Electrochemical Activation of Water

We start om discussion with the adsorption and activation of water over Pd(lll) and then advance to other metals. Filhol and Neurock l used the approach described above to examine the potential-dependent behavior of water over Pd(lll). In the absence of an electrochemical field or potential, water adsorbs on Pd(lll) with its oxygen end directed down towards the surface and its molecular plane tilted away from the surface normal vector by approximately 60°, as shown in Fig. 6.11. 

The phase diagram for the activation of water to form surface hydride and surface hydroxide phases is shown in Fig. 6.14. The comparison of different structures at a given potential can be compared in order to determine the free energy differences between these phases at a constant chemical potential. 

Similar calculations were carried out for water activation over Cu(lll), Ni(lll) and Pt(lll) surfacesl l. The calculations for water on Cu(lll) go into much more detail and show that the surface structure changes continuously as a function of applied potential [31.32] results show that various additional surface phases can form that were not 

The examples that follow below outhne how we can begin to extend the general mechanistic information gleaned from gas-phase surface reactions to more complex liquid phase systems and electrochemical systems. 

Vinyl acetate is synthesized via the selective oxidation of ethylene and acetic acid over supported Pd and PdAu catalysts via the reaction 

---

FiUiol JS, Neurock M. 2006. Elucidation of the electrochemical activation of water over Pd by first principles. Angew Chem Int Ed 45 402-406. 

Taylor CD, Janik MJ, Neurock M, Kelly RG. 2006c. Ah initio simulations of the electrochemical activation of water. Mol Sim 33 429-436. 

Tatsuma T, Hioki Y, Oyama N (1995) Dependence of swelling behavior and electrochemical activity of water-soluble polythiophene films on the nature of the electrolyte. J Electroanal... 

In contrast the oxo-ruthenium complex c ,c -[ (bpy)2Runl(0H2) 2(//-0)]4+ and some of its derivatives are known to be active catalysts for the chemical or electrochemical oxidation of water to dioxygen.464-472 Many studies have been reported473 181 on the redox and structural chemistry of this complex for understanding the mechanism of water oxidation. Based on the results of pH-dependent electrochemical measurements, the basic structural unit is retained in the successive oxidation states from Rum-0 Ru111 to Ruv O Ruv.466... 

Adsorption of third particles other than water molecules on metal electrodes influences the microstructure and the electrochemical activity of the electrode interface. For example, the interface of metal electrodes usually acts as a Lewis add in the adsorption of water molecules, but its Lewis add-base property is altered by the adsorption of third partides. Electronegative particles such as oi en molecules, if adsorbed, increase the local Lewis acidity of interfacial metal atoms around the adsorption sites whereas, electropositive particles such as sodium atoms, if adsorbed, increase the local Lewis basicity around their adsorption sites. Furthermore, the adsorption energy of water molecules is altered by the coadsorption of third partides on metal electrodes. 

Following this disclosure, several papers and patents appeared which we have found to be very instructive (121-124) under operating conditions the sometimes violently energetic reactions can be controlled, especially when the activity of water is kept low. However, conditions under which it is possible to redeposit electrochemically the alkali metal in a recharge require further study. 

The water electrolysis rest potential is determined from extrapolation to ideal conditions. Variations of the concentration, c, and pressure, p, from ideality are respectively expressed by the activity (or fugacity for a gas), as a = yc (or yp for a gas), with the ideal state defined at 1 atmosphere for a pure liquid (or solid), and extrapolated from p = 0 or for a gas or infinite dilution for a dissolved species. The formal potential, measured under real conditions of c and p can deviate significantly from the (ideal thermodynamic) rest potential, as for example the activity of water, aw, at, or near, ambient conditions generally ranges from approximately 1 for dilute solutions to less than 0.1 for concentrated alkaline and acidic electrolytes.91"93 The potential for the dissociation of water decreases from 1.229 V at 25 °C in the liquid phase to 1.167 V at 100 °C in the gas phase. Above the boiling the point, pressure is used to express the variation of water activity. The variation of the electrochemical potential for water in the liquid and gas phases are given by ... 

The actual mechanism of the anodic reaction prior to the onset of passivity demands greater attention. The electrochemical activity of the metallic ions may differ on different parts of a pure metal surface. The variations may be due to varying values of the ionic work function Y+ and/or different shapes of the Morse-type functions of Y4. and W+. The activation energies AFa and AFC depend not only on the values of Y+ and W+ but also at what point the work functions cross. Therefore the "geometric" or steric relationship between the surface ions and the water molecules or other complexing agent are... 

The most active samples have only local order (to 1.0 nm) and also have cage-like stmctures, or ones with micropores. The micropores (sized on the order of 0.4 to 1.0 nm) are likely filled with water or hydroxyl groups and are therefore conducive for proton conduction. The iron in the active samples also exists in mixed valence states, which is likely to contribute to the electrochemical activity of the Pt-FeOx. Furthermore, the Pt atoms are uniformly distributed throughout the stmcture. 

The fact that the product i x EOI (which represents the effective current density used for the oxidation of phenol) varies linearly with the current density (Fig. 12) indicates that the process is not limited by mass transfer at the anode and the oxidation occurs probably by active oxygen (OH") formed at the anode by the electrochemical oxidation of water. 

The above calculations have been extended to a number of other alloys, notably PtSn and PtMo as these have special interest in CO oxidation electrocatalysisOn PtSn, CO was found to interact only with the Pt sites, not with the Sn sites. This leaves the Sn sites available for the activation of water necessary to oxidize the chemisorbed CO, explaining why RSn is such an unusually good catalyst for the electrochemical oxidation of CO (at low potentials). ... 

Before electrochemical activation of the silver electrode, one observes the broad and intense stretching mode of water (3450 cm" ). This O—H scattering masks the vibrations of the 5 -AMP molecule in the NSRS spectrum (cf. Fig. 21 b). After activa-... 

Carbon itself has been successfully used as a biomaterial. Carbon based fibers used in composites are known to be inert in aqueous (even seawater) environments, however they do not have a track record in the biomaterials setting. In vitro studies by Kovacs [1993] disclose substantial electrochemical activity of carbon fiber composites in an aqueous environment. If such composites are placed near a metaUic implant, galvanic corrosion is a possibility. Composite materials with a polymer matrix absorb water when placed in a hydrated environment such as the body. Moisture acts as a plasticizer of the matrix and shifts the glass transition temperature towards lower values [Delasi and Whiteside, 1978], hence a reduction in stiffness and an increase in mechanical damping. Water immersion of a graphite epoxy... 

---