Atomic Structure and Valence State of Cobalt Nanocrystals on Carbon under Syngas Versus Hydrogen Reduction
Ofentse A. Makgae, Tumelo N. Phaahlamohlaka, Benzhen Yao, M. Schuster, Thomas J. A. Slater, Peter P. Edwards, Neil J. Coville, Emanuela Liberti, Angus I. Kirkland
Abstract
The composition of the reducing gas in the activation of Co Fischer-Tropsch synthesis catalysts determines the nature of the catalytically active Co species. This study reports on the effect of H2 versus syngas (H2/CO = 2) on the reducibility of Co3O4 nanoparticles supported on hollow carbon spheres, using ex situ and in situ high-resolution aberration-corrected analytical electron microscopy. High-resolution images revealed twinned fcc Co particles encapsulated in carbon from syngas treatment while H2-treated particles were mostly CoO. Moreover, the electron energy loss of the Co-L3,2 and O-K edge fine structures show improved reducibility in syngas than in H2 at 350 °C. The effect of high temperature on the reducibility of the Co3O4 nanoparticles is also explored. Carbon fiber encapsulation of twinned fcc Co particles observed during the syngas treatment provides sinter resistance at high temperatures. Both ex situ and in situ results indicate that syngas activation is efficient for obtaining highly reduced Co nanoparticles at lower temperatures.