect between Co and Ce species and the significant plasma-catalyst interaction. Specifically, the synergistic effect can decrease the catalyst crystallite size, enlarge the specific surface area and improve the amount of oxygen vacancies/mobility, providing more active sites for the adsorption of surface active oxygen species. Meanwhile, the plasma-catalyst interaction is able to generate the surface discharge and reinforce the electric field strength, thereby accelerating the plasma-catalytic reactions. In the end, the plasma-catalytic reaction mechanism and pathways of toluene conversion are demonstrated.

Full text of DOI:10.1039/c9ra00794f

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Enhanced plasma-catalytic decomposition of
toluene over Co–Ce binary metal oxide catalysts
with high energy efficiency†
Cite this: RSC Adv., 2019, 9, 7447
Zheng Bo,
Jinhui Zhu, Shiling Yang, Huachao Yang, * Jianhua Yan and Kefa Cen
In-plasma catalysis has been considered as a promising technology to degrade volatile organic compounds.
Heterogeneous catalysts, especially binary metal oxide catalysts, play an important role in further advancing
the catalytic performance of in-plasma catalysis. This work investigates the toluene decomposition
performance over Co–Ce binary metal oxide catalysts within the in-plasma catalysis. Co–Ce catalysts
with different Co/Ce molar ratios are synthesized by a citric acid method. Results show that the catalytic
activity of Co–Ce catalysts is obviously superior to those of monometallic counterparts. Especially,
Co0.75Ce0.25
x
O catalyst simultaneously realizes highly efficient toluene conversion (with a decomposition
ꢀ1
efficiency of 98.5% and a carbon balance of 97.8%) and a large energy efficiency of 7.12 g kW h
,
ꢀ1
among the best performance in the state-of-art literature (0.42 to 6.11 g kW h ). The superior catalytic
performance is further interpreted by the synergistic effect between Co and Ce species and the
significant plasma–catalyst interaction. Specifically, the synergistic effect can decrease the catalyst
crystallite size, enlarge the specific surface area and improve the amount of oxygen vacancies/mobility,
providing more active sites for the adsorption of surface active oxygen species. Meanwhile, the plasma–
catalyst interaction is able to generate the surface discharge and reinforce the electric field strength,
thereby accelerating the plasma-catalytic reactions. In the end, the plasma-catalytic reaction mechanism
and pathways of toluene conversion are demonstrated.
Received 30th January 2019
Accepted 28th February 2019
DOI: 10.1039/c9ra00794f
rsc.li/rsc-advances
downstream the plasma reactor). In IPC, the short-lived active
Introduction
þ
P
+
2ꢀ
3
species including O
2
, O and N2ðA
Þ produced by NTP can
Volatile organic compounds (VOCs) are atmospheric contami-
u
nations derived from industrial, commercial and agricultural reach the catalyst surface for efficient utilization, meanwhile
processes, which can be detrimental to human health and the the electric eld strength of plasma discharge is reinforced by
1,2
environment. Non-thermal plasma (NTP) consisting of ener- the integration of catalysts, thus yielding more highly energetic
6,7
getic electrons and active species has exhibited unique advan- species for accelerating the catalytic reactions. Catalysts are
tages for VOC decomposition, such as quick response, identied to play an important role in further advancing the
moderate capital cost and easy operation under ambient plasma-catalytic performance and energy efficiency of IPC
3,4
8
conditions. However, the formation of undesired by-products system. Heterogeneous catalysts have been employed for VOC
9
,10
11–14
and poor energy efficiency signicantly restrict the industrial decomposition, e.g., noble metals, metal oxides,
and their
15–21
application of NTP technology.
combinations.
Among these catalysts, metal oxide catalysts
11
12
13
14
Combining NTP with catalysts (i.e., plasma catalysis) has (e.g., CoO
,
x
MnO
,
x
CuO and CeO
) attract tremendous
2
been considered as a prospective approach to overcome these interest in consideration of the high catalytic activity, good
2,5
drawbacks. Based on the location of catalysts, the plasma economics and superior poisoning resistance. Especially,
1
6
17
catalysis can be classied into two congurations, i.e., in- binary metal oxide catalysts (e.g., MnCoO
x
,
CuCeO
x
,
18
plasma catalysis (IPC, placing the catalysts in the discharge CeMnO
x
), synthesized by two types of metal salt precursors,
zone) and post-plasma catalysis (PPC, placing the catalysts are being regarded as preeminent candidates for IPC because of
the potential complementary advantages from different
components for improving the physicochemical property and
catalytic activity.
Cobalt oxide (CoO ) with excellent reduction ability, abun-
x
dant oxygen vacancy and electrophilic oxide species has been
State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power
Engineering, College of Energy Engineering, Zhejiang University, Hangzhou,
Zhejiang Province, 310027, China. E-mail: huachao@zju.edu.cn; Fax: +86 571
8
†
1
7952438; Tel: +86 571 87953290
proved as one of the most efficient metal oxide catalysts for VOC
Electronic supplementary information (ESI) available. See DOI:
0.1039/c9ra00794f
22,23
decomposition.
x
Meanwhile, cerium oxide (CeO ) is being
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considered as a promising catalyst for VOC oxidation due to its Scientic) was used to gain the Raman spectra of catalysts. The
high oxygen mobility/storage capacity associated with the redox specic surface area, total pore volume and average pore
3
+
4+ 17,24
shi between Ce and Ce .
As a consequence, developing diameter of the Co–Ce binary metal oxide catalysts were deter-
Co–Ce binary metal oxide catalysts exhibits great potential to mined by N adsorption–desorption measurement. The oxida-
2
further promote the catalytic activity in consideration of the tion states of catalyst surface species were obtained by X-ray
possible synergistic effect between Co and Ce species. In fact, photoelectron spectroscopy (XPS) analysis using a VG Escalab
the collaborative interaction has been demonstrated in the Mark II equipment. The reducibility of Co–Ce binary metal
2
5,26
thermal catalytic oxidation of VOCs.
For example, Akram oxides was measured through hydrogen temperature-
26
et al. reported that the decomposition temperature of ethyl programmed reduction (H
2
-TPR) experiments with a chemi-
ꢁ
acetate using CoCeO catalyst (T ¼ 195 C) was exceptionally sorption analyser (Micrometrics, Autochem II 2920).
x
ꢁ
ꢁ
lower than those of CeO (T ¼ 215 C) and Co O (T ¼ 244 C).
2
3 4
However, the catalytic performance of VOC decomposition over Experimental setup
Co–Ce binary metal oxide catalysts in plasma catalysis has not
The experimental setup for catalytic decomposition of toluene
in IPC is presented in Fig. 1. The plasma-catalytic system con-
sisted of the gas supply system, the dielectric barrier discharge
yet been reported. Moreover, an in-depth insight into the reac-
tion mechanism in terms of Co–Ce synergistic effect and
plasma–catalyst interaction is also crucial to advance our
understanding towards VOC conversion process.
In this work, the toluene decomposition performance over
Co–Ce binary metal oxide catalysts in IPC was investigated. Co–
Ce catalysts with different Co/Ce molar ratios were synthesized
by the citric acid method. The catalytic activity in terms of
(DBD) reactor, the high voltage power supply as well as the gas
analysis system. The gaseous toluene was obtained with
a bubbler, which was diluted with pure air (99.999%, Jingong,
Hangzhou). The mass ow controller (MFC, Sevenstars D08-3F)
was used to maintain the gas ow rate constant during the test
ꢀ1
(
i.e., 0.5 L min ). The initial concentration of toluene was
toluene decomposition efficiency, CO
2
selectivity, carbon
210 ppm. The DBD reactor is illustrated detailedly in Fig. 1. An
balance and the formation of by-products was comprehensively
evaluated. Subsequently, a detailed characterization on the
structure and redox properties of Co–Ce catalysts was con-
ducted to unveil the synergistic effect between Co and Ce
species. Moreover, the inuence of catalysts on the plasma
discharge behavior (including current/voltage magnitude,
discharge mode and electric eld strength) was also explored. In
the end, the underlying reaction mechanism and pathways of
toluene decomposition over Co–Ce binary metal oxide catalysts
within IPC were demonstrated.
aluminium foil (40 mm long) wrapped around a cylindrical
quartz tube (10 mm outer diameter and 0.8 mm wall thickness,
respectively), which was used as the ground electrode. A coaxial
stainless steel rod (3.5 mm diameter) served as the high voltage
electrode. The catalyst (0.2 g) was packed into the gap between
the stainless steel rod and quartz wall (approx. 80% volume
fraction of the whole discharge zone) and a pair of polytetra-

uoroethylene (PTFE) heads were used to seal the plasma
reactor.
The high voltage power supply (CTP-2000K) connected with
the DBD reactor was used to generate the plasma discharge. The
electrical signals were recorded using the oscilloscope (MDO
Experimental
Preparation of Co–Ce binary metal oxide catalysts
3034). The discharge powers of DBD reactor were calculated by
a Q-U Lissajous method, and the specic input energy (SIE)
Co–Ce binary metal oxide catalysts were prepared by the citric
acid method. Cobalt nitrate and ceria nitrate were mixed in
deionized water under stirring. Then the citric acid (1.2 : 1
molar ratio of the acid to metal nitrate) was dissolved in the
solution as the complexing agent, followed by the addition of
polyglycol (10% weight of citric acid). The as-obtained solution
could be calculated as follows:
ꢀ
1
SIE (J L ) ¼ P/Q ꢂ 60,
(1)
ꢁ
was continuously stirred in the magnetic stirrer at 80 C until
ꢁ
a gel formed. The samples were dried at 110 C overnight and
ꢁ
subsequently calcined at 500 C during the next 4 h. Co/Ce
molar ratios (i.e., Co
y
Ce1ꢀy
O
x
, y ¼ 0, 0.25, 0.5, 0.75 and 1)
could be regulated with adjusting the molar ratio of cobalt
nitrate to ceria nitrate. Conceptually, y ¼ 0 and y ¼ 1 denoted
the CeO and CoO catalyst, respectively. All the samples were
x
x
sieved to 40–60 meshes before the catalytic performance
measurement.
Characterization
The X-ray diffraction (XRD) results were recorded by an XRD-
6000 Diffractometer with Cu Ka radiation. A DXR 532 Raman
spectrometer (532 nm excitation wavelength, Thermo Fisher Fig. 1 Schematic diagram of the experimental setup.
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ꢀ
1
ꢀ1
where P (W) is the discharge power in IPC and Q (L min ) is 0.5 lowest O
3
concentration (81 ppm at 401 J L ) with the Co0.75
-
ꢀ
1
L min in this work.
Ce0.25O catalyst (Fig. S1a†).
x
The analysis of outlet gas samples was continuously per-
The reaction rate of toluene oxidizing with atomic oxygen
ꢀ14
3
formed by gas chromatography (GC, GC9790 Plus, Fuli) equip- (7.6 ꢂ 10
cm per molecule per s) is much higher than that
ꢀ
22
3
28
ped with two ame ionization detectors. One was connected to with O
3
counterpart (3.9 ꢂ 10
cm per molecule per s).
to atomic oxygen is
was converted from carbon dioxide and carbon monoxide, and benecial for superior toluene oxidation performance. Besides,
the other one was used for toluene measurement. The concen- the concentration of NO is also measured (Fig. S1b†), in which
trations of O in the effluent were determined by an ozone the lowest NO content (19 ppm at 401 J L ) is recognized when
monitor (Models 106-MH). The concentrations of NOx were using the Co0.75Ce0.25O catalyst.
a nickel conversion furnace for measuring the methane, which Thus, the high conversion efficiency of O
3
x
ꢀ1
3
2
x
measured by a NOx analyzer (Testo 350XL). The analysis of
In addition to the efficient decomposition, the complete
organic by-products in the effluent was performed with gas conversion of toluene to CO , especially CO , is also highly
x
2
chromatography with mass spectrometry (GC-MS, Agilent desired. As presented in Fig. 2b and c, coupling the NTP with
890A GC/5957 MS). To ensure the reliability of tests, the Co–Ce catalysts signicantly improves the CO selectivity (by
plasma-catalytic system was run for 30 min before measure- 44.8%) and carbon balance (by 37.7%). Among the catalysts,
ments until a steady state of effluent compositions was Co0.75Ce0.25 catalyst exhibits the optimum CO selectivity of
achieved. 87.3% and carbon balance of 97.8% at 401 J L , demonstrating
The toluene decomposition efficiency (htoluene), selectivity of a high degree of toluene conversion to CO2 and CO . The
7
2
O
x
2
ꢀ
1
x
CO (SCO2), carbon balance (SCOx) and energy efficiency (henergy
could be calculated as:
)
durability and reusability of Co0.75Ce0.25O catalyst are further
x
2
ꢀ1
measured at 298 J L . The catalytic performance of Co0.75
catalyst decreases slightly aer 12 h test (Fig. S2a†),
demonstrating the excellent durability to resist deactivation.
Moreover, the Co0.75Ce0.25 catalyst could be reused for 5 times
-
Ce0.25O
x
h
toluene (%) ¼ (cinlet ꢀ coutlet)/cinlet ꢂ 100%
(2)
(3)
(4)
O
x
SCO2 (%) ¼ cCO /[7 ꢂ (cinlet ꢀ coutlet)] ꢂ 100%
2
with slight activity loss (less than 4%, Fig. S2b†), exhibiting
a superior reusability.
S
COx (%) ¼ (cCO2 + cCO)/[7 ꢂ (cinlet ꢀ coutlet)] ꢂ 100%
A comparison of plasma-catalytic performance with respect
to toluene decomposition efficiency and energy efficiency (i.e.,
the electrical energy input for per unit mass of VOCs) between
this work and previous studies is conducted (Fig. 2d). Detailed
boundary conditions of different catalytic systems for compar-
ison are available in Table S1.† Using binary metal oxide catalyst
ꢀ
1
h
energy (g kW h ) ¼ 3.6 ꢂ M ꢂ cinlet
ꢂ h /(SIE ꢂ 24.04),
toluene
(5)
where cinlet (ppm) and coutlet (ppm) are toluene concentrations
of inlet and outlet, respectively; cCO2 (ppm) and cCO (ppm) are
the concentrations of carbon dioxide and monoxide in the
effluent, respectively; M is the molar mass of toluene.
of Co0.75Ce0.25
O
x
, this work simultaneously realizes enhanced
toluene conversion (decomposition efficiency of 98.5%) and
ꢀ1
high energy efficiency of 7.12 g kW h , superior to those in the
ꢀ
1
18,29–37
state-of-art literatures (0.42 to 6.11 g kW h ).
Results and discussion
Structure and redox properties of catalysts
Plasma-catalytic performance of Co–Ce catalysts
The outstanding plasma-catalytic performance can be inter-
The catalytic activity of Co–Ce binary metal oxide catalysts preted by the synergistic effect between Co and Ce species and
towards toluene oxidation in IPC is evaluated. As presented in obvious plasma–catalyst interaction. A comprehensive charac-
Fig. 2a, the toluene removal efficiency increases monotonically terization on the structure and redox property of Co–Ce catalysts
with enlarging the SIE. The higher SIE can reinforce the electric is thus carried out. According to the XRD patterns in Fig. 3a, Co–

eld strength in the discharge region, thus generating more Ce binary metal oxide catalysts are composed of spinel phase of
highly energetic electrons and active particles to accelerate the Co (JCPDS 43-1003) and uorite structure of CeO (JCPDS 34-
3
O
4
2
27
reactions. Compared with NTP system, the introduction of 0394). In particular, the characteristic peak of CeO (111) crystal
2
catalysts remarkably improves the toluene decomposition effi- face in the Co–Ce binary metal oxide catalysts slightly shis to
ꢁ
ꢁ
ciency (by 35.0%), indicating the strong plasma–catalyst inter- higher Bragg angle region, i.e., from 28.7 of CeO
action. More importantly, the removal efficiency of Co–Ce Co–Ce catalysts (Fig. 3b). This phenomenon is primarily stim-
binary metal oxide catalysts is much higher than those of ulated by the incorporation of Co cations into CeO uorite
monometallic counterparts. To be specic, the decomposition lattice, leading to the formation of Co–Ce solid solution. When
2
to 28.95 of
2
4
+
3+
efficiencies of Co0.75Ce0.25
O
x
, Co0.5Ce0.5
O
x
ꢀ1
and Co0.25Ce0.75
O
x
Ce (0.097 nm in ionic radius) is replaced by Co (0.065 nm)
ꢀ1
2+
are 98.5% (at 401 J L ), 91.9% (at 394 J L ) and 86.5% (at 381 J and Co (0.082 nm), a contraction of CeO
2
lattice occurs, thus
developing plenty of defects in the lattice (Fig. 3c). These defects
and CoOx (82.5% at 400 J L ), demonstrating an optimum can increase the oxygen vacancies and contribute to the
catalytic performance when using the Co0.75Ce0.25 catalyst. generation of surface oxygen species for improved catalytic
ꢀ
1
ꢀ1
L
), respectively, superior to those of CeO (75.5% at 391 J L )
x
ꢀ
1
O
x
The excellent toluene oxidation can be further validated by the activity.
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Fig. 2 (a) Decomposition efficiency, (b) CO
2
selectivity, (c) carbon balance of Co–Ce catalysts as a function of SIE. (d) Comparison of toluene
x
decomposition efficiency and energy efficiency between this work (using Co0.75Ce0.25O catalyst) and previous studies.
2
The structural deformation in the crystallite lattice of Co–Ce affiliated with the suppressed growth of CeO crystallite by Co
catalysts can also be demonstrated by the Raman spectra. As doping.
shown in Fig. 3d, CeO As presented in Fig. 3e, N
cubic uorite structure at around 471.6 cm are further carried out to quantify the catalyst structure. Based
3 4
presents ve characteristic peaks of Co O spinel phase on this, the specic surface area, total pore volume and average
x
shows a characteristic F2g band of CeO
2
2
adsorption–desorption isotherms
ꢀ
1
38
and CoO
x
1
2
3
39
distinguished as F , E , F , F and A1g. In comparison with pore size of samples can be calculated (Table S2†). The specic
2
g
g
2g 2g
2
CeO , the peak of F bands in the binary metal oxide catalyst surface area of Co–Ce binary metal oxide catalysts (46.0–56.0 m
shis to lower frequency (452.3 cm ), conrming the incor-
poration of Co cations into the CeO uorite lattice and the
deformation of lattice framework. Similar phenomenon is sample exhibits the largest specic surface area (56.0 m g ),
recognized on the A1g bands of Co–Ce catalysts, highlighting the which is 3.6 folds greater than that of CoO . The higher surface
obvious Co–Ce interaction. area can provide abundant active sites for efficient oxidation
x
2g
ꢀ
1
ꢀ1
g
) is much larger than those of CoO
x
and CeO
x
(12.1 and 25.6
2
ꢀ1
2
m
g
, respectively). Among Co–Ce catalysts, Co0.75Ce0.25
O
x
4
0
2
ꢀ1
x
Moreover, the characteristic diffraction peaks of Co–Ce reaction, consistent with the as-obtained superior plasma-
oxides are much broader and weaker than those of mono- catalytic performance using Co0.75Ce0.25O catalyst.
x
metallic counterparts (Fig. 3a), implying the decrease of crys-
tallite size in the catalyst. To quantitatively characterize the catalyst surface species, which plays a crucial role in deter-
catalyst structure, the CeO crystallite size in Co–Ce catalysts is mining the oxygen vacancies and oxygen mobility for reac-
calculated by the Scherrer's equation:
XPS analysis is performed to unveil the oxidation state of
2
4
1
15,42
tion.
Fig. 4a), indexed to Co 2p3/2 and Co 2p1/2, respectively. The
spin–orbit splitting within Co 2p peaks (15.2 eV) reveals that
Two peaks located at 779.8 and 795.0 eV are recognized
(
d ¼ 0.89l/(b cos q),
(6)
26
Co
good accordance with XRD results. As shown in Fig. 4b, the Ce
d spectra are deconvolved into eight peaks, in which the spin–
orbit components of Ce 3d5/2 and Ce 3d3/2 are denoted by U
3 4
O is the primary phase of cobalt oxides in the catalyst, in
where d (nm) is the calculated crystallite size; l (nm) is the
wavelength of incident X-ray from Cu source; b is the peak width
3
at half peak height; q is the X-ray incidence angle. The CeO
crystallite sizes in Co0.75Ce0.25
and CeO are calculated to be 9.6, 11.9, 12.6 and 14.2 nm,
respectively, exhibiting a minimum value at Co0.75Ce0.25Ox
decreased by 32.4% in comparison with the monometallic
counterpart) (Table S2†). This observation is presumably
2
O
x
, Co0.5Ce0.5
O
x
, Co0.25Ce0.75
O
x
3+
3+
4+
and V, respectively. Ratios of Ce /(Ce + Ce ) are calculated by
integrating the peak area (Table S3†). The content of Ce
increases with incorporating the Co species, which are esti-
mated to be 12.7%, 16.2% and 19.2% for Co0.25Ce0.75
x
3+
(
x
O , Co0.5-
Ce0.5 and Co0.75Ce0.25 samples, respectively. The higher
O
x
O
x
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Fig. 3 (a) Angle patterns and (b) enlarged zone patterns from XRD analysis. (c) Schematic diagram of Co–Ce solid solution. (d) Raman spectra and
e) N adsorption–desorption isotherms.
(
2
3
+
ꢁ ꢁ
773.9 C, and CoO presents a distinct peak at 395.9 C with
x
46
Ce content is able to generate abundant defects in the CeO
2
43,44
ꢁ
47
lattice,
which facilitates more oxygen vacancies over catalyst a small shoulder of 341.4
C
(Fig. 4d). As for the Co–Ce
ꢁ
ꢁ
surface and adsorption centers for active oxygen species, thus composite, three peaks labeled as a (285.6 C), b (344.3 C) and
boosting the toluene conversion reaction.
As shown in Fig. 4c, O 1s spectra are tted into two peaks. tion of Co to Co cations while b and g peaks are predomi-
3 4
The peaks at around 532.0 and 529.8 eV can be identied as the nantly correlated with the reduction of Co to Co in the Co O
surface absorbed oxygen species (labeled by O ) and lattice phase and solid solution of Co–Ce, respectively. The temper-
oxygen (labeled by O ), respectively.
amount of Co dopant, the ratio of O /(Os + O ) increases lower than those of CoO (341.4 C) and CeO (483.6 C), indi-
ꢁ
g (484.7 C) are recognized. The a peak stems from the reduc-
3
+
2+
2
+
42
s
44,45
ꢁ
With increasing the ature of a peak in the Co–Ce catalyst (285.6 C) is obviously
l
ꢁ
ꢁ
s
l
x
x
remarkably from 20.6% to 31.0% (Table S3†). In consideration cating the improved reducibility. This phenomenon is associ-
of the high mobility of O , the larger ratio of O /(O + O ) is ated with the smaller crystallite size of Co–Ce samples, which
benecial for yielding improved catalytic activity. As expected, promotes the formation of easily reducible sites and improved
s
s
s
l
3
+
Co0.75Ce0.25
O
x
catalyst exhibits the highest Ce
and O
s
oxygen mobility for efficient toluene conversation. Detailed
concentration, agreeing well with the catalytic performance.
characterizations on the structure and redox property of used
H -TPR tests are carried out to evaluate the reducibility of Co0.75Ce0.25Ox catalyst aer plasma catalysis are available in
2
catalysts. CeO sample exhibits two broad peaks at 483.6 and Fig. S3, Tables S2 and S3.† It is found that the plasma-catalytic
x
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2
Fig. 4 (a) Co 2p, (b) Ce 3d and (c) O 1s spectra of XPS analysis. (d) H -TPR profiles of Co–Ce binary metal oxide catalysts.
reaction exhibits a negative impact on the structure and redox discharge and surface discharge (IPC), implying the enhanced
property of the Co0.75Ce0.25 catalyst, which is mainly ascribed ionization travelling along the catalyst surface for advanced
O
x
1
7
53
to the accumulation of intermediates on the catalyst surface,
catalytic reaction.
3
+
4+ 48
the partly oxidation from Ce to Ce
and the consumption of
The discharge voltage in the NTP and IPC system is also
measured and compared. As presented in Fig. 5d, the magni-
tude of discharge voltage increases with enlarging the SIE.
Importantly, the integration of catalysts can obviously extend
the voltage amplitude (by > 0.5 kV), e.g., from 9.45 kV at 419 J
49
s
O during the toluene decomposition.
Plasma–catalyst interaction
In IPC, catalysts are integrated in the discharge region, which
will affect the discharge behavior and thus determine the the DBD reactor will produce more highly energetic electrons
ꢀ
1
ꢀ1
L
to 9.97 kV at 401 J L . Enhanced electric eld strength in
50–52
catalytic activity.
Fig. 5a and b show the electrical signals for accelerating the plasma-catalytic oxidation, demonstrating
catalyst at 250 the signicance of plasma–catalyst interaction in boosting the
within the NTP and IPC (using the Co0.75Ce0.25
O
x
ꢀ
1
J L ). A typical lamentary discharge is observed for the NTP catalytic performance.
system, exhibiting obvious discharge current pulses in the
waveform. Importantly, introducing catalyst into the discharge
zone of the NTP noticeably affects the discharge behavior. On
Mechanism of plasma-catalytic reaction
one hand, the presence of catalyst suppresses the current In order to reveal the reaction mechanism and pathways, the by-
amplitude. The lamentary discharge can only be generated in products in the outlet of the NTP and IPC system over the
the small void gap between catalyst particles and quartz wall. As Co0.75Ce0.25O catalyst are measured by GC-MS (Fig. 6). Large
x
a consequence, the packed catalysts in the NTP will reduce the amounts of toluene and organic intermediates, e.g., aldehydes,
available discharge volume, thus decreasing the current hydrocarbons, acids and benzene series, are observed aer NTP
amplitude of lamentary discharge. On the other hand, the system treated. While types and amounts of by-products
integration of catalyst markedly increases the amount of signicantly decrease in IPC, in good accordance with the effi-
current pulses, which is predominantly attributed to the surface cient toluene conversion.
discharge generated over the catalyst surface. As schematically
shown in Fig. 5c, the discharge mode is changed from the identied into two parts:
The toluene decomposition process in IPC system can be
12,18
the gas-phase plasma reactions

lamentary discharge (NTP) to the combined lamentary and the catalytic reactions on catalyst surface. According to GC-
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Fig. 5 Electrical signals in (a) NTP and (b) IPC. (c) Schematic diagram of discharge behavior. (d) Discharge voltages within the NTP and IPC.
The destruction of C–H within methyl group might be primary
process of plasma reactions due to the lowest dissociation
energy, resulting in the generation of benzyl radicals. Aer that,
the formed benzyl radicals further react with oxygen species ($O
and $OH) from plasma discharge to produce benzaldehyde, as
demonstrated in GC-MS results.
Moreover, methyl and phenyl radicals can also be generated
from the breaking of C–C connecting the methyl group with the
aromatic ring. The reaction between methyl radicals and $NO
2
contributes to the formation of nitromethane. Phenyl radicals
could react with $CH , $H and $NO to produce o-xylene/p-
3
2
xylene, benzene and nitrobenzene. Subsequently, the aromatic
intermediates are attacked by highly energetic electrons (>5.5
eV) and active species in the plasma to produce ring-opening
2 2
intermediates, which are nally decomposed to CO and H O.
Noteworthy, the introduction of Co–Ce binary metal oxide
catalysts into the NTP system can yield stronger electric eld
and more highly energetic electrons to improve the catalytic
activity.
ꢀ
1
Fig. 6 Outlet gases of the NTP and IPC system at 250 J L measured
by GC-MS method.
Additionally, toluene and organic intermediates are also
adsorbed on the surface of Co–Ce catalysts placed in the
discharge region, which are further decomposed by surface
active oxygen species (Fig. 7). On one hand, the surface active
oxygen species are generated from adsorbed oxygen molecules
MS analysis, the reaction pathways of toluene degradation in
gas phase are presented (Fig. 7).
The dissociation energy of C–H (methyl group), C–H
(aromatic ring), C–C (methyl group-aromatic ring), C–C
attacked by highly energetic electrons (eqn (7)) and O
3
conver-
(
4
aromatic ring) and C]C (aromatic ring) are 3.7 eV, 4.3 eV,
56,57
54,55
sion reaction on catalyst surface (eqn (8) and (9)):
.4 eV, 5.0–5.3 eV and 5.5 eV, respectively.
In NTP system, the
energy of highly energetic electrons reaches up to 10 eV.
Therefore, these chemical bonds can be destroyed by electrons.
O
2
+ e / O + O
(7)
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Fig. 7 Mechanism of plasma-catalytic reaction over Co–Ce binary metal oxide catalysts.
O + * / O* + O
(8)
(9)
3
2
Conclusion
*
*
2
O þ O
3
/O
2
þ O ;
This work investigated the catalytic performance of toluene
where * represents active centers on catalyst surface. On the decomposition over Co–Ce binary metal oxide catalysts in IPC.
other hand, surface active oxygen species could also be supplied In comparison with the NTP system, the integration of Co–Ce
from the redox cycle between Co and Ce cations (eqn (10)) catalysts signicantly advances the toluene removal efficiency
1
8,25,58
according to the Mars-Van Krevelen mechanism:
(by 35.0%), CO
7.7%). Among the catalysts, the Co–Ce binary metal oxide
(10) catalysts present better catalytic activity than those of mono-
metallic counterparts. Especially, Co0.75Ce0.25 simultaneously
realizes high-performance toluene conversion (decomposition
2
selectivity (by 44.8%) and carbon balance (by
3
3
+
4+
2+
3+
Ce /Ce # Co /Co
O
x
During the catalytic reactions on the surface of catalysts,
active oxygen species are consumed by decomposition of
absorbed organic molecules, meanwhile the Co sites are
reduced to Co . The redox pair of Ce /Ce in CeO
the storage and release of lattice oxygen species, re-oxidizing
Co to Co . This redox cycle of Co /Co is able to transmit
the oxygen species to the surface of catalysts, thereby supplying
the consumed surface active oxygen species. Additionally, the
oxygen vacancies generated from lattice oxygen consumption
via Co–Ce redox cycle can be replenished by gas phase oxygen
species. Therefore, the synergistic effect between Co and Ce
species promotes the generation and mobility of active surface
oxygen species for superior catalytic performance.
efficiency of 98.5%, CO
2
selectivity of 87.3% and carbon balance
3
+
ꢀ1
of 97.8% at 401 J L ) and large energy efficiency of 7.12 g kW
2
+
3+
4+
facilitates
ꢀ1
2
h
, which is obviously higher than or comparable to those in
ꢀ1
the state-of-art literatures (0.42 to 6.11 g kW h ). The excellent
catalytic performance could be attributed to the synergistic
effect between Co and Ce species and crucial plasma–catalyst
interaction. On one hand, structural characterizations
2
+
3+
2+
3+
2 2
including XRD, Raman, N adsorption–desorption, XPS and H -
TPR reveal that the collaborative interactions can decrease the
catalyst crystallite size (by 32.4%), enlarge the specic surface
area (by 3.6 times), enhance the amount of oxygen vacancies/
3+
mobility (6.5% and 10.4% increase in the Ce
and Os
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concentration, respectively). These features are able to provide 15 N. Jiang, J. Hu, J. Li, K. F. Shang, N. Lu and Y. Wu, Appl.
more active adsorption centers for the surface oxygen species Catal., B, 2016, 184, 355–363.
and thus accelerate the toluene decomposition reactions. On 16 H. Song, F. Hu, Y. Peng, K. Li, S. Bai and J. Li, Chem. Eng. J.,
the other hand, the plasma–catalyst interaction can reinforce 2018, 347, 447–454.
the discharge voltage in the DBD reactor (by > 0.50 kV) and 17 X. B. Zhu, X. Gao, R. Qin, Y. X. Zeng, R. Y. Qu, C. H. Zheng
transform the discharge mode from lamentary discharge to and X. Tu, Appl. Catal., B, 2015, 170–171, 293–300.
the combined lamentary discharge and surface discharge, 18 B. W. Wang, C. M. Chi, M. Xu, C. Wang and D. Meng, Chem.
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GC-MS observations, the plasma-catalytic reaction mechanism 19 Z. Bo, M. Yuan, S. Mao, X. Chen, J. H. Yan and K. F. Cen,
and pathways of toluene decomposition are demonstrated. The Sens. Actuators, B, 2018, 256, 1011–1020.
as-obtained insights open an alternative way of employing Co– 20 Z. Bo, W. G. Zhu, W. Ma, Z. H. Wen, X. R. Shuai, J. H. Chen,
Ce binary metal oxide catalysts to realize enhanced toluene
conversion with high energy efficiency in the plasma catalysis.
J. H. Yan, Z. H. Wang, K. F. Cen and X. L. Feng, Adv. Mater.,
2013, 25, 5799–5806.
21 Z. Bo, S. Mao, Z. J. Han, K. Cen, J. Chen and K. K. Ostrikov,
Chem. Soc. Rev., 2015, 44, 2108–2121.
Conflicts of interest
2
2 A. Rokici n´ ska, P. Natka n´ ski, B. Dudek, M. Drozdek,
L. Lity n´ ska-Dobrzy n´ ska and P. Ku ´s trowski, Appl. Catal., B,
There are no conicts to declare.
2016, 195, 59–68.
2
3 A. Białas, M. Mazur, P. Natka n´ ski, B. Dudek, M. Kozak,
A. Wach and P. Ku ´s trowski, Appl. Surf. Sci., 2016, 362, 297–
303.
Acknowledgements
The authors acknowledge the nancial support from the
National Natural Science Foundation of China (No. 51576175), 24 Z. M. Li, X. L. Guo, F. Tao and R. X. Zhou, RSC Adv., 2018, 8,
the Zhejiang Provincial Natural Science Foundation of China 25283–25291.
No. LR17E060002) and the Fundamental Research Funds for 25 C. Wang, C. H. Zhang, W. C. Hua, Y. L. Guo, G. Z. Lu, S. Gil
(
the Central Universities (No. 2018XZZX002-17).
and A. Giroir-Fendler, Chem. Eng. J., 2017, 315, 392–402.
6 S. Akram, Z. Wang, L. Chen, Q. Wang, G. L. Shen, N. Han,
Y. F. Chen and G. L. Ge, Catal. Commun., 2016, 73, 123–127.
7 W. Abou Saoud, A. A. Assadi, M. Guiza, A. Bouzaza,
W. Aboussaoud, A. Ouederni, I. Soutrel, D. Wolbert and
S. Rtimi, Appl. Catal., B, 2017, 213, 53–61.
2
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