J . Phys. Chem. 1988, 92, 2561-2568
2561
Oxidation State of Cerium in Cerium-Based Catalysts Investigated by Spectroscopic Probes Franqois Le Normand,*+Lionel Hilaire: Koffi Kili,t Gerard Krill,* and Gilbert Mairet Laboratoire de Catalyse et Chimie des Surfaces, UA 423 du CNRS, Universitt Louis Pasteur, 4 rue Blaise Pascal, 67070 Strasbourg, France, and Laboratoire de Physique des Solides, Facultt des Sciences, Universitt? de Nancy I , BP 239, 54506 Vandoeuvre-Les-Nancy, France (Received: August 5, 1987; In Final Form: November 30, 1987) Pd-Ce catalysts, coimpregnated on y-alumina, have been prepared, calcined, and reduced according to a standard procedure. The oxidation state of cerium in these catalysts has been studied both by X-ray photoemission spectroscopy (XPS) on the 3d core level and by X-ray absorption spectroscopy (XAS) on the LIIIedge. Both techniques indicate a partial reduction of ceria in these systems. The reduction is more pronounced on the surface than in the bulk. The degree of reduction is influenced by (1) the concentration of cerium, (2) the presence of the transition metal, (3) the nature of the precursor salt of the transition metal, and (4) the experimental conditions of thereduction step. Implications concerning the role of ceria in the activity of transition metal-cerialy-alumina catalysts are discussed, including (1) the promotion of the catalytic activity, (2) the increase of the thermal stability of the support, and (3) the increase and stabilization of the dispersion of the transition metal.
Introduction
TABLE I: Characteristics of the Different Catalysts Studied
Cerium has proven to be a powerful modifying agent of catalysts for important industrial or environmental reactions such as the cracking of heavy oil on zeolites' or the automotive exhaust gas conversion. For the latter reaction, typical catalysts are transition metals (Pt, Rh, Pd)/y-A1203associated with cerium In this last reaction addition of ceria enhances the activity for the oxidation (CO and hydrocarbons) and the reduction (nitrogen oxides) steps, prevents the thermal loss of BET area, and stabilizes the transition metal in a finely dispersed state.4~~Investigations on the role of ceria have pointed out its capacity to provide oxygen for the oxidation step of the cycle and then remove it in the reduction stepss Thus ceria is expected to be in a more or less reduced state on these catalysts, in spite of the well-known difficulty of reducing it by hydrogen or other powerful reducing agents such as COS6 However the oxidation state of cerium in such catalysts has never been satisfactorily investigated in detail. W e present here spectroscopic results on palladium-cerium catalysts coimpregnated on y-alumina. The probes involved X-ray photoelectron spectroscopy (XPS) on the 3d core levels and X-ray absorption spectroscopy (XAS) on the LIIIedge of cerium. As previously reported7+ the spectra of I11 and IV cerium compounds are widely different, and therefore we expected to be able to characterize the oxidation state of cerium in such catalytic systems. We investigated the following parameters: concentration of cerium; presence of a transition metal; precursor salt of the transition metal; in situ reduction. In a future paper we will report on the characterization of the transition metal and the catalytic activity for hydrocarbon conversion of these systems.I0 Briefly summarized, the activity and selectivity are strongly modified by adding the rare earth and by changing the palladium precursor salt. Results are rationalized by assuming a modification of the hydrogen coverage and the creation of new catalytic sites at the interface between palladium and the rare earth.
Experimental Section Catalysts. In Table I we report the characteristics of the catalysts. They were prepared by coimpregnation of aqueous solutions (10-1 N) of Pd(NH3)&12 (series I) or Pd(N03)2(series 11) (purity 99.99%) with Ce(N0,)3 (purity 99.99%). All products came from Johnson-Matthey. For clarity's sake, series I and I1 catalysts will be often referred to hereafter as Pd(ch1oride) and Pd(nitrate) catalysts, respectively. C e 0 2 (ceria) support was prepared by precipitation of cerium nitrate into hydroxide at pH 9 followed by calcination ( 5 h) at 550 "C under 1 atm of synthetic 'Universitt Louis Pasteur. *UniversitC de Nancy I.
catalysta I- 1 1-2 1-3 1-4 1-5 1-6 1-7 1-8 11-1 11-2 11-3 11-4
Pd, wt %
Ce, wt 5%
9.3 8.1 6.9 7.8 8.5 8.5 7.0 6.4 8.5 5.8 8.1 8.9
0 0.02 0.33 0.52
111- 1
111-2 111-3 111-4 111-5 IV- 1
8.4
(Ce/Pd),,l,, 0 0.002 0.037 0.050 0.085
1.0 1.5 3.2 12.5 0 0.7 2.6 9.2 0.7 1.5 3.0 10.3 12.3 91.6
0.14
I
110 126 118 133 82 74 79 110
a a a
a a
17
Supports: series 1-111, y-Al,O,; series IV, Ce02. Palladium precursor salts: series I and IV, Pd(NH,),Cl,; series 11, Pd(N0,)2. air." Palladium and cerium loadings, measured on the pretreated catalysts, were determined at the Service Central de Microanalyse du CNRS (Vernaison, France). The palladium loading was kept nearly constant (8.0 f 1.5 wt %). BET surface areas of the pretreated catalysts were determined by krypton adsorption at 196 K (Table I). yA1203from Woelm (1) Tri, T. M.; Massardier, J.; Gallezot, P.; Imelik, B. Proceedings ofthe 7th International Congress on Catalysis, Tokyo; Seiyama, T., Tanabe, K., Eds.; Elsevier: Amsterdam, Netherlands, 1980, Vol. A, p 266. (2) Peters, A. W.; Kim, G. Industrial Applications of Rare Earth Elements; Gschneidner, K. A,, Ed.; ACS Symposium Series, No. 164; American Chemical Society: Washington, DC, 1964; Vol. 7, p 117. (3) Taylor, K. C. Catalysis, Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer Verlag: Berlin, 1984; Vol. 5 , p 119. (4) Summers, J. C.; Ausen, A. J . Catal. 1979, 58, 131. (5) Yao, Y. F. Y. J . Catal. 1984, 87, 152. (6) Bevan, J. M.; Kordis, J. J . Inorg. Nucl. Chem. 1964, 26, 1509. (7) Burroughs, P.; Hammet, H.; Orchard, A. F.; Thornton, G. T. J . Chem. Soc., Dalton Trans. 1976, 1686. (8) Beaurepaire, E.; Krill, G.; Le Normand, F. Proceedings of the 4th International Congress on EXAFS and Near Edge Structure, Fontevraud, France; Lagarde, P., Raoux, D., Petiau, J., Eds.; Les Editions de Physique: Paris, 1987; Vol. 2, Chapter 8, p 961. (9) Le Normand, F.; Kili, K.; Hilaire, L.; Bernhardt, P.; Krill, G.Proceedings of the 4th International Congress on EXAFS and Near Edge Structure, Fontevraud, France; Lagarde, P., Raoux, D., Petiau, J., Eds.; Les Editions de Physique: Paris, 1987; Vol. 1, Chapter 8, p 309. (10) Kili, K.; Le Normand, F.; Hilaire, L.; Maire, G., work in preparation. (1 1) Minachev, Kh.; Khodakov, Yu. S.; Nakhshuimov, V. S. J . Catal. 1977, 49, 207.
0022-3654/88/2092-2561$01.50/0
0.35 1.49 0 0.092 0.24 0.78
BET surface area, m2/g
0 1988 American Chemical Society
2562
The Journal of Physical Chemistry, Vol. 92, No. 9, 1988
and CeO, support have BET surface areas of 153 and 42 m2/g, respectively. We note a fast decrease of the BET surface area of the series I catalysts when the cerium loading increases up to ca. 1.5 wt %, as previously r e p ~ r t e d . ~ J ~ Each catalyst was pretreated in a similar manner, unless otherwise specified. After wetting at 110 OC for 12 h, the samples were calcined for 4 h at 200 OC under 1 atm of synthetic air, flushed with nitrogen at room temperature, then reduced under 1 atm of hydrogen for the catalytic test (linear temperature program 2.5 OC/min up to 400 OC, followed by 1 h at 300 "C), cooled to room temperature, and flushed again with nitrogen. Air was then slowly introduced to passivate the catalyst. In some cases (in situ LIIIedge XAS experiments), catalysts were transferred after cooling under H2 into a glovebox to prepare directly a sample for XAS measurement. The flow of gas was always 10 mL/min. The experimental apparatus for gas purification and catalyst pretreatments has been described e1~ewhere.l~ XPS. Spectra of the catalysts were recorded on a VG apparatus using A1 Kcu radiation. The base pressure during analysis was 1 X low9Torr. The resolution of the apparatus under usual conditions is 1.1 eV. The samples, pressed into pellets, were mounted on a sample holder that permitted reduction treatments (up to 400 "C, 1 atm of hydrogen of 99.99% purity, 4 h) in a preparation chamber directly connected to the analysis chamber of the spectrometer. The spectra were recorded in the accumulated mode and smoothed once. We checked before and after a long period of accumulation to ensure that no shift occurred due to variations in the degree of sample charging. The binding energies were referenced to A1 2p at 74.9 eV. In the case of Pd/Ce02 catalysts, some y A 1 2 0 3 was added mechanically to make use of the same reference. The binding energies reported here are accurate to f0.2 eV, unless otherwise noted. Relative surface compositions were calculated from photoelectron peak areas after correction for photoionization cross sectionI4 and difference in electron escape depth by using the square approximation
XAS. Most of the X-ray absorption experiments were done by using the synchrotron facility of LURE (Laboratoire d'Utilisation du Rayonnement ElectromagnEtique) in Orsay on the EXAFS I11 spectrometer. The accelerator was operating at 1.72 or 1.85 GeV, and the intensity of the beam line was typically 200 mA. The Bragg reflection of a silicon (220) or (311) monochromator was used to select the energies. Proportional counters were used to detect the incident (I,) and transmitted (I)beams. Some spectra have been recorded on a laboratory EXAFS spectrometer. A silver Rigaku rotating anode, working at a low voltage to minimize the presence of harmonics, was used for the production of X-rays, and a silicon (3 11) crystal was used to select the energies. Thus the resolution is found to be between 1.5 and 2.0 eV in that range of energies. Proportional and scintillation counters were used for the initial and transmitted beams, respectively. More experimental details are reported in ref 15 and 16. The energy position of the threshold was carefully checked by recording frequently the absorption spectrum of a reference (CeO2). XAS and XPS Spectroscopies of Cerium Compounds XPS. We report in Figure 1 the XPS cerium 3d 5 / 2 , 3 / 2 core level spectra. Origin of the Satellites. The 3d core levels of ceria and other cerium salts have been widely studied in the literature, from both
Le Normand et al. 3de 4f*
,
3d9 4f0
>
A-
885
905 BE in eV
925
Figure 1. XPS spectra of the cerium 3d transitions. Influence of the cerium content (compare A-E), the presence of palladium (compare B and F), the precursor salt of palladium (compare C and G), and in situ reduction (compare H and I, C and J): A, Ce02; B-E, Pd-Ce/yAl2O3(chloride),Ce content 12.5, 3.2, 1.5, and 0.3 wt %, respectively; F, 12.3 wt % Ce/y-A1203;G , Pd-2.6 wt % Ce/y-Al,O,(nitrate); H, 8.4 wt % Pd/Ce02; I = H reduced at 400 "C, calcined at 400 "C, and reduced 400 OC in situ; J = C reduced at 200 'C, H2.
theoretical and experimental points of view, and the presence of many satellites has long been recognized. Interpretations of these satellites are now controversial; however, we must always take into account the following effects: A multiplet effect occurs due to a coupling between spin and angular momentum of the single f electron and the 3d core hole, as in the case of the Ce3+electronic configuration 4f1(6d5s)"3d9. However, it is assumed that such an effect is of weak intensity for the 3d core level, unlike the 4d core level,I7 and results only in a broadening of the main structures. Due to the spatial extension of the 4f orbitals in light rare earths (La, Ce, Pr, etc.) these orbitals can be strongly hybridized with 0 2p orbitals. This results in a partly covalent character of the Ce-0 bonding and a partial occupation of the 4f orbitals which has been found to be around 0.55 in C e 0 2 from both theoretical and inverse photoemission result^.'^ The absorption process also induces afinal-state effect. The sudden creation of a core hole disturbs the arrangement of the valence electrons. Such final-state effects will be particularly important in XPS. The necessity of screening the positive core hole charge may induce an electronic transfer from 0 2p valence states to the more localized cerium 4f states in this light rare earth element. This effect has been well studied for metallic and intermetallic rare earth compounds.20 A double structure in the 3d core level spectrum is often observed that has been attributed to a well-screened level (with occupation of the 4f level) at low binding energy and a poorly screened level (without occupation of the 4f level) at high binding energy. Reference Compounds. CeOz. In the spectrum of pure C e 0 2 (Figure lA), apart from the spin-orbit coupling, three main 3d 5 / 2 features appear at respectively 882.5 i 0.3 (v), 888.6 f 0.3 (v"), and 898.5 f 0.2 eV (v"'), in good agreement with reported
(12) Dufaux, M.; Che, M.; Naccache, C. J. Chim. Phys. Phys.-Chim. Biol.
1970, 73, 527. (13) Garin,
F.; Gault, F. G. J . Am. Chem. SOC.1975, 97, 4466. (14) Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129. (15) Knapp, G. S.; Pan, H. K.; Georgopoulos, P.; Klippert, T. E. Proceedings of the 2nd International Congress on EXAFS and Near Edge Structure, Frascati, Italy; Springer Verlag: Berlin, 1982; 402. (16) Ravet, M. F.; Krill, G. Rigaku J. 1985, 2, 3.
(1 7) Signorelli, A. J.; Hayes, R. G. Phys. Reu. B Solid State 1973,8, 8 1. (18) Koelling, D. D.; Boring, A. M.; Wood, J. H. Solid State Commun. 1983, 47, 227. (19) Wuilloud, E.; Delley, B.; Schneider, W. D.; Baer, Y. Phys. Rev. Left. 1984, 53, 202. ( 2 0 ) Hillebrecht, V.; Fuggle, J. C. Phys. Rea. B: Condens. Matter 1982, 25, 3550.
Cerium-Based Catalysts literature spectra7J9,21-24 (we discuss here only the 3d 5 / 2 features of the 3d XPS core level, since for the 3d 3 / 2 spectrum there is some overlap with the line v"'). The higher binding energy v"' line and the well-defined 3d 3/2 u"' can be assigned to the final state 3d94P, in good agreement with theoretical calculation^.^^^^^ Moreover, these lines are not observed for purely trivalent ionic or metallic cerium compounds (CezO3,CeIn3, Ce, et^.).^,^^ In spite of some uncertainties concerning the assignment of the lines v and v" in the literature, they are generally attributed to the final configurations 3d94P and 3d94f1or a mixing of them.25 However, the broad v" feature is characteristic of cerium in the oxidation state IV (Ce02, etc.). The presence of a 3d94f2 final-state configuration (v) in the CeOz spectrum is somewhat intriguing, if we suppose that CeOz is a pure 4 p (IV) compound. This is strongly supported by its physical properties (conductivity of insulator type, diamagnetism, et^.).^^ Moreover, recent inverse photoemission spectroscopy results show that the 4f states are in the band gzp 6 eV above the 0 2p valence band.lg To screen the 3d core hole, we must suppose a charge transfer from 0 2p states of symmetry f with a great spatial extension to 4f states. Thus the v line intensity will be strongly dependent on the degree of hybridization between these orbitals. P d / C e 0 2 . When the 8.4 wt % Pd/Ce02 catalyst is reduced in situ up to 400 OC, new lines appear at around 880.0 f 0.5 eV (vo) and 885.5 f 0.4 eV (v') (Figure 11). The lower binding energy vo line is probably due to a shakedown of a trivalent cerium compound, either in an intermetallic or in an ionic state. It has been shown that the Occurrence of this peak for a reduced 8.4 wt % Pd/Ce02 catalyst is strongly dependent on the catalyst pretreatment.28 The new v' line is characteristic of Ce"' compounds such as Ce203.7pHowever, it could also be due to cerium hydrides (CeH2,, CeH2i9).29 Moreover, the poorly screened line (3d94f1) of metallic cerium is around 884.3 f 0.7 eV, difficult to separate from ionic (111) contributions. Since the well-screened line of metallic cerium is small, the occurrence of a vo line at 879-880 eV could be a signature of metallic cerium in an intermetallic compound. Ce"' Compounds. The 3d XPS core level of Ce"' ionic compounds has been much less studied due to difficulties in preparation and surface stabilization. Exposure of a cerium foil to controlled amounts of oxygen or moisture results in CezO3 formation initially.22*30Two well-defined lines are then obtained at 882.1 and 885.8 eV, which correspond to the v and v' lines respectively. X A S . Analysis Procedure. In Figure 2, we report the experimental cerium LIIIedge for some catalysts and for the reference Ce02. The threshold is arbitrarily chosen to be the inflection point of the more intense transition BI (5725.5 eV). After alignment to the threshold and correction for the background, the absorption intensity is normalized to absorption in the continuum states. To get more quantitative information, we analyzed the spectra with a program that takes into account (Figure 3) the following: an experimental resolution fitted by a Gaussian line shape (2.5 eV); the transitions toward the continuum states fitted by an arctangent function; the lifetime of the 2p hole fitted by a Lorentzian line shape of half-width 1.5 eY31 the density (21) Plateau, A.; Johansson, L.I.; Hagstrom, A. L.;Karlsson, S. E. Surf. Sci. 1977, 63, 153. (22) Praline, G.; Koel, B. E.; Hance, R. L.;Lee, H. I.; White, J. M. J . Electron Spectrosc. Relat. Phenom. 1980, 21, 17. (23) Krill, G.; Kappler, J. P.; Meyer, A.; Abadli, L.;Ravet, M. F. J . Phys.
F
1981,11, 1713.
(24) Beaurepaire, E. Thesis Docteur-IngCnieur, University of Strasbourg, France, July 1983. (25) Kotani, A.; Mizuta, H.; Jo, T.; Parlebas, J. C . Solid State Commun. 1985, 53, 805. (26) Kotani, A.; Parlebas, J. C. J . Phys. (Les V i s , Fr.) 1986, 46, 77. (27) Handbook on the Physics and Chemistry of Rare Earth; Gschneidner, K. A., Jr., Eyring, L., Eds.; Elsevier: Amsterdam, Netherlands, 1979; Vol. 3. (28) Le Normand, F.; Barrault, J.; Hilaire, L.; Kili, K.; Maire, G., to be submitted for publication. (29) Schlapbach, L.; Ostenvalder, J. Solid State Commun. 1982, 42, 271. (30) Koel, B. E.; Praline, G.; Lee, H. I.; White, J. M.; Hance, R. L. J . Electron Spectrosc. Relat. Phenom. 1980, 21, 31. Olivier, J. H. J . Phys. Chem. Re$ Data 1979,8, 329. (31) Krause, M. 0.;
The Journal of Physical Chemistry, Vol. 92, No. 9, 1988 2563
1
I
5715.00
571s .00
67
5735.00
5.0
CV
Figure 2. X-ray absorption on the cerium LIII edge. Spectra are deconvoluted from experimental resolution (2.5 eV); the absorption intensity is normalized to absorption in the continuum states. Influence of the cerium content (compare A-E, H), the presence of palladium (compare B and F), the precursor salt of palladium (compare C and G), and in situ reduction (compare B and I): A, CeO,; B-E, Pd-Ce/yAI2O3(chloride), Ce content 12.5, 3.2,0.5, and 0.3, respectively; F, 12.3 wt % Ce/y-A1203; G, Pd-2.6 wt % Ce/-pA1203(nitrate); H, 8.4 wt % Pd/Ce02; I = B after in situ reduction at 400 OC, H2. A0SORPTl (AU)
EV W
Figure 3. Cerium XAS LIIIedge analysis: (1) experimental spectrum deconvoluted by a Gaussian function of fwhm at 2.5 eV; (2) density of empty 5d states fitted by a cosine function with full-width W, (3) corehole lifetime fitted with a Lorentzian function of fwhm at 3 eV; (4) absorption in the continuum states fitted with an arctangent function (a = A/B).
of unoccupied 5d states fitted by a symmetrical cosine function of width W. LrrrAbsorption Edge of Reference Cerium Compounds. C e 4 . With these assumptions, the CeOz LIIIedge is fitted with five lines, denoted respectively A, B,,B2, CI, and C2 (Figure 2A). The last two are necessary to account for the broad C peak. The as-
2564 The Journal of Physical Chemistry, Vol. 92, No. 9, 1988
Le Normand et al.
In
I
.* ..
L.
x
C,.C*
x V”’
I
I 10
I I
C e r i u m content X
I
I
5720
5740
E(eV)
Figure 4. Cerium LllIedge of Ce(N03)3and Ce02,neither deconvoluted from experimental resolution nor normalized to the continuum states; experiments on in-lab spectrometer.
signment of these features is still a matter of discussion in the literature. Bianconi et al.,, suppose interactions in the final state with the core hole and suggest a charge transfer from valence orbitals 0 2p to Ce 4f. The lines B,, B2, and C are then respectively due to the 2p5(4f1,4f2,4fo)final states. To screen the core hole, Jo and Kotani,, add the interaction of 5d electrons with the core hole and 4f states to the dipole core hole-4f and f-f Coulombic interactions. For a reasonable value of the interaction potential Urd ( 5 eV), the assignment for the transitions 2ps(4f2,4f1,4p) are B2, B,, and C, respectively. Karnatak et al.34introduce in rare earth oxides of fluorite lattice (CeO,, Pro2, TbO,) a distinction between localized and delocalized 4f states, due to a partial covalent bonding between the rare earth cation and the oxygen anion. Thus the structures noted B,, BZ,and C are assigned to localized 2p54f1, delocalized 2p54f1, and 2p54F‘ final states, respectively. Beaurepaire et al.* explain, according to Kotani’s theory, the B and C structures by the 2p54f1and 2p54fo final states, respectively. The doublets B1,B2and C1,C2are probably due to crystal-field effects, since they are also observed for other cerium compounds of fluorite s t r ~ c t u r e . ,Whatever ~ the interpretation and the assignment of these lines, we must bear in mind that, in contrast to the screening of XPS core levels, in X-ray absorption the photoelectron is ejected just above the Fermi level and thus can participate in the screening of the core hole. It follows that the core-hole screening by occupation o f f states (f], fz) is less important in XAS than in XPS. While a complete assignment is doubtful due to uncertainties in the relative strength of these final-state interactions, the features that seem clear in the spectra are as follows (the prepeak A, usually observed for tetravalent cerium insulators with a weak relative intensity, will not be considered here): The high-energy transitions C, and C2 are mainly due to the 2p54f05d*3’ final state where cerium is in the oxidation state IV. As ascertained by Beaurepaire et al.,s the doublet in the transition C is probably due to a crystal-field effect. This splitting in the 5d band is not in disagreement with the published CeO, conduction band.19 Moreover, the spectral weight of this line as calculated from the fitting procedure is ca. 33 wt % for CeO,, close to the value determined for the analogous transition 3d94P in XPS (32 wt %) and in good agreement with reported analyses in the literat~re.~~J~ The low-energy transitions B, and B, are mainly due to the 2p54f15d*8’final-state configuration. (32) Bianconi, A.; Marcelli, A,; Tomellini, M.; Davoli, I. J. Magn. Magn. Mater. 1985, 47, 209. (33) Jo, T.; Kotani, A. Solid State Commun. 1985, 54, 45 1. (34) Karnatak, R. C.; Dexpert, H.; Esteva, J. M.; Gasgnier, M.; Caro, P. E.; Albert, L., submitted for publication in Phys. Reu. E Condens. Matter ( 3 5 ) Bauchspiess, K. R.; Bosch, W.; Holland-Moritz, E.; Launois, H.; Pott, R.; Wohlleben, D. Valence Fluctuation in Solids; Falicov, L. M., et al., Eds.; North-Holland: Amsterdam, Netherlands, 198 1; p 41 7.
0
5
10
8 . 4 %P d / C e O ,
CeO,
1
15
Figure 5. Relative intensity of the XPS 3d94f0line (0,% V’”)and XAS 2p54fo line (m, % C, + C,) as a function of the cerium content for Pd-Ce/y-Al,O,(chloride),8.4 wt % Pd/CeO, and CeO, support.
Ce”’ Compounds. On the contrary, the Llll edge of Ce”’ compounds such as Ce203, Ce(OH),, CePO,, and Ce(N03), always exhibits a single transition corresponding to the 2p54f’5d*J final c o n f i g u r a t i ~ n .The ~ ~ ~energy ~ ~ ~ ~of this transition is identical with the B, transition, which strengthens the assignment proposed (Figure 4). Thus, as for XPS and in spite of the difficulties in peaks assignment, XAS on the cerium LIIl edge for complex cerium materials may act as a fingerprint of electronic configurations and furthermore of the oxidation state.
Results and Interpretation Influence of the Cerium Content. XPS. Pd-Ce/y-Al,O, Catalysts. The spectra of Pd-Ce/ y-AlzO, catalysts (series I) reported in Figure 1 are substantially different from the pure CeO, spectrum. The growth of a line v’ and the disappearance of v” indicate that the electronic configuration is far from IV. Since we cannot detect any contribution of a line vo, cerium is probably mostly in an ionic form. The intensity of v’ relative to v increases weakly from the more (12.5 wt 5%) to the less (0.3 wt %) cerium-charged catalyst. The energy separation between the two features is ca. 3.0 eV, close to the values observed for Ce”’ compounds. The broadening of the lines probably results from the presence of several surface cerium species. A shift from ca. 1-1.5 and 2.0 eV occurs for the (3d 5/,)3d94fZ (v) and (3d 3/2)3d94f0 (u”’) well-resolved transitions, respectively, probably due to crystal-field effects. Quantitative Aspects. In a more quantitative way, the intensity of the 3d94fa lines (v”’ and u”) could be indicative of the oxidation state IV. Since u”’ is a well-resolved satellite, we can calculate a relative weight Y”’ of the v”’ transition with the following assumptions: linear approximation of the background due to secondary electrons; the ratio between the intensity Z of the 3d 3 / 2 and 3d 5 / 2 contributions, as measured by the surface areas, is given by the selection rule Y”’ = ~,Z(U”’)/[~~Z(U”’) I(3d %)I
+
where I(3d 5 / 2 ) and I(u”’) are respectively the intensity of the 3d 5 / 2 (except v’”) and u”’ features. Thus the “oxidation state”, as deduced from XPS, is given by v = 3 V”’ However, we must be careful in making use of this parameter since the spectral weight V”’ is not directly proportional to the 4p occupancy in the states as shown by F ~ g g l e . ~ ’ This parameter Y”’is reported in Figure 5 for Pd-Ce/y-Al,O, catalysts, Pd/Ce02, and Ce02. For CeO, and 8.4 wt % Pd/CeO,, a value for V’”of 32%is obtained, in good agreement with previous results r e ~ o r t e dfor ~ ~Ce02. , ~ ~ For the coimpregnated catalysts,
+
(36) Vainshtein, E. F.; Blokhin, S. M.; Bril, M. N.; Staryi, I. B.; Paderno, Y u . B. Russ. J . Inorg. Chem. (Engl. Transl.) 1965, 10, 64. (37) Fuggle, J. C. J . Less-Common Mer. 1983, 93, 159. (38) Le Normand, F.; Bernhardt, P.; Hilaire, L.; Kili, K.; Krill, G.; Maire, G. Stud. Surf. Sci. Catal., in press.
The Journal of Physical Chemistry, Vol. 92, No. 9, 1988 2565
Cerium-Based Catalysts
l a
5
0
10
"90
Figure 6. Surface ratios Ce/CI (m) and Pd/CI ( 0 )as a function of the cerium content for the Pd-Ce/y-AI2O3(chloride) catalysts and 8.4 wt % Pd/Ce02 (A and A', respectively).
5710.
5720.
PO
5730. BE (EV)
- CE12.4%
5740.
/
AL2?3_
1
TABLE 11: X-ray Diffraction of Pd-Ce/y-AI2O3(chloride) Catalysts
catalyst 1-7 1-8
Ce, wt % 3.2 12.5
I
5750.
phases CeOCl (diffuse), y-Alz03,Pd CeOCl (diffuse), C e 0 2 (diffuse), y-A1203, Pd
TABLE III: Parameters for the XAS Fittine Procedure"
E (10.3), eV W ( f 1 . 0 ) , eV
-6.2 2.5
0 4.0
4.2 6.0
10.3 3.5
13.2 3.5
"The energy reference is B, at 5729.5 eV; the ratio of transitions to the threshold versus transition to the continuum states u is 0.2-0.25.
V"' is fairly constant around 15%, far away from the value for pure CeO,, and decreases strongly for low cerium loadings. The relative surface concentrations have been calculated for Ce, Pd, and C1, present from the precursor salt Pd(NH3)4Cl, (Figure 6). They have been measured from the areas of the Ce 3d, Pd 3d, and C1 2p transitions, respectively. When the cerium content increases, the ratio Pd/C1 decreases continuously from 1.5 for Pd/y-Al2O3to 0.2 for 8.4 wt % Pd/Ce02. On the contrary and quite normally the ratio Ce/C1 increases at the same time. However, we must notice that this relative surface ratio is not very different from 1 when the cerium loading is low. XRD. The two catalysts of high cerium content (1-7 and 1-8) have been analyzed by X-ray diffraction in a Seeman-Boehlin focalization chamber (Table 11). Diffuse and large lines have been assigned to the reflections of the (loo), (1 lo), (1 11) planes of cerium oxychloride CeOCl, which crystallizes in the tetragonal system.39 Poorly crystallized CeO, has also been evidenced for the more cerium-loaded catalyst (1-8). Palladium reflections are also well defined, indicative of a particle size 1 3 0 A. X A S . Spectra of Pd-Ce/y-AI2O3and PdlCeO, Catalysts. The LIIIabsorption edge of Pd-Ce/y-A1203, and Pd/CeOz catalysts and CeO, are reported in Figure 2. They have been fitted by using the set of constant parameters (width of unoccupied cerium 5d states W a n d energy position E of the peak) reported in Table 111. In Figure 7 some characteristic fits obtained are shown. Agreement with experimental data is generally good, except for the low-cerium-content catalysts, where the statistics are rather poor. The striking feature of this study is a complete and progressive change in the shape of the absorption spectra with decreasing cerium content. All transitions decrease in intensity except B,, which corresponds to the final electronic configuration 2p54f15d*J with one localized f electron. This unique and intense absorption peak is characteristic of a Ce"' oxidation state. This is also in accord with our XPS results. Moreover, the width Wof the density of states is narrower than in cerium intermetallic compounds, and the shape of the transition (39) Templeton, D. M.; Dauben, C. H.J. Am. Chem. Soc. 1953,75,6069.
I
5710.
5720.
5730.
5740.
I
5750.
BE (EV)
PD - C E 3 . 2 % / AL203 I /
i
2.41
21. 2
I 1
5712.
5720.
5730.
5743.
I
5750.
a5 (EV)
Figure 7. Fit of cerium LIll edge from (a) CeO, support, (b) Pd-12.4 wt % Ce/y-A1203,and (c) Pd -3.2 wt % Ce/y-Alz03catalysts. Spectra are deconvoluted from experimental resolution (2.5 eV). The absorption intensity is normalized to absorption in the continuum states.
is clearly symmetrical. This strongly supports a preferential ionic state for cerium rather than a metallic one. Transitions occur then to discrete energy levels, broadened by a 4f or 5d degeneracy, rather than to the unsymmetrical conduction band of a metal. ( 2 ) Spectral Weight and Comparison with XPS. As for XPS, we may calculate the spectral weight corresponding to the final state 2p54f"5d (C, + Cz). The relative intensity of the C , C2 transitions is then an indication of the oxidation state (Figure 5). We obtain a sharp increase initially up to 1 wt % for cerium then a smooth increase to ca. 30 w t % for the P d / C e O , catalyst, very close t o the value of pure CeO,. We must note that for pure CeO,
+
2566
The Journal of Physical Chemistry, Vol. 92, No. 9, 1988
Le Normand et al. TABLE IV: Influence of the Precursor Salt of Palladium on the Surface Intensity of Ce/AI (3d/2p) catalyst Ce, wt o/c Precursor salt Ce/Al 1-6 1.5 Pd(NH3)4C12 0.08
I
ce Cenlcn,
0
5
10
1s
Figure 8. Relative intensity of the XAS 2ps4P 5d line (5% C, + C,) for Pd-Ce/y-A1,03, 8.4 wt % Pd/Ce02 catalysts, and CeOz support. Influence: cerium content (W), Pd-Ce/y-AI2O3(chloride)catalysts; absence of palladium (*),Ce/y-AI2O3catalysts; precursor salt of palladium (a), Pd-Ce/y-AI2O3(nitrate) catalysts; reduction ( O ) , Pd-Ce/y-AI2O3Pd-Ce/y-A1203(chloride)catalysts (chloride) catalysts; reduction (a), as obtained after in situ treatment (XPS % V"').
or 8.4 wt % Pd/Ce02 the spectral weights of the final states 3d94f0 (XPS) and 2p54P5d*J (XAS) are almost identical (ca. 30 wt %), in good agreement with previously reported data. This strongly supports the reliability of the two spectroscopic probes. Moreover, a comparison of the two techniques for the Pd-Ce/y-A120, (series I) is quite instructive. Up to ca. 1 wt % cerium content, the two spectral weights increase sharply. Then the 4fo spectral weight obtained by XAS technique increases more rapidly. This discrepancy can be. explained in two ways: A final-state effect occurs in the 3d XPS core level that favors transitions from an initial 4 P state to a final 4f" state in order to screen the 3d core hole. More probably, if we bear in mind that XPS is a surface-sensitive probe and XAS a volume-sensitive probe, the difference between the two results may proceed from a more reduced oxidation state of cerium (Ce"') at the surface of the catalyst. Moreover, this reduced surface state varies only weakly with the cerium content. Influence of the Presence of a Transition Metal. Some samples have been prepared by impregnation of cerium on y-alumina without the transition metal (series 111). In Figures 1 (B and F) and 2 (B and F), the sample 111-5 is compared with the PdCe/y-A1203catalyst 1-8, corresponding to nearly the same cerium content (ca. 12 wt %). Both spectra indicate an electronic configuration of cerium closer to the oxidation state IV in the absence of the transition metal. The XAS spectrum of sample 111-5 is close to that of CeO,. However, in the XPS spectrum we observed a weak v' line indicative of some reduced Ce"' compounds at the surface. Thus, as for the Pd-Ce/y-Alz03 series I catalysts, the rare earth is in a more reduced state in the first monolayers of the catalyst. The complete study has been achieved for series I11 samples by XAS expeiments only, and the spectral weight percents (C, + C2) are reported and compared with series I samples in Figure 8. For low cerium content ( I 1 wt %), we observe the same increasing intensity of the lines C1+ C,. Thus, quite surprisingly, cerium is in an oxidation state of nearly 111when dispersed initially on a y-A1203surface. The two curves differ notably for higher cerium content. As expected, cerium is in a more oxidized state in the Ce/y-A1203 system than in the Pd-Ce/y-A1203 system. Finally we must note that for 10.3 wt % Ce/y-A120,, we find by XPS a spectral weight 3d94P of ca. 22 wt % (Figure 8) instead of 30-35 wt % for the XAS 2p54P5d spectral weight. This is the same trend as observed above, which was interpreted as being due to enhanced reduction at the surface as compared to the bulk. Influence of the Precursor Salt of Palladium. Some samples have been prepared from Pd(N03)2instead of Pd(NH3)4C12as a precursor salt (series 11). Spectra C and G of Figures 1 and 2 give respectively the XPS and XAS features for Pd(ch1oride) and Pd(nitrate) catalysts of about the same cerium content, 3 wt %.
1-7 11-3
3.2
Pd(NH3)4C1Z
2.6
Pd(NO3),
0.13 0.03
Some discrepancies appear from these spectra. Although there is a large broadening of the structures in the nitrate precursor catalyst, it appears clearly from XPS that the state of cerium is close to 111. The satellite v"' (3d9(3/2)4fo) is very weak, and the calculated spectral weight 3d94P is ca. 8 wt %. On the contrary, the XAS analysis shows that cerium is in a more oxidized state for the nitrate precursor salt. This is confirmed by other XAS experiments on series I1 catalysts, the spectral weight 2p54f05d*J of which is reported in Figure 8. The difference in behavior between series I and I1 catalysts is also apparent for the ca. 1 wt % cerium content. This apparent discrepancy between the two results may be explained if we consider the surface intensity of cerium. As noted in Table IV, the relative surface intensity ratio Ce 3d/A12p is strongly affected by the precursor salt. Apparently, without chlorine, the surface cerium intensity decreases dramatically. This suggests that the majority of the cerium has either migrated toward the bulk of alumina support or grows up in large patches on the surface. In Situ Reduction. Some samples have been analyzed before and after in situ reduction (1 atm of H2, 200 "C, 4 h). XPS 3d core level spectra are reported in Figure 1 (H and I, C and J), and XAS LIIIedge spectra are reported in Figure 2 (B and I). As noted elsewhere, we observed a large reduction for the 8.4 wt % Pd/Ce02 catalyst, as evidenced by the decrease of the 3d9(3/2)4pv'" satellite and the appearance of the 3d9(,/,)4f1 satellite v'. For the Pd-3.2 wt % Ce/y-A1203,where cerium is already partially reduced, in situ reduction results also in an increase of the intensity of the v' line. However, dramatic changes are observed when the same sample is examined by XAS. Then cerium is almost completely reduced to the I11 state after in situ reduction. This effect is summarized in Figure 8, where we report the spectral weight 3d9(2p5)4fo(5d)for XPS (XAS) experiments before and after in situ reduction. We note in both cases a considerable influence of this parameter, especially in the XAS experiments. The small but significant discrepancy between XPS and XAS results for the in situ reduced samples is due in our opinion to a better reduction in the XAS furnace than in the preparation chamber of the XPS apparatus, where the samples are pressed into pellets.
Discussion We discuss first to what extent the spectroscopic probes applied in this work can give us a description of the oxidation state of cerium in such complex materials as catalysts. Then we consider some implications for the catalytic properties of these systems. Discussion of the Spectroscopic Results. The spectroscopic features in the reference oxide C e 0 2 are very puzzling, and there is considerable literature on this subject (see, for example, ref 7, 8, 18-26, 32-36,40,41), as there is for other rare earth compounds of the fluorite structure (Pro2,Tb02, Ce F4, et^.).^^,^^ The major question concerns the ground state: Is there a covalent mixing of 4f character in the filled 0 2p valence band? Recent SCF band calculations'* conclude that there is a partly covalent character of the Ce-0 bonding. Moreover, this calculated bond structure is in good agreement with valence (UPS) and inverse photoemission results in predicting the energy position for the 4f states above the Fermi 1 e ~ e l . IHowever, ~ Kotani et a1.25,33propose a model that reconciles the features in LIIIedge absorption and 3d (40) Bianconi, A.; Marcelli, A,; Dexpert, H.; Karnatak, R.; Kotani, A,; Jo, T.; Petiau, J. Phys. Reu. B Condens. Matter 1987, 35, 806. (41) Gasgnier, M.; Eyring, L.; Karnatak, R. C.; Dexpert, H.; Esteva, J. M.; Caro, P.; Albert, L. Presented at the 17th Rare Earth Research Conference, Hamilton, Canada, 1986. (42) Kaindl, G.; Wertheim, G. K.; Schmiester, G.; Sampathkumaran, E. V Phys. Rec. Lett. 1987, 58, 606.
Cerium-Based Catalysts core level photoemission with band structure in the ground state. They conclude that there is a mixed valent character in the Ce-0 bonding due to a large hybridization energy between the Ce 4f and 0 2p states. Whatever the interpretation, a partial occupancy of 0.55 in the 4f states is found, which corresponds to a mean ionicity of 3.45 for cerium in CeO,. We must note that this ionicity is not far from the 4fo spectral weight (3.30-3.35) as deduced from X-ray absorption and X-ray photoemission. The apparent discrepancy may be explained by final-state effects and large errors in the 4fo spectral weight determination. This points out how careful one must be when handling results of spectroscopic investigations, since they cannot give us directly the cerium ground state. In contrast to these rather complexproperties of cerium in the fourth valence state, spectroscopic investigations on cerium in the oxidation state 111 give a clear description of the electronic state in the ground state which is 4f', since the 4f states are localized well below the Fermi level. However, when the photoelectron is ejected into the vacuum states (XPS), the strong interaction between the core hole and the 4f states give rise to some mixing of 4f' and 4fz configurations, resulting in two main features. When we deal with a complex material where cerium species with different oxidation states may be present, spectroscopic investigations on cerium can give us roughly the mean oxidation state on the surface by X-ray photoemission and in the bulk by X-ray absorption, since Ce3+ and Ce4+ species have different spectroscopic features. This is clear for LIIIabsorption but more tentative for photoemission, since in addition to initial- and final-state effects, smaller effects such as multiplet or crystal-field effects can induce small shifts in the binding energy of the satellites. In spite of these difficulties of interpretation, some clear conclusions can be drawn from spectroscopic investigations on cerium-based catalysts: First, whatever the sample [Ce/y-A1203, Pd-Ce/y-Al,O,(chloride), or Pd-Ce/y-AI2O3(nitrate)], it seems quite obvious that cerium at low content is in a strongly reduced state (111). Moreover, the behavior of the three series is similar. This strongly supports the idea of an occupation of AI3+ cationic sites in the first several layers of the support by Ce3+cations. In spite of the difference of ionic radii (1.03 A for Ce3+ and 0.50 8, for A13+) the formation of CeA1203has been reported.44 However, more probably, Ce3+cations occupy vacant octahedrally coordinated sites on the surface. A rough calculation, considering the (1 10) face as the most probable plane on the surface of the spinel type y-A1203(S = 160 m2/g) with occupancy of only 3 / 4 of the octahedral sites by shows that they are filled up when the concentration of cerium reaches 1.5 wt %. For concentrations higher than ca. 1 wt %, there is a deviation in the cerium spectroscopic properties depending on the series of catalysts. Thus cerium is always in the less reduced state when alone on the surface and in a more reduced state for the PdCe/y-A120,(chloride) compounds. Since we find by XRD the lines characteristic of the tetragonal Ce"'OC1 and by XPS a molar ratio Ce/CI of nearly 1 for low cerium content, we conclude that cerium reacts quantitatively on the surface with chlorine coming from the precursor salt of the transition metal to give the oxychloride Ce"'OC1 or the hydroxychloride Ce111(OH)2C1. Such compounds are stable up to 400 O C 4 ' Other oxychlorides like LaOCl have recently been evidenced on the surface of Pd(ch1oride)/La,O, catalysts.48 On the other hand, it has been found that injections of dichloromethane destroy the interaction between iridium and CeO, support.49 Although the cerium LIIIedge and 3d core level of Ce1*'OC1 have not yet been reported, they are (43) Angelov, B. M. J . Phys. C 1981, 14, L757. (44) Kim, Y. S . Acta Crystallogr., Sect. 8: Struct. Crystallogr. Cryst. Chem. 1968, 824, 295. (45) Knozinger, H.; Ratnasamy, P. Catal. Reu.-Sci. Eng. 1978, 17, 31. (46) Soled, S. J . Catal. 1983, 81, 252. (47) Klevtzov, P. C. R . Seances Acad. Sci., Ser. C 1968, 266, 385. (48) Fleisch, T. H.; Hicks, R.; Bell, A. T. J . Catal. 1984, 87, 398. (49) Guenin, M.; Da Silva, P. N.; Frety, R. Appl. Catal. 1987, 27, 313.
The Journal of Physical Chemistry, Vol. 92, No. 9, 1988 2567 expected to be characteristic of purely trivalent compounds, Le., with a single transition in the XAS spectrum and double lines in the 3d core level. In the cases of Pd-Ce/y-A1,03(nitrate) and Ce/y-A120, samples, cerium is in a more oxidized state. This is probably due to the formation of CeO,, since nitrate anions are rapidly removed from the surface during the thermal treatments. However, we have noted above for the Pd-Ce/y-Al,O,(nitrate) samples, an apparent discrepancy with XPS results, since cerium is found to be reduced on the surface and the relative surface concentration Ce/A1 is low. This can be rationalized in two ways: (1) cerium is incorporated in the lattice of AI2O3 and is stabilized in the oxidation state IV (ionic radius 0.92 instead of 1.03 %, for Ce3+), or more probably (2) cerium oxide is formed in large three-dimensional patches on the surface. On the surface of these patches, cerium is in a reduced state by interaction with the transition metal. Hydrogen on Pd-Ce/y-A120, catalysts results in a dramatic reduction of cerium, as observed by LIIIabsorption. The cerium oxidation state then becomes roughly the same in the bulk and on the surface (Figure 8). On the other hand, the effect of in situ reduction on Ce/y-A1203and Ce0, support is weak, up to 300 0C.28950These two facts point out the importance of the transition metal in the reduction process. Palladium easily dissociates hydrogen which spills over the support y-A1203or more probably CeO, to form Ce(OH), species on the surface, which are stable under reducing condition^.^^ The ability of the support CeO, to favor the spillover of hydrogen is well established in the literature, where it is reported that the stoichiometries for hydrogen chemisorption or temperature-programmed reduction (TPR) are well above the expected hydrogen/transition metal atomic ratio.28,52,53This process can be summarized by the equation
+
+
+
Pd, - H 0,- + Ce?+ F= Pd, OH- CeS3+ (1) where Pd, and Ce, denote surface palladium and cerium atoms. This reduction occurs first in the neighborhood of the palladium particle, but due to the easy reducibility of cerium, the hydroxyl groups can propagate far away from the transition-metal particle. Moreover, this process must concern not only the surface cerium but also the bulk. An alternative explanation, as proposed by Fierro et al.,50considers the formation of a bulk "bronze" according to
+
+
xPd, - H CeO, xPd, CeO,H, (2) with substantial diffusion of hydrogen into the bulk lattice. However, the stability of these bronzes far below the stoichiometric is not well established. Ce/O ratio of Now if we return to unreduced catalysts, we note that when the cerium content increases, there is an increase in the mean oxidation state of cerium as evidenced by X-ray absorption results. This could be due to the growth of three-dimensional cerium dioxide or other mixed cerium oxides (Cen02n-2,n 2 4). They are formed only in significant amounts above 3 wt %, which corresponds to 2.6 kmol of (Ce02)/m2(BET), in good agreement with previously reported r e ~ u l t s .However, ~ on the surface cerium remains in a more reduced state, as evidenced by XPS results. This implies that cerium hydroxide or cerium oxychloride is always present on the surface. Implications for Catalysis. Cerium-based catalysts, in the presence of a transition metal, have revealed interesting properties for numerous reactions, most of them including redox steps: catalysis for exhaust gas depollution from automobiles (TM/ Ce/AI2O3; T M = Pt, Rh, Pd);4,5*54 methanation and methanol formation (TM/CeO, or TM-Ce/C; TM = Ni, Co, Pd, (50) Fierro, J. L. G.; Soria, J.; Sanz, J.; Rojo, J. M. Solid State Chem. 1987, 66, 154. (51) Barbezat, S.; Loriers, J. C.R.Acad. Sci. 1952, 234, 1978. ( 5 2 ) Gajardo, P.; Gleason, E. F.; Katzer, J. R.; Sleight, A. W. Proceedings of the 7th International Congress on Catalysis, Tokyo; Seiyama, T., Tanabe, K., Eds.; Elsevier: Amsterdam, Netherlands, 1980; Vol. B, p 1462. (53) Mitchell, M. D.; Vannice, M. A. Ind. Eng. Chem. Fundam. 1984,23, 88.
(54) Yao, M. D.; Yao, Yu.Y. E. J . Catal. 1984, 86, 254.
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The Journal of Physical Chemistry, Vol. 92, No. 9, 1988
Rh);5235541 water reduction at low temperatures (TM/Ce02; T M = R,Pd)t2 water gas shift (R/Ce02);63wet oxidation of ammonia (TM-Ce/oxides; T M = Co, Ni, Mn);64hydrocarbon conversion (TM/Ce02, TM-Ce/A1203 or SiO,; T M = Pt, Pd, Ni);28thioresistance to sulfurized hydrocarbon charges (Ni/Ce02).65 Among these reactions the role of cerium in the first one has been the most widely studied, due to both economic implications and the efficiency of cerium. It has been proven that this rare earth is a promoter of the catalytic activity for redox cycles, a dispersive agent for the transition metal, and a stabilizing agent inhibiting the loss of surface area of Al2O3. The promoter property is closely related to the oxygen-storage capacity of cerium, Le., the ability to insert oxygen during the oxygen-rich step of the cycle and to remove it during the oxygen-poor step. The removal of oxygen can be performed by the formation of a homologous oxide (eq 3) or a nonstoichiometric phase (eq 4), the former reaction
Ce,02, + Cen02n-2+ O2 C e 0 2 + CeO,
+ x/202
1.5
n24
(3)
