*2.2. Oxygen-Deficient SrFeO3*−<sup>δ</sup>

As discussed in our previous report [17], sample activation is an essential step to decompose CO2. An understanding of the reduction behavior is also needed because CO2 decomposition could mainly be affected by the number of oxygen vacancies and their mobility. CO2 decomposition is induced by the incorporation of O2<sup>−</sup> into oxygen vacancies. In-situ XRD was performed during high-temperature reduction with 3.5 vol% H2/Ar to identify structural changes occurring in the metal oxide. Figure 1 includes the in-situ X-ray powder patterns of SrFeO3−<sup>δ</sup> obtained at 500 ≤ T ≤ 800 ◦C.

SrFeO3−<sup>δ</sup> was observed in the perovskite phase at room temperature, and the pattern was very similar to that of SrFeO2.75 (PDF# 01-077-9154). As the temperature increased, the perovskite would lose more oxygen and change to a brownmillerite phase. Phase changes from SrFeO3−<sup>δ</sup> to SrFeO2.5 at 500 ◦C are attributed to the partial reduction of Fe4<sup>+</sup> to Fe3<sup>+</sup> [30]. An almost pure brownmillerite phase

(SrFeO2.5, PDF# 01-070-0836) was observed at 500–600 ◦C. The XRD pattern at 500 ◦C was completely indexed with an orthorhombic unit cell with lattice parameter a = 5.69(9) Å, b = 15.80(2) Å, c = 5.57(2) Å, and V = 501.8(0) Å3. This indicated that the cell volume increased by more than twice via the expansion of one side in the orthorhombic unit cell, especially the b-axis. Srn+1FenO3n<sup>+</sup><sup>1</sup> and Fe<sup>0</sup> peaks have been reported to appear when SrFeO2.5 was reduced further by increasing temperature and reaction time [25]. SrO and Fe<sup>0</sup> peaks are considered the final products of the reduction. In our patterns, only a trace of Fe metal (PDF# 01-080-3817) peaks appeared at 2<sup>θ</sup> <sup>≈</sup> 44.1◦ and 65.4◦ at <sup>≥</sup><sup>700</sup> ◦C. Typical Fe0 peaks could be observed at 2θ ≈ 44.0◦ and 65.3◦. The Sr3Fe2O6.14 phase might be possible; however, it overlaps with brownmillerite peaks. In addition, based on our thermogravimetric result (not shown), the change in the nonstoichiometric value (δ) was determined to be ~0.8 at 25 ◦C ≤ T ≤ 800 ◦C. Figure 1b shows the calculated unit cell volumes as a function of temperature. They demonstrate linearity over 600 ◦C [31]. It has been noted that SrFeO3−<sup>δ</sup> should be activated without complete structural collapse. If these phase changes are reversible, it would be beneficial for catalyst redox reactions or in a chemical looping system.
