Sabatier reaction

The Sabatier reaction or Sabatier process was discovered by the French chemist Paul Sabatier in the 1910s. It involves the reaction of hydrogen with carbon dioxide at elevated temperatures (optimally 300–400 °C) and pressures in the presence of a nickel catalyst to produce methane and water. Optionally, ruthenium on alumina (aluminium oxide) makes a more efficient catalyst. It is described by the following exothermic reaction:

CO₂ + 4 H₂ → CH₄ + 2 H₂O + energy

∆H = −165.0 kJ/mol

(some initial energy/heat is required to start the reaction)

Energy storage

Main article: Carbon-neutral fuel

It has been proposed in a renewable-energy-dominated energy system to use the excess electricity generated by wind, solar photovoltaic, hydro, marine current, etc. to make methane (natural gas) via water electrolysis and the subsequent application of the Sabatier reaction. In contrast to a direct usage of hydrogen for transport or energy storage applications, the methane can be injected into the existing gas network, which in many countries has one or two years of gas storage capacity. The methane can then be used on demand to generate electricity (and heat—combined heat and power) overcoming low points of renewable energy production. The process is electrolysis of water by electricity to create hydrogen (which can partly be used directly in fuel cells) and the addition of carbon dioxide CO₂ (Sabatier process) to create methane. The CO₂ can be extracted from the air or fossil fuel waste gases by the amine process, amongst many others. It is a low-CO₂ system, and has similar efficiencies of today's energy system. A 6 MW power-to-gas plant went into production in Germany in 2013, and powered a fleet of 1500 Audi A3s.

International Space Station life support

Oxygen generators on board the International Space Station produce oxygen from water using electrolysis; the hydrogen produced was previously discarded into space. As astronauts consume oxygen, carbon dioxide is produced, which must then be removed from the air and discarded as well. This approach required 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 using the Sabatier reaction to recover water from exhaled carbon dioxide and the hydrogen previously discarded from electrolysis on the International Space Station and possibly for future missions. The other resulting chemical, methane, is released into space. As half of the input hydrogen becomes wasted as methane, additional hydrogen is supplied from Earth to make up the difference. However, this creates a nearly-closed cycle between water, oxygen, and carbon dioxide which only requires a relatively modest amount of imported hydrogen to maintain.

Ignoring other results of respiration, this cycle looks like:

${\ce {2H2O->{O2}+2H2->[{\ce {respiration}}]{CO2}+{2H2}+\overbrace {2H2} ^{added}->{2H2O}+\overbrace {CH4} ^{discarded}}}$

The loop could be further closed if the waste methane was separated into its component parts by pyrolysis:

${\ce {CH4 ->[{\ce {heat}}] {C}+ 2H2}}$

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.

Alternatively, the loop could be partially closed (75% of H₂ from CH₄ recovered) by incomplete pyrolysis of the waste methane while keeping the carbon locked up in gaseous form:

${\ce {CH4 ->[{\ce {heat}}]}}\ 0.5{\ce {C2H2}}+1.5{\ce {H2}}$

The Bosch reaction is also being investigated by NASA for this purpose and is:

CO₂ + 2H₂ → C + 2H₂O

The Bosch reaction would present a completely closed hydrogen and oxygen cycle which only produces atomic carbon as waste. However, difficulties maintaining its temperature of up to 600°C 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 (coking), 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, Interplanetary Transport System) through In-Situ Resource Utilization. Hydrogen is combined with CO₂ from the atmosphere, with methane then stored as fuel and the water side product electrolyzed yielding oxygen to be liquefied and stored as oxidizer and hydrogen to be recycled back into the reactor. The original hydrogen could be transported from Earth or separated from martian sources of water.

A variation of the basic Sabatier methanation reaction may be used via a mixed catalyst bed and a reverse water gas shift in a single reactor to produce methane from the raw materials available on Mars, utilizing water from the Martian subsoil and carbon dioxide in the Martian atmosphere. A 2011 prototype test operation that harvested CO₂ from a simulated Martian atmosphere and reacted it with H₂, produced methane rocket propellant at a rate of 1 kg/day, operating autonomously for 5 consecutive days, maintaining a nearly 100% conversion rate. An optimized system of this design massing 50 kg "is projected to produce 1 kg/day of O₂:CH₄ propellant ... with a methane purity of 98+% while consuming 700 Watts of electrical power." Overall unit conversion rate expected from the optimized system is one tonne of propellant per 17 MWh energy input.

Detailed chemical reactions

The stoichiometric ratio of oxidizer and fuel is 2:1, for an oxygen:methane engine.

CH₄ + 2 O₂ → CO₂ + 2 H₂O

However, one pass through the Sabatier reactor produces a ratio of only 1:1. More oxygen may be produced by running the water gas shift reaction in reverse, effectively extracting oxygen from the atmosphere by reducing carbon dioxide to carbon monoxide.

Another option is to make more methane than needed and pyrolyze the excess of it into carbon and hydrogen (see above section) where the hydrogen is recycled back into the reactor to produce further methane and water. In an automated system, the carbon deposit may be removed by blasting with hot Martian CO₂, oxidizing the carbon into carbon monoxide, which is vented.

A fourth solution to the stoichiometry problem would be to combine the Sabatier reaction with the reverse water gas-shift reaction in a single reactor as follows:

3 CO₂ + 6 H₂ → CH₄ + 2 CO + 4 H₂O

This reaction is slightly exothermic, and when the water is electrolyzed, an oxygen to methane ratio of 2:1 is obtained.

Regardless of which method of oxygen fixation is utilized, the overall process can be summarized by the following equation:

2 H₂ + 3 CO₂ → CH₄ + 2 O₂ + 2 CO

Looking at molecular masses, we have produced sixteen grams of methane and 64 grams of oxygen using four grams of hydrogen (which would have to be imported from Earth unless Martian water was electrolysed), for a mass gain of 20:1; and the methane and oxygen are in the right stochiometric ratio to be burned in a rocket engine. This kind of in-situ resource utilization would result in massive weight and cost savings to any proposed manned Mars or sample return missions.

See also

• In-Situ Resource Utilization
• Timeline of hydrogen technologies
• Steam reforming
• Fischer–Tropsch process

External links

• A Crewed Mission to Mars
• Development of an improved Sabatier reactor
• Improved Sabatier Reactions for In Situ Resource Utilization on Mars Missions
• Catalytic methanation experimental instructions, videos, and theory