1.  Introduction

The combined pressures of climate change and the global energy transition have driven CO₂ reduction and utilization to the forefront of scientific and industrial efforts. As a major greenhouse gas, excess CO₂ contributes to global warming, while its conversion into fuels and value-added chemicals offers both environmental benefits and opportunities for a circular carbon economy [1]. Catalytic hydrogenation is among the most effective approaches for producing renewable fuels and chemical feedstocks from CO₂, and methane, methanol, and ethanol are the most widely studied target products because of their practical relevance.

Methane is the principal component of natural gas and benefits from a high calorific value together with mature storage and distribution infrastructure. CO₂ hydrogenation to methane proceeds mainly through methanation reactions and is most often catalyzed by Ni-based systems that combine low cost with high intrinsic activity [2]. Nevertheless, improving methane yield remains constrained by issues of selectivity and catalyst stability. Strategies such as tuning active-site structure, selecting appropriate supports, and controlling metal particle size have been shown to significantly improve catalytic performance [3].

Methanol is a versatile platform molecule for both chemical synthesis and energy applications. Its production from CO₂ typically proceeds via adsorbed C₁ intermediates such as formate and formaldehyde before full reduction to methanol. Cu-based catalysts continue to serve as the foundation for methanol synthesis because their activity and selectivity are strongly influenced by electronic structure and surface defects [4]. Synergies between bimetallic sites and oxide supports have further improved methanol yields and stability, underpinning efforts to extend these principles to higher alcohol synthesis.

Ethanol has received increasing attention due to its higher energy density relative to methanol and its direct use as a transportation fuel additive. Direct synthesis of ethanol from CO₂ requires C–C bond formation, which presents substantial catalytic and kinetic challenges. Multicomponent bimetallic catalysts such as Ru–In and Rh–Cu, together with metal–oxide composite systems, have been widely investigated to promote C–C coupling and improve ethanol selectivity [5]. Computational and mechanistic studies emphasize the importance of surface electronic structure and interfacial effects in steering pathways toward ethanol formation [6]. Despite these advances, high selectivity to ethanol remains limited by the barrier to C–C coupling and by catalyst deactivation, motivating continued research.

Although methane, methanol, and ethanol can all be obtained by CO₂ hydrogenation, their reaction pathways, mechanistic demands, and catalyst requirements differ markedly. Methanation emphasizes formation of C–H bonds, methanol synthesis requires controlled conversion of C₁ intermediates, and ethanol production hinges on efficient C–C bond formation. Controlling product selectivity therefore depends on careful catalyst design, optimized reaction conditions, and mechanistic insight [7]. Combining advanced in situ characterization with theoretical modeling helps reveal active sites and deactivation pathways and guides the rational design of next-generation catalysts.

2.  Catalytic hydrogenation of CO₂ to methane

Recent experimental work has examined CO₂ methanation in wall-cooled single-tube fixed-bed reactors loaded with commercial Ni-based catalysts. Studies conducted in reactors with 2 cm and 4 cm diameters under pressures of 1 to 4 bar and temperatures from 160 to 200 °C, with CO₂ feeds diluted in H₂ or N₂, provided detailed heat transfer and kinetic data [5]. These measurements enabled construction of a two-dimensional reactor model that accurately predicts CO₂ conversion and axial and radial temperature distributions. The model has been extended to applications such as upgrading biogas and designing processes for methane-free CO₂/H₂ feedstocks, and industrial reactor parameters capable of producing synthetic natural gas at rates on the order of 450 m³ h⁻¹ have been proposed [8]. Corrections to the static thermal conductivity further improved model fidelity across a wide temperature range, supporting scale-up and optimization of fixed-bed methanation reactors [9].

In addition to these engineering-oriented studies, significant effort has also been directed at understanding the intrinsic catalytic behavior of Ni-based systems, which remain the industrial standard for CO₂ methanation due to their relatively low cost and high activity. Thermodynamically, methanation is highly exothermic, which introduces challenges such as hot spot formation, catalyst sintering, and deactivation under high conversion conditions. These issues make precise reactor thermal management essential, especially in larger-scale systems. The reported advances in modeling and experimental validation therefore not only strengthen the fundamental understanding of transport and reaction phenomena but also provide critical guidance for process intensification and commercial deployment. Collectively, these efforts supply important experimental and theoretical foundations for CO₂-to-methane pathways and demonstrate the tight coupling between catalyst design, reactor engineering, and process scale-up.

3.  Catalytic hydrogenation of CO₂ to methanol

Progress in CO₂ hydrogenation to methanol has been substantial. Skrzypek’s early low-pressure studies of Cu–Zn–Al oxide catalysts identified formate hydrogenation as the rate-determining step and established mechanistic foundations for later work [10]. More recently, Witoon and colleagues reported that incorporating graphene oxide into Cu–Zn–Zr catalysts substantially enhanced CO₂ conversion and methanol selectivity, reaching 24.1% conversion and 75.8% methanol selectivity under a 3:1 H₂/CO₂ ratio; the improvement was attributed to enhanced hydrogen spillover mediated by graphene oxide [11]. Indium oxide supported on zirconia has been shown to deliver nearly complete methanol selectivity in some systems [12]. Transition metal carbides such as Mo₂C and Fe₃C have demonstrated promising activity, and Cu-doped carbide systems show markedly improved methanol selectivity [13]. For example, Cu/Mo₂C catalysts exhibited excellent performance at 135–200 °C and 3 MPa with turnover frequencies on the order of 10⁻⁴ s⁻¹ and methanol selectivity exceeding 90% [14].

The growing body of research highlights how methanol synthesis from CO₂ is not only a fundamental scientific challenge but also a strategically important target for sustainable energy and chemical industries. Compared with methane, methanol is a liquid fuel and chemical feedstock with broad applications, including direct use in fuel cells, blending with gasoline, and serving as a platform molecule for higher-value chemicals. Consequently, the search for catalysts with high selectivity, stability, and activity has become a central theme in this area. The introduction of new support materials, such as graphene oxide, has opened pathways for tailoring surface hydrogen transfer, while the deployment of metal–carbide interactions provides new electronic environments for CO₂ activation. These mechanistic insights deepen our understanding of how reaction intermediates such as formate and methoxy species govern the overall kinetics. Taken together, rational catalyst design and cooperative metal–support effects have advanced methanol synthesis significantly, and the lessons learned are increasingly being translated into strategies for higher alcohol formation, including ethanol.

4.  Catalytic hydrogenation of CO₂ to ethanol

Directly producing ethanol from CO₂ is particularly attractive because it couples carbon mitigation with production of a high-value liquid fuel. Thermodynamic analysis indicates that ethanol formation is exothermic and favored at high pressure and low temperature, while the kinetic requirement to activate the stable CO₂ molecule often necessitates elevated temperatures. Effective catalysts therefore need to combine CO₂ activation with sites that facilitate C–C bond formation.

Ethanol offers a high energy density, compatibility with gasoline blending, and lower emissions of CO and unburned hydrocarbons compared with some alternatives. Conventional production routes such as fermentation and ethylene hydration have limitations in efficiency and feedstock dependence, so CO₂ hydrogenation provides a sustainable alternative. Early homogeneous systems based on noble-metal complexes achieved good selectivity but suffered from sensitivity to air, limited stability, and high cost. Current research focuses on heterogeneous approaches including transition metals and bimetallic catalysts supported on oxides. Noble metals on oxide supports such as Rh, Pt, and Au and non-noble alternatives such as Cu, Fe, and Co have shown activity in higher-alcohol synthesis. For instance, modified Mo/SiO₂ catalysts have produced mixtures of C₁–C₅ alcohols, demonstrating the potential for multi-carbon alcohol synthesis from CO₂ [15]. Single-atom Pd catalysts have been reported to attain ethanol selectivity approaching 97.5% under 250–300 °C reaction conditions, offering a new route to C–C coupling [16]. In another study, a K-promoted Cu–Zn bimetallic catalyst delivered combined CO and ethanol selectivity figures in the high range, with ethanol comprising a measurable fraction of the product stream [17]. These results illustrate how tuned active sites and promoters can steer selectivity, although scaling such systems while preserving activity and stability remains a central challenge.

5.  Conclusion

Catalytic hydrogenation of CO₂ to methane, methanol, and ethanol offers distinct but complementary pathways for carbon utilization, each presenting specific mechanistic and catalytic challenges. Methanation has benefited from decades of reactor-scale research and is already close to industrial deployment for applications such as biogas upgrading and synthetic natural gas production, though challenges in heat management and catalyst stability remain. Methanol synthesis from CO₂, in contrast, has advanced through rational catalyst design and exploration of novel supports, carbides, and promoter effects, achieving high selectivity and activity in model systems. Ethanol formation, while still at an earlier stage of development, represents a particularly valuable target because it couples carbon mitigation with direct production of a high-energy-density liquid fuel, and recent studies on single-atom catalysts, bimetallic systems, and promoter-enhanced oxides provide compelling evidence that selective C–C bond formation is achievable.

Looking forward, continued progress will depend not only on optimized bimetallic and supported catalysts but also on deeper mechanistic understanding of intermediate formation, C–C coupling pathways, and catalyst deactivation phenomena. Improved catalyst stability, tolerance to impurities, and durability under realistic operating conditions will be essential for scaling. Equally important is the integration of advanced characterization techniques, such as operando spectroscopy and microscopy, with computational modeling to bridge atomic-level insights with macroscopic reactor behavior. Finally, coupling catalyst development with tailored reactor engineering and process design—including heat integration, modular reactors, and renewable hydrogen sources—will be critical to translate laboratory advances into industrial practice. With sustained research efforts across chemistry, catalysis, and process engineering, CO₂ hydrogenation can evolve into a viable component of sustainable energy systems, enabling carbon-neutral production of fuels and chemicals.