Thermal Kinetics of Aln- + O2 (n = 2-30): Measurable Reactivity of Al13. J Phys Chem A 2019;123:6123-6129. [PMID: 31251615 DOI: 10.1021/acs.jpca.9b03552]

Temperature-Dependent Kinetics for the Reactions of Fen- (n = 2-17) and FexNiy- (x + y = 3-9) with O2: Comparison of Pure and Mixed Metal Clusters with Relevance to Meteor Radio Afterglows and Surface Oxidation

Rate constants and product branching fractions were measured from 300-600 K for Fen- + O2 (n = 2-17) and for 300-500 K for FexNiy- + O2 (x + y = 3-9) using a selected-ion flow tube (SIFT) apparatus. Rate constants for 46 species are reported. All rate constants increased with increasing temperature, and several were in excess of the Langevin-Gioumousis-Stevenson (LGS) capture rate at elevated temperatures. As with previously studied transition metal anion oxidation reactions, the collision limit is treated as the sum of the LGS limit along with a hard-sphere contribution, allowing for determination of activation energies. These values are compared to each other along with previous results for Nin-. Measured rate constants for all three series (Fen-, Nin-, and FexNy-) vary over a relatively narrow range (1-5 × 10-10 cm3 s-1 at 300 K) being at least 15% of the collision rate constant. All reaction rate constants increase with temperature, described by small activation energies of 0.5-4 kJ mol-1. The data are consistent with an anticorrelation between the electron binding energy and rate constant, previously noted in other systems. The Fen- reaction produces a larger population of higher energy electrons than do the Nin- reactions, with FexNiy- producing an intermediate amount. The results suggest that the overall rate constant is limited by a small energetic barrier located at a large internuclear distance where electrostatic forces dominate, causing the potentials to be similar across systems, while the product formation is determined by the shorter-range, valence portion of the potential, which varies widely between systems.

Oxidative Activation of Small Aluminum Nanoclusters with Boron Atom Substitution prior to Completing the Endohedral B@Al12- Superatom

Elemental substitution and doping validate the optimization of chemical and physical properties of functional materials, and the composition ratio of the substituting atoms generally determines their properties by changing their geometric and electronic structures. For atomically precise nanoclusters (NCs) consisting of countable atom aggregates, the composition can be controlled accurately to provide an ideal model to study the heteroatom substitution effects. Since aluminum (Al) and boron (B) both belong to group 13 in the periodic table, the effect of B atom substitution on Aln NCs can be investigated while maintaining the total number of valence electrons in AlnBm NCs. In this study, oxidative reactivities of small Al NCs with B atom substitution are studied for AlnBm NCs (m = 1, n = 6-14 and m = 2, n = 11) supported on organic surfaces by using X-ray photoelectron spectroscopy and oxygen molecule (O2) exposure measurements. Before completing the endohedral B@Al12- superatomic NC, one B atom substitution in Al NCs (AlnB) enhances oxidative reactivities 3-20 times compared to those of Aln+1, particularly for n ≤ 11. When one Al atom of Al12B is further substituted by a B atom to form Al11B2, the reactivity drastically increases (6.6 × 102 times), showing that the B atom substitution makes the NC chemically active or inactive geometrically depending on the exohedral or endohedral site for the B atom in the Al NC. In addition, density functional theory calculations show that the electronegative B atom contributes to forming a locally positive Al site to facilitate O2 adsorption except in Al12B, in which the B atom is geometrically shielded by the surface of the Al12 cage in B@Al12.

Probing the binding and activation of small molecules by gas-phase transition metal clusters via IR spectroscopy

Isolated transition metal clusters have been established as useful models for extended metal surfaces or deposited metal particles, to improve the understanding of their surface chemistry and of catalytic reactions. For this objective, an important milestone has been the development of experimental methods for the size-specific structural characterization of clusters and cluster complexes in the gas phase. This review focusses on the characterization of molecular ligands, their binding and activation by small transition metal clusters, using cluster-size specific infrared action spectroscopy. A comprehensive overview and a critical discussion of the experimental data available to date is provided, reaching from the initial results obtained using line-tuneable CO₂ lasers to present-day studies applying infrared free electron lasers as well as other intense and broadly tuneable IR laser sources.

Al13− and B@Al12− superatoms on a molecularly decorated substrate

Aluminum nanoclusters (Al_n NCs), particularly Al₁₃⁻ (n = 13), exhibit superatomic behavior with interplay between electron shell closure and geometrical packing in an anionic state. To fabricate superatom (SA) assemblies, substrates decorated with organic molecules can facilitate the optimization of cluster–surface interactions, because the molecularly local interactions for SAs govern the electronic properties via molecular complexation. In this study, Al_n NCs are soft-landed on organic substrates pre-deposited with n-type fullerene (C₆₀) and p-type hexa-tert-butyl-hexa-peri-hexabenzocoronene (HB-HBC, C₆₆H₆₆), and the electronic states of Al_n are characterized by X-ray photoelectron spectroscopy and chemical oxidative measurements. On the C₆₀ substrate, Al_n is fixed to be cationic but highly oxidative; however, on the HB-HBC substrate, they are stably fixed as anionic Al_n⁻ without any oxidations. The results reveal that the careful selection of organic molecules controls the design of assembled materials containing both Al₁₃⁻ and boron-doped B@Al₁₂⁻ SAs through optimizing the cluster–surface interactions.

Anionic aluminium clusters are promising candidates for the fabrication of superatom-assembled nanomaterials. Here, the authors report enhanced stability for Al₁₃⁻ and boron-doped B@Al₁₂⁻ on a molecularly decorated p-type organic substrate.

Gas-Phase Anionic Metal Clusters are Model Systems for Surface Oxidation: Kinetics of the Reactions of Mn- with O2 (M = V, Cr, Co, Ni; n = 1-15)

The reactions of anionic metal clusters M_n^- with O₂ (M = V (n = 1-15), Cr (n = 1-15), Co (n = 1-12), and Ni (n = 1-14)) are investigated from 300 to 600 K using a selected-ion flow tube. All rate constants show a positive temperature dependence, well described by an Arrhenius equation. Rate constants exceed (or are extrapolated to exceed at higher temperatures) the Langevin-Gioumousis-Stevenson capture rate constant. Application of a capture model accounting for the finite size of the clusters reproduces the size-dependent trends in reactivity. The assumption that reactivity is further controlled by an energetic barrier early in the reaction coordinate is consistent with the experimental observations. An observed correlation of the derived barrier heights on the electron binding energy of M_n^- suggests the barrier may be formed at an avoided crossing between electronic states correlating to M_n^- + O₂ and M_n + O₂^- reactants, analogous to that previously proposed for Al_n^- + O₂ systems. The mechanism is analogous to that for reactions of O₂ with neutral metal surfaces, indicating that gas-phase reactions of anionic metal clusters can be an appropriate model systems for surface oxidation.

Redefining the Mechanism of O2 Etching of Aln- Superatoms: An Early Barrier Controls Reactivity, Analogous to Surface Oxidation

New insights into aluminum anion cluster reactivity with O₂ were obtained through temperature-dependent kinetics measurements. Overall reactivity is controlled by a barrier at an avoided crossing where charge is transferred from the cluster to the O₂, mechanistically similar to what occurs as O₂ approaches a bulk Al surface. Contrary to prior interpretations, spin conservation does not inhibit the reaction of clusters with an odd number of Al atoms. In fact, the only spin constraint in these systems is on the reactivity of even clusters due to repulsive surfaces, not previously recognized. Although the superatom nature of Al₁₃^- is manifest in its high electron binding energy (EBE), the mechanism of its reactivity is not special; it reacts with O₂ as if it were a small piece of bulk Al. These experiments highlight the sensitivity of Al cluster reactivity with O₂ to temperature and EBE, uncovering routes to industrial scale use of aluminum superatom-based materials.