Complex Interaction of Hydrogen with the RuO2(110) Surface - The

Mar 21, 2007 - Combined experiment and theory approach in surface chemistry: Stairway to heaven? Kai S. Exner , Franziska Heß , Herbert Over , Ari Pa...
0 downloads 0 Views 616KB Size
Download PDF
J. Phys. Chem. C 2007, 111, 5363-5373

5363

Complex Interaction of Hydrogen with the RuO2(110) Surface M. Knapp,† D. Crihan,† A. P. Seitsonen,‡ E. Lundgren,§ A. Resta,§ J. N. Andersen,§ and H. Over*,† Department of Physical Chemistry, Justus-Liebig-UniVersity, Heinrich-Buff-Ring 58, D-35392 Giessen, Germany, CNRS and IMPMC, UniVersite´ Pierre et Marie Curie, 4 Place Jussieu, Case 115, F-75252 Paris, France, and Department of Synchrotron Radiation Research, UniVersity of Lund, So¨lVegatan 14, S-22362 Lund, Sweden ReceiVed: October 13, 2006; In Final Form: February 9, 2007

Using a variety of dedicated surface sensitive techniques, we studied the interaction of hydrogen with bare and adsorbate modified RuO2(110) surfaces on the atomic scale. Hydrogen interacts strongly with the undercoordinated O atoms, thereby forming hydroxyl groups and passivating available oxygen species on the oxide surface, for instance, for the catalytic CO oxidation reaction. Temperature programmed reaction and desorption elucidate the complex reaction behavior of hydrogen with O precovered RuO2(110), including the hydrogen transfer reaction between the different kinds of undercoordinated surface oxygen atoms. Hydroxyl, water species, and hydrogen transfer are identified with high-resolution O1s core level spectroscopy by comparison with density functional theory (DFT) calculated O1s core level shifts. DFT calculations provide adsorption energies, atomic geometries, as well as diffusion barriers of H atoms on the RuO2(110) surface.

1. Introduction In recent years, the RuO2(110) surface has become one of the model systems for catalysis of late transition metal oxides.1 In particular, it has been shown that the catalytically active sites on the stoichiometric RuO2(110) are identified with regular surface sites rather than defect sites. More specifically, the 1-fold coordinatively unsaturated Ru sites (1f-cus Ru) and the undercoordinated bridging O (Obr) atoms determine the interaction of incoming molecules from the gas phase with the catalyst’s surface (see Figure 1). Undercoordinated surface atoms can nicely be resolved in core level spectroscopy2 and in scanning tunneling microscopy (STM).3 Most of the molecules studied so far on the RuO2(110) surface (CO,4 H2O,5,6 O2,7 N2,4 methanol,8 CO2,9 NO,10 ethylene,11 and NH312) adsorb from the gas phase directly onto the 1f-cus Ru atoms. The CO molecule, for instance, adsorbs first on the 1f-cus Ru site and then recombines with the neighboring bridging O atom to form CO2 above room temperature.3,13-20 In this oxidation process, the bridging O atoms are consumed rather than serving as true catalytically active sites. Quite in contrast, hydrogen has been shown to interact strongly with the bridging O atoms above 150 K, forming hydroxyl groups and a so-called dihydride species.21 The hydrogen uptake on RuO2(110) may be mediated by the adsorption of molecular hydrogen on the 1f-cus Ru sites. Since hydrogen atoms are released onto the oxide surface in any dehydrogenation reaction, the interaction of hydrogen with the RuO2 surface is of fundamental importance for the whole class of partial oxidation reactions of hydrocarbons,22 a topic that currently attracts much attention in the search for a green chemistry route for the aerobic oxidation of alcohols.23,24 In this paper, we comprehensively characterize the interaction of hydrogen with the stoichiometric RuO2(110) and oxygen * Corresponding author. Fax: ++49-641-9934559. † Justus-Liebig-University. ‡ Universite ´ Pierre et Marie Curie. § University of Lund.

Figure 1. (A) Ball-and-stick model of the clean RuO2(110) surface. Large balls (green) represent oxygen, and small balls (blue, red) represent ruthenium atoms of RuO2(110). The bridge-bonded oxygen atoms Obr and 1f-cus Ru atoms (red) are indicated. Both surface species are 1-fold undercoordinated with respect to bulk coordination. (B) Adsorption of CO and oxygen onto the stoichiometric RuO2(110) surface. Both adsorbates occupy the on-top positions above the 1f-cus Ru atoms.

covered RuO2(110) surfaces, employing a variety of techniques including mass spectrometry (temperature programmed desorption and reaction: TPD and TPR, respectively), high-resolution core level shift spectroscopy (HRCLS), and density functional theory (DFT) calculations. The seemingly simple H2/RuO2(110) system discloses many intriguing phenomena, including the passivation and reduction of the RuO2(110) catalyst as well as the hydrogen transfer reaction between the undercoordinated O atoms on the catalyst’s surface.

10.1021/jp0667339 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/21/2007

5364 J. Phys. Chem. C, Vol. 111, No. 14, 2007

Knapp et al.

2. Experimental and Computational Details The TPD/TPR experiments were conducted in an ultrahigh vacuum (UHV) chamber equipped with low energy electron diffraction (LEED) optics, a quadrupole mass spectrometer (QMS), and facilities for surface preparation and cleaning.25 The differentially pumped QMS was connected to the main chamber via a closed cone with a small aperture (d ) 2.5 mm) facing the sample at a distance of 1 mm. This ensures that only molecules released from the sample surface can reach the QMS. For the temperature programmed desorption reaction experiments, the heating rate was 10.0 K/s. The Ru(0001) sample was clammed in between two tungsten wires (diameter of 0.25 mm). The temperature of the sample could be varied from 100 K (by cooling with liquid N2) to 1530 K (by direct resistive heating). The sample temperature was measured by a Ni/NiCr thermocouple that was spot welded to the back side of the sample. The HRCLS measurements were conducted at beamline I311 at MAXII in Lund, Sweden.26 The photon energies for the measurements of the Ru-3d and O-1s core levels were chosen to be 330 and 580 eV, respectively, to enhance the surface sensitivity without having a too intense background in the spectra due to secondary electrons; the total energy resolutions of O-1s and Ru-3d spectra were 180 and 80 meV, respectively. All HRCL spectra were recorded at a sample temperature of 100 K. The experimental HRCL spectra were decomposed into a small number of Donjiac-Sunjic profiles convoluted with Gaussian distributions.27 The Ru(0001) sample was cleaned by argon ion bombardment at 1 keV followed by cycles of oxygen exposure at 1000 K to remove surface carbon. Final traces of oxygen were removed by flashing the surface to 1530 K, resulting in a sharp (1 × 1) LEED pattern. The ultrathin RuO2(110) film was produced by exposing a well-prepared single-crystal Ru(0001) to high doses of molecular oxygen at a sample temperature of 650 K. A typical oxygen dose for producing the RuO2(110) film is 2 × 106 Langmuirs (L); (1 L ) 1.33 × 10-6 mbar s). A glass capillary array was used to dose such high amounts of oxygen without breaking the ultrahigh vacuum (UHV) conditions. The local oxygen pressure in front of the sample was estimated to be about 1 × 10-2 mbar, while the background pressure did not exceed 1 × 10-4 mbar. To further improve the quality of the RuO2(110) surface and also for the restoration of the RuO2(110) surface when the surface had been in contact with reducing agents, we dosed 1 L of molecular oxygen at room temperature and flashed the surface to 600 K where the on-top oxygen has already desorbed.18 This treatment was repeated until the O2- TD signal of the weakly held on-top oxygen was maximized (desorption state at about 500 K). This preparation recipe ensures that neither Obr vacancy nor hydroxyl Obr-H are present on the surface (i.e., an ideally terminated (stoichiometric) RuO2(110) surface is accomplished as also confirmed by STM).28 We use this restoration procedure prior to every new adsorption experiment to ensure an ideal termination of the RuO2(110) surface. When we heated a CO or hydrogen exposed sample to higher temperatures than 600 K in TPD/TPR, we used a different restoration recipe. From STM, we know that annealing beyond 600 K causes severe restructuring of the oxide surface in that the RuO2(110) terraces form holes that are decorated by small Ru islands.28,29 These small Ru islands can only be reoxidized when oxygen is exposed to the sample at 650 K. Gases exposed to the sample surface were O2 (purity 99.998%, Messer

Figure 2. Oxygen thermal desorption spectra of RuO2(110), which was exposed to 5 L of O2 at various sample temperatures ranging from 155 to 350 K. For the 500 K spectrum, we kept RuO2(110) in 1 × 10-7 mbar of oxygen, while cooling the sample from 500 to 300 K with a cooling rate of 10 K/min. In the right inset, the on-top O coverage is shown as a function of the sample temperature during O2 exposure. The dashed line in this inset indicates the achieved saturation coverage on on-top O at room temperature. The infrared measurements (left inset) show the CO signal from a stoichiometric RuO2(110) surface (2123 cm-1, intensity 100%), from an on-top O covered surface RuO2(110) surface saturated at room temperature (2150 cm-1, intensity 20%), and from a 500 K O saturated RuO2(110) surface (thick solid line: intensity less than 1%).

Griesheim), H2 (purity 99.999%, Messer Griesheim), D2 (purity 99.7 atom %, Isotec), and CO (purity 99.97%, Messer Griesheim). For the DFT calculations, we used the Vienna ab initio simulation package (VASP),31 employing the projector augmented wave method32 with a plane wave cutoff of 37 Ry and a generalized gradient approximation for the exchange correlation term.33 The surface was modeled by five double layers of RuO2(110) (supercell approach). Consecutive RuO2(110) slabs were separated by a vacuum region of about 16 Å. Calculations of the adsorption energy and the geometry of the adsorbed hydroxyl group and water groups on RuO2(110) were performed using (1 × 1), (2 × 1), and (2 × 2) surface unit cells. The adsorbates were put only on one side of the slab. Dipole correction in the vacuum region was used to balance the different asymptotic electrostatic potentials. The core level energy levels were calculated by removing half of an electron from the core orbital and performing a self-consistent calculation of the electronic structure. Thus, screening effects (final state) of the core hole are included. The binding energy of the O1s core level was obtained as the energy difference of the Kohn-Sham eigenvalue of the core state from the Fermi energy. The experimental O1s core level shifts were assigned to specific O species on the surface on the basis of these DFT calculations. 3. Results and Discussion 3.1. Saturation Behavior of Oxygen on RuO2(110). Oxygen exposure to the stoichiometric RuO2(110) surface leads to the adsorption of atomic oxygen on-top of the 1f-cus Ru sites (referred to as on-top O or Oot).7 To saturate all 1f-cus Ru atoms by on-top O atoms, we cooled slowly the RuO2(110) film in an oxygen atmosphere of 10-7 mbar from 500 K down to 300 K. The corresponding 500 K thermal desorption (TD) spectrum of oxygen (cf. Figure 2) shows 20% more oxygen than a RuO2(110) surface that was saturated by oxygen at 300 K.34 With infrared spectroscopy, we were able to optimize the saturation O coverage by probing the on-top O vacancies with CO postexposure at 150 K. These infrared measurements in the CO stretching mode region (left inset in Figure 2) indicate clearly

Complex Interaction of H with RuO2(110) Surface

Figure 3. Thermal desorption spectrum of D2 when 1 L of D2 was exposed to a well-defined stoichiometric RuO2(110) surface at 114 K (stoichiometric RuO2(110)). No D2 signal is discernible in the temperature range of 250-350 K. Quite in contrast, the hydrogen desorption spectrum from a heavy reduced RuO2(110) with holes and small Ru islands at the rims shows hydrogen desorption in the temperature range of 200 to 350 K (heavily reduced RuO2(110)). A typical STM image of such a heavily reduced RuO2(110) surface is shown in the inset (taken from ref 30).

that the total coverage of on-top O vacancies is far below 1% when cooling the sample slowly in an oxygen atmosphere of 10-7 mbar. 3.2. Temperature Programmed Desorption and Reaction (TPD and TPR) of Hydrogen on RuO2(110). D2 TPD experiments show that molecular hydrogen desorbs around 110 K (cf. Figure 3), which is consistent with corresponding experiments of Wang et al.21 However, we do not observe any D2 desorption in the temperature range of 200-400 K, thus conflicting with recent experiments.21 To resolve this discrepancy, we purposely deteriorated the surface by dosing molecular hydrogen at room temperature and annealing the surface to 700 K. We know from previous STM work30 that this treatment leads inevitably to a RuO2(110) surface with small holes and small single atom high Ru islands decorating the rims of these holes (see inset in Figure 3). Recall that such a defected surface can only be restored by O2 exposure at temperatures above 650 K. Exposing such a non-stoichiometric RuO2(110) surface to 10 L of D2 at 115 K leads indeed to hydrogen desorption in the temperature range between 200 and 400 K (β-state in ref 21) as overlaid in Figure 3. The temperature range of the β-state is consistent with recent hydrogen adsorption/desorption experiments on the clean Ru(0001) surface.35 Therefore, we conclude that this β-state stems likely from the associative desorption of hydrogen atoms coming from the decorating Ru islands rather than from the decomposition of the so-called di-hydride species.21 In the next experiment, we exposed the stoichiometric RuO2(110) surface to 10 L of D2 at room temperature and recorded a TD spectrum of D2O. Subsequently, we exposed the stoichiometric RuO2(110) surface first to 10 L of D2 at room temperature and then saturated the surface by on-top O. Both D2O TD spectra are shown in Figure 4. Clearly, the production of D2O is increased by a factor of 20 when on-top O is present on the surface. In a further experiment, the sequence of the previous coadsorption experiment was reversed. The RuO2(110) surface was first saturated with on-top O at room temperature, thereby blocking 80% of the 1f-cus Ru atoms for further adsorption,7 and subsequently, the surface was exposed to 10 L of D2 at room temperature. If hydrogen enters the RuO2(110) surface exclusively via the 1f-cus Ru atoms, then blocking most of these sites by on-top O should suppress hydrogen adsorption

J. Phys. Chem. C, Vol. 111, No. 14, 2007 5365

Figure 4. Temperature programmed reaction of water D2O over RuO2(110) by (i) dosing only 10 L of D2 (dotted curve), (ii) dosing first 10 L of D2 and then 5 L of O2 (dashed-dotted line), and (iii) dosing first 5 L of O2 and then 10 L of H2 (solid line). Adsorption was in all cases performed at room temperature.

Figure 5. Hydrogen uptake is shown as a function of H2 dose. All exposure experiments were performed at room temperature. Gray curve: stoichiometric RuO2(110) surface was first exposed to H2, and then the surface was exposed to 5 L of O2. Black curve: 80% on-top O covered RuO2(110) surface was prepared by exposure of 5 L of O2, and then this surface was exposed to various doses of hydrogen.

and consequently the water formation. However, as indicated in Figure 4, the amount of produced D2O is even 3 times higher than for the sequence of adsorption (first 10 L of D2 and then 5 L of O2), suggesting that D2 adsorption may not necessarily require the presence of bare 1f-cus Ru atoms for its accommodation on the RuO2(110) surface. To produce substantial amounts of water, coadsorbed surface oxygen atoms in the form of on-top O need to be present on the RuO2(110) surface. These on-top O atoms pick up hydrogen atoms from the bridging O atoms, finally forming adsorbed water Oot-2H on top of 1f-cus Ru. It turns out that this hydrogen transfer mechanism is an important reaction step in the production of water over RuO2(110).36 We should note that even without an oxygen supply from the gas phase, some water was produced (cf. Figure 4, dotted line), of course, thereby reducing the RuO2(110) surface. This process of chemical reduction of RuO2(110) was studied in detail by a recent in situ surface X-ray diffraction experiment.37 Efficient reduction by hydrogen takes place above 400 K, the desorption temperature of adsorbed water. Below this temperature, reduction is inhibited by selfpoisoning due to the produced water that can be formed on the RuO2(110) surface even at room temperature.36 In a further set of TPD experiments, we studied the adsorption capacity of hydrogen on the stoichiometric RuO2(110) surface and the on-top O covered RuO2(110) surface as a function of the applied hydrogen exposure (cf. Figure 5). For these experiments, the total amount of produced water in TPR was used as a direct measure for the amount of adsorbed hydrogen on the surface. Clearly, the hydrogen uptake at saturation is doubled when the RuO2(110) surface is covered with on-top O. Assuming that the saturation coverage of hydrogen on the stoichiometric RuO2(110) surface is one (i.e., one hydrogen atom

5366 J. Phys. Chem. C, Vol. 111, No. 14, 2007

Figure 6. On-top O saturated RuO2(110) surface was exposed to 20 L of H2 at various surface temperatures. Arrhenius plot of the total amount of produced water is shown as a function of 1/T. From the slope, an activation energy Ea ) 6 kJ/mol is derived.

per bridging O atom), then the saturation H coverage on the 80% on-top O covered RuO2(110) is two. We may note that hydrogen adsorption on RuO2(110) can proceed via the 1f-cus Ru atoms if such active sites are available. This reaction pathway is at least important at low temperatures.21 However, we will show next that hydrogen does not need 1f-cus Ru atoms for its dissociative adsorption on RuO2(110).38 Even if all 1f-cus Ru sites are blocked by on-top O atoms, hydrogen adsorbs on the RuO2(110) surface. The dissociative adsorption of hydrogen over the on-top O saturated RuO2(110) surface is an activated process as demonstrated by the following experiment. We first blocked all the 1f-cus Ru sites by on-top O via O2 exposure at 10-7 mbar while slowly cooling the sample from 500 to 300 K. This on-top O saturated RuO2(110) surface was subsequently exposed to 20 L of H2 at various surface temperatures. After each hydrogen exposure, we took a water TPD and restored/reprepared the O saturated RuO2(110) surface thoroughly (cf. section 2). The integral water TPD signal is a direct measure of the total amount of hydrogen hosted on the RuO2(110) surface. The corresponding Arrhenius plot shown in Figure 6 implies an activation energy of 6 kJ/mol, indicating that the hydrogen adsorption is an activated process on the O saturated RuO2(110) surface that does not need free 1f-cus Ru atoms. A further question is related to the activity of the on-top O species in the uptake of hydrogen. To disentangle contributions from the bridging O and the on-top O species, the bridging O atoms of RuO2(110) were completely replaced by bridging CO molecules, exposing 10 L of CO at room temperature and flashing the sample briefly to 340 K.39 Subsequently, the 1f-cus Ru atoms were partly occupied by on-top O, dosing 5 L of oxygen at 170 K. At 170 K, bridging CO and on-top O were not able to recombine and form CO2.18 The on-top O coverage is only 0.3, as indicated in the right inset of Figure 2. After this chemical modification of the RuO2(110) surface, we exposed 50 L of H2 at 170 K. If the on-top O was covered by hydrogen, then the bridging CO could not recombine with this hydroxyl species (Oot-H) upon heating the sample, therefore not producing CO2. In fact, both the CO2 production and the CO desorption were not affected by H2 postexposure (cf. Figure 7). In addition, no water left the surface. In Figure 7, the CO and CO2 spectra are shown for the case where no H2 and where 50 L of H2 have been postexposed. These experiments indicate clearly that the on-top O species shows little tendency to adsorb hydrogen from the gas phase even if the free 1f-cus Ru sites are available. This result is quite surprising and conflicts partly with recent HREELS experiments.40 In this HREELS study, an efficient transfer of hydrogen from the 1f-cus Ru sites directly to the on-top O sites was proposed. Recall that in our experiment, 70% of the 1f-cus Ru sites still was available to absorb H2.

Knapp et al.

Figure 7. Tailoring the temperature programmed CO oxidation reaction over RuO2(110). In both cases, we started from a reduced RuO2(110) surface, where all bridging O atoms were replaced by CO molecules, and subsequently, 30% of the 1f-cus Ru sites were occupied by on-top O atoms by exposing the surface to 5 L of O2 at 170 K. The dotted lines indicate CO desorption (left) and CO2 production (right) during heating this surface to 650 K, while the solid line indicates the same spectra when this surface was also exposed to 50 L of H2 at 170 K.

3.3. O1s High-Resolution Core Level Spectroscopy (HRCLS). With HRCLS of O1s, one is able to discriminate OH groups from other oxygen species on the RuO2(110) surface. In Figure 8, we summarize the hydrogen adsorption experiments performed at room temperature. For the stoichiometric RuO2(110) surface, two O1s core levels are discernible (cf. Figure 8a). On the basis of DFT calculations, the O1s binding energies at 529.2 and 528.4 eV of the stoichiometric RuO2(110) surface have recently been assigned to bulkcoordinated O and bridging O atoms, respectively.2 The fitting parameters are the peak position, height, full width at halfmaximum (fwhm), and asymmetry of the profile. To reduce the number of fitting parameters, we used for all peaks the same asymmetry factor and the same fwhm (except for Oot-2H) as determined for the stoichiometric surface RuO2(110). Hence, for adsorbate covered RuO2(110) surfaces, already known O1s core levels contribute only one additional parameter, namely, the peak height, to the list of fitting parameters. O1s binding energy shifts are always referred to oxygen in the bulk environment of RuO2. The experimental O1s shifts are summarized in Table 1 and assigned to particular O species on the RuO2(110) surface based on DFT-calculated O1s shifts. Hydrogen adsorption on RuO2(110) leads to an additional feature at 530.3 eV in the O1s core level spectrum and a diminishing of the Obr component at 528.4 eV (cf. Figure 8a). This new feature at 530.3 eV is assigned to bridging O atoms capped by a single H atom (Obr-H: mono-hydride group, cf. Table 1). The O1s shift of Obr-H to a 1.1 eV higher binding energy is traced to a more covalent bonding of Obr-H (within the initial state model) than that of bulk O in RuO2. DFT calculations verify this assignment of Obr-H, determining the O1s core level shift to be 1.46 eV to a higher binding energy when a single hydrogen atom adsorbs on the bridging O atom (Obr-H complex). This value compares reasonably well with the experimentally found O1s shift of 1.1 eV. If, however, two hydrogen atoms occupy a single bridging O atom (forming a di-hydride species Obr-2H as proposed in ref 21), then DFT calculates an O1s core level shift of 4.63 eV to higher binding energies, a value that is not reconciled by any of our HRCLS experiments neither at room temperature nor at 110 K. Therefore, we presume that only one hydrogen atom can adsorb per bridging O atom. In Figure 8c, we compare the integral intensity of the O1s emissions of the Obr-species with that of the Obr-H species

Complex Interaction of H with RuO2(110) Surface

J. Phys. Chem. C, Vol. 111, No. 14, 2007 5367

Figure 8. O(1s) surface core level shift spectra of the stoichiometric RuO2(110) surface (A) in comparison with the RuO2(110) surface that was exposed to 20 L of H2 at 300 K (B). Clearly, an additional signal shows up at 530.3 eV, which is assigned to Obr-H; the bulk O1s peak of RuO2 appears at 529.2 eV. (C) Integral intensity of the O1s emission of the bridging oxygen (Obr) atoms and the hydroxyl group Obr-H as a function of H2 doses (sample temperature 300 K). Parallel to the decline of the Obr emission at 528.4 eV, the Obr-H emission at 530.3 eV increases. 100% corresponds to the oxygen signal of O bulk of RuO2(110). (d) O1s surface core level shift spectra of the RuO2(110) surface that was exposed to 20 L of H2 at 300 K and then exposed to 5 L of O2 at 300 K.

TABLE 1: Experimentally Observed O1s Shifts in Comparison with DFT-Calculated Onesa assignment Obulk Obr Oot Obr-H (1 × 1) O chemisorbed Oot-H Oot-H2 Obr-2H ObrsH‚‚‚Oot

exptl O1s binding energies (eV)

exptl O1s shift (eV)

theor O1s shift (eV)

529.2 528.4

reference -0.8

530.3 529.8

1.1 0.6

531.9

2.7

529.8

0.6

reference -0.87 -1.06 1.46 0.5 -0.80 2.96 4.86 0.72

a

On the basis of this comparison, the experimental O1s shifts are assigned to particular O and O-H species. Blank cells mean that this core level shift has not been observed experimentally. Except for Ru(0001)-(1 × 1)O, all other O species are from RuO2(110). For comparison with the O1s spectra, we also include the experimentally found binding energies of O1s.

as a function of the applied H2 doses. Parallel to the decline of the Obr emission at 528.4 eV, the emission of Obr-H at 530.3 eV increases. Somewhat surprising is that the intensity of Obr-H at saturation is significantly higher than that of the initial Obr

emission. This may be due to diffraction of the photoelectrons. Even for an exposure of 20 L of H2, there is still some emission of the bare Obr species left, indicating that not all bridging O atoms are capped by hydrogen. If the hydrogenated RuO2(110) surface (cf. Figure 8b: characterized by the Obr-H feature) is exposed to 5 L of O2 at room temperature (cf. Figure 8d), then the Obr-H related peak disappears almost completely in the O1s spectrum, and the Obr emission at 528.4 eV is almost recovered. This experiment indicates nicely that hydrogen atoms are transferred from the bridging O to the on-top O atoms, forming Oot-2H species and bare Obr atoms. The O1s spectrum in Figure 8d also reveals an additional feature at 531.9 eV, which is identified with adsorbed water sitting on top of the 1f-cus Ru atoms. This assignment is again based on DFT calculations (cf. Table 1), which indicate that the O1s peak of adsorbed water (Oot-2H) shifts by 2.96 eV with respect to bulk O of RuO2(110) in good agreement with the experimental value of 2.7 eV. These HRCLS results corroborate nicely with the proposed hydrogen transfer reaction30 in that the on-top O atoms pick up all the H atoms from the bridging O atoms via a hydrogen transfer reaction, forming Oot2H on the RuO2(110) surface. Annealing of the surface to 400 K leads to the disappearance of Oot-2H, which is consistent with corresponding TPD experiments (cf. Figure 4). We emphasize that according to the present HRCLS experiments, water formation takes place at a much lower surface temperature (below 200 K) so that annealing to 400 K is only required to desorb the produced water. This result is consistent with the first-order kinetics observed in the H2O TD spectra6,30 and is supported by DFT calculations. The DFT calculations in ref 30 indicate that the formation of water is activated by only 0.28 eV, while the adsorption energy of water is 1.0 eV. In Figure 9a,b, we present O1s core level shift experiments when the RuO2(110) surface is first exposed to 5 L of O2 and then postexposed to 17 L of hydrogen at room temperature. The O1s spectrum of a pure oxygen exposed RuO2(110) surface is practically identical to the stoichiometric RuO2(110) surface (cf. Figure 8a). The additional on-top O is not visible in the O1s spectrum since according to DFT calculations, the O1s core level of on-top O appears at the same binding energy as the bridging O atom. Recall that a room temperature exposure of 5 L of oxygen results in a RuO2(110) surface where about 80% of the 1f-cus Ru atoms is capped by on-top O atoms.7 If this on-top O covered RuO2(110) surface is exposed to 17 L of H2 at room temperature, then the O1s spectrum changes in two places. First, the emission of Obr at 528.4 eV is slightly reduced, and the Obr-H related emission appears, although with low intensity. Second, one peak appears at 531.9 eV, which is ascribed to Oot-2H (cf. Table 1). With an increasing H2 dose, the water peak at 531.9 eV increases, and two additional peaks appear in the high-energy flank at 530.3 and 529.8 eV at the expense of the Obr emission. For a H2 dose of 187 L, only a little Obr signal is left in the O1s spectrum. Two conclusions can be drawn from this experiment. First, hydrogen can readily adsorb on the RuO2(110) surface even if only 20% of the 1f-cus Ru atoms is available, which is consistent with corresponding TPD spectra in Figures 4 and 5. Second, low H2 exposures lead already to the production of adsorbed water (Oot-2H) and small amounts of Obr-H, while higher H2 exposures are able to form substantial amounts of Obr-H and an O species with an O1s emission at 529.8 eV. To clarify the chemical nature of the O1s emission at 529.8 eV (ObrsH‚‚‚Oot in Figure 9), we annealed the on-top O

5368 J. Phys. Chem. C, Vol. 111, No. 14, 2007

Knapp et al.

Figure 10. Structure model of hydrogen adsorption on RuO2(110). LEED determined values for the structural parameters are presented (small numbers). The reference plane (red line) consists of the Ru atoms attached to the bridging O atoms. The H-induced changes of the atomic coordinates of the 1f-cus Ru and bridging O are given with respect to stoichiometric RuO2(110) (large numbers above the gray arrows).

Figure 9. RuO2(110) surface was first exposed to 5 L of O2 and subsequently to (A) 17 L of H2 or to (B) 187 L of H2. Starting configuration: the RuO2(110) surface was exposed first to 5 L of O2 and subsequently to 187 L of H2. This surface was annealed to 500 K (C). In comparison with panel B, the O1s emission ObrsH‚‚‚Oot at 529.8 eV disappears together with the water species at 531.9 eV. Further annealing to 620 K also removes the Obr-H signal (D). O1s emission of Obr increases to about the intensity of the stoichiometric RuO2(110) surface.

saturated RuO2(110) surface exposed to 187 L of H2 to various temperatures. Up to 300 K, no changes in the O1s spectra occurred. However, upon annealing to 500 K (cf. Figure 9c), the O1s emission from water Oot-2H and also the unknown shoulder at 529.8 eV disappeared. Only the emission at 530.3 eV remained in the O1s spectrum. This experiment suggests that water or on-top O is responsible for the O1s emission at 529.8 eV since water desorbs around 400 K and oxygen desorbs around 450 K. We tentatively assign this feature at 529.8 eV to Obr-H, which forms a hydrogen bond (as the donor) to a (Oot, Oot-H, or Oot-2H) species sitting on the 1f-cus Ru atoms. The residual intensity at 529.8 eV is ascribed to a small fraction of (1 × 1) O areas on the surface. The assignment of ObrsH‚‚‚ Oot is further supported by DFT calculations, showing that the O1s binding energy of Obr-H decreases by 0.7 eV with respect to the isolated Obr-H species, when the bridging hydroxyl species performs a hydrogen bond to Oot-2H, Oot-H, or Oot (cf. Table 1). Finally, the sample was annealed to 620 K (cf. Figure 9d). Now, the intensity of the Obr-H related peak diminishes, while the Obr related emission increases in intensity. This behavior is consistent with the association of neighboring Obr-H groups to form water, Obr vacancies, and bare Obr. The (1 × 1) O related peak at 529.8 eV in Figure 9d increases a little upon increasing the temperature from 500 to 620 K. We suggest that this intensity increase is related to an increase of the (1 × 1) O

surface area. For a temperature as high as 620 K, the hydrogen covered RuO2(110) surface reduces in a way that metallic Ru islands appear on the surface (Figure 3). On these Ru islands, oxygen may adsorb, thus increasing the (1 × 1) O related peak intensity at 529.8 eV. 3.4. Hydrogen Adsorption on RuO2(110) at 110 K. 3.4.1. Surface Structure of H-RuO2(110): Quantitative LEED and DFT Calculations. LEED crystallography is not very sensitive to hydrogen; however, LEED is sensitive to hydrogen induced changes in the atomic structure of the RuO2(110) surface. Here, we present structural data for the case where we exposed the RuO2(110) surface to hydrogen at 110 K. At this low temperature, hydrogen adsorbs both on the 1f-cus Ru atoms and over the bridging O atoms as discussed in a recent HREELS study.21 The clean RuO2(110) surface is characterized by a strong inward relaxation of the bridging O atoms due to its reduced coordination.41 From simple chemical intuition, we expect that hydrogen adsorption over the bridging O position will partly remove the inward relaxation of the bridging O atom since the missing coordination is at least partly lifted by the coordination to H. In fact, this expected outward relaxation of the bridging O atom by 8 pm is found with a LEED structure analysis (cf. Figure 10). Also, the 1f-cus Ru atoms move 9 pm upward. This value is larger than that found by recent DFT calculations (3 pm) for H2 adsorption over the 1f-cus Ru atoms.42 But, we have to recall that in our study, both the 1f-cus Ru atoms and the Obr atoms are partly occupied by atomic hydrogen, while in the DFT study H2 molecules were put only on the 1f-cus Ru sites. This may rationalize the discrepancies in the 1f-cus Ru positions between the LEED and the DFT studies. 3.4.2. High-Resolution Core Level Spectroscopy of O1s and Ru-3d5/2. A recent HRCLS study2 indicated that the 1f-cus Ru atom of RuO2(110) generates a well-defined Ru-3d5/2 signature at 280.5 eV. When adsorbing H2 at low surface temperatures, this Ru-3d5/2 emission at 280.5 eV decreases with H2 exposure (cf. Figure 11). Therefore, this experiment is fully consistent with the molecular adsorption of hydrogen on the 1f-cus Ru atoms. We also measured O1s spectra when 4 L of D2 was exposed to RuO2(110) at 110 K and subsequently annealed to various temperatures (cf. Figure 12). Already at 110 K, the Obr-H emission is quite strong, while the O1s emission of bare Obr is significantly reduced. This measurement reveals that much of the hydrogen molecules has already dissociated at 110 K, forming Obr-H consistent with recent HREELS experiments.40

Complex Interaction of H with RuO2(110) Surface

Figure 11. HRCL spectrum of Ru-3d5/2 from the stoichiometric RuO2(110) surface in comparison with the RuO2(110) exposed to 4 L of D2 at 110 K. The difference plot in gray clearly indicates an emission at 280.5 eV, the binding energy of 1f-cus Ru atoms.

J. Phys. Chem. C, Vol. 111, No. 14, 2007 5369

Figure 13. Hydrogen induced deactivation of the recombination of CO with bridging O atoms from the RuO2(110) surface. With increasing pre-exposure of hydrogen up to 200 L, the CO2 production declines substantially, as indicated by the integral TPD CO2 intensity as a function of H2 pre-doses (inset).

Figure 12. RuO2(110) is exposed to 4 L of H2 at low surface temperature (110 K) following how the O1s core levels change with increasing annealing temperature. The small peak at 529.8 eV (gray curve) is related to chemisorbed O in the (1 × 1) O phase. There is an additional component at 531.9 eV that is assigned to adsorbed water from the residual gas.

Figure 14. Ball-and-stick model of the RuO2(110) surface, summarizing the microscopic steps in the hydrogen transfer reaction.30 On-top O sits on top of 1f-cus Ru, forming a hydrogen bond (broken line) with an adjacent hydroxyl group Obr-H (left). Via hydrogen transfer, the H atom of Obr-H moves to the on-top O atom forming a hydroxyl group Oot-H without a noticeable activation barrier. Oot-H can establish a hydrogen bond with a further Obr-H species (middle). By a further hydrogen transfer, this Oot-H transforms into Oot-2H (activated by 0.28 eV), a water molecule adsorbed on top of the 1f-cus Ru site (right), which is further stabilized by a hydrogen bond to Obr.

Upon annealing the surface to 150 K, both the Obr-H emission and the Obr emission do not change in intensity. The small shoulder at 531.9 eV is assigned to adsorbed water from the residual gas. 3.5. Controlling the Reaction Pathway on the Atomic Scale: Oxidation of CO. We are now able to use the atomic scale knowledge of the hydrogen interaction with the RuO2(110) surface to design specific reaction pathways for directing the fate of the adsorbed CO molecules. For instance, above room temperature, CO can easily recombine with bridging atoms Obr of the stoichiometric RuO2(110) surface to form CO2, thereby reducing the underlying RuO2(110) surface.3 However, preexposing molecular hydrogen to RuO2(110) transforms the bridging O atoms into hydroxyl groups that should not be prone to react with CO. In this way, we can control the actual activity of the RuO2(110) model catalyst in the oxidation of CO on the atomic scale. Figure 13 summarizes this kind of atomically controlled activity experiment. We exposed the stoichiometric RuO2(110) surface at room temperature to various H2 doses up to 200 L and saturated the surface by post-dosing 10 L of CO at 200 K. Without H2 preexposure, most of the CO was transformed to CO2 upon annealing the surface to 600 K, which is consistent with a previous study.18 With increasing the H2 dose, the CO2 production declines substantially (cf. Figure 13). At the same time, the desorption signal of CO increases (not shown), so that the total amount of adsorbed CO before the reaction is

practically independent of the H2 pre-doses. In the inset of Figure 13, the integral CO2 signal is shown as a function of the H2 pre-exposure. This experiment nicely corroborates the previous intuitive notion of the rational passivation of RuO2(110) by hydrogen exposure. 3.6. DFT Calculations. 3.6.1. Hydrogen Transfer Reaction. In Figure 4, we showed that the adsorption of on-top O onto a hydrogen covered RuO2(110) surface increases the efficiency of water formation by a factor of 20. The microscopic steps of this hydrogen transfer reaction30 were investigated by DFT calculations, and the results are summarized in Figure 14. For the DFT calculations, the initial state of this reaction is defined by hydrogen atoms capping the bridging O atoms. The coadsorbed on-top O atom is subject to a hydrogen bond by Obr-H, inducing a hydrogen transfer to form Oot-H without a noticeable activation barrier. Subsequently, the Oot-H bends over to a neighboring Obr-H group, establishing an accepting hydrogen bond with Obr-H. The reaction coordinate is defined as the separation of the O position in Oot-H and the hydrogen atoms sitting on the bridging O atom. DFT calculations determined the energy barrier to be 0.28 eV for the formation of Oot-2H (i.e., water adsorbed on top of the 1f-cus Ru atoms). The produced water molecule adsorbs quite strongly by 1.02 eV over the 1f-cus Ru atoms, in agreement with a recent DFT study.43 3.6.2. DFT-Calculated O1s Core Level Shifts. For a firm assignment of the experimentally observed O1s core level shifts,

5370 J. Phys. Chem. C, Vol. 111, No. 14, 2007

Knapp et al.

TABLE 2: Shifts of O1s Binding Energies as Determined by DFT Calculationsa structure

O3f

Obr-1Hbr

Obr

(1 × 1) Obr (1 × 1) Obr + Oot (1 × 2) Obr-H + Obr (B) (1 × 1) Obr-2H (D) (1 × 2) 2Obr-2Hzigzag (D) (1 × 2) 2Obr + Oot + Oot-2Hot (N)

+0.22 +0.08 +0.28 +0.46 +0.49 +0.07

-0.87 -0.96 -0.66

(1 × 2) 2(Oot-H + Obr-H ) (L)

+0.03

(1 × 2) 2Oot + Obr-H + Obr (E)

+0.05

-0.73

(1 × 2) 2Obr + Oot + Oot-H (F)

-0.03

(1 × 2) 2Obr + (2Oot-1H)zigzag (H)

-0.07

-0.76 -1.13 (H-bond to Oot-H -0.74

Obr-2Hbr

Oot-1Hot

Oot

Oot-2Hot

-1.06 +1.46

s -1.08 -0.72 (attached to Oot-2H)

s s

+4.80 +4.86 s

s -1.22

s s

s +2.96

-0.39 acceptor and donor s

s

-0.80

s

-0.69

s

+0.72 acceptor and donor +0.75 donor

s

s

s

s

s

-0.97 -1.1 1 -0.93

s

s

s

Reference O1s is bulk O of RuO2 (110). All structures are related to RuO2(110) ) (1 × 1)Obr. Details on the structures can be found in Figure 15A-N). (1 × 1)Obr corresponds to the stoichiometric RuO2(110) surface. (1 × 1)Obr + Oot corresponds to the on-top O saturated RuO2(110) surface. The nomenclature for the structure consists of the surface unit cell, the number, and the species of surface atoms in the unit cell. a

TABLE 3: Adsorption Energies of H-Complexes on Stoichiometric and On-Top Covered RuO2(110) Surface as Determined by DFT Calculationsa structure

adsorption energy w.r.t. H2 (eV)

(1 × 2) 2Obr+H2(cus) (A) (1 × 1) Obr-H (B) (1 × 2) Obr + Obr-H (B) (1 × 2) 2Obr-H zigzag (C) (1 × 1) Obr-2H dihydride (D) (1 × 2) Obr + Obr-2H dihydride (D) (1 × 1) (Oot + Obr-H) (E) (1 × 2) (2Oot + Obr + Obr-H) (E) (1 × 1) (Oot-H + Obr) (F) (1 × 2) (Oot + Oot-H + 2Obr) (F) (1 × 1) (Oot-H + Obr) without H-bond (1 × 2) (2Oot + 2Obr-H) zigzag (G) (1 × 2) (2Oot-H + 2Obr) zigzag (H) (1 × 1) (Oot-H + Obr-H) (I) (1 × 2) (Oot + Oot-H + Obr + Obr-H) (J) (1 × 2) (Oot + Oot-H + Obr + Obr-H) two H-bonds to one Oot (K) (1 × 2) (2Oot-Hot + 2Obr-Hbr) zigzag (L) (1 × 2) (2Obr + 2Oot-Hot) zigzag (M) (1 × 1) (Oot-2H + Obr) (N) (1 × 1) (Oot-2H + Obr) (N) without H-bond (1 × 2) (Oot + Oot-2H + 2Obr) (N) (1 × 2) (2Oot + Obr + Obr-2H) (D) (2 × 2) (2Oot + Obr + Obr-2H) (D)

+0.36 +1.92 +2.33 +1.97 +0.29 +0.70 +2.75 +3.07 +2.73 +2.86 +1.93 +2.76 +3.21 +2.79 +2.91 +2.94 +2.78 +3.21 +2.69 +2.51 +2.91 +0.68 +0.68

adsorption energy w.r.t. H2O (eV)

adsorption energy: literature +0.32 eV/H242 +1.88 eV/H242

+0.08 +0.67

+0.92 +1.23 +0.94 +1.07 +0.97 +0.93 +1.23 +0.84 +0.67 +0.94 +0.25 +0.83

+1.88 eV/H242 +0.56 eV/H242

+0.91 eV/H2O43 +0.56 eV/H242

a Reference energy is either H or water in the gas phase. For comparison, we indicate adsorption energies available from the literature.42,43 2 Positive energies mean exothermal adsorption. Except for the proper coverage, the schematic details of the structures can be found in Figure 15A-N.

we used DFT-calculated O1s core level shifts as summarized in Table 2. The reference (i.e., the O1s shift is set to zero) is O in a bulk environment of RuO2. Obr and Oot reveal similar O1s core level shifts, therefore they are difficult to discriminate in experimental O1s HRCL spectra. The hydroxyl species of the bridging O atom (Obr-H) is theoretically shifted by 1.46 eV, which is consistent with the corresponding experimental value of 1.1 eV. Generally, the shift of O1s to higher binding energies on hydrogen adsorption suggests a more covalent bond. This view is also reconciled with the Oot-H species (-0.4 to -0.8 eV) as compared to Oot (about -1.0 eV). The HRCLS experiments indicate that for very high H2 exposure, there is an additional O1s core level shift at +0.8 eV. From our DFT calculations, we infer that this O1s feature is related to Obr-H, which has an additional hydrogen bond to Oot, Oot-H, or Oot-2H. Table 2 discloses a general trend in that hydrogen bonding leads to a shift of the O1s core level of the bridging O species by more than 0.3 eV. But also, the O1s binding energy of the

O3f species in the surface plane (cf. Figure 1) changes by up to 0.3 eV upon hydrogen adsorption over the bridging O atoms. The bridging water species (Obr-2H: di-hydride species) reveals a very large shift of about 4.8 eV in the DFT calculations. Experimentally, this O1s feature has never been observed. The water species of the on-top O atom (Oot-2H) reveals a shift in O1s of +2.96 eV in DFT, which is reconciled with our HRCL experiment (of about +2.7 eV). 3.6.3. Stability and Kinetics of Various Hydrogen Adsorption Phases on RuO2(110). A collection of total energies for various hydrogen adsorption geometries on the stoichiometric and on-top O covered RuO2(110) surface is compiled in Table 3 and Figure 15. The surface structures are sketched by small balls for cus Ru atoms and big balls for oxygen atoms. The adsorption energies are given with respect to both molecular hydrogen and water in the gas phase. In a previous study,42 some of the adsorption energies on the stoichiometric RuO2(110) surface were already determined with respect to H2.

Complex Interaction of H with RuO2(110) Surface

J. Phys. Chem. C, Vol. 111, No. 14, 2007 5371

Figure 15. Schematic ball-and-stick model of the hydrogen structures of RuO2(110) used in Tables 2 and 3. Dashed lines indicate hydrogen bonds. The shown unit cell is repeated infinitely using periodic boundary conditions.

Here, we extend the DFT calculations to the case where on-top O is also present on the RuO2(110) surface. From inspection of Table 3, it is clear that the present total energy calculations agree quantitatively with those determined by previous DFT studies.42,43 Molecular hydrogen is stable on the 1f-cus Ru site by about 0.35 eV with respect to molecular hydrogen in the gas phase in agreement with a desorption temperature of about 100 K (cf. Figure 3). On the stoichiometric RuO2(110) surface, the most stable H configuration is Obr-H in an inclined configuration, which is stabilized by about 2 eV against molecular H2 in the gas phase. The dihydride species Obr-2H is stabilized by 0.68 eV against H2 release into the gas phase. This value is too small to explain the stability of a dihydride species up to 350 K and therefore to explain the β-state in the H2 desorption spectra of ref 21. The very low site specificity for the formation of hydroxyl groups on the on-top O covered RuO2(110) surface (Obr-H: 2.75 eV vs Oot-H: 2.73 eV with respect to H2 in the gas phase and about 1.0 eV against water in the gas phase) is quite puzzling, considering the large binding energy difference between bridging O and on-top O atoms of more than 1 eV7,17 with respect to molecular oxygen in the gas phase. This finding is consistent with a recent DFT study.43 The hydrogen bond of Oot-H to Obr and that of Obr-H to Oot amounts in both cases to 0.45 eV. The water species Oot-2H is stable by 1.0 eV against water in the gas phase in good agreement with corresponding H2O TPD.3 To estimate the diffusion barrier of hydrogen atoms along the bridging O rows and the on-top O rows, we determined total energies of the corresponding transition states. From these energies, we infer a diffusion barrier for H atoms along Obr

rows of 2.5 eV and one of only 0.22 eV along the on-top O rows. Therefore, hydrogen is practically immobile along the Obr rows, while hydrogen is very mobile along the on-top O row. This finding has a great impact on the kinetics of catalyzed reactions over RuO2 when H atoms are abstracted from the reacting molecules such as in the dehydrogenation of simple alcohols. The hydrogen transfer from Obr to Oot is activated by only 0.28 eV, allowing for a fast hydrogen transfer reaction. For the diffusion barriers of the Oot-H species along the 1f-cus rows, we determined a value of 0.5 eV. This value is much smaller than that of bare on-top O atoms, which amounts to be about 1 eV.7,17 4. Conclusion Detailed knowledge about the interaction of hydrogen with late transition metal oxide surfaces, such as RuO2(110), is mandatory for a comprehensive understanding of the whole class of oxide catalyzed dehydrogenation reactions. The interaction of hydrogen with RuO2(110) is of particular importance for the cold methanol combustion in fuel cells since Ru is used to improve the CO tolerance of the Pt catalyst. Under oxidizing conditions, one expects that Ru will quickly transform into RuO2 on the electrode surface. But, RuO2 is also used as an efficient cathode in the water electrolysis.44 Photoelectrochemical water splitting for the hydrogen production uses RuO2 as a photocatalyst.45 Therefore, the hydrogen interaction with RuO2 is not only fascinating from a fundamental point of view but is also of practical relevance. In this paper, we employed a variety of different techniques including TPD, TPR, LEED, HRCLS, and DFT calculations to comprehensively explore the interaction of hydrogen with

5372 J. Phys. Chem. C, Vol. 111, No. 14, 2007 RuO2(110). At low temperatures (below 120 K), hydrogen adsorbs partly molecularly as indicated by the alteration of the Ru-3d5/2 core level shifts in the energy range of the 1f-cus Ru atoms. This finding is consistent with recent HREELS experiments.21 However, even at 115 K, most of the hydrogen molecules dissociate and form Obr-H hydroxyl groups as evidenced by the O1s HRCL spectra. At room temperature, the hydrogen interaction is dominated by the bridging O atoms on the stoichiometric RuO2(110) surface. On the stoichiometric RuO2(110) surface, this process may be mediated by the molecular adsorption of H2 on top of the 1f-cus Ru sites as discussed in ref 21. However, the 1f-cus Ru atoms are not required for the adsorption of hydrogen, at least not at higher surface temperatures, above 250 K. This has been demonstrated by saturating all the 1f-cus Ru sites by on-top O and then exposing the surface to hydrogen. The H2 adsorption process turns out to be activated by 6 kJ/mol so that at 100 K, the uptake of hydrogen is very small. Therefore, the activated adsorption process of hydrogen on the on-top O saturated RuO2(110) surface is able to explain both recent HREELS experiments at 90 K40 and recent adsorption experiments at room temperature.3 For the on-top O covered RuO2(110) surface, we provided experimental evidence that the bridging O atoms harvest the hydrogen atoms from the gas phase (via the 1f-cus Ru atoms if available), while the on-top O atoms pick up these hydrogen atoms ultimately to form water. This process of hydrogen transfer has recently been identified3 and is nicely corroborated by the present HRCLS experiments of O1s. Therefore, under oxidizing reaction conditions, hydrogen is efficiently converted to water via this hydrogen transfer reaction. Hydrogen adsorption on RuO2(110) changes the active basic Obr sites into inactive Obr-H sites. To put this idea into practice, hydrogen coadsorption was used to control the CO oxidation reaction on the surface. We found that pre-exposure of hydrogen leads to a reversible deactivation of the CO oxidation over RuO2(110). The degree of deactivation can be controlled by the total amount of adsorbed hydrogen. There are a few discrepancies with a recent HREELS study40 that may call for some critical comments. First, the thermal desorption of hydrogen around room temperature from RuO2(110) that was exposed to 2 L of H2 at 90 K is debatable. In none of our experiments have we ever observed hydrogen desorption around room temperature from a well-prepared RuO2(110) surface. Rather, hydrogen always desorbs around 100 K when the surface has been exposed to it at low temperatures. Reducing the RuO2(110) surface purposely at temperatures above 650 K results in an oxide surface with small Ru islands decorating small holes in the RuO2(110) surface (cf. inset of Figure 3). From this reduced RuO2(110) surface, the H2 TPD is similar to that published in ref 21. Therefore, we conclude that the H2 desorption around 300 K21 stems from the Ru islands of an irreversibly reduced RuO2(110) surface. We emphasize that the particular preparation recipe applied in refs 21 and 40 will inevitably lead to such a reduced RuO2(110) surface and therefore to conclusions that may not be valid for the stoichiometric surface. We also have no experimental indication for the existence of a bridging water species Obr-2H (di-hydride in ref 21) when the stoichiometric RuO2(110) surface is exposed to hydrogen at low surface temperatures. From our DFT calculation, the O1s of Obr-2H should be shifted by more than 4 eV, but this shift has never been observed with HRCLS in the temperature range from 110 to 300 K. Rather, HRCLS indicates only an Obr-H species in O1s spectra and H2 on top of the 1f-cus Ru atoms in

Knapp et al. the Ru-3d5/2 spectra. Altogether, we dismiss the existence of the di-hydride species Obr-2H on RuO2(110). A still open question concerns the actual adsorption mechanism of H2 onto the RuO2(110) surface. Here, further DFT calculations are required. If the 1f-cus Ru atoms are free, hydrogen adsorption may take place via these sites, finally forming Obr-H species on the surface. For the stoichiometric RuO2(110) surface, DFT calculations indicate a high activation barrier (without providing a specific value) for the direct adsorption of hydrogen over the bridging O atoms.42 Therefore, the authors of ref 42 concluded that hydrogen adsorption on RuO2(110) needs 1f-cus Ru sites. Our experiments indicate, however, that the bridging O atoms are important for the adsorption of hydrogen on the on-top O saturated RuO2(110) surface, where practically no 1f-cus Ru atoms are available. Therefore, further DFT calculations are planned to elucidate the adsorption of H2 over the on-top saturated RuO2(110) surface. Only the close interplay of various techniques was able to unravel the complex interaction of hydrogen with the stoichiometric and O saturated RuO2(110) surface. In the future, this knowledge can be used to understand the dehydrogenation of NH3 and simple alcohols. But, this knowledge can also be used to purposely design reaction pathways on the RuO2(110) surface to improve the selectivity of chemical reactions. Acknowledgment. Enlightening discussions with Stefan Wendt are gratefully acknowledged. H.O. and D.C. gratefully acknowledge financial support from the Deutsche Forschungsgemeinschaft (DFG, SPP 1091). We thank Attila Farkas and Georg Mellau for there skillful assistance in the FTIR measurements shown in Figure 2. We thank the John von Neumann Institute for Computing for generous super-computing time. E.L. thanks the Swedish Research Council for financial support. Partial financial support is acknowledged from the European Union under Contract NMP3-CT-2003-505670 (NANO2). References and Notes (1) Over, H. Appl. Phys. A 2002, 75, 37. (2) (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. 2002, 504, 196. (3) Over, H.; Seitsonen, A. P.; Lundgren, E.; Schmid, M.; Varga, P. J. Am. Chem. Soc. 2001, 123, 11807. (4) Kim, Y. D.; Seitsonen, A. P.; Over, H. Phys. ReV. B 2001, 63, 5419. (5) Chu, Y. S.; Lister, T. E.; Cullen, W. G.; You, H.; Nagy, Z. Phys. ReV. Lett. 2001, 86, 3364. (6) Lobo, A.; Conrad, H. Surf. Sci. 2003, 523, 279. (7) Kim, Y. D.; Seitsonen, A. P.; Wendt, S.; Wang, J.; Fan, C. Y.; Jacobi, K.; Over, H.; Ertl, G. J. Phys. Chem. B 2001, 105, 3752. (8) Madhavaram, H.; Idriss, H.; Wendt, S.; Kim, Y. D.; Knapp, M.; Over, H.; Aβmann, J.; Lo¨ffler, E.; Muhler, M. J. Catal. 2001, 202, 296. (9) Wang, Y.; Lafosse, A.; Jacobi, K. J. Phys. Chem. B 2002, 106, 5476. (10) Wang, Y.; Jacobi, K.; Ertl. G. J. Phys. Chem. B 2003, 107, 13918. (11) Paulus, U. A.; Wang, Y.; Bonzel, H. P.; Jacobi, K.; Ertl, G. Surf. Science 2004, 566, 989. (12) Wang, Y.; Jacobi, K.; Scho¨ne, W.-D.; Ertl, G. J. Phys. Chem. B 2005, 109, 7883. (13) Over, H.; Kim, Y. D.; Seitsonen, A. P.; Wendt, S.; Lundgren, E.; Schmid, M.; Varga, P.; Morgante, A.; Ertl, G. Science 2000, 287, 1474. (14) Liu, Z.-P.; Hu, P.; Alavi, A. J. Chem. Phys. 2001, 114, 5957. (15) Kim, Y. D.; Over, H.; Krabbes, G.; Ertl, G. Top. Catal. 2001, 14, 95. (16) Fan, C. Y.; Wang, J.; Jacobi, K.; Ertl, G. J. Chem. Phys. 2001, 114, 10058. (17) (a) Seitsonen, A. P.; Over, H. Ruthenium Dioxide, a Versatile Oxidation Catalyst: First Principles Analysis. In High-Performance Computing in Science and Engineering in Munich 2002; Springer-Verlag: Berlin, 2003; pp 171-180. (b) Seitsonen, A. P. http://www.iki.fi/∼apsi/Science/ movies/.

Complex Interaction of H with RuO2(110) Surface (18) Wendt, S.; Seitsonen, A. P.; Kim, Y. D.; Knapp, M.; Idriss, H.; Over, H. Surf. Sci. 2002, 505, 137. (19) Reuter, K.; Scheffler, M. Phys. ReV. B 2003, 68, 45407. (20) Gong, X.-Q.; Liu, Z.-P.; Raval, R.; Hu, P. J. Am. Chem. Soc. 2004, 126, 8. (21) Wang, J.; Fan, C. Y.; Sun, Q.; Reuter, K.; Jacobi, K.; Scheffler, M.; Ertl, G. Angew. Chem., Int. Ed. 2003, 42, 2151. (22) Over, H.; Muhler, M. Prog. Surf. Sci. 2003, 72, 3. (23) (a) Yamaguchi, K.; Mizuno, N. Angew. Chem., Int. Ed. 2002, 114, 4720. (b) Ibid, Angew. Chem., Int. Ed. 2002, 41, 4538. (24) Zhan, B.-Z.; White, M. A.; Sham, T.-K.; Pincock, J. A.; Doucet, R. J.; Rao, K. V. R.; Robertson, K. N.; Cameron, T. S. J. Am. Chem. Soc. 2003, 125, 2195. (25) Over, H.; Bludau, H.; Skottke-Klein, M.; Moritz, W.; Ertl G.; Campbell, C. Phys. ReV. B 1992, 45, 8638. (26) (a) http://www.maxlab.lu.se/beamline/hd-bl.html. (b) Nyholm, R.; Andersen, J. N.; Johansson, U.; Jensen, B. N.; Lindau, I. Nucl. Instrum. Methods Phys. Res., Sect. A 2001, 520, 467. (27) Donjiac, S.; Sunjic, M. J. Phys. C 1970, 3, 285. (28) Over, H.; Seitsonen, A. P.; Lundgren, E.; Schmid, M.; Varga, P. Surf. Sci. 2002, 515, 143. (29) Over, H.; Knapp, M.; Lundgren, E.; Seitsonen, A. P.; Schmid, M.; Varga, P. Chem. Phys. Chem. 2004, 5, 167. (30) Knapp, M.; Crihan, D.; Seitsonen, A. P.; Over, H. J. Am. Chem. Soc. 2005, 127, 3236. (31) Kresse, G.; Furthmu¨ller, J. Comput. Mater. Sci. 1995, 6, 15. (32) Kresse, G.; Joubert, D. Phys. ReV. B 1998, 59, 1758.

J. Phys. Chem. C, Vol. 111, No. 14, 2007 5373 (33) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3365. (34) Crihan, D.; Knapp, M.; Seitsonen, A. P.; Over, H. J. J. Phys. Chem. B 2006, 110, 22947. Comment on ref. 40. (35) Kostov, K. L.; Widdra, W.; Menzel, D. Surf. Sci. 2004, 560, 130. (36) Knapp, M.; Crihan, D.; Resta, A.; Lundgren, E.; Andersen, J. N.; Seitsonen, A. P.; Schmid, M.; Varga, P.; Over, H. J. Phys. Chem. B 2006, 110, 14007. (37) He, Y. B.; Knapp, M.; Lundgren. E.; Over, H. J. Phys. Chem B 2005, 109, 21825. (38) We should emphasize that this adsorption process is different from an Eley Rideal-type mechanism, as Obr and Oot are considered as active surface sites of the oxide catalysts that are able to dissociate molecular hydrogen. The presence of basic (oxygen site) and acidic sites (metallic sites) renders oxide catalysts so viable in comparison with metal catylysts. The authors in ref 40 may be confused by a comparison with metallic catalysts where indeed the adsorption of H2 over an already adsorbed O atom would be considered as an Eley Rideal-type reaction. (39) Seitsonen, A. P.; Kim, Y. D.; Knapp, M.; Wendt, S.; Over, H. Phys. ReV. B 2002, 65, 5413. (40) Jacobi, K.; Wang, Y.; Ertl, G. J. Phys. Chem. B 2006, 110, 6115. (41) Kim, Y. D.; Seitsonen, A. P.; Over, H. Surf. Sci. 2000, 465, 1. (42) Sun, Q.; Reuter, K.; Scheffler, K. Phys. ReV. B 2004, 70, 235402. (43) Sun, Q.; Reuter, K.; Scheffler, K. Phys. ReV. B 2003, 67, 205424. (44) Burke, L. D.; Naser, N. S. J. Appl. Electrochem. 2005, 35, 931. (45) Dhere, N. G.; Jahagirdar, A. H. Thin. Solid Films 2005, 480, 462.