α-MnO2 nanofiber with high concentration of surface oxygen vacancy was obtained via vacuum deoxidation method. The activity of α-MnO2 strongly depends on the concentration and extent of oxygen vacancy, which can be adjusted by tuning the temperature and time of vacuum deoxidation. The formation of oxygen vacancy enhanced the ratio of Mn3+/Mn4+, which changed the charge distribution on the α-MnO2 nanofiber, resulting in a significant improvement of the adsorption of ozone on the surface of the catalyst. In the dry gas flow, the ozone removal rate at 20 h has increased from 32.6% to 95%. In the wet gas flow, the ozone removal rate was also enhanced thanks to more active sites offered by α-MnO2. What's more, we found that the deactivation caused by water vapor was temporary and the activity would recover once the humidity has decreased. Finally, DFT calculation revealed that surface oxygen vacancy was the adsorption and reaction site for ozone decomposition and a new mechanism of ozone decomposition in the presence of H2O also was proposed. This work developed a deeper understanding to the process of ozone decomposition and would promote manganese oxide catalyst for practical application.

Surface oxygen vacancy induced α-MnO2 nanofiber for highly efficient ozone elimination

Aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) as a bioplastics monomer is efficiently promoted by a simple system based on a nonprecious-metal catalyst of MnO2 and NaHCO3. Kinetic studies indicate that the oxidation of 5-formyl-2-furancarboxylic acid (FFCA) to FDCA is the slowest step for the aerobic oxidation of HMF to FDCA over activated MnO2. We demonstrate through combined computational and experimental studies that HMF oxidation to FDCA is largely dependent on the MnO2 crystal structure. Density functional theory (DFT) calculations reveal that vacancy formation energies at the planar oxygen sites in α- and γ-MnO2 are higher than those at the bent oxygen sites. β- and λ-MnO2 consist of only planar and bent oxygen sites, respectively, with lower vacancy formation energies. Consequently, β- and λ-MnO2 are likely to be good candidates as oxidation catalysts. On the other hand, experimental studies reveal that the reaction rates per surface area for the slowest step (FFCA oxidation to FDCA) decrease in the order of β-MnO2 > λ-MnO2 > γ-MnO2 ≈ α-MnO2 > δ-MnO2 > e-MnO2; the catalytic activity of β-MnO2 exceeds that of the previously reported activated MnO2 by three times. The order is in good agreement not only with the DFT calculation results, but also with the reduction rates per surface area determined by the H2-temperature-programmed reduction measurements for MnO2 catalysts. The successful synthesis of high-surface-area β-MnO2 significantly improves the catalytic activity for the aerobic oxidation of HMF to FDCA.

Effect of MnO2 Crystal Structure on Aerobic Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid.

Transition metal (Cerium, Cobalt, and Iron) doped cryptomelane-type manganese oxide (M-OMS-2) catalysts have been successfully synthesized and characterized. The different metal-ion-doped K-OMS-2 catalysts showed distinct differences in their ozone decomposition activity. Cerium-doped OMS-2 materials showed ozone conversion of 90% under RH = 90% and space velocity of 600,000 h−1. X-ray diffraction (XRD), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) data suggested that the as-synthesized M-OMS-2 materials were all crystalline with no segregated metal oxide impurities. ICP-OES and XAFS results confirmed that Co3+ and Fe3+ replace Mn3+ in the cryptomelane structure and Ce4+mainly replaces the K+ in the tunnel and partially replaces the Mn4+ in the framework of the cryptomelane structure. Because of the differences in the substitution sites and the ionic radius of dopants, the morphologies of the catalysts were different. The Mn3+content and number of surface defects play a key role during the decomposition of ozone. Ce-OMS-2 is a promising catalyst for purifying waste gases containing ozone under high-humidity conditions.

Transition metal doped cryptomelane-type manganese oxide catalysts for ozone decomposition

Heterogeneous catalytic ozonation (HCO) processes have been widely studied for water purification. The reaction mechanisms of these processes are very complicated because of the simultaneous involvement of gas, solid, and liquid phases. Although typical reaction mechanisms have been established for HCO, some of them are only appropriate for specific systems. The divergence and deficiency in mechanisms hinders the development of novel active catalysts. This critical review compares the various existing mechanisms and categorizes the catalytic oxidation of HCO into radical-based oxidation and nonradical oxidation processes with an in-depth discussion. The catalytic active sites and adsorption behaviors of O3 molecules on the catalyst surface are regarded as the key clues for further elucidating the O3 activation processes, evolution of reactive oxygen species (ROS) or organic oxidation pathways. Moreover, the detection methods of the ROS produced in both types of oxidations and their roles in the destruction of organics are reviewed with discussion of some specific problems among them, including the scavengers selection, experiment results analysis as well as some questionable conclusions. Finally, alternative strategies for the systematic investigation of the HCO mechanism and the prospects for future studies are envisaged.

Reactive Oxygen Species and Catalytic Active Sites in Heterogeneous Catalytic Ozonation for Water Purification

Manganese dioxide (MnO2 ) is a promising photo-thermo-electric-responsive semiconductor material for environmental applications, owing to its various favorable properties. However, the unsatisfactory environmental purification efficiency of this material has limited its further applications. Fortunately, in the last few years, significant efforts have been undertaken for improving the environmental purification efficiency of this material and understanding its underlying mechanism. Here, the aim is to summarize the recent experimental and computational research progress in the modification of MnO2 single species by morphology control, structure construction, facet engineering, and element doping. Moreover, the design and fabrication of MnO2 -based composites via the construction of homojunctions and MnO2 /semiconductor/conductor binary/ternary heterojunctions is discussed. Their applications in environmental purification systems, either as an adsorbent material for removing heavy metals, dyes, and microwave (MW) pollution, or as a thermal catalyst, photocatalyst, and electrocatalyst for the degradation of pollutants (water and gas, organic and inorganic) are also highlighted. Finally, the research gaps are summarized and a perspective on the challenges and the direction of future research in nanostructured MnO2 -based materials in the field of environmental applications is presented. Therefore, basic guidance for rational design and fabrication of high-efficiency MnO2 -based materials for comprehensive environmental applications is provided.

MnO2 -Based Materials for Environmental Applications.

Ozone is a ubiquitous pollutant and manganese dioxide (MnO2) has been widely used for ozone decomposition. However, the effect of MnO2 structure on ozone decomposition has never been investigated. Three tunnel-structure polymorphs, i.e., α-, β- and γ-MnO2 were prepared and characterized by BET, TEM, XRD, H2-TPR, O2-TPD, NH3-TPD, TGA-MS and XPS. The activity of three MnO2 polymorphs for ozone decomposition followed the order of α- > γ- > β-MnO2. The α-MnO2 owned the largest specific surface area and lowest average oxidation state of Mn. Furthermore, the adsorbed oxygen species on the surface of α-MnO2 were more easily reduced. In-situ Raman spectroscopy results showed that peroxide species formed during ozone decomposition, and over α-MnO2 they were more easily decomposed by increasing reaction temperature. It was found that the catalytic activity of MnO2 strongly depended on the density of oxygen vacancies. Accordingly, the ozone decomposition mechanism based on the involvement and recycling of oxygen vacancy (VO) is proposed. The decomposition of peroxide species is a rate-limiting step. These findings are helpful for designing more effective catalyst for ozone removal.

Long Chen

Papers

Catalytic decomposition of gaseous ozone over manganese dioxides with different crystal structures

The effect of morphology of α-MnO2 on catalytic decomposition of gaseous ozone