Mean Intrinsic Activity of Single Mn Sites at LaMnO₃ Nanoparticles Towards the Oxygen Reduction Reaction

Abstract

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 turn-over frequency of single active sites in these complex materials to elucidate appropriate structure-activity relationships.[a] Dr.

PrFeO3 Photocathodes Prepared Through Spray Pyrolysis

“…CasaXPS software was used to determine the Pr : Fe ratio for these PFO films which was 1 : (0.7–0.8), showing between 20 and 30 % A‐site surface enrichment at all temperatures (Table S1). A‐site surface segregation is commonly observed in perovskite oxide materials obtained by calcination of molecular precursors, showing a very complex dependence of the nature of the A and B sites and temperature . The Pr surface excess can play a role in charge extraction, although this issue is not assessed in this work.…”

High Interfacial Hole‐Transfer Efficiency at GaFeO3 Thin Film Photoanodes

“…[36,37] The main feature of the O 1s spectrum ( Figure 2c) is centered at 530.01 eV, which is assigned to lattice bound oxygen, [13] while the other contributions are linked to surface hydroxyl and carbonylated groups. [38] Fe 2p core-level ( Figure 2d) is significantly more complex to rationalize due to contribution from various phenomena such as multiple-splitting, multiple oxidation states, and charge transfer effects. As a first approximation, the most intense component in 2p 3/2 is at 710.8 eV, with a 13.5 eV splitting from the 2p 1/2 component, which is consistent with a Fe+3 oxidation state.…”

“…This behavior is rather unusual for transition metal oxides with the general ABO 3 formula (such as perovskites), which often exhibit different A:B ratio in the bulk and at the surface. [38,40] The optical properties of GFO thin films in the visible range are summarized in Figure 3. The absorption coefficient (α), extracted from transmittance and reflectance spectra of 350 nm films ( Figure S2, Supporting Information) are displayed as a function of the photon energy, revealing several optical transitions between 1 and 3 eV ( Figure 3a).…”

“…This behavior is rather unusual for transition metal oxides with the general ABO 3 formula (such as perovskites), which often exhibit different A:B ratio in the bulk and at the surface. [ 38,40 ]…”

Electrocatalytic Site Activity Enhancement via Orbital Overlap in A2MnRuO7 (A = Dy3+, Ho3+, and Er3+) Pyrochlore Nanostructures

“…14 However, numerous experimental observations have shown that these are only parts of a complex picture, in which surface structure and composition as well as orbital occupancy under electrochemical control also play hugely important roles. [15][16][17][18][19][20] Indeed, electrode potential can introduce substantial changes in surface oxide properties including metal coordination, composition and conductivity. In previous studies, we have linked changes in the oxidation state at the B-site at potentials close to the formal oxygen potential as a key descriptor towards the oxygen reduction reaction to OHunder alkaline conditions.…”