Pd/Al2O3-La2O3 catalysts prepared by sol–gel: characterization and catalytic activity in the NO reduction by H2

The 0.3%Pd/Al2O3-La2O3 (0, 6 and 15 wt.% La2O3) catalysts prepared by the sol–gel method were studied by N2 physisorption, X-ray diffraction (XRD), small angle X-ray scattering (SAXS), and temperature-programmed reduction (TPR). The catalysts were tested in the reduction of NO by H2. Results indicate that the texture and structure of Pd catalysts are modified by the addition of lanthana. The TPR profiles of the catalysts show reduction peaks at 100 and 150°C, suggesting that the reduction of crystalline PdO occurs in two stages: first to Pd2O, and then to Pd0. After recalcination of the samples, the reducibility properties are modified. One reduction peak appears at 360°C that can be assigned to the presence of a more stable surface PdO phase interacting strongly with lanthana species. The presence of metallic Pd and Pd3La alloy species in the Pd/Al2O3-La2O3 catalysts was detected after reduction at high temperature. These species present particle sizes of 77 and 130 Å. Pd3La alloy species are present also as crystallites of 20 Å. 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 lanthana reduced species. For the Pd/Al2O3-La2O3 catalysts, the selectivity to N2 is high at temperatures lower than 250°C. The strong interaction between Pd and lanthana species, forming Pd3LaOx species, may be responsible for the increase in activity and selectivity to N2.