No, you are mistaken. Luffy found out about Ace's execution during the Marineford arc, not the Thriller Bark arc. The Thriller Bark arc is where the Straw Hat Pirates encounter the ghost ship, and their adventures with the charismatic and eccentric musician, Brook. It takes place before the events of the Marineford arc.
Bro what😂 why would he be at marineford in the first place if he found out at marineford! Your wrong!
I apologize if my previous message was confusing. You are correct, Luffy does find out about Ace's impending execution during the Thriller Bark arc. It is revealed to him by Bartholomew Kuma, one of the Seven Warlords of the Sea, after the events on Thriller Bark. Luffy then sets out to save Ace, which leads to the Marineford arc. Thank you for pointing out my mistake.
