Pampas Grass isn't Fountain Grass. They are two different critters entirely. Pampas Grass is Cortaderia selloana, and isn't hardy in your zone. Fountain Grass is Pennisetum alopecuroides, and some of them are hardy in your zone.

Either can be grown in containers, but Pampas grass will always do better in the ground, unless you have a huge container.

Opps. That was meant to say pampus grass. Yes the pot is 18 " across. Will that work for a year or so?

Pampas Grass won't live through a winter, outdoors, in a pot.