Greenhouse Gas Conversion into Hydrocarbons and Oxygenates Using Low Temperature Barrier Discharge Plasma Combined with Zeolite Catalysts

: Global warming occurs as a result of the build-up of greenhouse gases in the atmosphere, causing an increase in Earth’s average temperature. Two major greenhouse gases (CH 4 and CO 2 ) can be simultaneously converted into value-added chemicals and fuels thereby decreasing their negative impact on the climate. In the present work, we used a plasma-catalytic approach for the conversion of methane and carbon dioxide into syngas, hydrocarbons, and oxygenates. For this purpose, CuCe zeolite-containing catalysts were prepared and characterized (low-temperature N 2 adsorption, XRF, XRD, CO 2 -TPD, NH 3 -TPD, TPR). The process of carbon dioxide methane reforming was conducted in a dielectric barrier discharge under atmospheric pressure and at low temperature (under 120 ◦ C). It was found that under the studied conditions, the major byproducts of CH 4 reforming are CO, H 2 , and C 2 H 6 with the additional formation of methanol and acetone. The application of a ZSM-12 based catalyst was beneﬁcial as the CH 4 conversion increased and the total concentration of liquid products was the highest, which is related to the acidic properties of the catalyst.


Introduction
The accumulation of greenhouse gases in the Earth's atmosphere has become a prominent environmental concern in recent decades due to its profound impact on our planet's atmosphere and climate.Greenhouse gases, such as carbon dioxide, methane, nitrous oxide, and fluorinated gases, play a crucial role in regulating the Earth's temperature through the greenhouse effect.However, human activities (burning of fossil fuels, deforestation, and industrial processes) have significantly increased the concentration of these gases, leading to a disbalance in the natural climate system.Consequently, the concentration levels of CO 2 and other greenhouse gases have reached unprecedented levels in the last century [1].
The joint processing of greenhouse gases, specifically methane and carbon dioxide, can be advantageous in terms of greenhouse gas reduction, energy, fuels and chemicals production [2,3].One of the main methods to convert both methane and carbon dioxide in a single process is the dry methane reforming (DRM) process [4,5].Dry methane reforming offers numerous opportunities for the production of valuable intermediates and chemicals.The product of this reaction is a syngas, which consists of CO and H 2 .The resulting syngas can be further processed to obtain various fuels and chemicals.For example, Fischer-Tropsch synthesis can convert syngas into synthetic hydrocarbons such as gasoline, diesel, and waxes.Alternatively, the syngas can serve as a precursor for the production of methanol, ammonia, or other chemical compounds [6,7].However, there are some limitations regarding DRM.The reaction is highly endothermic and requires elevated temperatures for efficient conversion.Carbon deposition is also a common issue, where carbonaceous species can accumulate on the surface of the catalyst, leading to catalyst deactivation [4].Therefore, effective catalyst design and reactor engineering are crucial to prevent carbon deposition and maintain catalytic activity over extended periods.Traditionally, transition metals like Ni or Co are widely studied in DRM due to their availability and low cost [8,9].However, its coking resistance ability is relatively low, so numerous catalyst modification attempts are being made.For example, Ni-Co bimetallic mullite catalysts [10], Zr-La-Ti oxide catalysts [11], as well as Ga-doped Ni/CeO 2 [12] are reported to show remarkable stability against carbon deposition.As an alternative to thermal-induced dry methane reforming, photocatalytic [13,14] and photo-thermal [15] DRM show great potential for reducing process temperature and enhancing gas conversion.
Another way to overcome the limitations of thermal-catalytic DRM is a plasmacatalytic approach [16].This process combines the advantages of plasma technology and catalysis to enhance the efficiency and selectivity of the reforming reaction [17].Plasma can activate the reactant molecules, enhancing their reactivity, and reducing the energy barrier for the reactions.The main advantage of non-thermal plasma is that it makes it possible to conduct thermodynamically unfavorable reactions at low temperatures due to the non-equilibrium character of plasma [18].In comparison to thermal-activated processes, in non-thermal plasma, the vibrational excitation and the energy transfer from one active species (i.e., electron, ion, radical) to the neutral molecule are the main pathways of molecule activation [19].The presence of the catalyst helps to improve overall reaction efficiency by providing active sites for the adsorption and activation of reactant molecules.Additionally, plasma can regenerate the catalyst by removing deposited carbon species, thus extending its lifetime.
One of the attractive ways to tune plasma-catalytic dry methane reforming is to use the process not only for syngas production but for oxygenate production via a single-stage process [20][21][22][23][24].In order to achieve that, the selection of the catalyst should be carefully performed.In the dry methane reforming reaction, methane decomposition occurs on the acidic part of the support [25], so one of the requirements for the catalyst support is the presence of acid centers on the surface.The most commonly used materials for supports are Al 2 O 3 , SiO 2 , and zeolites, which may be modified by TiO 2 , ZrO 2 , and CeO 2 [20,[26][27][28].Zeolites are structured aluminosilicates, which possess acid properties, and the acidity may be widely varied.With the variation of the acidity in the catalyst, different reaction products can be obtained.For example, a higher concentration of Brønsted acid centers in an HZSM-5-based catalyst for plasma-catalytic DRM leads to the formation of acetic acid, while a catalyst with less acidic properties exhibits higher activity towards methanol formation [29].As previously stated [30], the course of the reaction is also affected by the presence of oxygen vacancies on the surface; in particular, the addition of magnesiumaluminum oxide to the catalyst increases CO 2 conversion due to the formation of the oxygen vacancies.As reported in the literature [31,32], the textural properties of the supports are important as microporosity inhibits the formation of micro discharges inside the pores and thus decreases the conversion.Regarding the active phase, such metals as Ni, Co, Mo, V, Na, Cu, Zn, Pt, Ag, or their combination [33] are commonly used [34][35][36].It is reported that the adsorption and dissociation of methyl radicals and their subsequent recombination with other particles is particularly effective on copper and zinc centers [29,[37][38][39].The advantage of Cu-based catalysts in plasma-catalytic dry methane reforming is attributed to high activity in C-H bond splitting in CH 4 and the subsequent C-C coupling resulting in a higher oxygenate yield [40].
In the present study, we aimed to compare the activity of zeolite-based Cu catalysts for plasma-catalytic dry methane reforming.A zeolite of the MTW (ZSM-12) type was chosen for comparison with the widely used MFI (ZSM-5) zeolite due to its unique porous structure and tuning acidity availability [41].To the best of our knowledge, the MTW-type zeolite was not studied in plasma-catalytic dry methane reforming so far by other researchers.The zeolites were combined with Al 2 O 3 in order to increase the porous characteristics and to create a shaped body catalyst.A preliminary study was made by our group [42], where Niand Co-ZSM-12 based catalysts showed better plasma-catalytic performance compared to ZSM-5-based catalysts.In the current research, we focus on the Cu-containing catalysts and the evaluation of their activity and stability in the plasma-catalytic dry methane reforming process.

Materials and Methods
The reagents, which were used for the preparation of the catalysts, are summarized in Table 1.All the reagents were used without further purification.

Preparation of the Catalysts
The synthesis of the ZSM-12 zeolite was conducted according to the procedure reported before by our group [43].In a typical synthesis, a gel was prepared by mixing the solutions of Al 2 (SO 4 ) 3 •18H 2 O, NaOH, N-Methyldiethanolamine, and a colloid silica solution.The obtained gel was further hydrothermally treated at 155 • C for 120 h, then washed, dried, and calcined.After several cycles of an ion-exchange procedure, an H-form of the ZSM-12 zeolite was obtained.
For the preparation of the catalyst support, the obtained ZSM-12 zeolite and a commercial ZSM-5 zeolite were used.Boehmite (AlOOH) was used as a binder to obtain a shaped body support.The ratio of zeolite-to-binder was 70/30 by mass.A portion of boehmite (10.7 g) was mixed with 25 g of zeolite, then ground in a mortar to obtain a homogeneous mixture.After that, HNO 3 (21 cm 3 , 1 M) was added until a viscous mass was formed.The mass was further extruded, dried (60 • C for 2 h, 80 • C for 2 h, 110 • C for 2 h), and calcined (550 • C for 4 h).The obtained granules of support were additionally chopped to a size of 0.8-1.5 cm.Based on the zeolite used, two supports were obtained which were designated as ZSM-5/Al 2 O 3 and ZSM-12/Al 2 O 3 .
The catalyst support was impregnated with a solution of Cu(NO 3 ) 2 •3H 2 O and Ce(NO 3 ) 2 •6H 2 O salts with further drying and calcination.The catalyst supported on the ZSM-5/Al 2 O 3 support was designated as CuCe-5 and the catalyst supported on the ZSM-12/Al 2 O 3 support was designated as CuCe-12.Prior to the plasma-catalytic experiments, the catalysts were reduced in H 2 flow at 400 • C for 2 h.

Catalysts Characterization
The specific surface area, pore volume, and pore diameter of the prepared supports and the catalysts were determined by low-temperature N 2 adsorption analysis using a Belsorp miniX (Microtrac MRB, Osaka, Japan) surface area and pore size distribution analyzer.Before analysis, the catalysts (supports) were thermally degassed for 8 h (T = 300 • C, p = 10 Pa).The Brunauer-Emmett-Teller method was used for specific surface area calculation at p/p 0 = 0.05-0.2.
The elemental composition of the catalysts was determined by X-ray fluorescent spectroscopy using an ARL Perform's Sequental (Thermo Fisher Scientific, Ecublens, Switzer-land) analyzer (X-ray tube power is 2500 W).The sample, before analysis, was ground and pressed into a tablet with H 3 BO 3 .
The average size (D) of crystallites was calculated using the Scherrer equation: where D is the crystallite size, λ is the wavelength of the Cu-Kα radiation, K is a constant and its value is taken as 0.9, θ is the diffraction angle (rad), and β is the full-width at half maximum (FWHM) (rad).
The acidity of the samples was measured by NH 3 temperature-programmed desorption (NH 3 -TPD) using a USGA-101 (LLC "Unisit", Moscow, Russia) device.The sample (m = 0.15-0.2g) was treated in He flow in order to purify the surface from the adsorbed water and oxygen (T = 512 • C for 40 min).Then, the sample was saturated with NH 3 (5 vol% NH 3 -95 vol% He) at 60 • C for 24 min.The analysis was conducted in helium flow at 100-800 • C (heating rate 7 • C/min).Desorbed NH 3 registration was carried out using a thermal conductivity detector.The TPD profiles were deconvoluted using PeakFit (v4.12) software.
The temperature-programmed reduction of the catalysts was carried out using a USGA-101 device.Prior to analysis, the sample was calcined at 500 • C for 30 min in Ar flow, then cooled down to 60 • C. The reduction of the sample was conducted at the 10 • C/min rate up to 950 • C.

Plasma-Catalytic Gases Utilization
The process of carbon dioxide methane reforming was studied using a plasma-catalytic dielectric barrier discharge reactor unit (Figure 1).The mixture of CH 4 and CO 2 was fed into the reactor using RRG-20 precise mass-flow controllers (LLC "Eltochpribor", Zelenograd, Russia).The flow rate of the gases was set to 23.5 mL/min each and the total gas flowrate was 47 mL/min.A dielectric barrier discharge was formed in a quartz tube (16 mm outer diameter, 2 mm wall thickness) between the two electrodes.A steel rod (8 mm diameter) with a tread was placed inside the reactor (inner electrode) and a steel mesh (0.5 mm mesh size, 80 mm length) was placed on the outer wall of the quartz tube (ground electrode).The catalyst (1 g) was placed between the inner electrode and the inner wall of the tube and was fixed with mineral wool at both ends of the packed bed.A high voltage generator had a sine wave signal with a frequency of 23 kHz and was connected to the inner electrode.The electric signals were registered using a Tektronix TDS 2012B oscilloscope (Tektronix, Beaverton, OR, USA).Each experiment was conducted for 2 h with a gradual increase of voltage from 3.5 to 4.8 kV.
Based on the Volt-Columb characteristics of the discharge (Lissajous figure), the plasma absorbed power was calculated according to the equation: where C n is the value of the capacitor included in series with the discharge tube, f is a frequency of the applied voltage, and A is the area of a Lissajous figure.
The energy efficiency of the process (η) was calculated as a ratio of converted gas (CH 4 or CO 2 ) to the absorbed power using the following equation: where ν conv is the quantity of the gas converted (mol/min) and P is the absorbed power (W).
The energy efficiency of the process (η) was calculated as a ratio of converted gas (CH4 or CO2) to the absorbed power using the following equation: where  is the quantity of the gas converted (mol/min) and P is the absorbed power (W).The gaseous outlet mixture was analyzed using a PIA gas chromatograph (LLC "NPF MEMS", Samara, Russia) with a thermal conductivity detector and equipped with a Hayesep N adsorbent column (l = 2 m) for the determination of CO2 and C2H6 and a molecular sieve 13 Å column (l = 2 m) for the determination of H2, CO, CH4.The content of the gases was calculated based on the calibration curves.
Liquid products were trapped in an ice-chilled vessel which contained 20 mL of distilled water.The product flow from the reactor outlet bubbled through the vessel, so the liquid products were dissolved in water.The obtained solution was analyzed using a Crystallux-4000M (RPC "META-CHROM", Yoshkar-Ola, Russia) gas chromatograph with a Poraplot Q capillary column (25 m × 0.53 mm).
Based on the data obtained during the chromatographic analysis, the conversion (X), product selectivity (S), and product yield (Y) were calculated according to the equations: % 100% (10) The gaseous outlet mixture was analyzed using a PIA gas chromatograph (LLC "NPF MEMS", Samara, Russia) with a thermal conductivity detector and equipped with a Hayesep N adsorbent column (l = 2 m) for the determination of CO 2 and C 2 H 6 and a molecular sieve 13 Å column (l = 2 m) for the determination of H 2 , CO, CH 4 .The content of the gases was calculated based on the calibration curves.
Liquid products were trapped in an ice-chilled vessel which contained 20 mL of distilled water.The product flow from the reactor outlet bubbled through the vessel, so the liquid products were dissolved in water.The obtained solution was analyzed using a Crystallux-4000M (RPC "META-CHROM", Yoshkar-Ola, Russia) gas chromatograph with a Poraplot Q capillary column (25 m × 0.53 mm).
Based on the data obtained during the chromatographic analysis, the conversion (X), product selectivity (S), and product yield (Y) were calculated according to the equations: × 100% (10) where ν (in) is the quantity of gas (CO 2 /CH 4 ) which was injected into the reactor (mol), ν (out) is the quantity of the gas in the outlet stream (mol), ν (prod) is the quantity of the gas produced (mol), ν (conv) , and is the quantity of the gas (CO 2 /CH 4 ) which was converted to the products during the reaction.

Zeolite Catalyst Synthesis and Characterization
The prepared catalyst supports and the impregnated catalysts were analyzed with various methods to investigate their physicochemical characteristics.According to the low-temperature N 2 adsorption, the prepared zeolite supports had a relatively high specific surface area, which decreased slightly after the addition of the metals (Table 2).The pore volume was the same as in the support in case of the CuCe-5 and the CuCe-12 samples, which can indicate that the metal oxides were mostly located on the surface of the support rather than inside the pores.The studied adsorption isotherms of the ZSM-5 and ZSM-12 zeolites (Figure 2) according to the IUPAC classification belong to type I isotherms, which indicates the microporosity of the zeolites.The adsorption isotherms of the ZSM-5/Al 2 O 3 and ZSM-12/Al 2 O 3 supports as well as the CuCe-5 and CuCe-12 catalysts also belong to type I in the range of low relative pressures, and to isotherm IV in the range of high relative pressures.Thus, the supports and the catalysts contain both micro-and mesopores.The presence of the mesopores is explained by the addition of Al 2 O 3 in order to form a shaped body catalyst.
Gases 2023, 3, FOR PEER REVIEW where  () is the quantity of gas (CO2/CH4) which was injected into the reactor (m  () is the quantity of the gas in the outlet stream (mol),  () is the quantity of gas produced (mol),  () , and is the quantity of the gas (CO2/CH4) which was conver to the products during the reaction.

Zeolite Catalyst Synthesis and Characterization
The prepared catalyst supports and the impregnated catalysts were analyzed w various methods to investigate their physicochemical characteristics.According to low-temperature N2 adsorption, the prepared zeolite supports had a relatively high s cific surface area, which decreased slightly after the addition of the metals (Table 2).pore volume was the same as in the support in case of the CuCe-5 and the CuCe-12 s ples, which can indicate that the metal oxides were mostly located on the surface of support rather than inside the pores.The studied adsorption isotherms of the ZSM-5 and ZSM-12 zeolites (Figure 2) accord to the IUPAC classification belong to type I isotherms, which indicates the microporosit the zeolites.The adsorption isotherms of the ZSM-5/Al2O3 and ZSM-12/Al2O3 supports as w as the CuCe-5 and CuCe-12 catalysts also belong to type I in the range of low relative p sures, and to isotherm IV in the range of high relative pressures.Thus, the supports and catalysts contain both micro-and mesopores.The presence of the mesopores is explained the addition of Al2O3 in order to form a shaped body catalyst.The pore size distribution of the supports and the catalyst is shown on Figure 3. each sample, two peaks are present on the graph.The peak at 3.9 nm does not reflect actual porous characteristics of the samples, and corresponds to the tensile strength eff which is determined by the properties of the adsorptive [44].Thus, only the peaks at nm (for ZSM-5-based samples) and 6.7 nm (for ZSM-12-based samples) are taken into c sideration.It can be seen from the plots that the ZSM-5-based samples have a narro pore size distribution: the average pore diameter (6.1 nm for ZSM-5/Al2O3 and 6.4 nm The pore size distribution of the supports and the catalyst is shown on Figure 3.For each sample, two peaks are present on the graph.The peak at 3.9 nm does not reflect the actual porous characteristics of the samples, and corresponds to the tensile strength effect, which is determined by the properties of the adsorptive [44].Thus, only the peaks at 5.7 nm (for ZSM-5-based samples) and 6.7 nm (for ZSM-12-based samples) are taken into consideration.It can be seen from the plots that the ZSM-5-based samples have a narrower pore size distribution: the average pore diameter (6.1 nm for ZSM-5/Al 2 O 3 and 6.4 nm for CuCe-5) is close to the data from the BJH plot.For the ZSM-12 based samples, the pore size distribution is wider (peak at 6.7-7.0 nm), which means a less uniform distribution (average pore size 9.1-10.2nm).
CuCe-5) is close to the data from the BJH plot.For the ZSM-12 based samples, the pore size distribution is wider (peak at 6.7-7.0 nm), which means a less uniform distribution (average pore size 9.1-10.2nm).The acidity of the samples was determined by temperature-programmed NH3 desorption.It was stated that the ZSM-5-based samples had more acid sites, which is attributed to Si/Al ratio in the parent zeolites.As the Si/Al ratio tends to diminish (as in the ZSM-5 zeolite case), the acidity increases (Table 3).The TPD-patterns were deconvoluted to obtain three desorption peaks: the low-temperature (200-210 °C), the moderate-temperature (~250 °C) and the high-temperature peak at ~400 °C (Figure 4).According to the literature [45], the low-temperature peak can be ascribed to physically adsorbed ammonia (weak acid sites), so the true acid sites (medium and strong acid sites) can be described by the moderate-and high-temperature peak.It can be seen from the figure that with the addition of Cu and Ce to the support, the medium-temperature peak is shifted towards higher temperature and the area peak is increased compared to that in the TPD pattern of the corresponding supports.This can be related to the effect of the sequential desorption of the NH3 which is attached to Cu ions [46].The acidity of the samples was determined by temperature-programmed NH 3 desorption.It was stated that the ZSM-5-based samples had more acid sites, which is attributed to Si/Al ratio in the parent zeolites.As the Si/Al ratio tends to diminish (as in the ZSM-5 zeolite case), the acidity increases (Table 3).The TPD-patterns were deconvoluted to obtain three desorption peaks: the low-temperature (200-210 • C), the moderate-temperature (~250 • C) and the high-temperature peak at ~400 • C (Figure 4).According to the literature [45], the low-temperature peak can be ascribed to physically adsorbed ammonia (weak acid sites), so the true acid sites (medium and strong acid sites) can be described by the moderate-and high-temperature peak.It can be seen from the figure that with the addition of Cu and Ce to the support, the medium-temperature peak is shifted towards higher temperature and the area peak is increased compared to that in the TPD pattern of the corresponding supports.This can be related to the effect of the sequential desorption of the NH 3 which is attached to Cu ions [46].
Gases 2023, 3, FOR PEER REVIEW 7 CuCe-5) is close to the data from the BJH plot.For the ZSM-12 based samples, the pore size distribution is wider (peak at 6.7-7.0 nm), which means a less uniform distribution (average pore size 9.1-10.2nm).The acidity of the samples was determined by temperature-programmed NH3 desorption.It was stated that the ZSM-5-based samples had more acid sites, which is attributed to Si/Al ratio in the parent zeolites.As the Si/Al ratio tends to diminish (as in the ZSM-5 zeolite case), the acidity increases (Table 3).The TPD-patterns were deconvoluted to obtain three desorption peaks: the low-temperature (200-210 °C), the moderate-temperature (~250 °C) and the high-temperature peak at ~400 °C (Figure 4).According to the literature [45], the low-temperature peak can be ascribed to physically adsorbed ammonia (weak acid sites), so the true acid sites (medium and strong acid sites) can be described by the moderate-and high-temperature peak.It can be seen from the figure that with the addition of Cu and Ce to the support, the medium-temperature peak is shifted towards higher temperature and the area peak is increased compared to that in the TPD pattern of the corresponding supports.This can be related to the effect of the sequential desorption of the NH3 which is attached to Cu ions [46].On the X-ray diffractograms, it can be seen that the zeolite structure was not destroyed during metal loading (Figure 5).The diffraction patterns in the range of 2θ = 5 • -30 • are attributed to the zeolite phase and the intensity of these patterns does not change significantly.On the X-ray diffractograms, it can be seen that the zeolite structure was not destroyed during metal loading (Figure 5).The diffraction patterns in the range of 2θ = 5°-30° are attributed to the zeolite phase and the intensity of these patterns does not change significantly.The average size of the crystallites was determined for the CeO2 and CuO phases (Table 4).It should be noted that the CuO phase in CuCe-5 sample is not distinguished, which may be the result of the high dispersion of that phase on the ZSM-5/Al2O3 support surface.This support has a higher specific surface area, which leads to a better particle distribution.However, CeO2 crystallites are of a similar size in both catalysts, which indicates no effect of the support properties on the CeO2 crystallite size.The average size of the crystallites was determined for the CeO 2 and CuO phases (Table 4).It should be noted that the CuO phase in CuCe-5 sample is not distinguished, which may be the result of the high dispersion of that phase on the ZSM-5/Al 2 O 3 support surface.This support has a higher specific surface area, which leads to a better particle distribution.However, CeO 2 crystallites are of a similar size in both catalysts, which indicates no effect of the support properties on the CeO 2 crystallite size.The temperature-programmed H 2 reduction of the obtained samples was conducted in order to investigate the reducibility of the catalysts prior to the plasma-catalytic experiments.The reduction temperature of the pure CuO is in the range of 300-400 • C depending on the analysis conditions, as reported in the literature [47].The reduction of pure CeO 2 is observed in a temperature range of 350-580 • C and above 900 • C [48,49].It is seen from the TPR profiles (Figure 6) that the reduction temperature is lower than that of the pure CuO and CeO 2 substances.In the literature, it is stated that CuO-CeO 2 systems have a lower reduction temperature because of the interaction of the two oxides [50].In the TPR pattern of the CuCe-5 sample, only one high-intensity peak is present, which may indicate higher CuO particle dispersion [51], which confirms the XRD results.The TPR pattern of the CuCe-12 differs from the one of CuCe-5 and three peaks are distinguished in the TPR profile.According to the literature [52], the low-temperature peak (~150 • C) is attributed to the reduction of CuO x , which interacts strongly with CeO 2 , the peak at ~210 • C is attributed to the reduction of highly dispersed CuO x particles on the surface, and the high-temperature peak (~260 • C) is attributed to bulk CuO x particles.The latter is also consistent with the CuO crystallite size data obtained by the XRD analysis.
Gases 2023, 3, FOR PEER REVIEW 9 The temperature-programmed H2 reduction of the obtained samples was conducted in order to investigate the reducibility of the catalysts prior to the plasma-catalytic experiments.The reduction temperature of the pure CuO is in the range of 300-400 °C depending on the analysis conditions, as reported in the literature [47].The reduction of pure CeO2 is observed in a temperature range of 350-580 °C and above 900 °C [48,49].It is seen from the TPR profiles (Figure 6) that the reduction temperature is lower than that of the pure CuO and CeO2 substances.In the literature, it is stated that CuO-CeO2 systems have a lower reduction temperature because of the interaction of the two oxides [50].In the TPR pattern of the CuCe-5 sample, only one high-intensity peak is present, which may indicate higher CuO particle dispersion [51], which confirms the XRD results.The TPR pattern of the CuCe-12 differs from the one of CuCe-5 and three peaks are distinguished in the TPR profile.According to the literature [52], the low-temperature peak (~150 °C) is attributed to the reduction of CuOx, which interacts strongly with CeO2, the peak at ~210 °C is attributed to the reduction of highly dispersed CuOx particles on the surface, and the hightemperature peak (~260 °C) is attributed to bulk CuOx particles.The latter is also consistent with the CuO crystallite size data obtained by the XRD analysis.

Gas Conversion Using Plasma-Catalytic Process
In order to test the activity of the prepared catalysts, a dielectric barrier discharge plasma reactor was used.As a blank test, an experiment in an empty reactor without a catalyst was conducted.On Figure 7, the Lissajous figures, which were registered during the blank test, are presented.

Gas Conversion Using Plasma-Catalytic Process
In order to test the activity of the prepared catalysts, a dielectric barrier discharge plasma reactor was used.As a blank test, an experiment in an empty reactor without a catalyst was conducted.On Figure 7, the Lissajous figures, which were registered during the blank test, are presented.
Gases 2023, 3, FOR PEER REVIEW 9 The temperature-programmed H2 reduction of the obtained samples was conducted in order to investigate the reducibility of the catalysts prior to the plasma-catalytic experiments.The reduction temperature of the pure CuO is in the range of 300-400 °C depending on the analysis conditions, as reported in the literature [47].The reduction of pure CeO2 is observed in a temperature range of 350-580 °C and above 900 °C [48,49].It is seen from the TPR profiles (Figure 6) that the reduction temperature is lower than that of the pure CuO and CeO2 substances.In the literature, it is stated that CuO-CeO2 systems have a lower reduction temperature because of the interaction of the two oxides [50].In the TPR pattern of the CuCe-5 sample, only one high-intensity peak is present, which may indicate higher CuO particle dispersion [51], which confirms the XRD results.The TPR pattern of the CuCe-12 differs from the one of CuCe-5 and three peaks are distinguished in the TPR profile.According to the literature [52], the low-temperature peak (~150 °C) is attributed to the reduction of CuOx, which interacts strongly with CeO2, the peak at ~210 °C is attributed to the reduction of highly dispersed CuOx particles on the surface, and the hightemperature peak (~260 °C) is attributed to bulk CuOx particles.The latter is also consistent with the CuO crystallite size data obtained by the XRD analysis.

Gas Conversion Using Plasma-Catalytic Process
In order to test the activity of the prepared catalysts, a dielectric barrier discharge plasma reactor was used.As a blank test, an experiment in an empty reactor without a catalyst was conducted.On Figure 7, the Lissajous figures, which were registered during the blank test, are presented.It can be seen that with the increase in the input power, the shape of the figure (parallelogram) remains unchanged, but the figure itself is broadened, which is due to enhanced voltage and current signals.The observed shape of the Lissajous figure is typical for dielectric barrier discharge plasma [53].When the catalyst is introduced into the discharge zone, the input power is decreased Figure 8d.The discharge behavior in the absence of the catalyst differs from the one in the empty reactor due to the change in the dielectric properties of the matter between the inner and the outer electrodes.
Gases 2023, 3, FOR PEER REVIEW 10 It can be seen that with the increase in the input power, the shape of the figure (parallelogram) remains unchanged, but the figure itself is broadened, which is due to enhanced voltage and current signals.The observed shape of the Lissajous figure is typical for dielectric barrier discharge plasma [53].When the catalyst is introduced into the discharge zone, the input power is decreased Figure 8d.The discharge behavior in the absence of the catalyst differs from the one in the empty reactor due to the change in the dielectric properties of the matter between the inner and the outer electrodes.The supports (ZSM-5/Al2O3 and ZSM-12/Al2O3) along with the catalysts were also tested in plasma-catalytic CH4 reforming.From the results, it can be seen that the conversion of CO2 is decreased when the packing material is introduced.As carbon dioxide is a gas with acidic properties, a less acidic catalyst would be preferable.However, when material with less acidity was present (ZSM-12 based catalyst), no significant change in CO2 conversion was observed.Apparently, better adsorption would be achieved on strong basic sites which were not determined in the prepared catalysts.A minor increase in the CO2 conversion was achieved when the CeO2 was impregnated compared to the pure support.Cerium oxide possesses oxygen vacancies in the structure, which promote the splitting of CO2 into CO [54].The conversion of CH4 increased and was the highest in presence of the CuCe-12 sample.When relating the conversion to the input power, it was seen that in presence of CuCe-5 and CuCe-12, the input power was similar but CH4 conversion in the presence of CuCe-12 was higher, which is attributed to the synergy between the plasma and the catalyst.The activated methane molecule bridges to the Cu atom with subsequent recombination to gaseous and liquid products.Apparently, the higher surface The supports (ZSM-5/Al 2 O 3 and ZSM-12/Al 2 O 3 ) along with the catalysts were also tested in plasma-catalytic CH 4 reforming.From the results, it can be seen that the conversion of CO 2 is decreased when the packing material is introduced.As carbon dioxide is a gas with acidic properties, a less acidic catalyst would be preferable.However, when material with less acidity was present (ZSM-12 based catalyst), no significant change in CO 2 conversion was observed.Apparently, better adsorption would be achieved on strong basic sites which were not determined in the prepared catalysts.A minor increase in the CO 2 conversion was achieved when the CeO 2 was impregnated compared to the pure support.Cerium oxide possesses oxygen vacancies in the structure, which promote the splitting of CO 2 into CO [54].The conversion of CH 4 increased and was the highest in presence of the CuCe-12 sample.When relating the conversion to the input power, it was seen that in presence of CuCe-5 and CuCe-12, the input power was similar but CH 4 conversion in the presence of CuCe-12 was higher, which is attributed to the synergy between the plasma and the catalyst.The activated methane molecule bridges to the Cu atom with subsequent recombination to gaseous and liquid products.Apparently, the higher surface area and the higher distribution of the CuO species on the CuCe-5 surface did not provide much of an effect on CH 4 conversion compared to the bigger CuO crystallites in the case of CuCe-12.The products of CH 4 -CO 2 reforming included H 2 , CO, and C 2 H 6 .The yields of the products and their selectivities are presented in Figure 8b,c.It can be seen that the product yields are the highest in the case of an empty reactor, which may be attributed to the higher input power.As was mentioned before, in the presence of packing materials, the consumed power slightly decreases, which may affect product formation.It should be noted that the CuCe-12 sample exhibits the lowest selectivity for gaseous products, which may be related to the formation of liquid products.
The liquid products, which were produced during plasma-catalytic processing of the gases, were collected into a beaker containing distilled water to prevent evaporation.The resulting water solution was subjected to a gas chromatography analysis and the main products, which were methanol and acetone, were determined.From Figure 9 it may be observed that the total quantity of the oxygenated products was the highest in presence of the CeCe-12 catalyst.This is consistent with the suggestion that the decreased selectivity of the gaseous products (H 2 , CO, C 2 H 6 ) is attributed to the formation of liquid products.When comparing the two types of catalysts, it can be seen that more liquid products were produced in the presence of the ZSM-12-supported catalyst.A possible explanation can be the difference in acid properties.The catalyst with less acid centers promotes the formation of methanol rather than carboxylic acids [38].During the plasma activation of the CH 4 -CO 2 gas mixture, CO 2 *, CH 4 *, and CO* excited species are formed as well as CH 3 , CH 2 , CH, H, and O radicals [55].As a result of species recombination in the plasma and on the surface of the catalyst, OH, HCO ads , H 2 CO ads , and H 3 CO ads are produced, which further transform into molecules like CH 3 OH.According to the literature [56], the presence of acetone in the products is attributed to HCO ads species which couple with C 2 species to form a C 3 -intermediate resulting in acetone formation.
Gases 2023, 3, FOR PEER REVIEW 11 area and the higher distribution of the CuO species on the CuCe-5 surface did not provide much of an effect on CH4 conversion compared to the bigger CuO crystallites in the case of CuCe-12.The products of CH4-CO2 reforming included H2, CO, and C2H6.The yields of the products and their selectivities are presented in Figure 8b,c.It can be seen that the product yields are the highest in the case of an empty reactor, which may be attributed to the higher input power.As was mentioned before, in the presence of packing materials, the consumed power slightly decreases, which may affect product formation.It should be noted that the CuCe-12 sample exhibits the lowest selectivity for gaseous products, which may be related to the formation of liquid products.The liquid products, which were produced during plasma-catalytic processing of the gases, were collected into a beaker containing distilled water to prevent evaporation.The resulting water solution was subjected to a gas chromatography analysis and the main products, which were methanol and acetone, were determined.From Figure 9 it may be observed that the total quantity of the oxygenated products was the highest in presence of the CeCe-12 catalyst.This is consistent with the suggestion that the decreased selectivity of the gaseous products (H2, CO, C2H6) is attributed to the formation of liquid products.When comparing the two types of catalysts, it can be seen that more liquid products were produced in the presence of the ZSM-12-supported catalyst.A possible explanation can be the difference in acid properties.The catalyst with less acid centers promotes the formation of methanol rather than carboxylic acids [38].During the plasma activation of the CH4-CO2 gas mixture, CO2*, CH4*, and CO* excited species are formed as well as CH3, CH2, CH, H, and O radicals [55].As a result of species recombination in the plasma and on the surface of the catalyst, OH, HCOads, H2COads, and H3COads are produced, which further transform into molecules like CH3OH.According to the literature [56], the presence of acetone in the products is attributed to HCOads species which couple with C2 species to form a C3-intermediate resulting in acetone formation.The results obtained were compared to previously published investigations on plasma-catalytic DRM and CO2 decomposition (Table 5).It can be seen from the table that in the present work, the conversion of CO2 and CH4 was comparable to that in presence of the Cu/Al2O3 catalyst from the literature [57], but the energy efficiency in our system was considerably higher (0.8 kJ/mmol vs. 0.18 kJ/mmol).It can also be seen that in the major part of the Cu-involving DRM related works, methanol is the main product among the oxygenates produced and in our research, methanol content increases when the ZSM-12 zeolite is introduced into the catalyst composition.It should be noted that the Cu/CeO2 catalyst from the recent study [38] shows the worst CO2 conversion results among other The results obtained were compared to previously published investigations on plasmacatalytic DRM and CO 2 decomposition (Table 5).It can be seen from the table that in the present work, the conversion of CO 2 and CH 4 was comparable to that in presence of the Cu/Al 2 O 3 catalyst from the literature [57], but the energy efficiency in our system was considerably higher (0.8 kJ/mmol vs. 0.18 kJ/mmol).It can also be seen that in the major part of the Cu-involving DRM related works, methanol is the main product among the oxygenates produced and in our research, methanol content increases when the ZSM-12 zeolite is introduced into the catalyst composition.It should be noted that the Cu/CeO 2 catalyst from the recent study [38] shows the worst CO 2 conversion results among other catalysts.The authors explain this phenomenon by the interaction of Cu + species with CeO 2 oxygen vacancies, which led to CO + H 2 O reaction activation with subsequent CO 2 formation.Thus, controlling the valence state of the Cu species is essential for plasmacatalytic DRM reactions.In contrast, a CuO-based catalyst on CeAl support may lead to 13.5% CO 2 conversion if the CH 4 is absent, which means no side reactions occur.In the presence of our CuCe catalytic systems, the CO 2 conversion is lower due to the acidic nature of the zeolite-containing support.
From the literature, it is known that acetic acid is one of the desired products in plasma-catalytic dry methane reforming.However, in our system, no amount of this acid was observed when analyzing the liquid phase.One of the possible reasons for the absence of acetic acid in the liquid products can be the limited desorption of the acid from the zeolite pores, as reported in the literature [40].In order to overcome this limitation, reactor modification is needed in future work.For example, reactor cooling or co-reagent injection (such as O 2 or water vapor) may take place.
Reusability tests of the CuCe-12 were carried out in order to investigate the stability of the catalyst.Each run duration was 2 h in order to obtain three samples of the gas.No catalyst pretreatment was made between the runs.It was revealed that the obtained sample was stable in the current conditions within the investigated time (10 h) and no drastic changes in gas conversion and the yields of H 2 and CO were observed (Figure 10).It may be preliminarily concluded that catalyst deactivation did not occur in the present experiment during the 10 h of the total activity of the tests and thus the catalyst may be used in several cycles.The only problem with a continuous run is related to reactor heating, so additional technological solutions need to be implemented in future work.

Conclusions
In the present work, the conversion of greenhouse gases was conducted via the plasma-catalytic dry methane reforming approach using zeolite-containing catalysts.Under the studied conditions, the conversion of CH4 increases in the presence of the catalysts, but CO2 conversion slightly decreases which can be explained by the decrease of the input power in the presence of packing material and the acidic properties of the catalysts.Based on analysis results, the ZSM-5-based catalyst (CuCe-5) had a higher specific surface area, higher concentration of acid centers, and better CuO distribution.Nevertheless, CH4 conversion was higher and liquid phase content increased in the presence of CuCe-12.These effects are related to lower acidity as well as the bulk Cu phase which bridges the CH3 species and allows them to recombine with CH3OH and CH3C(O)CH3 molecules.There are still challenges to overcome in plasma-catalytic dry methane reforming.The design and optimization of the plasma-catalyst system requires careful consideration of factors such as plasma power, catalyst type, and reactor configuration.Further research and development efforts are ongoing for a better understanding of the complex interactions be-

Conclusions
In the present work, the conversion of greenhouse gases was conducted via the plasmacatalytic dry methane reforming approach using zeolite-containing catalysts.Under the studied conditions, the conversion of CH 4 increases in the presence of the catalysts, but CO 2 conversion slightly decreases which can be explained by the decrease of the input power in the presence of packing material and the acidic properties of the catalysts.Based on analysis results, the ZSM-5-based catalyst (CuCe-5) had a higher specific surface area, higher concentration of acid centers, and better CuO distribution.Nevertheless, CH 4 conversion was higher and liquid phase content increased in the presence of CuCe-12.These effects are related to lower acidity as well as the bulk Cu phase which bridges the CH 3 species and allows them to recombine with CH 3 OH and CH 3 C(O)CH 3 molecules.There are still challenges to overcome in plasma-catalytic dry methane reforming.The design and optimization of the plasma-catalyst system requires careful consideration of factors such as plasma power, catalyst type, and reactor configuration.Further research and development efforts are ongoing for a better understanding of the complex interactions between plasma and catalysts and to improve the overall performance of this process.

Figure 1 .
Figure 1.Scheme of the plasma-catalytic unit.

Figure 1 .
Figure 1.Scheme of the plasma-catalytic unit.

Figure 3 .
Figure 3. Pore size distribution based on desorption branch of the BJH plot.

Figure 3 .
Figure 3. Pore size distribution based on desorption branch of the BJH plot.

Figure 3 .
Figure 3. Pore size distribution based on desorption branch of the BJH plot.

Figure 4 .
Figure 4. NH3-TPD profiles of the prepared supports and catalysts.Figure 4. NH 3 -TPD profiles of the prepared supports and catalysts.

Figure 4 .
Figure 4. NH3-TPD profiles of the prepared supports and catalysts.Figure 4. NH 3 -TPD profiles of the prepared supports and catalysts.

Figure 5 .
Figure 5. X-ray diffractograms of the supports and obtained catalysts.

Figure 5 .
Figure 5. X-ray diffractograms of the supports and obtained catalysts.

Figure 8 .
Figure 8.The results of the plasma-catalytic processing of the CH4 and CO2 using the prepared catalysts: (a) the conversion of CH4 and CO2; (b) yield of the gaseous products; (c) selectivity of the gaseous products; (d) energy efficiency and power.

Figure 8 .
Figure 8.The results of the plasma-catalytic processing of the CH 4 and CO 2 using the prepared catalysts: (a) the conversion of CH 4 and CO 2 ; (b) yield of the gaseous products; (c) selectivity of the gaseous products; (d) energy efficiency and power.

Figure 9 .
Figure 9.The results of the obtained liquid phase chromatographic analysis.

Figure 9 .
Figure 9.The results of the obtained liquid phase chromatographic analysis.

Table 1 .
Reagents used for catalyst preparation.

Table 2 .
Elemental composition and the textural characteristics of the obtained supports and catalysts.

Table 2 .
Elemental composition and the textural characteristics of the obtained supports and catalys

Table 3 .
Quantity of the acid sites determined by NH 3 -TPD.

Table 3 .
Quantity of the acid sites determined by NH3-TPD.

Table 4 .
The average crystallite size of the CeO2 and CuO phases.
* n/d = no data

Table 4 .
The average crystallite size of the CeO 2 and CuO phases.

Table 5 .
DBD plasma-catalytic DRM and CO 2 decomposition performance data using Cu catalysts.

Table 5 .
Cont. n/d = no data, ** n/a = not applicable since only CO 2 decomposition to CO is considered in the study. *