Figure 5. (a) Optimized structure of the In2O3(110) surface, side view (upper), and top view (lower). (b) The D1 (upper) and D4 (lower). Reprinted with permission from ref. ^[38]. Copyright 2013 American Chemical Society. Various strategies have been adopted to improve the performance, including: (1) noble metal loading to facilitate H2 activation and formation of oxygen vacancies; (2) introduction of oxide support to elevate the dispersion and resist sintering of the active component; (3) adoption of different synthesis methods to achieve novel morphological, electronic, or interfacial effects. Supported catalysts, however, sometimes suffered from sintering even though the metal nanoparticles were separated physically by support materials ^[40][50]. Appropriate interaction between support materials and metal nanoparticles may hinder the sintering to some extent. Au/In2O3 catalyst prepared by deposition–precipitation method has exhibited the consequence of the interactions ^[51]. The Au/In2O3 catalyst yields a methanol selectivity of 67.8% with the STY of 0.47 gMeOH·gcat⁻¹·h⁻¹. After a 12 h on-stream test, the conversion of CO2 dropped by only 0.5%, and the mean particle size of the Au nanoparticles was slightly changed from 1.0 nm to 1.3 nm, indicating a nice structural stability that was not seen on other oxide support ^[45][51]. In addition, anchoring of Pd to the lattice of In2O3 leads to superior catalytic performance, as well as structural stability. By controlled coprecipitation, Pd was atomically dispersed in the lattice matrix of In2O3, and the growing up of the Pd cluster was efficiently hindered ^[40].

3. Nanoalloy Catalysts

Nanoalloy catalysts often exhibit unique electronic structures distinguished from either component ^{[11][52][53][54]}. As a result, the bonding properties of reactants, intermediates, or products can be tuned, which finally yields tunable activity and selectivity of the catalysts. Nanoalloy is formed either in the preparation process or under reaction conditions. The existence of nanoalloy can be validated by the finger-printed diffraction angle (2θ) in X-ray diffraction (XRD) ^[55], the alloy state of an element in X-ray photoelectron spectroscopy (XPS) ^[56], the specific vibrational mode of probe molecules binding to the alloy sites ^[57], or the lattice constant value shown in transmission electron microscopy (TEM) ^[58][59]. For a specific reaction, the turnover frequency (TOF) can be tuned by either the composition or the relative content ^[60]. Studt et al. compared the catalytic activities of three Ni-Ga catalysts via incipient wetness impregnation method and Cu/ZnO/Al2O3 catalyst via coprecipitation route as reference ^[7][60]. The better activity and methanol selectivity of the Ni5Ga3/SiO2 catalyst were attributed to the suppressed RWGS activity. While supported on mesoporous nitrogen-rich carbon, Ni5Ga3 was also found to have good activity for CO2 hydrogenation, though the activity is sensitive to the preparation method ^[59]. The author highlighted the freeze drying method that enables uniformly distributed metal nanoparticles with an average size of 2–5 nm and correlated the formation of NiGa alloy with the suitable local environment realized by the preparation method. Shi et al. ^[58] synthesized Cu-In intermetallic catalyst and investigated the effect of reduction temperature on the reaction activity. It was found that reduction temperature exerts a notable influence on the formation of alloy, the crystallite size, and the adsorption of gases. For instance, CuO was reduced to metallic Cu at 250 °C, and the Cu11In9 alloy appeared when the reduction temperature was above 300 °C. With higher reduction temperature, the crystallite size of Cu11In9 increases slightly, accompanied by an attenuated H2 adsorption ability. The CO2 adsorption ability varies notably with increasing the reduction temperature, and the maximum value was obtained with CuIn-300 (reduced at 300 °C), the trend of which agrees well with the STY of methanol tested at 240 °C and 3 MPa (for the same reason, the CO2 adsorption ability was thought to be the key factor for the design of Cu/In2O3 catalysts). Besides the temperature, reduction pressure also affects the formation of alloy. An X-ray absorption near edge structure (XANES) analysis on the reduction process of commercially available Cu/ZnO/Al2O3 catalyst at different pressure (1 mbar–10 bar) showed that Cu–Zn alloys were formed only under reduction pressure of 100 mbar or above ^[61]. The maximum reduction rate (simultaneous formation of copper (0)) is gradually shifted to low temperature by elevating the reduction pressure. The catalysts started to show methanol synthesis activity when the total pressure was above 1 bar, along with the increased formation of oxygen vacancies and other structural distortions in the ZnO phase. Note that, in this section, thwe researchers do not really attempt to emphasize the alloy formation in the activity modification for the metal supported on oxide since their interactions are not necessarily accompanied by the formation of nanoalloy. For Ni supported on In2O3, the authors emphasized the role of Ni–In alloy formation in determining its activity ^[41][54], while Hensen and coworkers ^[62] claimed the activity of Ni/In2O3 system stems from the synergy effect with no alloy formation evidenced. In any case, the formation of nanoalloy in modifying the electronic structure and hence the catalytic performance provides a novel idea for catalyst design.

4. Other Catalysts

MoS2 and MoS2-based materials have been widely used as lubricants ^[63], transistors ^[64], heterogeneous catalysts ^[65][66][67], and gas sensors ^[68]. Due to the special lattice structure, MoS2 is easily peeled into thin layers or even a single atomic layer ^[69]. The electronic structure of two-dimensional MoS2 is sensitive to the status of surface vacancies. The sulfur (S) vacancies especially located at edges or in-plane, exhibit completely different catalytic activities ^[70][71][72]. The edge S vacancies were thought to catalyze the CO2 hydrogenation to methane, while the in-plane S vacancies were proven to be ideal active sites for CO2 hydrogenation to methanol ^[70]. Over the in-plane sulfur vacancy-rich MoS2 nanosheets, a methanol selectivity of 94.3% at a CO2 conversion of 12.5% was achieved at 180 °C, and the catalyst was stable for over 3000 h without any deactivation. The findings enlightened the potential role of the in-plane vacancies in catalysis, and further modification of the vacancies-controllable MoS2 material or two-dimensional material is meaningful. Due to the unique structural characteristics, MOF materials were featured with highly ordered and tunable porous structures, high surface area, flexible organic linkers, and metal centers ^[73][74][75]. Accordingly, MOFs can be a template for porous material synthesis ^[30], supporting materials for nanocatalysts ^[75][76], or act as catalysts individually by introducing active metal centers as nodes or located on the MOF linkers ^[77][78]. Although many MOFs suffer from high temperature and moisture conditions, some selected MOF-based catalysts, including UiO-bpy ^[75], MOF-74 ^[76], UiO-67 ^[78], and UiO-66 ^[79], were reported to have good thermal stability even under moisture condition, of which further tuning of the catalytic properties is likely foreseen. Another promising catalyst catalog for CO2 hydrogenation to methanol is solid solution catalysts ^{[80][81][82][83]}. Wang et al. have prepared a series of ZnO-ZrO2 solid solution catalysts through the coprecipitation method ^[81]. Both methanol selectivity and CO2 conversion reach maximum over the catalyst when Zn/(Zn + Zr) ratio is around 13%. Methanol selectivity of 86% to 91% with CO2 conversion of 10% was achieved under the condition of 5.0 MPa, 24,000 mL·g⁻¹h⁻¹ and 320 °C. The catalyst was proved to show long-term thermal and chemical stabilities against sintering and poisoning by, e.g., SO2 or H2S. Density functional theory (DFT) simulation results suggested that Zn and Zr provide the adsorption sites for H2 and CO2, respectively. Therefore, a solid solution catalyst takes advantage of both components to achieve the synergetic effect. Such synergetic effect was also observed in other solid solution catalysts such as MaZrOx (Ma = Cd, Ga) ^[82]. Another interesting aspect of the ZnO-ZrO2 solid solution catalyst is that its activity was reported to be sensitive to the preparation method (the microstructure) rather than the ZnO/ZrO2 ratio. The 20% ZnO-ZrO2 catalyst prepared by the evaporation-induced self-assembly process exhibited better methanol synthesis activity than that of the coprecipitation method ^[80]. The enhanced activity of the former was ascribed to its larger specific surface area related to the mesoporous structure and more active sites for CO2 and H2 adsorption (which are possibly correlated with the larger surface area).