Mechanism of glycerol oxidation on bismuth vanadate photoanodes: Influence of tantalum doping

This study investigated the mechanisms underlying the photoelectrochemical oxidation of glycerol to dihydroxyacetone (DHA) and glyceraldehyde (GLAD) on BiVO₄ and Bi–O–Ta species using density functional theory calculations. The results revealed that for both products and metal-oxides, the first proton-coupled electron-transfer (PCET) step, which forms a carbon-centered radical, served as the rate-limiting step. The formation of DHA from a secondary-carbon-centered radical encountered lower energy barriers compared to the formation of GLAD from a primary-carbon-centered radical, indicating the favorability of the DHA production pathway. The differences between the energy inputs required for DHA and GLAD production were greater on the Bi–O–Ta species compared to the BiVO₄. Furthermore, the production of formic acid (FA) from glycerol via GLAD, involving three H₂O molecules and eight PCET steps, encountered greater energy barriers on Bi–O–Ta species than on BiVO₄. These computational results confirm that the Bi–O–Ta species enhances DHA generation from glycerol while suppressing the production of unfavorable byproducts such as GLAD and FA. Correspondingly, the Ta-doped BiVO₄ photoanode achieves a greater Faradaic efficiency than the BiVO₄ photoanode and approximately 100% selectivity for DHA in acidic media owing to the Bi–O–Ta species formed on the surface.