THERMAL INDUCED EVOLUTION OF CHLORINE-CONTAINING PRECURSORS IN IMPREGNATED PD AL2O3 CATALYSTS/

The results of a combined study using diffuse reflectance spectra, tem perature-programmed reduction, and extended X-ray absorption-edge fine structure of chlorine-containing precursors formed during preparation of Pd/Al2O3 catalysts are reported. The catalyst dried (385 K) after impregnation contains both PdCl42- species adsorbed electrostatically (about 60-70%) and strongly interacted PdClxOy species. Calcination at 775 and 925 K results in decomposition of the initial impregnated spe cies, formation of highly distorted PdClxOy structures, and nucleation of disordered PdO clusters. Growth of large PdO particles occurs duri ng calcination at 1075 K. The reduction of the oxide phase is more fac ile than that af chlorinated species in intimate interaction with alum ina. The concept of geometrical constraints during adsorption of squar e planar PdCl42- species on various configurations of positively charg ed hydroxyl groups on different crystal planes of alumina is discussed . It is proposed that electrostatic adsorption is favored on (100) and (110) planes, where the adsorbate matches the configuration-of opposi tely charged surface sites. The misfit between adsorbed species and su rface hydroxyls on the (111) planes leads to ligand-substituted specie s.