In comparison to Ru catalysts supported on activated carbon (AC), MgO, Al2O3, and TiO2 under similar reaction conditions, Ru catalyst using carbon nanotubes as support (Ru/CNTs) was found to show high NH3 conversion for the generation of COx-free hydrogen from NH3. The modification of the catalyst with KOH leads to a significant improvement in activity, either in term of NH3 conversion or TOE The Ru/CNTs catalyst prepared from acetone solvent is more active than that from water solvent, and the use of the former leads to an increase in Ru dispersion and a decrease in residual Cl concentration. The results of H-2-TPD and HRTEM characterization disclosed that the excellent catalytic activity of Ru/CNTs could be ascribed to the high dispersion of Ru particles, and to the high graphitization and high purity of the CNTs material. The residual Cl originated from the RuCl3 precursor was found to be a strong inhibitor for the decomposition reaction. Once the residual Cl is in the form of KCl, it is difficult to remove. The results of N-2-TPD studies imply that the desorption of N-2 is the rate-determining step, and the modification with KOH leads to a decrease in N-2 desorption temperature and in apparent activation energy of NH3 decomposition over Ru/CNTs. (C) 2004 Elsevier B.V. All rights reserved.