Oxygen Reduction Reaction at LaxCa1-xMnO3 Nanostructures: Interplay between A-site Segregation and B-site Valency (original) (raw)
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Effect of Ba Content on the Activity of La1‐xBaxMnO3 Towards the Oxygen Reduction Reaction
ChemElectroChem, 2018
The electrocatalytic activity of La1‐xBaxMnO3 nanoparticles towards the oxygen reduction reaction (ORR) is investigated as a function of the A‐site composition. Phase‐pure oxide nanoparticles with a diameter in the range of 40 to 70 nm were prepared by using an ionic liquid route and deposited onto mesoporous carbon films. The structure and surface composition of the nanoparticles are probed by XRD, TEM, EDX, and XPS. Electrochemical studies carried out under alkaline conditions show a strong correlation between the activity of La1‐xBaxMnO3 and the effective number of reducible Mn sites at the catalysts layer. Our analysis demonstrates that, beyond controlling particle size and surface elemental segregation, understanding and controlling Mn coordination at the first atomic layer is crucial for increasing the performance of these materials.
Oxygen Content and Structures of La1−xCaxMnO3+d as a Function of Synthesis Conditions
Journal of Solid State Chemistry, 1999
Using thermogravimetric analysis measurements, oxygen content was measured for La 1؊x Ca x MnO 3؉d (04x40.38) samples prepared at high oxygen pressure. The oxygen content was found to decrease uniformly with increasing x for samples annealed at 12 and 140 atm O 2 at 8003C, slowly cooled in 1, 20, and 100% O 2 , as well as for samples in thermal equilibrium at high temperatures of &1000 +14003C. Annealing at a speci5c temperature and oxygen pressure leads to approximately constant hole doping over an extended range of Ca substitution x. The most favorable synthesis conditions for obtaining stoichiometric, d ؍ 0, samples were determined. The data suggest that several studies reported in the literature were done on signi5cantly nonstoichiometric materials. Structural results show that, in addition to the orthorhombic Pbnm O (c/a<(2) and O* (a&b&c/(2) structures, the rhombohedral R3 c structure is stabilized at high oxygen contents for x40.14. A sequence of observed structures and a decrease of the unit cell volume with increasing d con5rm that the additional holes are doped uniformly by cation-vacancy defects for all studied x.
ChemElectroChem
LaMnO 3 has been identified as one of the most active systems towards the 4-electron oxygen reduction reaction (ORR) under alkaline conditions, although the rationale for its high activity in comparison to other perovskites remains to be fully understood. LaMnO 3 oxide nanoparticles are synthesised by an ionic-liquid based method over a temperature range of 600 to 950 8C. This work describes a systematic study of the LaMnO 3 properties, from bulk to the outermost surface layers, as a function of the synthesis temperature to relate them to the ORR activity. The bulk and surface composition of the particles are characterised by transmission electron microscopy, X-ray diffraction, X-ray absorption and X-ray photoemission spectroscopy (XPS), as well as low-energy ion scattering spectroscopy (LEIS). The particle size and surface composition are strongly affected by temperature, although the effect is non-monotonic. The number density of redox active Mn sites is obtained from electrochemical measurements, and correlates well with the trends observed by XPS and LEIS. ORR studies of carbon-supported LaMnO 3 employing rotating ring-disk electrodes show a step increase in the mean activity of individual surface Mn sites for particles synthesised above 700 8C. Our analysis emphasises the need to establish protocols for quantifying turnover frequency of single active sites in these complex materials to elucidate appropriate structure-activity relationships.
ACS Catalysis
In situ X-ray absorption and emission spectroscopies (XAS and XES) are used to provide details regarding the role of the accessibility and extent of redox activity of the Mn ions in determining the oxygen reduction activity of LaMnO 3 and CaMnO 3 , with X-ray absorption near-edge structure (XANES) providing the average oxidation state, extended X-ray absorption fine structure (EXAFS) providing the local coordination environment, and XES providing the population ratios of the Mn 2+ , Mn 3+ , and Mn 4+ sites as a function of the applied potential. For LaMnO 3 , XANES and XES show that Mn 3+ is formed, but Mn 4+ ions are retained, which leads to the 4e − reduction between 0.85 and 0.6 V. At more negative potentials, down to 0.2 V, EXAFS confirms an increase in oxygen vacancies as evidenced by changes in the Mn−O coordination distance and number, while XES shows that the Mn 3+ to Mn 4+ ratio increases. For CaMnO 3 , XANES and XES show the formation of both Mn 3+ and Mn 2+ as the potential is made more negative, with little retention of Mn 4+ at 0.2 V. The EXAFS for CaMnO 3 also indicates the formation of oxygen vacancies, but in contrast to LaMnO 3 , this is accompanied by loss of the perovskite structure leading to structural collapse. The results presented have implications in terms of understanding of both the pseudocapacitive response of Mn oxide electrocatalysts and the processes behind degradation of the activity of the materials.
Oxygen-storage behavior and local structure in Ti-substituted YMnO3
Journal of Solid State Chemistry
Hexagonal manganates RMnO 3 (R=Y, Ho, Dy) have been recently shown to exhibit oxygenstorage capacities promising for three-way catalysts, air-separation, and related technologies. Here, we demonstrate that Ti substitution for Mn can be used to chemically tune the oxygenbreathing properties of these materials towards practical applications. Specifically, Y(Mn 1-x Ti x)O 3 solid solutions exhibit facile oxygen absorption/desorption via reversible Ti 3+ Ti 4+ and Mn 3+ Mn 4+ reactions already in ambient air at ≈400 °C and ≈250 °C, respectively. On cooling, the oxidation of both cations is accompanied by oxygen uptake yielding a formula YMn 3+ 1-xy Mn 4+ y Ti 4+ x O 3+. The presence of Ti promotes the oxidation of Mn 3+ to Mn 4+ , which is almost negligible for YMnO 3 in air, thereby increasing the uptake of oxygen beyond that required for a given Ti 4+ concentration. The reversibility of the redox reactions is limited by sluggish kinetics; however, the oxidation process continues, if slowly, even at room temperature. The extra oxygen atoms are accommodated by the large interstices within a triangular lattice formed by the [MnO 5 ] trigonal bipyramids. According to bond distances from Rietveld refinements using the neutron diffraction data, the YMnO 3 structure features under-bonded Mn and even more severely under-bonded oxygen atoms that form the trigonal bases of the [MnO 5 ] bipyramids. The tensile bond strain around the 5-fold coordinated Mn site and the strong preference of Ti 4+ (and Mn 4+) for higher coordination numbers likely provide driving forces for the oxidation reaction. Reverse Monte Carlo refinements of the local atomic displacements using neutron total scattering revealed how the excess oxygen atoms are accommodated in the structure by correlated local displacements of the host atoms. Large displacements of the under-bonded host oxygen atoms play a key part in this lattice-relaxation process, facilitating reversible exchange of significant amounts of oxygen with atmosphere.
Catalysts, 2022
Perovskite oxides, being transition metal oxides, show promise as bifunctional catalysts being able to catalyze both oxygen evolution reactions (OER) and oxygen reduction reactions (ORR). These two reactions play a crucial role in energy storage and energy conversion devices. An important feature of perovskite catalyst is their ability to be tuned, as tuning can positively affect both reactivity and stability. In this study, Density Functional Theory (DFT) has been utilized to calculate both the equilibrium phase stability and the overpotentials (reactivity performance indicator of the catalysts) of La1−xSrxMnO3 (LSM) structures with different stoichiometry by introducing Mn and O vacancies for both the OER/ORR reactions. The electronic structures reveal that combined Mn and O vacancies can lead to higher catalytic activity for both OER and ORR due to the optimum filling of antibonding orbital electrons. Moreover, both O p-band centers and equilibrium phase stability plots show that...
Functional properties of LaxCe1−xO2−δ nanocrystals and their bulk ceramics
Journal of Materials Science: Materials in Electronics, 2018
Structure-property correlations were investigated for La x Ce 1−x O 2−δ (x = 0.05, 0.15) system (for both nanoparticles and ceramic). The nanoparticles were synthesized by the co-precipitation route and characterized for their structural, catalytic and visible light driven photocatalytic properties. Synthesised La x Ce 1−x O 2−δ nanopowders depicted phase purity with excellent compositional control as confirmed by various structural characterisation tools. The 15% La 3+-doped ceria nanoparticles portrayed superior photocatalytic properties; complete degradation (99%) of the methylene blue dye was observed within 60 min of visible light irradiation under an alkaline medium. High density (> 96%) of the ceramics (sintered at 1580 °C for 2 h) confirmed promising sinterability of the synthesised powders. The ionic conductivities of the La x Ce 1−x O 2−δ ceramics increased with temperature and frequency owing to enhanced oxygen vacancies in the ceria matrix as a result of doping (of La 3+-ions); a maximum conductivity of ~ 8.4 × 10 −2 S cm −1 was obtained at 900 °C for the 15% La 3+-doped ceria ceramics at 1 MHz frequency.
Journal of the American Chemical Society, 2022
The atomistic rationalization of the activity of transition metal oxides toward oxygen electrocatalysis is one of the most complex challenges in the field of electrochemical energy conversion. Transition metal oxides exhibit a wide range of structural and electronic properties, which are acutely dependent on composition and crystal structure. So far, identifying one or several properties of transition metal oxides as descriptors for oxygen electrocatalysis remains elusive. In this work, we performed a detailed experimental and computational study of LaMn x Ni 1−x O 3 perovskite nanostructures, establishing an unprecedented correlation between electrocatalytic activity and orbital composition. The composition and structure of the single-phase rhombohedral oxide nanostructures are characterized by a variety of techniques, including X-ray diffraction, X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and electron microscopy. Systematic electrochemical analysis of pseudocapacitive responses in the potential region relevant to oxygen electrocatalysis shows the evolution of Mn and Ni d-orbitals as a function of the perovskite composition. We rationalize these observations on the basis of electronic structure calculations employing DFT with HSE06 hybrid functional. Our analysis clearly shows a linear correlation between the OER kinetics and the integrated density of states (DOS) associated with Ni and Mn 3d states in the energy range relevant to operational conditions. In contrast, the ORR kinetics exhibits a second-order reaction with respect to the electron density in Mn and Ni 3d states. For the first time, our study identifies the relevant DOS dominating both reactions and the importance of understanding orbital occupancy under operational conditions.