Rj. Candal et al., Effects of pH and applied potential on photocurrent and oxidation rate of saline solutions of formic acid in a photoelectrocatalytic reactor, ENV SCI TEC, 34(16), 2000, pp. 3443-3451
Citations number
34
Categorie Soggetti
Environment/Ecology,"Environmental Engineering & Energy
A photoelectrocatalytic reactor containing titania-coated titanium electrod
es 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 p
urely photocatalytic experiment by operating at applied potentials of at le
ast +1.0 V (versus saturated calomel electrode). The kinetics of photodegra
dation at +1.0 V was modeled effectively using a Langmuir-Hinshelwood-Houge
n-Watson expression. Unexpected results were obtained when only the backgro
und electrolyte was passed through the reactor. During initial recirculatio
n 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 hyd
roxyl ions from the catalyst surface. However, when UV illumination was ini
tiated 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, a
lthough there are several explanations. Addition of formic acid to this sys
tem 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 adjust
ed to higher initial pH values, the reaction rate was unaffected until the
pH increased above 5 at which point the rate decreased.