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Abstract: Coordination pillared-layer metal-organic frameworks (CPL-MOFs), such as CPL-2, are interesting versatile and porous materials with the potential for gas adsorption and separation. CPL-2 shows the unusual and gradual linker rotation upon the adsorption of ethylene (C2H4) and ethane (C2H6), leading to a fully reversible adsorption isotherm specifically under the conditions studied. Grand canonical Monte Carlo (GCMC) simulations showed that it is impossible to accommodate the experimentally observed loadings of C2H4 and C2H6 in CPL-2 using the crystallographic structure reported in the literature. According to the simulation findings, the pore expansion might be initiated by the clockwise 4,4′-bipyridine (bpy) pillar linker rotation. The pillar rotation leads to the enlarged pore volume, rendering additional adsorption sites, which are not present in the pristine structure. In situ synchrotron PXRD experiments for C2H4 and C2H6 adsorption on CPL-2 confirmed the occurrence of pore expansion in CPL-2 MOF. The combined experimental and simulation study shows for the first time that the linker rotation in CPL-2 can result in a adsorption isotherm without hysteresis. This work developed a real insight into the nature of pillared-layer MOFs, and the revealed structural changes could be potentially exploited to enhance alkene and alkane working capacities of such microporous materials.

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Abstract: A series of Mn5Co1Ox catalysts calcined at different temperatures in the range of 400–800 °C were synthesized by coprecipitation of manganese and cobalt nitrates and tested in the oxidation of CO. The specific surface area, structure, and chemistry of the catalysts were studied. In addition, the reduction of the catalysts by hydrogen was studied using in situ X-ray diffraction and temperature-programmed reduction techniques. It was found that the low-temperature catalyst calcined at 400 °C displays the best catalytic activity, which is attributed to its high surface area, low-temperature reducibility, and a high surface content of Mn4+. The formation of highly disperse and active CoMnO3 species and excess oxygen in a Mn3−xCoxO4+δ spinel leads to excellent low-temperature redox properties. The elevated temperature calcination results in a decline in the catalytic activity in CO oxidation due to formation of a well crystalline Mn3−xCoxO4 spinel, a decrease in the surface area and reducibility.

Open Access

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Abstract: We use a symmetry-motivated approach to analyzing x-ray pair distribution functions to study the mechanism of negative thermal expansion in two ReO 3 -like compounds: ScF 3 and CaZrF 6 . Both average and local structures suggest that it is the flexibility of M -F- M linkages ( M = Ca, Zr, Sc) due to dynamic rigid and semirigid “scissoring” modes that facilitates the observed negative thermal expansion (NTE) behavior. The amplitudes of these dynamic distortions are greater for CaZrF 6 than for ScF 3 , which corresponds well with the larger magnitude of the thermal expansion reported in the literature for the former. We show that this flexibility is enhanced in CaZrF 6 due to the rocksalt ordering mixing the characters of two of these scissoring modes. Additionally, we show that in ScF 3 anharmonic coupling between the modes responsible for the structural flexibility and the rigid unit modes contributes to the unusually high NTE behavior in this material.

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Abstract: Solid-state ion conductors based on closo-polyborate anions combine high ionic conductivity with a rich array of tunable properties. Cation mobility in these systems is intimately related to the strength of the interaction with the neighboring anionic network and the energy for reorganizing the coordination polyhedra. Here, we explore such factors in solid electrolytes with two anions of the weakest coordinating ability, [HCB11H5Cl6]– and [HCB11H5Br6]–, and a total of eleven polymorphs are identified for their lithium and sodium salts. Our approach combines ab initio molecular dynamics, synchrotron X-ray powder diffraction, differential scanning calorimetry, and AC impedance measurements to investigate their structures, phase-transition behavior, anion orientational mobilities, and ionic conductivities. We find that M(HCB11H5X6) (M = Li, Na, X = Cl, Br) compounds exhibit order-disorder polymorphic transitions between 203 and 305 °C, and display Li and Na superionic conductivity in the disordered state. Through detailed analysis, we illustrate how cation disordering in these compounds originates from a competitive interplay among the lattice symmetry, the anion reorientational mobility, the geometric and electronic asymmetry of the anion, and the polarizability of the halogen atoms. These factors are compared to other closo-polyborate-based ion conductors to suggest guidelines for optimizing the cation-anion interaction for fast ion mobility. This study expands the known solid-state, poly(carba)borate-based materials capable of liquid-like ionic conductivities, unravels the mechanisms responsible for fast ion transport, and provides insights into the development of practical superionic solid electrolytes.

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Abstract: With the potential of delivering reversible capacities of up to 300 mAh/g, Li-rich transition metal oxides hold great promise as cathode materials for future Li-ion batteries. However, a cohesive synthesis-structure-electrochemistry relationship is still lacking for these materials, which impedes progress in the field. This work investigates how and why different synthesis routes, specifically solid-state and modified Pechini sol-gel methods, affect the properties of Li2MnO3, a compositionally simple member of this material system. Through a comprehensive investigation of the syn-thesis mechanism along with crystallographic, morphological and electrochemical characterization, the effects of different synthesis routes were found to predominantly influence the degree of stacking faults and particle morphology. That is, the modified Pechini method produced isotropic spherical particles with approx. 57% faulting and the solid state samples possessed heterogeneous morphology with approx. 43% faulting probability. Inevitably, these differences lead to variations in electrochemical performance. This study accentuates the importance of understanding how syn-thesis affects the electrochemistry of these materials, which is critical considering the crystallographic and electrochemical complexities of the class of materials more generally. The methodology employed here is extendable to studying synthesis-property relationships of other compositionally complex Li-rich layered oxide systems.