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 reaction)
Whether one would use the RWGS reaction or the Bosch reaction depends largely on whether carbon monoxide or elemental carbon is the preferred by-product.

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 at low temperature for this reaction is silica consisting of 5% copper by weight and a smaller amount of nickel. This catalyst is exclusively selective to CO (i.e., it only produces carbon monoxide) with 60% conversion of CO₂ to CO at 350^o C, 20 kPa (150 torr)(0,2 Bar), and a CO₂/H₂ feed ratio of 1/4.

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, methanol, and higher hydrocarbons such as ethylene and propylene, which are usable to produce further synthetic materials‎ and for energy storage. The higher hydrocarbons are manufactured via the Fischer-Tropsch reactions, which use carbon monoxide and hydrogen as feedstocks.

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 low equilibrium constant even at temperatures of 400^o C , so it must be fed with either a hydrogen-rich or a carbon dioxide-rich mixture to ensure satisfactory results, or you have to increase the operating temperature (equilibrium constant is 0.5 only at 750°C). Excess hydrogen (or excess carbon dioxide) is captured from the exhaust with a filtering membrane and fed back into the reactor. The effective catalyst depends on the operating temperature : Corean used ZnO-Al catalysts for the Camere Process at 600°C but also ZnO-Cr can be used above 600°C. The activity of both catalysts is however too low below this temperature.

References

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