## 5 Conclusions  
Through the research and exploration of the application status of steel slag in the fields of catalytic pyrolysis, organic degradation, electrocatalysis, photocatalysis, and transesterification, it is found that steel slag has great potential and advantages in the field of catalysis. Through acid modification, alkali modification, high temperature modification, composite modification, and physical modification, the surface area and pore diameter of steel slag can be increased by acid modification, alkaline modification, high temperature modification, compound modification, and physical modification, and the relative content of active substances can be increased. Through calcination and some composite modification methods, the stability of the active substance in steel slag can be enhanced, and the steel slag catalyst can then be reused many times. Steel slag is modified so that its catalytic performance is more efficient than that of similar types of catalysts, such aszeolite and some ceramic catalysts. At the same time, in some catalytic reactions, steel slag will react with certain substances (such as copper and nickel), forming a virtuous cycle, speeding up the catalytic process and improving the catalytic performance. At the same time, through the summary of steel slag in the above catalytic fields, it can be seen that steel slag will participate in the reaction in the catalytic process. It also shows great potential in lipid exchange to produce biodiesel and glycerol to produce hydrogen. At present, it is difficult to obtain the ideal steel slag catalyst by a single modification, as most methods are compound modifications. Therefore, efficiency, cost, and complexity should be considered in the selection of modification methods. At the same time, we can see that the modified steel slag has a good activity for the degradation of organic matter in some wastewater. In China and all over the world, various factories such as dye factories, textile factories, and printing factories discharge a large amount of wastewater organic pollutants every year, causing serious environmental pollution, so the demand for solid catalyst is very large. Considering the cost and performance, steel slag is expected to be widely used in wastewater treatment field. It not only realizes the purpose of \"using waste to treat waste\", and achieves energy saving, emission reduction, and pollution reduction, but also relieves the massive accumulation of steel slag and improves the additional utilization value of steel slag. In order to further improve and perfect the study of modified steel slag catalyst preparation, the following prospects are put forward, hoping to provide some references for the research in this field:  
(1) At present, catalysts were widely used in various fields. Selecting suitable catalysts with high catalytic performance and low costs had been a research hotspot. Modified steel slag had a good development prospect in the field of catalysis. Through the comparison of several catalytic fields, it can be found that the modified steel slag catalyst shows better catalytic performance in wastewater treatment and organic degradation than other fields. Nowadays, the treatment process of wastewater and sewage is complicated and the cost is high. If steel slag can be widely used in this area, it will greatly reduce the cost of sewage treatment. At the same time, it will provide a new direction for the resource utilization of steel slag:  
(2) The modification for steel slag using high concentrations of acid and alkali was easy to lead to leaching of other elements, which increased by-product treatment processes. At the same time, it was necessary to strengthen the analysis, including composition of the catalyzed steel slag, metal precipitates, and the effect on the catalytic reaction so as to prepare an efficient catalyst. Conceptualization, Y.-T.W. ; Writing--original draft preparation, F.-P.W. ; Writing--review and editing, T.-J.L., S.C., D.G., Q.Y., X.-M.W., Y.-T.W., Y.-N.Z., and J.-G.L.