FI104749B - Exhaust gas purification system to reduce hydrocarbon emissions during cold start of internal combustion engines - Google Patents

Abstract

The invention relates to an exhaust gas purification system for decreasing the emissions of hydrocarbons during the cold start of internal combustion engines. The exhaust gas purification system contains a hydrocarbon adsorber and a downstream catalyst system which can be composed of a single three-way catalyst or of a combination of oxidation, reduction and/or three-way catalysts in one or more beds. The hydrocarbon emissions during the cold start phase can be significantly decreased by using oxidation catalysts or three-way catalysts which, in comparison with conventional catalysts, have at least twice the loading of platinum and/or palladium.

Description

! 104749

Exhaust gas purification system to reduce hydrocarbon emissions during cold start of internal combustion engines

The invention relates to an exhaust gas cleaning system 5 for reducing hydrocarbon emissions during cold start of internal combustion engines. The exhaust gas purification system includes a hydrocarbon adsorbent and a catalyst system coupled thereafter, which may comprise a single tritium catalyst or a combination of oxidation, reduction and / or tri-carbon catalysts in one or more layers.

Future limit values for the emission of harmful substances in motor vehicles are laid down in Regulations TLEV / 1994 and LEV / 1997 (LEV = Low Emission Vehicle). They represent a tightening of the limit values, in particular 15 for hydrocarbons. As current exhaust catalysts have achieved high conversion rates of harmful substances at operating temperature, compliance with future limit values is only possible by improving the conversion of harmful substances during the cold start phase. Cold run of test cycles defined by regulation (eg US FTP-75). in fact, a large part of the hydrocarbons released in the process is discharged. At this stage, the catalysts have not yet reached the conversion condition of this operation temperature of 300-400 ° C. The hydrocarbons released during the cold start phase are mainly • C ^ C - compounds such as paraffins, isopa- raffins, olefins and aromatic compounds.

To reduce emissions of harmful substances ·· *. During the 30 cold-start phases, there are, for example, US patent -. ···. The exhaust gas purification system disclosed in 5,078,979, which consists of a hydrocarbon adsorbent and a catalyst coupled thereafter. The hydrocarbon adsorbent serves to adsorb during the cold start phase at relatively low temperatures, The hydrocarbons are desorbed again when the adsorber heats up more and then end up with the hot exhaust gas at a catalyst already close to the operating temperature, where it becomes harmless water 5 and carbon dioxide. which is also abundant in the exhaust.

The disadvantage of the solution described here is already the desorption of 10 hydrocarbons starting at relatively low temperatures, so that the next catalyst cannot yet achieve optimum conversion. Typically, there is a gaping gap between the catalyst start temperature TA of 300-400 ° C and the desorption temperature TD of the adsorbent 15 immediately preceding it, which is about 150-200 ° C, above 100 ° C, i.e. TA-TD> 100 ° C. In addition, there is a risk that the adsorber will be destroyed by heat since it will need to be installed near the engine in the exhaust gas cleaning system and will therefore be exposed to temperatures of up to 1000 eC in continuous operation.

A great deal has been done to eliminate these shortcomings. . There are a number of suggestions in the patent literature, such as [; * · 'DE-A-4 008 789, EP-A-0 460 542 and US-A-5 051 244]. *. · 25 documents also start with a combination of a hydrocarbon adsorbent and a catalytic converter, but propose to eliminate the described drawbacks for the exhaust exhaust.

U.S. Patent 5,051,244 has proposed a molar ·· *. 30 coupling of a silica screen adsorber to the actual catalyst, ···. to the front, which adsorbs in the cold conditions *** the harmful substances present in the exhaust gas, in particular: hydrocarbons, and removes them with additional heating of the exhaust gas purification system. To protect the Adsorber from 35 overheating caused by continuous operation of motor 3 104749, a short circuit switching motor from motor to catalyst is provided.

During the first 200 to 300 seconds after start-up, all exhaust gas is passed through an ad-5 sorbent and a catalyst. At this stage of use, the adsorber retains hydrocarbons. The adsorber and the catalyst are getting more and more warm due to the hot exhaust gas. The adsorber is short-circuited when desorption begins to be greater than that of ad-10 as a result of temperature rise. The exhaust then passes directly through the catalyst. When the operating temperature is reached, a portion of the hot exhaust gas is passed through an adsorber until complete desorption of the harmful substances which the catalyst is capable of converting at that stage with a good efficiency. After desorption has occurred, the adsorber is short-circuited again to protect it from destruction by thermal overload.

U.S. Patent Nos. 5,051,244 and 5,078,979 propose natural or synthetic zeolites with an atomic ratio of Si / Al of at least 2.4 as adsorbents.

Suitable zeolites include silicalite, fauzite, clinoptilolite, mordenite, cabazite, ultrasound zeolite Y, zeolite Y and ZSM-5 and mixtures thereof. The zeolite adsorbent may further contain finely divided, catalytically active metals such as platinum, palladium, rhodium, ruthenium, and mixtures thereof.

These prior art prior art solutions are either technically very demanding, expensive, and susceptible to interference, or, as in U.S. Patent 5,078,979. ···. 30, do not provide a solution to the desorption temperature of the adsorber · · -. ···. • ~ ~ * · * to close the temperature gap between the start and the catalyst. It is an object of this invention to provide an exhaust gas purification system that can overcome these prior art shortcomings.

»I

I I I

4 104749 while retaining very well hydrocarbons during the cold start phase.

This object is achieved by reducing hydrocarbon emissions during the cold start of internal combustion engines 5 by means of an exhaust gas purification system comprising a hydrocarbon adsorbent and a catalyst system coupled therewith comprising a single three-way catalyst or one or more multipliers and / or multipliers.

The present invention relates to an exhaust gas purification system for reducing hydrocarbon emissions during cold start of internal combustion engines, which system comprises a hydrocarbon adsorbent and a subsequent catalyst system comprising one or more layers of oxidation and chromium catalyst. The system is characterized in that the catalyst system coupled to the hydrocarbon adsorbent comprises an oxidation catalyst containing platinum and / or palladium and a three-way catalyst containing platinum and / or palladium and / or rhodium, whereby the oxidation catalyst is platinum and platinum. / or the palladium load is • · «.! * at least 3.5 g of platinum and / or palladium / dm3 catalyst volume, and the oxidation catalyst is fitted immediately after the adsorber.

: In the exhaust gas purification system according to the invention, the starting temperature TA of the oxidation or • ··: three-way catalyst and the desorption temperature of the adsorber directly connected to it. ***; The difference in 30 TD is less than 50 ° C, i.e. TA - TD <50 ° C.

Ml ··. The catalyst start-up temperature refers to the exhaust gas temperature before the catalyst at which the catalyst converts exactly 50% of the hydrocarbons.

The desorption temperature TD of the adsorber is a parameter that can only be determined when the engine is powered by power. For this purpose, the gross hydrocarbon emission of the engine is first measured as a function of time over the first 200 to 300 seconds after a cold start without using an ad-sorbent. This gross emission typically has a high and 5 wide peak during the first 60 to 100 seconds. Hydrocarbon emissions drop more and more as the engine warms to a level that is normal for an engine that has reached operating temperature. The second test run then measures the hydrocarbon emission after the coupled adsorber and the temperature before the adsorber as a function of time.

The adsorber initially provides a significant reduction in hydrocarbon emission through adsorption, but then increases as the exhaust gas warms up due to increased desorption from the adsorber and also lags the emission peak with respect to gross emission before it eventually drops to its operating temperature. As a result of the time difference between the gross emission peak and the emission peak using the adsorber, the intersection of the two emission curves will be encountered at some point within about 60 to 100 seconds 20 after a cold start.

'• S' At this point, the exhaust gas temperature t · · 'preceding the adsorber is called the desorption heat of the adsorber. It depends on the structure of the respective exhaust system: 25 and the adsorbent material itself, and is

* I

typically 150 to 200 ° C.

t · · • V Preferred adsorbent materials are zeolites.

li: However, as already disclosed in U.S. Patent No. 30,551,244, only those zeolites, * * • a 9. ··, which selectively adsorb hydrocarbons prior to water, are suitable for the adsorption of hydrocarbons from exhaust gas 1, f · · *. are hydrophobic and also have high heat and acid stability, ......

The hydrocarbon adsorbent should contain at least one 35 hydrophobic heat- and acid-stable zeolite having an ir / 104749 6 ratio of Si / Al greater than 20. In one particularly preferred embodiment of the invention, the adsorber combines two zeolites having changes in hydrocarbon adsorption capacity as a function of temperature different sorts of. In this case, at least two zeolites I and II should be combined, of which zeolite I has a higher adsorption capacity below 100 ° C than zeolite II and zeolite II has a higher adsorption capacity than zeolite 10 I. Zeolite I can be used for example with a Si / Al ratio greater than 40, and as zeolite II zeolite ZSM5 with a Si / Al ratio greater than 20. The ratio by mass of de-aluminized zeolite Y to zeolite ZSM5 in the adsorber should be 1:10 to 10 : 1. Zeolite Y belongs to large porous zeolites and has a pore diameter of 0.74 nm, a pore volume of 0.3 cm3 / g and a specific surface area of more than 700 m2 / g. Zeolite ZSM5 is a medium pore size zeolite with a pore diameter of about 0.5 nm. Zeolite Y initially has a high adsorption capacity for the aromatic compounds contained in the exhaust gas due to its large 20 pores.

.. However, the adsorption capacity decreases very rapidly

t I I

* when the temperature rises. In contrast, the zeolite ZSM5 initially has a lower adsorption capacity for aromatic compounds, but this capacity decreases with a smaller increase in temperature. • In addition, the latter zeolite has a good adsorption capacity on the remaining hydrocarbons still present in the exhaust gas: The combination of the two zeolites according to the invention results in optimal adsorption properties in the temperature range of interest, but not limited by the invention. * · 1 only for the mixture of the two zeolites, other zeolite mixtures may be used as long as their components meet the temperature dependence of the adsorption capacity and 35 pore diameter.

* (<

(I

i

f I

104749 7

The high Si / Al ratio of the zeolites used in accordance with the invention guarantees, on the one hand, highly selective adsorption of hydrocarbons relative to water adsorption and, on the other hand, good thermal stability above 1000 ° C and good acid resistance. Thermal stability is essential for the exhaust gas purification system of the invention, since the adsorber is installed close to the engine and is thus exposed to high temperatures.

The catalyst-10 system coupled to the hydrocarbon adsorber may comprise a single three-way catalyst or a combination of oxidation, reduction and / or three-way catalysts in one or more layers.

Such catalysts and their preparation are well known to those skilled in the art. They usually consist of an open-cell cellular carrier in the shape of a cell, which is a collector or metal. In order to incorporate the catalytically active noble metals, such cellular structures are provided with an activity-enhancing, high-surface, oxide-containing dispersion coating containing, for example, 100 to 400 g, usually 160 g, of gamma alumina per dm3 cell volume. Catalytically active precious metals can be applied thereto over the oxide <·,! by impregnating the chalkboard. Oxidation catalysts. platinum and / or palladium] ·] · 25 are preferably used in the pause. Three-way catalysts contain catalytically * · ·: ·! ! platinum and / or palladium as active precious metals

M I

: V and / or rhodium.

In the case of an exhaust gas purification system consisting of an adsorber, an oxidation catalyst and a three-way catalyst according to the invention, the oxidation catalyst is M »-. · *. the amount of platinum and / or palladium incorporated in the market, f ·· ..... - • t, relative to the amount l ΙΙΓ "contained in the oxidation catalysts, which is 0.01 to 1.8 g / dm3 of catalyst volume, at least twice , i.e., at least 3.5 g Plati-35 naa and / or palladium / dm3 catalyst volume.

I I

«Il 104749 8 The fraud catalyst is placed immediately after the adsorber.

The presence of a large amount of catalytically active elements results in a start temperature of less than about 50-100 ° C compared to conventionally charged catalysts.

In the case of a catalyst system coupled to an adsorber consisting of only one three-way catalyst containing platinum group metals platinum and / or 10 palladium and / or rhodium, the amount of platinum and / or palladium oxide present in this catalyst may be increased to lower the is from 0.01 to 1.8 g / dm 3 of the catalyst at 15 volumes, at least twice, i.e. to at least 3.5 g of platinum and / or palladium / dm3 of catalyst volume.

The adsorbent may be used as a bulk product in the form of tablets, extrudates or agglomerates. However, the use of an adsorbent on a dispersion coating with a monolithic honeycomb structure is preferred. . in the form of a coating of 100 - 400 g / dm3 • · «* volume of the cell structure. The actual amount of fuel used is determined by the hydrocarbon * of its internal combustion engine. 25 work to eliminate the toxins produced.

• I

A: Each professional can determine the most advantageous • · · • amount in a few tests.

: T: The dispersion coating is applied, for example, by dipping the honeycomb structure into an aqueous solution of the adsorbent mixture, followed by blowing, drying, and, optionally, calcining to adhere the coating to the honeycomb structure. In order to apply the desired amount of adsorbent, this coating process may be repeated several times if desired.

t <i i r 9 104749

Another possibility to achieve the object of this invention is to provide an exhaust gas purification system for reducing hydrocarbon emissions during cold start of internal combustion engines, which includes a hydrocarbon adsorbent directly in contact with the oxidation catalyst and one or more three-way catalyst coupled thereafter. This exhaust gas purification system is characterized in that the start temperature TA of the oxidation catalyst for hydrocarbon conversion and the desorption temperature TD of the adsorber in direct contact with the oxidation catalyst are less than 50 ° C, i.e. TA-TD <50 ° C.

Direct contact of the adsorber with the oxidation catalyst may be effected in the form of overlapping coatings with the monolithic honeycomb structure 15, with the adsorbing coating over the catalyst coating.

The selection and composition of the adrobant mixture and catalyst is as described above. In addition to reduced hydrocarbon emissions, carbon monoxide emissions from the latter exhaust gas cleaning systems are also significantly lower during cold start.

«« ·

The invention will now be further elucidated by means of a few prefixes.

Figure 1 shows the hydrocarbon emissions of the internal combustion engine of the US FTP-75 tests with the exhaust gas purification system of Comparative Example 3a according to Comparative Example 3a; during the phase.

Figure 1a is a schematic diagram of the structure of the exhaust gas purification system.

M1 - Fig. 2 shows the comparative example 3b according to • · · •. exhaust gas purification] hydrocarbon emissions from internal combustion engine r # p p * during the cold start phase of US FTP-75 tests.

r i i r l - i t • »« 104749 10

Figure 2a is a schematic diagram of the structure of the exhaust gas purification system.

Figure 3 shows the carbon-5 emission of a combustion engine equipped with an exhaust gas purification system according to Example 3 during the cold start phase of US FTP-75 tests.

Figure 3a is a schematic diagram of the structure of the exhaust gas purification system.

Example 1 10 Adsorption Properties of Zeolite Y and ZSM5 A DAY zeolite (dealuminated zeolite Y) with a Si / Al ratio greater than 100 and two zeolite 2SM5 with a Si / Al ratio greater than 500 was determined. and 58, adsorption capacity on toluene at 20 ° C and 80 ° C. 15 The results are shown in Table 1:

Table 1 Adsorption capacity T (° C) for DAY zeolite and ZSM5 Zeolite Si / Al Μ, 1 β ββη1 (g / 100 g) 20 20 DAY> 100 15.1 20 ZSM5> 500 6.2 20 ZSM5 58 7, 12 ^ «« 80 DAY> 100 0.8 • «:" 80 ZSM5> 500 1.4 «· 25 80 ZSM5 58 2.2« · • · · · · ·! 1 data applies to a toluene concentration of 1 g / m3 of air Mtoluene is the amount in grams of adsorbed toluene per 100 grams of zeolite at each of the stated temperatures · · · · 30 at equilibrium with the ambient atmosphere. clearly the different adsorption properties of the DAY zeolite and ZSM5 zeolite. When the DAY zeolite has a low temperature ·, the excellent adsorption capacity on toluene, which however, decreases very rapidly with the increase in temperature due to r 10474911, the corresponding curve for the ZMS5 zeolite is considerably slower. Even at 80 ° C, the zeolite ZSM5 is better than the DAY zeolite. The mixture of both zeolites provides more uniform adsorption properties over a wider temperature range.

Example 2

The starting temperature TA of the conversion of palladium oxidation catalysts containing various amounts of palladium and of a conventional platinum-rhodium three-way catalyst in fresh and aged condition with a gas flow rate through the catalyst of 1.0001 and 60,000 l / cm -1.

The catalysts consisted of an oxide-containing dispersion coating containing 160 g of gamma-alumina / dm3 in each case with ceramic honeycomb structures of cordierite, and the catalytically active noble metals precipitated thereon. The cell density of the cellular structures was 62 cells / cm 2. The trigger temperatures are shown in Table 2.

. . Table 2 Starting temperatures for various catalysts (lambda = 1.15)

• »I

·. : 25: .i: Incorporated TA (° C) Flow Rate Covered; volume (g Pd / dm3) Fresh Aged through the escutcheon (h "1) 3.53 226 237 75 000 5.30 227 232 75 000. · * ·. 30 7.06 220 235 75 000 • ·. ···. 10,59 219 220 75,000 ^ t «20,191 209 60,000 40,189,204 60,000

Conventional 3-way catalyst (5 Pt / 1 Rh) 35 1.41 g / dm3 251 286 60 000 104749 12

To measure the start-up temperatures of the aged catalysts, the catalysts were run for 100 hours with the engine connected, with the exhaust temperature before the catalyst being 1000 ° C.

The palladium oxidation catalysts of Table 2 have a significantly lower hydrocarbon conversion starting temperature than a conventional three-way catalyst. Also noteworthy is the aging stability of their onset temperature, which can be attributed to the large amount of palladium incorporated. Table 2 further shows that the starting temperature of the palladium oxidation catalyst decreases dramatically as the palladium content continues to rise.

Starting temperatures below 237 ° C for these palladium-rich oxidation catalysts are only slightly higher than typical desorption temperatures for adsorbers, with very high starting temperatures around 200 ° C. They are capable of converting hydrocarbons desorbable from adsorber 20 at approximately 200 ° C directly without complex exhaust gas connections known in the art. A conventional three-way catalyst does not

# I I

.1 * capable of doing so, especially in an aging state, due to its high • t ”start-up temperature.

• · · '·'! 25 Hydrocarbon emissions from a • «: motor vehicle (Mercedes 300 E; displacement 3 dm3, power ♦ · · V 162 kW) during the cold start period were subsequently measured using various exhaust gas purification systems corresponding to Examples 3 and Comparative Examples 3a and 3b. 30 Summary Residual Emission Measurements for US FTP-75 Tests • · «. ···. the results of the cases are shown in Table 3.

The exhaust gas purification systems consisted in each case of three sequentially connected honeycomb structures made of cordierite and 35 with a cell density of 62 cells / cm2. The motor-side 104749 13 cell structure had a length of 154 mm and a volume of 1.68 dm 3.

Both rear cells had a length of 102 mm and a volume of 1.2 dm 3.

In order to compare the exhaust gas purification system 5 according to the invention with the conventional systems, these honeycomb structures were coated as follows:

Comparative Example 3a (Figures 1, 1a) 1-3 Cell Structure: Coating with a conventional three-way catalyst as in Example 2; aged 10 Comparative Example 3b (Figures 2, 2a) Cellular structure 1: Coating with 100 g DAY zeolite (Si / Al> 100) / dm3 Cellular structure 2 to 3: Coating with a conventional three-way catalyst as in Example 2; aged 15 Example 3 (Figures 3, 3a) 1. honeycomb structure: The first honeycomb structure was replaced by two pieces. The engine side part, 52 mm long, was coated with 100 g DAY zeolite (Si / Al> 100) / dm 3. The second part was coated with 20 oxidation catalysts containing 7 g Pd / dm3 cell volume. Also these coatings. . was aged before the exhaust tests were carried out.

* 2-3. honeycomb structure: Coating with conventional three-way catalyst as in Example 2; Aged · 2 5 Figures 1-3 show curves depicting carbon: · · hydrocarbon emissions during the first 250 seconds of startup of various exhaust gas purification systems (Figures 1a to 3a); ) The reported hydrocarbon concentrations are based on one tenth of the diluted exhaust gas · ··· in accordance with US FTP-75 Test Guideline.

. ···. Figure 1 shows that the obsolete three-way data

• * *** lysers (as shown in Fig. 1a) begin to convert harmful substances in the exhaust after about 50 seconds. At this point, the exhaust gas temperature before the catalytic converter is 300 ° C. Figure 2 shows the same dependency ratios as Figure 1 but with an adsorber coupled to the three-way catalysts containing the DAY zeolite in the exhaust gas purification system (Figure 2a). Desorption of hydrocarbons from the adsorber begins already after about 30 seconds at a temperature before the adsorber of about 200 ° C. However, three-way catalysts are not yet able to convert most of the desorbed hydrocarbons. In contrast, Figure 3 shows a combination of an adsorber and a palladium-rich oxidation catalyst 10 to significantly reduce residual emissions with two conventional three-way catalysts (Figure 3a). The oblique shaded area in Fig. 3 shows the reduction in hydrocarbon emissions provided by the exhaust gas purification system of the invention according to Example 3 as compared to the conventional exhaust gas purification system according to Example 3a.

Table 3 shows the results of the measurement of the residual emissions from the exhaust gas cleaning system of Comparative Example 3b and Example 3. As these mit-20 background results show, the use of a palladium-rich oxidation catalyst in combination with a hydrocarbon adsorbent and conventional three-way catalysts has a positive effect on the ice content of the exhaust system.

• I

· _ '1 emissions. Not only are hydrocarbon emissions reduced «· r '·'! During the cold start phase, 30%, also carbon monoxide: emissions are reduced. This positive effect is also maintained by the · · · • V throughout the test.

• · · • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • e

• Q.

• · c ··· • · • • • t m f i 15 104749

Table 3 Residual emission measurement according to US FTP-75

Exhaust System First Bag Contents 5 (g / mile) (Cold Start Phase) CO HC NO "Example V 3b 1.00 0.51 0.83

2.05 0.37 0.89 according to Example 3

Exhaust system Total emissions (g / mile) 10 CO HC NOx according to Example V 3b 1.29 0.22 0.46

0.48 0.10 0.38 according to Example 3

Comparative Example • $ 9 $ I • · <• i ·

I

• «• · · I · · ···· .... ......

• • »• • • • • • • • tl • 0 · •….

• ♦ · · · · M1 · · · · · · · · · ·

Claims (6)

16 104749 An exhaust gas purification system for reducing hydrocarbon emissions during cold start of internal combustion engines 5 comprising a hydrocarbon adsorber and a catalyst system thereafter comprising an oxidation and three-way catalyst in one or more layers, characterized in that the hydrocarbon adsorber , and a three-way catalyst containing platinum and / or palladium and / or rhodium, wherein the oxidation catalyst has a platinum and / or palladium / dm3 loading of at least 3.5 grams of catalyst volume, and the oxidation catalyst is immediately adapted to the adsorber . Exhaust gas purification system according to claim 1, characterized in that the adsorber comprises a mixture containing dealuminized zeolite 20 Ys with a Si / Al ratio greater than 40 and zeolite ZSM5 with a Si / Al ratio greater than 1:10 to 10: 1 by weight. . the shape of the coating on the monolithic honeycomb structure • · <t '; · D, where the amount of coating is 100 - 400 g / dm3 cell: "volume of the structure. 3. Exhaust gas purification system to reduce hydrocarbon emissions during cold start of internal combustion engines: a system comprising a hydrocarbon adsorber and a sequential oxidation and three-way catalyst in one or more layers, characterized by:. The three-way catalyst contains a platinum group metal, platinum and / or palladium and rhodium, whereby the three-way catalyst has a platinum and / or palladium load of at least 3.5 g platinum and / or palladium / dm3 catalyst volume. <«I <17 104749 Exhaust gas purification system according to claim 3, characterized in that the adsorber comprises a mixture containing dealuminated zeolite Y with a Si / Al ratio greater than 40, a zeolite ZSM5 with a Si / Al ratio greater than 20 , in a 1:10 to 10: 1 weight ratio in the form of a coating on a monolithic honeycomb structure, wherein the amount of coating is 100 to 400 g / dm 3 of the volume of the honeycomb structure. An exhaust gas purification system for reducing hydrocarbon emissions during cold start of internal combustion engines, comprising a hydrocarbon adsorbent in direct contact with an oxidation catalyst and a three-way catalyst, characterized in that: the adsorber coating is on top of the catalyst coating and the oxidation catalyst has a platinum and / or palladium loading of more than 3.5 g of platinum and / or 20 palladium / dm3 of catalyst volume. Exhaust gas purification system according to claim 5, characterized in that the adsorber; II.i / comprises a mixture containing dealuminated zeolite • (I. Y with a Si / Al ratio greater than 40 and a zeolite ZSM5 with a ratio of Si / Al greater than 40). 20, in a weight ratio of 1: 10 to 10: 1., · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · • · • · · 1 - f {(f is 104749