A combination of time-resolved synchrotron μ-X-ray diffraction computed tomography (μ-XRD-CT) and μ-pair distribution function computed tomography (μ-PDF-CT) has been applied for the study of an individual Co/γ-Al2O3 catalyst pellet during reduction and the early stages of the Fischer–Tropsch synthesis (FTS) reaction, revealing insight into the solid-state changes occurring from within such crystalline materials. Both sets of data were of sufficient quality so as to be able to follow the spatial dependency of Co speciation evolution from Co3O4 to CoO to face-centered cubic (fcc) Co metal nanoparticles. These data revealed the samples to be highly heterogeneous and contain two types of Co species: small (≤6.5 nm) nanoparticles that interact strongly with the γ-Al2O3 support which are difficult to reduce (remaining as CoO) and nanoparticles that agglomerate and have little interaction or else are weakly interacting with the support but readily reduce in H2. The Co phase evolution under FTS conditions shows a strong dependence on the Co nanoparticle location; the complementarity between the observations made using μ-XRD-CT vs μ-PDF-CT allowed us to conclude that at the sample periphery a significant amount of agglomerated, weakly interacting with the support, small fcc Co metal nanoparticles (≤7.5 nm) oxidize to CoO/Co3O4 during FTS, most likely due to the presence of water vapor produced during the reaction. Catalytic tests demonstrated that this oxidation coincided with a decrease in CH4 selectivity and increased water-gas shift (WGS) activity. This oxidation of fcc Co nanoparticles (i.e., the removal of the contribution to the XRD signal) also explains the observation of sintering previously reported for such catalysts in the early stages of the FTS reaction.

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This publication highlights the effect of CO treatments on both calcined and reduced catalysts by using novel in situ techniques to track particle transformations, which are so vital to catalyst performance and activity. Cobalt carbide formation has been shown to occur readily with CO treatments on metallic cobalt particles, and in this study we have used in situ XRD and temperature‐programmed reduction (TPR) techniques to track the evolution of the carbide. We were able to show a dependence on CO partial pressures and the formation of a single hexagonal phase of cobalt as a result of the carbide step. The stability of cobalt carbide was studied and its reduction in hydrogen to cobalt metal was observed. CO was also used as the reducing gas and by using TPR and in situ XRD we were able to demonstrate the reduction of cobalt oxide (Co3O4) to cobalt carbide (Co2C) via both oxide (CoO) and metal (Co) intermediates.

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