national capital vs Phnom Penh

national capital and Phnom Penh both are nouns.

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

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