Gaseous titanium molybdates and tungstates: thermodynamic properties and structures.

Abstract

Titanium is a component of various construction materials, which is often used at high temperature and in an oxidizing atmosphere. Thermally stable even at high temperatures, titanium compounds may appear in the condensed phase. To predict the possibility of existence of gaseous associates formed by titanium oxides it is important to know their thermodynamic characteristics. Until the present investigation no gaseous salts of titanium were known. Measurements were performed by high-temperature Knudsen effusion mass spectrometry with a MS-1301 mass spectrometer. Vaporization was carried out using molybdenum and tungsten effusion cells containing samples of pure Au, Ti3O5 and SiO2. A theoretical study of gaseous titanium molybdates and tungstates in different spin states was performed by quantum chemical density functional theory (DFT) B3LYP and M06 methods. On the basis of the equilibrium constants of gaseous reactions, the standard formation enthalpies of gaseous TiMoO3 (-424 ± 28 kJ/mol), TiWO3 (-400 ± 22 kJ/mol), TiMoO4 (-795 ± 29 kJ/mol), TiWO4 (-750 ± 24 kJ/mol), TiMoO5 (-1146 ± 23 kJ/mol) and TiWO5 (-1125 ± 22 kJ/mol) at 298 K were determined. Energetically favorable structures were localized and vibrational frequencies were evaluated in the harmonic approximation. Natural atomic charges, bond orders, and valence indices were calculated for all relevant structures. The stability of gaseous species TiMoOn and TiWOn (n = 3, 4, 5) was confirmed by high-temperature mass spectrometry. A number of gas-phase reactions involving titanium-containing gaseous salts were studied. Enthalpies of reactions of gaseous TiXOn (X = Mo, W; n = 3, 4, 5) formation were evaluated theoretically and the obtained values are in agreement with the experimental ones.