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.