A combined HAADF STEM and density functional theory study of tantalum and niobium locations in the Mo–V–Te–Ta(Nb)–O M1 phases

The M1 phases of the Mo–V–Te–Nb mixed-metal oxides are highly promising catalysts for direct propane (amm)oxidation. Nb is beneficial to the activity and selectivity for acrylonitrile, but direct evidence for Nb location is lacking because structural methods cannot distinguish Nb (Z = 41) from Mo (Z = 42). We have employed high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) and density functional theory (DFT) to study the Ta (Z = 73) location in a Mo–V–Te–Ta–O M1 phase produced via hydrothermal synthesis, which possesses uniform Ta distribution compared to M1 phases previously synthesized via slurry evaporation. The HAADF STEM and DFT results indicate that Ta predominantly occupies the pentagonal bipyramidal site (S9). Moreover DFT results validate the previous hypothesis that Ta and Nb both prefer the S9 site.

Graphical abstractThe Ta location in the Mo–V–Te–Ta–O M1 phase was investigated by HAADF STEM and DFT calculations, which strongly indicated its location in the pentagonal bipyramidal site S9. The Nb location in the Mo–V–Te–Nb–O M1 phase was probed by the same theoretical methods, which supported the earlier hypothesis that Nb preferred the same pentagonal bipyramidal site S9.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Mo–V–Te–Ta–O M1 phase with uniform Ta distribution was synthesized hydrothermally. ► Ta was predominantly located in site 9 by HAADF STEM and DFT calculations. ► DFT calculations showed that Nb is predominantly located in site 9.