Nonthermal Synthesis of Three-Dimensional Metal Oxide Structures under Continuous-Flow Conditions and Their Catalytic Applications

Continuous-flow synthesis of one-dimensional (1D) metal oxide nanostructures and/or their integration into hierarchical structures under nonthermal conditions is still a challenge. In this work, a nonthermal, continuous-flow approach for the preparation of γ-manganese oxide (γ-MnO₂) and cerium oxide (CeO₂) microspheres has been developed. By this technique, γ-MnO₂ materials with surface areas of 240, 98, and 87 m²/g and CeO₂ microspheres with a surface area of 1 m²/g have been fabricated successfully. Characterization of the materials was carried out using powder X-ray diffraction, infrared and inductively coupled plasma optical emission spectrometer (ICP/OES), nitrogen sorption, scanning electron microscopy, transmission electron microscopy, and thermogravimetric analysis. The synthesized materials showed good catalytic activity in the oxidation of α-methyl styrene

Water Oxidation Catalysis using Amorphous Manganese Oxides, Octahedral Molecular Sieves (OMS-2), and Octahedral Layered (OL-1) Manganese Oxide Structures

Water oxidation is the bottleneck in artificial photosynthetic systems that aim to split water into hydrogen and oxygen. However, water oxidation occurs readily in plants, catalyzed by the Mn₄O₄Ca manganese cluster. In addition to this, manganese minerals are ubiquitous in nature displaying layered and tunnel structures. In this study, mixed valent porous amorphous manganese oxides (AMO), along with cryptomelane type tunnel manganese oxides (OMS-2) and layered birnessite (OL-1) have been used as water oxidation catalysts. Significantly higher turnovers were obtained with AMO (290 mmol O₂/mol Mn) compared to tunnel structure OMS-2 (110 mmol O₂/mol Mn) and layered structure OL-1 (27 mmol O₂/mol Mn) in water oxidation tests with Ce⁴⁺. Oxygen evolution was also confirmed under photochemical conditions using Ru­(bpy)₃ ²⁺ as a photosensitizer and persulfate as a sacrificial agent. The differences in catalytic activity among these catalysts have been probed using X-ray diffraction, transmission electron microscopy, Raman and Fourier transform infrared (FTIR) spectroscopy, average oxidation state, and compositional analyses. Comparison of AMO against prominent manganese catalysts described in literature shows AMO provided the highest turnover numbers. AMO catalyst was also reusable after regeneration. O-18 labeling studies proved that water was the source of dioxygen and IR proved the structural stability of AMO after reaction. AMO is related to hexagonal birnessites such as layered biogenic manganese oxides or H⁺-birnessite that have cation vacancies in the MnO₂ sheets rather than completely filled Mn³⁺/Mn⁴⁺ sheets, and this is influential in catalytic activity

Synthesis of Mesoporous Iron Oxides by an Inverse Micelle Method and Their Application in the Degradation of Orange II under Visible Light at Neutral pH

Mesoporous iron oxides (2-line ferrihydrite, α-Fe₂O₃, γ-Fe₂O₃, and Fe₃O₄) are successfully synthesized by modifying the reaction temperatures and calcination atmospheres of the sol–gel-based inverse micelle method. Different characterization techniques, such as PXRD, N₂ sorption, SEM, HRTEM, Raman spectroscopy, and XANES, are performed to determine the properties of the catalysts. Larger pore sizes can be obtained in mesoporous γ-Fe₂O₃ and Fe₃O₄ compared with mesoporous 2-line ferrihydrite and α-Fe₂O₃. The catalytic performance of mesoporous iron oxides are examined as Fenton catalysts in orange II degradation in the presence of oxidant H₂O₂ at neutral pH under visible light. Adsorption capacities of mesoporous iron oxides on orange II are greater than that of commercial Fe₂O₃. The greatest adsorption capacity is found to be 49.3 mg/g with mesoporous 2-line ferrihydrite. In addition, the degradation efficiency of orange II is found to be markedly improved by mesoporous iron oxides compared with the commercial catalyst. In the best case scenario, 2-line ferrihydrite shows the highest degradation rate constant (0.0258 min^–1) among all the catalysts tested. The excellent performance of 2-line ferrihydrite is mainly attributed to the larger surface area but also related to surface hydroxyl groups, acidic products, and possible additional adsorption sites. The recyclability of mesoporous 2-line ferrihydrite catalyst can be achieved up to 3 times without performance decay. At last, a discussion regarding the possible mechanisms of degradation of orange II over mesoporous 2-line ferrihydrite is proposed, based on the previous literature work and the observed reaction intermediates monitored by ESI/MS in this study