On the origin of the unique properties of supported Au nanoparticles

The unique catalytic activity of supported Au nanoparticles has been ascribed to various effects including thickness/shape, the metal oxidation state, and support effects. Previously, we reported the synthesis of ordered Au monolayers and bilayers on TiO(x), with the latter being significantly more active for CO oxidation than the former. In the present study, the electronic and chemical properties of ordered monolayer and bilayer Au films have been characterized by infrared reflection adsorption spectroscopy using CO as a probe and ultraviolet photoemission spectroscopy. The Au overlayers are found to be electron-rich and to have significantly different electronic properties compared with bulk Au. The common structural features of ordered Au bilayers and Au bilayer nanoparticles on TiO2(110) are described, and the exceptionally high catalytic activity of the Au bilayer structure related to its unique electronic properties.     

Catalytic activity of supported Au nanoparticles deposited from block copolymer micelles

Quasi-ordered, highly dispersed, gold nanoclusters of tightly controlled particle size were synthesized by dip-coating substrates with gold precursors encapsulated by block-copolymer micelles. By this method, gold particles (4.8 +/- 1.3 nm) were deposited on ITO-coated glass and shown to be catalytically active for electro-oxidation of carbon monoxide. XPS confirmed the catalytically active particles were predominantly Au0; however, a large fraction existed as Au3+. Whereas bulk gold is inert, these results demonstrate that catalytically active Au nanoparticles can be derived from micelle encapsulation.     

Pt on graphene monolayers supported on a Ni(111) substrate: relativistic density-functional calculations

The structural, energetic, and magnetic properties of Pt atoms and dimers adsorbed on a Ni-supported graphene layer have been investigated using density-functional calculations, including the influence of dispersion forces and of spin-orbit coupling. Dispersion forces are found to be essential to stabilize a chemisorbed graphene layer on the Ni(111) surface. The presence of the Ni-substrate leads not only to a stronger interaction of Pt atoms and dimers with graphene but also to a locally increased binding between graphene and the substrate and a complex reconstruction of the adlayer. The stronger binding of the dimer also stabilizes a flat adsorption geometry in contrast to the upright geometry on a free-standing graphene layer. These effects are further enhanced by dispersion corrections. Isolated Pt adatoms and flat dimers are found to be non-magnetic, while an upright Pt dimer has strongly anisotropic spin and orbital moments. For the clean C/Ni(111) system, we calculate an in-plane magnetic anisotropy, which is also conserved in the presence of isolated Pt adatoms. Surprisingly, upright Pt-dimers induce a re-orientation of the easy magnetic axis to a direction perpendicular to the surface, in analogy to Pt(2) on a free-standing graphene layer and to the axial anisotropy of a gas-phase Pt(2) dimer.     

Proton affinity of amino acids: Their interpretation with density functional theory-based descriptors

The calculation of group electronegativity and hardness for amino acid “functional groups,” considered as a biradical taken out of their protein environment, is performed for both the α-helix and β-sheet geometry of these amino acids. Group electronegativity and hardness are then used to interpret the experimental gas-phase proton affinity sequence of the amino acids. Group hardness was found to play the dominant role, whereas group electronegativity only had a minor influence on the sequence, thereby stressing the importance of the charged form in the acid-base equilibrium. An explanation for the deviations, seen for some of the amino acids, from the correlation between these group properties and the proton affinity was sought. © 1996 John Wiley & Sons, Inc.     

Structure‐activity relationship of supported Au catalysts with high catalytic activity by modifying the inactive supports

Three supported Au catalysts have been prepared by the deposition‐precipitation method by using the active carbon (AC), SiO₂‐AC, and SiO₂‐AC‐hollowed. The 3 supports were characterized by Brunauer‐Emmett‐Teller and scanning electron microscopy. Meanwhile, the supported Au nanoparticles were also characterized in detail by X‐ray powder diffraction, transmission electron microscopy, H₂‐TRP, and X‐ray photoelectron spectroscopy, and their catalytic activity and stability in CO oxidation was evaluated. The results demonstrated that Au supported on SiO₂‐AC‐hollowed exhibited much higher catalytic activity with acceptable stability for 72 hours than the other 2. We attributed to finer supported Au nanoparticles with abundant low‐coordinated Au atoms on the surfaces of hollowed supports with large special surface area and abundant pore structure. In summary, we successfully found an efficient and cheap method to prepare catalysts with high catalytic activity and acceptable stability by modifying the inactive supports.     

Generalized Stone-Wales transformation as the possible origin of ferromagnetism in polymeric C60: a density-functional theory study

Recently, there has been a proposal [Y.-H. Kim et al., Phys. Rev. B 68, 125420 (2003)] suggesting that ferromagnetic interactions in compressed and heated polymeric-C(60) solids could be due to the existence of triplet open cages resulting from successive generalized Stone-Wales transformations within the C(60) cage. Here, by performing B3LYP3-21G and B3LYP6-31G(d) optimizations, we carried out a systematic investigation of the thermodynamics and kinetics of the mechanism of generation of these open cages in their closed-shell singlet, open-shell singlet, and triplet states. We also computed the magnetic interactions induced by the open cages presenting a triplet ground state. Our results indicate that this mechanism is not appropriate to explain the ferromagnetism found in compressed and heated polymeric C(60) for the following reasons: (a) the formation of the only open cage presenting a triplet ground state requires overpassing a highest energy point of 318 kcal/mol, well above other competitive mechanisms reported in the literature; the triplet open cages formed are not stable against their transformation into a diamagnetic intermediate; (c) the magnetic interactions between two adjacent triplet open cages are antiferromagnetic.     

Geometric and magnetic properties of Pt clusters supported on graphene: relativistic density-functional calculations

The geometric and magnetic structures of small Pt(n) clusters (n = 1 - 5) supported on a graphene layer have been investigated using ab initio density functional calculations including spin-orbit coupling. Pt-Pt interactions were found to be much stronger than the Pt-C interactions promoting the binding to the support. As a consequence, the equilibrium structure of the gas-phase clusters is preserved if they are deposited on graphene. However, the clusters bind to graphene only via at most two Pt-C bonds: A Pt(2) dumbbell prefers an upright position, the larger clusters are bound to graphene only via one edge of the planar cluster (Pt(3) and Pt(5)) or via two terminal Pt atoms of a bent Pt(4) rhombus. Evidently, the strong buckling of the graphene layer induced by the Pt-C bonds prevents the formation of a larger number of cluster-support bonds. As the local spin and orbital magnetic moments are quenched on the Pt atoms forming Pt-C bonds, the magnetic structure of the supported clusters is much more inhomogeneous as in the gas-phase. This leads to noncollinear magnetic structures and a strongly reduced magnetic anisotropy energy.     

Reduction profiles of supported Ru, Au and Ru?Au

The reduction with hydrogen of Ru, Au and Ru–Au supported catalysts was followed by Differential Scanning Calorimetry (DSC). The supports used were MgO, SiO₂ and Al₂O₃. The differences in the reduction behavior of both metals on each carrier are correlated with the surface composition of the bimetallic clusters.

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Ru, Au Ru–Au . MgO, SiO₂ Al₂O₃. .

     

Unravelling the origin of intermolecular interactions using absolutely localized molecular orbitals

An energy decomposition analysis (EDA) method is proposed to isolate physically relevant components of the total intermolecular interaction energies such as the contribution from interacting frozen monomer densities, the energy lowering due to polarization of the densities, and the further energy lowering due to charge-transfer effects. This method is conceptually similar to existing EDA methods such as Morokuma analysis but includes several important new features. The first is a fully self-consistent treatment of the energy lowering due to polarization, which is evaluated by a self-consistent field calculation in which the molecular orbital coefficients are constrained to be block-diagonal (absolutely localized) in the interacting molecules to prohibit charge transfer. The second new feature is the ability to separate forward and back-donation in the charge-transfer energy term using a perturbative approximation starting from the optimized block-diagonal reference. The newly proposed EDA method is used to understand the fundamental aspects of intermolecular interactions such as the degree of covalency in the hydrogen bonding in water and the contributions of forward and back-donation in synergic bonding in metal complexes. Additionally, it is demonstrated that this method can be used to identify the factors controlling the interaction of the molecular hydrogen with open metal centers in potential hydrogen storage materials and the interaction of methane with rhenium complexes.     

The activity of Au supported on various types of carbon in the ring transformation reactions of methyloxirane

Summary  Vezetéknév     

Semiempirical SCF/CI interpretation of the origin of the catalytic activity of transition metal-macrocycle complexes

Semiempirical self-consistent field (SCF) and configuration interaction (CI) calculations of the intermediate neglect of differential overlap (INDO) type are applied to the analysis of the electronic transitions of the hexaazacyclophane base and its Ni and Cu complexes. The ground states (¹A_g for the ligand and Ni complex, ²B_1g for the Cu complex) are planar structures of D₂h symmetry. The low-energy region of the UV-visible spectra, whose analysis may help to recognize the catalytic active sites of the complexes is associated with d → d transitions in the Ni complex, and M → L charge transfer in the Cu complex.     

Electronic and chemical properties of supported Au nanoparticles

Oxidation and reduction behaviors of Au nanoparticles with different sizes on highly ordered pyrolytic graphite (HOPG) and silica were studied using X-ray photoelectron spectroscopy (XPS). For Au nanoparticles smaller than 6 nm in diameter, we found a novel oxygen species formed in Au nanoparticles, which is absent in larger particles and Au bulk crystals. This new oxygen species is attributed to the subsurface oxygen: for a complete understanding of the structures of catalytically active Au, the new oxygen species should be taken into account. In this context, it is worth mentioning that the subsurface oxygen species has been suggested to play an important role in heterogeneous catalysis. With decreasing Au particle size, a positive core level shift can be observed, which can be mostly attributed to the final state effects. Increase of the number of undercoordinated atoms with decreasing particle size is evidenced by a reduced splitting between 5d_3/2 and 5d_5/2 states and a band narrowing. Our results on electronic structures of Au nanoparticles on silica are compared to those on other substrates such as zirconia and titania to shed light onto the metal-support interactions.     

Unravelling the Relationship between Raman Enhancement and Photocatalytic Activity on Single Anisotropic Au Microplates

Anisotropic noble‐metal structures are attracting increasing attention because of interesting size‐ and shape‐dependent properties and have emerging applications in the fields of optics and catalysis. However, it remains a significant challenge to overcome chemical contributions and acquire molecular insight into the relationship between Raman enhancement and photocatalytic activity. This study gives visualized experimental evidence of the anisotropic spatial distribution of Raman signals and photocatalytic activity at the level of single nanometer‐thin Au microtriangles and microhexagons. Theoretical simulations indicate an anisotropic spatial distribution and sharpness‐dependent strength of the electric‐field enhancement. Analysis by using statistical surface‐enhanced Raman scattering (SERS) supports this view, that is, Raman enhancement is on the order of corner>edge>face for a single microplate, but SERS measurements at different depths of focus also imply a concentration‐dependent feature of SERS signals, especially at the corners and edges. Similarly, the SERS signals of product molecules in plasmonic photocatalysis also exhibit asymmetrical strengths at different corners of the same microplate. However, by examining the variations in the relative intensities of the SERS peaks, the difference in the photocatalytic activities at the corners, edges, and faces has been successfully calculated and is highly consistent with electric‐field simulations, thus indicating that an increased number of molecules adsorbed at specific sites does not necessarily lead to a higher conversion ratio in noble‐metal photocatalysis. Our strategy weakens the assumed impact of plasmonic local heating and, to a certain extent, excludes the influence of concentration effects and chemical contributions in noble‐metal photocatalysis, thus clearly profiling plasmon‐related characteristics. This study also promises a new research direction to understand the enhancement mechanism of SERS‐active structures.     

Selective modification of the acid-base properties of ceria by supported Au

Au supported on CeO(2) prepared by deposition-precipitation with urea leads to a basic catalyst. Au acts in two ways as surface modifier. First, Au selectively interacts with Ce(4+) cations by either blocking access to or reducing Ce(4+) to Ce(3+). Second, the resulting Au atoms (presumably as Au(+) ions) act as soft, weak Lewis acid sites stabilizing carbanion intermediates and enhancing hydride abstraction in the dehydrogenation of alcohols. In consequence, the thus-synthesized basic catalyst catalyzes the dehydrogenation of propan-2-ol to acetone with high efficiency and without notable deactivation. Additionally, the dehydration pathway of propan-2-ol is eliminated, as Au also quantitatively blocks access to strongly acidic Ce(4+) ions or reduces them to Ce(3+).     

CO oxidation over ceria supported Au22 nanoclusters: Shape effect of the support

CO oxidation over ceria-supported Au₂₂ nanoclusters shows strong dependence on the support shape: the lattice oxygen in CeO₂ rods is more reactive than in the cubes and thus make rods a superior support for Au nanoclusters in catalyzing low temperature CO oxidation.     

Electronic states and structural stability of supported Au clusters

The quantized energy levels of electrons in supported nanometer-size Au clusters have been resolved at room temperature using field emission techniques. By studying the time dependence of the electron emission current from an individual supported cluster, information about the structural stability of the cluster can also be obtained. Studies show abrupt jumps between different emission rates that are revisited as time progresses. This phenomenon can be attributed to a rearrangement of the cluster structure and/or orientation on the substrate and provides new evidence of multiple ‘isomeric’ structures for small clusters of metallic atoms.     

Model catalysts of supported Au nanoparticles and mass-selected clusters

In surface science, much effort has gone into obtaining a deeper understanding of the size-selectivity of nanocatalysts. In this article, electronic and chemical properties of various model catalysts consisting of Au are reported. Au supported by oxide surfaces becomes inert towards chemisorption and oxidation as the particle size became smaller than a critical size (2-3 nm). The inertness of these small Au nanoparticles is due to the electron-deficient nature of smaller Au nanoparticles, which is a result of metal-substrate charge transfer. Properties of Au clusters smaller than ～20 atoms were shown to be non-scalable, i.e., every atom can drastically change the chemical properties of the clusters. Moreover, clusters with the same size can show dissimilar properties on various substrates. These recent endeavours show that the activity of a catalyst can be tuned by varying the substrate or by varying the cluster size on an atom-by-atom basis.     

Interaction potentials from periodic density-functional theory calculations: molecular-dynamics simulations of Au clusters deposited on the TiN (001) surface

Molecular-dynamics simulations of gold particles deposited on a TiN (001) surface have been accounted for through classical pair potentials describing the atom force field. The interaction between Ti-N, Ti-Ti, N-N, Au-Au, Au-Ti, and Au-N pairs was estimated by following a procedure in which the interaction energy between two sets of atoms is estimated from density-functional calculations performed with periodic boundary conditions using plane waves as basis set. The pair potentials were expressed as the sum of two contributions: long range in a Coulomb form and a short-range term, which included the rest of the energy contributions. Simulations of the TiN (001) isolated surface reproduced the already described surface relaxation, with a rippling parameter in agreement with that found from a purely first-principles approach. Simulations of gold deposition on such surfaces showed the formation of metal clusters with well-defined fcc structure and epitaxially grown.     

Coverage effects and the nature of the metal-sulfur bond in S/Au(111): high-resolution photoemission and density-functional studies

The bonding of sulfur to surfaces of gold is an important subject in several areas of chemistry, physics, and materials science. Synchrotron-based high-resolution photoemission and first-principles density-functional (DF) slab calculations were used to study the interaction of sulfur with a well-defined Au(111) surface and polycrystalline gold. Our experimental and theoretical results show a complex behavior for the sulfur/Au(111) interface as a function of coverage and temperature. At small sulfur coverages, the adsorption of S on fcc hollow sites of the gold substrate is energetically more favorable than adsorption on bridge or a-top sites. Under these conditions, S behaves as a weak electron acceptor but substantially reduces the density-of-states that gold exhibits near the Fermi edge. As the sulfur coverage increases, there is a weakening of the Au-S bonds (with a simultaneous reduction in the Au --> S charge transfer and a modification in the S sp hybridization) that facilitates changes in adsorption site and eventually leads to S-S bonding. At sulfur coverages above 0.4 ML, S(2) and not atomic S is the more stable species on the gold surface. Formation of S(n)(n > 2) species occurs at sulfur coverages higher than a monolayer. Very similar trends were observed for the adsorption of sulfur on polycrystalline surfaces of gold. The S atoms bonded to Au(111) display a unique mobility/reactivity not seen on surfaces of early or late transition metals.     

Soluble-polymer supported synthesis of a prostanoid library: identification of antiviral activity

[formula: see text] The prostaglandins are potent natural products taking part in many biological processes. The "convergent generation of diversity" from a "toolbox" of prostanoid components, augmented with additional polymer-supported transformations, can enable construction of valuable libraries. A parallel-pool strategy was used to assemble a small library of prostanoids. The inhibition of a herpes-family virus demonstrated the potential for new drug discovery.