The mechanisms of hydrazine decompositions on Ir(1 1 1) have been investigated by using slab model based on periodic density functional theory (DFT). In order to shed light on the elementary radical reaction processes of hydrazine decomposition on Ir-based catalysts, three possible reaction pathways are considered. Through computational modeling we have investigated the adsorption characteristics, geometrical structures, activation energies, and reaction mechanisms. The initial reactants, transition states, and final products of each elementary step and various likely intermediates are discussed. We have found that the main reaction channel with relatively low energy barriers is the following: the thermal decomposition of hydrazine forms two NH(2) radicals, which attack an adjacent adsorbed hydrazine molecule or subsequent N(2)H(x) (x = 1-3) species and capture the H atoms step by step, finally leading to the formation of N(2) and NH(3) products. We show that the rate-determining step involves NH(2) interacting with a N(2)H species, with an energy barrier of 0.63 eV (or 14.5 kcal/mol). The overall reaction channel releases a large amount of thermal energies. The decomposition of hydrazine on Ir surfaces is therefore both thermodynamically and kinetically favorable. The other reaction channels investigated have much higher activation barriers with Ir catalysts. (c) 2011 Elsevier B.V. All rights reserved.