Surface coordinate geometry of iron catalysts: Distinctive behaviors of Fe/Al2O3 in CO hydrogenation

Two well-defined iron catalysts, Fe(II)/Al2O3 (4.5 wt %) and Fe2O3/Al2O3 (1.5 wt %), have been studied using Fe K-edge X-ray absorption fine structure (XAFS). The total coordination number of the nearest oxygen neighbors is found to be 4.9 for the former and 5.7 for the latter, but the average Fe-O bond length remains the same, 1.98 Angstrom. The nearest iron neighbors at 2.95-2.97 Angstrom are observed for both the catalysts, but for the Fe2O3/Al2O3, further two iron neighbors at 3.42 and 3.86 Angstrom are also present. The fact that the shell radius of 3.42 Angstrom is (2)/(3) root 3 times longer than that of the first nearest irons is consistent with what is expected from the para- and the meta-positions surrounding the same oxygen on the gamma-Al2O3 (111) plane but is at variance with the estimation made for the (110) plane. Two geometrical models proposed for the catalysts stoichiometrically explain why the higher metal loading only leads to one type of the iron neighbors but the lower metal loading unexpectedly leads to three types of iron neighbors.