Thermodynamics of gaseous strontium and calcium cerates studied by Knudsen effusion mass spectrometry and estimation of relative electron ionization cross-section for CeO2(g)

Abstract

Rationale

Ceria-based systems are of great interest because of their unique properties. Such systems may be used as anode materials for SOFCs or in oxygen sensors. The exploitation of these materials often requires high temperatures. In such conditions, the partial or complete evaporation of materials is possible. Therefore, knowledge of the values of partial pressures and thermodynamic properties is essential to predict and/or prevent possible consequences and accidents.

Methods

Knudsen effusion mass spectrometry was used to determine partial pressures of vapor species over the SrO–CeO₂ and CaO-CeO₂ systems. Measurements of partial pressures were performed with a MS-1301 mass spectrometer. Vaporization was carried out using molybdenum or tungsten effusion cells. A theoretical study of gaseous strontium and calcium cerates was performed by several quantum chemical methods: density functional theory (DFT) M06, DFT PBE0, and MP2.

Results

The minimum value of relative electron ionization cross-section for CeO₂ was estimated. In the temperature range of 2218–2249 K above the SrO-CeO₂ system and of 2128–2208 K above the CaO-CeO₂ system, gaseous M, MO, MO, CeO₂, O, O₂ and MCeO₃ (M = Sr, Ca) were found. Energetically favorable structures of gaseous SrCeO₃ and CaCeO₃ were found, and vibrational frequencies were evaluated in the “rigid rotor-harmonic oscillator” approximation. On the basis of the equilibrium constants of gaseous reaction MO + CeO₂ = MCeO₃, the standard formation enthalpy of gaseous SrCeO₃ (−913 ± 26 kJ/mol) and of gaseous CaCeO₃ (−917 ± 26 kJ/mol) at 298 K were determined.

Conclusions

The estimated value of relative electron ionization cross-section for CeO₂ is in agreement with the general rule applicable for dioxides. The stability of SrCeO₃ and CaCeO₃ gaseous species was confirmed by KEMS. Gas-phase reactions involving gaseous SrO (CaO) and CeO₂ with gaseous SrCeO₃(CaCeO₃) were studied. Enthalpy of formation reaction of gaseous SrCeO₃(CaCeO₃) from gaseous SrO (CaO) were evaluated theoretically, and the obtained value is in agreement with the experimental one.