The ortho-substituent effect regulating the separation of photogenerated carriers to efficiently photodegrade tetracycline on the surface of FeCo-based MOFs.

It is feasible to improve the photodegradation efficiency of organic pollutants by metal-organic frameworks (MOF)-based semiconductors via ligand engineering. In this work, three (Fe/Co)-XBDC-based MOFs were synthesized by introducing different ortho-functional groups X (X = -H, -NO2, -NH2) next to the carboxyl group of the organic ligand (i.e., terephthalic acid). The analysis focused on the influence mechanism of the adjacent functional group effect of the ligand on the physicochemical properties of the material and the actual photodegradation activity of TC. Multiple pieces of evidences suggested that the differences in electron-induced and photocharge-transfer mechanisms of the above ortho functional groups affect the crystal morphology and photocatalytic activity of FeCo-MOF during pyrolysis. Interestingly, (Fe/Co)-NH2BDC exhibited the highest photocatalytic activity under neutral conditions. The results of density functional theory show that the introduction of a strong donor-NH2 group can enhance light absorption and act as an "electron pump" to supply electrons to the iron center, accelerating the separation and efficient transport of photogenerated carriers on the ligand-metal bridge. In conclusion, this study is a proposal for a strategy of structural regulation for the enhancement of the catalytic activity of (Fe/Co)-MOFs in the photodegradation of TC.

Oxygen vacancy induction strategy to regulate the evolution of valence- and free-electrons for boosting visible photodegradation of tetracycline hydrochloride.

The generation, migration and reaction paths of electrons are the key steps for photodegradation of pollutants. However, efficient operation of the above pathways is still challenging. Herein, by strong coordination and slow pyrolysis, we constructed a narrow band Zn-Mn bimetallic photoactive core-shell material (Mn@Zn-N-C, Eg = 3.38 eV) with abundant oxygen vacancies for enhancing the above electronic paths. The photodegradation experiments of tetracycline hydrochloride (TCH) showed that the formation and transfer of vacancy-induced free electrons in the synthesized Mn@Zn-N-C was the key to improve the photocatalytic activity. The DFT calculation results revealed that the photogenerated electrons can transfer along the Mn-O-Zn bridge in Mn@Zn-N-C, and promote the formation of MnIV, which directly capture the free electrons and reset itself to MnII site. In this case, the introduction of Mn would enhance the separation of h+ and e-. The adjacent vacancies and defects then also trapped the above free electrons and hinder the recombination of photogenerated carriers. Simultaneously, the localized valence electron transfer between the above redox pairs (Mn4+/Mn2+ and Zn2+/Zn0) also promoted the long-term stability of the photocatalytic process. In summary, using vacancy induction strategy to regulate the evolution of valence- and free-electrons is a promising method to improve the production-transfer-utilization efficiency of photogenerated carriers.

Generating high-valent iron-oxo ≡FeIV=O complexes by calcium sulfite activation in neutral microenvironments for enhanced degradation of CIP.

Although alkaline sulfite activation of ferrate (Fe(VI)) has advantages of fast response and high activity for degradation of organic contaminants, the specific production pathways of active species and the pH conditions still hinder its widespread application. Based on this, our study constructed a novel advanced oxidation process of calcium sulfite (CaSO3) could activated Fe(VI) continuously by Ca2+ buffering and investigated the mechanism under different pH values and CaSO3 dosages with ciprofloxacin as a target organic pollutant. The results showed that Ca2+ stabilized the process at a neutral/weakly alkaline microenvironment of pH 7-8, which could alleviate the hydrolysis of ≡FeIV=O by protons and iron hydroxyl groups. Besides, the removal of pollutants occurred efficiently when sulfate (SO32-) was excessive and had a 3:1 ratio of SO32- to Fe(VI), achieving more than 99% removal of electron-rich phenolic organic pollutants within 2 min. By adding different radical scavengers and combining electrochemical analysis methods and electron paramagnetic resonance spectroscopy techniques to revealed that the main active species in Fe(VI)/CaSO3 process were ≡FeIV=O/≡FeV=O. Furthermore, the reactivity of various sulfate species (such as SO32-, SO3•-, SO4•-, SO5•-) with Fe(VI) was calculated using density functional theory (DFT), and it was found that Fe(VI)-SO32- reaction has a much lower energy barrier (-36.08 kcal/mol), indicating that SO32- can readily activate Fe(VI) and generate ≡FeIV=O to attack the atoms with high Fukui index (f -) in organic pollutants. The above results confirm the feasibility of Fe(VI)/CaSO3 process. Thus, this study can theoretically and practically prove that the main active species is ≡FeIV=O, rather than SO4•- or •OH in Fe(VI)/CaSO3 process.