Structural and Redox Properties of Ce_1–xZr_xO_2−δand Ce_0.8Zr_0.15RE_0.05O_2−δ(RE: La, Nd, Pr, Y) Solids Studied by High Temperaturein SituRaman Spectroscopy

Abstract

In situ Raman spectroscopy at temperatures up to 450 °C is used to probe the structural and redox properties of Ce 1−x Zr x O 2−δ solids (x = 0−0.8) prepared by the citrate sol−gel and coprecipitation with urea methods. The anionic sublattice structure of the solids is dependent on the preparation route. The composition effects exhibited by the Raman spectra are adequate for characterizing the phases present and/or eventual phase segregations. For x = 0.5 the pseudocubic t″ phase occurs for the solid prepared by the citrate sol−gel method, while phase segregation (cubic, tetragonal) is evidenced for the corresponding material prepared by the coprecipitation with urea method. A larger extent of defects and interstitial O atoms is evidenced for the materials prepared by the citrate sol−gel method. The well-known "defect" ("D") band around 600 cm −1 for CeO 2 as well as for Ce 1−x Zr x O 2−δ consists of at least two components: "D1" above 600 cm −1 and "D2" below 600 cm −1 . Doping of Ce 0.8 Zr 0.2 O 2−δ with rare earth cations (La 3+ , Nd 3+ , Y 3+ , Pr 3+ ) results in strengthening of the "D2" band that, however, is found to be insensitive under reducing conditions of flowing 5% H 2 /He at 450 °C. A novel approach based on sequential in situ Raman spectra under alternating oxidizing (20% O 2 /He) and reducing (5% H 2 /He) gas atmospheres showed that the "D1" band is selectively attenuated under reducing conditions at 450 °C and is therefore assigned to a metal−oxygen vibrational mode involving interstitial oxygen atoms that can be delivered under suitable conditions. A reversible temperature-dependent evolution of the anionic sublattice structures of Ce 1−x Zr x O 2−δ solids is evidenced by in situ Raman spectroscopy. The results are corroborated by powder XRD and oxygen storage capacity measurements, and observed structure/function relationships are discussed. It is shown that at low temperatures (e.g., 450 °C) the function of oxygen release and refill is based on a mechanism involving oxygen atoms in interstitial sites rather than on defects induced by hetrovalent M 4+ → RE 3+ doping, the latter improving the pertinent function at high (e.g., >600 °C) temperatures.

The Role of Neodymium in the Optimization of a Ni/CeO2 and Ni/CeZrO2 Methane Dry Reforming Catalyst

“…In the CeZrNd compositions, the overall degree of reduction of Ce 4+ to Ce 3+ was lower than that of the binary oxides. These results correlate to previous findings [25,26] and require further investigation to be fully understood. In these oxides, the addition of extra vacancies due to the presence of Nd 3+ may lead to a high vacancies concentration, which, in turn, may aggregate in less mobile clusters.…”

“…Moreover, it is clear that the Nd doping promoted CeNd reduction, which started at a lower temperature and covered a large range of temperatures with two broad bands centered at 300 and 700 • C. Quantitatively, the degree of reduction was higher than that of CeO 2 (see 3rd column on Table 3). The higher degree of CeO 2 reduction observed with the insertion of Nd 3+ into the fluorite lattice was a result of the formation of charge-balancing oxygen vacancies which contributed to a higher mobility of bulk oxygen anions [25]. In the case of CeZr, in which the mobility of bulk oxygen anions was intrinsically higher than that in CeO 2 , the introduction of extra vacancies due to Nd addition had a minor impact.…”

“…The addition of rare earth aliovalent ions (RE 3+ ) is an effective way to alter the structural and physochemical properties of ceria and CeZr solid solutions. The doping of CeZr oxides with Pr 3+ , Sm 3+ , La 3+ , and Gd 3+ , caused the formation of oxygen vacancies into the crystal lattice to balance the difference of charge; this generally improved their redox properties, oxygen storage capacity (OSC), and thermal stability [25,26]. Recently, neodymium (Nd 3+ ) has been demonstrated to be an effective dopant to stabilize the cubic phase of ceria-zirconia solid solutions in all range of compositions [27] and to improve the catalytic activity of CeZr in the removal of soot and other automotive exhausts [28,29].…”

The Reduction Properties of M–Doped (M=Zr, Gd) CeO2/YSZ Scaffolds Co–Infiltrated with Nickel

“…Figure (b) shows that there is a significant decrease in the intensity of both the F 2g and vacancy peak in the sample with Ni. The large decrease in the intensity of the F 2g mode indicates that the majority of the oxygen removed is from the crystal structure itself while the small decrease in the vacancy peak indicates there is some loss of interstitial oxygen . There is also a significant shift in the F 2g peak position – the peak shifts towards the laser by approximately 4.5 cm −1 – on reduction of the sample.…”

In Situ Raman Spectroscopy as a Tool for Discerning Subtle Structural Differences between Commercial (Ce,Zr)O2-Based OSC Materials of Identical Composition

“…The anionic (oxygen) sublattice structure of Ce x Zr 1−x O 2−δ mixed oxides is subject to reversible temperature-dependent evolution. Recently, for the first time, we used in situ Raman spectroscopy to capture pertinent temperature-dependent snapshots of Raman spectra reflecting the corresponding structural conformation at each temperature for Ce x Zr 1−x O 2−δ mixed oxides made by the citrate sol-gel and urea-coprecipitation methods [10]. The evolution of the sequential in situ Raman spectra obtained under oxidizing conditions at 50, 200, 450, and 50 • C were interpreted in terms of O atom relocations within the lattice [10].…”