Thermal genesis course and characterization of lanthanum oxide

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Abstract

La(NO3)3·6H2O was used as a precursor to produce La2O3 at 650°C in an atmosphere of air. Thermal processes occurred were monitored by means of thermogravimetry, differential thermal analysis, and gas-mass spectrometry. Infrared (IR)-spectroscopy, X-ray diffractometry and scanning electron microscopy characterized the intermediates and final solid products. The results showed that, La(NO3)3·6H2O decomposes through nine endothermic weight loss processes. Five dehydration steps occurred at 90, 105, 150, 175 and 215°C, leading to the formation of crystalline nitrate monohydrate, which decomposes to La(OH)(NO3)2 at 410°C. The latter, decomposes to La2O3 at 640°C, via two different intermediates; LaO(NO3) at 440°C, and non-stoichiometric unstable, La(O)1.5(NO3)0.5 at 570°C. The gaseous decomposition products as identified by gas-mass spectroscopy were water vapor, nitric acid and nitrogen oxides (NO, NO2 and N2O5). The final product La2O3 has a large crystalline containing pores, voids and cracks, with a surface area of 23 m2 g−1. Also it possessed Lewis acidic and basic sites, as indicated by Pyridine adsorption.

Introduction

Lanthanum oxide (La2O3) is known [1] to crystallize in a hexagonal structure at 2050°C [2]. It has diverse industrial and technological applications. It is used as an important component in automobile exhaust-gas convectors [3], [4]; as catalyst support [4], [5] in the formation of methanol [6]; as a refractory oxide for calcium lights and optical glass [7] in the formation of ceramics as a core for carbon-arc electrode [7].

Several studies [8], [9], [10], [11], [12], [13], [14] and a review article [15] have been published concerning with the thermal decomposition of rare earth metal salts. It was reported [15] that, rare earth metal oxides obtained from nitrate or acetate precursors leads to higher surface area compared with those obtained from oxalate precursors. Patil et al. [8] reported that, the formation of M2O3 from rare earth nitrates takes place via the oxy-nitrate (MONO3) with a low energy of activation. They also reported the possible formation of anhydrous nitrate, while Wendlandt and Bear [9] stated that the anhydrous nitrates are unstable. Stewart and Wendlandt [10] studied the decomposition of La(NO3)3·6H2O. They reported that, the dehydration takes place at 105°C to give La(NO3)3·3H2O, which decomposes at 200°C to form a basic nitrate; and at 500°C, La2O3 was the final product. Recently, the decomposition of Y(NO3)3·5H2O, was studied [11]. It was reported that a thermally stable monohydrate was formed. Also a mixture of Y(NO3)3 and Y(OH)(NO3)3 were obtained at 270°; and YONO3 was detected by XRD as crystalline at 325°C. Y2O3 was the final product at 500°C. The texture analysis by nitrogen adsorption method and scanning electron microscopy (SEM) [12], revealed that Y2O3 obtained from Y(NO3)3·5H2O at 500°C has a higher surface area (SBET=58 m2 g−1) compared with that obtained at 700°C (SBET=20 m2 g−1). They attribute the lower surface area as results of sintering.

Hussein et al. [13], [14] have studied the decomposition course of Th(NO3)4·5H2O [13] and Dy(NO3)3·6H2O [14] in air by thermogravimetry (TG), differential thermal analysis (DTA), infrared (IR), X-ray diffractometry (XRD) and SEM. They reported that, the unhydrous nitrates are thermally unstable and the decompositions gave ThO2 at 300°C and Dy2O3 at 600°C, respectively. Different intermediates of non-stoichiometric oxy-nitrates were also detected. The DyONO3 is crystalline and thermally stable, compared to ThO(NO3)2 which is unstable and amorphous.

The surface acidity measured by pyridine adsorption [16] for La2O3 obtained from La-acetate, indicated that La2O3 contains two types of Lewis acid sites of different strengths, as well as surface hydroxyl groups; most of these hydroxyls are anionic in nature.

The present investigation, is intended to characterize the thermal decomposition course of La(NO3)3·6H2O to the onset of La2O3 formation. The oxide formed at 650°C was subjected to electron optical method (SEM), surface area measurements by N2 adsorption and surface acidity by pyridine adsorption.

Section snippets

Materials

La(NO3)3·6H2O, abbreviated as LaNit, was 99.9% pure (Merck, Germany). It was used as received. Its calcination products were obtained by heating at various temperatures (150–800°C) for 1 h in a static air. The calcination temperatures were chosen on bases of the thermal analysis results. Prior to analysis, the calcination products were kept dry over Silica Gel. For simplicity, these products are donated in the text by LaNit, followed by the calcination temperature. Thus LaNit-400 indicates

Lanthanum nitrate hexahydrate, LaNit, La(NO3)3·6H2O

The DTA curve (Fig. 1) shows an endothermic weight invariant process located at 56°C. A direct visual observation revealed that, the material melts at 54°C. Therefore, process I is due to melting.

Conclusion

The thermal decomposition of LaNit in atmosphere of air involves the following pathways:

La(NO3)3·6H2O
90°→La(NO3)3·5H2O
105°→La(NO3)3·4H2O
150°→La(NO3)3·3H2O
175°→La(NO3)3·2H2O
215°→La(NO3)3·H2O
410°→La(OH)(NO3)2
440°→LaO(NO3)
570°→LaO1.25(NO3)0.5
640°C→La2O3

The monohydrate nitrate of La- is thermally stable, while the unhydrous of La is unstable. LaONO3 was formed as a stable and crystalline intermediate.

A non-stoichiometric form of oxynitrate was detected as an unstable intermediate.

The gases detected

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