Fueling the mission to Mars and Earth’s transition to renewable energy: Increasing catalyst performance for carbon dioxide methanation

Petersen, Elspeth
Major Professor
Jean-Philippe Tessonnier
Committee Member
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Chemical and Biological Engineering

Carbon dioxide methanation, also called the Sabatier reaction, has a wide range of applications. It can provide a means of carbon-neutral energy storage and transport by converting atmospheric or point-source carbon dioxide to methane using renewably generated hydrogen. This allows for energy transport and storage for intermittent renewable energy using the existing natural gas infrastructure. The Sabatier reaction will also play a large role in the human exploration of space. A reactor is currently under development by NASA to produce fuel on Mars for an ascent vehicle to return a crewed mission or planetary samples back to Earth. Synthesizing the fuel on-site eliminates the need to transport it from Earth, greatly decreasing the launch volume, weight, and cost requirements of the mission. The catalyst developed for this reactor can also be applied in crewed spacecraft to recycle oxygen from metabolic carbon dioxide and to produce fuel and water from trash.

However, issues related to catalyst deactivation currently limit the implementation of this technology. In order to ensure proper and long-term operation of a Sabatier reactor for such applications, catalyst durability and activity must be improved, and to do so, the effects of support properties, such as electronic interactions with the active phase, thermal conductivity, and behavior under typical reaction conditions must be understood. For this exothermic reaction, thermodynamics dictate that conversion and selectivity are favored at lower reactor temperatures, but supports with low thermal conductivity are not effective at transferring heat away from the active phase during the reaction. This buildup of heat results in localized elevated temperatures, higher than the overall bed temperature that detrimentally affect catalyst activity and longevity.

In order to increase catalyst longevity, beta silicon carbide, a material with exceptional heat conductivity and mechanical strength, has been chosen as a possible replacement for the traditional, low-thermal conductivity, metal-oxide supports currently in use. To test the effects of support thermal conductivity in the absence of other interfering variables, the thermal conductivity of silicon carbide was tuned via controlled oxidation to silicon dioxide. By strategically controlling the calcination duration, supports with a range of silicon carbide to silicon dioxide ratios were produced. As silicon dioxide has a much lower thermal conductivity, the increased calcination time resulted in a lower support thermal conductivity. Composition supports consisting of silicon carbide with a natural silicon dioxide washcoat were impregnated with an active Sabatier catalyst, ruthenium, and tested for conversion rates and methane selectivity in a packed bed reactor. The study found a trend of decreasing selectivity with an increasing proportion of silicon oxide in the support and thus a decrease in thermal conductivity. The trends in selectivity and conversion are likely due to localized hot spots and match those predicted by thermodynamics at elevated reaction temperatures, where carbon dioxide conversion shifts to the production of carbon monoxide at the expense of methane. These findings are also supported by reaction modelling of a catalyst particle, which shows increasing surface temperature on the supports with reduced thermal conductivity. In addition to the negative impact the elevated surface temperatures have on selectivity due to thermodynamic constraints, it can also reduce catalyst longevity by means of active phase and support sintering, fouling via coke formation, and physical degradation.

The use of silicon carbide for the Sabatier reaction to improve support thermal conductivity shows great promise for improving catalyst activity and longevity, but further study is still required to build on these initial findings to further enhance catalyst stability and activity. Incorporating metal oxides into the silicon carbide matrix will likely address some of the drawbacks seen with the silicon carbide supports. Metal oxides, such as alumina and titania, interact electronically with the active metal to increase catalyst dispersion, prevent sintering, and improve activity through participation in the reaction pathway. The knowledge gained in this and future studies will provide a more complete understanding of the deactivation pathways for Sabatier catalysts and enable the design of a new generation of robust, highly active catalysts. The development of a durable Sabatier reactor will enable the human exploration of Mars and the transition to renewable energy on Earth.