national capital vs Porto-Novo

national capital and Porto-Novo both are nouns.

Word	Noun	Adjective	Verb	Adverb
national capital	Yes	No	No	No
Porto-Novo	Yes	No	No	No

national capital (noun)	Porto-Novo (noun)
the capital city of a nation	the capital of Benin in southwestern part of country on a coastal lagoon