national capital vs Pnom Penh

national capital and Pnom Penh both are nouns.

Word	Noun	Adjective	Verb	Adverb
national capital	Yes	No	No	No
Pnom Penh	Yes	No	No	No

national capital (noun)	Pnom Penh (noun)
the capital city of a nation	the capital and largest city of Kampuchea