Reverse Water-Gas Shift Reaction

The Reverse Water-Gas Shift Reaction (RWGS reaction) was discovered in the 19th century as a method of producing water from carbon dioxide and hydrogen, with carbon monoxide as a side product. In the context of human missions to Mars, it has been proposed as a complement to the Sabatier/water electrolysis (SE) process to produce methane and oxygen from hydrogen and carbon dioxide on the surface. Alternatively, it can be used with water electrolysis to generate carbon monoxide and oxygen. The oxygen is used for breathing or as oxidizer, while the carbon monoxide can be used as a moderate specific-impulse fuel (with oxygen as the oxidizer) or as a feedstock to generate higher hydrocarbons (see Fischer-Tropsch reactions)

Process

In the presence of a suitable catalyst, the reaction takes place according to this equation:

CO₂ + H₂ → CO + H₂O (deltaH = +9 kcal/mole)

The reactor itself is very similar to a Sabatier unit; a simple steel pipe filled with catalyst. According to experiments done by Pioneer Astronautics in Lakewood, Colorado, the best catalyst for this reaction is an iron-chrome mixture.

Applications

Production of oxygen

The RWGS reaction’s chief attribute is that it, when used alongside water electrolysis, can generate any amount of oxygen from the equivalent amount of carbon dioxide with only a tiny amount of hydrogen. The hydrogen is recovered from the water via electrolysis and recycled back into the reactor’s feed end. When used with the Sabatier and water electrolysis reactions, the RWGS can provide an oxidizer/fuel (O/F) ratio of 3.5:1 (3.5 units of oxygen to 1 unit of methane) compared with 2:1 for the SE process alone. This is advantageous because a methane/oxygen engine reaches its highest specific impulse at this ratio.

However, the RWGS can be used in conjunction with water-electrolysis as an "infinite-leverage oxygen machine" to generate oxygen from carbon dioxide via a small amount of hydrogen.

Production of carbon monoxide

The side product carbon monoxide can be used to synthesize methane and methanol, usable to produce further synthetic materials‎ and for energy storage.

Advantages

For the purposes of CO₂ separation, the RWGS is far more efficient and requires a fraction of the power, compared to solid-oxide or molten carbonate electrolysis. It is also more rugged and reliable because it uses a simple steel pipe instead of multiple brittle tubes. For the same reason, a RWGS reactor can be scaled up (by adding more catalyst-filled pipes) to support a robotic sample return or human mission.

Disadvantages

This reaction has an equilibrium constant of 0.1 even at temperatures of 400 C or above, so it must be fed with either a hydrogen-rich or a carbon dioxide-rich mixture to ensure satisfactory results. Excess hydrogen (or excess carbon dioxide) is captured from the exhaust with a filtering membrane and fed back into the reactor.

References

R. Zubrin, The Case for Mars, pp. 153