The intercession will occur on Resurrection not in the Purgatory
The intercession (Shafa'at) refers to the act performed by prophets and Imams on Resurrection towards believers in order to release them from torment.
The following verses of the Koran confirm the intercession accruing on Resurrection:
Each soul is [held] as a pledge for anything he has earned, except for the companions on the Right who will be in gardens enquiring about the criminals: “whatever induced you to be scorched?” (74:38-42) No intercession by any mediators will benefit them. (74:48)
It is inferred from the last verse that the criminals will not be released from punishment by the intercession.
There are traditions (Hadiths) in which it is mentioned that act the time of death as well as the Prophet and Holy Imams will be present and help believers.
The following verses present the conversation between the companion on the Heights (A refers to the Holy Imams) and the inmates of the Fire:
Are you those who swore that God would not confer any mercy on them?
Enter the Garden, there is no [need] for you to fear nor should you feel saddened (7:49)
The following verses also indicate the intercession on Resurrection:
Some day we shall call forth all mankind along with whoever has led them. Those who are given their book in their right hand will read their book and not be harmed one bit (17:71)
“You all (Shiite) will enter the Paradise by the intercession on Resurrection but we (the Holy Imams) worry about the purgatory where there is no intercession.” Said Imam Sadeq (peace be upon him).
Sources :
1- "Recognition of Resurrection" (Allameh Tehrani)
2- "Acquaintance with the Koran" (Morteza Motahari)