X-ray Absorption Spectroscopy Applied to Mixed-metal Iron-based Fischer-Tropsch Catalysts

Synthetic fuels derived from methane, coal or biomass are essential in addressing future transportation fuel demands which are expected to exceed petroleum-derived capacities. The Fischer-Tropsch synthesis (FTS) is the most studied technique for the conversion of coal or biomass-derived syngas into transportation fuels. Fe-based catalysts are typically used for the FTS of biomass and/or coal-derived syngas due to: the relatively low cost of iron, water-gas shift activity, and low methane selectivity at industrial FTS conditions. Fe/Cu/SiO2 Fischer-Tropsch catalysts promoted with Cr, Mn, Mo, W, or Zr were studied in-situ, using Fe K-edge TPR XANES (temperature programmed reduction X-ray absorption near-edge structure), which was collected during reduction under flowing syngas. XANES analysis indicates that the phase transformations under syngas reduction are similar among the Fe/Cu/SiO2-containing catalysts. LCF (linear combination fitting) used an expanded model (original model: Fe2O3 --\u3e Fe3O4 --\u3e FexC) which included Fe2+ phases: FeO and Fe2SiO4. This expanded model was found to more closely account for the Fe-containing phases under syngas reduction. Fe K-edge XANES spectra were also collected during in-situ H2 reduction (25-300ºC, 2º C/min; 300ºC, 2 hr), followed by a syngas reaction (2:1 H2:CO ratio, 300ºC, 4 hr) of a Cr-promoted, Mn-promoted, and an unpromoted catalyst. XANES analysis indicates that the Fe phase transitions under flowing H2 (where bulk α-Fe is not observed) or syngas (before observable FexC formation) are very similar. During H2 reduction, it was shown from the Cr K-edge XANES analysis that Cr substituted as a trivalent species into Fe3O4, corresponding to the Fe3O4-Cr2O3 phase. Mn K-edge XANES analysis indicates that Mn substituted as a divalent species into Fe3O4, corresponding to a composition of (Fe1-yMny)3O4. The differences in the oxidation state of the substituted promoter (i.e., divalent Mn vs. trivalent Cr substitution into Fe3O4) likely contributed to the differences in the steady state activity of the catalyst. Mn promotion inhibited carbon deposition and had a higher steady state activity relative to the unpromoted catalyst. The deactivation of the Cr-promoted to the steady state CO hydrogenation levels of the unpromoted catalyst suggests that the formation of the Fe3O4-Cr2O3¬ phase does not prevent carbon deposition