Metal Oxide Sorbent Deactivation Study

The metal oxide (METOX) CO2 scrubber technology was developed by UTAS in the early 1990s as a replacement for non-regenerable LiOH canisters used in NASA’s Extravehicular Mobility Unit (EMU) system or space suit. METOX has been the main CO2 scrubber since 1998, utilizing a silver oxide sorbent and alkali metal salts to capture metabolically produced CO2 in the form of silver carbonate which is thermally regenerated to silver oxide after each use. In 2015 a performance reduction in one of the Metox canisters was observed and while the system still met the established CO2 removal requirements, the causes leading to the performance reduction have not been clearly justified. However, one of the most likely reasons is suspected to be the deactivation or degradation of the sorbent material within the system. Therefore, United Technologies Aerospace Systems, (UTAS) in collaboration with United Technologies Research Center (UTRC), is conducting a study to investigate the potential sorbent degradation mechanisms. The preliminary testing will focus on the identification of potential contaminants in addition to any changes in crystalline structure, morphology and surface area of sorbent samples extracted from the “as returned from orbit” METOX canisters and comparing the results to a prepared baseline Metox sorbent sample. Additionally, the effect of thermal desorption at a temperature above the nominal regeneration temperature will be evaluated on these sorbent samples while collecting the desorbed gases for further trace contaminant analysis. Potential oxidation of gas phase contaminants and implication on reaction kinetics will also be addressed. Some of the analytical techniques used in the study will include: X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) with Electron Dispersive Spectroscopy (EDS), Thermal Desorption with Gas Chromatograph Mass Spectroscopy (GC/MS), and Brunauer-Emmett-Teller (BET) for surface area determination. This paper summarizes the preliminary test results, and discusses potential mechanisms of sorbent degradation.