(403c) HAADF-STEM Study of Mo/V Distributions in Mo-V-Te-Ta-O M1 Phases and Their Correlations with Reactivity in Propane Ammoxidation

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

²Center
for Nanophase Materials Sciences, ORNL, Oak Ridge, TN 37831

Abstract

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 1^st 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.