Review of Mechanism Investigations and Catalyst Developments for CO₂ Hydrogenation to Alcohols

Abstract

Heterogeneous thermal-catalytic CO₂ hydrogenation to alcohols using renewable energy is a highly attractive approach for recycling greenhouse gases into high-value chemicals and fuels, thereby reducing the dependence on fossil fuels, while simultaneously mitigating the CO₂ emission and environmental problems. Currently, great advances have been made on the heterogeneous catalysts, but an in-depth and more comprehensive understanding to further promote this reaction process is still lacking. Herein, we highlight the thermodynamic and kinetic analysis of CO₂ hydrogenation reaction firstly. Then, various reaction pathways for CO₂ hydrogenation to methanol and higher alcohols (C₂₊ alcohols) have been discussed in detail, respectively, by combining the experimental studies and density functional theory calculations. On this basis, the key factors influencing the reaction performance, such as metal dispersion, support modification, promoter addition and their structural optimization, are summarized on the metal-based and metal-oxide-based catalysts. In addition, the catalytic performance of CO₂ hydrogenation to alcohols and the relationship between structure and properties are mainly summarized and analyzed in the past five years. To conclude, the current challenges and potential strategies in catalyst design, structural characterization and reaction mechanisms are presented for CO₂ hydrogenation to alcohols.

1. Introduction

Massive anthropogenic carbon dioxide (CO₂) emissions lead to major environmental challenges, such as global warming and ocean acidification, predominantly caused by the continuing consumption of fossil fuels worldwide for rapid industry development. It is urgent to take highly efficient measures in alleviating CO₂ emissions to protect the environment and build a sustainable society. Recently, some CO₂ mitigation approaches have focused on carbon capture, utilization and storage (CCUS) technology, because CO₂ is a renewable carbon resource with the characteristics of safety, non-toxicity and abundant reserves. From a chemical point of view, converting CO₂ into some high-value-added chemicals and fuels has been considered as a promising method, which is of great significance for achieving the balance of economy, energy and environment [1]. There are various chemical processes for effective CO₂ utilization, for example, thermal catalysis, electrocatalysis, photocatalysis and organic catalysis synthesis, etc. [2,3,4,5], which can obtain many valuable products, e.g., CO, methane, olefins, aromatics, gasoline, urea, alcohols and so on [6,7,8,9,10,11,12]. However, chemical conversion of CO₂ is usually formidably challenging in both scientific and technical areas, due to it being thermodynamically stable and chemically inert [13]. When using hydrogen with a higher Gibbs free energy molecule as a reactant, the conversion reaction between CO₂ and H₂ is more favorable in thermodynamics. In addition, hydrogen can be obtained by some clean and green routes, such as the water splitting by electrolysis using renewable energy sources and biomass steam reforming. Therefore, converting CO₂ to valuable products has become one of the most practical carbon dioxide utilization approaches, especially using hydrogen as reactant.

Currently, many methods have been developed to achieve effective CO₂ hydroconversion, such as thermochemical, photochemical and electrochemical routes, etc. Among them, the thermochemical CO₂ hydrogenation process has attracted widespread attention, because of its many unique advantages, including high catalytic performance and controllable reaction conditions. For instance, the CO, CH₄ and methanol syntheses from CO₂ hydrogenation reactions have gained great progress in large-scale applications in the recent decades. Among the various products, alcohols, including methanol, ethanol and other higher alcohols, are considerably desirable, owing to their broad range of applications. The methanol usually serves as one of the most important chemical raw materials, with high worldwide annual production of about 65 million tons, which is applied in pesticide, medicine, chemical, building materials, automotive and other industries. Moreover, methanol can be used as an intermediate and chemical building block for the synthesis of other commodities and specialty products, such as light olefins, formaldehyde, acetic acid, dimethyl ether, methyl tert-butyl ether and gasoline [14,15,16]. In addition, methanol is a good energy carrier for fuel and additive for gasoline, owing to its high volumetric energy density and octane rating, respectively. As the energy storage materials in the concept of “Methanol Economy”, renewable methanol will promote good development prospects in green and clean energy utilization in the future. As for the higher alcohols, such as ethanol, they display similar merits as methanol above. Importantly, the higher alcohols have a higher energy density and lower toxicity compared with methanol, leading to more suitable applications in fuel additives and pure fuels [17]. As a result, the direct synthesis of alcohols from CO₂ hydrogenation has attracted wide investigation interest as a promising strategy to mitigate the prominent environmental issues.

In fact, to achieve effective CO₂ hydrogenation, high temperatures (200–300 °C), elevated pressures (3–10 MPa) and catalysts are industrially necessary to overcoming the high energy barrier. In the CO₂ hydrogenation to alcohols process, the major byproducts (CH₄ and CO) are more easily generated via the methanation and reverse water–gas shift reaction (RWGS), respectively. In addition, the uncontrollable C–C bond coupling path is another key influencing factor in the synthesis of higher alcohols. Thus, the precise and highly efficient catalysis of alcohol product synthesis from CO₂ hydrogenation is a constant challenge. In the past few decades, numerous advances have been made for CO₂ hydrogenation to alcohols, including the development of efficient homo- and heterogeneous catalysts, in situ characterization methods and theoretical calculations to study intermediate species and reactor design. The catalyst design and preparation play an important role in catalytic performance, in which the various precious (Pt, Pd, and Rh) and non-precious metals (Cu, Co and Fe), supports (ZnO, Al₂O₃ and ZrO₂) and promoters (K, Ga and Na) have been used for this reaction. Nevertheless, the key factors in determining the catalytic performance are difficult to figure out, because there are many affecting factors. In order to improve the process of CO₂ hydrogenation to alcohols, it is necessary to provide an in-depth and more comprehensive understanding, from the reaction mechanisms to the effective preparation catalysts and their systematic investigations.

Herein, we firstly summarize the reaction mechanisms by the thermodynamic and kinetic analyses of CO₂ hydrogenation reactions. The discussion is next extended to the structural characteristics of the metal-based and metal-oxide-based catalysts for CO₂ hydrogenation to alcohols, in detail and the structure–activity relationship is mainly discussed. Finally, we put forward the current crucial challenges and potential strategies for the future design and characterization of new and more efficient heterogeneous catalysts, as well as the reaction mechanism, so as to further encourage future investigation for this promising field.

2. Reaction Mechanism

2.1. Thermodynamic and Kinetic Analysis

Thermodynamic analysis of CO₂ hydrogenation to alcohols is important as it can not only be used to understand chemical reaction mechanisms, but also to optimize reaction conditions. Thermodynamically, CO₂ is a quite stable molecule, owing to its high oxidation state and standard Gibbs free energy (−394.38 kJ·mol⁻¹). The introduction of other reactants with higher Gibbs free energy (such as H₂) can promote CO₂ conversion smoothly [18,19]. In terms of the hydrogenation of CO₂ to alcohols, the reaction process steps need to be discussed one by one, owing to the many products, including CO, CH₄ and CH₃OH and C₂₊ products (e.g., C₂₊ alcohols, C₂₊ alkanes). As shown in

Table 1. Possible reactions involved in CO₂ hydrogenation to alcohols.

Entry	Reaction	Reaction Equation	∆G298K (kJ/mol)	∆H298K (kJ/mol)
1	CO2 hydrogenation to methanol	CO2(g) + 2H2(g) ⇋ CH3OH(g) + H2O(g)	3.5	−49.3
2	Reverse water–gas shift reaction	CO2(g) + H2(g) ⇋ CO(g) + H2O(g)	28.6	41.2
3	CO hydrogenation to methanol	CO(g) + 2H2(g) ⇋ CH3OH(g)	/	−90.6
4	CO2 hydrogenation to alkane	nCO2 + (3n + 1)H2 ⇋ CnH2n+2 + 2nH2O	/	/
5	CO2 hydrogenation to olefin	nCO2 + 3nH2 ⇋ CnH2n + 2nH2O	/	/
6	CO2 hydrogenation to higher alcohols	nCO2 + 3nH2 ⇋ CnH2n+1OH + (2n − 1)H2O	/	/
7	CO2 hydrogenation to ethanol	2CO2 + 6H2 ⇋ C2H5OH(g) + 3H2O(g)	−65.62	−173.74

, the direct hydrogenation of CO₂ to methanol reaction is an exothermic and volume-reduction process, leading to it being more favorable for reactions in a lower temperature and higher pressure. However, there are some side reactions at the same time, for example, reverse water–gas shift reaction in Entry 2. Although the CO can be hydrogenated to methanol in Entry 3, it needs some certain reaction conditions. From Entry 4, the thermodynamic characteristics of CO₂ hydrogenation to C₂₊ alcohol are similar to the methanol synthesis reaction, which is suitable in a low temperature and high pressure. In addition, the equilibrium constant of alkane synthesis reaction in Entry 5 and 6 is higher than that of synthesis of higher alcohols from CO₂ hydrogenation, resulting in the byproduct alkanes [20]. Methanol may also be another byproduct in CO₂ hydrogenation to C₂₊ alcohol reaction.

In the actual catalysis reaction system, the performance of CO₂ hydrogenation may be mainly affected by the kinetics process. For instance, although the low reaction temperature is thermodynamically beneficial to methanol synthesis, it will decrease the reaction rate. When raising reaction temperature, the CO by-product will increase via reverse water–gas shift reaction. Given the intricate thermodynamics and kinetics results, there are various possible reaction paths for CO₂ hydrogenation to alcohols. Therefore, combing the recent theoretical and experimental advances in mechanistic studies [21,22,23], the main proposed reaction pathways are summarized and proposed as follows in Figure 1, the understanding of which can give important guidance for the rational design of highly efficient catalysts.

2.2. Methanol Synthesis Reaction Pathway

Despite numerous efforts, the reaction mechanism of CO₂ hydrogenation to methanol is rather complicated and is yet to be fully understood. For example, when activating CO₂ with hydrogen, the O or C atom in CO₂ can be hydrogenated into COOH* or HCOO* intermediates, respectively. This results in many different pathways and has aroused great controversy. Based the current research progress, four main routes and corresponding intermediates are discussed, as shown in Figure 1.

2.2.1. HCOO* Pathway

The formate (HCOO*, * denotes the adsorption state) is generated by the reaction between preabsorbed surface H species and the carbon atom in the CO₂ molecule at the beginning via a typical Eley–Rideal or Langmuir–Hinshelwood mechanism. Then, the further continuous hydrogenation of HCOO* can form dioxymethylene (H₂COO*), formaldehyde (H₂CO*), methoxy (H₃CO*) intermediate and methanol product, respectively (Figure 1 pathway A). The HCOO* pathway above has been proposed in extensive catalysts, such as partial Cu-based, Pd-based and metal oxide-based catalysts, which are supported by the density functional theory (DFT) calculations and experiments.

Chen et al. [24] proposed that methanol formation follows the formate pathway on the CuZnZr/CuBr₂ catalyst, as verified by in situ diffuse reflection infrared Fourier spectrum (DRIFTS) experiments. Lam et al. [25] reported CH₃OH formation via formate intermediates on the metal–oxide interface with some specific Lewis acid sites of Cu/Al₂O₃ catalysts, by means of the DFT calculation and operando spectroscopy experiment. As shown in Figure 2a, Chang et al. [26] also observed the band of HCOO* (1578 cm⁻¹) and H₃CO* (2845 cm⁻¹) by the infrared measurement on the Cu-ZrO₂ catalyst for CO₂ hydrogenation to methanol. Dang et al. [27] found the rate-determining step (RDS) for CH₃OH formation on In₂O₃ is the hydrogenation process of HCOO* to H₂COO* by virtue of the DFT calculation. (Figure 2b). Tezsevin et al. [28] investigated the hydrogenation of CO₂ to methanol on the surfaces of holmium (Ho)-doped Cu (211) by DFT calculations. The relative energies of H₂COO* formation are lower than that of the HCOOH*. This is because the doping of Ho could stabilize H₂COO* intermediate instead of HCOOH* intermediate.

2.2.2. R-HCOO* Pathway

In addition to the hydrogenation of HCOO* to H₂COO* above, the HCOO* can be hydrogenated to HCOOH*, hydroxymethoxy (H₂COO*), following breakage of the C–O bond to H₂CO*, to then form H₃CO* and methanol, denoted as the r-HCOO* pathway (Figure 1 pathway B). Grabow et al. [29] found that the activation barrier of the HCOO* hydrogenation to H₂COO* (1.59 eV) is higher than that of the HCOOH* (0.91 eV), and the formation of CH₂O* is inclined to the HCOOH* rather than the H₂COO* dissociation on the Cu (111) by DFT results. Cui et al. [30] found the HCOOH* path displays a lower energy barrier of 1.32 eV, which is much lower than that of the H₂COO* path on the Cu₁₃/γ-Al₂O₃ catalyst by DFT calculation. Cai et al. [31] thought the hydrogenation of HCOOH* thermodynamically is favorable on Pd/In₂O₃ catalysts due to the low Gibbs free energy of (−0.49 eV) the HCOOH* hydrogenation in comparison to that of CO* hydrogenation to CHO* (1.00 eV) in Figure 3. In addition to theoretical calculations, Wang et al. [32] found that the HCOOH hydrogenation to H₂CO* on the Cd/TiO₂ acts as a rate-determining step for CO₂ hydrogenation to methanol by in situ diffuse reflectance infrared Fourier transform spectroscopy and DFT calculations.

2.2.3. RWGS + CO-Hydro Pathway

Although there is the by-product CO of the RWGS reaction in Table 1, the CO can also be hydrogenated to methanol. The RWGS+CO-Hydro pathway is proposed. Specifically, CO₂ may be hydrogenated into carboxyl species (COOH*) following the breakage of the C–O bond to CO*, or the CO₂ could be directly dissociated to CO*. The methanol can be obtained by continuous CO* hydrogenation (Figure 1 pathway C). Michiels et al. [33] found the CO* intermediate would be converted into HCO*, HCOO* or H₂CO*, instead of the formate pathway by the microkinetic model on Cu(111). It has been shown in Figure 4 that Kattel et al. [34] reported the RWGS + CO-Hydro pathway is the main reaction path on Ti₃O₆H₆/Cu(111), because the stable combination of the formate species and surface active sites may poison the active site. Shen et al. [35] found the energy barrier of the CO* hydrogenation to HCO*, as the rate-determining step (RDS) of the RWGS + CO-Hydro pathway, is 1.24 eV, which is much lower than that of the HCOO pathway from the DFT study on Ni₄/In₂O₃ catalysts.

2.2.4. Trans-COOH Pathway

The trans-COOH pathway refers to the CO₂ hydrogenation to carboxyl (COOH*), dihydroxycarbene (COHOH*), followed by the breakage of the C–O bond to COH* and then the continuous hydrogenation to CH₃OH (Figure 1 pathway D). Tang et al. [36] found the HCOOH intermediate in the HCOO pathway is unstable, and the energy barrier of HCOO* hydrogenation to HCOOH* (1.34 eV) and CO* hydrogenation to HCO* (1.52 eV) are high. However, the energy barrier of the formation of COH* via three isomers of COHOH (0.57 eV) is much lower on Ga₃Ni₅ (221) surfaces by DFT calculations. Zhao et al. [37] found the CO₂ hydrogenation to hydrocarboxyl (trans-COOH) is more favorable than the formate path in the presence of H₂O, due to the low energy barrier in methanol synthesis on Cu (111) by DFT.

2.3. Higher Alcohol Synthesis Reaction Pathway

The synthesis of higher alcohols from CO₂ hydrogenation is a complex process due to the coexistence of the C–O activation and C–C coupling processes, with difficulty in determining the reaction intermediates. Three widely accepted mechanisms are proposed, including the CO-mediated, the formate-mediated, and the methanol-mediated pathways.

2.3.1. CO Mediated Pathway

The CO-mediated pathway refers to the CO* firstly generated by the RWGS reaction, and then hydrogenated to CH_x* and the remaining non-hydrogenated CO* reacts with CH_x* species (including CO* insertion into alkyl or alkyl insertion into CO*) for the high alcohols (Figure 1 pathway E). An et al. [38] reported the Co⁰-Co²⁺ site on Co/La₄Ga₂O₉ catalyst promoted the CO*, formed by RWGS reaction, dissociation and hydrogenation to CH_x*, as well as the insertion of CO* and CH_x* to ethanol. Xu et al. [39] recently revealed the Cu-ZnO dual sites were responsible for the non-dissociative activation of C–O to form CO*, and iron carbide improved the dissociation of the C–O bond to form alkyl, while the addition of Cs regulated the hydrogenation ability of the catalyst and promoted the insertion of CO* on the Cs-modified CuFeZn catalyst. According to the in situ DRIFTS analysis, they found that the important intermediates are CH₃CHO* (2754 cm⁻¹) and CH₃CH₂O (29,662,876 cm⁻¹), and the formation of CH₃CO follows the CO or CHO insertion mechanism via the CO-mediated pathway (Figure 5).

2.3.2. Formate-Mediated Pathway

The formate mediated pathway, which is different from the CO-mediated pathway, refers to the CH_x* reacting with HCOO* rather than the CO* (Figure 1 pathway F). Wang et al. [40] reported the hydrogenation of CO₂ to ethanol over a non-noble cobalt catalyst. An unsaturated C–H (CH_x*) signal was observed at 2904 cm⁻¹ and formate species at 1373 cm⁻¹ and 1594 cm⁻¹ by operando FT-IR spectroscopy (Figure 6). The bands belonging to CH_x* and HCOO* decrease, and the saturated C–H (acetic acid) bands appear. These changes indicate that ethanol is formed by the insertion of CH_x* and HCOO*, resulting in the high selectivity of ethanol. Moreover, Wang et al. [41] found the Cu-Cs-ZnO interface promoted the dissociation of HCOOH into CHO*, the C–C coupling of CHO* to ethanol on Cs/Cu/ZnO(000$\overline{1}$), combining DFT calculations and Kinetic Monte Carlo (KMC) simulations.

2.3.3. Methanol-Mediated Pathway

The methanol-mediated pathway refers to the methanol dehydroxylation to the CH_x* intermediate, which then reacts with CO* or HCOO* intermediates to form C–C bonds. This process is repeated to grow carbon chains to form the higher alcohols (Figure 1 pathway G). He et al. [42] used ¹³CH₃OH labeling experiments to investigate the reaction path of CO₂ hydrogenation to higher alcohols on Pt/Co₃O₄ catalyst. When adding ¹³CH₃OH, the peak (m/z = 47) attributed to either ¹³CH₃CH₂OH or CH₃¹³CH₂OH was found, indicating the carbon atom of ethanol might be formed from methanol. Not only that, the authors found that water could promote the generation of the higher alcohols. The protonated methanol formed by water could dissociate to CH₃*, OH* and H*, in which the CH₃* could be further coupled with CO* to CH₃CO*, followed by further hydrogenation to ethanol. Yang et al. [43] proposed a similar reaction path for the RhFeLi/TiO₂ catalyst, in which the protonate methanol formed by the surface hydroxyls promotes the breakage of the C–O bond and the formation of CH₃* by in situ DRIFTS. The CO* obtained by RWGS was inserted into CH₃* to form ethanol (Figure 7).

3. Metal-Based Catalyst

Since CO₂ is a symmetric linear molecule and a strong electron acceptor, the metal, with the unpaired d electrons in the outermost electron orbitals of metal, can provide electrons for CO₂ activation. Additionally, the metal has a unique metallic bond with the d-band holes, which can accept electrons from the hydrogen with an electron donor, so its activation requires the acceptance of electrons. Therefore, metal-based catalysts have been widely applied to the CO₂ hydrogenation reaction, especially in the synthesis of alcohols, obtaining good catalytic performance from fundamental research to industrial applications. Metal-based catalysts are mainly divided into the non-noble metal-based catalysts and noble metal-based catalysts.

3.1. Non-Noble Metal-Based Catalysts

The supported non-noble metal catalyst has the advantages of high economic benefit, abundant resources, being easy to obtain and having many preparation methods. Compared with precious metals, non-noble metal catalysts with lower hydrogenation activity may be more conducive to improving selectivity. However, their low activity and poor anti-toxicity are still puzzling researchers. In the process of CO₂ hydrogenation to alcohols, non-noble metals, such as Cu and Co, have received extensive attention because of their excellent hydrogen dissociation and C–O bond dissociation ability. However, it is susceptible to sintering and the ability to promote C–C coupling is poor. Therefore, researchers improve it by loading different carriers and doping, so as to achieve high selectivity and excellent stability.

3.1.1. Cu-Based

Generally, copper (Cu), as a common transition metal in nature, has advantages of a low price and good electromagnetic properties (3d¹⁰4s¹). When used for CO₂ hydrogenation to alcohols, the Cu usually displays a good ability in the hydrogen dissociate and CO₂ activation [44]. However, there are some challenges in Cu-based catalysts, such as easy aggregation and sintering of Cu via Ostwald maturation in harsh reaction conditions [45]. In order to further enhance the catalytic performance, many strategies for the regulation of specific active sites have been carried out, including different levels of copper loading and support, as well as the addition of other metals. Therefore, the metal dispersion, metal alloy and metal–support interaction around the main active sites in the past five years are summarized.

Cu-Based Catalyst for Methanol Synthesis

The preparation method, as an important factor, can usually affect the catalytic active sites and reaction performance. For example, the high Cu dispersion can be obtained by the modification of the preparation method, leading to enhancement of the catalytic activity. Many traditional preparation methods, such as coprecipitation, impregnation, hydrothermal and sol–gel, have been widely studied in the catalysts for methanol synthesis. In recent years, the development of effective catalysts by a simple preparation method has continued to attract the attention. Lei et al. [46] prepared a series of CuO/ZnO/Al₂O₃ catalysts by direct combustion of citric acid, oxalic acid and urea as fuel, respectively. Among them, CO₂ conversion could reach 16.2%, and the methanol yield was up to 10.3% in CuO/ZnO/Al₂O₃ catalysts with citric acid as fuel, owing to its large specific surface area exposed to Cu. Additionally, a suitable preparation method can regulate the electronic property of Cu, such as the ratio of both Cu⁰ and Cu⁺ species. The Cu⁰ is mainly used as the active site for H₂ adsorption and dissociation, while the Cu⁺ can promote the cleavage of the C–O bond, thus affecting the methanol yield. Yu et al. [47] synthesized the Cu/SiO₂ catalyst by ammonia evaporation (AE) and flame spray pyrolysis (FSP), respectively. The methanol selectivity of FSP at the same conversion reached up to 79.3%, which was much higher than that of AE (31.5%). This was because the content of Cu⁺ in FSP was about five times that of the AE sample. More Cu⁺ could inhibit the dissociation of CO and change the reaction path from formate to RWGS + CO-Hydro with a low energy barrier by the DRIFTS results (Figure 8). Arena et al. [48] prepared the Cu–ZnO/ZrO₂ catalysts with a high total surface area, dispersion and the exposure of the active Cu phase based on reverse co-precipitation under ultrasound irradiation. The optimized catalyst displays a good catalytic performance for the hydrogenation of CO₂ to CH₃OH in comparison with a commercial Cu–ZnO/Al₂O₃ methanol synthesis catalyst.

The support (ZnO, Al₂O₃, ZrO₂, etc.) can also change the Cu dispersion, Cu⁺, oxygen vacancies and the interaction between Cu and support, so as to influence the catalytic performance. Hu et al. [49] found that Al₂O₃ in supported Cu/Al₂O₃ catalyst could disperse Cu well and stabilize Cu⁺ species, leading to a strong CO₂ adsorption capacity and the formation of more carbonate, bicarbonate and formate species for CO₂ hydrogenation to methanol, in comparison with the Cu/ZnO catalyst. The oxygen vacancy has advantages of regulating the geometric structure and local electrons for CO₂ adsorption and activation, which can be formed in the reducible supports, such as TiO₂, ZrO₂ and ZnO. Zhang et al. [50] thought that the oxygen vacancy with abundant electrons induced a strong metal–support interaction (SMSI) between Cu and TiO₂. This could stabilize the Cu nanoparticles and promote the formation of the defective Ti³⁺ site. The electron was easily transferred to the electron-deficient carbon atom in CO₂, forming the HCOO* intermediate, resulting in a lower activation energy and higher CO₂ conversion (12.5%), relative to the Cu/TiO₂ catalyst without oxygen vacancies (5.73%).

Some recent studies have shown that the interface of composite metal oxides could increase the number of oxygen vacancies. Zuo et al. [51] synthesized a series of ZrO₂ and CeO₂ solid solutions using a solvent evaporation self-assembly method with the supported Cu-based catalysts, which displayed a higher methanol selectivity than Cu/ZrO₂. This is because the doping of Ce could enhance the interaction between Cu and solid solution for the adsorption and activation ability of CO₂. Wang et al. [52] took the view that the oxygen vacancy in the ZnO-ZrO₂ support could promote CO₂ adsorption and activation, and the Cu enhanced the H₂ dissociation, leading to the enhancement of carbonate hydrogenation to formate by the DFT calculations and in situ DRIFTS experiments. Moreover, Song et al. [53] prepared the Cu/ZnAl₂O₄ catalysts with the strong interaction between Cu and ZnAl₂O₄ support by wet impregnation method, whose CO₂ conversion (11.3%) and space-time yield (STY) of methanol (242 g_CH3OH kg_cat⁻¹ h⁻¹) were higher than Cu/ZnO/Al₂O₃ catalysts. The authors thought the hydrogen dissociation could occur on both the ZnAl₂O₄ support and the high dispersion of Cu, which contributed to the formation of HCOO* intermediates with a low energy barrier.

In addition, the additives may modify the active site structure and influence the reaction performance. Tezsevin et al. [28] found Ho metal enhanced the interaction between CO₂ and Cu surface, resulting in a low energy barrier of HCOO* formation (−1.02 eV) and the enhanced activation of CO₂ on the holmium-doped Cu (211) surface by DFT calculations. Yan et al. [54] found that W metal doping on Cu/CeO₂ catalysts could promote the irreversible reduction of Ce⁴⁺ to Ce³⁺ and inhibit the formation of oxygen vacancies, leading to a high STY of 12.3 mol_MeOH kg_cat⁻¹ h⁻¹ by the formate pathway. In the absence of W, the oxygen vacancies on the CeO₂ surface were responsible for CO₂ dissociation to CO* by the RWGS+CO-Hydro path, whose STY of methanol was only 1.26 mol_MeOH kg_cat⁻¹ h⁻¹. The influence of different doping ratios on the Cu-based catalyst could affect the catalytic performance. Liu et al. [55] found the variation of Ir doping content on the Cu surface changed the rate-determining step of CO₂ hydrogenation to methanol. The rate-determining step was the formation of CO+OH on Ir₃Cu₆(111), while it was the formation of HCO* on Ir₆Cu₃(111).

Cu-Based Catalyst for Higher Alcohol Synthesis

The key to the higher alcohol synthesis is the coupling C–C bond, alongside the hydrogen dissociate and CO₂ activation, compared with the CO₂ hydrogenation into methanol. Copper-based catalysts are more conducive to promoting the non-dissociative hydrogenation of the C–O bond than its cleavage, making it easier to produce methanol rather than higher alcohols. Therefore, a series of modified Cu-based catalysts have developed to improve the formation of CH_x* species as well as the growth of the carbon chain. An et al. [56] studied the cooperative Cu^Ⅰ sites on a Zr₁₂ cluster prepared by the metal–organic framework (MOF). The Cu^Ⅰ₂ could facilitate the hydrogen dissociation, while the Cu²⁺ and Cs⁺ provided an electron-rich environment to stabilize the formyl intermediate. They promoted C–C coupling of methanol and formyl to produce ethanol, leading to a high ethanol selectivity of >99% (Figure 9). Ding et al. [57] prepared a Cu@Na-Beta catalyst by embedding Cu nanoparticles into the Na-Beta zeolite, which displayed a good ethanol selectivity of 79% and yield of 14%. This is because the Cu nanoparticles promote CO₂ hydrogenation to form the CH₃* intermediate, while the zeolite skeleton limits the formation of byproducts such as methanol and formic acid. They promote the reaction of between CO₂ and methyl to form acetyl acid and hydrogenation to ethanol.

In addition, other metals have been introduced on the Cu-based catalysts to promote the adsorption of CO₂ by virtue of their interaction. Xu et al. [39] prepared a Cs-modified CuFeZn catalyst with a high C₂₊OH space-time yield of 73.4 mg g_cat⁻¹ h⁻¹. The authors concluded the Cu-ZnO was responsible for the non-dissociative activation of CO*, iron carbide promoted the C–O bond dissociation of CO₂ to alkyl intermediate and the addition of Cs improved the insertion of CO*. Zhang et al. [58] found the introduction of Ga could enhance the strong interaction between Cu and Co, and promote the formation of a rich Cu⁰/Cu⁺-CoGaO_x interface. This interface served as the main active center, leading to a high ethanol selectivity of 23.8% and the ethanol STY of 1.35 mmol_EtOH·g_cat⁻¹ h⁻¹ at CO₂ conversion of 17.8%.

3.1.2. Co-Based

In addition to Cu, other non-noble metals, such as cobalt (Co)-based catalysts, have also been applied for CO₂ hydrogenation to alcohols. The electronic configuration of transition metal Co is 3d⁷4s², which has a stronger electron variation ability compared with Cu. This leads to a good C–O dissociation activation ability, and it is more likely to form methane. Recently, various methods, such as the introduction of the supports and additives, have been taken on Co-based catalysts for CO₂ hydrogenation to alcohols.

Co-Based Catalysts for Methanol Synthesis

The interaction between the Co and the supports can adjust the electronic properties and increase the Co dispersion, which is used to inhibit CO₂ methanation and boost the methanol selectivity. As shown in the Figure 10, Wang et al. [59] reported Co@Si_x catalysts with abundant Co-O-SiO_n interfaces observed from in situ DRIFTS and XPS, which can inhibit the CH₃O* intermediate dehydrogenation of CO and the break of the C-O bond to methane, resulting in a high methanol selectivity of 70.5% and productivity of 3.0 mmol gcat⁻¹ h⁻¹. The Co alloying with other metals is also a significant method to change the Co dispersion and the electronic property. The study by Din et al. [60] prepared a zeolite-based Cu-Co bimetallic catalysts, which displayed the Cu and Co bimetals in zeolite support with good morphology and dispersion. With the increase of Cu content, the number of weakly and moderately basic sites increased; meanwhile, the methanol yield enhanced from 7.5 to 25.6 g Kg_cat⁻¹ h⁻¹. This was because the basic sites could significantly affect the adsorption and activation of CO₂.

Co-Based Catalyst for Higher Alcohol Synthesis

Given the strong hydrogenation ability of Co-based catalysts, these are more favorable to C–O dissociation activation, while the carbon chain growth becomes hard. The Co-based alloy catalysts by adding other metals, forming the existence of both Co⁰ and Co^δ+, which is considered a good method to inhibit methanation and promote chain growth for improving the catalytic higher alcohol synthesis performance. This is because the Co⁰ acts as the dissociative activation of CO₂ and H₂, and the non-dissociative activation of C–O will occur at Co^δ+. For example, An et al. [61] designed a series of CoGa_xAl_2-xO₄/SiO₂ (x = 0, 0.5, 1.0, 1.5, 2.0) catalysts for CO₂ hydrogenation to ethanol. The strong interaction between gallium oxide and cobalt induced the formation of the Co⁰-Co^δ+ active site, which promoted the process of ethanol synthesis by coupling the dissociation (CH_x*) and non-dissociation (CO*) of the intermediates. The ethanol selectivity reached up to 20.1% at 270 °C and 3.0 MPa on the CoGa_1.0Al_1.0O₄/SiO₂ catalyst. Zheng et al. [62] prepared a Co/La₂O₃-La₄Ga₂O₉ catalyst, which formed the Co⁰ and Co^δ+ due to the electron interaction between Ga and cobalt, based on the XPS and XRD results. A high CO₂ conversion of 9.8% and a selectivity of alcohol of 74.7% on Co/La₂O₃-La₄Ga₂O₉ catalyst were also obtained. The authors concluded that when CO₂ adsorbed on Co^δ+, the HCOO* intermediates formed, followed by decomposition into CO*. Meanwhile, CO₂ was dissociated and hydrogenated into CH₃* on the Co site. The coupling of CO* and CH₃* could occur and form CH₃CO*, and then it reacts with hydrogen into ethanol. In addition, Wang et al. [63] reported a cobalt catalyst incorporating a nickel species that could stabilize the CH_x* intermediates, thus avoiding the formation of methane. Not only that, the presence of Ni also made CO₂ hydrogenation form rich HCOO* species. Co promoted the insertion of CH_x* species into HCOO* (Figure 11), and then hydrogenation to ethanol with a high yield of 15.8 mmol g_cat⁻¹. Therefore, the addition of other metals to Co-based catalysts may also stabilize CH_x* intermediates, thereby improving the activity and inhibiting the formation of by-products, such as CH₄ and CO.

3.1.3. Others

Many other non-noble metal active components, such as Cd, Fe, and Ni, have been applied in CO₂ hydrogenation to alcohols because of their low cost and good catalytic hydrogenation ability. For example, Wang et al. [32] obtained TiO₂-supported Cd single atoms, clusters and nanoparticles using the different Cd loadings. Among them, the Cd clusters and TiO₂ support could help the stability of the HCOO* intermediate, leading to a lower energy barrier, good methanol selectivity (81%) and CO₂ conversion (15.8%), confirmed by the DFT calculations [64] and DRIFTS experiments. Fe has a unique ability to promote C–C coupling, which is usually used for higher alcohol synthesis. Xi et al. [65] prepared K-promoted bimetallic Fe- and In-based catalysts on a Ce-ZrO₂ support for CO₂ hydrogenation to higher alcohols. When Fe was added, both the RWGS reaction and C–C coupling ability were further enhanced. In addition, the alkaline structure additive K could inhibit the excessive hydrogenation of CO₂ to methane, thus improving the selectivity of higher alcohols. The optimized catalyst exhibited high CO₂ conversion of 29.6% and alcohol selectivity of 28.7%.

3.2. Noble Metal-Based Catalyst

Noble metals (Pd, Pt, Au, etc.) usually serve as good electron donors or acceptors due to the large atomic radius and the outer electrons being far away from the nucleus. The noble metal-based catalysts have abundant advantages in CO₂ activation via the electron interaction and hydrogen dissociation by metal–hydrogen bond, while facing the challenges of limited resources and high price. This results in a strong hydrogenation ability and C–C coupling ability for the hydrogenation of CO₂ to alcohols. Therefore, how to reduce the loading of noble metals while maintaining high catalytic activity has become the main research goal.

3.2.1. Pt-Based

The expensive Pt-based catalysts exhibit a strong adsorption and dissociation hydrogen activity and a weak activation ability of CO₂. Recently, the different Pt loading, support and additives have been used to further improve the catalytic performance and economic cost.

Pt-Based Catalyst for Methanol Synthesis

The interaction of Pt and supports (In₂O₃, TiO₂, MOF, etc.) has a synergic catalytic effect, which can offer unique active centers for CO₂ hydrogenation to methanol. Toyao et al. [66] prepared MoO_x/TiO₂-supported Pt nanoparticle catalysts for the selective hydrogenation of CO₂ to CH₃OH. The MoO_x with surface defect sites could activate CO₂, and Pt as an active metal promoted the formation of carbonate and formate, according to the in situ infrared analysis results, compared with MoO_x(30)/TiO₂ support, resulting in a high methanol yield of 73%. The author also analyzed the reaction of methanol synthesis with HCOOH and CO as reactants, respectively. As a result, CO hydrogenation to CH₄ occurred, and the reaction rate of HCOOH was higher than that of CO₂ hydrogenation to methanol. This indicated that CO₂ hydrogenation follows the formate pathway. Kuwahara et al. [67] found that a Pt-loaded molybdenum suboxide nanosheet structure showed a high CO₂ conversion and methanol yield, which was about 1.35 times higher than that of other molybdenum suboxide supports (including bulk, nanostrip and rod) under similar volume. This is because the high specific surface area of the Pt-loaded molybdenum suboxide nanosheet structure could form more oxygen vacancies for CO₂ adsorption and activation and inhibit Pt aggregation. Gutterod and colleagues [68] reported Zr-based UiO-67 MOF encapsulated Pt nanoparticle catalysts with good catalytic performance for CO₂ hydrogenation to methanol. Steady-state and transient kinetic studies involving H/D and ¹³C/¹²C exchange found that the interface derived from strong interaction between Pt and the linker-deficient Zr₆O₈ nodes acted as the main active centers to enhance the formation of methanol, in which the Pt was responsible for hydrogen dissociation, and the adsorbed CO₂ and hydrogen overflowed would form formate species at the Zr node (Figure 12a).

In addition, the different sizes of Pt have significant effects on the reactivity performance of synthesis methanol. Han et al. [69] found the atomically dispersed Ptⁿ⁺, formed by the interaction between In₂O₃ and Pt, could promote the CO₂ hydrogenation to methanol, due to the heterolysis of H₂ on the atomically dispersed Ptⁿ⁺ as the Lewis acid. Nevertheless, the Pt nanoparticles were inclined to the RWGS reaction, owing to the homolysis of H₂ and formation of CO. Furthermore, Li et al. [70] proposed adjacent Pt monomers on MoS₂ support, which exhibited a high catalytic activity compared with the isolated Pt monomers. Through theoretical study, it was found that the main intermediate, COOH* hydrogenation to C(OH)₂*, was the main reaction step on the isolated Pt monomer. However, the synergistic effect between two Pt atoms on adjacent Pt monomers could promote COOH* hydrogenation into HCOOH with a low activation energy, resulting in a good catalytic activity (Figure 12b,c).

Figure 12. (a) Schematic presentation of the postulated reaction mechanism of CO₂ hydrogenation to the formate intermediate in CH₃OH formation at the Pt–Zr node interface [68]. Steps for the addition of an H atom to COOH* over (b) isolated Pt monomers and (c) adjacent Pt on MoS₂ [70].

Figure 12. (a) Schematic presentation of the postulated reaction mechanism of CO₂ hydrogenation to the formate intermediate in CH₃OH formation at the Pt–Zr node interface [68]. Steps for the addition of an H atom to COOH* over (b) isolated Pt monomers and (c) adjacent Pt on MoS₂ [70].

Pt Base Catalyst for Higher Alcohols

The Pt-based catalyst shows excellent catalytic performance in the synthesis of methanol due to its excellent hydrogen dissociation ability. Not only that, Pt also has the ability to promote the RWGS reaction, and the resultant CO can be used as an intermediate to further generate higher alcohols, thus increasing the selectivity of higher alcohols in the CO₂ hydrogenation process. Ouyang and co-workers [71] prepared nanorod and nanoplate Co₃O₄-supported Pt catalysts (Pt/Co₃O₄-p and Pt/Co₃O₄-r), respectively. After hydrogen pretreatment at 200 °C, the part of Co₃O₄ on Pt/Co₃O₄-p was reduced to Co and CoO, while the Co₃O₄ and CoO co-existed on Pt/Co₃O₄-r. The synergistic effect of between Pt and Co₃O₄-p oxygen vacancy in Pt/Co₃O₄-p promoted a higher yield of higher alcohol of 0.56 mmol g_cat⁻¹ h⁻¹. Subsequently, Liu et al. [72] further studied the influence of the ordered mesoporous structure of Co₃O₄-m. In addition to the enhancement of RWGS reaction by means of Pt, the Co metal was conducive to the hydrogen dissociation, and the surface oxygen vacancies formed by unreduced Co₃O₄ could enhance CO₂ adsorption. The ordered mesoporous structure of Co₃O₄ could help the CH_x further form C₂₊ chains via the growth of carbon chains.

3.2.2. Pd-Based

Similarly, Pd-based catalysts show good dissociation of hydrogen but a weak binding ability between Pd and CO₂, which take advantage of the interaction between Pd and supports to regulate the electronic state and geometry structure of active site on Pd-based catalysts for CO₂ hydrogenation to alcohols.

Pd-Based Catalyst for Methanol Synthesis

A series of Pd/ZnO catalysts with different Pd particle sizes, varying from 1.6 to 7.9 nm, were synthesized by Zhang et al. [73] using the atomic layer deposition (ALD) technique. With the increment of Pd particle sizes, the Pd/ZnO catalysts displayed a strong metal–support interaction was found in the XPS and H₂-TPR results, followed by a higher CH₃OH selectivity. This was because the stronger bond between Pd and CO intermediates occurs on the larger the particle size catalysts through the RWGS+ CO-Hydro pathway. In a report by Tian et al. [74], the highly dispersed Pd species reduced the energy barrier of H₂ dissociation, thus promoting hydrogen dissociation and the strong metal–support interaction between Pd and MnO/In₂O₃ promoted hydrogen spillover. The CO₂ was adsorbed at the defective oxygen site, forming the formate intermediate, resulting in a good methanol selectivity of 70% at 280 °C. Since the oxygen vacancy is key to CO₂ conversion, the promoters were introduced into the Pd-based catalyst to improve the oxygen vacancy. Jiang et al. [75] added a series of promoters, including La, Zr, Cr, Ca, Co, Cd, Zn, Bi and K, on the Pd/CeO₂ catalyst by the co-impregnate method. Along with the oxygen vacancy increases, the CO₂ conversion showed an approximate linear enhancement, and the maximum presented on the Zr-modified Pd/CeO₂ catalyst, owing to the entrance of Zr into the CeO₂ lattice (Figure 13).

Pd-Based Catalyst for Higher Alcohols

The metallic Pd size on supported Pd based catalysts can not only influence H₂ dissociation, butalso have an important impact on the C–C coupling in the formation of higher alcohols. Caparros et al. [76] prepared a series of different Pd particle sizes on the Pd/Fe₂O₃ catalysts. For the single-atom Pd/Fe₃O₄ catalyst, the selectivity towards ethanol reached up to 97.5% at 300 °C and the activity was 413 mmol_EtOH g_Pd⁻¹ h⁻¹. But when the Pd single atoms evolved progressively into Pd nanoparticles, the ethanol selectivity decreased gradually. This was because the interaction between Pd single atoms and Fe₃O₄ assisted a particular architecture for C-C coupling, and the transformation of CO₂ into CO occurred on the Fe₃O₄ support through the RWGS reaction. In addition, Lou et al. [77] prepared Pd dimers with a uniform Pd₂O₄ configuration in the Pd₂/CeO₂ catalyst for CO₂ hydrogenation to ethanol. According to the DFT calculation, CO₂ inclined to dissociate into CO* on Pd dimer, instead of the COOH* and HCOO* from CO₂ hydrogenation with high energy barrier. Since the energy barrier of CO desorption was high, the CO hydrogenation to CH₂OH* occurred. However, the dissociation ability of CH₂OH* was stronger than in comparison to the CH₂OH* hydrogenation methanol, leading to the formation of a CH_x* intermediate. The coupling of the strong CO* adsorption and the presence of CH_x* intermediate would occur and produce the ethanol. The Pd₂/CeO₂ catalyst could achieve a high ethanol selectivity of 99.2% and yield of 45.6 g_ethanol g_Pd⁻¹ h⁻¹.

3.2.3. Rh-Based

Although rhodium-based catalysts are commonly used for hydrogenation of syngas to ethanol [78], in the hydrogenation of CO₂ to alcohols, their catalytic performance might depend largely on the interaction between Rh and supports.

Rh-Based Catalyst for Methanol Synthesis

A recent study by Wang et al. [79] found that the highly dispersed Rh species on the Rh/In₂O₃ catalyst enhanced the hydrogen dissociation and hydrogen spillover, which promoted the formation of oxygen vacancy and increased the alkalinity of the In₂O₃ surface, thus promoting the adsorption and activation of CO₂. This led to a high CO₂ conversion of 17.1%, methanol selectivity of 56.1% and space-time yield of 0.5448 g_MeOH h⁻¹ g_cat⁻¹ under 300 °C, 5 MPa. In order to enhance the CO₂ activation by means of increasing oxygen vacancy, Lu et al. [80] further prepared the In₂O₃-ZrO₂-supported Rh-based catalysts (Rh/In₂O₃-ZrO₂). The addition of ZrO₂ could inhibit the reduction of In₂O₃, boost oxygen vacancy, and stabilize the catalyst structure, resulting in a good 66.5% methanol selectivity and space-time yield of 0.684 g_MeOH h⁻¹ g_cat⁻¹ at a high CO₂ conversion of 18.1% at 300 °C and 5 MPa. Additionally, the oxygen vacancy can be adjusted by the different crystalline phases. Cao et al. [81] prepared a series of Rh-based catalysts with various crystalline phases of TiO₂ (p25, anatase and rutile). Among them, the rutile-supported Rh-based catalyst displayed a high methanol selectively, while the major products of p25 and anatase were the methane and CO, respectively (Figure 14). The authors found that the highest content of oxygen vacancy emerged on the rutile-supported Rh-based catalyst, which could be conducive to the formation of methanol by the CO* intermediate.

Rh-Based Catalyst for Higher Alcohols

Although Ru has good hydrogen dissociation ability, the recent studies have focused on the enhancement of the dissociation of C–O bonds and C–C bond coupling for the formation of higher alcohols by means of the additives (e.g., Ti, V, Na and Li) in Rh-supported catalysts. Zheng et al. [82] constructed a Rh₁/CeTiO_x single-atom catalyst by embedding monoatomic Rh onto a Ti-doped CeO₂ support. The catalyst characterization demonstrated that synergistic effects of Ti-doping and monoatomic Rh could generate oxygen-vacancy-Rh Lewis-acid–base pairs, which could improve CO₂ activation and C–O bond cleavage of CH_xOH* into CH_x* and COOH* into CO* species, respectively. This, consequently, led to the remarkable high TOF of 493.1 h⁻¹ activity and 99.1% ethanol selectivity. The Ti-doping also induced Rh–O bond structural reconstruction, which contributed to the improved catalytic stability. Wang et al. [83] prepared the Rh/MCM-41 catalyst with VO_x dopants forming the presence of Rh⁰-Rh^x+ groups. Notably, the Rh⁺ species deriving from the electronic interaction between VO_x and Rh could promote CO adsorption and dissociated into CH_x*, followed by the CH_x* carbonylation to CH₃CO* (Figure 15a). Given the synergistic effect on the high dispersion Rh species by the confinement MCM-41 effect and the formation of VO_x-Rh interface sites, a good CO₂ conversion ~12% and ethanol selectivity ~24% were obtained.

In addition, the introduction of alkali metals into the Rh-based catalysts has been considered as an effective method in regulating the electronic state and intermediate species. For example, Zhang et al. [84] reported the Na-modified Rh nanoparticles embedded in the zeolite Silicalite-1 (Na-Rh@S-1) catalyst showed a high space-time ethanol yield of 72 mmol g_Rh⁻¹ h⁻¹ for CO₂ hydrogenation to ethanol. The Na⁺ induced the formation of Rh⁺ and Rh⁰, which accelerated the CO* dissociation to CH_x*, along with the coupling of adsorbed CH_x* and CO* to form ethanol (Figure 15b), confirmed by in situ DRIFTS results. On the contrary, there was only the Rh⁰ species as the main product of the Rh@S-1 catalyst without Na⁺, which underwent the hydrogen dissociation and CO₂ hydrogenation to CO*, followed by the further dissociation and hydrogenation to CH_x* and CH₄. Kusama et al. [85] reported similar results to the above in the Rh/Al₂O₃ catalyst, in which the main product of the Rh/Al₂O₃ catalysts was the methane, while Rh/Al₂O₃ catalysts with Li modification changed the electronic state of the catalyst, resulting in the increase of intermediate bridge CO* to form the ethanol, with a good ethanol selectivity of 15.5% at 7% CO₂ conversion at 513 K and 5 MPa.

3.2.4. Others

Similarly, the other noble metals, such as Ru and Ir, also have a good ability of hydrogen dissociation and CO₂ activation. As their methanol selectivity is usually low with the common support, the In₂O₃ with good methanol activity was used for the support of methanol synthesis due to the strong metal–support interaction. Wu et al. [86] found that the oxygen vacancy derived from hydrogen spillover from the Ru/In₂O₃ catalyst could promote CO₂ adsorption and activation by the strong metal–carrier interaction between Ru and In₂O₃ support. Combined by the DFT and energy barrier analysis, this oxygen vacancy enhances the direct decomposition of CO₂ into CO*, accompanied by the continuous hydrogenation to CH₃OH with a methanol space-time yield of 0.57 g_methanol g_cat⁻¹ h⁻¹ at 300 °C. Xiong et al. [87] further improved Ru/In₂O₃ catalyst by adding ZrO₂ in order to form the solid solution of ZrO₂ and In₂O₃. The formed In-O_v-Zr oxygen vacancy structure serves as the main active site, leading to a higher the space-time yield, which increased by more than 20%, in comparison with the Ru/In₂O₃ catalyst. In addition, Shen et al. [88] designed a series of Ir/In₂O₃ catalysts with different Ir loading for methanol synthesis from CO₂ hydrogenation, which displayed a high dispersion Ir and oxygen vacancy of In₂O₃ by means of the interaction between iridium and In₂O₃. As the Ir content increased, the higher activity was obtained, because it could promote the formation of oxygen vacancies and stabilize them. The optimized Ir/In₂O₃ catalyst showed a high methanol selectivity over 70% and space-time yield of 0.765 g_MeOH h⁻¹ g_cat⁻¹ at CO₂ conversion of 17.7% under the reaction conditions of 300 °C and 5 MPa. The metal Ir promoted hydrogen dissociation and In₂O₃ reduction to produce oxygen vacancies, resulting in altering the formate route of In₂O₃ to the RWGS+CO-Hydro pathway with a lower energy barrier on Ir/In₂O₃ catalyst.

4. Metal Oxide-Based Catalyst

As a result of their oxygen vacancy and reducibility, the metal oxides are used to adsorb and activate CO₂ for CO₂ hydrogenation reaction. Recently, the metal oxide catalysts have been extensively studied in the hydrogenation of CO₂ to produce alcohols.

4.1. In₂O₃-Based

In₂O₃ catalysts are prone to form the oxygen vacancy for CO₂ adsorption and activation, as well as the InO_x for hydrogen dissociation, thus showing high activity and good stability in the CO₂ hydrogenation reaction.

4.1.1. In₂O_3-Based Catalyst for Methanol Synthesis

In the methanol synthesis process, the other support is usually introduced because their interaction can improve the dispersion and prevent sintering. Zhang et al. [89] investigated In₂O₃/m-ZrO₂ and the CO₂ hydrogenation reaction pathway under reaction conditions. They pointed out that m-ZrO₂ as support could promote the reduction of In₂O₃ to InO_x rather than In⁰. There are more oxygen vacancies on the In₂O₃/m-ZrO₂, owing to the weak In-O bond, weakened by the strong interaction between ZrO₂ and In₂O₃. Not only that, when the concentration of In on the surface was low, InO_x (0 < x < 1.5) would “dissolve” into the m-ZrO₂ subsurface, resulting in the formation of CO via RWGS. As the content of In increased, the content of InO_x increased, which promoted CO₂ hydrogenation to methanol by the HCOO* in Figure 16a,b. A study was reported by Shi et al. [90], where they synthesized rod In₂O₃ catalysts modified by different contents of graphene oxide (GO) by precipitation method for CO₂ hydrogenation to methanol. GO would increase the dispersion of In₂O₃ and convert cubic In₂O₃ (440) into hexagonal In₂O₃ (110). These two different forms of In₂O₃ would form c-In₂O₃(440)/h-In₂O₃(110) homojunction and interaction. The results of DFT showed that the energy barrier forming an oxygen vacancy could be reduced by homojunction. Hydrogenation of CO₂ on the oxygen vacancy would form formates and methanol. The selectivity of methanol (over 76%) and the conversion of CO₂ (10.4%) on the optimized In₂O₃/GO catalyst were both improved compared with the In₂O₃ catalyst.

In addition, the introduction of metals is used to increase hydrogen dissociation and improve the performance of the In₂O₃ catalyst for CO₂ hydrogenation due to its weak hydrogen dissociation ability. Fang et al. [91] prepared an In₂O₃/Co/C-N catalyst, in which the Co metal promoted the hydrogen activation, the oxygen vacancy acted on the active site for CO₂ adsorption, the presence of N enhanced the electron interaction between Co and In₂O₃, preventing the In₂O₃ sintering. As a result, a high CO₂ conversion rate of 9.4% and methanol selectivity rate of 88.4% were obtained via the HCOO* pathway. Tian et al. [92] found the addition of Pd to the In₂O₃ catalyst could promote H₂ dissociation, thus providing sufficient H atoms for CO₂ hydrogenation. Experiments confirmed that there were abundant oxygen vacancies in In₂O₃, which could be used as the active site for CO₂ activation, as well as prevent Pd sintering. The methanol yield of the Pd-based catalyst (331.7 g_MeOH kg_cat⁻¹ h⁻¹) was much higher than that of pure In₂O₃ (92.8 g_MeOH kg_cat⁻¹ h⁻¹). Therefore, the catalytic activity was significantly improved through the synergistic effect between In₂O₃ and the active metal Pd. Rui et al. [93] reported an In₂O₃-supported Au catalyst, which exhibited a high space-time yield of methanol of 0.47 g_MeOH/(h·g_cat) at 300 °C, 5 MPa, and 21,000 cm³ h⁻¹ g_cat⁻¹. The Au^δ+−In₂O_3−x interface formed by electron transfer between Au and In₂O₃ behaved as the active centers (Figure 16c,d), in which Au^δ+ was responsible for H₂ dissociation and In₂O_3−x with abundant oxygen vacancies promoted CO₂ activation.

Figure 16. (a) Schematic diagram and (b) performance diagram of CO₂ hydrogenation on In₂O₃/m-ZrO₂ catalyst [89]. (c) In Auger peaks and (d)Au 4f peaks for the Au/In₂O₃ catalyst [93].

Figure 16. (a) Schematic diagram and (b) performance diagram of CO₂ hydrogenation on In₂O₃/m-ZrO₂ catalyst [89]. (c) In Auger peaks and (d)Au 4f peaks for the Au/In₂O₃ catalyst [93].

4.1.2. In₂O_3-Based Catalyst for Higher Alcohol Synthesis

Although the In₂O₃-based catalyst has advantages of H₂ dissociation and oxygen vacancy for CO₂ adsorption toward CO₂ hydrogenation to methanol, both the C–O dissociation activation and carbon chain growth still remain challenges. Recently, some metals have been introduced into the In₂O₃ catalyst to solve the question above for the hydrogenation of CO₂ to higher alcohols. Ye et al. [94] designed a Ir₁−In₂O₃ single-atom catalyst with a high ethanol selectivity (>99%) and initial turnover frequency (481 h⁻¹). The authors thought that single-atom Ir, formed by the oxygen vacancy on In₂O₃, could promote the CO₂ and activation by the Ir-C bond and oxygen vacancy adsorption on the O in CO₂ molecule (Figure 17). According to DFT calculations, the energy barrier of CO₂ dissociation to CO* on Ir/In₂O₃ was 0.48 eV, which was lower than the pure In₂O₃ of 0.77 eV. In addition, the Ir could promote the hydrogen dissociation and form the CH₃O* intermediates, followed by the coupling of CO* and CH₃O* to form ethanol. Witoon et al. [95] found that adding K-Co in the In₂O₃ catalyst could form the KCoO₂ and CoO. The CoO could promote the non-dissociative activation of C–O to form CO*, while the formation of K–O–Co could inhibit the hydrogenation of C_xH_y* intermediates to form hydrocarbons by means of enhancing the strong adsorption of H₂. As a result, the K-Co-modified In₂O₃ catalyst showed a good higher alcohol space-time yield of 169.6 g kg_cat⁻¹h⁻¹.

4.2. Solid Solution

In recent years, bimetallic composite oxide catalysts (In₂O₃–ZrO₂, ZnO-ZrO₂, Ce_xZr_1-xO₂) have attracted much attention for CO₂ hydrogenation to alcohols because of their excellent stability, abundant oxygen vacancy and unique synergistic effect of metals. Tada et al. [96] studied the effect of different Zn contents on Zn_xZr_1−xO_2−x catalysts. With the increase of Zn content, the main active site converted from the Zn–O–Zr sites to the ZnO nanoparticles, confirmed by the results of catalytic performance evaluation and structural analysis. In the Zn–O–Zr site, the Zr could promote the CO₂ activation, while the Zn was responsible for the hydrogen dissociation. Their synergistic effect resulted in a high methanol yield of 1.9 mmol h⁻¹ g⁻¹ at 300 °C by the formed formates and methoxys. Furthermore, Sha et al. [97] prepared the Ga-modified ZnZrO_x (GaZnZrO_x) solid solution catalyst, which showed a 8.8% CO₂ conversion and 630 mg g_cat⁻¹ h⁻¹ methanol space-time yield at 320 °C. The addition of Ga promoted the adsorption and dissociation of H₂ and increased the amount of oxygen vacancy for CO₂ activation, thus accelerating the hydrogenation of HCOO* to CH₃O* by the formate pathway. In addition, the weak hydrogen dissociation ability of the ZnZrO_x could be enhanced by introducing metal, thus promoting the conversion of CO₂. Lee et al. [98] studied a Pd-promoted ZnZrO_x solid solution catalyst for CO₂ hydrogenation to methanol. The atomically dispersed Pd, induced by the ZnZrO_x anchoring, contributed to the production of more surface oxygen vacancies and hydrogen dissociation. Moreover, the presence of Pd accelerated the formation of HCOO* and CH₃O* intermediates through HCOO pathway, resulting in a low apparent activation energy and higher STY_MeOH of 0.69 g g_cat⁻¹ h⁻¹, in comparison to the ZnZrO_x of 0.55 g g_cat⁻¹ h⁻¹.

5. Conclusions

This overview highlights the latest research developments in the catalyst synthetic schemes and structural properties of metal-based and metal oxide-based catalysts for CO₂ hydrogenation reaction, as well as their effect on reaction mechanisms. First of all, we have reported and discussed the important role of reaction mechanisms and catalyst design for methanol and higher alcohol synthesis from CO₂ hydrogenation based on the main metal-based and metal oxide-based catalysts. The metal dispersion, support modification, promoter addition and their structural optimization in supported heterogeneous catalyst systems has great influence on the CO₂ and hydrogen molecule adsorption, activation, hydrogenation reaction and desorption of reactants and intermediates, thus determining the catalytic performance of catalysts in methanol and higher alcohol formation from CO₂ hydrogenation. Moreover, based on the thermodynamics analysis and reaction mechanism for methanol synthesis (HCOO*, r-HCOO*, RWGS+CO-Hydro and trans-COOH pathways), as well as the formation higher alcohols (CO-, formate- and methanol-mediated pathways) in detail, many advances in comprehension and design of active centers for CO₂ hydrogenation have been proposed, such as the metal dispersion, metal alloy, metal–support interaction, geometry and electronic properties, as well as the defects and coordination structure. However, there are still numerous challenges in accurately and comprehensively comprehending the active centers, owing to the complexity and obstacles of heterogeneous catalysts. Aiming at improving the performance of metal-based and metal oxide-based catalysts, several strategies in catalyst preparation, characterization techniques and theoretical calculations needs to be further developed in the future. Therefore, this review could put forward valuable guidelines for the design of efficient catalysts based on the great advances in understanding and improving active centers of heterogeneous catalysts, such as, enhancing CO₂ activation via controlling the oxygen vacancies and physicochemical features of supports, promoting the dissociative adsorption and spillover of H₂ by the metal size and stabilizing the key intermediates for the CO₂ hydrogenation to alcohols reaction.