Effects of oxygen vacancy defect on microwave pyrolysis of biomass to produce high-quality syngas and bio-oil: Microwave absorption and in-situ catalytic

This paper proposed to use ferric oxide (Fe 2 O 3 ) and ferroferric oxide (Fe 3 O 4 ) as catalysts with both microwave absorption and catalytic properties. Carbon dioxide (CO 2 ) was introduced as the reaction atmosphere to further improve the quality of biofuel produced by microwave pyrolysis of food waste (FW). The results showed the bio-gas yield and the syngas concentration (H 2  + CO) increased to 70.34 wt% and 61.50 mol%, respectively, using Fe 3 O 4 as the catalyst. The content of aliphatic hydrocarbons in bio-oil produced with the catalyst Fe 2 O 3 increased to 67.48% and the heating value reached 30.45 MJ/kg. Compared with Fe 2 O 3 catalyst, Fe 3 O 4 exhibited better microwave absorption properties and catalytic properties. Transmission electron microscopy (TEM) and Electron paramagnetic resonance (EPR) characterizations confirmed that the crystal surface of Fe 3 O 4 formed more oxygen vacancy defects and unpaired electrons. Additionally, according to the X-ray photoelectron spectroscopy (XPS) analysis, the content of lattice oxygen in Fe 3 O 4 was 14.11%, a value that was much lower than Fe 2 O 3 (38.54%). The oxygen vacancy defects not only improved the efficient utilization of microwave energy but also provided the reactive sites for the reaction between the volatile organic compounds (VOCs) and CO 2 to generate CO. This paper provides a new perspective for selecting catalysts that have both microwave absorption and catalytic properties during the microwave pyrolysis of biomass.