This is the accepted version of the journal article: Heras-Domingo, Javier; Sodupe Roure, Mariona; Solans Monfort, Xavier. «In- teraction between Ruthenium Oxide Surfaces and Water Molecules. Effect of Surface Morphology and Water Coverage». Journal of physical chemistry C, Vol. 123, Issue 13 (April 2019), p. 7786-7798. DOI 10.1021/acs.jpcc.8b06438 This version is available at https://ddd.uab.cat/record/280651 under the terms of the license Interaction Between Ruthenium Oxide Surfaces and Water Molecules. Effect of Surface Morphology and Water Coverage Javier Heras-Domingo, Mariona Sodupe, Xavier Solans-Monfort* Departament de Química, Universitat Autònoma de Barcelona (UAB), Cerdanyola del Vallès (Bellaterra), Spain 08193 1 Abstract RuO2 is a conducting transition metal oxide that has unique redox properties to be used as heterogeneous catalyst for oxidation reactions as well as in electrocatalysis. Furthermore, it has been reported to be an excellent catalyst for the oxygen evolution reaction, a key step for obtaining energy from water through environmentally friendly processes. In this context, a detailed knowledge of the RuO2-water interface is important for a better understanding of the electrochemical process, the water oxidation reaction and some oxidative reactions involving RuO2. Here, we use periodic boundary condition DFT (PBE-D2) calculations to analyze the influence of the surface morphology and water coverage in the adsorption energies and degree of water deprotonation. We have considered the four non-polar ((110), (011), (100) and (001)) most relevant surfaces and three degrees of water coverage: isolated molecules, half monolayer and full monolayer. Results indicate that three effects are crucial for determining the adsorption energy and degree of deprotonation: i) the intrinsic acidity of the unsaturated ruthenium cations and the intrinsic basicity of the Obr centers; ii) the presence of strong cooperative effects, already observed in the half monolayer situation of the (110) and (011) surfaces that favors 50% of deprotonation and leads to the formation of the (H3O2)- dimer; and iii) an increase of the surface basicity by the adsorption of water molecules on Ru centers bonded to Obr groups, which is more important in the (100) and (001) surfaces. 2 Introduction RuO2 is a conducting transition metal oxide that presents a rutile type structure as exclusively stable solid phase.1,2 It has unique redox properties to be used as heterogeneous catalyst for oxidation reactions.3–9 Moreover, it is an excellent material for electrocatalysis due to its high conductivity.1,10–14 A selected list of the oxidation reactions that RuO2 is able to catalyze includes CO, NH3 and methanol oxidations, water formation and the Deacon process in which Cl2 is formed from the oxidation of HCl.1,3–7,15–17 In electrocatalysis, RuO2 is used as anode in the chlor-alkali industrial process which forms Cl2 and NaOH.10,18 More recently, it has been shown to catalyze the hydrogen and the oxygen evolution reactions, which are key processes in the design of sustainable energy sources alternative to fossil fuels.11–14 The electrochemical processes involve solid – liquid (water) interfaces, which determines the nature of the active species.1 The study of RuO2 - water interaction is also relevant to understand the catalytic activity of hydrous RuO2 (RuO2-xH2O)19,20 as well as the origin of poisoning induced by water in the oxidation of CO.21 Indeed, in the last decades, considerable efforts have been devoted to study - water metal oxide interfaces9,22–27 due to their relevance in many scientific fields such as catalysis, electrochemistry, corrosion, geology, or atmospheric science.28–31 For the particular case of RuO2, most of the existing studies focus on the most stable (110) surface, which is characterized by the presence of unsaturated pentacoordinated ruthenium atoms (Ru5C) and bridged oxygens (Obr) (Figure 1). 3 Figure 1. Defect free RuO2 (110) surface. The water – oxide interface has been studied by using several spectroscopic techniques such as X-ray scattering measurements, High-resolution electron energy loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS). It has also been studied by means of scanning tunneling microscopy (STM) and DFT calculations.32–41 It is found that the interface nature highly depends on the experimental conditions, particularly on the applied voltage. Regarding studies without the addition of other species or external voltages, Lobo and Conrad concluded, from HREELS and TDS spectroscopies, that adsorption mainly involves non-dissociated water molecules.32 These molecules are located at Ru5C centers and stablish hydrogen bond interactions with Obr oxygens. Furthermore, water molecules desorb at around 400 K, which is consistent with a water adsorption energy of about -96 kJ mol-1. Similar adsorption energies were obtained by means of DFT by Siahrostami and Vojvodic, although they suggested that H2O on RuO2 is prone to dissociate.39 More recently, Dohnálek and co-workers combined STM microscopy and DFT calculations and analyzed the effect of water coverage on the (110) RuO2 surface.27,35–37 They showed that at very low coverages, single water adsorbs on Ru5C centers and that the Ru5C-H2O is in equilibrium with the deprotonated Ru5C-OH···H-Obr species, as a result of the Lewis acidity of 4 the ruthenium atoms. This study also shows that increasing the water coverage allows the formation of water dimers. The two water molecules of these dimers are adsorbed in contiguous Ru5C centers and one of them is deprotonated, which leads to the formation of the H3O2- motif. At even higher water coverages, hydrogen bonded chains of water molecules are formed. These chains are characterized by the presence of the same (H3O2-)n motif, which constitutes the fundamental building block. Computed adsorption energies per water molecule vary from -121 to -146 kJ mol-1, depending on the amount of water and the degree of deprotonation.34–36,39,40 At this point it is worth noting that, to the best of our knowledge, the adsorption of water molecules at different surfaces, besides the most stable (110) one, have not been addressed before. Only very recently during the revision process a combined experimental and computational work appeared in the literature. In this work, authors studied the interface between water and all relevant surfaces of RuO2 and they concluded that the degree of water deprotonation is higher for the (011) surface followed by the (110), the (001) and finally the (100).42. Thus, with the aim of analyzing how the surface morphology and the water coverage influence the H2O-RuO2 interface, present work analyzes the interaction of water molecules, starting from one molecule to a full monolayer, on the main crystallographic orientations (110), (100), (011) and (001) of flawless RuO2 surfaces. For that, we use periodic boundary DFT calculations and we focus on the degree of water deprotonation as a function of the surface topology, surface electronic structure and water coverage. Computational details 5 Calculations were carried out using density functional theory (DFT)43 with periodic boundary conditions as implemented the Vienna Ab Initio Simulation Package (VASP) code.44,45 All calculations were performed considering the projector augmented wave (PAW) pseudopotentials.46,47 The external electrons were expanded in plane waves with kinetic energy cut-off equal to 500 eV. The chosen functional was the PBE exchange correlation GGA functional.48 This functional is the most commonly used for studying RuO2 and correctly describes its conducting nature.8,34–37,39,42,49,50 The D2 Grimme’s dispersion correction was included in all the slab calculations since it is essential to properly describe water adsorption and hydrogen bonding.51 Test calculations with the most recent D3 approach were also performed with the aim of analyzing the influence of the Grimme’s parametrization in the adsorption energies and the relative stabilities between the H2O and the OH-/H+ deprotonated structures. Results are summarized in Table SX of the supporting information. They show that D3 parametrization marginally reduces the adsorption energy by less than 2.6 kJ mol-1. However, this diminution of the adsorption energy is very similar for the two forms (H2O or OH-/H+). Therefore, this leads to the same relative stability between the two forms regardless the Grimme’s parametrizations. Bulk calculations were performed considering a K-point mesh for the Brillouin zone (BZ) of (15,15,15) employing the Monkhorst- Pack grid (MP),52 while slab calculations were performed considering a Monkhorst-Pack K-point mesh of (6,6,1). The cut-off and K-point mesh were calibrated by ensuring the convergence of both cell parameters and cell energies. (Supporting Information S1- S2). The energy convergence criteria was fixed to 10-5 and 10-4 eV for electronic and geometry relaxations, respectively. Surface models of the main crystallographic orientations were built by cutting out the slab from the optimized bulk structure, with the MOLDRAW graphic program.53 The considered slabs were constructed considering a (2x1) supercell and a 4-layer thickness, the minimum slab 6 thickness for a surface energy convergence (Supporting Information S3). The c value was set to 35 Å ensuring an interlayer distance of at least 21 Å to minimize the interaction between replicas at the (h k l) perpendicular direction. The size of the models was large enough to represent the main properties and atoms positions were fully relaxed. Surface energies of the different facets were computed through the following equation: &!"#$ '((·&$%"& ) 𝛾("#$) = (1) *·+ where Eslab is the energy corresponding to the relaxed surface without optimizing the bulk cell parameters; Ebulk is the fully relaxed bulk energy; N is the number of formula units in the slab per units in the bulk unit cell, and (2A) the corresponding two cross-section area of the slab. Water adsorption on the surfaces at different coverages was studied by varying the number of water molecules per unit cell from one to four for surfaces (110), (100) and (001) and from one to eight for the (011) facet. In both cases, the largest number of water molecules corresponds to a water monolayer on the surface. Two type of water adsorption energies are reported along the text. The adsorption energy per water molecule, which is calculated through the following equation: &((&")*+, - '&((&") '(/+, - ·&+, - ) 𝐸,-. = /+, - (2) where the E(hkl)+H2O is the total energy of the slab with the already adsorbed water, E(hkl) is the total energy of the slab model, EH2O the total energy of an isolated water inside a 15x15x15 Å3 cubic 7 box and n (H2O) the number of adsorbed waters onto the surfaces. The adsorption energy per surface are, which is calculated through equation 3: 5(./0)*1, 2 '5(./0) '(61, 2 ·51, 2 ) E012/4 = (3) 4 Where all terms are equal to those of equation 2 except A, which stands for the surface area of the supercell. At this point it is worth mentioning that test calculations including additional water through the implicit PCM continuum model were performed on the adsorption of a single water molecule either deprotonated or not on the (110) and (100) surfaces as well as the most favorable monolayer structures with the aim of evaluating the effect of solvent. Results are reported in Table SX of the Supporting Information. They show that inclusion of additional water with a continuum model produce an important decrease of the global adsorption energy of about 27 to 39 kJ mol-1 for both the H2O and OH-/H+ forms. For the adsorption of a single water molecule, the decrease in the adsorption energy is larger for the form in which water is deprotonated, thus the preference for the deprotonated form in the (110) surface (see below) is reduced, while the preference for the non- deprotonated form is larger in the (100) (see below). The diminution of the Eads when using PCM is also observed for the monolayer structures. However, the differences between the two analyzed structures are smaller and thus the relative stabilities between them remain almost unaltered. Therefore, although the absolute values reported along the text may be slightly overestimated, we do not expect that describing additional water through a continuum model will change the major trends reported in this study Transition state geometries (TS) and energy barriers (DE) were studied through the climbing image nudged elastic band (CI-NEB)54,55 method, as implemented in the VASP Transition 8 State Tools (VTST) code. The climbing image variation was used in order to converge onto the saddle point and explore the minimum energy path (MEP). For that, we used 4 images between reagents and products and the Quick-Min algorithm as optimizer.56 As a convergence criteria, the maximum force at any atom in every image was required to be below the 0.05 eV/Å threshold. Transition states were finally verified by the presence of a single imaginary frequency through the harmonic vibrational analysis. Results and Discussion Results are divided into four subsections. First, we focus on the properties of RuO2 main crystallographic surfaces. Then, we address the adsorption of one single water molecule at each surface, with the main goal of understanding the intrinsic interaction between water and RuO2 facets. In the third subsection, we analyze the influence of cooperative effects on the water adsorption energies and on the degree of deprotonation at each surface, by analyzing the half monolayer coverage, which is associated with the formation of water chains35,36 (see below). Finally, we focus on the structure of the water monolayer in each surface and analyze the factors that determine this structure. In this section, we report the adsorption energies per surface area, which allows considering the density of adsorbed molecules in each surface. This is used to obtain the Wulff construction of a nanoparticle growth in thermodynamic equilibrium in water solution. Ruthenium Oxide bulk and surfaces. RuO2 crystallizes in a rutile-structure with space group P42/mmm, where Ru4+ exhibits a distorted octahedral coordination with four long Ru-O distances in the equatorial plane and two short Ru-O ones in the axial position.57,58 The oxygen atom displays a trigonal planar environment, with two long and one short Ru-O distances. The optimization of 9 the bulk crystal structure of RuO2 led to the following lattice parameters a=b=4.54 Å, c=3.13 and a=b=g=90º, the Ru-O distances being 1.996 Å and 1.945 Å for equatorial and axial oxygens, respectively (Bold values in Table S2 of the Supporting Information). The computed data are in good agreement with the experimental measurements for cell parameters (a=b=4.4910 ± 0.0004 Å, c=3.13± 0.0004 Å and a=b=g=90º) and for Ru-O equatorial and axial distances (1.984 and 1.942 Å).57,58 The full optimized unit cell was used to build the slab models of each crystallographic orientation, which are represented in Figure 2. The (110) and (011) facets have at the outermost layer two coordinated bridging oxygen Obr and 5-coordinated Ru5c atoms. It is worth pointing out that the Ru5C centers of (110) and (011) surfaces differ on the nature of the vacant site. The vacant site in the Ru5C center of the (110) surface is axial, while that of the (011) surface is equatorial. The surface (100) exhibits a saw morphology, with 2-coordinated oxygen atoms (O2c) and 3- coordinated O3c bonded to 5-coordianted Ru5c atoms, with the vacant site in axial position. Finally, the (001) facet displays all Ru atoms as a 4-coordinated Ru4c and all the oxygen atoms as 2- coordinated O2c, the vacant sites of Ru4C centers being both equatorial. 10 Figure 2. crystallographic surface models used in this work The Ru-O distances of outermost layer of the different slabs, the Bader charges of the oxygen bridge (Obr) and unsaturated ruthenium atoms RuCUS, as well as the computed surface energies are given in Table 1. This table also includes the values of the bulk for comparison. Regardless of the slab orientation, the internal 6-coordinated Ru6c atoms present Ru-O distances (1.90 – 2.05Å) that are close to the bulk values and are not discussed further. Regarding the surface atoms, the Ru-O distances vary depending on the coordination environment, the strongest bonds corresponding, as expected, to 4-coordinated Ru4c atoms, present at the (001) surface. Indeed, the 5-coordinated Ru5c, present at the (110), (100) and (011) surfaces, show significantly larger bond distances (average values ranging 1.96-1.97 Å compared with 1.89 Å for the (001) surface). Remarkably, the distance between unsaturated Ru-centers at the surface varies significantly from one surface to another. The shortest distances (3.173Å) are found for the (110) and (100) surfaces, while the Ru-Ru distances in the (001) surface are significantly larger (4.543Å). 11 Table 1. Computed surface energies (𝜸(𝒉𝒌𝒍) ) and Ru-O distances of the outermost layer of the main crystallographic orientations. Surface energies are in eV/Å2 and distances in Å. (h k l) 𝜸(𝒉𝒌𝒍) d(Ru-O)eq d(Ru-O)ax. d(Ru···Ru). qRua qOa Bulk -- 1.996 1.945 3.137 (1 1 0) 0.066 1.995 1.880 3.137 +1.60 -0.75 (0 1 1) 0.074 1.873-2.111 1.948 3.735 +1.64 -0.78 (1 0 0) 0.082 1.926-2.032 1.901 3.137 +1.65 -0.75 (0 0 1) 0.099 1.923 1.849 4.543 +1.54 -0.81 a Bader atomic charges of the Ru and O unsaturated atoms of the surface The surface energies of all stoichiometric orientations are similar to previous values reported in the literature.1,59–62 These values are used to get the equilibrium shape of a crystal (under vacuum conditions) through the Wulff construction approach,63 plotted with a python library called pymatgen64 (Figure 3). The Wulff construction is obtained by minimizing the global surface energy of the nanoparticle – medium interface, which implies that the lowest the surface energy of a facet is, the largest contribution it has in the equilibrium structure of a given material. In our case, the surface families with lower surface energies are the (1 1 0) and the (0 1 1) ones, which account for the 43.6% and 49.2%, respectively, of the total surface area (see Figure 3). Note that the (011) family present a higher contribution than the (110) one despite the higher surface energy of the former. This is associated to the fact that there are eight equivalent orientations of the (011) given by symmetry, while the (110) family only has four equivalent planes. Surfaces equivalent to the (100) facet slash the edge between the (110) family surfaces and account for a 7.2%. Finally, the (001) facet, with the highest surface energy has a negligible contribution to the equilibrium shape. 12 Remarkably, these contributions are in good agreement with experimental observations in single crystals grown by deposition from the vapor face through a process in which only the constituents of ruthenium oxides are in the reactor. The available data show that the (011) is the predominant facet followed by the (100) and (110) ones.1 Figure 3. Ruthenium Oxide Wulff construction obtained with the here computed surface energies in gas phase and solution (assuming a monolayer coverage). Adsorption of isolated water molecules at the different RuO2 surfaces (q = 1/4 or 1/8). A water molecule was adsorbed on to the clean surfaces with the aim of studying the intrinsic interactions between water and RuO2 surfaces as a function of the surface topology. This corresponds to a water coverage of 1/4 for surfaces (110), (100) and (001) and a coverage of 1/8 for the (011) facet. We considered the adsorption of the water molecule in both the non- deprotonated and the deprotonated forms. These two situations will be hereafter referred to wat and dpro, respectively. We also located the transition structure connecting both minima through the image climbing nudged elastic band (CI-NEB) method.54,55 The relative energies, the major structural parameters associated to this coverage and the Bader charges of the unsaturated Ru and 13 O atoms of the surface and the adsorbed water molecule are given in Table 2. Further details about the optimized structures are given in the Supporting Information (Section S4). In all considered surfaces, at least two different minima are found. One of these minima corresponds to the adsorption of water, the other is related with the formation of OH-/H+ pairs through water deprotonation. Regardless the surface morphology, the adsorbed water molecule interacts directly through its oxygen with the under-coordinated Ru4c and Ru5c to fill the empty coordination sites. Besides, a quite strong hydrogen bonding between one H atom of the water molecule and the nearest bridged oxygen Obr is also formed. The Ru···Owat distances are similar in all surfaces and they range from 2.172 to 2.180. Larger differences are observed for the Hwat···Obr distances, the values varying between 1.588 and 1.688 Å. The H···Obr distance seems to be related with the surface morphology and in particular the distance between Ru5C and Obr. Indeed, the shortest H···Obr distances tend to be associated with the smallest separation between the Ru5C and Obr centers. In the (001) surface, the water molecule establishes two hydrogen bonds with the closest Obr atoms in a bifurcated manner. Remarkably, another adduct with only one H-bond has also been located. This additional structure is less stable than that with two H-bonds by 17.1 kJ mol-1. The water adsorption energy at the different surfaces ranges between -115.4 to -133.8 kJ mol-1. The weakest adsorption (Eads = -115.4 kJ mol-1) corresponds to the (011) surface. In the other three surfaces the adsorption energy is significantly larger, ranging from -129.2 to -132.8 kJ mol- 1 . This stronger interaction of water with the (110), (100) and (001) surfaces is related to the number and strength of the H···Obr hydrogen bonds, which in turn depends on the morphology of the surface. 14 With the aim of further analyzing the intrinsic interaction between H2O and RuO2 surfaces, we computed the Bader atomic charges (Table 2) and analyzed the density of states (DOS) of the clean surface and the wat and dpro structures. Figure 4 shows the contribution to the DOS of the 4d orbitals of the unsaturated ruthenium center in which water is adsorbed, that of the p orbitals of the oxygen bridge receiving the proton in the dpro structure and that of the p orbitals of the oxygen of the water molecule. Bader atomic charges shows that interaction between H2O and the surface produce some electron transfer from the water molecule to the surface. Interestingly, this electron density does not accumulate on the Ru center (that becomes even more positively charged with respect to the clean surface (Table 1)) but the oxygen atoms of the surface and particularly the oxygen bridge. This agrees with the DOS which shows that the interaction between water and the RuO2 surface is stablished within an occupied p orbital of water and an empty d orbital of ruthenium (Figure SX of the supporting information). Remarkably, when comparing the contributions of the oxygen bridge p orbitals in the DOS of the different clean surfaces (Figure 4), it is observed that in the (110) surface the p orbital involved in the hydrogen bonding with water is at higher energies, thus suggesting higher basicity. Overall three key factors are identified: i) the smallest the separation between the Ru5C and the Obr, the shortest the H···Obr distance and the strongest the H-bond; ii) the higher basicity of the Obr sites in the (110) and (100) surfaces, the larger the strength of the H···Obr H-bond; and iii) the presence of two hydrogen bonds in the case of the (001) surface also increases the adsorption energy. 15 [A] [B] [C] -2 -2 -2 10 8 -4 -4 -4 6 -6 -6 -6 4 -8 -8 -8 2 0 1 2 0 1 2 0 1 2 0 -2 -2 -2 -2 -4 -6 -4 -4 -4 -8 -6 -6 -6 -10 0 1 2 3 4 -8 -8 -8 0 1 2 0 1 2 0 1 2 Figure 4. Projected density of states (pDOS) of the (110) clean surface including the pDOS of the isolated water molecule (Left). The black line corresponds to the 4d Ru5C orbitals, the red lined are the 2p orbitals of Obr and the blue one the orbitals of water. On the right the pDOS associated to the 2p orbitals of OBr in the clean surfaces (top) and in dpro structures (bottom) are reported: [a] (110) surface; [b] (100) and [c] (011). The red solid line 2py, the blue doted line 2pz and dashed black line 2px. Table 2. Water adsorption energies on the most favorable facets (kJ mol-1) at low water coverage q=(1/4 or 1/8) and the associated main structural parameters (Å). (h k l) Eads Ow-Hw Obr-Hw Ru-Ow qRua qOa qH2O wat -131.3 1.038 1.588 2.172 +1.67 -0.87 +0.10 (1 1 0) TS -128.3 1.212 1.252 2.106 dpro -139.6 1.754 1.011 1.993 +1.68 -1.08 -0.46/+0.66 wat -115.4 1.021 1.615 2.177 +1.73 -0.89 +0.10 (0 1 1) TS -104.5 1.317 1.137 2.059 Dpro -102.8 1.429 1.072 2.031 +1.79 -1.04 -0.49/+0.64 Wat -129.2 1.013 1.688 2.180 +1.70 -0.87 +0.10 (1 0 0) TS -111.6 1.333 1.136 2.074 Dpro -114.5 1.548 1.044 2.032 +1.76 -1.05 -0.46/+0.60 1.627 (0 0 1) Wat -133.8 1.018 2.172 +1.62 -0.93 +0.07 1.627 a Bader atomic charges of the Ru and O unsaturated atoms of the surface 16 Structures in which water dissociates upon adsorption on RuO2 show that the proton transfer toward Obr decreases the Ru-Ow distance by around 0.1 and 0.2 Å, as a result of the larger OH- basicity compared to that of H2O. Similarly, the Obr-Ru bond distance of the surface increases due to a decrease of the Obr basicity upon protonation (values ranging from 1.982 to 2.090 Å). Besides, the H+ and OH- moieties interact through hydrogen bonding. The Hbr···OW distance varies from 1.377 to 1.754 Å, indicating that the H-bond is stronger in the dprot structure than in the wat one with the exception of the (110) surface. Regarding the energetics, calculations indicate that deprotonation is a favorable process (DE = -8.3 kJ/mol) only in the case of the (110) surface, reaction energies at the (100), (011) and (001) surfaces being +14.6, +12.5 and +24.7 kJ mol-1 respectively. Remarkably, the thermodynamics of water deprotonation correlate well with the Ru- Ow distance after deprotonation. Indeed, the shortest Ru-Ow distance (1.993 Å) is found for the (110) surface, intermediate values (2.031-2.032) are obtained for the (100) and (011) facets and the longest distance is found for the (001) surface (2.066 Å). Furthermore, the basicity of Obr is expected to be larger in the case of the (110) surface. This can be related to three factors: the p orbital capturing the proton lies at higher energies in the (110) surface than the others (Figure 4), the Bader charge on the Obr upon protonation becomes more negative than in the other surface and iii) the Ru-O-Ru angle is smaller which it is usually associated with an increase of the basicity. Therefore, the water deprotonation thermodynamics does not seem to be related with the strength of the H-bonding formed between the H+ and OH- moieties but with the interaction of these two fragments with the surface. That is, surface deformation energy upon proton transfer is small for all facets and the geometry features suggest that both Ru-OH and Obr-H interactions are stronger for the (110) surface, the only one in which deprotonation is thermodynamically favorable. 17 The potential energy profiles corresponding to the water deprotonation process on the different surfaces are shown in Figure 5. Remarkably, in all cases, the energy barriers are low; i.e., they range from 2.9 kJ mol-1 for the (110) surface to 25.9 kJ mol-1 for the (001) one. Indeed, the lowest energy barrier is found for the thermodynamically preferred process (deprotonation on the (110) facet), while the highest one corresponds to the less favorable one (the (001) surface). For this latter surface, water deprotonation occurs from the wat structure presenting one single H-bond with Obr atoms of the surface.65 This structure is less stable than the one with two hydrogen bonds by 17.1 kJ mol-1. Therefore, most of the energy required for the water deprotonation arise from changing the coordination mode of the water molecule. TS TS P R P R TS P R Figure 5. Energy profiles associated to the water deprotonation at low coverages on the different surfaces. 18 In summary, at low water coverage (q=1/4 and (q=1/8), only the most stable (110) surface will exhibit deprotonation. Differences are related to the surface topology which influences the basicity of the Obr on each surface (see the associated DOS in Figure 4) and the interaction between the OH- and the unsaturated ruthenium center. At higher water coverages, however, cooperative H-bond interactions could lower the energy requirements to decompose water on each orientation. Water adsorption at the half monolayer coverage q = (2/4). Significant differences are observed upon adsorption of a second water molecule to the (110), (100) and (001) surface as well as three additional water molecules to the (011) surface. In both cases, the water coverage is 1/2, that is, half of the initially unsaturated ruthenium atoms have a water molecule adsorbed. For each surface we explore at least three potential situations: a) no deprotonated water molecules (wat/wat); b) half of the water molecules are deprotonated (wat/dpro); and c) all water molecules are deprotonated (dpro/dpro). Furthermore, with the aim of evaluating the cooperative effects (see below) for the case of the (110) and (001) facets, we also explored the possibility that the water molecules adsorb in the two closer Ru-centers (-c) or in two Ru-centers that are further separated (-f). Relative energies for both the non-deprotonated and deprotonated forms, as well as major structural parameters are given in Table 3. Figure 5 shows the optimized geometries of the structures associated to the (110) surface and Figure 6 presents the most stable structures associated to the other facets. First of all, we focus on the (110) surface. Here we explore the adsorption on adjacent Ru atoms as well as the adsorption on Ru centers separated by the Obr chain. The adsorption of two water molecules in Ru centers separated by the Obr chain (distance between Ru centers is more than 6.3 Å) leads to an adsorption energy per water molecule that it is almost equal to that 19 computed for an isolated water molecule. The structural parameters are also very similar to those of the adsorption of a single water molecule and no H-bonding between the two water molecules is formed (Figure 5a). Similarly, the adsorption energy per water unit of two deprotonated water molecules in two distant Ru5C centers is essentially that of a single deprotonated water molecule. Again, no hydrogen bonding between the two OH-/H+ fragments is stablished (Figure 5b). In this context, the adsorption of one water molecule and one OH-/H+ motive in distant Ru centers leads to an adsorption energy per H2O unit that is in between the adsorption energy of one H2O and that of a deprotonated water molecule (Supporting Information section S5). Table 3. Water adsorption energies on the most favorable facets (in kJ mol-1) at half monolayer coverages q=(2/4 or 4/8) and the associated main structural parameters (in Å). (h k l) Eads Ru···Ru Ow1-Hw1 Obr-Hw1 Obr-Hw2 Ru-Ow1 Ru-Ow2 Ow1-Hw2 wat/wat-ca -142.5 3.138 1.384 1.605 1.650 2.226 2.222 2.298 wat/dpro-ca -151.3 3.149 1.920 0.999 1.846 2.044 2.198 1.739 dpro/dpro-ca -149.6 3.139 1.864 1.005 1.009 2.002 2.002 2.234 (1 1 0) wat/wat-fb -131.7 6.380 1.030 1.662 1.689 2.203 2.204 --- wat/dpro-fb -139.2 6.427 1.765 1.010 1.606 1.996 2.181 --- dpro/dpro-fb -143.0 6.426 1.782 1.008 1.007 1.990 1.989 --- 4wat -126.7 3.666 0.974 1.022 3.035 2.097 2.202 1.463 (0 1 1) 2wat/2dpro -132.1 3.679 0.974 1.022 3.536 2.093 2.166 1.494 1wat/3dpro -127.5 3.674 0.975 1.023 3.530 2.092 2.207 1.481 wat/wat -128.3 3.144 1.007 1.711 1.676 2.204 2.204 2.346 (1 0 0) wat/dpro -127.1 3.069 1.603 1.044 2.233 2.073 2.179 1.717 dpro/dpro -112.9 3.081 0.977 1.016 1.023 2.012 2.027 2.275 20 wat/wat-ca -126.2 4.554 0.975 1.535 1.711 2.186 2.168 -- (0 0 1) wat/dpro-ca -122.9 4.483 0.975 1.077 1.673 2.052 2.167 -- wat/wat-fb -122.4 6.402 1.039 1.519 1.643 2.185 2.165 2.952 a Water adsorption takes place in adjacent unsaturated ruthenium centers. b Water adsorption takes place in distant unsaturated ruthenium centers. Figure 6. Optimized structures associated to water adsorption on the (110) surface with a half monolayer coverage (q = 2/4). The four resulting structures are: a) wat/wat-f; b) wat/dpro-f, c) wat/wat-c and d) wat/dpro-c. Distances are in Å The situation is completely different when the adsorption takes place on contiguous ruthenium centers (the distance between Ru centers in the pristine surface is 3.137 Å). The adsorption of two water molecules leads to an adsorption energy per water molecule of -142.5 kJ mol-1, which is larger (in absolute value) than that determined for the adsorption of one water molecule (-131.3 kJ mol-1). This indicates the presence of cooperative effects as a result of moderate hydrogen bond interactions between water molecules, the Hw1···Ow2 distance being 2.298 21 Å (Figure 5c). Note that, the structural parameters associated with the Ru···H2O interaction are similar to those of the isolated water and the subtle differences suggest that the interaction between the RuO2 surface and the adsorbed molecules is weaker at higher coverages (larger Ru5C···Ow and OBr···Hw distances). Similarly, the formation of two OH-/H+ species on contiguous Ru centers leads to an adsorption energy that it is also higher than that of one isolated OH-/H+ species (-149.6 vs. 131.6 kJ mol-1).66 The origin of this extra stabilization arises again from the formation of moderate hydrogen bonding between OH-/H+ fragments (2.234 Å) rather than from a stronger interaction with the surface. Finally, the situation in which one of the water molecules is deprotonated by a 2- coordinated bridged oxygen atom Obr of the oxide surface and the other remains as molecular water results in the formation of a formally H3O2- species (Figure 5d), with a strong intermolecular hydrogen bond (1.739 Å). In contrast, the H-bonding between the hydrogen atom of the OH- fragment and the subsequent water molecule is significantly larger (2.890 Å). As a result, units formed by formally one OH- and one H2O molecule; i.e., H3O2- species, are clearly identified. At this point it should be highlighted that, as already observed for the OH-/H+ form, the Ru-OH interaction has an important covalent behavior, which produces a significant electron donation from the formal OH- to the RuO2 surface that accumulates on the oxygen atoms of the surface and particularly the Obr. Overall the negative charge on Obr significantly increases and the Bader charge of the formal H3O2- unit is significantly smaller than -1. In any case, for simplicity we will refer to the here described pattern as H3O2- species, even though its charge indicates an intermediate situation. The adsorption energy of -151.3 kJ mol-1 is larger than the adsorption of two water molecules, either in their neutral and deprotonated forms. Overall, calculations suggest that the 22 majority of adsorbed water molecules in the (110) surface are in the form of H3O2- dimers due to the strength of the cooperative effects as recently reported by Dohnálek and co-workers.35,36 The situation for the remaining surfaces is similar to that of the (110) facet, but with some particularities associated to the morphology of each surface. In the (011) facet, the contiguous unsaturated Ru5C centers are still sufficiently close (3.731 Å in the clean surface) that cooperative effects between adsorbed water molecules, OH-/H+ species or combination of both are also present when four water molecules are adsorbed. This leads to an adsorption energy per water molecule that is higher than that of a single water (either in its neutral or deprotanated forms). Interestingly, stronger cooperative effects are again found when the formal H2O3- dimer is formed (Figure 6a). Because of that, structures such as 3wat/1dpro or 4dpro are not found as minima on the potential energy surface and evolve to 2wat/2dpro and 1wat/3dpro situations, respectively. Indeed, the gain achieved with the cooperative effects resulting from the formation of the H2O3- dimers are sufficient to compensate the 12.6 kJ mol-1 preference for the adsorption of molecular water with respect to water dissociation. Overall, the situation in which 50% of water molecules are deprotonated is preferred by 5.4 kJ mol-1. [A] [B] [C] 1.571 1.494 1.676 1.607 1.593 1.494 1.521 1.5711.559 1.847 1.711 Figure 7. Optimized geometries of the most stable structures associated to water adsorption with a half monolayer coverage (q = 2/4 or 4/8) on the: a) (011); b) (100) and c) (001) surfaces. Distances are in Å 23 In the (100) surface, the distance between close Ru5C centers is also short (3.137 Å). Nevertheless, the surface morphology avoids the formation of moderate or strong H-bonds between water molecules and OH-/H+ moieties. This is a consequence of relative orientation of the Hw···Obr and Ow···Hbr hydrogen bonds with respect to the adjacent water or OH-/H+ groups. As a consequence, the adsorption energies are almost invariant to the increase of water coverage both for the 2wat and 2dpro situations. Only for the case of the H2O3- dimer formation, cooperative effects seem to be relevant, but in this case, they are not sufficient to overcome the 14.7 kJ mol-1 preference for the adsorption of molecular water. Overall, the 2wat structure is preferred (Figure 6b) but, both the 2wat and 1wat/1dpro situations are very close in energy, the difference being less than 1.2 kJ mol-1. This suggests that both situations would be present in this surface and the amount of deprotonated water would be lower than 50%. Finally, Ru5C centers at the (001) surface are all separated by more than 4 Å. Therefore, no cooperative effects can be established between water molecules, when coverage increases. This turns in adsorption energies that are marginally lower than those of the isolated water molecule. Remarkably with respect to what is found in the smaller coverages, we were able to find a structure in which at least one water molecule deprotonates, but this situation is 3.3 kJ mol-1 less favorable than the adsorption of molecular water. Therefore, no deprotonation would occur at the (001) surface, the most favorable structure being represented in Figure 6c. Overall, at coverages q = 2/4 and regardless of the surface morphology, the preferred situation implies the adsorption of water molecules in contiguous Ru centers forming H-bond chains. The degree of water deprotonation varies significantly as a function of the considered surface and this is controlled by the strength of the cooperative effects determined by surface 24 morphology. In the (110) and (011) surfaces, morphology allows the formation of strong cooperative effects, which results in the formation of chains constituted by formal H3O2- dimers leading to a high degree of deprotonation (50%). For the other facets, cooperative effects are weaker and the degree of deprotonation tends to decrease. For instance, the preferred structure of the (100) is the adsorption of water molecules (0% deprotonation), although the structure with 50% of deprotonated water is only marginally less favorable. Finally, the preferred adsorption mode in the (001) surface is molecular water, as a result of a very large distance between Ru centers. The stronger hydrogen bonding at 50% of deprotonation can be understood by analyzing the variation of the acid and basic strength of the resulting species when interacting with the surface. Adsorbed water becomes more acidic when it interacts with Ru centers (positive Bader atomic charges), and more basic when it deprotonates (the OH fragment is negatively charged). Therefore, the interaction of one adsorbed molecule (acting as donor) with one OH- species (acting as an acceptor) forms a strong hydrogen bond, leading to the formation of the formal H3O2- dimer. The hydrogen bond is intermediate when the species involved are both water or OH- species since either the acceptor or the donor is not optimal. The worst situation is the interaction of an OH- group as a donor and a water molecule as acceptor, which leads to very long hydrogen bonds. This behavior perfectly correlates with the observed trends in H-bonding distances. (Supporting Information S5). Water adsorption at monolayer coverages (q = (4/4)). With the aim of analyzing the structure of a water monolayer on each surface, we considered the adsorption of four water molecules per unit cell for the (110), (100) and (001) facets and 8 water molecules in the (011) surface. In all cases, the water molecules occupy one unsaturated ruthenium center and this leads to a density of water molecules close to 5 per nm2 in the (110) and (001) surfaces and between 7 25 and 8 for the (011) and (100) (see Table 4). For each facet, we started from the situation in which there is no water deprotonation, and successively deprotonate water molecules until deprotonation becomes unfavorable. Table 4 reports the adsorption energies per water molecule, some selected distances, the Bader atomic charges and the adsorption energy per surface area (A2) of the most favorable degree of deprotonation. The reference conformation consisting in a monolayer without any deprotonation is added for comparison. Figure 7 shows the optimized geometry of the preferred structure in each facet. Further information on the structures and energetics of other water deprotonation degrees can be found in the Supporting Information (Section S6). The addition of water molecules to the most stable q = 2/4 coverage structures leads to the formation of a second water chain that it is separated from the initial one by the presence of the Obr chain. This results in a significant separation between the two water chains and thus, there is almost no interaction between water molecules of different chains. However, depending on the nature of the Obr, the facets can be divided into two groups. In the case of the (110), the Obr atoms involved in the hydrogen bonding with the adsorbed water molecules are not bonded to unsaturated Ru centers and thus, the increase of water coverage has very little influence in the relative stabilities of the non-deprotonated and deprotonated adsorption modes. In contrast, for the other surfaces ((011), (100) and (001)), the Obr centers are directly bonded to an unsaturated Ru atom that becomes the adsorption center of the additional water molecules. As a consequence, the adsorption energies and preferred structures are influenced by the increase of water coverage, favoring water deprotonation. In detail, the addition of two water molecules to the q = 2/4 coverage structures of the (110) leads to the formation of a second chain, which can be formed of either water molecules, H3O2- building blocks or OH- groups, without any relevant interaction with the first chain. In this context, 26 the water monolayer without any water dissociation corresponds to a structure formed by two parallel chains of water molecules and the corresponding adsorption energy per water molecule, albeit marginally smaller, is almost that of water adsorption without dissociation with the q = 2/4 coverage. The most favorable situation corresponds to the dissociation of two water molecules per unit cell (2dpro), one in each water chain, so that the resulting structure presents two parallel chains made of the formal (H3O2)- dimer as basic unit. Consequently, the adsorption energy per water molecule resembles that of a single chain of (H3O2)- species. Following the same argumentation, the structure with only one dissociated water molecule corresponds to the situation in which there are alternate water and (H3O2)- parallel chains, with an adsorption energy per water molecule that is in between the adsorption energies of the two chains (water and (H3O2)-) separately (-143.0 kJ mol-1). Finally, the structure of 3 dissociated water molecules per unit cell, corresponds to two parallel chains: one has OH- groups as building blocks and the other is formed by the (H3O2-) units. Results for the q = 2/4 coverage revealed that the chain with the (H3O2-)n is preferred over the chain formed by OH- groups, therefore, the situation with three dissociated water molecules is less favorable than the formation of two (H3O2-)n chains. Indeed, the associated adsorption energy per water molecule is again in between the values obtained for the two chains separately. 27 Table 4. Water adsorption energies on the most favorable facets (in kJ mol-1) at monolayer coverages q=(4/4 or 8/8) and the associated main geometrical structural parameters (in Å or degrees). d(OwH d(ObrH Ð(H2O d(RuOw d(OwHw (h k l) rH2Oa Eads Eads/A qObrb qH2Ob w ) w ) ) ) 2) 0.981 wat 4.96 -138.0 -27.4 1.728 105.9 2.231 2.281 -0.86 +0.08 1.031 (1 1 0) (4:2) 0.973 -0.71c 0.09c dpro 4.96 -147.7 -29.3 0.995 106.7 2.131 1.689 1.524 -1.11d -0.40/+0.62d (0 1 1) 0.986 dpro 7.97 -130.4 -82.9 1.024 106.6 2.096 1.475 (8:4) 1.034 0.980 wat 7.02 -135.4 -38.0 1.587 105.4 2.176 2.245 +0.08 1.018 (1 0 0) (4:1) 0.983 0.09 c dpro 7.02 -136.2 -38.2 1.023 106.4 2.137 1.750 1.151 -0.44/+0.65 d 0.984 wat 4.84 -126.9 -24.6 1.458 101.1 2.193 2.396 +0.07 1.039 (0 0 1) (4:1) 0.975 +0.09 dpro 4.84 -131.0 -25.4 1.036 109.4 2.133 1.965 1.568 -0.57/+0.64 a H2O/nm2 b Average values The situation is significantly different in the other surfaces. In the (011), (100) and (001) facets, the separating Obr chain is formed by oxygen atoms bonded to unsaturated Ru center and the increase of water coverage has an effect both on the adsorption energies and the degree of water deprotonation. In this context, although there is not direct interaction between the water molecules of different chains, the fact that the Ru center bonded to the Obr acting as hydrogen- bond acceptor accommodates the adsorption of the additional water molecule produces an important change in the basicity of the Obr. Consequently, the Hw···Obr becomes stronger, as evidenced by a decrease of the Hw···Obr distance of about 0.01 Å and a marginally increase of the 28 adsorption energy. The increase in the Obr basicity also favors the water deprotonation and thus, the amount of deprotonated water tends to increase. These effects are very minor on the (011). The adsorption energy per water molecule on the (011) surface at q = 4/4 coverages is very similar to that of the half monolayer coverage and the degree of deprotonation remains at 50%. In fact, the major difference with the half monolayer coverage in this (011) facet is that while for q = 2/4 several different minima were found with different degrees of deprotonation at the monolayer coverages only structures with 50% of deprotonation were achieved. All our attempts to localize structures with higher and lower degrees of deprotonation spontaneously evolved to structures with 4 water and 4 H+/OH- units. Regarding the (100) facet, the adsorption energy increases 7.9 kJ mol- 1 with respect to the q = 2/4 situation and the associated global minimum presents 25% of deprotonated water molecules instead of 0% as in the preferred structure of the half monolayer coverage. Finally, in the (001) facet, the preferred degree of deprotonation is also 25% and the adsorption energy per water molecule increases from -126.2 kJ mol-1 in the q = 2/4 to -131.0 kJ mol-1 in q = 4/4. All these data were used to determine the most favorable coverage at different temperatures through ab-initio thermodynamics (Supporting Information). Results show that from 0 to around 700 K the preferred coverage is the monolayer and this is regardless of the surface. At around this temperature, water desorption begins, and it does not take place stepwise. Once the monolayer coverage becomes unfavorable the preferred structure becomes the 0 coverage. Although the temperature of desorption may be too high, results suggest a general preference for the monolayer coverage. Overall, when increasing the water coverage from the most stable structures of the half monolayer situation to the full monolayer, cooperative effects between water molecules remain unchanged. This is due to the formation of parallel water chains that are too far apart for forming 29 hydrogen bonding interaction between them. However, the increase of Obr basicity due to the adsorption of water molecules in unsaturated Ru centers bonded to this Obr centers increases the degree of deprotonation and/or the adsorption energy per water molecule in the (100) and (001) surfaces. The final amount of deprotonated water in the monolayer is 50% in the (110) and (011) facets and 25% in the (100) and (001) surfaces. This degree of deprotonation trend within the different facets is in good agreement with results reported in a very recent contribution.42 [A] [B] 1.574 1.564 1.571 1.562 1.494 1.689 1.534 1.534 1.591 1.626 1.607 1.494 1.689 1.475 1.571 [C] [D] 2.081 1.548 1.569 Figure 8. Optimized geometries of the most stable structures associated to water adsorption with monolayer coverage (q = 4/4 or 8/8) on the: a) (110); b) (011); c) (100) and d) (001) surfaces. Distances are in Å At this point it is worth mentioning that while the adsorption energy per water molecule is an important descriptor for analyzing how strong is the interaction between water and RuO2 in the interface, other factors are also key. In particular, the density of unsaturated Ru centers on the surface is significantly different in the four studied facets. In fact, the number of water molecules adsorbed per unit area is significantly higher in the (011) and (100) surfaces and thus the adsorption 30 energy per surface area does not correlate with the strength of the RuO2-H2O interaction of each particular water molecule (higher for the (110) surface). Indeed, considering the water density over the surface, the strongest water-RuO2 surface interaction is observed in the (011) facet, followed by the (100). This suggest that these two surfaces will be the most stabilized ones in solution with respect with their surface energies in vacuum. As a consequence, the Wulff construction (See Figure 3) with the surface energies resulting after considering the H2O-RuO2 interface shows that the major facet would be the (011) with small contributions from the (100) and (110) surfaces. Despite the limitations of the current approach this suggest very different nanoparticle shapes depending on the synthetic procedure (vacuum or solution). Conclusions Understanding the RuO2 – water interface and the adsorption of water on the most relevant RuO2 surfaces is important for characterizing the nature of the surface species that can be involved in the catalytic activity of hydrous RuO2 in oxidation reactions or in the electrochemical processes. This includes the water oxidation reaction, which is seen as an alternative environmentally friendly source of energy. In this context, we analyzed the influence of water coverage and surface morphology in the adsorption energy per water molecule and the degree of water deprotonation. For that, we considered the four most relevant surfaces non polar ((110), (011), (100) and (001)) and three different coverages: isolated water molecules, half monolayer and the full monolayer. Present results suggest that the water adsorption energy and the degree of deprotonation are controlled by three main factors. The first one is the intrinsic acidity of the unsaturated ruthenium centers and the basicity of the Obr groups in the pristine surface. The former increases when decreasing the coordination number of the ruthenium center. The latter is highly influenced by the 31 energy levels of the p orbitals of the Obr, which are related with the Ru-Obr-Ru angle. This leads to the fact that deprotonation at very low coverages is only favorable for the (110) surface. The second factor is the presence of cooperative effects between the adsorbed water molecules, which generates a chain of interacting species adsorbed on contiguous ruthenium centers. The usual preferred structure when cooperative effects are important is the formation of the formally (H3O2)- unit that presents a strong internal hydrogen bond. Since the (110) surface is the flattest one and presents short distances between contiguous ruthenium centers, it also presents the strongest cooperative effects, maximizing a degree of deprotonation of the 50%. In contrast, the (001) surface, with the most separated Ru-centers, presents the weakest cooperative effects, the weakest adsorption energies and the smallest degree of deprotonated water. Finally, it has been observed that Obr groups become more basic when additional water molecules to the half monolayer structures are adsorbed on Ru centers bonded to these Obr groups. This occurs at the highest coverages of the (011), (100) and (001) resulting in stronger adsorption energies and slightly higher water deprotonation percentages when moving from the single chain of adsorbed water molecules to the monolayer. Interestingly, the density of unsaturated ruthenium centers in the different surface varies significantly, thus the interaction energies between water and RuO2 per surface area do not correlate with the adsorption energies per water molecule leading to a stronger H2O-RuO2 interaction for those surfaces with a higher density of defective centers ((011) and (100)) surfaces. Overall, the strength of the water adsorption and the degree of deprotonation (50% for the monolayers of the (110) and (011) but 25% for those of the (100) and (001)) significantly varies as function of the surface morphology. Therefore, the synthesis of materials with one particular predominant face could tune the material properties and, ultimately, could have an influence in the catalytic processes. 32 The authors declare no competing financial interests Acknowledgements. The authors gratefully acknowledge financial support from MINECO (CTQ2017-89132-P) and the Generalitat de Catalunya (2014SGR-482). 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