Microwave-catalyzed pyrolysis of waste engine oil using steel slag catalyst: Structure design and insights into the intrinsic mechanism of active catalytic sites and microwave absorption functions

As an industrial waste rich in metal elements, steel slag (SS) has the potential for application as a catalytic material. However, the presence of large amounts of silicates and ferrates is one of the main reasons for the low catalytic properties of SS. In this work, SS was converted into catalysts with multi-metal oxide fractions by regulating the pH of the pretreatment process and using microwave calcination technology. By targeted design of oxygen vacancy (OV) number, specific surface area, and grain size, the steel slag catalyst (SSC) showed catalytic and microwave absorption functions, which could quickly convert waste engine oil (WEO) into light fuel. When the pH of the pretreatment was 8, SSC contained an abundance of metal oxides (such as Fe2O3, Fe3O4, and MnFe2O4), and the specific surface area could reach 237.3 m2/g. The abundant OV defect structure also contributed to the enhanced catalytic properties of the SSC. The light hydrocarbon (C5-C20) composition of the WEO increased from 15.11% to 86.24% at 500℃ in the presence of the bifunctional catalyst. Density functional theory calculations showed that the OV was the key to enhancing the catalytic and microwave absorption functions of the SSC. OV could be used as the activity site to promote the decomposition of heavy hydrocarbons by breaking CH bonds. Meanwhile, defect-induced polarization caused by OV promoted the transfer of carriers in the microwave field, thus enhancing the microwave absorption properties of the catalyst. This work provided a new perspective on the design of bifunctional catalysts for the preparation of microwave pyrolysis.