I have bathed Copper coins in Peroxide on quite a few occasions and I just never had very good results. I also give fair warning that if you leave them in Peroxide too long, it will strip any and all patina from the coin, turn them to the pink Copper look we cringe at and will actually pit the metal. Other contaminants left on the Copper coin when it goes into the Peroxide bath, likely contribute to the results, so if hell bent on using Peroxide, I recommend a quick bath in Acetone and rinse with water in hopes of arresting and/or removing these other contaminants.