Adsorption and decomposition of NH3 on Ir(111): A density functional theory study

The adsorption and decomposition of NH3 on Ir(111) have been studied using the density functional theory calculations. The recombinative nitrogen desorption has also been investigated. The configurations and stability of NHx(x = 0-3) species have been performed using frequency analysis. The corresponding reaction energies, the activation energies and the structure of the transition states have been determined and analyzed in detail. Including the zero point energy correction, the calculated activation barrier energy for NHx(x = 1-3) dehydrogenation is between 0.94 eV and 1.05 eV, and that for the recombination desorption of N2 is 1.55 eV, which indicates that the N2 recombinative desorption is the rate-limiting step for the NH3 decomposition on Ir(111). The NH3 desorption energy (0.82 eV) is lower than the NH3 dehydrogenation barrier, which indicates that the ammonia rather desorbs than dissociates from a thermodynamic point of view, consistent with the experimental results. But the competition between desorption and dissociation can be controlled in practice via the applied pressure and temperature.