grasp vs catch on

grasp is a noun but catch on is not a noun.

Word	Noun	Adjective	Verb	Adverb
grasp	Yes	No	Yes	No
catch on	No	No	Yes	No

As verbs, catch on is a hyponym of grasp; that is, catch on is a word with a more specific, narrower meaning than grasp:

Other hyponyms of grasp include figure, cotton on, get it, get onto, get wise, latch on, tumble, twig, intuit, digest.

grasp (noun)	catch on (noun)
the act of grasping
an intellectual hold or understanding
understanding of the nature or meaning or quality or magnitude of something
the limit of capability

grasp (verb)	catch on (verb)
get the meaning of something	become popular
hold firmly	understand, usually after some initial difficulty