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Nov 7, 2014 - Atomic-Scale View on the H2O Formation Reaction from H2 on O‑Rich. RuO2(110). Yinying Wei, Umberto Martinez, Lutz Lammich, Flemming ...
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Atomic-Scale View on the H2O Formation Reaction from H2 on O‑Rich RuO2(110) Yinying Wei, Umberto Martinez, Lutz Lammich, Flemming Besenbacher, and Stefan Wendt* Interdisciplinary Nanoscience Center (iNANO) and Department of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark S Supporting Information *

ABSTRACT: The H2O formation reaction from H2 on O-rich RuO2(110) was studied by temperature-programmed desorption and reaction (TPD/TPR) and scanning tunneling microscopy (STM) measurements and density functional theory (DFT) calculations. On the one hand, following H2 adsorption at 270 K, our TPD/TPR measurements reveal that the on-top O species (Oot) enhances the sticking probability of H2, thus facilitating the H2 adsorption and dissociation on O-rich RuO2(110). On the other hand, for low H2 adsorption temperature (170 K), the limited mobility of Had species hinders H2 adsorption at a high coverage of preadsorbed Oot. To better understand the strong influence of the adsorption temperature and the interplay between coadsorbed species, we conducted DFT calculations and high-resolution STM measurements. Two distinct adsorbate configurations, Had−Oot and Oot−Had−Oot, are identified by STM. Mechanisms and molecular models for H2 dissociation and Had diffusion on O-rich RuO2(110) are proposed.

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saturation coverage,9,11−13 but much lower H2 exposure is needed at low substrate temperatures.9 Following our study of how H2 interacts with s-RuO2(110), we have now further investigated how H2 adsorbs and reacts on O-rich RuO2(110). Supplying O2 to s-RuO2(110) leads to RuO2(110) surfaces with weakly bonded on-top O atoms (Oot), which reside atop Rucus sites.14−16 In the following, we denote such O-rich RuO2(110) surfaces as o-RuO2(110). Figure 1a shows the side view and top down view of this surface, where Obr, Rucus, and Oot atoms are depicted in different colors (red, gray, and pink, respectively). After exposing o-RuO2(110) to H2, we explored the reaction between Oot and Had species, which leads to the formation of H2O.11−13 Studying the H2O formation reaction from coadsorbed Oot and Had species has a double-fold motivation. First, this reaction is inverse to H2O splitting, a reaction on which huge efforts have been dedicated to produce hydrogen via a photocatalytical route as a clean energy source. Mixed RuO2−TiO2 oxides have appeared as a promising candidate for this reaction;17 thus, valuable information can be gained by studying how the two elemental products H2 and O2 are interacting on RuO2. Second, since hydrocarbons release H during their oxidation (dehydrogenation), it is important to understand how hydrogen interacts with weakly adsorbed O species, Oot. In this article, we report on a series of temperatureprogrammed desorption and reaction (TPD/TPR) studies addressing the interaction of H2 with o-RuO2(110). We tested

INTRODUCTION Ruthenium dioxide (RuO2) is a very useful material for applications in catalysis, electrochemical cells, and photocatalysis.1,2 In photocatalytic water splitting systems, RuO2 is used as an efficient cocatalyst for both reduction and oxidation half-reactions.1−4 In catalysis, RuO2 is used because of its unique redox properties, making it efficient for catalyzing CO oxidation and dehydrogenation of methanol, HCl, and NH3.1,5 In electrocatalysis, RuO2 is used in particular as a component of dimensionally stable anodes to produce Cl2 and NaOH.1 Furthermore, from a fundamental research perspective, RuO2 serves as a versatile model system for studying the elemental steps of catalytic reactions,1,5 because well-ordered RuO2(110) films can be readily prepared on Ru(0001).6−8 In a previous paper, we have studied the interaction of H2 with the stoichiometric RuO2(110) surface (s-RuO2).9 The sRuO2(110) surface is characterized by alternating rows of two kinds of under-coordinated surface atoms, the bridging oxygen atoms, Obr, which are coordinated to only two Ru atoms underneath, and the so-called Rucus atoms, i.e., coordinatively unsaturated Ru sites.6,7 After exposing s-RuO2(110) to H2, we found three different adsorption configurations, depending on the sample temperature. At temperatures lower than 90 K, H2 adsorbs molecularly on-top on Rucus sites, as was directly imaged by STM,9 in agreement with previous results.10,11 At intermediate sample temperatures such as 150 K, H2 molecules adsorb dissociatively, leading to pairs of H species with one H adsorbed on-top on a Rucus site and the second at an adjacent Obr atom (Had−Rucus/Had−Obr).9 For H2 adsorption at room temperature, pairs of bridging hydroxyls (Had−Obr) occur on the surface.9,12 In addition, we reported that sticking of H2 on sRuO2(110) is strongly dependent on the temperature. At room temperature, a high exposure of H2 is required to reach © 2014 American Chemical Society

Received: September 19, 2014 Revised: November 6, 2014 Published: November 7, 2014 27989

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changes. An infrared pyrometer was used to calibrate the temperature of the sample during vacuum-annealing. The STM experiments were mainly performed in a commercial UHV chamber (SPECS, Germany) with a base pressure in the low 10−11 Torr range.19 This chamber was equipped with an Aarhus STM (SPECS, Germany), as well as with standard facilities for sample preparations. During imaging, the sample temperature was maintained by the cold STM block at a range between 100 and 150 K. All the STM images were obtained in the constant current mode (bias voltage: +0.7 to +1.2 V, tunneling current: ∼0.3 nA). Electrochemically etched tungsten tips were used in all the STM experiments. The Ru(0001) single crystals (Mateck, GmbH) were prepared using established protocols. Typically, we conducted 7 to 8 cycles of Ar+ sputtering (1.5 keV, 10 mA), followed by vacuum-annealing at 1000 K, as well as oxygen treatments to remove any traces of C contamination. Clean and atomically flat Ru(0001) surfaces were obtained, as judged by STM measurements. A short flash to ∼1200 K was applied directly before the surface oxidation was accomplished. RuO2(110) films of 10−20 Å in thickness were prepared by dosing 6 × 106 langmuir 18O2 to a clean Ru(0001) crystal at ∼660 K (1 L = 1 × 10−6 Torr × s). Oxygen was dosed via a doser, where a glass capillary array was used to enhance the local pressure by a magnitude of 102 at UHV conditions. During oxidation, the sample−doser distance was ∼1 mm, resulting to ∼1 × 10−2 Torr local oxygen pressure in front of the sample, while the background pressure did not exceed ∼1 × 10−4 Torr. 18O2 was used to exclusively probe the water that was formed on the sample. 18O2 (Sigma-Aldrich) and H2 (Air Liquide) were used as received. As-prepared RuO2(110) films were characterized by the presence of Obr vacancies. We thus used an “oxygen healing recipe”8,9 to guarantee the stoichiometry of the film. This procedure was conducted before each single trial, including TPD/TPR measurement and STM imaging. The o-RuO2(110) surfaces with different Oot coverage were prepared in a precise and consistent way, as described in Figure S1 in the Supporting Information. The water produced on the RuO2(110) surface was calibrated via a series of H2O-TPD experiments on sRuO2(110) in which the first and the second monolayer (ML) peaks of H2O were observed; see Figure S2 (Supporting Information). Exposing s-RuO2(110) to 3 L H2 at 90 K leads to the partial occupation of the Rucus sites by H2 (∼0.46 ML),9 with 1 ML being the density of the (1 × 1) units, 5.04 × 1014/ cm2. The DFT calculations were performed using the GPAW program,20,21 where the electrons are described using the projector augmented wave method in the frozen core approximation.22 The generalized gradient approximation with the Perdew−Burke−Ernzerhof functional was used to describe the exchange-correlation effects.23 The RuO2(110) surface was modeled using periodic slabs of four RuO2 trilayers preferentially with c(2 × 2) surface unit cells. All four trilayers and the adsorbates were fully relaxed. The slabs were asymmetric with the adsorbate on one side only. A grid spacing of 0.18 Å and a 2 × 2 × 1 grid of k-points was used. The climbing nudged elastic band method was used to calculate diffusion and dissociation barriers.24

Figure 1. (a) Stick-and-ball model of the o-RuO2(110) surface. Red and pink balls represent Obr and Oot atoms, respectively. Gray balls represent Ru atoms, and the one-fold undercoordinated Rucus atoms are indicated. (b) STM image (10 × 10 nm2) of s-RuO2(110). Two residual Obr vacancies are indicated by red squares. The RuO2(110) unit cell is indicated by black dotted lines. (c) STM image (10 × 10 nm2) of o-RuO2(110) with θ(Oot) ∼ 0.4 ML, acquired after cooling sRuO2(110) from 600 to 250 K in 10−8 Torr O2, followed by flashing to 396 K. The pink circle indicates a single Oot atom, and the RuO2(110) unit cell is indicated by black dotted lines. (d) STM image (10 × 10 nm2) of o-RuO2(110) with θ(Oot) ∼ 0.95 ML, acquired after cooling s-RuO2(110) from 600 to 250 K in 10−8 Torr O2. Two residual Oot vacancies are indicated by dashed pink circles.

different Oot coverages and H2 exposures at different adsorption temperatures, and the influence on the H2O formation reaction was tracked. We show that temperature plays an important role not only in the adsorption of H2 on o-RuO2(110) but also for the usability of reactive Rucus sites. On the one hand, Oot atoms on the reactive Rucus sites facilitate the adsorption and dissociation of H2 molecules; on the other hand, reduced diffusivity of Had species at a low adsorption temperature limits multiple uses of Rucus sites for H2 dissociation. Thus, a balanced interplay between various factors is needed to optimize the H2O production reaction on the o-RuO2(110) surface.

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EXPERIMENTAL AND COMPUTATIONAL DETAILS The TPD/TPR experiments were carried out in a UHV chamber with a base pressure in the low 10−11 Torr range equipped with a quadrupole mass spectrometer (QMS), a home-built Aarhus STM, and standard facilities for sample preparation and characterization.18 The QMS (Hiden Analytical, U.K.) was connected to the main chamber via a quartz shroud with an aperture diameter of ∼3 mm, facing the sample at a distance of ∼1 mm. The temperature of the sample could be varied from 100 K using liquid nitrogen to 1200 K by heating the back side of the sample with a filament and electron bombardment. The sample temperature was measured using a K-type thermocouple spot-welded to the Ru(0001) single crystal. The temperature of the sample was controlled and recorded with a Eurotherm temperature controller that contains an automatic compensation of ambient temperature

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RESULTS AND DISCUSSION STM Studies on o-RuO2(110). Figure 1b−d shows typical STM images of s-RuO2(110) as well as o-RuO2(110) surfaces 27990

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with coverages of θ(Oot) = 0.4 ML and θ(Oot) = 0.95 ML, respectively, with 1 ML being the density of the (1 × 1) units, i.e., the Rucus sites. On s-RuO2(110), the protruding Obr atoms are imaged bright, whereas the Rucus atoms are imaged dark.15,25 The Obr vacancies in the rows of Obr atoms appear as depressions in the STM images.9,12,15 The solid red squares, the solid pink circle, and the dashed pink circle indicate Obr vacancies, an individual Oot atom, and an Oot vacancy, respectively. Several features are noteworthy: First, the Obr rows are imaged with less topographic contrast the higher the coverage of Oot is; this trend can be clearly observed when comparing the STM images presented in panels (c) and (d) in Figure 1. At θ(Oot) = 0.95 ML, only the rows of Oot atoms are imaged clearly (Figure 1d), whereas, at θ(Oot) = 0.4 ML, both the Obr rows and Oot atoms can be imaged; i.e., the situation on the surface is much more clear at θ(Oot) = 0.4 ML (Figure 1c). Second, because each O2 molecule from the gas phase needs two adjacent Rucus sites for adsorption and dissociation, a trace amount of unoccupied Rucus sites is always present on the surface, even when we saturated the surface with O2 at p = 1 × 10−8 Torr while cooling down from 600 to 250 K. This procedure introduced by Knapp et al.11 was applied in an attempt to prepare a full layer of Oot atoms. As estimated from our STM images, the percentage of unoccupied Rucus sites, or the Oot vacancy density, was typically ∼5% ML, i.e., 5% of the total number of available Rucus sites. This density of Oot vacancies is in the range previously inferred from TPD/TPR studies,8 but somewhat larger than that reported by Knapp et al., who probed the Oot vacancy density in infrared measurements by CO adsorption.11 Last, at 396 Kthe maximum temperature at which the oRuO2(110) with θ(Oot) = 0.4 ML was preparedthe Oot atoms have substantial mobility, as is illustrated by the STM image shown in Figure 1c. The Oot atoms did not always appear in pairs, as expected if the diffusion of Oot is kinetically hindered. We observed occasionally also single Oot atoms and chains consisting of three or more Oot atoms, indicating that Oot atoms diffuse along the Rucus rows at ∼396 K, in good agreement with previous STM observation25 and computed diffusion barriers.14,26 TPD/TPR Studies on H2/o-RuO2(110). Next, we conducted two sets of TPD/TPR experiments on o-RuO2(110) with H2 adsorption temperatures of 170 and 270 K, respectively (see Figure 2). These experiments were conducted for testing how much H2O is produced depending on the Oot coverage, the H2 exposure, and the H2 adsorption temperature. In each individual TPD/TPR experiment, we first prepared oRuO2(110) with an Oot coverage of ∼0.4, ∼0.6, or ∼0.95 ML. Subsequently, the samples were cooled to either 170 or 270 K for the H2 post-exposures. For each Oot coverage, we collected TPD/TPR data for H2 exposures of 0.5, 1.5, and 4 L, respectively. Thus, in total, we conducted 18 TPD/TPR experiments on o-RuO2(110), and in each case, TPD/TPR data acquisition was done for m/z = 2 (H2), m/z = 20 (H218O), and m/z = 36 (18O2). In none of the TPD/TPR experiments did we find any trace of desorbing H2, indicating that all the adsorbed Had species either reacted to H2O or remained on the surface, forming highly stable Had−Obr species. Because we used at maximum 4 L H2, which is a comparatively low exposure, and no indications of a H2O recombination feature at 550−700 K10,12 are seen in our TPD/TPR spectra, we assume that all Had species react to H2O and desorb at ∼413 K. Consequently, remaining Had on the surface is negligible and the amount of

Figure 2. Water-TPR spectra (m/z = 20; black curves) and oxygenTPD spectra (m/z = 36; gray curves) acquired following H2 adsorption at 270 K (a) and 170 K (b), respectively. Coverages of preadsorbed Oot and the quantities of post-exposed H2 (in L) are listed in the columns on the right. 18O2 was used to prepare the RuO2(110) film and the various Oot coverages. The heating rate was 2 K/s throughout. Trial numbers are given in between the two series of TPD/TPR spectra.

formed H2O during the surface reaction (Table 1) is a good probe of how much H2 was adsorbed on the surface. The black curves in Figure 2 show the H2O-QMS signals, whereas gray curves show the traces of recombining oxygen (residual Oot atoms16). With the exception of two occasions where no oxygen recombination features appeared (see trials #7 and #8 on the right panel, where the H2O signal is peaking at ∼406 K), the H2O signal peaked at ∼413 K, irrespectively of the coverage. In contrast, the oxygen recombination peak shifted to higher temperatures depending on the amount of desorbing O2 (second-order desorption) as well as the amount of H2O formed during surface reaction (see the discussion below). For 0.4 and 0.6 ML Oot coverage and 4 L H2 exposure, only little or no O2 desorption was observed at 170 K adsorption temperature (Figure 2, right panel, trials #7 and #8). Table 1a shows the obtained integrated areas of the H2O signals extracted from the water-TPR spectra in ML, and the corresponding O2 integrated areas are shown in Table 1b. For comparison, we also list the integrated H2O and O2 areas obtained when s-RuO2(110) was first exposed to H2 at 170 and 270 K, respectively, followed by O2 saturation at the same temperatures. In addition, we show in Table 2 the ratios of the H2O integrated areas for 170 and 270 K H2 adsorption temperatures (denoted as γ). For o-RuO2(110) with various θ(Oot) and H2 exposures, we found that γ is lower than that found for s-RuO2(110). In Table 2, we differentiate between three regions: the yellow cells indicate γ values close to unity, the green region for γ values around 1.5−2.0, and the purple region (2.0 < γ ≤ 4.5) with values close to the one found for sRuO2(110). 27991

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Comparing the quantities of desorbing H2O when H2 was dosed at 270 K (red numbers in Table 1a), we find two trends: (i) the H2O integrated area increases monotonically with increasing Oot coverage; (ii) the H2O integrated area increases monotonically with the H2 exposure. The H2O integrated area is largest (∼0.51 ML) for θ(Oot) = 0.95 ML and 4 L H2 exposure. In addition, the H2O integrated areas obtained for oRuO2(110) are in almost all cases larger than those found on sRuO2(110). Only for 0.4 ML Oot coverage and 0.5 L H2 exposure, the H2O integrated area on o-RuO2(110) is slightly smaller than that found on s-RuO2(110). Remembering the low sticking probability of H2 at 270 K,9 the trends for H2 adsorption can be understood straightforwardly. When an incoming H2 molecule is hitting a Rucus site without any Oot in the proximity, the probability of this H2 molecule to stick on the surface is only ∼0.1.9 Instead, if the H2 molecule is hitting a Rucus site surrounded by Oot, the sticking probability is enhanced probably due to the more polarized environment provided by the Oot atoms.27 Accordingly, increasing the amount of H2 or the Oot coverage increases the probability that H2 molecules adsorb and dissociate on the surface, leading to more H2O formation. Because of the low H2 sticking probability on s-RuO2(110) at 270 K, the H2O integrated area increased nearly monotonically with increasing Oot coverage and H2 exposure, respectively. Quite in contrast, following H2 exposure at 170 K (see the blue numbers in Table 1a), the H2O integrated areas decreased with increasing Oot coverage for low (0.5 L) and medium (1.5 L) H2 exposures. The H2O integrated area is smallest (∼0.09 ML) for θ(Oot) = 0.95 ML and 0.5 L H2 exposure. In the series of TPD/TPR experiments using 4 L H2, the integrated H2O is largest (∼0.73 ML) at a medium Oot coverage [θ(Oot) = 0.6 ML]. Thus, regarding the Oot coverage, the trend at 170 K is generally opposite to that found for H2 exposures at 270 K, where the H2O integrated area scales with the Oot coverage. This is an important result because it confirms experimentally that H2 adsorption on RuO2(110) is precursor-mediated, i.e., that Rucus sites are required for the adsorption and dissociation of H2 molecules.9,10,28 The fact that the adsorption temperature needs to be lowered to observe a decreasing H2O signal with increasing Oot coverage indicates that facile diffusion of Had species occurs at 270 K, whereas Had diffusion is diminished at 170 K (see the discussions below). However, for a fixed Oot coverage, the H2O integrated area still scales up with the H2 exposure, as is also the case at 270 K. Accordingly, at 170 K adsorption temperature, the H2 sticking probability on Rucus sites is still a factor that needs to be considered. Generally, the H2 sticking probability at 170 K must be >0.1, since the H2O integrated areas were mostly larger at 170 K adsorption temperature than at 270 K. Thus, generally γ is larger than unity (see Table 2). Note also that, at 170 K adsorption temperature, the sum of desorbing O2 and H2O can occasionally be slightly higher than 1 ML (see Table 1). In two cases at 170 K adsorption temperature when H2 was the excessive reactant (4 L), all the preadsorbed Oot atoms are converted into H2O during temperature ramping (see Figure 2, right column, trials #7 and #8, and Table 1, lowest rows). In these cases, the amount of produced H2O is directly proportional to the Oot coverage. This situation occurs when all Oot atoms on the surface can recombine with two Had species so that H2O is formed when adequate thermal energy is provided during temperature ramping before any Oot atoms can recombine. This is found for θ(Oot) up to 0.6 ML. When,

Table 1. Integrated Areas Corresponding to the TPD/TPR Data Presented in Figure 2a

a

(a) H2O integrated areas (in ML) extracted from the TPR spectra for H2 adsorption at 270 K (red numbers) and 170 K (blue numbers), respectively. (b) Corresponding O2 integrated areas (in ML), quantifying residual Oot atoms on the o-RuO2(110) surface that did not react with Had species. Trial numbers (small black numbers) are given to facilitate identification of the corresponding TPD/TPR curves in Figure 2.

Table 2. Comparison of H2O Formation on o-RuO2(110) for H2 Adsorption at 170 and 270 K (see Figure 2 and Table 1)a

a

For comparison, four TPR measurements on s-RuO2(110), i.e., θ(Oot) = 0, were also considered (2nd column). Given γ values are the quotients of the H2O integrated areas corresponding to 170 and 270 K adsorption temperatures for each parameter set. Colors were chosen as follows: 2.0 ≤ γ ≤ 4.5 (purple); 1.5 ≤ γ ≤ 2.0 (green); 1.5 ≤ γ ≤ 0 (yellow). 27992

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intermediate state, an Had−Rucus/Had−Oot pair (see Figure 3b) before it finally forms two Had−Oot species, or one Had−Oot and one Had−Obr (see Figure 3c). The intermediate state sketched in Figure 3b is stabilized by −1.33 eV and is easily accessible by overcoming an activation barrier of only 0.25 eV. The adsorption energy of the Had−Rucus/Had−Oot pair and the activation barrier are similar to the corresponding values found for the formation of an Had−Rucus/Had−Obr pair on sRuO2(110), for which we reported −1.08 and 0.22 eV, respectively.9 If instead of an Had−Rucus/Had−Oot pair (see Figure 3b) an Had−Rucus/Had−Obr pair [as known already for H2/s-RuO2(110)] is the intermediate configuration (not shown), the activation barrier was computed to be 0.31 eV. Because of the small difference between the two activation barriers (0.25 eV versus 0.31 eV) we believe that both types of intermediate Had pairs can be formed. To reach a more stable configuration from the Had−Rucus/ Had−Oot intermediate configuration, we considered two possible diffusion pathways of the Had species on the Rucus site (see Figure 3c). If Had−Oot and Had−Obr are formed, the activation barrier is found to be only 0.23 eV. For the formation of two Had−Oot species next to the Rucus atom, the activation barrier is with 0.38 eV clearly higher (see Figure 3e). Accordingly, upon adsorption and dissociation of a H2 molecule at a single Oot vacancy on o-RuO2(110), the formation of Had−Oot and Had−Obr is the most likely scenario. An important aspect we need to take into consideration is the mobility of Had species on o-RuO2(110). According to previously published DFT studies, the barrier for Had diffusion from an Oot atom to a neighboring Obr atom along the [11̅0] direction is predicted to be extremely low, and a barrier of only ∼0.28 eV is faced for diffusion from Obr to an adjacent Oot that is already capped by an Had [forming (Had)2−Oot].11,13 Likewise, the activation barrier for the diffusion of Had from one Oot atom to the next along the [001] direction is calculated to be 0.22 eV.11 Thus, Had species are very mobile along the Oot rows. In contrast, Had diffusion along the Obr rows is generally quenched, since a barrier of ∼2.5 eV has been computed for such a diffusion pathway.11 Note also that the barrier for the diffusion of a complete Had−Oot species along the Rucus rows has been calculated to be ∼0.5 eV.11 This barrier is much smaller than that for the diffusion of bare Oot atoms along the Rucus rows, which amounts to be ∼1.1 eV, or even more.14,26,29 These various possible diffusion events along the Oot/Rucus rows and between Oot and Obr atoms we need to keep in mind when looking at the STM data presented in the following section. STM Studies on H2/o-RuO2(110). In our STM studies, we focused on two selected cases that appeared particularly interesting from our TPD/TPR studies. Figure 4a shows a typical STM image of the o-RuO2(110) with 0.4 ML Oot atoms following 1.5 L H2 exposure at 270 K. The superimposed white dotted lines indicate the RuO2(110) lattice (from Obr to Obr rows), with the darkest areas corresponding to bare Rucus sites. The very dim protrusions inside the Rucus rows arise from Oot atoms. The brighter (topographically higher) protrusions within the Rucus rows are newly formed species following H2 adsorption and dissociation. The STM image depicted in Figure 4b corresponds to an o-RuO2(110) surface (0.4 ML Oot) that was prepared in the same way as in the first STM experiment, but this time, the H2 exposure was performed at 170 K. In this STM image, a region on the surface is seen where one Rucus row is almost completely free of adsorbates.

however, the preadsorbed Oot coverage is higher than a critical value, the number of available Rucus sites for H2 adsorption and dissociation becomes a limiting factor. This can be seen when comparing the TPD/TPR data obtained for θ(Oot) = 0.95 ML (see Figure 2, right column, trials #3, #6, #9). In all of these experiments, the oxygen desorption peak is large and H2O production is only substantial for 4 L H2 exposure. DFT Calculations on H2/o-RuO2(110). To improve the understanding of the processes occurring on the H2 exposed oRuO2(110) surface, we conducted DFT calculations (see Figure 3). Considering the initial interaction of H2 molecules

Figure 3. Most stable configurations of H2 dissociation on oRuO2(110) with an Oot vacancy, computed using a c(2 × 2) unit cell. Pink, gray, and red balls represent Obr, Rucus, and Oot atoms, respectively. (a) Molecularly adsorbed H2 on Rucus. (b) Dissociation of H2 with one H remaining on Rucus (Had−Rucus) and the other H transferred to an adjacent Oot atom, thus forming Had−Oot. As indicated in (c), the Had on the Rucus site can diffuse either to an Obr atom (configuration c1), thus forming Had−Obr, or to the other Oot atom, forming a second Had−Oot species (configuration c2). In (b) and (c), the Oot atoms capped by Had are marked by crosses. (d, e) Potential energy curves of H2 dissociation on o-RuO2(110) at an Oot vacancy, starting with configuration (a) and ending with configurations c1 or c2, respectively [see (c)]. The Had−Rucus/Had−Oot pair configuration shown in (b) is a local minimum. Potential energies are with respect to H2 in the gas phase, H2(g); see the dashed straight lines. The barrier heights (in eV) are given directly in the plot.

with the o-RuO2(110) surface, we modeled the situation when a H2 molecule adsorbs at an Oot vacancy in the Oot rows. This scenario is motivated by our STM experiments in which we found that a small number of Oot vacancies always exist on oRuO2(110); see Figure 1d. In Figure 3a, a H2 molecule adsorbs associatively on top of a Rucus site with an adsorption energy of −0.44 eV, which is very close to the one found on s-RuO2(110) (−0.39 eV).9,28 As the most stable hydrogen adsorption configurations, we identified a pair of Had−Oot species and a configuration consisting of one Had−Oot and one Had−Obr species, respectively. For these configurations (see Figure 3c), we computed adsorption energies of −3.33 and −3.18 eV, respectively. To identify the intermediate configuration, we considered various possible H2 dissociation pathways starting from H2 adsorbed on the Rucus site (see Figure 3d). In Figure 3d,e, we report the reaction path with the lowest activation energy. We found that the dissociation of a H2 molecule occurs through an 27993

dx.doi.org/10.1021/jp509510j | J. Phys. Chem. C 2014, 118, 27989−27997

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Further Discussions and Peak Positions in TPD/TPR. The above observations allow us to conclude that the number of available Rucus sites is likewise decisive in H2 adsorption and dissociation on the o-RuO2(110) surface besides the sticking probability of H2, which is strongly temperature-dependent. Because on s-RuO2(110), the H2 sticking coefficient at 170 K is up to 4 times higher than that at 270 K (see Table 2 and ref 9.), it can be understood why, in the TPD/TPR experiments, the H2O integrated areas on o-RuO2(110) were generally larger at 170 K H2 adsorption temperature than at 270 K (Table 1). Only when the number of available Rucus sites is very small [θ(Oot) ≥ 0.95 ML] the H2O integrated areas following H2 exposure at 170 K are comparable or, eventually, smaller than those obtained following H2 exposure at 270 K (see Table 2). However, the number of the Rucus sites is only limiting the H2O formation reaction for low adsorption temperatures. This is because the mobility of Had species is decreased at these conditions, and thus, (partial) blocking of the few single Rucus sites is possible. In Figure 5a, an idealized schematic molecular model is shown for the case of θ(Oot) = 0.95, assuming that Had species cannot further diffuse on the surface. In this case, the Rucus sites are “blocked”, meaning that they cannot be further used for H2 adsorption and dissociation. At higher adsorption temperature (270 K), Had species have sufficient mobility to diffuse when Oot is present in the vicinity. Accordingly, we anticipate that each Rucus site is used multiple times for H2 adsorption and dissociation. Thus, the presence of Oot atoms facilitates the H2 interaction with the surface, as illustrated in Figure 5b. This scenario explains how multiple H2 dissociation events can occur on the few accessible Rucus sites. It also helps understanding as to why site-blocking experiments that could be conducted successfully on RuO2(110) with other adsorbates, such as CO and N2,34 are much more difficult to accomplish with H2. Figure 5c is a schematic presentation of the case where all the Oot atoms on the surface [θ(Oot) = 0.6 ML] are populated with Had species, so that no Oot recombination peak can be observed by TPD/TPR spectroscopy. This schematic presentation may illustrate the situations that were obtained in the TPD/TPR experiments corresponding to trials #7 and #8 in the right column of Figure 2. We now take a closer look at the peak positions in the series of TPD/TPR spectra in Figure 2 (see Table 3). From our control experiments with H2O on s-RuO2(110) (see Figure S2), we know that the temperature of maximum H2O desorption [Tmax (H2O)] is independent of the coverage (first-order desorption reaction). However, in the case of O2 desorption, Tmax (O2) varies with the Oot coverage because this is a second-order reaction.14,16,27 The lower the Oot coverage, the higher is Tmax. With our experimental settings, the peak maxima are roughly at 395 K for θ(Oot) ∼ 0.95 ML, 409 K for θ(Oot) ∼ 0.6 ML, and 423 K for θ(Oot) ∼ 0.4 ML; see Figure S1. We compare these positions of the O2 and H2O desorption features with those in the series of TPD/TPR data in Figure 2, i.e., when the o-RuO2(110) surface was exposed to H2 (Table 3). We first compare the positions of Tmax (O2) for oxygen in the control experiments and the coadsorption experiments for θ(Oot) ∼ 0.4 ML (brown boxes in Table 3a,b), whereby the focus is on the quantity of desorbing O2 and not on the initial Oot coverage (Table 3b). In the case when o-RuO2(110) is exposed to H2, the O2 feature is peaking at higher temperature compared to o-RuO2(110) without any H2 post-exposed [Tmax

Figure 4. STM images (10 × 10 nm2) acquired on o-RuO2(110) with θ(Oot) = 0.4 ML following 1.5 L H2 exposure at 270 K (a) and 170 K (b), respectively. The STM images were acquired at 150 K. Schematic molecular models of the areas indicated by black rectangles in (a) and (b) are shown in (c) and (d), respectively. Dashed white lines indicate the RuO2(110) lattice; red, pink, and gray balls represent Obr, Oot, and Had species, respectively. In (a), in the indicated area, two Oot atoms are marked by red circles.

Following H2 adsorption at 170 K, we also observed new bright protrusions on the Rucus rows. A primary difference exists between the new species observed in the STM images acquired after H2 exposure at 270 and 170 K. Whereas, in Figure 4a, all the protrusions appear in between two Rucus lattice sites, the new protrusions in Figure 4b are centered on top of the Rucus sites. In addition, in Figure 4a, the apparent height of the protrusions is slightly larger than that of the protrusion seen in Figure 4b, as shown in Figure S3 (Supporting Information). Such a small difference in apparent STM height can be caused by surface species of different O or H composition, as has been found for OxHy species (x = 1 or 2; y = 1, 2, 3, or 4) on rutile TiO2(110).30,31 On the basis of the different positions and apparent heights of the protrusions within the Rucus rows, we propose static molecular models for Had/o-RuO2(110) surfaces prevailing at different H2 adsorption temperatures at this Oot coverage. As illustrated in Figure 4c,d, these models were constructed according to the interpretation of the indicated areas in the STM images depicted in Figure 4a,b, which is assisted by the line profile shown in Figure S4 (Supporting Information). For the chosen H2 exposure, the predominant species formed on o-RuO2(110) at 270 K are Oot−Had−Oot species, where two adjacent Oot atoms share one Had species. In contrast, following the same H2 exposure at 170 K, a majority of the surface species are Had−Oot, where each Oot atom is capped by one Had species. Configurations with a higher packing density of Had−Oot are well possible when the surface is populated by more adsorbates. At locally highly packed Had/ o-RuO2(110) surfaces, we have occasionally observed water monomers, (Had)2−Oot species; see Figure S5 (Supporting Information). However, at the given experimental conditions, the (Had)2−Oot species are not stable. We frequently observed streaks appearing during STM imaging that are caused by vastly diffusing surface species. Both the diffusion of Had species mediated by (Had)2−Oot and the diffusion of (Had)2−Oot mediated by Had species32,33 can be anticipated to occur at 150 K, the temperature at which the STM image in Figure S5 (Supporting Information) was acquired. 27994

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Accordingly, extra thermal energy is needed to drive the recombination reactions. However, the slightly larger shift of Tmax (O2) at low H2 adsorption temperature is again an indication of different OxHy configurations, consistent with our STM observations. When 1.5 L H2 is dosed at 170 K onto oRuO2(110) with θ(Oot) ∼ 0.4 ML, the majority of species on the surface is terminal hydroxyls Had−Oot, whereas Oot−Had− Oot are dominant when the adsorption temperature is 270 K; see Figure 4. This possibly accounts for the extra energy needed to release the Oot for recombination, because Oot is more easily released from Oot−Had−Oot configurations than from Had−Oot configurations. We now compare the positions of Tmax (H2O) in our TPD/ TPR experiments with the control experiments where H2O was dosed onto s-RuO2(110). In our control experiments, Tmax (H2O) is ∼403 K for all H2O coverages (see Figure S2), in perfect agreement with previous results.35 However, in the reaction experiments addressing H2/o-RuO2(110), Tmax (H2O) is mostly at ∼413 K, and only in the two occasions where no oxygen was left on the surface the H2O signal is peaking at ∼405 K (see trials #7 and #8 on the right panel in Figure 2, and Table 3). These findings are again consistent with the need for extra thermal energy for driving the recombination reactions. In addition to reaction scenarios where solely Had species are diffusing on the surface until water is formed on Rucus sites, a scenario can be anticipated that includes the diffusion of Had− Oot species along the [001] direction.13 Furthermore, diffusion events of Had−Obr species to Rucus sites may also occur (diffusion in the [110̅ ] direction). Note that Knapp et al. found ∼5% ML Obr vacancies following a 100 L H2 exposure to sRuO2(110) at room temperature.12 The involvement of a low coverage of Had species originally residing on Obr atoms may also add to the observed shifts of the H2O signals in the TPD/ TPR experiments. Such a scenario explains naturally as to why the sum of desorbing O2 and H2O in our reaction experiments is in few cases higher than 1 ML (see Table 1). We believe that kinetic Monte Carlo simulations would be insightful to test the proposed reaction scenarios for the water formation from H2 on o-RuO2(110). Such an approach, eventually combining experimentally and theoretically derived results, would also allow one to simulate TPD/TPR spectra, as conducted successfully for the CO oxidation reaction on oRuO2(110).29

Figure 5. Schematic of idealized Had adsorption structures on oRuO2(110) for selected conditions: (a) θ(Oot) = 0.95 ML and T ≤ 170 K. The decreased mobility of Had species leads to indirect blocking of the Rucus sites (crosses), and hence, further H2 dissociation is inhibited. (b) θ(Oot) = 0.95 ML and T = 270 K. Because of the high mobility of the Had species, the Rucus sites are available for H2 dissociation multiple times. (c) θ(Oot) = 0.6 ML and T = 170 K. All Oot atoms are capped by Had species. Hence, no O2 feature will appear in the TPD/TPR spectra.

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CONCLUSIONS On the basis of our TPD/TPR, STM, and DFT studies, we found that both the adsorption temperature and the coverages of Oot and Had species are important for the initial adsorption of H2 and the formation of H2O on o-RuO2(110). The adsorption temperature has a strong influence on the sticking probability of H2 on reactive Rucus sites. The lower the temperature, the higher is the H2 sticking probability. This is the case for both sand o-RuO2(110) surfaces, with the sticking coefficient in most cases being larger on o-RuO2(110). Likewise, the mobility of Had species depends on the adsorption temperature, but it also depends strongly on the Oot coverage. Diffusion of Had species is likely to occur already at 170 K, but to a smaller extent than at 270 K. At high Oot coverages, the number of Rucus sites that are the active sites for H2 adsorption and dissociation is scarce. Thus, if additionally the mobility of Had species is limited, the few available Rucus sites are hindered from further accommodation of H2 molecules, leading to reduced water formation. Vice versa, if the mobility of Had species is high, as is the case at

(O2) is higher than 433 K compared to 423 K]. Additionally, we compared Tmax (O2) for H2/o-RuO2(110) surfaces when H2 adsorption was conducted at 170 and 270 K, respectively (green boxes in Table 3). Although at 170 K adsorption temperature, the integrated O2 area was larger than that at 270 K (0.19 ML versus 0.11 ML), Tmax (O2) was slightly higher in the first case (Table 3b). That is, at low H2 adsorption temperature, O2 desorption is peaking at higher temperature compared to the case when the H2 adsorption temperature was 270 K. A straightforward reason for these observed shifts of Tmax (O2) is that the two reactions, 2Had + Oot → H2O and Oot + Oot → O2, occur at similar temperatures, where Had species on the surface compete for Oot during temperature ramping. 27995

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Table 3. (a) Peak Positions, Tmax (O2) and Tmax (H2O) Corresponding to the TPD/TPR Data Presented in Figure 2a

a Trial numbers are given in the first column to facilitate the assignment of the peak positions to the corresponding TPD/TPR spectra. Experiments labeled by brown and green frames are discussed in the text. (b) Integrated O2 areas corresponding to the labeled coadsorption experiments in (a) along with the values from the control experiments.

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270 K and high Oot coverage, a few Rucus sites are sufficient for H2 adsorption and dissociation because they can be used many times. Accordingly, blocking of the Rucus sites by H2 can be achieved only at low temperatures. We anticipate that this effect can be observed even more clearly if the H2 adsorption temperature is reduced further. The various factors governing the water formation reaction (temperature and coverages) are clearly interconnected, underlining that temperature-dependent adsorption studies are essential for the understanding of the H2−o-RuO2(110) interaction. When aiming at maximizing the water formation reactivity, the reaction parameters need to be chosen with care.

ASSOCIATED CONTENT

S Supporting Information *

Additional TPD/TPR and STM data, respectively, addressing the preparation of o-RuO2(110) surfaces with different Oot coverages (Figure S1), the calibration of the water coverages on s-RuO2(110) (Figure S2), the apparent STM heights of Had− Oot and Oot−Had−Oot species (Figure S3), the apparent STM heights of Oot species in the neighborhood of Oot−Had−Oot (Figure S4), and the diffusion of (Had)2−Oot and/or Had species on the surface at 150 K (Figure S5). This material is available free of charge via the Internet at http://pubs.acs.org. 27996

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: ++45 8715 6731 (S.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge with thanks the support of this work by the Villum Kahn Rasmussen Foundation, the Carlsberg Foundation, the Danish Center for Scientific Computing, and the European Research Council through an Advanced ERC grant (F.B.).

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