45th International Conference on Environmental Systems ICES-2015-240 12-16 July 2015, Bellevue, Washington

Scaleable, High Efficiency Microchannel Sabatier Reactor

John O. Thompson1 UMPQUA Research Company, Myrtle Creek ,OR, 97457

An innovative Microchannel Sabatier Reactor System has been developed for the efficient production of oxygen (as water) and methane from carbon dioxide (CO2), a valuable in situ resource available in the atmosphere or as frozen deposits on Mars and other Near Earth Objects, which will help mitigate mission resupply logistics and risk. The Sabatier reaction benefits from inherently superior micro reactor heat and mass transfer characteristics compared to conventional reactor designs. Significantly, graded temperature micro reactors can readily be configured in a parallel arrangement to vastly improve conversion efficiencies. Process scale-up is easily achieved by increasing the number of equivalent, parallel micro reactors. High conversion rates require the deposition of a highly active, supported catalyst layer onto microchannel walls that is characterized by enhanced surface area, appropriate adsorption characteristics, and catalyst effectiveness factor. A core focus of the research effort was the design of a MSRS that optimizes residence time, thermal recovery, and most importantly, the achievement of near equilibrium reaction product composition at low temperature. A three stage reactor consisting of a supported noble metal catalyst on stainless steel tubing and sequentially heated to 365, 294, and 234 degrees centigrade demonstrated 96% single pass conversion efficiencies of CO2 to methane for an influent gas composition of 4:1 H2:CO2.

Nomenclature MSRS = microchannel Sabatier reactor system ISRU = in situ resource utilization ESM = equivalent system mass CRA = carbon dioxide reduction assembly ESA = European Space Agency JAXA = Japanese Aerospace Exploration Agency RWGS = reverse water-gas shift PNNL = Pacific Northwest National Laboratory EISA = Evaporation Induced Self Assembly

I. Introduction Oxygen is an important oxidant for high specific energy, chemical fuels such as hydrogen and methane, and oxygen and water are arguably two of the most critical resources for life-support. As such, means to extract O2 and H2O from in situ CO2 are needed to support future Mars exploration missions. Various techniques have been proposed and evaluated for oxygen extraction from the predominantly CO2 Martian atmosphere for ISRU. These include the reduction of Martian carbon dioxide using either the Sabatier or Bosch reactions. The Sabatier reaction is given by Equation 1, in which, methane and water vapor are formed, typically in a fixed- bed catalytic reactor. Using a variety of catalysts and reactor configurations, this reaction has been studied 1- extensively for the recovery of O2 (as water via electrolysis) from CO2 for employment aboard manned spacecraft. 10 The chief limitation to this approach for ISRU applications is the requirement to supply hydrogen lost in the methane byproduct, although methane may be used as an in situ generated propellant.

1 Principal Investigator, R&D, Address: UMPQUA Research Company PO Box 609 Myrtle Creek OR 97457. Ru/Al23 O CO22 + 4H → 2H2 O + CH 4 (1) The Bosch reaction,11-15 given by Equation 2, is another well-known process allowing the recovery of oxygen from carbon dioxide by reacting it with hydrogen at elevated temperature in the presence of appropriate catalysts forming water vapor and elemental carbon (C). Fe / Co / Ni CO22 + 2H → 2H2 O + C(s) (2) The production of carbon by the Bosch reaction limits its usefulness due to degradation of reactor performance through catalyst degradation or reactor plugging. This situation results in a clear advantage to the entirely gas phase Sabatier reaction. The Sabatier reaction enables the complete recovery of O2 from CO2 via electrolysis of the H2O product. Hydrogen represents the only consumable in that only ½ of the consumed H2 is recovered by H2O electrolysis. The H2 reduction of CO2 via the Sabatier reaction results in the formation of H2O and CH4 (Equation 1). Both products of this exothermic reaction are valuable to space exploration. Methane can be used either as a propellant or as a H2 source, and the recovered H2O may be consumed pure, used to reconstitute dehydrated food, or electrolyzed to recover O2 and H2. Many heterogeneous catalysts have been thoroughly investigated for the Sabatier reaction, including a wide variety of catalytic metals and support materials. Industrial applications favor nickel (Ni), cobalt (Co), iron (Fe), and molybdenum (Mo) catalysts due to their reduced cost although these metals generally require higher temperatures to achieve reasonable kinetics.16-22 At lower temperatures involving special applications, for which expense is less important than activity, ruthenium (Ru) is primarily utilized either by itself or in combination with secondary metals such as palladium (Pd), rhodium (Rh), or platinum (Pt).23-31 To enhance catalyst performance, various materials have been utilized as catalyst supports, with titania (TiO2), alumina (Al2O3), and zirconia (ZrO2) supported catalysts being the most active.23,24,26,28,29,31 Heat removal is particularly important for the exothermic Sabatier reaction because the equilibrium conversion of CO2 drops below 100% above 100°C, and at temperatures between 400 and 500°C the equilibrium conversion drops from ~85% to 65%. This is shown in Figure 1, which illustrates the equilibrium products for the Sabatier reaction with a starting 4:1 molar ratio of H2:CO2. Maintaining precise thermal control of an exothermic reaction in a packed bed or conventional monolithic reactor is difficult due to intrinsic heat transfer limitations. These large- scale reactors can also have reduced effectiveness factors due to mass transfer limitations imposed by pore diffusion resistance and the relatively large bulk diffusion distances. Utilization of a graded temperature micro reactor will circumvent the shortcomings common in packed bed reactor systems. Micro reactors benefit from inherently high heat and mass transfer coefficients due to the exceptionally short heat and mass transfer distances.32,33 Advantages inherent in micro reactor systems have led to a dramatic increase in the investigation of their application to a wide variety of chemical processes ranging from fuel cells to hydrogen peroxide synthesis. Many of the chemistries related to hydrocarbon processing including desulfurization, petroleum cracking, biodiesel esterification and transesterification reactions, Fischer-Tropsch reactions, and syngas production, have been incorporated into micro reactors of various designs.34-37

0.5

0.4

0.3

0.2

H2

Species Quantity (kmoles)Species Quantity H2O

0.1 CH4

CO2 CO 0.0 0.0 100.0 200.0 300.0 400.0 500.0 600.0 Temperature (oC) Figure 1 Equilibrium Product Distribution Versus Temperature at a 4:1 H2:CO2 Ratio.

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As previously mentioned, the Sabatier reaction is particularly well suited to a micro reactor because of the importance of temperature control. The thermodynamic data show that the equilibrium conversion declines from 100% at temperatures above ~100°C. The heat of reaction released during CO2 reduction will result in a significantly lower CO2 conversion if thermal control is not maintained and temperature increases. As the temperature rises, equilibrium increasingly favors secondary products such as CO. Unfortunately, the reaction kinetics are too slow to make operation at 100°C practical, and in fact, conversion rates do not approach equilibrium until approximately 400°C, depending on reactor activity and space velocities.23,24,26,29 This implies that conversion cannot exceed 85% in a single pass reactor operated above 400°C. The equilibrium may be shifted toward 100% by 38 increasing the H2 to CO2 ratio, but doing so reduces the savings to ESM because more H2 passes downstream and must be recovered or lost. In fact, the design for the Hamilton-Sundstrand developed CRA Sabatier reactor specifies 31,39 a 3.5:1 molar ratio of H2:CO2 to ensure maximum utilization of H2. This trade off between kinetics and equilibrium highlights the flexibility advantage of micro reactor systems, as a graded temperature micro reactor can operate with temperature profiles that maximize product yield and selectivity. In addition, a micro reactor array can be configured to provide the required process capacity by numbering up the system to meet process requirements. For example, a micro reactor with a channel cross-section of 250 µm by 1000 µm that consists of four U-shaped channels per layer and has ~30 layers has an internal volume of ~1.25 cm3. If this reactor were to see 2.0 L/min of CO2 mixed with H2 at a 1:4 molar ratio, the resultant residence time would be ~10 ms, which is insufficient for acceptable conversions at 400°C. A micro reactor array consisting of 40 parallel units would increase the residence time to 400 ms and enable the reaction to achieve equilibrium according to published micro reactor work.24,26,29 However, as previously indicated, the yield under these conditions is only 85%. A graded temperature micro reactor provides an alternative approach to overcoming these conversion limits by taking advantage of favorable high temperature kinetics in the first portion of the reactor and more favorable equilibria in the cooler length of the reactor. For example, if the CO2 flow rate is 1 mol/min at the reactor inlet, and reaction kinetics at 400°C are such that equilibrium is reached, then the outlet CO2 flow rate will be 0.15 mol/min. However, as the reactor temperature is reduced to 300°C, the equilibrium conversion improves to ~93%; resulting in an outlet CO2 flow of 0.07mol/min provided the contact time is sufficiently long. As the gas continues through the graded temperature micro reactor an exit temperature of 200°C could be achieved with nearly 100% conversion at equilibrium. Although reduction reaction kinetics diminish as temperature is reduced, significantly less reactant remains that must be consumed in the remaining contact time. This graded temperature configuration takes advantage of the typical exponential increase in reaction rate with temperature, and precludes the necessity of operating the micro reactors with a stoichiometric excess of H2. In addition to design flexibility, the MSRS concept dramatically enhances the reliability and safety of the reactor system. With a number of reactors arranged in parallel, one individual reactor can fail without compromising a significant fraction of the overall system performance, which facilitates the fulfillment of the safety requirements specified in the Human-Rating Requirements for Space Systems document.40 The benefit to long-term space habitation delivered by an efficient MSRS is sufficient that multiple designs have been developed by or with support from each of the world’s principal aerospace agencies including NASA, the ESA, the JAXA, and Russia.41 The NASA system developed at Hamilton Sundstrand has a reported 95% efficiency.42 However, the Hamilton-Sundstrand Sabatier reactor operates with a 3.5:1 molar ratio of H2:CO2 that is designed to 31,39 maximize H2 utilization and eliminate H2 from the effluent gas stream. This lean H2 reactant mixture increases the H2 to H2O conversion efficiency to 95% while the CO2 to CH4 conversion efficiency hovers between 60 and 42 70%. These Sabatier reactors contain packed beds of noble metal catalysts supported on alumina (Al2O3) substrates.43 Systems designed by other agencies are similar in both performance and design.41,44 The creation of a graded temperature micro reactor CO2 reduction system is directed at three possible areas of improvement over the traditional Sabatier packed bed reactor. First, micro reactor system design permits integration of reactor, temperature grading, and gas distribution components, which results in a compact form factor and subsequently reduced Equivalent System Mass (ESM). Second, current packed bed catalysts suffer from the production of fine particles when subjected to launch vibration and thermal cycling. In contrast, the micro reactor catalyst is supported by a thin film bonded to the microchannel walls. In comparison to catalyst supported on particles, these films are resistant to fracture induced by thermal cycling.45 The lack of close packed particles should further reduce vibration induced fines. The ability to preclude catalyst attrition will improve system reliability and reduce performance degradation due to loss of catalyst. Third, as each graded temperature reactor includes its own heat source, utilization of a micro reactor array introduces a level of design flexibility that is unachievable with macro-scale reactor systems.

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An example of a micro reactor system with extended capabilities is given by a previous NASA funded project undertaken by Pacific Northwest National Laboratory (PNNL).46 In this approach, a reticulated, metallic foam, and monolith was washcoated with ruthenium alumina catalyst particles forming a quasi-micro reactor consisting of multiple parallel microchannels. In this micro reactor system, the reverse water gas shift (RWGS) reaction shown in equation 3 was run countercurrent to the Sabatier reaction (1).

CO22 + H ƒ CO + H2 O ΔH350o C = 38.74 kJ/mole (3)

Using this reaction scheme, the exothermic Sabatier reaction (∆H = -179.13 kJ/mol) at 350°C provides heat to drive the endothermic RWGS reaction. By carefully balancing the quantity of CO2 provided to each micro reactor the capacity for in situ propellant production from Martian CO2 was maximized. Carbon dioxide conversions between 52 and 78% were achieved using this arrangement. Improvements were limited by the requirements of the RWGS reaction. The selected catalyst also suffered from deactivation over a period of a few experiments. The work done at PNNL demonstrated the potential for a microscale Sabatier reactor system, but the target application and the integrated approach prevented development of a CO2 reduction system that is relevant to in situ resource utilization. The apparent shortcomings in this approach were simultaneous control of the reaction temperature in each reactor as well as loss of catalyst activity. The three-stage micro reactor technology in our study features stainless steel micro reactors with the catalyst/support bonded directly to the microchannel walls. This arrangement significantly contrasts with the catalyst washcoat on metallic foam design of the PNNL study, and results in improved and independent thermal control of each micro reactor as well as preventing catalyst activity loss due to poorly controlled temperature excursions, which tend to sinter and deactivate the catalyst. Utilization of staged micro reactors also provides the opportunity to develop a scalable system that can rapidly and inexpensively advance in Technology Readiness Level, and be mated with other hardware with minimal redesign effort. A critical component of the MSRS technology is the development methods for the application of a highly active and robust catalyst layer to the wall of a microchannel. This catalyst layer consists of a stable, oxide support layer containing highly dispersed ruthenium catalyst particles. The catalyst support layer must strongly adhere to the metal surface and be insensitive to thermal expansion differences between the metal and the oxide, possess reasonable surface area, and exhibit adsorptive properties towards the reactants. Furthermore, the ruthenium catalyst particles must be homogeneously distributed on accessible oxide surfaces with a small and narrow catalyst particle size distribution. To meet these overall requirements, sol-gel methods were employed for the formation of the oxide support layers. Following the deposition of the oxide support with proper characteristics on the microchannel wall, ruthenium containing solutions were impregnated on the fired support.

II. Materials and Methods Two types of reactors were manufactured and coated with catalysts. The first type of reactor was a simple coil of stainless steel tubing with a 0.8 mm inside diameter. Although the stainless steel coils were inappropriate for forming a thermally monolithic reactor, they were commercially available and were used for rapid catalyst development. The second type of reactor was formed from channeled stainless steel plates. A manufacturing process was developed to produce the micro channel reactors from the stainless steel plates. These microchannel reactors consisted of a stack of stainless steel layers that were diffusion bonded together. The channel plate reactors assembled in this work were produced via diffusion bonding of mechanically channeled stainless steel plates. These plates were blind cut by a milling process with vias drilled through for gas flow to adjacent plates. The blanks for the individual plates were cut from 1.59 mm thick 316 stainless steel plate stock using a laser cutting technique. During the cutting process, 3.175 mm diameter alignment holes were cut in two corners of the blank for alignment during the diffusion bonding process. The reactors were milled with 1 mm wide by 1 mm deep channels and a 1 mm wide wall between the channels. The channels were milled using a computer controlled milling machine to ensure the consistency required for the diffusion bonding process. Figure 2 shows a typical channeled plate.

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Figure 2 Picture of Channel Plate.

After the milling process, burrs that line the edge of the channels produced during the milling process are removed by light sanding with 600 grit sand paper. The plates are washed with detergent, dried, and then inspected under magnification for debris and flaws. The two alignment holes are then reamed out to the diameter of the alignment dowel in both the channel plates and the end plates used to connect the reactor to external tubing. Holes are drilled through the plate at one end of the milled channel as a via to the next plate. The burrs from the drilling process are removed with a counter sink followed by a soak in a detergent bath, rinsing, drying, and inspection. The plates are then sonicated in a Citranox solution to remove any attached debris, rinsed, dried with high velocity nitrogen, and inspected before assembly into a reactor. In addition to the channel plates, three via plates and one blanking plate are used to create a connector port for the 1/8 inch diameter tubing as displayed in Figure 3. Three via plates with alignment holes and one 1/8 inch via hole are bonded to each side of the channel plate stack. After the bonding process, a 1/8 inch diameter tube is silver brazed inside the hole in the via plates. A blanking plate is bonded between the channel plates and via plates to prevent the introduction of braze and flux to the channel area. To complete the flow path, a hole is then drilled through the blanking plate using the inside of the 1/8 inch diameter tube for access.

ALIGNMENT DOWELS

1/8 INCH TUBE

VIA CHANNEL BLANKING PLATES PLATES PLATE

Figure 3 Assembly of the Micro Channel Reactor.

The diffusion bonding process is quite simple, but requires clean, flat surfaces to ensure intimate contact between the surfaces. After all foreign material has been removed from the surfaces as described previously. The plates are then soaked in acetone and then isopropanol followed by high velocity nitrogen removal of the isopropanol to ensure that no residue remains on the surface. The stack of stainless steel plates that will make up the reactor are mounted between two graphite plates with alignment holes that line up with the dowel pins. The graphite is coated with boron nitride powder to prevent graphite-metal bond formation. The assembly is then placed in the diffusion bonder. The vacuum oven is heated to approximately 1000 °C. After degassing of the metal is complete, the force on the plates is increased to approximately 1000 pounds per square inch of bonding surface. A reduction in surface area across the metal plate

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interfaces induces diffusion bonding. High pressure is required to induce the bond formation, so only regions that are solid throughout the metal plate stack will bond properly, so alignment becomes crucial. Three reactors were manufactured using the diffusion bonding process described above. Each reactor consisted of 13 channel plates, two blanking plates and 6 via plates and was 4.7 cm across. Each of these reactors was brazed with silver solder to attach 1/8 inch tubing to complete the flow path. One reactor was bisected after reactor testing to observe the alignment of the channels in the reactor. Figure 4 presents a cross section of one reactor and an external image of a completed reactor.

Figure 4 Cross Section and external surface of a diffusion bonded channel reactor.

Three reactors were used for catalyst testing and had 13 channel plates bonded between the blanking and via plates. Each channel plate had 80 cm of channels forming a volume of 0.8 cc and a geometric surface area of 32 cm2. This gives a total reactor volume of 10.4 cc and surface area of 416 cm2. Metal oxide supported ruthenium catalysts were deposited in the tubular reactors and the three channeled plate reactors. To increase the topographical surface area of the stainless steel channels, an acid etch was performed. Forty mL of a solution containing 10 wt% HCl and 10 wt% FeCl3 was flowed through the channels of the reactor for approximately 20 minutes. Similar exposure of thin plates of 316 stainless steel results in an etched surface with a mat finish that is characterized by increased surface area due to roughness generated by preferential dissolution of areas on the steel surface, as well as enhanced dissolution at grain boundaries. The deposition process for the porous alumina support is based on sol-gel chemistry and is formed from a precursor alkoxide mixed in solution with a pore former and a polymerization catalyst. To facilitate bonding of the oxide catalyst support to the etched micro reactor channel surface, an oxide layer was first formed on the etched surface by heating the reactor to 500 °C in a tube furnace for one hour with air flow through the channels. The bond between the stainless steel surface oxide layer and the catalyst support is strongly enhanced by this process. The deposition and thermal processing of a sol-gel layer was used to form a high surface area catalyst support layer. The sol gel process begins with a stable solution of an aluminum alkoxide mixed with a structure directing agent. As the reaction progresses (Figure 5), hydrolysis reactions produce hydroxy substitutions on the alkoxide with the loss of alcohol groups. Condensation occurs when the metal hydroxy groups react with other alkoxides forming polymeric chains. On heating, these polymers decompose to metal oxides.

OR OR HYDROLYSIS: RO Al OR + H2O RO Al OH + ROH OR OR

OR OR OR OR

RO Al OH + HO Al OR HO Al O Al OR + H2O OR OR OR OR CONDENSATION: OR OR OR OR RO Al OR + HO Al OR HO Al O Al OR + ROH OR OR OR OR Figure 5 The sol-gel process.

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Most transition metal alkoxides are highly reactive with even small amounts of moisture in air. At ambient temperature and humidity levels, these alkoxides rapidly progress through hydrolysis and condensation forming powders. These powders are unsuitable for forming metal oxide films. To inhibit the hydrolysis and condensation reactions, a significant amount of mineral acid was added to the alkoxide/alcohol solution, which slows hydrolysis reactions. The solution is generally stable until the acid is removed. When a thin film of a stable transition metal alkoxide solution is exposed to air at ambient temperature and humidity, the alcohol, water, and acid will slowly evaporate from the surface. Generally, the alcohol evaporates first leaving a concentrated water-acid-alkoxide film. Depending on the ambient humidity level and temperature, the water content of the film will change as the acid evaporates. Eventually, the pH of the film increases and hydrolysis and condensation occurs forming a gel layer. Thin, continuous alumina films produced by the sol-gel method will possess little porosity or will possess porosity at the molecular level. To create a mesoporous structure with large, accessible porosity, a structure- directing agent is introduced at the sol gel solution stage, which forms a template that creates voids in the sol-gel structure when the material is thermally processed. Removal of the template via thermal processing during or after densification results in pores in the metal oxide structure. Ideally, the templating agent will self-assemble in a regular orderly structure so the resulting porosity will consist of a repeatable pattern with a well-defined geometry. For transition metal alkoxide formulations, the rate of the gelation must be carefully controlled to form a repeatable mesoporous film. The templating agent used to make these mesoporous films, P123, consists of a poly(ethylene oxide)- poly(propylene oxide)-poly(ethylene oxide) triblock copolymer with a block pattern of EO20PO70EO20 and a mass of 5600 g/mol. Due to their large size, time is required for the P123 micelles to migrate and form into a pattern before the gel solidifies. The Evaporation Induced Self Assembly (EISA) process is a method of organizing micelles in a dip coated film. Figure 6 depicts the EISA process for P123 in a sol-gel precursor matrix for a substrate withdrawing from a precursor solution. During a dip coating process, the P123 micelles undergo several organizational transitions. As a substrate is withdrawn from the bulk solution, alcohol rich vapors evaporate from the surface concentrating the solution. As the film becomes richer in water and acid, a slower evaporation process dominates with the loss of acid and water from the film. As the P123 micelles form, they begin to self-organize into a pattern determined by the P123 to liquid volume ratio. The rate of the drying process can be critical to the self organization of the micelles. If drying is too rapid, the micelles will have insufficient time to migrate into their lowest energy sites in the organizational structure. Drying rate can be controlled by a combination of reduced temperature and increased atmospheric humidity. As the aging process continues under less humid conditions, residual water and acid leave the film and the film selectively contracts in a direction normal to the surface.

Figure 6 Organization of Micelles in a Sol-Gel Matrix.

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Sol-gel films deposited on the inside of a channel will follow a similar gelation process as the dip coated films with some exceptions. After the sol-gel had been pumped through the channel and airflow is introduced, the wave front of the air will be at a greater velocity compared to the average dip coating withdrawal process. Additionally, the downstream sol-gel coat will be exposed to the vapors emitted from the upstream surface, increasing the drying time. Most likely, the end result will be that the downstream channel surface will take longer to dry and the micelles will have more time to migrate. If the minimum migration time for the micelles in the film in the upstream part of the tube is sufficient, micelle organization should still occur. The tubular and channeled plate reactors were assembled, etched, and oxidized as described previously. A modified sol-gel process derived from the work of Ha47 was used to coat the channels of the stainless steel micro channel reactor. The P123 was initially dissolved in n-butanol followed by the addition of nitric acid. The aluminum sec-butoxide was then added while rapidly stirring the solution. Care was taken to minimize exposure of the aluminum source to the atmosphere, which will form insoluble precipitates. This procedure resulted in a clear solution after twelve hours of stirring. Molar ratios for the aluminum sec-butoxide, n-butanol, water, nitric acid, and P123 mixture was 1:20:2.65:1.76:0.0161, respectively. The homogeneous solution was flowed through each reactor’s channels for several minutes. Then, lab air was flowed through each reactor overnight. Each reactor was then thermally treated in a tube furnace. The furnace was ramped at 1º C/min to 400 ºC and a soak at 400 ºC for 120 minutes with active air flowing through the channels. Ruthenium was deposited onto the mesoporous alumina surface via the incipient wetness approach using a solution of ruthenium acetylacetonate dissolved in toluene. In the incipient wetness approach, the channels of the reactor are coated with a solution of 40 g/L ruthenium acetylacetonate in toluene that is pumped through the reactor for several minutes followed by drying and reduction in a tube furnace at 400 ºC under an H2/N2 atmosphere.

III. Results and Discussion The micro channel reactor test stand consists of pressurized hydrogen and carbon dioxide sources with flow rates regulated by two mass flow controllers. The two gases are mixed and flow through a 1/8 inch tube to the micro channel reactor located inside an insulated oven. Effluent gas simply exhausts through a stainless steel tube. Carbon dioxide and methane concentrations were quantified using a Matheson model 8430 gas chromatograph equipped with a thermal conductivity detector and column packed with molecular sieve 5A. The mobile phase was helium gas. Micro channel reactor conversion results are reported in percent efficiency. Efficiency is determined by the volume percentage of methane and carbon dioxide in the effluent gas by equation (4).

% Volume Methane Efficiency = (4) (% Volume Methane)+(%Volume Carbon Dioxide)

A channeled plate reactor was manufactured as described. The reactor has a total channel volume of 10.4 cc and 2 geometric surface area of 416 cm . For all reactor testing, the H2:CO2 ratio was maintained at the stoichiometric ratio of 4:1. The reactor was initially heated to 400 ºC and reactant gas flow initiated at 5 cc/min. The reactor temperature was varied between 350 and 450 ºC. Gas samples were collected and analyzed by the Matheson gas chromatograph. Sabatier conversion efficiency at five temperatures is presented in Figure 7. As expected, efficiency increases with temperature indicating that the reactor is kinetically limited at this flow rate. The reactant flow rate was lowered to 1.25 cc/min. With the longer residence time, the equilibrium limit to the reaction was achieved at 425 ºC and some activity persisted at 275 ºC.

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100

80

60 Flow Rate = 1.25 cc/min

40

Percent Conversion Conversion Percent 20 Flow Rate = 5 cc/min

0 280 300 320 340 360 380 400 420 440 Reactor Temperature in Centigrade

Figure 7 Efficiency of Channeled Plate Sabatier Reactor.

After this initial test, two additional channeled plate reactors were manufactured and coated with catalyst. Although the manufacturing process for the channeled reactors was a success, all three reactors showed rapid declines in catalytic activity over a span of two weeks. The cause of the deactivation is currently under investigation. The majority of the catalyst development and testing was performed on the tubular reactors due to commercial availability. A three stage reactor system was manufactured using 0.8 mm id tubular reactors and the catalyst deposition processes described previously. The entire three-stage reactor had a total of 1116 cm2 of surface area and volume of 22.6 cc with ¼ of the total volume in each of the first two reactors and ½ of the total volume in the third reactor. The three-stage reactor was operated with a gas flow rate of 0.6 cc/min and a gas composition hydrogen:carbon dioxide ratio of 4:1. The first reactor stage was maintained at 400°C. The temperature of the second and third stages was initially varied to observe the effect of temperature on conversion efficiency. Thermodynamically, the maximum efficiency for the Sabatier reaction with a 4:1 hydrogen:carbon dioxide ratio at 400°C is fixed at 85% and increases to 99% at 200°C. The efficiency numbers vary slightly due to small variations in the analysis and compositional variations of the influent gas stream. The temperature of the first stage reactor was set at 400°C and the third stage at 250°C. The temperature of the second stage reactor was varied between 400°C and 311°C. Table 1 presents the conversion efficiency data for each reactor stage at four stage two temperatures. At a stage two temperature of 396°C, the first stage has already brought the gas composition close to equilibrium so additional time at that temperature does not shift the composition equilibrium. As the stage two temperature is lowered, equilibrium shifts to the production of water and methane. The stage three reactor receives an influent with higher levels of methane and water as the stage two temperature is lowered and shifts the composition further toward the desired products. Both stage two and three are kinetically limited due to the reactor size and catalyst activity.

Table 1 Efficiency of Three-Stage Reactor System. Stage 1 Efficiency Stage 2 Stage 2 Efficiency Stage 3 Efficiency 400°C Temperature 250°C 83 396 82 89 82 369 85 92 82 341 86 92 82 311 87 94

The three-stage reactor system was operated for over 900 hours with no reduction in catalyst activity after a short induction period. The operating temperature for stage one, two, and three were 400°C, 317°C, and 250°C, respectively. The system was intermittently cooled down to check influent flow rate. Figure 8 displays the conversion efficiency for the system. There was no initial reduction in efficiency for the stage one reactor since it had previously been run and had achieved stable operation. Efficiency of the first stage remained at around 84% (effectively the equilibrium value) over the 900 hour test run.

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100

O 98 Efficiency of First Stage Reactor at 400 C Efficiency of Second Stage Reactor at 317 OC 96 Efficiency of Third Stage Reactor at 250 OC

94

92

90

88

86 to Methane in Percent Percent in Methane to 84

82 Conversion Efficiency of Carbon Dioxide of Efficiency Conversion

80 0 200 400 600 800 1000 Hours of Reactor Operation

Figure 8 Efficiency for Three-Stage Reactor System.

The composition of the effluent from the second stage reactor revealed a cumulative conversion efficiency of 88%, a three percent improvement over a single stage reactor. This reactor did experience a small initial efficiency reductions due primarily to loss of activity during the first 130 hours of operation. As the experiment continued, the catalytic activity showed a slight improvement over 900 hours. Sizing of the reactor and limited catalytic activity prevented the conversion efficiency from reaching equilibrium. The third stage reactor, which was operated at 250°C shifted the equilibrium even closer to the desired products of methane and water, starting at 95% conversion, and after an initial decline, stabilizing at 91% for 900 hours. This reactor also showed an initial loss of catalyst activity which then remained stable. At these lower temperatures, catalytic activity is significantly reduced and the equilibrium composition is not achieved. After 900 hours of stable operation, micro reactor flow rate and reactor temperatures were adjusted to maximize conversion efficiency. Gas flow rate was reduced to 0.1 cc/min increasing residence time 6 fold. At this lower flow rate, the first micro reactor reaches equilibrium well before the end of the reactor, and possibly the same situation exists in the second micro reactor. Therefore, the temperature of the first micro reactor was reduced to 365°C, the second micro reactor to 294°C, and the third micro reactor to 234°C. Under these conditions, the carbon dioxide conversion efficiency increased to 96% and remained stable.

IV. Summary and Conclusion The feasibility of an effective Microchannel Sabatier Reactor System that enables nearly complete single pass carbon dioxide reduction using the Sabatier reaction, allowing efficient recovery of oxygen from in situ carbon dioxide resources on Mars or other Near Earth Objects (NEOs), has shown potential. The focus of this work was on the development of manufacturing processes for microchannel reactors. These microchannel reactors were successfully produced via diffusion bonding of mechanically channeled stainless steel plates. The channels of completed reactors were etched, oxidized, coated with a mesoporous aluminum oxide catalyst support, and coated with catalytic ruthenium nuclei. These reactors were then tested for catalytic activity at operating temperatures around 400 °C. The channeled plate reactors produced in this work initially converted a 4:1 hydrogen to carbon dioxide gas stream to 85% methane and water but suffered a significant reduction in catalytic activity over a span of two weeks operation time. Operation of a three-stage reactor with similar tubular stainless steel reactors over a time span of 900 hours clearly demonstrated the feasibility of a multi-stage or graded temperature reactor system. Reduction of the second stage reactor from 400°C to 317°C increased the effectiveness of the stage 2 and 3 reactors due to the thermodynamic shift of the reaction equilibrium toward carbon dioxide reduction at the lower temperatures. A

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conversion efficiency of 96% was eventually achieved by reduction of the reactor temperatures for more favorable thermodynamics. To produce an efficient graded temperature MSRS, several limitations must be addressed. The catalytic activity of the reactors were unacceptably low. To improve efficiency, the mechanism of catalyst deactivation must be identified. Additionally, channel density must be increased through the reduction in channel wall dimensions. A reduction in channel dimensions will improve mass transport and flow turbulence. A graded temperature reactor with these design improvements will demonstrate the feasibility of the MSRS approach to carbon dioxide reduction.

Acknowledgments The author would like to acknowledge and thank Helen Vaccaro and Kris A. Lee of NASA – JSC for their oversight and knowledge on this project, the Microproducts Breakthrough Institute of Oregon State University for assistance in manufacturing the channeled reactors, and NASA’s SBIR program for funding this research.

References 1Akse, J.R., J.O. Thompson, High Efficiency Microchannel Sabatier Reactor System for In Situ Resource Utilization, Final Report, SBIR Phase I, Contract NNX12CE82P, NASA Johnson Space Center, 2012. 2 Alptekin, G., Hitch, B., Dubovik, M., Lind, J., and Smith, F., Prototype Demonstration of the Advanced CO2 Removal and Reduction System, SAE Paper No. 2005-01-2862, presented at the 35th International Conference on Environmental Systems, 2005. 3Jeng, F.F., Lafuse, S., Smith, F.D., Lu, S.-D., Knox, J.C., Campbell, M.L., Scull, T.D., and Green, S., Analyses of the Integration of Carbon Dioxide Removal Assembly, Compressor, Accumulator and Sabatier Carbon Dioxide Reduction Assembly, SAE Paper No. 2004-01-2496, presented at the 34th International Conference on Environmental Systems, 2004. 4 Murdoch, K.E., Scull, T.D., Carrasquillo, R.L., and Graf, J., Sabatier CO2 Reduction System Design Status, SAE Paper No. 2002-01-2531, presented at the 32nd International Conference on Environmental Systems, 2002. 5 Jeng, F.F., and Lin, C.H., A Trade Study on Sabatier CO2 Reduction Subsystem for Advanced Missions, SAE Paper No. 2001-01-2293, presented at the 31st International Conference on Environmental Systems, 2001. 6Lunde, P.J. Modeling, Simulation, and Operation of a Sabatier Reactor, Ind. Eng. Chem., Process Des. Dev., 13 (3), 226- 232, 1974. 7Strumpf, H., Chin, C., Lester, G., and Homeyer, S., Sabatier Carbon Dioxide Reduction System for Long-Duration Manned Space Application, SAE Technical Paper Series No. 911541, Society of Automotive Engineers, Warrendale, PA, 1991. 8Zubrin, R. Price, S., Mason, L., Report on the Construction and Operation of a Mars In-Situ Propellant Plant., AIAA-94- 2844, presented at the 30th AIAA/ASME Joint Propulsion Conference, Indianapolis, IN, June 1994. 9Zubrin, R. Price, S., Mason, L., Mars Sample Return With In-Situ Resource Utilization: An End-to-End Demonstration of a Full Scale Mars In-Situ Propellant Production Unit, Final Report NASA Contract NAS-9-19145, Jan. 1995. 10Nakata, A., Matsumoto, H., Hatano, S., Kita, Y., and Shimoda, T., Fundamental Study of Water Generation System on Mars, SAE 2001-01-2413, presented at the 31st International Conference on Enviromental Systems, July 2001. 11 Spina, L., and Lee, M.C., Comparison of CO2 Reduction Process – Bosch and Sabatier, SAE Paper No. 851343, presented at the 15st Intersociety Conference on Environmental Systems, July 1985. 12 Otsuji, K., et al, An Experimental Study of the Bosch and the Sabatier CO2 Reduction Processes, SAE Paper No. 871517, presented at the 17st Intersociety Conference on Environmental Systems, July 1987. 13Noyes, G.P., Carbon Dioxide Reduction Process for Spacecraft ECLSS: A Comprehensive Review, SAE Paper No. 881042, presented at the 18st Intersociety Conference on Environmental Systems, July 1988. 14 Wagner, R.C., Carrasquillo, R., Edwards, J., and Holmes, R., Maturity of the Bosch CO2 Reduction Technology for Space Station Application, SAE Paper No. 880995, presented at the 18st Intersociety Conference on Environmental Systems, July 1988. 15 Bunnell, C.T., Boyda, R.B., and Lee, M.G., Optimization of the Bosch CO2 Reduction Process, SAE Paper No. 911451, presented at the 21st International Conference on Environmental Systems, San Francisco, July 1991. 16Jozwiak, W.K., Szubiakiewicz, E., Góralski, J., Klonkowski, A., and Paryjczak, T., Physico-Chemical and Catalytic Study of the Co/SiO2 Catalysts, Kinet. Catal. , 45, 247-255, 2004. 17 Nagai, M., Kurakami, T., and Omi, S., Activity of Carbided Molybdena-Alumina for CO2 Hydrogenation, Catal. Today, 45, 235-239, 1998. 18 Tsuji, M., Kodama, T., Yoshida, T., Kitayama, Y., and Tamaura, Y., Preparation and CO2 Methanation Activity of an Ultrafine Ni(II) Ferrite Catalyst, J. Catal., 164, 315-321, 1996. 19 Fujita, S., Terunuma, H., Nakamura, M., and Takezawa, N., Mechanisms of Methanation of CO and CO2 over Ni, Ind. Eng. Chem. Res., 30, 1146-1151, 1991. 20 Schild, C., Wokaun, A., Koeppel, R.A., and Baiker, A., CO2 Hydrogenation over Nickel/Zirconia Catalysts from Amorphous Precursors: On the Mechanism of Methane Formation, J. Phys. Chem., 95, 6341-6346, 1991. 21Cratty, L.E., Jr. and Russel, W.W., Nickel, Copper and Some of their Alloys as Catalysts for the Hydrogenation of Carbon Dioxide, J. Am. Chem. Soc., 80, 767-773, 1958. 22Stowe, R.A. and Russel, W.W., Cobalt, Iron and Some of their Alloys as Catalysts for the Hydrogenation of Carbon Dioxide, J. Am. Chem. Soc., 76, 319-323, 1954. 11 International Conference on Environmental Systems

23Hu, J., Brooks, K.P., Holladay, J.D., Howe, D.T., and Simon, T.M., Catalyst Development for Microchannel Reactors for Martian In Situ Propellant Production, Catal. Today, 125, 103-110, 2007. 24Holladay, J.D., Brooks, K.P., Wegeng, R., Hu, J., Sanders, J., and Baird, S., Micro reactor Development for Martian In Situ Propellant Production, Catal. Today, 120, 35-44, 2007. 25Yu, M.Y., Fei, J.H., Zhang, Y.P., and Zheng, X.M., Immobilized Ruthenium Catalyst for Carbon Dioxide Hydrogenation, Chinese Chem. Lett., 17, 1097-1100, 2006. 26Brooks, K.P., Rassat, S.D., and TeGrotenhuis, W.E., Development of a Microchannel In Situ Propellant Production System, Final Report, U.S. Department of Energy Grant No.: DE-AC05-76RL01830, September, 2005. 27Yaccato, K., Carhart, R., Hagemeyer, A., Lesik, A., Strasser, P., Volpe, Jr., A.F., Turner, H., Weinberg, H., Grasselli, R.K., and Brooks, C., Competitive CO and CO2 Methanation Over Supported Noble Metal Catalysts in High Throughput Scanning Mass Spectrometer, Appl. Catal. A: Gen., 296, 30-48, 2005. 28 Novák, É., Fodor, K., Szailer, T., Oszkó, A., and Erdöhelyi, A., CO2 Hydrogenation on Rh/TiO2 Previously Reduced at Different Temperatures, Top. Catal., 20, 107-117, 2002. 29VanderWiel, D.P., Zilka-Mrco, J.L., Wang, Y., Tonkovich, A.Y., and Wegeng, R.S., Carbon Dioxide Conversions in Micro reactors, in: Proceedings of the Fourth International Conference on Microreaction Engineering, Atlanta, GA, , 187-193, 2000. 30 Solymosi, F., Erdöhelyi, A., and Kocsis, M., Methanation of CO2 on Supported Ru Catalysts, J. Chem. Soc., Faraday Trans. 1, 77, 1003-1012, 1981. 31Lunde, P.J. and Kester, F.L., Carbon Dioxide Methanation on a Ruthenium Catalyst, Ind. Eng. Chem., Process Des. Develop., 13, 27-33, 1974. 32Jensen, K.F., Microreaction Engineering – Is Small Better?, Chem. Eng. Sci., 56, 293-303, 2001. 33Quiram, D.J., Hsing, I., Franz, A.J., Jensen, K.F., and Schmidt, M.A., Design Issues for Membrane-Based, Gas Phase Microchemical Systems, Chem. Eng. Sci., 55, 3065-3075, 2000. 34Jähnisch, K., Hessel, V., Löwe, H., and Baerns, M., Chemistry in Microstructured Reactors, Angew. Chem. Int. Ed., 43, 406-446, 2004. 35Kiwi-Minsker, L. and Renken, A., Microstructured Reactors for Catalytic Reactions, Catal. Today, 110, 2-14, 2005. 36Kolb, G., Hessel, V., Cominos, V., Hofmann, C., Löwe, H., Nikolaidis, G., Zapf, R., Ziogas, A., Delsman, E.R., de Croon, M.H.J.M., Schouten, J.C., de la Iglesia, O., Mallada, R., and Santamaria, J., Selective Oxidations in Micro-Structured Catalytic Reactors – For Gas-Phase Reactions and Specifically for Fuel Processing for Fuel Cells, Catal. Today, 120, 2-20, 2007. 37 Voloshin, Y., Halder, R., and Lawal, A., Kinetics of Hydrogen Peroxide Synthesis by Direct Combination of H2 and O2 in a Micro reactor, Catal. Today, 125, 40-47, 2007. 38Brooks, K.P., Hu, J., Zhu, H., and Kee, R.J., Methanation of Carbon Dioxide by Hydrogen Reduction Using the Sabatier Process in Microchannel Reactors, Chem. Eng. Sci., 62, 1161-1170, 2007. 39Jeng, F.F., Lafuse, S., Smith, F.D., Lu, S., Knox, J.C., Campbell, M.L., Scull, T.D., and Green, S., Analyses of the Integration of Carbon Dioxide Removal Assembly, Compressor, Accumulator and Sabatier Carbon Dioxide Reduction Assembly, SAE Technical Paper 2004-01-2496, presented at the 34th International Conference on Environmental Systems, Colorado Springs, 2004. 40Human-Rating Requirements for Space Systems, NASA Procedural Requirements document NPR 8705.2B, Effective Date May 6, 2008. 41Chu, R.R., In Search of a Carbon Dioxide Removal Assembly for Crew Exploration Vehicle, SAE Technical Paper 2004- 01-2286, presented at the 34th International Conference on Environmental Systems, Colorado Springs, 2004. 42Knox, J.C., Campbell, M., Miller, L.A., Mulloth, L., Varghese, M., Luna, B., Integrated Test and Evaluation of a 4-Bed Molecular Sieve, Temperature Swing Adsorption Compressor, and Sabatier Engineering development Unit, SAE Technical Paper 2006-01-2271, presented at the 36th International Conference on Environmental Systems, Norfolk, VA, 2006. 43Goldblatt, L. and Murdoch, K., Control of Particulate Matter from a Sabatier Catalyst Bed, retrieved August 24, 2009, from Wolf Engineering, Web Site: http://wolf-engineering.com/sitebuildercontent/sitebuilderfiles/habitation2006.ppt 44Sakurai, M., Yoshihara, S., Nakayama, N., Oguchi, M., Ohnishi, M., Usuku, K., and Toda S., Subscale Air Revitalization th System by CO2 Reduction for Small Satellite Demonstration, SAE Technical Paper 2009-01-2507, presented at the 39 International Conference on Environmental Systems, Savannah, GA, 2009. 45Haanappel, V.A.C., van Corbach, H.D., Fransen, T., and Gellings, P.J., Cracking and Delamination of Metal Organic Vapour and Deposited Alumina and Silica Films, Mat. Sci. Eng. A-Struct., 167, 179-185, 1993. 46Brooks, K.P., Rassat, S.D., and TeGrotenhuis, W.E., Development of a Microchannel In Situ Propellant Production System, Final Report, U.S. Department of Energy Grant No.: DE-AC05-76RL01830, September, 2005. 47Ha, T., Park, H., Kang, E., Shin, S., Cho, H., Variations in Mechanical and Thermal Properties of Mesoporous Alumina Thin Films Due to Porosity and Ordered Pore Structure, J. Coll. Inter. Sci., 345, 120-124, 2010.

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