Some work has been done lately in nanotechnology to develop substances that act essentially as extremely efficient plants, harvesting energy from the sun at a rate much greater than photovoltaic solar panels. While I'm sure that the process that ensues is not the same as photosynthesis, I wonder if that would not also be possible.
Imagine this: On the way out of the exhaust of your car, your toxic or "evil" carbon dioxide is intercepted by a bunch of "nanoplants", and converted into oxygen and glucose. Of course, there would have to be an energy source (to replace what is normally sunlight) and a supply of water, as the chemical equation looks like this:
energy + 6CO2 + 6H2O = 6O2 + C6H12O6
Assuming that there would be plenty of waste energy from the engine that could somehow be captured and utilized, the only remaining difficulty is to procure a molecule of water for every one of carbon dioxide trying to pass through. This could be done via a storage tank for the water, or ideally using some system that would remove it from the air for use. If either of those could be efficiently done, the car would produce oxygen and sugar instead of carbon dioxide.
This is likely impossible for some reason, because at first glance it almost seems to defy entropy. On the other hand, is that not what biology often appears to do? The mere fact that respiration and photosynthesis both occur in nature is a bit inspiring, as it seems impossible for two opposite reactions to both have a productive outcome. I simply think that it is neat to imagine cars producing the stuff we breathe and the stuff we eat, instead of a greenhouse gas that heats our planet up. Also, this way the carbon doesn't disappear, because it can be consumed again.
I wonder if theres a way that we can do the same sort of thing to human waste.....