J.M. Trillo

Activation of rare earth oxide catalysts

Abstract It has previously been shown that even the heaviest rare earth oxides undergo partial conversion to bulk hydroxycarbonates when exposed to air. However, when rare earth oxides are used as catalysts, activation temperatures lower than those necessary to achieve the complete removal of H 2 O and CO 2 are often reported. To elucidate the actual active phases that operate in catalytic processes involving rare earth oxides, a systematic temperature-programmed desorption study of La 2 O 3 , Sm 2 O 3 and Yb 2 O 3 was carried out. Our results lend support to a particle structure for the activated powder “oxides” consisting of an oxycarbonate nucleus surrounded by an outer oxide layer.

A TPD technique for nonrestrictive kinetic studies

A TPD technique is described in which mass spectrometic detection and normal pressure of the carrier gas are employed. Using the thermal decomposition of MgCO3 as a test reaction, it is proved that kinetic studies of each desorbed product can be realized through the whole signal-time curve in a nonrestrictive way.AbstractОписывается техника ТПД с использованием MC детектирования и газа-носителя при нормальном давлении. Используя термическое разложение MgCO3 в качестве пробной реакции, было доказано, что кинетические исследования каждого десорбирующегося продукта могут быть проведены на основе всей кривой сигнал-время совсем не ограниченным путем.

Characterization of a lutetia-alumina catalyst by CO2 adsorption

Abstract Lutetia supported on X-alumina remarkably enhances its surface Lewis basicity strength, according to XPS and carbon dioxide volumetric adsorption measurements. From these data, a CO 2 /exposed Lu(III) ratio of almost 1:1 was estimated for the supported oxide activated at 773K, compared with 1:20 for the non supported one. The 10% alumina-supported lutetia interacts with gaseous CO 2 /H 2 mixtures at 625K leading to the formation of formate species, detected by FT-IR spectroscopy, an active intermediate for the conversion of synthesis gas towards oxygenated products.

Effect of the precursor salt on the reactivity of Lu2O3

Abstract The influence of the precursor salt on the surface behaviour of some Lu2O3 samples has been studied. They have been prepared by thermal decomposition of lutetium oxalate, basic carbonate, and hydroxide, and studied by means of TG, DTA, X-ray diffraction, SEM, SBET measurements and FT-IR spectroscopy. The surface reactivity of Lu2O3 samples obtained via hydroxide and carbonate is greater than that observed for the oxide obtained via the oxalate, as studied by the influence of thermal treatments on their hydration capacities and on their specific surface areas. The differences between the crystalline structures of the precursor salt and the final oxide satisfactorily accounted for the different surface behaviours of the oxides studied.

Hydration of Sm2O3 under mild conditions

The influence of pressure and temperature on Sm2O3 hydration, under mild conditions, are examined. The data and conclusions are referred to previous ones for Yb2O3. At the activation temperatures usually applied, as reported in the literature, to rare earth sesquioxides handled in air, 800–850 K, a non-pure-oxide phase is attained in the case of Sm2O3. Its complete dehydration is achieved at 823 K, but temperature programmed decomposition and Fourier transform-IR data lend support to the formation at this temperature of a dioxymonocarbonate phase which is transformed into the sesquioxide phase above 1100 K. Contrary to the finding for Yb2O3, hydration in bulk Sm2O3 has been observed over the whole pressure range 133–2666 Pa. In the absence of CO2, the temperature of evolution of H2O from bulk-hydrated Sm2O3, about 600 K, is considerably higher than that previously reported for Yb2O3, 440 K, following the cation polarizing pattern for the decomposition of metal hydroxides and oxosalts.

Influence of the method of preparation on Yb2O3 hydration

Preliminary studies have shown that in order to achieve the complete removal of the uptaken CO2 and H2O from a ytterbium oxide aged in air, thermal activation up to 1173 K is required. Parallel studies have shown that bulk hydration of Yb2O3 does occur, the CO2 acting as a kinetic inhibitor. The above results refer to a Yb2O3 prepared in our laboratory from hydroxicarbonate-like phases. In the present work, the influence of the preparation method on the hydration process is examined, by extending the study to two additional samples, a standard commercial one and another obtained by calcining a ytterbium oxalate. It is shown that the intensity of the hydration reaction depends strongly on the method of preparation. Certain applications of 4f oxides, such as their use as transition metal supports, involve thermochemical processes which may alter drastically the initial texture of the ytterbia from the oxalate. It makes this sample less useful although its initial specific surface area is high.

Lanthanide oxides: preparation and ageing

La2O3, CeO2, Sm2O3, Dy2O3, and Yb2O3 were obtained by calcining in air phases prepared by precipitation with ammonia from solutions of the corresponding nitrates. During the calcination to oxides, under the experimental conditions used, the nitrate ions are removed to an extent which could not be detected by temperature programmed desorption–mass spectrometry or i.r. spectroscopy, contradictory to the established literature. For comparative purposes, Yb2O3 has also been prepared by thermal decomposition of oxalate. The Brunauer–Emmett–Teller surface area was always lower for the oxide obtained from oxalate. This was so even after thermal treatment at 1 173 K. Independent of the preparation method, the heaviest 4f oxides undergo partial conversion to bulk carbonate hydroxides when exposed to air. According to our results and those reported in the literature, lanthanide oxides, when used as catalysts, are often activated at temperatures lower than those necessary for the complete elimination of CO2 and H2O.

Lanthanide oxides: Lu2O3 hydration☆

Abstract The hydration of diverse Lu2O3 samples, prepared from three different precursor salts, has been studied at temperatures up to 350 K and within the atmospheric range of pressures. The results are compared with the ones previously reported for Sm2O3 and Yb2O3. The Lu2O3 shows a peculiar reactivity towards H2O, regarding the one corresponding to the heavy rare earth sesquioxides. This is explained on the basis of the polymorphism of the Ln(OH)3 trihydroxides.

Study of Lanthanum Local Structure in Montmorillonite

SUMMARY: The local structure of the interlamellar lanthanide ions in montmorillonite upon heating is studied. The analysis of the EXAFS data has been carried out using two model environments, that of La3+ aquocomplex and that of the lanthanum oxide. The information thus obtained is crucial in elucidating the intercalation mechanism and demonstrates the important role that XAFS techniques can play in the knowledge of the location and environment of the exchangeable ions in layered silicates.

Influence of the textural properties on the catalytic activity of 4f oxides

Abstract Data corresponding to the catalytic decomposition of 2-propanol over La 2 O 3 , Eu 2 O 3 and two samples of Yb 2 O 3 (denoted (a) and (b)) are reported. Yb 2 O 3 (a) was prepared by the same method as La 2 O 3 and Eu 2 O 3 . Yb 2 O 3 (b), which was obtained by grinding Yb 2 O 3 (a), showed quite different textural properties from Yb 2 O 3 (a). The specific activity, with reference to 1 m 2 of the Brunauer-Emmett-Teller surface area, towards both dehydrogenation and dehydration of the alcohol is found to be about 12 times higher for Yb 2 O 3 (b) than for Yb 2 O 3 (a). When the specific activities of the three 4f oxides prepared in the same way are compared, the differences are smaller than those observed between Yb 2 O 3 (a) and Yb 2 O 3 (b). On the basis of these results some correlations relating to the recently reported catalytic behaviour of lanthanide oxides are discussed.