(351e) The Role of Surface-Bound Dihydropyridine Analogs in Pyridine-Catalyzed CO2 Reduction over Semiconductor Photoelectrodes
The (photo)electro-catalytic reduction of CO2 to useful fuels via energy from sunlight has received significant attention as a promising route for generating carbon-neutral fuels and value-added chemicals. Experimental evidence suggests that pyridine (Py) is an effective co-catalyst during CO2 reduction over GaP, CdTe, and CuInS2 semiconductor electrodes. Identifying the role of Py during catalytic reduction is essential for optimizing the design of such photocatalytic processes. We propose a general reaction mechanism for the Py-catalyzed reduction of CO2 over GaP(111), CdTe(111), and CuInS2(112) photoelectrode surfaces. This mechanism proceeds via formation of a surface-bound dihydropyridine (DHP) analog, 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 analog, which demonstrates that it is thermodynamically feasible to form this intermediate on all three investigated electrode surfaces under photo-electrochemical conditions. Hydride transfer barriers from the intermediate to CO2 demonstrate that the surface-bound DHP analog is as or even more effective at reducing CO2 to HCOO- as the DHP(aq) 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.