Reverse microemulsion synthesis promotes the formation of iron carbide in direct hydrogenation of CO2 to light hydrocarbons

Abstract

High specific surface area K-Fe/γ-Al2O3 was synthesized via the reverse microemulsion method and tested for direct hydrogenation of CO2 to light hydrocarbons (lower paraffins and olefins). The effect of the synthesis method was investigated by several characterization techniques and via reaction tests, while using wet impregnation-synthesized catalysts as a reference. The reverse microemulsion method resulted in superior catalytic performance, ascribed to the enhanced specific surface area, enhanced active phase-support interaction and reducibility, and facile formation of the active Hägg iron carbide (χ-Fe5C2) phase. The maximum obtained CO2 conversion and selectivity to C2+ hydrocarbons were 56 % and 52 %, respectively, attaining 7.4 mmol g−1 h−1 space time yield at 10 bar and 375 °C. Characterization results revealed the formation of χ-Fe5C2 and Fe3O4 phases under reaction conditions. Compared to the impregnation method that resulted in the formation of Fe3O4 nanoparticles and Fe/Fe3O4 core-shell nanoparticles, the reverse microemulsion-synthesized catalyst comprised of a mixture of Fe3O4 and χ-Fe5C2 nanoparticles with a relatively uniform particle size distribution. The superior catalytic activity of the reverse microemulsion-synthesized catalyst can be elucidated by the promoted magnetite → iron carbide transformation that results from the small initial nanoparticle size (below 10 nm).