Hmmm, chemical warfare get's tricky. Especially when you are dealing with nerve agents and what not. There is really no good reason anyone should ever deploy chemical weapons. The side affects of said weapons are usually more trouble than the offensive benefits. I can site examples for just about all of the common agents stockpiled today (VX Gas, Sarin, Soman, Mustard, etc). And who knows how zombies will be effected. They might even like it!
Bart