Fabrication of complex model oxide catalysts: Mo oxide supported on Fe3O4(111)

Abstract

Industrial catalysts for the oxidation of methanol to formaldehyde consist of iron molybdate [Fe₂(MoO₄)₃]. Using a variety of techniques we have previously shown that the surface of these catalysts is segregated in MoO₃, and in order to understand the relationship between surface structure and reactivity for these systems we have begun a surface science study of this system using model, single crystal oxides. Model catalysts of molybdenum oxide nanoparticles and films on an Fe₃O₄(111) single crystal were fabricated by the hot-filament metal oxide deposition technique (HFMOD), where molybdenum oxides were produced using a molybdenum filament heated in an oxygen atmosphere. Low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and scanning tunnelling microscopy (STM) have been used to investigate molybdenum oxide nanoparticles and films deposited on Fe₃O₄(111). The molybdenum oxide film forms in the highest oxidation state, +6, and is remarkably stable to thermal treatment, remaining on the surface to at least 973 K. However, above ∼573 K cation mixing begins to occur, forming an iron molybdate structure, but the process is strongly Mo coverage dependent.