Ru powder (< 0.3 µm, < 95%) was purchased from Tokuriki Honten Co. K/Ru was prepared using KNO3 aqueous solution (KNO3; 99.9%, Fujifilm Wako Pure Chemical) so that the molar ratio of K to Ru was 1:1. K/Ru was held in a vacuum at 200°C for 4 hours to evaporate water and NO2 completely.