Faradaic reactions involving adsorption

EMIRS studies of ethanol on platinum electrodes have demonstrated the presence of linearly bonded carbon monoxide on the surface [106]. An important problem in the use of EMIRS to study alcohol adsorption is the choice of a potential window where the modulation is appropriate without producing faradaic reactions involving soluble products. Ethanol is reduced to ethane and methane at potentials below 0.2 V [98, 107] and it is oxidized to acetaldehyde at c 0.35 V. Accordingly, a potential modulation would be possible only within these two limits. Outside these potential region, soluble products and their own adsorbed species complicate the interpretation of the spectra. The problem is more serious when the adsorbate band frequencies are almost independent of potential. In this case, the potential window (0.2-0.35 V) is too narrow to obtain an appropriate band shift and spectral features can be lost in the difference spectrum. 

In Chap. 4, the faradaic reaction involving the diffusion of redox species was presented. In this chapter reactions involving adsorption without diffusion will be presented, starting with simple electrosorption as in underpotential deposition, followed by adsorption/desorption involving one, two, or more adsorbed species. 

As the rate of electrocatalytic reaction is, by definition, dependent on the nature of electrode material, it immediately follows that reactants, intermediates and products of electrode reactions interact with the surface of the electrode. Hence, the state of the electrode surface in the course of electrode will reflect to the rate of the electrode reaction. Based on the definition of (electro)catalyst one can expect that the state of the surface after catalytic cycle should be the same as before reaction. Nevertheless, at least for metallic electrodes, the state of the electrode surface within the potential window where electrocatalytic reaction takes place is in the constant change and the electrode reaction itself cannot be considered independently on the potential-dependent surface processes taking place at the same time. These processes are typically investigated using cyclic voltammetry, allowing identification of various adsorption/desorption processes as well as pseudo-Faradaic reactions involving surface oxidation/reduction. 

Adsorption pseudocapacitance can come from two-dimensional surface reactions that involve faradaic desorption and adsorption of an electroactive species from the electrolyte at a metal surface [69], One good example is the adsorption/desorption of hydrogen (H) at Pt in the acid solution, following the reaction [41,69]... 

Similarly, using C03O4 as pseudocapacitive materials in the KOH electrolyte, the surface faradaic reaction is involved with OH- ions adsorption/desorption or inser-tion/extraction accompanied by the charge transfer process. This can be expressed as follows [127,128] ... 

Once faradaic current flows, the equilibrium between oxidized and reduced species is disturbed, and can be continually reestablished only if all the steps involved in the electrode process are rapid enough. (These steps include charge transfer, movement of depolarizer to the electrode and of product away from it (mass transport), and possibly adsorption or chemical reactions.) If there is a lag, then the electrode potential changes from its equilibrium value, the magnitude of the change being the overpotential or overvoltage. 

When the electroactive species or an intermediate adsorbs on the electrode surface, the adsorption process usually becomes an integral part of the charge transfer process and therefore cannot be studied without the interference of a faradaic current. In this situation, surface coverages cannot be measured directly and the role of an adsorbate must be inferred from a kinetic investigation. Tafel slopes and reaction orders will deviate substantially from those for a simple electron transfer process when an adsorbed intermediate is involved. Moreover the kinetic parameters, exchange current or standard rate constant, are likely to become functions of the electrode material and even the final products may change. These factors will be discussed further in the section on electrocatalysis (Section 1.4). 

The majority of easily detected compounds at solid anodes under constant applied potentials are self-stabUized via tt-resonance. Therefore, a desirable characteristic of electrodes in dc amperometry is inert. The electrode serves as a sink to provide and remove electrons with no direct involvement in the reaction mechanism. Since TT-resonance does not exist in polar ahphatic compounds (e.g., carbohydrates), stabilization of reaction intermediates is actively achieved via adsorption at clean noble metal electrodes. Faradaic processes that benefit from electrode surface interactions are described as electrocatalytic. Unfortunately, an undesirable consequence of this apiproach is the accumulation of adsorbed carbonaceous materials, which eventually foul the electrode surface. 

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