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

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.