A theory of the spin-crossover phenomena in the rare earth perovskite LaCoO3induced by temperature or a magnetic field

Published magnetic data for LaCoO3are successfully analyzed with coexisting5Dand low-spin (LS) cobalt states. Energy levels of the two states are derived in analytic forms. To this end, fictitious orbital angular momentumlof magnitude one defines the Γ5(5D) state. Our Hamiltonian includes the spin-orbit interaction, and a cubic crystal field embellished by a trigonal distortion9B20(lz2-2/3)-80B40(lz2-9/10). A singlet ground state with an energy gap to the first excited doublet is realized for certain values of the parameters. The temperature-independent paramagnetic susceptibility (TIPS) of the5Dstate has a finite value, which accords with the observation. Whereas, TIPS is symmetry forbidden in the LS state. A rigorous calculation is made of the excitation spectrum in the LS state. The elementary excitation is modeled as a creation of an electron-hole pair that results in an energy level scheme in which the first excited quartet lies above the singlet ground state. The electron spin resonance data are successfully equated with transitions within the excited quartet. Available magnetization data delineate parameters in the5DHamiltonian. The temperature dependence of the susceptibility of our coexisting model is qualitatively reasonable. To improve on a quantitative outcome, we are led to introduce a temperature dependent concentration for the5Dand LS states. Calculated Bragg diffraction patterns gathered with x-rays tuned to the CoK-edge reveal potential to refine the current crystal structure and to shed light on the origin of the coexisting states.

Enhancement of the Desorption Properties of LiAlH4 by the Addition of LaCoO3

The high hydrogen storage capacity (10.5 wt.%) and release of hydrogen at a moderate temperature make LiAlH₄ an appealing material for hydrogen storage. However, LiAlH₄ suffers from slow kinetics and irreversibility. Hence, LaCoO₃ was selected as an additive to defeat the slow kinetics problems of LiAlH₄. For the irreversibility part, it still required high pressure to absorb hydrogen. Thus, this study focused on the reduction of the onset desorption temperature and the quickening of the desorption kinetics of LiAlH₄. Here, we report the different weight percentages of LaCoO₃ mixed with LiAlH₄ using the ball-milling method. Interestingly, the addition of 10 wt.% of LaCoO₃ resulted in a decrease in the desorption temperature to 70 °C for the first stage and 156 °C for the second stage. In addition, at 90 °C, LiAlH₄ + 10 wt.% LaCoO₃ can desorb 3.37 wt.% of H₂ in 80 min, which is 10 times faster than the unsubstituted samples. The activation energies values for this composite are greatly reduced to 71 kJ/mol for the first stages and 95 kJ/mol for the second stages compared to milled LiAlH₄ (107 kJ/mol and 120 kJ/mol for the first two stages, respectively). The enhancement of hydrogen desorption kinetics of LiAlH₄ is attributed to the in situ formation of AlCo and La or La-containing species in the presence of LaCoO₃, which resulted in a reduction of the onset desorption temperature and activation energies of LiAlH₄.

Magnetic Field-Enhanced Electrocatalytic Oxygen Evolution on a Mixed-Valent Cobalt-Modulated LaCoO3 Catalyst

Extensive efforts to enhance the oxygen evolution reaction (OER) catalytic performance of transition metal oxides mainly concentrate on the extrinsic morphology tailoring, lattice doping, and electrode interface optimizing. Nevertheless, little room is left for performance improvement using these methods and an obvious gap still exists compared to the precious metal catalysts. In this work, a novel "mixed-valent cobalt modulation" strategy is presented to enhance the electrocatalytic OER of perovskite LaCoO₃ (LCO) oxide. The valence transition of cobalt is realized by ethylenediamine post reduction procedure at room temperature, which further induces the variation of magnetic properties for LCO catalyst. The optimized LCO catalyst with Co²⁺ /Co³⁺ of 1.98 % exhibits the best OER activity, and the overpotential at 10 mA cm^-2 current density is decreased by 170 mV compared pristine LCO. Impressively, the ferromagnetic LCO catalyst can perform magnetic OER enhancement. By application of an external magnetic field, the overpotential of LCO at 10 mA cm^-2 can be further decreased by 20 mV compared to that of under zero magnetic field, which arises from the enhanced energy states of electrons and accelerated electron transfer process driven by magnetic field. Our findings may provide a promising strategy to break the bottleneck for further enhancement of OER performance.

Amorphous Boron Dispersed in LaCoO3 with Large Oxygen Vacancies for Efficient Catalytic Propane Oxidation

Unsatisfactory oxygen mobility is a considerable barrier to the development of perovskites for low-temperature volatile organic compounds (VOCs) oxidation. This work introduced small amounts of dispersed non-metal boron into the LaCoO₃ crystal through an easy sol-gel method to create more oxygen defects, which are conducive to the catalytic performance of propane (C₃ H₈ ) oxidation. It reveals that moderate addition of boron successfully induces a high distortion of the LaCoO₃ crystal, decreases the perovskite particle size, and produces a large proportion of bulk Co²⁺ species corresponding to abundant oxygen vacancies. Additionally, surface Co³⁺ species, as the acid sites, which are active for cleaving the C-H bonds of C₃ H₈ molecules, are enriched. As a result, the LCB-7 (molar ratio of Co/B=0.93:0.07) displays the best C₃ H₈ oxidation activity. Simultaneously, the above catalyst exhibits superior thermal stability against CO₂ and H₂ O, lasting 200 h. This work provides a new strategy for modifying the catalytic VOCs oxidation performance of perovskites by the regulation of amorphous boron dispersion.

Activation of Peracetic Acid with Lanthanum Cobaltite Perovskite for Sulfamethoxazole Degradation under a Neutral pH: The Contribution of Organic Radicals

Advanced oxidation processes (AOPs) are effective ways to degrade refractory organic contaminants, relying on the generation of inorganic radicals (e.g., ^•OH and SO₄^•-). Herein, a novel AOP with organic radicals (R-O^•) was reported to degrade contaminants. Lanthanum cobaltite perovskite (LaCoO₃) was used to activate peracetic acid (PAA) for organic radical generation to degrade sulfamethoxazole (SMX). The results show that LaCoO₃ exhibited an excellent performance on PAA activation and SMX degradation at neutral pH, with low cobalt leaching. Meanwhile, LaCoO₃ also showed an excellent reusability during PAA activation. In-depth investigation confirmed CH₃C(O)O^• and CH₃C(O)OO^• as the key reactive species for SMX degradation in LaCoO₃/PAA system. The presence of Cl^- (1-100 mM) slightly inhibited the degradation of SMX in the LaCoO₃/PAA system, whereas the addition of HCO₃^- (0.1-1 mM) and humic aid (1-10 mg/L) could significantly inhibit SMX degradation. This work highlights the generation of organic radicals via the heterogeneous activation of PAA and thus provides a promising way to destruct contaminants in wastewater treatment.

Controllable P Doping of the LaCoO3 Catalyst for Efficient Propane Oxidation: Optimized Surface Co Distribution and Enhanced Oxygen Vacancies

The properties of LaCoO₃ are modified by a controllable P doping strategy via a simple sol-gel route. It is demonstrated that appropriate P doping is beneficial for forming a relatively pure perovskite phase and hinders the growth of perovskite nanoparticles. The combined results of density functional theory (DFT), extended X-ray absorption fine structure (EXAFS), X-ray absorption near-edge structure (XANES), temperature-programmed reduction of hydrogen (H₂-TPR), X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption of ammonia (NH₃-TPD) reveal that appropriate P doping gives rise to more oxygen vacancies, optimized distribution of Co ions, and improved surface acidity, which are beneficial for the adsorption of active oxygen species and the activation of propane molecules, resulting in an excellent catalytic oxidation performance. Especially, LaCo_0.97P_0.03O₃ exhibits more surface-active oxygen species, higher bulk Co³⁺ proportion, increased surface Co²⁺ species, and increased acidity, resulting in its superior propane oxidation performance, which is dominated by the Langmuir-Hinshelwood mechanism. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) confirms that the presence of P will accelerate oxygen mobility, which in turn promotes the oxidation rate. Moreover, the obtained LaCo_0.97P_0.03O₃ catalyst displays excellent thermal stability during the 60 h durability test at 400 °C and strong resistance against 5 vol % H₂O and/or 5 vol % CO₂ for prolonged 150 h.

Alternative pathways to α,β-unsaturated ketones via direct oxidative coupling transformation using Sr-doped LaCoO3 perovskite catalyst

A strontium-doped lanthanum cobaltite perovskite material was prepared, and used as a recyclable and effective heterogeneous catalyst for the direct oxidative coupling of alkenes with aromatic aldehydes to produce α,β-unsaturated ketones. The reaction afforded high yields in the presence of di-tert-butylperoxide as oxidant. Single oxides or salts of strontium, lanthanum and cobalt, and the undoped perovskite offered a lower catalytic activity than the strontium-doped perovskite. Benzaldehyde could be replaced by benzyl alcohol, dibenzyl ether, 2-oxo-2-phenylacetaldehyde, 2-bromoacetophenone or (dimethoxymethyl) benzene in the oxidative coupling reaction with alkenes. To our best knowledge, reactions between these starting materials with alkenes are new and unknown in the literature.

Structural Reversibility of LaCo1-x Cux O3 Followed by In Situ X-ray Diffraction and Absorption Spectroscopy

Combinations of perovskite-type oxides with transition and precious metals exhibit a remarkable self-regenerable property that could be exploited for numerous practical applications. The objective of the present work was to study the reversibility of structural changes of perovskite-type oxides under cyclic reducing/oxidizing atmosphere by taking advantage of the reducibility of LaCoO3 . LaCoO3±δ and LaCo0.8 Cu0.2 O3±δ were prepared by ultrasonic spray combustion and were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS) and temperature-programmed reduction (TPR). XRD and XAS data confirmed that copper adopted the coordination environment of cobalt at the B-site of the rhombohedral LaCoO3 under the selected synthesis conditions. The structural evolution under reducing atmosphere was studied by in situ XRD and XANES supporting the assignment of the observed structural changes to the reduction of the perovskite-type oxide from ABB'O3 (B'=Cu) to B'0 /ABO3 and to B'0 B0 /A2 O3 . Successive redox cycles allowed the observation of a nearly complete reversibility of the perovskite phase, i. e. copper was able to revert into LaCoO3 upon oxidation. The reversible reduction/segregation of copper and incorporation at the B-site of the perovskite-type oxides could be used in chemical processes where the material can be functionalized by segregation of Cu and protected against irreversible structural changes upon re-oxidation.

Independent Control of the Magnetization in Ferromagnetic La2/3Sr1/3MnO3/SrTiO3/LaCoO3 Heterostructures Achieved by Epitaxial Lattice Mismatch

We report the effect of interface symmetry-mismatch on the magnetic properties of LaCoO3 (LCO) thin films. Growing epitaxial LCO under tensile strain on top of cubic SrTiO3 (STO) produces a contraction along the c axis and a characteristic ferromagnetic response. However, we report here that ferromagnetism in LCO is completely suppressed when grown on top of a buffer layer of rhombohedral La2/3Sr1/3MnO3 (LSMO), in spite of identical in-plane and out-of-plane lattice deformation. This confirms that it is the lattice symmetry mismatch and not just the total strain, which determines the magnetism of LCO. On the basis of this control over the magnetic properties of LCO, we designed a multilayered structure to achieve independent rotation of the magnetization in ferromagnetic insulating LCO and half-metallic ferromagnet LSMO. This is an important step forward for the design of spin-filtering tunnel barriers based on LCO.

Dimensionality Controlled Octahedral Symmetry-Mismatch and Functionalities in Epitaxial LaCoO₃/SrTiO₃ Heterostructures

Epitaxial strain provides a powerful approach to manipulate physical properties of materials through rigid compression or extension of their chemical bonds via lattice-mismatch. Although symmetry-mismatch can lead to new physics by stabilizing novel interfacial structures, challenges in obtaining atomic-level structural information as well as lack of a suitable approach to separate it from the parasitical lattice-mismatch have limited the development of this field. Here, we present unambiguous experimental evidence that the symmetry-mismatch can be strongly controlled by dimensionality and significantly impact the collective electronic and magnetic functionalities in ultrathin perovskite LaCoO3/SrTiO3 heterojunctions. State-of-art diffraction and microscopy reveal that symmetry breaking dramatically modifies the interfacial structure of CoO6 octahedral building-blocks, resulting in expanded octahedron volume, reduced covalent screening, and stronger electron correlations. Such phenomena fundamentally alter the electronic and magnetic behaviors of LaCoO3 thin-films. We conclude that for epitaxial systems, correlation strength can be tuned by changing orbital hybridization, thus affecting the Coulomb repulsion, U, instead of by changing the band structure as the common paradigm in bulks. These results clarify the origin of magnetic ordering for epitaxial LaCoO3 and provide a route to manipulate electron correlation and magnetic functionality by orbital engineering at oxide heterojunctions.

Tuning the Spin State in LaCoO3 Thin Films for Enhanced High-Temperature Oxygen Electrocatalysis

The slow kinetics of oxygen surface exchange hinders the efficiency of high-temperature oxygen electrocatalytic devices such as solid oxide fuel cells and oxygen separation membranes. Systematic investigations of material properties that link to catalytic activity can aid in the rational design of highly active cathode materials. Here, we explore LaCoO₃ thin films as a model system for tuning catalytic activity through strain-induced changes in the Co spin state. We demonstrate that Raman spectroscopy can be used to probe the Co-O bond strength at different temperatures to determine the relative spin occupancies of LaCoO₃. We find that strain can be used to reduce the spin transition temperature and promote the occupation of higher spin states that weaken the Co-O bond. The decrease in Co-O bond strength and increased spin moment of the thin films result in significant enhancements of the oxygen surface exchange kinetics by up to 2 orders of magnitude.