Impact of Co/Mo ratio on the activity of CoMo/MgO catalyst for production of high-quality multi-walled carbon nanotubes from polyethylene waste

Abstract

A simple two-stage method was used to produce multiwalled carbon nanotubes (MWCNTs) from low-density polyethylene (LDPE) plastic waste using a group of CoMo/MgO catalysts with several Co/Mo molar ratios (14.5, 6.5, 2.5, 1 and 0.4). The catalytic degradation of LDPE waste was performed using the HZSM-5 catalyst at a low temperature of 400 °C to form non-condensable gases, which were subsequently used as a carbon source for the production of MWCNTs at 700 °C. The influence of Co/Mo ratio on the yield, morphological structure, thermal stability, purity, and quality of MWCNTs was studied. The freshly calcined catalysts were characterized using BET surface area, XRD, FTIR, and H2-TPR techniques. On the other hand, Raman spectroscopy, TEM, XRD, and TGA analyses were performed for full characterization of the as-deposited carbon materials. The TPR and FTIR profiles of the fresh catalysts illustrated that the interaction between Co, Mo, and MgO increased by decreasing the Co/Mo ratio from 14.5 to 0.4. Also, the CoMo(6.5)/MgO catalyst displayed the highest surface properties compared to the other catalysts with different Co/Mo ratios. The CoMo(6.5)/MgO catalyst was the most active in terms of MWCNTs production. Moreover, the morphology, purity, and quality of the produced MWCNTs were strongly associated with the catalyst composition. TEM images illustrated that pure MWCNTs were obtained using the CoMo(14.5)/MgO catalyst, while a mixture of MWCNTs, carbon nanofibers (CNFs) and carbon nano-onions (CNOs) was produced using the CoMo(0.4)/MgO catalyst. Raman spectroscopy and TGA results proved that MWCNTs with the best quality and purity were obtained using the catalysts with Co/Mo ratios of 14.5 and 6.5.