First-shell Coordination Research Articles

Alloying and Segregation in PdRe/Al2O3 Bimetallic Catalysts for Selective Hydrogenation of Furfural

X-ray absorption fine structure (XAFS) spectroscopy, temperature-programmed reduction (TPR), and temperature-programmed hydride decomposition (TPHD) were employed to elucidate the structures of a series of PdRe/Al2O3 bimetallic catalysts for the selective hydrogenation of furfural. TPR evidenced low-temperature Re reduction in the bimetallic catalysts consistent of the migration of [ReO4]− (perrhenate) species to hydrogen-covered Pd nanoparticles on highly hydroxylated γ-Al2O3. TPHD revealed a strong suppression of β-PdHx formation in the reduced catalysts prepared by (i) co-impregnation and (ii) [HReO4] impregnation of the reduced Pd/Al2O3, indicating the formation of Pd-rich alloy nanoparticles; however, reduced catalysts prepared by (iii) [Pd(NH3)4]2+ impregnation of calcined Re/Al2O3 and subsequent re-calcination did not. Re LIII X-ray absorption edge shifts were used to determine the average Re oxidation states after reduction at 400 °C. XAFS spectroscopy and high-angle annular dark field (HAADF)-scanning transmission electron microscopy (STEM) revealed that a reduced 5 wt.% Re/Al2O3 catalyst contained small Re clusters and nanoparticles comprising Re atoms in low positive oxidation states (~1.5+) and incompletely reduced Re species (primarily Re4+). XAFS spectroscopy of the bimetallic catalysts evidenced Pd-Re bonding consistent with Pd-rich alloy formation. The Pd and Re total first-shell coordination numbers suggest that either Re is segregated to the surface (and Pd to the core) of alloy nanoparticles and/or segregated Pd nanoparticles are larger than Re nanoparticles (or clusters). The Cowley short-range order parameters are strongly positive indicating a high degree of heterogeneity (clustering or segregation of metal atoms) in these bimetallic catalysts. Catalysts prepared using the Pd(NH3)4[ReO4]2 double complex salt (DCS) exhibit greater Pd-Re intermixing but remain heterogeneous on the atomic scale.

Engineering atomic Fe sites with asymmetrical nitrogen/sulfur coordination for stable and boosted sulfur conversion

Introducing nonmetallic heteroatoms into the conventional metal-N-C configuration is thought to be an effective strategy to regulate the coordinated chemistry environment of atomic metal centers and further improve the sulfur electrochemistry behavior in lithium-sulfur batteries. Nevertheless, the modulation of heteroatoms junction states (doped or coordinated) in carbocycle is rarely explored. Herein, based on the stable pre-coordination of Fe3+ and terminal sulfhydryl/amino groups in cysteine, an asymmetrical Fe-N3S1 coordination structure in cysteine-derived graphene (Fe/CG) is rationally designed and prepared. The intervention of S atom at the first-coordination shell of Fe centers could induce the distorted spatial coordination structure, which greatly reduces the Li2S decomposition barrier. Moreover, the bonded S atom increases the d-band center (Ɛda) of Fe and pronouncedly activates the surrounding atoms, enhancing the electronic conductivity and reaction activity/affinity of Fe/CG, which was evidenced through theoretical calculations. The serial electro-kinetic analysis further confirms the bidirectional catalytic effect of Li2S redox conversions on Fe/CG. Consequently, the S@Fe/CG cathodes exhibit outstanding cycling stability (718.8 mAh g−1 after 500 cycles at 1 C with a decay of 0.037 % per cycle) and rate performance (639 mAh g−1 at 4 C). This work provides a new perspective to regulate the local chemistry environment of metal centers via coordination engineering.

Characterization of the Coordination and Solvation Dynamics of Solvated Systems─Implications for the Analysis of Molecular Interactions in Solutions and Pure H2O.

The characterization of solvation shells of atoms, ions, and molecules in solution is essential to relate solvation properties to chemical phenomena such as complex formation and reactivity. Different definitions of the first-shell coordination sphere from simulation data can lead to potentially conflicting data on the structural properties and associated ligand exchange dynamics. The definition of a solvation shell is typically based on a given threshold distance determined from the respective solute-solvent pair distribution function g(r) (i.e., GC). Alternatively, a nearest neighbor (NN) assignment based on geometric properties of the coordination complex without the need for a predetermined cutoff criterion, such as the relative angular distance (RAD) or the modified Voronoi (MV) tessellation, can be applied. In this study, the effect of different NN algorithms on the coordination number and ligand exchange dynamics evaluated for a series of monatomic ions in aqueous solution, carbon dioxide in aqueous and dichloromethane solutions, and pure liquid water has been investigated. In the case of the monatomic ions, the RAD approach is superior in achieving a well separated definition of the first solvation layer. In contrast, the MV algorithm provides a better separation of the NNs from a molecular point of view, leading to better results in the case of solvated CO2. When analyzing the coordination environment in pure water, the cutoff-based GC framework was found to be the most reliable approach. By comparison of the number of ligand exchange reactions and the associated mean ligand residence times (MRTs) with the properties of the coordination number autocorrelation functions, it is shown that although the average coordination numbers are sensitive to the different definitions of the first solvation shell, highly consistent estimates for the associated MRT of the solvated system are obtained in the majority of cases.

Facilely Tuning the First-Shell Coordination Microenvironment in Iron Single-Atom for Fenton-like Chemistry toward Highly Efficient Wastewater Purification.

Precisely identifying the atomic structures in single-atom sites and establishing authentic structure-activity relationships for single-atom catalyst (SAC) coordination are significant challenges. Here, theoretical calculations first predicted the underlying catalytic activity of Fe-NxC4-x sites with diverse first-shell coordination environments. Substituting N with C to coordinate with the central Fe atom induces an inferior Fenton-like catalytic efficiency. Then, Fe-SACs carrying three configurations (Fe-N2C2, Fe-N3C1, and Fe-N4) fabricate facilely and demonstrate that optimized coordination environments of Fe-NxC4-x significantly promote the Fenton-like catalytic activity. Specifically, the reaction rate constant increases from 0.064 to 0.318 min-1 as the coordination number of Fe-N increases from 2 to 4, slightly influencing the nonradical reaction mechanism dominated by 1O2. In-depth theoretical calculations unveil that the modulated coordination environments of Fe-SACs from Fe-N2C2 to Fe-N4 optimize the d-band electronic structures and regulate the binding strength of peroxymonosulfate on Fe-NxC4-x sites, resulting in a reduced energy barrier and enhanced Fenton-like catalytic activity. The catalytic stability and the actual hospital sewage treatment capacity also showed strong coordination dependency. This strategy of local coordination engineering offers a vivid example of modulating SACs with well-regulated coordination environments, ultimately maximizing their catalytic efficiency.

Aqueous Structure of Lanthanide-EDTA Coordination Complexes Determined by a Combined DFT/EXAFS Approach.

Sustainable production of rare earth elements (REEs) is critical for technologies needed for climate change mitigation, including wind turbines and electric vehicles. However, separation technologies currently used in REE production have large environmental footprints, necessitating more sustainable strategies. Aqueous, affinity-based separations are examples of such strategies. To make these technologies feasible, it is imperative to connect aqueous ligand structure to ligand selectivity for individual REEs. As a step toward this goal, we analyzed the extended X-ray absorption fine structure (EXAFS) of four lanthanides (La, Ce, Pr, and Nd) complexed by a common REE chelator, ethylenediaminetetraacetic acid (EDTA) to determine the aqueous-phase structure. Reference structures from density functional theory (DFT) were used to help fit the EXAFS spectra. We found that all four Ln-EDTA coordination complexes formed 9-coordinate structures with 6 coordinating atoms from EDTA (4 carboxyl oxygen atoms and 2 nitrogen atoms) and 3 oxygen atoms from water molecules. All EXAFS fits were of high quality (R-factor < 0.02) and showed decreasing average first-shell coordination distance across the series (2.62-2.57 Å from La-Nd), in agreement with DFT (2.65-2.56 Å from La-Nd). The insights determined herein will be useful in the development of ligands for sustainable rare earth elements (REE) separation technologies.

Hydration of the neurotransmitter [formula omitted]-aminobutyric acid and its isomer [formula omitted]-aminobutyric acid

Room-temperature aqueous solutions of the neurotransmitter γ-aminobutyric acid (GABA) and its isomer α-aminobutyric acid (AABA) were studied with broad-band dielectric relaxation spectroscopy (DRS) and statistical mechanics (1D-RISM and 3D-RISM) calculations. Comparison of the first-shell coordination numbers from 3D-RISM with the total effective hydration numbers from DRS revealed that for both solutes only about half of the H2O molecules in direct contact with the solute are affected in their dynamics. Whereas for GABA ∼10 of them are moderately retarded by a factor of ∼2–3 compared to bulk water and only ∼1–2 effectively frozen at vanishing solute concentration, AABA appears to be more strongly hydrated with only ∼6 moderately retarded but ∼6 H2O frozen by the solute molecule. This is reflected in the effective solute dipole moments.

Nucleation Rate Theory for Coordination Number: Elucidating Water-Mediated Formation of a Zigzag Na2SO4 Morphology.

Predicting and controlling nanostructure formation during nucleation can pave the way to synthesizing novel energy materials via crystallization. However, such control over nucleation and crystallization remains challenging due to an inadequate understanding of critical factors that govern evolving atomistic structures and dynamics. Herein, we utilize coordination number as a reaction coordinate and rate theory to investigate how sodium sulfate, commonly known as a phase-change energy material, nucleates in a supersaturated aqueous solution. In conjunction with ab initio and force field-based molecular dynamics simulation, the rate theoretical analysis reveals that sodium sulfate from an initially dissolved metastable state transits to a heterogeneous mixture of prenucleated clusters and finally to a large cylindrical zigzag morphology. Measurements of Raman spectra and their ab initio modeling confirm that this nucleated morphology contains a few waters for every sulfate. Rate processes such as solvent exchange and desolvation exhibit high sensitivity to the evolving prenucleation/nucleation structures, providing a means to distinguish between critical nucleation precursors. Desolvation and forming the first-shell interionic coordination structure via monomer-by-monomer addition around sulfates are found to explain the formation of large nuclei. Thus, a detailed understanding of the step-by-step structure formation across scales has been achieved. This can be leveraged to predict nucleation-related structures and dynamics and potentially control the synthesis of novel phase-change materials for energy applications.

Near-Quantitative Predictions of the First-Shell Coordination Structure of Hydrated First-Row Transition Metal Ions Using K-Edge X-ray Absorption Near-Edge Spectroscopy.

The solvation structure of transition metal ions is important for applications in geochemistry, biochemistry, energy storage, and environmental chemistry. We study the X-ray absorption pre-edge and near-edge spectra at the K-edge of a nearly complete series of hydrated first-row transition metal ions with d orbital occupancy from d2 to d10. We optimize all of the structures at the density functional theory (DFT) level with explicit solvation and then compute the pre-edge X-ray absorption spectra with time-dependent DFT (TDDFT) and restricted active space second-order perturbation theory (RASPT2). TDDFT provides accurate results for spectra that are dominated by single excitations, while RASPT2 correctly distinguishes between singly and doubly excited states with quantitative accuracy compared with experiment. We analyze the pre-edge features for each metal ion to reveal the impact of the variations in d orbital occupancy on the first-shell coordination environment. We also report the lowest-energy ligand field d-d transitions using complete active space second-order perturbation theory.

X-ray absorption spectroscopic investigation of the Ca and Mg environments in CO2-bearing silicate glasses

Recent investigation showed that the CO2 solubility in silicate melts is strongly affected by the #Mg (MgO/MgO + CaO). CO2 solubility is decreasing with increasing #Mg implying that CO2 molecules dissolves in the vicinity of Ca atoms to form CO32– molecular groups. We have investigated several CO2-bearing (up to 17.2 wt%) silicate glasses using X-ray Absorption Spectroscopy (XAS) at the Mg and Ca K-edge in order to determine the structural environments of Mg2+ and Ca2+ cations. Analyses of the Mg XANES region show that the Mg environment is not affected by the presence of CO2 dissolved as CO32– groups regardless of the CO2 content. The Mg K-edge EXAFS simulations and XANES modelling show that the average MgO distance is close to 2 Å and the average Mg coordination is close to 6.On the contrary, the position of the Ca XANES peak main resonance related to first-coordination shell is negatively correlated with the CO2 content and shifts from 4051.4 to 4050.8 eV with CO2 increasing from 0 to 17.2 wt%. The Ca EXAFS simulations showed that CaO average distance is longer (~2.5 Å) than the MgO distance. Furthermore, we observed that the Ca coordination number is positively correlated with the CO2 content: Ca coordination number increases from 7 to 9 for CO2 content changing from 0 to 17.2 wt%.Mg and Ca XAS results suggest that CO32– groups are dissolved in the vicinity of Ca2+ cations and not in the surrounding of Mg2+ cations which environment remains unaffected by CO2 dissolution. The tighter Mg atomic environment as well as the lower coordination number for Mg cations as compared to Ca atomic environment could explain such behaviour. One implication of this result is the lower CO2 transport capacity for Mg-rich silicate melts such as komatiites or kimberlites.

Reverse Monte Carlo modeling for local structures of noble metal nanoparticles using high-energy XRD and EXAFS.

Reverse Monte Carlo (RMC) modeling based on the total structure factor S(Q) obtained from high-energy X-ray diffraction (HEXRD) and the k3χ(k) obtained from extended X-ray absorption fine structure (EXAFS) measurements was employed to determine the 3-dimensional (3D) atomic-scale structure of Pt, Pd, and Rh nanoparticles, with sizes less than 5 nm, synthesized by photoreduction. The total structure factor and Fourier-transformed PDF showed that the first nearest neighbor peak is in accordance with that obtained from conventional EXAFS analysis. RMC constructed 3D models were analyzed in terms of prime structural characteristics such as metal-to-metal bond lengths, first-shell coordination numbers and bond angle distributions. The first-shell coordination numbers and bond angle distributions for the RMC-simulated metal nanoparticles indicated a face-centered cubic (fcc) structure with appropriate number density. Modeling disorder effects in these RMC-simulated metal nanoparticles also revealed substantial differences in bond-length distributions for respective nanoparticles.

Structural properties of transition-metal clusters via force-biased Monte Carlo and ab initio calculations: A comparative study

We present a force-biased Monte Carlo (FMC) method for structural modeling of the transition-metal clusters of Fe, Ni, and Cu with sizes of 13, 30, and 55 atoms. By employing the Finnis-Sinclair potential for Fe and the Sutton-Chen potential for Ni and Cu, the total energy of the clusters is minimized using the local gradient of the potentials in Monte Carlo simulations. The structural configurations of the clusters, obtained from the biased Monte Carlo approach, are analyzed and compared with the same configurations from the Cambridge Cluster Database (CCD) upon relaxation of the clusters using the first-principles density-functional code nwchem. The results show that the total-energy value and the structure of the FMC clusters are essentially identical to the corresponding value and the structure of the CCD clusters. A comparison of the nwchem-relax FMC and CCD structures is presented by computing the pair-correlation function, the bond-angle distribution, the coordination number of the first-coordination shell, and the Steinhardt bond-orientational order parameter, which provide information about the two- and three-body correlation functions, the local bonding environment of the atoms, and the geometry of the clusters. An atom-by-atom comparison of the FMC and CCD clusters is also provided by superposing one set of clusters onto another, and the electronic properties of the clusters are addressed by computing the density of electronic states.

Structure of transition metal clusters: A force-biased Monte Carlo approach

We present a force-biased Monte Carlo (FMC) method for structural modeling of transition metal clusters of Fe, Ni, and Cu with 5 to 60 atoms. By employing the Finnis-Sinclair potential for Fe and the Sutton-Chen potential for Ni and Cu, the total energy of the clusters is minimized using a method that utilizes atomic forces in Monte Carlo simulations. The structural configurations of the clusters obtained from this biased Monte Carlo approach are analyzed and compared with the same from the Cambridge Cluster Database (CCD). The results show that the total-energy of the FMC clusters is very close to the corresponding value of the CCD clusters as listed in the Cambridge Cluster Database. A comparison of the FMC and CCD clusters is presented by computing the pair-correlation function, the bond-angle distribution, and the distribution of atomic-coordination numbers in the first-coordination shell, which provide information about the two-body and three-body correlation functions, the local atomic structure, and the bonding environment of the atoms in the clusters.

X-ray absorption study of 3d transition-metals and Mg in glasses and analogue crystalline materials in AFe3 + Si2O6 and A2X2 + Si5O12, where A = K, Rb, or Cs and X = Mg, Mn, Fe, Co, Ni, Cu, or Zn

Crystalline and glassy phases in the ‘leucite’ systems (K,Rb,Cs)Fe3+Si2O6 and (K,Rb,Cs)2(Mg,Mn,Fe,Co,Ni,Cu,Zn)2+–Si5O12 have been studied using K-edge X-ray Absorption Spectroscopy (XAS) for 3d transition elements and Mg. Crystalline samples of known structure are used as model compounds for deducing the local structures of the equivalent glasses. ‘Ideal’ tetrahedral framework X2+–O distances are: Mn2+–O 2.02, Fe2+–O 1.98; Co2+–O 1.96, Ni2+–O 1.95, Cu2+–O 1.92, Zn2+–O 1.93 and Mg2+–O 1.93Å. The quenched glasses have Extended X-ray Absorption Fine Structure (EXAFS) first-shell distances and coordination numbers consistent with Mg and 3d elements occurring in tetrahedral coordination in most cases. The X-ray Absorption Near Edge Structure (XANES) spectra of the glasses are basically the same as those for the crystalline samples showing that they have similar medium range order, consistent with most of the Mg and 3d elements acting as network formers along with the associated alkalis as X2Y2+O2 complexes. Anhydrous glasses in the system CaO–MgO–Al2O3–SiO2 together with a basalt glass from Hawaii show examples of Mg in octahedral- (2.07Å, basalt), tetrahedral- (~1.87Å, siliceous glass) and 5-coordinated (2.01–2.04Å) sites. Although most Earth scientists assume that Mg and Fe2+ act as network modifiers in natural magmas, ultra-potassic, peralkaline compositions ((Na+K)/(Al+Fe3+)>1) could have had some Fe2+ and Mg complexes with alkalis in the melt network as K2(Mg,Fe2+)O2 and the implications of this to melt density and viscosity are considered.

First-principles molecular dynamics investigation on Na3AlF6 molten salt

Abstract Local structure and transport properties of Na 3 AlF 6 molten salt were investigated by First-principles molecular dynamics (FPMD) simulation. For Na 3 AlF 6 molten salt, the local ionic structure is governed by five-coordinated [AlF 5 ] 2− and six-coordinated [AlF 6 ] 3− . Coulomb force dominates the interionic interactions for Na 3 AlF 6 molten salt. The first-shell average coordination number (CN) of Na-F, Al-F in the Na 3 AlF 6 molten salt is 6.03, 5.45, respectively and the F-Al-F bond angles are mainly located at 87°, 124° and 171°. The percentage of bridging F b is small about 1–2%, while the free F f is up to 26%, suggesting the polymerization degree of local structure is lower. Al-F bonds of the [AlF x ] 3−x groups in Na 3 AlF 6 molten salt have ionic characters as well as partial covalent characters due to the hybridization of F-2p and Al-3s (3p) orbitals, while the Na-F and F-F bonds are mainly ionic. The order of ion diffusion ability was found as Na + > F − > Al 3+ . Calculated results of viscosity and ionic conductivity are in good agreement with the experimental results, generally within 7%.

Antimonate and arsenate speciation on reactive soil minerals studied by differential pair distribution function analysis

The pentavalent oxyanions antimonate (Sb(V)) and arsenate (As(V)) are toxic metalloids in environmental systems. In this study, we compare their sorption and surface speciation on 2-line ferrihydrite (Fh), δ-MnO2 and Na-montmorillonite (Mont) at pH5.5. The Sb(V) and As(V) sorption affinity and the highest measured surface excess (mmol:g) show that mineral reactivity towards Sb(V) or As(V) sorption increases in the order of Mont<δ-MnO2<Fh. In addition, Sb(V) showed greater uptake relative to As(V) by both Fh and δ-MnO2. Using differential pair distribution function (d-PDF) analysis of high-energy X-ray scattering data, we found that the larger SbO6 octahedron (RSb–O=1.95Å) can bind in both single edge-(1E) and double corner-sharing (2C) geometries to Fh and δ-MnO2, whereas the smaller AsO4 tetrahedron (RAs–O=1.69Å) binds only in the 2C geometry. Thus, a greater number of surface sites support Sb(V) adsorption relative to As(V) adsorption on Fh and δ-MnO2. Conversely, Mont supports only 2C adsorption geometries, which is consistent with the similar uptake behaviors of Sb(V) and As(V) on this mineral. Finally, we indexed atomic pairs from adsorbed Sb(V) and As(V) at R>6Å in the d-PDFs. The analysis of the intermediate-ranged surface structure of our samples substantiated our interpretations of oxyanion adsorption geometries and provided information regarding changes in the underlying mineral surface upon ion adsorption. Our results show that the differences between Sb(V) and As(V) interactions with reactive soil minerals, including their sorption geometries, are linked to their distinct first-shell coordination environments.

Role of attractive forces in determining the equilibrium structure and dynamics of simple liquids

Molecular Dynamics simulations of a Lennard-Jones system with different range of attraction show that the attractive forces modify the radial distribution of the particles. For condensed liquids only, the forces within the the first coordination shell (FCS) are important, but for gases and moderate condensed fluids, even the attractive forces outside the FCS play a role. The changes in the distribution caused by neglecting the attractive forces, lead to a too high pressure. The weak long-range attractions damp the dynamics and the diffusion of the particles in gas-, super critical fluid- and in liquid states. The values of self-diffusion coefficients (SDC) agree qualitatively with a modified Cohen-Turnbull model. The SDC-s along the critical isotherm show no anomaly at the critical point in agreement with experimental data.

Pressure-induced phase transition of lead phosphate Pb3(PO4)2: X-ray diffraction and XANES

Lead phosphate Pb3(PO4)2 was investigated by in situ synchrotron radiation X-ray diffraction and X-ray absorption near-edge structure (XANES) combined with diamond anvil cell technique up to 60.9 GPa. A pressure-induced phase transition of Pb3(PO4)2 was observed from monoclinic (C2/c) to trigonal () phases at about 1.7 GPa at room temperature. The high-pressure annealing experiment indicates that the trigonal phase of Pb3(PO4)2 is stable at least up to 60.9 GPa. Isothermal pressure–volume relationship of trigonal Pb3(PO4)2 is well described by the third-order Birch–Murnaghan equation of state with V0 = 533(1) Å3, K0 = 89(4) GPa and K0′ = 5.8(2). In trigonal Pb3(PO4)2, the a-axis is more compressible than the c-axis, showing an anisotropy elasticity. XANES results reveal the evolution of Pb local environment, i.e. the first-shell coordination number of Pb1 changes from 9 to 12 and the average Pb–O bond length increases at the crossover of C2/c-to- phase transition.

Competition of the connectivity with the local and the global order in polymer melts and crystals

The competition between the connectivity and the local or global order in model fully flexible chain molecules is investigated by molecular-dynamics simulations. States with both missing (melts) and high (crystal) global order are considered. Local order is characterized within the first coordination shell (FCS) of a tagged monomer and found to be lower than in atomic systems in both melt and crystal. The role played by the bonds linking the tagged monomer to FCS monomers (radial bonds), and the bonds linking two FCS monomers (shell bonds) is investigated. The detailed analysis in terms of Steinhardt's orientation order parameters Ql (l = 2 - 10) reveals that increasing the number of shell bonds decreases the FCS order in both melt and crystal. Differently, the FCS arrangements organize the radial bonds. Even if the molecular chains are fully flexible, the distribution of the angle formed by adjacent radial bonds exhibits sharp contributions at the characteristic angles θ ≈ 70°, 122°, 180°. The fractions of adjacent radial bonds with θ ≈ 122°, 180° are enhanced by the global order of the crystal, whereas the fraction with 70° ~/< θ ~/< 110° is nearly unaffected by the crystallization. Kink defects, i.e., large lateral displacements of the chains, are evidenced in the crystalline state.

Using a Combined Theoretical and Experimental Approach to Understand the Structure and Dynamics of Imidazolium-Based Ionic Liquids/Water Mixtures. 2. EXAFS Spectroscopy

Extended X-ray absorption fine structure (EXAFS) spectroscopy is employed, in conjunction with molecular dynamics (MD) simulations, to investigate the interaction of water with the Br(-) ion in an imidazolium-based ionic liquid (IL). 1-Butyl-3-methylimidazolium bromide/water mixtures with molar ratios ranging from 1:3 to 1:200 have been analyzed, and a clear picture of the structural arrangements of the water molecules inside the IL has been obtained from the synergic interpretation of the EXAFS and MD data. At the lowest investigated water content, the presence of water is mainly detected around the Br(-) anion. Upon increasing the water fraction, more water molecules enter the Br(-) first-coordination shell but always in a lower number than what is needed to saturate the inner sphere. This suggests that interactions also exist between water and the imidazolium cation. The existence of tight ion pairs has been evidenced, even when water is present in the mixtures in great excess.

Ligand‐Stabilized and Atomically Precise Gold Nanocluster Catalysis: A Case Study for Correlating Fundamental Electronic Properties with Catalysis

We present results from our investigations into correlating the styrene-oxidation catalysis of atomically precise mixed-ligand biicosahedral-structure [Au25(PPh3)10(SC12H25)5Cl2](2+) (Au25-bi) and thiol-stabilized icosahedral core-shell-structure [Au25(SCH2CH2Ph)18](-) (Au25-i) clusters with their electronic and atomic structure by using a combination of synchrotron radiation-based X-ray absorption fine-structure spectroscopy (XAFS) and ultraviolet photoemission spectroscopy (UPS). Compared to bulk Au, XAFS revealed low Au-Au coordination, Au-Au bond contraction and higher d-band vacancies in both the ligand-stabilized Au clusters. The ligands were found not only to act as colloidal stabilizers, but also as d-band electron acceptor for Au atoms. Au25-bi clusters have a higher first-shell Au coordination number than Au25-i, whereas Au25-bi and Au25-i clusters have the same number of Au atoms. The UPS revealed a trend of narrower d-band width, with apparent d-band spin-orbit splitting and higher binding energy of d-band center position for Au25-bi and Au25-i. We propose that the differences in their d-band unoccupied state population are likely to be responsible for differences in their catalytic activity and selectivity. The findings reported herein help to understand the catalysis of atomically precise ligand-stabilized metal clusters by correlating their atomic or electronic properties with catalytic activity.