Enthalpic and Entropic Effects on Hydrogen and OH Adsorption on Pt(111), Pt(100), and Pt(110) Electrodes As Evaluated by Gibbs Thermodynamics

Gibbs thermodynamics have been used to characterize hydrogen and OH adsorption on Pt(111), Pt(100), and Pt(110) in 0.1 M HClO4. In this way, the Gibbs energy, enthalpy, and entropy of formation of the interface have been evaluated. The results demonstrate that hydrogen adsorption on the three platinum basal planes is mainly driven by entropic effects, while enthalpic effects are the driving force for OH adsorption. Remarkably, this thermodynamic analysis overcomes the limitations inherent to the more common treatment of data, employing a generalized isotherm. Nevertheless, the present work demonstrates that both approaches are adequate for the systems under study, and they provide complementary information. The relationships between the thermodynamic properties, obtained from both approaches, have been derived. Finally, the results are compared with experimental and theoretical data for hydrogen and OH adsorption on Pt(111), Pt(100), and Pt(110) under UHV conditions. It is found that the Pt-H bond energy is similar under electrochemical and UHV conditions, for the three basal planes, although the H-H lateral interactions seem to be more dependent on the environmental conditions. In the case of OH adsorption, it is concluded that coadsorbed water molecules play an important role on both the interaction of OH species with the platinum surface, and the OH-OH lateral interactions. This is related to the formation of hydrogen-bonds between neighboring water and OH species, and these bonds seem to be stronger for Pt(111) than for Pt(100).