Methane decomposition to produce COx-free hydrogen and nano-carbon over metal catalysts: A review

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Review Article

Methane decomposition to produce COx-free hydrogen and nano-carbon over metal catalysts: A review Jing Xia Qian a, Tian Wen Chen a, Linga Reddy Enakonda b, Da Bin Liu a, Gerard Mignani c, Jean-Marie Basset b, Lu Zhou a,* a

School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China KAUST Catalysis Center and Physical Sciences and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia c Consultant International Open Innovation, Kerfily 56580, France b

highlights

graphical abstract


Catalytic methane decomposition (CDM) was compared to other most common technologies.
Comparing the performance of Ni, noble metal, carbon and Fe catalysts for CDM.
Fixed, fluidized, plasma bed and molten-metal reactors were illustrated for CDM.
CDM would be a prospective technology for hydrogen production.

article info

abstract

Article history:

Catalytic decomposition of methane (CDM) is a promising technology for producing COx-

Received 2 November 2019

free hydrogen and nano-carbon, meanwhile it is a prospective substitute to steam

Received in revised form

reforming of methane for producing hydrogen. The produced hydrogen is refined and can

4 January 2020

be applied to the field of electronic, metallurgical, synthesis of fine organic chemicals and

Accepted 8 January 2020

aerospace industries. However, the CDM for COx-free hydrogen production is still in its

Available online xxx

infancy. The urgent for industrial scale of CDM is more important than ever in the current

Keywords:

metal, carbon and Fe-based catalysts, especially over cheap Fe-based catalyst to indicate

Methane decomposition

that CDM would be a promising feasible method for large hydrogen production at a

Fe-based catalysts

moderate cheap price. Besides, the recent advances in the reaction mechanism and kinetic

Molten-metal reactor

study over metal catalysts are outlined to indicate that the catalyst deactivation rate would

situation of huge COx emission. This review studies CDM development on Ni-based, noble

become more quickly with increasing temperature than the CDM rate does. This review

* Corresponding author. E-mail addresses: [email protected] (J.X. Qian), [email protected] (J.-M. Basset), [email protected] (L. Zhou). https://doi.org/10.1016/j.ijhydene.2020.01.052 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Qian JX et al., Methane decomposition to produce COx-free hydrogen and nano-carbon over metal catalysts: A review, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.052

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COx-free hydrogen

also evaluates the roles played by various parameters on CDM catalysts performance, such

Carbon

as metal loading effect, influences of supports, hydrogen reduction, methane reduction and methane/hydrogen carburization. Catalysts deactivation by carbon deposition is the prime challenge found in CDM process, as an interesting approach, a molten-metal reactor to continually remove the floated surface solid carbons is put forwarded in accordance to overcome the deactivation drawback. Moreover, particular CDM reactors using substituted heating sources such as plasma and solar are detailed illustrated in this review in addition to the common electrical heating reactors of fixed bed, fluidized bed reactors. The development of high efficiency catalysts and the optimization of reactors are necessary premises for the industrial-scale production of CDM. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Advantages and challenges of catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbon catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ni-based catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Noble metals catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fe-based catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Supported Fe based catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Non-supported Fe catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fixed bed reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fluidized bed reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Plasma bed reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Molten-metal reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Introduction The world’s two major energy challenges are sustainable and environmental pollution problems [1]. According to de Richter et al. [2], to serve human needs in 2030, the world will require over 55% more initial energy than in 2005, with an increment in worldwide carbon dioxide (CO2) discharges of 57%. Although the exploitation of fossil fuels currently satisfies the majority of the increasing world energy needs, they are doomed to run out relatively quickly. On the other hand, their combustion products such as COx and NOx are the main cause of some global problems, so their effects are very harmful [3]. Researches show that the consumption of fossil fuels for hydrogen production results in 500 million tonnes of CO2 discharges ever year, which equals to roughly 2% of the worldwide energy-associated CO2 discharges [4]. To resolve the problems of increasing fuel requirement and environmental pollutions, there is a desperate need for finding out ways for utilizing renewable energy. For most countries on earth, hydrogen is the second most important form of energy after electricity [5]. Advances in hydrogen

00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

production technologies, and the hydrogen in industrial applications, indicate that the developmental potential of hydrogen for decreasing the amounts of greenhouse gases (GHGS) in the atmosphere, will bring a positive influence on climate change effects [2]. Moreover, hydrogen is considered as an ideal energy carrier as its gravimetric energy density is very high, largely procurable in group form on the earth and its oxidation product (water) is pollution-free to the environment [5]. Currently, the most common technologies for hydrogen production are summarized as follows: steam reforming methane (SRM) [6,7], catalytic decomposition of methane (CDM) [8,9], partial oxidation of methane [10,11], gasification of coal and other hydrocarbons [12], electrolysis water splitting [13,14], photocatalytic water splitting [15e23], biomass gasification [24] and nuclear [5]. Compared to fossil fuels, other hydrocarbons and biomass, methane is the best source for the hydrogen production as it is easy to master and has a high hydrogen-to-carbon ratio [7,25]. Currently, commercial hydrogen is produced on a large scale by SRM, whereas the specific CO2 emissions by CDM to produce hydrogen are discovered to be much lower than that by SRM combined with

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CO2 capture from the syngas. Additionally, the production of by-product carbon with a value of $305/ton by CDM would make this process be economically competitive with SRM [4,26]. Compared to electrolysis water splitting [27e30], the main benefit of CDM is the feedstock usability by the present natural gas facilities, while electrolysis is primarily relied on the availability and cost of renewable electricity [4]. The usability of renewable electricity is expected to remain limited and this restrains the electrolysis utilization possibilities in the near future [4]. In addition, photocatalytic water splitting to hydrogen and oxygen with zero GHGS emissions by utilizing photoelectrodes is very valuable for the energetics and economics worldwide [31e33]. However, photoelectrode materials that have a suitable band gap, special catalytic characteristics and stability in a light environment, have not yet been found [34]. Under the condition of moderately endothermic, methane decomposed to hydrogen and solid carbon is a simple onestep reaction as shown in Eq. (1) [6]. However, methane is an inactive hydrocarbon because of the difficulty in breaking its CeH bond (440 kJ/mol) and high symmetric tetrahedral structure. Hence its pyrolysis can only react above 1200  C without a catalyst. Different metal and carbon catalysts have been applied to CDM researches. Furthermore, some researchers [35,36] studied the thermodynamics of CDM. From the perspective of thermodynamic, the conversion of methane increased with the reaction temperature increasing. Generally, it is clear from exploited catalysts that the activity increase as increasing the temperature up to a particular value then the catalysts starts deactivate [1,36,37]. CH4 / C þ 2H2

△H298 K ¼ 74:52 kJ=mol

(1)

In Refs. [7,38], the literature on the CDM has been reviewed. Comparison with these reviews, this work will provide the recent progress and prospect on metal and carbon catalysts with focusing on the carrier influences, reaction mechanism, reaction kinetics, reactor design and the future developing direction of CDM.

Advantages and challenges of catalysts Catalysts can reduce the activation energy and shorten the reaction time. Hence, selecting a suitable catalyst plays a crucial role in CDM process. Ni-based [39e44], doped noble metals [45e47], carbon [48,49] and Fe-based catalysts [50e52] are the mainstream researches of CDM catalysts. The characteristics of various kinds of CDM catalysts are listed in Table 1, which comprises the reaction condition, physical property, catalytic activity and carbon morphology.

Carbon catalysts In recent years, carbon catalysts [66] have been applied to CDM as their availability, durability, low cost, abundant porosity [67], molecular activation [68], and tolerance to high temperature. Wang et al. [69] reported the catalytic activity of the prepared ordered mesoporous carbons (OMCs) and some

3

commercial carbon materials for CDM. Results indicated that the catalytic activity was dependent on the chemical structure such as defects, whereas the stability was to rely on the physical properties such as the BET surface area and pore volume. Compared with disordered carbons, OMCs with relatively larger uniform pores could keep stable catalytic activity for long period. They concluded that it would be possible to improve the catalytic activity and stability of CDM by designing and preparing carbon materials with desired pore systems. Muradov et al. [48] investigated the elemental carbon, including a variety of activated carbons (ACs), carbon blacks (CBs), nanostructured carbons (including CNTS, graphite, and synthetic diamond powders) for their catalytic activity in CDM. They found that, ACs and CBs with suitable activity and stability at 850 C. Results indicated that the disordered carbons were in general more catalytic activity than ordered ones. Dufour et al. [49] evaluated the environmental performance of CDM by carbonaceous catalysts through life cycle assessment tools. Using the co-produced carbon as the carbonaceous catalysts, the author claimed that this “auto generated-catalyst CDM process” would be the more environmental-friendly, compared to CDM using metallic catalysts. Furthermore, its environmental characteristic was highly increased when the by-product carbon was applied to other commercial applications. For example, for a 70% methane conversion, the application of 50% of the by-product carbon would lead to an almost zero emissions process. Unfortunately, studies found that carbon catalysts gave poorer methane conversion than metal catalysts [6].

Ni-based catalysts For CDM over Ni-based catalysts, oxides supported [39e41] bimetallic/trimetallic [45,70] and carbon supported Ni catalysts [42,71] were first considered. Oxides supported Ni catalysts: Villacampa et al. [72] reported the results of properties and catalytic process of a 30%Ni/Al2O3 catalyst during CDM. They studied the effects of reaction and reduction temperatures (550e650 C) and feed composition (CH4/H2/N2) on the methane conversion, hydrogen production and coking rate. Results indicated that the catalyst was active at temperatures above 550 C. Besides, the hydrogen feed would compete with the methane on the Ni surface sites to inhibit the formation of both NCMs and encapsulating coke, which could deactivate the active form of the catalyst. Bayat et al. [73] investigated Ni/Al2O3 CDM catalysts with different Ni content at 570e700 C. Results indicated that the Ni content and reaction temperature both played a critical role of the catalytic characteristics of these catalysts, where the 50 wt% Ni/Al2O3 catalyst exhibited the highest activity and stability in comparison with other Ni catalysts at 625 and 650 C. Bimetallic/trimetallic Ni catalysts: It was known that Ni-based catalysts had a high activity in CDM, while they were sensitive to the operate temperature and would deactivate quickly at a high temperature. In the literature, researchers [39,47] tried to modify Ni catalysts with other metals to increase CDM activity and stability even at temperatures higher than 600 C.

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Table 1 e Performances of various catalysts recently researched. Catalysts Ni 40 wt%Ni/SiO2 75 wt%Ni/SiO2 35 wt%Ni/40 wt%Fe/SiO2 52.4 wt%Ni-25 wt%Fe/Al2O3 50 wt%Ni/MCM-22 50 wt%Ni/5 wt% Cu/MCM-22 50 wt%Ni/5 wt%Cu/5 wt% Zn/MCM-22 50 wt%Ni/10 wt% Cu/MCM-22 50 wt%Ni/10 wt% Zn/MCM-22 30 wt%Ni/SiO2 90 wt%Ni/SiO2 69 wt%Fe/Al2O3 10 wt%Fe/SiO2 12.3Fe/1Mo/6.15Al2O3(molar ratio) 12.3Fe/1Mo/6.15MgO(molar ratio) 1 wt%Rh-5wt%Ni/SiO2 1 wt%Pd-5wt%Ni/SiO2 1.8 wt%Ir-5wt%Ni/SiO2 1.8 wt%Ir-5wt%Ni/SiO2 23.33 wt%/Carbon Ordered mesoporous carbons CMK-5 Carbopack C Fluka 05120 CGNorit HS-50

Reactor Fixed bed Fixed bed Fixed bed Fixed bed Fixed bed Fixed bed Fixed bed Fixed bed Fixed bed Fixed bed Fixed bed Fixed bed Fluidized bed Fixed bed Rotary bed Rotary bed Fixed bed Fixed bed Fixed bed Fixed bed Fixed bed DSC-TGA thermobalance Fixed bed Fixed bed Fixed bed Fixed bed

GHSV Carbon deposition Carbon morphology Ref. Temperature [L∙g1cat.∙h1] [gC/gcat.] [ C] 500 500 650 650 650 750 750 750 750 750 600 600 700 800 750 750 550 550 550 550 850 1000

90 90 18 18 12 1.8 1.8 1.8 1.8 1.8 244 244 6 10.5 1.5 1.5 60 60 60 60 1.62 55

396 140 e e 562 3.63 4.26 5.68 5.5 5.45 14.2 0.48 3.6 13.8 1.92 8.26 10.1 26 3.45 7.5 1.5 25

CNFs CNFs Multi-walled carbon Bamboo-shaped carbon CNFs CNFs CNFs CNFs CNFs CNFs CNFs CNFs CNTs CNTs Bamboo-shaped carbon Tubular carbon CNFs CNFs CNFs CNFs CNFs Graphite

[53] [54] [55] [55] [56] [57] [57] [57] [57] [57] [58] [58] [59] [60] [61] [61] [62] [62] [62] [62] [63] [64]

850 850 850 850

38 38 38 38

0.28 0.65 0.45 0.28

e e Uniform carbon blacks CNFs

[65] [65] [65] [65]

Suelves et al. [74] studied the characterization of Ni/Al2O3 and Ni/Cu/Al2O3 catalysts prepared by different methods for CDM. The presence of Cu in Ni/Cu/Al2O3 catalyst had a strong effect on the degree of dispersion of Ni and Ni crystal domain sizes. Activity tests showed that the hydrogen production was not highly relied on the used preparation method but the existence of Cu as a dopant in Ni/Cu/Al2O3 catalyst improved the virtual active. Rastegarpanah et al. [40] investigated the La, Ce, Co, Fe, and Cu-promoted Ni/MgO·Al2O3 catalysts, and found that Ni/15 wt %Cu/MgO·Al2O3 exhibited the best activity at higher temperatures 675 C (>80% methane conversion). Anjaneyulu et al. [45] studied the influence of doping Ni/ Al2O3 by rare earth metals (La, Pr, Nd, Gd and Sm) on CDM activity. They found that, rare earth metals-doped Ni/Al2O3 exhibited the formation of hydrotalcite-like structures which greatly changed the activity of Ni particles. Results indicated that the Ni/Re/Al2O3 catalysts exhibited better methane conversions (Fig. 1) than Ni/Al2O3 due to a large Ni surface area and a strong connection between Ni and Re/Al2O3. Bayat et al. [39] prepared 50% Ni/Al2O3 and 50Ni/10Fe/nCu/ Al2O3 (n ¼ 0, 5, 10, 15, weight ratio) catalysts by wet impregnation method, and studied their CDM performances at 550e800 C. Among the investigated catalysts, 50Nie10Fee10Cu/Al2O3 presented the highest catalytic performance at 750 C to show 85% methane conversion. The rate of carbon diffusion was enhanced by Fe addition, while the adsorption of methane on the catalyst surface was increased by Cu addition. The high affinity of Cu between graphite structures would further restrain the formation of coated

carbon on the surface of Ni and thus inhibited the catalyst deactivation. Carbonaceous Ni catalysts: The carbon, with unique features such as porosity and surface chemistry [38], supported Ni catalysts were also extensively studied for CDM. Kang et al. [71] investigated the CDM performance over Ni/C/B2O3 coreshell catalysts. Although there was no catalytic reduction step by hydrogen, these catalysts showed an unparalleled activity for CDM. Results indicated that around 90% of methane conversion was achieved on 13 wt% Ni/C/B2O3 catalyst at 850 C. Furthermore, the spent Ni/C/B2O3 was successfully regenerated for 15 cycles without deactivation by oxidizing the produced carbon nano materials (CNMs) with CO2. Shen et al. [42] synthesized a series of NieCu based catalysts by polyol reduction method. They researched these catalysts for CDM, as well as studied the influences of surfactant polyvinylpyrrolidone, CNTs support and heterogeneous nucleation seed on the structural and catalytic characteristics of these catalysts. Results showed that the activity was closely associated with the Ni/Cu alloying degree and the types of support. Among the catalysts, the 74Ni/26Cu/ CNTs (actual Ni and Cu atomic ratio) showed the best activity with a stable methane conversion value of 80% at 700 C. The CNTs support was assumed to stabilize the quasi-spherical nanoparticles through its interactions. CDM mechanisms: In order to obtain a comprehensive understanding of the complex nature and involvement of many phases and steps, the mechanism and kinetics of CDM on Nibased catalysts have been studied [70,75]. Saraswat et al. [75] investigated the development of rate expressions of the

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Fig. 1 e Methane conversions with time-on-stream until complete deactivation of the catalyst [45].

Langmuir-Hinshelwood type followed by parameter estimation of the kinetic models based on the reaction mechanism proposed. The kinetics and modeling of CDM over 60Ni/5Cu/ 5Zn/Al2O3 (weight ratio) catalyst during 600e800 C were illustrated. Depending on dissociative adsorption and molecular adsorption of methane, reaction kinetic models were proposed (Table 2) to conclude that the adsorption of methane on the surface was the step to control the rate. Further, the author estimated the overall reaction activation energy as 73.2 ± 5.8 kJ/mol. Wei et al. [76] researched the isotopic and kinetic assessment of the CDM mechanism on n% Ir/ZrO2 (n ¼ 0.8, 1.6) catalysts at 600 C, and indicated that CeH bond activation was the only kinetically relevant step on Ir surface. The activation energy for CH4 decomposition was 81 kJ mol1 L1, and the kinetic isotope effect was 1.68 for CDM from initial CH4 and CD4 (Deuterium methyl) decomposition rates.

Dunker et al. [77] developed a kinetic model for CDM over carbon catalysts to predict that increasing the pressure from 0.1 to 3.0 MPa would reduce hydrogen yield by 48e60%.

Table 2 e Steps involved in the dissociative and molecular adsorption pathways for CDM [75]]. Dissociative adsorption 1. 2. 3. 4. 5. 6.

CH4 þ 2S % CH3S þ HS CH3S þ S % CH2S þ HS CH2S þ S % CHS þ HS CHS þ S % CS þ HS CS % C þ S H2 þ S % 2HS

Molecular adsorption 1. 2. 3. 4. 5. 6. 7.

CH4 þ S % CH4S CH4S þ S % CH3S þ HS CH3S þ S % CH2S þ HS CH2S þ S % CHS þ HS CHS þ S % CS þ HS CS % C þ S H2 þ S % 2HS

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Noble metals catalysts Some studies [47,78] showed that the addition of noble metals into supported-metal catalysts had better activity and stability in comparison with single metal in CDM. Therefore, some researchers [47,79] studied the relative catalytic activities of noble metals for CDM. Takenaka et al. [78] investigated the influence of the addition of different noble metals (Rh, Pd, Ir, and Pt) into 5 wt% Ni/SiO2 for CDM. They found that, Pd additive led to considerable increase in the stability and total hydrogen yields (2300 molH2/molNi over Ni/SiO2; 4500 molH2/molPdþNi over Pd/Ni/ SiO2), nevertheless modification with the other metals reduced the Ni/SiO2 activity (Fig. 2). That was because of the PdeNi alloys was formed. Pudukudy et al. [46] synthesized the n% Pd-promoted Ni/SBA-15 catalysts (n ¼ 0.2, 0.4) and investigated the performance of these catalysts. They found that, the addition of Pd increased the crystallinity of NiO and the

surface area of Ni/SBA-15 catalyst, allowing a better dispersion of NiO on the SBA-15 support. Moreover, the reduction temperatures of NiO were decreased by the hydrogen spillover influence. A maximum hydrogen yield 59% was obtained over the 0.4Pd/50Ni/SBA-15 (weight ratio) catalyst within 30 min, as well as no deactivation was found until 420 min. Bayat et al. [47] explored the CDM over 50Ni/nPd/Al2O3 (n ¼ 0, 5, 10, 15, 20, weight ratio) catalysts. Results indicated that the addition of up to 15 wt% Pd increased the catalyst properties because of the high activity of Ni/Pd and rapid diffusion of carbon over Ni/Pd. Nevertheless, further increase in Pd content reduced the catalytic activity, which could be lead to reduce in surface area and agglomeration of particles. Ultimately, 86% methane conversion was obtained at 800 C on 50Ni/15Pd/Al2O3 (weight ratio). Matsui et al. [80] examined the reactivity of by-product carbon formed from CDM on supported noble metal catalysts. They found that, the amount of CHx species on Ru-

Fig. 2 e Hydrogen yields in the methane decomposition over Ni/SiO2 modified with different metal species and Ni/SiO2 [78].

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Table 3 e Comparison of catalytic activity of inert oxides supported Fe catalysts. Reactor

Temperature [ C]

GHSV [L∙g1cat.∙h1]

Carbon deposition [gC/gFe]

Carbon morphology

Ref.

Fixed bed Fixed bed Vibrating flow Vibrating flow Vibrating flow Vibrating flow Vibrating flow Vibrating flow

800 800 700 700 700 700 625 625

105 105 8 8 8 8 45 45

7.5 22.5 45 13.5 14 17.4 53 104.8

CNTs CNTs CNTs CNTs CNTs CNTs CNTs CNTs

[86] [86] [87] [87] [87] [87] [88] [88]

Catalysts 26.5 wt% Fe/SiO2 26.5 wt% Fe/SiO2 85 wt% Fe/SiO2 85 wt% Fe/ZrO2 85 wt% Fe/Al2O3 85 wt% Fe/TiO2 50 wt% Fe/Al2O3 50 wt% Fe/6 wt% Co/Al2O3

loaded catalysts were independent of the kind of support, while the reactivity of CHx species over Ru intensely depended on the supports. Besides, the reactivity of CHx species over Ru/ La2O3 and Rh//La2O3 were studied, and it seemed that Rh brought about a higher rate of methane decomposition than Ru. The by-product carbons on Ru/La2O3 seemed to be more uniform and reactive than those over Ru/Al2O3. In conclusion, the addition of noble metals into catalysts can improve the activity and stability, while it is relatively expensive and there is no prospect of industrial application.

Fe-based catalysts In order to make the CDM truly green and economically, using a very cheap catalyst to decompose methane into hydrogen, is probably a promising approach [81]. Highly-efficient and ecofriendly Fe catalyst is effective alternative to address the problem in nowadays [82]. Furthermore, Fe is a good candidate for this purpose because of it also has the partially filled 3d orbitals [82,83] to facilitate the hydrocarbon dissociation via partially accepting electrons [81].

Supported Fe based catalysts Over the past few decades, many efforts have been devoted for developing high efficiency catalysts for CDM to realize the process with a moderate condition. Inert oxides supported Fe catalysts [84,85] have been researched widely. Especially, Al2O3 and SiO2 are the common inert oxides supports. Table 3 summarizes the performances of these catalysts for CDM process reported in the literature.

Supported Fe monometallic catalysts for CDM Fe/Al2O3 for CDM. Fe loading effect: Avdeeva et al. [88] studied the carbon deposition of CDM over nwt% Fe/Al2O3 (n ¼ 14e63) catalysts at 625 C and GHSV of 45 L∙g1cat.∙h1. Results indicated that the maximum carbon deposition of 28gC/gcat. was generated over 42 wt% Fe/Al2O3. Ibrahim et al. [89] explored the CDM on nwt% Fe/Al2O3 (n ¼ 14e63) catalysts at 700 C and GHSV of 6.6 L∙g1cat.∙h1. They found that, the hydrogen yield increased with the loading of Fe until reaching the maximum hydrogen yield of 77.2% obtained over 42 wt% Fe/Al2O3. Further increasing the Fe loading however decreased hydrogen yield, as high loadings of Fe reduced the surface area of the catalysts. Similarly, Zhou et al. [84] investigated the propertied over nwt% Fe/Al2O3 (n ¼ 3.5e70) catalysts at 750 C and GHSV of 7.5 L∙g1cat.∙h1. The catalysts activity was observed to increase with Fe loading until achieving a value of

about a Fe loading of 41 wt%. Exceeding this value, further increasing the Fe loading reduced the catalysts activity. Moreover, a stable methane conversion 80% was achieved for as long as 10 h over 41 wt% Fe/Al2O3 at 750 C and GHSV of 2.5 L∙g1cat.∙h1. They suggested that 41 wt% Fe/Al2O3 possibly has the best stoichiometric connection of Fe2O3 and Al2O3 among all catalysts that could be incorporated into each other’s lattice to form Fe2O3$Al2O3. It seemed as this kind of solid solution formation could augment the BET surface area and pore volume. Preparation methods influence: Fakeeha et al. [85] investigated the CDM over 20 wt% Fe/Al2O3 catalysts at 700 C and GHSV of 5 L∙g1cat.∙h1. They found that, irrespective of calcination temperatures, compared to catalysts prepared by adsorption methods, the Fe/Al2O3 impregnated catalysts presented an overall better catalytic performance. At 700 C, after 90 min CDM test, 65% methane conversion was obtained over Fe/ Al2O3 impregnated catalyst; whereas adsorption method prepared catalysts showed the highest methane conversion of 50%. Avdeeva et al. [88] investigated the carbon deposition of the catalysts by using two different precipitation methods: precipitation of Al2O3 support suspension with Fe(NO3)3 solution with NH4OH and co-precipitation aqueous solution of Fe and Al salts with NH4OH. At 625 C and GHSV of 45 L∙g1cat.∙h1, the better carbon deposition 26.5 gC/gcat. was observed for the sample of 50 wt% Fe/Al2O3 prepared by coprecipitation method. Fe/SiO2 for CDM. SiO2 and Fe loading effect: Takenaka et al. [86] carried out the CDM over n wt% Fe/SiO2 (n ¼ 5e54) at 800 C and GHSV of 105 L∙g1cat.∙h1. The size of Fe2O3 crystallites in Fe/SiO2 was found to become larger with loadings, as well as those of diameters less than ca. 30 nm in the fresh catalysts were translated into a-Fe metal and Fe3C (catalytic activity site) instantaneously after contact with methane, while those of larger diameters were translated into g-Fe metal saturated with carbon atoms. The carbon yields (molC/molFe) at 20 h were estimated to be 15, 34, 35, and 33 for Fe/SiO2 of 5, 10, 26.5, and 54 wt% loadings, respectively. It was concluded that the 26.5 wt% Fe/SiO2 catalyst possessed the best catalytic activity. For the effect of SiO2 loading on CDM activity, Ermakova et al. [87] studied the CDM performance over 60 wt% Fe/nwt% SiO2 (n ¼ 0e30) at 700 C and GHSV of 8 L∙g1cat.∙h1. They found that, Fe/15SiO2 showed the best catalytic activity, and 45gC/gFe carbon yield was obtained. They proposed the SiO2 can either inhibit or promote the CDM process, while SiO2 with the suitable specific surface area could make the interaction with Fe to be easy. Al-Fatesh et al. [90] also investigated

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the CDM over 20 wt% Fe/nSiO2 catalyst and concluded that methane conversion rapidly descended with the high content of SiO2. Preparation methods influence: Al-Fatesh et al. [90] studied CDM on 20 wt% Fe/SiO2 catalyst prepared by a wetimpregnation technique. Results indicated that 5% methane conversion was achieved at 800 C, GHSV of 4 L∙g1cat.∙h1. Ermakova and co-workers [87] synthesized 60 wt% Fe/SiO2 catalysts by precipitation of FeCl3$6H2O with NH4OH, over which 45gC/gFe carbon yield was obtained at 700 C, GHSV of 8 L∙g1cat.∙h1. Murata et al. [91] prepared the 10 wt% Fe/SiO2 by impregnation of iron nitrate into SiO2, and methane conversion was increased from 0 to 8% in the range temperature of 600e750 C.

Compare the catalysts activity of Fe/Al2O3 to Fe/SiO2. Murata et al. [91] compared the CDM performance of 10 wt% Fe/Al2O3 to 10 wt% Fe/SiO2 catalysts under strictly the same conditions. They found that, the methane conversion of Fe/ Al2O3 (75%) was much higher than Fe/SiO2 (10%). In addition, Al-Fatesh et al. [90] also tested 20 wt% Fe/Al2O3 and 20 wt% Fe/ SiO2 for CDM under the same conditions. Results indicated that the catalyst of 20 wt% Fe/SiO2 presented 5% methane decomposition at 700 C and GHSV of 3 L∙g1cat.∙h1, whereas 83% methane conversion was obtained over 20 wt% Fe/Al2O3 under the same reaction conditions. The reason was ascribed as the lower BET and pore volume of 20 wt% Fe/SiO2 catalyst than those of the 20 wt% Fe/Al2O3. Takenaka et al. [86] came to the same conclusions: the carbon yield in the CDM for Fe/ Al2O3 (22.5gC/gFe) was greater than that for Fe/SiO2 (7.5gC/gFe). They found that the catalytic activity relied intensely on the particle sizes of catalytically active kinds (a-Fe metal and Fe3C), which were formed during CDM by the reduction of Fe2O3 with methane. Among these catalysts, Fe2O3 particles with smaller sizes over (Fe/Al2O3) were translated into Fe3C, nevertheless larger ones (Fe/SiO2) were translated into g-Fe saturated with carbon atoms. Ermakova et al. [87] pointed to the opposite conclusion that SiO2 was the most appropriate textural promoter for Fe-based catalysts. They found that the most prospective results on extending catalyst lifetime were obtained with 60 wt% Fe/SiO2 rather than 60 wt% Fe/Al2O3. It was attributed to the strong interaction between the active component and SiO2.

Supported Fe binary catalysts for CDM. Many researchers [1,88,92] reported a promoter influence of certain transition metal additive on the CDM performance of inert oxides supported Fe-metal bimetallic catalytic formulation. Calafat et al. [93] investigated the CDM over 40%wt Fe/Ni/ ZrO2 at 650 C. Results indicated that the carbon yield of the bimetallic catalysts were higher than that of the monometallic catalysts, due to FeeNi alloys could stabilize the catalytic activity by reducing the deactivation rate of the catalysts. The carbon deposition rate of FeeNi alloys was lower than that of monometallic Ni and Fe phases. Pinilla et al. [1] reported the connection of Fe particles to Mo would help to prevent Fe particles from agglomerating under operation conditions when at a temperature higher than 800 C. Over Fe/Mo/MgO catalysts (respective molar ratio

of 50: 7.5: 42.5) during the temperatures of 600e950 C, GHSV of 1 L∙g1cat.∙h1, 87% methane conversion was obtained at 900 C. Al-Fatesh et al. [94] investigated the CDM on 15Fe/nNi/MgO (n ¼ 0e10, weight ratio) catalysts at 700 C, GHSV of 5 L∙g1cat.∙h1. Results obtained that 15Fe/3Ni/MgO catalyst displayed the best catalytic propertied, and 73% methane conversion was achieved. For the low methane conversion over the heavily loaded Ni catalysts (>3 wt%), it could be attributed to a decrease of the Ni dispersion/augment of the Ni-particles size, which resulted in a decrease of the amount of active sites. Besides, another possible explanation was the strong connection between Ni and Fe in NieFe solid solution could drop off the strength of the metal sites and result in a drop of activity of these catalysts in CDM. Therefore, the presence of suitable amounts of non-interacted NiO species as 3 wt% could be explained for the high performance of this catalyst in CDM. Pudukudy et al. [95] studied the CDM performance over 25Fe/25Co/SBA-15 (weight ratio) catalyst at 700 C, GHSV of 5 L∙g1cat.∙h1. The catalyst was highly active to show 51% of hydrogen yield and stable even after 300 min, because of the bimetallic alloys formation. The by-product carbons were observed to be in the form of a new type of hollow multiwalled nanotubes with open tips.

Carbon supported Fe catalysts for CDM. Besides oxides supported Fe catalysts for CDM, carbon supported Fe [96] have also been investigated to improve the performance by using the advantages of both metal and carbon catalysts. Due to their large surface area and pore volume, carbon materials would be expected to accommodate large amount of carbon deposit during the CDM process and possess excellent tolerance to sulfur and other poisonous impurities in the feedstock and resistance to high temperatures in CDM. CDM performance was conducted using Fe/AC catalyst at 750 C by Sivakumar et al. [97]. The author focused on using low-cost AC (derived from wood base material) as support for Fe catalysts with the aim to grow CNTs of different morphologies. At 750 C, 98% methane conversion was obtained, as well as thin-walled CNTs (diameter of 8 nm) were formed. It was noticed that Fe catalyst maintained its activity at 20% methane conversion even after 60 min. Wang et al. [96] successfully prepared nwt% Fe-doped carbon catalysts (n ¼ 5e30) from Shenmu sub-bituminous coal with addition of Fe(NO3)3 by KOH activation (Fig. 3), and tested these catalysts for CDM during the temperatures of 750e900 C, GHSV of 15 L∙g1cat.∙h1. They found that, with increasing amounts of Fe addition, the specific surface area of the resultant catalysts reduced, nevertheless the hydrogen output rate and methane conversion over these catalysts increased dramatically. When the amount of Fe added was 30 wt%, the catalyst showed the best catalytic activity, and the methane conversion reached 58% at reaction time of 9 h at 800 C̊ (Fig. 4). CO generation over inert oxides supported Fe catalysts. CDM is an ideal technology to produce pure hydrogen without any

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Fig. 3 e The preparation process of Fe-doped carbon catalysts [96].

Fig. 4 e CH4 conversion (a) and H2 output rate (b) over Fe doped carbons with different amounts of Fe added [96]. contamination of CO/CO2 compared to the traditional SRM technology [98,99]. However, in a real CDM process, the emission of CO/CO2 in very low concentration cannot be neglected because of the different reasons [81,100]: (I) the active metal is not reduced completely; (II) the reaction between methane and “oxygen” from the catalysts (support and/ or metal oxides) (III); the surface of carbonaceous catalysts contain OH groups, which is reacted with the methane directly. CO formed from metal-oxides reduction: According to Zhou et al. [101], over unsupported Ni bulk catalyst prepared by fusing Ni(NO3)2∙6H2O, without pre-reduction, a high initial methane conversion occurred from combined proceeding reactions of NiO reduction by CH4 (NiO þ CH4 / Ni þ COx þ H2O þ C þ H2) and CH4 steam reforming with this formed H2O at 800 C. They further used the mass spectrum to demonstrate the continual formation of H2, CO, CO2, and H2O during the initial 2 min, after that time, no signal of COx was observed. Similarly, without pre-reduction, Takenake et al. [102] conducted CDM over 77 wt% Fe/Al2O3, and reported the formation of H2O, CO and CO2 resulted from the reduction of iron oxides on Al2O3 with methane. The formation rates of CO and CO2 showed maxima value of 80 and 10 mmol/min at ca. 1 h of time on stream at 800 C, 1.5 L∙g1cat.∙h1. CO formed from oxides support: Choudhary et al. [103] studied the rate of CO formation in CDM process over 10 wt% Ni catalysts supported on different supports as H-ZSM-5((Code No.

DAZ-P; Si/Al ¼ 500), HY and SiO2. HY was produced from NaY (Code No. DAY-P; Si/Al ¼ 100) via aqueous ion exchange (ammonium hydroxide solution). They found that, although the catalysts were reduced with H2/Ar at 250 C for 0.5 h and at 450 C for 2.5 h, CO was found over all catalysts while the rates were high primordially but reduced quickly with time and then remained stable until catalyst deactivation. The amount of CO in the hydrogen stream stabilized at ca. 750, 350 and 150 ppm for H-ZSM-5, Ni/HY and Ni/SiO2 respectively at 450 C, 20 L∙g1cat.∙h1. They further suggested that the difference of CO formation rate may be connected with the amount/stability of the eOH groups present on the different supports. Tang et al. [50] synthesized a series of CeO2 supported Fe catalyst with different Fe loadings (14e56 wt%). For all catalysts, which were went through a pre-reduction with hydrogen (50 mL/min, 4.1% H2 in argon) at 750 C for 3 h, CO was detected throughout the CDM process and its formation was ascribed to the reaction between the lattice oxygen of CeO2 and carbon deposits (C þ O*(lattice oxygen) / CO). Even after 250 min CDM at 750 C, 4 L∙g1cat.∙h1, ca.100 ppm CO could still be detected over all Fe/CeO2 catalysts (Fig. 5), whereas bulk Fe catalyst showed no CO since the initial period. This further demonstrated the disadvantage of supports to bring about the CO formation. CO formed from carbonaceous catalysts: It is an accepted fact that CB, especially AC, contains great amounts of oxygen, in terms of RCOOH, ROCO, ROH and RO, which are reported to be

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Reaction mechanism and kinetic study of CDM over supported Fe catalysts. Activation treatment: Catalysts activity and sta-

Fig. 5 e CO production profile over all CeO2 catalysts [50]. very difficult to remove from the carbon surface by simple thermal treatment in an inert gas. As a result, in an CDM effluent gas over the original AC operated at 850 C and a residence time of approximately 0.1 s, N. Muradov et al. [104] observed 0.77 vol.% CO at the initial period, which was stabilized at c.a. 0.17 vol.% after 20 min until 70 min. Further, Moliner et al. [100] proposed two mechanisms of reaction could occur on carbon catalysts: (i) The oxygen groups on the surface react directly with the methane molecules like in a partial oxidation reaction or (ii) these groups release from the surface as CO or CO2, creating active reaction points. It would be easy to eliminate the first possibility by reducing the metal-oxides completely through an appropriate pre-reduction treatment. However, to eliminate the OH groups or lattice oxygen from catalysts especially the supports would be impossible, which thus would suggest a major drawback of the supported catalysts during an industrial operation as producing CO during CDM.

bility relies on the experimental factors such as reaction temperature, GHSV, which have been widely discussed and reviewed in literatures. Herein, besides these two common parameters, the effect of activation treatment on CDM performance is summarized as following. H2 reduction: Generally, pre-reduction of Fe-based catalysts with H2 is always applied in literature to reduce Feoxides into Fe0 to catalyze CDM process. To understand the Fe-oxides reduction mechanism, some related studies were summarized here. Zhou et al. [81] developed a comprehensive research about the Fe-based catalysts redox performance by H2-TPR technical over Fe supported with different supports such as TiO2, Al2O3, CaO, MgO and CeO2 (Fig. 6). For pure Fe sample, the sharp peak between 300 and 500 C was normally attributed to the reduction of Fe2O3 into Fe3O4. Further reducing Fe3O4/FeO/Fe0 can explain the broad peak present at the temperature ranged from 500 to 750 C. For supported Fe samples, although the author concluded that all catalysts followed the stepwise reduction mechanism as Fe2O3/Fe3O4/FeO/Fe0, the formation of solid solution between Fe and supports, such as Fe3þ-O-Ti (Fe2O3), Ca-O-Fe and Al-O-Fe, would result in additional H2-TPR peaks above 750e1000 C. Particularly, Zhou et al. [84] studied the reduction mechanism by H2-reduction over fused 41 wt% Fe/Al2O3 at different temperatures of 550, 750 and 900 C. At 550 C H2reduction, all Fe2O3 phases were reduced into Fe3O4 (3Fe2O3þH2/2Fe3O4þH2O), and part of Fe3O4 was further reduced into 26 wt% Fe0 (Fe3O4þ4H2/3Fe0þ4H2O). However, at 750 C H2-reduction, besides Fe2O3 was gradually reduced into Fe0 (Fe2O3/Fe3O4/FeO/Fe0), it was also accompanied by hercynite formation (Fe3O4þ Fe0þ4Al2O3/4FeAl2O4, FeO þ Al2O3/FeAl2O4; or Fe3O4þH2þ3Al2O3/3FeAl2O4þH2O). And that explained the sample was composed of 13.5 wt% Fe0 and

Fig. 6 e H2-TPR of Fe-based catalysts [81].

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Fig. 7 e Reduction process of Fe2O3 catalyst during CDM [105].

86.5 wt% FeAl2O4. After H2 reduction at 900 C, part of hercynite was reduced to Fe0 and g-Al2O3. It was found that a temperature has to be higher than 1000 C to fully reduce the hercynite, which explained the 6 wt% FeAl2O4 remaining even after H2 reduction at 900 C. CH4 reduction: Instead of H2, using methane feed to in situ reduce Fe-oxides catalysts was very interesting and extensively studied by many groups as an economic activation method to avoid additional H2 input. Geng et al. [105] studied the methane induced reduction process of 14 wt% Fe/Al2O3 catalyst for CDM. As shown in Fig. 7, with the consumption of Os (oxygen linked to the iron oxides), the Fe2O3 in catalyst would be reduced fully by methane. Meantime, as the active Fe0 accumulated to a certain

amount on catalyst, methane decomposition started occurring, and the amorphous carbon and H2 would produce at the same time. Meanwhile, Fe connected to amorphous carbon to form Fe3C, and amorphous carbon became graphite carbon under the action of Fe3C. They concluded that this period described above can be regarded as the step to activate the catalyst. Linga et al. [106] also confirmed the reduction of Fe2O3 by CH4 proceeded in three steps: Fe2O3/Fe3O4/FeO/Fe0 (Fig. 8). Once Fe0 was formed, it decomposed methane with formation of Fe3C, which was the important initial stage in the CDM process to initiate formation of CNTs. The author further demonstrated the catalyst reduced under methane shows excellent activity in comparison with the one reduced in H2 under similar

Fig. 8 e H2-TPR profiles of Fe2O3eAl2O3 catalysts by methane [106]. Please cite this article as: Qian JX et al., Methane decomposition to produce COx-free hydrogen and nano-carbon over metal catalysts: A review, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.052

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conditions. It was hypothesized that part of FeAl2O4 could be reduced at 750  C under the condition of H2, which may lead to the sintering of Fe0 and decreasing of BET surface area. CH4/H2 carburization: Shah et al. [107] studied the effect of catalyst pretreatment of carburization (after pre-reduction with H2 at 1000  C for 2 h, the sample was treated with 20% CH4-80% H2 at 700  C for 2 h to produce metal carbide) for CDM activity over 0.5%Mo-4.5%Fe/Al2O3 catalysts. Results indicated that not much difference in activity was observed between the reduced and the carburized catalyst. This suggested that the iron metal phase in the pre-reduced catalyst may be partially converted to the carbide phase under reaction conditions. Thus, the form of the catalysts may be similar in both reaction experiments, even though they were pretreated to produce different states. Reaction kinetics: Geng et al. [105] studied the kinetics of CDM and catalyst deactivation over 20 wt% Fe/Al2O3 catalyst during the temperatures of 750e900 C, and partial pressures of methane (PCH4) of 0.22, 0.50, 0.75, and 1.0atm. Due to the endothermic feature of CDM, results indicated that the initial methane decomposition rate was improved over the tested catalysts when increasing temperatures and PCH4. The calculated kinetics based on the generation rate of molar hydrogen showed that the reaction order of CDM was 2.27, and the value of activation energy Ea was 50 kJ/mol. Its deactivation order, methane concentration dependency, and activation energy were 1.8, 0.95, and 121.37 kJ/mol, respectively. The lower activation energy required for the CDM (50 kJ/mol) than that for catalytic deactivation (121.37 kJ/mol), indicated that the catalytic deactivation was more sensitive of temperature than the reaction of CDM. With increasing temperature, the catalyst deactivation rate would very more quickly than the methane decomposition rate does. Douven et al. [108] performed a kinetic study to describe experimental reaction rates of double-walled carbon nanotube (DWNT) synthesis by catalytic chemical vapor deposition over a Fe/Mo/MgO catalyst utilize methane as the carbon source. The best model was found to involve the irreversible dissociative adsorption of methane followed by the irreversible decomposition of the adsorbed methyl group, which was the rate-determining step. Furthermore, from parameter such as effect of temperature estimation, the activation energy Ea2 (k1 ¼ k1ref*exp[-Ea1/R(1/T-1/Tref)]) of irreversible decomposition of the adsorbed methyl group was found to be equal to 58 kJ/mol, while the activation energy Ea1 (k2 ¼ k2ref*exp[-Ea2/R(1/T-1/Tref)]) of irreversible dissociative adsorption of methane was not significantly different from zero (8 ± 6 kJ/mol). Where Ea1 and Ea2 are the activation energies of the first and the second elementary steps

respectively; k1ref and k2ref are the corresponding preexponential factors. T is the temperature and Tref is a reference temperature fixed to an intermediate value in the temperature domain studied (927 C).

Non-supported Fe catalysts In order to eliminate the formation of CO from catalysts especially the supports during the CDM process, some nonsupported Fe catalysts had been extensively reported in the literature, as shown in Table 4. Pudukudy et al. [109,110] also investigated the CDM over unsupported porous Fe catalysts (synthesized via a facile solid state citrate fusion method) during the temperature range of 700e900 C, and at a GHSV of 9 L∙g1cat.∙h1. The catalysts were found to be highly porous which may be induced from the release of bulk amounts of carbonaceous gases from the fused sample, during the calcination process. Over this catalyst, after pre-reduction at 600 C with pure hydrogen for 90 min, a CDM testing indicated an almost stable activity of hydrogen yield >50% and a total carbon yield of 6.62 gC/gcat. at 900 C for as long as 360 min. The formation of few layered graphene sheets was found over this spent iron catalyst. Raney-type catalysts were prepared by Cunha et al. [112] from MeeAl alloys (Me ¼ Fe or Cu) by a fast quenching method to leach Al using a concentrated NaOH solution at room temperature. Over Raney-type Fe35 catalysts (leaching out Al from Fe35Al65 alloy), after hydrogen pre-reduction at 600 C for 2 h, methane conversion decreased from initial 56%e19% after 5 h during a CDM reaction at 600 C, 18 L∙g1cat.∙h1. The author further studied the CDM over Raney-type FeeCu alloyed catalysts as Fe35Cu50 (leaching out Al from Fe35Al65 mixed with Cu50Al50 alloy) and found a good CDM stability from initial 40%e35% after 5 h. It was believed that the inactive Cu would alloy with the active Fe to dilute the particle size to small ensembles and thus improve catalyst stability. Here, it should be pointed out that residual Al2O3 is still present on reported Raney-type catalysts, due to incomplete leaching of Al. Allaedini et al. [111] investigated CDM over co-precipitated 4Fe/2Co/3Ni (molar ratio) catalyst to produce hydrogen and CNTs at 1000 C. After hydrogen pre-reduction at 600 C for 1 h, CDM was performed at 1000 C, 18 L∙g1cat.∙h1 for 3 h and the results showed a growth of CNTs at a yield of 2.37 gC/gcat. Furthermore, compared to the CNTs synthesis methods in which the catalysts were supported by materials such as silica, alumina, or magnesium oxide, FeeCoeNi catalyst did not need any complicated purification steps such as the removal of supports. This decreased the cost and increased the efficiency, as the catalyst was the only impurity to remove.

Table 4 e Recent studies on non-supported Fe catalysts for CDM. Catalysts Unsupported Fe Unsupported porous Fe 4Fe/2Co/ 3Ni(molar ratio)

Reactor Vibrating flow Vertical-up flow cracking tubular decomposition reactor

Temperature GHSV Catalyst particle Carbon deposition [ C] [L∙g1cat.∙h1] size [nm] [gC/gFe] 700 900

8 9

50e67 29

16.5 6.62

1000

18

42

5.5

Carbon morphology

Ref.

CNTs [87] Layered graphene [109,110] sheets CNTs [111]

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Reactors In the last decade, various reactors such as fixed bed reactor (FBR), fluidized bed reactor (FLBR), plasma bed reactor (PBR) and molten-metal reactor (MMR) have been studied for CDM. It is worth mentioning that the MMR was first proposed for CDM process in this review. Table 5 lists examples of recent researches using different reactor types and heating sources along with major findings.

Fixed bed reactor FBR is the most commonly used reactor for CDM [39,99]. Ibrahim et al. [89] explored the CDM over iron catalysts in a FBR with 48 cm height and 0.94 cm I.D (Fig. 9). Result indicated that 77.2% hydrogen yield was obtained over 60% Fe/Al2O3 catalyst. Paxman et al. [115] investigated an experiment set up which heated by solar flux for CDM to produce hydrogen. In short, the solar reactor for CDM was economic and environmentally friendly compared with the traditional heating

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method; the solar process avoided both CO2 emissions from fossil fuel combustion needed to conduct the endothermic CDM reaction and would thus avoid 13.9 kg-equivalent CO2/kg H2 produced [123]. Rodat et al. [116] also designed a pilot-scale solar reactor which operated at the 1 MW solar furnace for hydrogen and carbon black (CB) production from CDM. The reactor was composed of 7 tubular reaction zones and of a graphite cavity-type solar receiver behaving as a black-body cavity (Fig. 10). The influence of temperature (1335e1655  C) and residence time (37e71 ms) on methane conversion and hydrogen yield was studied, it was found that, when 900 g/h of 50% molar CH4/Ar injected at 1527  C, 200 g/h hydrogen (88% H2 yield) and 330 g/h CB (49% C yield) was produced in this reactor. Izadi et al. [114] used a vertical two pass-fixed bed tubular quartz reactor (150 cm height) for the production of CNTs over Co/Mo/MgO catalyst. According to the proposed model of three different mechanisms for description of the growing of CNTs. Activation energy was found to be about 56.4 kJ mol-1 and the catalyst deactivation rate was found second order.

Table 5 e Recent researches using different reactor types and heating sources for CDM. Reactor type C C C C C C C C C

Fixed bed reactor 48 cm height and 0.94 cm I.D Pilot-scale fixed bed reactor stainless steel 6.03 cm O.D, 0.874 cm thickness, 120 cm height Vertical two pass-fixed bed Tubular quartz reactor 150 cm height Solar flux reactor

C Pilot-scale solar reactor C 1 MW solar furnace C Composed of 7 tubular reaction zones C Temperature 1335e1655  C, residence time 37e71 ms C Fluidized bed reactor C 6.5 cm I.D. 80 cm height C Two chambers C Fluidized bed reactor C 2 cm diameter, 0.5 cm central tube C Plasma-driven reactor C 61 cm reactor section C Arc jet plasma reactor C Nanosecond pulsed plasma reactor C Molten metal bubble columns reactor C liquid metal tin bubble column reactor with a packed bed C Quartz glass and stainless-steel material C reactor tube length 126.8 cm, I.D 4.06 cm, and the steel cladding tube 11.5 cm long, I.D 4.925 cm

Heating source

Findings

Ref.

Electric furnace

77.2% hydrogen yield was obtained over 60% Fe/Al2O3 catalyst

[89]

Electric furnace

Rapid reduce in the catalyst pore mouth with the rate of carbon deposition gradually increased

[113]

Electric furnace

Activation energy 56.4 kJ mol-1 Catalyst deactivation rate second order CNTs was produced over Co/Mo/MgO catalyst Economic and environmentally friendly Avoided both CO2 emissions from fossil fuel combustion Avoid 13.9 kg-equivalent CO2/kg H2 produced 900 g/h of 50% molar CH4/Ar injected at 1527  C, 200 g/h hydrogen (88% H2 yield) and 330 g/h CB (49% C yield) was produced

[114]

Concentrated solar energy

Concentrated solar energy

[115]

[116]

Electric furnace

25%e40% methane conversion

[59]

Electric furnace

78% yield of CNTS

[117]

Electric furnace

[118]

Electric furnace Electric furnace

thermal efficiency was between 60% and 80% 30% carbon yield 79% methane conversion Discharge power of 9 W, 100% methane conversion PteRe/Al2O3

Electric furnace

Carbon can be removed successively

[121]

Electric furnace

Solid carbon clogging issues did not occur at the reactor wall The liquid metal temperature and the gas residence time have the most powerful effect on the CDM

[122]

[119] [120]

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Fig. 9 e Schematic diagram of FBR for CDM [89].

Fluidized bed reactor Compared to a FBR, the FLBR is the better prospective reactor for large-scale operation because of it is suitable for continuous addition of catalyst and withdrawal of solid carbon from the reactor [124,125]. Torres et al. [59] studied the CDM performance over Febased catalyst in a FLBR (Fig. 11), which made of 6.5 cm I.D. and 80 cm height. The reactor was divided into two chambers by using a horizontal perforated plate with holes of 1 mm diameter. 25%e40% methane conversion was obtained at the space velocities between 3 and 6 L∙g1cat$h1.

Allaedini et al. [117] studied the CNTs by CDM over Mo/Ce catalyst in a FLBR. The reactor was specifically designed for this process, its diameter was 2 cm and the central tube was 0.5 cm 78% yield of multi-walled CNT with an average diameter of 16 nm was obtained. They concluded that FLBR could be viewed as a promising reactor for CDM process to generate abundant of CNTs with desired structural performances. Pinilla et al. [126,127] investigated the parameters (reaction temperature, catalyst particle size, space velocity and the ratio of gas flow velocity to the minimum fluidization velocity) of the CDM process over Ni/Cu/Al2O3 catalyst in a FLBR. They proposed a compromise method with a relatively high

Fig. 10 e Schematic of the pilot-scale solar reactor [116]. Please cite this article as: Qian JX et al., Methane decomposition to produce COx-free hydrogen and nano-carbon over metal catalysts: A review, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.052

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Fig. 11 e Schematic diagram of FLBR for CDM [59].

persistence factor to inhibit the catalyst deactivation by analysis the parameters. Then, they compared the structural and textural characteristics of the CNFs produced by methane decomposition in FLBR with FBR. It was concluded that, compared with FBR, FLBR was an appropriate allocation to

carry out the CDM process to produce CNFs with amazing structural and textural characteristics.

Plasma bed reactor PLR, as a newly technology could be regarded as a clean alternative to produce hydrogen from CDM [128]. A large number of studies have been carried out for CDM by using PLR [118,119,129]. Fincke et al. [118] investigated the plasma-driven reactor for CDM to produce hydrogen and carbon black. The test apparatus with a 61 cm reactor section and the measured thermal efficiency of the plasma torch was between 60% and 80%. In addition, they also proposed a detailed kinetic model that included solid carbon nucleation and growth as well as this model was in comparison with experimental results and was used to examine process optimization. 30% carbon yield was achieved by increasing residence time in the PLR. Hwang et al. [119] employed an arc jet plasma reactor (Fig. 12) for CDM. The reactor was mainly operated by arc plasma mainly, and then glow-like jet region was used after the arc plasma. The result showed that 79% methane conversion was generated. Khalifeh et al. [120] investigated the CDM in a nanosecond pulsed plasma reactor (Fig. 13) over PteRe/Al2O3, they found that, 100% methane conversion was achieved at discharge power of 9 W. In conclusion, study and progress plasma technology have opened a new area for hydrogen production by CDM, but remains some challenges such as energy efficiency [128].

Molten-metal reactor Fig. 12 e Schematic diagram of the arc jet plasma reactor [119].

To avoid the catalyst deactivation due to the carbon deposition, MMR was proposed by some researchers [130,131].

Please cite this article as: Qian JX et al., Methane decomposition to produce COx-free hydrogen and nano-carbon over metal catalysts: A review, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.052

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Fig. 13 e Schematic diagram of the plasma reactor [120].

Fig. 14 e Hydrogen production with a NieBi molten catalyst. (A) Reactor (B) SEM of the carbon produced. (C) Raman spectrum of surface carbon. (D) Ab initio molecular dynamics simulation [121]. Please cite this article as: Qian JX et al., Methane decomposition to produce COx-free hydrogen and nano-carbon over metal catalysts: A review, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.052

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Upham et al. [121] studied molten metal alloy Ni/Bi catalysts for CDM. The melts were used in the molten-metal bubble columns reactor (Fig. 14), where carbon can be removed successively. Geißler et al. [122] investigated the CDM in a filled with liquid metal tin bubble column reactor with a packed bed at 1000 C and proposed a thermo-chemical model. The reactor was based on a quartz glass and stainless-steel material. All parts of the reactor in contact with Sn were made of quartz glass to avoid corrosion. The reactor tube was equipped with length of 126.8 cm, I.D 4.06 cm, and the steel cladding tube was 11.5 cm long, I.D 4.925 cm. Results found that solid carbon clogging issues did not occur at the reactor wall, and the model indicated that the liquid metal temperature and the gas residence time have the most powerful effect on the CDM process. In short, although the MMR could solve an enormous challenge in CDM, it will cause a great waste of energy because of the reaction temperature needs to be higher than 1000  C to melt the catalysts. Hence, developing a new kind of melt catalyst with lower reaction temperature (<1000  C) will have more widely industrial application prospects.

Conclusions and perspectives CDM has become a potential route for the production of COx-free hydrogen for fuel cell and further applications. Furthermore, the by-product carbon produced can be applied in the field of advanced materials. The purpose of this review is to provide a critical and evaluative perspective of catalysts, mechanism, kinetics, reactors for CDM. The catalytic activity and stability of Ni-based, noble metal, carbonaceous and Fe-based catalysts are fully researched and found that carbonaceous gave lower methane conversion than metal catalysts, noble metal catalysts were relatively expensive, Ni-based catalysts hold poor activity at a high temperature. Due to these drawbacks of Ni-based, noble metal, carbonaceous catalysts, a number of researchers focused on Fe-based catalysts because of its efficient, eco-friendly, cheap, readily available and it can also withstand high temperatures. In order to provide theoretical guidance for designing highly active Fe-based catalysts, the reaction mechanism and kinetic study are illustrated furtherly. The major challenge face during CDM process is the deactivation of the catalyst due to the reasons like sintering and blockage of the active site by by-product carbon. Eventually, a process to remove the accumulated carbon continuously is obligatory. This review summarizes the performances of CDM in many types of reactors such as FBR, FLBR, PBR and MMR, especially the MMR; it showes more promise for commercial applications as the by-product carbon can be removed successively to avoid the catalysts deactivation. Due to the CDM process is still in the laboratory-scale, further researches must to be carried out from the following aspects: - Highly active, long life and low cost Fe-based catalysts must be designed to improve the economy of the CDM process.

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- Selecting and optimizing the MMR is an important prerequisite for the industrial hydrogen production of CDM. Once the CDM process achieves industrial scale hydrogen production, it would be applied to the field of on-site demanddriven hydrogen production in small or medium industrial scale for clean-energy vehicles.

Acknowledgments This work was supported by the grant from the Independent Research Project of Nanjing University of Science and Technology (AE89891, AE89991). Thanks to the Analysis and Testing Center of Nanjing University of Science and Technology. Thanks to the Chemicals Testing Center of Nanjing University of Science and Technology. Thanks to the analysis and testing center of King Abdullah University of Science and Technology.

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Please cite this article as: Qian JX et al., Methane decomposition to produce COx-free hydrogen and nano-carbon over metal catalysts: A review, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.052