Plasma Promotes Dry Reforming Reaction of CH 4 and CO 2 at Room Temperature with Highly Dispersed NiO/ γ -Al 2 O 3 Catalyst

Plasma Promotes Dry Reforming Reaction of CH 4 and CO 2 at Room Temperature with Highly Dispersed -Al 2 3 Catalyst. Abstract: Plasma is an efﬁcient method that can activate inert molecules such as methane and carbon dioxide in a mild environment to make them reactive. In this work, we have prepared an AE-NiO/ γ Al 2 O 3 catalyst using an ammonia-evaporation method for plasma promoted dry reforming reaction of CO 2 and CH 4 at room temperature. According to the characterization data of XRD, H2-TPR, TEM, XPS, etc., the AE-NiO/ γ -Al 2 O 3 catalyst has higher dispersion, smaller particle size and stronger metal-support interaction than the catalyst prepared by the traditional impregnation method. In addition, the AE-NiO/ γ -Al 2 O 3 catalyst also exhibits higher activity in dry reforming reaction. This work provides a feasible reference experience for the research of plasma promoted dry reforming reaction catalysts at room temperature. In we found that the XRD peaks of the AE-NiO/ γ -Al 2 O 3 catalyst and TI-NiO/ γ -Al 2 O 3 catalyst are slightly larger compared to the support γ -Al 2 O 3 . That may be caused by impregnation and calcination during the preparation of the catalyst.


Introduction
Increasing global energy demand and fossil fuel consumption have brought a lot of carbon dioxide (CO 2 ) emissions and environmental pollution problems [1]. According to the latest data, the concentration of CO 2 in the atmosphere has reached 410 ppm [2]. The excessively high CO 2 concentration in the atmosphere has caused significant environmental problems such as the greenhouse effect and brings hidden dangers to the progress and development of human society [3,4].
Therefore, considerable efforts have been made to limit CO 2 emissions and reduce the concentration of CO 2 in the atmosphere, such as carbon capture, utilization and storage [5,6], catalytic conversion of CO 2 and eliminate and utilize carbon dioxide [7]. However, to optimize the structure of energy consumption and satisfy the demand of economic feasibility, the strategies of CCUS is not suitable for large-scale promotion. However, the catalytic conversion of CO 2 into value-added chemicals has attracted more and more attentions, especially the reaction of Dry Reforming of Methane (DRM) [8].
The DRM (Equation (1)) is use two main greenhouse gases (CO 2 and CH 4 ) as reaction materials, which can not only use the CO 2 in the atmosphere as a raw material, but it can also bring economic benefits. Moreover, the H 2 and CO were generated from the dry reforming process with a low H 2 /CO molar ratio about to 1, which is an ideal raw material for carbonylation and Fischer-Tropsch synthesis [9][10][11].
In the conventional dry reforming reaction research, since CO 2 and CH 4 are very stable inert molecules (E (C=O) = 803 kJ/mol, E (C-H) = 413 kJ/mol), the dry reforming reaction usually needs a high temperature (>700 • C) to overcome the thermodynamic limitations [12]. The high temperature reaction conditions are bound to cause high energy consumption and catalyst deactivation [13]. Han et al. reported that the conversion of CO 2 and CH 4 decreased to 51% and 42% from 81% and 64% using the Ni/γ-Al 2 O 3 as catalyst under the condition of 800 • C, and the flow of 20 mL/min after maintaining 5 h [14]. Traditional Ni-based catalysts are very prone to decrease in activity or even inactivation due to high temperature during the dry reforming reaction. Son et al. studied the coke formation over Ni/γ-Al 2 O 3 in the DRM reaction. In addition, they found the coking rate is 3.38 mg c /g cat ·h while the conversion of CO 2 and CH 4 decreased to 84.6% and 81.2% from 97.3% and 96.5% with 850 • C [15]. The carbon deposition of catalyst is an important factor hindering the stability of the catalyst, especially the carbon deposition on the active components caused by methane cracking reaction (Equation (2)) and Boudouard reaction (Equation (3)).
Despite the traditional thermal dry reforming reaction research is relatively extensive and there has been a small-scale industrial production, but they still suffer from problems such as carbon deposition and catalyst deactivation due to high temperature reaction conditions [16].
In recent years, researchers have taken many measures to reduce the temperature of the dry reforming reaction, such as nonthermal plasma technology (NTP). NPT can activate gas molecules at bulk gas temperature without additional heating to generate highly reactive electrons, ions, and various charged groups [17]. Currently, NTP mainly includes dielectric barrier discharges (DBD), gliding arcs discharge, glow discharge and corona discharges [18]. Among them, the ease of operation and easy upgradability of DBD plasma has attracted a lot of attention. DBD plasma can generate high-energy electrons at room temperature and atmospheric pressure due to its non-equilibrium character. The energy of high-energy electrons is usually between 1 to 10 eV, which is sufficient to activate some inert molecules such as CH 4 (4.5 eV) and CO 2 (5.5 eV) [19,20]. Accordingly, DBD plasma can activate some thermodynamically restricted reactions at a low temperature [21].
Generally, DBD plasma achieves the reaction enthalpy (∆H) required for DRM through the combination of low temperature and Gibbs energy [22], and can cooperate with the catalyst to promote the progress of CO 2 and CH 4 reforming reactions [23]. Song et al. combined 7 wt.% Ni/γ-Al 2 O 3 catalyst with DBD plasma into the DRM without any heating, the conversion of CO 2 and CH 4 were 32.6% and 55.5% at the discharge power of 130 W, total flow rate of 30 mL/min and the CH 4 /CO 2 ratio of 1 [24]. Tu et al. applied 10 wt.% Ni/γ-Al 2 O 3 into the reaction of methane reforming at the discharge power of 60 W, resulting the conversion of CO 2 and CH 4 were 26.2% and 44.1% [25]. In addition, many studies have shown that the reforming reaction of CO 2 and CH 4 combined with plasma at room temperature can also generate hydrocarbons, oxides and liquid hydrocarbons in addition to syngas [26][27][28][29]. Although the problems such as low conversion rate and complicated products still exist, the addition of plasma provides novel sights to lower the temperature of the methane reforming reaction, which deserves further research.
In this work, all catalysts were packed around the high-voltage electrode in the middle part of the discharge zone of the reactor in order to obtain the coupling effect of catalyst and plasma [30].

Effect of Preparation Methods on Catalyst Performance
TI-NiO/γ-Al 2 O 3 and AE-NiO/γ-Al 2 O 3 were all synthesized and evaluated under the same condition. As shown in Figure 1a,b, when there was only plasma with no catalyst filled, the conversion of CO 2 and CH 4 are very low. However, with the addition of catalyst, the coupling effect of catalyst and plasma can promote the conversion of CO 2 and CH 4 significantly. Many studies have proved that the combination of catalyst and plasma can significantly promote the dry reforming reaction of CO 2 and CH 4 at room temperature [25,31,32]. In this work, compared to the TI-NiO/γ-Al 2 O 3 , the AE-NiO/γ-Al 2 O 3 catalyst combined with plasma can better promote the conversion of CO 2 and CH 4 . It was worth noting that when the input power was controlled to 140 W, the plasma combined AE-NiO/γ-Al 2 O 3 can convert 80.3% and 86.4% of CO 2 and CH 4 while the plasma combined with TI-NiO/γ-Al 2 O 3 only were 23.4% and 34%. Whether the AE-NiO/γ-Al 2 O 3 catalyst or the TI-NiO/γ-Al 2 O 3 catalyst was used, the conversion of CO 2 and CH 4 under the effect of the plasma was significantly higher than that when only the plasma was used. It also showed that in the plasma-catalyzed dry reforming reaction, the catalyst is still the key to determining the reaction efficiency.
In addition to the conversion of CO 2 and CH 4 , the selectivity of AE-NiO/γ-Al 2 O 3 to the CO and H 2 was also slightly higher than that of TI-NiO/γ-Al 2 O 3 . As shown in Figure 1c,d, when the input power was 140 W, the selectivity of AE-NiO/γ-Al 2 O 3 to CO and H 2 reached at 44.3% and 43.6%, while the TI-NiO/γ-Al 2 O 3 38.1% and 38.6%, respectively. There are also individual exceptions, such as the selectivity of H 2 without catalyst was higher than that with catalyst, when the input power was 80 W. That may be caused by the cleavage of CH 4 to generate the H 2 and other free radicals (Equations (4)-(6)) in the absence of a catalyst. As shown in Figure S1a,b, when the input power was 140 W, the yield of CO and H 2 were reached at 37% and 37.7%, respectively. There was a H 2 /CO ratio of 1.11, which is suitable for F-T synthesis [33].
The symbols (*) and ( + ) correspond to the excited and ionic states, respectively. It can be seen from the Figure S1a, during the dry reforming reaction with plasma promoted, with the input power increasing, the decomposition of CO 2 (Equations (11)-(13)) leads to an increase in CO in the product. The yield of CO was 32.2% rising to 37%, when AE-NiO/γ-Al 2 O 3 catalyst combined plasma with 80 W input rising to 140 W input.
In addition, the free radicals produced by methane cracking combined to produce lowcarbon compounds, mainly C 2 H 6 and C 2 H 4 , and no obvious liquid products were detected in this work. As shown in Figure 1e,f, there is almost no generation of hydrocarbons when the input power was less than 120 W with the absence of a catalyst. However, in the presence of the catalyst, with the input power increasing, the selectivity to C 2 H 6 and C 2 H 4 increase significantly. Similar results were reported by Tu et al. [28]. They compared the use of a single plasma and a packed catalyst combined with plasma in the dry reforming reaction of CO 2 and CH 4 , the addition of the catalyst increased the hydrocarbon content in the product. The catalyst still plays a vital role in the dry reforming of CO 2 and CH 4 by plasma promoted. It is not difficult to find by comparing Figure 1e,f, C 2 H 6 is the main hydrocarbon products with the maximum selectivity was 17.3% when used the TI-NiO/γ-Al 2 O 3 catalyst combined with plasma 140 W input. However, when the catalyst was changed to AE-NiO/γ-Al 2 O 3 , the selectivity to C 2 H 6 decreased while the selectivity to C 2 H 4 increased, especially when the input power was 80 W, the selectivity to C 2 H 4 reached 11.3%. The generation of C 2 H 4 is an attractive phenomenon, which provides a very significant reference for plasma-assisted the reforming reaction of CO 2 and CH 4 to produce high value-added products at room temperature and ambient pressure.
Comprehensive analysis of the conversion, selectivity, and products in the dry reforming reaction by plasma promoted, the AE-NiO/γ-Al 2 O 3 catalyst is more active rather than the TI-NiO/γ-Al 2 O 3 catalyst. It can be seen from the Figure S1a, during the dry reforming reaction with plasma promoted, with the input power increasing, the decomposition of CO2 (Equations (11)-(13)) leads to an increase in CO in the product. The yield of CO was 32.2% rising to 37%, Figure 1. Effect of different reaction modes on the performance of the dry reforming reaction by plasma promoted: (a) conversion of CO 2 , (b) conversion of CH 4 , (c) selectivity to CO, (d) selectivity to H 2 , (e) selectivity to C 2 H 4 , (f) selectivity to C 2 H 6 (CO 2 /CH 4 = 1:1, total feed flow rate 20 mL/min, 600 mg catalyst, room temperature and ambient pressure). Table 1 lists the physical parameters of γ-Al 2 O 3 and catalysts prepared by the traditional impregnation method and the ammonia-evaporation method. The physical properties of the catalysts prepared by the two methods are not much different. It is worth noting that due to the difference in the preparation methods, the actual Ni loading of the catalyst prepared by the ammonia-evaporation method is lower than that prepared by the impregnation method. About the BET results, the occupation of the active metal leads to the decrease of the Specific surface area. Similar results are also mentioned in the report of Dieuzeide [34]. However, it is puzzling that the activity exhibited by the catalyst shows an opposite trend to the change of the BET area.  To confirm the size of the diameter crystallite, Scherrer calculation (Equati was performed (D is the diameter crystallite, is a dimensionless shape factor, X-ray wavelength, β is the line broadening at half the maximum intensity (FWH  To confirm the size of the diameter crystallite, Scherrer calculation (Equation (14)) was performed (D is the diameter crystallite, κ is a dimensionless shape factor, λ is the X-ray wavelength, β is the line broadening at half the maximum intensity (FWHM), and θ is the Bragg angle). According to the calculation of Scherrer calculation, the crystallite sizes of support Al 2 O 3 in the AE-NiO/γ-Al 2 O 3 and TI-NiO/γ-Al 2 O 3 catalysts are 10.5 nm and 11.4 nm, respectively.

Catalyst Characterization
H 2 -TPR profiles of the AE-NiO/γ-Al 2 O 3 and AE-NiO/γ-Al 2 O 3 catalysts are displayed in Figure 3. For all catalysts, one shoulder peak was observed between 400−550 • C and the other discrimination peak was located between 600−950 • C. The Shoulder peak at moderate temperature is attributed to the reduction of NiO particles that weakly interact with the support. The second significant peak at high temperature is related to NiO, which has a stronger interaction with the support. As can be seen from the TPR curve, the second H 2 consumption peak on AE-NiO/γ-Al 2 O 3 was higher than TI-NiO/γ-Al 2 O 3 , which are located at 810 • C and 700 • C, respectively. That may be due to the stronger interaction of NiO on the AE-NiO/γ-Al 2 O 3 than on the TI-NiO/γ-Al 2 O 3 . It can also be seen from the curves of AE-NiO/γ-Al 2 O 3 and TE-NiO/γ-Al 2 O 3 , the curve of AE-NiO/γ-Al 2 O 3 is smoother than TI-NiO/γ-Al 2 O 3 , that may be caused by the distribution of NiO on AE-NiO/γ-Al 2 O 3 is more uniform than that on TI-NiO/γ-Al 2 O 3.
Catalysts 2021, 11, x FOR PEER REVIEW  Figure 4 shows the TG results of AE-Ni/γ-Al2O3 and TI-Ni/γ-Al2O3. It's clear catalysts prepared by the ammonia evaporation method and the traditional impre method have approximately the same weight loss temperature, the weight loss ature in the first stage was about 140 °C while the second stage was 520 °C. In periment, according to the TG results, the calcination temperature of all the cataly set to 600 °C, and the catalysts prepared by the two methods have good thermal s  Figure 4 shows the TG results of AE-Ni/γ-Al 2 O 3 and TI-Ni/γ-Al 2 O 3 . It's clear that the catalysts prepared by the ammonia evaporation method and the traditional impregnation method have approximately the same weight loss temperature, the weight loss temperature in the first stage was about 140 • C while the second stage was 520 • C. In this experiment, according to the TG results, the calcination temperature of all the catalysts was set to 600 • C, and the catalysts prepared by the two methods have good thermal stability. Figure 4 shows the TG results of AE-Ni/γ-Al2O3 and TI-Ni/γ-Al2O3. It's clear t catalysts prepared by the ammonia evaporation method and the traditional impreg method have approximately the same weight loss temperature, the weight loss t ature in the first stage was about 140 °C while the second stage was 520 °C. In t periment, according to the TG results, the calcination temperature of all the cataly set to 600 °C, and the catalysts prepared by the two methods have good thermal st The TEM, HRTEM images and size distribution of AE-NiO/γ-Al2O3 catal shown in Figure 5. The catalyst particles are highly dispersed in Figure 5b,c. As seen from the size distribution chart, the average particle diameter of NiO was 2 When analyzing the exposed crystal plane, it was found that the crystal plane spa the exposed particles was 0.203 nm, which was consistent with the NiO (200) The TEM, HRTEM images and size distribution of AE-NiO/γ-Al 2 O 3 catalyst are shown in Figure 5. The catalyst particles are highly dispersed in Figure 5b,c. As can be seen from the size distribution chart, the average particle diameter of NiO was 2.5 nm. When analyzing the exposed crystal plane, it was found that the crystal plane spacing of the exposed particles was 0.203 nm, which was consistent with the NiO (200) crystal plane, indicating that the active component is NiO. Compared with Figure 6 (TEM images of TI-NiO/γ-Al 2 O 3 ), it is obvious that the distribution of NiO on AE-NiO/γ-Al 2 O 3 is more uniform and the particle diameter of NiO (2.5 nm) is smaller than that of TI-NiO/γ-Al 2 O 3 (4.5 nm). Compared with the TEM and HRTEM images of AE-NiO/γ-Al 2 O 3 , TI-Ni/γ-Al 2 O 3 are relatively unevenly distributed, with an average particle size of 4.5 nm. The lattice fringes are not clear, and the exposed crystal plane of NiO cannot be confirmed.
X-ray Photoelectron Spectroscopy (XPS) measurements were performed to determine the existing form of the Ni on the surface of the γ-Al 2 O 3 . The binding energy of Ni 2p are shown in Figure 7. It's observed that the binding energy of Ni in the AE-NiO/γ-Al 2 O 3 or TI-NOi/γ-Al 2 O 3 does not change before and after the reaction. The binding energy around 856.0 eV and 862.0 eV can be attributed to Ni 2p 3/2 and its satellite peak Ni 2p 3/2sat , respectively [35]. It has been proposed that the Ni 2p 3/2 peak with a binding energy of 856 eV and the corresponding shake-up satellite peak at 862 eV were characteristics of NiAl 2 O 4 [36]. The same conclusion was also reported by Huang et al. [37]. The energy band at 873 eV can be attributed to Ni 2p 1/2 of NiO. Furthermore, satellite peak Ni 2p 1/2sat can be observed at banding energy of 880 eV, which is due to the presence of Ni ions in NiO coming from the nickel configuration 2p 5 3d 9 4s [38]. Therefore, based on the XPS results, it can be concluded that nickel manly exists in the form of active phase NiO and a very small amount of inactive NiAl 2 O 4 on the surfaces of all prepared samples. plane, indicating that the active component is NiO. Compared with Figure 6 (TEM images of TI-NiO/γ-Al2O3), it is obvious that the distribution of NiO on AE-NiO/γ-Al2O3 is more uniform and the particle diameter of NiO (2.5nm) is smaller than that of TI-NiO/γ-Al2O3 (4.5 nm). Compared with the TEM and HRTEM images of AE-NiO/γ-Al2O3, TI-Ni/γ-Al2O3 are relatively unevenly distributed, with an average particle size of 4.5 nm. The lattice fringes are not clear, and the exposed crystal plane of NiO cannot be confirmed.  Figure 7. It's observed that the binding energy of Ni in the AE-NiO/γ-Al2O3 or TI-NOi/γ-Al2O3 does not change before and after the reaction. The binding energy around 856.0 eV and 862.0 eV can be attributed to Ni 2p3/2 and its satellite peak Ni 2p3/2sat, respectively [35]. It has been proposed that the Ni 2p3/2 peak with a binding energy of 856 eV and the corresponding shake-up satellite peak at 862 eV were characteristics of NiAl2O4 [36]. The same conclusion was also reported by Huang et al. [37]. The energy band at 873 eV can be attributed to Ni 2p1/2 of NiO. Furthermore, satellite peak Ni 2p1/2sat can be observed at banding energy of 880 eV, which is due to the presence of Ni ions in NiO coming from the nickel configuration 2p 5 3d 9 4s [38]. Therefore, based on the XPS results, it can be concluded that nickel manly exists in the form of active phase NiO and a very small amount of inactive NiAl2O4 on the surfaces of all prepared samples.

Preparation of Catalysts
AE-NiO/γ-Al2O3 catalyst was prepared by an ammonia-evaporation method (AE), in which Nickel nitrate hexahydrate (Damas-beta, 99%) was adopted as the metal precursor with the γ-Al2O3 balls (Diameter 1.5 mm) as the support. Synthetic method described as follows: 2.2 g Ni(NO3)2•6H2O was dissolved in 100 mL deionized water, stirring for 5 min until the nickel salt dissolving completely. Then added the appreciate volume of 28% ammonia aqueous solution to ensure that the pH value of mixed solution

Preparation of Catalysts
AE-NiO/γ-Al 2 O 3 catalyst was prepared by an ammonia-evaporation method (AE), in which Nickel nitrate hexahydrate (Damas-beta, 99%) was adopted as the metal precursor with the γ-Al 2 O 3 balls (Diameter 1.5 mm) as the support. Synthetic method described as follows: 2.2 g Ni(NO 3 ) 2 ·6H 2 O was dissolved in 100 mL deionized water, stirring for 5 min until the nickel salt dissolving completely. Then added the appreciate volume of 28% ammonia aqueous solution to ensure that the pH value of mixed solution is between 10-11 and stirring for 30 min. After that, taking 5.0 g γ-Al 2 O 3 into the nickel ammonia solution and stirring overnight, all the above steps were performed at room temperature (RT). Then suspension was heated at 70 • C to evaporate ammonia and deposited the nickel species on γ-Al 2 O 3 . The step of evaporation was stopped when the pH value of the suspension is decreased to 6-7. Next, washing the precipitate by filtration with deionized water for 3-5 times and drying the solid ball under the vacuum condition at 80 • C for 12 h. Finally, calcined at 600 • C for 4 h in air atmosphere to obtain the AE-NiO/γ-Al 2 O 3 catalyst.
TI-NiO/γ-Al 2 O 3 catalyst was prepared by the traditional impregnation method (TI) which was described as follows: 2.2 g Ni(NO 3 ) 2 ·6H 2 O was dissolved in 100 mL deionized water, stirring for 5 min until the nickel salt dissolved completely, then added 5.0 g γ-Al 2 O 3 balls (Diameter 1.5 mm) to stirring overnight. In addition, after washing the precipitate with deionized water for 3-5 times, drying the solid balls at 80 • C for 12 h in a vacuum condition. Finally, calcined at 600 • C for 4 h in air atmosphere to obtain the TI-NiO/γ-Al 2 O 3 catalyst.

Characterization of Catalysts
ICP: The content of loaded Ni was analyzed by Inductively Coupled Plasma (ICP), which was carried out on Ultima2 plasma emission spectrometer from Jobin Yvon (Paris, France).
PXRD: Powder X-ray diffraction (PXRD) patterns of the catalysts powder was collected by a Rigaku (Tokyo, Japan) Miniflex II diffractometer using a Cu-Kα radiation (λ = 1.5406 Å) with a scan speed of 0.5 • /min in the range of 5-80 • .
XPS: X-ray photoelectron spectroscopy (XPS) was performed on an Escalab 250 (Waltham, MA, USA) spectrometer (VG Systems) equipped with an Al-Kα (1486.6 eV) anode as X-ray source. The binding energies were corrected by C1s peak at 284.6 eV, providing experimental error is within ±0.1 eV.
BET: The surface area of samples was determined by N 2 adsorption-desorption isotherms which performed on Micromeritics ASAP 2020 (Shanghai, China) with the liquid nitrogen temperature (77 K).
TG: Thermogravimetric (TG) analysis was performed on a METTLER TOLEDO (Shanghai, China) thermogravimetric analyzer with a heating rate of 5 K/min from 298 to 1273 K in nitrogen (N 2 ).
TPR: H 2 temperature-programmed reduction experiments (H 2 -TPR) was carried on Autochem II 2920 (Shanghai, China). The sample was pretreated under a 30 mL/min argon purge at 160 • C for 60 min, then cooled to 50 • C, and then heated to 950 • C at 10 • C/min. The reducing gas was 10% H 2 /Ar.

Evaluation of Activity
As shown in Figure 8, the experiments were performed in a coaxial dielectric barrier plasma reactor. Catalytic activity was evaluated by a continuous flow fixed-bed plasma reactor with a power supply (CTP-2000K, Nanjing Suman Plasma Technology Co, Ltd., Nanjing, China) which the applied AC voltage varied from 0~30 kV with an adjustable frequency of 5~20 kHz and a Tektronix TBS-2102 two-channel digital oscilloscope to observe and record the electrical signals such as the applied voltage, the current and so on. The plasma was generated inside a coaxial alundum tube which is 656 mm long and has an outer diameter and an inner diameter of 16 mm and 10 mm, respectively. The discharge gap of plasma reactor is 3 mm. In addition, the diameter of the inner electrode is 4 mm. In the preparation stage of the dry reforming reaction, 600 mg catalyst were directly placed in the discharge region in contact with the plasma electrode. The reactant and product gases were analyzed by an online Gas Chromatograph (GC) (Shimadzu GC-2014) equipped with flame ionization detector (FID) and thermal conductivity detector (TCD). TCD is equipped with TDX-01 column for detecting hydrogen, carbon monoxide, methane, and carbon dioxide while FID is equipped with PLOT-Q column to detect low-carbon alkanes such as methane, ethane, and ethylene. The raw material gas is a 1:1 mixture of methane and carbon dioxide without dilution gas, and the mass flow meter is set to 20 mL/min.

Calculation of Parameters
The activity of DRM catalyst was calculated using Equations (15)-(23) as follows: The reactant and product gases were analyzed by an online Gas Chromatograph (GC) (Shimadzu GC-2014) equipped with flame ionization detector (FID) and thermal conductivity detector (TCD). TCD is equipped with TDX-01 column for detecting hydrogen, carbon monoxide, methane, and carbon dioxide while FID is equipped with PLOT-Q column to detect low-carbon alkanes such as methane, ethane, and ethylene. The raw material gas is a 1:1 mixture of methane and carbon dioxide without dilution gas, and the mass flow meter is set to 20 mL/min.

Calculation of Parameters
The activity of DRM catalyst was calculated using Equations (15)-(23) as follows: S CO (%) = CO produced CO 2 converted + CH 4 converted × 100 (18) S C 2 H 6 (%) = C 2 H 6 produced × 2 CH 4 converted + CO 2 converted × 100 (20) Y CO (%) = CO produced CO 2 input + CH 4 input × 100 The ratio of H 2 /CO molar in the product and carbon balance (based on major gas products) have been calculated as: The energy efficiency with respect to the conversion of CH 4 and CO 2 has been calculated as: where C i, S i , Y i represents conversions, selectivity and yield, respectively. 'i' is a variable which changes according to reactants and products whereas 'B carbon ' stands for carbon balance.
In this work, all discharge power calculations used the Q-V Lissajous graph. As shown in Figure 9, the input power of plasma was 120 W, the Y coordinate value of the CH1 output signal was taken as X Axis data, Y coordinate value of CH2 output signal was used as Y axis data, the discharge power was proportional to the area of the closed curve. Calculation formula such as Equation (24) [39].
which changes according to reactants and products whereas 'Bcarbon' stands for carbon balance.
In this work, all discharge power calculations used the Q-V Lissajous graph. As shown in Figure 9, the input power of plasma was 120 W, the Y coordinate value of the CH1 output signal was taken as X Axis data, Y coordinate value of CH2 output signal was used as Y axis data, the discharge power was proportional to the area of the closed curve. Calculation formula such as Equation (24) [39].

Conclusion
All of the above illustrate that the AE-NiO/γ-Al2O3 catalyst exhibits high catalytic activity due to its small particle size, high active metal dispersion and strong metal-support interaction. Compared with the catalyst prepared by the traditional impregnation method, the catalyst prepared by the ammonia-evaporation method is more suitable for plasma-promoted dry reforming reaction. Especially when the input power of DBD plasma is 140 W and without any additional heating, the conversion of CO2 and CH4 can reach 80.3% and 86.4%, respectively. What's more, the AE-NiO/γ-Al2O3 catalyst is beneficial to generate higher value-added products of C2H4 under the promotion of plasma, which has important reference significance for the plasma-promoted dry reforming reaction to generate high value-added products. Herein, this study provides a feasible catalyst synthesis method for the research of plasma catalytic dry reforming reaction catalyst.

Conclusions
All of the above illustrate that the AE-NiO/γ-Al 2 O 3 catalyst exhibits high catalytic activity due to its small particle size, high active metal dispersion and strong metal-support interaction. Compared with the catalyst prepared by the traditional impregnation method, the catalyst prepared by the ammonia-evaporation method is more suitable for plasmapromoted dry reforming reaction. Especially when the input power of DBD plasma is 140 W and without any additional heating, the conversion of CO 2 and CH 4 can reach 80.3% and 86.4%, respectively. What's more, the AE-NiO/γ-Al 2 O 3 catalyst is beneficial to generate higher value-added products of C 2 H 4 under the promotion of plasma, which has important reference significance for the plasma-promoted dry reforming reaction to generate high value-added products. Herein, this study provides a feasible catalyst synthesis method for the research of plasma catalytic dry reforming reaction catalyst.

Data Availability Statement:
The authors confirm that the data supporting the findings of this study are available within the article.

Conflicts of Interest:
The authors declare no conflict of interest.