Chemical Reaction
Chemical Reaction on Cluster
"Cluster Age" is just beginning in the field of energy, catalysts, electronics, medicine and so on. In catalysts, clusters and nanoparticles deposited on a substrate or protected by ligand molecules are researched for novel reactions, activity in specific environments, high yield and selectivity. It was found that the catalytic activity depends on the particle diameter, number of constituents, substrates and ligand molecules in these studies. These results should be basically attributed to the charge state and the electronic structure as well as the geometric structure. Actually these are tangled with each other, and it is not easy to derive a guiding principle. Here we are trying to elucidate the basis of their catalytic activity by use of an isolated, mass-selected cluster in the gas phase. Our scientific achievement contributes to the design of a novel catalyst.
We use a tandem-type mass spectrometer equipped with two reaction cells. Metal clusters are produced by using the Xe-ion sputtering method, and positively or negatively charged cluster ions are extracted by a series of ion optics. In the first reaction cell, the cluster ions adsorb reactant molecules, and the first quadrupole mass filter selects a specific cluster ion having reactant molecules. This mass-selected cluster is allowed to react with second molecules in the second reaction cell. Product ions are mass-analyzed by the second quadrupole mass filter, and their reaction cross sections and branching fractions are obtained.
For example, O2 adsorbed copper cluster anions, CunO2-, have been produced in the first reaction cell, and allowed to react with a carbon monoxide molecule in the second reaction cell. In this reaction, we found that one oxygen atom is removed from CunO2- and a product ion, CunO-, is obtained exothermically. This result suggests that negatively charged copper clusters can catalyze the oxidation of CO. Further, a theoretical analysis on Cu5- indicates that Cu5- deforms in this reaction process and this deformation reduces the activation energy barrier remarkably.
Low-Energy Cluster-Cluster Collision -For Spectroscopy of Cryogenic Cluster
We are developing a new experimental technique to produce cluster complexes by using a low-energy cluster-cluster collision. This technique enables a helium cluster to incorporate a size-selected metal cluster, and we can measure the spectrum of the cluster in a higher resolution. Furthermore, in the infrared photodissociation spectroscopy, we can measure the spectrum in a higher sensitivity by detecting the loss of helium atoms from the cluster complex since the interaction between a helium atom and the metal cluster is significantly weak. This technique is applicable to the molecule-adsorbed metal cluster ions, and we can unveil the reaction mechanism of molecules on the cluster by use of this technique.
Our experimental apparatus consists of a metal-cluster-ion source, neutral cluster source, low-energy collision region, and infrared photodissociation region.
In the collision between an argon cluster, ArN, and a cobalt dimer ion, Co2+, we have observed Co2+Arn (n ≤18) as product ions. Co2+ and ArN attract each other by the charge-induced dipole interaction, and Co2+ArN is produced at first. The excess energy of Co2+ArN is consumed through the evaporation of argon atoms, and Co2+Arn remains at last. The figure below shows the intensity of Co2+Arn (n ≤ 8) as a function of the relative velocity, and this intensity decreases inversely with the relative velocity.
Selected Publications
S. Hirabayashi, M. Ichihashi, and T. Kondow, "Reactions of copper cluster cations with nitrous oxide: Oxidation and N2O adsorption", Chem. Phys. Lett. 533, 15-19 (2012).
Most of the transition metal cluster ions are oxidized by N2O, however we observed that copper cluster ions adsorb N2O simply. Further, O2preadsorbed copper cluster ions can adsorb N2O more efficiently.
S. Hirabayashi, M. Ichihashi, and T. Kondow, "Enhancement of ammonia dehydrogenation by introduction of oxygen onto cobalt and iron cluster cations", J. Phys. Chem. A 114, 13040-13044 (2010).
We observed that cobalt tetramer and pentamer ions can dehydrogenate an ammonia molecule. Additionally Co4Om+ and Co5Om+ have larger total reaction cross sections, and especially Co4O+ and Co5O1,2+ facilitate the dehydrogenation of NH3.
M. Ichihashi, T. Hanmura, R. T. Yadav, and T. Kondow, "Adsorption and Reaction of Methanol Molecule on Nickel Cluster Ions, Nin+ (n=3-11)", J. Phys. Chem. A 104, 11885-11890 (2000).
S. Hirabayashi, R. Okawa, M. Ichihashi, T. Kondow and Y. Kawazoe, "Detection of OH Stretching Mode of CH3OH Chemisorbed on Ni3+ and Ni4+ by Infrared Photodissociation Spectroscopy", J. Phys. Chem. A, 111, 7664-7669 (2007).
In the reaction of nickel cluster ions with a methanol molecule, reaction pathway and reactivity depend on the cluster size significantly, and demethanation proceeds on the tetramer ion, and the carbide formation occurs on the octamer ion. We examined the structure of CH3OH on Nin+ by infrared photodissociation spectroscopy to elucidate this size-specific reactivity of Nin+. The measured spectra and the results of the density functional calculation suggest that the adsorbed CH3OH dissociates on Ni4+ and the demethanation proceeds efficiently via this dissociatively adsorbed species.
