Can you react to the trigger of Treasure Trove?

Yes, you can react to the trigger of Treasure Trove.

According to RiftJudge FAQ #2743, both the activated ability ("Kill this") and the "When this leaves the board" triggered ability create a chain, allowing players to respond with reactions to either.

How it works:

1. The Trigger: When Treasure Trove leaves the board, its triggered ability ("When this leaves the board...") is placed on the chain as a pending item (Rule 383.3).
2. Reaction Window: Because this triggered ability uses the chain, it creates a reaction window where players may add reactions before it resolves.
3. Resolution: Once the reaction window closes and no further reactions are added, the chain resolves in LIFO (Last-In, First-Out) order.

This is consistent with the general rule that triggered abilities use the chain and can be reacted to (Rule 376.3 and FAQ #8942).