Sulfur Tolerant Subnanometer Fe/Alumina Catalysts for Propane Dehydrogenation

A series of Al₂O₃-supported Fe-containing catalysts were synthesized by incipient wetness impregnation. The iron surface density was varied from 1 to 13 Fe atoms/nm² spanning submonolayer to above-monolayer coverage. Here, the resulting supported Fe-catalysts were characterized by N₂ physisorption, ex situ X-ray diffraction (XRD), X-ray pair distribution function (PDF), X-ray absorption spectroscopy (XAS), aberration corrected scanning transmission electron microscopy (AC-STEM) and chemically probed by hydrogen temperature-programmed reduction (H₂-TPR). The results suggest that over this entire range of loadings, Fe was present as dispersed species, with only a very small fraction of Fe₂O₃ aggregates, at the highest Fe loading in oxide phase. The in situ sulfidation of Fe/Al₂O₃ resulted in the formation of a highly active and selective PDH catalyst. The highest activity with 52% propane conversion and ~99% propylene selectivity at 560 °C was obtained for the 6.4 Fe/Al₂O₃-S catalyst, suggesting that this is the highest amount of Fe that could be fully dispersed on the support in sulfided form. XRD and AC-STEM indicated the absence of any crystalline iron sulfide aggregates after sulfidation and reaction. H₂-TPR results indicated that the amount of the reducible Fe sites in the sulfided catalyst remained constant above monolayer coverage, and increasing loading did not increase the number of reducible Fe sites. Consistent with these results, the reactivity per gram of catalyst showed no increase with Fe loading above monolayer coverage, suggesting that additional Fe remains conformal to the alumina surface.