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 structure were prepared by using a citric acid sol–gel method, and their catalytic performance for complete oxidation of methanol and ethanol was evaluated and compared with that of the γ-Al2O3-supported catalysts, Ag/γ-Al2O3, Pt/γ-Al2O3, and Pd/γ-Al2O3. The results showed that the Ag-modified La0.6Sr0.4MnO3-based catalysts with the perovskite-type structure displayed the activity significantly higher than that of the supported precious metal catalysts, 0.1%Pd/γ-Al2O3 and 0.1%Pt/γ-Al2O3 in the temperature range of 370–573 K. Over a 6%Ag/20%La0.6Sr0.4MnO3/γ-Al2O3 catalyst, the T95 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 the reaction exit-gas. However, for the 0.1%Pd/γ-Al2O3 and 0.1%Pt/γ-Al2O3 catalysts, the HCHO content in the reaction exit-gas reached ∼200 and ∼630 ppm at their T95 temperatures. Over a 6%Ag/La0.6Sr0.4MnO3 catalyst, the T95 temperature for ethanol oxidation can be as low as 453 K, with a corresponding content of CH3CHO in the exit-gas at 782 ppm; when ethanol oxidation is performed at 493 K, the content of acetaldehyde 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 temperature-programmed reduction (H2-TPR) and oxygen temperature-programmed desorption (O2-TPD) methods revealed that both the surface and the bulk phase of the perovskite La0.6Sr0.4MnO3 played important roles in the catalytic oxidation of the alcohols, and that γ-Al2O3 as the bottom carrier could be beneficial in creating a large surface area of catalyst. Moreover, a small amount of Ag+ doped onto the surface of La0.6Sr0.4MnO3 was able to partially occupy the positions of La3+ and Sr2+ due to their similar ionic radii, and thus, became stabilized by the perovskite lattice, which would be in 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 relative content of the surface O22−/O− species highly reactive toward the alcohols and aldehydes as well as CO. Besides, solution of low-valence metal oxides SrO and Ag2O with proper amounts in the lattice of the trivalent metal perovskite-type oxide LaMnO3 would also lead to an increase in the content of the reducible Mnn+ and the formation of anionic vacancies, which would be favorable for the adsorption-activation of oxygen on the functioning catalyst and the transport of the lattice and surface oxygen species. All these factors would contribute to the pronounced improvement of the catalyst performance.