mend vs patch up

mend is a noun but patch up is not a noun.

Word	Noun	Adjective	Verb	Adverb
mend	Yes	No	Yes	No
patch up	No	No	Yes	No

As verbs, patch up is a hyponym of mend; that is, patch up is a word with a more specific, narrower meaning than mend:

mend: restore by replacing a part or putting together what is torn or broken
patch up: mend by putting a patch on

mend (noun)	patch up (noun)
the act of putting something in working order again
sewing that repairs a worn or torn hole (especially in a garment)

mend (verb)	patch up (verb)
restore by replacing a part or putting together what is torn or broken	mend by putting a patch on
heal or recover	come to terms