Differential Pair Distribution Function Research Articles

Phosphate and phytate adsorption and precipitation on ferrihydrite surfaces

Transition of phosphate bidentate-binuclear surface complexation to precipitation with increasing P sorption loading was explored using differential pair distribution function (d-PDF) analysis.

Arsenate and Selenate Scavenging by Basaluminite: Insights into the Reactivity of Aluminum Phases in Acid Mine Drainage.

Basaluminite precipitation may play an important role in the behavior of trace elements in water and sediments affected by acid mine drainage and acid sulfate soils. In this study, the affinity of basaluminite and schwertmannite for arsenate and selenate is compared, and the coordination geometries of these oxyanions in both structures are reported. Batch isotherm experiments were conducted to examine the sorption capacity of synthetic schwertmannite and basaluminite and the potential competitive effect of sulfate. In addition, synchrotron-based techniques such as differential pair distribution function (d-PDF) analysis and extended X-ray absorption fine structure (EXAFS) were used to determine the local structure of As(V) and Se(VI) complexes. The results show that oxyanion exchange with structural sulfate was the main mechanism for removal of selenate, whereas arsenate was removed by a combination of surface complexes and oxyanion exchange. The arsenate adsorption capacity of basaluminite was 2 times higher than that of schwertmannite and 3 times higher than that of selenate in both phases. The sulfate:arsenate and sulfate:selenate exchange ratios were 1:2 and 1:1, respectively. High sulfate concentrations in the solutions did not show a competitive effect on arsenate sorption capacity but had a strong impact on selenate uptake, suggesting some kind of specific interaction for arsenate. Both d-PDF and EXAFS results indicated that the bidentate binuclear inner sphere was the most probable type of ligand for arsenate on both phases and for selenate on schwertmannite, whereas selenate forms outer-sphere complexes in the aluminum octahedral interlayer of basaluminite. Overall, these results show a strong affinity of poorly crystalline aluminum phases such as basaluminite for As(V) and Se(VI) oxyanions, with adsorption capacities on the same order of magnitude as those of iron oxides. The results obtained in this study are relevant to the understanding of trace element behavior in environments affected by acid water, potentially opening new research lines focused on remediation by natural attenuation processes or engineered water treatment systems.

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.

Thermal Stabilization of Metal-Organic Framework-Derived Single-Site Catalytic Clusters through Nanocasting.

Metal-organic frameworks (MOFs) provide convenient systems for organizing high concentrations of single catalytic sites derived from metallic or oxo-metallic nodes. However, high-temperature processes cause agglomeration of these nodes, so that the single-site character and catalytic activity are lost. In this work, we present a simple nanocasting approach to provide a thermally stable secondary scaffold for MOF-based catalytic single sites, preventing their aggregation even after exposure to air at 600 °C. We describe the nanocasting of NU-1000, a MOF with 3 nm channels and Lewis-acidic oxozirconium clusters, with silica. By condensing tetramethylorthosilicate within the NU-1000 pores via a vapor-phase HCl treatment, a silica layer is created on the inner walls of NU-1000. This silica layer provides anchoring sites for the oxozirconium clusters in NU-1000 after the organic linkers are removed at high temperatures. Differential pair distribution functions obtained from synchrotron X-ray scattering confirmed that isolated oxozirconium clusters are maintained in the heated nanocast materials. Pyridine adsorption experiments and a glucose isomerization reaction demonstrate that the clusters remain accessible to reagents and maintain their acidic character and catalytic activity even after the nanocast materials have been heated to 500-600 °C in air. Density functional theory calculations show a correlation between the Lewis acidity of the oxozirconium clusters and their catalytic activity. The ability to produce MOF-derived materials that retain their catalytic properties after exposure to high temperatures makes nanocasting a useful technique for obtaining single-site catalysts suitable for high-temperature reactions.

Ionic Liquid Character of Zinc Chloride Hydrates Define Solvent Characteristics that Afford the Solubility of Cellulose.

The recently described ionic liquid structure of the three equivalent hydrate of zinc chloride (ZnCl2·R H2O, R = 3, existing as [Zn(OH2)6][ZnCl4]) explains the solubility of cellulose in this medium. Only hydrate compositions in the narrow range of 3 - x < R < 3 + x with x ≈ 1 dissolve cellulose. Once dissolved, the cellulose remains in solution up to the R = 9 hydrate. Neutron diffraction and differential pair distribution function analysis of cellulose and model compound solutions (1 wt % cellulose in the R = 3 hydrate and 1 wt % ethanol in the R = 3 hydrate and the ZnCl2·3 ethanol liquid) coupled with detailed solubility measurements suggest that cellulose solubility occurs via coordination of the primary OH to the hydrated zinc cation with ring hydroxyls forming part of a second coordination shell around the cation of the ionic liquid.

Sorption selectivity of birnessite particle edges: a d-PDF analysis of Cd(ii) and Pb(ii) sorption by δ-MnO2 and ferrihydrite.

Birnessite minerals (layer-type MnO2), which bear both internal (cation vacancies) and external (particle edges) metal sorption sites, are important sinks of contaminants in soils and sediments. Although the particle edges of birnessite minerals often dominate the total reactive surface area, especially in the case of nanoscale crystallites, the metal sorption reactivity of birnessite particle edges remains elusive. In this study, we investigated the sorption selectivity of birnessite particle edges by combining Cd(ii) and Pb(ii) adsorption isotherms at pH 5.5 with surface structural characterization by differential pair distribution function (d-PDF) analysis. We compared the sorption reactivity of δ-MnO2 to that of the nanomineral, 2-line ferrihydrite, which exhibits only external surface sites. Our results show that, whereas Cd(ii) and Pb(ii) both bind to birnessite layer vacancies, only Pb(ii) binds extensively to birnessite particle edges. For ferrihydrite, significant Pb(ii) adsorption to external sites was observed (roughly 20 mol%), whereas Cd(ii) sorption was negligible. These results are supported by bond valence calculations that show comparable degrees of saturation of oxygen atoms on birnessite and ferrihydrite particle edges. Therefore, we propose that the sorption selectivity of birnessite edges follows the same order of that reported previously for ferrihydrite: Ca(ii) < Cd(ii) < Ni(ii) < Zn(ii) < Cu(ii) < Pb(ii).

Crystal engineering in 3D: converting nanoscale lamellar manganese oxide to cubic spinel while affixed to a carbon architecture

By applying differential pair distribution function analyses to MnOx-painted carbon nanofoam papers, we quantify the manganese oxide speciation as the nanoscale oxide transitions from disordered NaMnOxto spinel LiMn2O4.

High-energy X-ray diffraction of a hydrous silicate liquid under conditions of high pressure and temperature in a modified hydrothermal diamond anvil cell

In situ high-energy X-ray diffraction measurements were made for the first time on a water-saturated silicate melt at high pressure and temperature. A modified hydrothermal diamond anvil cell (HDAC), designed to minimize the path length of the X-ray beam within a diamond anvil and to increase the solid angle of the diffracted beam, was used to reduce high background contributions and extend X-ray diffraction data collection in Q space. Quantitative differential pair distribution function (PDF) analysis of X-ray diffraction data show that the first measurable (Si–O) peak is 0.095 Å greater in length in the hydrous melt than in the starting glass. Contributions from the H2O O–O correlations, as well as from the second nearest neighbor O–O correlations within the silicate melt, are evident within the second peak of the differential PDF. The procedure described opens new opportunities to directly investigate volatile-rich melts at high pressure and temperature.

Structure of Sulfate Adsorption Complexes on Ferrihydrite

Sulfate adsorption on mineral surfaces is an important environmental chemical process. However, the structure of sulfate adsorption complexes has remained uncertain. In this study, we have determined the S−Fe distance of sulfate inner-sphere adsorption complexes on iron (Fe) oxyhydroxide (ferrihydrite) surfaces under air-dried conditions using sulfur K-edge extended X-ray absorption fine structure spectroscopy and differential X-ray pair distribution function analysis. Both approaches indicate that the S− Fe interatomic distance of the sulfate adsorption complexes is 3.22−3.25 A, suggesting that sulfate forms bidentate−binuclear adsorption complexes on ferrihydrite surfaces. Outer-sphere complexes are also observed using infrared spectroscopic analysis. This finding clarifies the long-standing debate over the structure of sulfate adsorption complexes and has important implications for understanding the chemistry of sulfate at environmental interfaces.

Effect of Ferrihydrite Crystallite Size on Phosphate Adsorption Reactivity

The influence of crystallite size on the adsorption reactivity of phosphate on 2-line to 6-line ferrihydrites was investigated by combining adsorption experiments, structure and surface analysis, and spectroscopic analysis. X-ray diffraction (XRD) and transmission electron microscopy (TEM) showed that the ferrihydrite samples possessed a similar fundamental structure with a crystallite size varying from 1.6 to 4.4 nm. N2 adsorption on freeze-dried samples revealed that the specific surface area (SSABET) decreased from 427 to 234 m(2) g(-1) with increasing crystallite size and micropore volume (Vmicro) from 0.137 to 0.079 cm(3) g(-1). Proton adsorption (QH) at pH 4.5 and 0.01 M KCl ranged from 0.73 to 0.55 mmol g(-1). Phosphate adsorption capacity at pH 4.5 and 0.01 M KCl for the ferrihydrites decreased from 1690 to 980 μmol g(-1) as crystallite size increased, while the adsorption density normalized to SSABET was similar. Phosphate adsorption on the ferrihydrites exhibited similar behavior with respect to both kinetics and the adsorption mechanism. The kinetics could be divided into three successive first-order stages: relatively fast adsorption, slow adsorption, and a very slow stage. With decreasing crystallite size, ferrihydrites exhibited increasing rate constants per mass for all stages. Analysis of OH(-) release and attenuated total reflectance infrared spectroscopy (ATR-IR) and differential pair distribution function (d-PDF) results indicated that initially phosphate preferentially bound to two Fe-OH2(1/2+) groups to form a binuclear bidentate surface complex without OH(-) release, with smaller size ferrihydrites exchanging more Fe-OH2(1/2+) per mass. Subsequently, phosphate exchanged with both Fe-OH2(1/2+) and Fe-OH(1/2-) with a constant amount of OH(-) released per phosphate adsorbed. Also in this stage binuclear bidentate surface complexes were formed with a P-Fe atomic pair distance of ~3.25 Å.

Elucidation of structure and nature of the PdO–Pd transformation using in situ PDF and XAS techniques

The PdO-Pd phase transformation in a 4 wt% Pd/Al2O3 catalyst has been investigated using in situ X-ray absorption spectroscopy (XAS) and in situ X-ray total scattering (also known as high-energy X-ray diffraction) techniques. Both the partial and total pair distribution functions (PDF) from these respective techniques have been analysed in depth. New information from PDF analysis of total scattering data has been garnered using the differential PDF (d-PDF) approach where only correlations orginating from PdO and metallic Pd are extracted. This method circumvents problems encountered in characerising the catalytically active components due to the diffuse scattering from the disordered γ-Al2O3 support phase. Quantitative analysis of the palladium components within the catalyst allowed for the phase composition to be established at various temperatures. Above 850 °C it was found that PdO had converted to metallic Pd, however, the extent of reduction was of the order ca. 70% Pd metal and 30% PdO. Complementary in situ XANES and EXAFS were performed, with heating to high temperature and subsequent cooling in air, and the results of the analyses support the observations, that residual PdO is detected at elevated temperatures. Hysteresis in the transformation upon cooling is confirmed from XAS studies where reoxidation occurs below 680 °C.

Atomic ordering in nanosized PtxAu1–x(x= 0, 0.51, 1) by resonant X-ray diffraction and differential atomic pair distribution functions

Resonant X-ray diffraction and differential atomic pair distribution functions method is illustrated with results from a study of 4.5(3) nm Pt, Au and Pt0.51Au0.49 particles. By doing experiments at two energies close to the K absorption edge of Pt the atomic correlations relative to Pt and Au atoms in the alloy particles are differentiated. The experimental data show that Pt and Au atoms in all three samples arrange in a face centered cubic type structure while keeping their bond lengths of 2.76 Å and 2.86 Å, respectively, virtually unchanged. This leads to increased local structural distortions in the alloyed Pt0.51Au0.49 particles that may affect their catalitic properties substantially.

Total X-Ray Scattering of Spider Dragline Silk

Total x-ray scattering measurements of spider dragline silk fibers from Nephila clavipes, Argiope aurantia, and Latrodectus hesperus all yield similar structure factors, with only small variations between the different species. Wide-angle x-ray scattering from fibers orientated perpendicular to the beam shows a high degree of anisotropy, and differential pair distribution functions obtained by integrating over wedges of the equatorial and meridian planes indicate that, on average, the majority (95%) of the atom-atom correlations do not extend beyond 1 nm. Futhermore, the atom-atom correlations between 1 and 3 nm are not associated with the most intense diffraction peaks at Q=1-2 Å(-1). Disordered molecular orientations along the fiber axis are consistent with proteins in similar structural arrangements to those in the equatorial plane, which may be associated with the silk's greater flexibility in this direction.

Differential Pair Distribution Function Study of the Structure of Arsenate Adsorbed on Nanocrystalline γ-Alumina

Structural information is important for understanding surface adsorption mechanisms of contaminants on metal (hydr)oxides. In this work, a novel technique was employed to study the interfacial structure of arsenate oxyanions adsorbed on γ-alumina nanoparticles, namely, differential pair distribution function (d-PDF) analysis of synchrotron X-ray total scattering. The d-PDF is the difference of properly normalized PDFs obtained for samples with and without arsenate adsorbed, otherwise identically prepared. The real space pattern contains information on atomic pair correlations between adsorbed arsenate and the atoms on γ-alumina surface (Al, O, etc.). PDF results on the arsenate adsorption sample on γ-alumina prepared at 1 mM As concentration and pH 5 revealed two peaks at 1.66 Å and 3.09 Å, corresponding to As-O and As-Al atomic pair correlations. This observation is consistent with those measured by extended X-ray absorption fine structure (EXAFS) spectroscopy, which suggests a first shell of As-O at 1.69 ± 0.01 Å with a coordination number of ~4 and a second shell of As-Al at ~3.13 ± 0.04 Å with a coordination number of ~2. These results are in agreement with a bidentate binuclear coordination environment to the octahedral Al of γ-alumina as predicted by density functional theory (DFT) calculation.

Elucidating the Structure of Surface Acid Sites on γ-Al2O3

Differential pair distribution function analysis was applied to resolve, with crystallographic detail, the structure of catalytic sites on the surface of nanoscale γ-Al(2)O(3). The structure was determined for a basic probe molecule, monomethylamine (MMA), bound at the minority Lewis acid sites. These active sites were found to be five-coordinate, forming distorted octahedra upon MMA binding. This approach could be applied to study the interaction of molecules at surfaces in dye-sensitized solar cells, nanoparticles, sensors, materials for waste remediation, and catalysts.

Melting of Bi Sublattice in NanosizedBiFeO3Perovskite by Resonant X-Ray Diffraction

Free-standing BiFeO3 perovskite particles with a size ranging from polycrystalline bulk down to 5nm have been studied by high-energy resonant (Bi K edge) x-ray diffraction coupled to differential atomic pair distribution function analysis. Nanosized BiFeO3 particles are found to exhibit extra, i.e., beyond the usual thermal, structural disorder that increases progressively with diminishing their size. In particles of size smaller than approximately 18nm the disorder destroys the structural coherence of the Bisublattice and disturbs that of the Fe-based sublattice in the perovskite structure, substantially affecting the magnetoelectric properties it carries. The new structural information helps better understand the unusual behavior of perovskites structured at the nanoscale.

Radioactive Iodine Capture in Silver-Containing Mordenites through Nanoscale Silver Iodide Formation

The effective capture and storage of radiological iodine ((129)I) remains a strong concern for safe nuclear waste storage and safe nuclear energy. Silver-containing mordenite (MOR) is a longstanding benchmark for iodine capture; however, the molecular level understanding of this process needed to develop more effective iodine getters has remained elusive. Here we probe the structure and distribution of iodine sorbed by silver-containing MOR using differential pair distribution function analysis. While iodine is distributed between gamma-AgI nanoparticles on the zeolite surface and subnanometer alpha-AgI clusters within the pores for reduced silver MOR, in the case of unreduced silver-exchanged MOR, iodine is exclusively confined to the pores as subnanometer alpha-AgI. Consequently, unreduced silver-containing zeolites may offer a more secure route for radioactive iodine capture, with the potential to more effectively trap the iodine for long-term storage.

D-83 Element-Specific Structure of Nanosized Materials by High-Energy Resonant X-ray Diffraction and Differential Atomic Pair Distribution Functions

D-83 Element-Specific Structure of Nanosized Materials by High-Energy Resonant X-ray Diffraction and Differential Atomic Pair Distribution Functions

A new Al-rich hydroxylian pseudorutile from Kalimantan, Indonesia

A new type of Al-rich hydroxylian pseudorutile (HPR), containing 64.6 wt% TiO 2 , 12.1 wt% Al 2 O 3 , 10.9 wt% Fe 2 O 3 , and 12.4 wt% H 2 O, has been identified in detrital heavy mineral concentrates from Central Kalimantan, Indonesia. The unusual translucent orange-colored grains have been characterized using electron microprobe analyses, thermal analyses, 27 Al MAS NMR, and synchrotron XRD studies including the use of differential pair distribution function analysis, to determine how the Al is incorporated. The results support a model in which diaspore-form AlOOH is incorporated into nanopores in the solid hydrogel-like mineral. The HPR matrix structure comprises disordered unit-cell scale intergrowths of diaspore-type and rutile-type structure elements, which have a close dimensional match to the diaspore structure, thus providing a suitable template for epitactic nucleation and growth of the diaspore. The average composition of the grains is [FeTi 6 O 10.8 (OH) 5.4 ]·1.8AlOOH·H 2 O, where the formula within the square brackets represents the HPR matrix, and the AlOOH and H 2 O occupy the intragrain pore volume.

Investigation of Surface Structures by Powder Diffraction: A Differential Pair Distribution Function Study on Arsenate Sorption on Ferrihydrite

Differential pair distribution function (d-PDF) analysis of high energy powder X-ray diffraction data was carried out on 2-line ferrihydrite nanoparticles with arsenate oxyanions adsorbed on the surface to investigate the binding mechanism. In this analysis, a PDF of ferrihydrite is subtracted from a PDF of ferrihydrite with arsenate sorbed on the surface, leaving only correlations from within the surface layer and between the surface and the particle. As-O and As-Fe correlations were observed at 1.68 and 3.29 A, respectively, in good agreement with previously published EXAFS data, confirming a bidentate binuclear binding mechanism. Further peaks are observed in the d-PDF which are not present in EXAFS, corresponding to correlations between As and O in the particle and As-2nd Fe.