One way in which catalysts increase the rate of reaction is by forming a complex with one reactant. This complex reacts by a lower activation energy pathway than the reactant itself.
Thus if a reaction is viewed as a stepwise process, a catalyst is a reactant in the first step and a product in the second step as shown below.
Step 1: CH3CH2OH + H+ 
CH3CH2OH2+
Step 2: CH3CH2OH2+ CH2=CH2 + H2O + H+
Overall: CH3CH2OH CH2=CH2 + H2O
Neither H+ not CH3CH2OH2+ appear in the overall equation resulting from addition Step 1 and Step 2.
Think about: Why is CH3CH2OH2+ NOT a catalyst?
CH3CH2OH2+ Step 2: CH3CH2OH2+
CH2=CH2 + H2O + H+ Overall: CH3CH2OH
CH2=CH2 + H2O Neither H+ not CH3CH2OH2+ appear in the overall equation resulting from addition Step 1 and Step 2.
Think about: Why is CH3CH2OH2+ NOT a catalyst?