Green synthesis of transition metal nanocrystals encapsulated into nitrogen-doped carbon nanotubes for efficient carbon dioxide capture

The present study aims to explore the ability of transition metal encapsulated nitrogen-doped carbon nanotubes (M@NCNTs) as efficient CO2 adsorbent over a wide range of temperature and pressure. Initially, a single-step, economical, easily scalable and environment-friendly thermal decomposition technique was undertaken to synthesize the M@NCNTs. Their structure, surface morphology and composition were comprehensively studied using different characterization techniques. The mechanism of metal (M = Fe/Co/Ni) encapsulation inside the bamboo-shaped NCNTs was thoroughly investigated via X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM) analysis at different stages of pyrolysis. High pressure (≤20 bar) and low pressure (≤1 bar) CO2 uptake capacities reveal that M@NCNTs show much higher adsorption capacities in comparison to pristine multiwalled carbon nanotubes (MWNTs) and pure nitrogen doped carbon nanotubes (NCNTs). The uptake capacity was seen to follow a trend of Fe/Fe3C@NCNTs > Co@NCNTs > Ni@NCNTs in both high as well as low pressure adsorption studies and in this aspect the interaction of CO2 with transition metals has been discussed. The encapsulated metal nanoparticles along with high surface area NCNTs, therefore, play a synergistic role in enhancement of gas uptake properties. To the best of our knowledge, this is the first report on metal encapsulated NCNTs being investigated for CO2 capture.