J. Phys. Chem. 1996, 100, 8363-8368
8363
Surface-Chlorinated Ceria and Chlorine-Containing Reduced Pd/CeO2 Catalysts. A FTIR Study Ahmed Badri, Claude Binet,* and Jean-Claude Lavalley Laboratoire Catalyse et Spectrochimie, URA CNRS 414, ISMRA-UniVersite´ de Caen, 6, Bd du Mare´ chal Juin, 14050 Caen Cedex, France ReceiVed: October 12, 1995; In Final Form: February 2, 1996X
Adsorption of CO, used as infrared molecule probe, was performed at room temperature or at low temperature (140 K), on either chlorine-free or chlorine-containing ceria, in order to investigate both the chlorine effect and the ceria reduction state. On ceria and Pd/ceria catalysts, chlorine acts as an electronegative ligand. Its effect extends to the raft-like structure of palladium in Pd/CeO2 catalysts. Moreover, ceria chlorination prevents palladium from CO clustering. CO adsorption on the chlorine-containing Pd/CeO2 catalyst led to some water formation, tentatively assigned to an incomplete reduction of microporous ceria sites. Moreover, oxalatelike species were evidenced and considered as impurities on residual reduced carbonate sites.
Introduction Noble metals supported on ceria are components of the threeway catalysts (TWC) for gas exhaust treatment in an automotive converter. The precursors of the noble metals often contain chlorine, which can modify the chemisorption and redox properties of ceria.1 The aim of the present study, using probe adsorption followed by Fourier transform infrared spectroscopy (FTIR), is to evidence and specify these effects, Pd/ceria being chosen as the model catalyst. Carbon monoxide is currently used for characterizing cus cation sites on metal oxides.2,3 CO is adsorbed via the carbon end. It has been found that the hypsochromic shift of the ν(CO) wavenumber, measured from the gas phase value (2143 cm-1), depends upon the strength of the electrostatic field at the adsorption sites.4 Then the ν(CO) frequency not only depends on the formal cation charge but also on the electrostatic environmental contribution to the effective charge. Considering chlorine-free ceria, the wavenumber of the Ce4+(CO) species is 2170 cm-1, whereas the corresponding value for reduced (Ce3+) cations is still undetermined.5 The noble metal of the catalyst is also commonly investigated by adsorbing CO.6 In the case of palladium, it has been found that the ν(CO) frequency depended on the CO coordinative structure (1-, 2-, or 3-fold coordinated)7,8 on the adsorption sites, evidencing (100) and (111) faces of the face-centered cubic crystal.9-12 Moreover, the intensity of the ν(CO) bands can be used as a spectroscopic measure of the accessibility of the metal surface atoms to CO adsorption, as for example in the case of supported Pt13 and Pd metals.10,14,15 They are the reasons why CO has also been chosen as a probe to determine the chlorine effect on both the support (ceria) and the metal (Pd) properties. Experimental Section Ceria (from Rhoˆne Poulenc) had a high specific BET total area (120 m2‚g-1), including that (60 m2‚g-1) due to the microporosity. The chlorinated ceria sample was obtained by impregnating ceria with a 1 M aqueous HCl solution, then dried at 383 K. Palladium-loaded ceria was prepared by the impregnation procedure using either the Pd(NH3)4(OH)2 complex as the precursor for the chlorine-free samples or PdCl2 dissolved in water at 323 K for chlorine-containing catalysts. X
Abstract published in AdVance ACS Abstracts, May 1, 1996.
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After drying at 393 K, the catalysts were calcined under an oxygen flow (20 cm3‚min-1) for 1.5 h at 573 K. The palladium loading, determined by the CNRS microanalysis center, was 1.2 and 0.92 wt % for the unchlorinated and chlorinated catalysts, respectively. The calculated chlorine content of the chlorine-containing catalyst was 0.6 wt %. The catalysts were further denoted Pd(hy)/ceria or Pd(Cl)/ceria according to whether the precuror salt contained hydroxo or chlorine anions. The powdered samples were pressed into a disk (15 mg‚cm-2) under 600 bar pressure. Disks were pretreated in situ in the IR cell. Whatever the sample, a calcination in O2 at 673 K was performed under static conditions (P ) 13 kPa) to eliminate most of the carbonate species (residual species in commercial ceria and those resulting from the unavoidable atmosphere CO2 adsorption). In the case of chlorine-containing samples, no chlorine was considered to evolve during this treatment, since chlorine evolution was found to arise mostly at temperatures above 773 K.1 For reduced samples, they were obtained by three consecutive and identical treatments with hydrogen (P ) 20 kPa, t ) 20 min) at 423 K separated by evacuations at this temperature, the final evacuation time being 15 min and the pressure 3 × 10-3 Pa. The IR spectra were recorded at room temperature with a Nicolet 60SX FTIR spectrometer (resolution, 4 cm-1). The spectra reported in this paper are generally those obtained by subtracting the absorbances of the samples after and before adsorption of the probe molecule, with Figure 7, where background spectra are shown, being the exception. Results (1) Chlorine-Containing Ceria. Figure 1a shows the spectrum of the species formed by CO adsorption at room temperature on chlorinated ceria. The main band appears at 2187 cm-1 and is due to CO adsorbed on Lewis centers. It is 18 cm-1 shifted toward higher wavenumbers relative to that observed on unchlorinated ceria (Figure 1b) evidencing the improved acidity of ceria Ce4+ centers after chlorination. Only weak bands due to bidentate carbonate (1600, 1290 cm-1), tridentate carbonate, and carboxylate species (1507, 1355 cm-1) are observed when ceria is chlorinated (Figure 1a), while in the case of the unchlorinated sample, bands due to tridentate carbonate species are relatively strong (Figure 1b). This indicates a loss of surface basicity (O2- anions) upon chlorination. © 1996 American Chemical Society
8364 J. Phys. Chem., Vol. 100, No. 20, 1996
Badri et al. TABLE 1: Wavenumbers (cm-1) of CO Adsorbed Species Formed, at 140 K, on Chlorine-Free or Chlorine-Containing Ceria Either Unreduced or Reduceda species chlorine-free ceria unreduced reduced chlorine-containing ceria unreduced reduced
COW
COL
2148.5 2145.5
2169 2161
2166 2159
2187, 2175 2172
a
COW: species weakly interacting with the surface. COL: species adsorbed on Lewis sites. Figure 1. IR spectrum of species formed from adsorption (a) of 0.4 kPa CO, at room temperature, on ceria chlorinated by impregnation with a 1M HCl solution and (b) of 2 kPa CO on chlorine-free ceria.
CO adsorption was also studied at low temperature (140 K) on 1 M HCl-chlorinated ceria previously pretreated in O2 and then evacuated at 673 K. Introduction of a small amount of CO (Figure 2a) leads to the appearance of a band at 2190 cm-1, with a weak shoulder at ca. 2175 cm-1. When a larger amount of CO was introduced (Figure 2b), the band shifts to 2187 cm-1 without increasing its intensity whereas a broad band dominates at 2166 cm-1. Upon evacuation, the latter mainly vanishes, evidencing a weak band at 2175 cm-1 (Figure 2c). Taking as reference the ν(CO) value in the gas phase (2143 cm-1), we can assign the 2166 cm-1 band to CO species weakly interacting with the surface, noted COw, which is confirmed by their disappearance upon evacuation. The two other bands at 2187 and 2175 cm-1 are due to CO coordinated to cus Cez+ centers acting as Lewis sites and are noted COL and COL*, respectively. Note that the COL species are mainly predominant when introducing CO at room temperature (Figure 1a). Similar CO adsorption experiments have been performed on the chlorinated sample H2-reduced at 673 K. Introducing CO under a low pressure (0.13 kPa) leads to the appearance of a band at 2175 cm-1 (Figure 2d). For a higher CO pressure (1.3 kPa) this band is downshifted to 2172 cm-1 by intermolecular interactions, while a new band appears at 2159 cm-1 (Figure 2e). Upon evacuation, only the high-frequency band remains at 2177 cm-1 (Figure 2f). By comparison with results shown
in Figure 2b, the high-frequency band in Figure 2e is noted COL, the low-frequency one being COW. These results are gathered in Table 1 with those relative to CO adsorption under the same conditions (low temperature, 1.3 kPa CO pressure) on unreduced and reduced chlorine-free ceria.15 (2) Pd-Loaded Catalysts. The catalysts investigated were obtained from Pd impregnation of a high area and microporous ceria sample (see Experimental Section), explaining why high metal dispersions (ca. 90%) were spectroscopically measured from CO adsorption, as briefly reported below. (a) Chlorine-Free Pd(hy)/Ceria Catalyst. Introduction of a large amount of CO (P ) 0.6 kPa) on Pd(hy)/ceria H2-reduced at 423 K leads to a complex spectrum (Figure 3a), which can be divided into two ranges according to whether the band wavenumbers are situated below or above 1650 cm-1. Bands located above 1650 cm-1 are due to CO adsorption on palladium, whereas the others result from CO adsorption on the carrier. (i) The main bands at 1590 and 1370 cm-1 are assigned to νa and νs(OCO) modes of the formate species16-18 (a more detailed study indicates at least two types of formate species15). (ii) Those at 1470 and 1410 cm-1, of weak intensity, are due to the carbonate species. (iii) That at 1510 cm-1, as shown by introducing smaller doses of CO, may be associated with a component of that at 1370 cm-1, suggesting the formation of the carboxylate species.19 For the range above 1650 cm-1, it presents two strong bands at 2010 and 1960 cm-1 and broader ones at 2050, 1860, and
Figure 2. IR spectra of species formed from CO adsorption at low temperature (140 K) on the ceria sample chlorinated by impregnation with a 1M HCl solution: (a) 30 µmol CO‚g-1; (b) 1.3 kPa CO pressure; (c) evacuation; (d) 0.13 kPa CO pressure; (e) 1.3 kPa CO pressure; (f) evacuation. Spectra a-c are for the unreduced sample, and spectra d-f are for the reduced sample. For convenience, the intensities of spectra b and e were divided by 2.
Reduced Pd/CeO2 Catalysts
J. Phys. Chem., Vol. 100, No. 20, 1996 8365
Figure 3. IR spectra of species formed from CO adsorption on the chlorine-free Pd(hy)/ceria catalyst H2-reduced at 423 K. Adsorptions at room temperature were obtained (a) at 0.6 kPa CO pressure, (b) after evacuation, and (c) with subsequent addition of 0.1 kPa CH3OH. Adsorption at low temperature (140 K) was obtained at (d) 0.4 kPa CO pressure and after evacuating.
Figure 4. IR spectra of species formed from CO adsorption at room temperature on the chlorine-containing Pd(Cl)/ceria catalyst H2-reduced at 423 K: (a) adsorption of a small dose, 8 µmol‚g-1; (b) 0.6 kPa CO pressure; (c) after evacuation. For convenience, the intensity of spectrum a was multiplied by 2.
1780 cm-1 (Figure 3a). CO adducts by doses15 as well as evacuation at room temperature (Figure 3b) indicate that the main bands at 2010 and 1960 cm-1 are not sensitive to CO coverage. Therefore, their wavenumbers are free from intermolecular CO coupling contributions20 and they are related to isolated CO species. An activated process for the decomposition of the corresponding species has been inferred from their stability upon evacuation.15,21 Note that when CO at low temperature is adsorbed (Figure 3d), these two bands are hardly discernable, suggesting also an activated process for their formation. Finally, a comparison of spectra a and c of Figure 3 show that addition of methanol after CO at room temperature leads to the disappearance of the 2010 and 1960 cm-1 ν(CO) bands, showing that the corresponding species are destabilized by methoxy species formation on cus Ce3+ sites. Since similar ν(CO) bands have been observed on Pd/NaY zeolites and assigned to Pd-carbonyl clusters in zeolite supercages,22 we propose that the two bands at 2010 and 1960 cm-1 are due to PdCO (2010 cm-1) and Pd2CO (1960 cm-1) single carbonyl species, respectively. When CO was introduced on the oxidized catalyst, these species were only transiently observed.15,21 The other bands due to CO adsorption on Pd are clearly observed in Figure 3d and situated at 2050, 1865, and 1775 cm-1. As shown by introducing CO by doses, they are COcoverage dependent.15 A comparison of spectra a and b of Figure 3 confirms such a result: the bands at 1860 and 1780 cm-1 (Figure 3a) shift to 1820 and 1730 cm-1 (Figure 3b) upon evacuation, whereas that at 2050 cm-1 vanishes almost completely. Note that the same wavenumbers are observed after introducing CO (P ) 0.6 kPa) at room temperature (Figure 3a) or at 140 K after evacuation at the same temperature (Figure 3d), indicating a similar compacity of the adsorbed CO phase under the two experimental conditions. By analogy with dense adsorbed CO phases,7-11 the band at 2050 cm-1 is assigned to linear (on-top) CO adsorption and that at 1860 cm-1 to 3-fold coordinated CO molecules. For the lower wavenumber band (1775 cm-1 in the present case) it is generally observed for CO adsorbed on transition metals supported on reducible metal oxides and currently assigned to CO adsorbed by its two ends at the metal-support interface. This assignment deserves some comments as discussed below. The coverage-dependent broad bands at 2050 and 1860 cm-1 are indicative of an extended palladium structure.
The exposed metal fraction (or metal dispersion in the absence of encapsulation) is measurable through the total integrated intensity of the ν(CO) bands. CO was volumetrically introduced in doses at room temperature until the ν(CO) band intensity reached a stationary value corresponding to a dense adsorbed CO layer. If CO doses were further introduced, a multilayer adsorption built up from the equilibrium with the gas phase. For the dense adsorbed CO layer, the CO/Pd (exposed) ratio ) 1 was inferred.10 The palladium dispersion in the Pd/CeO2 sample was so evaluated to 90%. However, in the present case, the formation of Pd-carbonyl clusters may modify the accepted stoichiometry CO/Pd (exposed). To evaluate the incidence of such clusters formation on the so-measured dispersion value, integrated intensities of the ν(CO) bands (Figure 3) were ratioed in the absence (spectrum d) or in the presence of clusters (spectrum a). A 0.9 ratio was then obtained, showing that inclusion of CO palladium clusters in the total integrated intensity measurements has no practical effect on the evaluated dispersion. (b) Chlorine-Containing Pd(Cl)/Ceria Catalyst. CO was adsorbed, at room temperature, on the chlorine-containing catalyst H2-reduced at 423 K. Figure 4 shows spectra obtained after introduction of a small dose of CO (spectrum a), under a 0.6 kPa CO equilibrium pressure (spectrum b), and after subsequent evacuation (spectrum c). In the spectral range above 1650 cm-1, relevant to the CO adsorption on palladium, the striking difference between the chlorine-free (Figure 3a) and the chlorine-containing catalysts (Figure 4b) is the absence of sharp bands at 2010 and 1960 cm-1 due to carbonyl clusters in the first case. Only the three main bands due to CO adsorption on palladium crystallites are observed in Figure 4b. They are situated at 2090, 1945, and 1800 cm-1. Note that the 1800 cm-1 band undergoes an important downshift to 1740 cm-1 upon evacuation (spectrum c), inferring the great sensitivity of this band to intermolecular interactions between adsorbed CO molecules. In order to compare CO adsorption on Pd crystallites alone in chlorine-free and chlorine-containing catalysts, CO adsorption was performed at low temperature to avoid any important clustering (Figure 5). In fact, for other purposes (see below) 13CO was used instead of 12CO, leading to a 40 cm-1 12C f 13C downshift. A comparison of spectra a and b shows that chlorination of the carrier induces a 50 cm-1 upward shift for the well-defined band at 2003 cm-1. The evaluation of such
8366 J. Phys. Chem., Vol. 100, No. 20, 1996
Figure 5. IR spectra of species formed from adsorption of 13CO (0.3 kPa) at low temperature (140 K) on a Pd/ceria catalyst: (a) chlorinefree catalyst; (b) chlorine-containing catalyst.
a shift is made difficult for bands at lower wavenumbers because of their broad shape (1827 cm-1 band) and their poor resolution (1740 cm-1). An overall survey of the profile suggests a same (ca. 50 cm-1) hypsochromic shift for the three bands at 2003, 1827 and 1740 cm-1. From integrated band intensities and taking into account the slight difference of the Pd loading (see experimental), the same dispersion can be evaluated for the two catalysts (i.e., 90%). Two pairs of bands above 2090 cm-1 (Figure 5) are due to CO molecular adsorption on the carrier, either 13CO (strong bands only outlined for clarity) or 12CO (weak bands) as an impurity in 13CO. Note that the equivalent bands are absent in Figure 3d after outgassing. The 12CO band wavenumbers match very well the values reported in Table 1 for the reduced but Pd-unloaded carrier, while homologous 13CO bands undergo the expected 46-48 cm-1 isotopic downshift. A comparison of spectra a and b in Figure 5 clearly indicates the induced upward wavenumber shift of these bands, due to CO adsorption on the carrier, upon chlorination of ceria. In the spectral range below 1650 cm-1 (Figure 4), no formate species were observed on the chlorinated sample; only bands of low intensity were detected. They appeared as soon as CO was introduced (spectrum a), discarding a high activation energy for the formation of the corresponding species. Broad bands at about 1515-1360 cm-1 were already observed in Figure 3 relative to CO adsorption on the Pd(hy)/ceria catalyst and assigned to carbonate and carboxylate species. The formation of the latter species upon CO adsorption was not modified by ceria chlorination. The narrow band at 1610 cm-1 is located in the spectral range where the δ(HOH) deformation mode of water is expected. In order to specify its assignment, the Pd(Cl)/ceria catalyst was first reduced at 423 K by two successive 13 kPa H2 exposures separated by an evacuation stage and then exchanged by D2 at the same temperature. 13CO, instead of 12CO, was used as a probe (Figure 6) to complete the study of the carbonate species formation. D2O formation is clearly shown in Figure 6b by the presence of the δ(DOD) band at 1188 cm-1, whereas a weak shoulder at 1626 cm-1 may correspond to the δ(HOH) vibration. Note that D2O is not produced at 140 K (Figure 6a), since annealing to room temperature is necessary for its formation. Interestingly, evacuation at room temperature (Figure 6c) does not modify the 1188 cm-1 band intensity, whereas the intensity at 1626
Badri et al.
Figure 6. IR spectra of species formed from 13CO adsorption on the chlorine-containing Pd(Cl)/ceria catalyst H2-reduced at 423 K and then D2-exchanged at 423 K: (a) at 0.3 kPa 13CO exposure at low temperature (140 K); (b) after annealing to room temperature; (c) after evacuating at room temperature.
Figure 7. Spectral background of the Pd(Cl)/ceria catalyst H2-reduced and then D2-exchanged at 423 K.
cm-1 increases. A survey of the catalyst spectrum after H2 reduction and D2 exchange (Figure 7) shows that the hydroxyl surface species are almost exclusively OD (bands at ca. 2600 cm-1) and not OH (lack of well-apparent band near 3600 cm-1). This indicates that H2O formation does not imply common OH surface species (see discussion). When 13CO (0.3 kPa) was introduced at 140 K on the D-exchanged Pd(Cl)/ceria catalyst (Figure 6, spectrum a), bands due to the carbonate and carboxylate species appeared at 1487, 1455 (shoulder), 1372 and 1340 cm-1 (stretching modes), and 855 cm-1 (πCO3 out-of-plane deformation vibration). Upon annealing to room temperature under a 0.3 kPa 13CO pressure (spectrum b), the shoulder at 1455 cm-1 is resolved into two bands and is shifted to 1430 and 1405 cm-1. Concomitantly, bands initially at 1372 and 1340 cm-1 shift to 1315 and 1285 cm-1, while the πCO3 band shifts to 822 cm-1. Annealing does not modify the 1487 cm-1 band wavenumber. These results can be explained, assuming the concomitant formation at low temperature of a 13C-carboxylate species (band at 1487 cm-1) and a surface polydentate 12C-carbonate species, 13C-exchanged with gaseous 13CO upon annealing to room temperature. Two possible surface structures may be inferred for the carbonate species (bands at 1455, 1372, 1340, and 850 cm-1). The 13Ccarboxylate band at 1487 cm-1 is homologous to the 12Ccarboxylate band at 1515 cm-1 in Figure 4. The corresponding carboxylate-carbonate species may be shown as below, like an adsorbed head-to-tail oxalate species:
Reduced Pd/CeO2 Catalysts
This species possibly results from an easy insertion of 13CO in some defective carbonate sites with a very low activation energy, as may be judged from the complete formation of such species at low temperatures. Discussion Results for the nondissociative adsorption of CO on chlorinecontaining ceria are compared to those concerning chlorinefree ceria in Table 1. Let us remark first that two values (2187, 2175 cm-1) are reported for the COL species for unreduced chlorinated ceria. The 2175 cm-1 value, not far from 2169 cm-1 observed for the chlorine-free unreduced ceria (Table 1), could be assigned to the COL species adsorbed on Ce4+ sites where only long-range chlorine effects take place from an inhomogeneous chlorination of the surface. However, this value is very close to that observed for CO adsorption on the chlorinecontaining but reduced ceria, so the following more plausible assignment is inferred. It is conceivable that chlorination at room temperature by the impregnation method leads to chlorine chemisorption and that the oxygen and vacuum treatment at 673 K gives rise to some lattice O2- exchange with Cl- ions forming a CeOCl cerium(III) phase. Chlorine incorporation into the ceria surface would be more complete upon reduction. So, two types of sites would be considered on the H2-unreduced sample: (i) sites where Cl- is chemisorbed in the vicinity of Ce4+ ions characterized by ν(COL) at 2187 cm-1; (ii) sites belonging to a CeOCl phase where cerium is reduced to Ce3+, giving rise to ν(COL*) at 2175 cm-1. Results reported in Table 1 show that chlorination induces a 17 or 13 cm-1 upshift for the COW species in the unreduced or reduced ceria states, respectively. It is remarkable that the ν(CO)L - ν(CO)W differences (ca. 20 and 15 cm-1 for unreduced and reduced samples, respectively) do not appreciably depend on the presence of chlorine. If the shift of the ν(CO) band from the gas phase value (2143 cm-1) arises from an electric field at the CO molecule,4 this field appears as the sum of two components acting in the same direction (wavenumbers upward shift): (i) a uniform component over the surface that affects the COL and COW species, becoming important when chlorine is present; (ii) a very localized component at the cation sites depending only on the Ce4+ or Ce3+ oxidation degree of cerium. Thus, the difference ν(CO)L - ν(CO)W discriminates between the reduction degree of ceria even when chlorinated. From the ν(CO) upward shift upon chlorination due to the uniform component of the field, it may be inferred that chlorine reinforces the apparent Lewis acidity of the surface and acts like an electronegative ligand. Chlorination of the carrier led to a 50 cm-1 blue shift for ν(CO) bands corresponding to CO adsorbed on the metal crystallites. This shift is due to the above-mentioned effect of chlorine acting as an electronegative ligand. Recall that the shift was only 13 cm-1 for CO adsorbed on the reduced carrier. ν(CO) frequencies for CO adsorbed on the metal more or less depend on the electron back-donation from the metal to the antibonding 2π* orbital of CO.23 Chlorination of the carrier withdraws electrons from the metal, thus decreasing the π backdonation and reinforcing the CO bond. The ν(CO) wavenumber therefore increases. Transmission of the electronegative effect through the metal without an important screening can be related to the high palladium dispersion so that a raft-like structure may
J. Phys. Chem., Vol. 100, No. 20, 1996 8367 be postulated for the metal on the carrier. Metal clustering upon CO adsorption was not observed at room temperature for palladium on the chlorinated carrier. A possible explanation is that for the reduced chlorine-free carrier, the electron transfer to the metal weakens the palladium cohesive forces, whereas such an effect would be annihilated by an electronegative ligand like chlorine or the methoxy species formed upon methanol adsorption (see Figure 3, spectrum c). However, if oxygen vacancies on the reduced ceria surface are needed to stabilize carbonyl clusters, the no-clustering effect of CO may as well be due to the occupancy of such vacancies by chlorine in the case of the chlorine-containing catalyst. When the chlorinefree catalyst is considered, the methoxy species produced by adsorbing methanol would displace carbonyl clusters from O vacancies leading to their destabilization. With the exception of the general wavenumber shift, there is a striking similarity between the spectra due to CO adsorbed on palladium supported on chlorinated or on unchlorinated ceria (Figure 5). This allows us to consider possible reassignments of the lower wavenumber band (at around 1740 cm-1) generally considered for transition metals supported on reducible carriers, characterizing a two-end adsorbed CO molecule at the metalsupport interface (C bonded to the metal and O bonded to the carrier).24-28 In our work, it was expected that the carrier chlorination would induce a wavenumber shift for CO C bonded in 3-fold palladium sites (band at 1827 cm-1) different from that related to CO bound at the interface (band at 1740 cm-1). Moreover, it was expected that the relative 1827/1740 cm-1 band intensity ratio would reasonably be modified upon chlorination of the carrier sites. This is not observed. The rather large 1800 f 1740 cm-1 downshift observed in Figure 4 upon evacuation is also intriguing for CO molecules tilted at the interface and expected to be uncoupled from their nonparallel orientation with those adsorbed on terraces. If the attribution of the 1740 cm-1 band to an interfacial CO adsorption is rejected, other hypotheses may be considered. From quantum calculations, the adsorption energy of CO in 4-fold sites is not very significantly higher than that of bridged CO.29 Although not observed for CO adsorption on monocrystals, a band at 1745 cm-1 was assigned to CO adsorbed in 4-fold sites for thin Pd films deposited on Mo(100).30 For Pd/CeO2 catalysts, it was proposed that CO adsorption into 4-fold sites was induced by an increased π back-donation due to reduction of the chlorinefree ceria, but such an effect would be partly annihilated by the electron-withdrawing ceria chlorination. A plausible assignment of the band at 1740 cm-1 can be proposed by considering palladium crystallites anchored on ceria, with the possibility of bond formation between metals and cations on reducible carriers.31,32 Even at the low temperature of reduction used (523 K), interfacial interactions may produce palladium site distorsions such that either tilted adsorbed CO molecules should be considered or the long Pd-Pd distance would produced ketocarbonyl-like species with ν(CO) at around 1700 cm-1.31 In addition to σ, π-bonded CO species, ketocarbonyl species were proposed for Pd/SiO2 catalysts reduced at 673 K.32 When CO was adsorbed at low temperature (140 K) on H2reduced then D2-exchanged Pd(Cl)/CeO2 catalyst, D2O was first produced upon annealing to room temperature and then H2O was formed upon evacuation (Figure 6). In the case of Pdunloaded chlorine-containing ceria, adsorption of CO did not produce water, so palladium is necessary for water formation. Conceivable forms for hydrogen storage in ceria are hydroxyl (H+) species, dissolved hydrogen (H°) in bronze,33 and, possibly as in TiO2,34 a hydride (H-) form. Moreover, H may be stored at the surface or dissolved as H° species in Pd. Such a complexity, not clearly explored in the literature, makes the
8368 J. Phys. Chem., Vol. 100, No. 20, 1996 interpretation of water formation very speculative. We point out that H2O was formed (Figure 6c) although the D2-exchange was previously performed at 423 K, leading to a complete deuteriation for the surface hydroxyl species (Figure 7). The lack of complete H-D exchange may be due to chemical reasons but also to physical ones. For example, in the case of the Pd-free ceria samples, we observed that the OH species in ceria micropores were not exchanged by D2 at 473 K, although the external surface OH species were exchanged.15 This implies a diffusion-limited exchange process. In that sense we may raise the possibility that water is formed at the crystallite-carrier interface from hydrogen stored in micropores, CO being necessary for its formation. The more accessible micropores, storing D, would lead to D2O production upon annealing to room temperature (Figure 6b), with the less accessible micropores, storing not exchanging H, producing H2O upon evacuation (Figure 6c). In this interpretation, evacuation is thought to provoke some CO diffusion in the less accessible parts of the sample where H is stored. If CO completes the reduction of such defective microporous interfacial sites producing CO2, the water gas shift reaction is a possible mechanism for water production. For the chlorine-free Pd(hy)/CeO2 catalyst no water formation was observed on adsorbing CO; but since water dissociates into the OH species when adsorbed on unchlorinated ceria, this does not necessarily implys that water was not formed as an intermediate. CO adsorption, either on a chlorine-free or on a chlorinecontaining catalyst, led to the formation of carboxylate and carbonate groups assignable to the head-to-tail oxalate species. We infer that these species result from CO adsorption on defect sites resulting from the uncomplete decomposition of carbonate microphases as impurities in ceria (bands below 1600 cm-1 in the background spectrum in Figure 7 are related to such residual carbonate species). These sites led to the formation of carbonate CO22- species when the reduction operates at higher temperatures (823 K)35 and are destroyed through oxidoreductive cycles. Not every vacuum stable species would result from the CO adsorption on chlorine-containing ceria if ceria does not contain carbonate impurities. Volumetric measurements of the metal dispersion using CO adsorption can then be performed on such catalysts by keeping the carrier free from CO adsorption. For chlorine-free Pd/CeO2 catalysts, the formate species are produced upon adsorbing CO. Metal is not necessary to the formate species production (as long as ceria is H2-reduced), but special sites are needed16,18 like the monodentate surface hydroxyl species.16 Chlorination of such sites would prevent formate formation. Conclusion CO adsorption, at low temperature, on ceria allows us to determine both the chlorination and the ceria reduction states. Chlorination of ceria induces an upward frequency shift for the two ν(CO) bands due to CO either weakly interacting with the surface or adsorbed on Lewis cationic sites. The difference between these two ν(CO) band frequencies depends on the Ce4+ or Ce3+ oxidation states of ceria and is not modified by ceria chlorination. The possibility of the presence of cerium(III) in a CeOCl phase, even for the unreduced catalyst, was raised. The chlorine ligand effect over the carrier is transmitted to CO adsorbed in the various sites of a raft-like structure of palladium in a Pd/CeO2 catalyst, leading to an important (ca.
Badri et al. 50 cm-1) upward shift of ν(CO). It is observed that chlorination of the carrier for the reduced catalyst prevents palladium rafts from CO clustering. The reduced chlorine-containing Pd/CeO2 catalyst appears to be unreactive toward CO at room temperature, the only headto-tail oxalate-like species formed being attributed to defect sites resulting form the reduction of carbonate impurities. Consequently, the dispersion of palladium could be evaluated from volumetric measurements of CO chemisorption without interferences with some carrier contributions. Some water formation upon CO adsorption was tentatively assigned to an incomplete H2 reduction of the carrier at the micropores. References and Notes (1) Bernal, S.; Calvino, J. J.; Cifredo, G. A.; Gatica, J. M.; Perez Omil, J. A.; Laachir, A.; Perrichon, V. CAPoC-3, Brussels, 1994; Fresnet, A., Bastin, J. M., Eds.; 1994; Vol. 2, p 275. (2) Zaki, M.; Kno¨zinger, H. Spectrochim. Acta 1987, 43A, 1455. (3) Escalona Platero, E.; Scarano, D.; Spoto, G.; Zecchina, A. Faraday Discuss. Chem. Soc. 1985, 80, 183. (4) Bordiga, S.; Lamberti, C.; Geobaldo, F.; Zecchina, A.; Turnes Palomino, G.; Otero Arean, C. Langmuir 1995, 11, 527. (5) Zaki, M.; Vielhaber, B.; Kno¨zinger, H. J. Phys. Chem. 1986, 90, 3176. (6) Sheppard, N.; Nguyen, T. T. In AdVances in Infrared Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Heyden: London, 1978; Vol. 5. (7) Bradshaw, A.; Hoffmann, F. Surf. Sci. 1978, 72, 513. (8) Persson, B.; Tu¨shaus, M.; Bradshaw, A. J. Chem. Phys. 1990, 92, 5034. (9) Palazov, A.; Kadinov, G.; Bonev, C.; Shopov, D. Surf. Sci. 1987, 188, 505. (10) Binet, C.; Jadi, A.; Lavalley, J. C. J. Chim. Phys. Phys.-Chim. Biol. 1989, 86, 451. (11) Tessier, D.; Rakai, A.; Bozon-Verduraz, F. J. Chem. Soc., Faraday Trans. 1992, 88, 741. (12) Hicks, R.; Yen, Q.-J.; Bell, A. J. Catal. 1984, 89, 498. (13) Primet, M.; El Azhar, M.; Frety, R.; Guenin, M. Appl. Catal. 1990, 59, 153. (14) Duplan, J. L.; Praliaud, H. Appl. Catal. 1991, 67, 325. (15) Badri, A. Thesis, ISMRA-Universite´ de Caen, Caen, France, 1994. (16) Li, C.; Sakata, Y.; Arai, T.; Domen, K.; Maruya, K.-I.; Onishi, T. J. Chem. Soc., Faraday Trans. 1, 1989, 85, 1451. (17) Li, C.; Domen, K.; Maruya, K.-I.; Onishi, T. J. Catal. 1990, 125, 445. (18) Binet, C.; Jadi, A.; Lavalley, J. C. J. Chim. Phys. Phys.-Chim. Biol. 1992, 89, 1779. (19) Li, C.; Sakata, Y.; Arai, T.; Domen, K.; Maruya, K.-I.; Onishi, T. J. Chem. Soc., Faraday Trans. 1, 1989, 85, 929. (20) Uvdal, P.; Karlson, P.; Nyberg, C.; Anderson, S.; Richardson, N. Surf. Sci. 1988, 202, 167. (21) Badri, A.; Binet, C.; Lavalley, J. C. J. Chim. Phys. Phys.-Chim. Biol. 1995, 92, 1333. (22) Sheu, L. L.; Kno¨zinger, H.; Sachtler, W. M. H. J. Mol. Catal. 1989, 57, 61. (23) Fournier, R. J. Chem. Phys. 1993, 99, 1801. (24) Bredikhin, M.; Lokhov, Yu.; Zamaraev, K. Dokl. Akad. Nauk SSSR 1988, 301, 1124. (25) Sachtler, W.; Ichikawa, M. J. Phys. Chem. 1986, 90, 4752. (26) Ichikawa, M.; Fukishima, T. J. Phys. Chem. 1985, 89, 1564. (27) Boujana, S.; Demri, D.; Cressely, J.; Kiennemann, A.; Hindermann, J. P. Catal. Lett. 1990, 7, 359. (28) Alekseev, O.; Beutel, T.; Praukshtis, E.; Ryndin, Yu.; Likholobov, V.; Kno¨zinger, H. J. Mol. Catal. 1994, 92, 217. (29) Anderson, A.; Awad, M. J. Am. Chem. Soc. 1985, 107, 7854. (30) Heitzinger, J.; Gebhard, S.; Koel, B. Surf. Sci. 1992, 275, 209. (31) De La Cruz, C.; Sheppard, N. J. Mol. Struct. 1990, 224, 141. (32) De La Cruz, C.; Sheppard, N. Spectrochim. Acta 1994, 50A, 271. (33) Fierro, J.; Soria, J.; Sanz, J.; Rojo, J. J. Solid State Chem. 1987, 66, 154. (34) Munuera, G.; Gonzalez-Elipe, A.; Espinos, J.; Conesa, J.; Soria, J.; Sanz, J. J. Phys. Chem. 1987, 91, 6625. (35) Binet, C.; Badri, A.; Lavalley, J. C. J. Phys. Chem. 1994, 98, 6392.
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