Operando DRIFT and In situ Raman Spectroscopic Studies on Aspects of CO2 Fischer-Tropsch Synthesis Catalyzed by Bulk Iron Oxide-Based Catalysts

Hematite is a suitable precursor to obtain catalytically active iron (oxide) phases for CO2 Fischer-Tropsch synthesis after a reductive pretreatment. As concluded from in situ Raman spectroscopy, in hydrogen atmosphere the transformation from alpha-Fe2O3 to Fe3O4 is faster than the further reduction from Fe3O4 to Fe (metallic iron). The rate of these steps highly depends on the temperature. Starting from pure hematite, surface formate species are formed at a CO2/H-2 gas mixture at elevated temperatures, observable by DRIFT-spectroscopy. The exposure to CO2 and CO led to surface carbonate-carboxylate surface species. Reduced samples with varying contents of Fe3O4 and Fe did not show any observable adsorbates at reaction conditions. The same behavior was found during the dosage of the single gases CO2 and CO to these reduced catalysts. The formation of carbonaceous species, detected by Raman spectroscopy, could indirectly hint to the occurrence of carbidation and was especially observed for a good performing catalyst with a medium reduction degree.