HAADF-STEM Study of Mo/V Distributions in Mo-V-Te-Ta-O M1 Phases and Their Correlations with Reactivity in Propane Ammoxidation <>
Jungwon Woo1, Albina Borisevich2, Qian He 2 and Vadim V. Guliants1
1 Chemical Engineering, University of Cincinnati, Cincinnati OH 45221
2 Center for Nanophase Materials Sciences, ORNL, Oak Ridge, TN 37831
Selective ammoxidation of propane over the bulk mixed MoVTe(Nb,Ta)O catalysts containing so-called M1 phase has attracted significant attention of the catalysis community in recent years. The orthorhombic M1 phase has a large unit cell that contains 13 distinct metal lattice sites (S1-S13), several of which are mixed Mo/V sites and two are partially occupied by Te. Due to structural and compositional complexity of the M1 phase, high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) is particularly promising for the direct space analysis of the M1 phase because the atomic column contrast in HAADF-STEM images is highly sensitive to the atomic number (Z) of scattering species.
In this study, HAADF-STEM image simulations were performed in order to obtain accurate metal site distributions in the MoVTeTaO M1 phases, which were employed to validate three new probability models of the M1 phase reactivity in propane ammoxidation to acrylonitrile. The acrylonitrile yield and 1st order irreversible reaction rate constants for propane consumption normalized to the ab plane areas, k"ab, correlated with (1) the probability of 1-2 V5+ in the S3-S4-S4-S7-S7 center (Model 1); (2) increasing total V content in the S2-S4-S4-S7-S7 center (Model 2); and (3) the probability of more than 2 V cations in the S2-S4-S4-S7-S7 center (Model 3). The tentative correlations between the Mo/V site distribution in the M1 phase and its catalytic performance emphasize the importance of V5+ cation distribution in the S2-S3-S4-S4-S7-S7 center for its catalytic activity and selectivity in propane ammoxidation. Moreover, the fundamental relationships elucidated in this study are promising for the development of general rules of rational design of improved mixed metal oxide catalysts for propane ammoxidation and other selective oxidation reactions.