Recent Advances of VOCs Catalytic Oxidation over Spinel Oxides: Catalyst Design and Reaction Mechanism.

Volatile organic compounds (VOCs) harm the environment and human health and have been of wide concern and purified efficiently by catalytic oxidation. Spinel oxides, mainly composed of transition metal elements with low price and extensive sources, have been widely investigated as efficient and stable catalysts for VOCs oxidation due to their adjustable element composition, flexible structure, and high thermal/chemical stability. However, it is necessary to dissect the design of the spinel in a targeted way to satisfy the removal of different types of VOCs. This article systematically summarizes the recent advances regarding the application of spinel oxides for VOCs catalytic oxidation. Specifically, the design strategies of spinel oxides were first introduced to clarify their effect on the structure and properties of the catalyst. Then the reaction mechanism and degradation pathway of different kinds of VOCs on the spinel oxides were in detail summarized, and the characteristic requirements of the spinel oxides for various VOCs purification were analyzed. Furthermore, the practice applications were also discussed. Finally, the prospects were proposed to guide the rational design of spinel-based catalysts for VOCs purification and intensify the understanding of reaction mechanisms.

Acid Etching-Induced In Situ Growth of λ-MnO2 over CoMn Spinel for Low-Temperature Volatile Organic Compound Oxidation.

Surface lattice oxygen is crucial to the degradation of volatile organic compounds (VOCs) over transition metal oxides according to the Mars-van Krevelen mechanism. Herein, λ-MnO2 in situ grown on the surface of CoMn spinel was prepared by acid etching of corresponding spinel catalysts (CoMn-Hx-Ty) for VOC oxidation. Experimental and relevant theoretical exploration revealed that acid etching on the CoMn spinel surface could decrease the electron cloud density around the O atom and weaken the adjacent Mn-O bond due to the fracture of the surface Co-O bond, facilitating electron transfer and subsequently the activation of surface lattice oxygen. The obtained CoMn-H1-T1 exhibited an excellent catalytic performance with a 90% acetone conversion at 149 °C, which is 42 °C lower than that of CoMn spinel. Furthermore, the partially maintained spinel structure led to better stability than pure λ-MnO2. In situ diffuse reflectance infrared Fourier transform spectroscopy confirmed a possible degradation pathway where adsorptive acetone converted into formate and acetate species and into CO2, in which the consumption of acetate was identified as the rate-limiting step. This strategy can improve the catalytic performance of metal oxides by activating surface lattice oxygen, to broaden their application in VOC oxidation.

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

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.

Enhanced Acetone Oxidation over the CeO2/Co3O4 Catalyst Derived from Metal-Organic Frameworks.

A novel CeO2/Co3O4 catalyst with a high catalytic activity has been designed and prepared by pyrolysis of metal-organic frameworks, and its catalytic performance was evaluated by the acetone catalytic oxidation reaction. The Co3O4-M catalyst with T90 at 194 °C was prepared by pyrolysis of the MOF-71 precursor, which was 56 °C lower than that of commercial Co3O4 (250 °C). By the addition of cerium to the MOF-71 precursor, an enhanced CeO2/Co3O4 catalyst with T90 at 180 °C was prepared. Importantly, the CeO2/Co3O4 catalyst exhibited superior stability for acetone oxidation. After 10 cycle tests, the conversion could also be maintained at 97% for at least 100 h with slight activity loss. Characterization studies were used to investigate the influence of cerium doping on the property of the catalyst. The results showed that addition of cerium could facilitate the expansion of the surface area and enhance the porous structure and reducibility at low temperature. Furthermore, the surface ratio of Co3+/Co2+ and mobile oxygen obviously improved with the addition of cerium. Therefore, the metal oxides prepared by this method have potential for the elimination of acetone.