The characterisation of unsupported iron and manganese-promoted iron catalysts by x-ray photoelectron spectroscopy and temperature-programmed reduction

Abstract

X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR) have been used to characterise unsupported Fe and Mn-promoted Fe catalysts containing 1–4% Mn, and their precursor oxides/hydroxides. XPS characterisation has shown a considerable surface enrichment of Mn at the reduced catalyst surface while argon ion etching has indicated a substantial Mn concentration gradient over the first few nm into it. TPR studies using dried and calcined precursors have shown that the presence of Mn slightly helps the reduction of Fe 3O 4 to Fe o. Reduction profiles suggested that the fully reduced catalyst contains Fe o and MnO. The results strongly imply that the surface of the Fe o particles is substantially covered by a layer of MnO species which is complete at Mn concentrations of less than 1%; excess Mn probably forms separate MnO particles. This model also accounts for the manner in which alkene selectivity in Fischer-Tropsch synthesis varies with Mn concentration, and suggests a close analogy between promoter effects in this system and the strong metal-support interaction (SMSI) found with certain supported metal systems.