I don't think the introduction of a catalyst to a reaction would affect its enthalpy. The reason why I think this is that enthalpy is a state function meaning it does not matter what pathway is taken, the end result will always be the same. A catalyst only changes the reaction mechanism and the starting point and ending point should not change. If you look at a energy diagram of a reaction with and without a catalyst, you should see that the only thing that changed was the activation energy required to make the reaction happen. Not the enthalpy.