Competitive Molecular and Dissociative Hydrogen Chemisorption on Size Selected Doubly Rhodium Doped Aluminum Clusters

Hydrogen Storage Capacity of Single-Nb-Atom-Doped Al Clusters in the Gas Phase Revealed by Thermal Desorption Spectrometry

Hydrogen is a promising energy resource as a substitute for fossil fuels, and metal alloy hydrides are considered to be good candidates as hydrogen storage materials. In the hydrogen storage processes, hydrogen desorption is as important as hydrogen adsorption. In order to understand the hydrogen desorption features of those clusters, here, single-Nb-atom-doped Al clusters were prepared in the gas phase and their reaction with hydrogen was investigated using thermal desorption spectrometry (TDS). On average, six to eight H atoms were adsorbed in Al_nNb⁺ (n = 4-18) clusters, and most H atoms were released upon heating of the clusters to 800 K. Two types of desorption features of Al_nNb⁺ clusters were found, which related to the flexibility of the clusters. This study demonstrated the potential of Nb-doped Al alloy as an efficient hydrogen storage material with high storage capacity, thermal stability at room temperature, and hydrogen desorption ability upon moderate heating.

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.

Water Splitting by C60 -Supported Vanadium Single Atoms

Water splitting is an important source of hydrogen, a promising future carrier for clean and renewable energy. A detailed understanding of the mechanisms of water splitting, catalyzed by supported metal atoms or nanoparticles, is essential to improve the design of efficient catalysts. Here, we report an infrared spectroscopic study of such a water splitting process, assisted by a C₆₀ supported vanadium atom, C₆₀ V⁺ +H₂ O→C₆₀ VO⁺ +H₂ . We probe both the entrance channel complex C₆₀ V⁺ (H₂ O) and the end product C₆₀ VO⁺ , and observe the formation of H₂ as a result from resonant infrared absorption. Density functional theory calculations exploring the detailed reaction pathway reveal that a quintet-to-triplet spin crossing facilitates the water splitting reaction by C₆₀ -supported V⁺ , whereas this reaction is kinetically hindered on the isolated V⁺ ion by a high energy barrier. The C₆₀ support has an important role in lowering the reaction barrier with more than 70 kJ mol^-1 due to a large orbital overlap of one water hydrogen atom with one carbon atom of the C₆₀ support. This fundamental insight in the water splitting reaction by a C₆₀ -supported single vanadium atom showcases the importance of supports in single atom catalysts by modifying the reaction potential energy surface.

An octacoordinated Nb atom in the NbAl8H8+ cluster

The NbAl₈H₈⁺ cluster was formed in a molecular beam and characterized by mass spectrometry and infrared spectroscopy. Density functional theory calculations show the lowest-energy isomer is a high symmetry singlet with the Nb atom placed at the center of a distorted hexagonal Al ring and coordinated by two AlH moieties, therefore exhibiting octacoordination. The unprecedented high-symmetric geometry is attributed to the 20 valence electrons; the central Nb atom adheres to the 18-electron rule and two additional delocalized electrons stabilize the hexagonal ring.

Density Functional Theory Modeling of Reactions of Addition of H2 Molecules to Magnesium Clusters Mg17M Doped with Atoms M of Transition 3d Elements

Density functional theory calculations of potential energy surface (PES) have been performed for elementary hydrogenation reactions Mg₁₇M + H₂ → Mg₁₇MH₂ of magnesium clusters Mg₁₇M doped by transition 3d metals (M = Ti-Ni), and for consecutive reactions Mg₁₇Ni + nH₂ → Mg₁₇NiH_2n of addition of n hydrogen molecules to Ni-doped clusters Mg₁₇Ni and Mg₁₇NiH₂. Energetic, geometric, and spectroscopic features of intermediates and transition states along the minimum energy pathway have been found, and their trends were analyzed with dopants changing along the 3d series and with increasing number of atoms H attached to the surface positions of the magnesium backbone.

Argon tagging of doubly transition metal doped aluminum clusters: The importance of electronic shielding

The interaction of argon with doubly transition metal doped aluminum clusters, Al_nTM₂ ⁺ (n = 1-18, TM = V, Nb, Co, Rh), is studied experimentally in the gas phase via mass spectrometry. Density functional theory calculations on selected sizes are used to understand the argon affinity of the clusters, which differ depending on the transition metal dopant. The analysis is focused on two pairs of consecutive sizes: Al_6,7V₂ ⁺ and Al_4,5Rh₂ ⁺, the largest of each pair showing a low affinity toward Ar. Another remarkable observation is a pronounced drop in reactivity at n = 14, independent of the dopant element. Analysis of the cluster orbitals shows that this feature is not a consequence of cage formation but is electronic in nature. The mass spectra demonstrate a high similarity between the size-dependent reactivity of the clusters with Ar and H₂. Orbital interactions provide an intuitive link between the two and further establish the importance of precursor states in the reactions of the clusters with hydrogen.

Nanowire reconstruction under external magnetic fields

We consider the different structures that a magnetic nanowire adsorbed on a surface may adopt under the influence of external magnetic or electric fields. First, we propose a theoretical framework based on an Ising-like extension of the 1D Frenkel-Kontorova model, which is analyzed in detail using the transfer matrix formalism, determining a rich phase diagram displaying structural reconstructions at finite fields and an antiferromagnetic-paramagnetic phase transition of second order. Our conclusions are validated using ab initio calculations with density functional theory, paving the way for the search of actual materials where this complex phenomenon can be observed in the laboratory.

Hydrogen Chemisorption on Doubly Vanadium Doped Aluminum Clusters

The interaction of hydrogen with doubly vanadium doped aluminum clusters, Al n V2 + (n = 1–12), is studied experimentally by time-of-flight mass spectrometry and infrared multiple photon dissociation spectroscopy. The hydrogen binding geometry is inferred from comparison with infrared spectra predicted by density functional theory and shows that for the more reactive clusters the hydrogen adsorbs dissociatively. Three sizes, n = 4, 5 and 7, are remarkably unreactive compared to the other clusters. For larger sizes the reactivity decreases, a behavior that is similar to that of singly vanadium doped aluminum clusters, and that might be attributed to geometric and/or electronic shielding of the dopants. By examining the electronic structure of Al6V2 + and Al7V2 +, interactions between the frontier orbitals of the clusters and those of H2 that explain the size-dependent reactivity are identified.

Structure, stability, absorption spectra and aromaticity of the singly and doubly silicon doped aluminum clusters AlnSim0/+ with n = 3–16 and m = 1, 2

Structures of the binary Al_nSi_m clusters in both neutral and cationic states were investigated using DFT and TD-DFT (B3LYP/6-311+G(d)) and (U)CCSD(T)/cc-pvTZ calculations. Silicon-doped aluminum clusters are characterized by low spin ground states. For small sizes, the Si dopant prefers to be located at vertices having many edges. For larger sizes, the Si atom prefers to be endohedrally doped inside an Al_n cage. Relative stability, adiabatic ionization energy and dissociation energies of each cluster size were evaluated. A characteristic of most Si doped Al clusters is the energetic degeneracy of two lowest-lying isomers. Calculated results confirm the high stability of the sizes Al₄Si₂, Al₁₂Si and Al₁₁Si₂⁺ as “magic” clusters, that exhibit 20 or 40 shell electrons and are thermodynamically more stable as compared to their neighbors. Electronic absorption spectra of isoelectronic magic clusters Al₁₃⁻, Al₁₂Si, and Al₁₁Si₂⁺ that have two pronounced bands corresponding to blue and violet lights, have been rationalized by using the electron shell model. The magnetically included ring current density (MICD) analyses suggest that they are also aromatic structures as a result of the “magic” 40 shell electrons.

The isoelectronic “magic” clusters with 40 shell electrons have enhanced thermochemical stability.

Tuning the Reactivity of Small Metal Clusters by Heteroatom Doping

The reactivity of small metallic clusters, nanoparticles composed of a countable number of atoms (typically up to ∼100 atoms), has attracted much attention due to the fascinating properties these objects possess toward a variety of molecules. Cluster reactivity often is significantly different from the homologous bulk, with gold as prototypical example. Bulk gold is the noblest of all metals, whereas small gold clusters react with carbon monoxide, molecular oxygen, and hydrocarbons, among others. Furthermore, cluster reactivity is strongly size and composition dependent, allowing a wide range of tuning possibilities. The study of cluster reactivity usually follows two routes of investigation. In the first, research aims for fundamental understanding of mechanisms, mainly driven by curiosity. One consequence of the inherent small size of a cluster is that atoms can arrange themselves very differently from the crystallographic structure of the homologous bulk. In addition, quantum confinement effects dominate the electronic structure of a cluster with atom-like electronic shells instead of the electronic bands in bulk. These features result in a very rich and size-dependent interaction of a cluster with small molecules, governed by a fine interplay between the geometry and the electronic structure of the system. An alternative research approach uses the investigation of chemical reactions of isolated small clusters in the gas phase as model systems for the reactions taking place in more complex systems. This offers several advantages compared to more conventional methods and techniques used to study such complex systems. First, clusters can be produced under well-defined conditions, with control over size, composition, and charge state. Second, clusters in the gas phase solely interact with the molecule(s) chosen by the researcher, since contaminations are limited by the high vacuum conditions of the experiments. Third, due to the small number of atoms involved, detailed quantum chemical calculations can be performed on the systems under investigation. Thus, even though gas phase clusters differ significantly in size and in environmental conditions from those encountered, for example, in industrial catalysis, they can be used to unravel the complicated nature of a metal-molecule chemical bonding process. In this Account, both routes of investigation are discussed. The nature of the interaction between small gas phase clusters with diverse molecules is described, stressing the broader relevance of these studies. Particular emphasis is given to the effect of heteroatom doping. By adding a different element to a cluster, its geometric and electronic structure is modified, thereby altering its reactivity. Specifically, the effect of varying size and composition of doped gold, platinum, and aluminum clusters on their reactivity toward diverse molecules, relevant for catalytic applications, is discussed. Most studies presented here combine experiments based on mass spectrometric techniques with density functional theory calculations, allowing a deep understanding of the reaction mechanisms at a molecular level.