In order to help develop robust and deployable molecular electrocatalysts for the reduction of CO2 to CO, we must understand the effects of tuning their structure and catalytic conditions. To this end, we quantify how modifications to the catalyst fac-Re­(4,4′-R-bpy)­(CO)3X (bpy = 2,2′-bipyridine, R = OCH3, CH3, tBu, H, CN, CF3; X = Cl, Br, py­(OTf), or CH3CN­(OTf)) with and without an added proton source (phenol, acetic acid, 2,2,2-trifluoroethanol) affect the catalyst stability, activity, and overpotential. Through cyclic voltammetry experiments, we found that the substituents and proton source had a large effect on both overpotential and activity. Substituents with moderate electron-donating ability (tBu and CH3) increased activity and overpotential in comparison to the unsubstituted complex Re­(bpy)­(CO)3Cl. In contrast, substituents resulting in too much electron density distributed over the bpy ligand, either from too-strong electron-donating ability (OCH3) or from the requirement of a third reduction to activate the complex (CN and CF3), destabilized the catalyst. An added proton source both increased the activity and decreased the overpotential by 200 mV for all catalyst derivatives, shifting the catalytic mechanism from an electron-first pathway to a proton-first pathway. We used binding energies calculated via density functional theory to help understand the substituent effect on the catalyst affinity for CO2 and other intermediates relevant to the catalytic mechanism. Catalyst activity was quantified using intrinsic rate constants determined through the utilization of catalytic plateau currents, as well as the application of a foot of the wave analysis, which yielded incongruent values. Of those complexes tested, Re­(4,4′-tBu-bpy)­(CO)3Cl with an added 1 M phenol yielded the most active catalytic system (k cat = 6206 s–1) at an overpotential of 0.67 V.

Experimental evidence suggests that pyridinium plays an important role in photocatalytic CO2 reduction on p-GaP photoelectrodes. Pyridinium reduction to pyridinyl has been previously proposed as an essential mechanistic step for this reaction. However, theoretical calculations suggest that this step is not feasible in solution. Here, cluster models and accurate periodic boundary condition calculations are used to determine whether such a reduction step could occur by transfer of photoexcited electrons from the p-GaP photocathode and whether this transfer could be catalyzed by pyridinium adsorption on the p-GaP surface. It is found that both the transfer of photoexcited electrons to pyridinium and pyridinium adsorption are not energetically favored, thus making very unlikely pyridinium reduction to the pyridinyl radical and the proposed mechanisms requiring this reduction step. Given this conclusion, an alternative and energetically viable pathway for pyridinium reduction on p-GaP photoelectrodes is proposed. This pathway leads to the formation of adsorbed species that could react to form adsorbed dihydropyridine, which was proposed previously to play the role of the active catalyst in this system.

The role of the photoelectrode surface during pyridinecatalyzed CO 2 reduction on p-GaP photocathodes is currently under debate. Understanding the extent of the photoelectrode's direct participation in the catalytic CO 2 reduction mechanism is essential to improving the design of such photoelectrochemical systems. Here, we present new theoretical results demonstrating that the previously proposed pyridinyl radical intermediate is unlikely to form and that reduction of pyridinium to adsorbed pyridine and H species remains the most favorable reduction pathway, even when accounting for the aqueous environment. Furthermore, we conclude, based on recently reported experimental evidence and our new computational results reported herein, that the mechanism of CO 2 reduction operating in this system is likely heterogeneous. We also introduce a new heterogeneous mechanism involving a recently proposed radical species, which we predict will be stable on the electrode surface and that may serve as the active catalytic species in this system.

Pyridine (Py) is an effective cocatalyst during the photoelectrochemical reduction of CO 2 to methanol over GaP, CdTe, and CuInS 2 semiconductor surfaces. Identifying the role Py plays in the catalytic reduction mechanism is essential for optimizing the design of such photocatalytic processes. The Py-enhanced mechanism, however, is under considerable debate. Recent studies suggest that the semiconductor surface itself participates in a heterogeneous mechanism, and for this reason a detailed understanding of the interaction between Py and the surface is required. Additionally, surface reconstructions occurring during operation alter the nature of adsorption sites available for interaction with the solution, therefore impacting the performance of the electrode. To address this issue, we report a density functional theory investigation of the stability of GaP(111) and CdTe(111) surface reconstructions, as well as adsorption trends of intermediate species across sites created by such reconstructions. We also determine band edge positions of the solvated, reconstructed surfaces, which we compare to calculated reduction potentials involved in proposed elementary steps of the overall CO 2 reduction mechanism. This allows us to determine which reduction steps are thermodynamically feasible based on the energy of a photoexcited electron in the conduction band of the semiconductor. Given the band edge alignment of the GaP(111) surface, we determine that the 1e − reduction of the solvated pyridinium cation most favorably results in the formation of adsorbed Py* + H* species and that the formation of a 1-pyridinyl radical in solution is unlikely. Furthermore, we find that it is thermodynamically feasible to form a newly proposed adsorbed 2-pyridinyl intermediate adsorbed on the surface, which may act as a powerful hydride donor. On the CdTe(111) surface, we find that no 1e − reductions are thermodynamically feasible, leaving only 2e − reductions leading to the formation of dihydropyridine (DHP) as possible reduction steps. These results identify stable intermediate species along the CO 2 reaction path over reconstructed surfaces, thus lending insight into the Py-catalyzed reaction mechanism.

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