Dual Ce4+/Fe3+ Redox Phenomena into Nanocrystalline Ce1鈥?i>xFexO2鈥?i>x/2 Solid Solution

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• 作者：Iona Moog ; Carmelo Prestipino ; Santiago Figueroa ; J茅r么me Majimel ; Alain Demourgues
• 刊名：Journal of Physical Chemistry C
• 出版年：2014
• 出版时间：October 2, 2014
• 年：2014
• 卷：118
• 期：39
• 页码：22746-22753
• 全文大小：428K
• ISSN：1932-7455

文摘

The Ce_1鈥?i>xFe_xO_2鈥?i>x/2 solid solution was synthesized using a microwave-assisted hydrothermal route. The solubility limit corresponds to a Fe (x) content equal to 0.15. Our previous works showed also that isolated Fe³⁺ distorted octahedral sites and Fe³⁺ clusters are randomly distributed into the ceria network. Thermogravimetric analysis (TGA) under Ar/5%H₂ revealed a higher reduction rate for Fe-substituted ceria with changes of slope and the appearance of pseudoplateaus around 400 and 550 掳C. The first Fe K-edge X-ray absorption near-edge spectroscopy (XANES) spectra recorded at several temperatures up to 700 掳C shift gradually to lower energies as the temperature increases, with the formation of metallic iron starting at 550 掳C, as confirmed by X-ray diffraction analysis. Moreover, on the basis of the principal component factor analysis, three K-edge structures associated with three oxidation states of iron (Fe³⁺, Fe²⁺, and Fe⁰) have been distinguished during this temperature-programmed reduction. The second component attributed to Fe²⁺ is the most intense one around 500 掳C. Beyond this temperature, the coexistence of the three oxidation states Fe³⁺/Fe²⁺/Fe⁰ has to be mentioned up to 650 掳C, the temperature at which metallic iron is mainly stabilized. Ce L_III-edge and Fe K-edge XANES spectra recorded in quick extended X-ray absorption fine structure mode and at various temperatures show the progressive reduction of Ce⁴⁺ and Fe³⁺ starting beyond 250 掳C, which is in good agreement with the change of slope observed on the TGA curves. The Ce L_III-edge XANES spectrum of pure CeO₂ recorded at 700 掳C under reducing atmosphere exhibits much less Ce³⁺ stabilized into the fluorite network than Fe-substituted ceria. Fe³⁺ ions randomly distributed into ceria strongly contribute to enhancing the Ce⁴⁺ reducibility properties. The Ce⁴⁺ reduction seems to appear at lower temperatures and is slower than the Fe³⁺ reduction, whose speed decreases around 400 掳C. At this temperature, the Fe²⁺ and Fe³⁺ components dominate and the Ce³⁺ content remains high. However, a plateau with the stabilization of Ce⁴⁺/Ce³⁺ mixed valences around 500 掳C appears where the Fe²⁺ contribution is also a maximum and the Fe reduction is slower. Because of the large content of either Ce³⁺ and Fe²⁺, a charge-transfer equilibrium Ce⁴⁺ + Fe²⁺ 鈬?Ce³⁺ + Fe³⁺ may occur at this temperature at which a change of slope on TGA curve also is observed with the appearance of a plateau at 550 掳C associated with the Fe⁰ demixtion phenomenon. Then, after reaching the maximum Fe²⁺ rate, the Ce⁴⁺ reduction rate increases again up to 700 掳C, the temperature at which all Fe atoms have been transformed into metallic iron.