Think About This
Transition metals can be thought of as a sort of bank that holds electrons instead of money. Just like a normal bank, it doesn't matter if you deposit money or withdraw money, the bank will be there next time you need it. The restriction in this process is that for money to always be available money going into the bank has to equal money going out of the bank. Also, just as a lot of different people can use the same bank, a lot of different molecules can use the same catalyst. The bank is not used up by your money needs, and the catalyst is not used up by the molecules in the reaction!