Graphene Oxide Supported Ruthenium for CO Methanation

Open Access
Author:
Bhimanapati, Ganesh Rahul
Graduate Program:
Energy and Mineral Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
August 09, 2012
Committee Members:
  • Randy Vander Wal, Thesis Advisor
Keywords:
  • Graphene Oxide
  • Ruthenium
  • carbon monoxide
  • methanation
  • alumina.
Abstract:
The sustainable exploration of space requires minimization of re-supply from Earth through the implementation of In-Situ Resources Utilization (ISRU) strategies developed by NASA. One such strategy for lunar exploration is the production of oxygen from lunar regolith, a complex mix of minerals with large oxygen content. In the case of carbothermal-based oxygen production, reaction byproducts, i.e. carbon oxides, should be converted to methane for reintroduction into the carbothermal system. For this purpose, development of a methanation reactor to efficiently convert the mixed-carbon oxides into methane is essential to the integrity of the ISRU process. Nickel has been the favored catalyst for the methanation reaction because of its activity, selectivity and catalyst life. Yet Ni and Ni-based catalysts are particularly sensitive to poisoning by sulfur and coking by carbon. For these reasons Group VIII metals, in general far less prone to such detriments, have been in focus as alternatives to Ni, particularly Ru. Presently recent literature on methanation catalysts is concerned with preparative techniques and catalyst support, factors markedly affecting catalyst properties. The research reported here is focused on developing a new, more-efficient catalyst for the methanation component in NASA’s ISRU oxygen scheme. Efficiency generally translates into high catalyst exposure or high surface-area availability in heterogeneous catalysis. As a high surface area support graphene was chosen. Dispersion of Ru on graphene was achieved via a two-step decoration process using graphene oxide as a precursor. Notably GO production also facilitates graphene layer separation to create a high surface-area support. Yet agglomeration can occur during its reduction and deposition within a reactor bed, thereby lessening the effectiveness of the catalyst dispersion it supports. This is particularly problematic for nanoscale carbon materials as supports, given their high self-affinity between particles or platelets. Preventing this agglomeration will provide an increase in the accessibility to the surface metal sites on the support and ultimately could increase the performance activity of the catalysts. Towards this goal different Ru-based catalyst systems, using grapheme, were prepared and compared. The support of interest, grapheme, was first prepared as graphene oxide (GO) by the modified Hummers process and later decorated with ruthenium using a poly-ol process. A traditional catalyst system of ruthenium supported on alumina (Al2O3) was also prepared using the same process so as to facilitate comparisons. Another catalyst was synthesized in which the alumina was also introduced in the synthesis process along with GO to produce a mixed catalyst with the alumina helping to separate the graphene sheets, thereby partially alleviating their agglomeration.. These catalysts were then impregnated within hierarchical alumina foams and loaded into a microchannel methanation reactor based upon a simple pipe construction and Swagelok fittings. The use of alumina foams provided excellent contact area for the methanation gases (H2, CO) while maintaining a low pressure drop. The characterization techniques of FE-SEM and TEM identified the Ru catalyst particle size and dispersion on the graphene support. EDS identified the ruthenium elemental state with XPS measuring the GO elemental content. By TGA analysis the ruthenium content in the catalyst was determined. With this well characterized and quantified catalyst, methanation tests were performed at a temperature of 409oC. For these the catalyst was first pre-reduced at 300oC in the presence of H2. Reactant gas concentrations (i.e. the CO/H2 ratio) and space velocities were varied to test catalyst activity and selectivity, using gas chromatography to analyze the product gas mixture after characterization of the different catalysts for activity and selectivity, hypothesis related to both measurement. Amongst the catalyst systems of Ru/GO, Ru/Al2O3, Ru/ (GO+Al2O3), the mixed catalyst (Ru/ (GO+Al2O3)) performed the best with a CO conversion of 100 percent and selectivity of 100 percent towards methane. The comparisons made between the catalysts (Ru/GO)+bare Al2O3) and Ru/(GO+Al2O3) revealed that physical separation of the catalyst support layers, i.e. the graphene platelets, by the nanoalumina is the operative factor for highest activity. Graphene layer separation facilitates the Ru catalyst exposure, consequently the high Ru catalyst dispersion is effective. TEM images suggest this synergy of the mixed catalyst system with catalyst activity validating its impact.