There is no difference, however "certain of" is used most commonly, so if you're ever unsure, use that.

Keep in mind that prepositions create a relationship between their object and another word in the sentence, usually the main verb or the noun before the preposition. About and of are not always interchangeable, it depends on the relationship they are creating. I'm concerned about Max. *I'm concerned of Max. (*incorrect) I'm certain of Max. (A little unclear without more context but acceptable grammar and meaning) I'm certain about Max (A little unclear without more context but acceptable grammar and meaning) So sometimes different prepositions can create the same relationship (meaning) but not always.