An infrared spectroscopic study of surface reactions on lanthanum sesquioxide and a 1:1 mixture of lanthanum oxychloride and lanthanum sesquioxide

Abstract

Infrared spectroscopy was used to examine the surface properties of catalytic La₂Og₃ and a 1:1 mixture of LaOCl + La₂Og₃. Spectral studies of the dehydration/rehydration of the La(OH)₃/La₂O₃ system revealed the existence of two distinct kinds of bulk hydroxide ions in La(OH)₃, giving rise to infrared bands at 3610 and 3590 cm⁻¹. The former band is preferentially removed during the first stage of dehydration to LaOOH. The latter band, corresponding to the more strongly-bound type of OH⁻, is removed during the second-stage dehydration of the oxyhydroxide to La₂Og₃. No evidence was observed for either bulk or surface hydroxyls on La₂Og₃ above 300°C. CO₂ adsorption on La₂Og₃ results in the formation of unidentate and bidentate carbonate species. Formation of the latter species suggests the existence of anion vacancies on the oxide surface. CO²₃⁻ species formed on La₂Og₃ are completely removed by evacuation at (greater than or equal sign) 350°C. CO₂ adsorption on La(OH)₃ results in preferential hydrogen displacement from the type of surface hydroxide ion whose 0-H stretching band appears at 3590 cm⁻¹. Unidentate carbonate species similar to those observed on La₂Og₃ are formed on La(OH)₃..