Metal-solution interfaces that approach

More recently, the curvature at air/solution interfaces has been accounted for by Nikitas and Pappa-Louisi98 in terms of a specific molecular model that predicts a linear dependence of (lM/ ) on (1/0). The same model also reproduces the behavior at metal/solution interfaces, specifically Hg electrodes, for which most of the experimental data exist. Nikitas treatment provides a method for an unambiguous extrapolation of the adsorption potential shift to 0= 1. However, the interpretation of the results is subject to the difficulties outlined above. Nikitas approach does provide... 

The liquid metal mercury-solution interface presents the advantage that it approaches closest to an ideal polarizable interface and, therefore, it adopts the potential difference applied between it and a non-polarizable interface. For this reason, the mercury-solution interface has been extensively selected to carry out measurements of the surface tension dependence on the applied potential. In the case of other metal-solution interfaces, the thermodynamic study is much more complex since the changes in the interfacial area are determined by the increase of the number of surface atoms (plastic deformation) or by the increase of the interatomic lattice spacing (elastic deformation) [2, 4]. 

Ab initio MD studies were carried out to help understand the elementary processes that occur at the metal-solution interface [77]. At temperatures less than 300 K, acetic acid decomposed on Pd, leaving an adsorbed acetate intermediate along with a proton in solution. Above 300 K, however, surface acetate recombines with a proton in solution to form acetic acid. Acetic acid is then displaced from the surface by water. Once the acetic acid finds its way into solution, it redissociates to form acetate and protons in solution that are now more efficiently stabilized by water. A series of snapshots that portray some of the images from the simulation is shown in Figure 13. These results were further corroborated with a more conventional transition-state search approach, which showed that desorption of acetate from the surface was an activated process. The process is quite complicated, involving the simultaneous breaking of the acetate-metal bond, the formation of an... 

Differential capacitance measurements were used to determine the extent that DMSA adsorbsonto the metal surface as a function of its concentration in solution. In this approach the metal-solution interface is modelled as a resistor and capacitor in series and if the diffuse part of the double layer is neglected, the measured capacitance can be expressed as ... 

Infrared spectroscopy is frequently applied to investigate CO adsorption on electrodes, because CO is important as an intermediate and surface poison in many electrocatalytic reactions and the C-O stretching vibrational modes of the adlayer are sensitive to the chemical environment at the metal/solution interface. Infrared spectra of CO adsorbed on low-index surface planes of Pt single-crystal electrodes have become a benchmark for use in understanding the behavior of CO on other surfaces. Related approaches have been extended to bulk single-crystal metal electrodes that include Pd [66, 67], Ir [68-71], Rh [13, 70], Ru [72-74], Ni [75, 76] and Au [77]. 

The AIMD method, based on the Carr and ParrineUo approach [127], has also been applied in the study of electrochemistry [128]. Reactive Force Field approaches are now being used to study the ionomer/water/catalyst interfaces during an electrochemical reaction [129]. Neurock et al. developed a detailed first-principles approach that employs a double-reference method to simulate the influence of the electrochemical potential on the chemistry at the metal/solution interface [130]. hi this method the aqueous solution metal interface and the interfacial potential drop are explicitly treated. However the choice of an appropriate water surface structure is critical for establishing the appropriate electrochemical behavior at the atomistic scale. This method has been applied to smdy some electrochemical steps involved in the ORR and methanol oxidation on Pt (e.g. [131, 132]). 

It seems that certain additional information on this subject could be obtained from analysis and correlation of Volta and zero-charge potentials [155]. It is noteworthy that such an approach resulted in significant achievements in the study of metal-solution interfaces [156-159]. 

The approach of Chan and Eikerling (2011) emphasizes the importance of charging phenomena at the metal-solution interface, but it does not account for the intricate and largely unsettled effects that could arise from the progressive oxidation of Pt at high 0. It treats the potential of zero charge as a variable parameter that could attain values in the range from = 0.3 to 1.1 she-... 

Alteration of the electrical double layer. The adsorption of ions or species that can form ions on metal surfaces will change the electrical double layer at the metal-solution interface, and this in turn will affect the rates of the electrochemical reactions. The adsorption of cations, such as quaternary ammonium ions and protonated amines, makes the potential more positive in the plane of the closest approach to the metal of... 

Alternatively, the second barrel can be filled with the electrolyte and used in the same manner as in an ion conductance microscope [50]. It is possible to relate the solution conductance between tip and counter electrode to the normalized tip-to-substrate distance L. If the conductance is measured by a dc technique and the surface is impermeable to ions, a negative feedback effect (decrease of conductance as the tip approaches the surface) is observed. If an ac technique is used and the substrate is metallic, a positive feedback effect can be observed irrespective of whether the substrate is biased or not because the lowest impedance pathway for the current is through the metal via the metal-solution interface. In fact, after normalizing the conductance G(L) by the value with the tip far from the surface G , the distance dependence of the conductance is identical to that for faradaic currents in feedback SECM with a redox mediator ... 

In work by Bockris and his co-workers [1, 10, 119-123, 137, 138] the dependence of reversible adsorption of organic substances on the potential at platinum metals was interpreted on the basis of the assumption that there is competition between the organic molecules and the water molecules for sites on the surface and that the standard free energy of adsorption of water depends on the electric field at the electrode — solution interface. This approach has been criticized [9,139] from the point of view of the analysis made... 

Another progress in our understanding of the ideally polarizable electrode came from theoretical works showing that the metal side of the interface cannot be considered just as an ideal charged plane. A simple quantum-mechanical approach shows that the distribution of the electron gas depends both on the charge of the electrode and on the metal-solution coupling [12,13]. 

As mentioned above, the distribution of the various species in the two adjacent phases changes during a potential sweep which induces the transfer of an ion I across the interface when the potential approaches its standard transfer potential. This flux of charges across the interface leads to a measurable current which is recorded as a function of the applied potential. Such curves are called voltammograms and a typical example for the transfer of pilocarpine [229] is shown in Fig. 6, illustrating that cyclic voltammograms produced by reversible ion transfer reactions are similar to those obtained for electron transfer reactions at a metal-electrolyte solution interface. 

Ideal polarizable interfaces are critical for the interpretation of electrochemical kinetic data. Ideality has been approached for certain metal electrode-solution interfaces, such as mercury-water, allowing for the collection of data that can be subjected to rigorous theoretical analysis. 

General Observations About x. its Relationship to the Overall Partitioning Coefficient and to the Concept of Surface-Site Heterogeneity. One approach to metal/particle surface interactions which has been developed, historically, in a variety of forms, is a conceptual model that assumes only two conditions for surface sites occupied by an adsorbate or unoccupied. In applying this approach to the solid/aqueous solution interface, the adsorption... 

Macroscopic experiments allow determination of the capacitances, potentials, and binding constants by fitting titration data to a particular model of the surface complexation reaction [105,106,110-121] however, this approach does not allow direct microscopic determination of the inter-layer spacing or the dielectric constant in the inter-layer region. While discrimination between inner-sphere and outer-sphere sorption complexes may be presumed from macroscopic experiments [122,123], direct determination of the structure and nature of surface complexes and the structure of the diffuse layer is not possible by these methods alone [40,124]. Nor is it clear that ideas from the chemistry of isolated species in solution (e.g., outer-vs. inner-sphere complexes) are directly transferable to the surface layer or if additional short- to mid-range structural ordering is important. Instead, in situ (in the presence of bulk water) molecular-scale probes such as X-ray absorption fine structure spectroscopy (XAFS) and X-ray standing wave (XSW) methods are needed to provide this information (see Section 3.4). To date, however, there have been very few molecular-scale experimental studies of the EDL at the metal oxide-aqueous solution interface (see, e.g., [125,126]). 

Consider mercury as the liquid metal under study. One of the advantages of this metal is that the mercuiy/solution interface approaches closest to the ideal polarizable interface (see Section 6.3.3) over a range of 2 V. What this means is that this interface responds exactly to all the changes in the potential difference of an external source when it is coupled to a nonpolarizable interface, and there are no complications of charges leaking through the double layer (charge-transfer reactions). 

Similar to the molecular photosensitizers described above, solid semiconductor materials can absorb photons and convert light into electrical energy capable of reducing C02. In solution, a semiconductor will absorb light, and the electric field created at the solid-liquid interface effects the separation of photo-excited electron-hole pairs. The electrons can then carry out an interfacial reduction reaction at one site, while the holes can perform an interfacial oxidation at a separate site. In the following sections, details will be provided of the reduction of C02 at both bulk semiconductor electrodes that resemble their metal electrode counterparts, and semiconductor powders and colloids that approach the molecular length scale. Further information on semiconductor systems for C02 reduction is available in several excellent reviews [8, 44, 104, 105],... 

Another interface that needs to be mentioned in the context of polarized interfaces is the interface between the insulator and the electrolyte. It has been proposed as a means for realization of adsorption-based potentiometric sensors using Teflon, polyethylene, and other hydrophobic polymers of low dielectric constant Z>2, which can serve as the substrates for immobilized charged biomolecules. This type of interface happens also to be the largest area interface on this planet the interface between air (insulator) and sea water (electrolyte). This interface behaves differently from the one found in a typical metal-electrolyte electrode. When an ion approaches such an interface from an aqueous solution (dielectric constant Di) an image charge is formed in the insulator. In other words, the interface acts as an electrostatic mirror. The two charges repel each other, due to the low dielectric constant (Williams, 1975). This repulsion is called the Born repulsion H, and it is given by (5.10). 

The use of galvanostatic transients enabled the measurement of the poten-tiodynamic behavior of Li electrodes in a nearly steady state condition of the Li/film/solution system [21,81], It appeared that Li electrodes behave potentio-dynamically, as predicted by Eqs. (5)—(12), Section III.C a linear, Tafel-like, log i versus T dependence was observed [Eq. (8)], and the Tafel slope [Eq. (10)] could be correlated to the thickness of the surface films (calculated from the overall surface film capacitance [21,81]). From measurements at low overpotentials, /o, and thus the average surface film resistivity, could be measured according to Eq. (11), Section m.C [21,81], Another useful approach is the fast measurement of open circuit potentials of Li electrodes prepared fresh in solution versus a normal Li/Li+ reference electrode [90,91,235], While lithium reference electrodes are usually denoted as Li/Li+, the potential of these electrodes at steady state depends on the metal/film and film/solution interfaces, as well as on the Li+ concentration in both film and solution phases [236], However, since Li electrodes in many solutions reach a steady state stability, their potential may be regarded as quite stable within reasonable time tables (hours —> days, depending on the system s surface chemistry and related aging processes). 

Various chemical surface complexation models have been developed to describe potentiometric titration and metal adsorption data at the oxide—mineral solution interface. Surface complexation models provide molecular descriptions of metal adsorption using an equilibrium approach that defines surface species, chemical reactions, mass balances, and charge balances. Thermodynamic properties such as solid-phase activity coefficients and equilibrium constants are calculated mathematically. The major advancement of the chemical surface complexation models is consideration of charge on both the adsorbate metal ion and the adsorbent surface. In addition, these models can provide insight into the stoichiometry and reactivity of adsorbed species. Application of these models to reference oxide minerals has been extensive, but their use in describing ion adsorption by clay minerals, organic materials, and soils has been more limited. 

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