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Catalysts 2018, 8(3), 110; https://doi.org/10.3390/catal8030110

Review
Recent Scientific Progress on Developing Supported Ni Catalysts for Dry (CO2) Reforming of Methane
Department of Chemistry and Energy Engineering, Sangmyung University, Seoul 03016, Korea
Received: 20 February 2018 / Accepted: 9 March 2018 / Published: 11 March 2018

Abstract

:
Two major green house gases (CO2 and CH4) can be converted into useful synthetic gas (H2 and CO) during dry reforming of methane (DRM) reaction, and a lot of scientific efforts has been made to develop efficient catalysts for dry reforming of methane (DRM). Noble metal-based catalysts can effectively assist DRM reaction, however they are not economically viable. Alternatively, non-noble based catalysts have been studied so far, and supported Ni catalysts have been considered as a promising candidate for DRM catalyst. Main drawback of Ni catalysts is its catalytic instability under operating conditions of DRM (>700 °C). Recently, it has been demonstrated that the appropriate choice of metal-oxide supports can address this issue since the chemical and physical of metal-oxide supports can prevent coke formation and stabilize the small Ni nanoparticles under harsh conditions of DRM operation. This mini-review covers the recent scientific findings on the development of supported Ni catalysts for DRM reaction, including the synthetic methods of supported Ni nanoparticles with high sintering resistance.
Keywords:
dry reforming of methane; Ni; metal-oxide supports; surface basicity; confinement effect

1. Introduction

Human have largely relied on fossil fuels as an energy source for the last centuries and a large amount of CO2 gas has been emitted into atmosphere by burning fossil fuels. This extensive amount CO2 accumulated for centuries is considered to be the main reason for the global climate change, such as artic-sea ice melting and increase of sea-level [1,2]. Therefore, for last decades, a lot of attention has been paid for measures to reduce the amount of CO2 gas from the atmosphere and extensive scientific researches have been done on CO2 capturing, sequestration, and conversion of CO2 into other valuable chemical resources [3,4,5,6,7].
CO2 can be considered as a possible carbon source in chemical industry and CO2 reforming of methane (CH4), which is often referred as dry reforming of methane (DRM), has been attracted much attention [8,9,10,11,12,13]. In the DRM reaction, two major greenhouse gases (CH4 and CO2) are converted into H2 and CO. The generated H2 and CO are important syngas that can be used as reagents of other synthetic chemical reaction. In addition, DRM reaction can provide the syngas (H2 + CO) with a desirable ratio (H2/CO ratio < ~1) for the subsequent Fischer-Tropsh (FT) synthesis of liquid hydrocarbons and the synthesis of oxygenated chemical compounds [14,15].
DRM reaction can be assisted by heterogeneous metal catalysts and a variety of metal-based catalysts have been studied so far. The noble metals, such as Rh, Pt, Ir, Pd, and Ru, exhibit higher catalytic activity and stability for DRM reactions when compared to other transition metal-based catalysts (Co and Ni) [15,16]. It has been also proved that an addition of small amount of noble metals into transition metal-based catalysts resulted in enhanced catalytic performances [9,16,17,18,19,20,21,22]. However, to realize the industrial implementation of CRM reaction, the utilization of noble metals should be avoided due to their high cost and limited availability.
Among the non-noble metal-based catalysts, Ni-based catalysts are most practical choice in favor of their high catalytic activity, low cost and large abundance [10,23,24,25]. However, Ni-based catalysts have a critical drawback as DRM catalysts. Although initial catalytic activity of Ni-based catalyst is comparable to noble-based catalysts, its initial activity is easily deactivated during CRM reaction due to coke formation on active Ni surface and aggregation of catalytic Ni particles [23,24].
DRM is a highly endothermic reaction, and thus, it requires a lot of energy input as heat to move the reaction in forward direction. That is to say, inevitably DRM reaction should be performed at very high temperature (<700 °C), and at this high temperature catalytic Ni nanoparticles are easily aggregated together losing their initial activity and high resistance to coke formation.
There have been a lot of efforts devoted to improving the catalytic activity and stability of Ni-based DRM catalyst in the last decades. The formation of bimetallic states with noble-metals and other transition metal has been extensively studied [18,19,20,25], and it has also been demonstrated that catalytic performances of Ni catalysts are strongly related to the basicity of metal-oxide supports [26,27,28,29,30,31,32,33,34]. In addition, many scientific results indicate that coke formation on Ni catalysts is less favored when their size are in a nano meter scale [35,36,37,38]. Very recently, many efforts have been made towards the fabrication of small Ni nano particles with high thermal stability, which can retain their initial high reactivity and coke-resistance during long-term operation of DRM reaction.
The main scope of this mini-review is to discuss recent progress in developing supported Ni catalysts for DRM. The overview of DRM reaction is provided in the first part of this review (Section 2), which includes the thermodynamics nature of DRM, possible side reactions, and impacts of operating conditions on the performance DRM reactions. Then, recent scientific findings on supported Ni catalyst for DRM are covered in the second part (Section 3). A variety factors including parameters of DRM operation have been proved to have influence on catalytic performance of Ni-based catalysts. Catalytic activity and stability of Ni catalyst can be altered by chemical nature of supports, e.g., acidity, basicity, oxygen storing ability, and reducibility [23]. Recently, small Ni nanoparticles (<~10 nm) have been extensively studied as DRM catalyst and it has been suggested that chemical and geometrical structures of underlying supports play an important role in dispersion and thermal stability of Ni nanoparticles. In this mini-review, the influence of chemical and physical nature of metal-oxide supports on the catalytic performance of supported Ni catalysts will be mainly discussed. In addition, recent scientific findings showing the merit of utilization of Ni nanoparticles will be summarized together with the recently proposed synthetic methods of stable Ni nanoparticles.

2. Overview of DRM

DRM can be expressed as follow,
CH 4   +   CO 2     2   CO   +   2   H 2   ( Δ H 298 K O   = +   247   kJ   mol 1 )
Since DRM is a highly endothermic reversible reaction, it requires a large amount of heat energy to drive the reaction in the forward direction to achieve high conversion ratio of two green house gases (CH4 and CO2) into syngas (CO and H2). Figure 1 shows the influence of operating temperature on equilibrium conversion of DRM reaction [39].
Besides the main reaction (Equation (1)), there are a number of other reactions that can occur during the DRM process. Those possible reactions were well documented previously by Wang et al. [40] and Nikoo et al. [39] and recently revisited in the review paper by Aramouni et al. [41]. Among the possible side reactions, following 4 reactions (Equations (2)–(6) have been noticed as important ones by most of researchers, which are responsible for the carbon formation on catalysts surface during DRM reaction [39].
CH 4     C   +   2   H 2   ( Δ H 298 K O   = + 74.9   kJ   mol 1 ) ,   methane   decomposition
2 CO     C   +   CO 2   ( Δ H 298 K O   =   172.4   kJ   mol 1 ) ,   Boudouard   reaction
CO 2   +   2 H 2     C   +   2   H 2 O   ( Δ H 298 K O   = 90   kJ   mol 1 )
H 2   +   CO     C   +   H 2 O   ( Δ H 298 K O   =   131.3   kJ   mol 1 )
Among above four reactions, only the methane decomposition (Equation (2)) is an endothermic reaction that favored at high temperatures, whereas other reactions are exothermic disfavored at high temperature. Carbon formation is an undesired process during DRM process, which may lead to catalytic deactivation either by deactivating catalytic active sites or blocking the reactor [42]. The rapid carbon formation is a major cause of easy deactivation of Ni catalysts during DRM reaction. Thus, considering the thermodynamic natures of main and aforementioned side reactions of DRM, it is intuitive that operating DRM over Ni-based catalysts at high temperature is not only beneficial for obtaining higher syngas yield, but also minimizing carbon formation during DRM reaction. The temperature range of 870 to 1040 °C was suggested as an optimal temperature range where the deactivation of catalyst by carbon (coke) formation can be minimized [40]. The thermodynamic equilibrium analysis conducted by Nikoo et.al also showed that the carbon formation on catalyst surface can be suppressed at high temperature (>700 °C) (Figure 2) [39].
Many of recent studies on the catalytic DRM reaction have been performed at high temperature (>700 °C) and methane decomposition (Equation (2)) is a main pathway of carbon formation at such a high temperature whereas other reactions (Equations (3)–(5)) are less likely to proceed at the high temperature [24].
The dissociation of methane (Equation (2)) over Ni surface is sensitive to structure of Ni surface [43], and it leaves carbon atoms (Cα and Cβ) on Ni surface. Cα species are reactive that can be easily gasified by interacting with H2O, CO2, O, or H2, whereas Cβ atoms are not reactive [44]. The less reactive Cβ atoms can dissolve in Ni surface generating Ni3C species which may resulted in the growth of whisker carbon or nucleate forming coke layers on Ni surface. For the formation of coke layers, a large ensemble of Ni surface covered with carbon atoms is needed since carbon layers over more than ~ 80 atoms are thermodynamically stable [45,46,47,48].
The small Ni nanoparticles do not offer these large ensembles, therefore the deactivation of catalytic active Ni surface due to the coke formation can be suppressed by using small Ni nanoparticles as DRM catalysts. However, these small Ni nanoparticles are easily aggregated to each other forming larger particles at harsh operating conditions of DRM (>700 °C), which results in an increased carbon formation and catalyst deactivation [49].
There is another important side reaction which can be expressed as follows,
CO 2   +   H 2     CO   +   H 2 O   ( Δ H 298 K O   = + 41   kJ   mol 1 ) ,   reverse   water   gas   shift
This process is often referred as reverse water gas shift (RWGS) and it lowers the syngas ratio of H2/CO produced by the main reaction of DRM less than 1. Generally, the main reaction of DRM takes place with RWGS. However, RWGS is less endothermic than the main reaction of DRM, therefore its influence on syngas ratio is less significant with an increasing operating temperature of DRM. It has been reported that the produced syngas ratio (H2/CO) gets higher as temperature of DRM operation increased and approaches to 1 above 800 °C [39,40,50].
In addition to the temperature, operating pressure also has influence on the equilibrium conversion of reagents and products, the produced syngas ratio (H2/CO), and on the carbon formation. Nikoo and Amin performed thermodynamic analysis on DRM process based on direct minimization of Gibbs free energy method [39]. They reported that the equilibrium conversion of main reagents (CH4 and CO2) and products (H2 and CO) were reduced as the operating pressure increased from the atmospheric pressure in the temperature range of 700~1000 °C [39]. This can be explained by Le Chatelier’s principle; please note that the main reaction of DRM (Equation (1)) convert two equivalent reagents (CH4 + CO2) into four equivalent products (2CO + 2H2). In addition, the amount of carbon deposition on catalysts surface during DRM became larger at higher pressure.
The CO2/CH4 feed ratio also affect to equilibrium conversion of reagents and the syngas yields [50,51]. It has been reported that higher CO2/CH4 ratio resulted in higher CH4 conversion and H2 production, but lower CO2 conversion and CO generation at high temperature above 700 °C. Furthermore, the feed ratio influenced not only on the amount of deposited carbon, but also on the production of other side products, including water. 800 °C and ambient pressure with reagent mixture of CH4, CO2, O2 (feed ratio of 1:1:0.1) was suggested as an optimal operating conditions of DRM [39]. There are also other operating parameters that can influence on the performance of catalytic DRM reaction, including a reactor design and gas hourly space velocity (GHSV), thus one should be careful when comparing activity of catalysts reported in various literatures.

3. Ni Catalysts for DRM

A lot of scientific studies have been done on transition metal-based catalysts for DRM reaction and Ni catalysts supported on metal-oxides are most frequently investigated among them. Highly dispersed Ni nanoparticles supported on metal-oxide supports has been considered as a promising candidate as DRM catalyst since it can exhibit high catalytic activity and good long-term stability under the harsh operating conditions of DRM (>700 °C). Utilization of appropriate metal-oxide supports for Ni nanoparticles can not only increase thermal stability and coke-resistance of Ni nanoparticles at high temperature, but it can also promote the catalytic DRM reaction.

3.1. Reaction Mechanism of DRM over Supported Ni Catalysts; Influences of Metal-Oxide Supports

The DRM reaction over supported Ni catalyst mainly proceed via two steps: (1) decomposition of methane; and, (2) CO2 dissociation. It is generally agreed that dissociation of methane on Ni surface is the rate determining step of DRM reaction. Dissociated CH3 species are adsorbed on a top of Ni atom, whereas CH2 species prefer to be adsorbed between two adjacent Ni atoms. The dissociative adsorption of CH4 on Ni atoms is favored at step sites than that on terrace Ni atoms and this can explain higher catalytic activity of small Ni nanoparticles compared to bigger counterparts, which possess less amount of step Ni atoms per unit mass of Ni. Decomposition of a methane molecule leaves carbon atoms (atomic carbon, or CHx species) on Ni surface and these carbon atoms need to be oxidized into carbon monooxide that will be desorbed from Ni surface regenerating active Ni surface. The oxidation of the carbon atom on Ni surface is mainly proceeded by interaction with an atomic oxygen either on the surface of Ni or supports. Dissociate adsorption of CO2 is a major route for the generation of atomic oxygen on Ni surface during DRM and is generally considered as a fast process. When Ni particles are supported on inert metal-oxides, such as SiO2, the CO2 dissociation takes place on the surface of Ni particles. On the other hands, on the other active metal-oxide supports (e.g., MgO, La2O3, Ga2O3), chemisorption, and dissociation of CO2 can take place on the surface of supports or Ni-supports interfaces [29,52,53]. It has been suggested that higher basicity of metal-oxides supports can facilitate CO2 dissociation on the supports surface [26,54]. Some metal-oxides, such as CeO2 and ZrO2, can provide their lattice oxygen to oxidize carbon atoms on Ni surface into carbon monooxide [55,56,57]. The oxidation of carbon species from the Ni surface can also proceed via an interaction with hydroxyl groups on the surface, but this process has been suggested only significant at low temperature (<800 °C) [41,44].

3.2. Adjustment of Acidity and Basicity of Catalyst Surface

It has been generally believed that acidic supports (e.g., SiO2, Al2O3) facilitate the dissociation of methane leading to predominant methane cracking reaction over CO2 dissociation, which produce atomic oxygen and this, in turn, resulted in carbon formation on catalyst surface. In contrast, metal-oxide supports, such as MgO, La2O3 with high basicity promote dissociative adsorption of CO2, which can result in the suppression of carbon formation by producing higher number of oxygen atoms near the catalytic active metal surface [26,27,28,30,50]. The DRM reaction proceeds via three major step reactions (CH4 cracking, CO2 dissociation, and oxidation of carbon species on Ni surface) over Ni particles supported either by acidic metal-oxide (e.g., SiO2) or basic metal-oxides (e.g., MgO) are schematically described in Figure 3.
Influence of basicity of supports on catalytic performances have been studied either by comparing Ni catalyst supported by various metal-oxides or by comparing supported Ni catalysts with different promoters. Zhang et al. prepared a variety of Ni catalysts that are supported by SiO2, TiO2, ZrO2, Al2O3, MgO-modified Al2O3 (MA) [52]. In the presence of each catalyst inside a tubular fixed-bed continuous quartz reactor, DRM reaction was performed at 750 °C at ambient pressure with a reagent feed ratio of 1:1 (CH4:CO2). Ni catalyst supported by MA exhibited the best catalytic performances in terms of catalytic activity, stability, and anti-coking capability among them. They attributed the enhanced catalytic performances of MA-supported Ni catalysts to the promotion of dissociative CO2 adsorption on basic MgO sites, which led to the suppression of carbon formation. However, one should note that those supports studied by Zhang et al. also exhibited various degree of metal-support interaction which can affect size, dispersion, and reducibility of Ni particles, as authors also discussed in detail [52].
It has been demonstrated that utilization of alkali or alkaline metal oxides, such as Na, K, Mg, and Ca, onto the supported Ni catalyst, as promoters can suppressed the carbon formation by promoting dissociative CO2 adsorption on the basics sites of catalyst surface [58]. Recently, Pan et al. investigated promotion effects of Ga2O3 onto SiO2 supports on CO2 adsorption and catalytic DRM performance of supported Ni catalysts [53]. DRM was performed at 700 °C under ambient pressure with a feed ratio of 1:1:2 (CH4:CO2:Ar) over Ni/SiO2 and Ni/SiO2-Ga2O3, and they analyzed the states of adsorbed CO2 by means of Fourier transformation infrared spectroscopy (FT-IR) [53]. The results showed that Ga2O3 activated adsorbed CO2 on its surface, whereas CO2 only physically adsorbed on SiO2 surface, which resulted in the enhanced resistance of Ni/SiO2-Ga2O3 catalyst towards carbon formation during DRM reaction [53].
However, the promotion of CO2 dissociation on catalysts surfaces by increasing basicity can also cause the deactivation of catalysts. The Boudouard reaction (Equation (3)) can be facilitated even at high temperature due to increased amount of reagents (CO), generating larger numbers of carbon atoms on catalysts surface. In addition, an excess amount of atomic oxygen can oxidize neighboring Ni atoms into NiO, which are not catalytically active towards DRM reaction. Very recently it has been demonstrated that excessive surface basicity led to catalyst deactivation by carbon formation and formation of metal-oxides [59]. The excessive surface acidity also caused the catalytic deactivation by carbon deposition through the promoted decomposition of methane. Das et al. suggested that the moderate acidity and basicity on catalyst surface is important factor, together with their homogeneous distribution that determines catalytic conversion ratio of DRM reaction and long-term stability of supported metal catalysts (Figure 4) [59].
La2O3 has been also used either as a basic support or a basic promoter for supported Ni catalysts [60]. On the surface of Ni/La2O3 catalyst, methane dissociation takes place on Ni nanoparticles, whereas CO2 dissociation occurs on the surface of La2O3 supports. Interaction of La2O3 supports with CO2 can result in the formation of La2O2CO3 species that can oxidize carbon deposits on adjacent Ni atoms producing CO and regenerating active species (Ni and La2O3), as follows,
La 2 O 2 CO 3   +   C Ni     La 2 O 3   +   2 CO   +   Ni
This positive effects of the formation of La2O2CO3 on the removal of carbon deposits has been confirmed by previous transient studies performed using isotopic molecules (13CH4, 13CO2, 13CO, and C18O) [28]. Recently, it has been demonstrated that addition of Co on Ni/La2O3 catalyst facilitated the La2O2CO3 formation during DRM reaction, which resulted in enhanced catalytic activity and stability [61].

3.3. Enhancement of Dispersion and Thermal Stability of Ni Nanoparticles

It has been demonstrated that the size of Ni particle plays an important role in its resistance towards carbon formation as well as catalytic activity. Very recently, Aramouni et al. studied the size effect of catalytic metal particles on carbon formation during DRM by performing thermodynamic analysis [62]. They used Gibbs energy minimization method taking into account the Gibbs energy deviation of carbon formation during DRM on metal surface with different particle sizes from the thermodynamics of graphite formation [62]. It was found that the formation of carbon depends on the particle sizes; larger metal particles are more prone to yield higher amount of carbon at moderate operating conditions. High conversion of CH4 and CO2 with an optimal ratio of H2/CO (close to unity) can be achieved at higher temperature of 827–927 °C, however at this condition carbon formation can limit the efficiency of DRM process [62]. This highlighted the importance of developing highly dispersed small Ni nanoparticles (<5 nm) which possessing high resistance of carbon formation. This is in line with numerous experimental results previously performed by many researchers, indicating that the carbon formation on Ni surface during DRM reaction can be minimized by reducing the size of Ni particles [37,38,63,64].
Kim et al. reported the relationship between the carbon formation and size of Ni particles by performing DRM reaction over alumina supported Ni particles that were prepared via a sol-gel method and a subsequent supercritical drying process [63]. The average diameter of Ni particles drastically increased with increasing metal loading over 0.2 in Ni/Al mole ratio, while only the marginal change was found in the surface composition (Figure 5). This large increase of particle size with increasing metal loading attributed to the re-dispersion or aggregation of metal Ni particles during the thermal treatment. It was strongly correlated to the higher amount of filamentous carbon that formed on the surface Ni particles during DRM, as confirmed by TEM analysis performed after the DRM reaction at 773 °C for 30 h [63].
The thermal sintering of small Ni nanoparticles under harsh conditions of DRM operation (>700 °C) is a major problem that prevents the realization of their application as DRM catalysts. Many scientific results pointed that the size of Ni nanoparticles need to be below at least 8 nm to exhibit high resistance towards carbon formation [37,38,63,64]. However, those small Ni nanoparticles are prone to forming larger particles by the aggregation at high temperature (>700 °C).
The choice of metal oxides supports not only alters the catalytic activity of supported Ni particles towards DRM, but also influence on thermal stability of Ni nanoparticles. Strong metal-support interaction can stabilize the small Ni nanoparticles preventing them from thermal sintering, as well as enhance the catalytic activity by altering the electronic structure of Ni. The geometrical structure of metal-oxide supports is another parameter that can affect the catalytic performance of Ni nanoparticles; dispersion of Ni nanoparticles inside porous structure of metal-oxides supports has been demonstrated as a promising way to retain the size of small Ni nanoparticles.
The interaction between Ni and Al2O3 supports can lead to the formation of NiAl2O4 spinel structure, which can provide stable and highly dispersed metallic Ni particles via reduction [65]. The microstructural evolution of Ni-alumina catalyst at elevated temperature under reducing atmosphere has been investigated by in-situ XRD and TEM analysis [65]. Braidy et al. proposed the 3-step process of Ni particles formation from γ-NixAlyO4; (1) the formation of NiAl2O4 spinel structure from the Ni-rich zone generated by the diffusion of Ni atoms from γ-NixAlyO4, (2) the precipitation of Ni by the further Ni diffusion into those Ni-rich zones, (3) the formation of Ni nanoparticles and γ-Al2.67O4 [65]. Guo et al. reported the higher activity and better stability of Ni particles on MgO-γ-Al2O3 when compared to those on γ-Al2O3 supports and it was attributed to the enhanced stability of the small Ni crystallites on MgAl2O4 spinel structure [66]. Very recently, Chamoumi et al. demonstrated a highly active and stable Ni catalysts supported by an upgraded slag oxide (UGSO), which were prepared by an improved solid-state reaction [67]. The mining residue consisting of various metal (Fe, Mg, Al, Ca, Mn, and etc.) oxides was used as a support material. The high catalytic activity and stability of the Ni/UGSO catalyst was attributed to the high dispersion of Ni particles resulted from the formation of the NiAl2O4 and NiFe2O4 spinel structure, and the presence of various elements, including basic promoters, such as (MgO, CaO, and MnO) [67].
It was also found that poor dispersion of metal particles on La2O3 supports due to weak metal-support interaction can be improved by partial substitution of La with other metals such as Ca, Ce, Sr, etc. [60]. Chen et al. suggested that presence of small amount of Cu can also prevent deactivation of Ni particles due to aggregation or loss of Ni crystallites [67]. On the other hands, Dias et al. showed that addition of CaO could reduce the thermal stability of Ni particles that are supported by γ-Al2O3. Cu atoms competed with Ni atoms in interaction with Al2O3 supports, which reduced the metal-support interaction between Ni and Al2O3 supports. It, in turn, decreased the sintering resistance of the Ni particles although it improved the reducibility of Ni particles [68]. The degree of metal-support interactions was often measured by conducting temperature-programmed reduction (TPR) analysis with a constant flow of reduction gas (e.g., H2, CO2). The shift of TPR peak position to higher temperature is an indication of stronger interaction between metal and support, i.e., higher temperature or longer time is needed to activate the Ni particles prior to DRM reaction with increasing metal-supports interactions. It is also worth to mention that metal-oxides exhibiting a strong interaction with metals can cover the surface of metal particles either by the diffusion of oxides over the metal particles or diffusion of metal particles into the supports, resulting in a decrease of the number of active Ni atoms exposed to the reagent gas. These results pointed that there is an appropriate level of metal-support interaction for optimum catalytic performance of supported Ni nanoparticles. It was reported that the strong capability of CeO2 supports for metal-support interaction can lead to the reduced catalytic activity of metal particles [69,70].
Geometrical structure of metal-oxide supports also have a significant impact on the dispersion and sintering resistance of Ni nanoparticles. La2O3 supports derived from peroskite LaNiO3 precursors have been extensively studied recently and it has been proved that Ni dispersion on those supports can be significantly improved. X. Li et al. also reported that La2O3 nanorod can not only improve dispersion of Ni nanoparticles but also stabilize Ni nanoparticles (Figure 6) [60]. Recently, it has been demonstrated that Ni particles fabricated by exsolution from parent perovskite structures exhibited better dispersion and thermal stability when compared to the respective ones prepared by a conventional deposition method [71]. It is attributed to the stronger interaction between exsoluted Ni particles and supports to their nano-socket structure [71]. Singh et al. reported that the shape of parent perovskites affected the degree of exsoluted Ni/supports interactions and the defective anisotropic perovskites were suggested as ideal parent perovskite presursors [72]. The Ni particles that were exsoluted from LaNiO3 spheres and rods were highly dispersed on the supports and exhibited high catalytic activity and stability towards DRM reaction [72]. Recently, the fabrication Ni particles alloyed with Re and Fe by exsolution method has been also demonstrated [73]. The exsoluted Ni-alloy particles were firmly ‘socketed’ into a La2O3/LF supports and they exhibited high catalytic activity towards DRM reaction without showing carbon accumulation and particles sintering over 70 h of operation [73].
Recently, mesoporous silica and alumina has been received much attention as supports of Ni nanoparticles since Ni nanoparticles that are embedded in pores of those supports can exhibit good sintering resistance at high temperature [74,75,76,77,78,79]. Zhang et al. demonstrated that mesoporous structure of SBA-15 can prevent sintering of monometallic Ni particles by confining their size inside the pores [74].
Xu et al. prepared the ordered mesoporous alumina via improved evaporation-induced self-assembly (EISA) strategy and the synthesized mesoporous alumina exhibit large specific surface area, big pore volume, uniform pore size, and excellent thermal stability [80]. Ni particles were embedded inside the pores of the alumina via an incipient wetness impregnation method and their catalytic performances towards DRM reaction was investigated. The catalytic conversion of CH4 and CO2 at 700 °C reached the thermodynamic equilibrium conversion values and its high initial activity was retained for 100 h without a notable deactivation [80]. These results highlighted that the merit of utilization of mesoporous framework that can stabilize active Ni nanoparticles by the “confinement effect” and provide more accessible Ni active centers during DRM reaction [80].
Effective incorporation of Ni nanoparticles inside pores of mesoporous silica and alumina supports is a major key for the utilization of the confinement effect to retain the size of Ni nanoparticles. Xie et al. proposed a modified incipient wet process using polyols as new conveyors and removable carbon templates [77]. Polyol (ethylene glycol, EG) coordinated Ni2+ species can be effectively delivered into small channels of SBA-15 and polyol acted as carbon template during a subsequent pyrolysis process. As a result, highly dispersed Ni nanoparticles with an average diameter of 3.6 nm was synthesized inside the microchannel of silica SBA-15. The Ni/SBA-15-EG showed superior catalytic activity than Ni/SBA-15-H2O towards DRM reaction and its activity did not change significantly during long-term operation of DRM at 750 °C. This attributed to high dispersion and thermal stability of Ni nanoparticles of Ni/SBA-15-EG samples, which was confirmed by TEM images of spent Ni/SBA-15-EG [77].
Kaydouh et al. demonstrated that the “two solvent” method resulted in much selective dispersion of Ni nanoparticles at interior of the mesoporous structure of SBA-15 silica [81]. Ni/SBA-15 catalysts were prepared via a “two solvent” method where aqueous Ni precursor solution is mixed into silica suspended hydrophobic solvents and they exhibited superior catalytic performances when compared to Ni/SBA-15 prepared by a classical impregnation method [81]. It was explained by better dispersion of Ni nanoparticles of “two solvent”-prepared Ni/SBA-15 than the classically-prepared one, as evidenced by TEM and XRD analysis [81].
Recently, some research groups presented the modified impregnation method using various chelating ligands to immobilize Ni nanoparticles in mesoporous silica. Kang et al. used a polyethyleneimine (PEI) as a chelating ligand of Ni precursors during impregnation processes [82]. Ni-PET complexes were well distributed inside the pores and strongly bound the SBA-15 silica surface through the strong interaction between NH2 terminal groups of PEI and surface silanol groups of silica. Moreover, steric hindrance of PEI ligands prevented the aggregation of Ni-PEI complexes during the impregnation processes. It resulted in better dispersion of small Ni nanoparticles with narrower size distributions compared to the respective one prepared by classical impregnation method without using chelating ligands (Figure 7) [82]. The prepared catalysts showed stable catalytic activity towards DRM at 750 °C for 40 h and the size of Ni particles inside pores of SBA-15 silica was confined by pore structures of parent supports [82].
Zhang et al. studied the promoting impacts of various chelating ligands during modified impregnation methods on the dispersion of Ni nanoparticles [83]. Three ligands (ethylenediamine, citric acid, and acetic acid) were tested as chelating agents of Ni precursors during impregnation methods. Utilization of each ligand resulted in the formation of smaller Ni nanoparticles with better dispersion compared to non-ligand assisted impregnation methods. The catalysts prepared via ligands assisted method exhibit higher catalytic activity and stability than the respective one that was prepared via a classical impregnation method, which was attributed to better dispersion of Ni nanoparticles inside the pores of SBA-15 silica [83].
Although utilization of ligand during the impregnation method can improve dispersion of Ni nanoparticles inside the pore structure of supports, it has been reported that the formation of some Ni particles outside of pores during wet-chemical processes is still not avoidable, which are subject to aggregation during the DRM reaction [82,83]. Alternatively, small Ni nanoparticles can be embedded inside pores of porous supports using dry-methods. It can be more advantageous for preparation of small metal nanoparticles on porous supports since incorporation of gas phase precursor into the pore structure is much easier than aqueous precursors. Especially, atomic layer deposition (ALD) has been considered as a suitable technique for the preparation of small metal nanoparticles inside the pore structures of various supports. ALD is a self-limiting and self-terminating deposition technique where two gas phase precursors (metal precursor and oxidizing/reducing precursor) are injected onto the surface of substrates separately in a sequential manner. Gould et al. synthesized highly dispersed small Ni nanoparticles (particle size of ~4 nm) on alumina supports by ALD processes. The ALD-prepared Ni catalysts exhibited higher catalytic activity and carbon-formation resistance when compared to one prepared via incipient wetness (IW) process, which is attributed to smaller size of Ni particles with better dispersion of ALD-prepared sample than IW-prepared one [84]. Shang et al. prepared small Ni nanoparticles (mean diameter: ~3.6 nm) supported either by a porous γ-Al2O3 support using ALD process. The prepared catalysts showed high catalytic reactivity and exceptionally high stability towards DRM reaction performed in the range of 700 and 850 °C for 300 h (Figure 8) [85]. They also suggested that stronger bonding force between Ni nanoparticles and alumina supports was established for ALD-prepared sample than IW-prepared based on the larger size and lower catalytic performance of Ni nanoparticles that were prepared by IW method [86].

4. Conclusions

Catalytic DRM reaction has been attracted much attention since its main reaction converts two major green house gases (CO2 and CH4) into useful synthetic gas (H2 and CO). Due to high endothermic nature of the main reaction, it needs be operated at high temperature (>700 °C) to achieve high equilibrium conversion. The high temperature conditions are also beneficial to reduce the formation of carbon on the surface of active metal catalyst during DRM, which possibly deactivates the catalytic DRM reaction, since most of side reactions relating to carbon formation are exothermic.
Among many non-noble metal catalysts, supported Ni catalysts have been extensively studied due to high initial catalytic activity, which is comparable to other noble-based catalysts, and economic viability. However, easy deactivation of supported Ni catalysts due to coke-formation during the DRM operation remains as a main obstacle of its real application. Coke formation on active Ni surface at high temperature (>700 °C) mainly proceeds via methane cracking on Ni atoms and it has been suggested that this issue can be addressed either by adjusting the acidity-basicity of catalyst surface or reducing the size of Ni particles below ~8 nm.
Metal-oxide supports with higher basicity can alleviate the deactivation of Ni surface due to coke formation by promoting the CO2 dissociation at basic sites. It can provide oxygen atoms to neighboring Ni sites covered with carbon species, which can facilitate gasification of the carbon species from Ni surface regenerating catalytic active sites. Recent studies on the influence of chemical nature of supports on the catalytic performances of Ni catalyst pointed the importance of even dispersion of moderate acidic and basic sites near the Ni atoms.
It has been generally acknowledged that coke formation on Ni surface can be inhibited when the size of Ni catalyst is below ~8 nm since carbon atoms on more than ~80 atoms are not thermodynamically stable. Many experimental results indicated that small Ni nanoparticles are less prone to deactivated due to coke formation, and recent results of theoretical simulation also pointed the importance of small size of Ni nanoparticles. However, small Ni nanoparticles are prone to easy aggregation at such a high temperature (>700 °C) due to its thermodynamic instability. This low stability issue of small Ni nanoparticles can be addressed by increasing metal-support interaction or by immobilizing Ni nanoparticles inside porous metal-oxide supports, as pointed by recent studies. It has been demonstrated that small Ni nanoparticles can be highly dispersed on porous silica and alumina supports via modified wetness impregnation methods or atomic layer deposition and they exhibited high stability as well as high catalytic conversion of DRM.
Utilization of porous substrates is a promising strategy to develop highly active and stable Ni catalysts for DRM reaction with good economically viability. It is further desirable to investigate the synergic effect of “confinement effects” of porous supports and the optimized chemical nature of supports, e.g., even distribution of moderate acidic and basic sites. ALD process possesses high potential for this purpose since the various metals can be loaded into the nanopores and the amount of metal loading can be finely controlled by ALD. For instance, one may prepare Ni nanoparticles with basic MgO promoter, of which sizes are confined by porous structures via a subsequent ALD deposition of Ni and Mg elements inside the porous substrates. Besides, in order to fully exploit the advantages of Ni nanoparticles that are supported by porous substrates, it is also the desire to find the optimum pore structures (pore size, distribution, pore network) of support to maximize the accessibility of reagents to confined Ni nanoparticles inside the pores.

Acknowledgments

This research was supported by a 2018 Research Grant from Sangmyung University.

Author Contributions

H.O. Seo searched the scientific papers and prepared the manuscript.

Conflicts of Interest

The author declares no conflict of interest.

References

  1. Knutson, T.R.; Tuleya, R.E. Impact of CO2-Induced Warming on Simulated Hurricane Intensity and Precipitation: Sensitivity to the Choice of Climate Model and Convective Parameterization. J. Clim. 2004, 17, 3477–3495. [Google Scholar] [CrossRef]
  2. Notz, D.; Stroeve, J. Observed Arctic sea-ice loss directly follows anthropogenic CO2 emission. Science 2016, 354, 747–750. [Google Scholar] [CrossRef] [PubMed]
  3. Sood, A.; Vyas, S. Carbon Capture and Sequestration—A Review. IOP Conf. Ser. Earth Environ. Sci. 2017, 83, 012024. [Google Scholar] [CrossRef]
  4. Leung, D.Y.C.; Caramanna, G.; Maroto-Valer, M.M. An overview of current status of carbon dioxide capture and storage technologies. Renew. Sustain. Energy Rev. 2014, 39, 426–443. [Google Scholar] [CrossRef][Green Version]
  5. Arakawa, H.; Aresta, M.; Armor, J.N.; Barteau, M.A.; Beckman, E.J.; Bell, A.T.; Bercaw, J.E.; Creutz, C.; Dinjus, E.; Dixon, D.A.; et al. Catalysis Research of Relevance to Carbon Management:  Progress, Challenges, and Opportunities. Chem. Rev. 2001, 101, 953–996. [Google Scholar] [CrossRef] [PubMed]
  6. Kondratenko, E.V.; Mul, G.; Baltrusaitis, J.; Larrazabal, G.O.; Perez-Ramirez, J. Status and perspectives of CO2 conversion into fuels and chemicals by catalytic, photocatalytic and electrocatalytic processes. Energy Environ. Sci. 2013, 6, 3112–3135. [Google Scholar] [CrossRef]
  7. Aresta, M.; Dibenedetto, A. Utilisation of CO2 as a chemical feedstock: Opportunities and challenges. Dalton Trans. 2007, 28, 2975–2992. [Google Scholar] [CrossRef] [PubMed]
  8. Bradford, M.C.J.; Vannice, M.A. CO2 Reforming of CH4. Catal. Rev. 1999, 41, 1–42. [Google Scholar] [CrossRef]
  9. Özkara-Aydınoğlu, Ş.; Aksoylu, A.E. CO2 reforming of methane over Pt–Ni/Al2O3 catalysts: Effects of catalyst composition, and water and oxygen addition to the feed. Int. J. Hydrogen Energy 2011, 36, 2950–2959. [Google Scholar] [CrossRef]
  10. Rostrupnielsen, J.R.; Hansen, J.H.B. CO2-Reforming of Methane over Transition Metals. J. Catal. 1993, 144, 38–49. [Google Scholar] [CrossRef]
  11. Kathiraser, Y.; Thitsartarn, W.; Sutthiumporn, K.; Kawi, S. Inverse NiAl2O4 on LaAlO3–Al2O3: Unique Catalytic Structure for Stable CO2 Reforming of Methane. J. Phys. Chem. C 2013, 117, 8120–8130. [Google Scholar] [CrossRef]
  12. Ni, J.; Chen, L.; Lin, J.; Schreyer, M.K.; Wang, Z.; Kawi, S. High performance of Mg–La mixed oxides supported Ni catalysts for dry reforming of methane: The effect of crystal structure. Int. J. Hydrogen Energy 2013, 38, 13631–13642. [Google Scholar] [CrossRef]
  13. Sutthiumporn, K.; Maneerung, T.; Kathiraser, Y.; Kawi, S. CO2 dry-reforming of methane over La0.8Sr0.2Ni0.8M0.2O3 perovskite (M = Bi, Co, Cr, Cu, Fe): Roles of lattice oxygen on C–H activation and carbon suppression. Int. J. Hydrogen Energy 2012, 37, 11195–11207. [Google Scholar] [CrossRef]
  14. Centi, G.; Quadrelli, E.A.; Perathoner, S. Catalysis for CO2 conversion: A key technology for rapid introduction of renewable energy in the value chain of chemical industries. Energy Environ. Sci. 2013, 6, 1711–1731. [Google Scholar] [CrossRef]
  15. Kim, H.Y.; Park, J.-N.; Henkelman, G.; Kim, J.M. Design of a Highly Nanodispersed Pd–MgO/SiO2 Composite Catalyst with Multifunctional Activity for CH4 Reforming. ChemSusChem 2012, 5, 1474–1481. [Google Scholar] [CrossRef] [PubMed]
  16. Pakhare, D.; Spivey, J. A review of dry (CO2) reforming of methane over noble metal catalysts. Chem. Soc. Rev. 2014, 43, 7813–7837. [Google Scholar] [CrossRef] [PubMed]
  17. Li, L.; Zhou, L.; Ould-Chikh, S.; Anjum, D.H.; Kanoun, M.B.; Scaranto, J.; Hedhili, M.N.; Khalid, S.; Laveille, P.V.; D’Souza, L.; et al. Controlled Surface Segregation Leads to Efficient Coke-Resistant Nickel/Platinum Bimetallic Catalysts for the Dry Reforming of Methane. ChemCatChem 2015, 7, 819–829. [Google Scholar] [CrossRef]
  18. Wu, H.; Pantaleo, G.; La Parola, V.; Venezia, A.M.; Collard, X.; Aprile, C.; Liotta, L.F. Bi- and trimetallic Ni catalysts over Al2O3 and Al2O3-MOx (M=Ce or Mg) oxides for methane dry reforming: Au and Pt additive effects. Appl. Catal. B Environ. 2014, 156–157, 350–361. [Google Scholar] [CrossRef]
  19. Silverwood, I.P.; Hamilton, N.G.; McFarlane, A.R.; Kapitan, J.; Hecht, L.; Norris, E.L.; Mark Ormerod, R.; Frost, C.D.; Parker, S.F.; Lennon, D. Application of inelastic neutron scattering to studies of CO2 reforming of methane over alumina-supported nickel and gold-doped nickel catalysts. Phys. Chem. Chem. Phys. 2012, 14, 15214–15225. [Google Scholar] [CrossRef] [PubMed]
  20. Pawelec, B.; Damyanova, S.; Arishtirova, K.; Fierro, J.L.G.; Petrov, L. Structural and surface features of PtNi catalysts for reforming of methane with CO2. Appl. Catal. A Gen. 2007, 323, 188–201. [Google Scholar] [CrossRef]
  21. García-Diéguez, M.; Pieta, I.S.; Herrera, M.C.; Larrubia, M.A.; Alemany, L.J. Nanostructured Pt- and Ni-based catalysts for CO2-reforming of methane. J. Catal. 2010, 270, 136–145. [Google Scholar] [CrossRef]
  22. Jabbour, K.; El Hassan, N.; Casale, S.; Estephane, J.; El Zakhem, H. Promotional effect of Ru on the activity and stability of Co/SBA-15 catalysts in dry reforming of methane. Int. J. Hydrogen Energy 2014, 39, 7780–7787. [Google Scholar] [CrossRef]
  23. Kawi, S.; Kathiraser, Y.; Ni, J.; Oemar, U.; Li, Z.; Saw, E.T. Progress in Synthesis of Highly Active and Stable Nickel-Based Catalysts for Carbon Dioxide Reforming of Methane. ChemSusChem 2015, 8, 3556–3575. [Google Scholar] [CrossRef] [PubMed]
  24. Liu, C.; Ye, J.; Jiang, J.; Pan, Y. Progresses in the Preparation of Coke Resistant Ni-based Catalyst for Steam and CO2 Reforming of Methane. ChemCatChem 2011, 3, 529–541. [Google Scholar] [CrossRef]
  25. Bian, Z.; Das, S.; Wai, M.H.; Hongmanorom, P.; Kawi, S. A Review on Bimetallic Nickel-Based Catalysts for CO2 Reforming of Methane. ChemPhysChem 2017, 18, 3117–3134. [Google Scholar] [CrossRef] [PubMed]
  26. Alipour, Z.; Rezaei, M.; Meshkani, F. Effect of alkaline earth promoters (MgO, CaO, and BaO) on the activity and coke formation of Ni catalysts supported on nanocrystalline Al2O3 in dry reforming of methane. J. Ind. Eng. Chem. 2014, 20, 2858–2863. [Google Scholar] [CrossRef]
  27. Tsipouriari, V.A.; Verykios, X.E. Kinetic study of the catalytic reforming of methane with carbon dioxide to synthesis gas over Ni/La2O3 catalyst. Catal. Today 2001, 64, 83–90. [Google Scholar] [CrossRef]
  28. Tsipouriari, V.A.; Verykios, X.E. Carbon and Oxygen Reaction Pathways of CO2 Reforming of Methane over Ni/La2O3 and Ni/Al2O3 Catalysts Studied by Isotopic Tracing Techniques. J. Catal. 1999, 187, 85–94. [Google Scholar] [CrossRef]
  29. Verykios, X.E. Catalytic dry reforming of natural gas for the production of chemicals and hydrogen. Int. J. Hydrogen Energy 2003, 28, 1045–1063. [Google Scholar] [CrossRef]
  30. García, V.; Fernández, J.J.; Ruíz, W.; Mondragón, F.; Moreno, A. Effect of MgO addition on the basicity of Ni/ZrO2 and on its catalytic activity in carbon dioxide reforming of methane. Catal. Commun. 2009, 11, 240–246. [Google Scholar] [CrossRef]
  31. Osaki, T.; Mori, T. Role of Potassium in Carbon-Free CO2 Reforming of Methane on K-Promoted Ni/Al2O3 Catalysts. J. Catal. 2001, 204, 89–97. [Google Scholar] [CrossRef]
  32. Lemonidou, A.A.; Vasalos, I.A. Carbon dioxide reforming of methane over 5 wt % Ni/CaO-Al2O3 catalyst. Appl. Catal. A Gen. 2002, 228, 227–235. [Google Scholar] [CrossRef]
  33. Xu, L.; Song, H.; Chou, L. Carbon dioxide reforming of methane over ordered mesoporous NiO–MgO–Al2O3 composite oxides. Appl. Catal. B Environ. 2011, 108–109, 177–190. [Google Scholar] [CrossRef]
  34. Liu, S.; Guan, L.; Li, J.; Zhao, N.; Wei, W.; Sun, Y. CO2 reforming of CH4 over stabilized mesoporous Ni–CaO–ZrO2 composites. Fuel 2008, 87, 2477–2481. [Google Scholar] [CrossRef]
  35. Huo, M.; Li, L.; Zhao, X.; Zhang, Y.; Li, J. Synthesis of Ni-based catalysts supported on nitrogen-incorporated SBA-16 and their catalytic performance in the reforming of methane with carbon dioxide. J. Fuel Chem. Technol. 2017, 45, 172–181. [Google Scholar] [CrossRef]
  36. Zhang, J.; Li, F. Coke-resistant [email protected]2 catalyst for dry reforming of methane. Appl. Catal. B Environ. 2015, 176–177, 513–521. [Google Scholar] [CrossRef]
  37. Han, J.W.; Park, J.S.; Choi, M.S.; Lee, H. Uncoupling the size and support effects of Ni catalysts for dry reforming of methane. Appl. Catal. B Environ. 2017, 203, 625–632. [Google Scholar] [CrossRef]
  38. Han, J.W.; Kim, C.; Park, J.S.; Lee, H. Highly Coke-Resistant Ni Nanoparticle Catalysts with Minimal Sintering in Dry Reforming of Methane. ChemSusChem 2014, 7, 451–456. [Google Scholar] [CrossRef] [PubMed]
  39. Nikoo, M.K.; Amin, N.A.S. Thermodynamic analysis of carbon dioxide reforming of methane in view of solid carbon formation. Fuel Process. Technol. 2011, 92, 678–691. [Google Scholar] [CrossRef][Green Version]
  40. Wang, S.; Lu, G.Q.; Millar, G.J. Carbon Dioxide Reforming of Methane To Produce Synthesis Gas over Metal-Supported Catalysts:  State of the Art. Energy Fuels 1996, 10, 896–904. [Google Scholar] [CrossRef]
  41. Aramouni, N.A.K.; Touma, J.G.; Tarboush, B.A.; Zeaiter, J.; Ahmad, M.N. Catalyst design for dry reforming of methane: Analysis review. Renew. Sustain. Energy Rev. 2018, 82, 2570–2585. [Google Scholar] [CrossRef]
  42. Zhu, X.; Huo, P.; Zhang, Y.; Cheng, D.; Liu, C. Structure and reactivity of plasma treated Ni/Al2O3 catalyst for CO2 reforming of methane. Appl. Catal. B Environ. 2008, 81, 132–140. [Google Scholar] [CrossRef]
  43. Hu, Y.H.; Ruckenstein, E. Catalytic Conversion of Methane to Synthesis Gas by Partial Oxidation and CO2 Reforming. Adv. Catal. 2004, 48, 297–345. [Google Scholar]
  44. Papadopoulou, C.; Matralis, H.; Verykios, X. Utilization of Biogas as a Renewable Carbon Source: Dry Reforming of Methane BT—Catalysis for Alternative Energy Generation; Guczi, L., Erdôhelyi, A., Eds.; Springer: New York, NY, USA, 2012; pp. 57–127. [Google Scholar]
  45. Rostrup-Nielsen, J.R.; Sehested, J.; Nørskov, J.K. Hydrogen and synthesis gas by steam- and CO2 reforming. Adv. Catal. 2002, 47, 65–139. [Google Scholar] [CrossRef]
  46. Mortensen, P.M.; Dybkjær, I. Industrial scale experience on steam reforming of CO2-rich gas. Appl. Catal. A Gen. 2015, 495, 141–151. [Google Scholar] [CrossRef]
  47. Borowiecki, T. Nickel catalysts for steam reforming of hydrocarbons; size of crystallites and resistance to coking. Appl. Catal. 1982, 4, 223–231. [Google Scholar] [CrossRef]
  48. Seo, H.O.; Sim, J.K.; Kim, K.-D.; Kim, Y.D.; Lim, D.C.; Kim, S.H. Carbon dioxide reforming of methane to synthesis gas over a TiO2–Ni inverse catalyst. Appl. Catal. A Gen. 2013, 451, 43–49. [Google Scholar] [CrossRef]
  49. Bengaard, H.S.; Nørskov, J.K.; Sehested, J.; Clausen, B.S.; Nielsen, L.P.; Molenbroek, A.M.; Rostrup-Nielsen, J.R. Steam Reforming and Graphite Formation on Ni Catalysts. J. Catal. 2002, 209, 365–384. [Google Scholar] [CrossRef]
  50. Hassani Rad, S.J.; Haghighi, M.; Alizadeh Eslami, A.; Rahmani, F.; Rahemi, N. Sol–gel vs. impregnation preparation of MgO and CeO2 doped Ni/Al2O3 nanocatalysts used in dry reforming of methane: Effect of process conditions, synthesis method and support composition. Int. J. Hydrogen Energy 2016, 41, 5335–5350. [Google Scholar] [CrossRef]
  51. Abdullah, B.; Abd Ghani, N.A.; Vo, D.-V.N. Recent advances in dry reforming of methane over Ni-based catalysts. J. Clean. Prod. 2017, 162, 170–185. [Google Scholar] [CrossRef]
  52. Zhang, R.; Xia, G.; Li, M.; Wu, Y.; Nie, H.; Li, D. Effect of support on the performance of Ni-based catalyst in methane dry reforming. J. Fuel Chem. Technol. 2015, 43, 1359–1365. [Google Scholar] [CrossRef]
  53. Pan, Y.; Kuai, P.; Liu, Y.; Ge, Q.; Liu, C. Promotion effects of Ga2O3 on CO2 adsorption and conversion over a SiO2-supported Ni catalyst. Energy Environ. Sci. 2010, 3, 1322–1325. [Google Scholar] [CrossRef]
  54. Alipour, Z.; Rezaei, M.; Meshkani, F. Effects of support modifiers on the catalytic performance of Ni/Al2O3 catalyst in CO2 reforming of methane. Fuel 2014, 129, 197–203. [Google Scholar] [CrossRef]
  55. Su, Y.-J.; Pan, K.-L.; Chang, M.-B. Modifying perovskite-type oxide catalyst LaNiO3 with Ce for carbon dioxide reforming of methane. Int. J. Hydrogen Energy 2014, 39, 4917–4925. [Google Scholar] [CrossRef]
  56. Huang, T.-J.; Wang, C.-H. Roles of Surface and Bulk Lattice Oxygen in Forming CO2 and CO During Methane Reaction over Gadolinia-Doped Ceria. Catal. Lett. 2007, 118, 103–108. [Google Scholar] [CrossRef]
  57. Sun, N.; Wen, X.; Wang, F.; Peng, W.; Zhao, N.; Xiao, F.; Wei, W.; Sun, Y.; Kang, J. Catalytic performance and characterization of Ni–CaO–ZrO2 catalysts for dry reforming of methane. Appl. Surf. Sci. 2011, 257, 9169–9176. [Google Scholar] [CrossRef]
  58. Horiuchi, T.; Sakuma, K.; Fukui, T.; Kubo, Y.; Osaki, T.; Mori, T. Suppression of carbon deposition in the CO2-reforming of CH4 by adding basic metal oxides to a Ni/Al2O3 catalyst. Appl. Catal. A Gen. 1996, 144, 111–120. [Google Scholar] [CrossRef]
  59. Das, S.; Sengupta, M.; Patel, J.; Bordoloi, A. A study of the synergy between support surface properties and catalyst deactivation for CO2 reforming over supported Ni nanoparticles. Appl. Catal. A Gen. 2017, 545, 113–126. [Google Scholar] [CrossRef]
  60. Li, X.; Li, D.; Tian, H.; Zeng, L.; Zhao, Z.-J.; Gong, J. Dry reforming of methane over Ni/La2O3 nanorod catalysts with stabilized Ni nanoparticles. Appl. Catal. B Environ. 2017, 202, 683–694. [Google Scholar] [CrossRef]
  61. Tsoukalou, A.; Imtiaz, Q.; Kim, S.M.; Abdala, P.M.; Yoon, S.; Müller, C.R. Dry-reforming of methane over bimetallic Ni–M/La2O3 (M=Co, Fe): The effect of the rate of La2O2CO3 formation and phase stability on the catalytic activity and stability. J. Catal. 2016, 343, 208–214. [Google Scholar] [CrossRef]
  62. Aramouni, N.A.K.; Zeaiter, J.; Kwapinski, W.; Ahmad, M.N. Thermodynamic analysis of methane dry reforming: Effect of the catalyst particle size on carbon formation. Energy Convers. Manag. 2017, 150, 614–622. [Google Scholar] [CrossRef]
  63. Kim, J.-H.; Suh, D.J.; Park, T.-J.; Kim, K.-L. Effect of metal particle size on coking during CO2 reforming of CH4 over Ni–alumina aerogel catalysts. Appl. Catal. A Gen. 2000, 197, 191–200. [Google Scholar] [CrossRef]
  64. Zhang, Q.; Zhang, T.; Shi, Y.; Zhao, B.; Wang, M.; Liu, Q.; Wang, J.; Long, K.; Duan, Y.; Ning, P. A sintering and carbon-resistant Ni-SBA-15 catalyst prepared by solid-state grinding method for dry reforming of methane. J. CO2 Util. 2017, 17, 10–19. [Google Scholar] [CrossRef]
  65. Braidy, N.; Bastien, S.; Blanchard, J.; Fauteux-Lefebvre, C.; Achouri, I.E.; Abatzoglou, N. Activation mechanism and microstructural evolution of a YSZ/Ni-alumina catalyst for dry reforming of methane. Catal. Today 2017, 291, 99–105. [Google Scholar] [CrossRef]
  66. Guo, J.; Lou, H.; Zhao, H.; Chai, D.; Zheng, X. Dry reforming of methane over nickel catalysts supported on magnesium aluminate spinels. Appl. Catal. A Gen. 2004, 273, 75–82. [Google Scholar] [CrossRef]
  67. Chamoumi, M.; Abatzoglou, N.; Blanchard, J.; Iliuta, M.-C.; Larachi, F. Dry reforming of methane with a new catalyst derived from a negative value mining residue spinellized with nickel. Catal. Today 2017, 291, 86–98. [Google Scholar] [CrossRef]
  68. Chen, H.-W.; Wang, C.-Y.; Yu, C.-H.; Tseng, L.-T.; Liao, P.-H. Carbon dioxide reforming of methane reaction catalyzed by stable nickel copper catalysts. Catal. Today 2004, 97, 173–180. [Google Scholar] [CrossRef]
  69. Dias, J.A.C.; Assaf, J.M. Influence of calcium content in Ni/CaO/γ-Al2O3 catalysts for CO2-reforming of methane. Catal. Today 2003, 85, 59–68. [Google Scholar] [CrossRef]
  70. Wang, S.; Lu, G.Q. Role of CeO2 in Ni/CeO2–Al2O3 catalysts for carbon dioxide reforming of methane. Appl. Catal. B Environ. 1998, 19, 267–277. [Google Scholar] [CrossRef]
  71. Ay, H.; Üner, D. Dry reforming of methane over CeO2 supported Ni, Co and Ni–Co catalysts. Appl. Catal. B Environ. 2015, 179, 128–138. [Google Scholar] [CrossRef]
  72. Neagu, D.; Oh, T.-S.; Miller, D.N.; Ménard, H.; Bukhari, S.M.; Gamble, S.R.; Gorte, R.J.; Vohs, J.M.; Irvine, T.S. Nano-socketed nickel particles with enhanced coking resistance grown in situ by redox exsolution. Nat. Commun. 2015, 6, 8120. [Google Scholar] [CrossRef] [PubMed][Green Version]
  73. Singh, S.; Zubenko, D.; Rosen, B.A. Influence of LaNiO3 shape on its solid-phase crystallization into coke-free reforming catalysts. ACS Catal. 2016, 6, 4199–4205. [Google Scholar] [CrossRef]
  74. Zubenko, D.; Singh, S.; Rosen, B.A. Exsolution of Re-alloy catalysts with enhanced stability for methane dry reforming. Appl. Catal. B Environ. 2017, 209, 711–719. [Google Scholar] [CrossRef]
  75. Zhang, M.; Ji, S.; Hu, L.; Yin, F.; Li, C.; Liu, H. Structural Characterization of Highly Stable Ni/SBA-15 Catalyst and Its Catalytic Performance for Methane Reforming with CO2. Chin. J. Catal. 2006, 27, 777–781. [Google Scholar] [CrossRef]
  76. Sarkar, B.; Tiwari, R.; Singha, R.K.; Suman, S.; Ghosh, S.; Acharyya, S.S.; Mantri, K.; Konathala, L.N.S.; Pendem, C.; Bal, R. Reforming of methane with CO2 over Ni nanoparticle supported on mesoporous ZSM-5. Catal. Today 2012, 198, 209–214. [Google Scholar] [CrossRef]
  77. Moradi, G.; Khezeli, F.; Hemmati, H. Syngas production with dry reforming of methane over Ni/ZSM-5 catalysts. J. Nat. Gas Sci. Eng. 2016, 33, 657–665. [Google Scholar] [CrossRef]
  78. Xie, T.; Shi, L.; Zhang, J.; Zhang, D. Immobilizing Ni nanoparticles to mesoporous silica with size and location control via a polyol-assisted route for coking- and sintering-resistant dry reforming of methane. Chem. Commun. 2014, 50, 7250–7253. [Google Scholar] [CrossRef] [PubMed]
  79. Gálvez, M.E.; Albarazi, A.; Da Costa, P. Enhanced catalytic stability through non-conventional synthesis of Ni/SBA-15 for methane dry reforming at low temperatures. Appl. Catal. A Gen. 2015, 504, 143–150. [Google Scholar] [CrossRef]
  80. Li, S.; Gong, J. Strategies for improving the performance and stability of Ni-based catalysts for reforming reactions. Chem. Soc. Rev. 2014, 43, 7245–7256. [Google Scholar] [CrossRef] [PubMed]
  81. Xu, L.; Zhao, H.; Song, H.; Chou, L. Ordered mesoporous alumina supported nickel based catalysts for carbon dioxide reforming of methane. Int. J. Hydrogen Energy 2012, 37, 7497–7511. [Google Scholar] [CrossRef]
  82. Kaydouh, M.N.; El Hassan, N.; Davidson, A.; Casale, S.; El Zakhem, H.; Massiani, P. Highly active and stable Ni/SBA-15 catalysts prepared by a “two solvents” method for dry reforming of methane. Microporous Mesoporous Mater. 2016, 220, 99–109. [Google Scholar] [CrossRef]
  83. Kang, D.; Lim, H.S.; Lee, J.W. Enhanced catalytic activity of methane dry reforming by the confinement of Ni nanoparticles into mesoporous silica. Int. J. Hydrogen Energy 2017, 42, 11270–11282. [Google Scholar] [CrossRef]
  84. Zhang, Q.; Long, K.; Wang, J.; Zhang, T.; Song, Z.; Lin, Q. A novel promoting effect of chelating ligand on the dispersion of Ni species over Ni/SBA-15 catalyst for dry reforming of methane. Int. J. Hydrogen Energy 2017, 42, 14103–14114. [Google Scholar] [CrossRef]
  85. Gould, T.D.; Montemore, M.M.; Lubers, A.M.; Ellis, L.D.; Weimer, A.W.; Falconer, J.L.; Medlin, J.W. Enhanced dry reforming of methane on Ni and Ni-Pt catalysts synthesized by atomic layer deposition. Appl. Catal. A Gen. 2015, 492, 107–116. [Google Scholar] [CrossRef]
  86. Shang, Z.; Li, S.; Li, L.; Liu, G.; Liang, X. Highly active and stable alumina supported nickel nanoparticle catalysts for dry reforming of methane. Appl. Catal. B Environ. 2017, 201, 302–309. [Google Scholar] [CrossRef]
Figure 1. Equilibrium conversion of (a) CH4 and (b) CO2 as a function of temperature and CO2/CH4 ratio at atmospheric pressure for n(CH4 + CO2) = 2 mol. Reprinted with permission from Ref. [39]. Copyright (2011) Elsevier.
Figure 1. Equilibrium conversion of (a) CH4 and (b) CO2 as a function of temperature and CO2/CH4 ratio at atmospheric pressure for n(CH4 + CO2) = 2 mol. Reprinted with permission from Ref. [39]. Copyright (2011) Elsevier.
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Figure 2. The amount of produced carbon species (mol) during dry reforming of methane (DRM) reaction as a function of temperature and CO2/CH4 ratio at atmospheric pressure for n(CH4 + CO2) = 2 mol. Reprinted with permission from Ref. [39]. Copyright (2011) Elsevier.
Figure 2. The amount of produced carbon species (mol) during dry reforming of methane (DRM) reaction as a function of temperature and CO2/CH4 ratio at atmospheric pressure for n(CH4 + CO2) = 2 mol. Reprinted with permission from Ref. [39]. Copyright (2011) Elsevier.
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Figure 3. Schematic description of DRM reaction proceeds via three major steps (CH4 cracking, CO2 dissociation, and oxidation of carbon species) over Ni particles supported by either acidic (e.g., SiO2) or basic metal-oxide supports (e.g., MgO).
Figure 3. Schematic description of DRM reaction proceeds via three major steps (CH4 cracking, CO2 dissociation, and oxidation of carbon species) over Ni particles supported by either acidic (e.g., SiO2) or basic metal-oxide supports (e.g., MgO).
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Figure 4. Suggested mechanism of deactivation of Ni catalysts supported by metal-oxides with excessive acidity or basicity. Reprinted with permission from Ref. [59]. Copyright (2017) Elsevier.
Figure 4. Suggested mechanism of deactivation of Ni catalysts supported by metal-oxides with excessive acidity or basicity. Reprinted with permission from Ref. [59]. Copyright (2017) Elsevier.
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Figure 5. (a) Catalytic activity and the rate of coke formation, and (b) the particle size and the surface composition of Ni-alumina aerogel catalysts as a function of metal loading. The average particle size and surface composition were determined by TEM and XPS, respectively. The rate of coke formation was estimated from the total carbon contents in the spent catalysts. Reprinted with permission from Ref. [63]. Copyright (2000) Elsevier.
Figure 5. (a) Catalytic activity and the rate of coke formation, and (b) the particle size and the surface composition of Ni-alumina aerogel catalysts as a function of metal loading. The average particle size and surface composition were determined by TEM and XPS, respectively. The rate of coke formation was estimated from the total carbon contents in the spent catalysts. Reprinted with permission from Ref. [63]. Copyright (2000) Elsevier.
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Figure 6. TEM image of (a) Ni nanoparticles synthesized on La2O3 nanorod (denoted as 5Ni/La2O3-LOC in Ref. [60]) and (b) on La2O3 supports via a wet impregnation method (denoted as 5Ni/La2O3-C in Ref. [60]) before DRM reaction. TEM image of (c) Ni nanoparticles on La2O3 nanorod (denoted as 5Ni/La2O3-LOC in Ref. (denoted as 5Ni/La2O3-C in Ref. [60]) Insets of (c,d) present the size distribution of Ni particles on each support. Reprinted with permission from Ref. [60]. Copyright (2017) Elsevier.
Figure 6. TEM image of (a) Ni nanoparticles synthesized on La2O3 nanorod (denoted as 5Ni/La2O3-LOC in Ref. [60]) and (b) on La2O3 supports via a wet impregnation method (denoted as 5Ni/La2O3-C in Ref. [60]) before DRM reaction. TEM image of (c) Ni nanoparticles on La2O3 nanorod (denoted as 5Ni/La2O3-LOC in Ref. (denoted as 5Ni/La2O3-C in Ref. [60]) Insets of (c,d) present the size distribution of Ni particles on each support. Reprinted with permission from Ref. [60]. Copyright (2017) Elsevier.
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Figure 7. TEM image and size distribution of Ni nanoparticles synthesized on mesoporous silica via PET assisted impregnation method (a) before and after (b) DRM reaction (denoted as Ni/PST in Ref. [82]). TEM image and size distribution of Ni nanoparticles synthesized on mesoporous silica via classical impregnation method (c) before and (d) after DRM reaction (denoted as Ni/ST in Ref. [82]). Reprinted with permission from Ref. [82]. Copyright (2017) Elsevier.
Figure 7. TEM image and size distribution of Ni nanoparticles synthesized on mesoporous silica via PET assisted impregnation method (a) before and after (b) DRM reaction (denoted as Ni/PST in Ref. [82]). TEM image and size distribution of Ni nanoparticles synthesized on mesoporous silica via classical impregnation method (c) before and (d) after DRM reaction (denoted as Ni/ST in Ref. [82]). Reprinted with permission from Ref. [82]. Copyright (2017) Elsevier.
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Figure 8. CH4 conversion of DRM catalyzed by atomic layer deposition (ALD)-prepared Ni nanoparticles on mesoporous alumina at different temperatures (700~850 °C) (denoted as ALD Ni/𝛾-Al2O3 in Ref. [85]). Reprinted with permission from Ref. [85]. Copyright (2017) Elsevier.
Figure 8. CH4 conversion of DRM catalyzed by atomic layer deposition (ALD)-prepared Ni nanoparticles on mesoporous alumina at different temperatures (700~850 °C) (denoted as ALD Ni/𝛾-Al2O3 in Ref. [85]). Reprinted with permission from Ref. [85]. Copyright (2017) Elsevier.
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