Unveiling the Structural Origins of Dynamic Diversity in Pd-Based Metallic Glasses.

The ß-relaxation is one of the major dynamic behaviors in metallic glasses (MGs) and exhibits diverse features. Despite decades of efforts, the understanding of its structural origin and contribution to the overall dynamics of MG systems is still unclear. Here two palladium-based Pd─Cu─P and Pd─Ni─P MGs are reported with distinct different ß-relaxation behaviors and reveal the structural origins for the difference using the advanced X-ray photon correlation spectroscopy and absorption fine structure techniques together with the first-principles calculations. The pronounced ß-relaxation and fast atomic dynamics in the Pd─Cu─P MG mainly come from the strong mobility of Cu atoms and their locally favored structures. In contrast, the motion of Ni atoms is constrained by P atoms in the Pd─Ni─P MG, leading to the weakened ß-relaxation peak and sluggish dynamics. The correlation of atomic dynamics with microscopic structures provides a way to understand the structural origins of different dynamic behaviors as well as the nature of aging in disordered materials.

Anticoagulation Strategies in Pediatric Cardiopulmonary Bypass, Weight-Based vs. Concentration-Based Approaches.

Pediatric patients undergoing cardiopulmonary bypass (CPB) require adequate anticoagulation to combat hemostatic activation. Heparin is used to bind and catalyze antithrombin III (ATIII) that works to inhibit clot formation. To dose heparin, a weight-based (WB) or patient-specific concentration-based (PSCB) method can be used. The WB protocol calculates the dose based on the patients' weight and uses an activated clotting time (ACT) test to ensure anticoagulation. The ACT has limitations during CPB especially for pediatric patients who have immature hemostatic systems. The PSCB method predicts the patients' response to heparin by projecting a heparin dose-response (HDR) curve. Some investigators have found benefit to using the PSCB method but further investigation into how well the HDR predicts the heparin response is needed. A literature review was conducted for studies that looked at heparin management strategies in pediatric CPB patients between 1992 and 2020. Articles that focused on pediatric physiology, heparin management strategies, and anticoagulation were included. Articles older than 1990 were excluded. The literature review highlights that utilizing the PSCB approach more adequately anticoagulated patients. The WB protocol was found to have several flaws due to its reliance on the ACT, especially in infants. The results show that further investigation is needed to understand why there is benefit to using the PSCB approach. Observing the association between the HDR curve and subsequent heparin concentrations could determine how accurately it predicts the patients' response to heparin and why there is benefit to using this method.

On the effect of metal loading on the reducibility and redox chemistry of ceria supported Pd catalysts.

The effect of Pd loading on the redox characteristics of a ceria support was examined using in situ Pd K-edge XAS, Ce L3-edge XAS and in situ X-ray diffraction techniques. Analysis of the data obtained from these techniques indicates that the onset temperature for the partial reduction of Ce(IV) to Ce(III), by exposure to H2, varies inversely with the loading of Pd. Whilst the onset and completion temperatures of the reduction of Ce(IV) to Ce(III) are different, both samples yield the same maximal fraction of Ce(III) formation independent of Pd loading. Furthermore, the partial reduction of Ce is found to be concurrent with the reduction of PdO and demonstrated that the presence of metallic Pd is necessary for the reduction of the CeO2 support. Upon passivation by room temperature oxidation, a full oxidation of the reduced ceria support was observed. However, only a mild surface oxidation of Pd was identified. The mild passivation of the Pd is found to lead to a highly reactive sample upon a second reduction by H2. The onset of the reduction of Pd and Ce has been demonstrated to be independent of the Pd loading after a mild passivation with both samples exhibiting near room temperature reduction in the presence of H2.

X-ray Spectroscopic Quantification of Phosphorus Transformation in Saharan Dust during Trans-Atlantic Dust Transport.

Saharan dust is an important phosphorus (P) supply to remote and oligotrophic parts of the oceans and American lowland tropical rainforests. Phosphorus speciation in aeolian dust ultimately controls the release and bioavailability of P after dust deposition, but the speciation in Saharan dust and its change during the trans-Atlantic transport remains unclear. Using P K-edge X-ray absorption near edge structure (XANES) spectroscopy, we showed that with increasing dust traveling distance from the Sahara Desert to Cape Verde and to Puerto Rico, about 570 and 4000 km, respectively, the proportion of Ca-bound P (Ca-P), including both apatite and non-apatite forms, decreased from 68-73% to 50-71% and to 21-37%. The changes were accompanied by increased iron/aluminum-bound P proportion from 14-25% to 23-46% and to 44-73%, correspondingly. Laboratory simulation experiments suggest that the changes in P speciation can be ascribed to increasing degrees of particle sorting and atmospheric acidification during dust transport. The presence of relatively soluble non-apatite Ca-P in the Cape Verde dust but not in the Puerto Rico dust is consistent with the higher P water solubility of the former than the latter. Our findings provide insights into the controls of atmospheric processes on P speciation, solubility, and stability in Saharan dust.

Molecular-Scale Understanding of Sulfate Exchange from Schwertmannite by Chromate Versus Arsenate.

Schwertmannite effectively sorbs chromate (Cr(VI)), yet the sorption mechanisms remain elusive. We determined the Cr(VI) sorption mechanisms on schwertmannite at pH 3.2 and 5 using combined macroscopic sorption experiments with molecular-scale characterization and by comparing them to arsenate (As(V)) sorption. Cr(VI) adsorbs as bidentate-binuclear (BB) inner-sphere complexes through exchanging more sulfate and less >Fe-OH/OH2, with 0.59-0.71 sulfate released per Cr(VI) sorbed. While As(V) also forms BB complexes, it exchanges sulfate and >Fe-OH/OH2 equally with 0.49-0.52 sulfate released per As(V) sorbed. At high As(V) loadings, As(V) precipitates as amorphous FeAsO4, particularly at low pH. The abovementioned differences between Cr(VI) and As(V) can be related to their different ionic radii and binding strength. Moreover, Cr(VI) and As(V) preferentially exchange sulfate inner-sphere complexes, increasing the proportion of sulfate outer-sphere complexes in schwertmannite. In turn, the concentration of sulfate outer-sphere complexes increases and then decreases with increasing Cr(VI) loading. Results suggest that an oxyanion, which would form inner-sphere complexes on a mineral surface, preferentially exchanges inner-spherically bound oxyanions than outer-spherically bound ones on the surface, even though both are exchanged. This study improves our understanding of the sorption of oxyanions on schwertmannite and their capabilities to template schwertmannite formation and stabilize its structure.

Inhibition of Oxyanions on Redox-driven Transformation of Layered Manganese Oxides.

Layered manganese (Mn) oxides, such as birnessite, can reductively transform into other phases and thereby affect the environmental behavior of Mn oxides. Solution chemistry strongly influences the transformation, but the effects of oxyanions remain unknown. We determined the products and rates of Mn(II)-driven reductive transformation of δ-MnO2, a nanoparticulate hexagonal birnessite, in the presence of phosphate or silicate at pH 6-8 and a wide range of Mn(II)/MnO2 molar ratios. Without the oxyanions, δ-MnO2 transforms into triclinic birnessite (T-bir) and 4 × 4 tunneled Mn oxide (TMO) at low Mn(II)/MnO2 ratios (0.09 and 0.13) and into δ-MnOOH and Mn3O4 with minor poorly crystallized α- and γ-MnOOH at high Mn(II)/MnO2 ratios (0.5 and 1). The presence of phosphate or silicate substantially decreases the rate and extent of the above transformation, probably due to adsorption of the oxyanions on layer edges or the formation of Mn(II,III)-oxyanion ternary complexes on vacancies of δ-MnO2, adversely interfering with electron transfer, Mn(III) distribution, and structural rearrangements. The oxyanions also reduce the crystallinity and particle sizes of the transformation products, ascribed to adsorption of the oxyanions on the products, preventing their further particle growth. This study enriches our understanding of the solution chemistry control on redox-driven transformation of Mn oxides.

Structural evolution in a melt-quenched zeolitic imidazolate framework glass during heat-treatment.

A pronounced enthalpy release occurs around 1.38Tg in the prototypical metal-organic framework glass formed from ZIF-4 [Zn(C3H3N2)2], but there is no sign for any crystallization (i.e., long-range ordering) taking place. The enthalpy release peak is attributed to pore collapse and structural densification.

Evolution of Active Sites in Pt-Based Nanoalloy Catalysts for the Oxidation of Carbonaceous Species by Combined in Situ Infrared Spectroscopy and Total X-ray Scattering.

We present results from combined in situ infrared spectroscopy and total X-ray scattering studies on the evolution of catalytically active sites in exemplary binary and ternary Pt-based nanoalloys during a sequence of CO oxidation-reactivation-CO oxidation reactions. We find that when within a particular compositional range, the fresh nanoalloys may exhibit high catalytic activity for low-temperature CO oxidation. Using surface-specific atomic pair distribution functions (PDFs) extracted from the in situ total X-ray scattering data, we find that, regardless of their chemical composition and initial catalytic activity, the fresh nanoalloys suffer a significant surface structural disorder during CO oxidation. Upon reactivation in oxygen atmosphere, the surface of used nanoalloy catalysts both partially oxidizes and orders. Remarkably, it largely retains its structural state when the nanoalloys are reused as CO oxidation catalysts. The seemingly inverse structural changes of studied nanoalloy catalysts occurring under CO oxidation and reactivation conditions affect the active sites on their surface significantly. In particular, through different mechanisms, both appear to reduce the CO binding strength to the nanoalloy's surface and thus increase the catalytic stability of the nanoalloys. The findings provide clues for further optimization of nanoalloy catalysts for the oxidation of carbonaceous species through optimizing their composition, activation, and reactivation. Besides, the findings demonstrate the usefulness of combined in situ infrared spectroscopy and total X-ray scattering coupled to surface-specific atomic PDF analysis to the ongoing effort to produce advanced catalysts for environmentally and technologically important applications.

Liquid metal-organic frameworks.

Metal-organic frameworks (MOFs) are a family of chemically diverse materials, with applications in a wide range of fields, covering engineering, physics, chemistry, biology and medicine. Until recently, research has focused almost entirely on crystalline structures, yet now a clear trend is emerging, shifting the emphasis onto disordered states, including 'defective by design' crystals, as well as amorphous phases such as glasses and gels. Here we introduce a strongly associated MOF liquid, obtained by melting a zeolitic imidazolate framework. We combine in situ variable temperature X-ray, ex situ neutron pair distribution function experiments, and first-principles molecular dynamics simulations to study the melting phenomenon and the nature of the liquid obtained. We demonstrate from structural, dynamical, and thermodynamical information that the chemical configuration, coordinative bonding, and porosity of the parent crystalline framework survive upon formation of the MOF liquid.

Orbital Dimer Model for the Spin-Glass State in Y_{2}Mo_{2}O_{7}.

The formation of a spin glass generally requires that magnetic exchange interactions are both frustrated and disordered. Consequently, the origin of spin-glass behavior in Y_{2}Mo_{2}O_{7}-in which magnetic Mo^{4+} ions occupy a frustrated pyrochlore lattice with minimal compositional disorder-has been a longstanding question. Here, we use neutron and x-ray pair-distribution function (PDF) analysis to develop a disorder model that resolves apparent incompatibilities between previously reported PDF, extended x-ray-absorption fine structure spectroscopy, and NMR studies, and provides a new and physical explanation of the exchange disorder responsible for spin-glass formation. We show that Mo^{4+} ions displace according to a local "two-in-two-out" rule on each Mo_{4} tetrahedron, driven by orbital dimerization of Jahn-Teller active Mo^{4+} ions. Long-range orbital order is prevented by the macroscopic degeneracy of dimer coverings permitted by the pyrochlore lattice. Cooperative O^{2-} displacements yield a distribution of Mo-O-Mo angles, which in turn introduces disorder into magnetic interactions. Our study demonstrates experimentally how frustration of atomic displacements can assume the role of compositional disorder in driving a spin-glass transition.

Structure of Nano-sized CeO2 Materials: Combined Scattering and Spectroscopic Investigations.

The structure of several nano-sized ceria, CeO2 , systems was investigated using neutron and X-ray diffraction and X-ray absorption spectroscopy. Whilst both diffraction and total pair distribution functions (PDFs) revealed that in all of the samples the occupancy of both Ce4+ and O2- are very close to the ideal stoichiometry, the analysis using Reverse Monte Carlo technique revealed significant disorder around oxygen atoms in the nano-sized ceria samples in comparison to the highly crystalline NIST standard. In addition, the analysis revealed that the main differences observed in the pair correlations from various X-ray and neutron diffraction techniques were attributable to the particle size of the CeO2 prepared by the reported three methods. Furthermore, detailed analysis of the Ce L3 - and K-edge EXAFS data support this finding; in particular the decrease in higher shell coordination numbers with respect to the NIST standard, is attributed to differences in particle size.