The Catalyst is a dirt pedal made by Fulltone, depending on the setting, it can be used as a Clean Boost, Overdrive, Distortion or even Fuzz. Billy is seen using this during his Premier Guitar "Rig Rundown" video from the Oceania tour. During that video Billy states "The Catalyst I use as a pre-amp for a few songs".
[Rig Rundown Video]