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

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Recent Advances in Dry Reforming of Methane over Ni-based Catalysts

DOI: 10.1016/j.jclepro.2017.05.176

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Recent advances in dry reforming of methane over Ni-based catalysts

Bawadi Abdullaha,*, Nur Azeanni Abd Ghania, Dai-Viet N Vob

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

Article history:

Received 15 November 2016

Received in revised form

25 May 2017

Accepted 28 May 2017

Available online 29 May 2017

Keywords:

Catalyst development

Methane dry reforming

CO 2 utilization

Greenhouse gases

Catalysis

Coke-resistant catalysts

a b s t r a c t

A steady increase in atmospheric carbon dioxide (CO2) and methane concentrations in recent decades has sparked interest among researchers around the globe toﬁnd quick solutions to this problem One viable option is a utilization of CO2with 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

CO2reforming 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 well-deﬁned structure and superior mechanical strength could enhance the catalytic activity in the DRM reaction

Contents

1 Introduction 171

2 Method 172

3 Reaction thermodynamics 172

4 Ni-based catalysts for DRM 175

4.1 Catalyst support 175

4.2 Promoter 176

4.3 Bimetallic catalysts 177

4.4 Novel catalytic material 177

4.5 Recently developed catalysts for CO2reforming 177

5 Other technologies of CO2reforming of methane 178

5.1 Steam-CO2dual reforming of methane 178

5.2 Tri-reforming of methane 178

6 Kinetics and mechanistics of DRM 179

6.1 Influence of process variables on reaction rates 180

6.2 General applicable kinetic models 181

7 Conclusion 182

* Corresponding author.

E-mail address: bawadi_abdullah@utp.edu.my (B Abdullah).

Contents lists available atScienceDirect

Journal of Cleaner Production

j o u r n a l h o m e p a g e :w w w e l s e v i e r c o m / lo c a t e / j c l e p r o

http://dx.doi.org/10.1016/j.jclepro.2017.05.176

Journal of Cleaner Production 162 (2017) 170e185

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8 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

genera-tion of anthropogenic greenhouse gases Methane and CO2are 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)

Tradi-tionally, 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

pro-duced 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,

car-bon 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

de-pendency on crude oil and its undesirable inﬂuence on the

atmo-sphere, renewable energy is needed immediately for substituting

petroleum-based resources (Fayaz et al., 2016)

In order to reduce the amount of methane and CO2in the

at-mosphere, extensive research has been conducted toﬁnd effective

ways to convert methane and CO2into 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 ener-getics 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: CH4þ H2O/CO þ 3H2 DH298K¼ þ228 kJ=mol

(1) POM: CH4þ 1=2O2/CO þ 2H2 DH298K ¼ 22:6 kJ=mol

(2) DRM: CH4þ CO2/2CO þ 2H2 DH298K¼ 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/CH4ratio is required to attain a higher yield of H2which makes SRM process energetically unfavorable and accelerates catalysts deactivation (Carvalho et al., 2009) Moreover, SRM faces corrosion issues and requires a desulphurization unit (Djinovic et al., 2012) In the case of POM, this process is suitable for the production of heavier hydrocarbons and naphtha (Larimi and Alavi, 2012) Typi-cally, the POM process has very short residence time, high con-version rates and high selectivity (Ruckenstein and Hang Hu, 1999) However, the exothermic nature of the reaction causes the induc-tion of hot spots on the catalyst and makes the operainduc-tion difﬁcult to control (Asencios and Assaf, 2013) Besides, POM requires a cryo-genic unit to separate oxygen from the air (Djinovic 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 complicated gas separation of end products (San-Jose-Alonso et al.,

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 1shows the comparison between three processes in the CO2reforming 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

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

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2 Method

A previous review article of CO2reforming of methane was done

byWang 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 CO2reforming 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

deacti-vation that could lead to the low conversion of reactants Hence, it

is timely to provide a comprehensive review on the CO2reforming

of methane over Ni-based catalysts The literature was selected

based on current developments in the CO2reforming 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

re-action (cf Rxn 1 inTable 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

800C in the investigated temperature ranges from 100 to 900C 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 inTable 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 reverse-water-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/CO2feed ratio and the operating temperature and pressure, including the for-mation 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 dispropor-tionation 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))

CO2reforming 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 1

Comparison between the methods in DRM reaction.

Type of Reaction Steam Reforming of Methane (SRM) Partial Oxidation of Methane (POM) Dry Reforming of Methane (DRM) Advantages 1 High efﬁciency 2 High conversion of reactants

3 High selectivity of syngas

4 Short residence time

1 A technology that utilized two most abundant greenhouse gasses which are the CO 2 and CH 4

2 A clean and environmentally friendly fuel that is formed

Disadvantages 1 Requires high energy and very costly.

2 Requires high CO 2 /CH 4 ratio for greater yield of syngas cause the SRM reaction energetically unfavorable and lead to catalyst deactivation.

3 Complex system

4 Sensitive to natural gas qualities

1 Induction of hot spots on catalyst might occur due to the exothermic nature of reaction

2 Costly technology because it requires cryogenic unit to separate oxygen from the air

1 Carbon formation and sintering of catalyst

H 2 /CO ratio H 2 /CO ratio ¼ 3:1 H 2 /CO ratio ¼ 2:1 H 2 /CO ratio ¼ 1:1

Commercial Plant Topsoe Package Hydrogen Plants at Air Liquide, Belgium;

Plants in USA

Operating

Temperature,

Pressure and

Ratio

Temperature: 700 C to 1,000 C Pressure: 3e25 bar pressure Ratio: CH 4 /H 2 O ¼ 1:1

Temperature: 950 C to 1100 C Pressure: 100 bar pressure Ratio: CH 4 /O 2 ¼ 2:1

Temperature: 650 C to 850 C Pressure: 1 bar

Ratio: CH 4 /CO 2 ¼ 1:1

Table 2

Reactions in dry (CO 2 ) reforming of methane.

Main reaction

Side reaction that leads to the decrease in H 2 /CO ratio to <1

Side reactions that lead to formation of coke (carbon)

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transmission electron microscopy inFig 1 The pyrolytic carbon (cf.

Fig 1a) is usually formed due to the exposure of higher

hydrocar-bons 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

highﬁnal boiling point and low temperatures of the hydrocarbon mixture increase the rate of encapsulating carbon formation (Sehested, 2006) As shown inFig 1b, encapsulating carbon con-tains a thin CHxﬁlm covering the Ni particles that can lead to the catalyst deactivation Generally, encapsulating carbon occurs at temperatures below 500C (Bartholomew, 1984)

Theﬁnal type of carbon formation is whisker carbon, the most critical type of carbon formation in the DRM reaction The forma-tion of whisker carbon occurs when hydrocarbon or CO reacforma-tion 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 inFig 1c (Sehested,

2006) This type of carbon formation leads to the breakdown of catalyst, an increase in the pressure drop and signiﬁcant deactiva-tion of the Ni surface Whisker carbon is usually formed at tem-peratures above 450C (Bartholomew, 1984)

The effect of hydrogen and water in DRM was studied by Delgado et al (2015) The operating temperatures were set be-tween 100 and 900C (373 and 1173 K) at atmospheric pressure, and the inlet mixture was 1.6% CH4, 2.1% CO2,and 1.8% H2in N2

dilution FromFig 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

Fig 1 Electron microscopy images (Philips CM200 FEG TEM) of pyrolytic carbon on a

MgAl 2 O 4 carrier (A), encapsulating carbon (B), and whisker carbon (C) on Ni/MgAl 2 O 4

Elsevier.

(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 H 2 : (a) CH 4 and CO 2 ; (b) H 2 O, CO, H 2 , inlet gas composition of 1.6 vol.% CH 4 , 2.1 vol.% CO 2 , 1.8 vol.% H 2 in N 2 ; 1 bar; T inlet 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 H 2

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maximum water concentration at 400C (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

re-action temperatures between 100 and 300C (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/CH4feed ratio is higher than unity, carbon is normally formed

Less H2 available for hydrogenation reactions inRxns (5 and 6),

resulted in a decrease in carbon formation Based on the

thermo-dynamics calculations, temperatures higher than 900 C are

required for CO2/CH4feed 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 870C at 1 atm and CO2/CH4feed ratio of unity

On the other hand,Fig 3shows the H2/CO ratios produced from the DRM reaction at different temperatures with the pressure of

1 atm Based onFig 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 800C For instance, a H2/CO ratio of 1:1 that can be useful for F-T synthesis can be obtained at temperatures above 850C for CO2/CH4feed ratio being unity Fig 3b represents the inﬂuence of CO2/CH4feed ratios at 1 atm

to the product yields and H2/CO molar ratio in the end product Based on the study byHassani Rad et al (2016), the CO2increased, and the CH4decreased 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 CH4and 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 H 2 /CO molar ratio in product over NAC-I nanocatalyst (b) Effect of CH 4 /CO 2 ratio in feed on feed conversion,

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with constant temperature and inlet feed ratio FromFig 4a, CO2,

and CH4conversions are higher at atmospheric pressure than those

at higher pressures InFig 4b, carbon deposition signiﬁcantly

in-creases with increasing pressure Theseﬁndings 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 toNikoo and Amin (2011), CO2and CH4conversions

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 CO2reforming of

methane tends to shift to the reactant side

Fig 5illustrates the equilibrium constants of all possible

re-actions, presented as a function of time Based on the second law of

thermodynamics, the CO2reforming of methane is spontaneous if

the Gibbs free energy change of reaction (DGr) is negative while the

reaction is thermodynamically limited if theDGris positive For

each reaction temperature, the Gibbs free energy change of the

reaction (DGr) was calculated by Eq.(4):

DGr¼X i

wheregirepresents 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):

K¼ expDGr

RT

(5) The equilibrium constant (K) deﬁnes the extent to which the reaction occurs Based on Fig 5, DRM (Rxn (1)) is a thermody-namically favorable reaction that produces syngas at temperatures above 727C

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 re-action 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 coke-resistant 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 develop-ment 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 Ni-based catalysts toﬁnd the best way to improve the coke resistance

of Ni-based catalysts Moreover, recent attempts to improve cata-lytic 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 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 com-pounds 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 sup-ports 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’ ac-tivity The study was conducted using Ni/Al2O3, Ni/MgO, Ni/TiO2, Ni/SiO2, Ni/ZrO2, Ni/La2O3-ZrO2and Ni supported on mixed-metal oxides (Ni/Siral 10 and Ni/PuralMG30) at low temperature

Fig 4 The effect of pressure on a) equilibrium conversion of reactants and products

distribution for CO 2 /CH 4 ¼ 1, 1173 K and n 0

(CH4 þ CO2) ¼ 2 mol and on b) carbon deposition as a function of temperature; Reproduced with permission from Nikoo and

Fig 5 Equilibrium constants of reactions involving in CH 4 eCO 2 reaction at different

temperatures and atmospheric pressure; Reproduced with permission from Nikoo and

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(400C) (cf.Fig 6) From the experiment, catalysts that contain Zr

within the support showed the highest initial activities Ni/La2O3

-ZrO2 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

How-ever, it is remarkable for Ni/MgO to achieve an initial H2yield of

2.5% considering that the catalyst had low surface area In general,

the activity of the catalysts (based on H2yield) 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-ZrO2had the highest stability with only 9% loss of H2yield

from the initial state The least stability of catalyst with the loss of

20% of H2yield was Ni/ZrO2and 89% of H2yields 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 byGuo et al (2004)found that Ni/MgO-g-Al2O3

and Ni/MgAl2O4catalysts 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 tog-Al2O3, was attributed to the

characteris-tics of MgAl2O4, which has high resistance to sintering and has low

acidity The interactions between Ni and MgAl2O4produce 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-Al2O3to Ni/

MgOeAl2O3and Ni/MgAl2O4byGuo 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 byFauteux-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

pre-activation step, and no depre-activation was detected even at low H2O/C

molar ratio (1.9) and temperature below 760 C in diesel steam

reforming

4.2 Promoter 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 perfor-mance of the catalyst.Dias and Assaf (2003)discovered that sin-tering 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 inter-action aids in the formation of reducible Ni species The concen-tration of Ca affects the conversion of CO2and CH4in DRM reaction For instance, lower concentrations of Ca formed ionic oxides strongly and increased the conversion of CO2 At lower concentra-tion of Ca, the CO2is 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 CH4and CO2conversions (Dias and Assaf, 2003)

Besides promoting with CaO, the addition of potassium (K) as a promoter to Ni-based catalysts was also reported byJuan-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 sameﬁndings 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 CH4because 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 byChen et al (2004) They used both CO and CH4activities 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 CO2and CH4cracking 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

C-H bonds to CC-Hx 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 byValentini et al (2003)has shown that va-nadium promoted on alumina supported nickel catalyst gave a high conversion of CH4by 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/Al2O3catalyst (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 cata-lytic system The promoter weight percentage is important as it leads to the signiﬁcantly improved results of the reaction The

Fig 6 CO and H 2 yields after ﬁrst 10 h (black bars) and 100 h (gray bars) on DRM

stream at 400C and GHSV of 7200 mL h1g cat 1 ; Reproduced with permission from

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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

(X¼ 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 byZhang et al (2008), supported bimetallic

catalysts demonstrate high activity and stable DRM reaction

per-formances In an experiment to test the stability, bimetallic Ni-Co

catalyst supported on Al2O3eMgO, which was prepared by

co-precipitation 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

bime-tallic 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 in

ﬂu-ence 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

modi-fying 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

sin-tering of metal particles and strengthening the metal-support

interaction (Xu et al., 2011) This is due to the high speciﬁc

sur-face 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

be-tween 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-Al2O3and meso-ZrO2have demonstrated high

cata-lytic 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

produceﬁne chemicals Cellulose is biodegradable but possesses a

unique property as it provides a well-deﬁned structure, high

crys-talline 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

mem-brane (PEM) fuel cell electrodes Pretreatment and hydrolysis are

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, con-ventional methods of catalyst preparation Another less common method for catalyst preparation is sol-gel The sol-gel method produces aﬁne size distribution, decreases the deactivation rate, imparts high thermal resistance against agglomeration and pro-duces 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 distri-bution of Ni particles and enhance the activity and stability of the catalyst (Rahemi et al., 2013) However, plasma treatment is rela-tively expensive compared with other simple preparation methods (Usman et al., 2015) Thus, the combination of novel catalytic ma-terial and method would enhance the activity and stability of the catalyst in DRM reaction

4.5 Recently developed catalysts for CO2reforming The progress on developing catalysts for DRM reaction has been concentrated onﬁnding a new formulation of catalyst that can give higher activity and higher stability towards coke formation, sin-tering, 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 pro-moters 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 3shows 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 Reac-tion temperatures range from 600 to 800C, space velocities from

8000 to 60,000 mL/g.h Feed ratio of CH/CO is 1 in all cases Ni/

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Mg(Al)O catalyst prepared by co-precipitation method with

oper-ating temperature of 800C gave the highest conversion of CH4/

CO2 The conversion of CH4was 95% and CO2was 98% The 5%Ni/

ZrO2eC catalyst prepared by impregnation method with operating

temperature of 600C 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

interac-tion which is critical for the preveninterac-tion of coke formainterac-tion (Guo

et al., 2015) It can be concluded that catalyst composition,

prepa-ration method as well as operating temperature all impact greatly

on conversion

5 Other technologies of CO2reforming of methane

5.1 Steam-CO2dual reforming of methane

The steam-CO2dual 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/CO2in 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/ZrO2catalyst in the steam-CO2dual reforming of

methane in comparison with the classical Ni/ZrO2catalyst.Fig 7(a)

and (b) shows that the LA-Ni/ZrO2catalyst exhibits higher initial

catalyst activity which are CH4conversion is 94% and CO2

conver-sion is 95%.Fig 7(c) and (d) shows that both catalysts, LA-Ni/ZrO2

and Ni/ZrO2have the similar selectivity for H2and 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

5.2 Tri-reforming of methane

Recently, tri-reforming of methane (TRM) also has received

attention due to its ability to convert the CO2and 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):

CH4 þ H2O/CO þ 3H2

DH298 þ 206 kJmol1

CH4 þ O2

2 /CO þ 2H2

DH298 36 kJmol1

CH4 þ CO2/2CO þ 2H2

DH298 þ 247 kJmol1; (8)

CO þ H2O/CO2 þ H2 DH298 41 kJmol1

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

CH4 þ 2O2/CO2 þ 2H2O

DH298 þ 206 kJmol1

; (10)

CO is utilized in the methane dry reforming reaction during the

Table 3

Recently developed catalysts for the DRM reaction.

Catalyst Preparation method GHSV (mL/g

h)

CH 4 /CO 2 feed ratio

T (C)

Conv CH 4

(%)

Conv CO 2

(%)

H 2 /CO ratio Coke formation (wt%)

Refs.

15%Ni/ZrO 2 Combined co-precipitation and reﬂux

digestion

24,000 1 700 >85 >88 z1 NA ( Zhang et al., 2015 )

82.82

80 e 90 0.85

e0.90

NA ( Yu et al., 2015 )

e0.60

14 ( Mustu et al., 2015 )

10%Ni-7%CeO 2 /

MgO

2015 ) 2.33%Ni-4.66%Co/

ZSM5

2015 ) 1.2%Ni-1.8%Co/

CeZr

2015 ) 15%NiCeMgAl Co-precipitation 48,000 1.04 800 z98 z82.5 z0.79 NA ( Bao et al., 2015 )

3%(CoNi)/SiC-CeZrO 2

Fig 7 (a) CH 4 conversion, (b) CO 2 conversion, (c) H 2 selectivity, and (d) CO selectivity

as a function of time for stream for steamCO 2 dual reforming of methane over the developed LA-Ni/ZrO 2 catalyst (error bars equal 95% conﬁdence interval for conver-sion) Reaction conditions: mcat ¼ 50 mg, CH 4 /CO 2 /H 2 O ¼ 1:0.8:0.4, GHSV ¼ 48 000

mL h1g1, and performed at atmospheric pressure Reproduced with permission from

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