Study of Oxygen Vacancies in Ce_0.9Pr_0.1O_2-δ Solid Solution by in Situ X-ray Diffraction and in Situ Raman Spectroscopy

Abstract

A Ce 0.9 Pr 0.1 O 2-δ solid solution was prepared by a sol-gel method. Changes in microstructure of the solid solution under different atmospheres (O 2 , He, and H 2 ) and temperatures were characterized by an in situ X-ray diffraction (XRD) technique. Raman peaks at 460 cm -1 ascribed to the F 2g vibration mode of CeO 2 in the fluorite structure and at 570 cm -1 ascribed to oxygen vacancies in the solid solution were studied by in situ Raman spectroscopy using 785-and 514-nm excitation laser lines, providing bulk and surface information, respectively. With a 785-nm laser line, the A 570 /A 460 ratio reflecting the oxygen vacancies concentration increased under O 2 and He while it first increased and then decreased under H 2 with increasing temperature. With a 514-nm excitation laser line, the A 570 /A 460 ratio decreased with increasing temperature under all atmospheres. The growth of the A 570 /A 460 ratio under the 785-nm laser line was due to the positive effects of high temperature and high concentration of oxygen vacancies and the negative effect of reduction of the sample under reducing atmospheres (He and H 2 ), while the decline in the A 570 /A 460 ratio under 514 nm was due to the dominant negative effect of the migration of surface Pr from surface to bulk during the heating process.

Gold supported on metal-doped ceria catalysts (M=Zr, Zn and Fe) for the preferential oxidation of CO (PROX)

“…This behavior has been associated with structural modifications by the doping effect [37,44]. In our case, the observed decrease of the ceria particle size in the doped samples calculated by the Scherrer equation (table 2) previously discussed for Eu [37] and Pr [46] doped ceria, the detection and relative intensity of this Raman band are affected by many factors. In particular, those factors affecting the absorption and Raman scattering effect, such as the laser power and frequency, the particle size distribution and homogeneity of the material and the absorption properties of the solid, are relevant since they determine the penetration depth of the Raman radiation.…”

“…The second band at ca. 600 cm -1 , has been widely reported in literature and is associated to the presence of the oxygen vacancies preferentially placed on the surface of the cubic structure of the CeO 2 [11,37,38].…”

“…Raman active mode of CeO 2 fluorite structure, which can be viewed as a symmetric breathing mode of the oxygen atoms around Ce 4+ ions [37,38]. The second band at ca.…”

“…In this support, the F 2g band broads and shifts towards 440 cm -1 , while the oxygen vacancies band seems to disappear or is overlapped with the F 2g one. Several factors could be responsible of such behavior: changes in the structure, particle size, sizes distribution, number and nature of defects, nature and concentration of doping cations, among others [11,37,38]. As all these factors influence in a similar way both bands, it is well accepted that the correct way of evaluating the amount of oxygen vacancies of a sample is to measure the ratio between the areas of the oxygen vacancies band (Ov) and the F 2g band, in such a way that, the higher the Ov/F 2g ratio, the higher the number of oxygen vacancies [11,38].…”

Synergy between gold and oxygen vacancies in gold supported on Zr-doped ceria catalysts for the CO oxidation

“…As presented in recent works, a correct evaluation of the evolution of oxygen vacancies population in pure and doped cerium oxides must be carried out by comparing the area ratio of the oxygen vacancies band and the F 2g signal (Ov/F 2g ) [8,42,43] in order to reduce the possible influence of the particle size and other effects over the shape and intensity of the signals [8,43,46,47]. The Figure 5 inset shows how the population of oxygen vacancies increases with the Zr At.% in our materials.…”