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.

Herein, a series of rod-like MnCeOx are synthesized by pyrolyzing Mn/Ce-BTC for toluene oxidation. The 3Mn2Ce catalyst exhibits superior catalytic performance and stability under high humidity (10 % H2O). XRD, Raman, H2-TPR, O2-TPD, XPS and EXAFS results confirm that the formation of solid solution between Ce and Mn species and better reducibility play a key role in toluene oxidation. H2O-TPD, toluene-TPD, toluene-TPSR, in situ DRIFTS in different conditions and TD-GC–MS confirm that the introduction of water can promote toluene mineralization. H2O-TPD indicates that oxygen vacancies adsorbed H2O to provide HOH active sites and promote the conversion of Oads to Olatt. The degradation pathway of toluene was toluene→benzaldehyde→benzoate→CO2 and H2O. Additionally, the water promotion is also confirmed by DFT calculations. The promotion of water vapor is that the water vapor is conducive to the adsorption of toluene, benzaldehyde and oxygen, and promotes the activation of oxygen.

The promoting effect of H2O on rod-like MnCeOx derived from MOFs for toluene oxidation: A combined experimental and theoretical investigation

Natural illite microsheets were firstly utilized to induce oxygen vacancies into ultrafine cobalt oxide (Co3O4) for highly efficient Fenton-like catalysis via activation of peroxymonosulfate (PMS). The results indicated that presence of illite microsheets regulated multi-directional crystallization of Co3O4 nanospheres and resulted in reduced grain size and crystallinity. The smaller grain size provided more reactive edge sites for PMS catalysis. The numerous indistinct lattice boundaries caused by reduced crystallinity created abundant oxygen vacancies. Density functional theory (DFT) calculations illustrated that presence of oxygen vacancies significantly reduced adsorption energy and accelerated electron transfer, which further faciliated PMS activation. The oxygen vacancy-rich Co3O4/illite exhibited superior catalytic efficiency in real water matrix. Apart from sulfate and hydroxyl radicals, singlet oxygen generated from oxygen vacancy-based reaction pathway also played a significant role in atrazine degradation. This strategy provided a new insight for future designing of natural mineral-based catalysts for efficient wastewater treatment via Fenton-like process.

Natural illite-based ultrafine cobalt oxide with abundant oxygen-vacancies for highly efficient Fenton-like catalysis

Energy crisis and environmental problems urgently drive the proposal of new strategies to improve human wellbeing and assist sustainable development. To this end, scientists have explored many metal oxides-based photocatalysts with high stability, low cost, earth abundance, and potentially high catalytic activity relevant for key applications such as H2O splitting, CO2 reduction, N2 fixation, and advanced oxidation of pollutants. In these metal oxides, oxygen vacancies (OVs) are ubiquitous and intrinsic defects with pronounced impacts on the physicochemical properties of the catalysts, which may open new opportunities for obtaining efficient metal oxides. The thorough understanding of the structural and electronic nature of OVs is necessary to determine how they serve as catalytically active sites. In this review, we summarize the origin of OVs, the strategies to introduce OVs, as well as the fundamental structure-activity relationships to relate these crystal defects to catalyst properties including light absorption, charge separation, etc. We emphasize the mechanism of OVs formation and their effects on the intrinsic catalytic characteristics of the metal oxides. We also present some multicomponent catalytic platforms where OVs contribute to catalysis via synergy. Finally, opportunities and challenges on engineering defects in photocatalysts are summarized to highlight the future directions of this research field.

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Oxygen vacancies in metal oxides: recent progress towards advanced catalyst design

Catalytic ozonation of VOCs at low temperature: A comprehensive review

Transition metal (cerium and cobalt) doped γ-MnO2 (M-γ-MnO2, where M represents Ce, Co) catalysts were successfully synthesized and characterized. Cerium-doped γ-MnO2 materials showed ozone (O3) conversion of 96% for 40 ppm of O3 under relative humidity (RH) of 65% and space velocity of 840 L g–1 h–1 after 6 h at room temperature, which is far superior to the performance of the Co-γ-MnO2 (55%) and γ-MnO2 (38%) catalysts. Under space velocity of 840 L g–1 h–1, the conversion of ozone over the Ce-γ-MnO2 catalyst under RH = 65% and dry conditions within 96 h was 60% and 100%, respectively, indicating that it is a promising material for ozone decomposition. XRD and HRTEM data suggested that Ce-γ-MnO2 formed mixed crystals consisting of α-MnO2 and γ-MnO2 with specific surface area increased from 74 m2/g to 120 m2/g compared to undoped γ-MnO2, thus more surface defects were introduced. H2-TPR, O2-TPD, XPS, Raman, and EXAFS confirmed that Ce-γ-MnO2 exhibited more surface oxygen vacancies and surface defects, whi...

Oxygen Vacancies Induced by Transition Metal Doping in γ-MnO2 for Highly Efficient Ozone Decomposition.

Ozone (O3), as a harmful air pollutant, has been of wide concern. Safe, efficient, and economical O3 removal methods urgently need to be developed. Catalytic decomposition is the most promising method for O3 removal, especially at room temperature or even subzero temperatures. Great efforts have been made to develop high-efficiency catalysts for O3 decomposition that can operate at low temperatures, high space velocity and high humidity. First, this review describes the general reaction mechanism of O3 decomposition on noble metal and transition metal oxide catalysts. Then, progress on the O3 decomposition performance of various catalysts in the past 30 years is summarized in detail. The main focus is the O3 decomposition performance of manganese oxides, which are divided into supported manganese oxides and non-supported manganese oxides. Methods to improve the activity, stability, and humidity resistance of manganese oxide catalysts for O3 decomposition are also summarized. The deactivation mechanisms of manganese oxides under dry and humid conditions are discussed. The O3 decomposition performance of monolithic catalysts is also summarized from the perspective of industrial applications. Finally, the future development directions and prospects of O3 catalytic decomposition technology are put forward.

Recent advances in catalytic decomposition of ozone

Novel CeMnaOx (a is the molar ratio of Mn to Ce) catalysts for catalytic decomposition of ozone (O3) under high GHSV conditions were successfully synthesized by a facile homogeneous precipitation method. CeMn10Ox showed ozone conversion of 96 % for 40 ppm O3 under relative humidity (RH) of 65 % and space velocity of 840 L·g−1 ·h-1 after 6 h at room temperature, which is far superior to the performance of the α-Mn2O3 (38 %). The ozone decomposition activity of α-Mn2O3 increased by a factor of 2.5 with the addition of Ce. XRD, XPS, H2-TPR, O2-TPD, EXAFS and DFT calculations data confirmed that cerium-manganese complex oxides with enhanced surface area were formed, which played a key role in the decomposition of ozone. This study provides important insights for developing improved catalysts for gaseous ozone decomposition that can be prepared by a simple method, and promoting the performance of manganese oxides for practical ozone elimination.

Novel CeMnaOx catalyst for highly efficient catalytic decomposition of ozone

In the study, the catalyst precursors of Ce-modified γ-MnO2 were washed with deionized water until the pH value of the supernatant was 1, 2, 4 and 7, and the obtained catalysts were named accordingly. Under space velocity of 300,000 hr−1, the ozone conversion over the pH = 7 catalyst under dry conditions and relative humidity of 65% over a period of 6 hr was 100% and 96%, respectively. However, the ozone decomposition activity of the pH = 2 and 4 catalysts distinctly decreased under relative humidity of 65% compared to that under dry conditions. Detailed physical and chemical characterization demonstrated that the residual sulfate ions on the pH = 2 and 4 catalysts decreased their hydrophobicity and then restrained humid ozone decomposition activity. The pH = 2 and 4 catalysts had inferior resistance to high space velocity under dry conditions, because the residual sulfate ion on their surface reduced their adsorption capacity for ozone molecules and increased their apparent activation energies, which was proved by temperature programmed desorption of O2 and kinetic experiments. Long-term activity testing, X-ray photoelectron spectroscopy and density functional theory calculations revealed that there were two kinds of oxygen vacancies on the manganese dioxide catalysts, one of which more easily adsorbed oxygen species and then became deactivated. This study revealed the detrimental effect of surface acid ions on the activity of catalysts under humid and dry atmospheres, and provided guidance for the development of highly efficient catalysts for ozone decomposition.

Detrimental role of residual surface acid ions on ozone decomposition over Ce-modified γ-MnO2 under humid conditions

In this study, a series of Ce-OMS-2 catalysts were synthesized by a hydrothermal method, their physicochemical properties and catalytic activity for ozone decomposition were evaluated. The results ...

Xiaotong Li

Papers

Oxygen Vacancies Induced by Transition Metal Doping in γ-MnO2 for Highly Efficient Ozone Decomposition.

Recent advances in catalytic decomposition of ozone

Novel CeMnaOx catalyst for highly efficient catalytic decomposition of ozone

Detrimental role of residual surface acid ions on ozone decomposition over Ce-modified γ-MnO2 under humid conditions

Enhancing Oxygen Vacancies of Ce-OMS-2 via Optimized Hydrothermal Conditions to Improve Catalytic Ozone Decomposition