Patent Application
20080064913
These and other features and advantages of the invention will become better understood when considered in conjunction with the following detailed description and by making reference to the appended drawings, wherein:
According to a first aspect of the invention, a propellant decomposition catalyst is provided and comprises a platinum group metal (PGM) catalyst supported by a second catalyst selected from the group consisting of barium oxide, metal chromites, metal hafnates, metal zirconates other than calcium zirconate, and hydrates and mixtures thereof. Advantageously, the propellant decomposition catalyst can be loaded into a monopropellant thrust chamber or other reaction engine, for example, as a bed of catalyst powder, granules, or a porous monolith.
The PGM catalyst functions as the primary catalyst for propellant decomposition. However, due to the corrosive, high-temperature environment encountered in high-energy chemical propellant thruster chambers, the PGM eventually erodes and is lost during use, at least locally, downstream of the thruster chamber inlet. However, because the underlying support material (the “second catalyst”) is itself catalytic, it sustains the propellant decomposition reaction, and extends the operable life of the thruster. As a whole, the propellant decomposition catalyst is “self-adjusting;” it responds to the corrosive, high-temperature environment of a thrust chamber and sustains the propellant decomposition reaction (which is sometimes referred to as “combustion,” though O2 is typically not present), even as the PGM is depleted during use.
PGMs include platinum, palladium, iridium, rhodium, ruthenium, and osmium. As used herein, the term “platinum group metal catalyst” includes any of the individual metals, as well as mixtures and alloys thereof. Iridium is most preferred, followed by platinum and ruthenium.
The second catalyst, which supports the PGM, is itself capable of sustaining a high-energy chemical propellant decomposition reaction. Particularly useful are high melting point, catalytically active materials that can tolerate the combustion environment produced by high-energy chemical propellants. Because of their inherent oxidation resistance, certain oxides and oxide-like compounds are well suited to this role. In general, materials considered to be suitable for use as a second catalyst for high-energy chemical propellant decomposition include barium oxide, perovskites, materials having a perovskite-like structure, polymorphs of such materials, and hydrates and mixtures thereof. Nonlimiting specific examples include metal chromites, such as yttrium chromite and lanthanide chromites (e.g., lanthanum chromite, ytterbium chromite, erbium chromite, and neodymium chromite); metal hafnates, such as barium hafnate; and zirconates, such as barium zirconate. Calcium zirconate, however, did not perform well in tests and is not preferred. Also included are hydrates and mixtures of any of the aforementioned materials, as well as various polymorphs of the materials.
In some embodiments of the invention, the propellant decomposition catalyst includes a source of, and/or a sink for, oxygen radicals or ions. A nonlimiting example is cerium oxide (ceria), a nonstoichiometric compound. In cerium oxide, cerium exhibits two oxidation states, Ce3+ and Ce4+. Without being bound by theory, it is believed that cerium oxide stores and releases oxygen radicals or ions through the transformation of cerium from the +4 to the +3 oxidation state, and vice versa.
A rough ranking of selected catalytic materials useful for decomposition of a high-energy chemical propellant is presented in Table 1. Barium hexaaluminate and lanthanum hexaaluminate are included for comparison. Many of these materials were tested with AF-315 and shown to have catalytic activity, i.e., the ability to catalyze its decomposition. The relative rankings may change, depending on the choice of propellant.
Mixtures of these materials with cerium oxide (CeO2; Tmelt=2397° C.) are also catalytically active.
Although the dimensions and morphology of the catalytic material may be dictated by the choice of reaction engine and/or propellant with which the catalyst is to be used, in general, the catalytic material is provided as a powder, porous granules, or a porous monolith, with a surface area sufficiently high to sustain a high-energy chemical propellant decomposition reaction. In one embodiment of the invention, the catalytic material has a surface area of about 0.1 to 1000 m2/g, about 0.1 to 105 m2/g, about 3 to 105 m2/g, or about 10 to 105 m2/g, prior to supporting (e.g., being impregnated with) the platinum group metal catalyst. In general, catalytic activity increase with surface area.
For a powder, typical particle size is about 50 μm or less, and the bulk of the surface area (SA) is on the external surface of the particles, rather than internal (pores). Assuming a spherical particle, SA is given by the simple relationship between the radius of the sphere (r) and material density (ρ): SA=3/rρ. Powder beds are considered porous because of the numerous gaps or voids between adjacent particles.
For a porous granule, typical particle size is greater than about 50 μm, and the bulk of the SA is internal; external SA is the same as that of a particle, but internal surface area can be several thousand m2/g, depending on the material and the size and shape of the granule's inner passages (pores). In one embodiment, the catalytic material comprises granules of about 0.1 to 5 mm, preferably about 0.3 to 3 mm, more preferably about 0.6 to 1.41 mm in size. Overall pore volume ranges from about 0.1 to 1.0 cc/g, preferably about 0.2 to 0.6 cc/g, more preferably about 0.3 to 0.5 cc/g, prior to impregnating the granules with a PGM.
For a monolith (i.e., a single unit fabricated to match the desired catalyst bed size), nearly any range of sizes is possible. The nature of the surface area depends upon the type of material that is used to coat the monolithic support and the porosity of the support itself. SAs are generally reported for monoliths in terms of cm2/cm3, that is cm2 of surface area per total volume of monolith (cm3).
In general, the catalytic material (“second catalyst”) is coated or otherwise impregnated with an amount of PGM of from about 15 to 30% by weight of the second catalyst.
In one embodiment of the invention, the catalytic material is porous, and the PGM is provided as discrete particles carried within the pores of the second catalyst. A nonlimiting example of a suitable technique for depositing a PGM (i.e., iridium, and ruthenium) on and in a porous carrier (alumina), is provided in the U.S. Pat. No. 4,124,538 (columns 2-8, including Examples I-VI).
In a second aspect of the invention, a method of sustaining propellant decomposition is provided, and comprises passing a propellant having an adiabatic flame temperature, T1, over a platinum group metal catalyst supported by a second catalyst; and sustaining propellant decomposition as the platinum group metal catalyst is eroded away during use by passing a propellant over the second catalyst; wherein the second catalyst has a melting point higher than T1, and is selected from the group consisting of metal chromites, metal hafnates, metal zirconates other than calcium zirconate, barium oxide, and hydrates and mixtures thereof. More generally, the method comprises passing a propellant having an adiabatic flame temperature, T1, over a catalyst comprising (a) a catalytic material having a melting point higher than T1, selected from the group consisting of metal chromites, metal hafnates, and hydrates and mixtures thereof, or (b) a platinum group metal catalyst supported by a second catalyst selected from the group consisting of barium oxide, metal chromites, metal hafnates, metal zirconates other than calcium zirconate, and hydrates and mixtures thereof.
In general, the propellant comprises either a monopropellant or a bipropellant. The term “monopropellant” typically denotes a chemical substance, or a mixture of substances, typically liquid, that can be decomposed in a reaction engine (e.g., a thrust chamber) to generate hot gases and thrust, without the need for an external oxidizer. In some embodiments, the monopropellant also includes water as a stabilizer. In contrast, a “bipropellant” comprises a fuel and an oxidizer, which are carried separately and brought together in a reaction engine to generate thrust.
Nonlimiting examples of high-energy chemical monopropellants include hydrazine, hydrazine derivatives (e.g., monomethyl hydrazine, unsymmetrical dimethylhydrazine), dimethylaminoethyl azide (DMAZ), mixtures of hydrazinium nitrate (HN) and water, mixtures of hydrazine and hydrazinium nitrate, nitrous oxide, mixtures of nitrous oxide and one or more fuel components (e.g., hydrocarbons), and mixtures of propylene glycol dinitrate, 2-nitrodiphenylamine, and dibutyl sebacate (Otto Fuel II, a distinct-smelling, reddish-orange, oily liquid used by the U.S. Navy as a fuel for torpedoes and other weapon systems).
Also included are high-energy chemical monopropellants containing a generally oxygen-deficient fuel component and an oxidizer. Nonlimiting examples of fuel components for monopropellants include hydroxyethylhydrazinium nitrate (HEHN), diethylhydroxylammonium nitrate (DEHAN), triethanol amine nitrate (TEAN), methanol, glycerol, tris(aminoethyl)amine trinitrate (TRN3), glycine, and mixtures thereof.
Nonlimiting examples of oxidizers for high-energy chemical monopropellants include hydroxylammonium nitrate (HAN), hydrazinium nitrate (HN), hydrazinium nitroformate, ammonium nitrate (AN), 1,4-diazobicyclo-(2,2,2)-octane nitrate (DON), ethylamine nitrate (EAN), ethanolamine nitrate (EOAN), hydroxylamine perchlorate (HAP), isopropylamine nitrate (IPAN), methylamine nitrate (MAN), methyl hydrazine nitrate (N), glycolic materials such as propylene glycol nitrate (PGDN) and triethylene glycol dinitrate (TEGDN), piperidine nitrate (PN), trimethylamine nitrate (TMAN), ammonium dinitrimide, and mixtures thereof. In some cases, the oxidizer has a formula, AB, where A is selected from the group consisting of ammonium (NH4+), hydroxylammonium (NH3OH+), hydrazinium (N2H5+), piperidinium (C5H12N+), n-alkyl ammonium (H4−nNRn+, where n is 1 to 3, and R is C1-C3 alkyl) and quaternary ammonium (NR430 , where R is C1-C3 alkyl); and B is selected from the group consisting of nitroformate [C(NO2)3−], dinitrimide [N(NO2)2], and nitrate (NO3−).
Combinations of HAN and a fuel component are particularly useful. Nonlimiting examples include HAN/HEHN, HAN/DEHAN, HAN/TEAN, HAN/AAN, HAN/glycol (e.g., PGDN, TEGDN), HAN/glycerol, HAN/glycine, HAN/methanol (MeOH), and HAN/tris(aminoethyl)amine trinitrate (TRN3). Each blend typically contains 5-20% by weight water as a stabilizer.
The theoretical adiabatic flame temperature (sometimes referred to as the combustion temperature) for stoichiometric HAN/MeOH is 1927° C. For stoichiometric HAN/TRN3, the theoretical combustion temperature is coincidentally also 1927° C.
Table 2 presents theoretical flame temperatures for selected formulations of HAN (oxidizer)/HEHN (fuel component)/water (stabilizer) monopropellants. Temperature calculations assume a pressure of 100-200 psia, and are made using the NASA Glenn code.
Table 3 presents flame temperatures for a number of different monopropellants, including monomethylhydrazine (MMH), unsymmetrical dimethylhydrazine (UDMH) and various liquid gun propellants, Air Force and Navy liquid propellants, etc., typically denoted by an alpha-numeric code. Three separate formulas for each of XM46 and LP1898 are provided.
In another aspect of the invention, an improved reaction engine is provided, and comprises a reaction chamber provided with a catalyst bed comprising (a) a catalytic material selected from the group consisting of metal chromites, metal hafnates, and hydrates and mixtures thereof, or (b) a platinum group metal catalyst supported by a second catalyst selected from the group consisting of barium oxide, metal chromites, metal hafnates, metal zirconates other than calcium zirconate, and hydrates and mixtures thereof. Preferably, the catalytically-active support material (the “second catalyst”) comprises a material listed in Table 1, or a polymorph, hydrate, or mixture thereof, optionally impregnated with a PGM. The reaction engine can be used in rocket engines, gas generators, auxiliary power units, tank pressurization systems, and similar machines.
In one embodiment, a single-stage catalyst bed containing a single propellant decomposition catalyst—e.g., a combination of a PGM catalyst dispersed on and/or in a catalytically active support material as described above—is loaded into a thrust chamber. In this embodiment, the PGM is initially the primary catalyst throughout the entire length of the bed, but exposed areas of the supporting second catalyst can contribute to the overall catalytic activity of the bed. As the PGM is chemically attacked and removed from the bed, the amount of exposed second catalyst increases. Because the support material is catalytically active, it can sustain the combustion reaction in areas where the PGM is partially or completely removed.
In another embodiment, a catalyst as described herein is used as the second stage of a 2-stage catalyst bed. The upstream stage comprises a PGM (e.g., iridium or platinum) dispersed on and/or in a support that cannot tolerate the adiabatic flame temperature of the propellant, but which has a very high surface area, such as gamma alumina. The downstream stage comprises a PGM (e.g., iridium) dispersed on and/or in a catalytically-active, high-temperature-capable support, as described above. In this embodiment, it is desirable that the interface between the two stages is located in an area where the temperature is below the temperature at which gamma alumina loses surface area due to its conversion to alpha alumina.
In yet another embodiment, the invention is again used as the second stage in a 2-stage catalyst bed, but the upstream stage has a higher temperature capability. Lanthanum hexaaluminate and barium hexaaluminate are two examples of supports having moderate-to-low surface area and moderate temperature capability that could be used in the first stage. The downstream stage comprises a PGM (e.g., iridium) dispersed on and/or into a different catalytically-active, high-temperature-capable support (e.g., barium oxide, a perovskite, or a polymorph, hydrate, or mixture thereof). In this embodiment, it is desirable that the interface between the two stages is located in an area where the rate of loss of PGM is minimal. If this is not the case, the support in the upstream section of the bed, which is not catalytically active, will eventually lose enough PGM to cause propellant decomposition instabilities.
The catalyst bed constitutes a structure sufficient to hold the catalyst(s) in place in the reaction chamber. Such structures are well known in the art. In the case where the catalyst comprises a porous monolith, minimal structure may be required, as the monolith can be sized to fit snuggly within the reaction chamber. For catalysts having a granular or powdered form, the catalyst bed has a configuration that is adequate to hold the catalyst in place, and is made of a material sufficiently durable and temperature resistant to survive the propellant decomposition reaction.
Preferably, the catalytically-active support material and the PGM each have melting points above the adiabatic flame temperature of the propellant with which the thruster is to be used, and it is assumed that the PGM is more effective than the support in initiating the combustion reaction. In addition to being catalytically active, the support has sufficient chemical resistance to survive the propellant decomposition environment for the duration of the application. When a catalyst as described herein is used with a propellant such as AF-315 that will attack the PGM, localized loss of PGM will occur as described above, and a zone containing insufficient PGM to sustain the reaction will develop. But because the loss of PGM in this zone results in exposure of the underlying, catalytically-active support, the propellant decomposition reaction will be sustained.
Because the PGM-depleted zone develops at a location that is downstream of the catalyst bed inlet, the propellant decomposition reaction will have already been initiated on the PGM, and the largest activation-energy barrier will have been overcome. Consequently, a less-active catalyst will be able to sustain the combustion reaction and prevent propellant decomposition instabilities and other deleterious effects. The present invention is therefore distinct from a simple 2-stage or multi-stage catalyst bed that is assembled in a manner that yields two or more well-defined catalyst zones. However, the invention can be used as one or more of the stages in a 2-stage or multi-stage catalyst bed.
Pino Testing
Pino testing is a recognized method of evaluating propellant decomposition catalysts. Pino testing provides two valuable pieces of information: exhaust gas temperature rise rate (the time in milliseconds that it takes the exit gas temperature to rise a specified amount over the initial starting temperature) and overall temperature achieved. In general, the hotter the exhaust, the better the performance. A higher exit gas temperature indicates better catalytic performance, i.e., more complete decomposition of the propellant, and greater thrust. (The relationship between exhaust temperature, T, and specific impulse, Isp, is given by
where k is the specific heat ratio of the exhaust-gas mixture, R is the gas constant, T is the gas temperature, gc is the gravitational constant used to convert from weight to mass, and M is the mean molecular weight of the exhaust gas.) Similarly, the shorter the exit gas temperature rise rate, the better the performance. A short rise rate indicates faster ignition of the propellant.
Conventional Pino testing involves placing a catalyst sample into a holder, suddenly immersing it into a pool of propellant, and measuring the temperature-versus-time response of a thermocouple in the sample holder. While the temperature of the catalyst and propellant can be independently set prior to immersion, the most common procedure employs a heated catalyst sample and ambient-temperature propellant.
For the catalysts described herein, measuring the ignition-response time of the PGM-coated support is not sufficient. Additional testing must be done to quantify the performance of the support itself because, after some period of time, the support will be exposed and its catalytic performance will become important. But, because the support will be catalyzing a reaction that is already under way—it will have been initiated by the PGM in the upstream section of the bed—simply quantifying its activity with unreacted propellant is not an appropriate test. A more relevant test involves measuring the catalytic activity of the support on the products of incomplete combustion.
To perform this type of test, a modified Pino test set-up is used, as shown in
Iridium-coated material is placed in the bottom (upstream) portion 36 of the sample holder, catalytically active support material is placed in the downstream portion 38, and thermocouples are placed in both locations. Thus, when the sample is plunged into the propellant, the propellant will first encounter the iridium-coated material, and the reaction will begin. The fluid will then be forced to flow into the downstream portion of the catalyst bed, where the catalytically-active support will sustain the decomposition reaction.
Because the unimolecular and bimolecular reactions taking place in the downstream portion of the bed are different, and because the reactants will consist of hot, reactive intermediates, it is quite conceivable that the support will be a better catalyst than iridium. To determine if this is the case, three different test scenarios can be utilized. The first is as described above, where the downstream portion of the catalyst bed consists of the uncoated, catalytically-active support material. An alternative arrangement is to use the same iridium-coated catalyst upstream, with an inert support, such as ZrO2, used in the downstream portion of the bed (experiment control). The use of an inert support downstream will permit a comparison with the first experiment thus allowing quantification (qualification) of the catalytic performance of the support. Still another alternative is to use iridium-coated catalyst throughout the entire bed, and it too will act as a baseline (e.g., an all LCH-227 test). By comparing the results of the third experiment to that of the first, the activity of the iridium can be compared to that of the support. If the support is indeed a better catalyst for the intermediate reactions, this comparison will bear that out.
The following are nonlimiting examples of the invention.
The following is a representative procedure, using alumina: 0.3766 g IrCl3 is dissolved in a minimal amount of ethanol (ca. 10 ml), and 0.5057 g of alumina spheres are added to the solution. The mixture is allowed to stand for 40 h, during which time the solvent is slowly evaporated. The coated alumina spheres are dried for 2 h at 220° C. in air, and then calcined for 1 h at 700° C. in an air furnace. 0.6117 g of Ir-coated alumina spheres is recovered (17% Ir by weight).
Lanthanum chromite is prepared by adding lanthanum nitrate and chromium nitrate in a 1:1 mole ratio in sufficient water to dissolve the salts. A precipitate is formed by the slow addition of dilute ammonium hydroxide solution. The precipitate is then dried to yield a high-surface-area powder. The powder is then pressed into a wafer, sintered, broken into smaller pieces, and sieved to yield the desired particle size, typically 1-2 mm. The resulting porous granules of catalytically-active lanthanum chromite are then impregnated with an iridium chloride solution and calcined as described in Example 1.
Cerium oxide powder is added to a solution of lanthanum nitrate and chromium nitrate, where the mole ratio of cerium to lanthanum to chromium is 10:1:1. Ammonium hydroxide solution is added to the stirred suspension to form a precipitate, which is then calcined in air at 700° C. for 2-4 hours. The resulting mixture of LaCrO3 and CeO2 is ground into a powder, pressed into a wafer, sintered, broken into smaller pieces, and sieved to yield the desired particle size, typically 1-2 mm. The resulting porous granules, which consist of catalytically-active cerium oxide and lanthanum chromite in close proximity, are then impregnated with an iridium chloride solution and calcined as described elsewhere.
Commercially available barium oxide powder is formed into granules as described in Example 2 and then impregnated with iridium as described in Example 1.
Lanthanum nitrate, chromium nitrate and cerium nitrate are dissolved in water, where the molar ratio of cerium to lanthanum to chromium is 60:1:1. Ammonium hydroxide solution is added to the solution to form a precipitate, which is then calcined in air at 700° C. for 2-4 hours. The resulting mixture of LaCrO3 and CeO2 is ground into a powder, pressed into a wafer, sintered and broken into smaller pieces, and sieved to yield the desired particle size, typically 1-2 mm.
Using the catalyst preparation procedures described above, Examples 5-10 were prepared, without being impregnated with a PGM. To simulate a two-stage catalyst bed, each catalyst was separately evaluated for catalytic activity—the ability to decompose a high-energy chemical propellant—using a modified Pino Tester substantially as shown in
The following description is representative of the Pino Test procedure that was used to evaluate the catalysts. 0.5 g of a Control (LCH-227) was placed in the upstream portion of the catalyst holder in order to initiate the reaction. Two thermocouples were used, one in each portion of the bed (holder), and a thermocouple trace recorded data over the lifetime of the test. Each sample was separately tested by placing a given catalyst (0.5 g) in the downstream portion of the bed, adjacent to the upstream 0.5 g charge of the Control. An all-LCH-227 (Control) (1 g) sample was tested for comparison. The catalyst holder was preheated to 300° C. The holder assembly was immersed into the propellant cup to initiate contact with the catalyst. The bottom thermocouple registered the initial temperature response of the catalyst. The top thermocouple registered exhaust gas temperatures. Since exit-gas temperature and rise rate determine performance, the downstream temperature response is the primary variable of interest.
Each catalyst was preheated to about 320° C. in a heating mantle, and then pulled out and allowed to cool to about 300° C., at which time the catalysts and thermocouples were plunged into a pool of room temperature liquid propellant (AF-M315E). The samples' different starting temperatures reflect the materials' different heat capacities. It should also be noted that LCH-227 has a high heat capacity and is not quenched by incoming cold propellant to the extent that the other materials are.
Two-stage Pino test results are presented in Table 4 and
Examples 5 and 9 showed unexpectedly superior performance relative to the Control. In the case of Example 5 (LaCrO3 on 60 CeO2), both the rise time and overall temperature realized were better than that of the Control. The Control had an Exhaust Rise Rate of 0.219° C./ms, whereas Example 5's Rise Rate was 0.268° C./ms. The Control reached a maximum exhaust gas temperate of 500° C. in 1733 ms, which gives a ramp rate of 288.8° C./sec. In the same length of time, Example 5 was slightly higher at 525° C., giving a ramp rate of 302.3° C./sec. What is impressive about this catalyst is the overall exhaust gas temperature realized: 725° C., a 45% increase over the Control.
In the case of Example 9 (YCrO3 on 10 CeO2), the overall temperature achieved (423° C.) was somewhat less than that of the Control (500° C.). However, in the 1167 ms time it took Example 9 to reach 423° C., the Control was lagging at 375° C. The ramp rates for Example 9 and the Control are 362° C./sec and 289° C./sec, respectively. So the ramp rate of Example 9 bettered the Control by 25%. Furthermore, both Example 5 and Example 9 started at lower initial temperatures than the Control, due to quenching effects of the materials' heat capacities. Initial temperatures of LaCrO3 on 60 CeO2 and YCrO3 on 10 CeO2 were 107° C. and 114° C., respectively, compared to 158° C. for the Control, which means that the new catalysts also had to overcome lower starting temperatures.
The relative performance of the other catalysts was more or less comparable to that of the Control.
The invention has been described with reference to various examples and embodiments, but is not limited thereto. Various modifications can be made without departing from the invention, the scope of which is limited only by the appended claims and their equivalents. Throughout the claims, use of “a” and other singular articles is not intended to proscribe the use of plural components. Thus, more than one PGM may be deposited onto a catalytically active support material; more than one support material may be utilized, and so forth. Also, use of the word “about” in relation to a range of values is intended to modify both the high and low values recited, and reflects the penumbra of variation associated with measurement, significant figures, and interchangeability, all as understood by a person having ordinary skill in the art to which this invention pertains.
This invention was made with Government support of Contract No. F04611-03-M-3006, awarded by the United States Air Force (AFRL). The Government has certain rights in the invention.
