Morphologies and Electronic Structures of Calcium-Doped Ceria Model Catalysts and Their Interaction with CO2

WANG Yan, LI Xiong, HU Shanwei , XU Qian, JU Huanxin, ZHU Junfa

National Synchrotron Radiation Laboratory, Department of Chemical Physics, University of Science and Technology of China,Hefei 230029, P. R. China.

Abstract: CeO2-based catalysts are promising for use in various important chemical reactions involving CO2, such as the dry reforming of methane to produce synthesis gas and methanol. CeO2 has a superior ability to store and release oxygen, which can improve the catalytic performance by suppressing the formation of coke. Although the adsorption and activation behavior of CO2 on the CeO2 surface has been extensively investigated in recent years, the intermediate species formed from CO2 on ceria has not been clearly identified. The reactivity of the ceria surface to CO2 has been reported to be tuned by introducing CaO, which increases the number of basic sites for the ceria-based catalysts. However, the mechanism by which Ca2+ions affect CO2 decomposition is still debated. In this study, the morphologies and electronic properties of stoichiometric CeO2(111), partially reduced CeO2-x(111) (0 ＜ x ＜ 0.5), and calcium-doped ceria model catalysts, as well as their interactions with CO2, were investigated by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy, and synchrotron radiation photoemission spectroscopy. Stoichiometric CeO2(111) and partially reduced CeO2-x(111) films were epitaxially grown on a Cu(111) surface. STM images show that the stoichiometric CeO2 film exhibits large, flat terraces that completely cover the Cu(111) surface. The reduced CeO2-x film also has a flat surface and an ordered structure, but dark spaces are observed on the film. Different Ca-doped ceria films were prepared by physical vapor deposition of metallic Ca on CeO2(111) at room temperature and subsequent annealing to 600 or 800 K in ultrahigh vacuum. The different preparation procedures produce samples with various surface components, oxidation states, and structures. Our results indicate that the deposition of metallic Ca on CeO2 at room temperature leads to a partial reduction of Ce from the +4 to the +3 state, accompanied by the oxidation of Ca to Ca2+. Large CaO nanofilms are observed on CeO2 upon annealing to 600 K. However, small CaO nanoislands appear near the step edges and more Ca2+ ions migrate into the subsurface of CeO2 upon annealing to 800 K. In addition, different surface-adsorbed species are identified after CO2 adsorption on ceria(CeO2 and reduced CeO2-x) and Ca-doped ceria films. CO2 adsorption on the stoichiometric CeO2 and partially reduced CeO2-x surfaces leads to the formation of surface carboxylate. Moreover, the surface carboxylate species is more easily formed on reduced CeO2-x with enhanced thermal stability than on stoichiometric CeO2. On Ca-doped ceria films, the presence of Ca2+ ions is observed to be beneficial for CO2 adsorption; further, the carbonate species is identified.

Key Words: Calcium; Ceria; Scanning tunneling microscopy; X-ray photoelectron spectroscopy; Synchrotron radiation photoemission spectroscopy

1 Introduction

Carbon dioxide is of particular interests since it can be utilized in numerous chemical processes, such as dry reforming of methane (CH4+ CO2) to produce synthesis gas (CO + H2) and methanol synthesis1,2. However, the metal catalysts involving in these reactions undergo serious deactivation due to the coke formation3. The use of ceria-based catalysts can alter this situation. CeO2 has superior oxygen storage/release capacity,resulting in the self-cleaning properties of the catalysts that carbon is oxidized by the oxygen from the CeO2surface4. It is therefore understandable that the interaction between the CeO2surface and CO2has extensively been investigated in the literatures5–8. However, the intermediate species formed from CO2 on ceria is yet unclear. Moreover, it is reported that the reactivity of ceria surface to CO2 can be tuned by introducing CaO, which increases the basic sites for ceria-based catalysts9.It has been investigated that CO2could chemisorb at a regular surface site of CaO, leading to the formation of carbonates10–12.Although the presence of Ca improves the catalytic performance of ceria-based materials in reactions involving CO2, however,the mechanism of the effect of Ca2+ions on CO2 decomposition is still under debate. For example, Kang et al.13reported that the adding of Ca2+into ceria-based solid solutions could enhance the oxygen mobility which might account for the improved CO2splitting reactivity. However, the synergistic effect between Ca2+and Ce3+ions was identified on CaO-CeO2 based oxides to enhance the CO2 activation14. Moreover, the fundamental-level investigation of the interaction of CO2with CaO-CeO2surfaces is still lacking, which motivates our current study.

In this work, we investigated the morphologies and electronic structures of stoichiometric CeO2(111), reduced CeO2-x(111)(0 ＜ x ＜ 0.5), and Ca-doped CeO2 model catalysts and their interaction with CO2 by scanning tunneling microscopy (STM),X-ray photoelectron spectroscopy (XPS), and synchrotron radiation photoemission spectroscopy (SRPES). It is observed that CO2 adsorption on ceria (CeO2 and reduced CeO2-x) and Cadoped CeO2surfaces exhibits different adsorbed species. Cadoped CeO2films show different surface structures at different annealing temperatures, which results in different adsorption behaviors of CO2. Moreover, the presence of Ca in Ca-doped CeO2 films enhances the CO2 adsorption.

2 Experimental

The experiments were carried out in two separate ultrahigh vacuum (UHV) systems. STM measurements were performed in a system that comprises three chambers with base pressures all below 1 × 10-8Pa. The XPS and SRPES measurements were carried out on the Catalysis and Surface Science Endstation at BL11U beamline in the National Synchrotron Radiation Laboratory, Hefei. These two systems have been described in detail previously15,16. Briefly, the beamline is connected to an undulator and equipped with two gratings that cover photon energies from 20 to 600 eV with a resolution (E/ΔE) better than 104at 29 eV. All the STM images were recorded at room temperature with the bias of 3–4 V and tunneling current of 0.01–0.05 nA using an etched tungsten tip. The WSXM program17was used to process the STM images. Core level spectra of Ce 3d, Ca 2p, and O 1s were acquired with a monochromatic X-ray source using Al Kαradiation (1486.6 eV) (SAX 100, VG Scienta,Sweden). Moreover, Ca 2p and C 1s spectra were obtained with a photon energy of 420 eV, while valence band (VB) spectra were measured at hν = 115 eV. All spectra were measure at 0°with respect to the surface normal.

The CO2(99.999%, Wuhan Newradar Special Gas CO.,LTD.) adsorption was performed in 10 L doses (6.5 × 10-6Pa,260 s) at 180 K by backfilling the UHV chamber.

The Cu(111) single crystal (8 mm diameter and 2 mm thickness), purchased from Mateck GmbH, Germany, was used as the substrate. Cu(111) was cleaned by repeated cycles of Ar+sputtering and annealing until no contaminant could be found by XPS, and a sharp (1 × 1) LEED pattern was achieved. In this work, we prepared two ceria films, which were CeO2(111)/Cu(111) and reduced CeO2-x(111) (0 ＜ x ＜ 0.5), and three Ca-doped ceria films, including Ca/CeO2, Ca/CeO2-600 K and Ca/CeO2-800 K. The preparation procedures were as follows.

CeO2(111)/Cu(111). Stoichiometric CeO2(111) films were grown on a clean Cu(111) substrate at gradually increased substrate temperature by physical vapor deposition of Ce metal(99.9%, Alfa Aesar, USA) from an electron beam evaporator(EBE-4, Specs, Germany) in 4 × 10-5Pa oxygen (99.999%,Nanjing Shangyuan). After deposition, the films were postannealed at 850 K for 10 min in the same oxygen pressure. The monolayer (ML) of the ceria film is defined as O-Ce-O stack normal to the (111) plane with a thickness of 0.31 nm18.

CeO2-x(111) (0 ＜ x ＜ 0.5). The partially reduced CeO2-x(111)films were obtained at a lower oxygen pressure of 9 × 10-6Pa during the Ce deposition followed by annealing in UHV. The ceria films were 2–4 nm thick.

Ca/CeO2. The sample of Ca/CeO2was prepared by deposition of 1.2 ML Ca on CeO2(111) at room temperature. Ca (99.5%,Alfa Aesar, America) was physical vapor deposited on the CeO2surface from a homemade evaporator with the evaporation rate of 0.2 ML·min-1. The Ca flux was calibrated with a quartz crystal microbalance (QCM) (TMC 13, Prevac, Poland) before deposition. One ML of Ca is defined as 7.9 × 1014atoms·cm-2,which is the number of oxygen atoms per unit area in the topmost atomic layer of the CeO2(111) surface19.

Ca/CeO2-600 K and Ca/CeO2-800 K. The samples, Ca/CeO2-600 K and Ca/CeO2-800 K, were obtained by heating Ca/CeO2to 600 and 800 K in UHV, respectively.

3 Results and discussion

3.1 Characterization of Ceria-Based Model Catalysts

Firstly, we studied the surface morphologies and electronic structures of ceria-based catalysts. STM images of all samples are displayed in Fig. 1. As shown in Fig. 1a, the stoichiometric CeO2film exhibits large and flat terraces that completely cover the Cu(111) surface. The apparent height of CeO2islands is measured to be 0.30 nm, in agreement with the spacing of O-Ce-O trilayers in the fluorite bulk CeO2(111) structure (0.31 nm)18.As displayed in the atomic-scale resolution STM image (inset of Fig. 1a), CeO2displays a well-ordered atomic structure. The measured distance between two adjacent Ce atoms is 0.39 nm,which is consistent with 0.383 nm, the expected surface spacing for Ce-Ce in the bulk CeO2(111)20. Reduced CeO2-xfilm also has a flat surface and an ordered structure, however, dark spaces are observed on the surface (Fig. 1b). The nature of dark spaces is attributed to the surface defects, which are associated with the missing lattice oxygen atoms or holes18,20–22. After the CeO2surface was deposited 1.2 ML Ca at room temperature, the CeO2surface is fully covered by Ca islands, as seen in Fig. 1c. The line scan profile shown in the inset of Fig. 1c suggests that the heights of deposited Ca islands are less than 2.5 ? (1 ? = 0.1 nm). Fig.1d shows the morphological information of Ca/CeO2-600 K.Heating the room temperature deposited Ca on CeO2to 600 K leads to the aggregation of small Ca islands to large area Ca islands and part of CeO2substrate is exposed. As seen in the inset of Fig. 1d, a line scan profile across a large Ca island indicates that the Ca island is more than 10.0 ? high. On Ca/CeO2-800 K(Fig. 1e), more exposed CeO2is observed and small Ca islands are accumulated near terrace step edges. The line scan profile shows that the heights of Ca islands decrease to less than 2.0 ?(inset of Fig. 1e). Compared with that on Ca/CeO2-600 K, the amount of Ca nanoparticles on the surface of Ca/CeO2-800 K decreases. Furthermore, oxygen vacancies are observed on Ca/CeO2-800 K (Fig. 1f).

Fig. 1 STM images (130 nm × 130 nm) of samples: (a) CeO2(111), (b) CeO2-x(111), (c) Ca/CeO2, (d) Ca/CeO2-600 K, and (e) Ca/CeO2-800 K.The insets of (a) and (b) show the high-resolution STM images (4.8 nm × 4.8 nm) of CeO2(111) and CeO2-x(111), respectively. Panels (c)–(e) include the line scans along the corresponding Ca islands on ceria. (f) Magnified STM image (6.5 nm × 6.5 nm) of the black framed region in panel (e),showing the detailed structure of the CeO2 substrate for Ca/CeO2-800 K.

To identify the electronic structures of the ceria-based model catalysts, high resolution core-level spectroscopies were investigated. Fig. 2a–c display the Ce 3d and Ca 2p core-level spectra of the studied samples. As shown in Fig. 2a, six peaks are identified in the Ce 3d spectrum of the CeO2(111) film, which correspond to three pairs of spin-orbit split doublets arising from photoemission from Ce4+ions23,24. In the Ce 3d spectrum of reduced CeO2-x, four more peaks emerge and are labeled as u0,u’, v0, and v’, which are typical peaks for Ce3+ions23. By fitting the Ce 3d spectrum based on the procedure introduced by Skala25,26, the percentage of Ce3+in CeO2-xcan be determined,which is 0.259 (Fig. 2d). Thus this CeO2-xfilm is assigned to CeO1.87. The increase of Ce reduction in ceria suggested by XPS agrees well with the increase in the number of oxygen vacancies observed by STM.

On Ca/CeO2, the typical peaks for Ce3+are also visible in the Ce 3d spectrum. After fitting this Ce 3d spectrum, the fraction of Ce3+in Ca/CeO2is calculated to be 0.279 (Fig. 2d). This indicates that partial reduction of ceria occurs upon Ca deposition, which matches with the Ca 2p XPS study shown in Fig. 2b, c. As seen in Fig. 2b, the Ca 2p spectrum of Ca/CeO2can be easily fitted with one pair of spin-orbit split doublet that is 3.5 eV apart. The Ca 2p3/2at 346.9 eV is compatible with the reported values for CaO27-29, suggesting that Ca is oxidized to Ca2+on CeO2. This process can be depicted as Ca + 2CeO2→Ce2O3+ CaO, which is expected from thermodynamics. Based on ΔHf298of oxide formation energy30, this process is favored by 253 kJ·mol-1at room temperature. Therefore, the Ca islands on CeO2observed by STM image (Fig. 1c) are attributed to CaO nanoislands. However, CaO film is assembled by the repeating O-Ca-O trilayer31,32, and a complete O-Ca-O trilayer is 4.8 ? in height31, which is larger than the thickness of Ca islands (less than 2.5 ?). Therefore, the CaO nanoislands are proposed to be Ca terminated, forming a Ce-O-Ca mixed layer at the Ca-CeO2interface. This phenomenon is similar to the case of Zr16,33, Al34,Sn25,35, and Mn36on CeO2(111).

Compared with those from Ca/CeO2, the characteristic peaks related to Ce3+in the Ce 3d region of Ca/CeO2-600 K are attenuated, while those due to Ce4+are intensified (Fig. 2a).Although the Ca 2p peak of Ca/CeO2-600 K shifts to a lower binding energy (Fig. 2b), it still can be assigned to Ca2+ions,which bond to O2-. This shift may be attributed to the size and structure changes, as seen in STM images (Fig. 1c, d), after annealing. As mentioned above, the thickness of large Ca islands is two times higher than that of an O-Ca-O trilayer (Fig. 1d),suggesting the formation of CaO nanofilms with at least two layers thick. The oxygen in these CaO nanofilms should come from the lattice oxygen of CeO2. However, compared to Ca/CeO2, the Ce3+fraction in Ca/CeO2-600 K even decreases about ～14% (from 0.28 to 0.244) (Fig. 2d). Similar phenomena were observed on Pt-ceria37and W-ceria38interface upon heating, where the decrease of Ce3+concentration was attributed to the Ce3+migration into deeper layers37or the compensation of surface oxygen vacancies by bulk oxygen38. The fact that the heights of CaO nanofilms on Ca/CeO2-600 K are higher than those of CaO nanoislands on Ca/CeO2suggests that oxygen atoms diffuse from the bulk to the surface.

Fig. 2 Core level spectra of (a) Ce 3d and (b, c) Ca 2p from samples: CeO2, CeO2-x, Ca/CeO2, Ca/CeO2-600 K, and Ca/CeO2-800 K.(a) and (b) are collected with a photon energy of 1486.6 eV. (c) is acquired with a photon energy of 420 eV. (d) The values of Ce3+/(Ce3+ + Ce4+) in samples: CeO2-x(111), Ca/CeO2, Ca/CeO2-600 K, and Ca/CeO2-800 K. (e) The integrated Ca 2p intensities of samples: Ca/CeO2, Ca/CeO2-600 K, and Ca/CeO2-800 K collected at photon energies of 1486.6 and 420 eV.

On the Ca/CeO2-800 K sample, the Ce 3d intensity increases compared with that of Ca/CeO2-600 K. Moreover, Ce3+concentration in Ca/CeO2-800 K increases to 0.263 (Fig. 2d),suggesting that the CeO2substrate is more reduced. These results match well with the increase in the area of exposed ceria and surface defects indicated by STM images (Fig. 1e, f). On Ca/CeO2-800 K, the Ca 2p peak is again assigned to Ca2+. The Ca 2p intensity decreases compared with that of Ca/CeO2and Ca/CeO2-600 K (Fig. 2b, c), which is related to the decrease in the Ca amount on the CeO2surface observed by STM (Fig. 1e).In addition, the decrease extent is varied between the Ca 2p intensity collected at 1486.6 eV and that collected at 420 eV.

Fig. 2e displays the Ca 2p intensities of Ca-doped samples collected at photon energies of 1486.6 and 420 eV. All the Ca 2p intensities were normalized to the corresponding Ca 2p intensities of Ca/CeO2. As can be seen, the Ca 2p intensities collected at 1486.6 eV decrease to 0.993 on Ca/CeO2-600 K and 0.710 on Ca/CeO2-800 K, respectively. While the Ca 2p intensities collected at 420 eV decrease to 0.965 on Ca/CeO2-600 K and 0.543 on Ca/CeO2-800 K, respectively. It is obvious that the Ca 2p intensities collected at 420 eV decrease faster compared with those collected at 1486.6 eV. Given the fact that Ca 2p spectra collected at 420 eV are more surface sensitive than those collected at 1486.6 eV, this result is ascribed to the oxygen migration from ceria to the top of Ca and the diffusion of Ca into ceria for Ca/CeO2-600 K and Ca/CeO2-800 K. Particularly, after annealing the Ca/CeO2sample to 800 K, more Ca diffuses into the subsurface of CeO2to form mixed CeO2-CaO oxide, which matches with the STM images (Fig. 1) and the observed intensity increase of Ce 3d.

3.2 Interaction of CO2

Next we investigate the interaction of CO2with stoichiometric CeO2(111), reduced CeO2-x(111), Ca/CeO2-600 K, and Ca/CeO2-800 K. These samples were exposed to 10 L CO2at 180 K and stepwise heated to higher temperatures in UHV. Fig.3 illustrates the development of the C 1s spectra from CeO2,CeO2-x, Ca/CeO2-600 K, and Ca/CeO2-800 K exposed with 10 L CO2at 180 K and subsequent heating to different temperatures.As shown in Fig. 3a and 3b, only one peak at 289.5 eV emerges in the C 1s spectra after CO2exposed on CeO2and CeO2-xat 180 K. However, compared with CeO2, the intensity of C 1s is stronger on CeO2-x, suggesting much more CO2can be adsorbed on the reduced ceria, in agreement with our previous study using IRAS5. Although the formation of surface carbonate ()6,39and carboxylate ()40after CO2adsorption on ceria has been both reported in the literature, using IRAS we have confirmed the formation of carboxylate located over oxygen vacancies5.Therefore, we attribute the C 1s peak at 289.5 eV to the surface carboxylate. Even on the stoichiometric CeO2(111), very small amount of oxygen vacancies exists, as demonstrated by angledependent XPS21. Therefore, it is understandable that on the reduced ceria surface, the amount of adsorbed CO2increases.Upon annealing, the C 1s signal totally disappears at 260 K on CeO2(111), indicating the complete desorption of CO2from the surface. However, on partially reduced CeO2-x(111), the C 1s peak disappears until the temperature increases to 500 K. This suggests that carboxylates are more stable on reduced CeO2-xthan on the stoichiometric CeO2surface.

Fig. 3 C 1s spectra obtained from samples: (a) CeO2, (b) CeO2-x, (c)Ca/CeO2-600 K, and (d) Ca/CeO2-800 K upon 10 L CO2 adsorption at 180 K and subsequent heating to higher temperatures.

Similar to the observations on ceria surfaces, after 10 L CO2adsorption on Ca-doped ceria (i.e., Ca/CeO2-600 K and Ca/CeO2-800 K) at 180 K, also only one peak is observed in the C 1s region. As shown in Fig. 3c and 3d, the C 1s peak locates at 290.4 and 290.2 eV on Ca/CeO2-600 K and Ca/CeO2-800 K,respectively. According to the literature, carbonate species were observed upon CO2adsorption on CaO11,12,41and magnesiaceria model catalysts39,42. Moreover, carbonate was identified at 290 eV in C 1s spectra on Mg-doped ceria42. Based on the above discussion, this peak is attributed to the carbonate species.However, it should be mentioned that the nature of the surface species needs more straightforward experimental evidences such as IRAS or theoretical calculations. Nevertheless, since the binding energies of C 1s spectra from calcium carbonates are reported to be 289.7–291.9 eV43–46, we ascribe the peaks at 290.4 and 290.2 eV to CO32-formed near Ca2+ions. After heating Ca/CeO2-600 K to 500 K, the C 1s intensity slightly decreases,with no significant shift in binding energy. Further increasing the temperature to 700 K leads to a sharp decrease in the C 1s intensity and a shift of the peak position towards a lower binding energy of 290.2 eV. On Ca/CeO2-800 K, between 260 and 500 K,the intensity of the peak associated with CO32-species decreases monotonically and the binding energy shifts from 290.2 eV(180 K) to 290.3 eV (500 K). Up to 700 K, the peak vanishes in the C 1s region.

Fig. 4 O 1s spectra obtained from samples: (a) CeO2-x, (b) Ca/CeO2-600 K, and (c) Ca/CeO2-800 K upon 10 L CO2 adsorption at 180 K and subsequent heating to higher temperatures. The O 1s spectra of CeO2-x,Ca/CeO2-600 K, and Ca/CeO2-800 K are plotted as top curves in (a)-(c),respectively. (d) Integrated C 1s intensities of CeO2, CeO2-x, Ca/CeO2-600 K, and Ca/CeO2-800 K (black) and the intensity ratios of O 1s peak γ to peak α of CeO2-x, Ca/CeO2-600 K, and Ca/CeO2-800 K (red) with 10 L CO2 adsorption at 180 K as a function of temperature.

Fig. 4a–c show the O 1s spectra of CeO2-x, Ca/CeO2-600 K,and Ca/CeO2-800 K with CO2adsorption at 180 K and subsequent annealing to different temperatures, respectively.The O 1s spectra from CeO2-x, Ca/CeO2-600 K, and Ca/CeO2-800 K are shown at the top of each figure. As shown in Fig. 4a,the main peak obtained from CeO2-xat 530.0 eV (peak α)appears due to lattice oxygen23. A second component (peak β)emerges at 531.4 eV, which is attributed to O2-near Ce3+23.Exposing CO2to CeO2-xat 180 K triggers the attenuation of peak α and the emergence of a new peak at 531.8 eV (peak γ). As the binding energy of peak γ is consistent with the reported values of carboxylate5, we assign peak γ to the CO2-species. Upon heating, the intensity of peak γ diminishes monotonically and the peak α increases. After the temperature increases to 500 K, the O 1s spectrum is similar to that of pure CeO2-x, suggesting the CO2desorption from CeO2-x.As shown in Fig. 4b, one feature at 530.0 eV (peak α) appears on Ca/CeO2-600 K. This feature is associated with lattice oxygen of ceria. However, according to the literature47, the O 1s peak of CaO was reported to be at 530.2 eV, which is close to peak of lattice oxygen of ceria. Thus, we cannot exclude the contribution of O2-in CaO to peak α. Upon CO2adsorption at 180 K, a peak located at 533.0 eV (peak γ) emerges, which is related tonear Ca2+46. Accordingly, the peak α decreases. Upon annealing,the intensity of carbonate O peak (peak γ) slightly decreases between 180 K and 500 K and then sharply decreases up to 700 K. Similarly, after CO2adsorption on Ca/CeO2-800 K at 180 K(Fig. 4c), a new feature associated withnear Ca2+(peak γ)emerges at 532.2 eV. The different binding energies of carbonate O peaks for Ca/CeO2-600 K and Ca/CeO2-800 K are related to the adsorption of CO2on CaO and CeO2-CaO mixed oxide.Similar results have been reported for CO2adsorption on MgO/CeO2model catalysts42. Increasing annealing temperature leads to the decrease in the intensity of carbonate O peak. At 500 K, the intensity of the main peak at 530.0 eV (peak α) recovers to that of clean Ca/CeO2-800 K. The variations of peak α intensities of the studied samples upon CO2adsorption and subsequent annealing are due to the formation and decomposition of carboxylates on different ceria or carbonates on Ca-doped ceria.

In Fig. 4d, the C 1s and O 1s intensities for CeO2, CeO2-x,Ca/CeO2-600 K, and Ca/CeO2-800 K exposed with 10 L CO2are plotted as a function of temperature. It is seen that the intensities of C 1s after CO2adsorption formed on the four samples follows the order of Ca/CeO2-600 K ＞ Ca/CeO2-800 K ＞ CeO2-x＞ CeO2,suggesting that the doping of calcium in CeO2enhances the CO2adsorption. Upon heating, all C 1s intensities start to decrease,indicating that the surface carboxylates or carbonates start to decompose. Moreover, on ceria surfaces, the C 1s intensities decrease much faster upon annealing than those on Ca-doped ceria surfaces, indicating that the surface carboxylates are less stable than carbonates.

The Ca 2p spectra of Ca/CeO2-600 K and Ca/CeO2-800 K upon CO2adsorption at 180 K and subsequent annealing are also investigated. As shown in Fig. 5, after CO2exposed on Ca/CeO2-600 K and Ca/CeO2-800 K, the Ca 2p spectra are broad and consist of two pairs of spin-orbit split doublets that are 3.5 eV apart. The main Ca 2p3/2peak on both samples at 346.5 eV is assigned to Ca2+in Ca-doped ceria. The shoulders at higher binding energies of 347.9 eV on Ca/CeO2-600 K and 347.3 eV on Ca/CeO2-800 K are ascribed to Ca2+in CaCO329,48–50. This indicates that Ca2+ions on the Ca/CeO2-600 K and Ca/CeO2-800 K surfaces react with CO2to form CaCO3. Considering that more Ca2+diffuses into ceria in the sample of Ca/CeO2-800 K, the CaCO3-related Ca 2p peak is much weaker compared with that observed on Ca/CeO2-600 K. Upon annealing, the Ca 2p peaks related to CaCO3on both samples undergo slow decrease up to 500 K, followed by fast decrease. After 700 K annealing, almost no CaCO3-related Ca 2p peaks can be observed, suggesting that the CaCO3species completely decomposes. These results agree well with the C 1s and O 1s studies (Figs. 3, 4).

Fig. 5 Ca 2p spectra of (a) Ca/CeO2-600 K and (b) Ca/CeO2-800 K upon 10 L CO2 adsorption at 180 K and subsequent heating to higher temperatures.

Fig. 6 VB spectra obtained from samples: (a) CeO2, (b) CeO2-x, (c)Ca/CeO2-600 K, and (d) Ca/CeO2-800 K upon 10 L CO2 adsorption at 180 K and subsequent heating to higher temperatures. The VB of CeO2, CeO2-x, Ca/CeO2-600 K, and Ca/CeO2-800 K are plotted as top curves in (a)–(d), respectively.

Fig. 6a–d display the VB spectra from CeO2, CeO2-x,Ca/CeO2-600 K, and Ca/CeO2-800 K exposed with 10 L CO2at 180 K and subsequent annealing to different temperatures. The spectra of the studied samples are plotted as top curves in each figure. Note that the presented spectra were obtained with a photon energy of 115 eV, corresponding to the off-resonance condition51. The principle features of the VB spectrum measured in off-resonance mode from ceria have been discussed by Mullins et al.23. The authors attributed two broad features at 4.5 and 6.5 eV to the O 2p orbitals hybridized with Ce 4f and 5d orbitals, respectively. Moreover, a feature is observed at 2–3 eV in the VBs of the studied samples. Matolín et al. reported that the features between 0.1 and 4.5 eV in the VB of continuous CeO2(111)/Cu(111) films were assigned to Ce 5d, 6s, 4f states of CeO252,53. For CaO, no feature at 2–3 eV is reported to appear in the VB spectrum54. Thus, the feature at 2–3 eV is related to the ceria films grown on Cu(111), which was also observed in some literatures when studying the VBs of ceria films grown on Cu(111)25,34,35,39,52,53. Additional features appear in the VBs of CeO2and CeO2-xupon CO2exposure at 180 K. On CeO2, a feature appears at 9.7 eV (Fig. 6a); on CeO2-x, two features appear at 10.1 eV and 12.2 eV, respectively (Fig. 6b). According to the literatures55,56, they are related to the carboxylate species.After heating CeO2to 260 K, CO2desorbs from the surface,leading to the disappearance of the feature at 9.7 eV, in agreement with the observation of C 1s spectra shown in Fig. 3a.While the two features on CeO2-xgradually decrease upon annealing and completely disappear up to 500 K, indicating thatis much more stable on reduced CeO2-x.

On Ca/CeO2-600 K (Fig. 6c) and Ca/CeO2-800 K (Fig. 6d),the broad feature at 5.0 eV in the VBs is attributed to O 2p from oxygen species involved in Ca―O bonds46. The adsorption of CO2on both surfaces result in the appearance of three additional features: 6.5, 11.2 and 13.4 eV on Ca/CeO2-600 K and 6.2, 10.6 and 12.8 eV on Ca/CeO2-800 K; they are ascribed to the (1a2’;1e’; 4e’), (3e’; 1a2’), and 4a1’ molecular orbitals of CO32-,respectively54,57. Moreover, the evolution of these peaks with annealing temperatures agree well with those observations in the C 1s, O 1s and Ca 2p studies (Fig. 3–5). These results further support the formation of carboxylates on CeO2and reduced CeO2-xand carbonates on Ca-doped ceria upon CO2adsorption.Moreover, the carboxylates formed on ceria are less stable than carbonates formed on Ca-doped CeO2samples. In addition, the carbonates on Ca/CeO2-800 K exhibit a lower thermal stability compared with those on Ca/CeO2-600 K.

4 Conclusions

We investigate the morphologies and electronic structures of stoichiometric CeO2(111), partially reduced CeO2-x(111) (0 ＜ x ＜0.5), and Ca-doped ceria model catalysts and their interaction with CO2using STM, XPS, and SRPES. Deposition of 1.2 ML Ca on CeO2(111) leads to the immediate reduction of Ce4+to Ce3+and oxidation of metallic Ca to Ca2+. Upon heating to 600 K, large CaO nanofilms are observed on CeO2. Further increasing the temperature to 800 K, small CaO islands appear near the step edges and more Ca2+ions migrate into the subsurface of CeO2. CO2adsorption on both stoichiometric CeO2and reduced CeO2-xfilms at 180 K leads to the formation of surface carboxylate species. Moreover, the reduction of ceria enhances the CO2adsorption. On Ca-doped ceria films, CaCO3is identified on the surface. The presence of Ca2+located on the ceria surface is observed to enhance the CO2adsorption.