Dynamics, Stability, and Adsorption States of Water on Oxidized RuO2

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Dynamics, Stability, and Adsorption States of Water on Oxidized RuO(110) Manh-Thuong Nguyen, Rentao Mu, David C. Cantu, Igor Lyubinetsky, Vassiliki-Alexandra Glezakou, Zdenek Dohnalek, and Roger Rousseau J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b03280 • Publication Date (Web): 25 Jul 2017 Downloaded from http://pubs.acs.org on July 30, 2017

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The Journal of Physical Chemistry

Dynamics, Stability, and Adsorption States of Water on Oxidized RuO2(110) Manh-Thuong Nguyen,‡,a, b Rentao Mu,‡,a,b, † David C. Cantu,a,b, †† Igor Lyubinetsky,b,c VassilikiAlexandra Glezakou,a,b Zdenek Dohnálek,*,a, b, d Roger Rousseau*,a,b a

Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States b Institute for Integrated Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States c Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States d Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, Washington 99163, United States

ABSTRACT: Identifying and understanding how excess oxygen atoms affect the adsorption of water on metal oxides is crucial for their use in water splitting. Here, by means of high-resolution scanning tunneling microscopy and density-functional calculations, we show that excess oxygen atoms on the stoichiometric RuO2(110) significantly change the clustering, conformation, and deprotonation equilibrium of adsorbed water. We considered two reactive scenarios during which the stoichiometric surface was exposed to: (i) first to oxygen followed by water, and (ii) first to water followed by oxygen. In both cases the [OH-OH] complex on Ru rows is the dominant species, showing a significant difference from water-only adsorption on the stoichiometric surface in which the [OH-H2O] species is found to be prevalent. Surface reactivity at almost full O coverage is also addressed; there we show that site selectivity of the surface for H adsorption and dissociation of H2O is hindered, notwithstanding the increase of the dynamical motion of both species. We found that the workfunction of RuO2 can serve as a descriptor for the reactivity of this surface to water and its constituents.

1. INTRODUCTION Metal oxides are common materials used in (photo-)electrochemical water splitting.1-2 In different ways, a metal oxide can be integrated into electrochemical systems, and RuO2 is an interesting example. In numerous experiments this material has been proven to be an excellent co-catalyst in TiO2-based photo-catalytic oxidation reactions.3-5 It is also a good co-catalyst in photocatalytic half reaction of water splitting for O2 production,6 and it can cooperatively work with other co-catalysts to achieve stoichiometric water splitting.6-7 RuO2 capably operates with a wide variety of semiconductors in photocatalytic reactions.8-10 Additionally, RuO2 itself is a catalyst for both the oxygen11 and hydrogen12 evolution reactions that constitute key processes of water splitting into molecular hydrogen and oxygen. Understanding the catalytic functions of RuO2, regarding water decomposition, is an essential aspect of using RuO2 as a (co-) catalyst in water splitting. Despite its importance, surprisingly only few in vacuo studies of the adsorption of water on RuO2 surfaces have been reported.1317 In contrast, there are numerous theoretical and experimental studies employing various surface science techniques of the adsorption of oxygen and hydrogen on RuO2(110).18-29 To the best of our knowledge, the first ultrahigh vacuum (UHV) work for water on RuO2(110), using high-resolution electron energy loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS), was conducted more than a decade ago.13 In that study, Lobo and Conrad found that the first thermal desorption peak was observed between 350 and 425 K, and attributed it to the chemisorption of water at the coordinatively unsaturated Ru (Rucus) sites.13 They also found that adsorbed water forms hydrogen bonds with bridging oxygen (Ob) atoms, and that the stoichiometric RuO2(110) surface negligibly splits water. Directly related to the adsorption of water on this surface, the formation of water by surface hydrogenation were investigated by Knapp et al.20, 22 Using multiple techniques including temperature programmed reaction (TPR), highresolution core level spectroscopy (HRCLS), and scanning tunneling microscopy (STM), they showed that at room temperature, after the stoichiometric RuO2(110) was introduced to hydrogen, its Ob atoms become hydrogenated, forming hydroxyl groups. With more hydrogen dosing, some of these hydroxyl groups move to Rucus sites where they further hydrogenated, forming molecular water.20 Knapp et al. also observed that water was formed when RuO2(110) was exposed to gas-phase hydrogen and oxygen. By TPR and DFT calculations, the formation of water was elucidated as a process in which Ob and on-top oxygen (which is essentially

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an adsorbate) play different roles. While the former collects hydrogen from the gas phase, the latter picks up such the adsorbed hydrogen to form water.20 However, another mechanism, where H2 adsorption and dissociation occur at the Rucus sites, has also been put forward.29-30 As a step forward to a molecular-level understanding of intermolecular interactions of water on the stoichiometric RuO2(110) surface, some of us using STM and DFT calculations have investigated in detail the behavior of water at different temperatures and surface coverages.14-15 At low coverages, water monomers were observed in the molecular form, and mobile at 238 K. They diffuse along the Rucus rows to form water dimers, which are immobile at temperatures lower than 277 K.14 These dimers deprotonate rapidly, forming H3O2 structures. At high surface coverages, water forms dimers, trimers, tetramers, extended chains which are stabilized by deprotonating every two water molecules in clusters. Clusters of even numbers of water molecules were found to be more stable than that of odd numbers of molecules.15 At multilayer adsorption on RuO2(110), water displays two types of commensurate monolayers with different densities, Chu et al.26 showed, using the so-called the off-specular surface x-ray scattering-rod technique, that depending on cathodic or anodic potentials the bridging OH can form a low density water layer or together with supported water it forms a bilayer with bond distance similar to that of ice X. Note that in these studies a detailed mechanistic understanding of how water interacts with excess oxygen on RuO2(110) and consequences of such interactions were not discussed. An oxidized surface would provide a more realistic picture of water-RuO2 interactions under electrochemical conditions since atomic or molecular oxygen intermediates are generated in the oxygen evolution process. In water splitting cells, oxygen availability would make the oxygen electrode oxidized, as it has been shown for several semiconducting metal oxide surfaces including RuO2.27 Driven by thermodynamics, under electrochemical conditions, anode surfaces are likely to be fully covered with oxygen and/or hydroxyls.27-28 On this basis, knowledge of water interactions with oxidized RuO2 surfaces is needed to achieve a better understanding of water-RuO2 interfaces under electrochemical conditions. Here, high-resolution scanning tunneling microscopy (STM) and density functional theory (DFT) are used to investigate the adsorption of water on oxidized rutile-RuO2(110) (“rutile” is dropped henceforth). Similar to its rutile-TiO2 counterpart, the (110) facet is a frequently studied termination of RuO2.31-32 It is a natural growth direction,33 and for the polycrystalline RuO2 most of the exposed facets are (110), showing that it is a highly stable surface. 31 The stoichiometric termination of RuO2(110) is experimentally observed under UHV conditions, 34 and has been theoretically shown to have the lowest surface energy (i.e., being the most stable) in an oxygen, or oxygen and water, environment at room temperature and very low pressures.18-19 It is also known that when exposed to oxygen, it is oxidized with the oxygen atoms sitting on the Rucus sites.18 To elucidate effects of oxygen on water adsorption, we considered two reaction sequences at room temperature: (1) the stoichiometric surface is first exposed to water followed by reaction with oxygen and (2) the surface is first in part oxidized then water is dosed. We found that the adsorption sequence leads to the same surface intermediates, namely, paired OH species sitting on Rucus (HOt). In either case, water is dissociated and the paired HOt-HOt complex is the dominant species, with the hydroxyl groups sitting on top of Rucus. Consistent with previous study22 we found that oxygen adatoms on Rucus sites can extract H atoms from neighboring protonated bridging oxygen (HOb) species that are initially formed when H2O is dissociated on stoichiometric RuO2(110). We also show that the increasing oxidization level of the surface make the surface less reactive to O, OH, and H2O, and more reactive to H atoms. Acidity and basicity dependence of the surface on the oxygen coverage will also be discussed. 2. METHODS Spin-polarized density-functional calculations were performed with the CP2K package35 using the PBE exchange-correlation functional36 and the GTH norm-conserving pseudopotentials.37 We employed the Gaussian plane wave hybrid basis set scheme38 with molecular optimized (MOLOPT) double-ζ Gaussian basis sets39 for valance electron wave function expansion, and an energy cutoff of 500 Ry for the plane wave expansion of the auxiliary charge density. Only Γ-points were used for Brillouin zone sampling in self-consistent calculations. The force threshold convergence was set at 4×10-4 au. Zero point energy (ZPE) corrections to the total energy were computed from normal mode frequencies, which were determined using the frozen phonon technique with a displacement of atoms of 0.01 au. Only hydrogen atoms and their oxygen neighbors were considered for ZPE. All the energies reported in this work refer to total ZPE-corrected values. We adopted the Bader’s theory of atoms in molecules40-41 to determine the charge states of atoms. Surface workfunctions were calculated as the energy difference between the vacuum and Fermi levels of surface slabs. To prepare static structures of co-adsorbed species (O, OH, H2O) most structures were first equilibrated at 200 K for up to 2ps of ab initio molecular dynamics (AIMD) simulation, then slowly annealed to nearly 0 K. Transition states of the diffusion and dissociation events were determined using the climbing-image nudged elastic band (CI-NEB) technique42 with the initial replicas prepared with a series of constrained geometry optimizations. The convergence of band optimizations at threshold of 0.025 au was further verified with an imaginary frequency of the transition state. The rutile RuO2(110) surface was represented with six tri-layer slabs of the (2×6) periodicity and a vacuum slab of 30 Å separating the slabs and their periodic images. This (2×6) model has been shown to adequately reproduce both surface properties and water adsorption/deprotonation mechanism in our previous work.14-15 All atoms were allowed to relax in our calculations. Our theoretical approach leads to accurate properties of the stoichiometric RuO2(110) surface: a 6.41×3.13 Å2 primitive surface unit cell and a 5.7 eV workfunction are well in agreement with experimental counterparts of 6.38×3.11 Å2,43 and 5.8 eV,44 respectively. Experiments were carried out in an ultra-high vacuum (UHV) system, equipped with Omicron variable temperature STM, low energy electron diffraction, Auger electron spectroscopy and a home-built growth chamber. RuO2(110) thin films were prepared by oxidizing Ru(0001) surface in 2.5 × 10-5 Torr O2 at the temperature of ~ 600 K for 5 hours. All STM images were collected using a tungsten tip in the constant current mode (10~20 pA, if not stated) with a negative sample bias voltage (~ -1.0 V). The freshly prepared thin films are in general covered by clusters and adsorbates.15 To prepare a clean stoichiometric surface, several cycles of alternating O2 adsorption (~10 Langmuir, 1 L ≡ 1 × 10-6 Torr·s) at 300 K and UHV annealing at 750 K were carried out. Water or


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O2 molecules were dosed using a retractable tube doser mounted directly in the STM stage. The coverages of water or oxygen atoms are given in monolayers (MLs), where 1 ML is defined with respect to the coverage of Rucus sites, which have a surface density of 5.04 × 1014 cm−2. 3. RESULTS AND DISCUSSION 3.1. Preparation of oxidized (o-) RuO2(110) surface. Figures 1a and 1b display STM images of a stoichiometric (s-) RuO2(110) surface before and after O2 adsorption at 252 K. Clean sRuO2(110) in Figure 1a displays alternating bright and dark rows of (Ob (red) and Rucus (blue) atoms, respectively. After dosing a small amount of O2 at 252 K, STM image in Figure 1b shows pairs of new bright features on top of two adjacent Rucus atoms. Since the dissociation barrier of O2 on RuO2(110) is small (~0.25 eV),21 we assign these bright pairs to two adjacent oxygen adatoms (Oa, subscript “a” implies O adsorbed on Rucus) in accord with previous studies.45 Such an oxygen pair on the surface was also observed previously.46 Additionally, we find that Oa atoms are immobile at 252 K and start to diffuse above 350 K, consistent with a reported temperature of ~396 K24 and with the computed barrier of ~1.1 eV.24 As shown in Figure 1c, isolated Oa atoms are found on the top of Rucus atoms after O2 adsorption at 400 K. As the coverage of Oa increases, Oa rows are formed on neighboring Rucus sites. Figure 1d shows a surface with a small fraction (~12 %) of bare Rucus sites after a large O2 dose (~100 L) at room temperature. The surfaces with different levels of oxidation illustrated in Figure 1c and 1d are employed in our studies of water interactions with oxidized RuO2(110) as further described below. We will refer to these oxidized surfaces as o-RuO2(110). Note that similar approaches to prepare RuO2(110) with low and high oxygen coverage have also been successfully used in independent works. 24, 44-45, 47-48

Figure 1. Preparation of oxidized (o-) RuO2(110) surfaces. STM images of (a) clean stoichiometric RuO2(110) surface, (b) surface with oxygen adatom (~0.13 ML) pairs on Rucus rows prepared by dosing and dissociative adsorption of a small amount of O2 at 252 K, (c) surface with isolated oxygen adatoms (~0.16 ML) on Rucus rows prepared by dosing and dissociative adsorption of a small amount of O2 at 400 K, imaging was carried out at 295 K, and (d) surface with 0.88 ML of oxygen adatoms on Rucus rows after ~100 L O2 dose at 295 K. Image sizes: 4.5 × 4.5 nm2.

3.2. Reactions of water with isolated oxygen adatoms on RuO2(110) We first focus on the interaction of water molecules with a slightly oxidized RuO2(110) surface with isolated oxygen adatoms, Oa. The sequence of STM images observed during water adsorption at 257 K is shown in Figure 2. In the initial image (Figure 2a), three Oa atoms and three other brighter features are observed on Rucus sites. The smaller brighter feature (labeled H2O) is centered on the top of the Rucus site and is mobile, in contrast with the less bright Oa atoms that are immobile at 257 K. Based on our prior studies of water on s-RuO2(110)14 these features are assigned to water monomers. The larger bright features occupy two adjacent Rucus sites and have a "dumbbell-like" appearance as shown in the bottom of Figure 2c. This feature is similar to that of a water dimer observed previously.14 However, the larger bright feature is much less stable than a water dimer, see Supporting Information (SI), Figure S1. As illustrated in Figures 1b and 1c and discussed below, these larger brighter features are adjacent OHt-OHt pairs produced by H2O reaction with Oa.



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Figure 2. Top panels show consecutive STM images (2.5 × 2.5 nm2) of o-RuO2(110) (Oa coverage of 0.13 ML) after water adsorption at 257 K (a-c). The reaction of Oa species with the adsorbed water molecule is illustrated on the Rucus row highlighted by the white dashed line. Middle panels display schematics of the species observed in the STM images (assignments discussed in the text). In the schematics, Rucus is shown in brown, lattice O in red, Oa in magenta, and H in white. Bottom panels exhibit the line profiles along the white dash lines in the STM images.

From Figures 2a to 2b, we find that the water monomer on the Rucus row highlighted by the dashed line diffused two lattice constants upwards landing next to the Oa. As a result, a new larger feature is found, traversing two adjacent Rucus sites. Unlike OHt-OHt pairs with two identical maxima, the newly formed feature in Figure 2b is asymmetric (bottom panel, Figure 1b). This indicates that the H2O-Oa neighbors have not reacted within the time scale of the Figure 1b imaging. From Figures 2b to 2c, the newly formed feature becomes symmetric (bottom panel, Figure 1c), indicating the conversion of an Oa-H2O intermediate to an OHt-OHt pair. The corresponding line profiles in the bottom panels of Figure 2 clearly show that the Oa-H2O intermediate and OHt-OHt pair are different. An extended time lapse imaging of the same area further illustrates splitting of the OHt-OHt pair back to Oa and H2O, SI, Figure S1. The average lifetime of OHt-OHt pairs based on the statistics obtained from multiple images is determined to be 4000 and 300 s at 257 and 295 K, respectively. A simple estimate using an Arrhenius dependence of the temperature-dependent lifetime yields the apparent activation energy for splitting of the OHt-OHt pairs to spatially separated Oa and H2O of 0.44 eV. In our previous studies14 on s-RuO2(110), we have demonstrated that at temperatures where water monomer diffusion is facile (> 235 K), rapid dimerization occurs. In contrast, on o-RuO2(110) water monomers can be observed at higher temperatures as illustrated in Figure 2. We speculate that Oa atoms on the Rucus rows act as a barrier preventing diffusing water monomers from dimerization. Interestingly, the water monomers are present even at 295 K. While their high diffusion rates do not allow us to image them directly, the continuous appearance and disappearance of OHt-OHt pairs, provides a strong evidence. This is illustrated in Figure S2, which shows splitting of the OHt-OHt pair to Oa and H2O, OHt-OHt → Oa + H2O and formation of a new O’Ht-OHt pair, Oa' + H2O → O'Ht-OHt, on the same Rucus row. The OHt-OHt pair formation on adjacent Rucus rows (see Figure S3, SI) is also often observed, indicating facile cross-row diffusion of water monomers.


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Figure 3. Water diffusion and dissociation at an Oa, forming a HOt-HOt pair, and potential energy surface. Arrows indicate Ob and Rucus rows.

Results from DFT calculations further corroborate the STM observations shown in Figure 2. The system is slightly more stable (-0.07 eV) when Oa and H2O are adjacent, rather than when they are one Rucus site apart (Figure 3). The stabilizing forces are primarily due to the formation of a hydrogen bond (H-bond, HO-H...Oa 1.83 Å) between the Oa and one of the H2O hydrogen atoms. The small H-bond energy of -0.07 eV is partly due to the formation of this H-bond weakens the Oa-Ru bond. Upon H-bond formation, the Oa-Ru bond is 0.03 Å elongated and 5o tilted towards H2O. To reach this H-bonded structure, H2O needs to surpass the diffusion energy barrier of 0.60 eV. Note that, this barrier is consistent with that of a water monomer on s-RuO2(110), found to be 0.58 eV. 14 The system is further stabilized by the dissociation of H2O, that yields an OHt...OHt pair. Guided by hydrogen bonding, a proton is transferred to the Oa, leading to a structure that is 0.14 eV more stable. Here, the H-bond distance is 2.01 Å, leading to an Oa-Oa distance of 2.92 Å, which is smaller than the 3.15 Å distance between the two adjacent Rucus. The deprotonation barrier to form a HOt-HOt species was determined to be 0.1 eV, much smaller than the water diffusion barrier. This value of the dissociation barrier is consistent with low deprotonation barriers determined for monomers (H2O + Ob → HOt...HOb) and dimers ((H2O)2 + Ob → HOt...H2O + HOb) on s-RuO2(110).14 In light of these results, the observation of neighboring H2O...Oa species (Figure 2b) is rather surprising. The estimated deprotonation barrier based on the lifetime of H2O...Oa species is on the order of 0.4 eV, which is appreciably higher than the 0.1 eV estimated here. This theory-experiment discrepancy results from the underestimation of kinetics barriers by the generalized gradient approximation (GGA) PBE, which has also been seen in our recent study of water dissociation on rutile TiO2(110),49 or the diffusion of atomic oxygen on graphene.50 A possible reason for this limitation of GGA is that it overstabilizes stretched bonds due to the overdelocalization of electrons.50-51 3.3. Reactions of O2 with H2O on s-RuO2(110) In this section, we examine the results when water is adsorbed first on s-RuO2(110) followed by O2 adsorption at 295 K. We image the same area (Figure 4a-d) as a function of time during the course of this experiment. As shown in our previous studies, 14-15 water adsorption at 295 K results in dimerization of water molecules, 2H2O → H2O-H2O, followed by their deprotonation, H2OH2O + Ob → HOt-H2O + HOb. This process results in the energy stabilization of ~0.1 eV relative to two spatially isolated adsorbed H2O monomers. An example of such a surface with HOt-H2O (on Rucus rows, marked with circles) and HOb (on Ob rows) species is shown in two consecutive images in Figures 4a and 4b. The sequence also illustrates the low mobility of HOt-H2O species at 295 K. In a 3 min time interval, only a single feature (solid circle, Figure 4b) is found displaced by one lattice constant along the Rucus row in accord with our prior diffusion rate measurements.14-15 The statistical analysis (Figure 4e) further reveals that within the error of the measurement, the number of HOt-H2O and HOb species is the same.14-15 After O2 adsorption and dissociation to Oa atoms at 295 K, evident changes can be discerned (Figure 4c and 4d). The most striking difference is the significant increase of the bright features (rectangles, Figure 4c and 4d) on Rucus rows. The small amount of unreacted Oa atoms on Rucus rows can also be seen. The statistical analysis in Figure 4e shows a factor of two increase in the concentration of the bright features as compared to the original concentration of the HOt-H2O pairs in Figure 4a and 4b. The evidence for their distinct chemical makeup comes from their higher mobility. This is illustrated by a large number of solid rectangles in Figure 4d that mark the positions of species displaced relative to Figure 4c.



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Figure 4. (a, b) Consecutive STM images of the same area of the RuO2(110) dosed with H2O at 295 K. (b) = (a) + 3 min. Dashed circles illustrate the positions of HOt-H2O species characterized in our previous studies.14-15 Solid circle in (b) indicates a position of HOt-H2O species that became displaced from its original position in (a). (c, d) Same area as (a, b) after O2 adsorption at 295 K. (d) = (c) + 3 min. Dashed rectangles indicate the positions of new features (interpreted as HOt-HOt pairs) that formed as a result of the reaction with oxygen. Solid rectangles in (d) indicate position of HOt-HOt species that became displaced from its original position in (c). A small concentration of the excess oxygen atoms on Rucus rows is also seen. (a-d) Image sizes: 9 × 9 nm2. (e) The coverage of dissociated water dimers (HOtH2O) and bridging OH groups (HOb) on H2O/RuO2(110) surface before O2 adsorption, and the coverage of HOt-HOt pairs and HOb groups after O2 adsorption. The coverages of dissociated water dimers (HOt-H2O) and bridging OH groups (HOb) on H2O/RuO2(110) surface before O2 adsorption were determined by counting ~100 HOt-H2O and ~80 HOb features on the 30 × 30 nm2 area. The coverage of HOtHOt pairs and HOb groups after O2 adsorption were determined by counting ~200 HOt-H2O and 2 HOb features on the identical area. The error bars are determined from the counting statistics of different species.

In addition to the new features on the Rucus rows, we observe a complete disappearance of the HOb species on the Ob rows as shown in Figure 4c and 4d. The statistical analysis is presented in Figure 4e. The likely reaction responsible for the consumption of HOb species by Oa, is as follows: HOb + Oa → Ob + HOt.22 This is in analogy with TiO2(110).52 The combined evidence and the H and O atom inventory suggest that the resulting surface intermediate is the HOt-HOt pair as illustrated by the following overall reaction scheme, HOt-H2O + HOb + 2Oa → 2HOt-HOt + Ob. To further corroborate the feasibility of the surface reactions proposed based on the experimental findings, we turn to DFT calculations. We first address the formation of HOt species from HOb via the following reaction: HOb + Oa → Ob + HOt. In Figure 5a we show the optimized structures and relative energies of the HOb + Oa and Ob + HOt species, as well as the transition state geometry. In the initial ObH…Oa structure there is a 2.17 Å H-bond, while in the final Ob + HOt structure no hydrogen bond is observed. Despite the lack of hydrogen bonding, the final Ob + HOt configuration is energetically favored by 0.16 eV. We note that this energy is much larger than the one of 0.02 eV reported by Knapp et al.48 It could be attributed to the difference in unit cell and the ab intio approximations. This clearly exothermic reaction implies that Oa on Rucus sites provide stronger H adsorption sites. Furthermore, we find only a small interconversion barrier of ~0.2 eV, which is smaller than the value of 0.28 eV previously reported.48 This barrier indicates that the proton transfer process should not be kinetically hindered above ~70 K. Note that in the Ob + HOt configuration the structure in which the H points to Ob is indeed slightly less stable than the one shown here (see Figure S3, SI). The charge states of O and OH species were further analyzed to answer what makes an Oa site more reactive to H than an Ob site. Ob atoms are doubly coordinated and Oa atoms are singly coordinated, therefore Ob atoms may be more reduced than Oa atoms. Bader charge analysis supports this, as the charge state of Ob is -0.66 e while that of Oa is -0.46 e. However, Bader charge analysis suggests that after protonation the oxygen atom charge states are similar (-0.99 e for HOb and -0.90 e for HOt). To further understand the formation of the HOt-HOt pairs, we now turn to the reaction of HOt-H2O species with Oa. By analogy with the reaction of H2O with Oa (H2O +Oa → HOt-HOt) discussed above we explore the energetics of the following reactions: HOtH2O + Oa→ HOt-HOt-HOt → HOt-HOt + HOt. The atomic structures of these configurations are presented in Figure 5b. We find that the initial reaction of H2O with the Oa, leading to the chain of three HOt groups, is uphill in energy by ∆E1 = 0.1 eV, Figures 5b(i)-(ii). The next step leading to the separation of one HOt species from the HOt-HOt pair is energetically downhill by ∆E2 = 0.05 eV, Figure 5b(ii)-(iii). The total reaction energy, ∆E1 + ∆E2, is thus slightly endothermic by 0.05 eV. Note, however, that the dimerization of spatially separated HOt + HOt species is exothermic by 0.08 eV (Figure 5c(iv)-(v)), but overall, the dimerization of HOt is a downhill process. The reason why a HOt trimer (Figure 5b iii) is less stable than a dimer and a monomer (Figure 5b iv) is likely two-fold. (1) As shown in our previous study of water clusters on RuO2(110),14 adding an additional water molecule to a dimer does not result in stabilization. This is a consequence of relatively large Rucus lattice spacing that does not allow for strong continuous H-bonded chains. (2) Keeping that in mind, the adsorbate-surface dipole moments of the


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neighboring HOt groups counteract the hydrogen bonds. Quantitatively, we estimate an adsorbate-surface dipole moment, which is induced by the electron rearrangement upon HO adsorption on RuO2, as d Δ , where Δ is the induced charge density determined by the difference in the electron density between the adsorbate-surface system and its components. The value of is predicted by our calculations to be 0.36 eÅ. The interaction energy between two parallel dipoles is

, where is the

vacuum permittivity and d is the distance between the two dipoles. If d is given by the distance between two consecutive Rucus atoms in a row, 3.12 Å, the dipolar interaction energy is 0.06 eV. This energy is of the same order as the energy change when a trimer is transferred into a dimer and a monomer. Hence, the lack of strong hydrogen bonding and electrostatic destabilization disfavor the formation of trimers. To carry out a rigorous comparison of the (HOt)n chains, we show their relative energetics as a function of their length, n, in Figure 6. The adsorption energies per HOt group were calculated using the following expression, ∆Ea(n)= [E(surf+nHOt) - E(surf) – nE(HOt)]/n with n being the number of OH groups, E(surf+nHOt) the energy of the molecule-surface system, E(surf) the energy of the bare surface, and finally E(HOt), the energy of an OH group. The E(HOt) is computed using the energies of a water and a hydrogen molecule, E(HOt) = E(H2O) – E(H2)/2. In this comparison, the HOt dimer is indeed found to be energetically the most favorable configuration. From a monomer to a dimer, each HOt is stabilized by 0.04 eV due to the formation of a H-bond between the co-adsorbed species.

Figure 5. DFT structures and relative stability of different surface species resulting from the reactions of OHt-H2O and HOb species with Oa’s. (a) OHb + Oa → Ob + OHt, (b)(i-iii) OHt-H2O + Oa→ HOt-HOt-HOt → HOt-OHt + HOt, (b)(iv-v) HOt + OHt → HOt-OHt.

The adsorption energy is then decreased in the trimer and tetramer due to higher density of adsorbate-surface dipole moments (as discussed above), and due to a lower reduction level of the O in HOt species in these systems compared to the one in the dimer case. In the (OHt)n structures, the average net charge on each O atom is 0.94, 0.93, and 0.92 electrons for n = 2, 3, and 4, respectively. Another factor is that individual hydrogen bonds are weakened when more HOt groups are present within an extended network. As shown in Figure 6, in the case of the dimer, the H-bond length is slightly shorter than in the case of the trimer or tetramer. This would favor the inter-adsorbate interaction in the dimeric structure. However, the number of hydrogen bonds per OH increases with n (e.g., 0.5 for n=2, 0.75 for n=4), and as a result, each OH can further be stabilized with larger n. Surprisingly, this does not lead to a more stable structure, as observed with water on s-RuO2(110).23 From the calculated energies and experimental observations, it



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can be concluded that the stability of OH groups on Rucus sites is governed by a subtle competition between OH-surface (shown above as reduction state of O) and OH-OH (hydrogen bonding, and surface-mediated dipolar repulsion) interactions. Similarly to water on s-RuO2(110), this interplay determines that the optimal structure for HOt groups on o-RuO2(110) is the dimer. Interestingly, HOt-HOt hydroxyl dimers have also been observed on TiO2(110).52-53

Figure 6. Structures and adsorption energy (eV) per OHt in monomer (n=1), dimmer (2), trimer (3) and tetramer (4). The hydrogen bonds are marked with dash lines and are in Å.

3.4. Effects of surface oxidation levels on adsorption

Figure 7. (a) Close to fully-oxidized surface with several Oa vacancies, (b) water at oxygen vacancies, and (c) DFT model and dissociation energy of water at a vacancy (c).

We now focus on oxidized surfaces in the limit of nearly complete oxidation of the Rucus rows. As shown in Figure 1d, the initial bare almost fully-oxidized RuO2(110) surface is characterized by bright rows of Oa atoms with isolated Oa vacancy defects (dark) on Rucus rows. Figures 7a and 7b display two STM images of such surface exposed to a small amount of water, where new bright features are observed. In Figure 7a, some of the Oa vacancies are still present, while in Figure 7b all are covered. All bright features are highly mobile. To determine the preferred binding configuration of water in Oa vacancies, we turn to DFT. Energies were calculated for both molecularly and dissociatively-bound structures as shown in Figure 7c. Interestingly, we find that the dissociativelybound water is energetically less stable by 0.09 eV. This finding is consistent with the formation of water on oxidize RuO2(110)


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upon H2 adsorption.24 Note that this is in contrast with water dissociation on isolated Oa sites (Section 3.1), where the dissociatively-bound pair of HOt-HOt is preferred by 0.12 eV. The reversal of the stability is interpreted below. To further our understanding of this phenomenon, we focus on how surface oxidation affects the adsorption energy of different surface species (Oa, H on Ob and Ot, HOt, and H2O) involved in the reactions described above. Three levels of oxidation were considered: low (Figure 8a, top) with a single Oa in the slab, intermediate (Figure 8a, middle) with 50% of Rucus sites covered, and high (Figure 8a bottom), where all but one Rucus sites in the slab are covered by Oa atoms. The configurations of the Oa, H@Ob/Ot, HOt, and H2O species for different levels of oxidation are shown in Figure 8. Their adsorption energies are calculated as ∆Ea = E(ads+surf) - E(surf) - E(ads) and summarized in Figure 8. The reference states for H and O adsorption are with respect to gas phase H2 and O2 states respectively, whereas for OH the reference state is H2O - 1/2H2 as in Figure 6.

Figure 8. Pre-adsorption surface (a), adsorption O(b), H(c), OH(d), H2O(e) on low, medium, and high O coverages, here we adopt 1/2E(O2), 1/2E(H2), and E(H2O)-1/2E(H2) as the energy of H and OH, respectively. Additional O, OH, and H2O species are highlighted with black circles/ovals; additional H species at an Ob or Oa are highlighted with blue and green ovals. Energies are in eV. Arrows indicates the adsorption position of O, OH, H2O adsorbates.

For all species, we found that the adsorption energy decreases as the level of oxidation increase except for H atoms. For oxygen, TPD results have shown that the recombinative O2-TPD peak shifts to lower temperatures when the coverage of Oa increases,54 indicating the interactions between neighboring Oa should be repulsive. This is consistent with previous DFT calculations,23 where the repulsive interactions are also observed between neighboring Oa atoms. Adding an O atom to a Rucus site near to an Oa atom (e.g. Figure 8b low) releases -1.09 eV of energy, while addition at the Oa vacancy at high coverage (Figure 8b high) releases only -



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0.76 eV. For H, we found that the adsorption energy slightly increases (more negative) with higher coverage, and Oa sites are more reactive than Ob sites. This is consistent with the experimental findings presented in Figure 4, where HOb’s are consumed upon oxygen adsorption to form HOt species. Note that at higher O coverages HOb and HOt become almost isoenergetic (~0.1 eV), in full agreement with previous experimental and theoretical investigations that the site selectivity for H adsorption (Ob vs Oa) is almost the same.48 For H2O, we notice that the adsorption energy is reduced by 0.1 eV from low to high oxygen coverage. For OH, its stability on RuO2 is strongly decreased from low to high O coverage, as the adsorption energy increases from 0.27 eV to 0.51 eV, see Figure 8d. This is similar to the case of (HOt)n adsorption presented in Section 2 where the dimeric structure was determined to be the most stable one. In this context, we now consider why the RuO2(110) surface becomes less reactive to O-containing species at higher oxygen coverage. As surface reactivity is closely related to (local) electronic properties, we now inspect the work function, Φ, and charge states of surface atoms at different excess oxygen coverages. At extremely low oxygen coverage, the work function is 5.7 eV, in close agreement with the work-function previously reported (5.8 eV) in the literature.44 At high oxygen coverage the work function is 7.0 eV, and at about 50% oxygen coverage it is 6.6 eV. As shown in Figure 9a, higher Φ leads to electron transfer from the surface to adsorbates O, OH, and H2O being suppressed, thereby destabilizing adsorption. Conversely, highly oxidized surfaces will draw more electron density from H, consequently stabilizing H adsorption, but will impact acid-base properties. This can be further elaborated in simple chemical terms by considering the following thermodynamic cycles:

Scheme 1: Thermodynamic cycles of adsorption of H on HOt-Oa RuO2 surfaces: H is attached to Oa (left) or to HOt (right) in which: ∆ is the homolytic OH bond energy (H adsorption); ∆ is the difference between and the ionization potential of the H atom (1 Ry) and the electron affinity of the RuO2 surface decorated with HOt-Oa (Φ); ∆ is the proton affinity (basis strength) which can be calculated by ∆ ∆ ∆ . Note that ∆ , ∆ can be computed reliably using periodic DFT codes, but ∆ cannot be directly computed due to issues in comparing the total energies of charged and non-charged systems. Increasing the surface workfunction (or decreasing ∆ ) will increase the reducibility of the surface, meaning increasing ∆ (which gets more positive). On this basis, Φ can act as a measure for the surface acidity. In Figure 9b, we show ∆ for H adsorption at an Oa, and HOt site (thus resulting in HOt and H2O respectively, see Scheme 1, right). In both cases, ∆ becomes more positive at higher Φ (or higher oxygen coverage), however, the relative acidity of Oa and HOt changes against Φ. At Φ of about 6.6 eV (50 % oxygen coverage) ∆ is almost the same for Oa and HOt, while Oa, compared to HOt, is more acidic at higher Φ and more basic at lower Φ. This is consistent with the dissociation of water at the oxidized RuO2 surfaces, which occurs at low oxygen coverage and does not at high coverage. Our calculations predict that at about 50% oxygen coverage the downhill dissociation process of water is quenched, see Figure 9c. Next, by Bader analyses, the charge states of an open Rucus site are +1.45 e and +1.60 e at low and high oxygen overages, respectively. This implies that available electrons to transfer from the Rucus site to the adsorbates are less at high oxidation, thus reducing the ionic character of O-Rucus bonds (where O is the oxygen atom in H2O, OH, or O). The weakening of adsorbate-surface interactions is clearly indicated by the dependence of the adsorption energies on the work-function Φ (Figure 9a). Analyses of electronic properties suggest that a measurable quantity such as work function can be considered as a descriptor for surface reactivity/acidity of a metal oxide surface. For surface acidity, we also note that the surface workfunction can exponentially depend on the surface pH, as experimentally revealed in the case of carbon black.55 It seems that increasing the oxygen coverage means decreasing the overall stability of the oxygen overlayer. For oxygen coadsorption, this effect also occurs on various metal substrates.56-57 This will prevent the nucleation of oxygen at the surface, meaning that a mixture of oxidized and pristine patches are not likely to be observed on RuO2(110), in contrast with the oxidation of hematite 58 or graphene.59 For hydrogen adsorption, the change from high site-selective at low oxidation to low site-selective conditions at high oxidation (Ob vs Oa) is appealing. As RuO2 has been used as a catalyst for the dehydrogenation of small molecules such as ammonia or methanol,31 and site-requirement is an crucial issue in heterogeneous catalysis, it is evident that understanding these processes is of extreme importance for a wide class of reactions.60 Increasing Oa coverage also means blocking energetically favorable Rucus adsorption sites for water. Even if not every Rucus site is blocked, the adsorption energy of water at Rucus sites decreases.


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Figure 9. Versus work function: (a) adsorption energy of O, H (at Oa and Ob), HO, and water; (b) acidity of an Oa and HOt site; (c) dissociation energy of water.

We now comment on how Oa affects the dynamics of H2O and H on RuO2(110). In our previous experiments14-15 we found that on the stoichiometric surface at low temperatures (< 320 K) water only diffuses along the Rucus rows. Upon excess O adsorption, H2O also diffuses from one row to a neighboring row, see SI. On the stoichiometric surface, H diffusion along the Ob row is kinetically hindered, with the energy barrier of ~2.3 eV, while along an Oa row H is very mobile with a barrier is 0.22 eV.48 Moreover, the barrier for H to move from an Ob site to an Oa is also small: 0.18 eV from our calculation, or 0.28 eV from an independent work.48 Our studies thus further provide evidence that oxygen adatoms strongly elevate the dynamics of both H2O and H. We speculate that increasing the O content reduces the stability (thermodynamics) and enhances the mobility (kinetics) of H2O. The formation of water from hydrogen on RuO2(110) has been previously discussed. For example, at room temperature, molecular hydrogen on s-RuO2(110) is adsorbed and dissociated at Rucus sites, then transferred to Ob sites,29 once these sites are fully hydrogenated, additional hydrogen will combine with HOb functionalities to form molecular water.20 In another example, on oxygen rich surfaces hydrogen is adsorbed dissociatively on Oa sites and forms molecular water there.24 The dissociation of water at an Oa vacancy can be seen as the reverse process of water formation from hydrogen at full O coverage. This also indicates a significant difference in the chemistry of Ob and Oa vacancies: Ob vacancies are more reactive than Oa ones, which implies that it is easier to oxidize the surface than to reduce it. 4. CONCLUSIONS Using state of the art surface science techniques, we have computationally and experimentally investigated the co-adsorption effects of oxygen and water on o-RuO2(110). On the basis of our observations and analyses, we found that similar to being on the stoichiometric termination of RuO2, on slightly oxidized surfaces water is dissociatively adsorbed. At all oxidation states, no HOb species are present, as Oa atoms preferentially stabilize H, fully in agreement with previous literature.31 Moreover, HOt-OHt is found to be the most common species up to ~50% coverage, and energetically favorable, among several (HOt)n complexes. O, H, OH, and H2O adsorb less strongly at higher oxygen coverages. However, under higher oxidation conditions, the mobility of H and H2O on RuO2(110) also increases, and site selectivity of H adsorption on RuO2(110) becomes less pronounced. High oxygen coverage may also hinder water dissociation.

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional details about STM observations and DFT calculations for the formation of HOt-HOt, DFT calculations of OH on s-RuO2(110) are provided (SI.pdf) AUTHOR INFORMATION

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Corresponding Author * [email protected] (RR), [email protected] (ZD) Present Addresses † Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University; Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China ††Chemical and Materials Engineering, University of Nevada, Reno, NV 89557, United States

Author Contributions ‡These authors contributed equally.

ACKNOWLEDGMENT This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences under grant KC0301050-47319 and performed in EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogram national laboratory operated for DOE by Battelle.

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Dynamics, Stability, and Adsorption States of Water on Oxidized RuO2

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