(4c) Direct Hydrogenation of CO2 to Light Olefins and Paraffins over the Synergetic ?-Fe2O3-?-Fe5C2/Al2O3 Catalyst

Thermocatalytic hydrogenation of CO₂ into synthetic fuels is an attractive pathway to reduce our dependence on fossil fuels. Hydrogen can be generated via water electrolysis using renewable electricity. In this study, reverse microemulsion (RME) method was implemented to reduce the nanoparticle size in the Fe-based catalysts. In the RME system, nanosized water droplets containing reactants are surrounded by surfactant molecules and dispersed in a continuous oil phase. Nanodroplets act as microreactors, restraining the amounts of reactants and limiting the nanoparticle size.
A series of catalysts with alkali- and transition metal-promoters were synthesized. The RME synthesis resulted in a high specific surface area (ca. 200-250 m²/g) and well dispersed Al₂O₃-supported Fe₃O₄ nanoparticles (5-10 nm). Under reaction conditions Fe₃O₄ nanoparticles convert to a mixture of α-Fe₂O₃ and χ-Fe₅C₂ nanoparticles promoting reverse water gas shift (RWGS) and hydrogenation.
The synthesized catalysts were tested for direct, one-throughput CO₂ hydrogenation to lower olefins and paraffins (C2-C6). Fresh and spent catalysts were characterized by XRD, TPR, TPD, STEM-EDS, TGA-FTIR, and XPS. The α-Fe₂O₃-χ-Fe₅C₂/Al₂O₃ catalysts showed excellent performance, with C2-C6 selectivity and CO₂ conversion above 55% at 375 °C, 10 bar, 1000 mL/(kg h), and H₂/CO₂ = 4. Parameter sensitivity analysis was conducted investigating the effects of temperature, pressure, space velocity, and feed composition and optimizing the performance. Catalyst stability was also investigated showing only a minor decline over 100 h on stream.
The RME method resulted in superior catalytic performance due to the reduced inital nanoparticle size that promoted the formation of χ-Fe₅C₂ for hydrogenation, while also forming the α-Fe₂O₃ phase responsible for RWGS, resulting in a synergetic effect.