Dear Cristiane,
When you are assigning a item categatory 'N' material system will create purchase requisition. In IW31 in Component tab page if you come in extreme right you can see 'Res. /Purc. Req' and there are three options. 1 Never, 2 From release, 3 Immediately. This is the setting when the PR will be created automatically.
Regards,
Krishnendu.