The role of substrate electrons in the wetting of a metal surface

Finding Key Factors for Efficient Water and Methanol Activation at Metals, Oxides, MXenes, and Metal/Oxide Interfaces

Activating water and methanol is crucial in numerous catalytic, electrocatalytic, and photocatalytic reactions. Despite extensive research, the optimal active sites for water/methanol activation are yet to be unequivocally elucidated. Here, we combine transition-state searches and electronic charge analyses on various structurally different materials to identify two features of favorable O–H bond cleavage in H₂O, CH₃OH, and hydroxyl: (1) low barriers appear when the charge of H moieties remains approximately constant during the dissociation process, as observed on metal oxides, MXenes, and metal/oxide interfaces. Such favorable kinetics is closely related to adsorbate/substrate hydrogen bonding and is enhanced by nearly linear O–H–O angles and short O–H distances. (2) Fast dissociation is observed when the rotation of O–H bonds is facile, which is favored by weak adsorbate binding and effective orbital overlap. Interestingly, we find that the two features are energetically proportional. Finally, we find conspicuous differences between H₂O/CH₃OH and OH activation, which hints toward the use of carefully engineered interfaces.

Carbon Monoxide Tolerant Pt-Based Electrocatalysts for H2-PEMFC Applications: Current Progress and Challenges

The activity degradation of hydrogen-fed proton exchange membrane fuel cells (H2-PEMFCs) in the presence of even trace amounts of carbon monoxide (CO) in the H2 fuel is among the major drawbacks currently hindering their commercialization. Although significant progress has been made, the development of a practical anode electrocatalyst with both high CO tolerance and stability has still not occurred. Currently, efforts are being devoted to Pt-based electrocatalysts, including (i) alloys developed via novel synthesis methods, (ii) Pt combinations with metal oxides, (iii) core–shell structures, and (iv) surface-modified Pt/C catalysts. Additionally, the prospect of substituting the conventional carbon black support with advanced carbonaceous materials or metal oxides and carbides has been widely explored. In the present review, we provide a brief introduction to the fundamental aspects of CO tolerance, followed by a comprehensive presentation and thorough discussion of the recent strategies applied to enhance the CO tolerance and stability of anode electrocatalysts. The aim is to determine the progress made so far, highlight the most promising state-of-the-art CO-tolerant electrocatalysts, and identify the contributions of the novel strategies and the future challenges.

Impact of Water Coadsorption on the Electrode Potential of H-Pt(1 1 1)-Liquid Water Interfaces

Density functional theory molecular dynamics simulations of H-covered Pt(111)-H_{2}O interfaces reveal that, in contrast to common understanding, H_{2}O coadsorption has a significant impact on the electrode potential of and plays a major role in determining the stability of H adsorbates under electrochemical conditions. Based on these insights, we explain the origin behind the experimentally observed upper limit of H coverage well below one monolayer and derive a chemically intuitive model for metal-water bonding that explains an unexpectedly large interaction between coadsorbed water and adsorbates.

Structural and dynamical heterogeneities at glutamine-water interfaces

The behavior of water at the surfaces of solid amino acid crystals has received little attention despite its importance in nucleation processes. In this work, we take a first step to fill this gap by using molecular dynamics simulations to study the structural and dynamical properties of water near the (100), (010) and (001) surfaces of l-glutamine crystals. These highly hydrophilic surfaces serve as excellent model systems for interrogating the behavior of water. Despite having the same molecular composition, water at each surface displays characteristic structural, orientational and dynamical correlations. This behavior is tuned by how the different chemical groups of amino acids make contact with the liquid phase. All three surfaces yield a glassy layer of interfacial water which is reflected in different ways such as the presence of a rotationally arrested layer of water molecules and substantial slow down of the diffusion of water near the interface. By increasing the concentration of molecules in solution, we show that the binding of glutamine molecules to the crystal surface creates a crowded environment involving pockets of trapped water molecules altering the water dynamics in a highly non-trivial manner suggesting that the solvent dynamics may have important implications on crystal nucleation.

Activation of CO2 at the electrode–electrolyte interface by a co-adsorbed cation and an electric field

Electric polarization by the local microenvironment strongly affects the CO₂ activation at the electrode–electrolyte interface.

Coexistence of ice clusters and liquid-like water clusters on the Ru(0001) surface

Detailed RAIRS spectra reveal rich and varied local hydrogen bonding structures inside the two types of water clusters found on the Ru(0001) surface.

Modelling pH and potential in dynamic structures of the water/Pt(111) interface on the atomic scale

Modelling liquid structures averages of water in the interface with Pt(111) as grand canonical averages, that are functions of pH and electrode potential, using work functions as the absolute potential scale.

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.

Comparison of x-ray absorption spectra between water and ice: new ice data with low pre-edge absorption cross-section

The effect of crystal growth conditions on the O K-edge x-ray absorption spectra of ice is investigated through detailed analysis of the spectral features. The amount of ice defects is found to be minimized on hydrophobic surfaces, such as BaF2(111), with low concentration of nucleation centers. This is manifested through a reduction of the absorption cross-section at 535 eV, which is associated with distorted hydrogen bonds. Furthermore, a connection is made between the observed increase in spectral intensity between 544 and 548 eV and high-symmetry points in the electronic band structure, suggesting a more extended hydrogen-bond network as compared to ices prepared differently. The spectral differences for various ice preparations are compared to the temperature dependence of spectra of liquid water upon supercooling. A double-peak feature in the absorption cross-section between 540 and 543 eV is identified as a characteristic of the crystalline phase. The connection to the interpretation of the liquid phase O K-edge x-ray absorption spectrum is extensively discussed.

Interaction of H2O and H2S with Cu(111) and the impact of the electric field: the rotating & translating adsorbate, and the rippled surface

The interactions of H₂O and H₂S monomers with Cu(111) in the absence and presence of an external electric field are studied using density functional theory.

Hydration of metal surfaces can be dynamically heterogeneous and hydrophobic

We have applied molecular dynamics and methods of importance sampling to study structure and dynamics of liquid water in contact with metal surfaces. The specific surfaces considered resemble the 100 and 111 faces of platinum. Several results emerge that should apply generally, not just to platinum. These results are generic consequences of water molecules binding strongly to surfaces that are incommensurate with favorable hydrogen-bonding patterns. We show that adlayers of water under these conditions have frustrated structures that interact unfavorably with adjacent liquid water. We elucidate dynamical processes of water in these cases that extend over a broad range of timescales, from less than picoseconds to more than nanoseconds. Associated spatial correlations extend over nanometers. We show that adlayer reorganization occurs intermittently, and each reorganization event correlates motions of several molecules. We show that soft liquid interfaces form adjacent to the adlayer, as is generally characteristic of liquid water adjacent to a hydrophobic surface. The infrequent adlayer reorganization produces a hydrophobic heterogeneity that we characterize by studying the degrees by which different regions of the adlayers attract small hydrophobic particles. Consequences for electrochemistry are discussed in the context of hydronium ions being attracted from the liquid to the metal–adlayer surface.

A molecular perspective of water at metal interfaces

Water/solid interfaces are relevant to a broad range of physicochemical phenomena and technological processes such as corrosion, lubrication, heterogeneous catalysis and electrochemistry. Although many fields have contributed to rapid progress in the fundamental knowledge of water at interfaces, detailed molecular-level understanding of water/solid interfaces comes mainly from studies on flat metal substrates. These studies have recently shown that a remarkably rich variety of structures form at the interface between water and even seemingly simple flat surfaces. In this Review we discuss the most exciting work in this area, in particular the emerging physical insight and general concepts about how water binds to metal surfaces. We also provide a perspective on outstanding problems, challenges and open questions.

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).

Tailoring the structure of water at a metal surface: a structural analysis of the water bilayer formed on an alloy template

Recent studies show that structures based on the traditional "icelike" water bilayer are not stable on flat transition metal surfaces and, instead, more complex wetting layers are formed. Here we show that an ordered bilayer can be formed on a SnPt(111) alloy template and determine the structure of the water layer by low energy electron diffraction. Close agreement is found between experiment and the structure calculated by density functional theory. Corrugation of the alloy surface allows only alternate water molecules to chemisorb, stabilizing the H-down water bilayer by reducing the metal-hydrogen repulsion compared to a flat surface.

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.