CO removal from spacecraft gas streams was evaluated for three commercial, low temperature oxidation catalysts: Carulite 300, Sofnocat 423, and Hamilton Sundstrand Pt1. The catalysts were challenged with CO concentrations (1-100 ppm) under dry and wet (50% humidity) conditions using 2-3 % O2. CO removal and CO2 concentration were measured at constant feed composition using a FTIR. Water vapor affected the CO conversion of each catalyst differently. An initial screening found that Caulite 300 could not operate in humid conditions. The presence of water vapor affected CO conversion of Sofnocat 423 for challenge concentrations below 40 ppm. The conversion of CO by Sofnocat 423 was 80% at CO concentrations greater than 40 ppm under both dry and moist conditions. The HS Pt1 catalyst exhibited CO conversion levels of 100% under both dry and moist conditions

Monje, Oscar

NASA Technical Reports Server

45th International Conference on Environmental Systems ICES-2015-[insert submission number] 
12-16 July 2015, Bellevue, Washington 
Evaluation of Low Temperature CO Removal Catalysts 
 
Oscar Monje1 
Air Revitalization Lab, Kennedy Space Center, FL, 32821 
CO removal from spacecraft gas streams was evaluated for three commercial, low 
temperature oxidation catalysts: Carulite 300, Sofnocat 423, and Hamilton Sundstrand Pt1. 
The catalysts were challenged with CO concentrations (1-100 ppm) under dry and wet (50% 
humidity) conditions using 2-3 % O2. CO removal and CO2 concentration were measured at 
constant feed composition using a FTIR. Water vapor affected the CO conversion of each 
catalyst differently. An initial screening found that Caulite 300 could not operate in humid 
conditions. The presence of water vapor affected CO conversion of Sofnocat 423 for 
challenge concentrations below 40 ppm. The conversion of CO by Sofnocat 423 was 80% at 
CO concentrations greater than 40 ppm under both dry and moist conditions.  The HS Pt1 
catalyst exhibited CO conversion levels of 100% under both dry and moist conditions. 
Nomenclature 
ATCO = Ambient Temperature Catalytic Oxidizer 
HS = Hamilton Sundstrand 
HTCO = high temperature catalytic oxidizer 
ISS = International Space Station 
TCCS = trace contaminant control system 
RVCS = Regenerable VOC Control System 
 
I. Introduction 
Arbon monoxide is colorless, odorless, tasteless gas that is very difficult for people to detect. It is harmful to 
humans and symptoms of mild acute poisoning include light-headedness, confusion, headaches, vertigo, and 
flu-like effects; larger exposures can lead to significant toxicity of the central nervous system and heart, and even 
death. The NASA 7-day SMAC is set at 55 ppm and the 30-day and 180-day SMACs are set at 15 ppm.   
 In Shuttle, carbon monoxide was removed from cabin air using the Ambient Temperature Catalytic Oxidizer 
(ATCO) catalyst (2% platinum on carbon)3,4. The on-board trace contaminant control system (TCCS) for the 
International Space Station (ISS) U.S. On-orbit Segment consists of a granular activated carbon bed for the removal 
of high molecular weight contaminants and ammonia followed by a heated catalytic bed for low molecular weight 
hydrocarbons. The high temperature catalytic oxidizer (HTCO) of the TCCS removes low molecular weight 
compounds such as carbon monoxide (CO), formaldehyde (CH2O), and methane (CH4) operates at 400°C, requiring 
significant energy input1, 2. Heating the HTCO represents a considerable amount of power, which could be reduced 
if low temperature CO removal catalysts were identified. 
 In this study, three low temperature oxidation catalysts for removal of CO from spacecraft gas streams were 
evaluated for operation at 40% humidity. These catalysts have been previously studied for CO removal, however, 
experiments varied greatly for [CO], flow rates, and humidity levels making comparisons of the technology difficult. 
Low temperature oxidation catalysts may result in power savings to the TCCS, and may also be used in with post-
fire cleanup of cabin air3.     
II. Materials and Methods 
Three candidate low temperature CO removal catalysts were used in this study: 
 
• Carulite® 300, developed by Carus Corporation; it is a dark brown/black manganese dioxide/copper oxide 
catalyst with a bulk density of 0.8-0.9 g ml-1 and surface area of approximately 200 m2 g-1.  General applications 
include destruction of CO in compressed breathing air, respirators, escape masks, and in cryogenic gas purification.     
                                                          
1 Research Scientist, Air Revitalizayion Lab, and ESC-24, Kennedy Space Ceneter, Fl 32899. 
C 
https://ntrs.nasa.gov/search.jsp?R=20150022484 2019-08-31T05:02:36+00:00Z
 International Conference on Environmental Systems 
 
 
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• SofnocatTM 423, developed by Molecular Products, Inc.  SofonocatTM 423 is a platinum/palladium/tin 
oxide catalyst with a bulk density of 1.4-1.6 g ml-1 and is claimed to have high water tolerance for applications 
including destruction of CO in breathing gas filters, removal of hydrogen within enclosed environments, abatement 
of volatile organic compounds (VOCs), and gas purification.   
• HS Pt1, a CO oxidation catalyst developed by Hamilton Sundstrand. 
 
 
 
Figure 1. Three low temperature oxidation catalysts for removal of CO from spacecraft gas streams were 
evaluated for operation at 40% humidity. 
 
The candidate low temperature CO revoval catalysts differed in their composition ( Table 1) and each catalyst 
had its own particle size distribution (Table 2). The size of the particles is important as it may affect the flow of CO 
molecules into the active catalytic sites within the catalyst particles. The catalysts were sieved using several sieves 
(3.35 mm, 2.36 mm, 2 mm, 1.4 mm, 1 mm, 0.47 mm, and a bin to collect the <0.47 mm fraction. 
 
 
Table 1: Catalyst Characteristics 
Catalyst Manufacturer Active Metal Support 
Carulite® 300 Carus Corporation. MnO2/CuO Al2O3 
SofnocatTM 423 Molecular Products, Inc. Pt, Pd SnO2 
HSPt Hamilton Sundstrand Pt NA 
 
Particle size distribution was measured by sieving the catalysts. The particles were sieved using several sieves 
(3.35 mm, 2.36 mm, 2 mm, 1.4 mm, 1 mm, 0.47 mm, and a bin to collect the <0.47 mm fraction. 
 
Table 2: Catalyst Particle Size Distributions 
 HS Pt Sofnocat 423 Carulite 300 
3.35 mm  21.6 %  
2.36 mm  67.4 % 18.5 % 
2 mm  4.8 % 50.5 % 
1.4 mm 1.3 % 5.2 % 35.1 % 
1  mm 78.7 % 1.3 % 0.36 % 
0.47 mm 20.1 %   
< 0.47 mm 0.35 %   
 
 
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The CO catalysts were challenged in single pass mode with moist gas streams produced by blending 400 ppm of 
CO into a gas stream of breathing air, which could be humidified or not. The gas stream passed through the CO 
catalyst and was analyzed using a Gasmet DX-3030 FTIR (Fig 2).  
 
The apparatus used received close scrutiny by safety personnel. The tests were conducted only after the 
following CO Hazard Mitigation scheme was implemented: 
 
• Perform a system leak test prior to use 
• Contain CO within a cage vented via a snorkel 
• Deploy a calibrated CO meter with an alarm 
• Only utilize dilute concentrations of CO 
• Use the buddy system during the tests. 
 
 
 
CO 
Catalyst
 
Figure 2. Apparatus to measure CO removal 
 
Oxidation Catalyst Comparison 
 
The catalysts (~2-4g) were loaded onto a ½ inch-diameter sorbent bed of the Regenerable VOC Control System 
(RVCS) testbed. CO gas streams were mixed using a tank of 400 ppm CO with balance N2, and breathing to provide 
O2. This approach provided ~2-3 % O2 to the gas stream. The challenge gas stream was passed through the bed in a 
single pass mode and the CO concentration of the post-chamber air was analyzed using a Gasmet FTIR. CO 
concentration s from 100ppm down to < 1 ppm were used. The catalyst were evaluated using: Relative Humidity: 
0%, 40-50%,, Temperature: 23 °C, Flow rate of gas stream (L/min): 1500 - 1600 mL min-1. 
 
The oxygen excess with respect to the amount of oxygen required for the oxidation of CO to CO2 is commonly 
characterized by the process parameter, λ (Kalich et al, 1997): 
 
λ = 2[O2]/[CO] = 2PO2/PCO          (1) 
 
where¸ λ =1 is sufficient to provide for the complete oxidation of CO to CO2 in the absence of an oxygen-
consuming side reactions. The conditions used in this study ensured that λ ranged from 200 to 8000.  
The conversion of CO was defined as follows: 
 
XCO (%) = 100*( [CO]in − [CO]out ) /  [CO]in  (2) 
 
III. Results and Discussion 
 
An initial assessment of the performance of these three catalysts was conducted. This initial comparison of CO 
removal was conducted using 25 ppm CO and 40% relative humidity. The results of this effort identified that both 
Sofnocat and HS Pt performed well under humid conditions, but Carulite 300 did not (Fig 3). 
 
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Figure 3. Initial comparison of CO removal from moist gas stream by the 3 candidate catalysts. Sofnocat 423 
removed 80% and HS Pt1 removed 100%, but CO removal of Carulite 300 was only temporary.   
 
Oxidation of CO on Pt catalysts 
The CO oxidation reaction is generally believed to be described by a Langmuir-Hinshelwood mechanism of the 
following type for conditions where CO is the primary surface species:  
CO (g) ↔ CO (ads)         (3) 
O2 (g) ↔ 2 O (ads)         (4) 
CO (ads) + O (ads)    CO2 (ads)    CO2 (g)  (5) 
The reaction depends on the adsorption of CO and oxygen onto the surface of the catalyst. The reaction of adsorbed 
CO occurs with adsorbed O and release of produced CO2 is a fast reaction. Langmuir has described the reaction 
kinetics as first-order in O2 pressure and negative-first-order with respect to the CO pressure. The rate of the reaction 
is governed by desorption of CO or the lifetime of CO on the surface, depending on the reaction temperature, 
whereas the pressure dependence simply reflects the competition for adsorption sites between O2 and CO. 
 
HS Pt1 Catalyst 
 The low temperature CO oxidation capacity of the HS Pt1 catalyst was evaluated using several CO 
concentrations ranging between 0.5 to 140 ppm under both dry and moist (50% relative humidity) conditions. In 
each experiment, CO conversion (XCO) was measured at constant feed composition. The conversion of CO by HS 
PT1 was 100% under both dry and moist conditions (Fig 4).  
CO2 production occurs as soon as the CO stream surrounds the catalyst (Fig 5). A spike in CO2 concentration, much 
larger than would be expected from kinetics of CO oxidation, is observed the 1st time the catalyst is used. The 
second time the catalyst is exposed to CO, a smaller spike in CO2 concentration is observed and no spikes are 
observed in subsequent runs. This suggests that the spike occurs as CO2 adsorbed on the surface of the catalyst 
during storage is displaced by CO and becomes adsorbed (Equation 3). The CO2 produced by the oxidation of CO 
persists until CO is removed from the catalyst. The amount of CO2 produced during CO oxidation increases in 
linear fashion as the inlet CO concentration increases (Fig 6). 
 
 
Figure 4. The HS Pt1 catalyst exhibited CO conversion levels of 100% under both dry and moist (50% relative 
humidity) conditions. 
 
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Figure 5. The CO2 concentration rises as soon as CO surrounds the catalyst. The initial spike in CO2 concentration 
upon 1st exposure is probably caused by CO displacing CO2 already adsorbed on the catalyst surface. 
 
  
 
Figure 6. The amount of CO2 produced during the oxidation of CO rises linearly as the inlet CO concentration 
increases. The amount of CO2 is greater than the 1:1 line, probably due to the release of previously adsorbed CO2 
from the surface of the catalyst. 
 
  
Sofnocat 423 
The low temperature CO oxidation capacity of the Sofnocat 423 catalyst was evaluated using several CO 
concentrations ranging between 0.5 to 130 ppm under both dry and moist (50% relative humidity) conditions. In 
each experiment, CO conversion (XCO) was measured at constant feed composition. The conversion of CO by 
Sofnocat 423 was 80% at CO concentrations greater than 40 ppm under both dry and moist conditions (Fig 7).  The 
CO conversion of dry Sofnocat 423 decreased from 100% down to 80% as CO concentration was increased and it 
illustrates the negative-first-order kinetics with respect to CO pressure. The oxygen and carbon monoxide molecules 
in the surrounding gas compete with each other in reaching vacant spaces on the surface of the catalyst. If a space 
becomes filled with an oxygen molecule, this molecule (or each of the resulting oxygen atoms) can react with the 
carbon monoxide molecules which strike it and thus form carbon dioxide, which will be held by weaker chemical 
forces and be released rapidly. This process leaves the space still vacant so that another oxygen molecule can be 
adsorbed and a carbon dioxide molecule formed. This action continues until the space becomes filled by a carbon 
monoxide molecule. Under these conditions, the velocity of the reaction will be proportional to the pressure of 
oxygen and inversely proportional to that of the carbon monoxide. 
The effect of water vapor in reducing the rate of catalytic oxidation of CO may be accounted for in either of two 
ways: by assuming that the water vapor reduces the activity of the catalyst or that, by reversible adsorption, water 
molecules occupy active sites on the surface of the catalyst. It appears that the effect of water vapor at low CO 
concentrations is to compete with CO and O2 for active sites on the catalyst surface (Fig 7), but that this effect 
diminishes above 40 ppm CO. 
The concentration of CO2 produced during CO oxidation increased linearly as CO concentration increased (Fig 8). 
However, the concentration of CO2 produced at low CO concentrations falls below the 1:1 line thereby reflects the 
reduction in CO oxidation caused by water vapor.  
 
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Figure 7. In dry Sofnocat 423, CO oxidation is reduced as CO outcompetes oxygen for sites on the catalyst surface. 
In humid Sofnocat 423, the water vapor may outcompete CO at low CO. The effect of water vapor is diminished 
above 40 ppm CO. 
 
 
Figure 8. The CO2 concentration produced increased linearly as CO concentration increased in dry Sofnocat 423. In 
wet conditions, the CO2 produced fell below the 1:1 line indicating that water vapor may be responsible for reducing 
the rates of CO oxidation at CO concentrations below 40 ppm. 
 
Future Work 
The CO oxidation of Sofnocat 423 should be compared at a smaller particle size. Nearly 67% of the Sofnocat 423 
used in this study was composed of particles >23.6 mm. In contrast, the 70% of the HS Pt1 catalyst was composed 
of 1 mm particles. The Sofnocat 423 capacity should be evaluated after grinding it down to 1 mm particles to see if 
the 20% lower CO conversion of Sofnocat 423 vs Hs Pt1 is caused by mass transfer limitations due to its larger 
particle size. 
Acknowledgments 
This study was funded by the Atmosphere Resource Recovery & Environmental Monitoring (ARREM) Project 
of NASA’s Advanced Exploration Systems Program. 
References 
1Luna, B. P., James; Ehresmann, David; Howard, Jeanie; Salas, Louis; Mulloth, Lila; Perry, Jay, In International Conference 
on Environmental Systems; SAE International: San Francisco, CA, 2008. 
 
2Luna, B.; Somi, G.; Winchester, J.; Grose, J.; Mulloth, L.; Perry, J. Evaluation of Commercial Off-the-Shelf Sorbents & 
Catalysts for Control of Ammonia and Carbon Monoxide. In 40th International Conference on Environmental Systems; 
American Institute of Aeronautics and Astronautics: 2010. 
 
3Eldridge, C.; Nalette, T.; Graf, J.; Alptekin, G. CO Oxidation for Post-Fire Cleanup. In 41st International Conference on 
Environmental Systems; American Institute of Aeronautics and Astronautics: 2011. 
 
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4Nalette, T.; Eldridge, C.; Yu, P.; Alptekin, G.; Graf, J. Advanced Catalysts for the Ambient Temperature Oxidation of 
Carbon Monoxide and Formaldehyde. In 40th International Conference on Environmental Systems; American Institute of 
Aeronautics and Astronautics: 2010. 
 
5Carus Corporation Carulite 300 Information Page. http://www.caruscorporation.com/content.cfm/carulite-300 (April 15, 
2013). 
 
6Sofnocat 423 Technical Datasheet. http://www.molecularproducts.com/pdf/Sofnocat%20423%20UK%20DS.pdf (May 1, 
2013),  
 
 
 


Evaluation of Low Temperature CO Removal Catalysts

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