Preparation and characterization of catalytic lanthanum oxide
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Surface parameters and thermal dehydration/rehydration behavior of the La(OH)3/La2O3 system have been examined with a view toward establishing suitable methods for preparing and characterizing catalytic forms of the rare earth oxides. Relatively nonporous La(OH)3, prepared by hydrolysis of the pure oxide, undergoes thermal dehydration in two distinct stages. A well-defined oxyhydroxide (LaOOH) intermediate, formed by dehydration of the hydroxide at 200 C, decomposes in a second step at 300 C to generate the sesquioxide. A surface carbonate layer on the resulting oxide is only removed completely by subsequent thermal treatment at 700-800 C. Exposure of the oxide to water vapor at <200 C causes complete rehydration to the trihydroxide, rather than mere formation of surface hydroxyl groups, and results in complete recovery of surface area lost by hightemperature sintering. Spectroscopic studies reveal the existence of two structurally dissimilar kinds of bulk hydroxide ions in La(OH)3, differing in their relative extents of hydrogen bonding and giving rise to infrared bands at 3610 and 3590 cm-1. The latter band corresponds to the less numerous and more strongly bound type of OH- that is removed during second-stage dehydration of the oxyhydroxide. 1977.
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