278954 Aberration-Corrected High-Angle Annular Dark Field (HAADF) STEM Study of Bulk Ab Planes in Mo-V-Te-(Ta, Nb)-O M1 Phase Catalyst Prepared by Novel Synthesis Methods: Correlation Between Mo/V Distribution in Ab Planes and Catalytic Performance
Bulk mixed Mo-V-M-O (M= combination of Nb, Te, Sb, and Ta) oxides containing “M1” and “M2” phases are the highly promising catalysts for one-step propane ammoxidation. The crystal structure of the Mo-V-Te-(Ta, Nb)-O M1 phase has been elucidated by combined application of X-ray and neutron diffraction methods. However, questions still remain regarding the Nb location in the M1 phase lattice and its impact on the distribution of Mo and V cations among several sites in ab planes which has been proposed as the active and selective surface for propane ammoxidation. Current techniques, such as X-ray or neutron and electron diffraction, cannot distinguish Mo and Nb since they are next to each other in the Periodic Table. Previously, Ta locations in the Ta-substituted M1 phase (where much heavier Ta was used as a Nb model) were investigated by the aberration-corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). Although the STEM work successfully located the Ta and provided the distribution of Mo and V in the ab planes, there was a discrepancy between experimental and predicted reactivity based on the Mo and V occupancies in various lattice sites present in the ab planes. We suspect that one of possible reasons for this discrepancy is the observed segregation of Ta in the Ta-M1 phase catalyst prepared by slurry evaporation which is known for poor control over nucleation and growth of the desired M1 phase. Other possible reasons for this discrepancy are inconsistencies in the thickness of the Ta-M1 samples ground by mortar and pestle and limited number of data for STEM analysis.
In this study, the Ta-M1 phase catalysts were prepared by two different synthesis methods, standard hydrothermal and microwave-assisted hydrothermal synthesis. Hydrothermal and microwave-assisted synthesis resulted in the M1 phase that showed uniform Ta distribution across different M1 crystallites and no significant variation in Ta content from the surface region to the bulk of the M1 phase. Moreover, the M1 samples for STEM characterization were prepared by sectioning epoxy-embedded M1 crystals producing slices of uniform thickness (ca. 50 nm) which eliminated significant uncertainty in the atomic contrast of various metal lattice sites encountered in previous studies of mechanically crushed M1 crystals. Collected HAADF STEM images were quantified and analyzed statistically in terms of atomic contrast of metal lattice sites, atomic column by column, using Gatan Digital Micrograph (DM) software running DM scripts. We were able to collect and analyze large quantities of data from HAADF-STEM images for high accuracy using DM scripts. The statistically analyzed intensity of atomic columns suggested that the catalysts synthesized by novel synthesis methods show different Mo/V distribution in ab planes as compared to the results of the previous neutron diffraction study. The atomic contrasts of various metal lattice sites confirmed the location of Ta in the pentagonal bipyramidal site 9. The STEM-based partial occupancy data, used to estimate the fraction of the proposed active and selective surface sites, were correlated to the experimentally observed catalytic performance of those catalysts in propane ammoxidation.