Effect of hydrogen on carbon deposition on iron in COH2 mixtures

The establishment of structure-performance relationship of catalysts during a full life cycle is highly desirable for catalyst design and understanding of nature of active sites. As a case study, lower olefins synthesis directly from CO2 hydrogenation (CTO) was performed over iron catalyst and the catalyst structure evolution during its life cycle has been comprehensively studied. Particularly, operando techniques allow direct observation of the continuous phase transition during activation (Fe2O3 → Fe5C2), reaction and deactivation (Fe5C2 → Fe3O4), and regeneration (Fe3O4 → Fe5C2) process, respectively. After a combined CO2-CO regeneration, 91.8% of the initial activity and 97.7% of the initial C2–C4 olefins selectivity were recovered. Our results indicate that the Fe5C2 phase is the active phase for C2–C4 olefins production. The iron carbide phases are irreversibly oxidized to Fe3O4 under CTO conditions, which is the major factor responsible for the catalyst deactivation. This study not only reveals the dynamic structure evolution of iron catalyst during the full life cycle in CTO, but also endows deep insight into the underlying deactivation mechanism.