TPR-XANES/EXAFS carried out using a novel multi-sample holder provided key information for verifying the nature of the chemical transformations occurring during cobalt Fischer–Tropsch synthesis catalyst activation in hydrogen. In the past, assumptions had to be made regarding the nature of the cobalt species present along the trajectory of a standard TPR experiment. The new technique directly provided insight into (a) the nature of the reduction process of cobalt oxide species and (b) the resulting cobalt crystallite size, as a function of the strength of the catalyst support interaction with the cobalt oxide species. A two-step reduction process involving Co3O4 to CoO and CoO to Co0 transformations over standard calcined catalysts was observed and quantified over all catalysts exhibiting both weak interactions (e.g., Co/SiO2) and strong interactions (e.g., Co/Al2O3) with the support. Noble metal promoter (e.g., Pt) addition strongly improved the reducibility of cobalt oxide species, most likely via a H2 dissociation and spillover mechanism. Increasing cobalt loading, on the other hand, led to a measurable, but lesser, improvement on reducibility, due to the larger resulting particle size that resulted in less surface contact with the support. Higher reduction temperatures were needed to effectively reduce cobalt oxide particles deposited on strongly interacting surfaces in comparison with unsupported Co3O4 or only weakly interacting supported cobalt catalyst. Nevertheless, despite lower extents of reduction, the smaller resulting Co particles on the more strongly interacting catalysts generally led to higher Co0 active site densities. The addition of the noble metal promoter to strongly interacting supported catalyst significantly decreased the temperature required to reduce the cobalt oxides to Co0 particles; this allows one to take advantage of the higher Co0 surface areas arising from the combination of a smaller average Co0 particle size and a higher extent of reduction. Conclusions The use of the multiple-sample holder to carry out TPR-XANES/EXAFS provided key information for verifying the nature of the chemical transformations occurring during catalyst activation in hydrogen, as well as providing insight into the resulting cobalt crystallite size. The first peak was assigned to the Co3O4 to CoO transformation and occurred over a comparable temperature range for the unpromoted catalysts, while the second peak was due to the CoO to Co0 step, and depended strongly on the nature of the support, with the reduction of CoO in strongly interacting Co/Al2O3 extending beyond 700 °C. For the catalysts studied in this work, direct Co formation during step one of the reduction process, could be ruled out. Direct reduction of larger Co3O4 to Co0 crystallites during the first TPR peak was previously proposed by others. Another previously held view was that the first peak was assigned to reduction to Co0 of a cobalt oxide surface phase not stabilized by the interaction with the support and was therefore more easily reduced. We detected no Co0 formation during the first step, but only transformation from Co3O4 to CoO. Likewise, many authors have suggested that upon calcination, a mixed oxide of (CoO)X × (Al2O3)Y is formed as a separate entity in addition to Co3O4 crystallite formation and responsible for the broad peak. In this contribution, we did find that CoO is reduced in the broad peak, but it was first derived from the reduction of Co3O4 to CoO during the first peak. That is, the CoO phase was not already present as CoO in the catalyst directly after calcination (i.e., only Co3O4 was formed after calcination). The results underscore the complex inter-relationships among the support interaction, the reducibility of the cobalt oxide species, and the resulting average cobalt metal cluster size formed. Support type, and not surface area, was found to be the key factor in determining the strength of the interaction and the rate at which the cobalt oxides underwent reduction. In addition, H2 chemisorption/pulse reoxidation measurements and EXAFS indicated that the more weakly interacting support (in this case, SiO2) yielded a much larger cobalt crystallite size which is detrimental to the resulting cobalt active metallic surface area. In contrast, after a standard reduction treatment, despite a much lower extent of reduction, the strongly interacting support (Al2O3) yielded much smaller cobalt crystallites. In fact, despite its relatively lower reducibility in comparison with 20%Co/SiO2, unpromoted 15%Co/Al2O3 provided a higher cobalt metallic surface area after the 10 h reduction treatment, as measured by H2 chemisorption. Gains in the extent of reduction of the strongly interacting 15%Co/Al2O3 catalyst were achieved by either (a) increasing the loading to provide a larger particle size that weakened the interaction with the support or (b) utilizing a noble metal promoter (i.e., Pt) to facilitate reduction, most likely by a hydrogen dissociation and spillover mechanism. Increasing the extent of reduction by Pt addition to 15%Co/Al2O3 led to the reduction of the tiny CoO species interacting with the support and, as measured by H2 chemisorption, virtually doubled the number of active cobalt metal surface sites. Ideally, further development of the TPR-EXAFS technique would be beneficial. For example, findings ways to decouple the Debye–Waller factor from the coordination number as a function of temperature might allow for a determination of crystallite size during the TPR experiment. Acknowledgments The work carried out at the CAER was supported in part by funding from the Commonwealth of Kentucky. We would like to especially thank Professor Daniel E. Resasco at the University of Oklahoma's Department of Chemical Engineering and Materials Science (CEMS) for helpful discussions and insights. Argonne's research was supported in part by the U.S. Department of Energy (DOE), Office of Fossil Energy, National Energy Technology Laboratory (NETL). The use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. MRCAT operations are supported by the Department of Energy and the MRCAT member institutions.