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

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, Mo 5 O 14 -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 Mo 5 O 14 phase, contrast analysis provides evidence of V occupation primarily in the linking octahedra that interconnect the pentagonal {Mo 6 O 21 } units. Selected-area electron diffraction (SAED) studies suggest that the structure of the Mo 5 O 14 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.