3D Atomic Arrangement at Functional Interfaces Inside Nanoparticles by Resonant High-Energy X‑ray Diffraction

With current science and technology moving rapidly into smaller scales, nanometer-sized materials, often referred to as NPs, are produced in increasing numbers and explored for numerous useful applications. Evidence is mounting, however, that useful properties of NPs can be improved further and even new NP functionality achieved by not only controlling the NP size and shape but also interfacing chemically or structurally distinct entities into single, so-called “composite” NPs. A typical example is core–shell NPs wherein the synergy of distinct atoms at the core\shell interface endows the NPs with otherwise unachievable functionality. However, though advantageous, the concept of functional interfaces inside NPs is still pursued largely by trial-and-error. That is because it is difficut to assess the interfaces precisely at the atomic level using traditional experimental techniques and, hence, difficult to take control of. Using the core\shell interface in less than 10 nm in size Ru core–Pt shells NPs as an example, we demonstrate that precise knowledge of the 3D atomic arrangement at functional interfaces inside NPs can be obtained by resonant high-energy X-ray diffraction (XRD) coupled to element-specific atomic pair distribution function (PDF) analysis. On the basis of the unique structure knowledge obtained, we scrutinize the still-debatable influence of core\shell interface on the catalytic functionality of Ru core–Pt shell NPs, thus evidencing the usefulness of this nontraditional technique for practical applications

H₂ Reduction Annealing Induced Phase Transition and Improvements on Redox Durability of Pt Cluster-Decorated Cu@Pd Electrocatalysts in Oxygen Reduction Reaction

Hierarchical structures in shell with transition metal underneath is a promising design for high-performance and low-cost heterogeneous nanocatalysts (NCs). Such a design enables the optimum extent of synergetic effects in NC surface. It facilitates intermediate reaction steps and, therefore, boosts activity of NC in oxygen reduction reaction (ORR). In this study, carbon nanotube (CNT)-supported ternary metallic NC comprising Cucluster-in-Pdcluster nanocrystal and surface decoration of atomic Pt clusters (14 wt %) is synthesized by using the wet chemical reduction method with sequence and reaction time controls. By annealing in H2 environment (H2/N2 = 9:1, 10 sccm) at 600 K for 2 h, specific activity of Cu@Pd/Pt is substantially improved by ∼2.0-fold as compared to that of the pristine sample and commercial Pt catalysts. By cross-referencing results of electron microscopic, X-ray spectroscopic, and electrochemical analyses, we demonstrated that reduction annealing turns ternary NC into complex of Cu3Pt alloy and CuxPd1–x alloy. Such a transition preserves Pt and Pd in metallic phases, therefore improving the activity by ∼29% and the stability of NC in an accelerated degradation test (ADT) as compared to those of pristine Cu@Pd/Pt in 36 000 cycles at 0.85 V (vs RHE). This study presents robust H2 annealing for structure stabilization of NC and systematic characterizations for rationalization of the corresponding mechanisms. These results provide promising scenarios for facilitation of heterogeneous NC in ORR applications

Selenium Speciation in Coal Ash Spilled at the Tennessee Valley Authority Kingston Site

Selenium (Se) in coal ash spills poses a threat to adjacent ecosystems because of its potential to mobilize and bioaccumulate in aquatic organisms. Given that the mobility and bioavailability of Se is controlled by its valence states, we aimed to define Se speciation in coal ash solids and examine the relationships between Se speciation and the magnitude of its mobilization from coal ash. We used coal ash samples from the Tennessee Valley Authority (TVA)-Kingston fossil plant and the site of a coal ash spill that occurred in 2008 in Tennessee. Results of X-ray absorption spectroscopic analyses showed that Se in coal ash samples was a mixture of elemental Se⁰ and Se oxyanions. The amount of leachable Se increased with an increase of pH from 3 to 13. At the natural pH of coal ash samples (from pH 7.6 to 9.5), the leachable Se was comprised of Se oxyanions, mainly selenite. This was observed by both direct quantification of Se oxyanions in the leachate and the corresponding loss of Se oxyanions in the solid phase. At pH 12, however, the Se release appeared to derive from both desorption of Se oxyanions and oxidative dissolution of elemental Se⁰. Our results indicate that Se oxyanions are the most labile species; however, the magnitude of Se mobilization will increase if the waste material is subjected to alkaline conditions

Programming ORR Activity of Ni/NiO<i>_x</i>@Pd Electrocatalysts via Controlling Depth of Surface-Decorated Atomic Pt Clusters

Carbon nanotube supported ternary metallic nanocatalysts (NCs) comprising Ni_core–Pd_shell structure and Pt atomic scale clusters in shell (namely, Ni@Pd/Pt) are synthesized by using wet chemical reduction method with reaction time control. Effects of Pt⁴⁺ adsorption time and Pt/Pd composition ratios on atomic structure with respect to electrochemical performances of experimental NCs are systematically investigated. By cross-referencing results of high-resolution transmission electron microscopy, X-ray diffraction, X-ray absorption, density functional theoretical calculations, and electrochemical analysis, we demonstrate that oxygen reduction reaction (ORR) activity is dominated by depth and distribution of Pt clusters in a Ni@Pd/Pt NC. For the optimum case (Pt⁴⁺ adsorption time = 2 h), specific activity of Ni@Pd/Pt is 0.732 mA cm^–2 in ORR. Such a value is 2.8-fold higher as compared to that of commercial J.M.-Pt/C at 0.85 V (vs reversible hydrogen electrode). Such improvement is attributed to the protection of defect sites from oxide reaction in the presence of Pt clusters in NC surface. When adsorption time is 10 s, Pt clusters tends to adsorb in the Ni@Pd surface. A substantially increased galvanic replacement between Pt⁴⁺ ion and Pd/Ni metal is found to result in the formation of Ni@Pd shell with Pt cluster in the interface when adsorption time is 24 h. Both structures increase the surface defect density and delocalize charge density around Pt clusters, thereby suppressing the ORR activity of Ni@Pd/Pt NCs

Mechanism of Arsenic Adsorption on Magnetite Nanoparticles from Water: Thermodynamic and Spectroscopic Studies

Removal of arsenic (As) from water supplies is needed to reduce As exposure through drinking water and food consumption in many regions of the world. Magnetite nanoparticles (MNPs) are promising and novel adsorbents for As removal because of their great adsorption capacity for As and easy separation. This study aimed to investigate the adsorption mechanism of arsenate, As­(V), and arsenite, As­(III), on MNPs by macroscopic adsorption experiments in combination with thermodynamic calculation and microspectroscopic characterization using synchrotron-radiation-based X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). Adsorption reactions are favorable endothermic processes as evidenced by increased adsorption with increasing temperatures, and high positive enthalpy change. EXAFS spectra suggested predominant formation of bidentate binuclear corner-sharing complexes (²<i>C</i>) for As­(V), and tridentate hexanuclear corner-sharing (³<i>C</i>) complexes for As­(III) on MNP surfaces. The macroscopic and microscopic data conclusively identified the formation of inner-sphere complexes between As and MNP surfaces. More intriguingly, XANES and XPS results revealed complex redox transformation of the adsorbed As on MNPs exposed to air: Concomitant with the oxidation of MNPs, the oxidation of As­(III) and MNPs was expected, but the observed As­(V) reduction was surprising because of the role played by the reactive Fe­(II)

A Mechanism Study on the Synthesis of Cu/Pd Nanoparticles with Citric Complexing Agent

We have previously synthesized Cu/Pd nanoparticles with a citric complexing agent, demonstrating well the suspension and high catalytic ability of electroless copper deposition. Herein, we report the in situ investigation of the synthesis of Cu/Pd nanoparticles with a citric complexing agent by X-ray absorption near-edge structure (XANES). By characterizing the XANES spectra of Cu and Pd upon the stepwise addition of an alkaline solution, the reaction mechanism of Cu as well as Pd complexing ions was elucidated. Slow reduction of Pd ions and fast reduction of Cu ions induced by zerovalent Pd are found in XANES spectra. A three-stage formation mechanism of Cu/Pd nanoparticles was proposed in which a Pd reduction initial stage, a Cu-dominated reduction middle stage, and a Pd-dominated reduction final stage were indicated. As a result, a Pd-rich outer shell formed on the surface of synthesized Cu/Pd particles in the final stage. In summary, the formation mechanism and the Pd-rich outer shell structure of synthesized Cu/Pd nanoparticles were found in this citric complexing agent synthesis method

Stabilization of Natural Organic Matter by Short-Range-Order Iron Hydroxides

Dissolved organic matter (DOM) is capable of modifying the surfaces of soil minerals (e.g., Fe hydroxides) or even forming stable co-precipitates with Fe­(III) in a neutral environment. The DOM/Fe co-precipitation may alter biogeochemical carbon cycling in soils if the relatively mobile DOM is sorbed by soil minerals against leaching, runoff, and biodegradation. In this study, we aimed to determine the structural development of DOM/Fe co-precipitates in relation to changes in pH and C/(C + Fe) ratios using XRD, XPS, Fe K-edge XAS, FTIR, and C-NEXAFS techniques. The results showed that in the system with bulk C/(C + Fe) molar ratios ≤0.65, the ferrihydrite-like Fe domains were precipitated as the core and covered by the C shells. When the C/(C + Fe) molar ratio ranged between 0.71 and 0.89, the emerging Fe–C bonding suggested a more substantial association between Fe domains including edge- and corner-sharing FeO₆ octahedra and DOM. With C/(C + Fe) bulk molar ratios ≥0.92, only corner-sharing FeO₆ octahedra along with Fe–C bonding were found. The homogeneously distributed C and Fe domains caused the enhancement of Fe and C solubilization from co-precipitates. The C/(C + Fe) ratios dominated structural compositions and stabilities of C/Fe co-precipitates and may directly affect the Fe and C cycles in soils

Size Effect of Atomic Gold Clusters for Carbon Monoxide Passivation at Ru_core–Pt_shell Nanocatalysts

The surface of Pt_shell–Ru_core nanocatalysts was modified with an atomic-scaled Au cluster of different sizes by a polyol reduction technique using sequence and composition control. Our results, combining the structure, surface chemical analysis, and density functional theory calculation, elucidate that these clusters reduced the oxidation current of carbon monoxide to a maximum extent of ∼53%; consequently, the anti-CO poisoning factor of the NCs was doubled by increasing the Au/Pt ratios from 0 to 15 at%. Such substantial improvement is caused by steric shielding and the electron localization field that reject the sorption of electronegative ligands/molecules at the NC surface by Au clusters. Most importantly, this work clarifies the mechanistic insights of the charge relocation at core–shell nanoparticles by subnanoscaled cluster intercalation and the impacts of cluster size for the chemical durability of catalysts in fuel cell applications