J. CHEM. SOC. FARADAY TRANS., 1994, 90(4), 653-657 IR Studies of Cerium Dioxide: Influence of Impurities and Defects F. Bozon-Verduraz" and A. Bensalem Laboratoire de Chimie des Materiaux Divises et Catalyse, Universite Paris 7,2,place Jussieu, 75251 Paris Cedex 05,France An infrared study of CeO, is presented which allows discrimination of the bands due to residual carbonaceous impurities from the bands arising from multiphonon absorption and electronic transitions induced by atomic defects. CO adsorption experiments show the presence of coordinatively unsaturated (cus) Ce3' or Ce4+ surface ions depending on the nature of the pretreatment. The role of plasmon-phonon coupling in the infrared examination of semiconductors is also stressed. In recent years, much effort has been devoted to the prep- aration of cerium dioxide with high surface area, to the inves- tigation of its textural and structural changes upon increasing temperature and to the study of its surface chemistry.'-5 In addition, the role of cerium dioxide in automotive exhaust catalysts has led to renewed interest in fundamental studies concerning the interaction of oxygen, hydrogen, carbon mon- oxide and carbon dioxide with this Various experi- mental techniques were involved such as infrared spectroscopy, UV-VIS diffuse reflectance, X-ray photoelec- tron spectroscopy, magnetic susceptibility,* electrical conductivity" and temperature-programmed desorption or reduction.IR spectroscopy has been used to study the nature of the species formed upon adsorption of CO and CO, (carbonates) or of oxygen (superoxide and peroxide) upon ceria either heated in uacm or reduced by However, the propensity of ceria for non-stoichiometry ' and for strongly retaining carbonate impurities brings about some uncer-tainties, and the infrared spectra of virgin ceria and of ceria treated in reducing or oxidizing atmospheres need to be re- examined, taking into account the following points: (i) Removal of the carbonate impurities by outgassing at high temperature gives rise to defects such as oxygen vacancies and Ce3+ ions.(ii) Multiphonon bands may appear in the spectral range covered by carbonate species. (iii) In reducing conditions, ceria is an n-type semiconductor and its transmis- sion decreases as it is reduced because of the large absorption due to conduction electrons.(iv) As a consequence, chemi- sorption of electron acceptors leads to a significant decrease of the background absorbance, which induces some ambi- guities when difference spectra are recorded. The aim of this paper is to throw some light on the intri- cate influence of these factors, and provide a better under- standing of the interaction of ceria with metals in transition-metal catalysts. Experimental Materials Cerium dioxide CeO, was obtained from Rh6ne Poulenc. Its specific surface area and its mean particle size after various pretreatments are presented in Table 1. Apparatus IR spectra were recorded on a Perkin-Elmer 1730 Fourier-transform spectrometer with 30 scans at 4 cm-' resolution.The spectra presented are of the sample before adsorption, in the presence of the gas phase and upon decreasing the pres- sure. The sample was pressed under ca. 300 kg cm-, into a self-supporting disc weighing ca. 30 mg ern-,. The IR cell equipped with ZnSe windows was connected to a vacuum system with P < lop5Torr. Some experiments were carried out on a grease-free vacuum line to detect any sample con- tamination by the apparatus. Pretreatment Procedures The spectral features of cerium dioxide depend sharply on the temperature and on the atmosphere of the pretreatment. All samples were submitted to a calcination in flowing oxygen at a definite temperature T,, (673 < 7JK < 1073) for 2 h before outgassing for 15 h at a fixed temperature T, (573 < T,/K < 1173).Some samples were reduced in flowing hydrogen at T,(573 d T,IK f773) for 2 h (after the calcination at T,, and before outgassing at 7J. The samples not pretreated in H, will be referred to as unreduced. Results and Discussion Influence of the Pretreatment Procedure on the Spectrum of Ceria Because of its basic character, ceria strongly binds carbonate entities. Before performing any CO or CO, adsorption experiments, various bands due to these entities appear in the IR spectrum. The positions of the different bands observed after each type of pretreatment are collected in Table 2. U nreduced Samples After outgassing at 573 K for 15 h (Fig.1, Table 2), the spec- trum of the sample showed: (i) two intense bands at around 1460 cm-' (K) and 1390-1365 cm-' (L), (ii) two bands at 1067 and 1033 cm-' (M, N) of moderate intensity, (iii) two weak bands at 853 and 835 cm-' (P, Q) on the high- frequency tail of the phonon spectrum. After outgassing at 773 K (Fig. l), N disappeared and the above bands were less intense. Because of their thermal sta- bility, these bands must be ascribed to bulk polydentate Table 1 Textural properties of CeO, specific surface mean particle size pretreatment area/m2 g -lA 573 K, vacuum 110 100-120 173 K, hydrogen 31 1173 K, vacuum 5 1173 K, air 5 260-280 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Influence of the pretreatment procedure on the spectrum of ceria sample pretreatment temperature/K band position/cm -oxidation (TJK) reduction by H, (TJK) outgassing (T,/K) J K L M N PQ 673 -573 -1460 1390-1365 1067 1033 853 835 673 -773 -1472-1454 1390-1365 1067 857 835 1073 -1073 2127 --1063 -1073 -1173 2130 -1063 -673 573 573 21 19 1466 1370 1067 -855 841 673 673 573 21 19 1478 1368 1067 -859 835 673 773 573 2127 1466 1372 1067 -859 840 species rather than to monodentate carbonate." Although remained unchanged.It follows that the major contribution many bulk polydentate structures may be envisaged, the most to M does not come from carbonate species but from ceria stable should contain three cerium-oxygen bonds, in oppo- itself. As M was not modified upon contact of oxygen (160 sition with the bridged, bidentate, monodentate or carbox- Torr) with the sample either at 293 K or at 873 K (see Fig.2, ylate species (Scheme 1).l2 later) it should not be associated with oxygen vacancies or When the calcination and outgassing temperatures were as Ce3+ ions. It is therefore proposed that this band arises from high as 1073 K, K, L, P and Q disappeared, while M multiphonon processes. This view is supported by the results obtained on a ceria monocrystal by Mochiz~ki'~ who observed a band near 1030 cm-' on the high-frequency tail of the phonon spectrum (585, 465 and 280 cm-I). This attribution does not rule out a contribution of carbonate species (see below Table 3) when absorption bands between 1600 and 1300 cm-' show the presence of these entities, i.e.when the outgassing temperature is <873 K. In addition, a very weak band, J, appeared at 2127 cm-' (Fig. l), the intensity of this band growing when T, increased to 1173 K (Fig. 2). This peculiarity shows that J cannot be ascribed to occluded CO but is associated with a defect created by the drastic outgassing temperature. This is con-firmed by the decrease of J upon O2adsorption at 293 K and its disappearance after further heating in oxygen at 873 K for 15 h (Fig. 2). Experiments performed by diffuse reflectance spectroscopy (UV-VIS) also support this view ;outgassing at 1073 K leads to the formation of Ce3+ ions which are annihi- 2000 1350 700 lated by subsequent heating in oxygen at 873 K.9914wavenum ber/cm -' Fig.1 Unreduced samples. After oxidation at T,, for 2 h and out- gassing at T, for 15 h. (a) T,, =673 K, T, =573 K; (b)T,, =673 K, T,=773 K; (c) T,, =1073 K, T,=1073 K. Q) IJ bulk polydentate bridged bidentate -~~ 1--2000 1350 700 I wavenumber/cm-' Ce" Fig. 2 Unreduced samples. Successive treatments: (a)After oxida- monodentate carboxylate tion at 1073 K for 15 h and outgassing at 1173 K for 15 h; (b)after Scheme 1 Structure of some carbonate and carboxylate species contact with 130 Torr 0, at 293 K for 15 min; (c) at 873 K for 1.5 h Table 3 Attribution of bands of adsorbed CO and carbonaceous species (1 800-700 cm -I) ____~ ~ species band position/cm -~~~~ 'bulk 'carbonate 1480-1420 1400-1350 1060 880 monodentate carbonate 1480-1460 11380-1360 1070 850 bidendate carbonate 1610-1 580 1310-1250 1030 860-830 bridged carbonate 1750-1700 1180 carboxylate 1580-1560 1400-1350 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Samples reduced by Hydrogen In addition to the bands, K-Q, ascribed to carbonate species, the sample reduced at 573 K exhibited a very weak band, J, near 2118-2120 cm-' (Fig. 3). The intensity of J increased with T, and a shift to 2127 cm-' was noticed for T,= 773 K. This is the exact position of a band ascribed by Laachir et aL9 to occluded CO arising from the reduction by H, of car- bonate entities initially observed on a sample presenting a high surface area. According to these authors, this assignment was supported by the detection of CO in TPR experiments near 900 K. However, on a sample with a low surface area, these authors noticed that the same band grew with reduction temperature in the 873-1073 K range and is destroyed in 0, at room temperature.As long as bulk carbonates are present in the sample (absorption bands in the 1600-1300 cm-' range), the pres- ence of occluded CO cannot be discarded. However, if an occluded species could be expected to resist outgassing up to T,= 573 K, it should not be so sensitive to oxygen at room temperature. Hence we believe that the J band observed before CO adsorption in the 21 18-2127 cm-' range arises from an electronic transition from donor levels located near the conduction band such as Ce3+ ions or oxygen vacancies.Surface Ce3+ ions were detected by XPS9,'5 while the pres- ence of oxygen vacancies is deduced from electrical conduc- tivity measurements.'0*16 It is also relevant to note that a band near 2115 cm-' was already assigned to defects in cerium dioxide.6 Finally, the weakening of M when T, increases (Fig. 3) is ascribed to phonon-plasmon coupling' (see below). This coupling may also partly explain the marked intensity decrease of K and L. Oxygen Adsorption According to Li et a1.,6.'8 oxygen adsorption at 298 K on ceria outgassed at lo00 K gives rise to superoxide 0; species well characterized by v(0-0) = 1126 cm-' (checked by iso- topic experiments), but two bands, not discussed by the authors, appeared simultaneously at 1363 and 1342 cm-'.On a sample reduced by H, at 673 K and outgassed at lo00 K, a peroxide entity was detected [v(O-0) = 883 cm-'1 together with the superoxide. Aside from the bands ascribed to these entities, the spectra also showed two bands in the 1550-1200 cm-' range, not discussed by the authors, and two 'reverse bands' at 939 and 21 15 cm-' assigned to CeO, species (x < 2) present on the reference spectrum. The inten- sity of these four bands was shown to increase with the adsorption temperature between 200 and 473 K. While the 0; and 0;-entities are well characterized, the other bands 4 L KA a m e s % I I 2000 1350 700 ' wavenurnber/cm -Fig. 3 Samples reduced by H,. After oxidation at 673 K for 2 h and reduction by hydrogen for 2 h at (a) 573 K; (b)673 K; (c) 773 K.shown by Li et al. call for some comments. The peaks appearing between 1550 and 1200 cm-' upon oxygen adsorption may be ascribed to carbonates and carboxylates formed by reaction of oxygen with residual carbonaceous entities. To obtain ceria samples free of contaminants requires a pretreatment in oxygen at T,, > lo00 K, in order to burn out surface hydrocarbon impurities and occluded CO, before out- gassing at T,> lo00 K so as to remove all carbonates. This conclusion arises from experiments performed on two samples pretreated at T,, = 673 and 1073 K, respectively, and both outgassed at T,= 1073 K (Fig. 4). On the former sample, 0, adsorption gave rise to bands near 1570 and 1305 cm-',which did not appear with the latter sample.Concerning the so-called reverse bands at 2115 and 939 cm-', reported by Li et it is relevant to note that plot- ting only difference spectra may lead to difficulties, especially when oxygen adsorption is involved. In n-type semicon-ducting oxides, indeed, oxygen adsorption not only removes bands associated with defects previously created in a reducing atmosphere, but also gives rise to a marked diminu- tion of the background absorbance, due to the decrease of the concentration of conduction electrons (the oxygen species acting as electron traps). It must then be emphasized that absorption by conduction electrons is a function of vP2,l9 which implies that the decrease of the background absorb- ance upon oxygen adsorption is much more important at low wavenumbers. It follows that determining the position of absorbance maxima from difference spectra is not safe in this case and that examination of direct spectra has to be pre- ferred, at least in a preliminary way.A further complication arises from the particle size (6) dependence of the light intensity Id diffused by the sample which is expressed by: I, 2 d3v4.,0 As a consequence, Id decreases with v and the sample transmittance increases when v diminishes. Hence, it is important to check that the particle size does not vary during the course of adsorption-desorption experiments, which occurs when high pretreat- ment temperatures have been chosen.CO Adsorption Carbonate and Carboxylate Species As mentioned above, carbonate and carboxylate species are easily formed and various structures may coexist, even though their thermal stabilities are different, as discussed by 4 940 ........_.... 2000 1225 456 wavenumber/cm -' Fig. 4 Samples not reduced by H,. Influence of the calcination temperature on the IR spectrum of ceria upon oxygen adsorption (100 Torr) at 293 K. (a) T,, = 673 K, T,= 1073 K; (b)T,, = 1073 K, T,= 1073 K. Busca and Lorenzelli" in a well documented review on oxides. In fact, the nature of adsorption sites is expected to change with the atmosphere and the temperature of pretreatment. The frequency of the vibrational modes should indeed be sen- sitive to the degree of coordinative unsaturation of Ce"+ ions and to the oxidation state of cerium (+3 or +4) because of the role of the polarizing power of the metal ion.Hence the detailed assignment of all bands can only be tentative. The nature of the carbonate and carboxylate entities formed on ceria was discussed previously by several auth~rs~-~and our objective is essentially to discriminate the contribution of these species from the bands arising from ceria itself. Unreduced Samples As shown above, all the samples pretreated at T,< 773 K contain bulk polydentate carbonate species responsible for bands between 1800 and 700 cm-' (Table 2 and 3). Adsorp-tion of CO (100 Torr) at room temperature [Fig. 5 (b)] gave rise to: (i) a peak at 2165 cm- ' (H) with a shoulder (H') near 2150 cm-' assigned to linear CO adsorbed on coordinatively unsaturated Ce4+ ions, in agreement with Li et ~1.;~(ii) a set of new bands at 1607, 1546 and 1180 cm- ' ascribed mainly to carobxylate and carbonate entities (Table 3).While H and H' disappeared upon decreasing the CO pressure below 10 Torr, the other bands were stable in uucuo up to 473 K and were removed only by outgassing at 573 K for 2 h, the poly- dentate being unaffected. The behaviour of the samples pretreated at T,= 1073 K was completely different [Fig. 5(c)]. Before CO adsorption, these solids showed only the very weak band, J, at 2127 cm-',ascribed to an electronic transition (see above) and the peak, M, assigned to a multiphonon process.The poor inten- sity of J may be partly accounted for by the low surface area of aria after pretreatment at 1073 K (Table I). Upon adsorp- tion of 100 Torr CO at room temperature peak J was slightly enhanced, but no new band was detected in the 1800-700 cm-' range. It can then be concluded that many of the surface oxygen ions able to react with CO (to give the car- bonate entities) were removed during the pretreatment because of the drastic surface area decrease and of the reduction of surface Ce4+ ions to Ce3+. The formation of Ce3+ ions upon outgassing at high temperature was also shown by results obtained by diffuse refle~tance'~ and mag- netic susceptibility measurement~.~ The slight enhancement of J upon CO adsorption could be ascribed to the genesis of 1568 I1 II 2200 2100 1525 1250 975 wavenumber/cm -' Fig.5 Influence of the nature of the pretreatment on the spectrum of CO adsorbed under 100 Torr at 293 K. (-) Before CO adsorp-tion; (---) after CO adsorption. (a)Sample reduced by H, , T, = 773 K; (b)sample not reduced by H, , T,= 773 K; (c)sample not reduced byH2,T,=1073K. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 CO adsorbed on Ce3+ ions, in agreement with the wavenum- ber calculated by Zaki et al." Reduced Samples 2200-1800 cm-' Range. Before any CO adsorption, the solids reduced at T, < 673 K exhibited band J at 2119 cm-' (Table 2). Upon CO adsorption (100 Torr), a new band H (also observed on non-reduced samples) appeared at 2 165 cm-', still assigned to CO adsorbed on cus Ce4+ ions (spectrum not presented here).This band disappeared when the CO pressure was lowered to 10 Torr. For T, = 773 K, band J was observed at 2127 cm-' before CO adsorption [Fig. 5 (a)].After introduction of CO, H did not appear while J was significantly enhanced; its initial intensity was not recovered through short outgassing at room temperature, but it was restored after a prolonged evacuation (2 h) at room temperature. These results show first that the cus Ce4+ ions were transformed into cus Ce3+ ions through the H, pretreatment at 773 K. They also suggest that CO is more strongly adsorbed on the Ce3+ ions, generated in the course of the reduction pretreatment, than on the Ce4+ ions, which may be explained by a slight retrodonation from Ce3+ to co.1800-700 cm-' Range. While the K, L, M, P and Q bands initially present were enhanced, a new intense peak appeared between 1560 and 1580 cm-' together with weaker bands near 1030 and 893-900 cm-' [Fig. 5 (a)].These new bands were removed by outgassing at 573 K and are ascribed to bidentate carbonate and carboxylate entities (Table 3). Con- sidering the band intensities, the concentration of these species is much more important than in the case of non-reduced samples. In addition, the very weak bands at 1728 and 1180 cm-' show the presence of bridged carbonate entities. Conclusion The present work allows the discrimination of the bands due to residual carbonaceous impurities from the bands arising from multiphonon absorption and electronic transitions induced by atomic defects.The main results may be summarized as follows: (1) Complete elimination of carbonaceous impurities requires pretreatments at about 1073 K, first in oxygen then in uacuo. (2) The spectrum of thoroughly dehydroxylated ceria pre- sents a multiphonon band at 1063 cm- '. (3) Outgassing at temperatures d773 K creates coordi- natively unsaturated Ce4+ ions able to adsorb linear CO species vibrating near 2170-2 150 cm -(4) Outgassing at temperatures 21073 K or reducing by H, at T 2 573 K generates donor levels (Ce3+ ions or oxygen vacancies) which give rise to an electronic transition near 2120-2127 cm-' (0.26 eV).When the concentration of Ce3+ ions is large enough, (e.g. after reduction in H, at 773 K), CO adsorption gives rise to additional absorption in this range, which is ascribed to CO-Ce3+ species. (5) The distribution of carbonate-like entities generated by CO adsorption also appears to depend on the nature and the temperature of pretreatment. Carboxylate and bidentate entities are preferentially formed on samples prereduced by hydrogen. (6) Finally, it must be stressed that, on semiconducting materials, the intensity of absorption bands arising from adsorbed or bulk species depends sharply on the background absorbance. In the case of n-type semiconductors such as ceria, increasing the number of conduction electrons by reducing treatments leads to the predominance of plasmon- J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 657 phonon ~oupling'~ which hinders or even precludes the observation of purely vibrational bands. On the other hand, decreasing the concentration of conduction electrons through adsorption of an electron acceptor like oxygen allows the vibrational bands to reappear. 9 10 A. Laachir, V. Perrichon, A. Badri, J. Lamotte, E. Catherine, J. C. Lavalley, J. El Filleh, L. Hilaire, F. LeNormand, E. Quemere, G. N. Sauvion and 0. Touret, J. Chem. Soc., Faraday Trans., 1991,87, 1601. (a) R. N. Bluemental and E. K. Chang, J. Solid State Chem., 1988, 72, 330; (b) L. Eyring, Handbook ofthe Physics and Chem- istry of Rare Earths, ed. K. A. Gschneider Jr. and L. Eyring, The Rh6ne-Poulenc Company is gratefully acknowledged for the supply of cerium dioxide.11 12 North Holland, Amsterdam, 1982, Vol. 3, p. 337. G. Busca and V. Lorenzelli, Marer. Chem., 1982,7,89. K. Nakamoto, Infrared and Ramun Spectra of Inorganic and Coordination Compounds, Wiley, New York, 4th edn., 1986, References p. 252. (a)J. G. Fierro, S. Mendioroz and M. Olivan, J. Colloid Inter- face Sci., 1984, 100, 303; (b)J. L. G. Fierro, J. M. Rojo and J. M. Sanz, Colloids SurJ, 1985, 15, 75. J. L. G. Fierro, S. Mendioroz and A. M. Olivan, J. Colloid fnter- face Sci., 1985, 107, 60. J. L. G. Fierro and J. L. G. Soria, J. Solid State Chem., 1987,66, 154. J. M. Heintz and J. C. Bernier, J. Phys. C, 1986,47, 1. T. Yamaguchi, N. Ikeda, H. Hattori and K. Tanabe, J.Catal., 1981,67, 324. C. Li, K. Domen, K. Maruya and T. Onishi, J. Am. Chem. SOC., 1989,111,7683. (a) C. Li, Y.Sakata, T. Arai, K. Domen, K. Maruya and T. Onishi, J. Chem. SOC.,Faruday Trans. 1, 1989, 85, 929; (b)C. Li, Y. Sakata, T. Arai, K. Domen, K. Maruya and T. Onishi, J. Chem. Soc., Faruday Trans. I, 1989,85, 1451. A. Badri, S. Lamotte, J. C. Lavalley, A. Laachir, V. Perrichon, 0. 13 14 15 16 17 18 19 20 21 S. Mochizuki, Phys. Stat. Sol. B, 1982,114, 189. A. Rakai, A. Bensalem, J. C. Muller, D. Tessier and F. Bozon-Verduraz, 10th International Congress on Catalysis, Budapest, Elsevier, Amsterdam, 1992, p. 1875. T. Arai, K. I. Maruya, K. Domen and T. Onishi, J. Catal., 1993, 53, 117. J. M. Hermann, E. Ramaroson, J. F. Temp6re and M.F. Guil-leux, Appl. Catal., 1989,53, 117. F. Boccuzzi, C. Morterra, R. Scala and A. Zecchina, J. Chem. SOC., Faruday Trans., 1981,77,2059. C. Li, Q. Xin and X. Guo, Catal. Lett., 1992,12,297. J. T. Houghton and S. D. Smith, Infrared Physics, Oxford Uni- versity Press, Oxford, 1966. M. L. Hair, InJiared Spectroscopy in Surjbce Chemistry, Marcel Dekker, New York, 1967, p. 59. M. I. Zaki, B. Viekhaber and H. Knozinger, J. Phys. Chem., 1986,90,3 176 Tourret, G. N. Sauvion and E. Quemire, Eur. J. Solid State Inorg. Chem., 1991,28,445. Paper 3/03853J; Received 5th July, 1993