The diagram shows the change in activation energy brought about by a catalyst.

The reaction now occurs in stages on the catalyst, and the activation energy of any one stage will be lower than the activation energy of the uncatalysed reaction, as the next diagram shows.

General Catalytic Cycle

The diagram shows a general catalytic cycle. You can see from this cycle how the catalyst is not consumed in the process, but can be used again and again. In some reactions only one reagent will be held on the catalyst.