Ceria Reduction via Ce-Sn Bimetallic Bonding

Transcription

1 WDS'07 Proceedings of Contributed Papers, Part III, , ISBN MATFYZPRESS Ceria Reduction via Ce-Sn Bimetallic Bonding M. Škoda, M. Cabala, L. Sedláček, F. Šutara, V. Matolín Charles University, Faculty of Mathematics and Physics, Department of Surface and Plasma Physics, V Holešovičkách 2, Prague 8, Czech Republic. K. C. Prince and T. Skála Sincrotrone Trieste, Strada Statale 14, km 163.5, Basovizza-Trieste, Italy. V. Cháb Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnická 10, Prague 6, Czech Republic. Abstract. Tin was deposited in the amount of 0.4 and 0.8 nm onto CeO2(111) surface by molecular beam deposition at the temperature of 520 K. The interaction of tin with cerium oxide was studied with use of XPS, UPS and RPES (Resonant Photoelectron Spectroscopy). The strong tin ceria interaction led to the Ce(4-nx)+Snxn+O22- compound formation. It was accompanied with partial reduction of CeO2 observed as a giant 4f resonance enhancement of Ce3+ species referring to defects in ceria. CeO2 and SnO2 oxides were formed after oxygen treatment at 520 K. The strong Ce - Sn interaction and Sn Ce charge transfer that lead to a weakening of cerium oxygen bond and consequently to the formation of oxygen deficient active sites on the ceria surface can be a key of understanding the high catalytic activity of the SnOx/CeOx catalysts. Introduction Cerium oxide (ceria) has been a subject of increasing interest in recent years due to its technological importance in a wide field of various electronic and optical applications. Cerium oxide also constitutes important components of the catalysts used in commercially produced catalytic converters for CO oxidation, and have also shown significant activity in syngas reactions and hydrocarbon conversion. The CO oxidation activity has attracted wide interest due to the increasing number of applications, above all in the catalytic treatment of automobile exhaust emission. Ceria can be found in two stable stoichiometries: CeO 2 and Ce 2 O 3. The electronic structure of the CeO 2 dioxide is characterized by unoccupied 4f states of Ce 4+ (4f 0 ) whilst the Ce 2 O 3 trioxide has a Ce 3+ (4f 1 ) configuration [Fabris et al., 2005]. Different 4f configurations for Ce 4+ and Ce 3+ result in different core level and valence band (VB) structure. Photoelectron spectroscopy represents a powerful tool of Ce 4f state investigation. There are many spectroscopic data showing different 4f configurations using Ce 3d and Ce 4d core level and Ce VB spectra [Napetsching et al., 2004; Berner et al., 2002; Xiao et al., 2003, Fujimori, 1983; Mullins et al., 1998; Overbury et al., 1999] including resonant photoemission technique in the Ce 4d 4f photoabsorption region [Matsumoto et al., 1994; Shimada et al., 2002; Iwasaki et al., 2002; Witkowski et al. 1997; Kucherenko et al., 2002]. One of the important properties of ceria in terms of use in catalysis is named oxygen storage capacity (OSC), as it acts as an oxygen reservoir regulating the partial pressure of oxygen near the catalyst surface [Taylor, 1993; Trovarelli, 1996], which can provide oxygen to the gas mixture. The key factor for this property is the reversible transformation from Ce 4+ to Ce 3+ oxidation state resulting in a formation of oxygen vacancies on the cerium oxide surface. This transition corresponds in general to the crystal structure transition from the cubic fluorite lattice (Fm3m space group) of CeO 2 to the hexagonal lattice of Ce 2 O 3 [Berner et al., 2002; Xiao et al., 2003; Taylor, 1993]. The bulk Ce 2 O 3 has a hexagonal crystal structure (P-3m1) characterized by stacking of complete Ce and O layers with Ce 3+ O 2- Ce 3+ O 2- O 2- repeat in [0001] direction [Trovarelli, 1996]. However, Berner and Schierbaum [2002] proposed for Pt supported CeO 2 films the transformation of an O 2- Ce 4- O 2- layer sequence (CeO 2 ) into O 2-3/4 Ce 3+ O 2-3/4 (Ce 2 O 3 ) where O 2-3/4 indicates oxygen hexagonal layer if a quarter of O 2- ions is removed. The addition of a suitable metal atom to the cubic structure of ceria may promote reducible behavior, increase the OSC and consequently improve the catalytic activity for CO oxidation of the system, perhaps via the creation of active sites at the oxide metal boundary [Trovarelli et al., 1996; Boffa et al., 1994]. A number of studies of ceria-zirconia systems in order to increase the oxygen storage/relace capacity were done, for ex. [Gonzáles-Velasko et al., 1999; Fornasiero et al., 1996]. Ceria-zirconia solid solutions are reported to have three to five times the OSC than ceria-only systems [Hori et al., 1998]. 128

2 Mihaiu et al. [2006] dealt with the study of the thermal decomposition of tin and cerium precursors, in nonisothermal and isothermal conditions, in order to obtain Sn-Ce-O materials with different nominal Sn:Ce atomic ratio with use of simultaneous thermogravimetry and thermal analysis, powder X-ray diffraction, IR spectroscopy and BET surface area measurements. Preferential formation of the cubic fluorite-type structure for samples with Sn:Ce atomic ratio 1 could be evidence up to 600 C. Selective oxidation of CO in excess of hydrogen over CuO/Ce x Sn 1-x O 2 (1 x = ) mixed oxide catalysts was studied by Chen [2006]. This study showed a good activity and selectivity of the 7% CuO/Ce 0.9 Sn 0.1 O 2 catalyst. Shapovalov et al., [2007] presented density functional theory calculations for CeO2(111) surface doped with Au, Ag and Cu showing that the bond between the oxygen atoms and the oxide was weakened by presence of the dopant. This property can increase OSC of the catalyst. In order to understand these interactions at a fundamental level we have studied interaction with ceria overlayer prepared on single crystal surface Cu(111), which provides stable ceria overlayers [Šutara et al., to be published]. Experimental The experiments were carried out at the Materials Science Beamline (MSB) at the Elettra synchrotron light source in Trieste. The MSB is a bending magnet beamline using a plane grating monochromator with a tuning range from 40 to 900 ev. The ultra-high vacuum (UHV) experimental chamber was equipped with a multichannel 150 mm mean radius electron energy analyzer (Phoibos 150), rear view LEED optics, a dual Mg/Al X-ray source, Ce evaporation source and an ion gun. The photoelectron spectra were acquired at different photon energies: Al K-α (hν = ev) for Ce 3d and Cu 2p core levels and variable excitation energy in the interval ev for the resonant photoelectron spectroscopy of the Ce 4f states. The photoelectron spectra were taken at normal-emission with photons incident at 60 with respect to the surface normal. The sample was a copper crystal disc made be MaTeck GmbH of 8 mm diameter and 2 mm thickness, oriented to within 0.2 of the (111) plane. The cleaning procedure consisted of cycles of sputtering and flashing to 800 K in order to obtain sharp LEED pattern indicating a good crystalline surface quality. The surface cleanliness was checked by XPS. The Ce metal was deposited at constant deposition rate from an electron beam evaporator by using the Mo crucible onto the clean Cu(111) surface at 520 K in 5 x 10-7 mbar of O 2. The quantitative XPS analysis of Cu 2p intensity permitted to calibrate the deposition rate. Thickness of the CeO 2 film can be estimated from a relative decrease of the Cu 2p signal. Assuming the electron mean free path is 1.12 nm from the TPP formula [Tanuma et al., 1993] this gives the deposition rate of 0.03 nm per minute. For calculating the ceria film thickness in fractions of monolayer we considered 0.31 nm thickness of 1 ML of CeO 2 (taken for bulk cerium dioxide crystallographic data). Tin was deposited onto CeO 2 (111) surface by molecular beam evaporation at 520 K. The deposition rate was kept constant of 0.1 nm per minute. The deposition rate was calculated from a relative decrease of Cu 2p signal during deposition using formula [Tanuma et al., 1993] with assumption of the electron mean free path in CeO 2 of 1.5 nm. Results The CeO 2 (111) epitaxial films of thickness of 1.5 nm (5 ML of CeO 2 ) were grown on the Cu(111) substrate kept at 520 K in the oxygen pressure of 5 x 10-7 mbar. The preparation and characterization of the ceria layer are described in detail in [Šutara et al., to be published]. Disappearance of Cu(111) spots on LEED diffraction pattern indicated a total coverage of the copper substrate by the ceria layer. The LEED pattern corresponding to the (1.5 x 1.5) superstructure was interpreted as a formation of the continuous CeO 2 (111)/Cu(111) epitaxial overlayer [Šutara et al., to be published], analogous to Siokou and Nix [1999], who obtained continuous ceria films on Cu(111) substrate for 20 ML thick overlayers grown at room temperature. The electronic structure of CeO 2 is characterized by unoccupied 4f states of Ce 4+ (4f 0 ) whilst Ce 2 O 3 has a Ce 3+ (4f 1 ) configuration [Fabris et al., 2005]. A set of resonant photoelectron spectra in the Ce 4d 4f photoabsorption region at photon energies between 115 and 130 ev is presented in Fig. 1. At photon energy 115 ev there is no resonance. We recognize two resonant features located at binding energy (BE) 1.5 ev and 4.5 ev that reach their maxima at hν = 122 ev and ev, respectively. Valence band spectra obtained at these onresonance photon energies together with hν = 115 ev off-resonance excitation, are presented in Fig. 1. Tin in the amount of 0.8 nm was deposited onto CeO 2 (111) surface by molecular beam deposition. The substrate was kept at the temperature of 520 K. The interaction of tin with cerium oxide was studied with use of XPS, UPS and RPES. The RPES spectra obtained after tin deposition indicated a giant 4f resonance enhancement of the Ce 3+ species (maximum resonance at hν = 122 ev), see Fig

3 hν = 115 ev 122 ev ev Figure 1. Valence band spectra of the CeO2(111) thin film measured at different photon energies: 115 ev (offresonance); 122 ev (Ce,3+ resonance); (Ce4+ resonance). hν = 115 ev 122 ev ev Figure 2. Valence band spectra of tin deposited onto 5 ML CeO 2 (111) thin film measured at different photon energies: 115 ev (off-resonance); 122 ev (Ce 0, 3+ resonance); (Ce 4+ resonance). Ce 3d core level XPS spectra presented in Fig. 3, show considerable changes after tin deposition, first of all typical for partial reduction of CeO 2 [Overbury et al., 1999; Henderson et al., 2003]. The Ce 3d XPS spectra taken at different photoelectron escaping angles (with various surface sensitivity) showed that the Ce 4+ Ce 3+ transition occurred not only on the surface but also in the ceria film depth. The hypothesis of mixed Ce-Sn-O oxide formation is supported by high resolution Sn 4d spectrum taken at hν = 115 ev in Fig. 4, which consists of two doublets corresponding to tin metal and to a new chemical state giving a chemical shift to higher binding energy. This chemical state is not tin oxide as can be seen from Fig. 5, where the Sn 4d reference spectrum of SnO 2 measured at the same condition is presented. This reference SnO 2 sample was obtained by a radio-frequency (RF) oxygen plasma oxidation of the film presented in Fig. 4. This observation can be interpreted as mixed oxide formation caused by strong tin cerium oxide interaction. Because O 1s peak intensity does not change upon tin deposition, we can tentatively describe the mixed oxide as Ce (4-nx)+ Sn n+ x O 2-2 compound. Thus, the compound can be described as a product of the transition: 4+ 2 (4 nx) + n+ 2 Ce O Ce Sn O (1) 2 x 2 The quantitative XPS analysis shows that in this case x = 0.3. The comparison of high resolution Sn 4d spectrum of SnO 2 reference (Fig. 5) with spectrum for the Ce (4- nx)+ Sn x n+ O 2 2- compound, see Fig. 4, reveals that the Sn n+ intensity exhibits a narrower shape with higher binding energy. Evidently, the Sn electronic structure is different from SnO 2, probably due to strong hybridization of Ce 4f and Sn s,p orbitals which is characteristic for Ce x Sn y compounds [Holgado et al., 2005]. The compound formation should be accompanied by a charge transfer from Sn to unoccupied 4f 0 orbitals of Ce-O complex. This behavior is confirmed also by RPES results presented above. In order to study dependence of the Sn 4d spectrum on the quantity of deposited tin, half amount of 0.4 nm of tin was evaporated on clean CeO 2 (111) substrate. It can be clearly seen in Fig. 6b that the contribution of metallic tin is smaller than in Fig. 4 whilst the main part of the Sn 4d spectra corresponds to the state of the 130

4 formed compound. In order to investigate the mixed oxide reactivity towards oxygen adsorption, tin was heated at 520 K in oxygen atmosphere of 5 x 10-7 mbar for 5 min. Fig. 6c shows an increase of tin oxide character states and on the other hand an decrease of mixed oxide tin states after O 2 treatment. Metallic tin at 24 ev was completely oxidized. A great deal of the Sn-Ce bimetallic bonds was suppressed; CeO 2 and SnO 2 oxide were formed. Ce 3d 3/2 Ce 3d 5/ Figure 3. Ce 3d XPS spectra; 5 ML CeO 2 (111) film (dotted line), tin deposited onto CeO 2 (111) thin film (solid line). Sn hν = 115 ev Sn 4d 3/2 (Sn metal) Sn 4d 5/2 (Sn-Ce-O) Sn 4d Sn 4d 5/2 (Sn metal) 3/2 (Sn-Ce-O) Figure 4. Sn 4d spectra of tin deposited onto CeO 2 (111) thin film. hν = 115 ev Figure 5. Reference Sn 4d spectra of SnO 2 (111) thin film. O 1s spectra are presented in Figs. 6a,b,c. According to [Henderson et al., 2003] showing a trend of O 1s core level shift to higher binding energy after CeO 2 reduction, the peak component at ev in Fig. 7a,b,c can be associated to CeO 2, whilst the feature at 530 ev in Figs. 7b,c to surface reduced ceria Ce 2 O 3. The small intensity component at binding energy of ev on the main O 1s feature is typically assigned to water and CO 2 adsorption or to O 2- anions located near to oxygen vacancy sites [Henderson et al., 2003; Holgado et al., 2000]., The Ce 2 O 3 feature appeared after the deposition of tin (even in higher intensity than CeO 2 component - compare Figs. 7a and 7b), and the Ce 2 O 3 feature intensity remarkably fell after oxygen treatment. These facts confirm the hypothesis of reduction of ceria after tin deposition and oxidation of ceria after oxygen treatment. 131

5 Figure 6. Sn 4d spectra: tin deposited onto CeO 2 (111) thin film (dotted line) and after oxidation (solid line) (a); Sn 4d spectra deconvolution of tin deposited onto CeO 2 (111) thin film (b); Sn 4d spectra deconvolution of tin deposited onto CeO 2 (111) thin film after oxidation (c) Figure 7. O 1s XPS spectra of 5 ML CeO 2 (111) film (a); O 1s spectra of tin deposited onto CeO 2 (111) (b); O 1s spectra of tin deposited onto CeO 2 (111) after oxidation (c) Conclusion The model studies performed on a well defined Sn/CeO2(111)/Cu(111) system show that high catalytic activity of the SnOx/CeOx catalysts can be explained by strong Ce - Sn interaction accompanied by Sn Ce charge transfer that leads to weakening of cerium oxygen bond and consequently to the formation of oxygen deficient sites in the ceria film. Acknowledgements. This work is a part of the research programs No. MSM and LC06058 that are financed by the Ministry of Education of the Czech Republic. References Berner U., Schierbaum K., Cerium oxides and cerium-platinum surface alloys on Pt(111) single-crystal surfaces studied by scanning tunneling microscopy, Phys. Rev. B, 65, , Boffa A., Lin C., Bell A.T., Somorjai G.A., Promotion of CO and CO 2 Hydrogenation over Rh by Metal Oxides: The Influence of Oxide Lewis Acidity and Reducibility, J. Catal., 149, 149, Chen Y.-Z., Liaw B.-J., Huang C.-W., Selective oxidation of CO in excess hydrogen over CuO/Ce x Sn 1-x O 2 catalysts, Appl. Catal. A: General, 302, , Fabris S., de Gironcoli S., Baroni S., Vicario G., Balducci G., Taming multiple valency with density functionals: A case study of defective ceria, Phys. Rev. B, 71, (R), Fornasiero P., Balducci G., Di Monte R., Kaspar J., Sergo V., Gubitosa G., Ferrero A., Graziani M.J., Modification of Redox Behaviour of CeO 2 Induced by Structural Doping with ZrO 2, J. Catal., 164, 173, Fujimori A., Mixed-valent ground state of CeO 2, Phys. Rev. B, 28, 2281, Gonzáles-Velasko J.-R., Gutiérrez-Ortiz M.A., Marc J.-L, Botas J.A., Gonzáles-Marcos M.P., Blanchard G., Contibution of cerium/zirconium mixed oxides to the activity of a new generation of TWC, Appl. Catal. B. Environ., 22, 167, Goraus J., Slebarski A., Neumann M., Electronic structure of Ce x Sn y compouds, J. Of Alloys and Comp., 401, , Henderson M.A., Perkins C.L., Engelhard M.H., Thevuthasan S., Peden C.H.F., Redox properties of water on the oxidized and reduced surfaces of CeO 2 (111), Surf. Sci., 526, 1-18,

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