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.

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.

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.