**1. Introduction**

CeO2 has been used as catalysts and additives due to its high oxygen storage capacity and moderate basicity [1]. The inherent nature of CeO2 makes it attractive for chemical reactions involving oxygen removal and addition. The transformation of renewable energy to fuel gases often employs CeO2 in the process. The addition of CeO2 to catalysts suppresses the carbon deposition on the catalysts during the steam-reforming reactions of CH4 and tar to form syngas [2,3]. CeO2 also improves the resistance of Ni catalysts against H2S during methanation reaction [4] and reforming reaction [3,5] by accelerating the removal of sulfur from the surface of active species. Chemical looping reaction and thermochemical reaction also proceed over CeO2 [6–9]. Despite its various applications, it is preferable to reduce the total consumption of CeO2. The supply and price of CeO2 are volatile. Further, the environmental load during the mining and processing of rare earth elements is higher than those of base metals [10,11]. It is desired to use less CeO2 to make the catalysts, as well as the process, sustainable.

Various techniques have been employed to reduce the consumption of CeO2 by controlling its size and shape [12–14]. Fine CeO2 nanoparticles of high surface areas are prepared by surfactant-assisted methods [15,16]. The size of CeO2 particles also influences the oxygen mobility on the CeO2 surface [17,18]. The lattice constant of CeO2 increases as the size of nanoparticles decreased, leading oxygen vacancies to be more stable than on the coarse CeO2 particles [13,19,20]. The oxygen mobility on the CeO2 surface was also influenced by the plane indices of the surface. Among the low-index surfaces, CeO2(110) exhibits the highest reducibility while CeO2(111) is the least reducible [21–23]. The difference was attributed to the surface energy of the surface [24,25]. CeO2(111) is more stable than CeO2(110) and CeO2(100) because it is the non-polar, close-packed surface of CeO2. Meanwhile, polar CeO2(100) and less-densely packed CeO2(110) surfaces underwent reconstruction of their surface, forming facets containing reduced CeO2-x surface [12]. Such reconstruction of the

surface leads them to be more reducible than CeO2(111). Reportedly, nanorod composed of CeO2(110) surface and cubic CeO2 nanoparticles composed of CeO2(100) surface exhibited higher catalytic activity than conventional CeO2 nanoparticles mostly composed of CeO2(111) surface due to higher oxygen mobility on CeO2(110) and CeO2(100) surfaces at low temperatures [21–23]. However, such self-standing CeO2 particles sinter to ~20 m2/g or less, corresponding to 50 nm in diameter, after calcined at high temperatures >800 °C. Both nanorods and cubic CeO2 nanoparticles transform into conventional CeO2 nanoparticles after being exposed to such high temperatures. Al2O3 supports are widely employed to improve the thermal stability of CeO2 nanoparticles. CeO2 nanoparticles on Al2O3 remain ~10 nm in diameter after the heat treatment at >800 °C under the oxidizing atmosphere [26,27]. Contrary, CeO2 and Al2O3 react to form CeAlO3 and other oxides under reducing conditions [28–30]. Such reactions cause structural changes in the catalysts and sintering of CeO2 nanoparticles. High-temperature tolerant CeO2 nanoparticles are necessary to further reduce the consumption of CeO2 under reducing conditions.

Some studies employ MgO as a support to disperse CeO2 nanoparticles [31–34]. The melting point of MgO is ~2850 ◦C, which is higher than that of Al2O3 supports [31]. MgO is also known for its stability under reducing conditions [35]. Further, CeO2 nanoparticles are stabilized on MgO supports without forming any composite oxides [31–34,36]. Partial oxidation of CH4 was demonstrated over CeO2/MgO catalysts, suggesting their stability under the reducing conditions [34,37]. However, the average diameter of CeO2 increased to >5 nm after the calcination at 800 ◦C. Techniques to stabilize finer CeO2 nanoparticles are necessary to further reduce the consumption of CeO2.

MgO support of large surface area would contribute to stabilized CeO2 nanoparticles. Meanwhile, the stability of MgO depends on the atmosphere to which MgO is subjected [38–42]; relative surface energies of low-index surfaces of MgO vary depending on the humidity. Polar close-packed MgO(111) surface is stabilized by hydroxylation independent of the humidity [38]. On the contrary, non-polar MgO(100) is terminated by OH only under humid conditions since the OH-termination does not contribute to the stabilization as it is on the polar surface. Under humid conditions, therefore, OH-terminated MgO(111) is more stable than OH-terminated or pristine MgO(100). Meanwhile, pristine MgO(100) is more stable than OH-terminated MgO(111) under dry conditions [38]. Further, the sintering of MgO is accelerated in the presence of water vapor [39–42]. These reports sugges<sup>t</sup> that the morphology of MgO depends on the humidity of the atmosphere under which the MgO is prepared. These studies imply that the humidity can influence the catalytic activity of the catalysts containing MgO. However, no research assessed the effect of humidity on the catalytic activity and morphology of CeO2/MgO.

This research demonstrates that CeO2/MgO catalysts of fine CeO2 nanoparticles by controlling the atmosphere during the preparation. Calcination in dry air realized CeO2 nanoparticles smaller than 3 nm in diameter even after heating at 800 °C. The prepared CeO2/MgO catalyst outperformed pure CeO2 for dry reforming reaction although the mass ratio of CeO2 in CeO2/MgO was less than 1/5 of pure CeO2.

### **2. Materials and Methods**
