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The properties of H2O adsorbed on Ru(001) at 95 K have been studied using a variety of techniques. The geometrical properties of the overlayer, deduced from low‐energy electron diffraction and electron stimulated desorption ion angular distributions, indicate that layered, hydrogen‐bonded clusters of H2O molecules form which have specific orientati...
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... O-H stretch, the asymmetric O-H stretch, and the deformation or SCissoring mode v$ water adsorbed on a surface or coordinated with another atom has additional vibrational modes due to "frustrated" rotations (libra- tions) and translations. The vibrational modes of a single water molecule adsorbed on a surface with C 2u symmetry are shown in Fig. 5. The frequencies of these internal modes have been studied extenSively for isolated water molecules, 6,21,28 aqua-metallic complexes 18 ,29-S3 and crystalline ice. 6,22,34,35 In these compounds, vibra- tions in the spectral region between 3000 and 3750 cm-! are attributed to the symmetric and asymmetriC O-H stretching modes vO R (v! ...
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... 5. The frequencies of these internal modes have been studied extenSively for isolated water molecules, 6,21,28 aqua-metallic complexes 18 ,29-S3 and crystalline ice. 6,22,34,35 In these compounds, vibra- tions in the spectral region between 3000 and 3750 cm-! are attributed to the symmetric and asymmetriC O-H stretching modes vO R (v! and V4 of Fig. 5) and vibrational frequencies between 1500 and 1650 cm-! are attributed to the SCissoring mode V$(V8)' Absorption bands below 900 cm-! have been assigned to frustrated translations VT(V2, V8' V9) and librations V L (V5, Vs, ...
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... exact assignment of the v~ and V~' modes in terms of the librational modes of Fig. 5 (V5' V6, or vr) is not possible due to the fact that H 2 0 clusters form. The two librational modes which are resolved clearly between exposures of 0.1 and 2.2 L in Fig. 6 may represent vi- brations of two distinct types of H 2 0 admolecules or two different modes within the cluster. The assignment of multiple IR absorptions to specific H 2 0 ...
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... of the three uncoupled librations is unknown. The most specific in- formation available on the librational modes of water molecules in ice is from the Raman spectrum of ice VIII, where three modes are observed at 874, 548, and 494 cm-t.42 The most probable assignment is that the 874 and 494 cm-1 bands are due to coupled V5 and V6 modes (cf. Fig. 5), and the 548 cm-1 band is due to the vr mode. 35 The strong intensity of the librational modes in the EEL spectra relative to the loss feature due to the O-H stretching mode is surprising in view of the relative in- tensities of these features in ice, where the dynamic ef- fective charge e* associated with the O-H vibration, ...
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... proper- ties of the metallic substrate, this rule predicts that only vibrations with a dipole derivative perpendicular to the surface can be excited in dipolar scattering of elec- trons. For an adsorbed molecule with local C 211 sym- metry, only vibrations belonging to the totally sym- metric At representation would be dipole active, as shown in Fig. 5 for adsorbed H 2 0. The C 2v adsorption symmetry applies, e. g., to an H 2 0 molecule adsorbed in the first layer of Fig. 3(C) or 17(A) in the absence of any perturbation by neighboring or second-layer mole- cules. For such symmetry, only the Vtt V2' and v3 modes of Fig. 5 will be dipole active. If the symmetry is lower than C 2u • i. ...
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... totally sym- metric At representation would be dipole active, as shown in Fig. 5 for adsorbed H 2 0. The C 2v adsorption symmetry applies, e. g., to an H 2 0 molecule adsorbed in the first layer of Fig. 3(C) or 17(A) in the absence of any perturbation by neighboring or second-layer mole- cules. For such symmetry, only the Vtt V2' and v3 modes of Fig. 5 will be dipole active. If the symmetry is lower than C 2u • i. e., C$ ol,' C t , the remaining modes v,-v9 can become dipole allowed also. The modes which are not dipole allowed for C 2v symmetry include all the librational modes (vs, vs, and vr)' However, these modes cause the most intense features observed in the EEL spectra of Hp. ...
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Citations
... These measurements suggested that the first solvent contact layer adopts a stationary structure composed of water hexamers with maximized surface−water and water−water interactions. 30,31 Recent experiments and simulations have, however, largely disproven this model by demonstrating that on several metallic surfaces the interface is more diverse and disordered, exhibiting structurally more complex motifs and, most importantly, considerable solvent fluctuations. 19,23 Consequently, a reassessment of the computational hydrogen evolution activity of Pt(111) considering the significance of explicit interfacial dynamics is highly warranted. ...
The computational hydrogen evolution activity of Pt(111) remains controversial due to apparent discrepancies with experiments concerning rate-determining activation free energies and equilibrium hydrogen coverages. A fundamental source of error may lie within the static representations of the metal–water interface commonly employed in density functional theory (DFT)-based kinetic models neglecting important entropic effects on reaction dynamics. In this work, we present a dynamic reassessment of the Volmer–Tafel hydrogen evolution pathway on Pt(111) through DFT-based constrained molecular dynamics simulations and thermodynamic integration. Hydrogen coverage effects are gauged at two distinct surface saturations, while the critical potential dependence and constant potential conditions are accounted for using a capacitive model of the electrified interface. The uncertainty in the highly nontrivial treatment of the electrode potential is carefully examined, and we provide a quantitative estimation of the error associated with dynamically simulated electrochemical barriers. The dynamic description of the electrochemical interface promotes a substantial decrease of the Tafel free energy barrier as the coverage is increased to a full monolayer. This follows from a decreased entropic barrier due to suppressed adlayer dynamics compared to the unsaturated surface, a detail easily missed by static calculations predicting notably higher barriers at the same coverage. Due to observed endergonic adsorption of active hydrogen intermediates, the Tafel step remains rate-determining irrespective of the coverage as illustrated by composed Volmer–Tafel free energy landscapes. Importantly, our explicitly dynamic approach avoids the ambiguous choice of frozen solvent configuration, decreasing the reliance on error cancellation and paving the way for less biased electrochemical simulations.
... The high k 4 values in this case can be rationalized in terms of extremely short residence times of H 2 O molecules on Ru(0001) after their formation by eq 4, in agreement with previous reports by other authors. 53,54 The following set of differential equations was built to account for the spatiotemporal dependence of the surface concentration n i = n i (x,t) of all species involved in the reaction mechanism, based on the kinetic model presented in eqs 1−4. ...
We offer a comprehensive approach to determine how physical confinement can affect the water formation reaction. By using free-standing crystalline SiO2 bilayer supported on Ru(0001) as a model system, we studied the water formation reaction under confinement in situ and in real time. Low-energy electron microscopy reveals that the reaction proceeds via the formation of reaction fronts propagating across the Ru(0001) surface. The Arrhenius analyses of the front velocity yield apparent activation energies (E a app) of 0.32 eV for the confined and 0.59 eV for the nonconfined reaction. DFT simulations indicate that the rate-determining step remains unchanged upon confinement, therefore ruling out the widely accepted transition state effect. Additionally, H2O accumulation cannot explain the change in E a app for the confined cases studied because its concentration remains low. Instead, numerical simulations of the proposed kinetic model suggest that the H2 adsorption process plays a decisive role in reproducing the Arrhenius plots.
... Early literature proposed the so-called ice-like bilayer structure of water at electrochemical interfaces, which was often used to model the solvent environment in computational electrocatalysis [29,30]. The ice-like bilayer structure originated from low-energy electron diffraction patterns [31] and scanning tunneling microscopy images [32] observed in ultrahigh vacuum (UHV) conditions and was also supported by density functional theory calculations [33]. However, owing to such great difference between electrochemical condition and UHV, it is quite dubious to assume the bilayer structure at electrochemical interfaces. ...
The origin of the potential difference between the potential of zero charge of a metal/water interface and the work function of the metal is a recurring issue because it is related to how water interacts with metal surface in the absence of surface charge. Recently ab initio molecular dynamics method has been used to model electrochemical interfaces to study interfacial potential and the structure of interface water. Here, we will first introduce the computational standard hydrogen electrode method, which allows for ab initio determination of electrode potentials that can be directly compared with experiment. Then, we will review the recent progress from ab initio molecular dynamics simulation in understanding the interaction between water and metal and its impact on interfacial potential. Finally, we will give our perspective for future development of ab initio computational electrochemistry.
... Interaction of water with Ru(0001) has found particular interest. Early water adsorption experiments on Ru(0001), employing low energy electron diffraction (LEED), electron stimulated desorption -ion angular detection (ESDIAD) and high resolution electron energy loss spectroscopy (HREELS) revealed a rather poorly ordered adlayer after exposure at 95 K and a well ordered (√3×√3)R30° structure after adsorption at 165 K [5,6]. Based on LEED structure determination, the oxygen atoms in the water adlayer were found to be almost coplanar [7], in contrast to the so-called bilayer model proposed by Doering et al. [6], which was commonly accepted at that time and which also exhibits a (√3×√3)R30° structure, but with a relatively large difference in the vertical position of the oxygen atoms. ...
... Structurally, the previous studies agree that for water adsorption on Ru(0001) both, molecularly intact water and the partly dissociated water adlayer, result in the formation of ordered (√3×√3)R30° structures [5,9,10,15,[22][23][24]. For dissociated water on Ru(0001), previous STM studies revealed the formation of stripe structures oriented along the <1 -1 0 0> directions, which exhibit a (√3×√3)R30° structure, at least in their inner part [15]. ...
Aiming at an improved molecular scale understanding of the interaction of water with Ru surfaces and electrodes, in particular with respect to thermally induced water dissociation, we have performed a systematic study of the low temperature adsorption and structure formation behavior of water on Ru(0001) by scanning tunneling microscopy (STM). Typical structures formed upon adsorption at below 100 K indicate the presence of molecularly adsorbed water. For adsorption at 120 K, in contrast, the resulting structures are identical to phases which have been associated with partially dissociated adsorbed water, indicating that for continuous adsorption under these conditions thermally induced water dissociation is activated. This assignment is supported by the strongly hindered activity for further adsorption of H2O or CO (at 100 K) on surfaces where only half of the surface is covered by structures related to an adsorbed H2O/OH phase. This is attributed to passivation by a H adlayer on the remaining, apparently bare surface areas, which results from H2O dissociation. Consequences of these findings on the understanding of water interaction with Ru(0001) are discussed.
... Early low energy electron diffraction (LEED) [2,3] patterns and later scanning tunneling microscopy (STM) [4][5][6] images of transition metal surfaces like Ru(0001) in ultra-high vacuum (UHV) at cryogenic temperatures, were interpreted in terms of a √ 3 × √ 3 honeycomb bilayer structure. This bilayer structure was also confirmed later by density functional theory (DFT) calculations [7][8][9][10][11]. ...
We simulated a series of transition metal-water interfaces, namely Pt(111), Au(111), Pd(111) and Ag(111), by density functional theory based molecular dynamics, and found some common structural features for the surface water on different transition metal surfaces. Firstly, there exists a pronounced adsorption layer within ∼5 Å distance from metal surfaces, in which three main water species with different orientations (watA, watB-down and watB-up) could be identified. WatA and watB-down show a lower degree of hydrogen bonding, due to their interaction with the metal surface via one of the lone pairs of the oxygen atoms and via one of their H atoms, respectively. While, watB-up has an almost full coordination shell, indicating it not only forms hydrogen bonds in the adsorption layer, but also with the non-surface water. As expected, the honeycomb-like bilayer model used as the starting point of the simulation was destructed into irregular patterns after ∼10 ps of molecular dynamics simulations, and the surface water coverage concomitantly increases from 0.66 ML to ∼0.8 ML.
... 14 is usually found in crystalline ice structures around the freezing point. [15][16][17][18] This crystalline ice structure has a characteristic vibrational peak at 3220 cm −1 in IR spectra. 19,20 Above the freezing temperature, the liquid phase is the most thermodynamically stable structure, where the water molecule has two to three hydrogen bonds with other water molecules, 21 and gives a broad infrared absorbance peak around 3400 cm −1 . ...
The electrode potential dependence of the hydration layer on an n-Ge(100) surface was studied by a combination of in situ and operando electrochemical attenuated total reflection infrared (ATR-IR) spectroscopy and real space density functional theory (DFT) calculations. Constant-potential DFT calculations were coupled to a modified generalised Poisson-Boltzmann ion distribution model and applied within an ab initio molecular dynamics (AIMD) scheme. As a result, potential-dependent vibrational spectra of surface species and surface water were obtained, both experimentally and by simulations. The experimental spectra show increasing absorbance from the Ge-H stretching modes at negative potentials, which is associated with an increased negative difference absorbance of water-related OH modes. When the termination transition of germanium from OH to H termination occurs, the surface switches from hydrophilic to hydrophobic. This transition is fully reversible. During the switching, the interface water molecules are displaced from the surface forming a ``hydrophobic gap''. The gap thickness was experimentally estimated \revised{by a continuum electrodynamic model} to be ≈2 Å. The calculations showed a shift in the centre of mass of the interface water by ≈0.9 Å due to the surface transformation. The resulting IR spectra of the interfacial water in contact with the hydrophobic Ge-H show an increased absorbance of free OH groups, and a decreased absorbance of strongly hydrogen bound water. Consequently, the surface transformation to a Ge-H terminated surface leads to a surface which is weakening the H-bond network of the interfacial water in contact.
... Besides, the controllable adsorption of water molecules on metal surfaces also sheds lights on solutions to practical application of hydrogen energy, next generation of fuel cells, and water-gas-shift reaction. As a result, many experimental [1][2][3][4][5][6][7][8][9][10][11][12][13] and theoretical investigations [14][15][16][17] are carried out in studying the adsorption structures of single to layers of water molecules on different metal surfaces. Through scanning tunneling microscopy (STM) observations, cluster structures like water monomer, dimer, and hexamer have successfully been watched on different metal surfaces like Ag(111), 6 Cu(111), 6 and Pd(111). ...
... By applying low-energy electron diffraction (LEED) detections, ordered 2D water networks can be directly seen while adsorbing on metal surfaces like Pt(111) 3 and Ru(0001). 5 The 2D networks serve as wetting layers on these metal substrates. It is found that hydrogen bonds between adsorbed water molecules are key to understand the various configurations from 1D chains to 2D networks. ...
By using density functional theory calculations, we systematically investigate the adsorption of water molecules at different coverages on the Be(0001) surface. The coverage dependence of the prototype water structures and energetics for water adlayer growth are systematically studied. The structures, energetics, and electronic properties are calculated and compared with other available studies. Through our systematic investigations, we find that water molecules form clusters or chains on the Be(0001) surface at low coverages. When increasing the water coverage, water molecules tend to form a 2 × 2 hexagonal network on the Be(0001) surface.
... The other frequencies (f 4-f8) are observed due to the liberation and frustrated translation modes. We see that (Table II) 54 This we believe should be the reason for the decrease in the stretching and bending vibrational frequencies in this system. In addition, DFT studies on H 2 O on Ni(100) and Ni(111) surfaces also showed similar decrease in the frequencies of these two modes. ...
Water adsorption and dissociation on Ni(110) surface is studied in detail and compared with its close packed counterparts using density functional theory calculations. Water adsorption occurs on the top site as found on Ni(100) and Ni(111) but the adsorption is stronger on Ni(110). H and OH preferably adsorb on the short bridge sites (brgshort) opposed to hollow sites on (100) and (111) surfaces. Energy barriers for water molecule dissociation on Ni(110) as obtained from the transition state (TS) calculations were low compared to other Ni low indexed surfaces. TS geometries at different positions of the lattice coordinate, Q, were obtained to study the effect of surface temperature on dissociation of H2O molecules. These calculations revealed that second layer atoms were also involved in the TS. Dissociation probabilities are obtained using a semi-classical approximation by sampling Q for a Boltzmann distribution at different temperatures. Results showed that the increasing surface temperature significantly increases the dissociation probabilities at lower energies and saturates near the barrier for dissociation. Although the contribution from both top and second layers is similar at low surface temperatures, motion of top layer atoms contribute more towards dissociation probability at higher surface temperatures. Dissociation probabilities obtained are more than one order of magnitude higher than that on Ni(100) and Ni(111) surfaces suggesting Ni(110) to be more reactive among the low indexed Ni surfaces.
... In addition, the great abundance of water, ice and water-covered solid surfaces in the biosphere explains the attention devoted to the study of water adsorption on single crystal surfaces by means of modern surface science techniques [31,32]. In particular, for H 2 O/Ru(0001), the possible structures water can form, from adsorbed isolated molecules to small clusters, periodic bilayers or ice multilayers were extensively studied during 20 years since the late 1970s through a wide variety of experimental techniques [27,[33][34][35][36][37][38][39]. The basic findings were: (i) three distinct water peaks in the thermal desorption spectra (TDS), one at low temperature (150 ∼ 160 K) attributed to ice multilayers, and two at higher temperatures (170 ∼ 180 K, 210 ∼ 220 K) attributed to water aggregates (periodic bilayers, clusters) in more direct contact with the metal surface; (ii) an ordered ( ...
Recent years have witnessed an ever growing interest in theoretically studying chemical processes at surfaces. Apart from
the interest in catalysis, electrochemistry, hydrogen economy, green chemistry, atmospheric and interstellar chemistry, theoretical
understanding of the molecule–surface chemical bonding and of the microscopic dynamics of adsorption and reaction of adsorbates
are of fundamental importance for modeling known processes, understanding new experimental data, predicting new phenomena,
controlling reaction pathways. In this work, we review the efforts we have made in the last few years in this exciting field.
We first consider the energetics and the structural properties of some adsorbates on metal surfaces, as deduced by converged,
first-principles, plane-wave calculations within the slab-supercell approach. These studies comprise water adsorption on Ru(0001),
a subject of very intense debate in the past few years, and oxygen adsorption on aluminum, the prototypical example of metal
passivation. Next, we address dynamical processes at surfaces with classical and quantum methods. Here the main interest is
in hydrogen dynamics on metallic and semi-metallic surfaces, because of its importance for hydrogen storage and interstellar
chemistry. Hydrogen sticking is studied with classical and quasi-classical means, with particular emphasis on the relaxation
of hot–atoms following dissociative chemisorption. Hot atoms dynamics on metal surfaces is investigated in the reverse, hydrogen
recombination process and compared to Eley–Rideal dynamics. Finally, Eley–Rideal, collision-induced desorption, and adsorbate-induced
trapping are studied quantum mechanically on a graphite surface, and unexpected quantum effects are observed.
... [3][4][5] Because of the close match in lattice constants between Ru(0001) (4.68 Å) and the basal plane of I h -phase crystal water ice (4.50 Å), an overlayer structure based on p( 3 × 3)R30°was observed and depicted using the basal water bilayer in I h ice, in early investigations. [6][7][8] Lately, a com-mensurate p( 3 × 3)R30°structure assigned to the first D 2 O bilayer was clearly detected, and the separation between two vertically different D 2 O layers in the bilayer was estimated to be 0.10 ( 0.02 Å (nearly coplanar), by low-energy electron diffraction (LEED) intensity analysis. 9 However, details of the water layer structures on the Ru(0001) surface are still under debate. ...
The initial growth of a water (D2O) layer on (1 x 1)-oxygen-covered Ru(0001) has been studied in comparison with that on bare Ru(0001) by means of temperature-programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRAS). Although water molecules adsorbed on both bare and (1 x 1)-oxygen-covered Ru(0001) commonly tend to form hydrogen bonds with each other when mobility occurs upon heating, the TPD and IRAS measurements for the two surfaces exhibit distinct differences. On (1 x 1)-oxygen-covered Ru(0001), most of the D2O molecules were desorbed with a peak at 160 K, even at submonolayer coverage, as condensed water desorption. The vibration spectra of adsorbed D2O also showed broad peaks such as a condensed water phase, from the beginning of low coverage. For submonolayer coverage, in addition, we found a characteristic O-D stretching mode at around 2650 cm(-1), which is never clearly observed for D2O on bare Ru(0001). Thus, we propose a distinctive water adsorption structure on (1 x 1)-oxygen-covered Ru(0001) and discuss its influence on water layer growth in comparison with the case of D2O on bare Ru(0001).
