| In situ Raman spectroscopy is a powerful technique because it can provide fundamental molecular-level information about catalyst surface structure and reactive surface intermediates under practical reaction conditions (temperatures, atmospheres, etc.). In this work, in situ Raman spectroscopy was employed to study the surface property of a Ce0.9Pr0.1O2-δ solid solution under different atmospheres and temperatures. By use of various excitation laser lines, information of different surface layers was obtained, which may give new insight on the property of oxygen vacancies in doped CeO2 solid solution.A Ce0.9Pr0.1O2-δ solid solution was prepared by a sol-gel method. Changes in microstructure of the solid solution under different atmospheres (O2, He, and H2) and temperatures were characterized by an in situ X-ray diffraction (XRD) technique. Raman peaks at 460 cm"1 ascribed to the F2g vibration mode of CeO2 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 A570/A460 ratio reflecting the oxygen vacancies concentration increased under O2 and He while it first increased and then decreased under H2 with increasing temperature. With a 514-nm excitation laser line, the A570/A460 ratio decreased with increasing temperature under all atmospheres. The growth of the A570/A460 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 H2), while the decline in the A570/A460 ratio under 514 run was due to the dominant negative effect of the migration of surface Pr from surface to bulk during the heating process.Moreover, the CeO2-δ,Ce0.9Pr0.1O2-δ,Ce0.95Cu0.05O2-δ,Ce0.9Pr0.05Cu0.05O2-δ mixed oxides were prepared by sol-gel method. These catalysts were characterized by XRD, in situ Raman and in situ IR techniques. Only the diffraction peaks due to cubic CeO2 were observed in these mixed oxides. The peak at 570 cm-1 ascribed to oxygen vacancies was obviously observed in the Pr-containing catalysts. The value of A570/A460 in Ce0.9Pr0.1O2-δ mixed oxide was higher than that in Ce0.9Pr0.05Cu0.05O2-δ mixed oxide, which is likely due to the difference of Pr content in the catalysts. In situ Raman spectroscopies under reaction gases (CO-O2-N2,O2-N2 and CO-N2) indicate that the surface of Ce0.9Pr0.1O2-δ and Ce0.9Pr0.05Cu0.05O2-δ mixed oxides were close to the oxidation state. Combined with the results of in situ Raman, in situ IR and CO oxidation, the presence of oxygen vacancies and Cu ions both can enhance the catalytic ability. This is likely due to that oxygen vacancies provide sites for O2 adsoprtion and Cu ions provide sites for CO adsorption. The Ce0.9Pr0.05Cu0.05O2-δ mixed oxide contains oxygen vacancies and Cu ions simultaneously, so its activity was the best. The in situ IR also confirmed the adsorption of CO on Cu ions, and the oxygen vacancies complicate the formation of carbonate species which is hard to be decomposed.
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