AVS 58th Annual International Symposium and Exhibition
    Surface Science Division Tuesday Sessions
       Session SS-TuA

Paper SS-TuA1
La1-xCexMnO3 Perovskites: Structural Features and Performance for Preferential CO Oxidation Reaction

Tuesday, November 1, 2011, 2:00 pm, Room 109

Session: Catalysis on Metals and Alloys
Presenter: Silvia Maluf, Federal University of Sao Carlos, Brazil
Authors: S.S. Maluf, Federal University of Sao Carlos, Brazil
B.E. Hayden, University of Southampton, UK
C.R.M. Afonso, Federal University of Sao Carlos, Brazil
E.M. Assaf, University of Sao Paulo, Brazil
P.A.P. Nascente, Federal University of Sao Carlos, Brazil
Correspondent: Click to Email
The perovskite structure is characterized by a large capacity to stabilize unusual valence states of different metal ions and can also accommodate variable amounts of different lattice defects. Several lanthanum transition metal based perovskites, represented as La1-xAxMO3, have been known as very good oxidation catalysts in a variety of reactions. Particularly, LaMnO3 perovskites have shown to be the most active. The catalytic activity of these compositions in different reactions can be further enhanced by the substitution of lanthanum by cerium. The catalysts were prepared by co-precipitation method, from aqueous solutions of La, Mn, and Ce nitrates with Na2CO3/NaOH solution (pH 10). The prepared samples were La1-xCexMnO3 (x = 0, 0.05, and 0.10) and were characterized by EDS-SEM, surface area-BET method, X-ray diffraction (XRD), and temperature programmed reduction (TPR). The catalytic activity was tested over the temperature range of 130–230°C in a fixed-bed tubular glass micro-reactor, with 200mg of catalyst. The reaction products were analyzed in-line by gas chromatography. The amounts of the constituent metals obtained by EDS were in agreement with expected values; the presence of cerium did not change the surface are of the samples (26-27m2 g-1). The X-ray diffraction lines corresponded to cubic LaMnO3 structure; the replacement of La did not change the diffractograms. Peaks of cerium oxide were not observed, suggesting the incorporation of Ce ions into the LaMnO3 lattice. The sample with the highest content of cerium presented an increase on thermal stabilization of Mn3+ ions, and Mn4+ species were reduced to lower temperatures. The SEM results indicated that the presence of cerium decreased slightly the grain size (from 56 to 43 nm) and the range of distribution sizes is homogeneous and the most grains are spherical. All samples presented the same catalytic behavior, starting their activities at 130°C, and their activities increased with the temperature. At 150°C, the sample with 5% of Ce presented a slight advantage compared to other samples, and this was more significant at 230°C, indicating that the promoter effect of cerium can be observed for higher reaction temperatures. The results suggested that the replacement of La by Ce caused an increase of cationic/anionic vacancies and also in the Mn4+/Mn3+ ratio change, and consequently the catalytic behavior for CO oxidation changed, facilitating the CO adsorption.

Acknowledgements

The authors thank FAPESP for financial assistance, Ilika Technologies and University of Southampton for the XRD analyses.