Hydrogen Production from Methane Cracking in Dielectric Barrier Discharge Catalytic Plasma Reactor Using a Nanocatalyst

: The study experimentally investigated a novel approach for producing hydrogen from methane cracking in dielectric barrier discharge catalytic plasma reactor using a nanocatalyst. Plasma-catalytic methane (CH 4 ) cracking was undertaken in a dielectric barrier discharge (DBD) catalytic plasma reactor using Ni / MgAl 2 O 4 . The Ni / MgAl 2 O 4 was synthesised through co-precipitation followed customised hydrothermal method. The physicochemical properties of the catalyst were examined using X-ray di ﬀ raction (XRD), scanning electron microscopy—energy dispersive X-ray spectrometry (SEM-EDX) and thermogravimetric analysis (TGA). The Ni / MgAl 2 O 4 shows a porous structure spinel MgAl 2 O 4 and thermal stability. In the catalytic-plasma methane cracking, the Ni / MgAl 2 O 4 shows 80% of the maximum conversion of CH 4 with H 2 selectivity 75%. Furthermore, the stability of the catalyst was encouraging 16 h with CH 4 conversion above 75%, and the selectivity of H 2 was above 70%. This is attributed to the synergistic e ﬀ ect of the catalyst and plasma. The plasma-catalytic CH 4 cracking is a promising technology for the simultaneous H 2 and carbon nanotubes (CNTs) production for energy storage applications.


Introduction
The atmosphere is heavily polluted due to the urbanisation and commercialisation throughout the globe. It causes serious greenhouse gases (GHGs) emissions, more specifically, the carbon dioxide (CO 2 ) and methane (CH 4 ) along with other volatile compounds. Various techniques have been applied to treat the GHGs to reduce harmful emissions for sustainable development. One of the exciting techniques is to utilise the GHGs for producing zero-emission fuel, which is currently under investigation throughout from the last couple of decades. It is an essential step to reduce the GHG concentration in the atmosphere as well as a sustainable approach for fuel synthesis [1][2][3]. Previous studies revealed that CH 4 is one of the prominent components of GHG with a total share of 16% in the environment usually emitted from petroleum processing, waste management and agriculture activities [4].
On the other hand, CH 4 is also the principal constituent (76 wt%) of natural gas (NG) which reserves are abundantly available in underground. The utilisation of CH 4 has various routes as fuel both in domestic and industrial processes. One of the most sustainable and attractive ways to utilise CH 4 is to produce syngas and hydrogen (H 2 ) along with co-reactants such as O 2 , H 2 O, and CO 2 [5,6]. The popular routes for CH 4 mitigation are thermocatalytic processes such as thermal decomposition of methane as shown in the below reaction (R1), methane partial oxidation [7], methane dry reforming [8][9][10] and methane steam reforming [11] in thermal reactors. The higher energy input for elevated temperatures makes the thermal reactors economically challenging for this process [12,13]. Various techniques have been employed to overcome the shortfalls to make the process viable [14,15].
In recent days, various plasma systems are used for the processing of the methane carking as well as other oxidative reactions using microwave plasma, spark plasma [8,10,16] and nonthermal plasmas (NTPs) like dielectric barrier discharge (DBD) and silent discharges. NTP seeks attention for gas processing, especially the DBD cold plasma reactor is one of the promising techniques [8,12]. The DBD plasma reactor has some useful characteristics from low-temperature operation to accessible upscaling opportunities as compared to thermal plasma [8,17]. More significant aspects of the DBD plasma for gas processing has been reported in an extensive review by Ramses and Bogaerts [12]. In addition, the DBD plasma has been successfully utilised for CH 4 cracking with efficient conversion and significant H 2 yield [18][19][20]. The hydrogen is the next-generation future fuel due to the recent developments in hydrogen-based fuel cell technologies [21]. The DBD plasma-based methane cracking has been reported in several studies aiming for cleaner production of H 2 . However, the conversion efficiency and cleaner H 2 is always challenging in the DBD plasma reactor for a longer time on streams [6,22].
To improve the conversion of CH 4 , the various catalysts have been employed in the catalytic DBD plasma. The most valuable catalysts for plasma catalytic DBD methane cracking are Ni/γ-Al 2 O 3 , γ-Al 2 O 3 , Pd/SiO 2 , Pd/TiO 2 , Pd/Al 2 O 3 [23], Pt/γ-Al 2 O 3 [24], ZnO, ZnCr 2 O 4 , Cr 2 O 3 [25]. The improvement in the conversion of CH 4 as well as enhanced product selectivity been a witness in various referenced studies [25]. Plasma-catalysis drives scope on improving the selectivity of targeted products which is very important for CH 4 cracking process. The magnesium aluminate (MgAl 2 O 4 ) as a catalyst has been investigated for various reforming process [9,26,27] as well as plasma catalytic methane dry reforming in previous studies. It demonstrated a substantial improvement in conversion of reactants and product distribution, especially on the H 2 selectivity [28,29]. The nickel (Ni) impregnated MgAl 2 O 4 can improve the CH 4 conversion and H 2 selectivity suppressing the recombination of methyl radicals [30]. The MgAl 2 O 4 based catalyst has not been previously reported as its distinct properties such as high resistance to temperature, and mild plasma conditions are much suitable to use in plasma-based methane cracking processes. Therefore, it is seems meaning to incorporate the Ni impregnated MgAl 2 O 4 in the DBD plasma reactor for methane cracking for hydrogen production and simultaneously it produces carbon nanotubes (CNTs) which are essential material for energy storage applications [31]. Plasma produces a very clean and well-structured CNTs for further application reported in various studies.
In this work, an experimental study has been conducted to the synthesis of a nanocatalyst (Ni/MgAl 2 O 4 ) for CH 4 cracking in fixed bed DBD plasma reactor for H 2 and CNTs production. The catalyst was synthesised using the co-precipitation method followed by hydrothermal process. The catalyst is further characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX) and thermogravimetric analysis (TGA). Furthermore, the stability of the catalyst was examined for 16 h reaction time or time on stream (TOS). Finally, spent catalyst is further characterised using SEM, TGA and differential thermogravimetric (DTG) to investigate the formed CNTs over catalyst surface.

Synthesis of Ni/MgAl 2 O 4
The support MgAl 2 O 4 was prepared through co-precipitation process supported by the hydrothermal method presented in Figure 1. Briefly, magnesium nitrate hexahydrate (Mg(NO 3 ) 2 ·6H 2 O) (99.5 %, Sigma, St. Louis, MI, USA) and aluminium nitrate nonahydrate (Al(NO 3 ) 3 ·9H 2 O) (99.5 %, Merck, NJ, USA) was dissolved in ACS reagent, ammonia solution (28.0%) with the 2:1 molar ratio of Mg:Al. The nitrate solution was then combined to 0.01 molar citric acid (CA) solution using pipette at 60 • C on continuous stirring at a speed of 350 rpm. The ammonia is acting as a precipitating agent while citric acid is assisting control crystal growth and morphology. The nitrate solution is transferred to a polytetrafluoroethylene (PFTE) Teflon autoclave and kept in the furnace for 24 h at a temperature of 160 • C. Further, the sample has been washed using ethanol numerous times and deionised (DI) water for the removal of impurities. The prepared samples dried in the oven at a temperature of 120 • C for 24 h to remove the moisture. The received derived sample was crushed and kept for calcination at 700 • C for 4 h. the stability of the catalyst was examined for 16 h reaction time or time on stream (TOS). Finally, spent catalyst is further characterised using SEM, TGA and differential thermogravimetric (DTG) to investigate the formed CNTs over catalyst surface.

Synthesis of Ni/MgAl2O4
The support MgAl2O4 was prepared through co-precipitation process supported by the hydrothermal method presented in Figure 1. Briefly, magnesium nitrate hexahydrate (Mg(NO3)2·6H2O) (99.5 %, Sigma, St. Louis, MI, USA) and aluminium nitrate nonahydrate (Al(NO3)3·9H2O) (99.5 %, Merck, NJ, USA) was dissolved in ACS reagent, ammonia solution (28.0%) with the 2:1 molar ratio of Mg:Al. The nitrate solution was then combined to 0.01 molar citric acid (CA) solution using pipette at 60 °C on continuous stirring at a speed of 350 rpm. The ammonia is acting as a precipitating agent while citric acid is assisting control crystal growth and morphology. The nitrate solution is transferred to a polytetrafluoroethylene (PFTE) Teflon autoclave and kept in the furnace for 24 h at a temperature of 160 °C. Further, the sample has been washed using ethanol numerous times and deionised (DI) water for the removal of impurities. The prepared samples dried in the oven at a temperature of 120 °C for 24 h to remove the moisture. The received derived sample was crushed and kept for calcination at 700 °C for 4 h.
For Ni impregnation, wetness incipient impregnation technique has been employed. The precursor (10 wt%). nickel nitrate hexahydrate (Ni(NO3)2.6H2O) (99%, Merck) was added to DI water to get 0.01 molar nitrate solution. The nitrate solution stirred for 10 min at 60 °C. The required amount of support MgAl2O4 was then combined to the nickel nitrate solution and stirring for three (3) hours at 110 °C. The sample was kept in a furnace (CSF 1100, Carbolite, Cheshire, UK) for overnight about 10 to 12 h for drying. The dried sample was crushed and preserved in the furnace for 5 h at 700 °C to achieve the final catalyst for methane cracking application.  for drying. The dried sample was crushed and preserved in the furnace for 5 h at 700 • C to achieve the final catalyst for methane cracking application.

Materials Characterisation
The physicochemical properties of the synthesised catalyst are examined by several methods i.e., XRD, SEM-EDX and TGA. XRD was accomplished employing Bruker's X-ray Diffractometer (D8-Advance, MA, USA), using Cu-kα radiation (40 kV, 200 mA). The crystallite size was analysed by Scherrer's equation [32]. After that, SEM was carried out using TESCAN VEGA 3 (Czech Republic), conducted at 20 kV HV and integrated with the beam of X-MaxN by Zeiss optics [13]. TGA 5500 (TA Instruments, Newcastle, DC, USA) was used to analyses the weight loss (%) and differential thermogravimetric analysis (DTG) of the fresh catalyst. The sample (10 mg) was loaded in a platinum pan and placed in the furnace at a heating rate of 10 • C min −1 under the N 2 flow of 40 mL min −1 [30,33]. Spent catalyst was characterised by SEM and TGA-DTG ((TA Instruments, Newcastle, DC, USA)) after 16 h TOS to investigate the morphological changes and CNT formation.

Plasma-Catalytic Methane Cracking System
The experimental setup for the catalytic-DBD reactor for CH 4 cracking is as shown in Figure 2. The reactant CH 4 (99.9 %) flow rate was regulated by a mass flow meters/controller (MFC) (Alicat, Tucson, AZ, USA). The plasma power supply model CTP-2000K (Nanjing, China) incorporated with the high voltage regulator was used to produce plasma in the DBD reactor. The input voltage and input current were also monitored by Tektronix TDS 2012B oscilloscope (Beaverton, OR, USA) coupled with voltage probe Tek P6015A (Beaverton, OR, USA) [28]. The plasma reactor consists of an alumina tube having an inner diameter (ID) of 10 mm and the outer diameter (OD) of 12 mm. The stainless steel rod with an inner diameter of 4 mm and 20 cm in length was utilised as a HV electrode while a mesh of aluminium is wrapped serving as a ground electrode. The prepared catalyst is loaded in the centre of the alumina tube hold by quartz wool. The gases were analysed by GC-TCD/FID (Agilent 6890N, Santa Clara, CA, USA). The GC column details are given in details here [34]. The HP PlotQ capillary column with configuration of 40 m × 0.53 mm ID, 40 µm was used to detect CO 2 while Molsieve column with the configuration of 30 m × 0.530 mm ID, 25 µm used for detecting H 2 and CH 4 , both the columns were connected to TCD. Another column HayeSep Q-Supelco with the configuration of 6 ft × 1/8 in. ID × 2.1 mm OD, 80/100 mesh was employed as TCD C 2 -C 6 back flashing. To separate the hydrocarbons ranging from C 1 -C 6 were analysed by GS-Gaspro column having the configuration of 60 m × 0.32 mm ID) detected to FID. Agilent supplied all the columns. The process parameters such as feed flow rates, power input and loading of prepared catalyst were maintained constant.
Energies 2020, 13, 5921 5 of 15 detect CO2 while Molsieve column with the configuration of 30 m × 0.530 mm ID, 25 µm used for detecting H2 and CH4, both the columns were connected to TCD. Another column HayeSep Q-Supelco with the configuration of 6 ft × 1/8 in. ID × 2.1 mm OD, 80/100 mesh was employed as TCD C2-C6 back flashing. To separate the hydrocarbons ranging from C1-C6 were analysed by GS-Gaspro column having the configuration of 60 m × 0.32 mm ID) detected to FID. Agilent supplied all the columns. The process parameters such as feed flow rates, power input and loading of prepared catalyst were maintained constant.  The plasma-catalytic performance was monitored for methane conversion, H 2 selectivity, specific input energy (SIE) and DBD energy efficiency (EE) using the following equations (Equations (1)-(4)).
Total feed flow rate mL min −1 where n = molar fraction of the gases. Feed flow rate was quantified in mL min −1 was transformed into mmol min −1 applying the conditions; temperature T = 25 • C, p = 1 atm along with a conversion factor, 1 mmol = 24.04 mL [34]. The calculation of the P in calculation is reported elsewhere [34]. The experiments were replicated to minimise experimental errors.  [35]. It also shows the space group (227:Fd-3m), the crystallite sized (average) is recorded at 10.3 nm. In addition, hexagonal structure NiO is detected for the JCPDS # 44-1159 having major peaks at 37.5 • , 43.9 • and 63 • with miller indices of (101), (012) and (110) correspondingly [36]. The space group for hexagonal NiO is R-3m(166) and active phase is NiO 2+ [37]. The surface morphology of MgAl2O4 and Ni/MgAl2O4 is examined using the SEM with magnifications of 5 µm and 500 nm and presented in Figure 4. The MgAl2O4 shows the fine particles with spherical structure, and some particles exhibited the worm-like shapes Figure 4a,b [38]. The two different morphologies of the MgAl2O4 offers a comprehensive and uniform distribution of Ni over the surface depicted in Figure 4c,d. The porous structure of MgAl2O4 offers to diffuse the Ni inside the pores and create actives sites. It may also assist the reactant gas and plasma species interaction later in the plasma-catalytic process.

Physicochemical Properties of the Catalyst
The elemental analysis of MgAl2O4 and 10 wt% Ni/MgAl2O4 are demonstrated in Figure 5a,b. The significant elements O, Mg and Al, were found, and the composition is exhibited inset table and spectrum of Figure 5a. While 10 wt% Ni/MgAl2O4 shows Ni along with O, Al and Mg, which is evident in the presence of Ni in the reported catalyst. The extra peaks without identification are due to the carbon tape and gold coating before the SEM/EDX analysis.
The TGA for 10 wt% Ni/MgAl2O4 is undertaken to analyse the thermal stability of the prepared samples, as shown in Figure 6. The 6% weight loss under 300 °C is observed, and it is ascribed to the moisture and volatile matters depicted in Figure 6, column A. In column B, which temperature is more significant than 300 °C demonstrated no weight loss further to 900 °C. This analysis revealed that the synthesised catalyst is stable for the plasma-catalytic operation for methane cracking in mild conditions [39]. The unstable catalyst may lead to phase modification and sintering later in the methane cracking reactions [40]. The TGA for 10 wt% Ni/MgAl 2 O 4 is undertaken to analyse the thermal stability of the prepared samples, as shown in Figure 6. The 6% weight loss under 300 • C is observed, and it is ascribed to the moisture and volatile matters depicted in Figure 6, column A. In column B, which temperature is more significant than 300 • C demonstrated no weight loss further to 900 • C. This analysis revealed that the synthesised catalyst is stable for the plasma-catalytic operation for methane cracking in mild conditions [39]. The unstable catalyst may lead to phase modification and sintering later in the methane cracking reactions [40].

Plasma and Plasma-Catalytic Test and Reaction Mechanism
The CH 4 cracking is undertaken for performance analysis of plasma and plasma-catalysis presented in Figure 7. The CH 4 conversion for plasma, MgAl 2 O 4 and Ni/Mgal 2 O 4 is recorded as 65%, 73% and 80% respectively at the same experimental conditions (Figure 7a). The plasma only CH 4 conversion is lower as compared to the plasma-catalytic reaction. Plasma only reaction occurs due to the electron-induced dissociation of CH 4 , which is independent of reaction temperature [41]. The CH 4 molecules collide with an energetic electron in the plasma discharge zone at discharge volume (V D ) of 13.5 cm 3 and start to dissociate while overcoming the required dissociation energy of 4.5 eV [22,42]. In plasma only electron-CH 4 interaction is induced, which led to the dissociation reactions and product formation reactions are as follows: Dissociation reactions ((R2)-(R4))  The proposed reaction mechanism for plasma-catalytic CH4 cracking is demonstrated in Figure  8. It can be observed from the H2 selectivity about the reaction mechanism. The activation of CH4 to methyl radical CH3 * and further breakdown in the presence of plasma while attachment to the metal (M, Ni). Similarly, further breakdown leads to the complete dissociation of the C-H bond to form C * and H * . While the recombination of H * formed H2 and released metal (M) [46]. At the same time, the traces of C2H6 has produced from the recombination of CH3 * radicals. There are other possible routes for the formation of HCs, but the analysis of the product is more suitable for proposed pathways.  While loading the catalyst, CH 4 conversion is improving for MgAl 2 O 4 (73%) and Ni/MgAl 2 O 4 (80%). The catalyst loading improves the CH 4 conversion in both cases. In Ni loaded MgAl 2 O 4 shows the highest conversion of CH 4 . The plasma produces hot spots on the catalyst, assist the Ni reduction, also changes catalyst functions, and reduce activation barrier due to gas heating effect [43]. While catalyst enriches the electric field, boost micro discharges and alters the discharge behaviour of DBD plasma. The catalyst-plasma interaction gives surplus effects called synergistic effect, which improves the conversion of CH 4 and EE of DBD catalytic reactor. The MgAl 2 O 4 as a support material is mechanically stable and has porous structure confirmed by SEM, assist in activating CH 4 , and improve the DBD plasma discharge behaviour. Ni further assists the CH 4 activation due to active sites, activated by plasma give more surplus effect and enhanced the conversion by 15%. The plasma only and catalyst loaded DBD system shows the difference in the conversion of CH 4 and activity at certain level justifying by the synergistic effect. Unlike thermal catalysis, plasma-catalysis is not purely temperature dependent reaction. The energetic electron effect on the activation of reactant contributes more than catalytic effect [44]. However, the product selectivity in many cases is improved more as compare to conventional catalysis [45].

Gaseous product formation reactions ((R5)-(R8))
The H 2 and C x H x formation after the recombination of H* and CH X * in governing steps [22]. The H 2 selectivity is noted 62% (Figure 7b), and some traces of C 2 H 6 (1.5%) and C 2 H 4 (1%) are also analysed in GC-FID for plasma only reaction. The H 2 selectivity of MgAl 2 O 4 and Ni/MgAl 2 O 4 is 68% and 75% respectively (Figure 7b). The enhanced H 2 selectivity is explained in the plasma-catalyst interaction mechanism. The undetected C x H x might be the balance for the H 2 and carbon balance in the product analysis due to the limitation of the analysis technique. The EE is lowest for plasma only (0.105 mmol kJ −1 ) while MgAl 2 O 4 (0.115 mmol kJ −1 ) and Ni/MgAl 2 O 4 (0.13 mmol kJ −1 ) shows improvement in the EE due to the higher conversion of CH 4 at constant input power (Figure 7c). The combined effect of plasma and catalyst enhances the EE of the reaction, and hence it is suitable for CH 4 cracking in plasma-catalytic systems to improve EE over MgAl 2 O 4 stable catalyst in mild conditions.
The proposed reaction mechanism for plasma-catalytic CH 4 cracking is demonstrated in Figure 8. It can be observed from the H 2 selectivity about the reaction mechanism. The activation of CH 4 to methyl radical CH 3 * and further breakdown in the presence of plasma while attachment to the metal (M, Ni). Similarly, further breakdown leads to the complete dissociation of the C-H bond to form C * and H * . While the recombination of H * formed H 2 and released metal (M) [46]. At the same time, the traces of C 2 H 6 has produced from the recombination of CH 3 * radicals. There are other possible routes for the formation of HCs, but the analysis of the product is more suitable for proposed pathways. The proposed reaction mechanism for plasma-catalytic CH4 cracking is demonstrated in Figure  8. It can be observed from the H2 selectivity about the reaction mechanism. The activation of CH4 to methyl radical CH3 * and further breakdown in the presence of plasma while attachment to the metal (M, Ni). Similarly, further breakdown leads to the complete dissociation of the C-H bond to form C * and H * . While the recombination of H * formed H2 and released metal (M) [46]. At the same time, the traces of C2H6 has produced from the recombination of CH3 * radicals. There are other possible routes for the formation of HCs, but the analysis of the product is more suitable for proposed pathways.  The stability of the plasma-catalytic CH 4 cracking on Ni/MgAl 2 O 4 catalyst is presented in Figure 9. The CH 4 conversion and H 2 selectivity being partially declining along with the TOS. The CH 4 conversion above 75% while sustaining the EE above 0.125 mmol kJ −1 . Along with the TOS the total reduction in the conversion of CH 4 , and H 2 selectivity is only −5% and −4%, respectively. The negative sign indicates the reduction in the conversion and selectivity. Similar trends can be observed for EE in the 16 h TOS. The stability is mostly attributed to the activation of NiO particles due to plasma species and instant heating. The impurities in the catalyst are also removed by plasma in catalyst expose to plasma [47]. The catalyst activation assists in the CH 4 activation as proposed in the possible reaction mechanism routes. Further, the breakdown of the methyl radical is also assisted by the plasma-catalyst interface while inhibiting the recombination of methyl radical, which is also observed the product analysis of in basic screening [43]. The plasma-catalyst interface improves many aspects since MgAl 2 O 4 is mechanically stable support material and NiO also assist the Ni dispersion. The selectivity of H 2 is also ascribed to the highly basic nature of the MgAl 2 O 4, which improves the CH x * adsorption and assist in the activation and further breakdown [48][49][50]. The CH 4 cracking on plasma-catalytic to CH x heavily depends on the Ni/MgAl 2 O 4 interaction providing the higher coordinate sites in the plasma-catalytic interface, which is expected to achieve in the case for longer TOS. The plasma-catalytic interface gave reasonable stability and improved EE for CH 4 cracking in catalytic-DBD reactor condition.
selectivity of H2 is also ascribed to the highly basic nature of the MgAl2O4, which improves the CHx * adsorption and assist in the activation and further breakdown [48,49,50]. The CH4 cracking on plasma-catalytic to CHx heavily depends on the Ni/MgAl2O4 interaction providing the higher coordinate sites in the plasma-catalytic interface, which is expected to achieve in the case for longer TOS. The plasma-catalytic interface gave reasonable stability and improved EE for CH4 cracking in catalytic-DBD reactor condition.

Characterisation of Spent Catalyst and Reaction Mechanism
The morphology and TGA-DTG analysis of the spent Ni/MgAl2O4 after 16 h TOS is given in Figure 10. The CNTs were observed in SEM analysis (Figure 10a) of spent catalyst along carbon fibres [51]. Mostly the CNTs formed are useful for further utilisation in energy storage application [31]. The TGA analysis (Figure 10b) shows the weight less than 200 °C is ascribed to the volatile matters, while weight lost from 200-400 °C is ascribed to the amorphous carbon. The weight loss beyond 500 is ascribed to the multiwall CNTs [52]. The CNTs can also be seen in SEM micrographs. The nature of the carbon formed is analysed using DTG profile (Figure 10b). The DTG curve at 355 °C, the peak is

Characterisation of Spent Catalyst and Reaction Mechanism
The morphology and TGA-DTG analysis of the spent Ni/MgAl 2 O 4 after 16 h TOS is given in Figure 10. The CNTs were observed in SEM analysis (Figure 10a) of spent catalyst along carbon fibres [51]. Mostly the CNTs formed are useful for further utilisation in energy storage application [31]. The TGA analysis (Figure 10b) shows the weight less than 200 • C is ascribed to the volatile matters, while weight lost from 200-400 • C is ascribed to the amorphous carbon. The weight loss beyond 500 is ascribed to the multiwall CNTs [52]. The CNTs can also be seen in SEM micrographs. The nature of the carbon formed is analysed using DTG profile (Figure 10b). The DTG curve at 355 • C, the peak is ascribed to the amorphous and fibrous carbon formed and ioxidized at less than 400 • C [51]. The DTG curve at 690 • C is ascribed to multiwall CNTs with low defects and low curvature with pure sp 2 structure [53,54]. The formed carbon is ascribed to a stable material for energy storage applications and discharge while increasing the temperature without surface modification [55]. This technology for methane cracking for simultaneous hydrogen and CNT formation is a beneficial process [56]. ascribed to the amorphous and fibrous carbon formed and ioxidized at less than 400 °C [51]. The DTG curve at 690 °C is ascribed to multiwall CNTs with low defects and low curvature with pure sp 2 structure [53,54]. The formed carbon is ascribed to a stable material for energy storage applications and discharge while increasing the temperature without surface modification [55]. This technology for methane cracking for simultaneous hydrogen and CNT formation is a beneficial process [56].

Conclusions
The CH4 cracking in catalytic DBD plasma fixed bed reactor has been studied and found that the plasma-catalytic process enhances the CH4 conversion (80%), improved the EE of the catalytic DBD reactor. The possible interaction between plasma-catalyst enhances the discharge behaviour, active species and improve the contact time between electrons and gas molecules to dissociate and formed the products. The selectivity for H2 is improved to 75% in plasma-catalytic-DBD systems as compared to plasma only CH4 cracking (62%). While EE also improved in such manner 0.13 mmol kJ −1 . The 16 h TOS stability shows a slight declined in the CH4 conversion due to the fibrous carbon and CNT formation confirmed from TGA-DTG analysis. The spent catalyst shows the formation of CNTs which are beneficial for further utilisation for energy storage systems.
The CH4 utilisation in non-thermal DBD plasma for H2 and CNTs formation is a highly desirable route for the simultaneous H2 production and storage for fuel cell applications. Further study is recommended on the cleaning of H2 in cold plasma catalytic systems via membrane or monolith reactor systems.