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Characteristics of back corona discharge in a honeycomb catalyst and its application for treatment of volatile organic compounds Fada Feng, Yanyan Zheng, Xinjun Shen, Qinzhen Zheng, Shaolong Dai, Xuming Zhang, Yifan Huang, Zhen Liu, and keping yan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b00447 • Publication Date (Web): 05 May 2015 Downloaded from http://pubs.acs.org on May 10, 2015

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Environmental Science & Technology

1

Characteristics of back corona discharge in a honeycomb catalyst and

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its application for treatment of volatile organic compounds

3

Fada Feng1,2, Yanyan Zheng1, Xinjun Shen1, Qinzhen Zheng1, Shaolong Dai1, Xuming

4

Zhang1, Yifan Huang1*, Zhen Liu1, Keping Yan1

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1. Key Laboratory of Biomass Chemical Engineering of Ministry of Education,

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College of Chemical and Biological Engineering, Zhejiang University, Hangzhou

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310007, People’s Republic of China

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2. School of Chemistry and Environment, Jiaying University, Meizhou 514015, People’s Republic of China

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Abstract: The main technical challenges for the treatment of volatile organic

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compounds (VOCs) with plasma-assisted catalysis in industrial applications are large

12

volume plasma generation under atmospheric pressure, by-product control, and

13

aerosol collection. To solve these problems, a back corona discharge (BCD)

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configuration has been designed to evenly generate non-thermal plasma in a

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honeycomb catalyst. Voltage-current curves, discharge images, and emission spectra

16

have been used to characterize the plasma. Grade particle collection results and flow

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field visualization in the discharge zones show not only that the particles can be

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collected efficiently, but also that the pressure drop of the catalyst layer is relatively

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low. A three-stage plasma-assisted catalysis system, comprising a dielectric barrier

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discharge (DBD) stage, BCD stage, and catalyst stage, was built to evaluate toluene

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treatment performance by BCD. The ozone analysis results indicate that BCD 1

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enhances the ozone decomposition by collecting aerosols and protecting the Ag-Mn-O

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catalyst downstream from aerosol contamination. The GC and FTIR results show that

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BCD contributes to toluene removal, especially when the specific energy input is low,

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and the total removal efficiency reaches almost 100%. Furthermore, this removal

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results in the emission of fewer by-products.

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TOC Art

28 29

1 Introduction

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It is proposed that the future applications for plasma-assisted catalysis will most

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likely be in the treatment of gaseous waste1-4 and in the use of plasma to prepare and

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modify catalysts5-6. In conventional catalysis, the required activation energy for

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reactions is supplied in the form of heat, while in plasma-assisted catalysis, the catalyst

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can be activated directly by plasma, in which only the electrons are energized, and the

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bulk gas is not signiﬁcantly heated7. Therefore, the key requirement for these systems

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is plasma-induced activation of the catalyst at relatively low temperatures. Besides, 2


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plasma-assisted catalysis has the potential to enhance global reaction rates, improve

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reaction selectivity, and increase energy efficiency2. Most of the researches in this field

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mainly focus on plasma generation8, plasma activation of catalysts9, configuration of

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plasma combined with catalysts10,11, by-product generation and control12, mechanism

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of synergistic effects13-16, and reaction kinetics17. Plasma-assisted catalysis for the

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treatment of VOCs has been studied over two decades, and several excellent reviews

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have been published in recent years1-3,9,18,19. However, several technical challenges

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still remain before large-scale industrial application of plasma-assisted catalysis can

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become practical. Understanding the complex mechanism of synergistic effects and

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elucidation of the decomposition roadmap of VOCs are widely acknowledged

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examples of these challenges, but problems associated with large volume plasma

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generation under atmospheric pressure, by-product control, and aerosol collection

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must also be addressed.

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In a one-stage plasma-catalysis configuration such as that seen in a packed-bed

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reactor, the catalyst is placed in the discharge zone. This allows for a significant

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synergistic effect, but the relatively high-pressure drop across the catalyst layer is

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problematic. Employing a honeycomb structured catalyst is a way to lessen the

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pressure drop, but generating plasma evenly in a honeycomb is difficult. In this work,

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a hybrid honeycomb configuration comprising a packed bed and capillaries has been

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used to generate a discharge so that a larger surface area can be obtained with a lower

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pressure drop20,21. It was also found that this system allowed back corona discharge 3



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(BCD) to be successfully employed22-24, which represents an effective method for

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non-thermal plasma generation and a synergistic plasma catalysis effect25-28. Our

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research has demonstrated that BCD not only has the potential to activate the catalyst,

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but also has a further advantage in that aerosols, the major by-product of VOC

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treatment by the plasma method, can be captured on the catalyst surface and broken

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down into CO2 and H2O by plasma and catalytic oxidation26. BCD can be easily

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induced using normal high-voltage DC power sources, which are very reliable and

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inexpensive, and therefore, it has the potential for application in industrial-scale VOC

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treatment.

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In this paper, a BCD reactor capable of evenly generating plasma in a

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honeycomb catalyst is described. The plasma produced is characterized by reference

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to voltage-current curves, discharge images, and emission spectra. Grade particle

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collection efficiency and flow field visualization in the discharge zones are also

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investigated to gain further understanding of BCD. A three-stage plasma-assisted

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catalysis system, comprising DBD, BCD, and catalyst, is proposed to evaluate the

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BCD performance using toluene as a sample VOC. Since DBD is an ideal method to

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generate ozone that can be used to decompose VOCs29, a DBD is applied in the first

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stage of the hybrid reactor to generate a high concentration of ozone and partially

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decompose the toluene. In the second stage, a BCD collects and decomposes the

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aerosols produced by the DBD, which has the dual purpose of removing aerosol

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pollution and promoting the treatment in the following catalytic stage. The 4


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performance of BCD for ozone generation and decomposition are also described.

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Finally, the total removal efficiency and product composition are discussed in order to

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evaluate the hybrid plasma-assisted catalysis method.

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2 Experimental Section

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Figure 1(a) shows the experimental setup used in this work, in which the reactor can

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be regarded as the BCD reactor alone (Figure 1(b)) for plasma characterization,

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particle collection measurement, and flow field visualization, or as the full three-stage

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plasma-assisted catalysis system (Figure 1(c)) for toluene decomposition. The

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experimental setups of particle collection measurement and flow field visualization

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are described in S1 and S2 of Supporting Information, respectively. As shown in

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Figure 1(b), the main BCD reactor is arranged in a needles-mesh-honeycomb-mesh

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conﬁguration. The distance between the needle electrodes and middle mesh, and the

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middle mesh and bottom mesh is 16 mm and 14 mm, respectively. The discharge

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electrode is a stainless steel needle with a tip radius of about 0.25 mm, and the mesh is

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made of nickel foam with a pore density of 110 pores per inch (PPI). The Ag-Mn-O

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catalyst supported on the honeycomb for BCD generation is prepared by

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oxidoreduction precipitation. A 400 cell/in2 mesh alumina honeycomb support with a

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diameter of 100 mm and a thickness of 10 mm was coated with a mixture of Ag-Mn-O

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and alumina gel, and then calcined at 723 K in air for 4 h. The BCD reactor is placed in

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a quartz tube with an inner diameter of 120 mm and powered by a negative high-

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voltage DC power supply. 5



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Figure 1(c) shows a schematic diagram of the three-stage plasma-assisted

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catalysis system, comprising DBD, BCD, and catalyst. The DBD reactor consists of 12

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cylindrical quartz tubes of around 7 mm inner diameter, 1 mm wall thickness, and 20

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cm length wrapped in a high-voltage electrode made of copper mesh. Each quartz tube

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is mounted inside a stainless steel rod to form annular discharge gaps of about 1 mm

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radial width. The DBD reactor is driven by an AC power source at a frequency of 50 Hz

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and peak voltage of ±11 kV. In the BCD stage, the configuration is the same as in

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Figure 1(b). For the catalyst stage, a 400-mesh honeycomb alumina support of 100

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mm diameter and 20 mm thickness supports 2 g of Ag-Mn-O catalyst. The Ag-Mn-O

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catalyst here is prepared by a previously described impregnation method26,30. The

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toluene decomposition experiment was carried out in air with a relative humidity of

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around 10% at atmospheric pressure and ambient temperature (around 293 K). The

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initial concentration of toluene was 100 ppm, and the total gas flow rate was 5 L/min.

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The total discharge volume was around 27.1 cm3, which corresponds to a residence

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time of 0.33 s. The details of the instruments for electric measurement and gas

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composition measurement can be found in S3 of Supporting Information. The

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definitions of toluene removal efficiency, CO selectivity, CO2 selectivity and carbon

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balance, specific input energy (SIE) and BCD discharge current density are also

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described in S4 of Supporting Information.

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Figure 1. (a) Schematic diagram of our experimental setup; (b) BCD reactor and (c)

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hybrid plasma-assisted catalyst for toluene decomposition.

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3 Results and Discussion

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BCD discharge images

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Figure 2 shows the images of the discharge zones for two BCD configurations:

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needle-mesh-honeycomb-mesh and needle-mesh-capillary quartz tube mesh. Figure

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2(a) corresponds to the reactor depicted in Figure 1(b). A negative corona discharge

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and BCD-induced streamers can be observed in the zones near the needle electrodes

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and between the middle mesh and honeycomb catalyst surface, respectively. To enable 7



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visual observation of the discharges inside the honeycomb channels, a bundle of quartz

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capillaries were used in place of the honeycomb (as shown in Figure 2(b)). The image

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shows that the plasma is evenly distributed inside the channels of the quartz capillaries.

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For the similar discharge configuration, it is believed that plasma can also be

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produced evenly inside the honeycomb channels where the catalysts are supported

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Figure 2. Images of the discharge zones for two BCD configurations: (a)

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Needle-mesh-honeycomb-mesh (corresponds to the configuration shown in Figure

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1(b); applied voltage: 30 kV; current density: 11.4 µA/cm2), and (b)

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needle-mesh-capillary quartz tube mesh (channel diameter: 0.8 mm; applied voltage:

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26 kV; current density: 6.2 µA/cm2).

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Voltage-current curves

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Figure 3 shows the voltage-current curves for BCD and typical corona discharge,

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in which the discharge occurs at the high-voltage electrode tips without the floated 8


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mesh and honeycomb placed downstream. Before BCD occurs, the discharge current is

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low due to charge accumulation on the honeycomb, which serves as a dielectric layer.

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The discharge current increases significantly after BCD occurs. When the applied

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voltage is 30 kV, the discharge current is about 358 µA, which is twice that of a typical

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corona discharge. This is attributed to the large volume of plasma produced by BCD

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inside the honeycomb channels.

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Figure 3. Voltage-current curves of normal corona discharge and BCD with a

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honeycomb.

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Emission spectra

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In order to characterize the active species and energy level of the discharges, the

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emission spectra of the discharge between the middle mesh and the honeycomb surface,

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as well as the BCD in the honeycomb channels, were measured. The results are shown

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in Figure 4. Strong emission lines of the second positive system of N2 in the discharge

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zone were observed at 337.13 nm, 357.69 nm, and 380.49 nm. These lines are similar

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to those observed in BCD on a fly ash layer31. The emission spectrum from O3

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(315.61 nm and 336.52 nm, 1B2–X1A1)32-34 can also be observed in this discharge 9



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zone. As shown in Figure 4(b), the emission intensity from the honeycomb channels

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is relatively low, because less light can cross the narrow channels. The emission

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spectrum of OH (A2Σ → X2Π, (0−0)) is obscured by the emission spectrum of the ∆v

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= +1 vibration transition band of N2 (C3Πu → B3Πg)31,32,35. Additionally, more

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reactive radicals such as OH, O, and O3 are produced due to BCD inside the

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honeycomb channels.

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Figure 4. Emission spectra of the (a) streamer discharge between the middle mesh

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and the catalyst surface, and (b) BCD in the honeycomb channels. Applied voltage, 30

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kV; discharge current, 358 µA.

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Particle collection from BCD

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In an electrostatic precipitator, BCD is a detrimental phenomenon that decreases the

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particle collection efficiency36. Fly ash particles collected on the plate will be

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spattered from the layer and reenter the gas flow when BCD occurs23,24,37. In the

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treatment of VOCs with BCD, organic aerosols, the major by-product, can adhere to

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the surface of the catalyst and be decomposed by the plasma-assisted catalysis

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process26. Figure 5 shows the effect of BCD on the grade collection efficiency of fly 10


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ash samples. The initial total particle concentration was 30.9 mg/m3. When BCD

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occurs, the concentration downstream decreases to 1.86 mg/m3, and the total particle

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weight removal efficiency is about 94%. It is apparent that most of the particles in the

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gas phase are collected in the honeycomb, especially the particles with diameters

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under 1 µm, which matches the diameter range of aerosol particles generated by

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toluene decomposition with non-thermal plasma, as reported in our previous work38.

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Figure 5. Fine particle collection in the BCD reactor. Applied voltage, 30 kV;

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discharge current, 358 µA; initial total particle concentration, 30.9 mg/m3; gas flow

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rate, 10 L/min. The line with solid square points represents the grade weight

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concentration distribution without BCD. The lines with solid triangle points and soft

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diamond points represent grade weight concentration downstream and the

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corresponding grade collection efficiency with BCD, respectively.

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Flow field distribution

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The PIV method was used for instantaneous measurement of velocity field

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distribution without any disturbance to the flow39,40. Figure 6 shows the images of the

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flow field inside the BCD reactor. Three conditions, i.e., (a) no particles, (b) particles 11



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with no discharge, and (c) particles with BCD, are compared. For no discharge, the

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gas flow is a laminar flow, while intense turbulence with a peak velocity of around 2

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m/s is generated when BCD occurs. As shown in Figure 6(c), moxa smoke particles

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are treated by the honeycomb as the area below it is dark, which indicates fewer

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particles in that area. As the diameter of moxa smoke particles is close to that of the

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aerosols generated by non-thermal plasma, it can be concluded that the aerosols can

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also be efficiently treated by BCD. Figure 6(d) shows velocity field distribution

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averaged over 20 images, and indicates a distinct secondary flow that moves from the

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needle electrodes toward the honeycomb surface and then passes through the channels.

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The velocities of the secondary flow upstream and downstream of the honeycomb are

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around 1–2 m/s and 0.6 m/s, respectively. Compared with other works20,21, it is

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apparent that the pressure drop of the catalyst bed in this BCD reactor is quite small.

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The simulation result with computational fluid dynamics software indicates that the

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pressure drop is around 6 Pa.

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Figure 6. Images of the flow field inside the BCD reactor, (a) with no particles, (b)

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with particles but no discharge, and (c) particles with BCD. Image (d) shows the flow

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field distribution calculated with PIV software. Applied voltage, 30 kV; discharge

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current density, 12.7 µA/cm2.

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Ozone generation and decomposition

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Figure 7 shows ozone generation and decomposition in the three-stage

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plasma-assisted catalysis system vs. the SIE of DBD. The SIE of BCD is fixed at

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128.9 J/L. The results indicate that the total ozone concentration is linearly dependent

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on the SIE. Meanwhile, BCD produces additional ozone, but at a much lower rate

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than DBD. For example, BCD adds only extra 208 ppm ozone when the SIE is 128.9

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J/L. However, BCD makes a significant contribution to ozone decomposition. For

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example, when the SIE of DBD is 35 J/L, the ozone decomposition efficiency 13



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increases from 97.5% to 99.1% after BCD is turned on. This is because BCD collects

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the aerosols produced in the previous stage, and, since the aerosols cause deactivation

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of the catalyst, high catalyst activity is maintained downstream.

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Figure 7. Ozone generation and decomposition of DBD vs. the SIE, where the SIE of

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BCD is fixed at 128.9 J/L.

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Toluene removal

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Figure 8 shows the toluene removal efficiency of the three-stage plasma-assisted

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catalysis system under four conditions: DBD alone, DBD-BCD, DBD-catalyst, and

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DBD-BCD-catalyst. The toluene removal efficiency with the DBD-BCD-catalyst

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system is significantly higher than that achieved with DBD alone or DBD-catalyst,

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especially when the SIE is low. For example, when the SIE is at 35 J/L, the toluene

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removal efficiencies in the DBD, DBD-BCD, DBD-catalyst, and DBD-BCD-catalyst

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are around 37%, 72%, 68%, and 95%, respectively. Furthermore, the energy

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consumption is almost one order smaller than that reported elsewhere10.

14


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Figure 8. Toluene removal efficiency vs. the SIE of DBD under four conditions: DBD

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alone, DBD-BCD, DBD-catalyst, and DBD-BCD-catalyst. The horizontal ordinate

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refers the SIE of DBD and the SIE of BCD is 128.9 J/L.

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With regards to plasma induced ozone generation and toluene decomposition, there

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are three fundamental reactions: R1, R2 and R341,42. According to the results of ozone

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generation and toluene removal induced by the DBD reactor, they are linearly

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dependent on each other (Details in S5 of Supporting Information). So, it can be

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concluded that the two parallel gaseous reactions R2 and R3 almost independently

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consume O radicals.

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O radical production:

e + O2 → e + O + O

(R1)

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O3 generation:

O + O2 + M → O3 + M

(R2)

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Toluene decomposition: O + C6 H 5 ⋅ CH 3 → product

249

where, M refers to O2 and N2 in air. By using the equations (S1) and (S2) in

250

Supporting Information, the energy yields (in the form of G-values) for ozone

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generation G(O3) and toluene decomposition G(-toluene) can be calculated about 4.08 15


(R3)


252

(molecules/100 eV) and 0.40 (molecules/100 eV), respectively.

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In the DBD-BCD-catalyst system, as shown in emission spectra, BCD produces lots

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of active species, such as OH, O and N2*. They can be used for toluene and aerosol

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decomposition. In this stage, aerosol produced in previous DBD stage can be charged

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and collected on the surface of honeycomb catalyst, then can be decomposed in situ

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by plasma and catalyst because of the back corona generated in the channel of

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honeycomb26. So, the catalyst downstream can be protected from aerosol

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contamination and the three-stage system performs best in toluene removal as shown

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in Figure 8. The ozone produced in both DBD and BCD is actively used in the

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catalyst stage. Catalytic ozone oxidation is the most important process for toluene, CO,

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and other by-products removal. The mechanism of plasma catalysis degradation has

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already been well studied in our previous works30,41 and others’ works9,12,13,17-19.

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Gas phase products

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Figure 9 presents the FTIR spectra of the gaseous product of toluene

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decomposition using both DBD alone and the DBD-BCD-catalyst system. The results

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show that by-products such as formaldehyde, formic acid, NOx, ozone, and carbon

268

monoxide are produced during the degradation of toluene by the DBD. However, most

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of the incompletely decomposed VOCs, carbon monoxide, and ozone are removed

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when the BCD is turned on and the downstream Ag-Mn-O catalyst works. In the

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DBD-BCD-catalyst system, the main product of toluene decomposition is CO2 and

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H2O. 16


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Figure 9. FTIR spectra of the gas phase product of toluene decomposition with (a)

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DBD alone and (b) DBD-BCD-catalyst.

276

For plasma-assisted catalysis system, the by-product of NOx can reach several

277

hundred ppm at high SIE of plasma9,11, so it should receive more attention. However,

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the average SIE of single discharge unit of DBD in this work is quite small, even less

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than 3 J/L when the total SIE of DBD reaches 35 J/L (12 quartz tubes discharge units).

280

As a result, the NOx produced by DBD is negligible that the concentration of NO2 is

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0.1 ppm and 0.2 ppm when the SIE of DBD reaches 35 J/L and 64 J/L, respectively.

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With regards to BCD, the NOx production can also be neglected because the

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concentration of NO2 is only 1.1 ppm when the SIE of BCD is 128.9 J/L.

284

CO selectivity, CO2 selectivity and carbon balance

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Figure 10 shows the CO selectivity, CO2 selectivity and carbon balance of the

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three-stage system in four conditions. The results show that CO2 selectivity and

287

carbon balance are improved when the BCD and catalyst are used. For DBD alone,

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DBD-BCD, DBD-catalyst, and DBD-BCD-catalyst, the CO2 selectivity is 67 %, 73 %,

289

86 % and 87 %, respectively, and the carbon balance is 41 %, 48 %, 71 % and 82 %,

290

respectively. The higher CO2 selectivity and carbon balance in DBD-BCD-catalyst 17



process can be attributed to the aerosol decomposed by BCD and the catalytic ozone

292

oxidation of incompletely decomposed VOCs by the catalyst downstream. CO, CO2 selectivity and carbon balance (%)

291

100

CO selectivity CO2 selectivity

Carbon balance

80

60

40

20

0

293

DBD

DBD-BCD

DBD-catalyst DBD-BCD-catalyst

Plasma-assisted catalysis system

294

Figure 10. CO selectivity, CO2 selectivity and carbon balance of the three-stage

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system in four different conditions: DBD alone, DBD-BCD, DBD-catalyst and

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DBD-BCD-catalyst. The SIE of DBD and BCD is fixed at 64 J/L and 128.9J/L,

297

respectively.

298

Catalyst degradation

299

For VOCs decomposition with plasma-assisted catalysis, especially two-stage

300

plasma catalysis reactor, the aerosol produced by plasma stage will induce catalyst

301

degradation. The XPS spectrum analysis of the aerosols deposited on catalyst surface

302

show that most carbon in aerosols are in the forms of C-C and/or C-H (59.6 %) and

303

COOH and/or COOR (28.2 %). Details can be found in S6 of Supporting Information.

304

For the three-stage system, BCD placed between DBD and catalyst plays an important

305

role in aerosol collection and decomposition. As shown in Figure 11, the toluene

306

removal efficiency without BCD decreases to below 70 % when operation time 18


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increases to 10 hours, but the removal efficiency with BCD remains stable at high

308

level around 98 %. So, the three-stage system is proven to be feasible for industrial

309

application not only for less by-products emission, but also for stable removal

310

efficiency and prolonged lifetime of catalyst.

311 312

Figure 11. The comparison of toluene removal efficiency in DBD-catalyst and

313

DBD-BCD-catalyst with operation time. The SIE values of DBD and BCD are fixed

314

at 64 J/L and 128.9J/L, respectively.

315

Author information:

316

Corresponding author

317

Tel: +86-571-88273897, Fax: +86-571-88210786, E-mail: [email protected]

318

Notes: The authors declare no completing financial interest

319

Acknowledgments

320

This work is supported financially by the Natural Science Foundation of China (No.

321

21276232,41476080&51377145),

322

Development

323

2013AA065001),

Program

of

Natural

National China

Science

High

(863

Technology

Program)

Foundation

of

19


Research

and

(No.2013AA065005, Zhejiang

Province


324

(No.LY13E070002& LQ14D060004) and Natural Science Foundation of Guangdong

325

Province (No. 2014A030310196).

326

Supporting Information Available:

327

The experimental setup of the particle collection measurement system for the BCD

328

reactor (S1 text and Figure S1); the experimental setup of the 2D-PIV system for flow

329

field measurement in BCD reactor (S2 text and Figure S2); the details of the electric

330

measurement and gas composition measurement instruments (S3 text); the definitions

331

of toluene removal efficiency, CO and CO2 selectivity, carbon balance, SIE and BCD

332

discharge current density (S4); the analysis of ozone production and toluene

333

decomposition in DBD reactor (S5, Figure S5, Figure S6 and Figure S7); the XPS

334

spectrum analysis of the aerosols deposited on catalyst surface (S6, Figure S8 and

335

Table S1). This information is available free of charge via the Internet at

336

http://pubs.acs.org/.

337

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