CO₂ decomposition in a packed DBD plasma reactor: influence of packing materials

Abstract

Carbon dioxide (CO₂) decomposition has drawn significant interest over the years due to its global warming potential. A packed bed dielectric barrier discharge reactor has been designed and tested for the conversion of CO₂ into carbon monoxide (CO). The discharge volume was filled with different packing materials (glass beads, alumina, anatase titania, ceria) so as to understand the influence of dielectric constant, porosity and ultraviolet light. Typical results indicated that the packed bed DBD promotes CO₂ conversion into CO and oxygen and CeO₂ packing showed the highest conversion (10.6%) at a specific input energy of 4.8 J mL⁻¹. The best performance of CeO₂ may be due to oxygen vacant sites, which stabilize the atomic oxygen formed in the reaction and thereby promoting CO₂ conversion. During the present study, CO₂ decomposition has been achieved at ∼0.139 eV per molecule.

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1. Introduction

The contribution of carbon dioxide (CO₂) towards the greenhouse effect is well-established and there is an immediate need to address this issue.¹ Although various approaches like carbon capture and storage (CCS), carbon capture and utilization (CCU), and catalytic/photocatalytic conversion have been tested, their scope is limited either due to the environmental regulation and/or cost considerations.^2,3 When compared with the widely practiced chemical/physical methods, direct decomposition of CO₂ into CO has an advantage, in terms of CO formation, which is a feed stock in the chemical industry.^2,4 However, the enthalpy change of CO₂ decomposition indicates that this process is highly endothermic and demands high operating temperature [eqn (1) and (2)].⁵

CO₂ → CO + O, ΔH = 5.5 eV per molecular 532 kJ mol⁻¹ (2)

Thermal decomposition of CO₂ demands temperature excess of 2000 K for the formation of CO and O₂.⁶ Among the alternative processes for CO₂ decomposition, non-thermal plasma (NTP) activation has advantages like operation under ambient conditions, presence of energetic electrons capable of initiating the reaction under mild conditions, etc. Various NTP configurations like glow discharge, corona discharge,^7–9 microwave discharge,¹⁰ radio frequency discharge¹¹ and gliding arc discharge^5,12 have been tested for this purpose. Recently, dielectric barrier discharge (DBD) was also tested for CO₂ conversion.^13–16 DBD plasma generates the high energy electrons (1–10 eV), capable of initiating the chemical reactions while keeping the background under ambient conditions.^17–19 In addition to the energetic electrons, DBD also produces UV-Vis light, oxygenated species, etc. Hence, integration of a suitable catalyst/photocatalyst is expected to increase the efficiency of the process. Especially integrating porous solids like alumina improves the interaction of the CO₂ with the active species of plasma, and thereby facilitating the vibrational relaxation of the excited CO₂ molecules, leading to the dissociation of CO₂ molecule under energetically feasible conditions.

In general, the performance of DBD reactor depends on the configuration of the reactor, background gas, flow rate of the gas, catalyst/packing material and the input power.^12,20,21 It was reported that among argon, nitrogen and helium, nitrogen is more effective for CO₂ decomposition reaction.¹² Bogaerts et al., have reported that microwave plasma is energy efficient (23%) than DBD plasma (5%).¹⁰ Xin Tu et al., reported that BaTiO₃ packing is more effective than glass beads, which was assigned due to the high dielectric constant of BaTiO₃, whereas, David Yap et al., reported the best conversion while using the glass beads.^6,13 In general, the effect of packing material may either be due to the surface activation (dielectric, porous solids), catalytic action promoted by the discharge (metal oxides)²² and/or by the radiation (TiO₂).^23–28 However, to understand this, a detailed study is warranted. With this background, the present study is aimed at understanding the influence of packed bed DBD reactor during the CO₂ decomposition and to achieve the best CO selectivity. For this purpose, Al₂O₃ beads were chosen to understand the influence of the surface, whereas TiO₂ packing is to understand the influence of the UV-radiation. CeO₂ packing is to understand the oxygen storage/release capacity.^29,30

2. Experimental set-up

The experimental setup used in the present study has been shown in Fig. 1.

Fig. 1 Schematic diagram of Experimental set up.

Discharge was generated in a cylindrical DBD reactor, which has 22 and 19 mm outer and inner diameter, respectively. A wire mesh wrapped outside of the quartz tube acts as the outer electrode (9 cm in length), whereas a stainless steel rod of 10 mm diameter serves as the inner electrode. The effective discharge gap is 4.5 mm and discharge volume is ∼73.79 mL. The inner electrode was connected to a high voltage source and the outer electrode was grounded through a capacitor (4 μF) (Fig. 2).

Fig. 2 DBD reactor set up.

100% CO₂ (without dilution) gas was sent through the DBD reactor at a fixed flow rate of 20 mL min⁻¹. The DBD plasma was ignited by varying the AC voltage in the range 14 to 22 kV at a fixed frequency 50 Hz. A high voltage probe (Agilent 34136A HV probe) was used to measure the applied voltage. Charge–voltage (Q and V) signals were measured with a digital oscilloscope (Tektronix TDS2014B) and plotted to get a typical Lissajous figure (Fig. 3), whose area multiplied with frequency gives the power (W), which on dividing with the flow rate gives the energy (J) dissipated in the discharge.

Fig. 3 Lissajous figure for DBD reactor as a function of voltage.

The CO₂ and CO gases were analyzed by using a VARIAN 450 gas chromatography (GC) equipped with a packed column (HAYASEP A, 80/100 mesh, 2 m) and a TCD detector. N₂ was used as the carrier gas. The reactor outlet was connected to GC with a long Teflon tube (3 mm diameter) and every 15 minutes an online injection was carried out. Then three injected results were collected corresponding to each voltage and the average area of three gradual injection data has been considered for further calculations. The emission spectrum of the discharge was measured by using an emission spectrometer (Princeton Instrument Action SpectraPro® SP-2300), equipped with three gratings (600 g mm⁻¹ with 500 nm Blaze, 600 g mm⁻¹ with 750 nm Blaze and 1200 g mm⁻¹ with 500 nm Blaze) which uses an optical fiber to collect the spectrum. The emission experiment has been conducted with 600 g mm⁻¹ with 500 nm Blaze. The packed materials used in the present study are: glass beads (3.5–4.5 mm, SD fine-chem limited), TiO₂ (3–4 mm, Alfa Aesar), γ-Al₂O₃ (3–4 mm, Sumitomo Chemical), CeO₂ (∼2 mm, Sigma-Aldrich). The surface area of packed materials was estimated by using a NOVA 2200e sorption apparatus. CO₂ conversion, CO yield, Specific Input Energy (SIE), energy efficiency and Specific Energy Required per molecule (SER) were calculated by using the following relations:

SIE is the energy consumption per mL of the gas flow, which was calculated by using eqn (5).

where ΔH is the enthalpy of reaction (1) (2.9 eV per molecule for CO formation from CO₂) and SER is the specific energy requirement, which is defined as the total energy consumption per unit of reaction product. SER was calculated by using eqn (8)

3. Results and discussion

3.1. Effect of packing materials on discharge characteristics

Fig. 3 presents the V–Q Lissajous figure as a function of the voltage, from which the total charge for various packing materials was calculated. As seen in Fig. 3, the area of Lissajous figure and thereby power increases with increasing the voltage. Fig. 4 presents the influence of various packing materials on the discharge characteristics at 22 kV, which indicates that the input power dissipated is different for various packed DBD configurations.

Fig. 4 Lissajous figure for different packed material at 22 kV.

3.2. Effect of packing material on discharge power

Fig. 5 presents the influence of various packing materials on the power dissipated in the discharge. Among the packing materials studied, TiO₂ packing showed the highest power and the discharge power followed the trend: TiO₂ > Al₂O₃ > CeO₂ ≈ glass beads > DBD. It is known that materials with high dielectric constant generate more number of microdischarges and create strong electric field at the corners and edges. The dielectric constant for various packed materials follows the order: TiO₂ > CeO₂ > Al₂O₃ > glass beads (Table 1). Although CeO₂ has higher dielectric constant (25)^31,32 than Al₂O₃, probably due to more uniform shape, the power dissipated is lesser than Al₂O₃. The power variation data complements the charge variation, as presented in Fig. 6, which also confirms the TiO₂-DBD has the highest charge dissipated per half cycle. Fig. 5 and 6 confirm the highest power and charge for TiO₂-DBD. Table 2 summarizes the power, specific input energy (SIE) and CO₂ conversion for various configurations.

Fig. 5 Average power dissipated as a function of the voltage for various packed materials (gas flow rate: 20 mL min⁻¹; without dilution; frequency: 50 Hz).

Table 1 The physical properties of packed materials

Packed material	Diameter of packed material (mm)	Surface area (m^2 g^−1)	Dielectric constant
Glass beads	3.5–4.5	0	4.1
TiO2	3–4	64	85
Al2O3	3–4	133	9.1
CeO2	2	30	25

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Fig. 6 Total charge transferred by the micro discharges per half cycle as a function of applied voltage (gas flow rate: 20 mL min⁻¹; without dilution; frequency: 50 Hz).

Table 2 CO₂ conversion with power and SIE variation for different packed bed DBD

Voltage (kV)	Without packing			Glass beads			TiO2			Al2O3			CeO2
	Power (W)	SIE (J mL^−1)	Conv. (%)	Power (W)	SIE (J mL^−1)	Conv. (%)	Power (W)	SIE (J mL^−1)	Conv. (%)	Power (W)	SIE (J mL^−1)	Conv. (%)	Power (W)	SIE (J mL^−1)	Conv. (%)
14	0.1	0.3	1.2	0.2	0.6	3.5	0.7	2.1	5.1	0.6	1.8	3.5	0.2	0.6	2.6
16	0.3	0.9	2.8	0.4	1.2	4	1	3	6.3	0.9	2.7	4.6	0.4	1.2	4.4
18	0.5	1.5	4.2	0.6	1.8	5.4	1.3	3.9	6.8	1.2	3.6	6.1	0.6	1.8	6.6
20	0.8	2.4	5.9	0.9	2.7	6.5	1.7	5.1	7.5	1.5	4.5	7.8	0.9	2.7	9.1
22	1.1	3.3	6.3	1.2	3.6	7.9	2.2	6.6	8.2	1.9	5.7	9.1	1.6	4.8	10.6

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It is known that dielectric packing improves the electric filed strength, due to which more charge will be transferred to the microdischarges.⁶ Fig. 6 shows the total charge transferred by the microdischarges per half cycle for various packed bed DBD configurations. For a DBD reactor without any packing, the charge transferred was only between 0.06 μC and 0.51 μC, corresponding to the voltage variation between 14 and 22 kV, respectively. Under the same experimental conditions, 0.41 μC to 0.72 μC was transferred for TiO₂ packing. Porous alumina packed DBD showed a slightly lower charge of 0.34 to 0.68 μC, at 14 to 22 kV, respectively, whereas, glass beads and CeO₂ packing showed the lowest charge transfer between 0.15 and 0.6 μC. This observation can be explained based on the high dielectric constant (85) of TiO₂ beads and alumina (9.1), due to which the strength of microdischarges will be highest for TiO₂ than other packing materials.^33,34

3.3. Packed bed DBD for carbon dioxide decomposition

Fig. 7 represents the conversion of CO₂ as a function of the discharge power for various packed bed configurations. Interestingly, packed bed DBD shows higher conversion than DBD without packing. This may be due to more homogeneous discharge in the presence of packing, surface activation and/or due to improved filed strength. General observation is that the CO₂ conversion increases with increasing the input power for all configurations. As shown in Table 2, the discharge power varies in each configuration. With DBD reactor, the maximum discharge power was around 1.1 W at 22 kV, where it showed the conversion of only 6%. Glass beads showed a gradual increase in CO₂ conversion with discharge power and the conversion of CO₂ reached 8% at 1.2 W. TiO₂ packing showed better conversion than DBD and glass beads packing. This may be due to surface activation of the photocatalyst. Since the presence of UV-Vis light is confirmed, it may be assumed that photocatalytic/photoassisted CO₂ decomposition may be taking place.³⁵ With TiO₂, 8.2% CO₂ conversion was observed at ∼2.2 W.

It was reported by Holzer et al., that porous solids like γ-Al₂O₃ improves the effective residence time of the reactant molecules in the discharge zone and thereby improves the interaction of the reactant molecules with activated species of the discharge.^36,37 In order to understand the combined surface area, porosity and dielectric effect, γ-Al₂O₃ beads (surface area 133 m² g⁻¹, macroporous and dielectric constant 9.1) were packed. As seen from the Fig. 7, at 1.8 W, 9% conversion was observed. It is worth mentioning that power dissipated was not the same at any voltage (Table 2). For example, at 22 kV, Al₂O₃ packing shows higher conversion than glass beads. However, at 22 kV, the power dissipated with glass beads packing was only 1.2 W, whereas, it is 1.9 W for Al₂O₃ packing. The better performance of Al₂O₃ packed plasma reactor may be due to the high discharge power and enhanced residence time of the gas in the discharge zone due to porous nature of Al₂O₃. Hence, due to the combined effect of porous nature and dielectric constant, CO₂ conversion of ∼9% was observed. This is in agreement with the observations made earlier.^9,37

Fig. 7 CO₂ conversion with respect to discharge power (gas flow rate: 20 mL min⁻¹; without dilution; frequency: 50 Hz).

Interesting observation is that CeO₂-DBD showed the highest conversion of CO₂ at any power. The highest conversion of 10.6% was achieved at 1.6 W (22 kV), whereas, under the same input power conditions, DBD and Al₂O₃ packed DBD showed only 6.3% and 7.8% conversion, respectively. The best performance of CeO₂ packed reactor may be due to its ability to stabilize the nascent oxygen formed during CO₂ decomposition. It is worth mentioning that nascent oxygen is a highly reactive species, which can either re-oxidize CO or may recombine to form O₂ molecule. It has been reported that CeO₂ has oxygen storage and release properties.³⁸ So the nascent oxygen formed due to CO₂ decomposition may be stabilized on CeO₂ surface and thereby preventing the re-oxidation of CO. It is known that atomic oxygen can promote the decomposition of second CO₂ molecule.³⁹ In addition, the carbon was forming due to Boudouard reaction may be oxidized by nascent oxygen. As a result, CO yield also improves with CeO₂ packing. As seen from the data presented in Table 2, CO₂ conversion followed the trend: DBD < glass beads < TiO₂ < Al₂O₃ < CeO₂.

The CO yield with respect to SIE has been shown in Fig. 8. The CO yield also increases with the increasing of SIE. ∼10.5% CO yield achieved with CeO₂ packing at 1.6 W, whereas, 6.5% and 5.8% yield was obtained with glass beads and DBD respectively. Al₂O₃ and TiO₂ packing also showed higher CO yield than DBD and glass beads packed DBD. The loosely bound oxygen on CeO₂ readily oxidizes the carbon and increases the CO yield.

Fig. 8 Yield of CO with respect to specific input energy (gas flow rate: 20 mL min⁻¹; without dilution; frequency: 50 Hz).

3.4. Effect of packed materials on the reaction rate

The CO₂ decomposition varies with the packing materials. The decomposition rate constant was calculated by using the following eqn (9).⁴⁰

ln(C_in/C_out) = (SIE) × K + C (9)

where K is the decomposition rate constant and C is the intercept. C_in and C_out symbolized for CO₂ input and CO₂ output concentration, respectively.

ln(C_in/C_out) as a function of specific input energy (SIE) for different packing materials have been shown in Fig. 9 and the corresponding decomposition rate constant was 0.020, 0.015, 0.005, 0.016 and 0.018 for CeO₂, Al₂O₃, TiO₂, glass beads and DBD, respectively, indicating that CeO₂-DBD has the best CO₂ decomposition capability.

Fig. 9 Relationship between ln(C_in/C_out) and SIE (linear fits) for different packed materials reactors.

3.5. Energy efficiency

Fig. 10 presents the energy efficiency as a function of SIE. It may be concluded that DBD alone is not effective for CO₂ decomposition and packed bed DBD improves the energy yield. Energy efficiency decreases on increasing the SIE for all reactors.⁴¹ CeO₂ packing shows the energy efficiency of 1.91 mmol kJ⁻¹ at SIE 0.6 J mL⁻¹, whereas under the same experimental conditions, TiO₂ and Al₂O₃ shows 1.09 mmol kJ⁻¹ and 0.86 mmol kJ⁻¹, respectively. Interestingly, glass beads packing shows the best energy efficiency (2.58 mmol kJ⁻¹) at the lower SIE (0.6 J mL⁻¹). It is worth mentioning that the observed result with TiO₂ packed system shows 30.3% energy efficiency (calculated by eqn. (7)) which is comparable with gliding arc discharge and atmospheric thermal arc reactors which has the energy efficiency of 25% and 30%, respectively.^12,39.

Fig. 10 Energy efficiency with respect to SIE at different packed materials (gas flow rate: 20 mL min⁻¹; without dilution; frequency: 50 Hz).

3.6. Specific energy requirement for decomposition of single CO₂ molecule

The specific energy requirement (SER) was calculated by using eqn (8). Fig. 11 shows SER as a function of SIE, which highlights that CeO₂ packed DBD reactor demands lowest energy than other configurations. The highest energy demand was 0.187 eV per CO₂. CeO₂ is also one of the most efficient packing materials, which showed energy requirement of 0.0534 eV per molecule. It signifies that the role of surface is an important factor in CO₂ dissociation. CeO₂ prevents the recombination of oxygen atoms and/or prevents the CO oxidation to CO₂. As a result, CeO₂ packed DBD showed the best CO₂ conversion. TiO₂ and Al₂O₃ packed reactor demands 0.095 to 0.187 eV per molecule and 0.1194 to 0.1456 eV per molecule, respectively at SIE of 0.488 to 1.534 eV per molecule and 0.418 to 1.325 eV per molecule.

Fig. 11 Specific Energy Requirement (SER) with respect to Specific Input Energy (SIE) at different packed materials.

3.7. Optical emission spectroscopy

The emission spectrum of the discharge was recorded for various packed DBD reactors to understand the nature of the activate species formed in plasma (Fig. 12). The signature peaks corresponding to excited CO and CO₂ were observed. Very intense peak at 335 nm was due to CO₂ asymmetric V₂–V₁₆ transition indicating that the dissociation may be due to vibrational relaxation.^10,23,42

Fig. 12 Emission spectra of CO₂ plasma. (Applied voltage 18 kV; grating: 600 glue at 500 nm.)

Robby Aerts et al. reported that the vibrational excitation is responsible for CO₂ splitting in DBD plasma. Peak centered at 297 nm is due to the third positive system of CO, as reported by Martin Kraus et al.^23,43 In the present study, CO₂⁺ peak at 290 nm was not detected, probably due to the fast conversion of CO₂⁺ to CO. CH emission peak at 314 nm (C ²∑–X ²Π) may be due to the presence of 0.05% moisture in the CO₂ gas. The typical CO₂ bands are found at 357 nm and 375 nm, which are assigned due to the transition of V₃–V₁₆ and V₆–V₁₉ respectively, whereas, the peak found at ∼414 nm may be the transition between V₁–V₁₂ and V₈–V₂₀ due to CO₂⁺. The peak at 391.2 nm is due to the ¹B₂–X¹∑⁺ transition of CO₂.⁴⁴ One more peak observed at 435 nm is due to the vibrational transition of CO₂. The peaks around 400 and 406 nm are due to the O₂ emission.^44,45 The peaks observed around 483 nm, 519.8 nm and 559.4 nm are due to CO emission.²⁵

4. Conclusions

DBD plasma assisted conversion of CO₂ into CO has been carried out by using a packed DBD plasma reactor. Present study highlights the promising nature of DBD plasma reactor in converting CO₂ into CO. The performance of the DBD plasma reactor was found to increase on integrating various packing materials. It has been confirmed that CeO₂ packing showed the best performance and appears to be economical. The best CO₂ conversion of ∼10.6% has been achieved at 1.6 W, while using CeO₂-DBD, which is assigned due to the ability of CeO₂ to stabilize the product atomic oxygen and thereby preventing the re-oxidation of CO.

Acknowledgements

The authors gratefully acknowledge to Ministry of New and Renewable Energy, New Delhi, India for supporting to this project (CHY/2014-15/019/MNRE/CHS/0140). Debjyoti thanks UGC India for junior research fellowship.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra27085e

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