**1. Introduction**

Climate change caused by global warming has emerged as a problem worldwide owing to the increase in greenhouse gas (GHG) emissions. Carbon dioxide (CO2) emissions account for more than 90% of global GHG emissions [1,2]. According to the Intergovernmental Panel on Climate Change report in 2018, CO2 emissions were estimated to be at 32 and 40 billion tons in 2010 and 2020, respectively [3]. Carbon capture and storage technologies have been developed to capture CO2 emissions, especially those from power plants [4–6]. However, CO2 storage is vulnerable to earthquakes and can cause pollution and is, therefore, recognized as only a temporary method [7]. Therefore, there is an urgent need to develop and implement techniques for utilizing captured CO2. For example, CO2 reforming of methane has been proposed; however, it has high cost and energy requirements [8,9].

One of the treatment methods for captured CO2 is catalytic decomposition using oxygen-deficient metal oxides. Tamauara et al. reported that oxygen-deficient magnetite (Fe3+<sup>δ</sup>O4, δ=0.127) decomposed up to 100% of CO2 and H2O at 290 ◦C [10]. Subsequently, CO2 decomposition using ferrites with divalent metals such as Ni2<sup>+</sup> and Cu2<sup>+</sup> was investigated [11]. Mn2<sup>+</sup>- and Zn2+-based ferrites were also reported as having CO2 decomposition efficiencies of 66% and 90%, respectively [12,13]. Even trivalent, Ni-Cu ferrites were tested for an identical purpose [14]. It was reported that nickel and copper substitutions at the A-site of ferrites were the most beneficial for reduction and oxidation reactions and demonstrated meaningful results. However, all these approaches are difficult to apply in practical applications because the experimental data were obtained from a small stagnant batch-type reactor.

The first attempt to go beyond the batch system was made in 2001. Shin et al. reported CO2 decomposition data obtained through thermogravimetric analysis using activated CuFe2O4 [11]. Furthermore, Kim et al. investigated CO2 decomposition using activated Ni0.5Zn0.5Fe2O4−<sup>δ</sup> in a continuous flow of 10% CO2-balanced N2 [15,16]. They reported that trivalent ferrites (i.e., (NixZn1−x)Fe2O4, x = 0.3, 0.5, 0.7, and 1) showed a higher CO2 decomposition efficiency than divalent NiFe2O4 ferrite. They ascertained that the ferrites could completely decompose 10% CO2 for 5 to 7 min. They also asserted that Ni/Zn-ferrite synthesized by the hydrothermal method displayed better CO2 decomposition performance than that synthesized by the coprecipitation method. However, they did not perform a blank test and quantitative analysis. Although their results were elementary and had some weaknesses, their trials were invaluable in that they can be applied in practical applications. Therefore, the accumulated data of CO2 decomposition in a continuous system should be obtained for realizing economically efficient CO2 treatment.

In our previous work [17], we demonstrated the possibility of continuous CO2 decomposition by using oxygen-deficient metal oxides and suggested its reaction mechanism. Compared to Ni-ferrites, a nonperovskite-type metal oxide (i.e., SrFeCo0.5Ox) was much more effective for CO2 decomposition: Ni-ferrites decomposed only up to 20% of CO2, whereas SrFeCo0.5Ox displayed a CO2 decomposition efficiency of up to 90%. These results were obtained based on our suggested mechanism that high electrical and ionic conductivities affect CO2 decomposition. Currently, the obtainment of suitable isothermal and regeneration data will be more helpful for practical applications. In our ongoing research project, we have found that another material, SrFeO3−δ, shows greater promise for this purpose.

Originally, SrFeO3−<sup>δ</sup> was used as an oxygen transport material [18–23] and as a catalyst for methane combustion and chemical looping processes [24,25]. Perovskite-type SrFeO3−<sup>δ</sup> (0 ≤ δ ≤ 0.5) is a nonstoichiometric metal oxide containing Fe ions in a mixed valence, such as Fe4<sup>+</sup> and Fe3<sup>+</sup> [26]. Under reducing conditions, SrFeO3−<sup>δ</sup> produces oxygen vacancies; the number of oxygen vacancies depends on the temperature and the oxygen partial pressure [27]. Recently, Marek et al. reported the stable use of SrFeO3−<sup>δ</sup> in chemical looping systems; the material reduced above δ = 0.5 could be reoxidized with either CO2 or air, resulting in SrFeO3−<sup>δ</sup> (0 ≤ δ ≤ 0.5) [25]. Therefore, we consider it a promising material for CO2 decomposition. Several studies have reported on the use of SrFeO3−<sup>δ</sup> in various fields. However, no study has reported the use of SrFeO3−<sup>δ</sup> for CO2 decomposition in a continuous-flow system. In this report, we describe the reduction behavior and redox reaction of SrFeO3−δ. Furthermore, through cyclic experiments, we demonstrate that it exhibits consistently high CO2 decomposition performance under isothermal conditions. We also demonstrate its structural stability as a catalytic material for practical applications.

## **2. Results**

#### *2.1. Characterization*

The crystal structure of SrFeO3−<sup>δ</sup> was analyzed by X-ray powder diffraction (XRD) at 40 kV and 200 mA. The XRD powder patterns of the samples were obtained in 0.02◦ steps over the range of 20◦ ≤ 2θ ≤ 80◦. It has been reported that the structure of SrFeO3−<sup>δ</sup> could be changed susceptibly by δ values, namely, cubic at δ = 0–0.12, tetragonal at δ = 0.16–0.24, and orthorhombic at δ = 0.25 [28,29]. The lattice constant of SrFeO3−<sup>δ</sup> obtained from XRD data was a=5.479(5) Å, b=7.729(8) Å, c = 5.521(2) Å, and V = 233.8(5) Å3. It was determined to be an orthorhombic perovskite, which is in good agreement with the reported value (PDF# 01-077-9154). Figure 1a shows XRD powder patterns of as-synthesized SrFeO3−δ. We also obtained secondary electron images. These are discussed with those measured

after CO2 decomposition tests at the end of this section. The chemical composition was reasonably acceptable, and the surface area of SrFeO3−<sup>δ</sup> was determined to be 3.19 m2/g.

**Figure 1.** In-situ XRD results of SrFeO3−δ: (**a**) In-situ XRD powder pattern and (**b**) unit cell volume at 500 ≤ T ≤ 800 ◦C. The symbols indicate Fe metal (x) and brownmillerite (o).
