Gas-Phase Reactions of Copper Oxide Cluster Cations with Ammonia: Selective Catalytic Oxidation to Nitrogen and Water...

Reactions of copper oxide cluster cations, Cu _nO _m⁺ ( n = 3-7; m ≤ 5), with ammonia, NH₃, are studied at near thermal energies using a guided ion beam tandem mass spectrometer. The single-collision reactions of specific clusters such as Cu₄O₂⁺, Cu₅O₃⁺, Cu₆O₃⁺, Cu₇O₃⁺, and Cu₇O₄⁺ give rise to the release of H₂O after NH₃ adsorption efficiently and result in the formation of Cu _nO _m-1NH⁺. These Cu _nO _m⁺ clusters commonly have Cu average oxidation numbers of 1.0-1.4. On the other hand, the formation of Cu _nO _m-1H₂⁺, i.e., the release of HNO, is dominantly observed for Cu₇O₅⁺ with a higher Cu oxidation number. Density functional theory calculations are performed for the reaction Cu₅O₃⁺ + NH₃ → Cu₅O₂NH⁺ + H₂O as a typical example of H₂O release. The calculations show that this reaction occurs almost thermoneutrally, consistent with the experimental observation. Further, our experimental studies indicate that the multiple-collision reactions of Cu₅O₃⁺ and Cu₇O₄⁺ with NH₃ lead to the production of Cu₅⁺ and Cu₇O⁺, respectively. This suggests that the desirable NH₃ oxidation to N₂ and H₂O proceeds on these clusters.