Using Aberration-Corrected STEM Imaging to Explore Chemical and Structural Variations in the M1 Phase of the MoVNbTeO Oxidation Catalyst

We report results of aberration-corrected STEM imaging of the orthorhombic M1 phase in the MoVNbTeO selective oxidation catalyst prepared using two different solution techniques. Atomic coordinates and cation site occupancies for both sample preparations are compared with a previously reported Rietveld-based model for this structure. The high angle annular dark-field (HAADF) images from the two preparations varied significantly only in the occupancy of the heptagonal channels. The M1 sample prepared under ambient conditions exhibited partial occupancy, whereas the hydrothermally prepared sample had heptagonal channels that were primarily vacant. Compared to the Rietveld model, both the atomic positions and the occupancies were found to be consistent, with the exception of the Mo4 and the two Te sites. The Mo4 site shows a lower HAADF intensity than expected, suggesting that appreciable V content may be present, whereas both Te sites had low Te occupancy as compared with the refined model, likely due to e-beam sublimation. Z-contrast imaging may become a valuable tool for rapidly obtaining initial model parameters for the Rietveld refinement of complex structures.