As crewed space missions extend beyond low Earth orbit, the need to reliably recover potable water is critical. Aboard the International Space Station (ISS), the water is recycled from cabin humidity condensate, urine distillate, and hygiene wash wastes. In spacecraft cabin air environments, off-gassing from equipment, human metabolism, and human personal care products contributes to significant airborne concentrations of volatile organic compounds (VOCs). These polar and water-soluble compounds ultimately dissolve into the humidity condensate and stress the process load, logistics costs, and lifecycle requirements of the water processing systems. The aim of this effort was to develop the High Performance Photocatalytic Oxidation Reactor System (HPPORS) technology for the destruction of airborne VOCs prior to reaching the water processing systems. This innovation will reduce the logistics costs and lifecycle requirements of water processing systems, and help extend NASA missions to include long-duration space habitation and lunar and Mars colonization missions.
The HPPORS is an innovative photocatalytic oxidation system that combines long-lifetime, high-intensity light emitting diodes (LEDs) with efficient, visible light-activated photocatalysts for the destruction of polar VOCs and other airborne contaminants under ambient conditions of temperature and pressure. The basis of the technology is the integration of visible photocatalysts with robust blue LEDs, uniform side emission fiber optics, and efficient catalyst surface illumination technologies to create a photocatalytic oxidation unit for spacecraft cabin air purification. This combined approach leads to numerous performance benefits including high VOC conversion efficiency, compact reactor volume, low pressure drop, and the elimination of conventional ultraviolet (UV) mercury lamp logistics and hazards.
The technique of photocatalytic oxidation is a viable approach for volatile organic chemical destruction. A simple but necessary condition is that the photocatalyst is active only if photons reach and illuminate the catalyst surface. Partial illumination or intensity shadowing using external light sources such as mercury lamps is known to be significant in packed bed substrates such as monoliths, micro-beads, and zeolites. In these structures there are significant regions devoid of photon illumination, which leads to diminished photocatalytic activity and efficiency. The use of a side emitting fiber optic (SEFO) bundle to transmit, illuminate and support the photocatalyst overcomes this shortfall. The SEFO bundle not only generates uniform, close-coupled illumination of the photocatalyst but also creates a high surface-area-to-reactor volume for a compact footprint and low-pressure-drop system.
Titanium dioxide (TiO2) is the most widespread photocatalyst for VOC removal in air and aqueous systems. It is inert, stable, inexpensive, and poses no harm to the environment or humans. However, its action spectrum lies in UV and does not extend into the visible portion of the spectrum. This unfortunately prevents use of higher power, long-lived visible LEDs or solar radiation to activate the photocatalyst. To shift the photocatalyst activity from UV to visible wavelengths, the TiO2 electronic structure can be modified by introducing metal and non-metal materials into the catalyst structure. In this project, a process was developed to modify the TiO2 structure by introducing nanostructured noble metals into the catalyst framework. This had the effect of extending the action spectrum well into the visible spectrum, which allowed the efficient coupling of blue light into the photocatalyst. To take advantage of the surface illumination properties presented by the SEFO, the optical fiber bundle was then carefully coated with a thin surface film of the nanostructured noble metal photocatalyst.
The feasibility of the HPPORS was investigated using a blue, 470 nm LED array coupled to a polymer-based side emitting fiber bundle coated with a noble metal nanoparticle- TiO2 photocatalyst. A representative VOC sample (ethanol) was diluted in air and passed over the catalyst coated fiber bundle. The reactor was tested in continuous, single pass operation at ambient pressure and temperature and demonstrated over 95 percent ethanol destruction.
The primary application of the HPPORS is to provide a compact, low-maintenance, high-performance air purification device for spacecraft environmental control and life support systems. The technology can boost efficiency and performance of the ISS and can help to extend NASA’s mission beyond low Earth orbit to include long-duration space habitation, lunar, and Mars colonization missions. The technology has numerous terrestrial air purification applications in commercial and private sectors including homes, schools, commercial offices, hospitals, and public transportation. The technology can be extended to liquid systems for applications in water purification, oil spill remediation, and cleanup of water produced from oil and gas production.