Read the following paragraph and answer the question. Give ONLY the answer, no other words or explanation.

For example:

Answer: <most likely answer, as short as possible>.

Paragraph: 
Many transition metals and their compounds are used as catalysts. Given MnO 2, FeCl3, Pt, and Ni, which would you select for each purpose and why?.


Question: Many transition metals and their compounds are used as what?
catalysts
Catalysts.