Studies on MnO2—III. The kinetics and the mechanism for the catalytic decomposition of H2O2 over different crystalline modifications of MnO2

The catalytic decomposition of aqueous H2O2 solution over MnO2 of various crystalline modifications has been studied in the temperature and the pH ranges of 20–35°C and 3–14 respectively. The active site in the pH range of 3–8 is considered to be Mn4+ and in the range 8–14 is Mn3+ ion. Catalytic activity attains maximum at pH of about 11.5–12.0 and then decreases with further increase in pH. The experimental rate data agrees satisfactorily with the proposed mechanism based on the above considerations. Catalytic activity has been correlated with some physico-chemical characteristics of the samples such as surface oxygen, surface OH groups, lattice parameters and in somewhat complex manner with electrochemical or discharge capacity of MnO2. Although the discharge capacity increases with increase in catalytic activity for some samples (mostly γ-MnO2) a number of samples belonging to α, β and δ-variety show sharp deviation from the above behaviour. This is explained from the state of occurrence of MnOOH in the host lattice.