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 (PR 3, X) ligands. The picture has emerged from analysis of large-scale density functional theory calculations of structurally characterized compounds, namely Au 102(SR)44, Au39(PR3)14X6 ؊ , Au 11(PR3)7X3, and Au13(PR3)10X2 3؉ , 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.density functional theory ͉ monolayer-protected cluster
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.
Density-functional theory computations on a cluster Au 144 (SR) 60 with an icosahedral Au 114 core with 30 RS-Au-SR units protecting its surface yield an excellent fit of the structure factor to the experimental X-ray scattering structure factor measured earlier for 29 kDa thiolate-protected gold clusters. This cluster has a special combination of atomic and electronic structure that provides explanations for the observed stability and capacitive charging properties with several available oxidation states in electrochemistry and optical absorption extending well into the infrared region.
How thiols and disulfides bind to gold surfaces to form self-assembled monolayers is a long-standing
open question. In particular, determining the nature itself of the anchor groups and of their interaction with the
metal is a priority issue, which has so far been approached only with oversimplified models. We present ab
initio calculations of the adsorption configurations (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. A complete characterization of their structure, binding energies, and type of bonding
is obtained. It is established that 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.
Density functional theory calculations are used to explore phosphine- and thiolate-protected gold nanoclusters, namely, Au(39)(PH(3))(14)Cl(6) and Au(38)(SCH(3))(24). For Au(38)(SCH(3))(24), a novel structural motif is predicted, consisting of ringlike (AuSCH(3))(4) units protecting a central Au(14) core. The calculated optical spectrum of this species features a large optical gap (about 1.5 eV) and a prominently peaked structure, correlating with experimental findings of "molecular-like spectra" of thiolate-protected 1.1 nm gold nanoparticles. Ligand-ligand interactions and steric effects in the ligand shell are suggested as possible driving forces toward an ordered gold core structure. A novel mechanism for ligand-exchange reactions on gold clusters is proposed.
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.
The
dynamic character of the active centers has made it difficult
to unravel the reaction path for NH3-assisted selective
catalytic reduction (SCR) of nitrogen oxides over Cu-CHA. Herein,
we use density functional theory calculations to suggest a complete
reaction mechanism for low-temperature NH3-SCR. The reaction
is found to proceed in a multisite fashion over ammonia-solvated Cu
cations Cu(NH3)2
+ and Brønsted
acid sites. The activation of oxygen and the formation of the key
intermediates HONO and H2NNO occur on the Cu sites, whereas
the Brønsted acid sites facilitate the decomposition of HONO
and H2NNO to N2 and H2O. The activation
and reaction of NO is found to proceed via the formation of nitrosonium
(NO+) or nitrite (NO2
–) intermediates.
These low-temperature mechanisms take the dynamic character of Cu
sites into account where oxygen activation requires pairs of Cu(NH3)2
+ complexes, whereas HO–NO
and H3N–NO coupling may occur on single complexes.
The formation and separation of Cu pairs is assisted by NH3 solvation. The complete reaction mechanism is consistent with measured
kinetic data and provides a solid basis for future improvements of
the low-temperature NH3-SCR reaction.
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