Reduction of High Surface Area CeO2â ZrO2 Mixed Oxides

9186

J. Phys. Chem. B 2000, 104, 9186-9194

Reduction of High Surface Area CeO2-ZrO2 Mixed Oxides Marco Daturi,*,† Elisabetta Finocchio,†,⊥ Claude Binet,† Jean-Claude Lavalley,† Fabienne Fally,‡ Vincent Perrichon,‡ Hilario Vidal,§,# Neal Hickey,§ and Jan Kasˇpar§ Laboratoire de Catalyze et Spectrochimie, UMR 6506, ISMRA, 6, Bd du Mare´ chal Juin, 14050 Caen Cedex, France, Laboratoire d’Application de la Chimie a` l’EnVironnement, UMR 5634, CNRSsUniVersite´ C. Bernard-Lyon 1, 43, Bd. du 11 NoVembre 1918, F-69622 Villeurbanne Cedex, France, and Dipartimento di Scienze Chimiche, UniVersita` di Trieste, Via Giorgieri, 1, 34127 Trieste, Italy ReceiVed: February 18, 2000; In Final Form: June 26, 2000

The objective of this study was to examine the mechanism of the reduction by hydrogen of ceria-zirconia (CZ) mixed oxides having a high BET surface area (100 m2 g-1). Three methods were used in parallel to assess the Ce3+ content, the surface and bulk oxygen vacancy concentrations, and the resulting oxygen storage capacity (OSC): temperature programmed reduction, Fourier transform infrared (FT-IR) measurements of methanol adsorbed on the reduced surfaces, and a Faraday microbalance to determine the magnetic susceptibility of the reduced oxides. The three methods conclude that the introduction of zirconium into the ceria lattice has a positive influence on the OSC. Compared to pure ceria, the CZ mixed oxides exhibit better redox properties, with a lower temperature of initial reduction and a higher reduction percentage for all compositions. The reducibility increases with the zirconium content, however the OSC per gram of solid is practically the same for Zr contents between 20% and 50%. The reduction process very rapidly involves the bulk, but a treatment at room temperature under oxygen of the reduced samples oxidizes them almost completely. However, the FT-IR results underline the differing behavior of ceria for the distinct surface and bulk reduction processes.

Introduction Since the middle of the 1990s, CeO2-ZrO2 mixed oxides have become the state of art as oxygen storage and release promoters in advanced three-way catalyst (TWC) technology.1 The oxygen storage capacity (OSC), i.e., the ability of CeO2based promoters to act as an oxygen buffer under exhaust conditions by releasing/acquiring oxygen through redox processes involving the Ce4+/Ce3+ couple, provides a way to increase the efficiency of the TWC by enlarging the air-to-fuel window. CeO2-ZrO2 mixed oxides provide higher performances compared to CeO2 even after severe aging, due to their high thermal stability and enhanced reducibility.2-4 The exact origin of the enhanced redox properties of the CeO2-ZrO2 mixed oxides and, particularly, the mechanism of the reduction process are still a matter of debate.5,6 Nevertheless, there is general agreement that reduction in the bulk of these materials is improved by the insertion of ZrO2 into the lattice, which creates mobile lattice oxygen available for reduction.7 In the present investigation, the mechanism of reduction and the OSC of a series of high surface area (HS) CeO2-ZrO2 mixed oxides is investigated by means of FT-IR, oxygen uptake, and magnetic balance techniques. The aim is to add knowledge to the mechanism of CeO2-ZrO2 reduction. * Corresponding author. E-mail: [email protected]. † ISMRA. ‡ CNRSsUniversite ´ C. Bernard-Lyon 1. § Universita ` di Trieste. ⊥ Permanent address: Istituto di Chimica, Facolta ` di Ingegneria, Universita` di Genova, P. le J. F. Kennedy, 16129 Genova, Italy. # Permanent address: Departamento de Ciencia de los Materiales e Ingenierıá Metalu´rgica y Quı´mica Inorgańica, Universidad de Ca´diz, Puerto Real, 11510, Spain.

It is important to remark that within the time frames of the experiments performed it is not possible to reach a thermodynamic equilibrium state, so the processes observed are governed by kinetics, leading to a quasi-stationary state after undergoing a relatively fast process.8 This is more critical for the lower temperatures at which the reduction begins to be observed because the process is hence slow. Each technique here used (TPR, magnetic balance, FT-IR) has its own experimental requirements, so experimental conditions may differ. For example, contact time or hydrogen pressure in a flow or in a static process can differ. Thus we have preferred to compare the overall trends observed with each technique (independently considered) more than directly rely on experimental values. Experimental Section High surface (HS) ceria-zirconia mixed oxides with different molar composition were synthesized by Rhodia using a precipitation route from nitrate precursor. Their nature has been described in detail recently.9 Hereafter they will be identified as CZ-X/Y, where X and Y indicate the molar percentage of CeO2 and ZrO2, respectively. Their BET surface area was between 97 and 108 m2g-1.9 Temperature programmed reduction (TPR) and oxidation (TPO) measurements were performed in a conventional system equipped with a thermal conductivity detector as previously reported.10 The experiments were carried out as follows:10 the mixed oxide was pretreated in situ in Ar at 823 K for 2 h, with pulsing pure O2 (100 µL) every 30 s. Such a treatment ensures a clean CeO2-ZrO2 surface, providing TPR profiles free from artifacts.11 The sample was then cooled to room temperature with pulsing O2, the flow switched to 5% H2 in Ar, and the temperature ramped at rates of 10 K min-1 up to 1273 K. The

10.1021/jp000670r CCC: $19.00 © 2000 American Chemical Society Published on Web 09/09/2000

High Surface Area CeO2-ZrO2 Mixed Oxides sample was held at this temperature for 30 min, then the gas was switched to Ar and the sample cooled to 700 K, where O2 uptake (total-OSC) was measured by a pulse method. The OSC measured at lower temperatures was carried out in a similar way, with the exception of the isothermal reduction, which was carried out for 1 h. Magnetic susceptibility measurements were carried out with a Faraday microbalance in order to measure the Ce3+ content during the reduction. The apparatus and the experimental details have already been already described.12 Prior to the measurements, the instrument was calibrated with cobalt mercury thiocyanate. The specific magnetic susceptibility values (χ) were corrected for ferromagnetic impurities by extrapolation to infinite magnetic field. The uncertainty on the extrapolated value was about 0.1 × 10-6 emu cgs g-1 (to be multiplied by 12.56 to obtain SI units in m3 g-1). About 200 mg of the oxides was placed onto quartz wool in a silica bucket for analysis. A standard cleaning pretreatment similar to that described above for TPR was applied. After 2 h at 823 K under flowing O2(5%)/He, the sample was cooled to room temperature, evacuated, and then heated for 1 h at 773 K under high vacuum (0.1 mPa). The reduction study consisted of successive 1 h treatments under flowing H2(5%)/He between room temperature and 973 K. After reduction at one given temperature, the sample was cooled under the same atmosphere (10 K min-1) to 298 K, at which temperature the magnetic susceptibility measurements were performed. The percentage of Ce3+ was calculated using the Curie-Weiss equation determined on a well-defined Ce2O3 sample13 which gives a χ value of 11.4 × 10-6 emu cgs g-1 at 298 K. The reduction percentage could also be estimated from the mass loss during reduction and from the oxygen uptake determined by gravimetry during a subsequent oxidation. For FT-IR studies, the powdered sample was pressed into a disk of ∼10 mg cm-2 and activated in situ in a quartz cell. All the samples were submitted to a standard cleaning pretreatment for the elimination of impurities. This pretreatment has been described elsewhere.11 For the reduction at a fixed temperature, samples were exposed to 13 kPa of H2 for 0.5 h and then evacuated at the same temperature for 0.25 h; this was repeated twice more. Spectra were recorded at room temperature with a Nicolet Magna 550 FT-IR spectrometer and treated by the Nicolet OMNIC software. Results and Discussion 1. Temperature Programmed Reduction and OSC Measurements by O2 Uptake. The TPR profiles of all the CeO2containing catalysts employed in this study are reported in Figure 1. The TPR profile of CeO2 shows the well-known two-peak pattern due to respectively surface and bulk reduction.14 In contrast, the TPR profiles of the mixed CeO2-ZrO2 oxides show essentially a main broad reduction feature, in agreement with the promotion of the reduction in the bulk of the mixed oxide upon doping with ZrO2. As previously discussed,10,15 surface and bulk reduction cannot be distinguished by the conventional TPR technique, since both processes occur almost simultaneously during the TPR experiment. A perusal of Figure 1 reveals, however, some additional features: (i) the apparent H2 consumption continues even after the main peaks; (ii) small, but significant shifts in the peak temperatures are apparent; (iii) a strong tailing of the peaks toward low temperatures is particularly evident in the CZ-15/85 sample. With regard to the first point, recent investigations have suggested that single-phase high surface area CeO2-ZrO2 products feature a single reduction peak, additional high temperature peaks being mainly due to

J. Phys. Chem. B, Vol. 104, No. 39, 2000 9187

Figure 1. TPR profiles of (a) CZ-100/0-HS, (b) CZ-80/20-HS, (c) CZ-68/32-HS, (d) CZ-50/50-HS, and (e) CZ-15/85-HS.

the presence of nonincorporated ceria or other features such as bulk carbonates etc.1,15,16 Due to the application of the cleaning procedure, the latter effects are not expected in the present data. However, sample sintering does occur during the TPR, which could easily give rise to broad H2 consumption at high temperatures and/or some types of buoyancy effects frequently found in thermogravimetric studies. The shift of the reduction peaks to low temperatures upon increasing the ceria content from 15 to 80 mol % (point ii) is in agreement with the suggestion that a high concentration of ions with redox character (i.e., Ce4+ ions) favors oxygen mobility.17 The tailing observed in CZ-15/85 could be related to a sample transformation occurring during the TPR experiment. We recall that this sample contained some nonincorporated meta-ZrO2 which was incorporated into the lattice upon thermal treatment.9 To avoid artifacts due to H2 adsorption/desorption,18,19 the O2 uptake at 700 K was taken as the measure of the amount of oxygen vacancies created upon reduction. These uptakes were measured after an isothermal reduction as described in the Experimental Section. The results are summarized in Table 1. These values correspond to the overall amount of transferable oxygen as a function of reduction temperature, and they were carried out by running the isothermal reduction at selected temperatures for 30-60 min, i.e., until completion of the H2 consumption. Note that under our experimental conditions full reoxidation of the support is ensured and there is no significant contribution from O2 adsorption.3 In addition, previous studies have shown a good correspondence between the degree of reduction as determined by this methodology and that measured by other techniques such as EXAFS and magnetic susceptibility measurements.18,20 Comparison of the values reported in Table 1 reveals a number of interesting features. Despite comparable textural properties, the total-OSC is higher for the mixed oxides compared to CeO2, with exception of the CZ-15/85 sample. This is observed irrespective of the reduction temperatures. This behavior is particularly remarkable for the data at 773 K, i.e.,

9188 J. Phys. Chem. B, Vol. 104, No. 39, 2000

Daturi et al.

TABLE 1: O2 Uptakes Measured at 700 K over the CeO2-ZrO2 Mixed Oxides after Reduction at the Indicated Temperature

a

sample

peak temp (K)

773 K

CZ-100/0-HS CZ-80/20-HS CZ-68/32-HS CZ-50/50-HS CZ-15/85-HS

790, 1168 828 833 836 656, 845

135 435 425 400 180

O2 uptakea (µmol g-1) 973 K 1273 K 365 565 580 610 245

550 670 720 740 260

773 K

Ce3+ b (%) 973 K

1273 K

9 35 39 47 62

25 46 53 72 84

38 54 66 87 90

Standard deviation ( 10 µmol g-1. b Estimated from the O2 uptake.

at a temperature which has been associated with the reduction of surface of CeO2. Bearing in mind that previous characterization of the present samples showed that surface composition is consistent with the bulk one,9 we infer that partial reduction in the bulk of the mixed oxides occurs even at 773 K. Note that if we assume the nonreducibility of the Zr4+ sites,3,21 reduction of as many as four subsurface layers can be estimated for the CZ-15/85 sample. The OSC measured at the three temperatures investigated depends on both the sample composition and reduction temperatures. In fact, at 773 K, we find a maximum amount of transferrable oxygen for the CZ-80/20 sample; however, when the reduction temperature is increased to 973 and 1273 K, this maximum shifts to the composition of the sample CZ50/50. On the other hand, it appears clear that the differences in oxygen uptakes measured for the intermediate compositions (50-80 CeO2 mol %) are relatively small compared to the values measured for either CZ-15/85 or CZ-100/0 sample. In the CZ-15/85 case, the limiting factor is the total content of reducible CeO2, since 85% of Ce4+ sites are reduced already at 973 K. Finally, it is of interest to note that the effectiveness of Ce4+ reductionsexpressed as percent Ce3+sincreases with the ZrO2 content at all investigated reduction temperatures. Similar behavior has been observed in Rh/CeO2-ZrO2 mixed oxides prepared by a solid-state synthesis,3 i.e., highly sintered samples. However, the degrees of reduction here obtained are remarkably high at moderate temperatures and, furthermore, they are obtained in the absence of the supported metal, indicating the importance of the textural properties on the reduction behavior of these mixed oxides. However, the OSC per gram of solid is practically the same for zirconium contents between 20% and 50%. 2. Ce3+ Magnetic Susceptibility Measurements. First, it must be mentioned that any increase in the paramagnetic susceptibility can be ascribed to the formation of Ce3+, no significant reduction of the zirconium ions having been observed on a pure zirconia treated 1 h at 973 K under hydrogen.22 Thus, from the χ values measured after the successive treatments, the reduction percentages for each oxide are calculated according to the ratio [Ce3+]/[Ce3+ + Ce4+]. The susceptibilities χ of the initial samples under vacuum were in the range -0.05 to -0.09 10-6 emu cgs g-1, values close to that expected for a diamagnetic ceria sample containing only Ce4+ ions. These initial values have been taken as a reference corresponding to 0% reduction of the mixed oxide. After the cleaning pretreatment, there is a slight increase of χ compared to the initial value, suggesting that some mild reduction of the oxide samples happens after desorption under vacuum at 773 K. However, the calculated reduction extent remains lower than 0.5% (see Table 2, last column). The reduction percentages are shown in Figure 2 as a function of the reduction temperature for each oxide. Compared to CeO2, the reducibility of the mixed oxides is much higher and it

TABLE 2: Oxidation of the CeO2-ZrO2 Mixed Oxides after Reduction and Evacuation at 973 K

sample CZ-80/20-HS CZ-68/32-HS CZ-15/85-HS a

oxygen uptake residual Ce3+ Ce3+ (%) after (%) after O2 initial cleaning (µmol g-1) equiv at 298 Ka Ce3+ (%) at 298/973 Kb treatmentb 623 650 242

50.4 59.7 83.3

1.3/0.8 1.5/0.7 0.1/0.0

0.5 0.4c 0.0

6 kPa of O2. b Calculated from χ. c Under 5% H2/He.

Figure 2. Reduction percentages [Ce3+]/[Ce3+ + Ce4+] of the mixed oxides, estimated from the magnetic susceptibility measurements at 298 K, as a function of the reduction temperature. The measurements were performed under H2(5%)/He.

increases with zirconium content. The beginning of Ce3+ formation is observed at about 470 K for the CZ-80/20 and CZ-68/32 samples and at lower temperature for CZ-50/50 and CZ-15/85, close to 373 K in the latter case. Up to 700 K, the curves for the CZ-80/20 and CZ-68/32 samples are very close. They are clearly distinct at higher temperatures. The reducibility enhancement appears independent of the structure of the mixed oxide. It is most pronounced for the CZ-15/85, which has a tetragonal structure.9 For this solid, the reduction percentage obtained at 973 K is 93.4%, evidencing an almost complete reduction of the cerium ions. For ceria, the reducibility appears lower than observed in a previous work where reduction percentages higher than 50% were obtained at 973 K.23 Several factors can explain this difference: higher H2 pressure (100 kPa instead of 5 kPa in this study) and longer reduction times (2 h instead of 1 h at each temperature and more intermediate reduction temperatures), combined with a poorer textural stability under hydrogen above 700 K.24 However, the curve related to ceria in Figure 2 does exhibit an inflection point, which is normally expected for CeO2 in this type of magnetic study and corresponds to the reduction of one surface layer. It is observed between 770 and 870 K at a reduction percentage of about 15%. According to the magnetic data obtained on ceria with different BET surface areas,23 this reduction percentage would lead to a BET surface area of 95

High Surface Area CeO2-ZrO2 Mixed Oxides

J. Phys. Chem. B, Vol. 104, No. 39, 2000 9189

TABLE 3: Ce3+ Percentages (%) of the CeO2-ZrO2 Mixed Oxides after Reduction at 973 K and a Subsequent Treatment under Vacuum at 298 and 973 K Ce3+ a (%) evacuation at sample

cooling at 298 K under H2(5%)/He

CZ-100/0-HS CZ-80/20-HS CZ-68/32-HS CZ-50/50-HS CZ-15/85-HS

21.2 54.6 65.1 77.2 93.4

298 K

973 K

19.7 54.3 64.4

15.4 50.7 60.6

86.2

82.8

973 K (∆m) 16.0 50.8 60.5 72.9 87.4

a

Calculated from (except for the last column where it is calculated from ∆m.

m2

g-1,

m2

g-1

very close to the 100 value of the present sample. The surface reduction process may be represented by

2Ces4+ + 4Os2- f 2Ces3+ + 3Os2- + 0 + 1/2O2(g) (1) The symbol 0 represents a surface O vacancy. For generality, the eliminated O-surface species are written as O2 gas, although this would be H2O or CO2 for H2 and CO reducing agents. In contrast, no inflection point is observed on the reducibility curves of the mixed oxides, indicating that no clear distinction can be made between surface and bulk reduction, in good agreement with the TPR results. Variations of mass were also monitored during the successive treatments under hydrogen. Upon introduction of hydrogen at room temperature, there is a very small mass increase. However, considering the spurious changes due to buoyancy and mass flow effects which can happen under these conditions, it is not possible to give a correct estimation of the hydrogen mass adsorbed at room temperature. Thus, for all the samples, H2 chemisorption is clearly evidenced at 373 K. The mass increase reaches a maximum at 550 K and then decreases owing to the elimination of water. In the case of pure ceria, the maximum is shifted to higher temperature i.e., 625 K. We have used the mass changes to calculate the reduction percentage at each temperature. The obtained values are smaller than those calculated from the magnetic susceptibilities, the difference between the two curves decreasing with increasing temperature. This discrepancy between the two sets of values can be explained by the fact that measurements were performed after cooling under 5% H2/He, without desorption at high temperature. Under these conditions, some hydrogen and water remain adsorbed on the solid and contribute to diminish the reduction percentage calculated from the mass loss, whereas the presence of hydrogen adsorbed on the oxide would result in a higher content of paramagnetic Ce3+ ions. Thus, to get the same reduction percentages starting from the mass loss or the magnetic susceptibility, it seems necessary to desorb the reduced solids at high temperature. In the case of pure ceria, temperatures higher than 700800 K are required to desorb H2.25 In the case of the CZ-50/50 mixed oxide reduced between room temperature and 873 K,22 it has been shown that the Ce3+ content is not very different before and after desorption for T < 773 K. A more significant although limited effect of the reversible H2 adsorption/desorption was observed after reduction at 873 K, where the difference in Ce3+ after desorption was 3.7%. For this reason, we have studied, for the samples reduced at 973 K, the influence of a vacuum desorption at 973 K on the mass and the susceptibility, and Table 3 gives the obtained Ce3+ percentages. It appears that a slight decrease in the Ce3+ content is already observed after evacuation of hydrogen at room

Figure 3. OSC (µmol of O2 g-1 of oxide) of the mixed oxides, estimated from the Ce3+ content obtained from the magnetic susceptibility measurements at 298 K, as a function of the reduction temperature. The measurements were performed under H2(5%)/He.

temperature. The difference is higher after desorption at 973 K, between 3.9% and 5.8%, and even 10.6% for the CZ-15/85. However, the cerium content of this oxide being low, the uncertainty on this value is high. Table 3 clearly shows that the final reduction percentage values calculated from χ after reduction and desorption at 973 K are in very good agreement with those deduced from the total mass loss assuming the global reduction scheme given by eq 1. To reinforce this result, we have performed the oxidation of these samples reduced and desorbed at 973 K and followed the evolution of the Ce3+ content as a function of the mass of adsorbed oxygen. O2 was introduced at 298 K by small doses through a leak valve. The reoxidation started at very low pressure (0.1 Pa). The magnetic susceptibility decrease followed an almost linear relationship with the oxygen mass adsorbed at room temperature. The final oxygen pressure was 6 kPa. Then the oxide was heated at 973 K to achieve reoxidation. Table 2 gives the oxygen uptakes at room temperature. As shown in Table 2, the residual Ce3+ content measured after oxidation at room temperature and 973 K are very small and close to those measured initially after the cleaning treatment. It is thus possible to conclude that the initial cleaned state has been restored and the reoxidation can be considered complete. Thus, for the solids reduced and desorbed at 973 K, Tables 2 and 3 provide evidence of a very good agreement between the Ce3+ percentages calculated from the magnetic susceptibility and those deduced from the mass changes, after either reduction or oxidation. In other words, the OSC deduced from the magnetic measurements is very reliable provided that the reversibly adsorbed hydrogen has been eliminated or that it is negligible. As outlined above, previous measurements on the reversibility of hydrogen adsorbed on a CZ-50/50-HS oxide22 have shown that the amount of hydrogen so adsorbed does not seem significant below 773 K. Consequently, the Ce3+ concentration can be considered with a good approximation as representative of the OSC under the experimental conditions here used. The corresponding curves are shown in Figure 3 as a function of reduction temperature. It must be noted that for the protocol of reduction used in IR measurements, Fs and Fs+ surface defects were also considered at 573 K (see belowseqs 2-4). 3. OSC Measurements Using OCH3 Species as a Surface IR Probe. Methoxy species produced by the dissociative adsorption of methanol on the surface of the crystallites have been used as an IR probe for the reoxidation process.26 They can be considered as more directly representative of the real surface state than the above used TPR/TPO and magnetic measurements. A forbidden electronic Ce3+ transition in the IR


Daturi et al. TABLE 4: ν(OC) Wavenumbers (cm-1) for OCH3 Species Formed by Methanol Adsorption at Room Temperature on CZ-100/0-HS Sample Pre-treated by H2 (3 (0.5 h) and outgassed at the Indicated Temperatures T (K) vibration

573

623

673

773

873

ν(OC(I)) ν(OC(II-A)) ν(OC(II*))

1107 1062 b

1106 1072 b

1112a 1078 1083

1114a

1103a

b

1081

1070

1080

a

1073

Weak band. b No band.

Figure 4. Spectra of CZ-100/0-HS sample H2 pretreated at 573 (a), 623 (b), 673 (c), 773 (d), 873 (e), and 1073 K (f) after subsequent introduction of 130 Pa of methanol at room temperature: ν(OC) stretching region of adsorbed OCH3 species.

SCHEME 1

Figure 5. Adsorbed OCH3 species: trend of the ν(OC(II-A)) wavenumber vs the temperature of reduction for CZ-100/0-HS (a) and CZ50/50-HS (b).

spectral range may also be used to follow the sample’s reduction state.27 This transition becomes allowed, albeit very weakly, by the action of the local crystal field when defects are created in the bulk. Therefore it may detect only a reduction which is not limited to the surface of the crystallites. Because of experimental requirements, conditions for the in situ reduction of the samples in the IR cell differ from those used for TPR/TPO and magnetic measurements. Comparison of the IR results with those obtained by the two other techniques is then qualitative. However, a quantitative bridge may be established between the present IR results and magnetic measurements in the literature,22 which were performed under similar experimental conditions of reduction. 3.1. Methoxy Species Adsorbed on CZ-100/0-HS Sample. Upon methanol adsorption on CZ-100/0-HS treated under H2 at 573 K, the resulting ν(OC) bands for methoxy species resulting from CH3OH dissociation (Figure 4, spectrum a) are identical with those observed in the case of unreduced ceria.26 The bands are ascribed to methoxy species adsorbed on surface cerium ions coordinatively unsaturated to a greater or lesser extent, as indicated in Scheme 1. The II-A and II′ notations, here used to distinguish between the two doubly bridging OCH3 species, are analogous to those used for OH species,28 as the same cationic sites can be involved in both OCH3 and OH species adsorption. While the spectroscopic behavior of bands due to OCH3(II-A) and OH(II-A) favors assignment of the adsorption of OCH3 and OH species to the same II-A cationic site, we propose to identify the OCH3(II′) site with the previously defined 28 OH(II-B) site, even if the only argument

that supports our statement is the frequency order with respect to the coordinative unsaturation of the sites. As methoxy species are adsorbed on coordinatively unsaturated surface cations, they complete the coordination sphere of these cations. Thus, the ν(OC) value is very sensitive to the cationic site arrangement. Conversely, the CH3 group in methoxy speciessmore distant from the surfacesis thought to experience an average electric field and consequently to be sensitive only to electronic surface properties, and not to the geometry of the adsorption site. Therefore, we do not further take into account ν(CH3) vibrations in this work. It is worth remarking that upon reduction between 623 and 673 K (Figure 4, spectra b,c) a conversion between OCH3(I) and OCH3(II*) species is apparent [the ν(OC(II*)) band at ∼1080 cm-1 is on the high-frequency side of the ν(OC(II-A)) one]. This conversion is reversible upon reoxidation by adsorbing O2 at room temperature.26,29 Upon reducing between 773 and 873 K, the overall intensity of ν(OC) bands falls abruptly indicating a sintering of the material which occurs under the reducing environment.30 The sample was in fact pretreated at 873 K under oxidizing conditions, which does not affect its texture.11 Reduction temperature effects on the frequencies for pure ceria adsorbed methoxy species are reported in Table 4. Upon reduction between 623 and 673 K the ν(OC(I)) band nearly vanishes, its wavenumber being increased. The ν(OC(II-A)) wavenumber increases smoothly from 573 to 773 K, but a downshift is observed after reduction at 873 K. The upper-limit value (1080 cm-1) is recovered at 1073 K. Conclusions similar to those obtained from a parallel study of the ν(OH) vibrations of OH adsorbed species 22 may be drawn (see Figure 5, curve a): (i) a progressive surface reduction beginning at 573 K and reaching a limit at 773 K; (ii) a surface-subsurface reorganization at 873 K, leading to a transient surface partial reoxidation and allowing the reduction of the bulk to begin;


J. Phys. Chem. B, Vol. 104, No. 39, 2000 9191 TABLE 5: ν(OC) Wavenumbers (cm-1) for OCH3 Species Formed by Methanol Adsorption at Room Temperature on CZ-50/50-HS Sample Pretreated under H2 (3 × 0.5 h) and Outgassed at the Indicated Temperaturesa T (K) 473

523

573

673

773

873

ν(OC(I)

vibration

1104

1104

1119b

1119b

1119b

ν(OC(II-A))

1064

1065

1104 1119b 1074

1079

1079

1079

a

Only OCH3(I, II) species adsorbed on cerium ions are reported here. b Weak band.

Figure 6. Spectra of CZ-50/50-HS sample H2 pretreated at 473 (a), 523 (b), 573 (c), 673 (d), 773 (e), and 873 K (f) after subsequent introduction of 130 Pa of methanol and evacuation at room temperature: ν(OC) stretching region of adsorbed OCH3 species.

(iii) the obtaining at 1073 K of the reduced surface state already observed at 773 K, while the reduction progresses deeply into the bulk. 3.2. Methoxy Species Adsorbed on CZ-50/50-HS Sample. Methanol adsorption on the CZ-50/50-HS sample results in coordination of methoxy species to both zirconium and cerium cations. On zirconia, ν(OC) bands were observed at 1154 (type I) and 1052 cm-1 (type II).31 ν(OC) bands for bridged OCH3(II) species adsorbed either on zirconium or on cerium cations were not resolved, but the wavenumber of the maximum of the composite band was found to be governed by the OCH3(II-A) species adsorbed on cerium ions. For OCH3(I) species, adsorbed on either zirconium or cerium cations, the ν(OC) bands were well resolved. When OCH3(I) species were adsorbed on Zr4+, the ν(OC) band was found not to depend on the reductive treatment, indicating then that Zr4+ ions were not reduced at any stage of the treatment. For simplicity, only bands due to OCH3 species adsorbed on cerium ions are considered below. Spectra of adsorbed OCH3 species in the ((OC) region are shown in Figure 6. Comparing these spectra (CZ-50/50-HS sample) with those in Figure 4 (CZ-100/0-HS sample) two striking differences are evident (beyond others considered below): (i) Between 773 and 873 K the overall intensity of the ν(OC) bands is unchanged for the mixed compounds (Figure 6, curves e,f), while for pure ceria, as discussed above, a great intensity loss is observed in this temperature range (Figure 4, curves d,e). Indeed for the CZ-50/50-HS sample there is no sintering at 873 K,11 inferring that zirconia has a stabilizing effect for the surface area. (ii) When the temperature of the H2 treatment increases, an upward shift of the ν(OC) band for the OCH3(II-A) species is observed both for CZ-50/50-HS and CZ-100/0-HS samples (Figures 4 and 6), but for CZ-50/50-HS sample no distinct OCH3(II*) species are observed. When CZ-100/0-HS is reduced at 773 K, the OCH3(II*) species overlap with OCH3(II-A). In the case of the CZ-50/50-HS sample, the transition is not observed in two stages and it occurs at lower temperature. Thus, the ionic surface structure obtained for pure ceria is also reached by the reduction of CZ-50/50-HS sample, but already at ca. 600 K (interpolated temperature in Figure 5b) and without a distinct transient subsurface-surface reorganization. Wavenumbers concerning ν(OC) vibrations of OCH3 species adsorbed on cerium ions are reported in Table 5 for CZ-50/50HS sample. Looking at the ν(OC(I)) and ν(OC(II-A)) wave-

Figure 7. Oxygen storage capacity for samples CZ-100/0-HS (a) and CZ-50/50-HS (b) versus H2 treatment temperature, from IR measurements.

numbers for increasing temperatures, it clearly appears that the reduction of the CZ-50/50-HS sample begins at 573 K and that a very stable surface state is maintained at least up to 873 K. The results for the ν(OC(II-A)) bands are reported in Figure 5 (curve b), clearly illustrating: (i) a lowering by about 50 K of the initial reduction temperature of the CZ-50/50-HS sample (curve b), compared to that of CZ-100/0-HS (curve a) (ii) the stabilization of the surface state of CZ-50/50-HS sample for reduction temperatures above 573 K (curve b). 3.3. OSC of CZ-100/0-HS and CZ-50/50-HS Samples. The OCH3(I) T (OCH3(II* or II-A) reversible conversion of sites upon reduction/oxidation of the samples is used as a measure of the reduction state upon adding known volumes of oxygen at room temperature.26 OSC is thus measured by the amount of oxygen necessary to reoxidize a sample previously submitted to a reduction treatment.29 Under such conditions instantaneous surface reoxidation occurs, while simultaneous deep reoxidation into the bulk is questionable. In fact, ceria crystallites, highly reduced after reduction at T > 1073 K, were found to be not completely reoxidized by exposing them to O2 at room temperature.23 The oxygen uptakes at room temperature for low or moderately reduced samples are a full measure of the OSC associated with that reduction temperature, while for highly reduced samples the utilized OSC may be only indicative of the tendency to be reoxidized.32 In Figure 7 the OSC measured in this way (in µmol g-1 of sample) is plotted against the temperature of the H2 pretreatment for CZ-100/0-HS (curve a) and CZ-50/50-HS samples (curve b). In the case of CZ-100/ 0-HS, application of the method to samples reduced at temperatures higher than 773 K was not possible because of the concomitant oxidation of the sample and of the OCH3 surface probe into formate species. We suggest that, as the bulk of ceria becomes notably reduced, the local thermal effect in the reoxidation process is high enough to allow the methoxy oxidation.


Daturi et al.

The OSC for pure ceria measured by infrared versus the temperature of reduction (Figure 7, curve a) is comparable with the value obtained from magnetic experiments (Figure 3). The general trend is the same for the corresponding curves, but the OSC values measured by IR are somewhat higher. This is almost certainly because of milder reductions performed during the experiment in the magnetic balance (5 kPa of H2 in He versus 13 kPa of pure H2 used during the IR pretreatment). Nevertheless, the value of 280 µmol of O2 g-1 reached at 773 K (Figure 7a) is close to that reached at 873 K in magnetic experiments (Figure 3). As outlined previously, this approximately corresponds to the reduction of the first surface layer for pure ceria. In the case of CZ-50/50-HS sample reduced at 673, 773 and 873 K the corresponding OSC values obtained through either IR (Figure 7, curve b) or magnetic measurements (Figure 3) are similar. However, for the sample reduced at 673 K the OSC measured by IR is higher than that obtained by magnetic balance. It seems that the experimental conditions of reduction have a more critical influence for the lower temperatures. In fact, a previous study has shown that a CZ-50/50-HS sample treated in the magnetic balance according to the protocol used in IR (see Experimental Section) has a reduction percentage of 42% at 673 K.22 The corresponding OSC is 355 µmol of O2 g-1, in reasonable agreement with 320 µmol of O2 g-1 obtained by the IR measurements. By contrast, a very important discrepancy between results from the two techniques is observed for reduction at about 573 K (300 µmol of O2 g-1 in Figure 7b, compared to 50 µmol of O2 g-1 in Figure 3). If, as above, we consider the reduction percentage but at 573 K measured from the magnetic balance (16%) using the same experimental conditions of reduction as in IR experiments,22 we obtain an OSC value of 135 µmol of O2 g-1 i.e., about half the present IR value (300 µmol of O2 g-1). For the beginning of the reduction (in the experimental conditions here used during IR measurements), we propose that reduced features on the surface are not only Ce3+ ions. The production of surface defects such as Fs and Fs+ centers (anionic vacancies containing two or one electrons) must also be considered, being here observed in a transient state of the surface at the lower temperature of reduction (573 K). It is noteworthy that the OSC measured at 573 K (Figure 7, curve b) corresponds to the reduction of about one surface layer in the case of pure ceria (Figure 7, curve a). Then two assumptions are made: (i) The surface reduction process is the same as for pure ceria, without distinction between O atoms bound to either Zr4+ or Ce3+ ions, but in the case of CZ-50/50-HS sample half the produced electronic charges are stabilized not as Ce3+ ions but as Fs or Fs+ surface centers. (ii) Near subsurface oxygen ions are involved in the reduction allowing the first subsurface layer of Ce3+ ions to be reoxidized. The two assumptions may be combined in the following mechanism:

Ce4+, Zr4+(s) + 4O2-(s) f Ce3+, Zr4+(s) + 3O2-(s) + ~(s) + 1/2O2 (2) O2-(sb) + 2~(s) f O2-(s) + 0(s) + ~~(sb)

(3)

~~(sb) + 2Ce4+(sb) f 0(sb) + 2Ce3+(sb)

(4)

where the used notations mean O vacancy with no electrons 0, one electron, ~ (F+ center), or two electrons ~~ (F center); (s) indicates surface and (sb) near subsurface species. Step 2 is suggested to take place at 573 K, the elimination of one O

surface atom producing one Fs+ center and one Ce4+/Ce3+ reduction. Step 3 is the migration of a near-subsurface oxygen atom to the surface, leading to a subsurface F center, the electrons of which are further stabilized as Ce3+ in step 4. In steps 3 and 4 no oxygen is evolved from the sample, hence they may correspond to the 573-673 K temperature range in which no OSC variation is observed (Figure 7b). Obviously, whatever the sample (either pure ceria or mixed compounds) the overall reduction occurs through the surface; then a surface reduction is followed by a subsurface-surface reorganization from O bulk migration. The difference between pure ceria and mixed compounds is the following: (i) In the case of pure ceria an oxygen vacancy with no electrons penetrates in the subsurface layer during the near subsurface-surface reorganization. By contrast, in the case of the mixed oxides, an electronically charged vacancy is left in the subsurface allowing the subsurface Ce4+ ions to be reduced. (ii) In the case of pure ceria, the O bulk mobility is low in comparison with the surface reduction process rate below ca. 1000 K. Thus (see Figure 5, curve a), subsurface-surface reorganization is observed between the temperature at which the surface is notably reduced (623 K) and that at which O bulk mobility becomes relatively important (ca. 1000 K). In the case of mixed compounds O bulk mobility is already high at ca. 600 K, so that no such subsurface-surface reorganization is observed (Figure 5, curve b). Only the IR-measured OSC (Figure 7, curve b) evidences a feature that is interpreted here as being due to a subsurface-surface reorganization, but in the narrow temperature range 573-673 K (instead of 623-1000 K for pure ceria). Using the TPR technique (Figure 1) and because of the temperature ramping, the surface and bulk reductions may be either temperature resolved in the case of pure ceria or unresolved for mixed compounds. The main qualitative conclusion is that the surface and the bulk reductions are nearly simultaneous in the case of mixed compounds. 3.4. Characterization of Superoxide O2- Species on CeO2ZrO2 Mixed Compounds. Surface electronically charged defects on irreducible oxides may produce superoxide O2- species upon O2 adsorption at room temperature.33-35 For reducible oxides reduced by H2, O2 adsorption leads to O2- (i.e., to oxide reoxidation) and not to O2-. Nevertheless, the kinetics of superoxide transformation are often the slow step in the surface reoxidation, which allows investigation of the ability of the samples to stabilize anionic defects created by evacuation at high temperature. A great deal of literature is devoted to the formation and evolution of complexes of molecular oxygen adsorbed on metal oxides. If we exclude the physical adsorption measured on some compounds such as NiO and Cr2O336,37 at low temperatures, it is commonly accepted that the formation of superoxide and peroxide species needs textural defects in order to maintain a relative stability. In the redox process peroxides O22- are intermediates between superoxides O2- and anionic oxygen O2- coordinated with surface cationic sites; their mean lifetime is very short and they are detectable almost only at low temperature, when the kinetics of the decay reaction are suppressed. Superoxides have slow kinetics of transformation, which allows them to be measurable even at room temperature. In literature there are many examples of O2- species formation over the surface of several oxides, such as MgO, CaO, SrO, Co2O3, Cr2O3, and ZnO,38-44 when O2 is adsorbed after a reducing treatment under hydrogen. Other oxides, such as Fe2O3 or CeO2, show the presence of superoxides after a thermal treatment under dynamic vacuum, severe enough


Figure 8. ν(O-O) band for superoxide O2- species over equivalent amounts of CZ-100/0-HS (a), CZ-80/20-HS (b), CZ-68/32-HS (c), CZ50/50-HS (d), CZ-15/85-HS (e), and CZ-0/100-HS (f). They result from 13 kPa O2 adsorption at room temperature on samples purified by the standard pretreatment, except for CZ-0/100-HS sample (purified here by heating quickly to 873 K).

to create anionic vacancies on the oxide surfaces.33,45-50 Ceria in particular presents a band correlated with the O-O vibration of O2- species at 1126 cm-1, having the corresponding first overtone at 2237 cm-1.33 By contrast, zirconia (irreducible at a mild temperature) rarely shows superoxides on its surface, unless defects are produced and fixed by high-temperature treatment under hydrogen.34 Defective sites are possibly created through anchored impurity decomposition when the thermal treatment is fast enough to avoid surface reequilibration.35 All CexZr1-xO2 samples (except ZrO2) show the formation of superoxide species when oxygen (13 kPa) is adsorbed on their surfaces at room temperature (Figure 8), after the standard cleaning pretreatment.11 For ZrO2 quick heating to 873 K under vacuum was applied. The band assignment has been confirmed by 18O2 isotopic substitution, the fundamental vibration being then observed at ca. 1060 cm-1 and the overtone at 2109 cm-1. On ceria the amount of O2- species is very low, but it becomes strikingly high for the mixed compound having the higher cerium content (CZ-80/20-HS), decreasing then as the cerium content does (Figure 8). Defects on which O2 adsorbs forming O2- species should be O vacancies having one or two electrons (Fs or Fs+ surface reduced centers). Our results indicate that such defects around Ce cations are more easily formed and/or stabilized when zirconia is present. This is in accordance with very recent literature results.51 Conclusion The TPR technique has shown that surface and bulk reduction processes occur almost simultaneously. The oxygen uptakes measured at 700 K after an isothermal reduction at 773, 973, and 1273 K also show that the total-OSC is higher in the mixed oxides compared to CeO2 with the exception of the CZ-15/85 sample, and irrespective of the reduction temperatures. It depends on both the sample composition and reduction temperatures. However, if expressed as % Ce3+, the effectiveness of Ce4+ reduction increases with ZrO2 content at all investigated reduction temperatures. The magnetic balance results confirm the higher reducibility of the mixed oxides compared to CeO2. It increases with the zirconium content. The beginning of the Ce3+ formation is observed at about 470 K for the CZ-80/20 and CZ-68/32 samples and at lower temperature for CZ-50/50 and CZ-15/85, close to 373 K in the latter case. In contrast with CeO2, no inflection point is observed on the reducibility curves of the mixed oxides,

J. Phys. Chem. B, Vol. 104, No. 39, 2000 9193 indicating that no clear distinction can be made between surface and bulk reduction, in good agreement with the TPR results. Moreover, the contribution of the reversible H2 adsorption process on the formation of Ce3+ ions was estimated after desorption of the solids reduced at 973 K. On these desorbed oxides, a very good agreement has been found between the Ce3+ percentages calculated from the magnetic susceptibility and those deduced from the mass changes, after either reduction or reoxidation. It is inferred that the OSC deduced from the magnetic measurements is very reliable provided that the reversibly adsorbed hydrogen has been eliminated or that it is negligible, which is probably the case below 773 K. Using OCH3 species as a surface probe, FT-IR specifically investigates the surface state of the sample either reduced or reversibly reoxidized by O2 added in doses. Correlation between the surface state of reduction and the OSC is thus obtained. The determination of the temperature at which the reduction begins depends on the sensitivity of the technique. Thus the beginning of the H2 reduction of CZ-50/50-HS sample is noticeable at 523 K (a value higher than both obtained by magnetic balance and detected in the TPR profile). However it is 50 K lower than the beginning of the reduction for pure ceria. A strikingly stable surface completely reduced state of cerium ions in CZ-50/50-HS sample is already reached at ca. 600 K, being observed at least for reduction up to 873 K, whatever the bulk reduction of the sample was. Conversely, in the case of pure ceria, a surface partial reoxidation of cerium ions was observed for reduction at 873 K through a surface-subsurface reorganization. For CZ-50/50-HS sample reduced at 573 K, the OSC measured by IR largely exceeds the amount of Ce3+ ions. Thus it is proposed that, at least in the experimental conditions of reduction used in IR measurements, both surface Fs (or Fs+) center formation and surface Ce4+/Ce3+ reduction take place for CZ-50/50-HS sample; for reduction between 573 and 673 K the reduction as Fs (or Fs+) centers is completely converted into a reduction of all the near-subsurface Ce4+ ions. This last reduction state may be compared with the complete reduction of pure ceria surface observed at 773 K. The reasons why the presence of Zr in ceria-based materials improves OSC is still controversial. Pure ceria has a cubic structure. When completely reduced, it shows a transition to a hexagonal phase. However, in mixed compounds zirconium, which coordinates only in the cubic (or in the slightly distorted tetragonal and monoclinic) structure, impeaches this transition and enhances the stability. Thus O vacancies are formed in the framework. Beyond the above structural considerations, electronic reasons may explain the easier extraction of oxygen from mixed compounds. Zr4+ not being reducible, the electronic charge transferred in the reduction process is partly and primarily trapped in electronic surface defects (Fs centers) and then stabilized through cerium reduction into the bulk. Acknowledgment. The authors are grateful to the European Commission for the financial support received from the TMR Program (Contract No. FMRX-CT-96-0060(DG12-BIUO)). References and Notes (1) Kasˇpar, J.; Fornasiero, P.; Graziani, M. Catal. Today 1999, 50, 285. (2) Balducci, G.; Fornasiero, P.; Di Monte, R.; Kasˇpar, J.; Meriani, S.; Graziani, M. Catal. Lett. 1995, 33, 193. (3) Fornasiero, P.; Di Monte, R.; Ranga Rao, G.; Kasˇpar, J.; Meriani, S.; Trovarelli, A.; Graziani, M. J. Catal. 1995, 151, 168. (4) Cuif, J. P.; Blanchard, G.; Touret, O.; Seigneurin, A.; Marczi, M.; Que´mere´, E. SAE 1996, No. 961906.

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Reduction of High Surface Area CeO2â ZrO2 Mixed Oxides