Insights into the Mechanism of Ammonia Decomposition on Molybdenum Nitrides Based on DFT Studies

Ammonia can be used to produce carbon-free hydrogen for fuel cells and is considered as a key player in a noncarbon economy. In the present study, we investigated ammonia decomposition on Mo2N(100) and (111) surfaces using density functional theory calculations. The stepwise dissociation of ammonia over the surface Mo atom on Mo2N(100) has activation barriers >1.0 eV for all three N-H bonds. In contrast, the activation barriers for dissociating the first and second N-H bond in 3-fold Mo sites on the (111) surface are only 0.64 and 0.22 eV, respectively, whereas the activation barrier for breaking the last N-H bond is 1.12 eV. Coupling of NHx species with intrinsic surface N atoms does not always promote N-H bond activation but the formation of NlatNHx species needs to overcome a high activation barrier. Consequently, N-N coupling is not expected to have significant contributions to overall ammonia dissociation. Our results also showed that recombinative desorption of N atoms involves the formation of highly activated meta-stable dinitrogen species and its subsequent desorption as the N-2 molecule. The overall ammonia decomposition is limited by this recombinative desorption process, with an overall energy cost of similar to 2.61 eV on Mo2N(100) and similar to 1.29 eV on Mo2N(111). On both surfaces, the intrinsic surface N atoms of the nitride actively participate in the formation of desorbed nitrogen, making the process follow a Mars-van Krevelen mechanism.