(465b) CO-CO Coupling on Cu Facets: Coverage, Strain and Field Effects

Authors: 
Sandberg, R. B., Stanford University
Chan, K., SUNCAT Center for Interface Science and Catalysis, Stanford University and SLAC National Accelerator Laboratory
Nørskov, J. K., Stanford University and SLAC National Laboratory
The electrochemical CO2 reduction reaction has the potential to produce industrial chemicals and to mitigate the increasing CO2 concentrations in the atmosphere. Polycrystalline copper, the only transition metal catalyst to produce hydrocarbons at reasonable Faradaic efficiency, produces a reasonable 1mA/cm2 current density at ~1V overpotential.1 Polycrystalline Cu has shown similar onset potentials for C1 and C2 hydrocarbons at -0.75V vs. RHE2, suggesting both C1 and C2 pathways to be limited by the initial hydrogenation of *CO to form *CHO. However, the (100) facet and nanostructured Cu have been shown to have an earlier onset for C2 products in alkaline conditions.4-7 These results suggest a C-C coupling pathway that proceeds prior to *CO hydrogenation. Recent calculations have found CO-CO coupling to be feasible in the presence of a solvent and cation, which induces a field in the Helmholtz plane.8 We present a DFT study on the effect of coverage, strain, and electric field on CO-CO coupling energetics on Cu (100), (111), and (211). Our calculations indicate that CO-CO coupling is facile on all three facets in the presence of a cation-induced electric field in the Helmholtz plane, with the lowest barrier on Cu(100). The CO dimerization pathway is therefore expected to play a role in C2 formation at potentials negative of the Cu potential of zero charge, corresponding to CO2/CO reduction conditions at high pH. Both increased *CO coverage and tensile strain further improve C-C coupling energetics on Cu (111) and (211). Since CO dimerization is facile on all 3 Cu facets, subsequent surface hydrogenation steps may also play an important role in determining the overall activity towards C2 products. Adsorption of *CO, *H, and *OH on the 3 facets were investigated with a Pourbaix analysis. The (211) facet has the largest propensity to co-adsorb *CO and *H, which would favor surface hydrogenation following CO dimerization. These results suggest that the (100) and (211) facets should be the most active due to field and coverage effects.

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