49th International Conference on Environmental Systems ICES-2019-353 7-11 July 2019, Boston, Massachusetts

Metal Oxide Sorbent Deactivation Study

James Auman Jr1, Sandra Guerrero2, Thomas Chase3 Collins Aerospace , Windsor Locks,CT, 06096

Tim Nalette4 United Technologies Aerospace Systems retiree, Windsor Locks , CT, 06096

Daniel Goberman5 United Technologies Research Center, East Hartford, CT, 06108

Robert Boyle6 NASA Johnson Space Center, Houston, TX, 77058

Brian Macias7 NASA Johnson Space Center, Houston, TX, 77058

Collins Aerospace in collaboration with United Technologies Research Center (UTRC), conducted a study to investigate the potential degradation mechanisms for the sorbent used in NASA’s Metal Oxide (METOX) canisters. The preliminary testing focused on the identification of potential contaminants present in the gas phase, changes in crystalline structure, morphology and surface area of sorbent. Samples from METOX canisters returned from orbit were analyzed and compared to a ground prepared baseline METOX sorbent. Investigation of differences in reactor performance were addressed by conducting equilibrium loading tests using sorbent sheets from a METOX canister used in simulated ground EVAs. Preliminary data suggest that sorbent deactivation may be due to the potential migration, and redistribution of alkali metal salts within the sorbent sheet, which results from the normal operation of the system- specifically the chemical cycling that occurs during the adsorption/regeneration cycles.

Nomenclature ALPM = Actual Liters per Minute EMU = Extravehicular Mobility Unit EVA = Extra Vehicular Activity ACFM = Actual Cubic Feet per Minute BTU = British Thermal Unit (~1055 Joules) BTC = Breakthrough Curve METOX = Metal Oxide ISS = International Space Station PLSS = Primary Life Support System

1 Staff Engineer, Collins Aerospace, 1 Hamilton Road MS 1A-2-W62 Windsor Locks CT 06096-1010 2 Sr.Research Engineer, Collins Aerospace, 1 Hamilton Road MS 1A-2-W66, Windsor Locks CT 06096-1010, 3 System Engineer, Collins Aerospace, 1 Hamilton Road MS 1A-2-W66, Windsor Locks CT 06096-1010 4 United Technologies Aerospace Systems, Retired, 4991 Shimerville Road, Emmaus, PA 18049. 5Associate Director, Discipline Leader-Materials Characterization, United Technologies Research Center, 411 Silver Lane, MS 129-22, East Hartford, CT 06108 6 NASA Extravehicular Mobility Unit (EMU) System Manager 7 NASA Extravehicular Mobility Unit (EMU) Life Support System Hardware Manager

Copyright © 2019 Collins Aerospace Nomenclature (continued) LiOH = Lithium Hydroxide CO2 = Carbon Dioxide pph = Pound per hour pp CO2 = Partial Pressure CO2, mmHg PTFE = Polytetrafluoroethylene LCVG = Liquid Cooling and Ventilation Garment BET = Brunauer-Emmett-Teller Surface Area TGA = Thermogravimetric Analysis XRD = X-ray Diffraction TD-GC = Thermal Desorption Gas Chromatography SEM = Scanning Electron Microscope EDS = Energy Dispersive Spectroscopy XPS = X-ray Photoelectron Spectroscopy Analysis

I. Introduction

Th e Metal Oxide (METOX) carbon dioxide (CO2) scrubber technology was originally developed by Collins Aerospace in the early 1990s as a replacement for non-regenerable lithium hydroxide (LiOH) canister in NASA’s Extravehicular Mobilty Unit (EMU) system and has been the main CO2 scrubber since 1998. The system utilizes a silver oxide sorbent to capture metabolically produced CO2 in the form of silver carbonate, which is thermally regenerated to silver oxide. The sorbent is prepared by dissolving alkali metal salts in water to form an homogeneous solution which is then added to the silver compound to form a mixture with a paste-like consistency. The sorbent material is then formed into a rectangular “sheet” which is subsequently wrapped in a microporous Polytetrafluoroethylene (PTFE) film which keeps the metal oxide sorbent from being released into the air stream and allows for the diffusion of water vapor and CO2 into, and out of the sorbent during the adsorption and regeneration processes.

A METOX canister is composed of approximately 90 sorbent sheets and the canister was designed to support an eight-(8) hour Extravehicular Activity (EVA), at an average metabolic rate of 850 BTUs per hour (equivalent to a 1,3 CO2 generation rate of 0.17 pph) resulting in a total required CO2 removal capacity of 1.48 pounds. In recent years, NASA has found that during various EVAs, some canisters exhibited signs of earlier than expected breakthrough, leading to premature terminations of EVA activity.4 While some sorbent degradation was anticipated in the original design due to irreversible reactions with ISS contaminants3 and the slow decomposition of silver oxide to silver metal, the impact of some trace contaminants, like siloxanes, are not yet well understood.

The objective of this study is to characterize samples of METOX sorbent material extracted from canisters used during ground or on orbit EVAs to identify potential changes in the sorbent crystalline structure, presence of contaminants, and determine any resulting changes in reaction kinetics or total sorbent capacity that would address the apparent loss of performance seen on orbit. While all flight canisters still meet the “as designed” requirements, NASA had historically been accustomed to utilizing excess capacity.5 Therefore this effort seeks to determine the cause and mechanisms that are believed to have negatively affected this excess capacity. Analysis of charcoal capacity used for trace contaminant control during an EVA, and incorporated into the METOX canister design, was not in the scope of this investigation.

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II. METOX Background

A. METOX Canister Assembly

The METOX canister assembly consists of a stainless-steel housing, approximately 90 metal oxide sheet assemblies, 0.25 pounds of activated charcoal bed, a 150 mesh particulate filter and a state, or status, indicator. The overall dimensions are 13.46 inches wide, 9.78 inches high and 3.42 inches thick. The maximum assembly weight is 32 pounds in the regenerated state. A center support is welded between the front and back walls to provide additional structural support dividing the internal cavity into two equal volumes. Figure 1 shows the METOX canister assembly. Approximately 45 metal oxide sheet assemblies are loaded into each of the two volumes of the canister. Figure 1. Metal Oxide Canister

Each stack sheet consists of the following components: PTFE wrapped silver oxide sorbent: 8.625 inches (wide) x 2.625 inches (long) x 0.083 inches (thick), two perforated stainless steel sheets (0.001-inches thick with one on each side of the sorbent sheet), and a section of stainless steel heat exchanger fin stock (0.020 inches thick) to provide the airflow passages through the bed. After the sheets are installed, two Teflon bags filled with activated charcoal are placed in the approximately 0.25 inch space on the Inlet side of the Metox canister (one bag for each side). Figure 2 Figure 2. Metal Oxide Canister Components shows the canister’s major components.

B. Canister Operation and Regeneration

During operation, the gas enters the left header (Inlet header) and is distributed across the Inlet side of the canister via air flow channels. The gas passes through the charcoal bed first where metabolically produced trace contaminants are removed. Gas then passes through the metal oxide bed where carbon dioxide (CO2) and some water (H2O) vapor are removed.

The refreshed Oxygen (O2) is then collected on the Outlet side and directed to the right header (Outlet header) via air flow channels. Then passes through a 150-micron particulate filter before exiting the canister. Because the process gas flows “by” the sorbent sheets, with reactive gases diffusing into the sheets, the flow path for the system is considered a “flow by” bed rather than a more traditional packed bed configuration. Each of the longer dimensions (8.625 inches) of a sorbent sheet will then represent either the Inlet or Outlet edge of each sheet.

The sorbent is thermally regenerated on orbit by flowing ISS air between 150-205 Celsius degrees (oC) for 10 hours in the direction opposite to the flow direction during adsorption, or EVA. The flow is reversed so that any contaminants adsorbed in the charcoal during the EVA are not pushed into the canister sorbent during the regeneration. However, it is also true that as a result the canister is not protected from potential contaminants in the process air during the regeneration process. Additionally, as a result of this flow reversal, the outlet edge of each sorbent sheet in the canister will be exposed to the highest temperature for a longer period of time . During the 3 International Conference on Environmental Systems

original sizing of the Metox sorbent this was an important factor to consider which resulted in the incorporation of additional sorbent material to accommodate some level of degradation due to this exposure.

C. Metal Oxide Chemistry

Experimental data has shown that carbon dioxide (CO2) will not react directly with metal oxides unless there is 8,9 enough moisture available to react with the oxide to form a hydroxide. The reactions of CO2 with silver oxide to form silver carbonate is presented in equations (1) to (7) below. There are two alkali metal salts in the METOX sorbent formulation: a carbonate and a halide. These salts provide the necessary basic pH to increase the reaction kinetics for adsorption of CO2 and also allow for operation at lower relative humidities (within the sorbent sheets) which is typically encountered during the low pressure EVA environment.

+ − 퐴푔2푂 → 퐴푔 + 퐴푔푂 (1) 퐴푔푂− + 퐻 푂 → 퐴푔푂퐻 + 푂퐻− 2 (2) + − 퐴푔푂퐻 → 퐴푔 + 푂퐻 (3) − − 퐶푂2 + 푂퐻 → 퐻퐶푂3 (푟푎푡푒 푙푖푚푖푡푖푛푔 푠푡푒푝) (4) − − = 퐻퐶푂3 + 푂퐻 → 퐻2푂 + 퐶푂3 (5) + = 2퐴푔 + 퐶푂3 → 퐴푔2퐶푂3 (6) (7) 퐴푔2푂 + 퐶푂2 → 퐴푔2퐶푂3 (표푣푒푟푎푙푙)

While water is not consumed in the overall reaction, it does act as a “catalyst” for the reaction by combining with the silver oxide to maintain a supply of hydroxide ions for the rate limiting reaction per equation (4), and silver ions per equation (6). Migration of the alkali metal salts may potentially reduce reaction kinetics, however, if only related to water content it may not be readily observed at atmospheric pressure.

Sorbent regeneration is accomplished by increasing the temperature between 150 and 205 degrees Celsius (°C). At this temperature range, the silver carbonate will decompose directly to silver oxide and CO2.

퐴푔2퐶푂 3 ↔ 퐴푔2푂 + 퐶푂2 푟푒푔푒푛푒푟푎푡푖표푛 (8)

During early development of the METOX technology, some of the proposed mechanisms for potential performance degradation of the metal oxide sorbent included:

- Thermal decomposition of the sorbent material - Chemical decomposition/poisoning of the sorbent - Migration of the sorbent constituents

Silver oxide will start decomposing at 200oC, but at a rate significantly slower than the decomposition of silver carbonate. Theoretically, the silver oxide will present a more pronounced decomposition at temperatures in the range of 350-400oC. During the flight qualification process, the canister was cycled for 100 cycles, and the observed degradation in performance against the flight acceptance test requirements was approximately 7.5%.5 None of the ground-only and on-orbit canisters have seen anywhere near this number of cycles, and it is therefore unlikely that decomposition of the oxide is the primary cause of the observed performance shift.

The proposed mechanisms were further investigated by conducting several tests on samples of used on-ground and on-orbit sorbent material and making comparisons to a newly made baseline sample of sorbent. A description of the samples tested as well as the analytical techniques used are presented in the next sections.

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III. Test Procedure

A.Sorbent Samples

METOX Canister Samples and Baseline Sample

Samples of sorbent material were extracted from several METOX canisters that were returned to the ground for refurbishment. Canister SN0002 is the Certification Canister and had been exposed to 100 cycles on the ground under controlled laboratory conditions in the 1998 timeframe. This canister could be considered relatively clean since it has not been exposed to the environmental conditions of the other canisters, specifically the ISS trace contaminants. Canister SN0006 and SN0009 have been used for ground EVAs in a manned vacuum chamber.

Canister SN0005 was the only available canister with some exposure to on-orbit EVAs. Table 1 shows the list of canisters included in this study and the corresponding number of cycles for each prior to disassembly.

A baseline sorbent sample was prepared in the Advanced Engineering Laboratory at Collins Aerospace as defined by Engineering Change 17D144 titled “ Define a New Metal Oxide Container Configuration with New Sorbent Materials” and which created the SV821799-04-00 configuration of Metal Oxide Containers. The baseline sample was initially activated/regenerated by flowing hot nitrogen at 30 alpm and 205oC for six hours followed by a two hour cooling period until the temperature was approximately 25oC. The prepared baseline sample was not subjected to any CO2 adsorption cycling.

Table 1. METOX Canisters Tested METOX Canister Serial Number Number of Cycles Cycles Description SN0002 100 Ground-lab-unmanned SN0006 52 Ground-vacuum chamber-manned SN0005 34 2 ISS EVAs, 32 ground EVAs SN0009 18 Ground-vacuum chamber-manned

Sample Nomenclature

In the early phases of the METOX canister refurbishment process, the sheet orientation was not always documented during disassembly. Only samples extracted from canister SN0005 were clearly identified by location within the canister (e.g., closer to left or right header ports) and flow orientation during an adsorb cycle (e.g., Inlet or Outlet flow). In an attempt to evaluate samples representative of different sections the canister, when the information was available, sorbent samples were identified according to:

1. The proximity of the sorbent sheet, within the assembly, to the canister header port (e.g., closer to Inlet/Outlet Header) 2. The flow direction within the sheet during an adsorb cycle: (Inlet), (Center), (Outlet) or Edge-1, Center , Edge-2 3. The specific area where the subsample was taken (e.g., Top, Mid, Bottom section, Surface or Core section). Figure 3 shows a schematic of sample retrieval and the locations sampled within the sorbent sheet. Table 2 Figure 3. Sample retrieval and location sampled summarizes the sample list and characterization tests within sorbent sheet performed.

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Table 2. Sample list and characterization tests performed Sample ID METOX Sample TGA BET Thermal XRD SEM XPS Canister Description Desorption EDS Sample-01 NA Engineering       Baseline Sample -03 SN0002 Edge-1  Closer to Inlet Header Sample-09 SN0006 Edge-1   Closer to Inlet Header Sample -09A SN0006 Edge-1  Closer to Inlet Header Sample -09B SN0006 Center   Closer to Inlet Header Sample -09C SN0006 Edge-2  Closer to Inlet Header Sample-15 SN0009 Edge-1  Closer to Inlet Header Sample-15B SN0009 Center  Closer to Inlet Header Sample-27A SN0005 Inlet    Closer to Inlet Header Sample-27B SN0005 Center  Closer to Inlet Header Sample-27C SN0005 Outlet  Closer to Inlet Header

B. Sorbent Characterization

Several analytical techniques were used to investigate the potential sorbent degradation mechanisms and characterize the available sorbent material. Results were compared to engineering baseline sample 01 representative of uniformly mixed starting sorbent material prepared in the Advanced Engineering Laboratory at Collins Aerospace. The baseline material was regenerated under controlled laboratory conditions prior to analysis.

Characterization techniques included: Brunauer-Emmett-Teller (BET) for surface area determination, Thermogravimetric Analysis (TGA) for thermal decomposition evaluation, X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) with Electron Dispersive Spectroscopy (EDS) for crystalline structure identification. X- Ray Photoelectron Spectroscopy (XPS) to determine surface elemental composition and Thermal Desorption with Gas Chromatograph and Mass Spectroscopy (GC-MS), for trace contaminant identification. The main findings are discussed below.

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Brunauer-Emmett-Teller Surface Area (BET)

Brunauer-Emmett-Teller testing was conducted to examine the surface area of the different sorbent samples. In general, the sample surface areas were relatively small compared to most sorbents and catalysts. While the surface area was slightly higher in sections closer to one of the edges, the fact that the baseline surface area was low (0.59 m2/g), and its performance is known to be acceptable, led us to conclude that BET surface area was not a meaningful evaluation tool for this material. The results are presented in Table 3. At the time of this analysis, samples from SN0005 were not available.

Table 3. METOX sorbent BET results Sample METOX Surface Area (m2/g) Sample Description Canister Sample-01 NA 0.59 Engineering Baseline regenerated, uncycled Sample -03a SN0002 0.15 100 Lab cycles Sample -03c SN0002 0.20 100 Lab cycles Sample -09a SN0006 0.99 52 Ground cycles Sample -09c SN0006 1.19 52 Ground cycles Sample -15a SN0009 0.80 18 Ground cycles Sample -15c SN0009 1.80 18 Ground cycles

Thermogravimetric Analysis (TGA)

Thermogravimetric Analysis, TGA, was conducted to investigate the degassing behavior of the sorbent material. Samples-01, 09(SN0006), 15(SN0009) and 27(SN0005) were run in dry nitrogen gas with a ramp rate of 10°C per minute from ambient temperature to 250°C holding at this temperature for 1 hour. The results indicated in general that the baseline was properly regenerated however additional regeneration was necessary for samples from flight canisters for adequate sample preparation for both BET and Thermal Desorption Gas Chromatography testing.

X-ray Diffraction (XRD)

X-ray diffraction (XRD) was used to characterize the crystalline structure of the sorbent material in samples 09B (SN0006), 15B (SN0009) and baseline sample-01. While the objective was to be able to identify potential silver oxide reduction to elemental silver, this was not possible as the relative concentration of silver was likely too small to detect compared to silver oxide (Ag2O) and silver carbonate (Ag2CO3) present in the samples. The XRD results revealed significant presence of silver carbonate indicating again the incomplete regenerated state of the ground- only and on-orbit samples. After conducting an additional regeneration cycle at 200°C on the samples, the XRD profiles revealed large compositions of silver oxide (Ag2O) with a small amount of silver carbonate (Ag2CO3), similar to the baseline profile.

Thermal Desorption Gas Chromatography (TD-GC)

Thermal desorption of sorbent sample-27A (SN0005) and baseline sample-01 was conducted to identify organic contaminants present in the effluent as a result of the regeneration cycle. Samples were subjected to the following GC program temperatures using helium as carrier gas: 5 min hold at 35 ºC, 35 ºC to 80 ºC at 10 ºC/min, 80 ºC to 200 ºC at 4 ºC/min, 200 ºC to 260 ºC at 10 ºC/min. This procedure resulted in a much quicker than usual desorption cycle, and the test revealed the presence of many oxygen containing compounds and nitrogen based compounds in sample-27A compared to the baseline. Several volatile organics (alkanes, alkenes, alcohols, aldehydes, cycloalkanes, aromatics, heterocyclic acids, esters, amides, etc.) and siloxane based compounds were also found in both samples in concentrations which are significantly less than the maximum allowed for all of the identified constituents.3,6 While it is possible for these contaminants to be present in the sorbent material, the mechanisms for adsorption of these compounds is not well understood. Furthermore, these contaminants did not have a significant impact in total adsorption capacity for the ground canister SN0006 as presented further in the study. Testing of canister SN0005 which has seen orbit EVA is yet to be tested.

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Scanning Electron Microscope (SEM) images with EDS analysis

Samples 09A, 09B and 09C from SN0006 were analyzed by SEM with EDS to attempt to identify any potential reduction from silver oxide to elemental silver which could have an impact on performance. The microstructure of the sorbent was found to be primarily made of relative large (~1-10um), angular crystals with significant porosity between the crystals. These crystals were identified as mostly silver and oxygen, likely representing Ag2O. Small crystallites were identified widely distributed in general, on the scale of 500nm, and often appeared to contain Ag, C and O and likely represent Ag2CO3. No indication of elemental silver was detected consistent with the previously discussed XRD work.

X-ray Photoelectron Spectroscopy Analysis (XPS)

X-Ray Photoelectron Spectroscopy Analysis (XPS) was conducted to determine the surface composition of the constituents present in the METOX sorbent and to identify any trends within the sorbent sheet. Sample -27(SN0005) was chosen because orientation and flow direction were identified during canister disassembly, and it was the only sample available at this stage of our evaluation with any ISS atmosphere exposure.

Representative samples were taken from the mid-section of a sorbent sheet and subsamples representing the flow direction during an adsorption cycle (e.g., Inlet to Center to Outlet) were selected for XPS analysis. After analysis of the surface section, some material was manually scrapped to expose the core to repeat the analysis. Figure 4 shows the sorbent samples retrieved for XPS analysis.

Before conducting XPS analysis, the samples were submitted to an argon ion cleaning procedure to remove adventitious carbon and oxygen from the surface to avoid possible increase in these two elements in the semi-quantitative analysis. Figure 5 shows the trend Figure 4. Samples for XPS analysis. plot for XPS results for the sections sampled within sample-27. Top: Surface samples. Bottom: Core exposed

Figure 5. Trend Plot of XPS results-Sample-27 Surface versus Core

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The XPS results in Figure 5 revealed a clear trend in increasing concentrations of both alkali metal salts moving from Inlet side to Outlet side. The increase was more pronounced for surface samples than for samples taken from the sorbent’s core. This would be expected as the bulk of the reactions and mass transfer is occurring at the surfaces closest to the process air stream.

Silver distribution was relatively uniform across the samples tested which is expected given the relatively insoluble nature of silver compounds. Oxygen, since primarily associated with silver oxide (Ag2O), was also expected to be uniformly distributed in the material as confirmed by the XPS data.

Carbon, inherently associated with one of the alkali metal carbonate in the system, was expected to trend similarly. However, the carbon data might not be accurate due to issues associated with the calibration of the analytical instrumentation, possible background CO2 and suspected incomplete desorption of the material. However, this does not affect the observed trend in alkali salts composition discussed above.

To further support the XPS findings, we devised a simple pH measurement test to validate the observed differences, since the pH of one of the alkali metal salts is considerably higher than the balance of the other sorbent constituents. This approach is discussed further in the next section.

C. Sorbent pH Testing

The reaction mechanisms for the adsorption of carbon dioxide (CO2) by silver oxide (Ag2O) assume that the rate determining step is the reaction of CO2 with an aqueous phase hydroxide ion. Thus, maintaining a highly basic pH by using alkali metal carbonate in the paste formulation is crucial to obtaining adequate reaction rates of CO2 with the sorbent. To determine any potential changes in pH within different locations a sorbent sheet, testing of several sorbent samples was conducted. Sorbent sheets representative of sections closer to both header ports of the canisters SN0005 and SN0006 were evaluated. Samples from SN0005 were confirmed to have been regenerated, whereas samples from SN0006 were evaluated in the “as available” condition. The prepared Baseline Sample-01 Figure 7. Sample Location and Preparation (regenerated, uncycled) was also tested. for pH Testing Approximately ¼ inch subsamples representing Inlet, Center and Outlet sections of each sorbent sheet (in theadsorption flow direction) were dissolved in DI water for pH testing. Figure 7 shows the location where subsamples were taken and a photo of sample preparation.

The volume of water was adjusted to keep the ratio of water to sorbent mass constant in all samples. In theory, this would have resulted in equal concentration for soluble constituents if the sorbent was still uniform. Figure 8 shows the data trends with respect to location within the canister and specific sections within the sorbent sheet. Table 4 summarizes the pH results.

Sorbent from SN0006 and SN0005 showed pH values increasing in the direction of the adsorption flow from Inlet to Outlet edge independently of the section within the canister (e.g., sorbent closer to Inlet/Outlet header ports). The prepared baseline material presented a pH of 10.3.

Due to the logarithmic pH scale, the measured pH values in various locations of the sheets (SN0005 in particular) correlate to a two-fold shift in concentrations between the Inlet and Outlet edges of the sorbent sheet. This could be a result of redistribution of the alkali metal salts within the sheets.

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Figure 8. pH trend within sorbent sections

Table 4. Summary of pH results Sample Subsample pH Sample Subsample pH Section Section SN0005 Inlet 10.57 SN0005 Inlet 10.12 Closer to Outlet Closer to Inlet Header Header SN0005 Center 10.61 SN0005 Center 10.31 Closer to Outlet Closer to Inlet Header Header SN0005 Outlet 10.71 SN0005 Outlet 10.37 Closer to Outlet Closer to Inlet Header Header SN0006 Inlet 10.74 SN0006 Inlet 10.11 Closer to Outlet Closer to Inlet Header Header SN0006 Center 10.67 SN0006 Center 10.30 Closer to Outlet Closer to Inlet Header Header SN0006 Outlet 10.72 SN0006 Outlet 10.37 Closer to Outlet Closer to Inlet Header Header

D. Potential Sorbent Constituents re-distribution Causes

There are a couple of potential hypotheses identified that might be causing this redistribution of the water soluble salts over time:

1.While water is required for the reaction with CO2, the water adsorption wave progresses through the bed much quicker than the CO2, therefore the bed is essentially saturated with water for much of the EVA duration. As the crewmember’s metabolic rate changes, the LCVG set point temperature may be increased or decreased as desired for crew comfort. At high metabolic rates, the crewmember may decrease the LCVG temperature to increase comfort. Since water vapor generation is highest at the high metabolic rates, the reduction in LCVG may result in condensation at the LCVG cooling tubes.

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When the metabolic rate is then reduced following this potential condensation event, the crewmember starts to experience evaporative cooling as the condensed water on the LCVG begins to evaporate. To offset this cooling effect, the LCVG temperature may be increased for crew comfort, resulting in an increased dew point as more water is evaporated. At this point, the METOX canister will see a wave of increased water vapor, which will affect the water loading of the hygroscopic components of the sorbent. With a very complex equilibrium due to the different reactions with CO2 and water, as well as the different metabolic profiles for different crewmembers and missions is feasible to envision potential migration of water-soluble compounds over time.

2. During the sorbent regeneration process, the flow is reversed so that the hot regeneration air enters the outlet of the canister. As the thermal wave progresses through the bed, one can envison an evaporation front whereby water is diffusing towards the front, essentially dragging any water soluble compounds with it. Due to the high solubility of the alkali salts, they would tend to diffuse towards the evaporation front, resulting in redistribution. Due to the considerable differences in temperature and relative humidity during regeneration and normal EVA use, the process is most likely not completely reversible, resulting in the observed differences in composition between inlet and outlet edges of a sorbent sheet.

Migration of the water-soluble constituents would cause a reduction of the reaction kinetics due to maldistribution of the salts within the sorbent structure; however the equilibrium capacity of the sorbent should remain about the same. To determine if the aforementioned sorbent changes result in changes in chemical performance, a subscale test was run to investigate potential changes in the rate of reaction and equilibrium loading, with cycling, and compared to a baseline sorbent sample.

E. CO2 Removal Subscale Testing

Subscale testing was conducted to estimate CO2 equilibrium capacity and determine any potential changes in reaction kinetics due to this salt migration theory. Four (4) sorbent sheets were extracted from canister SN0006 and loaded in a mini-METOX test reactor shown in Figure 9, following the usual stack configuration. Canister SN0006 has not been exposed to ISS on orbit conditions. However, this canister has been used in several ground manned vacuum chamber tests and therefore, may have potentially been exposed to some metabolic contaminants that might have some impact on performance.

Additionally, sorbent samples from this canister have shown that the apparent pH has shifted between Inlet and Outlet potentially as a result of the slow redistribution of salts. Therefore, there is still value in testing the performance of the sorbent from this canister and comparing it to a baseline sample. A baseline stack with four (4) Metox sheets was prepared as defined by Engineering Change 17D144 titled “ Define a New Metal Oxide Container Configuration with New Sorbent Materials” and which created the SV821799-04-00 configuration of Metal Oxide Container using legacy PTFE material. Figure 9. Mini Metox test article

Typical EMU flow conditions (6 ACFM or 170 ALPM) and residence time of 1.33 seconds were scaled with an initial metabolic rate equivalent to 1000 BTU/hr equivalent to 3.8 mmHg Inlet partial pressure of CO2. Single pass adsorption cycles were run at ambient pressure and temperature with an Inlet dew point of approximately 19oC. Figure 10 shows the METOX Subscale test rig schematic.

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RIG AEL01 P1 T1 Humidifier Portable T206 Logic Controller Metox Subscale DP1 Reactor N2 FM-1 CA1 F T204

Humidifier CO2 FM-2 Bypass BYPASS F (1% cal gas or 100% process gas) DP2 CA2 T2 SCROLL PUMP VENT or LAB VACUUM V-1 Figure 10. METOX Subscale test rig AEL 001 schematic

The effluent CO2 concentration was continuously recorded with a HORIBA VIA-510 IR gas analyzer. Data collection was achieved through a Labview-based data-acquisition system Adsorption cycles were conducted until >96% breakthrough was achieved. Regeneration of the subscale test bed was carried out for six hours with nitrogen at 30 alpm and 218oC Inlet gas temperature followed by a 2 hr-cooling time until the outlet temperature reached 25 oC. The weight of the reactor was recorded before the adsorption cycle and after the regeneration cycle to ensure complete regeneration.

The sorbent mass present in both stacks (Baseline and SN0006) was approximately the same, resulting in an expected theoretical CO2 adsorption capacity of 55.05g CO2. The CO2 and H2O breakthrough curves (BTC) are shown in Figures 11,12, and 13. Table 5 summarizes the loadings per sorbent mass at 50% and 96% BT, and Figure 14 compiles the CO2 and H2O total loadings normalized per sorbent mass. Each adsorption cycle was conducted for 45 hours to reach equilibrium ( >96% breakthrough). At the end of the cycle, the test article was weighed to estimate total CO2 and H2O loadings. These values were compared then to the loadings obtained via data integration. The difference was typically less than 1.5%.

The BT curves for SN0006 and Baseline did not show a significant difference in CO2 removal capacity. In fact, at the 50% BT level (Outlet ppCO2=1.9mmHg), the difference in CO2 removal capacity between the Baseline and SN0006 is only 5% (38.06 g CO2 vs. 36 g CO2 respectively). This difference was only 4% at the 96% BT( Outlet ppCO2=3.43mmHg): 52.67g CO2 vs. 50.96g CO2. These results are in alignment with the amount of degradation in performance seen during canister certification testing (7.5% degradation over 100 cycles).5 The removal efficiency based on the expected capacity of 55.05 g CO2, was 92% for SN 0006 and 96% for the Baseline.

A reaction rate constant (k), for CO2 reaction was also Figure 11. CO2 Breakthrough curves. Scaled estimated based on a model developed for single pass 7 test flow 7.45alpm, 3.8mmHg ppCO2 inlet, 19 adsorption testing in the early METOX development work. o C Inlet dew point, Ambient P,T

The values obtained for SN0006 and Baseline were k= 2.06 and 2.17 sec-1, respectively. These values and the slopes of the CO2 BT curves show that the rate of reaction during the early part of the operation, is very similar with slight differences towards the end of the cycle. This trend can be observed clearly in Figure 14.

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Figure 12. H2O Breakthrough curves. Scaled test Figure 13. H2O Breakthrough curves, showing o flow 7.45alpm, 3.8mmHg ppCO2 Inlet, 19 C Inlet up to 10 hours of test time. Scaled test flow o dew point, Ambient P,T 7.45alpm, 3.8mmHg ppCO2 Inlet, 19 C Inlet dew point, Ambient P,T

Figure 12 shows the total H2O BT curves for the Baseline and SN0006, and Figure 13 shows the first 10 hours of test for easy comparison. The baseline stack adsorbed 33.14g H2O at >96% BT (Outlet ppCO2=1.9 mmHg), while SN0006 stack adsorbed only 26.48 at the same 96% BT level.

The difference in water loading between the two stacks (25% difference at 50% BT and 22 % difference at 96% BT) is unlikely to be the result of redistribution of salts, but more likely due to slight differences in dew points during the adsorption cycle, or slight error in dew point sensor readings. Since the the reported values are integrated using values from two dew point sensors this could explain the differences in calculated water loadings. Another potential influence on the calculated water loadings is due to running the test at near saturation temperatures as slight changes in ambient temperature can result in a change in water adsorption characteristics.

Figure 14. Compiled CO2 and H2O Loadings per sorbent mass. SN0006 vs Baseline

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Table 5. CO2 and H2O loadings per mass of sorbent at 50 % and 96% Breakthrough (BT) Reaction # Sorbent CO loading (grs CO grs CO loading (grs CO grs 2 2/ 2 2 / rate sheets/total sorbent) sorbent) constant

METOX Sorbent at 50% BT (1.9mmHg -1 TEST at 96% BT (3.6 mmHg ppCO2) k sec Reagents mass (grs) ppCO2)

SV821799 SN0006 4/289.8 0.121 (at time=11.87 hrs) 0.174 (at time=44.79 hrs) 2.06 -03-00 SV821799 Baseline 4/289.0 0.132 (at time=12.29 hrs) 0.182 (at time=44.79 hrs) 2.17 -04-00 # Sorbent H O loading (grs H O/grs 2 2 H O loading (grs H O/ grs sorbent) sheets/total sorbent) 2 2 METOX Sorbent at 50% BT (1.9mmHg TEST at 96%BT (3.6 mmHg ppCO2) Reagents mass (grs) ppCO2)

SV821799 SN0006 4/289.8 0.073 (at time=11.87 hrs) 0.091(at time=44.79 hrs) -03-00 SV821799 Baseline 4/289.0 0.095 (at time=12.29 hrs) 0.115 (at time=44.79 hrs) -04-00

IV. Conclusions The objective of this study was to investigate the potential degradation mechanisms for the sorbent used in NASA’s Metal Oxide (METOX) canisters. Based on SEM/EDS, XRD and XPS analysis there does not appear to be any significant changes in morphology, structure or composition between a ground- prepared baseline and samples from canister (s) used in simulated ground EVAs. Results from SEM analysis with EDS show (and confirmed by XRD) the absence of elemental silver (Ag) across multiple samples, which suggests the absence of silver oxide (Ag2O) thermal decomposition.

While the samples from on-orbit canisters have been insufficient to date to adequately investigate on-orbit poisoning, based on the TD-GCMS results there is no significant foreign contamination or material changes in morphology that would indicate a diffusion limiting phenomena. However XPS and pH results show that there is a potential indication of alkali salt migration from Inlet side to Outlet side of a sorbent sheet which may impact the performance. The increase is more pronounced for surface samples than for samples taken from the sorbent’s core. Which would be expected as the bulk of the reactions and mass transfer is occurring at the surfaces closest to the process air stream.

Equilibrium loading data obtained with canister SN0006 (ground EVAs) and prepared baseline show similar kinetics and CO2 adsorption capacity but comparatively less H2O loading for SN0006. While the differences in observed water loading could be attributable to slight differences/errors in dew points sensor readings, the amount of degradation in CO2 performance observed for SN0006 is the same order of magnitude of that seen during the certification testing done 20 years ago. Canisters from orbit could not be tested due to schedule constraints, but should be included in future work to provide a more accurate comparison of the data, and more thorough investigation into the potential impact of on-orbit gas phase contaminants. Additionally, sub-ambient testing may indicate a performance shift that wouldn’t be seen at ambient pressures since one functions of the added alkali salts is to maintain sufficient water loading at reduced pressures.

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Future work

In order to fully understand the sorbent degradation mechanisms, future work should include testing of:

1. CO2 equilibrium loading capacity with sorbent samples from used on orbit canister SN0005 using Mini METOX test fixture.

2. Test kinetics and equilibrium performance of a Metox sorbent stack prepared with modified sorbent paste containing different molar concentrations of alkali metal salt within each sheet-prepared to simulate a redristribution of salts-. After equilibrium capacity testing, verification of pH, and XPS analysis are recommended.

3. Conduct Acceptance Test, AT, on an available “used on orbit” canister prior to disassembly. Compare this data to previous available data. If necessary, include sub-atmospheric testing to further investigate the impact of metal alkali halide migration.

4. Dissassemble canister properly and extract 4 sorbent sheets to assemble into the small mini METOX fixture. Conduct equilibrium capacity tests, pH tests, XPS and other pertinent analysis.

5. Add the above additional data results to the current study.

Acknowledgements

The authors would like to give special thanks to Tim Nalette for his excellent technical support and guidance throughout this study.

References

1 Allen, G., Baker, G., Nalette, T., Mankin, M., et al. “Performance Characteristics of the Regenerable CO2 Removal System for the NASA EMU” SAE Technical Paper 1999-01-1997, 1999, https://doi.org/10.4271/1999-01-1997 2 Peter, B., Westheimer, D“EMU METOX Performance Testing” ICES Paper 2018-180 3 SVHSER18044 Qualification Report DRL#33 for ITEM METOX/200 CANISTER HS PART NUMBER SV821799-1, April 1998. 4 Nalette, T., “Metal Oxide Trace Contaminat Considerations- Canister and Regeneration System” Analysis Memo#95-078 5 SVHSER19435Addendum to Qualification Report DRL#33 for the ITEM METOX/2000 CANISTER HS PART NUMBER SV821799-1, October 1998. 6 NASA WHITE SANDS TEST FACILITY,METOX canister regeneration effluent characterization, WSTF#02-37316, January 2003 7 UTC Hamilton Standard Division” Metal Oxide Technology Review and Concept Selection Analysis”, September 1987 8 P. A. Barnes, M. F. O'Connor. and F. S. Stone. “Reactivity of Silver Oxide in the Absorption of Carbon Dioxide” Inorg. Phys. Theor., J. Chem. Soc. (A), p. 3395, 1971. 9 J.R.Aylward. C. R.Russell, S. Russell and J.I. Smith “Research and Development Program for a Combined Carbon Dioxide Removal and Reduction System” CR-111885 SVHSER 5863 May 1971. U.S. Government Agencies and Contractors Only. Chem. Soc. (A), p. 3395, 1971.

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