Low Temperature Synthesis of NH₃ from Atomic N and H at the Surfaces of FeS₂{100} Crystals

We have investigated the interaction of H 2 and NH 3 with {100} surfaces of iron pyrite (FeS 2 ) in ultrahigh vacuum (UHV) and the synthesis of NH 3 from the reaction of preadsorbed atomic N with atomic H, the H either preadsorbed on or arriving at the surface, all at surface temperatures of 170 K or above. Hydrogen adsorbs and reacts only when it arrives at the surface as atomic H, after cracking at a hot filament. Recombinative desorption of H 2 occurs with a peak temperature of 225 K and is accompanied by desorption of H 2 S, formed by surface reaction of H ads with S, with a peak temperature of 260 K. NH 3 adsorbs molecularly from the gas phase, albeit with some degree of partial dissociation, and desorbs with a peak temperature of 215 K. NH 3 can be synthesized when the surface, after pre-exposure to activated nitrogen, is exposed to atomic H. The synthesized NH 3 desorbs with a peak temperature of 230 K, demonstrating that all hydrogenation steps are feasible under these conditions; the process appears to be desorption-limited. The results are discussed in comparison with both industrial Haber-Bosch NH 3 synthesis and natural NH 3 synthesis by the enzyme nitrogenase. The potential significance of the low-temperature hydrogenation and desorption, in the context of a possible bioinspired approach toward ammonia synthesis, is outlined.