The redox properties of various Co catalysts were investigated in order to understand a deactivation process that has been reported for small Co particles in Fischer-Tropsch synthesis (FTS). Coulometric titration was used to measure the equilibrium isotherms for a series of Co catalysts, including bulk CoOx, 2-wt% Co on SBA-15, and several zirconia-supported Co (2- to 20-wt% Co) catalysts. TEM results demonstrate that the 2-wt% Co/SBA-15 consisted of particles that were less than 5 nm in diameter; but the thermodynamic properties were indistinguishable from that found for bulk Co, showing that the redox properties are not a function of particle size alone. For zirconia-supported samples with 2-wt% and 5-wt% Co, about half of the Co exhibited a significant shift to lower P(O-2) for the equilibrium state associated with the coexistence of CoO and Co, implying that interactions between CoOx and zirconia can affect the redox properties of Co and suggesting that a large fraction of the Co in these catalysts would be in the form of CoO on these samples under FTS conditions. With higher Co loadings, the equilibrium data was identical to that for bulk CoOx. For the 5-wt% Co/zirconia catalyst, the addition of 0.5-wt% Pd shifted the equilibrium back to that found with bulk CoOx, implying that Pd promoters are able to break up interactions between Co and zirconia. (C) 2011 Elsevier B.V. All rights reserved.