When you have a chemical reaction, you start out in one energy state and end up in a lower energy state. However, there is an "energy barrier" between the states. This means that you can mix, for example, hydrogen and oxygen gasses without them reacting, until something adds enough energy to go over the barrier. You get that energy back as part of the reaction; it just keeps the reaction from starting. Catalysts work by lowering the energy barrier, usually by providing a bunch of intermediate states that have lower energy barriers than the original reaction. In quantum mechanics, there are two ways of getting past a barrier. You can go over, which requires adding enough energy to climb over the barrier. However, you can also "tunnel" through the barrier *without* adding energy. The probability of tunneling depends on the area (integral) of the barrier, and not on its height. So theoretically, a catalyst could work by reducing the *width* of the barrier instead of its height.