Hydrogen bonding in mixed OH+H2O overlayers on Pt(111)

Adsorbate Resonance Induces Water-Metal Bonds in Electrochemical Interfaces

This study delves into the intricate interactions between surface-near species, OH and H2O, on electrodes in electrochemical interfaces. These species are an inevitable part of many electrocatalytic energy conversion reactions such as the oxygen reduction reaction. In our modeling, we utilize high statistics on a dataset of complex solid solutions with high atomic variability to show the emergence of H2O-metal covalent bonds under specific conditions. Based on density functional theory (DFT) calculations of adsorption energies on many thousands of different surface compositions, we provide a quantifiable physical understanding of this induced water covalency, which is rooted in simple quantum mechanics. Directional hydrogen bonding between surface-near H2O and OH, enables surface bonding electrons to delocalize, mediated by near-symmetrical adsorbate resonance structures. The different adsorbate resonance structures differ by surface coordination explaining the induced H2O-metal bonding.

Electrowinning for Room-Temperature Ironmaking: Mapping the Electrochemical Aqueous Iron Interface

A promising route toward room-temperature ironmaking is electrowinning, where iron ore dissolution is coupled with cation electrodeposition to grow pure iron. However, poor faradaic efficiencies against the hydrogen evolution reaction (HER) is a major bottleneck. To develop a mechanistic picture of this technology, we conduct a first-principles thermodynamic analysis of the Fe110 aqueous electrochemical interface. Constructing a surface Pourbaix diagram, we predict that the iron surface will always drive toward adsorbate coverage. We calculate theoretical overpotentials for terrace and step sites and predict that growth at the step sites are likely to dominate. Investigating the hydrogen surface phases, we model several hydrogen absorption mechanisms, all of which are predicted to be endothermic. Additionally, for HER we identify step sites as being more reactive than on the terrace and with competitive limiting potentials to iron plating. The results presented here further motivate electrolyte design toward HER suppression.

Hydroxyl on Stepped Copper and its Interaction with Water

We describe the hydroxyl and mixed hydroxyl-water structures formed on a stepped copper surface following the reaction of adsorbed O with water at a low temperature and compare them to the structures found previously on plane copper surfaces. Thermal desorption profiles, STM, and low-energy electron diffraction show that water reacts with O at temperatures below 130 K on Cu(511). Two well-defined phases appear as the OH/H2O layer is heated to desorb excess water, a 1OH:1H2O phase and a pure OH phase. The 1OH:1H2O structure consists of 1D chains binding across two adjacent copper steps, with a double period along the step. Electronic structure calculations show that the structure has a zigzag chain of water along the terrace, stabilized by hydrogen bonds to OH groups adsorbed in the step bridge sites. This structure binds OH in its favored site and is similar to the structure observed on other open faces of Cu and Ni, suggesting that this structural arrangement may be common on other surfaces that have steps or rows of close packed metal atoms. The hydroxyl/water chains decompose at 210 K to leave OH adsorbed in the Cu step bridge site, with some forming H-bonded trimers that bridge between two Cu steps. Heating the surface causes hydroxyl to disproportionate near 300 K, desorbing water to leave chemisorbed O.

Elucidation of the mechanism of melamine adsorption on Pt(111) surface via density functional theory calculations

The oxygen reduction reaction (ORR) activity of Pt catalysts in polymer electrolyte fuel cells (PEFCs) should be enhanced to reduce Pt usage. The adsorption of heteroaromatic ring compounds such as melamine on the Pt surface can enhance its catalytic activity. However, melamine adsorption on Pt and the consequent ORR enhancement mechanism remain unclear. In this study, we performed density functional theory calculations to determine the adsorption structures of melamine/Pt(111). Melamine was coordinated to Pt via two N lone pairs on NH2 and N- in the triazine ring, resulting in a chemisorption structure with slight electron transfer. Four types of adsorption structures were identified: three-point adsorption (two amino groups and a triazine ring: Type A), two-point adsorption (one amino group and a triazine ring: Type B), two-point adsorption (two amino groups: Type C), and one-point adsorption (one amino group: Type D). The most stable structure was Type B. However, multiple intermediate structures were formed owing to the conformational changes from the most stable to other stable adsorption structures. The resonance structures of the adsorbed melamine stabilise the adsorption, as increased resonance allows for more electron delocalisation. In addition, the lone-pair orbital of the amino group in the adsorbed melamine acquires the characteristics of an sp3 hybrid orbital, which prevents horizontal adsorption on the Pt surface. We believe that understanding these adsorption mechanisms will help in the molecular design of organic molecule-decorated Pt catalysts and will lead to the reduction of Pt usage in PEFCs.

Identifying Adsorbed OH Species on Pt and Ru Electrodes with Surface-Enhanced Infrared Absorption Spectroscopy through CO Displacement

OH adspecies are involved in numerous electrocatalytic reactions, such as CO, H2, methanol, and ethanol oxidation and oxygen reduction reactions, as a reaction intermediate and/or reactant. In this work, we have, for the first time, identified the OH stretching band of OH adspecies on Pt, Ru, and Pt/Ru electrodes with surface-enhanced infrared absorption spectroscopy (SEIRAS) in a flow cell through potential modulation and CO displacement. We found that while Ru had a relatively constant OH coverage at potentials between 0.1 and 0.8 V, Pt had a maximum OH coverage at 0.6 V in 0.1 M HClO4 and 0.7 V in 0.1 M KOH. CO oxidation kinetics on Ru were sluggish, although adsorbed OH appeared on Ru at very low potentials. Binary Pt/Ru electrodes promote CO oxidation through a synergistic effect in which Ru promotes OH adsorption and Pt catalyzes the reaction between the CO and OH adspecies. In addition, water coadsorbed with CO at Ru sites of Pt/Ru also plays an important role. These new spectroscopic results about OH adspecies could advance the understanding of the mechanism of fuel cell related electrocatalysis.

Platinum-Catalysed Selective Aerobic Oxidation of Methane to Formaldehyde in the Presence of Liquid Water

The aerobic, selective oxidation of methane to C1 -oxygenates remains a challenge, due to the more facile, consecutive oxidation of formed products to CO2 . Here, we report on the aerobic selective oxidation of methane under continuous flow conditions, over platinum-based catalysts yielding formaldehyde with a high selectivity (reaching 90 % for Pt/TiO2 and 65 % over Pt/Al2 O3 ) upon co-feeding water. The presence of liquid water under reaction conditions increases the activity strongly attaining a methane conversion of 1-3 % over Pt/TiO2 . Density-functional theory (DFT) calculations show that the preferential formation of formaldehyde is linked to the stability of the di-σ-hydroxy-methoxy species on platinum, the preferred carbon-containing species on Pt(111) at a high chemical potential of water. Our findings provide novel insights into the reaction pathway for the Pt-catalysed, aerobic selective oxidation of CH4 .

Nucleation and growth of water ice on oxide surfaces: the influence of a precursor to water dissociation

We have investigated how nucleation and growth processes of ice are influenced by interfacial molecular interactions on some oxide surfaces, such as rutile TiO2(110), TiO2(100), MgO(100), and Al2O3(0001), based on the diffraction patterns of electrons transmitted through ice crystallites under the experimental configuration of reflection high energy electron diffraction (RHEED). The cubic ice Ic grows on the TiO2(110) surface with the epitaxial relationship of (110)Ic//(110)TiO2 and [001]Ic//[11[combining macron]0]TiO2. The epitaxial ice growth tends to be disturbed on the TiO2(110) surface under the presence of oxygen vacancies and adatoms. The result is not simply ascribable to small misfit values between TiO2 and ice Ic lattices (∼2%) because ice grains are formed randomly on TiO2(100). No template effects are identified during ice nucleation on the pristine MgO(100) and Al2O3(0001) surfaces either. The water molecules are chemisorbed weakly on these surfaces as a precursor to dissociation via the acid-base interaction. Such anchored water species act as an inhibitor of epitaxial ice growth because the orientation flexibility of physisorbed water during nucleation is hampered at the interface by the preferential formation of hydrogen bonds.

Mechanistic Insights into the Hydrogen Oxidation Reaction on PtNi Alloys in Alkaline Media: A First-Principles Investigation

The promising alkaline anion exchange membrane fuel cell suffers from sluggish kinetics of the hydrogen oxidation reaction (HOR). However, the puzzling HOR mechanism hinders the further development of highly active catalysts in alkaline media. In this work, we conducted detailed first-principles calculations to acquire a deep understanding of the alkaline HOR mechanism on PtNi bulk alloys [Pt₃Ni(111), Pt₂Ni₂(111), and PtNi₃(111)] and its surface alloy [PtNi_surf(111)]. The full free energy profiles suggest that the HOR on PtNi alloys proceeds via the Tafel-Volmer mechanism, that is, the direct decomposition of H₂ into two adsorbed H, followed by its reaction with OH^- in the electrolyte, as the rate-determining step, to form H₂O. Therefore, the HOR activity of PtNi alloys is solely impacted by the adsorption of hydrogen, rather than hydroxyl species, though the oxophilicity is also enhanced by alloying Pt with Ni. Thermodynamically, a moderate H adsorption free energy, ΔG_H* ≈ 0.414 eV, is calculated to be an optimal candidate for the HOR at pH = 13. Alloying Pt with Ni can elevate the d-band center (ε_d), push the value of ΔG_H* closer to 0.414 eV, and thus lower the free energy barrier (E_a) of the rate-determining Volmer reaction, leading to the highest HOR activity of PtNi₃(111) among all considered PtNi alloys. This situation is further confirmed by both the microkinetic model and the Tafel plot, where PtNi₃(111) exhibits the highest reaction rate (r = 9.42 × 10³ s^-1 site^-1) and the largest exchange current density (i₀ = 1.42 mA cm^-2) for HOR in alkaline media. This work provides a fundamental understanding of the HOR mechanism and theoretical guidance for rational design of electrocatalysts for HOR in alkaline media.

Reflection high energy electron diffraction (RHEED) study of ice nucleation and growth on Ni(111): influences of adspecies and electron irradiation

How interfacial molecular interactions influence nucleation and growth processes of water ice is explored using pristine, oxygenated, and CO-adsorbed Ni(111) substrates based on RHEED, together with the effects of high-energy electron irradiation on the crystallization kinetics. A monolayer of amorphous solid water deposited onto the pristine Ni(111) substrate crystallizes into ice Ic at ca. 150 K, whereas ice Ih (Ic) is formed preferentially during water vapor deposition at 135 K (125 K). The ice nucleation tends to be hampered on the oxygenated Ni(111) surface because of the hydrogen bond formation with chemisorbed oxygen, leading to the growth of randomly-oriented ice Ic crystallites via spontaneous nucleation. The amorphization and recrystallization of initially crystalline ices are observed during prolonged RHEED measurements at 20 and 70 K, respectively, signifying that high-energy electron irradiation has both thermal and non-thermal effects on the water phase transition. The epitaxial growth (non-epitaxial growth) of ice occurs during electron irradiation of amorphous solid water formed on the pristine and oxygenated Ni(111) substrates (CO-adsorbed Ni(111) substrate) even at 100 K (120 K) because nucleation and growth are initiated at the substrate interface (in the ASW film interior).

Crystallization kinetics of thin water films on Pt(111): effects of oxygen and carbon-monoxide adspecies

This paper describes nucleation, epitaxial growth, and wettability of water on Pt(111) and how they are influenced by oxygen and carbon-monoxide adspecies, based on reflection high energy electron diffraction (RHEED), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and temperature-programmed desorption (TPD). Amorphous solid water deposited onto the pristine Pt(111) substrate crystallizes into ice Ih together with a 2D layer at 150 K, whereas ice Ic (stacking disordered ice or a mixture of ice Ic and Ih) is formed preferentially onto oxygenated Pt(111) (CO-adsorbed Pt(111)) at 155-160 K (150 K). The ice nucleation and epitaxial growth tend to be hampered on the oxygenated Pt(111) surface via hydrogen bond formation with chemisorbed oxygen. The CO-adsorbed Pt(111) surface is hydrophobic, as evidenced by the fact that water forms a complex with CO during evaporation of crystallites at 160-165 K. A disordered 2D layer remains on pristine Pt(111) up to 175 K, whereas an ordered 2D layer exhibiting the (√3 ×√3)R30° structure formed on oxygenated Pt(111) up to 200 K.

Unusual strain effect of a Pt-based L10 face-centered tetragonal core in core/shell nanoparticles for the oxygen reduction reaction

Nanoparticles with a low-Pt content core and a few-layer thick Pt skin are attractive catalysts toward the oxygen reduction reaction (ORR) not only for their low cost, but also because their activity can be enhanced by judiciously choosing the core alloy.

Effect of hydrophobic cations on the oxygen reduction reaction on single‒crystal platinum electrodes

Highly active catalysts for the oxygen reduction reaction are essential for the widespread and economically viable use of polymer electrolyte fuel cells. Here we report the oxygen reduction reaction activities of single‒crystal platinum electrodes in acidic solutions containing tetraalkylammonium cations with different alkyl chain lengths. The high hydrophobicity of a tetraalkylammonium cation with a longer alkyl chain enhances the oxygen reduction reaction activity. The activity on Pt(111) in the presence of tetra‒n‒hexylammonium cation is eight times as high as that without this cation, which is comparable to the activities on Pt₃Co(111) and Pt₃Ni(111) electrodes. Hydrophobic cations and their hydration shells destabilize the adsorbed hydroxide and adsorbed water. The hydrophobic characteristics of non‒specifically adsorbed cations can prevent the adsorption of poisoning species on the platinum electrode and form a highly efficient interface for the oxygen reduction reaction.

Highly active catalysts for the oxygen reduction reaction are valuable for fuel cells. Here the authors evaluate catalytic activity of single-crystal platinum electrodes in acidic solutions that contain hydrophobic cations, which prevent the adsorption of poisoning species and form an efficient interface.

Communication: Proton exchange in low temperature co-mixed amorphous H2O and D2O films: The effect of the underlying Pt(111) and graphene substrates

Isotopic exchange reactions in mixed D₂O and H₂O amorphous solid water (ASW) films were investigated using reflection absorption infrared spectroscopy. Nanoscale films composed of 5% D₂O in H₂O were deposited on Pt(111) and graphene covered Pt(111) substrates. At 130 K, we find that the reaction is strongly dependent on the substrate with the H/D exchange being significantly more rapid on the Pt(111) surface than on graphene. At 140 K, the films eventually crystallize with the final products on the two substrates being primarily HOD molecule on Pt(111) and a mixture of HOD and unreacted D₂O on graphene. We demonstrate by pre-dosing H₂ and O₂ on Pt(111) that the observed differences in reactivity on the two substrates are likely due to the formation of hydrogen ions at the Pt(111) surface that are not formed on graphene. Once formed the mobile protons move through the ASW overlayer to initiate the H/D exchange reaction.

The reactivity of water and OH on Pt-Ni(111) films

Bimetallic Pt catalysts are of interest as water redox catalysts in low temperature fuel cells. Here we compare water and hydroxyl adsorption on Pt-Ni(111) films and a PtNi(111) alloy surface with the behaviour on the pure metals. Whereas water adsorbs and desorbs intact from close packed Pt and Ni, it dissociates on PtNi surfaces to form adsorbed hydroxyl and hydrogen. Reactivity to water increases in the order Pt(111) < monolayer Pt-Ni(111) < multilayer (2-6 ML) Pt-Ni(111) ∼ PtNi(111) surface alloy and does not scale directly with the Pt strain. Hydroxyl can also be formed by reaction with pre-adsorbed O and is less stable than on pure Pt, decomposing to water and O in a broad peak near 180 K, 20 K lower than on Pt(111). The reduced stability of OH on Pt-Ni(111) films is common to all the PtNi surfaces and consistent with bimetallic PtNi surfaces showing less blocking by OH during the oxygen reduction reaction.

Intramolecular hydrogen bonding in malonaldehyde and its radical analogues

High level Brueckner doubles with triples correction method-based ab initio calculations have been used to investigate the nature of intramolecular hydrogen bonding and intramolecular hydrogen atom transfer in cis-malonaldehyde (MA) and its radical analogues. The radicals considered here are the ones that correspond to the homolytic cleavage of C-H bonds in cis-MA. The results suggest that cis-MA and its radical analogues, cis-MA_RS, and cis-MA_RA, both exist in planar geometry. The calculated intramolecular O-H⋯O=C bond in cis-MA is shorter than that in the radical analogues. The intramolecular hydrogen bond in cis-MA is stronger than in its radicals by at least 3.0 kcal/mol. The stability of a cis-malonaldehyde radical correlates with the extent of electron spin delocalization; cis-MA_RA, in which the radical spin is more delocalized, is the most stable MA radical, whereas cis-MA_RS, in which the radical spin is strongly localized, is the least stable radical. The natural bond orbital analysis indicates that the intramolecular hydrogen bonding (O⋯H⋯O) in cis-malonaldehyde radicals is stabilized by the interaction between the lone pair orbitals of donor oxygen and the σ^* orbital of acceptor O-H bond (n → σ^*_OH). The calculated barriers indicate that the intramolecular proton transfer in cis-MA involves 2.2 kcal/mol lower barrier than that in cis-MA_RS.

Thermodynamic assessment of the oxygen reduction activity in aqueous solutions

The hydrogen bonding of hydrophilic oxygen reduction intermediates to water has large effects on scaling relations and volcano plots.

Theoretical Analysis of Electrochemical Formation and Phase Transition of Oxygenated Adsorbates on Pt(111)

The electrochemical oxygenation processes of Pt(111) surface are investigated by combining density functional theory (DFT) calculations and Monto Carlo (MC) simulations. DFT calculations are performed to construct force-field parameters for computing the energy of (√3 × √3)R30°-structured OH*-H2O* hydrogen-bonding networks (differently dissociated water bilayer) on the Pt(111) surface, with which MC simulations are conducted to probe the reversible H2O* ↔ OH* conversion in OH*-H2O* networks. The simulated isotherm (relation between electrode potential and OH* coverage) agrees well with that predicted by the experimental cyclic voltammetry (CV) in the potential region of 0.55-0.85 V (vs RHE). It is suggested that the butterfly shape of CV in this region is due to different variation trends of Pt-H2O* distance in low and high OH* coverages. DFT calculation results indicate that the oxidative voltammetry in the potential region from 0.85 V to ca. 1.07 V is associated with the dissociation of OH* to O*, which yields surface structures consisting of OH*-H2O* networks and (√3 × √3)-structured O* clusters. The high stability of the half-dissociated water bilayer (OH*-H2O* hydrogen-bonding network with equal OH* and H2O* coverages) formed in the butterfly region makes OH* dissociation initially very difficult in energetics, but become facile once starts due to the destabilization of OH* by the formed O* nearby. This explains the experimentally observed nucleation and growth behavior of O* phase formation and the high asymmetry of oxidation-reduction voltammetry in this potential region.

Water at Interfaces

The interfaces of neat water and aqueous solutions play a prominent role in many technological processes and in the environment. Examples of aqueous interfaces are ultrathin water films that cover most hydrophilic surfaces under ambient relative humidities, the liquid/solid interface which drives many electrochemical reactions, and the liquid/vapor interface, which governs the uptake and release of trace gases by the oceans and cloud droplets. In this article we review some of the recent experimental and theoretical advances in our knowledge of the properties of aqueous interfaces and discuss open questions and gaps in our understanding.

Generation of highly reactive oxygen species on metal-supported MgO(100) thin films

A series of highly reactive oxygen species are formed with the assistance of water on an insulating surface.

Oxygen-induced changes to selectivity-determining steps in electrocatalytic CO2 reduction

The state of the electrocatalyst surface—including the oxidation state of the catalyst and the presence of spectator species—is investigated on Cu surfaces with density functional theory in order to understand predicted ramifications on the selectivity of CO₂ reduction between CH₄ and CH₃OH.

Water clustering on nanostructured iron oxide films

The adhesion of water to solid surfaces is characterized by the tendency to balance competing molecule-molecule and molecule-surface interactions. Hydroxyl groups form strong hydrogen bonds to water molecules and are known to substantially influence the wetting behaviour of oxide surfaces, but it is not well-understood how these hydroxyl groups and their distribution on a surface affect the molecular-scale structure at the interface. Here we report a study of water clustering on a moiré-structured iron oxide thin film with a controlled density of hydroxyl groups. While large amorphous monolayer islands form on the bare film, the hydroxylated iron oxide film acts as a hydrophilic nanotemplate, causing the formation of a regular array of ice-like hexameric nanoclusters. The formation of this ordered phase is localized at the nanometre scale; with increasing water coverage, ordered and amorphous water are found to coexist at adjacent hydroxylated and hydroxyl-free domains of the moiré structure.

First principles investigation of zinc-anode dissolution in zinc-air batteries

With surging interest in high energy density batteries, much attention has recently been devoted to metal-air batteries. The zinc-air battery has been known for more than a hundred years and is commercially available as a primary battery, but recharging has remained elusive, in part because the fundamental mechanisms still remain to be fully understood. Here, we present a density functional theory investigation of the zinc dissolution (oxidation) on the anode side in the zinc-air battery. Two models are envisaged, the most stable (0001) surface and a kink surface. The kink model proves to be more accurate as it brings about some important features of bulk dissolution and yields results in good agreement with experiments. From the adsorption energies of hydroxyl species and experimental values, we construct a free energy diagram and confirm that there is a small overpotential associated with the reaction. The applied methodology provides new insight into computational modelling and design of secondary metal-air batteries.

Strain relief and disorder in commensurate water layers formed on Pd(111)

Water adsorbs and desorbs intact on Pd(111), forming a hydrogen-bonded wetting layer whose structure we examine by low energy electron diffraction (LEED) and He atom scattering (HAS). LEED shows that water forms commensurate (√3 × √3)R30° clusters that aggregate into a partially ordered, approximately (7 × 7) superstructure as the layer completes. HAS indicates that the water layer remains disordered on a local (approximately 10 Å) scale. Based on workfunction measurements and density functional theory simulations we propose that water forms small, flat domains of a commensurate (√3 × √3)R30° water network, separated by disordered domain boundaries containing largely H-down water. This arrangement allows the water layer to adapt its density and relieve the lateral strain associated with adsorbing water in the optimum flat atop adsorption site. We discuss different possibilities for the structure of these domain walls and compare this strain relief mechanism to the highly ordered, large unit cell structures formed on surfaces such as Pt(111).

The Pt(111)/electrolyte interface under oxygen reduction reaction conditions: an electrochemical impedance spectroscopy study

The Pt(111)/electrolyte interface has been characterized during the oxygen reduction reaction (ORR) in 0.1 M HClO(4) using electrochemical impedance spectroscopy. The surface was studied within the potential region where adsorption of OH* and O* species occur without significant place exchange between the adsorbate and Pt surface atoms (0.45-1.15 V vs RHE). An equivalent electric circuit is proposed to model the Pt(111)/electrolyte interface under ORR conditions within the selected potential window. This equivalent circuit reflects three processes with different time constants, which occur simultaneously during the ORR at Pt(111). Density functional theory (DFT) calculations were used to correlate and interpret the results of the measurements. The calculations indicate that the coadsorption of ClO(4)* and Cl* with OH* is unlikely. Our analysis suggests that the two-dimensional (2D) structures formed in O(2)-free solution are also formed under ORR conditions.

c(2×2) water-hydroxyl layer on Cu(110): a wetting layer stabilized by Bjerrum defects

Understanding the composition and stability of mixed water-hydroxyl layers is a key step in describing wetting and how surfaces respond to redox processes. Here we show that, instead of forming a complete hydrogen bonding network, structures containing an excess of water over hydroxyl are stabilized on Cu(110) by forming a distorted hexagonal network of water-hydroxyl trimers containing Bjerrum defects. This arrangement maximizes the number of strong bonds formed by water donation to OH and provides uncoordinated OH groups able to hydrogen bond multilayer water and nucleate growth.