Synergistic effect of non-thermal plasma–catalysis hybrid system on methane complete oxidation over Pd-based catalysts

Synergistic effect of non-thermal plasma–catalysis hybrid system on methane complete oxidation over Pd-based catalysts

Chemical Engineering Journal 259 (2015) 761–770 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevie...

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Chemical Engineering Journal 259 (2015) 761–770

Contents lists available at ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Synergistic effect of non-thermal plasma–catalysis hybrid system on methane complete oxidation over Pd-based catalysts Heesoo Lee a, Dae-Hoon Lee b, Young-Hoon Song b, Won Choon Choi c, Yong-Ki Park c, Do Heui Kim a,⇑ a

School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-742, Republic of Korea Korea Institute of Machinery and Materials, 156 Gajeongbuk-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea c Korea Research Institute of Chemical Technology, 141 Gajeong-ro, Yuseong-gu, Daejeon 305-600, Republic of Korea b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Complete oxidation of CH4 proceeded

in the plasma–catalysis hybrid system.  Plasma–catalysis hybrid system had much lower CO selectivity compared to plasma only reaction.  Only Pd/Al2O3 catalyst demonstrated the synergistic effect of plasma and catalyst on CH4 oxidation.  The hybrid system was found to be a good way to lower the light-off temperature of CH4 oxidation.

a r t i c l e

i n f o

Article history: Received 7 May 2014 Received in revised form 28 July 2014 Accepted 30 July 2014 Available online 14 August 2014 Keywords: Methane oxidation Plasma–catalyst hybrid reaction Pd/c-Al2O3 Dielectric barrier discharge Synergistic effect

⇑ Corresponding author. Tel.: +82 2 880 1633. E-mail address: [email protected] (D.H. Kim). http://dx.doi.org/10.1016/j.cej.2014.07.128 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.

a b s t r a c t The complete oxidation of methane was carried out in a dielectric barrier discharge (DBD) quartz tube reactor where both catalyst and plasma were hybridized into one in-plasma catalysis system. The palladium-based catalysts such as Pd/Al2O3, Pd/SiO2, and Pd/TiO2 were used as oxidation catalyst. Input voltage of the plasma–catalyst reactor varied from 2 kVp-p to 4 kVp-p to investigate which input voltage offered the best circumstance for plasma–catalyst interaction. In the absence of catalyst, methane began to be oxidized to CO and CO2 even at room temperature, and the conversion increased with the increment of temperature and the input voltage since the active radicals were generated more abundantly under those conditions. However, large amount of CO were also produced in addition to CO2, especially at low temperature below 200 °C when plasma was only used. In the presence of both plasma and catalyst, methane was oxidized to produce mostly CO2 with low CO selectivity at room temperature, indicating that the complete oxidation was successfully performed with the aid of catalyst. The role of plasma was to oxidize CH4 to produce CO, which was subsequently oxidized to CO2 over catalyst at low temperature. Hence, in most cases, the methane conversion of plasma–catalysis hybrid system was almost equal to the summation of two separate systems. Interestingly, it was found that the synergistic effect of plasma–catalysis hybrid system on methane oxidation existed substantially only under specific condition. For example, 2 wt% Pd/Al2O3 presented higher methane conversion than the summation of conversion for catalyst only reaction and plasma only reaction when 4 kVp-p was applied. Ó 2014 Elsevier B.V. All rights reserved.

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1. Introduction There are large reserves of methane in the form of natural gas and shale gas. The world-wide presence of methane and the fact that methane is regarded as clean energy source compared to other fossil fuels make it an attractive alternative energy source. Besides this, natural gas has potential technical and economic advantages due to achievable high fuel efficiencies and attractive cost, respectively [1]. Despite of these advantages, the emission of unburned natural gas makes it difficult to use since the methane is recognized as the major portion of global warming gas. It contributes 21 times more to global warming than CO2 at equivalent emission rate [2,3] and has quite long lifetime. After Euro III, the regulation on CH4 emissions from NGVs in urban areas, especially from heavy-duty vehicles, is currently being implemented in most industrialized countries [4]. Therefore, complete oxidation of methane is one of the critical problems to solve for widening the use of methane without worrying about the environmental concern such as global warming. Many studies have been performed to develop methane complete oxidation catalysts, for example, palladium-based catalysts [5–8], platinum-based catalysts [8], and non-noble metal catalysts such as mixed oxide catalysts [9–13]. However, it was difficult to achieve high activity at low temperature (i.e. below 200 °C) compared to other exhaust gases such as CO or hydrocarbons. Hence, the exhaust gas requires additional heat to reach the operating temperature of catalyst, which will have a negative effect on the economic point of view. An alternative way to catalytic oxidation of methane at lower temperature would be to use plasma. There are various thermal and non-thermal plasma sources such as dielectric barrier discharge (DBD), corona, gliding arc, rotating arc, spark, microwave, glow discharge and pulsed discharge with catalyst or without catalyst [14,15]. In the non-thermal plasma, high-energy electrons (1–20 eV) are produced and they can initiate the formation of other various radicals. Since electron mass is very light, non-thermal plasma gives rise to the increase in temperature by only few degrees [16]. Hence, non-thermal plasma is also referred to as ‘‘cold plasma’’ or ‘‘non-equilibrium plasma’’ [17]. As such, partially ionized gas can be created using simple reactor configuration and relatively inexpensive power source, while almost isothermal reaction condition is maintained. Various plasma sources are currently being used for solving environmental problems. Among those, DBD requires alternating voltage and at least one dielectric barrier between high voltage electrode and ground electrode [18]. Because of the dielectric barrier, the DBD is a non-equilibrium discharge, which means that high temperature electrons are created in the micro-discharges. They can initiate plasma-induced chemical reactions without increasing the temperature substantially. Plasma and catalyst have their own advantages and disadvantages. The catalyst is highly selective while it is active only at high temperature since reactants must overcome the activation energy. On the other hand, the plasma can be highly reactive even at room temperature although it is nonselective and requires the high energy. Hence, the hybridization of plasma and catalyst into one system can provide complementary or even synergistic results for the activation at low temperature [19]. Previous studies have shown the positive effect of using a plasma–catalysis combination system on various reactions such as oxidation, dissociation, reduction, and reforming [20–24]. Heterogeneous catalyst can be combined with non-thermal plasma in two ways: the catalyst in the discharge zone (in-plasma catalysis, IPC) or the catalyst after the discharge zone (post-plasma catalysis, PPC) as shown in Fig. 1. Catalyst can be put into a reactor as a packed bed in several ways, such as a layer of catalyst material or coating on the reactor wall or electrodes [25]. In order to maximize an influence of short-lived radical

Fig. 1. Schematic overview of two plasma–catalysis hybrid system configurations; (a) in-plasma catalysis (IPC) and (b) post-plasma catalysis (PPC).

on catalytic reactions, it is desirable for plasma to take place near the catalyst surface. Hence, in-plasma catalysis hybrid system was mostly used in this study and PPC was compared to see which configuration offered the better condition for methane complete oxidation. In this contribution, methane complete oxidation was performed in the DBD plasma reactor loaded with various palladium-based catalysts and at different plasma operating conditions. Holzer et al. [26] reported that c-Al2O3 and SiO2 which were widely used for Pd-based catalysts’ support showed synergistic effect with NTP (non-thermal plasma) on oxidation of VOCs (volatile organic compounds). In addition, TiO2 was known to have higher dielectric constant than the others. Therefore, c-Al2O3, SiO2, and TiO2 were used as model support material for loading Pd. Reaction tests were conducted in a simple noble gas condition to examine the hybridization effect of plasma and catalyst on complete methane oxidation. 2. Experimental 2.1. Preparation of catalyst Commercial supports such as c-Al2O3 (Sasol), SiO2 (Sigma Aldrich), TiO2 (Millennium Inorganic Chemicals) were used. The support materials were pre-calcined at 500 °C for 2 h in air. Palladium supported catalysts were synthesized by incipient wetness impregnation method. The solution of Pd(NO3)22H2O precursor (Sigma Aldrich) was impregnated into the supports. The loading amount of palladium in its metallic state was 2 wt% of the catalysts. The catalysts were dried and subsequently calcined in air (N2:O2 = 85:15) at 500 °C for 2 h. The catalysts were sieved to the particle size of 425–600 lm. Solid-state phases was identified with XRD (RIGAKU SMARTLAB) operated at 40 kV and 50 mA. Specific surface area of catalysts was also determined by the BET method using ASAP 2010 instrument. The CO chemisorption using Micromeritics ASAP 201C instrument was carried out to measure metallic dispersion of the catalysts as well. 2.2. Plasma–catalysis hybrid system A schematic view of plasma–catalysis system and equivalent circuit of DBD reaction system are presented in Fig. 2(a) and (b), respectively. It consisted of a 1000:1 high voltage probe (Tektronics P6015A), a current probe (Pearson electronics 6585), and a capacitor (1000 pF) for measuring voltage (V), current (A), and transferred electric charge (Q), respectively. All output signals were transmitted to a 100 MHz digital oscilloscope (Tektronics DPO 2014) which was utilized to calculate discharge power by the V–Q Lissajous figure method described later [27,28]. Configurations of in-plasma catalysis reactor and post-plasma reactor are displayed in Fig. 3(a) and (b), respectively. The catalysts

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(a) MFC MFC Vent MFC

Gas chromatography

Reactor with furnace

Funcon generator

Capacitor

CH4

O2

He

High voltage amplifier

H2O trap

(b) AC

DBD reactor Plasma

Barrier Capacitor

Fig. 2. (a) Schematic view of plasma–catalysis system and (b) equivalent circuit of DBD reaction system.

in bead form (U = 425–600 lm) were placed on the quartz filter at the middle of reactor, inside the plasma discharge zone. The height of catalyst bed varied from 5 mm to 15 mm since the weight of catalyst was 0.5 g while their density differed from each other. The catalyst bed temperature was measured and recorded using K type thermocouple which was protected by another quartz tube just below the filter in the reactor, in order to minimize the interference between the thermocouple and plasma. As a result, it can measure the temperature just after catalyst bed during in-plasma reaction without interfering with plasma. Another thermocouple in the furnace was used to control the setting temperature. The DBD reactor was made of a quartz tube with an inner diameter of 10 mm a thickness of 1.3 mm and a length of 500 mm. The filter was located at the middle of reactor and its thickness was 2.5 mm. In this reactor, catalyst zone and plasma zone were combined at same position. Electrical discharge of the reactor was DBD. It consisted of a stainless steel rod of which thickness was 3 mm. The rod was centered in a quartz tube reactor, fixed with a groove

at the filter and Teflon ferrule. The outer surface of the reactor was surrounded by copper foil with a length of 20 mm and a thickness of 0.05 mm. The length of discharge zone was 200 mm, and the gap was 5 mm, resulting in a reaction volume of about 2.5 cm3. To create the discharge, an AC high voltage was generated with a maximum of 4 kVp-p by arbitrary function generator (GW INSTEK AFG-2012) and amplified by high voltage amplifier (TREK 20/ 20C-HS) with a maximum peak voltage of 20 kV and a variable frequency up to 20 kHz. All experiments were carried out under identical conditions of waveform (sinusoid) and driving frequency (4 kHz) with the varying amplitude of the applied voltage from 2 kVp-p to 4 kVp-p with the internal of 1 kV. 2.3. Lissajous method According to the gas discharge theory, the load characteristics of a reactor are equivalent to that of a capacitor, and the DBD process is equal to the charge and discharge transitions of the

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(a)

(b)

High votlage electrode

High votlage electrode

Ground electrode

Ground electrode 20mm

Catalyst layer

Catalyst layer

Filter

Filter

Quartz reactor

Quartz reactor

T/C

T/C

Quartz tube

Quartz tube

Fig. 3. (a) Configuration of in-plasma catalysis reactor and (b) post-plasma catalysis reactor.

capacitor [27,29]. The capacitor was placed between the ground electrode of the reactor and the ground to examine the transported charges. According to the Eqs. (1) and (2) in the following, the plasma power can be obtained by measuring the area of the V–Q Lissajous cyclogram. The slope of discharge transitions is equal to dielectric capacitance, and that of capacitive transitions is equal to the total capacitance of the discharge reactor [27].



1 T

Z

T

UðtÞIðtÞdt ¼

0

1 T

I

UðQÞdQ ¼ C M f

I

mole ðproduced COÞ  100½% mole ðconverted CH4 Þ

Selectivity ðCO2 Þ ¼

mole ðproduced CO2 Þ  100½% mole ðconverted CH4 Þ

ð4Þ

ð5Þ

3. Results and discussion

UðU C ÞdU C ¼ f  s

P ðWÞ Specific input energy ðJ=LÞ ¼ gas flow rate ðL=SÞ

ð1Þ 3.1. Effect of input plasma energy on methane oxidation under plasma only condition

ð2Þ

where P = discharge power, T = time, I = current, U = voltage, Q = charge, CM = measuring capacitor, UC = voltage of measuring capacitor, f = frequency, S = area of Lissajous figure. 2.4. Activity measurement The catalytic activity was measured at atmospheric pressure in the same quartz reactor. The reactants were 2500 ppm CH4, 2.5% O2, balanced in He. The total gas flow was maintained to 200 cm3/min. The space velocity was about 24,000 h1. Each gas mixture was controlled by using mass flow controllers (SIERRA). Outflow gases passing through the discharge region were analyzed by gas chromatography (AGILENT GC 6890N) equipped with a thermal conductivity detector (TCD) and a 60/80 Carboxen-1000 packed column, which allowed us to measure H2, O2, N2, CO, CO2, CH4, C2H4, C2H6 and C2H2. Methane conversion was examined at steady-state condition in the presence and in the absence of catalyst, while increasing the temperature from room temperature to 400 °C by 50 or 100 °C. Note that CO and CO2 are the only products under these reaction conditions due to the presence of excessive oxygen. Hence, methane conversion and selectivity of CO, and CO2 species are defined as:

Conversion ðCH4 Þ ¼

Selectivity ðCOÞ ¼

mole ðconsumed CH4 Þ  100½% mole ðintroduced CH4 Þ

ð3Þ

First set of methane complete oxidation was carried out under plasma only condition to compare with the reaction involving both plasma and catalyst. It must be pointed out that CO and CO2 were the only products of all the reactions in our study, although our GC system could detect acetylene, ethylene and ethane. The methane conversion, the CO concentration, and the CO/CO2 ratio results were obtained as function of temperature and energy deposition. It is well known that plasma input power is one of the important parameters in DBD plasma–catalyst reaction [30–32]. The specific input energy was measured by Lissajous figure as presented in Fig. 4. In the same input voltage and frequency, the plasma current varied slightly with increasing the temperature because the more active radicals like electrons and ions were easily generated at higher temperature. Consequently, the plasma power went up with the temperature increment. The specific input energy ranged from 32 to 41 J/L at 2 kVp-p input voltage, from 91 to 104 J/L at 3 kVp-p input voltage, from 165 to 231 J/L at 4 kVp-p input voltage. Fig. 5 displayed the effect of the specific input energy on methane conversion as function of temperature. Since the electrical energy at 5 kVp-p gave rise to the reactor damage due to arc discharge, we did not increase the input voltage above 4 kVp-p. The methane conversion at room temperature as a function of specific input energy is displayed in Fig. 6. As expected, methane conversion increased with the increment of specific input energy. This result can be explained by the phenomenon that active species like radical are more generated at higher specific input energy.

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80

4kVp-p 60

3kVp-p CO concentration(ppm)

3kVp-p 2kVp-p

40

Charge(nC)

4kVp-p

(a)

500

20 0 -20 -40

2kVp-p

400

300

200

100

-60 0

-80 -2000

-1000

0

1000

0

2000

100

200

300

400

Temperature(ഒ

Input voltage(V) 70

Fig. 4. Lissajous figure obtained at different applied voltages of plasma and at room temperature.

4kVp-p

(b)

3kVp-p

60

2kVp-p CO selectivity(%)

100

4kVp-p 3kVp-p

Methane conversion(%)

80

2kVp-p

60

50 40 30 20 10

40

0 0 20

100

200

300

400

Temperature(ഒ

0 0

100

200

300

400

Fig. 7. Effect of input voltage on the (a) CO concentration (filled symbols) and (b) CO selectivity (open symbols) during methane oxidation under plasma only condition.

Temperature(ഒ Fig. 5. Methane conversion under plasma only condition with various applied voltages.

35

Room temp.

Methane conversion(%)

30

25

20

15

10

5 20

40

60

80

100

120

140

160

180

Specific input energy(J/L) Fig. 6. Effect of specific input energy on the methane conversion obtained under plasma only condition.

Fig. 7 demonstrated the CO concentration and the CO selectivity. CO concentration and selectivity began to drop above 200 °C because the gas phase CO oxidation to CO2 began to operate in this condition. However, as the input voltage increased, larger amount of CO was produced due to the dominant partial oxidation of CH4 to CO in DBD plasma, so the complete CO oxidation to CO2 was retarded up to around 300 °C. In the case of plasma only condition, CO selectivity was about 50% below 100 °C, which can be rationalized by the plasma chemistry in the following. Although most abundant He gas transferred energy to other molecules such as methane or oxygen via collisions under plasma condition, however, their contribution to the overall reaction could be neglected because the number of He molecule did not change during the plasma-assisted reaction. On the other hand, methane and oxygen were dissociated through the contact with energy-rich plasma electrons to form methyl, and hydrogen, and oxygen radicals, respectively [33]. Oxygen radicals were more reactive and 10 times abundant at this condition compared to methane, thus these oxygen radicals reacted with methyl, hydrogen, or hydroxyl radicals, resulting in the formation of CO, CO2 and H2O. At low temperature, CO was produced more selectively, and as the temperature increased, CO2 was generated more plentifully originating from gas phase oxidation of CO to CO2. According

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4kVp-p

3

3kVp-p 2kVp-p

1 0 -1 -2

Methane conversion(%)

80

Input voltage

Current(mA)

2

-3 -300

T50

100

-200

-100

0

100

200

2wt% Pd/TiO2

(343ഒ

2wt% Pd/SiO2

(227ഒ

2wt% Pd/γ−Al2O3(253ഒ

60

40

20

300

0

Time(μs)

0

100

200

300

400

Temperature(ഒ

Fig. 8. Input voltage shape (black curve) and consequent current characteristics during plasma only reaction obtained at room temperature.

Fig. 9. Methane light-off curves of Pd-based catalysts in the absence of DBD plasma.

to Zhou et al. [34], in the low temperature condition, the following reactions were proposed:

e þ O2 ! O þ O þ e e þ CH4 ! CH3 þ H þ e CH4 þ 3O ! CO þ 2H2 O

based catalysts in the absence of plasma is presented in Fig. 9. In the absence of plasma, all catalysts began to be reactive above 200 °C. Pd/SiO2 showed the best performance on the methane combustion, followed by Pd/c-Al2O3 and Pd/TiO2. The complete oxidation of methane proceeded for all catalysts as evidenced by the production of only CO2. 3.3. Methane oxidation under in plasma–catalysis hybrid condition

CH4 þ O ! CO þ 2H2 2H þ O ! 2H2 O CO þ O ! CO2 As the temperature increased, the last reaction became more predominant, thus the CO selectivity decreased abruptly as observed in Fig. 7. The shape of input voltage and consequent current is presented in Fig. 8. This figure shows that most of the discharge current peak existed at ascending and descending part of the applied voltage curve. As the input voltage increased, the consequent current peaks also grew bigger since active species like electrons, ions and radicals were created in larger quantities. The asymmetric shape of current peak was arising from the single dielectric barrier which was located at ground electrode.

The catalysts were placed inside the discharge zone of plasma to investigate the effect of plasma–catalysis hybridization. This concept was based on other plasma–catalysis applications such as toluene decomposition [35,36], methane reforming [37–40] in which the combination of plasma and catalyst resulted in a better reactivity and selectivity than catalyst alone or plasma alone. Vandenbroucke et al. [41] introduced a synergistic effect factor f for the methane abatement, CO formation, and CO2 formation to evaluate the synergistic effect in the process. They were calculated as follows:

f methane ¼

ð6Þ f CO ¼

3.2. Methane oxidation on various Pd catalysts only The specific surface area and Pd dispersion of the catalysts are summarized in Table 1. Pd/c-Al2O3 had the highest surface area among the catalysts. XRD results (not shown) demonstrated that the only peaks, assigned to the support, existed without any palladium-related peaks, so it was confident that the palladium species were well dispersed. Pd dispersion is in the order of Pd/c-Al2O3, followed by Pd/SiO2 and Pd/TiO2, although there was not much difference among them. The methane conversion over various Pd-

Table 1 Specific surface area and Pd dispersion of the catalysts. Catalyst

Specific surface area (m2/g)

Pd dispersion (%)

Pd/SiO2 Pd/TiO2 Pd/c-Al2O3

151 80 240

4.8 4.2 5.7

ðMethane conversionÞplasma—catalyst ðMethane conversionÞplasma þ ðMethane conversionÞcatalyst

ðYield of COÞplasma—catalyst ðYield of COÞplasma þ ðYield of COÞcatalyst

f CO2 ¼

ðYield of CO2 Þplasma—catalyst ðYield of CO2 Þplasma þ ðYield of CO2 Þcatalyst

ð7Þ

ð8Þ

The synergistic effect factor showed the interrelation among plasma–catalyst hybrid reaction, plasma only reaction, and catalyst only reaction. The value of fmethane exceeding 1 implied that a synergistic effect existed on methane oxidation when plasma and catalyst were both used. Table 2 summarizes the synergistic effect factors of each catalyst in plasma–catalysis hybrid system at room temperature. It indicated that the plasma–catalysis system did not provide apparent synergistic effect with respect to the methane conversion except the 2 wt% Pd/Al2O3 case. However, in terms of CO or CO2, the synergistic effect factor for CO decreased and the factor for CO2 increased in almost all cases, which indicated the Pd-based catalysts enhanced the selectivity to CO2 regardless of the support materials.

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Table 2 Synergistic effect factors for plasma–catalysis hybrid system at room temperature.

Compared with each of catalyst only and plasma only reactions, the plasma catalytic oxidation of methane under various palladium-based catalysts were carried out as functions of temperature and input voltage. For all cases, CO selectivity, which should be maintained at zero percent, was much lower over plasma–catalysis hybrid condition than over the plasma only one, implying the beneficial role of catalyst. First of all, 2 wt% Pd/TiO2 reaction results are shown in Fig. 10. Catalytic performance of 2 wt% Pd/TiO2 was relatively low compared to other Pd-based catalysts. Thus, at low input voltage, the synergistic effect between plasma and catalyst was hardly seen. At even 4 kVp-p, plasma–catalyst hybrid reaction had similar methane conversion value to that of plasma only reaction. TiO2 had higher dielectric constant than SiO2 or Al2O3, thus it was expected to have some promotional effect. However, it was not a ferroelectric material and had much lower dielectric constant than common ferroelectric materials. Thus, Pd/TiO2 did not show the enhancement effect on methane oxidation in spite of the relatively high dielectric constant. Activity results of 2 wt% Pd/SiO2 catalyst is demonstrated in Fig. 11. Because of non-porosity [26] and low dielectric constant of SiO2 support material, the current and consequent specific input energy were quite low at low input voltage (2 kVp-p and 3 kVp-p). Therefore, plasma–catalysis hybridization had antagonistic effect. This is also confirmed in low fmethane (0.40 and 0.62) as presented in Table 2. Though, at 4 kVp-p, the methane conversion value became similar to that of plasma only reaction. 2 wt% Pd/SiO2 had relatively low plasma energy at low input voltage, and as the input voltage increased the plasma energy recovered to that of plasma only reaction, which is shown in Table 3. Finally, the activity result of 2 wt% Pd/c-Al2O3 is plotted in Fig. 12. In the case of 2 kVp-p and 3 kVp-p, the slopes of the graphs were changed in the range of light-off temperature where catalyst began to operate as in the cases of Pd/TiO2 and Pd/SiO2. The low power DBD plasma (2 kVp-p and 3 kVp-p) could not change overall reaction, as evidenced by the same termination temperature of reaction. However, at 4 kVp-p, strong synergistic effect between plasma and 2 wt% Pd/c-Al2O3 catalyst on methane oxidation was obviously observed. Even at room temperature, methane conversion value of plasma–catalyst hybrid reaction exceeded that of plasma only reaction, as explained by the synergistic effect factor of this condition exceeding 1 (1.67). It is notable to point out that

the XRD and BET results of all the catalysts did not change at all after reaction, thus it was confirmed that the DBD plasma did not affect the textual property of catalyst under this operating condition. In the cases of Pd/c-Al2O3 and Pd/SiO2 which had low light-off temperature, they were activated above input voltage of 4 kVp-p since they had low activation energy so the plasma energy could easily overcome the activation energy barrier. However, except for the case of Pd/c-Al2O3, the combination of plasma and catalyst demonstrated similar or lower activity than plasma only reaction which meant that all the catalysts did not enhance the methane conversion. In other words, among the catalysts with different supports, only c-Al2O3 supported catalyst showed higher methane conversion when combined with DBD plasma. According to Holzer et al. [26], porous c-Al2O3 which had high surface area provided synergistic ability of oxidation with non-thermal plasma, which was consistent with our result. 3.4. Hybridization effect of Pd/c-Al2O3 with DBD plasma When the 2 wt% Pd/c-Al2O3 catalyst was placed in discharge zone, the methane conversion was highly enhanced at specific condition since the catalyst was activated above the certain input voltage (i.e. energy). However, as described above, length of the discharge zone surpassed that of catalyst layer, thus it is unclear whether the synergistic effect stemmed from in-plasma catalysis (IPC) system or not. Hence post-plasma catalysis (PPC) was applied to investigate the origin of synergistic effect. The ground electrode surrounding quartz reactor was moved to the upper side, so it was located about 20 mm from the top of catalyst layer. Such postplasma catalysis configuration is displayed in Fig. 3(b). Note that the discharge voltage and current of PPC process were equal to those of IPC blank process, as shown in Fig. 4. As shown in Fig. 13, under this condition, CO selectivity was retained at low level like in-plasma catalysis reaction, however, methane conversion in PPC system hardly demonstrated any synergistic effect between plasma and catalyst. In other words, both plasma and catalyst were required to be in the same zone to promote the synergistic effect on methane oxidation. This fact can be explained by the characteristics of PPC that the active radicals formed by DBD plasma abruptly disappeared and could not reach the catalyst surface, although gas phase CO could. These results are in consistent

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100

100

(a) 2kVp-p

C+P C P

80

C+P C P

80

Methane conversion(%)

Methane conversion(%)

(a) 2kVp-p

60

40

20

0

60

40

20

0 0

100

200

300

400

0

100

Temperature(ഒ

400

300

400

300

400

100

(b) 3kVp-p

(b) 3kVp-p

C+P C P

C+P C P

80

Methane conversion(%)

80

Methane conversion(%)

300

Temperature(ഒ

100

60

40

20

0

60

40

20

0 0

100

200

300

400

0

100

Temperature(ഒ

200

Temperature(ഒ 100

100

(c) 4kVp-p

(c) 4kVp-p

Methane conversion(%)

C+P C P

80

Methane conversion(%)

200

60

40

20

0

C+P C P

80

60

40

20

0 0

100

200

300

400

Temperature(ഒ

0

100

200

Temperature(ഒ

Fig. 10. Methane conversion over 2 wt% Pd/TiO2 in the plasma–catalysis hybrid system with various input voltages. (a) 2 kVp-p, (b) 3 kVp-p, (c) 4 kVp-p. C = Catalyst, P = Plasma, C + P = In-plasma catalysis.

Fig. 11. Methane conversion over 2 wt% Pd/SiO2 in the plasma–catalysis hybrid system with various input voltages. (a) 2 kVp-p, (b) 3 kVp-p, (c) 4 kVp-p. C = Catalyst, P = Plasma, C + P = In-plasma catalysis.

with the past studies [42–45]. For the post-plasma catalysis, the catalytic decomposition of O3 on c-Al2O3 and the subsequent consumption of the atomic oxygen were identified as the only relevant oxidation process [45]. However, for in-plasma catalysis, other consumption mechanisms including the formation of short-lived radicals operated, resulting in enhancement of overall methane

conversion. Furthermore, the electric properties of the catalysts were found to influence the non-thermal plasma discharge. This result led to the conclusion that specific plasma energy condition and configuration could help to improve the catalyst activation for methane oxidation so light-off temperature of the catalyst could be shifted toward lower temperature.

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H. Lee et al. / Chemical Engineering Journal 259 (2015) 761–770 Table 3 Various parameters of plasma–catalyst hybrid reaction.

Methane conversion(%)

100

(a) 2kVp-p C+P C P

80

Catalyst

Input voltage (Vp-p) (specific input energy (J/L))

Dielectric constant of support (e) [46,47]

Pd/TiO2

2 (23–40) 3 (101–110) 4 (172–227)

80

Pd/SiO2

2 (8–32) 3 (83–94) 4 (169–193)

3.9

Pd/c-Al2O3

2 (18–34) 3 (88–97) 4 (163–191)

9

60

40

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Fig. 13. Methane conversion over 2wt% Pd/Al2O3 in the post-plasma catalysis system. C = Catalyst, P = Plasma, C + P(PPC) = Post-plasma catalysis.

Temperature(ഒ only reaction and had far lower CO selectivity compared to plasma only reaction. The catalyst only reaction and hybrid reaction ended at almost same temperature in most cases. It is remarkable that only 2 wt% Pd/c-Al2O3 demonstrated the synergistic effect on methane complete oxidation in in-plasma catalysis hybrid system among the Pd-based catalysts with various supports. In the case of 2 wt% Pd/c-Al2O3, the plasma and the catalyst showed synergistic effect on complete methane oxidation even at room temperature. However, post-plasma catalysis (PPC) system did not show synergistic effect, only to demonstrate the sum effect. It was evidently concluded that the sufficient input voltage (i.e. 4 kVp-p in this work) in combination with the right catalyst (Pd/c-Al2O3) is required to enhance the synergistic effect on methane oxidation in in-plasma catalysis hybrid system.

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Temperature(ഒ Fig. 12. Methane conversion over 2 wt% Pd/Al2O3 in the plasma–catalysis hybrid system with various input voltages. (a) 2 kVp-p, (b) 3 kVp-p, (c) 4 kVp-p. C = Catalyst, P = Plasma, C + P = In-plasma catalysis.

4. Conclusions The combined use of plasma and catalyst was proved to be an ideal route for lowering the light-off temperature of methane oxidation. In plasma–catalysis hybrid system, the methane oxidation reaction started at much lower temperature compared to catalyst

The authors appreciate the financial support from ‘‘Hybrid technology of nano catalyst–plasma for low carbon/emission’’ of MOTIE (Ministry of Trade, Industry and Energy) and ISTK (Korea Research Council for Industrial Science and Technology) with the grant number of B551179-11-03-00. References [1] L. Zhenhua, G.B. Hoflund, A review on complete oxidation of methane at low temperatures, J. Nat. Gas Chem. 12 (2003). [2] A. Bhatia, H. Pathak, P.K. Aggarwal, Inventory of methane and nitrous oxide emissions from agricultural soils of India and their global warming potential, Curr. Sci. 87 (2004) 317–324.

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