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Thermodynamics of gaseous barium cerate studied by Knudsen effusion mass spectrometry.

RATIONALE
BaCeO3 species, which are known to be excellent proton conductors, are potential candidates as electrolytes in hydrogen concentrators and fuel cells. Oxides of barium and cerium, with their reactivity can, in turn, form gaseous associates - complex molecules with two different types of atoms (not including oxygen). To predict the possibility of the existence of gaseous associates formed by barium and cerium oxides it is important to know their thermodynamic characteristics. Until the present investigation, gaseous cerates were unknown.


METHODS
High-temperature Knudsen effusion mass spectrometry was used to determine the partial pressures of vapor species over the BaO-CeO2 system, and the formation enthalpies of gaseous CeO2 and BaCeO3 were derived. Measurements of partial pressures and reaction enthalpies were performed with a MS-1301 mass spectrometer. Vaporization was carried out using molybdenum and tungsten effusion cells containing the samples under study and pure gold as a reference substance. A theoretical study of gaseous cerium dioxide and barium cerate was performed by several quantum chemical methods: DFT M06, DFT PBE0 and MP2.


RESULTS
In the temperature range 1900-2120 K, CeO and CeO2 were found to be the main vapor species over the solid CeO2 . Ba, BaO, CeO, CeO2 and BaCeO3 species were found in the vapor over the BaO-CeO2 mixture. On the basis of the equilibrium constant of the gaseous reaction BaO + CeO2  = BaCeO3 , the standard formation enthalpy of gaseous BaCeO3 (-1065 ± 25 kJ/mol) at 298  K was determined. Energetically favorable structures of gaseous CeO2 and BaCeO3 were found and vibrational frequencies were evaluated in the harmonic approximation.


CONCLUSIONS
The stability of BaCeO3 gaseous species was confirmed by high-temperature mass spectrometry. Gas-phase reactions involving gaseous barium and cerium oxides with gaseous barium cerate were studied. The enthalpies of the formation reactions of gaseous barium cerate from gaseous BaO and CeO2 were evaluated theoretically and the obtained values were in agreement with the experimental ones. Copyright © 2016 John Wiley & Sons, Ltd.