Axial N Ligand-Modulated Ultrahigh Activity and Selectivity Hyperoxide Activation over Single-Atoms Nanozymes.

Learning and studying the structure-activity relationship in the bio-enzymes is conducive to the design of nanozymes for energy and environmental application. Herein, Fe single-atom nanozymes (Fe-SANs) with Fe-N5 site, inspired by the structure of cytochromes P450 (CYPs), are developed and characterized. Similar to the CYPs, the hyperoxide can activate the Fe(III) center of Fe-SANs to generate Fe(IV)O intermediately, which can transfer oxygen to the substrate with ultrafast speed. Particularly, using the peroxymonosulfate (PMS)-activated Fe-SANs to oxidize sulfamethoxazole, a typical antibiotic contaminant, as the model hyperoxides activation reaction, the excellent activity within 284 min-1 g-1 (catalyst) mmol-1 (PMS) oxidation rate and 91.6% selectivity to the Fe(IV)O intermediate oxidation are demonstrated. More importantly, instead of promoting PMS adsorption, the axial N ligand modulates the electron structure of FeN5 SANs for the lower reaction energy barrier and promotes electron transfer to PMS to produce Fe(IV)O intermediate with high selectivity. The highlight of the axial N coordination in the nanozymes in this work provides deep insight to guide the design and development of nanozymes nearly to the bio-enzyme with excellent activity and selectivity.

General Carbon-Supporting Strategy to Boost the Oxygen Reduction Activity of Zeolitic-Imidazolate-Framework-Derived Fe/N/Carbon Catalysts in Proton Exchange Membrane Fuel Cells.

The oxygen reduction reaction (ORR) activity of the Fe/N/Carbon catalysts derived from the pyrolysis of zeolitic-imidazolate-framework-8 (ZIF-8) has been still lower than that of commercial Pt-based catalysts utilized in the proton exchange membrane fuel cells (PEMFCs) due to low density of accessible active sites. In this study, an efficient carbon-supporting strategy is developed to enhance the ORR efficiency of the ZIF-derived Fe/N/Carbon catalysts by increasing the accessible active site density. The enhancement lies in (i) improving the accessibility of active sites via converting dodecahedral particles to graphene-like layered materials and (ii) enhancing the density of FeNx active sites via suppressing the formation of nanoparticles as well as providing extra spaces to host active sites. The optimized and efficient Fe/N/Carbon catalyst shows a half-wave potential (E1/2) of 0.834 V versus reversible hydrogen electrode in acidic media and produces a peak power density of 0.66 W cm-2 in an air-fed PEMFC at 2 bar backpressure, outperforming most previously reported Pt-free ORR catalysts. Finally, the general applicability of the carbon-supporting strategy is confirmed using five different commercial carbon blacks. This work provides an effective route to derive Fe/N/Carbon catalysts exhibiting a higher power density in PEMFCs.

Porous Carbon Membrane-Supported Atomically Dispersed Pyrrole-Type FeN4 as Active Sites for Electrochemical Hydrazine Oxidation Reaction.

The rational design of catalytically active sites in porous materials is essential in electrocatalysis. Herein, atomically dispersed Fe-Nx sites supported by hierarchically porous carbon membranes are designed to electrocatalyze the hydrazine oxidation reaction (HzOR), one of the key techniques in electrochemical nitrogen transformation. The high intrinsic catalytic activity of the Fe-Nx single-atom catalyst together with the uniquely mixed micro-/macroporous membrane support positions such an electrode among the best-known heteroatom-based carbon anodes for hydrazine fuel cells. Combined with advanced characterization techniques, electrochemical probe experiments, and density functional theory calculation, the pyrrole-type FeN4 structure is identified as the real catalytic site in HzOR.

Speciation and location of arsenic and antimony in rice samples around antimony mining area.

Arsenic (As) and antimony (Sb) are considered as priority environmental pollutants and their accumulation in crop plants particularly in rice has posed a great health risk. This study endeavored to investigate As and Sb contents in paired soil-rice samples obtained from Xikuangshan, the world largest active Sb mining region, situated in China, and to investigate As speciation and location in rice grains. The soil and rice samples were analyzed by coupling the wet chemistry, laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS), synchrotron-based micro X-ray fluorescence mapping (µ-XRF) and micro X-ray absorption near-edge structure (µ-XANES) spectroscopy. The results of field survey indicated that the paddy soil in the region was co-polluted by Sb (5.91-322.35 mg kg-1) and As (0.01-57.21 mg kg-1). Despite the higher Sb concentration in the soil, rice accumulated more As than Sb indicating the higher phytoavailability of As. Dimethylarsinic acid (DMA) was the predominant species (>60% on average) in the rice grains while the percentage of inorganic As species was 19%-63%. The µ-XRF mapping of the grain section revealed that the most of As was distributed and concentrated in rice husk, bran and embryo. Sb was distributed similarly to As but was not in the endosperm of rice grain based on LA-ICP-MS. The present results deepened our understanding of the As/Sb co-pollution and their association with the agricultural-product safety in the vicinity of Sb mining area.

Cd(II) retention and remobilization on δ-MnO2 and Mn(III)-rich δ-MnO2 affected by Mn(II).

Birnessite owing to its negative surface charge and defective structure exhibits high sorption affinities for Cd(II). However, Mn(II) can not only compete for the sorption sites with Cd(II), but also react with structural Mn(IV) in birnessite to form Mn(III), and thus, affect Cd(II) immobilization by birnessite. Herein, we investigate effects of Mn(II) on Cd(II) retention and remobilization on two birnessite δ-MnO2 and Mn(III)-rich δ-MnO2 (denoted as HE-MnO2). At pH 5.5, Cd(II) sorption to birnessite was inhibited by Mn(II) addition. Mn(II) addition to δ-MnO2 led to Cd(II) migration from vacant sites to edge sites, forming double-corner sharing (DCS) complexes. Mn(II) introduction to δ-MnO2 led to less stable Cd(II) species formed on birnessite, indicating that Cd(II) was more firmly bound to vacant sites than edge sites of birnessite. Cd(II) formed double-edge sharing (DES) and DCS complexes on HE-MnO2. Mn(II) addition to HE-MnO2 increased the CdMn distance in DES complexes. The stability of adsorbed Cd(II) on HE-MnO2 was slightly elevated due to Mn(II) addition. At pH 7.5, Mn(II) had no effect on Cd(II) sorption and desorption amounts on birnessite. However, low concentration of Mn(II) added to δ-MnO2 induced partial migration of Cd(II) from vacant sites to edge sites while high concentration of Mn(II) added to birnessite led to the formation of amorphous Cd(II)-Mn(III) coprecipitate. These findings imply that aqueous Mn(II) is an important factor in influencing Cd(II) immobilization by birnessite in the environment.

Antimony oxidation and sorption behavior on birnessites with different properties (δ-MnO2 and triclinic birnessite).

Birnessites are abundant naturally occurring minerals with high sorption and oxidation capacity that could therefore play an important role in antimony (Sb) migration and transformation. There are various types of birnessites in the environment. However, little is known about the similarities and differences in Sb oxidation and sorption on birnessites with different properties. In this study, the behavior of Sb oxidation and sorption on two contrasting birnessites (δ-MnO2 and triclinic birnessite (TrBir)) were investigated via batch and kinetic experiments and various spectroscopic techniques. Our results showed that the reaction mechanisms between Sb and the two birnessites were similar. The edge sites of birnessites were responsible for Sb(III) oxidation. Mn(IV) was reduced to Mn(III) and Mn(II), bound with birnessites and released to the solution, respectively. Because of the rapid rate of electron transfer of adsorbed Sb(III) to birnessites, the only Sb species on δ-MnO2 after the oxidation reaction was Sb(V). Sb(V) was adsorbed at the edge sites of birnessites by replacing the OH group of birnessites, forming corner-sharing complexes with birnessites. However, the Sb sorption and oxidation capacities of the two birnessites were significantly different. Poorly-crystallized δ-MnO2 exhibited a much higher oxidation and sorption capacity than well-crystallized TrBir because the former had many more edge sites than the latter. This study reveals the general mechanism of the reaction between Sb and birnessite and indicates that birnessite with a high number of edge sites would exhibit a huge capacity in Sb oxidation and sorption.

Biochar decreased the bioavailability of Zn to rice and wheat grains: Insights from microscopic to macroscopic scales.

Zn deficiency is a critical problem for many crops and human populations worldwide. Soil biochar amendment has recently been promoted as a sustainable agricultural practice. However, its effect on the bioavailability of micronutrients (especially Zn) to crops has not been fully addressed. This study investigated the impact of long-term biochar application in soils on Zn bioavailability to rice and wheat, using field experiments, and batch sorption/desorption experiments, in combination with extended X-ray absorption fine structure spectroscopy (EXAFS). In field soils biochar amendment increased total Zn content, but significantly decreased CaCl2-extractable Zn concentrations. Intriguingly, the uptake of Zn to wheat and rice grains was decreased. At high biochar application rates of 124 and 270t/ha the Zn concentrations in wheat grains (36.6 and 37.5mg/kg) reached a deficient level, lower than the recommended concentration of 45mg/kg. The batch experiments showed that biochar application at a cumulative rate of 10.5, 15.8, 31.5, 124, and 270t/ha significantly increased soil pH and soil organic matter (SOM) content, resulting in greater sorption and lower desorption of Zn. The EXAFS results demonstrated that the main forms of sorbed Zn were outer-sphere Zn complexes, Zn-illite, Zn-kaolinite and Zn-OM. The proportion of Zn-OM increased with increasing biochar application rates, suggesting that higher SOM might be more effective in immobilizing Zn and thus decreasing the Zn bioavailability. These results on the microscopic and macroscopic scales improved our understanding of the Zn bioavailability to crops, and raised potential concerns on the Zn deficiency in agricultural soils with long-term biochar application.