Collision‐induced dissociation of Ni+n (n=2–18) with Xe: Bond energies, geometrical structures, and dissociation pathways

Orbital-Dependent Photodynamics of Strongly Correlated Nickel Oxide Clusters

The ultrafast electronic relaxation dynamics of neutral nickel oxide clusters were investigated with femtosecond pump-probe spectroscopy and supported with theoretical calculations to reveal that their excited state lifetimes are strongly...

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.

Catalytic CO2 reduction to valuable chemicals using NiFe-based nanoclusters: a first-principles theoretical evaluation

The catalytic performance and possible mechanisms of CO₂ hydrogenation on noble-metal-free NiFe bimetal nanoparticles are theoretically evaluated.

Gas-phase perspective on the thermodynamics and kinetics of heterogeneous catalysis

Gas-phase studies of small transition metal cluster cations provide thermochemistry of utility to surface science and heterogeneous catalysis.

Influence of Single Impurity Atoms on the Structure, Electronic, and Magnetic Properties of Ni5 Clusters

With a gradient-corrected density functional method, we have studied computationally the influence of single impurity atoms on the structure, electronic, and magnetic properties of Ni5 clusters. The square-pyramidal isomer of bare Ni5 with six unpaired electrons was calculated 23 kJ/mol more stable than the trigonal bipyramid in its lowest-energy electronic configuration with four unpaired electrons. In a previous study on the cluster Ni4, we had obtained only one stable isomer with an O or an H impurity, but we located six minima for ONi5 and five minima for HNi5. In the most stable structures of HNi5, the H atom bridges a Ni-Ni edge at the base or the side of the square pyramid, similarly to the coordination of an H atom at the tetrahedral cluster Ni4. The most stable ONi5 isomers exhibit a trigonal bipyramidal structure of the Ni5 moiety, with the impurity coordinated at a facet, (micro3-O)Ni5, or at an apex edge, (micro-O)Ni5. We located four stable structures for a C impurity at a Ni5 cluster. As for CNi4, the most stable structure of the corresponding Ni5 complex comprises a four-coordinated C atom, (micro4-C)Ni5, and can be considered as insertion of the impurity into a Ni-Ni bond of the bare cluster. All structures with C and five with O impurity have four unpaired electrons, while the number of unpaired electrons in the clusters HNi5 varies between 3 and 7. As a rough trend, the ionization potentials and electron affinities of the clusters with impurity atoms decrease with the coordination number of the impurity. However, the position of the impurity and the shape of the metal moiety also affect the results. Coordination of an impurity atom leads to a partial oxidation of the metal atoms.

Thermochemistry of the activation of N2 on iron cluster cations: Guided ion beam studies of the reactions of Fen+ (n=1–19) with N2

The kinetic energy dependences of the reactions of Fe(n)+ (n = 1-19) with N2 are studied in a guided ion beam tandem mass spectrometer over the energy range of 0-15 eV. In addition to collision-induced dissociation forming Fe(m)+ ions, which dominate the product spectra, a variety of Fe(m)N2+ and Fe(m)N+ product ions, where m < or = n, is observed. All processes are observed to exhibit thresholds. Fe(m)+ - N and Fe(m)+ - 2N bond energies as a function of cluster size are derived from the threshold analysis of the kinetic energy dependences of the endothermic reactions. The trends in this thermochemistry are compared to the isoelectronic D0(Fe(n)+ - CH), and to bulk phase values. A fairly uniform barrier of 0.48+/-0.03 eV at 0 K is observed for formation of the Fe(n)N2+ product ions (n = 12, 15-19) and can be related to the rate-limiting step in the Haber process for catalytic ammonia production.

Methane activation by nickel cluster cations, Ni[sub n][sup +] (n=2–16): Reaction mechanisms and thermochemistry of cluster-CH[sub x] (x=0–3) complexes

The kinetic energy dependences of the reactions of Ni+(n) (n=2-16) with CD(4) are studied in a guided ion beam tandem mass spectrometer over the energy range of 0-10 eV. The main products are hydride formation Ni(n)D+, dehydrogenation to form Ni(n)CD+(2), and double dehydrogenation yielding Ni(n)C+. These primary products decompose at higher energies to form Ni(n)CD+, Ni(n-1)D+, Ni(n-1)C+, Ni(n-1)CD+, and Ni(n-1)CD+(2). Ni(n)CD(2) (+) (n=5-9) and Ni(n-1)CD(2) (+) (n > or =4) are not observed. In general, the efficiencies of the single and double dehydrogenation processes increase with cluster size. All reactions exhibit thresholds, and cross sections for the various primary and secondary reactions are analyzed to yield reaction thresholds from which bond energies for nickel cluster cations to C, CD, CD(2), and CD(3) are determined. The relative magnitudes of these bond energies are consistent with simple bond order considerations. Bond energies for larger clusters rapidly reach relatively constant values, which are used to estimate the chemisorption energies of the C, CD, CD(2), and CD(3) molecular fragments to nickel surfaces.

The thermochemistry of adsorbates on transition metal cluster ions: relationship to bulk-phase properties

In previous work, guided ion beam tandem mass spectrometry has been used to study the reactions of the cluster cations of several transition metals (V, Cr, Fe and Ni) with D2, O2, CO2 and CD4. By examining the kinetic energy dependence of these reactions and interpreting thresholds observed for various reactions, bond energies for D, O, C, CD, CD2 and CD3 can be obtained. The results of these studies are reviewed with an emphasis on the relationship between the thermochemistry obtained from this work with that for adsorbates on bulk-phase metal surfaces. It is found that in cases where quantitative comparison can be made, for example D and O atoms, modest sized clusters bind these species to approximately the same extent as the bulk-phase surface of the same metal. As there is little information available for the molecular fragments investigated here, the cluster bond energies provide some of the first experimental values for such adsorbates on surfaces.

Mass spectrometry--not just a structural tool: the use of guided ion beam tandem mass spectrometry to determine thermochemistry

Guided ion beam tandem mass spectrometry has proved to be a robust tool for the measurement of thermodynamic information. Over the past twenty years, we have elucidated a number of factors necessary to make such thermochemistry accurate. Careful attention must be paid to the reduction of the raw data, ion intensities versus laboratory ion energies, to a more useful form, reaction cross sections versus relative kinetic energy. Analysis of the kinetic energy dependence of cross sections for endothermic reactions can then reveal thermodynamic data for both bimolecular and collision-induced dissociation (CID) processes. Such analyses need to include consideration of the explicit kinetic and internal energy distributions of the reactants, the effects of multiple collisions, the identity of the collision partner in CID processes, the kinetics of the reaction being studied, and competition between parallel reactions. This work provides examples illustrating the need to consider this multitude of effects along with details of the procedures developed in our group for handling each of them.

Reactions and thermochemistry of small transition metal cluster ions

This review discusses the reactivities and thermodynamics of small-size-specific transition metal clusters and focuses on thermodynamic information, which has not been comprehensively discussed before. Because of this focus, guided-ion-beam mass spectrometry was used to acquire much of the data. The details of this technique and the associated data analysis methods are provided. Results on the stabilities of bare transition metal clusters are provided for neutral, cationic, and anionic species. Implications for the electronic and geometrical structures are discussed, as well as the extrapolation of these values to bulk phase behavior. Detailed results for reactions of transition metal clusters with D2 and the oxygen donors O2 and CO2 are reviewed. Available bond energies between size-specific clusters and one D atom and one and two O atoms are compiled, and their implications are evaluated and favorably compared with bulk phase analogs. Several additional thermodynamic studies of various cluster systems are also discussed.