STUDY OF LANTHANUM-BASED COLLOIDAL SOLS FORMATION

Lanthanum-based colloidal sols have been studied in order to synthesize lanthanum oxychloride inorganic coatings or thin layers for catalytic applications. The singularity of this process leading to microporous coatings is based on the polymerization of lanthanum acetate species in aqueous solution. Sols are prepared from lanthanum chloride modified by acetate ions. The sol formation mechanism can be explained by La3+ hydrolysis, giving basic species (La (OH)x with x = 1 or 2) which condense and lead to polycondensed hydroxo ions. In the pH range of sol formation, OH-and CH3COO- are ligands competitive towards La3+ ;when acetate ions are present, the condensation rate is limited by lanthanum acetate complexation. Several distributions of lanthanum hydroxide and acetate species are given and related to experimental results (pH, Fourier transform infrared (FTIR) spectroscopy, turbidity and stability of sols, SEM and TEM analysis of coatings). FTIR spectroscopy has been revealed as useful to evidence a polymerization of acetate species in sols leading to translucent gels. These gels allow the preparation of almost fully dense LaOCl coatings after thermal treatment above 450-degrees-C. These results confirm the possible polymerization of lanthanum acetate complexes during the drying step and emphasize the great effect of acetate species on the final material texture.