ABSTRACT
To determine the effect of F- on the electrochemical formation of Zr, the reduction mechanism, kinetic properties, and nucleation mechanism of Zr(IV) were compared in the LiCl-KCl-K2ZrF6 system before and after the addition of F- at different concentration ratios of F-/Zr(IV). As indicated by the results, when the ratio of F-/Zr(IV) ranged from 7 to 10, the intermediate state Zr(III) was detected, and the reduction mechanism of Zr(IV) was converted into Zr(IV) â Zr(III) â Zr. The diffusion coefficients of Zr(IV), Zr(III), and Zr(II) decreased with an increase in the value of F-/Zr(IV). The exchange current density (j0) of Zr(II)/Zr exceeded that of Zr(III)/Zr, and the j0 and α values of Zr(III)/Zr decreased with the increase of F-/Zr(IV). The nucleation mechanism at different ratios of F-/Zr(IV) was investigated through chronoamperometry. The result suggested that the nucleation mechanism of Zr varied with the overpotential at F-/Zr(IV) = 6. The addition amount of F- led to the variation of the nucleation mechanism of Zr, i.e., progressive nucleation when F-/Zr(IV) = 7 and instantaneous nucleation when F-/Zr(IV) = 10. Zr was prepared through constant current electrolysis at different concentrations of F- and then analyzed through X-ray diffraction (XRD) and scanning electron microscopy (SEM), suggesting that the concentration of F- can exert a certain effect on the surface morphology of products.
ABSTRACT
The rational design of photocatalysts for efficient nitrogen (N2) fixation at ambient conditions is important for revolutionizing ammonia production and quite challenging because the great difficulty lies in the adsorption and activation of the inert N2. Inspired by a biological molecule, chlorophyll, featuring a porphyrin structure as the photosensitizer and enzyme nitrogenase featuring an iron (Fe) atom as a favorable binding site for N2via π-backbonding, here we developed a porphyrin-based metal-organic framework (PMOF) with Fe as the active center as an artificial photocatalyst for N2 reduction reaction (NRR) under ambient conditions. The PMOF features aluminum (Al) as metal node imparting high stability and Fe incorporated and atomically dispersed by residing at each porphyrin ring promoting the adsorption and the activation of N2, termed Al-PMOF(Fe). Compared with the pristine Al-PMOF, Al-PMOF(Fe) exhibits a substantial enhancement in NH3 yield (635 µg g-1cat.) and production rate (127 µg h-1 g-1cat.) of 82% and 50%, respectively, on par with the best-performing MOF-based NRR catalysts. Three cycles of photocatalytic NRR experimental results corroborate a stable photocatalytic activity of Al-PMOF(Fe). The combined experimental and theoretical results reveal that the Fe-N site in Al-PMOF(Fe) is the active photocatalytic center that can mitigate the difficulty of the rate-determining step in photocatalytic NRR. The possible reaction pathways of NRR on Al-PMOF(Fe) were established. Our study of porphyrin-based MOF for the photocatalytic NRR will provide insight into the rational design of catalysts for artificial photosynthesis.
