Mechanistic Insight into C–C Coupling over Fe–Cu Bimetallic Catalysts in CO2 Hydrogenation

Density functional theory (DFT) calculations were carried out to investigate the mechanism of CO 2 hydrogenation in production of C 1 and C 2 hydrocarbons over Cu–Fe bimetallic catalyst. CH* is found to be the most favorable monomeric species for production of CH 4 and C 2 H 4 via C–C coupling of two CH* species and subsequent hydrogenation. On the bimetallic Cu–Fe(100) surface at 4/9 ML Cu coverage, the energetically preferred path for CH* formation goes through CO 2 * → HCOO* → HCOOH* → HCO* → HCOH* → CH*, in which both the HCOO* → HCOOH* and HCO* → HCOH* steps have substantial barriers. The bimetallic surface suppresses CH 4 formation and is more selective to C 2 H 4 due to the higher hydrogenation barrier of CH 2 * species relative to those for C–C coupling and CH–CH* conversion to C 2 H 4 . On monometallic Fe(100) surface, CH* formation goes through a path of CO 2 * → CO* → HCO* → HCOH* → CH*, different from that identified on Cu–Fe(100). The hydrogenation of HCO* to HCOH* is the rate-limiting step that controls CO 2 conversion to CH 4 and C 2 H 4 . CH 4 formation is kinetically more favored, with a 0.3 eV lower energy barrier, than C 2 H 4 formation. The bimetallic combination of Cu and Fe enhances CO 2 conversion by reducing the kinetic barriers, and alters the selectivity preference to more valuable C 2 H 4 from CH 4 on monometallic Fe surface. C 2 H 6 can be produced from further hydrogenation of C 2 H 4 with moderate barriers.