Enhancement of Ammonia Dehydrogenation by Introduction of Oxygen onto Cobalt and Iron Cluster Cations

Anomalously Efficient Dehydrogenation of NH3 on Ir4+ and Ir5. J Phys Chem A 2022;126:4451-4455. [PMID: 35786880 DOI: 10.1021/acs.jpca.2c03316] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Gas-phase reactions of iridium cluster cations, Ir_n⁺ (n = 1-8), with ammonia are studied at near-thermal energies. In single collision reactions, dehydrogenation of NH₃ proceeds at n = 1-5, and in particular, Ir₄⁺ and Ir₅⁺ are found to be significantly reactive. This size dependence is quite different from those of other platinum group metal cluster cations, where usually only the dimers are able to dehydrogenate NH₃. Moreover, the sequentially dehydrogenated products, Ir_4,5(NH)_m⁺ (m = 2-5), are chiefly observed under multiple collision conditions. This observation suggests that the NH species on Ir_4,5⁺ possibly encourages, or at least does not prohibit, the adsorption of the coming NH₃ molecule and the dehydrogenation.

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 _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.

Gas-phase perspective on the thermodynamics and kinetics of heterogeneous catalysis

Gas-phase studies of small transition metal cluster cations provide thermochemistry of utility to surface science and heterogeneous catalysis.