Henrik Grönbeck

A unified view of ligand-protected gold clusters as superatom complexes

Synthesis, characterization, and functionalization of self-assembled, ligand-stabilized gold nanoparticles are long-standing issues in the chemistry of nanomaterials. Factors driving the thermodynamic stability of well documented discrete sizes are largely unknown. Herein, we provide a unified view of principles that underlie the stability of particles protected by thiolate (SR) or phosphine and halide (PR3, X) ligands. The picture has emerged from analysis of large-scale density functional theory calculations of structurally characterized compounds, namely Au102(SR)44, Au39(PR3)14X6−, Au11(PR3)7X3, and Au13(PR3)10X23+, where X is either a halogen or a thiolate. Attributable to a compact, symmetric core and complete steric protection, each compound has a filled spherical electronic shell and a major energy gap to unoccupied states. Consequently, the exceptional stability is best described by a “noble-gas superatom” analogy. The explanatory power of this concept is shown by its application to many monomeric and oligomeric compounds of precisely known composition and structure, and its predictive power is indicated through suggestions offered for a series of anomalously stable cluster compositions which are still awaiting a precise structure determination.

On the Structure of Thiolate-Protected Au25

Density functional theory is used to explore the structure of Au25(RS)18. The preferred structure consists of an icosahedral Au13 core protected by 6 RS-Au-RS-Au-RS units. The enhanced stability of the structure as an anion is found to originate from closure of an eight-electron shell for delocalized Au(6s) electrons. The evaluated XRD pattern and optical spectra are in good agreement with experimental data.

Gold and platinum microclusters and their anions: comparison of structural and electronic properties

Abstract We present a study of Au 2 to Au 5 and Pt 2 to Pt 5 clusters within density-functional theory in the neutral and anionic states. Results obtained using two exchange-correlation (xc) functionals, local spin density approximation and spin-polarized Becke–Lee–Yang–Parr (BLYP), are compared. The structural characteristics and relative stabilities of different isomers as well as electron affinities and vertical detachment energies are calculated. The latter compare generally well with experimental data, especially in the BLYP case. However, strong dependence of the vertical electron detachment energies on the isomer and on the xc-functional scheme indicates that special care must be taken for any comparison of theoretical results with experiment. Both for platinum and gold tetramers and pentamers, 3D geometries are unfavored. Triplet states are predicted to be more stable than singlets for platinum, for all sizes considered. The relative stability of different geometrical isomers is strongly affected by the addition of one electron. Marked differences emerge in the electronic structure between the clusters of these two metals.

Density functional theory approach to thiols and disulfides on gold: Au(111) surface and clusters

We discuss our recent ab initio calculations of the adsorption configurations (both dissociative and not) of methanethiol and dimethyl disulfide on Au(111) at low coverage, which are based on density functional theory using gradient-corrected exchange–correlation functionals (both BLYP and PBE). A complete characterization of their structure, binding energies, and type of bonding is obtained. Dissociation is clearly favored for the disulfide with subsequent formation of strongly bound thiolates, in agreement with experimental evidence, whereas thiolates resulting from S–H bond cleavage in thiols can coexist with the adsorbed “intact” species and become favored if accompanied by the formation of molecular hydrogen. New calculations are also presented for the thiolates and the disulfide on a 38-atom gold cluster. Differences and similarities of the bonding on the cluster and on the surface are described in detail.

Low Temperature CO Oxidation over Supported Ultrathin MgO Films

Density functional theory is used to investigate CO oxidation over an ultrathin MgO film supported on Ag(100). O(2) is found to be activated on MgO/Ag(100) whereas CO is only weakly bonded to the surface. These adsorption properties together with a low activation barrier render the MgO/Ag system an efficient catalyst for CO oxidation at low temperatures. As the predicted mechanism is general in nature, the result is suggested to have implications for a wide range of oxidation reactions.

The Active Phase of Palladium during Methane Oxidation

The active phase of Pd during methane oxidation is a long-standing puzzle, which, if solved, could provide routes for design of improved catalysts. Here, density functional theory and in situ surface X-ray diffraction are used to identify and characterize atomic sites yielding high methane conversion. Calculations are performed for methane dissociation over a range of Pd and PdOx surfaces and reveal facile dissociation on either under-coordinated Pd sites in PdO(101) or metallic surfaces. The experiments show unambiguously that high methane conversion requires sufficiently thick PdO(101) films or metallic Pd, in full agreement with the calculations. The established link between high activity and atomic structure enables rational design of improved catalysts.

Harmonic heat flow in isotropic layered systems and its use for thin film thermal conductivity measurements

A theoretical model is presented describing harmonic heat flow in a two layer system heated by a modulated Gaussian laser beam. Amplitude and phase of the modulated temperature rise in the layers, as well as in the backing substrate and adjacent atmosphere, are calculated by solving the three‐dimensional heat conduction equation with a source term including exponential absorption of the laser light in one or two layers. Heat conduction is assumed to be isotropic throughout the system, however, a thermal contact resistance between the two layers can be taken into account. Results are presented for single and double layer systems of gold and various dielectric thin film materials on glass substrates. Amplitude and phase of the harmonic temperature variation are calculated either as a function of position in the sample system or at the surface as a function of the laser beam modulation frequency. It is found that both amplitude and phase of the calculated temperature rise exhibit typical thin film features i...

Geometric and electronic properties of small vanadium clusters: A density functional study

The geometric and electronic properties of vanadium clusters in the range from V2 to V8 have been investigated using the density functional theory, and an LCAO approach for the expansion of the electronic wavefunctions. The optimized low energy isomers are found to be three dimensional for clusters larger than the tetramer, and the evaluated bond dissociation energies, ionization potentials and electron affinities are in good agreement with experimental results. All cluster sizes are found to possess low magnetic moments as ground states, which is in contrast with previous suggestions. In the case of V3− and V4−, a comparison with photo-electron spectra is done by computing the self consistent excitation spectra.

Analysis of Porphyrines as Catalysts for Electrochemical Reduction of O2 and Oxidation of H2O

Bioinspired structures are promising as improved catalysts for various redox reactions. One example is metal hangman-porphyrines (MHP), which recently have been suggested for oxygen reduction/evolution reaction (ORR/OER). The unique properties of the MHPs are attributed to both the hangman scaffold and the C6F5 side groups. Herein, the OER/ORR over various transition metal MHPs is investigated by density functional theory calculations within an electrochemical framework. A comparison of the reaction landscape for MHP, metal porphyrine (MP) and metaltetrafluorophenyloporphyrines (MTFPP), allow for a disentanglement of the different roles of the hangman motif and the side groups. In agreement with experimental studies, it is found that Fe and Co are the best MHP metal centers to catalyze these reactions. We find that the addition of the three-dimensional moiety in the form of hangman scaffold does not break the apparently universal energy relation between *OH and *OOH intermediates. However, the hangman motif is found to stabilize the oxygen intermediate, whereas addition of C6F5 groups reduces the binding energy of all reaction intermediates. Our results indicate that the combination of these two effects allow new design possibilities for macromolecular systems with enhanced catalytic OER/ORR activity.

Noble gas temperature control of metal clusters: A molecular dynamics study

We use classical molecular dynamics simulations to investigate temperature control of unsupported clusters using a noble gas atmosphere. The simulations are performed using a many-body interaction scheme for the intra-cluster potential, while a pairwise Lennard-Jones potential is used to model the interaction between the noble gas and the clusters. In order to isolate different parameters determining the energy exchange efficiency, we have studied the energy transfer with respect to (i) impact parameter, (ii) cluster temperature, (iii) noble gas temperature, (iv) gas–metal interaction strength, (v) metal potential, and (vi) noble gas mass. With these results, we are able to estimate the number of collisions needed to equilibrate a cluster at a given gas temperature. Our estimates are confirmed by simulations of cluster cooling in a noble gas atmosphere.