DBD Plasma Assisted CO₂ Decomposition: Influence of Diluent Gases

Abstract

Carbon dioxide (CO₂) partial reduction to carbon monoxide (CO) and oxygen has been conducted in a dielectric barrier discharge reactor (DBD) operating a packed bed configuration and the results are compared with that of no packing condition. The effect of diluent gas is studied to understand the influence on dielectric strength of the plasma gas on CO₂ splitting, with the objective of obtaining the best CO selectivity and high energy efficiency. Typical results indicated that among N₂, He and Ar gases, Ar showed the best decomposition efficiency. Glass beads packing has a strong influence on the performance, probably due to the enhanced field strength due to dielectric nature of the packed material. In a similar manner, Ar mole ratio in the gas mixture also played a significant role, where the maximum CO₂ conversion of 19.5% was obtained with packed DBD at CO₂:Ar ratio 1:2. The best CO yield (16.8%) was also obtained under the same conditions. The highest energy efficiency was found to be 0.945 mmol/kJ. The activated species formed inside the CO₂ plasma were identified by optical emission spectroscopy.

1. Introduction

The continuous rise of carbon dioxide (CO₂) in the atmosphere has a negative impact on the environment, as it significantly contributes to global warming. Short-term processes such as CO₂ sequestration, carbon storage and capture cannot offer a permanent solution [1,2]. Efforts have been done to develop cost-effective and energy-efficient technologies to reduce CO₂. In this context, CO₂ conversion into value-added products like CO is of great importance, as CO is one of the major feedstocks for various chemical processes. Several technologies such as electrochemical, thermochemical and photochemical processes have been investigated for this reaction [3]. However, these processes are limited either due to high input cost or poor selectivity to CO. In this context, non-thermal plasma (NTP) activation of CO₂ seems to be an interesting approach. NTP is characterized by the presence of high energetic electrons that are capable of promoting chemical reactions under ambient conditions. NTP has reported applications in water treatment [4,5], surface treatment [6], VOC removal [7] reforming reactions [8,9], etc. Among the various NTP configurations, dielectric barrier discharge (DBD) has attracted considerable attention due to the ability to produce high energetic electrons (1–10 eV), uniform distribution of microdischarges, capable of initiating the chemical reactions under ambient conditions, etc. [10,11,12,13,14,15,16].

Recently Mei et al. reported the conversion of CO₂ in a NTP reactor, where the diameter and shape of the inner electrode and the type of outer electrode influenced the efficiency of the process [17]. The effect of the gas flow rate, discharge power, discharge length and photocatalytic effect towards CO₂ conversion and energy efficiency also have been reported by Mei et al. [18,19]. Zeng et al. studied the effect of Ni/Al₂O₃ on CO₂ hydrogenation with Ar as a diluent gas [20]. They reported almost 56.1% CO₂ conversion in 60% Ar mixture. Ramakers et al. studied CO₂ decomposition with diluent gases He and Ar. They showed that at up to 70% diluent gas mixture, He and Ar both have same effect but at higher gas percentage, Ar was more effective than He [21]. Wang et al. investigated N₂ and Ar dilution on CO₂ decomposition with 5A molecular sieve packing material and it was reported that N₂ dilution shows better results than Ar, up to 50% diluent gas mixture [22]. Yap et al. reported almost 13.5% conversion at CO₂/He is 1/1 [23]. In a similar manner, Debjyoti et al. reported that packing material also influences the conversion of CO₂ [24]. As there is no systematic study to explain the influence of diluent gas and influence of packed bed configuration, this study was aimed at understanding the effect of diluent gases (N₂, Ar, He) on CO₂ decomposition in packed bed configuration. This study also involves understanding the influence of glass beads packing on CO₂ conversion.

2. Results and Discussion

2.1. Discharge Characterization

Figure 1 represents the power dissipated in the discharge on varying the applied voltage for various plasma gases. Figure 1 indicates that N₂ dilution has the lowest discharge power at low voltages, while Ar dilution shows the highest power at high voltages. This difference is due to the higher dielectric strength of nitrogen than argon [25], because of which the breakdown voltage is higher for nitrogen than for argon gas.

Figure 2 represents Lissajous figures for diluent gases at 24 kV with glass beads-packed DBD system. The shape of figures changes with the changing of diluent gas, which is due to the change in dielectric strength of the gas mixture. The charge transferred per half cycle [26] is given in

Table 1. Discharge characteristics at different diluent gases.

Diluent Gas	Applied Voltage (kV)	Discharge Power (W)	Charge Transfer per Half Cycle (μC)
N2	24	2.0	10.61
He	24	2.2	12.60
Ar	24	2.4	14.28

, which confirms that N₂ needs higher voltage for the breakdown than He and Ar gases and the charge transferred per half cycle is less for N₂ dilution. Ar dilution shows the highest charge transferred per half cycle, because of which the break down voltage of Ar+CO₂ mixture is the lowest. As a result, at any fixed voltage, higher conversion of CO₂ may be expected for Ar when compared to N₂ and He gases.

It is well known that the electric field strength varies with the dielectric packing and, because of that, high charge will be transferred to the microdischarges [27]. Figure 3 shows the charge transferred to the microdischarges per half cycle on increasing the voltage. The DBD system with Ar diluent gas showed the highest charge transferred at every applied voltage, where the charge increased from 3.46 μC to 10.61 μC, with increase from 16 kV to 24 kV, respectively for N₂ dilution, whereas under the same conditions, He dilution resulted the charge transfer in an increase from 4.61 μC to 12.61 μC and for Ar dilution, it was highest at 5.37 μC to 14.28 μC.

2.2. Effect of CO₂:Ar Mole Ratio on CO₂ Decomposition

As the previous studies confirm that Ar dilution is beneficial over N₂ and He dilution, further studies are carried out by varying the mole ratio of Ar and CO₂ in the gas mixture. Figure 4a confirms the effect of Ar dilution on CO₂ conversion for DBD reactor without packing. As seen in Figure 4a, the CO₂ conversion increases with the increase of specific input energy (SIE) at any CO₂:Ar ratio. The maximum CO₂ conversion observed was 10.6% for CO₂:Ar ratio 1:2 at the SIE 4.8 J/mL. CO₂ conversion increases with the increasing Ar mole ratio, which is due to the decreasing dielectric strength of the gas mixture, due to which more energy is available in the discharge for CO₂ molecules. In addition, due to the lower number of CO₂ molecules than Ar atoms, the available electrons may excite CO₂. The possible reactions that favor CO₂ conversion are shown in Equations (1)–(3) [28].

$${\mathrm{CO}}_2+{\mathrm{e}}^{-}\to {\mathrm{CO}}_2\left({\mathrm{B}}_2^1\right)+{\mathrm{e}}^{-}$$

(1)

$${\mathrm{CO}}_2+{\mathrm{e}}^{-}\to {{\mathrm{CO}}_2}^{+}\left({\mathrm{A}}^2\mathsf{\pi }\right)+{\mathrm{e}}^{-}$$

(2)

$${{\mathrm{CO}}_2}^{*}+{\mathrm{e}}^{-}\to \mathrm{CO}+\mathrm{O}+{\mathrm{e}}^{-}$$

(3)

The CO selectivity with respect to the discharge power is shown in Figure 4b, which confirms the increasing selectivity with increasing the discharge power and maximum selectivity achieved ~98% with CO₂:Ar ratio 2:1, indicating ~100% carbon balance. The competing Boudouard reaction may take place at 1:2 argon ratio due to higher CO₂ conversion. Figure 4c represents the product yield vs. SIE at different CO₂:Ar mole ratio. The CO yield increases with the increase of Ar in the mixture and the maximum yield was ~10% at SIE 4.8 J/mL.

2.3. Effect of Glass Beads on CO₂ Decomposition

Figure 5a represents the CO₂ conversion with the change of SIE for glass beads-packed DBD for CO₂:Ar mole ratio varied from 2:1 to 1:2. Figure 5a confirms that CO₂ conversion increases with the increasing SIE as well as Ar percentage. Due to the dielectric effect of glass beads, more energetic electrons will be formed in the discharge, due to which higher conversion is achieved. Generally dielectric material acts as a bridge between two electrodes and facilitates the transfer of charge between two electrodes. As seen from Figure 5b, discharge power has almost no influence on the selectivity to CO. The highest selectivity was observed at CO₂:Ar ratio 2:1, which may be due to the low CO₂ conversion to CO, which may not favor Boudouard reaction. This is also consistent with the lowest CO selectivity at CO₂:Ar ratio 1:2. Figure 5c shows the CO yield with respect to SIE, which confirms that the glass beads-packed DBD show the best CO yield of 16.8% at CO₂:Ar mole ratio 1:2.

2.4. Effect of Diluent Gases with Packed Bed DBD

The effect of diluent gases has been shown in Figure 6. Figure 6a indicates that the CO₂ conversion increases with the increasing discharge power for all diluent gases. The maximum conversion has been observed with Ar dilution, especially at higher discharge power. A similar observation was reported by Ramakers et al. [21]. During the present study, ~19.5% CO₂ conversion was observed with Ar dilution. The diluent gas effect follows the order Ar > He > N₂. One of the plausible reasons for this observation may be due to facile energy transfer from metastable species to CO₂ molecules (Equations (4)–(9)).

The following electron impact dissociation may be responsible for the CO₂ decomposition reaction.

The electron energy transfers through excited He atoms to CO₂ molecule [23].

$$\mathrm{He}+{\mathrm{e}}^{-}\to {\mathrm{He}}^{*}+{\mathrm{e}}^{-}$$

(4)

$${\mathrm{He}}^{*}+{\mathrm{CO}}_2\to \mathrm{He}+\mathrm{CO}+\mathrm{O}$$

(5)

Similarly for Ar dilution, the following reactions may be involved [28].

$${\mathrm{Ar}}_{\mathrm{g}}+{\mathrm{e}}^{-}\to {\mathrm{Ar}}_{\mathrm{i}}+{\mathrm{e}}^{-}$$

(6)

$${\mathrm{Ar}}_{\mathrm{i}}+{\mathrm{e}}^{-}\to {\mathrm{Ar}}_{\mathrm{j}}+{\mathrm{e}}^{-}$$

(7)

$${\mathrm{Ar}}_{\mathrm{j}}+{\mathrm{e}}^{-}\to {\mathrm{Ar}}_{\mathrm{i}}+{\mathrm{e}}^{-}$$

(8)

$${\mathrm{Ar}}_{\mathrm{j}}\to {\mathrm{Ar}}_{\mathrm{i}}+\mathrm{h}\mathsf{\vartheta }$$

(9)

where, Ar_g, Ar_i and Ar_j represent the ground, excited or metastable state of Ar atom respectively.

Figure 6b represents the selectivity of CO with respect to SIE, which shows He dilution is better than other gases. There is almost zero carbon loss with He dilution. Although Ar and N₂ showed 80–90% selectivity towards CO, He gave the best result. Figure 6c shows the CO yield with respect to discharge power, which increases with the increasing power. Helium dilution shows the highest CO yield (16.8%) at any power, which may be due to high thermal conductivity of He, which facilitates the transfer of energy to CO₂ molecules.

2.5. Effect of Diluent Gases on the Reaction Rate

The rate constant of CO₂ decomposition has been calculated through the following equation [24].

$$\ln \left({\mathrm{C}}_{\mathrm{in}}/{\mathrm{C}}_{\mathrm{out}}\right)=\left(\mathrm{SIE}\right)\times \mathrm{K}+\mathrm{C}$$

where, C_in and C_out represent CO₂ input and CO₂ output concentration respectively. K is decomposition rate constant and C is the intercept. Figure 7 shows the plot of ln(C_in/C_out) Vs SIE for various gases and the decomposition rate constants are 0.02802, 0.02408 and 0.03452, respectively for N₂, He and Ar, which indicates that Ar dilution provides the best CO₂ decomposition condition. The rate constant for He and N₂ diluent system is almost the same.

2.6. Energy Efficiency

Figure 8 shows the energy efficiency with respect to discharge power for CO₂ decomposition for the diluent gases with glass beads-packed reactor. The energy efficiency was calculated for the best conditions (CO₂:diluent gas = 1:2). It was reported that the energy efficiency of DBD plasma is lesser than gliding arc and microwave plasma system [29]. The curve indicates decreasing energy efficiency with increasing discharge power, which is in agreement with earlier observations [24,27]. The maximum energy efficiency of 0.945 mmol/kJ was found at the applied power 0.8 W with Ar dilution and the order of energy efficiency is Ar > He > N₂.

2.7. Optical Emission Spectra (OES)

Bogaerts et al. reported that vibrational excitation contributes to nearly 6% towards CO₂ dissociation [30]. CO₂ splitting may be initiated by the electron impact excitation from lowest vibrational level to highest vibrational level followed by vibrational-vibrational collision [29,30]. The optical emission during reaction with glass beads-packed condition was recorded to understand the excited species formed inside the plasma (Figure 9). Under all conditions, nearly the same peaks were observed. The activated species like CO, CO₂⁺, and OH have been clearly identified and the peak assignment is shown in Figure 9. The CH and OH bands observed are due to the traces of moisture impurity present in CO₂ cylinder. The peak observed around 483 nm is due to CO⁺ [24]. The significance peak for CO was found around 519.8 nm [31]. According to Kraus et al., the peak observed at 297 is due to the third positive system of CO [32]. The peak at 290 nm due to CO₂⁺ [33] is missing in our study.

3. Experimental Set-Up

The schematic of the experimental setup used for the study is shown in Figure 10. Briefly, plasma was generated in a co-axial cylindrical quartz tube, which has ~23 and 20 mm outer and inner diameter, respectively. Similarly, a stainless steel mesh wrapped around the quartz and a stainless steel rod (10 mm diameter) placed at the center of the quartz tube acts as the outer electrode and inner electrode, respectively. The discharge gap was 5 mm and the resulting discharge volume was ~21.2 mL. The AC high voltage was applied to the stainless steel inner electrode, whereas the outer electrode was grounded via a capacitor (300 nF). A high-voltage probe connected to the inner electrode was used to record the applied high voltage (V). The flow rate of gas mixture was regulated by using mass flow controllers (MFCs) and the total flow rate was kept constant at 30 mL/min. The mixture of gases (CO₂ with diluent gas, i.e., Argon, Helium or Nitrogen) was supplied to the plasma reactor and discharge plasma was generated by applying AC high voltage (16 kV to 24 kV, 50 Hz) between the electrodes. A digital oscilloscope (Tektronix TDS 2014B) was used to monitor the charge-voltage (Q-V) signals, which are plotted to obtain a Lissajous figure. The area of the Lissajous figure multiplied by frequency estimates the power dissipated in the discharge [24]. The progress of the reaction was monitored by using a Thermo Fischer Scientific-Trace 1030 gas chromatograph (GC) equipped with a Porapak Q column and TCD detector. The emission spectrum was recorded to understand the active species formed in the discharge. For this purpose, Princeton Instrument Action SpectraPro^® SP-2300 emission spectrometer with three different gratings (1200 g mm⁻¹ with 500 nm Blaze, 600 g mm⁻¹ with 750 nm Blaze and 600 g mm⁻¹ with 500 nm Blaze) was used [24].

The CO₂ conversion, CO selectivity, CO yield, energy efficiency and specific energy input have been calculated through the following equations.

$${\%\mathrm{of}\mathrm{CO}}_2\mathrm{conversion}=\frac{\left({\mathrm{CO}}_2\mathrm{input}-{\mathrm{CO}}_2\mathrm{output}\right)\left(\mathrm{mmol}/\min \right)}{{\mathrm{CO}}_2\mathrm{input}\left(\mathrm{mmol}/\min \right)}\times 100$$

(10)

$$\%\mathrm{of}\mathrm{CO}\mathrm{selectivity}=\frac{\mathrm{CO}\mathrm{produced}\left(\mathrm{mmol}/\min \right)}{\left({\mathrm{CO}}_2\mathrm{input}-{\mathrm{CO}}_2\mathrm{output}\right)\left(\mathrm{mmol}/\min \right)}\times 100$$

(11)

$$\%\mathrm{of}\mathrm{CO}\mathrm{yield}=\frac{\mathrm{CO}\mathrm{produced}\left(\mathrm{mmol}/\min \right)}{{\mathrm{CO}}_2\mathrm{input}\left(\mathrm{mmol}/\min \right)}\times 100$$

(12)

$$\mathrm{Specific}\mathrm{input}\mathrm{energy}\left(\mathrm{J}/\mathrm{mL}\right)=\frac{\mathrm{power}\left(\mathrm{W}\right)}{\mathrm{total}\mathrm{flow}\mathrm{rate}\left(\mathrm{mL}/\min \right)}\times 60$$

(13)

$$\mathrm{Energy}\mathrm{efficiency}\left(\mathrm{mmol}/\mathrm{kJ}\right)=\frac{1000\times {\mathrm{CO}}_2\mathrm{converted}\left(\mathrm{mmol}/\min \right)}{60\times \mathrm{discharge}\mathrm{power}\left(\mathrm{W}\right)}$$

(14)

4. Conclusions

The present study highlights the diluent gas effect on CO₂ decomposition in a DBD-packed bed reactor. The best diluent gas is Ar, which shows almost 19.5% CO₂ decomposition towards CO with glass beads-packed DBD at discharge power 2.4 W. This study also presents the best diluent ratio for effective CO₂ decomposition i.e., CO₂:Ar = 1:2. The best conditions indicated near zero carbon loss. The experimental results are explained on the basis of dielectric behavior of gas mixture. CO₂ conversion increased with the packing of glass beads due to its dielectric behavior.