Adsorption in Presence of a Double Layer

We consider the adsorption of a single molecule, j, on a metal film M. The film is deposited on a solid electrolyte, e.g. YSZ, or is partly covered by a promoter, or simply has a significant coverage of adsorbed reactants and products on its surface, so that we may consider (Chapter 5) that an effective double layer is present at the the metal-gas interface (Fig. 6.15). 

The double layer is described by its effective thickness, d, and by its field strength E (Fig. 6.15). The adsorbed moleculeJias a dipole moment P. It is well documented100 that the local field strength E can affect strongly not only the chemisorptive bond strength but also the preferred orientation of the adsorbate (Fig. 6.16). 

In the case of electrochemically promoted (NEMCA) catalysts we concentrate on the adsorption on the gas-exposed electrode surface and not at the three-phase-boundaries (tpb). The surface area, Ntpb, of the three-phase-boundaries is usually at least a factor of 100 smaller than the gas-exposed catalyst-electrode surface area Nq. Adsorption at the tpb plays an important role in the electrocatalysis at the tpb, which can affect indirectly the NEMCA behaviour of the electrode. But it contributes little directly to the measured catalytic rate and thus can be neglected. Its effect is built in UWr and D, 

We first assume Langmuir-type adsorption (E=0 or P,=0) which implies negligible lateral interactions between adsorbed molecules and negligible inherent or induced heterogeneity of the catalyst-electrode surface  

Equation (6.23) is obtained assuming equilibrium between gaseous and adsorbed species Sp 

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