Why and How Carbon Dioxide Conversion to Methanol Happens on Functionalized Semiconductor Photoelectrodes

Functionalization of semiconductor electrode surfaces with adsorbed 2-pyridinide (2-PyH^-*) has been postulated to enable selective CO₂ photoelectroreduction to CH₃OH. This hypothesis is supported by recent estimates of sufficient 2-PyH^-* lifetimes and low barriers for hydride transfer (HT) to CO₂. However, the complete mechanism for reducing CO₂ to CH₃OH remained unidentified. Here, vetted quantum chemistry protocols for modeling GaP reveal a pathway involving HTs to specific CO₂ reduction intermediates. Predicted barriers suggest that HT to HCOOH requires adsorbed HCOOH* reacting with 2-PyH^-*, a new catalytic role for the surface. HT to HCOOH* produces CH₂(OH)₂, but subsequent HT to CH₂(OH)₂ forming CH₃OH is hindered. However, CH₂O, dehydrated CH₂(OH)₂, easily reacts with 2-PyH^-*, producing CH₃OH. Further reduction of CH₃OH to CH₄ via HT from 2-PyH^-* encounters a high barrier, consistent with experiment. Our finding that the GaP surface enables HT to HCOOH* explains why the primary CO₂ reduction product over CdTe photoelectrodes is HCOOH rather than methanol, as HCOOH does not adsorb on CdTe and so the reaction terminates. The stability of 2-PyH^-* (vs its protonation product DHP*), the relative dominance of CH₂(OH)₂ over CH₂O, and the required desorption of CH₂(OH)₂* are the most likely limiting factors, explaining the low yield of CH₃OH observed experimentally.