national capital vs Ndjamena

national capital and Ndjamena both are nouns.

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

national capital (noun)	Ndjamena (noun)
the capital city of a nation	the capital and largest city of Chad; located in the southwestern on the Shari river