The Role of Surface-Bound Dihydropyridine Analogues in Pyridine-Catalyzed CO Reduction over Semiconductor Photoelectrodes.

We propose a general reaction mechanism for the pyridine (Py)-catalyzed reduction of CO over GaP(111), CdTe(111), and CuInS(112) photoelectrode surfaces. This mechanism proceeds via formation of a surface-bound dihydropyridine (DHP) analogue, which is a newly postulated intermediate in the Py-catalyzed mechanism. Using density functional theory, we calculate the standard reduction potential related to the formation of the DHP analogue, which demonstrates that it is thermodynamically feasible to form this intermediate on all three investigated electrode surfaces under photoelectrochemical conditions. Hydride transfer barriers from the intermediate to CO demonstrate that the surface-bound DHP analogue is as effective at reducing CO to HCOO as the DHP molecule in solution. This intermediate is predicted to be both stable and active on many varying electrodes, therefore pointing to a mechanism that can be generalized across a variety of semiconductor surfaces, and explains the observed electrode dependence of the photocatalysis. Design principles that emerge are also outlined.