Methanol, cyclic carbonates, dimethyl carbonates, isocyanate, carboxylic acid, hydrocarbons, CO, and olefins are valuable common industrial products converted via CO₂ catalytic conversion ^[1][54]. Inspired by those homogeneous and heterogeneous catalytic systems, increasing attention has been given to the design synthesis and evaluation of reticular frameworks and their derived materials for CO₂ capture and conversion because of their unprecedented porosities with CO₂-philic groups and active centers ^[2][8]. The thermocatalysis of CO₂ into fine chemicals can be conducted via the hydrogenation of CO₂ to chemicals and fuels as CO₂ molecules dissociate by activated hydrogen atoms in hydrogenation, which requires hash conditions (low temperature and pressure).

In general, the catalytic conversion of CO₂ into added-value chemicals and syngas can be affected by many factors. For instance, a high density of active sites leads to enhanced activity and efficiency for CO₂ conversion. Catalysts with high density of active sites, high surface areas, and large pore sizes are essential for efficient mass diffusion. In addition, increasing the CO₂ pressure might facilitate the conversion by increasing CO₂ concentration or reducing the melting point of organic compounds ^[3][4]. However, high CO₂ pressures may also decrease the yield by forming two phases, which are associated with contact problems between the substrate and the catalyst, and disrupt the catalyst’s composition. High temperatures enhance the reaction rate but may decrease selectivity, especially for enantioselective catalytic systems.

In general, CO₂ conversion can be achieved using a multitude of reactions. In CO₂ hydrogenation, these reactions include the reverse water–gas shift (RWGS) (Equation (1)), Sabatier reaction (Equation (2)), CO₂-to-methanol (Equation (3)), and Fischer–Tropsch synthesis (Equation (4)) ^[4][5][77,78]:

The RWGS reaction has become a research hotspot owing to its capability toward CO₂ recycling and production of 1:1 syngas. This reaction is the key intermediate step in any CO₂ hydrogenation because it effectively produces CO from CO₂. It consumes H₂ and produces CO, which is much more reactive as a feedstock for C1 chemistry than CO₂ molecules with carbon double bonds, thereby facilitating the reaction at low temperatures. Equations (3) and (4), which undergo CAMERE and FT processes, enhance catalytic efficiencies when the CO generated from RWGS reactions Equation (1) is used as the raw feedstock. 

2.1. CO₂ Methanation

CO₂ methanation, a process of methane production via CO₂ hydrogenation, is a pioneering technology that has received extensive recognition for providing solutions toward CO₂ emission reduction, renewable energy utilization, and natural gas market reliever ^[6][79]. Abundant greenhouse gases (GHGs) in the atmosphere can be utilized to yield clean and green fuels via CO₂ methanation. Methane (CH₄) is the main natural gas formed with higher combustion value, and its combustion products are clean and safe relative to other fossil fuels. CH₄ has high energy density and is easy to store, making it an efficient renewable hydrogen energy carrier. The Sabatier reaction (Equation (2)) is a highly exothermic reaction that only occurs completely at low temperatures between 423 K and 573 K and high atmospheric pressure (1 atm) in the presence of a catalyst ^[7][72] to obtain optimum CO₂ conversion and CH₄ selectivity. However, the standard heat of CO₂ formation is −394.38 kJ/mol, and its high chemical inertness suppresses its activation and molecular dissociation. In addition, at temperatures above 300 °C, RWGS takes over the activity, thereby increasing CO selectivity, decreasing CH₄ selectivity, and promoting CO₂ conversion. Similar to other CO₂ hydrogenation reactions, methanation faces challenges, such as catalyst deactivation due to carbon deposition (fouling, Equation (5)) and decreased activity, resulting in a short catalyst lifetime. In order to prolong the catalyst lifespan and prevent catalyst degradation, CO₂ methanation catalysis has received huge attention to develop high-activity catalysts at low temperatures ^[8][5]. Gao et al. designed an adequate ratio of CO₂/H₂ and found that a sufficient amount of H₂ can significantly influence the production of water vapor (H₂O), increase methanation activity rather than CO formation, and inhibit carbon deposition. Various catalyst preparation methods, including conventional impregnation, sol–gel, and coprecipitation methods, have been developed to discover ways to achieve high activity and selectivity under mild conditions, good stability, and service life of CO₂ methanation catalysts. Therefore, suitable active metals, such as Cu, Ni, Fe, Pd, and Co, have been reported for CO₂ methanation, and Ni-based catalysts have been selected for industrial commercialization. However, these conventional metal-based catalysts often show low reaction activity at low temperature (180–300 °C) and tend to form particle agglomeration and sintering at high temperatures. For instance, Ni/Al₂O₃ catalysts prepared by wetness impregnation exhibit low metal dispersion and sinter in the presence of water at high temperatures, thereby increasing CO selectivity and energy consumption. Thus, active Ni metal with low loading is weakly dispersed and becomes less active at low temperatures, whereas the catalyst deactivates at high temperatures due to sintering. Catalyst design has been improved by adding promoters (Fe, Pd, Rh), additives, and support metal oxides (SiO₂, ZrO₂, CeO₂) to enhance activity at low temperatures. However, addition of these components in the conventional method is detrimental toward efficient CO₂ adsorption.

Core-Shell Nanostructured Catalysts for CO₂ Methanation

Core-shell confinement of structure has garnered interest as a potential nanostructure support for CO₂ methanation. The activity and selectivity of core-shell structured catalysts are mainly affected by the type of metal used. Among porous shell supports ^[9][13], metal-organic frameworks (MOFs) are a potential porous shell for Ni confinement because of their porous crystalline materials, high specific surface area, and tunable uniform elemental distribution. Ni precursors were treated solvothermally in a solvent solution at 136 °C to induce crystallization, and the Ni-MOF-74 suspension is calcined and then reduced in 5% hydrogen to form the Ni_xFe@C catalyst. The obtained catalyst achieves a CO₂ conversion of 72.3% with 99.3% CH₄ selectivity at 350 °C. Encapsulation of Ni-Fe alloy within carbon porous structure facilitates high CO₂ adsorption and effectively prevents the aggregation of active metal NPs during the reaction, thereby conferring the core-shell catalyst with superior stability. Moreover, the homogeneity of Ni-Fe NP elemental distribution can be preserved, which improves Ni dispersion. New core-shell nanostructure based on cobalt (Co) catalysts have been successfully fabricated by Cui et al. ^[5][78] to study catalytic performance of low temperature methanation. MnO-heterostructured NPs injected into porous graphitic carbon (Co/MnO@PGC) were synthesized via a single-step pyrolysis of bimetal CoMn@MOF-74. The resulting nanocomposite features an enriched Co/MnO heterointerface and exhibits excellent catalytic performance for low-temperature CO₂ methanation. The synthesized Co/MnO@PGC catalyst allowed CO₂ molecules to activate faster at a low heat of 160 °C over 99% selectivity with high STY_CH4 of 0.14 μmolCH₄⋅s ⁻¹ gcat⁻¹. As the temperature reached 240 °C, CO₂ conversion and space–time yield (STY_CH4) rose to 32.1% and 13.34 μmol_CH4⋅gcat⁻¹⋅s⁻¹, respectively. At a high pressure (30 bar), STY_CH4 can reach up to 5.60 μmol_CH4⋅s⁻¹⋅gcat⁻¹ at 160 °C, which is even comparable to that of the optimal level of Ru-based catalysts. These results indicate that the synergistic interactions between Co and MnO NPs at the Co-MnO heterointerface are responsible for enhancing the catalytic activity toward CH₄ production at a low temperature. In addition, the Co/MnO heterostructured NPs encapsulated into PGC play an important role in preventing metal particle aggregation and improving thermal stability. High TOF_CH4 suggested that the Co/MnO heterointerface formed inside the PGC of Co/MnO@PGC can significantly boost its activity of low-temperature (160–220 °C) CO₂ methanation. Core-shell metal@metal oxide particles can be promising as high thermal-conducting support materials owing to the high thermal conductivity of metal and excellent surface structural properties of the metal oxide itself. Various supports, such as Al₂O₃ ^[10][80], CeO₂ ^[11][81], ZrO₂, SiO₂ ^[12][60], or zeolites ^[12][60] have been proposed as metal oxide shell to protect the active metals at the core. In addition, Le et al. ^[13][82] synthesized a Ni/Al@Al₂O₃ CSN catalyst for CO and CO₂ methanation by using hydrothermal surface oxidation (HTSO). Ni/Al@Al₂O₃ has selectively yielded carbonate and formate species, which suppress the CO intermediate. The confinement effect helped the CSN catalyst lower the activation energy barriers (74 kJ/mol), which outperforms the activation energy of conventional catalysts, namely, Ni/Al₂O₃ (80 kJ/mol) and Ni/SiO₂ (89 kJ/mol). Apparently, Ni/Al@Al₂O₃ CSN can successfully enhance the catalytic CO₂ adsorption owing to its high Ni dispersion and strong CO₂ binding. Meanwhile, Ilsemann et al. ^[12][60] prepared Co@SiO₂ and Co@Silicalite-1 catalysts via a solvothermal method to encapsulate the Co NPs inside two mesoporous structures of silica shells. They found that Co@SiO₂ improves the catalytic activity in low-temperature CO₂ methanation (230°C–400 °C) by suppressing the side reaction (RWGS), which results in highly selective CO₂ hydrogenation to methane. The thermal stability provided by mesoporous silica could preserve the active Co metals at elevated temperatures. However, CO methanation causes slight coking, resulting in a shift of kinetic stability and reduction in methane yield. Similarly, a silicalite-1-confined Ni catalyst was prepared through the selective desilication of the molecular sieve to produce extra voids and pore channels to cage Ni in the crystal ^[14][83]. The Ni@Silicalite-1 catalyst is characterized by higher CO₂ conversion and CH₄ selectivity than conventional Ni/Silicalite, which can be attributed to the higher Ni fine dispersion in the void of silicate. The catalyst maintains stable performance over 50 h at 450 °C. CSNs are expected to provide an anti-sintering effect by anchoring the active metal NPs in the porous channel. The formation of metal–metal oxide compounds modulates strong interaction to realize the low deactivation rate of reaction. However, this core-shell design has some shortcomings, such as complex preparation, expensive instrumentation, and lack of exposed defect surface, all of which restrict its industrial applications. Therefore, Yang et al. ^[11][81] prepared Ni-phyllosilicate@CeO₂ CSN by using a hydrothermal method, to create Ni fine dispersion (3.3–6.3 nm). The anchored Ni phyllosilicate could further increase the H₂ and CO₂ uptake, contributing to high CO₂ conversion rate (65%), thus exhibiting high catalytic activity and stability for 100 h lifetime CO₂ methanation. Le et al. ^[15][84] continued their work on the Ni/Al@M-Al₂O₄ core-shell catalyst by promoting various transition metals (M = Mg, Ni, Co, Zn, or Mn) to develop a synergistic interaction between M/Al and enhance the catalytic activity of the CSN in low-temperature CO₂ methanation. Different influences of thermal conductivity on shell NP dispersion were observed as an anti-sintering property. Ni/Al@M-Al₂O4 CSNs were prepared using deposition-precipitation (DP) and wet impregnation (WI) methods to facilitate superior heat conductivity and surface properties for highly exothermic and endothermic reactions and control the effect of parameters on metal particle size. Morphological analyses showed that 9 wt% Ni/Al@MnAl2O4 (DP) has a significant BET surface area of 129 m²/g and the highest Ni metal dispersion (9.7%) among other synthesized catalysts. Introduction of Mg into the spinel Ni/Al@MgAl₂O₄ CSN has provided a larger BET surface area of 171 m²/g with the same dispersion quality to Ni/Al@MnAl₂O₄. Ni/Al@MnAl₂O₄ (DP) and Ni/Al@MgAl₂O₄ (DP) have demonstrated better catalytic performance than the catalyst prepared using the WI method. Both catalysts selectively produce high methane yield by hindering more chain of hydrocarbon while facilitating 90% CO₂ conversion under 300 °C, thereby promoting high catalytic activity for CO₂ methanation. Bimetallic Ni-Al has facilitated functional heat transfer across Ni/Al@M-Al₂O₄ CSN catalyst particulates as the Al metal releases high heat conductivity. Interestingly, Ni/Al@MnAl₂O₄ shows superior catalytic stability because it has a lifetime of 50 h while preventing coke deposition and Ni particle agglomeration during CO₂ methanation. Considering the sharp rise in temperature of methanation and rapid catalyst deactivation by Ni particles, Wang et al. ^[16][85] suggested the interaction of Mg with Ni as a bimetallic core in a Ni/Mg@MCM-41 duo-core@shell catalyst prepared using an in situ hydrothermal method with different Mg contents. Wang et al. also synthesized a conventional Ni/MCM-41 by using WI for comparison of catalytic performance. They found that Ni/MCM-41 features a larger BET surface area of 622.5 m²/g than 0.05 wt% Ni/Mg@MCM-41, and the specific surface area continues to decrease as an additional 0.05 wt% Ni is incorporated. This result can be ascribed to the blockage of Mg particles on the pores when Mg²⁺ has less tendency to replace Si⁴⁺ ions in the SiO₂ lattice. However, the Ni/Mg@MCM41 core-shell catalyst produces better atomic composition than the conventional Ni/MCM41 catalyst. Under an optimal temperature of 360 °C, 0.05 wt% Ni/Mg@MCM-41 exhibits the highest CO₂ conversion at 88% during CO₂ methanation. The selectivity of CH₄ gradually decreases with increasing temperature, proving that high temperatures are not advantageous to CH₄ production because CO₂ methanation is an exothermic reaction. Regardless, the 0.05 wt% Ni/Mg@MCM-41 catalyst shows high catalytic activity at low temperatures (below 360 °C) and a large specific surface area (606.3 m²/g), which are suitable for CO₂ methanation. A well-defined nanostructured Ni@SiO₂ core-shell catalyst (diameter size of 27.1 nm) was synthesized with distinct metal–metal oxide interfaces in proximity to each other to carry out CO₂ hydrogenation ^[17][86]. Noteworthy, the Ni@SiO₂ interface in the catalyst is responsible for RWGS reaction to form CO selectively. The strong interaction between Ni core and SiO₂ shell effectively restrains NP growth (agglomeration) and carbon deposition. Thus, the Ni@SiO₂ core-shell catalyst, yields 89.8% CH₄ and successfully converts 99.0% of CO molecules. Moreover, it retains high catalytic stability in CO methanation under a 100 h lifetime condition, which surpasses the stability of the conventional Ni/SiO₂ catalyst, whose CO conversion collapses after 12 h lifetime. Ni@mpCeO₂ CSN was synthesized using nanocasting, followed by strong electrostatic adsorption for CO₂ methanation ^[18][74]. The turn of frequency (TOF) for the Ni/mpCeO₂ CSN catalyst (0.183 s⁻¹) at 225 °C is threefold higher than that of the Ni catalyst supported on conventional CeO₂ prepared using the same method. Compared with the Ni catalyst, the Ni/mpCeO₂ CSN provides a rich NiCeO₂ interface with more oxygen vacancies, playing a key role in CO₂ activation. CO₂ activation over the Ni/mpCeO₂ CSN catalyst occurs through combined associative and dissociative mechanisms that have been observed through DRIFT mechanism study. Ni NPs are highly dispersed in the channels of mpCeO₂, which enhance H₂ dissociation, thereby supplying sufficient *H species for the formation of CO and *HCO intermediate species owing to high CH₄ selectivity. In addition to enhanced low-temperature activity and selectivity, the Ni/mpCeO₂ catalyst maintains its stability 70 h on stream because Ni sintering has been suppressed by the confinement effect of mesoporous CeO₂ structure. This study demonstrates the importance of the Ni-CeO₂ interface, at which high oxygen vacancy concentration facilitates CO₂ adsorption and activation while the adjacent Ni active sites accelerate H₂ dissociation.

2.2. CO₂ Hydrogenation to Methanol

Among all possible products in the CO₂ hydrogenation reaction mentioned in the previous section, methanol (MeOH) is the most attractive (Equation (3)). MeOH is a clean, biodegradable, high-energy fuel and is highly versatile as it can easily generate other valuable fuels, such as DME, olefins, hydrocarbons, and long-chain alcohols ^[19][20][21][27,87,88]. Furthermore, combustion of MeOH generates few carbon side products owing to its freezing point (−96 °C). Thus, MeOH is suitable as a hydrogen carrier without producing a huge number of SO_x or NO_x. Unlike methanation, MeOH synthesis has thermodynamic and kinetic limitations, such as its high-pressure requirement for complete CO₂ activation and low reaction temperature to selectively enhance the methanol yield ^[22][23][46,89] Nevertheless, this reaction still produces a competing side reaction, RWGS. Therefore, a potential route to suppress CO production must be discovered to maintain high methanol selectivity. CO₂ hydrogenation to MeOH is favored at high pressures. Therefore, stable catalysts that are resilient to high temperatures and pressures are required. Active transition metals (Ni, Ru, Ga, Cu, and Co) are usually used for CO₂ hydrogenation to MeOH because of their high activity at certain temperatures, abundance, low cost, and different oxidation states/phases to improve catalytic stability and selectivity. However, such active metals can encounter rapid deactivation (sintering, fouling, and poisoning) because methanol synthesis is a naturally structure-sensitive reaction, thus limiting CO₂ activation. Cu-based thermal catalysts are those used most often and hold many advantages for effective commercialization at the industrial scale. Cu catalysts are strongly active toward high methanol selectivity because of their three oxidation states (Cu⁰, Cu¹⁺, and Cu²⁺) ^[24][25][26][90,91,92]. Cu provides active sites for H₂ dissociation, and metal oxides increase the number of active sites for CO₂ activation. Wang et al. reported that methanol selectivity corresponds to the proportion of strong basic sites to the total basic sites. Despite their benefits, Cu NPs agglomerate into large particles at elevated temperatures, which decrease MeOH yield. Therefore, understanding the changes in different surface atom arrangements on Cu NPs is important because the homogeneity of the catalyst structure affects the catalytic activity.

Core-Shell Nanostructured Catalyst for CO₂ Hydrogenation to Methanol

CSN catalysts are attractive for these conversions because of the impact of the shell materials on reaction selectivity and catalyst stability ^[27][93] The core-shell nanostructure and surface have many advantages, such as enhancement of the essential properties of conventional catalysts. For example, commercialized Cu/ZnO/Al₂O₃ catalysts are unstable under high partial pressure at elevated temperatures, accelerating steam in the reaction atmosphere, hence causing rapid agglomeration and undesired crystal growth ^{[19][28][29][30]}[3,27,94,95] A core-shell arrangement may optimize the interaction between the metal and porous support and minimize Cu sintering by creating unique multifunctionalities of catalytic sites. An et al. ^[31][96] investigated Cu NPs coated with Zn to boost the surface electronic concentration and surface adsorption from high bimetallic synergy. The Cu/Zn bimetallic particles were anchored within the MOF network, and CuZn@UiO-bpy CSN was suggested to enhance the active metal dispersion and the SMSI. Cu NPs form within a diameter of 0.5–2 nm, depicting that the NPs are homogeneously dispersed, thus spreading more active sites and catalytic activity. Such structural properties result in remarkable CO₂ conversion (17.4%) and methanol selectivity (85.6%). In addition, CuZnO@UiO-bpy CSN shows three times higher methanol yield at 250 °C than the conventional catalyst. This report agrees with the findings of Tisseraud et al.’s compilation study ^[32][33][34][97,98,99] where Cu@ZnO_x CSN catalysts exhibit 100% methanol selectivity as the oxygen deficiency formed by Zn migration provides active sites and hinders CO formation by side reactions (RWGS or MeOH decomposition). A recent study has prepared a CuIn@mSiO₂ core-shell catalyst using a two-step solvothermal synthesis ^[35][57]. This catalyst was compared internally with Cu@SiO₂, In@SiO₂, and conventionally prepared CuIn/SiO₂ catalysts. A perfect core-shell shape has been successfully synthesized. The activity results showed that CuIn@SiO₂ outperforms the others in terms of CO₂ conversion but exhibits the second lowest methanol selectivity (21.8%) owing to CO formation at 250 °C. Nevertheless, the most stable performance over 100 h is dominated by Cu@SiO₂ with zero sign of carbon deposits. Considering that mesoporous silica (mSiO₂) shells are highly effective in limiting metal agglomeration and preserving the original metal particle sizes by providing a layer of thermally stable surface, Yang et al. ^[36][100] embedded Cu/ZnO within a layer of mSiO₂ and achieved stable CO₂ conversion and methanol yield over 160 h lifetime at a low temperature (260 °C). They observed that Cu/ZnO@mSiO₂ shows better catalytic performance than the conventional impregnated catalyst Cu/ZnO/SiO₂, which is deactivated after only 20 h on stream. Apart from tuning the product selectivity and preventing active sites sintering, the core-shell catalyst can help fix and activate CO₂ molecular activation. Hydroxyl species, which can be obtained from transition metal phyllosilicate (TM@SiO₂p), enhance CO₂ hydrogenation. Jangam et al. ^[29][94] prepared Cu-SiO₂p via a hydrothermal method and compared its performance with conventionally impregnated Cu-SiO₂ for CO₂ hydrogenation to MeOH at 200–350 °C. Cu-SiO₂p reduced at 225 °C produces a stable CO₂ conversion and methanol selectivity of 3.5% and 77%, respectively. Recently, hollow Cu@ZrO₂ derived from a MOF network has been developed through pyrolysis for selective CO₂ hydrogenation to methanol ^[37][101]. The hollow structure provides easy access of CO₂ and H₂ to diffuse on active sites. Han et al. found that the basic sites of Cu-ZrO₂ interfaces are responsible for the main adsorption and activation sites of CO₂. The core-shell confinement structure yields a high methanol selectivity of 85% at 220 °C. Other than Cu-based catalysts, noble metals, such as Pd-based heterogeneous catalysts also received recognition for CO₂ hydrogenation. The electronic structure of Pd-based catalysts plays significant roles in the reaction because its metallic sites can be tuned to obtain high-stability catalysts. However, their applications for large-scale plants are limited by sintering and expensive source. Xiao et al. ^[38][102] have recently designed stale Pd NPs in a confined environment. Pd@Cu core-shell was confined within a layered double hydroxide through modified coprecipitation. They conducted a catalytic test on formate species formation to identify the feasible CO₂ hydrogenation pathway without forming CO intermediates. Pd_0.4@CuMgAlO_x with a CO₂/H₂ composition of 20:20 successfully yielded 5.68 mmol∙g1∙h-1 of formate, whereas the core-shell catalyst showed no significant loss in formate yield after the fourth cycle, confirming its excellent stability. Kinetically, the Pd metallic sites govern the H₂ dissociation by forming active Pd-H.

3. CO₂-Reforming Reactions

Although renewable H₂ production via electrolytic, photoelectrochemical (PEC), and solar thermochemical methods is promising, cost effective, abundant, and sustainable, it is still not viable for industrial commercialization. Thus, thermocatalytic hydrogen production, which is a nonrenewable route that is effective for rich H₂ production from biomass, is still relevant in the gas production market. The reforming of fossil fuels, especially natural gasses, via decomposition of hydrocarbon molecules to release H₂ is the most common source for H₂ production globally. Traditionally, H₂ can be produced via several processes, such as steam methane reforming, partial oxidation reforming, methane pyrolysis, coal gasification, and DRM.

3.1. CO₂ Dry Reforming of Methane

Unlike the abovementioned methods, DRM offers low operating cost, utilizes two hazard greenhouse gases (CO₂ and CH₄) to produce highly pure gas (CO/H₂), and allows easy processing of value-added hydrocarbons and chemicals via the Fischer–Tropsch process ^[39][104]. DRM is a more economical process relative to other methods because it eases the gas separation of final products. CO₂ utilization has a significant influence on DRM performance, considering that the adsorption isotherm and its activation are the main steps to achieve optimal H₂ production. DRM is an endothermic reaction that requires excessive heating (>700 °C) driven by the following main reaction (Equation (6)). Poisoning, catalyst deactivation, and coke deposition are the common issues faced in DRM at high temperature because of methane decomposition (Equation (7)), Boudouard reaction (Equation (5)), and CO₂ hydrogenation (Equation (1)), whereas the RGWS reaction usually occurs in DRM at temperatures below 800 °C. To limit the RWGS reaction, DRM must operate at high temperatures, approximately 900 °C, to achieve high yields of H₂ and CO. Hence, key factors of efficient and feasible DRM reaction are optimized temperature, pressure, CH₄/CO₂ ratio, and catalyst design and composition. An effective DRM reaction mechanism theoretically involves multidisciplinary transitional states, such as methane dissociative adsorption, CO₂ dissociative adsorption, hydroxyl group formation, and intermediate oxidation and desorption ^[40][105]. In detail, CH₄ gases must dissociate on the catalyst surface sites to complete their tetravalency, whereby CH₃ molecules are adsorbed on top of active metal atoms while another CH₂ molecule occurs between two metal atoms, called step sites. Then, another greenhouse gas, CO₂, breaks its double bond, where C-O is adsorbed on the surface between the metal and support, leaving one oxygen atom exposed. Later, the H₂ molecules migrate from the metal particles to the support atoms to form hydroxyl (⁻OH) species at temperatures below 800 °C. Finally, the metal-surface oxygen provided by high oxygen mobility support reacts with the S-CH_x species to form new S-CH_xO intermediates, which potentially form as CO and H₂. This kinetic reaction of DRM is influenced by surface electronic properties. Efforts have been exerted to develop novel catalysts that can increase CO₂ and CH₄ activities at low temperatures. Nickel is the most prevalent metal-based catalyst because of its abundancy and low cost, making it suitable for industrial catalytic processes; however, Ni is rapidly deactivated because of high carbon formation from side reactions, either methane cracking or CO disproportionation ^[40][41][105,106] Noble metals, such as Pd ^[41][106], Ru, and Pt ^{[42][43][44][45]}[107,108,109,110], are suggested to replace Ni because they are highly resistant to carbon formation. However, their expensiveness restricts their promising properties in the larger market. Hence, catalysts that minimize coke formation and preserve the active sites in DRM need to be developed. Core-shell catalysts may offer high thermal stability, sintering resistance, and several functionalities, which aid in reducing the rate of carbon deposition ^[46][47][111,112]. The core-shell structure also promotes good control of dispersion and preservation from metal agglomeration, resulting in enhanced catalyst stability. Porous materials with high thermal resistance and optimum porous size channels that could anchor the active metal core are ideal to stabilize the metal NPs and minimize metal sintering at elevated temperatures ^[48][113].

Core-Shell Nanostructured Catalyst for CO₂ Reforming of Methane

Several studies on CSN catalysts for DRM have been published. Metals confined with mesoporous channels are the most extensively investigated catalysts in this field. Metal–support interaction is important in driving high activity of DRM; thus, a strong interface relationship is often demanded to inhibit active metal agglomeration ^[49][114]. Ni NPs embedded into zeolitic materials, such as silicalite, have been widely accepted as a promising structure for Ni confinement, but over 20 wt% Ni loading decreases the dispersal and encapsulation. Recently, Liu et al. have developed a controlled “dissolution–fractional crystallization” method of confining high loading and uniform Ni NPs into the hollow silicalite-1 (S-1) shell ^[50][115]. Active species of Ni as core has maintained its particle size to ca. 4–5 nm with optimized Ni loading from 3% to 20%, and the TOF remains at ca. 60 s⁻¹ (800 °C) in the dry reforming of methane reaction. The augmented density of active sites with Ni loading renders an outstanding reaction rate of 20.0 mol_CH4/g_cat/h over 20% Ni@S-1. A multiple core-shell structure using indium–nickel (In-Ni) intermetallic alloy as core and SiO₂ as porous shell has been successfully synthesized using the Stober method ^[51][116]. The InNi@SiO₂ CSN displays superior coking resistance for DRM reaction by minimizing the amount of carbon formation in DRM reaction. Even for the sample with only 0.5 wt% In doping, In_0.5Ni@SiO₂ CSN has been achieved based on the balance of coke deposition resistance and DRM reactivity. Hypothetically, lowering in loading confers coke resistance, whereas high In loading leads to low catalytic activity because of the formation of InNi₃C_0.5 species. Moreover, the specific surface area of the core-shell catalysts does not change in structural behavior even after reduction during the reaction for 20 h, indicating that no observed coke blocks the pore channels during the long-term thermal operation. The increase in electron-cloud density on Ni can weaken the ability of Ni to activate the C–H bond and decrease the deep cracking of methane. The binding energy of Ni_2p^3/2 in the InxNi@SiO₂ catalyst decreases with increasing in loading, which means the interaction between Ni and Ni is weakened, and the interaction between Ni and In is enhanced, indicating that Ni particles are not easy to be sintered. Lu et al. ^[52][117] have developed a novel structure catalyst of Ni@S2-T with Ni NPs highly dispersed in silicalite-2 zeolite (S2) via a two-step method involving the microemulsion method followed by solvent-free crystallization. S2 is a silica analog of aluminosilicate zeolite (ZSM-11), which consists of four- to six-member rings chained to create a porous channel system with 10-member ring openings. The unique pore structure and channel of S2 lattice can improve the confinement efficiency by strengthening the metal–support interaction caused by the formation of Ni phyllosilicate intermediate in the shell, which is regarded as the main reason for the superb catalytic performance of Ni@S2-T for DRM. Compared with Ni-SiO₂ prepared by microemulsion, Ni/S2 by impregnation, and Ni@S2-O by direct crystallization, Ni@S2-T catalyst exhibits optimal catalytic activity and stability for DRM. The catalyst achieves long-term stability, over 100 h, as the conversion of CH₄ maintains its value of 25%. Furthermore, after the Ni@S2-T catalyst is overspent, hardly any coke can be found after the prolonged test, which indicates the remarkable anti-coking ability of Ni@S2-T. Kong et al. controlled the porous size of SiO₂ channel under intrinsic hydrothermal treatment to allow precise control over the Ni surface ^[53][118]. The synthesized Ni@SiO₂ CSNs show that the confinement structure can destroy large Ni ensembles and form metastable Ni−O⋅Si centers. CH₄ activates on the small fraction of Ni to form CHx rather than carbon. Moreover, the Ni−O⋅Si center stabilized by interfacial confinement provides labile oxygen to oxidize CH_x. This Ni catalyst exhibits highly stable activity under 800 °C, CH₄: CO₂ = 2:1, and 5 bars without carbon deposition for 100 h, where carbon formation is thermodynamically much favorable. Another breakthrough of core-shell structures defined as hollow nanostructures has been intensely researched for their delimited cavity and enclosed shell ^[54][119], which could manifest tunable focal properties aside from well-defined active sites, thus enhancing the catalytic functionality. Herein, Kosari et al. modified Ni-SiO₂ hollow spheres (HSs) with different shell thicknesses and interior cavity sizes via a hydrothermal method ^[55][120]. As a result, final hollow Ni-SiO₂ exhibits varied shell thicknesses from 11 nm up to double-shell morphology with 77 nm being the inner shell distance. NiHS-SiO₂-S renders CH₄ conversion equal to 69%, which is higher than those of other low Ni-loaded catalysts. However, DRM activity rate decreases as the shell thickness of SiO₂-derived samples is increased further. Interestingly, no coke formation during the reactivity tests indicates that the formulated NiHS-SiO₂ catalyst is a promising candidate to catalyze the DRM reaction while acting as a strong carbon resistance material. Recently, Marinho et al. have developed a core-shell confined structure, a Ni-based mesoporous mixed CeO₂-Al₂O₃ oxide catalyst by top–down synthesis, and an evaporation-induced self-assembly (EISA) method to overcome Ni particle sintering in high-temperature DRM reaction ^[10][80]. EISA utilizes mesoporous bimetallic oxides to control the shape and robustness of the Ni@CeO₂/Al₂O₃ catalyst by presenting as mesoporous structures with highly dispersed Ni in the form of NiAl₂O₄ spinel clusters. Small (<5 nm) and homogeneous metallic Ni particles form after reduction steps. In addition, the Ce species in the structure reinforce the strong metal–support interaction with Al₂O₃, which enhances oxygen mobility and acts as spatial sites for CO₂ adsorption, thereby increasing the catalytic activity and promoting the carbon removal mechanism. Therefore, Ni@CeO₂-Al₂O₃ catalysts prepared by one-pot EISA exhibit high activity and stability for DRM owing to the successful encapsulation of Ni particles and coke resistance. Wang et al. ^[56][121] prepared basic metal oxide (MgO and La₂O₃)-modified Ni confined in dendritic mesoporous silica catalysts (Ni-MgO@DMS and Ni-La₂O₃@DMS) via a sol–gel method. The Ni-MgO@DMS CSN formed exhibits a completely confined structure and yields optimum conversion rates of CH₄ and CO₂ up to 35% and 40%, respectively, in a low-temperature DRM reaction of 550 °C. Remarkably, only a few insignificant carbon depositions are observed during the 8 h time-on-stream stability evaluation, which can be attributed to the fast alkaline oxides adsorption and CO₂ activation. Moreover, the modified Ni-MgO@DMS and Ni-La₂O₃@DMS CSNs show high Ni sintering and carbon resistance owing to the high oxygen vacancy facilitated by the presence of magnetic oxide species Mg and La, contributing to its hydrocarbon species (CHx) activation. As the reaction time is extended to 50 h, the spent Ni-La₂O₃@DMS catalyst has a negligible amount of carbon formation.