Masahiko Ichihashi

Comparison of adsorption probabilities of O2 and CO on copper cluster cations and anions.

Reactions of size-selected copper cluster cations and anions, Cu(n)(±), with O(2) and CO have been systematically investigated under single collision conditions by using a tandem-mass spectrometer. In the reactions of Cu(n)(±) (n = 3-25) with O(2), oxidation of the cluster is prominently observed with and without releasing Cu atoms at the collision energy of 0.2 eV. The reactivity of Cu(n)(+) is governed to some extent by the electronic shell structure; the relatively small reaction cross sections observed at n = 9 and 21 correspond to the electronic shell closings, and those at odd sizes in n ≤ 16 match with the clusters having no unpaired electron. On the other hand, the reactivity of Cu(n)(-) exhibits no remarkable decrease by the electronic shell closings and the even-numbered electrons. These behaviors may be due to an influence of the electron detachment of the reaction intermediate, Cu(n)O(2)(-). Both the cations and anions show the dominant formation of Cu(n-1)O(2)(±) in n ≤ 16 and Cu(n)O(2)(±) in n ≥ 17 in the experimental time window. By contrast, Cu(n)(-) (n = 3-11) do not react with CO at the collision energy of 0.2 eV, while Cu(n)(+) (n = 3-19) adsorb CO though the cross sections are relatively small. The difference in the reactivity between the charge states can be understood in terms of the frontier orbitals of the Cu cluster and O(2) or CO.

Oxidation of CO and NO on Composition-Selected Cerium Oxide Cluster Cations

The collisional reactions of composition-selected cerium oxide cluster cations, CenOm(+) (n = 2-6; m ≤ 2n), with CO and NO have been investigated under single collision conditions using a tandem mass spectrometer. At near-thermal energy, oxidation of CO and NO is observed only for the stoichiometric clusters, CenO2n(+) (n = 3-5), and the cross sections for the NO oxidation are found to be larger than those for the CO oxidation. In addition, the collision-energy dependence of the reaction cross sections reveals that the CO oxidation has a small activation barrier, whereas the NO oxidation is a barrierless process. These experimental findings are supported by density functional theory calculations.

Reaction cross section for incorporation of ND3 into NH4+(NH3)n−1 (n=3–9) at very low energy collision

Cross sections for the collision-induced reactions between protonated ammonia cluster ions, NH4+(NH3)n−1 (n=3–9), with ND3 were measured at a collision energy ranging from 0.02 to 1.4 eV in the center-of-mass frame with an extremely narrow energy distribution of 0.02 eV. Two types of reaction, incorporation and dissociation, were observed at the same collision energy. The incorporation cross section increased drastically with a decrease in the collision energy, especially in the collision energy region below about 0.2 eV. The incorporation cross section at a collision energy of about 0.02 eV was larger than twice the geometrical reaction cross section. It is suggested that the increase of the incorporation cross section corresponds to the increase of the collision cross section between the cluster ion and the neutral molecule at the low collision energies used in this study because of the presence of the electrostatic attractive force. The reaction probability for incorporation also increased with a decre...

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(+).

Enhancement of ammonia dehydrogenation by introduction of oxygen onto cobalt and iron cluster cations.

Reactions of oxygen-chemisorbed cobalt and iron cluster cations (Co(n)O(m)(+) and Fe(n)O(m)(+); n = 3-6, m = 1-3) with an NH(3) molecule have been investigated in comparison with their bare metal cluster cations at a collision energy of 0.2 eV by use of a guided ion beam tandem mass spectrometer. We have observed three kinds of reaction products, which come from NH(3) chemisorption with and without release of a metal atom from the cluster and dehydrogenation of the chemisorbed NH(3). Reaction cross sections and branching fractions are strongly influenced by the number of oxygen atoms introduced onto the metal clusters. Oxygen-chemisorbed metal clusters with particular compositions such as Co(4)O(+), Co(5)O(2)(+), and Fe(5)O(2)(+) are extremely reactive for NH(3) dehydrogenation, whereas Co(4)O(2)(+) and Fe(4)O(2)(+) exhibit high reactivity for NH(3) chemisorption with metal release. The enhancement of dehydrogenation for specific compositions can be interpreted in terms of competition between O-H and neighboring Co-H (or Fe-H) formation.

Structures and reactions of methanol molecules on cobalt cluster ions studied by infrared photodissociation spectroscopy

Structures of methanol molecules chemisorbed on cobalt cluster ions, Co(n)(+) (n=2-6), were investigated by infrared photodissociation (IR-PD) spectroscopy in the wavenumber range of 3400-4000 cm(-1). All the IR-PD spectra measured exhibit an intense peak in the region of the OH stretching vibration. In the IR-PD spectra of Co(2)(+)(CH(3)OH)(2,3) and Co(3)(+)(CH(3)OH)(3), weak peaks were observed additionally in the vicinity of 3000 cm(-1), being assignable to the CH stretching vibration. The comparison of the experimental results with the calculated ones leads us to conclude that (1) molecularly chemisorbed species, Co(n)(+)(CH(3)OH)(m) (m=1-3), and dissociatively chemisorbed species, Co(n)(+)(CH(3)OH)(m-1)(CH(3))(OH), are dominant and (2) the methanol dehydrogenation proceeds via an intermediate, Co(n)(+)(CH(3))(OH).

Chemisorption of Nitrogen Monoxide on Size‐Selected Cobalt Cluster Ions

Collisional reactions of NO with Con+ (n = 2–10) were studied by measuring the absolute cross sections for creating the product ions in a beam-gas geometry at different collision energies and internal temperatures of Con+. The dominant reaction is chemisorption of NO on Con+ (formation of ConNO+ and Con–1NO+). As n increases, the chemisorption cross section starts to rise sharply at n = 4 and soon reaches the highest possible value, which is the Langevin cross section. The chemisorption cross section practically does not change with the internal temperature, while the branching fraction for the production of ConNO+ with respect to the sum of ConNO+ and Con–1NO+ decreases with the internal temperature in the size range of n = 6–10. A statistical model based on the RRK theory explains the dependences of the chemisorption cross section and the branching fraction on the size (n), the collision energy, and the internal temperature. The energies of the NO chemisorption derived from the dependences of the chemisorption cross section and the branching fraction on the collision energy and the internal temperature show that (1) the NO chemisorption is dissociative for n ≥ 4 and (2) more than one isomer is involved in the chemisorption of NO on Co9+.

Theoretical study on ammonia cluster ions: Nature of kinetic magic number

We theoretically investigated collision cross sections due to the attractive forces between NH4+(NH3)n−1 (n=2–8) and ND3. We found that the dependence of the collision cross sections on collision energy and cluster size are comparable to those of measured fusion cross sections. The kinetic magic number, n=5, is related to the structure of the pentamer. Namely, the center ion in the pentamer is surrounded by first-shell ammonia molecules.

Reactions of Ti- and V-Doped Cu Cluster Cations with Nitric Oxide and Oxygen: Size Dependence and Preferential NO Adsorption

Reactions of copper cluster cations doped with an early transition metal atom, CunTi(+) (n = 4-15) and CunV(+) (n = 5-14, 16), with NO and O2 were investigated at a near-thermal collision energy using a guided ion beam tandem mass spectrometer. Most of the clusters adsorb NO and O2 under single collision conditions, and this reaction is often followed by the release of Cu atoms. For both Ti- and V-doped Cu clusters, the total cross sections for the reaction with NO increase gradually with the cluster size up to n ≈ 11 and then decrease rapidly, whereas those with O2 are almost constant in n ≤ 12 and then decrease. The size dependence of the reactivity toward NO is found to correlate with that of the adsorption energy calculated by the density functional theory method; CunTi(+) clusters exhibit the larger reaction cross sections when they have the larger adsorption energies. The calculations of CunTi(+) also show that a structural transition from a Ti-exposed structure to Ti-encapsulated one occurs around n = 12. This indicates that a geometric property of the clusters, i.e., the position of the dopant atom, is a determining factor of reactivity. In addition, the Ti- and V-doping dramatically improves the reactivity of Cu cluster cations toward NO but it does not affect that toward O2 significantly. As a result, most of the Ti- and V-doped Cu clusters are more reactive toward NO than toward O2. We also studied the multiple-collision reaction of Cu7Ti(+) with NO and obtained the cluster dioxide, Cu3TiO2(+), as a product ion, which suggests that the dissociation of NO and the subsequent formation/release of N2 take place.

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.