What is the catalyst in the following mechanism?

Question

Step 1: Ce4+ + Mn2+ => Ce3+ + Mn3+

step 2: Ce4+ + Mn3+ => Ce3+ + Mn4+

step 3: Mn4+ + Tl+ => Tl3+ + Mn2+

Answer 1

The catalyst can be identified by the fact that it is not used up or consumed in the reaction. So, we want to look for something that is present as a reactant, and also as a product, i.e. is not consumed or changed.

In the present example, we have Mn²⁺ as a reactant in Step 1 but it appears as a product in Step 3 so it has not been consumed or changed, thus making it a catalyst.

As a side note, you can see than Mn³⁺ is a product is Step 1 but is used as a reactant in Step 2. This qualifies Mn³⁺ as an intermediate.