Electrochemical oxide growth on polycrystalline platinum
Date
2000
Authors
Huang, Yin
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Abstract
Sweep-hold voltammetry and solution exchange experiments have been used to study the electrochemical oxide growth on platinum. The effect of some anions on Pt electrochemistry was also investigated.
Sweep-hold methods were employed to study the kinetics of the oxide growth for different sweep rates and different hold potentials. Nonlinear regression results of the sweep-hold data indicated agreement with the direct logarithmic rate law. The relationship of the parameters used in fitting with the oxide charge density and electrode potential were explored in new empirical forms although the theoretical justification for them is unclear. The results imply that at least two different processes take place for platinum oxidation: one for the earlier stage and another for later oxide growth. Conway's model was compared to the data. The model could correctly predict the constancy of the OH adsorption charge density, but at higher potentials the estimated value of the charge was unreasonable, indicating a problem with this model.
Hydrogen was found to be a very good indicator for the oxide growth study. The number of bare platinum sites inferred from the hydrogen oxidation current was compared with the charge of oxide growth, and it was concluded that the oxide formed in the earlier stage must be a Pt(I) compound. Combined with the previous EQCM experimental results, the oxide was suggested to be PtzO. A corresponding mechanism for the oxide growth with this intermediate involved was then proposed.
The effect of some anions and small organic molecules was also studied by solution exchange experiments. The adsorption of bisulfate was found to involve electron transfer. Chloride was found to dissolve the oxide already formed on the surface. er, SCN- and thiol were found to adsorb oxidatively on platinum.