Dynamic structural evolution of iron catalysts involving competitive oxidation and carburization during CO 2 hydrogenation.

Identifying the dynamic structure of heterogeneous catalysts is crucial for the rational design of new ones. In this contribution, the structural evolution of Fe(0) catalysts during CO 2 hydrogenation to hydrocarbons has been investigated by using several (quasi) in situ techniques. Upon initial reduction, Fe species are carburized to Fe 3 C and then to Fe 5 C 2 . The by-product of CO 2 hydrogenation, H 2 O, oxidizes the iron carbide to Fe 3 O 4 . The formation of Fe 3 O 4 @(Fe 5 C 2 +Fe 3 O 4 ) core-shell structure was observed at steady state, and the surface composition depends on the balance of oxidation and carburization, where water plays a key role in the oxidation. The performance of CO 2 hydrogenation was also correlated with the dynamic surface structure. Theoretical calculations and controll experiments reveal the interdependence between the phase transition and reactive environment. We also suggest a practical way to tune the competitive reactions to maintain an Fe 5 C 2 -rich surface for a desired C 2+ productivity.