Effect of Ni incorporation in cobalt oxide lattice on carbon formation during ethanol decomposition reaction

Abstract

In this work, we report the results of catalytic decomposition of ethanol over a porous mixed metal oxide of Ni-Co (NiCoO2) and analyze the effect of Ni incorporation in Co3O4 lattice on the activity and hydrogen selectivity from ethanol decomposition reaction. In-situ FTIR analysis was conducted between 50 °C to 400 °C for both, Co3O4 and NiCoO2, catalysts to understand the reaction mechanism leading to gas phase product distribution. On cobalt surface the reaction pathway proceeds via the formation of surface ethoxy intermediate that subsequently transforms to aldehyde and acetate intermediates before decomposing to release CO2, H2 and CH4 gases at high temperature. A similar pathway is followed on NiCoO2 surface, leading to aldehyde intermediate, which is relatively unstable and decomposes to release CO along with CH4 and H2 at 400 °C. The NiCoO2 catalyst shows relatively low selectivity for CO2, possibly due to the unstable aldehyde species providing little acetate intermediate that decomposes to release gaseous CO2. The addition of Ni improves the activity for ethanol decomposition by achieving a complete ethanol conversion at 350 °C as compared to 420 °C for cobalt alone. The crystallinity, morphology and particle analysis of the spent catalyst after the reaction was studied using XRD, SEM, and TEM respectively. The XRD shows a phase change of porous NiCoO2 to NiCo alloy, whereas SEM indicates the presence of fibrous structure on the surface with 91.7% of carbon while keeping 1:1 ratio of Ni and Co after the reaction. The detailed analysis of carbon structure using HRTEM-STEM shows the simultaneous growth of carbon nanofibers (CNFs) and multiwalled carbon nanotubes (MWCNTs) that were favored by larger and smaller crystallites respectively. A detailed mechanism of carbon deposition is also proposed.