What kind of freon is used in a refrigerator?

It's only Freon if it was manufactured by DuPont and sold under the name Freon - it's a trademark of theirs.

Pure ammonia is still used in some industrial refrigerators. From the mid-1930s to the mid-1990s, R12 (CFC12) was used. Afterwards, R134a (HFC134a) was used in home refrigerators. Some newer units now use R600.