Characterization of Active Species in Rb Promoted Iron Catalyst during Fischer-Tropsch Synthesis: an Exafs, Xanes and Mossbauer Study

摘要

Group I alkali metal promoted iron-based catalysts for Fischer-Tropsch (FT) synthesis offer a number of benefits regarding product selectivity. Rubidium promoted iron catalysts are prepared with two Rb/Fe atomic ratios (1.44 and 5) using rubidium nitrate and rubidium carbonate as rubidium precursors. Results of catalytic activity and deactivation studies in a CSTR revealed that Rb promoted catalyst results in a steady conversion with a lower deactivation rate than the corresponding unpromoted catalyst although the initial activity of the promoted catalyst is almost half that of the unpromoted catalyst. Mossbauer spectroscopic measurements of CO activated and working catalyst samples indicate that the composition of the iron carbide phase formed after carbidation is X-Fe5C2 for both promoted and unpromoted catalysts. However, in the case of the rubidium promoted catalyst,' EI-Fe2.2C becomes the predominant carbide phase as FTS continues and the overall catalyst composition remains carbide. The carbide content of the unpromoted catalyst is found to decline very quickly with synthesis time. EXAFS and XANES synchrotron techniques are used to determine the chemical compound that most closely represents the state of the Rb promoter in the working catalyst under FTS environment. Results of XANES measurements suggest that Rb is present in the oxidized state and that the compound most prevalent in the active catalyst samples closely resemble rubidium carbonate. Relative intensity of the XANES white line indicate that the catalyst with a Rb/Fe atomic ratio of 5 is more oxidized than the catalyst with a Rb/Fe atomic of 1.44. Fourier transformed magnitude spectra for the Fe K-edge from EXAFS measurement indicate that the Fe-Fe coordination peaks are in good agreement with those observed for the iron carbide reference sample at approximately 2.2 and 2.5 A. Furthermore, there are low R peaks (e.g., -1.5 A) that exhibit good agreement with the iron carbide reference, and may correspond to Fe-C coordination. However, it is not possible to rule out the Fe-0 coordination from the contribution of iron oxide in the samples.