Insights into the Redox Behavior of Pr0.5Ba0.5MnO3−δ-Derived Perovskites for CO2 Valorization Technologies

In situ temperature-programmed (TP) analyses in a multianalytical approach including X-ray diffractometry (XRD), temperature-programmed reduction (TPR), thermogravimetry (TGA), near-edge X-ray absorption fine structure spectroscopy (NEXAFS) are used to study the relationship between redox properties and structural changes in Pr0.5Ba0.5MnO3−δ (m-PBM), PrBaMn2O5+δ (r-PBM), and PrBaMn2O6−δ (o-PBM) when exposed to reduction/oxidation cycles. TP-XRD analysis shows that under reducing conditions, between 300 and 850 °C, the biphase perovskite m-PBM turns into the monolayered perovskite r-PBM. Stabilization of the latter phase at room temperature requires early oxidation in air at a high temperature (850 °C) to avoid segregation, resulting in the formation of the oxidized layered phase (o-PBM). The o-PBM layered perovskite is characterized by the H2-TPR profile, showing two reduction peaks at temperatures below 500 °C. TP-NEXAFS characterization reveals the copresence of Mn(IV) (60%), Mn(III) (30%), and Mn(II) (10%) and helps to interpret the reduction profile: Mn(IV) converts to Mn(III) at ∼300 °C (I pk), Mn(III) to Mn(II) at ∼450 °C (II pk). The TGA characterization confirms the reversibility of the o-PBM ↔ r-PBM process at 800 °C; in addition, it shows that the r-PBM can be oxidized almost completely (∼99%) also by CO2 without accumulation of carbonates. This study sheds light on the peculiar redox behavior of PBM-based materials and paves the way for their application as oxygen carriers and catalytic promoters in different CO2 enhancement technologies. Here, we discuss the results obtained to develop versatile and redox-resistant electrodes for solid oxide electrochemical cell/solid oxide fuel cell applications.