[Xenon54]

Assuming you have a reliable energy source, how would you turn CO2 back into C and O2? Also, how much energy would it take?

[Arkcon]

Where have you found free elemental carbon?

[Xenon54]

Where have you found free elemental carbon?

I was planning on turning it into a non harmful compound or even just turn it directly into a fuel such as ethanol.

[Arkcon]

Hi there, Xenon54:, you seem to have missed the point of where I was going when I answered your question, with a question. 

OK.  Time for me to put on my Moderator: hat.  Some new people hate this, but I wouldn't be doin' ma jerb if I don't let people know ...

Greetings, Xenon54:, I'd like to welcome you to the Chemical Forums, I can see you're new here, but I want to ask you to review our Forum Rules{click}.  You already accepted the rules when you signed up for our forum, and you have to follow them, whether you agree with them or not, or even if you're unaware of them.

We want to see you do some work, on your own, and we'll help.  We don't dump complete answers, for anyone, not for a high school student, nor a hard-working professional, nor even the home chemist working on their newest invention.

*Ahem*

You want to turn carbon dioxide into free, elemental carbon.  Well, on this planet, where can we find free elemental carbon?  What do we call that stuff?  And where did it come from?

I was planning on turning it into a non harmful compound or even just turn it directly into a fuel such as ethanol.

This is also a good idea.  Where does ethanol usually come from?  And our feed-stock for ethanol, where does that come from?

[Corribus]

You are basically looking for a way to unburn coal. Burning coal is an oxidation process, so that should give you a start. As for energy requirements, that's easily calculated from standard thermodynamic benchmarks. Of course, the calculation would assume a 100% efficient process. Moreover, "how much energy" in the primary reaction is not always (or often) the primary concern in an industrial process. I'm sure you can think of some other hurdles (energetic and otherwise) that would have to be overcome.

[Enthalpy]

If willing (do you?) to do this at the scale of our atmosphere, the difficulty is huge. Many people thought at it, nobody has a solution that I knew of.
- CO₂ must be collected on the right scale. As opposed, all trials collect concentrated CO₂ where it is produced: at lime factories, power plants.
- Unburning CO₂ takes as much energy as was gained when burning the fossil fuels over decades. We don't have a clean energy source of proper size yet. As opposed, all trials leave the CO₂ as is or nearly.

These were investigated and prototyped:
- Liquefy CO₂ where it's produced, inject it in natural reservoirs in the ground, like depleted oil and gas reservoirs. Even building reservoirs would be too expensive, and human technology isn't durable enough.
- Find silicates in big amounts, convert them into silica and carbonates:
MSiO₃ + CO₂ SiO₂ + MCO₃ where M is Ca, Na₂ or anything available.

Looks dirt-cheap, especially as compared with other proposals? Even these were abandoned when the carbon emission bonds became too cheap during the last crisis. They attempted to reconcile capitalism and ecology by materializing CO₂ emissions as bonds traded at a stock exchange, but varying exchange rates don't encourage a sustained effort.