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-ge l alumina-lanthana mixed oxides, was characterized by X-ray diffractio n (XRD), scanning electron microscopy (SEM), high-resolution transmiss ion electron microscopy (HRTEM) and X-ray photoelectron spectroscopy ( XPS), The investigation of the structure of lanthana-promoted Pd catal ysts, prepared by different methods, is of great importance for the im provement of palladium-only three-way catalysts, The data obtained sho w the presence of a crystalline phase, different from alumina, lanthan a or alumina-lanthana reported compounds, in Al2O3-La2O3 and Pd/Al2O3- La2O3. Attempts to assign this crystalline phase to a known one were u nsuccessful. According to XPS data, in lanthanum-containing samples, t he state of lanthanum is different from that in La2O3. After calcinati on in air at 1273 K for 3 h this phase vanishes and large crystals of LaAlO3, metastable for studied composition, appear. The sol-gel prepar ation method, in comparison with traditional preparation methods, lead s to stabilization of LaAlO3 phase under the considered conditions and the increase of the stability of gamma-Al2O3 phase at high temperatur e. The improvement of the catalytic activity at medium temperature and the increase of the selectivity to NH3 at high temperatures for lanth anum-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 reser ved.