(222c) Microstructural Characterization of Bulk Ab Planes In Mo-V-Te-(Ta, Nb)-O M1 Phase Catalyst by Aberration-Corrected High Angle Annular Dark Field (HAADF) STEM
Microstructural characterization of bulk ab planes in Mo-V-Te-(Ta, Nb)-O M1 phase catalyst by aberration-corrected high angle annular dark field (HAADF) STEM
Jungwon Woo1 and Albina Borisevich3, Miaofang Chi3, and Vadim Guliants1,2*
1Chemical Engineering, University of Cincinnati, Cincinnati, OH 45221-0012
2Energy and Materials Engineering, Cincinnati, OH 45221-0012
3Oak Ridge National Laboratory, Materials Science & Technology Division, Oak Ridge, TN 37831-6064
There has been growing interest in developing highly active and selective catalysts for direct propane ammoxidation to acrylonitrile (ACN) in order to replace the current industrial technology that relies on conversion of olefins and aromatics. Mo-V-M-O (M= combination of Nb, Te, Sb, and Ta) based mixed metal oxide catalyst consisting of “M1” and “M2” phases is the most promising system being investigated by academic and industrial research groups for the one-step propane ammoxidation. The crystal structures of the Mo-V-Te-(Ta, Nb)-O M1 phases have been reported that were solved 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 the ab planes proposed to contain the active and selective surface sites for propane ammoxidation. These questions arise because Mo and Nb are located next to each other in the Periodic Table and therefore, are virtually indistinguishable by X-ray, neutron and electron diffraction techniques. The main objective of this study was to employ Ta cations (that are heavier than Nb but chemically similar) as a probe for Nb to investigate its location and impact on the partial occupancy of various Mo and V at sites in ab planes by the aberration-corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM).
The phase pure M1 catalysts were prepared by two different synthesis methods, standard hydrothermal and microwave-assisted hydrothermal synthesis. 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 that produced M1 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 atomic column, using the Gatan Digital Micrograph (DM) software running DM scripts. The statistically analyzed intensity of atomic columns suggested that the Mo site 4 in the ab planes of Ta-containing M1 has some V occupancy unlike the bulk crystal structure of the M1 phase based on the X-ray and neutron diffraction data. The atomic contrasts of various metal lattice sites confirmed the location of Ta in the pentagonal bipyramidal site 9 and suggested a low level of Ta occupancy at site 11. These results suggested that chemically similar Nb may also partially occupies sites 9 and 11 in the ab planes of the Nb-containing M1 phase. The STEM-based partial occupancy data were used to estimate the fraction of the proposed active and selective surface sites that were correlated to experimentally observed performance of this catalyst in propane ammoxidation.