Supporting electrolytes active metal ions

Electrochemical promotion (EP) denotes electrically controlled modification of heterogeneous catalytic activity and/or selectivity. This recently discovered phenomenon has made a strong impact on modem electrochemistry/ catalysis/ and surface science. Although it manifests itself also using aqueous electrolytes/ the phenomenon has mainly been investigated in gas-phase reactions over metal and metal oxide catalysts. In the latter case, the catalyst, which is an electron conductor, is deposited in the form of a porous thin film on a solid electrolyte support, which is an ion conductor at the temperature of the catalytic reaction. Application of an electric potential on the catalyst/support interface or, which is equivalent, passing an electric current between catalyst and support, causes a concomitant change also in the properties of the adjacent catalyst/gas interface, where the catalytic reaction takes place. This results in an alteration of the catalytic behaviour, controllable with the applied potential or current. 

In an electrolyte, the activity coefficient of the metal ions cannot be neglected and deviates considerably from one. The activity coefficient depends on the ionic strength /, defined in Eq. (1.12). Eor diluted solutions (c<10 mol kg i), it can be calculated by the Debye-Hiickel theory in the first approximation (Eq. (1.15)). In practice, if working with an excess of supporting electrolytes, the activity coefficient is approximately constant. The variation of the equilibrium potential with the concentration is determined by the Nemst factor 2.303 RTIF=59.2 mV at 25 °C for z+ = l (decadal logarithm). 

There is no doubt that specific adsorption of anions and organic species occurs in a wide potential range as the experiments were carried out in 0.1 mol dm HCIO4 supporting electrolyte at low concentrations (10 -10 mol dm ) of the species studied. It was found that the adsorption of anions exerts some influence on the anodic dissolution rate however, this effect is very far from that observed in the case of thiourea. In the latter case, the rate of anodic dissolution was significantly influenced by the presence of thiourea, even at its very low concentration. This behavior was ascribed to the strong interaction of thiourea with the metal ions and the formation of complexes both on the metal surface and in the solution phase. The phenomena presented were considered as a result of the adsorption competition of the species studied with C104 ions. Three fundamental mechanisms were formulated for the active dissolution ... 

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