(17d) DFT Studies of CO2 Electrochemical Reduction Reactions On Cu-Based Catalysts

Direct conversion of solar or electrical energy to chemical energy using abundant sources of CO₂ and water presents opportunities to produce high energy density fuels and reduce atmospheric CO₂ levels. Large overpotentials and poor understanding of the factors that affect the selectivity are significant barriers to the application of CO₂ electrochemical reduction to produce liquid fuels. Recent experimental results from our collaborators at Louisiana State University (LSU) show that Cu/ZnO and oxidized Cu catalysts produce methanol instead of methane ^[1]. This result is the inverse of the findings from Hori and co-workers for Cu metal ^[2], where they observed methane production but no detectable methanol. Based on density functional theory (DFT) calculations, Nørskov and co-workers have reported a pathway for methane versus methanol production on Cu surfaces through formation of a surface methoxy (CH₃O*) ^[3,4]. To probe the differences between the selectivity observed on Cu metals versus Cu oxide or Cu/ZnO surfaces, we performed DFT calculations to map out the energetics of CO₂ electrochemical reduction to methane versus methanol on several surface models including Cu metals, Cu/ZnO(1010), and Cu₂O(111). The methane and methanol branching from the key methoxy intermediate species on the various surface models will be presented. There are strong differences in the stability of species in comparison of Cu metal with Cu oxide. In particular, on one monolayer of Cu on ZnO(1010), we found a facile downhill pathway to methoxy species (CH₃O* + H*) on the surface. Our DFT results on Cu₂O(111) surface suggest that the hydrogenation of O* versus H* on the CH₂O* intermediate might lead to the experimentally observed selectivity to methanol on Cu oxide surfaces ^[1], however, the real surface character (e.g. hydride phase; reduced phases) of Cu oxides under experimental conditions should play an important role in affecting the reaction. We will also discuss our ongoing work to examine the role of kinetics (i.e. barriers to reaction steps) and other factors (e.g. solvation; applied electrode potential) on CO₂reduction on the Cu(111) surface.

References

(1) Le, M.; Ren, M.; Zhang, Z.; Sprunger, P. T.; Kurtz, R. L.; Flake, J. C. Journal of the Electrochemical Society 2011, 158, E45.

(2) Hori, Y.; Murata, A.; Takahashi, R. Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases 1989, 85, 2309.

(3) Peterson, A. A.; Abild-Pedersen, F.; Studt, F.; Rossmeisl, J.; Nørskov, J. K. Energy & Environmental Science 2010, 3, 1311.

(4) Durand, W. J.; Peterson, A. A.; Studt, F.; Abild-Pedersen, F.; Nørskov, J. K. Surface Science 2011, 605, 1354.