X‑ray Absorption Spectroscopic Study of a Highly Thermally Stable Manganese Oxide Octahedral Molecular Sieve (OMS-2) with High Oxygen Reduction Reaction Activity

The development of catalysts with high thermal stability is receiving considerable attention. Here, we report manganese oxide octahedral molecular sieve (OMS-2) materials with remarkably high thermal stability, synthesized by a simple one-pot synthesis in a neutral medium. The high thermal stability was confirmed by the retention of the cryptomelane phase at 750 °C in air. Mechanistic studies were performed by X-ray absorption near-edge structure (XANES) spectroscopy and <i>ex situ</i> X-ray diffraction (XRD) to monitor the change in oxidation state and the phase evolution during the thermal transformation. These two techniques revealed the intermediate phases formed during the nucleation and growth of highly crystalline cryptomelane manganese oxide. Thermogravimetric analysis, Fourier transform infrared spectroscopy (FTIR), time-dependent studies of field emission scanning electron microscopy (FE-SEM), and high-resolution transmission electron microscopy (HR-TEM) techniques confirm the formation of these intermediates. The amorphous phase of manganese oxide with random nanocrystalline orientation undergoes destructive reformation to form a mixture of birnessite and hausmannite during its thermal transformation to pure crystalline OMS-2. The material still has a relatively high surface area (80 m²/g) even after calcination to 750 °C. The surfactant was used as a capping agent to confine the growth of OMS-2 to form short nanorods. In the absence of the surfactant, the OMS-2 extends its growth in the <i>c</i> direction to form nanofibers. The particle sizes of OMS-2 can be controlled by the temperatures of calcination. The OMS-2 calcined at elevated temperatures (400–750 °C) shows high remarkable catalytic activity for oxygen reduction reaction (ORR) in aqueous alkaline solution that outperformed the activity of synthesized solvent-free OMS-2. The activity follows this order: OMS-2_500 °C > OMS-2_750 °C > OMS-2_400 °C. The developed method reported here can be easily scaled up for synthesis of OMS-2 for use in high-temperature (400–750 °C) industrial applications, e.g., oxidative dehydrogenation of hydrocarbons and CO oxidation

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

<i>In Situ</i> Characterization of Mesoporous Co/CeO₂ Catalysts for the High-Temperature Water-Gas Shift

Mesoporous Co/CeO₂ catalysts were found to exhibit significant activity for the high-temperature water-gas shift (WGS) reaction with cobalt loadings as low as 1 wt %. The catalysts feature a uniform dispersion of cobalt within the CeO₂ fluorite type lattice with no evidence of discrete cobalt phase segregation. <i>In situ</i> XANES and ambient pressure XPS experiments were used to elucidate the active state of the catalysts as partially reduced cerium oxide doped with oxidized cobalt atoms. <i>In situ</i> XRD and DRIFTS experiments suggest facile cerium reduction and oxygen vacancy formation, particularly with lower cobalt loadings. <i>In situ</i> DRIFTS analysis also revealed the presence of surface carbonate and bidentate formate species under reaction conditions, which may be associated with additional mechanistic pathways for the WGS reaction. Deactivation behavior was observed with higher cobalt loadings. XANES data suggest the formation of small metallic cobalt clusters at temperatures above 400 °C may be responsible. Notably, this deactivation was not observed for the 1% cobalt loaded catalyst, which exhibited the highest activity per unit of cobalt