CA2079122A1 - Treating emissions from engines fueled by compressed natural gas - Google Patents
Treating emissions from engines fueled by compressed natural gasInfo
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- CA2079122A1 CA2079122A1 CA 2079122 CA2079122A CA2079122A1 CA 2079122 A1 CA2079122 A1 CA 2079122A1 CA 2079122 CA2079122 CA 2079122 CA 2079122 A CA2079122 A CA 2079122A CA 2079122 A1 CA2079122 A1 CA 2079122A1
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Abstract
Abstract of the Invention A three-way catalyst system for treating the exhaust stream of a compressed natural gas (CNG) fueled engine operating slightly rich of stoichiometry, the said catalyst system providing high conversions and low light-off temperature for the removal of HC, CO, and NOX. A high surface area gamma alumina support is impregnated with an intimate mixture of 0.2-30% Pd and 0.5-20% La2O3, the Pd being in crystalline form with a particle size in the range of 5-500 angstroms.
A method of making such catalyst system comprising sequentially impregnating a high surface area gamma alumina support with lanthana and palladium by incipient wetness techniques, such techniques comprising the procedure of contacting alumina first with a lanthanum nitrate solution of desired concentration to obtain 0.5-20% lanthana, contacting the lanthana impregnated alumina with a palladium nitrate solution of desired concentration to obtain a 0.2-30% palladium content in the catalyst, each of said steps being separated by drying and calcination.
A method of treating exhaust gas from a CNG
fueled engine with the catalyst system described above, comprising: (a) operating said engine at slightly rich of stoichiometry; (b) exposing such catalyst to the exhaust gases in the range of 400-750°C and at a space velocity in the range of 0-100 K hr-1, said exhaust gas being converted at an efficiency greater than 90% for each of NO, CO, and CH4.
A method of making such catalyst system comprising sequentially impregnating a high surface area gamma alumina support with lanthana and palladium by incipient wetness techniques, such techniques comprising the procedure of contacting alumina first with a lanthanum nitrate solution of desired concentration to obtain 0.5-20% lanthana, contacting the lanthana impregnated alumina with a palladium nitrate solution of desired concentration to obtain a 0.2-30% palladium content in the catalyst, each of said steps being separated by drying and calcination.
A method of treating exhaust gas from a CNG
fueled engine with the catalyst system described above, comprising: (a) operating said engine at slightly rich of stoichiometry; (b) exposing such catalyst to the exhaust gases in the range of 400-750°C and at a space velocity in the range of 0-100 K hr-1, said exhaust gas being converted at an efficiency greater than 90% for each of NO, CO, and CH4.
Description
1 - 2~
TREATING EMISSIONS FROM ENGINES FUELED
BY COMPRESSED NATURAL GAS
Background of the Invention Technical field This invention relates to the technology of catalytically converting emissions of a compressed natural gas (CNG) fueled engine, and more particularly to catalytic conversion of exhaust gases containing saturated hydrocarbons, including methane.
Discussion of the Prior Art Natural gas (essentially 85% methane~ is an attractive source of fuel for vehicles because it provides for a lower fuel cost, longer engine life, lower maintenance, and reduced oil consumption. Development of catalysts for high efficiency removal of saturated hydrocarbons, including methane, by oxidation within an exhaust stream is of strategic importance; it may be crucial in view of the emission control requirements promulgated by the U.S. Government. In the past, oxidation of methane has received little attention in automotive catalysis. Extreme difficulty of removal of methane is experienced because a C-H bond must be ruptured. In the oxidation of higher alkanes, oxidation is easily achieved by the cleavage of C-C bonds. Since the C-H bond is stronger, methane is more difficult to oxidize.
The prior art has investigated the use of noble metals and base metals as catalysts for stimulating the oxidation of methane by cleavage of the C-H bond.
Alumina, silica, thoria, and titania supported platinum and palladium catalysts were evaluated in 1983 and 1985 (see C.F. Cullis and B.M. Willatt, Journal of Catalysis, - 2 - 2~ f ~
Vol. 83, p. 267, 1983; and V.A. Drozdov, P.G.
Tsyrulnikov, V.V. Popovskii, N.N. Bulgakov, E.M. Moroz, and T.G. Galeev, Reaction Kinetic Catalysis Letters, Vol.
27, p. 425, 1985). These studies showed that an alumina supported palladium catalyst is the most active, followed by an alumina supported platinum catalyst. A reduction in catalytic activity is observed when silica and titania are used as supports. A systematic study of the use of alumina supported base metal catalysts for methane oxidation was conducted in 1963; chromium was found to be the most active. At a metal loading of 3.1 weight percent, chromium was found to be comparable to palladium (see K.C. Stein, J.J. Feenan, L.J. Hofer, and R.B.
Anderson, Bureau of Mines Bulletin, No. 608, U.S.
Government Printing Office). However, use of only Cr2O3 on A12O3 is disadvantageous because of the volatile and toxic nature of Cr2O3 and the poor durability of the Cr2O3-A12O3 catalyst- In another article evaluating base metal catalysts for methane oxidation, unsupported Co3O4 was found most active (see R.B. Anderson, K.C. Stein, J.J. Feenan, and L.J. Hofer, Industrial Engineering Chemistry, Vol. 53, p.
809, 1961). However, use of only Co3O4 on A12O3 is disadvantageous because of the volatile and toxic nature of Co3O4 and the tendency of Co to form a low surface area spinel with A12O3 resulting in poor durab;lity.
The prior art has found that the deactivation of a palladium on alumrna catalyst can occur by the reaction of water vapor with~palladium oxide to form Pd(OH)2.
It is desirable to retard the mobility of the adsorbed water vapor species and thereby reduce Pd~OH)2 formation. Such prior art has also found that palladium oxide is less active than palladium, and therefore it is desirable to retain palladium in the metallic state and _ 3 _ 2~ f ~
inhibit the formation of palladium oxide.
In the course of examining Pd on A12O3 at an effective three-way catalyst (converting methane, CO and NOX), the prior art has demonstrated a negative teaching to the use of lanthana with palladium (see H.
Muraki, "Performance of Palladium Automotive Catalyst", SAE Technical Paper Series No. 910842, 1991). This work resulted in a conclusion that total hydrocarbon conversion of CH4, being the most difficult hydrocarbon to oxidize, by palladium/lanthanum catalysts, near stoichiometric conditions, is lower than that of a palladium catalyst by itself; La2O3 is believed to suppress hydrocarbon oxidation activity. The prior art has also demonstrated that the use of La2O3 with palladium increases the hydrocarbon light-off temperature (see H. Muraki et al, "Palladium-Lanthanum Catalysts for Automotive Emission Control", Ind. Eng. Chem. Prod. Res.
Dev., 25 (1986) 202-208).
Lanthana has been used by the prior art with Pd/A12O3 catalysts in ways not related to catalyst conversion enhancement, namely, to thermally stabilize the alumina support (see U.S. patent 4,906,176). This patent teaches the use of other catalytic components, i.e., manganese, chromium, zirconium, rare earth elements, tin, zinc, copper, magnesium, barium, strontium, and calcium to promote catalytic activity.
However, patent '176 fails to appreciate the conversion enhancement role La2O3 may play during CH4 oxidation because the disclosure used the wrong and undesirable form of oxide support (i.e., La2O3.11A12O3), the lanthana was not deposited correctly, operated under genelally lean conditions, and never measured the conversion efficiency attained using the above catalyst because of their primary interest in measuring thermal b 35 stability.
SummarY of the Invention The invention pertains to a three-way catalyst system for heating the exhaust of a compressed natural gas fueled engine operated slightly rich of stoichiometry. This catalyst system achieves simultaneous removal of saturated hydrocarbons, particularly methane, nitric oxide, and carbon monoxide at enhanced rates. Also, the light-off temperatures are lower. The catalyst system comprises a high surface area gamma alumina support impregnated with an intimate mixture of 0.2-30% palladium and 0.5-20% lanthana, the palladium being in a crystalline form and having a particle size in the range of 5 to 1000 angstroms. The intimate presence of La2O3 permits Pd to adsorb CH4 and 2 instead of Pd becoming oxidized.
The intimate mixture is assured by providing a substantially continuous contact between the palladium and lanthana. ~he exhaust gas composition is slightly rich (of stoichiometry), preferably in the redox ratio, R, range of 1.1-1.2 (R being the ratio of reducing components to oxidizing components in the exhaust gas.
Enhanced o~idation rate for methane by use of this invention allows conversions greater than 90% when the exhaust gas is maintained in the temperature range of 400-750C, and at a space velocity of 5-100 K hr 1.
The light-off ternperature for methane (at 50% conversion) is no greater than 450C and can be as low as 300C.
The catalyst of this invention has also achieved increased aging resistance by retaining a CH4 conversion efficiency at or above 80% after 100 hours at 550C.
Other aspects of this invention comprise a method of making such catalyst and a method of treating CNG emissions. The method of making involves: (a) sequentially impregnating gamma alumina with lanthanum and palladium by incipient wetness techniques that involve contacting a desired amount of alumina first with a lanthanum nitrate solution of desired concentration to obtain 0.5-20% lanthana by weight of the catalyst and thereafter contacting the lanthana impregnated alumina with a palladium nitrate solution of desired concentration to obtain a 0.2-30% palladium content in the catalyst, the impregnated alumina, after each stage of impregnation, being dried and calcined prior to the next stage. The method of treating comprises: (a) operating the CNG fueled engine slightly rich of stoichiometry; and (b) exposing a catalyst constituted of a high surface area gamma alumina support impregnated with an intimate mixture of .5-20% La2O3 and .2-30~
Pd, to exhaust gases in the range of 400-750C, and at a space velocity of 5-100 K hr 1, the exhaust gas being converted at an efficiency greater than 90% for each of NO, CO, and CH4.
Brief Description of the Drawings Figure 1 is a graphical illustration of oxidation efficiency of different catalysts, including the catalyst of this invention, as a function of temperature to provide light-off information for oxidation of methane.
Figure 2 is a graphical illustration of conversion efficiency as a function of temperature for the catalyst of this invention; it illustrates the conversion efficiency for the various gaseous species using a gas that simulates emissions from a CNG engine.
Figure 3 is a graphical illustration of conversion efficiency as a function of redox ratio using simulated CNG emissions; this illustrates the difference in methane, C0, and NO conversion efficiency for an - 6 ~
embodiment of this invention compared to a palladium/rhodium prior art catalyst.
Figures 4 and 5 are graphical illustrations similar to that of Figure 3, but illustrating the effects of aging after about 80 and 100 hours, respectively, of use of the catalyst of this invention.
Figure 6 is a graphical illustration of the conversion efficiency as a function of the redox ratio using simulated CNG emissions; this illustrates the difference in CH4, CO, and NO conversion efficiency resulting from the addition of La to a Pd/A12O3 catalyst.
Detailed Description and Best Mode The catalyst functions three-way (CO, HC, and NO) to cleanse the exhaust of a CNG fueled internal combustion engine when operated under slightly rich conditions, although such catalyst system can be used to convert HC and CO from any (fuel-lean or stoichiometric) gaseous stream that contains saturated hydrocarbons, particularly methane, provided the gas stream is maintained in a desired temperature range. When used for cleansing the automotive exhaust of a CNG fueled engine, such catalyst system will provide a lower light-off temperature compared to the prior art catalysts and will provide methane conversion equal to or greater than 90%
at slightly rich conditions.
The catalyst is comprised of a high surface area gamma alumina support impregnated with 0.5-20% lanthanum oxide (La2O3) and 0.2-30% palladium, by weight of the catalyst.
The exhaust, which this catalyst is designed to treat, is slightly rich and contains saturated hydrocarbons, particularly methane. This means that the redox ratio R is generally in the range of 1.1-1.2, --7-- 2. ~ J ~
although the catalyst will perform with less efficiency at redo~ ratios outside such range. The temperature of such an exhaust gas should be in the range of 400-750C
in order to obtain optimum methane conversion, such temperature being reached during normal operating conditions rather than during start-up of the engine.
Optimum conversion is facilitated when the catalyst has a space velocity in the range of 2-100 K hr . At space velocities outside of this range, the conversion efficiency of the catalyst will be detrimentally affected. Methane conversion is lowered particularly when the space velocity increases; however, CO and NO
conversions are not significantly affected when the space velocity increases above 100 K hr 1. The exhaust gas from a CNG fueled engine will typically have a hydrocarbon content in the range of 60-3000 ppm, a CO
content in the range of 450-22,500 ppm, a NO content in the range of 85-4250 ppm, and a H2 content in the range of 150-7500 ppm. Oxygen will generally be about 320-16,000 ppm. To simulate such an e~haust gas, the ratio of hydrogen to CO should be in the range of 0.25-0.75.
The support must be of gamma alumina rather than delta or alpha forms of alumina because delta and alpha forms of alumina have, among other factors, low surface areas. With gamma alumina, the surface area will be significantly increased and be in the range of about 50-400 m2/gm. The particle size of the gamma alumina should be less than 200 angstroms, and the monolith carrier should have a cell size in the range of 100-600 cells per square inch. Gamma alumina may also be modified with oxides of base, rare earth and alkali metals such as barium, cerium, titanium, and nickel.
The lanthana impregnation is carried out to load the support with lanthana in the weight range of 0.5-20%. If lanthana is added in an amount less than such range, then the beneficial effect of increase in activity due to lanthana addition is not observed. If lanthana exceeds such range, then the lanthana surface area decreases and no additional benefit is derived.
~ Lanthana, as used herein, provides a unique chemical union with the palladium metal to obviate or delay the oxidation of the palladium and thereby leads to a synergistic relationship for catalysis. Elements that are equivalent to the function of lanthana for purposes of this invention may include tungsten oxide and molybdenum oxide.
Palladium is impregnated in a manner to provide the presence of large crystalline particles, preferably in the particle range of 5-1000 angstroms. With palladium weight loadings below 0.2%, there will be an insufficient catalysis effect and therefore not promote the objects of this invention. If the palladium loading is in excess of 30%, the palladium surface area decreases and no additional benefit from palladium addition is derived.
Other elements that may be present in the catalytic impregnation may include elements that avoid retention of water for improving the long life stability of catalysts. This may include elements such as tungsten oxide (incorporated by using ammonium meta tungstate during the impregnation process) or chromium oxide, both of which tend to prevent oxidation of palladium by reducing the mobility of water and thereby keeping it away from the palladium.
Some of the chemical reactions that take place as a result of using the catalyst of this invention are indicated below:
2 ~ J. .~
CH~ + 202--~C02 ~ 2H20 CO + 1/202 ~ C2 H2 + 1/202 ~3 H20 In additional to these steps, several reactions such as those between CH4 and NO, CO and NO, and NO and H2 occur. The stoichiometric reactions are complicated and vary with exhaust gas composition.
Performance As shown in Figure 1, the Pd/La2O3/A12O3 catalyst of this invention is compared with prior art catalysts to indicate the improvement in light-off characteristics when operating on the exhaust gas from a CNG fueled engine. Light-off temperature is that temperature at which 50% conversion efficiency is achieved. The flow study used a simulated exhaust gas having 1500 ppm CH4, 3000 ppm 2' and balance of N2 (at 30 K hr 1 SV).
The temperature obtained for light-off USing a 1~ palladium and 3-10% lanthanum catalyst was about 4gO-445C; when the palladium was increased to as much as 10% with 3% lanthana, the light-off temperature occurred as low as 340C. This is in striking contrast to the light-off temperatures for the prior art catalysts which range from 490-580C.
Figure 2 shows the light-off characteristics for methane, carbon monoxide, and nitric oxide when the invented catalyst is used for treating the simulated CNG
vehicle exhaust mixture. The light-off temperature for CH4, NO, and CO are 450C, 205C, and 170C, respectively, when the redox ratio is 1.15. AS shown in Figure 2, the optimum temperature for achieving maximum methane conversion is in the range of S50-650C for the invented catalyst herein. Thus, the catalyst of this invention achieves extremely high three-way conversion efficiencies when operated in such temperature range and at a redox ratio (slightly rich) around 1.15. Under similar conditions, the light-off temperatures for the invented catalyst are lower than those of the Pd/Rh prior art catalyst.
As shown in Figure 3, all of NO, CO, and methane will be converted at a level equal to or greater than 90%
when such conditions are met (temperature 550-650C, R = 1.1-1.2) and the catalyst of this invention is deployed. The optimum methane conversion efficiency obtained when the prior art catalyst (palladium/rhodium shown in broken line and solid symbols) is employed does not exceed 70% at a redox ratio of about 1.2; this is substantially lower than about 90% achieved by the catalyst of this invention. Equally important is the conversion of NO which drops to about 80% for the prior art catalyst (palladium/rhodium) at a redox ratio of about 1.2; a conversion efficiency of about 100% for N0 is achieved when using the catalyst of this invention.
An examination was made of the aging characteristics of the catalyst of this invention after a period of about 80 hours at 550C at a redox ratio of 1.02 (Figure 4) and after about 100 hours (Figure 5).
Methane conversion efficiency dropped only about 3% from that for a fresh catalyst; and as shown in Figure 5, the conversion efficiency for methane dropped only an additional 2-5% when aging was carried out for about 100 hours at the same temperature and redox ratio. This is a significant improvement in resistance to aging.
Comparison of the conversion efficiencies for CH4 reported for Pd/La203/A12O3 and Pd/A12O3 catalysts (Figure 6) shows that the CH4 conversion for the former is approximately 30% higher in the redox ratio window 1.2 to 1.4. The NO conversion for the two catalysts is comparable. The addition of La2O3 lowers the CO conversion by about 7% when R is between 1.2 and 1.4.
This invention also provides a method of making a catalyst system in order to obtain optimum methane conversion for the treatment of the exhaust from a CNG
fueled internal combustion engine. This comprises the steps of sequentially impregnating a gamma alumina support with lanthana and palladium by incipient wetness. This included: (a) contacting an amount of A12O3 first with lanthanum nitrate solution of desired concentration to obtain 0.5-20% lanthana by weight of the catalyst and thereafter contacting the lanthana impreqnated alumina with a palladium nitrate solution of desired concentration to obtain a 0.2-30% (by weight of the catalyst) palladium loading, the first and second stages of this method being separately followed by drying and calcination.
Lanthanum nitrate may be substituted by use of other lanthanum compounds soluble in water, acids, and organic solvents, examples of which include lanthanum isopropoxide, lanthanum oxalate, lanthanum acetate, lanthanum halides, lanthanum hydroxide, lanthanum carbonate. The palladium nitrate solution may be substituted by other palladium compounds such as palladium chloride, palladium acetate, palladium 2.4 pentane dionate, that are soluble in either polar or nonpolar solvents. Drying is preferably carried out at a temperature of 100C (373 K) for about one hour and calcination is preferably carried out at a temperature of 600~ (873 K) for about six hours. It is desirable that the method be one in which the impregnations are - 12 - ~ , ;~?
sequential although simultaneous impregnation may also be carried out, but the latter leads to coverage of active Pd metal by amorphous l.a2O3 and this leads to lower activity.
In most of the examples used to generate data for Figures 1-6, the catalyst of this invention was made by contacting 5 grams of alumina with 6 cc's of lanthanum nitrate solution of desired concentration. The resulting precursor was dried at 100C for one hour and calcined at 10 600C for six hours to form a 10% La2O3/A12O3 composite oxide. Fivé grams of such composite oxide were contacted with 6 cc's of palladium nitrate solution of desired concentration to obtain a 1% Pd/10%
La2O3/A12O3 precursor. The latter precursor was 15 dried at 100C for one hour and calcined at 600C for six hours to form the catalyst.
This invention also comprehends a method of treating exhaust gas from a CNG fueled engine with the catalyst system of claim 1, comprising: (a) operation of said engine at slightly rich of stoichiometryi (b) exposing such catalyst to the exhaust gases in the range of 400-750C and at a space velocity in the range of 2-100 X hr 1, said exhaust gas being converted at an efficiency greater than 90% for each of ~O, CO, and CH4. It may also be possible to close-couple this catalyst to the engine to ensure the exposure to exhaust gases in the temperature range of 400-750C so that the time required to attain the light-off temperature is shortened. This results in lower tail pipe emissions.
TREATING EMISSIONS FROM ENGINES FUELED
BY COMPRESSED NATURAL GAS
Background of the Invention Technical field This invention relates to the technology of catalytically converting emissions of a compressed natural gas (CNG) fueled engine, and more particularly to catalytic conversion of exhaust gases containing saturated hydrocarbons, including methane.
Discussion of the Prior Art Natural gas (essentially 85% methane~ is an attractive source of fuel for vehicles because it provides for a lower fuel cost, longer engine life, lower maintenance, and reduced oil consumption. Development of catalysts for high efficiency removal of saturated hydrocarbons, including methane, by oxidation within an exhaust stream is of strategic importance; it may be crucial in view of the emission control requirements promulgated by the U.S. Government. In the past, oxidation of methane has received little attention in automotive catalysis. Extreme difficulty of removal of methane is experienced because a C-H bond must be ruptured. In the oxidation of higher alkanes, oxidation is easily achieved by the cleavage of C-C bonds. Since the C-H bond is stronger, methane is more difficult to oxidize.
The prior art has investigated the use of noble metals and base metals as catalysts for stimulating the oxidation of methane by cleavage of the C-H bond.
Alumina, silica, thoria, and titania supported platinum and palladium catalysts were evaluated in 1983 and 1985 (see C.F. Cullis and B.M. Willatt, Journal of Catalysis, - 2 - 2~ f ~
Vol. 83, p. 267, 1983; and V.A. Drozdov, P.G.
Tsyrulnikov, V.V. Popovskii, N.N. Bulgakov, E.M. Moroz, and T.G. Galeev, Reaction Kinetic Catalysis Letters, Vol.
27, p. 425, 1985). These studies showed that an alumina supported palladium catalyst is the most active, followed by an alumina supported platinum catalyst. A reduction in catalytic activity is observed when silica and titania are used as supports. A systematic study of the use of alumina supported base metal catalysts for methane oxidation was conducted in 1963; chromium was found to be the most active. At a metal loading of 3.1 weight percent, chromium was found to be comparable to palladium (see K.C. Stein, J.J. Feenan, L.J. Hofer, and R.B.
Anderson, Bureau of Mines Bulletin, No. 608, U.S.
Government Printing Office). However, use of only Cr2O3 on A12O3 is disadvantageous because of the volatile and toxic nature of Cr2O3 and the poor durability of the Cr2O3-A12O3 catalyst- In another article evaluating base metal catalysts for methane oxidation, unsupported Co3O4 was found most active (see R.B. Anderson, K.C. Stein, J.J. Feenan, and L.J. Hofer, Industrial Engineering Chemistry, Vol. 53, p.
809, 1961). However, use of only Co3O4 on A12O3 is disadvantageous because of the volatile and toxic nature of Co3O4 and the tendency of Co to form a low surface area spinel with A12O3 resulting in poor durab;lity.
The prior art has found that the deactivation of a palladium on alumrna catalyst can occur by the reaction of water vapor with~palladium oxide to form Pd(OH)2.
It is desirable to retard the mobility of the adsorbed water vapor species and thereby reduce Pd~OH)2 formation. Such prior art has also found that palladium oxide is less active than palladium, and therefore it is desirable to retain palladium in the metallic state and _ 3 _ 2~ f ~
inhibit the formation of palladium oxide.
In the course of examining Pd on A12O3 at an effective three-way catalyst (converting methane, CO and NOX), the prior art has demonstrated a negative teaching to the use of lanthana with palladium (see H.
Muraki, "Performance of Palladium Automotive Catalyst", SAE Technical Paper Series No. 910842, 1991). This work resulted in a conclusion that total hydrocarbon conversion of CH4, being the most difficult hydrocarbon to oxidize, by palladium/lanthanum catalysts, near stoichiometric conditions, is lower than that of a palladium catalyst by itself; La2O3 is believed to suppress hydrocarbon oxidation activity. The prior art has also demonstrated that the use of La2O3 with palladium increases the hydrocarbon light-off temperature (see H. Muraki et al, "Palladium-Lanthanum Catalysts for Automotive Emission Control", Ind. Eng. Chem. Prod. Res.
Dev., 25 (1986) 202-208).
Lanthana has been used by the prior art with Pd/A12O3 catalysts in ways not related to catalyst conversion enhancement, namely, to thermally stabilize the alumina support (see U.S. patent 4,906,176). This patent teaches the use of other catalytic components, i.e., manganese, chromium, zirconium, rare earth elements, tin, zinc, copper, magnesium, barium, strontium, and calcium to promote catalytic activity.
However, patent '176 fails to appreciate the conversion enhancement role La2O3 may play during CH4 oxidation because the disclosure used the wrong and undesirable form of oxide support (i.e., La2O3.11A12O3), the lanthana was not deposited correctly, operated under genelally lean conditions, and never measured the conversion efficiency attained using the above catalyst because of their primary interest in measuring thermal b 35 stability.
SummarY of the Invention The invention pertains to a three-way catalyst system for heating the exhaust of a compressed natural gas fueled engine operated slightly rich of stoichiometry. This catalyst system achieves simultaneous removal of saturated hydrocarbons, particularly methane, nitric oxide, and carbon monoxide at enhanced rates. Also, the light-off temperatures are lower. The catalyst system comprises a high surface area gamma alumina support impregnated with an intimate mixture of 0.2-30% palladium and 0.5-20% lanthana, the palladium being in a crystalline form and having a particle size in the range of 5 to 1000 angstroms. The intimate presence of La2O3 permits Pd to adsorb CH4 and 2 instead of Pd becoming oxidized.
The intimate mixture is assured by providing a substantially continuous contact between the palladium and lanthana. ~he exhaust gas composition is slightly rich (of stoichiometry), preferably in the redox ratio, R, range of 1.1-1.2 (R being the ratio of reducing components to oxidizing components in the exhaust gas.
Enhanced o~idation rate for methane by use of this invention allows conversions greater than 90% when the exhaust gas is maintained in the temperature range of 400-750C, and at a space velocity of 5-100 K hr 1.
The light-off ternperature for methane (at 50% conversion) is no greater than 450C and can be as low as 300C.
The catalyst of this invention has also achieved increased aging resistance by retaining a CH4 conversion efficiency at or above 80% after 100 hours at 550C.
Other aspects of this invention comprise a method of making such catalyst and a method of treating CNG emissions. The method of making involves: (a) sequentially impregnating gamma alumina with lanthanum and palladium by incipient wetness techniques that involve contacting a desired amount of alumina first with a lanthanum nitrate solution of desired concentration to obtain 0.5-20% lanthana by weight of the catalyst and thereafter contacting the lanthana impregnated alumina with a palladium nitrate solution of desired concentration to obtain a 0.2-30% palladium content in the catalyst, the impregnated alumina, after each stage of impregnation, being dried and calcined prior to the next stage. The method of treating comprises: (a) operating the CNG fueled engine slightly rich of stoichiometry; and (b) exposing a catalyst constituted of a high surface area gamma alumina support impregnated with an intimate mixture of .5-20% La2O3 and .2-30~
Pd, to exhaust gases in the range of 400-750C, and at a space velocity of 5-100 K hr 1, the exhaust gas being converted at an efficiency greater than 90% for each of NO, CO, and CH4.
Brief Description of the Drawings Figure 1 is a graphical illustration of oxidation efficiency of different catalysts, including the catalyst of this invention, as a function of temperature to provide light-off information for oxidation of methane.
Figure 2 is a graphical illustration of conversion efficiency as a function of temperature for the catalyst of this invention; it illustrates the conversion efficiency for the various gaseous species using a gas that simulates emissions from a CNG engine.
Figure 3 is a graphical illustration of conversion efficiency as a function of redox ratio using simulated CNG emissions; this illustrates the difference in methane, C0, and NO conversion efficiency for an - 6 ~
embodiment of this invention compared to a palladium/rhodium prior art catalyst.
Figures 4 and 5 are graphical illustrations similar to that of Figure 3, but illustrating the effects of aging after about 80 and 100 hours, respectively, of use of the catalyst of this invention.
Figure 6 is a graphical illustration of the conversion efficiency as a function of the redox ratio using simulated CNG emissions; this illustrates the difference in CH4, CO, and NO conversion efficiency resulting from the addition of La to a Pd/A12O3 catalyst.
Detailed Description and Best Mode The catalyst functions three-way (CO, HC, and NO) to cleanse the exhaust of a CNG fueled internal combustion engine when operated under slightly rich conditions, although such catalyst system can be used to convert HC and CO from any (fuel-lean or stoichiometric) gaseous stream that contains saturated hydrocarbons, particularly methane, provided the gas stream is maintained in a desired temperature range. When used for cleansing the automotive exhaust of a CNG fueled engine, such catalyst system will provide a lower light-off temperature compared to the prior art catalysts and will provide methane conversion equal to or greater than 90%
at slightly rich conditions.
The catalyst is comprised of a high surface area gamma alumina support impregnated with 0.5-20% lanthanum oxide (La2O3) and 0.2-30% palladium, by weight of the catalyst.
The exhaust, which this catalyst is designed to treat, is slightly rich and contains saturated hydrocarbons, particularly methane. This means that the redox ratio R is generally in the range of 1.1-1.2, --7-- 2. ~ J ~
although the catalyst will perform with less efficiency at redo~ ratios outside such range. The temperature of such an exhaust gas should be in the range of 400-750C
in order to obtain optimum methane conversion, such temperature being reached during normal operating conditions rather than during start-up of the engine.
Optimum conversion is facilitated when the catalyst has a space velocity in the range of 2-100 K hr . At space velocities outside of this range, the conversion efficiency of the catalyst will be detrimentally affected. Methane conversion is lowered particularly when the space velocity increases; however, CO and NO
conversions are not significantly affected when the space velocity increases above 100 K hr 1. The exhaust gas from a CNG fueled engine will typically have a hydrocarbon content in the range of 60-3000 ppm, a CO
content in the range of 450-22,500 ppm, a NO content in the range of 85-4250 ppm, and a H2 content in the range of 150-7500 ppm. Oxygen will generally be about 320-16,000 ppm. To simulate such an e~haust gas, the ratio of hydrogen to CO should be in the range of 0.25-0.75.
The support must be of gamma alumina rather than delta or alpha forms of alumina because delta and alpha forms of alumina have, among other factors, low surface areas. With gamma alumina, the surface area will be significantly increased and be in the range of about 50-400 m2/gm. The particle size of the gamma alumina should be less than 200 angstroms, and the monolith carrier should have a cell size in the range of 100-600 cells per square inch. Gamma alumina may also be modified with oxides of base, rare earth and alkali metals such as barium, cerium, titanium, and nickel.
The lanthana impregnation is carried out to load the support with lanthana in the weight range of 0.5-20%. If lanthana is added in an amount less than such range, then the beneficial effect of increase in activity due to lanthana addition is not observed. If lanthana exceeds such range, then the lanthana surface area decreases and no additional benefit is derived.
~ Lanthana, as used herein, provides a unique chemical union with the palladium metal to obviate or delay the oxidation of the palladium and thereby leads to a synergistic relationship for catalysis. Elements that are equivalent to the function of lanthana for purposes of this invention may include tungsten oxide and molybdenum oxide.
Palladium is impregnated in a manner to provide the presence of large crystalline particles, preferably in the particle range of 5-1000 angstroms. With palladium weight loadings below 0.2%, there will be an insufficient catalysis effect and therefore not promote the objects of this invention. If the palladium loading is in excess of 30%, the palladium surface area decreases and no additional benefit from palladium addition is derived.
Other elements that may be present in the catalytic impregnation may include elements that avoid retention of water for improving the long life stability of catalysts. This may include elements such as tungsten oxide (incorporated by using ammonium meta tungstate during the impregnation process) or chromium oxide, both of which tend to prevent oxidation of palladium by reducing the mobility of water and thereby keeping it away from the palladium.
Some of the chemical reactions that take place as a result of using the catalyst of this invention are indicated below:
2 ~ J. .~
CH~ + 202--~C02 ~ 2H20 CO + 1/202 ~ C2 H2 + 1/202 ~3 H20 In additional to these steps, several reactions such as those between CH4 and NO, CO and NO, and NO and H2 occur. The stoichiometric reactions are complicated and vary with exhaust gas composition.
Performance As shown in Figure 1, the Pd/La2O3/A12O3 catalyst of this invention is compared with prior art catalysts to indicate the improvement in light-off characteristics when operating on the exhaust gas from a CNG fueled engine. Light-off temperature is that temperature at which 50% conversion efficiency is achieved. The flow study used a simulated exhaust gas having 1500 ppm CH4, 3000 ppm 2' and balance of N2 (at 30 K hr 1 SV).
The temperature obtained for light-off USing a 1~ palladium and 3-10% lanthanum catalyst was about 4gO-445C; when the palladium was increased to as much as 10% with 3% lanthana, the light-off temperature occurred as low as 340C. This is in striking contrast to the light-off temperatures for the prior art catalysts which range from 490-580C.
Figure 2 shows the light-off characteristics for methane, carbon monoxide, and nitric oxide when the invented catalyst is used for treating the simulated CNG
vehicle exhaust mixture. The light-off temperature for CH4, NO, and CO are 450C, 205C, and 170C, respectively, when the redox ratio is 1.15. AS shown in Figure 2, the optimum temperature for achieving maximum methane conversion is in the range of S50-650C for the invented catalyst herein. Thus, the catalyst of this invention achieves extremely high three-way conversion efficiencies when operated in such temperature range and at a redox ratio (slightly rich) around 1.15. Under similar conditions, the light-off temperatures for the invented catalyst are lower than those of the Pd/Rh prior art catalyst.
As shown in Figure 3, all of NO, CO, and methane will be converted at a level equal to or greater than 90%
when such conditions are met (temperature 550-650C, R = 1.1-1.2) and the catalyst of this invention is deployed. The optimum methane conversion efficiency obtained when the prior art catalyst (palladium/rhodium shown in broken line and solid symbols) is employed does not exceed 70% at a redox ratio of about 1.2; this is substantially lower than about 90% achieved by the catalyst of this invention. Equally important is the conversion of NO which drops to about 80% for the prior art catalyst (palladium/rhodium) at a redox ratio of about 1.2; a conversion efficiency of about 100% for N0 is achieved when using the catalyst of this invention.
An examination was made of the aging characteristics of the catalyst of this invention after a period of about 80 hours at 550C at a redox ratio of 1.02 (Figure 4) and after about 100 hours (Figure 5).
Methane conversion efficiency dropped only about 3% from that for a fresh catalyst; and as shown in Figure 5, the conversion efficiency for methane dropped only an additional 2-5% when aging was carried out for about 100 hours at the same temperature and redox ratio. This is a significant improvement in resistance to aging.
Comparison of the conversion efficiencies for CH4 reported for Pd/La203/A12O3 and Pd/A12O3 catalysts (Figure 6) shows that the CH4 conversion for the former is approximately 30% higher in the redox ratio window 1.2 to 1.4. The NO conversion for the two catalysts is comparable. The addition of La2O3 lowers the CO conversion by about 7% when R is between 1.2 and 1.4.
This invention also provides a method of making a catalyst system in order to obtain optimum methane conversion for the treatment of the exhaust from a CNG
fueled internal combustion engine. This comprises the steps of sequentially impregnating a gamma alumina support with lanthana and palladium by incipient wetness. This included: (a) contacting an amount of A12O3 first with lanthanum nitrate solution of desired concentration to obtain 0.5-20% lanthana by weight of the catalyst and thereafter contacting the lanthana impreqnated alumina with a palladium nitrate solution of desired concentration to obtain a 0.2-30% (by weight of the catalyst) palladium loading, the first and second stages of this method being separately followed by drying and calcination.
Lanthanum nitrate may be substituted by use of other lanthanum compounds soluble in water, acids, and organic solvents, examples of which include lanthanum isopropoxide, lanthanum oxalate, lanthanum acetate, lanthanum halides, lanthanum hydroxide, lanthanum carbonate. The palladium nitrate solution may be substituted by other palladium compounds such as palladium chloride, palladium acetate, palladium 2.4 pentane dionate, that are soluble in either polar or nonpolar solvents. Drying is preferably carried out at a temperature of 100C (373 K) for about one hour and calcination is preferably carried out at a temperature of 600~ (873 K) for about six hours. It is desirable that the method be one in which the impregnations are - 12 - ~ , ;~?
sequential although simultaneous impregnation may also be carried out, but the latter leads to coverage of active Pd metal by amorphous l.a2O3 and this leads to lower activity.
In most of the examples used to generate data for Figures 1-6, the catalyst of this invention was made by contacting 5 grams of alumina with 6 cc's of lanthanum nitrate solution of desired concentration. The resulting precursor was dried at 100C for one hour and calcined at 10 600C for six hours to form a 10% La2O3/A12O3 composite oxide. Fivé grams of such composite oxide were contacted with 6 cc's of palladium nitrate solution of desired concentration to obtain a 1% Pd/10%
La2O3/A12O3 precursor. The latter precursor was 15 dried at 100C for one hour and calcined at 600C for six hours to form the catalyst.
This invention also comprehends a method of treating exhaust gas from a CNG fueled engine with the catalyst system of claim 1, comprising: (a) operation of said engine at slightly rich of stoichiometryi (b) exposing such catalyst to the exhaust gases in the range of 400-750C and at a space velocity in the range of 2-100 X hr 1, said exhaust gas being converted at an efficiency greater than 90% for each of ~O, CO, and CH4. It may also be possible to close-couple this catalyst to the engine to ensure the exposure to exhaust gases in the temperature range of 400-750C so that the time required to attain the light-off temperature is shortened. This results in lower tail pipe emissions.
Claims (10)
1. A method of treating exhaust gas from a CNG
fueled engine, comprising:
(a) operating said engine at slightly rich of stoichiometry;
(b) exposing such catalyst to the exhaust gases in the range of 400-750°C and at a space velocity in the range of 2-100 K hr-1, said exhaust gas being converted at an efficiency greater than 90% for each of NO, CO, and CH4.
fueled engine, comprising:
(a) operating said engine at slightly rich of stoichiometry;
(b) exposing such catalyst to the exhaust gases in the range of 400-750°C and at a space velocity in the range of 2-100 K hr-1, said exhaust gas being converted at an efficiency greater than 90% for each of NO, CO, and CH4.
2. The method of treating as in claim 1, in which said catalyst is close-coupled to said engine to provide a shorter light-off time and thereby lower tail pipe emissions.
3. The method of treating as in claim 1, in which said method is carried out for at least 100 hours at temperatures up to 750°C while preventing degradation of conversion efficiency for CH4 by no more than 3%.
4. The method of treating as in claim 2, in which the light-off temperature for CH4 is lower than 450°C.
5. A method of making a catalyst for cleansing the exhaust gas of a CNG fueled engine operated under slightly rich conditions, the method comprising sequentially impregnating a high surface area gamma alumina support with lanthana and palladium by incipient wetness techniques, such techniques comprising contacting sequentially a desired amount of alumina first with a lanthanum nitrate solution of desired concentration to obtain 0.5-20% lanthana, contacting the lanthana impregnated alumina with a palladium nitrate solution of desired concentration to obtain a 0.2-30% palladium content in the catalyst, each of said steps being separated by drying and calcination.
6. The method of making as in claim 5, in which the high surface area gamma alumina support contains trace amounts (< 1%) of oxides of base metals, rare earth metals, and alkali metals such as Ba, Ce, Ti, and Ni.
7. The method of making as in claim 5, in which said drying is carried out in the temperature range of 20-200°C and calcination is carried out in the temperature range of 200-1200°C, each being respectively carried out for periods up to 48 hours.
8. The method of making as in claim 5, in which the particle size of said support is controlled to be c 200 angstroms, and the particle size of said palladium is controlled to be in the range of 20-1000 angstroms, said palladium being comprised of large crystalline particles
9. The method of making as in claim 8, in which said support has a surface area of 50-400 m2/gm.
10. The method of making as in claim 5, in which a proportion of said lanthana is replaced by tungsten oxide or molybdenum oxide.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2079122 CA2079122A1 (en) | 1991-10-07 | 1992-09-25 | Treating emissions from engines fueled by compressed natural gas |
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US722,317 | 1991-06-27 | ||
| CA 2079122 CA2079122A1 (en) | 1991-10-07 | 1992-09-25 | Treating emissions from engines fueled by compressed natural gas |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2079122A1 true CA2079122A1 (en) | 1993-04-08 |
Family
ID=4150458
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2079122 Abandoned CA2079122A1 (en) | 1991-10-07 | 1992-09-25 | Treating emissions from engines fueled by compressed natural gas |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2079122A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN105715337A (en) * | 2014-12-19 | 2016-06-29 | 罗伯特·博世有限公司 | Method for operating methane oxidation catalyst and exhaust gas aftertreatment system |
-
1992
- 1992-09-25 CA CA 2079122 patent/CA2079122A1/en not_active Abandoned
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN105715337A (en) * | 2014-12-19 | 2016-06-29 | 罗伯特·博世有限公司 | Method for operating methane oxidation catalyst and exhaust gas aftertreatment system |
| CN105715337B (en) * | 2014-12-19 | 2021-08-27 | 罗伯特·博世有限公司 | Method for operating a methane oxidation catalyst and exhaust gas aftertreatment system |
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