I recently read about a pyrolysis process that separates methane into Carbon and Hydrogen. The basic technology/process is you have a vertical column of super-heated liquid metal and you inject methane into the bottom of the column. The methane breaks down into hydrogen and solid carbon. Are there any similar processes that separate CO2 into solid carbon and oxygen using liquid metals and/or salts etc.?

  • 2
    $\begingroup$ chemistry.stackexchange.com/questions/915/… $\endgroup$
    – Mithoron
    Jan 15 '21 at 2:05
  • $\begingroup$ Not energetically feasible at all. $\endgroup$
    – M. Farooq
    Jan 15 '21 at 5:06
  • $\begingroup$ The current modeling of future power system operations suggests there will be a large amount of excess generation which is "spilled" and/or dumped because there is no practical use for the electricity. This is called over-build and spill. It's a new/old strategy in power system operations. There will be many cases where we can dump this power into a thermal reservoir of some sort and then gradually draw off the reservoir to accomplish some task. It could be drying wood or making powdered milk or possibly pyrolysis. I'm asking if there's a known pyrolysis process that accomplishes this. $\endgroup$
    – Lee
    Jan 15 '21 at 7:35
  • $\begingroup$ By this I mean a pyrolysis process that breaks up CO2 into carbon and oxygen using a liquid metal bath and/or a salt and or some other thermal + catalyst driven process. $\endgroup$
    – Lee
    Jan 15 '21 at 7:36
  • $\begingroup$ The answer is No ! Unfortunately ! $\ce{CO2}$ cannot be transformed into $\ce{C + O2}$ using liquid metals or salts. If it would have been feasible, somebody would have already done it somewhere. $\endgroup$
    – Maurice
    Jan 15 '21 at 9:57

Standard enthalpy of formation of carbon dioxide is -393.5 kJ/mol. Standard enthalpy of formation of both oxygen and graphite is 0 J/mol. Standard entropy of carbon dioxide is 213.8 J/mol-K (NIST). Standard entropy of oxygen is 205 J/mol-K (Chem LibreTexts). Standard entropy of graphite is 5.69 J/mol-K (ChemLibreTexts).

Everything below is a lot of fairly inexact approximations/assumptions because I didn't want to use a better model or search literature for empirical adjustments and/or constants.

Assuming constant T,P (which I doubt would be true under reactor conditions) G = H - TS. Approximating $\Delta H, \Delta S$ as constant with T, $\Delta G = (0 - (-393500)) - (T \cdot (5.69 + 205 - 213.8)) = 393500 + T \cdot (3.11)$. Then $\Delta G < 0$ regardless of temperature. Catalysts also do not affect thermodynamics, so this process can never occur spontaneously. Increasing pressure of the reaction chamber would not help appreciably either, as the reaction has 1 mol gas on each side. While assuming $\Delta H, \Delta S$ are constant with T is quite incorrect as conditions become more extreme, the approximation should suffice to show that the reaction does not occur easily. Additionally, even if one were to account for the perturbations in $\Delta H, \Delta S$ with T and P, these conditions would be expensive to maintain in a reactor. (Note that this analysis does not even account for the activation energy/kinetics problem! I did not look up the activation energy of the problem, but it is entirely possible that the extreme conditions would still not allow the process to occur at a reasonable rate.)

As a result, this reaction is (probably) practically only driven forward by direct energy input (aka a coupled reaction/external energy). This often, if not always, involves some energy input, such as light or a coupled chemical reaction, that directly impacts the electron states of the participant compounds. These changes in electron states change the identity/properties and stability of compounds, and, when applied effectively, they can help drive the reaction forward. However, relevant changes to electron states using heat not only often involve very high temperatures, but the effect of heat on electron states is also a highly statistical process. Thus, it may be relatively inefficient. Thus, pyrolysis, while it may be theoretically possible, would be incredibly impractical at achieving separation of carbon dioxide. (Also, to compare to your methane example, methane combusts readily, carbon dioxide does not. Methane has a standard enthalpy of formation of -74.87 kJ/mol.)

Finally, if you still need more reasons to look at a different way to drive this reaction, I have not mentioned: energy/cost to continuously scrape/collect the produced carbon, purify/collect/pressurize an inlet carbon dioxide stream, (if you actually want to run the reaction, you'll probably want) catalytic surfaces to encourage carbon deposition, possible packed/fluidized bed catalysts, heat exchangers, and much, much more.

ELI5: Methane combusts/reacts readily and is an "unstable" compound. Carbon dioxide is very stable and does not react easily. Higher temperatures can make some reactions that do not happen at a lower temperatures occur, but this particular reaction receives no such benefit. Heat and catalysis can also drive forward the speed of a reaction, but the reaction has to be able to occur first. Pyrolysis, as you described it, involves catalysts and heat. As a result, pyrolysis cannot be used to separate carbon dioxide into carbon and oxygen.

tldr: Excepting conditions extreme enough to perturb $\Delta H_{rxn}, \Delta S_{rxn}$, conditions of constant T,P never favor breakdown of carbon dioxide into elemental carbon and diatomic oxygen. Pyrolysis cannot drive this reaction.

  • $\begingroup$ Thanks for all of that. How about this? What if you had two stages of reactions. Stage 1: Fe3O2 is heated to 1500C driving off oxygen. The resulting FeO is moved to CO2 chamber where it absorbs oxygen from the CO2. Result is CO and cooled Fe3O2. $\endgroup$
    – Lee
    Jan 17 '21 at 2:36
  • $\begingroup$ Stage 2: Pyrolyze the carbon monoxide. I realize this may sound like a silly waste of fuel but I'm only asking if it's technically possible? This may sound strange but there's a high probability we'll "spill" upwards of 25 to 40% of our total electricity production in the future. This won't really start to happen for 20 years but this is where we are headed. Dumping this production into a thermal reservoir and then pulling off the thermal energy could be a good way to utilize some of this spill. $\endgroup$
    – Lee
    Jan 17 '21 at 2:45
  • $\begingroup$ Aha.... Here is the sort of thing I was looking for. thechemicalengineer.com/news/turning-co2-into-carbon-black You make methane first by combining CO2 and hydrogen. Pyrolyze the methane into carbon and hydrogen. Repeat cycle. $\endgroup$
    – Lee
    Jan 17 '21 at 3:19

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