Adsorption from Solution on Platinum: An in Situ FTIR and Radiotracer Study

FTIR and radiotracer methods have been used to examine the adsorption of H₃PO₄and CF₃SO₃^-from aqueous solution onto Pt electrodes at various temperatures and concentrations. The FTIR measurements were calculated by assuming coincidence of the measurements with those of the radiotracer approach at the adsorption maximum on 0-potential plots. In the radiotracer method, three independent approaches were used to identify the potential region of negligible adsorption for CF₃SO₃^-. The time constant of the H₃PO₄adsorption is in minutes, that of CF₃SO₃^-in seconds. IR peaks for CF₃SO₃^-are assigned. The Bockris-Swinkels isotherm (modified for H₃PO₄) is found consistent with the data. ΔH_ads°(CF₃SO₃^-) is -44 kJ mol^-1; ΔH_ads° (H₃PO₄) is -97 kJ mol^-1. The AS_ads° values are respectively -97 and -220 J mol^-1 K^-1. Error analysis shows the coverage data to be better than ±17% (except for CF₃S0₃^- at extremes of the potential). Anomalous radiotracer results may arise as a result of decomposition reactions. The standard state in adsorption measurements from solution is discussed in extenso: a method for the comparison of ΔH° and ΔS° for different systems using different standard states is derived. The time dependence of H₃PO₄adsorption was found to be consistent with control of adsorption by diffusion in the solution. A detailed examination of two models of adsorption for H₃PO₄leads to the conclusion that the parabolic potential dependence is influenced by both oxide layer formation and H₂0 adsorption. Corresponding analysis of CF₃S0₃^- adsorption shows this to be consistent with a single imaging model up to θ = 0.25. An analysis of the entropy data indicates that CF₃S0₃^- adsorbs with two degrees of translational freedom, but for H₃PO₄calculations are consistent only with a model for zero translational degrees of freedom on adsorption. The orientation of adsorbed CF₃SO₃^-is with its CF₃^- group and that of H₃PO₄is with its -OH groups toward the metal.