Beneficial effect of cerium excess on in situ grown Sr0.86Ce0.14FeO3-CeO2 thermocatalysts for the degradation of bisphenol A.

Ce-doped SrFeO3 perovskite-type compounds are known as good thermocatalysts for the abatement of wastewater contaminants of emerging concern. In this work, Sr0.86Ce0.14FeO3-CeO2 perovskite-oxide systems with increasing amounts of cerium excess (0, 5, 10 and 15 mol% Ce), with respect to its maximum solubility in the perovskite, were prepared in one-pot by solution combustion synthesis and the effects of cerium excess on the chemical physical properties and thermocatalytic activity in the bisphenol A degradation were evaluated. The powders were characterized by powder X-ray diffraction combined with Rietveld refinement, X-ray photoelectron spectroscopy, thermal gravimetry, temperature programmed reduction, nitrogen adsorption, scanning electron microscopy and energy dispersive X-ray spectroscopy techniques. Results highlight that the perovskite structural, redox, surface, and morphological properties are affected by the in situ co-growth of the main perovskite phase and ceria and that a larger cerium excess has a beneficial effect on the thermocatalytic performance of the perovskite oxide-ceria biphasic system, although ceria is not active as a thermocatalyst itself. Perovskite properties and performance are enhanced by the tetragonal distortion induced by the introduction of cerium excess in the synthesis. It is supposed that a larger oxygen mobility and an easier reducibility are among the most relevant features that contribute to superior thermocatalytic properties of these perovskite oxide-based systems. These results also suggest new perspectives in the nanocomposite preparation and their catalytic applications.

Structure Dependence of Poisson's Ratio in Cesium Silicate and Borate Glasses.

In glass materials, Poisson's ratio (ν) has been proposed to be correlated with a variety of features, including atomic packing density (Cg), liquid fragility (m), and network connectivity. To further investigate these correlations in oxide glasses, here, we study cesium borate and cesium silicate glasses with varying modifier/former ratio given the difference in network former coordination and because cesium results in relatively high ν compared to the smaller alkali modifiers. Within the binary glass series, we find positive correlations between ν on one hand and m and Cg on the other hand. The network former is found to greatly influence the correlation between ν and the number of bridging oxygens (nBO), with a negative correlation for silicate glasses and positive correlation for borate glasses. An analysis based on topological constraint theory shows that this difference cannot be explained by the effect of superstructural units on the network connectivity in lithium borate glasses. Considering a wider range of oxide glasses from the literature, we find that ν generally decreases with increasing network connectivity, but with notable exceptions for heavy alkali borate glasses and calcium alumino tectosilicate glasses.

Metal-Organic Framework Glasses Possess Higher Thermal Conductivity than Their Crystalline Counterparts.

The thermal conductivity (κ) of glasses is known to always be lower than that of their corresponding crystals due to the stronger phonon-phonon scattering in the former. However, it is unknown whether this relation holds for metal-organic frameworks. Here, we report our discovery of an inverse relation in κ between glass and crystal for two zeolitic imidazolate frameworks (ZIFs), ZIF-4 and ZIF-62, that is, melt-quenched ZIF-4 and ZIF-62 glasses possess higher thermal conductivities than their crystalline counterparts. We find that the ZIF crystal pellets exhibit ultralow κ (∼0.1 W m-1 K-1) and that the higher κ of the ZIF glasses is due to the collapse of internal cavities and higher atomic number density in the latter. For other systems like oxides, vitrification causes higher free volume, but the opposite is found for the ZIFs, that is, lower free volume owing to the partial collapse of the crystalline framework upon melting.

Revisiting the Dependence of Poisson's Ratio on Liquid Fragility and Atomic Packing Density in Oxide Glasses.

Poisson's ratio (ν) defines a material's propensity to laterally expand upon compression, or laterally shrink upon tension for non-auxetic materials. This fundamental metric has traditionally, in some fields, been assumed to be a material-independent constant, but it is clear that it varies with composition across glasses, ceramics, metals, and polymers. The intrinsically elastic metric has also been suggested to control a range of properties, even beyond the linear-elastic regime. Notably, metallic glasses show a striking brittle-to-ductile (BTD) transition for ν-values above ~0.32. The BTD transition has also been suggested to be valid for oxide glasses, but, unfortunately, direct prediction of Poisson's ratio from chemical composition remains challenging. With the long-term goal to discover such high-ν oxide glasses, we here revisit whether previously proposed relationships between Poisson's ratio and liquid fragility (m) and atomic packing density (Cg) hold for oxide glasses, since this would enable m and Cg to be used as surrogates for ν. To do so, we have performed an extensive literature review and synthesized new oxide glasses within the zinc borate and aluminoborate families that are found to exhibit high Poisson's ratio values up to ~0.34. We are not able to unequivocally confirm the universality of the Novikov-Sokolov correlation between ν and m and that between ν and Cg for oxide glass-formers, nor for the organic, ionic, chalcogenide, halogenide, or metallic glasses. Despite significant scatter, we do, however, observe an overall increase in ν with increasing m and Cg, but it is clear that additional structural details besides m or Cg are needed to predict and understand the composition dependence of Poisson's ratio. Finally, we also infer from literature data that, in addition to high ν, high Young's modulus is also needed to obtain glasses with high fracture toughness.