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

[Display omitted] • The specific catalytic properties of oxygen vacancy were systematically explored. • The defect of metal oxide improved the utilization efficiency of microwave energy. • Syngas concentration in the bio-gas could reach 61.5 mol% with Fe 3 O 4 as a catalyst. • The lower heating value of light bio-oil was up to 30.45–31.21 MJ/kg. • Iron oxide provided reactive sites for CO 2 reduction to CO in microwave pyrolysis. 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. [ABSTRACT FROM AUTHOR]