Low-temperature hydrogenation of CO₂ to methanol: progress in constructing catalytic active sites

2.1 Formation of active sites with a high negative charge density

Electron transfer is critical in CO₂ activation, forming a surface carboxylate species (CO₂^δ−) (Graciani et al. 2014; Zheng et al. 2018). Studies have shown that alloying facilitates electron transfer from metals with a lower electronegativity to metals with a higher electronegativity, generating active metals with a high negative charge density that promote CO₂ activation. Khan et al. (2016) have reported Pt–Co alloy nanocrystal preparation using a chemical solvothermal method for application in the liquid-phase hydrogenation of CO₂ to methanol. The resultant Pt₃Co octopods delivered a turnover frequency (TOF) of 758 h⁻¹ at 150 °C and 32 bar CO₂/H₂, which was 6.1 times higher than that recorded for Pt octopods. As shown in Figure 1, the photoelectron Pt 4f_7/2 binding energy of the Pt₃Co octopods was shifted by 0.9 eV (71.3 → 70.4 eV) when compared with Pt octopods as a result of electron transfer from Co to Pt, increasing the negative charge associated with the Pt sites. Infrared reflection absorption spectroscopy (IRRAS) and density functional theory (DFT) calculations have indicated that Pt atoms at the apex of the Pt₃Co octopods bear a high negative charge density, which promotes the activation of CO₂ to generate CO₂^δ−. The formation of some alloy phases serves to enhance methanol synthesis, notably the Ni₅Ga₃ alloy which was shown to increase methanol selectivity by inhibiting the RWGS reaction and circumventing CO as by-product (Studt et al. 2014; Men et al. 2020). The application of a colloidal Pd₂Ga alloy in CO₂ hydrogenation delivered a methanol STY that was 4 times higher than Cu/ZnO/Al₂O₃ (García-Trenco et al. 2017).

Figure 1:

XPS spectra for Pt nanocrystals (Khan et al. 2016); reproduced with permission from John Wiley and Sons (copyright 2016).

A higher negative charge density facilitates transfer of electrons to the antibonding orbitals of adsorbed CO₂ and facilitates the adsorption of the (HCOO*) intermediate for further hydrogenation to methanol. Zhang et al. (2017a) have reported the synthesis of a Rh₇₅W₂₅ alloy NSs/C by chemical solvothermal, which was used to promote the liquid-phase hydrogenation of CO₂ at 150 °C and 32 bar CO₂/H₂. The resultant TOF_Rh (592 h⁻¹) was 4 times higher than that obtained with Rh NSs/C, with an increased methanol selectivity (from 95 % to 97 %). The electron transfer from W to Rh resulted in a lower Rh 3d_5/2 XPS binding energy for Rh₇₅W₂₅ NSs (Figure 2).

Figure 2:

XPS spectra for Rh nanocrystals (Zhang et al. 2017a); reproduced with permission from the American Chemical Society (copyright 2017).

The FTIR spectrum recorded following CO₂ adsorption on Rh₇₅W₂₅ NSs at 150 °C is shown in Figure 3 where the absorption bands at 1,500−1,600 cm⁻¹ are attributed to CO₂^δ− asymmetric stretching vibrations associated with CO₂ activation.

Figure 3:

In situ DRIFT spectra of CO₂ adsorption on Rh nanocrystals at 150 °C (Zhang et al. 2017a); reproduced with permission from the American Chemical Society (copyright 2017).

Contacting Rh₇₅W₂₅ NSs with CO₂/H₂ at 150 °C resulted in CO₂^δ− hydrogenation to the intermediate HCOO*, generating a higher associated FTIR peak intensity (at 2,961, 1,684, 1,453, and 1,047 cm⁻¹) relative to the CO₂^δ− (1,500−1,600 cm⁻¹) signals observed for Rh NSs (Figure 4). In addition, the C = O vibration peak (1,684 cm⁻¹) exhibited a red-shift (to 1,693 cm⁻¹) in the transition of CO₂^δ− to HCOO*, indicating a stronger adsorption of HCOO*. The negatively charged active site facilitates CO₂ activation to generate CO₂^δ−, and enhances adsorption of the HCOO* intermediate, inhibiting the formation of formaldehyde as by-product, which enables selective hydrogenation to methanol.

Figure 4:

In situ DRIFT spectra for Rh nanocrystals exposed to CO₂/H₂ at 150 °C (Zhang et al. 2017a); reproduced with permission from the American Chemical Society (copyright 2017).

2.2 Increased exposure of active site

The metal-carrier interface is viewed as the catalytically active site (Han et al. 2022) where the characteristics of the interface determine the level of selective CO₂ hydrogenation at low temperatures (Han et al. 2021). Alloys with grain boundaries present more accessible active sites to facilitate CO₂ activation. Grain boundary-rich RhCo NSs/C synthesized using a chemical solvothermal procedure, and employed in the liquid-phase hydrogenation of CO₂ at 150 °C and 32 bar CO₂/H₂ (Zheng et al. 2018), generated a TOF_Rh of 612 h⁻¹, 6.1 times and 2.5 times higher than Rh/C and RhCo NPs/C, respectively, with an associated methanol selectivity of 96.3 %, attributed to the high-density accessible grain boundaries. In situ XPS characterization has confirmed a high negative charge associated with the surface Rh atoms due to Co→Rh electron transfer that promoted the activation of CO₂ to generate CO₂^δ− and the intermediate H₃CO*.

Catalysts with faceted surfaces, rich in step and edge sites, facilitate effective CO₂ hydrogenation. A “zigzag” Pt₄Co NWs/C reported by Bai et al. (2017), exhibiting a Pt-rich surface with abundant steps/edges was prepared using an ultrasound-hydrothermal method. In the liquid-phase hydrogenation of CO₂ at 150 °C and 32 bar CO₂/H₂, the catalyst delivered a TOF_Pt of 1773 h⁻¹ with 100 % methanol selectivity and a STY of 81.4 mmol/g/h. This level of activity was superior to that of Pt NWs/C (TOF_Pt = 515 h⁻¹ and methanol STY = 28.7 mmol/g/h), and a commercial Pt/C (TOF_Pt = 17 h⁻¹ and methanol STY = 17.3 mmol/g/h). The Pt 4f_7/2 XPS binding energy for Pt₄Co NWs/C (70.7 eV) was significantly lower (by 0.7 eV) than that (71.4 eV) recorded for Pt NWs/C (Figure 5), indicating an electron transfer from Co to Pt that increased the negative charge on the supported Pt which enhanced CO₂ activation and subsequent hydrogenation (Khan et al. 2016).

Moreover, DRIFTS analysis has shown that the absorption peaks associated with the intermediate bidentate carbonate on Pt₄Co NWs/C exhibited a red-shift relative to Pt NWs/C, indicating a stronger CO₂ interaction with Pt atoms in Pt₄Co NWs. The results confirm that the surface structure rich in steps/edges exposes an abundance of active sites which, combined with the alloying effect, promote CO₂ activation.

Figure 5:

XPS Pt spectra for Pt–Co NWs (Bai et al. 2017); reproduced with permission from John Wiley and Sons (copyright 2017).

The application of Cu-based catalysts in CO₂ hydrogenation has been the subject of a number of studies (Chen et al. 2016; Chang et al. 2023; Huang et al. 2024). The combination of Cu and Au in the formation of alloy catalysts has improved performance in the low-temperature hydrogenation of CO₂ to methanol. Zinc oxide supported CuAu was employed to promote CO₂ liquid-phase hydrogenation at 200 °C and 50 bar CO₂/H₂ (Mosrati et al. 2023). When the Cu₉₃Au₇ alloy cluster loading was increased from 1 wt% to 10 wt%, the CO₂ conversion increased from 0.1 % to 3.4 %, but methanol selectivity was 100 % regardless of loading. The dependence of CO₂ conversion on loading demonstrates the low-temperature catalytic hydrogenation capability of the surface alloy sites. In contrast, Au/ZnO did not exhibit any activity. Commercial Cu/ZnO/Al₂O₃ with a Cu loading of 50.6 % delivered a CO₂ conversion of 3.5 % and lower methanol selectivity (88 %). When compared with Cu/ZnO/Al₂O₃, Cu₉₃Au₇/ZnO (with 10 wt% Cu loading) exhibited a five-fold higher intrinsic activity for methanol synthesis with inhibited CO formation. The addition of Au inhibited Cu particle agglomeration (from 30−40 nm to 2.7−4.4 nm), facilitating the exposure of more active sites. Characterization by XPS has demonstrated that, during CO₂ hydrogenation, metallic Cu was segregated in the outer shell of the nanoalloy whereas negatively charged Au represented the core of the nanoalloy. The copper segregation on the nanoalloy surface facilities better interaction with reactants and intermediates in promoting the reaction.

So far mere Ni₅Ga₃ non-precious metal alloy catalyst has been reported for low-temperature CO₂ hydrogenation to methanol. The preparation of Ni₅Ga₃ requires high-temperature reduction in H₂, which results in metal particle aggregation or surface reconstruction, thereby diminishing the active sites. By comparison, the preparation of noble metal alloys needs only mild reduction temperatures. To lower the reduction temperature, strategic modifications could involve tailoring the local atomic environment of Ni₅Ga₃ (e.g., by engineering coordination geometries, optimizing hydrogen spillover dynamics at interfaces, or incorporating noble metal species). Another approach is re-engineering the synthesis protocol to avoid the high-temperature reduction procedure, using sodium borohydride wet-chemical reduction. Furthermore, it is worth noting that the low-temperature CO₂/H₂ activation capability of non-precious metal alloys is relatively low, limiting their applications. Nevertheless, non-precious metal alloy catalysts are cheaper for industrial use, worthing further development.

Conventional co-precipitation, impregnation, and sol-gel methods cannot make metal salts completely into alloy phases. Alloy catalysts are currently synthesized using chemical solvothermal methods, which necessitate solvents, surfactants, and linking agents. Merely precious metal-containing alloys (Pt–Co, Rh–Co, and Rh–W) have shown promising low-temperature catalytic activity correlating to electron transfer between the metals, as demonstrated by XPS and Bader charge analysis. Current mechanistic characterization relies mainly on in situ DRIFTS and DFT calculations. Discrepancy sometimes arises because in situ spectroscopic conditions do not reach the actual reaction circumstances due to hardware limitations (e.g., inadequate pressure), rendering the analyses approximate rather than definitive. It is expected that the application of in situ EPR and operando infrared spectroscopy monitoring (steady-state versus transient analysis) (Gau et al. 2021) could provide valuable characterization information, but such exploration is lacking.

3.1 Non-metal doping

Doping with nitrogen can introduce H adsorption sites and lower the reaction activation energy, promoting CO₂ hydrogenation. N-doped Co₄N NSs were prepared by ammonia pyrolysis (Wang et al. 2017) and used in the liquid-phase hydrogenation of CO₂ to methanol at 150 °C and 32 bar CO₂/H₂. The resultant TOF (25.6 h⁻¹) was 64 times higher than that achieved with Co NS (0.4 h⁻¹) and 2.2 times higher than Cu/ZnO/Al₂O₃ (11.5 h⁻¹). This response was attributed to the lower hydrogenation activation energy due to N doping (43.3 kJ/mol for Co₄N NSs compared with 91.4 kJ/mol for Co NSs). An in situ DRIFTS analysis of Co₄N NSs in a CO₂ atmosphere (Figure 6a) exhibits peaks due to CO₂^δ− stretching vibrations (at 1,489 and 1,346 cm⁻¹) when the temperature was raised to 120 °C, representing the lowest temperature for CO₂ activation on Co₄N NSs. The in situ DRIFTS recorded at 40 °C (Figure 6b) reveals stretching vibration peaks for CO₂^δ− and N–H (3,140 cm⁻¹). An increase in temperature to 60 °C was accompanied by the appearance of C–H (2,955 cm⁻¹) and O–C–O (1,626 cm⁻¹) vibration peaks due to HCOO*. At 120 °C, the intensity of these peaks decreases, and peaks associated with the CH₂O* species appear. The results suggest that Co₄NH_x is beneficial for the activation and transformation of CO₂ at lower temperatures than observed for Co₄N NSs. The mechanistic analysis suggests that H₂ is dissociated on the Co, generating H atoms that are adsorbed on N sites associated with Co₄N NSs, which restructs to form the active Co₄NH_x phase, where the H of the amino group interacts directly with CO₂ to form the intermediate HCOO*. Moreover, the adsorbed H₂O* activates the H of the amino group through hydrogen-bonding interactions, promoting hydrogenation.

Figure 6:

In situ DRIFTS of Co₄N NSs in (a) CO₂ (b) CO₂/H₂ atmospheres (Wang et al. 2017); reproduced with permission from Springer Nature (copyright 2017).

Li et al. (2024) have reported the preparation of Co–N/C catalysts rich in pyridine N–Co bonds by ammoniation of a ZIF-67 precursor. A TOF of 47.8 h⁻¹ (methanol selectivity of 99 %) was recorded for the liquid-phase hydrogenation of CO₂ at 180 °C and 20 bar CO₂/H₂, which was 7 times higher than that achieved with non-ammoniated Co/C (mainly containing pyrrole-type Co-N_x sites). After five reaction cycles, the activity of Co–N/C exceeded 96 % of the original value. This stable performance is attributed to a significant reduction of electron localization at the Co center (transferring electrons from Co to adjacent N atoms) as a result of the incorporation of pyridine N. The methane side-product was inhibited. Nitrogen doping served to enhance CO₂ and H₂ adsorption on the Co–N/C surface, lowering the energy barrier for methanol formation (93.7 kJ/mol on Co–N/C versus 106.6 kJ/mol on Co/C). Hydrogen is dissociatively adsorbed on the Co–N/C surface and interacts with N atoms to form amino groups. Theoretical and experimental studies have shown that pyridine-N has a higher affinity for bonding with Co when compared with pyrrole-N and graphitized-N. Pyridine-N–Co serves as the active site during CO₂ hydrogenation. This study has established the critical role of the N configuration in determining CO₂/H₂ adsorption and activation.

Moreover, N doping enhances catalyst basicity for CO₂ adsorption (Donphai et al. 2023). The introduction of amino groups on the surface of activated carbon (AC) increased the CO₂ adsorption capacity with an increase in the surface N content to 1.38 % (Zhang et al. 2013). At 25 °C and 36 bar, CO₂ adsorption on AC-NH₂ and AC was 19.07 mmol/g and 16.67 mmol/g, respectively, indicating an effective adsorption on the basic N sites. An increase in temperature resulted in a lower CO₂ adsorption capacity that was more pronounced for AC relative to AC-NH₂. The latter exhibited CO₂ uptake at room temperature (25 °C), establishing a strong affinity of AC-NH₂ for CO₂ adsorption that enables low-temperature reaction. The doped N species can improve the dispersion of active metals, where Cu–ZrO₂/CNTs–N prepared by deposition-precipitation (Sun et al. 2018) exhibited strong interaction between pyridine-N and Cu. The addition of N promoted the dispersion and reduction of CuO, decreasing the grain size of Cu, and enhancing dissociative adsorption of H₂. The results have demonstrated that pyridine-N contributes to effective CO₂ adsorption and activation, promoting methanol formation.

3.2 Metal doping

3.2.1 Electron transfer promotion

Active sites with a high negative charge density are effective in the low-temperature adsorption activation of CO₂ (Khan et al. 2016; Zhang et al. 2017a). Adding promoters (Mg, Ga, La, etc.) facilitates electron transfer to the active sites (Xie et al. 2021; Cored et al. 2022; Guo et al. 2022). Furthermore, the strong electronic metal-support interaction (EMSI) between the metal and support enables electrons to transfer from the support to the active metal, related closely to the Fermi level (Frost 1988). Strong EMSI can alter the d-band center of transition metals, increasing the local electron density at the active interface (Jackson et al. 2017) which affects catalytic activity. Huang et al. (2024) prepared Er_0.2CuZnO by co-precipitation and achieved 8.5 % CO₂ conversion with 89.8 % selectivity to methanol for reaction at 170 °C and 50 bar, superior to the conversion (5.6 %) and selectivity (84.7 %) recorded for CuZnO. The improved performance was attributed to a decrease in ZnO particle size due to Er doping that serves to increase the Cu–ZnO interface with EMSI between Cu and ZnO. Doping Er promotes the transfer of electrons from ZnO to Cu to generate Cu^δ−. As shown in Figure 7, the Cu 2p_3/2 binding energy in Er_0.2CuZnO is lower (by 0.19 eV) relative to CuZnO, indicating a greater electron charge density associated with Cu NPs in Er_0.2CuZnO. In situ DRIFTS has demonstrated that Er doping promotes CO₂ adsorption activation and enhances the activation of surface carbonate species and hydrogenation of the intermediate CO* to HCO*.

Figure 7:

Copper XPS spectra for ErCuZnO and CuZnO (Huang et al. 2024); reproduced with permission from the American Chemical Society (copyright 2024). Selectivity (44.3 %), and methanol STY (0.80 mmol/g/h) on 1Pd–10Cu/CeO₂, were higher than the values (1.8 %, 84 %, and 0.31 mmol/g/h, respectively) recorded for Cu/CeO₂. This response was ascribed to the interaction between highly dispersed Pd and Cu that inhibited the migration and encapsulation of the active site by reduced CeO₂, resulting in a higher Cu concentration on the 1Pd–Cu/CeO₂ surface. The Pd component promotes the reduction of Cu species and surface CeO₂ by transferring electrons to Cu and CeO₂, generating more active Cu species. In addition, the reduction of surface CeO₂ (Ce⁴⁺→Ce³⁺) promotes the generation of O_v, enhancing the catalytic activation of CO₂.

The use of metal dopants to promote the formation of active sites is an effective method to enhance catalytic performance. The CeO_x/Cu (111) surface can stabilize the intermediate COOH*, promoting dissociation to generate CO*, and intermediate HCO* hydrogenation to methanol (Graciani et al. 2014). Palladium-doped Cu/CeO₂ increased CO₂ conversion and methanol yield (Choi et al. 2017). At 190 °C and 30 bar CO₂/H₂, the CO₂ conversion (8.8 %), methanol selectivity (44.3 %), and methanol STY (0.80 mmol/g/h) on 1Pd-10Cu/CeO₂, were higher than the values (1.8 %, 84 %, and 0.31 mmol/g/h, respectively) recorded for Cu/CeO₂. This response was ascribed to the interaction between highly dispersed Pd and Cu that inhibited the migration and encapsulation of the active site by reduced CeO₂, resulting in a higher Cu concentration on the 1Pd-Cu/CeO₂ surface. The Pd component promotes the reduction of Cu species and surface CeO₂ by transferring electrons to Cu and CeO₂, generating more active Cu species. In addition, the reduction of surface CeO₂ (Ce^{4+ →} Ce³⁺) promotes the generation of O_v, enhancing the catalytic activation of CO₂.

3.3 Metal oxide doping

4.1 Oxygen vacancy-promoted electron transfer

Oxygen vacancies (O_v) facilitate electron transfer, activating the linear CO₂ molecules with a consequent alteration to the structural geometry. These vacancies promote methanol desorption from active sites, increasing overall methanol production (Chang et al. 2023). Moreover, the O_v can induce SMSI, promoting the formation of active interfaces for CO₂ activation, and contributing additional stability to the dispersed active metals (Zhang et al. 2022). Toyao et al. (2019) have prepared Pt(3)/MoO_x(30)/TiO₂ by sequential impregnation of MoOx/TiO₂ with Pt NPs. Application in the liquid-phase hydrogenation of CO₂ at 150 °C (10 bar CO₂ and 50 bar H₂) resulted in a methanol yield of 73 %, approaching the equilibrium value. This was superior to yields recorded for other metals on MoO_x(30)/TiO₂ and Pt loaded on other supports (including Cu/Zn/Al₂O₃). In situ XANES analysis (Figure 8) has revealed that CO₂ adsorption on Pt(3)/MoO_x(30)/TiO₂ resulted in a positive shift in the Mo K-edge signals, indicating Mo oxidation by CO₂ with a consequent O_v reduction. This process serves to activate CO₂ and promote the subsequent hydrogenation reaction. The O_v sites formed during the H₂ pretreatment play an important role in CO₂ hydrogenation. This study has established that the combination of Pt and Mo is crucial for achieving high catalytic activity, where the reduced and O_v-containing MoO_x species are key to achieving low-temperature hydrogenation of CO₂ to methanol.

Figure 8:

In situ Mo K-edge XANES spectra of Pt(3)/MoO_x(30)/TiO₂ (Toyao et al. 2019); reproduced with permission from the American Chemical Society (copyright 2019).

Ultrafine tetragonal ZrO₂ rich in O_v (Chang et al. 2023) was used as a Cu support to catalyze CO₂ hydrogenation to methanol. The Cu/ZrO₂ (20CZ-OG) was prepared by oxalic acid-gel (OG) coprecipitation, which produced abundant O_v (36.2 %) generated by the self-reduction and thermal decomposition of oxalic acid complexes. By comparison, the O_v content of Cu/ZrO₂ (20CZ-AC) prepared by ammonium bicarbonate (AC) co-precipitation was 23.4 %. In situ ESR (Electron Spin Resonance) analysis has shown that the O_v can accept electrons from metal clusters, resulting inelectron enrichment. Electrons transfer from O_v to CO₂ serves to weaken the ESR signal, suggesting activation. In situ DRIFTS has demonstrated the contribution of O_v to CO₂ adsorption and the formation of COOH* intermediates. In addition, O_v facilitates methanol desorption, increasing overall yields. At 200 °C and 40 bar CO₂/H₂, a 90 % methanol selectivity was achieved using 20CZ-OG with a methanol STY of 5.62 mmol/g/h, significantly higher than that obtained with 20CZ-AC (2.81 mmol/g/h) and Cu/ZnO/Al₂O₃ (1.56 mmol/g/h and ⁓80 % methanol selectivity).

4.2 Sulfur vacancy (S_v) regulation of C–O dissociation and H₂ activation

Metal oxides such as In₂O₃ (Martin et al. 2016) and ZnZrO_x (Wang et al. 2023b) can catalyze CO₂ hydrogenation to methanol through an H-assisted dissociation mechanism at temperatures above 300 °C. In this case, the involvement of the RWGS reaction (Wang et al. 2020; Ye et al. 2024) results in low methanol selectivity. The introduction of transition metals (such as Cu, Pd, and Ni) to metal oxides can promote H₂ activation and enhance catalytic activity at relatively low temperatures (<260 °C) (Danaci et al. 2016; Rui et al. 2017; Zhang et al. 2023). However, an over-hydrogenation of CO₂ to CH₄ and the RWGS reaction is promoted, lowering methanol selectivity (Kattel et al. 2017a; Yin et al. 2018). The interplay between C–O dissociation and H₂ activation restricts a synchronous improvement in CO₂ conversion and selectivity. Research has found that S_v-rich FL-MoS₂ nanosheet catalysts (Hu et al. 2021) can address this limitation by promoting the dissociation of CO₂ into surface-bound CO* and O*, and activating H₂ on S_v. This serves to suppress deep hydrogenolysis of methanol to produce CH₄, achieving enhanced methanol selectivity. Reaction at 180 °C resulted in a 12.5 % CO₂ conversion over FL-MoS₂, with a methanol selectivity of 94.3 %, superior to the 4.2 % CO₂ conversion and methanol selectivity of 80.9 %, obtained with the commercial Cu/ZnO/Al₂O₃ catalyst. In addition, the FL-MoS₂ catalyst exhibited stable activity over 3,000 h, demonstrating the effectiveness of S_v in promoting the low-temperature hydrogenation of CO₂ to methanol.

Plane molecular structure causes MoS₂ challenging to generate in-plane S_v. (Zhang et al. 2024a) synthesized MoS₂-NS with in-plane S_v using NaCl-assisted method and applied it to CO₂ hydrogenation to methanol. Na facilitates not only the formation of in-plane S_v, but also the adsorption of CO₂ on S_v, following methanol production through the COOH* pathway, versus the dissociation pathway of CO₂–CO* on S_v in absence of Na. At 200 °C and 30 bar reaction, Na-modified MoS₂-NS-60 achieved methanol STY of 17.82 mmol/g/h with CO₂ conversion of 4.8 % and methanol selectivity of 96 %. TOF based on total S_v reached 170 h⁻¹. The main reason for the enhancement is the increased in-plane S_v and the promoted H₂ adsorption/dissociation.

O_v and S_v can both serve as active sites to promote the adsorption and activation of reactants, and both affect the stability of reaction intermediates by altering the surface electronic state of the catalyst. O_v enhances CO₂ adsorption and activation by exposing unsaturated metal sites or altering electronic structure, which requires synergistic activation of H₂ through metal sites such as Cu or Pt. O_v promotes the hydrogenation of CO₂ to form HCOO*/COOH* intermediates, reduces the subsequent hydrogenation energy barrier, and inhibits RWGS reaction. S_v directly adsorbs and activates CO₂ and H₂ by forming coordination unsaturated Mo atoms, forming CO and H atoms, avoiding over hydrogenation to produce CH₄, and improving methanol selectivity. It achieves efficient catalysis through the confinement effect of two-dimensional structure and optimization of S_v density.

CO₂ activation by O_v also has a limited impact in terms of low-temperature hydrogenation. A combination of precious metals (notably Pt) and partially reducible carriers (MoO_x(30)/TiO₂) is required to enhance the O_v contributions. The in-plane S_v associated with FL-MoS₂ represents a stable catalytic but the level of CO₂ conversion to methanol is still far removed from the equilibrium conversion.

5.1 Role of size (cluster) in promoting methanol formation

The dispersion of metals, within a specific range, is directly related to particle size. A significant number of studies have shown that the dispersion/size of active metals affects catalytic performance (Men et al. 2019; Zhu et al. 2020a; Zhou et al. 2023). If the interface between the metal and carrier represents an active site, the high dispersion/small metal size increases the number of active interfaces and enhances catalytic activity (Fujiwara et al. 2019; Zhang et al. 2024b). A low metal dispersion (or large particle size) limits catalyst activation and reaction rate. Furthermore, different degrees of dispersion produce active sites with different properties, affecting product selectivity. Ting et al. (2019) prepared a Re(1)/TiO₂ catalyst using ultrasound and impregnation. The catalyst was used to promote the liquid-phase hydrogenation of CO₂ at 150 °C and 60 bar CO₂/H₂, delivering a TON of 44 with a methanol selectivity of 82 %. An increase in Re loading (0.2−20 wt%) was accompanied by an initial increase and subsequent decrease in activity and methanol selectivity. At 1 wt% Re, the catalytic performance was optimal. At low Re loading, a high density of isolated Re atoms increased the generation of CO. At higher loading, the formation of larger Re NPs resulted in the formation of CH₄. At a Re loading of 1 wt%, highly dispersed and sub-nanometer-sized Re clusters were generated that promoted the formation of methanol. XANES characterization (Figure 9) has indicated that the Re valence state decreased with increasing reduction temperature. A low and high reduction temperatures did not result in methanol production. At the lower reduction temperature, Re exhibited a +6 valence. At higher reduction temperatures, Re exhibited a 0 valence, which resulted in Re sintering. Sample reduction temperature at 500 °C generated sub-nanometer-sized Re with a valence state between 0 and +4 that served as the active site for methanol synthesis.

Figure 9:

XANES spectra results for Re(1)/TiO₂ at different reduction temperatures: (a) L3-edge intensity and (b) binding energy chemical shifts (Ting et al. 2019); reproduced with permission from the American Chemical Society (copyright 2019).

In a comparative study conducted at 150 °C and 56 bar CO₂/H₂ (Phongprueksathat et al. 2023), Re/TiO₂ delivered a 4 % CO₂ conversion with a methanol selectivity of 65 %, whereas Cu/ZnO/Al₂O₃ exhibited a CO₂ conversion below 1 % and a methanol selectivity of 85 %. A higher Re loading resulted in larger Re particles that promoted by-products (CH₄ and HCOOCH₃) with lower CO₂ conversions and methanol selectivities. At a 3 wt% Re loading, highly dispersed Re^δ+ clusters with a high number of single atom Re⁰ species were generated. The Re⁰ sites act as an H₂ activator to transfer H to nearby Re^δ+ species responsible for activating CO₂ to form the HCOO* intermediate followed by hydrogenation to produce methanol. Both Re⁰ and Re^δ+ are required for Re/TiO₂ to efficiently promote methanol formation. Insufficient H provided by Re⁰ gives rise to a decomposition of formic acid into CO, whereas an excess results in over-hydrogenation to CH₄. In contrast, Re^δ+ activates CO₂, stabilizes HCOO*, and promotes hydrogenation to methanol.

The activity of catalysts is related to the size of active metal particles, resulting in structural sensitivity (Sun et al. 2017). Wu et al. (2017) have examined the effect of different Au particle sizes on CO₂ hydrogenation activity at 180 °C and 40 bar CO₂/H₂, demonstrating that the TOF for Au(1.6)/ZrO₂ (Au cluster size of 1.6 nm) was 20 h⁻¹ with a methanol selectivity of 75 %, much higher than the values (8 h⁻¹ with 55 % selectivity) for Au(2.4)/ZrO₂. The superior selective hydrogenation performance was attributed to the formation of smaller Au particles and more Au–ZrO₂ interfaces (where Au and ZrO₂ interact synergistically). Xiao et al. (2017) produced a series of Cu/ZnO/Al₂O₃/ZrO₂ catalysts using a co-precipitation hydrothermal method, applying different pH values; the samples are denoted as RHT-6, 7, 8, 9, 10, 11. An increase in pH was accompanied by an initial decrease and subsequent increase in Cu particle size, CO₂ conversion, and methanol STY, indicating a negative correlation between Cu particle size and activity. The RHT-9 catalyst, prepared at a pH of 9.0, exhibited the smallest Cu particle size, stronger interaction between Cu and ZnO, and the highest active Cu dispersion. Moreover, RHT-9 delivered the highest CO₂ conversion (10.7 %), methanol STY (2.72 mmol/g/h), and methanol selectivity (81.8 %), with a TOF of 7.34 × 10⁻³ (at the 190 °C and 50 bar CO₂/H₂). The results indicate a dependency of activity on Cu particle size. A smaller Cu particle size provides more active sites to convert the feed-gas, where a surface synergism between Cu and ZnO facilitates H₂ activation and dissociation. Smaller active metal particles lower the energy barrier for CO₂ activation, stabilizing intermediates, and improving methanol selectivity. In the case of Cu–TiO₂^HT and Cu–TiO₂^IMP catalysts synthesized using hydrothermal and initial wet impregnation methods, respectively (Sharma et al. 2021), Cu–TiO₂^HT exhibited smaller Cu NPs (3−5 nm) whereas a larger Cu NP size (approximately 15 nm) was associated with Cu–TiO₂^IMP. The Cu–TiO₂^HT delivered a superior catalytic performance with 9.4 % CO₂ conversion and a methanol selectivity of 96 % (at 200 °C and 30 bar CO₂/H₂), whereas CO₂ conversion over Cu–TiO₂^IMP was 6 % with a methanol selectivity of 62 %. The application of DFT calculations have shown that the activation barrier for CO₂ on Cu–TiO₂^HT with a smaller Cu size (93 kJ/mol) was significantly lower than that (127 kJ/mol) determined for Cu–TiO₂^IMP with a larger Cu size. A higher catalytic activity and methanol selectivity for Cu–TiO₂^HT were attributed to an enhanced dispersion of small-sized Cu NPs, which converted CO₂ to methanol via the RWGS pathway. Highly dispersed Cu particles increase the number of active sites for activation and dissociation of CO₂ into CO, improving CO₂ conversion. Smaller Cu NPs facilitate the formation of a greater number of Cu–TiO₂ interfaces, stabilizing the reaction intermediates (COOH*, CO*, and HCO*), inhibiting desorption, and achieving high methanol selectivity.

5.2 Enhanced catalytic activity and stability due to higher metal dispersion

Improving the dispersion of active metals, preventing their aggregation, and stabilizing interfacial active sites can enhance catalyst stability. The Cu–ZnO interface plays an important role in Cu/ZnO catalyzed CO₂ hydrogenation to methanol (Sun et al. 2020; Cui et al. 2021). The aggregation of Cu NPs diminishes the active Cu–ZnO interface, directly affecting catalytic stability and methanol selectivity. Bimetallic organic framework (bi–MOF) templates have been utilized to produce catalysts with well-distributed and well-arranged elements with an increased number of interfacial active sites (Qi et al. 2021). Han et al. (2022) prepared a CuZnO–MOF-74-350 using a Cu–Zn bi–MOF template that showed a preponderance of Cu–ZnO interfaces, which prevented the aggregation of Cu particles. In the CO₂/H₂ reaction conducted at 190 °C and 40 bar, the catalyst delivered a 7.5 % CO₂ conversion with a 78.2 % methanol selectivity and stable activity for more than 100 h. In contrast, the methanol selectivity for a commercial Cu/ZnO/Al₂O₃ declined from 62.7 % to 44.3 % over 15 h due to a sintering of Cu and ZnO phases (An et al. 2017). The better performance shown by CuZnO-MOF-74-350 was attributed to greater availability of Cu–ZnO interfaces, which facilitated CO₂ adsorption and stabilized the formate intermediate formate. The catalyst exhibited high Cu dispersion (52.0 %) when compared with the physically mixed MOF-based CuZnO–MM-350 (28.2 %), inhibiting sintering of the active metal particles.

The function of the carrier is to disperse the active metals, ideally achieving separation of single active metal atoms. When compared with clusters/NPs or large particles, single-atom metals exhibit superior catalytic performance associated with distinct sizes and valence states. Research has shown that the particle sizes (Karelovic and Ruiz 2015; Zhu et al. 2020b) and valence states (Samson et al. 2014; Yu et al. 2020; Liu et al. 2024) of active metals have a significant impact on catalytic performance. The activity of a series of Cu/ZrO₂ catalysts (CAZ-X, X = 1−15, representing Cu loading) is shown in Figure 10 for reactions conducted at 180 °C and 30 bar CO₂/H₂ (Zhao et al. 2022). The Cu in CAZ-1/2 was present in a highly dispersed single atom form, resulting in a methanol selectivity of 100 %. The loading amount affected the activity through changing the metal dispersion or size. When the loading was below 8 wt%, the CO₂ conversion exhibited a linear increase with Cu loading. The single Cu atom sites in the Cu₁–O₃ units are highly effective in dissociating H₂ with the assistance of O atoms, activating CO₂ to generate HCOO*.

Figure 10:

Catalytic activity of CAZ-X at 180 °C and 30 bar CO₂/H₂ (Zhao et al. 2022). Reproduced with permission from Springer Nature (copyright 2022).

A small number of reduced Cu clusters or NPs were formed in CAZ-4, which serve as active sites for RWGS. A greater number of small Cu clusters or NPs were formed in CAZ-8, resulting in increased CO selectivity. The large Cu particles associated with CAZ-12 and CAZ-15 did not activate CO₂ or contribute to CO₂ conversion and product selectivity. The CAZ-15-H catalyst was prepared by pretreating CAZ-15 with HNO₃, where the large Cu NPs were partially removed, generating an activity and actual Cu loading comparable to CAZ-8. This response suggests that the active hydrogenation sites are very stable, and not affected by treatment with a strong acid. A Cu loading of 8 wt% is required for effective hydrogenation activity. To date, no direct correlation has been observed between metal loading and reaction mechanisms. The loading level dictates the formation of active site configurations (e.g., single atoms, clusters, NPs), consequently altering product selectivity.

Correlating the EXAFS analysis data and DFT theoretical calculations, it was concluded that Cu in CAZ-1 before and after the reaction was present in a monodisperse state, with high thermal stability. The local structure of CAZ-1 consists of an isolated Cu and three O atoms (Cu₁–O₃ units) where the copper present as Cu⁺ is responsible for converting CO₂ into methanol. The presence of copper in CAZ-15 is in the form of small Cu⁰ clusters or particles and Cu^δ+ species (δ ≈ 1.4) serves to convert CO₂ into CO and methanol, respectively.

The reaction pathway for CO₂ hydrogenation to methanol at the Cu^δ+ (1 < δ < 2) active sites, based on DFT calculations, is shown in Figure 11. The energy barrier for CO₂ hydrogenation to form HCOO* is low (0.08 eV), whereas the energy barrier for forming COOH* is high (0.43 eV). Therefore, CO₂ hydrogenation to methanol at Cu^δ+ (1 < δ < 2) active sites follows the HCOO* pathway. This study has confirmed that, when Cu species are dispersed as single atoms and the ZrO₂ surface exhibits uniform Cu₁–O₃ active sites, CO₂ is selectively (100 %) converted to methanol. When the Cu species exist in clusters or small NPs, they inhibit the formation of methanol rather than serving as active sites for producing methanol. Large Cu particles are not active sites in CO₂ hydrogenation. Low loadings easily form single atom Cu sites serving as efficient active sites whereas high loadings easily form Cu clusters, small Cu NPs or large Cu particles, which are not conducive to methanol generation. Single atom Cu shows being the most effective active site. However, its activation ability of CO₂ or H₂, hydrogenation rate, and even stability need to be further explored to raise the activity. Generally, a lower loading is advantageous from the viewpoint of practical application, for considering both the cost and the maximizing utilization of metals. However, the metal loading must be controlled to achieve a single-atom state while maximizing the loading of active metal, thereby enhancing catalytic activity. There is currently no consensus on the optimal Cu loading for commercialization.

Figure 11:

Gibbs free energy for CO₂ hydrogenation at Cu^δ+ sites (Zhao et al. 2022); reproduced with permission from Springer Nature (copyright 2022).

An appropriate active site size effectively promotes selective CO₂ hydrogenation, but the requisite low loading to achieve a smaller size and higher dispersion limits this promotional effect. Developing active sites with the requisite size at high loading is very challenging. The use of MOF materials as supports is effective in obtaining higher loadings of well-dispersed active metals, but the feasible temperature window for catalytic operation has restricted practical applications.

7.1 Hydrophobic modification

The accumulation of water on the catalyst surface and the competitive RWGS reaction affect methanol synthesis. Surface water can contribute to the sintering of the active copper species and result in CuO formation, which decreases stability and methanol selectivity (Prašnikar et al. 2019). Hydrophobic modification of the catalyst surface can impede sintering and improve the catalytic performance. Wang et al. (2024) have reported the synthesis of Cu/ZnO by oxalic acid-assisted solid-phase grinding with treatment using stearic acid as a hydrophobic functionalization agent. The results indicate that the hydrophobic treatment with appropriate concentrations of stearic acid can remove residues from the decomposition of oxalic acid, increasing specific surface area, surface basicity and reducibility of Cu/ZnO, while hindering Cu aggregation. The surface modification also increased the H₂ adsorption capacity of Cu/ZnO with the formation of a surface H-rich region that enhanced the selective hydrogenation rate. Reaction at 200 °C reaction, generated a methanol selectivity and STY over Cu/Zn-2 mM of 78.8 % and 2.72 mmol/g/h, respectively, which was higher than the values (71.7 % and 2.15 mmol/g/h) obtained with Cu/ZnO. Furthermore, the Cu/Zn-2 mM was stable for over 100 h.

The influence of applicable to other supports (such as ZrO₂, Al₂O₃) for hydrophobic modification and different types of hydrophobic modifiers on the catalyst surface and the mass transfer of CO₂ have yet to be fully elucidated.

7.2 Hydrophilic modification

The surface-OH groups of catalysts determine hydrophilicity. By adjusting the surface hydrophilicity, the catalytic performance can be improved (Li and Zhao 2017). Surface hydrophilicity can influence the concentration of reactants on the catalyst surface. Peng et al. (2018) investigated the effect of surface –OH on SiC quantum dots (QDs) in CO₂ hydrogenation at the molecular level. The surface of SiC QDs exhibited a large number of hydroxyl groups, contributing to hydrophilicity. At 150 °C and 32 bar CO₂/H₂, the methanol STY reached 169.5 mmol/g for hydrophilic SiC QDs, more than three orders of magnitude higher than the STY (0.1 mmol/g/h) obtained with hydrophobic commercial SiC (Figure 15a). The TOF of SiC QDs was 0.27 mmol/m²/h, much higher than the commercial SiC (0.01 mmol/m²/h, Figure 15b). In addition, the methanol selectivity in both cases was 100 %.

Figure 15:

(a) Activity and (b) TOF_methanol for commercial SiC and synthetic SiC QDs (Peng et al. 2018); reproduced with permission from Elsevier (copyright 2018).

The reaction activation energy associated with hydrophilic SiC QDs was 48.6 kJ/mol, significantly lower than that (94.7 kJ/mol) recorded for the hydrophobic commercial SiC. Mechanistic studies have shown that the activity enhancement of SiC QDs is closely related to hydrophilicity. The abundant active sites for H₂ dissociation on the SiC (111) surface generate reactive H atoms. The H atoms of –OH on the SiC (111) surface react with CO₂ to form HCOO*, releasing O atoms that combine with dissociated H atoms to regenerate –OH. This reaction pathway serves to lower the energy barrier for HCOO* formation and promotes CO₂ activation, enhancing hydrogenation activity.

Furthermore, the study explored the use of Ni(OH)₂, NiO, NiCo LDHs, NiCo₂O₄, and other hydroxyl-rich catalysts for the low-temperature CO₂ hydrogenation to methanol. As expected, the metal hydroxides and LDHs exhibited significant enhancement to the activity versus their metal oxide counterparts. Specifically, the mass activity of Ni(OH)₂ and NiCo LDHs, were 18.4 and 14.4 times higher than that of NiO and NiCo₂O₄, respectively. Besides hydroxyl groups, the enhanced activity also possibly resulted from other factors such as hydrogen dissolution, spillover, reducibility, and surface oxygen atoms as a result of different phases of metal oxides and hydroxides. The surface hydroxyl group density may be regarded as an activity descriptor for CO₂ hydrogenation. Hydroxyl-rich nanocrystals were regarded as the candidate catalysts for CO₂ hydrogenation. The key challenge lies in unambiguously identifying the role of hydroxyl groups and systematically excluding confounding variables. Also, the effect of modifiers on the surface hydroxyl density of nanocrystalline catalysts remains unclear.

7.3 Physical hybridization of additives

The formation of methanol can be promoted by an appropriate water coverage of the Cu single atoms on ZnO (Wu et al. 2022b). Introducing 0.11 vol% water in CO₂ hydrogenation over Cu₁/ZnO at 170 °C and 30 bar resulted in an increase in CO₂ conversion to a maximum of 4.9 % after reaction for 3 h, with a subsequent decrease and stabilization at 1.8 %; the associated methanol selectivity was 99.1 %. In the absence of the water, CO₂ conversion and methanol selectivity were 1.0 % and 93.5 %, respectively. If the water concentration is too low to disrupt the dynamic reaction equilibrium, methanol production remains unchanged. Mechanistic studies have shown that water promotes the transformation of CO₂ into the COOH* and CH₂O* intermediates. An increase in water content promotes the water gas shift (WGS) reaction with CO, with an increase in methanol selectivity. DFT calculations (Figure 16a) have shown that the CO₂ hydrogenation reaction pathway on Cu₁/ZnO under anhydrous conditions involves the reaction of CO₂ with H to form HCOO* or COOH*. Formation of HCOO* is thermodynamically more favorable than COOH*, but still requires crossing an energy barrier (1.25 eV) that is higher than the barrier (0.81 eV) associated with COOH* formation. The COOH* pathway is more favorable, where COOH* is hydrogenated to form HCOOH*, which in turn generates H₂COOH*, CH₂O*, CH₃O*, and CH₃OH*. In this pathway, the apparent energy barrier for CO₂ hydrogenation to methanol is 1.49 eV.

Figure 16:

CO₂ hydrogenation pathways on Cu₁/ZnO (a) without and (b) with water (Wu et al. 2022b); reproduced with permission from John Wiley and Sons (copyright 2022).

In the case of CO₂ hydrogenation on Cu₁/ZnO in the presence of water, the energy barriers for the formation of COOH* and HCOO* are lower (Figure 16b), and the COOH* pathway is kinetically favorable. The apparent energy barrier mediated by water is 0.42 eV, much lower than that (1.49 eV) observed in the absence of water, confirming that water is favorable in increasing CO₂ hydrogenation rate over Cu₁/ZnO. Water provides H atoms to surface CO₂ and reaction intermediates, lowering the activation energy barrier and increasing activity.

The hydrophilic SiC QD surface bearing –OH groups can promote methanol synthesis. However, the applicability of this strategy in the case of metal oxides and even active metals has not been fully explored. Following surface hydrophobic/hydrophilic modification, the role of the available functional groups in determining the energy barrier associated with intermediate formation and the stability of the intermediates requires further investigation. The addition of trace water can serve as a source of H to generate the COOH* intermediate, lowering the activation energy barrier, as has been demonstrated for the Cu/ZnO catalyzed hydrogenation of CO₂. The addition of trace amounts of water for other catalysts has not been reported.

8.1 Catalytically active solvents

Fan et al. (1999) have reported a one-pot three-step reaction pathway for low-temperature methanol synthesis. The first step involves CO₂ hydrogenation to formic acid. In Step 2, formic acid reacts with an ethanol solvent to produce ethyl formate. In Step 3, ethyl formate hydrogenation produces methanol and ethanol. Using ethanol as the solvent, reaction at 200 °C, 30 bar CO₂/H_2, and for 20 h, the conversion of CO₂ over Cu–Cr–O was 15 % with a methanol selectivity of 80 %. The reaction when ethanol was not used as solvent proceeded at a lower rate, generating a lower methanol yield (by a factor of 10), confirming an alcohol-assisted pathway via the ethyl formate intermediate. Higher alcohols as co-catalysts promote methanol synthesis through the formic acid-ester intermediate route. Nieminen et al. (2018) reported methanol STY over Cu/ZnO of 0.26 mmol/g/h that reaction at 180 °C and 60 bar using 2-butanol as solvent. The control test using n-hexane as a solvent did not produce methanol, confirming the promotional effect of higher alcohol solvents.

Water as a product in CO₂ hydrogenation to methanol blocks the surface Cu active sites, lowering reaction rate (Sahibzada et al. 1998). Utilizing molecular sieves (porous crystalline aluminosilicate compounds) as dehydrating agents has promoted methanol synthesis over Cu/ZnO. Reaction over Cu/ZnO at 150 °C and 50 bar using ethanol as the solvent, generated a methanol selectivity and yield of 87.22 % and 33.80 %, respectively (Boonamnuay et al. 2021). With the addition of a 3A molecular sieve, the in situ adsorption of water generated, during the reaction served to shift the equilibrium in favor of methanol generation, increasing the methanol selectivity and yield to 91.16 % and 42.8 %, respectively.

8.2 Cascade catalysis

The reduction of CO₂ to methanol is a multi-electron (6e⁻) conversion process, and achieving enhanced reaction performance using a single catalyst is challenging (Huff and Sanford 2011). Cascade catalysis in a single reactor utilizes a catalytic step sequence conversion to achieve high feedstock conversion. In a cascade catalytic low-temperature CO₂ hydrogenation (Chen et al. 2015), a Cu–Cr catalyst was used to promote CO₂ hydrogenation to the formate intermediate, whereas Cu/Mo₂C catalyzed the hydrogenation of the formate to methanol. Operating the process at 135 °C and 40 bar, the selectivities of methanol and ethyl formate over Cu/Mo₂C (Figure 17a) were 74 % and 20 %, respectively. The ethyl formate selectivity over Cu–Cr (Figure 17b) was 97 %, where subsequent hydrogenation to methanol was largely inhibited. The cascade reaction utilizing Cu/Mo₂C and Cu–Cr (Figure 17c) resulted in an increase in methanol production by approximately 60 % compared with the total product formed in the single catalysis process. The selectivities of methanol and ethyl formate in the cascade reaction were 77 % and 20 %, respectively. The TOF associated with methanol synthesis in this cascade reaction reached 4.7 × 10⁻⁴ s⁻¹, demonstrating that Cu/Mo₂C and Cu–Cr catalysts synergistically catalyzed the hydrogenation of CO₂ to methanol via the ethyl formate intermediate.

Figure 17:

Cascade hydrogenation of CO₂ over (a) Cu/Mo₂C, (b) Cu-Cr, and (c) Cu-Cr +Cu/Mo₂C. Reaction conditions: 135 °C; 10 bar CO₂; 30 bar H₂; 2 mL ethanol; 35.5 mL 1,4-dioxane. (Chen et al. 2015); reproduced with permission from the American Chemical Society (copyright 2015).

Huff and Sanford (2011) have examined the application of homogeneous cascade catalysis to convert CO₂ to methanol via formic acid and formate ester intermediates at 135 °C (Figure 18). The reaction involves three steps: (a) CO₂ hydrogenation to formic acid; (b) esterification of formic acid to produce a formic acid-ester; (c) formic acid-ester hydrogenation releases methanol. The methanol TON generated by the cascade reaction over the three steps was 2.5, far lower than that (21) achieved by a stepwise cascade catalytic reaction, due to the incompatibility of the catalysts, where catalyst B (Sc(OTf)₃) served to deactivate catalyst C ((PNN)Ru(CO)(H)). The application of a heterogeneous process can facilitate effective separation with improved performance.

Figure 18:

Cascade hydrogenation of CO₂ to methanol via formic acid and formate ester (Huff and Sanford 2011); reproduced with permission from the American Chemical Society (copyright 2011).

8.3 Coupled CO₂ capture and hydrogenation to methanol

Kothandaraman et al. (2018) have studied the low-temperature hydrogenation of CO₂ to methanol using tertiary amines and ethanol as solvents. Operating a Cu/ZnO/Al₂O₃ catalyst at 170 °C and 60 bar CO₂/H₂, with ethanol and NEt₃ as promoters, the methanol yield reached 100 % (calculated based on the molar NEt₃ content). As shown in Figure 19, the reaction involves three steps. In step (i), H₂ is dissociated to generate surface reactive H atoms. In step (ii), CO₂ reacts with NEt₃ and EtOH to form an alkyl carbonate intermediate. In step (iii), the alkyl carbonate is hydrogenated to produce a formate salt that reacts with EtOH to form ethyl formate, which is then hydrogenated and dehydrated to produce methanol.

Figure 19:

Coupled CO₂ capture-hydrogenation to methanol (Kothandaraman et al. 2018); reproduced with permission from the Royal Society of Chemistry (copyright 2018).

Cascade hydrogenation of CO₂ via the formate ester intermediate pathway requires at least two catalysts with the requirement for effective separation. The associated reaction kinetics for stepwise reactions and integrated cascade systems warrant further study.