The increase in ammonia synthesis activity due to an increase in the oxygen content of CaH2 can be explained as follows. During the reaction, CaH2 became CaH2-x(e -)x , resulting in intense electron donation to Ru . Too much electron donation can induce more severe hydrogen poisoning . The oxygen in Ru/CaH2-2xOx weakened the electron donation and facilitated the release of hydrogen, as well as the previous report that the electronegative element, F facilitates the release of H in CaFH . Therefore, the introduction of oxygen improved hydrogen poisoning and enhanced activity. It is evidenced by the larger reaction order of hydrogen as the oxygen content increased in Ru/CaH2-2xOx. However, excessive oxygen caused the lower electron donation and decreased the activity found in Ru/CaH1.12O0.44, indicated by the high app Ea. It matches that the N2 order is high, and N2 dissociation is more difficult when the oxygen content is x = 0.44. Therefore, Ru/CaH2-2xOx had the plateau of catalytic activity against the oxygen amount. When we focus on the XRD patterns around 2θ = 27-33°, the CaH2-derived peak shifts to a wider angle with increasing CaO content (Figure , Figure ). The plausible explanations are given below. The increase in CaO means an increase in electrophilic O relative to CaH2: O prefers electrons rather than H. Therefore, the increase in CaO weakens the ionic bonds between Ca 2+ and H -, which facilitates the replacement of H -in CaH2 by O 2-and the release of hydrogen. The defects in the crystal structure due to the release of hydrogen cause Ca to attract H more strongly, shortening the bond lengths and shrinking the crystal lattice, and the CaH2-derived peak shifts to the wide-angle (Figure , Figure ). When the oxygen content exceeds x = 0.15, the peak conversely shifts to a narrower angle because the oxygen itself expands the unit lattice. The introduction of oxygen to Ru/CaH2 enhanced the catalytic activity, in the same way that Ru/MH3 (M = La, Ce) was activated by oxygen in the previous study . In Ru/CaH2-2xOx, the CaH2-derived unit lattice shrank; in Ru/MH3-2xOx the MH3-derived unit lattice expanded. These differences in unit lattice when oxygen is introduced can be explained as follows. MH3 is in the same space group (Fm3 ̅ m) as MH2, but the lattice is larger for the dihydrides than for trihydrides. Therefore, in the case of Ru/LaH3-2xOx and Ru/CeH3-2xOx, the crystal lattice is more extensive as the hydrogen is more easily released by oxygen. Because of this different behavior of crystal lattice stretching, the origin of activity increased by oxygen in Ru/LaH3-2xOx and Ru/CeH3-2xOx can be different from the mechanism in Ru/CaH2-xO2.