NO on copper clusters

Reaction of nitric oxide molecules on transition-metal-doped silver cluster cations: size- and dopant-dependent reaction pathways

We report size- and dopant-dependent reaction pathways as well as reactivity of gas-phase free Ag_nM⁺ (M = Sc-Ni) clusters interacting with NO. The reactivity of Ag_nM⁺, except for M = Cr and Mn, exhibits a minimum at a specific size, where the cluster cation possesses 18 or 20 valence electrons consisting of Ag 5s and dopant's 3d and 4s. The product ions range from NO adducts, Ag_nM(NO)_m⁺, and oxygen adducts, Ag_nMO_m⁺, to NO₂ adducts, Ag_nM(NO₂)_m⁺. At small sizes, Ag_nMO_m⁺ are the major products for M = Sc-V, whereas Ag_nM(NO)_m⁺ dominate the products for M = Cr-Ni in striking contrast. In both cases, these reaction products are reminiscent of those from an atomic transition metal. However, the reaction pathways are different at least for M = Sc and Ti; kinetics measurements reveal that the present oxygen adducts are formed via NO adducts, while, for example, Ti⁺ is known to produce TiO⁺ directly by reaction with a single NO molecule. At larger sizes, on the other hand, Ag_nM(NO₂)_m⁺ are dominantly produced regardless of the dopant element because the dopant atom is encapsulated by the Ag host; the NO₂ formation on the cluster is similar to that reported for undoped Ag_n⁺.

Metal Cluster Models for Heterogeneous Catalysis: A Matrix-Isolation Perspective

Metal cluster models are of high relevance for establishing new mechanistic concepts for heterogeneous catalysis. The high reactivity and particular selectivity of metal clusters is caused by the wealth of low-lying electronically excited states that are often thermally populated. Thereby the metal clusters are flexible with regard to their electronic structure and can adjust their states to be appropriate for the reaction with a particular substrate. The matrix isolation technique is ideally suited for studying excited state reactivity. The low matrix temperatures (generally 4-40 K) of the noble gas matrix host guarantee that all clusters are in their electronic ground-state (with only a very few exceptions). Electronically excited states can then be selectively populated and their reactivity probed. Unfortunately, a systematic research in this direction has not been made up to date. The purpose of this review is to provide the grounds for a directed approach to understand cluster reactivity through matrix-isolation studies combined with quantum chemical calculations.

The adsorption and activation of NO on silver clusters with sizes up to one nanometer: interactions dominated by electron transfer from silver to NO

Evidence for NO unitary adsorption, the formation of (NO)₂ and the reduction to form N₂O is observed on silver clusters with sizes up to one nanometer. The adsorption and activation of NO are enhanced by electron transfer from silver to NO.

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.

Infrared photodissociation spectroscopy of mass-selected silver and gold nitrosyl cation complexes

The [M(NO)n](+) cation complexes (M = Au and Ag) are studied for exploring the coordination and bonding between nitric oxide and noble metal cations. These species are produced in a laser vaporization supersonic ion source and probed by infrared photodissociation spectroscopy in the NO stretching frequency region using a collinear tandem time-of-flight mass spectrometer. The geometric and electronic structures of these complexes are determined by comparison of the distinctive experimental spectra with simulated spectra derived from density functional theory calculations. All of these noble metal nitrosyl cation complexes are characterized to have bent NO ligands serving as one-electron donors. The spectrum of [Au(NO)2Ar](+) is consistent with 2-fold coordination with a near linear N-Au-N arrangement for this ion. The [Au(NO)n](+) (n = 3-4) cations are determined to be a mixture of 2-fold coordinated form and 3- or 4-fold coordinated form. In contrast, the spectra of [Ag(NO)n](+) (n = 3-6) provide evidence for the completion of the first coordination shell at n = 5. The high [Au(NO)n](+) and [Ag(NO)n](+) (n ≥ 3 for Au, n ≥ 4 for Ag) complexes each involve one or more (NO)2 dimer ligands, as observed in the copper nitrosyl cation complexes, indicating that ligand-ligand coupling plays an important role in the structure and bonding of noble metal nitrosyl cation complexes.

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(+).

On the copper oxide neutral cluster distribution in the gas phase: Detection through 355 nm and 193 nm multiphoton and 118 nm single photon ionization

The distribution of neutral copper oxide clusters in the gas phase created by laser ablation is detected and characterized through time-of-flight mass spectroscopy (TOFMS). The neutral copper oxide clusters are ionized by two different approaches: Multiphoton absorption of 355 and 193 nm radiation; and single photon absorption of 118 nm radiation. Based on the observed cluster patterns as a function of experimental conditions (e.g., copper oxide or metal sample, ablation laser power, expansion gas, etc.) and on the width of the TOFMS features, one can uncover the true neutral cluster distribution of CumOn species following laser ablation of the sample. Ablation of a metal sample generates only small neutral CumOn clusters for m less, similar 4 and n approximately 1, 2. Ablation of copper oxide samples generates neutral clusters of the form CumOm (m < or = 4) and CumO(m-1) (m > 4). These clusters are directly detected without fragmentation using single photon, photoionization with 118 nm laser radiation. Using 355 and 193 nm multiphoton ionization, the observed cluster ions are mostly of the form Cu2mOm+ for 4 < or = m < or = 10 (193 nm ionization) and CumO1,2 (355 nm ionization) for copper oxide samples. Neutral cluster fragmentation due to multiphoton processes seems mainly to be of the form CumO(m,m-1) --> CumO(m/2,m/2+1). Neutral cluster growth mechanisms are discussed based on the cluster yield from different samples (e.g., Cu metal, CuO powder, and Cu2O powder).