The supported cobalt catalysts are widely used in Fischer-Tropsch synthesis. Catalysts are activated by reduction of Co₃O₄ in hydrogen. It is important to understand how the reduction conditions influence the structural parameters of catalyst. We investigated Co₃O₄ supported on g-Al₂O₃ and model samples to compare their behavior during reduction process. In situ X-ray powder diffraction was used to characterize the structure of Co catalyst supported on g-Al₂O₃ and model samples.

Initial structure of both types of samples is similar according to XRD structural analysis and contains vacancies in the cation positions. In situ X-ray diffraction investigations show that reduction process is different for supported and non-supported samples in hole. But there are some features which are the same for both types of the samples. The reduction process for the both types of samples begins at the same temperature (T=180^oC). The structure mechanism of the reduction process beginning is also the same: we observed decreasing the quantity of cation vacancies and filling the non-spinel octahedral position by Co ions that indicates on the appearance of clusters with the structure of CoO inside the structure of Co₃O₄. Appearance of CoO clusters is common feature for both types of samples. But further reduction process is different for supported and non-supported samples. Non-supported sample is reduced to metal Co (hcp) at the temperature T=180^oC without formation intermediate CoO phase i.e. non-supported sample is reduced in one step. In contrary supported cobalt oxide is reduced in two steps. Reduction of Co₃O₄ to CoO clearly identified by XRD is the first step. The second step (CoOàCo) begins at 260^oC and it is not fully completed at the temperature 400^oC. At this temperature two phases CoO and Co (fcc) coexist. Metallic Co (hcp) and Co (fcc) have very high concentration of staking faults. The diffraction pattern simulation showed that Co(hcp) and Co(fcc) structures contained stacking faults with concentration of about 0.2 and 0.1, correspondingly.

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