J . Phys. Chem. 1988, 92, 4964-4970
4964
The rdf for the 21.4 wt % sample differs from the ones for the samples with lower loading, especially with respect to the relatively high intensity of the peak at 0.3 nm. This points to a high degree of polymerization, which is in accordance with the expected formation of precipitates. This rdf also differs from that of the A H M reference compound. This suggests that the precipitate has a different structure. N o attempt was made to unravel this structure. The amount of Mo(1V) present in this sample is still consistent with the adsorption model, however. During the preparation of catalysts by impregnation, the time allowed to reach equilibrium is often about 1 h or even less. In the case of the AHM/y-Al2O3 system, the EXAFS results indicate that 1 h is too short for equilibrium to be attained, since it was found that in the impregnated sample less Mo(1V) is present than in the material obtained in an adsorption experiment.
Conclusions The present EXAFS study has resulted in a modification of the model describing the adsorption of A H M onto y-A1203. It appears that two adsorption sites are necessary: (1) basic O H groups, leading to adsorption of Mo( IV); (2) coordinatively unsaturated A13+ sites, leading to adsorption of Mo(V1). Of the site-blocking reactions employed here, the F exchange of basic O H groups gives results in line with the above adsorption model, but the reaction of AI3+ sites with acacH leads to more ambiguous results. A normal catalyst impregnation time of about 1 h is too short for the reaction of AHM with the basic O H groups to proceed to completion. Registry No. AHM, 12027-67-7; MO, 7439-98-7.
Surface Characterization of Alumina-Supported Ceria J. Z. Shyu,* W. H. Weber, and H. S. Gandhi Research Staff; Ford Motor Company, P. 0. Box 2053, Dearborn, Michigan 481 21 (Received: November 18, 1987)
CeOz/A1203samples were characterized by XPS, Raman spectroscopy, and TPR. Three cerium species were detected, namely, a CeA103 precursor in the dispersed phase, small CeOZcrystallites, and large Ce02 particles. The CeAlO, precursor is so named because it gives a Ce(II1)-like XPS spectrum and a completely different Raman spectrum from that of CeO,, while TPR reveals that this species is more easily reduced to CeA10, than the Ce0, particles on alumina. It was also found that ceria in the CeA10, precursor and in a small crystallite form can be transformed to surface CeAIO, upon Hzreduction at temperatures higher than 600 "C, while even a partial conversion to CeA10, for bulk CeOz requires a temperature above 800 "C. CeAlO, thus formed shows good thermal stability in air in the temperature range up to 600 " C .
1. Introduction In this paper we investigate the chemical states of Ce on a high-surface-area y-alumina substrate and determine the dependence of these chemical states on ceria (CeOz) loading under a variety of oxidation-reduction treatments. Group VI11 metals and certain rare-earth-metal oxides dispersed on alumina have been widely studied because of their potential applications in catalysis. In this regard ceria has been used in automotive catalysts because of its ability to release and store oxygen',2 and to improve thermal stability of alumina.'" For ceria to serve as an oxygen storage component, it is essential that the reversible reaction between Ce4+ and Ce3+ takes place e a ~ i l y . ' * ~Ceria * ~ - ~ forms nonstoichiometric oxides upon thermai treatment in H2,' under vacuum,8 and in oxygen.'-" Upon exposure to oxidizing environments, even at room temperature, these oxides can be reoxidized to Ce02.798 Also, it has been reportedlOJ1that, in the presence of alumina, CeA10, may be formed in H2 or under vacuum at elevated temperatures. The techniques employed in this study were X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and temperatureprogrammed reduction (TPR). Bulk structures of the samples were examined by X-ray diffraction (XRD). 2. Experimental Section 2.1. Materials. CeOl was prepared by decomposing (NH,)2Ce(N03)6.6H20 at 500 "C for 4 h, which resulted in a BET area of = l o m2/g.' CeA103 (cerium aluminate) on alumina was made by heating the 7 wt % CeO2/AI2O3sample in flowing 10% H2/Ar at 1000 "C for 80 h. The formation of CeA103 with residual A1203was confirmed by XRD. The alumina support was obtained from Degussa (alumina-C) and had an initial BET area of 100 f 20 m2/g. *To whom correspondence should be addressed.
TABLE I: XPS Data for CeO1/AI2O1 Samples C e 0 2 loading XPS data wt %
Mmol/rn2
1.o 2.5 5.0 7.0
0.58 1.42 2.76 3.79 5.23 8.20
10.0 17.0
IO3 X Ce/AI ratio 4.0 10.0 19.0 20.6 21.3 25.7
% u"' in Ce 3d
2.2 3.0 4.2 4.8 6.2 9.3
The Ce02/A1203samples were prepared by mixing an alumina slurry with the appropriate amount of an aqueous Ce(N03), solution, followed by drying at 75 "C and calcination at 800 "C for 4 h. A list of these samples is given in Table I. In this table and the rest of the paper, both commonly used units for ceria loadings are employed, namely, weight percent (wt %) and micromoles of ceria per m2 of surface area of alumina (pmol/m*). 2.2. Spectroscopic Techniques. All XPS spectra were recorded on a Surface Science Laboratory SSX-100 XPS spectrometer with monochromatized A1 K a X-rays. A 600-pm spot size and 100-eV (1) Yao, H. C.; Yu-Yao, Y. F. J. Catal. 1984,86, 254. (2) Gandhi, H. S.; Piken, A. G.; Shelef, M.; Delosh, R. G. SAE paper, No. 760201, Warrendale, PA, 1976. (3) Yao, H. C. J. Catal. 1984, 19, 398. (4) Yu-Yao, Y. F.; Kummer, J. T. J. Catal. 1987, 106, 307. (5) Su, E. C.; Rothschild, W. G. J. Catal. 1986, 99, 506. (6) Su,E. C.; Montreuil, C. N.; Rothschild, W. G. Appl. Catal. 1985, 17, 75. (7) Rosynek, M. P. Catal. Rev.-Sci. Eng. 1977, 16, 11 1. (8) Kaufberr, N.; Mendelovici, L.; Steinberg, M. J. Less Common Metal 1985, 107, 281. (9) Yamaguchi, T.; Ikeda, N.; Hattori, H.; Tanabe, K. J. Catal. 1981.67,
_774 _ ..
(IO) Mizuno, M.; Berjoan, R.; Coutures, J. P.; Feox, M. Yagyo-KyokaiShi 1975, 83, 50. (1 1) Geller, S.; Raccah, P. M. Phys. Reu. B 1970, 2, 1167.
0022-365418812092-4964$01.50/0 0 1988 American Chemical Society
Surface Characterization of Alumina-Supported Ceria
The Journal of Physical Chemistry, Vol. 92, No. 17, 1988 4965 TABLE 11: XPS Data for Reference Cerium Compounds sample
treatment history
800 OC calcined
Ce02 Ce02
920 OC reduction in H, and exposed to ambient air Ce(OH), as received 7% CeA10,/AI2O3 as prepared“
% u”’ in Ce 3d 13.4 13.7 12.4 0.0
“See Experimental Section.
922
BINDING ENERGY ( r V )
072 ( b l 7 % CeAP03/ARz03
Figure 1. C e 3d XPS spectra of CeA1O3/AI2O3 (a) and CeO, (b); see text for the assignments for each peak.
analyzer pass energy were typically used for the analysis. This pass energy gives a peak width of the Au 4f7/2 line of approximately 1.0 eV. Sample charging during the measurement was compensated by a low-energy electron flood gun operated at approximately 2 eV. XPS Ce/Al intensity ratios were calculated by using the integrated peak area ratios of the Ce 3d and A1 2p lines, respectively, for Ce and Al. All samples were exposed to air before analysis. Reduction of samples for XPS analysis was conducted in flowing Hz in a quartz tube at the given temperature for 2 h. Samples were exposed to air while they were transferred to the XPS spectrometer for analysis. Raman scattering measurements were done using the 514.5-nm line from an Ar ion laser, a SPEX Triplemate spectrometer, and an EG&G PAR Model 1420 intensified solid-state array detector. The laser beam intensity was typically 100 mW and the spectrometer resolution 5-6 cm-’. Additional details of the Raman scattering apparatus are given elsewhere.l2 Samples were pressed into pellets for Raman analyses. TPR experiments were carried out on an Altamira temperature-programmed system. For all the experiments, 10% H2/Ar at a flow rate of 30 mL/min was used as a reducing gas, and the heating rate was 15 “C/min.
3. Results 3.1. Reference Cerium Compounds. 3.1 . I . XPS Analysis. XPS spectra of CeA103/A1203and C e 0 2 are shown in Figure 1. The complex spectrum of CeOz can be resolved into eight components by least-squares fitting with the assignment as defined in Figure 1 (v’s represent the Ce 3d5 contribution and u’s represent the Ce 3d3 contribution). d ~ j i m o r i ’ ~ has - ’ ~ attributed the peaks v”’ (u”’) to the transitions to the final state 4f“ from the initial state 4f“, v (u) to the transitions to the 4f’ final state from the 4f’ initial state, and v’ and v” (u’ and u”) to the 4P final states arising from ligand-to-4f shake-up transitions from the 4f’ initial state. The presence of 4fI initial state of the formal Ce4+ in CeOz is due to partial occupancy of 0.6 e in the 4f orbital resulting from valence mixing with the oxygen ligand.13J5s16 Compared with CeOz, CeAlO, exhibits a different spectrum in the Ce 3d region. In particular, for CeA103the u”’ peak is absent and the relative intensity of v‘ with respect to v (u’ vs u) is increased substantially. The absence of the u”’ peak for CeA103 can be interpreted as the lack of 4 p configuration in the formal Ce3+state. Thus, any transitions involving the 4f” configuration will not appear in the spectra.
It has been suggested that the Ce 3d spectrum from a partially oxidized Ce species can be approximated as a linear combination (12) Weber, W. H.; Baird, R. J.; Graham, G. W. J . Rumun Specfrosc. 1988, 19, 239.
(13) Fujimori, A. J. Mugn. Mugn. Muter. 1985, 47,48, 243. (14) Allen, J. W. J . Mugn. Mugn. Muter. 1985, 47,48, 168. (15) Fujimori, A. Phys. Reu. E 1983, 28, 2282. (16) Fujimori, A. Phys. Rev. E 1983, 27, 3992.
I
100
200
300
400 500 600 RAMAN SHIFT (cm-’)
700
800
900
Figure 2. Raman spectra of CeO, (a) and CeAIOS (b) in arbitrary units; 514.5 nm, 300 mW.
of spectra recorded from the oxides.17 Assuming no preferential enrichment of Ce4+over Ce3+or vice versa, the percent area for the u”’ peak in the total Ce 3d region (Ce 3d5/2 and Ce 3d3/2) can be used to describe the relative amount of Ce4+in the sample. Other peaks were also displayed in the present paper to demonstrate the basis that we have employed for estimating the background and the total peak areas in the Ce 3d region. However, to obtain a consistent estimate of the background (nonlinear Shirley background), a small contribution of v”’ was included for the Ce 3d s ectrum for CeAlO, even in the absence of the corresponding 5i2 u p,” in the Ce 3d3/2 region. Since only the percent area in the u”’ region and the remaining area in the Ce 3d region are of importance to determine the % u’”, a consistent background subtraction should enable one to obtain, within reasonable accuracy, the % u”’ in the Ce 3d region. Thus, the incorporation of the v”’ peak should not influence the validity of utilizing the percent u”’ as a measure of the relative amount of Ce4+ in the sample. XPS data for other reference Ce compounds are listed in Table 11. The u”’ for Ce(OH)4 amounts to 12.4% in the Ce 3d region, which is similar to that for Ce02. Also, after reduction at 920 “ C followed by exposure to air, CeOz gives -13.7% in the u”’ region. Although formation of Ce203 or other nonstoichiometric Ce oxides may happen after the r e d ~ c t i o n ,reoxidation ~.~ of these reduced oxides into C e 0 2 may have occurred in ambient air,7,8 at least to the depth probed by XPS. 3.1.2. Raman Analysis. Raman spectra for CeOz and CeA1O3/Al2O3 are shown in Figure 2. CeOz gives a strong narrow peak at 465 cm-’ that is due to the FzgRaman active mode characteristic of fluorite structure materials. This is a triply degenerate mode (at the Brillouin zone center) associated with the oxgygen atoms vibrating against each other.18 In addition, a weak band around 265 cm-’ and a shoulder near 600 cm-’ are observed. The weak band coincides with the location of the normally Raman inactive (but infrared active) zone-center t12~sverse optical (TO) mode and the shoulder with the location of the zone-center longitudinal optical (LO) phonon mode.Ig (17) Platau, A. Ph.D. Thesis, Linkoping University, Sweden, 1982. (18) Keramidas, V. G.; White, W. B. J . Chem. Phys. 1973, 59, 1561.
Shyu et al.
4966 The Journal of Physical Chemistry, Vol. 92, No. 17, 1988
I I
0
1
”
2
’
I
1
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I
0
’
3 4 5 6 7 8 9 CERIA LOADING [ p rnole/rn2)
1
0
Figure 4. XPS data plot for CeO2/Al20,;% u”’ plot (dashed) and Ce 3d/AI 2p intensity ratio (solid). k
(
e
) 5 23prnole/rn2
K-
( d ) 3.79 p r n o l e / m 2
I
I
;
I
I
I
1
I
---+ -1 I
872
922 B I N D I N G ENERGY ( e V i
Figure 3. Ce 3d XPS spectra of CeO2/AI20,: (a) 0.58 pmol/m2; (b)
1.42 pmol/m2; (c) 2.76 pmol/m2. Perturbations to the fluorite-structure lattice may produce these weak Raman features. CeA10, has a perovskite-like structure that is slightly distorted from the simple cubic perovskite form.20 The nature of the distortion, however, is not completely understood. Kaufherr et aL8 suggest a tetragonal structure with a. = 3.760 f 0.004 A and co = 3.787 f 0.004 A. In a much earlier paper, Kim21suggests a rhombohedral structure with a. = 5.327 f 0.004 A and a = 60° 15’ f 2’. In both cases the deviation from cubic symmetry is extremely small, but in neither case have the precise locations of the atoms within the unit cell been determined. For a cubic perovskite structure there are no Raman active vibrational modes. For the tetragonal structure, assuming it is like that of tetragonal BaTiO,, there would be eight Raman-active modes: I’ = 3A1 + B1 4E.22 For the rhombohedral structure, assuming it is like that of rhombohedral LaAlO,, there would be five Raman-active modes: r = AI, + 4E,.23 However, because of the very small distortion from cubic symmetry, all of the Raman modes should be very weak. In fact, we have been unable to observe any Raman lines from our CeA10, samples; only a strong and broad fluorescence background was observed as shown in Figure 2. The y-alumina also shows no Raman lines, and its fluorescence background is much less than that from CeAlO,. The strong fluorescence for CeA10, and the narrow Raman line for CeO, will thus be used to help identify the cerium species on Ce02/ A1203. 3.2. CeOz/Al2O3Samples. 3.2.1. XPS Analysis. Ce 3d XPS spectra of three selected Ce02/A1203are shown in Figure 3. The u”’ peak, which is used as a measure of the % of Ce4+ (or Ce3+) in the sample, amounts to only 2.2% in the total Ce 3d region for 0.58 wmol/m2 Ce02/A1203(Table I). As the ceria loading increases, the % u”’ in the Ce 3d region is also increased. The low % u”’ for the 0.58 wmol/m2 Ce02/A1203suggests that this Ce species on alumina resembles Ce3+ at low ceria loadings, presumably due to dispersed Ce that strongly interacts with alumina. Due to the resemblance of the dispersed Ce species on alumina to CeA103 in the Ce 3d region, we will refer to this species as a
+
(19) Mochizuki, S. Phys. Status Solidi B 1982, 114, 189. (20) Wyckoff, R. W. G. Crystal Structure; Wiley-Interscience: New York, 1963; Vol. 2, pp 390-422. (21) Kim, Y. S. Acta Crystallogr. B 1968, 24, 295. (22) Rousseau, D. L.; Bauman, R. P.; Porto, S. P. S. J . Raman Spectrosc. 1981, 10, 253. (23) Scott, J . F. Phys. Reu. 1969, 183, 823.
100
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300
400
500 600 RAMAN SHIFT (ern-')
700
800
900
Figure 5. Raman spectra of CeO2/AI20,;514.5 nm, 300 mW, 7 cm-’ resolution, 15 min total integration time, -lo-’ cps full scale.
CeA10, precursor and this is discussed in more detail in section 4. The XPS intensity ratio of Ce 3d/A1 2p plotted as a function of ceria loading is depicted in Figure 4. The ratio is linear with ceria loading for low values, but there is a decrease in the slope of the plot at higher ceria loading. The breakpoint is around 2.7 pmol/m2, which is similar to that in the plot of oxygen chemisorption vs loading reported by Yao and Yu-Yao.’ We can thus take this breakpoint as a working definition of the boundary between dispersed and bulk phases. Below this point, the dispersed phases are the major species, and above it particulate phase or large particles will start to form.24 The particle size of the latter is estimated by XRD to be about 20 nm, whereas no ceria phase is seen by XRD for the former. A plot of the % u’” in the Ce 3d region as a function of ceria loading is also shown in Figure 4 (dashed curve). The % u”’ increases monotonically with ceria loading, indicating an increasing relative amount of Ce4+. This result suggests that there are two different Ce species in the dispersed phase (below 2.7 wmol/m2), namely, the CeAlO, precursor and another Ce species. The former dominates at very low loadings, while the latter grows more rapidly than the former with loading. The latter phase is probably associated with small C e 0 2 crystallites dispersed on the alumina. These small C e 0 2 crystallites are similar to bulk C e 0 2 in XPS, probably due to weak interaction with the alumina support. 3.2.2. Raman Analysis. Raman spectra of a series of Ce02/A1203samples are shown in Figure 5 . All samples show a peak around 460 cm-I, characteristic of C e 0 2 (F2g mode). Additional features are also seen in the Raman spectra at approximately 250, 268, 288, 383, 558,623, 739, 772, and 840 cm-l, (24) Wu, M.; Hercules, D. M. J . Phys. Chem. 1978, 1 1 , 615
The Journal of Physical Chemistry, Vol. 92, No. 17, 1988 4967
Surface Characterization of Alumina-Supported Ceria 460
(0)
C E R I A LOADING
UNSUPPORTED
(prno1e/m2)
0.58
I .42
200
100
300
400
500
700
600
800
900
RAMAN SHIFT (ern-')
Figure 6. Raman spectrum of 0.58 Mmol/rnz; curve a in Figure 5.
3.79
- 140
Y)
>
5.23
-
8.20
0
2
p
4
6
mole /m2 C e O t
Figure 7. Raman intensity plot for CeO2/AI20,.
0
200 400 6 0 0 800 1000 TEMPERATURE *C
as indicated in Figure 6, an expanded plot of the bottom spectrum in Figure 5. These new spectral features are presumably due to the interacting Ce species dispersed on the alumina (the CeA103 precursor), and we will take the strength of the 250 cm-' line to be a measure of the amount of this species present. Peak intensities of the 250- and 460-cm-' lines, plotted as a function of ceria loading, are depicted in Figure 7. For 0.58 pmol/m2 CeOz/A1203, the ratio of the strength of the 250-cm-' peak to that of the 460-cm-' peak is largest. As more ceria is added, the intensity of the 250-cm-' peak increases very slowly while the intensity of the 460-cm-' F2gpeak increases much more rapidly. This finding is in good agreement with that derived from XPS. The C e 0 2 FZgpeak in Figure 5 is several cm-' lower in frequency and about 50% broader than the same line for pure CeOz shown in Figure 2. Both of these changes are probably caused by smaller particle sizes in the CeOz/Al2O3samples. For very small crystallites (20-200 A), the lack of long-range order relaxes the momentum conservation selection rule in the Raman scattering process.25 The Raman line shape in this case will be given by a weighted average of the phonon frequencies extending out into the Brillouin zone, away from q = 0. The result will be a shifted and broadened line shape, and the amount of shifting and broadening can be calculated from the dispersion of the phonon mode and the frequency and width of the mode at q = 0 for a large crystal. Unfortunately, none of these data are known for CeO,. However, the magnitude of the shift and broadening are comparable to those observed in Si and GaAs for particle sizes in the 20-200-A range.2s-z6 3.3. Reduction Study. 3.3.1. TPR Analysis. TPR profiles for unsupported CeOz and a series of CeO2/Al2O3are shown in Figure 8. The results are similar to those obtained previously from this laboratory,' but a slight improvement in temperature
resolution is seen. This improvement is presumably due to higher heating and gas flow rates as well as a smaller reactor volume. For Ce0, (Figure sa), three peaks are detected at approximately 450, 580, and above 800 "C. The TPR profile can be interpreted as a stepwise reduction of CeOz. The peak at 450 OC is probably associated with reduction of the surface oxygen of Ce02.' The peak at r 5 8 0 OC is likely due to formation of nonstoichiometric Ce oxides (CeO, with x ranging from 1.9 to 1.7, or the p phase, as defined in ref 28).7,9,27328The last peak above 800 OC is attributed to the total reduction of ceria to ce203.1,9,27,28 TPR data for CeOz/A1203 are shown in Figure 8b-f. In general, there are five groups of peaks observed in TPR which , above are the peak below 300 OC, the peaks at 410, ~ 6 0 0 , 7 2 0and 800 OC. The peak below 300 OC is probably associated with adsorbed oxygen on alumina.' For 0.58 pmol/m2 CeOz/AlZO3, two major peaks are observed at 410 and 720 OC. Since the Ce species on this sample is mostly the CeA103 precursor, the peak at 410 "C is assigned to reduction of the surface oxygen of this species, whereas the peak at 720 OC is assigned to conversion of a partially reduced CeAlO, precursor to CeA103. Details of these assignments are discussed later. As also shown in Figure 8, an increase in ceria loading results in two additional TPR peaks at ~ 6 0 and 0 above 800 OC, and the peak intensities increase along with ceria loading. However, for ceria loading below 3.79 pmol/m2, the peaks at 410 and 720 OC still represent the major reducible species. At 8.20 pmol/m2 ceria loading, the peak above 800 OC becomes the predominant species, which is essentially the reduction characteristic of bulk CeOz. The peak at 600 "C for Ce02/A1203is shifted from 580 "C for the
(25) Richter, H.; Wang, Z. P.; Ley, L. Solid State Commun. 1981, 39, 625. (26) Tiong, K. K.; Arnirtharaj, P. M.; Pollak, F. H. Appl. Phys. Lett. 1984, 44, 122.
(27) Bevan, D. J. M.; Kordis, J. J . Inorg. Nucl. Chem. 1964, 26, 1509. (28) Bevan, D. J. M. J . Inorg. Nucl. Chem. 1955, I, 49. (29) Coenen, J. W. E. In Preparation of Catalysts I& Delmon, B., Grange, P., Jacobs, P., Poncelet, G., Eds.; Elservier: New York, 1979; p 89.
Figure 8. TPR profiles for CeO2/AI2O3.
4968
Shyu et al.
The Journal of Physical Chemistry, Vol. 92, No. 17, 1988
I C )R E D U C E D AT 571.C
\
H I
1200
600 800 1000 REDUCTION TEMPERATURE ( K )
200
400
Figure 10. The % u'" plotted as a function of reduction temperature for 8.20 and 2.02 pmol/m2 CeO2/AI2O3. 8.20 p m o l e / m z ceo~/Ap203(Calcined 1
2 BINDING ENERGY ( e V )
Figure 9. Ce 3d XPS spectra of 8.20 pmol/m2 CeO2/AI20,.
TABLE 111: XPS Data for 8.20 pmol/m2 CeO2/AI2O3 % u"' in Ce
treatment history 800 "C calcination 408 OC reduction" 576 OC reduction 820 O C reduction 920 OC reduction 920 OC reduction, followed by calcination at 600 OC 920 OC reduction, followed by calcination at 920 OC
3d region 9.3
7.0 7.6 4.4 3.4 5.5 8.6
All samples after reduction were exposed to ambient air before analysis.
unsupported ceria. This shift is likely caused by a difference in the ceria particle size. The appearance of characteristic peaks of CeO, (peaks at 600 and above 800 "C) in the TPR profiles for ceria loadings higher than 0.58 kmol/m2 is consistent with the Raman and XPS data shown previously. 3.3.2. XPS and Raman Study. Two Ce02/A1203samples are selected for the reduction study, namely, 2.02 and 8.20 pmol/m2 ceria loadings. The former has a ceria loading below the breakpoint of the XPS Ce 3d/A12p intensity ratio plot (Figure 4) where ceria is expected to be in the dispersed phase (the CeA10, precursor and small CeO, crystallites). In contrast, the latter should possess mostly large CeO, particles along with the dispersed ceria. Under the experimental conditions used in this study, CeAlO, should be the only stable Ce3+ species that can be observed, and other reduction products such as Cez03 and nonstoichiometric Ce oxides will be reoxidized when exposed to ambient air during sample The difference in the chemical state of Ce between the calcined and reduced samples is indeed due to chemical transformation of CeOz/A1203into CeA103/A1,0,. Figure 9 shows the Ce 3d XPS spectra for 8.20 kmol/m2 Ce02/A1203after calcination at 800 "C (Figure 9a) and reduction in H 2 at 920 and 576 "C for 2 h followed by exposure to air (Figure 9b,c). In comparison to the calcined sample, a slight decrease is seen in the % ' u"' of the Ce 3d spectra for the sample after 576 OC reduction and a greater change is observed for the sample reduced at 920 OC. These data are also tabulated in Table 111. A plot of the % u"' in the Ce 3d region for the 8.20 kmol/m2 CeO2/A1,O3 sample as a function of reduction temperature is shown in Figure 10. It is apparent that significant reduction of
100
I
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I
I
I
I
I
1
200
300
400
500
600
700
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RAMAN SHIFT (cm-l)
Figure 11. Raman spectra of 8.20 pmol/m2 CeO2/AI2O3;after calcination (at 800 "C), and reduction at 920 OC followed by exposure to air; 514.5 nm, 100 mW, 400 s total integration time, -lo3 cps full scale.
TABLE IV: XPS Data for 2.02 pmol/m2 CeO2/AI2O3 treatment history % u"' in Ce 3d region
800 OC calcination 192 OC reductiona 402 OC reduction 594 'C reduction 920 OC reduction
4.1 4.2 4.8 0.0 0.0
All samples after reduction were exposed to ambient air before analysis.
Ce02/A1203to CeAIO, starts to occur at temperatures above 800 OC. Transformation of ceria on alumina to CeAlO, is also investigated by Raman spectroscopy. As shown in Figure 11, after 920 OC reduction, Raman data show a peak at r 4 6 0 cm-' on top of a broad fluorescence background that is essentially a mixture of Ce02 and CeA103. This result is consistent with the XPS data presented previously. Thermal stability of the CeAlO, thus formed is also studied by XPS. Figure 9d,e and Table I11 show that 600 "C calcination in air causes only a slight increase in the % u"' in the Ce 3d region (from 3.4 to 5.5%). However, calcination in air at 920 OC results in nearly a total recovery of the C e 0 2 phase, as evidenced by the great increase in the % u"' in the Ce 3d XPS spectra. This indicates that the CeA10, on alumina has good thermal stability up to temperatures much in excess of 600 OC, in agreement with Mizuno et a1.I0 Similar reduction studies are also conducted on the 2.02 kmol/m2 Ce02/A1203and the XPS results are shown in Figure 12 and Table IV. Reduction at temperatures below 400 O C gives no change in the % u"' in the Ce 3d spectra, and when the re-
Surface Characterization of Alumina-Supported Ceria
The Journal of Physical Chemistry, Vol. 92, No. 17, 1988 4969
- 6 b, -8
0
l b l C*O2 + 2 8 H, IC1 C?O,+.S
.
C*On +.28 HzO
A9,0,+.5
jdj/, 400
200
I
600
,
,
800
, I
1
HZ * h A 9 , 0 ~ + . 5 H ~ O
Id1 & 0 2 + . 5 H 2 . . 5 & & + . S ~ O
,
I
1000 1200
,
I 1400
TEMPERATUREIK
Figure 13. Thermodynamic analysis of reduction of ceria.
872
922 B I N D I N G ENERGY ( r V )
Figure 12. Ce 3d XPS spectra of 2.02 pmol/m2 CeO2/AI2O3
duction temperature reaches 594 OC, nearly a total disappearance of the u"' peak is seen. A plot of the % u'" as a function of reduction temperature is shown in Figure 10. This result indicates that ceria in the CeA103 precursor and as small crystallites can be converted to CeA103 at temperatures above z600 OC. 4. Discussion 4.1. Ceria-Alumina Interactions. The dispersed Ce species
that shows a Ce3+-likespectrum in XPS and gives an entirely different Raman spectrum from that of C e 0 2 has been referred to as a CeA103 precursor. The Ce3+-like property of this Ce species has previously been reported by Kummer using magnetic measurement^.^^ However, as it is reducible in TPR, this Ce species cannot be due to Ce3+, since this is the lowest oxidation state of cerium that can be achieved under the experimental c o n d i t i o n ~ . l - ~Moreover, *~ this species is not Ce(OH)4, since Ce(OH)4 generates a Ce 3d XPS spectrum very similar to CeO,. Therefore, we designate this species as a CeA103 precursor probably stabilized in the cation vacancies of the alumina surI
The additional Ce species on alumina in the loading range below 2.76 Mmol/m2 is small C e 0 2 crystallites. XPS, Raman spectroscopy, and TPR give evidence for the Ce species as CeO,. In fact, this Ce species may be confined in the alumina support; but it must be similar to Ce02, and differs from C e 0 2 mainly in crystallite size. The small crystallite nature of this Ce species is supported by the lack of X-ray diffraction patterns and by Raman line broadening and shift of the CeO, phase in these samples. The last Ce phase that can form on alumina is the bulk or large particle phase. This phase is formed when the alumina surface (30) Kummer, J. T., unpublished results in Technical Report No. SR-84134(1984), Ford Motor Co. (31) Che. M.: Kibblewhite, J. F. J.; Tench, A. J. J . Chem. SOC.,Faraday Trans.' 1 1973, 69, 857. (32) Barin, I.; Knacke, 0.;Kubaschewski, 0.;Thermodynamical Properties of Inorganic Substances; Springer-Verlag: New York, 1911.
is saturated (>2.8 pmol/m2) and has a similar reduction behavior to unsupported Ce02. It is noteworthy that, unlike the XPS intensity ratio plot, the CeO F Raman intensity plot shows no discontinuity around 2.7 pmol/m2! . This lack of a breakpoint in the Raman plot is due to larger sampling depth of the technique (-10-100 Mm) which is far greater than the crystallite sizes of both small crystallites and large CeO, particles. On the other hand, the presence of a discontinuity in the XPS intensity ratio plot indicates that the small crystallites possess a size within the sampling depth of XPS, whereas that for the large CeO, particles is beyond the sampling depth (-30 A). 4.2. Reduction of Ce02/A1203.Equilibrium constants ( K ) for H2 reduction of ceria plotted as a function of temperature are shown in Figure 13. The thermodynamic data are adapted from ref 32. At temperatures below 600 K, reduction of ceria into CeO, and CeA103 is not thermodynamically favorable. However, within this temperature range, formation of nonstoichiometric CeOl,83 and CeO1.72 is the most favorable r e a c t i ~ n . ~At, ~temperatures ~ above 800 K, formation of CeA103 becomes the most favorable reaction. Also, it should be noted that formation of Ce203 is less favorable than that for CeA103, and this is especially true at temperatures above 800 K. The thermodynamic analysis helps interpret the TPR data. For instance, the three peaks observed in the TPR profile for unsupported Ce0, (Figure 8a) can be interpreted as follows. The peak at 450 OC amounts to a small percentage of the total area in the profile, and it is reasonable to assign this peak to a surface reduction of CeO, that has a BET area of r 1 0 m2/g.' The peak at 580 "C amounts to z20-30% of the total area which can be assigned to be the formation of the nonstoichiometric cerium oxides (the CeO, phase as defined earlier)9*28which is the most thermodynamically favorable reduction product at this temperature. This assignment is also supported by the closeness in the area ratio between the peak at 580 "C versus that above 920 OC and the theoretical ratio in hydrogen consumption between the formation of the CeO, phase and Ce2O3. The latter is believed to be the final reduction product.1~7*927,2s Although during the dynamic TPR process reduction of ceria can be kinetically controlled, it appears that the relative peak areas due to formation of CeO, and Ce203 agree well with the thermodynamic analysis. Reduction of CeO, on alumina differs from that for unsupported CeO,, due to Ce0,-A1203 interactions. For ceria in the CeA103 precursor, the surface oxygen is first removed by reduction,' followed by formation of CeA103. Ce203would not form because the formation of CeA103 is a more favorable thermodynamic process. Also, no CeO, phase formation is seen for Ce in the CeA103 precursor. This is probably due to the strong interaction such that no further removal of oxygen can occur until a temperature is reached for the formation of CeA103.' XPS and TPR can provide some mechanistic information for the CeA103 formation from Ce02/A1,03. Kinetically, reduction at 600 OC in H2 of these small crystallites should lead to mainly the CeO, phase, as seen in the TPR profile. Prolonged heating in H, may lead to a solid solution, which eventually forms CeA103, the major Ce species observed in XPS. Formation of a solid solution between ceria and alumina as an intermediate step for
J . Phys. Chem. 1988, 92, 4970-4973
4970
CeA10, formation has been proposed by Kaufherr et a1.* The fact that higher reduction temperature is required for the large C e 0 2 particles to form CeA10, is probably due to diffusional limitations of Ce to form the solid solution with alumina. As also shown in TPR, during the formation of CeA10, from the large C e 0 2 particles, the CeO, phases (CezO, when reduction temperature is above 800 "C) are formed. Some portions of the CeO, species may form the solid solution with alumina, and the remaining CeO, species are reoxidized to CeO, when exposed to ambient air before XPS analysis. It appears that formation of CeA10, from C e 0 2 particles (both small crystallites and large particles) on alumina involves an intermediate state in which ceria is, at least, partially reduced.
5. Conclusions Surface characterization of Ce02/A1203 by XPS, Raman spectroscopy, and T P R leads to the following conclusions: 1. Three ceria phases can be found on CeO,/A1,O3, depending on ceria loading. These phases are the CeA10, precursor, intermediate (small crystallite, not detectable by XRD), and large C e 0 2 particles ( r 2 0 nm in size). 2. The CeA10, precursor shows a Ce(II1)-like feature in XPS and gives a completely different spectrum from C e 0 2 in Raman
scattering experiments, while TPR data reveal that this Ce species is more easily reduced into CeA10, than the CeO, particles on alumina. 3. Ceria in the CeAIO, precursor and in a small crystallite form on alumina can be transformed into CeAlO,, upon reduction in H, at temperatures higher than 600 "C, while even a partial conversion to CeA10, from the large C e 0 2 particles requires a temperature above 2800 "C. 4. CeAlO,, formed by reduction of CeOz/Al20,, shows good thermal stability in air in the temperature range up to 600 "C. Note Added in Proof: Further studies of the y-alumina material used in this work indicate that some of the Raman lines assigned to the dispersed phase of Ceria may be due to transitional alumina phases instead. Acknowledgment. We acknowledge the following colleagues at Ford Research: R. K. Belitz for XRD analysis; R. J. Baird, G. W. Graham, M. Bettman, and K. Otto for useful discussions; L. C. Davis for useful information regarding the theoretical aspects in photoemission of Ce oxides; and M. Shelef for a review of the manuscript and helpful discussions. Also, a useful discussion with Professor John C. Hemminger of the University of California, Irvine, is acknowledged.
Molecular Orbital Study of Nickel-Oxygen Interactions Henry Castejdn, Antonio J. HernPndez, Departamento de Qufmica, Universidad Sim6n Bolhar, Apartado 80659, Caracas 1080- A , Venezuela
and Fernando Ruette* Centro de Qufmica, Instituto Venezolano de Investigaciones Cient$cas, IVIC, Apartado 21 827, Caracas I O 1 0- A , Venezuela (Received: October 12, 1987)
A modified CNDO-UHF procedure is used to study nickel-oxygen interactions in NiO and [Ni02]9 (q = 0, 2 + ) systems. Some relevant properties are compared with those obtained in a previous study on analogous cobalt-oxygen adducts. The nickel orbitals that most contribute to the bonding in these molecules are the 4s and 3d, with relatively little contribution from the 4p orbitals. The ground state of NiO is calculated to be a 32with A holes localized in Ni(3d) orbitals. The most stable deformation state for the [NiO2I0system corresponded to a peroxo 'A, state with a IA' superoxo state 10 kcal/mol higher in energy. This is reversed in the [Ni021z+system, where a superoxo 'A' state is the most stable distortion geometry. The dioxygen activation, measured by Boca's x(a) index, in these 0, adducts decreased when the oxidation state of nickel increased from Ni(0) to Ni(I1). The lowest activation energy calculated to break the 0-0bond corresponded to 112 kcal/mol, suggesting a nondissociative chemisorption of O2 on a single Ni atom.
Introduction In a previous paper' we reported CNDO-UHF calculations for the distortion, dioxygen activation, and dissociation properties of [Co02]q as a preliminary step in the calculation of the properties of O2on transition metals anchored to metal oxides29and metallic surface^.^ Dioxygen complexes of this type have recently become of considerable interest to chemists because of their importance for both homogeneous and heterogeneous catalysis. We studied the effect of the net charge q on the stabilization of the electronic states of the adduct to simulate the interaction of transition metals with different kinds of chemical environments. The results obtained clearly showed that one may accurately study the behavior of a given electronic state along the distortion and dissociation coordinates using semiempirical methods. They constitute an adequate framework for the explanation of experimental data provided that the state of minimum energy is fixed along the reaction path. (1) Hernindez,A.; Ruette, F.; Ludeiia, E. V . J . Mol. Catal. 1987, 39, 21. (2) Beran, S.; Jiru, P.; Wichterlova, B. J . Phys. Chem. 1981, 85, 1951. (3) Ruette, F.; Ludefia, E . V . J . Catal. 1981, 67, 266. (4) Ruette, F.; HernBndez, A,; Ludeiia, E. V. Surf, Sci. 1985, 151, 103.
0022-3654/88/2092-4970$01.50/0
TABLE I: Parameters for Nickel and Oxygen
orbital !J
-'/A111
+ A!J),
eV
-PO,
eV
&I
nickel 4s 4P 3d
2s 2P
4.306" 1.260' 6.182" oxygen 25.39017* 9.111b
2.55 1.05 2.95 2.275b 2.215b
5.0 3.0 8.5 23.0 22.3
Reference 1 1. Reference 12. In the present work we report similar studies on [Ni02]q,where we have chosen q = 0 and 2+ as the representative cases. Experimental data show that binary dioxygen complexes of nickel are capable of existing under conditions of matrix is~lation.~The poor CNDO estimates of the binding energies are improved by an adequate parametrization that reproduces the equilibrium bond lengths and bond energies of 0, and NiO. This thermodynamic ( 5 ) Huber, H.; Ozin, G . A. Can. J . Chem. 1972, 50, 3746.
Q 1988 American Chemical Society
