Fischer-Tropsch synthesis (FTS) is a pivotal technology for the conversion of syngas (CO/H₂) into hydrocarbons. Pioneered by German scientists Fischer and Tropsch in 1923, the first fixed-bed FTS plant was commissioned in 1935. While early FTS processes relied predominantly on Fe-based catalysts, researchers have since developed other catalytic systems. These include Co-based catalysts, known for their high selectivity toward long-chain hydrocarbons; Ru-based catalysts, distinguished by their superior catalytic activity; and Mo-based catalysts, which exhibit notable resistance to sulfur poisoning^[37-39].

Pristine unmodified transition metal nanoparticles suffer from significant drawbacks, including severe particle agglomeration and sintering, as well as facile coverage of active sites by carbon deposits. Furthermore, their uniform electronic and geometric configurations impede precise regulation of carbon chain propagation, resulting in low CO conversion, poor selectivity toward target products, and abundant byproducts during FTS. These limitations render these catalysts incapable of meeting the requirements for high-efficiency, oriented catalysis, highlighting the critical need for catalyst modification and optimization^[40,41]. Current modification strategies center on tuning the catalyst microstructures. Specifically, metal doping and crystal phase engineering modulate the electronic and lattice structures of nanoparticles, while support confinement and surface functionalization immobilize active nanoparticles and optimize particle size and dispersion. Together, these approaches improve catalytic performance and boost FTS activity and selectivity toward desired products^[42,43].

Figure 2A provides a comparative overview of two typical CO reduction pathways. Electrochemical CO reduction over Cu catalysts under ambient conditions proceeds via direct coupling of two adsorbed CO species; this CO-CO coupling acts as the rate-determining step, with a high energy barrier dominating the production of short-chain C₂ products^[44]. Conversely, thermocatalytic Fischer-Tropsch synthesis generates adsorbed hydrogen via H₂ dissociation, followed by C-O bond cleavage of CO to form CH_x intermediates^[45]. Subsequent low-barrier CH_x coupling enables continuous carbon chain propagation to afford long-chain hydrocarbons, yet high temperatures are required to facilitate CO dissociation. Fundamentally, this process converts syngas over transition-metal catalysts (e.g., Fe, Co, Ni, Ru) [Figure 2B] through a multifaceted sequence of adsorption, dissociation, intermediate transformation, and product formation^[46]. This yields hydrocarbons (alkanes, alkenes) and oxygenates (alcohols, aldehydes), alongside byproducts such as H₂O and CO₂. The reaction is inherently synergistic, involving surface-catalyzed polymerization and hydrogenation. Furthermore, the pivotal step in FTS is carbon-carbon bond formation, which critically depends on well-dispersed transition metal nanoparticle catalysts. Both catalytic activity and product selectivity are intrinsically linked to the electronic and geometric structures of the nanoparticles, which are governed by key parameters such as particle size, morphology, and crystal phase structure^[46-48].

Fe-based catalysts exhibit dual functionality in both RWGS and FTS, achieving C₅₊ hydrocarbon selectivity exceeding 50%^[49]. However, promoters are essential for accelerating carbide formation. Conversely, Co/Ru-based catalysts are characterized by high methane selectivity (> 90%) but are constrained by the thermodynamic limitations of the RWGS reaction, which limit CO partial pressure and necessitate advanced techniques, such as membrane reactors, to enhance CO coverage^[50,51]. Wang et al.^[52] doped cobalt into iron to fabricate a tandem FeCo alloy carbide/CuZnAl catalyst for CO₂ hydrogenation to C₂₊ alcohols. The CuZnAl (CZA) component activates CO₂ and produces abundant CH₂O^* oxygen-containing intermediates via the reverse water-gas shift reaction, which subsequently spill over onto the FeCo active sites. Co incorporation generates (Fe_3/4Co_1/4)₅C₂ alloy carbide and modulates its electronic structure to moderate oxygen affinity, thereby eliminating the drawbacks of monometallic iron carbide, including susceptibility to water oxidation and excessive cleavage of C-O bonds that favors hydrocarbon formation. Two distinct active sites on the alloy carbide surface simultaneously supply CH₂O^* and CH₂^* species, which undergo C-C coupling with a low 0.52 eV barrier. The coupled intermediates are further hydrogenated to preferentially yield ethanol. This catalytic system achieves high CO₂ conversion, superior C₂₊ alcohol selectivity, and outstanding long-term stability [Figure 2C]^[52]. The resulting FTS products can be further refined into a diverse array of high-value chemicals, including light olefins, gasoline, diesel, waxes, oxygenates, and α-olefins. Consequently, these products serve as vital precursors to bulk commodities such as synthetic fibers, rubbers, plastics, lubricants, surfactants, and detergents.

Consequently, modifications to Fischer-Tropsch catalysts have been proposed. Modified Fischer-Tropsch synthesis represents an optimized advancement over conventional FTS. Its core strategy addresses the bottlenecks of traditional FTS, such as poor product selectivity, limited feedstock scope, and insufficient catalyst stability, through catalyst modification, feedstock adjustment, or process parameter optimization. This approach enables more efficient, targeted conversion of syngas (or CO₂ combined with green hydrogen).

The regulatory mechanism governing the strong interaction between metals/metal oxides and zeolites, which affects catalyst structure, reducibility, mass transfer performance, and catalytic activity, is the primary reason for the enhanced catalyst performance^[53]. Furthermore, as illustrated in Figure 2D^[54], the gas recycle ratio, a key process parameter, also plays a crucial role in regulating the reaction behavior of CO and H₂ conversion to C₅-C₁₈ olefins during Fischer-Tropsch synthesis^[55].

Smith and Anderson first described the chain-growth process for alcohol synthesis from syngas over modified methanol catalysts in 1982 as a 1-2 carbon-addition mechanism involving α- and β-carbons^[78]. As shown in Figure 4A, the synthesis of liquid fuels from CO₂ and H₂ follows various catalytic pathways that cover core reactions, key catalysts, intermediates, and applications. This forms a classic technical roadmap for CO₂ resource utilization^[79]. Figure 4B systematically illustrates the reaction pathways of CO hydrogenation to C₂₊ alcohols over the catalysts^[80]. Differences in product distribution for syngas hydrogenation to alcohols over four CuZn-based catalysts (CZ: CuZn, CZA, CZC:CuZnCr, and CZG: CuZnGa) clarify the synergistic catalytic mechanism involving multiple active sites. From the perspective of the microscopic reaction process, Cu⁰ mediates CO dissociation to generate CH_x species; oxygen vacancies cooperate with Cu⁺ to capture CO and form CH_xO intermediates via non-dissociative adsorption. Zn^δ+ serves as the functional site for C-C coupling, driving the combination of CHₓ and CHₓO to produce C₂ intermediates, which are further hydrogenated into ethanol. When Zn^δ+ coupling sites are scarce, CH_xO^* tends to be directly hydrogenated to methanol. These findings establish a progressive structure-activity relationship: promoters modulate the concentration of oxygen vacancies; oxygen vacancies coordinate the synergistic behavior of multiple active sites; and the spatial distribution of active sites regulates the carbon-chain-length distribution of alcohol products.

Historically, modified methanol synthesis catalysts can be classified into two main categories. High-temperature catalysts are primarily composed of ZnCrO_x and MnCrO_x systems doped with alkali metal promoters, which deliver a higher ratio of higher alcohols to methanol and eliminate the need for methanol recycling. Conversely, low-temperature catalysts represented by Cu/ZnO and Cu/Zn/Cr systems have emerged as the industrial mainstream for CO₂ hydrogenation to methanol, benefiting from mild reaction conditions and the cost-effectiveness of copper-based components. Among them, Cu/ZnO/Al₂O₃ is the most widely adopted low-temperature copper-based catalyst, and the dynamically reversible synergistic interaction between Cu and Zn species is the primary origin of its superior methanol catalytic activity, distinguishing it from high-temperature Cr-based catalysts^[81]. As illustrated in Figure 4C, the in situ generated H₂O during catalysis drives the re-oxidation of transient CuZn alloys, which restores the Cu lattice parameter to the benchmark value of pure Cu and enables the continuous reversible interconversion between CuZn alloys and Cu-ZnO interfacial sites, thereby stabilizing the dynamic equilibrium required for catalytic cycles^[82]. This unique dynamic redox cycle mechanism microscopically explains why low-temperature copper-based CZA catalysts are more suitable for directional methanol synthesis than high-temperature Cr-based counterparts, which in turn sustains stable catalytic activity toward CO₂ hydrogenation over long-term operation and is fully consistent with their outstanding methanol synthesis performance under steady-state conditions.

Historically, modified methanol synthesis catalysts were divided into two categories. High-temperature catalysts, primarily ZnCrO_x and MnCrO_x+K/Cs, are noted for their high higher-alcohol-to-methanol ratio and the absence of a methanol recycling requirement. Conversely, low-temperature catalysts, such as Cu/ZnO and Cu/Zn/Cr+Cs/K, benefit from milder reaction conditions and the cost-effectiveness of Cu-based components.

He et al.^[83] prepared a quaternary Cu/ZnO/ZrO₂/MgO catalyst via a co-precipitation method. The introduction of Mg significantly optimized the catalyst’s microstructure, improved the dispersion of active components, and created appropriate basic sites. In terms of microstructure, Mg inhibited the agglomeration of CuO and ZnO particles, reducing particle size, increasing specific surface area and pore volume, and enhancing active-site exposure^[84]. Furthermore, it regulated surface properties by constructing medium-strong basic sites, which strengthened CO₂ adsorption and activation. Additionally, Mg promoted inter-species interactions, specifically enhancing the synergy between Cu and ZnO, which improved the dispersion and reducibility of Cu species, increased the content of chemisorbed oxygen, and ultimately optimized catalytic activity.

Traditional Cu/ZnO/Al₂O₃ catalysts still have room for performance improvement, as water produced during the reaction can induce the sintering and deactivation of Cu and ZnO. Therefore, it is imperative to develop Cu-based catalysts with both high activity and high stability. The catalytic activity of bifunctional hybrid catalysts is closely linked to the reducibility of copper oxide in the calcined products. Introducing ZrO₂ into the Cu-ZnO catalyst system can effectively enhance the reducibility of copper oxide and significantly lower its reduction temperature^[85]. Chang et al.^[86] fixed the mass fraction of Cu at 30% to investigate the regulatory effect of the ZnO/ZrO₂ mass ratio on catalytic performance. The results revealed that the ratio drastically modulates the catalyst microstructure (CuZn alloy formation and Cu dispersion) and surface adsorption properties (the number of active CO₂ adsorption sites). According to the data summarized in Table 2, the CO₂ desorption capacity, surface metallic Zn⁰ (CuZn alloy) content, and methanol space-time yield all exhibited typical volcanic trends, with the optimal catalytic performance achieved over the C3Z2Z5 catalyst at a ZnO:ZrO₂ mass ratio of 2:5. In Zn-poor and Zr-rich systems, increasing the ZnO proportion facilitates CuZn alloy generation, enriches Cu-ZrO₂ interfaces and CO₂ activation sites, and thus continuously elevates catalytic activity. Once the ratio exceeds 2:5, insufficient ZrO₂ content drastically reduces Cu-ZrO₂ interfaces and aggravates Cu particle agglomeration, simultaneously lowering CO₂ adsorption capacity and CuZn alloy content, which deteriorates catalytic activity. These volcanic profiles arise from the synergistic balance between two types of active sites: CuZn alloys and Cu-ZrO₂ interfaces. The maximum catalytic performance is achieved when the synergistic effect of these two sites reaches an optimal balance. The synergistic effect between Cu and oxides (ZnO, ZrO₂), particularly through the CuZn alloy and Cu-ZrO₂ interface, is crucial for enhancing the catalytic performance of CO₂ hydrogenation to methanol and provides a theoretical basis for designing high-efficiency catalysts^[87].

Zr does not incorporate into the precursor lattice; instead, it integrates into the ZrO₂ lattice to form a solid solution after calcination. As shown in Figure 4D, modification with CeO₂ and ZrO₂ lowers the reduction temperature of CuO and improves catalyst reducibility^[88]. Among all samples, CZA/Zr exhibits the lowest reduction peak temperature. The strong interaction between MnO₂ and Cu species significantly shifts the reduction peak toward higher temperatures, inhibiting CuO reduction. The reducibility follows the order: CZA/Zr > CZA/Ce > CZA > CZA/Mn, which directly corresponds to the differences in Cu dispersion over various catalysts and elucidates the intrinsic origin of their distinct low-temperature catalytic activities. Furthermore, López-Luque et al.^[89] found that the methanol formation rate shows a volcanic trend with the surface spacing of Cu nanoparticles. Thus, in addition to mass ratio and doping elements, particle spacing is another critical design parameter for metal-oxide interface catalysts.

Mo-based catalysts exhibit remarkable sulfur tolerance during the conversion of CO to higher alcohols. They can tolerate trace sulfur impurities in feedstocks, thereby avoiding the need for stringent desulfurization pretreatment and broadening their practical applicability. Furthermore, their selectivity can be precisely tailored through modification strategies, including alkali metal doping, transition metal incorporation, and support optimization^[90]. However, these reactions typically require harsh operating conditions (high temperature and pressure), and selectivity toward target products remains suboptimal, limiting their industrial viability^[91].

In the direct synthesis of higher alcohols from syngas, Mo-based catalysts face the dual challenges of harsh reaction conditions and suboptimal product selectivity. Conventional Mo-based catalytic systems typically operate at temperatures exceeding 300 °C and pressures above 80 bar, where methanol is the dominant product, severely limiting the effective generation of C₂₊ alcohols^[92]. Consequently, developing Mo-based catalysts that achieve high C₂₊ alcohol selectivity under milder conditions, especially at low pressure, holds substantial scientific significance and practical value.

In industrial practice, implementing low-pressure reaction conditions not only significantly reduces energy consumption but also simplifies facility design and decreases operational and maintenance costs. This approach simultaneously enhances both the environmental sustainability and economic competitiveness of the process, providing critical technical support for the green, large-scale production of higher alcohols^[93]. Mo-based catalysts hold significant promise for converting syngas to higher alcohols because of their abundant availability and remarkable sulfur tolerance. However, their product selectivity and space-time yield still necessitate further improvement^[94]. Molybdenum sulfide-based catalysts have long been a primary research focus for mixed alcohol synthesis owing to their outstanding sulfur tolerance and high activity in the water-gas shift reaction, making them appealing candidate catalysts for this reaction^[95,96]. Because mixed alcohol synthesis is often coupled with the reverse water-gas shift process, efficient CO₂ conversion helps tune product distribution. Related investigations on molybdenum-based catalysts further reveal that unsaturated molybdenum oxide (Mo₁₇O₄₇) delivers high activity toward the reverse water-gas shift reaction without pre-carburization, while maintaining stable operation at 600 °C over 2,000 h^[97]. Accordingly, this review focuses on two prominent catalyst classes: Mo-S and Mo-C systems.

Among various catalysts for CO hydrogenation, Rh-based catalysts are considered highly promising for synthesizing C₂ oxygenates from syngas, owing to their unique capability. However, the commercial application of rhodium (Rh) is significantly constrained by its high cost as a noble metal^[119,120]. While traditional homogeneous Rh-based catalysts exhibit high activity and selectivity, they pose challenges for product separation. Conversely, heterogeneous catalysts, though easily recoverable, often suffer from insufficient activity and selectivity due to low ligand concentrations and limited mobility of active sites^[121]. These limitations can be mitigated by using supports with high specific surface area, hierarchical pore structures, and high nitrogen content^[122]. Metal particle size is a critical parameter governing the performance of heterogeneous catalysts; typically, the specific atomic activity increases markedly as particle size decreases. When the metal particle size reaches the ultra-small limit, single-atom catalysts (SACs) form^[123,124].

Arevalo et al.^[125] found that deep hydrogenation to produce methane readily occurs on stepped Rh surfaces; therefore, modification and promoter tuning are required to modulate the kinetic competition between CH_x hydrogenation and CO insertion. Figure 6A illustrates the full reaction pathway for higher alcohol synthesis from syngas on stepped Rh (211) surfaces. The reaction is initiated by CO hydrogenation to form CHO (Formyl), from which multiple competing reaction branches diverge. The CHO intermediate can be hydrogenated to methanol or undergo dehydroxylation to yield CH_x species. C₁ fragments react with CO/CHO via insertion to form ethanol; ethanol can further undergo dehydroxylation to generate ethyl species, which sequentially produce n-propanol. Continuous hydrogenation of CH_x species leads to methane formation. Colored arrows in the figure distinguish insertion, hydrogenation, dehydroxylation, and desorption processes, explicitly revealing the competitive mechanism that governs the formation of target alcohols and methane byproducts. Figure 6B presents catalyst structural models and density functional theory (DFT)-calculated Gibbs free energy profiles to compare the reaction energy barriers at high-coordination terrace sites of Rh (111) and low-coordination step sites of Rh (221)^[126]. Theoretical calculations confirm that low-coordination Rh (221) step sites facilitate C-O bond cleavage and ^*CH_x hydrogenation, promoting the formation of a methane byproduct, while boron oxides can spontaneously and selectively anchor and block such step sites^[127]. Conversely, high-coordination Rh (111) terrace sites favor the coupling of ^*CH₂ with CO/^*CHO to produce C₂ oxygenates^[126]. Collectively, boron species shield active sites for methane formation and raise the energy barrier for C-O cleavage and hydrogenation while preserving sites for carbon chain coupling. This clarifies the intrinsic mechanism by which boron modification suppresses methane generation and boosts ethanol selectivity at the molecular scale.

Structural distortion facilitates the incorporation of Rh atoms and the formation of shorter Rh-O bonds [Figure 6C], while significantly increasing the aggregation energy barrier of Rh single-atom sites (Rh SASs)^[128]. This confirms a strong interaction between Rh single atoms and surface O atoms, which endows the Rh₁/CeTiO_x catalyst with exceptional structural stability, thereby ensuring high catalytic performance and outstanding long-term durability for ethanol production. Consequently, regulating support doping is pivotal for optimizing single-atom catalyst performance. Notably, maintaining a low Rh loading is crucial for achieving high ethanol selectivity. Conversely, high loading tends to induce Rh cluster formation, shifting the reaction pathway toward methanol or methane formation^[129].

The active centers of SACs consist of isolated metal atoms, which provide single metal sites but cannot inherently form metal-metal bonds^[130]. To address this limitation, Gong et al.^[131] fabricated RhFeO_x/TiO₂ catalysts via drying and calcination treatments using P25 (anatase/rutile mixed-phase TiO₂) as the support, with iron introduced as a promoter. Benefiting from a fourfold synergistic effect, the Fe promoter constructs composite Rh⁰-Rh^δ+-O_v-Ti³⁺ active sites at the anatase-rutile phase boundaries of P25. Specifically, Fe generates abundant O_v-Ti³⁺ defects, synchronously regulates the coexistence of Rh⁰ and Rh^δ+ species, strengthens the SMSI at the phase junctions, and immobilizes ultrafine Rh nanoparticles at the anatase/rutile interfaces. This strategy addresses the inherent drawbacks of conventional single-atom catalysts and markedly accelerates the C-C coupling reaction.

Although introducing two promoters simultaneously can greatly improve overall selectivity toward oxygenates and C₂ hydrocarbons, systematic modulation of the deposition sequence and metal molar ratio is required; random doping weakens the synergistic effect. Figure 6D compares the carbon selectivity distribution of all products across monometallic Rh, binary Rh-Fe/Rh-Mn catalysts with different molar ratios, and ternary Rh-Fe-Mn catalysts prepared via varied deposition sequences^[132]. Stepwise controllable surface deposition enables independent tuning of the surface coverage of Fe and Mn sites on Rh. Fe sites continuously convert acetaldehyde to ethanol, while Mn sites selectively facilitate C₂ olefin formation. Monometallic Rh mainly produces methane and acetaldehyde. Binary catalysts reduce methane selectivity and increase the proportion of ethanol or olefins, respectively. The ternary system simultaneously boosts the selectivity of ethanol and light hydrocarbons. Conversely, binary catalysts with a single promoter show competition between Fe and Mn for active sites on Rh, leading to a mutual offset of the two promoters' functions and only a limited reduction in methane selectivity^[133].

Interface modulation between metal-support and metal-promoter interactions is a core strategy to tailor catalytic activity and product selectivity in CO₂ hydrogenation^[134]. Figure 6E compares the atomic arrangement and charge distribution between the confined dual-site model of Rh⁺-Rh⁰/UiO-67 and the oxide-supported model of metallic Rh⁰/t-ZrO₂(101)^[135]. Rh dimers anchored inside UiO-67 cavities serve as Rh⁺ sites, whereas intracavity Rh clusters act as Rh⁰ sites with direct interfacial connections. Positively charged Rh⁺ atoms construct a charge-differentiated dual active interface. Conversely, Rh nanoparticles on the t-ZrO₂ surface are nearly zero-valent metallic Rh, lacking cationic Rh sites and heterogeneous charge interfaces^[136]. Bader charge analysis confirms that the metal-organic framework (MOF) framework modulates the charge distribution of Rh atoms to intrinsically generate adjacent Rh⁺/Rh⁰ sites. Oxide supports only immobilize homogeneous Rh clusters and cannot spontaneously form neighboring dual sites with separated valence states. Metal doping modification of Rh/ZrO₂ merely slightly adjusts the electron density of Rh, failing to reproduce the atomic-level valence-separated interface enabled by MOF confinement. Precise spatial confinement is a unique strategy for constructing charge-heterogeneous dual active sites.

In comparison with MOF-derived Cu/ZrO₂-DM catalysts for CO₂ hydrogenation to methanol, Cu-based catalysts enhance methanol yield by virtue of their large specific surface area and highly dispersed Cu active sites^[137]. Conversely, Rh-based catalysts precisely manipulate reaction pathways via interfacial electronic effects. Together, these catalytic systems show that reactant adsorption and activation dictate product distribution, providing a theoretical framework for interfacial modulation in the rational design of catalysts for CO₂ hydrogenation to methanol and ethanol.

In recent years, 85% of global energy and most chemical production have relied on fossil fuels. The resulting massive CO₂ emissions have significantly exacerbated global warming. This necessitates implementing technologies such as CO₂ capture^[103] and utilization (CCU)^[138,139] to achieve carbon cycling. Although catalysts can significantly improve target-product yields, current transition-metal- and noble-metal-based catalysts suffer from critical limitations, including low ethanol yields and unsatisfactory selectivity. These challenges have prompted researchers to explore novel strategies for ethanol synthesis via CO₂ hydrogenation^[140,141].

Figure 7A illustrates the tandem reaction pathway over solid Co FTS catalysts and molecular Co reductive hydroformylation (RHF) catalysts, whose performance is predominantly governed by the spatial distance between the two components^[142]. When active sites are closely adjacent and molecular catalysts can diffuse into the pores of solid supports, the in situ formed 1-alkenes are rapidly trapped and converted into higher alcohols. Conversely, a large separation between active sites facilitates re-adsorption and hydrogenation of alkenes to paraffins, and only a small fraction of linear alcohols is produced directly via FTS^[142]. Short inter-site distances reduce the diffusion path of alkene intermediates and suppress undesired saturation side reactions, thereby markedly boosting alcohol selectivity. Artificially isolating active sites leads to a substantial decline in alcohol yield. Although ligands enable fine-tuning of the linear-to-branched alcohol ratio, intimate synergism within confined pore channels is a prerequisite for efficient alcohol synthesis in this tandem system; simple physical mixing fails to deliver optimal catalytic performance. Accordingly, suitable spatial separation between the two components of bifunctional catalysts is critical to optimize intermediate migration, match reaction kinetics, and improve overall catalytic performance^[143-145]. For supported Cu-based catalysts applied in ethanol conversion, an unbalanced spacing between Cu dehydrogenation sites and ZrO₂ acid sites impairs catalytic efficiency. Excessively large separation extends the diffusion pathway of reaction intermediates and accelerates side reactions, thereby lowering the selectivity toward ethyl acetate and substrate conversion. Conversely, an overly short distance causes metallic Cu to encapsulate zirconia acid sites, triggers sintering of Cu nanoparticles and deterioration of metal dispersion, and mismatches the reaction rates of dehydrogenation and condensation, ultimately hindering the formation of target products^[146]. Figure 7B presents the Zr X-ray Photoelectron Spectroscopy (XPS) high-resolution spectra and the proportion of oxygen-deficient Zr^x+ (Zr₁ species) over CuCoAl, CuCoAl|m-ZrO₂, and CuCoAl|t-ZrO₂ catalysts^[147]. The composite system incorporating tetragonal ZrO₂ generates more oxygen-deficient Zr^x+ sites, whereas fewer oxygen-deficient sites are formed in the monoclinic ZrO₂ composite system. Oxygen vacancies facilitate the water-gas shift reaction and stabilize oxygen-containing intermediates. The disparity in oxygen-deficient-site concentration directly governs CO₂ production and higher-alcohol selectivity. Accordingly, oxygen defects act as critical surface sites for modulating product distribution.

To address the challenges of low selectivity and insufficient stability in the conversion of syngas to (C₂₊OH), Luan et al.^[148] adopted a “reaction coupling strategy” to develop a bifunctional system composed of ZnCrAlO_x (zinc-chromium-aluminum mixed oxide) and a K/Ni-promoted Mo-based sulfide (KNiMoS-MMO). Through component synergy, promoter modulation, and structural optimization, they achieved efficient conversion of syngas into low-carbon alcohols. Furthermore, leveraging proximity regulation, Lin et al.^[149] engineered a catalyst comprising CoMn oxide and CuZnAlZr oxide. During extended operation, this catalyst maintained stable CO conversion rates, CO₂ selectivity, and product distribution (specifically the ratio of oxygenates to olefins) without significant deactivation, thereby satisfying the rigorous stability requirements for industrial applications [Figure 7C]. In another advancement, Jiang et al.^[73] developed a multifunctional catalytic system centered on carbon matrix-encapsulated iron-based catalysts (Fe@NC) through a triple strategy involving precursor screening, K-promoter modification, and the co-addition of organic ligands. This design not only ensures efficient coupling between the RWGS and FTS reactions but also suppresses by-product formation, substantially improving selectivity toward long-chain hydrocarbons. Figure 7D compares the chemical states of surface carbon on catalysts modified by different alkali metals. Four types of carbon characteristic peaks, namely graphitic carbon, C-Mo bonds, C-O, and C=O, can be fitted for all samples^[150]. The C-Mo peak verifies the stable existence of the molybdenum carbide phase. Doping with Li, Na, K, and Cs does not alter the category of carbon species but only slightly adjusts the relative intensity of each carbon peak; the electronic effect of alkali metals exerts negligible influence on the surface carbon skeleton structure of catalysts. Compared with the unmodified catalyst, the proportion of oxygen-containing impurity carbon species in alkali-modified samples shows no obvious variation, and the peak position of the C-Mo characteristic peak remains nearly unchanged^[151]. This indicates that alkali metals cannot directly regulate the electronic structure of molybdenum carbide, and the carbon carrier acts merely as a skeleton without modulating Mo valence states. Although alkali metals can tune metallic Mo active sites, they do not change the distribution of surface carbon species. Hence, carbon spectra cannot be adopted to screen the optimal promoters for alcohol synthesis. Overall catalytic performance is synergistically governed by multiple intrinsic preparation parameters, including fabrication procedures, thermal treatment temperature, and the physicochemical properties of carbon supports. Specifically, the nitrogen doping level, degree of graphitization, and porous structure of carbon supports can efficiently modulate electron-transfer efficiency and reactant mass-transport rates within the catalytic system.

In summary, multifunctional catalyst design does not follow a single, fixed paradigm. Instead, catalytic systems with target performance can be constructed by precisely combining and structurally regulating multiple components tailored to specific reaction requirements. Nevertheless, moderate proximity is imperative to achieve efficient synergy among various active sites.

Currently, research on the catalytic hydrogenation of carbon dioxide to methanol has gradually focused on developing novel catalysts that are low-cost, environmentally friendly, and highly active. By regulating the interfacial contact between active metals and supports, as well as metal-support strong interactions, catalyst preparation protocols can simultaneously modulate the dispersion, reducibility, surface electronic structure of active components, and the adsorption-dissociation capacity of reactants. Meanwhile, these strategies enable precise control over the amount of target active phases formed during the reaction and improve catalyst sintering resistance^[152]. Originating from the intrinsic physicochemical properties of catalysts, these approaches ultimately achieve simultaneous optimization of catalytic activity, methanol selectivity, and long-term operational stability. Accordingly, precise tuning of key physicochemical parameters (e.g., crystallite size, oxygen vacancy concentration, and the proportion of high-valence active species) via customized preparation strategies can systematically enhance the overall catalytic efficiency of CO₂ hydrogenation to methanol^[153-154]. Table 5 systematically compares the core characteristics of four conventional catalyst synthesis routes, namely co-precipitation, ammonia evaporation, impregnation, and hydrothermal method, and summarizes the merits, drawbacks, and optimal applicable reaction domains of each technique. Precise modulation of catalytic performance can be achieved via these versatile synthetic strategies^[164]. Based on practical reaction requirements, researchers can rationally select or further modify synthetic protocols to maximize advantages and mitigate deficiencies. Catalyst physicochemical properties can be optimized along multiple dimensions, including metal dispersion, pore structure, and metal-support interaction, thereby accommodating the distinct demands of diverse catalytic reactions in terms of activity, selectivity, and stability. For example, metal-organic frameworks (MOFs) are commonly prepared via six routes: solvothermal synthesis, microwave-assisted synthesis, direct pyrolysis, the sacrificial template method, post-synthetic modification, and supercritical fluid deposition (SFD). Comparative analyses reveal that MOFs prepared via these distinct methods have unique advantages, making them suitable for specific applications^[53]. Similarly, San et al.^[165] investigated Cu/ZrO₂ catalysts synthesized through four different methods and reached the same conclusion. Notably, compared with catalysts prepared by traditional methods, the Cu/ZrO₂-DM catalyst - synthesized via ligand replacement (partial substitution of terephthalic acid with amino terephthalic acid) combined with incipient wetness impregnation for Cu loading - exhibits superior catalytic performance^[165]. In the broader context of catalyst optimization, refining preparation methods remains a pivotal approach to enhancing overall efficacy. By precisely regulating process parameters (e.g., reaction temperature, precursor ratios, and post-treatment conditions), the morphological structure, active site distribution, and electronic properties of catalysts can be optimized, thereby significantly improving their catalytic activity, selectivity, and stability^[153].

Catalytic performance can be significantly enhanced through synergistic elemental interactions, optimized structural design, and strategic selection of supporting materials^[166]. Metal supports play a multifaceted role in catalysis: they regulate the dispersion and valence state of active components to optimize active site architecture; facilitate reactant adsorption, activation, and intermediate transformation to streamline reaction pathways; strengthen metal-support interactions to bolster stability; and fine-tune pore structures and surface acid-base properties to improve mass transfer and selectivity^{[94,104,167-169]}. Collectively, these effects synergistically elevate catalytic activity, selectivity, and durability. Carbon modification, a widely adopted support engineering strategy, effectively promotes the generation of surface oxygen vacancies, thereby strengthening CO₂ adsorption and activation. More importantly, carbon species intimately bound to metal components serve as efficient channels for hydrogen spillover. This saturates surface hydrogen adsorption and renders the H₂ reaction order near zero, effectively overcoming the kinetic limitations inherent in CO₂ hydrogenation to methanol^[170]. Given the positive linear correlation between total alcohol selectivity and oxygen vacancy concentration, increasing the density of surface oxygen vacancies is a proven approach to enhance total alcohol selectivity^[171]. For example, in comparative studies of monometallic (Pt, Ni, Co) and bimetallic (PtNi, PtCo, PdNi) catalysts supported on CeO₂ (a reducible support) versus γ-Al₂O₃ (an irreducible support), the support type was found to primarily dictate catalytic activity and the extent of bimetallic bond formation^[172]. As a representative reducible support, the morphology of CeO₂ directly influences oxygen vacancy concentration as well as the dispersion and stability of active metal species. Furthermore, particle size, modulated by calcination temperature, alters metal-support interactions and ultimately determines overall catalytic performance. Notably, CeO₂ can stabilize Rh in the +2 oxidation state, enabling efficient conversion of CO₂ to methane^[173,174]. Conversely, irreducible supports like γ-Al₂O₃ generally exhibit lower intrinsic activity but favor higher CO selectivity. Loading metals onto γ-Al₂O₃ also modifies the catalyst’s basicity, yielding greater energy efficiency for syngas conversion and product formation^[175].

Although product selectivity in catalytic reactions is intrinsically governed by supported active metal components, supports act far beyond simple physical scaffolds. Instead, they participate deeply in catalytic cycles and modulate reaction behavior across multiple dimensions, as shown by the Cu-based modified methanol synthesis catalysts and noble-metal single-atom Rh catalytic systems discussed in the foregoing sections. Previous literature has systematically summarized four core mechanisms by which supports regulate catalytic processes: in situ generation of auxiliary active sites, stabilization of key adsorbed reaction intermediates, construction of metal-support interfaces with electronic synergies, and modulation of reaction kinetics for the conversion of reactants and intermediates^[142]. Collectively, these four mechanisms alter the intrinsic catalytic properties of catalysts.

Taking the Cu/ZrO₂ catalytic system for CO₂ hydrogenation to methanol elaborated in earlier chapters as an example, multiple regulatory effects originating from the support directly determine CO₂ conversion activity, target methanol selectivity, and long-term operational stability. ZrO₂ possesses moderate Lewis acidity and outstanding high-temperature anti-sintering capacity, which can anchor Cu nanoparticles and preserve mesoporous pore structures, rendering it the most suitable support for this reaction system^[176]. Furthermore, the distance between the catalyst-relevant periphery (CRP) regions on the support surface and Cu active particles must be precisely matched to generate interfacial synergy^[177]. Mismatched particle spacing directly disrupts the kinetic balance between CO₂ activation and formate intermediate transformation, and only optimized spacing can maximize catalytic efficiency. Such interfacial synergy rules serve as a critical prerequisite for high-efficiency production of oxygenates via CO₂ hydrogenation.

Extending to the single-atom noble metal catalytic system for selective ethanol synthesis through CO₂ hydrogenation discussed previously, the Rh₁/CeTiO_x catalyst with Ti-doped CeTiO_x support further highlights the multi-functional regulatory capacity of supports^[125]. This composite oxide support is not merely an inert substrate for immobilizing isolated Rh single atoms; rather, it comprehensively reconstructs the catalytic system via four coupled core effects: (1) reversible surface structural reconstruction of the support to create high-density oxygen vacancy defects; (2) generation of SMSI between the support and Rh single atoms to precisely tune the electronic states of active sites; (3) fabrication of bifunctional active centers relying on defect sites and doped heterogeneous interfaces; (4) lattice confinement effect to suppress the migration and agglomeration of Rh single atoms and significantly improve the thermal stability of catalysts. These multi-layer regulatory mechanisms jointly determine the electronic distribution, spatial configuration of active sites, and complete reaction pathway covering CO₂ activation and C-C coupling, ultimately governing the overall catalytic activity, ethanol selectivity, and long-cycle durability of catalysts. This also provides clear theoretical guidance for the rational design of supports for single-atom noble metal catalysts.

Consistent studies show that the performance of reactions represented by higher-alcohol synthesis from syngas and RWGS largely depends on modulating the surface chemical properties of catalysts^[178-180]. Introducing secondary-metal or alkali-metal promoters into Mo-based (molybdenum carbide or molybdenum sulfide) catalysts is an effective way to precisely regulate the chemical structure and electronic environment of active sites^[181].

The conventional RWGS reaction typically operates at elevated temperatures (400-700 °C), which not only predisposes catalysts to deactivation but also requires complex heat-exchange systems. Conversely, at lower temperatures (< 600 °C) and high pressure, CO₂ hydrogenation tends to favor methane and methanol formation, resulting in extremely low CO selectivity. Therefore, there is a critical need to develop catalysts with both high activity and selectivity, and this demand can be effectively met by modifying catalysts with metal promoters. The synergistic interaction between the two metals alters the binding energies of reactants and reaction intermediates, fundamentally influencing the reaction’s chemical behavior^[182-184].

Alkali metal K represents the most widely adopted modifier/promoter in catalytic systems for higher alcohol synthesis from syngas. In recent years, K has emerged as a core tuning strategy to optimize catalytic activity, C₂₊ alcohol selectivity, and long-term operational stability by precisely modulating the electronic environment of active sites, surface defects, and reactant adsorption configurations^[185]. It is compatible with various mainstream catalytic systems, including Mo-based catalysts and bimetallic Cu-Fe catalysts, providing an efficient regulatory route to overcome the intrinsic performance bottlenecks of different catalyst families.

As summarized from the foregoing comparative investigations of MoS₂ and Mo₂C molybdenum-based catalysts, under identical K and Mo loadings, MoS₂ sulfide catalysts construct abundant interlayer sulfur vacancies and heterogeneous MoS₂/KMoS₂ interfaces^[104], which substantially strengthen CO insertion and C-C coupling kinetics. Accordingly, MoS₂ delivers superior C₂₊ alcohol selectivity and space-time yield relative to Mo₂C carbides. Conversely, the intrinsic active sites of Mo₂C favor the single-carbon hydrogenation pathway, yielding methanol and light alkanes as dominant products accompanied by abundant surface acidic sites^[106]. Standard K doping dosages fail to deliver satisfactory regulation effects for Mo₂C; higher K loadings are therefore required to neutralize surface acidic sites, restructure CO adsorption modes, and ultimately elevate the fractions of total alcohols and long-chain alcohols in product distribution.

For the Mo₂C/γ-Al₂O₃ catalytic system described in previous sections, single-K modification improves the crystallinity of the β-Mo₂C phase and optimizes the valence distribution of Mo species^[91]. When combined with Ni to form a dual-promoter system, K and Ni generate synergistic electronic effects that simultaneously tune the non-dissociative CO adsorption strength and balance the competing reactions of carbon chain propagation and hydrogenation. This suppresses excessive C-O bond hydrogenolysis from a kinetic perspective, mitigates the formation of hydrocarbon byproducts such as methane, and markedly boosts the yield of liquid alcohol products.

Within the bimetallic Cu-Fe catalyst category belonging to modified Fischer-Tropsch systems discussed earlier, the K promoter exerts multiple regulatory functions. On one hand, K accelerates the RWGS reaction over catalyst surfaces, continuously supplying active CO intermediates. Conversely, it modulates the electron density at metal-metal interfaces to enhance linear CO adsorption while weakening bridged CO adsorption, thereby delicately balancing the two competing pathways of CO dissociative and non-dissociative activation. Meanwhile, K lowers the energy barrier for C-C coupling and inhibits over-activation of H₂, restraining alkylation side reactions at the source to reduce alkane and alkene byproducts and selectively enrich C₂₊ alcohols^[186]. Such regulatory effects effectively alleviate the inherent drawbacks of Cu-Fe catalysts, including facile phase segregation and overly broad product distribution.