Comparison of the Surface State of Ni Nanoparticles Used for Methane Catalytic Decomposition

Nickel nanoparticles were synthesized by two different routes and their reactivity was assessed during methane catalytic decomposition (MCD) conducted in situ in a thermogravimetric analyzer at atmospheric pressure from room temperature to 930 degrees C. Commercially available nickel nanoparticles were also evaluated for comparative purposes. Carbon buildup was monitored continuously to provide a qualitative comparison of the rate of the reaction for all the systems, while residues composed of nickel nanoparticles and carbon nanotubes (CNTs) were collected at different stages for further evaluation by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The surface composition of the fresh nickel nanoparticles was clearly sensitive to the preparation method and was presumably responsible for the trends observed during the catalytic evaluation. On one hand, an early onset in the temperature of the catalytic reaction at 400 degrees C is distinguished by Ni nanoparticles prepared in a one-step direct thermal decomposition of nickel acetate tetrahydrate while, under the same conditions, Ni nanoparticles produced by an alternative emulsion-mediated synthesis reacted at T > 480 degrees C. When MCD is finished at 930 degrees C, multiwalled carbon nanotubes (MWCNTs) and bamboo-shaped carbon nanotubes (BSCNTs) structures are collected selectively in the systems catalyzed by particles prepared by one-step and emulsion-mediated synthesis, respectively. Commercial nickel nanoparticles, on the other hand, produced low quality CNTs in a comparatively much slower fashion. Methane catalytic decomposition conducted in a thermogravimetric analyzer provides a complementary tool to compare in situ reactivity of nanoparticles and to isolate them for further ex situ characterization.