Catalyst deactivation is an important topic for industrial catalyst development. Sintering of small cobalt crystallites is one of the deactivation mechanisms of cobalt-based Fischer-Tropsch synthesis (FTS) catalysts. This study investigates the mechanism of cobalt sintering at low-conversion FTS conditions. A Co/SiO2/Si(100) model catalyst is exposed to 20 bar dry synthesis gas (H-2/CO: 2/1) at 230 degrees C for 10 h. Cobalt nanoparticles were characterized before and after treatment using transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). TEM images of identical locations on the model catalyst showed a loss of some small crystallites and decrease in size of some crystallites. Sintering is dominated by an Ostwald ripening mechanism using our model catalyst under the present conditions. Complementary XPS measurements confirm the loss of Co dispersion. Therefore, the loss of small Co nanoparticles causes a rapid loss of metal surface area when exposed to model FTS conditions. (C) 2015 Elsevier Inc. All rights reserved.