Activation of hydrogen peroxide in copper(II)/amino acid/H2O2 systems: effects of pH and copper speciation

Activation of hydrogen peroxide by different Cu(II)-amino acid complexes is performed and compared, with quinaldine blue as an oxidation indicator. Parameters such as pH and concentrations of Cu(II), hydrogen peroxide, and amino acids (L = glycine, alanine, and lysine) are examined to understand the activation mechanism of hydrogen peroxide. The experimental rate law determined is first order in Cu(II)-amino acid complexes and variable order in hydrogen peroxide, by Michaelis-Menten kinetics. It indicates that the formation of ligand-Cu(II)-peroxide complex may be responsible for the activation of hydrogen peroxide. The oxidation rate is also substantially enhanced in Cu(II)/amino acid/H2O2 systems with increasing pH from 6 to 9. The trend is consistent with the formation of hydroxyl radical (
- OH), whose formation is favored in alkaline solutions. A mechanistic pathway that includes the formation of ligand-Cu(II)-peroxide complex and
- OH is proposed. For glycine, alanine, and lysine, the maximum activation efficiencies appear at a ligand/copper molar ratio of 1.5-2.0, regardless of the change in pH values or ligand concentrations. According to the stability constants for Cu(II)-amino acid complexes, it is predicted that CuL and not CuL2 is the dominant species forming the active copper complex catalyst.