Electrochemical studies at low temperatures .9. Barrierless mechanism of hydrogen evolution on Ag and Cu electrodes.

The temperature dependence of hydrogen evolution between 263 and 228 K was studied on Ag and (Su electrodes in 4.2 m HClO4 solutions in the range of overpotentials eta: -0.2 divided by -0.4 V. We have found that in te case of both metals the eta - lgj curves obtained can be divided into two sections: in the eta: -0.2 divided by -0.3 V interval the slopes of the curves are smaller (0.06 V), while at eta < -0.3 V they are greater (0.10 V). In the range between -0.2 divided by -0.3 V the values of the charge transfer coefficient beta are approximate to 0.75 at both metals, suggesting that here the barrierless mechanism of the hydrogen evolution reaction prevails, In this interval of overpotentials above the freezing point of the solution, between 263 and 243 K the value of the activation energy for Ag electrode equal 58 kJ mol(-1), while for Cu electrodes it is 51 kJ mol(-1). According to these values the bonding enthalpy for Ag-H-a is 155 kJ mol(-1) and for Cu-H-a 163 kJ mol(-1). A comparison of these data shows that a decrease in the activation energy causes an increase in the M-H-a bond enthalpy. These latter experimental results suggest that the r.d.s. is the electron transfer. Below the freezing point of the solution (between 238 - 228 K) at low values of eta(-0.2 divided by -0.3 V) the eta - lgj curves with beta approximate to 0.75, show that the barrierless process of hydrogen evolution is not affected by the aggregate state of the electrolyte.