Impact of Boric Acid on the Stability of Hydrogen Peroxide in Aqueous Solution

The reactivity of H₂O₂ is of importance for its role as an oxidant in many processes. It is an important oxidant produced radiolytically in nuclear reactors, and knowing its stability is of key importance when predicting corrosion of fuel cladding and structural materials. Boric acid is commonly used as pH buffer in many systems, and in pressurized water reactors, large amounts are also added to the coolant to control the fission. In general, boric acid has long been assumed to have a minor impact on the radiation chemistry of aqueous systems. In this work, we have studied how boric acid impacts heterogeneous catalytic decomposition, thermal decomposition and radiation-induced decomposition of H₂O₂. ZrO₂ was used as a heterogeneous catalyst. The presence of boric acid slowed down the decomposition of H₂O₂ on ZrO₂ by competing for adsorption sites on the catalyst surface. For the two homogeneous cases, the effect was the opposite; boric acid increased the rate of H₂O₂ decomposition. This is attributed to the formation of peroxoborate in various forms from complexation of H₂O₂ and boric acid. The rationale for this is suggested to be enhanced reactivity of peroxoborate toward superoxide resulting in a more efficient chain-reaction driving the peroxide decomposition.