Nanostructured manganese doped ceria solid solutions for CO oxidation at lower temperatures

Noble metal-free nanostructured Ce0.7Mn0.3O2-delta (CM) solid solutions were prepared by a simple, facile, and high yielding coprecipitation method from aqueous solutions of cerium and manganese nitrates. The influence of calcination temperature from 773 to 1073 K on the structural features and catalytic behaviour of the CM solid solutions has been thoroughly investigated. The synthesized materials were characterized using XRD, ICP-OES, BET surface area, TEM and HRTEM, UV-vis DRS, Raman, XPS, EPR, and H-2-TPR techniques. The XRD results indicated that the samples calcined at various temperatures from 773 to 973 K did not show any diffraction peaks corresponding to manganese oxides and thereby ensured the formation of Ce0.7Mn0.3O2-delta solid solution. However, the diffraction peak corresponding to Mn2O3 was clearly observed at 1073 K due to the occurrence of phase segregation between Mn-oxide and ceria. The TEM and HRTEM observations showed the nanocrystalline nature of the solid solutions with a particle size in the range of similar to 7-20 nm. The doping of Mn into the ceria lattice increased the concentration of structural oxygen vacancies, which are beneficial for oxidation reaction as revealed by UV-vis DRS and Raman results. XPS analysis confirmed that Ce and Mn exist in +4 or +3 and +3 or +2 oxidation states, respectively on the surface of the samples. The EPR studies showed the presence of isolated Mn2+ and Mn3+ species in different structural locations such as ceria-lattice defect sites, framework of Ce4+ locations, interstitial locations, and at the surface of the ceria. H-2-TPR measurements showed a pronounced enhancement in the reduction of ceria by manganese doping. Among the various catalysts examined, the CM catalyst calcined at 773 K exhibited superior CO oxidation activity at much lower temperature with T-50 =390 K and 100% CO conversion at similar to 470 K in comparison to pure ceria and MnOx. The observed remarkable enhancement in the reactivity for CO oxidation at lower temperatures has been attributed to a highly dispersed state of Mn2+/Mn3+ in the ceria matrix, facile redox behaviour, and a synergistic Mn-Ce interaction. (C) 2014 Elsevier B.V. All rights reserved.