The superbasic ZrO2 (ZrO2-KOH) with high surface area was prepared by reflux-digestion of ZrO(OH)(2) gel in mother liquor and examined as a support promoting ammonia synthesis by particulate Ru. The TEM image of Ru/ZrO2-KOH catalyst indicates that the size of Ru particles is in the range of 1.5-3.2 nm, which is the optimum size of Ru particles for the maximum number of B-5-type sites. Additionally, the characterizations including CO2-TPD, XPS and N-2-TPD demonstrate that the Ru particles supported on ZrO2-KOH exhibit higher electron density compared to that of Ru/ZrO2 catalyst. As a result, the dissociative adsorption of N-2 over Ru/ZrO2-KOH is more facile than that over Ru/ZrO2 catalyst. Thus, the Ru/ZrO2-KOH catalyst shows an excellent activity for ammonia synthesis especially under the conditions of low temperature and pressure. The highest reaction rate of Ru/ZrO2-KOH is 16.91 mmol/(g h) at 698 K and 3.0 MPa, which is much higher than that of various Ru catalysts supported on other supports. Crown Copyright (c) 2013 Published by Elsevier B.V. All rights reserved.