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Article

Plasma-Enhanced Chemical Looping Oxidative Coupling of Methane through Synergy between Metal-Loaded Dielectric Particles and Non-Thermal Plasma

by
Shunshun Kang
1,2,3,4,
Jinlin Deng
1,2,3,4,
Xiaobo Wang
1,2,3,4,
Kun Zhao
1,2,3,4,*,
Min Zheng
5,
Da Song
1,2,3,4,
Zhen Huang
1,2,3,4,
Yan Lin
1,2,3,4,
Anqi Liu
1,2,3,4,
Anqing Zheng
1,2,3,4,* and
Zengli Zhao
1,2,3,4
1
Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China
2
CAS Key Laboratory of Renewable Energy, Guangzhou 510640, China
3
Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China
4
University of Chinese Academy of Sciences, Beijing 100049, China
5
State Key Laboratory of Complex Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming 650031, China
*
Authors to whom correspondence should be addressed.
Catalysts 2023, 13(3), 557; https://doi.org/10.3390/catal13030557
Submission received: 9 January 2023 / Revised: 27 February 2023 / Accepted: 3 March 2023 / Published: 10 March 2023
(This article belongs to the Special Issue Catalysts in Chemical Looping Technology for Energy Storage and Carbon Emission Reduction)
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Abstract

:
A plasma–catalyst hybrid system has been developed for the direct conversion of methane to C2+ hydrocarbons in dielectric barrier discharge (DBD) plasma. TiO2 presented the highest C2+ yield of 11.63% among different dielectric materials when integrated with DBD plasma, which made us concentrate on the TiO2-based catalyst. It was demonstrated that MnTi catalyst showed the best methane coupling performance of 27.29% C2+ yield with 150 V applied voltage, without additional thermal input. The catalytic performance of MnTi catalyst under various operation parameters was further carried out, and different techniques, such as X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and H2-temperature-programmed reduction were used to explore the effect of Mn loading on methane oxidative coupling (OCM) performance. The results showed that applied voltage and flow rate had a significant effect on methane activation. The dielectric particles of TiO2 loaded with Mn not only synergistically affected the coupling reaction, but also facilitated charge deposition to generate a strong local electric field to activate methane. The synergy effects boosted the OCM performance and the C2+ yield became 1.25 times higher than that of the undoped TiO2 under identical operating conditions in plasma, which was almost impossible to occur even at 850 °C on the MnTi catalyst in the absence of plasma. Moreover, the reaction activity of the catalyst was fully recovered by plasma regeneration at 300 °C and maintained its stability in for at least 30 consecutive cyclic redox tests. This work presents a new opportunity for efficient methane conversion to produce C2+ at low temperatures by plasma assistance.

1. Introduction

Light olefins, especially ethylene, are important organic raw materials, which are widely used in the production of plastics, drugs, fibers, etc. Ethylene is also regarded as a leading indicator in measuring the level of a country’s petrochemical industry and plays a pivotal role in the national economy. Because of its abundant reserves and clean characteristics [1,2], it is considered that methane will replace oil as the primary energy in the future. Therefore, a significant amount of manpower and material resources have been invested in the comprehensive utilization of natural gas [3,4,5]. The production of low-carbon olefins from methane has attracted the attention of researchers [6]. The oxidative coupling of methane (OCM) offers a feasible approach for the direct conversion of methane to hydrocarbons [7,8,9]. However, because of the high reactivity of oxygen, which easily reacts with reactants and products, the generation of by-products of CO and CO2 at high temperatures is unavoidable, and the C2+ yield of the OCM reaction is still limited to no more than 30%; thus, it is difficult to provide economic benefits in industry. On this basis, a replaceable technique known as chemical looping oxidative coupling of methane (CL-OCM) is investigated. As shown in Figure 1, the main advantage of CL-OCM is the replacement of gaseous oxygen with lattice oxygen in the catalyst (also known as the oxygen carrier), thus avoiding an excessive oxidation of methane. CL-OCM has become a hot research topic, since it can attain a higher C2 yield (between 20 and 30%) compared to traditional methods, including the direct conversion of methane and the non-oxidative coupling of methane (NCM) [10]. However, CL-OCM typically requires high-temperature reaction conditions due to the high C–H bond activation barrier of methane, resulting in a low catalyst stability and a high energy consumption. Therefore, it is necessary to develop more modest technologies to assist the CL-OCM strategy.
As a system containing high-energy electrons and exhibiting low-temperature properties, plasma can effectively activate stable molecules at relatively low temperatures, thus providing a new approach toward CL-OCM. Plasma can be considered as a quasi-neutral system consisting of electrons, ions, radicals, and excitations of gas molecules formed by the ionization and dissociation of gas molecules under external energy, such as an electric field. Using plasma, energetic electrons collide with the reactants and activate them, producing large quantities of highly reactive substances to initiate the reaction [11]. Thus, some stable small molecules, such as CH4 [12,13,14], CO2 [15,16,17,18] and N2 [19,20], can be effectively activated by plasma. Compared with the traditional thermal activation of methane, plasma shows a strong activation ability on methane [21]. Methane and products can avoid deep oxidation reactions at low temperatures (lower than 600 °C), which is beneficial to the selectivity of C2 hydrocarbon products. Therefore, research on the plasma activation of methane has become increasingly active in recent years. However, the gas-phase non-catalytic reaction initiated by plasma is mainly carried out through the free radical mechanism, which has little controllability of the product distribution [22].
To control the selectivity of products and enhance the performance of methane activation, catalysts are usually introduced into the plasma system. The addition of catalysts in the plasma can combine the characteristics of low-temperature activation of plasma with the characteristics of product orientation of the catalysts, which provides a high potential to produce value-added products. At present, several dielectric materials, such as TiO2 [23], Al2O3 [24,25], SiO2 [13,24], and ZSM [26,27], have been used for methane activation in the presence of plasma. Microelectrodes formed under an external electric field inside dielectric particles will enhance the electric field, thus improving the methane conversion [24]. Moreover, the adsorption of porous material can also make the reactants relatively concentrated, which is expected to significantly improve the discharge energy efficiency [25,28]. Liu et al. [29] found that the presence of plasma vibrationally excited methane appeared to be primarily responsible for the improvement of methane conversion in the presence of catalysts. Vibrationally excited methane made a significant contribution to the formation of the products without catalysts, even though a large amount of electric field energy was consumed [30]. This was mainly due to the high internal energy; the vibrationally excited methane was more easily dissociated on the catalyst to produce methyl radicals, thus increasing the methane conversion. Therefore, the energy to overcome the methane activation barrier originates from the electric field rather than the external heat during the plasma reaction, thus avoiding the high-temperature condition.
In order to integrate the advantages of various catalysts, a more ideal approach is to form a composite of high-performance catalysts and dielectric materials. In this way, the local electric field generated by the high permittivity of catalysts can enhance the activation of methane, while simultaneously improving the selectivity of C2 hydrocarbons [31,32]. However, so far, most of the studies on catalysts for methane oxidative coupling catalysts have been focused on thermal reactions at high temperatures, and few studies have reported their methane oxidation coupling activity under plasma conditions. Related studies have confirmed that catalysts with methane oxidation coupling performance at high temperatures also exhibit excellent OCM activity in the plasma [33]. Keller et al. [34] investigated the OCM activity of various metal oxides and found that manganese oxides presented excellent performance for C2 formation with a low cost and environmental friendliness. Cheng et al. [35] employed Mg6MnO8 with the use of a low concentration of Li dopant as the catalyst for CL-OCM. The experiment showed that Mn-based oxides with Li dopant attained C2 hydrocarbons with a selectivity of 50.4%. DFT calculations demonstrated that oxygen vacancies caused by Li-doping in Mg-Mn composite oxygen carriers were unfavorable for the adsorption of methyl radicals and, hence, increased the activation barrier of methyl radicals, thus significantly improving the selectivity of C2 hydrocarbon. Jiang et al. [36] applied Na-doped LaMnO3 as a catalyst in the CL-OCM process and found that Mn-based catalysts were well-suited catalysts, which could attain an ideal C2 selectivity and yield. Moreover, Na2WO4 is considered as a suitable catalyst for the OCM reaction [37]. Previously, transition metal oxides impregnated with Na2WO4 were experimentally found to inhibit the formation of COx in the reaction, indicating that the introduction of Na2WO4 could result in the growth of C2 selectivity [38]. Kiani et al. [39] prepared a Na2WO4/SiO2 catalyst via a modified impregnation method and found that the OCM reaction occurred at the dispersed phase Na-WO4 site. Due to the high thermal stability and superior C2 yield, Mn/Na2WO4/SiO2 is considered to be a promising catalyst to achieve commercial application [40,41]. Kim et al. [42] investigated the OCM activity of Mn/Na2WO4/TiO2, and found that a superior C2 yield was attained on Mn/Na2WO4/TiO2 compared to on Mn/Na2WO4/SiO2.
In this study, we investigated the effect of active metal loadings on different types of dielectric materials for methane conversion and C2+ production in dielectric barrier discharge (DBD) plasma at atmospheric pressure in chemical looping mode, with no oxygen co-feed. The regeneration process was carried out at 300 °C in an atmosphere of air under plasma. The impact of different input flows, temperatures, and applied voltages on the performance of the catalyst in the DBD reactor was also investigated. The catalysts were measured by X-ray diffraction (XRD) to confirm the crystal phases. H2-temperature programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS) were performed to investigate the properties of oxygen species on catalysts. TEM–EDS was carried out to determine the element distributions. Finally, it was found that the MnTi catalyst resulted in a clear C2+ hydrocarbons enhancement in plasma when compared with a catalysis-only system. This discovery enables the room-temperature conversion of methane and provides an effective guideline for the study of methane activation in low-temperature plasma.

2. Results and Discussion

2.1. Effect of Dielectric Materials

All experiments were performed in chemical looping mode, meaning that no oxygen co-feed was present. The metal amount was not varied. According to the literature reported on plasma-catalytic systems, methane conversion and product selectivity would vary with different dielectric materials. It was found that the microelectrodes were generated inside the dielectric materials under the external electric field, which could improve the capability of charge transfer [43]. In terms of chemical reactions, the charge transfer capacity can be converted into the ability to activate methane, meaning that various dielectric materials can enhance the activity of methane conversion. Figure 2 shows the catalytic performance of SiO2, Al2O3, and TiO2 dielectric materials for CL-OCM in DBD plasma. SiO2 exhibits the highest methane conversion of 37.75%, but contains CO and CO2 as the main products (with a CO and CO2 selectivity of 36.95% and 36.03%), resulting in a low C2+ yield of 9.12%, with a C2+ selectivity lower than 30%. While Al2O3 and TiO2 exhibit an improvement for C2+ selectivity (mainly ethane and propane as the main products), C2+ yields of 9.85% and 11.63% are obtained, respectively. Compared to Al2O3, TiO2 displays more remarkable oxidative coupling properties in plasma, which is illustrated by the further improvement in the selectivity of ethane and propane, and exhibits the highest C2+ yield among the used dielectric materials in this study. Thus, TiO2 is considered as an ideal support for CL-OCM in plasma.
In general, there is a positive correlation between the performance of catalysts and dielectric materials in the plasma–catalyst system. Since TiO2 exhibited the best catalytic performance among the supports under plasma conditions, different metal loadings on TiO2 support were further investigated. The catalytic performance of different metal loadings on TiO2 support for CL-OCM in DBD plasma is shown in Figure 3. It can be seen that the loading of active metals on TiO2 support contributes greatly to the increase in methane conversion and C2+ selectivity compared to pure TiO2 in the plasma-catalytic system. The most striking improvement is nearly 100% of C2+ selectivity, with dominant products being ethane and propane, which account for more than 70% of the total products. As stated in previous research, manganese oxide and Na2WO4 would take part in the dehydrogenation reaction of methane and result in the formation of methyl radicals, which was the essential initial step in the OCM reaction [40,44,45]. However, a remarkable synergy between Mn(NO3)2 and Na2WO4 in the methane conversion could not be observed in the plasma system. The catalytic performance of different metal loadings on TiO2 support in DBD plasma exhibits no obvious difference, and the order of activity of methane conversion and C2+ yield under different catalysts is MnTi > NaWTi > MNWTi. MnTi is chosen as the best catalyst judging from the C2+ yield.
To investigate the role of plasma in methane conversion at low temperatures, comparison experiments were conducted, as shown in Figure 4. The C2+ yield of 19.78% can be obtained in the presence of plasma without additional thermal input, while the methane oxidative coupling performance of the MnTi catalyst is almost negligible, even at 850 °C on the MnTi catalyst without plasma. It generally known that C–H bond breakage of CH4 cannot occur at ambient temperatures due to the high activation barrier; however, this can be enabled under plasma conditions. For the reaction under plasma conditions, electron collisions result in the formation of many excited state species with high internal energy. These excited state species have a lower C–H activation barrier, resulting in C–H breakage on the catalyst surface at a lower temperature. Thus, plasma plays a major role in activating the methane reaction, thus allowing the methane coupling reaction to occur at low temperatures.

2.2. Effect of Flow Rate

Figure 5 shows the effect of different flow rates on the catalytic performance of the MnTi catalyst in the presence of DBD plasma. We can see that C2+ selectivity increases continuously with increasing input gas flow rate, but the selectivity of different products varies with the flow rate in a different trend. The selectivity of C2H4 and C2H2 increases with the increase in flow rate, while the selectivity of C2H6 and C3H8 remains stable. It is worth noting that the by-products of CO and CO2 clearly decrease as the input flow increases, demonstrating the opposite effect of high input flow on C2+ selectivity. It could be inferred that under the condition of a high input flow, the free radicals formed by the ionization of CH4 molecules under an electric field are easier to desorb from the surface of the catalyst into the gas phase, and avoid the subsequent excessive oxidation, resulting in the improvement of C2+ selectivity. However, from Figure 5a it can also be seen that methane conversion decreases with the increase in input flow, which is mainly because the high space velocity decreases the gas residence time and the conversion [31,46,47]. A higher residence time would increase the probability of collision between methane and high-energy electrons, thus improving the conversion of methane. Comprehensively considering the methane conversion and C2+ selectivity, the flow rate of 60 mL/min is found to be the optimum flow rate on the MnTi catalyst. The methane conversion reaches a maximum of 20.72% and C2+ products achieve the highest yield of 20.03%.

2.3. Effect of Temperature

The variation of methane conversion and product selectivity with different temperatures is shown in Figure 6. A temperature of 200 °C is chosen as the upper limit to guarantee a reliable carbon balance. In light of the experimental results, the conversion of methane shows no significant difference. According to the previous literature, many efforts have been made to explore the effect of temperature on methane activation, but the relationship between temperature and catalytic performance is still controversial [33,34,48,49]. A similar result was observed by Liu et al., and the negative influence of temperature on methane conversion was attributed to the fact that a temperature rise favored the dehydration of the catalyst, which resulted in a decrease in OH− concentration [33]. It was believed that hydroxyl species played a positive role in the activation of methane under an electric field. Similarly, from Figure 6b, it can also be found that the rise in temperature has little consequences for the distribution of the products. Therefore, the effect of temperature below 200 °C on methane conversion in DBD plasma can be considered negligible. This is because the dissociation energy of the C–H chemical bond is high up to 434 kJ/mol, which is almost impossible to be activated below 500 °C in traditional thermal reactions. Therefore, the breakage of the C–H bond in methane and subsequently generated methyl radicals is mainly initiated by high-energy electron collisions of plasma under 500 °C, rather than by additional thermal input.

2.4. Effect of Applied Voltage

Figure 7 shows the catalytic performance of the MnTi catalyst in DBD plasma at different applied voltages, and 200 V is chosen as the upper limits to guarantee a reliable carbon balance. The conversion of methane increases with the increase in applied voltage. Lissajous plots of different applied voltages are shown in Figure 8, and the discharge power is calculated by the integral area of the Lissajous plots. According to the calculation, the increase in applied voltage will increase the discharge power, thus generating more high-energy electrons. The increased number of high-energy electrons may lead to a higher probability of bond breakage between carbon and hydrogen in the methane molecules, thus improving the degree of methane dissociation and conversion, which appears to be primarily responsible for a higher CH4 activation [47]. However, the product distribution is independent of applied voltage and shows little difference with the variation of applied voltage, as shown in Figure 7b. Noticeably, the C2+ yield reaches a maximum of 30.04% at an applied voltage of 200 V, but the carbon balance of 85.28% is not perfect (the carbon balance of different applied voltages is displayed in Figure 9), indicating that the coke is formed due to methane cracking [31]. However, we still could obtain a 27.29% C2+ yield at a reliable carbon balance (91.4%) at 150 V applied voltage.

3. Characterization

3.1. XRD

The XRD patterns of the as-prepared catalysts are shown in Figure 10. According to the XRD patterns, all the characteristic diffraction peaks of TiO2 correspond well with the anatase phase in the JCPDS database (PDF-#84-1286). Moreover, new diffraction peaks also appear in the fresh MnTi catalyst with a structural change, indicating the emergence of a new phase after the loading of manganese nitrate. By comparing with the JCPDS database, these new diffraction peaks can be assigned to the rutile phase, which becomes the most dominant crystalline phase of the catalyst, judging from the intensity of the diffraction peaks. This change in crystalline phase structure is consistent with the thermal behavior of titanium dioxide [50]. The rutile phase is still the most dominant crystalline phase structure of the catalyst when the MnTi catalyst is reduced by CH4, confirming the stability of the catalyst structure. It is worth noting that no characteristic diffraction peak for Mn in XRD patterns is observed, which might be due to low Mn loading or the high dispersion of Mn in the catalyst.

3.2. XPS

According to previous research, oxygen species play an important role in the process of oxidative coupling of methane [48]. Thus, it is necessary to explore the oxygen species on the catalyst surface, which can further explain the reasons for the improvement in methane activation and C2 hydrocarbon selectivity through the synergetic effects between the catalyst and the plasma. The surface compositions and chemical states of the oxygen species were explored by XPS measurements, as displayed in Figure 11. For each TiO2-based catalyst, four peaks are used for the fitting of the O 1s spectra. The peak at a lower binding energy between 528.69 eV and 529.61 eV is assigned to surface lattice oxygen (O2−) [51]; the second peak existing between 531.65 eV and 529.68 eV is attributed to the surface adsorbed oxygen species (O22−, O−) [36]; the third peak with a binding energy of about 533.11–531.48 eV results from hydroxyls species OH− [52]; and the forth peak with a binding energy higher than 534.0 eV is ascribed to adsorbed molecular water species H2O [52]. The composition of the surface O species is displayed in Table 1. Among them, lattice oxygen species are considered as non-selective oxygen species, which contribute to the complete oxidation of methane and the generation of COx by-products, thus limiting C2+ selectivity [36]. Compared with TiO2, it can be seen that Mn loading clearly increases the concentration of hydroxyl species, whilst simultaneously reducing the concentration of lattice oxygen. The decrease in lattice oxygen species indicates the increase in selective oxygen species, which favors the oxidative coupling of methane. Moreover, evidence from the literature also shows that hydroxyl species OH− would react with different oxygen species to form catalytic active sites for OCM, which could significantly improve methane activation and C2 selectivity. Moreover, the presence of hydroxyl species could create a micro-discharge on the catalyst surface, which further facilitates the activation of methane [30,33,45]. Compared to fresh MnTi catalyst, the concentration of the surface adsorbed oxygen species (O22−, O−) decreases after reduction with CH4, confirming that the surface adsorbed oxygen species (O22−, O−) takes part in the oxidative coupling of methane. Moreover, it can be observed that there is a peak shift of the oxygen species towards a low binding energy when compared to pure TiO2 catalyst, indicating the improvement in oxygen mobility and the formation of oxygen vacancies on the MnTi catalyst. The existence of oxygen vacancies can reduce the adsorption energy of methyl radicals, thereby increasing the selectivity for C2+ hydrocarbons.

3.3. TEM–EDS

TEM-EDS was carried out to explore the dispersion of Mn on the fresh MnTi catalyst, as displayed in Figure 12. The TEM images of the fresh MnTi catalyst reflect that Mn nanoparticles are dispersed uniformly on rod TiO2 and no obvious Mn clusters can be found, confirming the high dispersion of Mn element on the catalyst surface. EDS mapping also demonstrates that Mn is distributed uniformly on the fresh MnTi catalyst. This explains the absence of Mn element in the XRD pattern. As stated in previous research, the stability and reaction activity of catalysts are closely related to the dispersion of the loaded metal. A higher dispersion of the metal would expose more active site edges or corners, which are more reactive on the metal surface, and hence be more conducive to improving the reactivity of the catalyst in different reactions [53]. Therefore, a higher Mn dispersion will be more favorable for the formation of C2+ hydrocarbons due to the large number of unsaturated active sites that are more active for the breakage of C-H bonds.

3.4. H2-TPR

The reduction properties correlated with oxygen release characteristics of catalysts were tested by H2-TPR technology, as shown in Figure 13. It can be observed that TiO2 presents the low reductive property between 50 to 300 °C, and an obvious reduction peak of TiO2 emerges from 300 °C and exhibits a peak at 625 °C, owing to the reduction of O22− species at 387 °C and coordinatively unsaturated Ti3+ at 625 °C, respectively [51]. However, in the MnTi catalyst profile, a new reduction peak region emerges at ~551 °C, which contributes to the reduction process of Mn2O3 [36,54]. It was reported that Mn2O3 played a crucial role in enhancing the OCM activity of catalysts, which could take part in the hydrogen abstraction from CH4 and trigger CH4 activation. Moreover, the total reduction peak area increases as Mn is induced, confirming that the catalytic performance of the catalyst is enhanced. It is important to note that the reduction peaks of Ti3+ shift from 625 °C to 551 °C after Mn is loaded, demonstrating the improvement in oxygen release. This also proves, to some extent, that the formation of oxygen vacancies facilitates the formation of C2 hydrocarbon products.
A possible reaction mechanism for CL-OCM on the MnTi catalyst is displayed in Figure 14. Because of the high relative permittivity, TiO2 particles are polarized in the plasma discharge and a local electric field is formed around the particle contact point, causing an increase in methane conversion. Additionally, manganese oxide will take part in the hydrogen abstraction of methane, which is the first step of CL-OCM. In the traditional thermo-catalytic reaction, methane activation requires a high temperature due to the high activation barrier of the C-H bond. For the reaction in plasma, CH4 activation is initiated by electronic collision to form substantial species, including radicals, ions, and vibrationally or electronically excited species. The existence of vibrationally excited methane appears to be primarily responsible for the improvement in methane conversion and C2+ selectivity in the presence of catalysts. Compared to the thermo-catalytic reaction, vibrationally excited methane molecules with elevated internal energy have a lower C-H activation barrier, and can, thereby, be activated on the Mn site at a much lower temperature, demonstrating that the combination of TiO2 with the Mn species has a synergistic effect for CL-OCM reaction. Moreover, Mn loading can increase the amounts of hydroxyl species and oxygen vacancies, which can create a micro-discharge on the catalyst surface, thus facilitating the activation of methane. The increase in oxygen vacancies can reduce the adsorption energy of methyl radicals, thereby increasing the selectivity for C2+ hydrocarbons. It is noteworthy that coking becomes severe with increasing reduction time, which indicates that the redox-active lattice is consumed relatively quickly.

3.5. Chemical Looping Redox Experiments

Since the stability and recyclability of catalysts are of utmost importance for catalysts in industrial chemical looping applications, 30 consecutive cyclic redox tests were explored on the MnTi catalyst. The catalytic performance of the catalyst on methane conversion, C2+ selectivity, and C2+ yield in the plasma–catalyst hybrid system is displayed in Figure 15. It can be seen that methane conversion and C2+ yield remain stable at average values of ~21% and ~20%, respectively, while the C2+ products selectivity simultaneously stays above 95% during 30 consecutive cyclic redox tests, demonstrating that the reactivity of the MnTi catalyst is fully recovered after the regeneration by plasma, and it presents good regeneration ability in the plasma–catalyst hybrid system. Moreover, compared to thermal regeneration, plasma regeneration could realize catalyst regeneration at a lower temperature (300 °C). According to relevant literature, the good regeneration of catalysts in plasma could be ascribed to the fact that reactive oxygen species were formed [48]. Reactive oxygen species, on the one hand, reacted more easily with carbon deposition to generate CO and CO2, avoiding catalyst deactivation; on the other hand, they oxidized the reduced catalyst to recover OCM performance.

4. Experimental

4.1. Preparation of the Catalyst

5%Mn(NO3)2-TiO2 (MnTi), 10%Na2WO4-TiO2 (NWTi), and 5%Mn(NO3)2 + 10%Na2WO4-TiO2 (MnNWTi) used in this study were prepared by the impregnation method, using aluminum oxide (Al2O3, Maclin), silica sand (SiO2, 6–10 mesh, Maclin), anatase titanium oxide (TiO2, 99.0% pure, Maclin), manganese nitrate hydrate (Mn(NO3)2, 50 wt% in water, Maclin), and sodium tungstate dihydrate (Na2WO4·2H2O, 99.5% pure, Maclin). To obtain the MnTi catalyst, TiO2 was impregnated with aqueous solutions of Mn(NO3)2 as precursors, yielding 5 wt% Mn(NO3)2, and then dried at 105 °C until the water evaporated completely. Finally, the catalyst was calcined at 850 °C for 6 h and the MnTi catalyst was obtained. The production process of MnNWTi and NWTi followed the same procedure as described above.

4.2. Material Characterization

The prepared catalysts were measured by X-ray diffraction (XRD) using a Japan Science D/max-R Diffractometer (Tokyo, Japan) with Cu Kα radiation (40 kV, 40 mA, λ = 0.15406 nm) to confirm the structural features and crystal phases. The catalysts were scanned with a 2θ range of 10~80° at a rate of 10°/min.
The valence state of the oxygen species of fresh and reduced catalysts was investigated by X-ray photoelectron spectroscopy (ESCALAB 250Xi, Thermo Fisher Scientifific Inc, Waltham, MA, USA) with an Al Ka X-ray source (hv = 1486.6 eV) operating under the conditions of 20 kV and 10 mA.
The microstructure and lattice parameters of the catalyst were determined by transmission electron microscopy (JEO-2100F, JEOL, Akishima, Japan), and elemental distribution was confirmed by energy dispersive spectroscopy. Before analysis, the catalyst was treated in ethanol by ultrasonic dispersion for 20 min to obtain a uniform suspension.
H2-temperature programmed reduction (H2-TPR) technology was employed to determine the reducibility of catalysts with a Quantachrome ChemStar™ (Graz, Austria) instrument rigged with a thermal conductivity detector (TCD). Catalyst powder with a weight of 50 mg was loaded in a U-shape quartz tube. In order to remove water molecules and air molecules adsorbed on the catalyst surface, the catalyst was kept at 300 °C for 1 h under a helium atmosphere as a pretreatment. The heating rate of the catalysts was 10 °C/min and the flow rate of He carrier gas was 35 mL/min. After pretreatment, the catalyst was heated from 50 °C to 900 °C at a heating rate of 10 °C/min, in 10% H2 and 90% Ar at a flow rate of 30 mL/min.

4.3. Catalytic Test

The CL-OCM experiment was tested with a plasma-catalytic DBD reactor system at atmospheric pressure, as displayed in Figure 16. The DBD reactor was an alundum tube (O.D: 25 mm, I.D: 20 mm) consisting of a stainless-steel rod as the inner electrode centered in the reactor. The high-frequency discharge probe of the plasma generator (CTP-2000K, Nanjing, China), whose panel could read the input voltage, and voltage regulator were directly connected to the stainless-steel rod, and the outer electrode was connected to the outer capacitor (or resistance). The discharge parameters were recorded using a digital oscilloscope (DPO 2024B, Tektronix) to draw Lissajous plots for the calculation of the output power of plasma. The calculation formula is as follows:
P(out) = f × C × k × kx × ky × S
In this experiment, f = 10.04 kHz, C = 0.47 µF, k = 1000, kx, ky = 1, and S is the integral area of the Lissajous plots.
The catalyst pellets with a particle size of 20~50 mesh (2.0 g) were charged into the stainless mesh (>300 mesh) at the bottom of the stainless-steel rod. The stainless-steel mesh was placed in the grid area outside the alumina tube to ensure that the catalyst was within the discharge area. All experiments were conducted at atmospheric pressure. During the reaction step, the DBD reactor was set at the experimental temperature and pure N2 was used for purging. The reaction gas (40% volume fraction CH4, balance Ar) flowed through the reactor using a mass flow controller. After the introduction of CH4, the plasma was turned on for 10 min and the gaseous products were collected. The regeneration process was carried out at 300 °C for 10 min in conditions of 21% O2 balanced with N2 under plasma with an applied voltage of 100 V. The gaseous products were analyzed by a gas chromatograph (GC-2014, Shimadzu in Japan).
The conversion of methane, selectivity, and the yield of the C2 product were determined by the following Equations (1)–(3):
X ( C H 4 ) ( % ) = m o l e s   o f   C H 4   c o n v e r t e d m o l e s   o f   C H 4   f e e d   ×   100
S ( Cx ) ( % ) = x × m o l e s   o f   C x   p r o d u c e d m o l e s   o f   C H 4   c o n v e r t e d   ×   100
Y ( C x ) ( % ) = x × m o l e s   o f   C x   p r o d u c e d m o l e s   o f   C H 4   f e e d   ×   100

5. Conclusions

In this study, the catalytic performance of different dielectric materials on methane oxidative coupling reaction was investigated in a DBD reactor. TiO2 presented the best methane coupling performance among the various dielectric materials studies, such as SiO2, Al2O3, and TiO2. Hence, different active metal loadings on TiO2 were tested. In light of the experimental results, metal loadings were capable of increasing methane conversion while simultaneously improving the C2+ selectivity. Among all tested catalysts, the MnTi catalyst was considered as the desirable catalyst judging from methane conversion and C2+ production. The high Mn dispersion, which was more conducive to improving the reactivity of the catalyst, was confirmed by TEM–EDS and XRD analysis. Moreover, the XPS spectra for O 1s revealed that Mn loading increased the concentration of hydroxyl species and reduced the concentration of non-selective oxygen species, which resulted in the growth of methane conversion and C2+ selectivity. Finally, H2-TPR spectra showed that Mn loading supplied more oxygen species, which was also beneficial for the formation of methyl radicals and C2+ hydrocarbons.
We further optimized various operation parameters to explore the OCM activity of the MnTi catalyst in plasma under different flow rates, applied voltages, and temperatures. A higher methane conversion was realized by increasing applied voltage and lowering flow rates. Moreover, the variation of operation parameters showed little effect on the product distribution, indicating that the selectivity of products was mainly affected by the properties of the catalysts rather than the operation parameters. Finally, excellent regenerative properties of the MnTi catalyst during chemical looping were confirmed through 30 consecutive cyclic redox tests, showing a methane conversion of about 21%, a 96% C2+ hydrocarbons selectivity, and a 20% C2+ hydrocarbon yield. Overall, the combination of TiO2 with a high Mn metal dispersion in plasma reactors shows excellent OCM activity, which is a novel technology that achieves high catalytic properties and recyclability in methane oxidation coupling reactions.

Author Contributions

S.K.: Data curation, Formal analysis, Writing—original draft. J.D.: Data curation. X.W.: Writing—Review and editing. K.Z.: Conceptualization, Methodology, Funding acquisition, Writing—review and editing. M.Z.: Data curation. D.S.: Formal analysis. Z.H.: Writing—Review and editing. Y.L.: Formal analysis. A.L.: Formal analysis. A.Z.: Writing—Review and editing. Z.Z.: Writing—Review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (51876205, 22279144, 51866003); the Youth Innovation Promotion Association, CAS (2019341); and the Yunnan Basic Research Program Project (2019FB071).

Data Availability Statement

Not applicable.

Conflicts of Interest

There is no conflict of interest.

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Catalysts 13 00557 g001 550
Figure 1. CL-OCM process.
Figure 1. CL-OCM process.
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Figure 2. Performance of different dielectric materials in DBD plasma. (a) Oxidative coupling performance; (b) product distribution (temperature: ambient temperature, flow rate: 60 mL/min, applied voltage: 100 V).
Figure 2. Performance of different dielectric materials in DBD plasma. (a) Oxidative coupling performance; (b) product distribution (temperature: ambient temperature, flow rate: 60 mL/min, applied voltage: 100 V).
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Catalysts 13 00557 g003 550
Figure 3. Performance of different metal loadings in DBD plasma. (a) Oxidative coupling performance; (b) product distribution (temperature: ambient temperature, flow rate: 60 mL/min, applied voltage: 100 V).
Figure 3. Performance of different metal loadings in DBD plasma. (a) Oxidative coupling performance; (b) product distribution (temperature: ambient temperature, flow rate: 60 mL/min, applied voltage: 100 V).
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Catalysts 13 00557 g004 550
Figure 4. Catalytic performance of the MnTi catalyst with plasma and without plasma (flow rate: 60 mL/min, applied voltage: 100 V).
Figure 4. Catalytic performance of the MnTi catalyst with plasma and without plasma (flow rate: 60 mL/min, applied voltage: 100 V).
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Catalysts 13 00557 g005 550
Figure 5. Effect of flow rate on the catalytic performance of the MnTi catalyst in DBD plasma. (a) Oxidative coupling performance; (b) product distribution (temperature: ambient temperature, applied voltage: 100 V).
Figure 5. Effect of flow rate on the catalytic performance of the MnTi catalyst in DBD plasma. (a) Oxidative coupling performance; (b) product distribution (temperature: ambient temperature, applied voltage: 100 V).
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Catalysts 13 00557 g006 550
Figure 6. Effect of temperature on the catalytic performance of the MnTi catalyst in DBD plasma. (a) Oxidative coupling performance; (b) product distribution (flow rate: 60 mL/min, applied voltage: 100 V).
Figure 6. Effect of temperature on the catalytic performance of the MnTi catalyst in DBD plasma. (a) Oxidative coupling performance; (b) product distribution (flow rate: 60 mL/min, applied voltage: 100 V).
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Catalysts 13 00557 g007 550
Figure 7. Effect of applied voltage on the catalytic performance of the MnTi catalyst in DBD plasma. (a) Oxidative coupling performance; (b) product distribution (temperature: ambient temperature, flow rate: 60 mL/min).
Figure 7. Effect of applied voltage on the catalytic performance of the MnTi catalyst in DBD plasma. (a) Oxidative coupling performance; (b) product distribution (temperature: ambient temperature, flow rate: 60 mL/min).
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Catalysts 13 00557 g008 550
Figure 8. Lissajous plots of different applied voltages on the MnTi catalyst.
Figure 8. Lissajous plots of different applied voltages on the MnTi catalyst.
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Figure 9. Carbon balance of different applied voltages on the MnTi catalyst.
Figure 9. Carbon balance of different applied voltages on the MnTi catalyst.
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Figure 10. XRD patterns of (a) TiO2, (b) fresh MnTi catalyst, and (c) reduced MnTi catalyst.
Figure 10. XRD patterns of (a) TiO2, (b) fresh MnTi catalyst, and (c) reduced MnTi catalyst.
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Figure 11. XPS scans on O 1s for (a) TiO2, (b) fresh MnTi catalyst, and (c) reduced MnTi catalyst.
Figure 11. XPS scans on O 1s for (a) TiO2, (b) fresh MnTi catalyst, and (c) reduced MnTi catalyst.
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Figure 12. EDS mapping of the fresh MnTi catalyst (a) TEM images of MnTi catalyst, (b) Ti element distribution, and (c) Mn element distribution.
Figure 12. EDS mapping of the fresh MnTi catalyst (a) TEM images of MnTi catalyst, (b) Ti element distribution, and (c) Mn element distribution.
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Catalysts 13 00557 g013 550
Figure 13. H2-TPR profiles.
Figure 13. H2-TPR profiles.
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Figure 14. Possible reaction mechanism for CL-OCM on the MnTi catalyst.
Figure 14. Possible reaction mechanism for CL-OCM on the MnTi catalyst.
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Figure 15. CLOCM of the MnTi catalyst (temperature: ambient temperature, flow rate: 60 mL/min, applied voltage: 100 V).
Figure 15. CLOCM of the MnTi catalyst (temperature: ambient temperature, flow rate: 60 mL/min, applied voltage: 100 V).
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Figure 16. Reaction equipment diagram (the part on the bottom right is a magnification of the catalyst placement).
Figure 16. Reaction equipment diagram (the part on the bottom right is a magnification of the catalyst placement).
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Table
Table 1. Composition of the surface O species for fresh and spent MnTi catalyst and TiO2.
Table 1. Composition of the surface O species for fresh and spent MnTi catalyst and TiO2.
SampleOxygen Distribution/%
O2−O22−/O−OH−H2O
TiO221.9437.9636.054.06
Fresh 5%Mn(NO3)2-TiO217.0228.4145.338.63
Reduced 5%Mn(NO3)2-TiO217.8222.7349.2610.18
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Kang, S.; Deng, J.; Wang, X.; Zhao, K.; Zheng, M.; Song, D.; Huang, Z.; Lin, Y.; Liu, A.; Zheng, A.; et al. Plasma-Enhanced Chemical Looping Oxidative Coupling of Methane through Synergy between Metal-Loaded Dielectric Particles and Non-Thermal Plasma. Catalysts 2023, 13, 557. https://doi.org/10.3390/catal13030557

AMA Style

Kang S, Deng J, Wang X, Zhao K, Zheng M, Song D, Huang Z, Lin Y, Liu A, Zheng A, et al. Plasma-Enhanced Chemical Looping Oxidative Coupling of Methane through Synergy between Metal-Loaded Dielectric Particles and Non-Thermal Plasma. Catalysts. 2023; 13(3):557. https://doi.org/10.3390/catal13030557

Chicago/Turabian Style

Kang, Shunshun, Jinlin Deng, Xiaobo Wang, Kun Zhao, Min Zheng, Da Song, Zhen Huang, Yan Lin, Anqi Liu, Anqing Zheng, and et al. 2023. "Plasma-Enhanced Chemical Looping Oxidative Coupling of Methane through Synergy between Metal-Loaded Dielectric Particles and Non-Thermal Plasma" Catalysts 13, no. 3: 557. https://doi.org/10.3390/catal13030557

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