Hydrogenation of Carbon Dioxide to Methanol over Non-Noble Catalysts: A State-of-the-Art Review

: The malignant environmental changes caused by the ever-increasing amount of anthropogenic CO 2 emissions have been particularly prominent in recent years. To achieve carbon mitigation and carbon neutrality, CO 2 hydrogenation to methanol is regarded as a promising and sustainable route. However, the development of catalysts with exceptional performance and the establishment of a clear structure–activity relationship remain formidable challenges. Considering the lack of a state-of-the-art review on the catalytic progress of CO 2 hydrogenation to methanol over non-noble catalysts, we conducted a detailed review in terms of the thermodynamic analysis, catalytic development, and reaction mechanism. In this work, we mainly reviewed the latest research progress of different catalysts including Cu-based, In 2 O 3 -based, bimetallic, solid solution, and other catalysts. Meanwhile, we summarized the effects of the support materials, promoters, and preparation methods on the catalytic performance. In addition, we also summarized the possible reaction mechanisms of direct hydrogenation of CO 2 to methanol. Overall, this work would be of importance for the researchers to obtain a comprehensive understanding of the design and development of efﬁcient catalysts for CO 2 hydrogenation to methanol.


Introduction
Fossil fuels such as coal, oil, and natural gas have been consumed on an unprecedented scale to meet the increasing energy demands, which have caused massive CO 2 emissions in the environment.To date, global anthropogenic CO 2 emissions have reached 37 Gton CO 2 per year, which can cause a series of environmental problems, such as serious greenhouse effects, ocean acidification, and glacier melting [1][2][3].Therefore, the control and mitigation of CO 2 emissions in the atmosphere has become an urgent task to protect the environment.Carbon capture, utilization, and storage (CCUS) strategies have been proposed to reduce CO 2 emissions, which mainly can be divided into two categories: carbon capture and storage (CCS) and carbon capture and utilization (CCU) [4,5].Compared with CCS technology, CCU technology can convert the waste CO 2 into various value-added liquid fuels and platform chemicals, such as methanol, carbon monoxide, methane, dimethyl ether, and polycarbon, and is considered as a potential and sustainable pathway to achieve carbon neutrality [6][7][8].Among various products, methanol is an important platform compound, which can be further converted into low-carbon olefins, aromatics, gasoline, and other high value-added chemicals and fuels [9][10][11].Therefore, CO 2 hydrogenation to methanol using green hydrogen has received wide attention due to its great economic value and industrial application prospects.

Thermodynamic Analysis
Large-scale commercial methanol production is mainly derived from syngas.Compared with CO as a raw material (Equation ( 1)), the conversion and activation of CO 2 generally require numerous energy due to its highly stable molecule and low Gibbs free energy (Equation ( 2)) [34].Therefore, high temperature is beneficial for promoting CO 2 activation and accelerating the reaction rate.However, it should be noted that different from CO hydrogenation, the reverse water gas shift (RWGS) reaction (Equation ( 3)) generally competes with CO 2 hydrogenation reactions, where the RWGS reaction is endothermic, whereas the CO 2 hydrogenation reaction is exothermic [35,36].Therefore, as shown in Equation (2), the operating conditions of low temperatures and high pressures are favorable.In addition, unlike CO hydrogenation, CO 2 hydrogenation generally consumes more H 2 and generates more water due to higher oxygen content.The presence of water accelerates the sintering of the active site in the catalyst, resulting in catalyst deactivation and the reduction of methanol production in subsequent steps [37][38][39].Therefore, it is of importance to design an efficient catalytic system to promote CO 2 activation and suppress the poisonousness of the by-products for the catalysts at suitable temperatures and pressures [40].The CO hydrogenation to methanol reaction is as follows: CO + 2H 2 = CH 3 OH ∆H 298k = −90.6 kJ mol −  [54] 3Mg-C-ZZ SSC 3 (G) 2000 3 12.9 81.5 N.A. [55] a (W) = WHSV, mL•gcat ; N.A.:Not available.

Supports
The type of support has a great influence on the physical properties of the catalyst, and the activity of the catalyst for CO 2 hydrogenation to methanol is proportional to the surface area of Cu.In Cu-based catalysts, the support with high specific surface area can disperse the active components well and prevent the catalyst from deactivation due to sintering [35].The traditional supports primarily include Al 2 O 3 [56][57][58], ZnO [59,60], and ZrO 2 [61][62][63].However, the water generated by the side reaction can accelerate the sintering of the active sites, which leads to the catalyst being relatively easy to deactivate, and Al 2 O 3 has hydrophilic properties, so the development of other carriers to make up for this is necessary.Some emerging supports have been gradually successfully developed, such as mesoporous aluminosilicate support, Mg-Al layered double hydroxide (LDH) [43,45].For example, Kubovics et al. [46] synthesized a novel Cu-ZnO multicomponent catalyst as a 3D aerogel, using reduced graphene oxide (rGO) as a support.It was found that the addition of rGO to the catalytic system significantly increased the methanol production rate by fourfold compared to pristine Cu-ZnO NPs at 220 • C.This was mainly because the intrinsic activity of Cu was enhanced due to the strain imposed at the ZnO interphase, causing a misfit at the contact surface.Recently, MOF and zeolite have been potential supports due to their abundant pore structure and tremendous surface area.Generally, MOF and zeolite are beneficial for confining the growth of Cu particles, obtaining high Cu dispersion, maximizing interface sites, and strengthening their interaction with Cu by regulating electron transfer [64].For example, Duma et al. [47] prepared Cu-ZnO catalysts supported on an aluminum fumarate metal-organic framework (AlFum MOF) with high surface area and good porosity, which further promoted the homogenous, uniform dispersion of Cu and Zn active sites.As a result, the catalysts exhibited good activity, with a doubling loading of Cu and Zn over the AlFum MOF.Therefore, the CO 2 conversion increases from 10.8 to 45.6%, and the methanol production increases from 0.034 to 0.056 g MeOH •g cat −1 •h −1 .SiO 2 is widely used as a support for various heterogeneous cata- lysts due to its low price and high specific surface area.For instance, Shawabkeh et al. [65] studied the CO 2 adsorption on SiO 2 at different Cu loading to investigate the CO 2 interaction with the surface (mainly oxygen atom) via the sol-gel method from both experimental and theoretical points of view.The results suggested that an increase in the amount of CuO on the surface of SiO 2 improved the basicity of the adsorbent, resulting in more CO 2 uptake.Dalia Santa Cruz-Navarro et al. [66] synthesized Cu-based catalysts with ammonium salt and acidic ZSM-5 zeolite as support using the liquid phase (LPIE) and solid state (SSIE) ion exchange methods and compared their catalytic performances.The results showed that the catalyst prepared using the SSIE method had higher copper loading than the catalyst prepared using the LPIE method, which was attributed to the SSIE method promoting the diffusion of the volatile copper metal complexes through the internal channels of molecular sieves.

Promoters
The addition of promoters can change the acidity and alkalinity of the catalyst surface, enhance the interaction between the active components, form more defect sites on the surface, and change the product composition distribution.Cu-ZnO catalysts with various promoters have received wide attention.ZrO 2 and Ga 2 O 3 are considered important catalysts for the modification of Cu-ZnO [67].For example, Sun et al. [48] designed Cu-ZnO catalysts with different molar ratios of ZrO 2 using the co-current precipitation method and controlled the pH to regulate the distributions of Cu 0 and Cu + species.It was found that the CuZn10Zr (10 mol% ZrO 2 ) catalyst had the highest space-time yield to methanol with 0.65 g MeOH •g cat −1 •h −1 under the reaction conditions of 220 • C and 3 MPa.Their research showed that the addition of an appropriate amount of ZrO 2 was beneficial to promoting the dispersion of Cu to provide more active sites for H 2 and CO 2 adsorption and activation.Cored et al. [68] studied two CuO/ZnO/Ga 2 O 3 catalysts prepared using the co-precipitation method and compared the promoting effect of Ga 3+ -doped in the wurtzite ZnO lattice of a Cu/ZnO/Ga 2 O 3 catalyst with that of a zinc gallate (ZnGa 2 O 4 ) phase.The study indicated that Ga 3+ -doped ZnO has been considered as a more efficient promoter than ZnGa 2 O 4 owing to the presence of surface vacancies with loosely bounded electrons, increasing the conductivity of the material and enhancing methanol selectivity versus CO formation.Among the different interfaces between the samples, the interaction of Cu and Ga is more favorable in the Ga 3+ -doped ZnO sample by increasing the number of surface basic centers, which is important for the stabilization of intermediate species.Sang et al. [69] systematically studied activation mechanisms of CO 2 on the Ga-modified Cu surface (Figure 1) with different forms (Cu, Cu8Ga1, Cu6Ga3, and Ga 2 O 3 @Cu) using density functional theory (DFT) calculations combining the thermodynamics with chemical kinetics.DFT results and the Mulliken atomic charge of CO 2 indicated that CO 2 was transformed into chemisorbed CO 2 * with a lower reaction energy barrier, and two oxygen atoms of CO 2 obtained different charges at the interface between the metal surface and Ga 2 O 3 , suggesting that the charge imbalance of the CO 2 molecule was more favorable to the activation of C=O.For Cu, Cu8Ga1, and Cu6Ga3 catalysts, CO 2 behaved as physical adsorption, and the oxygen atoms of CO 2 gained the same charges.
formation.Among the different interfaces between the samples, the interaction of Cu and Ga is more favorable in the Ga 3+ -doped ZnO sample by increasing the number of surface basic centers, which is important for the stabilization of intermediate species.Sang et al. [69] systematically studied activation mechanisms of CO2 on the Ga-modified Cu surface (Figure 1) with different forms (Cu, Cu8Ga1, Cu6Ga3, and Ga2O3@Cu) using density functional theory (DFT) calculations combining the thermodynamics with chemical kinetics.DFT results and the Mulliken atomic charge of CO2 indicated that CO2 was transformed into chemisorbed CO2* with a lower reaction energy barrier, and two oxygen atoms of CO2 obtained different charges at the interface between the metal surface and Ga2O3, suggesting that the charge imbalance of the CO2 molecule was more favorable to the activation of C=O.For Cu, Cu8Ga1, and Cu6Ga3 catalysts, CO2 behaved as physical adsorption, and the oxygen atoms of CO2 gained the same charges.

Preparation Methods
It is well known that the preparation methods of catalysts have significant influences on the catalytic performances, which mainly include coprecipitation [22,23,49], impregnation [47,70,71], and sol-gel [65,70,72].However, to further enhance the activity, selectivity, water tolerance, and stability of Cu-based catalysts, some emerging methods have appeared in recent reports [25,46,50,73,74].For example, metal nanoparticles (NPs) attracted growing interest due to their outstanding applications in numerous fields, but so far, no large-scale synthesis of CuNPs has been reported using the disproportionation route and the reductive pathway.Ouyang et al. [75] report a method for obtaining Cu (0) nanoparticles (CuNPs) from readily available organocopper reagents (Figure 2).The method can be used to synthesize spherical CuNPs with excellent control of their size and shape on the decigram scale.Overall, the study offers many prospects for the rapidly developing field of copper plasmonic catalysis.Kubovics et al. [46] prepared its precursor CuOZnO NPs via conventional chemistry and designed rGO aerogels-supported CuZnO NPs with water repulsion as efficient 3D catalysts.Then, the supercritical CO2 was applied for the

Preparation Methods
It is well known that the preparation methods of catalysts have significant influences on the catalytic performances, which mainly include coprecipitation [22,23,49], impregnation [47,70,71], and sol-gel [65,70,72].However, to further enhance the activity, selectivity, water tolerance, and stability of Cu-based catalysts, some emerging methods have appeared in recent reports [25,46,50,73,74].For example, metal nanoparticles (NPs) attracted growing interest due to their outstanding applications in numerous fields, but so far, no large-scale synthesis of CuNPs has been reported using the disproportionation route and the reductive pathway.Ouyang et al. [75] report a method for obtaining Cu (0) nanoparticles (CuNPs) from readily available organocopper reagents (Figure 2).The method can be used to synthesize spherical CuNPs with excellent control of their size and shape on the decigram scale.Overall, the study offers many prospects for the rapidly developing field of copper plasmonic catalysis.Kubovics et al. [46] prepared its precursor CuOZnO NPs via conventional chemistry and designed rGO aerogels-supported CuZnO NPs with water repulsion as efficient 3D catalysts.Then, the supercritical CO 2 was applied for the nano-structuration and the fabrication of 3D devices with macro-and mesoporosity, and the targeted sample was obtained through H 2 reduction.nano-structuration and the fabrication of 3D devices with macro-and mesoporosity, and the targeted sample was obtained through H2 reduction.Santa et al. [66] studied the effect of the synthesis method on the physicochemical properties of Cu-based catalysts through two ion exchanges.The advantage of the ion exchange method in the liquid phase and the solid phase is that the compensating cation of the zeolite can be well exchanged with the metal ions with catalytic activity, so that the metal content in the pores reaches a high level and has good metal dispersion.The results show that both ion exchange methods are suitable for the incorporation of uniformly distributed Cu into the zeolite structure, and the solid ion exchange method has the highest exchange percentage.Ali et al. [76] prepared 30 wt%CuO49.65 wt%ZnO20.35wt%Al2O3 catalyst at glycine to nitrates (G/O) ratios between 0.1 and 1.23 through the solution combustion synthesis (SCS) method.In brief, in the experimental procedure, varying quantities of glycine were used as a fuel into a mixed nitrate precursor solution, and the generated mixture was subjected to continuous stirring.The mixture gradually transformed into a soft gel upon heating on a hotplate and underwent spontaneous combustion at 150 °C.Subsequently, the synthesized powder was subjected to calcination in a muffle furnace using static air at different temperatures.The heating and cooling rates during calcination were, respectively, set at +1 and −1 °C•min −1 for three hours.APrašnikar et al. [51] also prepared several perovskite-containing Cu/Sr/Ti materials using the one-step solution combustion method with different fuels and conducted a long-term stability test of the catalyst with the best catalytic performances (Cu5Sr3Ti2OX, citric acid, calcination at 650 °C).The results showed that the catalyst retained 91% of initial activity after 122 h and even preserved 71% of the activity.Han et al. [52] prepared hollow Cu@ZrO2 catalysts through the pyrolysis of Cu-loaded Zr-MOF and found that a lower pyrolysis temperature can enhance the porous structure and the metal-support interaction of Cu and ZrO2 (Figure 3).More specifically, low-temperature pyrolysis generated highly dispersed Cu nanoparticles with balanced Cu 0 /Cu + sites, more surface basic sites, and abundant Cu-ZrO2 interface in the hollow structure, which contributed to enhancing the catalytic ability of CO2 adsorption/activation and selective hydrogenation to methanol.Qu et al. [77] anchored a small amount of Cu on the surface of a ZnAl2O4 support using the ammonia evaporation method.The interaction between Cu formed through ammonia evaporation and Zn-O structure derived from the surface of ZnAl2O4 was stronger, and it tended to form a higher proportion of Cu + , which stabilized the methoxy group and improved the methanol production efficiency.Santa et al. [66] studied the effect of the synthesis method on the physicochemical properties of Cu-based catalysts through two ion exchanges.The advantage of the ion exchange method in the liquid phase and the solid phase is that the compensating cation of the zeolite can be well exchanged with the metal ions with catalytic activity, so that the metal content in the pores reaches a high level and has good metal dispersion.The results show that both ion exchange methods are suitable for the incorporation of uniformly distributed Cu into the zeolite structure, and the solid ion exchange method has the highest exchange percentage.Ali et al. [76] prepared 30 wt% CuO 49.65 wt% ZnO 2 0.35 wt% Al 2 O 3 catalyst at glycine to nitrates (G/O) ratios between 0.1 and 1.23 through the solution combustion synthesis (SCS) method.In brief, in the experimental procedure, varying quantities of glycine were used as a fuel into a mixed nitrate precursor solution, and the generated mixture was subjected to continuous stirring.The mixture gradually transformed into a soft gel upon heating on a hotplate and underwent spontaneous combustion at 150 • C. Subsequently, the synthesized powder was subjected to calcination in a muffle furnace using static air at different temperatures.The heating and cooling rates during calcination were, respectively, set at +1 and −1 • C•min −1 for three hours.APrašnikar et al. [51] also prepared several perovskite-containing Cu/Sr/Ti materials using the one-step solution combustion method with different fuels and conducted a long-term stability test of the catalyst with the best catalytic performances (Cu 5 Sr 3 Ti 2 O X , citric acid, calcination at 650 • C).The results showed that the catalyst retained 91% of initial activity after 122 h and even preserved 71% of the activity.Han et al. [52] prepared hollow Cu@ZrO 2 catalysts through the pyrolysis of Cu-loaded Zr-MOF and found that a lower pyrolysis temperature can enhance the porous structure and the metal-support interaction of Cu and ZrO 2 (Figure 3).More specifically, low-temperature pyrolysis generated highly dispersed Cu nanoparticles with balanced Cu 0 /Cu + sites, more surface basic sites, and abundant Cu-ZrO 2 interface in the hollow structure, which contributed to enhancing the catalytic ability of CO 2 adsorption/activation and selective hydrogenation to methanol.Qu et al. [77] anchored a small amount of Cu on the surface of a ZnAl 2 O 4 support using the ammonia evaporation method.The interaction between Cu formed through ammonia evaporation and Zn-O structure derived from the surface of ZnAl 2 O 4 was stronger, and it tended to form a higher proportion of Cu + , which stabilized the methoxy group and improved the methanol production efficiency.

In2O3-Based Catalyst
The traditional Cu-based catalysts have been extensively studied in CO2 hydrogenation to methanol [14,78].However, its further application is limited by the high activity of the RWGS reaction and the poor stability and sintering of the active phase induced by H2O.Compared with the aforementioned Cu-based catalysts, In2O3 has moderate adsorption capacity of CO2 and CO and significantly better methanol selectivity than Cu, Co, and noble metal catalysts.Therefore, it has attracted extensive attention from researchers.In addition, In2O3 is easy to load and modify the surface, which can further promote the CO2 and H2 activation, and stabilize key intermediates, providing great potential for the design and preparation of efficient methanol synthesis catalysts [20,28,79].

Metal Promoters
Compared with traditional catalysts, In2O3 demonstrates better ability to adsorb and activate CO2 owing to its abundant oxygen vacancy on the surface.However, the weak H2 dissociation ability of In2O3 hinders the hydrogenation of carbon species, resulting in a significantly low CO2 conversion rate of approximately 6%.It is a common strategy to introduce other metal elements into the In2O3 system and form M/In2O3 structure to enhance H2 dissociation, adsorption, and overflow capacity.
As a metal with strong H2 dissociation ability, Pd is often used to further enhance the performance of In2O3-based catalysts.For example, Tian et al. [80] prepared Pd-In2O3 catalysts with high activity using the solid phase method.Pd species have high dispersibility on the prepared Pd/In2O3 catalyst, which significantly promoted H2 dissociation and provided sufficient H adatoms for CO2 hydrogenation.In addition, partial Pd 2+ species can exist stably during CO2 hydrogenation, which facilitated methanol synthesis.Rui et al. [81] also prepared Pd/In2O3 catalysts by mixing the Pd/peptide composite and In2O3, followed by the removal of peptide through thermal treatment.In contrast, the catalyst was also synthesized using the conventional incipient wetness impregnation method.It is notable that the CO2 conversion of the former catalyst was higher than 20% at 300 °C, and the maximum STY was 0.89 gMeOH•gcat −1 •h −1 , which was higher than that of the catalyst prepared using the conventional method.This is mainly because the catalyst obtained using the former preparation method can obtain Pd-NPs with small particle size and high dispersion.The Pd NPs has a high hydrogen dissociation adsorption capacity, which can provide the hydrogen for the hydrogenation reaction and maintain the density of oxygen vacancies.Similar effects are also found for Pt, Rh, Au, and Ru supported on In2O3 [39].For instance, Sun et al. [82] prepared Pt-In2O3 catalysts using the deposition precipitation

In 2 O 3 -Based Catalyst
The traditional Cu-based catalysts have been extensively studied in CO 2 hydrogenation to methanol [14,78].However, its further application is limited by the high activity of the RWGS reaction and the poor stability and sintering of the active phase induced by H 2 O. Compared with the aforementioned Cu-based catalysts, In 2 O 3 has moderate adsorption capacity of CO 2 and CO and significantly better methanol selectivity than Cu, Co, and noble metal catalysts.Therefore, it has attracted extensive attention from researchers.In addition, In 2 O 3 is easy to load and modify the surface, which can further promote the CO 2 and H 2 activation, and stabilize key intermediates, providing great potential for the design and preparation of efficient methanol synthesis catalysts [20,28,79].

Metal Promoters
Compared with traditional catalysts, In 2 O 3 demonstrates better ability to adsorb and activate CO 2 owing to its abundant oxygen vacancy on the surface.However, the weak H 2 dissociation ability of In 2 O 3 hinders the hydrogenation of carbon species, resulting in a significantly low CO 2 conversion rate of approximately 6%.It is a common strategy to introduce other metal elements into the In 2 O 3 system and form M/In 2 O 3 structure to enhance H 2 dissociation, adsorption, and overflow capacity.
As a metal with strong H 2 dissociation ability, Pd is often used to further enhance the performance of In 2 O 3 -based catalysts.For example, Tian et al. [80] prepared Pd-In 2 O 3 catalysts with high activity using the solid phase method.Pd species have high dispersibility on the prepared Pd/In 2 O 3 catalyst, which significantly promoted H 2 dissociation and provided sufficient H adatoms for CO 2 hydrogenation.In addition, partial Pd 2+ species can exist stably during CO 2 hydrogenation, which facilitated methanol synthesis.Rui et al. [81] also prepared Pd/In 2 O 3 catalysts by mixing the Pd/peptide composite and In 2 O 3 , followed by the removal of peptide through thermal treatment.In contrast, the catalyst was also synthesized using the conventional incipient wetness impregnation method.It is notable that the CO 2 conversion of the former catalyst was higher than 20% at 300 • C, and the maximum STY was 0.89 g MeOH •g cat −1 •h −1 , which was higher than that of the catalyst prepared using the conventional method.This is mainly because the catalyst obtained using the former preparation method can obtain Pd-NPs with small particle size and high dispersion.The Pd NPs has a high hydrogen dissociation adsorption capacity, which can provide the hydrogen for the hydrogenation reaction and maintain the density of oxygen vacancies.Similar effects are also found for Pt, Rh, Au, and Ru supported on In 2 O 3 [39].For instance, Sun et al. [82] prepared Pt-In 2 O 3 catalysts using the deposition precipitation method (DP).The synergistic effect between Pt nanoparticles and In 2 O 3 effectively modulated the relationship between H 2 activation and surface oxygen vacancy content of In 2 O 3 , promoting the efficient hydrogenation of CO 2 to methanol and maintaining good stability.Rui et al. [83] prepared the Au/In 2 O 3 catalyst using the sedimentation precipitation method.The catalyst exhibits excellent catalytic properties with 100% selectivity of the reaction below 225 • C, and the selectivity is still as high as 67.8% at 300 • C.This excellent performance is due to the Au δ+ -In 2 O 3-X interface used as the active site, which gives gold the ability to activate hydrogen by regulating the electronic structure of gold.In addition, Sun et al. [84] prepared a Ag/In 2 O 3 catalyst in the same manner.They analyzed the interaction between the surface of Ag and In 2 O 3 containing oxygen vacancy based on DFT.The results showed that the interface site could promote the activation and hydrogenation of CO 2 .Moreover, the introduction of Ag can promote the formation of surface oxygen vacancy, thus promoting the increase of oxygen vacancy sites and the adsorption and dissociation of CO 2 .
Recently, a study indicated that a Ni-promoted In 2 O 3 catalyst was used to produce methanol via CO 2 hydrogenation.Hensen et al. [85] prepared a Ni-promoted In 2 O 3 catalyst using the one-step flame spray pyrolysis (FSP) method.The reduced Ni is conducive to the activation of H 2 molecules, and NiO-In 2 O 3 will form oxygen vacancies and Ni-O-In during the reaction.Specifically, the metal nitrate solution is dissolved in a mixed solution of ethanol and 2-ethylhexanoic acid at room temperature, and then, the resulting solution is injected into the nozzle of the flame synthesis device, and the catalyst is collected in a quartz filter.Frei et al. [86] synthesized the catalysts with different morphology of Ni using the coprecipitation and impregnation methods.Compared with the coprecipitation method, more stable and active catalysts were obtained using the dry impregnation method.The Ni content of different impregnating catalysts has an obvious influence on the selectivity of products.To be specific, the RWGS reaction promoted methanol synthesis to some extent, and no methane was produced when Ni content was less than 10 wt%.Considering that the special wettability of the Ni-promoted In 2 O 3 catalyst is beneficial for forming layered structures rather than aggregate particles, their strong anchoring on oxides through alloying can obtain high catalyst stability.DFT simulation results showed that the In 2 O 3 -modulated Ni layer easily provided homogeneous cracking hydrogen to In 2 O 3 , enhancing the formation of oxygen vacancy and promoting the hydrogenation of CO 2 .However, it hardly can activate CO 2 on its own, which overall explains the beneficial effects and the lack of methane generation.The catalyst consisting of 1 wt% Ni provided the best balance between charged and free radical hydrogen atoms, resulting in a twofold increase in the methanol space time yield (STY) compared to that achieved with pure In 2 O 3 .Wu et al. [87] reported the experimental and theoretical results of CO 2 hydrogenation to methanol over the Ru-prompted In 2 O 3 catalyst (Ru/In 2 O 3 ).The results showed that the methanol selectivity of Ru/In 2 O 3 catalyst with 1wt% Ru loading can reach 69.7%, and the STY can reach 0.57 g MeOH •g cat −1 •h −1 at 300 • C and 5 MPa (Figure 4).Compared with In 2 O 3 , the Ru/In 2 O 3 catalyst has better stability.The characterization analysis showed that Ru and In 2 O 3 interacted with each other.Ru increased the content of oxygen vacancy on the catalyst surface, stabilized the surface structure of the catalyst, and inhibited the excessive reduction of In 2 O 3 .DFT calculation showed that the In 2 O 3 surface makes Ru clusters more stable, and there is an obvious strong interaction between metal and support, which makes the surface structure of the catalyst more stable.Recently, Liu et al. [88] reported that Ir/In 2 O 3-X catalysts prepared using the traditional impregnation method show high activity, selectivity, and stability for CO 2 hydrogenation to methanol.It was found that there was a linear positive correlation between the amount of Ir and the catalytic activity within a certain range.The Ir/In 2 O 3 catalyst with 10% Ir loading showed a methanol selectivity of 70% and methanol STY of 0.765 g MeOH •g cat −1 •h −1 at 300 • C. The addition of Ir not only effectively avoids the excessive reduction of In 2 O 3 but also stabilizes the oxygen vacancy on the surface of the In 2 O 3 support, thus further improving the stability of the

Oxide Support
The composite oxide catalyst composed of In2O3 and other oxides can also promote the production of highly selective methanol.For example, Liu et al. [89] added ZrO2 into Pt/In2O3 catalyst to explore the effect of ZrO2 on catalytic performances of CO2 hydrogenation.The introduction of ZrO2 achieved high activity and stability of CO2 hydrogenation to methanol in the presence of high CO content.This is mainly because the addition of ZrO2 resulted in stronger electron transfer between Pt and In2O3-ZrO2 support, weakening the CO adsorption and inhibiting the overreduction of In2O3 and CO poisoning at the Pt site.In addition, it was found that the synergistic effect between the Zr-modified oxygen vacancy (In-OV-Zr) and the Pt site promoted the hydrogenation of CO2 to methanol through the formate route.Sharma et al. [19] investigated the catalytic performances of ZrO2 and CeO2-supported In2O3 catalysts for the synthesis of methanol from CO2 hydrogenation.The selectivity of In13/CeO2 for methanol is higher than that of In13/ZrO2 at 553 K, and the methanol selectivity decreases with the increase of temperature.However, it found that the ZrO2-supported In2O3 catalyst was very stable, while In2O3 supported by CeO2 was easy to be inactivated, as shown in Figure 5.This is mainly due to the hydrophilic properties [90], redox properties, and the agglomeration of CeO2 support further inhibiting the CO2 hydrogenation.In addition, it was found that the In13/CeO2 catalyst could be regenerated by washing the catalyst with Ar to a certain extent.However, repeated reaction-regeneration cycles showed that the conversion continued to decline after regeneration and was even lower in the following cycles.In summary, the adsorption of water for In13/CeO2 can recover some activity due to the partial reversibility, while it is irreversible for the structural change leading to sustaining deactivation.For comparison, In13/ZrO2 showed an excellent stability under the reaction conditions.Monoclinic ZrO2 as a support can greatly improve the activity of In2O3 for CO2 hydrogenation to methanol.Javier and Cecilia et al. [30] recently investigated electron, geometric, and interfacial phenomena, and the special role of Zr as a carrier in the catalytic hydrogenation of CO2 to

Oxide Support
The composite oxide catalyst composed of In 2 O 3 and other oxides can also promote the production of highly selective methanol.For example, Liu et al. [89] added ZrO 2 into Pt/In 2 O 3 catalyst to explore the effect of ZrO 2 on catalytic performances of CO 2 hydrogenation.The introduction of ZrO 2 achieved high activity and stability of CO 2 hydrogenation to methanol in the presence of high CO content.This is mainly because the addition of ZrO 2 resulted in stronger electron transfer between Pt and In 2 O 3 -ZrO 2 support, weakening the CO adsorption and inhibiting the overreduction of In 2 O 3 and CO poisoning at the Pt site.In addition, it was found that the synergistic effect between the Zr-modified oxygen vacancy (In-OV-Zr) and the Pt site promoted the hydrogenation of CO 2 to methanol through the formate route.Sharma et al. [19] investigated the catalytic performances of ZrO 2 and CeO 2 -supported In 2 O 3 catalysts for the synthesis of methanol from CO 2 hydrogenation.The selectivity of In 13 /CeO 2 for methanol is higher than that of In 13 /ZrO 2 at 553 K, and the methanol selectivity decreases with the increase of temperature.However, it found that the ZrO 2 -supported In 2 O 3 catalyst was very stable, while In 2 O 3 supported by CeO 2 was easy to be inactivated, as shown in Figure 5.This is mainly due to the hydrophilic properties [90], redox properties, and the agglomeration of CeO 2 support further inhibiting the CO 2 hydrogenation.In addition, it was found that the In 13 /CeO 2 catalyst could be regenerated by washing the catalyst with Ar to a certain extent.However, repeated reactionregeneration cycles showed that the conversion continued to decline after regeneration and was even lower in the following cycles.In summary, the adsorption of water for In 13 /CeO 2 can recover some activity due to the partial reversibility, while it is irreversible for the structural change leading to sustaining deactivation.For comparison, In 13 /ZrO 2 showed an excellent stability under the reaction conditions.Monoclinic ZrO 2 as a support can greatly improve the activity of In

Bimetallic Catalysts
Bimetals have unique electronic effects due to their unique chemical composition and geometric configuration, which can regulate the electronic, geometric, and chemical properties of their active components, thereby enhancing catalytic performance.Because of this, bimetallic catalysts have been widely used in CO2 hydrogenation to methanol in recent years, and catalysts with high catalytic activity and methanol selectivity can be obtained by adjusting the composition ratio and grain size of the two metal elements.
The metal Cu is widely used in the hydrogenation of CO2 to methanol, and Cu-containing bimetallic catalysts have been widely studied in recent years.For example, Xu et al.
[91] compared the catalytic performances and surface properties of Cu-Pd bimetallic catalysts (Cu/Pd = 33.5)over different supports (SiO2, CeO2, TiO2, and ZrO2) and explored the effect of the reduction temperature on the catalytic performances.The results showed that the catalytic performances of SiO2 and ZrO2-loaded catalysts were more noticeably influenced by the reduction in temperature compared to the CeO2 and TiO2-loaded catalysts.This can be attributed to the varying degrees of interaction between Pd and Cu caused by the support and reduction in temperature.Lin et al. [92] prepared TiO2-CeO2 and TiO2-ZrO2 binary supports using the precipitation method in different ratios and then loaded Pd and Cu impregnated on the binary supports to form bimetallic catalysts.It was found that the production and selectivity of methanol were improved by the binary supports catalysts compared with the single support catalysts.This is probably because the surface area of the binary supports was significantly increased, which enhanced both the dispersion of the metal phase and gas adsorption.Meanwhile, the presence of more oxygen vacancies in the binary supports also promoted CO2 conversion.In addition, the binary carriers improved the CO2 adsorption behavior of weakly bonded species, which contributed to improving catalytic performances.Compared to the Ti-Ce binary support, the Ti-Zr binary support showed a greater improvement in the adsorption characteristics of weakly bonded CO2 and the moderately strong metal-support interaction (SMSI)

Bimetallic Catalysts
Bimetals have unique electronic effects due to their unique chemical composition and geometric configuration, which can regulate the electronic, geometric, and chemical properties of their active components, thereby enhancing catalytic performance.Because of this, bimetallic catalysts have been widely used in CO 2 hydrogenation to methanol in recent years, and catalysts with high catalytic activity and methanol selectivity can be obtained by adjusting the composition ratio and grain size of the two metal elements.
The metal Cu is widely used in the hydrogenation of CO 2 to methanol, and Cucontaining bimetallic catalysts have been widely studied in recent years.For example, Xu et al. [91] compared the catalytic performances and surface properties of Cu-Pd bimetallic catalysts (Cu/Pd = 33.5)over different supports (SiO 2 , CeO 2 , TiO 2 , and ZrO 2 ) and explored the effect of the reduction temperature on the catalytic performances.The results showed that the catalytic performances of SiO 2 and ZrO 2 -loaded catalysts were more noticeably influenced by the reduction in temperature compared to the CeO 2 and TiO 2 -loaded catalysts.This can be attributed to the varying degrees of interaction between Pd and Cu caused by the support and reduction in temperature.Lin et al. [92] prepared TiO 2 -CeO 2 and TiO 2 -ZrO 2 binary supports using the precipitation method in different ratios and then loaded Pd and Cu impregnated on the binary supports to form bimetallic catalysts.It was found that the production and selectivity of methanol were improved by the binary supports catalysts compared with the single support catalysts.This is probably because the surface area of the binary supports was significantly increased, which enhanced both the dispersion of the metal phase and gas adsorption.Meanwhile, the presence of more oxygen vacancies in the binary supports also promoted CO 2 conversion.In addition, the binary carriers improved the CO 2 adsorption behavior of weakly bonded species, which contributed to improving catalytic performances.Compared to the Ti-Ce binary support, the Ti-Zr binary support showed a greater improvement in the adsorption characteristics of weakly bonded CO 2 and the moderately strong metal-support interaction (SMSI) effects.The methanol generation rate of Pd-Cu/Ti 0.1 Zr 0.9 O 2 bimetallic catalyst reached 0.62 µmol•g cat −1 •s −1 .Han et al. [93] prepared Cu/ZnO catalysts with abundant Cu-ZnO interface using Cu-Zn bimetallic organic frameworks (MOFs) template strategy ( X represents the Cu/Zn ratios), named (Cu X ZnO-MOF-74-350), which exhibited high methanol selectivity of 80% and long-term durability of 100 h under the condition of 190 • C and 4.0 MPa (Figure 6).The good catalytic performances were mainly due to the abundant Cu-ZnO interface, which facilitates the adsorption of CO 2 and the formation of intermediates.Yu et al. [94] prepared Cu-Zn catalysts coated with UiO-66 using the depositionprecipitation method.The results showed that the Cu-Zn@UiO-66 catalysts with a Cu-Zn loading of 35 wt% and a Cu/Zn molar ratio of 2.5 had excellent catalytic activity.The catalyst had a maximum methanol yield of 9.1% at 240 • C and good reaction stability after a time on stream of 100 h due to the ultra-small nanoparticles (NP) confined in UiO-66 and the abundant Cu/ZnO X interface leading to the enhancement of synergistic effect of Cu and ZnO, which promotes the conversion of CO 2 and the formation of methanol.In recent studies, Ni and In-based catalysts have also been used to form bimetallic oxide catalysts with Cu, both of which exhibit excellent catalytic performances.Wang et al. [95] studied the effect of Cu-Ni loading on different graphites (GO, rGO, NGO) for the synthesis of methanol from CO 2 hydrogenation.The results showed that the CuNi-rGO catalyst showed 7.87% CO 2 conversion and 98.7% methanol selectivity at 498 K and 4.0 MPa.This was attributed to the promotion of Cu 2+ reduced by Ni and the strong chemisorption activation of CO 2 caused by the CuNi-rGO catalyst.Shi et al. [96] prepared bimetallic catalysts with different Cu and In ratios via the co-precipitation method.When the molar ratio of Cu to In was 1:2, the catalysts exhibited the excellent performance due to the high dispersion and the improvement of synergistic effect of active sites.Under the condition of 260  In bimetallic catalysts with Ni as the main active component, the electronic structure of the Ni atom is changed by the neighboring metal atoms, resulting in unique activity for In bimetallic catalysts with Ni as the main active component, the electronic structure of the Ni atom is changed by the neighboring metal atoms, resulting in unique activity for methanol production.Studt et al. [97] synthesized and tested a series of catalysts with different Ni-Ga ratios (including Ni-rich sites and Ga-rich sites), and the results showed that Ni 5 Ga 3 was particularly active and selective.Notably, they had superior methanol production due to their better ability to reduce the RWGS activity in favor of methanol production compared with the conventional Cu/ZnO/Al 2 O 3 catalyst.Rasteiro et al. [98] prepared Ni 5 Ga 3 catalysts with SiO 2 , ZrO 2 , and CeO 2 as supports using the incipient wetness impregnation method.Among these three catalysts, the Ni 5 Ga 3 catalyst supported by CeO 2 was poisoned at the active site due to excessive adsorption of intermediates at the alloy-carrier interface, which resulted in the failure of intermediates to hydrogenate to methanol.The Ni 5 Ga 3 catalyst supported by SiO 2 had a poor affinity to adsorb CO 2 , which limited the methanol production through the alloy surface route.In contrast, the interface of the Ni 5 Ga 3 catalyst supported by ZrO 2 contributed to forming the stable intermediates and promoting hydrogen overflow, thus exhibiting the most excellent catalytic activity.This study illustrates the importance of alloy-support synergy.Otherwise, Duyar et al. [99] introduced a third metal (Au, Cu, Co) as a promoter in Ni-Ga/SiO 2 , which enhanced the activity of CO 2 hydrogenation to methanol by weakening the interaction between the surface and the adsorbate with Au and Cu compared to Ni 5 Ga 3 /SiO 2 .The experimental results showed that the activity of Au-Ni-Ga increased by nearly four times, and Cu-Ni-Ga showed the highest specific activity among the three catalysts.
Pd is also often used as a bimetallic catalyst with other metals such as In, Zn, and Ga for CO 2 hydrogenation to methanol.The formation of the bimetallic system facilitates the overflow of H 2 to the active site, which reacts with the adsorbed CO 2 and promotes methanol production.Therefore, the Pd-M (In, Zn, Ga) bimetallic system has great potential for CO 2 hydrogenation to methanol.As seen in the work of Ojelade's team [100], they conducted a comparative study of PdZn/CeO 2 catalysts with different Pd/Zn composition ratios for methanol synthesis.When the Pd/Zn molar ratio was close to 1, the PdZn alloy phase on the CeO 2 support was maximum, and the methanol STY maximum was 0.114 g MeOH •g cat −1 •h −1 .Yin et al. [101] studied PdZn alloy catalysts prepared from Pd@ZIF-8 precursors, which exhibited excellent methanol STY up to 0.65 g MeOH •g cat −1 •h −1 and TOF of 972 h −1 .This may be attributed to the SMSI effects between Pd and ZnO support, which are facilitated by the sub-nano-Pd being confined within the ZIF-8 pore structure.Snider et al. [102] synthesized InPd/SiO 2 catalysts with different In/Pd ratios using the incipient wetness impregnation method.The catalysts showed the highest activity at 4 MPa and 300 • C when the molar ratio of In to Pd was 2:1.More specifically, the methanol selectivity and yield were 61% and 0.588 g MeOH •g cat −1 •h −1 , respectively.Experimental results and DFT calculations indicated that the synergistic interaction between bimetallic In-Pd particles enriched with In on the surface of In 2 O 3 enhances the reaction activity.Tian et al. [103] used TCPP(Pd)@MIL-68(In) as a sacrificial template to synthesize the loaded Pd@In 2 O 3 catalysts, and the obtained catalysts showed a high 81.1 g MeOH •g Pd −1 •h −1 within 50 h STY and 81% methanol selectivity at 295 • C and 3.0 MPa.DFT calculations showed that the InPd clusters over-reduced In 2 O 3 , which was detrimental to the improvement of the catalytic activity for methanol production by regulating the electronic interactions between Pd and In 2 O 3 .Collins et al. [104] prepared GaPd bimetallic catalysts by loading different ratios of Ga and Pd atoms (2,4,8 atom/atom) on SiO 2 .The results showed that the turnover frequency to methanol based on surface palladium was 200-folds higher than that of the monometallic Pd/SiO 2 catalyst, because the apparent activation energy of methanol synthesis on the surface palladium is much lower than that on the monometallic Pd/SiO 2 catalyst.The characterization results showed that Pd 2 Ga bimetallic particles play a significant role in dissociating H 2 and providing hydrogen atoms to Ga 2 O 3 in the reaction.Close contact between Pd 2 Ga and Ga 2 O 3 may be the key to optimizing the performances of the bifunctional mechanism.A new Rh-In bimetallic catalyst was first reported by Li et al. [105].They prepared a series of Rh-containing catalysts with different In/Al compositions using the wet-impregnation method.The result indicated that the optimal ratio of In to Al was 1, and the methanol yield was 0.001 g MeOH •g cat −1 •h −1 .The catalysts were characterized by their ability to maintain high H 2 conversion in H 2 -deficient feed gas and inhibit the RWGS reaction.Geng et al. [106] prepared SiO 2 -loaded In-Ru bimetallic catalysts and investigated the promotion of In on Ru in CO 2 hydrogenation to methanol.The catalyst achieved 85% methanol selectivity without methane at 240 • C and 3.4 MPa.It enhanced the charge transfer from the surface to the formic acid state, stabilized the formic acid and methoxy on the surface, and inhibited H 2 activation and the conversion of CO hydrogenation to CH 4 and the violent decomposition of methanol on Ru to CO.

Solid Solution Catalysts
In addition to the catalysts mentioned above, solid solution catalysts have a broad application prospect for the hydrogenation of CO 2 to methanol [10].For example, Li et al. [107] prepared a series of Ga-promoted ZnZrOx solid solution catalysts to improve the methanol activity and keep high methanol selectivity.They found that when Ga content varied from 0 to 10% (the mole fraction of Ga in the total metal molar content), the CO 2 conversion fluctuated between 7.7 and 8.8%, while the methanol selectivity remained unchanged (87.5% vs. 87.4%)at 320 • C. The introduction of Ga in the ZnZrO X solid solution catalyst improved the adsorption and activation ability of the catalyst for H 2 and CO 2 and promoted the hydrogenation of HCOO* to CH 3 O* (Figure 7).Li's team also prepared ZnO-ZrO 2 solid solution catalysts with ordered mesoporous structure using the evaporation-induced self-assembly (EISA) method and compared them with the catalysts prepared using the co-precipitation method [108].The results showed that the methanol producing rate of 20% ZnO-ZrO 2 catalyst synthesized using the EISA method was 0.707 g MeOH •g cat −1 •h −1 , which was 1.35 times that of the co-precipitation catalyst at 320 • C and 5.5 MPa.This is mainly because the EISA catalyst has a larger specific surface area and more active sites for adsorbing CO 2 and H 2 .The preparation method mainly dissolves the triblock copolymer P123 in ethanol and adds n-butanol zirconium and ZnCl 2 .
After stirring overnight, the solvent is evaporated to form a gel and then calcined.The surface of the calcined catalyst needs to be washed with water and ethanol, respectively, and finally dried.
Atmosphere 2023, 14, x FOR PEER REVIEW 13 of 25 was 1, and the methanol yield was 0.001 gMeOH•gcat −1 •h −1 .The catalysts were characterized by their ability to maintain high H2 conversion in H2-deficient feed gas and inhibit the RWGS reaction.Geng et al. [106] prepared SiO2-loaded In-Ru bimetallic catalysts and investigated the promotion of In on Ru in CO2 hydrogenation to methanol.The catalyst achieved 85% methanol selectivity without methane at 240 °C and 3.4 MPa.It enhanced the charge transfer from the surface to the formic acid state, stabilized the formic acid and methoxy on the surface, and inhibited H2 activation and the conversion of CO hydrogenation to CH4 and the violent decomposition of methanol on Ru to CO.

Solid Solution Catalysts
In addition to the catalysts mentioned above, solid solution catalysts have a broad application prospect for the hydrogenation of CO2 to methanol [10].For example, Li et al. [107] prepared a series of Ga-promoted ZnZrOx solid solution catalysts to improve the methanol activity and keep high methanol selectivity.They found that when Ga content varied from 0 to 10% (the mole fraction of Ga in the total metal molar content), the CO2 conversion fluctuated between 7.7 and 8.8%, while the methanol selectivity remained unchanged (87.5% vs. 87.4%)at 320 °C.The introduction of Ga in the ZnZrOX solid solution catalyst improved the adsorption and activation ability of the catalyst for H2 and CO2 and promoted the hydrogenation of HCOO* to CH3O* (Figure 7).Li's team also prepared ZnO-ZrO2 solid solution catalysts with ordered mesoporous structure using the evaporation-induced self-assembly (EISA) method and compared them with the catalysts prepared using the co-precipitation method [108].The results showed that the methanol producing rate of 20% ZnO-ZrO2 catalyst synthesized using the EISA method was 0.707 gMeOH•gcat −1 •h −1 , which was 1.35 times that of the co-precipitation catalyst at 320 °C and 5.5 MPa.This is mainly because the EISA catalyst has a larger specific surface area and more active sites for adsorbing CO2 and H2.The preparation method mainly dissolves the triblock copolymer P123 in ethanol and adds n-butanol zirconium and ZnCl2.After stirring overnight, the solvent is evaporated to form a gel and then calcined.The surface of the calcined catalyst needs to be washed with water and ethanol, respectively, and finally dried.Furthermore, Tada et al. [109] explored the effect of Zn content on the hydrogenation of CO 2 to methanol over Zn X Zr 1-X O 2-X catalyst and ensured the structure of active sites through theoretical calculation and experiment.When Zn content was low, the Zn X Zr 1-X O 2-X catalyst was mainly composed of Zn clusters (i.e., isolated [ZnOa] clusters and [ZnbOc] oligomers), which meant the formation of Zn-O-Zr sites with specific activity for CO 2 hydrogenation to methanol.However, the excessive addition of Zn can result in the aggregation of ZnO clusters and the domain formation of ZrO 2 and ZnO phases.Similarly, Huang et al. [110] synthesized a series of ZnO-ZrO 2 composite oxides via the co-deposited method and overall studied their phase structural evolution and catalytic properties in the CO 2 hydrogenation reaction.It was found that with the increase in Zr content, the phase structure of ZnO-ZrO 2 composite oxides evolved from the mixture of hexagonal ZnO phase and Zn-doped ZrO 2 solid solution phases to pure Zn-doped ZrO 2 solid solution phase, which showed high selectivity for the formation of CH 3 OH.Wang et al. [111] prepared a Cu/CeZrO X solid solution catalyst using the microimpingement flow reactor with ultrasound, which had a better micro-mixing efficiency compared with the conventional batch method (Figure 8).Under reaction conditions of 240 • C, 3 MPa, 30,000 mL•g cat −1 •h −1 , the Cu/CeZrO X solid solution catalyst exhibited methanol selectivity of 55.3% and STY of 0.222 g MeOH •g cat −1 •h −1 at CO 2 conversion of 4.67%, which was higher than other traditional Cu-based catalysts [112][113][114].This was ascribed to the synergistic interaction between the oxygen vacancies and a higher proportion of Cu + .
Atmosphere 2023, 14, x FOR PEER REVIEW 14 of 25 CO2 hydrogenation to methanol.However, the excessive addition of Zn can result in the aggregation of ZnO clusters and the domain formation of ZrO2 and ZnO phases.Similarly, Huang et al.
[110] synthesized a series of ZnO-ZrO2 composite oxides via the co-deposited method and overall studied their phase structural evolution and catalytic properties in the CO2 hydrogenation reaction.It was found that with the increase in Zr content, the phase structure of ZnO-ZrO2 composite oxides evolved from the mixture of hexagonal ZnO phase and Zn-doped ZrO2 solid solution phases to pure Zn-doped ZrO2 solid solution phase, which showed high selectivity for the formation of CH3OH.Wang et al. [111] prepared a Cu/CeZrOX solid solution catalyst using the micro-impingement flow reactor with ultrasound, which had a better micro-mixing efficiency compared with the conventional batch method (Figure 8).Under reaction conditions of 240 °C, 3 MPa, 30,000 mL•gcat −1 •h −1 , the Cu/CeZrOX solid solution catalyst exhibited methanol selectivity of 55.3% and STY of 0.222 gMeOH•gcat −1 •h −1 at CO2 conversion of 4.67%, which was higher than other traditional Cu-based catalysts [112][113][114].This was ascribed to the synergistic interaction between the oxygen vacancies and a higher proportion of Cu + .

Other Catalysts
Recently, some other catalysts have been widely studied as well.For example, Li et al. [115] synthesized a Mo-Co-C-N catalyst with a ZIF-67 precursor under reaction conditions of 2 MPa, 6000 mL•gcat −1 •h −1 , and 275 °C, and the CO2 conversion, methanol selectivity, and STY reached 9.2%, 58.4%, and 0.106 gMeOH•gcat −1 •h −1 , respectively.This is mainly because the addition of N provided abundant oxygen vacancies, facilitating the dissociation and adsorption of CO2 and further increasing methanol selectivity [116].Zeng et al. [117] developed boxlike assemblages of quasi-single-layer MoS2 nanosheets, which were edgeblocked by ZnS crystallites (denoted as h-MoS2/ZnS) via a MOF-engaged method.Under reaction conditions of 260 °C, 5 MPa, and 15,000 mL•gcat −1 •h −1 , the h-MoS2/ZnS catalyst can achieve STYMeOH of 0.93 gMeOH•gcat −1 •h −1 , CO2 conversion of 9.0%, and methanol selectivity of 67.3%.Li et al. [118] reported a novel Cd/TiO2 catalyst, with methanol selectivity of 81%, CO2 conversion of 15.8%, and little CH4 selectivity.The unique electronic properties of Cd clusters on the TiO2 substrates were the main reason for the high selectivity of CO2 hydrogenation to methanol through the HCOO* pathway at the interface catalytic site.

Other Catalysts
Recently, some other catalysts have been widely studied as well.For example, Li et al. [115] synthesized a Mo-Co-C-N catalyst with a ZIF-67 precursor under reaction conditions of 2 MPa, 6000 mL•g cat −1 •h −1 , and 275 • C, and the CO 2 conversion, methanol selectivity, and STY reached 9.2%, 58.4%, and 0.106 g MeOH •g cat −1 •h −1 , respectively.This is mainly because the addition of N provided abundant oxygen vacancies, facilitating the dissociation and adsorption of CO 2 and further increasing methanol selectivity [116].Zeng et al. [117] developed boxlike assemblages of quasi-single-layer MoS 2 nanosheets, which were edge-blocked by ZnS crystallites (denoted as h-MoS 2 /ZnS) via a MOF-engaged method.Under reaction conditions of 260 • C, 5 MPa, and 15,000 mL•g cat −1 •h −1 , the h-MoS 2 /ZnS catalyst can achieve STY MeOH of 0.93 g MeOH •g cat −1 •h −1 , CO 2 conversion of 9.0%, and methanol selectivity of 67.3%.Li et al. [118] reported a novel Cd/TiO 2 catalyst, with methanol selectivity of 81%, CO 2 conversion of 15.8%, and little CH 4 selectivity.The unique electronic properties of Cd clusters on the TiO 2 substrates were the main reason for the high selectivity of CO 2 hydrogenation to methanol through the HCOO* pathway at the interface catalytic site.

Mechanism Studies
For CO 2 hydrogenation to methanol, various metal-based catalysts have been studied, and Cu-based catalysts are widely employed because of their higher catalytic activity as well as better selectivity to methanol.However, the active center and the underlying catalytic mechanism are still inconclusive.It is generally recognized that metallic Cu is the active phase, which mainly includes three types of recognized active sites: the interface of Cu and ZnO, Cu 0 , and Cu + species.Many researchers have proposed various mechanisms of CO 2 hydrogenation to methanol over Cu-based catalysts using experiments and DFT calculation methods.Three main mechanisms have been put forward based on the nuance of interface models and chosen pathways, namely the formate (HCOO*) mechanism, the RWGS + CO-Hydro mechanism, and the trans-COOH mechanism [119].In the following content, we mainly focus on the recent theoretical advances in reaction pathways of the three mechanisms on Cu-based catalysts and their extension to other catalyst systems for guiding the design of efficient catalysts.

The HCOO Mechanism and r-HCOO Mechanism
The majority of researchers prefer that the most possible pathway for CO 2 hydrogenation to methanol is the HCOO mechanism, in which formate (HCOO*) is the key intermediate formed by CO 2 reacting with preadsorbed surface atomic H via either an Eley-Rideal (ER) [120,121] or Langmuir-Hinshelwood (LH) mechanism [33,122].HCOO is then hydrogenated to dioxymethylene (*HCOOH), followed by further hydrogenation to *H 2 COOH.The obtained *H 2 COOH is cleaved to formaldehyde (*H 2 CO) which is further hydrogenated to methoxy (*H 3 CO) and methanol (*H 3 COH) [123].
Yang et al. [120] explored the possible reaction pathway for CO 2 hydrogenation to methanol on Cu (111) and Cu nanoparticle surfaces through a combination of experimental and theoretical methods.DFT calculations show that the hydrogenation of CO 2 on the Cu surface is carried out by formate intermediates, and the overall reaction rate is limited by the hydrogenation of formate and dioxymethylene.However, CO hydrogenation was easy to be hindered, which was mainly because CO tended to be converted to formyl, and the obtained formyl is unstable and preferred to dissociate into CO and H atoms on Cu.Sun et al. [60] proposed a possible reaction mechanism of CO 2 hydrogenation to methanol, which followed the formate pathway on the dual active sites of CuZn10Zr through in situ diffuse reflectance infrared Fourier transform spectra.The exposed Cu 0 took to participate in the H 2 activation, while on the Cu + species, the intermediate of formate from the CO 2 and H 2 co-adsorption was more inclined to produce methanol rather than other by-products, improving the methanol selectivity.Qu et al. [124] explored the SMSI between Cu and ZnAl 2 O 4 spinel and found that the spinel catalyst had a double H 2 activation center.The interstitial H atoms in the spinel promoted the conversion of CO 2 to formate species, while the H 2 dissociated by Cu could accelerate the formation of methanol from formate.Wang et al. [125] investigated the effects of transition metal doping on CO 2 hydrogenation on Cu (211) surfaces using the DFT method and found that the doping of Rh, Ni, Co, and Ru can promote CO 2 hydrogenation to COOH.Studies have also found that the hydrogenation of HCOO*/H 2 COO* is assumed to be the rate-controlling step in the CO 2 hydrogenation to methanol process for the formate-intermediated route [126].Han et al. [74] studied the mechanistic pathway of CO 2 hydrogenation to methanol over hollow Cu@ZrO 2 catalysts using in situ FTIR tests.The results revealed that methanol formation followed the formate-intermediated pathway, and the synergistic effect between Cu and ZrO 2 at the interface was crucial in the reaction mechanism.
In addition, the H atoms spilled over to the support surface at the metal-support interface region, participated in the formation of the carbonaceous intermediate species, and were finally converted to HCOO*, CH 2 O*, CH 3 O*, and CH 3 OH.Liu et al. [127] systematically explored three possible paths of CO 2 hydrogenation to methanol over a single-atom Zr-doped Cu-based catalyst from the kinetic and thermodynamic aspects of each elementary reaction by the DFT method (Figure 9).The results showed that methanol was produced by the HCOO pathway, which involved the intermediates of bi-HCOO*, HCOOH*, H 2 COO*, H 2 COOH*, H 2 CO*, and H 3 CO*.In addition, the H 2 COO*, H 2 CO*, and H 3 CO* hydrogenation were proven to be the rate-limiting steps.Liu et al. [89] studied the pathway of methanol synthesis on Pt/In 2 O 3 catalysts with ZrO 2 addition through theoretical calculations and found that CO 2 activated at the In-Ov-Zr site tends to form methanol through the formate pathway.They also calculated the Gibbs free energy of CO adsorption on the catalyst model with or without ZrO 2 by DFT, and the results showed that the Gibbs free energy of CO adsorption on the model with ZrO 2 was lower.This is mainly because the addition of ZrO 2 significantly reduced the interaction between CO and Pt sites, making it easier for CO to desorb from Pt sites to the gas phase under reaction conditions and inhibiting the poisoning of CO on Pt sites.Wang et al. [128] found that both Cu/ZnO and trace Au-added Cu/ZnO catalysts were carried out in the formate path in the methanol synthesis reaction through in situ DRIFTS characterization and DFT calculations.The metal-oxide interface with oxygen vacancy modification is considered as the active site, and the introduction of Au produces more active sites, which promotes the activation of CO 2 and the modulation of intermediate species, thereby significantly improving the catalytic performance of methanol synthesis.
Atmosphere 2023, 14, x FOR PEER REVIEW 16 of 25 atom Zr-doped Cu-based catalyst from the kinetic and thermodynamic aspects of each elementary reaction by the DFT method (Figure 9).The results showed that methanol was produced by the HCOO pathway, which involved the intermediates of bi-HCOO*, HCOOH*, H2COO*, H2COOH*, H2CO*, and H3CO*.In addition, the H2COO*, H2CO*, and H3CO* hydrogenation were proven to be the rate-limiting steps.Liu et al. [89] studied the pathway of methanol synthesis on Pt/In2O3 catalysts with ZrO2 addition through theoretical calculations and found that CO2 activated at the In-Ov-Zr site tends to form methanol through the formate pathway.They also calculated the Gibbs free energy of CO adsorption on the catalyst model with or without ZrO2 by DFT, and the results showed that the Gibbs free energy of CO adsorption on the model with ZrO2 was lower.This is mainly because the addition of ZrO2 significantly reduced the interaction between CO and Pt sites, making it easier for CO to desorb from Pt sites to the gas phase under reaction conditions and inhibiting the poisoning of CO on Pt sites.Wang et al. [128] found that both Cu/ZnO and trace Au-added Cu/ZnO catalysts were carried out in the formate path in the methanol synthesis reaction through in situ DRIFTS characterization and DFT calculations.The metal-oxide interface with oxygen vacancy modification is considered as the active site, and the introduction of Au produces more active sites, which promotes the activation of CO2 and the modulation of intermediate species, thereby significantly improving the catalytic performance of methanol synthesis.In addition to the aforementioned traditional formate pathway, a revised formate (r-HCOO) pathway has been proposed.In the work of Grabow et al. [129], the partial species proposed in previous mechanisms were considered.Meanwhile, the new intermediates such as HCOOH* and CH3O2* were also involved.It was found that the CH2O, HCOOH, and HCOOCH3 were determined to be crucial byproducts through an extensive set of periodic, self-consistent DFT calculations.Moreover, the DFT results showed that the HCOO* can be converted to HCOOH*preferentially, rather than H2CO2*, as suggested in previous mechanisms.HCOOH* was then further hydrogenated to form CH3O2*, which then transformed into CH2O* by cracking its OH group.CH3O* was the final intermediate before the formation of CH3OH*.Recently, Chen et al. [130] also found that the optimal mechanism on the W-doped Rh (111) surface for CO2 hydrogenation to methanol was the r-HCOO pathway.Liu et al. [31] systematically studied the mechanisms and kinetics of CO2 hydrogenation over a serial of MOF-808-coated single-atom metal catalysts via DFT calculation methods.Two plausible CO2 hydrogenation pathways on CuII-MOF-808 In addition to the aforementioned traditional formate pathway, a revised formate (r-HCOO) pathway has been proposed.In the work of Grabow et al. [129], the partial species proposed in previous mechanisms were considered.Meanwhile, the new intermediates such as HCOOH* and CH 3 O 2 * were also involved.It was found that the CH 2 O, HCOOH, and HCOOCH 3 were determined to be crucial byproducts through an extensive set of periodic, self-consistent DFT calculations.Moreover, the DFT results showed that the HCOO* can be converted to HCOOH*preferentially, rather than H 2 CO 2 *, as suggested in previous mechanisms.HCOOH* was then further hydrogenated to form CH 3 O 2 *, which then transformed into CH 2 O* by cracking its OH group.CH 3 O* was the final intermediate before the formation of CH 3 OH*.Recently, Chen et al. [130] also found that the optimal mechanism on the W-doped Rh (111) surface for CO 2 hydrogenation to methanol was the r-HCOO pathway.Liu et al. [31] systematically studied the mechanisms and kinetics of CO 2 hydrogenation over a serial of MOF-808-coated single-atom metal catalysts via DFT calculation methods.Two plausible CO 2 hydrogenation pathways on CuII-MOF-808 catalysts were studied, namely formate path and carboxyl path.As a result, the formate pathway was more favorable, and the conversion of H 2 COOH*-to-H 2 CO* was the key step from the kinetic perspective over CuII-MOF-808 (Figure 10).The study of Li et al. [131] showed that H 2 dissociation on the ZrO 2 sample needed to overcome the high activation energy, which was the rate-determining step of the reaction.While the introduction of Zn in the ZnZrO x catalyst promoted the heterolytic dissociation of H 2 , making the dissociation of H 2 COOH* the rate-determining step of the reaction, the Zn-O-Zr asymmetric sites on ZnZrO x catalyst showed a synergistic effect at the atomic level, which promoted the synthesis of methanol.
Atmosphere 2023, 14, x FOR PEER REVIEW 17 of 25 catalysts were studied, namely formate path and carboxyl path.As a result, the formate pathway was more favorable, and the conversion of H2COOH*-to-H2CO* was the key step from the kinetic perspective over CuII-MOF-808 (Figure 10).The study of Li et al. [131] showed that H2 dissociation on the ZrO2 sample needed to overcome the high activation energy, which was the rate-determining step of the reaction.While the introduction of Zn in the ZnZrOx catalyst promoted the heterolytic dissociation of H2, making the dissociation of H2COOH* the rate-determining step of the reaction, the Zn-O-Zr asymmetric sites on ZnZrOx catalyst showed a synergistic effect at the atomic level, which promoted the synthesis of methanol.

The RWGS + CO-Hydro Mechanism
For the RWGS + CO-Hydro pathway, CO2 is hydrogenated to form the CO* intermediate, followed by CO hydrogenation to methanol with HCO (formyl), H2CO (formaldehyde), and H3CO (methoxy) intermediates.The RWGS + CO-Hydro mechanism has been suggested for Cu-based systems and bimetallic catalysts [7,124].Zhang et al. [75] designed Cu + /CeZrOx interfaces by employing a series of ceria-zirconia solid solution catalysts with different Ce/Zr ratios to form a Cu + -Ov-Ce 3+ structure in which Cu + atoms were combined with the oxygen vacancies of Ce.The in situ DRIFTS results showed that the oxygen vacancies in the solid solutions can not only effectively assist the CO2 activation, modulate the electronic state of copper, and promote the formation of Cu + /CeZrOX interfaces, but also contribute to stabilizing the key *CO intermediate compound to inhibit its desorption and facilitate its further hydrogenation to methanol through the RWGS + CO-Hydro pathway.Rasteiro et al. [132] found that the reaction pathway on the Ni-Ga alloy involved both the RWGS + CO-Hydro and formate routes employing DRIFTS analyses, as confirmed by the presence of formate, methoxy, and CO intermediates.Yang et al. [133] considered both the formate pathway and the RWGS + CO-Hydro route to investigate the CO2 hydrogenation process on metal-doped Cu (111) surfaces using DFT calculations and Kinetic Monte Carlo (KMC) simulations.The calculation results indicated that the addition of Pd, Rh, Pt, and Ni can promote the formation of methanol on Cu (111).Meanwhile, the conversion of

The RWGS + CO-Hydro Mechanism
For the RWGS + CO-Hydro pathway, CO 2 is hydrogenated to form the CO* intermediate, followed by CO hydrogenation to methanol with HCO (formyl), H 2 CO (formaldehyde), and H 3 CO (methoxy) intermediates.The RWGS + CO-Hydro mechanism has been suggested for Cu-based systems and bimetallic catalysts [7,124].Zhang et al. [75] designed Cu + /CeZrO x interfaces by employing a series of ceria-zirconia solid solution catalysts with different Ce/Zr ratios to form a Cu + -Ov-Ce 3+ structure in which Cu + atoms were combined with the oxygen vacancies of Ce.The in situ DRIFTS results showed that the oxygen vacancies in the solid solutions can not only effectively assist the CO 2 activation, modulate the electronic state of copper, and promote the formation of Cu + /CeZrO X interfaces, but also contribute to stabilizing the key *CO intermediate compound to inhibit its desorption and facilitate its further hydrogenation to methanol through the RWGS + CO-Hydro pathway.Rasteiro et al. [132] found that the reaction pathway on the Ni-Ga alloy involved both the RWGS + CO-Hydro and formate routes employing DRIFTS analyses, as confirmed by the presence of formate, methoxy, and CO intermediates.Yang et al. [133] considered both the formate pathway and the RWGS + CO-Hydro route to investigate the CO 2 hydrogenation process on metal-doped Cu (111) surfaces using DFT calculations and Kinetic Monte Carlo (KMC) simulations.The calculation results indicated that the addition of Pd, Rh, Pt, and Ni can promote the formation of methanol on Cu (111).Meanwhile, the conversion of CO and HCO to CH 3 OH through the RWGS + CO-Hydro pathway was found to be much faster than that via the formate pathway.Liu et al. [134] employed the DFT calculation to study the reaction energy and activation barriers of all elementary steps involved in three possible hydrogenation mechanisms (Figure 11a).The calculated results indicated that the activation barrier of the rate-determining step of the RWGS + CO-Hydro mechanism was the lowest, which can be completed through the CO 2 * → trans-COOH* → cis-COOH* → CO* → HCO* → HCOH* → H 2 COH* → CH 3 OH* pathway (Figure 11b).Ou et al. [135] also found that methanol was generated over Pd/TiO 2 catalyst following the RWGS + CO-Hydro pathway rather than the formate pathway due to its lower activation barrier.Sun et al. [136] found through in situ DRIFTS experiments that Cu + species in Cu/SiO 2 catalysts prepared using the FSP method can inhibit the desorption of CO* by stabilizing CO* intermediates and further promote the hydrogenation of CO* to CH 3 OH, promoting the RWGS + CO-hydro pathway.
Atmosphere 2023, 14, x FOR PEER REVIEW 18 of 25 CO and HCO to CH3OH through the RWGS + CO-Hydro pathway was found to be much faster than that via the formate pathway.Liu et al. [134] employed the DFT calculation to study the reaction energy and activation barriers of all elementary steps involved in three possible hydrogenation mechanisms (Figure 11a).The calculated results indicated that the activation barrier of the rate-determining step of the RWGS + CO-Hydro mechanism was the lowest, which can be completed through the CO2* → trans-COOH* → cis-COOH* → CO* → HCO* → HCOH* → H2COH* → CH3OH* pathway (Figure 11b).Ou et al. [135] also found that methanol was generated over Pd/TiO2 catalyst following the RWGS + CO-Hydro pathway rather than the formate pathway due to its lower activation barrier.Sun et al. [136] found through in situ DRIFTS experiments that Cu + species in Cu/SiO2 catalysts prepared using the FSP method can inhibit the desorption of CO* by stabilizing CO* intermediates and further promote the hydrogenation of CO* to CH3OH, promoting the RWGS + CO-hydro pathway.

The Trans-COOH Mechanism
The earliest study of the trans-COOH mechanism with the hydrocarboxyl (COOH*) species as the first hydrogenated species was proposed by Zhao and co-workers [137] on the basis of the experiment of Yang et al. [138].They found that in the presence of H 2 O, the hydrogenation of CO 2 to produce hydrocarboxyl (trans-COOH) is kinetically more favorable than formate via a unique hydrogen transfer mechanism (Figure 12).The obtained trans-COOH was then converted to hydroxymethylidyne (COH) via dihydroxycarbene (COHOH) intermediates and then continuously hydrogenated for three steps to form hydroxymethylene (HCOH), hydroxymethyl (H 2 COH), and methanol, respectively.

The Trans-COOH Mechanism
The earliest study of the trans-COOH mechanism with the hydrocarboxyl (COOH*) species as the first hydrogenated species was proposed by Zhao and co-workers [137] on the basis of the experiment of Yang et al. [138].They found that in the presence of H2O, the hydrogenation of CO2 to produce hydrocarboxyl (trans-COOH) is kinetically more favorable than formate via a unique hydrogen transfer mechanism (Figure 12).The obtained trans-COOH was then converted to hydroxymethylidyne (COH) via dihydroxycarbene (COHOH) intermediates and then continuously hydrogenated for three steps to form hydroxymethylene (HCOH), hydroxymethyl (H2COH), and methanol, respectively.It was found that the reduction of CO 2 to CH 3 OH was more likely to occur completely through the LH mechanism, and trans-COOH was proved to be the most favorable pathway.Similarly, Liu et al. [34] also considered the three possible reaction pathways according to DFT calculations to explore the reaction mechanisms on PdCu 3 (111) surface.The results showed that the CO 2 hydrogenation to CH 3 OH preferred to occur via the COOH pathway.The adsorbed CO 2 reacted with H atoms to form COOH*, which was further hydrogenated to produce COHOH* and hydroxymethylidyne.And methanol was formed by the HCOH and H 2 COH intermediates.

Summary and Outlook
The combination of CO 2 and renewable H 2 can be used to synthesize valuable methanol, which is not only conducive to mitigating carbon emissions but also bene-ficial to achieving the artificial carbon cycle.In this work, we summed up the advances in studies of reaction thermodynamics, catalyst development, and mechanism study for the hydrogenation of CO 2 to methanol.In terms of reaction thermodynamics, the lowtemperature and high-pressure operating conditions are proven to be favorable.In terms of catalyst development, we mainly review the research progress of Cu-based catalysts, In 2 O 3 -based catalysts, bimetallic catalysts, solid solution catalysts, and other novel catalysts.For Cu-based catalysts, the suitable supports, promoters, and preparation methods contribute to improving the number or activity of active sites to enhance the H 2 adsorption and the CO 2 activation and promote the formation of methanol.For In 2 O 3 -based catalysts, several strategies are employed to improve the catalytic performances.More specifically, the introduction of metal promoters can promote H 2 activation and oxygen vacancy formation.The oxide support is used to improve the dispersion of the active component and prevent the active component from sintering as well.For the bimetallic catalysts, the second metal can not only inhibit the aggregation of active species but also regulate the electronic structure around the active components.For the solid solution catalysts, their application further improves the thermal stability and structural performances of traditional catalysts.In addition, the catalytic performances of novel catalysts (e.g., MoS 2 , MOFs, and Cd/TiO 2 ) are briefly introduced.In terms of the mechanism study, we summarized three possible reaction pathways, namely the formate (HCOO*) mechanism, the RWGS + CO-Hydro mechanism, and the trans-COOH mechanism, and reviewed their recent theoretical advances on Cu-based catalysts and their extension to other catalytic systems.
As an important carbon-neutral technology, CO 2 hydrogenation to methanol has shown great promise for industrial application.In order to further improve its process performances and technical competitiveness, it is crucial to design efficient catalysts, explore reaction mechanisms, and conduct deep kinetic study and reactor design.From the perspective of the catalyst design, increasing the dispersion and the exposed surface of Cu, regulating the interaction between Cu and ZnO, and balancing the surface H/C ratio of active sites are the main focus of future research.From the perspective of the reaction mechanisms, exploring the role of the active site and investigating the factors of the rate-determining steps through in situ catalyst characterization techniques and theoretical simulation calculations can further guide the rational design of efficient catalysts.In addition, the reaction kinetics of the CO 2 and CO hydrogenation involved in the methanol synthesis process is the foundation for the subsequent reactor design and optimization of reactors, which provides constructive suggestions for large-scale industrial applications.
catalyst.In turn, In 2 O 3 stabilizes the highly dispersed metal Ir, avoiding the Ir aggregation observed on other oxides.Atmosphere 2023, 14, x FOR PEER REVIEW 9 of 25 catalyst.In turn, In2O3 stabilizes the highly dispersed metal Ir, avoiding the Ir aggregation observed on other oxides.

Figure 4 .
Figure 4. CO 2 hydrogenation activity of In 2 O 3 and Ru/In 2 O 3 .(a) CO 2 conversion and methanol selectivity, (b) methanol STY, (c) apparent activation energy of CO 2 conversion, and (d) methanol formation rate versus time of the Ru/In 2 O 3 catalyst for 40 h on stream [87].
2 O 3 for CO 2 hydrogenation to methanol.Javier and Cecilia et al. [30] recently investigated electron, geometric, and interfacial phenomena, and the special role of Zr as a carrier in the catalytic hydrogenation of CO 2 to methanol.According to the kinetic analysis, monoclinic ZrO 2 -based catalysts can activate reactants better than Al 2 O 3 or CeO 2 oxide-supported In 2 O 3 , possibly due to the superior nature of oxygen vacancy on the carrier In 2 O 3 and the direct contribution of ZrO 2 to CO 2 activation through its oxygen vacancy.Atmosphere 2023, 14, x FOR PEER REVIEW 10 of 25methanol.According to the kinetic analysis, monoclinic ZrO2-based catalysts can activate reactants better than Al2O3 or CeO2 oxide-supported In2O3, possibly due to the superior nature of oxygen vacancy on the carrier In2O3 and the direct contribution of ZrO2 to CO2 activation through its oxygen vacancy.
• C, 3.0 MPa, and 7500 mL•g cat −1 •h −1 , the catalyst obtained CO 2 conversion of 10.3%, CH 3 OH selectivity of 86.2%, and CH 3 OH STY of 0.190 g MeOH •g cat −1 •h −1 .Atmosphere 2023, 14, x FOR PEER REVIEW 11 of 25 effects.The methanol generation rate of Pd-Cu/Ti0.1Zr0.9O2bimetallic catalyst reached 0.62 μmol•gcat −1 •s −1 .Han et al. [93] prepared Cu/ZnO catalysts with abundant Cu-ZnO interface using Cu-Zn bimetallic organic frameworks (MOFs) template strategy (X represents the Cu/Zn ratios), named (CuXZnO-MOF-74-350), which exhibited high methanol selectivity of 80% and long-term durability of 100 h under the condition of 190 °C and 4.0 MPa (Figure 6).The good catalytic performances were mainly due to the abundant Cu-ZnO interface, which facilitates the adsorption of CO2 and the formation of intermediates.Yu et al. [94] prepared Cu-Zn catalysts coated with UiO-66 using the deposition-precipitation method.The results showed that the Cu-Zn@UiO-66 catalysts with a Cu-Zn loading of 35 wt% and a Cu/Zn molar ratio of 2.5 had excellent catalytic activity.The catalyst had a maximum methanol yield of 9.1% at 240 °C and good reaction stability after a time on stream of 100 h due to the ultra-small nanoparticles (NP) confined in UiO-66 and the abundant Cu/ZnOX interface leading to the enhancement of synergistic effect of Cu and ZnO, which promotes the conversion of CO2 and the formation of methanol.In recent studies, Ni and In-based catalysts have also been used to form bimetallic oxide catalysts with Cu, both of which exhibit excellent catalytic performances.Wang et al. [95] studied the effect of Cu-Ni loading on different graphites (GO, rGO, NGO) for the synthesis of methanol from CO2 hydrogenation.The results showed that the CuNi-rGO catalyst showed 7.87% CO2 conversion and 98.7% methanol selectivity at 498 K and 4.0 MPa.This was attributed to the promotion of Cu 2+ reduced by Ni and the strong chemisorption activation of CO2 caused by the CuNi-rGO catalyst.Shi et al. [96] prepared bimetallic catalysts with different Cu and In ratios via the co-precipitation method.When the molar ratio of Cu to In was 1:2, the catalysts exhibited the excellent performance due to the high dispersion and the improvement of synergistic effect of active sites.Under the condition of 260 °C, 3.0 MPa, and 7500 mL•gcat −1 •h −1 , the catalyst obtained CO2 conversion of 10.3%, CH3OH selectivity of 86.2%, and CH3OH STY of 0.190 gMeOH•gcat −1 •h −1 .

Figure 7 .
Figure 7. Proposed reaction mechanism for CO2 hydrogenation to methanol over 5%GaZnZrOx catalyst [107].Furthermore, Tada et al.[109] explored the effect of Zn content on the hydrogenation of CO2 to methanol over ZnXZr1-XO2-X catalyst and ensured the structure of active sites through theoretical calculation and experiment.When Zn content was low, the ZnXZr1-XO2-X catalyst was mainly composed of Zn clusters (i.e., isolated [ZnOa] clusters and [ZnbOc] oligomers), which meant the formation of Zn-O-Zr sites with specific activity for

Figure 9 .
Figure 9. Potential energy profiles of the HCOO pathway in the process of methanol synthesis from CO2 hydrogenation on the Zr1-Cu surface [128].

Figure 9 .
Figure 9. Potential energy profiles of the HCOO pathway in the process of methanol synthesis from CO 2 hydrogenation on the Zr1-Cu surface [128].

Figure 11 .
Figure 11.(a) Reaction networks for CO 2 hydrogenation to CH 3 OH on the Cu@Pd core-shell surface.(b) Potential energy profiles of the HCOO pathway, the COOH pathway, and the RWGS + CO-Hydro pathway [134].

Figure 12 .
Figure 12.Potential energy surfaces for CO2 hydrogenation to methanol on Cu (1 1 1) via the formate and hydrocarboxyl mechanisms [137].In addition, Tang et al.[33] discussed three possible reaction pathways of CO2 hydrogenation to CH3OH on Ga3Ni5 (221) surfaces based on formate (HCOO), hydrocarboxyl (COOH) formations, and RWGS reaction with CO hydrogenation through DFT calculations.It was found that the reduction of CO2 to CH3OH was more likely to occur completely through the LH mechanism, and trans-COOH was proved to be the most favorable pathway.Similarly, Liu et al.[34] also considered the three possible reaction pathways according to DFT calculations to explore the reaction mechanisms on PdCu3 (111) surface.The results showed that the CO2 hydrogenation to CH3OH preferred to occur via the COOH pathway.The adsorbed CO2 reacted with H atoms to form COOH*, which was further hydrogenated to produce COHOH* and hydroxymethylidyne.And methanol was formed by the HCOH and H2COH intermediates.

Figure 12 .
Figure 12.Potential energy surfaces for CO 2 hydrogenation to methanol on Cu (1 1 1) via the formate and hydrocarboxyl mechanisms [137].In addition, Tang et al.[33] discussed three possible reaction pathways of CO 2 hydrogenation to CH 3 OH on Ga 3 Ni 5 (221) surfaces based on formate (HCOO), hydrocarboxyl (COOH) formations, and RWGS reaction with CO hydrogenation through DFT calculations.It was found that the reduction of CO 2 to CH 3 OH was more likely to occur completely through the LH mechanism, and trans-COOH was proved to be the most favorable pathway.Similarly, Liu et al.[34] also considered the three possible reaction pathways according to DFT calculations to explore the reaction mechanisms on PdCu 3(111) surface.The results showed that the CO 2 hydrogenation to CH 3 OH preferred to occur via the COOH pathway.The adsorbed CO 2 reacted with H atoms to form COOH*, which was further hydrogenated to produce COHOH* and hydroxymethylidyne.And methanol was formed by the HCOH and H 2 COH intermediates.