Microstructural Invesigation of CeO2 formation in the Pd0.97Ce0.03 Alloy V.M. AZAMBUJA1, D.S. DOS SANTOS1,L. PONTONNIER2, M. MORALES2, D. FRUCHART2

(1)PEMM-COPPE/UFRJ, P.O. Box 68505, 21945-970 Rio de Janeiro, RJ, Brazil (2)Laboratoire de Cristallographie, CNRS BP 166, 38042 Grenoble Cedex 09, France

When small amounts of metallic alloying elements, which have interact strongly with oxygen (such as Al, Zr, Ce, Cu), are inserted into the matrix of noble metals or alloys, such as Pd, Ag, it is possible to perform treatments to obtain ceramic-metals composite materials[1]. This occurs when these alloys are exposed to an oxidizing atmosphere and high temperature for a long time, sufficient to permit the oxygen diffuse through the metallic matrix. These treatments generate oxide precipitates by internal oxidation of these solute elements. Such oxides can contribute significantly to the improvement of the chemical and physical properties of these alloys, as well as promoting an increase in hydrogen solubility [2]. Excellent catalytic properties may also result in such alloys, as are observed for the widely-used Pd-ceramic composite catalysts [3]. The aim of this work is to study the morphology and microstructure of an internally oxidized Pd0.97Ce0.03 alloy, and the formation of the ceramic precipitates by TEM and X-ray diffraction analysis. A button containing 5 g of the Pd0.97Ce0.03 alloy was prepared by arc melting under argon from pure elements (99.99% purity). The button was re-melted several times to homogenize the alloy. The product of the melt was cold-rolled to a foil of about 100 µm in thickness. Subsequently, the alloy was oxidized in air at 1073 K for 72 h to promote internal oxidation, and to guarantee to complete the formation process of the oxide precipitates. The microstructure of the internally oxidized alloy was analyzed by TEM, using a Philips CM 300 microscope operating at 300 kV and equipped with an energy disperse X-ray spectrometry for microanalysis (EDS), λ = 0.0196 Å. TEM revealed the precipitation of acicular cerium oxide in the Pd matrix. Figure 1 shows a high resolution micrograph and a selected area electron diffraction pattern corresponding to the Pd/oxide interface. The rotation of the electron diffraction patterns presented was calibrated using MoO3. The diffraction pattern in figure 1, obtained for a precipitate with [011] zone axis parallel to the electron beam, shows its relationship to the superimposed fcc Pd lattice with its [001] zone axis parallel to the beam, revealing the coherence between the cerium oxide precipitate and the palladium matrix. The diffraction pattern shows an angle of 10° between a* Pd and [ _

111 ]* of Ce oxide. This difference between palladium [020] direction and the cerium oxide [ _

111 ] direction is also observed in the high resolution figure (HRTEM). This is confirmed that the values and angle are observed for the d020 of palladium and d __

111 of CeO2, both of which are close to the know values [4]. The cerium oxidation can occur with different oxide stochiometries, the more common of which are: CeO; CeO2; Ce2O3 e Ce7O12 [4]. However, by electron diffraction, it was possible to index the precipitate as being cubic with the lattice parameter 5.4 Å, which correspond to CeO2 with the zone axis [011]. This was confirmed by the EDS analyses. The [

_

111 ]* direction of CeO2 was found to be alligned with the [a*] axis of the palladium lattice within an average orientation difference of 10º, this suggests a degree of coherence between the precipitates and the palladium matrix. Figure 2 shows arrangement of precipitates of cerium oxide observed precipitated along the (111) planes of the palladium matrix. The needle-shaped precipitates are located along the [110] direction of the palladium sublattices. This suggests the adjustment between lattice parameter of palladium matrix (3.89 Å) and CeO2 precipitate (5.4 Å) such is observed in the schematic diagram of figure 2. References [1]- X. Huang, W. Mader, R. Kirchheim, Acta Metallurgica Materialia, vol. 39, pp.893-907, 1991.

[2]- V.M. Azambuja, D.S. Dos Santos, L. Pontonnier, S. Miraglia, D. Fruchart, Journal of Alloys and

Compounds, vol. 346 (1-2), pp.142-146, 2002.

[3]- C.S. Satterfield, “Heterogeneous Catalysis in Industrial Practice”, 2ª edition, McGraw-Hill, 1991.

[4]- P. Villars, Pearson’s Handbook, Desk Edition Crystallographic Data for Intermetallic Phases, ASM,

1995.

Figure 1: The high resolution micrograph and a selected area electron diffraction pattern corresponding to the

Pd/oxide interface.

Figure 2: Arrangement of precipitates of CeO2 observed along the (111) planes of the Pd matrix and the schematic

diagram of the adjustment between lattice parameter of palladium matrix and CeO2 precipitate.

Pd- 3,88 Å

CeO2 = 5,37 Å

60°

Matrix Pd zone axis [001] Precipitate CeO2 zone axis

b*Pd

a*Pd

c*CeO2// [220]Pd

[111]CeO2

[111]CeO2

100 nm