Probing the Local Structure of Na in NaNO3-Promoted, MgO-Based CO2 Sorbents via X-ray Absorption Spectroscopy.

This work provides insight into the local structure of Na in MgO-based CO2 sorbents that are promoted with NaNO3. To this end, we use X-ray absorption spectroscopy (XAS) at the Na K-edge to interrogate the local structure of Na during the CO2 capture (MgO + CO2 ↔ MgCO3). The analysis of Na K-edge XAS data shows that the local environment of Na is altered upon MgO carbonation when compared to that of NaNO3 in the as-prepared sorbent. We attribute the changes observed in the carbonated sorbent to an alteration in the local structure of Na at the NaNO3/MgCO3 interfaces and/or in the vicinity of [Mg2+···CO32-] ionic pairs that are trapped in the cooled NaNO3 melt. The changes observed are reversible, i.e., the local environment of NaNO3 was restored after a regeneration treatment to decompose MgCO3 to MgO. The ex situ Na K-edge XAS experiments were complemented by ex situ magic-angle spinning 23Na nuclear magnetic resonance (MAS 23Na NMR), Mg K-edge XAS and X-ray powder diffraction (XRD). These additional experiments support our interpretation of the Na K-edge XAS data. Furthermore, we develop in situ Na (and Mg) K-edge XAS experiments during the carbonation of the sorbent (NaNO3 is molten under the conditions of the in situ experiments). These in situ Na K-edge XANES spectra of molten NaNO3 open new opportunities to investigate the atomic scale structure of CO2 sorbents modified with Na-based molten salts by using XAS.

Photoelectrochemical glycerol oxidation on Mo-BiVO4 photoanodes shows high photocharging current density and enhanced H2 evolution.

Mo-doped BiVO4's lower efficiency can be attributed in part to exciton recombination losses. Recombination losses during photoelectrochemical water oxidation can be eliminated by using glycerol as a hole acceptor. This results in an enhanced photocurrent density. In this research, we present the synthesis of a Mo-doped BiVO4 photoelectrode with a greater photocurrent density than a traditional pristine photoanode system. Increased photon exposure duration in the presence of glycerol leads to 8 mA cm-2 increase in photocurrent density due to the creation of a capacitance layer and a decrease in charge transfer resistance on the photoelectrode in a neutral-phosphate buffer solution thus confirming the photo charging effect. Glycerol photooxidation improves the photoelectrode's rate of hydrogen evolution. Research into the effects of electrolyte and electrode potential on photoelectrodes has revealed that when the applied potential increases, the light absorbance behaviour changes following its absorption distribution over the applied potential. Under a transmission electron microscope (TEM), a unique dynamical crystal fringe pattern is found in the nanoparticles scratched from the photoelectrode.