We all know how CO2, as a greenhouse gas affects the climate and causes global warming. I was thinking of a small device, which could be attached to the exhaust pipe of an automobile. It could convert the CO2 into something having negligible greenhouse effect, such as water. Is this feasible? Thanks!

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    $\begingroup$ Water doesn't contain carbon; where will the carbon go? More importantly, though, the question we face here is not from the realm of chemistry. The real question is "where is the money". $\endgroup$ Feb 4 at 14:32
  • $\begingroup$ I found out that CO2 reacts rapidly with monoisopropanolamine to give amine carbamate. Could this be used? $\endgroup$
    – Math_Whiz
    Feb 4 at 14:43
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    $\begingroup$ The reaction surely does happen, as do many, many others. But what will you do with all that amine carbamate? Where will you get all that monoisopropanolamine? See, I was not kidding. It is all about money. $\endgroup$ Feb 4 at 14:47
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    $\begingroup$ There are many other ways, and they have one thing in common: they all cost money. $\endgroup$ Feb 4 at 14:57
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    $\begingroup$ Many organisms use photochemistry to convert CO2 to useful molecules. They are called plants. But planting many trees isn't practical in a car. $\endgroup$
    – matt_black
    Feb 4 at 21:36

Unfortunately this is not feasible

Current catalytic converters are designed to remove noxious pollutants from exhausts. Things like carbon monoxide (poisonous) and nitrogen oxides (noxious and irritating).

The trouble with doing this for carbon dioxide is fundamental. Carbon dioxide and water are the ultimate products of combustion of hydrocarbons because they are the thermodynamically deepest products of combustion (ie forming them releases the most energy from combustion). We are not worried about water. But the trouble with being at the bottom of the thermodynamic well is that, to turn the compound into something else, you need to add energy.

There are known chemical reactions that convert carbon dioxide into other, more useful and less greenhouse-contributing products. We can go the whole hog back to methane or, more commonly, methanol. But these reactions are very energy intensive and require fairly special catalysts and conditions. To do them as a way of reducing carbon dioxide emissions would end up using most of the energy from hydrocarbon burning to run the reaction not to move the vehicle. And there are always energy losses in such reactions so we might end up greatly increasing fuel consumption overall to get a small reduction in emission. It is probably far better to make engines and vehicles more efficient.

On an industrial scale the preferred alternative to carbon dioxide conversion is carbon capture and storage (CCS). Separating carbon dioxide is much cheaper than converting it to something else. Even so, CCS plants are in their infancy and none are remotely economical even when subsidised (or at any feasible carbon price). and these reactions require large chemical plants and could never be viable in a car.

  • $\begingroup$ Downgrade (-1): Per my EDIT and cited source, my answer is NOT ONLY prescient, it is uncannily similar to actually recently developed technology. $\endgroup$
    – AJKOER
    Feb 5 at 3:58
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    $\begingroup$ @AJKOER Your answer would be a good answer if the question was "are there potential future technologies for turning CO2 into useful products". But that isn't the question. The question is "can a small device attached to exhaust pipe of a vehicle reduce the emitted CO2". And your answer is irrelevant to that for simple thermodynamic reasons. Feel free to post the other question and your answer should get a positive score. $\endgroup$
    – matt_black
    Feb 5 at 11:59
  • $\begingroup$ Actually, one should ask Math-Whiz of his conception of "small" and whether a battery cell (as alluded to in the breakthrough) would constitute an acceptable answer. Even if he concurs, why do I suspect that it will have no impact on grading here even though it would clearly refute your speculated affirmation of not possible now (ostensibly wrong) or sometime in the future (more likely wrong). The word of the day is integrity. $\endgroup$
    – AJKOER
    Feb 5 at 15:20
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    $\begingroup$ @AJKOER Get a grip,man. The question was about things that might work in a car. That implies the scale. If your battery tech worked on that scale and could be recharged, it would likely be at least as big as the battery in an electric car (on thermodynamic grounds alone) which completely defeats the point. Again: there is nothing wrong with the chemistry in your answer but not in this context where the solution has to fit in a car. Post it as an answer to a different question and harvest the upvotes. But not thisquestion. $\endgroup$
    – matt_black
    Feb 5 at 17:32
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    $\begingroup$ @AJKOER Typical catalytic converters for a car have a volume of a few litres. Do some calculations–if you want to prove me wrong–and tell me the electrical storage capacity of any possible battery that size and the potential volume of CO2 it could absorb. Then convert to the distance such a battery could drive a car and the amount of CO2 consumed and compare to the distance a car could drive on a single tank of gasoline. $\endgroup$
    – matt_black
    Feb 5 at 20:31

While not likely easily created, other products with air, $\ce{CO2}$ and water, in the presence of an electrical source, are conceivable in my opinion.

For example, there is some discussion around NAIs per this government source to quote:

One of the examples is the negative ion generator using an ultraviolet source to irradiate electrically conductive material, which was patented as early as 1964 (Patent No. US 3128378 A). In this patent, an ultraviolet lamp was used to irradiate metal materials, which photo-electrically eject electrons. The electrons then collide with air molecules and generate NAIs.

Assuming the chemistry involves superoxide radical anions created as follows:

$\ce{O2 + e- -> .O2-}$

And, in a medium of moist air, the superoxide is present as the hydroperoxide anion (see Wikipedia discussion here):

$\ce{H2O <=> H+ + OH-}$

$\ce{H+ + .O2- -> .HO2}$

In an albeit slow reaction, this is a precursor to hydrogen peroxide:

$\ce{.HO2 + .HO2 -> O2 + H2O2 }$

although this source notes an acceleration of the self-reaction in the presence of $\ce{CO2}$. It is also known that:

$\ce{H2O2 + CO2 -> H2CO4}$

as per a source: The Mechanism of Carbon Dioxide Catalysis in the Hydrogen Peroxide N-Oxidation of Amines, to quote:

"The reactivity of the peroxymonocarbonate ion, HCO4- (an active oxidant derived from the equilibrium reaction of hydrogen peroxide and bicarbonate), has been investigated in the oxidation of aliphatic amines"

Also, see Figure 2 at this source. Possibly, a more direct (and rapid) postulated radical reactions (which is in accord with the above):

$\ce{.HO2 + HCO3- -> .HCO4 + OH-}$

$\ce{.HCO4 + e- -> HCO4-}$

Here is an interesting supporting reference: Chemical effects of large impacts on the Earth's primitive atmosphere providing support for a prior significant presence of H2CO4 in the primitive earth's atmosphere, to quote:

Intense bombardment of the moon and terrestrial planets approximately 3.9-4.0 x 10(9) years ago could have caused the chemical reprocessing of the Earth's primitive atmosphere. In particular, the shock heating and rapid quenching caused by the impact of large bodies into the atmosphere could produce molecules such as HCN and H2CO4 which are important precursors for the abiotic synthesis of complex organic molecules.

Bottom line, some more research, and work may provide a path in the future.

[EDIT] The future is near. See The O2-assisted Al/CO2 electrochemical cell: A system for CO2 capture/conversion and electric power generation. To quote from the abstract:

Economical and efficient carbon capture, utilization, and sequestration technologies are a requirement for successful implementation of global action plans to reduce carbon emissions and to mitigate climate change. These technologies are also essential for longer-term use of fossil fuels while reducing the associated carbon footprint. We demonstrate an O2-assisted Al/CO2 electrochemical cell as a new approach to sequester CO2 emissions and, at the same time, to generate substantial amounts of electrical energy... On this basis, we demonstrate that an electrochemical cell that uses metallic aluminum as anode and a carbon dioxide/oxygen gas mixture as the active material in the cathode provides a path toward electrochemical generation of a valuable (C2) species and electrical energy. Specifically, we show that the cell first reduces O2 at the cathode to form superoxide intermediates. Chemical reaction of the superoxide with CO2 sequesters the CO2 in the form of aluminum oxalate, Al2(C2O4)3, as the dominant product. On the basis of an analysis of the overall CO2 footprint, which considers emissions associated with the production of the aluminum anode and the CO2 captured/abated by the Al/CO2-O2 electrochemical cell, we conclude that the proposed process offers an important strategy for net reduction of CO2 emissions.

As such, my opening sentence is completely accurate. Further, if one re-reads my proposed answer, there are some uncanny similarities.

What is quite amazing is that the cited reaction path also involves the presence of electrons, oxygen, the superoxide radical anion, and CO2 leading to a composite compound.

Assertions of a definitive negative answer here, is patently incorrect!

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    $\begingroup$ How can this be a solution to CO2 emission from engines? Will cars need to have huge electrical generators or UV lamps in future? $\endgroup$
    – matt_black
    Feb 4 at 21:34
  • $\begingroup$ Yes, UV emission processing perhaps leading to less likely than complete CO2 conversion. However, there is already an apparent battery cell CO2 capture system. See my EDIT. $\endgroup$
    – AJKOER
    Feb 5 at 4:12

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