I would pronounce it "KAH-tyuh, " just like
Katya /
Katia. I don't like many K names for a girl, but
Katya /
Katia is one of them.
Unless you can tell me where
Catia is from, I can't really get into it as a name, sorry. Obviously it's a variant. But is it made up, or from a background that I'm not familiar with?