Formation of AgFeO2, α-FeOOH, and Ag2O from mixed Fe(NO3)3–AgNO3 solutions at high pH

Transformation and removal of imidacloprid mediated by silver ferrite nanoparticle facilitated peroxymonosulfate activation in water: Reaction rates, products, and pathways

Imidacloprid (IMI) is one of the most extensively used chlorinated organic pesticides and its widespread occurrence makes it attract increased public concern and scientific interest. Peroxymonosulfate (PMS) activation has been widely studied for the elimination of organic pollutants from water. But few studies are focused on their heterogeneous catalytic performance towards imidacloprid especially with the presence of silver ferrite nanoparticles (nAgFeO₂)-based catalysts. Herein, the catalyst, nAgFeO₂, was prepared via a co-precipitation method, and further applied to activate PMS for the removal of imidacloprid (IMI). Our results demonstrated that the prepared nAgFeO₂ significantly promoted the activation of PMS for removing IMI, and the removal of IMI followed a pseudo first-order kinetics model with the corresponding nAgFeO₂ dosage. Electron paramagnetic resonance (EPR) and quenching tests revealed the singlet oxygen (¹O₂)-mediated nonradical pathway, instead of hydroxyl radical (•OH) or sulfate radical (SO₄^•-), played the dominant role in the degradation of IMI. Eight products were identified and the degradation pathways of IMI were proposed. It is postulated that the primary site at the C-1 position of IMI was more easily attacked by the •OH yielding (6-chloropyridin-3-yl) methanol). While the site at the amidine nitrogen (2) of IMI was more likely attacked by the ¹O₂, and then reacted with •OH to produce 5-hydroxy imidacloprid. Overall, this study provides insights into the mechanisms of nonradical oxidation processes based on PMS for the elimination of pesticides from water, broadening the application of silver ferrite nanoparticles in wastewater treatment.

Achieving delafossite analog by in situ electrochemical self-reconstruction as an oxygen-evolving catalyst

Development of novel and robust oxygen evolution reaction (OER) catalysts with well-modulated atomic and electronic structure remains a challenge. Compared to the well-known metal hydroxides or (oxyhydr)oxides with lamellar structure, delafossites (ABO₂) are characterized by alternating layers of A cations and edge-sharing BO₂ octahedra, but are rarely used in OER due to their poor electron conductivity and intrinsic activity. Here, we propose a delafossite analog by mutation of metal oxyhydroxide and delafossite based on first-principles calculations. Modulation on the electronic structure due to distortion of the original crystal field of the BO₂ layers is calculated to enhance electron conductivity and catalytic activity. Inspired by the theoretical design, we have experimentally realized the delafossite analog by electrochemical self-reconstruction (ECSR). Operando X-ray absorption spectroscopy and other experimental techniques reveal the formation of delafossite analog with Ag intercalated into bimetallic cobalt-iron (oxyhydr)oxide layers from a metastable precursor through amorphization. Benefitting from the featured local electronic and geometric structures, the delafossite analog shows superior OER activity, affording a current density of 10 mA⋅cm^-2 at an overpotential of 187 mV and an excellent stability (300 h) in alkaline conditions.

Impact of Crystal Types of AgFeO2 Nanoparticles on the Peroxymonosulfate Activation in the Water

A simple co-precipitation method was developed to synthesize AgFeO₂ nanoparticles (NPs) with hexagonal 2H and 3R polytypes coexistence. The ratio of 2H and 3R types in AgFeO₂ NPs were regulated by controlling the calcination temperature (300, 400, and 500 °C). Such AgFeO₂ NPs were used as heterogeneous catalysts to activate peroxymonosulfate (PMS) for the removal of Orange I (OI) in the water. External water conditions effects and the stability of AgFeO₂ NPs were investigated. The catalytic performance of AgFeO₂ NPs was found to be significantly enhanced with the increasing content of 2H-AgFeO₂. ¹O₂, O₂^•-, SO₄^•-, and •OH were identified as the dominating reactive oxygen species (ROSs) participated in the catalytic process. The electron transfer of Ag⁰/Ag⁺ and Fe²⁺/Fe³⁺ cycles facilitated the decomposition of PMS to generate ROSs. The surface hydroxyl groups (-OH) were regarded as the catalytic active sites. The higher 2H-AgFeO₂ content in AgFeO₂ NPs promoted the concentration of surface hydroxyl groups ( C_-OH) and the reactivity of AgFeO₂ NPs for PMS activation. Based on theoretical calculations, the 2H-AgFeO₂ (004) plane with more Fe sites was more conducive to binding with the -OH compared to the 3R-AgFeO₂ (012) plane, ascribed to the stronger adsorption energy and shorter Fe-O bond length between 2H-AgFeO₂ and -OH.

Electrochemical (de)lithiation of silver ferrite and composites: mechanistic insights from ex situ, in situ, and operando X-ray techniques

The structure of pristine AgFeO₂ and phase makeup of Ag_0.2FeO_1.6 (a one-pot composite comprised of nanocrystalline stoichiometric AgFeO₂ and amorphous γ-Fe₂O₃ phases) was investigated using synchrotron X-ray diffraction. A new stacking-fault model was proposed for AgFeO₂ powder synthesized using the co-precipitation method. The lithiation/de-lithiation mechanisms of silver ferrite, AgFeO₂ and Ag_0.2FeO_1.6 were investigated using ex situ, in situ, and operando characterization techniques. An amorphous γ-Fe₂O₃ component in the Ag_0.2FeO_1.6 sample is quantified. Operando XRD of electrochemically reduced AgFeO₂ and Ag_0.2FeO_1.6 composites demonstrated differences in the structural evolution of the nanocrystalline AgFeO₂ component. As complimentary techniques to XRD, ex situ X-ray Absorption Spectroscopy (XAS) provided insight into the short-range structure of the (de)lithiated nanocrystalline electrodes, and a novel in situ high energy X-ray fluorescence nanoprobe (HXN) mapping measurement was applied to spatially resolve the progression of discharge. Based on the results, a redox mechanism is proposed where the full reduction of Ag⁺ to Ag⁰ and partial reduction of Fe³⁺ to Fe²⁺ occur on reduction to 1.0 V, resulting in a Li_1+yFe^IIIFe^II_yO₂ phase. The Li_1+yFe^IIIFe^II_yO₂ phase can then reversibly cycle between Fe³⁺ and Fe²⁺ oxidation states, permitting good capacity retention over 50 cycles. In the Ag_0.2FeO_1.6 composite, a substantial amorphous γ-Fe₂O₃ component is observed which discharges to rock salt LiFe₂O₃ and Fe⁰ metal phase in the 3.5-1.0 V voltage range (in parallel with the AgFeO₂ mechanism), and reversibly reoxidizes to a nanocrystalline iron oxide phase.

Synthetic control of composition and crystallite size of silver ferrite composites: profound electrochemistry impacts

A new paradigm for concomitant control of crystallite size and composition of bimetallic (Ag_xFeO₂) composites increases lithium battery capacity ∼200%.