O2 in a Nanoceria-Supported

J. Phys. Chem. B 2004, 108, 17983-17991

17983

Interfacial Redox Processes under CO/O2 in a Nanoceria-Supported Copper Oxide Catalyst A. Martıńez-Arias,*,† A. B. Hungrıá,† M. Ferna´ ndez-Garcıá,† J. C. Conesa,† and G. Munuera‡ Instituto de Cata´ lisis y Petroleoquı´mica, CSIC, C/Marie Curie, Campus Cantoblanco, 28049 Madrid, Spain and Departamento de Quı´mica Inorga´ nica, UniVersidad de SeVilla, 41092 SeVilla, Spain ReceiVed: July 30, 2004; In Final Form: September 7, 2004

A catalyst of copper oxide supported on cerium oxide has been characterized by XRD, Raman, XAFS, XPS, and EPR techniques. Its redox properties under CO/O2 have been explored with XPS and EPR. The results reveal the presence of mainly CuO-related species highly dispersed over the nanosized ceria support and essentially forming two types of species as a function of their degree of aggregation while isolated Cu2+ cations constitute only a residual part of the copper component. Analysis of the redox processes evidences that reduction starts from interface positions followed by the copper oxide component and extends later to ceria support positions not in contact with the copper oxide. Involvement of each catalyst component in the oxidation processes strongly depends on the starting degree of reduction attained by the system. A simultaneous or sequential, with support oxidation occurring first, oxidation of both oxide components is shown to occur as a function of the initial reduction degree of the system. The sequential mechanism prevails when a relative large reduction degree, leading to stabilization of reduced states of copper, is achieved. A model of the redox processes taking place upon interaction with CO/O2 is presented on the basis of the results obtained.

Introduction Catalysts based on ceria present a wide range of applications such as three-way catalysts for automobile exhaust gas emission control, removal of SOx-NOx from fluid catalytic cracking flue gases, electrocatalysts over fuel cell electrodes, and catalysts for various oxidation and hydrogenation reactions.1-4 For most of these applications, ceria-related compounds are thought to operate mainly as redox state or structural promoters of the active metal (or metal oxides) with which they are in contact as well as a bifunctional promoter.1,2 A noteworthy aspect of ceria-related oxides is that they can promote the activity of catalysts with different functionalities such as precious metals (for instance, Rh, Pt, and Pd, typically present in TWC) and base metals or metal oxides.1,2,5-13 Among the latter, outstanding activities, comparable to those exhibited by precious metal catalysts, are shown by copper (or copper oxide) catalysts in reactions of high technological interest like CO oxidation, preferential CO oxidation in H2-rich streams, methanol steam reforming, methanol synthesis from CO and H2, or water-gas shift.5-14 In CO oxidation, different hypotheses have been made to account for the synergetic effects observed upon establishment of interactions between highly dispersed copper oxide species and ceria-related oxides.5-7,15-18 It is well recognized the existence of a correlation between the reducibility of the dispersed copper oxide species and the catalytic activity.6,15-20 Furthermore, recent results have shown that the copper oxide active sites exhibit a facile redox interplay with both reducing and oxidizing reactants,7,18,20 suggesting that the reaction follows a redox mechanism,7,17,18 in contrast with earlier reports suggesting a stabilizing effect of ceria on certain redox states of copper as mainly responsible for the catalytic enhancement.5 Besides this, it appears that such redox activity involves * Author to whom correspondence should be addressed. E-mail: [email protected]. † Instituto de Cata ´ lisis y Petroleoquı´mica. ‡ Universidad de Sevilla.

concomitant redox changes in the catalyst support,7,18,21 strongly suggesting that the interfacial region plays the most relevant role in the catalytic activity,17,18 in fair agreement with previous postulations.5,11 On these bases, differences between the activities of copper oxide catalysts supported on different ceria-based oxides have been mainly related to the different redox activities of the O2-/VO¨ (where VO¨ denotes a doubly ionized oxygen vacancy, using the Kro¨ger-Vink notation) interface sites in each case.7 A somewhat different mechanistic proposal has been recently put forward by Wang et al.16 In their approach, the reaction is proposed to take place in two general sets of processes involving, respectively, an induction and a light-off period. Although a role for interface vacancies at the support is recognized, this role appears to be mainly limited to facilitating oxygen activation during the induction steps prior to light-off while sites active during the light-off period are proposed to be related to oxygen vacancies on top of the copper oxide component.16 However, a recent work on a CuOx/CeZrO4 system suggests that the CO oxidation reaction takes place exclusively at interface sites with a certain reductive preactivation of the copper oxide component apparently occurring just prior to reaction onset.18 The existence of induction periods within processes related to copper oxide reduction has been also observed during reduction under CO of bulk CuO and Cu2O;22 the same study proposes that the magnitude of the induction time decreases with the introduction of defects and imperfections in the copper oxides.22 On the other hand, recent proposals based on kinetic modeling of redox changes under CO and O2 suggest, as proposed earlier,7 redox changes between Cu2+ and Cu+ (and concomitantly Ce4+ and Ce3+) as the main steps involved in the CO oxidation reaction mechanism.17 Within this context, the present work attempts to provide further insights into redox processes taking place in a CuO/ CeO2 catalyst upon interaction with CO/O2. For this purpose, interactions of the catalyst with CO and O2 are analyzed with EPR and XPS spectroscopies. Such techniques, along with

10.1021/jp0465837 CCC: $27.50 © 2004 American Chemical Society Published on Web 10/23/2004

17984 J. Phys. Chem. B, Vol. 108, No. 46, 2004 XAFS, XRD, and Raman, are in turn employed to characterize the initial catalyst. Experimental Section Synthesis. The CeO2 support was prepared by precipitation within a reverse microemulsion. For this, two microemulsions of similar characteristics containing aqueous phases prepared by dissolving cerium(III) nitrate hexahydrate for the first and tetramethylammonium hydroxyde pentahydrate for the second were mixed; n-heptane was used as organic solvent, Triton X-100 (Aldrich) as surfactant, and hexanol as cosurfactant in the microemulsions. Following centrifugation, decanting, and rinsing of the resulting solid with methanol, it was dried at 383 K for 24 h and finally calcined under air at 773 K for 2 h. Details of the preparation parameters employed during the synthesis of this support are similar to those employed previously for related materials.23 The copper oxide catalyst supported on CeO2 (hereafter referred to as CuC) was prepared by incipient wetness impregnation of the CeO2 support using an aqueous solution of Cu(NO3)2‚3H2O (to give a final copper load of 1 wt. %, representing ca. 157 µmol of Cu per gram of catalyst). The resulting material was dried overnight at 383 K and subsequently calcined under air at 773 K for 2 h. Techniques. X-ray photoelectron spectra (XPS) were recorded with a Leybold-Heraeus spectrometer equipped with an EA-200 hemispherical electron multichannel analyzer (from Specs) and a 120 W, 30 mA Mg KR X-ray source. A computer was used for controlling the instrument and recording the spectra. Ar+-etching treatments were carried out with a current of 6 mA and an aceleration voltage of 3.5 kV (ion current, 8 µA). This gave a sputtering yield rate of ca. 12 Å/min, according to a calibration made using a standard Ta2O5 thin film electrochemically grown on a Ta foil; the ion damage (penetration) depth in these conditions was estimated as ca. 5 nm by TRIM-90 using Monte Carlo standard calculations.24 The CuC sample (0.2 mg) was slightly pressed into a small (4 × 4 mm2) pellet and then mounted on a sample rod and introduced into the pretreatment chamber where it was outgassed at 473 K for 2-3 h until a pressure of less than 2 × 10-8 Torr was achieved, the sample then being reoxidized under 1 Torr of O2 at the same temperature. Subsequently, it was reduced under 1 Torr CO at 300 K and then further outgassed at 473 K. From this point, the sample was either treated under 1 Torr CO at 300 K or reoxidized with 1 Torr O2 doses up to 473 K. Following each treatment, the sample was moved into the ion-pumped analysis chamber where it was further outgassed until a pressure less than 2 × 10-9 Torr was attained (2-3 h.). This low pressure was maintained during all the data acquisition by ion pumping of the chamber. After each treatment, XP spectra in the relevant energy windows were collected for 20-90 min, depending on the peak intensities, at a pass energy of 44 eV (1 eV ) 1.602 × 10-19 J) which is typical of high-resolution conditions. The intensities were estimated by calculating the integral of each peak after subtraction of an S-shaped Shirley-type background with the help of UNIFIT for Windows (version 3.2) software;25 atomic ratios were then derived using the appropriate experimental sensitivity factors. All binding energies (BE) were referenced to the adventitious C1s line at 284.6 eV.26 This reference gave BE values with an accuracy of (0.1 eV; the peak u′′′ characteristic of Ce4+ was thus obtained at 917.0 ( 0.1 eV. In Ce(3d) spectra, factor analysis (also known as principal component analysis) was used to calculate the Ce3+/ Ce4+ ratios in each set of spectra recorded, using the methodology developed in a previous work.27

Martıńez-Arias et al. Electron paramagnetic resonance (EPR) spectra were recorded at 77 K with a Bruker ER 200 D spectrometer operating in the X-band and calibrated with a DPPH standard (g ) 2.0036). Portions of about 30 mg of sample were placed inside a quartz probe cell with greaseless stopcocks using a conventional high vacuum line (capable of maintaining a dynamic vacuum of ca. 6 × 10-3 N m-2) for the different treatments. In all cases, the sample was pretreated in 300 Torr (1 Torr ) 133 N m-2) of pure oxygen at 773 K for 2 h. CO reduction treatments at a specific reduction temperature were made under static conditions using 100 Torr of CO, heating for 1 h at the corresponding temperature, and subsequently outgassing at the same temperature for 0.5 h. Quantitative estimation of the amount of species present in the spectra was performed by double integration of the corresponding EPR spectra and comparison with a copper sulfate standard. Computer simulation has been used to determine spectral parameters or to determine specific contributions of signals to a determinate spectrum. XAFS measurements were performed on station 9.3 at the Daresbury Laboratory (Warrington, U.K.), equipped with a Pdcoated plane mirror and a Si(220) double crystal monochromator set at 50% harmonic rejection detune. Data were collected at the Cu K-edge in fluorescence mode using a 13-element Ge solid-state detector with semiautomatic windowing and dead time correction. The sample was precalcined under diluted oxygen flow at 773 K and then, after cooling to room temperature under the same flow, taken out from the treatment cell and positioned at ca. 45° to the incident beam to maximize the solid angle seen by the detector situated perpendicular to the beam. A total of 12 scans were recorded with a total accumulation time of 2.5 h. Correction due to potential selfabsorption effects was made on the basis of the work of Schroeder et al.28 The energy scale was calibrated with the measurement of a copper foil and giving a value of 8979.0 eV to the first inflection point. Powder XRD patterns of the samples were recorded on a Seifert XRD 3000P diffractometer using nickel-filtered Cu KR radiation operating at 40 kV and 40 mA, using a 0.02° step size and 10 s counting time per point. Analysis of the diffraction peaks was done with the computer program ANALYZE Rayflex Version 2.293. Raman spectra (at 4 cm-1 resolution) were obtained with a Bruker FT-Raman instrument using the 1064-nm exciting line (200 mW beam) and taking 100 scans for every spectrum. Results XRD. Figure 1 displays the X-ray diffractograms of the CeO2 support and the CuC sample. Diffraction peaks attributable to the fluorite phase of CeO2 are observed in both cases (lattice parameters of 5.412 ( 0.001 and 5.413 ( 0.001, in close agreement with the value expected for such phase,29 are estimated for CeO2 and CuC, respectively). On the other hand, linear fitting of the data obtained in the Williamson-Hall plot (Figure 2), for which the Cu KR1 component of the peaks shown in Figure 1 (except for the incomplete (331) one appearing at ca. 77° 2θ value) has been employed, following an approach similar to that employed by Zhou and Huebner,30 yields particle size values of 7.7 ( 0.2 Å and 8.3 ( 0.3 Å and root-meansquared microstrains of (1.9 ( 0.3) × 10-3 and (2.6 ( 0.3) × 10-3 for CeO2 and CuC, respectively. Raman. Raman spectra of the CeO2 support and the CuC sample are shown in Figure 3. A strong peak at ca. 462 cm-1 corresponding to the triply degenerate F2g mode of fluorite CeO2 (the only one allowed in first order)31,32 is observed for both

Interfacial Redox Processes under CO/O2

Figure 1. X-ray diffractograms of the ceria-supported copper oxide (CuC) and the ceria support (CeO2) samples.

Figure 2. Williamson-Hall plot of data extracted from the X-ray diffractograms of the indicated samples. Dashed and dotted lines correspond to the fittings of CeO2 and CuC samples, respectively.

Figure 3. Raman spectra of the indicated samples.

samples. The main differences between them concern the position of the maximum (463.2 and 461.4 cm-1 for CeO2 and CuC, respectively) and the peak width (16.4 and 19.8 cm-1

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Figure 4. Cu K-edge XANES spectrum of the CuC sample. The inlet compares the second derivative of CuC with those of CuO (gray line) and Cu2+-ZSM5 (dotted line) references.

fwhm values for CeO2 and CuC, respectively, with the latter showing a somewhat higher asymmetry, related to a higher red half-width). XAFS. The Cu K-edge XANES spectrum of the initial calcined CuC sample is displayed in Figure 4. A comparison with a bulk CuO reference and a Cu-ZSM-5 sample, which displays the exclusive presence of isolated Cu2+ species,33 provides evidence that Cu is mostly inserted in a CuO-type matrix, obviously, in a relatively high dispersion state. This is shown in the inset of Figure 4; the comparison of the second derivatives displays modulations at the edge region (8975-9000 eV) peaking at rather close energies in the sample and the CuO reference while the isolated Cu2+ species showed all such peaks with maxima shifted to higher energy. This difference in the energy location of second derivative peaks between aggregated and isolated species is a constant which has been previously reported by others.34 XPS. A Cu/Ce atomic ratio of 0.084 has been calculated from the XP spectrum of the initial calcined CuC sample, which indicates a relatively high dispersion state of copper, compatible with the absence of peaks because of Cu-related phases in the X-ray diffractogram. Ar+-sputtering experiments performed over CuC yield the profile of Cu/Ce atomic ratio evolution shown in Figure 5. Following a small sharp decline at short sputtering time, a relatively broad maximum is observed at intermediate sputtering time followed by a slow decrease reaching a Cu/Ce value of 0.064 after 8 min of sputtering. XPS analysis of the chemical state of copper has been done by means of Wagnertype diagrams (considering the energies Eb and Ek of the Cu 2p3/2 and Cu(L3M45M45, 1G) peaks, respectively, and the modified Auger parameter R′ ) EB + EK, as shown in Figure 6). According to Wagner,35 the shifts in modified Auger parameter, which can be accurately measured in the presence of static sample charging and are shown as lines of slope -1 in the diagrams, are related to differences in the extra-atomic relaxation energy (∆R′ ) 2∆Rea) for the core photohole according to a nonlocal screening mechanism (final state effects). On the other hand, within Moretti’s electrostatic model,36 which assumes a nonlocal core-hole screening relaxation mechanism and neglects exchange contributions to the XP observables, the lines of slope -3 in these diagrams correspond to species with the same chemical state (i.e., similar initial state effects). This model has been successfully applied to analyze the properties of Cu, Cu2O, and CuO films grown on SiO2 and

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Martıńez-Arias et al.

Figure 5. Profile of the Cu/Ce atomic ratio as a function of the Ar+sputtering time for CuC. Figure 7. EPR spectra of CuC. (a) Initial calcined sample; (b) reduced under CO at 300 K; (c) reduced under CO at 373 K. Asterisk marks correspond to residual Mn2+ impurities.

Figure 6. Wagner diagram showing the evolution of Cu (2p and AES) XP parameters during redox treatments performed over CuC. Values for reference samples can be found elsewhere.36,38

ZrO2 supports.37 From the Wagner’s plot in Figure 6, we may conclude, in agreement with XAFS results (Figure 4), that Cu in the calcined CuC sample (treated under vacuum at 473 K followed by treatment under O2 at 473 K) is chemically similar to Cu2+ in bulk CuO. However, the shift toward a lower value of R′ within the dashed line of slope -3 crossing through the reference point for bulk CuO indicates a much smaller extraatomic relaxation of the photohole, according to the ∆R′ ) 2∆Rea relationship. This suggests a strong effect of CuO size and of the CeO2 interface to the relaxation process, in accordance with a recent report by Espino´s et al.37 However, highly dispersed species in Cu2+-overexchanged ZSM-5 zeolite, studied by Gru¨nert et al.,38 experience a much lower core-hole extra-atomic relaxation, according to its position in Figure 6. This can be related to the smaller polarizability of the oxide ions within the zeolitic framework compared with that of oxide ions in bulk CuO.36,39

As can be observed in Figure 6, treatment of the oxidized catalyst with a dose of CO at 300 K produces a displacement in the Wagner chemical state diagram which, upon subsequent outgassing at 473 K, progresses to a position close to the line of slope -3 corresponding to Cu+ states. This evolution indicates that ca. 70% of the copper is reduced to Cu+ by CO even at 300 K. The reduction level apparently achieved in the ceria component during this latter process amounts only to about 10%. Interestingly, treatment under CO at 300 K (and subsequently under O2 up to 373 K) of the sample which has been reduced under CO at 300 K and outgassed at 473 K reveals changes in the spectra (shifts along the -3 slope line corresponding to Cu+ chemical state) without change in Cu/Ce ratio, thus indicating that the chemical state of copper (Cu+) and the copper oxide structure remains unchanged. According to the Moretti’s electrostatic model described above, such changes must be related to variations in the degree of interaction of the corresponding Cu+ entities with the ceria support. Consistent with this, the changes induced upon reduction or oxidation within the Cu+ line are accompanied by concomitant reduction or oxidation (this latter being apparently complete already upon interaction with O2 at 300 K) of the ceria support. Furthermore, recovery of the initial position in the Wagner diagram upon increasing the oxidation temperature to 473 K strongly suggests that particle size changes are not taking place during the redox cycle, in agreement with the constancy of the Cu/Ce ratio. EPR. Figure 7 shows the EPR spectra of the initial calcined CuC sample along with those of the sample reduced under CO at 300 and 373 K. The spectrum of the initial calcined sample is formed by the overlapping of different Cu2+ signals (according to the attributions below): a major featureless broad signal showing extremes at g ≈ 2.20 and g ≈ 2.04, signal B (following the nomenclature adopted in previous reports),20 and two other superimposed minor axial signals in which four-line hyperfine splittings can be resolved in each of its components (signals type C). These latter show g| ) 2.233 (A| ) 16.0 × 10-3 cm-1) and g⊥ ) 2.036 (A⊥ ) 1.8 × 10-3 cm-1), signal C1, and g| ) 2.274 (A| ) 17.4 × 10-3 cm-1) and g⊥ ) 2.041 (A⊥ ) 2.3 × 10-3 cm-1), signal C2. Only approximately 35% of the whole copper content is detected in the EPR spectrum of the initial calcined CuC sample (Figure 7a). A significant decrease of the

Interfacial Redox Processes under CO/O2 overall Cu2+ intensity is produced upon reducing the sample under CO up to 473 K, ca. 16, 2, and 0% of the whole copper content being observed after reduction at 300, 373, and 473 K, respectively. A differential behavior of the signals toward the reduction under CO is inferred from comparison between Figure 7a and b. Thus, the treatment under CO at 300 K induces a strong decrease of signal C1 and affects significantly to signal B (although a small rest of it still remains after this treatment, according to the shape of the spectrum in Figure 7b) while signal C2 appears almost unaffected. Type C signals are typical of isolated Cu2+ ions in, to a first approximation, tetragonally expanded octahedral environments with the unpaired electron essentially residing in the dx2-y2 orbital of the Cu ion.40-42 Analysis with expressions given by Kivelson and Neiman for d9 ions in D4h symmetry,40 further simplified by assuming that the only significant covalency effects are related to mixing between the dx2-y2 orbital of the Cu ion and the corresponding symmetry-adapted combination of ligand (assumed to be oxide ions) orbitals,43 suggests a higher ligand bond covalency and ligand field in signal C1 while the magnitude of the tetragonal distortion appears fairly similar. Signal B, showing average 〈g〉 value close to those of type C signals, must also be due to Cu2+ ions; its larger line width (leading to unresolved hyperfine splitting) can be attributed to dipolar broadening effects caused by mutual interactions with neighboring paramagnetic Cu2+ ions, indicating that the corresponding ions are located in a Cu2+-containing aggregate phase of oxidic type.7,20 Taking into account that antiferromagnetic couplings between Cu2+ ions in well-crystallized CuO phases produce EPR-silent species,39 Cu2+ ions yielding signal B can be considered as belonging to small copper oxide clusters where this antiferromagnetic character is not fully developed. Relatively larger CuO-type particles (considering the above-presented studies of copper valence state by XAFS and XPS) would also be present in both samples, accounting for the portion of copper which remains undetected by EPR in the initial calcined sample. Reoxidation of the sample previously reduced under CO up to 473 K has been analyzed in detail by introducing doses of O2 at 77 K followed by warming to 300 K (typically during 15 min) and further outgassing at 300 K. An increase in the intensity of Cu2+ species (affecting mainly to major signal B) is observed after this latter treatment, the evolution observed being strongly dependent on the initial degree of reduction of the sample, as shown in Figure 8. In any of the cases, new signals related to chemisorbed oxygen species (mainly O2-Ce4+ species, as exposed below) are observed after the first step of introduction of O2 at 77 K; they strongly decrease or disappear upon the subsequent warming to 300 K. The rate of such decrease has been observed to depend strongly on the initial redox state of the sample: as the sample becomes more oxidized (with increasing the amount of O2 dosed), a diminishing rate is observed until a point is attained at which the O2--Ce4+ species become stable toward such warming treatment. Analysis of the chemisorbed oxygen species formed upon O2 chemisorption at 77 K allows to extract conclusions on the state of the ceria component after each reduction treatment as well as to follow the reoxidation of such component.45,46 To analyze copper effects, spectra of the copper-free ceria support are compared with those of the CuC sample. Figure 9 compares such spectra for the samples reduced under CO at 300 K. In the case of the ceria support, a weak signal at g| ) 2.036 and g⊥ ) 2.011 (signal O1, see Table 1 for a summary of the signals observed) is formed. In contrast, CuC shows a spectrum composed (on the basis of computer simulation analysis

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Figure 8. Evolution of the overall Cu2+ intensity in the EPR spectra as a function of the dose of O2 contacted at RT for CuC prereduced under CO at the indicated temperatures.

Figure 9. EPR spectra following oxygen adsorption at 77 K on the samples indicated prereduced under CO at 300 K. Computer simulations are overlapped as gray lines. Cu2+-related signals have been canceled in CuC through computer subtraction.

according to which the spectrum could not be satisfactorily simulated with only one signal) by the overlapping of two signals at gz ) 2.030, gx ) 2.018, and gy ) 2.0130 (signal OC) and gz ) 2.030, gx ) 2.014, and gy ) 2.011 (signal O2) and which displays an appreciably higher overall intensity than observed for the ceria support. Differences between both samples are revealed in the behavior of the signals toward subsequent warming to 300 K. Thus, signal O1 observed for the ceria support is not strongly modified by such treatment; in contrast, the superoxide signals observed in CuC disappear after this treatment. Subsequent introduction of a second O2 dose at 77 K on the latter restores only partially (ca. 8-fold overall intensity decrease) a spectrum similar to that of Figure 9b. The results of the oxygen adsorption experiments performed over the samples reduced under CO at 473 K are shown in Figure 10. Introduction of a first O2 dose at 77 K over the reduced copper-free CeO2 sample leads to a spectrum (Figure 10a) constituted by the overlapping of a signal at gz ) 2.034, gx ) 2.014, and gy ) 2.011 (signal O3) and a broad axial signal at g⊥ ) 2.048 and g| ) 1.978 (signal OM). Brief warming to 300 K leads to the disappearance of both signals while

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TABLE 1: Characteristics of the EPR Signals Obtained upon Oxygen Adsorption on the CuC or Ceria Samples Reduced under COa signal

EPR parametersb

O1 O2 O3

g| ) 2.036, g⊥ ) 2.011 gz ) 2.030, gx ) 2.016-2.014, gy ) 2.011 gz ) 2.034, gx ) 2.014, gy ) 2.011

Ce4+-O2- species at support positions

OC

gz ) 2.030, gx ) 2.018 gy ) 2.013

Ce4+-O2- species at copper oxide-ceria interfacial positions

OM

g⊥ ) 2.048, g| ) 1.978

Ce3+-O- species (tentative, see text)

proposed assignment

a Axes attribution follows criteria of previous works.45,46 For assignment details, see text. b A difference between signals O1-O3 and signal OC concerns their widths which on the average amounts to 〈D〉 ∼ 8 and 15 G, respectively.

Figure 10. EPR spectra following consecutive adsorption of 50 µmol g-1 doses of O2 at 77 K (with intermediate warming to 300 K treatments). (a-c) CeO2; (d-f) CuC. Computer simulations are shown as gray lines. For the CuC sample, Cu2+ contributions were canceled by computer subtraction.

subsequent introduction of a second O2 dose at 77 K leads to the spectrum shown in Figure 10b in which signal OM appears also present (only on the basis of computer simulation), although with a lower contribution than observed after introduction of the first O2 dose. The spectrum is in this case mainly formed by signal O3 with a smaller contribution of signal O2 (at gz ) 2.030, gx ) 2.015, and gy ) 2.011). This latter signal is the only one present after subsequent introduction of a third O2 dose at 77 K; in this case, in contrast to results obtained for the previous doses, only a slight decrease of the superoxide signal is produced upon warming the sample to 300 K. Results of similar experiments performed on CuC (Figure 10d-f) show spectra mainly formed by the overlapping of signals O2 and OC, although small contributions of signal OM are also required for a satisfactory simulation (in particular, at the low and high field tails of the spectra) of spectra obtained after introduction of the first and second O2 doses (Figure 10d and e). The spectrum obtained after the third O2 dose (Figure 10f) resembles much the one observed upon O2 adsorption on the sample reduced at 300 K (Figure 9b) in which the main contribution is due to signal OC; this contrasts with the spectra obtained after chemisorption of the two first O2 doses (Figure 10d and e) in which signal O2 predominates. As it occurred for the ceria

support, warming to 300 K after introduction of the first and second O2 dose at 77 K produces in CuC the disappearance of the oxygen-derived signals; even though a strong decrease is produced upon this treatment after the third dose, a small superoxide signal could however be observed in that case. Signals O1, O2, O3, and OC display parameters (Table 1) similar to those observed for superoxide species chemisorbed on cerium cations (formally O2--Ce4+ species), on the basis of previous experiments of chemisorption of 17O-enriched oxygen mixtures.45-47 Differences between them reflect variations in the chemical environment of the corresponding superoxide species, as will be discussed below. On the other hand, signal OM has not been, to the best of our knowledge, reported to occur in experiments of oxygen chemisorption on ceria-related samples, in which exclusively superoxide species have been observed.46 Its EPR parameters, with g⊥ > g| (Table 1), are inverted with respect to those expected for chemisorbed superoxide species,47 thus indicating a different nature for the radical species. Such parameters are, however, compatible with its attribution to ionically bonded O- species,48,49 although employment of 17O-enriched oxygen would be required for a more definitive assignment. The magnitude of the g⊥ value, the absence of covalency effects, and the relatively large line width of signal OM would suggest a Ce3+ adsorption center for the O- species;46,49 such a configuration would, however, be strongly unstable with respect to a O2--Ce4+; however, it cannot be fully discarded that it occurs as a metastable state at the relatively low adsorption temperature employed. Further insight into this is out of the scope of this work and will not be thus attempted. Discussion Characterization of Initial Calcined CuC. The bulk characteristics of the CeO2 sample employed as support in this work have been analyzed in detail in a former work.50 HREM results showed the presence of fairly rounded nanoparticles with ca. 8-nm average particle size (in good agreement with XRD results) within polycrystalline aggregates of variable size in the 0.1-1 µm range. The oxide nanoparticles showed a relatively low dispersion in the size distribution (ca. 2-nm standard deviation) and displayed only a residual level of nonstoichiometry (the cell constant of 5.412 Å agrees with that expected for stoichiometric CeO2 although a very weak Raman band at ca. 595 cm-1 reveals the presence of a residual amount of oxygen vacancies) and a small microstrain (in accordance with analysis of Figure 2 and contributing to a small red shift and broadening of the first-order Raman band at ca. 463 cm-1 with respect to the values observed for thermally sintered microsized CeO2).50 Small changes are observed in the X-ray diffractogram upon incorporation of copper over CeO2 (Figures 1 and 2). Although the synthesis method chosen for copper incorporation (impregnation) would not normally favor copper introduction into the ceria lattice (in contrast to other methods in which the copper

Interfacial Redox Processes under CO/O2 and cerium precursors are reacted together during the first preparation steps, like IGC, coprecipitation, sol-gel, or combustion-based ones),17,21,51,52 a previous report by Xiaoyuan et al. proposes that some Cu2+ could enter the CeO2 lattice in impregnated (and further calcined at 773 K) systems,53 which has prompted us to analyze this possibility in the CuC sample. In this respect, the practical absence of differences in the respective lattice parameters of the fluorite cell suggests that no large amount of copper has been incorporated to the lattice. In such a case, a certain lattice contraction appears to be produced as a result of compensating effects between isomorphic replacement of Ce ions by the smaller Cu ones (leading to lattice contraction) and the formation of oxygen vacancies for maintenance of charge neutrality (leading to lattice expansion).21,51-53 Nevertheless, the differences observed in the Raman spectra (Figure 3) indicate that a small amount of copper may have been introduced into the ceria lattice. Thus, the small red shift and broadening in the F2g mode band indicate a certain higher phonon confinement in CuC, which according to comparison with recent experimental results on a series of CeO2 samples would roughly be similar to that shown by a quasi-stoichiometric ceria of ca. 6.5-nm particle size.50 This could be attributed to anion vacancies formation as a consequence of the introduction of a small amount of copper at lattice positions, probably relatively close to the sample surface. It must be noted in the joint analysis of these samples by XRD and Raman the major sensitivity of these techniques to details of the cationic and anionic sublattice, respectively, in materials of this nature. On these bases, differences observed between the samples in the microstrain values can be attributed to such copper incorporation although the influence of copper presence at the sample surface on interparticle interactions can also play a role on this. On the other hand, the slightly higher particle size value of CuC (as determined from analysis of the Williamson-Hall plot and also in accordance with the slightly lower fwhm observed in general for the diffraction peaks of this sample, Figure 2) can be related to a small sintering produced upon contact of the support with the impregnating aqueous solution and subsequent calcination treatment. Regarding characterization of the copper oxide component, let us first analyze aspects related to the valence state of copper. Earlier works have suggested the possibility that interactions with ceria could induce a certain stabilization of copper in a reduced Cu+ state in the calcined catalyst;5 additionally, the nanostructured nature of the copper oxide component could also favor, as observed for other oxide compounds,54 a certain stabilization of oxygen vacancies in this component (which would, in principle, be accompanied by the presence of reduced states of copper because of charge balance reasons; these effects appear, however, limited for CuO nanoparticles).55 Nevertheless, both XAFS and XPS (Figures 4 and 6) indicate that the valence state of the copper cations in the initial calcined CuC sample must be on its majority similar to that present in CuO, with the latter technique revealing the existence of relatively strong interactions of the well-dispersed copper oxide component with the underlying ceria support. Further details on the dispersed copper oxide component are given by the more sensitive EPR technique (Figure 7a), which indicates the presence of three distinct copper oxide entities in the form of isolated Cu2+ species (as a minor component giving rise to signals type C), as clustered CuO-type species (giving rise to signal B), and probably as relatively larger (even though in an overall well-dispersed state, considering the relatively high Cu/Ce ratio observed by XPS) CuO microcrystals (escaping EPR detection). This latter inter-

J. Phys. Chem. B, Vol. 108, No. 46, 2004 17989 pretation is in fair agreement with previous work on systems of this kind suggesting (mainly on the basis of TPR evidence) the presence of essentially two types of copper oxide entities differing in their particle size and degree of interaction with the ceria support.5,53 However, the absence of EPR detection of isolated Cu2+ cations (or the presence of signals broadened beyond detection) may occur for certain symmetry configurations.41,43 However, analysis of the Ar+-sputtering profile (Figure 5) is in agreement with an essentially multimodal distribution of CuO particles, as clustered and relatively large particles (with the latter predominating) rather than with the presence of a major component of isolated Cu2+ cations. This is based on comparison of the Ar+-sputtering profile with previously established models,56,57 by which the observed profile appears to arise from superimposition of a curve from dispersed species (responsible for the small initial sharp decline) with that from large particles (for which an initial increase in Cu/Ce, responsible for the observed maximum, would be expected). Nevertheless, the high initial Cu/Ce ratio, along with the low values observed for the Auger parameter, suggests the presence of rather flat copper oxide particles strongly interacting with the CeO2 support. Redox Properties. Both XPS and EPR results (Figures 6 and 7) indicate that CO easily reduces copper in the CuC catalyst with ca. 70% of the copper being reduced (from Cu2+ to Cu+) upon CO interaction at 300 K, according to the XPS estimation. The fact that CuO is reduced at room temperature by CO is not usual. It does not occur for bulk CuO or for many supported CuO catalysts,22 although a small degree of reduction has been reported to occur in some cases for the latter type of systems.58 Certainly, as proposed by Wang et al.,22 the amount of defects or imperfections present in the initial CuO particles (which can strongly depend on the preparation parameters employed in each case) can have a crucial role in explaining these discrepancies. The copper reduction in CuC is accompanied by a slight support reduction according to XPS results. Furthermore, the comparison of the EPR intensities of O2--Ce4+ species formed upon oxygen adsorption over the support and CuC samples reduced at 300 K (Figure 9) indicates a copper oxide promotion of ceria reduction, taking into account that such intensity must be proportional to the amount of reduced cerium centers present at the sample surface prior to oxygen adsorption.46 Support centers which are involved in this Cu-catalyzed reduction process would be expected to be located at or close to the oxide-oxide borderline. Some differential features could be then expected in the O2--Ce4+ species formed on such centers. Indeed, signal OC is formed exclusively for the CuC sample under such conditions and presents differences with all the other related signals formed on the copper-free ceria sample, affecting both its parameters (particularly, higher gx and gy) and width (appreciably larger). The former suggests, according to previous analyses,46 a higher covalency degree in the O2--Ce4+ bond while the latter can be related to effects of chemical heterogeneity in the radical environment or alternatively to a faster relaxation of the spin system for these centers. Therefore, as it has been proposed previously for other ceria-supported systems,59 signal OC can be attributed to O2--Ce4+ species formed at copper oxide-ceria interface positions. It is tempting to attribute the location of these centers to the bridging positions between both oxides. This would indicate a more facile reduction of the corresponding bridging oxygen that would give support to the existence of a simultaneous copper oxide and ceria reduction process. However, the obtained results do not allow to discard the possibility that reduction of the copper oxide clusters occurs first (a recent report by Carnes and Klabunde

17990 J. Phys. Chem. B, Vol. 108, No. 46, 2004 suggests a higher facility for reduction of nanostructured CuO with respect to the extended system)60 and then acts as promoter of the support reduction. It must be noted also the difficulties to demonstrate this from an experimental point of view given the absence of adequate experimental references for the copper oxide clusters (not only in terms of achieving a comparable particle size but also considering the possible support structural or electronic effects eventually induced on the dispersed copper oxide entities). In this respect, the observed shift within the chemical state line of slope -3 corresponding to Cu+ species upon mild reduction at 300 K (Figure 6) strongly suggests an important promotion of ceria reduction once such reduced state of copper is attained for the whole component. This is in favor of a reduction mechanism by which the copper oxide component is reduced first and then acts as promoter of ceria reduction. According to XPS results, evolution of the reduction process in the CuC system apparently proceeds toward stabilization of the copper oxide component in a Cu+ valence state with redox changes taking place essentially in the ceria component. It is interesting in this respect that once such reduced state of copper is reached, reoxidation of the copper component requires T > 373 K (Figure 6) while the support is readily reoxidized upon O2 contact at 300 K. Such relatively high reoxidation temperature which is required for oxidation of the copper component is, however, higher than that observed in a previous work to be required to reoxidize the copper oxide component when the initial CuC sample was reduced under milder conditions (CO at 300 K);7 in that case, mainly on the basis of EPR results, an important copper oxidation took place upon interaction with O2 at 300 K.7 It certainly happens that prevalent operation of one (copper oxide and ceria oxidation taking place practically simultaneously) or the other (oxidation affecting almost exclusively to the ceria component) oxidation process depends on the starting degree of reduction attained by the system. In agreement with this, the EPR results of Figure 8 evidence the increasing difficulty for copper oxidation with increasing the reduction degree of the system. In other words, when the system attains a relatively large reduction degree, oxidation of the system occurs sequentially with the ceria component becoming oxidized most preferentially. In turn, as noted above, the reversibility of the redox cycle, according to XPS results, suggests no significant effects of copper sintering or restructuring (likely producing modifications in the interaction with the support) during the system reduction. Evolution of the copper oxide and ceria components during the oxidation, according to the XPS results, suggests a simple model by which the oxidation process affects first to support positions far from the interface or not affected by copper oxide presence extending later toward oxidation of interface sites and finally the copper oxide component. Analysis of the evolution of the superoxide species during reoxidation of the CuC sample reduced at 473 K (Figure 10), which shows the presence of increasing contributions of signal OC as the degree of oxidation of the sample is increased, gives support to this hypothesis. For this analysis, the oxidation process progresses with both increasing the oxygen dose or the oxygen adsorption temperature (from 77 to 300 K). The latter usually leads to disappearance of the superoxide species which is related to formation of more reduced oxygen-derived diamagnetic species such as peroxide or (finally) oxide anions, as nicely shown recently for a ceria sample (reduced under CO) on the basis of Raman experiments.61 On the whole, to summarize these analyses, a model of the redox processes taking place upon interaction of the system with


Figure 11. Model of redox processes in CuC. Reduced regions are represented by dashed zones (see text for details).

CO/O2 can be proposed on the basis of the results obtained, as depicted in Figure 11. First stages in the reduction process involve copper oxide reduction (which may preferentially occur at interface positions to account for reported results showing ceria promotion of copper oxide reduction,16,52,53 however, pure copper oxide references employed for this purpose may not reflect the exact properties of CuO entities present in the catalyst) and support reduction at interface positions, the latter being promoted by copper oxide. Further progress of the reduction process appears to involve preferentially the reduction of the copper oxide component extending later to support positions far from the interface. Reoxidation of the system from this latter stage involves preferentially the ceria component starting from positions far from the interface and progressing toward the interface positions. A certain stabilization of the copper oxide component in a reduced Cu+ state during the redox cycles is inferred from the higher oxidation temperature required to restore the initial fully oxidized state (most easily attainable when the reduction degree is relatively lower). Conclusions A study of characterization of a CuO/CeO2 catalyst and redox processes taking place on it upon interactions with CO/O2 is presented. The characterization results (by XRD, Raman, XAFS, XPS, and EPR) reveal the presence in the initial catalyst of essentially fully oxidized CuO-type entities highly dispersed over a nanostructured ceria support. Analysis of the evolution of the reduction and oxidation upon interaction with CO and O2 as a function of the catalyst component involved in each step allows establishing a model of the redox processes taking place in the sample. According to this model, reduction involves first-stage copper oxide and ceria interface positions. Reoxidation from this state is relatively easy being already produced upon interaction with O2 at 300 K. Further progress in the reduction process is proposed to involve first the copper oxide component and to extend later to ceria positions far from the interface. Reoxidation from this more highly reduced state involves first the ceria component (starting at positions far from the interface and progressing toward the oide-oxide borderline), while copper oxidation occurs later and is somewhat damped by stabilization of a reduced state of copper (Cu+) during the redox cycling. Acknowledgment. Thanks are given to Ms. A. Iglesias-Juez and to the scientific and technical staff at Daresbury Laboratory station 9.3 (Drs. I. Harvey, A.R. Lennie) for the help given during recording of the XAFS spectra. Financial help by CICyT project MAT2003-03925 is acknowledged. References and Notes (1) Trovarelli, A. Catal. ReV.sSci. Eng. 1996, 38, 439. (2) Catalysis by ceria and related materials; Trovarelli, A., Ed.; Catalytic Science Series, Vol. 2; Imperial College Press: London, 2002. (3) Murray, E. P.; Tsai, T.; Barnett, S. A. Nature 1999, 400, 649.

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O2 in a Nanoceria-Supported

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