Since the only byproduct of the H-Cat operation is water, it is possible to have a completely closed-cycle with no exhaust.
From: Eric Walker I wrote: In light of these points about tetra-carbonyl nickel and dioxin, the hobbyist attempting to carry out an experiment involving a reaction between H and nickel adsorbed onto the surface of a catalytic converter should be careful about dealing with the offgas, especially if copper turns out to be a significant byproduct of the reaction. On second thought, these byproducts are probably not too much of a concern, as they probably require a hydrocarbon to form (e.g., gasoline). Eric
