(572a) CO2 Methanation with Ru Decorated Carbon Nanotubes-Supported Catalysts

The sustainable exploration of space requires the minimization of re-supply from Earth by implementation of In-Situ Resources Utilization (ISRU) strategies developed by NASA (National Aeronautic & Space Administration). One of the strategies for the exploration of the Moon is the production of oxygen from the lunar regolith, which is a complex mix of minerals with large oxygen content. In the case of carbothermal based oxygen production, carbon oxides should be converted to methane for reintroduction in the carbothermal system. For that reason, the development of a methanation system that will efficiently convert mixed carbon oxides to methane turn out to be highly necessary.

Many authors have studied the catalytic synthesis of methane and other hydrocarbons from mixture of CO and H₂^[1],[2],[3]. It is well established that CO dissociation occurs readily on Ni^[4], Co^[5] and Ru⁵. Multi-walled carbon nanotubes (MWCT) can be efficiently used as to support metal catalysts for many catalytic heterogeneous reactions^[6],[7],[8]. The high specific area of CNTs significantly contributes to improve the final catalytic performance of the system since the reactions are governed by mass and heat transfer phenomena. This reflects the catalytic activity depending on the metal particle distribution as well the particle size on supporting materials.

As a result the key strategy to fabricate a high performance catalytic system is to reach high dispersion levels of metal precursors upon the MWCTs in parallel with high metal loading. 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. In summary, 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 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 tecnhiques 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 over different density foams for use in the reaction, enhancing heat and mass transfer while maintaining a low-pressure drop. A new microchannel reactor was developed based on the nanofabricated catalysis to evaluate the chemical activity and selectivity towards reduction of mixed carbon oxides.

Ruthenium deposited carbon nanotubes showed superior methane formation comparing to the Co/CNTs. TEM analysis of these catalytic systems illustrated highly dispersed ruthenium nanoparticles on the support, while the cobalt loading on the support has metal particles of 10-25 nm of diameter, 10 times bigger than the ruthenium ones. Different synthesis protocols were tested and the best results will be presented.

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