The outer skin of a fluid region must be bounded, so inlet, outlet, wall etc. Assuming there's a wall the default material is aluminium. Which you've got the collision pair for: hence my question.
The material choice only really matters if you have heat transfer, or DEM collisions.
Depending on what you're trying to do, you may want to look at Ansys Rocky. Fluent's DEM code is based on the DPM model, whereas Rocky is a full DEM solver which connects into Fluent for the flow solution. I'm not sure what the status is on Rocky in Student or the other academic licences.