Performance and mechanism of CO2 reduction by DBD-coupled mesoporous SiO2

Jiangming Wang; Yongshi Yang; Jinxin Yu; Zhongzhou Ye; Zhen Li; Zhaolian Ye; Songjian Zhao

doi:10.1515/ntrev-2023-0577

Article Open Access

Performance and mechanism of CO₂ reduction by DBD-coupled mesoporous SiO₂

Jiangming Wang , Yongshi Yang , Jinxin Yu , Zhongzhou Ye , Zhen Li , Zhaolian Ye and Songjian Zhao

Published/Copyright: September 5, 2023

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From the journal Nanotechnology Reviews Volume 12 Issue 1

Abstract

In the process of CO₂ reduction with dielectric barrier discharge (DBD)-coupled catalysis, the existing material presents unsatisfactory synergy, such as high cost, complicated preparation processes, and low conversion rates. An inexpensive and environmentally friendly mesoporous SiO₂ with different morphologies by gel–sol method was synthesized and then introduced for synergistic conversion of CO₂ with DBD. The physicochemical properties of the synthesized mesoporous SiO₂ materials were analyzed using X-ray diffraction, thermogravimetric analysis, scanning electron microscopy and Brunauer-Emmett-Teller method, indicated the prepared mesoporous materials manifested large specific surface areas, ordered pore channels and pore size, and good stability. The CO₂ reduction performance, CO selectivity, and energy efficiency of DBD alone and DBD-coupled mesoporous SiO₂ were investigated at different input powers. The SiO₂ prepared with 1.05 g cetyltrimethylammonium bromide addition had the highest activity, in which the conversion of CO₂, CO yield and energy efficiency were increased by 56.73, 68.41, and 122.31%, respectively, compared with DBD alone. The primary CO₂ conversion mechanism of the mesoporous SiO₂-coupled DBD was analyzed. It is shown that the suitable pore capacity structure, the large specific surface area, and the presence of filament discharge within the pore size of suitable mesoporous material can promote the decomposition of CO₂ on its surface.

Keywords: DBD plasma; mesoporous SiO2 ; CO2 conversion

1 Introduction

With the rapid advancement of human civilization, global energy resources are becoming increasingly scarce, leading to an increased reliance on fossil fuels and a consequent rise in fossil combustion year after year. According to the World Energy Outlook 2022, global energy demand is projected to increase by about 0.8% annually until 2030, and burning fossil fuels caused by energy consumption has resulted in significant CO₂ emissions. The US National Oceanic and Atmospheric Administration has reported that atmospheric CO₂ levels have risen from 280 ppm in the industrial era to 405 ppm in 2017 and continue to rise [1]. A significant catastrophe has resulted from excessive CO₂ emissions, which have upset nature’s natural carbon balance and caused ecological harm [2], sea level rise [3], global warming [4], and other problems. Therefore, reducing CO₂ emissions has become an essential strategic concern for all nations across the globe. Today’s CO₂ problem is usually solved by carbon capture and storage (CCS) and carbon capture and utilization (CCU), which can be captured from point sources in industrial processes such as power plants. In CO₂ storage, environmentally friendly chemical additives have recently been found to accelerate the process of repeated recycling, such as hydrate-based CO₂ sequestration [5]. Unlike CCS, CCU uses captured CO₂ to convert it into valuable chemicals [6], such as syngas [7], methane [8], and methanol [9]. Carbon monoxide (CO) is an important chemical raw material that has been used in many reactions, such as feedstock for Fischer–Tropsch synthesis and hydrocarbons, as well as a primary feedstock for fuel synthesis and other chemicals. Utilizing CO₂ as a carbon source and selectively converting it into CO is a promising way to reduce CO₂ and generate new clean energy sources.

The stability of CO₂ as the final form of carbon and oxygen compounds makes it difficult to activate. Thermodynamic analysis shows that CO₂ is difficult to decompose into O₂ and CO at reaction temperatures below 2,000°C, making conventional methods such as thermocatalytic decomposition of CO₂ difficult and uneconomical, limiting their large-scale industrial applications [10]. To overcome the disadvantages of these traditional methods, various new technologies have been developed, such as photocatalysis [11], electrocatalysis [12], photoelectric reduction, and plasma technology [13]. Photocatalysis and electrocatalysis have a low selectivity of the products, unstable reactions, and high electrical potential required, and they usually have low energy efficiency. In contrast, plasma technology can overcome thermodynamic barriers at room temperature and pressure and is characterized by mild operating conditions, compactness, fast reaction rates and reaction times, relatively low cost, and environmental friendliness [14]. Dielectric barrier discharge (DBD) is a standard gas discharge method with a more consistent and stable discharge performance, and the discharge area can produce a large number of high-energy electrons (1–10 eV) that can effectively excite molecules to chemical reactions [15]. Coaxial cylindrical DBD reactors are often used as reaction devices for the treatment of gases due to the advantages of uniform gas distribution, good confinement, and easy adjustment of the reaction process [16], and they can produce a uniform and stable low-temperature plasma [17]. As a result, it is a promising technology for large-scale CO₂ conversion at atmospheric pressure. Materials are required to create a plasma-catalytic synergy to boost the conversion and selectivity of CO₂ due to the low conversion and energy efficiency of a single DBD.

The material filled in the DBD is a critical factor in the conversion of CO₂, and the addition of a material is an effective way to regulate and optimize the plasma discharge process to improve the conversion and energy efficiency of CO₂. The material in the discharge zone alters the plasma discharge pattern, increasing the local electric field at the contact point and the average electric field within the reactor, among other relevant chemical properties. Michielsen et al. [18] found that adding BaTiO₃ to a DBD-filled bed reactor promoted the conversion of CO₂. Mei and Tu [19] found that adding Ni/γ-Al₂O₃ to a BaTiO₃-filled bed reactor increased CO₂ conversion. Li et al. [20] found that adding a mixed ZrO₂–CeO₂ filler to a coaxial reactor promoted CO₂ conversion due to oxygen vacancies. Wang et al. [21] investigated the effect of various media materials on CO₂ discharge performance. The highest conversion rate of 23.6% was found for reactors with filled MgO.

Nowadays, the reported materials are expensive, difficult to prepare and have low conversion rates, necessitating the development of more cost-effective, eco-friendly, and highly efficient materials to expand the use of CCU. SiO₂ is an attractive non-toxic, odorless, non-polluting non-metallic material, often used as a new catalyst carrier, selective adsorbent, etc. [22]. It is also economically efficient and easy to prepare the material. Cho et al. [22] investigated SiO₂-based adsorbents with high CO₂ capture capacity and regeneration performance. Shawabkeh et al. [23] found that the adsorption of CO₂ by SiO₂ at different Cu loadings. Its morphological regularity and pore size can be tuned, where both parameters, material size, and morphology, greatly influence the performance effect of the material. The effect of the addition of filler material depends to a large extent on the type of material used and the corresponding size and gap. Uytdenhouwen et al. [15] reported that in SiO₂, ZrO₂, and Al₂O₃ microspheres as well as glass wool reactors, SiO₂ and glass wool were the best in terms of conversion. The available data suggest that porous material can generate plasma within the pores, increase gas residence time through their adsorption properties, and react synergistically in catalytic plasma reactions. The mesoporous SiO₂ has microporous discharge, surface defects, and property modifications such as CO₂ adsorption that favor the plasma system. Therefore, mesoporous SiO₂ also has potential applications in plasma-catalyzed CO₂ conversion. Mesoporous SiO₂ has been studied in many different fields, yet people have yet to investigate the mechanism of action of a single SiO₂ in CO₂ conversion in plasma.

In this study, large specific surface area mesoporous SiO₂ material with ordered pore channels and pore sizes were prepared by a simple and economical method using cetyltrimethylammonium bromide (CTAB) as a template and ethyl orthosilicate (TEOS) as a silicon source. Different CTAB additions were adjusted to change the morphology, pore size, and other structural characteristics of the material and then characterized by X-ray diffraction (XRD), TGA, scanning electron microscope (SEM), and Brunauer-Emmett-Teller method. The performance of DBD alone and DBD-coupled mesoporous SiO₂ for CO₂ reduction, CO selectivity, and energy efficiency were investigated at different input powers, and the related mechanisms involved in the reaction were also analyzed.

2 Experimental

2.1 Material preparation

Weigh different masses (0.65, 0.85, 1.05, and 1.25 g) of CTAB (purchased from Sinopharm Chemical Reagent Co., Ltd., China) and mix with 200 mL of deionized water, stirring at 50°C in a magnetic mixer until the solution is clarified. After clarification, 15 mL of ammonia solution (provided by Shanghai Ling Feng Chemical Reagent Co., Ltd.) is added, stirring for 30 min at room temperature, 5 mL of n-ethyl silicate (TEOS, purchased from Macklin) dropwise is then slowly (1 mL/min) added, and then the reaction is allowed to stand for 3 h at 85°C. The sample is washed by centrifuging (at 6000 rpm for 5 min) and three times with deionized water and ethanol, dry at 80°C and calcine at 600°C (heating rate 5℃/min) for 8 h to obtain a white powder, which is ground and pressed, and the sample is taken between 40 and 60 mesh. The samples are labeled as Si-a, Si-b, Si-c, and Si-d according to the content of CTAB.

2.2 Characterization

The prepared materials were characterized by XRD performed on an X pert Powder with Cu Kα radiation (30 mA, 40 kV) in a 2θ range from 10° to 80° with a scanning speed of 10° per minute and the step size is 0.02°. The surface morphologies of the material were analyzed using an electron microscope (SEM, Sigma 300, Germany), and energy-dispersive X-ray spectroscopy (EDS) tests (Xplore, Oxford) were performed to determine the elemental distribution of the products. Specific surface area analysis of the samples was obtained from N₂ physisorption isotherms at −196℃, using Quantachrome Instruments, and the specific surface area and the average pore size of the adsorbent were determined; the pressure range is Relative pressure P/P ₀ = 0–1. Thermal analysis was conducted with a NETZSCH STA 2,500 thermal analyzer at N₂ flow from 20 to 900℃ with 10℃/min. Fourier-transform infrared spectroscopy (FTIR) analysis was done in the range of 500–4,000 cm⁻¹, with a resolution of 4 cm⁻¹ using TENSOR 27, the scanning mode and number of scans are KBr compression test, 32 times. Measurements of X-ray photoelectron spectroscopy (XPS) were recorded on Thermo Scientific K-Alpha using Al Kα (hv = 1486.6 eV) radiation, with charge correction by C 1s (284.8 eV) done for the obtained spectra.

2.3 Experimental methods

2.3.1 Reaction procedure

The experiments are carried out at atmospheric pressure. The reaction gas during the experiments is high-purity CO₂, supplied from a cylinder, which is fed into the plasma reactor at a controlled gas flow rate of 30 mL/min by means of a mass flow meter. The material is filled to a volume of 4 mL. The products of the reaction are determined analytically using a meteorological chromatograph (GC-9860-5C-NJ) equipped with a TCD detector and a double FID detector.

The plasma reactor used in the experiments is a cooled coaxial DBD reactor with the structure shown in Figure 1(a). The material used is stainless steel 316 as the inner electrode (high voltage electrode) of the reactor. The outer electrode is a quartz tube (120 mm long) with an outer diameter of 25 mm and an inner diameter of 20 mm, with a one-sided discharge gap of 3 mm. To investigate the effect of different materials on CO₂ conversion, the material is filled uniformly and flatly in the discharge gap, and the lower end is fixed with quartz wool. The unit is equipped with a circulating water cooling device to control the temperature of the reaction system. The principle of the cooling device is shown in Figure 1(b).

Figure 1

(a) The detailed structure of the coaxial DBD reactor; (b) the schematic diagram of the cooling device.

2.3.2 Calculation of discharge parameters

The oscilloscope (TBS1102C) can detect and display the current and voltage waveforms during the experiment. The Lissajous graph method [24] can be used to calculate discharge power and other discharge characteristics. The discharge power (P) is calculated in the following manner:

(1) P = fW = f ∫ t 0 − T 2 t 0 + T 2 u ( t ) i ( t ) d t = f C n ∫ t o − T 2 t 0 + T 2 u ( t ) d u c ( t ) = f C n S ,

where u ( t ) is the applied voltage, i ( t ) is the reactor current, u c ( t ) is the voltage across the external capacitor, T is the applied voltage discharge period, f is the applied voltage discharge frequency, C n is the external capacitance value, and S is the area of the Lissajous figure.

The low voltage signal at both ends of the capacitor and the voltage measured by the high voltage probe at the high voltage end of the reactor is input into the CH1 and CH2 channels of the oscilloscope, and a closed curve, namely, the Lissajous graph, is obtained as shown in Figure 2. The discharge charge Q _d, peak-to-peak charge Q _pk-pk, and transferred charge Q _trans in one cycle can be obtained from the Lissajous graph.

Figure 2

Lissajous figure.

2.3.3 Calculation of reaction parameters

Gas chromatography was used to determine the content of various gases before and after the reaction, as well as to analyze the CO₂ conversion ( C co 2 ), CO selectivity ( S CO ), CO yield ( Y CO ), energy density ( SEI ), and energy efficiency ( η ) of the reaction process after the reaction, as shown below:

(2) C co 2 ( % ) = CO 2 converted ( mmol / min ) CO 2 input ( mmol / min ) × 100 ,

(3) S CO ( % ) = CO produced mmol min CO 2 converted mmol min × 100 ,

(4) Y CO ( % ) = CO produced mmol min CO 2 input mmol min × 100 ,

(5) SEI ( J / mL ) = Input power ( W ) CO 2 flow rate mL min × 60 ,

(6) η ( % ) = Δ H ( kJ / L ) × C co 2 ( % ) 22 . 4 × SEI ( J / mL ) .

Of which, the reaction enthalpy Δ H of CO₂ decomposition is 283 kJ/mol [17].

3 Results and discussion

3.1 Material characterization

3.1.1 XRD

The XRD results (Figure 3a) show that all four samples obtained by adding different quantities of CTAB show a broad diffraction peak at 2θ = 20°–30°, which belongs to the quartz crystalline phase [25], and the high width and low intensity of this peak confirm the amorphous structure of the formed phase. The addition of CTAB caused a change in the pore size and structure of the samples, resulting in a slight shift in the characteristic diffraction peaks in the range of 0.5°–1°, which may be related to the varying pore size and morphology of the samples. In addition, no other peaks were observed in this range, which is proof of the high purity of the synthesized samples.

Figure 3

(a) XRD patterns, (b) TGA spectra.

3.1.2 TGA

Figure 3b shows the results of the thermogravimetric analysis of material Si-a, Si-b, Si-c, and Si-d. The weight loss below 200℃ is attributed to the removal of physically adsorbed water and interlayer water, and the weight loss of the four materials is different, indicating that SiO₂ particles of different morphologies adsorb different amounts of water molecules. After 200℃, with the increase in temperature, the four SiO₂ materials of different morphologies all experience gradual weight loss, mainly due to dehydroxylation, and the weight loss is not significant [26].

3.1.3 SEM

Based on the principle of synergistic self-assembly, it is known that CTAB plays a controlling role in the synthesis of silica samples. During the reaction, the silicate anion and CTAB cation interact with each other, which can cause a change in the charge density of SiO₃ ²⁻ [26]. When the additional amount of CTAB was 0.65 g (as in Figure 4a), the silica nanoparticles basically showed irregular patterns; when the additional amount of CTAB was 0.85 g (as in Figure 4b), the silica nanoparticles gradually formed rods with a size of 20–30 nm; when the addition amount of CTAB was 1.05 and 1.25 g, the silicon dioxide nanoparticles appear in the shape of rods with small uneven spheres on top. This is because the excessive addition of CTAB causes part of the CTAB to be wrapped in the skeleton of SiO₂ without reaction, and part of it cannot be removed by high-temperature calcination. The energy spectrum analysis graph (Figure 4e and d) shows that the sample was synthesized as homogeneous SiO₂ particles of high purity. Figure S1 shows the histograms of the particle size distribution for all samples (Si-a, Si-b, Si-c, and Si-d), and Table S1 shows the particle sizes of the synthesized materials.

Figure 4

(a–d) SEM Images of SiO₂ with different morphologies, (e and f) EDS mapping of SiO₂-c.

3.1.4 BET

Figure 5 shows the physical adsorption/desorption isotherms and pore size distribution of N₂ for the four material Si-a, Si-b, Si-c, and Si-d. The specific data of the specific surface area, pore size, and pore capacity are shown in Table 1, in which it can be seen that the specific surface area of SiO₂ increases with the addition of CTAB. The addition of different amounts of CTAB resulted in the formation of pores of varying degrees of order in the SiO₂ particles, with a uniform distribution of pore size between 2 and 3 nm. Figure 5a shows the N₂ adsorption–desorption profiles for SiO₂ added with different amounts of CTAB, which is a typical type IV adsorption–desorption profile, indicating the presence of an excellent mesoporous structure of the material. Due to the relatively small cavity pore size and pore capacity on the particle surface, the isotherm has almost no hysteresis loop [27]. Moreover, due to gaps between the SiO₂ particles, the isotherm rises rapidly under high-pressure conditions. Figure 5b shows the pore size distributions of the four types of SiO₂ added with different amounts of CTAB, confirming the uniformity of the pore distribution. At the same time, material Si-c (Table 1) has the largest pore capacity.

Figure 5

(a) BET isotherms, (b) porosity distribution.

Table 1

Specific surface area, pore size, and pore capacity

Sample	S _BET (m²/g)	Pore volume (cc/g)	Pore diameter (nm)
Si-a	741.0	0.86	2.47
Si-b	798.7	0.86	2.733
Si-c	859.7	1.10	2.73
Si-d	878.6	0.87	2.46

3.2 Catalytic performance analysis

The CO₂ conversion performance, CO yield, and energy efficiency of four mesoporous SiO₂ with different CTAB additions in the DBD plasma system were investigated, as shown in Figure 6. All other reaction conditions being the same, the CO₂ conversion of the mesoporous SiO₂ material filled with different CTAB additions showed an increase compared to unfilled. The CO₂ conversion and CO yield gradually increased as the input power increased from 20 to 60 W. This is likely due to the fact that as the input power increases, the intensity of the discharge region increases, the density of activated particles and energetic electrons increase, and the energetic electrons in the discharge region bombard CO₂ more readily, generating excess CO₂ ⁺ and O₂ ⁻, thereby increasing the CO₂ conversion rate [18]. However, increasing the input power increases the energy in the reaction region, but the magnitude of the energy input is greater than the increase in CO₂ conversion, resulting in a decrease in energy efficiency as the input power increases.

Figure 6

(a) CO₂ conversion rate, (b) yield of CO, and (c) energy efficiency with different packing materials.

Figure 6a shows the CO₂ conversions of samples Si-a, Si-b, Si-c, and Si-d in the DBD plasma reactor and without filling. In this case, the best CO₂ conversion without filling was 9.57% at an input power of 60 W. Under the same conditions, samples Si-a, Si-b, Si-c, and Si-d improved by 35.37, 12.88, 56.73, and 36.81%, respectively.

Figure 6b shows the CO yields of the four samples Si-a, Si-b, Si-c, and Si-d in the plasma reactor, with the best CO yield of 7.8% for unfilled and an increase of 65.38, 42.3, 68.41 and 52% for filled samples Si-a, Si-b, Si-c, and Si-d, respectively. The previous results demonstrate the expected CO₂ conversion reaction activity of the mesoporous SiO₂ synergistic DBD reactor in which the mesoporous SiO₂ is all activated by the plasma.

It was also found that the material filling contributed to the energy use efficiency of the reaction process (Figure 6c). At an input power of 20 W, the material was all filled with higher energy efficiency than unfilled, with filled samples Si-a, Si-b, Si-c, and Si-d improving the energy efficiency by 70.81, 72.98, 122.31, and 88.84% compared to unfilled (2.33%). This leads to the conclusion that sample c shows the best conversion of CO₂ in collaboration with DBD. The material Si-c was put through a performance stability test, as shown in Figure S2, and was found to have good performance stability, with no tendency for the CO₂ conversion to decrease under long periods of discharge (4 h). The low initial CO₂ conversion is due to the fact that the plasma power supply has just been switched on, and there has not been sufficient time for the CO₂ to react with the plasma. Table 2 summarizes the recent literature on energy efficiency with DBD-assisted CO₂ decomposition. It can be seen that the material Si-c has a high CO selectivity and energy efficiency (mmol/KJ). Although the performance in terms of CO₂ conversion is not as good, this is only the performance of a single silicon-based material, and it is believed that the performance will be even better with the subsequent addition of active components. The next step will be to focus on the reaction mechanism of CO₂ conversion brought about by sample Si-c in terms of FTIR and XPS.

Table 2

Comparison of DBD plasma-assisted conversion of CO₂

Plasma	Flowrate (mL/min)	Power (W)	Diluent gas	Packed material	C co 2 ( % )	S CO ( % )	Y CO ( % )	Energy efficiency (mmol/kJ)	Reference
DBD	50	50	—	BaTiO₃	28	96	—	0.254	[28]
DBD	39	30	—	SiO₂@TiO₂-0.25 M with LUDO	15	—	—	—	[29]
DBD	55	150	—	Molecular sieves 5 A	25	63	—	—	[30]
DBD	30	2.4	Ar	Glass beads	19.5	86	16.8	0.6	[31]
DBD	30	2.4	—	15% CuO/CeAl	13.5	59	7.8	1.373	[32]
DBD	30	2.4	—	5% ZnO + g-C₃N₄	12	70	—	1.1	[33]
DBD	30	60	—	Si-c	15	87	13.13	0.697	This work

3.3 FTIR analysis of material

Figure 7 shows the FTIR spectra of sample Si-c before and after the reaction. The IR spectrogram of mesoporous SiO₂ before the reaction shows that the peak at 467 cm⁻¹ appears due to the bending vibration of the Si–O–Si bond [34]; the symmetric stretching vibration peak of the Si–O–Si bond appears at 802 cm⁻¹. The complete vibration of the Si–OH bond causes an absorption peak at 962 cm⁻¹, which is relatively weak. The antisymmetric stretching vibration peak of Si−O−Si appears around 1,090 cm⁻¹ [35,36]. The peak at 3450 cm⁻¹ in the spectrum is the antisymmetric stretching vibration peak of the –OH group with a broad absorption band. The absorption peaks at 2,860 and 2,930 cm⁻¹ are due to the presence of the –CH₂− bond in CTAB [25], which also supports the elaboration of sample c in the SEM, where there is too much CTAB to allow complete removal of CTAB. Comparing the FTIR plots of sample Si-c before and after the reaction, it can be seen that the peak intensity of the sample after the plasma experiment has become more prominent, likely due to the fact that high-energy electrons in the plasma bombard the sample, thus exposing more of the active components in the sample.

Figure 7

FTIR Spectrum of Si-c before and after the reaction.

3.4 XPS analysis

XPS was used to further understand the changes in mesoporous SiO₂ before and after the experimental reaction and to provide a mechanism for plasma enhancement. Figure 8 shows the XPS spectral profile of sample Si-c before and after the reaction, the XPS fine spectrum of Si 2p, and the fine spectrum of O 1s. XPS analysis (Figure 8a) shows that material Si-c contains C 1s, O 1s, Si 2p, and Si 2s peaks. The characteristic peak of Si 2p was observed at 103.38 eV, which also indicates that sample Si-c is in the form of SiO₂ as shown in Figure 8b, Si 2p_1/2 and Si2p_3/2 in the Si 2s spectrum correspond to the Si–C and Si–O bonds, respectively [37]. A broad peak of O 1s can be observed at 532.88 eV (Figure 8c). We can see that the Si 2p spectrum and O 1s spectrum of sample Si-c after the reaction, where the higher binding energy (BE) of the nanoparticles before the reaction shifts to a lower BE, undergo a slight shift (not significant), which could be due to the fact that the plasma discharge contains many reactive species that lead to the formation of free radicals, and the complexation of free radicals reduces the surface charging effect of the sample, such that the reduction leads to a shift to a lower BE [38].

Figure 8

XPS Spectrum of Si-c before and after the reaction: (a) XPS survey spectra; (b) Si 2p; (c) O 1s.

3.5 Discharge characteristics

In the material-filled DBD plasma reactor, the discharge frequency, input power, and CO₂ flow rate were 9.0 kHz, 60 W, and 30 mL/min. Compared to the unfilled DBD plasma, the material-filled DBD had a more vigorous discharge intensity and produced more active components, such as high-energy electrons, to carry out the reaction. As can be seen in Figure 9, due to the filling of the material, the Lissajous diagram of DBD is a parallelogram with an increased area than when it is not filled, which indicates a change in the discharge state. The increase in discharge charge (Q _d = 0.5181) for the filled material compared to the unfilled discharge charge (Q _d = 0.4903) indicates that the DBD of the filled material facilitates the generation of more discharge charge, thus increasing the reactivity of the discharge region. The DBD filled with material also increases the peak-to-peak charge during the discharge process from Q _pk-pk = 0.6429–0.7076, which shows that the introduction of dielectric materials can effectively promote charge transfer in the discharge region and facilitate the physical and chemical reaction changes in the DBD [39].

Figure 9

Lissajous graphs of discharge system with or without material packing (material selection: Si-c, circulating water temperature = 20℃, input power = 60 W).

3.6 Mechanism analysis

In the case of no material filling, DBD only showed filamentary discharge distributed in the discharge area and whistling noise, while with material filling, DBD showed surface discharge and faint filamentary discharge. As shown in Table 1, the specific surface area increased with the addition of CTAB; the average pore size of all samples was in the mesopore range of 2–3 nm, while the pore volume of sample Si-c was larger than that of the other samples, reaching 1.095 cc/g. The larger specific surface area and more mesopore distribution may provide more surface active centers, which is favorable for CO₂ adsorption on the material surface, thus further accelerating the conversion of CO₂. The adsorption capacity of SiO₂ for CO₂ is weak compared with most materials. Therefore, we investigate some new studies on the properties of adsorption factor and specific surface area. In terms of adsorption factors, the sample Si-c-doped Al [40] was selected to study the effect of its enhanced CO₂ adsorption capacity on the CO₂ reduction performance of DBD, as can be found in Figure S3, the addition of Al reduces the performance activity of CO₂ reduction; this is probably because the addition of Al disrupts the characteristic pore structure of the material Si-c. On the other hand, it also proves that the CO₂ adsorption effect brought by Al is not sufficient for the change of properties brought by the pore structure of the material. In terms of specific surface area, the reported conventional silicas [34] was chosen. The specific surface area of this material is 345.93 m²/g, and the pore volume is 1.128 cm³/g, which is similar to the Si-c material in this article. After performance testing, Figure S4 reveals that the performance of conventional SiO₂ is much inferior to that of Si-c, indicating that the specific surface area has a significant effect on the performance effect of DBD synergistic materials. The depth of plasma penetration into the pore is influenced by a variety of factors, such as the size of the pore volume, the shape of the pore, and the density of the plasma. Suitable pores have a relatively large specific surface area, and the deposition of electrons on the dielectric surface creates a strong regional electric field at the bottom of the pore, resulting in ionization occurring mainly near or inside the pore [41,42]. The mechanism present in the DBD synergistic mesoporous SiO₂ reduction of CO₂ is as follows: the conversion of CO₂ in DBD plasma (Figure 10) is mainly excited by high-energy electrons, which are the main active particles in the decomposition of CO₂ in DBD plasmas, where CO₂ is decomposed into CO and O₂ by electron collisional dissociation reactions. After the DBD is filled with a dielectric material, the discharge mode changes from filament discharge to a combination of surface discharge and filament discharge, the intensity of the discharge area is enhanced, and the discharge is uniform and stable, which is conducive to the conversion of CO₂. In addition, the appropriate pore volume and specific surface area may have formed an intra-pore discharge, which further changes the regional electric field increasing the effective ionization volume and changing the ionization rate, which enhances the conversion reaction of CO₂.

Figure 10

Schematic diagram of mesoporous SiO₂ with DBD to decompose CO_2.

4 Conclusion

The synthesis of mesoporous SiO₂ with different morphologies by gel–solution method and construction of a material-assisted fixed-bed DBD system to study the mesoporous SiO₂ in DBD for the conversion of CO₂ was carried out. Characterization by XRD, FTIR, XPS, and SEM revealed that, as predicted, SiO₂ with mesoporous characteristics contributed to the conversion of CO₂. Among the SiO₂ prepared with different additions of CTAB, the addition of 1.05 g of SiO₂ had the highest conversion performance, and compared to the unfilled system, the conversion of CO₂, energy efficiency, and CO yield were increased by 56.73, 68.41, and 122.31%, respectively. The filling of mesoporous SiO₂ changed the discharge form and intensity of the discharge region, and the richer surface structure promoted the decomposition of CO₂ on its surface and the presence of filament discharge with suitable mesoporous material pore size, all three of which contributed to the performance of CO₂ conversion in DBD. This study provides an experimental basis for further understanding the mechanism of material synergy for CO₂ conversion in DBD. Although there is currently no capability to investigate the phenomenon of plasma discharge in pores in mesoporous materials, the synergistic reduction of CO₂ by mesoporous materials and DBD proves that it is feasible to probe the direction of mesoporous materials. This preliminary study provides a basis for selecting an additional material carrier for plasma catalytic technology in CO₂ conversion applications and, furthermore, for realizing the industrial application of plasma material on a large scale with cheap and active carriers and accelerating the use of CCU.

Acknowledgments

The authors acknowledge the support from the Natural Science Foundation of Jiangsu Province (BK20210983, BK20221405), the Changzhou Science and Technology Bureau (CQ20210103, CM20223017), the Zhongwu Scientific Research Air Pollution Detection Team (11830512201), the Talent Introduction Program of Jiangsu University of Technology (KYY19022, KYY19040), and Shuangchuang Ph.D award (from World Prestigious Universities).

Funding information: This study was supported by the Natural Science Foundation of Jiangsu Province (BK20210983, BK20221405), the Changzhou Science and Technology Bureau (CQ20210103, CM20223017), the Zhongwu Scientific Research Air Pollution Detection Team (11830512201), the Talent Introduction Program of Jiangsu University of Technology (KYY19022, KYY19040), and Shuangchuang Ph.D award (from World Prestigious Universities).
Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
Conflict of interest: The authors state no conflict of interest.
Supporting information: Catalyst preparation, Fig S1-S4.
Data availability statement: The authors confirm that the data supporting the findings of this study are available within the article.

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Received: 2023-03-10

Revised: 2023-06-20

Accepted: 2023-07-08

Published Online: 2023-09-05

This work is licensed under the Creative Commons Attribution 4.0 International License.

Supplementary material

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https://doi.org/10.1515/ntrev-2023-0577

Keywords for this article

DBD plasma; mesoporous SiO2; CO2 conversion

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BY 4.0

Performance and mechanism of CO2 reduction by DBD-coupled mesoporous SiO2

Article

Abstract

1 Introduction

2 Experimental

2.1 Material preparation

2.2 Characterization

2.3 Experimental methods

2.3.1 Reaction procedure

2.3.2 Calculation of discharge parameters

2.3.3 Calculation of reaction parameters

3 Results and discussion

3.1 Material characterization

3.1.1 XRD

3.1.2 TGA

3.1.3 SEM

3.1.4 BET

3.2 Catalytic performance analysis

3.3 FTIR analysis of material

3.4 XPS analysis

3.5 Discharge characteristics

3.6 Mechanism analysis

4 Conclusion

Acknowledgments

References

Supplementary Material

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Performance and mechanism of CO₂ reduction by DBD-coupled mesoporous SiO₂