Effects of rhodium and platinum on the reactivity of lanthanum phases

doi:10.1016/j.matchemphys.2004.04.017

Materials Chemistry and Physics

Volume 86, Issues 2–3, August 2004, Pages 440-447

https://doi.org/10.1016/j.matchemphys.2004.04.017 Get rights and content

Abstract

Rhodium and platinum catalysts were prepared by wet impregnation of Anedra lanthanum oxide. The polymorph forms of La₂O₂CO₃ have also been prepared by an appropriate treatment of lanthanum compounds in carbon dioxide. The characterisation of the phases present in these solids was performed by XRD, FT-IR, LRS, TPD and TGA. One of the untreated supports (Aldrich) was La(OH)₃, and the other (Anedra) consisted mainly of II-La₂O₂CO₃ with traces of type Ia oxycarbonate and lanthanum hydroxide. Catalysts with a low rhodium loading showed an increase of Ia-La₂O₂CO₃ already present on the support, whereas those with a low platinum loading would not modify the phases of the lanthanum solid. A different behaviour was observed in high metal loading catalysts. In this case, metals would catalyse the decomposition of type Ia polymorph and also prevent the hydration of the support. The treatment with CO₂ provides additional information about the effect of the gas upon the metal containing solids. Pt would favour the carbon dioxide interaction with the support to form monoclinic polymorph (type Ia), while rhodium would prevent the further formation of the oxycarbonate phase.

Introduction

Numerous applications of lanthanum oxide have been recently found in various fields. In catalysis, it is used as support for metals active for different reactions. Due to its refractory nature, it is an important ingredient in the manufacturing of superconductors and ceramics. La₂O₃ is also used in calcium lights, optical glass, and in the formation of ceramic cores for carbon arc electrodes [1].

The simple manipulation of lanthana in air induces its hydration and its carbonation in bulk. Therefore, carbonation should not be disregarded in the preparation of metal/La₂O₃ solids. Lanthanum oxide carbonates exist in three crystalline modifications (I, Ia and II); the three polymorphs hold in an arrangement of (La₂O₂²⁺) layers separated by CO₃²⁻ ions. Type I has square layers and is tetragonal, while type Ia is described as a monoclinic distortion of form I [2]. II-La₂O₂CO₃ is completely indexed on the hexagonal unit cell. An exact description of this polymorph is given by Olafsen et al. [3]. Improved understanding of these materials is of importance in relation to the use of lanthanum oxides as catalyst supports in CO₂-containing atmospheres.

The incorporation of noble metals to the lanthanum oxide matrix induces phase transformations and catalytic chemical reactions between gas phase components and the solid.

On the other hand, it is well known that the physicochemical properties of the support strongly affect the catalytic behaviour of the metal on oxide materials [4], [5].

The goal of this study was to characterise the structure of solids obtained when either Rh or Pt are supported on lanthanum oxide and are subjected to different heat treatments in the presence of several gas atmospheres. XRD, FT-IR, LRS; TPD and TGA were the main tools used to achieve this goal.

Section snippets

Preparation methods

To prepare II-La₂O₂CO₃, La₂O₃ (Anedra) was heated under flowing dry CO₂ using a temperature program consisting of a linear 5 K min⁻¹ ramp from 300 to 923 K, a linear 2 K min⁻¹ ramp from 923 to 973 K, and finally an isothermal plateau at 973 K for 30 min [6]. The Ia-La₂O₂CO₃ was obtained from the Aldrich La₂O₃ (99.99% Gold Label). The oxide was heated at 10° min⁻¹ from 298 to 773 K, under flowing He. The flow was then switched to dry CO₂ and the temperature kept at 773 K for 1 h.

Metal deposition on the

The support

The XRD pattern of the Aldrich support exposed to ambient conditions indicates that La(OH)₃ is the only phase present in this solid (Fig. 1a). Two phases, La(OH)₃ (ASTM No. 6-585) and La₂O₂CO₃ (ASTM No. 37-804), were identified in the lanthanum oxide used as support (Anedra).

Table 1 shows the IR band assignments. The FT-IR spectrum of La₂O₃ (Aldrich) is shown in Fig. 2a. A sharp band at 3608 cm⁻¹ (not shown) and a broad band at 643 cm⁻¹ are observed. They are characteristic of stretching and

Lanthanum phases

A variety of compounds may coexist with the lanthanum oxide exposed to different gases. The actual composition of the solid depends on the preparation method and the pretreatment. There is also a particular sensitivity to the presence of H₂O and CO₂ in the gas phase [12]. La₂O₃ was shown to undergo hydration and carbonation. This latter process was found to involve the formation of a hydroxycarbonate material on the outer layers of the bulk oxide. Fleisch et al. [13] found that on exposure to

Conclusions

•
The initial phase composition as well as the heating treatment and atmosphere are crucial in determining the reactivity of lanthanum solids.
•
A low rhodium loading would increase the amount of type Ia polymorph on the support, whereas a low platinum loading would not modify those phases.
•
A high metal loading would catalyse the decomposition of type Ia-La₂O₂CO₃ and prevent the hydration of the lanthanum support.
•
Pt would favour the carbon dioxide interaction with the support to form monoclinic

Acknowledgements

The authors wish to acknowledge the financial support received from UNL, CONICET and ANPCyT. They are also grateful to the Japan International Cooperation Agency (JICA) for the donation of the major instruments used in this study. Thanks are finally given to Prof. Elsa Grimaldi for the edition of the English paper.

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Effects of rhodium and platinum on the reactivity of lanthanum phases

Abstract

Introduction

Section snippets

Preparation methods

The support

Lanthanum phases

Conclusions

Acknowledgements

J. Solid State Chem.

J. Catal.

J. Catal.

J. Catal.

Appl. Catal.: A Gen.

Appl. Catal.

J. Catal.

Powder Technol.

Thermochem. Acta

Thermochim. Acta

Thermochim. Acta

J. Catal.