Những tiến bộ gần đây trong reforming khô của mêtan trên chất xúc tác Ni

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RecentAdvancesinDryReformingofMethane
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DOI:10.1016/j.jclepro.2017.05.176

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Journal of Cleaner Production 162 (2017) 170e185

Contents lists available at ScienceDirect

Journal of Cleaner Production
journal homepage: www.elsevier.com/locate/jclepro

Review

Recent advances in dry reforming of methane over Ni-based catalysts
Bawadi Abdullah a, *, Nur Azeanni Abd Ghani a, Dai-Viet N. Vo b
a

Biomass Processing Laboratory, Centre for Biofuel and Biochemical Research, Chemical Engineering Department, Universiti Teknologi PETRONAS, Bandar
Seri Iskandar, 32610, Perak, Malaysia
b

Faculty of Chemical and Natural Resources Engineering, Universiti Malaysia Pahang, Lebuh Raya Tun Razak, 26300, Gambang, Kuantan, Pahang, Malaysia

a r t i c l e i n f o

a b s t r a c t

Article history:
Received 15 November 2016
Received in revised form
25 May 2017
Accepted 28 May 2017
Available online 29 May 2017

A steady increase in atmospheric carbon dioxide (CO2) and methane concentrations in recent decades
has sparked interest among researchers around the globe to find quick solutions to this problem. One
viable option is a utilization of CO2 with methane to produce syngas via catalytic reforming. In this paper,
a comprehensive review has been conducted on the role and performance of Ni-based catalysts in the
CO2 reforming of methane (sometimes called dry reforming of methane, DRM). Coke-resistance is the
key ingredient in good catalyst formulation; it is, therefore, paramount in a choice of catalyst supports,
promoters, and reaction conditions. Catalyst supports that have a strong metal-support interaction
created during the catalyst preparation exhibit highest stability, high thermal resistance and high coke
resistance. In addition, the outlook of the Ni-based catalysts has been proposed to provide researchers
with critical information related to the future direction of Ni-based catalysts in industrial settings.
Among others, it has been a great interest among researchers to synthesize catalyst supports from
cellulosic materials (plant-based materials). The unique properties of the cellulose which are a welldefined structure and superior mechanical strength could enhance the catalytic activity in the DRM
reaction.
© 2017 Elsevier Ltd. All rights reserved.

Keywords:
Catalyst development

Methane dry reforming
CO2 utilization
Greenhouse gases
Catalysis
Coke-resistant catalysts

Contents
1.
2.
3.
4.

5.

6.

7.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Reaction thermodynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ni-based catalysts for DRM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.
Catalyst support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.
Promoter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.
Bimetallic catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.
Novel catalytic material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.5.
Recently developed catalysts for CO2 reforming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Other technologies of CO2 reforming of methane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.
Steam-CO2 dual reforming of methane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.
Tri-reforming of methane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Kinetics and mechanistics of DRM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.
Influence of process variables on reaction rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.
General applicable kinetic models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

* Corresponding author.
E-mail address: (B. Abdullah).
/>0959-6526/© 2017 Elsevier Ltd. All rights reserved.

171
172
172
175
175
176
177
177
177
178
178
178

179
180
181
182


B. Abdullah et al. / Journal of Cleaner Production 162 (2017) 170e185

8.

171

Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182

1. Introduction
Over the past decade, there has been an increase in energy
consumption, mainly due to a rapid growth in human population
(Li, 2005). This growing demand for energy has shifted the energy
scenario over the years by industrialization (Tanksale et al., 2010).
Moreover, energy demand is expected to keep increasing in the
future despite the current low oil price. At present, the dependence
on fossil fuels which consist of oil, natural gas, and coal to meet
energy demand have created environmental issues by the generation of anthropogenic greenhouse gases. Methane and CO2 are the
most abundant greenhouse gases and are the main contributors to
the recent climate-change issues (Noor et al., 2013). Even though
the concentration of methane in the atmosphere is lower compared
to CO2 (Talyan et al., 2007), surprisingly it has caused about 20% of
the overall global warming (Wuebbles and Hayhoe, 2002). Traditionally, methane is produced from two sources; first, it comes from

natural sources such as termites, grasslands, wildfires, lakes and
wetlands and second, from human activities such as coal mining,
landfills, oil and gas processing and agricultural activities (Yusuf
et al., 2012). According to the U.S. Environmental Protection
Agency (EPA) (Agency, 2011), the production of methane from
landfill contributes to about one-third of all emitted methane in the
US alone in which, landfill gas consists of 40e45% of methane and
55e60% of CO2 by volume (Raco et al., 2010). Apart from that,
methane is also a major component of natural gas but most natural
gas reservoirs are located far from industrial areas and often produced offshore, and thus, the limitation in technology and cost for
transporting this valuable natural gas from offshore to potential
market has led to the flaring of a large volume of natural gas
globally (Lunsford, 2000). These actions resulted in the wastage of
an important hydrocarbon source and contributed to global
warming by releasing a greenhouse gas to the atmosphere (Elvidge
et al., 2009). Due to the pressure of fighting against the global
climate change and ensuring the continuous energy sources, carbon dioxide capture and storage (CCS) was introduced around the
world with the objective to minimize the carbon dioxide emissions
(Yang and Wang, 2015). Moreover, to reduce the substantial dependency on crude oil and its undesirable influence on the atmosphere, renewable energy is needed immediately for substituting
petroleum-based resources (Fayaz et al., 2016).
In order to reduce the amount of methane and CO2 in the atmosphere, extensive research has been conducted to find effective
ways to convert methane and CO2 into other valuable products. The
most common option is the conversion of CO2 and methane to
syngas owing to a low cost and relatively established technology
(Bahari et al., 2016). It is an important process to transform the
hydrocarbons, usually in the chemical industries for the production
of syngas (Alirezaei et al., 2016). Syngas is considered a building
block that can be used as reactants for other applications such as
Fischer-Tropsch (F-T) oil, methanol, and other valuable liquid fuels
and chemicals (Pen~

a et al., 1996). Reforming is the most common
method used in industries to produce syngas, via one of the three
reforming processes, via steam reforming of methane (SRM), partial
oxidation of methane (POM) and dry reforming methane (DRM)
(Asencios and Assaf, 2013). SRM is the conventional technology for
production of hydrogen from hydrocarbon fuels due to the highest

hydrogen yield compared to the other two methods (Palma et al.,
2016). Approximately 75% of hydrogen produced is derived from
SRM process (Fan et al., 2016). The differences between these
techniques are based on the oxidant used, the kinetics and energetics of the reaction, and the ratio of the syngas produced (H2/CO).
The details of the main reactions for reforming processes are
summarized as followings:

SRM:

CH4 þ H2 O/CO þ 3H2

DH298K ¼ þ228 kJ=mol
(1)

POM:

CH4 þ 1=2O2 /CO þ 2H2

DH298K ¼ À22:6 kJ=mol
(2)

DRM:

CH4 þ CO2 /2CO þ 2H2

DH298K ¼ þ247 kJ=mol
(3)

From Rxn. (1), SRM reaction produces a higher H2/CO ratio
which is 3:1 (Gangadharan et al., 2012) compared to the ratio
required for F-T synthesis which is 2:1 (Oyama et al., 2012). SRM
requires intensive energy input due to the endothermic nature and
caused it is very expensive (Nieva et al., 2014). In addition, a higher
H2O/CH4 ratio is required to attain a higher yield of H2 which makes
SRM process energetically unfavorable and accelerates catalysts
deactivation (Carvalho et al., 2009). Moreover, SRM faces corrosion
issues and requires a desulphurization unit (Djinovi
c et al., 2012). In
the case of POM, this process is suitable for the production of
heavier hydrocarbons and naphtha (Larimi and Alavi, 2012). Typically, the POM process has very short residence time, high conversion rates and high selectivity (Ruckenstein and Hang Hu, 1999).
However, the exothermic nature of the reaction causes the induction of hot spots on the catalyst and makes the operation difficult to
control (Asencios and Assaf, 2013). Besides, POM requires a cryogenic unit to separate oxygen from the air (Djinovi
c et al., 2012).
Of all other technologies, DRM is the most promising one as it
utilizes two abundant greenhouse gases (CO2 and methane) to
produce syngas that is important for industries, and at the same
time can reduce the net emission of greenhouse gases to the
environment (Selvarajah et al., 2016). In addition, the DRM process
is also cheaper than other methods since it eliminates the
-Alonso et al.,
complicated gas separation of end products (San-Jose
2009). DRM produces a H2/CO ratio of unity that can be used for the
synthesis of oxygenated chemicals (Wurzel et al., 2000) and higher

hydrocarbons for F-T synthesis (Nieva et al., 2014). Moreover, DRM
can be extended to biogas (CO2, CO and CH4) as a feedstock to
produce clean and environmentally friendly fuels (Xu et al., 2009).
Besides that, syngas from DRM is considered as solar or nuclear
energy storage (Fraenkel et al., 1986). Table 1 shows the comparison
between three processes in the CO2 reforming of methane.
The use of catalysts in DRM reaction is important to maximize
the production of syngas as it helps to alter and enhance the rate of
reaction without being used up in the process. Catalyst works by
providing an alternative mechanism that lowers the activation
energy resulted in less energy required to reach the transition state.
Even though DRM requires high temperature to operate due to its
endothermic nature, the presence of catalysts could lower the
temperature of the reaction significantly.


172

B. Abdullah et al. / Journal of Cleaner Production 162 (2017) 170e185

Table 1
Comparison between the methods in DRM reaction.
Type of Reaction

Steam Reforming of Methane (SRM)

Advantages

1 A technology that utilized two most
abundant greenhouse gasses which are

the CO2 and CH4
2 A clean and environmentally friendly
fuel that is formed
1 Induction of hot spots on catalyst 1 Carbon formation and sintering of
1 Requires high energy and very costly.
catalyst
might occur due to the exothermic
2 Requires high CO2/CH4 ratio for greater yield of syngas cause
nature of reaction
the SRM reaction energetically unfavorable and lead to
2 Costly technology because it requires
catalyst deactivation.
cryogenic unit to separate oxygen
3 Complex system
from the air
4 Sensitive to natural gas qualities
H2/CO ratio ¼ 3:1
H2/CO ratio ¼ 2:1
H2/CO ratio ¼ 1:1
Topsoe Package Hydrogen Plants at Air Liquide, Belgium;
NIL
NIL
Plants in USA
Temperature: 700 C to 1,000 C
Temperature: 950  C to 1100  C
Temperature: 650  C to 850  C
Pressure: 3e25 bar pressure
Pressure: 100 bar pressure
Pressure: 1 bar
Ratio: CH4/H2O ¼ 1:1

Ratio: CH4/O2 ¼ 2:1
Ratio: CH4/CO2 ¼ 1:1

Disadvantages

H2/CO ratio
Commercial Plant
Operating
Temperature,
Pressure and
Ratio

1 High efficiency

2. Method
A previous review article of CO2 reforming of methane was done
by Wang et al. (1996) which presented a comprehensive review of
the thermodynamics, catalyst selection and activity, reaction
mechanism, and kinetics of this reaction. Since then, there is
extensive research on the CO2 reforming of methane, in particular
on the Ni-based catalysts. It is mostly common catalyst used in the
industries. This review was conducted to identify the gap in the
reaction process and the ways to overcome the problems especially
coke formation associated with Ni-based catalysts. Coke formation
and sintering of catalyst are the primary causes of catalyst deactivation that could lead to the low conversion of reactants. Hence, it
is timely to provide a comprehensive review on the CO2 reforming
of methane over Ni-based catalysts. The literature was selected
based on current developments in the CO2 reforming of methane to
improve the catalytic performance and increase the conversion of
CO2 and CH4. The review also comprises of the development of

catalyst, thermodynamic analysis for the reaction process and the
outlook for future research associated with DRM.
3. Reaction thermodynamics
The thermodynamic behavior of DRM is essential to determine
the most suitable reaction temperature, pressure and feed ratio to
produce a high yield of syngas. DRM requires high energy for the
reaction to take place as it is a highly endothermic reversible reaction (cf. Rxn 1 in Table 2) (Lavoie, 2014). A very high temperature
is needed to drive the reaction in the forward direction to obtain a
high conversion to produce syngas (Liu et al., 2009). All Ni-based
catalysts used in experiments show their highest conversion at

Partial Oxidation of Methane (POM)

Dry Reforming of Methane (DRM)

2 High conversion of reactants
3 High selectivity of syngas
4 Short residence time

800  C in the investigated temperature ranges from 100 to 900  C.
While, the use of high temperatures can avoid the production of
secondary products but it requires more energy. In this case, the
aim of using catalysts is to reduce the energy needed to obtain a
high yield of syngas.
The reactions which may occur in DRM are considered in Table 2
(Nikoo and Amin, 2011). Rxn (1) shows that DRM produces H2/CO
ratio of unity. However, in general, DRM has a H2/CO ratio of <1
because there is a simultaneous production of CO from reversewater-gas-shift (RWGS) reaction (cf. Rxn (2)) which causes an
increasing amount of CO compared to H2 (Nikoo and Amin, 2011).
Although H2/CO ratios <1 may seem undesirable, this syngas ratio

can, in fact, be used for F-T synthesis for the production of higher
hydrocarbons (Pakhare and Spivey, 2014). Apart from RWGS, other
side reactions can also occur depending on the CH4/CO2 feed ratio
and the operating temperature and pressure, including the formation of carbon (coke).
Coke is an undesired product as it inhibits the catalyst activity
by causing physical blockage of the reformer tubes, the collapse of
the catalyst support, encapsulation of the metal crystals and pore
blockage (Rostrup-Nielsen, 1997). There is a consensus that carbon
is formed by the decomposition of CH4 (Rxn (3)) and disproportionation of CO (Rxn (4)) (Ginsburg et al., 2005). However, two
other reactions are also believed to contribute to the formation of
coke: hydrogenation of CO2 (Rxn (5)) and hydrogenation of CO (Rxn
(6)) (Nikoo and Amin, 2011). All reactions are exothermic reactions
except for decomposition of methane (Rxn (3)).
CO2 reforming of methane involves a risk of carbon formation
that may reduce the performance of the catalyst. There are three
types of carbon formation that are usually observed in a reformer,
namely pyrolytic, encapsulating and whisker carbon, as imaged by

Table 2
Reactions in dry (CO2) reforming of methane.
Rxn Num

Rxn Name

Main reaction
1
Dry reforming of methane
Side reaction that leads to the decrease in H2/CO ratio to <1
2
Reverse water-gas-shift (RWGS)

Side reactions that lead to formation of coke (carbon)
3
Decomposition of methane
4
Disproportionation of CO
5
Hydrogenation of CO2
6
Hydrogenation of CO

Rxn Equation

DH298K (kJmol-1)

CH4 þ CO2 4 2CO þ 2H2

þ247

CO2 þ H2 4 CO þ H2O

þ41

CH4 4 C þ 2H2
2CO 4 C þ CO2
CO2þ2H2 4 C þ 2H2O
H2 þ CO 4 H2O þ C

þ74.9
À172.4
À90

À131.3


B. Abdullah et al. / Journal of Cleaner Production 162 (2017) 170e185

Fig. 1. Electron microscopy images (Philips CM200 FEG TEM) of pyrolytic carbon on a
MgAl2O4 carrier (A), encapsulating carbon (B), and whisker carbon (C) on Ni/MgAl2O4
reforming catalysts. Reproduced with permission from Sehested, J. Copyright 2006
Elsevier.

transmission electron microscopy in Fig. 1. The pyrolytic carbon (cf.
Fig. 1a) is usually formed due to the exposure of higher hydrocarbons to high temperature. The sintering or sulfur poisoning of the
catalyst can lead to low activity and cause the higher hydrocarbons
to reach high temperatures in the reformer (Sehested, 2006). This
type of carbon formation usually occurs at temperatures above
600+C, and the critical parameters are high temperature, high void
fraction, high pressure and acidic catalyst (Bartholomew, 1984).
Carbon encapsulation occurs during heavy hydrocarbon feed
reforming higher content of aromatic compound (cf. Fig. 1b). The

173

high final boiling point and low temperatures of the hydrocarbon
mixture increase the rate of encapsulating carbon formation
(Sehested, 2006). As shown in Fig. 1b, encapsulating carbon contains a thin CHx film covering the Ni particles that can lead to the
catalyst deactivation. Generally, encapsulating carbon occurs at
temperatures below 500  C (Bartholomew, 1984).
The final type of carbon formation is whisker carbon, the most
critical type of carbon formation in the DRM reaction. The formation of whisker carbon occurs when hydrocarbon or CO reaction on
one side of the Ni particle results in the growth of carbon whiskers,

while the nucleation of graphitic carbon as carbon whiskers on the
other side of the nickel particle as illustrated in Fig. 1c (Sehested,
2006). This type of carbon formation leads to the breakdown of
catalyst, an increase in the pressure drop and significant deactivation of the Ni surface. Whisker carbon is usually formed at temperatures above 450  C (Bartholomew, 1984).
The effect of hydrogen and water in DRM was studied by
Delgado et al. (2015). The operating temperatures were set between 100 and 900  C (373 and 1173 K) at atmospheric pressure,
and the inlet mixture was 1.6% CH4, 2.1% CO2, and 1.8% H2 in N2
dilution. From Fig. 2, it shows that at a lower temperature, there is
an increase in water with the addition of hydrogen compared to dry
reforming. The water was produced through RWGS and getting a

(c)
Fig. 2. Comparison of experimentally determined (symbols) and numerically predicted (lines) concentrations as a function of temperature for catalytic dry reforming of methane
with co-feed H2: (a) CH4 and CO2; (b) H2O, CO, H2, inlet gas composition of 1.6 vol.% CH4, 2.1 vol.% CO2, 1.8 vol.% H2 in N2; 1 bar; Tinlet ¼ 373 K; total flow rate of 4 slpm; dashed
lines ¼ equilibrium composition at given temperature. (c) Computed surface coverage of adsorbed species as function of the temperature for methane dry reforming with H2 cofeed: inlet gas composition of 1.6 vol.% CH4, 2.1 vol.% CO2, 1.8 vol.% H2 in N2; 1 bar; total flow rate 4 slpm. Reproduced with permission from Delgado et al. Copyright 2015 Elsevier.


174

B. Abdullah et al. / Journal of Cleaner Production 162 (2017) 170e185

maximum water concentration at 400  C (673 K) (cf. Fig. 2b). The
water was used up together with unconverted methane by the
steam reforming reaction when the temperature increased
(Delgado et al., 2015).
A high coverage with hydrogen and CO at low and medium
temperatures is represented by the computed surface coverage
respectively (cf. Fig. 2c). Maximum carbon formation occurs at reaction temperatures between 100 and 300  C (373 and 573 K), and
this carbon formation is mainly formed by the reaction between
CO(s) and H(s). The total coverage with adsorbed species is rather

low at higher temperatures (Delgado et al., 2015). Moreover, when
CO2/CH4 feed ratio is higher than unity, carbon is normally formed.
Less H2 available for hydrogenation reactions in Rxns (5 and 6),
resulted in a decrease in carbon formation. Based on the thermodynamics calculations, temperatures higher than 900  C are
required for CO2/CH4 feed ratio of unity to obtain a syngas mixture
ratio of 1:1 with a small amount of carbon (Nikoo and Amin, 2011).
This outcome is in agreement with a study conducted by Wang
et al. (1996) which suggests that carbon deposition is possible

only up to 870  C at 1 atm and CO2/CH4 feed ratio of unity.
On the other hand, Fig. 3 shows the H2/CO ratios produced from
the DRM reaction at different temperatures with the pressure of
1 atm. Based on Fig. 3a, with increasing temperature, the ratio of
H2/CO increases due to the endothermic nature of the DRM reaction
(Hassani Rad et al., 2016). H2/CO molar ratio gets closer to unity at
higher temperatures, typically above 800  C. For instance, a H2/CO
ratio of 1:1 that can be useful for F-T synthesis can be obtained at
temperatures above 850  C for CO2/CH4 feed ratio being unity.
Fig. 3b represents the influence of CO2/CH4 feed ratios at 1 atm
to the product yields and H2/CO molar ratio in the end product.
Based on the study by Hassani Rad et al. (2016), the CO2 increased,
and the CH4 decreased with the increasing of feed ratios. Moreover,
with increasing CH4 concentration in the feed, the H2/CO ratio
approached unity while the product yield reduced proportionally
(Hassani Rad et al., 2016). However, the required H2/CO ratio is not
fixed as it depends on industrial needs.
Fig. 4a portrays the conversion of CH4 and CO2, the main product
distributions, and the H2/CO ratio at different system pressures

(a)

(b)
Fig. 3. (a) Effect of temperature on feed conversion, products yield and H2/CO molar ratio in product over NAC-I nanocatalyst. (b) Effect of CH4/CO2 ratio in feed on feed conversion,
products yield and H2/CO molar ratio in product over NAC-I nanocatalyst; Reproduced with permission from Hassani Rad et al. Copyright 2016 Elsevier.


B. Abdullah et al. / Journal of Cleaner Production 162 (2017) 170e185

175

reaction (DG r) was calculated by Eq. (4):


DGr ¼

X



gi DGfig

(4)

i

where gi represents the stoichiometric coefficient for species i. To
define the possible range of the spontaneous occurrence of the
reactions, the equilibrium constant (K) is calculated by using the Eq.
(5):

. 


K ¼ exp À DGr RT

(a)

(b)

Fig. 4. The effect of pressure on a) equilibrium conversion of reactants and products
distribution for CO2/CH4 ¼ 1, 1173 K and n0(CH4 þ CO2) ¼ 2 mol and on b) carbon
deposition as a function of temperature; Reproduced with permission from Nikoo and
Amin. Copyright 2011 Elsevier.

with constant temperature and inlet feed ratio. From Fig. 4a, CO2,
and CH4 conversions are higher at atmospheric pressure than those
at higher pressures. In Fig. 4b, carbon deposition significantly increases with increasing pressure. These findings agrees well with
the outcome shown by Nematollahi et al. (2012) whereby
increasing pressure results in decreasing of conversion rates and
syngas yields. It is preferable to operate DRM at atmospheric
pressure to obtain high conversions and high yield of syngas.
According to Nikoo and Amin (2011), CO2 and CH4 conversions
are usually greater at low pressure than at higher pressures, since
the effect of temperature on reaction conversion was suppressed by
high pressure. This phenomenon can be explained well based on
LeChatelier’s principles, where the endothermic CO2 reforming of
methane tends to shift to the reactant side.
Fig. 5 illustrates the equilibrium constants of all possible reactions, presented as a function of time. Based on the second law of
thermodynamics, the CO2 reforming of methane is spontaneous if
the Gibbs free energy change of reaction (DGr) is negative while the

reaction is thermodynamically limited if the DGr is positive. For
each reaction temperature, the Gibbs free energy change of the

(5)

The equilibrium constant (K) defines the extent to which the
reaction occurs. Based on Fig. 5, DRM (Rxn (1)) is a thermodynamically favorable reaction that produces syngas at temperatures
above 727 C.
4. Ni-based catalysts for DRM
Numerous studies have been published for the development of
active and coke-resistant catalysts for the DRM reaction (Bahari
et al.; Selvarajah et al., 2016). The common catalysts for DRM reaction are supported noble metal catalysts such as Ru, Rh, and Pt
and supported transition metal catalysts such as Ni and Co (Niu
et al., 2016). First principle calculations have proven that noble
metals Ru and Rh show higher activity than that of Ni at the same
particle size and dispersion (Jones et al., 2008). Although noble
metals such as Ru, Rh and Pt are very active and more cokeresistant towards DRM reaction than other transition metals, they
have limited availability and are expensive (Kehres et al., 2012).
Among these catalysts, Ni-based catalysts are the most frequent
catalysts used at industrial scales (Nair and Kaliaguine, 2016).
To commercialize DRM reaction in the industries, the development of cheap and cost-effective catalysts that have high activity
and high resistance to carbon deposition is the prime concern.
Researchers have conducted investigations on the type of support
used (Pompeo et al., 2007) and the effect of adding promoters to Nibased catalysts to find the best way to improve the coke resistance
of Ni-based catalysts. Moreover, recent attempts to improve catalytic activity and inhibit carbon formation have been carried out by
combining two or three metals as active sites (Zhao et al., 2016).
Preparation technique and catalyst pretreatment process (Chang
et al., 1994) also play a major role in the change of structural
properties, the reduction behavior, and also the catalytic
performance.

4.1. Catalyst support

Fig. 5. Equilibrium constants of reactions involving in CH4eCO2 reaction at different
temperatures and atmospheric pressure; Reproduced with permission from Nikoo and
Amin. Copyright 2011 Elsevier.

Typically, a catalyst consists of more than a single component,
whereby the components are constructed into the desired shape
and structure. The active metal is usually embedded in the support
material to produce a supported metal catalyst. These support
materials play several important functions to the activity of the
catalysts. By providing a large surface area where metallic compounds may disperse, the support materials maximize the surface
area of the active sites which then allows the coarse geometry of
the catalyst to be customized for the reactor. Typically, these supports were inactive on their own but would take part in the total
reaction when interacting with the active metal sites (bi-functional
mechanism) (Ferreira-Aparicio et al., 2000).
Sokolov et al. (2012) prepared a series of supported Ni catalysts
to observe the effect of the support materials on the catalysts’ activity. The study was conducted using Ni/Al2O3, Ni/MgO, Ni/TiO2,
Ni/SiO2, Ni/ZrO2, Ni/La2O3-ZrO2 and Ni supported on mixed-metal
oxides (Ni/Siral 10 and Ni/PuralMG30) at low temperature


176

B. Abdullah et al. / Journal of Cleaner Production 162 (2017) 170e185

4.2. Promoter

Fig. 6. CO and H2 yields after first 10 h (black bars) and 100 h (gray bars) on DRM
stream at 400  C and GHSV of 7200 mL hÀ1 gÀ1

cat; Reproduced with permission from
Sokolov et al. Copyright 2012 Elsevier.

(400  C) (cf. Fig. 6). From the experiment, catalysts that contain Zr
within the support showed the highest initial activities. Ni/La2O3ZrO2 yielded CO and H2 that is close to equilibrium, and they
showed the highest stability, followed by Ni/ZrO2. Although Ni/SiO2
catalyst had the highest specific surface area, the initial yield of H2
was the lowest followed by Ni/Al2O3, Ni/MgO, and Ni/TiO2. However, it is remarkable for Ni/MgO to achieve an initial H2 yield of
2.5% considering that the catalyst had low surface area. In general,
the activity of the catalysts (based on H2 yield) can be represented
as: Ni/La2O3-ZrO2 > Ni/ZrO2 > Ni/PuralMG30 > Ni/Siral 10 > Ni/
TiO2 > Ni/MgO > Ni/Al2O3 > Ni/SiO2. To have a better understanding
of the resistance of the catalyst towards deactivation, the yield of H2
at 0 h and at 100 h time-on-stream were compared. It is found that
Ni/La2O3-ZrO2 had the highest stability with only 9% loss of H2 yield
from the initial state. The least stability of catalyst with the loss of
20% of H2 yield was Ni/ZrO2 and 89% of H2 yields was Ni/TiO2. The
improved Ni-support interaction on mesoporous La2O3-ZrO2
probably emerges from partial encapsulation of NiOx species by
mesopores during the preparation of the catalyst which resulted in
a formation of strong chemical bonding that has a greater portion of
each Ni particle in following steps (Sokolov et al., 2012).
Another study by Guo et al. (2004) found that Ni/MgO-g-Al2O3
and Ni/MgAl2O4 catalysts demonstrate better stability and higher
activity compared to Ni/g-Al2O3. The good stability of the catalyst
was attributed to the MgAl2O4 spinel layer in Ni/MgO-g-Al2O3
which efficiently suppressed the phase change to form NiAl2O4
spinel phases and can make the tiny Ni crystallites stable. The high
activity of the catalyst, as well as the high coke and sintering
resistance compared to g-Al2O3, was attributed to the characteristics of MgAl2O4, which has high resistance to sintering and has low

acidity. The interactions between Ni and MgAl2O4 produce a highly
dispersed active Ni species (Guo et al., 2004).
Moreover, recent studies on Ni-based catalysts for DRM reaction
also reveals that catalysts which are based on supported-Ni-Al
spinels show excellent results with respect to catalyst activity and
performance. Based on the comparison study of Ni/g-Al2O3 to Ni/
MgOeAl2O3 and Ni/MgAl2O4 by Guo et al. (2004) it was proven that
formation of carbon was 7e8 fold higher in the case of larger Ni
particles (Ni/g-Al2O3) compared to Ni/MgAl2O4.
A similar study was conducted by Fauteux-Lefebvre et al. (2010)
on the Ni-Al spinel phase (NiAl2O4) catalyst. They found that this
formulation was well dispersed in a ceramic support composed of
Al2O3eYSZ. It was evident that the catalyst is active without a preactivation step, and no deactivation was detected even at low H2O/C
molar ratio (1.9) and temperature below 760  C in diesel steam
reforming.

One way to avoid the formation of carbon deposition is by the
addition of promoters such as the alkaline and earth metals
(Valentini et al., 2004). An alkaline promoter such as CaO can
prevent sintering from occurring which provides better performance of the catalyst. Dias and Assaf (2003) discovered that sintering also causes the catalyst to deactivate. During the calcination
of the catalyst, when calcium is integrated as a promoter in Al2O3
supported Ni catalysts, their structure is changed, thereby affecting
the catalyst performance. The interaction of Ca from the promoter
CaO with the support at a structural level lowers the sintering
resistance. The competition between Ca and Ni during the interaction aids in the formation of reducible Ni species. The concentration of Ca affects the conversion of CO2 and CH4 in DRM reaction.
For instance, lower concentrations of Ca formed ionic oxides
strongly and increased the conversion of CO2. At lower concentration of Ca, the CO2 is attracted to the surface of the catalyst which
then also increased the conversion of CH4. On the other hand,
higher concentrations of Ca increased the Ni electron density which
then resulted in the decline of CH4 and CO2 conversions (Dias and

Assaf, 2003).
Besides promoting with CaO, the addition of potassium (K) as a
promoter to Ni-based catalysts was also reported by Juan-Juan et al.
(2006). After undergoing pretreatment with hydrogen, adding K to
the catalyst modified the NiO-Al2O3 support interaction and
improved the Ni species reducibility. Besides, potassium also acts as
a catalyst for the gasification of coke formed during the reaction
without changing its structure. The size and structure of the Ni
particles remain the same when potassium is used. Luna and Iriarte
(2008) also reported the same findings whereby the formation of
carbon on the surface of the catalyst is prevented when the catalyst
is promoted with potassium. Mostly, the reducibility of the catalyst
is increased when potassium modifies the interaction of metal and
support. It is suggested that the transfer of potassium from the
support to Ni surface in a promoted potassium catalyst decreases
the conversion of CH4 because a portion of the most active sites for
the DRM reactions are neutralized (Luna & Iriarte, 2008).
Other than Ca and K, the role of Cu as a promoter over silica
supported Ni catalyst was investigated by Chen et al. (2004). They
used both CO and CH4 activities and catalyst characterizations as
the basis to evaluate the results. The addition of Cu can stabilize the
active site structure and prevent the Ni catalyst from deactivating
due to loss of Ni crystallites or sintering. The incorporation of Cu
onto the Ni catalyst formed Cu-Ni species which can change the
catalytic activity. These Cu-Ni species are responsible for balancing
the coke removal by CO2 and CH4 cracking and hindering carbon
accumulation on the Ni particles. Nonetheless, when the Cu-Ni
species are enclosed by carbon accumulation, they are still able to
catalyze the primary step to activate the DRM, i.e. the splitting of CH bonds to CHx species.
It has also been reported in the literature that incorporating

vanadium as a promoter can reduce the deposition of carbon on the
active sites and increased the overall performance of DRM reaction.
A study conducted by Valentini et al. (2003) has shown that vanadium promoted on alumina supported nickel catalyst gave a high
conversion of CH4 by limiting the formation of the inactive phase of
Ni/Al2O3, namely NiAl2O4. Moreover, from the interpretation of H2
chemisorption, XRD, and XPS analysis, vanadium was found to
cause changes in the microstructure by hindering aluminate spinel
phase from forming on the Ni/Al2O3 catalyst (Valentini et al., 2003).
In addition, promoters are used in small amounts, usually from
0.01 to 10 wt percent (wt%), according to the corresponding catalytic system. The promoter weight percentage is important as it
leads to the significantly improved results of the reaction. The


B. Abdullah et al. / Journal of Cleaner Production 162 (2017) 170e185

optimum amount of promoter is different according to the type of
promoters, which have different ability to modify the catalyst
structure. Daza et al. (2010) studied the performance of modified
Ni/Mg-Al (mixed oxides, MO) by different Ce weight percentage
(X ¼ 0, 1, 3, 5, and 10 wt%). They found that promoter weight
percentage is an important criterion to avoid coke deposition. For
example, the catalyst (Ni/Mg-Al) modified by 3 wt% of Ce showed
higher CH4 (99%) and CO2 (95%) conversion without any decrease in
stability up to 100 h of reforming reaction (CH4/CO2/He:10/10/80).
Filamentous type carbon deposition was present in the catalyst
promoted by 1 wt% Ce, but it was absent in the catalyst promoted
with 3wt% Ce.
4.3. Bimetallic catalysts
Based on a study by Zhang et al. (2008), supported bimetallic
catalysts demonstrate high activity and stable DRM reaction performances. In an experiment to test the stability, bimetallic Ni-Co

catalyst supported on Al2O3eMgO, which was prepared by coprecipitation method, demonstrated little deactivation after
2000 h on stream (Zhang et al., 2007). One of the key factors
responsible for the excellent catalytic performance of this bimetallic catalyst is the preparation method. The high calcination
temperature used during the preparation of the catalyst formed
strong interactions between metal and support which then caused
the catalyst to convert into stable spinel-like framework structures.
In general, the formation of carbon is efficiently hindered by the
formation of Ni-Co alloy during the catalyst reduction compared to
the single Ni sites. Different catalyst synthesis methods also influence the reaction performance. For example, the co-precipitation
method can produce smaller metal particle sizes as compared to
wet impregnation method.
4.4. Novel catalytic material
Other than developing the Ni-based catalyst with some modifying agents during the catalyst preparation, incorporating the Ni
particles within the mesoporous support could also increase the
conversion of reactants and yield of products by avoiding the sintering of metal particles and strengthening the metal-support
interaction (Xu et al., 2011). This is due to the high specific surface area of mesoporous materials that can improve the dispersion
of Ni particles onto the supported catalyst (Zhang et al., 2015).
Moreover, the strong metal-support interaction stabilizes the Ni
particles which are incorporated into the mesoporous matrix.
Multiple contact areas created between the Ni-particle and support
could enhance the thermal stability and assist cooperativity between the metal and support (Gnanamani et al., 2011). As examples
reported in the literature, development of Ni-based catalysts
incorporated into mesoporous supports such as MCM-41, SBA-16,
TUD-1, meso-Al2O3 and meso-ZrO2 have demonstrated high catalytic activity and high resistance to carbon formation in DRM
(Zhang et al., 2015).
Catalyst supports also can be synthesized from plants in order to
improve the performance of the catalyst for DRM. In recent years,
polymers from trees have been an area of interest for researchers to
speed up the chemical reactions. Catalysts mounted on widely
available cellulose could provide efficiently; low cost means to

produce fine chemicals. Cellulose is biodegradable but possesses a
unique property as it provides a well-defined structure, high crystalline order, a controlled surface chemistry, and high mechanical
strength which apparently extends to catalysis. For example,
Guilminot et al. (2007) the use of cellulose acetate-based carbon
aerogels as promising catalyst support for proton exchange membrane (PEM) fuel cell electrodes. Pretreatment and hydrolysis are

177

the main steps to synthesize the catalyst support. Pretreatment
includes the use of a physical technique such as size reduction and
ultrasonic, chemical process, physico-chemical techniques such as
liquid hot water, biological methods and some combination of
those techniques in order to fractionate the lignocellulose from its
component. (Bensah and Mensah, 2013). The pretreatment step
helps to increase the surface area (Lee et al., 2008) and porosity
(Harmsen et al., 2010; Lee and Jeffrles, 2011) that will lead to the
increasing of hydrolysis rate. Cellulose and hemicelluloses are
converted into monomeric sugars in hydrolysis step through the
addition of cellulase such as acids and enzymes (Bensah and
Mensah, 2013). The enzymatic hydrolysis gives more advantages
compared to acids hydrolysis. Enzymatic hydrolysis required low
energy consumption due to the mild process requirement produces
high sugar yields, and no unwanted wastes. Pretreatment is costly
among various techniques. However, the result of hydrolyzing
lignocellulose without pretreatment is far less favorable as there is
only 20% of native biomass is hydrolyzed (Mosier et al., 2005).
Ni-based catalysts have been commercially used as the metal
precursor in DRM, yet improvement on the metal is needed to
enhance the performance of the catalyst. Nowadays, nanoparticles
have received increasing interest among researchers as they have

promising physical and chemical properties and high potential in
technological applications (Du et al., 2004). A study reported that
NiCoB catalyst with average particle size of 10 nm and prepared by
chemical reduction showed higher catalytic activity than Raney
nickel in the hydrogenation in benzene. It is advisable to develop
nano-sized nickel metal precursor for the DRM reaction in order to
improve the catalytic activity and increase the conversion of the
reactants and yield of the products.
Preparation method greatly influences the physico-chemical
properties and performance of a catalyst (Jeong et al., 2013).
Impregnation and co-precipitation are the most widely used, conventional methods of catalyst preparation. Another less common
method for catalyst preparation is sol-gel. The sol-gel method
produces a fine size distribution, decreases the deactivation rate,
imparts high thermal resistance against agglomeration and produces a product with high purity as compared to the conventional
methods (Gonçalves et al., 2006; Gonzalez et al., 1997). Recently, a
new method, non-thermal glow discharge plasma was developed
to improve the metal-support interaction, give the higher distribution of Ni particles and enhance the activity and stability of the
catalyst (Rahemi et al., 2013). However, plasma treatment is relatively expensive compared with other simple preparation methods
(Usman et al., 2015). Thus, the combination of novel catalytic material and method would enhance the activity and stability of the
catalyst in DRM reaction.
4.5. Recently developed catalysts for CO2 reforming
The progress on developing catalysts for DRM reaction has been
concentrated on finding a new formulation of catalyst that can give
higher activity and higher stability towards coke formation, sintering, the formation of inactive chemical species and metal
oxidation (Takanabe et al., 2005; Zhang et al., 2008). Modifying the
active sites of catalysts by adding some catalyst supports and promoters during catalyst preparation could enhance the catalytic
performance thereby resulting in higher conversion and selectivity.
Several recently developed catalysts for DRM are considered in
Table 3.
Table 3 shows a summary of nine Ni-based catalysts recently

applied to the DRM reaction. The catalysts differ in terms of the
types of supports and promoters, and preparation method. Reaction temperatures range from 600 to 800  C, space velocities from
8000 to 60,000 mL/g.h. Feed ratio of CH4/CO2 is 1 in all cases. Ni/


178

B. Abdullah et al. / Journal of Cleaner Production 162 (2017) 170e185

Table 3
Recently developed catalysts for the DRM reaction.
Catalyst

Preparation method

GHSV (mL/g CH4/CO2 feed
h)
ratio

15%Ni/ZrO2
10%Ni/CeO2

Combined co-precipitation and reflux 24,000
digestion
Impregnation
13,400

5%Ni/ZrO2eC

Impregnation

NA

Ni/Mg(Al)O
10%Ni-7%CeO2/
MgO
2.33%Ni-4.66%Co/
ZSM5
1.2%Ni-1.8%Co/
CeZr
15%NiCeMgAl
3%(CoNi)/SiCCeZrO2

Co-precipitation
Impregnation

T
Conv. CH4
( C) (%)

Conv. CO2 H2/CO
ratio
(%)

Coke formation
(wt%)

Refs.

1

700 >85

>88

z1

NA

(Zhang et al., 2015)

1

80 e 90

(Yu et al., 2015)

34

14

(Mustu et al., 2015)

8000
12,000

1
1

800 95

700 z45

98
z89

0.85
e0.90
0.55
e0.60
z0.93
NA

NA

1

760 67.05 e
82.82
600 24

NA
NA

Wet impregnation

60,000

1

700 z56

z63

z0.84

NA

Ethylene Glycol

NA

1

750 78

84

z0.84

0.24

Co-precipitation
Deposition precipitation

48,000
NA

1.04
1

800 z98
750 66

z82.5
75

z0.79
0.77

NA
0.4

(Li et al., 2015a)
(Khajenoori et al.,
2015)
(Estephane et al.,
2015)
(Djinovi
c et al.,
2015)
(Bao et al., 2015)
(Aw et al., 2015)

Mg(Al)O catalyst prepared by co-precipitation method with operating temperature of 800  C gave the highest conversion of CH4/
CO2. The conversion of CH4 was 95% and CO2 was 98%. The 5%Ni/
ZrO2eC catalyst prepared by impregnation method with operating
temperature of 600  C gave the lowest conversion. The preparation
method is one of the most significant factors affecting catalyst
performance due to the important role it plays in controlling the
size of Ni particles and modification of the metal-support interaction which is critical for the prevention of coke formation (Guo

et al., 2015). It can be concluded that catalyst composition, preparation method as well as operating temperature all impact greatly
on conversion.

5. Other technologies of CO2 reforming of methane
5.1. Steam-CO2 dual reforming of methane
The steam-CO2 dual reforming of methane has been considered
as the alternative technology for the production of syngas, in which
the H2/CO ratio can be adjusted by controlling H2O/CO2 in the feed
and the introduction of steam in the dual reforming of methane
helps to minimize the coke deposition on the catalyst (Li et al.,
2015b).
Li et al. (2015b) had investigated the catalytic stability of
developed LA-Ni/ZrO2 catalyst in the steam-CO2 dual reforming of
methane in comparison with the classical Ni/ZrO2 catalyst. Fig. 7 (a)
and (b) shows that the LA-Ni/ZrO2 catalyst exhibits higher initial
catalyst activity which are CH4 conversion is 94% and CO2 conversion is 95%. Fig. 7 (c) and (d) shows that both catalysts, LA-Ni/ZrO2
and Ni/ZrO2 have the similar selectivity for H2 and CO and there
were no visible changed can be identified along with the time on
stream. The excellent performance of LA-Ni/ZrO2 is due to the
intensified Ni-support interaction, increased Ni dispersity,
improved the reducibility of NiO and enlarged oxygen vacancies.

Fig. 7. (a) CH4 conversion, (b) CO2 conversion, (c) H2 selectivity, and (d) CO selectivity
as a function of time for stream for steamÀCO2 dual reforming of methane over the
developed LA-Ni/ZrO2 catalyst (error bars equal 95% confidence interval for conversion). Reaction conditions: mcat ¼ 50 mg, CH4/CO2/H2O ¼ 1:0.8:0.4, GHSV ¼ 48 000
mL hÀ1 gÀ1, and performed at atmospheric pressure. Reproduced with permission from
Li et al. Copyright 2015 ACS Publication.



CH4 þ H2 O /CO þ 3H2

CH4 þ

O2
/CO þ 2H2
2









DH298 À 36 kJmolÀ1 ;



CH4 þ CO2 /2CO þ 2H2





DH298 þ 206 kJmolÀ1 ;

(7)




DH298 þ 247 kJmolÀ1 ;




(6)

DH298 À 41 kJmolÀ1 ;

(8)

5.2. Tri-reforming of methane

CO þ H2 O /CO2 þ H2

Recently, tri-reforming of methane (TRM) also has received
attention due to its ability to convert the CO2 and methane into
syngas with desired ratio of H2/CO ratio for methanol and F-T
synthesis. TRM combines the three basic technologies in methane
reforming process in a single reactor which are methane steam
reforming (6), methane partial oxidation (7), CO2 reforming of
methane (8) and also water-gas shift reaction (9):

The highly exothermic complete oxidation (10) can also take
place that increases energy efficiency:

CH4 þ 2O2 /CO2 þ 2H2 O





(9)



DH298 þ 206 kJmolÀ1 ;
(10)

CO2 is utilized in the methane dry reforming reaction during the


B. Abdullah et al. / Journal of Cleaner Production 162 (2017) 170e185

tri-reforming reaction (Rxn. 8). In addition, typical flue gas from the
combustion process of power plants which has the average
composition of CO2 3e16%. O2 2e13%, H2O 6e8% and N2 75e76%
can be used as a CO2 source for TRM (Minutillo and Perna, 2009).
The addition to these effluents CH4, H2O and air resulted in a reaction mixture which proceed with adequate rate in a temperature
range of 700e900  C with the presence of suitable catalyst (Pino
et al., 2011). The TRM process is more energy efficient compared
to the SRM or DRM and also can be applied for transformation of
low quality, CO2-rich natural gas into syngas (Halmann and
Steinfeld, 2006).
Coke formation can occur during the reforming process of
methane, usually during methane cracking (11), the Boudouard
reaction (12) and reduction of CO to carbon (13). Hence, TRM can

help to reduce the coke formation problem.

CH4 /C þ 2H2
2CO 4C þ CO2







DH298 þ 75 kJmolÀ1 ;



(11)





DH298 À 172 kJmolÀ1 ;

CO þ H2 4C þ H2 O





(12)



DH298 À 131 kJmolÀ1 ;

(13)

Appropriate catalysts are required in the TRM reaction in order
to convert CO2 in the presence of H2O and O2. Usually, Ni-based
catalysts is used due to its good activity and selectivity in the
reforming reaction. However, it tends to deactivate due to coke
formation during the reaction. Thus, the development of catalyst is
needed to enhance the performance of the catalyst by the use of
supports with low concentration of Lewis sites and also supports
that can build the strong metal-support interaction.
Pino et al. (2011) had studied the effect of Ni-CeO2 catalyst in the
TRM reaction. The experiments were carried out with a feed contains molar ratio (H2O þ CO2 þO2)/CH4 of 1.02. Fig. 8 indicates the
influence of CO2/H2O molar ratio at constant O2 content. They
discovered that the conversion of CH4 is stable about 93% with the
increasing of the CO2/H2O molar ratio. In addition, the conversion
of CO2 is increasing at high ratio of CO2/H2O ratio which is from 67%
to 86%. Moreover, the H2/CO ratio in the product decreases from 2.8
to 1.3.
Song and Pan (2004) also had investigated the performance of

Fig. 8. Effect of CO2/H2O molar ratio on the performance of the NieCeO2 catalyst in the
tri-reforming of CH4 carried out at 800  C with a feed containing (H2O þ CO2 þO2)/CH4
ratio of 1.02 at constant O2 content (O2/CH4 ¼ 0.1). DHr represents the heat of reaction.
Reproduced with permission from Song and Pan. Copyright 2011 Elsevier.

179

Ni-MgO catalyst in the TRM reaction. They found out that Ni/MgO
improved the conversion of CO2 due to more interfaces between Ni
and MgO resulting from the formation of NiO/MgO solid solution
and the strong interaction of CO2 with MgO.

6. Kinetics and mechanistics of DRM
Kinetic studies are performed to find a suitable reaction rate
model, be it empirical or based on a theoretical reaction mechanism, which gives a best-fit with the corresponding experimental
data and potentially is used to describe the rate of reaction and
define the chemical process (Wang and Lua, 2014). The understanding of this matter can further optimize the catalyst’s design
and chemical systems (reactor design) which can further improve
the overall development of DRM with more cost effective technology (Hoang et al., 2005). Steam reforming has received by far the
largest amount of attention from a mechanistic point of view.
However, since there is a revival of interest in dry reforming over
the past decades, a series of catalysts have been studied for DRM
resulting in a number of mechanistic steps for DRM published in
the literature.
Aldana et al. (2013) investigated the DRM reaction mechanism
over the Ni-based catalyst. Based on Fig. 9, H2 was discovered to
dissociate on Ni0 sites while carbon dioxide was activated on the
ceriae zirconia support to produce carbonates that could be hydrogenated into formate and further into methoxy species. This
mechanism includes weak basic sites of the support for the
adsorption of carbon dioxide and involves a stable metal-support
interface. It can be explained as the much better activity of these
catalysts as compared to Niesilica on which both carbon dioxide
and hydrogen are activated on Ni0 particles. Pan et al. (2014)
findings also support this mechanism.
Akamaru et al. (2014) conducted a DFT analysis of the DRM over
Ru nanoparticle supported on TiO2 (101). Fig. 10 shows the potential

energy diagram. The adsorbed carbon dioxide on each site can
transform into carbon monoxide through different reaction paths
with nearly the same potential energy barriers.

Fig. 9. Reaction mechanism proposed on NieCZ solegel sample for: (a) carbon dioxide
methanation and (b) carbon monoxide formation. Reproduced with permission from
Aldana et al. Copyright 2013 Elsevier.


180

B. Abdullah et al. / Journal of Cleaner Production 162 (2017) 170e185

Fig. 10. Potential energy diagram for carbon dioxide methanation on the Ru surface
slab structure. Each reactant, product and intermediate structures are also shown in
the inset of the figure. Reproduced with permission from Akamaru et al. Copyright
2014 Elsevier.

6.1. Influence of process variables on reaction rates
Extensive investigations have been made to study the effects of
changing the process variables on the catalyst performance for
DRM reaction, as different process variables can result in the
different performance of the catalyst. Activation energy is an

important concept that must be considered as this will determine
the reaction rate. Several activation energy (EA) values of CH4 and
CO2 in DRM reaction obtained over different types of Ni-based
catalysts are considered in Table 4. For most catalysts, the activation energy of CH4 is higher than CO2 since CH4 molecules are more
stable than CO2. Thus, more energy is required to activate the rather
stable molecules (Nagaoka et al., 2000). Moreover, the basicity of

the support used for the catalysts resulted in the variation in the
activation barrier. Kathiraser et al. (2015) believe that the activation
energy in DRM reaction strongly depends on the type of catalyst
support, promoter and bimetallic interactions of the catalyst.
Cui et al. (2007) have carried a thorough study on the mechanism of DRM over Ni/a-Al2O3 using steady-state and transient kinetic methods at temperatures of 550e750  C. They found that the
EA values of CH4 dissociation and CO2 conversion could be split into
low (550e575  C), middle (575e650  C) and high (650e750  C)
temperature regions. At the low and high temperature region, the
reaction was steady but fluctuated in the middle temperature region. It is suggested that at a temperature above 650  C, the CH4
dissociation into CHx and hydrogen species on the Ni active sites
have reached equilibrium state (Cui et al., 2007).
Apart from the activation energy, it is crucial to accurately
formulate the intrinsic kinetic models of the appropriate catalyst
based on elementary steps to achieve a compromise between
economic feasibility and efficiency in the process. However, this
reaction kinetics are influenced by mass transport of reactants. If
the effect of mass transport is eliminated, the observed conversions
can be directly attributed to the intrinsic kinetics of the catalysts.
According to Kathiraser et al. (2015)[5], in order to eliminate
external mass transport resistance, there is a need to test different
gas hourly space velocities (GHSV) to confirm that the conversions
have reached a steady state value, such that a further change in
GHSV does not affect the reactant conversions. Another thing to
consider is the contact time. Contact times play an important role in
CO2 and CH4 conversions. At a high value of contact time, the
conversions of CO2 or CH4 remain unaffected. In order to eliminate
internal mass transport resistance, the particle size of a catalyst
should be as small as possible, such that a further decrease in size
does not affect the conversions (Kathiraser et al., 2015).
Generally, a high GHSV and low catalyst amount with small

particle size can minimize the amount of external and internal

Table 4
EA values over several Ni-based catalysts for DRM reaction.
Catalyst

Reactor type

Nickel supported catalyst
8Ni/Al2O3 (550 e 650 C) Quartz Tube
(ID:4 mm)
8Ni/Al2O3 (650 e 750 C) Quartz Tube
(ID:4 mm)
Micro Fixed Bed
Ni/Al2O3 (400 - 650 C)
(ID: 8mm)
Micro Fixed Bed
4.82Ni/Al2O3 (750 850 C)
Micro Fixed Bed
Ni/TiO2 (400 - 650 C)
(ID:8mm)
7Ni/MgO (550 e 750 C) Micro Fixed Bed
5Ni/MgAl2O4 (600 e
Quartz Fixed Bed
800 C)
(ID:4 mm)
Promoted Nickel supported catalyst
Fixed Bed (ID:
13.5Ni-2K/5MnO-Al2O3
(550 - 800 C)

16mm)
Bimetallic Ni-based catalyst
0.3 Pt-10Ni/Al2O3 (580 e Micro Fixed Bed
620 C)
(ID:6 mm)

Preparation method

Total flow rate
(mL/min)

Cat. amount Cat. particle
(mg)
size (mm)

ECH4 (kJ/ ECO2 (kJ/ Refs.
mol)
mol)

Wet impregnation

360

40

e

31.1

40.5

(Hoang et al., 2005)

Wet impregnation

360

40

e

89.1

88.6

(Hoang et al., 2005)

28

500

e

e

64.4

(Nagaoka et al., 2000)

100-980

e

e

242.67

115.86

(Abreu et al., 2008)

28

500

e

e

59.8

(Nagaoka et al., 2000)

Incipient wetness

Incipient wetness
impregnation
Co-precipitation

e

10

250 e 425

105

99

(Wei & Iglesia, 2004)

30

20

0.0104
e0.0118

26.39

40.43

(Guo et al., 2004)

Impregnation

400

50

200 e 1000

113.8

e

(Pechimuthu et al., 2006)

Sequential
impregnation

100

5

250 e 425

112.55

98.74

€
lu and Erhan
(Ozkara-Aydıno
g
Aksoylu, 2013)


B. Abdullah et al. / Journal of Cleaner Production 162 (2017) 170e185

mass transfer limitations, and oppositely low GHSV is expected to
lead to severe mass transfer limitations. However, there is a study
reported in the literature where high EA values were obtained even
though low mass flow rates (i.e. low GHSV) were utilized, which is
in contrast to the expected low EA values due to mass transfer effect.
Kim et al. (2007) explored the usage of CO2-photoacoustic signal
(PAS) to kinetically analyze the DRM reaction of Ni catalyst supported on Al2O3 and TiO2. This may be the reason for the utilization
of low mass flow rates as this method generates heat periodically
because the photoacoustic signal is created when a material absorbs a modulated laser beam. It is important to note the features of
kinetic curves which function as the fingerprints for reaction
mechanism. These features include the inflection point, a short
induction period or breakpoints (Kim et al., 2007). From all the
findings, no specific GHSV can be concluded to eliminate the effect
of mass transfer limitation. This suggests that preliminary studies
are critical in the development of the intrinsic kinetic models.
6.2. General applicable kinetic models
Three types of models are typically used for DRM reaction: the
power law model, Eley Rideal (ER) model and Langmuir
Hinshelwood-Hougen-Watson (LHHW) model. Of all the models,
many researchers have used power law models to study the reaction mechanism for DRM, perhaps because of their simplicity in
application and parameter estimation. It is useful for initial guess
estimates to solve more complex models which require a larger
amount of data. Power-law models support the kinetic rate for DRM
reaction in the form of:

Â
Ãn
r ¼ k½PCH4 Šm PCO2

(14)

However, Power laws models have a limitation in explaining the
various reaction mechanistic steps that take place on the catalyst
surface over different mechanistic schemes and a wider range of
partial pressure data. Therefore, more rigorous models are used.
Based on the ER model, a kinetic model was developed over Ni/
CeO2-ZrO2 catalyst by Akpan et al. (2007). They assumed that the
rate-determining step (RDS) is the dissociative adsorption of
methane. Their proposed model was validated by fitting the data
obtained from the experiment. The steps of the proposed mechanism, as well as the rate of reaction, are considered in Table 5,
where * and Ox denote the unoccupied active sites and lattice O2 on
their support surface, respectively.
Verykios (2003) conducted a study on Ni/La2O3 catalyst for DRM
reaction using various methods to explain the reaction mechanism
based on the findings from the previous literature. In the literature,
it has been reported that XRD and FTIR detect the formation of
relatively stable La2O2CO3 species from CO2 and the La2O3 support
(Zhang and Verykios, 1996). Another study has indicated that either

Rate of reaction

Mechanism
Adsorption

CH4 þ MðasÞ ƒƒƒ
ƒƒƒ
ƒ!
ƒ MðasÞ À CH3 þ H2

Surface reaction

NiðasÞ À CH4 ƒƒƒƒ!NiðasÞ À C þ 2H2

Adsorption

CO2 þ La2 O3 ƒƒƒ
ƒƒƒ
ƒ!
ƒ La2 O2 CO3

Surface reaction

La2 O2 CO3 þ NiðasÞ À C!La2 O3 þ 2CO þ NiðasÞ

Rate of reaction

RCH4 ¼ K

K1

k2

K3

k4

K1 k2 K3 k4 PCH4 PCO2
1 k2 K3 PCH4 PCO2 þK1 k2 PCH4 þK3 k4 PCO2

the La2O3 support or the La2O2CO3 species is responsible for the CO

formation by acting as an efficient source of oxygen (Tsipouriari and
Verykios, 1999). However, the CO2 dissociation rate on the crystalline Ni was very low. This suggests that the carbon that collected
on the metal surface was mostly derived from methane (Tsipouriari
and Verykios, 2001). From these findings, Verykios (2003) proposed the following mechanism for DRM in the production of
syngas mixture (CO and H2) using Ni/La2O3. Initially, methane was
adsorbed onto the surface of the active metal sites of the Ni catalyst,
followed by carbon deposition and hydrogen formation from the
adsorbed methane cracking. Then, after the reversible methane
adsorption achieves steady state, the carbon deposited on the
active metal site could react with La2O2CO3 species. Thus, the activity of the catalyst is not affected by the methane cracking as the
catalyst is highly stable. The reaction rate was also determined
based on the proposed mechanism, and the kinetic model was
shown to fit well with the kinetic data obtained from experimental
data. The steps of the proposed mechanism, as well as the rate of
reaction, are presented in Table 6.
K1 and K3 are the equilibrium constants for methane adsorption
and the reaction between CO2 and La2O3, as a function of temperature. k2 and k4 are rate constants, as a function of temperature and
their respective activation energies.
A different kinetic model for DRM reaction was proposed by
Quiroga and Luna (2007) with an assumption that the adsorption
and decomposition of methane are followed by non-dissociative
CO2 adsorption on the catalyst support active site. The RDS in this
mechanism is the surface reaction between the adsorbed species.
The steps of the proposed mechanism, as well as the rate of reactions, are considered in Table 7.
Based on the two proposed reaction mechanisms for DRM, the
similarity between the two is that methane adsorption on the
metal surface (including Pt (Topalidis et al., 2007), Ni (Xu et al.,
1999) or Rh (Múnera et al., 2007)) occurred before the methane
cracking, carbon deposition and hydrogen formation. In the
mechanism proposed by Verykios (2003), cracking of methane is

the RDS of the reaction, while Quiroga and Luna (2007) suggest that
the adsorption and methane cracking steps are in steady state.

Mechanism

Mechanism

Surface reaction
Surface reaction
Surface reaction

Table 6
Reaction mechanism of DRM proposed by Verykios (2003).

Table 7
Reaction mechanism of DRM proposed by Quiroga & Luna (2007).

Table 5
Reaction mechanism of DRM proposed by Akpan et al. (2007).

Adsorption

181

CH4þ2*4CH3(*)þH(*)
CH3(*)þ*4CH2(*)þH(*)
CH2(*)þ*4CH(*)þH(*)
CH(*)þ*4C(*)þH(*)
C(*)þOx4CO þ Ox-1þ*
CO2þOx-14Ox þ CO

4H(*)42H2þ4*
H2þOx4Ox-1þH2O

ÀrA ¼

N2 N2
NA À KCp ND
B
1 5
1þ34:3ND2

222800
2:1Â1017 eÀ RT





Adsorption
Adsorption

KCH4

CH4 þ MðasÞ ƒƒƒƒƒƒƒ!
ƒƒƒƒƒƒƒ MðasÞ À CH3 þ H2
KCo2

CO2 þ SupðasÞ ƒƒƒƒƒƒƒ!
ƒƒƒƒƒƒƒ CO2 À SupðasÞ

k1
Surface
MðasÞ À CH3 þ CO2 À SupðasÞ ƒƒƒƒ!2COðgÞ þ 2H2ðgÞ þ MðasÞ þ SupðasÞ
reactiona


Rate of
P 0:5 P 2
P
P
k1 KCH4 KCO2 CH40:5CO2 À H2 b CO2
P
reactionb
H2

RCH4 ¼ 
PCH4
þPCO2 KCO2
P 0:5 KCH4
H2

1þ


a
b

Sup(as) ¼ active site of catalyst support.
Kref ¼ reference constant.

182

B. Abdullah et al. / Journal of Cleaner Production 162 (2017) 170e185

Therefore, the significant contrast in these two mechanisms happens to be the existence of RDS in the proposed steps.
7. Conclusion
Catalytic DRM is a promising technology in future fuel industries
for the production of syngas. As both reactants of DRM (CO and
CO2) are found in the final product, it is very favorable in the
context of the carbon economy. However, the process is impractical
to be used commercially as it poses certain limitations. Thermodynamically, the DRM reaction is favored at very high temperatures
(above 700  C) which are energy consuming. Moreover, the catalysts may also deactivate because of sintering and deposition of
coke from the side reactions of DRM that form carbon. Ni-based
catalysts have been studied widely using different supports to
catalyze the DRM reaction in order to investigate the different interactions between the metal and support and the influence of that
interaction on the catalyst performance. Although they may be
more cost effective compared to noble metals and other transition
metals, Ni-based catalysts tend to deactivate very fast due to the
formation of carbon. However, Ni that is combined with other
transition metals e.g. Co metal gives high activity and very slow
deactivation rate by the formation of carbon.
The three standard kinetic models used for DRM are the power
law model, LHHW model, and ER model. Most of the kinetic models
are based on the reversible dissociative adsorption of CH4 on the
catalyst active site to produce H2, or on reversible and dissociative
adsorption of CO2 on the support to yield CO. The preparation
method and process parameters also play a major role in the
catalyst performance.
8. Outlook

Over the past decades, many investigations have been conducted on DRM over Ni-based catalysts to understand better the
reaction mechanism and ways to improve resistance towards carbon deposition. Several techniques have been suggested to lower
the coke formation tendency of Ni-based catalysts such as: using
suitable catalyst preparation method to prevent coke formation;
employing metal oxides with strong Lewis basicity as supports or
promoters (since Lewis acidity is identified to encourage coke
buildup); addition of a second metal which can improve the
transport of hydrogen and/or oxygen between the active catalyst
and the support by spillover and could alter the mechanism of
carbon deposition; sulfur deposition which can inhibit the coke
buildup initiator by blocking the step edge sites; restricting the
carbon deposition steps by introducing steam to deliver hydrogen
and oxygen to the surface which alters the rate of coke forming
steps.
Future research in this field is likely to concentrate on bimetallic
nickel based catalysts such as incorporated Co with Ni catalyst, as
these bimetallic catalysts have demonstrated stable activity and
high resistance toward deactivation although there is carbon
deposited. Combined reforming reactions should also be considered such as DRM and partial oxidation as the heat released from
partial oxidation can be the supplied heat for DRM as it can minimize the operating cost. Therefore, future research on these combined technologies should also be considered.
The particle size of the catalyst is also important to improve the
catalytic performance, hence the high conversion of reactants and
yield of products would be increased. Further research also can be
focused on improving the metal dispersion by incorporating the
metal particles into high surface area amorphous materials. This is
due to the unique surface chemical properties of the amorphous
structure that could enhance the durability and activity of the

catalysts.
In addition, bio-based materials that are synthesized from

plants also can be used as the catalyst support for DRM reaction.
The unique characteristic of the bio-based support could enhance
catalytic performance in DRM. The bio-support catalyst is also
cheaper than that of the other type of catalyst supports and
abundantly available. Pretreatment and hydrolysis steps are
important in order to extract the cellulose from the plant. Pretreatment step helps to increase the porosity and surface area that
will lead to the increasing of hydrolysis rate. Cellulose can be harvested through acid or enzymes hydrolysis depending on the
conditions at which the amorphous parts are selectively hydrolyzed. It can be considered as a promising catalyst support for DRM
in the future. Due to the cost of pretreatment that is relatively high,
so more intensive research should be directed to this area. Moreover, nanoparticle sizes for the Ni metal also can be synthesized to
enhance the catalytic performance of the DRM reaction. It can
improve the dispersion of Ni metal on the support so that the
catalyst will perform better in the DRM reaction.
The preparation method of the catalyst also plays an important
role in the performance of the catalyst. The suitable and proper
preparation method could give higher Ni dispersion on support,
strong metal-support interaction, high catalytic activity, stability,
and high resistance to carbon formation. The future research of
suitable and proper preparation method of the catalyst should also
be considered to prevent coke formation and also enhance the
catalyst performance for DRM reaction.
Acknowledgement
The authors gratefully acknowledge Universiti Teknologi PETRONAS, Malaysia in providing the necessary facilities to conduct the
work.
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