Concentration dependent effect of CuCl2 on the photocatalytic degradation of phenol over anatase, rutile and brookite TiO2

Abstract

Effect of Cu²⁺ ions on the TiO₂-photocatalyzed degradation of organic pollutants in water has been reported in the literature, but the results are controversial. In this work, the effects of Cu²⁺ concentration, TiO₂ structures, and light intensity have been re-investigated, by using phenol degradation in an acidic aqueous suspension as a model reaction. As the concentration of CuCl₂ increased, the rates of phenol degradation over anatase, rutile, and brookite increased, and then decreased, but the reaction rate measured with P25 TiO₂ always decreased. Such unusual behavior of P25 was also observed when CuCl₂ was replaced with NaCl and MgCl₂, ascribed to the salt-induced particle aggregation. In all cases, Cu⁺ species were formed, followed by reoxidation to Cu²⁺ by O₂. However, at a high CuCl₂ loading, metallic Cu was also detected, due to the competitive disproportionation of Cu⁺ ions. An open circuit potential measurement with anatase and P25 film electrodes revealed that the interfacial electron transfer from the irradiated TiO₂ to Cu²⁺ was faster than that to O₂. Furthermore, the reaction rate was proportional to light intensity. The relevant quantum yield of phenol degradation over rutile upon the addition of CuCl₂ was increased by approximately 4 times.