Functionalization of semiconductor electrode surfaces with adsorbed 2-pyridinide (2-PyH - ) has been postulated to enable selective CO 2 photoelectroreduction to CH 3 OH. This hypothesis is supported by recent estimates of sufficient 2-PyH - ∗ lifetimes and low barriers for hydride transfer (HT) to CO 2 . However, the complete mechanism for reducing CO 2 to CH 3 OH remained unidentified. Here, vetted quantum chemistry protocols for modeling GaP reveal a pathway involving HTs to specific CO 2 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 2 (OH) 2 , but subsequent HT to CH 2 (OH) 2 forming CH 3 OH is hindered. However, CH 2 O, dehydrated CH 2 (OH) 2 , easily reacts with 2-PyH - , producing CH 3 OH. Further reduction of CH 3 OH to CH 4 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 2 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 2 (OH) 2 over CH 2 O, and the required desorption of CH 2 (OH) 2 ∗ are the most likely limiting factors, explaining the low yield of CH 3 OH observed experimentally.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry