Effect of MnOx Loading on Structural, Surface, and Catalytic Properties of CeO2-MnOx Mixed Oxides Prepared by Sol-Gel Method

Nanosized CeO2-MnOx mixed oxides with different Ce/Mn ratios were prepared by a citric acid-assisted sol-gel method. The effect of the MnOx content on structural, redox, and surface properties of the CeO2-MnOx was investigated by various techniques, namely, BET surface area, SEM, XRD, ICP-OES, Raman and FT-IR spectroscopy, H-2-TPR, and XPS. Catalytic activity was evaluated with soot-catalyst mixture under tight contact conditions. BET surface area analysis indicated that Mn doping favors an enhancement in the surface area of pure CeO2. Raman active peaks for both pure CeO2 and CeO2-MnOx mixed oxides were determined at ca. 460 cm(-1). The observed Raman shift can also be assigned to Raman active modes of CeO2 that are shifted from the original position due to different Mn doping. Formation of Ce1-xMnxO2-delta solid solutions was further confirmed by XRD and FT-IR analysis. H-2-TPR results demonstrate that Ce1-xMnxO2-delta solid solutions significantly improved the redox property over the pure CeO2 or MnOx. XPS analysis reveal that Ce is in the form of Ce4+ and Ce3+, and Mn existed in the form of Mn4+, Mn3+, and Mn2+ on the surface of the samples. Catalytic activity results indicate that the oxidation temperature of soot can be significantly reduced by CeO2-MnOx mixed oxides, which render higher Ce3+ concentration and more surface oxygen vacancies compared with pure CeO2. Particularly, the Ce0.7Mn0.3O2-delta sample exhibited highest activity (a shift of T-50 by ca. 122 K) compared to pure ceria.