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Fuel Processing Technology

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Research article

Cold plasma dielectric barrier discharge reactor for dry reforming ofmethane over Ni/ɤ-Al2O3-MgO nanocomposite

Asif Hussain Khojaa,b, Muhammad Tahira, Nor Aishah Saidina Amina,⁎

a Chemical Reaction Engineering Group (CREG), Faculty of Chemical and Energy Engineering, Universiti Teknologi Malaysia (UTM), 81310 Skudai, Johor Bahru, Johor,MalaysiabUS-Pakistan Centre for Advanced Studies in Energy (CAS-EN), National University of Sciences & Technology (NUST), H-12 Sector, 44000 Islamabad, Pakistan

A R T I C L E I N F O

Keywords:Dry reforming of methaneNon-thermal plasmaɤ-Al2O3-MgOSynergistic effectEnergy efficiencySyngas

A B S T R A C T

Dry reforming of methane (DRM) to syngas in a dielectric barrier discharge (DBD) plasma reactor over Ni-loadedɤ-Al2O3-MgO nanocomposite catalysts has been investigated. The catalysts are prepared by modified incipientwetness impregnation method, assisted by cold plasma treatment. The samples are characterized by XRD, N2

adsorption-desorption, H2-TPR, CO2-TPD, FESEM and EDX. The performance of the catalyst for DRM is eval-uated at various specific input energy (SIE Jml−1) and gas hourly space velocity (GHSV, h−1). The maximumconversion achieved are 74.5% and 73% for CH4 and CO2 respectively, over 10% Ni/ɤ-Al2O3-MgO at specificinput energy (SIE)= 300 Jml−1 and gas hourly space velocity (GHSV)=364 h−1. The main reaction productsare H2 (29.5%), CO (30.5%) with H2/CO=1 inferring RWGS reaction is suppressed for 12 h operation time. Theenhanced conversion and yield are due to the strong metal-support interaction, high Lewis basicity and stable10% Ni/ɤ-Al2O3-MgO catalyst as well as the plasma-catalyst interface. The energy efficiency (EE) of the plasma-catalytic DRM is higher (0.117mmol kJ−1) compared to plasma only (0.087mmol kJ−1) demonstrating thesynergy between catalyst and plasma. The reaction mechanism is also proposed to postulate the steps involved inthe DRM.

1. Introduction

The world is facing imminent energy crises due to urbanization andrapid increase in population. The energy supply is mainly based onfossil fuels (82%) while the rest 18% is produced from non-conven-tional energy resources [1]. It is essential to focus on renewable or non-conventional energy resources to reduce energy insecurity and endorsethe concept of sustainable development. Utilization of fossil fuelscaused emissions of greenhouse gases (GHGs), especially the majorcontributor, CO2 (76%) [1,2]. The second highest contributor is CH4

(16%), mainly emitted from agricultural activities, waste management,coal management and biomass burning or handling [3,4]. Utilization ofCO2 and CH4 to produce syngas (CO, H2) and other valuable chemicalsin dry reforming of methane (DRM) (Eq. (1)) could improve the tech-nology for increasing carbon efficiency. DRM produced syngas with thecomposition of H2/CO=1, which is the feedstock for Fischer-Tropsch(FT) synthesis to produced liquid fuels [5].

Several technologies in DRM such as photochemical, electro-chemical and thermochemical pathways with or without catalyst, andall the possible combinations have been accomplished during the lasttwo decades [5–7]. Recently, non-thermal plasma (NTP) for DRM has

attracted many researchers. DRM is usually carried out using dielectricbarrier discharge (DBD), but microwave plasma (MW) and gliding arc(GA) NTPs are uncommon in DRM [8,9]. However, MW and GA havehigher energy efficiency (EE, mmol kJ−1) as compared to DBD plasmain DRM as well as in CO2 splitting [2,10]. Nevertheless, the EE can beimproved by using different potential heterogeneous catalysts for DBDplasma DRM, specifically called plasma-catalysis [11–13]. DBD plasmahas some advantages for DRM since the energy input is low, with easyoperation including catalyst loading and upscaling. DBD has a modestreactor design and also lower installation cost [10,14]. However, themajor problems with DBD plasma DRM are lower energy efficiency andthe deposition of surplus carbon due to the possible reactions such asmethane cracking (Eq. (2)) and Boudouard reaction (Eq. (3)). The de-posited carbon blocks the active site of catalyst, causing sintering ofmetal particles and deactivating the catalyst, as well as decreases theDRM process performance [15].

+ ⇔ + ° =°−CH CO CO H H kJ mol2 2 Δ 2474 2 2 25 C

1 (1)

⇔ + ° =°−CH C H H kJ mol2 Δ 75C4 2 25

1 (2)

⇔ + ° = −°−CO C CO H kJ mol2 Δ 172C2 25

1 (3)

https://doi.org/10.1016/j.fuproc.2018.05.030Received 7 March 2018; Received in revised form 23 May 2018; Accepted 23 May 2018

⁎ Corresponding author.E-mail address: noraishah@cheme.utm.my (N.A.S. Amin).

Fuel Processing Technology 178 (2018) 166–179

Available online 01 June 20180378-3820/ © 2018 Elsevier B.V. All rights reserved.

T

Different types of catalysts have been synthesized for NTP DRM toenhance the plasma-catalytic performance and stability by inhibitingcarbon deposition [16–18]. Noble metals and transition metals (Pt, La,Ce, Ru, Rh) catalysts have seemed preferable due to their better stabi-lity in DRM, but the high cost makes the noble metals unsuitable forcommercial practice [19–21]. Ni-based catalyst is widely considered forplasma DRM due to availability and low cost. Regrettably, the Ni cat-alyst inevitably deactivated due to metal sintering and carbon deposi-tion in NTP catalytic DRM [22,23]. The most common supports used inNi-based catalysts are ɤ-Al2O3, MgO, ZnO, TiO2, ZrO2 and multi-ele-mental supports [2,24,25]. Ni supported Al2O3 has been extensivelyused and reported unstable in NTP catalytic DRM owing to the rapidcoke formation and sintering of active phase [26]. The activity andstability of the Ni-based (Ni/Al2O3) catalyst can be improved with theaddition of alkaline-earth metal oxides such as MgO and CaO [17].Combination of Al-Mg based support has high potential to process feedgases in thermal DRM [26]. The addition of MgO to Ni/Al2O3 as apromoter assist to increase the basicity of the support as MgO is a strongbasic oxide. MgO can also enhance the interaction of metal (Ni) andsupport (Al2O3) affects the reducibility and stability. With ɤ-Al2O3

having a high surface area and combined with MgO, a strong basicoxide, the composite catalyst can be suitable for NTP catalytic DRM[27]. DRM process is acidic in nature owing to the acidic properties ofCO2; hence, the addition of MgO to ɤ-Al2O3 enhances the basicity andstability of the catalyst. It can further suppress the reverse water gasshift reaction (RWGS) (Eq. (4)) as well as the hydrogenation of CO2

(Sabatier reaction) towards methane and water formation (Eq. (5)). Thestrong basic sites allow adsorption and activation of high CO2 con-centration on the support surface, thus increasing the CO2 conversion.Meanwhile, CH4 activation and dissociation are usually subject toplasma generated energetic species and active metal [28,29].

+ ⇔ + ° =°−CO H CO H O H kJ molΔ 41.1C2 2 2 25

1 (4)

+ ⇔ + ° = −°−CO H CH H O H kJ mol4 2 Δ 164.9C2 2 4 2 25

1 (5)

Catalyst treatment and activation methods significantly affect thecatalyst morphology and catalytic performance. Most recently, there isa certain focus on the synthesis of green catalyst by applying coldplasma for treatment to reduce metal ions into metal oxide and also toactivate the catalyst other than by the conventional thermal calcination[30]. Liu et al. [30] studied the application of cold (DBD) plasma forcatalyst treatment resulting in reduced particle size, improved metaldispersion and enhanced the metal-support interaction. DBD plasmatreatment eliminated high energy consumption during high-tempera-ture calcination and the reduction process [30]. In another develop-ment, Estifaee et al. [31] studied the effect of plasma treatment (glow

discharge) on catalyst morphology and DRM activity. It was reportedthat the plasma treated sample has 11% higher surface area, betteractivity and significant resistance to carbon deposition than a conven-tional calcined catalyst.

In light of the recent advancement in DBD plasma reactor for DRM,the synergy between catalyst and DBD reactor is still a major challenge.Zhang et al. [32] studied various material loading and the effect oncatalytic activity, but the relation with discharge volume (VD) andcatalyst volume (Vcat) in a packed bed DBD system have not beenconsidered. Recently, Tu et al. studied the promotional effect of Mg onNi/Al2O3 in temperature controlled DBD reactor placed in a tube fur-nace, however, the insights of the reactor configuration based on Vcat

and VD was not elucidated as the discharge behaviour majorly based oncatalyst loading and material behaviour. The DBD reactor configurationcorresponding to a catalyst volume (Vcat) and discharge volume (VD) isproportional to the feed flow rate of a packed-bed catalytic DBD plasmareactor. The relation between Vcat and VD with corresponding to gashourly space velocity (GHSV) in a packed-bed DBD reactor are useful inthe selection of the appropriate catalyst loading, as it directly con-tributes to the energy efficiency (EE) of the catalytic DBD plasma DRM.

The aim of this work is to investigate the catalyst-plasma synergy indifferent DBD reactor configurations to enhance the energy efficiency.Ni/ɤ-Al2O3-MgO (Ni-AM) nanocomposites were synthesized by mod-ifying incipient wetness impregnation method. The samples were thentreated by DBD plasma to activate the catalyst for DBD plasma DRM.The catalysts were characterized by X-ray Diffraction (XRD),Brunauer–Emmett–Teller (BET), H2 temperature-programmed reduc-tion (H2-TPR) and CO2 temperature-programmed desorption (CO2-TPD), field emission scanning electron microscopy (FE-SEM) and en-ergy dispersive X-ray (EDX). The catalytic activity test was carried outin a packed-bed DBD plasma reactor. The effect of various Ni loadings,gas hourly space velocity (GHSV, h−1) and specific input energy (SIE,ml kJ−1) were further investigated for Ni-AM along with time onstream and carbon balance. The different reactor configurations basedon Vcat and VD corresponding to GHSV in a DBD plasma reactor hasbeen critically analysed and reported. Furthermore, reaction me-chanism has been proposed to rationalize the enhanced performance ofNi-AM.

2. Experimental

2.1. Catalyst preparation

Ni-based catalyst was prepared by modifying incipient wetnessimpregnation method followed by cold plasma treatment and activation

Fig. 1. Schematic of catalyst preparation and activation: (a) preparation of Ni/ɤ-Al2O3, Ni/MgO and Ni/ɤ-Al2O3-MgO catalyst; (b) DBD plasma setup for catalysttreatment and activation.

A.H. Khoja et al. Fuel Processing Technology 178 (2018) 166–179

167

as illustrated in Fig. 1(a–b). The ɤ-Al2O3 (99% Sigma Aldrich) and MgO(98% Sigma Aldrich) powder with a weight ratio of 2:1 were used ascatalyst supports. Nickel nitrate hexahydrate (Ni(NO3)2·6H2O) (99%,Sigma Aldrich) was dissolved in citric acid (99%) (Sigma Aldrich) so-lution and stirred for 10min at 60 °C. ɤ-Al2O3 and MgO were added to ametal-citrate solution to enhance the metal dispersion over supportsurface and kept in vigorous stirring for 3 h at 110 °C [33]. Finally, theslurry was dried overnight in the oven at 110 °C.

The prepared samples were treated and calcined using cold DBDplasma (Fig. 1(b)). The prepared sample (2 g) was loaded in a quartztube having 10mm inner diameter wrapped with stainless steel mesh asa ground electrode. The stainless steel rod was inserted in the middle ofquartz tube functioning as high voltage (HV) electrode. DBD plasmawas supplied for 45min, applying input power 100W (SIE, 150 Jml−1)with a frequency of 7.5 kHz for calcination and activation. The N2 at aflow rate of 40mlmin−1 was used for calcination and activation of thecatalyst. The measured temperature was in the range of 280–300 °C atconstant power. The temperature of the DBD plasma reactor wasmeasured by using an infrared (IR) thermometer, Fluke 572-2 (FlukeCorporation USA). The catalysts turned blackish from bluish-white afterplasma treatment and activation. The 10wt% Ni/ɤ-Al2O3, 10 wt% Ni/MgO and 10wt% Ni/ɤ-Al2O3-MgO were coded as Ni/A, Ni/M and Ni/AM respectively.

2.2. Material characterization

The XRD patterns were obtained by the Rigaku Smart Lab, X-raydiffractometer (Rigaku Corporation, Japan) using Cu-kα radiation(40 keV, 40mA) at a scanning rate of 1.2° min−1 and scanning from5°–90° of 2θ. The diffractometer was equipped with a Ni-filtered Cu-kαradiation source (l= 1.54056 Å), operated at 40 kV and 200mA.

The morphology of the prepared samples were examined before andafter the DRM reaction by field emission scanning electron microscopy(FE-SEM) and energy-dispersive X-ray spectroscopy (EDX). FE-SEM andEDX were conducted in Hitachi SU8020 (Hitachi High-TechnologiesCorporation, Japan) integrated with the beam of X-MaxN by OxfordInstrument optics and full control of the probe current from 1 pAto> 5 nA [34].

N2 adsorption-desorption isotherms were recorded at 77 K on aThermo Scientific Surfer Analyzer (Thermo Fisher Scientific, Italy).Prior to each analysis, the samples were degasified at 200 °C for 4 h toensure complete removal of moisture. The Barrett–Joyner–Halenda(BJH) method was used to calculate the average pore diameter.

H2-TPR was carried out to analyse the reduction profile of Ni inMicromeritic AutoChem II 2920 Chemisorption Analyzer(Micromeritics Instrument Corporation, USA). The catalyst (50mg) wasdried at 200 °C in the flow rate of 30mlmin−1 helium gas (99.99%) in aquartz tube. The temperature was allowed to decrease to 50 °C and H2/Ar (5%) gases flowed through the sample at a flow rate of 30mlmin−1.Then, the catalyst was continuously heated at the rate of 10 °Cmin−1 to1000 °C. The signals for H2 (m/e= 2) and H2O (m/e= 18) were noted.

CO2-TPD was conducted in a quartz tube on Al2O3 and Ni-AM freshcatalyst using Micromeritic AutoChem II 2920 Chemisorption Analyzer(Micromeritics Instrument Corporation, USA). 50mg sample was pre-heated with He gas (30mlmin−1) at 700 °C for 1 h and left to cool atroom temperature. CO2 was adsorbed at 50 °C for 1 h and weakly ad-sorbed CO2 was removed by purging with He gas at a flow rate of30mlmin−1. The sample was heated from 50 to 900 °C at the rate of10 °Cmin−1 in the flow of He gas and desorption of CO2 was detectedby TCD. CO2 desorption amount was estimated by integrating TPDprofile with distributing fitting software (BEL Japan).

2.3. DBD-plasma reactor setup

The schematic DBD plasma system is demonstrated in Fig. 2. TheDBD system consists of four major parts: gas feeding system, alternating

current (AC) power supply, plasma reactor and gas analysis system. Thegas feeding system consisted of CH4 and CO2 gas cylinders, a gas mixer,and mass flow controllers (ALICAT Scientific 200 SCCM, USA) for bothreactant gases. The AC power supply (CTP-2000K, China) was in-tegrated with a voltage regulator (1–30 kV) to generate the plasma. TheTektronix TDS 2012B two-channel digital oscilloscope with a highvoltage probe (Tek P6015A) was used to assess the input voltage, fre-quency and total current. The alumina dielectric tube has a length of35 cm with inner diameter 10mm and outer diameter 12mm. Thecatalyst was incorporated in the middle of the discharge zone packedwith glass wool. A high voltage stainless steel electrode having a dia-meter of 4mm and elongated ground electrode aluminium with length20 cm was used to generate the plasma and DBD effect. The gas analysissystem comprised of an online Gas Chromatograph (GC) (Agilent6890N) equipped with thermal conductivity detector (TCD) and flameionization detector (FID). A HP-PlotQ capillary column (Agilent,40m×0.53mm ID, 40 μm) for detecting air and CO2, one unit ofMolsieve capillary column (Agilent, 30 m×0.530mm ID, 25 μm) forseparating H2, O2, N2, CH4 and CO were connected to the TCD detector.In addition, one unit of HayeSep Q-Supelco, (6 ft× 1/8 in.ID× 2.1mm OD, 80/100mesh) for backflushing C2 to C6 was con-nected to the TCD. For separating C1 to C6 hydrocarbons, a unit of GS-GasPro (Agilent, 60m×0.32mm ID) capillary column was connectedto the FID detector.

CH4 and CO2 with purity of (99.9%) were used as feed gases withthe feed ratio 1:1. Plasma power supply integrated with voltage reg-ulator produced the plasma. Digital mass flow controllers measured theflow rates of the input gases. The reactor consisted of a high tempera-ture and frequency resistant alumina tube, HV electrode inserted in themiddle of an alumina tube and wrapped with the ground electrode. Twopolytetrafluoroethylenes (PTFE) end pieces were fitted at both ends ofthe reactor. Each end piece has a hole as gas inlet and outlet and onecentral hole for passing the high-voltage electrode. In the reactor con-figuration, two different approaches were made to study the relationbetween the length of the ground electrode, DL, VD and Vcat. DL wasvaried (DL=5–20 cm) to achieve different VD at the constant dischargegap (Dgap) 3mm. The effect of GHSV was studied in three differentapproaches. (i) Varying total flow rate at constant VD and Vcat (GHSV);(ii) VD was adjusted at constant Vcat and total flow rate (GHSV′); and(iii) catalyst bed volume (Vcat) varied at different catalyst loadings (0.5,1, 2 g) at constant flow rate of reactant gases (GHSV″). The output flowrate was measured by a digital bubble flow meter and analysed by anonline Gas Chromatograph (GC) equipped with TCD and FID.

2.4. Plasma-catalyst activity test

The plasma-catalytic activity tests were performed to analyse theperformance of catalysts in DBD plasma DRM at atmospheric pressureand room temperature. The responses comprised of reactant conver-sion, selectivity, yield, carbon balance, and EE and were calculatedaccording to Eqs. (6)–(16). The time on stream was tested to analyse thestability of the selected catalyst.

⎜ ⎟= ⎛⎝

− × ⎞⎠

CH conversion X nCH nCHnCH

( )% ( ) ( )( )

100CHin out

in4

4 4

44 (6)

⎜ ⎟= ⎛⎝

− × ⎞⎠

CO conversion X nCO nCOnCO

( )% ( ) ( )( )

100COin out

in2

2 2

22 (7)

⎜ ⎟= ⎛⎝ − + −

× ⎞⎠

CO selectivity S nCOnCH nCH nCO nCO

( )% ( )[( ) ( ) ] [( ) ( ) ]

100COout

in out in out4 4 2 2 (8)

⎜ ⎟= ⎛⎝ × −

× ⎞⎠

H selectivity S nHnCH nCH

( )% ( )2 [( ) ( ) ]

100Hout

in out2

2

4 42 (9)

A.H. Khoja et al. Fuel Processing Technology 178 (2018) 166–179

168

⎜ ⎟= ⎛⎝

×− + −

× ⎞⎠

C H selectivity S nC HnCH nCH nCO nCO

( )% 2 ( )[( ) ( ) ] [( ) ( ) ]

100C Hout

in out in out2 6 2 6

2 6

4 4 2 2

(10)

⎜ ⎟= ⎛⎝ ×

× ⎞⎠

H yield Y nHnCH

( )% ( )2 ( )

100Hout

in2

2

42 (11)

⎜ ⎟= ⎛⎝ +

× ⎞⎠

CO yield Y nCOnCH nCO

( )% ( )( )

100COout

in4 2 (12)

⎜ ⎟= ⎛⎝ +

× ⎞⎠

Y nC HnCH nCO

C H yield( )% ( )( )

100C Hout

in2 6

2 6

4 22 6 (13)

⎜ ⎟⎛⎝

⎞⎠

= ⎛⎝

× ⎞⎠

−

−SIE JmL

Power JTotal flow rate ml

( sec )( min )

60 sec1 min

1

1 (14)

⎜ ⎟= ⎛⎝

+ + ++

× ⎞⎠

Carbon balance, C (%) (nCH ) (nCO ) (nCO) 2(nC )(nCH ) (nCO )

100outB

4 out 2 out out 2

4 in 2 in

(15)

⎜ ⎟⎛⎝

⎞⎠

= ⎛⎝

− + − ⎞⎠

−

−EE mmolkJ

nCH nCH nCO nCO mmolPower kJ

[( ) ( ) ] [( ) ( ) ]( min )( min )

in out in out4 4 2 2 1

1

(16)

whereas the ‘n’ is a molar fraction. The flow rate was measured inmlmin−1 and converted to mmol min−1 using conditions T=25 °C,P=1 atm and conversion factor, 1 mmol= 24.04ml for calculating SIEand EE. The input power (P) was calculated by measuring the inputpeak voltage (V) and total current (I) at a constant frequency(f=7.5 kHz) as explained in Eq. (17) [35,36].

∫=P f t I t dtV( ) ( )t

t

0

1

(17)

3. Results and discussion

3.1. Catalyst characterization

XRD was carried out to identify crystallography of the preparedcatalysts. Fig. 3 and Table 1 shows XRD patterns with detail crystal-linity of Ni-A, Ni-M and Ni-AM after plasma treatment and activation ofcatalyst samples. The XRD patterns confirmed the formation of cubiccrystalline phases of NiO, ɤ-Al2O3 and MgO. The ɤ-Al2O3 cubic phase(space group, fd-3m (227)) is detected in at 19.07°, 32.55°, 45.06° and66.05° for faces (hkl) (100), (311), (400) and (440), respectively(JCPDS#50-0741). The NiO index peak appeared in 37.24°, 43.99° and63.1° for face (hkl) (111), (200) and 220 respectively (JCPDS No. 44-1159) (space group, Fm-3m (225)) [37,38]. The crystallinity of MgOhas been decreased with the addition of ɤ-Al2O3. MgO peaks, detectedat 37.24°, 43.99°, 63.1° and 78.6°, correspond to hkl (200), (211) (222)and (311) (JCPDS No, 45-0946) (space group, fm-3m (225)) and are ingood compromise with literature [12,27]. The weak peak concentrationof Al2O3 in Ni-AM is perhaps due to cold plasma treatment (< 300 °C)[26]. The d-spacing and crystallite size are listed in Table 1 corre-sponding to the faces of Al2O3, NiO and MgO [39,40]. The averagecrystallite size is 12.3 nm, 13 nm and 15.5 nm for NiO, Al2O3 and MgOrespectively.

Fig. 2. Experimental setup of catalytic-DBD plasma reactor system.

Fig. 3. XRD patterns of 10wt% Ni-A, 10 wt% Ni-M and 10 wt% Ni/AM cata-lysts.

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Fig. 4 demonstrates the FE-SEM micrographs of 10 wt% Ni-AM na-noparticles with different magnification to analyse the morphology andmetal dispersion over catalyst support. Fig. 4(a) displays the nano-particle uniform particle size in Ni-AM catalyst at 1.0 μmmagnification.Ni-AM nanocatalyst demonstrates the homogeneous structure of theparticles. Fig. 4(b) reveals uniform particle size and it assists in themetal dispersion to increase the active sites. The successful preparationof controlled morphology is due to the addition of citric acid as sur-factant and cold plasma treatment [27,30,41].

N2 adsorption-desorption isotherms of Ni-A, Ni-M and Ni-AM sam-ples are demonstrated in Fig. 5(a). The samples of Ni-A and Ni-AMdisplays type IV isotherm with a hysteresis loop of H3 type, an in-dicative of mesoporous material [42]. However, Ni-M sample displaystype II isotherm. The volume adsorbed at higher P/Po caused by ad-sorption occurs by mesopores. The adsorption of N2 is lower in low P/Po, which interprets the minimal formation of micropores in the sam-ples. The Ni-AM samples depict the hysteresis (H3) loop. The hysteresisloop for Ni-AM is higher (non-identical) to that of Ni-A and Ni-M due tothe distortion of Al and Mg crystal lattice. At higher P/Po in type IVisotherms, the higher N2 is adsorbed due to the capillary condensationbelow the expected condensation pressure of the N2 [43]. According tothe classification of IUPAC, the material shows H3 type hysteresis loophaving mesoporous structure and the pores are interconnected to eachother [44]. The pore size distribution of Ni-A, Ni-M and Ni-AM arepresented in Fig. 5(b). The Ni-AM nanocomposite demonstrates thepore diameter of the sample is mostly between 5 and 30 nm with a highintensity of volume uptake, compared to Ni-A and Ni-M.

Table 2 depicts the specific surface area, pore volume, pore dia-meter and average crystallite size of the prepared samples. The porevolume for Ni-A, Ni-M and Ni-AM samples were 0.242, 0.118 and0.196 cm3 g−1, respectively. Furthermore, the majority of the porediameter culminates< 30 nm for all prepared samples. The pore dia-meter of Ni-M is higher as compared to Ni-A, while the nanocompositeNi-AM displays a moderate BJH pore diameter. The reported samplesare categorized as mesoporous since the distribution of the pore size is

mostly between 2 and 50 nm [43]. The surface area of Ni-A(119m2 g−1) is higher that Ni-M (36m2 g−1), whereas after combiningin composite, the surface area of Ni-AM is 74m2 g−1. The low surfacearea of Ni-AM as compared to Ni/A is ascribed to the Ni dispersion oversupport surface and it may block the pores, consequently, the MgO hasalso a lower surface area, which definitely reduced the surface area ofcomposite Ni-AM.

H2-TPR was carried out to determine the interaction between Ni andsupport, as well as the reducibility of the active phase of the species andcorrelates to the DBD plasma treatment and activation. Fig. 6(a) showsTPR profile for the Ni-AM nanocomposite catalyst. The first two peaksat low temperature are attributed to the reduction of Ni+2 crystallites at485 °C and 572 °C to NiO and interacting with support [12]. The low-temperature signals can also be attributed to the β-NiO interaction withAl2O3 and reduction of NiO-MgO. The second peak at high temperatureis associated with ɤ-NiO at 904 °C to NiO and interprets the strong in-teraction with support [45,46]. The high-temperature peak is moreresemblance to the ɤ-NiO having stronger interaction as compared to β-NiO [43]. The factors usually affect the nickel reduction are Ni loading,calcination temperature and interaction of NiO and support. In thiscase, samples are treated and activated using cold plasma rather thanthe conventional calcination and reduction methods to reduce the bulknickel oxides. The reducibility of the Ni is improved by cold plasmadecomposition and Ni atoms can diffuse into the support uniformly dueto the cold plasma effect reported elsewhere [47]. The total H2 uptakeby Ni-AM at three different peaks are 40.19 μmol g−1, 72.54 μmol g−1

and 430.5 μmol g−1 at 485.4 °C, 572 °C and 904.2 °C, respectively(Table 3).

The basicity of catalyst has significant influence in the catalyticactivity in NTP DRM due to the acidic nature of CO2 in the reaction.Strong basic sites can enhance the catalytic activity as CO2 is acidic innature and it increases the chemisorption and activation of reagent gas[29]. CO2-TPD was carried out to analyse the basic sites of fresh Ni-AMand Al2O4 (control) catalyst as depicted in Fig. 6(b–c). A weak peak hasbeen analysed at 50–200 °C against the TCD signal in column (I) forAl2O3 and Ni-AM. The lower temperature peaks indicate that the ad-sorbed CO2 can be easily disabled from basic sites, inferring the Al2O3

has more weak basic sites than Ni-AM catalyst. The peak intensity ofstrong basic sites is much higher than lower temperature, weak basicsites in Ni-AM, as the CO2 uptake by strong basic sites is much higherfor Ni-AM as depicted in Table 3. The distribution of the basic sites, i.e.weak, intermediate, strong and very strong, on the catalyst correspondsto the peak within the temperature in the ranges of 20–150 °C,150–300 °C, 300–450 °C and>450 °C in CO2-TPD profile [12,48]. Thepeaks show the strong basic sites in temperature ranges up to200–500 °C in Ni-AM, while weak sites were analysed at 20–150 °C forAl2O3 indicating lower basicity. Furthermore, an elbow (peak) has beendetected in both samples at 825 °C and 833.3 °C with CO2 uptake washigher for Ni-AM than Al2O3 which has no significant CO2 uptake. The

Table 1XRD analysis of the prepared fresh Ni-AM nanocomposite.

2 θ (degree) hkl indices d-Spacing (nm) Crystallite size (nm)

NiO 37.49 (111) 0.288 1244.5 (200) 0.25 1363.99 (220) 0.176 12

Al2O3 19.07 (100) 0.50 1432.55 (311) 0.28 1845.06 (400) 0.20 1066.05 (440) 0.14 10

MgO 37.24 (200) 0.25 2243.99 (211) 0.20 1363.1 (222) 0.14 1278.6 (311) 0.12 20

Fig. 4. FE-SEM micrograph of the 10wt% Ni-AM with different magnifications (a) 1 μm (b) 500 nm.

A.H. Khoja et al. Fuel Processing Technology 178 (2018) 166–179

170

strong Lewis basic sites have the capability of high CO2 uptake and caninhibit carbon deposition in DRM.

3.2. Performance analysis of prepared catalysts

The performance of the Ni-A, Ni and Ni-AM catalysts were carried

Fig. 5. (a) Nitrogen adsorption–desorption isotherms of prepared catalyst 10 wt% Ni-A, 10wt% Ni-M and 10wt% Ni-AM (b) Pore size distribution (B.J.H. des) ofprepared samples 10wt% Ni-A, 10 wt% Ni-M and 10wt% Ni-AM.

Table 2BET specific surface area and porous characteristics of prepared samples.

Sample (BET)specific surfacearea (m2 g−1)

BJH adsorptionpore volume(cm3 g−1)

BJH porediameter(nm)

Crystallite size(nm)

Ni-A 119 0.242 13.6 10.9Ni-M 36 0.118 23.2 13.6Ni-AM 74 0.196 16.2 11.8

Fig. 6. (a) H2 consumption by H2-TPR for fresh 10 wt% Ni-AM (5% H2/Ar flow 30mlmin−1 at 10 °C from 50 to 1000 °C); (b) CO2-TPD profile for fresh Al2O3 (c) CO2-TPD profile for fresh 10wt% Ni-AM.

Table 3Analysis of reducibility and basicity of the prepared catalysts.

H2-TPR Peaks Temperature °C H2-uptake μmol g−1 cat Peak conc. (%)

Ni/AM 1 485.4 40.2 692 572 72.5 68.93 904.2 430.5 69.8

CO2-TPD Peaks Temperature °C CO2-uptake μmol g−1 cat Peak conc. (%)

Al2O3 1 87.7 135.4 88.42 825.5 81.2 89.9

Ni-AM 1 140 83.7 99.12 360 354 99.83 833.3 92.5 99.9

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out to analyse the DRM activity in the DBD plasma reactor and reportedthe results with 3% error margin. The conversion of CO2 and CH4 overdifferent catalysts and product selectivity is depicted in Fig. 7(a). TheCH4 conversion of 74%, prevailed over 10 wt% Ni-AM nanocompositecompared to Ni/A, Ni/M and plasma only reaction. Ni-A and Ni-Mconferred CH4 conversion of 67.5% and 66%, respectively while forplasma only registers the lowest conversion (63%). Moreover, the CO2

conversion was much higher in Ni-AM (73.5%) compared with Ni-A(64%) and Ni-M (65%), respectively. The CO2 conversion in plasmaonly was recorded the lowest (59%). The higher conversions of CH4 andCO2 in Ni-AM nanocomposite as compared to Ni-A, Ni-M and plasmaonly DRM is attributed to the well-dispersed Ni on the catalyst surfaceand the basic nature of the composite catalyst due to the addition ofMgO as a promoter [17]. As discussed in the CO2-TPD analysis, strongbasic sites exist in Ni-AM, that could enhance the chemisorption andactivation of reactant gas resulting in high conversion. The presence ofNi in Ni/AM increased the active sites for the activation and dissocia-tion of CH4 in the plasma-catalyst DRM.

The H2 selectivity is the highest over Ni/AM (46%) as compared toNi/A (37%), Ni/M (37.5%) and plasma only (Fig. 7(a)). The CO se-lectivity is the following order: Ni/AM (47.5%) > Ni/A (43%) > Ni/M (40%) > plasma only (38%). Consequently, the ratio of H2/CO wasmuch lower than unity for Ni/A, Ni/M and plasma only. The H2 and COratio obtained in Ni-AM was closed to unity (0.98) as shown inFig. 7(b), suitable for FT synthesis for synthetic liquid fuels production[11,29,49]. Higher selectivity of CO in plasma only and Ni/A indicatesthe occurrence of RWGS (Eq. (4)) which utilizes H2 and CO2 to produceCO and H2O [50]. The H2/CO ratio in Ni-AM indicates RWGS is sup-pressed due to the highly basic nature of the catalyst. The formation ofC2 hydrocarbons (C2H6) is higher in plasma only and Ni-A which in-terprets the bulk formation and recombination of methyl radicals.However, Ni-AM enhances the H2 production owing to the suppressedrecombination of CH3 and H species or possibly a further breakdown ofC2H6 due to the electric field enhancement in the presence of a Ni/AMcatalyst. The H2 yield might be enhanced due to the chemical kineticchanges, as a consequence of significant heating of gas mixture in the

Fig. 7. Performance analysis (a) plasma-catalytic DRM activity; (b) H2:CO ratio and EE; plasma only, 10% Ni-A, 10% Ni-M and 10% Ni-AM; SIE= 300 Jml−1,catalyst loading=0.5 g, Dgap= 3mm, DL= 20 cm, feed ratio= 1:1, GHSV=364 h−1.

Fig. 8. Effect of Ni loading on the plasma-catalytic activity of Ni-AM (a) conversion of CH4 and CO2 (b) selectivity of H2, CO and C2H6; SIE=300 Jml−1,GHSV=364 h−1, catalyst loading=0.5 g, Dgap= 3mm, DL= 20 cm, feed ratio= 1:1, VD=10.5 cm3.

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presence of a catalyst, since product distribution is highly dependent ongas temperature [51]. The EE is higher in Ni-AM (0.11mmol kJ−1) thanNi-A, Ni-M and plasma only DRM at the same experimental parameters.Thus, the Ni/AM catalyst is selected as the best catalyst for furtherparametric and reactor configuration study.

Various Ni loadings (5, 10, 15 and 20wt%) in AM catalyst supportwas tested to examine the effect of Ni content on the catalytic activity inDBD plasma for DRM as presented in Fig. 8. Increasing Ni loading,partly increases the conversions of CH4 and CO2 (Fig. 8(a)). The con-versions of reactant gases in 5% Ni are lower than that of 10–20wt% Niloading. However, there is no significant improvement in the conver-sion of reactants in 15% Ni and 20% Ni as compared to 10% Ni loading.H2 selectivity increases with Ni loading from 5 to 10wt%, while mar-ginally decreases over 15–20wt% of Ni loading as shown in Fig. 8(b).More importantly, CO selectivity gradually increases probably due tooxidation of deposited carbon (Fig. 8(b)). The deposited carbon may beoxidized to CO by interacting with oxygen species that are generatedfrom CO2 and it is usually referred to as carbon gasification [52,53].The conductive Ni active sites dispersed on the catalyst surface con-tributes to CH4 dissociation to form a reactive carbon, which takes partin carbon gasification [28]. The selectivity of C2H6 is not significantlyaffected. The higher Ni content may lead to the formation of carbonsince high Ni loading inhibits the dispersion of metal over supportsurface, and rate of monatomic carbon formation over gasification isexpected to accumulate and promote filamentous carbon. Coke de-position is also dependent upon the different carbon formation routesand its oxidative removal [54,55]. Hence, the loading of active metaland its dispersion on support play a substantial role in the plasma-catalysis system.

3.3. Time on stream and carbon balance

The effect of the time on a stream of Ni-A, Ni-M and Ni-AM catalysthas been carried out for 12 h operation time at constant GHSV, feedratio, Vcat, and VD in DBD cold plasma reactor for DRM as shown inFig. 9(a). After 12 h of operating time, the conversions of CH4 and CO2

decreased for Ni-A and Ni-M (Fig. 9(a)) due to coke formation throughmethane cracking and Boudouard reactions [31]. Ni-AM displays betterstability compared to Ni/A, and Ni/M in 12 h operation time.Fig. 9(b–c) indicated no severe reduction in selectivity and yield of COand H2 in Ni-AM. The high activity and stability of Ni-AM may be at-tributed to the high Lewis basicity as well as the high dispersion ofactive metal on ɤ-Al2O3-MgO due to the cold plasma treatment andactivation [30]. The carbon balance (CB) explains the carbon formationduring the DRM process (Eq. (15)). Carbon deposition is lower (15%)for Ni/AM as compared to Ni/A and Ni/M during the operation time(12 h) for Ni-AM (Fig. 9(d)). Meanwhile, CB for Ni-A and Ni-M sig-nificantly dropped owing to the deposition of filamentous carbon [28].The EE of the Ni-AM is higher than that of Ni-A and Ni-M, as depicted inFig. 9(d), confers Ni-AM gave a better plasma-catalytic performancecompared to Ni-A and Ni-M. Table 4 summarizes the comparison of thepresent study with previously reported literature for catalytic DBD re-actor. The plasma-treated catalyst registers high conversion, selectivityand H2: CO ratio close to unity – a requisite for subsequent syngasprocessing. The previous studies reported superior conversions and EEbut the syngas composition (H2: CO < 1) is less suitable for the gas toliquid technology (GTL) [18]. Furthermore, the different experimentalconditions such as catalyst loading, reactor configuration (dischargevolume, discharge gap, electrode morphology, etc.), and power inputplay a significant role in feed processing. The comparison in Table 4presents the various experimental conditions affect the EE of DBD re-actor.

The elemental distribution of spent alumina tube and catalyst (Ni-AM) are demonstrated using EDX. Fig. 10(a) exhibits carbon formationon the surface of the alumina reactor tube in plasma only DRM. Thecarbon formation in plasma only DRM was higher than coupling with

nanocomposite Ni/AM depicted in elemental analysis. This interpreta-tion is also supported by carbon balance (CB). The carbon deposited onthe catalyst is insignificant after the reaction (Fig. 10(b)). The samplecomprised of O, Al and Mg with higher peak intensity in EDX analysis.While Ni and carbon showed lower intensity peaks in EDX chromato-gram. MgO resisted carbon formation due to the strong basic nature,which promoted chemisorption and activation of CO2 [56]. Activemetal generated carbonaceous species, possibly via methane cracking,reacted with oxygen produced from CO2 or CO and contributed to thelow carbon formation.

3.4. Effect of specific input energy (SIE)

The effect of SIE was investigated for Ni-AM catalyst by varying theinput power from 50 to 150W adjusting the input voltage through avoltage regulator at constant applied frequency 7.5 kHz. The inputpower was adjusted by applying different applied voltages from 10 to20 kV at constant GHSV, Vcat and VD. SIE is inversely proportional toEE. Fig. 11 depicts the effect of SIE (Jml−1) on the catalytic activity ofNi-AM nanocomposite. The conversion of CH4 and CO2 increased whileincreasing the SIE by adjusting the input power at a constant feed flowrate. Increasing the input power could enhance electric field and elec-tron density in discharge zone, hence endorsed the generation of activespecies, promote collision of active species with the reactant gases byaltering the discharge behaviour and boost the dissociation process ofCH4 [28,57,58]. Furthermore, the conversion of CH4 was partiallyhigher than CO2 due to the lower dissociation energy of CH4 (4.5 eV)compared to CO2 (5.5 eV) and higher threshold energy, 10 eV and11.9 eV respectively [36]. The EE is decreasing while increasing the SIEin same experimental conditions. At SIE 150 Jml−1 the EE is0.128mmol kJ−1, while at 300 and 450 Jml−1, the EE is recorded0.117mmol kJ−1 and 0.105mmol kJ−1 respectively. The increment inthe SIE also increases the conversion of reactant gases but the net effecton EE is not straightforward.

The selectivity of H2 and CO increased with higher SIE; however,the C2H6 selectivity decreased. This suggests that the increase in SIE canchange the reaction pathways and enhances the H2 yield by suppressingthe recombination CH3 and H, or may assist the breakdown of methylradicals [28]. The discharge behaviour and the conversion efficiencymay also be changed due to the catalyst-plasma synergy. The sy-nergistic effect can be easily achieved in plasma-catalysis as the plasmahelps to intensify a surplus effect [59]. Plasma establishes a strongelectric field and generates the reactive species, ions, electrons andphotons to the surface and catalyst lowering the activation barrier forcertain reactions [60]. The interaction between plasma-catalyst is in-terdependent, the effect of plasma changed the catalyst physio-che-mical properties, while the catalyst modified the surface reactions bylowering the activation barrier and the discharge behaviour of plasma[59]. The plasma modifies catalyst properties such as dielectric con-stant, surface faceting and its morphology. Meanwhile, catalyst modi-fies the electric field generation, distribution and electron energy dis-tribution, which consequently affects the generation of reactive speciesand promote the DRM activity [16]. Plasma-catalysis is a complexphenomenon to understand the effect of plasma and catalyst in-dividually, but the synergistic effect is significant to distinguish thecontribution of catalyst in DBD plasma catalysis [59].

3.5. Effect of GHSV

Feed flow rate is a significant operating process parameter for cat-alytic DBD plasma DRM, as it is directly related to the processing ca-pacity of the DBD reactor and inversely related to the retention time ofthe reactants in the discharge zone [36]. The corresponding GHSV fromfeed flow rate can be calculated using Eq. (18). GHSV can be variedeither by adjusting the total feed flow rate or reactor volume. However,in the plasma gas phase system, GHSV can also be calculated via two

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approaches: based on reactor volume configuration; (i) discharge vo-lume (VD) and (ii) catalyst bed volume, (Vcat). Eq. (18) can be modifiedinto (Eqs. (19)–(20)) for GHSV′ (h−1) and GHSV″ (h−1). The VD cal-culation for the DBD plasma reactor is reported in our previous work[36]. However, with the employment of catalyst, the VD will bechanged and also affects the discharge behaviour of DBD plasma (Eq.(21)) [61]. The Vcat is calculated using Eq. (22). The bulk density (ρb)values are calculated by Eqs. (23) and (24).

⎜ ⎟= ⎛⎝

× ⎞⎠

−−

GHSV (h ) Total flow rate (cm min )Reactor volume (cm )

60 min1 h

13 1

3 (18)

⎜ ⎟′ = ⎛⎝

× ⎞⎠

−−

GHSV (h ) Total flow rate (cm min )V (cm )

60 min1 h

13 1

D3 (19)

⎜ ⎟″ = ⎛⎝

× ⎞⎠

−−

GHSV (h ) Total flow rate (cm min )V (cm )

60 min1 h

13 1

cat3 (20)

= − −V V VV (cm ) ( )tube e catD3 (21)

⎜ ⎟⎜ ⎟= ⎛⎝

⎞⎠

= ⎛⎝

⎞⎠

Mass of catalyst g mρ

V (cm )( )

Bulk density (g/cm )cat

bcat

33 (22)

= −−ρ ρ ε(g‐cm ) [ (1 )]catb3 (23)

Fig. 9. Effect of time on stream (TOS): (a) conversion (b) selectivity (c) yield (d) carbon balance, H2/CO and EE; GHSV 364 h−1, SIE=300 Jml−1, catalystloading= 0.5 g, Vcat = 2.75 cm3, Dgap= 3mm, DL=20 cm VD=10.5 cm3.

Table 4Experimental performance studies of DBD plasma DRM using different catalysts.

Catalyst Cat loading(g)

Flow rate(ml min−1)

Power(W)

SIE(J ml−1)

XCH4

(%)XCO2

(%)SH2(%)

SCO(%)

H2/CO EE(mmol kJ−1)

Ref.

10%Ni/Al2O3-MgO 0.5 20 100 300 74.5 73 47 48 0.98 0.117 This studya

26%Ni/Al2O3 1.86 50.4 97 116 56.4 30.2 31 52.4 0.59 0.033 [28]10%Ni/Al2O3 NA 50 50 60 38 21 27.6 45.3 0.61 0.72 [68]10%Ni/Al2O3 1.0 40 24 30 38 46 38 45 0.84 0.32 [69]12%Cu-Ni/Al2O3 0.1 60 60 60 56 30.2 31 52.4 0.60 NA [70]10%Ni/Al2O3 1.0 80 120 90 65 68 35 41 0.88 NA [71]

a Experimental conditions: CO2/CH4= 1, GHSV 364 h−1, SIE=300 Jml−1, catalyst loading= 0.5 g, Vcat= 2.75 cm3, Dgap= 3mm, DL=20 cm.

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⎜ ⎟= ⎛⎝

⎡⎣⎢

− ⎤⎦⎥

× ⎞⎠

Voidage ε V VV

( ) 100T cat

T (24)

where ‘VT’ is total makeup volume and ‘Vcat’ is the volume of catalyst.In the first case, the effect of total feed flow rate (10–50mlmin−1)

corresponding to GHSV (h−1) was investigated for Ni-AM in DBDplasma reactor at constant SIE, Vcat, VD and feed ratio 1:1 and the re-sults are presented in Fig. 12. Increasing GHSV at constant Vcat and VD

decreased the conversion of CH4 and CO2 abruptly. The maximumconversion was achieved at the lowest GHSV. Increasing the GHSV atconstant Vcat and VD led to less interaction between reactants and re-active species due to shorter retention time. Evidently, the conversionof CO2 and CH4 declined from 75% to below than 50%, when increasingthe feed flow rate from 10 to 50 (ml min−1) for both reactant gases. Theretention time is calculated by dividing the reactor volume over totalflow rate [36].

Mass transfer limitations in gas phase catalytic reactions could beovercome using higher feed flow rates corresponding to GHSV at con-stant Vcat and VD. The effect of higher feed flow rate is more significantin the catalytic activity than that of mass transfer limitations due to

shorter residence time [62]. The mass transfer limitation may also bereduced by surface modification of catalyst due to reactive speciesgenerated by plasma. The adsorbed reactive species of C and methylradicals can be further oxidized by oxygen species to form CO and H2

[32]. The specific energy density decreased with increasing feed flowrate at constant input power. Hence, the conversion was heavily af-fected by the lower electron density and retention time. The influenceof feed flow rate on the selectivity of H2 and CO was insignificant. Athigher GHSV, the dissociation probability of reactants and energeticspecies may reduce and drops the conversions of reactant gases. How-ever, CO and H2 are mainly produced from CO2 dissociation and H atomrecombination [63]. The formation of methyl radicals increased at ashorter residence time, consequently increased the C2H6 selectivity.

In the second case, GHSV is calculated based on Vcat and VD ex-pressed in Eqs. (19)–(20). Two different reactor configurations havebeen made in this study to investigate the effect of GHSV correspondingto VD (GHSV′) and Vcat (GHSV″) as illustrated in Figs. 13–14 andsummarized in Table 5. Meanwhile, variation in GHSV correspondingto VD marked lower conversions and selectivity (Fig. 13(a)) due toshorter residence time and limited surface reaction, since Vcat (catalyst

Fig. 10. (a) EDX of alumina reactor (tube) plasma only reaction; (b) EDX of spent 10wt% Ni-AM.

150 300 4500.40.60.81.0

3456789

30405060708090

100H

2/CO

rat

ioSe

lect

ivity

(S x)

)%

noisrevnoC

(Xn)

)%

SIE (J ml-1)

XCH4, XCO2

, SH2

SCO, SC2H6, H2/CO

Fig. 11. Effect of SIE on the catalytic activity using 10wt% Ni-AM in DBDplasma DRM;GHSV (364 h−1), feed ratio= 1:1, catalyst loading= 0.5 g, Vcat = 2.75 cm3,Dgap= 3mm, DL=20 cm, VD= 10.5 cm3.

182 364 546 728 9100.40.60.81.0

2468

30

40

50

60

70

80

90

100 XCH4

, XCO2, SH2

SCO, SC2H6, H2/CO

yt ivitcacitylat a

C

GHSV (h-1)

Fig. 12. Effect of GHSV (h−1) corresponds to total feed flow rate(10–50mlmin−1) on the conversion and selectivity using Ni-AM;SIE=300 Jml−1, catalyst loading= 0.5 g, Vcat= 2.75 cm3, Dgap= 3mm,DL=20 cm, VD= 10.5 cm3.

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loading) is constant. The electric field generated at constant Vcat is alsonot uniform. Furthermore, the effect of Vcat and VD variation on theconversion of reactants were studied simultaneously. The conversiondecreased while increasing GHSV for both cases (Fig. 14). However,high Vcat increases the conversion more than VD due to more activesites, longer residence time and effective surface reactions. Higher Vcat

led to the formation of strong electric field and electron density thatexhibits different discharge behaviour [32]. Conversions of reactantgases at various GHSV were monitored for both reactor configurationsbased on Vcat and VD respectively. Thus, it is imperative to understandthe contribution of catalyst and plasma in a DBD reactor to get thesynergistic effect. The EE in the reactor configuration based on Vcat ishigher than those values in Table 5 There is a remarkable influence ofcatalyst loading on EE as the plasma discharge behaviour, electrondensity and electric field varied with different reactor configuration.The synergistic effect between plasma-catalyst gives additional effectand has been validated by EE in DBD reactor.

3.6. Reaction mechanism

Dry reforming of methane (DRM) at NTP is highly endothermicreaction requiring high energy input to activate, ionize and dissociatestable GHGs i.e. CO2 and CH4. DRM process can be further elaborated tounderstand the reaction mechanism during the application of coldplasma. The electron impact reactions such as electron impact excita-tion, ionization and dissociation are responsible for the gas breakdownin DBD plasma. In DBD plasma, threshold energy for electron impactionization is too high to generate secondary electron, hence the electronimpact excitation and dissociation play key role in DRM [30]. Thethreshold energy for direct dissociation of CH4 (10 eV) and CO2

(11.9 eV), certainly enough for vibrational excitation and dissociationof CH4 and CO2 to radicals because the average electron energy of DBDis lower. The electron dissociation energy of C-H (4.5 eV) and C-O(5.5 eV) are lower than the energetic electron energy (> 10 eV), whichproposes the temperature of the electron (Te) is much higher than gastemperature (Tg) (Te≫ Tg) [64]. The energetic electron collision is a

Fig. 13. Effect of GHSV′ (h−1) correspond to the VD variation at constant Vcat; feed flow rate=20mlmin−1, SIE= 300 Jml−1, feed ratio= 1, Dgap= 3mm, catalystloading= 0.5 g.

Fig. 14. Effect of GHSV″ (h−1) correspond to Vcat and VD variation in Ni-AM (a) 0.5 g (b) 1.0 g (c) 2 g; feed flow rate= 20mlmin−1, feed ratio= 1,SIE=300 Jml−1, Dgap= 3mm.

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key factor to initiates the vibrational excitation and direct dissociationprocess of gas molecules and the formation of radicals CH3

⁎, CH2⁎, CH⁎,

H* and C species in the presence of a catalyst. The CO2 dissociatesmostly into CO*, CO and O* species [51,65]. The possible reactionsaccording to product distribution are stated in Eqs. (25)–(33):

Methane dissociation

+ → + +− ∗ ∗ −e CH CH H e4 3 (25)

+ → + +− −e CH C 2H e4 2 (26)

+ → + +− ∗ −e CH CH H e4 2 2 (27)

+ → + + +− ∗ ∗ −e CH CH H H e4 2 (28)

+ → + +− ∗ −e CH C H e (29)

+ → +∗ ∗CH CH C H H4 2 4 (30)

+ →∗ ∗H H H2 (31)

+ →∗ ∗CH CH C H3 3 2 6 (32)

+ → +∗ ∗H C H C H H2 6 2 5 2 (33)

The formation reactions can occur and formed higher hydrocarbonslike C2-C4 during this process [58]. The recombination of H⁎ speciesgenerates H2 (Eq. (31)).

Carbon dioxide dissociation

+ → +− ∗ −e CO CO O2 (34)

+ → + +− −e CO CO O e2 (35)

+ → +− + −e CO CO e2 2 (36)

The incorporation of catalyst in the discharge gas alters the dis-charge behaviour of the plasma by enhancing the electric field andpromoting the reactive species generation. The plasma generated re-active species can also modify the catalyst surface. The excited speciesof reactant gases are randomly adsorbed on the catalyst surface andeasily recombined to further generate H2 and CO. The surface chemistryin plasma-catalysis (Eqs. (37)–(42)) (M= active metal and S= supportsurface) on basic catalyst support can be attributed to high CO2 ad-sorption. Consequently, the adsorbed CO2 on the surface can react withO−2 forms CO3

2– that is highly reactive species. CO32−can react with H

atoms dissociated from CH4 [66]. The CH4 adsorption and dissociationtake place on the surface of active metal as the excited species increasesthe vibrational temperature of CH4 (Eq. (33)). It enhances the dis-sociative chemisorption of CH4 on metal (Ni) surface to CHx (x=1–3)and H. CHx further dissociates into H and reactive carbon, which can beoxidized by oxygen atoms. The complete reaction mechanism scheme ofplasma-catalysis is shown in Fig. 15 for Ni-AM catalyst and Eqs.(38)–(42). The proposed reaction paths follow the Langmuir-Hinshel-wood-Hougen-Watson (LHHW) mechanism whereby the CH4 activationis on Ni and deposited C⁎ may gasify on MgO [48,67]. According to theproposed mechanism, the dissociation, or activation of CH4 and CO2

initiated by plasma, followed by the adsorption of elemental and in-termediates containing C, H, O and oxy-carbonates on the active sitesand surface of the support. The surface reactions formed the desiredproducts in the plasma-catalytic DRM.

+ − → + −CH Ni S NiCH Ni H4 3 (37)

− → +H Ni H Ni2 2 (38)

+ → −CO S CO S2 2 (39)

− + → −− −CO S O CO S22

32 (40)

− + − → − + −− − −CO S H S HCO S OH S232

2 (41)

− → +CO S CO S (42)

The application of high Lewis basic support enhanced the CO2 che-misorption capacity and potentially higher CO2 conversion was observed[19,50]. It also adsorbs the excited methyl radicals, which may later leadto the formation of H and reactive carbon. Moreover, carbon depositionreduced due to the oxidation of highly reactive oxygen species withadsorbed reactive carbon. The oxidative capability of basic supports isbetter than acidic and inert supports in NTP based DRM process. Thehigher Lewis basicity of the surface may lead to the absorption of CO2

and higher oxidation ability to reduce carbon deposition.

4. Conclusion

The cold plasma treated Ni-AM catalyst is highly resistant to cokeformation and prevailed high performance in a packed-bed DBD plasmaDRM as compared to plasma only DRM. The low carbon deposition in Ni/AM as compared to Ni/A and Ni/M is attributed to high Lewis basicity,surface faceting and high dispersion of active metal. The H2/CO ratio isabout unity for Ni-AM catalyst indicated the lower formation of carbon bysuppressing RWGS and carbon gasification. The enhanced H2 is attributedto the suppression of recombination of CH3 and H to form CH4.Furthermore, Ni loading, SIE and GHSV (w.r.t reactor volume) are in-vestigated as process parameters. The GHSV is exclusively studied in twodifferent reactor configurations based on Vcat and VD at a constant flowrate and its effect on the DRM activity is critically reported. The differentconversion behaviour has been observed in both reactor configurationsdue to different discharge behaviour; selectivity was significantly affectedwhile changing the GHSV in both approaches. The incorporation of cat-alyst in a packed bed DBD plasma reactor caused synergistic effect asevident in the study of GHSV corresponding to Vcat and VD. EE achievedfor Ni-AM is 0.117mmol kJ−1 as compared to non-catalytic DBD DRMattested the synergy between plasma and catalyst. The outcome of thisstudy is useful for evaluating and determining the contribution of GHSV inenhancing the EE of the catalytic DBD plasma reactor.

Table 5Reactor configuration using Ni-AM with different VD and Vcat corresponding to GHSV′ and GHSV″ (h−1); Dgap= 3mm, SIE=300 Jml−1, feed ratio= 1.

Reactor configuration DL

(cm)VD

(cm3)Catalyst(g)

Vcat

(cm3)GHSV′(h−1)

EE(mmol kJ−1)

DL variation for VD at constant Vcat. GHSV′ (h−1) corresponds to VD. 5 3.3 0.5 2.75 364 0.08310 6.6 0.5 2.75 181 0.09220 13.2 0.5 2.75 91 0.12

Reactor configuration DL

(cm)VD

(cm3)Catalyst(g)

Vcat

(cm3)GHSV″(h−1)

EE(mmol kJ−1)

Catalyst loading and variation in Vcat and VD. GHSV″ (h−1) correspond to Vcat. 5 3.3 0.5 2.75 436 0.08410 6.6 1 5.5 218 0.09920 13.2 2 11 109 0.165

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Acknowledgement

The authors would like to extend their deepest appreciation to theUniversiti Teknologi Malaysia and Ministry of Higher Education(MOHE) for the financial support of this research under RUG (ResearchUniversity Grant, (Vot13H35) and FRGS-MRSA grant (Vot 4F988) re-spectively.

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