**4. Conclusions**

This study demonstrated that stable CeO2 nanoparticles were deposited on MgO by the simple impregnation method. When CeO2/MgO was prepared under the dry atmosphere, the CeO2 nanoparticles remained ~3 nm in diameter even after being heated at 800 ◦C, which is much smaller than ~5 nm of CeO2/MgO prepared under ambient air. The difference was attributed to the higher surface area of the catalysts prepared under the dry atmosphere. H2-TPR, CO2-TPO, XPS, and in situ XRD showed that CeO2/MgO(dry) exhibited higher oxygen mobility than CeO2/MgO(air) due to the higher dispersion of CeO2 nanoparticles. The higher catalytic activity of CeO2/MgO(dry) than CeO2/MgO(air) was also demonstrated by the dry reforming reaction.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/ 10.3390/en14237922/s1, Table S1: Chemical composition of CeO2 estimated by X-ray fluorescence, Figure S1: Scanning Transmission Electron Microscope images (a,d) and corresponding energy dispersive X-ray spectroscopy mapping of Ce Lα (b,e) and Mg Kα (c,f) for Ce0.01Mg0.99O1.01 (a–c), Ce0.05Mg0.95O1.05 (d–f), Table S2: Volume-weighted average diameter of CeO2 nanoparticles estimated from TEM and XRD results, Figure S2: TEM images of (above) MgO(air) and (bottom) MgO(dry), Figure S3: H2-TPR profile of MgO(air), Figure S4: XRD patterns of CeO2 during in situ XRD measurement, Figure S5: The first derivative of a0/a0 shown in Figure 8 of the main manuscript, Figure S6: Time course profiles of the CH4 conversion rates during dry reforming reactions, Figure S7. Time course profiles of the outlet gas composition during dry reforming reactions, Table S3: Compositions of the inlet and outlet gases during the dry reforming reaction.

**Author Contributions:** Conceptualization, K.T.; methodology, K.T.; validation, K.T. and R.M.; formal analysis, K.T. and R.M.; investigation, K.T. and R.M.; resources, K.T.; data curation, K.T.; writing— original draft preparation, K.T.; writing—review and editing, K.T.; visualization, K.T.; supervision, K.T.; project administration, K.T.; funding acquisition, K.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** I would like to show my appreciation to Hiroshima and Tani from Nippon Steel Corporation for the maintenance and troubleshooting of TEM measurements.

**Conflicts of Interest:** The authors declare no conflict of interest.
