Deterioration of the oxygen storage and release properties of CeZrO₄ by incorporation of calcium

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• 作者：M. Ló ; pez Granados ; A. Gurbani ; R. Mariscal ; J.L.G. Fierro
• 关键词：TWC converters ; Chemical deactivation ; Fouling ; OSR ; O⁻ ; ₂ superoxide radical ; CeZr mixed oxides
• 刊名：Journal of Catalysis
• 出版年：2008
• 期刊代码：97_00219517
• 类别：ce
• 出版时间：10 June 2008
• 卷：256
• 期：2
• 页码：172-182
• 文件大小：477 K

摘要

Calcium was added to CeZrO₄ mixed oxide by wet impregnation with a Ca nitrate solution to achieve solids with nominal Ca/(Ce + Zr) atomic ratios of 0.01, 0.02, 0.05, 0.1, 0.2, and 0.5. The solids were calcined at 873 K to decompose the Ca nitrate to CaO. The determination of oxygen storage capacity complete (OSCC) and FTIR studies of the formation of surface superoxide radicals (O⁻₂) were conducted to study the oxygen storage and release (OSR) properties of the solids. Both measurements indicate that the OSR properties clearly deteriorated for samples with nominal Ca/(Ce + Zr) at. ratios >0.01 (0.9 atoms of Ca nm⁻²). The degree of deactivation increased with increasing Ca loading; for the sample with the largest Ca loading, the OSCC was ca. 3-fold lower than that of bare CeZrO₄. The solids also were characterized by TGA, chemical analysis by TXRF, XRD, N₂ isotherms, and XPS. The results indicate that the Ca remained on the surface of the CeZrO₄ as supported CaO; the formation of large bulky crystals of CeZrCa mixed oxides by solid-state reaction between the Ca phase and the CeZrO₄ did not occur. For Ca loading below the amount required to completely cover the surface (which, experimentally, is ca. 18 atoms of Ca per nm² of surface), CaO existed as very well-dispersed CaO domains few atoms thick. Above this value, the surface was completely covered by CaO, and three-dimensional crystals of CaO were formed. Given these results, we propose that the main mechanism for the deterioration of OSR properties of CeZrO₄ by incorporation of CaO on the surface by impregnation is fouling: CaO physically prevents contact between the O₂ gas and the surface sites of the CeZrO₄. It also is likely that a very limited solid-state reaction occurs between the latter two phases, with a CaZrO₃-like amorphous oxide formed at the grain boundaries. The formation of this amorphous oxide also can chemically contribute to the deactivation. The relevance of these results in the understanding of the deactivation of TWC by Ca is discussed.