Distribution Of Surface Sites Research Articles

Determining the chemical ordering in nanoalloys by considering atomic coordination types.

The energetically most favorable chemical ordering of bimetallic nanoparticles can be characterized by combining global optimization algorithms and surrogate energy models. The latter approximate the energy of nanoalloys relying on structural descriptors, training models, and data. Here, we systematically evaluate the performance of highly data-efficient topological descriptors [Kozlov et al., Chem. Sci. 6, 3868 (2015)] for predicting the energies of metal nanoalloys with different chemical orderings. We also introduce a new descriptor based on atomic coordination types, which results in a less data-efficient and interpretable approach, but improves the general accuracy and the quantification of orderings in the inner parts of nanoparticles. The capacity of both the original and new approaches in combination with a basin hopping algorithm is illustrated by generating convex hulls of PdZn nanoalloys and predicting the resulting active surface site distribution as a function of particle composition. Finally, we show how these approaches can be combined with machine-learning adsorption models in electrocatalysis studies for a fast evaluation of the reactivity landscape of targeted nanoalloys.

Deciphering the Phillips Catalyst by Orbital Analysis and Supervised Machine Learning from Cr Pre-edge XANES of Molecular Libraries.

Unveiling the nature and the distribution of surface sites in heterogeneous catalysts, and for the Phillips catalyst (CrO3/SiO2) in particular, is still a grand challenge despite more than 60 years of research. Commonly used references in Cr K-edge XANES spectral analysis rely on bulk materials (Cr-foil, Cr2O3) or molecules (CrCl3) that significantly differ from actual surface sites. In this work, we built a library of Cr K-edge XANES spectra for a series of tailored molecular Cr complexes, varying in oxidation state, local coordination environment, and ligand strength. Quantitative analysis of the pre-edge region revealed the origin of the pre-edge shape and intensity distribution. In particular, the characteristic pre-edge splitting observed for Cr(III) and Cr(IV) molecular complexes is directly related to the electronic exchange interactions in the frontier orbitals (spin-up and -down transitions). The series of experimental references was extended by theoretical spectra for potential active site structures and used for training the Extra Trees machine learning algorithm. The most informative features of the spectra (descriptors) were selected for the prediction of Cr oxidation states, mean interatomic distances in the first coordination sphere, and type of ligands. This set of descriptors was applied to uncover the site distribution in the Phillips catalyst at three different stages of the process. The freshly calcined catalyst consists of mainly Cr(VI) sites. The CO-exposed catalyst contains mainly Cr(II) silicates with a minor fraction of Cr(III) sites. The Phillips catalyst exposed to ethylene contains mainly highly coordinated Cr(III) silicates along with unreduced Cr(VI) sites.

Colloidal Synthesis and Optical Properties of Perovskite-Inspired Cesium Zirconium Halide Nanocrystals.

Optoelectronic devices based on lead halide perovskites are processed in facile ways, yet are remarkably efficient. There are extensive research efforts investigating lead-free perovskite and perovskite-related compounds, yet there are challenges to synthesize these materials in forms that can be directly integrated into thin film devices rather than as bulk powders. Here, we report on the colloidal synthesis and characterization of lead-free, antifluorite Cs2ZrX6 (X = Cl, Br) nanocrystals that are readily processed into thin films. We use transmission electron microscopy and powder X-ray diffraction measurements to determine their size and structural properties, and solid-state nuclear magnetic resonance measurements reveal the presence of oleate ligand, together with a disordered distribution of Cs surface sites. Density functional theory calculations reveal the band structure and fundamental band gaps of 5.06 and 3.91 eV for Cs2ZrCl6 and Cs2ZrBr6, respectively, consistent with experimental values. Finally, we demonstrate that the Cs2ZrCl6 and Cs2ZrBr6 nanocrystal thin films exhibit tunable, broad white photoluminescence with quantum yields of 45% for the latter, with respective peaks in the blue and green spectral regions and mixed systems exhibiting properties between them. Our work represents a critical step toward the application of lead-free Cs2ZrX6 nanocrystal thin films into next-generation light-emitting applications.

Thermal Emission Spectroscopy of Single, Isolated Carbon Nanoparticles: Effects of Particle Size, Material, Charge, Excitation Wavelength, and Thermal History

Results are presented for thermal emission from individually trapped carbon nanoparticles (NPs) in the temperature range from ∼1000 to ∼2100 K. We explore the effects on the magnitude and wavelength dependency of the emissivity, ε(λ), of the NP size and charge and of the type of carbon material, including graphite, graphene, diamond, carbon black, and carbon dots. In addition, it is found that heating the NPs, particularly to temperatures above ∼1900 K, results in significant changes in the emission properties, which is attributed to changes in the distribution of surface and defect sites caused by annealing and sublimation.

Dissolution rate spectra of β-dicalcium silicate in water of varying activity

The surface normal dissolution flux of beta dicalcium silicate is quantified using in situ digital holographic microscopy. The dissolution flux in highly undersaturated flowing solutions depends on the water activity to an empirically determined power of 1.7. A heterogeneous distribution of reactive surface sites (e.g., crystalline defects) at the surface leads to a distribution of local fluxes. Accounting for the nominally unreactive and slowly reactive surface sites, the median of the macroscopic forward reaction dissolution flux distribution is −1.0 μmol m−2 s−1 with an interquartile range of 0.3 μmol m−2 s−1. Ex situ X-ray photoelectron spectroscopy indicates that the surface calcium-to-silicon ratio decreases by 10% after some dissolution.

Palladium, Iridium, and Rhodium Supported Catalysts: Predictive H₂ Chemisorption by Statistical Cuboctahedron Clusters Model.

Chemisorption of hydrogen on metallic particles is often used to estimate the metal dispersion (D), the metal particle size (d), and the metallic specific surface area (SM), currently assuming a stoichiometry of one hydrogen atom H adsorbed per surface metal atom M. This assumption leads to a large error when estimating D, d, and SM, and a rigorous method is needed to tackle this problem. A model describing the statistics of the metal surface atom and site distribution on perfect cuboctahedron clusters, already developed for Pt, is applied to Pd, Ir, and Rh, using the density functional theory (DFT) calculation of the literature to determine the most favorable adsorption sites for each metal. The model predicts the H/M values for each metal, in the range 0–1.08 for Pd, 0–2.77 for Ir, and 0–2.31 for Rh, depending on the particle size, clearly showing that the hypothesis of H/M = 1 is not always confirmed. A set of equations is then given for precisely calculating D, d, and SM for each metal directly from the H chemisorption results determined experimentally, without any assumption about the H/M stoichiometry. This methodology provides a powerful tool for accurate determination of metal dispersion, metal particle size, and metallic specific surface area from chemisorption experiments.

Modelling realistic TiO2 nanospheres: A benchmark study of SCC-DFTB against hybrid DFT.

TiO2 nanoparticles (NPs) are nowadays considered fundamental building blocks for many technological applications. Morphology is found to play a key role with spherical NPs presenting higher binding properties and chemical activity. From the experimental point of view, the characterization of these nano-objects is extremely complex, opening a large room for computational investigations. In this work, TiO2 spherical NPs of different sizes (from 300 to 4000 atoms) have been studied with a two-scale computational approach. Global optimization to obtain stable and equilibrated nanospheres was performed with a self-consistent charge density functional tight-binding (SCC-DFTB) simulated annealing process, causing a considerable atomic rearrangement within the nanospheres. Those SCC-DFTB relaxed structures have been then optimized at the DFT(B3LYP) level of theory. We present a systematic and comparative SCC-DFTB vs DFT(B3LYP) study of the structural properties, with particular emphasis on the surface-to-bulk sites ratio, coordination distribution of surface sites, and surface energy. From the electronic point of view, we compare HOMO-LUMO and Kohn-Sham gaps, total and projected density of states. Overall, the comparisons between DFTB and hybrid density functional theory show that DFTB provides a rather accurate geometrical and electronic description of these nanospheres of realistic size (up to a diameter of 4.4 nm) at an extremely reduced computational cost. This opens for new challenges in simulations of very large systems and more extended molecular dynamics.

Local Structures and Heterogeneity of Silica-Supported M(III) Sites Evidenced by EPR, IR, NMR, and Luminescence Spectroscopies.

Grafting molecular precursors on partially dehydroxylated silica followed by a thermal treatment yields silica-supported M(III) sites for a broad range of metals. They display unique properties such as high activity in olefin polymerization and alkane dehydrogenation (M = Cr) or efficient luminescence properties (M = Yb and Eu) essential for bioimaging. Here, we interrogate the local structure of the M(III) surface sites obtained from two molecular precursors, amides M(N(SiMe3)2)3 vs siloxides (M(OSi(OtBu)3)3·L with L = (THF)2 or HOSi(OtBu)3 for M = Cr, Yb, Eu, and Y, by a combination of advanced spectroscopic techniques (EPR, IR, XAS, UV-vis, NMR, luminescence spectroscopies). For paramagnetic Cr(III), EPR (HYSCORE) spectroscopy shows hyperfine coupling to nitrogen only when the amide precursor is used, consistent with the presence of nitrogen neighbors. This changes their specific reactivity compared to Cr(III) sites in oxygen environments obtained from siloxide precursors: no coordination of CO and oligomer formation during the polymerization of ethylene due to the presence of a N-donor ligand. The presence of the N-ligand also affects the photophysical properties of Yb and Eu by decreasing their lifetime, probably due to nonradiative deactivation of excited states by N-H bonds. Both types of precursors lead to a distribution of surface sites according to reactivity for Cr, luminescence spectroscopy for Yb and Eu, and dynamic nuclear polarization surface-enhanced 89Y NMR spectroscopy (DNP SENS). In particular, DNP SENS provides molecular-level information about the structure of surface sites by evidencing the presence of tri-, tetra-, and pentacoordinated Y-surface sites. This study provides unprecedented evidence and tools to assess the local structure of metal surface sites in relation to their chemical and physical properties.

Modeling Clovis Landscape Use and Recovery Bias in the Southeastern United States Using the Paleoindian Database of the Americas (PIDBA)

In eastern North America, there are few stratified sites dating to the Late Pleistocene epoch (> 11,700 cal B.P.). Instead, researchers have relied on the distribution of surface sites and isolated finds to make inferences about how the early inhabitants of the region used the landscape. While proxies for modern recovery bias have been found to affect artifact recovery at a national scale, in the southeastern United States, I argue, they are poor predictors for the frequency of Clovis type bifaces (ca. 13,250–12,850 cal B.P.) from counties in the southeastern United States as reported in the Paleoindian Database of the Americas (PIDBA). Instead, counties with the highest density of Clovis bifaces are near sources of lithic raw material, and in particular the intersection of major rivers, physiographic boundaries, and lithic raw material sources. I contend that these locations could represent seasonal aggregation loci. Alternatively, they could reflect areas where lithic raw material is more readily available, which may have resulted in higher rates of artifact discard. Determining which of these hypotheses is a more accurate reflection of past human behavior requires that we untangle the degree to which the amount of time people spent at these locations was successive versus coeval.

Modeling Clovis Landscape Use and Recovery Bias in the Southeastern United States Using the Paleoindian Database of the Americas (PIDBA)

In eastern North America, there are few stratified sites dating to the Late Pleistocene epoch (> 11,700 cal B.P.). Instead, researchers have relied on the distribution of surface sites and isolated finds to make inferences about how the early inhabitants of the region used the landscape. While proxies for modern recovery bias have been found to affect artifact recovery at a national scale, in the southeastern United States, I argue, they are poor predictors for the frequency of Clovis type bifaces (ca. 13,250–12,850 cal B.P.) from counties in the southeastern United States as reported in the Paleoindian Database of the Americas (PIDBA). Instead, counties with the highest density of Clovis bifaces are near sources of lithic raw material, and in particular the intersection of major rivers, physiographic boundaries, and lithic raw material sources. I contend that these locations could represent seasonal aggregation loci. Alternatively, they could reflect areas where lithic raw material is more readily available, which may have resulted in higher rates of artifact discard. Determining which of these hypotheses is a more accurate reflection of past human behavior requires that we untangle the degree to which the amount of time people spent at these locations was successive versus coeval.

Effect of Surface and Salt Properties on the Ion Distribution around Spherical Nanoparticles: Monte Carlo Simulations.

Nanoparticle surface charge properties represent key parameters to predict their fate, reactivity, and complexation in natural, biological, and industrial dispersions. In this context, we present here an original approach to better understand the surface charge electrostatic properties of spherical nanoparticles (NPs). The ion distribution around one nanoparticle is investigated using Monte Carlo simulations and by adjusting a wide range of parameters including NP properties (surface charge density and site distribution), salt concentration (ionic strength and cation concentration), and salt valency (mono-, di-, and trivalent salt). A canonical Metropolis Monte Carlo method is used to reach equilibrium states and a primitive Coulomb model is applied to describe the electrostatic interactions between explicit discrete sites, counterions, and salt particles. Our results show that the presence of explicit surface charges on the NP and in solution has a strong influence on the local ion distribution and on the effective surface charge of the nanoparticles. The increase of surface charge density reduces the NP effective charge by the formation of a condensation layer around the nanoparticle. However, a limit of condensation is achieved due to steric effects and electrostatic repulsions. The presence of di- and trivalent cations is also found to strongly modify the effective charge and improve condensation state as long as electrostatic repulsion between the cations close to the surface are not so strong. At high trivalent cation concentration, the NP effective charge is greatly reduced and the local environment around the nanoparticle becomes more structured with the formation of a multi layer structure composed by anions and cations.

Correlating Hydrogen Oxidation/Evolution Reaction Activity with the Minority Weak Hydrogen-Binding Sites on Ir/C Catalysts

Fundamental understanding of the active sites mediating hydrogen oxidation/evolution reaction (HOR/HER) is critical to the design of an efficient HOR/HER electrocatalyst for affordable hydrogen exchange membrane fuel cells and electrolyzers. Here we report the existence of the most active sites on carbon supported iridium nanoparticles (Ir/C) for HOR/HER in alkaline electrolyte by investigating activities of Ir/C with varying particle sizes in the range of 3–12 nm. The distribution of surface sites is quantified by deconvoluting the Hupd desorption peak in cyclic voltammograms. The portion of the sites with the lowest hydrogen binding energy (HBE) increases with the increase of the particle size or the decrease of the total electrochemical active surface area (t-ECSA). The HOR/HER activity normalized to t-ECSA decreases as t-ECSA increases while it remains constant when normalized to the surface area of the sites with an average HBE of −0.33 eV, accounting for only about 15−30% of the total sites, shoulde...

Removal of Sr(II) from aqueous solutions using polyacrylamide modified graphene oxide composites

In this paper, polyacrylamide modified graphene oxide (PAM/GO) composites were synthesized and characterized by scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray diffraction. The surface site distribution of PAM/GO was simulated with the aid of FITEQL 3.2 code. The synthesized PAM/GO composites were applied to adsorb Sr(II) from aqueous solutions. The adsorption conditions, such as contact time, pH value, and ionic strength, were investigated, which indicated pH and ionic strength dependent adsorption process. The adsorption isotherm agreed well with the Langmuir model, with a maximum adsorption capacity of 2.11mmol/g at pH=8.5±0.1 and T=303K. Thermodynamic parameters calculated from the temperature-dependent adsorption isotherms suggested that the adsorption of Sr(II) on PAM/GO composites was an endothermic and spontaneous process. The efficient adsorption suggested that PAM/GO might be a potential candidate for the preconcentration and separation of Sr(II) from wastewater.

Adsorption of zwitterionic fluoroquinolone antibacterials to goethite: A charge distribution-multisite complexation model

Fluoroquinolone (FQ) antibacterials are aquatic contaminants of emerging concern (CEC), and adsorption to mineral surfaces is expected to play an important role in the fate, transport, and treatment of FQs. This study characterizes and models the adsorption of a zwitterionic FQ, ofloxacin (OFX), to goethite (α-FeOOH) over a wide range of pH (3–11), OFX concentration (20–500μM), and electrolyte compositions (0.001–0.1M NaCl and NaClO4). Comparing OFX adsorption to structural analogues demonstrates that the carboxylate group is essential for binding to goethite. ATR-FTIR measurements indicate that FQs complex to goethite surfaces through carboxylate and carbonyl oxygen atoms, and that ClO4− co-adsorbs with OFX. Adsorption of the zwitterionic OFX increases with increasing ionic strength and is enhanced in NaClO4 relative to NaCl electrolyte, whereas adsorption of a non-zwitterionic analogue is insensitive to ionic strength. A CD-MUSIC (charge distribution-multisite complexation) model, incorporating multiple modes of surface complexation constrained by spectroscopic measurements and the crystallographic distribution of goethite surface sites, yields accurate predictions over wide-ranging solution conditions. According to the model, OFX adsorbs predominantly by inner-sphere complexation on terminal surfaces of the rod-shaped goethite crystals in NaCl electrolyte, and OFX-ClO4− ion pairing in NaClO4 induces formation of additional inner- and outer-sphere surface complexes on multiple crystal faces of goethite.

In situ analysis of optimum surface atom coordination for Pt nanoparticle oxygen reduction electrocatalysts

Highly dispersed Pt nanoparticles have been extensively studied for the electrocatalytic oxygen reduction reaction (ORR). Pt bulk and supported-nanoparticle electrodes have exhibited varying degrees of surface structure sensitivity toward the ORR for two main reasons: first, preferential adsorption of supporting electrolyte or water; and second, intrinsic variation of reaction kinetics on different Pt(hkl) surfaces or atomic scale imperfections on the Pt surface (e.g. steps, kinks, edges, and corners). The impact of surface atom coordination on ORR activity is seldom reported because there are few techniques that lend themselves to detailed, in situ assessment of catalyst surface site distribution. Surface active sites on ORR electrocatalysts have been inferred from application of bulk crystal structure data to specific nanoparticle geometries that account for electrocatalytically active surface area, ECA (cm2/gPt). This approach fails to capture the wide variety of active sites present on electrocatalyst surfaces under operating conditions, particularly at nanoparticle sizes that span the atomic cluster to nanocrystal transition. In this paper, we apply the techniques developed by Feliu et al. to determine surface site distribution in situ and, for the first time in the field, correlate these observations with ORR mass activity, MA (A/gPt), and surface activity, SA (μA/cm2Pt) on Pt nanoparticle catalysts. This approach indicates that the predominant active site available for ORR on nanoparticles in the size range of 1.8–6.9nm is (110) or (311). This observation is confirmed by using perchloric acid, sulfuric acid, and potassium hydroxide to demonstrate that the supporting electrolyte has little influence on ORR kinetics for these nanoparticles. Such behavior suggests that the Pt nanoparticle surfaces investigated consist of stepped adlayers on (111) or (100) facets that eliminate the (111) terraces historically associated with ORR activity. The predominance of such a stepped surface on Pt ORR electrocatalysts is unexpected and demonstrates the need for in situ characterization of active site distribution.

Misleading information on homogeneity and heterogeneity obtained from sorption isotherms

In this paper, the applications and the differences among the widely applied sorption isotherms (Langmuir isotherm for adsorption, competitive adsorption, ion exchange, Freundlich isotherm) are shown. The misleading information obtained by the formal applications of the isotherms is demonstrated using a simple mathematical model of homovalent ion exchange on homogeneous surface. This model calculation clearly reveals that when studying surface accumulation processes, the mechanism of the sorption process has to be determined. The correct thermodynamic interpretation of the data of sorption experiments is possible only if the isotherm is adapted to the sorption mechanism. It is emphasized that the regression values of the applied models themselves provide correct information neither on the surface heterogeneity nor the interactions among the sorbed species. The curved shape of an inadequate applied isotherm gives no information about the heterogeneity. In order to study the energy distribution of surface sites, the thermodynamic equations, including isotherms, has to be selected on the basis of sorption mechanism. All variable quantities involved in the given model (concentrations of the competing substances in all phases) have to be measured experimentally and included into the isotherm.

Surface properties of Ru particles supported on carbon materials: A microcalorimetric study of the effects over the CO chemisorptions of residual anionic species

Chemisorption of CO combined with microcalorimetry has been applied to study the nature, number and adsorption strength distribution of surface sites exposed by carbon-supported Ru catalysts. A comparative analysis of the CO chemisorption on different Ru catalysts, prepared using two different metal precursors, RuCl3·xH2O and Ru(NO)(NO3)3, has been carried out. An activated carbon and the corresponding derivative where oxygen surface groups were incorporated, as well as carbon nanotubes and a high surface area graphite, were used as catalytic supports. Based on previous temperature programmed reduction studies, all the catalysts were reduced under hydrogen flow at 523K or at 573K. The CO adsorption differential enthalpy profiles show that Ru(NO)(NO3)3 precursor produces more homogeneous surface site distribution in the Ru nanocrystals, in comparison with those prepared from RuCl3, as well as higher values of enthalpies in the medium range of coverage. As a possible explanation for this effect, residual chloride species remaining after reduction treatment in the ex-chloride catalysts, that can be anchored to the Ru nanoparticles weakening the CO adsorption, have been considered. This behavior occurs for the three studied carbon supports. On the other hand, the oxygen surface groups incorporated on the activated carbon seem not to modify the CO adsorption properties of the catalysts, independently of the precursor employed.

Isoparaffin production by aqueous phase processing of sorbitol over the Ni/HZSM-5 catalysts: Effect of the calcination temperature of the catalyst

Ni/HZSM-5 catalysts calcined at different temperatures were used in the isoparaffin production by aqueous phase processing of sorbitol and characterized by N2 physical adsorption, temperature-programmed reduction (H2-TPR), temperature-programmed desorption of ammonia (NH3-TPD) and Raman techniques. The effect of calcination temperature of the catalysts on the catalytic performance for the reaction was investigated. The activity test results indicated that the maximal i-C6H14 selectivity of 45.4% and the total i-C6H14 and i-C5H12 yield of 32.3% were obtained over the catalyst calcined at 500°C, which exhibited the optimum surface area and pore structure with 100% of the reducibility of Ni species on the surface of HZSM-5. In addition, the amount and the strength distribution of acidic surface sites of the catalyst decreased with the increase of calcination temperature at 500°C above. All these factors result in an increase in the formation of isoparaffin.

A Critical Assessment of Methods for the Intrinsic Analysis of Liquid Interfaces. 1. Surface Site Distributions

A Critical Assessment of Methods for the Intrinsic Analysis of Liquid Interfaces. 1. Surface Site Distributions

A Critical Assessment of Methods for the Intrinsic Analysis of Liquid Interfaces. 1. Surface Site Distributions

Substantial progress in our understanding of interfacial structure and dynamics has stemmed from the recent development of algorithms that allow for an intrinsic analysis of fluid interfaces. These work by identifying the instantaneous location of the interface, at the atomic level, for each molecular configuration and then computing properties relative to this location. Such a procedure eliminates the broadening of the interface caused by capillary waves and reveals the underlying features of the system. However, a precise definition of which molecules actually belong to the interfacial layer is difficult to achieve in practice. Furthermore, it is not known if the different intrinsic analysis methods are consistent with each other and yield similar results for the interfacial properties. In this paper, we carry out a systematic and detailed comparison of the available methods for intrinsic analysis of fluid interfaces, based on a molecular dynamics simulation of the interface between liquid water and carbon tetrachloride. We critically assess the advantages and shortcomings of each method, based on reliability, robustness, and speed of computation, and establish consistent criteria for determining which molecules belong to the surface layer. We believe this will significantly contribute to make intrinsic analysis methods widely and routinely applicable to interfacial systems.