J. MATER. CHEM., 1994, 4( 12j, 1927-1928 MATERIALS CHEMISTRY COMMUNICATIONS Behaviour of Ceria under Hydrogen Treatment: Thermogravimetry and in sifuX-Ray Diffraction Study C. Lamonier, G. Wrobel" and J. P. Bonnelle Laboratoire de Catalyse Heterogene et Homogene, URA CNRS No 402, Universite des Sciences et Technologies de Lille, 59655 Villeneuve d'Ascq Cedex, France Ceria reduction under hydrogen leads to a fluorite lattice expansion between 573 and 1073 K. Both bulk reduction of Ce4' ions and insertion of hydrogen are thought to be responsible for this expansion in the 573-843 K temperature range. Cerium oxide, CeO,, crystallizes in a fluorite structure in which each cerium ion is coordinated by eight oxygen neigh- bours. When treated in a reducing atmosphere, CeO, is known to form non-stoichiometric oxides, CeO, -x.These oxides upon exposure to oxidizing environment, even at room tem- perature, can be reoxidized to CeO,. These properties make CeO, a very interesting component of catalysts in the treat- ment of automotive exhaust gas, as well as in hydrogenation or other oxidation reactions. Numerous experimental tech- niques have been used to study the interaction of reducing agents such as hydrogen with CeO,: gravimetry, H NMR,' in situ magnetic susceptibility, temperature-programmed reac-tions (TPR, TPO).2p4 Most authors reported the presence of two main peaks of hydrogen consumption at 773 or 853 K and above 1073 K in TPR. Several crystallographic studies of the Ce-0 system have also been made.2~5~6 Nevertheless, none of them was carried out under reducing conditions.Our purpose is to characterize the evolution of ceria during the reduction of the oxide under hydrogen by using both thermo- gravimetry and in situ X-ray diffraction (XRD) in the 300-1073 K temperature range. A cerium hydroxide gel was prepared by dropwise addition of a 0.5 mol dm-3 cerium nitrate solution to an excess of hydroxide potassium solution (1.5 mol dmP3) with constant stirring. The resulting precipitate was washed with hot distilled water, dried overnight at 373 K and calcined at 573 K for 4 h. XRD showed the presence of crystalline CeO, with the fluorite structure. The BET surface area was 135 m2 g-'. Reduction experiments were carried out under hydrogen with a Sartorius microbalance connected to a flow gas system for the gravi- metric measurements and in an Anton Paar chamber adapted to a Siemens D5000 diffractometer for the XRD in situ analysis.The heating and flow rates chosen were 100K h-' and 5 dm3 h-', respectively. Fig. 1 shows the weight loss of the CeO, sample when in contact with H, as a function of temperature (black curve). The derivative curve (in grey) exhibits three inflexion points at 573, 736 and 863 K. Before 473 K, the weight variation is attributed to the loss of chemisorbed water. The reduction is performed up to 1173 K and no plateau is attained. Three diffraction patterns are reported in Fig. 2 of ceria heated to 1073 K under various atmospheres.Owing to the use of a platinum holder, spectra present platinum diffraction peaks. In Fig. 2(u) the diffraction lines correspond to CeO,. When ceria is reduced under pure hydrogen, neither the hexagonal Ce203 nor the rhombohedra1 Ce01,82 phase reported by Bevan5 is formed, even at 1073 K. The whole spectrum has the same trend as in Fig. 2(u);however, each ceria diffraction line is shifted towards lower angles from Fig. 2(a) to (b), corresponding to a lattice expansion of the Or 10 --2 h v -82 -4--0 E.$ 4-3 -8-1-0.020 273 473 673 873 1073 reduction TIK Fig. 1 Thermogravimetric profile and relative derivative curve (in grey) of CeO, treated under pure hydrogen 30 40 50 60 2tYdegrees Fig. 2 XRD patterns of CeO, heated to 1073 K in (a) flowing 20% O,-N,, (b) flowing pure hydrogen, (cj flowing pure hydrogen and cooled to room temperature.(a) CeO,, (A) platinum, (H)expanded phase. fluorite structure. In Fig. 2(c) two compounds are obtained: one is that evidenced in Fig. 2(h), the other is CeO, which comes from the partial reoxidation of the latter compound. In order to specify the conditions in which the expanded phase has been formed during the hydrogen treatmcnt, the fluorite peak positions for the most intense lines have been plotted as a function of the reduction temperature (T,). In Fig. 3, A( 28) is the difference between the initial peak position (300 K) and its position at T,, corrected from the thermal expansion factor.Above 973 K, it is not possible to follow the (220) line because of the superposition of this diffraction peak with the (200) peak of platinum. Note that each curve 1928 1.2- tone I zone II ,, n zone 111 L3 .-I A I 400 600 800 1000 reduction TIK Fig.3 Shift of the main CeO, diffraction peaks during the in situ reduction treatment under hydrogen. 0,(311); *, (220); 17,(111). has the same profile and three zones can be distinguished. In the first zone (I, 300-593 K) the cell parameter is unmodified, while in zones IT (593-843 K) and I11 (843-1073 K) there is lattice expansion. Indeed, A(28) values can be considered to be within the experimental error (from 0.1" for T,<673 K to 0.03" for T,>873 K) for zone I, but this is not the case for zones I1 and 111 where the angular deviations are important enough, especially for the (220) and (31 1) lines, to be linked to a bulk phenomenon.So the expansion coefficient of the fluorite structure reaches 0.6% in zone I1 at 723 K, increases abruptly in zone I11 (1.6%) and reaches 2.1% at 1073 K. When compared with results reported in literature, it appears that the two peaks deduced from TPR experiments correspond to the derivative peak at 863 K of Fig. 1 and to the beginning of another one near 1170 K. On the other hand, the analysis of the structure under H, at temperatures up to 1070K, which reveals the presence of a cubic fluorite phase only, is largely in agreement with the results reported in ref. 2. However, this study reveals that the expansion of the CeO, lattice has already commenced by 570 K, which corresponds to the first-derivative peak in Fig.1. As it is generally admitted that ceria reduction begins at 473 K,2 it can be deduced that zone I is concerned mainly with superficial reduction and zones IT and I11 with bulk reduction. Moreover, taking into account the results of Fierro et uL1 who show the incorpor- ation of hydrogen into ceria during reduction under H, in the 573-773 K range, and considering previous studies on cerium-nickel oxides' extended to cerium-copper oxides, in which we have measured the amount of hydrogen occluded in the hydride form in doped ceria treated at 573-673 K under H,, we propose that the expansion of the lattice in J.MATER. CHEM., 1994, VOL. 4 zone I1 is due to a bulk reduction of Ce4+ ions and insertion of hydrogen as H-ions, according to the following scheme: 2Ce4++2O2-+H2+2Ce3++20I1-(1) 20H-+H,O +0,---0 (2) H,+02-+U+OH-+H-(3) in which 0,-, OH-and H- species hold anionic positions and represents anionic vacancies within the fluorite structure. In zone I, reactions (1) and (2) are thought to take place at the surface leading to the creation of the vacancies necessary to dissociate molecular hydrogen. In zone IT, owing to migration phenomena, the reduction spreads to the bulk, giving rise to the expansion of the ceria lattice up to 723 K, the temperature at which the dehydroxylation [reaction (2)] becomes important.'.' Therefore, the second peak in Fig.1 is related to this last phenomenon, the next peak (863 K) undoubtedly being connected with a fresh increase in the cell parameter due to a further reduction in zone 111. In conclusion, over the whole range of temperature studied, cerium oxide retains the fluorite structure and can be described as Ce4+xCe3+1-x02-y(OH-)zH-tEl", keeping in mind th:t 02-,OH- and H-are about the same size (1.32, 1.76, 1.54 A, respectively). This formulation accounts for the easy reoxi- dation of the solid when subjected to an oxygen atmosphere. Moreover, in situ XRD analysis of ceria reduction under H, allows the zone in which the insertion of hydrogen into the structure occurs to be delimited. This is particularly important in hydrogenation catalysis, when ceria is doped or supported, as will be shown in a following paper. References 1 J. L. G. Fierro, J. Soria, J. Sanz and J. M. Rojo, J. Solid State Chem., 1987,66,154. 2 V. Perrichon, A. Laachir, G. Bergeret, R. Frety, L. Tournayan and 0.Touret, J. Chem. Soc., Faraduy Trurts., 1994, YO, 773. 3 H. C. Yao and Y. F. Yu Yao, J. Catal., 1984,86,154. 4 L. Tournayan, N. R. Marcilio and R. Frety, Appl. Cutal., 1991, 78,31. 5 D. J. M. Bevan, J. Inorg. Nucl. Chem., 1955,1,49. 6 J. Barrault, A. Alouche, V. Paul-Boncour, L. Hilaire and A. Percheron-Guegan, Appl. Cutal., 1989,46,261). 7 M. P. Sohier, G. Wrobel, J. P. Bonnelle and J. P. Marcq, Appl. Catal., 1992,84, 169. 8 C. Binet, A. Jadi and J. C. Lavalley, J. Chim. Phjs., 1992.89, 31. Communication 4/06133K: Received lOih October, 1994