Effect of the Operating Conditions on the Growth of Carbonaceous Nanomaterials over Stainless Steel Foams. Kinetic and Characterization Studies

This work is an advance on the development of structured catalytic reactors. Here, we present the results of the effect of the main operational variables (reaction temperature, % H2 and % C2H6) on the kinetics of carbonaceous nanomaterials (CNMs) formation by catalytic decomposition of ethane over stainless steel foams. Some of the main drawback problems that occur during the operation of chemical structured reactors are related to the preparation of long term stable coatings. The washcoating is the most used technique to deposit the catalytic layer over the substrate. The application of this procedure is quite complex in the case of geometries such as foams or cloths. In the case of the deposition of layers of carbonaceous nanomaterials, an alternative route, avoiding the washcoating, is their direct growth by catalytic decomposition of light hydrocarbons over the surface of the metallic substrate. In the case of structured steel foams, the substrate already contains the catalytic active phases for this reaction, like Fe and Ni, among of the minor components (Cr, Mn, Mo) that can act as promotors/stabilizers. The nanomaterials obtained after reaction were characterized by Raman spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The characterization results indicate that there is a maximum, obtained at ca. 900 °C, 33.3 % of C2H6 and 1.7 % of H2, in the quality of the carbonaceous nanomaterials grown. Under these conditions, the CNMs consist mainly of few layer graphene (FLG) and graphite nanolayers (GNL) encapsulating the metallic nanoparticles. In addition, the kinetic results indicate the existence of another optimum, at ca. 800 °C, 33.3 % of C2H6 and 1.7 % of H2, in the productivity to the carbonaceous nanomaterials. The existence of these optimums is due to the driving force for the diffusion of the carbon atoms through the Fe-Ni nanoparticles (NPs) obtained at high temperatures (e. g. above 800 °C) caused by the competence between two opposite phenomena: the increase of the rate of carbon diffusion through the metallic nanoparticles of Fe-Ni and the deactivation of these nanoparticles. The deactivation is the consequence of the encapsulation and reconstruction of the nanoparticles during the formation of the several types of CNMs. The evolution of the carbon mass during the reaction time was analyzed using a phenomenological kinetic model that takes into account the main stages involved during the formation of carbonaceous nanomaterials: hydrocarbon decomposition, carburization, diffusion, precipitation and deactivation. The results obtained from the kinetic model, along with the characterization results, enable quantify the influence of the operating variables on each stage of the carbonaceous nanomaterial formation and therefore open the way to optimize the process.