If the oxides of alkali or alkaline earth metals are reacted with oxygen, the corresponding peroxides are obtained. The barium peroxide is particularly important.
2 BaO + O2 -> 2 BaO2
Formally, we have the barium salt of hydrogen peroxide here. If we add a stronger acid to this salt, the weak acid H2O2 is expelled. Water is also an acid, just an even weaker one. When water is added, a hydrolysis equilibrium is established.
BaO2 + 2 H2O -> Ba(OH)2 + H2O2
With strong acids, the equilibrium is far on the right. You can even use dilute sulfuric acid or phosphoric acid. This will form the poorly soluble barium sulfate/phosphate, which can easily be filtered off. This way, the equilibrium shown above is shifted completely to the right, and more or less pure hydrogen peroxide is obtained.
In addition to the electrolysis of sulfuric acid solutions, which produces hydrogen peroxide anodically, the anthraquinone process is used almost exclusively today.
The anthraquinone is catalytically converted into anthrahydroquinone. Anthrahydroquinone is then oxidized with oxygen under pressure. In this process, the molecular oxygen is being hydrogenated to hydrogen peroxide.
Anthraquinone process for the production of H2O2 (R = ethyl)
The anthraquinone molecule is reformed after each cycle, and plays therefore the role of a catalyst in this process.