(552f) Development of a Microchannel Methanation Reactor Using Metal Decorated Carbon Nanotubes-Supported Catalyst

NASA's plan for sustainable exploration of space requires independence from re-supply by implementation of In-Situ Resources Utilization (ISRU) strategies. One of the strategies for lunar exploration is oxygen production from lunar regolith, a complex mix of minerals with high oxygen content. In carbothermal based oxygen production, carbon oxides (CO, CO₂) should be converted to methane for reintroduction in the carbothermal system. For that reason, the development of a methanation system that will efficiently convert these mixed carbon oxides to methane will be critical to closing the carbon cycle. .

Many authors have studied the catalytic synthesis of methane and other hydrocarbons from mixtures of CO and H₂^1,2,3. It is well established that CO dissociation occurs readily on Ni⁴, Co⁵ and Ru⁵ as catalysts. As catalyst supports, multi-walled carbon nanotubes (MWCT) can be efficiently used for these metals under reducing conditions ^6,7,8. The high CNT specific area significantly contributes to improving the catalytic performance since the reactions are governed by mass and heat transfer phenomena. This reflects the catalytic activity depending on the metal particle size, dispersion and exposure on supporting materials.

Hence the strategy to fabricate a high performance catalytic system is to reach high dispersion and loading levels of metal precursors upon the MCNTs. Synthesis variables such as the use of surface modified CNTs, impregnation and reduction techniques are critical to both controlling the decoration process and to anchoring the catalyst nanoparticles. With these parameters and goal, the project objective is the development of a nanostructured catalytic system based on dispersed ruthenium and cobalt particles supported on multi-wall carbon nanotubes. These nanostructed catalysts were deposited within a foam structure to fully expose the decorated MWCT catalysts to the reactant (carbon oxide) gases for methanation.

Ruthenium and cobalt supported MWCT catalysts were prepared using different metal precursors, ruthenium trichloride (RuCl₃.xH₂O), cobalt nitrate hexahydrate [Co(NO₃)₂.6H₂O], diverse preparation techniques and different MWCT surface chemistry. Results of Thermal Gravimetric Analysis and Transmission Electron Microscopy indicated that the acid treatment by concentrated nitric acid generated additional carboxylic and hydroxyl functionalities on the nanotubes, resulting to a superior amount of metal decoration and better dispersion of the nanoparticles. The effect of chemical reduction process and the addition of cobalt were investigated and compared with a typical single metal catalytic system. The metal/MCNTs were deposited within foams of different cell density to enhance heat and mass transfer while maintaining a low-pressure drop. A microchannel reactor was developed based on the nanofabricated catalysts to evaluate the chemical activity and selectivity towards reduction of mixed carbon oxides.

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