Abstract
The structure of X-ray amorphous precursors of MoVTeO and MoVTeNbO oxide catalysts and their conversion at high-temperature treatment and under exposition in the reaction medium is studied by analytical transmission electron microscopy (TEM) in combination with the local energy dispersive X-ray (EDX) microanalysis of the elemental composition. At thermal treatment in the range of 300–450 °C the presence of both true amorphous phases and nanosized crystallites of the orthorhombic M1 and pseudohexagonal M2 phases is detected in the MoVTe(Nb)O catalysts. An increase in the annealing temperature results in the formation of well-crystallized particles of the M1 and M2 phases in the MoVTeNbO catalyst. The four-component M1 phase is stable at 550 °C and it is maintained under the reaction conditions. At an increase in the temperature of thermal treatment the successive crystallization of nanosized particles occurs in the three-component MoVTeO system with the formation of mainly the M2 phase. The nanostructured three-component M1 phase formed at low temperatures cannot crystallize into the full M1 phase in the absence of Nb and decomposes with the formation of the M2 phase that is inactive in the conversion of propane. The calculation procedure for EDX, which was developed specially for these catalysts, enabled the refinement of the chemical composition of separate phases.
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