Unusual Process of Water Formation on RuO2(110) by Hydrogen

14007

2006, 110, 14007-14010 Published on Web 06/29/2006

Unusual Process of Water Formation on RuO2(110) by Hydrogen Exposure at Room Temperature M. Knapp,† D. Crihan,† A. P. Seitsonen,‡ A. Resta,§ E. Lundgren,§ J. N. Andersen,§ M. Schmid,| P. Varga,| and H. Over*,† Department of Physical Chemistry, Justus-Liebig-UniVersity, Heinrich-Buff-Ring 58, D-35392 Giessen, Germany, IMPMC, CNRS and UniVersite´ Pierre et Marie Curie, 4 place Jussieu, case 115, F-75252 Paris, France, Department of Synchrotron Radiation Research, UniVersity of Lund, So¨lVegatan 14, S-22362 Lund, Sweden, and Institut fu¨r Allgemeine Physik, Vienna UniVersity of Technology, Wiedner Hauptstr. 8-10, A-1040 Wien, Austria ReceiVed: May 1, 2006; In Final Form: June 5, 2006

The reduction mechanism of the RuO2(110) surface by molecular hydrogen exposure is unraveled to an unprecedented level by a combination of temperature programmed reaction, scanning tunneling microscopy, high-resolution core level shift spectroscopy, and density functional theory calculations. We demonstrate that even at room temperature hydrogen exposure to the RuO2(110) surface leads to the formation of water. In a two-step process, hydrogen saturates first the bridging oxygen atoms to form (Obr-H) species and subsequently part of these Obr-H groups move to the undercoordinated Ru atoms where they form adsorbed water. This latter process is driven by thermodynamics leaving vacancies in the bridging O rows.

The chemical reduction of late transition metal oxides by molecular hydrogen exposure is an important process in the preformation of practical oxide catalysts.1,2 For instance, assupplied RuO2 powder catalysts deactivate quickly during the oxidation of CO. However, when RuO2 powder is first fully reduced in a H2 atmosphere at 770 K and then mildly reoxidized at lower temperature, the resulting catalyst is extraordinarily active and structurally stable during the oxidation of CO.3 More generally, the process of chemical reduction of oxide catalysts comes into play whenever reducing agents are involved in the catalyzed reaction. The first elementary reaction step in the partial oxidation of alcohols, for instance, is the hydrogen transfer from the OH group of alcohol to the undercoordinated O atoms of the oxide surface. Eventually, this process may even cause the reduction of the oxide surface in an oxygen deficient environment. Despite its practical importance in heterogeneous catalysis, the microscopic details of the chemical reduction of late transition metal oxide surfaces by molecular hydrogen are only poorly understood. On the basis of a combined temperature programmed reaction (TPR), high-resolution core level spectroscopy (HRCLS), scanning tunneling microscopy (STM), and density functional theory (DFT) study, we demonstrate here that the reduction of the RuO2(110) surface at room temperature proceeds via a two-step process (cf. Figure 1). Hydrogen exposure at room temperature caps first the bridging O atoms, forming Obr-H groups on the RuO2(110) surface. When most * Corresponding author. E-mail: Herbert.Over@ phys.chemie.uni-giessen.de. Fax: ++49-641-9934559. URL: http://www.chemie.uni-giessen.de/home/over. † Justus-Liebig-University. ‡ CNRS and Universite ´ Pierre et Marie Curie. § University of Lund. | Vienna University of Technology.

10.1021/jp0626622 CCC: $33.50

Figure 1. (a) Ball-and-stick model of the stoichiometric RuO2(110) surface, indicating the undercoordinated O atoms (Obr) and Ru atoms (1f-cus Ru). (b) Top view of RuO2(110) which was exposed to 5 L of H2. Part of the Obr atoms are transformed to Obr-H. (c) The RuO2(110) surface is exposed to 100 L of H2: all Obr form Obr-H and some of these Obr-H groups are shifted to 1f-cus Ru atoms, forming the water species Oot-2H. The Oot-2H species establishes a H-bond to Obr-H (dotted red line). Oot stands for a terminal oxygen bonded on top of the 1f-cus Ru atom.

of the bridging O atoms are saturated by a single H atom, further hydrogen uptake leads to the formation of water sitting on top © 2006 American Chemical Society

14008 J. Phys. Chem. B, Vol. 110, No. 29, 2006 of the 1f-cus Ru atoms, thereby creating vacancies in the bridging O rows. The TPR experiments were conducted with a differentially pumped quadrupole mass spectrometer (QMS), which was connected to the main chamber via a closed cone with a small aperture (d ) 5 mm) facing the sample at a distance of 1 mm. This ensures that only molecules released from the sample surface can reach the QMS. The HRCLS measurements were conducted at the beam line I311 at MAXII in Lund, Sweden.4 The photon energy for the measurements of the O1s core levels was chosen to be 580 eV with a total energy resolution of 250 meV. The experimental O1s spectra were decomposed into a small number of Donjiac-Sunjic profiles5 convoluted with Gaussian distributions. STM images were recorded at the TU Vienna in constant current mode and at room temperature, using a commercial Omicron scanning tunneling microscope. For the ab initio calculations,6 we employed the projector augmented wave method7 with a plane-wave cutoff of 37 Ry and generalized gradient approximation.8 The surface was modeled by five double layers of RuO2(110) (supercell approach) with adsorbates only at one side of the slab. Consecutive RuO2(110) slabs were separated by a vacuum region of about 16 Å. Calculations of the adsorption energy and the geometry of adsorbed hydroxyl and water groups on RuO2(110) were performed using a (2×1) or a (2×2) surface unit cell with a uniform k point mesh of 4×4 and 4×2 k points, respectively. The core level energies were calculated by removing half of an electron from the core orbital and performing a self-consistent calculation of the electronic structure.9 Thus, the screening effects (“final state”) of the hole are included. The energy is obtained as the energy difference between the Kohn-Sham eigenvalue of the core state and the Fermi energy. On the basis of these DFT calculations, the experimental O1s core level shifts were assigned to specific O species on the surface. The RuO2(110) surface is produced by exposure of a wellprepared Ru(0001) surface to 2 × 106 L of O2 (1 L ) 1 Langmuir ) 1.33 × 10-6 mbar‚s) at a sample temperature of 650 K. In the bulk structure of RuO2, the Ru atoms bind to six oxygen atoms, forming a slightly distorted RuO6 octahedron, while the O atoms are coordinated to three Ru atoms in a planar sp2 configuration. On the stoichiometric RuO2(110) surface (cf. the ball-and-stick model of the bulk-truncated RuO2(110) surface, Figure 1a), two kinds of undercoordinated surface atoms are present which are arranged in rows along the [001] direction: (i) the bridging oxygen atoms (Obr), which are coordinated only to two Ru atoms underneath (instead of three), and (ii) the so-called 1f-cus Ru atoms (1f-cus stands for 1-fold coordinatively unsaturated sites).10 The 1f-cus Ru and Obr can clearly be resolved both in the core level spectroscopy11 and in STM.12 The TPR measurements are summarized in Figure 2. The RuO2(110) surface was exposed to various doses of D2 keeping the sample at room temperature. Subsequently, the desorption rate of heavy water was monitored by a QMS. After each run, the RuO2(110) was restored by oxygen exposure at 650 K followed by several cycles of oxygen exposure at room temperature and annealing to 600 K. This procedure ensures a stoichiometric RuO2(110) surface as checked by HRCLS. Clearly, two desorption states are discernible in the TPR spectra of Figure 2. One water signal, R, appears at 410 K whose temperature position is nearly independent of the applied D2 dose, that is, first-order desorption. The second water feature, β, is much broader than feature R, and it shifts with the D2 doses according to second-order kinetics. The water formation in the β peak is due to the recombination of neighboring Obr-H

Letters

Figure 2. Temperature programmed reaction (TPR) spectra of heavy water (D2O) after the stoichiometric RuO2(110) surface has been exposed to 0 L of D2, to 5 L of D2, and to 100 L of D2. Clearly, the TPR state, R, around 420 K reveals first-order kinetics, while the broad desorption feature, β, beyond 500 K is of second order. The heating rate was 10.0 K/s.

groups.13 The total amount of produced water in the R peak of Figure 2 is 0.02 and 0.10 ML for the case of 5 and 100 L D2 exposure, respectively. During D2 dosing, no D2O is detected with the mass spectrometer, so we dismiss D2O adsorption from the gas phase as a possible reason for the R state in TPR. Rather, the TPR measurements in Figure 2 suggest that heavy water is produced on the RuO2(110) surface and leaves the surface around 410 K. From the first-order kinetics of the R peak, we imply that the desorption of D2O rather than the formation of D2O is the rate determining step in this TPR process. The maximum of water desorption at 410 K agrees well with a previous water desorption spectra of RuO2(110) exposed to water.14,15 In Figure 3, high-resolution O1s core level spectra of RuO2(110) are shown before and after H2 exposure. First, the stoichiometric RuO2(110) surface reveals two O species at binding energies of 529.2 and 528.4 eV which are attributed to bulk oxygen of RuO2(110) (main peak) and the bridging O atoms on the RuO2(110) surface, respectively.11 After exposure of 5 L of H2 at room temperature, the O1s spectrum changes significantly (compare Figure 3b). The intensity of the bridging Obr atoms decreases substantially, while an additional feature appears at a binding energy of 530.3 eV. According to our DFT calculations, this new feature is assigned to Obr-H (cf. also Table 1). Exposure of the stoichiometric RuO2(110) surface to 100 L of H2 results in the O1s spectrum shown in Figure 3c. The intensity of the Obr signal vanishes, while that of the Obr-H signal increases. It should be noted that the intensity increase of Obr-H and the corresponding decrease in the Obr signal are complementary in a way that the weighted sum of both intensities is independent of the H2 exposure over a wide exposure range (0-100 L H2). In addition, a small additional O1s emission at 532.1 eV can be discerned in Figure 3c concomitant with an additional feature at 529.8 eV. DFT calculations indicate that these two additional O1s features are related to adsorbed water over 1f-cus Ru atoms (532.1 eV) and to Obr-H which forms an additional H-bonding with Oot-2H. Oot indicates terminal oxygen adsorption on top of the 1f-cus Ru atoms, and Oot-2H is synonymous with absorbed water on top of the 1f-cus Ru atoms. These assignments are further supported by an experiment where the sample was annealed to 550 K (cf. Figure 3d), leading to the desorption of water and the loss of both of these O1s features. Together with the TPR

Letters

J. Phys. Chem. B, Vol. 110, No. 29, 2006 14009

Figure 4. STM images (15 nm × 15 nm) of the stoichiometric RuO2(110) surface (a) which was exposed to 3 L of H2 (b) and 100 L of H2 (c) at room temperature. In panel (d), the RuO2(110) and 100 L of H2 was subsequently annealed to 423 K. Vbr indicates a vacancy in the Obr row, Obr-H corresponds to a bridging hydroxyl group, and Oot-2H is water in the on-top position above the 1f-cus Ru atoms. Figure 3. High-resolution O1s core level spectra of (a) the stoichiometric RuO2(110) surface in comparison with O1s spectra in which the RuO2(110) surface is exposed to 5 L (b) and 100 L (c) of D2 at room temperature. (d) Annealing the 100 L H2 exposed surface to 550 K. The O1s spectra were recorded at a sample temperature of 100 K. The various profiles indicate the decomposition of the experimental O1s spectrum in distinct oxide-related O1s emissions. The open circles are experimental data.

measurements, we infer that water is already formed at room temperature by simply exposing the RuO2(110) surface to molecular hydrogen. The DFT-calculated O1s shifts of O and O-H species on RuO2(110) are compared to experimental ones in Table 1. The O1s reference is bulk O of RuO2(110). Clearly, the Obr-H and the Oot-2H species can be identified. The Obr-2H species would result in an O1s shift of 4.9 eV which has not been observed in any of our experimental O1s spectra, thus conflicting with a recent high-resolution energy electron loss study.18,19 According to the DFT calculations, the formed water species Oot-2H establishes a hydrogen bond with the adjacent Obr-H species, thereby shifting the O1s spectrum of Obr-H by 0.7 eV to lower binding energies. Also, this shift is reconciled in the experimental HRCL spectra, showing that the position of Obr-H shifts from 530.3 to 529.8 eV.

However, there is still one appealing question left: How is this water produced on the RuO2(110) surface upon H2 exposure? To clarify this point, we performed the STM experiments summarized in Figure 4. In Figure 4a, we present the pristine RuO2(110) surface. Small amounts of H2 were exposed while the STM tip was still in tunnel contact with the surface. Hydrogen adsorption leads to the appearance of distinct modulations along the bridging O rows in the same surface region as in Figure 4a (cf. Figure 4b). Shadowing effects due to the STM tip may lead to lower hydrogen coverage. Together with the HRCLS data and DFT-simulated STM images (not shown), we interpret these modulations along the bridging O rows as being due to hydroxyl groups (Obr-H). The H2 exposure of 100 L to the RuO2(110) surface at room temperature causes a pronounced modification of the oxide surface. Before dosing H2, we retracted the STM tip from the surface to avoid shadowing the image area. The HRCLS data indicate that the bridging Obr atoms are completely saturated by single hydrogen atoms. The bridging O rows reveal in the STM image (cf. Figure 4c) about 5% vacancies. Concomitant with the 5% vacancies in the bridging O rows, about 4-5% new bright features appear along the 1f-cus Ru rows, that is, between the adjacent Obr-H rows. Since the number of

TABLE 1: Experimentally Observed O1s Shifts (exptl) in Comparison with DFT-Calculated Ones (theor)a assignment

exptl O(1s) shift/eV

theor O(1s) shift/eV

Obulk Obr Obr-H Oot-H with HB Oot-2H Obr-2H

reference -0.8 1.1 -2.8 --

reference -0.9 1.4 -0.8 2.9 4.9

Obr-H‚‚‚Oot

0.6

0.7

a

DFT adsorption energy

literature: theor adsorption energy

1.17 eV/1/2H2 1.43 eV/1/2H2 1.02 eV/H2O (2×2) 0.83 eV/H2O (2×2) 0.68 eV/H2 1.53 eV/1/2H2

0.94 eV/1/2H216 --0.91 eV/H2O17 --0.56 eV/H216 ---

Based on this comparison, the experimental O1s shifts are assigned to particular O and O-H species. “- -” means that this O1s shift has not been observed experimentally. “- - -” means that this adsorption energy is not available. “HB” means hydrogen bond. In addition, the adsorption energies are indicated with respect to molecular 1/2H2, H2, and H2O in the gas phase. When available, these values are compared to values from the literature.16,17

14010 J. Phys. Chem. B, Vol. 110, No. 29, 2006 vacancies equals those of the newly formed protrusions, we argue that part of the Obr-H species transforms into adsorbed water molecules on 1f-cus Ru sites, leaving vacancies behind in the Obr rows. This assignment is fully consistent with the evolution of a water signal in the O1s spectrum of Figure 3c. Upon annealing the surface to 423 K (Figure 4d), the desorption temperature of water, all bright features along the 1f-cus Ru rows disappear in the STM image, further supporting the assignment of the bright features in STM to adsorbed water. The vacancies in the Obr rows persist at 423 K. To substantiate the assignment of the depressions along the Obr rows as Obr vacancies, we annealed the surface to 607 K. Here, we observe that the RuO2(110) surface rearranges in a way that the isolated vacancies agglomerate into holes of ∼10 nm2 size (cf. Supporting Information Figure S1). At the rim of such holes, small flat clusters are formed which we ascribe to Ru islands. This reduction-induced restructuring of the RuO2(110) surface has already been identified with the chemical reduction of RuO2(110) by CO exposure.12 The reason for this restructuring process is the nonexistence of solid Ru suboxides. Partial reduction of the RuO2(110) surface leads at temperatures above 600 K instead to the separation into stoichiometric RuO2(110) areas and single-layer Ru(0001) islands on the surface.12 Table 1 compiles also the adsorption energies of the various hydroxyl and water species with respect to H2 and water in the gas phase. The adsorbed water species Oot-2H over 1f-cus Ru atoms are stabilized by 1.02 eV against gaseous water. The Obr-2H species is stabilized by 0.83 eV in the (2×2) unit cell against gaseous water. Applying a Born-Haber cycle with the final state Vbr + H2O(gas) (Vbr corresponds to a vacancy in the Obr row of RuO2(110)), the Oot-2H species is by 0.19 eV (1.02-0.83 eV) more stable than a hypothetic Obr-2H species. Therefore, we propose that the water molecule is produced by shifting Obr-H to the 1f-cus Ru site, taking up an additional H atom. As a consequence, vacancies are created in the Obr rows which are seen in STM (cf. Figure 4c). This means that the observed water formation on the RuO2(110) surface at room temperature is driven by thermodynamics. In situ surface X-ray diffraction (SXRD) experiments20 indicate that the reduction of RuO2(110) by molecular hydrogen exposure proceeds efficiently only at temperatures beyond the desorption temperature of water (>400 K). However, the reduction of polycrystalline RuO2 films (thickness 2 nm) by atomic hydrogen can take place even at temperatures below 320 K.21 A similar effect was observed in our laboratory in Giessen: upon exposure to 100 mbar of H2 at room temperature, the RuO2(110) film was completely removed after a few seconds. We suggest that the water production mechanism identified here may be the key process to understanding the room temperature reduction of RuO2(110) by hydrogen exposure. The water molecules sitting on the 1f-cus Ru sites are then removed from the surface by hydrogen-induced destabilization, for instance, by breaking the hydrogen bond of the Oot-2H species to the Obr-H species; the strength of this hydrogen bond is 0.2 eV. This finding is of some importance for the extreme ultraviolet lithography (EUVL) technology where Ru-based capping layers are considered as promising candidates to protect the EUVL mirrors against oxidation and carbonization.22 In an EUV environment, the Ru capping layer gradually oxidizes,23 thereby degrading the optical reflectivity of RuO2-coated EUV mirrors. Accordingly, the Ru capping layer has to be restored routinely by chemical reduction with hydrogen at room temperature in order to maintain the high optical reflectivity required for the EUV mirrors.

Letters In summary, we have presented ample experimental and theoretical evidence that water forms on the RuO2(110) surface already at room temperature. First, hydrogen exposure leads to hydroxyl groups (Obr-H) along the bridging O rows. After saturation of the Obr rows by hydrogen, part of the Obr-H species react with additional hydrogen, thereby forming water Oot-2H over the 1f-cus Ru atoms and vacancies in the Obr rows of the RuO2(110) surface. The second reaction step is driven by thermodynamics. Acknowledgment. We would like to thank the John Von Neumann Institute for Computing for the generous supercomputing time as well as the Fonds zur Fo¨rderung der wissenschaftlichen Forschung (FWF) and the DFG (SPP 1091) for financial support. The MAX-lab staff is acknowledged for technical support. Supporting Information Available: Figure showing how the morphology of the hydrogen exposed RuO2(110) surface changes upon annealing to 603 K. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Thomas, J. M.; Thomas, W. J. Principles and Practice of Heterogeneous Catalysis; VCH: Weinheim, 1996. (2) Ertl, G., Kno¨zinger, H., Weitkamp, J., Eds. Handbook of Heterogeneous Catalysis; Wiley: New York, 1997. (3) Aβmann, J.; Crihan, D.; Knapp, M.; Lundgren, E.; Lo¨ffler, E.; Muhler, M.; Narkhede, V.; Over, H.; Schmid, M.; Varga, P. Angew. Chem., Int. Ed. 2005, 116, 939. (4) Nyholm, R.; Andersen, J. N.; Johansson, U.; Jensen, B. N.; Lindau, I. Nucl. Instrum. Methods Phys. Res., Sect. A 2001, 467, 520. (5) Donjiac, S.; Sunjic, M. J. Phys. C 1970, 3, 285. (6) We employed the Vienna ab initio simulation package (VASP). Kresse, G.; Furthmu¨ller, J. Comput. Mater. Sci. 1995, 6, 15. Kresse, G.; Joubert, D. Phys. ReV. B 1998, 59, 1758. (7) Blo¨chl, P. Phys. ReV. B 1994, 50, 17953. (8) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. ReV. B 1992, 46, 6671. (9) Pehlke, E.; Scheffler, M. Phys. ReV. Lett. 1993, 71, 2338. (10) Over, H.; Kim, Y. D.; Seitsonen, A. P.; Wendt, S.; Lundgren, E.; Schmid, M.; Varga, P.; Morgante, A.; Ertl, G. Science 2000, 287, 1474. (11) (a) Over, H.; Seitsonen, A. P.; Lundgren, E.; Wiklund, M.; Andersen; J. N. Chem. Phys. Lett. 2001, 342, 467. (b) Over, H.; Seitsonen, A. P.; Lundgren, E.; Smedh, M.; Andersen, J. N. Surf. Sci. 2001, 504, 196. (12) Over, H.; Knapp, M.; Lundgren, E.; Seitsonen, A. P.; Schmid, M.; Varga, P. ChemPhysChem 2004, 5, 167. (13) Jacobi, K., Wang, Y., Ertl, G. J. Phys. Chem. B 2006, 110, 6115. (14) Lobo, A.; Conrad, H. Surf. Sci. 2003, 523, 279. (15) Knapp, M.; Crihan, D.; Seitsonen, A. P.; Over, H. J. Am. Chem. Soc. 2005, 127, 3236. (16) Sun, Q.; Reuter, K.; Scheffler, K. Phys. ReV. B 2004, 70, 235402. (17) Sun, Q.; Reuter, K., Scheffler, K. Phys. ReV. B 2003, 67, 205424. (18) Wang, J.; Fan, C. Y.; Sun, Q.; Reuter, K.; Jacobi, K.; Scheffler, M.; Ertl, G. Angew. Chem., Int. Ed. 2003, 42, 2151. (19) For keeping the RuO2(110) film stoichiometric while working with reducing agents, repeated oxygen-exposure/annealing cycles are required after each experiment (Surf. Sci. 2002, 505, 137). Otherwise, the RuO2(110) will progressively be reduced. Therefore, the preparation/ restoration method used in refs 13 and 18 will inevitably lead to clean but heterogeneous surfaces, consisting of RuO2(110) with metallic Ru islands decorating holes in the oxide (cf. the STM image: Figure 10 in Surf. Sci. 2002, 515, 143). Both the hydrogen evolution up to 300 K and the Obr-2H species are related to hydrogen adsorption on these Ru islands. (20) He, Y. B.; Knapp, M.; Lundgren, E.; Over, H. J. Phys. Chem. B 2005, 109, 21825. (21) Nishiyama, I.; Oizumi, H.; Motai, K.; Izumi, A.; Ueno, T.; Akiyama, H.; Namiki, A. J. Vac. Sci. Technol., B 2005, 23, 3129. (22) Madey, T. E.; Faradzhev, N. S.; Yakshinskiy, B. V.; Edwards, N. V. Appl. Surf. Sci., in press (and references therein). (23) Bajt, S.; Chapman, H. N.; Nguygen, N.; Alameda, J.; Robinson, J. C.; Malinowski, M.; Gullikson, E.; Aquila, A.; Tarrio, C.; Grantham, S. Appl. Opt. 2003, 42, 5750.