Ultrathin MnO2-Coated FeOOH Catalyst for Indoor Formaldehyde Oxidation at Ambient Temperature: New Insight into Surface Reactive Oxygen Species and In-Field Testing in an Air Cleaner.

Herein, we tailored a series of ultrathin MnO2 nanolayers coated on the surface of commercial goethite (α-FeOOH) by a facile in situ chemical precipitation method. α-FeOOH inhibited the MnO2 crystal growth via the incorporation of K+ ions between MnO2 and α-FeOOH interfaces during the synthesis process. The hybrid design of MnO2 with an ultrathin nanolayer structure could reduce the electron transfer resistance and bring abundant oxygen vacancies, accelerating the activation of molecular O2 to generate more oxygen-free radical species and favoring the thermodynamic HCHO oxidation. The ROS quenching in gas/aqueous systems and DRIFTS results demonstrated that •O2- was responsible for HCHO oxidization, which assisted the preliminary intermediate dioxymethylene dehydrogenation into formate species. The 25%MnO2@FeOOH(25wt% of MnO2) catalyst was subsequently loaded into the filter substrates of a commercial air cleaner and tested in an indoor room with actual application conditions. As a result, the composite filter could eliminate different initial concentrations of HCHO (150-450 ppb) to the WHO guideline value (≈81 ppb) within 60 min. Furthermore, the 25%MnO2@FeOOH sample was also effective against the representative bacteria and mold in indoor air. This study provides new insight into the role of the chemisorbed ROS for HCHO oxidation at ambient temperature.

The pH-dependent degradation of sulfadiazine using natural siderite activating PDS: The role of singlet oxygen.

Occurring naturally siderite (FeCO3) was used as the heterogeneous catalyst to activate peroxodisulfate (PDS) for the degradation of sulfadiazine under different initial pH values. The findings of this system exhibited various ROS (e.g. 1O2, SO4- and OH) present during a wide range of pH values. Among them, 1O2 could significantly facilitate the initial degradation rate, and the increased pH enhanced the role of 1O2. The factors including initial pH values, siderite dosage, PDS concentration, initial contaminants concentration, and water matrix were discussed. The role of each ROS was investigated through quenching test and electron paramagnetic resonance (EPR). Furthermore, the comprehensive degradation process was proposed based on the LC-MS results. And the cycle test demonstrates the reusability of siderite at a pH of 3. Accordingly, this study is of great significance for understanding the degradation of such sulfonamide pollutants in the siderite/PDS system.

Effect of iron minerals during coaling on the transformation of NO in the presence of NH3: Take pyrite as an example.

Pyrite, a naturally occurring mineral, can be found extensively in coal. The change in the pyrite structure that occurs during coaling process, the ability of the pyrite-derived α-Fe2O3 to convert NO in the presence of NH3 before catalyst bed and the kinetic study were investigated in this work. The pyrite-derived α-Fe2O3 was obtained by calcining at 500, 600, 700, 800 °C and was characterized by the X-ray diffraction (XRD), N2 physisorption, the X-ray photoelectron spectrometer (XPS), the scanning electron microscope (SEM), UV-visible near-infrared spectroscopy (UV-vis DRS), the temperature-programmed desorption of ammonia (NH3-TPD) and the in situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFTS). The results indicated that the α-Fe2O3 derived from natural pyrite exhibited an affirmative effect on NO conversion in the presence of NH3 at reaction temperatures of 200-450 °C, particularly at 350 °C, the pyrite-derived α-Fe2O3 displayed the best efficiency for the NO conversion. In addition, the formed sulfate derived from the oxidation of pyrite enhanced the NO conversion at the temperature of 300-450 °C, while hinder the NO conversion at 200-275 °C. The in-situ DRIFTS and kinetic studies demonstrated that both the Eley-Rideal and Langmuir-Hinshelwood mechanism contributed to the selective catalytic reduction (SCR) of NO when the reaction temperature was over 200 °C, while selective catalytic oxidization (CO) happened over 300 °C. This study favored the understanding of the NO behavior in flue gas pipeline after sprawling NH3 and the mechanism of NO conversion before the catalyst bed.

The positive effect of siderite-derived α-Fe2O3 during coaling on the NO behavior in the presence of NH3.

Siderite is a naturally occurring mineral that can be found extensively in coal. The structural evolution of siderite in the process of coaling and its performance in the transformation of NO in the presence of NH3 were investigated in this work. In addition, the effects of the coexisting component, including vapor, SO2, and the alkali metal K, were also discussed. Heat treatment was performed at 450, 500, 550, 600, and 700 °C to obtain siderite-derived α-Fe2O3, which was then evaluated in de-NOx via the selective catalytic reduction (SCR) of NO with NH3 in a fixed bed. The X-ray diffraction (XRD), the X-ray fluorescence spectrometer (XRF), N2 adsorption-desorption (BET), the X-ray photoelectron spectrometer (XPS), the scanning electron microscope (SEM), and the transmission electron microscope (TEM) were used to investigate the variations in the morphology and structure of the thermally treated siderite. The results showed that siderite was gradually oxidized and decomposed into α-Fe2O3 with a nanoporous structure and large surface area of 27.27 m2 g-1 after calcination under an air atmosphere. The α-Fe2O3 derived from siderite at 500 °C (H500) exhibited an excellent SCR performance, where the NO conversion rate was great than 90% between 250 and 300 °C due to the pore structure and high specific surface area, additional adsorbed oxygen states, abundant oligomeric Fe oxide clusters, and large amount of acid sites. Regardless of the vapor content, SO2 concentration, and reaction temperature, the α-Fe2O3 derived from siderite at 500 °C (H500) still favored the conversion of NO. When the reaction temperature was lower than 350 °C, H500 favored the conversion of NO even in the presence of an alkali metal (K). The experimental data demonstrated the positive effect of siderite-derived α-Fe2O3 in SCR technology and provided insight into NO behavior in coaling flue gas after NH3 injection.

A novel discovery of a heterogeneous Fenton-like system based on natural siderite: A wide range of pH values from 3 to 9.

Natural iron-bearing minerals have been proven to be effective for activating H2O2 to produce OH, which can be used to degrade organic pollutants. In this study, the performance of siderite to degrade sodium sulfadiazine via catalytic H2O2 degradation was investigated at different solution pH values from 3 to 9. An interesting discovery was made: the performance of the siderite-H2O2 system was excellent under acidic, neutral, and even alkaline conditions. The influence of various factors (e.g. initial concentration, anions, natural organic matters, etc.) on the system under different pH conditions was investigated, which confirmed that siderite exhibited an excellent catalytic performance. By combining EPR characterization with scavenger research, it was proposed that dissolved iron (Fe2+) mainly initiated the homogenous Fenton reaction to degrade pollutants under acidic conditions, while structural Fe2+ species present in siderite triggered Fenton-like reactions under neutral or even alkaline conditions. From the SEM and XPS characterizations, oxidation and dissolution of Fe2+ on the surface were also observed, confirming our inference concerning the different reaction mechanisms. The experimental findings show that this siderite-H2O2 system can be used in solutions with pH values from 3 to 9 and that siderite plays a positive role in soil and groundwater remediation when H2O2 is used as an oxidant.