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

(Chemical Equation Presented) Density functional theory (DFT) calculations were carried out to investigate the mechanism of CO₂ hydrogenation in production of C₁ and C₂ hydrocarbons over Cu-Fe bimetallic catalyst. CH∗ is found to be the most favorable monomeric species for production of CH₄ and C₂H₄ 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 CO₂∗ → HCOO∗ → HCOOH∗ → HCO∗ → HCOH∗ → CH∗, in which both the HCOO∗ → HCOOH∗ and HCO∗ → HCOH∗ steps have substantial barriers. The bimetallic surface suppresses CH₄ formation and is more selective to C₂H₄ due to the higher hydrogenation barrier of CH₂∗ species relative to those for C-C coupling and CH-CH∗ conversion to C₂H₄. On monometallic Fe(100) surface, CH∗ formation goes through a path of CO₂∗ → 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 CO₂ conversion to CH₄ and C₂H₄. CH₄ formation is kinetically more favored, with a 0.3 eV lower energy barrier, than C₂H₄ formation. The bimetallic combination of Cu and Fe enhances CO₂ conversion by reducing the kinetic barriers, and alters the selectivity preference to more valuable C₂H₄ from CH₄ on monometallic Fe surface. C₂H₆ can be produced from further hydrogenation of C₂H₄ with moderate barriers.