Cobalt sulfide catalysts for single-walled carbon nanotube synthesis

Abstract

Catalysts play a critical role in carbon nanotube (CNT) synthesis. The challenge in precisely controlling CNT structures for various applications creates a long-standing goal to develop new and efficient CNT synthesis catalysts. Although sulfur-containing compounds are often used as promoters in CNT synthesis, metal sulfides have not been directly used as catalysts in CNT synthesis. Here, we study a series of cobalt sulfide catalysts supported on porous silica (CoS/SiO2) for CNT synthesis. Catalysts were first reduced in H2 at different temperatures and then exposed to ethanol at 900 °C to grow CNTs. We found the reduction temperature in H2 and the Co mass loading can strongly influence their catalytic performance. After the CoS/SiO2 catalyst with 1 wt% Co was reduced in H2 at 700 °C, it yields single-walled CNTs (SWCNTs) with a relatively narrow diameter distribution from 0.8 to 1.1 nm. We found that Co9S8 in the catalyst was partially reduced to metallic Co. The coexistence of Co9S8 and metallic Co nanoparticles can stabilize metallic Co nanoparticles and limit their fast aggregation. The Co mass loading in CoS/SiO2 catalysts can be further increased to 5 wt% while retaining catalysts' selectivity towards SWCNTs. When the Co mass loading increases to 7 or 10 wt%, aggregated large Co metal particles result in mainly multi-walled CNTs and graphite. Overall, our results show that CoS/SiO2 catalysts are a potential catalyst candidate for CNT synthesis.