Production of nanostructured carbon materials using Fe–Mo/MgO catalysts via mild catalytic pyrolysis of polyethylene waste

Abstract

A series of MgO supported bimetallic Fe–Mo catalysts with various Fe:Mo weight ratios (such as 50:0, 45:5, 40:10, 30:20, 20:30 and 10:40, respectively) have been prepared and evaluated for production of carbon nanomaterials (CNMs) via catalytic pyrolysis of low-density polyethylene (LDPE) plastic waste. Highly acidic HZSM-5 zeolite was used as a degradation catalyst during the pyrolysis process. To investigate the structure and physicochemical properties of the fresh catalysts, several characterization tools such as XRD, TPD, BET, TPR, and FTIR were employed. While to clarify the nature and morphology of as-produced CNMs, the spent catalysts were characterized by XRD, TEM, Raman spectroscopy and TPO. The XRD and TPR results confirmed that the non-interacted metal oxides species were dominant in the structure of the catalysts with higher Fe2O3 or MoO3 contents, whereas several mixed oxide species were extensively present in the catalysts with moderate Fe2O3 and MoO3 contents. The obtained results revealed that the Fe/Mo ratio was found to play a crucial role on the catalytic growth activity as well as the type and morphology of as-deposited CNMs. All results showed that the type and morphology structure of carbon deposited can be adjusted by changing the Fe/Mo ratio in the MgO support. The activities of catalysts were significantly improved with the successive addition of Mo contents up to 30 wt% due to the presence of a large number of diverse active sites as a result of interaction between all catalyst components. The catalysts with higher Fe or Mo contents promote the formation of both CNTs and CNFs, while the catalysts with intermediate loadings of Fe and Mo produce mainly CNTs, CNFs accompanied with GNSs as hybrid materials.