Sabatier reaction

Overview
The Sabatier reaction or Sabatier process involves the reaction of hydrogen with carbon dioxide at elevated temperatures and pressures in the presence of a nickel catalyst to produce methane and water. Optionally ruthenium on alumina makes a more efficient catalyst. It is described by the following reaction:


 * CO2 + 4H2 → CH4 + 2H2O

It was discovered by the French chemist Paul Sabatier.

Space Station Life Support
Currently, oxygen generators onboard the International Space Station produce oxygen from water using electrolysis and dump the hydrogen produced overboard. As astronauts consume oxygen, carbon dioxide is produced which must then be removed from the air and discarded as well. This approach requires copious amounts of water to be regularly transported to the space station for oxygen generation in addition to that used for human consumption, hygiene, and other uses—a luxury that will not be available to future long duration missions beyond low Earth orbit.

NASA is currently investigating the use of the Sabatier reaction to recover water from exhaled carbon dioxide, for use on the International Space Station and future missions. The other resulting chemical, methane, would most likely be dumped overboard. As half of the input hydrogen becomes wasted as methane, additional hydrogen would need to be supplied from Earth to make up the difference. However, this creates a nearly closed cycle between water, oxygen, and carbon dioxide which only requires relatively modest amount of imported hydrogen to maintain.

Ignoring other results of respiration, this cycle would look like:


 * 2H2O → O2 + 2H2 → (respiration) → CO2 + 2H2 + 2H2 (added) → 2H2O + CH4 (discarded)

The loop could be completely closed if the waste methane was simply pyrolyzed into its component parts:


 * CH4 + heat → C + 2H2

The released hydrogen would then be recycled back into the Sabatier reactor, leaving an easily removed deposit of pyrolytic graphite. The reactor would be little more than a steel pipe, and could be periodically serviced by an astronaut where the deposit is chiselled out.

The Bosch reaction is also being investigated for this purpose. Though the Bosch reaction would present a completely closed hydrogen and oxygen cycle which only produces atomic carbon as waste, difficulties maintaining its higher required temperature and properly handling carbon deposits mean significantly more research will be required before a Bosch reactor could become a reality. One problem is that the production of elemental carbon tends to foul the catalyst's surface, which is detrimental to the reaction's efficiency.

Manufacturing Propellant on Mars
The Sabatier reaction has been proposed as a key step in reducing the cost of manned exploration of Mars (Mars Direct) through In-Situ Resource Utilization. After producing methane and water by combining hydrogen transported from Earth and carbon dioxide taken from the atmosphere of Mars, oxygen would be extracted from the water by electrolysis and used as a rocket propellant along with the methane.

The stoichiometric propulsion fuel mix ratio is 3.5:1 oxygen:methane by weight, however one pass though the Sabatier reactor produces a ratio of only 2:1. More oxygen may be produced by running the water gas shift reaction in reverse. Another option is to take some of the methane produced by the Sabatier reaction and pyrolyze it into carbon and hydrogen (see above section) where the hydrogen is recycled back into the reactor to produce further methane and water. When the water is electrolyzed, the extra oxygen needed is obtained. In an automated system, the carbon deposit may be removed by blasting with hot Martian CO2, oxidizing the carbon into carbon monoxide.

A third, and perhaps more elegant solution to the stoichiometry problem, would be to simply combine the Sabatier reaction with the reverse water gas-shift reaction in a single reactor as follows:

3CO2 + 6H2 → CH4 + 2CO + 4H2O

This reaction is slightly exothermic, and when the water is electrolyzed, an oxygen to methane ratio of 4:1 is obtained, resulting in a large backup supply of oxygen. With only the light hydrogen transported from Earth, and the heavy oxygen and carbon extracted locally, a mass leveraging of 18:1 is afforded with this scheme. This in-situ resource utilization would result in massive weight and cost savings to any proposed manned Mars missions (or robotic sample return missions).