Catalytic Conversion of CO₂ to Methanol: Advances in Catalyst Design and Plasma-Assisted Technology

Abstract

The hydrogenation of CO₂ to methanol is a crucial route for achieving carbon recycling. Among the extensively studied catalysts, copper-based catalysts suffer from insufficient activity and stability, while noble metal catalysts are limited by prohibitively high cost. In contrast, metal–organic framework (MOF) materials demonstrate unique advantages due to their designable architectures and high dispersion. Conventional thermal catalysis relies on high temperature and pressure; photocatalysis suffers from low efficiency; and electrocatalysis shows poor selectivity. These limitations motivate the exploration of new catalytic approaches. Plasma catalysis, particularly dielectric barrier discharge (DBD) technology, can efficiently activate CO₂ via high-energy electrons and reactive species at ambient temperature and pressure, and generate a synergistic effect with catalysts, significantly enhancing methanol production efficiency and selectivity. Studies have shown that plasma–catalyst synergistic systems, such as those employing Cu/γ-Al₂O₃ or Pt/In₂O₃, exhibit superior performance to individual processes under mild conditions. Future research should focus on elucidating the plasma–catalyst interface mechanism, optimizing reactor design, and developing compatible, high-efficiency catalysts to establish a novel pathway for CO₂ conversion with low energy consumption and high efficiency.

1. Introduction

With the development of the industrialization process of mankind, the demand for energy has increased significantly. As one of the most important resources for modern industry, fossil energy releases a large amount of CO₂ gas during its combustion, which is then emitted into the atmosphere (Figure 1). As a predominant greenhouse gas, CO₂ exerts a radiative forcing that is the principal factor for global warming. This phenomenon subsequently catalyzes a series of environmental issues, including the melting of polar glaciers and the concomitant rise in sea levels, thereby presenting a profound threat to human existence. Empirical data indicate that the atmospheric CO₂ concentration attained 409 ppm in 2019, marking a 25% rise during the last 50-year period. From an alternative perspective, CO₂ itself constitutes an important carbon resource applicable in chemical and food industries, with utilization methods encompassing both direct use and catalytic conversion. Currently, CO₂ has been employed in the production of light olefins, aromatics, gasoline, and related commodities. Maximizing the utilization of CO₂ resources can mitigate the greenhouse effect while providing sufficient carbon resources for the chemical industry and reducing dependence on fossil fuels, which is essential for future human development. Therefore, fully harnessing atmospheric CO₂ resources to reduce its concentration in the air is particularly critical for sustainable human development.

CO₂ is abundant in the atmosphere, and its resources can be captured through collection technologies. Due to the inert nature of CO₂ molecules, energy input is required when CO₂ serves as the sole reactant. However, when CO₂ reacts with molecules possessing higher Gibbs free energy (e.g., hydrogen), the conversion of CO₂ becomes thermodynamically favorable. Consequently, the hydrogenation of CO₂ to produce clean fuels and chemicals constitutes an environmentally sustainable process, wherein CO₂ is effectively recycled as a carbon source. Among all CO₂ utilization strategies, CO₂ hydrogenation to synthesize various fuels demonstrates significant potential. As shown in Figure 2, CO₂ hydrogenation can yield various chemicals, including carbon monoxide, methane, methanol, formates, formamides, dimethyl ether, olefins, long-chain alcohols, and hydrocarbons, under different operating conditions using distinct catalytic systems. Among these, methanol has gained particular favor due to its wide range of applications.

Methanol (chemical formula CH₃OH), referred to as wood alcohol or methyl hydrate, is the simplest saturated monohydric alcohol in structure, and its extensive applications make it indispensable in modern industry [2,3,4]. In the chemical industry, methanol is utilized to produce various chemical products, including formaldehyde, acetic acid, and dimethyl ether, and can also be employed in manufacturing cellulose acetate, plastic resins, synthetic rubber, and so on. Furthermore, methanol can serve as a solvent, be used in organic synthesis, and act as a raw material for pharmaceuticals and pesticides. In agriculture, methanol can be employed as a wood preservative and cleaning agent; industrially, it can be utilized for metal degreasing and polyvinyl chloride plastic production; it can also be used as a food additive and preservative, though strict adherence to relevant safety standards and regulations is required. In the energy sector, methanol is gaining increasing attention. Primarily, methanol’s combustion products differ from those of conventional chemical feedstocks, which generate pollutants such as sulfur oxides, nitrogen oxides, and particulate matter; they consist mainly of CO₂ and water, making methanol a recognized clean energy source. Moreover, methanol boasts abundant sources, with diverse production feedstocks including coal, natural gas, and biomass, which eliminates dependence on a single resource constraint for its production. Regarding storage and transportation, methanol is a liquid at ambient temperature and pressure with high stability, minimal leakage risk, and ease of handling, avoiding the challenges associated with high-pressure storage and transport like those encountered with hydrogen. Furthermore, methanol exhibits high combustion efficiency with a volumetric energy density of 15.6 MJ·L⁻¹, approximately half that of gasoline (34.2 MJ·L⁻¹), and when combined with its advantage of clean combustion products, is considered to possess potential for replacing gasoline and diesel as fuel [5].

The chemical process of CO₂ hydrogenation to methanol primarily comprises three core reaction steps: first, direct CO₂-to-methanol conversion; second, reverse water-gas shift (RWGS); and third, CO hydrogenation to methanol [6]. The first reaction is an exothermic molecular reduction reaction; although low temperatures favor methanol synthesis, they simultaneously constrain CO₂ activation efficiency. As a competitive reaction, the endothermic RWGS reaction does not become particularly active under high-temperature conditions, leading to a sharp decline in methanol selectivity. Regarding pressure parameters, increasing reaction pressure not only enhances CO₂ conversion rate and methanol selectivity but also has minimal impact on the RWGS side reaction. However, in practical applications, pressure increase is limited by factors such as equipment pressure-bearing capacity, economic cost, and safety considerations, with pressure typically maintained around 5 MPa. Additionally, under constant-pressure conditions, the molar ratio of hydrogen to carbon dioxide (H₂/CO₂) in the feed gas significantly influences the reaction process. Although increasing the H₂/CO₂ molar ratio can enhance the conversion rates of both carbon monoxide and carbon dioxide, thereby improving methanol yield, excessive hydrogen proportion will reduce the equilibrium conversion rate of hydrogen and consequently increase economic costs. The H₂/CO₂ molar ratio is generally regulated to approximately 3 based on stoichiometric equilibrium and engineering economic constraints.

The activity and selectivity of the CO₂ hydrogenation to methanol reaction are not only influenced by the reaction temperature and pressure, but more crucially depend on the performance of the catalyst employed. The rational design of efficient catalysts coupled with the optimization of reaction conditions can enhance the reaction efficiency and economic viability to a certain extent, thereby providing robust support for the resource utilization of CO₂.

Understanding the reaction mechanism of CO₂ conversion to methanol at the molecular level is of great guiding significance for the rational design of catalysts with high activity, high selectivity, and high stability. However, the discussions on the reaction mechanism and key reaction intermediates remain highly intense so far. Regarding the process of methanol synthesis from syngas containing CO₂, the prevailing view suggests that methanol is primarily generated from CO₂, while CO is converted to CO₂ via the water-gas shift (WGS) reaction [7,8,9,10], and acts as an adsorbent to remove water byproducts and the resulting oxygen species, which would otherwise inhibit the function of active metal sites. Isotope tracing experiments have further confirmed that CO₂ serves as the primary carbon source for methanol synthesis from either syngas or CO₂ [7,8].

In summary, advancing the technology for CO₂ hydrogenation to methanol is conducive to achieving carbon cycling and sustainable development. However, this process still faces challenges such as insufficient catalyst activity and selectivity, as well as the high energy consumption of traditional catalytic methods. Based on this, this article focuses on emerging technologies such as innovative catalyst design and plasma-assisted catalysis, reviewing their research progress and analyzing their synergistic mechanisms, with the aim of providing direction for breaking through existing technical bottlenecks and achieving efficient resource utilization of CO₂.

2. Catalysts for CO₂ Hydrogenation Conversion

2.1. Copper-Based Catalysts

As shown in Figure 3a, the research history of Cu-based catalysts can be traced back to the 1930s [11]. Currently, Cu-based catalysts are the most widely used catalysts for methanol synthesis and a research focus in heterogeneous catalysis for CO₂ hydrogenation to methanol, primarily because the active copper metal component can effectively suppress C-O bond cleavage, thereby promoting methanol formation. However, the catalytic activity of pure copper catalysts is relatively low. Therefore, their modification is required. As the primary active component, copper can first be loaded and dispersed onto various oxide supports, such as ZnO [12], Al₂O₃ [13], ZrO₂ [14], CeO₂ [15], TiO₂ [16], SiO₂ [17], etc. Studies have shown that these oxides can enhance the dispersion of Cu, thus improving the stability of the catalyst. Currently, Cu/ZnO catalysts have gained extensive attention and research. As the most fundamental Cu-based catalyst, ZnO in Cu/ZnO effectively enhances the dispersion and stability of Cu, while the oxygen vacancies in ZnO promote CO₂ adsorption [18,19]. Furthermore, ZnO participates in the formation of active sites and can stabilize key intermediates such as formate; acting as a physical spacer, it prevents the sintering and agglomeration of Cu particles, thereby contributing to enhancing catalyst stability [20]. However, an excessively high ZnO content can lead to the adsorption of excessive CO₂ and the generation of CO, thus reducing the selectivity towards methanol in the products [21]. Recent research findings have demonstrated that supporting Cu/ZnO-based catalysts on metal oxide carriers can increase the number of active sites, maintain the stability of surface copper, enhance catalyst stability, improve copper dispersion, and promote the adsorption of CO₂ and H₂ [22,23], which can enhance catalyst activity. As shown in Figure 3b, modifications of Cu-based catalysts are generally studied based on the Cu/ZnO system, such as Cu/ZnO/Al₂O₃, Cu/ZnO/ZrO₂ [18,22], Cu/ZnO/CeO₂ [24], Cu/ZnO/Ga₂O₃ [23], etc. Among these, Al₂O₃ is the most widely used support for Cu/ZnO-based catalysts. On one hand, the incorporation of Al₂O₃ helps stabilize the active phase; on the other hand, its acidic properties facilitate the dissociation of reactants [25]. However, the hydrophilic nature of Al₂O₃ can lead to the sintering of Cu particles, thereby reducing catalyst stability. ZrO₂ is another widely used support for copper-based catalysts. Owing to its good thermal stability, mechanical stability, high specific surface area, and semiconductor properties, Cu/ZrO₂-based catalysts exhibit excellent performance in CO₂ hydrogenation to methanol. Moreover, because of the lower hydrophilicity of ZrO₂, Cu/ZrO₂-based catalysts show higher catalytic performance than Al₂O₃-based catalysts.

2.2. Noble Metal Catalysts

Noble metal-based catalysts, recognized as promising alternatives to copper-based catalysts, exhibit excellent stability, resistance to sintering, and poisoning tolerance. In contrast to the reaction conditions of 200–300 °C and 3–5 MPa required for Cu-based catalysts, noble metal catalysts demonstrate superior catalytic performance under lower temperatures and pressures. Currently, Pd and Au are the most commonly employed precious metals in CO₂ hydrogenation to methanol systems. Various materials, including metal oxides (e.g., ZnO [27], Ga₂O₃ [28], CeO₂ [29], In₂O₃ [30]), mesoporous silica [31], and carbon-based materials [32] have been explored as supports for noble metal-based catalysts. Research by Matsumura et al. [33] demonstrated that Pd/CeO₂ catalysts effectively convert syngas to methanol at 443 K and 3 MPa, whereas Cu-based catalysts require higher reaction temperatures (≥503 K). Currently, Pd/ZnO catalysts have garnered significant research attention due to their superior performance. Bowker et al. [34] proposed that PdZn alloys can stabilize formate intermediates, thereby facilitating methanol formation. Simultaneously, the introduction of metallic Pd significantly suppresses the RWGS reaction, reducing CO formation. However, the active sites of Pd/ZnO catalysts remain controversial. Some studies suggest that the formation of PdZn alloys under high-temperature reduction conditions leads to a decline in catalytic activity [35]. Conversely, other researchers propose that either PdZn alloys or Pd species covered by partially reduced ZnO_X serve as active sites for methanol synthesis [29,34]. As shown in Figure 4, the formation of PdZn alloys alters the intermediates during the reaction process, shifting the reaction pathway toward the formate (HCOO *) route and effectively suppressing CO formation [36].

Behm et al. [37] reported on the influence of Au particle size and support properties on the performance of catalytic CO₂ hydrogenation. The Au/ZnO catalyst exhibited higher methanol formation activity than the Au/ZrO₂, Au/TiO₂, and Au/Al₂O₃ catalysts. For the Au/ZnO catalyst, as the diameter of the Au NPs increased, the methanol formation rate decreased, while the methanol selectivity increased from 56% to approximately 82%. Isotopic labeling experiments revealed that as the reaction temperature increased from 240 °C to 300 °C, the carbon source for methanol synthesis shifted from CO₂ to CO. Furthermore, through kinetic measurements and in situ IR spectroscopy, the study elucidated that methanol is produced on the Au/ZnO catalyst via the direct hydrogenation of CO₂ from formate and methoxy intermediates.

Although noble metal catalysts can achieve efficient CO₂ hydrogenation to methanol under relatively mild conditions, their high cost and the scarcity of raw materials limit their industrial application.

2.3. Other Catalysts

Bimetallic alloy catalysts: Over the past few decades, owing to the unique structural, chemical, and electronic properties as well as synergistic effects of bimetallic catalysts compared to their monometallic counterparts, researchers have developed a variety of bimetallic catalytic systems for CO₂ hydrogenation to methanol, including alloys like Cu-Zn [38,39], Pd-Zn [40], Cu-Pd [41], Pd-Ga [42] and etc. The Pd-Cu catalysts supported on amorphous silica or mesoporous silica, as reported by Song et al. [41] exhibited a high methanol formation rate, which is attributed to the formation of PdCu and PdCu₃ alloys that promote the chemisorption of CO₂. Furthermore, the team discovered that catalysts containing the PdCu alloy exhibited higher methanol selectivity compared to those with PdCu₃.

Intermetallic compounds (IMCs): The entire nanoparticle of an intermetallic compound possesses a homogeneous distribution of metallic elements. The significant increase in methanol synthesis activity following the formation of IMCs in metal-oxide-supported catalysts has attracted considerable interest from researchers, particularly in systems such as Pd-Ga [28], Pd-Zn [28], Pd-In [43,44], Cu-In [45], Ni-Ga [46] and etc. IMCs exhibit high activity and methanol selectivity in the CO₂ hydrogenation reaction. Wu et al. [45] demonstrated that the interface between Cu₁₁In₉ and In₂O₃ provides the catalytic sites, where H₂ undergoes dissociative adsorption on the Cu₁₁In₉ surface, and CO₂ adsorbed on the oxygen vacancies of In₂O₃ migrates to the Cu₁₁In₉-In₂O₃ interface, where it reacts with the adsorbed H₂ to form methanol.

Oxide catalysts: In recent years, novel oxide and hybrid oxide catalysts (such as In₂O₃/ZrO₂ [47] and ZnO/ZrO₂ [48]) have been further developed, demonstrating excellent catalytic activity in the hydrogenation of CO₂ to methanol. The In₂O₃/ZrO₂ catalyst reported by Perez-Ramirez et al. [49] exhibited excellent methanol selectivity (100%) and stability (1000 h) during CO₂ hydrogenation to methanol. The Zn-ZrO₂ solid solution catalyst reported by Li et al. [48] also showed excellent single-pass CO₂ conversion (>10%) and high methanol selectivity of 86–91%. Furthermore, this catalyst demonstrated remarkable resistance to sulfur. Characterization results indicated that H₂ adsorption and dissociation occur on Zn sites, while CO₂ adsorption takes place on coordinatively unsaturated Zr sites. The strong synergistic effect between Zn and Zr facilitates the activation of H₂ and CO₂ on adjacent Zn and Zr sites, thereby endowing the Zn-ZrO₂ solid-solution catalyst with its superior performance. The catalytic activities of selected catalysts are presented in

Table 1. Methanol synthesis performance from CO₂ hydrogenation over selected reported catalysts.

Catalyst	Temp (°C)	H2/CO2	P (MPa)	GHSV (h−1)	XCO2 (%)	SMeOH (%)	Ref.
Cu/ZnO/Al2O3	270	2.2	4.5	8000	10.9	72.7	[50]
Cu/ZnO	250	3	3	18,000	2.3	100	[51]
CuZnGa	270	3	4.5	18,000	20	49	[52]
Cu/ZnO/ZrO2	220	3	3	3600	12.0	71.1	[53]
Cu/ZnO/ZrO2	240	3	3	3600	17.0	56.2	[54]
La-Cu/ZrO2	220	3	3	3600	5.8	72.0	[55]
Cu/ZrO2	240	3	2	5400	6.3	48.8	[56]
Pd/ZnO	250	3	2	3600	10.7	60	[34]
Pd/In2O3	300	4	5	21,000	20	70	[30]
Au/ZnO	240	3	0.5	4800	0.3	82	[37]
Cu11In9-In2O3	280	3	3	7500	11.4	80.5	[45]
In2O3/ZrO2	300	4	5	20,000	5.2	99.8	[49]
ZnO-ZrO2	320	4	5	24,000	10	91	[57]
CuZn@UiO-bpy	250	3	4	18,000	3.3	100.0	[58]
CuZnOx@UiO-66	250	3	4	18,000	3.5	86.1	[59]

below.

3. Metal–Organic Frameworks (MOFs)

Metal–organic frameworks (MOFs) are a class of grid-like porous organic/inorganic hybrid materials assembled from metal ions or clusters and organic ligands. Commonly encountered metal clusters or atoms include zinc, copper, chromium, iron, aluminum, and zirconium, among others. The ligands are typically composed of polycarboxylates or nitrogen-containing aromatic molecules [60,61] and can be extended into one-dimensional, two-dimensional, and three-dimensional materials. Based on the size of the molecular channel voids, MOFs can be classified as microporous, mesoporous, and macroporous materials. Figure 5 illustrates the fundamental principles of MOF assembly. MOFs typically possess characteristics desirable for catalyst materials, such as high specific surface area, high thermal stability, chemical stability, high catalytic activity, and selectivity. Simultaneously, they offer the advantage of flexible tunability in their pore structure and surface functional groups. Moreover, they can further serve as supports for loading other materials such as metal atoms or clusters, metal oxides, etc. [62]. In recent years, MOFs have garnered increasing attention, and numerous MOFs have been synthesized, such as UiO-66, MOF-808, MOF-5, MOF-74, HKUST-1, and ZIF-8. They often possess significant commercial potential and considerable application value. Currently, with a space-time yield as high as 2000 kg·m⁻³·day⁻¹, large-scale production is achievable [63,64]. In summary, due to the versatility of their nanostructures and the unique properties of nanomaterials, scientific research based on MOFs holds broad application prospects and is of great significance and value.

Currently, MOF materials possess significant potential application value in the field of CO₂ hydrogenation, capable of catalytically synthesizing various high-value-added products, including methane, alcohols, and C²⁺ hydrocarbons, particularly methanol. MOF materials inherently feature well-developed pore structures, high specific surface area, and multi-dimensionally stable architectures. On one hand, this facilitates the formation of bimetallic or even multimetallic catalytic active sites; on the other hand, it provides a substantial contact area between reactants and catalytic active sites to enhance catalytic efficiency. Simultaneously, some MOF materials exhibit excellent stability and hydrophobicity, enabling them to withstand high-temperature and high-pressure reaction environments. Their hydrophobic nature can prevent the sintering of catalytic active sites. Moreover, MOF materials inherently contain metal centers that can exert a synergistic catalytic effect during the reaction process. The ligands of MOF materials may interact with CO₂ and H₂, enhancing the adsorption capacity for reactants and even providing a certain degree of activation.

The integration of MOFs into catalytic systems to enhance the carbon dioxide hydrogenation process, particularly the conversion to high-value products such as methanol and formic acid, has attracted widespread attention. Numerous studies have highlighted the potential of MOF-derived catalysts and MOF-based composite materials in this field.

Based on the aforementioned descriptions of Cu-based catalysts and the relevant characteristics of MOF materials, some researchers have attempted to combine the two to prepare catalysts. For instance, An et al. [58] first modified the MOF material by attaching bpy (2,2-bipyridine) to the MOF. Subsequently, they encapsulated Cu/Zn nanomaterials within the MOF pores, forming a Cu/ZnO@MOF structure. The confinement effect of the porous structure and the strong metal-support interaction between organic ligands and metal oxides were utilized to suppress the phase separation of Cu/ZnO and the agglomeration of Cu particles, as shown in Figure 6a. Compared to the conventional commercial catalyst Cu/ZnO/Al₂O₃, this catalyst demonstrated over three times higher methanol yield under reaction conditions of 250 °C and 4 MPa, with selectivity reaching 100%. Meanwhile, the catalyst maintained excellent stability during continuous operation for 100 h. Furthermore, it was demonstrated that the metal-support interaction formed between the bpy organic linkers and the zirconia clusters serving as secondary building units of the MOF is crucial for stabilizing CuZn nanoparticles and suppressing their phase separation. The stability and anti-agglomeration properties of the nanoparticles also heavily depend on the pore structure of the MOF [58].

Liu et al. [65] utilized in situ reconstructed Cu@UiO-66 with a size confinement effect as a precursor to prepare a Cu@ZrO_x catalyst with a three-dimensional porous framework structure, which demonstrated exceptional catalytic activity and stability during the reaction, as shown in Figure 6b. Under reaction conditions of 260 °C, 4.5 MPa, and 21,600 h⁻¹, the CO₂ conversion reached 13.1%, methanol selectivity was 78.8%, and the space-time yield achieved 796 g/kg_cat/h. Moreover, it exhibited excellent stability during a continuous 105 h reaction test. These achievements are attributed to the abundance of Cu-ZrO_x interfaces and the stable three-dimensional ZrO_x structure within the catalyst.

Constructing heterostructures based on the MOF-on-MOF architecture has been demonstrated as an effective strategy for enhancing catalytic performance. Xu et al. [66] successfully synthesized an Fe₂O₃@ZrO₂ composite catalyst derived from MOF-on-MOF by pyrolyzing a bimetallic organic framework (NH₂-MIL-88B@UiO-66). Under specific reaction conditions (340 °C, 3.0 MPa, CO₂/H₂ = 1:3, GHSV = 10,000 mL·g⁻¹·h⁻¹), this catalyst exhibited an excellent space-time yield of light olefins (12.7 mmol·g⁻¹·h⁻¹). It showed good activity in the direct CO₂ hydrogenation reaction, indicating that the epitaxial growth of the heterostructure contributes to the modulation of catalytic performance.

MOF-derived catalysts have certain applications in CO₂ hydrogenation to methanol. Han et al. [67] utilized a Cu/ZnO nanocatalyst obtained by calcining Cu_xZnO-MOF-74 in static air for CO₂ hydrogenation to methanol. Liu et al. [65] used confined Cu@Zr-MOFs (UiO-66) prepared via the NaBH₄ reduction method as a precursor, where UiO-66 was synthesized hydrothermally and subsequently treated with H₂ reduction to obtain a Cu@ZrO_X catalyst with a three-dimensional porous framework structure for methanol synthesis. Cai et al. [68] utilized In-MOFs (MIL-68(In)) nanorods (synthesized hydrothermally) as precursors to prepare hollow In₂O₃ nanotube supports. Subsequently, Pd was loaded via NaBH₄ reduction, resulting in a high-performance Pd/In₂O₃ catalyst for methanol synthesis. Qi et al. [69] synthesized a CuZn-BTC precursor on hydrated alumina via a hydrothermal method, subsequently preparing a Cu-ZnO/Al₂O₃ catalyst for CO₂ hydrogenation to methanol.

MOF-derived catalysts provide novel approaches for CO₂ hydrogenation to methanol. Based on previous research findings mentioned above, MOF-derived catalysts play a significant role in CO₂ hydrogenation to methanol. One approach involves calcining MOF-on-MOF precursor materials to decompose them into bimetallic oxides, creating novel morphologies that alter crystal facet exposure. Another method involves loading metals onto MOF materials to create confined catalysts by leveraging their unique pore structures. These two distinct methods each possess specific characteristics and advantages. Furthermore, currently MOF materials are primarily applied in the preparation of single-metal catalysts for methanol synthesis reactions. As mentioned in the previous section on Cu-based catalysts, constructing Cu-based bimetallic systems has been demonstrated as an effective strategy for enhancing catalytic performance in CO₂ hydrogenation to methanol [70,71,72]. Therefore, combining bimetallic systems with MOFs possessing unique structures can further enhance methanol synthesis performance. On one hand, MOF materials can serve as precursors to derive oxides that are subsequently modified with metallic Cu. On the other hand, leveraging the confinement effect of MOFs, metallic Cu can be loaded into MOF materials to create highly dispersed Cu-based/MOF-confined catalysts, simultaneously improving metal utilization.

The prominent advantages of MOFs in the field of CO₂ hydrogenation catalysis are rooted in their uniquely designable crystalline structures, which create a sharp contrast with conventional catalysts. Traditional catalysts, such as copper-based materials, while low-cost, suffer from poor thermal stability and complex, difficult-to-control active sites. Noble metal catalysts, despite their high activity, are expensive and present challenges in selectivity control. The core advantage of MOFs lies in their highly customizable structures: precise selection of metal node types (e.g., copper clusters, zirconium clusters) and organic ligand functional groups (e.g., amino, hydroxyl) at the atomic level, thereby obtaining catalytic centers with both high activity and selectivity, and achieving “shape-selective catalysis” by controlling pore dimensions. Secondly, the well-defined crystalline structure of MOFs makes them an ideal bridge connecting heterogeneous and homogeneous catalysis. It provides an unprecedented platform for directly studying the reaction mechanism of CO₂ hydrogenation at the molecular level, contributing to fundamental guidance for catalyst design. Moreover, the power of MOFs also lies in their multifunctional integration capability, enabling the synergy of efficient CO₂ adsorption/enrichment and catalytic hydrogenation within a single material, significantly enhancing reaction efficiency and even allowing tandem multi-step reactions. Finally, MOFs themselves can serve as excellent precursors or supports, yielding highly dispersed, sintering-resistant metal nanocatalysts through controlled pyrolysis, an approach that significantly advances practical applications of high-performance catalysts. In summary, MOFs are not only promising catalytic materials but also powerful research tools and design platforms, profoundly transforming how we understand and develop CO₂ conversion catalysts.

4. Catalytic Approaches for CO₂ Hydrogenation Conversion

4.1. Thermal Catalysis

Thermal catalysis is a process that utilizes thermal energy as the power source to drive catalytic reactions and is widely employed in the basic chemical industry for synthesizing chemical feedstocks, including ammonia, olefins, hydrocarbons, etc. [73]. The sufficient utilization of renewable energy sources like solar and wind power for water electrolysis to produce hydrogen has significantly advanced thermal catalytic reduction technologies. The thermal catalytic process can reduce CO₂ to carbon monoxide, methane, ethylene, propylene, and other aromatic compounds. C₁-series small molecules can be directly obtained through hydrogenation conversion of carbon dioxide molecules, whereas multi-carbon hydrocarbon products require initial conversion of CO₂ to methanol followed by transformation into olefins via methanol-to-hydrocarbons (MTH) processes. However, due to the inherent high stability of CO₂ molecules, this reaction requires high temperature and high-pressure conditions. To overcome the energy barrier to carbon-carbon coupling processes, the selection of highly selective catalysts is crucial. Homogeneous catalysts such as Rb-, Rh- [74], and Ir-based [75] systems exhibit high selectivity toward formic acid, yet their practical application is limited due to high cost and scarcity. In contrast, heterogeneous catalysts (e.g., Fe- [76], Ni-based [77]) are cost-effective and readily available, yet suffer from instability and significantly inferior catalytic performance compared to homogeneous systems, necessitating further research.

Given the relative maturity of thermocatalytic process equipment, current research on CO₂ thermocatalytic reduction primarily focuses on catalyst optimization, with metal-oxide systems being the most extensively studied catalytic systems [78,79,80,81]. Through advanced in situ characterization techniques and theoretical calculations, it has been revealed that catalytic active sites are typically located at metal-oxide interfaces, where bifunctional or electronic effects directly influence the adsorption/desorption kinetics of reactants and intermediates, consequently affecting reaction activity and selectivity [82].

Current thermocatalytic CO₂ conversion has achieved high levels of catalytic activity and target product selectivity. However, the high-temperature and high-pressure conditions required by conventional thermocatalysis still rely on carbon-based fossil fuels as the primary energy source, which limits their effectiveness in CO₂ emission reduction and makes it challenging to achieve genuine zero-carbon or even negative carbon emissions.

4.2. Photocatalysis

Photocatalysis, also termed artificial photosynthesis, utilizes solar energy to drive catalytic reactions that convert water and carbon dioxide into substances like carbon monoxide, formic acid, and methane while releasing oxygen, primarily involving energy and environmental fields [83,84,85,86]. The photocatalytic process employs solar energy as a power source, where the solar radiation reaching Earth per second equates to energy from burning millions of tons of coal, thus offering advantages of simplicity, convenience, energy efficiency, and environmental friendliness compared to other CO₂ conversion technologies. Currently, photocatalytic technology has been applied in multiple aspects, including light-induced water splitting for hydrogen production, CO₂ photoreduction, and solar fuel generation.

The photocatalytic reduction process primarily consists of three steps: (1) Light absorption: The photocatalytic reduction process requires photon energy exceeding the bandgap of semiconductor catalysts, with absorption edges located in the near-infrared and visible regions. Upon light absorption, semiconductor catalysts generate photoexcited electrons and simultaneously produce holes. (2) Migration of photogenerated electrons and holes: As the reduction process occurs at the catalyst surface, the generated electrons and holes need to migrate from the catalyst interior to the surface, where successfully arrived charge carriers will be trapped by surface defects. (3) Surface redox reactions: CO₂ adsorbed on the catalyst surface undergoes reduction with photogenerated electrons and holes, requiring precise alignment between the conduction/valence band positions and reaction energy levels to reduce CO₂ to products like carbon monoxide, formic acid, methanol, and methane. (The working principle of semiconductor photocatalysis is illustrated in Figure 7 [87]). Environmental factors, including light source intensity [88], reaction temperature [89], reactant concentration, and system acidity [90] significantly influence the reaction efficiency. Furthermore, intrinsic characteristics of catalytic materials such as defects and morphology serve as dominant factors governing the reaction.

Photocatalytic CO₂ hydrogenation utilizes solar energy to drive the catalytic reaction between CO₂ and H₂. However, the photocatalytic process is prone to charge carrier recombination, resulting in low catalytic efficiency. Furthermore, conventional photocatalysts can only absorb ultraviolet and partial visible light, wasting significant amounts of energy [91,92,93]. Moreover, due to the high thermodynamic stability of linear CO₂ molecules, photocatalytic CO₂ reduction typically exhibits low energy conversion efficiency.

4.3. Electrocatalysis

Electro CO₂ Reduction Reaction (ECR) is a promising CO₂ utilization technology that can convert CO₂ into various fuels and chemicals through electrochemical reactions under relatively mild conditions. In recent years, the utilization of renewable energy sources such as solar and tidal power can provide sufficient electrical energy support for the electrocatalytic reduction process, thereby promoting the development of electrocatalytic CO₂ reduction technology [94]. The electrocatalytic process offers advantages, including safe and controllable reaction procedures, mild operating conditions, recyclable electrolytes, and compact, scalable systems, rendering it with broad development prospects. ECR products can be categorized into C₁ products (e.g., carbon monoxide, methane, and methanol) and C²⁺ multi-carbon products (e.g., ethylene, ethanol, and propanol) based on carbon atom count. Existing research indicates that studies on C₁ products have reached considerable maturity, with Faradaic efficiencies exceeding 95%, whereas for multi-carbon products like ethylene, selectivity remains poor due to high energy barriers for C-C coupling and substantial overpotentials [95]. Among various products currently achievable through ECR, CH₃OH represents a potential liquid product that can serve as an intermediate feedstock for fuel and chemical production, garnering extensive attention.

The ECR process typically involves four fundamental steps: (1) CO₂ molecules undergo chemical adsorption on the active sites of electrocatalysts; (2) CO₂ is activated to form CO₂^•− radical anions; (3) through electron and proton transfers, chemical reactions occur that break C-O bonds and form C-H bonds to generate products; (4) finally, the products desorb from the active sites [96].

As shown in Figure 8, during electrolysis, the anode reaction produces O₂ from water while releasing electrons and protons; subsequently, electrons travel to the cathode via an external circuit, while protons migrate to the cathode through the electrolyte. At the cathode, electrons combine with protons to either generate H₂ or reduce CO₂ to form various products. The proportion of electrons allocated to H₂ production versus CO₂ reduction depends on the activation energy barriers of each respective reaction [97]. Furthermore, the hydrogen evolution reaction (HER) constitutes one of the primary competing reactions for CO₂ reduction, significantly impacting the selectivity of target products. Since both reduction reactions require electrons at the cathode, factors including catalyst type, solvent selection, operating conditions (e.g., temperature and pressure), and cell parameters also affect CO₂ reduction performance. Additionally, the separation and purification of products demand substantial time and economic costs, which hinder the industrial application of current electrocatalytic technologies.

4.4. Comparison of Various Catalytic Technologies

Regarding the catalytic reduction technologies discussed above, this paper provides a concise summary of their principles and advantages/disadvantages in CO₂ hydrogenation to methanol processes through

Table 2. Comparison of various catalytic techniques.

Name	Principle	Advantage	Disadvantage	Ref.
Thermal Catalysis	By using catalysts to reduce the activation energy required for chemical reactions, reaction rates and selectivity can be significantly enhanced at relatively lower temperatures.	High efficiency and energy saving; High selectivity; Fast reaction rate; Mature technology, easy industrialization	Requires high temperature and pressure operation; Relatively high energy consumption; Catalyst deactivation issues	[73,98]
Photoinduced Catalysis	By photoexciting electrons on the photocatalyst surface, electron-hole pairs are generated, which react with surrounding molecules or ions to catalyze chemical reactions.	Mild operating conditions required; Environmentally friendly	Low production efficiency	[86,99]
Electrochemical Catalysis	By applying a specific potential to electrodes, electrons flow into or out of chemical species in solution, thereby inducing chemical reactions.	Simple equipment	High energy consumption, low current efficiency	[97]

.

In summary, conventional CO₂ hydrogenation technologies generally suffer from either high energy consumption or low efficiency, while traditional processes constrained by high-temperature/high-pressure conditions often lead to excessive energy consumption, demanding stability requirements, and slow response in practical applications. With the advancement of reforming technologies, plasma technology demonstrates significant potential for chemical reactions due to its flexible operating conditions and rapid switching capability. Consequently, research on applying plasma technology to CO₂ hydrogenation for methanol production shows increasingly promising prospects.

5. Plasma Catalysis

5.1. Plasma

Plasma is a high-energy-density substance composed of electrically coupled multivalent ions and atoms. It can generate high-energy gas at relatively low temperature conditions, exhibiting strong chemical reactivity. Based on discharge pressure, plasma can be classified into low-pressure plasma and high-pressure plasma. Classification by electron temperature divides plasma into thermal plasma and non-thermal plasma, with the latter further subdivided based on local thermodynamic equilibrium state into thermal and non-thermal plasma [100].

Atmospheric thermal plasma refers to high-temperature plasma generated under atmospheric pressure. Atmospheric thermal plasma can be classified into non-equilibrium plasma and thermodynamic equilibrium plasma. Non-equilibrium atmospheric pressure plasma exhibits electron temperatures significantly higher than ion temperatures, typically ranging from 3000 K to 30,000 K. The electrons in such plasma possess high reactivity, enabling broad applications in chemical reactions and material surface modification. Thermodynamic equilibrium plasma maintains equal electron and ion temperatures, existing in a state of thermal equilibrium. This type of plasma has relatively low energy density and is commonly employed in analytical and detection applications such as spectral analysis, mass spectrometry, and atomic absorption spectroscopy [101].

Atmospheric non-thermal plasma refers to plasma generated at atmospheric pressure where electron temperature exceeds gas temperature, while ion temperature approximates gas temperature. In gas discharge processes, electrons are accelerated to energies sufficient for atom ionization, subsequently causing gas ionization and excitation to form non-thermal plasma. Such plasma typically includes plasma clouds and gas discharges, with electron temperatures generally ranging from 1000 K to 20,000 K, while gas temperatures remain between room temperature and several hundred degrees Celsius. Compared to thermal plasma, atmospheric non-thermal plasma contains relatively fewer ions and atoms but higher electron density, endowing electrons with substantial energy for broad applications in chemical reactions, material surface modification, biomedical fields, etc. The high-energy electron intensity and low-temperature characteristics of non-thermal plasma effectively address challenges in CO₂ reduction and accommodate the thermodynamic properties (exothermic nature) of CO₂-to-methanol conversion. Different gases and discharge conditions generate varied non-thermal plasma types, such as plasma clouds and glow discharges [102]. Atmospheric non-thermal plasma primarily includes Corona Discharge (CD), Spark Discharge (SD), Gliding Arc Discharge (GA), and Dielectric Barrier Discharge (DBD).

5.2. Plasma-Assisted CO₂ Hydrogenation

In conventional thermal catalytic processes, achieving high conversion and selectivity for CO₂ hydrogenation to CH₃OH requires substantial energy input to activate CO₂ molecules. Research indicates that approximately 783 kJ/mol energy is required to dissociate the C=O double bond during thermal activation. At a 300 K temperature, the energy efficiency is merely 4.4% [103]. Simultaneously, high-temperature and high-pressure conditions may induce catalyst deactivation through sintering. Photocatalysis also suffers from low energy efficiency issues. Electrocatalysis demonstrates higher energy efficiency and CO₂ conversion rates with relatively scalable reaction systems yet remains constrained by low product selectivity.

Plasma technology has garnered extensive attention due to its easy handling, mild operating conditions (ambient pressure and low temperature (<100 °C)), strong activation capability, and high product selectivity. In the field of CO₂ conversion, plasma technology has also demonstrated remarkable advantages [104]. In recent years, increasing numbers of researchers have devoted efforts to plasma-catalytic CO₂ conversion, with products mainly including CO, CH₄, CH₃OH, etc., among which CH₃OH, as an important organic chemical feedstock, has attracted particular attention.

Plasma possesses non-equilibrium characteristics, low power requirements, and the ability to induce physical/chemical reactions at relatively low temperatures, demonstrating potential to drive thermodynamically unfavorable chemical reactions [105]. Currently, numerous research teams have conducted extensive investigations on plasma technology applications in CO₂ conversion. Nunnally et al. [106] reported energy efficiency and conversion rate of CO₂ decomposition as 43% and 18%, respectively, suggesting that non-equilibrium CO₂ excitation and rapid quenching induced by high temperature gradients around gliding arc plasma contributed to these conversion levels. Tu et al. [107] reported that when using gliding arc plasma for CH₄ reforming reactions, energy efficiency can exceed 60% with conversion rates ranging from 8% to 16%. Experimental results demonstrated that the energy efficiency of gliding arc plasma in CH₄ reforming was superior to that of DBD or CD plasma, attributable to its higher electron density (approximately 1023 counts/m⁻³). Lu et al. [108] investigated the application of DBD plasma in CH₄ reforming under low-temperature atmospheric pressure conditions. Results showed that without catalyst packing at a reactor temperature of 600 °C, the system achieved 41% CH₄ conversion and 55% CO₂ conversion.

According to reaction thermodynamics, CO₂ hydrogenation to CH₃OH is an exothermic reaction, favored under lower temperatures and higher pressures [109]. However, lower temperatures face limitations from thermodynamic equilibrium in CO₂ activation processes. While elevated temperatures promote CO₂ activation, they lead to substantial CO formation, detrimental to CH₃OH production. Therefore, dielectric barrier discharge (DBD) plasma technology with non-thermal equilibrium characteristics demonstrates significant advantages for activating thermodynamically stable molecules like CO₂ at atmospheric pressure and low temperatures [101,110]. These ique plasma characteristics indicate substantial potential for overcoming barriers in catalytic CO₂ hydrogenation to CH₃OH. Furthermore, plasma processes enable rapid startup, energy savings, and fast reaction rates, providing an attractive approach for CH₃OH production under ambient temperature and pressure.

5.3. Plasma–Catalyst Synergy for CO₂ Hydrogenation

Compared to pure plasma systems, catalyst-packed plasma demonstrates higher efficiency in CO₂ conversion (As shown in Figure 9) [111,112,113]. In plasma-catalytic CO₂ hydrogenation to CH₃OH, the reaction zone contains abundant high-energy electrons that dissociate reactant molecules (CO₂ and H₂) through collisions, generating radicals like CO, O, and H. These active species interact with catalysts, adsorb onto catalyst surfaces, and undergo rapid recombination coupled with continuous hydrogenation to form CH₃OH molecules.

Plasma-catalysis synergy is an emerging discipline combining catalytic chemistry, plasma physics, and plasma chemistry, gaining increasing attention from researchers [114,115]. Plasma-catalysis synergy primarily involves plasma activation of reactive species and catalyst-mediated adsorption/transformation of active components. Under mild conditions, inert gases are excited to activated states by plasma, and the activated species undergo selective recombination on catalyst surfaces to yield desired products.

Compared to thermal catalysis, the primary advantage of plasma-assisted catalysis lies in its ability to operate at low temperatures, while the plasma itself acts as a source of heat, electrons, and free radicals. Since the dissociation probability of excited states is higher than that of ground states, radicals generated by the plasma can directly interact with adsorbed molecules via the Eley–Rideal mechanism to form the target product when reactants are adsorbed on the catalyst surface [116]. In each fundamental step, the plasma can function both as a heat source to create high-temperature conditions for electrons and as a supplier of active species for the catalytic reaction [117,118,119]. On the other hand, low-temperature plasma can also modulate the surface acidity/basicity of the support, enhance metal dispersion, and alter the microstructure at the active component-support interface, thereby influencing the intrinsic properties of the catalyst [120,121,122,123,124]. Moreover, the persistent bombardment of the catalyst surface by high-energy electrons and active species (radicals, excitons, ions, etc.) from the plasma can modify the surface properties of the catalyst.

Under ambient pressure and room temperature conditions, the synergistic effect between plasma and catalyst can efficiently facilitate the synthesis of methanol from CO₂ and H₂. Zhang et al. [125] investigated the catalytic effect by placing a catalyst-loaded tray downstream of the gliding arc plasma. Experimental results demonstrated that this catalyst placement method significantly enhanced CO₂ decomposition efficiency. Research indicated that the incorporation of TiO₂ substantially increased both CO₂ conversion rate and energy efficiency, with the mechanism being that high-energy electrons generated by the gliding arc plasma induced electron-hole pairs on the TiO₂ surface, thereby achieving plasma-TiO₂ synergistic catalysis.

5.3.1. Transition Metal Catalysts

The earliest research on plasma-catalytic CO₂ hydrogenation to CH₃OH was conducted by Eliasson et al. [126] in 1998. They performed experiments on direct CO₂ hydrogenation to CH₃OH by utilizing the synergy between plasma and a CuO-ZnO-Al₂O₃ catalyst, achieving a CO₂ conversion of 12.4% and a CH₃OH selectivity of 0.4%. By comparing the three types of experiments—catalyst only, plasma discharge only, and plasma catalysis—it was found that the optimal reaction temperature was 493 K without plasma, while the optimal temperature for plasma catalysis was 373 K, demonstrating that DBD plasma can effectively reduce the required reaction temperature.

Wang et al. [112] proposed a plasma-driven catalytic process for the highly selective synthesis of methanol via CO₂ hydrogenation at room temperature and atmospheric pressure. Using a unique water electrode in a DBD reactor, they investigated the effects of the H₂/CO₂ feed ratio and the catalyst’s role in the plasma hydrogenation process. Moreover, a comparative analysis of CO₂ conversion and CH₃OH yield over Cu/γ-Al₂O₃ and Pt/γ-Al₂O₃ catalysts was conducted using the water-electrode DBD reactor. The study found that Cu/γ-Al₂O₃ exhibited superior catalytic activity, achieving a maximum CO₂ conversion to methanol of 11.3% and a methanol selectivity of 53.7%. These values represent the highest reported methanol yield and selectivity for the plasma-catalytic CO₂ hydrogenation process to date. Furthermore, they investigated the effects of three different reactor configurations on the plasma-catalytic hydrogenation of CO₂ to CH₃OH. The results indicated that CH₃OH production is closely correlated with the structure of the DBD plasma reactor. Simultaneously, the energy efficiency for CH₃OH generation was also found to be strongly dependent on both the DBD reactor structure and the catalyst composition. The combination of the water-electrode DBD reactor with the Cu/γ-Al₂O₃ catalyst exhibited the highest energy efficiency (306 mmol/kWh), approximately 84 times greater than that of the aluminum foil reactor (3.6 mmol/kWh).

Joshi and Loganathan [127] achieved CO₂ hydrogenation to CH₃OH by combining plasma and thermal catalysis technologies. They used quartz wool as a support and compared the effects of CuO/QW, NiO/QW, Fe₂O₃/QW, NiO/Fe₂O₃/QW, and CuO/Fe₂O₃/QW on the performance of CO₂ hydrogenation to CH₃OH, finding that 5 wt.% CuO/Fe₂O₃/QW exhibited superior CO₂ conversion (16.7%) and CH₃OH selectivity (32.7%). Compared to the thermal catalytic process under identical temperature conditions, the approach combining plasma and thermal catalysis increased the CO₂ conversion rate by 8-fold. The remarkable enhancement can be attributed to the lowered catalyst reduction temperature under the plasma-induced reductive atmosphere and the high activity of Fe₂O₃ toward the RWGS reaction [128]. Furthermore, they investigated the CO₂ hydrogenation to CH₃OH reaction using Ni as the active center. The results showed that when NiO/QW was used as the catalyst, the main products were CO and CH₄, with very low selectivity toward CH₃OH. However, when Fe₂O₃ was incorporated to form NiO/Fe₂O₃/QW, the CH₃OH selectivity increased significantly. Through catalyst characterization, they identified that the active sites for CH₃OH formation are the interaction between mixed NiO and Fe₂O₃ formed in situ, and the NiFe₂O₄ spinel possessing hydrogenation and RWGS capabilities [128,129]. This study provides a novel strategy for achieving superior performance in CO₂ hydrogenation to CH₃OH by integrating plasma technology with thermal catalysis.

Although plasma catalysis exhibits excellent catalytic performance, the reactive species generated by the plasma have very short lifetimes and may undergo ineffective collisions and be converted into byproducts before contacting the catalyst; therefore, enhancing the CO₂ adsorption capacity of the catalyst surface is crucial. Based on this, Zhang et al. [130] developed β-Mo₂C as a catalyst for plasma synergistic catalysis; this catalyst exhibits a one-dimensional rod-like microstructure, composed of densely packed tiny crystals. The one-dimensional structure of this β-Mo₂C material not only helps to increase the specific surface area but also facilitates the occurrence of catalyst surface discharge under plasma conditions, enhancing the local electric field at the material surface during this process. Under ambient temperature and pressure conditions, efficient activation of CO₂ and its selective conversion to CO were achieved. This demonstrates a strong synergistic effect between the plasma and the one-dimensional material, effectively enhancing the energy efficiency of the plasma synergistic catalytic system.

5.3.2. Noble Metal Catalysts

Men et al. [131] prepared a highly dispersed Pt/film/In₂O₃ catalyst, which, combined with plasma technology, achieved 37% CO₂ conversion and 62.6% CH₃OH selectivity under ambient pressure conditions. This performance is superior to that of Pt/In₂O₃ prepared by conventional H₂ reduction (24.9% and 36.5%) and to that of the commercial Cu/ZnO/Al₂O₃ catalyst (25.6% and 35.1%). The reasons for the high CH₃OH selectivity are mainly as follows: (1) high dispersion of Pt nanoparticles; (2) the film effectively suppresses the aggregation of Pt nanoparticles and facilitates electron transfer from the catalyst to CO₂; (3) FT-IR studies revealed that Pt/film/In₂O₃ adsorbs CO₂ and activates it to CH₃O*, which is a key intermediate for hydrogenating CO₂ to CH₃OH; (4) the use of the film as a support enhances the catalyst’s CO₂ adsorption capacity, contributing to the increased CO₂ conversion and CH₃OH selectivity. Similarly, Liao et al. [132] established a two-dimensional axisymmetric fluid model and conducted an in-depth theoretical investigation into the reaction mechanism of DBD plasma-catalytic CO₂ hydrogenation at atmospheric pressure, finding that the hydrogenation of the intermediate species CH₃O* is the primary pathway for CH₃OH production in the plasma-catalytic CO₂ hydrogenation to CH₃OH.

5.3.3. Other Catalysts

Since plasma excitation generates substantial ultraviolet light, many relevant studies have overlooked the impact of UV radiation on reaction activity in plasma-catalytic processes. Therefore, Lu et al. [133] proposed integrating photocatalysts into plasma-catalytic systems, preparing K-intercalated g-C₃N₄ supported on γ-Al₂O₃ using copper as the high-voltage electrode. They investigated the catalytic performance of this material in CO₂ decomposition using DBD plasma. Initially, they examined temperature effects on the catalytic reaction, discovering that reducing the temperature from 120 °C to 50 °C enhanced both CO₂ conversion and energy efficiency by approximately 50%. Compared with the pure γ-Al₂O₃ catalyst, the CO₂ conversion, CO yield, and energy efficiency increased from 11.1%, 10.0%, and 9.41% to 19.3%, 18.1%, and 20.6%, respectively. Oscilloscope measurements revealed that catalyst packing transformed the discharge mode into partial surface discharge, thereby enhancing reaction activity. The catalyst’s morphology and structure were characterized using SEM, XRD, FT-IR, and XPS, confirming the maintained porous structure and uniform distribution of K elements between g-C₃N₄ layers.

6. Conclusions and Future Perspectives

This article systematically reviews the research progress on catalyst systems and various catalytic approaches for CO₂ hydrogenation to methanol. Regarding catalysts, copper-based catalysts are widely used due to their low cost and relatively high selectivity, but their activity and stability still require further improvement; Noble metal catalysts (e.g., Pd, Au) exhibit excellent performance under mild conditions, but their application is constrained by high costs; Bimetallic alloys, intermetallic compounds, and novel oxide catalysts (e.g., In₂O₃/ZrO₂, ZnO-ZrO₂) have significantly enhanced catalytic performance through synergistic effects. Furthermore, metal–organic frameworks (MOFs), by virtue of their high specific surface area, tunable pore structures, and excellent stability, serve as ideal catalyst supports or precursors, enabling highly active and selective methanol synthesis through the encapsulation of active components (e.g., Cu/ZnO). In terms of catalytic methods, thermal catalysis is mature but energy-intensive, photocatalysis operates under mild conditions but with low efficiency, electrocatalysis offers controllable reactions but poor product selectivity, while plasma catalysis demonstrates unique advantages due to its efficient activation capability at low temperatures and atmospheric pressure, particularly when synergistically combined with catalysts, significantly enhancing CO₂ conversion and methanol selectivity.

Currently, the primary challenges in CO₂ hydrogenation to methanol lie in overcoming the limitations imposed by harsh reaction conditions such as high temperature, high pressure, and high energy consumption, improving product selectivity and catalytic activity, and addressing the issue of catalyst deactivation. By utilizing low-temperature plasma technology to generate abundant high-energy electrons and reactive species under ambient temperature and pressure to drive chemical reactions, while simultaneously modulating the intrinsic coordination environment of the catalyst, efficient CO₂ hydrogenation to methanol can be achieved [112]. Concurrently, the continuous bombardment of the surface by energetic particles within the plasma can be mitigated by integrating the catalyst with the plasma, thereby reducing coke formation and catalyst poisoning. However, the catalytic reaction for CO₂ hydrogenation to methanol under plasma conditions still faces the challenge of poor product selectivity. To reduce catalytic costs and enhance catalyst stability and selectivity, it is necessary, on one hand, to refine the research on plasma discharge characteristics and the mechanism by which they influence catalyst properties during synergistic catalysis. On the other hand, research and development on single-atom catalysts should be pursued to minimize the size of active metal species and thereby improve catalytic selectivity [134]. Currently, most catalysts employed for CO₂ hydrogenation to methanol utilize noble metals (Pt, Rh, Ru, Ir, Au, and Pd) as the active metal components [135,136,137,138,139,140]. However, their scarcity and high cost hinder large-scale application. Research has revealed that non-precious metals such as Cu, Fe, Co, Ni, and Mn, when used as active components in catalysts, also exhibit considerable activity for catalytic hydrogenation reactions [141,142]. Consequently, research in this field is increasingly shifting towards non-precious metal catalysts. Therefore, investigating the synergistic effects of low-temperature plasma coupled with catalysts, and developing cost-effective, highly stable, and selective non-noble metal catalysts with high dispersion represent a key research focus for future plasma-assisted CO₂ hydrogenation to methanol.

Future research should focus on developing low-cost catalysts that exhibit high activity, high selectivity, and good stability. Concurrently, further exploration of the synergistic mechanisms between plasma and catalysts, along with optimization of reactor design and process parameters, is essential to provide novel technological pathways for carbon resource utilization under carbon neutrality goals.