Tuning combined steam and dry reforming of methane for “metgas” production: A thermodynamic approach and state-of-the-art catalysts

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Tuning combined steam and dry reforming of methane for “metgas” production: A thermodynamic approach and state-of-the-art catalysts Karam Jabbour PII: DOI: Reference:

S2095-4956(19)30938-6 https://doi.org/10.1016/j.jechem.2019.12.017 JECHEM 1043

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Journal of Energy Chemistry

Received date: Revised date: Accepted date:

29 September 2019 13 December 2019 16 December 2019

Please cite this article as: Karam Jabbour , Tuning combined steam and dry reforming of methane for “metgas” production: A thermodynamic approach and state-of-the-art catalysts, Journal of Energy Chemistry (2020), doi: https://doi.org/10.1016/j.jechem.2019.12.017

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Tuning combined steam and dry reforming of methane for “metgas” production: A thermodynamic approach and state-of-the-art catalysts Karam Jabbour * Department of Chemical Engineering, Faculty of Engineering, University of Balamand, POBox 100, Tripoli, Lebanon * Corresponding author. E-mail address: [email protected] (K. Jabbour)

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ABSTRACT Nowadays, combined steam and dry reforming of methane (CSDRM) is viewed as a new alternative for the production of high-quality syngas (termed as “metgas”, H2:CO of 2.0) suitable for subsequent synthesis of methanol, considered as a promising renewable energy vector to substitute fossil fuel resources. Adequate operation conditions (molar feed composition, temperature and pressure) are required for the sole production of “metgas” while achieving high CH4, CO2 and H2O conversion levels. In this work, thermodynamic equilibrium analysis of CSDRM has been performed using Gibbs free energy minimization where; i) the effect of temperature (range: 200–1000 °C), ii) feed composition (stoichiometric ratio as compared to a feed under excess steam or excess carbon dioxide), iii) pressure (range: 1–20 bar) and, iv) the presence of a gaseous diluent on coke yields, reactivity levels and selectivity towards “metgas” were investigated. Running CSDRM at a temperature of at least 800 °C, a pressure of 1 bar and under a feed composition where CO2+H2O/CH4 is around 1.0, are optimum conditions for the theoretical production of “metgas” while minimizing C(s) formation for longer experimental catalytic runs. A second part of this work presents a review of the recent progresses in the design of (principally) Ni-based catalysts along with some mechanistic and kinetic modeling aspects for the targeted CSDRM reaction. As compared to noble metals, their high availability, low cost and good intrinsic activity levels are main reasons for increasing research dedications in understanding deactivation potentials and providing amelioration strategies for further development. Deactivation causes and main orientations towards designing deactivation-resistant supported Ni nanoparticles are clearly addressed and analyzed. Reported procedures based on salient catalytic features (i.e., acidity/basicity character, redox properties, oxygen mobility, metal-support interaction) and recently employed innovative tactics (such as confinement within mesoporous systems, stabilization through core shell structures or on carbide surfaces) are highlighted and their impact on Ni0 reactivity and stability are discussed. The final aspect of this review encloses the major directions and trends for improving synthesis/preparation designs of Nibased catalysts for the sake of upgrading their usage into industrially oriented combined reforming operations.

Keywords: Combined steam and dry reforming of methane; Thermodynamic equilibrium analysis; “Metgas” production; Nickel-based catalysts; Heterogeneous catalysis; Structure-activity relationship

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Dr. Karam Jabbour obtained his Bachelor degree in Chemical Engineering in 2011 and his Msc degree in Petroleum Engineering in 2013 from the University of Balamand (UOB, Lebanon). After, he obtained a scholarship from the Agence Universitaire de la Francophonie (AUF) to pursue his doctorate studies between the Sorbonne University (Paris) and UOB. Three years later, he obtained his Ph.D. degree in Chemical Engineering, specialization in Materials Science and Catalysis. He joined the department of Chemical Engineering at UOB as a Researcher at the end of 2017. His research interests are heterogeneous catalysis particularly the design and in depth characterization of porous-based catalysts for subsequent application in gaseous conversion processes aiming for renewable energy production. He is actively engaged in several European collaborations dealing with waste to energy conversion processes.

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1. INTRODUCTION Apart from its energy carrying properties and its sustainable need as a basic building block in the C1 chemistry, methanol (MeOH) stands as one of the most promising renewable energy vectors to substitute fossil fuels [1]. Currently, it is applied in the direct synthesis of various chemical intermediates such as formaldehyde, methyl tertiary-butyl ether (MTBE), dimethyl terephthalate (MTP), acetaldehyde and, acetic acid [2–8]. Nowadays, more than 80% of the worldwide need of MeOH is supplied from the catalytic conversion of synthesis gas (syngas, gaseous mixtures of H2 and CO) based on methane-rich feed streams [9]. Indeed, CH4 emitted from natural gas reserves and biomass decomposition processes is being extensively used in power generation systems as well as in gas to liquid (GTL) processes (i.e., the conversion of syngas to MeOH) for production of useful chemical compounds from versatile syngas mixtures [10,11]. Generally, there are two steps involved in GTL procedures: the first one is syngas generation following a catalytically assisted CH4 reforming path via either steam reforming of methane (SRM, Eq. (1)) [12], dry reforming of methane (DRM, Eq. (2)) [13], partial oxidation of methane (POM, Eq. (3)) [14] or autothermal reforming (ATR, Eq. (4)) [15]. The second stage involves the chemical transformation of syngas, via for instance the Fisher-Tropsch (FT) process, into an extensive range of chemical and petrochemical derivatives. Steam reforming

CH4 + H2O

CO + 3H2

H2:CO=3

H0 (298 K) = 206 kJ mol-1

(1)

Dry reforming

CH4 + CO2

2CO + 2H2

H2:CO=1

H0 (298 K) = 247 kJ mol-1

(2)

Partial oxidation

CH4 +1/2O2

CO + 2H2

H2:CO=2

H0 (298 K) = -38 kJ mol-1

(3)

Autothermal reforming

CH4 + 1/3O2 + 1/3H2O

H2:CO=2.34

H0 (298 K) = 45.6 kJ mol-1

(4)

CO + 7/3H2

The molar ratio of hydrogen to carbon monoxide in syngas is around three for SRM (Eq. (1)) and close to one for DRM (Eq. (2)). However, a stoichiometric H2:CO product ratio close to 2.0, called “metgas” [1619], is preferred when syngas is to be applied directly for methanol production targeting high conversion rate values. Moreover, such a ratio is also mandatory in some GTL operations ((2n+1)H2 + nCO CnH(2n+2) + nH2O) aiming the preparation of long hydrocarbon chains [20,21]. The POM with oxygen (Eq. (3)) can produce the H2:CO ratio with a value of 2.0 but, the process is difficult to control due to the formation of local hot spots inside the reformer with associated risk of explosions [22]. On the other hand, the ATR of methane (Eq. (4)), as applied industrially, is capable of producing the desired ratio after further separation and adjustment steps, adding to the overall cost and complexity of the process [18,23]. In fact, the ATR of methane is an equilibrium limited reaction (even in case of complete fuel conversion) and produces a hydrogen-rich gaseous mixture containing carbon oxides and other by-products. Therefore, in order to produce selective “metgas” mixtures, ATR processes are carried out in high

5 temperature reformers coupled to high and low temperature shift reactors followed by pressure swing adsorption towers [23]. The ATR of methane suffers from technical drawbacks associated with (i) the generation of hot spots inside the reactor (as for POM due to the exothermic nature of both processes) [24], (ii) the excessive production of solid carbonaceous deposits on the catalyst as well as inside the flow lines of the reformer [25] and, (iii) fuel evaporation and their inappropriate mixing [26,27]. In a typical ATR process, the temperature profile of catalytic bed rises to a maximum owing to the reaction of methane with O2 then cools down because in the course of the endothermic methane steam reforming reaction. The non-uniform axial temperature distribution could cause some evaporation of fuels to their superheated states (under exothermic oxidation conditions) followed by their re-condensation (under endothermic conditions) resulting in zones poorly mixed and non-homogeneously distributed inside the reformer [27]. In contrast to these conventional routes, the Noble prizewinner George A. Olah [28] developed a technology for the direct production of “metgas” in a single step process, without the need of additional separation units or membrane reactors. The basis of this technique is a combination of SRM (Eq. (1)) and DRM (Eq. (2)) termed bi-reforming or combined steam and dry reforming of methane (CSDRM), beginning with an initial stoichiometric CH4:CO2:H2O molar ratio of 3:1:2 (Eq. (5)). Steam reforming

2x (CH4 + H2O

CO + 3H2)

H2:CO=3

H0 (298 K) = 206 kJ mol-1

(1)

Dry reforming

1x (CH4 + CO2

2CO + 2H2)

H2:CO=1

H0 (298 K) = 247 kJ mol-1

(2)

Combined steam and dry reforming

3CH4 + CO2 + 2H2O

H2:CO=2

H0 (298 K) = 659 kJ mol-1

4CO + 8H2

(1)+(2)=(5)

Adding to the energetic advantages of CSDRM, this reaction consumes major greenhouse gases reflecting simultaneously a positive environmental impact. Indeed, such gases are water vapor (excluding clouds) with an abundance range between 36%–70%, CO2 ranging between 9%–26%, CH4 occupying a percentage array of 4%–9% and ozone (O3) accounting for the remaining 3%–7% [29]. Therefore, interests in the exploitation of such detrimental gases via CSDRM for the generation of added-value chemicals is a necessity among both industrial and scientific communities. Moreover, the CSDRM technology could be considered as a basis for upgrading the calorific value of biogas (composed of CH4, CO2 and H2O) via solar reforming as one of the most advantageous “solar thermochemical processes” [30,31]. However, the industrial implementation of CSDRM suffers drawbacks owing to the high endothermic nature of the reaction (Eq. (5)) necessitating complex conditions of steam and high temperature in order to reach desirable conversion levels. These considerably severe practices result in rapid and drastic catalytic deactivation by i) thermal agglomeration of metallic sites, ii) coke deposition and/or iii) potential re-oxidation of reduced nanoparticles under reaction medium [32–79]. Solid carbon deposited on catalytic

6 active sites can also cause serious reactor blockage and accumulation in production lines. Such carbonaceous species could originate from either CO disproportionation (Eq. (6)), CH4 decomposition (Eq. (7)), hydrogenation of carbon monoxide (Eq. (8)) and/or hydrogenation of carbon dioxide (Eq. (9)). The thermodynamic extent of such coke formation reactions vary strongly according to reaction

CO disproportionation

2CO

CO2 + C(s)

H0 (298 K) = -172 kJ mol-1

(6)

CH4 decomposition

CH4

2H2 + C(s)

H0 (298 K) = 75 kJ mol-1

(7)

CO hydrogenation

CO + H2

H0 (298 K) = -131 kJ mol-1

(8)

CO2 hydrogenation

CO2 + 2H2

H0 (298 K) = -90 kJ mol-1

(9)

C(s) + H2O C(s) + 2H2O

conditions (i.e., operating temperature and inlet feed composition) [80–87]. An additional shortcoming of CSDRM is the selectivity towards the sole production of “metgas”, an aspect vaguely discussed in the available literature. This feature is significantly important since the purpose of CSDRM is the synthesis of “metgas” as an intermediate feed for MeOH generation [3-8,1618]. An alteration from the production of “metgas” via CSDRM yields H2:CO ratios with wide distributions (range: one to values exceeding 10) [80–83] depending on the inlet feed composition and its deviation from the stoichiometric ratio (Eq. (5)). Moreover, the lack in selectivity towards “metgas” could originate from non-coke formation reactions, occurring concurrently along reforming reactions, consuming either CO or H2 products via respectively the water gas shift and the reverse water gas shift side-reactions (Eqs. (10) and (11)). Water gas shift Reverse water gas shift

CO + H2O CO2 + H2

CO2 + H2 CO + H2O

H0 (298 K) = - 41.1 kJ mol-1

(10)

H0 (298 K) = 46.1 kJ mol-1

(11)

In view of that, in-depth analysis of thermodynamic simulations associated to CSDRM must receive a fair amount of attention and should provide a basis for experimental and computational investigations for possible scaling-up operations upon understanding the various pathways linked to the reaction. In this context, thermodynamic equilibrium studies using Gibbs free minimization for dry reforming of methane [84,85] and others combining multiple reforming types have been broadly reported in literature [81– 83,86–88]. It was shown that the addition of steam (to an equimolar CH4:CO2 feed) has several practical advantages over CH4 reforming with CO2 alone. The formation of C(s) is reduced due to oxidative desorption of carbonaceous precursors (partially hydrogenated CHx compounds) and (ii) syngas products with versatile H2:CO ratios are obtained via adjusting CO2 and H2O concentrations [81-83,86]. Jafarbegloo et al. [82] provided comparative thermodynamic to experimental results emphasizing

7 furthermore on the positive effect of steam on coke reduction and on the impact of CO2:H2O molar ratio on products (H2 and CO gases) distribution under conventional DRM conditions (molar ratio CH4:CO2 = 1:1). Yet, excess steam rises serious cost and practicability concerns requiring controlled amending of inlet feed composition depending on the subsequent application of syngas [89,90]. On the other hand, thermodynamic studies of DRM merged with steam and oxygen have been lately conducted to evaluate the potential of overcoming the steam surplus limitation by integrating it with other commercial reforming technologies involving co-presence of supplementary oxidants (air and oxygen) as tools against carbon deposition and its accumulation [26,85–87,91]. Nevertheless, using pure oxygen at high reaction temperature (range: 700–900 °C) requires extensive safety precautions and the need of air separation units (ASU) which significantly influences the cost of the syngas plant [48]. Compared to methane trireforming (CSDRM in presence of O2), an economic and feasibility assessment conducted by Chen and coworkers [91] demonstrated promising benefits for combining dry and steam reforming in a single unit. Such benefits covered total capital investment, heating and, electricity costs. Moreover, when CSDRM was proposed to occur in a parallel-series system, a design suggested by Yang et al. [89] and developed on Aspen Plus V8.4®, improved technological, economic and environmental metrics are expected compared to previous reforming assessment studies. Moreover, the CSDRM process is considered as an alternative to the commercially available SRM technology owing to its lower global warming footprint [91,92]. Nonetheless, thermodynamic studies about precise prerequisites for efficient “metgas” producing-operations are scarcely reported and (the existing few ones) are lacking systematic and deep interpretation of preferable reaction conditions for maximum conversion and satisfying selectivity results. In fact, reported thermodynamic studies are either assumed under carbon-free operations (neglecting a practical aspect of the process) [92,93] or simulated for diverse feed compositions yet not those particularly interesting for the sole production of “metgas” [30,81–83,86–87]. As recently mentioned in a study by Jang et al. [90], the main purpose of conducting thermodynamic CSDRM simulations is finding the optimum conditions that will i) suppress C(s) deposition, ii) provide high conversion yields while iii) maintaining a selective production of a H2:CO ratio of 2.0 without increasing operating costs (preventing excess steam). Besides thermodynamic considerations, much effort has been devoted towards the preparation of catalytic systems with features allowing them to operate steadily on stream under drastic CSDRM operations. The main direction of catalyst development is centered on the use of nickel as active site rather than noble metals owing to its good intrinsic activity levels, lower cost and, wider availability [32–74,77]. However, the stability and selectivity of supported Ni-based catalysts remain critical concerns and reports addressing deactivation causes and corresponding improvement tactics are still under vigorous investigation. Thus, catalyst development is becoming a key aspect of research in this area. Compared to

8 DRM and SRM, bibliographic reports on the CSDRM catalysts are less numerous due to the high complexity of this reaction occurring at elevated temperature and in steam-rich medium. Based on this state-of-the-art, the first part of this review covers salient thermodynamic features modulated to provide key elements for optimal “metgas” production. The effect of several parameters on i) conversion levels of CH4, CO2 and H2O, ii) on H2 and CO yields and, iii) on the variation in the production profiles of H2:CO ratios are investigated to the fullest. Theoretical simulations include: the effect of i) temperature (range: 200–1000 °C), ii) feed composition (CH4:CO2:H2O equal to 3:1:2, 3:2:2 and, 3:1:4), iii) pressure (range: 1–20 bar), iv) solid C(s) deposits (comparison between C(s)-free and C(s)assisted operations) and, v) the presence of a diluent gas (chemically inert). Analysis is performed for each parameter independently and when several co-exist together. The stoichiometric ratio (CH4:CO2:H2O equal to 3:1:2, Eq. (5)) is used as reference in order to compare the effect of excess carbon dioxide and that of water (by doubling their stoichiometric molar contents), based on their potentiality to provide oxidative properties during DRM [81–83,86–88]. Compared to the study of Jang et al. [90]; the wider temperature range and the versatility in the (CO2+H2O)/CH4 molar ratio (ranging from 1 to 1.7/CO2:H2O ratio varying from 0.25 to 1) considered here offer practical guidelines for the estimation of suitable reaction condition especially if model biogas mixtures are potential feed streams for “metgas” generation. Besides the influence of solid carbon deposition and that of pressure, lots of CSDRM studies are conducted in presence of a diluent such as He, N2 (or Ar) for practical and security purposes [32– 35,37–38,47,50–58,62–63,65,67–76]. The presence of diluent does not modify activation mechanisms but improves catalytic performances in methane reforming reactions [94]. Consequently, it becomes necessary to understand and point out associated theoretical limitations for accurate comparison of catalytic performances with thermodynamics. The goal of the second part of this review is to illuminate recent innovative developments in the synthesis of suitable catalysts for CSDRM. A critical examination of several emerging technologies and measures adopted to stabilize catalysts along with corresponding data displaying activity and stability levels are reported. Additionally, available literature dealing with reaction mechanism and kinetic modeling are reviewed. Finally, future trends and outlooks regarding the design of efficient catalytic systems are addressed. As far as I know, such approach combining (i) thermodynamic interpretation of simulated CH4:CO2:H2O mixtures for the sole production of “metgas” under practical conditions and (ii) critical assessment of the available literature dealing with the catalysis of the CSDRM process was not yet considered.

2. SIMULATION METHODOLOGY

9 Thermodynamic analysis consists on the determination of phases and equilibrium compositions of a particular system at specified operating conditions. A minimization of the total Gibbs free energy is viewed as a suitable tool for defining the equilibrium status of any reacting system [95,96]. The system becomes thermodynamically favorable when the value of the total Gibbs free energy is at a minimum and its differential is essentially zero for specified temperature, pressure and, concentration of species, as shown in

(Eq.

(12))

(12)

(d

below:

For a mixture of

species, the total Gibbs free energy of the system is expressed by the sum of the

chemical potential of all components as shown in (Eq. (13)) where:

is the moles of species and

is

(13)

∑ the chemical potential of component

which is

defined by (Eq. (14) below:

(14)

Where: constant,

is the standard Gibbs free energy of the formation of compound , is the system temperature,

is the system fugacity and

is the universal gas

is the standard-state fugacity of

component . Combination of (Eq. (13)) and (Eq. (14)) yields (Eq. (15)) that represents the total Gibbs

(15)

∑ free energy of a single phase of a gas:

∑

10

Here:

can be expressed as

coefficient of component , and

̂ , where

is the molar faction of component , ̂ is the fugacity

is the pressure of the system. Since the standard state is defined as the

pure ideal gas state at 1 atm, the standard fugacity (

) is equal to the standard pressure (

= 1 atm). To

calculate the equilibrium of the gas composition, the right-hand side of the equation ( should be minimized under the constraint of

)

= zero. The Lagrange multiplier is a mathematical

optimization method used to find the minimum of a function subject under some specified constraints. (Eq. (16)) is obtained upon substituting the above variables and using the Lagrange multiplier method where;

th

is the number of atoms of the

(

̂

element in each molecular species and

is the Lagrange

(16)

∑

multiplier [92].

Prior to conducting thermodynamic models, it is mandatory to determine the phases and components of the system. Considered gaseous species are CH4, CO2, H2O, CO, H2 and Ar (to highlight the impact of dilution). To account for potential solid carbon deposits (for C(s)-assisted CSDRM operations), C(s) was introduced separately a solid phase with an initial composition set at zero. The total Gibbs free energy for the solid phase is presented by (Eq. (17)). Since coke is a pure solid phase, its partial fugacity is equal to the standard fugacity (

). The vapor-solid equilibrium should be then applied to consider the

equilibrium of coke formation. Thus, (Eq. (18)) for the solid C(s) phase is obtained by substituting with the above variables where: ̅ is the partial Gibbs free energy of gaseous coke, ̅ is the partial Gibbs free energy of solid coke,

is the molar Gibbs free energy of solid coke and,

is the standard Gibbs

free energy of coke formation. The boundary of the system is defined by (Eq. (19)) where; the total mass of the

th

stands for

element in reactants. Consequently, (Eq. (20)) can be adopted to predict

(17)

∑ ̅

̅

∑ =0

∑ equilibrium compositions for mixed gas-solid systems [97]:

(18) (19)

11

∑

(

(

The equilibrium state at specified

̂

and

(20)

∑

)

is determined by minimizing the Gibbs free energy for a given

set of species without any specification of the possible reactions that will ultimately take place in the system. The presented thermodynamic plots (section 3) are created using the HSC 7.1 Chemistry software (where H, S and C stand for enthalpy, entropy and heat capacity, respectively) which principle is to provide equilibrium simulations based on defined chemical species without specifying the number and nature of involved chemical reactions. The aim and detailed conditions of the performed simulations are grouped in Table 1. First, (CO2+H2O)/CH4 varied from 1 to 1.7 under wither C(s)-assisted or C(s)-free operations. Then, pressure varied from 1 to 10 bar at a fixed CH4:CO2:H2O ratio under both C(s)-free and C(s)-assisted operations (conditions under which pressure range has been extended to cover simulations at 15 and 20 bar). Finally, the effect of having CH4 and CO2 diluted in argon is studied in absence and in presence of carbonaceous deposits. Equilibrium compositions (unconverted reactants, expected H2(g), CO(g) and C(s) products along with H2(g) and CO(g) yields) are defined according to (Eqs. (21)-(27)): (

(21)

(

(22)

(

(23)

(24) (25)

(26)

( (

3. REACTION THERMODYNAMICS

(27)

12 3.1. Effect of the (CO2+H2O)/CH4 molar ratio on reactivity and selectivity levels (C(s)-free and C(s)assisted operations) The effect of temperature and reactant molar ratio on the conversion of methane, carbon dioxide and water along with the produced hydrogen to carbon monoxide ratio are displayed in Fig. 1. For complementary purposes, Table 2 presents respective H2 and CO yields expected under conducted theoretical simulations. The stoichiometric ratio (CH4:CO2:H2O equal to 3:1:2, (Eq. 5)) is used as reference to evaluate the effect of excess CO2 (CH4:CO2:H2O equal to 3:2:2) and that of H2O (CH4:CO2:H2O equal to 3:1:4), respectively. Both SRM (Eq. (1)) and DRM (Eq. (2)) are endothermic reactions and hence, are favored at high temperature. During CSDRM, the contribution of DRM (inducing significant CO2 conversions) becomes mostly dominant at elevated temperatures, exceeding 650 °C, compared to SRM (requiring lower temperatures: 450–600 °C) owing to its higher endothermic character (higher enthalpy change value) [12,13,81]. Thus, the conversion of carbon dioxide is critical in CSDRM forcing thus high temperature requirements (Fig. 1). Indeed, this is revealed by thermodynamic simulations displaying negative CO2 conversion values (insignificant to display) at temperatures of 520 °C, 580 °C and 670 °C for the respective inlet feed ratios of 3:2:2, 3:2:1 and 3:2:4 (Fig. 1a and b), compared to significant CH4 and H2O conversion levels. Negative or no CO2 conversion indicates the suppression of DRM and the dominance of low temperature CO2-producing reactions such as WGS (Eq. (10)) generating CO2 rather than consuming it (a main reagent in the medium). When CSDRM operates under excess steam conditions, (CO2+H2O)/CH4 ratio of 1.7, a delay in the initiation of CO2 conversion is recorded (orange dashed-curve compared to black and violet-dashed curves, Fig. 1a). The presence of excess steam promotes the manifestation of WGS that consumes H2O and produces in return CO2. In contrast to low CO2 consumption, CH4 and H2O conversions are noteworthy within the 200-670 °C zone (XCH4 around 90% and XH2O in the range of 60%, Figs. 1a,b). Based on Figs. 1c and d, the variation in the concentration profile of CO2 is quite different from those of CH4 and H2O (constantly decreasing). Indeed, the concentration profile of CO2 is displaying an increasing trend within some temperature regions (T range: 450–550 °C) followed by a decreasing behavior at a higher temperature range (T

550 °C) for the three

simulated inlet feed compositions. The decreasing concentration profile of CO2 at high reaction temperatures is expected for CO production via DRM, as CO2 is one of the main reagents (along with CH4 and H2O) in CSDRM. On the other hand, the increasing concentration profile of CO2 attests for the negative conversion values recorded at lower temperatures where CO2 concentration has been increasing (CO2 is being produced) as a result of its production via WGS and/or through combined WGS and SRM (CH4 + 2H2O

CO2 + 4H2) reactions. In fact, the maximum increase in CO2 concentration (expressed in

13 percent and calculated with respect to the initial content) is 18.6% (at T = 500 °C) for the stoichiometric feed composition, 46.4% (at T = 510 °C) in presence of excess H2O (CH4:CO2:H2O = 3:1:4) and around 10% (T = 510 °C) for CH4:CO2:H2O of 3:2:2 (Fig. 1c). The in situ production of CO2 depends on the initial molar proportion of CO2 with respect to those of CH4 and H2O (termed as the CO2/(CH4+H2O) molar ratio). The higher the CO2/(CH4+H2O) molar ratio is, the lesser the extent of CO2 production will be (Le Châtelier principle). In presence of both CO2 and H2O oxidants, methane preferentially reacts with steam due to the more stable chemical nature of CO2 necessitating activation at higher temperatures [81,82]. Extra steam in feed increases CH4 conversion (orange curve in Fig. 1a) due to its consumption by water (Eq. (1)) till 800 °C where complete CH4 conversion is reached unrelatedly to the amount of steam in the feed (Fig. 1a). At temperatures exceeding 800 °C, CH4 becomes the limiting reactant and undergoes consumption in both steam and dry reforming routes. As expected, CO2 conversion follows an increasing trend with increasing temperature (dashed-curves in Fig. 1a) due to the pronounced impact of both DRM (Eq. (2)) and RWGS (Eq. (11)), being both CO2-consuming and highly endothermic. Although an increase in steam enhances CH4 conversion, increasing steam content will not ultimately favor an increase in its consumption. The conversion of steam increases with increase in temperature due to its consumption via WGS and SRM reactions until 700 °C (orange dotted-curve in Fig. 1b) where; it reaches its maximum and begins damping as a result of the dominance of both RWGS (producing additional H2O) and DRM (non-water consuming) reactions. Furthermore, as the fraction of steam increases in the system, the maximum conversion of steam is attained at lower temperatures (shift to the left in the XH2O profile, Fig. 1b). Accordingly, H2 yield for the lower (CO2+H2O/CH4) of 1.0 increases with rising temperature (Table 2). This indicates that H2 is produced by SRM, DRM and WGS reactions. Likewise, H2 yield for the higher (CO2+H2O/CH4) ratio of 1.7 increases considerably with increase in temperature up to 800 °C, where it starts to show a decreasing trend (Table 2). This is mainly due to the occurrence of the RWGS side-reaction (favorable at high temperatures) that consumes H2 thus, decreasing its theoretical content at elevated reaction temperatures. Alternatively, the maximum value of H2 yield decreases as (CO2+H2O)/CH4 ratio increases owing to the fact that CH4 is the limiting reactant and excess in H2O (oxidizing agent) reduces the expected yield of H2 (according to Eq. (26)). On the other hand, CO yield monotonically increases with increasing temperature because of the endothermic nature of both dry and steam reforming reactions. Below 600 °C, CO yield follows an increasing trend for all simulated inlet feed compositions (Table 2). However, between 600 and 1000 °C, the degree of increase in the CO yield for the stoichiometric feed composition is sharper than those obtained under excess carbon dioxide and excess steam simulations; conditions under which CO yield tends to increase in a gradual manner (Table 2). Such a variation in CO yield follows that of CO2 conversion (higher CO2 conversion leading to higher

14 CO yields, Fig. 2a), both parameters being highly dependent on CO2-consuming reactions, particularly the thermodynamic extent of DRM (and RWGS). Concerning selectivity measures, molar H2:CO ratio decreases from around 8 to much smaller values with increasing temperature for the three molar compositions considered in this study (Fig. 1b). More hydrogen than carbon monoxide is generated at low temperatures due to the dominance of WGS and SRM over considerably endothermic DRM and RWGS reactions. Excess CO2 in the feed stream hampers the WGS side-reaction since the total molar product number becomes larger than that of reactants, shifting the reaction to the opposite direction (towards formation of reactants). In fact, lower H2:CO values are recorded, over the entire temperature range, for the simulated CH4:CO2:H2O composition of 3:2:2. As the temperature reaches 800 °C, H2:CO molar ratio converges towards fixed values and becomes dependent on both steam and dry reforming reactions. Accordingly, it is reasonable that intermediate ratio values between 1 and 3 are produced upon combining simultaneously SRM and DRM. For the sole production of “metgas”, an inlet CH4:CO2:H2O feed composition equal to the stoichiometric one and temperatures of 800 °C are necessary requirements. Simulated thermodynamic plots show that an increase in carbon dioxide content is of benefit for DRM (H2:CO ratio trends to values close to 1.5) whereas an increase in water feeding strongly favors the advance of SRM leading higher H2:CO molar ratios (Fig. 1b). However, when a separate solid phase of carbon is considered to accommodate for CO disproportionation, CH4 decomposition, CO hydrogenation and, CO2 hydrogenation reactions (Eqs. (6)– (9)), simulated plots display different trends (Fig. 2). Solid carbon deposition, a major concern in reforming reactions, is significantly affected by operating temperature and inlet feed composition. Usually, equations (6) till (9) contribute to solid carbon formation yet, during DRM and CSDRM; carbon is mainly generated via CH4 decomposition being the most endothermic C(s)-forming reaction (Eq. (7)). Fig. 2(a) shows that carbon generation follows a decreasing trend with increasing reaction temperature owing to the fact that exothermic CO disproportionation is favored at lower temperature where; an increase in temperature causes an overall reduction in carbon formation (Fig. 2a). Exceeding the stoichiometric content of CO2 (CH4:CO2:H2O equal to 3:2:2) and that of H2O (CH4:CO2:H2O equal to 3:1:4) effectively enhances the system towards carbon formation and creates (almost) carbon-free operations beginning from 700 °C (Fig. 2a). The surplus of CO2 promotes the RWGS (producing additional H2O oxidizing species), that of H2O restricts CO, and CO2 hydrogenation reactions, resulting in less (overall) amounts of deposited carbonaceous residues. Nevertheless, under excess CO2 conditions, small contents of carbon still form above 700 °C compared to carbon-free zones for the simulated CH4:CO2:H2O mixture of 3:1:4 (Fig. 2a) since steam is more efficient than CO2 in oxidizing carbon at high temperatures [98].

15 Regarding reactivity levels, no detectable methane conversion is noticed within the 200–450 °C range for any given feed stream composition (Fig. 2a) owing to the dominance of exothermic CO disproportionation, CO hydrogenation and CO2 hydrogenation reactions (Eqs. (6,8,9)). Carbon is then generated, in majority, within such temperature range depending on the amount of oxidants species (CO2 and H2O) and on the availability of carbon sources (CH4 and CO2). As the carbon to steam molar ratio (CH4+CO2)/H2O increases, carbon formation becomes prominent as evidenced from the C(s)-formation profile for CH4:CO2:H2O of 3:2:2 compared to that simulated for CH4:CO2:H2O of 3:1:4 (purple and orange curves, Fig. 2a). With reference to CO2 it converges under extremely higher temperatures; conditions under which CO2 producing side-reactions (Eqs. (6,10)) become thermodynamically unfavorable. The maximum CO2 conversion value at 1000 °C is 38% for the simulation conducted under excess CO2 (CH4:CO2:H2O = 3:2:2) whereas, a conversion value of 93% is obtained under stoichiometric conditions (CH4:CO2:H2O = 3:1:2). Such differences noted on the level of XCO2 could be mainly ascribed to higher amounts of C(s) generated under excess CO2 conditions (purple C(s) formation curve, Fig. 2a) through (for instance) CO disproportionation. This reaction will result in CO2 production (Eq. (6)) and therefore increases its concentration in the medium. So, CO2 conversion will not become possible until its amount diminishes back to a value below the initial one (inlet concentration). In case of excess steam (CH4:CO2:H2O of 3:1:4), CO2 conversion is not detected even at T

800 °C (Fig. 2a). The majority of

CO2 molecules (lowest content compared to other reactants) have been already consumed and converted into C(s) and/or CH4 reacted preferentially with H2O via SRM (at low operating temperature) having higher reaction kinetics than DRM [81,82]. As for the selectivity of the process, H2:CO ratio is greater throughout the 200 till 750 °C range in presence of solid carbon (Fig. 1a) compared to simulated values obtained in absence of C(s) (Fig. 1a). Above 800 °C, H2:CO ratio approaches theoretical values close to those under carbon-free conditions (Fig. 1b) because DRM and SRM become simultaneously codominant. With respect to H2 and CO yields, it is worth noting that their variation follows the same trend as that recorded under C(s)-free operations in the sense that H2 yield values are slightly lower for the feed composition simulated under excess CO2 compared to the stoichiometric one and to that operated under excess H2O (Table 2, explanation provided previously). Additionally, H2 yield values are much higher than those of CO especially within the 400–600 °C range independently on the simulated inlet feed composition (Table 2). According to Fig. 2(a), C(s) formation profiles are accentuated within such temperature range due to the occurrence of C(s)-forming reactions consuming CH4 and producing H2 (such as methane decomposition reaction) as well as those consuming CO as being generated (Eqs. (6)-(9)). This explains the high H2 than CO yields. Once the thermodynamic extent of C(s)-producing sidereactions becomes less favorable at high reaction temperatures (above 600 °C), CO yield ultimately increases due to the dominance of highly endothermic (and CO producing) dry and steam reforming

16 reactions (Table 2). Even if excess steam effectively inhibits carbon formation, extra costs are required to supply pure water, vaporize it and, separately pump it into the reformer unit. Therefore, fixing inlet feed composition close to the stoichiometric one while maintaining selective “metgas” productions, considerably accepted C(s)-amounts (compared to the amount expected under excess CO2 stream, Fig. 2a) upon neglecting additional costs are key factors to consider for industrialization of CSDRM. 3.2. Effect of pressure on reactivity and selectivity levels (C(s)-free and C(s)-assisted operations) As most GTL processes require elevated pressures (range: 5–20 bar) to reduce compression steps for downstream application of syngas, it becomes practical and economical to carry out reforming reactions at elevated pressures [99,100]. Therefore, thermodynamic simulations are investigated to study the effect of pressure on reactivity and selectivity performances in CSDRM under stoichiometric feed composition (molar CH4:CO2:H2O composition of 3:1:2). Footprints of carbon formation profiles as a function of temperature for five operating pressures (P = 1, 5, 10, 15 and 20 bar) are also considered. Simulations conducted under C(s)-free and C(s)-assisted reforming are drawn as a function of temperature (range: 200– 1000 °C) and shown on Figs. 3 and 4, respectively and H2 and CO yields are presented in Table 2. It is worth mentioning that under C(s)-assisted operations, simulations at pressure values equal to 15 and 20 bar are performed to account for industrial scenarios under which high pressure is a pre-requisite in reformers for subsequent utilization of pressurized syngas in liquid fuel production. Equilibrium conversions of CH4, CO2 and H2O are significantly suppressed, over the entire range between 200 and 1000 °C, by increasing pressure from 1 bar passing to 5 then 10 bar (Fig. 3). Nematollahi et al. [31] reported alike thermodynamic trends for reactivity levels in dry reforming of methane (CH4:CO2= 1:1) with increasing pressure. Liu et al. [101] also found that conversion of both CH4 and H2O are inhibited when passing from 1 bar to higher pressures and that atmospheric pressure is required to avoid efficient losses in H2 yields. So, a decrease in conversion with increase in pressure is anticipated upon combining SRM to DRM. Fig. 3(a) shows that a rise in operating pressure from 1 to 10 bars, at any given temperature (low to intermediate range: 200–750 °C), leads to a decrease in CH4, CO2 and H2O conversions (Figs. 3a,b) along with an increase in H2:CO values (Fig. 3b). This could result from contributions from side-reactions such as the WGS (Eq. (10)), which consumes CO2 and produces CO. On the other hand, at elevated temperatures (T 800 °C), conversion of reactants approaches similar values (high conversion values are recorded) at any given operating pressure with H2:CO ratios equal to 2.0 at 1, 5 and at 10 bar (Figs. 3a,b). Complete conversions and selective “metgas” productions show that the effect of pressure is almost negligible under highly endothermic conditions. One should also note that slightly higher CO2 than CH4 conversion is obtained, for the range between 800 and 1000 °C, for operating pressures of 5 and 10 bar compared to equal reactivity levels at P = 1 bar (Fig. 3a).

17 Additionally, at P = 10 bar and T = 1000 °C, H2O conversion is 80% being 20% lower than that achieved at P = 1 bar (XH2O is complete at 1000 °C, Fig. 3b). Higher CO2 than CH4 conversions and lower H2O reactivity levels noted as a function of increasing pressure could be tentatively associated to (some) thermodynamic limitations associated to SRM whereas, DRM continues to predominate at elevated temperatures esteeming desired hydrogen to carbon monoxide ratio of 2.0. Similar observations are reported in the study of Özkara-Aydınoğlu [81] on thermodynamic simulations of CSDRM. The author observes, under the effect of pressure, lower CH4, CO2 and H2O conversions compared to simulation data conducted at atmospheric pressure yet, those of methane and steam appear to decrease at a slightly sharper rate than those of CO2. Additional details are in progress to clarify such behaviours. Since the total number of mole of products is larger than that of reactants for both steam and dry reforming reactions, a variation similar to those of CH4, CO2 and H2O conversions is expected for H2 and CO yields under typical CSDRM conditions. Indeed, yields of products are significantly suppressed, at any given temperature, upon increasing operation pressure from 1 to 10 bar (Table 2). To sum up, CSDRM is a volume increasing reaction (reactants have lower moles of gas than products) where: low pressure potentially favors the formation of desired products; conditions under which methane, carbon dioxide and steam react in the most desired way. Besides, from industrial and commercial perspectives, the consideration of solid carbon is mandatory for any type of reforming reaction. For a fixed ratio of CH4:CO2:H2O = 3:1:2, carbon formation increases with increasing pressure from 650–850 °C (Fig. 4a). Within such temperature range, both CO disproportionation (Eq. (6)) and methane decomposition (Eq. (7)) are thermodynamically favored and lead to solid C(s) generation. Yet, both reactions show opposite tendencies with increasing pressure. In fact, an increase in pressure promotes the forward CO disproportionation reaction (Eq. (6)) towards generation of C(s) since reactants (two moles of CO) have larger moles of gas than products (1 mole of CO2). On the opposite, a rise in pressure shifts the equilibrium of the CH4 decomposition reaction to the reverse reaction (towards formation of methane) owing to larger moles of gas. For instance, the amount of carbon increases from 0.2 to 0.52 kmol as pressure increases from 1 to 20 bar at 800 °C (Fig. 4a). Similar behavior is detected in the study of Jang et al. [90] for similarly conducted thermodynamic simulations. However, the effect of pressure on carbon formation follows an opposite trend at temperatures lower than 650 °C for the five pressure values considered in the simulations. Indeed, more carbon is produced at P = 1 bar than at P = 10 bar and this amount is even much higher than at P = 20 bar (Fig. 4a). It is worth noting that CO disproportionation, CO hydrogenation and CO2 hydrogenation reactions favored within the 400–650 °C are all volume decreasing reactions and therefore become thermodynamically favored with increasing pressure. The trends of C(s) formation show different behaviors for all pressure values considered in simulations (Fig. 4a). Nevertheless, both CO hydrogenation and CO2 hydrogenation

18 reactions are also steam-generating reactions (Eqs. (6,8,9)) and so, as they occur (to a high extent under high pressure), steam (strong oxidant) is being in situ generated and is potentially causing the oxidation of carbonaceous deposits as they develop. This could explain the lower amount of C(s) generated at P = 20 bar compared to the amount formed at P = 1 bar within 400–650 °C temperature range. Moreover, Fig. 4(a) shows that CH4 and CO2 conversions are postponed in presence of solid carbon. Indeed, SRM and DRM are volume-expanding reactions hence; a delay in the initiation of reactants conversion is probable when syngas along with solid carbon are expected products. Once methane conversion initiates, that of steam shows a sudden increase at any given pressure (Fig. 4a,b) while that of carbon dioxide becomes noticeable at much elevated temperatures (i.e., at P = 10 bar, XCO2 begins at around 950 °C, Fig. 4a). For higher-pressure values (i.e., P = 15 and 20 bar), methane begins to converge at around 650 °C for P = 15 bar and at a temperature much higher (close to 680 °C) for P = 20 bar whereas, no carbon dioxide conversion is detectable over the entire temperature range (Fig. 4a). Such observations accentuate on the fact that for volume-expanding reactions, an increase in pressure delays the chemical activation of reactants in the direction of products formation. For P = 1, 5 and 10 bar, CO2 will not converge towards syngas (via DRM) until all CO2 consuming and producing side-reactions become disadvantaged with respect to operating temperature and pressure. This validates the fact that dry reforming is the most challenging and energy demanding reaction in CSDRM. Complementary to the consumption trends of CH4, CO2 and H2O and the production trends of H2, CO and C(s), theoretical H2 and CO yields follow the same decreasing trend of conversion noted with increasing pressure (Table 2). These results come in accordance with the main conclusions deduced previously and indicate as well that at temperatures above 800 °C, reforming reactions become predominant (owing to increasing H2 and CO yields, Table 2) and the majority of side-reactions (C(s) -or non C(s)-producing ones) become thermodynamically unfavorable with the exception of methane decomposition. To sum up on the effect of pressure on coke formation: an increase in pressure increases the minimum temperature at which coke formation can be avoided. The minimum temperature increases up to 950 °C with increasing pressure from 1 to 10 bar. Thus, a low C(s)-producing CSDRM operation yielding selective production of “metgas” is best operated at 800 °C and 1 bar unless pressure is an industrial requirement then, the temperature should be raised to a least 950 °C. 3.3. Effect of inert diluent on reactivity and selectivity levels (C(s)-free and C(s)-assisted operations) In addition to thermodynamic analysis on the optimization of operating conditions for sole production of “metgas”, effects of diluting the feed (by argon) on reactants conversion and products distribution are presented in Tables 2 and 3. In small (or semi-pilot) academic laboratories, safety measures are major constraints. If leaked in sealed environments, pure methane could cause serious risks due to its explosive

19 potentials whereas CO2 buildup is more critical than oxygen shortage. Diluting reforming gases with a chemically insert gas such as He (or Ar) is viewed as a potential solution. Some DRM studies conducted in plasma reactors have investigated the effect of diluting CH4 and CO2 with He [102,103]. They found that in situ generated He+ species are energy transfer ions contributing in the enhancement of reactants conversion and consequently H2 and CO yields. In CSDRM, adding a diluent could positively improve the performance since the presence of an additional gaseous component in the medium could ultimately force a shift in the direction of syngas formation. In fact, much elevated conversion values for CH4, CO2 and H2O along with higher H2 and CO yields are obtained for the diluted feed stream compared to the pure one for both C(s)-free and C(s)-assisted scenarios (Tables 2 and 3). For instance, CO2 conversion under diluted feed is almost five folds the value achieved under pure feed stream at 600 °C. Regarding selectivity, H2:CO molar ratio approaches the desired “metgas” value at a temperature as low as 600 °C when argon is considered in the simulation. A ratio equal to 2.7 under pure CH4:CO2:H2O mixture becomes 2.3 upon diluting CH4 and CO2 with Ar (Table 3). Moreover, raises (in the range of 20%) in both H2 and CO yields at 600 °C are recorded when non C(s)-assisted CSDRM is simulated in presence of argon (Table 3). Similarly, increasing percentages in H2 and in CO yields (26% for the former and 30% for the latter) are also noticed at 600 °C when the simulation is assumed to occur in presence of solid carbon (Table 3). However, for a reaction temperature of 800 °C (and higher), dilution effect becomes negligible (where the system is entirely driven by highly endothermic SRM and DRM reactions. Therefore, for correct and concrete interpretation of catalytic performances for identification of possible side-reaction contributions and/or deactivation mechanisms, experimental results should be correlated to thermodynamic values at fixed temperature and pressure for C(s)-simulated conditions upon accounting for dilution effects (when applicable). 3.4. Combined parameters ensuring selective CSDRM operation for subsequent MeOH production From all above theoretical considerations, an accentuation on the concept of “methanol economy” [23,19,23,87], an approach based on “metgas” (H2:CO = 2.0) to methanol via bi-reforming or combined steam and dry reforming of shale, natural gas or diverse biogas mixtures, is of significant industrial, environmental and economic interests. This selective process, a hypothetical model like the “hydrogen economy” [104], enables the straightforward synthesis of methanol (partly from renewable energy vectors) that could eventually lead to a sustainable solution for our dependency on fossil fuels. The generation of MeOH from syngas is not feasible in a single step rather; it requires two separate (independent) reaction stages. The first one is based on reforming of methane (most studied hydrocarbon) into a fixed H2:CO ratio of two followed then by the chemical transformation of the as-produced “metgas” into methanol (Eq. (28)). Both stages reflect a quite diverse mirror of operation conditions. Fig.

20 5 displays the Gibbs free energy change for all reactions involved in a typical bi-reforming reaction including that of methanol synthesis from “metgas”. As previously discussed and according to Fig. 5, the CSDRM reaction (Eq. (5)) is strongly endothermic where its

increases with increasing temperature

(line 4, Fig. 5a), the reaction responsible for MeOH production (Eq. (23)) is thermodynamically favored at much lower temperatures (line 1, Fig. 5a). Methanol synthesis

4x (CO + 2H2

CH3OH)

H0 (298 K) = 4x (90.7) kJ mol-1

(28)

The reforming process must be then operated at high temperatures (at least 700 °C) since under these conditions, the Gibbs free energy starts to become negative for DRM, SRM and consequently CSDRM (Fig. 5a) and shifts to positive values for CO-involving side-reactions such as hydrogenation, dehydrogenation and disproportionation despite the occurrence of the CH4 decomposition coke-forming reaction (Fig. 5b). Besides, knowing that the bi-reforming is a combination of SRM and DRM, it is not highly recommended to increase pressure because conversion of reactants is reduced (as detailed in section 3.2) together with the disadvantage of working under elevated temperature for the generation of optimal H2:CO molar ratio proceeding liquid methanol production. Contrarily, MeOH synthesis is a decrease in volume reaction (twelve to four moles, Eq. (28)) and so, it will not become thermodynamically possible unless operated under considerably elevated pressures (P = 20 bar) and low temperatures (around 200 °C, Fig. 5a).

4. STATE-OF-THE-ART CATALYSTS OF CSDRM REACTION 4.1. Increasing interest for transition metal-based catalysts Thermodynamic analysis shows that CSDRM requires at least a reaction temperature of 800 °C to attain high syngas selectivity. One of the key factors accounting in the success of the process is the development of a catalytic system capable of accelerating the different chemical reactions involved within the process while being resistant enough to withstand harsh reaction conditions. The catalyst to be developed should be capable of activating methane, carbon dioxide and water with high rates and for long runs by avoiding all possible C(s)-forming and non C(s)-forming side-reactions. CSDRM is unavoidably accompanied by carbon deposition where the employed catalyst must resist coke formation/accumulation, active phase sintering and, potential re-oxidation of metallic sites (deactivation modes highly pronounced in steam-rich atmospheres) [32,33]. So far, bibliographic reports on CSDRM are less numerous compared to those of DRM and SRM owing to the high complexity of the reaction. In this section, the current state-of-the-art on catalytic CSDRM is reviewed. Table 4 summarizes the different heterogeneous catalysts already investigated in CSDRM coupled to their corresponding reactivity results (expressed in terms of CH4 and

21 CO2 conversion levels) and produced H2:CO molar ratios as a function of time of stream (TOS) under applied conditions of temperature, pressure and, inlet feed composition. For the sake of completion, Table 5 presents (for each catalytic combination) main reasons that lead to active/stable performances and those responsible for deactivation along with the tactics employed to enhance their TOS performance. Amongst already tested active phases, a patent by Yagi et al. [106] devoted to noble-metals (Ru /or Rh) supported on a basic support revealed steady performances (up to 1000 h) at elevated pressures. Qin et al. [75,76] tested several MgO-based noble metal catalysts (Pt, Pd, Ir, Ru and Rh) and found all samples stable on stream with 0.5 wt% Rh impregnated on MgO being the most reactive (Tables 4 and 5). Moreover, Soria et al. [83] conducted a thermodynamic study and then performed the reaction over a 4 wt% Ru/ZrO2-La2O3 catalyst in order to verify the applicability of their simulation results. Experimental data were in line with theoretical ones and the catalyst was found stable (Tables 4 and 5) for 10 h on stream. For instance, at 500 °C and for a CH4:CO2:H2O inlet feed composition of 3:3:1.5, thermodynamic H2 and CO yields (maximum expected values) are 32 and 19%, respectively. Experimentally, H2 and CO yields over Ru/ZrO2-La2O3 (under conditions similar to those adopted for simulation) are 30% and 17% respectively, indicating thus the great correlation between theoretical and experimental outcomes. Furthermore, Khani et al. [67] recently showed that 3 wt% Ru/LaZnAlO4 is active and selective in CSDRM at 800 °C (Table 5) but their study was (somewhat) incomplete lacking steady-state evaluation for better assessment of catalyst performance (Table 4). The main features of noble-metal catalysts are their high carbon-carbon, hydrogen-carbon, hydrogen-oxygen and hydrogen-hydrogen activation bonds, low carbon tolerance and, high resistance towards sintering and re-oxidation [107]. However, in consideration of high cost and limited availability of noble metals, their substitution by transition ones (such as Ni or Co) is attractive and highly desirable for large-scale applications. In fact, Table 4 shows that the majority of developed CSDRM catalysts are transition metal-based (Ni being predominantly the most applied) and analogous works report promising intrinsic activities. In this context, Olah et al. [4–8] studied various metals such as V, Ti and Mo dispersed on SiO2 and Al2O3 and catalysts were efficient but preferentially nickel deposited on MgO [18] was ideal for “metgas” generation (Tables 4 and 5). Indeed, performances of Ni/MgO were significantly higher (even at lower temperatures) than those obtained on Ru/MgO catalyst [75–76]. Additionally, research groups of Choudhary et al. [52,74] and Zhang et al. [73] reported high activities (XCH4 exceeding 90%, Table 4) for Ni-CaO [52], Ni-MgOSA-525 [74] and Ni/ZrO2 [73] with intrinsic data comparable to those obtained over Ru/LaZnAlO4 [67], all tested under similar reaction conditions (Table 4). Nevertheless, Ni catalysts are susceptible to deactivation (Table 5) necessitating needs for suitable supports, promoters (or co-metals), and/or modification in their preparation practices to counteract extensive loss of activities (Table 5).

22 4.2. Salient catalyst features for high CSDRM activity and deactivation resistance In this section, significant features in the design of highly active, stable and “metgas” selective catalysts are discussed. These features cover aspects associated to the importance of i) surface acidity-basicity, ii) lattice oxygen species, iii) redox properties, iv) hydroxyl groups, v) metal-support interactions (MSIs) and vi) bimetallic synergetic effects. 4.2.1 Surface acidity-basicity The chemical characteristics of the support such as its acidic or basic potential has a major impact not only on the extent of MSI but also on the rates of surface occurring reactions such as adsorption of CH4, CO2 and H2O gases. Supports having a strong acidic character may be unfavorable to minimize C(s)formation. Alumina is often used as a catalytic carrier in almost all reforming reactions because of its high surface area, a property that enables a facile and homogenous dispersion of metallic particles either on external or internal surfaces. Moreover, Al2O3 is mechanically and chemically stable yet, it is mildly acidic and therefore, it tends to promote carbon deposition through methane decomposition reaction, a side-reaction favored to occur on acid sites [108,109]. Indeed, a comparative study between two traditional “standard” oxides such as Al2O3 and ZrO2 reveals that Pt nanoparticles dispersed on ZrO2 present improved catalytic performances. This is due to the ability of ZrO2 to activate CO2 (owing to its O2 defects, as will be discussed in upcoming sections) to a higher extent than when dispersed on alumina over which carbon accumulation from CH4 cracking decreases reactivity levels [110]. Expanding the comparative interpretation and including SiO2 as another “standard” oxide, a recent study shows that Ni impregnated on ZrO2 (or Al2O3) catalysts present higher population of weak acid sites as compared to Ni over SiO2 indicating an enhancement of acidic strengths. According to CO2-temperature programmed desorption (TPD) profiles, CO2 is shown to be weakly adsorbed over the Ni/ZrO2 and Ni/Al2O3 catalysts attesting on their stronger acidic character as compared to Ni/SiO2 displaying weak to medium strength adsorption (basic) sites [111]. Thus, on Al2O3 and ZrO2-supported catalysts, the acidic support is proven to play a minor role in improving coke removal rather in promoting its surface accumulation since reactants are only activated on metallic sites [111]. Normally, activation of acidic gases (CO2) will advance through either hydrogen-assisted dissociation or direct dissociation for generation of surface-adsorbed OH groups following the pathway of CO2(ads)+ H(ads) CO(ads) + OH(ads) [112]. On the other hand, oxides with basic properties (or modified by basic additives) are anticipated to promote surface CO2 adsorption leading in increased concentrations of surface-adsorbed OH groups. The addition of promoters, such as alkali or alkaline oxides, into the catalyst composition is an effective methodology to improve overall catalytic performance during CSDRM (Tables 4 and 5) by increasing the basic nature of the catalyst. Consequently, an increase in the basicity of the support

23 increases the concentration of adsorbed CO2 which in turn reduces carbon formation through CO disproportionation (i.e., CO2 + C(s)

2CO) [33,36,40,46-47,62,69].

For instance, in our recent work, we found that the introduction of MgO (or CaO) basic modifiers via “one-pot” in the course of alumina precipitation results in Ni5%Mg(or Ca)5%Al2O3 catalysts that are remarkably active, stable and “metgas” selective (ref. [33], Table 4). Compared to non-promoted Ni5% and Ni10%Al2O3, the amount of C(s)-deposited after 40 h of catalysis was significantly reduced and promoted catalysts presented improved reactivity levels and high-purity “metgas” productions (Tables 4 and 5). Fig. 6 is a graphical representation of the state of spent catalysts showing C(s)-accumulation and C(s)-free surfaces for non-MgO containing catalysts and those housing 5 wt% MgO, respectively. In presence of MgO, surface activated CO2 generates free O* species (Fig. 6) that react with neighbor C(s)deposits (and transform them into CO) preventing their accumulation on Ni0 sites and Al2O3 surfaces. Similarly, several researchers have investigated the utilization of basic additives in CSDRM for their potential positive effects. The groups of Koo et al. [36] and Mehz et al. [40] conducted CSDRM under both MgO-free and MgO-assisted Ni impregnated on Al2O3 catalysts and deduced that the addition of MgO has a beneficial effect on reactivity, selectivity and coke suppression (refs. [36,40], Tables 4 and 5). The coverage of Ni0 sites with higher amounts of O(ads) prevents the adsorption of hydrogen-deficient hydrocarbon species CHx,ads, which decompose into surface carbon, owing to the fast reaction of oxygen with CHx,ads to form CO. The influence of La2O3 as an additive has been also considered for reactivity improvement and coke resistance of Ni0 species deposited on Al2O3 (ref. [46], Tables 4 and 5). According to their findings, methane dissociates on Ni0 sites to form carbon and hydrogen whereas CO2 (and H2O) are preferentially adsorbed on La2O3 sites to form in situ La oxycarbonate species. Such intermediates are proposed to interact with C(s)-species from CH4 decomposition (Eq. (7)), to form CO gas, which eases the removal of carbonaceous residues. Despite oxidative carbon removal properties, addition of MgO is not always beneficial especially in case of partial in situ re-oxidation of Ni0 into NiO, a deactivation mode highly encountered over silica-based catalysts (refs. [32,49], Tables 4 and 5). Usually, alternative improvements methods are applied such as substitution of silica by another metal oxide (i.e., alumina) characterized by a higher resistance towards steam, as will be discussed in following parts of this review (ref. [33], Tables 4 and 5). The utilization of basic supports such as MgO and CaO has been widely examined for almost all reforming reactions including CSDRM [18,52,74] and authors report promising performances (ref. [74], Tables 4 and 5). However, due to harsh CSDRM operation conditions, the presence of a basic character is not the only feature responsible for stable performances [47,69] rather combined parameters are mandatory for designing efficient CSDRM catalysts. Strong deactivation was detected over Ni/MgO [47]

24 and “one-pot” synthesized Ni-MgO [69] catalysts when tested for “metgas” production at T

750 °C

(Table 4). Deactivation causes and corresponding improvement tactics are detailed in Table 5 and will be addressed in upcoming sections. 4.2.2 Lattice oxygen species (perovskite and mixed-oxide structures) A catalytic support with high oxygen storage capacity and high oxygen mobility are vital features for the design of supported catalysts with enhanced reactivity and stability in reforming reactions involving hydrocarbon cracking associated with heavy C(s) deposition. Perovskite-type catalysts are characterized by the formula ABO3 and have been widely used in high-temperature applications because of their stable structure inducing high thermal stability [113,114]. Perovskite structures offer the possibility of altering the dimension of the unit cell by substituting the A ion by another metallic element. This ion is generally located in the cavity made between the octahedra of the octahedrally arranged B-site cation. Upon substitution, the covalence of the B–O bond in the structure will ultimately change depending on the oxygen vacancy of the substituent [115]. The consequence of such a metal-metal replacement will result in the formation of lattice structural defects reported to prompt catalytic activity in methane reforming reactions [116]. Enhancement of catalytic reactivity results from higher capability in O2 adsorption potentials and subsequently improved oxygen mobility (in other terms, higher coke removal capacity) within the crystal assembly. Additionally, an isomorphic substitution of catalytically active transition Ni metal at the B-site cation is remarkably shown to increase sintering resistance under drastic SRM conditions [117]. It is worth mentioning that the utilization of perovskite-derived structures as catalysts in CSDRM is limited when compared to their application in either dry or steam reforming reactions nevertheless, interests are increasing in this direction (Tables 4 and 5). In this context, there have been studies dealing with partial substitution of elements such as Sr [55,61] in La nickelate based-perovskite structures. For example, Yang et al. [61] found that the incorporation of Sr within the perovskite matrix decreases the oxidation state of Ni and facilitates its reduction by shifting it to lower temperature ranges. The improved reduction potential is mainly attributed to an increase in vacancies within the spinel-like structure of the perovskite catalyst which facilitates the mobility of oxygen and hence the capability of Ni to undergo reduction and/or oxidation [55,61]. Such redox processes within perovskites do not undergo successive cycles rather they occur irreversibly via in situ generated intermediate species [118]. Although Srsubstituted LaNiO3 developed larger Ni particles that agglomerate during reduction and catalysis because of weaker MSI which result in lower intrinsic activity levels (ref. [61], Table 4) yet; these catalysts present stable performances (compared to deactivating one for non Sr-modified catalyst, Table 4) along with high resistances towards carbon deposition. Fig. 7(a) represents selected transmission electron

25 microscopy (TEM) images coupled to their corresponding energy dispersive spectroscopy (EDS) analyses performed over spent CSDRM catalysts. These results establish the relationship between Ni0 particle sizes and Sr content where nickel particles are shown to grow significantly (>100 nm) with increasing Sr content. On the contrary, carbon content is shown to decrease with increasing Sr content (Fig. 7b). Indeed, spent LaNiO3 catalyst (LN, Fig. 7b) presents higher C(s)-related weight losses (close to 20 wt%) compared to almost no losses for Sr-modified catalysts (Fig. 7b). Additionally, results from temperatureprogrammed surface reactions (TPSR, under CH4 stream) of reduced catalysts (Fig. 7c) show that CH4 activation shifts to higher temperatures (T

650 °C) with increasing Sr concentration emphasizing on the

high coking resistance of Sr-substituted perovskite catalysts [61]. To sum up, stable performances and low amounts of carbonaceous deposits are attributed to the presence of nickel in its desired metallic state (Ni0) and the formation of La2O2CO3 species. These intermediates tend to facilitate the oxidation of methane, the regeneration of basic La2O3 and consequently the prevention against C(s)-accumulation. Likewise, Kim et al. [55] studied the promotional effect of Sr in LaNiO3 perovskite catalysts upon improving the dispersion of LaSrNiOx species over SiC-modified Al2O3 supports. Non-dispersed and SiCAl2O3 modified perovskites are extremely active and stable while LaSrNiOx/SiC-Al2O3 exhibit higher CH4 and CO2 conversions (Table 4). Higher reactivity levels are attributed to higher amounts of accessible Ni0 sites and wider availability of mobile lattice oxygen species. The enhanced dispersion of catalytically active centers on SiC-Al2O3 particles increases the formation of La2NiO4-Al2O3 resulting in improved catalytic activity by suppressing thermal aggregation of active nickel-containing metals (perovskite-like La2NiO4 crystallites). Similarly to Ni-containing perovskites, Co-based ones are also promising candidates in various reforming reactions including CSDRM (ref. [77], Table 4). When tested at 850 °C under an ideal “metgas”-producing inlet feed composition (CH4:CO2:H2O of 3:1:2), Co-modified Nd2O3 catalysts operate selectively with a product molar ratio slightly higher than 2.0 (Table 4) due to an intervention from CH4 decomposition (Eq. (7)) generating excess H2. Supplementary examples of lattice oxygen contribution include Ni nanoparticles deposited on oxides having redox properties such those based on (or containing) CeO2 and ZrO2 elements (refs. [4748,63,66,73,78,83], Tables 4 and 5). Such oxides are popular for their ease in liberating oxygen species (as will be highlighted in section 4.2.3) and their importance in methane reforming reactions is well established in literature. Zhang et al. [73] conducted CSDRM over Ni impregnated on ZrO2 and the catalyst was reactive under various inlet feed compositions where CH4 conversion reached values higher than 85% (Table 4). Additionally, the positive effect resulting from O2 lattice species for carbon removal (and accumulation prevention) were also addresses in the work of Itkulova et al. [66] dealing with mixed Pt-Co impregnated on ZrO2-modified Al2O3 support (Table 4). Authors have shown that the amendment

26 of Al2O3 by ZrO2 is favorable from both reactivity and coking limitation perspectives (Table 5) because of an intimate interaction (and contact) between Ni0 and ZrO2 that enables intermediate CHx species to react with oxygen atoms (from ZrO2 lattice) and generate CO and H2 products. The consequence of which is the formation of surface oxygen vacancies with high affinity for oxygen atoms [119] that are compensated from O2 supplied by CO2 and H2O surface decomposition reactions [66]. 4.2.3 Redox properties Several investigations on CeO2 and/or ZrO2 promoted transition metal-based catalysts (mostly added to Ni nanoparticles) have been studied for CSDRM and corresponding literature is summarized in Tables 4 and 5 [38,42,44-45,70]. CeO2 and ZrO2 act as oxygen scavengers on catalytic surfaces where improved coking resistances in their presence is mainly attributed to their cyclic redox patterns. Non-stoichiometric Ce (or Zr)O2-x (x ranges between 0 and 0.5) species, resulting from the oxidative removal of coke into CO, are constantly regenerated by in situ oxidation steps, assisted by oxygen-rich species (CO2 and H2O), into Ce (or Zr)O2. Due to their active oxygen storage and release capacities, transport of free O* species is significantly hastened within the catalytic system. Owing to CeO2 and ZrO2 stable ionic transition states (such as Ce3+, Ce4+ and Zr4+); their oxidized phases shift easily between CeO2 (or ZrO2) and CeO2-x (or ZrO2-x). Such an oxygen buffering effect discharges free oxygen species, in the course of their reduction stage, which promotes reaction with CH4 and CO under O2-rich conditions. Detailed mechanistic aspects of reactant dissociation on CeO2 (or ZrO2) sites are not reported for CSDRM however, such features have been previously considered for DRM [120–122] and SRM [123,124]. Here, they will be presented for combined reforming taking into consideration the dissociative reactions of the three main gaseous feeds (CH4, CO2 and H2O) present in reaction medium. Neglecting the influence of Ni, surface reactions over CeO2 (similar paths could be adopted over ZrO2) proceed as shown in (Eqs. (29)-(31)). At high reaction temperature, the gas-solid reaction between ceria and CH4 generates H2 and CO (Eq. (29)) whereas dissociation reactions of CO2 and H2O over CeO2 are only induced over (partially) reduced surfaces (termed as CeO2-n, (Eqs. (30) and (31)) yielding CO and H2, respectively. CeO2 + nCH4

CeO2-n + nCO + 2nH2

(29)

CeO2-n + nCO2

CeO2 + nCO

(30)

CeO2-n + nH2O

CeO2 + nH2

(31)

27 The reactions involving C(s), originating from CH4 decomposition and CO disproportionation reactions, with lattice O* atoms at CeO2 surface are presented in (Eqs. (32) and (33)). (

+ +

(32)

+ +

(33)

The reaction between C(s) species and lattice oxygen on catalyst surface promotes the remarkable redox characteristics between Ce3+/Ce4+ of CeO2 which aids the regeneration process of O2-deprived CeO2-x species into O2-stored (CeO2) ones. All studies dealing with the investigations of CeO2 effects on catalytic performances in CSDRM report enhanced dispersion, stronger MSI and, lower coke deposition (Tables 4 and 5). An example of which is the increase in reactivity levels, selectivity performance and coke resistance detected (at three different temperatures) over Ni-Ce/MgO-Al2O3 as compared to non-promoted Ni/MgO-Al2O3 (ref. [42], Tables 4 and 5). All these improvements originate from the enhancement of MSI (as a result of an electronic effect that chemically modifies metallic atoms [125]) reflecting improved dispersion of active Ni0 and CeO2 nanoparticles and effective oxygen transfer through intimate Ni-Ce contact. Similar observations are reported in other studies and results are detailed in Tables 4 and 5. On the other hand, several studies show that partial substitution of Ce4+ for Zr4+ in CeO2 lattice structure induces thermal stability if the mixed oxide is applied as catalytic support or added separately as a promoter. Indeed, superior thermal resistance, boosted oxygen storage capacity and enhanced metal dispersion have been obtained by simply substituting supports of type CeO2 (ref. [48], Table 4) or ZrO2 (refs. [48,78], Table 4) by mixed CeO2-ZrO2. The cubic CexZr1-xO2 phase is shown to maintain higher redox potentials because of anticipated oxygen storage capacities and ease in formation of oxygen vacancies, known as active sites for dissociative adsorptions of CO2 and H2O molecules [37,48,78]. When used as support, carbon species generated by CH4 decomposition on active metallic centers become more prone to be oxidized (rather than deposited) by lattice oxygen species available all around the contact perimeter of dispersed Ni0 particles increasing thus their anti-coking abilities which enable them to catalyze stably and selectively CSDRM (refs. [37,48,78,90], Tables 4 and 5). Specifically, Bae et al. [45] studied the significant importance of co-impregnating Ni and mixed CeO2-ZrO2 oxides onto MgAl2O4 surfaces rather than post-impregnating Ni on surfaces (CeO2-ZrO2/ MgAl2O4) previously coated by CeO2ZrO2 species (Tables 4 and 5). Improved catalytic performances (reactivity and selectivity levels, Table 4) are attributed to the different segregation mechanisms of Ni and mixed CeO2-ZrO2 (depending on preparation method), as shown in Fig. 8. In fact, larger concentration of CeO2-ZrO2 with adjacent contact

28 with nickel crystallites is responsible for enhancing CO2 activation due to higher oxygen capacity generated by the enhanced dispersion of CeO2-ZrO2 crystallites. Such physico-chemical characteristics induced by preparation method (Fig. 8) are factors that positively affected Ni0 anti-sintering and anticoking properties. 4.2.4 Hydroxyl groups Oxidation of intermediate CHx species through the participation of surface OH groups has not been reported for CSDRM and discussion regarding the importance of such groups in C(s)-removal during DRM are also scarcely mentioned although, reports dealing with their positive impacts are well established for SRM [126,127]. The reason for limited investigations on the effect of OH groups in DRM is most probably due to the low coverage of catalytic surfaces by OH species whereas their potential surface coverage is an important feature in SRM owing to H2O surface dissociation reactions and, this effect should be considered in combined steam and dry reforming. In fact, a proposed mechanism (Fig. 9) displaying all possible surface dissociation reactions taking place on Ni 0 sites (excluding the probable impact of support acidic or basic character, discussed in section 4.2.1) during CSDRM highlights on the vital role of surface OH groups in products (H2 and CO) formation. Therefore, one should provide a literature analysis on available CSDRM data in which OH groups could have been involved in catalyzing the reaction even if not clearly discussed. It could be established that upon altering the support (or catalyst) acidity/basicity by addition of rare earth metals (i.e., La2O3) or basic modifiers (MgO or CaO), the concentration of surface OH groups increases due to improved CO2 activation, as previously discussed in section 4.2.1. In addition to CO production (Fig. 9), the presence of surface-attached OH groups (in excess) facilitates C(s)-removal due to their reaction with adsorbed CHx intermediates yielding oxygenated formate-type CHxO species that subsequently decompose into CO. Such an effect could be probably witnessed in B2O3-promoted Ni/SBA-15 catalysts (refs. [61,68], Tables 4 and 5). A significant reduction in carbon formation is reported for bore-promoted catalysts (Table 5) with simultaneous promotion in reactivity levels (Table 4). A potential reaction mechanism over promoted samples can be proposed based on the various physicochemical characterizations performed on fresh and spent catalysts. Potentially, two separate reactions occur concurrently: the first is that happening over Ni0 sites responsible for the activation of CH4, CO2 and H2O (Fig. 9) followed by an oxidation one, initiated by in situ generated surface OH groups, involving CHx conversion into CHxO then into CO thus, limiting C(s)-accumulation and maintaining catalytic stability (Table 4). 4.2.5 Metal-support interactions

29 According to the study of Roh et al. [47], the design of efficient CSDRM catalysts is dependent on metalsupport interaction which plays vital roles in reaction initiation, reactivity maintenance and selectivity orientation. From their study on Ni impregnated on various “standard” oxides including Al2O3, CeO2, ZrO2, MgO and mixed MgO-Al2O3, they found that activity levels and “metgas” production depend strongly on the chemistry of the support. Catalysts are ranked in the following increasing order of performance: Ni/MgO-Al2O3> Ni/MgO> Ni/CeO2> Ni/ZrO2> Ni/Al2O3 (classification is based on reactivity and stability evaluations, Table 4). The high catalytic performance of Ni/MgO-Al2O3 accompanied by low C(s) deposition and limited Ni0 sintering (ref. [47], Table 5) is due to stronger Nisupport interactions established in the course of metal impregnation and thermal treatment. During in situ reduction, the hydrotalcite-like MgO-Al2O3 decomposes into mixed Al2O3 and MgO layers, which aid in improving the dispersion of Ni0 centers and consequently strengthens metal interaction with the support matrix. Another example is the experiment carried out by Ashok et al. [69] showing that the addition of an acidic (aqueous) siliceous solution during Ni-MgO synthesis improves significantly the dispersion of Ni upon generating smaller and well-dispersed nanoparticles (Table 4) because of an improved thermal stability resulting from reinforced interactions of Ni with MgO and SiO2-MgO surfaces. On another note, supports such as MgO induce the establishment of solid solutions with metallic ions such as those found in impregnation solutions (Ni2+), according to similarities between their cationic radii (values are close to 0.78 Å) and therefore fulfilling the interstitial solid solution rules founded by HumeRothery [128]. The formation of a strong Ni-Mg solid solution combined with the basicity character of the MgO support are the factors that lead to the remarkable 320 h catalytic stability of Ni/MgO under various simulated inlet feed compositions for “metgas” production (ref. [18], Tables 4 and 5). In addition, reports regarding the effect of thermally resistant MgAl2O4 spinel oxide structures generated during thermal treatments of hydrotalcite MgO-Al2O3 materials has been considered as factors affecting the longterm stability of supported Ni0 species during CSDRM (ref. [34], Table 4). The beneficial impact of this structure resides in the following aspects: i) strong MSI with Ni that generates highly dispersed metallic sites on the magnesium aluminate spinel-derived structure and ii) the merits of the basic properties of the supports that enhances CO2 chemisorption which helps against coke deposition and its accumulation (as discussed earlier). Nonetheless, very strong metal-support interactions lead to the necessity of operating the in situ pre-treatment reduction step at high temperatures, which adds to the complexity and cost of the overall reforming operation. Thus, appropriate MSI levels are required by adjusting the configuration of the spinel MgAl2O4 phase upon incorporating optimum amounts of MgO (neither too low not too high) in Al2O3 for achieving positive influences in catalytic behaviors (ref. [36], Table 4). It is worth mentioning that the utilization of MgAl2O4 spinel structures as catalytic supports in CSDRM is far from reaching industrial implementation and their usage is still limited to laboratory-based catalytic evaluations. On

30 another hand, MgO is being applied commercially for “metgas” production in the „„George Olah renewable methanol plant‟‟ of carbon recycling international (CRI) in Iceland [129]. The application of hydrotalcite based-materials could be as promising as that of MgO owing to advantageous effects discussed above in addition to the fact that industrial CSDRM conditions are similar to those of SRM (both processes are conducted at elevated temperatures and under steam-rich atmospheres) where MgAl2O4 are very popular candidates for catalyzing methane steam reforming. The strategy of enhancing the dispersion of metallic nanoparticles and improving their interaction with the support is a proficient tactic for developing effective monometallic-supported catalysts for CSDRM (Tables 4 and 5) as has been recently reviewed by Li et al. [130] in their work on reforming reactions especially those conducted in presence of steam where sintering becomes highly favored. In this context, an improved conventional Ni/Al2O3 catalyst is emerged by enhancing the interaction of Ni with their commercial alumina support after a steam-assisted treatment session (H2/H2O) proceeding the reduction one (ref. [35], Table 4). Fig. 10(a) shows the temperature programmed reduction (TPR) profiles of the conventionally pre-treated (in H2-rich medium) Ni/Al2O3 and that subjected to the stream pre-treatment session. As a result of steam treatment, Ni is reduced at much higher temperatures (range: 700–1000 °C) indicating the formation of Ni-aluminate that result in the generation of tiny Ni0 nanoparticles, being more reactive (ref. [35], Table 4) than bigger ones deposited over non-thermally treated Ni/Al2O3 (ref. [35], Table 5). The higher reactivity of the thermally treated Ni/Al2O3 catalyst induces higher H2 and CO yield values where, H2 yield is around 92.4% over thermally treated Ni/Al2O3 (recorded after 200 h of CSDRM) compared to 88.9% over the non-thermally treated catalyst. Moreover, a higher CO yield value (91.5% compared to only 81.5%) is measured over the stable alumina-based catalyst owing to the hightemperature thermal treatment (H2/H2O) session generating highly accessible Ni0 sites that are strongly interacting with Al2O3. Consequently, weight loss ascribed to deposited carbonaceous species is much greater for non-thermally treated Ni/Al2O3 (15.4 wt% of coke over the spent catalyst, Fig. 10b) as compared to thermally treated one developing only 3.6 wt% of coke (Fig. 10b), in line with ameliorated reactivity and catalytic stability levels (Table 4). 4.2.6 Bimetallic synergetic effect Besides the design of monometallic Ni-based catalysts with various support modifications and alteration of properties by addition of metals having basic or redox properties; alloying with a second metal (either noble or transition one), acting as co-catalyst, results in modification of catalytic properties through a synergistic effect between both metals. The second metal offer advantages in terms of activity, stability and “metgas” selectivity on the first metal (typically nickel species) (refs. [39,50,53,66,71,72], Table 4). Bimetallic catalysts are receiving significant attention in CSDRM based on their technological impacts

31 and intrinsic scientific significance. When coupled to an oxide (such as catalytic support), these catalysts develop bifunctional properties due to a change in metal-oxygen bond strength that eventually reflects differences in their catalytic properties and performances [131,132]. For instance, García-Diéguez et al. [39] attest that the addition of just 0.04 wt% of Rh2O3, which modifies the Ni ensemble environment (particularly Ni electronic properties), results in an overall amelioration in CSDRM performance (Table 4) with inhibition in Ni0 sintering and carbon deposition/accumulation (Table 5). Fig. 11 displays representative TEM images of the various evaluated Ni, Rh and, Ni-Rh catalysts before (Fig. 11A) and after catalysis (Fig. 11B). A comparison between TEM images of the non-Rh containing catalyst (inset figures a, Fig. 11A,B) and those housing trace amounts of Rh (inset figures b and c, Fig. 11A,B) show an apparent reduction in carbon formation in presence of rhodium nanoparticles. In fact, the amount of C(s)-detected by elemental analysis over Rh-containing catalysts is less than 0.3 wt% whereas, higher amounts of carbonaceous deposits are clearly detected (appearing in the form of short carbon nanotubes) on the surface of spent Ni4%Al2O3 (Fig. 11). Yet, these catalysts display poor selectivity towards “metgas” production owing to the dominance of the side RWGS reaction decreasing the produced H2:CO molar ratio to values lower than the desired value of two. Moreover, several researchers provided experimental data on the synergistic effect demonstrated by Ni-Co bimetallic-based catalysts as a proficient tactic for insuring high activity and stability under harsh CSDRM conditions (ref. [50], Table 4). Alloying Ni with Co decreases average Ni0 particle size to an extent where nanoparticles display sizes below the critical limit (size of Ni0 nanospecies around 6 ( 2) nm [133]) viewed essential for initiation of C(s) nucleation and their subsequent growth. Such a reduction in Ni0 size upon coupling with Co is attributed to an amending in the integrity of surface Ni assembly. An improvement in catalytic reactivity along with increased resistance to C(s) deposition can be furthermore achieved upon incorporating trace amounts of Rh2O3 (0.2 wt%) to bimetallic Ni-Co-Al2O3-ZrO2 systems (ref. [50], Tables 4 and 5). It is worth mentioning that when RhCo-Ni-Al2O3-ZrO2 catalyst is operated under DRM conditions, lower H2 and higher CO yields (values: 46.2% and 49.6%, respectively) are obtained as compared to the results (H2 yield: 49.5%; CO yield: 50.3%) measured under CSDRM conditions (ref. [50], Table 5). Authors attributed this behavior to an intervention of the WGS side-reaction in case of CSDRM (due to the presence of steam) yielding excess H2 upon consuming the in situ generated CO. Introducing small amounts of PtO2 to Ni have been also found to positively affect combined methane reforming due to an increase in the dispersion of transition metals resulting from the intimate contact of Ni (or Co) with the noble metal [66,67]. In a recent investigation, Itkulova et al. [66] developed a facile route to synthesize a series of Co-Pt bimetallic nanoparticles with controlled surface composition and structure based on the preferential reduction of Pt precursor. The authors conducted reforming reactions in absence (standard dry reforming operation) and presence of steam (combined dry and steam reforming)

32 and reported almost complete methane and carbon dioxide conversion levels under combined reforming conditions (Table 4). Introducing steam into the CH4-CO2 feed is beneficial to the performance because of its oxidative potential that helps in carbon removal (as previously discussed in section 3.1 of this review) thus, preventing the development of deactivating (graphitic-based) carbonaceous deposits. Besides reaction medium, high conversion levels reported over Co-Pt/Al2O3 are attributable to the synergistic eff ect resulting from the combination of the two metals and such effect is shown to be accentuated upon modifying the Al2O3 support by small amounts (5 and 10 wt%) of zirconium oxide (ref. [66], Tables 4 and 5). Catalytic stability for 50 h on stream is ascribed to the selective deposition of Pt atoms on Co surfaces, leading in a homogeneous coverage of Pt centers. Based on Table 4, it can be seen that the catalyst with a smaller content of Pt (0.25 wt%) performs with the same activity as the catalyst with 0.5 wt% Pt. However, a difference on the level of H2 production (expressed in terms of H2 yield, mmol g-1 s-1) is detected between both Pt-containing catalysts. A higher H2 yield value (2.7 mmol g-1 s-1) is observed over the catalyst with higher contents of Pt attesting for the higher H2:CO molar ratio (value: 1.47; ref. [66], Table 4) compared to a H2:CO ratio of 1.42 over the 0.2 wt% Pt-containing catalyst (H2 yield: 2.4 mmol g-1 s-1). This difference in hydrogen yields is not addressed in their study but it could be mainly attributed to an excessive production of H2 resulting from methane decomposition being (slightly) accentuated in presence of Pt. It is worth mentioning that in Pt-containing catalysts, catalyst activation often leads to a restructuration of the catalytic assembly towards a core-shell derived-structure characterized by a Ni-rich core and a bimetallic alloy of Ni-Pt nanoparticles acting as the shell [71]. Catalytic evaluations reveal that increasing Pt content from 0.2 to 0.3 wt% leads to a greater tendency towards CO2 activation into CO (ref. [71], Table 4) and a lower one towards CHx dehydrogenation into C(s), in agreement with the smaller Ni0 crystalline domains (Table 5) having higher selectivity towards reforming reactions rather C(s)-forming side-reactions. Accordingly, the lower amount of deposited carbonaceous residues over the 0.3 wt% Pt-promoted catalyst influenced positively H2 production with a yield of 79% compared to a H2 yield 73.5% over the 0.2 wt% Pt-containing sample. No data regarding CO yields are reported for any of Pt containing catalysts [71]. As for Pt-coverage and formation of core-shell like derived structures, bimetallic systems of Ni-Ru also tend to interact in the form of clusters with surfaces mainly covered by Ni leading in increased dispersion of Ni0 particles shown to favor the formation of reactive intermediate C(s)-based species (known to initiate CO generation) under combined reforming reactions [72]. These structures, with optimum synergetic effects, are obtained upon manipulating the order of metal deposition during catalyst synthesis. Simultaneously introducing Ni and Ru precursors over MgO-Al2O3 oxide is more beneficial for the design of more reactive bimetallic catalysts than a consecutive impregnation of Ru on Ni-containing hydrotalcite materials (ref. [72], Tables 4 and 5). The sample prepared by simultaneous post-

33 impregnations demonstrates lower tendency to coke deposition because of an improved Ru-Ni interaction making it more active for the in situ gasification process of carbon deposits. 4.3. Innovative development in stabilization of Ni0 nanoparticles for selective CSDRM operations Based on the previous section, it can be concluded that several key factors, such as catalyst structure and synthesis method, are crucial for the design of catalytic configurations having specific properties (i.e., catalyst resistance to carbon and thermal agglomeration) and therefore particular functionalities. Many studies have been conducted with the purpose of designing Ni-based catalysts in the most appropriate manner for providing considerably high reactivity levels. Part of these methodologies have been covered in the proceeding part where additional (recent) innovative tactics (including the stabilization of Ni0 nanoparticles (i) on carbon/carbide-based supports, (ii) inside supports with organized porous arrangements, (iii) inside core-shell derived-oxides and, (iv) within well-defined oxide structures) will be addressed in this part of the review. 4.3.1 Stabilization of Ni nanoparticles on carbon/carbide supports Several carbon-based materials like activated carbon, carbon black, graphite, carbon nanotubes and, carbon nanofibers have been considered as promising supports in heterogeneous catalysis [134,135]. When low metal loadings (range: 1–10 wt%) are used as active phases, the large specific surface area and the high porosity of carbonaceous oxides are advantageous for the dispersion of active metallic centers which in turn improves the resistance of Ni0 to sintering preventing as the consequence the formation of (deactivating) encapsulating C(s)-types. Moreover, in carbon-based supports characterized by mesoporous structures, catalytic reactions proceed differently over confined nanoparticles (occur at a faster and more selective rate) than those supported on external surfaces [136]. When compared to “standard” (conventional) supports such as Al2O3 and SiO2, carbon-based materials offer supplementary advantages when applied as catalytic supports. Such benefits consist of (i) an improved resistance to acidic and basic environments, (ii) a greater tolerance to harmful impurities (such as sulfur gases), (iii) a higher thermal stability even at temperatures exceeding 750 °C, (iv) an easier reducibility of oxides to metallic phases (lower temperature requirements for in situ reduction treatments), (v) a versatility in the preparation of porous carbon supports (i.e., granules, pellets and, fibers) with tailored pore size distributions depending on the catalytic application, (vi) a lower cost and a wider availability and, (vii) an easier recovery of active phases after catalysis using typical separation processes such as leaching. Despite these encouraging characteristics, some carbon-based oxides could possibly suffer from a drawback associated with their tendency to gasify when subjected to high-temperature hydrogenation, reforming and oxidation reactions making them critical options when applied as catalytic supports [137].

34 When thermally stable carbon materials are used for the dispersion of Ni species, their high surface area and surface groups make them ideal supports that result in active and stable CSDRM catalysts (refs. [54,79], Table 4). It is proposed that the CSDRM reaction mechanism over activated carbon-based catalysts is established on CH4 adsorption and cracking followed by simultaneous adsorption of CO2 and H2O leading consequently to gasification of (solid) surface carbon by adsorbed oxidants (CO2 and H2O) to produce CO [54]. In their study, Brush et al. [54] conducted CSDRM under various inlet feed compositions with a special focus on carbon types developed in the course of catalysis. They conducted their mechanistic analysis by stopping the reaction at several stages (100 min into catalysis, immediately after deactivation and, 60 min after deactivation) to characterize their spent samples. They show, based on high-resolution TEM images (Fig. 12), that carbon deposition (specifically graphitic-deposits) is not at the origin of deactivation (even upon running the reaction under conditions favoring C(s) deposition) where, the external surfaces of the (deactivating) spent catalyst are completely free of any graphitic accumulations (Figs. 12c,d). Such an accumulation present on the fresh catalyst (Fig. 12a), resulting from the adopted synthesis methodology, undergoes in situ gasification by CO2 and H2O when subjected to reaction medium (Fig. 12b). Likewise, the high coking resistance of non-noble metals (i.e., molybdenum and tungsten) dispersed on carbon/carbide supports is highlighted in the work of Claridge et al. [79]. Authors evaluate the reactivity of Mo and W impregnated on carbide supports in multiple methane-based reactions (such as dry, partial oxidation, steam and combined dry and steam reforming reactions) and report active (reactivity levels close to expected thermodynamic ones) and stable performances with (almost) no carbon deposition over spent samples. Even if they exist (in small amounts), carbonaceous residues are present under non-deactivating graphitic forms rather as, easily oxidizable, carbon nanotubes. The coking resistance follows a cyclic route where first carbon species, from shallow carbide layers, react with the dissociative adsorption products of CO2 and H2O (specifically activated O*) to leave a vacant structural defect. This vacancy is then filled with CHx species, resulting from methane cracking, where excess O* will react with such carbon intermediates (instead of the carbon present within outer carbide layers) facilitating thus carbon removal. 4.3.2 Confinement of Ni0 nanoparticles within porous walls of ordered mesoporous structures Structured supports with ordered arrangement and well-defined porous channels, such as oxides based on SiO2 and/or Al2O3, are interesting candidates when applied as supports owing to their exceptional physical characteristics including high specific surface areas, narrow pore size distributions and large porous volumes [138]. The design of Ni deposited on (or into) porous supports can be conducted by several active phase introduction methods (i.e., post-impregnation over the support or directly via “onepot” method in the course of support precipitation) or simply by varying the type and chain length of the

35 structuring (template) agent used in the preparation. Both tactics alter pore size distributions, internal porous connectivity, porous shapes (hexagonal or cubic arrangements) and active phase localization in resulting catalysts. Indeed, Ni0 nanoparticles (after reduction sessions) can be either grafted on the support surface or else embedded within the porous matrix of the housing oxide [139,140]. In any case, a highly structured catalyst with a well-defined porous network is characterized by a uniformity in its active sites distribution than other type of catalysts and is therefore more “easily” defined than those with noncontrollable distribution of active sites. From design and development perspectives, porous-based catalysts are more predictable which makes them more reproducible for large-scale industrial applications [141]. In this context, some (very recent) studies dealt with the development of Ni on porous materials as potential catalytic systems for “metgas” production via CSDRM [32-33,49,58,60,68]. Yet, such reports are scarce compared to those already reporting performances of porous-based materials in either dry [13] or steam reforming [12] of methane reactions. Owing to the unique physical properties of SBA-15, its usage as a support for Ni0 species in CSDRM is gaining substantial attention in recent literature [49,58,60,68]. The high thermal and mechanical stabilities, distinctive 2D hexagonal arrangement of the porous structure, large pore size and, high internal surface area are the main criteria that result in active Ni/SBA-15 catalysts along with minimal Ni0 sintering and low amounts of carbon deposition. Such advantages could be accomplished over non-porous based catalysts yet after incorporation of secondary elements (or promoters) having particular properties (as previously discussed in sections 4.2.1-2-3-5-6). Because of their high specific surface area and interconnected porous channels, a homogeneous dispersion of small nanoparticles is achieved through conventional post-impregnation (Ni deposition) methods prior to and after high-temperature catalytic reaction [49,58,60,68]. For instance, average size of NiO nanoparticles over freshly synthesized Ni10%/SBA-15 is close to 16 nm where well-dispersed nanoparticles retracing the porous channels of the SBA-15 matrix (Fig. 13b) remain in the same size range after catalysis [60]. Moreover, the physical features of SBA-15 (such as hexagonal arrays and cylindrical mesopores in longitudinal arrangement) are clearly visible before (Fig. 13a) and after Ni impregnation, (Fig. 13b) highlighting on the high mechanical and thermal stability of the mesoporous SiO2 withstanding aqueous solution impregnation and subsequent calcination step. The performance of Ni/SBA-15, associated to the physical properties of mesoporous SBA-15, as catalysts for CSDRM is very promising where reactivity levels are elevated along with selective production of “metgas” mixtures (ref. [49], Tables 4 and 5). Coke resistance is enhanced upon alteration of catalyst basicity via the addition of MgO oxides (ref. [49], Table 5). Likewise, it is found that the addition of boron (1 or 3 wt%) to Ni/SBA-15 catalysts is beneficial towards CH4 and CO2 conversion but exceeding the 3 wt% content induces opposite effects (ref. 60, Tables 4 and 5). Complementary, product yields obtained at boron loading of 3 wt% are optimal and reached 55% with respect to CO and

36 85% with respect to H2. Increasing boron content to 5 wt% (or higher) induces negative effects due to larger activation barriers of C–H bonds (leading to coke deposition) and significant decline in active surface due to Ni0 sintering on stream (ref. [60], Table 5). Moreover, Ni impregnated on SBA-15 are shown to be versatile systems capable of catalysing, with high reactivity, various CSDRM inlet feed compositions with trends close to theoretical (thermodynamic) ones (previously discussed in section 3 of this review). For example, under stoichiometric conditions (CH4:CO2:H2O equal to 3:1:2) and at 800 °C, CH4 and CO2 conversion levels are close yet slightly lower than theoretical values (Table 4). Decreasing CH4/(CO2+H2O) ratio increases CH4 conversion while decreasing that of CO2 because of thermodynamic dominance of steam reforming over methane dry reforming. On the other hand, increasing CO2/(CH4+H2O) ratio elevates CO2 conversion while maintaining high CH4 conversion value (in the range of 60%) because of dominance of dry reforming consuming both CH4 and CO2 gases (ref. [58], Table 4). Additionally, increasing CO2/(CH4+H2O) ratio considerably enhances H2 and CO yields because of rising in CO2 gasification rate of partially dehydrogenated intermediate species. In fact, H 2 yield rises from 50% to 55% and that of CO passes from 43% to 55% as a result of increasing the CO2/(CH4+H2O) ratio from 0.15 (CH4:CO2:H2O = 3:1:3.6) to 0.20 (CH4:CO2:H2O = 3:1:2). Nonetheless, selectivity values under the three evaluated inlet feed compositions (Table 4) deviate from thermodynamic ones because of the occurrence of side-reactions mainly RWGS under stoichiometric conditions, CH4 decomposition under excess methane stream and WGS in presence of excess CO2. Siang et al. [60,68] have also reported lacks for “metgas” production over Ni/SBA-15 catalysts (even those promoted by boron element) but corresponding reasons are not clearly addressed in their reports. It can be tentatively associated to the occurrence of side-reactions especially CH4 decomposition (producing C(s) and H2, Eq. (7)) increasing therefore H2:CO product ratio to values higher than 2.0 even under conditions favoring “metgas” generation (Table 4). Despite some C(s) deposition, the majority of Ni0 particles remain confined inside the support assuring stable performances on stream (refs. [49,60,68], Table 4). Contrarily to the reported stable performances of Ni/SBA-15 in CSDRM (even for those displaying poor selectivity), our team tested the reactivity of a 5 wt% Ni impregnated on mesoporous SBA-15 and noted strong deactivation behavior even after few hours on stream at 800 °C (ref. [32], Table 4). The catalyst displays high intrinsic CH4 and CO2 conversion levels but low selectivity outcome. After a short exposure to reaction medium, it experiences a rapid deactivation where conversion values drop by about 50% initial ones after only 12 h of catalysis. Losses in reactivity with time on stream are attributed to partial reoxidation of active Ni0 into inactive NiO crystals (detected by wide-angle X-ray diffraction) under steamrich CSDRM atmosphere. In the same study, we obtained similar deactivation trends also associated to in situ re-oxidation of Ni0 over another SiO2-based catalyst (Ni impregnated on macroporous diatomite, Ni5%/CeliteS). Representative TEM micrographs of spent Ni5%/SBA-15 and Ni5%/CeliteS catalysts are

37 displayed in Figure 14. The TEM image of Ni5%/CeliteS reveals extremely large pores (in line with the macroporous character of the support) along with big Ni nanoparticles (difficult to distinguish whether Ni0 or NiO species) and some C(s) deposition (see arrows in Fig. 14a). In the case of the mesoporous sample, the hexagonal array of SBA-15 silica is still clearly visible and partially occupied by Ni nanoparticles while others are found on the external surface together with few (very short) nanotubes (Fig. 14b). Deactivation of these siliceous-based supports under CSDRM is therefore mainly associated with partial re-oxidation and sintering rather than with coking. As an alternative to limit re-oxidation and design stable CSDRM catalysts, silica is substituted by alumina where Ni impregnated on mesoporous (or non-porous) Al2O3 based-catalysts are found to be more active and stable (particularly Ni impregnated on mesoporous Al2O3) than Ni5%/SBA-15 (ref. 32, Table 4). However, deactivation is noted over both Al2O3based samples because of their unstable structure resulting from (i) partial shrinkage of the alumina matrix during post-impregnation of aqueous Ni solution and (ii) non-ordered macroporous structure of the non-porous Ni5%Al2O3 catalyst prepared in absence of structuring agent. The partial collapse of the alumina structure causes severe agglomeration of Ni0 nanoparticles on the external surface of grains (Fig. 15c,d) and sintering is accentuated in the case of the non-porous catalyst (Fig. 15a,b). Conversely, the substantial effect of metal confinement inside well-defined Al2O3 matrix is verified upon introducing Ni via “one-pot” approach, in presence of a templating agent, during alumina synthesis. The Ni 5%Al2O3 catalyst prepared by improved evaporation-induced self-assembly (EISA), presents the highest reactivity with conversion values close to expected thermodynamic ones and is an ideal candidate for “metgas” generation (ref. [32], Table 4). The occlusion of metallic Ni0 nanoparticles inside the 2D hexagonal mesoporous arrangement of the alumina support enables particles to remain fully accessible and protects them from sintering and subsequently from coke formation/accumulation (Fig. 15e,f). 4.3.3 Enclosure of Ni0 nanoparticles as a composite metal in core-shell structures Among the various categories of composite nano-catalytic structures such as core-shell, yolk-shell and Janus nanostructures, former ones are the most adopted routes for synthesizing composites designed for heterogeneous catalysis processes [142,143]. In core-shell structures, oxide precursors are hydrolyzed then condensed into concentric core-shell nanospecies with conformal shells precipitating around metallic nanoparticles [144,145]. For core-shell catalytic systems, diffusion of gaseous reactants through the shell towards the active metal (i.e., reduced Ni0 particles) should be rapid and easy so that conversion of reactants and generation of products is a continuous process with minimal mass transfer limitations [146]. Core-shell based-catalysts for application in CSDRM are rarely considered whereas their utilization is popular for catalyzing methane dry and steam reforming reactions. This is mainly due to the high

38 complexity of combined reforming and the risk of not withstanding extensive operation conditions (high temperature and steam-rich atmosphere), as already discussed in the thermodynamic part of this review. Nevertheless, a recent work by Kang et al. [41] dealt with the preparation and testing (for “metgas” production) of Ni-based Al2O3 and MgO-Al2O3 core-shell structured composites. Catalysts are prepared under a multi-bubble sonoluminescence (MBSL) conditions [147,148] to obtain core-shells with high thermal stability and well-defined structures. Figure 16A displays the blue light from a bubble cloud of several thousand microbubbles at MBSL conditions. This indicates that synthesized materials are emitting light and are assembled in their desired shapes. In fact, the inset TEM image of Ni@Al2O3 catalyst (Fig. 16a) shows that the darker part (resulting from the diffusion of higher density elements) of the core-shell is the center occupied by Ni0 nanoparticles and the lighter (gray) layers are Al2O3 shells. The core-shell catalyst shows excellent activity as well as stability in CSDRM (ref. [41], Table 4) for different temperature values (range: 700–850 °C) indicating high resistances towards C(s) deposition and Ni0 sintering for 50 h on stream. Yet, the catalyst displays low selectivity for “metgas” despite a stoichiometric inlet CH4:CO2:H2O feed composition ideal for the production of a H2:CO molar ratio of two. The production of higher than expected H2:CO values (range: 3.2–2.6, Table 4) at the various operating temperatures (expected thermodynamic values are previously discussed in sections 3.1-3.3) is tentatively ascribed to the excess contribution of the RWGS reaction yielding additional H2. As a way to improve CO2 adsorption for sufficient production of CO (and therefore to minimize H2:CO values), authors extended their core-shell preparation method by incorporating elements having basic properties (principally MgO). The Ni-based MgO-Al2O3 catalyst displays reactivity and stability levels as promising as those recorded over Ni@Al2O3 (Table 4). Selected scanning electron microscopy (SEM) images of spent Ni@MgO-Al2O3 catalyst after 50 h of combined reforming at 800 °C and 850 °C (Figs. 6b and c, respectively) show smooth homogeneous external morphologies without any cracks and defects. Such observation highlight their high thermal stability. Additionally no carbonaceous residues nor metallic nanoparticles are deposited on the external surface justifying the resistance of core-shell materials to both coking and sintering. Nevertheless, the presence of the basic modifier was not beneficial for selectivity improvement where H2:CO values remained higher than expected under tested temperatures. Such observations lead authors to suggest that CH4 and/or H2O molecules, coexisting in the same reactional medium, block active CO2 sites minimizing therefore CO2 adsorption and consequently CO production rate. 4.3.4 Stabilization of Ni0 nanoparticles within well-defined (non-ordered) oxide structures In addition to the incorporation of promoters with basic/redox properties, usage of mesoporous oxides for confinement purposes and synthesis of thermally stable core-shell derived materials, some of the newest

39 strategies for the development of stable and selective CSDRM catalysts involve the usage of “unusual” supports capable of improving MSI. Such supports include (i) porous halloysite derived SiO2-Al2O3 oxides [51], (ii) Ni ribbon plates [56,57], (iii) Ni foams [65] and, (iv) macroporous Ni/La2O3-SiO2 composites [62]. The utilization of halloysite nanotubes (specific surface area = 83.3 m2 g-1, porous volume= 0.33 cm3 g-1) as templates for the design of (natural) Ni-based catalyst is found beneficial for the high temperature reforming reaction (ref. [51], Tables 4 and 5). Compared to Ni impregnated on a mixed SiO2-Al2O3 (Ni/SA-P) prepared by co-precipitation, the catalyst derived from halloysite (after successive purification and thermal treatment sessions) presents higher stability for 30 h run (Table 4) owing to an improved dispersion of smaller Ni0 nanoparticles and stronger MSI. Thermogravimetric profiles of spent catalysts (Fig. 17a) confirm that small nanoparticles strongly interacting with the support matrix are less prone to coking. In fact, no visible weight loss ascribed to C(s) deposition is detected over spent Ni/(SANR-H) whereas, an oxidative removal of around 30 wt% of carbonaceous deposits is noticed over spent Ni/(SAP). Moreover, TEM image of the spent halloysite-derived catalyst (Fig. 17b) attests the absence of carbonaceous species demonstrating the remarkable resistance to coking. It is worth mentioning that selectivity measures are absent over both catalysts although the adopted inlet feed composition and operation temperature are adequate for “metgas” production (Table 4). Besides natural silica-alumina composites, “original” MgO-coated Ni ribbon plates [56,57] or Ni foams [65] appear as promising candidates for “metgas” production via CSDRM. The catalysts with MgO underlayer (treated under H2 or air streams) completely covering nickel ribbon exhibit stable performances under combined reforming conditions (Tables 4 and 5). After reaction, nickel crystals remain well-dispersed and epitaxially bounding with MgO inhibiting thus their contact with reaction medium and preventing their drastic carbonization and sintering. Intrinsic activity levels (refs. [56,57], Table 4) are found to depend on the type of MgO treatment. In fact, heating MgO under hydrogen stream rather than in flowing air improves the dispersion of Ni0 species thus creating smaller metallic particles with higher resistances to deactivation (Table 5). Yet, data concerning CO2 conversion and H2:CO molar ratio are not reported and therefore selectivity towards “metgas” is not clearly addressed over such catalytic systems. Regarding Ni foam catalysts, impregnation of Ni/Al2O3 (prepared by post-impregnation) rather than Ni-free Al2O3 over a designed Ni foam induces positive enhancement in both CH4 and CO2 conversion values (ref. [65], Table 4). The high catalytic performance is attributed to a uniform distribution of Ni over Al 2O3 and over the Ni foam support. The optimized dispersion of active Ni centers facilitates heat transfer and creates a uniform temperature distribution across the catalyst bed (ref. [65], Table 5).

40 Synthesizing porous Ni/La2O3-SiO2 catalysts is conducted for purposes including the investigation of (i) the positive effect of La2O3 (as basic additive) and (ii) of different preparation methods (type of structuring agent applied in the course of preparation) on reactivity and stability in CSDRM. The addition of 3 wt% La2O3 is found ideal for a stable performance and for a selective production of “metgas” (ref. [62], Tables 4 and 5). In addition to the presence of La2O3, the use of either ethylene glycol (EG) or polyethylene glycol (PEG) in catalyst preparation enhances activity, stability and coke resistance. The utilization of both structuring agents in the same preparation batch is not necessary for stabilizing Ni0 against coking and sintering. However, the absence of either one results in a catalyst that deactivates rapidly on stream (Table 4) because of a non-stable structure associated with severe carbon deposition (Table 5).

5. REACTION MECHANISM AND KINETIC MODELING FOR CSDRM In this section of the article, a brief review on the mechanism of CSDRM is presented along with some literature findings dealing with the design of theoretical models for appropriate simulation of the kinetic aspects of the combined reforming reaction. 5.1. Potential reaction pathways during CSDRM reaction A reaction mechanism for combined steam and dry reforming of methane over a Rh/MgO-based catalyst has been proposed by Qin at al. [76] upon following an in situ isotope-labelled 13CO2 transient experiment (conducted as temperature programmed adsorption experiments) during the course of combined reforming (CH4:CO2:H2O = 3:1:2). In their experiments, 13C isotope-labelled 13CO2 was used to replace the normal CO2 in mixed reforming. Interpretation of their results was followed on a mass spectrometer (MS) upon calculating the relative amounts of labelled and unlabeled species based on their relative peak heights. The overall CSDRM process is expressed in Table 6 in terms of various mechanisms classified into four consecutive reaction paths involving: i) methane activation, ii) decomposition of CO2 and H2O, iii) surface reactions of adsorbed species and, finally iv) “metgas” generation. The in situ labelled 13CO2 transient experiments showed that C(s) formed from CO2 and/or CO decomposition reactions was less reactive than carbonaceous intermediates (CHx species) originating mainly from methane decomposition (methane cracking reaction). On the basis of their temperature programmed spectra and isotopic transient techniques, they suggested that both steam and dry reforming reactions start at the same time (CH4, CO2 and H2O molecules are activated simultaneously) and proceed towards syngas generation by sharing same kind of reaction intermediates specifically activated O* (termed as O-M in Table 6) generated from the dissociation of oxygen-rich H2O and CO2 species. The reaction mechanism suggested that CO is in situ generated from the reaction between O* and (chemically active) CHx species (x= 0, 1, 2 or 3). In the same

41 study involving Ru, Pt, Ir and Pt, authors stated that the reactivity of each catalyst is strongly dependent on the rate of CH4 activation and on the relative stability of the oxygen-metal (O-M) bond influenced by the nature of the active site. 5.2. Kinetic modeling for CSDRM reaction Owing to the fact that reforming reactions are generally heterogeneous reactions, kinetic limitations are always seen to affect the performance of reforming systems. It has been established that the condition of the system in the bulk phase is always different from that actually occurring inside the porous arrangement of a catalyst due to some transport limitations that influence product selectivity and therefore H2 and CO distributions [87]. Several studies have been reported in literature aiming to understand the influence of such limitations in order to accurately predict selectivity measures under actual reforming operation conditions. In this context, Xu et al. [149] developed a kinetic modeling for SRM accompanied by the side WGS reaction on a Ni impregnated on MgAl2O4 catalyst. Authors derived three rate expressions by using rigorous model discrimination and parameter estimation techniques for about 120 independent theoretical runs. These rate expressions are expressed in Table 7 and were derived assuming negligible external diffusion and heat transfer limitations, as evidenced by their experimental judgements. As for SRM Verykios et al. [150] established a rate expression for DRM (shown in Table 7) over a Ni impregnated on La2O3 catalyst. In the kinetic model on DRM, it was assumed that methane activation is a surface-induced reaction that is also the rate-determining step, meaning that all other intermediate steps are believed to occur at a faster rate than that responsible for the C–H cleavage bond. Compared to SRM and DRM reactions, scarce are the studies dealing with the kinetic modeling of CSDRM where a recent investigation by Challiwala et al. [87] was performed to check the feasibility of a combined SRM/DRM process based on available kinetic models reported for SRM and DRM (Table 7). It is worth mentioning that in both steam and dry reforming of methane kinetic studies, integral and differential examinations were performed on similar setups making thus the coupling of the developed rate expressions (derived from both studies, Table 7) a suitable approach to understand the regimes of kinetic simulations and to predict original models for CSDRM (CH4:CO2:H2O = 3:3:3). Authors integrated and validated the combined SRM/DRM kinetic parameters with thermodynamic data and found a good agreement between both sets of values in terms of CO and H2 distributions over a wide range of temperature (227–1227 °C) and pressure (P ranging from 1 to 25 bar). However, they noticed that at lower temperature values (range: 227–627 °C), H2 trend deviated by a small extent from the expected equilibrium values because of a tuning in (some) kinetic parameters that were adopted from literature [149,150] and applied as developed for kinetic evaluation of CSDRM. Other features that contributed to the overlapping between the two predicted trends (kinetic and thermodynamic profiles) are the effects of heat, mass and momentum

42 transfer limitations that were not considered to arise inside the bulk and the porous medium of the catalytic bed within the reformer unit. Such limitations should be considered for appropriate data interpretation upon adopting appropriate transport equations that are pertinent with the type of the catalyst used.

6. SUMMARY AND FUTURE OUTLOOKS The first part of this review deals with an evaluation of thermodynamic aspects for tuning combined steam and dry reforming of methane (CSDRM, 3CH4 + CO2 + 2H2O

4CO + 8H2) towards “metgas”

(H2:CO = 2.0) production. Such an intermediate syngas mixture can be used directly as feeding stream for subsequent generation of methanol, viewed as one of the most promising renewable energy vectors for substitution of non-renewable fossil fuels resources. The molar CH4:CO2:H2O composition, operating temperature, process pressure and, dilution by an inert gas (i.e., He or Ar) have significant influences on conversion levels, coke yields and selectivity (H2:CO product ratio) in CSDRM. Theoretical simulations have shown that (CO2+H2O)/CH4 ratio strongly affects the temperature at which maximum conversion of gaseous reactants is attained. Conversion profiles and H2:CO production ones attest that CSDRM is a complex network of multiple reactions (i.e., reforming of methane, non C(s)-forming reactions and C(s)forming ones) where syngas producing ones are normally initiated at high temperatures (above 600 °C) justifying the strong endothermic nature of the reaction. Moreover, data have shown that the activation of CO2 is critical in CSDRM and cannot be initiated unless operating the reaction at elevated temperatures. Thus, both facts force the applicability of the reaction under harsh temperature conditions, which adds to the complexity of the process. Thermodynamic plots under C(s)-assisted simulations have shown that coke formation is not likely to be avoided at low temperatures even under excess oxidizing agents (excess H2O or CO2) yet, such formation could be inhibited at high temperatures (exceeding 750 °C) and especially under rich-oxidation atmospheres (CH4:CO2:H2O = 3:1:4). In addition to limiting C(s) accumulation for long lasting catalytic runs, a main industrial focus associated to CSDRM should be the generation of high-purity “metgas” mixtures projected for methanol production and for applications in FT processes aiming for long hydrocarbon chains. Among the evaluated inlet feed compositions, simulating CSDRM under a stoichiometric feed composition (CH4:CO2:H2O = 3:1:2) is ideal for selective production of “metgas” within 750–800 °C range. Increasing the pressure from 1 to 10 bar reduces product yields and increases coke formation rates (particularly CH4 decomposition) since equilibrium will shift to reactional directions with larger moles of reactants. Therefore, a low C(s)-producing CSDRM reaction yielding selective production of “metgas” is best operated at 800 °C and at one bar unless pressure is an industrial requirement then, the temperature should be raised to a least 950 °C in order to minimize potential carbon accumulation. If a diluent is considered in the simulation, increase in reactivity levels are detected at

43 relatively low temperatures (600 °C) however, its effect is negligible at around 800 °C (and higher), where the system becomes entirely driven by highly endothermic SRM and DRM reactions. The second part of the review summarizes the recent innovative developments in terms of Ni-based catalysts for “metgas” production via CSDRM. Catalyst design and preparation technique play vital roles in the optimization of catalytic properties that influence catalytic stability in CSDRM. Numerous salient tactics are summarized in order to highlight on the main factors that lead to the development of stable Nibased catalysts. Such features include surface acidity and basicity, mobility of oxygen species, redox properties, presence of active hydroxyl groups, degree of MSI and, metallic synergism. For instance, bimetallic catalysts for particularly designed synergistic effects are presently gaining attention since the added metal complements the desirable properties of Ni to provide active and coke-resistant sites in CSDRM. Moreover, there exist a general agreement (based on reported data in various reforming reactions) that nano-based catalysts are characterized by an improved dispersion of active sites, which constitutes a vital requirement in the prevention of thermal agglomeration and subsequent coke deposition/accumulation. In this context, several innovative Ni-based catalysts have been considered in previous literature and their promising performances are reported in this review. Amongst the suggested means, stabilizing active sites on (or into) a high surface area porous oxide or designing the catalyst in the form of a core-shell composite structure have been proven as efficient tactics for developing stable systems for long catalytic runs. Particularly, confining nanoparticles within a porous matrix and establishing a strong MSI stood as a favorable method for limiting sintering and coking. The utilization of structuring agents within the course of synthesis and introducing Ni precursors directly in the course of support precipitation achieved the desired goal of forming mesoporous Ni-based catalysts for selective “metgas” productions. Then again, the encapsulation of Ni nanoparticles (as the core) by a homogeneous thin shell is a unique design that provides a great prospect of exponentially increasing the rate of conversion in addition to improving metal sintering, especially under harsh operation conditions. Regarding mechanistic aspects, SRM and DRM reactions are believed to run simultaneously through several surface-adsorption reactions generating H2, CO as well as C(s) deposits. The removal of carbonaceous deposits (expressed as CHx intermediates) is achieved through in situ generation of activated O* species resulting from dissociation reactions of oxygen-rich molecules (such as H2O and CO2). Such surface-induced oxidation reactions will generate incompletely oxidized CO molecules and hydrogen atoms, ultimately leading in syngas generation. On the other hand, a good agreement is found between thermodynamic data and kinetic modeling for CSDRM upon adopting available SRM and DRM models (simulated in absence of heat, mass and momentum transport limitations) and arranging them into one suitable approach for estimating kinetic parameters for CSDRM. Yet, efforts devoted in the direction

44 of designing complete analysis upon accounting for all possible transport limitations is mandatory for accurate interpretation of kinetic mechanisms. Nonetheless, the major drawback associated to nano-based catalysts is the up scaling of catalyst synthesis from laboratory batches to continuous industrially oriented scales. Thus, developing economical and reproducible synthetic strategies is mandatory for linking all of the reported advantages of nano-based catalysts to large-scale “metgas” generation facilities. Regarding the establishment of strong MSI (metaloxide bounding) and core-shell framed structures, details on the geometric and electronic aspects of the catalytic design should be a required knowledge for a better understanding of reactivity evaluation and for providing accurate correlations between structural properties and catalysis. In-depth examination into the formation of intermediates in the course of preparation and their impact of the characteristics of the catalyst are possible through in situ experimental techniques, like steady-state isotropic kinetic analysis upon using mass spectrometry. Additional characterization tools can also serve for such a purpose, such as (i) diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), (ii) TEM (for visualizing embedded nanoparticles within the oxide matrix and possible orientation behaviours) and, (iii) X-ray absorption spectroscopy (XAS) accompanied by high spatial resolution (for identification of chemical states, local structural information and, bond strength coordination). Such fundamental (advanced) data will provide grounds for rational optimization of selectivity towards “metgas” since this is an issue for the majority of CSDRM catalysts despite their high activity and stability. On another aspect, formation of supported Ni nanoparticles by plasma treatment is attracting much attention since specific “plasma” Ni clusters with particular structure and improved interaction with the support can be formed. In terms of catalysis, plasma technology generates homogenous surfaces that are concentrated in active metal phases, which significantly initiate high intrinsic activity levels along with improved carbon resistance assuring thus stable catalytic operation. The utilization of plasma techniques for catalyst synthesis and subsequent application under harsh CSDRM conditions is still not considered and efforts should be devoted for its implementation in catalytic reactions aiming towards large-scale “metgas” productions.

ACKNOWLEDGMENTS The ERANET EU-FP7 initiative, the national ANR (France) and CNRS-L (Lebanon) agencies are highly appreciated for their financial support through the SOL-CARE (Energy-065, 2016-2019) project (JCENERGY-2014 first call). Dr. Nissrine El Hassan (UOB), Pr. Pascale Massiani (Laboratoire de Réactivité de Surface, Paris) and Dr. Anne Davidson (Laboratoire de Réactivité de Surface, Paris) are sincerely valued for their productive scientific discussions and constructive comments.

45 Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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50

Table 1. Summary of conditions adopted to perform CSDRM thermodynamic simulations. Variables

CH4:CO2:H2O molar ratio

Set 1 Set 2 Set 3 Set 4 Set 5

Solid C(s)

P (bar)

Aim of the simulation

3:1:2, 3:1:4 and 3:2:2 3:1:2, 3:1:4 and 3:2:2

Temperature range (°C) 200–1000 200–1000

n.c. c.

1 1

3:1:2 3:1:2

200–1000 200–1000

n.c. c.

1, 5 and 10 1, 5, 10, 15 and 20 1

Effect of initial molar composition Effect of initial molar composition under C(s)assisted operations Effect of operation pressure Effect of operation pressure under C(s)-assisted operations Effect of dilution under C(s)-free and C(s)assisted operations

3:1:2 (pure) and 3:1:2 600–1000 n.c. and c. (diluted)* n.c. not considered in the simulation, c. considered in the simulation * Dilution is performed as follows: 10 vol% CH4 diluted in argon and 10 vol% CO2 diluted in argon

51

Fig. 1. Effect of initial CH4:CO2:H2O feed ratio (for C(s)-free operations) on the thermodynamic equilibrium plots of (a) CH4 and CO2 conversion levels, (b) conversion levels of H2O and variation profiles of H2:CO molar ratios, (c) CH4 and CO2 concentration profiles and, (d) H2O, H2 and CO concentration profiles as a function of temperature (range: 200 –1000 °C) at 1 bar. Arrows are added next to XH2O, H2:CO, H2 and CO profiles to indicate their corresponding X-axis.

52

53

54

Table 2. Equilibrium H2 and CO yields during combined steam and dry reforming of methane under the various simulation conditions listed in Table 1. H2 yield (%) CO yield (%) Temperature (°C) Temperature (°C) 400 600 800 1000 400 600 800 Effect of initial molar composition for C(s)-free operations CH4:CO2:H2O = 3:1:2 10 49 89 98 1 40 89 CH4:CO2:H2O = 3:1:4 13 51 86 82 1 31 78 CH4:CO2:H2O = 3:2:2 9 44 90 91 2 36 82 Effect of initial molar composition for C(s)-assisted operations CH4:CO2:H2O = 3:1:2 16 53 88 98 0.6 18 81 CH4:CO2:H2O = 3:1:4 17 59 85 88 0.9 27 85 CH4:CO2:H2O = 3:2:2 13 51 90 91 0.7 21 91 Effect of operating pressure for a CH4:CO2:H2O molar ratio of 3:1:2 (under C(s)-free operations) P = 1 bar 10 49 89 98 1 40 89 P = 5 bar 5 22 72 93 0.5 13 71 P = 10 bar 3 16 59 88 0.3 9 58 Effect of operating pressure for a CH4:CO2:H2O molar ratio of 3:1:2 (under C(s)-assisted operations) P = 1 bar 16 53 88 98 0.6 18 81 P = 5 bar 6 28 74 93 0.2 7 66 P = 10 bar 4 21 61 87 0.2 5 52 P = 15 bar 4 19 51 82 0.1 5 40 P = 20 bar 3 17 46 78 0.1 4 35 Effect of dilution for a CH4:CO2:H2O molar ratio of 3:1:2 (under C(s)-free operations) Pure feed 10 49 89 98 1 40 89 10 vol% CH4 in Ar and 10 vol% CO2 in Ar 21 69 98 100 4 61 98 Effect of dilution for a CH4:CO2:H2O molar ratio of 3:1:2 (under C(s)-assisted operations) Pure feed 16 53 88 98 0.6 30 81 10 vol% CH4 in Ar and 10 vol% CO2 in Ar 28 79 89 100 3 65 90 Aim of simulation

1000 99 85 88 99 91 94 99 93 87 99 94 88 84 80 99 100 99 100

55

Table 3. Equilibrium conversions levels of CH4, CO2 and H2O for combined steam and dry reforming of methane under pure and diluted CH4 and CO2 inlet streams as a function of temperature (P = 1 bar, inlet CH4:CO2:H2O feed ratio of 3:1:2) for C(s)-free and C(s)- assisted reforming operations. Feed composition 3:1:2 (pure) 3:1:2 (diluted)a 3:1:2 (pure) + Cs a

3:1:2 (diluted) + Cs a

X CH4 (%) Temperature (°C) 600 800 1000 39 89 99 67 98 100

X CO2 (%) Temperature (°C) 600 800 1000 10 89 99 46 98 100

X H2O (%) Temperature (°C) 600 800 1000 54 90 98 77 98 100

H2:CO molar ratio Temperature (°C) 600 800 1000 2.7 2.0 2.0 2.3 2.0 2.0

52

98

---b

40

93

54

94

99

5.8

2.1

2.0

99

b

47

93

76

98

100

3.6

2.0

2.0

81

91 97

---

Gases dilution is considered on volume basis (10 vol% CH4 diluted in argon, 10 vol% CO2 diluted in argon). b Negative values for CO2 conversion.

56

Fig. 5. Gibbs free energy change as a function of temperature (range: 200 –1000 °C) for: (a) main CSDRM reactions (including methanol production from “metgas” and (b) C(s)-producing and non C(s)-producing side-reactions. Part (a) covers the following reactions: (1) methanol production from “metgas”, (2) dry reforming of methane, (3) stream reforming of methane and, (4) combined steam and dry reforming of methane. Part (b) covers (i) C (s)-producing sidereactions including: (5) methane decomposition, (6) carbon monoxide dehydrogenation, (7) carbon monoxide hydrogenation and, (8) carbon monoxide disproportionation and (ii) non C (s)-producing side-reactions such as: (9) reverse water gas shift and (10) water gas shift.

57

Table 4. Bibliographic listing of the different types of already tested mono and bi-metallic-supported catalysts and their performances in CSDRM. Catalytic support

Metal (wt%)

Co-metala (wt%)

MgO

15%Ni

---

T (°C) 830

P (bar)

Rrb (molar)

TOSc (h)

GHSVd (mL gcat-1 h-1)

7

3:1.2:2.4 3:2.4:4.8 3:1.2:2.4

320

60000

1

3:1.2:2.4

910

HDL (30%)* MgO ZrO2 CeO2 α-Al2O3 Ce0.2Zr0.8O2 Ce0.8Zr0.2O2

12%Ni

15%Ni 12%Ni

HDL (30%)*

12%Ni

γ-Al2O3

10%Ni

12%Ni

α-Al2O3

12%Ni

SBA-15

10%Ni

HDL (30%)* HDL (50%)* HDL (70%)* HDL (30%)* HDL (50%)* HDL (70%)* HDL (30%)* HDL (50%)* HDL (70%)* CeO2 ZrO2 Ce0.8Zr0.2O2

MgO

800

20

265000

5

α-Al2O3

HDL (30%)*

---

---

800

6% CeO2 --6% CeO2 ----2.5% CeO2 --2.5% CeO2 --2.5% CeO2 --20% MgO --20% MgO --6% CeO2 6% CeO2 --3% MgO 3% MgO

700 800 800

1

3:1.2:2.4

20

265000

1

3:1.2:2.4

15

265000

1

3:1.2:2.4

20

265000

1

3:1.2:2.4

5

530000

1

3:1.2:2.4

600 650 700 650

20

700

530000 7

650

1

850

1

3:1.2:2.4 3:1.5:2.25 3:2.25:1.5

20

530000

120 600 600

27000

650

12%Ni

---

700

1

3:1.2:2.4

5

530000

750

15%Ni 0.5%Ru 0.5%Rh 0.5%Ir 0.5%Pt 0.5%Pd

@Al2O3 @MgOAl2O3 @Al2O3 10%Ni @MgOAl2O3 @Al2O3 Table 4. (continued)

---

---

800

800

1

1

3:1.2:2.4

3:1:2

20

8

265000

550000

700 ---

1 750 800

3:1:2

50

30000

Initial conversione (%) CH4 CO2 71 72 85 --86 87 92 60 64 --57 72 86 76 97 80 76 54 89 --97 --92 95 28 9 43 10 56 31 67 35 77 62 81 70 69 --76 --87 64 91 77 62 40 69θ 54θ 73θθ 58θθ 98 86 99 93 98 92 74 33 72 42 75 51 89 65 90 72 90 78 97 71 97 84 97 84 97 79 96 --96 80 46 47 --40 --25 24 69 62

Final conversionf (%) CH4 CO2 71 72 84 --86 87 89 60 73 --46 35 62 48 97 80 76 54 85 --97 --92 94 28 9 43 10 56 31 67 35 77 62 81 70 64 --75 --87 64 91 77 18 12 38θ 18θ 72θθ 48θθ 86 50 97 78 96 77 74 32 72 42 75 51 89 65 90 72 90 78 97 71 97 84 97 84 97 --70 --96 --46 47 40 --25 24 69 62

H2:COg (molar ratio) Initial Final 1.99 1.99 1.99 1.99 1.97 1.97

---

18

47

--1.9

Ref.

1.9

37

--2.0 2.0 2.0 3.0 2.8 2.4 2.4 2.2 2.1

--2.0 2.0 3.0 2.8 2.4 2.4 2.2 2.1

70 34

42

--2.1 2.0

2.1 2.0 ---

1.74 1.74 1.66

36

38 2.0 1.86 1.78

49

--43 2.0 2.0 2.0 1.9

2.0 2.0 2.0 1.9 ---

48

1.9 2.3 1.9 1.9 2.1 2.1 2.6

1.9

---

75, 76

2.6

70

60

70

60

2.5

2.5

91

65

91

65

3.2

3.2

89

77

89

77

2.9

2.9

95

76

95

76

2.7

2.7

41

58

Catalytic support

Metal (wt%)

@MgOAl2O3 @Al2O3 @MgOAl2O3

Co-metala (wt%)

T (°C)

P (bar)

Rrb (molar)

TOSc (h)

GHSVd (mL gcat-1 h-1)

800 10%Ni

---

1

3:1:2

50

30000

850

HDL (10%)* 15%Ni

--10% TiO2 10% La2O3

750

1

3:3:0.48

---

20000

Initial conversione (%) CH4 CO2 94

77

94

77

2.8

2.8

97

76

97

76

2.9

2.9

96

80

96

80

2.6

2.6

100 100 100

90 93 95

Al2O3(10%)La2O3

10% MgO

100

94

α-Al2O3

15%Ni

--5% MgO 7.5% MgO 10% MgO 15% MgO

750

1

3:3:0.48

---

20000

Nd2O3

29%Co

---

850

1

3:1:2 3:1.6:1.6 3:2:1

1

20000

95 96 97 100 100 73 82 83 8 30 21 56 82 100 98 94 99.8 98.5 99 99.3 94 85 86 86 79 80 74 80⍺ 81⍺⍺ 98 96 97 98 41 27 50 54 69 70 82 85 94.1 98.3 81! 67!!

90 92 93 92 94 86 85 87 19 22 30 49 85 85 98 98

23%Ni 81%Ni 23%Ni 81%Ni 23%Ni 81%Ni 23%Ni 81%Ni

CaO

600 700 3:1.7:1.7 ---

81%Ni

HDL (30%)

HDL (50%)* HDL (70%)* γ-Al2O3

12%Ni

HDL (30%)*

15%Ni

SA-5205

ZrO2 Ce0.18Zr0.82O2 γ-Al2O3 γ-Al2O3+ γ-Al2O3 γ-Al2O3+ γ-Al2O3 γ-Al2O3+ γ-Al2O3 γ-Al2O3+ γ-Al2O3

850

---

1 3:3.5:1.6 3:0.15:2.8 3:0.75:2.5 3:2.1:1.1 3:0.9:1.65

850

19700 10

32250

---

*

^

31500

800

4% CeO2 --15% Ce0.8Zr0.2O2

850

10

3:1.2:3

20

5000#

850

10

3:1.2:3

20

5000#

20

25100

22

12000

13.6%Ni

5.1% MgO

850

1

3:1.2:2.3 3:1.7:1.7 3:2.1:1.4 3:2.3:1.2

1.5%Pt

---

800

1

1:0.5:0.5

1

3:1.2:2.4

700 750 7%Ni

---

--50666

800 850 5%Ni

Table 4. (continued)

---

800

200 1

3:1.2:2.4

40

69000

H2:COg (molar ratio) Initial Final

Final conversionf (%) CH4 CO2

---

---

73 82 83

86 85 87

---

---

48 58 54 40 47 35 41⍺ 44⍺⍺ --46 44 28 44 46 55 62 71 76.1 82.4 80! 68!!

97 81 83 84 80 80 77 79⍺ 81⍺⍺ 98 96 97 98 10 25

--44 52 49 35 47 32 34⍺ 41⍺⍺ --20 44

---

90.8 97.1 83! 50!!

73.3 81.2 80! 41!!

Ref.

41

46

1.10 1.10 1.10 1.10 1.05 2.40 1.70 1.80 2.50 2.23 1.80 1.77 1.60 1.65 1.70 1.60 2.13 2.80 2.20 1.40 1.80 2.27 2.15 2.10 2.30 2.23 2.22 2.31⍺ 2.29⍺⍺ 2.25 1.70 1.40 1.30 0.93 0.69 2.62 2.20 2.41 2.20 2.22 2.10 2.08 2.01 1.9! 2.1!!

---

40

2.40 1.70 1.80

77

---

52

1.80 2.28 2.20 2.24 2.40 2.23 2.19 2.39⍺ 2.24⍺⍺

44

45

---

74

0.43 0.49

78

--35 2.16 2.03 2.1! 2.4!!

32

59

Catalytic support

Metal (wt%)

Co-metala (wt%)

T (°C)

P (bar)

Rrb (molar)

γ-Al2O3 SBA-15 CeliteS$

5%Ni

---

800

1

3:1.2:2.4

800

1

3:1.2:2.4

40

138000

10%Ni

--5% CaO 5% MgO ---

SBA-15

10%Ni

---

800

1

3:1:2 3:1:3.6 3:1:2.5

---

36000

-Al2O3

10%Ni

0.2% PtO2 0.3% PtO2

650

1

3:3:3

4

15600

α-Al2O3

1%Ni 3%Ni 5%Ni

1% MgO

750

1

3:3:0.48

---

20000

γ-Al2O3

SBA-15

TOSc (h)

GHSVd (mL gcat-1 h-1)

40

5%Ni

10%Ni

La2O3-NiO3

30%Ni

SiO2

17.5%Ni

--1% B2O3 3% B2O3 5% B2O3 3% B2O3 --10% SrO 30% SrO 50% SrO --1% La2O3 3% La2O3 7% La2O3

800

1

3:1:2

12

---

69000

36000

24 650

1

3:3:1.5

20

70000

800

1

3:1.2:2.4

60

158400

800

2

3:0.75: 2.25

5 ---

18000

3% La2O3 γ-Al2O3

10%Ni 10%Ni

-----

3:1.2:0.42

0.5% RuO2 ---

HDL (10%)*

15%Ni

0.5% RuO2

750

1

3:1.2:0.63

24

120000

--3:1.2:1.68

0.5% RuO2 γ-Al2O3 ZrO2 γ-Al2O3 ZrO2 γ-Al2O3 ZrO2

13%Ni 12%Ni

12%Ni

---

800

1

5%Co

Table 4. (continued)

3:2.1:0.9

---

70 77

---

77 40 23.5 22 10 40 60 7262 40-52--62---60 --10 22/ 20// 36 55/ 28// 14 33/ 20//

1% PtO2

--3:3:0.6

Ref.

32

33

58 71 59

60, 68

61

62

64

72

1.40

---

---

1.55

---

2.0 1.2 1.3 1.0 1.2

---

73

---

3:2:0.6 1

26 43/ 25// 21 50/ 35// 22 60/ 53//

---

3:2.6:0.4

β

0.5% PtO2

67 31 32.5 18 8 45 70 8579 46-75--70------

---

88.5

800 760 750 720

-----

12000

β

0.25% PtO2 α-Al2O3

---

H2:COg (molar ratio) Initial Final 2.6!!! 1.1!!! 1.9 1.2 1.2 0.8 2.1 2.1 2.1 2.1 2.0 2.0 2.3 2.3 2.15 1.70 --1.85 1.75 1.42 1.87 1.66 1.09 1.1 --1.1 2.3 2.4 --2.6 2.7 2.6 2.6 1.44 1.27 --1.29 1.42 2.05 2.05 2.03 2.03 2.012.012.03 2.03 2.06-2.08-1.98--- 2.00--2.03---- 2.0---1.00 --2.10= --1.60 1.50 1.07/ 1.05/ // 1.25 0.90// 2.20 2.00 1.90/ 2.00/ 2.20// 2.10// 1.60 1.55 1.30/ 1.25/ 1.50// 1.50//

--88

13%Ni 12%Ni

Final conversionf (%) CH4 CO2 38!!! 20!!! 48 55 ----63 59 66 64 79 77 82 80

3:1.3:1.2

β

13%Ni

Initial conversione (%) CH4 CO2 53!!! 39!!! 78 91 30 50 57 51 62 59 79 78 81 80 62 59 80 46 66 56 76 --76 54 52 65 61 74 71 60 60 65 62 78 68 60 50 78 68 50 35 24.5 33.4 23 18.2 11.3 8.6 73 87 85 90 779075 90 74-86-86--90-----85 92---77 --92= 13 33 24/ 55/ 23// 27// 40 22 57/ 60/ 32// 39// 17 25 33/ 60/ 20// 53//

1000

88.8 100 100 100 100

--100 98 68.5 71.9

---

---

53

60

Catalytic support

Metal (wt%)

Co-metala (wt%)

5%Co

0.5% PtO2 0.25% PtO2 0.5% PtO2

Al2O3 Al2O3(5%)ZrO2 Al2O3(10%)ZrO2 Al2O3(20%)ZrO2

2.5%Co2.5%Ni

ZrO2

10%Ni

La2O3-SrONiO3

Ni foam

T (°C)

P (bar)

Rrb (molar)

12%Ni

---

700

1

3:3:0.6

3%Ni

50

1000

--700

1

3:3:0.43

6.5

51000

850

1

3:1.2:2.4

20

60000

850

800

1

1

3:1.02:3.6

3:3:6

---

pNirb+MgOΨ

GHSVd (mL gcat-1 h-1)

---

0.5% PtO2 --0.2% Rh2O3 --0.5% MoCarbide --5% Al2O3SiC 10% Al2O3SiC 20% Al2O3SiC 20% Al2O3 20% Ni/Al2O3

TOSc (h)

10

---

18000

10000

4 ---

750

1

3:1.95:3.3

---

950

1

3:0.75:1.5 3:1.2:1.2 3:1.5:0.75

1.5

30%Ni 10%Ni

---

700

1

3:1.2:2.4

30

48000

4%Ru

---

500

1

15%Ni

10% MgO

850

---

---

20

62500

4%Ni

Mo-carbide

SA-Pϕ SANR-Hϵ ZrO2(7%)La2O3 Ce0.18Zr0.82O2 γ-Al2O3 ZnLaAlO4 Carbide

3.5

120000

10

25000

1

3:3:0.3 3:3:1.5 3:1.2:2.4

500

20000

800

1

3:1.5:2.25

---

10500

900

8.7

3:1:2

75

5250

10%Ni 3%Ru 3%Pt 5%Mo 5%W

Initial conversione (%) CH4 CO2 100 68.5 100 100 100 100

Final conversionf (%) CH4 CO2 --100

100

100

100

1.43

---

55.5 70.9 90

52.9 59.9 71

55.5 70.9 83

52.9 59.9 55

1.52 1.45 1.90

1.52 1.45 2.10

95

79

94

77

2.0

2.1

83.7

55.8

83

21

2.10

2.00

94.8

34

94

33.5

2.06

2.06

96.4

47.8

96

47

1.84

1.83

84.4

27.8

84

27

2.07

2.08

67

30

100 8 50ϐ 60ϐϐ 57ϐ 57ϐϐ 34 60 55 61 70 20 25 100 78 92 93 88 91 92 89

---

H2:COg (molar ratio) Initial Final 1.0 --1.42 1.47 1.47

100 100 100 40 50 17 10 83 85 63 60 78 ---

66

50 63

55

1.5 ---

43

---

Ref.

0 37ϐ 43ϐϐ 50ϐ 57ϐϐ 10 10 10 33 70 20 25 100

10 10 15 20 45 17 10 83

---

1.5 1.5 1.0

2.0 1.5 0.8 ---

0.90 1.77 2.0 1.55 2.00 2.20 1.90 2.00 1.97 1.49

65

55, 57

---

--91 92

---

2.0

54 51

0.90 1.77 2.0

83 90

---

67

2.00 1.97

79

--42 0.04% 750 1 3:1:2 --6000 89 44 --1.63 --39 Rh2O3 0.4%Rh --93 39 1.78 --80 67 50 40 2.0 2.1 MgO 140 15%Ni 30% SiO2 750 1 3:1.5:3 160000 80 60 80 60 2.0 2.0 69 SiO2 --24 80 60 40 22 1.8 1.8 Co-metal (or secondary metal) could be added on the support prior to/post to or/along the addition of the active site (metal). Its addition could be conducted on the upport via post-impregnation or directly (“one-pot” approach) during its course of synthesis. Details regarding catalyst synthesis are mentioned in Table 4 and in orresponding references. Rr is the molar ratio of the feeding reactants (CH4:CO2:H2O- mol/mol/mol). TOS stands for time on stream (h) GHSV stands for gas hourly space velocity (mL gcat-1 h-1) CH4 (%) is defined as: , [CH4]in: % concentration in inlet flow, [CH4]out: % concentration in outlet flow γ-Al2O3

4%Ni

CO2 (%) is defined as:

, [CO2]in: % concentration in inlet flow, [CO2]out: % concentration in outlet flow

H2:CO is the molar ratio (mol/mol) in the outlet products stream HDL stands for hydrotalcite-like MgO-Al2O3 mixed oxide (%: corresponding weight percent of MgO to Al 2O3)

61

@

Stands for core/shell like-materials The GHSV was reported based on the flow of methane (mL gcat-1 h-1) MgO-SA-5205 stands for low surface-area macroporous support composed mainly of alumina (86.1 wt%) and silica (11.8 wt%) and is pre-coated with a sub-layer f MgO oxide (5.1 wt%) Steam pre-treated Ni/Al2O3 sample (molar H2:H2O ratio of 1:3, 850 °C/2h, 7 °C min-1). Additional details regarding sample preparation are found in the orresponding reference Performances of Ni/Ce/Al2O3 catalyst synthesized by impregnation of Ni on a previously CeO2-loaded Al2O3 support. θθ Performances of Ni-Ce/Al2O3 catalyst ynthesized by simultaneous impregnation of Ni and CeO2 on Al2O3 support Reactivity levels of Ni/CeO2-ZrO2/MgO-Al2O3 prepared by sequential impregnation of Ni and mixed CeO2-ZrO2 metals. ⍺⍺ Reactivity levels of Ni-CeO2ZrO2/MgO-Al2O3 prepared by co-impregnation of Ni and mixed CeO2-ZrO2 metals Performances of Ni-Al2O3 sample synthesized via a direct “one-pot” method during alumina precipitation in presence of a structuring agent to obtain a porous atalyst. !! Performances of Ni/Al2O3 sample prepared by post-impregnation of Ni on mesoporous Al2O3 support. !!! Performances of Ni-Al2O3 sample prepared via a irect one-pot” method during alumina precipitation in absence of a structuring agent to obtain a non-porous catalyst. $ Commercial diatomite natural silica. Additional details are found in the corresponding reference Reactivity levels of Ni-La2O3-SiO2 sample synthesized via a direct “one-pot” method during silica precipitation in presence of polyethylene glycol (PEG) and of thylene glycol (EG) structuring agents. -- Reactivity levels of Ni-La2O3-SiO2 sample synthesized via a direct “one-pot” method during silica precipitation in absence f PEG and of EG structuring agents. --- Reactivity levels of Ni-La2O3-SiO2 sample synthesized via a direct “one-pot” method during silica precipitation in absence of PEG structuring agent. ---- Reactivity levels of Ni-La2O3-SiO2 sample synthesized via a direct “one-pot” method during silica precipitation in absence of EG tructuring agent. Additional details are found in the corresponding reference Ni/Al2O3 sample prepared in combination with porous membrane reactor technology [105] Performances of Ni-Ru/HDL sample synthesized by a simultaneous post-impregnation of Ni and Ru over the support. // Performances of Ru/Ni/HDL sample ynthesized by a consecutive post-impregnation of Ru on a previously Ni-impregnated HDL support. Additional details are found in the corresponding reference Reactivity levels of Ni/ZrO2 sample prepared via washing ZrO(OH)2 hydrogel with anhydrous ethanol. Additional details are found in the corresponding reference pNirb+MgO stands for porous nickel ribbon coated with an MgO layer (6 wt% MgO coating). ϐ Reactivity levels of Ni/pNirb+MgO sample prepared by calcination f the coating MgO layer in air. ϐϐ Reactivity levels of Ni/pNirb+MgO sample prepared by calcination of the coating MgO layer in hydrogen. Additional details egarding sample preparation are found in the corresponding reference Performances of Ni/(SA-P) sample prepared by post-impregnation of Ni on a silica-alumina mixed oxide synthesized by a precipitation method of silica and lumina precursors. ϵ Ni/(SANR-H) sample prepared by post-impregnation of Ni on a silica-alumina mixed oxide derived from natural halloysite nanorod (after alcination to 1000 °C). Additional details regarding sample preparation are found in the corresponding reference

Table 5. Categorization and main physico-chemical characteristics of already tested mono and bi-metallic-supported catalysts in CSDRM. Refer to Table 4 for

62 additional detail regrading catalyst composition and corresponding performance. Catalytic support

Catalyst compositiona

Metal(s) depositiona Active Cophase metal Imp.

---

Generation of “metgas” for subsequent methanol production

Imp.

---

Evaluate the effect of support chemistry on Ni reactivity in CSDRM

Imp.

---

Compare the reactivity of various noble-metal based catalysts under dry, steam and CSDRM

---

Determine the effect of incorporating SiO2 and MgO oxides on the reactivity of Ni0 particles in CDSRM

Ni/MgO

Rh,Ru,Ir, Pd, Pt/MgO

Aim of study

MgO#

NivSiO2– MgO

O.P.

Evaluate the effect of support chemistry on Ni reactivity and stability Ni/MgO– Al2O3

Imp.

--Assess the effect of calcination temperature of the support on Ni reactivity and stability

Hydrotalcite derived oxides: MgO–Al2O3

Ni–Ce/MgO– Al2O3

Ni/MgO– Al2O3

Table 5. (continued)

Imp.

Imp.

Imp.

---

Evaluate the effect of Ce addition on Ni reactivity and stability

Study the effect of Mg/Al molar ratio on the amount of coke deposition

Remarks: Tactics leading to catalyst improvement/ reason for good performance or deactivationb  Formation of NiO-MgO solid solution during synthesis stabilizes Ni0 against sintering and C(s) deposition  Stable CH4 conversion (Table 4) yet excessively large Ni0 particles (311 nm) leading to severe C(s) deposition on spent catalyst  Substitute MgO by MgO-Al2O3  Noble metal-based catalysts present stable performances on stream  Little C(s) deposition is detected over spent catalysts resulting from low dissolution rates of carbon into the bulk of noble metals  The addition of SiO2 enhances significantly reactivity, stability and, selectivity of Ni-MgO catalyst (Table 4)  The high basicity strength, moderate acidity sites and high structural stability (support did not decompose on stream) are combined factors ensuring catalytic stability  Compared to non SiO2-containing catalyst, Ni0 particles are smaller and did not sinter during catalysis (Ø Ni0= 11 nm vs. 18 nm over spent Ni-MgO)  Incorporation of MgO results in i) enhanced steam adsorption, ii) the addition of basic properties leading to low C(s) deposition, iii) enhanced dispersion of Ni0 nanoparticles (8 nm) and iv) stronger Ni to support interaction and prevention of sintering  Calcining the support at 800 °C prevents its collapse enhancing thus the dispersion of Ni0 species and improving their interaction with the support  Small Ni0 nanoparticles with strong MSI are resistant towards coking and sintering  The addition of CeO2 improves overall catalytic activity compared to non-Ce promoted catalyst (Table 4)  The presence of CeO2 enhances metal-support interaction and generates well-dispersed Ni0 sites (8 nm compared to 11 nm for Ni/MgO-Al2O3)  The intimate Ni-Ce contact provides oxygen transfer that facilitates C(s) removal (C(s) content: 8% compared to 25.5% for Ni/MgO-Al2O3)  An enhanced dispersion of smaller Ni0 nanoparticles with stronger MSI are found over the support containing 30 wt% MgO (27.5 % compared to 91% over the 70 wt% MgOcontaining hydrotalcite)  An amount of 1.75% of C(s) is detected over the optimum catalyst in comparison to values as high as 6% over other spent catalysts

“Metgas” selectivityc

Ref.

Yes

18

N.R.

47

Yes

75, 76

Yes

69

N.R.

47

Yes

34

Yes

42

Yes

43

63

Catalytic support

Hydrotalcite derived oxides: MgO–Al2O3

ZrO2#

Catalyst compositiona

Metal(s) depositiona Active Cophase metal

Aim of study

Provide a comparative study on the catalytic properties of coreshell like-materials in CSDRM and autothermal reforming

Ni–MgO– Al2O3

O.P.

---

Ni–Ti (or La)–MgO– Al2O3

O.P.

O.P.

Evaluate the effect TiO2 and La2O3 addition on catalytic performance of Ni species

Ni–Ce/MgO– Al2O3

Imp.

Imp.

Elucidate the positive effect of CeO2 as a co-metal for the promotion of Ni reactivity and stability

Ni–CeO2– ZrO2/MgO– Al2O3

Imp.

Imp.

Evaluate the role of CeO2-ZrO2 distribution and addition method on the performance of Ni species

 Study the effect of steam content on “metgas” production  Check the influence of Ru deposition method on the resistance of Ni0 species towards C(s) deposition

Ni–Ru/MgO– Al2O3

Imp.

Imp.

Ni/ZrO2

Imp.

---

Evaluate the effect of support chemistry on Ni reactivity

Ni–ZrO2

O.P.

---

Interpret the effect of applying CeO2-ZrO2 mixed oxides on Ni0 dispersion and reactivity

---

Compare catalytic performances of Ni supported on ZrO2 or mixed CeO2-ZrO2 oxide under dry, steam and CSDRM reactions

Pt/ZrO2 Table 5. (continued)

Imp.

Remarks: Tactics leading to catalyst improvement/ reason for good performance or deactivationb  Core-shell structures result in catalysts with high thermal stability along with reactive and stable performances on stream with high resistance towards coking and sintering  Even in the presence of MgO, catalyst suffers from poor “metgas” selectivity due to blockage of CH4 activation sites by CO2 and H2O  The addition of both co-metals have no effect on neither selectivity nor reactivity levels  Due to their oxygen storage capacities, free surface oxygen atoms oxidize coke once deposited (around 50% less of C(s) is found over promoted spent catalysts compared to Ni-MgOAl2O3)  Ce-promoted catalyst presents higher CH4 and CO2 conversion levels than non-promoted catalyst (Table 4)  The presence of CeO2 improves the reactivity of Ni0 particles and limits their aggregation due to strong MSI with MgAl2O4 support  The interaction between Ni and Ce metals enhances the resistance of Ni0 towards coke deposition due to the surplus of free surface oxygen from CeO2  The lack in selectivity towards “metgas” is not clearly discussed  Compared to sequential impregnation, coimpregnation of Ni and CeO2-ZrO2 metals improves overall performance especially CO2 conversion (Table 4)  The homogeneous formation of CeO2-ZrO2 phases result in small Ni0 particles acting as active sites for reactant activation  The desired H2:CO ratio = 2 is achieved thought the addition of water to a model biogas with a CO2/CH4 ratio of 0.4, using a molar ratio CH4:CO2:H2O of 3:1.2:1.68 (Table 4)  The sample prepared by simultaneous impregnation presents the highest catalytic activity (Table 4) and lowest tendency to C(s) deposition due to good Ru-Ni interaction  Stable CH4 conversion (Table 4) yet excessively large Ni0 particles (200 nm) leading to severe C(s) deposition on spent catalyst  Substitute ZrO2 by MgO-Al2O3  Strong deactivation on stream (Table 4) due to low dispersion of Ni0 species (2.11% compared to 6.6% for Ni-CeO2-ZrO2) and weak Ni-support interaction leading to metal sintering under CSDRM  Substitute ZrO2 by CeO2-ZrO2  Catalyst deactivates rapidly on stream due to oxidation of partially reduced ZrO2 by H2O thus decreasing the density of oxygen vacancies  Substitute ZrO2 by CeO2-ZrO2

“Metgas” selectivityc

Ref.

No

41

N.I.

46

No

44

N.I.

45

Yes

72

N.R.

47

N.R.

48

N.I.

78

64

Catalytic support

ZrO2#

Catalyst compositiona

Metal(s) depositiona Active Cophase metal

Aim of study

Ni/ZrO2

Imp.

---

Evaluate the performance of a nano-composite Ni/ZrO2 catalyst under SRM and CSDRM

Ni–Mo/ZrO2

Imp.

Imp.

Test the effect of molybdenum carbide on Ni stability in CSDRM

Ru/ZrO2– La2O3

Imp.

---

Compare CSDRM thermodynamic simulations to experimental values (effect of water addition on DRM)

Ni/CeO2

Imp.

---

Evaluate the effect of support chemistry on Ni reactivity

Ni/CeO2

Imp.

---

Interpret the effect of applying CeO2-ZrO2 mixed oxides on Ni0 dispersion and reactivity

CeO2

CeO2–ZrO2 mixed oxides*

Ni–CeO2– ZrO2

O.P.

---

Optimize the CeO2/ZrO2 molar ratio for enhancement of Ni reactivity and selectivity towards “metgas”

Ni–CeO2– ZrO2

O.P.

---

Interpret the effect of applying CeO2-ZrO2 mixed oxides on Ni0 dispersion and reactivity

---

Evaluate catalytic performances of Ni supported on ZrO2 or mixed CeO2-ZrO2 oxide under dry, steam and CSDRM operations

O.P.

Compare CSDRM thermodynamic simulations to experimental values estimated for “metgas” producing conditions

Pt/CeO2–ZrO2

Ni–MgO– CeO2–ZrO2 Table 5. (continued)

Imp.

O.P.

Remarks: Tactics leading to catalyst improvement/ reason for good performance or deactivationb  Catalyst is active under various CH4:CO2:H2O inlet feed compositions (Table 4)  The good reactivity levels in CSDRM is attributed to high BET surface area, welldispersed and small Ni0 nanoparticles  Mo2C modified Ni/ZrO2 presents higher and stable performance than Ni/ZrO2 (Table 4)  The addition of 0.5 wt% Mo2C improves Ni0 dispersion (22 nm over spent Ni-Mo2C/ZrO2 compared to 24 over spent Ni/ZrO2) and influences the type of deposited carbon from deactivating shell-like into non-deactivating whisker-like deposits  The catalyst presents experimental data that are consistent with thermodynamic simulations under chosen conditions (T, P, feed composition)  The catalyst operates stably on stream (under various inlet feed compositions, Table 4)  Strong deactivation on stream (Table 4) due to formation of large Ni0 particles over spent sample (245 nm) and heavy C(s) deposition  Substitute CeO2 by MgO-Al2O3  The catalyst exhibits stable CH4 performance yet its reactivity level was slightly lower than that of Ni-Ce0.8Zr0.2O2 catalyst (Table 4)  The catalyst displays similar Ni0 dispersion value as that over Ni-CeO2-ZrO2 (6.6%)  Substitute CeO2 by CeO2-ZrO2  Ni-Ce0.8Zr0.2O2 exhibits higher activity and stability than Ni-Ce0.2Zr0.8O2 with selective production of “metgas” (Table 4)  Better catalytic performances are attributed to i) enhanced dispersion of NiO particles (2.16% compared to 1.04% for Ni-Ce0.2Zr0.8O2), ii) stronger MSI and iii) higher O2 storage capacity which prevents C(s)-accumulation on stream  Ni-Ce0.8Zr0.2O2 displays the highest performance compared to Ni-ZrO2 and Ni-CeO2 catalysts (Table 4)  Smaller Ni0 particles are formed over the mixed-oxide (below detection limit of XRD apparatus) resulting in stronger MSI and enhanced O2 transfer during CSDRM  Ni-Ce0.18Zr0.82O2 is more stable than Pt/ZrO2 (Table 4)  The stability is ascribed to higher amounts of O2 vacancies of the support which favors the cleaning mechanism of solid coke deposits during catalysis  The catalyst presents experimental data that are consistent with thermodynamic simulations under chosen conditions (T, P, feed composition)  The catalyst operates stably on stream and is selective for “metgas” synthesis (Table 4)

“Metgas” selectivityc

Ref.

N.I.

73

Yes

63

N.I.

83

N.R.

47

Yes

48

Yes

37

Yes

48

N.I.

78

Yes

90

65

Catalytic support

Catalyst compositiona

Ni/Al2O3

Metal(s) depositiona Active Cophase metal

Imp.

---

Aim of study

Evaluate the effect of support chemistry on Ni reactivity

Ni–Ce/Al2O3

Imp.

Imp.

Study the effect of CeO2 addition on reactivity and coking of Ni0 particles supported on Al2O3

NiMgO/Al2O3

Imp.

Imp.

Test the effect of MgO on coke deposition over Ni impregnated on Al2O3

Ni–Ce/Al2O3

Imp.

Imp.

Demonstrate the effect of Ni and CeO2 loading procedures on reactivity and stability of bimetallic Ni-Ce supported on Al2O3 catalysts

Ni–Al2O3

O.P.

---

Provide a comparative study on the catalytic properties of coreshell like-materials in CSDRM and autothermal reforming

Ni–MgO– La2O3–Al2O3

O.P.

Imp.

Evaluate the effect TiO2 and La2O3 addition on catalytic performance of Ni species

Ni/MgO/ Al2O3

Imp.

Imp.

Examine the effect (and optimum amount, wt%) of MgO on reduction of coke from Ni0 sites

Imp.

Elucidate the positive effect of CeO2 as a co-metal for the promotion of Ni reactivity and stability

Al2O3^

Ni–Ce/Al2O3

Table 5. (continued)

Imp.

Remarks: Tactics leading to catalyst improvement/ reason for good performance or deactivationb

“Metgas” selectivityc

Ref.

 Strong deactivation on stream (Table 4) due to formation of large Ni0 particles over spent sample (128 nm) associated with heavy C(s) deposition  Substitute Al2O3 by MgO-Al2O3

N.R.

47

Yes

70

Yes

36

N.R.

38

No

41

N.I.

46

N.I.

40

No

44

 Ce-promoted Ni/Al2O3 is more active and stable than Ni/Al2O3 catalyst (Table 4)  The addition of cerium enhances Ni dispersion (1.2% compared to 0.9% over Ni/Al2O3) and provides anti-coking properties due to the high oxygen storage capacity of CeO2  The presence of CeO2 provides free O2 atoms to neighbor Ni0 for oxidation of C(s)-deposits  Addition of MgO enhances reactivity levels particularly CO2 conversion (Table 4)  20 wt% MgO suppresses 40% wt% of C(s) deposition (C(s) amount: 2.84% over spent Ni/Al2O3 vs. 1.16% over spent Ni-MgO/Al2O3)  Ni-Ce/Al2O3 prepared by co-impregnation results in higher reactivity and selectivity levels than Ni/Al2O3 and Ni/Ce/Al2O3 (Table 4)  Co-impregnation strengthens Ni and CeO2 interaction which promotes Ni0 dispersion (0.56% over Ni/Ce/Al2O3, 1.24% over NiCe/Al2O3 and 0.9% over Ni/Al2O3) and lowers C(s) deposition  Core-shell structures result in catalysts with high thermal stability along with reactive and stable performances on stream with high resistance towards coking and sintering  Catalysts suffer lack in “metgas” production due to blockage in CH4 activation sites  Catalyst shows high reactivity levels with values comparable to those obtained on similarly-composed samples yet prepared differently (Table 4)  The Ni-MgO-La2O3-Al2O3 sample prepared by co-impregnation presents lower amounts of C(s)deposits (0.14 mmol C h-1) compared to that prepared by post-impregnation of MgO over NiLa2O3-Al2O3 (0.18 mmol C h-1)  Increasing MgO content have no influence on reactivity levels (Table 4)  Adding 15 wt% MgO reduces C(s) deposits (0.3 mmol C h-1 over Ni/Al2O3 vs. 0.002 mmol C h-1 over Ni/MgO15%/Al2O3) upon increasing CO2 adsorption and dissociation into free O2 atoms  Catalyst displays stable performance yet reactivity levels are lower than those of MgOAl2O3-based catalysts (Table)  Bigger Ni0 particles seen over spent catalyst (24 vs. 16 nm over Ni-Ce/MgO-Al2O3) associated with higher amounts of carbonaceous deposits (12% vs. 2% over Ni-Ce/MgO-Al2O3)  Substitute Al2O3 by MgO-Al2O3

66

Catalytic support

Catalyst compositiona

Ni/MgO– SiO2–Al2O3

Metal(s) depositiona Active Cophase metal

Imp.

Aim of study

---

Examine the reactivity of Ni0 supported on mixed MgO-SiO2Al2O3 commercial support in various methane reforming reactions including CSDRM

Ni/Al2O3

Imp.

---

Investigate the effect of steam treating Ni/Al2O3 prior to catalysis on dispersion, reactivity and, selectivity of Ni0 particles in CVSDRM

Ni–Al2O3

O.P.

---

Optimize deposition method into (or on) porous Al2O3 (or SiO2) for improved reactivity of Ni0 species

O.P.

Improve coke resistance of Ni0 particles incorporated in mesoporous alumina by MgO (or CaO) incorporation

Imp.

Test the reactivity of bimetallic Ni-Pt supported Al2O3 catalysts in various methane reforming reactions including CSDRM

Imp.

Demonstrate the effect Ni-Mg particle size on the activity of Ni0 particles in CSDRM and the impact of such on coke formation

---

Provide a comparative interpretation on the performance of conventional Ni/Al2O3 catalyst in CSDRM to that of a Ni/Al2O3 coupled to porous membrane reactor technology

Ni–MgO– Al2O3

O.P.

^

Al2O3

Pt/Ni/Al2O3

Ni/MgO/ Al2O3

Ni/Al2O3

Table 5. (continued)

Imp.

Imp.

Imp.

Remarks: Tactics leading to catalyst improvement/ reason for good performance or deactivationb  The catalyst is active and operates stably on stream under various inlet feed compositions (Table 4)  Coating SiO2-Al2O3 by MgO results in a catalyst with high resistance towards coking  The formation of NiO-MgO solid solution during synthesis stabilizes Ni0 against sintering  Steam treatment improves activity, stability and selectivity of Ni/Al2O3 in CSDRM (Table 4)  Ni-alumina interaction becomes stronger and smaller Ni0 particles (Ø Ni0 = 6.5 nm vs. 21.8 nm over non-thermally treated Ni/Al2O3) with higher resistance towards sintering and coking are formed on the surface of Al2O3  Introducing Ni via “one-pot” during alumina precipitation (in presence of surfactant) improves catalytic performances (Table 4)  Confinement of well-dispersed Ni0 particles (Ø Ni0 = 3 nm vs. 9.5 and 25 nm over others) within structured Al2O3 and strengthened Ni-support interaction are factors for stable catalysis  Addition of MgO (or CaO) improves reactivity and selectivity levels for same nickel content 95 wt%) and approaches those recorded over twicericher (10wt%) Ni-Al2O3 catalyst (Table 4)  Reduction in C(s) content from 19 wt% over Ni10%Al2O3 to 3 (or 4) wt% over Ni5%Mg(or Ca)Al2O3 is achieved due to enhanced CO2 dissociative adsorption by basic oxides and generation of O2 atoms that oxidize C(s)-deposits  The 0.3 wt% PtO2 loaded catalyst is capable of operating under a variety of feed compositions without noticeable deactivation (Table 4)  PtO2 content influences Ni0 particles size where higher PtO2 content leads to smaller Ni0 particles (12.8 vs. 20.1 nm for the 0.3 and 0.2 wt% PtO2 containing samples, respectively)  Smaller Ni0 particles with enhance dispersion lead to stable CH4 reactivity levels (Table 4)  An increase in Ni content from 1 to 5 wt% enhances the dispersion of NiO-MgO mixed phases (13.7% over 1 wt% Ni/MgO/Al2O3 vs. 29.7% for 5 wt% Ni/MgO/Al2O3) which results in improved CH4 conversion levels (Table 4)  Coke formation rate follows a trend opposite to those of reactivity and dispersion: less C(s) is formed over the sample with the highest reactivity and NiO-MgO dispersion value  Ni/Al2O3 catalyst combined with porous membrane reactor enhance catalytic activity (Table 4)  Enhanced reactivity is attributed to the characteristics of the membrane, which provide a fast permeation rate of H2 (smallest molecule) compared with that of the other (heavier) gases

“Metgas” selectivityc

Ref.

Yes

74

Yes

35

Yes

32

Yes

33

N.I.

71

N.I.

59

N.I.

64

67

Catalytic support

Catalyst compositiona

Metal(s) depositiona Active Cophase metal

Aim of study

Ni/Al2O3

Imp.

---

Evaluate the performance of a nano-composite Ni/ZrO2 catalyst under SRM and CSDRM

Co–Pt/Al2O3

Imp.

Imp.

Test the reactivity of Co0 species in CSDRM upon coupling with noble Pt as co-metal

Imp.

Check the effect of addition of ZrO2 to γ-alumina-supported bimetallic on the reactivity of bimetallic Co-Pt catalysts in CSDRM

O.P.

Investigate the reactivity of Rhpromoted bimetallic Ni-Co incorporated within Al2O3-ZrO2 catalysts in various methane reforming reactions including CSDRM

Co–Pt/Al2O3– ZrO2

Rh–Co–Ni– Al2O3–ZrO2

Imp.

O.P.

Al2O3^ Ni/SiO2– Al2O3

Ni/Al2O3

Ni–Rh/Al2O3

SiO2

Ni/MgO/ SBA-15

Table 5. (continued)

Imp.

Imp.

Imp.

Imp.

---

Test the reactivity of Ni0 particles supported on new mixed silicaalumina oxide derived from natural halloysite nanotubes

---

Search for new catalytic formulations based on noble or transition metals to achieve high catalytic performance in various methane reforming reactions including CSDRM

Imp.

Investigate the effect of bimetallic Ni-Rh supported on Al2O3 in dry and combined steam and dry reforming of methane reactions

Imp.

Study the effect of MgO promoter on the reactivity, stability and coke resistance of Ni0 supported on mesoporous SBA-15 support

Remarks: Tactics leading to catalyst improvement/ reason for good performance or deactivationb  Ni/Al2O3 is used for comparison purposes and data associated to its performance in CSDRM are not reported  Bimetallic Co-Pt/Al2O3 catalysts are stable in CSDRM (Table 4) and are resistant to coke formation (few C(s) nanotubes are seen over 5 months spent catalysts)  Pt is responsible for the formation and sustainability of highly dispersed and reduced Co0 nanoparticles  The addition of zirconia into bimetallic CoPt/Al2O3 catalysts improves CH4 and CO2 conversions in conjunction with catalyst stability (Table 4)  No sintering of metallic particles or coke deposition were observed over spent catalysts  The addition of Rh2O3 influenced positively reactivity levels compared to non-Rh2O3 promoted catalyst with both remaining stable on stream (Table 4)  Rh-Co-Ni-Al2O3-ZrO2 catalyst is resistant towards coke deposition where only 3.8 wt% of C(s) is formed after catalysis  Compared to synthesized SiO2-Al2O3 mixed oxide, the catalyst derived from natural halloysite nanotubes improved Ni0 resistance towards sintering and coking resulting in an active and stable catalyst for CSDRM (Table 4)  Total C(s) deposition is around 40 wt% over spent Ni/SA-P compared to almost no C(s) deposition over spent Ni/SANR-H  Ni/Al2O3 presents the lowest CH4 conversion value compared to Ni or noble metals supported on ZnLaAlO4 (Table 4)  The average Ni0 particles size is around 60 nm over Al2O3 being twice bigger than those of Ru0 supported on ZnLaAlO4 showing less dispersion of Ni0 over commercial Al2O3 oxide  Substitute Al2O3 by ZnLaAlO4 or else, use noble metals rather than nickel as active sites  Bimetallic Ni-Rh/Al2O3 catalyst shows the highest performance in CSDRM (Table 4)  The addition of RhO2 modifies the electronic properties of Ni0 species inhibiting their sintering and subsequently C(s)-formation  The lack in selectivity even over the most reactive Ni-Rh/Al2O3 catalyst is ascribed to the excess occurrence of the RWGS side-reaction  Ni/MgO/SBA-15 exhibits more stable performances than Ni/SBA-15 (Table 4)  The preserved reduction state of Ni0 (during catalysis) over Ni/MgO/SBA-15 compared to reoxidation of Ni0 to NiO over spent Ni/SBA-15 along with minimal C(s) deposition ensure catalytic stability

“Metgas” selectivityc

Ref.

N.I.

73

N.I, N.R.

53

N.I, N.R.

66

N.I.

50

N.R.

51

Yes

67

No

39

N.I.

49

68

Catalytic support

Catalyst compositiona

Metal(s) depositiona Active Cophase metal

Aim of study

Ni/SBA-15 Ni/CeliteS

Imp.

---

Optimize deposition method into (or on) porous Al2O3 (or SiO2) for improved reactivity of Ni0 species

Ni/SBA-15

Imp.

---

Test the reactivity of Ni/SBA-15 under various CSDRM feed compositions

Imp.

Inspect the influence of boron content as a promoter to Ni0 species supported on mesoporous SBA-15 in CSDRM

O.P.

Assess the effect of La2O3 content and synthesis method (in absence or presence of structuring agent) on the reactivity and stability of Ni0 deposited inside silica oxide

B/Ni/SBA-15

Imp.

SiO2

Ni–La–SiO2

Ni–SiO2

Table 5. (continued)

O.P.

O.P.

---

Determine the effect of incorporating SiO2 and MgO oxides on the reactivity of Ni0 particles in CDSRM

Remarks: Tactics leading to catalyst improvement/ reason for good performance or deactivationb  Ni/SBA-15 presents higher catalytic reactivity than Ni/CeliteS yet, both samples deactivate strongly on stream (Table 4)  Deactivation of SiO2-based catalysts is attributed to re-oxidation of Ni0 into catalytically inactive NiO species on stream  Substitute SiO2 (SBA-15 or CeliteS) by mesoporous Al2O3 and introduce Ni via “onepot” method rather than post-impregnation  Under stoichiometric feed composition, selectivity is slightly higher than “metgas” composition due to the dominance of SRM over DRM while increasing steam content causes a sudden decrease in H2:CO molar ratio (Table 4)  An inlet feed with composition close to stoichiometry remains an ideal condition for “metgas” production  Due to steric effects of SBA-15 and welldispersion of Ni0 particles, Ni/SBA-15 are highly resistant towards sintering and coking  Adding 3 wt% of boron influences positively reactivity and stability levels of Ni/SBA-15 (Table 4). Increasing its content to 5 wt% induces negative effects due to larger activation barrier of C–H bonds (leading to coke deposition) and significant decline in active surface area owing to Ni0 sintering on stream  Due to the incorporation of boron at Ni0 octahedral sites at first sublayer surface (preventing total blockage), sintering and C(s) deposition can be suppressed  The lack in selectivity towards “metgas” is not clearly discussed  The addition of La2O3 improves activity, selectivity, stability, coking and sintering resistances of Ni0 embedded within SiO2 oxide (synthesized in presence of both PEG and EG structuring agents, Table 4)  Adding up 3 wt% La2O3 is found optimal for stable and selective performances on stream (Table 4)  Using both PEG and EG structuring agents in the course of catalyst preparation influences positively reactivity and stability levels (Table 4) owing to the generation of catalyst with larger specific surface area and thus, higher dispersion of Ni and La nanoparticles along with minimized coking potentials  Ni-SiO2 deactivates strongly on stream (Table 4, 40% loss in reactivity levels after 24 h compared to longer runs for Ni-MgO and NiSiO2-MgO catalysts)  Low catalytic performances are attributed to lack of basic character, poorer dispersion of Ni0 and subsequently high sintering tendency  Substitute SiO2 by SiO2-MgO

“Metgas” selectivityc

Ref.

No

32

No

58

No

60, 68

Yes

62

No

69

69

Catalytic support

CaO

Catalyst compositiona

Ni–CaO

NdCoO3

Perovskite type oxides

Ni foam

La2O3–SrO– NiO3

Metal(s) depositiona Active Cophase metal

O.P.

O.P.

O.P.

Aim of study

---

Study the performance of Ni-CaO catalysts (for different Ni contents) in various methane reforming reactions including CSDRM (for multiple reaction conditions)

---

Study the performance of NdCoO3 perovskite-like materials in various methane reforming reactions including CSDRM (for multiple reaction conditions)

O.P.

Investigate the effect of partial substitution by SrO within perovskite type oxides on reactivity and stability of Ni0 species in CSDRM

La2O3–SrONiO3/Al2O3– SiC

Imp.

---

Elucidate the role of SiCmodified Al2O3 supports on dispersion, reactivity and stability of La2O3-SrO-NiO3 during CSDRM

Ni/Al2O3/Ni foam

Imp.

---

Scrutinize the effect of activating Ni foam by post-impregnating Al2O3 or Ni/Al2O3 on activity and selectivity in CSDRM

Table 5. (continued)

Remarks: Tactics leading to catalyst improvement/ reason for good performance or deactivationb  Increasing Ni content from 23% to 81% improves reactivity levels (Table 4) owing to the higher availability of active sites (Ni0)  Owing to the high endothermic character of CSDRM, increasing reaction temperature from 600 to 800 °C results in higher reactivity levels yet, 800 °C remains an optimum temperature choice for “metgas” production (Table 4)  “One-pot” Ni-CaO catalysts are capable of operating under wide inlet feed compositions (Table 4)  Formation of NiO-CaO solid solution during synthesis stabilizes Ni0 against sintering and C(s) deposition  H2:CO product molar ratio can be significantly altered by manipulating CO2:H2O ratio in inlet feed composition (Table 4)  NdCoO3 catalysts are promising candidates for long CSDRM operations since they present stable performances for 60 min on stream (Table 4)  Bigger Ni0 particles are formed with increasing SrO concentration (Ø Ni0= 13.5 nm over La2O3NiO3 vs. 30.8, 27.6 and 29.9 nm over 10, 30 and 50 wt% SrO-containing La2O3-NiO3 catalysts, respectively)  Even if Ni0 are bigger and reactivity levels are lower for SrO-containing catalysts (Table 4), their resistance to C(s) deposition is significantly higher (80 wt% coke over spent La2O3-NiO3 vs. few percentages over spent SrO-containing catalysts)  High resistance to coke formation is attributed to better CO2 activation capacity leading to free O2 atoms necessary for C(s) oxidation  The dispersion of perovskite derived oxides over 20 wt% SiC-modified Al2O3 supports promotes catalytic reactivity (Table 4)  The increased dispersion of Al2O3 on the SiC support can increase the formation of intimately interacted La2NiO4-Al2O3 particles with higher accessibility to CH4, CO2 and H2O gases  The strong formation of perovskite-like La2NiO4 with Sic-modified Al2O3 suppresses aggregation of Ni0 during catalysis  Post-impregnating Ni/Al2O3 (rather than Al2O3) over Ni foam induces positive changes in both CH4 and CO2 conversion levels (Table 4)  Higher reactivity and selectivity levels are attributed to additional presence of Ni within the coating layer that plays the role of active sites for CH4, H2O and CO2 reactants  The selective performance is mainly due to the uniform distribution of Ni over Al2O3 and over Ni foam, the latter facilitating heat transfer and creating a uniform temperature distribution

“Metgas” selectivityc

Ref.

Yes

52

No

77

N.I.

61

Yes

55

Yes

65

70

Catalytic support

Catalyst compositiona

Metal(s) depositiona Active Cophase metal

Aim of study

Remarks: Tactics leading to catalyst improvement/ reason for good performance or deactivationb

“Metgas” selectivityc

Ref.

 Post-impregnation of Ni (3 or 4 wt%) over porous Ni ribbon pre-coated by MgO is beneficial for the development of active and stable CSDRM catalysts (Table 4)  The effect of Ni content on reactivity and Study the effect of treatment (in stability of Ni/pNirb+MgO catalysts is negligible air or H2) of MgO coating on the in CSDRM (Table 4) Ni/pNirb+ 56, pNirb+MgO Imp. --reactivity and stability of Ni N.R.  Treatment of MgO coating (either in air or H2) MgO 57 impregnated on porous Ni ribbon prior to Ni impregnation is found to have an in CSDRM effect on Ni reactivity (Table 4)  Improved dispersion of Ni0 species along with higher resistances towards coking and sintering lead to higher reactivity levels and selective “metgas” production over the H2-treated MgO coated catalyst  H2:CO product ratios formed from different ratios of CO2:H2O follows an expected trend: the ratio increases as the amount of oxidant increases (CH4 producing H2 is catalyzed by H2O Investigate the performance of and CO2) Ni-Mo2C in steam, dry and combined steam and dry  For different inlet feed compositions, catalyst Ni–Mo2C O.P. --N.I. 54 reforming of methane reactions: exhibit similar performance: high activity Special focus on deactivation followed by sharp deactivation after short modes in CSDRM exposure on stream (Table 4) Carbide  Deactivation is not attributed to neither coking nor sintering but rather to partial re-oxidation of active Mo2C into catalytically inactive MoO2  Both non-noble metal based-carbide catalysts Evaluate the performances of (molybdenum and tungsten) are active and stable non-noble metal based-carbide catalysts for CSDRM with performances being Mo (or W)– O.P. --catalysts in various methane close to thermodynamic ones Yes 79 Mo2C reforming reactions including  The catalysts developed no carbonaceous CSDRM deposits and metals did not agglomerate under reaction conditions a Catalyst composition is identified based on the way the active phase and/or the co-metal are introduced onto the support: “/” indicates post-impregnation (Imp.) on the support and “–“ indicates direct deposition via “one-pot” (O.P) approach of the metal(s) in the course of synthesis. b Reported improvements/characteristics of the most active/stable/selective catalyst and deactivation causes of the one(s) presenting low reactivity and/or stability and/or selectivity under CSDRM conditions (Table 4). c Yes if the optimum catalyst was selective towards “metgas” production under conditions favoring the production of H 2:CO around 2.0. Otherwise, it is No. In case the used molar ratio of the feeding reactants (Rr, Table 4) or temperature (Table 4) were not ideal for “metgas” production, the term “N.I.” is used instead. If the molar product ratio was not reported (Table 4), the term “N.R.” is employed. # In some cases, MgO or ZrO2 oxides are mixed with other oxides yet, each remain the main constituent of the catalytic support (accounting for 55 wt% (or more) of the overall composition, Table 4). * The appropriate composition (wt%) of CeO2-ZrO2 mixed oxides is mentioned in Table 4. ^ The various crystalline phases of alumina are present in Table 4. In some cases, Al2O3 is mixed with other oxides yet, alumina remains the main constituent of the catalytic support (accounting for 70 wt% (or more) of the overall composition, Table 4).

71

Fig. 6. A graphical representation comparing two series of CSDRM spent catalysts: (left image) in absence of basic MgO modifier, carbon polymerization takes place resulting in growth of carbon nanotubes on external Ni0 particles. Contrarily, in presence of MgO (right image), carbon formation and accumulation are completely inhibited owing to the formation of activated free O* species which react with adsorbed C* deposits leading to their oxidative removal. Reproduced with permission from ref. [33].

72

Fig. 7. (A) TEM images coupled to EDX analyses and (B) thermogravimetric analysis of: spent non Sr-modified (LN) and Srmodified (LSr0.1N, LSr0.3N and LSr0.5N) La nickelate based-perovskite catalysts. (C) TPSR (under CH4 stream) of reduced non Sr-modified (LN) and Sr-modified (LSr0.1N, LSr0.3N and LSr0.5N) La nickelate based-perovskite catalysts. Reproduced with permission from ref. [61].

73

Fig. 8. Proposed aggregation mechanism of Ni and CeO2-ZrO2 over co-impregnated Ni and CeO2-ZrO2 on MgAl2O4 (NCZMA(C)) and sequentially impregnated CeO 2-ZrO2 followed by Ni on MgAl2O4 (NCZMA(SI)). Reproduced with permission from ref. [45].

Fig. 9. Proposed sketches of possible reaction mechanisms encountered in the course of CSDRM over Ni 0 supported catalysts. Reproduced with permission from ref. [92].

74

Fig. 10. (A) TPR profiles and (B) thermogravimetric (TG) and differential thermogravimetric (DTG) profiles of nonthermally treated Ni/Al2O3 (NiAl) and thermally treated Ni/Al2O3 (WNiAl) catalysts. Reproduced with permission from ref. [35].

Fig. 11. TEM images of (a) Ni4%/Al2O3, (b) Rh0.04%Ni4%/Al2O3, (c) Rh0.4%/Al2O3 catalysts (A) before catalysis and (B) after catalysis. Reproduced with permission from ref. [39].

75

Fig. 12. High-resolution TEM images of Ni30%Mo2C catalyst at various stages: (a) after synthesis (before catalytic reaction), (b), 100 min into reaction (CH4:CO2:H2O = 3:1.2:1.2), (c) immediately after deactivation (CH4:CO2:H2O= 3:1.2:1.2) and, (d) 60 min after deactivation (CH4:CO2:H2O = 3:1.2:1.2). Reproduced with permission from ref. [54].

76

Fig. 13. TEM micrographs of freshly synthesized (a) SBA-15 support and (b) Ni10%/SBA-15 catalysts. Reproduced with permission from ref. [60].

Fig. 14. TEM micrographs of spent (a) Ni5%/SBA-15 and (b) Ni5%/CeliteS catalysts. Reproduced with permission from ref. [32].

77

Fig. 15. TEM micrographs of spent (a,b) Ni5%Al2O3 (non-porous), (c,d) Ni5%/Al2O3 and (e,f) Ni5%Al2O3 (mesoporous) catalysts. Reproduced with permission from ref. [32].

Fig. 16. (a) Sonoluminescence from a bubble cloud of several thousands of micro bubbles along with TEM micrograph (inset image) of the core-shell Ni@Al2O3 catalyst. (b,c) SEM images of spent Ni@MgO-Al2O3 after CSDRM at 800 and 850 °C, respectively. Reproduced with permission from ref. [41].

78

Fig. 17. (a) Thermogravimetric curves of spent Ni/(SA-P) and Ni/(SANR-H) catalysts and (b) TEM micrograph of spent Ni/(SANR-H) catalyst. Reproduced with permission from ref. [51].

Table 6. Adsorption activated mechanisms (with all successive steps) under standard combined steam and dry reforming of methane reaction (CH4:CO2:H2O = 3:1:2) over Rh/MgO catalyst [76]. Reaction type

Mechanism* CH4 + 2M CH3-M + H-M CH4 activation CH3-M + 2M CH-M + 2H-M CH-M + M C-M + H-M H2O + 3M O-M + 2H-M CO2 and H2O decomposition CO2 + 2M O-M + CO-M Surface adsorption CHx-M + O-M + (x-1) M CO-M + xH-M CO-M CO + M H2 and CO generation 2H-M H2 + 2M * M stands for the active metal site of the catalyst in all the listed equations.

79 Table 7. Kinetic parameters of SRM and DRM following Langmuir Hinshelwood Hougen Watson mechanism Rate expressions

Kinetic parameters SRM model [149]a ⁄

(

(

(

(

⁄

(

⁄

(

( ⁄ ( ⁄

(

( ⁄ (

(

(

( DRM model [150]b ⁄

(

( ⁄ ( a

⁄

stands for: CH4 + H2O CO + 3H2, stands for: CO + H2O CO2 + H2 and, stands for: CH4 + 2H2O CO2 + 4H2. b is the equilibrium constant of methane adsorption, is the rate constant of the decomposition (cracking) of methane on the surface of Ni, is the equilibrium constant of the reaction between CO2 and La2O3 and, is the rate constant of the reaction between oxycarbonate species and carbon deposited on the surface of Ni.

80 Graphical Abstract

Representation of: (left) typical thermodynamic simulation plots under combined steam and dry reforming of methane (CSDRM) conditions (CH4:CO2:H2O:Ar = 3:1:2:12) including C(s) formation and the presence of an inert gaseous diluent and (right) salient catalyst features and the innovative catalytic designs already investigated for testing Ni-based catalysts in CSDRM.