The Rapid Formation of La(OH)_3 from La_2O_3 Powders on Exposure to Water Vapor

摘要

The reactivity of various lanthana powders in air was studied. The materials rapidly hydroxylate to form a stable hydroxide, La(OH)_3, at room temperature. Smaller amounts of an oxy-carbonate species (La_2O_2CO_3) are also observed following air exposure. This oxycarbonate phase is stable to hydroxylation. All oxide materials synthesized here show rapid reactions so that 24 h of exposure to atmosphere is generally sufficient to cause complete hydroxylation at room temperature. The rate of reaction was related to the crystallite size as determined by XRD. The reaction was found to follow a two-stage kinetic process, a relatively slow surface reaction followed by a rapid bulk reaction. The relevance of the reactivity of these powders is discussed.