The genesis of carbon-supported FeMn and KFeMn catalysts from stoichiometric mixed-metal carbonyl clusters: II. Characterization by Mössbauer spectroscopy and [formula omitted

Abstract

Mössbauer effect spectroscopy (MES) and transmission electron microscopy/energy-dispersive X-ray spectroscopy ( TEM EDS ) were used to study the chemistry of the iron particles produced by the thermal decomposition of Stoichiometric KFe, FeMn, and KFeMn mixed-metal carbonyl clusters on a high surface area carbon. Intermediate chemical states during the cluster decomposition process below 473 K were identified using MES, and they indicated that decarbonylation occurred via the formation of Fe(CO) 5 and [Fe 4(CO) 13] − during heating in H 2. Following decomposition at 473 K, the principal final phase was the D-structure, which has been associated with superparamagnetic Fe combined with an Fe 2+ state. Additionally, a doublet characteristic of Fe 3+ oxide and/or superparamagnetic carbide appeared in the spectrum. No evidence for a mixed spinel such as Fe 2MnO 4 was obtained, but the Fe and Mn appeared to remain in contact, presumably as MnO x on top of small Fe crystallites. A subsequent treatment in H 2 at 673 K caused the K-promoted catalysts to sinter and form separate phases of Mn oxide and large particles of α-Fe, as detected by MES and TEM EDS . For the unpromoted FeMn sample, little sintering occurred under H 2 at 673 K, and the particles existed in a phase which has been previously found in Fe-only Carbon-supported catalysts.