Coulostatic potential transients induced by laser heating of a Pt(111) single-crystal electrode in aqueous acid solutions. Rate of hydrogen adsorption and potential of maximum entropy

Short pulses (similar to10 ns) of high-power laser light produce a sudden increase in the surface temperature of a Pt(111) single-crystal electrode in acidified potassium perchlorate or sulfate solutions. The change of the electrode potential at open circuit was monitored during the relaxation of the temperature. At low pH, the potential transients in the hydrogen adsorption region exhibit a bipolar shape, which can be explained considering that the relaxation is influenced by the rate of hydrogen adsorption. With this assumption, we have estimated a value of the rate constant for hydrogen adsorption around 8 x 10(6) M-1 s(-1). By increasing the pH, the rate of hydrogen adsorption is reduced, making it possible to decouple the double-layer response from the hydrogen adsorption process. In this case, the potential where the double-layer response to the temperature increase is zero can be identified with the potential of maximum entropy of formation of the double layer. This potential is located in the double-layer region (ca. 0.43 V vs Pd/H-2) for a solution of pH 3. The relevance of this measurement in terms of the location of the potential of zero charge is discussed.