Structural properties of Pd catalysts supported on Al2O3-La2O3 prepared by sol-gel method

The structure of palladium catalysts (0.3 wt% Pd), supported on sol-gel alumina-lanthana mixed oxides, was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS), The investigation of the structure of lanthana-promoted Pd catalysts, prepared by different methods, is of great importance for the improvement of palladium-only three-way catalysts, The data obtained show the presence of a crystalline phase, different from alumina, lanthana or alumina-lanthana reported compounds, in Al2O3-La2O3 and Pd/Al2O3-La2O3. Attempts to assign this crystalline phase to a known one were unsuccessful. According to XPS data, in lanthanum-containing samples, the state of lanthanum is different from that in La2O3. After calcination in air at 1273 K for 3 h this phase vanishes and large crystals of LaAlO3, metastable for studied composition, appear. The sol-gel preparation method, in comparison with traditional preparation methods, leads to stabilization of LaAlO3 phase under the considered conditions and the increase of the stability of gamma-Al2O3 phase at high temperature. The improvement of the catalytic activity at medium temperature and the increase of the selectivity to NH3 at high temperatures for lanthanum-promoted Pd catalyst are ascribed to a lanthanum-containing phase. The increase in NH3 production decreases its practical value for an NO reduction catalyst. (C) 1998 Elsevier Science B.V. All rights reserved.