Electronic Structure of Magnesia−Ceria Model ... - ACS Publications

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Electronic Structure of Magnesia
Ceria Model Catalysts, CO2 Adsorption, and CO2 Activation: A Synchrotron Radiation Photoelectron Spectroscopy Study Thorsten Staudt,† Yaroslava Lykhach,*,† Nataliya Tsud,‡ Tom
as Sk
ala,§ Kevin C. Prince,§ Vladimír Matolín,‡,§ and J€org Libuda†,|| †

)

Lehrstuhl f€ur Physikalische Chemie II, Department Chemie und Pharmazie, Friedrich-Alexander-Universit€at Erlangen-N€urnberg, Egerlandstr. 3, 91058 Erlangen, Germany ‡ Faculty of Mathematics and Physics, Department of Surface and Plasma Science, Charles University in Prague, V Holesovick
ach 2, 1800 Prague 8, Czech Republic § Sincrotrone Trieste SCpA, Strada Statale 14, km 163.5, 34149 Basovizza-Trieste, Italy Erlangen Catalysis Resource Center, Friedrich-Alexander-Universit€at Erlangen-N€urnberg, Egerlandstr. 3, 91058 Erlangen, Germany ABSTRACT: We have studied the electronic properties of single crystal based ceria and magnesia
ceria model catalysts, the CO2 adsorption, and the CO2induced reoxidation of these systems by synchrotron radiation photoelectron spectroscopy (SR-PES). All model systems were prepared starting from a fully stoichiometric and well-ordered CeO2(111) film grown on Cu(111). Different magnesia
ceria mixed oxide films were prepared by physical vapor deposition (PVD) of magnesium, oxygen treatment, and subsequent annealing. The preparation procedure was varied to obtain samples with different oxidation state, structure, and surface composition. Different carbon-containing species were identified, including surface carbonates formed in the vicinity of Mg2þ and Ce3þ/4þ and surface carboxylates. The presence of Mg2þ was observed to strongly enhance carbonate formation but suppress the formation of carboxylates. Changes in the oxidation state of ceria upon CO2 exposure were monitored with highest sensitivity by resonant photoelectron spectroscopy (RPES). Reoxidation of Ce3þ was observed to be suppressed on magnesia-containing samples, even in the presence of a high surface concentration of Ce3þ. The findings suggest that carboxylates are an intermediate step of reoxidation, whereas the generation of stable surface carbonates inhibits formation of this intermediate and, therefore, CO2induced reoxidation.

1. INTRODUCTION Carbon dioxide can potentially be utilized in various largescale chemical processes. One example is the dry reforming of methane using CH4/CO2 mixtures available from coal, natural gas, carbonaceous wastes, or biogas.1,2 The reaction yields synthesis gas (H2/CO) from which many chemicals are produced, including, e.g., liquid hydrocarbons, olefins, methanol, formaldehyde, or acetic acid.1 Certainly, the transformation of two greenhouse gases into valuable feedstock is appealing from an environmental point of view. In general, dry reforming catalysts suffer from rapid catalyst deactivation by carbon formation. The carbon originates from methane dehydrogenation and CO2 disproportionation with a general thermodynamic tendency to form coke.1
3 Here, the use of reducible oxide supports such as ceria (CeO2) may provide a possible solution.4,5 Ceria has the remarkable capability to store and release large amounts of oxygen, giving rise to a self-cleaning functionality of the catalyst if the liberation of oxygen and its reaction with carbon are rapid in comparison to carbon aggregation. The processes of release and uptake of oxygen are accompanied by a reversible transformation between the oxidation states Ce4þ and Ce3þ which can be traced spectroscopically. Recently, we have r 2011 American Chemical Society

investigated the removal of carbonaceous sediments on a Pt/CeO2/ Cu(111) model catalyst, formed by activation of methane and other hydrocarbons.6 The reaction leads to high surface concentration of Ce3þ ions in the surface region of the support. The universal mechanism of CO2 activation on ceria is yet unclear and currently widely discussed in the literature.7
14 The process involved dissociation of CO2 into CO and an oxygencontaining surface species.7
11,14 It is suggested that surface Ce3þ ions may become active sites for CO2 activation via formation of either carbonates or inorganic carboxylates.12,13 In our recent communication,15 we have demonstrated that dissociation of CO2 occurs on pure and strongly reduced ceria films leading to partial reoxidation of ceria even at room temperature. It was shown that the reaction rate strongly depends on the degree of the ceria reduction. High initial rates of CO2 dissociation rapidly decay as reoxidation proceeds. Finally, partially reoxidized ceria becomes inert toward CO2. However, the reoxidized ceria shows poor thermal stability Received: January 13, 2011 Revised: March 30, 2011 Published: April 11, 2011 8716

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The Journal of Physical Chemistry C resulting in a loss of recovered lattice oxygen during annealing to 700 K in an ultrahigh vacuum (UHV). It is speculated that the reactivity of ceria surfaces toward CO2 may be tuned by introducing additional functionalities at the nanoscale. Basic sites, such as alkaline earth metal oxides (MgO), facilitate the formation of surface carbonates.1,16
18 It is generally accepted that CO2 chemisorbs on the defect surface sites of MgO, such as edges and corners, whereas the terrace sites remain unreactive.18 The reaction of CO2 with low-coordinated oxygen ions at room temperature yields stable unidentate and bidentate carbonates. Recently, we have studied CO2 activation on mixed MgO
CeO2 model catalysts, with different concentrations and degrees of intermixing between Mg2þ, Ce3þ, and Ce4þ ions.19 The obtained results suggested that the activation pathway of CO2 can indeed be modified by carbonate formation. Simultaneously, reoxidation of ceria can be effectively suppressed. Unfortunately, the work suffered from the comparably low sensitivity of conventional X-ray photoelectron spectroscopy (XPS) to changes of the surface oxidation state. Here, we present the results of a study using Synchrotron Radiation Photoelectron Spectroscopy (SR-PES) and, in particular, Resonant Photoemission Spectroscopy (RPES). These methods allow us to monitor changes in the oxidation state of ceria-based catalysts with high sensitivity and, thus, to identify intermediates and possible reaction mechanisms.

2. EXPERIMENTAL SECTION All SR-PES and RPES measurements have been performed at the Materials Science Beamline of the Elettra synchrotron facility in Trieste, Italy. The source was a bending magnet that produces linearly polarized light in the energy range of 21
1000 eV. Photoelectron spectra were acquired with a high luminosity electron energy analyzer (Specs Phoibos 150), equipped with a nine-channel detector. The experimental end station was equipped with a rearview LEED optics, a quadrupole mass spectrometer, an ion gun, and a gas inlet system. The basic setup of the chamber included a dual Mg/Al X-ray source used for energy calibration of the synchrotron light and measuring of the core levels beyond the reach of the synchrotron light. The background pressure in the analysis chamber was better than 2  10
10 mbar during all measurements. The resonant photoelectron spectra were recorded at photon energies (PEs) between hν = 115 and 124.8 eV. Core level spectra of O 1s, C 1s, and Mg 2p were acquired using synchrotron radiation at hν = 650, 410, and 115 eV, respectively. All spectra were normalized with respect to the ring current and acquisition time. Additionally, Ce 3d, Cu 2p3/2, O 1s, Mg 1s, and C 1s core level spectra were measured with an X-ray source using Al KR (1486.6 eV) radiation. All spectra were taken at constant pass energy, at emission angles of the photoelectrons (γ) of 20° and 60° for Al KR and of 0° for synchrotron radiation with respect to the sample normal. The escape depths of the photoelectrons were approximated by a product of the Inelastic Mean Free Path (IMFP)20 and cos(γ): for the emission from Ce 3d by Al KR radiation, they were 1.1 nm (γ = 20°) and 0.6 nm (γ = 60°), while for the synchrotron light they were less than 0.5 nm. The total spectral resolutions achieved with Al KR and synchrotron radiation were 1 eV and 150
200 meV, respectively. The Ce 3d, O 1s, C 1s, Mg 2p, and Mg 1s core level spectra were fitted with Voigt profiles after subtraction of a Shirley background. For systematic studies of CO2 activation, we used six different samples each fabricated starting from a common CeO2(111)/

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Cu(111) template: (I) CeO2(111)/Cu(111), (II) CeO2
x, (III) 0.4 nm MgO/CeO2
x, (IV) 1.0 nm MgO/CeO2
x, (V) MgO
Ce2O3 mixed, and (VI) MgO
CeO2 mixed. A single-crystal Cu(111) disk (MaTecK) was used as a substrate for all samples discussed in this work. Cu(111) was cleaned by several cycles of Arþ ion sputtering (at 300 K for 60 min) and annealing (723 K for 5 min) until no traces of carbon or any other contaminant were found in the photoelectron spectra. Epitaxial layers of CeO2(111) were grown on clean Cu(111) by physical vapor deposition (PVD) of Ce metal (Goodfellow, 99.99%) at an oxygen partial pressure of 5  10
7 mbar (Linde, 99.995%) at 523 K, followed by annealing of the films at 523 K in an oxygen atmosphere of the same pressure for 10 min. The thickness of the prepared films determined from the attenuation of the Cu 2p3/2 signal, acquired using the Al KR X-ray source at γ = 20°, was typically 1.5
1.8 nm, corresponding to about 5 to 6 monolayers (MLs) of ceria. Here we define 1 ML as one O
Ce
O trilayer of CeO2(111), corresponding to a thickness of 0.313 nm. For further details concerning the preparation procedure, we refer to the literature.21
23 The fully stoichiometric CeO2 film (sample I) was used as a starting point for the preparation of reduced and mixed magnesia
ceria layers. Toward this end, Mg metal (Goodfellow, 99.9%) was evaporated from a resistively heated Mo crucible either in an oxygen O2 atmosphere (5  10
7 mbar) or in UHV at a sample temperature of 300 K. The amount of the deposited material was estimated by the attenuation of the intensity of the Cu 2p3/2 photoemission signal, acquired using an Al KR X-ray source at γ = 20°. The CO2 (Linde, 99.995%) exposure was performed in successive doses of 1 L (1.3  10
8 mbar, 100 s), 10 L (5.3  10
8 mbar, 250 s), 200 L (5.3  10
7 mbar, 500 s), 4000 L (6.7  10
6 mbar, 800 s), 10 000 L (6.7  10
6 mbar, 2000 s), and 20 000 L (6.7  10
6 mbar, 4000 s) at 300 K by backfilling the UHV chamber. During annealing of the exposed samples, the sample temperature was controlled by a DC power supply passing a current through the Ta wires holding the sample. The actual temperature was measured by a K-type thermocouple attached to the rear surface of the sample. 2.1. Sample Preparation. (I). CeO2(111)/Cu(111). The stoichiometric CeO2 film contains cerium ions exclusively in the oxidation state Ce4þ. LEED measurements revealed a wellordered CeO2(111) film with a (1.5  1.5) superstructure.22,23 With respect to their surface morphology, the prepared films typically exhibit flat terraces separated by steps, with the top terraces often covered with small ceria aggregates.21 The thickness of the film was 1.8 nm. (II). CeO2
x. The sample of partially reduced ceria was prepared by exposing the well-ordered CeO2(111) film to 50 L of methanol at a sample temperature of 120 K, followed by annealing to 700 K in UHV (see ref 24 for details). As a result, the sample contains both Ce4þ and Ce3þ ions, with Ce3þ ions located predominantly at the surface of the film. The stoichiometry of the sample (CeO1.79) was determined from intensity contributions of both Ce4þ and Ce3þ ions to the Ce 3d region (see, e.g., ref 15 for details). The properties of this sample with respect to CO2 activation have been discussed earlier.15 (III
IV). 0.4 and 1.0 nm MgO/CeO2
x. Samples were prepared by the deposition of Mg onto the stoichiometric CeO2(111) film at room temperature under oxygen atmosphere (5  10
7 mbar). Different thickness of MgO overlayers (0.4 and 1.0 nm) corresponds to different deposition times. Growth and morphology of the MgO layers on ceria has been investigated in a previous study, 8717

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Figure 1. Core level spectra of Ce 3d (A), O 1s (B), and Mg 2p (C) acquired at photon energies of 1486.6 eV (Al KR, γ = 60°), 650 eV, and 115 eV, respectively, on the samples: (I) stoichiometric CeO2(111)/Cu(111); (II) reduced CeO2
x; (III) 0.4 nm MgO/CeO2
x; (IV) 0.1 nm MgO/CeO2
x; (V) MgO
Ce2O3 mixed oxide; (VI) MgO
CeO2 mixed oxide.

and we refer to the latter for details.19 Briefly, the MgO deposited at room temperature forms a rough film consisting of a high density of small three-dimensional aggregates. The exact composition of these deposits is unknown. The MgO
CeO2 phase diagram suggests that both oxides should not form ternary compounds.25 However, formation of nonequilibrium MgO
CeO2 solid mixtures was reported by several authors.26,27 Nonequilibrium MgxCe1
x/2O2 solid solution exhibits a large number of oxygen vacancies, facilitating diffusion of oxygen.27 (V). MgO
Ce2O3 Mixed. The sample of MgO
Ce2O3 mixed oxide containing Ce ions exclusively in the Ce3þ state was prepared by the deposition of Mg onto CeO2 film at room temperature under UHV conditions. The estimated thickness of Mg was about 0.6 nm. The deposition was followed by annealing at 700 K. After preparation, angle-resolved XPS measurement revealed a homogeneous depth distribution of Mg and Ce ions. (VI). MgO
CeO2 Mixed. The sample of stoichiometric MgO
CeO2 mixed oxide, containing Ce ions exclusively in the oxidation state Ce4þ, was obtained by exposing sample V to oxygen at 523 K (5  10
7 mbar, 1000 s).

3. RESULTS AND DISCUSSION 3.1. Electronic Structure of Ceria-Based Model Catalysts. Core level spectra of Ce 3d (A), O 1s (B), and Mg 2p (C) obtained from samples I
VI are shown in Figure 1. For the stoichiometric CeO2 (sample I), the characteristic shape of the Ce 3d spectrum is composed of three spin
orbit doublets arising from photoemission

from Ce4þ ions (see ref 6 for more details). The Ce 3d spectra obtained at different photoemission angles, i.e., 20° and 60°, were practically identical (data not shown). The corresponding O 1s peak reveals a single component located at 529.4 eV. Reduction of the stoichiometric CeO2 layer causes dramatic changes in both the Ce 3d and the O 1s regions. Two additional spin
orbit doublets emerge in the Ce 3d spectrum on the partially reduced CeO2
x sample (sample II), which originate from photoelectron emission from Ce3þ ions.6 The O 1s spectrum now contains two peaks located at 529.7 and 532.1 eV. The two components correspond to the oxygen ions located near Ce4þ and Ce3þ, respectively. A small shift of the major component in the O 1s spectrum toward higher binding energy is most likely caused by the change of a static charge distribution within the sample. Depositing Mg onto stoichiometric CeO2 either under an atmosphere of oxygen (samples III
IV) or in vacuum (sample V) causes severe reduction of ceria. We conclude that a fast redox reaction occurs upon deposition of Mg leading to immediate conversion of Ce4þ to Ce3þ according to Mg0 þ 2Ce4þ f Mg2þ þ 2Ce3þ, even at room temperature. Evidently, the degree of ceria reduction depends on the amount of Mg deposited. Thus, the Ce 3d spectra from sample III (0.4 nm MgO/CeO2
x) and sample IV (1.0 nm MgO/CeO2
x) reveal different degrees of reduction. The corresponding Ce 3d spectra obtained from the samples III
IV at photoemission angles 60° and 20° were very similar (data not shown). Cerium ions in both oxidation states, Ce3þ and Ce4þ, are uniformly distributed within sample III. The corresponding O 1s spectrum reveals two peaks at 529.9 and 8718

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Valence band (VB) spectra of the studied samples measured at three different PEs are shown in Figure 2. Specific spectral features are resonantly enhanced in these spectra at certain PEs, which is the basis of RPES. RPES is recognized as the most sensitive tool to monitoring small changes in the electronic structure of ceria29
31 and allows determining the surface concentration of Ce4þ and Ce3þ with great precision. Before discussing specific features in the VB, we briefly reiterate the idea of RPES. In cerium oxide, the two oxidation states of cerium, Ce4þ and Ce3þ, differ with respect to occupation of their 4f levels (Ce4þ: 4f0; Ce3þ: 4f1). At the PEs corresponding to the resonant 4d f 4f transition in either Ce3þ or Ce4þ, an indirect two-step photoemission event occurs including an effective decay of the intermediate states via Super
Coster
Kronig transitions Ce3þ : 4d10 4f 1 þ hν f 4d9 4f 2 f 4d10 4f 0 þ e
Ce4þ : 4d10 4f 0 þ hν f 4d9 4f 1 f 4d10 4f 0 L þ e


Figure 2. Valence band photoelectron spectra obtained at photon energies corresponding to resonant enhancements in Ce3þ (121.4 eV, red) and Ce4þ (124.8 eV, black) and to off-resonance (115 eV, green) on samples: (I) stoichiometric CeO2(111)/Cu(111); (II) reduced CeO2
x; (III) 0.4 nm MgO/CeO2
x; (IV) 0.1 nm MgO/CeO2
x; (V) MgO
Ce2O3 mixed oxide; (VI) MgO
CeO2 mixed oxide. The corresponding resonant enhancement features in Ce3þ and Ce4þ, denoted as D(Ce3þ) and D(Ce4þ), emerge at 1.5 and 4.1 eV, respectively.

533.0 eV. A single Mg 2p peak is located at 50.25 eV. For 1.0 nm MgO/CeO2
x (sample IV), Ce ions are found in the oxidation state Ce3þ, exclusively. The corresponding O 1s and Mg 2p spectra show single components located at 530.8 and 50.6 eV, respectively. Both values are close to those reported for pure MgO (O 1s, 531.0 eV; Mg 2p, 50.8 eV).28 Deposition of Mg onto stoichiometric CeO2 under UHV conditions results in immediate oxidation of Mg and reduction of ceria. Subsequent annealing of the sample at 700 K yields a homogeneous layer of MgO
Ce2O3 mixed oxide (sample V) with uniform depth distribution of the ions (as indicated by angle-resolved XPS). The shape of the Ce 3d spectrum suggests that the cerium ions are found in the oxidation state Ce3þ. The O 1s and Mg 2p spectra contain single components at 530.3 and 50.26 eV, respectively. Exposing sample V to oxygen atmosphere at 523 K leads to conversion of all Ce3þ ions to Ce4þ. The corresponding Ce 3d spectrum of the MgO
CeO2 mixed oxide (sample VI) shows three spin
orbit doublets, resembling the sample of perfectly stoichiometric CeO2 (sample I). The O 1s spectrum contains a dominant peak at 529.13 eV and a small shoulder at 531.7 eV.

(L indicates a hole in the valence band). The corresponding maxima of the resonance enhancement in Ce3þ and Ce4þ ions are found at PEs of 121.4 and 124.8 eV, respectively.31 The resonance-related features appear in the VBs at binding energies (BE) of 1.5 eV (Ce3þ) and 4 eV (Ce4þ). Additionally, one VB spectrum is measured at a PE of 115 eV, corresponding to an off-resonance condition. For quantitative analysis, the so-called resonant enhancement ratio (RER) is calculated. This is done by first determining the individual resonant enhancements for Ce4þ (denoted as D(Ce4þ)) and Ce3þ (denoted as D(Ce3þ)) as intensity differences between the corresponding features in- and off-resonance (see, e.g., ref 31). The RER, determined as the ratio D(Ce3þ)/D(Ce4þ), is a direct measure for the Ce3þ/Ce4þ ion ratio. A typical set of the valence band spectra obtained in- and offresonance mode on the stoichiometric CeO2 film (sample I) is displayed in Figure 2I. The principal features of the spectrum measured in off-resonance mode have been discussed by Mullins et al.32 The authors distinguished two features in the band at 4.5 and 6.5 eV and associated them with O 2p orbitals hybridized with Ce 4f and 5d orbitals, respectively. In the spectra acquired in in-resonance conditions, we observe a significant increase of the intensity at around 4.1 eV (D(Ce4þ)) due to Ce4þ-related resonant emission. The corresponding RPE spectra obtained from reduced CeO2
x (sample II) are displayed in Figure 2II. With regard to the sample I, the difference in the shape of the valence bands becomes noticeable in the spectra acquired under the resonant conditions. Here, a new feature emerges at 1.5 eV which is assigned to emission from Ce 4f levels in Ce3þ ions.31,32 The increase of D(Ce3þ) intensity from the sample II indicates a massive increase of surface concentration of the Ce3þ ions. Additionally, we observe a moderate decrease of the D(Ce4þ) intensity as compared to sample I. The RERs determined for samples I and II are 0.03 and 4.1, respectively. On the basis of a calibration using the direct comparison with XPS (see ref 15), this yields surface compositions of CeO2.00 (sample I) and CeO1.79 (sample II). As discussed above, addition of Mg causes partial reduction of ceria. In the VB spectra for 0.4 nm MgO/CeO2
x (sample III), the presence of both Ce3þ and Ce4þ ions is indicated by the resonant enhancement of the corresponding features. However, due to the damping effects and overlapping emission from O 2p states in MgO, a quantitative determination of the degree of reduction from the RER is no longer straightforward. Therefore, 8719

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Figure 3. Core level spectra of C 1s acquired at a photon energy of 410 eV on the samples I
VI exposed to 14 200 L of CO2 at room temperature. The suggested assignments of the carbonate species are displayed schematically.

we restrict the further discussion to the resonant enhancement D(Ce3þ) which does not overlap with MgO-related states. Noteworthy, a weaker resonant enhancement of Ce3þ is found for sample IV (1.0 nm MgO/CeO2
x) as compared to sample III. At first glance, this observation may appear to be in conflict with the results of the SR-PES studies discussed above. However, the discrepancy can be easily explained by the stronger damping of the Ce3þ signal by the thick (1.0 nm) MgO overlayer. The VB spectral features from sample IV reveal considerable similarities with pure MgO films,33 showing the O 2p features at 5.6 and 8.3 eV. Together with the O 1s and Mg 2p binding energies being similar to bulk MgO, we conclude that the surface of sample IV is covered by a MgO layer. Comparing samples III and IV, we deduce that the near-surface region of the 0.4 nm MgO/CeO2
x (sample III) is composed of both magnesium and cerium ions, whereas for sample IV the cerium ions are located below a MgO surface layer. With respect to the O 1s and Mg 2p region, the characteristics of sample III are rather similar to those of sample V (compare binding energies in Figure 1). The VB spectra of sample V are displayed in Figure 2V. Again, we observe a pronounced resonant D(Ce3þ) feature. It is noteworthy that for the annealed sample V the D(Ce4þ) resonance is practically absent. We conclude that hardly any Ce4þ ions remain in the surface region of the sample, at least within the information depth of the RPES experiment. The shape of the O 2p emission is similar to that observed for the thick MgO layer (sample IV) but is slightly broader and shifted by 0.5 eV to lower binding energies. This shift may be caused in part by changes of the chemical environment when the mixed oxide is formed. For the spectra acquired under in-resonance conditions, a small increase in the VB intensities near 5 and 6 eV could be associated with the features found by Mullins et al.

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on completely reduced Ce(III) oxide samples.32 In addition, changes of the electrostatic potential within the oxide film may occur upon formation of the mixed oxide. The latter effect leads to additional band shifts, as found for instance in ultrathin BaO films on alumina.34 Finally, VB spectra from a completely oxidized mixed MgO
CeO2 film (sample VI) are shown in Figure 2VI. Most importantly, the D(Ce3þ) resonant feature is completely absent. This suggests full reoxidation of the mixed film. The O 2p valence band emission changes drastically, with the main feature appearing at 4 eV and broad emissions reaching up to 8.5 eV BE. The shape of the valence band remains unchanged, while the position shifts are similar to the stoichiometric CeO2 (sample I). However, the resonant D(Ce4þ) enhancement is significantly weaker than for the pure ceria film. This indicates formation of a mixed oxide containing a substantial fraction of Mg2þ ions. 3.2. Interaction with CO2. In the next step, we investigate the interaction of CO2 with the different ceria and ceria
magnesia samples I
VI at room temperature. C 1s spectra obtained on the samples after exposure to 14 200 L of CO2 are compared in Figure 3. For the stoichiometric CeO2 (sample I) (Figure 3I), no carbon signal is observed. In contrast, two peaks emerge in the C 1s spectrum of reduced CeO2
x (sample II) at 290 and 287 eV. In earlier studies on adsorption of CO2 on ceria,5,12,13 CaO,35,36 and MgO,16 formation of carbonate species has been proposed in most cases. Carbonate forms as a result of a Lewis acid
base adduct with lattice oxygen sites (compare ref 17). Senanayake and Mullins identified surface carbonate (CO32
) species at 290 eV in C 1s spectra of a CeO2(111)/Ru(0001) sample at low temperature.37 Senanayake et al. also observed a similar peak in the C 1s spectrum of reduced ceria nanoparticles.38 However, the majority of surface carbonates was found to be bound rather weakly and to desorb below 300 K. In agreement with these studies, we attribute the feature at 290 eV to carbonate and conclude that more strongly bound carbonates are formed on the reduced CeO2
x (sample II) only. If not adsorbed in form of a carbonate, CO2 can in principle coordinate to a metal ion, leading to formation of a carboxylate species. Such species have been suggested to exist on ceria powders and have been confirmed by means of IR spectroscopy.13 We tentatively attribute the dominant peak at 287 eV to the formation of a carboxylate, as already discussed in a previous publication.19 Future IR spectroscopy experiments on the present model catalysts may help to verify this hypothesis. Using this assignment, we conclude that carboxylates are only formed on the strongly reduced ceria surface. Next we proceed to the samples III
IV (0.4 nm MgO/ CeO2
x and 0.1 nm MgO/CeO2
x). The corresponding C 1s spectra after exposure to 14 200 L of CO2 are displayed in Figure 3III
IV. On both samples, only one feature is identified at approximately 291 eV BE. According to the above discussion, the peak can be attributed to the formation of a surface carbonate. The positive BE shift of 1 eV with respect to the surface carbonate on CeO2
x indicates, however, a different chemical environment. Considering the binding energies of C 1s reported for the magnesium carbonates (290.5
292 eV),28,39 we attribute the peak at 291 eV to CO32
species formed in the vicinity of Mg2þ ions. The C 1s spectra of the mixed ceria
magnesia samples after interaction with CO2 are displayed in Figure 3 (samples V and VI). Only one peak is found in both cases, which is located at 291 eV for the reduced mixed MgO
Ce2O3 film (sample V) and at 290 eV for the fully oxidized mixed MgO
CeO2 film (sample 8720

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Figure 4. Development of C 1s spectra on the samples III, V, and VI after exposure to increasing doses of CO2 at room temperature. The calculated integral area is displayed as a function of integral CO2 exposure in the upper right part of the figure.

VI). On the basis of the above discussion, we attribute these features to surface carbonates formed in the vicinity of Mg2þ (291 eV BE) and Ce4þ (290 eV BE) centers, respectively. This assignment is also supported by the position of the carbonaterelated feature in the O 1s spectrum (data not shown). There we identified carbonate related features at 533 and 531.8 eV on the samples III
V and samples II and VI, respectively. The positions of these peaks are similar to those reported for carbonates near Mg2þ (533 eV)27 and Ce3þ/Ce4þ (532.3 eV).37 In Figure 4, additional information on the kinetics and propensity for carbonate formation is provided for different MgOcontaining samples. For the samples III (0.4 nm MgO/CeO2
x), V (MgO
Ce2O3 mixed), and VI (MgO
CeO2 mixed), C 1s spectra as a function of CO2 exposure and the corresponding integral intensities in the carbonate C 1s region are displayed. We find that the largest amount of carbonates is formed on sample III (not mixed). For the reduced mixed oxide (sample V), the amount of carbonates decreases by a factor of 2. A further decrease is observed for the oxidized mixed oxide (sample VI). On all samples, the initial rate of carbonate formation is high before it decreases and, finally, saturates at exposures around 1000
10 000 L of CO2. Two points are noteworthy with respect to these results. First, we can conclude that stable surface carbonates are most efficiently formed in the vicinity of Mg2þ centers. Apparently, formation of the mixed oxide layer (sample V) leads to a decrease in carbonate formation, even though the surface cerium ions are reduced to Ce3þ. Those surface carbonates that are still formed on the MgO
Ce2O3 mixed oxide are preferentially located in the vicinity of the Mg2þ and not the Ce3þ centers.

Figure 5. (A) Development of the Ce3þ resonant enhancement (D(Ce3þ)) on samples II
VI as a function of CO2 exposure. (B) Valence band photoelectron spectra of samples II and III acquired photon energy 121.4 eV (Ce3þ resonance) after exposure to different doses of CO2.

The second point concerns the formation of carbonates on the stoichiometric MgO
CeO2 mixed oxide (sample VI). According to the valence band spectra discussed in section 3.1, the surface region is dominated by Ce4þ ions. In sharp contrast to the freshly prepared CeO2(111) film (sample I), we do observe formation of carbonates on this surface, and these carbonates are mainly located in the vicinity of Ce4þ ions. This finding strengthens the argument that the surface defect structure has a large influence on carbonate formation: On the nearly perfect surface of the freshly prepared CeO2(111), no carbonate species are formed which are stable up to room temperature. Small amounts, which may be adsorbed at defects and steps, are below the detection limit of the present PES experiment. On structurally distorted ceria films, room temperature carbonates can be formed, in the vicinity of both Ce3þ centers (sample II) and Ce4þ centers (sample V). If both cerium and magnesium ions are present at the surface, carbonate formation preferentially occurs in the vicinity of Mg2þ. The last observation concerns the carboxylate-like carbon species which is observed for the reduced CeO2
x film. This species is only found for the pure ceria film containing Ce3þ ions. The presence of Mg2þ completely suppresses formation of this species. Even for the intermixed MgO
Ce2O3 film (sample V) which contains exclusively Ce3þ and no Ce4þ ion in the surface region, this species is not observed. We conclude that formation 8721

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The Journal of Physical Chemistry C of the surface carbonate is strongly preferred in Mg2þ-containing samples and, in fact, suppresses the formation of the carboxylate. 3.3. Reoxidation of Different Samples with CO2. Next we investigate the reoxidation of ceria and ceria
magnesia samples by CO2 using RPES. In a previous communication, we have shown that reduced CeO2
x films on Cu(111) can indeed be reoxidized by CO2 at room temperature and under UHV conditions.15 Here, we present the corresponding data for the MgO-modified ceria films. Due to the overlapping emissions from the O 2p valence band between 4 and 8 eV BE, we primarily focus on the resonant enhancement of the Ce3þ-related feature at 1.5 eV BE. The development of the spectra and the corresponding resonant enhancement D(Ce3þ) as a function of CO2 exposure are displayed in Figure 5. Close inspection of the data reveals that substantial reoxidation by CO2 is observed for the MgO-free and reduced CeO2
x film only (sample II). As discussed in the previous communication,15 the rate for reoxidation is highest during the initial stages of CO2 exposure and decreases at a later stage. Yet, we observe that CO2-induced reoxidation continues up to very large CO2 exposure, i.e., even above 15 000 L. At the largest exposures used in the present study (∼35 000 L), D(Ce3þ) has already dropped to less than half of its initial value. For the MgO-modified surfaces, the situation is very different. After deposition of 0.4 nm of MgO at 300 K (sample III), we initially observe only a very small decrease in D(Ce3þ) upon CO2 exposure, but no further reaction is detected at exposures exceeding 200 L. It has been previously reported that if MgO contains Fs centers (charged oxygen vacancies)40 the dissociation of CO2 releases gaseous CO, whereas atomic oxygen may stay trapped in vacancies.41 Additionally, formation of carbonates may partially cause the damping of D(Ce3þ) intensity. Comparing the evolution of D(Ce3þ) on the samples II
VI, we conclude that MgO strongly suppresses ceria reoxidation. Of special interest is the behavior of the MgO
Ce2O3 mixed oxide film (sample V). Here, we again observe only a very small decrease in D(Ce3þ) immediately after the CO2 exposure. Again this effect may be due to partial dissociation of CO2 either on Fs centers40 in MgO or on surface Ce3þ ions and, in part, due to damping by surface carbonates. Upon continued exposure no further change of the Ce3þ concentration is detected. This behavior is noteworthy as the valence band spectra for this sample indicate a substantial amount of cerium ions in the surface region. In addition, these surface cerium ions are exclusively in the oxidation state Ce3þ. In contrast, we are dealing with a mixture of Ce3þ and Ce4þ ions for the MgO-free reduced ceria layer (sample II). This finding shows that reoxidation is not a simple function of the ceria oxidation state. In fact, we have to conclude that the surrounding Mg2þ ions efficiently suppress CO2-induced reoxidation. This effect is potentially related to the rapid formation of inactive surface carbonates formed in these centers. At this point we may speculate on the mechanism of the reoxidation process. In view of the present results it appears that the formation of the carboxylate species, observed on the pure reduced ceria film, is related to the reoxidation mechanism. In the first step, coordination of CO2 in form of an electronegative carboxyl ligand may occur at coordinatively unsaturated Ce3þ centers. We may invoke that already this step may give rise to a partial electron transfer from Ce3þ to the ligand, simultaneously leading to an attenuation of the Ce3þ resonance. Close inspection of the spectra reveals, however, that formation of the carboxylate occurs rapidly during the initial stages of exposure, whereas reoxidation continues

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Figure 6. (A) Development of the Ce3þ resonant enhancement (D(Ce3þ)) on samples II
VI exposed to the 34 200 L CO2 at 300 K and subsequent annealing to different temperatures. (B) Ce 3d core level spectra obtained at photon energy of 1486.6 eV (Al KR, γ = 60°) from the sample V before and after the exposure to 34 200 L of CO2 and annealing to 700 K.

up to large CO2 exposures. This observation implies not only that the CO2 activation process involves adsorption in the form of a carboxylate species but also that, in a second step, this species dissociates to release CO. Therefore, we suggest that the surface carboxylate is a true intermediate of CO2-induced reoxidation. In contrast, the formation of surface carbonates does not have a positive effect on CO2 activation. In fact, the results for the MgOmodified mixed oxide samples even suggest that stable surface carbonates are not just pure spectator species, but their presence actually suppresses the formation of the reactive carboxylates and, therefore, reoxidation. 3.4. Thermal Stability. Finally, the thermal stability of the samples I
VI has been investigated in the temperature region of carbonate decomposition (see Figure 6). Earlier15 we discussed the observation that the reduced CeO2
x film (sample II) exhibits a relatively poor thermal stability. The loss of oxygen that was recovered during CO2 exposure occurs already at temperatures above 500 K, as indicated by an increasing Ce3þ concentration. The corresponding development of D(Ce3þ) is plotted in Figure 6A as a function of annealing temperature for the samples II
VI. In contrast to sample II, we detect no significant variation of D(Ce3þ) on all magnesium-containing samples (samples III
VI) in the temperature region between 300 and 700 K. This observation is noteworthy as it indicates that 8722

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The Journal of Physical Chemistry C the small amount of oxygen that is taken up by these samples is efficiently stored in a more stable fashion than for the Mg-free sample II. Selected Ce 3d spectra obtained from sample VI (MgO
Ce2O3 mixed) are shown in Figure 6B. As discussed above, this sample contains cerium ions nearly exclusively in the oxidation state Ce3þ. Therefore, the corresponding Ce 3d spectrum before adsorption of CO2 is dominated by the two Ce3þ-related doublets. Additionally, three doublets of low intensity can be resolved originating from small amounts of Ce4þ ions in the film. The intensity of these three doublets increases moderately after exposure to large doses of CO2 (34 200 L). Since no carboxylate species are found on the surface, the mechanism of CO2 dissociation on these samples may be different and not involve the formation of carboxylates. One alternative mechanism may involve activation at Fs centers in the proximity of Mg2þ ions. The recovered oxygen can eventually migrate to the proximity of cerium centers, where it gives rise to oxidation of Ce3þ to Ce4þ. Further increase of the Ce4þ concentration is observed after annealing of the sample to 700 K. Since RPES indicates no change in the surface Ce3þ concentration (D(Ce3þ)) in this temperature interval, this observation indicates that oxygen diffuses into the bulk and gives rise to bulk reoxidation. We may speculate that in the mixed magnesia
ceria sample the mobility of oxygen and, therefore, propensity for bulk reoxidation is facilitated. This may be the origin of the improved thermal stability of the magnesia
ceria samples, in contrast to the pure ceria films, for which reoxidation is more efficient but mainly restricted to the surface region.

4. CONCLUSIONS Using SR-PES, RPES, and XPS, we have studied (i) the preparation and electronic structure of different ceria and ceria
magnesia model catalysts on Cu(111) and (ii) their interaction with CO2. 1. Six different types of thin oxide films were investigated: (I) a fully stoichiometric CeO2(111) film, (II) a reduced CeO2
x film prepared by methanol adsorption and annealing, (III, IV) MgO layers of different thickness on ceria films, (V) reduced mixed ceria
magnesia films prepared by annealing, and (VI) fully oxidized mixed ceria
magnesia films prepared by annealing and postoxidation. 2. We have investigated the valence band structure of the different samples, with a special focus on using resonant enhancement effects to monitor the concentration of Ce3þ and Ce4þ in the surface region. It is found that PVD of Mg onto stoichiometric CeO2, even in the presence of an O2 atm, leads to immediate reduction of Ce4þ to Ce3þ and oxidation of magnesium. For larger amounts of deposited Mg, exclusively Ce3þ is detected in the surface region. Upon annealing to 700 K, intermixing of the ceria layer and the magnesia overlayer occurs. For the mixed oxide, only Mg2þ and Ce3þ ions, but no Ce4þ, are present in the near surface region. By high-temperature oxidation (523 K), Ce3þ is quantitatively converted into Ce4þ. 3. The interaction of CO2 with the ceria and magnesia
ceria films was studied by SR-PES. At room temperature, no stable carbon-containing species is observed on fully stoichiometric CeO2. On reduced CeO2
x films, two species are observed which are associated with surface carbonates and, tentatively, with surface carboxylates located at coordinatively unsaturated Ce3þ centers. On magnesia-containing samples, a second surface carbonate can be identified, which is located in the

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vicinity of Mg2þ ions. Deposition of MgO, for both the intermixed and nonintermixed films, enhances the formation of surface carbonates. In contrast, the formation of the surface carboxylates is suppressed, even on intermixed and reduced samples which expose Ce3þ ions at a high concentration. 4. Substantial reoxidation of reduced CeO2
x films occurs upon extended exposure to CO2 at 300 K. Reoxidation of ceria is, however, hindered or even completely suppressed on ceria
magnesia samples. We conclude that the formation of surface carboxylates is an intermediate reaction step in CO2-induced reoxidation. Formation of surface carbonates, on the other hand, does not lead to reoxidation but actually hinders the formation of carboxylates and, therefore, the reoxidation process. 5. Thermal stability of the ceria-based catalyst has been investigated in the temperature interval 300
700 K. It is shown that oxygen originating from CO2 activation is easily lost on pure ceria samples, whereas formation of mixed magnesia
ceria oxides prevents thermal release of lattice oxygen.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: þ499131-8528867.

’ ACKNOWLEDGMENT The authors gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft (DFG) within the ERACHEM program (“NanoFunC” project) and by the Ministry of Education of the Czech Republic (LA08022 and LC06065). We acknowledge additional support from the DFG within the Excellence Cluster “Engineering of Advanced Materials” in the framework of the excellence initiative. We are also grateful for additional support by the Fonds der Chemischen Industrie, the DAAD, and the European Union (COST D-41). Close cooperation with M. A. Schneider, L. Hammer, H.-P. Steinr€uck, A. Bayer, R. Streber, and M. P. A. Lorenz (Erlangen) is acknowledged concerning the characterization of the ceria films. ’ REFERENCES (1) Hu, Y. H.; Ruckenstein, E. Adv. Catal. 2004, 48, 297–345. (2) Bradford, M. C. J.; Vannice, M. A. Catal. Rev. - Sci. Eng. 1999, 41, 1–42. (3) Rostrup-Nielsen, J. R. Catal. Today 1997, 37, 225–232. (4) Trovarelli, A. Catalysis by Ceria and Related Metals; Imperial College Press: London, 2002. (5) Wang, X.; Gorte, R. J. Appl. Catal., A 2002, 224, 209–218. (6) Lykhach, Y.; Staudt, T.; Lorenz, M. P. A.; Streber, R.; Bayer, A.; Steinr€uck, H. P.; Libuda, J. ChemPhysChem 2010, 11, 1496–1504. (7) Valenzuela, R. X.; Bueno, G.; Corberan, V. C.; Xu, Y. D.; Chen, C. L. Catal. Today 2000, 61, 43–48. (8) Bernal, S.; Blanco, G.; Gatica, J. M.; Larese, C.; Vidal, H. J. Catal. 2001, 200, 411–415. (9) Demoulin, O.; Navez, M.; Mugabo, J. L.; Ruiz, P. Appl. Catal., B 2007, 70, 284–293. (10) Otsuka, K.; Wang, Y.; Sunada, E.; Yamanaka, I. J. Catal. 1998, 175, 152–160. (11) Jin, T.; Zhou, Y.; Mains, G. J.; White, J. M. J. Phys. Chem. 1987, 91, 5931–5937. 8723

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