Transition Metal Clusters Research Articles

Structural and Magnetic Properties of Small 4d Transition Metal Clusters: Role of Spin–Orbit Coupling

The spin and orbital magnetic moments, as well as the magnetic anisotropy energy (MAE), of small 4d transition metal (TM) clusters are systematically studied by using the spin-orbit coupling (SOC) implementation of the density-functional theory (DFT). The effects of spin-orbit interactions on geometrical structures and spin moments are too weak to alternate relative stabilities of different low-lying isomers. Remarkable orbital contributions to cluster magnetic moments are identified in Ru, Rh, and Pd clusters, in contrast to immediate quenching of the atomic orbital moment at the dimer size in other elemental clusters. More interestingly, there is always collinearity between total spin and orbital moments (antiferromagnetic or ferromagnetic coupling depends on the constituent atoms whose 4d subshell is less or more than half-filled). The clusters preserve the validity of Hund's rules for the sign of orbital moment. The calculations on MAEs reveal the complicated changes of the easy axes in different structures. The perturbation theory and the first-principles calculations are compared to emphasize how MAEs evolve with cluster size. Finally, large orbital moments combined with strong spin-orbit coupling are proposed to account for large MAEs in Ru, Rh, and Pd clusters.

Structure, dynamic and energetic of mixed transition metal clusters

Classical molecular dynamics simulation (MD) with Sutton-Chen potential has been used to generate the minimum energy and to study the thermodynamic and dynamic properties of mixed transition metal cluster motifs of AgnNi(13−n) for n ≤ 13. Literature results of thirteen particle clusters of neat silver and nickel atoms were first reproduced before the successive replacement of the silver atom by nickel. Calculation was repeated for both silver-centred and nickel-centred clusters. It was found that the nickel-centred clusters were more stable than the silver-centred clusters. Heat capacities and hence the melting points of silver and nickel-centred clusters were determined by using the Histogram method. Species-centric order parameters developed by Hewage and Amar were used to understand the dynamic behaviour in the transition of silver-centred clusters to more stable nickel-centred clusters. This species-centric order parameter calculation further confirmed the stability of nickel-centred clusters over those of silver-centred species.

Probing the structural and electronic properties of small vanadium dioxide clusters by density functional theory and comparison with experimental photoelectron spectroscopy

The structural evolution and bonding of a series of early transition-metal dioxide clusters, V(n)O(2)(q)(n = 3-9, q = 0, -1), have been investigated using density functional theory (DFT) calculations and the results are compared with experimental literature data. For each vanadium dioxide cluster, many low-lying isomers are generated using the Saunders "Kick" global minimum stochastic search method. Theoretical electron detachment energies (both vertical and adiabatic) were compared with the experimental measurements to verify the ground states of the vanadium dioxide clusters obtained from the DFT calculations. Five kinds of dissociative adsorption configurations of ground-state structure of V(n)O(2)(q) are identified. The dissociative adsorption of O(2) on V(n)(-1, 0) is more favorable than O(2) molecular adsorption. Furthermore, the adsorption energy of O(2) is higher than that of a single atom on the bare V(n)(-1, 0) clusters, but less than twice the adsorption energy for an atom, indicating that O(2) being adsorbed on vanadium clusters are more difficult than single O atom adsorbed on vanadium clusters.

Initial- and final-state effects on screening and branching ratio in2px-ray absorption of size-selected free3dtransition metal clusters

A comprehensive study of size-dependent initial- and final-state effects in $2p$ x-ray absorption of free, homonuclear $3d$ transition metal clusters from titanium through nickel in a size range of 1--200 atoms is presented. From atom to bulk the $2p\ensuremath{\rightarrow}3d$ excitation in $3d$ transition metals shifts to higher photon energy, which can be explained by increasing valence electron delocalization in the initial state. A cluster-size-dependent decrease of the ${L}_{2,3}$ branching ratio at a constant $2p$ spin-orbit splitting can be attributed to core-hole screening effects for all investigated $3d$ transition metal clusters. For small manganese cluster ions, highly localized $3d$ orbitals with ferromagnetic spin coupling and a filled $3d$ majority spin subband are suggested by the experimental $2p$ x-ray absorption spectra.

Adsorption of NO on the Rh13, Pd13, Ir13, and Pt13Clusters: A Density Functional Theory Investigation

The adsorption of NO on transition-metal (TM) surfaces has been widely studied by experimental and theoretical techniques; however, our atomistic understanding of the interaction of nitrogen monoxide (NO) with small TM clusters is far from satisfactory, which compromises a deep under- standing of real catalyst devices. In this study, we report a density functional theory study of the adsorption properties of NO on the TM13 (TM = Rh, Pd, Ir, Pt) clusters employing the projected augmented wave method. We found that the interaction of NO with TM13 is much more complex than that for NO/TM(111). In particular, for low symmetry TM13 clusters, there is a strong rearrangement of the electronic charge density upon NO adsorption and, as a consequence, the adsorption energy shows a very complex dependence even for adsorption sites with the same local effective coordination. We found a strong enhancement of the binding energy of NO to the TM13 clusters compared with the TM(111) surfaces, as the antibonding NO states are not occupied for NO/TM13, and the general relationship based on the d-band model between adsorption energy and the center of gravity of the occupied d-states does not hold for the studied TM13 clusters, in particular, for clusters with low symmetry. In contrast with the adsorption energy trends, the geometric NO/TM13 parameters and the vibrational N−O frequencies for different coordination sites follow the same trend as for the respective TM(111) surfaces, while the changes in the frequencies between different surfaces and TM13 clusters reflect the strong NO−TM13 interaction.

Strong-field ionization of small niobium and tantalum clusters

Strong-field ionization and concomitant Coulomb explosion studies are performed on small (<30 atoms) homogeneous transition metal clusters composed of niobium or tantalum. Enhanced ionization of the clusters results in the formation of charge states reaching M11+ [M=Nb, Ta] under irradiation with 100fs pulses of 624nm light at intensities of approximately 1PW/cm2. The same maximum charge states were created upon expansion of the pulse width to 350fs with consistent pulse energy. These results are compared to previous studies involving metal oxide and carbide clusters of similar size, yielding identical ion states. This suggests that the enhanced ionization behavior of metal-containing clusters is driven by the delocalized electrons on the metal species.

Spin in density‐functional theory

AbstractThe accurate description of open‐shell molecules, in particular of transition metal complexes and clusters, is still an important challenge for quantum chemistry. Although density‐functional theory (DFT) is widely applied in this area, the sometimes severe limitations of its currently available approximate realizations often preclude its application as a predictive theory. Here, we review the foundations of DFT applied to open‐shell systems, both within the nonrelativistic and the relativistic framework. In particular, we provide an in‐depth discussion of the exact theory, with a focus on the role of the spin density and possibilities for targeting specific spin states. It turns out that different options exist for setting up Kohn–Sham DFT schemes for open‐shell systems, which imply different definitions of the exchange–correlation energy functional and lead to different exact conditions on this functional. Finally, we suggest possible directions for future developments. © 2012 Wiley Periodicals, Inc.

Scalable properties of metal clusters: A comparative study of modern exchange-correlation functionals

The performance of eight generalized gradient approximation exchange-correlation (xc) functionals is assessed by a series of scalar relativistic all-electron calculations on octahedral palladium model clusters Pd(n) with n = 13, 19, 38, 55, 79, 147 and the analogous clusters Au(n) (for n up through 79). For these model systems, we determined the cohesive energies and average bond lengths of the optimized octahedral structures. We extrapolate these values to the bulk limits and compare with the corresponding experimental values. While the well-established functionals BP, PBE, and PW91 are the most accurate at predicting energies, the more recent forms PBEsol, VMTsol, and VT{84}sol significantly improve the accuracy of geometries. The observed trends are largely similar for both Pd and Au. In the same spirit, we also studied the scalability of the ionization potentials and electron affinities of the Pd clusters, and extrapolated those quantities to estimates of the work function. Overall, the xc functionals can be classified into four distinct groups according to the accuracy of the computed parameters. These results allow a judicious selection of xc approximations for treating transition metal clusters.

Communication: The structures of small cationic gas-phase platinum clusters

The structures of small platinum clusters Pt(3-5)(+) are determined using far-infrared multiple photon dissociation spectroscopy of their argon complexes combined with density functional theory calculations. The clusters are found to have compact structures, and Pt(4)(+) and Pt(5)(+) already favor three-dimensional geometries, in contrast to a number of earlier predictions. Challenges in applying density functional theory to 3rd row transition metal clusters are addressed. Preliminary calculations suggest that the effects of spin-orbit coupling do not change the favoured lowest-energy isomers.

ChemInform Abstract: Chemical Transformations Supported by the [Re6(μ3‐Se)8]2+ Cluster Core

AbstractReview: 62 refs.

Encapsulation of small magnetic clusters in fullerene cages: A density functional theory investigation within van der Waals corrections

The encapsulation of magnetic transition-metal (TM) clusters inside carbon cages (fullerenes, nanotubes) has been of great interest due to the wide range of applications, which spread from medical sensors in magnetic resonance imaging to photonic crystals. Several theoretical studies have been reported; however, our atomistic understanding of the physical properties of encapsulated magnetic TM $3d$ clusters is far from satisfactory. In this work, we will report general trends, derived from density functional theory within the generalized gradient approximation proposed by Perdew, Burke, and Ernzerhof (PBE), for the encapsulation properties of the TM${}_{m}$@C${}_{n}$ (TM $=$ Fe, Co, Ni; $m=2\ensuremath{-}6$, $n=60,70,80,90$) systems. Furthermore, to understand the role of the van der Waals corrections to the physical properties, we employed the empirical Grimme's correction ($\mathrm{PBE}+\mathrm{D}2$). We found that both PBE and $\mathrm{PBE}+\mathrm{D}2$ functionals yield almost the same geometric parameters, magnetic and electronic properties, however, $\mathrm{PBE}+\mathrm{D}2$ strongly enhances the encapsulation energy. We found that the center of mass of the TM${}_{m}$ clusters is displaced towards the inside C${}_{n}$ surfaces, except for large TM${}_{m}$ clusters ($m=5$ and 6). For few cases, e.g., Co${}_{4}$ and Fe${}_{4}$, the encapsulation changes the putative lowest-energy structure compared to the isolated TM${}_{m}$ clusters. We identified few physical parameters that play an important role in the sign and magnitude of the encapsulation energy, namely, cluster size, fullerene equatorial diameter, shape, curvature of the inside C${}_{n}$ surface, number of TM atoms that bind directly to the inside C${}_{n}$ surface, and the van der Waals correction. The total magnetic moment of encapsulated TM${}_{m}$ clusters decreases compared with the isolated TM${}_{m}$ clusters, which is expected due to the hybridization of the $d\ensuremath{-}p$ states, and strongly depends on the size and shape of the fullerene cages.

Possible valence-bond condensation in the frustrated cluster magnet LiZn2Mo3O8

The emergence of complex electronic behaviour from simple ingredients has resulted in the discovery of numerous states of matter. Many examples are found in systems exhibiting geometric magnetic frustration, which prevents simultaneous satisfaction of all magnetic interactions. This frustration gives rise to complex magnetic properties such as chiral spin structures orbitally-driven magnetism, spin-ice behavior exhibiting Dirac strings with magnetic monopoles, valence bond solids, and spin liquids. Here we report the synthesis and characterization of LiZn2Mo3O8, a geometrically frustrated antiferromagnet in which the magnetic moments are localized on small transition metal clusters rather than individual ions. By doing so, first order Jahn-Teller instabilities and orbital ordering are prevented, allowing the strongly interacting magnetic clusters in LiZn2Mo3O8 to probably give rise to an exotic condensed valence-bond ground state reminiscent of the proposed resonating valence bond state. Our results also link magnetism on clusters to geometric magnetic frustration in extended solids, demonstrating a new approach for unparalleled chemical control and tunability in the search for collective, emergent electronic states of matter.

Study of the electrocatalytic properties of Ir4(CO)12 for the oxygen reduction and hydrogen oxidation reactions, in the absence and presence of fuel cell contaminants

Carbonyl cluster compounds are promising potential substitutes for platinum as catalysts in fuel cells. In this work, we have used Ir4(CO)12 as a new electrocatalyst to study its activity for the Oxygen Reduction Reaction (ORR) in the absence and presence of methanol in different concentrations (1.0 and 2.0 mol L−1), and the Hydrogen Oxidation Reaction (HOR) with pure hydrogen and hydrogen/carbon monoxide mixtures ([CO] = 100 ppm and 0.5%). This iridium cluster can perform both the ORR and HOR, and it exhibits the important property of being tolerant to the aforementioned contaminants to an important extent, in contrast to platinum, which is easily deactivated even by traces of CO or methanol. The compound was electrochemically studied by rotating disk electrode (RDE) measurements, using the cyclic and linear sweep voltammetry (CV and LSV) techniques. The ORR temperature dependence (293 K–333 K) was studied and kinetic parameters such as the Tafel slope, charge transfer coefficient, exchange current density and activation energy were calculated from the LSV polarization curves. This carbonyl complex is one of the few transition metal clusters whose catalytic activity for these reactions has been reported.

Influence of spin–orbit coupling on electronic structures of TM@Au12 ([formula omitted], 4d, and 5d atoms)

Influence of spin–orbit coupling (SOC) on electronic structures of TM@Au12 (TM=3d, 4d, and 5d transition-metal atoms) is studied by using density functional theory. The SOC can disperse much the frontier energy levels of the clusters, especially for the clusters with TM atoms at the end of each series. The SOC generally decrease the gaps of the frontier orbitals and the spin magnetic moments due to the orbital dispersion and increase of hybridization between the TM and the host atoms, respectively. Considerable orbital magnetic moments can be obtained. Our work provides imperative understanding on SOC effects in transition-metal clusters.

Interactions and Reactions of Transition Metal Clusters with the Interior of Single-Walled Carbon Nanotubes Imaged at the Atomic Scale

Clusters of transition metals, W, Re, and Os, upon encapsulation within a single-walled carbon nanotube (SWNT) exhibit marked differences in their affinity and reactivity with the SWNT, as revealed by low-voltage aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM). Activated by an 80 keV electron beam, W reacts only weakly with the SWNT, Re creates localized defects on the sidewall, and Os reacts readily causing extensive defect formation and constriction of the SWNT sidewall followed by total rupture of the tubular structure. AC-HRTEM imaging at the atomic level of structural transformations caused by metal-carbon bonding of π- and σ-character demonstrates what a crucial role these types of bonds have in governing the interactions between the transition metal clusters and the SWNT. The observed order of reactivity W < Re < Os is independent of the metal cluster size, shape, or orientation, and is related to the metal to nanotube bonding energy and the amount of electronic density transferred between metal and SWNT, both of which increase along the triad W, Re, Os, as predicted by first-principles density functional theory calculations. By selecting the appropriate energy of the electron beam, the metal-nanotube interactions can be controlled (activated or precluded). At an electron energy as low as 20 keV, no visible transformations in the nanotube in the vicinity of Os-clusters are observed.

Ultrafast ionization and subsequent coulomb explosion of zirconium oxide and tungsten carbide “superatomic” cluster species and comparison to group 10 metals

Ultrafast pulses centered at 624 nm are used to explore the ionization behavior of small zirconium oxide, palladium oxide, tungsten carbide, and platinum clusters. ZrxOy and WxCy have been identified as “superatom” species as they are isovalent with PdxOy and PtxCy clusters, respectively. Intensity selective scanning (ISS) technique is used to study the high charge states of each metal as a function of laser intensity. The ion signals of each atomic charge state are compared to those predicted from semi-classical atomic tunneling theory using sequential ionization potentials. A clear difference in ion signal appearance potentials is observed in early-group transition metal clusters vs. late transition metal clusters, indicating the significance of the d-orbitals in the overall ionization behavior of these clusters. The ionization behavior of each cluster species is explored in the context of transition metal clusters.

Reverse hydrogen spillover on and hydrogenation of supported metal clusters: insights from computational model studies

"Reverse" spillover of hydrogen from hydroxyl groups of the support onto supported transition metal clusters, forming multiply hydrogenated metal species, is an essential aspect of various catalytic systems which comprise small, highly active transition metal particles on a support with a high surface area. We review and analyze the results of our computational model studies related to reverse hydrogen spillover, interpreting available structural and spectral data for the supported species and examining the relationship between metal-support and metal-hydrogen interactions. On the examples of small clusters of late transition metals, adsorbed in zeolite cavities, we showed with computational model studies that reverse spillover of hydrogen is energetically favorable for late transition metals, except for Au. This preference is crucial for the chemical reactivity of such bifunctional catalytic systems because both functions, of metal species and of acidic sites, are strongly modified, in some cases even suppressed - due to partial oxidation of the metal cluster and the conversion of protons from acidic hydroxyl groups to hydride ligands of the metal moiety. Modeling multiple hydrogen adsorption on metal clusters allowed us to quantify how (i) the support affects the adsorption capacity of the clusters and (ii) structure and oxidation state of the metal moiety changes upon adsorption. In all models of neutral systems we found that the metal atoms are partially positively charged, compensated by a negative charge of the adsorbed hydrogen ligands and of the support. In a case study we demonstrated with calculated thermodynamic parameters how to predict the average hydrogen coverage of the transition metal cluster at a given temperature and hydrogen pressure.

Method/basis set dependence of NICS values among metallic nano-clusters and hydrocarbons

The influence of various all-electron basis sets and effective core potentials employed along with several DFT functionals (B3LYP, B3PW91, BLYP, BP86 and M06) on the magnitude of nucleus independent chemical shift (NICS) values in different metallic nano-clusters and hydrocarbons is studied. In general, it is demonstrated that the NICS values are very sensitive to the applied method/basis set; however, the method/basis set dependence is more prominent for computed NICS values in transition metal clusters. In hydrocarbons, medium-size basis sets perform roughly similar to large basis sets in most cases. It is also found that NICS(0) values are more sensitive to the method/basis set variation compared to the NICS values computed at 1 or 2 Å above the ring plane. However, in many cases, no broad-spectrum regulation is found for the effect of basis set/method on the magnitude of NICS values. A detailed study showed that bond length alternation in a molecule has an insignificant effect on the magnitude of NICS values so the influence of method/basis sets on the magnitude of NICS values mostly arises from the different predicted ring current intensities at various computational levels.

Two wheel-like polytungstates obtained by incorporation of two {Cu4} clusters and two molybdenum centers in the {W48} wheel

Two new {P(8)W(48)} wheel-based compounds, Na(12)Li(16){[Cu(H(2)O)](2)[Cu(4)(OH)(4)(H(2)O)(8)](2)P(8)W(48)O(184)}·55H(2)O (1), and K(4)Na(24)Li(10){(MoO(2))(2)(P(8)W(48)O(184))}·61H(2)O (2) have been synthesized by a conventional aqueous solution method, and characterized by UV, IR, TG analysis, XPRD, (31)P NMR, XPS, single-crystal X-ray diffraction analyses, magnetic study and electrochemistry study. In compound 1, a wheel-type {P(8)W(48)} containing two {Cu(4)} clusters and two isolated Cu cations results in a 10-Cu-containing polyoxotungstate, which represents the first {P(8)W(48)}-based compound trapping two transition metal (TM) clusters in its inner cavity. Further, the polyoxoanion was connected by Na(+) and Li(+) cations into a 3D framework. Compound 2 is a 2-Mo-containing {P(8)W(48)}-based polyoxotungstate. Magnetic study indicates that antiferromagnetic interactions exist in compound 1.

Probing the structural and electronic properties of small vanadium monoxide clusters

The structural evolution and bonding of a series of early transition-metal oxide clusters, V(n)O(q) (n = 3-9, q = 0,-1), have been investigated with the aid of previous photoelectron spectroscopy (PES) and theoretical calculations. For each vanadium monoxide cluster, many low-lying isomers are generated using the Saunders "Kick" global minimum stochastic search method. Theoretical electron detachment energies (both vertical and adiabatic) were compared with the experimental measurements to verify the ground states of the vanadium monoxide clusters obtained from the DFT calculations. The results demonstrate that the combination of photoelectron spectroscopy experiments and DFT calculation is not only powerful for obtaining the electronic and atomic structures of size-selected clusters, but also valuable in resolving structurally and energetically close isomers. The second difference energies and adsorption energies as a function of the cluster size exhibit a pronounced even-odd alternation phenomenon. The adsorption energies of one O atom on the anionic (6.64 → 8.16 eV) and neutral (6.41 → 8.13 eV) host vanadium clusters are shown to be surprisingly high, suggesting strong capabilities to activate O by structural defects in vanadium oxides.