**4. Conclusions**

There is an urgent need to control CO2 emissions in order to mitigate their negative impact on the environment. The catalytic conversion of CO2 is an encouraging approach to mitigate CO2 emissions by producing chemicals and fuels. A highly promising route is selective CO2 hydrogenation to produce light olefins. The huge market demand for the lower olefins offers a great opportunity for the target technology to profoundly impact the scale of CO2 utilization once it is developed with renewable hydrogen. Currently, there are two primary pathways (the CO2−FTS route and the MeOH-mediated route) to produce light olefins from CO2 hydrogenation in a one-step process. In the CO2–FTS path, Fe is one of the most widely used components, while in the MeOH path, Cu/zeolite has been used the most. Even though significant efforts have been made, considerable challenges remain in the development of highly efficient catalysts with selective pathways to light olefins due to the thermodynamically stable nature of the CO2 molecule, the complexity of the reaction networks, and catalyst deactivation. During CO2 hydrogenation, the primary causes for catalyst deactivation are the sintering (or agglomeration) of metal particles, phase transformation at the catalyst's surface, and catalyst poisoning by water or carbonaceous deposits (i.e., coke). A firm grasp of the causes for deactivation is essential in order to develop a mitigation strategy and sustain a high selectivity toward the desired olefins during CO2 hydrogenation. In this review, we summarized the reports published within five years on the effect of the promotors, metal oxide support, bifunctional composites and structure on the catalyst design in order to minimize catalyst deactivation.

*Promoter effect:* Alkali metals such as K and Na have been broadly used as promotors to control the electronic properties. Mn, Ce, and Ca metals have been used as structural promotors. Transition metals such as Zn, Co, Cu, V, Zr, etc., have been used as both electronic and structural promotors. With the inclusion of alkali promoters, Fe-based catalysts can possess higher olefin selectivity. The alkali metals act as electron donors to Fe metal centers, fostering CO2 adsorption while decreasing their affinity with H2, and consequently leading to a higher olefin yield. Some studies show that doping the catalyst with a second metal improves the olefin yield by forming a highly active interface. The second metal promoters may provide a way to tune the CO2 and H2 adsorption and activation, shifting the product distribution towards the desired hydrocarbons.

*Support effect:* Supporting the Fe-based species on supports such as SiO2, CeO2, m-ZrO2, γ-Al2O3, TiO2, ZSM-5, MgO, NbO HPCMs, MOFs, and β-Mo2C may enhance the catalytic performance by improving the active metal dispersion and retarding the sintering of the active particles. The surface area, basicity, reducibility, oxygen vacancies, and morphology of the support played important roles—in most cases, with the presence of promoters (K, Zr, Cs)—in affecting the amount and particle size of the active carbide species; the synergy effect; the metal–support interaction; the strength and capacity of CO, CO2, and H2 adsorption on support; and the surface C/H ratio for CO2 hydrogenation. By tuning the above-mentioned characteristics properly, the physically deposited carbon species, coke generation and metal sintering could be mitigated.

*Bifunctional composite catalyst effect*: The catalysts tested for CO2 hydrogenation to light olefins via the MeOH-mediated route mainly involve two active components (metal oxides and zeolite), and so are called bifunctional composite catalysts. In this review, multiple variations (acidity, particle size, proximity, oxygen vacancy) of the combination of methanol synthesis catalysts (Cu, Zn, In, Ce, Zr, etc. metal oxides) with various zeolites (SAPO-34 and ZSM5) have been reported for enhanced olefin selectivity and catalyst stability by mitigating coke formation, reducing the particle size growth of active carbide species, and inhibiting inactive species formation for CO2 hydrogenation to light olefins.

*Structure effect*: The structure of the catalysts plays a pivotal role in CO2 hydrogenation to light olefins. The structures and properties (for example, LMO, the surface modification of zeolite from hydrophilic to hydrophobic, Fe@NC, the pore sizes of Al2O3, the defective structure of MgH2/CuxO and carbon-confined MgH2/C nano-lamellae, the 3D architecture of the porous HSG, and core–shell CZZ@SAPO-34) could be tuned to mitigate catalyst

deactivation by retarding the sintering of active species and coke deposition, tolerating water formation and enabling favorable phase transformation for an enhanced light olefin yield and catalyst stability.

Despite the many advances made in catalytic development, especially with light olefin yield and stability, a novel catalytic system that is both economically viable and resistant to deactivation has not yet been achieved. Most research efforts have focused on the development of catalytic materials and the adjustment of properties and metal interactions for the desired catalyst activity and long-term stability. Future research directions for CO2 hydrogenation should consider: (1) the further modification of the catalytic surface H/C molar ratio and the fostering of C-C coupling; (2) tuning the basicity and oxygen vacancies of the catalyst support to facilitate the CO2 adsorption and activation; (3) examining more novel catalytic materials/structures to boost the catalyst stability; and (4) exploring more energy-saving catalysts for CO2 hydrogenation to light olefins. In addition, in situ measurements using synchrotron-based techniques, such as X-ray adsorption spectroscopy (XAS), should be performed in order to understand the ways in which the local environment of the catalysts affects their activity, stability and efficient mitigation.

**Author Contributions:** Conceptualization, C.Z.; writing—original draft, C.Z., N.J.R., T.H., D.W., M.W., C.M., A.Z. and N.F.; writing—review and editing, C.Z. and N.J.R.; funding acquisition, C.Z.; resources, C.Z.; supervision, C.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the National Science Foundation under Grant No. 1955521 (C.Z.).

**Acknowledgments:** The authors are grateful for the U.S. Department of Energy, the Office of Science, and the Office of Workforce Development for Teachers and Scientists under the Science Undergraduate Laboratory Internships Program (T.H. and A.Z.) and Visiting Faculty Program (C.Z.).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Abbreviations**

