High-temperature transport properties, thermal expansion and cathodic performance of Ni-substituted LaSr2Mn2O7−δ

The substitution of manganese with nickel in LaSr₂Mn₂O_7−δ, where the solubility limit corresponds to approximately 25% Mn sites, enhances the Ruddlesden-Popper phase stability at elevated temperatures and atmospheric oxygen pressure. The total conductivity of LaSr₂Mn_2−yNi_yO_7−δ (y=0-0.4) decreases with nickel additions, whilst the average thermal expansion coefficients calculated from dilatometric data in the temperature range 300-1370 K increase from (11.4-13.7)×10⁻⁶ K⁻¹ at y=0 up to (12.5-14.4)×10⁻⁶ K⁻¹ at y=0.4. The conductivity and Seebeck coefficient of LaSr₂Mn_1.6Ni_0.4O_7−δ, analyzed in the oxygen partial pressure range 10⁻¹⁵-0.3 atm at 600-1270 K, display that the electronic transport is n-type and occurs via a small polaron mechanism. Reductive decomposition is observed at the oxygen pressures close to Ni/NiO boundary, namely ∼2.3×10⁻¹¹ atm at 1223 K. Within the phase stability domain, the electronic transport properties are essentially p(O₂)-independent. The steady-state oxygen permeability of dense LaSr₂Mn_1.6Ni_0.4O_7−δ membranes is higher than that of (La,Sr)MnO_3−δ, but lower if compared to perovskite-like (Sr,Ce)MnO_3−δ. Porous LaSr₂Mn_1.6Ni_0.4O_7−δ cathodes in contact with apatite-type La₁₀Si₅AlO_26.5 solid electrolyte exhibit, however, a relatively poor electrochemical performance, partly associated with strong cation interdiffusion between the materials.     

Structural and magnetic properties of pyrochlore solid solutions (Y,Lu)2Ti2−x(Nb,Ta)xO7±y

The synthesis and characterization of the pyrochlore solid solutions, Y₂Ti_2−xNb_xO_7−y, Lu₂Ti_2−xNb_xO_7−y, Y₂Ti_2−xTa_xO_7−y and Lu₂TiTaO_7−y (−0.4<y<0.5), is described. Synthesis at 1600 °C, and 10⁻⁵ Torr yields oxygen deficiency in all systems. All compounds are found to be paramagnetic and semiconducting, with the size of the local moments being less, in some cases substantially less, than the expected value for the number of nominally unpaired electrons present. Thermogravimetric analysis (TGA) shows that all compounds can be fully oxidized while retaining the pyrochlore structure, yielding oxygen rich pyrochlores as white powders. Powder neutron diffraction of Y₂TiNbO₇-based samples was done. Refinement of the data for oxygen deficient Y₂TiNbO_6.76 indicates the presence of a distribution of oxygen over the 8b and 48f sites. Refinement of the data for oxygen rich Y₂TiNbO_7.5 shows these sites to be completely filled, with an additional half filling of the 8a site. The magnetic and TGA data strongly suggest a preference for a Ti³⁺/(Nb,Ta)⁵⁺ combination, as opposed to Ti⁴⁺/(Nb,Ta)⁴⁺, in this pyrochlore family. In addition, the evidence clearly points to Ti³⁺ as the source of the localized moments, with no evidence for localized Nb⁴⁺ moments.     

Decorating Single-Atomic Mn Sites with FeMn Clusters to Boost Oxygen Reduction Reaction

The regulation of electron distribution of single-atomic metal sites by atomic clusters is an effective strategy to boost their intrinsic activity of oxygen reduction reaction (ORR). Herein we report the construction of single-atomic Mn sites decorated with atomic clusters by an innovative combination of post-adsorption and secondary pyrolysis. The X-ray absorption spectroscopy confirms the formation of Mn sites via Mn-N₄ coordination bonding to FeMn atomic clusters (FeMn_ac/Mn-N₄C), which has been demonstrated theoretically to be conducive to the adsorption of molecular O₂ and the break of O−O bond during the ORR process. Benefiting from the structural features above, the FeMn_ac/Mn-N₄C catalyst exhibits excellent ORR activity with half-wave potential of 0.79 V in 0.5 M H₂SO₄ and 0.90 V in 0.1 M KOH as well as preeminent Zn-air battery performance. Such synthetic strategy may open up a route to construct highly active catalysts with tunable atomic structures for diverse applications.     

Environmental remediation by hydroxyapatite: Solid state synthesis utilizing waste chicken eggshell and adsorption experiment with Congo red dye

This study reports the adsorption efficacy of hydroxyapatite (HAp) for removing Congo Red (CR) dye from aqueous solution. HAp was synthesized utilizing chicken eggshell as a precursor of Ca source. Solid state synthesis method was implemented which comprised calcination at 950 °C (E-HAp950). XRD analysis confirmed the formation of bi-phasic HAp with 15.5% of β-TCP. Elemental composition was evaluated by XPS and EDX analysis. FESEM analysis revealed the particles are of plate and spherical shaped also confirmed by the TEM images. DLS particle size, zeta potential, BET surface area and point of zero charge were also evaluated. Adsorption efficacy of E-HAp950 for removing CR was evaluated by batch adsorption experiment. Maximum adsorption capacity (q_max) was found to be 9.64 mgg⁻¹ which was best explained by the non-linear fitting (R² = 0.98) of Langmuir isotherm. Adsorption kinetics profusely followed pseudo second order kinetic model (R² = 0.999) with q_{e (experimental)} being very much closer to q_{e (calculative)} for this model. Thus, hydroxyapatite prepared by utilizing eggshell waste through solid state method has the potential to remove toxic dyes.     

Mechanistic insights into methylene blue removal via olive stone-activated carbon: A study on surface porosity and characterization

Global attention is increasingly focused on the adverse health and environmental impacts of textile dyes, marking the necessity for effective and sustainable dye remediation strategies in industrial wastewater. This study introduces a novel, eco-friendly activated carbon produced from olive stones (OLS), a readily available by-product of the olive oil industry. The OLS was chemically activated with H₃PO₄ and KOH, creating two materials: OLS-P and OLS-K, respectively. These were then utilized as cost-effective adsorbents for the removal of methylene blue (MB) dye. The activated materials were characterized via X-ray diffraction (XRD), Fourier transform infra-red spectroscopy (FTIR), iodine number, and pHpzc analysis, with the zero-point charge determined as approximately pH 1 for OLS-P and pH 8 for OLS-K. Batch adsorption experiments conducted at various temperatures revealed that adsorption process followed the pseudo-second-order kinetic model and the Langmuir isotherm model. Temperature was found to significantly impact adsorption performance, with OLS-K demonstrating a substantial increase in adsorption capacity (q_e) from 6.27 mg/g at 23˚C to 94.7 mg/g at 32 ˚C. Conversely, OLS-P displayed a decrease in q_e from 16.78 mg/g at 23 ˚C to 3.67 mg/g at 32 ˚C as temperature increased. The study highlights the potential of KOH-treated olive stones as a promising, cost-efficient adsorbent for methylene blue remediation from wastewater.     

Durable Electrocatalytic Reduction of Nitrate to Ammonia over Defective Pseudobrookite Fe2TiO5 Nanofibers with Abundant Oxygen Vacancies

We propose the pseudobrookite Fe₂TiO₅ nanofiber with abundant oxygen vacancies as a new electrocatalyst to ambiently reduce nitrate to ammonia. Such catalyst achieves a large NH₃ yield of 0.73 mmol h⁻¹ mg⁻¹_cat. and a high Faradaic Efficiency (FE) of 87.6 % in phosphate buffer saline solution with 0.1 M NaNO₃, which is lifted to 1.36 mmol h⁻¹ mg⁻¹_cat. and 96.06 % at −0.9 V vs. RHE for nitrite conversion to ammonia in 0.1 M NaNO₂. It also shows excellent electrochemical durability and structural stability. Theoretical calculation reveals the enhanced conductivity of this catalyst and an extremely low free energy of −0.28 eV for nitrate adsorption at the presence of vacant oxygen.     

Understanding Synergistic Catalysis on Cu-Se Dual Atom Sites via Operando X-ray Absorption Spectroscopy in Oxygen Reduction Reaction

The construction and understanding of synergy in well-defined dual-atom active sites is an available avenue to promote multistep tandem catalytic reactions. Herein, we construct a dual-hetero-atom catalyst that comprises adjacent Cu-N₄ and Se-C₃ active sites for efficient oxygen reduction reaction (ORR) activity. Operando X-ray absorption spectroscopy coupled with theoretical calculations provide in-depth insights into this dual-atom synergy mechanism for ORR under realistic device operation conditions. The heteroatom Se modulator can efficiently polarize the charge distribution around symmetrical Cu-N₄ moieties, and serve as synergistic site to facilitate the second oxygen reduction step simultaneously, in which the key OOH*-(Cu₁-N₄) transforms to O*-(Se₁-C₂) intermediate on the dual-atom sites. Therefore, this designed catalyst achieves satisfied alkaline ORR activity with a half-wave potential of 0.905 V vs. RHE and a maximum power density of 206.5 mW cm⁻² in Zn-air battery.     

Stepwise Engineering the Pore Aperture of a Cage-like MOF for the Efficient Separation of Isomeric C4 Paraffins under Humid Conditions

The separation of isomeric C4 paraffins is an important task in the petrochemical industry, while current adsorbents undergo a trade-off relationship between selectivity and adsorption capacity. In this work, the pore aperture of a cage-like Zn-bzc (bzc=pyrazole-4-carboxylic acid) is tuned by the stepwise installation methyl groups on its narrow aperture to achieve both molecular-sieving separation and high n-C₄H₁₀ uptake. Notably, the resulting Zn-bzc-2CH₃ (bzc-2CH₃=3,5-dimethylpyrazole-4-carboxylic acid) can sensitively capture n-C₄H₁₀ and exclude iso-C₄H₁₀, affording molecular-sieving for n-C₄H₁₀/iso-C₄H₁₀ separation and high n-C₄H₁₀ adsorption capacity (54.3 cm³ g⁻¹). Breakthrough tests prove n-C₄H₁₀/iso-C₄H₁₀ can be efficiently separated and high-purity iso-C₄H₁₀ (99.99 %) can be collected. Importantly, the hydrophobic microenvironment created by the introduced methyl groups greatly improves the stability of Zn-bzc and significantly eliminates the negative effect of water vapor on gas separation under humid conditions, indicating Zn-bzc-2CH₃ is a new benchmark adsorbent for n-C₄H₁₀/iso-C₄H₁₀ separation.     

Enhanced Four-Electron Oxygen Reduction Selectivity of Clamp-Shaped Cobalt(II) Porphyrin(2.1.2.1) Complexes

The molecular structure, electrochemistry, spectroelectrochemistry and electrocatalytic oxygen reduction reaction (ORR) features of two Co^II porphyrin(2.1.2.1) complexes bearing Ph or F₅Ph groups at the two meso-positions of the macrocycle are examined. Single crystal X-ray analysis reveal a highly bent, nonplanar macrocyclic conformation of the complex resulting in clamp-shaped molecular structures. Cyclic voltammetry paired with UV/Vis spectroelectrochemistry in PhCN/0.1 M TBAP suggest that the first electron addition corresponds to a macrocyclic-centered reduction while spectral changes observed during the first oxidation are consistent with a metal-centered Co^II/Co^III process. The activity of the clamp-shaped complexes towards heterogeneous ORR in 0.1 M KOH show selectivity towards the 4e⁻ ORR pathway giving H₂O. DFT first-principle calculations on the porphyrin catalyst indicates a lower overpotential for 4e⁻ ORR as compared to the 2e⁻ pathway, consistent with experimental data.     

Radical-Mediated Photocatalysis for Lignocellulosic Biomass Conversion into Value-Added Chemicals and Hydrogen: Facts,Opportunities and Challenges

Photocatalytic biomass conversion into high-value chemicals and fuels is considered one of the hottest ongoing research and industrial topics toward sustainable development. In short, this process can cleave C_β−O/C_α−C_β bonds in lignin to aromatic platform chemicals, and further conversion of the polysaccharides to other platform chemicals and H₂. From the chemistry point of view, the optimization of the unique cooperative interplay of radical oxidation species (which are activated via molecular oxygen species, ROSs) and substrate-derived radical intermediates by appropriate control of their type and/or yield is key to the selective production of desired products. Technically, several challenges have been raised that face successful real-world applications. This review aims to discuss the recently reported mechanistic pathways toward selective biomass conversion through the optimization of ROSs behavior and materials/system design. On top of that, through a SWOT analysis, we critically discussed this technology from both chemistry and technological viewpoints to help the scientists and engineers bridge the gap between lab-scale and large-scale production.