The Kinetics of Oxygen Reduction in Molten Phosphoric Acid at High Temperatures

Abstract

The solubility and diffusivity of oxygen in concentrated (98%) phosphoric acid, and the kinetics of its reduction on platinum within the 25°–150°C temperature interval have been investigated utilizing glass insulated Pt microelectrodes with radii of 12.7 and 250 μm. The enthalpy of solution of oxygen in phosphoric acid was found to be , and the entropy of solution is . Fundamental entropy calculations indicate that the large negative entropy change upon solution is due to the loss of three degrees of freedom in translation (free translation becoming hindered) and two degrees of freedom in rotation (only libration occurs in solution). A hydrogen bonded model for the state of oxygen in phosphoric acid is found to be consistent with the experimental values obtained for enthalpy of solution, entropy of solution, and heat of activation for diffusion, . The electrochemical transfer coefficient for the oxygen reduction reaction was found to remain constant with temperature, at . The exchange current density at 25°C in 98% is , increasing with temperature with an apparent heat of activation of 17.3 kcal mol−1.