Application of combined plasma-catalytic method for carbon particulate matter (PM) removal

Article abstract of DOI:10.1039/c4ra13662d

The carbon particulate matter (PM) generated by diesel engine emissions has attracted world-wide attention because it has a remarkable impact on air quality and the human body. Therefore, research on the removal of carbon PM has attracted increasing interest recently. Additionally, oxidative removal of carbon PM requires high temperature due to its high activation energy. In this research, a promising technology of nonthermal plasma (NTP) combined with catalytic oxidation is reported. The effective removal of carbon PM under NTP conditions by the synergy of combining plasma with MnO_x/CeO₂ catalysts at low temperatures was reported in this paper. The removal efficiency of carbon PM on MnO_x/CeO₂ catalysts (Mn loading 5.0 wt%) can be as high as 85.2% and 94.3% at 20°C and 200°C respectively, at the discharge power of 18.0 W and air flow rate of 30 mL min^-1. Moderate reaction conditions with low temperature but high removal efficiency are the advantages of the decomposition of carbon PM in a dielectric barrier discharge (DBD) reactor. It is proposed that reactive oxygen species produced under NTP conditions are responsible for carbon PM converting into CO₂. Furthermore, various parameters such as temperature, discharge power and air flow rate under NTP were investigated in the present paper, as well as the reaction process of NTP. Meanwhile, the MnO_x/CeO₂ catalysts were also characterized by XRD and TEM techniques.

Full text of DOI:10.1039/c4ra13662d

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Application of combined plasma-catalytic method
for carbon particulate matter (PM) removal
Cite this: RSC Adv., 2015, 5, 40012
Lu-Jie Liu, Xiang-Xiang Li, Hui Wang, Bo Xue, Xiao-Ming Zheng and Min Chen*
The carbon particulate matter (PM) generated by diesel engine emissions has attracted world-wide
attention because it has a remarkable impact on air quality and the human body. Therefore, research on
the removal of carbon PM has attracted increasing interest recently. Additionally, oxidative removal of
carbon PM requires high temperature due to its high activation energy. In this research, a promising
technology of nonthermal plasma (NTP) combined with catalytic oxidation is reported. The effective
removal of carbon PM under NTP conditions by the synergy of combining plasma with MnO
x 2
/CeO
catalysts at low temperatures was reported in this paper. The removal efficiency of carbon PM on MnO
x
/
ꢀ
ꢀ
CeO
2
catalysts (Mn loading 5.0 wt%) can be as high as 85.2% and 94.3% at 20 C and 200
C
ꢁ1
respectively, at the discharge power of 18.0 W and air flow rate of 30 mL min . Moderate reaction
conditions with low temperature but high removal efficiency are the advantages of the decomposition of
carbon PM in a dielectric barrier discharge (DBD) reactor. It is proposed that reactive oxygen species
Received 5th November 2014
Accepted 24th April 2015
produced under NTP conditions are responsible for carbon PM converting into CO
2
. Furthermore,
various parameters such as temperature, discharge power and air flow rate under NTP were investigated
DOI: 10.1039/c4ra13662d
in the present paper, as well as the reaction process of NTP. Meanwhile, the MnO
also characterized by XRD and TEM techniques.
x 2
/CeO catalysts were
www.rsc.org/advances
been reported as the most efficient transition-metal oxide
catalysts for catalytic disposal of pollutants and CeO plays an
Introduction
2
Solid carbon particles such as soot, char and coke generated by important role as a support owing to its storage capacity of
20–22
gasication and combustion of coal, heavy oil, and diesel fuel, oxygen and thermal stability.
Accordingly, of these two
require high activation energy compared to that of a gaseous or substances, manganese oxides and cerium oxide were chosen
1
liquid product. In particular, carbon PM, emitted from diesel and prepared as catalysts in this work.
engine, is considered to be one of the most hazardous materials
Furthermore, nonthermal plasma could activate molecules
2–4
23,24
to air quality and human health. However, removal of carbon including PM, hydrocarbons, and NO
x
at room temperature.
PM is quite difficult based on current techniques, since the Thus, extensive application of NTP has been reported in
25
oxidative removal of carbon PM is usually carried out under an chemical synthesis, removal of environmental pollutants such
2
6,27
oxygen-rich environment in which the temperature can be as volatile organic compounds (VOCs),
and fabrication of
ꢀ
28
reached as high as 600–700 C due to its high activation energy. materials. Consequently, moderate reacting conditions with
Three kinds of catalysts, the transition metal catalysts, the low temperature and high removal efficiency are the advantages
5–7
8–10
11–14
spinel-type catalysts,
and the perovskite-type catalysts
of decomposition of carbon PM in NTP. It is proposed that
, active molecule of N and
produced under
show the best catalytic performance, but problems associated NTP conditions are responsible for carbon PM converted into
have been applied in the catalytic combustion for removal of reactive oxygen species such as (O
carbon PM. Among those catalysts, the perovskite-type catalysts
3
2
+
ꢁ
29–32
2
O , O , O , O(1D), O(3P), and electrons)
ꢀ
with a high temperature of 400 C towards the formation of CO
2
. Catalytic oxidation of carbon PM with plasma assisted is
spinel structure have hindered its further development in the most economic approaches because of its lower tempera-
1
5
removal of carbon PM. Recently, solid oxides such as cerium ture and excellent selectivity towards the formation of CO
oxide have been investigated in the catalytic oxidation at lower
Therefore, a combined method of nonthermal plasma-
temperature. Some modied CeO
oxides catalyze oxidation of catalytic reaction has been used in the removal of carbon PM.
carbon PM mainly because of its function of promoting evolu- In this article, we present the results of catalytic oxidation
.
2
2
16–19
tion of lattice oxygen.
x 2
Moreover, manganese oxides have carbon PM on MnO /CeO catalysts under NTP conditions. DBD
is chosen as the reactor because it can produce high concen-
trations of reactive oxygen species which could oxidize the
Institute of Catalysis, Department of Chemistry, Zhejiang University (Xixi Campus),
Hangzhou 310028, China. E-mail: chenmin@zju.edu.cn
carbon PM to CO
2
efficiently at lower temperature. Additionally,
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both a combined plasma-catalytic method and reaction process discharge began. This discharge process just lasted several
of NTP were investigated.
minutes at ambient temperature.
For comparison, the catalytic mode for the removal of carbon
PM was carried out under the same conditions without
Experimental
discharge. The products of the reaction, such as CO and CO
were detected on a line gas chromatography (GC-1690, keqiao,
catalysts with different manganese ratio, china) with a FID attachment. The PM removal on the
2
2
,
1.
Preparation of catalyst
A series of MnO /CeO
x
were prepared by impregnation method. The support of CeO
was dried for 2 h at 110 C, manganese acetate solution was
2
measurements was calculated as follows:
ꢀ
PM conversion [%] ¼ [c(CO) + c(CO )]/c(PM) ꢂ 100%
(1)
dropwise added. Then it was aged at ambient temperature for
2
ꢀ
1
2 h, followed by calcination at 500 C for 4 h in the air atmo-
sphere. The obtained catalysts were named as MnO /CeO .
x
2
In this paper, Printex U carbon powder was used as the 3. Catalysts characterization
substitute for carbon PM in diesel engine emissions. The
Structural characterizations of the samples were performed
using X-ray diffraction (XRD). The XRD patterns were recorded
on a Rigaku D/max-IIIB instrument using CuKa radiation (40 kV
and 40 mA). The phase analysis was determined from XRD
patterns using the MDI-JADE-5 program. Transmission electron
microscopy (TEM) images were acquired with a HT-7700 elec-
tron microscope.
carbon powder was made up of small spherical carbon particles
10–80 nm). It was subjected to chemical analyses in order to
(
assess the solid phase emissions such as solid particulate or
soot from diesel. The sample ratio of catalyst and U carbon
powder was 2 : 1000 and these were prepared by mechanical
grind method. All the samples were passed by the procedures
of grounding fully, pressing, and sieved to 20–40 mesh size
pellets.
Results and discussion
2.
Combined plasma-catalytic reactions
1. Removal efficiency of carbon PM over different catalysts
The experiments were carried out in a dielectric barrier Table 1 exhibits the removal efficiency of carbon PM over
discharge (DBD) reactor connected to a high-voltage power different catalysts such as Al , CeO , MnO /Al and MnO
supply (Fig. 1). In the conguration of the DBD reactor, a quartz CeO at the temperature range from 20 to 200 C. Under NTP
2
O
3
2
x
2
ꢀ
O
3
x
/
2
tube (i.d. 8.0 mm) was used as the reactor as well as the condition, as shown in Table 1, it demonstrates no carbon PM
dielectric barrier, and also a stainless steel rod (o.d. 3.0 mm) removal efficiency on those catalysts in the absence of NTP
was applied as the high-voltage electrode. At the same time, the condition during this temperature range. This result shows that
stainless steel net wrapped on the outer surface of the quartz it is difficult to oxide carbon PM only by the catalytic combus-
tube and was employed as the grounding electrode. A high- tion since oxidation of PM requires a high temperature.
voltage power was used to control the discharge in the However, with the presence of nonthermal plasma (NTP) tech-
reactor. Discharge power was measured by the V-Q Lissajous nologies and MnO
program. The temperature of the reactor was measured by a cant PM removal efficiency can be observed. On the MnO
thermocouple attached at the surface of the quartz tube reactor (Mn loading 5.0 wt%) catalyst, the carbon PM removal efficiency
x
/CeO
2
catalyst applied in plasma, a signi-
x
/CeO
2
ꢀ
ꢀ
wall and controlled by an outer electrical heater. The sample reaches to 85.2% at 20 C and 94.3% at 200 C, respectively. This
pellets (20–40 mesh, 0.5 g) were lled in the interior of the result indicates that by using NTP, the reactant molecules can
discharge zone. Thus, a combined discharge plasma catalytic be easily activated and dissociated at ambient temperature.
process can be carried out in this equipment. 0.5 g sample (20– Similarly, over other catalysts, effective carbon PM removal
ꢀ
4
0 mesh) was pretreated at 500 C for 60 minutes in N
2
ow efficiency is obtained as well during combined plasma-catalytic
ꢀ
before experiment, and then cooled it to ambient temperature. process. At 200 C, the carbon PM removal efficiency over MnO
x
/
Then a owing mixed gas (21% O
2
and 79% N
2
) was introduced Al
2
O
3
catalyst is about 76.2%, and reaches 71.0% for CeO
2
and
into the dielectric DBD reactor in which the total ow rate was 25.5% for Al
controlled at 30 mL min ,and simultaneously, the stage of
2
O
3
. Among all catalysts, MnO /CeO catalyst shows
x
2
ꢁ1
Table 1 The removal efficiency of carbon PM over different catalysts
a
at different temperature
Catalyst
ꢀ
Temperature/ C
Al
2
O
3
CeO
2
MnO
x
/Al
2
O
3
x 2
MnO /CeO
2
1
2
0
00
00
23.8%
24.9%
25.5%
23.4%
64.1%
71.0%
35.1%
68.0%
76.2%
85.2%
87.4%
94.3%
a
Reaction conditions: mcat. ¼ 0.50 g, total ow rate at 30 mL minꢁ1
,
Fig. 1 The schematic diagram of the experimental setup.
discharge power at 18.0 W.
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the highest carbon PM removal efficiency. This combined Fig. 3. On the CeO
2
sample (A), the diffraction peaks due to
plasma-catalytic process is possible due to the fact that various cubic CeO phase are detected. On the 5% MnO /CeO catalysts
2
x
2
oxygen atoms generated by plasma discharges play an impor- before and aer reaction (B and C), no diffraction peak, char-
tant function in breaking C–C bonds, which results in the acteristic of the manganese oxide could be observed except the
oxidation of graphite. These oxygen atoms are the main reac- CeO
tants that can react with carbon PM to convert them into CO CeO
2
phase, indicating the MnO
surface. This could be explained by the fact that the XRD
x
was in high dispersion on the
2
33
ꢁ
and CO
2
.
In the reaction process, the O
2
is converted to O
technique could not detect very small crystallites (usually <3
3
5
that reacts with O
2
to generate O . The observed synergistic nm). Meanwhile, XRD patterns of MnO /CeO before and aer
3
x 2
catalytic effect is owing to the fact that with manganese oxide reaction showed that the catalyst still kept its crystalline struc-
supported on CeO , more oxygen can be joined in the redox ture aer the plasma reaction.
2
34
cycle. On the other hand, the active species generated in the
Moreover, TEM was used for direct observations of the 5%
discharge zone by the nonthermal plasma (NTP) are much MnO /CeO catalysts. The surface morphology of the 5% MnO /
x
2
x
2
easier for adsorption and dissociation on the surface of catalyst. CeO catalysts before and aer NTP reaction is clearly shown in
As a result, radical species can reach on the surface of catalyst Fig. 4. It can be seen that the catalyst shows no evident changes
and participate in the reaction.
aer nonthermal plasma reaction, indicating the 5% MnO
can be CeO catalyst has high stability and can be recycled. Both results
/CeO
2
x
/
Furthermore, during the experiment, only CO
detected with no other products such as CO detected by chro- (XRD and TEM) suggest that the performance of 5% MnO
matography in the effluence, indicating the main component of catalyst is stable under the plasma discharge conditions.
carbon PM is pure graphite structure. It may be due to the fact
that active species generated in the combined plasma-catalytic
process are contributing to accelerate the reaction of carbon
2
2
x
24
4.
Effect of discharge power and ow velocity in plasma-
catalytic process
2
converted into CO and CO .
Since the parameter affects the reaction greatly, the character-
istic information such as ow rate and discharge power was
investigated. Fig. 5(a) shows the conversion of carbon PM in O2
2.
The activity of series of CeO
Fig. 2 demonstrates the effect of the Mn loading (2.5%, 5.0%,
.5%) on the catalytic performance of MnO /CeO catalysts. The
2 x
-supported MnO catalysts
plasma on 5% MnO /CeO as a function of ow rate at the range
x
2
ꢁ
1
from 30 to 90 mL min . It is clear that the conversion of carbon
PM to CO shows a declining trend with the increasing of ow
rate. It reaches the maximum at the lower ow rate of 30 mL
7
x
2
removal efficiency of carbon PM increases with Mn loaded, but
it reaches a highest value on 5.0% MnO /CeO catalyst. This
trend may be attributed to the formation of bulk manganese
particles which will decrease catalytic activity when the MnO
2
x
2
ꢁ
1
ꢁ1
min , and then drops dramatically at 90 mL min , indicating
ꢁ
1
the condition of 30 mL min in ow rate is proved to be the
suitable condition for the removal of carbon PM in NTP-catalytic
reaction, which can be interpreted that more and more active
particles are generated in the discharge zone of plasma under
appropriate ow rate conditions. Moreover, with the increase of
x
34
content is over 7.5%.
x 2
3. The morphology of the 5% MnO /CeO catalyst
XRD was used for direct observations of the catalyst. Three ow rate, the effective collisions between active particles and
patterns, representing of CeO , 5% MnO /CeO catalyst before reactant molecules are inevitably reduced as the ow rate of
and aer combined plasma-catalytic reaction are presented in
2
x
2
Fig. 2 Removal properties of carbon PM as a function of different Mn
loadings on MnO /CeO catalysts (reaction conditions: room temper- Fig. 3 XRD patterns of (a) CeO
ature, 30 mL min flow rate of the air, discharge power at 18.0 W). and (c) after reaction.
x
ꢁ
2
2 x 2
, 5% MnO /CeO catalyst (b) before
1
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reactant gas increased. Thus, the following experiments were
ꢁ1
carried out at the condition of ow rate of 30 mL min
.
Additionally, the discharge power is an important parameter
for plasma chemistry. The inuence of the discharge power on
carbon PM oxidation in NTP is also shown in Fig. 4(b). It shows
an opposite trend in that of ow rate. Obviously, at the range of
discharge power from 10.0 to 18.0 W, the conversion of carbon
2
PM to CO greatly improves with the increasing of discharge
power. This is due to the fact that higher discharge power is able
to supply more electrons with sufficient energy to activate the
carbon PM particle. As a result, the active particle density can be
improved and then the oxidation of carbon PM will be sped up.
5. Effect of reaction temperature and time in plasma-
catalytic process
The effect of reaction temperature and time on plasma removal
of PM over CeO and 5% MnO /CeO catalyst are illustrated in
2
x
2
Fig. 6. As can be seen from Fig. 6, carbon PM removal efficiency
is clearly inuenced by both temperature and time. The carbon
PM removal efficiency over CeO
quite low and with the time increasing to 25 minutes, the PM
removal efficiency is still lower to 20%. However, for 5% MnO
CeO catalyst, at the same reaction time, the PM removal rate
2
at ambient temperature are
x
/
2
reaches 80%, indicating the carbon PM removal is improved
signicantly. This result suggests that by supported manganese
oxide on CeO , it may be enter into ceria lattice and improves
2
the oxygen storage capacity of ceria as well as the oxygen
21
mobility on the surface of the mixed oxides. It is obviously on
ꢀ
5% MnO
x
/CeO
2
catalyst, when temperature is higher to 200 C,
the reaction happens more rapidly. The removal of carbon PM is
higher than 90% in 10 minutes.
x 2
Fig. 4 TEM photograph of 5% MnO /CeO catalyst (a) before and (b)
ꢁ1
after reaction; (reaction condition: total flow rate at 30 mL min
,
ꢀ
discharge power at 18.0 W and temperature at 200 C).
6. Reaction process for carbon PM removal under plasma
discharge conditions
The mechanism of carbon PM removed under plasma discharge
conditions is not clear. From the fact that oxidation products is
carbon dioxide, the nonthermal plasma reaction processing for
Fig. 5 Removal efficiency of carbon PM as a function of (a) flow rate
and (b) discharge power over MnO /CeO catalyst (reaction condition
a): ambient temperature, discharge power at 18.0 W; (b): ambient
x
2
Fig. 6 PM removal as a function of reaction time on catalysts: (a) CeO
2
(
ꢁ1
at ambient temperature, (b) 5% MnO
x
/CeO
2
at ambient temperature,
temperature, 30 mL min flow rate of the air).
ꢀ
(
c) 5% MnO
x
/CeO
2
at 200 C.
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x
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1
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Conclusions
x
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1
The combined plasma-catalytic process may be an efficient way
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demonstrated to be an efficient catalyst in the plasma-catalytic
reaction for carbon PM converted into CO . The synergistic
1
x
2
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ꢁx
x
3
2
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lower than that in traditional catalytic combustion mode.
2
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1
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Acknowledgements
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5 Z. Q. Li, M. Meng, Q. A. Li, Y. N. Xie, T. D. Hu and J. Zhang,
x
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The support of 973 Program (2013CB228104) of China is greatly
appreciated.
1
Fe-substituted
nanometric
La0.9
K
x 3
0.1Co1ꢁxFe O -delta
perovskite catalysts used for soot combustion, NO storage
and simultaneous catalytic removal of soot and NOx,
x
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