We will now investigate how the C2-proton affects the reduction of CO2. Pathway i (Figure 3.4 a) considers electron transfer to the cation to form a cation radical as proposed by Wang, et al 4. For this pathway, the reduced cation acts as an initiator and reduces CO2 through a nucleophilic attack to form an imidazolium-CO2 adduct (Supporting Information Scheme S3.1). After the nucleophilic attack, the C2-proton is transferred via an isomerization step, which is considered the rate-determining step (RDS). However, DFT calculations for the isomerization step do not show any noticeable difference in the activation energy for several selected cations (Supporting Information Section VII). Besides, pathway i implies an electrochemical response in the absence of CO2 (the electron transfer to the imidazolium cation), which is not observed in Figure 3.1. If an initial electron transfer to the imidazolium cation were the case, the same onset potentials would be observed for the reduction of the cation (CVs under He purging) and reduction of CO2 (CVs under CO2 purging). This situation is typical when pyridinium-type molecules are used as the promoter but this is not the case with imidazolium cations. (For more arguments see Supporting Information Section VIII). Thus, even though the mechanism proposed in pathway i might be considered in homogeneous solution electron transfer reactions, Au catalyzed surface transformation appears to follow another mechanism. Pathway ii (Figure 3.4 b) depicts the initial electron transfer to CO2 upon its adsorption. Upon electron transfer, the imidazolium cation donates its C2-proton to the negatively charged *CO2 ¯ being formed on the Au surface. Such a charge transfer is also confirmed by differential charge analysis from the interaction between MM and *CO2 ¯ (Figure 3.3, Supporting Information Figure S3.10). This pathway highlights a concerted coupled electron-proton transfer (CEPT) mechanism. For this mechanism, the first electron transfer should be still rate-determining, and this was confirmed with the obtained Tafel slope of 119 mV/dec (Supporting Information Figure S3.4). CEPT reactions are of significant importance
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