Reaction Sites of CO on Size-Selected Silicon Oxide Cluster Anions: A Model Study of Chemistry in the Interstellar Environment

Reaction of size-selected iron-oxide cluster cations with methane: a model study of rapid methane loss in Mars' atmosphere

We report gas-phase reactions of free iron-oxide clusters, FenOm+, and their Ar adducts with methane in the context of chemical processes in Mars' atmosphere. Methane activation was observed to produce FenOmCH2+/FenOmCD2+ and FenOmC+, where the reactivity exhibited size and composition dependence. For example, the rate coefficients of methane activation for Fe3O+ and Fe4O+ were estimated to be 1 × 10-13 and 3 × 10-13 cm3 s-1, respectively. Based on these reaction rate coefficients, the presence of iron-oxide clusters/particles with a density as low as 107 cm-3 in Mars' atmosphere would explain the rapid loss of methane observed recently by the Curiosity rover.

Exploring s-d, s-f, and d-f Electron Interactions in AgnCe+ and AgnSm+ by Chemical Reaction toward O2

We investigate gas-phase reactions of free Ag_nCe⁺ and Ag_nSm⁺ clusters with oxygen molecules to explore s-d, s-f, and d-f electron interactions in the finite size regime; a Ce atom has a 5d electron as well as a 4f electron, whereas a Sm atom has six 4f electrons without 5d electrons. In the reaction of Ag_nCe⁺ (n = 3-20), the Ce atom located on the cluster surface provides an active site except for n = 15 and 16, as inferred from the composition of the reaction products with oxygen bound to the Ce atom as well as from their relatively high reactivity. The extremely low reactivity for n = 15 and 16 is due to encapsulation of the Ce atom by Ag atoms. The minimum reactivity observed at n = 16 suggests that a closed electronic shell with 18 valence electrons is formed with a delocalized Ce 5d electron, while the localized Ce 4f electron does not contribute to the shell closure. As for Ag_nSm⁺ (n = 1-18), encapsulation of the Sm atom was observed for n ≥ 15. The lower reactivity at n = 17 than at n = 16 and 18 implies that an 18-valence-electron shell closure is formed with s electrons from Ag and Sm atoms; Sm 4f electrons are not involved in the shell closure as in the case of Ag_nCe⁺. The present results suggest that the 4f electrons tend to localize on the lanthanoid atom, whereas the 5d electron delocalizes to contribute to the electron shell closure.

Preadsorption Effect of Carbon Monoxide on Reactivity of Cobalt Cluster Cations toward Hydrogen

We report gas-phase reactions of free Co_n(CO)_m⁺ (n = 3-11, m = 0-2) with H₂, expecting a catalytic reaction of coadsorbed CO and H₂ on Co_n⁺. Preadsorption of CO molecules is found to promote H₂ adsorption, in particular, on Co_n(CO)⁺ (n = 5, 8-10). Density functional theory (DFT) calculations reveal that the reactivity is governed by the molecular-orbital energy of Co_n⁺, which is tuned by preadsorbed CO molecules. Collision-induced-dissociation experiments performed on Co_nCOH₂⁺ (n = 8-10) imply that at least some of the CO and H₂ molecules are bound together on Co_n⁺.

Adsorption Kinetics of Nitrogen Molecules on Size-Selected Silver Cluster Cations

We present adsorption processes of dinitrogen on size-selected silver cluster cations, Ag n + (n = 1–10), studied by kinetics measurement using an ion trap. The cluster ions showed sequential adsorption of N2 molecules when the ion trap was cooled down to 105 K, excluding n = 8 and 9 that were exceptionally inactive at this temperature. Termolecular rate coefficients of each adsorption step are determined by analyzing time-dependent changes in the reactant and product ion signals. The first-step rate coefficients were found to increase exponentially from n = 1 to 7 due to increased internal degrees of freedom at larger sizes, which are favorable for accommodating the adsorption energy in a free cluster. In contrast, the adsorption rate turned to decrease for n > 7 due to weaker binding of dinitrogen as revealed by density-functional-theory (DFT) calculation. Adsorption sites on Ag n + are further discussed on the basis of the maximum number of adsorbing N2 molecules observed in the experiment.

Under what conditions does (SiO)N nucleation occur? A bottom-up kinetic modelling evaluation

Silicon monoxide (SiO) is a structurally complex compound exhibiting differentiated oxide-rich and silicon-rich nano-phases at length scales covering nanoclusters to the bulk. Although nano-sized and nano-segregated SiO has great technological potential (e.g. nano-silicon for optical applications) and is of enormous astronomical interest (e.g. formation of silicate cosmic dust) an accurate general description of SiO nucleation is lacking. Avoiding the deficiencies of a bulk-averaged approach typified by classical nucleation theory (CNT) we employ a bottom-up kinetic model which fully takes into account the atomistic details involved in segregation. Specifically, we derive a new low energy benchmark set of segregated (SiO)_N cluster ground state candidates for N ≤ 20 and use the accurately calculated properties of these isomers to calculate SiO nucleation rates. We thus provide a state-of-the art evaluation of the range of pressure and temperature conditions for which formation of SiO will or will not proceed. Our results, which match with available experiment, reveal significant deficiencies with CNT approaches. We employ our model to shed light on controversial issue of circumstellar silicate dust formation showing that, at variance with the predictions from CNT-based calculations, pure SiO nucleation under such conditions is not viable.

The role of electronegativity on the extent of nitridation of group 5 metals as revealed by reactions of tantalum cluster cations with ammonia molecules

The electronegativity of the metal (V > Ta) plays a key role in determining the composition of the metal nitrides.