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.
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NO2 adsorbed on MgO(100) supported by Ag or Pt is explored by density functional theory calculations. NO2 is weakly adsorbed on MgO(100), with a bond involving minor oxide to adsorbate charge ...transfer. However, if MgO is supported, then the adsorption energy is considerably enhanced and NO2 is adsorbed as a nitrite (N ). Analysis reveals that the NO2 excess charge originates from the oxide side of the oxide/metal interface and that the electron abstraction increases the oxide/metal adhesion. The proposed mechanism is general and should apply for oxidizing surface species.
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Single-atom alloys, which are prepared by embedding isolated metal sites in host metals, are promising systems for improved catalyst selectivity. For technical applications, catalysts based on ...nanoparticles are preferred thanks to a large surface area. Herein, we investigate hydrogenation of acetylene to ethylene using kinetic Monte Carlo simulations based on density functional theory and compare the performance of Pd/Cu nanoparticles with Pd(111) and Pd/Cu(111). We find that embedding Pd in Cu systems strongly enhances the selectivity and that the reaction mechanism is fundamentally different for nanoparticles and extended surfaces. The reaction mechanism on nanoparticles is complex and involves elementary steps that proceed preferentially over different sites. Edge and corner sites on nanoparticles are predicted to lower the selectivity, and we infer that a rational design strategy in selective acetylene hydrogenation is to maximize the number of (111) sites in relation to edge sites for Pd/Cu nanoparticles.
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High-pressure X-ray photoelectron spectroscopy, mass spectrometry, and density functional theory calculations have been combined to study methane oxidation over Pd(100). The measurements reveal a ...high activity when a two-layer PdO(101) oriented film is formed. Although a one-layer PdO(101) film exhibits a similar surface structure, no or very little activity is observed. The calculations show that the presence of an oxygen atom directly below the coordinatively unsaturated Pd atom in the two-layer PdO(101) film is crucial for efficient methane dissociation, demonstrating a ligand effect that may be broadly important in determining the catalytic properties of oxide thin films.
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Understanding how nanostructure and atomic-scale defects of the support affect metal catalyst nanoparticle sintering is of crucial importance to minimize thermal deactivation, as well as to ...understand the origin of widely observed but still unexplained phenomena, such as transient multimodal particle size distributions and nanoparticle redispersion. To shed light on these issues, we present a generic experimental approach that relies on nanofabrication to introduce controlled structural heterogeneity in a chemically homogeneous model catalyst support. This is achieved by fabricating arrays of nanocone structures separated by flat areas, where both are homogeneously sputter-coated with a thin amorphous alumina layer. Using ex situ aberration-corrected scanning transmission electron microscopy (STEM) to analyze Pt model catalyst nanoparticles on such nanostructured supports prior and after exposure to 4% O2 in Ar carrier gas at 600 °C, we find that the initial particle size distributions and their time evolution during sintering to be different on the cones and the flat areas. On the cones, redispersion of Pt into highly abundant particles of about 1 nm occurs very rapidly. In contrast, particle shrinkage and growth combined with redispersion occur on the flat areas, leading to a broader and bimodal size distribution. These processes are amplified and efficiently demonstrated by the nanostructured surface because of (i) higher support defect density on the nanocones compared to the flat surfaces in between and (ii) initially different Pt particle size distributions on the cones and on the flat surfaces. Hence, the nanostructured surface facilitates the clear identification of catalyst redispersion in oxidizing conditions and experimentally identifies a mechanism that gives rise to (transient) bi- or multimodal particle size distributions during sintering.
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It is well documented that different surface structures of catalytically active metals may exhibit different catalytic properties. This is typically examined by comparing the catalytic activities ...and/or selectivities of various well-defined smooth and stepped/kinked single crystal surfaces. Here we report the direct observation of the heterogeneity of active polycrystalline surfaces under reaction conditions, which is manifested by multifrequential oscillations during hydrogen oxidation over rhodium, imaged in situ by photoemission electron microscopy. Each specific surface structure, i.e. the crystallographically different µm-sized domains of rhodium, exhibits an individual spiral pattern and oscillation frequency, despite the global diffusional coupling of the surface reaction. This reaction behavior is attributed to the ability of stepped surfaces of high-Miller-index domains to facilitate the formation of subsurface oxygen, serving as feedback mechanism of the observed oscillations. The current experimental findings, backed by microkinetic modeling, may open an alternative approach towards addressing the structure-sensitivity of heterogeneous surfaces.
Density functional theory calculations are used to evaluate Au4f core level shifts of methyl thiolate protected Au
25
, Au
102
and Au
144
nanoparticles. The shifts are found to provide sensitive ...fingerprints of the chemical environment. In particular, Au atoms in protective gold-thiolate complexes have higher binding energies than Au atoms with solely metal neighbors. The core level shifts for the nanoparticles are compared to the corresponding results for methyl thiolates adsorbed on Au(111) and implications for the understanding of the gold-sulfur bond is discussed.
Density functional theory calculations are used to evaluate Au4f core level shifts of thiolate-protected gold nanoparticles.
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•Modulation excitation spectroscopy applied to methane oxidation over Pd-alumina.•In situ characterization with X-ray absorption fine structure.•EXAFS fitting of demodulated spectra ...used to quantify the Pd oxidation state.•At lean conditions conversion increases as Pd becomes more oxidized.•Low oxidation states of Pd are not very active for methane oxidation.
Rich/lean cycling is used to study the total oxidation of methane over a Pd/Al2O3 catalyst at different oxidation states. Time-resolved energy-dispersive X-ray absorption fine structure is used in a modulation excitation approach to monitor the chemical state of the palladium nanoparticles during the cycling, and the resulting spectra are demodulated using phase sensitive detection. Cycling is performed using oxygen pulses with a concentration of 0.15, 0.25, and 1.5% over a constant flow of 0.1% methane. For the two lowest oxygen concentrations the methane conversion is generally low but increases at the switches between the rich and the lean periods, while for the highest oxygen concentration the conversion is highest during the lean periods. The oxidation state of Pd changes rapidly but to a limited extent for the two lowest oxygen concentrations, whereas for the high oxygen concentration the oxidation appears to proceed via a two-step process, where the first step is rapid and the second step is slower. EXAFS fitting of the demodulated spectra is used to quantify the Pd oxidation state, the first rapid oxidation step is assigned to surface oxidation, while the second step is assigned to bulk oxidation of Pd. A low methane conversion is observed when the Pd nanoparticles are covered with chemisorbed oxygen or surface oxide, the methane conversion is higher when the Pd nanoparticles are bulk oxidized.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
The specific CO oxidation activity of palladium versus palladium oxide is still controversially discussed. In this study, 5 wt. % Pd-γ-Al2O3 catalysts were utilized to investigate the effect of the ...palladium oxidation state on the CO oxidation activity. Comprehensive in situ and ex situ characterization of different alumina supported PdO x (x = 0−1) phases (by HR-TEM, XRD, and FTIR spectroscopy), combined with kinetic measurements and DFT calculations of CO adsorption, allowed us to assess the catalytic activity of the different PdO x (x = 0−1) species: Supported Pd0 and substoichiometric PdO x<1 exhibited comparable high activities, due to a rapid reduction of PdO x<1 to Pd upon CO exposure. PdO nanoparticles showed a lower catalytic activity resulting from their higher stability against reduction by CO. The limited extent of oxidation of Pd under typical reaction conditions together with the facile reduction of substoichiometric PdO x<1 suggest that the active phase present under the current reaction conditions is oxygen-covered metallic Pd.
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Janus graphene stacks as artificial graphite allow the reversible intercalation of sodium ions.
Sodium, in contrast to other metals, cannot intercalate in graphite, hindering the use of this cheap, ...abundant element in rechargeable batteries. Here, we report a nanometric graphite-like anode for Na
+
storage, formed by stacked graphene sheets functionalized only on one side, termed Janus graphene. The asymmetric functionalization allows reversible intercalation of Na
+
, as monitored by operando Raman spectroelectrochemistry and visualized by imaging ellipsometry. Our Janus graphene has uniform pore size, controllable functionalization density, and few edges; it can store Na
+
differently from graphite and stacked graphene. Density functional theory calculations demonstrate that Na
+
preferably rests close to -NH
2
group forming synergic ionic bonds to graphene, making the interaction process energetically favorable. The estimated sodium storage up to C
6.9
Na is comparable to graphite for standard lithium ion batteries. Given such encouraging Na
+
reversible intercalation behavior, our approach provides a way to design carbon-based materials for sodium ion batteries.
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