Catalytic Decomposition of 2% Methanol in Methane Over Metallic Catalyst by Fixed-Bed Catalytic Reactor

energies

Article Catalytic Decomposition of 2% Methanol in Methane over Metallic Catalyst by Fixed-Bed Catalytic Reactor

Ali Awad 1,2,*, Israr Ahmed 3, Danial Qadir 2, Muhammad Saad Khan 4 and Alamin Idris 5,*

1 Department of Chemical Engineering, University of Faisalabad, Faisalabad 38000, Pakistan 2 Chemical Engineering Department, Universiti Teknologi PETRONAS, Seri Iskandar 32610, Malaysia; [email protected] 3 School of Chemical and Material Engineering, National University of Science and Technology, Islamabad 44000, Pakistan; [email protected] 4 Mechanical Engineering Department, Texas A&M University at Qatar, Al Rayyan 5270, Qatar; [email protected] 5 Department of Engineering and Chemical Sciences, Karlstad University, 651 88 Karlstad, Sweden * Correspondence: [email protected] (A.A.); [email protected] (A.I.); Tel.: +46-54-700-1785 (A.I.)

Abstract: The structure and performance of promoted Ni/Al2O3 with Cu via thermocatalytic decomposition (TCD) of CH4 mixture (2% CH3OH) were studied. Mesoporous Cat-1 and Cat-2 were synthesized by the impregnation method. The corresponding peaks of nickel oxide and copper oxide in the XRD showed the presence of nickel and copper oxides as a mixed alloy in the calcined catalyst. Temperature program reduction (TPR) showed that Cu enhanced the reducibility of the catalyst as the peak of nickel oxide shifted toward a lower temperature due to the interaction strength of the metal particles and support. The impregnation of 10% Cu on Cat-1 drastically improved the catalytic

performance and exhibited 68% CH4 conversion, and endured its activity for 6 h compared with Cat-1, which deactivated after 4 h. The investigation of the spent carbon showed that various forms

Citation: Awad, A.; Ahmed, I.; of carbon were obtained as a by-product of TCD, including graphene fiber (GF), carbon nanofiber Qadir, D.; Khan, M.S.; Idris, A. (CNF), and multi-wall carbon nanofibers (MWCNFs) on the active sites of Cat-2 and Cat-1, following Catalytic Decomposition of 2% various kinds of growth mechanisms. The presence of the D and G bands in the Raman spectroscopy Methanol in Methane over Metallic confirmed the mixture of amorphous and crystalline morphology of the deposited carbon. Catalyst by Fixed-Bed Catalytic Reactor. Energies 2021, 14, 2220. Keywords: alloys; carbon nanofiber; catalyst; clean energy; premixed gas https://doi.org/10.3390/en14082220

Academic Editor: Bashir A. Arima 1. Introduction Received: 5 March 2021 The need for less harmful and more efﬁcient sources of fuel has gained interest in Accepted: 14 April 2021 Published: 16 April 2021 recent years [1]. The continuous combustion of fossil fuels used in industries to satisfy energy demands has contributed to the emission of contaminants and greenhouse gases,

Publisher’s Note: MDPI stays neutral such as COX, NOX, and SOX. Thus, it is imperative to find an alternative renewable source with regard to jurisdictional claims in of energy, i.e., hydrogen (H2). H2 is considered one of the most promising and energy- published maps and institutional affil- efficient fuels that can be converted into clean energy effectively [2,3]. Presently, H2 is used iations. in various engineering applications, such as NH3 production, oil refineries, and CH3OH production plants [4]. Various methods to produce H2 have been developed, which include dry reforming, partial oxidation, and steam reforming of CH4 [5–7]. Ammonia decomposition was also proposed as an effective technique for hydrogen production [8]. However, the low purity of Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. H2 and substantial emissions of CO2 restrict their practical applications [9]. Thermocatalytic This article is an open access article decomposition (TCD) of methane has gained interest among researchers over the last distributed under the terms and decade, as it is a single-step decomposition process. Still, high operating parameters and conditions of the Creative Commons poor catalyst stability limit its industrial applications. Catalytic decomposition of CH4 is an Attribution (CC BY) license (https:// endothermic technique that produces hydrogen without COX emissions, which simplifies creativecommons.org/licenses/by/ the process significantly and reduces the overall process cost [10]. Furthermore, the value- 4.0/). added carbon in the form of carbon nanofiber (CNF), bi-wall carbon nanofiber (BWCNF),

Energies 2021, 14, 2220. https://doi.org/10.3390/en14082220 https://www.mdpi.com/journal/energies Energies 2021, 14, 2220 2 of 12

carbon nanotube (CNT), and multi-wall carbon nanotube (MWCNT) is the only by-product of this process [11,12]. The TCD of CH4 is a non-oxidative decomposition technique that faces many challenges in the form of high reaction temperature due to the high bond −1 energy of CH4, which has an enthalpy of 74.52 kJmol [13]. The mechanistic preview of the reaction state is a four-step reaction, where the attachment/diffusion of methane on the surface of the catalyst is the binary step, followed by the breakage of C-H atoms as the main step, as high energy is required to break the sp3-hybridized carbon hydrogen bond. The next step is the conversion of hydrogen atoms to a stabilized molecule, and in the last stage, the carbon attached to the surface of the catalysts is transformed into various forms, such as GF, CNF, and MWCNF, depending on the material composition and reaction parameters. One of the effective methods to reduce this high reaction temperature is by using transition metals (Ni, Cu, Co, and Fe) as catalysts [14]. However, carbon produced as a by-product encapsulates the active sites of the catalyst, which causes rapid deactivation and opens new horizons for research in this field [15]. The selection of catalysts is crucial in the TCD process for H2 generation, so the improvements in the composition and properties have a substantial effect on the sustainability of the current process [16]. Various solutions have been presented for the optimization of TCD. This can be achieved by altering the physicochemical properties and composition of the catalysts, and amending the reaction conditions, such as reaction temperature, gas hourly space velocity, and the weight of the catalyst. Moreover, the practice of using admixtures and premixed gases as feedstocks was also suggested as an active solution to enhancing the effectiveness of TCD [17]. The types of catalysts that are used in the catalytic decomposition of methane processes are metallic and carbonaceous materials [18]. However, carbonaceous catalysts exhibit lower CH4 consumption and reaction rates compared with metallic catalysts; hence, they are not employed frequently [19]. The most common metals used for the catalytic decomposition of CH4 are the transition metals with partially filled d orbitals. These metals are mixed with porous supports, such as Al2O3, MgO, Fe2O3, and CeO2, using various synthesis methods, including impregnation, co-precipitation, and sol-gel [20,21]. Of all the transition metals, Ni is frequently employed in reforming reactions due to its reactive nature, easy availability and because it is relatively less expensive, but its early deactivation limits its applications. Therefore, the modification of Ni-based catalysts with other transition metals is essential for improving the stability of the catalysts. Various metals such as Cu, Pd, Mg, Mo, Fe, Co, and Cr are impregnated on a Ni-based catalyst, thus forming mixed-metal alloys [22]. Thus, they proved to be very effective catalysts in maintaining the equilibrium of the TCD. The study of the thermocatalytic decomposition of CH4 with other gases was also proposed as an active method of improving catalyst stability and activity [23]. Hence, the TCD of CH4 with various hydrocarbons as feedstock has been widely explored [24]. The study of TCD with 2% CH3OH/CH4 was explored in our previous publication over Ni/Al2O3 [25]. We found that the CH4 decompositions improved by increasing the metal loadings and reaction temperatures. The existence of 2% CH3OH improved the activity of the material, and its catalytic stability was significantly enhanced. Similarly, the effect of bimetallic catalysts on TCD was investigated in our previous study [26]. The study showed that the addition of Pd on a Ni-based catalyst improved the catalyst stability and activity significantly when we used 2% CH3OH/CH4 premixed feedstock. In our previous publication, we reported the results of 15% Cu on a Ni-based catalyst for hydrogen production over TCD of methane and methanol mixture [27]. We concluded that the addition of 2% methanol enhanced the catalytic activity of the material. Thus, we decided to test another Cu-based catalyst over premixed feedstock. Hence, in this study, we investigated the use of Cu as an effective promoter on the effectiveness of a Ni-based catalyst by using a novel premixed feedstock 2% CH3OH/CH4 over a high temperature fixed-bed catalytic reactor. Furthermore, the physicochemical properties of fresh and spent promoted catalysts were also explored using various microscopy techniques. Although Energies 2021, 14, 2220 3 of 12

several metallic catalysts have already been tested for the TCD of methane, this research work focused on the testing of the TCD over premixed gas, where only a handful of research studies have been accomplished.

2. Materials and Methods 2.1. Materials

The materials, which included 2% CH3OH/CH4,H2, and N2 with a purity of 99.99%, were purchased from Malaysia Linde Limited (Ipoh, Malaysia). H2 was used as a pre- treatment gas for catalyst activation, and N2 was used as purging gas with the feedstock. Nickel nitrate hexahydrate and copper nitrate trihydrate were used as nickel and copper precursors, whereas γ-Al2O3 was used as the material support. Ni (NO3)2.6H2O was purchased from Bendosen Chemicals (Johor, Malaysia); Cu (NO3)2.3H2O and γ-Al2O3 were purchased from R&M Chemicals (Selangor, Malaysia).

2.2. Catalyst Synthesis Monometallic catalyst Cat-1 was prepared by the wet impregnation technique and treated as a reference catalyst. The bimetallic catalyst Cat-2, containing 10% Cu supported on γ-Al2O3, was also synthesized by the same technique at an overall 50% concentration of Ni in the catalyst weight, as per calculated amounts of Ni (NO3)2.6H2O, Cu (NO3)2.3H2O used. The metal precursors were dissolved in H2O and, later, the calculated amount of γ-Al2O3 as support was added. The mixture was gently mixed and evaporated at 363 K using a magnetic stirrer at 400 rpm for 3 h, dried at 383 K overnight, and calcined at 873 K for 4 h [27]. The catalyst tagging is explained in Table1.

Table 1. The names and composition of the materials prepared.

Sr. No Material Tag Material Composition

1. Cat-1 50% Ni/Al2O3 2. Cat-2 50% Ni−10% Cu/Al2O3

2.3. Catalyst Characterization The analytic properties of the catalyst were studied via BET, X-ray diffraction (XRD), H2 temperature-programmed reduction (H2-TPR), ﬁeld emission electron microscope (FESEM), transmission electron microscope (TEM), and Raman spectroscopy. The detailed information on the speciﬁcations of the analytical equipment employed in this research work can be found in [27,28].

2.4. Catalyst Evaluation The catalytic decomposition runs over the synthesized catalysts were carried out by using a fixed-bed catalytic reactor, as previously performed in our group [23]. A catalyst weight of 0.1 g was appropriately sieved and sandwiched in the center of the steel pipe of the reactor by employing quartz wool. The catalyst was activated with H2 at −1 −1 15,000 mL gcat h and 873 K for 2 h. The catalytic runs were carried out at atmospheric pressure and 1023 K, using a premixed feedstock of 2% CH3OH/CH4 diluted with N2. The composition of gaseous products was analyzed using Agilent 7820A series gas chromatog- raphy. The calibrated data were used to calculate the concentrations of H2,N2, CH3OH, and CH4 in the post-reaction gaseous mixture and, finally, the reactor was purged with N2 and cooled to room temperature. The reactor configuration can be found in our previous paper [29]. Energies 2021, 14, x FOR PEER REVIEW 4 of 12

Energies 2021, 14, 2220 4 of 12 3. Results 3.1. Characterization of Fresh Catalyst 3.1.1.3. Results BET Study 3.1. Characterization of Fresh Catalyst The textural characteristics of Cat-1 and Cat-2, as determined by BET, showed a no- ticeable3.1.1. BET variation Study in the textural properties in the promoted Ni-based catalyst. This obser- vationThe is rational textural due characteristics to the successful of Cat-1 impregnation and Cat-2, of NiO as determined and CuO in by the BET, porous showed struc- a turenoticeable of γ− Al variation2O3. The surface in the area textural of the properties commerciall in they available promoted alumina Ni-based that we catalyst. employed This inobservation our research is rationalgroup was due 7.66 tothe m2g successful−1. The BET impregnation surface area ofof NiOCat-1 and (5.15 CuO m2g in−1) the dropped porous bystructure adding 10% of γ -AlCu2 (4.96O3. Them2g− surface1) due to area the partial of the obstruction commercially of pores available on the alumina surface that of the we − − supportemployed by inmetal our researchprecursors group [30]. was The 7.66 pore m 2volumeg 1. The and BET pore surface diameter area of also Cat-1 changed (5.15 m from2g 1 ) − 0.011dropped to 0.018 by addingcm3 g−1 and 10% 8.90 Cu (4.96to 14.80 m2 gnm1 )by due adding to the Cu partial on the obstruction Cat-1. The of change pores in on the the texturesurface properties of the support of Cat-2 by metal can be precursors related to [30the]. agglomeration The pore volume of particles, and pore diameterVery similar also 3 −1 findingschanged are from also 0.011 reported to 0.018 in cm[23],g whereand the 8.90 surface to 14.80 area nm bydecreased adding Cuafter on successful the Cat-1. im- The pregnationchange in theof Cu texture over propertiesNi-based catalysts. of Cat-2 can be related to the agglomeration of particles, Very similar ﬁndings are also reported in [23], where the surface area decreased after 3.1.2.successful XRD Study impregnation of Cu over Ni-based catalysts.

3.1.2.The XRD XRD Study diffractograms of Cat-1 and Cat-2 were employed to identify various phases in the calcined catalysts, as shown in Figure 1, over a range of 2θ = 0–90°. For both cata- The XRD diffractograms of Cat-1 and Cat-2 were employed to identify various phases lysts, the reflections at 2θ of 25°, 35°, 44°, 55°, 58°, and 66° were associated with the γ− in the calcined catalysts, as shown in Figure1, over a range of 2 θ = 0–90◦. For both Al2O3 phase [10]. The relative intensity of all peaks of γ−Al2O3 lessened after the Ni and catalysts, the reflections at 2θ of 25◦, 35◦, 44◦, 55◦, 58◦, and 66◦ were associated with the Cu impregnations. This observation can be linked with the enhancement of the metal dis- γ-Al2O3 phase [10]. The relative intensity of all peaks of γ-Al2O3 lessened after the Ni persion on the γ-Al2O3 support. Moreover, the peaks at 2θ of 37.4°, 64.3°, 75.3°, and 79° and Cu impregnations. This observation can be linked with the enhancement of the metal appeared as a result of the cubic NiO phase for both Ni-based catalysts. In the XRD dif- dispersion on the γ-Al O support. Moreover, the peaks at 2θ of 37.4◦, 64.3◦, 75.3◦, and fractograms of Cat-2, no2 independent3 diffraction peaks of CuO were observed; rather, the 79◦ appeared as a result of the cubic NiO phase for both Ni-based catalysts. In the XRD relative intensities of NiO increased. The increase in the intensity of the NiO peaks showed diffractograms of Cat-2, no independent diffraction peaks of CuO were observed; rather, the that in the promoted catalyst, CuO and NiO possess identical peaks overlapping each relative intensities of NiO increased. The increase in the intensity of the NiO peaks showed other.that in Thus, the promoted they form catalyst, mixed CuOoxides and (Ni NiO(1−x)Cu possessxO) [31]. identical Similar peaks findings overlapping were reported each other. in the literature [32] where the authors explained that, after the successful impregnations of Thus, they form mixed oxides (Ni(1−x)CuxO) [31]. Similar findings were reported in the Cu,literature the relative [32] where intensities the authors of NiO explainedincreased, that, thus after confirming the successful the formation impregnations of mixed of ox- Cu, ides.the relative intensities of NiO increased, thus confirming the formation of mixed oxides.

Figure 1. X-ray diffraction for a fresh catalyst. Figure 1. X-ray diffraction for a fresh catalyst.

Energies 2021, 14, x FOR PEER REVIEW 5 of 12

3.1.3. H2-TPR Study

The H2-TPR study of Cat-1 and Cat-2, as shown in Figure 2, was carried out to inves- tigate the reducibility of active metal oxides. A broader peak at a range of 600–800 K of Cat-1 exhibited the reduction of strongly interacted NiO with γ−Al2O3 support. The whole reduction of NiO into metallic Ni° was achieved here. The reduction of NiO is shown in Equation (1) [33].

NiO+ H2 → Ni° + H2O (1)

Cat-2 shows three H2 consumption peaks at 560, 700, and 840 K., The first peak at 500 K corresponds to the interaction of the Cu and Al2O3 supports, while the second sharp

Energies 2021, 14, 2220 peak at 650 K ascribes to the reduction of the Ni–Cu alloy. The last peak at 900 K can5 ofbe 12 associated to the reduction of Ni that had a strong interaction with the support. It can be seen (c.f. Figure 2) that the Cu addition shifted the TPR profile of NiO to a lower temperature as the addition of a promoter on the Cat-1 catalyst improved its degree of reducibility.3.1.3. The H same2-TPR reduction Study improvement was also reported when a Cu promoter was used

in a Cat-1The Hcatalyst2-TPR [34,35]. study of The Cat-1 reduction and Cat-2, of CuO as shownat 500 K in converted Figure2, wasa H2 carriedmolecule out into to atomicinvestigate hydrogen, the reducibility which assisted of active the metalreduction oxides. of NiO A broader at a lower peak temperature at a range of range 600–800 by a K spilloverof Cat-1 effect, exhibited hence the the reduction shifting of the strongly H2-TPR interacted peaks of NiONiO withto lowerγ-Al temperatures.2O3 support. The The ◦ Hwhole2-TPR reductionof CuO is shown of NiO in into Equation metallic (2). Ni was achieved here. The reduction of NiO is shown in Equation (1) [33]. CuO+ H2 → Cu°◦ + H2O (2) NiO+ H2 → Ni + H2O (1)

FigureFigure 2. 2. TheThe temperature-programmed temperature-programmed reduction reduction for for fresh fresh catalyst. catalyst. Cat-2 shows three H consumption peaks at 560, 700, and 840 K., The ﬁrst peak at 3.1.4. FESEM Images 2 500 K corresponds to the interaction of the Cu and Al2O3 supports, while the second sharp peakThe at 650surface K ascribes morphology to the of reduction the Ni-bas ofed the material Ni–Cu and alloy. the The Cu-promoted last peak at Cat-2 900 Kcata- can lysts,be associated as shown toin theFigure reduction 3, shows of the Ni diss thatemination had a strong of uneven interaction and nonuniform with the support. particles It oncan γ− beAl seen2O3 supports. (c.f. Figure The2) FESEM that the images Cu addition show that shifted the theparticle TPR distribution proﬁle of NiO of toboth a lower cata- lyststemperature was in the as nanoscale, the addition while of amore promoter tightly on aggregated the Cat-1 nanoparticles catalyst improved were itsseen degree in Cat- of 2.reducibility. Moreover, Thethe particle same reduction size of the improvement prepared catalysts was also reportedranged from when 25–100 a Cu promoternm. The wasag- glomerationsused in a Cat-1 and catalyst larger particles [34,35]. Thewere reduction observed of more CuO often at 500 in Kpromoted converted catalysts a H2 molecule due to theirinto high atomic metal hydrogen, loadings, which and assistedfrom the thesuccessful reduction addition of NiO of at Cu a lower on the temperature Cat-1 [36]. More- range over,by a FESEM spillover analysis effect, henceconfirmed the shifting that larger of the pa Hrticle2-TPR size peaks and more of NiO aggregated to lower temperatures. distribution ofThe the H nanocatalyst2-TPR of CuO particles is shown would in Equation result in (2). lower BET surface area.

◦ CuO+ H2 → Cu + H2O (2)

3.1.4. FESEM Images The surface morphology of the Ni-based material and the Cu-promoted Cat-2 catalysts, as shown in Figure3, shows the dissemination of uneven and nonuniform particles on γ- Al2O3 supports. The FESEM images show that the particle distribution of both catalysts was in the nanoscale, while more tightly aggregated nanoparticles were seen in Cat-2. Moreover, the particle size of the prepared catalysts ranged from 25–100 nm. The agglomerations and larger particles were observed more often in promoted catalysts due to their high metal loadings, and from the successful addition of Cu on the Cat-1 [36]. Moreover, FESEM analysis conﬁrmed that larger particle size and more aggregated distribution of the nanocatalyst particles would result in lower BET surface area. Energies 2021, 14, x FOR PEER REVIEW 6 of 12

Energies 2021, 14, x 2220 FOR PEER REVIEW 6 6of of 12 12

Figure 3. FESEM images of fresh catalysts. FigureFigure 3. 3. FESEMFESEM images images of of fresh fresh catalysts. catalysts. 3.2. Catalytic Cracking Reforming Evaluation 3.2. Catalytic Cracking Reforming Evaluation 3.2.The Catalytic TCD Crackingof 2% CH Reforming3OH/CH 4Evaluation over Cat-1 and Cat-2 was studied at a temperature of 1023 KThe and TCDa feedstock of 2% CHflow3OH/CH rate of 18,0004 over mL Cat-1 gcat− and1h−1 using Cat-2 0.1 was g of studied material, at a as temperature shown in The TCD of 2% CH3OH/CH4 over Cat-1 and Cat-2− was1 − 1studied at a temperature of Figureof 1023 4. The K and TCD a feedstockof CH4 is an ﬂow endothermic rate of 18,000 process, mL gandcat theh performanceusing 0.1 g of of the material, catalyst as 1023 K and a feedstock flow rate of 18,000 mL gcat−1h−1 using 0.1 g of material, as shown in is shownenhanced in Figureby an 4increase. The TCD in reaction of CH 4temperatureis an endothermic [37]. Moreover, process, Cat-1-type and the performance materials Figure 4. The TCD of CH4 is an endothermic process, and the performance of the catalyst areof extensively the catalyst used is enhanced in hydrocarbon by an increase cracking in and reaction reforming temperature reactions [ 37[38];]. Moreover, however, they Cat-1- is enhanced by an increase in reaction temperature [37]. Moreover, Cat-1-type materials aretype sensitive materials to reaction are extensively temperatures used in and hydrocarbon are subject cracking to rapid and deactivation reforming at reactions high tem- [38]; are extensively used in hydrocarbon cracking and reforming reactions [38]; however, they peratureshowever, [39]. they In are the sensitive TCD reaction, to reaction along temperatures with H2 as and the are desired subject product, to rapid deactivationan ample are sensitive to reaction temperatures and are subject to rapid deactivation at high tem- amountat high of temperatures carbon was also [39]. deposited In the TCD on reaction,the surface along of the with catalyst. H2 as Hence, the desired the inability product, of an peratures [39]. In the TCD reaction, along with H2 as the desired product, an ample Ni°ample to diffuse amount coke of leads carbon to the was formation also deposited of the encapsulating on the surface layer of the of catalyst.carbon around Hence, the the amount of carbon◦ was also deposited on the surface of the catalyst. Hence, the inability of activeinability sites, of thus Ni deactivatingto diffuse coke the leadscatalyst to the[35]. formation A similar of observation the encapsulating was made layer in a of recent carbon Ni° to diffuse coke leads to the formation of the encapsulating layer of carbon around the work,around in which the active Cat-1 sites, was thus deactivated deactivating after the 4 h, catalyst resulting [35 in]. A26% similar CH4 conversion observation at was 1023 made K active sites, thus deactivating the catalyst [35]. A similar observation was made in a recent andin aGHSV recent 18,000 work, mL in which gcat−1h Cat-1−1 using was 0.1 deactivated g of catalyst after (c.f. 4 Figure h, resulting 4). in 26% CH4 conversion work, in which Cat-1 was deactivated−1 after−1 4 h, resulting in 26% CH4 conversion at 1023 K at 1023 K and GHSV 18,000 mL gcat h using 0.1 g of catalyst (c.f. Figure4). and GHSV 18,000 mL gcat−1h−1 using 0.1 g of catalyst (c.f. Figure 4).

− − FigureFigure 4. 4.MethaneMethane conversions conversions over over 6 h 6 (1023 h (1023 K, K,1 bar, 1 bar, and and gas gas velocity velocity 18 18 K KmL mL g− g1 h1−1).h 1). Figure 4. Methane conversions over 6 h (1023 K, 1 bar, and gas velocity 18 K mL g−1 h−1). VariousVarious efforts efforts were were made made to to synthesize synthesize st stableable and and high high carbon carbon resistant resistant bi-metallic bi-metallic promotedpromoted Ni-based Ni-based catalysts. catalysts. Th Thee alloying alloying and and synergistic synergistic effect effect between between Ni Ni and and pro- pro- Various efforts were made to synthesize stable and high carbon resistant bi-metallic motermoter boosted boosted the the CH CH4 conversions4 conversions and and carbon carbon resistance resistance ab abilityility of ofthe the catalyst catalyst drastically. drastically. promoted Ni-based catalysts. The alloying and synergistic effect between Ni and pro- TheThe addition addition of ofmetal metal additive additive Cu Cu (10%) (10%) on on the the performance performance of of Cat-1 Cat-1 was was explored explored in in moter boosted the CH4 conversions and carbon resistance ability of the catalyst drastically. recent work (c.f. Figures4 and5). The formation of mixed oxides during calcination of The addition of metal additive Cu (10%) on the performance of Cat-1 was explored in NixCu(1−x)O and Ni-Cu alloy after H2 reduction considerably affects the performance of

Energies 2021, 14, 2220 7 of 12

the promoted catalyst [40]. It was explained that carbon has a higher afﬁnity with Cu◦, and the rate of carbon diffusion in Cu◦ is relatively higher compared with Ni◦. Hence, instead of forming an encapsulation layer, it diffuses in the promoter [32]. In Cat-2, Cu acted like a carbon reservoir to maintain the equilibrium between formed and diffused carbon and kept ◦ the surface of Ni fresh for the conversion of CH4 into H2 and elemental carbon. Hence, the addition of 10% Cu on Cat-1 increased the CH4 conversions from 26% to 68%, and the catalyst sustained its stability for 6 h (c.f. Figure4). It has also been observed that small methanol plays a signiﬁcant role in maintaining the catalytic stability of the catalyst. Generally, apart from the decomposition of CH3OH into the mixture of CO and H2 and a series of reactions, including a reverse water gas reaction, a methanation reaction also occurred, as shown in Equations (3)–(6) [41,42].

CH3OH → CO + 2H2 (3)

CO + 3H2 → CH4+ H2O (4)

CO + H2O → CO2 +H2 (5)

C + CO2 → 2CO (6)

The role of CH3OH is to maintain the catalytic ability by linking it to the partial regeneration of the catalyst [43]. There are various approaches to catalyst regeneration, in which the carbon encapsulated on the catalyst can be oxidized into CO2 and CO [44]. The mechanism of CH3OH decomposition is complex as various reactions occur concurrently, resulting in an assortment of products. CH3OH primarily decomposes into H2 and CO as synthesis gas (c.f. Equation (3)). Additionally, as per the methanation reaction (c.f. Equation (4)), CO and H2 react with each other to produce steam that further converts into H2 and CO2 (c.f. Equation (5)). The produced CO2 assists in the fractional regeneration of the material by converting the C into CO (c.f. Equation (6)). Hence, along with TCD, a fractional cycle of regeneration also occurred that kept the surface of Ni fresh of CH4 dissociation. Although it can be seen in the BET analysis that the incorporation of the Cu in Ni/Al2O3 decreased the surface area of the material, the performance of the catalyst showed that the CH4 conversions and catalyst stability improved. The results show that the TCD of CH4 primarily depends upon the availability and dispersion of active sites on the catalyst support that overcomes the surface area deficiencies [45]. The effects of TCD temperature on the performance of the promoted catalysts with Cu over a range of 950–1100 K, evaluated as methane conversions, are shown in Figure5. The conversions increased from 58% to 73% by increasing the reaction temperature from 973–1073 K due to the endothermic nature of the reaction. It was discovered that TCD is a temperature- dependent process, and the appropriate temperatures significantly affect the reaction yield. The study of thermocatalytic decomposition of methane over premixed gas feedstock has not yet been explored much. The motivation of the current study was researchers employing various hydrocarbons as feedstock together with methane, obtaining some promising results. The authors explained that the activity of the carbon obtained as a by-product seemed to affect the kinetics of the process as they used propane, ethylene, ethanol, and acetylene over a carbonaceous catalyst, and reasonable conversions were obtained. Moreover, the results depicted that the activity of carbon obtained as a by- product of benzene was highest, followed by ethylene, propane, and methane [27]. Thus, we carried out the study using premixed gas over metallic catalysts as the results were quite promising. Still, some extra work needs to be conducted in this research field, where the reaction parameters and reaction mechanism need to be further explored. Energies 2021, 14, x FOR PEER REVIEW 7 of 12

recent work (c.f. Figures 4 and 5). The formation of mixed oxides during calcination of NixCu(1−x)O and Ni-Cu alloy after H2 reduction considerably affects the performance of the promoted catalyst [40]. It was explained that carbon has a higher affinity with Cu°, and the rate of carbon diffusion in Cu° is relatively higher compared with Ni°. Hence, instead of forming an encapsulation layer, it diffuses in the promoter [32]. In Cat-2, Cu acted like a carbon reservoir to maintain the equilibrium between formed and diffused carbon and kept the surface of Ni° fresh for the conversion of CH4 into H2 and elemental carbon. Hence, the addition of 10% Cu on Cat-1 increased the CH4 conversions from 26% to 68%, and the catalyst sustained its stability for 6 h (c.f. Figure 4). It has also been observed that small methanol plays a significant role in maintaining the catalytic stability of the catalyst. Generally, apart from the decomposition of CH3OH into the mixture of CO and H2 and a series of reactions, including a reverse water gas reaction, a methanation reaction also occurred, as shown in Equations (3)–(6) [41,42].

CH3OH → CO + 2H2 (3)

CO + 3H2 → CH4+ H2O (4)

CO + H2O → CO2 +H2 (5)

C + CO2 → 2CO (6)

The role of CH3OH is to maintain the catalytic ability by linking it to the partial regeneration of the catalyst [43]. There are various approaches to catalyst regeneration, in which the carbon encapsulated on the catalyst can be oxidized into CO2 and CO [44]. The mechanism of CH3OH decomposition is complex as various reactions occur concurrently, resulting in an assortment of products. CH3OH primarily decomposes into H2 and CO as synthesis gas (c.f. Equation (3)). Additionally, as per the methanation reaction (c.f. Equa- tion (4)), CO and H2 react with each other to produce steam that further converts into H2 and CO2 (c.f. Equation (5)). The produced CO2 assists in the fractional regeneration of the Energies 2021, 14, 2220 material by converting the C into CO (c.f. Equation (6)). Hence, along with TCD, a frac- 8 of 12 tional cycle of regeneration also occurred that kept the surface of Ni fresh of CH4 dissociation.

FigureFigure 5. The 5. Theeffect effectof TCD oftemperature TCD temperature on methane co onnversions methane (973–1073 conversions K, 1 bar, gas (973–1073 velocity K, 1 bar, gas velocity 1818 K KmL mL g−1 h g−1−).1 h−1).

3.3. Post Reaction Analysis of Spent Catalyst

3.3.1. Morphology Analysis The formation and arrangement of coke deposited on the surface of both materials tested for TCD of the methanol/methane mixture are shown in Figure6. The hierarchy of the carbon nanoﬁber produced as a by-product of TCD is dependent on the composition of the metal in the catalyst. The morphology of the carbon encapsulated on both catalysts varied in shape and size. It can be observed in Figure6 that both graphene carbon (GC) and carbon nanoﬁbers (CNFs) having no distinct shape, but rather various shapes (straight, zigzag, curved, and interwoven) are produced as a by-product of the catalytic decomposition of feedstock. Wider CNFs were observed on the surface of the Cu-promoted catalyst due to agglomerated particles and tip and base growth mechanisms. It can be easily compared in the FESEM images that the morphology of the carbon produced as Energies 2021, 14, x FOR PEER REVIEW 9 of 12 a by-product improved, as wider CNFs in excess were deposited on the surface of the Cu-promoted catalyst compared with Ni/Al2O3.

FigureFigure 6. 6. FESEMFESEM images images of ofcarbon carbon deposited deposited on onthe the catalyst. catalyst.

3.3.2. TEM Images

The nature of the carbon deposited on the surface of Cat-2 was examined by TEM images, as shown in Figure 7, over the TCD of 2% CH3OH/CH4 mixture at 1023 K. It can be observed in Figure 7 that both amorphous and crystalline forms of coke formed on the surface of Cat-2. Moreover, the carbon growth mechanism can also be studied by carefully analyzing the TEM images. The black dots in the CNF show the presence of Ni, both on the tip and in the body of the fiber. It was reported that the formation of CNF follows tip growth and base growth mechanisms [46]. In the present study, some of the Ni particles disconnected from the support, and they remained on the tip of CNF, therefore assisting in CH4 dissociations. Additionally, some of the metal can be observed in the CNF body that helped in the growth of carbon nanofibers and multi-wall carbon nanofibers. Hence, we concluded that the carbon formation during the TCD of CH3OH/CH4 occurred simul- taneously by tip and base growth mechanisms, as shown in Figure 7 [47].

Figure 7. TEM images of carbon deposited on the catalyst.

3.3.3. Raman Spectroscopy The notch of crystallization and graphitization of the coke deposited over the Cu- promoted Cat-2 catalyst is shown in Figure 8 by Raman spectroscopy. The results show the existence of two characteristic bands, D and G bands, located at 1360 and 1527 cm−1,

Energies 2021, 14, x FOR PEER REVIEW 9 of 12

Energies 2021, 14, 2220 9 of 12

Moreover, the catalytic activity of Cat-2 was better and remained active for 6 h TOS. This ﬁnding shows that although carbon was produced, it did not block the active sites of the catalyst; instead, they acted as catalyst support. This ﬁnding is per the study by [39], where the authors reported that after successful impregnation of the metallic promoter on

Ni-based catalyst, wider CNFs were produced that acted as catalyst support, improving Figurethe stability. 6. FESEM images of carbon deposited on the catalyst.

3.3.2.3.3.2. TEM TEM Images Images TheThe nature nature of the carbon depo depositedsited on the surface of Cat-2 was examined by TEM images,images, as as shown shown in in Figure Figure 7,7, over over the the TCD TCD of of 2% 2% CH CH3OH/CH3OH/CH4 mixture4 mixture at 1023 at 1023 K. It K. can It becan observed be observed in Figure in Figure 7 that7 boththat bothamorphous amorphous and crystalline and crystalline forms forms of coke of formed coke formed on the surfaceon the surfaceof Cat-2. of Moreover, Cat-2. Moreover, the carbon the growth carbon mechanism growth mechanism can also be can studied also be by studied carefully by analyzingcarefully analyzingthe TEM images. the TEM The images. black Thedots black in the dots CNF in show the CNF the showpresence the presenceof Ni, both of Ni,on theboth tip on and the in tip the and body in theof the body fiber. of It the was fiber. reported It was that reported the formation that the of formation CNF follows of CNF tip growthfollows and tip growth base growth and base mechanisms growth mechanisms [46]. In the [ 46present]. In the study, present some study, of the some Ni ofparticles the Ni disconnectedparticles disconnected from the support, from the and support, they rema and theyined remainedon the tip onof theCNF, tip therefore of CNF, thereforeassisting inassisting CH4 dissociations. in CH4 dissociations. Additionally, Additionally, some of somethe metal of the can metal be observed can be observed in the inCNF the body CNF thatbody helped that helped in the growth in the growth of carbon of carbonnanofibers nanofibers and multi-wall and multi-wall carbon nanofibers. carbon nanofibers. Hence, weHence, concluded we concluded that the that carbon the carbonformation formation during during the TCD the of TCD CH3 ofOH/CH CH3OH/CH4 occurred4 occurred simul- taneouslysimultaneously by tip byand tip base and growth base growth mechanisms, mechanisms, as shown as shown in Figure in Figure 7 [47].7 [47 ].

Figure 7. TEM images of carbon deposited on the catalyst.

3.3.3.3.3.3. Raman Raman Spectroscopy Spectroscopy TheThe notch notch of of crystallization and and graphiti graphitizationzation of of the the coke coke deposited deposited over over the the Cu- Cu- promoted Cat-2 Cat-2 catalyst catalyst is is shown in in Figure 88 byby RamanRaman spectroscopy.spectroscopy. TheThe resultsresults showshow −1 thethe existence existence of of two characteristic bands, D and G bands,bands, locatedlocated atat 13601360 andand 15271527 cmcm−1,, respectively. The G band indicates the existence of highly ordered carbon structures, while the D band shows the defective structures of carbon originated as a by-product of TCD [48]. The ID/IG ratio over Cat-2 is estimated to be 1.0, which shows the presence of crystalline carbon as a by-product of TCD. The presence of both bands shows the existence of amorphous and crystalline carbon on the surface of the Cu-promoted catalyst. The vital difference between the relative intensities of both the D and G bands conﬁrms the minor imperfections in the structure of carbon (c.f. Figure8). Energies 2021, 14, x FOR PEER REVIEW 10 of 12

respectively. The G band indicates the existence of highly ordered carbon structures, while the D band shows the defective structures of carbon originated as a by-product of TCD [48]. The ID/IG ratio over Cat-2 is estimated to be 1.0, which shows the presence of crystalline carbon as a by-product of TCD. The presence of both bands shows the existence of amorphous and crystalline carbon on the surface of the Cu-promoted catalyst. The vital Energies 2021, 14, 2220 difference between the relative intensities of both the D and G bands confirms the minor10 of 12 imperfections in the structure of carbon (c.f. Figure 8).

Figure 8. Raman spectra of deposited carbon on the catalyst. Figure 8. Raman spectra of deposited carbon on the catalyst. 4. Conclusions 4. Conclusions Catalytic decomposition of methane is an attractive technique for the production of cleanCatalytic hydrogen decomposition and carbon of nanofiber methane (CNF),is an attractive but high technique reaction temperaturefor the production and rapid of cleancatalyst hydrogen deactivation and carbon limit itsnanofiber industrial (CNF), applications. but high In reaction this research temperature work, we and attempted rapid catalystto optimize deactivation the reaction limit its by industrial employing app 2%lications. methanol In premixedthis research in methanework, we as attempted feedstock, toand optimize by running the reaction the reaction by employing over Cu-promoted 2% methanol Ni-based premixed metallic in methane catalysts as in feedstock, a fixed-bed andcatalytic by running reactor. the The reaction materials over wereCu-promoted synthesized Ni-based by the metallic wet impregnation catalysts in method,a fixed-bed and catalyticcharacterized reactor. by The TPR, materials XRD, BET,were FESEM,synthesized TEM, by and the Ramanwet impregnation spectroscopy. method, The results and characterizedshowed that dueby TPR, to the XRD, impregnation BET, FESEM, of metal TEM, on theand support, Raman thespectroscopy. surface area The was results reduced showeddue to that the agglomerationdue to the impregnation of particles of and meta thel on blockage the support, of alumina the surface pores, area as confirmed was re- ducedby BET due and to FESEMthe agglomeration analyses. Similarly, of particles the and reducibility the blockage of the of catalyst alumina was pores, enhanced as con- by firmedimpregnating by BET Cuand as FESEM the TPR analyses. profiles shifted Similarly, to a lowerthe reducibility temperature, of with the Nicatalyst◦ and Cuwas◦ beingen- hancedthe outcomes by impregnating of H2 activation Cu as the of both TPR catalysts. profiles shifted The catalytic to a lower activity temperature, of Cat-2 significantly with Ni° andimproved Cu° being due the to outcomes the presence of H2 of activation metallic of Cu, both and catalysts. the conversions The catalytic further activity increased of Cat- by 2 increasingsignificantly reaction improved temperature. due to the The presence post-reaction of metallic analysis Cu, ofand the the spent conversions carbon confirmed further increasedGF, CNF, by and increasing MWCNF. reaction Although temperature. the addition The ofpost-reaction methanol improved analysis of the the TCD spent process car- bonand confirmed the early GF, deactivation CNF, and of MWCNF. the catalyst Although was stopped, the addition extra of research methanol work improved is needed the to TCDelaborate process the and reaction the early mechanism deactivation of the of conversion the catalyst of was mixed-gas stopped, feedstock. extra research In the work future, is variousneeded compositionsto elaborate the of reaction premixed mechanism gas feedstock of the should conversion be tested of mixed-gas for TCD for feedstock. hydrogen Inproduction the future, overvarious bi-metallic compositions catalyst, of soprem thatixed the processgas feedstock can be should optimized. be tested for TCD for hydrogen production over bi-metallic catalyst, so that the process can be optimized. Author Contributions: Conceptualization, A.A. and I.A.; methodology, A.A.; formal analysis, D.Q.; Authorinvestigation, Contributions: A.I.; resources, Conceptualization, A.I.; data curation,A.A. and D.Q.;I.A.; methodology, writing—original A.A.; draft formal preparation, analysis, D.Q.; A.A.; investigation,writing—review A.I.; andresources, editing, A.I.; A.I.; data visualization, curation, D.Q.; M.S.K.; writing—original supervision, M.S.K.; draft preparation, project administra- A.A.; writing—reviewtion, I.A.; funding and acquisition,editing, A.I.; A.I.visualization, All authors M. haveS.K.; readsupervision, and agreed M.S.K.; to theproject published administration, version of the manuscript. Funding: This research received no external funding. Data Availability Statement: Not applicable. Acknowledgments: The authors would like to acknowledge Karlstad University and Universiti Teknologi PETRONAS for providing the support and necessary facilities to conduct the research work. Conflicts of Interest: The authors declare no conflict of interest. Energies 2021, 14, 2220 11 of 12

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