Study of Ag/La0.6Sr0.4MnO3 catalysts for complete oxidation of methanol and ethanol at low concentrations

Ag-modified La0.6Sr0.4MnO3-based catalysts with the perovskite-type structu re were prepared by using a citric acid sol-gel method, and their catalytic performance for complete oxidation of methanol and ethanol was evaluated a nd compared with that of the gamma-Al2O3-supported catalysts, Ag/gamma-Al2O 3, Pt/gamma-Al2O3, and Pd/gamma-Al2O2. The results showed that the Ag-modif ied La0.6Sr0.4MnO3-based catalysts with the perovskite-type structure displ ayed the activity significantly higher than that of the supported precious metal catalysts, 0.1%Pd/gamma-Al2O3 and 0.1%Pt/gamma-Al2O3 in the temperatu re range of 370-573 K. Over a 6%Ag/20%La0.6Sr0.4MnO3/gamma-Al2O3 catalyst, the T-95 temperature for methanol oxidation can be as low as 413 K. Even at such low reaction temperature, there were little HCHO and CO detected in t he reaction exit-gas. However, for the 0.1%Pd/gamma-Al2O3 and 0.1%Pt/gamma- Al2O3 catalysts, the HCHO content in the reaction exit-gas reached similar to 200 and similar to 630 ppm at their T-95 temperatures. Over a 6%Ag/La0.6 Sr0.4MnO3 catalyst, the T-95 temperature for ethanol oxidation can be as lo w as 453 K, with a corresponding content of CH3CHO in the exit-gas at 782 p pm; when ethanol oxidation is performed at 493 K, the content of acetaldehy de in the exit-gas can be below 1 ppm. Characterization of the catalysts by X-ray diffraction (XRD), TEM, XPS, laser Raman spectra (LRS), hydrogen tem perature-programmed reduction (H-2-TPR) and oxygen temperature-programmed d esorption (O-2-TPD) methods revealed that both the surface and the bulk pha se of the perovskite La0.6Sr0.4MnO3 played important roles in the catalytic oxidation of the alcohols, and that gamma-Al2O3 as the bottom carrier coul d be beneficial in creating a large surface area of catalyst. Moreover, a s mall amount of Ag+ doped onto the surface of La0.6Sr0.4MnO3 was able to par tially occupy the positions of La3+ and Sr2+ due to their similar ionic rad ii, and thus, became stabilized by the perovskite lattice, which would be i n favor of preventing the aggregation of the Ag species on the surface and enhancing the stability of the catalyst. On the other hand, modification of the Ag+ to the surface of La0.6Sr0.4MnO3 resulted in an increase in relati ve content of the surface O-2(2-)/O- Species highly reactive toward the alc ohols and aldehydes as well as CO. Besides, solution of low-valence metal o xides SrO and Ag2O with proper amounts in the lattice of the trivalent meta l perovskite-type oxide LaMnO3 would also lead to an increase in the conten t of the reducible Mnn+ and the formation of anionic vacancies, which would be favorable for the adsorption-activation of oxygen on the functioning ca talyst and the transport of the lattice and surface oxygen species. All the se factors would contribute to the pronounced improvement of the catalyst p erformance. (C) 2000 Elsevier Science B.V. All rights reserved.