Published: April 11, 2011

r 2011 American Chemical Society 8716 dx.doi.org/10.1021/jp200382y | J. Phys. Chem. C 2011, 115, 8716–8724

ARTICLE

pubs.acs.org/JPCC

Electronic Structure of Magnesia�Ceria Model Catalysts, CO2

Adsorption, and CO2 Activation: A Synchrotron RadiationPhotoelectron Spectroscopy StudyThorsten Staudt,† Yaroslava Lykhach,*,† Nataliya Tsud,‡ Tom�a�s 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 Hole�sovi�ck�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

1. INTRODUCTION

Carbon dioxide can potentially be utilized in various large-scale chemical processes. One example is the dry reforming ofmethane using CH4/CO2 mixtures available from coal, natural gas,carbonaceous wastes, or biogas.1,2 The reaction yields synthesis gas(H2/CO) fromwhichmany chemicals are produced, including, e.g.,liquid hydrocarbons, olefins, methanol, formaldehyde, or aceticacid.1 Certainly, the transformation of two greenhouse gases intovaluable feedstock is appealing from an environmental point of view.In general, dry reforming catalysts suffer from rapid catalystdeactivation by carbon formation. The carbon originates frommethane dehydrogenation and CO2 disproportionation with ageneral thermodynamic tendency to form coke.1�3 Here, the useof reducible oxide supports such as ceria (CeO2) may provide apossible solution.4,5 Ceria has the remarkable capability to store andrelease large amounts of oxygen, giving rise to a self-cleaningfunctionality of the catalyst if the liberation of oxygen and itsreaction with carbon are rapid in comparison to carbon aggregation.The processes of release and uptake of oxygen are accompanied by areversible transformation between the oxidation states Ce4þ andCe3þ which can be traced spectroscopically. Recently, we have

investigated the removal of carbonaceous sediments on a Pt/CeO2/Cu(111) model catalyst, formed by activation ofmethane and otherhydrocarbons.6 The reaction leads to high surface concentration ofCe3þ ions in the surface region of the support.

The universal mechanism of CO2 activation on ceria is yetunclear and currently widely discussed in the literature.7�14 Theprocess involved dissociation of CO2 into CO and an oxygen-containing surface species.7�11,14 It is suggested that surface Ce3þ

ions may become active sites for CO2 activation via formation ofeither carbonates or inorganic carboxylates.12,13 In our recentcommunication,15 we have demonstrated that dissociation of CO2

occurs on pure and strongly reduced ceria films leading to partialreoxidation of ceria even at room temperature. It was shown that thereaction rate strongly depends on the degree of the ceria reduction.High initial rates of CO2 dissociation rapidly decay as reoxidationproceeds. Finally, partially reoxidized ceria becomes inert towardCO2. However, the reoxidized ceria shows poor thermal stability

Received: January 13, 2011Revised: March 30, 2011

ABSTRACT: We have studied the electronic properties of single crystal basedceria and magnesia�ceria model catalysts, the CO2 adsorption, and the CO2-induced reoxidation of these systems by synchrotron radiation photoelectronspectroscopy (SR-PES). All model systems were prepared starting from a fullystoichiometric and well-ordered CeO2(111) film grown on Cu(111). Differentmagnesia�ceria mixed oxide films were prepared by physical vapor deposition(PVD) of magnesium, oxygen treatment, and subsequent annealing. Thepreparation procedure was varied to obtain samples with different oxidationstate, structure, and surface composition. Different carbon-containing specieswere identified, including surface carbonates formed in the vicinity of Mg2þ andCe3þ/4þ and surface carboxylates. The presence of Mg2þ was observed to strongly enhance carbonate formation but suppress theformation of carboxylates. Changes in the oxidation state of ceria upon CO2 exposure were monitored with highest sensitivity byresonant photoelectron spectroscopy (RPES). Reoxidation of Ce3þ was observed to be suppressed on magnesia-containingsamples, even in the presence of a high surface concentration of Ce3þ. The findings suggest that carboxylates are an intermediatestep of reoxidation, whereas the generation of stable surface carbonates inhibits formation of this intermediate and, therefore, CO2-induced reoxidation.

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resulting in a loss of recovered lattice oxygen during annealing to700 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 thenanoscale. Basic sites, such as alkaline earth metal oxides (MgO),facilitate the formation of surface carbonates.1,16�18 It is gen-erally accepted that CO2 chemisorbs on the defect surface sites ofMgO, such as edges and corners, whereas the terrace sites remainunreactive.18 The reaction of CO2 with low-coordinated oxygenions at room temperature yields stable unidentate and bidentatecarbonates.

Recently, we have studied CO2 activation on mixed MgO�CeO2 model catalysts, with different concentrations and degreesof intermixing betweenMg2þ, Ce3þ, andCe4þ ions.19The obtainedresults suggested that the activation pathway of CO2 can indeed bemodified by carbonate formation. Simultaneously, reoxidation ofceria can be effectively suppressed. Unfortunately, the work sufferedfrom the comparably low sensitivity of conventional X-ray photo-electron spectroscopy (XPS) to changes of the surface oxidationstate. Here, we present the results of a study using SynchrotronRadiation Photoelectron Spectroscopy (SR-PES) and, in particular,Resonant Photoemission Spectroscopy (RPES). These methodsallow us to monitor changes in the oxidation state of ceria-basedcatalysts with high sensitivity and, thus, to identify intermediates andpossible reaction mechanisms.

2. EXPERIMENTAL SECTION

All SR-PES and RPES measurements have been performed atthe Materials Science Beamline of the Elettra synchrotron facility inTrieste, Italy. The source was a bending magnet that produceslinearly polarized light in the energy range of 21�1000 eV.Photoelectron spectra were acquiredwith a high luminosity electronenergy analyzer (Specs Phoibos 150), equipped with a nine-channeldetector. The experimental end station was equipped with a rear-view LEEDoptics, a quadrupolemass spectrometer, an ion gun, anda gas inlet system. The basic setup of the chamber included a dualMg/Al X-ray source used for energy calibration of the synchrotronlight and measuring of the core levels beyond the reach of thesynchrotron light. The background pressure in the analysis chamberwas better than 2 � 10�10 mbar during all measurements. Theresonant photoelectron spectra were recorded at photon energies(PEs) between hν= 115 and 124.8 eV. Core level spectra ofO 1s, C1s, and Mg 2p were acquired using synchrotron radiation at hν =650, 410, and 115 eV, respectively. All spectra were normalized withrespect to the ring current and acquisition time. Additionally, Ce 3d,Cu 2p3/2, O 1s, Mg 1s, and C 1s core level spectra were measuredwith an X-ray source using Al KR (1486.6 eV) radiation. All spectrawere taken at constant pass energy, at emission angles of thephotoelectrons (γ) of 20� and 60� for Al KR and of 0� forsynchrotron radiation with respect to the sample normal. Theescape depths of the photoelectrons were approximated by aproduct 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 theywere less than 0.5 nm. The total spectral resolutions achieved withAl 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 levelspectra were fitted with Voigt profiles after subtraction of a Shirleybackground.

For systematic studies of CO2 activation, we used six differentsamples each fabricated starting from a common CeO2(111)/

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 asubstrate for all samples discussed in this work. Cu(111) wascleaned by several cycles of Arþ ion sputtering (at 300 K for 60min) and annealing (723 K for 5min) until no traces of carbon orany other contaminant were found in the photoelectron spectra.Epitaxial layers of CeO2(111) were grown on clean Cu(111) byphysical 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 filmsat 523 K in an oxygen atmosphere of the same pressure for 10min. The thickness of the prepared films determined from theattenuation of the Cu 2p3/2 signal, acquired using the Al KRX-ray source at γ = 20�, was typically 1.5�1.8 nm, correspondingto about 5 to 6 monolayers (MLs) of ceria. Here we define 1 MLas one O�Ce�O trilayer of CeO2(111), corresponding to athickness of 0.313 nm. For further details concerning thepreparation procedure, we refer to the literature.21�23 The fullystoichiometric CeO2 film (sample I) was used as a starting pointfor the preparation of reduced and mixed magnesia�ceria layers.Toward this end, Mgmetal (Goodfellow, 99.9%) was evaporatedfrom a resistively heated Mo crucible either in an oxygen O2

atmosphere (5� 10�7mbar) or inUHV at a sample temperatureof 300 K. The amount of the deposited material was estimated bythe attenuation of the intensity of the Cu 2p3/2 photoemissionsignal, acquired using an Al KR X-ray source at γ = 20�.

The CO2 (Linde, 99.995%) exposure was performed in succes-sive 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 tem-perature was controlled by a DC power supply passing a currentthrough the Ta wires holding the sample. The actual temperaturewas measured by a K-type thermocouple attached to the rearsurface of the sample.2.1. Sample Preparation. ( I). CeO2(111)/Cu(111). The stoi-

chiometric CeO2 film contains cerium ions exclusively in theoxidation state Ce4þ. LEED measurements revealed a well-ordered CeO2(111) film with a (1.5 � 1.5) superstructure.22,23

With respect to their surface morphology, the prepared filmstypically exhibit flat terraces separated by steps, with the topterraces often covered with small ceria aggregates.21 The thick-ness 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 Lof methanol at a sample temperature of 120 K, followed byannealing to 700 K inUHV (see ref 24 for details). As a result, thesample contains both Ce4þ and Ce3þ ions, with Ce3þ ionslocated predominantly at the surface of the film. The stoichiom-etry of the sample (CeO1.79) was determined from intensitycontributions of both Ce4þ and Ce3þ ions to the Ce 3d region(see, e.g., ref 15 for details). The properties of this sample withrespect to CO2 activation have been discussed earlier.15

(III�IV). 0.4 and 1.0 nm MgO/CeO2�x. Samples were preparedby the deposition of Mg onto the stoichiometric CeO2(111) film atroom temperature under oxygen atmosphere (5 � 10�7 mbar).Different thickness of MgO overlayers (0.4 and 1.0 nm) corre-sponds to different deposition times. Growth and morphology ofthe MgO layers on ceria has been investigated in a previous study,

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and we refer to the latter for details.19 Briefly, the MgO deposited atroom temperature forms a rough film consisting of a high density ofsmall three-dimensional aggregates. The exact composition of thesedeposits is unknown. TheMgO�CeO2 phase diagram suggests thatboth oxides should not form ternary compounds.25 However,formation of nonequilibrium MgO�CeO2 solid mixtures wasreported by several authors.26,27 Nonequilibrium MgxCe1�x/2O2

solid solution exhibits a large number of oxygen vacancies, facilitat-ing diffusion of oxygen.27

(V). MgO�Ce2O3 Mixed. The sample of MgO�Ce2O3 mixedoxide containing Ce ions exclusively in the Ce3þ state wasprepared by the deposition of Mg onto CeO2 film at roomtemperature under UHV conditions. The estimated thickness ofMg was about 0.6 nm. The deposition was followed by annealingat 700 K. After preparation, angle-resolved XPS measurementrevealed 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 inthe oxidation state Ce4þ, was obtained by exposing sample V tooxygen 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), andMg 2p (C) obtainedfrom samples I�VI are shown in Figure 1. For the stoichiometricCeO2 (sample I), the characteristic shape of the Ce 3d spectrum iscomposed of three spin�orbit doublets arising from photoemission

from Ce4þ ions (see ref 6 for more details). The Ce 3d spectraobtained at different photoemission angles, i.e., 20� and 60�, werepractically identical (data not shown). The correspondingO1s peakreveals 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 additionalspin�orbit doublets emerge in the Ce 3d spectrum on the partiallyreduced CeO2�x sample (sample II), which originate from photo-electron emission from Ce3þ ions.6 The O 1s spectrum nowcontains two peaks located at 529.7 and 532.1 eV. The twocomponents correspond to the oxygen ions located near Ce4þ

and Ce3þ, respectively. A small shift of the major component in theO1s spectrum toward higher binding energy ismost likely caused bythe 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 (sampleV) causes severe reduction of ceria. We conclude that a fast redoxreaction occurs upon deposition of Mg leading to immediateconversion of Ce4þ toCe3þ according toMg0þ 2Ce4þfMg2þþ2Ce3þ, even at room temperature. Evidently, the degree of ceriareduction depends on the amount of Mg deposited. Thus, theCe 3d spectra from sample III (0.4 nm MgO/CeO2�x) andsample IV (1.0 nm MgO/CeO2�x) reveal different degrees ofreduction. The corresponding Ce 3d spectra obtained from thesamples III�IV at photoemission angles 60� and 20� were verysimilar (data not shown). Cerium ions in both oxidation states,Ce3þ and Ce4þ, are uniformly distributed within sample III. Thecorresponding O 1s spectrum reveals two peaks at 529.9 and

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 nmMgO/CeO2�x; (IV) 0.1 nmMgO/CeO2�x;(V) MgO�Ce2O3 mixed oxide; (VI) MgO�CeO2 mixed oxide.

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533.0 eV. A single Mg 2p peak is located at 50.25 eV. For 1.0 nmMgO/CeO2�x (sample IV), Ce ions are found in the oxidationstate Ce3þ, exclusively. The corresponding O 1s and Mg 2pspectra show single components located at 530.8 and 50.6 eV,respectively. Both values are close to those reported for pureMgO (O 1s, 531.0 eV; Mg 2p, 50.8 eV).28

Deposition of Mg onto stoichiometric CeO2 under UHVconditions results in immediate oxidation of Mg and reductionof ceria. Subsequent annealing of the sample at 700 K yields ahomogeneous layer of MgO�Ce2O3 mixed oxide (sample V)with uniform depth distribution of the ions (as indicated byangle-resolved XPS). The shape of the Ce 3d spectrum suggeststhat the cerium ions are found in the oxidation state Ce3þ. The O1s and Mg 2p spectra contain single components at 530.3 and50.26 eV, respectively.Exposing sample V to oxygen atmosphere at 523 K leads to

conversion of all Ce3þ ions to Ce4þ. The corresponding Ce 3dspectrum of the MgO�CeO2 mixed oxide (sample VI) showsthree spin�orbit doublets, resembling the sample of perfectlystoichiometric CeO2 (sample I). The O 1s spectrum contains adominant peak at 529.13 eV and a small shoulder at 531.7 eV.

Valence band (VB) spectra of the studied samples measured atthree different PEs are shown in Figure 2. Specific spectralfeatures are resonantly enhanced in these spectra at certainPEs, which is the basis of RPES. RPES is recognized as the mostsensitive tool to monitoring small changes in the electronicstructure of ceria29�31 and allows determining the surfaceconcentration of Ce4þ and Ce3þ with great precision. Beforediscussing specific features in the VB, we briefly reiterate the ideaof 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 resonant4d f 4f transition in either Ce3þ or Ce4þ, an indirect two-stepphotoemission event occurs including an effective decay of theintermediate states via Super�Coster�Kronig transitions

Ce3þ: 4d104f 1 þ hν f 4d94f 2 f 4d104f 0 þ e�

Ce4þ: 4d104f 0 þ hν f 4d94f 1 f 4d104f 0Lþ e�

(L indicates a hole in the valence band). The correspondingmaximaof the resonance enhancement in Ce3þ and Ce4þ ions are found atPEs of 121.4 and 124.8 eV, respectively.31 The resonance-relatedfeatures appear in the VBs at binding energies (BE) of 1.5 eV(Ce3þ) and 4 eV (Ce4þ). Additionally, one VB spectrum ismeasured at a PE of 115 eV, corresponding to an off-resonancecondition. For quantitative analysis, the so-called resonant enhance-ment ratio (RER) is calculated. This is done by first determining theindividual resonant enhancements for Ce4þ (denoted asD(Ce4þ))and Ce3þ (denoted as D(Ce3þ)) as intensity differences betweenthe corresponding features in- and off-resonance (see, e.g., ref 31).The RER, determined as the ratio D(Ce3þ)/D(Ce4þ), is a directmeasure for the Ce3þ/Ce4þ ion ratio.A typical set of the valence band spectra obtained in- and off-

resonance mode on the stoichiometric CeO2 film (sample I) isdisplayed in Figure 2I. The principal features of the spectrummeasured in off-resonance mode have been discussed by Mullinset al.32 The authors distinguished two features in the band at 4.5 and6.5 eV and associated themwithO 2p orbitals hybridized with Ce 4fand 5d orbitals, respectively. In the spectra acquired in in-resonanceconditions, we observe a significant increase of the intensity ataround 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 tothe sample I, the difference in the shape of the valence bandsbecomes noticeable in the spectra acquired under the resonantconditions. Here, a new feature emerges at 1.5 eV which isassigned to emission from Ce 4f levels in Ce3þ ions.31,32 Theincrease of D(Ce3þ) intensity from the sample II indicates amassive 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 forsamples I and II are 0.03 and 4.1, respectively. On the basis of acalibration using the direct comparison with XPS (see ref 15),this yields surface compositions of CeO2.00 (sample I) andCeO1.79 (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 theresonant enhancement of the corresponding features. However,due to the damping effects and overlapping emission from O 2pstates in MgO, a quantitative determination of the degree ofreduction from the RER is no longer straightforward. Therefore,

Figure 2. Valence band photoelectron spectra obtained at photonenergies 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) reducedCeO2�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. Thecorresponding resonant enhancement features in Ce3þ and Ce4þ,denoted as D(Ce3þ) and D(Ce4þ), emerge at 1.5 and 4.1 eV,respectively.

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we restrict the further discussion to the resonant enhancementD(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 sampleIII. At first glance, this observation may appear to be in conflictwith the results of the SR-PES studies discussed above. However,the discrepancy can be easily explained by the stronger dampingof the Ce3þ signal by the thick (1.0 nm)MgO overlayer. The VBspectral features from sample IV reveal considerable similaritieswith pure MgO films,33 showing the O 2p features at 5.6 and 8.3eV. Together with the O 1s and Mg 2p binding energies beingsimilar to bulk MgO, we conclude that the surface of sample IV iscovered by a MgO layer. Comparing samples III and IV, wededuce that the near-surface region of the 0.4 nmMgO/CeO2�x

(sample III) is composed of both magnesium and cerium ions,whereas for sample IV the cerium ions are located below a MgOsurface layer.With respect to the O 1s and Mg 2p region, the characteristics

of sample III are rather similar to those of sample V (comparebinding energies in Figure 1). The VB spectra of sample V aredisplayed in Figure 2V. Again, we observe a pronounced resonantD(Ce3þ) feature. It is noteworthy that for the annealed sample Vthe D(Ce4þ) resonance is practically absent. We conclude thathardly 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 thethick MgO layer (sample IV) but is slightly broader and shiftedby 0.5 eV to lower binding energies. This shift may be caused inpart by changes of the chemical environment when the mixedoxide is formed. For the spectra acquired under in-resonanceconditions, a small increase in the VB intensities near 5 and 6 eVcould be associated with the features found by Mullins et al.

on completely reducedCe(III) oxide samples.32 In addition, changesof the electrostatic potential within the oxide film may occurupon formation of the mixed oxide. The latter effect leads toadditional band shifts, as found for instance in ultrathin BaO filmson alumina.34

Finally, VB spectra from a completely oxidized mixedMgO�CeO2 film (sample VI) are shown in Figure 2VI. Mostimportantly, the D(Ce3þ) resonant feature is completely absent.This suggests full reoxidation of themixed film. TheO 2p valenceband emission changes drastically, with the main feature appear-ing at 4 eV and broad emissions reaching up to 8.5 eV BE. Theshape of the valence band remains unchanged, while the positionshifts are similar to the stoichiometric CeO2 (sample I). How-ever, the resonant D(Ce4þ) enhancement is significantly weakerthan for the pure ceria film. This indicates formation of a mixedoxide 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�magnesiasamples I�VI at room temperature. C 1s spectra obtained on thesamples after exposure to 14 200 L of CO2 are compared inFigure 3. For the stoichiometric CeO2 (sample I) (Figure 3I), nocarbon signal is observed. In contrast, two peaks emerge in the C1s 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 beenproposed in most cases. Carbonate forms as a result of a Lewisacid�base adduct with lattice oxygen sites (compare ref 17).Senanayake and Mullins identified surface carbonate (CO3

2�)species at 290 eV in C 1s spectra of a CeO2(111)/Ru(0001)sample at low temperature.37 Senanayake et al. also observed asimilar peak in the C 1s spectrum of reduced ceria nanoparticles.38

However, the majority of surface carbonates was found to be boundrather weakly and to desorb below 300 K. In agreement with thesestudies, we attribute the feature at 290 eV to carbonate and concludethat more strongly bound carbonates are formed on the reducedCeO2�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 carboxylatespecies. Such species have been suggested to exist on ceria powdersand have been confirmed by means of IR spectroscopy.13 Wetentatively attribute the dominant peak at 287 eV to the formation ofa carboxylate, as already discussed in a previous publication.19 FutureIR spectroscopy experiments on the present model catalysts mayhelp to verify this hypothesis. Using this assignment, we concludethat carboxylates are only formed on the strongly reduced ceriasurface.Next we proceed to the samples III�IV (0.4 nm MgO/

CeO2�x and 0.1 nm MgO/CeO2�x). The corresponding C 1sspectra after exposure to 14 200 L of CO2 are displayed inFigure 3III�IV. On both samples, only one feature is identified atapproximately 291 eV BE. According to the above discussion, thepeak can be attributed to the formation of a surface carbonate. Thepositive BE shift of 1 eV with respect to the surface carbonate onCeO2�x indicates, however, a different chemical environment.Considering the binding energies of C 1s reported for the mag-nesium carbonates (290.5�292 eV),28,39 we attribute the peak at291 eV to CO3

2� 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 andVI). Only one peak is found in both cases, which is located at 291eV for the reduced mixed MgO�Ce2O3 film (sample V) and at290 eV for the fully oxidized mixed MgO�CeO2 film (sample

Figure 3. Core level spectra of C 1s acquired at a photon energy of410 eV on the samples I�VI exposed to 14 200 L of CO2 at roomtemperature. The suggested assignments of the carbonate species aredisplayed schematically.

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VI). On the basis of the above discussion, we attribute thesefeatures to surface carbonates formed in the vicinity of Mg2þ

(291 eV BE) and Ce4þ (290 eV BE) centers, respectively. Thisassignment is also supported by the position of the carbonate-related feature in the O 1s spectrum (data not shown). There weidentified carbonate related features at 533 and 531.8 eV on thesamples III�V and samples II and VI, respectively. The positionsof these peaks are similar to those reported for carbonates nearMg2þ (533 eV)27 and Ce3þ/Ce4þ (532.3 eV).37

In Figure 4, additional information on the kinetics andpropensity for carbonate formation is provided for different MgO-containing samples. For the samples III (0.4 nmMgO/CeO2�x), V(MgO�Ce2O3mixed), and VI (MgO�CeO2mixed), C 1s spectraas a function of CO2 exposure and the corresponding integralintensities in the carbonate C 1s region are displayed. We find thatthe largest amount of carbonates is formed on sample III (notmixed). For the reduced mixed oxide (sample V), the amount ofcarbonates decreases by a factor of 2. A further decrease is observedfor the oxidized mixed oxide (sample VI). On all samples, the initialrate 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 effi-ciently formed in the vicinity of Mg2þ centers. Apparently,formation of the mixed oxide layer (sample V) leads to a decreasein carbonate formation, even though the surface cerium ions arereduced to Ce3þ. Those surface carbonates that are still formedon the MgO�Ce2O3 mixed oxide are preferentially located inthe vicinity of the Mg2þ and not the Ce3þ centers.

The second point concerns the formation of carbonates on thestoichiometricMgO�CeO2mixed oxide (sampleVI). According tothe valence band spectra discussed in section 3.1, the surface regionis dominated byCe4þ ions. In sharp contrast to the freshly preparedCeO2(111) film (sample I), we do observe formation of carbonateson this surface, and these carbonates are mainly located in thevicinity ofCe4þ ions. This finding strengthens the argument that thesurface defect structure has a large influence on carbonate forma-tion: On the nearly perfect surface of the freshly prepared CeO2-(111), no carbonate species are formed which are stable up to roomtemperature. Small amounts, which may be adsorbed at defects andsteps, are below the detection limit of the present PES experiment.On structurally distorted ceria films, room temperature carbonatescan be formed, in the vicinity of both Ce3þ centers (sample II) andCe4þ centers (sample V). If both cerium and magnesium ions arepresent at the surface, carbonate formation preferentially occurs inthe vicinity of Mg2þ.The last observation concerns the carboxylate-like carbon

species which is observed for the reduced CeO2�x film. Thisspecies is only found for the pure ceria film containing Ce3þ ions.The presence of Mg2þ completely suppresses formation of thisspecies. Even for the intermixed MgO�Ce2O3 film (sample V)which contains exclusively Ce3þ and no Ce4þ ion in the surfaceregion, this species is not observed. We conclude that formation

Figure 4. Development of C 1s spectra on the samples III, V, and VIafter exposure to increasing doses of CO2 at room temperature. Thecalculated integral area is displayed as a function of integral CO2

exposure in the upper right part of the figure.

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 acquiredphoton energy 121.4 eV (Ce3þ resonance) after exposure to differentdoses of CO2.

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of the surface carbonate is strongly preferred inMg2þ-containingsamples 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 samplesby CO2 using RPES. In a previous communication, we haveshown that reduced CeO2�x films on Cu(111) can indeed bereoxidized by CO2 at room temperature and under UHVconditions.15 Here, we present the corresponding data for theMgO-modified ceria films. Due to the overlapping emissionsfrom the O 2p valence band between 4 and 8 eV BE, we primarilyfocus on the resonant enhancement of the Ce3þ-related featureat 1.5 eV BE. The development of the spectra and the corre-sponding resonant enhancement D(Ce3þ) as a function of CO2

exposure are displayed in Figure 5.Close inspection of the data reveals that substantial reoxida-

tion by CO2 is observed for the MgO-free and reduced CeO2�x

film only (sample II). As discussed in the previous communi-cation,15 the rate for reoxidation is highest during the initialstages of CO2 exposure and decreases at a later stage. Yet, weobserve that CO2-induced reoxidation continues up to very largeCO2 exposure, i.e., even above 15 000 L. At the largest exposuresused in the present study (∼35 000 L), D(Ce3þ) has alreadydropped 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), weinitially observe only a very small decrease inD(Ce3þ) uponCO2

exposure, but no further reaction is detected at exposuresexceeding 200 L. It has been previously reported that if MgOcontains Fs centers (charged oxygen vacancies)40 the dissocia-tion of CO2 releases gaseous CO, whereas atomic oxygen maystay trapped in vacancies.41 Additionally, formation of carbonatesmay partially cause the damping of D(Ce3þ) intensity. Compar-ing the evolution ofD(Ce3þ) on the samples II�VI, we concludethat 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 smalldecrease in D(Ce3þ) immediately after the CO2 exposure. Againthis effect may be due to partial dissociation of CO2 either on Fscenters40 in MgO or on surface Ce3þ ions and, in part, due todamping by surface carbonates. Upon continued exposure nofurther change of the Ce3þ concentration is detected. This behavioris noteworthy as the valence band spectra for this sample indicate asubstantial 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 amixture of Ce3þ andCe4þ ions forthe MgO-free reduced ceria layer (sample II). This finding showsthat reoxidation is not a simple function of the ceria oxidation state.In fact, we have to conclude that the surrounding Mg2þ ionsefficiently suppress CO2-induced reoxidation. This effect is poten-tially related to the rapid formation of inactive surface carbonatesformed 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 theformation of the carboxylate species, observed on the pure reducedceria film, is related to the reoxidation mechanism. In the first step,coordination of CO2 in form of an electronegative carboxyl ligandmay occur at coordinatively unsaturated Ce3þ centers. We mayinvoke that already this step may give rise to a partial electrontransfer from Ce3þ to the ligand, simultaneously leading to anattenuation of the Ce3þ resonance. Close inspection of the spectrareveals, however, that formation of the carboxylate occurs rapidlyduring the initial stages of exposure, whereas reoxidation continues

up to large CO2 exposures. This observation implies not only thatthe CO2 activation process involves adsorption in the form of acarboxylate species but also that, in a second step, this speciesdissociates to release CO. Therefore, we suggest that the surfacecarboxylate is a true intermediate of CO2-induced reoxidation. Incontrast, the formation of surface carbonates does not have apositive effect on CO2 activation. In fact, the results for the MgO-modified mixed oxide samples even suggest that stable surfacecarbonates are not just pure spectator species, but their presenceactually 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 ofcarbonate decomposition (see Figure 6). Earlier15 we discussedthe observation that the reduced CeO2�x film (sample II)exhibits a relatively poor thermal stability. The loss of oxygenthat was recovered during CO2 exposure occurs already attemperatures above 500 K, as indicated by an increasing Ce3þ

concentration. The corresponding development of D(Ce3þ) isplotted in Figure 6A as a function of annealing temperature forthe samples II�VI. In contrast to sample II, we detect nosignificant variation of D(Ce3þ) on all magnesium-containingsamples (samples III�VI) in the temperature region between300 and 700 K. This observation is noteworthy as it indicates that

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

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the small amount of oxygen that is taken up by these samples isefficiently stored in a more stable fashion than for the Mg-freesample II. Selected Ce 3d spectra obtained from sample VI(MgO�Ce2O3mixed) are shown in Figure 6B. As discussed above,this sample contains cerium ions nearly exclusively in the oxidationstate Ce3þ. Therefore, the corresponding Ce 3d spectrum beforeadsorption of CO2 is dominated by the two Ce

3þ-related doublets.Additionally, three doublets of low intensity can be resolvedoriginating from small amounts of Ce4þ ions in the film. Theintensity of these three doublets increasesmoderately after exposureto large doses of CO2 (34 200 L). Since no carboxylate species arefound on the surface, the mechanism of CO2 dissociation on thesesamples may be different and not involve the formation ofcarboxylates. One alternative mechanism may involve activation atFs centers in the proximity ofMg2þ ions. The recovered oxygen caneventually migrate to the proximity of cerium centers, where it givesrise 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 indicatesthat oxygen diffuses into the bulk and gives rise to bulk reoxidation.We may speculate that in the mixed magnesia�ceria sample themobility of oxygen and, therefore, propensity for bulk reoxidation isfacilitated. This may be the origin of the improved thermal stabilityof the magnesia�ceria samples, in contrast to the pure ceria films,for which reoxidation is more efficient but mainly restricted to thesurface region.

4. CONCLUSIONS

Using SR-PES, RPES, and XPS, we have studied (i) thepreparation and electronic structure of different ceria andceria�magnesia model catalysts on Cu(111) and (ii) theirinteraction with CO2.1. Six different types of thin oxide films were investigated: (I)

a fully stoichiometric CeO2(111) film, (II) a reducedCeO2�x film prepared by methanol adsorption and anneal-ing, (III, IV) MgO layers of different thickness on ceriafilms, (V) reducedmixed ceria�magnesia films prepared byannealing, and (VI) fully oxidized mixed ceria�magnesiafilms prepared by annealing and postoxidation.

2. We have investigated the valence band structure of thedifferent samples, with a special focus on using resonantenhancement effects to monitor the concentration of Ce3þ

and Ce4þ in the surface region. It is found that PVD of Mgonto stoichiometric CeO2, even in the presence of an O2

atm, leads to immediate reduction of Ce4þ to Ce3þ andoxidation of magnesium. For larger amounts of depositedMg, exclusively Ce3þ is detected in the surface region.Upon annealing to 700 K, intermixing of the ceria layer andthe magnesia overlayer occurs. For the mixed oxide, onlyMg2þ and Ce3þ ions, but no Ce4þ, are present in the nearsurface region. By high-temperature oxidation (523 K),Ce3þ is quantitatively converted into Ce4þ.

3. The interaction of CO2 with the ceria and magnesia�ceriafilms was studied by SR-PES. At room temperature, no stablecarbon-containing species is observed on fully stoichiometricCeO2. On reduced CeO2�x films, two species are observedwhich are associated with surface carbonates and, tentatively,with surface carboxylates located at coordinatively unsaturatedCe3þ centers. On magnesia-containing samples, a secondsurface carbonate can be identified, which is located in the

vicinity of Mg2þ ions. Deposition of MgO, for both theintermixed and nonintermixed films, enhances the formationof surface carbonates. In contrast, the formation of the surfacecarboxylates is suppressed, even on intermixed and reducedsamples which expose Ce3þ ions at a high concentration.

4. Substantial reoxidation of reduced CeO2�x films occursupon extended exposure to CO2 at 300 K. Reoxidation ofceria is, however, hindered or even completely suppressedon ceria�magnesia samples. We conclude that the forma-tion of surface carboxylates is an intermediate reaction stepin CO2-induced reoxidation. Formation of surface carbo-nates, on the other hand, does not lead to reoxidation butactually hinders the formation of carboxylates and, there-fore, the reoxidation process.

5. Thermal stability of the ceria-based catalyst has beeninvestigated in the temperature interval 300�700 K. It isshown that oxygen originating fromCO2 activation is easilylost on pure ceria samples, whereas formation of mixedmagnesia�ceria oxides prevents thermal release of latticeoxygen.

’AUTHOR INFORMATION

Corresponding Author*E-mail: yaroslava.lykhach@chemie.uni-erlangen.de. Fax: þ49-9131-8528867.

’ACKNOWLEDGMENT

The authors gratefully acknowledge financial support by theDeutsche Forschungsgemeinschaft (DFG) within the ERA-CHEM program (“NanoFunC” project) and by the Ministry ofEducation of the Czech Republic (LA08022 and LC06065). Weacknowledge additional support from the DFG within theExcellence Cluster “Engineering of Advanced Materials” in theframework of the excellence initiative. We are also grateful foradditional support by the Fonds der Chemischen Industrie, theDAAD, and the European Union (COST D-41). Close coopera-tion withM. A. Schneider, L. Hammer, H.-P. Steinr€uck, A. Bayer,R. Streber, and M. P. A. Lorenz (Erlangen) is acknowledgedconcerning the characterization of the ceria films.

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