Probing the Influence of Acidity and Temperature to Th(IV) on Hydrolysis, Nucleation, and Structural Topology

Systematic control of the molar ratio between thorium hydroxides and selenic acid and their reaction temperature under hydrothermal conditions results in four novel thorium-based selenate complexes, namely, [Th₈O₄(OH)₈­(SeO₄)₆(H₂O)₁₆]·(SeO₄)₂·13H₂O (<b>Th</b>-<b>1</b>), [Th₈O₄(OH)₈­(SeO₄)₈(H₂O)₁₃]·7H₂O (<b>Th</b>-<b>2</b>), Th­(OH)₂­(SeO₄)­H₂O (<b>Th</b>-<b>3</b>), and Th₃(SeO₄)₆(H₂O)₆·2.5H₂O (<b>Th</b>-<b>4</b>), as well as the thorium mixed selenite selenate compound Th­(SeO₃)­(SeO₄) (<b>Th</b>-<b>5</b>). Smaller [H₂SeO₄]/[Th­(IV)] ratio or lower temperature give rise to the formation of octameric [Th₈(μ₃-O)₄­(μ₂-OH)₈]¹⁶⁺ cores in <b>Th-1</b>/<b>Th-2</b> and infinite [Th­(μ₂-OH)₂H₂O]²⁺ chains in <b>Th-3</b>, respectively. Increasing the [H₂SeO₄]/[Th­(IV)] ratio or elevating the temperature generates a microporous (11.3 Å voids) open-framework <b>Th-4</b>, a monomeric thorium species without oxo/hydroxyl ligands, and a three-dimensional thorium structure <b>Th-5</b>. Formation of these compounds suggests that variables including acidity and temperature play a critical role in the hydrolysis and oligomerization of Th^IV ions. Increasing acidity limits the deprotonation of water molecules and formation of nucleophilic hydroxo/oxo-aquo Th species, and high temperature appears to suppress the olation/oxolation hydrolysis reactions, which in both ways limit the formation of the thorium oligomers

Surface Structural Reconstruction for Optical Response in Iodine-Modified TiO₂ Photocatalyst System

We report an alternative mechanism for the optical response of an iodine-modified anatase TiO₂ photocatalyst material. Unlike the general realization that the iodine atom provides impurity levels within the optical band gap, we suggest that a distorted surface structure plays a dominant role. Anatase pure TiO₂ and iodine-modified TiO₂ (I-TiO₂) nanocrystals were synthesized by a simple solvothermal method, and I-TiO₂ exhibited an extended absorption edge up to 550 nm. Employing iodine K-edge X-ray absorption fine structure (XAFS), we demonstrate that iodine is not incorporated into the nanoparticle interior but exists in the form of IO₃^– groups at the surface. Furthermore, this IO₃^– cluster adsorption largely induces a disordered structure, as revealed by Ti 2p_3/2 X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and Ti K-edge XAFS data fitting. Density functional theory calculation indicates that this distorted structure can form a midgap state, which should be responsible for its excellent optical property. This finding represents a promising route for improving the optical response of the nanophotocatalyst system via surface treatment

Th(H₂O)(I^VO₃)₂[I^VII_0.6V_1.76O₇(OH)]: A Mixed-Valent Iodine Compound Containing Periodate Stabilized by Crystallographically Compatible Lattice Sites

Periodate is a strong oxidant and is often reduced to IO₃^– or I₂ under hydrothermal conditions. Here, we present a rare case of a mixed-valent iodate­(V)/periodate­(VII) compound, Th­(H₂O)­(I^VO₃)₂[I^VII_0.6V_1.76O₇(OH)], prepared with a hydrothermal method starting from periodic acid. Crystallographic results demonstrate that heptavalent iodine adopts I^VIIO₆ distorted octahedral geometries, which are stabilized on the crystallographically compatible crystal lattice sites of VO₆ octahedra through an aliovalent substitutional disorder mechanism. X-ray photoelectron and synchrotron radiation X-ray absorption spectroscopes both quantitatively confirm the presence of mixed valent iodine oxoanions with a molar ratio (I^V/I^VII) of 4:1, consistent with the single crystal X-ray analysis. The crystallization of mixed-valent products with compatible lattice site can be fancily utilized for stabilizing the uncommon oxidation states of other elements in general

Atomically Dispersed Dual Metal Sites Boost the Efficiency of Olefins Epoxidation in Tandem with CO₂ Cycloaddition

Tandem catalysis provides an economical and energy-efficient process for the production of fine chemicals. In this work, we demonstrate that a rationally synthesized carbon-based catalyst with atomically dispersed dual Fe–Al sites (ADD-Fe-Al) achieves superior catalytic activity for the one-pot oxidative carboxylation of olefins (conversion ∼97%, selectivity ∼91%), where the yield of target product over ADD-Fe-Al is at least 62% higher than that of monometallic counterparts. The kinetic results reveal that the excellent catalytic performance arises from the synergistic effect between Fe (oxidation site) and Al sites (cycloaddition site), where the efficient CO2 cycloaddition with epoxides in the presence of Al sites (3.91 wt %) positively shifts the oxidation equilibrium to olefin epoxidation over Fe sites (0.89 wt %). This work not only offers an advanced catalyst for oxidative carboxylation of olefins but also opens up an avenue for the rational design of multifunctional catalysts for tandem catalytic reactions in the future

Highly Active Surface Structure in Nanosized Spinel Cobalt-Based Oxides for Electrocatalytic Water Splitting

Spinel cobalt-based oxides are a promising family of materials for water splitting to replace currently used noble-metal catalysts. Identifying the highly active facet and the corresponding coordinated structure of surface redox centers is pivotal for the rational design of low-cost and efficient nanosized catalysts. Using high-resolution transmission electron microscopy and advanced X-ray techniques, as well as ab initio modeling, we found that the activity of Co³⁺ ions exhibits the surface dependence owing to the variability of its electronic configurations. Our calculation shows that the Co³⁺ site in {100} facet of nanosized Li₂Co₂O₄ exhibits an impressive intrinsic activity with low overpotential, far lower than that of the {110} and {111} facets. The unique, well-defined CoO₅ square-pyramidal structure in this nonpolar surface stabilizes the unusual intermediate-spin states of the Co³⁺ ion. Specially, we unraveled that oxygen ion anticipates the redox process via the strong hybridization Co 3d–O 2p state, which produces a 3d_<i>z</i>^₂1.1 filling orbit. Finally, a spin-correlated energy diagram as a function of Co–O distance was devised, showing that the covalency of Co–O significantly affects the spin state of Co³⁺ ions. We suggest that the nonpolar surface that contains CoO₅ units in the edge-sharing systems with the short Co–O bond distance is a potential candidate for alkaline water electrolysis

Regulation of Magnetic Behavior and Electronic Configuration in Mn-Doped ZnO Nanorods through Surface Modifications

In this research, Mn-doped ZnO nanorods were synthesized by a solvothermal method and their magnetic behavior was tuned from paramagnetism to ferromagnetism via a change in their surface environment. The structure and electronic configuration of Mn ions were investigated by means of X-ray diffraction and X-ray absorption fine structure to understand the surface-controlled magnetic properties. The surface electronic configuration was studied by Mn L_3,2-edge XANES, which were simulated using the ligand field multiplet theory and the ligand-to-metal charge transfer effects. The results pointed out that Mn³⁺ ions occupy distorted tetrahedronal sites near the surface region and different surface modifications produce changes in the Mn 3d-anion p hybridization strength. A midgap state with strong O-2p character has been also recognized at the O K-edge XANES, and the density of such a state is strongly related to the observed ferromagnetism. This research represents a novel promising route for tuning the magnetic behavior of nano-dilute magnetic semiconductor systems via surface atomic changes

Improving the Solubility of Mn and Suppressing the Oxygen Vacancy Density in Zn_0.98Mn_0.02O Nanocrystals via Octylamine Treatment

Zn_0.98Mn_0.02O nanocrystals were synthesized by the wet chemical route and were treated with different content of octylamine. The environment around Mn and the defect type and concentration were characterized by photoluminescence, Raman, X-ray photoelectron spectroscopy, and X-ray absorption fine structure. It is found that N codoping effectively enhances the solubility of Mn substituting Zn via reducing donor binding energy of impurity by the orbital hybridization between the N-acceptor and Mn-donor. On the other hand, the O atoms released from MnO₆ and the N ions from octylamine occupy the site of oxygen vacancies and result in reduction of the concentration of oxygen vacancies in Zn_0.98Mn_0.02O nanocrystals

Immobilization of Alkali Metal Fluorides via Recrystallization in a Cationic Lamellar Material, [Th(MoO₄)(H₂O)₄Cl]Cl·H₂O

Searching for cationic extended materials with a capacity for anion exchange resulted in a unique thorium molybdate chloride (TMC) with the formula of [Th­(MoO₄)­(H₂O)₄Cl]­Cl·H₂O. The structure of TMC is composed of zigzagging cationic layers [Th­(MoO₄)­(H₂O)₄Cl]⁺ with Cl^– as interlamellar charge-balancing anions. Instead of performing ion exchange, alkali thorium fluorides were formed after soaking TMC in AF (A = Na, K, and Cs) solutions. The mechanism of AF immobilization is elucidated by the combination of SEM-EDS, PXRD, FTIR, and EXAFS spectroscopy. It was observed that four water molecules coordinating with the Th⁴⁺ center in TMC are vulnerable to competition with F^–, due to the formation of more favorable Th–F bonds compared to Th–OH₂. This leads to a single crystal-to-polycrystalline transformation via a pathway of recrystallization to form alkali thorium fluorides

A Large Family of Centrosymmetric and Chiral f‑Element-Bearing Iodate Selenates Exhibiting Coordination Number and Dimensional Reductions

The exploration of phase formation in the f-element-bearing iodate selenate system has resulted in 14 novel rare-earth-containing iodate selenates, Ln­(IO₃)­(SeO₄) (Ln = La, Ce, Pr, Nd; <b>LnISeO-1</b>), Ln­(IO₃)­(SeO₄)­(H₂O) (Ln = Sm, Eu; <b>LnISeO-2</b>), and Ln­(IO₃)­(SeO₄)­(H₂O)₂·H₂O (Ln = Gd, Dy, Ho, Er, Tm, Yb, Lu, Y; <b>LnISeO-3</b>), as well as two new thorium iodate selenates, Th­(OH)­(IO₃)­(SeO₄)­(H₂O) (<b>ThISeO-1</b>) and Th­(IO₃)₂(SeO₄) (<b>ThISeO-2</b>). <b>LnISeO-3</b> and <b>ThISeO-2</b> crystallize in the chiral space group <i>P</i>2₁2₁2₁, while <b>LnISeO-1</b>, <b>LnISeO-2</b>, and <b>ThISeO-1</b> crystallize in the centrosymmetric space group <i>P</i>2₁/<i>c</i>. The numbers of both coordinating and hydrating water molecules crystallized in <b>LnISeO-1</b>, <b>LnISeO-2</b>, and <b>LnISeO-3</b> increase along these three series, in line with the increasingly negative values of hydration enthalpies of heavier trivalent lanthanide ions. Such a systematic change in compositions, especially the first coordination sphere of Ln, further induces structural rearrangements, including coordination number and dimensional reductions. More specifically, the structures of <b>LnISeO-1</b>, <b>LnISeO-2</b>, and <b>LnISeO-3</b> have undergone transitions from 2D Ln–oxo layers with 10-coordinate Ln centers to 1D Ln–oxo chains with 9-coordinate Ln centers and then to 0D Ln–oxo monomers with 8-coordinate Ln centers, respectively. The formation and characterization of this large family of Ln/Th iodate selenates suggest that such a mixed-anion system not only exhibits richer structural chemistry but also can be capable of generating intriguing properties, such as the second-harmonic generation (SHG) effect

Selenium Sequestration in a Cationic Layered Rare Earth Hydroxide: A Combined Batch Experiments and EXAFS Investigation

Selenium is of great concern owing to its acutely toxic characteristic at elevated dosage and the long-term radiotoxicity of ⁷⁹Se. The contents of selenium in industrial wastewater, agricultural runoff, and drinking water have to be constrained to a value of 50 μg/L as the maximum concentration limit. We reported here the selenium uptake using a structurally well-defined cationic layered rare earth hydroxide, Y₂(OH)₅Cl·1.5H₂O. The sorption kinetics, isotherms, selectivity, and desorption of selenite and selenate on Y₂(OH)₅Cl·1.5H₂O at pH 7 and 8.5 were systematically investigated using a batch method. The maximum sorption capacities of selenite and selenate are 207 and 124 mg/g, respectively, both representing the new records among those of inorganic sorbents. In the low concentration region, Y₂(OH)₅Cl·1.5H₂O is able to almost completely remove selenium from aqueous solution even in the presence of competitive anions such as NO₃^–, Cl^–, CO₃^2–, SO₄^2–, and HPO₄^2–. The resulting concentration of selenium is below 10 μg/L, well meeting the strictest criterion for the drinking water. The selenate on loaded samples could be desorbed by rinsing with concentrated noncomplexing NaCl solutions whereas complexing ligands have to be employed to elute selenite for the material regeneration. After desorption, Y₂(OH)₅Cl·1.5H₂O could be reused to remove selenate and selenite. In addition, the sorption mechanism was unraveled by the combination of EDS, FT-IR, Raman, PXRD, and EXAFS techniques. Specifically, the selenate ions were exchanged with chloride ions in the interlayer space, forming outer-sphere complexes. In comparison, besides anion exchange mechanism, the selenite ions were directly bound to the Y³⁺ center in the positively charged layer of [Y₂(OH)₅(H₂O)]⁺ through strong bidentate binuclear inner-sphere complexation, consistent with the observation of the higher uptake of selenite over selenate. The results presented in this work confirm that the cationic layered rare earth hydroxide is an emerging and promising material for efficient removal of selenite and selenate as well as other anionic environmental pollutants