Carbon monoxide is a key intermediate in the electrochemical reduction of carbon dioxide to methane and ethylene on copper electrodes. We investigated the electrochemical reduction of CO on two single-crystal copper electrodes and observed two different reaction mechanisms for ethylene formation: one pathway has a common intermediate with the formation of methane and takes place preferentially at (111) facets or steps, and the other pathway involves selective reduction of CO to ethylene at relatively low overpotentials at (100) facets. The (100) facets seem to be the dominant crystal facets in polycrystalline copper, opening up new routes to affordable (photo)electrochemical production of hydrocarbons from CO(2).

The product selectivity in the electrochemical reduction of carbon dioxide and carbon monoxide strongly depends on the atomic configuration of the copper electrode surface. On Cu(111), methane formation is favored, whereas on Cu(100), ethylene formation is favored, with selective ethylene formation at low overpotentials. To distinguish the reactivity of (100) terraces vs (100) steps, we have studied carbon monoxide reduction on Cu(322), with the [5(111) × (100)] orientation, and Cu(911), with the [5(100) × (111)] orientation. Only on Cu(911) is the selective ethylene formation at low overpotentials observed, indicating that this reaction pathway occurs only on (100) terraces. We also show that the reduction of ethylene oxide to ethylene is significantly faster on Cu(100) compared with Cu(111), giving further evidence to the importance of the associated intermediate for ethylene formation. On Cu(110), the potential dependence of methane and ethylene formation is similar to Cu(111), and we have observed a primary alcohol among the products.

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