Spectroscopic study of hydration and carbonation effects of La2O3-based materials

Abstract

One of the main applications of Lanthanum oxide is as support of catalysts for methane oxidation. In this field, a great effort has been made in establishing the thermal evolution of La2O3 and La2O3-related phases, such as La(OH)3 and La carbonates, in varying atmospheric conditions. The goal of these studies is twofold: to predict, on the one hand, the aging behavior of the catalysts, and to test the conditions yielding the best performance. In this work we have focused in the identification of phase content and thermal mechanisms involving hydration and carbonation effects of La2O3-based materials by means of Raman spectroscopy. It is known that, in air, La2O3 hydrates easily to form La(OH)3 together with small amounts of Lanthanum carbonates. The thermal evolution of La(OH)3, however, is highly dependent upon environmental conditions, such as presence of humidity and CO2 partial pressure, as well as on the heating rates and retention times at each temperature. In vacuum La(OH)3 evolves to LaOOH and this phase dehydrates first to a mixture of C- and ALa2O3 until, upon further heating, only A-La2O3 phase remains. Experiments suggest that A phase forms directly from La(OH)3 whereas C phase forms preferably from LaOOH. In air, the path of La(OH)3 dehydration to A-La2O3 is a function of heating rates: the first dehydration step yields LaOOH, as in vacuum, but under slow heating this oxyhydroxide is not allowed to decompose to La2O3. Instead, carbonation occurs, resulting in a full conversion of LaOOH to several types of oxycarbonates, usually denoted as I, Ia and II-La2O2CO3. Thermal stability of these phases increases in the sequence I-Ia-II, so that at 670 ºC only II-La2O2CO3 remains. In our in-situ experiments II-La2O2CO3 converts to La2O3 at 700 ºC.