national capital vs Baku

national capital and Baku both are nouns.

Word	Noun	Adjective	Verb	Adverb
national capital	Yes	No	No	No
Baku	Yes	No	No	No

national capital (noun)	Baku (noun)
the capital city of a nation	a port city on the Caspian Sea that is the capital of Azerbaijan and an important center for oil production