Controlled Synthesis, Mechanism and Degradation Property of Birnessite-MnO2 Nanoflowers and Nanoflakes

Birnessite-MnO2 nanoflakes were synthesized via an aqueous oxidation method at 90 °C using Mn(CH3COO)2, NaOH, and KMnO4. The samples’ morphology, crystalline structure, and optical property were determined by field emission scanning electron microscopy, X-ray powder diffraction and UV-Vis spectrophotometry. The birnessite-MnO2 nanoflakes were converted to KxMn8O16 and Mn suboxides following a decrease in the concentration of KMnO4 in the reaction. The amount of NaOH in the reaction determined the type of precursor. Without NaOH, the precursor was converted from Mn(OH)2 to Mn2+ (from Mn(CH3COO)2), thereby enabling the synthesis of birnessite-MnO2 nanoflowers. The formation mechanism of birnessite-MnO2 nanoflowers and nanoflakes was clarified via the corresponding simulated crystal structures. Evaluation of the synthesized samples confirmed that the birnessite-MnO2 nanoflakes and nanoflowers exhibited excellent degradation properties.

Aqueous phase oxidation of bisulfite influenced by nitrate photolysis

Nitrate aerosol is ubiquitous in the atmosphere, and it can exit in both solid aerosol particles and fog and cloud droplets. Nitrate in the aqueous and particulate phase can undergo photolysis to produce oxidizing active radicals, which will inevitably affect various atmospheric chemical processes. However, the role of nitrate aerosols in these atmospheric photochemical processes remains unclear. In this study, the effects of nitrate photolysis on the aqueous phase oxidation of bisulfite under different conditions were investigated. Results show that nitrate photolysis can significantly promote the oxidation of bisulfite to sulfate. It is found that pH plays a significant role in the reaction, and ammonium sulfate has significant impacts on regulating the pH of solution and the enhancement of sulfate production. We also found an apparent synergism among halogen chemistry, nitrate and its photochemistry and S(IV) aqueous oxidation, especially the oxidation of halide ions by the nitrate photolysis and by the intermediate peroxymonosulfuric acid (HSO5−) produced by the free radical chain oxidation of S(IV) in acidic solution leads to the coupling of the redox cycle of halogen with the oxidation of bisulfite, which promotes the continuous aqueous oxidation of bisulfite and the formation of sulfate. In addition, it is also found that O2 is of great significance on nitrate photolysis for the conversion of HSO3−, and H2O2 generation during the nitrate photolysis is verified. These results provide a new insight into the heterogeneous aqueous phase oxidation pathways and mechanisms of SO2 in cloud and fog droplets and haze particles.

Formation of a Pd/MgO Structured Catalyst for the Aqueous Oxidation of Silane to Silanol

The catalytic oxidation of silanes to produce silanols using water as an oxidant at mild temperatures is a major challenge in Si-H activation. Highly efficient and easy-to-recycle catalysts based on Pd nanoparticles are in high demand. In this study, Pd nanoparticles embedded in an MgO porous overlayer on an Mg plate as a structured catalyst was prepared by the plasma electrolyte oxidation (PEO) technique. The Pd/MgO catalyst is strongly anchored to the MgO plate, building a structured catalyst. Fabrication parameters such as the temperature of the electrolyte and applied voltage significantly influenced the structure of the obtained Pd/MgO catalyst and in turn its catalytic activity. The catalytic activities of Pd/MgO were evaluated by activation of a Si-H bond for catalyzing the aqueous oxidation of silanes to silanol at mild temperatures. The catalytic activity of Pd nanoparticles is favored by their electro-deficient state due to influence from the MgO substrate. The Pd/MgO catalyst exhibits good performance stability during recycling. This work paves the way for fabricating structured catalysts with long-term stability and enhanced metal–oxide interaction.

Directional Oxidation of Pyrite in Acid Solution

This study aimed to investigate the oxidation mechanism of pyrite crystallographic direction by cutting pyrite samples to expose their (100), (110), and (111) planes. Differences in the oxidation rates of pyrite planes in acid solution were determined. The morphological changes of pyrite were evaluated by scanning electron microscopy and hyperdepth-3D microscopy. The oxidation products of pyrite were examined by Raman spectroscopy and X-ray photoelectron spectroscopy. Results showed that the aqueous oxidation of pyrite produced Fe(OH)3, Fe2O3, Fe2(SO4)3, and S8 on the surface. Moreover, the morphologies of corrosion patterns differed from one crystal plane to another: square, rectangular, and triangular etch pits were found on the (100), (110), and (111) planes, respectively. The corrosion patterns reflected the symmetrical arrangement of the crystallographic planes in the lattice on which they formed.