Evidence for dissolved hydrogen in the mixed ionic–electronic conducting perovskites La0.6Sr0.4FeO3−δ and SrTi0.7Fe0.3O3−δ

Abstract

Two mixed ionic–electronic conducting, Fe-containing perovskites were investigated regarding their reducibility in dry H₂, namely lanthanum strontium ferrite (LSF4, La_0.6Sr_0.4FeO_3−δ) and strontium titanium ferrite (STF3, SrTi_0.7Fe_0.3O_3−δ). Upon treatment under comparable reduction conditions, LSF4 is by far more affected by reduction and is reduced more deeply than STF3. Thermal treatments of fully oxidized or slightly reduced LSF4/STF3 at decreased O₂ partial pressure lead to spontaneous desorption of O₂. Temperature-programmed desorption (TPD) spectra of H₂ reveal distinct differences in H₂ and H₂O desorption. A simple mass balance of H₂ reveals that oxygen vacancies formed on STF3 are more resilient towards O₂ re-oxidation compared to those on LSF4. The results also imply that substantial amounts of hydrogen are dissolved in the bulk of LSF4 or STF3. 4.9 × 10⁻² mol H₂ per mol LSF4 and 1.6 × 10⁻² mol H₂ per mol STF3 are incorporated if the specimens are heated in flowing/dry H₂ up to 550 °C and 612 °C, respectively. For LSF4 this equals about 13 hypothetical ML of H₂ and for STF3 about 20 hypothetical ML of H₂. This conclusion is also supported by Fourier-transform infrared spectroscopy (FT-IR). FT-IR reveals water formation during static H₂ treatment of LSF4/STF3, which indicates perovskite reduction. Furthermore, both samples behave extraordinarily hydrophobic and no chemistry involving surface hydroxy groups was observed.