Unraveling the Nature of Sites Active toward Hydrogen Peroxide Reduction in Fe-N-C Catalysts

Fe-N-C catalysts with high O₂ reduction performance are crucial for displacing Pt in low-temperature fuel cells. However, insufficient understanding of which reaction steps are catalyzed by what sites limits their progress. The nature of sites were investigated that are active toward H₂O₂ reduction, a key intermediate during indirect O₂ reduction and a source of deactivation in fuel cells. Catalysts comprising different relative contents of FeN_xC_y moieties and Fe particles encapsulated in N-doped carbon layers (0–100 %) show that both types of sites are active, although moderately, toward H₂O₂ reduction. In contrast, N-doped carbons free of Fe and Fe particles exposed to the electrolyte are inactive. When catalyzing the ORR, FeN_xC_y moieties are more selective than Fe particles encapsulated in N-doped carbon. These novel insights offer rational approaches for more selective and therefore more durable Fe-N-C catalysts.     

Hyperfine interactions and lattice dynamics of Sn21O6Cl16(OH)14

In this study, the tin(II) oxy-hydroxychloride Sn₂₁O₆Cl₁₆(OH)₁₄ has been synthesised and investigated. This compound is the synthetic equivalent of mineral abhurite, which was discovered in 1985 as a tin corrosion product formed on the surface of tin ingots after long immersion in seawater. The Mössbauer parameters of Sn₂₁O₆Cl₁₆(OH)₁₄ determined at various temperatures are reported and discussed for the first time. At room temperature, the isomer shift and the quadrupole splitting are, respectively, δ=3.22 mm s⁻¹ and Δ=1.71 mm s⁻¹, relative to the centroid of the spectrum of BaSnO₃. The Mössbauer recoil-free fraction has been also evaluated over a wide range of temperature. At 300 K, the recoil-free fraction of Sn₂₁O₆Cl₁₆(OH)₁₄ is f₃₀₀=0.09±0.02.     

Local ordering and magnetism in Ga0.9Fe3.1N

Prior investigations of the ternary nitride series Ga_1-xFe_3+xN (0≤x≤1) have indicated a transition from ferromagnetic γ′-Fe₄N to antiferromagnetic “GaFe₃N”. The ternary nitride “GaFe₃N” has been magnetically and spectroscopically reinvestigated in order to explore the weakening of the ferromagnetic interactions through the gradual incorporation of gallium into γ′-Fe₄N. A hysteretic loop at RT reveals the presence of a minority phase of only 0.1-0.2 at%, in accord with the sound two-step synthesis. The composition of the gallium-richest phase “GaFe₃N” was clarified by Prompt Gamma-ray Activation Analysis and leads to the berthollide formula Ga_0.91(1)Fe_3.09(10)N_1.05(7). Magnetic measurements indicate a transition around 8 K, further supported by Mössbauer spectral data. The weakening of the ferromagnetic coupling through an increasing gallium concentration is explained by a simple Stoner argument. In Ga_0.9Fe_3.1N, the presence of iron on the gallium site affects the magnetism by the formation of 13-atom iron clusters.