energies

Review
Recent Advances in Methane Pyrolysis: Turquoise Hydrogen
with Solid Carbon Production
Tamás I. Korányi * , Miklós Németh, Andrea Beck and Anita Horváth

Department of Surface Chemistry and Catalysis, Institute for Energy Security and Environmental Safety,
Centre for Energy Research, Konkoly-Thege M. u. 29-33, 1121 Budapest, Hungary
* Correspondence: koranyi.tamas@ek-cer.hu

Abstract: Beside steam reforming, methane pyrolysis is an alternative method for hydrogen pro-
duction. ‘Turquoise’ hydrogen with solid carbon is formed in the pyrolysis process, contrary to
‘grey’ or ‘blue’ hydrogen via steam methane reforming, where waste carbon dioxide is produced.
Thermal pyrolysis is conducted at higher temperatures, but catalytic decomposition of methane
(CDM) is a promising route for sustainable hydrogen production. CDM is generally carried out
over four types of catalyst: nickel, carbon, noble metal and iron. The applied reactors can be fixed
bed, fluidized bed, plasma bed or molten-metal reactors. Two main advantages of CDM are that
(i) carbon-oxide free hydrogen, ideal for fuel cell applications, is formed and (ii) the by-product can
be tailored into carbon with advanced morphology (e.g., nanofibers, nanotubes). The aim of this
review is to reveal the very recent research advances of the last two years achieved in the field of this
promising prospective technology.

Keywords: methane pyrolysis; catalytic decomposition of methane; turquoise hydrogen

Citation: Korányi, T.I.; Németh, M.;

Beck, A.; Horváth, A. Recent 1. Introduction
Advances in Methane Pyrolysis:
One of the long-term effects of the chemical industry on the environment is the accu-
Turquoise Hydrogen with Solid
mulation of greenhouse gases, leading to climate change. Reducing carbon dioxide (CO2 )
Carbon Production. Energies 2022, 15,
emissions by prioritizing the diversification of energy resources is a great challenge for
6342. https://doi.org/10.3390/
the world economy. Production of hydrogen from natural gas by a smaller environmental
en15176342
impact method and its application as a clean source of energy is a possible solution for
Academic Editor: Devinder these problems [1].
Mahajan Recently, steam methane (CH4 ) reforming (SMR: CH4 + H2 O ↔ CO + 3H2 ) followed
Received: 28 June 2022
by the water–gas shift reaction (WGS: CO + H2 O ↔ CO2 + H2 ) is the most common,
Accepted: 30 August 2022
dominant process for large-scale hydrogen production, with a simultaneous long life of
Published: 30 August 2022
the used catalysts. Despite its long-time commercial optimization, the endothermic SMR
process is expensive due to its high capital costs and energy consumption, and it produces
Publisher’s Note: MDPI stays neutral
CO2 in a significant amount. In addition, the burning of methane that ensures the heat
with regard to jurisdictional claims in
necessary for the process produces even more carbon dioxide [2]. The production of every
published maps and institutional affil-
cubic meter of hydrogen is accompanied by the emission of a half m3 of carbon dioxide.
iations.
Pure H2 must be separated from CO2 , CH4 and other impurities in effluent gas streams
using energy-intensive (adsorption-, cryogenic-, or membrane-) separation processes. The
development of new processes to produce hydrogen without concomitant formation of CO2
Copyright: © 2022 by the authors.
is necessary to overcome the drawbacks of steam reforming [3]. The term ‘grey hydrogen’
Licensee MDPI, Basel, Switzerland. refers to carbon dioxide emissions being released to the atmosphere, and ‘blue hydrogen’ is
This article is an open access article used when emissions are captured through carbon capture and storage (CCS). Hydrogen
distributed under the terms and produced from renewable energy sources is often referred to as ‘green hydrogen’.
conditions of the Creative Commons Thermal dissociation or pyrolysis of methane is a technologically simpler, one-step
Attribution (CC BY) license (https:// process (CH4 → C + 2H2 ) compared with steam reforming, and it does not produce car-
creativecommons.org/licenses/by/ bon oxides. Per mole of methane, only half as much hydrogen is produced compared to
4.0/). SMR+WGS, but less energy input is required (74.5 kJ for methane pyrolysis, 164.6 kJ for

Energies 2022, 15, 6342. https://doi.org/10.3390/en15176342 https://www.mdpi.com/journal/energies

Energies 2022, 15, 6342 2 of 14

SMR+WGS), and solid carbon is coproduced (rather than CO2 ). Carbon oxides produced
during the SMR process would poison the catalysts for ammonia synthesis and must be
completely removed [2]. Following pyrolysis, carbon oxides are not present as pollutants,
and small amounts of unconverted CH4 can be tolerated in the hydrogen used, e.g., in a
fuel cell or in ammonia production. In addition, pure carbon is a valuable by-product; it
can be used as electrodes or additives (in, e.g., concrete, asphalt, tires) or in microelectron-
ics (carbon nanotubes or graphitic nano-fibres) [3]. Hydrogen produced using methane
pyrolysis is often referred to as ‘turquoise hydrogen’.
Methane is one of the most stable organic molecules; therefore, its thermal dissoci-
ation occurs at very high, 1000–1100 ◦ C, temperatures, which can be decreased signifi-
cantly (500–1000 ◦ C) using catalytic pyrolysis or in other words catalytic decomposition of
methane (CDM). The order of methane decomposition activity by transition metals is as
follows: Co, Ru, Ni, Rh > Pt, Re, Ir > Pd, Cu, W, Fe, Mo [4]. Reaction equilibrium favours
high temperature and low pressure to achieve high CH4 conversion, but catalysts can be
sintered and therefore deactivated at high temperatures. However, the main challenge for
the practical application of these catalysts is their rapid deactivation due to carbon accumu-
lation on their surfaces. Cyclic regeneration of the catalyst by gasification or combustion
in a fluidized bed reactor can extend its lifetime of operation [4]. If it occurs with steam
or oxygen, it results in significant CO2 emissions, which contradicts the idea of producing
turquoise hydrogen. Therefore, instead of burning the deposited carbon by-product, it
must be removed and valourized [5].
The aim of this review is to summarize the progress in catalyst development and
reactor types in methane pyrolysis and to reveal the recent advances of this promising
prospective technology. Many reviews [3–11] have examined the CDM literature, and here
we try to refresh them with the latest (2021–2022) results.
The survey of the last two year’s publications in the field of methane pyrolysis research
led us to note a frequent problem concerning the evaluating points of catalytic performance.
CH4 conversion, H2 yield/selectivity and carbon productivity (gcarbon /gcatalyst , called fre-
quently as carbon yield) are used generally to describe CDM performance. Indeed, CH4
conversion and H2 yield can be followed during the reaction via analysing the gas phase
products, while the carbon yield can be directly determined only after the reaction by
measuring all carbon accumulated during the time-on-stream. Space-time yield (STY)
of hydrogen is a good measure of the reaction. In most cases, all of these measurable
characteristics are not given, only some of them. Moreover, the way of their determina-
tion/calculation is sometimes not clearly described. If significantly lower H2 yield is listed
along with a relatively high CH4 conversion, it indicates that H2 selectivity is much lower
than 1. In this case, the appearance of a product other than hydrogen and carbon should be
mentioned. Sometimes, H2 selectivity equal to 1 is supposed for calculations. Occasionally,
only the H2 vol% is given instead of the H2 yield. In general, the examination of hydrogen
and also carbon balance is recommended, and the missing part (if it is significant, such as
H2 selectivity ~0.5) should be traced in both the gas phase and in solid products to gain
reliable catalytic results and to better understand the process.

2. Conventional Catalyst Development

Generally, four types of material have been applied as conventional catalysts in the
CDM process, related to their increasing preferred temperature ranges: nickel-, iron-, doped
noble metal- and carbon-based catalysts [9]. Mostly, carbon filaments are formed on nickel
and iron catalysts, graphitic- and turbostratic carbon on noble metals and turbostratic
carbon and carbon filaments on carbon-based catalysts (Figure 1) [12].
Energies 2022,15,
Energies2022, 15,6342
6342 33 of
of 14
14

Figure 1. Graphical representation

Figure 1. representation of
of the
the bulk
bulk of
of literature data onon catalysts, preferred
preferred temperature
temperature
rangeand
range andcarbon
carbonproducts
products related
related to catalytic
to catalytic methane
methane decomposition
decomposition reaction.
reaction. Catalysts:
Catalysts: 1-Ni-
1-Ni-based,
based, 2-Fe-based,
2-Fe-based, 3-carbon-based,
3-carbon-based, 4-summary
4-summary of datatorelated
of data related Co, Ni,toFe,
Co,Pd,
Ni,Pt,
Fe,
Cr,Pd,
Ru,Pt,Mo,
Cr,W Ru, Mo, W
catalysts,
catalysts, 5-non-catalytic
5-non-catalytic decomposition.
decomposition. CarbonCF-carbon
Carbon products: products: filaments,
CF-carbonTC-turbostratic
filaments, TC-turbostratic
carbon, GC-
graphitic carbon, AmC-amorphous carbon. (Copied from Ref. [12] with Elsevier permission). per-
carbon, GC-graphitic carbon, AmC-amorphous carbon. (Copied from Ref. [12] with Elsevier
mission).
2.1. Nickel-Containing Catalysts
2.1. Nickel-Containing
Nickel-containingCatalysts
catalysts can be classified as oxide (Al2 O3 , SiO2 , MgO and their mix-
Nickel-containing
tures) and carbon supported, catalysts
one-canandbe classified as
two-metallic oxide
(Ni, NiFe, (Al 2O3, SiO
NiCo, NiCu,2, MgOetc.) and their
catalysts.
mixtures)
The and carbonofsupported,
main advantage one- is
these catalysts and two-metallic
that they are working(Ni, NiFe, NiCo,
at the lowestNiCu, ◦ C)
etc.) cata-
(500–750
lysts. The
reaction main advantage
temperatures, with of these methane
80–85% catalysts conversion,
is that theybut are they
working
becomeat the lowest (500–
deactivated fast
750higher
at °C) reaction temperatures,
temperatures [9]. Dipu with
[4]80–85%
overviewedmethane conversion, but(from
the advancements they become deac-
2015 to 2020)
in CDMfast
tivated forat Ni-based catalysts. The
higher temperatures influence
[9]. Dipu [4]of promoter, the
overviewed metal composition,
advancements support,
(from 2015
admixture,
to 2020) in synthesis
CDM for methodNi-based and operating
catalysts. Theparameters
influence were discussed.
of promoter, metal composition,
Undoped oxide-supported Ni catalysts were efficient in the 500–750 ◦ C temperature
support, admixture, synthesis method and operating parameters were discussed.
rangeUndoped
with highoxide-supported
(40–90 wt%) metal loading over
Ni catalysts wereSiO 2 , Al2 in
efficient O3the
, MgO, MCM-22
500–750 and CeO2
°C temperature
supports
range with [9,10].
highNi/MgO
(40–90 wt%) catalysts
metal with mesoporous
loading over SiO structures
2, Al2O3, synthesized
MgO, MCM-22 by hydrother-
and CeO2
mal method
supports presented
[9,10]. Ni/MgOincreasing methane
catalysts with conversion
mesoporous (28 → synthesized
structures 49%) and hydrogen yield
by hydrother-
◦ reaction temperature
mal→
(33 53%) with
method increasing
presented nickelmethane
increasing (10 → 40%)
contentconversion (28at→600 49%)Cand hydrogen yield (33
(Table
→ 53%) 1).with
Only the NiO-MgO
increasing nickel phase
contentas(10a solid
→ 40%) solution
at 600was formed temperature
°C reaction in the calcined cat-
(Table
alysts [13]. Xu et al. [14] prepared a series of 10 wt% nickel-containing
1). Only the NiO-MgO phase as a solid solution was formed in the calcined catalysts Ni/Al O and
2 3 [13].
Ni/MgAl 2 O4prepared
Xu et al. [14] catalysts awith varying
series synthesis
of 10 wt% and pre-treatment
nickel-containing Ni/Al2methods
O3 and Ni/MgAland examined
2O4 cat-
their
alystsinitial methanesynthesis
with varying turnoverand andpre-treatment
carbon co-product methods selectivity in the CDM
and examined theirreaction
initial me- at
650 ◦ (Table 1). They found that methane turnover increases with Ni particle size and
thaneCturnover and carbon co-product selectivity in the CDM reaction at 650 °C (Table 1).
small (<10 nm)
They found thatNimethane
particlesturnover
are selective toward
increases with theNiformation
particle size of graphitic
and small carbon
(<10 nm)layers,
Ni
while large (>20 nm) Ni particles are selective toward the formation
particles are selective toward the formation of graphitic carbon layers, while large (>20 of carbon nanotubes
(CNTs). Catalyst are
nm) Ni particles deactivation is due to
selective toward thethe fragmentation
formation of carbon of Ni particles(CNTs).
nanotubes into smaller Ni
Catalyst
particles, followed by their encapsulation with graphitic carbon layers
deactivation is due to the fragmentation of Ni particles into smaller Ni particles, followed (but eventually all
the samples were deactivated within an hour of reaction). Chen
by their encapsulation with graphitic carbon layers (but eventually all the samples were and Lua [15] prepared
Ni/SBA-15
deactivatedcatalysts
within anbyhourelectroless nickel plating.
of reaction). Chen and The 32 [15]
Lua wt%prepared
Ni/SBA-15 catalyst exhibited
Ni/SBA-15 catalysts
the best catalytic performance and the highest resistance to deactivation, with a stable 44%
by electroless nickel plating. The 32 wt% Ni/SBA-15 catalyst exhibited the best catalytic
methane conversion at 575 ◦ C (Table 1).
performance and the highest resistance to deactivation, with a stable 44% methane con-
Modified (doped) bi- and trimetallic oxide-supported nickel catalysts were also tested
version at 575 °C (Table 1).
to increase the stability and catalytic activity at higher temperatures. The second metal is
Modified (doped) bi- and trimetallic oxide-supported nickel catalysts were also
expected to change the nickel properties if sufficient interaction is provided between the
tested to increase the stability and catalytic activity at higher temperatures. The second
two metals (for example, via alloy formation), which can influence the methane cracking
metal is expected to change the nickel properties if sufficient interaction is provided be-
rate, carbon migration rate or simply the dispersion of the catalyst or interaction of metal
tween the two metals (for example, via alloy formation), which can influence the methane
components with the support. Ahmed et al. [16] investigated pre-reduced 40% Ni-10%
cracking rate, carbon migration rate or simply the dispersion of the catalyst or interaction
Energies 2022, 15, 6342 4 of 14

Mo/CeO2 and 40% Ni-10% Mo/CeO2 -SiO2 catalysts in pure CH4 stream at 700 ◦ C and
revealed that the mixed support results in poorer catalytic properties in terms of hydrogen
and carbon yield due to larger particle size (Table 1). This behaviour was explained by
the greater sintering of metal particles on the SiO2 -containing support caused by the weak
metal-support interaction. The XRD, TEM, TGA and Raman findings of spent catalysts
demonstrated the production of MWCNTs with a higher yield, quality, graphitization
degree and thermal stability over Ni–Mo/CeO2 catalysts.
Ni–Cu/Al2 O3 alloy catalysts were prepared from Ni–Cu–Al hydrotalcite-like com-
pounds and tested for methane decomposition at 650 ◦ C (Table 1). Alloying Ni with Cu
caused a decrease in methane conversion, but significantly enhanced the catalytic life and
carbon yield. The highest carbon yield was obtained at an atomic ratio of Ni:Cu = 7:3,
which is approximately 78 times that of the Ni/Al2 O3 catalyst. Carbon morphology was
changed from thin carbon nanotubes (CNTs) to thick fishbone-carbon nanofibers (CNF)s
and platelet-CNFs, depending on the copper content [17].
Bimetallic Ni-Co/γ-Al2 O3 catalysts were more active than monometallic Ni/γ-Al2 O3 ,
presenting 86 and 15% methane conversions and 51 and 12% hydrogen yields at 600 ◦ C
temperature, respectively (Table 1). Due to the formation of surface bimetallic Ni-Co
alloys, the reducibility of the catalyst was facilitated, while the dispersion of particles
increased [18]. Cu–Zn-promoted Ni–Co/Al2 O3 catalysts with fixed 50 wt% Ni loading
gave the highest methane conversion of 85% at 700 ◦ C (Table 1). Zn addition improved
the stability of the catalyst by retaining the active metal size during the CDM reaction. Zn
promoted the growth of reasonably long and thin carbon nanotubes [19].
A multi-metallic nickel-based catalyst with an optimal composition (60%Ni-5%Cu-
5%Zn/Al2 O3 ) was used for CDM in a fluidized bed reactor under bubbling conditions.
Higher than 90% methane conversion was achieved with high-quality CNTs (Table 1).
The carbon product separation was effectively performed via ultrasonication of the spent
catalyst and by using a strong magnet to collect the ferromagnetic nickel-loaded catalyst
material in ethanol suspension. The complete carbon removal was carried out by a further
recalcination step, and the regenerated catalyst regained its full activity [20].
The effects of Ni loading and gas hourly space velocity (GHSV) over Ni supported on
palm oil fuel ash (Ni-POFA with high SiO2 content) catalysts were investigated in CDM at
550 ◦ C for 6 h (Table 1). Over optimum 15 wt% Ni loading and 7000 mL/g h GHSV, the
Ni-POFA catalyst performed at 87% initial CH4 conversion and 27% initial H2 yield [21].
Graphene-encapsulated nickel nanoparticles (Ni@G) prepared from nickel nitrate and
kraft lignin showed high catalytic activity for methane decomposition at temperatures of
800 to 900 ◦ C and exhibited long-term stability of 600 min time-on-stream (TOS) without
apparent deactivation. The methane conversion was 88% and the hydrogen production
was 95 vol% at 900 ◦ C over a 25 wt% Ni@G catalyst (Table 1). During the CDM process,
graphene shells over Ni@G nanoparticles were cracked and peeled off the nickel cores.
Both the exposed nickel nanoparticles and the cracked graphene shells may participate in
the CDM reaction, explaining the high activity of the dual (metallic nickel and graphenic
carbon) catalytic system. Graphene nanoplatelets, fluffy graphene, 3D fluffy graphene, and
3D graphitic nanochips were formed as the main solid products, depending on the reaction
time [22].
Ni and Ni–Pd alloy supported on CNTs with various atomic ratios were synthesized
and tested for CDM. The addition of Pd to Ni stabilized the metal particles and terminated
the CNT growth due to coking. Increasing the CH4 molar fraction from 30 to 100% over the
NiPd/CNT catalyst with optimum 10:1 Ni:Pd atomic ratio, the conversion decreased from
55 to 35%, but the hydrogen production rate increased and remained stable at 600 ◦ C for
6 h (Table 1). A self-sustained cyclic reaction–regeneration process was demonstrated by
leaching out the metal from the produced CNTs/CNFs catalyst mixture and re-synthesizing
the 10Ni–1Pd/CNT catalyst [23].
Energies 2022, 15, 6342 5 of 14

2.2. Iron Containing Catalysts

Iron-based catalysts are cheaper, but they generally work at higher temperatures than
nickel-containing ones (Figure 1). Natural iron ores (Tierga and Ilmenite) were used as
CDM catalysts. Tierga (iron oxide) exhibited high stability and activity (70 vol% H2 ) at
850 ◦ C when methane also acted as a reducing agent (Table 1). The activation with CH4
led to the initial fragmentation of the α-Fe phase and inhibition of large amounts of Fe3 C
formation. Hybrid nanomaterials composed of graphite sheets and carbon nanotubes with
a high degree of graphitization were obtained [24].
Carbonaceous nanomaterials (CNMs) were obtained over stainless-steel foams via
CDM. The maximum productivity attained was 0.116 g C/g foam h operating at 950 ◦ C
with a feed ratio of CH4 /H2 = 3 (Table 1). The formation of graphene-related materials
(GRMs, mainly few-layer graphene and graphene) was favoured above 900 ◦ C; at lower
temperatures carbon nanotubes were produced [25].
Carbon-encapsulated iron nanoparticles (CEINPs) with varying iron concentrations
(20, 30 and 40 wt%) were investigated for methane decomposition between 700 and 800 ◦ C
in a semicontinuous flow fixed-bed reactor. The graphitic shell prevented atmospheric
oxidation and sintering at high temperature, improving the thermal stability of the catalysts.
The highest initial methane conversion (96%) was reached over the 30 wt% Fe containing
catalyst at 800 ◦ C and dropped to 37% after 180 min of reaction (Table 1). Four different
active sites were suggested in the catalyst: graphite, graphite encapsulated iron nanoparti-
cles, uncovered iron nanoparticles and activated carbon. Catalyst deactivation was due
to carbon deposition from CH4 in the form of coke and graphite on non-encapsulated Fe
nanoparticles [26].

2.3. Noble Metal Catalysts

The addition of noble metals into supported-metal catalysts provided better activity
and stability in comparison with the single metal in CDM [9]. Only the Ni-Pd/CNT cata-
lyst [23] discussed above among Ni-based catalysts appeared during the period 2021–2022.

2.4. Carbon Based Catalysts

Due to their availability, durability, low cost, abundant porosity, molecular activa-
tion and tolerance to high temperatures, carbon catalysts are frequently used in CDM [9].
Metal-free carbon catalysts (activated carbon (AC), mesoporous carbon (MC) and carbon
black (CB)) have been investigated for CDM. CB showed greater stability over the course
of the reaction than AC and MC. The lifetime of CB catalysts and the possibility of their
regeneration are fundamental for their potential applicability in industry. A strong relation-
ship between the number and type of oxygen-containing functional groups and methane
conversion was found. The action of epoxy and quinone functional groups for methane
activation was confirmed: epoxy groups gently activate methane, quinone groups tend to
accelerate coke formation [27].
Boretti published two papers on the perspectives of solar-driven CDM producing
“aquamarine” hydrogen over carbon black [28] and other [29] catalysts. The basic idea of
the thermochemical reactor setup was developed by a group led by BASF: the reaction
occurs at about 1000 ◦ C in a moving carbon-bed reactor. According to the solar-driven
approach described by Boretti (Figure 2) [28], the thermal energy is provided by molten
salt, MgCl2 -KCl, flowing from a hot thermal energy storage tank to the cold thermal energy
storage tank through the reactor, and from the cold tank to the hot tank through a higher-
concentration solar receiver. The perspective could be extended to supported metal-based
catalysts such as Fe, Ni, Co and Cu on metal oxide supports such as SiO2 , Al2 O3 and
TiO2 , and carbon-based catalysts such as carbon blacks, carbon nanotubes and activated
carbons [29].
Energies 2022, 15, 6342 6 of 14
Energies 2022, 15, 6342 6 of 14

Figure2.2.Sketch
Figure Sketchofofaanovel
novelconcentrated
concentrated solar
solar energy-driven
energy-driven version
version of aoftheoretical
a theoretical plant
plant for cata-
for catalytic
lytic thermal pyrolysis of CH 4 with CB as a catalyst, with the addition of a gas heater. (Adapted
thermal pyrolysis of CH4 with CB as a catalyst, with the addition of a gas heater. (Adapted from
from Ref. [28] with Wiley permission).
Ref. [28] with Wiley permission).

2.5. Other
2.5. Other Catalysts
Catalysts
Othernew
Other newcatalysts
catalystsalsoalso appeared
appeared in the
in the literature
literature of CDM
of CDM besidebeside the above
the above men-
mentioned
tioned
four four groups
groups of catalysts.
of catalysts. Cobalt-containing
Cobalt-containing catalystscatalysts are discussed
are discussed in threein three papers.
papers. Co-
Co-loaded
loaded biomass
biomass fly ashfly(10%
ash Co/BFA)
(10% Co/BFA) was employed
was employed for methane
for methane at 700 ◦ C
decomposition
decomposition at
in
700a fixed-bed reactor with
°C in a fixed-bed reactor a stable
with a63% stableCH63%4 conversion
CH4 conversionand greater than 30%
and greater hydrogen
than 30% hy-
yield
drogen foryield
330 min
for time-on-stream
330 min time-on-stream (Table 1) [30].
(Table The 1) same group
[30]. The of authors
same group of reported
authors71% re-
methane
ported 71% conversion
methanewith 45% hydrogen
conversion with 45% yield at 850 ◦ C
hydrogen for 34
yield at h850
on °C
stream
for 34activity using
h on stream
aactivity
5% Co/CeOusing2a-BFA catalyst2-BFA
5% Co/CeO (Tablecatalyst
1) [31]. (Table
Henao1)et[31]. al. [32]
Henaostudied
et al.the
[32]influence of the
studied the in-
reaction temperature (650–950 ◦ C) and feed composition (7%–43% of CH and H ) on the
fluence of the reaction temperature (650–950 °C) and feed composition 4(7%–43% 2 of CH4
yield
and H and CNTs
2) on quality
the yield andof CNTs
CDM using quality a Co-Cu/cellulose-derived carbon catalyst. carbon
of CDM using a Co-Cu/cellulose-derived Below
800 ◦ C, the reaction wasthe selective ◦ C,
catalyst. Below 800 °C, reactiontowards the formation
was selective towardsof theCNTs, whileofabove
formation CNTs,800while
the obtained
above 800 °C,nanomaterial
the obtainedexhibited
nanomaterial a graphite-like
exhibited amorphology
graphite-likesimilar to Ref. similar
morphology [25]. The to
best
Ref. operating parameters
[25]. The best operating for parameters
growing CNTs for with
growinghighCNTsproductivity
with high (0.29 gC/gcat·h) (0.29
productivity and
quality wereandfound at 750 ◦ C under 29% CH : 14% H : 57%N (Table 1).
gC/gcat∙h) quality were found at 750 °C4 under 29% 2 CH4 :2 14% H2 : 57%N2 (Table 1).
Awadallah et al. [33] prepared 20–50%
Awadallah et al. [33] prepared 20–50% Mo/MgO catalysts Mo/MgO catalystsby by impregnation
impregnation andandde-
declared that mainly Mo, reduced from
clared that mainly Mo, reduced from the MgMoO4 phase, the MgMoO 4 phase,
was the catalytically activeactive
was the catalytically site in
site in methane
methane decomposition.
decomposition. The presence
The presence of non-interacting
of non-interacting MoO3 was MoO not3 was not beneficial
beneficial concern-
concerning activity and longevity at 50% Mo-loading. Multi-walled
ing activity and longevity at 50% Mo-loading. Multi-walled carbon nanotube bundles were carbon nanotube
bundles
deposited were deposited
at 800 °C withatvery800 ◦uniform
C with very and uniform and narrowThe
narrow diameters. diameters.
maximum Thecarbon
maximumyield
carbon yield was 180% for the 40% Mo/MgO catalyst with
was 180% for the 40% Mo/MgO catalyst with 68% final methane conversion after 120 min 68% final methane conversion
after 120 min
reaction timereaction
(Table 1).timeThe (Table 1). The
highest highest
initial initialconversion
methane methane conversion
was 75% was 75%50%
for the for
the 50% Mo/MgO catalyst, but it decreased to 59% final conversion,
Mo/MgO catalyst, but it decreased to 59% final conversion, resulting in a 140% carbon yield resulting in a 140%
carbon yield due to the presence of MoO3 species.
due to the presence of MoO3 species.
Kreuger et al. [34] studied methane pyrolysis over nonporous α-Al2 O3 surfaces in
Kreuger et al. [34] studied methane pyrolysis over nonporous α-Al2O3 surfaces in the
the range of 900–1300 ◦ C in single-particle and fixed-bed reactors. The selectivity towards
range of 900–1300 °C in single-particle and fixed-bed reactors. The selectivity towards car-
carbon (and hydrogen) was initially low (38% at 1000 ◦ C) with 20% methane conversion
bon (and hydrogen) was initially low (38% at 1000 °C) with 20% methane conversion over
over the fresh catalyst, indicating an activation process for the formation of carbon and
the fresh catalyst, indicating an activation process for the formation of carbon and hydro-
hydrogen from the intermediate products (e.g., benzene), but later a temperature-dependent
gen from the intermediate products (e.g., benzene), but later a temperature-dependent
maximum in carbon loading was observed. They were able to predict carbon growth and
maximum in carbon loading was observed. They were able to predict carbon growth and
CH4 conversion as a function of temperature, specific bed area, carbon loading and gas
CH4 conversion as a function of temperature, specific bed area, carbon loading and gas
composition based on the parametrization of kinetic models.
composition based on the parametrization of kinetic models.
Energies 2022, 15, 6342 7 of 14

Table 1. Results on best-performing solid catalysts in methane pyrolysis in 2021–2022 publications.

Feed Rate * Max. CH4 Carbon Carbon

Catalyst Preparation Pretreatment Temp. (◦ C) React. Time H2 Productivity Ref.
(mL/min) Conversion Productivity Morphology
200 mg 40% Ni/MgO hydrothermal 850 ◦ C 1 h red. 200 (33%/N2 ) 600 4.5 h 49% 53% yield [13]
300 mg 10% Ni/Al2 O3 sol-gel 650 ◦ C 3 h red. 70 (30%/N2 ) 650 1h 46% [14]
140 mg 32% Ni/SBA-15 electroless plating 450 ◦ C 4 min red. 15 (50%/N2 ) 575 750 min 44% [15]
500 mg 40% Ni-10%
coimpregnation 700 ◦ C 1 h red. 50 (100% CH4 ) 700 3h 73% yield 537 wt% MWCNT [16]
Mo/CeO2
10 mg Ni-Cu (7:3)/Al2 O3 coprecipitation 800 ◦ C 30 m red. 25 (20%/N2 ) 650 19 h 68% 133 wt% CNT, CNF [17]
2 mL 1% Ni-2% Co/Al2 O3 impregnation 600 ◦ C 6 h red. 160 (6%/N2 ) 600 5h 86% 51% yield 11 wt% [18]
100 mg 50% 42 L/g/h
coprecipitation 750 ◦ C 3 h red. 700 80 h 85% 265 wt% CNT [19]
Ni-CoCuZn/Al2 O3 (50%/N2 )
5g 60% Ni-5% Cu-5%
impregnation 550 ◦ C 5 h red. 180 (25%/N2 ) 750 180 min >90% CNT [20]
Zn/Al2 O3
80 mg 15% Ni/POFA (SiO2 ) combustion no pretreatment 60 (20%/N2 ) 550 6h 87% 27% yield filament [21]
10 g 25% Ni@G coprecipitation 300 ◦ C 0.5 h in N2 150 (67%/Ar) 900 10 h 88% 95 vol% H2 4.46 g/g Ni G, graphite [22]
200 mg 10% Ni-1% Pd/CNT solvothermal 400 ◦ C 4 h red. 30 (30%/N2 ) 600 6h 55% 90% select. CNT, CNF [23]
600 mg iron ores natural 900 ◦ C 1 h in CH4 20 (100% CH4 ) 850 3h 56% 70 vol% H2 1.63 g/g cat. CNT, graphite [24]
700 (43%/14%
200 mg stainless steel foam commercial 900 ◦ C ox. +
red. 950 <5% 116 g/(g foam * h) GRM, CNT [25]
H2 /N2 )
500 mg 30% Fe@C impregnation 1000 ◦ C in N2 100 (5%/N2 ) 800 3h 96% 0.39 g/g cat. coke, graphite [26]
1.1 mL Active Carbon (AC) commercial no pretreatment 60 (50%/N2 ) 1000 45 min 37% [27]
200 mg 10% Co/BFA impregnation 700 ◦ C 5 h calc. 40 (100% CH4 ) 700 330 min 63% 30% yield whisker [30]
500 mg 5% Co/CeO2 -BFA impregnation 700 ◦ C 3 h calc. 20 (100% CH4 ) 850 34 h 71% 45% yield [31]
700 (29%/14%
25 mg 24% Co-6% Cu/C coimpregnation 850 ◦ C 75 m red. 750 11 h 0.29 g/g cat. CNT [32]
H2 /N2 )
500 mg 40% Mo/MgO impregnation 700 ◦ C 1 h red. 60 (100% CH4 ) 800 2h 68% 68% yield 180 wt% MWCNT [33]
* Methane content (vol%)/inert gas in brackets. + Fluid-bed reactor; all other references used fixed-bed reactors.
Energies 2022, 15, 6342 8 of 14

The great challenge in methane pyrolysis is to obtain very pure hydrogen with high
yield and gain separated, precious, possibly nanostructured carbon such as carbon nan-
otubes at the same time. If the process heat of the endothermic reaction is provided from
the combustion of H2 product, the efficiency of hydrogen production is lowered. Metal-
containing catalysts can provide graphitic, structured carbon, but the product separation
and catalyst regeneration is demanding. Moreover, the exact conditions of its formation
and tailoring of a desired carbon structure with the use of the appropriate catalyst is again
a future task that should be solved. In the case of purely carbon-based catalysts, separation
of pyrolytic carbon from the carbon catalyst is of less importance, but we should keep in
mind that the structure of carbon products is usually less ordered. Environmental issues
may suggest a preference for the application of Fe-based and carbon-supported systems,
with special attention on the future utilization of the carbon product. We must note that
catalyst regeneration without CO2 formation remains the next rather challenging issue of
the methane pyrolysis process. Because all the above-mentioned case studies are still at
research level, expertise is needed from a technological point of view to plan the scale-up
steps and study the influence of process parameters if a prospective catalyst family is found.
However, the responsibility and role of basic research is inevitable at the present stage of
catalytic methane pyrolysis to clarify the interaction of catalyst structure and performance
influencing the purity of H2 and carbon products.

3. Reactors Used in Methane Pyrolysis

Four different reactor configurations can be generally applied for CDM: fixed-bed,
fluidized-bed, plasma-bed and molten-metal reactors [9]. Fixed-bed reactors are the most
commonly used ones, but are usually preferred on a laboratory scale only. Their main
drawback is the filling of the reactor with the carbon product during long-term experiments.
A better prospective reactor for large-scale operation is the fluidized-bed reactor because it
is suitable for the continuous addition of catalyst particles and withdrawal of solid carbon
coproducts. A pressure drop does not increase significantly, and the operation for longer
times is possible. The vigorous movement of the particles allows efficient heat and mass
transfer between the gas and the solid catalyst. Two parallel reactors can operate in a cyclic
mode by switching the natural gas feed and the regeneration agent stream (air, steam)
between the two reactors. The plasma bed reactor is mainly operated by arc plasma, where
a glow-like jet region is used. Its drawback is its low energy efficiency. The molten metal or
liquid–bubble-column reactor operates with molten media, such as molten metals (Ti, Pb,
Sn, Ga), molten-metal alloys (Ni−Bi, Cu−Bi) or molten salts (KBr, NaBr, NaCl, NaF, MnCl2 ,
KCl). The main advantage of liquid–bubble-column reactors is the easy separation of the
carbon by-product from the liquid medium due to density differences. Their drawbacks
are the limited stability of the molten media at the required high operating temperatures
(>900–1000 ◦ C) and the corrosion at such high temperatures [35]. The schematic diagrams
of these reactor types are given in Ref. [9].

4. Molten Media Pyrolysis of Methane

Methane pyrolysis in molten metals/salts to prevent both reactor coking and rapid
catalyst deactivation is the most promising alternative to conventional pyrolysis. This
strategy can fundamentally solve the problem of carbon deactivation because, at the liquid–
solid interface between the molten catalyst and the carbon product, the generated carbon
may be separated from the active site and removed continuously from the reactor [36]. As
a recent review [11] discusses this process in detail, we present here only the very latest
results in this area.
Methane pyrolysis was performed in a quartz bubble column using molten gallium
as a heat transfer agent and catalyst. A maximum conversion of 91% was achieved at
1119 ◦ C and ambient pressure, with a residence time of the bubbles in the liquid of 0.5 s
(Table 2) [37].
Energies 2022, 15, 6342 9 of 14

Table 2. Results of methane pyrolysis 2021–2022 publications in molten media.

Molten Ni/Bi = 27/73, Ni/Bi = 27/73,

Ga Sn 5% Ni/Sn KCl NaBr:KBr Co-Mn (2:1)
Medium NaBr ZrO2 , NaBr
Temperature
1119 1000 1050 1000 1000 850–1000 985 985
(◦ C)
25–250
Feed Rate, 450 mL/min, 70 mL/min, 20 mL/min, 15 mL/min, 45 mL/min, 9 mL/min, 9 mL/min,
mL/min, 100%
Composition 50% CH4 /Ar 35% CH4 /N2 50% CH4 /Ar 100% CH4 33% CH4 /Ar 67% CH4 /Ar 67% CH4 /Ar
CH4
Residence
0.5 n.a. n.a. 0.3 0.69–0.76 n.a. n.a. n.a.
Time (s)
Operation
n.a. n.a. 5, steady state 40, stable conv. 24 24, stable conv. 50, stable conv. 50, stable conv.
Time (h)
Max. CH4
Conversion 91 n.a. 19 1.8 5.85 10.52 32 38
(%)
Carbon graphite + graphite + 70 wt% 74 wt%
carbon black soot graphite amorphous
morphology amorphous amorphous graphite graphite
Reactor
Quartz n.a. Alumina Quartz Quartz Quartz Quartz Quartz
Material
Reactor
Diameter 36 35 30 15 16 16 8 8
(mm)
Reactor
n.a. 100 450 250 250 250 650 650
Length (mm)
Reactor Filled
50 23 100 75 190 190 65 86
Height (mm)
Bubble
0.2 mm porous porous porous
Generator 0.5 mm nozzle n.a. 2 mm orifice 2 mm orifice 2 mm orifice
distributor membrane membrane
Diameter
Reference [37] [38] [39] [40] [41] [42] [43] [43]

Methane bubbled through molten tin at 1000 ◦ C and ambient pressure with an increas-
ing flow rate from 25 to 250 mL/min (Table 2) decreased the mole fraction of hydrogen
from 12 to 4.4% in the resulting gas mixture due to the shortening contact between the gas
bubbles and the tin melt. A unique float-type structure placed inside the reactor solved the
problem of continuous removal of the carbon deposits generated during methane pyrolysis
and controlled the metal melt level [38].
Nitrogen-diluted methane was bubbled through molten tin, tin-nickel, and tin-copper
alloys in an alumina tube reactor at 950–1050 ◦ C. A maximum 19% conversion was achieved
over the 5% Ni containing alloy at 1050 ◦ C in a 10 cm molten-metal column (Table 2) [39].
Methane conversion was measured using a bubble column reactor at 700–1000 ◦ C
temperatures by injecting 1:1 Ar:CH4 reactant gas mixture into molten KCl (Table 2). The
melt acted as a carbon-separating agent and as a pyrolytic catalyst and enabled 40 h of
continuous running without catalytic deactivation, with an apparent activation energy of
277 kJ/mole. Heating the solid product at 1200 ◦ C produced the highest purity carbon
(97.2 at%), with some salt residues [40].
Alkali halides (NaBr, KBr, KCl, NaCl) and an eutectic mixture of NaBr:KBr (49:51 mol%)
as the liquid media were tested for CH4 pyrolysis and characterized the properties of the
generated carbons. Significantly lower activation energies (224–278 kJ/mol) were found
than those measured during non-catalytic gas-phase methane pyrolysis (~422 kJ/mol).
The purity of the washed carbon samples was in the range of 92–97% [41]. The same
group of authors [42] tested alumina-supported La, Ni, Co and Mn catalysts as particle
suspensions in molten NaBr-KBr at 850–1000 ◦ C for CDM. The increase in the molar Co:Mn
ratio from zero to two increased the CH4 conversion at 1000 ◦ C from 5 to 10%, and they
observed a stable performance over ca. 24 h of methane pyrolysis and product selectivities
for hydrogen near unity. An enhanced conversion was measured using the smaller catalyst
particle size ranges (Table 2).
Noh et al. [43] applied bubble-column reactors containing molten Ni-Bi alloy (bot-
tom), molten NaBr (top) in two-stage, and zirconia (between them) in three-stage reactors
 bubble column reactor with liquid metal alloy catalysts.
The two main problems in this field are high process temperatures, which require
high energy usage, and corrosion of reactor material due to high temperatures. Neverthe-
less, the advantages of this process, such as prevention of reactor coking and rapid catalyst
Energies 2022, 15, 6342 deactivation, separation of carbon by-product from the catalyst, improvement of 10 of 14
heat
transfer (and thermal inertia) owing to the high heat capacity of molten media and the
enhancement of the gas residence time due to the liquid viscosity, compensate for the
mentioned
(Figure 3) problems.
for CDM Molten
with 32salts
and are
38%weaker
stable catalysts
methanethan molten metals,
conversion but their ap-at
rates, respectively,
985 ◦ C up
plication reduces the
to 50 h investment
operation cost2).
(Table of The
CDM. Salts melt
enhanced at a relatively
methane lower
pyrolysis temperature
rates with the use
compared
of zirconiawith metals,
beads theythat
indicate have low
they can vapor pressure to
be employed diminishing
control the salt evaporation
bubble behaviourandand
they are less the
to enhance expensive
contactand
arealess densebubbles
between than metals, and
and the carbon
liquid contaminated
catalyst surface forwith salt is
an efficient
bubble
much column
easier reactor
to purify with liquid
because metal
the salt alloy catalysts.
is flushable by dissolution in water [11].

Figure 3. The structures of the two-stage and three-stage bubble-column reactors implemented for
Figure 3. The structures of the two-stage and three-stage bubble-column reactors implemented for
methane decomposition reaction. (Copied from Ref. [43] with Elsevier permission).
methane decomposition reaction. (Copied from Ref. [43] with Elsevier permission).
5. Reaction Mechanism
The two and Aspects
main problems of Industrialization
in this field are high process temperatures, which require high
The usage,
energy reaction mechanisms
and corrosion ofand industrialization
reactor material due possibilities of methane pyrolysis
to high temperatures. are
Nevertheless,
summarized
the advantagesin Ref.
of [35]. It is generally
this process, such accepted that the
as prevention rate-limiting
of reactor cokingstep is rapid
and the splitting
catalyst
ofdeactivation,
methane intoseparation
a methyl radical and aby-product
of carbon hydrogen atom frominthenoncatalytic methane pyrolysis.
catalyst, improvement of heat
Some works
transfer (andhave proposed
thermal a molecular
inertia) owing to adsorption
the high heat mechanism,
capacity ofwhereas
molten amedia
dissociative
and the
adsorption
enhancementmodel of has
the been described time
gas residence in other
duestudies
to the for catalytic
liquid methane
viscosity, pyrolysis.for
compensate Me-the
mentioned
thane problems.onMolten
is first adsorbed salts are
the catalyst weaker
surface andcatalysts than molten
then dissociates metals,
following but their
a series of
application
stepwise reduces
surface the investmentreactions
dehydrogenation cost of CDM. Salts melt
according atmolecular
to the a relativelyadsorption
lower temperature
mech-
compared with metals, they have low vapor pressure diminishing salt evaporation and
they are less expensive and less dense than metals, and carbon contaminated with salt is
much easier to purify because the salt is flushable by dissolution in water [11].

5. Reaction Mechanism and Aspects of Industrialization

The reaction mechanisms and industrialization possibilities of methane pyrolysis
are summarized in Ref. [35]. It is generally accepted that the rate-limiting step is the
splitting of methane into a methyl radical and a hydrogen atom in noncatalytic methane
pyrolysis. Some works have proposed a molecular adsorption mechanism, whereas a
dissociative adsorption model has been described in other studies for catalytic methane
pyrolysis. Methane is first adsorbed on the catalyst surface and then dissociates following a
series of stepwise surface dehydrogenation reactions according to the molecular adsorption
mechanism. Methane dissociates upon adsorption on the catalytic active sites, generat-
ing chemisorbed CH3 and H fragments according to the dissociative adsorption model,
which is followed by the same surface dissociation reactions described by the molecular
adsorption mechanism. The vapor−liquid−solid (VLS) model was applied for the growth
of filamentous carbon, which includes carbon nanotubes and nanofibers. It supposes
Energies 2022, 15, 6342 11 of 14

carbon diffusion through the metal (assumed to have the properties of a liquid) particles
as the rate-limiting stage. Nevertheless, there is no general agreement, and the reaction
mechanism involved in methane pyrolysis, as well as the overall rate-limiting step, is still
unclear and must depend on the type of catalyst material, reactor design, temperature and
other process parameters [35].
The quality and sale of the carbon coproduct may improve the economic efficiency of
the industrial pyrolysis of methane. The characteristics of the carbon depend on the catalyst
used and the reaction conditions. The formation of carbon nanotubes and nano-fibres
usually occurs over metal catalysts. At high operation temperatures, as the diameter and
length of the carbon nanofilaments decrease, their crystallinity and graphitization degree
increase. Carbon blacks are formed over activated carbons, and they produce amorphous
turbostratic structures. The use of carbon nanotubes as a catalyst favours the growth of their
walls, leading to the formation of multiwalled carbon nanotubes. A suitable experimental
setup must still be found to industrialize the process, and the possible commercialization
or storage of the purified carbon coproduct still remains a challenge [35].
The industrialization of methane pyrolysis is limited, and the presented works are
mainly at the research stage. High processing temperatures, deactivation of catalytic
systems (e.g., by sintering of active sites and/or by carbon deposition) and difficulties with
removing the formed carbon hinder the application possibilities. Catalyst structure and
process parameters control the morphology of carbon co-products. The aim of regeneration
is to prolong the lifetime of the catalyst and to remove the poisoning carbon by gasification
without destroying the useful carbon products. Gasification can be catalysed by the
CDM catalyst itself into carbon oxides (CO2 or CO) using oxygen, water or CO2 as a
reactant. Generally, the most deactivating carbon forms, which are in close contact with
the catalytic active sites, are of the most reactive ones. The selective removal of such
carbons can be enhanced by optimizing the temperature and the oxygen/water/carbon-
dioxide concentration. However, all these types of gasification produce carbon oxides,
which corrupt the eco-friendly nature of CDM and may also cause catalyst oxidation. The
hydrogenation of deactivating carbon to CH4 is also a promising, but less studied type of
gasification [10]. With this method, carbon oxide formation and also the oxidation of the
active metal(s) can be fully avoided; furthermore, this method is technologically more facile
compared to the oxidative regeneration methods, because no change between reductive
and oxidative steps is needed.

6. Summary, Conclusions and Perspectives

As can be seen in the number of reviewed publications, nickel-containing catalysts
and molten media pyrolysis attracted the highest attention regarding CDM during the
period 2021–2022. The main advantage of nickel-containing catalysts in conventional
methane pyrolysis is their relatively low working temperature, but their drawback is
their generally fast deactivation, especially at higher temperatures. A possible solution
could be the application of graphene-encapsulated nickel nanoparticles as catalysts, which,
according to Ref. [22], show long-term stability without apparent deactivation at 900 ◦ C
with high methane conversion and hydrogen production. The outer graphene shell breaks
up and the naked nickel core will be a new continuously regenerated active site during
the reaction, and the graphene products also serve as carbon-based catalysts and valuable
solid products in the CDM reaction. A similar but iron-based catalyst (carbon-encapsulated
iron nanoparticles [26]) also presented high initial methane conversions, but this catalyst
deactivated quickly.
In agreement with one of the latest reviews [11], solar energy utilization can be a
promising option for CDM. The two papers of Boretti [28,29] outline such a perspective: the
usage of solar-derived thermal energy provided by a molten-salt flow for methane pyrolysis
is a very promising project for the future. This solar–thermal driven catalytic decomposition
of methane produces ‘aquamarine’ hydrogen and carbon particles of commercial interest at
Energies 2022, 15, 6342 12 of 14

a reduced cost. The design and development of a high-efficiency reactor for the proposed
temperature range capable of continuous operation is the most critical aspect of the project.
Due to its many advantages over conventional pyrolysis and technological novelty,
CDM in molten media reached the highest publication activity in the present and last
year. Papers on pure molten metals through to metal alloys and oxide-supported metal
catalysts suspended in molten salt mixtures were published, showing the development of
this research area. In the latest review [11], molten salts and Ni-Bi alloys were declared as
the most promising and most efficient for methane pyrolysis in molten media. Recently,
a molten NaBr-KBr mixture [41,42] and Ni-Bi alloys [43] as CDM catalysts were devel-
oped even further: alumina-supported Co-Mn oxide catalysts increased and stabilized the
methane conversion in the salt mixture [42], zirconia beads enhanced methane pyrolysis
rates in a three-stage bubble-column reactor containing molten Ni-Bi alloy, zirconia and
molten NaBr [43].
Optimizing the CDM process to produce both the desired grade of carbon and fuel
cell-grade hydrogen, particularly if a carbon with very tight product specification is being
produced, could be a major challenge. The high reaction temperatures required for CH4
conversion limit the choice of materials of construction, adversely impact catalyst life, and
exacerbate heat losses. Most catalysts produce both amorphous and structured carbons.
Amorphous carbons such as activated carbon and carbon black are more active for CDM
than structured carbons such as graphite and diamond. Graphite used in lithium-ion
batteries and nanotube- and nanofiber carbons are high-value products. The yield of high-
value forms of carbon is critical for successful commercial implementation. Separation of
the catalyst and the carbon by-product remains a challenge. Catalysts should be recovered
and fully regenerated without burning the highly valued carbon products. The major
benefit of molten-metal technology is the easy separation of carbon from molten metal due
to density differences; however, high temperature for conversion is still required [44].
Conclusively definite progress has been reached in this research area, but even the
BASF plant developed for industrial application of methane pyrolysis is still in its infancy.
According to a BASF report in 2021 [45], methane pyrolysis for large-scale production
should be available by 2030 at the latest. The existing large conventional and unconven-
tional methane deposits (methane clathrates, natural gas hydrates occurring in deep seas
and permafrost) are a precious source for hydrogen energy. Their exploitation is highly
required by optimized methane pyrolysis processes in an environmentally friendly manner
in the near future.

Author Contributions: Writing—original draft preparation, T.I.K.; writing—review and editing,

T.I.K., A.B. and A.H.; visualization, M.N.; supervision, T.I.K. and A.H. All authors have read and
agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments: Support by Hungarian-Egyptian bilateral project # NKM 2019-12 is acknowledged.
Conflicts of Interest: The authors declare no conflict of interest.

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