The catalytic reaction of iridium nanoparticles was carried out over gamma-alumina granules to investigate the factors controlling the catalytic activities. The evaluation of iridium nanoparticle activity in the laboratory reactor was done for support grains with different diameter between 1 mm to 4 mm. The characterization of three iridium catalysts was evaluated by XRD, FESEM and EDX before and after catalytic activity. The rate of hydrazine decomposition increased to 306 h-1 with the better distribution of nanoparticles, regardless of the grain size of catalyst support, whereas the hydrazine decomposition rate was larger for smaller supported catalyst. The probability of hydrazine monohydrate molecules in contact with active sites becomes higher and the movement of reactants and gas products will be easy inside and outside the pores, leading to the increase in reactivity. The rate of the hydrazine monohydrate decomposition and H2 selectivity decreased to 216 h-1 and 26% with the lower catalyst grains and the higher size of support along with worse distribution on the surface. The remarkable results of the results prove that support granule size is a dominant factor in the catalytic decomposition. پرونده مقاله