(606f) Fischer-Tropsch Synthesis: EXAFS/XANES Characterization of Iron-Based Catalysts Promoted by Group I Alkali Metals

Alkali metal promoters, especially potassium, are known to decrease the catalyst deactivation rate and increase the wax selectivity of iron Fischer-Tropsch synthesis. In previous studies, CAER researchers have shown that the promotion of the activity of iron / silica catalysts depends on the conversion level, showing that K promotes the catalytic activity over a wide range of conversion levels. Furthermore, K, Rb and Cs promoted catalysts produces lower CH₄ selectivities, whereas higher ethylene selectivities were produced in the order Cs = Rb = K > Na > Li > unpromoted. In an attempt to correlate such observations with possible modifications in the structure of iron/silica catalysts due to the presence of different group I alkali metal promoters, the activation (reduction/carburization under CO flow) of alkali metal promoted iron/silica catalyst samples (Fe:Si:group I metal atomic ratio 100:4.6:1.5) was examined using in-situ EXAFS/XANES. Successive x-ray absorption spectra were recorded as the calcined sample was slowly being heated under CO flow from room temperature to 290^oC and held at that temperature for 10 h. Linear combination fitting of the XANES spectra showed an increase in the rate of formation of Fe₃O_4-x and iron carbides, according to the order: unpromoted oC, though, the catalyst contained nearly 100% iron carbides in the K, Rb and Cs promoted samples, whereas the Li and the unpromoted samples had respectively 8 and 12 % of oxides remaining in the sample. Additionally, structural changes during and after carburization were examined by EXAFS analysis.

The U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, supported the use of the Advanced Photon Source under Contract DE-AC02-06CH11357; MRCAT operations are supported by the Department of Energy and the MRCAT member institutions.