Surface coordinate geometry of iron catalysts: Formation of tetrahedral/octahedral site on silica surface with/without sodium promoter

摘要

Sodium-modified, silica-supported iron oxide catalysts (Na-Fe/SiO2) calcined at 300, 500, 650, and 800 degrees C have been compared with unmodified Fe/SiO2 treated in the same conditions by using Fe ii-edge XANES, EXAFS, XRD, and Mossbauer techniques. Each iron on Fe/SiO2 is found to have 5.0 nearest oxygen neighbors at an average distance of 1.94 Angstrom; there is no change after calcination at 800 degrees C, However, remarkable changes have been observed for Na-Fe/SiO2 calcined at temperatures higher than 500 degrees C, After calcination at 500 degrees C, only tetrahedrally coordinated Fe3+ is observed, Accordingly, the coordination number of the nearest oxygens decreases to 4.1 with a significantly shortened Fe-O bond length of 1.89 Angstrom. Supporting irons on sodium-pretreated silica (Fe/Na-SiO2) and then calcinating the catalyst obtained at 800 degrees C results in a similar phenomenon. (C) 1998 Academic Press. [References: 17]