Hydrogenation of Carbon Dioxide to Value-Added Chemicals by Heterogeneous Catalysis and Plasma Catalysis

Abstract

Due to the increasing emission of carbon dioxide (CO₂), greenhouse effects are becoming more and more severe, causing global climate change. The conversion and utilization of CO₂ is one of the possible solutions to reduce CO₂ concentrations. This can be accomplished, among other methods, by direct hydrogenation of CO₂, producing value-added products. In this review, the progress of mainly the last five years in direct hydrogenation of CO₂ to value-added chemicals (e.g., CO, CH₄, CH₃OH, DME, olefins, and higher hydrocarbons) by heterogeneous catalysis and plasma catalysis is summarized, and research priorities for CO₂ hydrogenation are proposed.

1. Introduction

Climate changes are mostly induced by the greenhouse effect, and carbon dioxide (CO₂) accounts for a dominant proportion of this greenhouse effect. Indeed, one can barely ignore the connection between the emission of CO₂ and climate changes [1]. The CO₂ concentration in the atmosphere has actually climbed to 405 ppm in 2017, as shown in Figure 1a. As the global energy consumption is still mainly based on burning coal, oil, and natural gas, and this situation will last until the middle of the century (see Figure 1b), experts predict that the CO₂ concentration in the atmosphere will continue to rise to ~570 ppm by the end of the century if no measures are taken [2]. Hence, it is urgent to control the CO₂ emissions by taking effective measures to capture and utilize CO₂.

In principle, there are three strategies to reduce CO₂ emissions, i.e., reducing the amount of CO₂ produced, storage of CO₂, and utilization of CO₂ [3]. Recently, some comprehensive reviews have discussed the technological state-of-the-art of carbon capture and storage (CCS) [4,5,6]. Direct air capture technology (DAC) draws people’s attention to mitigate climate change by taking advantage of chemical sorbents (e.g., basic solvents, supported amine and ammonium materials, etc.) [4]. In addition, Bui et al. also considered the economic and political obstacles in terms of the large-scale deployment of CCS [6]. Significantly reducing the amount of CO₂ produced is unrealistic in view of the current energy structure dominated by fossil energies, as shown in Figure 1b. CO₂ storage seems to be a potential approach, but there are some challenges, such as efficiency of capture and sequestration of CO₂, cutting down the operation costs for capture and separation of CO₂, and the long-term stability of underground storage [7,8]. Utilization of CO₂ is a promising approach, since CO₂ is a cheap and attractive carbon source, which can be used to yield a variety of industrial raw materials, which can be further converted into value-added chemicals and fuels. Interestingly, CO₂ can also be used as a desired trigger for stimuli-responsive materials. Darabi et al. summarized the synthesis, self-assembly, and applications of CO₂-responsive polymeric materials [9].

One of the options to convert CO₂, is catalytic hydrogenation, as illustrated in Figure 2. The production of H₂, however, is also a vital problem to realize direct hydrogenation of CO₂ to oxygenates and hydrocarbons. Hydrogen production can be either based on renewable or non-renewable sources, such as electrical, thermal, photonic, and hybrid [10,11]. The main methods of hydrogen production include electrolysis, thermolysis, photo-electrolysis, and hybrid thermochemical cycles [10]. According to some evaluation criteria—such as global warming potential (GWP), social cost of carbon (SCC), acidification potential (AP), energy and exergy efficiencies, and production cost—the hybrid hydrogen production methods seems a promising route. On the other hand, the production cost evaluation shows that coal gasification ($0.92/kg H₂) and fossil fuel reforming ($0.75/kg H₂) are relatively low-cost methods compared to early R&D phase methods (e.g., photo-electrochemical: $10.36/kg H₂ from water dissociation). However, fossil fuel is non-renewable, being a major drawback. Therefore, reducing the cost of hydrogen production is an urgent and challenging issue, since we have to balance both the economy and sustainability.

Various methods have been adopted—including photo-catalysis, electro-catalysis, heterogeneous catalysis, and plasma catalysis [12,13]—to realize hydrogenation of CO₂. Dalle et al. systematically presented the activity of the first-row transition metal complexes (i.e., Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) in CO₂ reduction by electro-catalysis, photo-catalysis and photoelectron-catalysis [14]. Li et al. discussed the important roles of co-catalysts (e.g., biomimetic, metal-based, metal-free, and multifunctional) in selective photo-catalytic CO₂ reduction [15]. Besides thermochemical approaches, Mota et al. made a detailed retrospective based on electrochemical and photo-chemical approaches for CO₂ hydrogenation to oxygenates and hydrocarbons [16]. In this review, we focus on the latter two methods. Heterogeneous catalysis has been widely studied, while plasma catalysis is still an emerging technology. Some excellent reviews were recently published for heterogeneous catalytic CO₂ hydrogenation [12,17,18]. Jadhav et al. focused on methanol production [17], Porosoff et al. on the synthesis of CO, CH₃OH, and hydrocarbons [18], while Alvarez et al. discussed the greener preparation of formates (formic acid), CH₃OH, and DME [12]. With regard to plasma catalysis for CO₂ hydrogenation, there are only a handful reports (as discussed below). Nevertheless, it exhibits great potential since plasma can operate at ambient temperature and atmospheric pressure.

In this review, we summarize the progress of mainly the last five years in CO₂ hydrogenation to value-added chemicals (e.g., CO, CH₄, CH₃OH, DME, olefins, and higher hydrocarbons) driven by both heterogeneous catalysis and plasma catalysis. The literature bibliography range is shown in Figure 3. Indeed, on the one hand, the insights obtained by heterogeneous catalysis can be useful for the further development of the emerging field of plasma catalysis. On the other hand, we also want to pinpoint the differences between heterogeneous and plasma catalysis, and thus the need for dedicated design of catalytic system tailored to the plasma environment.

2. Heterogeneous Catalysis

In heterogeneous catalysis, support materials, active metals, promoters, and the preparation methods of catalysts are major adjustable factors determining the catalytic activity. As far as the support materials are concerned, the catalytic performance is influenced by the metal-support interaction since it usually induces specific physicochemical properties for catalysis, e.g., active cluster, active oxygen vacancy, and acid-based property. Metal oxides and zeolites with special channel structures are usually selected. For the active metal components, research focuses on searching cheap and available metals to replace precious metals or to reduce the amount of precious metals in industrial heterogeneous catalysis. Promoters (structural-type and electron-type) can also significantly influence the catalytic activity by regulating the adsorption and desorption behavior of molecules (reactant, intermediate, and product) on the catalyst surface. Preparation methods usually determine the catalyst morphology including metal dispersion (particle size distribution), specific surface area, and channel structure, which also influences the catalytic performance [12]. In this section, we summarize recent progress of CO₂ hydrogenation to CO, CH₄, CH₃OH, and some other products in terms of rational design of heterogeneous catalysts.

2.1. CO₂ to CO

CO can be used as feedstock to produce liquid fuels and useful chemicals by Fischer–Tropsch (F–T) synthesis reaction. Therefore, the catalytic hydrogenation of CO₂ to CO via the reverse water–gas shift (RWGS) reaction, reaction (1), is promising to account for the shortage of energy. Accordingly, catalyst design for the RWGS reaction have attracted extensive attention.

CO₂ + H₂ ↔ CO + H₂O, △H_298K = 41.2 kJ mol⁻¹

(1)

Generally, noble metal catalysts (e.g., Pt, Ru, and Rh) exhibit effective ability towards H₂ dissociation, and thus precious metals have been investigated extensively for the RWGS reaction. However, these catalysts yield, methane as dominant product, mainly caused by the higher rate of C-H bond formation than CO desorption [19]. To change the product distribution, Bando et al. applied Li-promoted Rh ion-exchanged zeolites for CO₂ hydrogenation, achieving 87% CO selectivity for an atomic ratio of Li/Rh higher than 10/1 [19]. Although the catalytic performance of noble metals can be improved via promoters, the high price and instability of noble metals (aggregation of nano-particles) limit the industrial applicability. In order to properly decrease the operation cost and improve the life cycle of catalysts, non-noble metal carbides have been developed. Porosoff et al. synthesized a molybdenum carbide (Mo₂C) catalyst for the RWGS reaction [20], yielding 8.7% CO₂ conversion and 93.9% CO selectivity (at 573 K reaction temperature). The catalytic performance was much better than those of bimetallic catalysts (e.g., Pt-Co, Pt-Ni, Pd-Co, Pd-Ni) supported on CeO₂ (~5% CO₂ conversion and 83.3% CO selectivity at the same reaction temperature). The catalytic mechanism of Mo₂C in the RWGS reaction was further studied by ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and in situ X-ray absorption near edge spectroscopy (XANES), which demonstrate that Mo₂C not only broke the C=O bond, but also dissociated hydrogen. Hence, Mo₂C has a dual functional and is an ideal catalytic material for the RWGS reaction. Interestingly, the authors also found that by modifying the catalyst with Co, the catalytic activity of the Co-Mo₂C catalyst was further improved (9.5% CO₂ conversion, 99% CO selectivity at 573 K), probably attributed to the existence of the CoMoC_yO_z phase confirmed by X-ray Diffraction (XRD) (see Figure 4).

The support plays an important role in CO₂ hydrogenation to CO through the RWGS reaction. Kattel et al. prepared Pt, Pt/SiO₂ and Pt/TiO₂ materials as catalysts for the RWGS reaction [21], and showed that Pt nanoparticles alone cannot catalyze the RWGS reaction, while using SiO₂ and TiO₂ as support, the overall CO₂ conversion can be significantly improved, pointing towards a synergy effect between Pt and the oxide support. To reveal the synergy effect, they combined density functional theory (DFT), kinetic Monte Carlo (KMC) simulations and other experimental measurements. They found that the hydrogenation of CO₂ to CO is promoted once CO₂ is stabilized by the Pt-oxide interface. In the case of Pt nanoparticles (NP) alone, the conversion was close to 0, since the ability of Pt NP in binding CO₂ is weak. When SiO₂ and defected TiO₂ with oxygen vacancies served as supports, however, the CO₂ conversion was enhanced (to 3.35% and 4.51%, respectively) on the Pt-oxide interface. The synergy effect between Pt and oxide supports in activating and hydrogenating CO₂ is shown in Figure 5, and possible reaction pathway [21] for the RWGS reaction is illustrated in Scheme 1.

Yan et al. investigated the effect of the Ru-Al₂O₃ interfaces on the catalytic activity of the RWGS reaction, and proposed Ru₃₅/Al₂O₃ and Ru₉/Al₂O₃ catalyst models to explain the experimental observations [22]. The product selectivity switched between CH₄ and CO over the Ru/Al₂O₃ catalyst, i.e., monolayer Ru sites favored the production of CO, while Ru nano-clusters preferred the production of CH₄, during CO₂ reduction reaction. Confirmed by kinetic analysis, characterization of the surface structures and real-time monitoring of the active intermediate species, the product selectivity of CH₄ and CO was regulated by the Ru sites and Ru-Al₂O₃ interfacial sites. Furthermore, based on the combination of theoretical calculations and isotope-exchange experimental results, the authors found that the O* species derived from the dissociative adsorption of CO₂ at interfacial Ru sites easily bridge with the Al sites from the γ-Al₂O₃ support, and new Ru-O-Al bonds are formed via the oxygen-exchange process. The interfacial O species existing in Ru-O-Al bonds was responsible for the CO₂ activation via oxygen-exchange with the O atoms of CO₂. Therefore, this experimental work is a good inspiration to further explore the influence of metal-support interfaces for the effective activation of CO₂.

Besides the widely used impregnation method, Yan et al. also reported a doping-segregation method for the preparation of Rh-doped SrTiO₃ [23]. Precursors with a molar ratio of Sr:Ti:Rh = 1.10:0.98:0.02. First, TiO₂ was suspended in deionized water, and then Sr(OH)₂·8H₂O and Rh(NO₃)₃ were introduced. Subsequently, the sample was poured in a stain steel acid digestion vessel, which was kept at 473 K for 24–48 h. Finally, the reaction product was dried at 343 K overnight after it was centrifuged, and washed via deionized water. As confirmed by in situ X-Ray-Diffraction (XRD) and X-ray Absorption Fine Structure (XAFS) measurements, the Rh-doped SrTiO₃ catalysts produce sub-nanometer Rh clusters, which are highly active for the conversion of CO₂ compared to the supported Rh/SrTiO₃ prepared by wetness impregnation. The better catalytic performance (7.9% CO₂ conversion and 95% CO selectivity at 573 K) could be ascribed to the cooperative effect between sub-nanometer Rh clusters and the reconstructed SrTiO₃ which is active for dissociation of H₂ and is favorable for adsorption/activation of CO₂. Therefore, the novel approach, doping-segregation method, maybe a novel strategy to tune the size of active metals and the physicochemical properties of supports for rational design of catalysts for the RWGS reaction.

Dai et al. studied CeO₂ catalysts which were prepared by the hard-template method (Ce-HT), the typical complex method (Ce-CA), and the typical precipitation method (Ce-PC) for the RWGS reaction [24]. The experimental results show that catalysts prepared by Ce-CA, Ce-HT, and Ce-PC methods exhibit a 100% CO selectivity and the CO₂ conversions were 9.3, 15.9, and 12.7% respectively at 853 K. Obviously, the hard-template (Ce-HT) method is beneficial for the RWGS reaction in view of the catalytic performance. The Ce-HT method comprises the following steps: (1) Ce(NO₃)₃·6H₂O was dissolved in ethanol, and KIT-6 mesoporous silica was added; (2) The mixture was stirred until a dry power was obtained; (3) The powder was calcined; (4) The obtained samples were treated with NaOH to remove the template. To reveal the relationship between the preparation method and the catalytic activity, the authors carried out XRD, Transmission Electron Microscopy (TEM) and Brunauer–Emmett–Teller (BET) characterization, and the characterization results show that CeO₂ catalysts which were prepared by the Ce-HT method have a porous structure and a high specific surface area, while CeO₂ catalysts prepared by the other methods (Ce-CA, Ce-PC) have an agglomerated structure (Ce-CA) and overlapped bulk structure (Ce-PC) with low porosity. Moreover, in the CeO₂ catalysts, oxygen vacancies as active sites were formed by H₂ reduction at 673 K, which were confirmed by in situ X-ray photoelectron spectroscopy (XPS) and H₂-temperature-programmed reduction (H₂-TPR). Therefore, the improvement of the catalytic activity for the RWGS reaction can be ascribed to the change of catalyst structure and oxygen vacancies.

Except for the above monometallic catalysts, some bimetallic catalysts—i.e., Pt-Co, Fe-Mo and Ni-Mo [25,26,27]—were also prepared and tested in the RWGS reaction. In general, the preparation method of bimetallic catalysts generally can be divided into two categories: (1) the bi-metal was first prepared and then supported on the carrier [25]; (2) the bi-metal as formed in the preparation progress [26,27]. Compared with pure Co catalyst, Pt-Co catalyst showed a better catalytic activity (mainly CO, close to 100%) for the RWGS reaction. Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and environmental transmission electron microscopy (ETEM) showed that Pt migrates on the catalyst surface, and Pt aids the reduction of Co to its metallic state under appropriate reaction conditions confirmed by near edge X-ray absorption fine structure (NEXAFS) spectroscopy [25]. Abolfazl et al. investigated Mo/Al₂O₃ and Ni–Mo/Al₂O₃ catalysts prepared by the impregnation method used for the RWGS reaction. The experimental results showed that Ni–Mo/Al₂O₃ catalyst is a promising catalyst (35% CO₂ conversion, i.e., close to the 38% equilibrium conversion) compared with Mo/Al₂O₃ catalyst (15% CO₂ conversion). As demonstrated by XPS, the electronic effect, which transfers electrons from Ni to Mo and leads to an electron-deficient state of the Ni species, is beneficial for CO₂ adsorption, and thus improves the CO₂ conversion. Furthermore, the XRD and H₂-TPR profiles indicated that the Ni–O–Mo structure crystallizes into the NiMoO₄ phase which can improves the adsorption and dissociation of H₂ on the Ni-Mo/Al₂O₃ catalyst [27]. Similarly, they also found that Fe-Mo/Al₂O₃ catalyst synthesized by the impregnation method is an efficient catalyst with high CO yield, almost no by-products and relatively stable (60 h) for the RWGS reaction. The enhancement of the catalytic activity may be attributed to better Fe dispersion with the addition of Mo and smaller particle size of active Fe species, which was confirmed by the BET method and scanning electron microscopy (SEM) [26].

Overall, to some extent, the reaction activity and stability of bimetallic catalysts for the direct hydrogenation of CO₂ to CO are excellent compared to mono-metallic catalysts. The reduction of the active metal, the formation of alloy metal, the dispersion and particle size of the catalyst are possible factors for the improvement of the catalytic performance in the RWGS reaction. Details of the conversion and product selectivity, along with the reaction conditions of several representative RWGS catalytic systems, are compared in

Table 1. Catalytic performance of several catalytic systems for CO₂ hydrogenation into CO, in terms of CO₂ conversion and CO selectivity, along with the reaction conditions

Catalyst	H2:CO2	GHSV	Temperature (°C)	Pressure (MPa)	CO2 Conversion (%)	CO Selectivity (%)
Mo2C [20]	3	36,000 a	300	0.1	8.7	93.9
Co-Mo2C [20]	3	36,000 a	300	0.1	9.5	~99.0
Pt-TiO2 [21]	1	119.7 b	300	0.1	4.5	99.1
Pt-SiO2 [21]	1	24.7 b	300	0.1	3.3	100
Rh-SrTiO3 [23]	1	12,000 a	300	N/A	7.9	95.4
Co-MCF-17 [25]	3	60,000 b	200-300	0.5	~5.0	~90.0
Pt-Co-MCF-17 [25]	3	60,000 b	200-300	0.5	~5.0	~99.0
Fe-Mo-Al2O3 [26]	1	30,000 a	600	1	~45.0	~100.0
Mo-Al2O3 [27]	1	30,000 a	600	0.1	16	N/A
Ni-Mo-Al2O3 [27]	1	30,000 a	600	0.1	34	N/A
La-Fe-Ni [28]	2	24,000 a	350	N/A	16.3	96.6

^a mL g_cat⁻¹ h⁻¹; ^b h⁻¹; N/A: not available.

. Obviously, precious metal catalysts (e.g., Pt, Rh, Ni) are still advantageous for the formation of CO compared to non-noble metal catalysts (e.g., Fe, Co, Mo), and upon increasing the gas hourly space velocity (GHSV), the selectivity of CO slightly decreases to some extent.

2.2. CO₂ to CH₄

Methane, a high value carbon source, is used to produce syngas via steam reforming, and subsequently the syngas is usually converted into chemicals and/or fuels through F–T synthesis. The direct hydrogenation of CO₂ to CH₄ (also called CO₂ methanation), reaction (2), is a feasible approach, if the production technology of H₂ becomes widespread and at low-cost.

CO₂ + 4H₂ ↔ CH₄ + 2H₂O, △H_298K = −252.9 kJ mol⁻¹

(2)

Ni-based [29], Co-based [30], and Ru-based [31] catalysts have been used in CO₂ methanation, and Ni-based catalysts are considered to be the most effective and the lowest cost alternative. However, coke formation is a serious problem for Ni-based catalytic systems [2]. Therefore, researchers are trying to seek appropriate promoters to improve the activity and stability of Ni-based catalytic systems for CO₂ methanation. Yuan et al. investigated the effect of Re on the catalytic activity of Ni-based catalysts in CO₂ methanation [32]. Based on DFT calculations, they found that, attributed to the strong affinity of Re to O (see Figure 6), the presence of Re markedly lowered the energy barrier of C-O bond cleavage, which benefits the activation of CO₂. Moreover, CH₄ selectivity can be enhanced owe to the presence of Re, which was confirmed by analysis of surface coverage of the adsorbed species on Ni (111) and Re@Ni (111). In addition, micro-kinetic analysis showed that, in addition to CO* and H*, a suitable amount of O_ad atoms were present on Re@Ni (111), and thus a possible reaction network of CO₂ methanation was proposed as shown in Scheme 2, in which a red line represents the preferable steps in each pathway.

Besides Re, La is also an excellent promoter of Ni-based catalysts for CO₂ methanation. Quindimil et al. [29] applied Ni-La₂O₃/Na-BETA catalysts for CO₂ methanation. They found that the presence of La₂O₃ created more CO₂ adsorption sites and more hydrogenation sites, mainly attributed to the improved surface basicity and Ni dispersion by the co-catalyst effect of La. Under the optimized reaction conditions, 65% CO₂ conversion and nearly 100% CH₄ selectivity were achieved over a Ni-10%La₂O₃/Na-BETA catalyst with a good stability for more than 24 h at 593 K.

CeO₂, TiO₂, and SiO₂ have been used as the supports of methanation catalysts [33]. Reactions over Ni/CeO₂ catalyst performed full selectivity to CH₄ with higher TOF (up to forty-fold) compared to TiO₂ and SiO₂ supported Ni nanoparticles, and in CO₂ methanation, the catalytic stability of Ni/CeO₂ catalyst lasted for 50 h at 523 K. HRTEM analysis indicated that different supports induced distinctive crystal structure. Ni/CeO₂ catalyst presented hexagonal Ni nanocrystallites, while TiO₂ and SiO₂ favored the formation of pseudo-spherical Ni nanoparticles. Demonstrated by TPR, XPS, and UV Raman analysis, characterization results revealed partial reduction of the CeO₂ surface, and the partial reduction of the CeO₂ surface contributed to the generation of oxygen vacancies, which is beneficial for the formation of a strong metal-support interaction (SMSI) between Ni and CeO₂, while no SMSI was observed over Ni/SiO₂ and Ni-TiO₂ catalyst. Furthermore, pulse reaction by temporal analysis products (TAP) demonstrated the capacity of CO₂ adsorption following the order: Ni/SiO₂ < Ni/TiO₂ < Ni/CeO₂. Therefore, metal particle morphology and surface oxygen vacancies were used to anchor/stabilize Ni nanoparticles, and SMSI of Ni/CeO₂ catalyst contributed to the remarkable catalytic activity for CO₂ methanation.

Lin et al. investigated the influence of TiO₂ phase structure on the degree of dispersion of Ru nanoparticles [31]. Experiments showed that Ru/r-TiO₂ (rutile-type TiO₂) catalysts have a fast rate of CO₂ conversion, more than twice as fast as Ru/a-TiO₂ (anatase-type TiO₂) for CO₂ methanation. Meanwhile, compared to Ru/a-TiO₂ catalysts, Ru/r-TiO₂ catalysts exhibited a much higher thermal stability. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images showed that r-TiO₂ supported Ru nanoparticles displayed a narrower particle size distribution (1.1 ± 0.2 nm) compared with a-TiO₂ supported Ru nanoparticles (4.0 ± 2.4 nm). As confirmed by XRD measurements and H₂-TPR experiments, a strong interaction existed in RuO₂ and r-TiO₂ contributed to the formation of the Ru–O–Ti bond. Experimental tests, revealed that the strong interaction between RuO₂ and r-TiO₂, not only promotes the highly dispersion of Ru nanoparticles, but also prevents nanoparticles’ aggregation, which is responsible for the enhancement of the catalytic activity and thermal stability.

Furthermore, the influence of Al₂O₃, ZrO₂, SiO₂, KIT-6, and GO supports on Ni-based catalysts [30,34,35] has also been reported recently (

Table 2. Catalytic performance of several catalysts for CO₂ methanation, in terms of CO₂ conversion and CH₄ selectivity, along with the reaction conditions

Catalyst	H2:CO2	GHSV	Temperature (°C)	CO2 Conversion (%)	CH4 Selectivity (%)
Ni-La/Na-BETA [29]	4	10,000 b	350	65	100
Co/meso-SiO2 [30]	4.6	60,000 a	280	40	94.1
Co/KIT-6 [30]	4.6	22,000 a	280	48.9	100
Ru/BF4/SiO2 [30]	4	2400 b	250	70.5	N/A
Re-Ni(111) [32]	4	N/A	250	N/A	100
Ni/Al2O3-ZrO2 [34]	4	40,000 a	400	~70.0	N/A
Ni-SiO2/GO [35]	4	500 b	470	54.3	88
Ni-ZrO2 [36]	4	75 a	350	~40.0	~95.0
Ni-Ce/USY [37]	4	N/A	350	65	95
Co/KIT-6 [38]	4	60,000 a	340	40	86.7
Co/SiO2 [39]	4	60,000 a	360	44.3	86.5
Ni-Nb2O5 [40]	4	750 b	325	81	~99.0

^a mL g_cat⁻¹ h⁻¹; ^b h⁻¹; N/A: not available; pressure: 0.1 MPa.

). The specific area of support, the metal-support interaction and particle size of active sites are still mainly adjustable factors. Interestingly, Ni-SiO₂/GO-Ni-foam catalyst which was synthesized via intercalation of graphene oxide (GO) exhibited excellent activity compared with Ni-SiO₂/Ni-foam catalyst (see Figure 7) at 743 K for CO₂ methanation. Furthermore, the formation of nickel silicates on GO is responsible for the uniform dispersion of Ni active sites, which is favorable for inhibition of catalyst sintering.

Besides the traditional methods (co-impregnation method and deposition–precipitation method), some novel preparing methods, such as the sequential impregnation and a change of the preparation progress (e.g., the metal incorporation order, the reduction temperature, and the selection of different types of precursors), have been recently reported for preparing methanation catalysts.

Romero-Sáez et al. synthesized Ni-ZrO₂ catalysts supported on CNTs via the sequential and co-impregnation methods for CO₂ methanation [36]. The catalyst prepared by co-impregnation was apparently less active and selective to CH₄, compared with the catalyst synthesized by the sequential impregnation method. The preparation approach of the sequential impregnation method is as follows: (1) the appropriate amount of ZrO(NO₃)₂ xH₂O was dissolved in acetone; (2) the CNTs were added to the solution; (3) removal of the solvent, drying and heat treatment at 623 K; (4) in a rotatory evaporator, the same procedure for Ni(NO₃)₂ impregnation, followed drying and heat treatment was applied. As characterized by TEM analysis, NiO nanoparticles surrounded by ZrO₂ in core–shell structures were formed via co-impregnation method.The existence of core–shell structure reduced reactant access to Ni and Ni–ZrO₂ interface. However, when the catalyst was prepared via a sequential impregnation method, NiO nanoparticles were available and deposited either on the surface or next to the ZrO₂ nanoparticles, which improved the extent of the Ni–ZrO₂ interface. The ratio of Ni–O–Zr exposed species thus increases in the sequential impregnation method, and the interaction between H atoms (produced upon H₂ dissociation on the Ni surface) and the CO₂ molecule (activated by ZrO₂), can also be enhanced. The schematic representation is shown in Figure 8.

Interestingly, Bacariza et al. investigated the influence of the metal incorporation order in the preparation of Ni-Ce/Y(USY) zeolite catalysts [37]. In their experiments, three ways were used for the preparation of catalysts: Ni before Ce (Ce/Ni), Ce before Ni (Ni/Ce), and co-impregnation (Ni-Ce). Experimental tests showed that the catalytic activity follows the order: Ce/Ni ≈ Ni/Ce < Ni-Ce. TEM and H-TPR characterizations demonstrated that the Ni⁰ average size decreases to approximately 2.5 nm, and in terms of Ni/Ce or Ni-Ce catalysts, stronger interactions between Ni and Ce species are established. However, the CO₂ adsorption capacity is smaller for Ni/Ce catalyst. In contrast, even though larger Ni⁰ particles (13.3 nm) are formed for Ce/Ni catalyst, CO₂ adsorption capacity can be enhanced. Finally, Ce-Ni was found as the best preparation method for CO₂ methanation using Ni-based catalysts supported on CeO₂.

In addition, it has been reported that the reduction temperature and a variety of different precursors (e.g., nitrate, chlorate, and oxalate) affect the number of active centers for CO₂ methanation [30,38]. As discussed above, the preparation methods showed beneficial influences on the catalytic performance, suggesting that reasonable adjustment of the preparation process is a strategy to optimize the experiments for the conversion of CO₂ to CH₄. Details of CO₂ conversion and product selectivity, along with the reaction conditions of several representative catalytic systems, are compared in Table 2. From Table 2, we can see that Ni-based catalysts are the main catalytic systems for the conversion of CO₂ to CH₄. Furthermore, the optimal temperature for CH₄ production is 300–400 °C.

2.3. CO₂ to CH₃OH

Methanol is not only an important industrial raw material, but also a stable hydrogen storage compound, which is convenient for transport and reserve. Recently, the concept of “methanol economy” advocated by the Nobel Laureate George Olah revealed the importance of methanol in energy structure [41]. Hence, a large amount of heterogeneous catalysts for the direct hydrogenation of CO₂ to CH₃OH, reaction (3), have emerged in recent years. Herein, we summarize some catalytic systems for the direct hydrogenation of CO₂ to CH₃OH (see Figure 9).

CO₂ + 3H₂ ↔ CH₃OH + H₂O, △H_298K = −49.5 kJ mol⁻¹

(3)

Cu-ZnO-based catalysts, developed by ICI (Imperial Chemical Industries) in the 1960s, are still widely used in CH₃OH synthesis from syngas (CO and H₂), and Cu⁰ is considered to be the active site. Li et al. reported the synthesis of Cu/ZnO catalysts using a unique method, i.e., facile solid-phase grinding, using mixture of oxalic acid, copper nitrate, and zinc nitrate as raw materials [42]. Appropriate amount of these compounds were physically mixed, and then manually ground for 0.5 h. Subsequently, at 393 K, the obtained precursor was dried for 12 h, and at 623 K, calcined for 3 h in N₂ flow, followed by passivation in 1% O₂/N₂ flow for 5 h. In contrast, by the following steps: (1) at 623 K, calcining the dried precursor at for 3 h in air; (2) at 503 K, reducing the oxide for 10 h in 5% H₂/N₂ flow; and (3) at room temperature, passivating the catalyst in 1% O₂/N₂ flow for 5 h, the catalyst can be obtained via H₂ reduction. As demonstrated by XRD, H₂-TPR and thermal-gravity-differential thermal analysis (TG-DTA), in the facile solid-phase grinding process, the decomposition of oxalate complexes and the reduction of CuO took place simultaneously when the samples were calcined in N₂, which prevented the growth of active Cu⁰ species and the aggregation of catalyst particles. In contrast, in the conventional H₂ reduction process, the growth of active species or the aggregation of catalyst particles were inevitable. Notably, Cu/ZnO catalysts prepared by a facile solid-phase grinding method achieved 29.2% CO₂ conversion (at 523 K and 3 MPa), which is even higher than the thermodynamic equilibrium conversion. Indeed, the equilibrium conversion of CO₂ at 473 K and 3 MPa is 25.78% according to the Benedict–Webb–Rubin equation [43], and it is even lower at 523 K, since CO₂ hydrogenation to CH₃OH is an exothermic reaction. Generally, tandem reaction can break the limit of thermodynamic on reaction result. Inspiring from which, the above unusual experimental results (higher than thermal equilibrium value) could be most likely achieved through tandem reactions, in which every stepwise reaction was accelerated by different metal sites. Therefore, the simple and solvent-free method based on solid-phase grinding, opened a new approach to synthesize bimetallic or multimetallic catalysts without further reduction.

To improve the catalytic activity of Cu/ZnO/Al₂O₃ catalysts, ZnO was replaced by g-C₃N₄-ZnO hybrid material, as reported by Deng et al. [44]. The experimental results showed that, at 12 bar and 523 K, the methanol space time yield (STY) reached 5.73 mmol h⁻¹ g_Cu⁻¹ for Cu- g-C₃N₄-ZnO/Al₂O₃, which is superior to the methanol yield (5.45 mmol h⁻¹ g_Cu⁻¹) of industrial catalyst (Cu-ZnO-Al₂O₃) under the same reaction pressure. As confirmed by time-resolved photoluminescence (TRPL) and electronic spin resonance (ESR), the electron-richness of ZnO was enhanced via the formation of type-II hetero-junction between g-C₃N₄ and ZnO. In addition, in the TPR curve, enhanced SMSI was observed, and the SMSI between electron-rich ZnO and Cu could boost the catalytic performance in CH₃OH production. The study provided a viable and economic method to modify traditional catalysts for improving CH₃OH production.

For CuO-ZnO-ZrO₂ catalysts, doping graphene oxide (GO) is a good strategy for improving the activity of CO₂ to CH₃OH [45]. The highest methanol selectivity reached up to 75.88% (473 K, 20 bar) at 1 wt % GO content, while under the same conditions, the methanol selectivity was found to be 68% over the GO-free catalyst. Furthermore, CuO-ZnO-ZrO₂-GO catalyst exhibited an almost constant space-time yield (STY) of methanol after 96 h time-on stream experiment. As demonstrated by H₂-TPD and CO₂-TPD, the CO₂ and H₂ adsorption capacity is enhanced over the CuO-ZnO-ZrO₂-GO catalysts. Additionally, to explain the improvement of catalytic activity, the authors proposed that GO nano-sheet can serve as a bridge between mixed metal oxides, which strengthens a hydrogen spillover (see Figure 10). That is, H species migrating from the copper surface to the carbon species are adsorbed on the isolated metal oxide particles.

It is also reported that modification by small amounts (2 and 5 at %) of WO₃ can improve the CO₂ conversion and CH₃OH selectivity of the CuO-ZnO-ZrO₂ catalyst [46]. The optimal catalytic performance can be attributed to the specific surface area of metallic Cu, basic sites, and the reducibility of catalysts. However, if we want to effectively tune the catalytic reaction, a better understanding of the reaction mechanisms is essential. Up to now, the possible reaction pathways for the hydrogenation of CO₂ to methanol over Cu-ZnO-based catalysts are shown in Scheme 3.

Besides Cu-based catalysts, In₂O₃ catalyst with surface oxygen vacancies has attracted more and more attention from researchers. Using density functional theory (DFT) calculations, Ye et al. investigated a In₂O₃ catalyst for the hydrogenation of CO₂ to CH₃OH [47]. On a perfect In₂O₃ (110) surface, six possible surface oxygen vacancies (O_v1 toO_v6) were investigated (see Figure 11a), and the D4 surface with the O_v4 defective site was found to be most beneficial for CO₂ activation and further hydrogenation. Potential energy profiles of CO₂ hydrogenation and protonation on the D4 defective In₂O₃ (110) surfaces are shown in Figure 11b. In addition, the simulation results showed that the formation of CH₃OH replenishes the oxygen vacancy sites, while H₂ contributes to generate the vacancies, and this cycle between perfect and defective states of the surface is responsible for the formation of CH₃OH from CO₂ hydrogenation. To demonstrate this hypothesis, In₂O₃ catalyst was used for the hydrogenation of CO₂ to CH₃OH in practice, which showed the superior catalytic activity (7.1% CO₂ conversion, 39.7% CH₃OH selectivity at 603 K). This experimental result is better than for many other reported catalytic systems, which generally show low selectivity of CH₃OH at 603 K. Confirmed by thermo-gravimetric analysis (TGA), XRD, and HR-TEM, the authors found that In₂O₃ catalyst had satisfactory thermal and structural stability for CO₂ conversion to CH₃OH below 773 K [48]. Furthermore, to reveal the effect of oxygen vacancies, Martin et al. synthesized bulk In₂O₃ catalyst, and at a wide range of reaction conditions, the CH₃OH selectivity could be tuned up to 100%. XRD, H₂-TPR, CO₂-TPD, XPS, operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and electron paramagnetic resonance (EPR) characterization confirmed that oxygen vacancies on the In₂O₃ catalyst are active sites for the reduction of CO₂. Additionally, to further improve the stability of In₂O₃ catalyst for the production of CH₃OH, the authors investigated a variety of supports (i.e., ZrO₂, TiO₂, ZnO₂, SiO₂, Al₂O₃, C, SnO₂, MgO). ZrO₂ showed the best catalytic performance since it prevented the sintering of the In₂O₃ phase, which was demonstrated by the enduring stability of In₂O₃/ZrO₂ catalyst over 1000 h [49]. Overall, In₂O₃ is a potential catalyst for CO₂ hydrogenation to CH₃OH with high selectivity.

Interestingly, Wang et al. synthesized a series of x% ZnO-ZrO₂ solid solution catalysts (x% represents the molar ratio of Zn) for CO₂ direct hydrogenation to CH₃OH [50]. Under the specified reaction conditions (5.0 MPa, H₂/CO₂ = 3:1 to 4:1, 593 K to 588 K), ZnO-ZrO₂ catalyst can achieve 86–91% methanol selectivity with single-pass CO₂ conversion more than 10%, better than the results reported by other researchers. Moreover, in the presence of 50 ppm SO₂ or H₂S in the reaction stream, no deactivation was observed. Experimental results confirmed that ZrO₂ and ZnO alone showed little activity in methanol synthesis, while the catalytic performance was significantly enhanced and CO₂ conversion reached the maximum value when the Zn/(Zn + Zr) molar percentage is close to 13%. Demonstrated by CO₂-TPD, most of the CO₂ adsorbed on the Zr sites of ZnO-ZrO₂ catalyst, and the H₂-D₂ exchange experiment indicated that ZnO had much higher activity than ZrO₂. Therefore, in ZnO-ZrO₂ solid solution catalyst, the synergetic effect between the Zn and Zr sites markedly promotes the activation of H₂ and CO₂. To explain the reaction mechanism on the solid solution catalyst (ZnO-ZrO₂), in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations were conducted. It was concluded that CO₂ direct hydrogenation to CH₃OH is dominated via the formate pathway, and CH₃OH was formed by H₃CO* protonation on the surface of ZnO-ZrO₂ catalyst.

Although Cu-ZnO-based catalysts are highly selective for CO₂ conversion to methanol, there are still some serious problems during industrial operation, such as deactivation of active sites at high temperature and agglomeration of catalytic particles under industrial reaction conditions. To overcome these issues, researchers designed a series of bimetallic catalysts for the direct hydrogenation of CO₂ to CH₃OH. Jiang et al. prepared a series of Pd-Cu bimetallic catalysts supported on SiO₂ with a wide range of total metal loading (i.e., 2.4–18.7 wt %) and evaluated the catalytic performance for the reduction of CO₂ to CH₃OH. With a decrease in metal loadings, the CO₂ conversion dropped stepwise, namely from 6.6 to 3.7%. However, the CH₃OH STY was slightly enhanced by 31% from 0.16 to 0.21 mmol mol⁻¹ s⁻¹ [51]. To unclose the reaction mechanisms, the authors carried out in situ diffuse reflectance FT-IR (DRIFTS) measurements on Pd(0.34)-Cu/SiO₂ catalyst [52]. The resulting spectra identified that the dominant species on a bimetallic surface were formate and carbonyl species on a bimetallic surface, which were dependent on the catalyst composition. Therefore, they proposed that on Pd-Cu catalysts, the surface coverage of formate species was correlated to the methanol promotion, implying its vital role in CH₃OH synthesis. Bahruji et al. prepared PdZn/TiO₂ bimetallic catalysts by a solvent-free chemical vapor impregnation method [53]. According to the order in which the metals were impregnated, the catalysts could be classified as ²Pd-¹Zn-TiO₂, ²Zn-¹Pd-TiO₂, and PdZn/TiO₂ (1 and 2 represent the order of sequential metal impregnations). The experimental results showed that PdZn/TiO₂ catalyst achieved optimal catalytic performance, 10.1% CO₂ conversion and 40% CH₃OH selectivity. The formation of ZnTiO₃ and PdZn nanoparticles was confirmed via XPS, XRD, and TEM. Combining the experimental results, the authors proposed that PdZn nanoparticles were beneficial for methanol formation and catalytic stability. Although it is hard to assign the formation of the PdZn alloy to the differences between the preparation methods, an interaction between Pd(acac)₂ and Zn(acac)₂ precursors might be responsible for the production of the PdZn active site in terms of PdZn/TiO₂ catalyst. Additionally, Yin et al. reported the preparation of Pd@zeolitic imidazolate framework-8 (ZIF-8) catalyst for the conversion of CO₂ to CH₃OH [54]. The optimal methanol yield can reach 0.65 g g_cat⁻¹ h⁻¹ over a PdZn catalyst. As demonstrated by XPS, XRD, TEM and electron paramagnetic resonance (EPR), after H₂ reduction, PdZn alloy particles formed, and abundant oxygen defects existed on the ZnO surface. The experimental results revealed that the active site is a PdZn alloy rather than metallic Pd, in terms of CH₃OH formation.

Numerous studies suggested that promoters have an indispensable role in the catalytic performance of CuZn catalysts [55], although the mechanism of action still remains unclear. Pd-doped CuZn catalysts were prepared to evaluate the surface modification effect. An interesting observation was made from the volcano-shaped relationship between CH₃OH STY and Pd loading, which implied that an appropriate amount of Pd loading is beneficial for methanol synthesis. The chemisorption analyses further revealed a strong interaction between Pd and Cu surface, where a hydrogen spillover has taken place, generating more activated Cu sites. Hence, the CH₃OH STY and the methanol turnover frequency (TOF) improved a lot with Pd loading of 1 wt % [55]. In addition, a variety of bimetallic catalysts (e.g., Cu-Fe, Cu-Co, Cu-Ni, Co-Ga, Ni-Ga) have been evaluated by many researchers [56,57,58,59]. A possible catalytic reaction mechanism of CO₂ hydrogenation over Cu−Ni/CeO₂−NT is shown in Figure 12. Details of the CO₂ conversion and product selectivity, along with reaction conditions of several representative catalytic systems are compared in

Table 3. Catalytic performance of several catalytic systems for CO₂ hydrogenation into CH₃OH, in terms of CO₂ conversion and CH₃OH selectivity, along with the reaction conditions.

Catalyst	H2:CO2	GHSV	Temperature (°C)	Pressure (MPa)	CO2 Conversion (%)	CH3OH Selectivity (%)
Cu-ZnO [42]	2.9	2160 a	250	3	29.2	83.6
Cu/g-C3N4-Zn/Al2O3 [44]	3	6800 a	250	1.2	~7.0	~55.0
CuO-ZnO-ZrO2-GO [45]	3	15,600 a	240	2	N/A	75.8
W-Cu-Zn-Zr [46]	2.7	2400 a	240	3	19.7	49.3
In2O3 [48]	3	15,000 a	270	4	1.1	54.9
In2O3 [48]	3	15,000 a	330	4	7.1	3
In2O3 [49]	4	16,000–48,000 b	300	5	N/A	100
ZnO-ZrO2 [50]	3-4	24,000 a	315-320	5	>10	86–91
Pd-Zn-TiO2 [53]	3	916 b	250	2	10.3	61
Pd-Zn-ZIF-8 [54]	3	21,600 a	270	4.5	~22.0	~50.0
Pd-Cu-Zn [55]	3	10,800 a	270	4.5	~8.0	~65.0
Co5Ga3 [58]	3	N/A	250	3	1	63
CuNi2/CeO2-NT [59]	3	6000 b	260	3	17.8	78.8
Cu-Zn-SiO2 [60]	3	2000 a	220	3	14.1	57.3
Ni5Ga3/SiO2/Al2O3/Al [61]	3	3000 a	210	0.1	~1.0	86.7
Cu-Zr-SiO2 [62]	3	N/A	230	5	N/A	77
Cu/Mg/Al [63]	2.8	2000 b	200	2	3.6	31
Cu-Ce-Zr [64]	3	7500 a	250	3	14.3	53.8
Cu-TiO2 [65]	3	3600 a	260	3	N/A	64.7
Cu-Zn-Mn-KIT-6 [66]	3	120,000 a	180	4	8.2	>99.0
Cu-SBA-15 [67]	3	N/A	210	2.2	13.9	91.3
Au-CuO/SBA-15 [68]	3	3600 b	250	3	24.2	13.5
Cu-Zr-SBA-15 [69]	3	N/A	250	3.3	15	N/A
Pd/In2O3 [70]	4	>21,000 a	300	5	>20.0	>70.0

^a mL g_cat⁻¹ h⁻¹; ^b h⁻¹; N/A: not available.

. In terms of hydrogenation of CO₂ to CH₃OH, In-based and Pd-based catalysts have gradually drawn researchers’ attention, besides traditional Cu-based catalysts. Additionally, many researchers demonstrated that 250 °C is the optimal temperature to produce CH₃OH with the high selectivity. According to the current experimental results, increasing the reaction pressure is also a good strategy to improve CO₂ conversion, but a high pressure means a high cost of operation and equipment.

2.4. CO₂ to Other Products

Besides CO, CH₄, and CH₃OH, dimethyl ether (DME), light olefins [71,72,73,74], alcohol [75], isoparaffins [76], and aromatics [77,78] have also been produced by CO₂ hydrogenation. Clearly, in terms of CO₂ conversion, the coupling of C-C bond to produce higher hydrocarbons and oxygenates is a technological barrier. However, taking advantage of tandem catalysis to realize one-step synthesis of hydrocarbons via hydrogenation of CO₂ is feasible. The extensive studies can be mainly categorized into two categories: methanol-mediated and non-methanol-mediated reactions [79,80]. In the methanol-mediated approach, DME is usually produced as main product, while for the non-methanol-mediated approach, alkenes and alkanes are generally produced as main products. The possible reaction mechanism for CO₂ hydrogenation to DME and light olefins is shown in Scheme 4.

Cu-based catalysts are in practice used for methanol synthesis, and HZSM-5 zeolites are widely employed for methanol dehydration due to its solid acid catalysis. Therefore, a variety of studies have been reported with regard to the bi-functional catalysts consisting of Cu and HZSM-5 used for the direct hydrogenation of CO₂.

Zhang et al. reported a Cu-ZrO₂/HZSM-5 catalyst promoted by Pd/CNT for direct synthesis of DME from CO₂/H₂ [79]. Although a minor amount of the Pd-decorated CNTs into the CuZr/HZSM-5 catalytic system caused little change in the activation energy for CO₂ conversion compared with the CuZr/HZSM-5 catalyst, the former created a micro-environment including higher concentration of active H species and adsorbed CO₂ species on the surface of the catalyst system. Owing to the increase of hydrogenation reactions, under reaction conditions of 5 MPa and 523 K, the specific rate of CO₂ hydrogenation-conversion was 1.22 times higher than the pure CuZr/HZSM-5 catalyst. Furthermore, Zhang et al. reported a series of V-modified CuO-ZnO-ZrO₂/HZSM-5 catalysts which were prepared via an oxalate co-precipitation method for the synthesis of DME from CO₂/H₂ [81]. The catalytic performances of the catalysts were strongly dependent on the content of V, which was confirmed by XRD, N₂O chemisorption, and X-ray photoelectron spectroscopy (XPS). Similarly, the influence of the promoter—i.e., La and W—has also been examined. The optimal catalytic activity was obtained (43.8% CO₂ conversion and 71.2% DME selectivity) when the amount of La was 2 wt %. In contrast, the Cu-ZnO-ZrO₂ catalyst admixed with WO_x obtained 18.9% CO₂ conversion and 15.3% DME selectivity. Obviously, La is a better promoter compared with W, and this can be explained by the fact that the reducibility and dispersion of bi-functional catalysts (CuO-ZnO-Al₂O₃-La₂O₃/HZSM-5) were largely dependent on the modification of La, while the hybrid catalyst (Cu-ZnO-ZrO₂-WO_x/Al₂O₃) modified by W strongly adsorbed water molecules, resulting in a lower catalytic performance. Moreover, as shown in Figure 13, the STY of DME of different WO_x/Al₂O₃ catalysts (i.e., on supports with different pore sizes) as a function of W surface density shows a volcanic curve relation. Details of CO₂ conversion and DME selectivity reported recently are shown in

Table 4. Catalytic activity of several catalysts for CO₂ hydrogenation into other products (e.g., DME, olefins, alcohol, isoparaffins, gasoline, aromatics), in terms of CO₂ conversion and product selectivity, along with the reaction conditions.

Catalyst	H2:CO2	GHSV	Temperature (°C)	Pressure (MPa)	CO2 Conversion (%)	Selectivity (%)
Fe-Zn-K [71]	3	1000 b	320	0.5	51.03	olefins (53.58)
K-Fe [72]	3	1200 a	340	2	38	light olefins (78)
In/HZSM-5 [73]	3	9000 a	340	3	13.1	liquid fuels (78.6)
Ce-Pt@mSi-Co [74]	3	N/A	250	N/A	~3.0	C2–C4 (60)
K/Cu-Zn-Fe [75]	3	5000 b	300	6	42.3	alcohol (56.43)
Na-Fe/HMCM-22 [76]	2	4000 a	320	3	26	isoparaffins (74)
ZnZrO-HZSM-5 [77]	N/A	1200 a	320	4	14	aromatic (73)
ZnAlOx-HZSM-5 [78]	3	2000 a	320	3	9.1	aromatics (74)
Cu-Zr-Pd/HZSM-5 [79]	3	25,000 a	250	5	18.9	DME (51.8)
Fe-K/HSG [80]	3	26,000 a	340	20	N/A	olefins (59)
V-Cu-Zn-Zr/HZSM-5 [81]	3	4200 b	270	3	32.5	DME (58.8)
Cu-Zn-Al-La/HZSM-5 [82]	3	3000 b	250	3	43.8	DME (71.2)
ZnO-ZrO2-SAPO-34 [95]	N/A	3600 a	380	2	12.6	olefins (80–90)
Cu-ZnO-Al2O3 [96]	3	10 b	270	5	9	DME (31)
In-Zr/SAPO-34 [97]	3	9000 a	380	3	26.2	C2+ (36.1)
Cu-Zn-kaolin-SAPO-34 [98]	3	1800 a	400	3	50.4	C2–C4 (65.3)
Fe/C-Bio [99]	3	N/A	320	1	31	C4–18 alkenes (50.3)
Cu-Zn-Zr/HZSM-5 [100]	3	10,000 a	220	3	9.6	DME (46.6)
Cu-Zn-Zr/HZSM-5 [101]	3	9000 a	240	3	~30	DME (~35)
Cu-Zn-Al/HZSM-5 [102]	3	4200 b	270	3	30.6	DME (49.02)
Cu-Zn-Al/HZSM-5 [103]	3	1800 a	262	3	46.2	DME (45.2)
Cu-Zn-Zr-zeolite [104]	3	10,000 b	240	3	24	DME (38.5)
Cu-Zn-Al/HZSM-5 [105]	3	1800 a	270	3	48.3	DME (48.5)
Fe-K/HPCMs-1 [106]	3	3600 a	400	3	33.4	olefins (47.6)
Na-Fe/HZSM-5 [107]	3	4000 a	320	3	22	C5–C11 (78)
Fe5C2-/a-Al2O3 [108]	3	3600 a	400	3	31.5	C2+ (69.2)
Fe-Zr-Ce-K [109]	3	1000 b	320	2	57.34	C2–C4 (55.67)
Fe/C+K [110]	3	24,000 a	320	30	24	C2–C6 (36)
Co-Cu/TiO2 [111]	3	3000 a	250	3	18.4	C5+ (42.1)

^a mL g_cat⁻¹ h⁻¹; ^b h⁻¹; N/A: not available.

[82,83]. The above studies indicate that the current catalysts used for DME preparation from CO₂/H₂ are still dominated by Cu-based-HZSM-5 catalysts. Some other zeolite catalysts (SBA-15, SAPO-5, SUZ-4) with similar acid properties (acid content and acid strength) as HZSM-5 zeolite may be a direction of future research.

Among the large-scale technologies with industrial potential, the conversion of CO₂ to DME is promising and relatively mature [84]. Korea Gas Corporation (KOGAS) has developed a DME plant, with CO₂ as a raw material. The schematic diagram of the KOGAS tri-reforming process is shown in Figure 14 [85,86]. Kansai Electric Power Co. and Mitsubishi Heavy Industries have realized a bench-scale (100 cm³ catalyst loading) experiment for DME synthesis [87,88]. However, during the reaction process, water formation decreased the yield of DME [89,90,91]. Catizzone et al. and Bonura et al. evidenced that zeolites or ferrierite could effectively mitigate the influence of water and avoid catalyst sintering [89,90]. In a fixed bed reactor, kinetic modeling confirmed the negative effect of water (formed during the reaction process) on DME production, which is consistent with the experimental results. Therefore, Falco et al. suggested that hydrophilic membranes could be promising for industrial production of DME [92]. Recently, Fang et al. advocated CO₂ capture and conversion by using a membrane reactor system, where a high-temperature mixed electronic and carbonate-ion conductor (MECC) membrane was used for CO₂ capture and a solid oxide electrolysis cell (SOEC) was used for CO₂ reduction [93]. Furthermore, based on membrane technology, Sofia et al. carried out a techno-economic analysis project of power and hydrogen co-production via an integrated gasification combined cycle (IGCC) plant with CO₂ capture [94]. The development of membrane technology would be an advantageous factor for the capture and utilization of CO₂.

Li et al. synthesized a novel ZnZrO/SAPO tandem catalyst, combined by a ZnO-ZrO₂ solid solution and a Zn-modified SAPO-34 zeolite, which achieved 80–90% olefin selectivity among the hydrocarbon products [95]. Based on the surface reaction kinetics, they proposed that the tandem reaction process proceeded as follows: (1) generation of CH_xO species on ZnZrO via CO₂ reduction; (2) olefins production from the derived CH_xO species which migrate/transfer onto SAPO zeolite pore structure. Moreover, experimental results confirmed that the excellent selectivity can be ascribed to the effective synergy between ZnZrO and SAPO for the tandem catalyst. In addition, this catalyst (ZnZrO and SAPO-34) shows an excellent stability toward thermal and sulfur treatments, indicating the potential value for industrial application.

Recently, CO₂ was converted to aromatics with a selectivity up to 73%, at 14% CO₂ conversion over ZnZrO/HZSM-5 catalyst [77]. Demonstrated by operando infrared (IR) characterization, Li et al. proposed that CH_xO, as an intermediate species, transformed from the ZnZrO surface into the pore structure of HZSM-5, which is responsible for C-C bond formation for aromatics production. The reaction scheme is shown in Figure 15. Interestingly, the presence of H₂O and CO₂ markedly suppressed the generation of polycyclic aromatics, consequently, enhanced the stability (100 h in the reaction stream) of the tandem catalyst, which showed potential industrial application prospects. Similarly, Ni et al. synthesized a tandem catalyst of ZnAlO_x and HZSM-5, which yields 73.9% aromatics selectivity and 0.4% CH₄ selectivity among the carbon products without CO [78]. Confirmed by XRD, SEM, TEM, and element distribution analysis, ZnAlOx was formed and uniformly dispersed in the tandem catalyst. Furthermore, demonstrated by 2,6-di-tert-butyl-pyridine absorption (DTBPy-FTIR), the external Brønsted acid of HZSM-5 can be shielded by ZnAlO_x, which is beneficial to aromatization. According to the operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) study, they proposed the following possible reaction mechanism: MeOH and DME, produced by hydrogenation of formate species, are transmitted to HZSM-5 and then converted into olefins and finally aromatics.

Non-methanol-mediated reactions, i.e., the direct hydrogenation of CO₂ to light olefins is even more significant than CH₃OH synthesis from CO₂ hydrogenation, since a large proportion of methanol is used for the synthesis of olefins in industry, through the methanol-to-olefins (MTO) reaction by using SAPO-34 zeolite catalysts. CO₂ to lower olefins can be realized by coupling of the RWGS reaction and F–T synthesis, as shown in Scheme 1 and Scheme 3.

Iron-based catalysts has been considered as an excellent option for the synthesis of light olefins from CO₂/H₂, mainly because of their excellent activity, high selectivity, and low price. Using honeycomb-structure graphene (HSG) as the support and K as a promoter, Wu et al. prepared an iron-based catalyst [80]. They found out that the confinement effect of the porous HSG was beneficial for the sintering of the Fe active sites, and within 120 h stability test, no significant deactivation occurred. In addition, as confirmed by CO₂ temperature programmed desorption (CO₂-TPD) and H₂-TPD, they found out that potassium as a promoter effectively enhanced the chemisorption and activation of the reactants CO₂ and H₂. Moreover, as revealed by ⁵⁷Fe Mossbauer absorption spectroscopy, for the Fe/HSG catalyst, there were two doublets with isomer shift (IS) of 0.96 mm s⁻¹ and quadrupole splitting (QS) of 0.28 mm s⁻¹ and IS of 1.21 mm s⁻¹ and QS of 1.82 mm s⁻¹, which corresponded to the Fe (II) species in low- and high-coordination environments. Instead, for the FeK1.5/HSG catalyst, only one doublet with IS of 0.31 mm s⁻¹ and QS of 1.05 mm s⁻¹ ascribed to the Fe (III) species, which implies that K is capable of stabilizing high valence-state iron (Fe (III)) during CO₂ hydrogenation to light olefins (CO₂–FTO). The above mentioned three factors contributed to the excellent catalytic performance, i.e., 56% olefins selectivity and a 120-h stability testing experiment (as shown in Figure 16).

Zhang et al. used an impregnation method to prepare Fe-Zn-K catalysts, in which K was used as a promoter [71]. The experimental results showed that the Fe-Zn-K catalyst with H₂/CO reduction showed the best catalytic activity, with 51.03% CO₂ conversion and 53.58% C₂–C₄ olefins selectivity, at the reaction conditions of 593 K and 0.5 MPa. XRD, H₂-TPR, and XPS characterization results revealed that, in the Fe-Zn-K catalyst, ZnFe₂O₄ spinel phase and ZnO phase were formed. Among them, ZnFe₂O₄ spinel phase strengthens the interaction between iron and zinc, and changed the reduction and CO₂ adsorption behaviors. In addition, the H₂-TPR profiles show that the catalyst modified by K contributed to a slight shift of the initial reduction peak to higher temperature, indicating the reduction of Fe₂O₃ and formation of Fe phase was inhibited by K modification.

You et al. investigated the catalytic activity of non-supported Fe catalysts (bulk Fe catalysts) modified by alkali metal ions (i.e., Li, Na, K, Rb, Cs) for the conversion of CO₂ to light olefins [72]. By calcining ammonium ferric citrate, non-supported Fe catalyst was prepared. Compared with Fe catalyst without modification (5.6% CO₂ conversion, 0% olefins), the modification of the Fe catalyst with an alkali metal ion markedly enhanced the catalytic activity for CO₂ conversion to light olefins. For Fe catalyst modified by K and Rb, the conversion of CO₂ and the olefin selectivity (based on only the hydrocarbon compounds, without CO) increased to about 40% and 50%, respectively, and the yield of light (C₂–C₄) olefins can reach 10–12%. Further investigation via XRD showed that over the alkali-metal-ion-modified Fe catalysts, Fe₅C₂ was formed, while in the unmodified catalyst, iron carbide species were not observed after the reaction. Accordingly, they proposed that in the presence of an alkali metal ion, the generation of iron carbide species was one possible reason for the enhanced catalytic activity. Therefore, modification by alkali metal ions remains a good strategy to tune the product distribution.

Additionally, Wei et al. reported a highly efficient, stable and multifunctional Na–Fe₃O₄/HZSM-5 catalytic system [76], for direct hydrogenation of CO₂ to gasoline-range (C₅–C₁₁), in which the selectivity of C₅–C₁₁ hydrocarbons reached 78% (based on total hydrocarbons), while under industrial conditions only reached 4% methane selectivity at a 22% CO₂ conversion. Characterization by high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and Mossbauer spectroscopy showed that two different types of iron phase—i.e., Fe₃O₄ and x-Fe₅C₂—were discerned in the spent Na–Fe₃O₄ catalyst, which cooperatively catalyzed a tandem reaction (RWGS and FT). The possible reaction scheme for CO₂ hydrogenation to gasoline-range hydrocarbons is shown in Figure 17. Furthermore, acid sites existing in the HZSM-5 zeolite were favorable for acid-catalyzed reactions (oligomerization, isomerization, and aromatization). Notably, the multifunctional catalyst exhibited a significant stability for 1000 h on stream, showing the potential as promising industrial catalyst material for CO₂ conversion to liquid fuels. Similarly, Gao et al. investigated a bi-functional catalytic system composed of In₂O₃ and HZSM-5, which can achieve 78.6% C₅₊ selectivity with only 1% CH₄ at a 13.1% CO₂ conversion [73]. As demonstrated by Ye et al. [47], CO₂ and H₂ can be activated in the oxygen vacancies on the In₂O₃ surfaces, and catalyzed CH₃OH formation. Subsequently, C−C coupling occurred inside the zeolite pores structure (HZSM-5) to synthesize gasoline-range hydrocarbons with a relatively high octane number (C₅₊).

Details of the conversion and selectivity for the hydrogenation of CO₂ into other products, along with the reaction conditions of several representative catalytic systems, are compared in Table 4. Fe-based catalysts are beneficial for the production of olefins, while Cu-based + HZSM-5 is still a dominant catalytic system for CO₂ conversion to DME. Similarly, 250 °C is the optimal temperature for the production of DME based on the current experimental results, and 300–400 °C, which is close to industrial production conditions, seems to be better for the direct conversion of CO₂ to olefins. On the other hand, the selectivity of DME can be maintained upon increasing GHSV (>10,000 mL g_cat⁻¹ h⁻¹), but the high selectivity of olefins generally relies on a relatively low GHSV (<5000 mL g_cat⁻¹ h⁻¹).

2.5. Opportunities of Heterogeneous Catalysis for CO₂ Conversion

The relationship between products selectivity and CO₂ conversion by heterogeneous catalytic hydrogenation is shown in Figure 18. It is obvious that, in the RWGS reaction, although the CO selectivity reaches up to nearly 100%, the CO₂ conversion is low. In the methanation reaction of CO₂, the CH₄ selectivity is high enough, and the CO₂ conversion also exceeds 50%. With regard to the synthesis of CH₃OH, DME, and light olefins, the relationship between conversion and selectivity is less clear.

In the RWGS reaction, CO is mainly produced over Fe-, Co-, Mo-, and Pt-based catalysts (Table 1), while CO₂ methanation is typically carried out on Co- and Ni-based catalysts (Table 2). Although CO is a main component of syngas and CH₄ is an important energy resource, neither is the best product for CO₂ conversion from a relatively economic point of view. Furthermore, the conversion and utilization of CO and CH₄ is also an important research field, which also implies that CO and CH₄ are not the optimal choice as end-products of CO₂ conversion. Therefore, the coupling reaction is a potential direction for CO₂ conversion to some higher value products, i.e., CH₃OH and light olefins.

Cu-ZnO-based catalyst is still the main catalytic material for direct conversion of CO₂ to CH₃OH under industrial reaction condition, while highly innovative methodologies are awaited to achieve low-pressure and low-temperature CH₃OH synthesis processes. Considering the industrial application value of methanol, exploring novel catalytic systems and designing rational reactors to improve the catalytic activity should be targeted in the future. DME, up to now, is synthesized via Cu-based and H-ZSM5 catalyst (Table 4), and this process essentially remains a two-step tandem reaction. Hence, the bottleneck of DME production could be the deactivation of CH₃OH dehydration catalyst, due to water poisoning. The formation of coke is also the major reason of catalyst deactivation, especially in a relatively long-term operation process. To maintain high activity of catalyst for the production of DME, water- and coke-resistant catalysts need to be developed for CH₃OH dehydration.

As shown in Table 4, the direct hydrogenation of CO₂ to light olefins is mainly catalyzed by Fe-based catalysts. However, the starting point of current research work is mainly the coupling of RWGS, F–T and/or methanol-to-olefins (MTO) reactions. Therefore, suitable catalytic materials are sought for the coupling of C-C bonds, which is an effective strategy for improving the catalytic activity.

3. Plasma Catalysis

Plasma, the ‘fourth state of matter’, consists of electrons, neutral species (i.e., molecules, radicals, and excited species) and ions. Plasma can be in so-called thermal equilibrium or not, based on which it is subdivided into ‘thermal plasma’ and ‘non-thermal plasma’ (NTP) [13,112,113]. In non-thermal plasma, the gas temperature remains near room temperature, while electrons temperature is extremely high, usually in the range of 1–10 eV (~10,000–100,000 K). The latter is enough to activate stable gas molecules into reactive species (e.g., radicals, excited atoms, molecules, and ions). These reactive species, especially the radicals, can trigger reactions at low temperature. That is, NTP offers a unique approach to enable thermodynamically unfavorable chemical reactions to proceed at low temperature by breaking thermodynamic limits. Nevertheless, the control of selectivity of desired products in plasma is extremely difficult, since the reactions in plasma are mainly triggered through nonselective collision between active species (radicals, molecules, atoms, and ions).

To improve the desired product selectivity of the reactions in plasma, the combination of catalysts with plasma technology (so-called plasma catalysis) is a promising strategy, since catalysts usually have a special feature of regulating product distribution.

NTP can be generated through various types of discharges—i.e., microwave discharges, glow discharges, gliding arc discharges, dielectric barrier discharges (DBD), etc.—but DBD are the best option to be used in plasma catalysis. Indeed, DBDs are usually operated at atmospheric pressure and ambient temperature, and the integration of DBD plasma with catalysts has the advantages of simple operation and low cost. Thus plasma-driven direct hydrogenation of CO₂ is mostly based on DBD plasmas. The possible plasma/catalyst synergism is illustrated in Figure 19 [13].

A lot of research has been performed for pure CO₂ splitting, in various types of plasma reactors, including DBD, microwave discharge and gliding arc discharge, without catalysts [114,115,116,117]. A typical experimental set-up of a DBD plasma for CO₂ decomposition is shown in Figure 20. Paulussen et al. studied the conversion of CO₂ to CO and oxygen in DBD [114], and they found that the gas flow rate is the most crucial parameter affecting the CO₂ conversion. At 0.05 L min⁻¹ flow rate, 14.75 W cm⁻³ power density and 60 kHz discharge frequency, 30% CO₂ conversion was achieved. The performance might be further enhanced by optimizing the discharge parameters (i.e., power, frequency, dielectric material) or by implementing parallel reactors.

Dry reforming of methane (DRM), i.e., the combined conversion of CH₄ and CO₂ by plasma and/or plasma catalysis, has attracted extensive attention in recent years. For instance, Li et al. studied CO₂ reforming of CH₄ by taking advantage of atmospheric pressure glow discharge plasma [118]. Liu et al. reported high-efficient conversion of CO₂ and CH₄ in AC-pulsed tornado gliding arc plasma [119]. Kolb et al. investigated DRM in a DBD reactor [120]. In the above-mentioned studies [118,119,120], syngas (CO and H₂) was produced as the main product. Recently, Wang et al. reported a novel one-step reforming of CO₂ and CH₄ into liquid oxygenate products, dominated by acetic acid, at room temperature by the coupling of DBD plasma and catalysts [121]. They examined the effect of CH₄/CO₂ molar ratio and of various catalysts (γ-Al₂O₃, Cu-γ-Al₂O₃, Au-γ-Al₂O₃, Pt-γ-Al₂O₃). Interestingly, compared with plasma-only mode, the coupling of plasma and catalysts tuned the selectivity of liquid chemicals, and oxygenates selectivity was achieved up to approximately 60% when Cu catalyst was used. Details about the effects of operating mode and catalysts on the CO₂ conversion reaction results are shown in Figure 21. Although much efforts need to be made to reveal the unknown mechanisms, the results are attractive and show an excellent application prospect.

Besides the above-mentioned metal catalysts, zeolites have also been used in plasma catalytic DRM. Zhang et al. studied the catalytic performance of zeolite catalysts (i.e., NaA, NaY, and HY) for the direct conversion of CH₄ and CO₂ at relatively low temperature range and ambient pressure via DBD plasma [122], and the products were dominated by syngas and C₄ hydrocarbons. Form the investigated catalysts (NaA, NaY, and HY), HY zeolite catalyst exhibited the best performance (26.7% CO₂ conversion and 52.1% C₄ hydrocarbon selectivity), which was mainly attributed to the appropriate pore size and electrostatic properties of HY zeolite.

Although direct hydrogenation of CO₂ to CH₃OH is an exothermic reaction, studies of heterogeneous catalysis for CO₂ hydrogenation to CH₃OH were usually operated at high temperature and high pressure, mainly caused by the high stability of CO₂ and the low equilibrium constant at atmospheric pressure. Plasma catalysis, however, is a promising approach to enable CO₂ conversion to CH₃OH at ambient conditions, and has gradually attracted more and more interest. For instance, Eliasson et al. reported the direct hydrogenation of CO₂ to CH₃OH by coupling of DBD plasma and a discharge-activated catalyst (CuO-ZnO-Al₂O₃) [123]. By comparison of the experiments, with catalyst only, discharge only, and discharge + catalyst, they found that DBD plasma effectively lowered the optimal reaction temperature corresponding to the best catalytic performance. Indeed, the optimal reaction temperature was 493 K for catalyst only, while in terms of discharge only and discharge + catalyst, the optimal reaction temperature for CO₂ conversion was 373 K. The maximum selectivity for the methanol formation (10%) was achieved at a temperature of 373 K, which implies that the plasma improved the catalytic activity of CuO-ZnO-Al₂O₃ catalyst at low temperature. Additionally, the authors found that low input power and high pressure are beneficial for the improvement of the methanol selectivity.

Zeng et al. studied CO₂ hydrogenation by combining various catalysts (i.e., Cu/γ-Al₂O₃, Mn/γ-Al₂O₃, and Cu–Mn/γ-Al₂O₃) with DBD plasma in a coaxial packed-bed [124]. The experimental results showed that the addition of catalysts in the reactor improved the conversion of CO₂. At the same time, with the increase of the H₂/CO₂ molar ratio, the CO₂ conversion was improved, and the CO yield was also enhanced. It is also worth mentioning that, compared with plasma only experiments, the energy efficiency was enhanced by adding catalysts, although the synergetic mechanism between catalysts and plasma is still unknown.

Recently, Wang et al. examined the influence of plasma reactor structure and catalysts on CO₂ conversion and CH₃OH selectivity for plasma catalytic CO₂ hydrogenation [125], and a schematic diagram of the experimental setup and images of the H_2/CO₂ discharge are shown in Figure 22. They investigated three kinds of reactors, i.e., a cylindrical reactor (aluminum foil sheet as ground electrode), a double dielectric barrier discharge reactor (water as ground electrode), and a single dielectric barrier discharge reactor (water as a ground electrode). The single DBD reactor equipped with a special water-electrode showed the optimal reaction performance (21.2% CO₂ conversion and 53.7% CH₃OH selectivity). In addition, they tested the catalytic performance of Pt/γ-Al₂O₃ catalysts and Cu/γ-Al₂O₃ catalysts in the optimized reactor for direct conversion of CO₂ to CH₃OH. As shown in Figure 23, both Pt/γ-Al₂O₃ catalysts and Cu/γ-Al₂O₃ catalysts improved not only the CO₂ conversion, but also the CH₃OH selectivity, and Cu/γ-Al₂O₃ catalysts showed a better catalytic performance (21.2% CO₂ conversion and 53.7% methanol selectivity). That is, a strong synergistic effect between the plasma and Cu/γ-Al₂O₃ catalysts promoted the hydrogenation of CO₂ to methanol, although the reaction temperature in the optimized reactor remained near room temperature.

It is obvious that the combination of the plasma and catalysts can enhance the catalytic reaction at room temperature and atmospheric pressure. The maximum methanol selectivity of 53.7% was achieved with a CO₂ conversion of 21.2% via the plasma-catalysis process (see Figure 23). All the above studies show a strong synergistic effect between plasma and catalysts.

Using one-dimensional fluid modeling [126], De Bie et al. proposed a chemical reaction network in the plasma (i.e., without catalysts) for conversion of CO₂ to value-added chemicals, i.e., CO, CH₄, CH₂O, CH₃OH, and hydrocarbons. The simulation results indicated the dominant reaction pathways for the conversion of CO₂ and H₂, as illustrated in Scheme 5. According to the model, the combination between H atoms and CHO radicals is the most important reaction to form CO, while this reaction is counterbalanced by the reorganization of H with CO into CHO radicals. Therefore, the most effective net CO formation reaction is dissociation of CO₂ influenced by electrons. The production of CH₄ was generally driven by two reactions, i.e., three-body recombination reaction between CH₃ and H radicals, and charge transfer reaction between CH₅⁺ and H₂O. However, the latter reaction is partly balanced by the loss of CH₄, resulting from a charge transfer reaction with H₃⁺. The production of CH₂O is closely related to the initial CO₂ fraction in the gas mixture. At a low initial fraction of CO₂, the reaction between CO₂ and CH₂ radicals seem to be the most important channel for the formation of CH₂O, while at higher initial CO₂ fractions, CH₂O is also produced out of two CHO radicals to some extent. In addition, as predicted by the model, the most important channel for the formation of CH₃OH is the three-body reaction between CH₃ and OH radicals, while the three-body reaction between CH₂OH and H radicals is also an effective production channel for CH₃OH. Furthermore, they also found that a higher density of CH₃ and CH₂ radicals would be essential to tune the distribution of end products. Therefore, it can be predicted that the degree of hydrogenation in the reaction has a significant influence for the targeted products.

Figure 24 summarizes the selectivity of CO, CH₄, or CH₃OH, as a function of the CO₂ conversion, obtained from the recent reports discussed above. Several main trends are clear. Firstly, the main products are CO and CH₄ for direct hydrogenation of CO₂ in plasma catalysis, while the selectivity of CH₃OH is relatively low. Moreover, it is obvious that researchers have paid more attention to the reduction of CO₂ to CO and CH₄ up to now. However, the production of other hydrocarbons, such as olefins and gasoline hydrocarbons, would also be a promising direction, to achieve the maximum utilization of CO₂ by plasma catalysis. Therefore, some insights from heterogeneous catalysis, especially the combination of metal catalysts, metal oxide catalysts, and zeolite catalysts could be helpful to develop a methodology for plasma catalytic hydrogenation of CO₂. On the other hand, there is no guarantee that good catalysts in thermal processes would also perform well in plasma catalysis, because of the clearly different operating conditions (e.g., lower temperature, abundance of reactive species, excited species, charges, and electric field present in plasma).

Up to now, in view of the complexity of this interdisciplinary field, the development of plasma catalysis still needs major research efforts. On one hand, plasma catalysis has potential for industrial application, because it drives the CO₂ conversion reaction at ambient temperature and atmospheric pressure, breaking thermodynamic equilibrium to make full use of feedstock. On the other hand, the excessive energy consumption—caused by high input power and heat loss—is an important disadvantage, but it can be mitigated with the further development of renewable energy (i.e., wind, solar, and tidal energy). Additionally, to improve the reaction activity for CO₂ conversion, it is crucial to explore the reaction mechanisms by in situ characterization and computer modeling, to improve the synergistic effect between plasma and catalysts. Finally, we need to search for suitable catalytic materials to strength the reaction performance and decrease the production costs.

4. Outlook and Conclusions

Currently, fuels and base chemicals are nearly all produced from non-renewable fossil energy (oil, natural gas, and coal), and CO₂ is generally the end product (e.g., upon burning fossil fuels) or a waste product in chemical industry. This indicates that we should use CO₂ as the main carbon source when fossil energy would get depleted in the future. Thus, in the long term, hydrogenation of CO₂ (as well as CO₂ conversion with other H-sources) into value-added chemicals and fuels is very significant, since it can close the carbon cycle, as shown in Figure 25. However, some crucial issues should be addressed in advance for application of CO₂ hydrogenation in industry.

From a relatively economical point of view, the direct hydrogenation of CO₂ is not yet a viable approach. On one hand, CO₂ of a certain purity generally depends on the development of capture and separation techniques. On the other hand, compared with the price of the feedstock (H₂, 10,000 $/ton), the price of the main products (i.e., liquefied natural gas, 770 $/ton, and CH₃OH, 340 $/ton) obtained by CO₂ hydrogenation is too low to make this an economically viable process. However, once the production of H₂ can be realized with fully-fledged solar power technology, the production cost for CO₂ hydrogenation will be greatly reduced. Meanwhile, the CO₂ conversion driven by solar energy (artificial photosynthesis) is also a promising routing.

Technologically, the direct hydrogenation of CO₂ to CO is an endothermic reaction (ΔH = 41.2 kJ mol⁻¹), and a higher reaction temperature is beneficial for the production of CO according to Le Chatelier’s principle. Based on the available experimental results (Table 1), the CO selectivity can reach nearly 100% under conditions of 873 K and 1 MPa. On the other hand, the production of other products (i.e., CH₄, CH₃OH, DME, and light olefins) from CO₂ is exothermic, and in theory a low temperature favors the equilibrium conversion. However, the inert CO₂ molecule generally needs relatively high reaction temperature to be activated. The competition between these two factors makes the reaction performance not very satisfactory. Up to now, from the perspective of technology readiness level, methanol synthesis is a far more advanced production process, with a high selectivity up to 80–90% with Cu-based catalyst (Table 3). However, there are some limitations in heterogeneous catalysis for direct hydrogenation of CO₂, such as catalyst sintering at high temperature, the influence of water produced in the reaction process and low CO₂ conversion caused by thermo-dynamical restriction in terms of the formation of CH₃OH and hydrocarbons. Hence, we should explore novel catalytic materials and reactors to break the thermodynamic equilibrium, as well as design bi-functional and/or multi-functional catalysts (metal, metal oxides, and zeolites) to couple the chemical reactions (RWGS, methanation, methanol synthesis, and/or MTO).

Plasma catalysis, a new field of catalysis, has attracted sufficient attention in recent years, due to its simple operating conditions (ambient temperature and atmospheric pressure) and unique advantages in activating inert molecules. However, the energy efficiency is still too high for commercial exploitation. On the other hand, renewable energy (e.g., wind, solar, and tidal energy) will further develop in the near future, and it will be a perfect match with plasma catalysis (because plasma is generated by electricity and can simply be switched on/off—allowing storage of fluctuating energy), hence making the problem of limited energy efficiency less dramatic. Nevertheless, significant efforts must be devoted to elucidate the reaction mechanisms in plasma catalysis, to improve not only the energy efficiency, but certainly also the selectivity towards value-added products. Indeed, plasma catalysis is complicated from chemistry and physics point of view, but the potential of plasma technology for industrial applications deserves major research efforts. A combination of computer simulations with experiments will be needed for an in-depth understanding of the reaction mechanisms, responsible for the synergy between plasma and catalysts. Although the development process in plasma catalysis may be slow due to its complex character, it would bring great benefits to human society, once it is developed mature enough, and once appropriate catalysts for plasma catalysis can be designed. Therefore, future research should definitely emphasize on a better understanding and rational screening of highly active catalysts.

Compared with the numerous studies on CO₂ hydrogenation by heterogeneous catalysis, much more research should be carried out in the field of plasma catalysis, to improve the CO₂ conversion and target products selectivity (and even to exploit new products, such as olefins, gasoline hydrocarbons, and aromatics), since the results achieved by plasma catalysis (Figure 24) are far from those of heterogeneous catalysis (Figure 18). To achieve this goal, the advantage of plasma should be exploited in full, and at the same time, insights from heterogeneous catalysis (e.g., catalyst combination, reaction combination, active sites design, etc.) can help to further improve the potential of this promising field of catalysis.