Anomalously Efficient Dehydrogenation of NH3 on Ir4+ and Ir5.

Gas-phase reactions of iridium cluster cations, Irn+ (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, Ir4,5(NH)m+ (m = 2-5), are chiefly observed under multiple collision conditions. This observation suggests that the NH species on Ir4,5+ possibly encourages, or at least does not prohibit, the adsorption of the coming NH3 molecule and the dehydrogenation.

Dissociative Adsorption of N2 onto Size-Selected Tin+ and TinO+ (n ≤ 16) toward Nitrogen Fixation.

The reactivity of titanium cluster ions and their oxides with molecular dinitrogen was examined using a tandem-type mass spectrometer at a low collision energy of 0.5 eV. The clusters can adsorb dinitrogen and release a titanium atom to consume the obtained excess energy. This indicates that N2 adsorption energy is large enough to break the titanium cluster. While the smaller clusters have relatively low reactivity, the measured reaction cross section increases with the cluster size and reaches nearly one-third of the Langevin cross section at the larger clusters. Density functional theory calculations indicate that the reaction proceeds exothermically and that molecular dinitrogen adsorbs on the clusters dissociatively. It is suggested that the energy levels of the highest occupied molecular orbitals of the titanium clusters are decisively important for N2 activation.

Activation of Methane by Tungsten Carbide and Nitride Cluster Cations.

Gas-phase reactions of tungsten carbide and nitride cluster cations, WnCm+ (n = 1-5, m ≤ 5) and WnNm+ (n = 1-6, m ≤ 2), with methane are investigated at near thermal energies. Most of the clusters react readily with CH4 to form WnCm+1H2+ or WnNmCH2+ under single collision conditions, in contrast to the almost no reactivity of pure tungsten clusters. This result indicates that the introduction of carbon or nitrogen atoms can enhance the reactivity of tungsten clusters toward the CH4 dehydrogenation. In addition, the formation and the release of an ethylene molecule are strongly suggested in the reaction of WC+ with CH4 as a minor reaction channel. Nearly all the nitride cluster cations, WnNm+ (n ≥ 2), exhibit higher reactivity than their corresponding carbides, WnCm+, whereas WN+ is less reactive than WC+. Furthermore, the multiple collision reactions of the highly reactive tungsten nitride species such as WN+ and W4N+ lead to the formation of WnNmCxH2x+ (x = 2, 3), which shows that the dehydrogenation of more than one CH4 molecule occurs.

Dehydrogenation of Methane by Partially Oxidized Tungsten Cluster Cations: High Reactivity Comparable to That of Platinum Cluster Cations.

Gas-phase reactivity of pure and partially oxidized tungsten atomic and cluster cations, Wn+ (n = 1-6) and WnOm+ (n = 1-5, m ≤ 6), with methane is studied at the collision energies from 0.1 to 1.0 eV under single collision conditions. The dehydrogenation of CH4 (i.e., the release of H2) is observed for most of WnOm+, whereas Wn+ (n ≥ 2) are almost unreactive. This result indicates that the reactivity of tungsten clusters can be enhanced by the addition of oxygen atoms. Moreover, the reaction cross section of WnOm+ strongly depends on the cluster composition, and some clusters such as W2O3+, W3O+, W3O5+, and W5O3+ exhibit high reactivity. It turns out that the reactivity of these clusters is roughly comparable to that of the typical platinum cluster cations, which are one of the most reactive clusters toward methane dehydrogenation. The reactivity of Wn+ and WnOm+ toward CH4 can be explained by a simple model of their orbital energies and the potential energy diagrams obtained by using the density functional theory calculations. The calculations also suggest that the oxygen atom(s) in WnOm+ is like a spectator and the formation of a hydroxyl group is not necessary for the cleavage of C-H bonds in CH4.

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.

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, RhnX+ (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 RhnX+ (n = 3-6) adsorb NO. For the specific clusters RhnAl+ (n = 3 and 4) and RhnV+ (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 RhnX+ clusters studied.

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.

Catalytic oxidation of CO with N2O on isolated copper cluster anions.

A catalytic redox reaction involving N2O and CO on size-selected copper cluster anions, Cun(-), was investigated in the gas phase using a guided ion-beam tandem mass spectrometer. When Cun(-) is exposed to a mixture of N2O and CO, CunO(-) is produced via the decomposition of N2O. Increase of the CO partial pressure results in the reproduction of Cun(-) and decrease of CunO(-) through the oxidation of CO. The present results demonstrate that a full catalytic cycle for the reaction, N2O + CO → N2 + CO2, takes place on copper cluster anions. Furthermore, in the investigations of the elementary reactions of Cun(-) + N2O and CunO(-) + CO, we found that the catalytic oxidation of CO with N2O on Cun(-) proceeds most efficiently at n = 7 in the size range of n = 5-16.

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(+).

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.

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.

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).

Detection of OH stretching mode of CH3OH chemisorbed on Ni3+ and Ni4+ by infrared photodissociation spectroscopy.

Structures of nickel cluster ions adsorbed with methanol, Ni3+ (CH3OH)m (m = 1-3) and Ni4+ (CH3OH)m (m = 1-4) were investigated by using infrared photodissociation (IR-PD) spectroscopy based on a tandem-type mass spectrometer, where they were produced by passing Ni3,4+ through methanol vapor under a multiple collision condition. The IR-PD spectra were measured in the wavenumber region between 3100 and 3900 cm-1. In each IR-PD spectrum, a single peak was observed at a wavenumber lower by approximately 40 cm-1 than that of the OH stretching vibration of a free methanol molecule and was assigned to the OH stretching vibrations of the methanol molecules in Ni3,4+ (CH3OH)m. The photodissociation was analyzed by assuming that Ni3,4+ (CH3OH)m dissociate unimolecularly after the photon energy absorbed by them is statistically distributed among the accessible modes of Ni3,4+ (CH3OH)m. In comparison with the calculations performed by the density functional theory, it is concluded that (1) the oxygen atom of each methanol molecule is bound to one of the nickel atoms in Ni3,4+ (defined as molecular chemisorption), (2) the methanol molecules in Ni3,4+ (CH3OH)m do not form any hydrogen bonds, and (3) the cross section for demethanation [CH4 detachment from Nin+ (CH3OH)] is related to the electron density distribution inside the methanol molecule.

Reactions of nitrogen monoxide on cobalt cluster ions: reaction enhancement by introduction of hydrogen.

Absolute cross sections for NO chemisorption, NO decomposition, and cluster dissociation in the collision of a nitrogen monoxide molecule, NO, with cluster ions Con+ and ConH+ (n=2-5) were measured as a function of the cluster size, n, in a beam-gas geometry in a tandem mass spectrometer. Size dependency of the cross sections and the change of the cross sections by introduction of H to Con+ (effect of H-introduction) are explained by a statistical model based on the RRK theory, with the aid of the energetics obtained by a DFT calculation. It was found that the reactions are governed by the energetics rather than dynamics. For instance, Co3+ does not react appreciably with NO because the reactions are endothermic, while Co3H+ does because the reaction becomes exothermic by the H-introduction.

Ab initio molecular dynamics study of methanol adsorption on copper clusters.

The preferential structures of small copper clusters Cun (n=2-9) and the adsorption of methanol molecules on these clusters are examined with first principles, molecular dynamics simulations. The results show that the copper clusters undergo systematic changes in bond length and bond order associated with altering their preferential structures from one-dimensional structures, to two-dimensional and three-dimensional structures. The results also indicate that low coordination number sites on the copper clusters are both the most favorable for methanol adsorption and have the greatest localization of electronic charge. The simulations predict that charge transfer between the neutral copper clusters and the incident methanol molecules is a key process by which adsorption is stabilized. Importantly, the changes in the dimensionality of the copper clusters do not significantly influence methanol adsorption.

How many metal atoms are needed to dehydrogenate an ethylene molecule on metal clusters?: Correlation between reactivity and electronic structures of Fen+, Con+, and Nin+.

The absolute cross section for dehydrogenation of an ethylene molecule on Mn+ [Fen+ (n = 2-28), Con+ (n = 8-29), and Nin+ (n = 3-30)] was measured as a function of the cluster size n in a gas-beam geometry at a collision energy of 0.4 eV in the center-of-mass frame in an apparatus equipped with a tandem-type mass spectrometer. It is found that (1) the dehydrogenation cross section increases rapidly above a cluster size of approximately 18 on Fen+, approximately 13 and approximately 18 on Con+, and approximately 10 on Nin+ and (2) the rapid increase of the cross section for Mn+ occurs at a cluster size where the 3d electrons start to contribute to the highest occupied levels of Mn+. These findings lead us to conclude that the 3d electrons of Mn+ play a central role in the dehydrogenation on Mn+.

Polymerization of ethylene molecules chemisorbed on CrOH+ as a model system of chromium-containing catalyst.

The reaction process of the production of CrOH(C2H4)2(+) was studied in connection with the ethylene polymerization on a silica-supported chromium oxide catalyst (the Phillips catalyst). Cluster ions CrOH(C2H4)2(+) and CrOH(C4H8)+ were produced by the reactions of CrOH+ with C2H4 (ethylene) and C4H8 (1-butene), respectively, and were allowed to collide with a Xe atom under single collision conditions. The cross section for dissociation of each parent cluster ion was measured as a function of the collision energy (collision-induced dissociation, or CID). It was found that (i) the CID cross section for the production of CrOH+ from CrOH(C2H4)2(+) increases sharply at the threshold energy of 3.16 +/- 0.22 eV and (ii) the CID cross section for the production of CrOH+ and C4H8 from CrOH(C4H8)+ also increases sharply at the threshold energy of 3.26 +/- 0.21 eV. In comparison with the calculations based on a B3LYP hybrid density functional method, it is concluded that two ethylene molecules in CrOH(C2H4)2(+) are polymerized to become 1-butene. The calculation also shows that the dimerization proceeds via CrOH(C2H4)+ (ethylene complex) and CrOH(C2H4)2(+) (ethylene complex), in which the ethylene molecules bind with CrOH+ through a pi-bonding.

Size-specific reactions of copper cluster ions with a methanol molecule.

Chemisorption of a methanol molecule onto a size-selected copper cluster ion, Cu(n)+ (n = 2-10), and subsequent reactions were investigated in a gas-beam geometry at a collision energy less than 2 eV in an apparatus based on a tandem-type mass spectrometer. Mass spectra of the product ions show that the following two reactions occur after chemisorption: dominant formation of Cu(n-1)+(H)(OH) (H(OH) formation) in the size range of 4-5 and that of Cu(n)O+ (demethanation) in the size range of 6-8 in addition to only chemisorption in the size range larger than 9. Absolute cross sections for the chemisorption, the H(OH) formation, and the demethanation processes were measured as functions of cluster size and collision energy. Optimized structures of bare copper cluster ions, reaction intermediates, and products were calculated by use of a hybrid method (B3LYP) consisting of the molecular orbital and the density functional methods. The origin of the size-dependent reactivity was explained as the structural change of cluster, two-dimensional to three-dimensional structures.