Astronaut crews aboard the International Space Station (ISS) rely on recycled water from cabin condensation and urine to sustain their daily water intake. A Water Recovery System (WRS) uses a series of sedimentation, multifiltration beds, and a thermal catalytic oxidation reactor (COR) to reclaim nearly all crewmembers’ daily water supply. However, operation of the COR is an energy intensive process requiring high pressure and temperature to eliminate remaining volatile organics. The ISS and deep space travel represent an environment that is sensitive to the water-energy nexus and could benefit from a low-cost, low-energy alternative process.
Interest in photocatalytic TiO2 microfluidic reactors for applications in volatile organic compound removal has grown considerably over the past decade. Advantages of microfluidic devices over traditional packed-bed thermal oxidation systems include increased surface area to volume ratios and greatly reduced mass diffusion lengths, which can translate to enhanced performance on a miniature scale. TiO2-based photocatalysts operate at standard pressure and temperature and have been functionalized for novel use in water purification, water-splitting, and photochemistry. However, low volumetric throughput remains a critical limitation in many applications, as does the difficulty associated with integrating TiO2 uniformly within complex microfluidic device geometries.
Herein, we present our recent efforts to optimize growth of nanoporous TiO2 (NPT) for use within Ti-based microfluidic devices. NPT is grown in situ, directly from the Ti channel surfaces, using a H2O2-based oxidation process. Advantages of this approach include: a) conformal catalyst coverage of complex geometries; b) high porosity yielding increased surface area and fluidic accessibility; and c) potential for scalable fabrication of large area photocatalytic devices with increased volumetric throughput. Additionally, we propose that incorporating a novel high-density, high-aspect-ratio micropillar array within the reaction chamber to serve as a scaffold for NPT oxidation will yield significant performance enhancements to overcome the majority of microreactor limitations.
Using a multi-criteria Taguchi design of experiments study, optimal NPT growth conditions were determined based upon methylene blue degradation efficiency and NPT film crack area. Our studies identified the key oxidation parameters responsible for photocatalytic performance. These ideal growth conditions were applied to a Ti deep reactive ion etched (DRIE) micropillar array to form NPT in situ and demonstrated a notable improvement in photocatalytic response compared flat NPT films. Collectively, these results represent important steps towards our goal of developing robust, high-performance multi-scale photocatalytic microreactors with complex channel geometries for increased mass and photon transfer efficiency.