Structural Investigation of the Thermal Decomposition of Ammonium Heptamolybdate by in situ XAFS and XRD

Abstract

AbstractThe decomposition of ammonium heptamolybdate (AHM, (NH4)6Mo7O24×4H2O) was studied in situ by X‐ray diffraction and X‐ray absorption spectroscopy, as well as by thermal analysis (TG/DTA). Decomposition conditions such as reactant atmospheres, (20% oxygen, 5% propene, 5% hydrogen, pure helium, and static air), heating rates, and gas flow rates were varied to investigate their influence on the decomposition process. The results obtained show that the reaction pathway is affected by the partial pressures of the gas‐phase decomposition products. The partial pressures of the decomposition products, water and ammonia, at a given temperature, is influenced mainly by the reactant gas flow rate and the heating rate. Lowering the partial pressures of ammonia and water inhibits the crystallization of the intermediate ammonium tetramolybdate (ATM), and promotes the formation of the intermediate hexagonal MoO3. The decomposition pathway under low gas phase product partial pressure is: (i) AHM; (ii) (ca. 335 K) X‐ray amorphous phase; (iii) (ca. 520 K) hexagonal MoO3; (iv) (ca. 650 K) the products, which depend on the reactant atmosphere, are mixtures of highly disordered Mo4O11, and/or α‐MoO3. Under different conditions the decomposition pathway is: (i) AHM; (ii) (ca. 350 K) X‐ray amorphous phase; (iii) (ca. 470 K) ATM; (iv) (ca. 570 K) hexagonal MoO3 + α‐MoO3; (ca. 650 K) α‐MoO3. Under a hydrogen containing atmosphere, a peculiar decomposition pathway is observed: an intermediate MoO3 with an unusual texture is formed prior to the reduction to MoO2 and the consecutive formation of orthorhombic Mo4O11. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)