특별한 is adjective. You're right. And speaking of 특별, it is listed as a noun like 특별함 in dictionary. 특별함 is used itself as a noun meaning specialty or distinction. Meanwhile, I have never seen people using 특별 alone as a noun. It's rather used like adjective with noun and comes before noun. Examples 특별구역, 특별만남, 특별활동...etc