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

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journal contribution
posted on 25.05.2017, 00:00 by Xiaowa Nie, Haozhi Wang, Michael J. Janik, Yonggang Chen, Xinwen Guo, Chunshan Song
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.