CeO 2 -Based Heterogeneous Catalysts in Dry Reforming Methane and Steam Reforming Methane: A Short Review

: Transitioning to lower carbon energy and environment sustainability requires a reduction in greenhouse gases such as carbon dioxide (CO 2 ) and methane (CH 4 ) that contribute to global warming. One of the most actively studied rare earth metal catalysts is cerium oxide (CeO 2 ) which produces remarkable improvements in catalysts in dry reforming methane. This paper reviews the management of CO 2 emissions and the recent advent and trends in bimetallic catalyst development utilizing CeO 2 in dry reforming methane (DRM) and steam reforming methane (SRM) from 2015 to 2021 as a way to reduce greenhouse gas emissions. This paper focus on the identiﬁcation of key trends in catalyst preparation using CeO 2 and the effectiveness of the catalysts formulated.


Introduction
In the 21st century, global warming and climate change are widespread issues that plague our planet due to the increase in greenhouse gas (GHGs) emissions. Carbon dioxide (CO 2 ) and methane (CH 4 ) are the most plenteous greenhouse gases that contribute to today's climate change issues which have brought catastrophic changes to the global weather [1]. These issues are related to the burning of fossil fuels including oil, coal and gas to meet the demands of energy consumption which are driven by population and economic growth [2]. The world is now shifting to lower carbon energy to combat this issue. Governments and private sectors around the globe are making collective efforts to reduce emissions of greenhouse gases.
Six years have passed since the Paris Agreement target which was signed in December 2015 by 195 countries to fight climate change [3]. The goal of the Paris Agreement was to contain the rise of the global average temperature at well below 2 • C above pre-industrial levels and to limit the temperature increase even further to 1.5 • C by 2100 [4]. Recently, many oil and gas global companies such as Shell, BP and Petronas have stated they are moving towards net zero carbon emissions by 2050 [5][6][7]. The EU, Japan and South Korea, together with more than 110 countries, have vowed to obtain carbon neutrality by 2050 while China says it will do so before 2060 [8]. This change is one of the consequences of the Paris Agreement's ratification.
When the COVID-19 pandemic hit the world in 2020 and 2021, the world experienced a record decline in global CO 2 emissions due to the unprecedented cessation of human activities. The decreases in industry resulted in decreases in emissions (157.9 Mt CO 2 , 7.1% compared to 2019), followed by road transportation (145.7 Mt CO 2 , −8.3%) and power generation (131.6 Mt CO 2 , −3.8%) [9]. Recently, in September 2021, the world lockdown measures were lifted and energy demand is expected to continue growing which will undoubtedly increase greenhouse gas emissions.
In this paper, we will discuss a method of CO 2 emission management, the methane reforming process, which focuses on dry reforming and steam reforming and utilizes Dry reforming methane (DRM) is acknowledged as an alternative for steam reforming (SRM) because it can directly utilize raw natural gas and does not require subsequent gas separation and purification to remove CO 2 [29].
Dry reforming methane (DRM) has been in the spotlight as this reaction has the advantage of solving two issues related to the reduction in greenhouse gas (GHG) emissions since it uses (CH 4 and CO 2 ) as feed stocks to produce valuable syngas (CO and H 2 ). Over the years, dry reforming of methane has been investigated critically and extensively by many scientists [25,[30][31][32][33][34][35][36]. Syngas is an important precursor for the production of highchained hydrocarbon fuels and value-added oxygenated compounds [37]. DRM yields the H2 to CO unit ratio (1:1) needed for Fischer-Tropsch synthesis [32]. This unit ratio is desirable for many gas to liquid (GTL) applications. Apparently, the indirect routes of methane conversion to liquid fuels (via syngas) are more efficient than direct conversion and are the most widely used routes in the GTL industry [38].
Coke depositions are unavoidable in DRM reactions as the deposits will be generated from two side reactions, namely the Boudard reaction (3) and methane cracking (4): CH 4 → C + 2H 2 (∆H298K = +75 kJ mol −1 ) DRM is seen as a high potential process technology in the current fluctuating low oil price regime due to its higher CO selectivity and low H 2 /CO ratio, which are suitable for the synthesis of long chain hydrocarbons or oxygenated chemicals such as acetic acid, dimethyl ether and oxo-alcohols [4].
The further conversion of syngas is very important for chemical industries that produce commodity goods for human lives and comfort which are essential in agriculture, hygiene, health, food science, pharmaceuticals, construction, vehicles and petrochemicals.

The General Strategy for Formulating the Catalyst
The development of catalysts is a critical component of the ongoing search for novel methods of enhancing the yield and selectivity of chemical reactions. A catalyst makes it possible to obtain an end product by different pathways with lower energy barriers [39]. Previously, noble metals (e.g., Rh, Pt, Pd and Ru) and base metals such (e.g., Iron, Nickel and Zinc) were studied as catalysts for DRM reactions [40]. Noble metals have a superior capability to break the C-H bond and suppress carbon deposition [41]. However, due to the high costs of noble metals, Nickel-based catalysts are more preferable in the DRM process as they are cheap, abundant and demonstrate a high catalytic activity and availability. Nevertheless, Nickel-based catalysts have weaknesses in terms of their proneness to sintering and coking at high temperatures [42,43].
The main driving force for DRM reactions is the presence of active sites for the dissociation of CH 4 and CO 2 . Bifunctional active sites utilize both loaded metals and supports as sites for reactant activation [42]. There is evidence that the presence of a base metal in the catalysts can suppress the formation of coke during the decomposition of CH 4 and the Boudard reaction [44]. The basicity of a catalyst contributes to its activity by facilitating the chemisorption, activation and decomposition of acidic CO 2 gas in the presence of an active metal phase. This increases the surface coverage of CO 2 on the catalyst, reduces the carbon deposition from the Boudard reaction and lowers the reactant activation barrier [42]. Nanocrystals, which are only a few nanometers in size, present the best catalytic efficiencies. As explained by Vedrine [45], nanoscience has encouraged bottom-up strategies. The performance of a solid catalyst relies on its grain size, shape, composition and preparation. Usman et al. [46] agreed that the preparation methods play an important role Catalysts 2022, 12, 452 4 of 22 in the synthesis of smaller particle sizes and in the higher dispersion of the active metal. Metal particles produced in the small size range (1-10 nm) experiences difficulties related to their application in the reactor; therefore, support bodies are required. Supports play a key role in the enhancement of catalytic activity and the suppression of carbon deposition in dry reforming methane [31].
For decades, various catalyst configurations, morphologies and topologies have been tested to determine how synergistic component interactions influence active metal dispersion, basicity, redox property, oxygen mobility, particle size, size distribution, reducibility and the mass transfer limitations of catalysts [42,47]. The most common strategy is to include the application of supports and promoters with a high basicity in order to increase the CO 2 adsorption capacity, as well as to improve the oxidation of carbonaceous species via the Boudard reaction. Another approach is to increase the interactions between the active phase of the nickel and support and thus inhibit sintering [48].

Ceria
Cerium (Ce) is a versatile and important rare earth element that has been involved in many areas of heterogeneous catalysis for several years [49][50][51][52]. It is the most reactive element of the lanthanide series with reserves that are much higher than copper and tin (66.5 and 60 ppm, respectively) [53]. As shown in Figure 1, it has a fluorite structure (FCC) with a space group Fm3m and it consists of a simple cubic oxygen sub-lattice with cerium ions occupying alternate cubes [53,54]. Each Ce cation is surrounded by eight oxygen atoms and the coordination number of the oxygen atoms is four [55]. CeO 2 contains an oxygen deficient (CeO 2-x , with (0 < x ≤ 0.5) in a reducing condition which is considered as a partially reduced oxide. This property facilitates readily oxidized to CeO 2 by capturing oxygen in an oxidizing condition [56,57]. Metal particles produced in the small size range (1-10nm) experiences difficulties related to their application in the reactor; therefore, support bodies are required. Supports play a key role in the enhancement of catalytic activity and the suppression of carbon deposition in dry reforming methane [31]. For decades, various catalyst configurations, morphologies and topologies have been tested to determine how synergistic component interactions influence active metal dispersion, basicity, redox property, oxygen mobility, particle size, size distribution, reducibility and the mass transfer limitations of catalysts [42,47]. The most common strategy is to include the application of supports and promoters with a high basicity in order to increase the CO2 adsorption capacity, as well as to improve the oxidation of carbonaceous species via the Boudard reaction. Another approach is to increase the interactions between the active phase of the nickel and support and thus inhibit sintering [48].

Ceria
Cerium (Ce) is a versatile and important rare earth element that has been involved in many areas of heterogeneous catalysis for several years [49][50][51][52]. It is the most reactive element of the lanthanide series with reserves that are much higher than copper and tin (66.5 and 60 ppm, respectively) [53]. As shown in Figure 1, it has a fluorite structure (FCC) with a space group Fm3m and it consists of a simple cubic oxygen sub-lattice with cerium ions occupying alternate cubes [53,54]. Each Ce cation is surrounded by eight oxygen atoms and the coordination number of the oxygen atoms is four [55]. CeO2 contains an oxygen deficient (CeO2-x, with (0 < x ≤ 0.5) in a reducing condition which is considered as a partially reduced oxide. This property facilitates readily oxidized to CeO2 by capturing oxygen in an oxidizing condition [56,57]. There are many factors that explain why CeO2 rare-earth oxides have dominated in technological applications in the field of catalysts [58]. The reasons include their environmental friendliness, surface-bound defects, excellent redox ability and remarkable oxygen storage capacity and release ability. They have strengths such as the ability to stabilize metal dispersion and promote the water-gas shift [59,60]. CeO2 can be used as an oxygen reduction reaction (ORR) catalyst due to its ability to switch between Ce 4+ and Ce 3+ oxidation states which leads to decent ionic conductivity and good oxygen sorption [55,61]. The presence of oxygen vacancies on the surface often dramatically alters the adsorption and subsequent reactions of various adsorbates, either on a clean surface or on metal particles supported on the surface [62]. The modification oxidation state of the metal catalyst cab influence the absorption behavior of the reactants and the subsequent conversion of the There are many factors that explain why CeO 2 rare-earth oxides have dominated in technological applications in the field of catalysts [58]. The reasons include their environmental friendliness, surface-bound defects, excellent redox ability and remarkable oxygen storage capacity and release ability. They have strengths such as the ability to stabilize metal dispersion and promote the water-gas shift [59,60]. CeO 2 can be used as an oxygen reduction reaction (ORR) catalyst due to its ability to switch between Ce 4+ and Ce 3+ oxidation states which leads to decent ionic conductivity and good oxygen sorption [55,61]. The presence of oxygen vacancies on the surface often dramatically alters the adsorption and subsequent reactions of various adsorbates, either on a clean surface or on metal particles supported on the surface [62]. The modification oxidation state of the metal catalyst cab influence the absorption behavior of the reactants and the subsequent conversion of the reaction intermediates and the reaction path. Therefore, CeO 2 is a highly tunable material with great potential for CO 2 catalysis due to its unique properties [63]. CeO 2 recorded its first appearance in 1979 when it was observed to have the ability to promote dispersion in comparison with conventional supports such as Al 2 O 3 [64]. Since the first discovery, it has been identified in various roles such as promoting noble metal dispersion, promoting the water-gas shift (WGS) and the steam reforming reactions, favoring catalytic activity at the interfacial metal-support sites, promoting CO removal through oxidation by employing a lattice oxygen as well as storing and releasing oxygen (oxygen storage capacity, OSC) under lean and rich conditions, respectively in three-way catalysts (TWCs) [64].

Ceria as a Support
Supports play important roles in the dispersion, activity and stability of active sites. The role of a support is to provide a high surface area for the dispersion of metals, to give resistance to sintering and to stabilize and promote active sites. They may participate in the reaction itself and can modify the catalytic properties of the active phase and increase resistance to coking [11,65]. The support also plays an active role in the catalytic reaction. It provides certain physiochemical properties such as basicity (CaO, La 2 O 3 and MgO), oxygen storage capacity (CeO 2 , CeO 2 -ZrO 2 and TiO 2 ) and reducibility (CeO 2 and ZrO 2 ) [4]. Acidic supports directly affect the mechanism which favors carbon deposition while basic supports facilitate good effects such as high affinity for CO 2 chemisorption and oxygen mobility [66].
CeO 2 is a good support for a noble metal catalyst and ensures long-term use due to its unique properties that have been mentioned (strong metal support, OSC, reducibility (Ce 4+/ Ce 3+) and soot resistance. These properties of CeO 2 enhance the ability of noble metals in a reaction compared to a support with an inert nature such as Al 2 O 3 , especially for SRM, DRM and WGS reactions [67]. Generally, a high surface area provides a greater tendency for active species to make contact with reactants; therefore, they enhance catalytic performance [53].
Luisetto et al. [65] explained that the oxygen vacancies on the CeO 2 surface may adsorb the oxygen formed by the dissociation of CO 2 , improving the reforming activity and the removal of carbon deposits. CeO 2 -Al 2 O 3 combinations are excellent supports for reforming reactions due to their lower acidity in comparison to bare alumina and the greater oxygen capacity storage (OCS) of CeO 2 . Adding CeO 2 to form a CeO 2 -Al 2 O 3 support improves reducibility and enhances the oxygen mobility and metal dispersion [68]. The redox properties of ceria facilitate the oxidation of carbon deposits which expand the lifetime of the catalyst [69]. CeO 2 can modify the metal-support interactions of the Ni catalyst, which can improve the reducibility of the Ni/Al 2 O 3 catalyst [70]. Several studies have revealed that ceria modifies metal-support interactions, increasing the active phase dispersion and improving the stability of alumina at high temperatures [71].
Farooqi et al. [72] made a comparison of three synthesis catalysts namely Ni/Al 2 O 3 , Ni/Al 2 O 3 -CeO 2 and Ni/Al 2 O 3 -La 2 O 3 . It was discovered that the addition of CeO 2 as a combined support with Al 2 O 3 improved the dispersion, increased the active metal content on the catalyst surface and enhanced the reducibility and the catalyst basicity. Similar findings by Chein [73] revealed that high carbon-resistant Ni/Al 2 O 3 with a CeO 2modified catalyst was produced due to the fact that the CeAlO 3 phase suppressed coke formation without damaging the catalytic activity, inhibited the growth of graphitic carbon, decomposed CO 2 and formed active surface oxygen.
Huang et al. [74] indicated that adding CeO 2 to a Ni/Mo/SBA-15 catalyst used for DRM was beneficial. Catalysts with low CeO 2 amounts (up to 2 wt%) showed more stability than Ni/Mo/SBA-15 during dry reforming methane. The addition of CeO 2 helped small metallic Ni particles to be stably dispersed on the composite support and also enhanced the reducibility of the catalysts and adsorption of CO 2 during the reaction. Huang et al. [74] also stated that CeO 2 actually has dual roles in preventing carbon deposition in the CO 2 reforming of methane. The addition of CeO 2 decreased the acidity of the support. This Strong metal-support interactions and abundant oxygen vacancies are very important to prevent the sintering of nickel particles as depicted in research by Xianjun et al. [75]. As shown in Figure 2, methane first absorbs on the active Ni particles and then decomposes into CHx, while carbon dioxide absorbs Ni particles and decomposes into carbon monoxide and O *. Active intermediates containing CHx and O * can react with each other to produce carbon monoxide and hydrogen. Lattice oxygen replenishment results from CO 2 dissociation and oxygen mobility.
Catalysts 2022, 12, x FOR PEER REVIEW 6 of 25 the CO2 reforming of methane. The addition of CeO2 decreased the acidity of the support. This stopped the formation of pyrolytic carbon and the basic CeO2 helped the chemisorption and dissociation of CO2 and subsequently accelerated carbon elimination. Strong metal-support interactions and abundant oxygen vacancies are very important to prevent the sintering of nickel particles as depicted in research by Xianjun et al. [75]. As shown in Figure 2, methane first absorbs on the active Ni particles and then decomposes into CHx, while carbon dioxide absorbs Ni particles and decomposes into carbon monoxide and O *. Active intermediates containing CHx and O * can react with each other to produce carbon monoxide and hydrogen. Lattice oxygen replenishment results from CO2 dissociation and oxygen mobility. Supports can influence the chemical state of noble metal nanoparticles other than simply acting as a substrate for the dispersion of the loaded metal. Controlling the size and uniformity of a supported metal catalyst is a major challenge for the catalytic structure-property relationship. To generate monodisperse metal catalysts, molding agents or surfactants with a significant adhesion for the metal surface are typically used, masking the catalyst's intrinsic catalytic behavior [76].

Ceria as a Promoter
The function of chemical promoters is to present new, supplementary active sites or to reinforce the chemical property relating to the reactivity of the catalyst such as basicity and redox properties [4]. The addition of promoters can aid in the reduction in carbon deposition and sintering, as well as the oxidation of carbonaceous species, resulting in an improved reaction conversion.
The addition of CeO2 into catalysts demonstrates the positive effects on catalytic activity and stability and carbon suppression when it is used as a promoter instead of a support with a strong metal-support interaction, which reduces Ni sintering and carbon deposition [74]. The work by Li et al. [77] found that adding 3 wt% of Ce could suppress the sintering of Ni particles on SBA-15 by promoting the oxidation of coke formed on the nickel catalyst based on its internal oxygen transfer to coke. Applying a Ce promoter has its own advantages. Ce doping inhibits reactions (3), increases the catalyst basicity and CO2 adsorption and favors the oxidation of deposited carbon species [48].
Arora and Prasad [32] stated that promoters such as Sn, Sr, Ca, Ce, K and Zr are employed to prevent carbon accumulation and the combination of Ce, Zr and transition metals has garnered interest due to their oxygen storage abilities. They give an oxygen lattice in the Ce oxide phase during reducing conditions and generate anionic vacancies which enhance the activity of the catalyst. Supports can influence the chemical state of noble metal nanoparticles other than simply acting as a substrate for the dispersion of the loaded metal. Controlling the size and uniformity of a supported metal catalyst is a major challenge for the catalytic structureproperty relationship. To generate monodisperse metal catalysts, molding agents or surfactants with a significant adhesion for the metal surface are typically used, masking the catalyst's intrinsic catalytic behavior [76].

Ceria as a Promoter
The function of chemical promoters is to present new, supplementary active sites or to reinforce the chemical property relating to the reactivity of the catalyst such as basicity and redox properties [4]. The addition of promoters can aid in the reduction in carbon deposition and sintering, as well as the oxidation of carbonaceous species, resulting in an improved reaction conversion.
The addition of CeO 2 into catalysts demonstrates the positive effects on catalytic activity and stability and carbon suppression when it is used as a promoter instead of a support with a strong metal-support interaction, which reduces Ni sintering and carbon deposition [74]. The work by Li et al. [77] found that adding 3 wt% of Ce could suppress the sintering of Ni particles on SBA-15 by promoting the oxidation of coke formed on the nickel catalyst based on its internal oxygen transfer to coke. Applying a Ce promoter has its own advantages. Ce doping inhibits reactions (3), increases the catalyst basicity and CO 2 adsorption and favors the oxidation of deposited carbon species [48].
Arora and Prasad [32] stated that promoters such as Sn, Sr, Ca, Ce, K and Zr are employed to prevent carbon accumulation and the combination of Ce, Zr and transition metals has garnered interest due to their oxygen storage abilities. They give an oxygen lattice in the Ce oxide phase during reducing conditions and generate anionic vacancies which enhance the activity of the catalyst.
Adding promoters (e.g., Co, ZrO 2 , CeO 2 , MgO and CaO) to a nickel-based catalyst is an effective way to promote Ni/Al 2 O 3 . CeO 2 can modify the metal-support interaction of Ni catalysts, which can improve the reducibility of the Ni/Al 2 O 3 catalyst [78]. Mallikarjun et al. [79] suggested that the promotion of alkali metals can enhance the carbon resistance in a Ni-based catalyst. The higher activities of catalysts modified with Ce and CO 2 can be explained on the basis of the CeAlO 3 formed, which interacts with Ni differently when compared to CeO 2 and Al 2 O 3 alone. The highest catalytic activity was attributed to the formation of an interface between Ni and Ce, which acted as an active site for methane activation [46].
Many researchers have reported on the use of bimetallic systems, as doping with other metals can improve catalytic performance in the DRM reaction [80,81]. The doping of a second metal can enhance the adsorption properties of CO 2 and H 2 O which is beneficial for reactions with carbon and for reducing carbon deposition [81].

Summary of DRM with CeO 2 -Based Catalysts from 2015 to 2021
Ay et al. [82] found that the Co/CeO 2 catalyst exhibited a much lower performance than the Ni/CeO 2 and Ni-Co/CeO 2 catalysts due to strong metal-support interactions. The activities of ceria-based catalysts decreased with an increase in the calcination temperature accompanied by a decrease in coke deposition.
T. Stroud et al. [83] highlighted that through the addition of small quantities of dopants such as Sn and CeO 2 , the DRM performance can be improved. As shown in Figure 3, the optimum amount of the Ni/Sn molar ratio was identified to be 0.02. This multicomponent catalyst, Sn0.02Ni/Ce-Al, remains active for long periods of 92 h with 85% CO 2 conversion. Sn atoms were found occupying C nucleation sites in the vicinity of Ni atoms which slowed down the carbon formation, whereas the presence of ceria provided a high oxygen storage capacity and modified the acid/base properties of the support lead with alumina.
Adding promoters (e.g., Co, ZrO2, CeO2, MgO and CaO) to a nickel-based catalyst is an effective way to promote Ni/Al2O3. CeO2 can modify the metal-support interaction of Ni catalysts, which can improve the reducibility of the Ni/Al2O3 catalyst [78]. Mallikarjun et al. [79] suggested that the promotion of alkali metals can enhance the carbon resistance in a Ni-based catalyst. The higher activities of catalysts modified with Ce and CO2 can be explained on the basis of the CeAlO3 formed, which interacts with Ni differently when compared to CeO2 and Al2O3 alone. The highest catalytic activity was attributed to the formation of an interface between Ni and Ce, which acted as an active site for methane activation [46].
Many researchers have reported on the use of bimetallic systems, as doping with other metals can improve catalytic performance in the DRM reaction [80,81]. The doping of a second metal can enhance the adsorption properties of CO2 and H2O which is beneficial for reactions with carbon and for reducing carbon deposition [81].

Summary of DRM with CeO2-Based Catalysts from 2015 to 2021
Ay et al. [82] found that the Co/CeO2 catalyst exhibited a much lower performance than the Ni/CeO2 and Ni-Co/CeO2 catalysts due to strong metal-support interactions. The activities of ceria-based catalysts decreased with an increase in the calcination temperature accompanied by a decrease in coke deposition.
T. Stroud et al. [83] highlighted that through the addition of small quantities of dopants such as Sn and CeO2, the DRM performance can be improved. As shown in Figure  3, the optimum amount of the Ni/Sn molar ratio was identified to be 0.02. This multicomponent catalyst, Sn0.02Ni/Ce-Al, remains active for long periods of 92 h with 85% CO2 conversion. Sn atoms were found occupying C nucleation sites in the vicinity of Ni atoms which slowed down the carbon formation, whereas the presence of ceria provided a high oxygen storage capacity and modified the acid/base properties of the support lead with alumina. Mallikarjun et al. [79] developed Ni/CeO2-SrO by the impregnation method and pronounced that the catalyst with 12wt% Ni and equal mole ratios of CeO2-SrO showed better activity and showed extraordinary stability over a period of 250 h. The Ni dispersion and metal reducibility were improved by the addition of CeO2.
Akiki et al. [84] revealed that a catalyst based on 5 wt% of Ni can be used as an optimal concentration for DRM and the effect of Ce is more beneficial than La as a promoter. The 1.5Ce-Ni5/MgAl2O4 catalyst exhibited the best catalytic activity and stability for DRM Mallikarjun et al. [79] developed Ni/CeO 2 -SrO by the impregnation method and pronounced that the catalyst with 12wt% Ni and equal mole ratios of CeO 2 -SrO showed better activity and showed extraordinary stability over a period of 250 h. The Ni dispersion and metal reducibility were improved by the addition of CeO 2 .
Akiki et al. [84] revealed that a catalyst based on 5 wt% of Ni can be used as an optimal concentration for DRM and the effect of Ce is more beneficial than La as a promoter. The 1.5Ce-Ni5/MgAl 2 O 4 catalyst exhibited the best catalytic activity and stability for DRM with 96% CO 2 conversion and 92% CH 4 conversion. Strong interactions between the CeO 2 and the support enhanced the structure of the catalyst, resulting in the creation of more oxygen vacancies.
Jin et al. [85] deposited Ni nanoparticles on the four channels of the α-Al 2 O 3 hollow fibers catalyst support by using atomic layer disposition (ALD). The CeNi/Al 2 O 3 NP catalyst was prepared by the incipient wetness (IW) method and was stable for 360 h which was 7.5 times longer than Ni/Al 2 O 3 NP-ALD and produced an excellent performance after regeneration. The higher stability for the Ni-based catalyst was achieved due to the strong oxygen storage and release properties of CeO 2 which improved the CO 2 dissociative adsorption reaction and lead to reduced carbon formation.
Jawad et al. [68] investigated a series of Ni-based Al 2 O 3 -CeO 2 composite catalysts which showed a significant improvement with the addition and doping of third metal particles such as Pt, Fe and Mo within the bimetallic catalyst due to enhanced metal dispersion and catalyst reducibility. The Ce and MOx-modified catalysts demonstrated increased redox properties and abundant oxygen vacancies among the Ni-based composite catalysts which provided supplement active oxygen and more active sites for the activation of CO 2 and CH 4 .
Karemore et al. [86] studied the influence of reaction conditions (temperature, space time, feed composition and time-on-stream) and reaction kinetics on a mixed reforming methane reaction using Ni-K/CeO 2 -Al 2 O 3 to facilitate catalyst development and a reactor design for the reaction. The reactant conversion and product yield increased with the increase in space, time and temperature. The presence of the promoters K and CeO 2 oxidized the carbon formed on the catalyst surface and caused the carbon deposition rate over 50 h to be low (2.45 mgC/gcat-h). The syngas (H 2 /CO) ratio at 800 • C significantly increased from 1.32 to 2.14 mol/mol with the increase in the S/C ratio of 0.2-0.5 mol/mol.
Chein et al. [73] studied the reactant composition to determine the performance and stability of the catalyst using a modified Ni/Al 2 O 3 catalyst. The carbon resistance increased on the catalyst's surface with the modification of CeO 2 on the Ni catalyst and the addition of O 2 . As the number of CeO 2 loading increased, the CH 4 conversion increased. However, as shown in Figure 4, the CH 4 conversion obtained from 10Ni15Ce catalyst was lower than that from the 10Ni10Ce and 10Ni5Ce catalysts. The conversion of 10Ni15Ce was almost the same with 10Ni0Ce. Hence, it was concluded that the optimum amount of CeO 2 loading was in the range of 5 to 10% for the best DRM performance due to the decrease in the specific surface area as the CeO 2 loading increased and the Ni particle aggregation increased. The addition of O 2 significantly suppresses the RWGS reaction in DRM due to the dominance of the CH 4 oxidation reaction. These findings concur with the research by Arora and Prasad [32] which stated that the addition of oxygen to SRM and DRM can improve the energy efficiency or synergistic effects in the processing and mitigation of coking.  Swirk et al. [87] studied DRM double-layered hydroxides modified with cerium (coprecipitation method) and with yttrium prepared by the incipient wetness impregnation method with 0.2, 0.4 and 0.6 wt%. Ni reducibility decreased, basicity increased, Ni dispersion enhanced and smaller Ni crystallites were observed with the promotion of both Ce and Y, as compared with a hydrotalcites catalyst due to the formation of a yttria-doped ceria (YDC) phase. Highlighted in the studies was the modification of the smallest loading Swirk et al. [87] studied DRM double-layered hydroxides modified with cerium (coprecipitation method) and with yttrium prepared by the incipient wetness impregnation method with 0.2, 0.4 and 0.6 wt%. Ni reducibility decreased, basicity increased, Ni dispersion enhanced and smaller Ni crystallites were observed with the promotion of both Ce and Y, as compared with a hydrotalcites catalyst due to the formation of a yttria-doped ceria (YDC) phase. Highlighted in the studies was the modification of the smallest loading of yttrium (0.2 wt%) which led to an increase in both CO 2 and CH 4 conversions for 5 h.
Zhang et al. [88] investigated the effect of adding Zr dopants into the ceria support, Ni/CeZrO 2 , on the DRM reaction performance and revealed that the conversion, reaction rate and H 2 selectivity substantially increased with the addition of a Zr dopant. A larger Ce 3+ substance was noticed in the mixed-oxide support upon the reaction with pure CH 4 or during DRM, when doping the Zr into the ceria support implied a higher reducibility of the mixed-oxide support. Moreover, Zr prevented Ni migration from the surface into ceria forming a Ce 1−x Ni x O 2−y solid solution which maintained the active NiO on the Ni/CeZrO 2 surface.
Hassani Rad SJ, et al. [89] prepared Ni/Al 2 O 3 -CeO 2 and Ni/Al 2 O 3 -MgO nanocatalysts using the impregnation and sol-gel methods. The sol-gel method produced a better performance compared to the impregnation method as it provides a much higher surface area, a better dispersion of metals, a more homogenous morphology and a smaller nanoparticle size leading to modified adsorption properties. Among all, the sol-gel method synthesized with the ceria promoter catalyst and the Ni/Al 2 O 3 -CeO 2 nanocatalyst emerged as the best choice as it exhibited a H2/CO ratio of 1 and a H2 yield of 94% at 850 • C.
Farooqi [72] compared the performances of three catalysts, namely Ni/Al 2 O 3 , Ni/Al 2 O 3 -CeO 2 and Ni/Al 2 O 3 -La 2 O 3 which were prepared by the sol-gel method. The catalyst with the addition of CeO 2 on the support showed the highest and most stable conversion due to the fact that it enhanced dispersion, increased the active metal content on the catalyst surface and improved the reducibility and basicity of the catalyst.
Price et al. [90] highlighted that encapsulation techniques with incorporations of 8 wt% Ni/ZnO cores in silica (SiO 2 ) can lead to advantageous conversions of CO 2 and CH 4 at high temperatures compared to uncoated traditional catalytic materials: Ni/CeO 2 and Ni/Al 2 O 3 . Encapsulating a catalytic core increases the surface area and reaction kinetics which results in a high level of reactant conversion.
Das et al. [91] developed a novel core-shell structured Ni-SiO 2 @ CeO 2 catalyst with Ni nanoparticles sandwiched between SiO 2 and CeO 2 layers as shown in Figure 5, applied for the dry reforming of bio-gas (CH 4 /CO 2 = 1.5) at low temperatures (600 • C). Ni-SiO 2 @CeO 2 produced a higher Ni dispersion which resulted in a superior performance compared to its bare structure with negligible coke formation during a 72-h stability test and high dry reforming activity (~0.12 mol CH 4 min −1 gNi −1 ). The dual confinement effect provided by the encapsulation of Ni nanoparticles between SiO 2 and CeO 2 layers prevents Ni sintering and the redox capacity of CeO 2 and the higher RWGS activity of ceria leads to a high coke resistance for the catalysts.
K. Han et al. [92] synthesized Ni@SiO 2 @CeO 2 by coating ceria on the surface of Ni@SiO 2 which was initially prepared by the reverse microemulsion method. Ni@SiO 2 @CeO 2 exhibited a bi-functional mechanism compared to the mono-functional mechanism of Ni@SiO 2 which resulted in a one and a half times higher catalytic performance and a reduced carbon deposition at low temperatures of 400 to 600 • C which indicated high stability.
Cardenas-Arenas et al. [93] demonstrated that nanoparticle catalysts, designated NiO-CeO 2 , synthesized by the reversed microemulsion process were capable of lowering 63% of carbon deposition during the DRM test. Triton X-100, n-heptane and hexanol were used to create the microemulsion. The nanoparticles' small size facilitates the participation of cerium cations in the redox reactions that occur during DRM and stabilizes the nickel cationic species.
SiO2@CeO2 produced a higher Ni dispersion which resulted in a superior performance compared to its bare structure with negligible coke formation during a 72-h stability test and high dry reforming activity (~0.12 mol CH4 min −1 gNi −1 ). The dual confinement effect provided by the encapsulation of Ni nanoparticles between SiO2 and CeO2 layers prevents Ni sintering and the redox capacity of CeO2 and the higher RWGS activity of ceria leads to a high coke resistance for the catalysts. K. Han et al. [92] synthesized Ni@SiO2@CeO2 by coating ceria on the surface of Ni@SiO2 which was initially prepared by the reverse microemulsion method. Ni@SiO2@CeO2 exhibited a bi-functional mechanism compared to the mono-functional mechanism of Ni@SiO2 which resulted in a one and a half times higher catalytic performance and a reduced carbon deposition at low temperatures of 400 to 600 °C which indicated high stability.
Cardenas-Arenas et al. [93] demonstrated that nanoparticle catalysts, designated NiO-CeO2, synthesized by the reversed microemulsion process were capable of lowering 63% of carbon deposition during the DRM test. Triton X-100, n-heptane and hexanol were used to create the microemulsion. The nanoparticles' small size facilitates the participation of cerium cations in the redox reactions that occur during DRM and stabilizes the nickel cationic species.
Marinho et al. [94] highlighted that the Ni catalyst synthesized by the EISA method produces materials with a high surface area and well defined mesopores. It favors the formation of the NiAl2O4 spinel phase with very well dispersed Ni particles on the support and inhibits sintering at high temperatures. The addition of cerium promotes oxygen mobility when interacting strongly with Al2O3 and enhances the carbon resistance and catalytic performance.
Luisetto et al. [65] synthesized a Ni catalyst supported by a solid solution of CeO2 with Zr, Sm and La dopands using the one-step citric acid method. The most promising catalysts in terms of low carbon formation were Ni/Sm-DC and Ni/La-DC, as the nature of the dopants influenced the Ni-support interaction and the electronic state of the metal catalyst.
Padi et al. [95] suggested that the exsolution process in the nanoscale NiO-CeO2 solid solution with a fluorite structure could produce the supported Ni/CeO2 catalyst. In Figure  6, the elemental mapping by STEM-EDX highlights the grain boundaries and stacking faults after 90 h of the DRM reaction which provides nucleation sites for nanoparticle growth. This Ni/CeO2 catalyst demonstrated an active and stable performance in DRM at Marinho et al. [94] highlighted that the Ni catalyst synthesized by the EISA method produces materials with a high surface area and well defined mesopores. It favors the formation of the NiAl 2 O 4 spinel phase with very well dispersed Ni particles on the support and inhibits sintering at high temperatures. The addition of cerium promotes oxygen mobility when interacting strongly with Al 2 O 3 and enhances the carbon resistance and catalytic performance.
Luisetto et al. [65] synthesized a Ni catalyst supported by a solid solution of CeO 2 with Zr, Sm and La dopands using the one-step citric acid method. The most promising catalysts in terms of low carbon formation were Ni/Sm-DC and Ni/La-DC, as the nature of the dopants influenced the Ni-support interaction and the electronic state of the metal catalyst.
Padi et al. [95] suggested that the exsolution process in the nanoscale NiO-CeO 2 solid solution with a fluorite structure could produce the supported Ni/CeO 2 catalyst. In Figure 6, the elemental mapping by STEM-EDX highlights the grain boundaries and stacking faults after 90 h of the DRM reaction which provides nucleation sites for nanoparticle growth. This Ni/CeO 2 catalyst demonstrated an active and stable performance in DRM at 800 • C for 50 h which verified the strong metal-support interaction with no coking at all. The outstanding result was due to the combination effect of the strong metal-support interaction derived by the exsolution method and the existence of a highly mobile oxygen lattice within the ceria support. Coking on a CeO 2 -supported material can be prevented without the need to add a second oxide to the final support phase. The composition of catalyst supports attributable to the effects of size and charge balance is noted in this method.
Zhang Q et al. [96] introduced a novel photoactivation using UV-Visible Infrared (UV-Vis-IR) illumination to improve the solar-light-driven thermocatalytic activity of a Ni/CeO 2 catalyst in DRM. The synergetic effect among the Ni nanoparticles and CeO 2 exists for DRM on the catalyst derived from the migration of the oxygen lattice at the Ni-CeO 2 interface. The improved catalytic activity of the Ni metal was confirmed by the DFT calculation in which the irradiation reduces the activation energy of the dominant steps of C and CH oxidation.
Using a plasma decomposition method, X. Yan et al. [70] synthesized two types of catalysts with distinct interfacial structures and interactions between Ni and CeO 2 . The first catalyst, designated Ni/CeO 2 -SiO 2 -P, included CeO 2 with a higher concentration of reactive oxygen species in close proximity to Ni NP, whereas the second catalyst, designated Ni/CeO 2 -SiO 2 -C, contained CeO 2 that was separated from Ni NP. The Ni/CeO 2 -SiO 2 -P catalyst outperformed the Ni/CeO 2 -SiO 2 -C catalyst in terms of performance and stability in DRM. The superior performance of Ni/CeO 2 -SiO 2 -P is due to the unique interface structure, which promotes the formation of formate species and the reaction of original and active Cα species via more available oxygen species and more accessible hydrogen sites on the metal-support interface, whereas in Ni/CeO 2 -SiO 2 -C, the insufficient conversion of Cα results in the accumulation of less active Cβ species and Cγ, deactivating the catalytic performance.
Catalysts 2022, 12, x FOR PEER REVIEW 11 of 25 800 °C for 50 h which verified the strong metal-support interaction with no coking at all. The outstanding result was due to the combination effect of the strong metal-support interaction derived by the exsolution method and the existence of a highly mobile oxygen lattice within the ceria support. Coking on a CeO2-supported material can be prevented without the need to add a second oxide to the final support phase. The composition of catalyst supports attributable to the effects of size and charge balance is noted in this method. Zhang Q et al. [96] introduced a novel photoactivation using UV-Visible Infrared (UV-Vis-IR) illumination to improve the solar-light-driven thermocatalytic activity of a Ni/CeO2 catalyst in DRM. The synergetic effect among the Ni nanoparticles and CeO2 exists for DRM on the catalyst derived from the migration of the oxygen lattice at the Ni-CeO2 interface. The improved catalytic activity of the Ni metal was confirmed by the DFT calculation in which the irradiation reduces the activation energy of the dominant steps of C and CH oxidation.
Using a plasma decomposition method, X. Yan et al. [70] synthesized two types of catalysts with distinct interfacial structures and interactions between Ni and CeO2. The first catalyst, designated Ni/CeO2-SiO2-P, included CeO2 with a higher concentration of reactive oxygen species in close proximity to Ni NP, whereas the second catalyst, designated Ni/CeO2-SiO2-C, contained CeO2 that was separated from Ni NP. The Ni/CeO2-SiO2-P catalyst outperformed the Ni/CeO2-SiO2-C catalyst in terms of performance and stability in DRM. The superior performance of Ni/CeO2-SiO2-P is due to the unique interface structure, which promotes the formation of formate species and the reaction of original and active Cα species via more available oxygen species and more accessible hydrogen sites on the metal-support interface, whereas in Ni/CeO2-SiO2-C, the insufficient conversion of Cα results in the accumulation of less active Cβ species and Cγ, deactivating the catalytic performance.
Tu PH et al. [97] invented a new two-step hydrothermal process which produced flowerlike Ce0.5Zr0.5O2 with an OSC of 536µmolO2g −1 which was double the OSCs of pure flowerlike CeO2 (284µmolO2g −1 ). The function of ceria (CeO2) as a support material for the Ni catalyst in DRM was improved by producing a solid solution (SS) with zirconia (ZrO2) to raise the OSC. The flowerlike Ce0.5Zr0.5O2 synthesized by a two-step hydrothermal process as shown in Figure 7 produced the highest catalytic performance for DRM at 750 °C with an initial methane conversion of 88.4% compared with the Ce0.5Zr0.5O2 synthesized by the one-step hydrothermal process with methane conversion of 83.7%. The author Tu PH et al. [97] invented a new two-step hydrothermal process which produced flowerlike Ce 0.5 Zr 0.5 O 2 with an OSC of 536 µmolO 2 g −1 which was double the OSCs of pure flowerlike CeO 2 (284 µmolO 2 g −1 ). The function of ceria (CeO 2 ) as a support material for the Ni catalyst in DRM was improved by producing a solid solution (SS) with zirconia (ZrO 2 ) to raise the OSC. The flowerlike Ce 0.5 Zr 0.5 O 2 synthesized by a two-step hydrothermal process as shown in Figure 7 produced the highest catalytic performance for DRM at 750 • C with an initial methane conversion of 88.4% compared with the Ce 0.5 Zr 0.5 O 2 synthesized by the one-step hydrothermal process with methane conversion of 83.7%. The author concluded that the petals of the flowerlike structure elevated the sintering resistance of the Ni metal and the coking resistance due to a high OSC.
Simonov et al. [98] prepared mixed ceria-zirconia oxides including those doped by Nb and Ti by the traditional citrate method and by continuous solvothermal flow synthesis in supercritical alcohols. It was observed that the catalyst prepared in supercritical alcohols was the most active and stable from the rest with the specific activity doubling after the addition of Nb and Ti due to the strengthening of the metal-support interaction and the increase in OSC.
Fedorova et al. [99] investigated the effect of Ni loading methods prepared by incipient wetness impregnation and the one-pot technique on the catalytic behavior of DRM. The results established that the catalytic activity is dependent on the composition of the support and the method of Ni deposition. The effective activation energy of DRM over the impregnated sample was demonstrated to be lower than that of the one-pot series. The TOF increased three times when titanium and niobium cations were added to nickelcontaining catalysts based on ceria-zirconia. The author found that the "one-pot" method in a supercritical medium for the preparation of catalysts is advantageous due to its pace of production and scalability. concluded that the petals of the flowerlike structure elevated the sintering resistance of the Ni metal and the coking resistance due to a high OSC. Simonov et al. [98] prepared mixed ceria-zirconia oxides including those doped by Nb and Ti by the traditional citrate method and by continuous solvothermal flow synthesis in supercritical alcohols. It was observed that the catalyst prepared in supercritical alcohols was the most active and stable from the rest with the specific activity doubling after the addition of Nb and Ti due to the strengthening of the metal-support interaction and the increase in OSC.
Fedorova et al. [99] investigated the effect of Ni loading methods prepared by incipient wetness impregnation and the one-pot technique on the catalytic behavior of DRM. The results established that the catalytic activity is dependent on the composition of the support and the method of Ni deposition. The effective activation energy of DRM over the impregnated sample was demonstrated to be lower than that of the one-pot series. The TOF increased three times when titanium and niobium cations were added to nickel-containing catalysts based on ceria-zirconia. The author found that the "one-pot" method in a supercritical medium for the preparation of catalysts is advantageous due to its pace of production and scalability.
Bin Li et al. [100] prepared porous silica-supported nickel catalysts (Ni-CeX-Y/SiO2) with different contents of CeO2 by the "one-pot" method and discovered that the addition of CeO2 hindered the formation of the 1:1 Ni-phyllosilicate species and weakened the interaction between Ni and Si but could efficiently prevent the sintering of Ni nanoparticles and therefore Ni-CeX-Y/SiO2 catalysts possess excellent anti-sintering ability. Moreover, the kinetic study revealed that the introduction of CeO2 is this method could decrease the activation energy of CH4 decomposition and CO2 dissociation. The active oxygen species from CeO2 and the increasing number of O* derived from CO2 dissociation resulted in the decrease in carbon deposition in CH4 decomposition which verified that the useful effect on the gasification rate was more powerful than the rate of carbon formation. Hence, the stability of the Ni-CeX-Y/SiO2 catalyst improved exceptionally by increasing CeO2 loading. Bin Li et al. [100] prepared porous silica-supported nickel catalysts (Ni-CeX-Y/SiO 2 ) with different contents of CeO 2 by the "one-pot" method and discovered that the addition of CeO 2 hindered the formation of the 1:1 Ni-phyllosilicate species and weakened the interaction between Ni and Si but could efficiently prevent the sintering of Ni nanoparticles and therefore Ni-CeX-Y/SiO 2 catalysts possess excellent anti-sintering ability. Moreover, the kinetic study revealed that the introduction of CeO 2 is this method could decrease the activation energy of CH 4 decomposition and CO 2 dissociation. The active oxygen species from CeO 2 and the increasing number of O* derived from CO 2 dissociation resulted in the decrease in carbon deposition in CH 4 decomposition which verified that the useful effect on the gasification rate was more powerful than the rate of carbon formation. Hence, the stability of the Ni-CeX-Y/SiO 2 catalyst improved exceptionally by increasing CeO 2 loading.
Jeon et al. [101] observed that when CeO 2 was added to a Ni-MgO catalyst constructed at various titration rates, the OSC effect changed and had an effect on not only the catalytic activity, but also the stability. Without adding CeO 2 , the influence of the titration rate on Ni-MgO catalysts is limited to changes in Ni crystallite size and dispersion, which are capable of altering the catalytic activity during DRM. Thus, the Ni-MgO-CeO 2 catalyst synthesized at a rapid titration rate demonstrated the greatest DRM performance at 800 • C and a high gas hourly space velocity of 720,000 mL·g −1 ·L −1 .
Lustemberg et al. [102] elucidated the nature of the active sites in Ni/CeO 2 catalysts for DRM and direct methane to methanol conversion. Due to the discovery that Ni at low loadings on CeO 2 (111) is particularly active in DRM, Lustemberg and coworkers correlated experimental observations on the CeO 2 (111) surface with clusters of tiny cationic Ni atoms at sharp edges with the highest Ni potential. Calculations based on Density Functional Theory (DFT) were utilized to elucidate the reasons underlying the discovery. By examining the activation barrier for C-H bond breaking during the dissociative adsorption of CH 4 , it was determined that the size and shape of the supported Ni nanoparticles, as well as the strength of the Ni support bonding and the charge transfer at the step edge, were critical for the high catalytic activity. Table 1 summarizes the Dry Reforming Methane (DRM) utilizing CeO 2 -based catalysts from 2015 to 2021.

Summary of SRM with CeO 2 -Based Catalysts from 2015 to 2021
Cifuentes et al. [103] discovered that 33% Si content in the support (RhPt/CeSi-33, Si:Ce ratio of 1:2) was the best for the catalyst as it decreased the basicity of the support, reduced the crystalline size of CeO 2 , increased the catalyst surface area and decreased the active particle size. Moreover, it produced a maximum H 2 yield of 5.2+ 0.2 mol H 2 /mol EtOH. The addition of Si reduced the relative basicity of CeO 2 ; hence, this composite catalyst provides an equilibrium between the basicity (to maximize H 2 and excess of ethylene and coke formation) and acidity (to promote CH 4 formation and H 2 O activation).
Iglesias I, et al. [104] compared nickel-based catalysts supported on pure or doped ceria (5% Zr, Pr, or La doping) produced through the co-precipitation urea technique in SRM reactions at 600, 750, and 900 • C and evaluated them under various feed conditions. At 600 • C, an increase in the vapor/methane ratio resulted in an increased hydrogen yield and decreased carbon creation and it was revealed that an intermediate calcination temperature (750 • C) maximized the nickel-support interaction, resulting in maximum methane conversion. OSC levels decrease in high-temperature and reductive environments. Zr stabilizes ceria, forming a ceria-zirconia solid solution in all composition ranges and improving textural features, thermal resistance, catalytic activity at lower temperatures and most importantly, oxygen storage/transport properties [105].
Zhang et al. [106] investigated the effect of doping different metals, namely Ti, Sn, Zr and Ce with Yttrium by the co-precipitation method to be used as supports for Nibased catalysts in methane steam reforming. The structures of Y 2 Zr 2 O 7 and Y 2 Ce 2 O 7 compounds became defective fluorites and the surfaces of Ni/Y 2 TiO 7 and Ni/Y 2 CeO 7 had more abundant active oxygen species to suppress carbon formation. Ni/Y 2 TiO 7 exhibited the highest activity, stability and coking resistance among the rest of the tested catalysts due to the largest amount of active surface oxygen species and the strongest Ni interaction with the support.
Iglesias I, et al. [107] optimized the nickel catalyst supported in zirconium-doped ceria in SRM at low temperatures and with a stoichiometric water/methane feed ratio. The formation of the zirconium tetragonal phase during synthesis was harmful to the reducibility of the solid and oxygen mobility, resulting in a better selectivity for low oxidation products. The effect of nickel loading on Ce 0.85 Zr 0.15 O 2-δ and pure ceria was evaluated, and it was determined that dispersion remained nearly constant up to 5%wt, was greater for the Zr-doped catalyst and decreased below 1% for the 10% catalyst. Figure 8 demonstrates that for each nickel loading, there were reduction events in the low temperature area (α and β) and the high temperature region (δ), demonstrating that the overall hydrogen consumption rose as the nickel loading increased. This study is consistent with Montini's comment on steam reforming, which indicated that the most investigated systems utilize ceria-zirconia mixed oxides due to their enhanced redox characteristics. The CeO 2 -ZrO 2 oxides act as active supports for group 8, 9 and 10 metal nanoparticles, which serve as the catalytically active phases for hydrocarbon activation [108].
Lai et al. [109] developed approximately 6nm Ni-CeO 2 -Al 2 O 3 hybrid nano catalysts to achieve a low starting temperature (400 • C ), high activity and high stability SRM by aerosol-based evaporation-induced self-assembly. This two-stage gas-phase method produced an ideal H 2 yield (~3x of the converted methane) and the amount of coke formation was reduced by >3x and had a high operation stability for 8 h. Fibril carbon, which has been identified as a non-deactivating carbon, was not found in the SEM images of Figure 9 (3 and 4) which proves that the addition of CeO 2 to the nanocomposite efficiently reduced carbon formation. The whisker carbon fibers were formed on the surface of the nanocomposite without the addition of CeO 2 , as shown in the SEM images of Figure 9 (1 and 2).
Palma et al. [110] applied the Al 2 O 3 -CeO 2 catalysts to structured catalysts using washcoat slurries with loaded Ni which was prepared by wet impregnation for SRM. The methane conversion increased with the ceria content at the same temperature with XCH 4 = almost 100% selectivity to hydrogen. No coke formation was registered at temperatures higher than 700 • C due to the oxygen transfer capacity that promotes the gasification of carbon deposits. Lai et al. [109] developed approximately 6nm Ni-CeO2-Al2O3 hybrid nano catalysts to achieve a low starting temperature (400 °C ), high activity and high stability SRM by aerosol-based evaporation-induced self-assembly. This two-stage gas-phase method produced an ideal H2 yield (~3x of the converted methane) and the amount of coke formation was reduced by >3x and had a high operation stability for 8 h. Fibril carbon, which has been identified as a non-deactivating carbon, was not found in the SEM images of Figure  9 (3 and 4) which proves that the addition of CeO2 to the nanocomposite efficiently reduced carbon formation. The whisker carbon fibers were formed on the surface of the nanocomposite without the addition of CeO2, as shown in the SEM images of Figure 9 (1 and 2).   Lai et al. [109] developed approximately 6nm Ni-CeO2-Al2O3 hybrid nano catalysts to achieve a low starting temperature (400 °C ), high activity and high stability SRM by aerosol-based evaporation-induced self-assembly. This two-stage gas-phase method produced an ideal H2 yield (~3x of the converted methane) and the amount of coke formation was reduced by >3x and had a high operation stability for 8 h. Fibril carbon, which has been identified as a non-deactivating carbon, was not found in the SEM images of Figure  9 (3 and 4) which proves that the addition of CeO2 to the nanocomposite efficiently reduced carbon formation. The whisker carbon fibers were formed on the surface of the nanocomposite without the addition of CeO2, as shown in the SEM images of Figure 9 (1 and 2). Torimoto et al. [111] investigated the support effects of CeO 2 , Nb 2 O 5 and Ta 2 O 5 over Pd catalysts at low temperature SRM in an electric field to identify the factors controlling the activity of the catalyst support. All catalysts demonstrated activity at low temperatures exceeding the thermal equilibrium when tested in the electric field with the order of activity Pd/CeO 2 > Pd/Nb 2 O 5 > Pd/Ta 2 O 5 and the surface proton conduction was measured using electrochemical impedence spectroscopy (EIS) with the order of proton ability as CeO 2 > Nb 2 O 5 > Ta 2 O 5 . This work testified that as the adsorbed and activated amounts of H 2 O became larger, the proton conductivity became higher, then the catalyst was able to achieve high activity in the electric field for low temperature SRM.
Ghungrud SA et al. [112] developed multifunctional hybrid materials consisting of Ni, Co (in varying proportions 0-40%) and hydrotalcite using the co-precipitation method for sorption-enhanced SRM. Then, these materials were promoted with Ce species to improve the basicity for CO 2 adsorption and thermal stability and ultimately improved H 2 production. Ce-HM1 exhibited the maximum adsorption capacity, a better cyclic stability and a lower regeneration energy requirement.
Moogi et al. [113] compared the H 2 production and carbon formation of three types of Ni-based catalyst (Ni, Ni-La 2 O 3 and Ni-La 2 O 3 -CeO 2 ) on mesoporous silica supports (SBA-15 and KIT-6) in glycerol steam reforming. It was highlighted during the N2-physisorption test that the addition of La 2 O 3 increased the surface area of the catalyst by preventing pore mouth plugging in SBA-15. Ni-La 2 O 3 -CeO 2 /SBA-15 gave the highest hydrogen concentrations of 62 mol% and less carbon formation on/near the nickel sites during the reforming reaction while the Ni-La 2 O 3 /SBA-15 catalyst experienced severe coke formation. The addition of CeO 2 to the catalyst increased the catalytic stability by facilitating the oxidative gasification of the carbon formed on the Ni active sites of catalyst during the reaction. The Ni-La 2 O 3 -CeO 2 /KIT-6 formed a gaseous product with a lower H 2 concentration due to active methanation.
Liao et al. [114] investigated the catalytic activity of several supports in a microreactor for methanol steam reforming. The author demonstrated that a one-step hydrothermal method on the Al 2 O 3 support produced CuO/ZnO/CeO 2 /ZrO 2 nanoflowers with the maximum Ce 3+ , oxygen vacancies and outstanding redox characteristics. At 310 • C, the nanoflowers catalyst on H-Al 2 O 3 was composed of multiple nanosheets that grew aggressively on the ceramic surface, resulting in the highest loading strength of 99.8% methanol conversion and 0.16 mol/h H 2 production.
Salcedo et al. [115] used DFT to undertake a complete investigation of the SRM reaction on the surface of model Ni/CeO 2 (111) catalysts. The results indicated that lowloaded Ni/CeO 2 catalysts had distinct sites due to the metallic phases and the natures and interactions of the supports, which facilitate the easy activation of CH and OH bonds formed by CH 4 and H 2 O, respectively. Therefore, the objective of improving the ceriasupported metal catalyst should be to modify it in such a way that the barrier to oxidation processes to create CO is reduced, which might be accomplished by employing Ni-based bimetallic catalysts. The results shed light on the molecular interactions between C and OH species during methane steam reforming on low-loaded Ni/CeO 2 catalysts, where metal support interactions are critical for binding and activating methane and water.
Wu et al. [116] demonstrated that adjusting the oxidation state of nickel using DFT simulations was efficient at regulating the activity and stability of partially oxidized Ni/CeO 2 (Ni-NiO/CeO 2 ) for the SRM reaction. This was accomplished by fine-tuning the metal/oxide ratio to ensure an optimal interaction with ceria. The predicted catalyst was validated experimentally, with NiO/CeO 2 -364 • C demonstrating a higher performance for SMR, with methane conversion and H 2 production being stable throughout a 1500-min period at 700 • C.
Varkolu et al. [117] used the one-step evaporation-induced self-assembly (EISA) method to construct mesoporous Ni-CeO 2 -ZrO 2 -SiO 2 composite catalysts with high surface areas (approximately 200 m 2 /g). The composite catalyst exhibited a strong interaction between the nickel and metal oxide supports, resulting in the creation of a CeO 2 -ZrO 2 solid solution, which enhanced the catalyst's stability. The optimal catalyst, with a mole ratio of 1:2 CeO 2 /ZrO 2 and a nickel loading of 20%, demonstrated constant catalytic activity for 30 h with a hydrogen yield of 75%. The optimal conditions for the reaction were a steam/carbon ratio of 2.5 and a temperature of 873 K. Table 2 summarizes the Steam Reforming Methane (SRM) utilizing CeO 2 -based catalysts from 2015 to 2021.

Conclusions
As the global energy transition accelerates, the creation of new and optimized DRM and SRM technologies is critical for long-term sustainability in the fight against climate change. A high carbon resistance, high-performance, long-lasting, and low-cost improved catalyst composition is critical to advance this agenda of commercializing reforming technology.
Over the last six years, researchers have concentrated on enhancing formulations for building CeO 2 -based heterogeneous catalysts in DRM and SRM. Due to its extraordinary qualities, this rare earth has consistently functioned admirably as a heterogeneous catalyst, resulting in the production of a more desired product. The CeO 2 -supported catalyst is stable and the increase in stability has been attributed to an increase in the oxygen storage capacity, which enhances the CO 2 dissociative adsorption reaction and results in reduced carbon production. CeO 2 also performs admirably as a catalyst support, preventing the creation of carbon, which is reliant on the manner of catalyst preparation and the surface reactivity features caused by the substitution of Ce 4+ with another cation. The optimal amount of CeO 2 loading was discovered to be between 5% and 10% for the optimal DRM performance.
A high-quality catalyst should be capable of lowering the temperature necessary to initiate the reaction, hence reducing energy consumption and cost. CeO 2 has been used more in DRM technologies than in SRM technologies over the last six years. Most researchers have focused on developing transition and rare earth metal catalysts with small particle sizes and a high sintering resistance. The encapsulation of metal nanoparticles, Evaporation Induced Self Assembly (EISA), Exsolution and Two-step hydrothermal methods are emerging new approaches for catalyst preparation that have significantly reduced the carbon deposition and have increased CH 4 and CO 2 conversion in DRM. The exsolution method could be extended to solid solutions containing reducible metal cations and to the creation of a variety of catalyst supports. The "one-pot" method of catalyst synthesis in a supercritical medium has also been discovered to be an effective method due to its ease of preparation and scalability. Theoretical calculations can provide direction in designing CeO 2 -based catalysts through the control of the density and nature of the oxygen vacancies.
With so many features and advantages, it is easy to see why CeO 2 remains the most favored chemical element in catalyst formulation. Scientific advancements have shed new light on the relationship between metal-support interactions and carbon resistance in DRM and SRM. In the future, greater emphasis should be placed on the durability of the composite catalyst being created for immediate practicality and usefulness.

Conflicts of Interest:
The authors declare no conflict of interest