According to the wiki, MnO2 catalyzes the decomposition of hydrogen peroxide, but in your original post you said you want to purify the MnO2. Is this a single step in a multi-step process or do you just want to purify the MnO2?
MnO2 is insoluble in water, carbon & zinc more so, zinc is also a Lewis acid & could be expected to hydroxylate & thus solubize carbon, so I would think you could filter & dry aqueous solutions to wind up with successively purer concentrations, up to a point.
You can also buy relatively pure MnO2 at ceramic stores.