Crystal plane dependent dispersion of cobalt metal on metastable aluminas

Abstract

Metallic Co nanoparticles, widely used and studied as supported heterogeneous catalysts for Fischer-Tropsch synthesis (FTS), display catalytic properties that can vary significantly depending on their size and crystal structure. In this work, we used 59Co Internal Field NMR (59Co IF NMR) complemented by high-resolution transmission electron microscopy (HRTEM) to demonstrate the influence of strong metal-support interaction on two noticeably different metastable alumina phases - γ-Al2O3 and χ-Al2O3. According to 59Co IF NMR and HRTEM, the metallic particles supported on χ-Al2O3 were larger and displayed a significantly higher content of Co in its hcp phase, which is known to be more active and selective to C5+ in FTS. The 1H NMR chemical shifts of hydroxyl groups anchored to the (110) and (111) spinel crystal planes were calculated by DFT. It revealed that the hydroxyl coverage of γ-Al2O3 facilitates the dispersion of Co precursor over the surface of the support, ultimately leading to the formation of smaller metal Co nanoparticles on γ-Al2O3 than on χ-Al2O3.