Well, it's a little more complex than the way you put it.
Keep in mind that a catalyst doesn't execute a reaction itself, it simply acts to lower the transitional state energy "hump," thereby allowing reactions to require less energy, and therefore occur much faster than otherwise.
Also, because of the laws of thermodynamics, part of the heat energy generated by combusting the H2 will be lost as waste heat, and that's even if you could capture the rest of the heat and cycle it back into the system, which you probably can't since the system works on hydrolysis (requiring electrical energy, not just "any" old ebnergy).
It would probably be better to use the same catalyst and convert the Hydrogen and Oxygen back into electricity, and run your vehicle on that. You could all or most of the same components already in use for Hybrid and all electric vehicles. In fact it would be much like a hybrid, except the electricity would be generated by the fuel cell, rather than an internal combustion engine turning a generator. I suspect you'd still need a battery, because the peak output of the fuel cell wouldn't be high enough for acceleration or hill climbing at highway/freeway speeds. But I could be wrong about that, since I don't know all the details of fuel cell dynamics.