*3.2. CO2 Decomposition Experiments*

We used a continuous-flow reactor to investigate the products of the CO2 decomposition reaction in the tests. Sample activation and CO2 decomposition were performed with 3.5 vol% H2/N2 and 1 vol% CO2/He,respectively. Approximately 1.5 g of sample powder with zirconia balls (ϕ2–3 mm, 10 g) was placed at the center of a quartz tube (I.D.: 12 mm, O.D.: 16 mm, height: 600 mm). The flow rate for the sample activation and CO2 decomposition was 50 mL min<sup>−</sup>1. The CO2 decomposition experiments were performed under increasing temperature at 25 ≤ T ≤ 800 ◦C (nonisothermal). The ramp rate was 3 ◦C min<sup>−</sup>1. The same measurements were performed at certain fixed temperatures between 500 and 800 ◦C (isothermal). The residence time was calculated to be 3.393 s. CO2 decomposition cycle tests were also performed to check the regeneration, reproducibility, and stability of the sample. The temperature was selected with the results based on the isothermal CO2 decomposition results. Five cyclic tests under a fixed condition were performed at 700 ◦C. The blank tests were carried out under identical conditions by using only zirconia balls instead of the sample powder. Table 3 summarizes the experimental conditions for both activation and decomposition. The produced gas concentrations were measured with an Agilent 8890 gas chromatograph (GC, ShinCarbon ST 100/120 micropacked column, Bellefonte, PA, USA). The details of this experiment have been described in a previous report [27].
