Abstract
N2 dissociation on a selection of rare earth (RE) elements (Nd, Sm, Eu, Yb) is studied to better understand a recent experimental observation that some rare earth nitrides (RENs) are able to form at ambient temperature and pressure in the presence of N2 gas. This observation implies facile dissociation of N2 and, therefore, points to RE elements as potential low temperature ammonia synthesis catalysts. In the present work, the mechanism and energetics of N2 dissociation on these materials is probed with computational techniques to better understand this experimental observation.
The DFT+U method was utilised due to the strongly correlated 4f electrons in the REs. Systematic study of the effects of varying the magnitude of the Hubbard U correction to select the optimum value for each of the materials and properties of interest was undertaken.
The study of the mechanism of N2 dissociation on the RE surfaces was initiated from the widely-accepted mechanism utilised by transition metals. However, the calculations showed that the adsorbed species had a propensity to partially absorb into the surfaces, such that the transition metal mechanism is unlikely operative on these materials.
Thereafter, initial and final states required for N2 dissociation on both stepped and flat RE surfaces were investigated. It is found that when N2 adsorbs on a RE surface, the stepped feature is important, as evidenced by stronger binding of N2 near the step compared to the flat surface for all of the REs. However, the different REs utilised the step in a number of different ways, with stable configurations of adsorbed N2 varying between REs. Final states were found to be particularly favourable when atomic N was absorbed in the first subsurface layer, for all REs. Several particularly interesting pairs of initial and final states were identified that could possibly lead to low-energy dissociation paths. However, further study is required to elucidate these pathways. The partial absorption of N2 and N into the surface supports the experimental observations of REs being a promising new class of low temperature ammonia synthesis catalysts.