I tried to research the difference beween particle and preposition in phrasal verb, but the information on this website is not very clear. According to the website, in "She is making up excuses" up is called particle, but in the sentence "stop picking on your brother " on is preposition because the information comes immediatey after the phrasal verb completes a prepositional phrase. Without these prepositional phrase, the sentence would be incomplete. Can't the same be said about "she is making up excuses"? Without the preposition, the sentence would also be incomplete. I would appreciate greatly a more detailed explanation.
One test is to use a pronoun for the object. If the order verb object particle is acceptable, then the particle is an adverb, not a preposition. If the order verb particle object is acceptable, then the particle is a preposition.
1a. Stop picking on him.
1b. *Stop picking him on.
2a. *Stop leading on him.
2b. Stop leading him on.
So here, in "pick on", "on" is a preposition, but in "lead on", "on" is an adverb.
(All this assumes that it's meaningful at all to assign a part of speech to a phrasal verb's particle.)