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 CO photoelectroreduction to CHOH. This hypothesis is supported by recent estimates of sufficient 2-PyH* lifetimes and low barriers for hydride transfer (HT) to CO. However, the complete mechanism for reducing CO to CHOH remained unidentified. Here, vetted quantum chemistry protocols for modeling GaP reveal a pathway involving HTs to specific CO 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 CH(OH), but subsequent HT to CH(OH) forming CHOH is hindered. However, CHO, dehydrated CH(OH), easily reacts with 2-PyH*, producing CHOH. Further reduction of CHOH to CH 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 CO 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 CH(OH) over CHO, and the required desorption of CH(OH)* are the most likely limiting factors, explaining the low yield of CHOH observed experimentally.