ABSTRACT:The interfacial properties of Pt(111) single crystal electrodes have been investigated in the pH range 3

The laser-induced temperature jump method is used to determine the potential of maximum entropy (pme) of a Pt(111) single crystal electrode in contact with an aqueous solution, in a wide pH range and in the absence of specifically adsorbed anions. For this purpose, buffer solutions composed of a mixture of NaF and HClO4 are used. The results are compared with those from non-buffered perchlorate solutions. The use of the NaF/HF buffer allows extending the pH range from 3 to 6, approaching the situation of a neutral pH. Laser experiments show that the pme appears located at nearly the same potential position in the whole pH range between 3 and 6. This value is around 300 mV vs SHE, in agreement with previous works. Moreover, the potential response to the fast thermal perturbation at high potentials, above the pme, indicates the presence of a slower contribution, resulting in a bipolar, non-monotonous, profile. Such behaviour is strongly affected by the presence of fluoride anion and/or hydrofluoric acid, thus evidencing a strong structural effect on interfacial water from these species. These results bring some light to the understanding of the interfacial properties in acid-neutral conditions, and also show the influence of different non-specifically adsorbed anions on the interfacial properties that cannot be evidenced only by cyclic voltammetry.

Peroxodisulfate reduction has been investigated on Pt(111) on a broad range of pH values as a way to test interfacial charge. The reaction always proceeds in a potential window that is almost pH independent, regardless changes in the presence of adsorbed species. Comparison with other available information for these interphases allows to conclude that this reaction is very sensitive to the free charge on the metal, being inhibited when the latter is negative. This explains the complete current inhibition when the potential is lower than the potencial of zero free charge. Most remarkably, the inhibition at high potentials can be related with the existence of a second potential of zero charge, above which the surface becomes again negatively charged. This conclusion agrees with classical results for polycrystalline platinum (which has been reexamined), results from laser induced T-jump experiments and recent theoretical models of the interphase. In this way, this reaction is postulated as a new probe to locate the values of the potential of zero free charge.

Abstract:Carbonate and bicarbonate adsorption on Pt(111) electrodes from CO 2 saturated acidic solutions is investigated by cyclic voltammetry and Fourier Transform Infrared Reflection Absorption Spectroscopy (FT-IRRAS). Spectroscopic results show carbonate and bicarbonate adsorption even at pH=1, where bulk concentration of these anions is negligible. Moreover, analysis of the potential dependence of band intensities corresponding to adsorbed carbonate and bicarbonate reveals an effect of the electrode potential on the surface acid-base equilibrium. In this regards, increasing potentials favor bicarbonate deprotonation, leading to carbonate formation. A tentative thermodynamic analysis is given to rationalize these trends.

The concepts of total and free charge of platinum single crystal electrodes are revised in this paper, together with the associated concepts of potential of zero total and free charge. Total charges can be measured from CO displacement method. Results on solution of different pH are described. A novel buffer composition is used to attain pH values close to neutrality while avoiding interferences from anion adsorption processes. Stress is made on the fact that free charges are not accessible through electrochemical measurement for systems at equilibrium since adsorption processes (hydrogen and hydroxyl) interfere with free charge determination. Still, a model is described that allows, under some assumptions, extract free charge values and the corresponding potential of zero free charge for Pt (111) electrodes. On the other hand, fast measurement outside equilibrium can separate free charges from adsorption processes based on their different time constant. In this way, the laser induced temperature jump experiment allows determination of the potential of maximum entropy, a magnitude that is intimately related with the potential of zero free charge. Values of the potential of maximum entropy as a function of pH are given for the different basal planes of platinum.