Reduction of CeO, by Hydrogen

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J. CHEM. SOC. FARADAY TRANS., 1991, 87(10), 1601-1609

Reduction of CeO, by Hydrogen Magnetic Susceptibility and Fourier-transform Infrared, Ultraviolet and X-Ray Photoelectron Spectroscopy Measurements Ahmidou Laachir and Vincent Perrichon" lnstitut de Recherches sur la Catalyse, Laboratoire Propre du CNRS conventionne a Wniversite Claude Bernard Lyon I , 2 avenue Einstein, 69626 Villeurbanne Cedex, France Ahmed Badri, Jean Lamotte, Eugene Catherine and Jean Claude Lavalley Laboratoire de Catalyse et Spectrochimie, URA 414 du CNRS, ISMRA, 14050 Caen Cedex, France Jaafar El Fallah, Lionel Hilaire and Franqois le Normand Laboratoire de Catalyse et Chimie des Surfaces, URA 423 du CNRS, Universite Louis Pasteur, 4 rue Blaise Pascal, 67070 Strasbourg Cedex, France Eric Quemere, Guy Noel Sauvion and 0. Touret RhGne-Poulenc Recherches, 12 rue des Gardinoux, 93308 Aubervilliers Cedex, France

The reduction of CeO, by hydrogen has been studied from 300-1200 K by several complementary techniques: temperature-programmed reduction (TPR) and magnetic susceptibility measurements, Fourier-transform infrared (FTIR), UV-VIS diffuse reflectance and X-ray photoelectron (XP) spectroscopy. Two CeO, samples were used with B.E.T. surface areas of 115 and 5 m2 g-' , respectively. The concentration of Ce3+ was determined in situ by measuring the magnetic susceptibility and the Ce"' photoemission line. The reduction began at 473 K, irrespective of the initial surface area of the ceria. In the case of the low-surface-area sample, an intermediate reduction step was observed between 573 and 623 K, corresponding to the reduction of the surface. This intermediate step was less easily observed in the case of the high-surface-area ceria. In both cases, the reduction led to a stabilised state with the formal composition CeO,.,, . Temperatures higher than 923 K were required to reduce the ceria further. The surface Ce"' content determined by XPS was close to that determined by magnetic susceptibility measurements. The intensity of the 17000 cm-' band in the UV-VIS reflectance spectrum also varied with the degree of reduction. Finally, the evolution of the surface species observed by IR spectroscopy was in good agreement with the results from the other techniques. The IR results indicated large changes in the concentration and nature of both the hydroxyl and the polydentate carbonate species during the reduction process. The adsorption of oxygen on samples previously reduced to the composition CeO,.,, led to almost complete reoxidation at room temperature. The state of the initial B.E.T. surface did not influence the oxidation process. A slight excess adsorption of oxygen was evident on the surface. This was thermodesorbed at 380 K under vacuum .

When heated to high temperatures or treated in reducing atmospheres, cerium dioxide is known to show large deviations from its CeO, stoichiometric composition. '-' However, subsequent treatment with oxygen leads to rapid re-establishment of its normal stoichiometry. These redox properties of CeO, are utilised in catalytic devices developed for the automotive post-combustion process, since the cerium oxide is a regulator of the oxygen partial pressure over the ~atalyst.'.~Several experimental techniques have been used to study the interaction of reducing agents such as hydrogen or carbon monoxide with CeO,: gravimetry, TPR, FTIR, , ~ - ~ there is a need for more H-NMR, EPR e t ~ . ~However, quantitative results concerning the extent of nonstoichiometry on well characterised samples. For example, in the TPR experiments, some artifacts can interfere in the interpretation of the measurements, e.g. the effect of the hydrogenation of carbonates on the H, consumption. Also some techniques require certain experimental conditions (temperature, atmosphere) which make it impossible to characterise the Ce4+-Ce3 valence changes under the reaction conditions. Finally, although the extent of the B.E.T. surface area has been recognised to be an important parameter in the H, reduction proce~ses,'~~ the discrimination between the surface and the bulk reduction is still not fully understood. This work is a comprehensive study of CeO, reduction by H, using several complementary techniques. The magnetic +

susceptibility method is particularly suitable in monitoring the reduction since it allows one to follow the valence changes of the cerium ion in situ. The method utilises the fact that the Ce4+ ion is diamagnetic whereas Ce3+ is paramagnetic, the latter having a 4f unpaired electron with a magnetic moment close to 2.5 p B . The UV-VIS diffuse reflectance method is also an appropriate technique since it is able to detect any changes in the bulk. FTIR and XP spectroscopies, however, are more sensitive to changes in the surface species. In particular, XPS is a semi-quantitative method for determining the oxidation state changes by curve fitting of the photoemission spectra.

Experimental Materials

The two CeO, samples were obtained from Rh8ne Poulenc. The first (Ce0,-400) was prepared at 673 K and had a B.E.T. surface area of 115 m2 g-' with a high proportion of microporous area (surface = 61 m2 g-'). By calcination of this high-surface-area sample at 1123 K, a second sample (Ce0,-850) was obtained, with a B.E.T. surface area of 5 m2 8-l. The purity was 99.5%, with La,O, as the main impurity (0.3%). Before each run, after being heated at a rate of 5 K min- ',the samples were standardised for 1 h at 673 K under

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a flow of oxygen and then treated for 1 h at the same temperature either under vacuum or under rare gas before cooling to room temperature. After this standardisation treatment at 673 K, no water remained in the solid, as shown by vacuum thermodesorption up to 1073 K. However, residual carbonate species were still present and could be thermodesorbed as CO, with a maximum at 810 K. The total weight loss of CO, was 42 pmol g- '. TPR The apparatus is described in ref. 10. The H, consumption was measured by means of a catharometric detector. The reactants (1% H, in Ar) were passed over the catalyst (200 mg) with a 1.1 dm3 h-' flow rate. The water produced by the reduction was trapped on a molecular sieve. The heating rate was 8 K min-'. FTIR and U V Spectroscopy IR spectra were obtained with a Nicolet 60 SX spectrometer using the absorbance mode. The UV-VIS diffuse reflectance spectra were recorded on a Beckmann 5240 spectrometer equipped with an integrating sphere.

Since CeO, is a strong insulator, spectra were recorded both with and without a flood gun to compensate for the positive charge created by the photoelectron emission process. It was established that a broadening of the spectra occurs and that some supplementary reduction also occurs (as determined by the curve-fitting analysis) when spectra were recorded without a flood gun. Thus all spectra were recorded with flood gun compensation. It was also established that no reduction of cerium occurs when the sample is submitted to the X-ray flux up to 4 h. The binding energies corresponded to the 3d3/2 4 f V" (V = valence state) state, denoted by u"', at 916.7 eV. After subtraction of the background contribution due to inelastic processes, the Ce 3d photoemission lines were determined using a curve-fitting procedure. The experimental curve was approximated by a Gaussian curve and also by a Lorentzian curve (due to the finite life time of the core hole). The experimental curve accounted for the nonmonochromatised nature of the A1-Ka emission and the broadening due to the electron detector. The peak was at 1.1 eV as measured from the broadening of the 4f7/2 line of gold foil. The lifetime of the core hole was taken as 0.9 eV and 1.0 eV for the 3d,,, and 3d3/, states of cerium respe~tively.'~ The background contribution B(E) was obtained by a Shirley function :

Gravimetric and Magnetic Susceptibility Measurements These were performed in a Faraday microbalance built in the laboratory" and connected to a 50 dm3 s-' ion pump which allowed pressures < 10- ' Torr.7 The magnetic susceptibility x was obtained using the Faraday method in the range 294lo00 K. The calibration was carried out using (NH,),Fe(SO,), - 6H,O as a standard and corrections for the sample holder and ferromagnetic impurities were made as explained in ref. 12. A quadrupole mass spectrometer (MQ 63 Riber) was used to analyse the gases during the thermodesorption experiments. The sample (100-150 mg) was deposited in the sample holder on a quartz wool bed. The reduction experiments were performed under a flow of purified hydrogen (4 dm3 h- '). The magnetic susceptibility of the initial sample under emu g-' (to be multiplied by vacuum, -0.03 12.56 x l o p 3to obtain the SI units in m3 kg-I), corresponded to a slightly diamagnetic sample, which is to be expected for Ce4+ ions. It was not changed by the standardisation nor by a treatment at 1073 K at Torr. Thus, even though there may be a slight initial Ce3+ content, it is clear that the dehydroxylation or the decarbonation of the ceria does not result in the formation of Ce3+ ions to any significant extent. Note also that the ferromagnetic impurity content was low (3-6 ppm) and did not vary significantly during the reduction treatments. XPS Spectra were recorded on a VG spectrometer using Al-Ka radiation, the power of the X-ray source being 200 W. The base pressure during analysis was lo-' Torr. The samples were pressed into pellets (at 10 kg ern-,) after the standardization treatment and were mounted on a nickel sample holder that allowed reduction treatments up to 1173 K. Unless otherwise stated, the samples were treated with H, (purity 99.99%) at 1 atml for 4 h in a preparation chamber directly connected to the analysis chamber. Samples were cooled to room temperature before evacuating the hydrogen. 1 Torr = 101 325/760 Pa.

1 1 atm = 101 325 Pa.

B(E) = K

Jr:

F(E) dE

(1)

where K is a constant. The spectra were resolved using a fitting procedure with the different CeIV and Ce"' states as shown in Fig. 8 (later). The agreement between the calculated and the experimental spectra is satisfactory when cerium is in the IV state but when the cerium is reduced to the I11 state a clear discrepancy appears at the trough (890-895 eV) between the two main contributions. This discrepancy and further details on the limitations of the Ce 3d photoemission curve-fitting procedure are discussed elsewhere.', An estimation of the amount of Ce'" can be obtained from the intensity of the vo (uo) and v' (u') lines, according to:

Errors are estimated to be ca. f5% in the pure Ce" state and ca. +7% at higher reduction states [Ce"'(%) > 30%].

Results TPR Fig. 1 shows the hydrogen TPR curves obtained with Ce0,-400 and Ce0,-850 after standardisation at 673 K under 0,and cooling under argon. For Ce0,-400 [Fig. l(a)], after a small shoulder at 600 K, a large peak was noted at 820 K, followed by a negative peak at 890 K, and then again the positive slope of another peak at T > 1100 K. For Ce0,-850 [Fig. l(b)], the hydrogen consumption is comparatively lower. There was a small shoulder at 820 K and a broad peak with a maximum at T > 1100 K. In a separate experiment, the reaction products were analysed by gas chromatography using similar conditions (10% H2 in He, 2 K min-I). The negative peak at 890 K must be related to a release of C O which could result from the reduction of carbonates present in the bulk of the ceria and not eliminated by the standardisation treatment at 673 K. This was confirmed below by the IR study. The peak at 820 K depends upon the B.E.T. surface area since the peak almost disappeared in the case of Ce0,-850 (5

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cm-' and another band close to 1330 cm-' which was diffcult to detect due to the strong absorption variation of the sample in the 22o(r1000 cm- range. Standardised samples were reduced by heating for 1 h under pure hydrogen ( P = 100 Torr), in static conditions, at different temperatures, followed by evacuation at the same temperature. Almost no change occurred up to 473 K [Fig. 2(a)-2]. Reducing the Ce0,-400 sample at higher temperatures shifted the v(0H) bands due to type I1 OH groups to higher frequencies: they were situated at 3670 and 3636 cm - ' (very weak) or at 3680 and 3642 cm - when reducing at 573 or 673 K respectively. Reduction at higher temperatures also created a broad band near 3450 cm-' superimposed on that due to the tridentate O H groups; its intensity increased with the reduction temperature up to 673 K [Fig. 2(a)-3]. Similar results were obtained on Ce0,-850 by treatment with H, up to 673 K [Fig. 2(b)-3]. After reducing at 873 K only one sharp v(0H) band appeared at 3658 cm-' whereas that at 3450 cm-' was less intense but broadened towards lower wavenumbers down to 3200 cm-' [Fig. 2(b)-4]. The latter band partly persisted after reducing at 1073 K, in contrast to the sharp band which disappeared [Fig. 2(b)-5]. Treatment at such a high temperature led to the appearance of a new broad, strong band, at ca. 3620 cm-' [Fig. 2(b)-5]. Shifts in the wavenumber of the type I1 O H groups showed that changes in the ceria surface state occur when reducing at T 2 473 K, which corresponded to the beginning of the reduction, as observed from magnetic susceptibility measurements (see below). At temperatures > 473 K, the hydrogen dissociated and led to the formation of supplementary OH groups characterised by the broad band at 3450 cm-'. This could be assigned either to hydrogen-bonded hydroxyl groups as shown by Li et uZ.,~' or to water formation. H,O adsorption on CeO, shows that molecularly adsorbed water is desorbed at temperatures < 473 K. Under our experimental conditions, it was therefore impossible to detect water itself since after reduction, the samples had been evacuated at the reduction temperature. We therefore assign the 3450 cm- ' band to hydrogen-bonded hydroxyl groups resulting from H, dissociation. The observation of these groups even after treatment at 873 K or higher suggests that some of them could be internal groups. The broad, strong band at 3620 cm-' observed when reducing Ce0,-850 by H, at 1073 K does not correspond to hydroxyl groups since it is not sensitive to the change of H, into D, . Moreover, it persists after evacuation at 1073 K. We assign this band to an electronic transition, as already proposed on ZnO.,' Such an assignment is confirmed by the effect of the introduction of 0, at room temperature; as soon as 0, is introduced on the sample reduced at 1073 K, the 3620 cm- band disappears. Bands due to the carbonate species were slightly modified by reduction by H,. For instance, those near 1490, 1375 and 855 cm-' tended to increase in intensity when the reduction temperature is >473 K. This was due to a change in the state of the environment of the polydentate carbonate species. Of more significance was the appearance of a band at 2127 cm - ', observed on Ce0,-850 after reducing at temperatures > 873 K [Fig. 2(b)-4 and 5). Introduction of 0, at room temperature on such samples caused the 2127 cm-' band to disappear while bands due to polydentate carbonates increased in intensity [Fig. 2(b)-6]. This allowed the establishment of a relation between the 2127 cm-' band and the carbonate species. The 2127 cm-' band was assigned to occluded CO, resulting from a partial reduction of the carbonates. On Ce0,-850, reduced at high temperature, the 2127 cm- band

'

'

1

400

1

1

1

I

600

800

1

I

1

1000

TIK Fig. 1 TPR curves of standardised (a) Ce02-400and (b) Ce0,-850. (H2/Ar:1/99; 8 K min-') m2 g-'). Finally, the last peak at T > 1100 K was associated with the bulk reduction of the ceria.

FTIR The IR spectrum of the residual species on the standardised Ce0,-400 sample is shown in Fig. 2(a)-1. Hydroxyl and carbonate species remained on the surface. Comparing the results with those obtained on T h o , l 5 and by comparing the peaks with those assigned to methoxy groups,I6 it is suggested that the v(0H) high-frequency band, split into two components at 3650 and 3634 cm-', corresponds to type I1 species : H

Ce / * h c e

and that the band at 3500 cm-' is due to tridentate OH groups H

I Ce

Type I OH groups were characterised by a band at 3710 cm-'; they were less stable than the type I groups which prevented their observation when activation was performed at 473 K or more, as already observed on thoria." Below 1600 cm-', weak absorbances were noted at 1490 (shoulder), 1468, 1380, 1356, 1073, 1013, 954, 852 and 838 cm-'. Li et a1." observed some of these by adsorbing CO or CO, on CeO, activated at loo0 K. They assigned them to bridged, bidentate and monodentate carbonates. In our case, the high thermal stability and the relatively low value of the Av3 splitting favour polydentate carbonate formation,'8 followed by bulk carbonate formation, as already proposed on Tho,. The spectrum of the standardised Ce0,-850 sample [Fig. 2(b)-1] showed bands due to type I1 OH groups only. Carbonate species lead to the appearance of a weak band at 1068

'

'

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2.6

2.0

1.4

1.0

0.8

-

0.2 4000

3400

2800

1600

2200

1000

4000

3400

v/cm -

2800

2200

1600

1000

v/cm -

Fig. 2 IR spectra of standardised (a)Ce0,-400 and (b) Ce02-850 samples. (1) Before reduction (2) reduced by H, at 473 K ; (3) 673 K ; (4) 873 K ; (5) 1073 K and then (6) reoxidised with 0, at room temperature

was relatively strong suggesting that some of the C O remained in the sample. However, on CeO2-400, the band due to occluded C O decreased in intensity after the sample reduced at the same temperature was evacuated at 873 K, showing that C O desorption then occurred. This was in agreement with the TPR results which showed C O desorption at 890 K on CeO2-400 whereas C O desorption did not occur on Ce0,-850. UV-VIS Diffuse Reflectance Spectroscopy Allen et ~ 1 . ~ reported ' the appearance of a band at 17000 cm-' due to a Ce@)-Ce(Iv) interaction when ceria was reduced by H, . Fig. 3 shows that it occurred when CeO, was heated under H, at temperatures >473 K. Its intensity notably increased at 573 K and 673 K in the case of CeO2-400 [Fig. 3(a)]. However, the band was hardly noticeable on Ce0,-850 at such reduction temperatures, heating at 973 K under H, being necessary to make it strong in this case. However, its intensity depended on the reduction time as shown in Fig. 3(b). The addition of oxygen at room temperature completely eliminated the band, in both ceria samples. The intensity of the 17000 cm-' band appeared to vary with the reduction state of the ceria since the results are completely in agreement with those reported below, obtained from magnetic susceptibility measurements.

2-

Q,

1.5-

0

m

2 n

1-

0.60.2-

I 4

v/cm-'

x

10-3

v/cm-l

x

Fig. 3 UV-VIS diffuse reflectance spectra of (a) Ce02-400 and (b) Ce02-850 treated under H, (Pe = 100 Torr) at (1) 473 K, (2) 573 K, (3) 673 K, (4) 873 K, (5) 973 K for 3 h and (6) 973 K for 63 h

Gravimetric and magnetic Susceptibility Measurements Reduction ofCe0,-400 by Hydrogen When the standardised sample was put under hydrogen flow at room temperature, a slight increase of mass was detected and due to buoyancy and flow effects, it was difficult to deduce the actual quantity of H, adsorbed on the sample. In the susceptibility experiments, no change was observed even after 20 h. The CeO, sample was heated in a stream of hydrogen up to 673 K with incremental steps of 50 K. The heating rate was 4 K min-'. At each temperature, the sample was held for 2 h before being cooled to 294 K for measurement of the magnetic susceptibility, this always being carried out under H,. The maximum temperature was kept to 673 K in order to avoid textural modifications. A mass loss was observed for temperatures higher than 473-523 K, in agreement with the results of Fierro et aL3 This was due to the elimination of water molecules resulting from the reduction of ceria according to the reaction:

2Ce0,

+ H, -+ Ce,O, + H,O

(3)

Thus, the reduction percentages could be deduced from the values of the mass losses. However the calculations were erroneous since some of the hydroxyl groups remain attached to the solid at the reduction temperature. For example, after 2 h of reduction at 623 K, 230 pmol (H,O) g-' were desorbed from the sample by heating under vacuum at 673 K. FTIR spectroscopy has also shown the high thermal stability of some hydroxyls [v(OH) = 3450 cm- '1. Furthermore, at higher temperatures, the mass losses include the reduction of the carbonates species, as verified by the analysis of CH, and C O in the reaction products. It was concluded that gravimetric measurements alone cannot give a correct estimate of the degree of reduction and therefore, the Ce3+ ion content. In the magnetic susceptibility experiments, no change occurred below 423 K. The first increase was observed after the treatment at 473 K. After the reduction at 673 K, the value of x at 294 K was 2.36 x lop6 emu g-'. To calculate the corresponding percentage reduction according to eqn. (3), we assumed that C e 2 0 3 was paramagnetic and obeyed the Curie-Weiss law:

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/-. 10 0 400

500

600

700

800

TIK

Fig. 4 Reduction percentage obtained from the magnetic susceptibility us. the reduction temperature, 2 h isothermal treatment for each temperature. (a) Ce0,-400, (b) Ce02-850, (c) Ce02-400, 20 h isotherm

600

700

800

900

1000

TIK

where x is the specific susceptibility, C is the Curie constant, p is the magnetic moment of the Ce3+ ion, i.e. 2.5 pB, M is

the molecular mass of CeO,., and 6 is the Weiss temperature. The latter was determined experimentally on two different ceria samples reduced, at 673 and 10oO K, respectively by plotting the reciprocal susceptibility against temperature. The plots were nearly linear and 6 was found to be ca. -160. Thus the susceptibility of Ce203 can be expressed by x = 4.8 x 10-3/(T + 160). This expression was used to calculate the reduction percentages from the susceptibility data obtained at the actual reduction temperature. Fig. 4(a) shows the variation of reduction percentage with temperature. The reduction began at 473 K and increased with temperature. It reached a plateau at around 700 K, the reduction level being close to 25%. The degrees of reduction calculated from the values of x at 294 K and after cooling the sample to room temperature differed from the previous one only by a few percent. This verifies the validity of the susceptibility equation used. It also indicates that, in the reaction conditions, the reduction state of ceria is not greatly modified by cooling the sample under H, . Reduction of Ce0,-850 by Hydrogen The results obtained on Ce0,-850 are shown in Fig. 4(b).The reduction process began at 473 K, as for Ce0,-400 but with a slow rate (2.8% reduction at 673 K). A sharp increase was noted at ca. 750 K, followed by a plateau at ca. 900 K for 3 5 3 6 % reduction. The corresponding formal composition is CeO1.82 *

Influence of the Isothermal Reduction Time By calculating the number of ions present at the surface using a cubic model,' one can estimate that a reduction limited only to the surface would lead to a 20% reduction extent for Ce0,-400 (115 m2 g- ',including microporosity) and 1YOfor Ce0,-850 (5 m2 g-'). These values are not very different from those obtained at 650 K, i.e. 25 and 3%, respectively. This may suggest that at this temperature, the reduction is limited to the surface without major bulk reduction. To verify this, we studied the reduction process on CeO2-400 as before, but holding each intermediate temperature for 20 h. The results are given in Fig. 4(c). We observe that the extent of the reduction exceeds that of the surface without an inflexion point in the curve. There is a plateau at 673 K (32%) which corresponds to the composition CeO,.,, . These results indicate that for this high-surface-area ceria, there is not a clear

Fig. 5 Analysis of the gases thermodesorbed from a Ce0,-400 sample reduced at 673 K (CeO,.,,) and desorbed at 673 K. 0 , H,O; CO; 4 H, 4

9

distinction between the reduction of the surface and that of the bulk. Study of the Reduced Phase by Thermodesorption under Vacuum Ce0,-400 reduced at 673 K (CeO,.,,) was put under vacuum at 673 K and then thermodesorbed up to 1030 K (8 K min- '). As shown in Fig. 5 the analysed products were H,O, H, and CO, with maxima at 780, 840 and 850 K, respectively. The mass loss was 1.2 mg g- ',which corresponded to 26 pmol (CO) g-', 26 pmol (H,O) g-' and ca. 3 pmol (H,) g-', This quantity of hydrogen is low but detectable. Note that 42 pmol (CO,) g-' were desorbed from standardised Ce0,-400 at ca. 810 K. Thus we can assume that some of the polydentate carbonate species were partially hydrogenated during the reduction at 673 K, to give intermediates which further decomposed into C O and H,O. Oxygen Adsorption on the Reduced Samples Oxygen adsorption was carried out at room temperature on Ce0,-400 reduced to the formal CeO,.,, composition as determined by magnetic susceptibility. Increasing pressures of oxygen were introduced and the susceptibility was measured for each equilibrium value of the adsorbed oxygen. Most of lo-' Torr). the oxygen was adsorbed at low pressure The saturation was observed for pressures around 10 Torr and the adsorbed mass corresponded to 360 pmol (0,)g-', the value being 350 pmol (0,)for complete reoxidation. At 294 K, no desorption was detected when the sample was put under vacuum. Fig. 6 shows the variation of the susceptibility us. the number of 0, chemisorbed molecules. The variation is linear. The emu mol-' 0,. This corresponding slope is -6.4 x value is slightly lower than the theoretical one (-7.3 x obtained for the chemisorption of one oxygen atom per two Ce3+ ions. It could indicate that some excess oxygen was adsorbed. This excess oxygen was confirmed by a thermodesorption experiment under vacuum up to 673 K (Fig. 7). The main peak was CO,, with two additional peaks of 0, (383 K) and H,O (583 K). The excess oxygen was also revealed by a small reduction peak at 408 K detected during a TPR on a reoxidized Ce0,-400 sample. This could correspond to a paramagnetic superoxide species 0; as shown by EPR23*24or

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N,Jprnol g-'

Fig. 6 Magnetic susceptibility changes as a function of the number of oxygen molecules adsorbed at 294 K on a reduced Ce0,-400 (CeO 1.88)

more probably to peroxide species 0;-which were observed recently by FTIR after adsorption on partially reduced ceria.' These species are diamagnetic. Their desorption would result in no significant change of the magnetic susceptibility, which was effectively observed in our conditions. However, the IR study was unable to detect this species on our samples. Compared with the initial Ce02-400 sample, the maximum for the CO, peak at 653 K indicates that the reduction-reoxidation processes lead to less stable carbonate species. The origin of the water desorption at 583 K is less certain. One may suppose that it comes from the oxidation of hydrogen retained in the ceria, as shown above. In the case of Ce0,-850. reduced at 970 K to CeO,.,,, the adsorption of oxygen at room temperature also results in complete oxidation of the Ce3+ ions. That means that the reoxidation process does not depend on the texture of the initial ceria.

XPS Assignments of the Ce 3d Lines Fig. 8 shows the XPS Ce 3d and 3d3/, core-level spectra for a mixture of Ce" and Ce and the corresponding deconvolution. The assignment of the Ce 3d photoemission lines, since the has been the subject of initial work of Burroughs et much work, both from an experimental and a theoretical point of The spin-orbit coupling is indicated by v and u for the 3d,/, and 3d3,, states respectively and the peaks denoted by v and v" were assigned to a mixing of 3d9 4f2

TIK

.,

Fig. 7 Analysis of the thermodesorbed gases after the reoxidation at 294 K of a reduced Ce0,-400 sample. 0,; 0 ,H,O; A,CO,

S/b/882.3

888.6

898.1

916.7

88b.6 885:2'

Q20

Fig. 8 Ce 3d photoemission of a mixture of Ce" and Ce"' peaks. Peaks denoted by the subscripts v and u are assigned to 3d,,, and 3d,,, states, respectively. The fitting procedure (-) exhibits both CeIV(v, v", vm,u, un, u"') and Ce"' (vo, v', uo, u') states. For the assignment of these lines, see text

and 3d9 4f V"- CeIVfinal states and v"' to the 3d9 4f" V" Ce"' final state. V" represents the full valence band which corresponds here to 0 2p6. The occurrence of 4f1 V"-' and 4f? VNp2final states was due to a lowering of the f states to screen the core hole created during the photoemission process. As clearly seen at the end of the reduction process, the appearance of vo (poorly resolved from the v line) and v' lines is associated with Ce''' states. They were assigned to 3d9 4f2 V"-' and 3d9 4f' V" Ce"' final states, respectively, as with the Ce" states, the creation of a 3d9 core hole results in a lowering of the binding energy of the more localized 4f states. When they were assigned to the 3d&, 4f" V" Ce"' state at 916.7 eV, the binding energies of the Ce 3d,/2 lines, as determined by the curve-fitting procedure, were 880.6 & 0.5, 882.3 f 0.2, 885.2 & 0.3; 888.6 & 0.3 and 898.1 & 0.2 eV for the vo, v, v', v" and v'" states, respectively, which are in good agreement with reported literature data.3

vn-2

1933

Reference Spectrum Fig. 9 shows the spectrum of the Ce0,-850 sample following an in situ treatment of dry air at 1123 K for 1 h and then an evacuation under a very low pressure Torr). The curvefitting procedure revealed some initial reduction (up to 10%). We believe that this reduction was due to the high mobility of the oxygen atoms of ceria following the oxidation treatment. However, when 10 Torr of CO, at room temperature were introduced just after the oxidation treatment, no detectable Ce"' states could be found. Furthermore, a match between this spectrum and a spectrum of decontaminated CeO, 34 was satisfactory. Thus we believe that CO,, acting as a weak oxidant at room temperature (as found by UV-VIS reflectance s p e c t r o ~ c o p y ~stabilises ~) the CetV state by passivation. However, the highly active surface generated by the oxidation treatment is probably partly reduced under high vacuum.

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