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Titanium-Based Microreactors for Water Purification


A new era of space exploration is on the horizon. Over the past decade interest in crewed missions to Mars and beyond has rapidly developed in the private sector. Innovation in spaceflight has taken center stage but to achieve these ambitious goals improvements to the Environmental Control and Life Support Systems (ECLSS) will be a necessity as well. The system aboard the International Space Station (ISS) can be viewed as a model to assess the demands for sustaining a long-term crew on deep space voyages. Currently, as part of the Water Recovery System (WRS), a thermal catalytic reactor is used to eliminate dissolved low molecular weight volatile organic compounds (VOC). This catalytic reactor uses high temperature and oxygen to oxidize VOCs into harmless by-products. However, components used to maintain the high temperature and pressure fail for more frequently that would be feasible for eventual use in deep space missions. We propose that by replacing the catalytic reactor with photocatalytic microfluidic reactors, which operate at standard temperature and pressure, reliability can be greatly improved.

Herein, we present a multi-faceted study to develop a high-density photocatalytic microfluidic reactor with design enhancements to mitigate common limitations associated with this technology. The first aim was to apply a design of experiments (DOE) approach to optimize nanoporous titania (NPT) for use within the microfluidic reactor concept. NPT is a unique form of titanium dioxide (TiO2) that directly forms on titanium (Ti) surfaces through a hydrogen peroxide (H2O2) based oxidation process. The Taguchi method and grey relational analysis (GRA) were applied to the oxidation conditions (H2O2 concentration, temperature, and time) to maximize the reaction rate constant, k, and NPT film quality. The second aim applied Ti microelectromechanical systems (MEMS) fabrication techniques to create the first, high-density microfluidic reactor system utilizing a high aspect ratio Ti micropillar array for enhanced catalyst loading and reduced diffusion distance. The micropillar reactors outperformed conventional flat planar reactors and achieved 2-fold or greater photocatalytic activity than all other devices reported in the literature thus far. These results demonstrate the significant performance enhancements of the micropillar array and identify future directions for further validating the concept for use in ECLSS applications.

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