Could we seed Earth's atmosphere with hydrogen?

Question

Suppose the chemical reaction: $$3CO_2+2H_2\rightarrow 2H_2O+2O_2+3C$$ This reaction could be used to filter out $CO_2$ from the atmosphere, slowing down or reversing climate change. The solar system is rich in hydrogen.

There's a problem though, for this reaction to take place requires the catalyst of iron in the form of an aerosol.

Though this might work on a planet without life, it would most certainly kill off all life on earth.

What kind of future technology could make this a possible method taking $CO_2$ out of the atmosphere at a global scale with or without iron aerosol?

Or is there another realistic way of taking $CO_2$ out of the atmosphere with technology within the span of a few decades? Assuming carbon neutral energy generation of no more than an order of magnitude higher than earth currently has.

Answer 1

Hydrogen floats and escapes

Hydrogen cannot be contained in the atmosphere, because it will float to the top, and then escape the Earth.

http://www.livescience.com/28466-hydrogen.html

Already today we can break up carbon dioxide. There are several ways of doing it. For instance we can pewpew it with a laser to mention one of the more exotic examples.

But the major problem with breaking up carbon dioxide is that all of these processes require energy. And the excess $CO_2$ that we have a problem with now was created when we were generating energy. So in order to split $CO_2$, we need to generate at least as much energy as was gained when the $CO_2$ was formed in the first place, and this time without having any $CO_2$ as a waste product.

This of course means that we should just skip the $CO_2$-generating step all-together and just use the carbon dioxide free energy directly instead.

So to answer your question: what (future) technology can be used to break up carbon dioxide? Well you have a buffet to choose from already.

The break-through you are looking for is a method of generating carbon dioxide free energy in such abundance that we can cover all our energy consumption needs, and have enough left over to start breaking up carbon dioxide too.

EDIT: The one technology that would be really cool and futuristic in this context — that would break up carbondioxide using sunlight as its source of energy — is one that emulates photosynthesis, in other words: artificial photosynthesis. If humanity can get artificial photosynthesis to work, then we will have a very cool technology indeed, a cool technology that will open the door to many solutions to woes that we face today.

EDIT by OP: This technology could be used as an decentralized energy source. For it to work it requires further advances in "light harvesting, charge separation, catalysis, semiconductors, nanotechnology, modelling from synthetic biology and genetic engineering, photochemistry and photophysics, photoelectrochemistry, catalysis, reaction mechanisms and device engineering."

Answer 2

Your equation is not thermodynamically plausible. It will actually operate in the reverse direction.

One tool for determining if a reaction works is to use the Standard Gibbs Free Energy of Formation, which is a measure of how much energy it takes to create a particular molecule from its raw components in their lowest energy form. You can sum these to see if a reaction will create or consume free energy. Reactions that create free energy (by releasing it from the bonds of their molecules) will occur naturally in the forward direction. Reactions that consume it will not (mostly... there's a few details that I don't believe matter here). A table of values for your compounds would be:

• $CO_2$ : -394 kJ/mol
• $H_2$ : 0 kJ/mol
• $H_2O_{(g)}$ : -237 kJ/mol
• $O_2$ : 0 kJ/mol
• $C$ : 0 kJ/mol

For an example of how this works, consider a reaction we know occurs ferociously: $2H_2+O_2\rightarrow 2H_2O + \Delta E$, where E is energy released. If we plug in those Gibbs free energy numbers we get $2\cdot0 + 0 \rightarrow 2\cdot-237 + \Delta E$ which shows $\Delta E=474kJ$ per mole of oxygen burned.

In your equation, we can do the same: $$3CO_2+2H_2\rightarrow 2H_2O+2O_2+3C$$ $$3\cdot -394+2\cdot 0\rightarrow 2\cdot-237+2\cdot 0+3\cdot 0 + \Delta E$$ $$-1182\rightarrow -474 + \Delta E$$ $$\Delta E = -708$$

Your reaction is highly endothermic, and will not occur without a very large amount of energy from an outside source.

The issue, of course, is that $CO_2$ is a very stable compound, hence its very negative free energy of formation. Once you've got it, there's not many reasons it will let go. This is related to Oxygen's ability to... well.. oxidize. $C+O_2$ is quite energetic compared to $CO_2$.