Ni-Co-Mg-Al catalysts for hydrogen and carbonaceous nanomaterials production by CCVD of methane

This paper reports the characterization and catalytic results of a bimetallic Ni-Co/Mg-Al catalyst during the methane decomposition reaction for the co-production of H2 and nanocarbonaceous materials (e.g. carbon nanotubes-CNTs). The catalyst has been characterized before and after reaction with the aim of observing the changes in its structure and also the type of nanocarbonaceous materials formed. The influence on CNT growth rate, of the temperature during the calcination and reaction stages has been studied. The activity, stability and selectivity to the production of hydrogen and carbon nanotubes greatly depend on the catalyst composition and on the activation and reaction conditions used. The bimetallic Ni-Co/Mg-Al catalyst, prepared by controlled coprecipitation, shows higher stability that the monometallic catalysts Ni/Mg-Al and Co/Mg-Al. TEM and TPO observations indicate that the type of nanocarbonaceous material obtained when the catalyst does not suffer deactivation, corresponds to multi-wall carbon nanotubes (MWNT), with average diameter of ca. 20 nm. The kinetic results of CNT formation, obtained in a thermobalance, were evaluated using a Phenomenological Kinetic Model that includes the main relevant steps involved in CNT growth by catalytic decomposition of hydrocarbons (CCVD): (i) hydrocarbon decomposition at the gas side; (ii) carburization of the metallic nanoparticle surface; (iii) atom carbon diffusion (bulk and/or surface) (iv) nucleation and CNT growth; and finally (v) growth termination by catalyst deactivation or by the effect of steric hindrance of the CNTs formed. In this paper we have applied the Phenomenological Kinetic Model to study the influence of the calcination and reaction temperature. The obtained values for the kinetic parameters have realistic physical meaning in good agreement with the mechanism considered during the formation and growth of carbon nanotubes.