Due to its high overpotential and low efficiency, the conversion of water to O(2) using solar energy remains a bottleneck for photocatalytic water splitting. Here the microscopic mechanisms of the oxygen evolution reaction (OER) on differently structured anatase surfaces in aqueous surroundings, namely, (101), (001), and (102), are determined and compared systematically by combining first-principles density functional theory calculations and a parallel periodic continuum solvation model. We show that OER involves the sequential removal of protons from surface oxidative species, forming surface peroxo and superoxo intermediates. The initiating step, the first proton removal, dictates the high overpotential. Only at an overpotential of 0.7 V (1.93 V vs SHE) does this rate-controlling step become surmountable at room temperature: the free energy change of the step is 0.69, 0.63, and 0.61 eV for (101), (102), and (001) surfaces, respectively. We therefore conclude that (i) OER is not sensitive to the local surface structure of anatase and (ii) visible light (<∼590 nm) is, in principle, capable of driving the photocatatlytic OER on anatase kinetically. By co-doping high-valent elements into the anatase subsurface, we demonstrate that the high overpotential of the OER can be significantly reduced, with extra occupied levels above the valence band.
Dark
teas are prepared by a microbial fermentation process. Flavan-3-ol
B-ring fission analogues (FBRFAs) are some of the key bioactive constituents
that characterize dark teas. The precursors and the synthetic mechanism
involved in the formation of FBRFAs are not known. Using a unique
solid-state fermentation system with β-cyclodextrin inclusion
complexation as well as targeted chromatographic isolation, spectroscopic
identification, and Feature-based Molecular Networking on the Global
Natural Products Social Molecular Networking web platform, we reveal
that dihydromyricetin and the FBRFAs, including teadenol A and fuzhuanin
A, are derived from epigallocatechin gallate upon exposure to fungal
strains isolated from Fuzhuan brick tea. In particular, the strains
from subphylum Pezizomycotina were key drivers for these B-/C-ring
oxidation transformations. These are the same transformations seen
during the fermentation process of dark teas. These discoveries set
the stage to enrich dark teas and other food products for these health-promoting
constituents.
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