November 17, 2020. By Kolemann Lutz
A novel approach to the molecular separation of carbon dioxide provides a lightweight, low energy solution at room temperatures and pressures, which could slash costs for CO2 reduction and improve chemical manufacturing on three planets in the inner Solar system.
Challenges. The conversion from CO2 to carbon monoxide and later oxygen typically requires high‐temperature electrolysis in solid‐oxide electrolysis cells (SOECs) that operate at temperatures typically between 500 and 850 °C. Additionally, there is the risk of electrolyte and electrode–electrolyte interface degradation issues and high energy water electrolysis that requires 285.83 kJ of energy per mole. The splitting of pure CO2 in the absence of water is of particular interest, as it kickstarts the anthropogenic carbon cycle without constraints on water availability.
Experiment Discovery. Researchers at the National Institute of Standards and Technology (NIST) built and employed an electron microscope that pulses an adjustable electron beam to initiate the reduction of CO2 to CO (CO2+C = 2CO) at room temperature and pressure. The electron beam employs one narrow beam that is one nanometer in diameter and another beam 1000X wider in a CO2 environment at a partial pressure of 50 Pa within a column of an 80 kilovolt (kV) environmental scanning transmission electron microscopy (ESTEM).
The narrow beam bombards nanoparticles of aluminum (Al), Earth’s most abundant metal, with energy from traveling waves of electrons, also known as localized surface plasmons (LSPs), that surf on individual Al nanoparticles. The nanoparticles were deposited onto a prepositioned layer of graphite, a crystalline form of Carbon, placed six nanometers away where the plasmons latch onto. The graphite acted as a magnet to pluck the oxygen atoms and to break one of the polar covalent bonds between C and O. The team mentioned that the commercially available aluminum nanoparticles should be evenly distributed to maximize contact with the carbon source and the incoming CO2.
Afterward, the same electron beam also acted as a microscope that imaged structures as few as one billionth of a meter inside the modified gas cell holder with the modified CO2 environment, enabling the team to estimate how much CO2 was removed. By using EELS electron energy loss spectroscopy (EELS), the researchers measured carbon depletion, the temperature of Al nanoparticles as well as the spatial distribution of LSP modes and carbon gasification near nanoparticles, which helped estimate the reaction rate. To support this hypothesis, in situ gas chromatography-mass spectrometry (GCMS) measurements during electron beam illumination were used to detect CO directly as a reaction product.
“For the LSP-driven reduction of CO2 with the electron beam in the aloof mode, the average measured carbon etching rate of ~0.22nmmin−1 appears to be low for practical applications. The low etching rate can be attributed to the weak electric field amplitude associated with the LSP mode at the nanoparticle–graphite interface when the aloof-mode excitation is used.”
The team accomplished a major feat: getting rid of the carbon dioxide without the need for a source of high heat. This light beam oxygen plucking process could also complement recent improvements in carbon monoxide storage. Inspired from carbon monoxide that loves the iron in our blood, a metal-organic framework (MOF) incorporates chains of iron atoms tuned to attract CO, which would require significantly less energy than other capture or storage technologies, such as cryogenic distillation of CO.
CO is an essential intermediate in iron and steel production and is used in a variety of industrial processes, including as a component of synthesis gas – a mix of CO and hydrogen used to make synthetic fuel or to synthesize other chemicals.
Implications. NASA's current plan for oxygen production primarily revolves around the idea of scaling up the Mars Oxygen ISRU Experiment (MOXIE), a model that will produce 6 to 10g of O per hour using solid oxide electrolysis onboard the Mars 2020 Perseverance rover. MOXIE is 1% the scale of the intended oxygen processing plant capable of producing consumable grade (99.6 percent) oxygen.
If liquid hydrocarbon fuels are the final target, the separate splitting of CO2 and H2O for producing both CO and H2 is advantageous as it enables tight control of the syngas's purity and quality required for gas-to-liquid processing. O2 is the second most important industrial gas. The reduction of CO2 produces CO and Oxygen byproducts that are the building blocks for synthesizing CH4, ethanol, a slew of other chemical reactions and other downstream carbon compounds. This carbon filtration and separation technology greatly lowers the energy required to produce valuable chemicals such as methanol, formaldehyde, formic acid, which are all important to manufacture adhesives, foams, plywood, cabinetry, flooring, building and insulation materials, disinfectants, and even other raw materials for the synthesis of medicines, detergents, fertilizers, and textiles.
Applied to Mars. “In addition to applying this nanotechnology to exhaust systems, we could also apply this light beam splitting technology to CO2 intake systems on the Martian surface to create O2 and liquid feedstocks for chemical manufacturing”, mentioned by Kolemann Lutz, Cofounder at MarsU.
If this light beam technology can reliably produce quantities of oxygen with greater performance than electrochemically splitting CO2 molecules with solid oxide electrolysis, it could become a complementary method to produce oxygen for propellant and life support while preserving valuable liquid hydrogen and water. “This technique could bring the in-situ manufacturing and 3D printing of industrial-scale anhydrous CO2 splitting and direct air carbon capture DAC units on other planets closer to reality.”
ISRU-produced O2 could also be used as an oxidizer to burn the fuel onboard rovers, rockets, and other machinery. Hydrogen would still need to be imported or extracted from ice water using electrolysis to convert a mixture of gaseous CO and H into liquid hydrocarbons. If the targeted propellant is CH4, the splitting of H2O followed by the exothermic Sabatier reaction (4H2 + CO2 = CH4 + 2H2O, ΔH273K = −164 kJ mol−1 CO2) may be the preferred path because of the higher solar-to-fuel energy conversion ( ηsolar-to-fuel) efficiency and fewer steps.
This approach opens a path forward for continued research to open the frontier for limitless quantities of oxygen to make full use of carbon dioxide, the most accessible and abundant resource currently on Mars. It holds the potential to improve the efficiency of large-scale commercial CO2 reduction and light beam technology through the application of geoengineering carbon neutrality on planet Earth first.
Research Paper. Canhui Wang, Wei-Chang D. Yang, David Raciti, Alina Bruma, Ronald Marx, Amit Agrawal, and Renu Sharma. Endothermic Reaction at Room Temperature enabled by Deep-Ultraviolet Plasmons. Nature Materials. Nov. 2, 2020. DOI: 10.1038/s41563-020-00851-x