[_joel]

Sabtier doesn't make H2, it makes CH4. You'd need to convert it at about 300degC with a catalyst (so that takes lots of energy). Then the byproduct would be CO, whilst that isn't a bad direct greenhouse gas it interact with hydroxyl radicals in the atmosphere. Hydoxyls ineract with other greenhouse gases to reduce their power, so it would have no net effect, if anything it could be worse. Just combust the CH4 directly, it's got better energy storage capacity anyway.

[Tabular-Iceberg]

You would take the resulting CH₄ and split off the hydrogen using pyrolysis and put that in the car. Which is likely how you would get the H₂ to feed the Sabatier reaction in the first place, from CH₄ out of the ground. I don't know what is sillier, this or hydrolysis of water.

I suppose the benefit of removing and adding and removing carbon again would be that at each removal step you end up with a pile of solid carbon that you can put in a landfill. But I'm always wary of schemes to spend energy to sequester carbon, even if it is renewable energy. That's energy you probably could have used to greater effect elsewhere, like charging an electric car or supplanting a coal power plant.