Pd/Al2O3-La2O3 catalysts prepared by sol-gel: characterization and catalytic activity in the NO reduction by H-2

The 0.3%Pd/Al2O3-La2O3 (0, 6 and 15 wt.% La2O3) catalysts prepared by the s ol-gel method were studied by N-2 physisorption, X-ray diffraction (XRD), s mall angle X-ray scattering (SAXS), and temperature-programmed reduction (T PR). The catalysts were tested in the reduction of NO by H-2. Results indic ate that the texture and structure of Pd catalysts are modified by the addi tion of lanthana. The TPR profiles of the catalysts show reduction peaks at 100 and 150 degreesC, suggesting that the reduction of crystalline PdO occ urs in two stages: first to Pd2O, and then to Pd-0. After recalcination of the samples, the reducibility properties are modified. One reduction peak a ppears at 360 degreesC that can be assigned to the presence of a more stabl e surface PdO phase interacting strongly with lanthana species. The presenc e of metallic Pd and Pd3La alloy species in the Pd/Al2O3-La2O3 catalysts wa s detected after reduction at high temperature. These species present parti cle sizes of 77 and 130 Angstrom. Pd3La alloy species are present also as c rystallites of 20 Angstrom. The modification of the reducibility properties of the catalysts influences the activity in the NO reduction by hydrogen. The selectivity depends on the extent of interaction between Pd and lanthan a reduced species. For the Pd/Al2O3-La2O3 catalysts, the selectivity to N2 is high at temperatures lower than 250 degreesC. The strong interaction bet ween Pd and lanthana species, forming Pd3LaOx species, may be responsible f or the increase in activity and selectivity to N-2. (C) 2001 Elsevier Scien ce B.V. All rights reserved.