Single wall carbon nanotubes growth over cobalt-iron mesoporous MCM-41 bimetallic catalyst under methane chemical vapor deposition, an experimental and DFT evaluation

Abstract

Cobalt and iron MCM-41 catalysts were synthesized through an in-situ incorporation process starting from commercial iron and cobalt nitrates. The incorporation was confirmed by diffuse reflectance UV spectroscopy (DRS-UV) inspecting the cobalt and iron silicate-like photon absorption features and comparing with pure MCM-41-Co and MCM-41-Fe catalysts. Additionally it was found that the incorporation of cobalt and iron does not compromise the mesoporous structure of MCM-41 as confirmed by N2 adsorption isotherms. All catalysts showed high surface areas (∼1100 m2 g −1 ). Catalysts performance was conducted in a simple methane chemical vapor deposition (CVD) set up at 800 °C to produce single wall carbon nanotubes (SWCNT) under a constant flow of methane for 30 min. CVD products were characterized by thermogravimetric analysis (TGA) and Raman spectroscopy, finding that the iron content in the catalysts favors the selectivity and yield of graphitic-like structures, and confirming the presence of SWCNT by the appearance of a characteristic radial breathing mode (RBM) signals. These results were supported by Density Functional Theory (DFT) simulations of the methane dissociation (CH4 +TM → H3C –TMH) over Con (n = 1–5) and ComFe (m = 1–4), finding a different activation energy trend where ComFe (m = 1–4) clusters have the lower activation energy. The DFT study also revealed a charge difference (δC − δTM) higher in the case of dissociation over ComFe (m = 1–4) which may lead to an electrostatic stabilization of the transition metal, diminishing the activation energy of those clusters and leading to a faster carbon uptake.

Keywords

• dft; raman; swcnt; uv-vis.