Patent Application
20080064914
In the course of the following detailed description, reference will be made to the attached drawings in which:
The catalysts of this disclosure can be formulated in several different ways in order to provide the catalytic activity and high-temperature stability that enables successful use in liquid propellant applications. The presence of one or more metals selected from Ag, Au, Co, Cr, Cu, Fe, Ir, Mn, Ni, Os, Pd, Pt, Re, Ru, Rh, and V (in a metallic state) can promote the decomposition of HAN-based monopropellant blends. Further, because several of these metals are not thermally or chemically stable at the conditions produced by monopropellant decomposition, the metals can be combined with other metals, such as Ag, Al, Au, Ba, Ca, Ce, Co, Cr, Cu, Dy, Er, Eu, Fe, Gd, Hf, Ho, Ir, La, Lu, Mg, Mn, Mo, Nb, Nd, Ni, Os, Pd, Pr, Pt, Re, Ru, Rh, Sc, Sm, Sr, Ta, Th, Ti, Tm, V, W, Y, Yb, Zr and mixtures thereof, to form alloys possessing enhanced stability without significantly reducing catalytic activity. In some cases, incorporating additional metallic or oxidic species may also increase catalytic reactivity.
The presence of one or more oxides selected from oxides of Al, Ce, Co, Cr, Cu, Dy, Er, Eu, Fe, Gd, Hf, Ho, La, Lu, Mg, Mn, Nb, Nd, Ni, Pr, Sc, Sm, Th, Ti, Tm, V, Y, Yb, Zr and mixtures thereof can promote the decomposition of HAN-based monopropellant blends. The oxides can be mixed with other oxide compositions to improve the thermal or chemical stability of the catalyst at the conditions produced by monopropellant decomposition. In some cases, incorporating additional oxidic species may also increase catalytic reactivity.
By dispersing these active phases on a thermally stable, porous ceramic carrier, a large number of surface-active sites are exposed and a catalyst with a high specific reactivity, defined as moles of propellant decomposed per unit mass catalyst per unit time, is realized. The inventive catalysts may include from 0.05 to 40% of active phase material based upon the total weight of the catalyst.
The catalyst can be utilized in conjunction with propellant and propellant decomposition hardware in order to produce hot gases that can be used to provide thrust, drive turbines, inflate devices, and the like.
In addition to exhibiting high reactivity and stability, it is advantageous for the catalyst to possess high mechanical strength and thermal conductivity in order to function well in practical applications. The high mechanical strength allows the catalyst to withstand the stresses imposed by the hot gases generated within the catalyst and passing over the surface of the catalyst. The thermal conductivity of the propellant decomposition catalyst plays a role in distributing the heat generated by the exothermic decomposition reaction throughout the catalyst bed. A catalyst comprising a low loading of metal on a porous ceramic carrier or a metal oxide supported on a porous ceramic carrier may not exhibit sufficient thermal conductivity. An additional thermally conductive metallic phase may be added to the catalyst (as described, e.g., in Example 9, infra). Conductive metallic phases that can be added include Co, Cr, Fe, Ir, Mo, Nb, Ni, Os, Pd, Pt, Re, Ru, Rh, Ta, V, W and mixtures thereof.
The selection and processing of the porous ceramic carrier play an important role in determining the stability and activity of the catalyst. Carriers that are able to retain a high surface area in an oxidizing environment at temperatures above 1400° C. are preferred. Examples of suitable carriers include metal hexaaluminates, aluminas, lanthanide oxides, metal zirconates, metal hafnates and metal carbides. Metal oxide-based carriers have generally not been applied to catalytic processes at temperatures above 1600° C. because thermally induced sintering reduces the carrier surface area below values practical for catalytic applications. While the slow anisotropic crystalline growth of metal hexaaluminates has allowed the production of useful carriers at temperatures up to 1600° C., little investigation of this material above 1600° C. has been conducted.
The carrier-active-catalytic phase combination and proportions are selected to minimize carrier-catalyst interactions that could reduce carrier stability and/or catalyst activity. At the temperatures of relevance for propellant decomposition, reaction of the supported phase with the carrier can easily occur, forming unwanted compounds of reduced catalytic utility. For instance, at high loadings, vanadium oxide can leach barium out of barium hexaaluminate to produce highly sinterable α-alumina.
In one embodiment of the present invention, barium hexaaluminate is used as the porous ceramic carrier. The carrier can be prepared by the base-catalyzed hydrolysis of aluminum sec-butoxide and barium methoxyethoxide in methoxyethanol. By removing the solvent at a temperature and pressure above the supercritical point of methoxyethanol, a needle-like microstructure is developed that provides increased ability to resist sintering as the carrier is heated. Particular processes for producing such a carrier are outlined in Examples 1 and 2, infra.
The inventive catalyst can be prepared in various ways. One suitable method comprises impregnating the porous carrier with solutions of precursors to the phases desired to be present in the final catalyst. The impregnation can be carried out with multiple solutions including different precursors, or with a single solution including multiple precursors. The impregnation can be carried out by adding to the porous carrier enough solution to fill the pores, then drying and calcining. Alternatively, the impregnation can be carried out by soaking the porous carrier in an excess of solution from which the required amounts of precursors are adsorbed by the carrier, after which the porous carrier is dried and calcined. Better results are obtained by repeatedly impregnating the porous carrier with precursor solutions of lower concentrations followed by drying and calcining. By using solutions with low precursor concentrations, highly dispersed metal and metal-oxide precursors are deposited on the porous carrier. Drying and calcining prior to the next impregnation step fixes the metal or metal oxide to the carrier and prevents redissolution of the precursor into the impregnating solution during the subsequent impregnation step. Repeated impregnation steps may also be conducted when it is desired to deposit larger amounts of the active catalytic species onto the porous carrier. Solutions of the precursors may be made up in alcohol, water, or other suitable solvents. Additional details for producing embodiments of the catalysts are found in the Examples, infra.
Any soluble form of the desired precursor can be employed in making the catalysts. In particular embodiments, the catalyst is one that can be decomposed to the metal by heating at a temperature below 600° C. or those that can be converted to the metal oxide by heating at a temperature below 600° C. Nitrates, chlorides, alkoxides and the like are examples of suitable precursors.
The porous carrier containing the solution of precursors can be dried by heating in an oxidizing, reducing or inert atmosphere. The dried impregnated carrier can then be heated to produce the desired active phase and to thermally condition the catalyst prior to use for propellant decomposition. Heating in an oxidizing atmosphere, in a reducing atmosphere and/or in an inert atmosphere to different temperatures may be conducted to retain the desired porous carrier characteristics, to produce the desired active catalytic phase and/or to produce the desired metal phase for thermal conductivity promotion. Parameters such as atmosphere, heating rate and duration of the heat treatment influence the properties of the final product.
The catalyst can also be produced by forming the porous carrier and supported phases at the same time in a wet chemical process. Carrier and supported-phase precursors, such as metal alkoxides, metal nitrates, metal chlorides, and the like, can be dissolved in a solvent, such as alcohol, and hydrolyzed with water in the presence of a catalyst to produce a gel or precipitate that can be collected, dried and heated to produce a catalyst of the desired composition.
The catalyst should retain a surface area in excess of 10 m2/g when heated in an oxidizing atmosphere at 1400° C. for more than one hour. Alternatively, the catalyst should retain a surface area in excess of 1 m2/g when heated in an oxidizing atmosphere at 1650° C. for more than one hour. The catalyst may be in the form of granules, pellets or structured elements, such as monoliths or foams.
The catalyst can be used in practice by placing the catalyst into an enclosure equipped with inlet and outlet connections. It is advantageous to maintain the catalyst at a temperature between −100 and 400° C. prior to introduction of the propellant. Propellant is admitted to the catalyst via the enclosure inlet. Propellant flows through the porous catalyst, reacts to form low molecular weight gases and exits the system through the enclosure outlet. The propellant and gaseous product may pass around catalyst particles, and/or through catalyst particles via internally connected pores.
The following examples illustrate formulations of the inventive catalysts and methods of synthesizing and using the catalysts.
A carrier comprising barium-oxide-doped alumina was prepared by a wet chemical process. 0.80 g of barium was mixed with 10 mL of methoxyethanol to produce a solution of Ba(CH3OC2H4O)2. This solution was mixed with another solution of 7.37 g aluminum sec-butoxide in 10 mL methoxyethanol. A solution of 1.90 g water in 10 mL methoxyethanol was added dropwise with rapid stirring. Then 7.20 mL of glacial acetic acid was diluted with 5 mL of methoxyethanol and added to the solution.
The sample was dried by venting and purging solvent from the sample heated to 330° C. at 2000 psig. The powder at the top of the dried sample was physically separated from the denser powder at the bottom of the sample. The denser powder was then heated in flowing air at a ramp rate of 3° C./min to 1400° C., held for 5 hours at 1400° C. and then cooled down. The resulting surface area of this sample was 65 m2/g.
A carrier comprised of barium hexaaluminate was prepared by a wet chemical process. 8.10 g of Ba was mixed with 180 mL of methoxyethanol to produce a solution of Ba(CH3OC2H4O)2. This solution was mixed with another solution of 174.6 g aluminum sec-butoxide in 180 mL methoxyethanol. 18.45 g ethyl acetoacetate was added to the solution. 90.1 mL of 1.81 M ammonia in methoxyethanol solution was slowly added to the solution. To this solution was added dropwise a solution of 38.16 g water in 180 mL methoxyethanol. The solution gelled within thirty minutes and the gel was aged for an additional 16 hours at 55° C. to yield a translucent product.
The sample was dried by venting and purging solvent from the sample heated to 330° C. at 2000 psig. The sample was then heated in flowing air at a ramp rate of 3° C./min to 1650° C., held for 5 hours at 1650° C. and then cooled down. The resulting surface area of this sample was 8 m2/g.
A catalyst comprising iridium, platinum and barium hexaaluminate with an Ir:Pt:Ba:Al atomic ratio of 0.85:0.84:1:12 was prepared by impregnating porous barium hexaaluminate granules. The carrier granules were prepared by pelletizing a mixture of 75 wt % barium hexaaluminate powder with a surface area of 6 m2/g with 25 wt % polyethylene glycol. After calcining the pellets at 600° C. in air, the pellets were crushed into granules and sieved to a −12/+20 mesh fraction. An impregnation solution including 0.25 mol/L H2IrCl6 and 0.25 mol/L H2PtCl6 dissolved in 2-propanol was prepared. An amount of impregnation solution sufficient to fill the pores of the granular carrier was mixed with the granular carrier and then dried at 70° C. The impregnated granules were then heated to 380° C. in air. After cooling, the impregnation and heat treatment procedure was repeated five times in order to reach the desired catalyst Ir and Pt content. Following the final impregnation, the catalyst was reduced in a flowing stream of hydrogen at 600° C. and then heated to 1650° C. in a flowing stream of argon. The resulting surface area of the catalyst was 4 m2/g.
Three parts by mass of the catalyst was ground and mixed with 1 part AF-M315E, which includes 44.5% by weight stabilized HAN, 44.5% by weight hydroxyethylhydrazine nitrate and 11.0% by weight water. The monopropellant decomposition activity of the catalyst was measured with a temperature programmed technique in which the catalyst-propellant mixture is slowly heated. The temperature at which a significant reaction exotherm is detected is termed the decomposition onset temperature and is a good measure of the catalytic activity. The mixture was heated at 10° C./min in a differential scanning calorimeter to measure the onset temperature of the decomposition reaction. The onset of the exothermic reaction was observed to be 123° C.
A catalyst of the prior art including 32 wt % Ir supported on aluminum oxide (i.e., a Shell 405 catalyst) was reduced in a flowing stream of hydrogen at 600° C., and then portions of the catalyst were heated to 1000 and 1650° C. in a flowing stream of argon.
Three parts by mass of each sample of catalyst were ground and mixed with 1 part AF-M315E. Each mixture was heated at 10° C./min in a differential scanning calorimeter to measure the onset temperature of the decomposition reaction. The onset of the exothermic reaction and surface area of each catalyst are listed in TABLE I.
A series of catalysts were prepared by impregnating a porous ceramic carrier composed of barium hexaaluminate powder that possessed a surface area of 7 m2/g. Impregnation solutions were prepared by dissolving nitrate and chloride salts in isopropanol. Thirteen impregnations were conducted for each metal salt in order to attain a target loading of 25 wt % based upon the impregnated metal content. These catalysts were then heated to 1000° C. in argon after reducing a portion of each sample in flowing hydrogen at 600° C. and oxidizing a portion of each sample in flowing air at 600° C.
The AF-M315E monopropellant decomposition activity of each catalyst and of the Shell 405 catalyst heated to 1000° C. in argon was determined by measuring the decomposition onset temperature as listed in TABLE II.
One embodiment of the inventive catalyst includes an active catalytic phase comprising a metal oxide mixed with additional metal oxides to produce an active phase with improved thermal and chemical stability. V2O5, with a melting point of 695° C., was mixed and ground with Y203 and Tm203 and then heated to 1650° C. in air to produce active catalyst phases of YVO4 (melting point: 1810° C.), TmVO4 (melting point: 1800° C.) and Tm8V2O17 (melting point: 1900° C.). Upon reduction in flowing hydrogen at 1100° C., active catalyst phases of YVO3 and TmVO3 were produced.
The AF-M315E monopropellant decomposition activities of these unsupported active phases and that of unheated V205 and V203 were measured via temperature programmed reaction, with the results summarized in TABLE III.
A catalyst comprising yttrium oxide, vanadium oxide and barium hexaaluminate with a Y:V:Ba:Al atomic ratio of 0.42:0.42:1:12 was prepared by impregnating porous barium hexaaluminate powder. The barium hexaaluminate powder possessed a surface area of 6 m2/g. An impregnation solution including 0.25 mol/L V205 dissolved in concentrated HCl was prepared. An amount of impregnation solution sufficient to fill the pores of the carrier was mixed with the carrier and then dried at 60° C. The impregnated carrier was then heated to 380° C. in air. After cooling, the impregnation and heat treatment procedure was repeated a second time in order to reach the desired catalyst V content. An impregnation solution including 0.22 mol/L Y(NO3)3 dissolved in isopropanol was prepared. An amount of impregnation solution sufficient to fill the pores of the carrier was mixed with the carrier and then dried at 60° C. The impregnated carrier was then heated to 380° C. in air. After cooling, the impregnation and heat treatment procedure was repeated two additional times in order to reach the desired catalyst Y content. Following the final impregnation, the catalyst was oxidized in a flowing stream of air at 600° C. and then heated to 1650° C. in a flowing stream of air.
Three parts by mass of the catalyst were ground and mixed with 1 part AF-M315E. The mixture was heated at 10° C./min in a differential scanning calorimeter to measure the onset temperature of the decomposition reaction. The onset of the exothermic reaction was observed to be 91° C.
A catalyst comprising vanadium oxide supported on barium hexaaluminate with a V:Ba:Al atomic ratio of 0.1:1:12 was prepared by impregnating porous barium hexaaluminate powder. The barium hexaaluminate powder possessed a surface area of 6 m2/g. An impregnation solution including 0.05 mol/L V205 dissolved in concentrated HCl was prepared. An amount of impregnation solution sufficient to fill the pores of the carrier was mixed with the carrier and then dried at 90° C. The impregnated carrier was then heated to 200° C. in air. After cooling, the impregnation and heat-treatment procedure was repeated a second time in order to reach the desired catalyst V content. Following the final impregnation, the catalyst was oxidized in a flowing stream of air at 600° C. and then heated to 1650° C. in a flowing stream of air.
Three parts by mass of the catalyst was ground and mixed with 1 part AF-M315E. The mixture was heated at 10° C./min in a differential scanning calorimeter to measure the onset temperature of the decomposition reaction. The onset of the exothermic reaction was observed to be 86° C.
A catalyst comprising iridium, platinum and vanadium oxide supported on barium hexaaluminate with an Ir:Pt:V:Ba:Al atomic ratio of 0.36:0.35:0.17:1:12 was prepared by impregnating porous barium hexaaluminate powder. The barium hexaaluminate powder possessed a surface area of 7 m2/g. An impregnation solution comprising 0.25 mol/L V205 dissolved in concentrated HCl was prepared. An amount of impregnation solution sufficient to fill the pores of the carrier was mixed with the carrier and then dried at 90° C. The impregnated carrier was then heated to 200° C. in air. After cooling, the impregnation and heat-treatment procedure was twice repeated in order to reach the desired catalyst V content. An impregnation solution comprising 0.13 mol/L H2IrCl6 and 0.13 mol/L H2PtCl6 dissolved in 2-propanol was prepared. An amount of impregnation solution sufficient to fill the pores of the granular carrier was mixed with the granular carrier and then dried at 60° C. The impregnated granules were then heated to 200° C. in air. After cooling, the impregnation and heat-treatment procedure was repeated four times in order to reach the desired catalyst Ir and Pt content. Following the final impregnation, the catalyst was reduced in a flowing stream of hydrogen at 600° C. and then heated to 1650° C. in a flowing stream of argon.
One part by mass of the catalyst was ground and mixed with three parts AF-M315E. The mixture was heated at 10° C./min in a differential scanning calorimeter to measure the onset temperature of the decomposition reaction. The onset of the exothermic reaction was observed to be 107° C.
The catalyst of Example 9 was heated to 1850° C. in an argon atmosphere. The catalyst was then heated to 1650° C. in a flowing stream of argon.
One part by mass of the catalyst was ground and mixed with three parts AF-M315E. The mixture was heated at 10° C./min in a differential scanning calorimeter to measure the onset temperature of the decomposition reaction. The onset of the exothermic reaction was observed to be 120° C.
The catalyst of Example 3 was loaded into a test stand similar in design to that depicted in
While this invention has been shown and described with references to particular embodiments thereof, those skilled in the art will understand that various changes in form and details may be made therein without departing from the scope of the invention.
This application claims the benefit of U.S. Provisional Application No. 60/666,274, filed on Mar. 28, 2005, the entire teachings of which are incorporated herein by reference.
This invention was made with Government support under Contract F33615-01-C-5200 awarded by the US Air Force Research Laboratory. The Government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 60666274 | Mar 2005 | US |
