Structure of the Rh2O3(0001) surface

Surface Science 606 (2012) 1416–1421

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Structure of the Rh2O3(0001) surface S. Blomberg a,⁎, E. Lundgren a, R. Westerström a, E. Erdogan a, N.M. Martin a, A. Mikkelsen a, J.N. Andersen a, F. Mittendorfer b, c, J. Gustafson a a b c

Division of Synchrotron Radiation Research, Lund University, Box 118, SE-221 00, Sweden Institute of Applied Physics, Vienna University of Technology, Austria Center for Computational Materials Science, Vienna University of Technology, Austria

a r t i c l e

i n f o

Article history: Received 29 February 2012 Accepted 4 May 2012 Available online 14 May 2012 Keywords: HRCLS XPS STM LEED Rh(111) Rh2O3

a b s t r a c t We have studied the (0001) surface termination of Rh2O3 on a Rh(111) single crystal using a combination of high resolution core level spectroscopy, low energy electron diffraction, scanning tunneling microscopy and density functional theory. By exposing the Rh(111) to atomic oxygen we are able to grow Rh2O3 layers exposing the (0001) surface. The experiments support the theoretical predictions stating that the surface is terminated with an O–Rh–O trilayer yielding a RhO2 termination instead of a bulk Rh2O3 termination. The structural details as found by the DFT calculations are presented and reasons for the previously observed strong differences in catalytic activity between the structurally similar RhO2 surface oxide, and the Rh2O3(0001) surface are discussed. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Driven mainly by its importance for catalysis, the interaction between Pt-group metals and different gasses has been extensively studied within the field of surface science. The result is a deep understanding of the adsorption processes as well as the catalytic reactions between co-adsorbed molecules. Traditionally, however, these studies are performed on very simplified model systems, such as perfect single crystal surfaces in UHV compatible pressures. Such conditions are significantly different from industrial catalysts that generally consist of nanoparticles of the active metal, dispersed inside a porous oxide support, and function at pressures of 1 bar or above. Consequently, much effort has been devoted recently to bridge these so-called pressure and material gaps between surface science measurements and industrial catalysis [1,2]. Due to its relative simplicity, in combination with industrial relevance, one of the most studied reactions in this context is the oxidation of CO through 2CO+O2 →2CO2. Although this reaction is highly exothermic, it is very slow without a catalyst due to the required three-body interaction [3]. When catalyzed, the reaction is most often described using the Langmuir–Hinshelwood mechanism, where O2 adsorbs dissociatively on a suitable metal surface and is available for reaction with a single coadsorbed CO molecule. If, however, the surface is to a large extent covered by CO, there are not enough sites available for the dissociative O2 adsorption, and the catalyst is CO poisoned. Similarly, in most cases, CO cannot

⁎ Corresponding author. Tel.:+46 46 222 87 15; fax: + 46 46 222 42 21. E-mail address: [email protected] (S. Blomberg). 0039-6028/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2012.05.004

adsorb on an oxidized surface, which leads to oxygen poisoning. Since CO should not dissociate upon adsorption, a layer of chemisorbed oxygen usually leaves enough space for CO to adsorb, yielding an active surface [4,5]. These findings led to the general belief that the active phase of a CO oxidation catalyst is a metallic oxygen dominated surface [6–8]. Several recent studies have shown that, under more realistic conditions, a thin oxide film is formed on Ru, Pt, Pd and Rh when the catalysts switch from low (CO poisoned) to high activity [9–18]. Although there is still a debate concerning these observations, it is clear that the oxidation and reduction of these surfaces under more realistic conditions are of major importance for understanding CO oxidation catalysis on a fundamental level [2,19–27]. Previous oxidation studies have shown that a RhO2 surface oxide with a trilayer stacking is formed on all Rh surfaces independently on the structure [28,29]. This is true also for the Rh nanoparticles [30], vicinal surfaces [31] and PtRh alloys [27]. In the case of Rh, it has previously been observed that while the RhO2 surface oxide is active in CO oxidation, the Rh2O3 bulk oxide poisons the reaction [8,27,32]. In this paper we report on a combined high resolution core level spectroscopy (HRCLS), low energy electron diffraction (LEED), scanning tunneling microscopy (STM) and density functional theory (DFT) study of the oxidation of the Rh(111) surface using atomic oxygen. We show that we can follow the formation of an ultra-thin RhO2 surface oxide, as well as the subsequent bulk oxidation at higher temperatures. DFT calculations are used to simulate the experimental findings and we confirm previous predictions [33] of the formation of a trilayer terminated Rh2O3(0001) bulk like structure on the Rh(111) surface. The reasons for the difference in catalytic activity between the RhO2 surface

S. Blomberg et al. / Surface Science 606 (2012) 1416–1421

oxide and the trilayer terminated Rh2O3(0001) surface, despite their structural similarities, are discussed. 2. Experimental and computational setup The Rh(111) single crystal was cleaned in a standard way by cycles of Ar + sputtering and subsequent annealing. By annealing the crystals in an O2 pressure of 10− 7 mbar, carbon contaminations were removed and any remaining oxygen was removed by flashing to 800 °C in UHV. The clean surface was confirmed by XPS. To oxidize the surface an MBE-Komonenten Gmbh Oxygen Atom Beam Source OBS-40 was used. At a temperature of 1800 °C an oxygen flux of 10 15 atoms/s was generated, which was still below the limit for sample contamination. This was checked with an overview spectrum when the HRCLS measurements where performed. The optimized condition for the oxygen source was also used for the STM and LEED studies. However, in order to obtain domains large enough to observe a LEED pattern and to perform STM, the sample was post annealed at 560 °C. The HRCLS measurements were carried out at a beamline I311 at MAX-lab in Lund, Sweden. The Rh 3d5/2, C 1s and O 1s core levels were probed and the spectra were deconvoluted using the Doniach– Šunjić lineshape [30] convoluted with a Gaussian lineshape. The Gaussian FWHM was varied due to the different photon energies used. A linear background was subtracted from the spectra and the calibration of the spectra was done using the Fermi edge. The STM images were recorded using a commercial Omicron STM1, operated at room temperature in Lund, Sweden. The STM is positioned inside an UHV system with a base pressure of 10 − 10 mbar. STM tips used in these experiments were all chemically etched tungsten tips. All images shown were recorded in a constant current mode. The calculations were performed with the Vienna ab-initio simulation package (VASP) [34,35] using the PBE exchange correlation

a

1417

functional [36], PAW potentials [37,38] and a cut-off energy of 400 eV. The core level shifts were determined in the final state approximation [39] in a cell containing 108 Rh atoms and 189 O atoms, corresponding to a Rh2O3 supercell repeated three times in both directions of the surface unit cell. A 3 × 3 × 1 k-point mesh was used for the integration of the Brillouine zone. 3. Results and discussion The oxidation of Rh(111) using atomic oxygen is followed by HRCLS in Fig 1. The bottom spectra in Fig. 1(a) and (b) show the clean Rh(111) surface where a negative core level shift in Rh 3d5/2, relative to the Rh bulk at 307.1 eV, is characteristic for the clean Rh(111) surface [28,33]. When the crystal is exposed to atomic O at a pressure of 5·10 − 7 mbar and a sample temperature of 450 °C for 10 min an ultra-thin trilayer surface oxide is formed. It consists of two oxygen layers separated by a single layer of Rh [28], as shown in Fig. 1(c). In XPS, the surface oxide is identified by two components in the O 1s spectrum separated by 0.91 eV, corresponding to the two O layers. As reported in [28], the peak at the lower binding energy corresponds to the surface layer and the one at the higher binding energy originates from the oxygen layer below the surface. The small component at about 531 eV, is probably due to H adsorption from the residual gas, forming OH groups on surface. In the Rh 3d5/2 spectrum a component shift of +0.75 eV relative to the bulk is attributed to the Rh layer within the oxide film. The other new component, at −0.25 eV, corresponds to the interface layer between the metal bulk and the surface oxide. Dosing atomic oxygen for 60 min with the same cracker parameters described above but now at a slightly lower sample temperature (380 °C), results in a shoulder developing on the high-energy side of the surface oxide peak in Rh 3d5/2 (upper spectra and model in Fig. 1). Simultaneously a significant decrease of the metal bulk signal is observed, indicating that the surface is covered by a thicker oxide

b O 1s hν=650 eV

Rh 3d5/2 hν=390eV

c thin bulkoxide

bulk

hν=402eV

surface oxide

hν=402eV

metal surface

533

531

529

527

309

308

307

306

Binding Energy (eV) Fig. 1. UHV XPS spectra of the oxidation of Rh(111) using atomic oxygen. The O 1s spectra are shown in a) and Rh 3d5/2 in b). The bottom spectra measured at 10− 10 mbar at LN temperature show a clean Rh(111) crystal where the negative shifted surface component is observed in the Rh 3d5/2 spectrum. When the crystal is exposed to an atomic oxygen pressure of 5·10− 7 mbar in 10 min at a sample temperature of 450 °C, a trilayer surface oxide is observed (middle spectra). The top spectra show a thin Rh2O3(0001) oxide formed when the sample is exposed to 5 · 10− 7 mbar in 60 min at 380 °C. c) Models of the clean (111) crystal surface, the trilayer surface oxide and the bulk oxide. Red balls represent oxygen and the blue ones represent Rh.

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component denoted to the oxide surface. This confirms our assignment that the high binding energy component in the O 1s is due to electrons emitted from the bulk of the Rh2O3. In the case of the Rh 3d5/2 level, both the component assigned to the bulk oxide and the component assigned to the surface of the Rh2O3 decrease relative to the metal bulk, as the kinetic energy of the photoelectrons increase. However, the decrease is observed to be more pronounced in the surface component than in the bulk component, again confirming the original assignment based on the growth of the oxide as observed in Fig. 1. Photoelectron diffraction effects may have an influence on the intensities of the photoemission peak in the spectrum. This effect has been shown to be less pronounced at a considerably high kinetic energy of the detected electrons [43], and the consistent trends in Fig. 2(c) and (d) confirm the component assignment. Additional information comes from calculated core level binding energy shifts, and a comparison between the measured and calculated core level shifts is shown in Fig. 3. The calculations show a pronounced difference in the core level shifts of the bulk-truncated oxide surface [Fig. 3(c)] and the trilayer termination [Fig. 3(d)]. Starting with the O 1s, we find an experimental shift of − 0.91 eV, compared to the calculated values of −1.32 for the bulk terminated surface and −0.93 for the trilayer terminated surface. Thus the O 1s shifts favor the trilayer terminated surface. For the Rh 3d5/2 level, the calculations predict a shift of 1.2 eV for the bulk truncated Rh2O3(0001) surface, while the calculated shift for the trilayer terminated surface (0.88 eV) is slightly smaller than the shift in the bulk Rh2O3 (0.93 eV). The latter termination is in better agreement with the experimentally observed shifts of 0.75 eV and 1.04 eV, respectively, in particular the direction of the shift agrees with the experiment. The difference between the calculated and the measured core level shifts (~150 meV) is slightly larger than for related metallic systems, presumable due to reduced screening stemming from the low charge

than the surface oxide. The broadening of the components in the spectrum is probably due to a slightly disordered oxide. Despite the broadening of the components, the spectrum cannot be fitted with only two components within the limits of a physical reasonable line shape. A component with a surface core level shift of +1.04 eV with respect to the Rh bulk is therefore fitted to the Rh 3d5/2 spectrum. A peak at a similar binding energy has previously been associated with Rh bulk oxide [19,40,41]. In the O 1s spectrum we find a general broadening together with an intensity increase of the component at a higher binding energy, associated with oxygen below the surface, also indicating that the oxide is growing thicker. In addition, previous DFT results predict that the surface of the bulk oxide is reconstructed such that the top three layers are similar to the surface oxide [42]. At the chemical potential corresponding to the experimental preparation conditions (μ = − 1.4 eV), the reconstruction leading to the formation of the trilayer results in a gain in the free surface energy of ΔG=30 meV/A2 compared to the truncated Rh2O3 surface. The Rh layer within the surface oxide, as well as the surface of this reconstructed bulk oxide, is 50% denser than the Rh layers in the oxide bulk. To compensate, the DFT results predict that the second Rh layer of the bulk oxide is instead 50% less dense. This will yield a relatively high intensity of the surface component as compared to the bulk oxide component. A surface partly covered by the thin surface oxide would also explain the ratio of the photo emission peaks. To validate the assignment of the different components of the bulk oxide we performed energy dependent measurements, shown in Fig. 2. By utilizing the change of the photoelectrons' mean free path, as their kinetic energy is changed, we are able to assign the different components in the spectra as originating from the surface or bulk regions. Starting with the O 1s spectrum [Fig. 2(a) and (c)] we find that the high binding energy component denoted Rh2O3 bulk decreases less in intensity with increasing photon energy as compared to the

b

O1s hν: 900eV

c

Rh 3d5/2 hν: Rh2O3 bulk 710eV

oxide surface interface/ Rh2O3 bulk

bulk 307.1 eV

hν: 809eV

0.8 0.6 oxide surface

0.4 0.2

hν: 502eV

650

d

809

900

Rh 3d5/2

3.5

hν: 390eV

hν: 650eV

interface/Rh2O3 bulk

1

peak area ratio

Intensity (arb. units)

Intensity (arb. units)

hν: 606eV

peak area ratio

O1s

surface oxide/ surface on Rh2O3

peak area ratio

a

3 2.5

surface oxide/ surface on Rh2O3

2 1.5

Rh2O3 bulk bulk

1 535

533

531

529

527

310

309

Binding Energy (eV)

308

307

306

390

502

606

710

Photon energy (eV)

Fig. 2. a) Energy dependence of the HRCL spectra from Rh(111) after 60 min of exposure to atomic oxygen at a sample temperature of 380 °C. a) O 1s and b) Rh 3d5/2. In c) and d) are the ratios of the integrated area of the different components as a function of photon energy plotted. In c) the O 1s level relative to the area of the interface/Rh2O3 bulk oxide component and in d) the Rh 3d5/2 level relative to the metal bulk component area, are plotted. The plots show that the area of the surface related components are decreasing relative to the bulk components with increasing photon energy. This is expected as the measurements are more bulk sensitive with increasing photon energy.

S. Blomberg et al. / Surface Science 606 (2012) 1416–1421

b Rh2O3 0eV

O 1s hν=650 eV

Rh2O3surface -0.91eV

OH

533

531

529

527

Intensity (arb. units)

Intensity (arb. units)

a

1419

Rh2O3 surface +0.75eV Rh2O3 +1.04eV

309

Rh 3d5/2 hν=390eV

bulk 0eV

308

307

306

Binding Energy (eV) DFT calculated shifts -1.32 eV

c

bulk terminated surface

-0.93 eV

d trilayer terminated surface

0.88 eV

1.2 eV

0 eV

0.93 eV

0.93 eV

0 eV

Fig. 3. a) HRCL spectra from Rh(111) after 60 min of exposure to atomic oxygen at a sample temperature of 380 °C. a) O 1s and b) Rh 3d. Calculated binding energy shifts from c) a bulk terminated model and d) from a trilayer terminated model. Red values indicate O 1s shifts in which the high binding energy component is referenced to as zero, and black values indicate shifts in Rh 3d in which the binding energy is referenced to the Rh 3d binding energy in the Rh metal bulk.

carrier concentration in the oxide. In summary, a comparison between measured and calculated core level binding energy shifts from the Rh2O3(0001) surface clearly favors a trilayer terminated surface in agreement with previous theoretical predictions [42]. Tolia et al. [41] reported a previous XPS study of bulk oxide Rh films. For Rh 3d5/2, their results are in agreement with the results in this study. For O 1s, they also report two components, but the component corresponding to the surface is shifted toward a higher binding energy, relative to the peak from layers below the surface, instead of toward a lower binding energy as found in the present study. In addition, the C 1s spectra in [41] always show some amount of C contamination on the surface. Hence we assume that the disagreement between the two studies originates from C contamination, and that

a

Rh(01)

the surface oxygen peak is shifted toward a higher binding energy when coordinated to C. The above results are confirmed by STM and LEED as shown in Fig. 4. In this case the surface was annealed at 560 °C for 600 s after the exposure of atomic oxygen at 5·10− 5 mbar and a sample temperature of 500 °C, in order to obtain larger domains of ordered areas. The resulting LEED pattern is shown in Fig. 4(a), where the strongest spots agree well with the inplane lattice constant of the RhO2 surface oxide [33]. Surrounding these spots, we find the well-known morié pattern due to the (9×9) coincidence periodicity between the surface oxide and the Rh(111) substrate. In addition, a pattern closely corresponding to a (√3×√3)R30° periodicity relative to the surface oxide can be seen. This is in perfect agreement with the presence of the bulk oxide [12,33]. Thus the LEED

b

c

Rh(10) (9x9) 12Åx12Å

d

Rh2O3(0001)

500Åx500Å

20Åx20Å

12Åx12Å

Fig. 4. a) LEED pattern from a Rh(111) surface at 70 eV after exposure to atomic oxygen at 500 °C and post-annealing for 600 s at 560 °C. The spots from the Rh(111) substrate are indicated, as well as the unit cells for the (9 × 9) surface oxide and Rh2O3(0001). b) The STM image shows areas with the Rh2O3 bulk oxide, the (9 × 9) surface oxide and flat, probably small domains of a (2 × 2) structure. The tunneling parameters were 1.90 V and 500 pA for the 50 × 50 nm image and 1.90 V and 575 pA for the 2 × 2 nm image. The inset is a zoom-in of the marked area. Simulated STM images from c) a bulk terminated and d) a trilayer terminated Rh2O3(0001) surface.

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pattern reveals a coexistence of the bulk and surface oxides. The same LEED pattern has been observed previously by LEEM [8] after NO2 exposures of the Rh(111) surface. An STM image from a similar experiment is shown in Fig. 4(b), where we can identify three different surface structures. In the top left corner we find a relatively flat area, probably with small domains of a (2 × 2) structure with chemisorbed oxygen. To the right we find a long range periodicity, which we recognize as the (9 × 9) surface oxide. Most of the surface, however, shows a hexagonal pattern with a periodicity of about 5 Å. The surface appears rather disordered with almost flat areas mixed with darker areas with long periodicity which is in good agreement with the 5.12 Å expected for the bulk oxide. Hence, the STM image agrees well with a surface that, since some oxygen has desorbed in the annealing, exposes metallic, surface oxide and bulk oxide covered areas. Concerning the surface structure of the bulk oxide, Fig. 4(c) and (d) shows the simulated STM images corresponding to the bulk terminated and trilayer reconstructed surfaces respectively, as discussed above. Comparing these to the zoom-in shown as an inset in Fig. 4(b) the best agreement is for the trilayer reconstruction, thus supporting the HRCLS and DFT results. A model of the calculated final structure is shown in Fig. 5. In the top view (panel (a)) the hexagonal character of the in-plane structure can be seen. In the side view in panel (b), we also indicate the distance between the atomic layers, showing a significantly larger distance between the trilayer and the next Rh layer. It is interesting to note that different corundum structured materials exhibit different (0001) surface terminations. The corundum type oxides with a (0001) surface have also been observed on Cr2O3, V2O3, Al2O3 and Fe2O3. Experimental investigation using LEED proposed a half-metal termination for the α-Al2O3(0001) [44] and Cr2O3(0001) [45], even though other terminations have been posted for the Cr2O3(0001) [46]. STM studies supported by spin density functional theory (s-DFT) calculations report that two domains, Fe- and O-terminated α-Fe2O3(0001) surfaces, are stable under typical experimental conditions but the terminations are dependent on the method of surface preparation [47]. The halfmetal termination has also been suggested for the V2O3(0001) [48] surface but a more recent study using medium-energy ion scattering and low-energy noble gas impact-collision ion scattering spectroscopy report on a trilayer [49], as we propose in this study, termination. This model is supported by theoretical investigation as the most stable structure [50]. The similar surface structures of the bulk and surface oxides are interesting from a catalysis' point of view. A number of previous reports [8,27,32] clearly indicate that the formation of the Rh2O3(0001) bulk oxide poisons the surface for CO oxidation. This can be understood as the surfaces do not expose any suitable sites for CO adsorption. The RhO2 surface oxide, however, has been reported to be present on Rh surfaces while the activity is high

a

b 0.97 1.10 1.40 0.93 1.03

Fig. 5. Calculated surface structure of the Rh2O3(0001) surface. a) Top-view. b) Side view. The distances between three layers are indicated in Ångström. Red and white balls correspond to oxygen and Rh atoms, respectively.

[8,11,12,20]. This is however surprising, as the formation of the ideal surface oxide layer leads to a passivation of the surface, in agreement with e.g. the formation of a surface oxide on Pd [51]. To address this point we have evaluated the removal energy for an oxygen atom in the topmost layer. Indeed we find that the removal energy is 0.8 eV higher for the trilayer surface, indicating a reduced activity of the surface. At least three different surface scenarios, or combinations of them, could explain these observations. The first involves the presence of oxygen defects at the RhO2 surface oxide which would allow for CO adsorption and a high reactivity. To explain the inactivity of the Rh2O3(0001) surface, such defects would not be allowed on this surface. This could in turn be understood if the existence of an oxygen vacancy in the surface is energetically unfavorable such that any vacancy formed is immediately healed by O atoms from the bulk. Such a process would not be possible for the surface oxide since there is no bulk O available. The second explanation would involve a border type of reaction between the RhO2 surface oxide and the metal Rh surface as has been proposed previously by DFT [27] and by UHV measurements [52]. Finally, a third explanation could be patches with chemisorbed oxygen which co-exist with the RhO2 surface oxide, and are solely responsible for the high CO2 production and that the RhO2 surface oxide is inactive as the Rh2O3(0001) surface. Now that the oxide surface structures, as well as their corresponding core level shift, are established, it is possible to explore this question further, e.g. using high pressure X-ray photoelectron spectroscopy. 4. Conclusions In summary, we have studied the oxidation of the Rh(111) single crystal surface using a combination of HRCLS, LEED and STM. The experimental studies were complemented by theoretical DFT simulations, for the interpretation of the experimental data. Our results show that it is possible to form a fairly well ordered bulk-like Rh2O3(0001) film on Rh(111) using atomic oxygen. Furthermore, our studies show that the best agreement between theory and calculations is obtained for a trilayer terminated Rh2O3(0001) surface. The differences in catalytic activity between the Rh2O3(0001) surface and the RhO2 surface oxide were discussed. Acknowledgment This work was financially supported by the Swedish Research Council, the Crafoord Foundation, the Knut and Alice Wallenberg Foundation, the Foundation for Strategic Research (SSF), the Anna and Edwin Berger Foundation and the Austrian Science Funds (FWF): F4511-N16 (FOXSI). References [1] E. Lundgren, H. Over, J. Phys. Condens. Matter 20 (2008) 180302. [2] A. Stierle, A.M. Molenbroek, MRS Bull. 32 (2007) 1001. [3] H.J. Freund, G. Meijer, M. Scheffler, R. Schlogl, M. Wolf, Angew. Chem. Int. Ed. 50 (2011) 10064. [4] I. Chorkendorff, J.W. Niemantsverdriet, Concepts of Modern Catalysis and Kinetics, 2nd ed. Wiley-VCH, Weinheim, 2007. [5] G.A. Somorjai, Chemistry in Two Dimensions: Surfaces, Cornell University Press, Ithaca, 1981. [6] G. Rupprechter, C. Weilach, Nano Today 2 (2007) 20. [7] F. Gao, Y. Wang, Y. Cai, D.W. Goodman, J. Phys. Chem. C 113 (2009) 174. [8] J.I. Flege, P. Sutter, Phys. Rev. B 78 (2008) 153402. [9] B.L.M. Hendriksen, J.W.M. Frenken, Phys. Rev. Lett. 89 (2002) 046101. [10] B.L.M. Hendriksen, S.C. Bobaru, J.W.M. Frenken, Surf. Sci. 552 (2004) 229. [11] J. Gustafson, R. Westerström, A. Mikkelsen, X. Torrelles, O. Balmes, N. Bovet, J.N. Andersen, C.J. Baddeley, E. Lundgren, Phys. Rev. B 78 (2008) 045423. [12] J. Gustafson, R. Westerström, A. Resta, A. Mikkelsen, J.N. Andersen, O. Balmes, X. Torrelles, M. Schmid, P. Varga, B. Hammer, G. Kresse, C.J. Baddeley, E. Lundgren, Catal. Today 145 (2009) 227. [13] H. Over, Y.D. Kim, A.P. Seitsonen, S. Wendt, E. Lundgren, M. Schmid, P. Varga, A. Morgante, G. Ertl, Science 287 (2000) 1474. [14] Y.B. He, M. Knapp, E. Lundgren, H. Over, J. Phys. Chem. B 109 (2005) (1830) 21825. [15] H. Over, O. Balmes, E. Lundgren, Catal. Today 145 (2009) 236.

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