Preparation of Phase-Pure M1 MoVTeNb Oxide Catalysts by Hydrothermal Synthesis-Influence of Reaction Parameters on Structure and Morphology

摘要

This work presents a detailed investigation of the preparation of MoVTeNbO_x catalysts by hydrothermal synthesis.Phase-pure synthesis of M1 has been achieved applying the metals in a molar ratio Mof V/Te/Nb=1/0.25/0.23/0.12.Raman,UV/Vis spectroscopy,and SEM/EDX analysis show that the elements are inhomogeneously distributed in the initial suspension that is formed after mixing the metal salts in an aqueous medium.Iso-and heteropoly anions of molybdenum,free telluric acid as well as supra-molecular polyoxometalate clusters are observed in the solution,whereas all metals have been found in the precipitate.Complete rearrangement of molecular building blocks under hydrothermal conditions is essential for formation of phase-pure materials.Optimized synthesis conditions with respect to temperature and time result in the formation of a precursor consisting of nano-structured M1 characterized by an extended periodic organization in the[001]direction and a fairly homogeneous distribution of the elements.Residual ammonium containing supra-molecular species in the precursor result in the formation of phase mixtures during the subsequent crystallization by heat treatment in inert gas.Phase-pure M1 exhibits a distinct degree of flexibility with respect to the chemical composition that becomes obvious by incorporating Nb not exclusively into pentagonal bi-pyramidal units,but also into octahedral coordinated positions as shown by EXAFS.Anisotropic growth of the needle-like M1 crystals has been observed during the final heat treatment performed at 873-923 K in inert atmosphere disclosing a potential method to control the catalytic properties of MoVTeNbO_x catalysts.