What Is the Role of Pyridinium in Pyridine-Catalyzed CO₂Reduction on p-GaP Photocathodes?

Abstract

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.

Hydride Shuttle Formation and Reaction with CO2on GaP(110)

“…[19] Bocarsly and co-workerso bserved that the GaP(110) surface is more active towards CO 2 reduction than the GaP(111)s urface. [16] The hypothesis that the precursors needed for the adsorbed catalystf ormation are generated via PyH + reduction [10] is consistent with the experimental observation that an acidic pH is an essential condition for CO 2 reduction to occur. However, we found that the conduction band minima( CB min )o ft he two surfaces lie very close, with ad ifference of less than 0.1 V. [16] Therefore, we concluded that ah omogeneous mechanism cannote xplain these experimental observations, and that the mechanism must involvea dsorbed intermediates.…”

“…We used the same GaP(110) cluster model employedi np revious studies [10,11,13,16,17] and built following the procedure reportedi nr ef. [11].T he clusters tructure includes 24 Ga atoms, 24 Pa toms, and 40 Ha toms.…”

“…[19] If the CO 2 reduction mechanism in this system were homogeneous,t hen this observation could be explained only by as ubstantial differencei nt he energy of the photoexcited electrons provided by the two surfaces. [2] The computed reductionp otential for PyH + reduction to Py* and H* (À0.85 Vv s. SCE) [10] again lies well below the GaP CB min ,t hus suggesting that the process is thermodynamically feasible on p-GaP photocathodes under illumination. Several adsorptionf ree energy studies found that Py and DHP favorably bind to the GaP surface, whereas CO 2 does not.…”

Organic, Organometallic and Bioorganic Catalysts for Electrochemical Reduction of CO2

“…The use of semiconductors as electrodes and catalysts in solution was investigated by several groups. These studies yielded a range of products, from CO and H 2 95, 96, 97 to methanol 98…”

Recent progress on advanced design for photoelectrochemical reduction of CO2 to fuels

“…1a). Numerous p-type semiconductors have been exploited and investigated, including silicon [29,30], metal oxides [31][32][33], sulfides [34,35], tellurides [36,37], phosphides [38,39] and others [40,41]. In addition, profiting from the remarkable activity for CO 2 adsorption and activation, metal materials (e.g., Pd, Au, Ag and Cu) usually serve as the cathodes in electrochemical CO 2 reduction [42].…”