I dont know about iProcurement but in Purchasing.. the charge account comes from Organization Parameter form in inventory.
Shahzad M. Saleem
You may consider using Expense Charge Account Rules available in Purchasing.
The detail is available in User's Guide under "Expense Charge Account Rules", or
you can also reference note How to Set Up and Use Category-based Account Default in Purchasing (Doc ID 293046.1)
Hope this helps.