(35a) Nanoengineered Methanation 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].  It
is well established that CO dissociation occurs readily on Ni [1], Co [1] and Ru [5]. Multi-walled carbon nanotubes (MWCT) can be
efficiently used as to support metal catalysts for many catalytic heterogeneous
reactions.  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.

References

1. H. Pichler,
A. Hector., Kirk-Othmer Encyclopedia Chem. Tech. IV, 446 (1964).

2. H.-J.
Jung, P. L. Walker, Jr., M. A. Vannice, J. Catal. 75,
416-422 (1982)

3. M. A. Vannice,
Cat. Rev.-Sci.
Eng. 14, 153 (1976).

4. M. Araki, V. Ponec, J. Catal. 44, 439 (1976).

5. J. W. A. Sachtler, J. M. Kool, V. Ponec, J. Catal. 56, 284 (1979).

6. H. Tong, H.-L. Li, X.-G. Zhang, Carbon 45:2424-2432 (2007).

7. J. Garcia, H. T. Gomes, Ph. Serp,
Ph. Kalck, J. L. Figueiredo,
J. L. Faria, Carbon
44:2384-2391 (2006).

8.
W. Li, X. Wang, Z. Chen, M. Waje, Y. Yan, J. Phys. Chem. B 110:15353-