Investigating the role of Ca in NiMoCa catalyst on carbon nanotubes growth from face mask via one-pot synthesis

Abstract

The annual rise in plastic waste generation, particularly face mask, has led to considerable environmental consequences due to improper management. Herein, mesoporous NiMoCa-X catalysts with different Ca loadings (X = 0, 7, 15, and 32) were prepared to investigate the role of Ca in promoting the stability and performance of NiMoCa-X for carbon nanotubes (CNTs) growth during face mask pyrolysis. Increasing Ca loading within the NiMoCa-X catalysts resulted in the decrease of the NiMoO4 weight loading followed by the increase in the CaMoO4 and CaCO3 phases’ weight loading. This study reveals that the presence of CaMoO4 catalyzes surface coke oxidation reactions, while the presence of the optimized loads of CaCO3 promotes aromatization and good quality CNTs. The influences of catalyst-to-face mask ratio, pyrolysis temperature, duration, and cycle on the characteristics of the CNTs were also studied. The optimal performer was NiMoCa-15, exhibiting the highest carbon yield. This is attributed to (i) the smaller NiO crystallite size, (ii) the presence of the surface coke oxidizer CaMoO4 phase, and (iii) the emergence of high surface basicity of the CaCO3 phase. Typically, the CaMoO4 phase oxidizes surface coke via surface oxygen vacancies, ensuring a sustainable carbon diffusion pathway into Ni for CNTs production. Simultaneously, the CaCO3 phase catalyzes aromatization reactions of hydrocarbons to produce hydrocarbons with simpler structures, facilitating the production of high-quality CNTs over irregular coke. However, excessive Ca loading can render the catalyst surface hydrophilic, triggering aggressive surface coke gasification and subsequently producing short CNTs with low carbon yield. Considering these, the NiMoCa-15, characterized by balanced loading of CaMoO4 and CaCO3 phases, emerges as a promising catalyst for CNTs growth from discarded face mask plastics via one pot synthesis.