Acetate formation on metals via CH4 carboxylation by CO2: A DFT study

Density functional theory calculations have been performed to investigate CH4 activation and coupling to CO2 forming C2 carboxylates such as acetate on the close-packed (111) or (0001) surfaces of ten mid-to-late transition and coinage metals (Co, Ni, Cu, Ru, Rh, Pd, Ag, Ir, Pt, and Au). Consistent with the literature, the activation energy (Ea) for the initial C-H bond scission in CH4 is mild, being ca. 1 eV or less on all but the coinage metals, of which Ag exhibits the highest Ea at 2.13 eV, followed by Au and Cu. Ea for the CH3-CO2 coupling step is 0.8 - 1.1 eV on Co, Ru, Rh, and Ag, 1.2 - 1.5 eV on Ni, Cu, Pd, and Ir, and 1.8 - 2.1 eV on Pt and Au. While the two Ea are comparable for several metals in terms of DFT total energies, free energy analysis indicates CH3-CO2 coupling to be much more rate-limiting than CH4 activation. Overcoming it would require over 800 K even on the most active of the metals considered, Ru, which makes the formation of acetate not feasible on the monometallic metal surfaces. Instead, we propose that single atom alloys based on early transition metals doped into a host metal such as Ni(111) could be viable catalysts. The dopant sites serve to stabilize the transition state of C-C coupling while Ni sites continue to activate CH4, thereby significantly lowering the required temperature for acetate formation.

Automated summary

Density functional theory calculations were used to study the activation and coupling reactions of CH4 and CO2 on metal surfaces. The results showed that the activation energy for C-H bond scission in CH4 was low on most metals except for the coinage metals. The coupling reaction between CH3 and CO2 was found to have higher activation energies on most metals. However, the formation of acetate was not feasible on monometallic surfaces.