A new mechanism for the selectivity to C1 and C2 species in the electrochemical reduction of carbon dioxide on copper electrodes

We have investigated the reaction mechanism of the electrochemical reduction of carbon dioxide to hydrocarbons on copper electrodes. This reaction occurs via two pathways: a C-1 pathway leading to methane, and a C-2 pathway leading to ethylene. To identify possible intermediates in the reduction of carbon dioxide we have studied the reduction of small C-1 and C-2 organic molecules containing oxygen. We followed the formation and consumption of intermediates during the reaction as a function of potential, using online mass spectrometry. For the C-1 pathway we show that it is very likely that CHOads is the key intermediate towards the breaking of the C-O bond and, therefore, the formation of methane. For the C-2 pathway we suggest that the first step is the formation of a CO dimer, followed by the formation of a surface-bonded enediol or enediolate, or the formation of an oxametallacycle. Both the enediol(ate) and the oxametallacycle would explain the selectivity of the C-2 pathway towards ethylene. This new mechanism is significantly different from existing mechanisms but it is the most consistent with the available experimental data.