Figure shows the rate of ammonia synthesis over Ru/CaH2-2xOx at 0.1 MPa and 300°C. When the oxygen content was 29 mol% against Ca, the rate of ammonia synthesis reached a maximum and was about twice as high as that of 14 mol%. Too much oxygen, greater than 29 mol%, reduced the activity and raised the app Ea (Figure , Table ). Figure shows the XRD (Powder X-ray diffraction) patterns of Ru/CaH2-2xOx after ammonia synthesis. Ru-derived peaks were not identified, indicating Ru was loaded as amorphous. The catalyst Ru/CaH2 (x = 0.00) showed only CaH2-derived peaks and no CaO-derived peaks. The catalysts exposed to oxygen (x = 0.14-0.44) showed a stronger peak around 2𝜃 = 37.6° derived from CaO (2 0 0) as the oxygen content increased. The addition of oxygen shifts the CaH2-derived peak to the wide-angle side.