UC Berkeley

Heterogenized cobalt phthalocyanine (CoPc) catalyzes electrochemical CO2 reduction (CO2R) to methanol. While it is clear that CO is a reaction intermediate, the role of formaldehyde, a sometimes-observed trace product, is less well-understood. Here, we gain insight into the methanol formation mechanism by selectively trapping formaldehyde. Using a simple preparation of CoPc heterogenized on multiwalled carbon nanotubes as the electrocatalyst, we employed three hydroxylamines which react with formaldehyde in the electrolyte to form stable, unreactive oximes at different rates: pentafluorobenzylhydroxylamine [PFBHA] (slowest), ethylhydroxylamine [EHA], and hydroxylamine [HA] (fastest). PFBHA has only a small effect on the methanol/formaldehyde selectivity, while EHA and HA (and higher concentrations of EHA) both reduce methanol molar selectivity by 25%, with a corresponding carbon-balanced increase in formaldehyde selectivity. Unbound formaldehyde thus is a relatively (compared to CO) short-lived intermediate responsible for only a quarter of the observed methanol. Extended CO2R in PFBHA-containing electrolyte scrubs the solution of CH2O without impacting MeOH selectivity, and we saw no noticeable difference in the rate of MeOH selectivity decline compared to control. This suggests that CH­2O is not a poison. 2D reaction-diffusion-convection simulations performed in a simplified geometry are consistent with the observed trends in methanol/formaldehyde selectivity, although derivatization rate constants larger than our experimental estimates need to be employed. We predict that hydroxylamine derivatization could be used for mechanistic analysis in other electrocatalytic networks with aldehyde intermediates.