Volatile organic compounds (VOCs) are one of the main sources of air pollution, which are of wide concern because of their toxicity and serious threat to the environment and human health. Catalytic oxidation has been proven to be a promising and effective technology for VOCs abatement in the presence of heat or light. As environmentally friendly and low-cost materials, manganese-based oxides are the most competitive and promising candidates for the catalytic degradation of VOCs in thermocatalysis or photo/thermocatalysis. This article summarizes the research and development on various manganese-based oxide catalysts, with emphasis on their thermocatalytic and photo/thermocatalytic purification of VOCs in recent years in detail. Single manganese oxides, manganese-based oxide composites, as well as improving strategies such as morphology regulation, heterojunction engineering, and surface decoration by metal doping or universal acid treatment are reviewed. Besides, manganese-based monoliths for practical VOCs abatementare also discussed. Meanwhile, relevant catalytic mechanisms are also summarized. Finally, the existing problems and prospect of manganese-based oxide catalysts for catalyzing combustion of VOCs are proposed.

Recent Progress of Thermocatalytic and Photo/Thermocatalytic Oxidation for VOCs Purification over Manganese-based Oxide Catalysts

A Fe doped TiO2 catalyst (Fe-TiO2) was evaluated for NO oxidation reaction under visible light irradiation. The Fe-TiO2 not only exhibited better catalytic activity and stability, but also suppressed the production of nitrogen dioxide (NO2) than the undoped TiO2. The results of in situ DRIFTS indicated that visible light benefited the adsorption and activation of NO at Fe sites (Fe-(NO)2) and Ti sites (Ti3+-NO) as well as the formation of active intermediates (e.g. Fe3+-NO3− and Ti4+-NO−). After EPR, DRS, PL and XPS testing, it was proposed that incorporating Fe ions into TiO2 induced the formation of more oxygen vacancies (OVs), while visible light further facilitated this process, resulting in an enhanced adsorption & activation of NO and also a promoted formation of reactive oxygen species (ROS). Thus, a strengthened photo-driven effect for oxidizing NO into NO2− and NO3− occurred on Fe-TiO2.

Visible-light-driven deep oxidation of NO over Fe doped TiO2 catalyst: Synergic effect of Fe and oxygen vacancies

Unraveling the decisive role of surface CeO2 nanoparticles in the Pt-CeO2/MnO2 hetero-catalysts for boosting toluene oxidation: Synergistic effect of surface decorated and intrinsic O-vacancies

Cobalt-manganese composite oxides have high-activity potential for catalytic oxidation of volatile organic compounds (VOCs). However, low defect content and poor oxygen mobility of the spinel structure make it difficult to efficiently activate C H bonds of propane at low-temperature. Herein, novel amorphous Co-Mn binary oxides with enhanced defect were fabricated by a facile self-redox approach. Highly defective amorphous Co Mn binary oxides with low-temperature reducibility, weak Mn O bond strength and good mobility of the surface lattice oxygen remarkably improved the catalytic activity for propane oxidation. Highly defective amorphous Co1Mn3Ox catalyst showed the best catalytic activity for propane mineralization and the T90 was 207℃ under high space velocity (18 000 mL·g−1 h−1). Moreover, in situ spectroscopy technologies were utilized to explore the structure-activity relationship and the degradation mechanism of propane oxidation under real reaction condition. Significantly, as an effective defect engineering strategy, amorphous binary oxides exhibit excellent activity and can provide some enlightenments for developing efficient VOCs catalysts.

Efficient defect engineering in Co-Mn binary oxides for low-temperature propane oxidation

The development of catalysts for volatile organic compound (VOC) treatment by catalytic oxidation is of great significance to improve the atmospheric environment. Size-effect and oxygen vacancy engineering are effective strategies for designing high-efficiency heterogeneous catalysts. Herein, we explored the in situ carbon-confinement-oxidation method to synthesize ultrafine MnOx nanoparticles with adequately exposed defects. They exhibited an outstanding catalytic performance with a T90 of 167 °C for acetone oxidation, which is 73 °C lower than that of bulk MnOx (240 °C). This excellent catalytic activity was primarily ascribed to their high surface area, rich oxygen vacancies, abundant active oxygen species, and good reducibility at low temperatures. Importantly, the synthesized ultrafine MnOx exhibited impressive stability in long-term, cycling and water-resistance tests. Moreover, the possible mechanism for acetone oxidation over MnOx-NA was revealed. In this work, we not only prepared a promising material for removing VOCs but also provided a new strategy for the rational design of ultrafine nanoparticles with abundant defects.

Defective Ultrafine MnOx Nanoparticles Confined within a Carbon Matrix for Low-Temperature Oxidation of Volatile Organic Compounds.

Nanofiber catalysts are potentially promising to be equipped in the lean-burned vehicle exhaust emission system considering its facile synthesis, low-cost, long durability in contrast to the traditionally complicated powder based catalyst. Here, via electrospinning method, we synthesized YMn2O5 nanofibers (average diameter: ˜93 nm) including nanowire stacked with particles, and hollow tubes with or without inside nanowires. The morphologies remained even after hydrothermal aging at 800  ℃ for 10 h with 10% H2O stream. X-ray photoelectron spectrum revealed Mn4+/Mn3+ ratio 0.589 on the surface of YMn2O5 catalyst (YMO), indicating the existence of oxygen vacancies. The highest conversion of electrospun mullite fibers at 310  ℃ is ˜18% higher than powder-based YMO at 352 ℃ in the presence of 5% H2O at WHSV = 240,000 ml g−1 h−1. The extracted activation energy from NO-to-NO2 conversion curves of YMO nanofiber (61.68 kJ/mol) is slightly lower than that of powder-based mullite (91.45 kJ/mol), consistent with calculated 68.16 kJ/mol on the fiber YMO surface (121) with a rate-limiting step of the NO2 desorption. The catalytic activity can be attributed to the unit occupy of the eg (dz2 and dx2-y2) orbitals of Mn-dimer atoms around fermi level based on the combination of the theoretical calculations and DRIFTS spectra analysis. Importantly, after the hash hydrothermal-aging, the highest NO oxidation conversion of powder-based mullite is less than 50%, while the fiber maintains a conversion of 62%. Moreover, the fresh YMO fiber maintained superior oxidation stability for 12 h at its maximum conversion of 66%. This work provides the possibility for mullite oxide fibers to replace powder-based precious metal catalyst coating on monolith converter.

Electrospun YMn2O5 nanofibers: A highly catalytic activity for NO oxidation

Revealing the catalytic oxidation mechanism of volatile organic compounds (VOCs) is insightful for the development of efficient catalysts. However, because of the complicated interactions and a lar...

Difference of Oxidation Mechanism between Light C3–C4Alkane and Alkene over Mullite YMn 2 O 5 Oxides’Catalyst

Mn-based mullite catalyst GdMn2O5 is developed via the hydrothermal method to efficiently remove volatile organic compound acetone with a high efficiency and stability in total oxidation. The T90 over GdMn2O5 approaches to 160 s°C in contrast to 191 °C over 1 wt % Pt/Al2O3. The catalytic performance maintains after reaction at 200 °C for 260 h with 5.0 % H2O. The surface labile oxygen is the key to superior catalytic performance. According to the in situ DRIFTS and the oxidation pathway proposed by DFT calculations, the acetate species decomposition is assigned as the rate-limiting step. And the calculated kinetic barrier for the rate-limiting step, 151.7 kJ/mol (1.58 eV), is in line with the apparent activation energy from acetone oxidation over GdMn2O5 of 150.0 kJ/mol (1.56 eV). This work provides insights into developing highly efficient and hydrothermally stable volatile organic compounds removal catalysts via surface oxygen activation.

Tong Zhang

Papers

Electrospun YMn2O5 nanofibers: A highly catalytic activity for NO oxidation

Difference of Oxidation Mechanism between Light C3–C4Alkane and Alkene over Mullite YMn 2 O 5 Oxides’Catalyst

Labile oxygen promotion of the catalytic oxidation of acetone over a robust ternary Mn-based mullite GdMn2O5

Low-temperature removal of aromatics pollutants via surface labile oxygen over Mn-based mullite catalyst SmMn2O5