I have a tub deck that was covered with Formica. I removed the formica and want to resurface with ceramic tile 8x8. The adhesives I see all say not to install over particle board. Could I use a thin set? Using the existing deck would save me a lot of work and time. Could I seal the particle board or do I need to some sort of backer board?

I would suggest using Hardibacker or Wonderboard as a backer over the particle board, then sealing it with a product like Redgard. Then you should be able to tile over it, as long as the substructure is strong enough to support all this additional weight. Good luck.