Retracted Article: Methane conversion for hydrogen production: technologies for a sustainable future

Abstract

Industrial and technological developments have resulted in a rapid increase in the rate of global energy consumption with the energy demand expected to increase by 33% by 2035. Fossil fuel resources are the dominant source of energy and due to the increased use of fossil fuels, the amount of carbon emissions have grown the highest in the past several years. This also led to an increase in the emission of greenhouse gases (GHGs) like CO₂, NO_x, and SO_x, which contribute to climate change. Moreover, with the world's fossil reserves getting depleted, it is necessary to move towards a sustainable and environmentally genial source of energy. Various technologies are available for the production of hydrogen, among them catalytic decomposition of methane (CDM) to produce hydrogen as a CO_x free technology have gained the highest importance in recent years due to the CO free production of hydrogen. In this study we will analyze and discuss different carbon nano materials (CNMs) to be recycled as valuable byproduct for different purposes. Moreover, the financial and ecological assessment of CDM will be compared with methane steam reforming in terms of CO₂ emission, coal gasification, productivity, and cost of hydrogen production. Catalytic decomposition of methane may be a reliable and convenient technology for on-site maximum production of hydrogen on a small or moderate industrial scale. Ni-based, Fe-based, carbonaceous catalyst and noble metals for the CDM process will be addressed. Prioritizing hydrogen/carbon yield and production costs, iron based catalysts are ideal for catalytic decomposition of methane. Though catalysts based on nickel exhibit a much higher hydrogen production with 0.39 mol H₂ per g cat. per h compared to Fe-based ones with 0.22 mol H₂ g cat. per h, hydrogen cost of the former is assumed to be 100-fold higher ($0.89/$0.009). Furthermore, CDM performances on various types of reactors will be addressed, the molten-metal catalyst/reactor may be a suitable path for the commercialization of CDM. Lastly, the synthesis mechanism, characterization, and application of carbon byproducts with diverse structures and morphologies will be described in this article. Unlike other studies, the current review will show that economical Fe-based catalysts (10 tons H₂/1 ton iron ore) and novel molten-metal reactors (with 95% methane conversion) for CDM are the more suitable research directions for the basic understanding of CDM and moreover carbon nanomaterials synthesized by CDM could be also exploited for supercapacitors, as oil for lubrication and for wastewater purification.

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1. Introduction

Global energy consumption (about 86%) is still ruled by fossil fuels; only 14% is coming from alternative renewable energy resources.¹ Nevertheless, till now, the great increase in utilization of natural gas, petroleum and heavy oil in transportation and industry has generated huge environmental complications,^1,2 and produces large quantity of greenhouse gases (GHGS) like CO_x, SO_x, NO_x and C_xH_y into the earth's atmosphere.^3,4 The amount of CH₄, another strong greenhouse gas, in earth's atmosphere increased from 722 ppb before 1750 to 1859 ppb in 2018.⁵ This increased release of CH₄ and CO₂ resulted in climate change, global warming, acid rain and air pollution,^3,4,6 and may have a tragic impact on agriculture and natural ecosystem.⁷ Therefore, a hydrogen economy free of carbon is strongly needed.^8–10 Solar, wind, geothermal, nuclear power and tidal (energy obtained from ocean tides) energy cannot fully meet the energy needs because of their safety concern, high prices, and less developed technology.^4,8 These serious environmental issues may be addressed using hydrogen due to its environmentally friendly and sustainable nature. Hydrogen plays a role as an energy carrier both to deliver and store available energy. Hydrogen is significant as an industrial gas and raw material with extensive usages like in oil industry, methanol production, ammonia production and fuel cells etc.^11,12 Hydrogen can be quickly transformed into energy by using fuel cells and can be used in vehicles directly. It is expected that in fuel cells powered cars, hydrogen usage could replace petroleum demand, with yearly hydrogen production estimated to be about 150 million tons in 2040.¹³

H₂ is not a prime energy source as it should be produced from its compounds. Methane remains the prime source for H₂ production.¹⁴ Nowadays, about 90% of the overall world's total hydrogen productivity is obtained from CH₄.¹⁵ The steam reforming of methane (SRM) is considered the utmost popular method, and around 50% of the global need for hydrogen is achieved through this technology.¹⁶ Though, expecting a large quantity of carbon dioxide discharges because of SRM, the process of CDM may produce carbon dioxide free H₂. In ref. 17 an ecological and economic study proved that CDM process may work at very low temperatures (600 °C), as compared to SRM. This would lead to a substantial savings of energy. Moreover, other researchers¹⁸ evaluate that the CDM-H₂ economy might be a bridge for sustainable H₂ production.⁶

Various research studies are investigating catalytic decomposition of methane and few are concentrating on hydrogen production, whereas others particularly on the production of carbon materials. Carbon catalyst and metals^19–25 can significantly control carbon structure and hydrogen yield.²⁶ Other researchers²⁷ studied carbon materials as catalysts for methane decomposition by evaluating the role of carbon materials and by deliberating their textural properties, effect and O₂ surface group and catalyst surface area. The early activity of catalytic methane decomposition was associated with the presence of oxygen surface group and catalyst surface area; however, the long-standing permanency of the catalyst was associated with the Braunauer Emmett Teller (BET) micro pore volume and surface area. Additionally, study demonstrated that activated carbon black and carbon catalyst were the ones with the best performances among carbon-based materials, thanks to their lower crystalline features.

The growth of carbonaceous and metal catalysts for catalytic methane decomposition with enhanced stability and conversion rate have been studied.²⁸ The highly considered catalysts having carbonaceous nature are carbon black and activated carbon, whereas the most common catalysts made from metals are copper, iron and nickel. The speed of hydrogen production and the properties of carbon formed have been evaluated. The types of reactors, source of heating, methods of formation of the catalyst, regeneration operation and conditions have also been studied. In terms of industrialization, fluidize bed reactors are considered among the best ones for catalytic methane decomposition. The current developments in the reaction mechanism of CDM have been studied⁴ together with the kinetics on metal catalysts, particularly the function of metal particles during the reaction and inactivation mechanism of the catalysts.^8,10 They proposed that the catalyst having the maximum metal loading can reduce more carbon monoxide produced from the support. Moreover, to the earlier stated topics, other groups³ studied entire progress in research or laboratory scale and conferred the effect of co-feeding with propylene, ethylene, ethanol, alkanes, CO₂, H₂O and catalyst generation for increasing the SDM production. The researchers encouraged additional investigations towards integrated membrane reactors to perform both decomposition renewal and separation processes for reliable, simple, clean and constant production of H₂. On these bases, CDM is proven to be a capable exciting technology and its industrialization would be important for the future economy of H₂. Though various researches have obtained good performances on CDM over reactor designation and preparation of catalyst, further steps and time are necessary to reach industrialization. Iron based catalysts are considered perfect for CDM, due to their environmental and economic benefits, high temperature resistance, stability, and the production of valuable carbon nano materials (CNMs). Hence, this study reviews iron based catalytic decomposition of methane. The commercial assessment of numerous technologies to produce hydrogen, followed by the function of iron based catalyst, laboratory or pilot reactors, reaction kinetics, particularly molten melt designing, and reaction mechanism are discussed. Moreover, the structures, mechanism of formation and uses of the CNMs byproduct is presented.

2. Economic assessment

Table 1 illustrates numerous types of H₂ production methodologies comprising SRM,²⁹ methanol reforming of steam (MSR),³⁰ gasification of coal,³¹ splitting of water³² and CDM.³³ While Table 2 illustrates a summary of basic hydrogen production cost, all-inclusive expenditure of hydrogen production on these methodologies and CO₂ taxes. The hydrogen cost for the primary production consists of the material cost, maintenance, fixed assets, electricity, water, workers and steam. The CO₂ tax is estimated according to $7.2 per ton CO₂, whereas hydrogen production cost estimation is based on primary cost of hydrogen production and tax of CO₂. In the case of catalytic CDM, complete H₂ manufacturing expenditure is considered based on key production cost of H₂, tax on CO₂, and financial benefits of the CNMs byproduct.

Table 1 Reaction of some hydrogen production technologies

Technology	Reaction	Ref.
Coal gasification	C + H2O → CO + H2	29
	CO + H2O → CO2 + H2
SRM	CH4 + H2O → CO + 3H2O	30
	CH4 +2H2O → CO2 + 4H2O
	CO + H2O → CO2 + H2
MSR	CH3OH + H2O → CO2 + 3H2O	31
CDM	CH4 → C + 2H2	32
Water splitting	Cathode (−): 2H^+ + 2e^− → H2	33
	Anode (+): 2H2O → overall reaction 2H2O → 2H2 + O2

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Table 2 List of hydrogen production technologies and costs^a

Production technology	Productivity (N m^3 h^−1)	Primary H2 production cost ($ per t H2)	CO2 emission (t CO2 per t H2)	CO2 tax ($)	Total H2 production cost ($)	Ref.
a The CO2 tax is calculated based on $7.2 per ton CO2. CDM is feasible with product carbon value $570.3 per ton (methane decomposition to produce one ton hydrogen, meanwhile produce three tons carbon material).
Coal gasification	20 000	2020.8	29.01	208.8	2229.6	34 and 35
SRM	20 000	1934	11.04	79.5	2013.5	36 and 37
	1000	2693	12.49	89.9	2782.9
Reforming of methanol	1200	3402.6	29.10	209.4	3612	38
Electricity-water splitting	1000	6138.7	44.94	323.4	6462	39
Hydroelectric-water splitting	1000	1977.6	14	100.8	2078.4	35
Wind power-water splitting	1000	3490.4	23.6	170	3660	38 and 39
Photovoltaic-water splitting	1000	4624.6	31.5	226.7	4851.2	40
CDM	1000	2167–3764	6.6	47.5	503.6–2100.6	41

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3. SRM vs. gasification of coal

For SRM process, clearly the entire cost of H₂ production and CO₂ discharge increases as the production reduces from 20 000 to 1000 N m³ h⁻¹. Having a similar output target of 20 000 N m³ h⁻¹, the cost of entire H₂ formation of methane steam reforming ($2013.5 per t H₂) is less compared to gasification of coal ($2229.6 per t H₂), however the release of carbon dioxide of coal gasification is nearly three times as higher compared to steam reforming of methane.

3.1. SRM vs. MSR

Though the lesser operation temperature ≈ 300 °C makes steam reforming of methane a fascinating technology to produce H₂ with lower energy consumption, the entire cost of hydrogen production ($3612 per t H₂) and emission of carbon dioxide (29.10 t CO₂ per t H₂) of MSR remain high compared to SRM ($2013.5 per t H₂, 11.04 t CO₂ per t H₂), at a productivity of H₂ c. a. 1000 N m³ h⁻¹.^33,36

3.2. SRM vs. water splitting

Splitting of water is an ecofriendly procedure beside a further H₂ refinement unit.³⁷Table 2 illustrate that wind, photovoltaic and electricity using splitting of H₂O can't contest with SRM at hydrogen production level of 1000 N m³ h⁻¹. In addition, for photovoltaic splitting of H₂O, the materials as a photocatalyst must have an appropriate band gap and be active, stable, cheap, abundant, and efficient.³⁸ There are no substances today that can satisfy all these parameters.³⁹ Water splitting through hydroelectric was an alternative mode for SRM to produce hydrogen at the similar expenditure (entire production cost of hydrogen: $2078.4 per t H₂vs. $2782.9 per t H₂; emissions of CO₂: 14 t CO₂/ t H₂vs. 12.49 t CO₂ per t H₂). Though, because of the limitation of industrially present electrolyzers†⁴⁰ (H₂ production of polymer electrolyte membrane up to 100 N m h⁻³; alkaline electrolyzers up to 1000 N m³ h⁻¹), hydroelectric-water splitting production is considered to have little chance of being scaled up to as large as that of SRM.

3.3 SRM vs. CDM

At the level of hydrogen productivity of 1000 N m³ h⁻¹, the complete cost of H₂ formation of SRM ($2782.9 per t H₂) is high as compared to CDM ($503.6 to 2100.6 per t H₂); the emissions of CO₂ from CDM (6.6 t CO₂ per t H₂) are less compared to SRM (12.49 t CO₂ per t H₂) technology. Research studies⁴¹ compared catalytic decomposition of methane H₂ formation expenses and showed that a breakeven worth for the byproduct of catalytic decomposition of methane carbon nanomaterials was found to be $503.6 to 2100.6 per t H₂, above which catalytic decomposition of methane would be an inexpensive technology competing with steam reforming of methane.⁴¹ They additionally determined that the small or average hydrogen formation (1000 N m³ h⁻³) that happens on the spot on a commercial scale would be the main advantage for CDM process as online hydrogen formation by catalytic decomposition of methane can overcome the expenses of hydrogen transport. Few researchers and different industries are working for the commercialization of systems for CDM. Another group in ref. 42 has effectively produced hydrogen and graphite by methane catalytic decomposition on iron-ore catalyst.

Iron ore it's inexpensive compared to Ni. The energy required for the system was obtained from the produced H₂ while the extra one was used as output. In the meantime, to maximize the economic viability, graphite powder byproduct can be wholesaled. Hazer group limited⁴² can manufacture hydrogen amount of 10 along with 1 ton of Fe ore catalyst. The procedure is combined with the processing of gas liquid to yield graphite and H₂ by catalytic decomposition of methane. The researchers intended to manufacture 100 kg maximum pure hydrogen per day, which can be practically implemented by a tiny project of limited supply of hydrogen vehicle. The Karlsruhe Institut fur Technologie (KIT) and Institute of Advanced Sustainability (IASS)⁴² investigated the CDM process and produced a novel reactor that relies on the technology of liquid metal to eliminate the attached graphite in situ and confirmed the constant and effective reactor operation. This certainly proved the capacity of catalytic decomposition of methane for a huge hydrogen generation. Based on primary evaluation, the starting expenses has been estimated around $2.2 to 3.7 per kg H₂, which did not consider the worth of the byproduct of graphite. Linde Engineering and ThyssenKrupp Steel combined with Badische Anilin-und-Soda-Fabrik (BASF) to establish a two-phase process to get the CO, syngas and H₂. The very first step contained the production of H₂ and C by an advanced technology of decomposition of methane at high temperatures without using a catalyst. H₂ gas was then treated with large concentration of carbon dioxide, from additional commercial processes, to attain syngas. Because catalytic decomposition of methane without using a catalyst needs much more heat, BASF has built effective ways that, within the system, recycle the fuel, to significantly reduce the quantity of energy required.

4. Fe-based catalysts for CDM

A large variety of catalysts has been utilized for methane catalytic decomposition. Here, after introducing common Ni- and carbonaceous-based catalysts, CDM over Fe-based catalysts will be emphasized. The stability and catalytic activity of numerous catalysts have been analyzed and compared, comprising the operation conditions, type of reactors, activity, served stability, carbon yield, initial and final methane conversion, and amount of catalyst. The studies showed that hydrogen production with carbonaceous catalysts is less efficient compared to iron-oriented catalysts.⁴³ Nickel and nobel metals may increase methane conversion but are comparatively more costly and less investigated. Some groups have reported iron as an economical catalyst, the cost of iron is about 1/140 with respect to that of nickel.⁴³ Also, iron is considered much stable than nickel catalyst at elevated temperature. This may lead to a comparatively good thermodynamical conversion for a catalytic decomposition of methane (an endothermic process). Iron-based catalysts are an interesting option for catalytic decomposition of methane.

4.1 Supported Fe catalysts

Various researches confirmed a successful carbon decomposition of methane over iron catalysts.^44,45 Inert oxides, which favor the catalytic activity of iron, have gained increasing interest^46,47 as they showed good stability and high catalytic activity for CDM. Single-oxides (such as Al₂O₃, MgO, TiO_2, CaO, CeO₂, SiO₂ and ZrO₂) are used as catalytic supports for methane decomposition. Ref. 48 reports the use of various oxides supports (Al₂O₃, CaO, TeO₂, TiO₂ and MgO) with iron showing that Fe/Al₂O₃ is the most performing one. By a gas hourly space velocity (GHSV) of 1.875 L per g cat. per h, at 750 °C, 70% conversion of methane was achieved for 400 minutes for 65 wt% Fe/Al₂O₃. Moreover, infused 20 wt% iron catalysts were also produced with various additives (g-Al₂O_3, a-Al₂O₃, MCM41, SiO₂/TiO_2, SiO₂, MgSiO₃, zeolite and CeO₂/ZrO₂) finding that CH₄ conversion varies from 2 to 23%. The 20 wt% Fe/g-Al₂O₃ ratio was considered the best one, with a 23% CH₄ conversion. They suggested that Al₂O₃ support increases the Fe surface area for the selective deposition of carbon nanotubes (CNTs).

The influence of composition and structure of 59.5 wt% iron catalysts on TiO_2, Al₂O₃, ZrO₂ and SiO₂ on the production and morphology of filamentous carbon formed by CDM was studied at 600–800 °C and GHSV of 48 L per g cat. per h.⁴⁹ Fe/SiO₂ showed the best performances: 45 gC/gFe was produced at 680 °C. Non uniform and random filaments were produced on Fe/TiO₂, while thick-walled bamboo-like tubes over Fe/SiO₂. The filaments formed over Fe/Al₂O₃ and Fe/ZrO₂ were containing hollow tubes.

Furthermore, common supports of rare or binary inert oxides reinforced catalysts were also studied. Cunha et al.⁵⁰ measured catalytic decomposition of methane on n wt% La₂O₃-promoted iron catalyst of Raney type (n ¼ 5, 10, 20, 50) at 600 °C (physical mixtures of La₂O₃ and Raney Fe). Fe/80Al₂O₃/20SiO₂ catalyst shows a methane conversion profile that quickly inclined in 300 min from 44% to 5%; while the one of Fe/90Al₂O₃/10SiO₂ reduced from 74% to 47%. The increase in SiO₂ content decomposition of methane was reducing the pore volume and BET.

Murata et al.^50,51 reported the performance of CDM in terms of catalytic stability over Fe/Al₂O₃ and Fe/MgO/Al₂O₃ at 800C in the presence of CH₄/O₂/CO₂/N₂ ¼ 80/10/5/5; W/F ¼ 41.9 g cat. per h per mol. Conversion for Fe/Al₂O₃ catalyst was reduced in 6 h from 95% to 79%. Meanwhile, for Fe/MgO/Al₂O₃ catalyst (Mg/Fe ¼ 1/1 wt. ratio) it remains stable at 95% for CDM for around 6 hours.

4.2 Non-supported iron catalysts

CDM has been presented as an environmentally friendly technology to produce hydrogen free from CO_x by various researchers nonetheless in opinion of Aparicio et al.⁵² generation of carbon monoxide in actual catalytic decomposition of methane cannot be ignored and should be taken into account for various reasons: (a) the impurities due to oxygen inside the reaction mixture; (b) the partial reduction of metal oxide (i.e., substantial quantity of CO may be produced by methane reaction with metal oxides in the early phases of the decomposition if the catalysts are not reduced during the deactivation or disintegration step); or (c) reaction of the carbonaceous adsorbed species originated by dissociative chemisorption of methane with surface OH groups (mostly from oxides supports)^53,54via a redox mechanism. To eliminate the “O” from oxide supports, the non-supported or bulk Fe catalysts for CDM were investigated by many researchers,^55,56 although the supports could help to improve CDM performance. There are several benefits in using non supported catalysts as reported in ref. 57. Firstly, the prevention of CO production due to the chemical reactions occurring between methane and/or deposited nano-carbon and lattice/surface oxygen. Moreover, carbon produced nanomaterials can be simply separated from the catalyst using magnetic field or acid treatments. Ashik et al.⁵⁸ recently reported methane decomposition over iron NPs produced by co-precipitation treating FeNO₃ solution with NH₄ (calcination at 350 °C for the 3 hours). The catalyst was active at 595 °C and showed the highest H₂ production of 9% which reduced to 2% in a few minutes.

Ref. 58 reported CDM performances of magnetite (Fe₃O₄) at an operating temperature ranging from 800 to 900 °C and GHSV of 7.5 L per g cat. per h. Fe₃O₄ was triggered by CH₄ reducing at 900 °C for 2 h in comparison with H₂O₂. CH₄ conversion of 98% was achieved, and remained constant for 75 h. Other researchers³⁷ produced non-supported Fe oxides by solution combustion utilizing the fuel in the form of citric acid. After combining a specific quantity of citric acid with iron nitrate solution (corresponding molar ratio of 0.5 : 1.0), the obtained mixture was then slowly evaporated by continuous stirring at ∼80 °C till a paste was formed. Afterward, calcination was done in air at 600 °C for 3 hours to form an iron oxide catalyst. Then the catalyst was reduced in situ at 800 °C for 1 hour utilizing concentrated H₂ with a 50 mL min⁻¹ flow rate. Catalyst was assessed for catalytic decomposition of methane at 900 °C and a GHSV of 10.8 L per g cat. per h. The output showed that graphene nano-platelets of few layers were observed at 3.2 gC g cat⁻¹. Zhang et al.⁵⁹ through a reduction substitution methodology, prepared a non-supported Ni–Fe (with 1 : 1 molar ratio) bimetallic catalyst. After the pre-reduction by H₂ at 400 °C, Fe–Ni modified from NiFe₂O₄ and α-Fe (Ni) to supplement of α-Fe (Ni) and γ-Fe-Ni (Fe_0.64Ni_0.36) alloy particles. No CO_x formation and larger catalytic decomposition of methane were observed. This may be a novel method for an iron-oriented catalyst of CDM with a way to eliminate oxygen from ordinary supports or attempt distinct supports with less oxygen like BN and SiC.

4.3 Wasted Fe-based catalysts

Lately, a group investigated a few iron rich wastes for the process of CDM. Iron wastes come from red mud (RM, Fe₂O₃-30–50 wt%),⁶⁰ goethite (Fe₂O₃-51.28 ± 1.49 wt%),⁶¹ Fe ore tailings (Fe₂O₃-26.8 wt%),⁶² copper flotation waste (Fe₂O₃-67.68 wt%),⁶³ hematite (Fe₂O₃-97.3 wt%)⁶⁴ and residues of nickel-leaching (Fe₂O₃-38.57 wt%).⁶⁵ Inside India, almost 18e20 million tons of tailings were generated per annum⁵⁸ and 3 million tons of red mud⁶⁶ were produced each year. Around 9 million tons of red mud was formed per year globally. Catalyst from red mud common RM composition was 30 wt%e50 wt% Fe₂O₃, SiO₂ and Al₂O₃. It also has few other metallic elements like Zr, Mg, V and Cr. The main mineral constituents comprised sodalite, hematite, anatase goethite, quartz, and rutile. Obviously, red mud is a potential source of many valuable metals. Hence, red mud is among the best sources for the catalytic decomposition of methane due to environmental and economic reasons. Fang et al.⁶⁷ studied the CDM for GHSV of 4.8 L per g cat. per h and over nano-mesoporous modified red mud catalyst at 800 °C. The distribution of modified red mud (MRM) pore size was 3 × 10¹² nm, whereas the total pore volumes and surface area were 0.39 cm³ g⁻¹ and 190.61 m² g⁻¹ correspondingly. A pore structure like a wormhole was also produced, which was positively affecting the performances of catalytic decomposition. The outcomes showed a conversion of methane of 25.99% that was reduced to 20% after 250 min of reaction time. Balakrishnan et al.⁶⁸ studied carbon oxidation and deposition over RM catalyst in a novel two steps process, formed to transport, store, and utilize the associated petroleum gas (APG) lost in petroleum exploration in offshore platforms (Fig. 1). It was transformed catalytically to H₂ and fuel in the form of carbon, with byproducts C deposited around 27 wt%. After the deposition of carbon in the platform, it can be securely transported and simply stored to the shore. Likewise, metal and carbon can be oxidized in controlled environments to produce CO/H₂ in the form of steam. Hydrogen fuel produced can be preserved as a hydride or electricity in batteries, Balakrishnan et al.⁶⁹ investigated the catalytic decomposition of methane over red mud at 800 °C, and GHSV of 7.2 L per g cat. per h with a supreme H₂ production speed 3.80 10⁻⁵ mol H₂ g⁻¹ s⁻¹ over a catalyst comprising 36.7 wt% Fe₂O₃. Researchers concluded that there is a crucial role of alkali metal content (alkaline metals are famous to be extremely active poisons in decomposition reaction).

Fig. 1 Flow chart of CDM for the conversion of offshore associated petroleum gas (APG) into hydrogen using the RM as a catalyst.

4.4. Iron ochre catalysts

Zhou et al.²⁷ investigated ores of Fe (minerals and ores comprising sufficient amounts of iron oxides in term of hematite (Fe₂O₃) with some quantity of Al₂O₃, Na₂O and SiO₂ impurities) for CDM as a bulk catalyst. After the reduction with 100 mL min⁻¹ H₂ at 850 °C for 3.5 h, CH₄ decomposition was performed with 100 mL min⁻¹ pure methane (5 bar, 850 °C, and 3.75 L per g cat. per h) in a reactor. This led to a production of hydrogen of 3.2 mol g cat⁻¹, free of CO_x impurities. The used catalyst sample was characterized as mixtures of Fe₃C and graphite carbon with many defects. Alharthi et al.⁷⁰ observed the catalytic decomposition of methane over Fe ochre achieved from a river in an automated temperature experimentation at a GHSV of 1.4 L per g cat. per h in between 600 and 800 °C. They explained the formation of two valuable products, magnetic and hydrogen carbon containing composites from the two wasted Fe ochre and CH₄. Methane is observed as a leftover from the operation of refineries and from landfill dumping. Hydrogen productivity increased with temperature and carbon and graphite bound Fe and/or Fe₃C were formed.

4.5. Catalyst regeneration

As great quantities of C are produced the catalysts decrease their reactivity. Various compounds (O_2, H₂O and CO₂) can produce this deactivation.^28,52 Wen et al.⁷¹ demonstrated that, by in situ injecting a minute quantity of O₂ during the decomposition of methane, the growth of amorphous Fe/Mo/MgO catalyst can be observed. Another study⁵² effectively reused catalysts of 40 wt% Fe/Al₂O₃ for five cycles by CO₂ oxidation. The activation time ranges from 20 to 5 min, and thanks to catalyst regeneration the methane conversion was enhanced from 70% to 75%.

5. Reactor

Designing a reactor plays an important part for the commercialization of CDM. For a methane deposition frequently, used reactors are plasma reactor (PLR), rotary bed reactor (RBR), fixed bed reactor (FBR) and molten melt reactor (MMR).

5.1 Fixed-bed reactorhn

Fixed bed reactor has been utilized widely for catalytic decomposition of methane over Ni, Fe, C and noble metal-based catalyst to evaluate the impact of different parameters such as GHSV, temperature, catalyst preparation, partial pressure and feed composition on conversion of methane and yield of carbon or hydrogen.^72–74 FBR was extensively used in the research of carbonaceous catalysts. Lee et al.⁷⁵ analyzed catalytic decomposition of methane over carbon black in a fixed bed vertical shape height of 45 cm, and 2.8 cm I. D (6.05 cm O. D) carbon flow steel reactor at 1020 and 1170 °C with cyclone and at the bottom a bag-filter. The reaction zone was heated through an electric tube furnace. Carbon steel plates with holes and crakes' wool were applied for supporting the carbon black catalyst. Approximately 100% conversion of methane was achieved at 1170 °C by⁷⁶ using a fixed bed reactor of three-zone with outstanding temperature uniformity. Tube reactor was coupled with 120 cm height, 6.03 cm O. D, 8.74 cm thickness and stainless-steel tube. The apparent kinetics, catalyst deactivation was calculated with a simple model of catalyst activity for CDM. The deactivation and reaction order were 0.5 and 2 and the experimental data well fitted the simulations.

Few catalysts of metal were also applied to catalytic decomposition of methane. Konieczny et al.⁴⁵ examined catalytic decomposition of methane in a FBR (length of 45.7 cm and 1.3 cm O. D) with a thermocouple of K-type. They observed 98% methane conversion that lasted for around 75 hours. Pinilla et al.⁴⁶ investigated Fe-based catalysts in a fixed bed reactor, 1.8 cm I. D and 60 cm height, heated by using an electric furnace, with 93 vol% H₂ achieved at 1 L gat⁻¹ h⁻¹ and 900 °C.

5.2. Fluidized bed reactor

Fluidized bed reactors are largely functional for petroleum, metallurgical and chemical industries. In recent years, experimental results^77–80 demonstrated that it can be utilized as a quick reactor for CDM eliminating solid particles in the by-products.⁸¹ In ref. 82 a model consisting of catalyst and CDM activation and deactivation through the elimination of carbon for CDM in fluidized bed reactors is described. The outcomes of this unique model identify the key parameters for catalyst renewal (temperature, size, types of the catalyst and fluidizing gas velocity) and their optimal values. Łamacz et al.⁸³ studied the catalyst of Ni/CeZrO₂ for the very first time utilized in a FLBR (Fig. 2) for catalytic decomposition of methane to produce hydrogen and CNTs.

Fig. 2 The scheme of the catalyst preparation (a), and installation used for experiments (b).⁸³

The deactivated catalyst was regenerated in a micro-reactor with 4.15H₂O/Ar accompanied by hydrogen. A 93% conversion of methane was accomplished on Ni/CeZrO₂ catalyst at 700 °C. Another study⁸⁴ proved catalytic decomposition of methane in FLBR with Fe-based catalyst. The reactor system was coupled with a fluidized bed made of quartz (50 cm long, 1.85 cm I. D) and heated by a resistive heater. Numerous researches have examined the production of carbon materials in a fluidized bed reactor for hydrogen production in the catalytic decomposition of methane.⁸⁴ Other researchers⁸⁵ investigated the catalysts nature based on iron at a temperature between 700 °C and 900 °C in FLBR with height of 8 cm and 6.5 cm I. D. The FLBR was separated into two compartments by a perforated horizontal plate having the holes of 1 mm diameter. A variety of multiwall CNTs and carbon nanofibers (CNFs) with exciting properties related to structure was formed. Ref. 86 presented the capability of internal metal to graphitize CH₄ decomposition-based carbon nanofibers, and the formation of carbon nanofibers with different proportions of nickel and silicon in the decomposition of methane over the Ni and Cu dependent catalyst in a fluidized bed reactor. The structural order of the prepared carbon materials gradually improved by increasing the Si/Ni weight ratio in CNFs. Hence, compared with FBR, FLBR is a more promising CDM reactor for large-scale production because it offers sufficient growing space, heat transfer, good mass, and it allows easy handling of solid particles and ongoing removal of carbon formed from the reactor.

5.3 Molten-metal reactor

To overcome the critical issues of catalytic decomposition such as catalyst deactivation or plugging of reactor through carbon deposition, few investigators^87,88 studied a unique molten metal reactor for in situ separation of the carbon byproduct from the solid catalysts. In the molten system of this reactor, the insoluble carbon float to the surface where it can be skimmed off. Researchers also investigated molten-melt magnesium as a catalyst for CDM.⁸⁹ CH₄ was decomposed to carbon and hydrogen in a reactor system made of stainless steel. Into the reactor one or two stage reaction may happen due to the fluidity of the molten magnesium, the byproducts of carbon can be transferred away without covering the active part of the catalyst and hence its lifespan was greatly prolonged. Upham et al.^89,90 produced a sequence of firm molten metal alloy catalytic for CDM. They also implemented the density functional theory to study different physical parameters of atoms introduced into melts together with their effect on the catalytic activity of the melt itself. The melts were used inside the reactor of molten metal bubble columns where carbon can be continuously removed (Fig. 3). Almost 95% methane transformation was achieved in a 1.1 m bubble column containing molten Ni_0.27 Bi_0.73 (27 mol% of Ni dissolved in molten Bi) at 1065 °C.

Fig. 3 Hydrogen production with a NieBi molten catalyst. (A) Reactor, (B) SEM of the carbon produced, (C) Raman spectrum of surface carbon, and (D) ab initio molecular dynamics simulation.⁸⁹

Another research⁹¹ proved catalytic methane decomposition inside the reactor made from liquid tin at temperatures around 700 °C to 950 °C and measured the actual kinetic data confirming a kinetic equation for the methane conversion also using other experimental data.⁹² The outcomes showed that for all the investigated temperatures and the entire methane deposition process, liquid tin had no effect in the catalytic activity; though, the midway products were minimized by the existence of liquid tin. Additionally, most of the methane conversion observed in the bubble column reactor happened in the gas phase section above the liquid phase.

KIT and IASS⁸⁶ found that conventional methods were inappropriate for large industrial scale uses for CDM as carbon byproducts accumulated in the walls of heated reactor and so blocked the reactors quickly. Hence, they utilized molten tin as a liquid medium and transfer of heat in a reactor of bubble column. Researchers⁹² looking at the financial side of SRM technology in the catalytic decomposition of methane for large scale manufacturing of CNMs and hydrogen. Zeng et al.⁹² proved that the conversion of methane with a furnace having electric arc was not in competition with steam reforming of methane without the presence of a carbon tax. Nonetheless, catalytic decomposition of methane could become a competitor with steam reforming of methane without the cost of CO₂ in case that the molten metal is active catalytically and attained conversion to near-equilibrium at 1000 °C or lesser. Overall, providing a selective and functional molten metal catalyst in catalytic methane decomposition could be beneficial at skimming the byproduct of carbon from the top of the metal surface in the molten melt reactor.

6. Applications of carbon produced by CDM

The carbon obtained from catalytic decomposition of methane can be divided in CNFs, CNOs and CNTs, whereas the texture, morphology and structure of these C materials largely depend on reaction parameters²⁸ like pressure, catalyst, gas phase composition and temperature. The CNMs chemical and physical properties (pore volume, surface chemistry, pore size and porosity) make them suitable for many applications.^8,10,27

Although several carbon materials have been tested, activated carbon catalysts and carbon blacks are the most commonly used carbon supports.^93–98 The typically large surface area and high porosity of activated carbon catalysts favor the dispersion of the active phase over the support and increase its resistance to sintering when the quantity of metal loaded is low. The pore size distribution can also be adjusted to suit the requirements of each reaction. The surface chemistry of carbon catalysts also influences their performance as catalyst supports, especially during the synthesis stage. Carbon materials are normally hydrophobic and they usually show a low affinity towards polar solvents, such as water, and a high affinity towards non-polar solvents, such as acetone. Although their hydrophobic nature may affect the dispersion of the active phase over the carbon support, the surface chemistry of carbon materials can easily be modified, for example by oxidation, to increase their hydrophilicity to favor ionic exchange. Apart from an easily tailorable porous structure and surface chemistry, carbon materials have other advantages:⁹³ (i) metals on the support can be easily reduced; (ii) the carbon structure is resistant to acids and bases; (iii) the structure is stable to high temperatures (even above 750 °C); (iv) porous carbon catalysts can be prepared in different physical forms (as granules, cloth, fibers, pellets, etc.); (v) the active phase can be easily recovered; and, (vi) the cost of carbon supports is usually lower than that of conventional supports, such as alumina and silica. Nevertheless, carbon supports have some disadvantages, such as, they can be easily gasified, which makes them difficult to use in hydrogenation and oxidation reactions,⁹⁶ and their reproducibility can be poor, esp. activated carbon catalysts, since different batches of the same material can have varying ash content. Carbon supported metal catalysts are employed in a number of applications including hydrodesulfurization of petroleum, hydrodenitrogenation, dehydrohalogenation, hydrogenation of CO, hydrogenation of halogenated nitroaromatics compounds and nitrocompounds, hydrogenation of unsaturated fatty acids, hydrogenation of alkenes and alkynes, oxidation of organic compounds and organic pollutants, and for fuel cells.^95–97 As well as acting as catalyst supports, carbon materials themselves can be used as catalysts for different heterogeneous reactions.⁹⁸ A large application of these CNMs would be significant for CDM process industrialization and business interest. Here, we will focus on CNTs, CNOs and CNFs.

6.1 Characteristics of CNTs

Carbon nanotubes show huge thermal⁹⁹ and electronic conductivity¹⁰⁰ and mechanical strength.¹⁰¹ Henceforth, it is not strange that they obtain a huge attraction for important applications like catalysts,¹³⁰ electrodes,¹⁰² fuel cells¹⁰³ and electrical devices.¹⁰⁴ Many researchers^105,106 investigated CDM to form carbon nanotubes over iron catalysts. Zhou et al.²⁹ presented a model of carbon deposition (Fig. 4) over Fe catalysts based on DFT. Firstly, methane decomposes to amorphous carbon and hydrogen (Fig. 4a). Fe₃C is then produced from the reorganization of iron atoms when the amount of deposited carbon surpasses the limitation of carbon solubility (Fig. 4b and c). The oversaturated deposition of Fe₃C produces the amorphous carbon originating from decomposition of methane on graphite (Fig. 4d). Carbon accumulates on the catalyst and crystallizes in a cylindrical network and eventually grows into tubular structures (Fig. 4e). This group studied the morphology of the byproduct carbon nano materials from catalytic decomposition of methane over iron catalysts at 750 °C, finding bamboo-like CNTs.

Fig. 4 CNTs formation mechanism over Fe catalyst. Methane decomposition to hydrogen and amorphous carbon (a). Rearrangement of iron atoms (b and c). The supersaturated Fe₃C decomposition deposited the amorphous carbon deriving from methane decomposition to graphitic one (d). Carbon deposited out to grew into tubular structures (e).

Others¹⁰⁷ examined catalytic decomposition of methane to get CNTs and H₂ over Al₂O₃ supporting iron catalysts. A research group⁵⁷ showed a huge manufacturing of bamboo shaped multi-walled CNT on the nickel/cobalt/iron catalyst and produced carbon nanotubes with an average diameter of ∼20 nm. The results of thermal studies showed that the CNTs exhibited a very good oxidation stability. Avdeeva et al.⁵⁹ studied the higher shelf-life iron-containing catalysts like Fe/Co/Al₂O₃ and FeAl₂O₃ for the decomposition of methane.

6.2 Characteristics of CNFs

Nanofiber technology is the division of nanotechnology that focuses on producing materials in the form of nanoscale fibers in order to obtain better functionalities. The unique combination flexibility, superior directional strength and high specific surface area of these fibers make them ideal materials for clothing and supports for aerospace structures. Decomposition of methane is a novel technology to form CNFs.^36,137 CNFs over CNTs formation was selected by Simon et al.¹⁰⁸ thanks to the correlation between reaction temperature and structural type of the carbon products on Pd/Al₂O₃. Herringbone CNFs were obtained at 700 °C, and the inclination angle of the graphene planes was 136°.¹⁰⁹ Elevated reaction temperature (750 °C) produces bamboo shaped herringbone CNFs, and the inclination angle was 114. At 800 °C, bamboo-shaped CNFs were rarely achieved; the angle of graphene planes was less than the one at 700 °C and 750 °C. At 850 °C, the lowest number of CNTs/CNFs was obtained; additionally, the inclination angle of CNFs was 24 and CNTs was 0°. Kim et al.^110,111 produced nanosheets and metal organic farmwork (MOF) through direct-carbonization for the advancement of batteries and supercapacitors for the diverse nanoarchitecture.

6.3 Characteristics of CNOs

Two different types of CNOs growth techniques were planned by He and co-workers.¹⁰³ In the vapor-solid mechanism, Ni NPs are produced firstly, and after the graphitic layer carbon adsorption occurs on the surface of catalytic particles. Lastly, CNOs with an encapsulated nickel nanoparticle were found. In another vapor–liquid mechanism, nickel NPs was produced first and after that the entire liquid nickel particle was covered by a graphitic layer. Eventually, a very small liquid nickel particle was produced.

Fakeeha et al.¹¹² showed that CNOs are produced at 700 °C on Fe/Al₂O₃ catalyst through decomposition of catalytic CH₄. Ibrahim et al.¹¹³ mentioned that CNO_S were obtained over 40% Fe/Al₂O₃ catalyst at 700 °C by CDM. Zhou et al.⁵³ examined the CDM outcomes over iron ore catalysts.

6.4 Applications of CNMs prepared by the CDM process

Wang et al.¹¹⁴ used Ni/Fe/SiO₂ and Fe/Al₂O₃ catalysts by nitric acid reflux to prepare isolated CNTs with 96% purity. Though most of the samples exhibited a decent dispersion instantly after sonication, in a few hours unprocessed CNTs settled down and in the solvent purified samples continued well-dispersed even after being stored for 60 days. This highlighted the acid purification effect on the dispersibility of CNTs.

Zhou et al.⁵³ achieved CNMs from catalytic methane decomposition on iron ore catalysts at 750 °C, 850 °C, and 900 °C. They correlated the efficacy of formed carbon to commercial PACS on removal of pollutant from wastewater. The outcomes showed that though the CNMs did not show a better removal than commercial PACs, the average removal of three tested micro pollutants were higher than 98%.

Ge et al.¹¹⁵ produced CNMs from catalytic decomposition of methane on stainless steel wire at 800 °C. They concluded that the CNMs in purified form could be utilized as a conductive agent for supercapacitors, which had a maximum cycling stability and high specific capacity.

Utilizing the gas deposition process of the coal-bed, Shanxi Zhongxing Environmental and Energy Technology Co., Ltd¹¹⁶ measured the formation of CNOs. The manufactured CNOs have been effectively applied in supercapacitor and lubricating oil fields. The availability of CNMs from CDM may expand their limit and produce new market opportunities leading to a CNM industry.

7. Conclusion and perspectives

Catalytic decomposition of methane is a one-step procedure to produce CO_x-free hydrogen and CNMs like CNOs, CNFs and CNTs. This review article presented the cost of total hydrogen production and CO_x emissions for technologies like MSR, SRM, CDM, water splitting and coal gasification to evaluate their economic impacts and environmentally friendly nature. These evaluations are required to support and promote less greenhouse gasses emissions. The catalytic decomposition of methane would be financially inexpensive with SRM technology together with an extensive recycling of byproducts such as CNMs. Iron, nickel, carbon and noble metal-based catalysts have been broadly and carefully investigated and from the industrial costs point of view, iron-based catalysts, particularly bulk or wasted iron catalysts, were suggested as an effective material for catalytic methane decomposition as it is well studied and less costly compared to the other catalysts and these characteristics make it an ideal candidate.

The primary challenge faced during catalytic decomposition of methane is the separation of deactivated catalysts but to overcome this challenge this process was investigated in different reactors (specially MMR, FLBR and FBR). The MMR exhibited outstanding potentials for commercial applications, as carbon byproducts can be easily separated from the solid catalysts uninterruptedly and this avoids the reactor clogging and catalyst deactivation. Recently, huge cumulative effort has been made to go towards its commercialization, and the influence of different parameters like reactor, catalyst and mechanism has been studied. Yet, numerous challenges must be overcome, the most important are the following: (1) from an environmental and economic point of view, wasted iron-based catalysts for catalytic decomposition of methane would be a highly important challenge to overcome (2) the synthesis of molten-metal catalyst at low temperature would be a great challenge for CDM but researchers are working to use different type of nanocatalyst such as nanocomposites and some other encapsulated structures to tackle these challenges.

Although based on the initial studies, catalytic decomposition of methane is definitely a promising technology. For hydrogen economy its commercialization would be a positive aspect for the future. Many studies have been done on CDM over reactor designing and catalyst preparation, nevertheless further work is needed for its reliable commercialization. Additional research is needed on the catalytic decomposition of methane to make it more practical and scalable. Especially further work and development on molten-metal reactors is required to efficiently remove the byproduct avoiding any catalytic deactivation (Fig. 4). Hopefully, according to the economical evaluation, CDM will become an economical technology on a small or medium industrial scale such as demand-driven onsite H₂ and byproduct formation. This in turn will generate new marketing opportunities leading to the foundation of new CNM-based industries and a new era for the emerging sustainable energies.

Conflicts of interest

There is no conflict to declare.

Acknowledgements

We are thankful to MUR PON DM 1061 for funding this study.

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Footnote

† Hydrogen manufacturing of membrane produced from polymer electrolyte membrane up to 100 N m h⁻³; alkaline electrolyzers up to 1000 N m³ h⁻¹.

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