Pd/Al_2O_3-La_2O_3 catalysts prepared by sol-gel:characterization and catalytic activity in the NO reduction by H_2

摘要

The 0.3%Pd/Al_2O_3-La_2O_3 (0,6 AND 15wt.% La_2O_3) catalysts prepared by the sol-gel method were studied by N_2 physisorption,Xpray diffraction(XRD),small angle X-ray scattering (SAXS),and temperature-porogrammed reduction(TPR).The catalysts were tested in the reduction of NO by H_2.Results indicate that the texture and structure and structure of Pd catalysts are modified by the addition of lanthana.The TPR profiles of the catalysts show reduction peaks at 100 and 150degC,suggesting that the reduction of crystalline PdO occurs in two stages:first to Pd_2O,and then to Pd~0.After recalcination of the samples,the reducibility properties are modified.One reduction peak appears at 360degC tha tcan be asigned to the presence of a more stable surface PdO phase interacting strongly with lanthana species.The presence of metallic Pd and Pd_3La alloy species in the Pd/Al_2O_3-La_2O_3 catalysts was detected after reduction at high temperature.These species present particle sizes of the catalysts inlfuences the activity in the NO reduction by hydrogen.The selectivity depends on the extent of interaction between Pd and lanthana reduced species.For the Pd/Al_2O_3-La_2O_3 catalysts,the selectivity to N_2 is high at temperatures lower than 250degC.The strong interaction between Pd and lanthana species,forming Pd_3LaO_x species,may be responsible for the increase in activity and selectivity to N_2.