Research progress on the cobalt-based catalysts for the hydrogenation of carbon dioxide to ethanol

Abstract

The catalytic hydrogenation of CO₂ to ethanol represents a pivotal technology for achieving carbon neutrality and producing high-value fuels and chemicals, where developing the catalysts of high activity, high ethanol selectivity, and long-term stability is vitally important. Currently, the catalysts for the CO₂ hydrogenation to produce ethanol mainly include noble metal-based catalysts (Pt, Pd, Rh, etc.) and Fischer-Tropsch synthesis catalysts (Fe, Co, Mo, etc.); among them, the cobalt-based catalysts have emerged as an ideal candidate for the thermocatalytic hydrogenation of CO₂ to ethanol, owing to their unique electronic structure and tunability. However, the inherent chemical inertness of the relatively stable CO₂ molecule poses a significant barrier to its activation. Furthermore, under high-temperature and high-pressure conditions, the hydrogenation of CO₂ is highly prone to forming undesirable byproducts such as methane or CO, whereas the selective activation of CO₂ and its subsequent conversion into ethanol remain particularly challenging. Accordingly, current processes for the CO₂ hydrogenation to ethanol are greatly limited by the low single-pass conversion of CO₂, insufficient selectivity toward the target product (ethanol < 60%), and the tendency of catalysts to rapid deactivation. This review aims to conduct a systematic and in-depth analysis of the recent research progress of the cobalt-based catalysts used in the conversion of CO₂ into ethanol. We first establish the basic reaction framework, clarify the thermodynamic boundary conditions, and determine the kinetic steps that control the overall reaction rate. When delving into the molecular-level events, we analyze the complex microscopic mechanisms responsible for the two most critical steps: the initial formation of C–C bonds and the subsequent controlled removal of oxygen. The formation of C–C bonds is a necessary and particularly delicate step for generating ethanol and other C₂₊ products and the reaction mechanism of this step, whether through CO insertion, CO dimerization, or hydroxyl carbonyl (CHOH) coupling, is subject to intense debate and strongly influenced by the properties of the active sites. We then critically assess the synergistic effects among various active sites such as metallic Co, Co₂C, CoO_x and bimetallic configurations on the reaction mechanism; in particular, the structure-activity relationships influenced by the support effects and promoter modifications are thoroughly discussed. Lastly, the application of inverse catalysts and tandem catalytic systems in ethanol synthesis is reviewed. The role of water as a hydrogen source and its impact on the reaction are analyzed in depth. This review tries to integrate current knowledge and identifies existing shortcomings, which presents a forward-looking outlook on the innovative research directions for the cobalt-based catalysts and emphasizes the necessity of conducting complex in situ/operational characterization and theoretical modeling in the design of next generation multifunctional catalysts. All these may provide a valuable reference framework to stimulate and guide the development of future efficient cobalt-based catalytic systems, deepen the basic understanding of reaction processes and mechanisms, and ultimately accelerate the progress of CO₂ hydrogenation technology for sustainable ethanol production.

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