Insights into CNT synthesis process and mechanism in terms of the composition and transformation of hydrocarbon cracked gas

Abstract

Recycling of waste plastic to produce carbon nanotubes (CNTs), which involves a complex hydrocarbon mixture, is attracting increasing attention. This study aimed to explore the process of CNT formation on Fe/Al₂O₃ catalysts from hydrocarbon cracked gas, which was mainly composed of C1–C4 alkanes, C2–C4 alkenes, C6–C8 aromatics, and C5–C7 (excluding aromatics). The composition and transformation of hydrocarbon cracked gas during CNT synthesis were monitored by on-line gas chromatography. The morphology, quality and quantity of carbon products were analyzed in detail by SEM and TG-DSC. Amorphous carbon and CNTs were detected on the spent Fe/Al₂O₃ catalyst, which exhibited an irregular lump shape and a filamentous shape with a high aspect ratio, respectively. The CNT yield was 3.5–278.1 mg g_cat⁻¹, and the amorphous carbon yield was 11.3–197.5 mg g_cat⁻¹. An increase in CNT yield was accompanied with a decrease in amorphous carbon yield, indicating different mechanisms for the formation of amorphous carbon and CNTs on the Fe/Al₂O₃ catalyst. In addition, increasing the content of C2–C4 alkenes and C6–C8 aromatics promoted the formation of amorphous carbon, and increasing the content of C1–C4 alkanes and C5–C7 promoted the formation of CNTs. It was concluded that Fe₂O₃ acts as the active site for amorphous carbon and CNT formation. (1) Fe₂O₃ enhanced the polycondensation of alkenes and aromatics and the formation of polycyclic aromatic hydrocarbons, which were converted into amorphous carbon via the de-branched chain and dehydrogenation. (2) Fe₂O₃ enhanced the C–C/C–H bond cleavage of C1–C4 alkanes and C5–C7 hydrocarbons and the formation and deposition of carbon atoms, the successive accumulation of which led to carbon over-saturation on the surface and the precipitating out of CNTs.