Low-Temperature Oxygen Storage of CrIV-CrV Mixed-Valence YCr1-xPxO4-δ Driven by Local Condensation around Oxygen-Deficient Orthochromite.

ABSTRACT

The oxygen storage capability and related defect structure of tetrahedral orthochromite(V) compound YCr1-xPxO4 (x = 0, 0.3, 0.5, and 0.7) were investigated by employing thermal gravimetry and in situ X-ray spectroscopy for reversible oxygen store/release driven by heating-cooling cycles in the temperature range from 50 to 600 °C. YCr1-xPxO4 started releasing oxygen as heated from 50 °C under ambient atmosphere, with reduction of CrV to CrIV, while the reduced YCr1-xPxO4-δ phase was significantly reoxidized via absorbing oxygen by cooling to 50 °C under ambient atmosphere, recovering the original stoichiometric phase. Operando X-ray adsorption spectroscopy and first-principles calculations demonstrate that nonstoichiometric YCr1-xPxO4-δ phases were stabilized by forming linking polyhedral CrIV2O76- via corner sharing between oxygen-deficient CrIVO32- and adjacent CrIVO44-. YCr1-xPxO4 was found to have an extremely low reduction enthalpy of about 20 kJ mol-1 probably due to the relatively high reduction potential of high-valence-state Cr(V)/Cr(IV) redox pairs, thereby resulting in reversible oxygen storage in such a low-temperature region.