Are prepositional verbs transitive verbs?
I ask that because some prepositional verbs can become passive verbs and the “object of preposition” can become the “subject” of passive prepositional verb.
A verb is transitive if it takes at least a direct object and possibly an indirect object.
A verb must have a direct object before it can have an indirect object.
Direct objects of verbs can appear right after the verb. There is no preposition you can put between the verb and the object.
I hit the ball
Indirect objects can be expressed with a prepositional phrase lead by 'to' or 'for, but don't have to.
I gave the ball to Mike.
I gave Mike the ball.
The prepositional phrase part of a prepositional verb is therefore not an object because there isn't a "second object".
I approved of the expenditure.
This would be passively expressed like this:
The expenditure was approved of by me.
Note that there is a difference between approved and approved of, so the of is needed in the passive voice construction.