Atomic-Scale Investigation of Two-Component MoVO Complex Oxide Catalysts Using Aberration-Corrected High-Angle Annular Dark-Field Imaging

Abstract

High-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) imaging was used to study various hydrothermally prepared MoVO phases with either M1-type orthorhombic symmetry, Mo5O14-type tetragonal symmetry, or trigonal symmetry. For the M1-type phase, HAADF analysis suggests atomic positions consistent with existing M1 models having mixed Mo and V occupancy in the octahedral sites linking the pentagonal units. For the V-substituted Mo5O14 phase, contrast analysis provides evidence of V occupation primarily in the linking octahedra that interconnect the pentagonal {Mo6O21} units. Selected-area electron diffraction (SAED) studies suggest that the structure of the Mo5O14 analog is likely to be tetragonal, rather than orthorhombically distorted as has previously been proposed. HAADF analysis of the trigonal phase provides reasonable support for previously reported structural models. High-resolution images provide strong support for the existence of atomic disorder within the octahedral sites connecting the three heptagonal channels that characterize this structure.