Reactions of cationic iron clusters with ammonia, models of nitrogen hydrogenation and dehydrogenation

The gas phase reactivities of small cationic iron clusters, Fe(n)(+) (n = 1-20), towards ammonia were investigated using Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Sequential addition of ammonia molecules to the clusters was observed to be the dominating process for n > 4. In the case of n = 4 we observed addition of ammonia accompanied by dehydrogenation. This reaction was modelled using hybrid density functional theory. Clusters with n < 4 do not react with ammonia. Clusters Fe(n)(+) (n = 1-20) react with neither N(2) nor H(2) at around 10(-8) mbar. When dinitrogen was seeded into the expanding helium, mixed clusters of the type Fe(n)N(m)(+) were observed. These ions react with H(2),either by addition, or by substitution of N(2). The clusters with m = 1 were isolated in separate experiments and reacted with H(2), which showed that mixed clusters with n = 5-13 add up to 5 molecules of dihydrogen in successive slow reactions.