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

Reactions of copper oxide cluster cations, Cu n O m + ( n = 3–7; m ≤ 5), with ammonia, NH 3, are studied at near thermal energies using a guided ion beam tandem mass spectrometer. The single-collision reactions of specific clusters such as Cu 4 O 2 +, Cu 5 O 3 +, Cu 6 O 3 +, Cu 7 O 3 +, and Cu 7 O 4 + give rise to the release of H 2 O after NH 3 adsorption efficiently and result in the formation of Cu n O m –1 NH + . These Cu n O m + clusters commonly have Cu average oxidation numbers of 1.0–1.4. On the other hand, the formation of Cu n O m –1 H 2 +, i.e., the release of HNO, is dominantly observed for Cu 7 O 5 + with a higher Cu oxidation number. Density functional theory calculations are performed for the reaction Cu 5 O 3 + + NH 3 → Cu 5 O 2 NH + + H 2 O as a typical example of H 2 O 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 Cu 5 O 3 + and Cu 7 O 4 + with NH 3 lead to the production of Cu 5 + and Cu 7 O +, respectively. This suggests that the desirable NH 3 oxidation to N 2 and H 2 O proceeds on these clusters.