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

Abstract

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.

A Critical Look at Direct Catalytic Hydrogenation of Carbon Dioxide to Olefins

“…[85,86] The authors proposed formates as reaction intermediates if aF e ÀCu catalyst was used. The latest studies in this field make use of computational calculations (DFT) to elucidate the reactionp athway on Fe and FeÀCu catalysts.…”

“…The latest studies in this field make use of computational calculations (DFT) to elucidate the reactionp athway on Fe and FeÀCu catalysts. [85,86] The authors proposed formates as reaction intermediates if aF e ÀCu catalyst was used. On the other hand, Satthawong et al concluded that CO was the reaction intermediate with aFe ÀCo catalyst.…”

“…Recent computational work indicated an alternative reaction pathway,w ithout going through CO as intermediate if using FeÀCu(100) surface, as shown in Equation (3). [57,85,86] The main path over am onometallic Fe (100) surface is showni nE quation (4).…”

CO2 hydrogenation to high-value products via heterogeneous catalysis

“…Nie et al also investigated the mechanism of CO 2 hydrogenation to methane and C 2 hydrocarbons over Fe(100) and Cu–Fe(100) surfaces and showed that the hydrogenation barrier of CH 2 * species is higher than those for C–C coupling and CH–CH* conversion to ethylene on Cu–Fe bimetallic catalysts (see Fig. 3) 108 , which results in a more selective process to ethylene. The addition of K to Fe–Cu/Al 2 O 3 catalysts can further inhibit methanation and enhance the production of olefin-rich C 2 –C 4 hydrocarbons by increasing the surface coverage of carbon 85 .…”

Preparation and Performances of ZIF-67-Derived FeCo Bimetallic Catalysts for CO2 Hydrogenation to Light Olefins

“…To our knowledge, Fe-based catalysts have been extensively applied in the CO 2 hydrogenation reaction due to similar characteristics to the traditional Fischer-Tropsch (FT) synthesis process [9][10][11][12]. In general, the mechanism of the CO 2 hydrogenation reaction over Fe-based catalysts is based on a revised Fischer-Tropsch path, in which CO is firstly generated by the reverse water gas reaction (RWGS), and then hydrocarbon compounds were generated by the FT reaction [13,14].…”