Manganese dioxide (MnO2) is added to the decomposition of hydrogen peroxide (H2O2) reaction to act as a catalyst. A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the reaction.

In the decomposition of H2O2, MnO2 provides a surface for the H2O2 molecules to adsorb onto. This adsorption allows the H2O2 molecules to be in close proximity to each other, which facilitates the reaction between them. The MnO2 also helps to break down the H2O2 molecules into water (H2O) and oxygen (O2).

The overall reaction for the decomposition of H2O2 with MnO2 as a catalyst is:

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2 H2O2 (l) -> 2 H2O (l) + O2 (g)

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MnO2 is a commonly used catalyst for the decomposition of H2O2 because it is inexpensive, readily available, and highly efficient. It is also relatively safe to handle and does not pose a significant environmental or health risk.