Effect of boron promotion on the stability of cobalt Fischer–Tropsch catalysts

Density functional theory (DFT) calculations indicate that boron atoms are thermodynamically stable at step, p4g clock, and subsurface sites of a Co catalyst under Fischer–Tropsch synthesis (FTS) conditions. Moreover, the presence of boron at step and clock sites is calculated to destabilize the adsorption of carbon atoms at neighboring sites by +160 and +108 kJ/mol, respectively. The calculations hence suggest that boron promotion can selectively block the deposition, nucleation, and growth of resilient carbon species. To experimentally evaluate this concept, the deactivation of a 20 wt.% Co/γ-Al2O3 catalyst promoted with 0.5 wt.% boron was studied for 200 h during FTS at 240 °C and 20 bar. Boron promotion was found to reduce the deactivation rate more than six-fold, without affecting the initial activity or selectivity. Characterization with X-ray photoelectron spectroscopy (XPS) and temperature-programmed hydrogenation (TPH) confirms that boron promotion reduces the deposition of resilient carbon species.