Mechanism of microwave-assisted iron-based catalyst pyrolysis of discarded COVID-19 masks

Inexpensive iron-based catalysts are the most promising catalysts for microwave pyrolysis of waste plastics, especially a large number of disposable medical masks (DMMs) with biological hazards produced by spread of COVID-19. However, most synthesized iron-based catalysts have very low microwave heating efficiency due to the enrichment state of iron. Here, we prepared FeAlOx catalysts using the microwave heating method and found that the microwave heating efficiency of amorphous iron and hematite is very low, indeed, these materials can hardly initiate pyrolysis at room temperature, which limits the application of iron-based catalysts in microwave pyrolysis. By contrast, a mixture of DMMs and low-valent iron oxides produced by hydrogen reduction at 500 °C can be heated by microwaves to temperatures above 900 °C under the same conditions. When the hydrogen reduction temperature was incerased to 800 °C, the content of metallic iron in the catalyst gradually increased from 0.34 to 21.43%, which enhanced the microwave response ability of the catalyst, and decreased the gas content in the pyrolysis product from 78.91 to 70.93 wt%;corresponding hydrogen yield also decreased from 29.03 to 25.02 mmolH2·g-1DMMs. Moreover, the morphology of the deposited solid carbon gradually changed from multi-walled CNTs to bamboo-like CNTs. This study clarifies the pyrolysis mechanism of microwave-assisted iron catalysts and lays a theoretical foundation for their application in microwave pyrolysis. © 2022 Elsevier Ltd

Enhanced microwave absorption capacity of iron-based catalysts by introducing Co and Ni for microwave pyrolysis of waste COVID-19 Masks

Inexpensive iron-based catalysts are the most promising catalysts for microwave-assisted deconstruction of waste plastics. However, the microwave heating efficiency of most of the synthesized iron-based catalysts is very low, in particular, the FeAl catalyst was prepared by microwave combustion method, and its mixture with disposable medical masks (DMMs) was only heated to about 150℃ within 10 min. Here, we introduce the second-phase metals (Co or Ni) into the FeAl catalyst, resulting in the rearrangement of the catalyst structure and electrons to give the catalyst good microwave absorption ability. The mixture of the catalyst and DMMs can be quickly heated to above 900℃ in 10 min, especially after reaching the melting point of plastic, the instantaneous heating rate reaches 350 ℃·min−1. under the unique microwave hot-spot pyrolysis mechanism, DMMs can be rapidly pyrolyzed into carbon nanotubes (19.65 wt%) and gas (77.65 wt%) within 14 min due to the efficient dehydrogenation efficiency and activity of Co. The corresponding H2 yield is up to 38.66 mmolH2·g−1DMMs, and the percentage of CO and H2 in the gas is as high as 90 wt%. This work improves the microwave conversion efficiency of iron-based catalysts by introducing second phase metals, and waste DMMs were efficiently converted into CO, H2 and CNTs, which can also be extended to other polymer or biomass chemical cycles.