Fig. 4. Origin of the pH effects on the C₂ Oxy/HC selectivity for COR.

Analyses of DSC for a ethylene, b ethanol + acetaldehyde (denoted as ethanol*), and c acetic acid. Similar to the degree of rate control conceptualized by Campbell et al.⁶⁰, DSC serves as a powerful tool to quantify the magnitude of selectivity controlling by a certain intermediate or a TS. A positive (negative) value of DSC indicates that the corresponding reaction intermediate or TS needs to be stabilized (destabilized) in order to enhance the selectivity. The boundary values of 1 and −1 represent full selectivity control by the intermediates. Only the energy states having significant contribution to the DSC, i.e. absolute value of DSC > 0.01, are shown. Key intermediates and TSs associated with the CHCOH, OCHCH, and CH₂CO pathways are shown in reddish, bluish, and greenish colors, respectively. From the upper to lower panels, the pH of bulk electrolyte increases from 7 to 13. Analytical approximation of $ln\left(\frac{r_{{\mathrm{C}}_2\mathrm{Oxy}}}{r_{{\mathrm{C}}_2\mathrm{HC}}}\right)$ as a function of d U_RHE and e U_SHE at pH7 and pH13, in comparison with the theoretical results obtained through microkinetic modeling. The slopes of the two legs are given by Eq. (1) and Eq. (2), respectively. β^Oxy could be either β^OCCH-H or β^CHC-HO depending on the dominant C₂ Oxy formation pathway. Here β^OCCH-H was employed for Cu(100). Source data are provided as a Source Data file.