Effects of pH and Applied Potential on Photocurrent and Oxidation Rate of Saline Solutions of Formic Acid in a Photoelectrocatalytic Reactor

A photoelectrocatalytic reactor containing titania-coated titanium electrodes was employed to degrade solutions of formic acid (2 mmol dm^-3) in 0.01 mol dm^-3 NaCl. Reaction rates were increased above that observed for a purely photocatalytic experiment by operating at applied potentials of at least +1.0 V (versus saturated calomel electrode). The kinetics of photodegradation at +1.0 V was modeled effectively using a Langmuir-Hinshelwood-Hougen-Watson expression. Unexpected results were obtained when only the background electrolyte was passed through the reactor. During initial recirculation of this solution with no UV illumination and no applied potential, the pH increased from 6.5 to 9, suggesting ion exchange of chloride ions with hydroxyl ions from the catalyst surface. However, when UV illumination was initiated with an applied potential, the pH decreased to 3.5-4.2, depending on the magnitude of the potential. The cause of this behavior is not known, although there are several explanations. Addition of formic acid to this system buffered the pH near 3, producing the highest rate of degradation at an applied potential of +1.0 V. When the formic acid test solution was adjusted to higher initial pH values, the reaction rate was unaffected until the pH increased above 5 at which point the rate decreased.