Promotion of manganese on Fe-based catalyst for the production of carbon nanotubes (CNTs) from plastics

Abstract

Manganese (Mn) is an effective promoter of Fe-based catalysts to increase the production of carbon nanotubes (CNTs) from waste plastics but suffers from the lack of insights into how Mn affects the catalytic performance of CNTs production. Herein, we systematically investigate the promotion mechanism of Mn in Fe-based catalysts by varying the adding order of Mn and Fe during one or two-step impregnation methods. The results show that α-(Fe1-xMnx)2O3 is formed and uniformly distributed in Fe crystals when Mn and Fe are simultaneously added by the one-step impregnation method ((Mn + Fe)/MgO), resulting in the highest gas (66.3 mmol/gplastic), and CNTs (235 mg/gplastic) yields. While α-(Fe1-xMnx)2O3 in FeMn/MgO (Fe is added first and followed by Mn by two-step impregnation method) exhibited a poor distribution resulting in reduced gas and CNTs yields compared to (Mn + Fe)/MgO. For MnFe/MgO prepared by the two-step impregnation method and Mn is added first, the formed α-(Fe1-xMnx)2O3 is entirely not inserted into Fe crystals, and the corresponding catalyst exhibits lower gas and CNTs yields. The best catalytic performance of (Mn + Fe)/MgO is owing to the well-dispersed α-(Fe1-xMnx)2O3, which increases the carbon solubility capacity of the Fe crystals and regulates the equilibrium of carbon dissolution and precipitation, promoting the thermal cracking and carbon formation reactions, thereby improving the catalytic performance of CNTs and gas production.