High-temperature redox chemistry of Pr_(0.5)Sr_(1.5)Cr_(0.5)Mn_(0.5)O_(4-δ) investigated in situ by neutron diffraction and X-ray absorption spectroscopy under reducing and oxidizing gas flowsf

摘要

The structural and redox stability of the n = 1 Ruddlesden-Popper (RP) oxide Pr_(0.5)Sr_(1.5)Cr_(0.5)Mn_(0.5)O_(4-δ), synthesized by the citrate-gel method, has been investigated over the temperature range 25-700 °C under reducing (5% H2 flow) and oxidizing (O2 or air flow) conditions by means of in situ neutron powder diffraction (NPD) and X-ray absorption near-edge structure spectroscopy (XANES). Sequential Rietveld refinement of the NPD patterns collected under hydrogen revealed de-intercalation of oxide ions from the equatorial anion positions with retention of IAImmm symmetry. The reduction from Pr_(0.5)Sr_(1.5)Cr_(0.5)Mn_(0.5)O_(4.00(2)) to Pr_(0.5)Sr_(1.5)Cr_(0.5)Mn_(0.5)O_(3.81(2)) is accompanied by an expansion of both the a and c lattice parameters. When the reduced sample is heated in air, oxygen refills the equatorial sites and the unit cell contracts; the interlayer interstitial site remains unoccupied. XANES showed the oxidation states in the as-prepared composition to be Pr~(3+), Cr~(3+) and Mn~(4+). When the material is heated under dilute hydrogen, the oxidation states Pr~(3+) and Cr~(3+) are retained whereas Mn~(4+) is reduced to Mn~(3+). These observations constitute the first direct evidence that the d-block element, and not praseodymium, is responsible for the electrocatalytic activity of Pr-containing RP oxides. When the reduced material is heated Under oxygen, Mn~(3+) is reoxidised to Mn~(4+) and a low concentration of tetrahedrally-coordinated Cr(VI) forms, suggesting a possible poisoning mechanism in fuel-cell applications.