Mechanism of H2O2 Decomposition on Transition Metal Oxide Surfaces

We performed an experimental and density functional theory (DFT) investigation of the reactions of H2O2 with ZrO2, TiO2, and Y2O3. In the experimental study we determined the reaction rate constants, the Arrhenius activation energies, and the activation enthalpies for the processes of adsorption and decomposition of H2O2 on the surfaces of nano- and micrometer-sized particles of the oxides. The experimentally obtained enthalpies of activation for the decomposition of H2O2 catalyzed by these materials are 30 ± 1 kJ·mol–1 for ZrO2, 34 ± 1 kJ·mol–1 for TiO2, and 44 ± 5 kJ·mol–1 for Y2O3. In the DFT study, cluster models of the metal oxides were used to investigate the mechanisms involved in the surface process governing the decomposition of H2O2. We compared the performance of the B3LYP and M06 functionals for describing the adsorption energies of H2O2 and HO• onto the oxide surfaces as well as the energy barriers for the decomposition of H2O2. The DFT models implemented can describe the experimental reaction barriers with good accuracy, and we found that the decomposition of H2O2 follows a similar mechanism for all the materials studied. The average absolute deviation from the experimental barriers obtained with the B3LYP functional is 6 kJ·mol–1, while with the M06 functional it is 3 kJ·mol–1. The differences in the affinity of the different surfaces for the primary product of H2O2 decomposition, the HO radical, were also addressed both experimentally and with DFT. With the experiments we found a trend in the affinity of HO• toward the surfaces of the oxides, depending on the type of oxide. This trend is successfully reproduced with the DFT calculations. We found that the adsorption energy of HO• varies inversely with the ionization energy of the metal cation present in the oxide.