ABSTRACT
Production of biodiesel generates glycerol as a 10 wt% byproduct. Therefore, efficient and selective glycerol upgrading is critical for the sustainable production of biodiesel as well as for the production of chemicals from renewable feedstocks. In this study, the photoelectrochemical glycerol oxidation reaction (GOR) was investigated using a nanoporous BiVO4 photoanode in pH 9.3 and pH 2 buffer solutions. In both solutions, glycolaldehyde (GCAD), a C2 species, was the major product, which has never been the major product in any previous electrochemical or photoelectrochemical GOR study. To produce GCAD from the C3 species glycerol, C-C cleavage should occur to produce C2 and C1 species with a 1:1 ratio. Intriguingly, our results show that, during photoelectrochemical GOR on BiVO4, more GCAD is produced than can be explained by simple C-C cleavage, meaning that GCAD is also produced from C-C coupling of two C1 species produced from C-C cleavage. This is equivalent to converting two glycerol molecules to three GCAD molecules, which offers an extraordinary way to maximize GCAD production. To gain further insight into the nature of this unprecedented C-C coupling during GOR, photoelectrochemical oxidation of intermediate oxidation products (glyceraldehyde and 1,3-dihydroxyacetone) and glycerol-1,3-13C2 was compared to that of standard glycerol. Photoelectrochemical GOR was also compared with electrochemical GOR on BiVO4 to interrogate whether light is critical for the observed C-C coupling. Results obtained from comprehensive control experiments revealed critical information about C-C cleavage and C-C coupling during GOR on BiVO4.
ABSTRACT
Photoanodes used in a water-splitting photoelectrochemical cell are almost always paired with an oxygen evolution catalyst (OEC) to efficiently utilize photon-generated holes for water oxidation because the surfaces of photoanodes are typically not catalytic for the water oxidation reaction. Suppressing electron-hole recombination at the photoanode/OEC interface is critical for the OEC to maximally utilize the holes reaching the interface for water oxidation. In order to explicitly demonstrate and investigate how the detailed features of the photoanode/OEC interface affect interfacial charge transfer and photocurrent generation for water oxidation, we prepared two BiVO4(010)/FeOOH photoanodes with different Bi:V ratios at the outermost layer of the BiVO4 interface (close to stoichiometric vs Bi-rich) while keeping all other factors in the bulk BiVO4 and FeOOH layers identical. The resulting two photoanodes show striking differences in the photocurrent onset potential and photocurrent density for water oxidation. The ambient pressure X-ray photoelectron spectroscopy results show that these two BiVO4(010)/FeOOH photoanodes show drastically different Fe2+:Fe3+ ratios in FeOOH both in the dark and under illumination with water, demonstrating the immense impact of the interfacial composition and structure on interfacial charge transfer. Using computational studies, we reveal the effect of the surface Bi:V ratio on the hydration of the BiVO4 surface and bonding with the FeOOH layer, which in turn affect the band alignments between BiVO4 and FeOOH. These results explain the atomic origin of the experimentally observed differences in electron and hole transfer and solar water oxidation performance of the two photoanodes having different interfacial compositions.
ABSTRACT
Some post-translationally modified tyrosines can perform reversible redox chemistry similar to metal cofactors. The most studied of these tyrosine modifications is the intramolecular thioether-crosslinked 3'-(S-cysteinyl)-tyrosine (Cys-Tyr) in galactose oxidase. This Cu-mediated tyrosine modification in galactose oxidase involves direct electron transfer (inner-sphere) to the coordinated tyrosine. Mammalian cysteine dioxygenase enzymes also contain a Cys-Tyr that is formed, presumably, through outer-sphere electron transfer from a non-heme iron center ~6Å away from the parent residues. An orphan protein (BF4112), amenable to UV spectroscopic characterization, has also been shown to form Cys-Tyr between Tyr 52 and Cys 98 by an adjacent Cu2+ ion-loaded, mononuclear metal ion binding site. Native Cys-Tyr fluorescence under denaturing conditions provides a more robust methodology for Cys-Tyr yield determination. Cys-Tyr specificity, relative to 3,3'-dityrosine, was provided in this fluorescence assay by guanidinium chloride. Replacing Tyr 52 with Phe or the Cu2+ ion with a Zn2+ ion abolished Cys-Tyr formation. The Cys-Tyr fluorescence-based yields were decreased but not completely removed by surface Tyr mutations to Phe (Y4F/Y109F, 50%) and Cys 98 to Ser (25%). The small absorbance and fluorescence emission intensities for C98S BF4112 were surprising until a significantly red-shifted emission was observed. The red-shifted emission spectrum and monomer to dimer shift seen by reducing, denaturing SDS-PAGE demonstrate a surface tyrosyl radical product (dityrosine) when Cys 98 is replaced with Ser. These results demonstrate surface tyrosine oxidation in BF4112 during Cys-Tyr formation and that protein oxidation can be a significant side reaction in forming protein derived cofactors.
