Fischer-Tropsch Synthesis: Investigation of the Metal-Support Interactionof Cobalt-Based Catalysts by Standard Tpr and Synchrotron-Based Tpr-Xanes/Tpr-Exafs Techniques

摘要

The application of a multi-sample holder to carry out TPR-XANES/EXAFS [1] provided keyinformation for verifying the nature of the chemical transformations occurring during theactivation of cobalt-based Fischer-Tropsch catalysts activation in H2, as well as providinginsight into the resulting crystallite size as a function of the catalyst support interaction with theCo- oxide species. A 2-step reduction process involving Co3O4 to CoO to Co0 transformationsover 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 loadingand reduction promoter addition (e.g., Pt) on extent of reduction and Co crystallite size werealso 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 contrastwith reduction models that assume direct reduction of Co oxide species to Co0 in both TPRpeaks [3,4]. Support type, and not surface area, was found to be a key factor in determiningthe 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 largercobalt crystallite size which is detrimental to the resulting active Co metallic surface area (alinear correlation exists between Co0 surface and CO conversion rate [5]). In contrast, after astandard reduction treatment, despite a much lower extent of reduction, the strongly interactingsupport (Al2O3) yielded much smaller Co crystallites, which in fact provided a higher cobaltmetallic 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 thesupport or (b) utilizing a noble metal promoter (I.e., Pt) to facilitate reduction, most likely by ahydrogen dissociation and spillover mechanism [6].