Chemistry of (and on) transition metal clusters: a Fourier transform ion cyclotron resonance study of the reaction of niobium cluster cations with nitric oxide

What Determines the Drastic Reactivity of Nbn+ Clusters with Nitric Oxide under Thermalized Conditions?

We report an in-depth study of the adsorption and reaction of NO with cationic Nb_n⁺ (n = 1-20) clusters under thermalized conditions in a laminar flow tube reactor in tandem with a customized triple quadrupole mass spectrometer (FT-TQMS). It is found that the small-sized Nb_n⁺ clusters (2 ≤ n ≤ 7) readily react with NO giving rise to dominant fragmentation products pertaining to the loss of a stable diatomic molecule NbO or NbN. In contrast, the reaction products of larger-sized clusters (n ≥ 10) proceed through diverse channels, including NO adsorption, N₂/N₂O release, and even NO₂ formation. These experimental observations provided the incentive for us to dig deep into the reaction mechanism with the help of DFT calculations. In contrast to the NO-donation coordination in transition metal complexes, here the cationic Nb_n⁺ clusters exhibit dominant electronic donation in initiating the reactions with NO molecules. We fully demonstrated the reaction rate constants, compared the reaction energy diagram of typical Nb_n⁺ clusters, and unveiled the distinct interaction mechanism of niobium clusters available for NO activation and conversion.

Effects of Second-Metal (Al, V, Co) Doping on the NO Reactivity of Small Rhodium Cluster Cations

Reactions of pure and doped rhodium cluster cations, Rh_nX⁺ (n = 2-6; X = Al, V, Co, Rh), with NO molecules were investigated at near-thermal energy using a guided ion beam tandem mass spectrometer. We found that the doping with Al and V increases the total reaction cross section mostly. Under single-collision conditions, Rh₂X⁺ reacts with NO to produce Rh₂N⁺ with release of metal monoxide, XO, whereas Rh_nX⁺ (n = 3-6) adsorb NO. For the specific clusters Rh_nAl⁺ (n = 3 and 4) and Rh_nV⁺ (n = 4-6), the NO adsorption is often accompanied by the release of one Rh atom. In addition, we examined the reactions of Rh₅X⁺ (X = Al, V, Co, Rh) with NO under multiple-collision conditions and observed the cluster dioxide formation and the N₂ release, i.e., NO decomposition. Particularly, the V-doping is most effective for the NO decomposition. One possible explanation for the present results is that the formation of a stable dopant metal-oxygen bond directly leads to the increase of NO dissociative adsorption energy and the reduction of the energy barrier between the molecular and dissociative adsorption, thereby encouraging the NO decomposition on the small Rh_nX⁺ clusters studied.

Stability of Aluminum-Doped Copper Cluster Cations and Their Reactivity toward NO and O2

Aluminum-doped copper cluster cations, CunAl(+), were produced via an ion sputtering method and analyzed by mass spectrometry. The measured size distributions show that Cu6Al(+) and Cu18Al(+) are highly stable species, which can be understood in terms of the electronic subshell 1P and 2S closings, respectively. Furthermore, the reactions of size-selected CunAl(+) (n = 4-6 and 8-16) with NO and O2 were studied at near thermal energies by using a tandem-type mass spectrometer. The doping of an Al atom improves the reactivity of the clusters toward NO in particular for n = 9, 11, 13, and 15, whereas it does not change the reactivity toward O2 significantly. Consequently, it was found that CunAl(+) (n = 9, 11, 13 and 15) are more reactive toward NO than toward O2. The high reactivity of Cu9Al(+) toward NO compared to that of Cu10(+) is explained in terms of the increase of the adsorption energy and the lowering of the barrier to dissociative adsorption, with the aid of calculations based on density functional theory. Moreover, the multiple-collision reactions of CunAl(+) (n = 9, 11, and 13) with NO result in the production of cluster dioxides, Cun-3AlO2(+), (i.e., release of N2), which clearly indicates that NO decomposition proceeds on these clusters.

Reactions of size-selected copper cluster cations and anions with nitric oxide: enhancement of adsorption in coadsorption with oxygen

Reactions of size-selected Cu(n)(±) and Cu(n)O(m)(±) (n = 3-19, m ≤ 9) clusters with NO were investigated in the near-thermal energy region under single collision conditions using a tandem-type mass spectrometer with two ion-guided cells. Oxygen atoms preadsorbed on the cluster can significantly enhance the NO adsorption probability and cause additional reactions. NO adsorption is observed particularly for anionic copper cluster dioxides, Cu(n)O2(-) (n ≥ 8), followed by the release of a Cu atom from Cu(n)O2(-) (n = 8, 10, and 12), which suggests that NO adsorbs strongly, i.e., dissociatively on these clusters. Density functional theory calculations support that dissociative adsorption of NO occurs in the reaction of Cu8O2(-) under the present experimental conditions. On the other hand, NO oxidation proceeds in reactions of oxygen-rich cluster cations such as Cu4O3(+), Cu6O5(+), Cu9O7(+), and Cu11O8(+).