[arigatoni]

TI+1+2CE+4 = TI+3 +2CE+3
Which species acts as a catalyst? I am really confused.

[mjc123]

It is a reaction between two species. Both are converted to different species. Neither can be considered a catalyst. Look up the definition of catalyst.

[arigatoni]

Makes sense. I know what a catalyst is thats why the question wasn't making any sense to me. Turns out i misread what i was asked...

The question gives me 3 mechanism and asks me to find the reaction
These are the steps I am given
Ce+4 + Mn+2 = Ce+3 + Mn+3
Ce+4+Mn+3 = Ce+3 +Mn+4
TI+4+Mn+4 = TI+3 + Mn+3
I am asked which one acts as a catalyst. But I am still having trouble finding the answer. Is the correct answer Mn+4 ?

[mjc123]

Your third reaction is wrong because both ions are being reduced!

[arigatoni]

typo. Ok here is a picture. http://prntscr.com/vis65q (dont worry about the link its from an app called lightshot which is used for taking screenshots if you feel uncomfortable clicking the link tell me so i can upload a picture via the forum)

[mjc123]

Oh, it's thallium. I thought you meant titanium, but were spelling Ti with a capital I. (Especially as you also wrote CE for cerium at first.) Do make an effort to be accurate in typing.

The first two reactions overall give
2Ce⁴⁺ + Mn²⁺  2Ce³⁺ + Mn⁴⁺
(though writing it as two reactions implies that it occurs in two steps, not one.)
Combined with the third reaction, the overall process is
2Ce⁴⁺ + Tl⁺  2Ce³⁺ + Tl³⁺
Note that there are no Mn species in this equation because you start with Mn²⁺ and end with Mn²⁺ (though it gets oxidised to Mn⁴⁺ and reduced back to Mn²⁺).
What do you think is the catalyst in all this?