*2.1. Catalyst Preparation*

MgO was prepared by the thermal decomposition of Mg (OH)2 (FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan, 0.07 μm, >99.9%). Although the thermal decomposition of Mg(OH)2 is a common technique to prepare MgO, this study employed ambient air conditions and dry air conditions to see the effect of humidity. The Mg(OH)2 was dried at 110 ◦C for 1 h and then calcined at 800 ◦C for 5 h in ambient air or under a dry gas flow of 20% O2 and N2 balance. The samples were heated at a heating rate of 4 ◦C /min. The flow rate of the dry gas was fixed at 100 cm3/min for 0.3 g of the Mg(OH)2. The O2 and N2 were supplied from gas cylinders (Taiyo Nippon Sanso Corporation, Tokyo, Japan, O2 > 99.99995%, N2 > 99.99995%). MgO supports prepared in ambient air and under the dry gas flow are denoted by MgO(air) and MgO(dry), respectively. CeO2 was

deposited on the MgO supports by impregnation. An acetone solution of Ce(NO3)3 6H2O (Kanto Kagaku, >98.5%) was added dropwise to the MgO supports. The samples were dried at 60 ◦C with continuous stirring in ambient air. Then, the samples of MgO(air) and MgO(dry) were calcined at 800 ◦C for 5 h in the ambient air and under the dry gas flow, respectively. The molar ratio of CeO2 to MgO was adjusted to 0.01/0.99 or 0.05/0.95 by changing the amount of Ce(NO3)3 6H2O used for impregnation. The obtained CeO2/MgO catalysts were denoted by 0.01- or 0.05-CeO2/MgO(air) and 0.01- or 0.05-CeO2/MgO(dry) corresponding to the content of CeO2 and the atmosphere for the heat treatments. A portion of the MgO was subjected to characterizations without the deposition of CeO2. A pure CeO2 catalyst was also prepared via a citrate method as a reference [43]. Aquatic solutions of the Ce(NO3)3 6H2O and citric acid monohydrate (Kanto Kagaku, Tokyo, Japan, >99.5%) were prepared separately and mixed into a solution. The molar ratio between Ce(NO3)3 6H2O and citric acid monohydrate was kept at 12 . The pH of the solution was adjusted to ~7.0 by adding an aquatic solution of ammonia (Kanto Kagaku, Tokyo, Japan, 28.0–30.0% as NH3) dropwise. After stirring continuously for 1 h at room temperature, the solution was dried on a rotary evaporator at 60 ◦C to become a viscous gel. The gel was further dried at 120 ◦C for 3 h and calcined at 800 ◦C for 5 h in ambient air. The high purity of the obtained CeO2 was confirmed by X-ray fluorescence (Table S1 can be find in the supplementary materials). Before the reaction, the obtained powdery catalysts were pelletized by a compression molding machine and then crushed to a particle size between 500 and 750 μm by stainless steel sieves.
