Dihydrogen Phosphate Clusters: Trapping H₂PO₄^– Tetramers and Hexamers in Urea-Functionalized Molecular Crystals

Co-crystallization of two urea-functionalized ligands with tetrabutylammonium (TBA) dihydrogen phosphate resulted in the isolation of discrete (H₂PO₄^–)₄ and (H₂PO₄^–)₆ clusters stabilized in the crystalline state by multiple urea hydrogen bonds. Structural analysis by single-crystal X-ray diffraction, combined with a Cambridge Structural Database survey of (H₂PO₄^–)_<i>n</i> aggregates, established that these clusters display unique topologies and hydrogen-bonding connectivities

Dihydrogen Phosphate Clusters: Trapping H₂PO₄^– Tetramers and Hexamers in Urea-Functionalized Molecular Crystals

Co-crystallization of two urea-functionalized ligands with tetrabutylammonium (TBA) dihydrogen phosphate resulted in the isolation of discrete (H₂PO₄^–)₄ and (H₂PO₄^–)₆ clusters stabilized in the crystalline state by multiple urea hydrogen bonds. Structural analysis by single-crystal X-ray diffraction, combined with a Cambridge Structural Database survey of (H₂PO₄^–)_<i>n</i> aggregates, established that these clusters display unique topologies and hydrogen-bonding connectivities

Compound Defects and Thermoelectric Properties of Self-Charge Compensated Skutterudites Se_<i>y</i>Co₄Sb_{12–<i>x</i>}Se_<i>x</i>

In the past two decades, many studies have focused on the effects of electropositive guest fillers on the electrical and thermal transport properties in CoSb₃-based skutterudites. Recently, some electronegative elements such as S, Se, Cl, and Br have been filled into the voids in CoSb₃ with a small amount of n-type dopant Te on the Sb sites. In this report, self-charge compensated skutterudites Se_<i>y</i>Co₄Sb_{12–<i>x</i>}Se_<i>x</i> (0 < <i>x</i> + <i>y</i> < 0.9) with Se occupying two different atomic sites have been fabricated by a traditional melting–annealing process combined with a spark plasma sintering method. Phase purity was determined by X-ray diffraction, and the microstructures were examined by scanning electron microscopy. The temperature dependencies of the electrical and thermal transport properties were characterized. Se could enter both the void and Sb sites in CoSb₃ with a solubility limit around 0.6. The Se content has little effect on bandgap. Similar to Ga dual-site occupied Ga_<i>y</i>Co₄Sb_{12–<i>x</i>}Ga_<i>x</i> (<i>y</i> = 2<i>x</i>), a typical semiconducting electrical property with a low carrier concentration as well as a large Seebeck coefficient is observed. A correlation between the large Seebeck coefficient and the carrier scattering mechanism has been proposed. In addition, a largely reduced room temperature lattice thermal conductivity is obtained with a minimum value of 2.1 Wm^–1 K^–1 for Se_0.2Co₄Sb_11.6Se_0.4. The effects of Se on lattice thermal conductivity and filler resonant frequency are discussed

Sodium Sulfate Separation from Aqueous Alkaline Solutions via Crystalline Urea-Functionalized Capsules: Thermodynamics and Kinetics of Crystallization

The thermodynamics and kinetics of crystallization of sodium sulfate with a tripodal tris-urea receptor (L1) from aqueous alkaline solutions have been measured in the 15–55 °C temperature range for a fundamental understanding of the elementary steps involved in this sulfate separation method. The use of radiolabeled Na₂³⁵SO₄ provided a practical way to monitor the sulfate concentration in solution by β liquid scintillation counting. Our results are consistent with a two-step crystallization mechanism, involving relatively quick dissolution of crystalline L1 followed by the rate-limiting crystallization of the Na₂SO₄(L1)₂­(H₂O)₄ capsules. We found that temperature exerted relatively little influence over the equilibrium sulfate concentration, which ranged between 0.004 and 0.011 M. This corresponds to 77–91% removal of sulfate from a solution containing 0.0475 M initial sulfate concentration, as found in a typical Hanford waste tank. The apparent pseudo-first-order rate constant for sulfate removal increased 20-fold from 15 to 55 °C, corresponding to an activation energy of 14.1 kcal/mol. At the highest measured temperature of 55 °C, 63% and 75% of sulfate was removed from solution within 8 and 24 h, respectively. These results indicate the capsule crystallization method is a viable approach to sulfate separation from nuclear wastes

Synthesis and Thermoelectric Properties of Charge-Compensated S_<i>y</i>Pd_<i>x</i>Co_4–<i>x</i>Sb₁₂ Skutterudites

Recently, the electronegative elements (e.g., S, Se, Cl, and Br) filled skutterudites have attracted great attention in thermoelectric community. Via doping of some electron donors at the Sb sites, these electronegative elements can be filled into the voids of CoSb₃ forming thermodynamically stable compounds, which greatly extends the scope of filled skutterudites. In this study, we show that doping appropriate elements at the Co sites can also stabilize the electronegative elements in the voids of CoSb₃. A series of S_<i>y</i>Pd_<i>x</i>Co_4–<i>x</i>Sb₁₂ compounds were successfully fabricated by a traditional solid state reaction method combined with a spark plasma sintering technique. The phase composition and electrical and thermal transport properties were systematically characterized, and the related mechanisms were deeply discussed. It is found that the charge compensation between Pd doping and S filling is the main reason for the formation of thermodynamically stable S_<i>y</i>Pd_<i>x</i>Co_4–<i>x</i>Sb₁₂ compounds. Filling S element in the voids of CoSb₃ provides additional holes to reduce the carrier concentration while scarcely affecting the carrier mobility. However, doping Pd at the Co sites not only changes the carrier scattering mechanism but also deteriorates the carrier mobility. Low lattice thermal conductivities are observed in these S_<i>y</i>Pd_<i>x</i>Co_4–<i>x</i>Sb₁₂ compounds, which are attributed to the low resonant frequency of the S element. Finally, a maximal figure of merit of 0.85 is obtained for S_0.05Pd_0.25Co_3.75Sb₁₂ at 700 K

Synthesis of Ultrafine and Highly Dispersed Metal Nanoparticles Confined in a Thioether-Containing Covalent Organic Framework and Their Catalytic Applications

Covalent organic frameworks (COFs) with well-defined and customizable pore structures are promising templates for the synthesis of nanomaterials with controllable sizes and dispersity. Herein, a thioether-containing COF has been rationally designed and used for the confined growth of ultrafine metal nanoparticles (NPs). Pt or Pd nanoparticles (Pt NPs and Pd NPs) immobilized inside the cavity of the COF material have been successfully prepared at a high loading with a narrow size distribution (1.7 ± 0.2 nm). We found the crystallinity of the COF support and the presence of thioether groups inside the cavities are critical for the size-controlled synthesis of ultrafine NPs. The as-prepared COF-supported ultrafine Pt NPs and Pd NPs show excellent catalytic activity respectively in nitrophenol reduction and Suzuki–Miyaura coupling reaction under mild conditions and low catalyst loading. More importantly, they are highly stable and easily recycled and reused without loss of their catalytic activities. Such COF-supported size-controlled synthesis of nanoparticles will open a new frontier on design and preparation of metal NP@COF composite materials for various potential applications, such as catalysis and development of optical and electronic materials

New Porous Crystals of Extended Metal-Catecholates

New Porous Crystals of Extended Metal-Catecholate