Thermodynamic study of gaseous tin molybdates by high-temperature mass spectrometry

Molybdenum and tin are components of various construction materials which are often used at high temperature and in an oxidizing atmosphere. Oxides of molybdenum and tin, with their high reactivity, can, in their turn, form a number of gaseous compounds. To predict the possibility of the existence of gaseous associates formed by tin and molybdenum oxides it is important to know their thermodynamic characteristics. Until the present investigation only a few gaseous salts of tin were known.

High-temperature Knudsen effusion mass spectrometry was used to determine the partial pressures of vapor species over the SnO(2) -MoO(3) system. The formation enthalpies of gaseous SnMoO(4), Sn(2) MoO(5) and SnMo(2)O(7) were derived. Measurements were performed with a MS-1301 mass spectrometer. Vaporization was carried out using a molybdenum effusion cell containing the samples under study and pure gold as the reference substance. A theoretical study of gaseous tin molybdates was performed by several quantum chemical methods: wave function based explicitly correlated F12 methods and DFT M0(6) methods.

In the temperature range of 1200-1400 K, SnO, Sn(2)O(2), SnMoO(4), Sn(2) MoO(5), SnMo(2)O(7), MoO(3), Mo(2)O(6) and Mo(3)O(9) were found to be the main vapor species over the samples studied. On the basis of the equilibrium constants of gaseous reactions, the standard formation enthalpies of gaseous SnMoO(4) (-699 ± 29 kJ/mol), Sn(2) MoO(5) (-1001 ± 38 kJ/mol) and SnMo(2)O(7) (-1456 ± 60 kJ/mol) at 298 K were determined. Energetically favorable structures were found and vibrational frequencies were evaluated in the harmonic approximation.

The stability of gaseous species, SnMoO(4), Sn(2) MoO(5) and SnMo(2)O(7), was confirmed by high-temperature mass spectrometry. A number of gas-phase reactions involving tin-containing gaseous salts were studied. The enthalpies of reactions of gaseous tin molybdates were evaluated theoretically and the obtained values are in agreement with those obtained experimentally.