High performance Ni–CNTs catalyst: synthesis and characterization

Abstract

In this study, a Ni–CNTs nano-composite was synthesized via mechanical milling. The phase composition and morphology of the samples were investigated by employing X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). The effects of powder particle characteristics on the catalytic performance of the samples have also been systematically studied using various techniques such as temperature programmed reduction (TPR), N₂ adsorption–desorption, and CO hydrogenation experiments. The results showed that by increasing the milling time and also the CNTs content, the mean crystallite size of Ni decreases from 73 nm in an un-milled Ni sample to 35 nm for the sample processed with 30 wt% CNTs and milled for 15 hours. Moreover, addition of CNTs prevents Ni oxidation and also agglomeration of the nano-composite particles. FESEM micrographs showed that CNTs are gradually embedded in the Ni particles and disperse more homogeneously on increasing the milling time. TEM images showed that adding CNTs up to 10 wt% resulted in the refinement of Ni particles to 25 nm size. BET results indicated that addition of CNTs up to 30 wt% to the Ni sample increased the surface area from 0.9 to 57 m² g⁻¹. Furthermore, enhancement in the reducibility of Ni by increasing CNTs was observed from the TPR results. Consequently, the activity of the prepared nano-composite samples as a CO hydrogenation catalyst was improved at all temperatures. Ni–10 wt% CNTs, as the best nano-composite sample, exhibits complete CO conversion at 340 °C with the highest CH₄ selectivity attributed to the smaller particle size and high surface area confirmed by TEM and BET techniques, respectively.