Mutual influence of cupric cations and several anions in anatase and rutile TiO2 photocatalysis

Copper ions in aqueous solution are known to promote organic oxidation in semiconductor photocatalysis, but the counter anions seem to be important as well. In this work, the performance of Cu(ClO₄)₂ in presence of several anions in sodium forms (F^-, Cl^-, ClO₄^-, NO₃^-, and SO₄^2-) has been examined. Phenol oxidation in aqueous solution (pH 4) under UV light was used as model reaction and TiO₂ in the forms of anatase (AT) and rutile (RT) as photocatalysts. On the addition of 0.1-5 mM Cu²⁺, the reactions on AT and RT all increased. On the addition of 1 mM anions, reactions on AT increased by F^-and SO₄^2-, but reactions on RT all decreased. In presence of 3 mM Cu²⁺, however, reactions on AT and RT all decreased by 1 mM anions except NO₃^-. Such anion effects were also observed for H₂ production on AT and RT in presence of Cu²⁺ and 10% methanol. A possible mechanism for the positive and negative anion effects is discussed. This work indicates that the formation of a Cu(II)/Cu(I) complex with anions weakens the positive effect of copper ions on organic oxidation in TiO₂ photocatalysis.