| 1. |
M. Arakawa, K. Kono, Y. Sekine, and A. Terasaki, Reaction of Size-Selected Iron-Oxide Cluster Cations with Methane: A Model Study of Chemical Processes in Mars’ Atmosphere, 21th International Symposium on Small Particles and Inorganic Cluster (ISSPIC XXI), 2023.09, Small particles and clusters of minerals such as silicate (e.g., (Mg,Fe)SiO3 and (Mg,Fe)2SiO4), and silica (SiO2) are among the most abundant materials in space. It is prevalent hypothesis that, in the early stage of planetary formation, such materials contribute to chemical processes such as formation of organic molecules. Size-selected gas-phase clusters provide a good model for this chemistry, because it is possible to investigate reactions step by step with precise control in the number of atoms and molecules involved in the reaction. Furthermore, it has recently been suggested that silica and silicate clusters could be highly abundant in the interstellar medium [1,2]. In this context, we have reported reactions of gas-phase free silicate, MglSiOm−, and silica, SinOm−, cluster anions with CO and H2O molecules [3,4]. In addition, coadsorption and subsequent reaction of CO and H2 molecules on cobalt cluster cations, Con+, has been examined to discuss formation of organic molecules on the cluster [5].
Another chemical process attracting much attention recently is rapid methane loss in the atmosphere of Mars [6]. Observation by the Curiosity rover found temporary spikes of methane and its rapid loss, but the mechanism of the loss has not been clarified yet. Mars’ soil is rich in iron oxide, and storms of iron-oxide particles (dust devils) occur very frequently. Because the iron-oxide cluster, Fe2O2, is known as an active center of enzyme, methane monooxygenase, we hypothesized that iron-oxide particles/clusters were responsible for the rapid loss. Numerous theoretical studies on the interaction of Fe2O2 with methane have been reported [7]. As for an experimental study, FeO+ was reported to mediate activation of methane [8]. In the present study, we report gas-phase reaction of size-selected iron-oxide cluster cations, FenOm+, with methane, CH4, and deuterated methane, CD4, molecules to verify our hypothesis, where methane activation was observed to produce FenOmCH2+ and FenOmC+. The reactivity exhibited size dependence. For example, the rate coefficients of the methane activation for Fe3O+ and Fe4O+ were estimated to be 1 × 10−12 and 3 × 10−12 cm3 s−1, respectively [9]. Based on these values, the presence of iron-oxide clusters/particles of 4 × 106 cm−3 (10−14 Pa) in Mars’ atmosphere would explain the loss of methane.. |
| 2. |
M. Arakawa, M. Horioka, K. Minamikawa, T. Kawano, and A. Terasaki, Reaction kinetics of NO on Agn+ and AgnM+ (M = Sc–Ni): Size- and dopant-dependent reaction pathways, The Symposium on Size-Selected Clusters S3C, 2023.03, Nitric oxide (NO) is one of the toxic gases generated during combustion processes, causing environmental issues such as photochemical smog and acid rain. In this context, catalytic reduction of NO is an important subject in industrial chemistry. Here we report reaction of gas-phase free silver cluster cations, Agn+, and transition-metal-doped silver cluster cations, AgnM+ (M = Sc–Ni), with NO molecules studied by kinetics measurement using an ion trap.
For the reaction of Agn+, AgnO(NO2)m−1+ and Agn(NO2)m+ are observed as major products after multiple collisions. By analysing the kinetics data, two different reaction mechanisms are found for formation of the major reaction products depending on cluster size as follows:1
(1) As found for n = 4, 6, and 9, AgnO+ is formed by reaction with two NO molecules followed by release of neutral N2O. Further reaction of AgnO+ with another NO molecule produces AgnNO2+. Agn(NO2)m+ (m ≥ 1) is thus successively formed via the intermediate, AgnO(NO2)m−1+.
(2) Both AgnNO2+ and AgnO+ are formed concurrently from Agn+ exposed to NO molecules as found for n = 7, 8, 10, 11, 12, and 15; AgnO+ does not serve as an intermediate for AgnNO2+. AgnO(NO2)m−1+ and Agn(NO2)m+ (m ≥ 2) are formed by successive addition of NO2 to AgnO+ and AgnNO2+, respectively. It is speculated that the successive addition of NO2 proceeds via disproportionation, i.e., three NO molecules are converted to NO2 and N2O.
Doping a transition metal atom to Agn+ changes the reaction products: the product ions range from NO adducts, AgnM(NO)m+, and oxygen adducts, AgnMOm+, to NO2 adducts, AgnM(NO2)m+. The rich variety of reaction products is explainable by change in the reaction site, which is either the dopant atom or the silver atom, depending on the cluster size.2. |
| 3. |
M. Arakawa, M. Horioka, K. Minamikawa, T. Kawano, and A. Terasaki, Reaction kinetics of nitric oxide molecules on silver cluster cations: Size-dependent reaction pathways, International Congress on Pure & Applied Chemistry Kota Kinabalu (ICPAC Kota Kinabalu 2022), 2022.11. |
| 4. |
M. Arakawa and A. Terasaki, Elementary processes in chemical evolution studied by size-selected cluster chemistry, The 23rd East Asian Workshop on Chemical Dynamics (EAWCD), 2019.09, Size-selected clusters in the gas phase provide us with an ideal approach to elucidate how the chemical and physical properties of matter appear as atoms associate together one by one toward bulk solids and liquids. For example, we are studying silver clusters to show emergence of collective excitation of electrons in the course of cluster growth into a nanoparticle by optical absorption spectroscopy. X-ray absorption spectroscopy has been employed to investigate chemical state of each constituent atom in the cluster ion. Furthermore, reaction experiments on gas-phase clusters offer opportunities to probe reactions step by step with precise control in the number of atoms and molecules involved in the reaction as a model for bulk chemical processes. Here we present two topics focusing on size-selected cluster chemistry.
The first topic is tantalum nitride, which is an attractive material with a potential for various applications, e.g., copper diffusion barriers in microelectronics, an interlayer in magnetic random-access memories, and photocatalysts for H2 evolution from water under visible light. Elucidation of nitridation mechanism at the molecular level would supply a useful recipe for fabricating high-quality tantalum-nitride materials. In the present study, nitridation of tantalum cluster cations, Tan+ (n = 1–10), by ammonia molecules is investigated,7 since it is known that nitridation does not take place by N2. The reaction of the monomer cation, Ta+, with two NH3 molecules leads to formation of TaN2H2+ along with release of two H2 molecules. The dehydrogenation occurs until the formal oxidation number of Ta reaches +5. On the other hand, all the tantalum clusters, Tan+, react with two NH3 molecules and form TanN2+ after release of three H2 molecules. Further exposure to ammonia showed that TanNmH+ and TanNm+ are produced through successive reactions; a pure nitride and three H2 molecules are formed for every other NH3 molecule. The nitridation occurred until the formal oxidation number of Ta reaches +5 as in the case of TaN2H2+ in contrast to other group 5 elements, i.e., vanadium and niobium, which have been reported to produce nitrides with lower oxidation states.9,10 The present results on small gas-phase clusters show correlation with their bulk properties: tantalum is known to form bulk nitrides in the oxidation states of either +5 (Ta3N5) or +3 (TaN), whereas vanadium and niobium form nitrides in the oxidation state of +3 (VN and NbN). Along with DFT calculations, these findings reveal that nitridation is driven by the electron-donating ability of the metal, i.e., electronegativity of the metal plays a key role in determining the composition of nitrides.. |
| 5. |
M. Arakawa and A. Terasaki, Reaction of gas-phase metal and mineral clusters with H2O, CO, and H2 molecules related to chemistry in space, International Congress on Pure & Applied Chemistry Yangon (ICPAC Yangon 2019), 2019.08, Small particles and clusters of minerals such as silicate (e.g., (Mg,Fe)SiO3 and (Mg,Fe)2SiO4) and silica (SiO2) as well as metals such as iron and cobalt, are among the most abundant materials in space. It is prevalent hypothesis that, in the early stage of planetary formation, such materials contribute to chemical processes such as water delivery to the Earth and formation of organic molecules. Size-selected gas-phase clusters provide a good model for this chemistry, because it is possible to investigate reactions step by step with precise control in the number of atoms and molecules involved in the reaction. In this context, we present reactions of gas-phase free silicate, MglSiOm−, and silica, SinOm−, cluster anions with CO, and H2O molecules, to investigate chemical processes during the early stage of planetary formation. Furthermore, coadsorption of CO and H2 molecules on cobalt cluster cations, Con+, will be presented to discuss formation of organic molecules on the cluster.. |
| 6. |
Masashi Arakawa, G. Naresh Patwari, Akira Terasaki, The origin of bulk-nitride composition of group 5 metals as revealed by nitridation of tantalum cluster cations by ammonia molecules, International Congress on Pure & Applied Chemistry Langkawi (ICPAC Langkawi 2018), 2018.10. |
| 7. |
M. Arakawa, K. Ando, S. Fujimoto, S. Mishra, G. Naresh Patwari, and A. Terasaki, Successive nitridation of tantalum cluster cations by ammonia molecules: The origin of bulk-nitride composition of group 5 metals, 19th International Symposium on Small Particles and Inorganic Cluster (ISSPIC XIX), 2018.08, Tantalum nitride is an attractive material with a potential for various applications, e.g., copper diffusion barriers in microelectronics, an interlayer in magnetic random access memories, and photocatalysts for H2 evolution. Elucidation of nitridation mechanism of tantalum at the molecular level would supply a useful recipe for fabricating high-quality tantalum-nitride materials. In this context, gas-phase clusters provide an ideal approach to probe reactions step by step with precise control in the number of atoms and molecules involved in a reaction [1,2]. In the present study, nitridation of free tantalum cation, Ta+, and tantalum cluster cations, Tan+, by ammonia molecules is investigated [3], since it is known that nitridation does not proceed by nitrogen molecules [4].
In the experiment, Tan+ (n = 1–10) was generated by a magnetron-sputter cluster-ion source. They were mass-selected and guided into a reaction cell filled with NH3 molecules. The ions produced by the reaction were identified by a quadrupole mass analyzer.
The reaction of monomer cation, Ta+, with two molecules of NH3 leads to formation of TaN2H2+ along with release of two H2 molecules. The dehydrogenation occurs until the formal oxidation number of the tantalum atom reaches +5. On the other hand, all the tantalum cluster cations, Tan+, react with two molecules of NH3 and form TanN2+ with the release of three H2 molecules. Further exposure to ammonia showed that TanNmH+ and TanNm+ are produced through successive reactions; a pure nitride and three H2 molecules are formed for every other NH3 molecule. The nitridation occurred until the formal oxidation number of tantalum atoms reaches +5 as in the case of TaN2H2+. These reaction pathways of tantalum atom and cluster cations are in contrast to those of other group 5 elements, i.e., vanadium and niobium cluster cations, which have been reported to produce nitrides with lower oxidation states [5,6]. The present results on nitridation of small clusters illustrate correlation with their bulk properties: Tantalum is known to form bulk nitrides in the oxidation states of either +5 (Ta3N5) or +3 (TaN), whereas vanadium and niobium form only an oxidation state of +3 (VN and NbN) [7]. Along with DFT calculations, these findings reveal that electronegativity of the metal (V > Nb > Ta) plays a key role in determining the composition of metal nitrides. In contrast to nitrides, all of vanadium, niobium and tantalum form most stable oxides in the oxidation state of +5 due to high electronegativity of oxygen compared with nitrogen. The present study thus revealed that electronegativity of group 5 metal is crucial for nitridation.. |
| 8. |
M. Arakawa, G. Naresh Patwari, and A. Terasaki, Nitridation mechanism of tantalum cluster cations by ammonia molecules: contrast to other group 5 metals, Gas Phase Model Systems for Catalysis – GPMC 2018, 2018.06, Tantalum nitride is an attractive material with a potential for various applications such as photocatalysts for H2 evolution from water and copper diffusion barriers in microelectronics. Elucidation of nitridation mechanism of tantalum at the molecular level would supply a useful recipe for fabricating high-quality tantalum-nitride materials. In the present study, reactions of free tantalum cation, Ta+, and tantalum cluster cations, Tan+, with ammonia molecules were investigated to probe nitridation reactions step by step with precise control in the number of atoms and molecules involved in the reaction [1].
In the experiment, Tan+ (n = 1–10) was generated by a magnetron-sputter cluster-ion source. They were thermalized through collisions with helium molecules cooled by liquid nitrogen, and were mass-selected and guided into a reaction cell filled with NH3 molecules. The ions produced by the reaction were identified by a quadrupole mass analyzer, and the yield of each reaction product was measured as a function of the cluster size, n.
The reaction of monomer cation, Ta+, with two molecules of NH3 leads to formation of TaN2H2+ along with release of two H2 molecules. The dehydrogenation occurs until the formal oxidation number of the tantalum atom reaches +5. On the other hand, all the tantalum cluster cations, Tan+, react with two molecules of NH3 and form TanN2+ with the release of three H2 molecules. Further exposure to ammonia showed that TanNmH+ and TanNm+ are produced through successive reactions; this is achieved by alternate single and double dehydrogenation upon adsorption of every other NH3 reactant molecule. The nitridation occurred until the formal oxidation number of tantalum atoms reaches +5 as in the case of TaN2H2+. These reaction pathways of tantalum atom and cluster cations are in contrast to those of other group 5 metals, i.e., vanadium and niobium clusters, which have been reported to produce nitrides with lower oxidation states [2,3]. The present results on nitridation of small clusters of group 5 metals illustrate correlation with their bulk properties: Tantalum is known to form bulk nitrides in the oxidation states of either +5 (Ta3N5) or +3 (TaN), whereas vanadium and niobium form only an oxidation state of +3 (VN and NbN) [4]. Along with DFT calculations, these findings reveal that nitridation is driven by the electron-donating ability of the group 5 element, i.e., electronegativity of the metal plays a significant role in determining the composition of metal nitrides.. |
| 9. |
M. Arakawa, Application of cluster chemistry to astrochemistry: Molecular evolution involving mineral clusters, Kaleidoscope: A Discussion Meeting in Chemistry, 2018.07. |
| 10. |
Masashi Arakawa, Reaction of silicate clusters related to chemistry in the interstellar environment, International Symposium on Molecular Science -Physical Chemistry/ Theoretical Chemistry, Chemoinformatics, Computational Chemistry-,, 2018.03. |
| 11. |
Masashi Arakawa, Ryo Yamane, Akira Terasaki, Reaction site of a CO molecule on silicon-oxide cluster anions, Symposium on Size Selected Clusters, 2016.02. |
| 12. |
Masashi Arakawa, Ryo Yamane, Akira Terasaki, Adsorption of a CO molecule on silicon-oxide cluster anions toward elucidation of reaction processes on mineral surfaces in proto-planetary nebulae, PACIFICHEM 2015, 2015.12. |
| 13. |
Masashi Arakawa, Ryo Yamane, Akira Terasaki, Reaction sites of a CO molecule on silicon-oxide cluster anions as a model of mineral surfaces, Workshop on Nanoscale Atomic and Molecular Systems, 2015.08. |
| 14. |
Masashi Arakawa, Kei Kohara, Akira Terasaki, Formation of hydrated-alumina clusters toward elucidation of generation process of organic molecules on mineral surfaces, Workshop on Interstellar Matter, 2014.10. |
| 15. |
Masashi Arakawa, Kei Kohara, Akira Terasaki, Dissociation, oxidation, hydroxylation, and hydration of aluminum cluster cations upon reaction with H2O and O2, Gas Phase Model Systems for Catalysis – GPMC 2014, 2014.04. |
| 16. |
Masashi Arakawa, Kei Kohara, Akira Terasaki, Formation of stable aluminum hydroxide clusters in aluminum-cluster ion beam exposed to O2 and H2O, Trombay Symposium on Radiation and Photochemistry, 2014.01. |
