Passive Voice with intransitive verbs?

With a sentence like, "He sat on the couch," is sat still being used as an intransitive verb since, in general, prepositional phrases cannot act as direct objects? To me, the prepositional phase "on the couch" seems to be behaving like a direct object. If it is not behaving like a direct object, then I should not be able to construct a passive voice version of the sentence (i.e., "The couch was sat on by him"). To be sure, no one would actually use this passive voice construction, but it was nonetheless possible to create.

What am I misunderstanding here?

Solution 1:

In your example, "on the couch" is actually functioning as an adverbial phrase modifying "sat". It answers the question "where?"

If it were functioning as a direct object, your passive construction would be something like, "On the couch was sat by me," which is nonsensical. As it is, in the passive voice, the word "on" is in proximity to the verb it modifies, "sat".