Development of the local and average structure of a V–Mo–Nb oxide catalyst with Mo₅O₁₄-like structure during synthesis from nanostructured precursors

Abstract

A combination of X-ray and neutron PDF measurements with powder diffraction and EXAFS data was used to determine the structures of a V—Mo—Nb-oxide catalyst and its poorly crystallized precursors that exhibit the strongest catalytic activities. The crystalline material belongs to space group P-4 2₁m, a = 22.8, c = 4.002, and is build up of pentagonal MeO₇ bipyramids surrounded by edge sharing Me-octahedrons (Me = Mo, V, Nb). In the average structure all MeO₇ units are at the same z-level, while the local structure analysis shows systematic shifts along [001]. Samples synthesized at 300 °C and 400 °C exhibit a nanostructure, whose local structure predates the final crystalline structure. Initial nanoparticles are spherical and grow predominantly along the c-axis. The successful analysis required a reverse analysis that took the crystalline material as starting model for the samples synthesized at lower temperatures.