Been a while since I studied chemistry, but you can create H2 from CO2 and CH4. However this isn't so simple. H2 as the H2 produced has a nasty habit of reacting with the CO2 at low energies. Good for power stations etc,not so good as a fuel source. At a push (although I've never seen it) I'm guessing you could use Sabatier reaction conditions. and end up with CH4 but use a load of CO2 in the process. Not very efficient though.