The structure of small nickel clusters. I. Ni3–Ni15

Co-Promoted CoNi Bimetallic Nanocatalyst for the Highly Efficient Catalytic Hydrogenation of Olefins

Bimetallic catalysts, especially non-noble metals, hold great potential for substituting for noble metals in catalytic hydrogenation. In present study, a series of Co_xNi_y (x + y = 6) bimetallic catalysts were prepared through the impregnation-reduction method and cyclohexene was chosen as probe-molecule to study the promotion effect of Co on the catalytic olefin hydrogenation reactions. Meanwhile, density functional theory (DFT) was utilized to investigate the formation energies and the charge distribution of CoNi bimetals, as well as the transition state (TS) searches for hydrogen dissociation and migration. The results suggest that bimetals tend to have superior catalytic performance than pure metals, and Co₃Ni₃ shows the highest catalytic activity on the cyclohexene hydrogenation. It was found that the charge transfer from Co to Ni and the alloying give rise to the refinement of CoNi grains and the improvement of its catalytic activity and stability. Thus, it may be possible to obtain better catalytic performance by tuning the metal/metal atomic ratio of bimetals.

Structure and size-dependent vibrational and thermal properties of Ni clusters: A systematic ab initio approach

There is scarce information on the vibrational and thermal properties of small Ni clusters. Here, the outcomes of ab initio spin-polarized density functional theory calculations on the size and geometry effects upon the vibrational and thermal properties of Ni_n (n = 13 and 55) clusters, are discussed. For theses clusters a comparison is presented between the closed shell symmetric octahedral (O_h) and the icosahedral (I_h) geometries. The results indicate that the I_h isomers are lower in energy. Besides, ab initio molecular dynamics runs at T = 300K show that Ni₁₃ and Ni₅₅ clusters transform from their initial O_h geometries towards the corresponding I_h ones. For Ni₁₃, we also consider the lowest energy less symmetric layered 1-3-6-3 structure, and the cuboid, recently observed experimentally for Pt₁₃, which is competitive in energy but is unstable, as phonon analysis reveals. We calculate their vibrational density of states (νDOS) and heat capacity, and compare with the Ni FCC bulk counterpart. The characteristic features of the νDOS curves of these clusters are interpreted in terms of the clusters' sizes, the interatomic distance contractions, the bond order values as well as the internal pressure and strains of the clusters. We find that the softest possible frequency of the clusters is size and structure-dependent, being the smallest for the O_h ones. We identify mostly shear, tangential type displacements involving mainly surface atoms for the lowest frequency of the spectra of both I_h and O_h isomers. For the maximum frequencies of these clusters the central atom shows anti-phase movements against groups of nearest neighbor atoms. An excess of heat capacity at low temperatures with respect to the bulk is found, while at high temperatures a constant limiting value, close but lower to the Dulong and Petit value, is determined.

Systematic cluster growth: a structure search method for transition metal clusters

The systematic cluster growth (SCG) method is a biased structure search strategy based on a seeding process for investigating the structural evolution and growth pattern of transition metal clusters.

On the influence of exact exchange on transition metal superatoms

The electronic structure of A7C (A = Hg, Pd, V, Cr, Mn, Fe, Ni, Cu; C = 0, ±1, ±2) clusters has been determined using density functional theory methods. The A7C (A = Hg, Pd, Cr, Cu; C = 0, ±1, ±2) clusters all conform to the existing superatomic model, with a sufficiently stabilised local structure to prevent perturbation upon the introduction of exact exchange to the exchange correlation functional. For the A7C (A = Mn, Fe, Ni; C = 0, ±1, ±2) clusters the incorporation of exact exchange separates the atomic s- and d-electrons, leading to a net increase in the number of superatomic electrons. Conversely the incorporation of exact exchange into the exchange correlation functional decreases the number of superatomic electrons for the V7C (C = 0, ±1, ±2) clusters, owing to the radial extension of the d-orbitals influencing their ability to contribute into superatomic shells.

Embedding tetrahedral 3d transition metal TM4 clusters into the cavity of two-dimensional graphdiyne to construct highly efficient and nonprecious electrocatalysts for hydrogen evolution reaction

Coupled with the high structural stability and good conductivity, all the new 2D composite nanostructures TM₄@GDY (TM = Sc, Ti, Mn, Fe, Co, Ni and Cu) can uniformly exhibit considerably high catalytic activity for hydrogen evolution reaction.

Study of odd-even effects in physisorption and chemisorption of Ar, N2, O2 and NO on open shell Ag11-13+ clusters by means of self-consistent van der Waals density functional calculations

We have studied the adsorption and coadsorption properties of one or more X = Ar, N₂, O₂, and NO adsorbates on cationic silver clusters Ag_11-13⁺, whose sizes are in the open shell region of metal clusters, aiming to understand the observed odd-even effects in the abundance spectra of Ag_11-13⁺·mX complexes. All calculations were performed self-consistently using a non-local van der Waals correlation functional, covering the different nature of the interactions between the silver substrate and the several adsorbates, which range from dispersion (London) forces for Ar, non covalent π-π interactions for N₂, charge-transfer interactions for O₂ and NO, and the covalent Ag-Ag bond in the nude silver cluster. Despite the wide interval of adsorption energies, spanning two orders of magnitude, we have been able to explain the following experimental facts. For X = Ar, N₂, and O₂ reactions with Ag_11-13⁺, it was observed in the mass spectra an abundance peak at n = 12 [M. Schmidt, et al., ChemPhysChem, 2015, 16, 855]. In addition it was observed the competitive adsorption of two or more N₂ molecules, and the cooperative effect of adsorbing N₂ together with O₂ molecules. For X = NO, an abundance peak at n = 12 has been also observed [J. Ma, et al., Phys. Chem. Chem. Phys., 2016, 18, 12819]. We find that the main factors determining these properties are the different core motifs of the cluster geometry (pentagonal bipiramid for Ag₁₁⁺ and Ag₁₃⁺, but triangular prism for Ag₁₂⁺) and, on the other hand, the odd number of valence electrons for Ag₁₂⁺, leading to a smaller HOMO-LUMO gap than those of its neighbours. Further details about the preferred adsorption sites, dipole moments, and dipole polarizabilities are also discussed.

Tuning the Catalytic Activity of Pd x Ni y (x + y = 6) Bimetallic Clusters for Hydrogen Dissociative Chemisorption and Desorption

Density functional theory was used to study dissociative chemisorption and desorption on Pd _x Ni _y (x + y = 6) bimetallic clusters. The H₂ dissociative chemisorption energies and the H desorption energies at full H saturation were computed. It was found that bimetallic clusters tend to have higher chemisorption energy than pure clusters, and the capacity of Pd₃Ni₃ and Pd₂Ni₄ clusters to adsorb H atoms is substantially higher than that of other clusters. The H desorption energies of Pd₃Ni₃ and Pd₂Ni₄ are also lower than that of the Pd₆ cluster and comparable to that of the Ni₆ cluster, indicating that it is easier to pull the H atom out of these bimetallic catalysts. This suggests that the catalytic efficiency for specific Pd _x Ni _y bimetallic clusters may be superior to bare Ni or Pd clusters and that it may be possible to tune bimetallic nanoparticles to obtain better catalytic performance.

The Behavior of Magnetic Properties in the Clusters of 4d Transition Metals

The current focus of material science researchers is on the magnetic behavior of transition metal clusters due to its great hope for future technological applications. It is common knowledge that the 4d transition elements are not magnetic at their bulk size. However, studies indicate that their magnetic properties are strongly dependent on their cluster sizes. This study attempts to identify magnetic properties of 4d transition metal clusters. Using a tight-binding Friedel model for the density of d-electron states, we investigated the critical size for the magnetic-nonmagnetic transition of 4d transition-metal clusters. Approaching to the critical point, the density of states of the cluster near the Fermi level is higher than 1/J and the discrete energy levels form a quasi-continuous band. Where J is correlation integral. In order to determine the critical size, we considered a square shape band and fcc, bcc, icosahedral and cuboctahedral close-packed structures of the clusters. We also investigated this size dependent magnetic behavior using Heisenberg model. Taking some quantum mechanical approximations in to consideration, we determined magnetic behavior of the clusters. For practicality, we considered three clusters of transition metals (Ru, Rh and Pd) and the obtained results are in line with the results of previous studies.

Superatomic states in nickel clusters: Revising the prospects for transition metal based superatoms

The geometries and electronic structures of small Ni_n^z clusters (n = 8, 9, 10) (z = 0, ±1, 2) have been elucidated for a range of multiplicities for each cluster size and charge, using density functional theory methods. These clusters have been found to conform in part to the existing superatomic model, with each cluster having a filled superatomic S-orbital, filled or partially filled superatomic P-orbitals, and empty or partially filled superatomic D-orbitals. Despite local states of mixed symmetry being present in the immediate vicinity of the Fermi energy, the addition or removal of a single electron from these systems causes a significant shift in the relative energies of the superatomic orbitals. In addition, this study demonstrates the possibility for d-electrons to contribute into superatomic orbitals to a greater or lesser extent, depending on the local environment. In summary, these observations lead to the prospect of a predictive model for electronic shell closings in some transition metal cluster systems.

Enhancing 4-propylheptane dissociation with nickel nanocluster based on molecular dynamics simulations

In the present work, a 0.4nm nickel cluster has been theoretically studied. Its equilibrium structural parameters have been calculated by the DFT method based on the PBEH1PBE hybrid functional and split-valence basis set Lanl2DZ including effective core potentials. We have systematically considered diverse spin states of this cluster and find out its ground state. The relative stability of these states depends on the HOMO-LUMO gap. The interaction of the Ni₆ with 4-propylheptane С₁₀Н₂₂ has been studied to simulate the process of catalytic cracking of hydrocarbons. The optimization of this structure has been performed by the ωPBE/Lanl2DZ_ecp method (the TeraChem V.1.9 program package) with no symmetry restrictions; the electron shells of the metal were described by effective core pseudopotentials. For visualization and quantitative estimation of the bonding bonds between the nickel nanocluster and 4-propylheptane, the analysis of weak interactions based on RGD has been performed. To confirm the proposition about the formation of Ni-H bonds, we have scrutinized critical points of electronic density. Values of laplasian of electronic density and Bader atomic charge distribution in the global minimum of the total energy have been estimated by the AIMAll 15.05.18 program suite. Finally, we have simulated interaction of Ni₆ with 4-propylheptane in terms of the Born-Oppenheimer ab initio molecular dynamics. The results of the molecular dynamics simulation provide pair radial distribution function CH at 1500°C and a detailed picture of the processes occurring in the system.

DFT studies of Ni cluster on graphene surface: effect of CO2 activation

The activation of CO₂ can be significantly enhanced by Ni cluster deposited onto monovacancy graphene surface.

The electronic structures of small Ni(n) (n=2-4) clusters and their interactions with ethylene and triplet oxygen: a theoretical study

Density functional theory (DFT) calculations of small nickel clusters and their interacting systems are carried out using the BLYP and B97-2 methods, after DFT calibration. All bare nickel clusters in this study have high multiplicities and are paramagnetic. Our results for the interactions between ethylene and oxygen with Ni(n) (n=2-4) clusters at different adsorption modes show that for ethylene, π-orientation is preferred, and that oxygen adsorption in a bridge mode is stronger than on-top coordination. Vibrational frequency analysis reveals that the vibrational modes of ethylene π-coordinated to nickel clusters converge toward the corresponding value for surface-bound ethylene, as the cluster size increases from two to four, showing that finite clusters can be used as localized models for ligand adsorption on nickel surfaces. We also calculate DFT global reactivity descriptors, chemical potential and hardness, and use these to predict the relative stability and reactivity of each bare cluster.

Single-molecule sensing using carbon nanotubes decorated with magnetic clusters

First-principles and nonequilibrium Green's function techniques are used to investigate magnetism and spin-polarized quantum transport in metallic carbon nanotubes (CNT) decorated with transition metal (Ni(13), Pt(13)) magnetic nanoclusters (NC). For small cluster sizes, the strong CNT-NC interaction induces spin-polarization in the CNT. The adsorption of a benzene molecule is found to drastically modify the CNT-NC magnetization. Such a magnetization change should be large enough to be detected via magnetic-AFM or SQUID magnetometry, hence suggesting a novel approach for single-molecule gas detection.

Spin fluctuations of paramagnetic Rh clusters revealed by x-ray magnetic circular dichroism

The magnetic moment induced on Rh atoms, forming 1.6 nm average diameter clusters, embedded in an Al(2)O(3) matrix, has been determined using x-ray magnetic circular dichroism measurements. The magnetic moment varies linearly with the applied magnetic field. At 2.3 K and under 17 T, the spin magnetic moment amounts to 0.067(2) μ(B)/Rh atom. The orbital moment does not exceed 2% of the spin moment. The susceptibility is highly temperature dependent. This is in agreement with a prediction due to Moriya and Kawabata, that in itinerant electron systems, close to the onset of magnetism, the renormalization of the magnetic susceptibility by electron correlations, leads to a Curie-like behavior.

A density-functional study of the structures, binding energies and magnetic moments of the clusters Mo(N) (N = 2-13), Mo(12)Fe, Mo(12)Co and Mo(12)Ni

We report the results of density-functional calculations of the structures, binding energies and magnetic moments of the clusters Mo(N) (N = 2-13), Mo(12)Fe, Mo(12)Co and Mo(12)Ni that were performed using the SIESTA method within the generalized gradient approximation for exchange and correlation. For pure Mo(N) clusters, we obtain collinear magnetic structures in all cases, even when the self-consistent calculations were started from non-collinear inputs. Our results for these clusters show that both linear, planar and three-dimensional clusters have a strong tendency to form dimers. In general, even-numbered clusters are more stable than their neighbouring odd-numbered clusters because they can accommodate an integer number of tightly bound dimers. As a consequence, the binding energies of pure Mo(N) clusters, in their lowest-energy states, exhibit an odd-even effect in all dimensionalities. Odd-even effects are less noticeable in the magnetic moments than in the binding energies. When comparing our results for pure Mo clusters with those obtained recently by other authors, we observe similarities in some cases, but striking differences in others. In particular, the odd-even effect in three-dimensional Mo clusters was not observed before, and our results for some clusters (e.g. for planar Mo(3) and Mo(7) and for three-dimensional Mo(7) and Mo(13)) differ from those reported by other authors. For Mo(12)Fe and Mo(12)Ni, we obtain that the icosahedral configuration with the impurity atom at the cluster surface is more stable than the configuration with the impurity at the central site, while the opposite occurs in the case of Mo(12)Co. In Mo(12)Co and Mo(12)Ni, the impurities exhibit a weak magnetic moment parallely coupled to the total magnetic moment of the Mo atoms, whereas in Mo(12)Fe the impurity shows a high moment with antiparallel coupling.

H2 adsorption on 3d transition metal clusters: a combined infrared spectroscopy and density functional study

The adsorption of H2 on a series of gas-phase transition metal (scandium, vanadium, iron, cobalt, and nickel) clusters containing up to 20 metal atoms is studied using IR-multiple photon dissociation spectroscopy complemented with density functional theory based calculations. Comparison of the experimental and calculated spectra gives information on hydrogen-bonding geometries. The adsorption of H2 is found to be exclusively dissociative on Sc(n)O+, V(n)+, Fe(n)+, and Co(n)+, and both atomic and molecularly chemisorbed hydrogen is present in Ni(n)H(m)+ complexes. It is shown that hydrogen adsorption geometries depend on the elemental composition as well as on the cluster size and that the adsorption sites are different for clusters and extended surfaces. In contrast to what is observed for extended metal surfaces, where hydrogen has a preference for high coordination sites, hydrogen can be both 2- or 3-fold coordinated to cationic metal clusters.

Assessment of density functional theory optimized basis sets for gradient corrected functionals to transition metal systems: The case of small Nin (n⩽5) clusters

Density functional calculations have been performed for small nickel clusters, Ni(n), Ni(n) (+), and Ni(n)(-) (n<or=5), using the linear combination of Gaussian-type orbital density functional theory approach. Newly developed nickel all-electron basis sets optimized for generalized gradient approximation (GGA) as well as an all-electron basis set optimized for the local density approximation were employed. For both neutral and charged systems, several isomers and different multiplicities were studied in order to determine the lowest energy structures. A vibrational analysis was performed in order to characterize these isomers. Structural parameters, harmonic frequencies, binding energies, ionization potentials, and electron affinities are reported. This work shows that the employed GGA basis sets for the nickel atom are important for the correct prediction of the ground state structures of small nickel clusters and that the structural assignment of these systems can be performed, with a good resolution, over the ionization potential.

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.

Homonuclear transition-metal trimers

Density-functional theory has been used to determine the ground-state geometries and electronic states for homonuclear transition-metal trimers constrained to equilateral triangle geometries. This represents the first application of consistent theoretical methods to all of the ten 3d block transition-metal trimers, from scandium to zinc. A search of the potential surfaces yields the following electronic ground states and bond lengths: Sc3(2A1',2.83 A), Ti3(7E',2.32 A), V3(2E",2.06 A), Cr3(17E',2.92 A), Mn3(16A2',2.73 A), Fe3(11E",2.24 A), Co3(6E",2.18 A), Ni3(3A2",2.23 A), Cu3(2E',2.37 A), and Zn3(1A1',2.93 A). Vibrational frequencies, several low-lying electronic states, and trends in bond lengths and atomization energies are discussed. The predicted dissociation energies DeltaE(M3-->M2+M) are 49.4 kcal mol(-1)(Sc3), 64.3 kcal mol(-1)(Ti3), 60.7 kcal mol(-1)(V3), 11.5 kcal mol(-1)(Cr3), 32.4 kcal mol(-1)(Mn3), 61.5 kcal mol(-1)(Fe3), 78.0 kcal mol(-1)(Co3), 86.1 kcal mol(-1)(Ni3), 26.8 kcal mol(-1)(Cu3), and 4.5 kcal mol(-1)(Zn3).

Shell and subshell periodic structures of icosahedral nickel nanoclusters

Using the modified analytic embedded atom method and molecular dynamics, the binding energies and their second order finite differences (stability functions) of icosahedral Ni clusters with shell and subshell periodicity are studied in detail via atomic evolution. The results exhibit shell and subshell structures of the clusters with atoms from 147 to 250,000, and the atomic numbers corresponding to shell or subshell structures are in good agreement with the experimental magic numbers obtained in time-of-flight mass spectra of threshold photoionization, and Martin's theoretical proposition of progressive formation of atomic umbrellas. Clusters with size from 147 to 561 atoms are energetically investigated via one-by-one atomic evolution and their magic numbers are theoretically proved. For medium-size Ni clusters with 561 to 2057 atoms, the prediction of magic numbers with atomic numbers is performed on the basis of umbrella-like subshell growth in near face-edge-vertex order. The similarity of the energy curves makes it possible to extend the prediction to even larger Ni nanoclusters in hierarchical Mackay icosahedral configurations.

Enhancement of nitrogen physisorption in coadsorption with oxygen on free, positively charged silver clusters

The coadsorption of molecular nitrogen and oxygen on small cationic silver clusters in the gas phase is experimentally studied. The presence of oxygen enhances the adsorption probabilities of N2. This indicates a partial charge transfer out of the finite free electron reservoir of the small silver particles into the chemisorbed oxygen molecule.