Thermal N-H Bond Activation on Anionic and Cationic Platinum Clusters: Non-Predetermined Reaction Pathways Indicate Transitions to a Bulk Surface Reactivity

Reactions of cationic and anionic platinum clusters Pt-n(+/-), n=1-5, with NH3 are studied by FT-ICR mass spectrometry and DFT calculations. With cationic clusters, radiative association of an intact NH3 is the dominant reaction channel. On anionic clusters, NH3 undergoes reductive elimination of molecular hydrogen, with nitride and hydride bound at remote sites of the cluster. Nitride assumes a bridge position, while hydride is bound atop a single platinum atom. On the [Pt-n,NH3](-) potential energy surface, for n=4 fifteen local minima and connecting transition states were identified with relevance to the hydrogen formation reaction, and 12 local minima were identified for n=5. These potential energy surfaces offer a rich variety of pathways, illustrating a key feature of a successful bulk catalyst: The variety of pathways helps the catalyst to work over a wide range of temperatures and pressures.