(30d) Fischer-Tropsch Synthesis: Investigation of the Metal-Support Interaction of Cobalt-Based Catalysts by Standard Tpr and Synchrotron-Based Tpr-Xanes/Tpr-Exafs Techniques

Authors: 
Jacobs, G., University of Kentucky
Ji, Y., University of Kentucky
Davis, B. H., University of Kentucky, Center for Applied Energy Research
Marshall, C. L., Argonne National Laboratory
Cronauer, D., Argonne National Laboratory
Kropf, A. J., Argonne National Laboratory


The application of a multi-sample holder to carry out TPR-XANES/EXAFS [1] provided key information for verifying the nature of the chemical transformations occurring during the activation of cobalt-based Fischer-Tropsch catalysts activation in H2, as well as providing insight into the resulting crystallite size as a function of the catalyst support interaction with the Co- oxide species. A 2-step reduction process involving Co3O4 to CoO to Co0 transformations over standard calcined catalysts was quantified over catalysts exhibiting both weak (e.g., Co/SiO2) and strong interactions (e.g., Co/Al2O3) with the support. The impacts of Co loading and reduction promoter addition (e.g., Pt) on extent of reduction and Co crystallite size were also investigated. Results were in good agreement with, and assisted in the interpretation of, standard H2-TPR and H-chemisorption / pulse reoxidation data [2]. The results are in contrast with reduction models that assume direct reduction of Co oxide species to Co0 in both TPR peaks [3,4]. Support type, and not surface area, was found to be a key factor in determining the strength of the interaction and the rate at which the cobalt oxides underwent reduction. Yet not only that, the more weakly interacting support (in this case, SiO2) yielded a much larger cobalt crystallite size which is detrimental to the resulting active Co metallic surface area (a linear correlation exists between Co0 surface and CO conversion rate [5]). In contrast, after a standard reduction treatment, despite a much lower extent of reduction, the strongly interacting support (Al2O3) yielded much smaller Co crystallites, which in fact provided a higher cobalt metallic surface area. Further gains in extent of reduction were managed 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 [6].

Acknowledgements: Argonne's research supported by U.S. Department of Energy (DOE), Office of Fossil Energy. Use of the Advanced Photon Source was supported by the U. S. DOE, 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

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