Mechanism of hydrogen peroxide decomposition by manganese dioxide.

Application: Manganese can decompose hydrogen peroxide under alkaline conditions, but the right combination of chemicals can make the solution more stable.

Hydrogen peroxide is widely used in the pulp and paper industry, especially in the brightening of mechanical pulps. Catalytic processes that involve transition metal ions, particularly manganese, lead to the decomposition of peroxide under alkaline conditions. Because manganese dioxide is one of the more stable species among the different oxidation states of manganese, it is important to understand the chemistry involving Mn[O.sub.2] and hydrogen peroxide. The researchers studied the mechanism of Mn[O.sub.2]induced hydrogen peroxide decomposition by following the development of [Mn.sup.2+], [Mn.sup.3+], and [H.sub.2][O.sub.2] concentration during the course of reactions. They found that a small amount of Mn[O.sub.2] is converted to [Mn.sup2+] and [Mn.sup3+] as an initiation reaction. Afterwards, a redox cycle involving [Mn.sup.2+] and [Mn.sup.3+] is responsible for the decomposition of peroxide. This initiation reaction is limited to the surface area of Mn[O.sub.2] particles, and an increase in the particle surface area leads to an increase in peroxide decomposition.

At the same manganese concentration, [Mn.sup.2+] is more effective in catalyzing peroxide decomposition than is Mn[O.sub.2]. By converting [Mn.sup.2+] to Mn[O.sub.2], mills can reduce the Mn[O.sub.2]-induced decomposition of peroxide. Also effective in decreasing Mn[O.sub.2]-induced peroxide decomposition are additives such as silicates, EDTA, and DTPA, which suppress the concentrations of [Mn.sup.2+] and [Mn.sup.3+]. S!

Wekesa and Ni are with the Limerick Pulp and Paper Research and Education Centre, University of New Brunswick, Fredericton, NB, Canada, E3B 6C2. Email Ni at yonghao@unb.ca