The reactivity of CeO2 towards MoO3 in air atmosphere – reinvestigation

Abstract It is known that both the type and possibilities of application of cerium molybdates(VI) largely depend on the methods of their synthesis. Despite this, information on the type of molybdates(VI) formed as a result of a waste-free and environmentally friendly reaction occurring in the solid state between CeO 2 and MoO 3 in the air atmosphere, are divergent. The conducted research indicates that CeO 2 and MoO 3 react in air and in the temperature range of 500–650 °C to form two compounds, i.e. Ce 5 Mo 8 O 32 and/or Ce 2 Mo 4 O 15 . Only the Ce 5 Mo 8 O 32 compound, for the first time, was obtained as a pure phase. The synthesis of Ce 5 Mo 8 O 32 takes place through an intermediate stage. In this stage, with the evolution of oxygen, the compound Ce 2 Mo 4 O 15 is formed, which then reacts with excess CeO 2 to Ce 5 Mo 8 O 32 . The obtained compound was characterized by XRD, DTA–TG, FTIR and UV–Vis/DRS methods. Ce 5 Mo 8 O 32 has a green-olive colour and a density of 4.82 ± 0.05 g cm −3 . It was found that this compound melts incongruently at the temperature of 960 ± 5 °C with the separation of solid CeO 2 . The value of the energy gap E g ~ 2.59 eV allows the Ce 5 Mo 8 O 32 compound to be classified as a semiconductor. The previously unknown properties of the compound with mixed cerium valence (Ce 4 3+ Ce 4+ Mo 8 6+ O 32 ), characterized in this work, will constitute the basis for its application research.