Amorphous superbasic ZrO2-KOH of high-surface-area was prepared by reflux-digestion of ZrO(OH)(2) gel in aqueous KOH (0.5 M), followed by calcination at 873 K. Incipient wetness impregnation of the high-surface-area superbasic ZrO2-KOH with Ru acetylacetonate [Ru(acac)(3)] and RuCl3 (dissolved in ethanol), respectively, leads to nanosized Ru (2-7 nm) catalysts. We found that the Ru catalyst performs better when the Cl-free Ru precursor is adopted. Due to the high surface area and superbasicity of the support material, the Cl-free Ru/ZrO2-KOH catalyst is highly effective for the generation of COx-free hydrogen from NH3 decomposition. The high dispersion of Ru is related to the high surface area of ZrO2 and to the presence of K+. We deduce that the presence of Cl has direct as well as indirect (by weakening the promotional effect of KOH) effects on the electronic state of Ru nanoparticles. The apparent activation energy of ammonia decomposition on Cl-free Ru supported on the superbasic ZrO2-KOH is higher than that on conventional ZrO2. N-2-TPD results disclosed that the superbasicity of the support material favors the combinative desorption of surface N atoms, a step generally regarded as the rate-determining in catalytic cycle of ammonia decomposition. (c) 2006 Elsevier B.V. All rights reserved.