DE3853003T2 - Heat-resistant catalyst support fittings and combustion catalysts. - Google Patents

Description

The present invention relates to supports made of silicon dioxide and aluminum oxide for catalysts.

As carrier or support materials for catalysts used under high temperature and humidity conditions, such as catalysts for high temperature combustion such as for gas turbines, kerosene and gas-based heaters, oil and gas-based hot water generators, heavy oil-based burners and reformers for fuel cells, catalysts for steam reformers, and catalysts for purifying odorous substances and exhaust gases, cordierite (2MgO-2Al₂O₃5SiO₂), mullite (3Al₂O₃-2SiO₂) and the like have been used as ceramic substrates or supports. However, before use, these must be coated on their surfaces with a porous material since they themselves are not porous. Alumina is generally used as a porous support to support a catalytically active metal component; however, when used under high temperature conditions, alumina is converted to alpha-alumina, which has a specific surface area so reduced that the active metal component is hardly dispersed and is rapidly deactivated.

It has also been reported that lanthanum beta-alumina (LaO₃-11Al₂O₃) and barium-alumina (BaO-6Al₂O₃) have high heat resistance (see JP-A-62-153158); however, these materials cause problems in that their specific surface areas after firing or calcination at high temperature are insufficient and their commercial or industrial application is expensive because of an increased amount of precious metal required as a catalyst component.

It has also been reported that an aqueous solution of a mixture of an alumina hydrate with colloidal silica can be thickened and gelled by adding a basic compound, and the resulting gel can be dried and aged to form a heat-resistant silicon-alumina catalyst support composition (JP-A-61-287446). A monolithic catalyst support has also been reported comprising a structural phase of a suitably integrated sintered ceramic material and a high surface area support phase of a porous inorganic oxide consisting of 50 to 93 wt.% alumina and 7 to 50 wt.% silicon dioxide. However, a problem common to these compositions is that their specific surface areas are sharply reduced when used at high temperatures for a prolonged period of time. A further problem with the above-mentioned monolithic catalyst support is that it is difficult, particularly for applying a coating layer for a honeycomb structure, to form a uniform and thick coating layer within the closed cells of a honeycomb.

EP-A-0235411 discloses catalysts for the conversion of hydrocarbons which contain nickel and tungsten and are supported on a porous support of gamma-alumina, wherein at least 60% of the pore volume is in Pores of diameter from 1800 to 2400 nm and preferably substantially all of them are in pores of diameter greater than 1000 nm. Example 1 describes a catalyst made from alumina and having pores of size 1200 nm and above. Silica-alumina catalysts can contain up to 10% silica.

EP-A-0105435 describes a catalyst support for hydrogen supply processes, which is represented by a cogel of alumina and 5 to 10 wt.% silica, which has a surface area in pores below 6000 nm diameter of 350 to 450 m²/g and a pore volume in the pores below 6000 nm diameter of 1.0 to 1.35 cc/g. It is prepared by coprecipitations of sodium silicate and aluminum sulfate and then of sodium aluminate. The proportion of the total pore volume of pores with a diameter of at least 1000 nm is not disclosed.

US-A-424236 describes catalyst supports for catalysts for the desulfurization of oil, which are composed of aluminum oxide and silicon dioxide with an average pore radius of 650 to 1300 nm, with a total pore volume of 0.75 to 1.3 ml/g and less than 10% of the pore volume being composed of pores with a diameter of 10,000 nm and above.

None of these documents discloses an aging measure for the carrier substrate or its NMR spectrum.

According to the present invention, there is provided a heat-resistant catalyst support having a composition of silica and alumina and formed by process steps in which a Forming alumina hydrogel at a pH outside the range of 6.0 to 8.0, coating the hydrogel with silica, and aging the coated hydrogel at a final pH of 6.0 to 7.7 at a temperature of 40 to 90ºC for 1 to 5 hours and then heating at a higher temperature to allow the formed silica-alumina crystals to grow into crystals of a uniform size, the support comprising:

a silicon dioxide content in the range of 2 to 30 wt.%,

a total pore volume of at least 0.3 ml/g, of which at least 60% of the pore volume is in pores with a diameter of 1000 nm (100 A) or more (as determined by measuring the pore distribution by the nitrogen absorption method) and

an NMR spectrum exhibiting a peak area (measured by ²⁹Si NMR, with silicone rubber after firing at 500ºC in air as a reference peak (-21.9 ppm)) at -50 to -130 ppm, with 0 to 10% of the peak area comprising a peak area at -100 to -130 ppm.

Preferably, the volume of pores having a diameter of 1000 to 4000 nm (100 to 400 Å) is at least 0.20 ml/g.

The catalyst support thus comprises an inorganic oxide of high surface area capable of maintaining its pore volume and specific surface area even when fired under high temperature and humidity conditions or under corresponding reaction conditions. A catalyst comprising this support exhibits reactivity for the combustion of combustible components over an extended period of time and proves to be resistant to poisoning by impurities. Catalytic reactions, activity, lifetime, selectivity or economic efficiency of catalysts used are strongly influenced by the quality of the catalyst supports used. Catalyst supports should have a suitable pore volume; it is assumed that an increase in the pore diameter causes an increase in the rate of inward and outward diffusion of reactants and reaction products, ie catalyst particles, thereby improving the catalytic efficiency. Since the reactions take place on the catalyst surface, the surface area should not be reduced to the utmost; it is necessary that the mutual correlation between the two physical properties lies within a specific range.

The inventors have now found that silica-alumina compositions having small pores are more easily sintered and tend to lose their pores when fired compared to silica-alumina compositions having large pores. More specifically, when a support of silica-alumina compositions having large pores is subjected to firing at a low temperature to accommodate a catalytically active metal component and then used at a high temperature, a portion of the active metal component sinters into the support, resulting in a decrease in the amount of the active metal component. On the other hand, even when the support is subjected to firing at a high temperature to accommodate a catalytically active metal component and used at a high temperature, the active metal component tends to coalesce because the specific surface area of the support is already reduced, which in turn leads to a decrease in the amount of the active metal component. in the amount of active metal. On the other hand, if the support is used for a diffusion-controlled oxidation reaction, it is necessary to improve the effect on the free diffusion of molecules involved in the corresponding reactions. For this purpose, it is again desirable to use a support which has relatively large pores, in a large volume fraction in each case.

It is also desirable that in the silica-alumina composition, the coalescence of silica is suppressed, even when the oxide is kept under conditions of high temperature and humidity for a prolonged period of time; also, the silica or silicon dioxide phase bonded to alumina should be homogenized, thereby enabling the said composition to have a large specific surface area so that the catalyst has a long service life.

Preferably, the corresponding NMR spectra also have a peak area at -50 to -80 ppm which amounts to 0 to 25%, based on the peak area at -50 to -130 ppm, measured by the said nuclear magnetic resonance method.

If the silica is not well dispersed on the alumina, as in Comparative Example 4 and Fig. 6 to be described later, the composition has poor heat resistance. Although the exact reason for this is not clear, from the observation of a product obtained by merely fusing, without reaction, silica with alumina, it is considered that if silica is not dispersed on alumina at all, alumina crystals are converted into an α-phase, which results in a reduction in the specific surface area and hence in the heat resistance. If the NMR peak area at -50 to -80 ppm exceeds 25% based on the peak area at -50 to -130 ppm, as shown in Fig. 4 below, silica is dispersed on alumina to a greater extent than required, so that the composition is inferior in heat resistance.

The exact reason for this is also not entirely clear. However, in the case of a silica-alumina composition containing an alumina source having 2 to 30 wt.% silica, silica is very uniform and highly dispersed on alumina because 4 Al atoms are generally bonded to 1 SiO. In such a case, the resulting composition is improved in heat resistance over a composition in which silica is not entirely dispersed on alumina; however, that composition is considered to be so unstable that a severe phase transformation occurs when silica-alumina is converted to mullite (3Al₂O₃-2SiO₂) upon firing. Therefore, to produce the catalyst supports having the above-mentioned NMR spectrum or the preferred peak area at -50 to -130 ppm, the silica is deposited on an alumina hydrogel after precipitation. Then, an ageing agent such as a hydroxycarboxylic acid or its salt is added to the product while controlling the pH, thereby obtaining a composition having high heat resistance in which silica is dispersed on alumina in a suitably uniform manner. It is believed that such uniform dispersion of silica on alumina is the reason why the composition is so stabilized that no appreciable phase change takes place when the silica-alumina is converted into mullite (3Al₂O₃-2SiO₂) upon firing. which results in an increase in heat resistance.

The silica-alumina catalyst support of the present invention is designed for use as a layer on an inorganic oxide as a substrate; or a sintered ceramic product having the catalyst support as a layer thereon or uniformly dispersed therein. A catalytically active metal component is incorporated into the support.

The catalyst support moldings and catalysts for combustion reactions according to the invention are molded in the above-mentioned manner, whereby the catalyst supports are able to maintain their high pore volume and their specific surface area even when they are used under conditions of high temperature and humidity for an extended period of time, whereby the catalysts also retain their high reactivity with reduced or limited deactivation.

In the attached figures represent:

Fig. 1 is an X-ray diffraction pattern of the catalyst support prepared in Example 1 of the invention,

Fig. 2 is an X-ray diffraction pattern of the catalyst support prepared in Example 3 of the invention,

Fig. 3 is an X-ray diffraction pattern of the catalyst carrier prepared in Comparative Example 3,

Fig. 4 shows an NMR spectrum measured from the catalyst support prepared in Example 1 according to the invention,

Fig. 5 shows an NMR spectrum measured from the catalyst support prepared in Example 5 according to the invention,

Fig. 6 shows an NMR spectrum measured from the catalyst support prepared in Comparative Example 4,

Fig. 7 shows an NMR spectrum measured from the catalyst support prepared in Comparative Example 7 and

Fig. 8 is a diagram explaining the evaluation of the measured NMR spectra.

The catalyst supports according to the present invention will now be explained in further detail.

The catalyst carrier has a silica content of 2 to 30 wt.%, preferably 2.5 to 25 wt.%. If the silica content is less than 2 wt.%, the heat-resistant inorganic composition according to the present invention cannot be obtained, and the number of fine pores with a small pore diameter to be destroyed during high-temperature firing increases, resulting in a reduction in the total pore volume. On the other hand, if the silica content exceeds 30 wt.%, then there is an increase in the component ratio of mullite, which in turn leads to a reduction in the total pore volume and thus in the reactivity.

A preferred total pore volume is 0.4 to 1.5 ml/g, especially 1.3 ml/g or less. One of the problems with a total pore volume of 0.3 ml/g or less is that a sufficient diffusion rate of reactants and reaction products cannot be obtained, as will be described later. On the other hand, one of the problems with a total pore volume of more than 1.5 ml/g is that strength cannot be maintained and a reduction in bulk density occurs, resulting in a reduction in the feed weight to the reactor. More preferably, the volume of pores having a diameter in the range of 100 to 400 Å is 0.20 ml/g or more. The catalyst carrier of the present invention has a specific surface area of 20 m2/g or more after firing at 1200°C in air, but should preferably have a specific surface area of at least 50 m2/g, particularly at least 70 m2/g.

To prepare the catalyst supports of the invention, a colloidal gel of each of the components thereof is first prepared and then subjected to aging, drying, shaping and/or firing processes. In this preparation process, however, the aging process produces a considerable influence on the physical and chemical properties, i.e. the pore structure and silica dispersibility of the products.

The process for preparing the catalyst support compositions of the invention includes steps in which:

(1) forms an alumina hydrogel,

(2) Apply silica and

(3) the alumina-silica crystals age.

The starting material for use in the alumina hydrogel formation step may include an inorganic salt such as a sulfate, nitrate or chloride of aluminum and a soluble aluminum salt such as alkali metal aluminates.

The alumina source may preferably be used in an aqueous state, and its aqueous salt concentration may range from about 0.1 to 4.0 moles, preferably from about 0.3 to 2.0 moles. A basic compound such as aqueous ammonia is added to the aqueous salt solution to form the alumina hydrogel.

In the alumina hydrogel formation step, it is essential to form as many reduced alumina hydrogel particles as possible. The pH in this formation step is important and can be 8 or more or, on the other hand, 6 or less. In a pH range of 6 to 8, the solubility of the alumina hydrogel is increased too much, and large crystals are then formed.

The alumina hydrogel is then transferred to the silica coating step.

In this step of coating with silica, an aqueous solution (with a soluble salt concentration in the range of about 0.1 to 10 mol, preferably about 0.3 to 5.0 mol) of an alkali metal silicate (with preferably a ratio of Na₂O:SiO₂ = 1:2 to 1:4) is used and added dropwise to the alumina hydrogel formed in the previous step, whereupon it is well dispersed for coating. When co-precipitating alumina and silica in the same pH range, alumina can be formed and deposited on silica, which acts as a nucleus. However, such cases are not preferred because alumina is to grow in the aging stage to be described later, and it is then converted to α-alumina in the growth process.

The silica-alumina crystals thus formed are now transferred to the aging stage.

The purpose of this aging step is to dissolve small crystals to thereby develop into crystals of uniform size. In this step, the silica-alumina crystals formed in the aforementioned coating step undergo crystal growth as formed. However, if the aging step is carried out near pH 8, the silica-alumina crystals can be rearranged into silica-alumina crystals having alumina-coated layers. However, it has been found that it is preferable to lower the pH to 6.0 in the aging step for the purpose of influencing the growth of the silica-alumina crystals as they are formed. By such a measure, it is possible to suppress the rearrangement of the silica-alumina crystals and to grow small-diameter silica-alumina crystals in an as-formed state but with increases in pore volume. The aging step is carried out at 40 to 90°C, preferably at 50 to 80°C, for 1 to 5 hours, and preferably 1.5 to 3 hours.

If the small diameter silica-alumina crystals are subjected to the firing step in the as-formed state without performing such an aging step, the alumina is sintered by firing because of the small pore diameter of the alumina, which lowers the heat resistance. Thus, it is makes it possible to obtain a composition having a large pore diameter by increasing the pore diameter of the crystals in the aging step.

The silica-alumina crystals developed in the ageing stage are filtered off, then washed with an aqueous solution of ammonium carbonate, chloride and nitrate and then dried in air at 80 to 200°C, preferably at 90 to 150°C.

After drying, the silica-alumina compositions may be fired in air at 400 to 1400°C, preferably at 500 to 1200°C, to form a catalyst support according to the present invention.

It is considered that by preparing the silica-alumina compositions by the above-mentioned preparation method, the alumina hydrogel is coated with silica, thereby stabilizing porous θ-alumina to suppress its conversion to α-alumina. On the other hand, mullite (a composite material of alumina with silica) is formed by the addition of silica to cause a reduced specific surface area. However, by optimizing the amount of silica to be added to control the growth of the silica-alumina crystals, it becomes feasible to suppress the conversion of θ-alumina to α-alumina, obtain pores due to the stabilization of θ-alumina, and achieve specific surface areas at high temperatures.

In general, the bonding states of silica with alumina in alumina-silica compositions are represented by the formulas:

According to the above-mentioned NMR measurement of such silica-alumina compositions, the structures represented by the above formulas (I) to (IV) have their absorption peaks at -80 ppm, -88 ppm, -97 ppm and -105 ppm, respectively. It should be noted that the structure (V) having no aluminum substituent has its absorption peak at -111 ppm.

Fig. 8 is a graph showing the results of 29Si-NMR measurement of a silica-alumina composition with the shift values from the reference peak as abscissa and the relative absorption strength as ordinate.

In Fig. 8, the peak position of the above formulas (I) to (V) are indicated at a to e. As shown by the dashed lines, the curves for the bonding states represented by the above formulas (I) to (V) show a decreased relative strength in terms of shifts compared with the reference peak. Therefore, the measurement results are given as the sum of absorption values related to the respective structures; the maximum value of the relative absorption strength of the absorption curves corresponding to the above formulas (II) to (IV) showing the compositions of the present invention cannot be clearly indicated.

However, by such a measurement, the relative amount of the present amount of the respective bonding states, which correspond at least to the above-mentioned formulas (I) to (V), can be found out. Since the curve of the total relative strength shifts to the absorption peak position of the bonding state represented by the above-mentioned formula (I), on the one hand, Al/Si approaches 4, since the curve of the total relative strength shifts to the peak position of the bonding state represented by the above-mentioned formula (V), on the other hand, Al/Si approaches 0, due to incorrect dispersion of silica and aluminum oxide.

For this reason, the second catalyst support of the present invention can be defined by the ratio of s' and s", relative to S, where:

s' is the area defined by a curve section exceeding the displacement value at which the Al/Si ratio is 4 and a reference line A,

s" is the area defined by a curve section of not higher than the displacement value at which the Al/Si ratio is 0 and the reference line A, and

S is the total area defined by a corresponding curve and the reference line A.

Note that the ²⁹Si NMR method was carried out using a JNM-GX 270 model manufactured by JEOL, Ltd. and the measurement of ²⁹Si nuclei (53.54 MHz) was carried out by using the CP/MAS (Cross Polarization/Magic Angle Spinning) method. Silicon rubber was used as the standard substance with the proviso that the corresponding value is fixed at -21.90 ppm.

To prepare the silica-alumina compositions for forming the catalyst supports, a silica source is added to an alumina source in the process of preparing the said silica-alumina compositions. Precipitation of the alumina hydrogel is carried out at a pH equal to or higher than 8 or equal to or lower than 6. Further, the silica source is added to deposit silica. Thereafter, a hydroxycarboxylic acid or its salt is added as an aging agent for the hydrogel, and an aging process is carried out while lowering the pH to 6 to obtain the silica-alumina compositions.

Hydroxycarboxylic acids or their salts which can be used for this purpose include tartaric acid, citric acid or other hydroxyl group-containing polyvalent carboxylic acids and their alkali metal salts, aminodicarboxylic acids or their salts such as glutamic or aspartic acid or other amino group-containing polyvalent carboxylic acids and their alkali metal salts can be used. These compounds can be added to the aluminum compound at 0.0002 to 0.1 mole per mole.

In the process for preparing the silica-alumina compositions, the hydroxycarboxylic acid or its salt is added and the pH is lowered to 6 in the aging step, thereby adjusting the s'/s ratio to a value of 0 to 25% while stably controlling the s"/s ratio at a value of 0 to 10%.

Without resorting to any ageing agent, an increase in the s'/s ratio may result due to insufficient dispersibility of silica. In such a case, it may be impossible to produce catalyst supports in which the peak area at -50 to -80 ppm is 0 to 25% based on the peak area at -50 to -130 ppm as measured by a nuclear magnetic resonance method (29Si-NMR) with silicone rubber as a reference peak. However, in the process for producing the silica-alumina compositions, the ratios s'/s and s"/s can be stably controlled to a value of 0 to 25% and 0 to 10%, respectively, while controlling the pore distribution by causing the precipitation of the alumina hydrogel at pH = 8, adding a silica source and carrying out the aging step by adding ammonium tartrate (used in a ratio of 0.01 mole per mole of alumina) while lowering the pH to 6. In this way, it is possible to produce silica-alumina compositions in which the peak area at -110 to -130 ppm is 0 to 10% based on the peak area at -50 to -130 ppm, and the peak area at -50 to -80 ppm is 0 to 25% based on the peak area at -50 to -130 ppm, as measured by a nuclear magnetic resonance method (29Si-NMR) using silicone rubber as a reference peak.

The shape of the catalyst carrier of the present invention will now be described. That is, the silica-alumina compositions obtainable in the above-described production process are collected by filtration, dried and, if necessary, pulverized, then water is added by firing or not, followed by moisture conditioning and mixing. Alternatively, the precipitates (hydrogel) may be mixed together. Although molded bodies will be explained in detail with reference to the catalyst carrier molded bodies to be described later, in general, the resulting mixture is extruded by an extruder into, for example, an extrudate or a honeycomb or particle shape, which is then subjected to firing in air at 400 to 1400°C, preferably 500 to 1200°C. For catalysts for purifying exhaust gases from automobiles, the firing operation can be carried out at 550 to 1000°C. On the other hand, when a combustion catalyst is prepared by a wax coating process on a support such as mullite, silica-alumina, cordierite or a corresponding honeycomb product, said support is immersed in a suspension comprising a mixture of the aforementioned powders and an acid (such as acetic, nitric or hydrochloric acid) or a base (such as ammonium hydroxide) and/or added hydrogel. After the excess deposits have been flushed off with an inert gas (such as nitrogen), the carrier base is dried and fired. The drying measures are carried out in air at a temperature of 80 to 200ºC, preferably 90 to 150ºC, and the firing measures are carried out by heating in air at a temperature of 400 to 1400ºC, preferably 500 to 1200ºC.

To design catalyst supports of the present invention in catalyst supports for purifying exhaust gases from automobiles, the supports are treated with at least one metal component selected from metals of group VIII of the periodic table, i.e. with a metal component of the iron group, iron, nickel and cobalt, and the noble metals platinum, palladium, iridium, rhodium, ruthenium and osmium, as a catalytically active metal component. In addition to the metal components of group VIII, metal components of group V such as vanadium and metal components of group VI such as chromium can be used as catalytically active metal components. Preferred active metal components are at least one selected from platinum, rhodium and iridium. These active metal components are present in the form of oxides on the supports in a catalytically effective amount which is preferably in the range of 0.001 to 5% by weight, in particular 0.005 to 2% by weight.

There may be added to and used for the compositions of the present invention one or two or more oxides of an alkaline earth metal such as barium, calcium, strontium, zirconium and magnesium, a metal of Group III of the Periodic Table of the Elements such as boron, scandium and yttrium, a metal of Group IV such as titanium, zirconium and hafnium and/or a rare earth metal such as lanthanum, cerium and thorium in an amount of from 0.01 to 30 wt.%, preferably from 0.05 to 10 wt.%. Although the method of addition is not critical, the corresponding Substances can be suitably coprecipitated simultaneously with the precipitation of aluminium oxide or silica.

According to the present invention, the pore volume, pore distribution and specific surface area of the catalyst supports as well as the characteristic NMR values are not influenced by exposure to the catalytically active metal components. The catalysts obtained are therefore identical to the supports with regard to these special features and properties.

The catalyst supports and combustion catalysts of the present invention will now be explained in detail with reference to the following Examples and Comparative Examples.

EXAMPLE 1

0.15 mol of an aqueous aluminum sulfate solution was initially charged, which in turn was adjusted to pH 8 by the addition of 1 N ammonia water. Then, an aqueous solution of No. 3 water glass (JISK-1408) was added to the resulting solution so that the silica content of the product became 10 wt%, thereby forming silica-alumina precipitates.

The precipitates thus formed were aged at 60ºC for 3 h and the final pH was reduced to 6.2. After aging, the precipitates were filtered and washed with water. The product was then dried in air at 120ºC for 12 h and subjected to firing in air at 500ºC for 3 h, at 1000ºC for 15 h and at 1200ºC for 15 h to obtain 3 catalyst supports.

EXAMPLE 2

Catalyst supports were obtained in a similar manner to Example 1, except that the final pH upon aging was reduced to 7.5.

EXAMPLE 3

Catalyst supports were obtained in a similar manner to Example 1, except that an aqueous solution of No. 3 water glass was added so that the silica content of the product was 5 wt.% and the final pH upon aging was reduced to 6.0.

EXAMPLE 4

Catalyst supports were obtained in a similar manner to Example 1, except that an aqueous solution of No. 3 water glass was added such that the silica content of the product was 5 wt.% and the final pH upon aging was reduced to 7.7.

EXAMPLE 5

0.15 mol of an aqueous aluminum sulfate solution was initially charged, which in turn was adjusted to pH = 8 by the addition of 1 N ammonia water. An aqueous solution of No. 3 water glass (JISK-1408) was added to the resulting solution so that the silica content of the product became 10 wt.%, thereby forming precipitates of silica-alumina. Furthermore, 0.0015 mol of ammonium tartrate was added to the precipitates.

The precipitates thus formed were aged at 60ºC for 3 hours, the final pH was reduced to 6. After After aging, the precipitates were filtered off and washed with water. The product was then dried in air at 120ºC for 12 hours and subjected to firing in air at 500ºC for 3 hours, at 1000ºC for 15 hours and at 1200ºC for 15 hours to obtain 3 catalyst supports.

COMPARISON EXAMPLE 1

An aqueous solution of aluminum sulfate was premixed with an aqueous solution of No. 3 water glass (JISK-1408) so that the silica content of the product was 10 wt%, then ammonia water was added to the mixed solution while adjusting the PH to 8, whereby silica and alumina were precipitated simultaneously. Without aging, the precipitates were filtered off from the aqueous solution and then washed with water. Thereafter, the product was dried at 120°C for 12 hours in air and subjected to firing at 500°C for 3 hours in air.

COMPARISON EXAMPLE 2

An aqueous solution of aluminum sulfate was adjusted to pH 8 by adding ammonia water, and an aqueous solution of No. 3 water glass (JISK-1408) was added to the solution so that the silica content of the product became 5 wt%, thereby forming silica/alumina precipitates, which were then subjected to firing without aging in a similar manner to Comparative Example 1.

COMPARISON EXAMPLE 3

Powders of alumina were obtained in a similar manner to Comparative Example 1, except that only aluminum sulfate was used as the starting material and no silica were used, and the powders were then subjected to firing in a similar manner as in Comparative Example 1.

COMPARISON EXAMPLE 4

The alumina powders prepared in Comparative Example 3 were used as a starting material and mixed with commercially available silica gel powders, and the mixture was then subjected to firing in a similar manner to Comparative Example 1.

COMPARISON EXAMPLE 5

Alumina sol (manufactured by Nishio Kogyo Co., Ltd.) and colloidal silica (manufactured by Nissan Chemical Industries, Ltd.) were suspended in water and mixed together so that 10 wt% of silica was contained in the final composition. While maintaining the pH at 7, a silica-alumina composition was precipitated. After filtration and washing with water, the precipitates were dried at 120°C in air for 2 hours and then subjected to firing at 500°C in air for 3 hours to obtain a 500°C fired product. Further firing at 1200°C for 15 hours, 100 hours and 300 hours gave 3 products subjected to firing at 1200°C.

A catalytically active metal component can be applied to the carriers by an impregnation method, a kneading method and similar other methods. However, the impregnation method is preferred because of the simple procedure. For impregnation with the active metal component, a soluble compound of the said metal component is prepared and the carrier is then impregnated with it. A concentration of the impregnation solution is in the range of about 0.005 to about 0.1 wt.%. A preferred impregnation solution is aqueous solution. After impregnation, the carrier impregnated with the active metal component is transferred to the drying and firing stages. Drying is carried out at approx. 80 to approx. 200ºC and firing at approx. 500 to approx. 1400ºC.

According to the kneading method, a solution of the active metal component is added to and introduced into the silica-alumina composition during its precipitation.

The X-ray diffraction analysis results of the catalyst supports obtained by firing at 1200°C for 15 hours in Examples 1 and 3 and Comparative Example 3 are shown in Figures 1, 2 and 3, respectively.

As can be seen from the X-ray diffraction patterns, the crystal structure of the silica-alumina compositions of the present invention comprises mullite (3Al₂O₃-2SiO₂) and θ-Al₂O₃, whereas that of the Comparative Example is converted to α-Al₂O₃.

Then, the catalyst supports obtained by firing at 500°C for 3 hours, at 1000°C for 15 hours and at 1200°C for 15 hours in the above Examples and Comparative Examples were measured for specific surface area (m²/g), pore volume (ml/g) and pore distribution (%), as well as the proportion of pores having a diameter of 100 Å or more based on the total pore volume and the volume (ml/g) of pores having a diameter of 100 to 400 Å. The measurement results are shown in Tables 1 to 3 below. Table 1 also shows the NMR results. TABLE 1 (After firing at 500ºC for 3 hours) Example Silica (wt%) Alumina (wt%) Specific surface area (m²/g) Pore volume (ml/g) Pore distribution (%) Å diameter 400 or larger Volume of pores with diameter 100 Å or more / total pore volume Volume of pores with diameter 100-400 Å TABLE 2 (After firing at 1000ºC for 15 hours) Silica (wt%) Alumina (wt%) Specific surface area (m²/g) Pore volume (ml/g) Pore distribution (%) Å diameter 400 or larger Volume of pores with diameter 100 Å or more / total pore volume Volume of pores with diameter 100-400 Å TABLE 3 (After firing at 1000ºC for 15 hours) Silica (wt%) Alumina (wt%) Specific surface area (m²/g) Pore volume (ml/g) Pore distribution (%) Å diameter 400 or larger Volume of pores with diameter 100 Å or more / total pore volume Volume of pores with diameter 100-400 Å

Regarding the above physical properties, the specific surface area (m²/g) was measured with a BET type specific surface area meter using nitrogen gas as an absorption gas. The pore distribution was determined by finding an absorption isotherm with an Ominisorp manufactured by Omicron Co., Ltd. and performing calculation according to the method of Cranston-Inkley/BJH [Advances in Catalysis, 9, 143 (1970); J. A. C. S. 73, 373 (1951)], using a relative pressure.

Then, heat resistance estimation was carried out based on the specific surface areas of the above catalyst supports subjected to firing at 1200 °C in air for 15 h, 100 h and 300 h, respectively. The results are shown in Table 4. TABLE 4 Beam No. Comparison Example Example

It should be clear that although Table 4 relates to results obtained on products fired at 1200ºC in air, similar degradation trends are expected for products which have been treated at lower temperatures of not more than 1200ºC.

EXAMPLE 6 PRODUCTION OF CATALYSTS

The precipitates prepared in Example 1 were filtered, washed, dried at 120°C for 20 hours and pulverized in a ball mill. The product was kneaded with water into a clay, which was then extruded with an extruder equipped with a die having a plurality of holes each having a diameter of 9.15 cm to obtain a round rod. Thereafter, the rod was dried at 120°C in air and then subjected to firing at 500°C in air for 3 hours to obtain a catalyst carrier.

Commercially available premium nitric acid was added to deionized water to prepare a 0.1 N aqueous nitric acid solution, and 1.6 g of palladium chloride was added and heated and dissolved in 1 L of this aqueous solution to prepare a palladium-containing solution.

Twenty (20) ml of the above rod-shaped catalyst support was immersed in 25 ml of this solution, and then palladium was supported on the support in an amount of 0.024 g/20 ml by slowly stirring for about 2 hours. After washing with deionized water, the product was dried in air at 120°C for 12 hours and further fired in air at 500°C for 3 hours.

In a similar manner as above, the compositions of Examples 2 to 5 and Comparative Examples 1 to 5 were Carriers were used to deposit palladium, thereby producing corresponding catalysts.

ACTIVITY TESTS

A mixture of 1% CO, 4% O₂ and the balance N₂ was used as a reaction gas, and it was passed through a reaction vessel made of a quartz glass tube having an inner diameter of 12.7 mm (and a volume of 1.9 cm³) filled with each of the catalysts shown in Table 5 under conditions of a volume velocity of 30,000 V/h/V and a reaction temperature of a tube heating furnace of 250 °C. After the inlet port of the reaction vessel was raised to a temperature of 250 °C and kept stable at that temperature, the gases emitted from the outlet of the reaction vessel were analyzed by gas chromatography. Note that catalysts subjected to firing at 500 °C for 3 hours, at 1000 °C for 100 hours and at 1200 °C for 100 hours were used, respectively. The measured results of CO conversion are given in Table 5. TABLE 5 Beam No. Comparison Example Example

From Table 5, it can be seen that the catalysts of the present invention show no significant difference in reactivity over comparative catalysts when subjected to only low temperature treatment such as firing at 500°C for 3 hours, but still show high CO conversion when subjected to high temperature treatment such as firing at 1000 and 1200°C.

To investigate the durability of the catalysts, accelerated durability tests were carried out on the products fired at 1000 and 1200°C. According to the accelerated durability test, each catalyst was placed in an electric furnace, heated to 1000°C while passing steam having a water/air mole fraction of 0.029 and kept at this temperature for 200 to 500 hours to determine the specific surface area (ml/g) of the catalyst. The catalysts used were obtained with the catalyst supports prepared in Examples 1, 3 and 5 and Comparative Examples 2 and 3. The results are shown in Table 6. TABLE 6 1000ºC without water supply 1000ºC with water supply Holding time Comparison example Example

There was also no significant change in the pore distribution, which means that the catalysts can withstand extended periods of use under high temperature conditions.

Preferred embodiments of the catalyst supports of the present invention will now be described.

The silica-alumina composition (hereinafter referred to as composition A) having a main peak at a position where the aluminum to silicon atomic ratio of a portion having a silica bond formed therein was 4 to 3 and having a silica content of 10 wt% was obtained in Example 1 above. The results of 29Si NMR measurement of this composition are illustrated in Fig. 4. The measurement was carried out by measuring the 29Si nucleus at 53.54 MHz according to the CP/MAS (Cross Polarization/Magic Angle Spinning) method. Silicone rubber was used as a standard substance. From Fig. 4, the following results were obtained.

COMPOSITION A

Area (-50 to -80 ppm) / Area (-50 to -130 ppm) = 0.27

Area (-110 to -130 ppm) / Area (-50 to -130 ppm) = 0.05

Three combustion products were also obtained by firing for 15, 100 and 200 hours at 1200ºC.

The silica-alumina composition of Example 2 has a main peak in a position where the aluminum to silica ratio of a subset having a formed silica bond is 4 to 3, while the composition of Example 3 has a peak at 4 to 3 and the composition of Example 4 has a main peak at 4 to 3.

The alumina-silica compositions of Examples 1, 2, 3 and 4 and Comparative Example 4 were washed with water, dried at 120 °C in air for 12 hours and then subjected to firing at 500 °C in air for 3 hours to obtain 500 °C fired products.

29Si NMR measurement was carried out on the products fired at 500°C on the catalyst supports prepared above. The results are shown in Figs. 4 to 7, respectively. Based on the results of NMR measurement, the total area S defined by the reference line A and the curve, and the areas s' and s", defined by the absorption strength curves and the reference line A were determined to be from -50 to -80 ppm and from -110 to -130 ppm, respectively, and the s'/S and s"/S ratios were calculated. TABLE 7 Example

(The measurement was carried out using silicone rubber (-21.9 ppm) as a reference peak.)

It should be noted that the s'/S and s"/S values are obtained from Fig. 4 for Example 1, from Fig. 5 for Example 5, from Fig. 6 for Comparative Example 4 and from Fig. 7 for Comparative Example 5. It should also be recognized and understood that with respect to the s'/S and s"/S values, the For the 3 firing products produced by firing at 1200ºC, the results obtained were similar to those obtained with the product fired at 500ºC.

The relationships between the manufacturing conditions and the s'/S and s"/S values in Examples 3 and 5 are shown below. EXAMPLE 3 (without ageing agent) pH during precipitation of alumina hydrogel pH during ageing EXAMPLE 5 (with ageing agent) pH during precipitation of alumina hydrogel pH during ageing

As can be seen from the above, a high specific surface area can be obtained with the catalyst supports of the present invention in a more preferable range according to the present invention.

A description will now be given of catalyst support molded bodies of the present invention.

Catalyst carrier molded bodies of the present invention are obtained by a first process of molding the silica-alumina compositions prepared in the aforementioned first catalyst carrier or by forming on the obtained molded body further forms a catalyst support layer with a high surface area which comprises a silica-alumina composition; a second method is based on subjecting a mixture of a ceramic material such as cordierite or mullite with said silica-alumina compositions to a molding process; or a third method is carried out in which a catalyst support layer with a high surface area which comprises said silica-alumina compositions is formed on a ceramic molded body which is obtainable by molding cordierite, mullite, etc.

As for the first method, silica-alumina precipitates prepared in connection with the preparation of the aforementioned first catalyst support are dried and, if necessary, pulverized to obtain powders, to which water is then added with or without firing, followed by conditioning and mixing by exposure to moisture. Alternatively, the precipitates (hydrogel) may be mixed together. The silica-alumina composition may then be finely pulverized and molded with the addition of a binder such as methyl cellulose, polyvinyl alcohol or starch. To facilitate molding, the powders may also be treated and mixed with water and an agent for controlling the evaporation of moisture during drying, such as glycerine or ethylene glycol. If the molded body has a honeycomb shape, the powders are extruded using an extruder with a grid nozzle, then dried and finally subjected to firing in air at 400 to 1400°C, preferably at 500 to 1200°C. Furthermore, the silica-alumina composition can be applied to the molded bodies thus obtained by a wax coating process in a manner to be described later.

According to the second method, a finely pulverized ceramic material that has been subjected to a pre-reaction is mixed with the finely pulverized silica-alumina composition that was prepared in the preparation of the above-mentioned first catalyst carrier. As the ceramic materials that can be used in the present invention, use can be made of any sinterable material that can impart mechanical and thermal strength and durability to the heat-resistant catalyst carrier molded bodies. Cordierite, mullite, clay, talc, zirconia, zirconia spinel, alumina, silica, lithium aluminum silicate or alumina-zirconia composite materials are preferred.

In the above-mentioned ceramic materials, the proportion of the silica-alumina composition is preferably in a range of 30 to 60 wt.%. The purpose of using the pre-reacted ceramic material is partly to produce a cordierite crystal structure in advance and to mix it with the silica-alumina powders only at a later stage, thereby achieving further growth of the cordierite crystals at a relatively low temperature (1000 to 1100°C) in the firing stage.

The mixture can then be molded by adding a binder such as methyl cellulose, polyvinyl alcohol or starch. To facilitate molding, the mixture can also be treated with water and an agent for controlling the evaporation of moisture during drying, such as glycerin or ethylene glycol, and subjected to a kneading process. If the molded body has a honeycomb shape, the kneaded product is then extruded with an extruder with a grid nozzle, then dried and finally subjected to firing in air at 400 to 1400ºC, preferably at 500 to 1200ºC.

According to the third method, a coating layer of silica-alumina according to the present invention is formed on a preformed honeycomb ceramic molded article by a wash-coating method to be described later. Honeycomb ceramic molded articles are prepared by adding a binder, water and a moisture evaporation control agent, etc. to a ceramic material to be converted into cordierite upon firing, kneading them, and extruding the kneaded product with an extruder having a mesh die, followed by drying and firing.

Regarding the washing coating method used in the first and third methods, the precipitates of the silica-alumina composition are first dried at 120°C and then subjected to firing at 500°C for 5 hours and at 1000°C for 5 hours to prepare various types of powders for slurry. These powders are used in combination. Powders fired at lower temperatures have an advantage in terms of specific surface area over powders fired at higher temperatures, whereas powders fired at higher temperatures have an advantage in terms of the degree of sintering progress over powders fired at lower temperatures, and the post-sintering heat shrinkage of a coating layer is correspondingly reduced so that the applied layer adheres firmly and is essentially irremovable.

Regarding the particle size of the coating material powders, it is important that they be finely pulverized to a particle size sufficient to penetrate 4 µm pores of a cordierite substrate. In other words, it is preferred that the products fired at 500 and 1000°C be reduced to an average particle size of up to 5 µm and that particles of 1 to 10 µm account for 80% or more.

On the other hand, as the solvent for slurrying, a mixed liquid of water and glycerin can be used. Glycerin has an effect of controlling the evaporation rate of water when drying the coating layer. The higher the concentration of glycerin, the slower the evaporation of water from the coating layer and the smaller the difference in the amount of moisture between the deeper and the surface portions of the coating layer. Thus, a higher concentration of glycerin is effective in preventing microcracks of the coating layer, which tend to occur in the drying process. The glycerin-to-water ratio is suitably in a range of 1/20 to 1/3. It is also preferable that the amount of glycerin is increased with the increase in the thickness of the coating film formed by 1 dipping process. In addition to water and glycerin, organic binders, surfactants, etc. may be added, including quaternary ammonium silicate, aluminum compounds and phosphates and nitrates of magnesium or alumina sol, silica sol, polyvinyl alcohol and methyl cellulose, which are commonly used as binders for slurry particles.

The above-mentioned molded body is dipped in or sprayed with a slurry which Slurry powder containing solvents for slurrying, organic binders and the like for coating with the silica-alumina composition of the present invention. After excess deposits are purged with an inert gas (nitrogen, etc.), the coated molded article is dried and subjected to a firing operation. It is dried at a temperature of 80 to 200°C, preferably 90 to 150°C, in air, and the firing operation is carried out by heating to 400 to 1400°C, preferably 500 to 1200°C, in air. Molded articles coated from products fired at 1000°C were similar to those obtained from a product fired at 500°C.

It should be understood that the molded articles may have any of monolithic shapes such as columnar and spherical pellet, honeycomb, foam and fiber shapes. The honeycomb shape is particularly preferred.

If catalyst carrier molded bodies are designed to become combustion catalysts, they are produced by immersing the molded bodies in a suspension to which a mixture of an acid (acetic acid, nitric acid, hydrochloric acid, etc.) or a base (ammonium hydroxide, etc.) has been added, the corresponding molded bodies and/or the corresponding hydrogel being used within the scope of the present invention. When designed to become catalysts for cleaning exhaust gases from automobiles, at least one metal component from metals of group VIII of the periodic table, ie a metal component from the iron group, iron, nickel or cobalt, or the noble metals platinum, palladium, iridium, rhodium, ruthenium or osmium, is applied to the carrier as a catalytically active metal component. A preferred active metal component is at least one from platinum, rhodium and iridium. These active Metal components are applied to the supports in the form of oxides in a catalytically effective amount which is preferably in the range of 0.001 to 5 wt.%, particularly 0.005 to 2 wt.%.

Also added to or used in silica-alumina compositions according to the present invention may be one or two or more oxides of an alkaline earth metal such as barium, calcium, strontium, zirconium or magnesium, a metal of Group III of the Periodic Table of the Elements such as boron, scandium or yttrium, a metal of Group IV of the Periodic Table of the Elements such as titanium, zirconium or hafnium, and a rare earth metal such as lanthanum, cerium or thorium. Although the method for their addition is not critical, they may be suitably coprecipitated simultaneously with the precipitation of alumina or silica.

In the following, catalyst carrier moldings and combustion catalysts in which these carriers are used are described in more detail with reference to comparative examples.

EXAMPLE 7 - FIRST PROCEDURE

The precipitates prepared in Example 1 were filtered, washed with an aqueous solution of ammonium carbonate, and dried at 120°C for 20 hours. Thereafter, one hundred (100) parts by weight of the dried product was subjected to firing at 500°C for 3 hours and then pulverized to 3 to 20 µm in a ball mill. The pulverized product was charged and kneaded with 5 parts by weight of methyl cellulose, 10 parts by weight of glycerin, and 30 parts by weight of water in a screw kneader for 30 minutes to prepare an embryonic clay for extrusion molding.

This product was fed to a screw type extruder to form a honeycomb structure of 50 mm diameter x 1000 mm length including a surface grid with a wall thickness of 0.2 mm and 1 side of 1.8 mm. Then, this structure was very gently dried at 80ºC for 12 hours, then at 100ºC for 24 hours and finally at 120ºC for 24 hours and then heated to 500ºC at a heating rate of 60ºC/h, at which temperature it was subjected to firing for 3 hours to obtain a honeycomb catalyst support molded body of the silica-alumina composition.

COMPARISON EXAMPLE 6

While alumina sol (ASK 110, manufactured by Nishio Kogyo Co., Ltd.) was stirred in a kneader heated to 80°C, colloidal silica (Snowtex, manufactured by Nissan Chemical Industries, Ltd.) was added thereto and a 1N aqueous solution of ammonium carbonate was further slowly added dropwise until a pH of 8.5 was reached. After the dropping was completed, kneading was continued for 20 minutes. The product was dried at 120°C for 12 hours and pulverized into 10 µm powders, which were then fired at 1000°C for 5 hours and at 1200°C for 5 hours. The resulting composition was found to contain 10 wt% of silica. Using this composition, a honeycomb catalyst carrier was obtained in a similar manner to Example 7.

COMPARISON EXAMPLE 7

Alumina sol (ASK 110, manufactured by Nishio Kogyo Co., Ltd.) and colloidal silica (Snowtex, manufactured by Nissan Chemical Industries, Ltd.), both sufficiently dried at 120ºC, were independently pulverized and finely divided. Both powders were put into a mortar so as to obtain a SiO₂/Al₂O₃ weight ratio of 10/90, wherein they were sufficiently mixed together and dried. After adding water to the mixture, the mixture was further mixed and then dried to a particle form. Pulverization gave powders of 10 µm. The obtained powder formulation was subjected to firing at 1000 and 1200°C for 5 hours each. Using the obtained composition, a honeycomb catalyst carrier was obtained in a similar manner to Example 7.

The specific surface area (m²/g), pore volume (ml/g) and pore distribution (%), as well as the proportion of pores having a diameter of 100 Å or more based on the total pore volume and the volume (ml/g) of pores having a diameter of 100 to 400 Å were measured for the products of Example 7 fired at 1200 °C for 15 hours and the products of Comparative Examples 6 and 7. The results are shown in Table 9 below. TABLE 9 Example Silica (wt%) Alumina (wt%) Specific surface area (m²/g) Pore volume (ml/g) Pore distribution (%) Å diameter 400 or larger Volume of pores with diameter of 100 Å or more / total pore volume Volume of pores with diameter of 100-400 Å

An estimate of the heat resistance of the above compositions was made by determining their specific surface areas after firing at 1200ºC in air for 15, 100 and 300 hours. The results are given in Table 10 below. TABLE 10 Beam Nos. Comparison Example Example

It should be noted that although Table 10 concerns results on products fired at 1200ºC in air, similar degradation trends are also expected at lower temperatures not higher than 1200ºC.

EXAMPLE 8 - SECOND METHOD

Kaolin, talc and ball clay powder in respective amounts of 3.8, 4.0 and 1.2 kg were mixed with each other sufficiently by a Spat flow mixer (manufactured by Kawada, Ltd.) so that the chemical composition of the main amount was 51 wt% of silica, 35 wt% of alumina and 14 wt% of magnesia to formulate a cordierite-forming material. 10 parts of methyl cellulose and an appropriate amount of deionized water were added to the formulation, followed by mixing and kneading in a kneader to plasticize the product to an extrusion-moldable state. The product was extruded by a piston type extruder having a nozzle with a plurality of holes each having a diameter of 1.5 mm to obtain a round rod, which was then heated in an electric furnace heated to a temperature of 1300°C. was subjected to firing for 4 hours. The firing product was finely divided in a ball mill into cordierite powders which had undergone a pre-reaction and had an average particle size of 10 µm.

On the other hand, the precipitates prepared in Example 1 were filtered, washed with an aqueous solution of ammonium carbonate and dried at 120°C for 3 hours. Then, 100 parts by weight of the dried product was fired at 1500°C for 3 hours and then pulverized to 3 to 20 µm in a ball mill. Then, 4 components comprising 3 parts by weight of methyl cellulose, 0.3 parts by weight of stearic acid, 60 parts by weight of pre-reacted cordierite and 40 parts by weight of the above-mentioned silica-alumina composition were rotated in a ball mill until they were well mixed with each other. Then, the mixture was removed from the ball mill and kneaded with 40 parts by weight of deionized water in a screw kneader for 30 minutes to prepare an extrusion-moldable embryonic clay. Then, the clay was fed into a screw extruder to form a honeycomb mold of 45 mm in diameter and 100 mm in height, which had a planar cell structure with a wall thickness of 0.40 mm and a cell density of 100/2.54 cm2.

Then, this honeycomb shape was wrapped with an aluminum foil and dried with steam at 110ºC for 24 hours. After that, the product was heated to 1000ºC at a heating rate of 60ºC/h and then to 1200ºC at a heating rate of 30ºC/h, at which temperature it was further heated to sinter the ceramic material, thus obtaining a honeycomb catalyst carrier.

COMPARISON EXAMPLE 8

A honeycomb catalyst carrier was obtained in a similar manner as in Example 8, except that the precipitates of Comparative Example 6 were used instead of the precipitates prepared in Example 1.

COMPARISON EXAMPLE 9

A honeycomb catalyst carrier was obtained in a similar manner as in Example 8, except that the precipitates of Comparative Example 7 were used instead of the precipitates prepared in Example 1.

Physical properties of the honeycomb catalyst supports thus obtained and fired at 1200 °C for 15 h are given in Table 11 below. TABLE 11 Porosity Axial fracture strength Specific surface area Example Comparative example

It should be noted that the porosity is given in %, the axial breaking strength in g/cm and the specific surface area in m²/g.

The specific surface area (m²/g), pore volume (ml/g) and pore distribution (%) as well as the proportion of pores with a diameter of 100 Å or more, based on the Total pore volume and the volume (ml/g) of pores with a diameter of 100 to 400 Å were determined. The results are shown in Table 12 below. TABLE 12 Example Silica (wt%) Alumina (wt%) Specific surface area (m²/g) Pore volume (ml/g) Pore distribution (%) Å diameter 400 or larger Volume of pores with diameter of 100 Å or more / total pore volume Volume of pores with diameter of 100-400 Å

The specific surface area values of the honeycomb catalyst supports prepared in Example 8 and Comparative Examples 8 and 9 and fired at 1200ºC for 15, 100, 200 and 300 hours were determined. The results are given in Table 13 in m²/g. TABLE 13 Firing time Example Comparison example

EXAMPLE 9 - THIRD METHOD

The precipitates prepared in Example 1 were filtered off, washed with an aqueous solution of ammonium carbonate, and dried at 1200°C for 20 hours. Thereafter, the dried product was fired at 950°C for 5 hours and then finely pulverized for 3 hours with an automatic mortar manufactured by Nitto Science Co., Ltd. to obtain coating powders having an average particle size of 10 µm. Three hundred (300) g of these powders, 10 g of ammonium nitrate, 40 g of glycerin, and 650 g of deionized water were mixed and stirred with a Labo stirrer manufactured by Yamato Co., Ltd. for 3 hours or more to prepare a slurry.

Then, a honeycomb monolithic support (manufactured by NGK Insulators, Ltd., diameter: 150 mm, length: 15 mm, 220.5 g and 0.26 L) formed of cylindrical cordierite with 210 cells per 2.54 cm2 was immersed in water to supply it with water, then it was taken out while blowing away the water remaining in the cells by an air stream.

The honeycomb support, which had been supplied with water, was immersed in the slurry prepared previously for 15 minutes and then taken out while blowing away excess slurry remaining in the cells by an air stream. The support on which the slurry was coated in this manner was dried at 130°C for 5 hours and further subjected to firing at 1000°C for 5 hours in air using an electric furnace to form 18 g of coating film on the support.

COMPARISON EXAMPLE 10

A honeycomb catalyst carrier was obtained in a similar manner as in Example 9, except that the precipitates of Comparative Example 6 were used instead of the precipitates prepared in Example 1.

COMPARISON EXAMPLE 11

A honeycomb catalyst carrier was obtained in a similar manner as in Example 9, except that the precipitates of Comparative Example 7 were used instead of the precipitates prepared in Example 1.

The specific surface area (m²/g), pore volume (ml/g) and pore distribution (%), as well as the proportion of pores with a diameter of 100 Å or more based on the total pore volume and the volume (ml/g) of pores with a diameter of 100 to 400 Å were measured for the products fired at 1000ºC for 15 hours in this example and the comparative examples. The results are shown in Table 14 below. TABLE 14 Example Silica (wt%) Alumina (wt%) Specific surface area (m²/g) Pore volume (ml/g) Pore distribution (%) Å diameter 400 or larger Volume of pores with diameter of 100 Å or more / total pore volume Volume of pores with diameter of 100-400 Å

Measurements were made on the honeycomb catalyst supports prepared in Example 9 and Comparative Examples 10 and 11 and fired at 100°C for 15, 100, 200 and 300 hours by the nitrogen absorption method. The results are shown in Table 15 below in m²/g. TABLE 15 Firing time Example Comparison example

EXAMPLE 13 PRODUCTION OF CATALYSTS

The catalyst support moldings prepared in Example 10 were immersed in an aqueous solution of chloroplatinate in nitric acid (H2PtCl6 x 6H2O, 0.47 g/200 ml 0.1 N HNO3) to support platinum-containing metal catalyst on the catalyst support. The product was then dried at 80°C for 1 hour with warm air and then at 250°C for 1 hour. Thereafter, the dried catalyst support was immersed in an aqueous solution of rhodium chloride (RhCl6 x 3H2O, 0.045 g/200 ml of 0.1 N HNO3) to support rhodium-containing metal catalyst on the support, followed by warm air drying at 80°C to thereby obtain a catalyst A. Quantitative analyses of the noble metals contained in this catalyst revealed that platinum was present at 1.0 g/l of catalyst and rhodium was present at 0.1 g/l of catalyst.

In a similar manner as above, the compositions obtained in Examples 8 and 9 and Comparative Examples 6 to 9 were used as a carrier to be loaded with palladium, thereby preparing further catalysts.

ACTIVITY TESTS

A mixture of 1% CO, 4% O₂ and the balance N₂ was used as a reaction gas and passed through a reaction vessel made of a quartz glass tube having an inner diameter of 12.7 mm (and a volume of 1.9 cm³) filled with each of the catalysts shown in Table 16 under conditions of a volumetric velocity of 30,000 V/h/V and a reaction temperature of a tube heating furnace of 250°C. After the inlet portion of the reaction vessel was raised to a temperature of 250°C and kept stable at that temperature, the gases emitted from the outlet of the reaction vessel were analyzed by gas chromatography. It should be noted that catalysts subjected to firing at 500°C for 3 hours and at 1200°C for 100 hours were used and examined. The results obtained for CO conversion are given in Table 16. TABLE 16 Example Comparison example

From Table 16, it can be seen that the catalysts prepared using the supports of the present invention show no significant difference in reactivity from comparative catalysts when subjected to low temperature treatment such as firing at 500°C for 3 hours, but they still show a high value in CO conversion even when subjected to high temperature treatment such as firing at 1200°C.

To examine the durability of the catalysts, accelerated durability tests were conducted on products fired at 1000 and 1200°C. According to the accelerated durability test, each catalyst was placed in an electric furnace, heated at 1000°C while passing steam having a water/air mole fraction of 0.029 therethrough, and kept at that temperature for 200 to 500 hours to determine the specific surface area (ml/g) of the catalysts. The compositions of Examples 8 and 9 were used as the products of the invention and the compositions of Comparative Examples 9 and 10 were used as comparative products to prepare the respective catalysts. The results are shown in Table 17 below. No significant change was observed in the pore distribution of the catalyst support moldings of the invention, thereby demonstrating that the moldings of the invention can withstand extended periods of operation under conditions of high humidity. TABLE 17 1000ºC without water supply 1000ºC with water supply Holding time Example Comparison example

Claims (9)

1. A heat-resistant molded catalyst support made of a composition of silica and alumina, which is formed by the steps of forming an alumina hydrogel at a pH outside the range of 6.0 to 8.0, coating the hydrogel with silica, and aging the coated hydrogel at a final pH of 6.0 to 7.7 at a temperature of 40 to 90°C for 1 to 5 hours and then heating at a higher temperature to grow the formed silica-alumina crystals into crystals of uniform size, the support comprising: a silicon dioxide content in the range of 2 to 30 wt.%, a total pore volume of at least 0.3 ml/g, of which at least 60% of the pore volume is in pores of a diameter of 1000 nm (100 Å) or more (as determined by measuring the pore distribution by the nitrogen absorption method) and an NMR spectrum showing a peak area (measured by ²⁹Si-NMR, with silicone rubber after firing at 500ºC in air as reference peak (-21.9 ppm)) at -50 to -130 ppm, with a peak area of 0 to 10% at -100 to -130 ppm. 2. A catalyst support according to claim 1, wherein the volume of pores having a diameter of 1000 to 4000 nm (100 to 400 Å) in the pore distribution in said support is at least 0.20 ml/g. 3 A catalyst support according to claim 1 or 2, wherein said composition has a peak area at -50 to -80 ppm which accounts for 0 to 25% based on the peak area at -50 to -130 ppm measured by said NMR method. 4. A catalyst support according to any preceding claim, wherein the coating of the hydrogel with silica was carried out by adding dropwise to the hydrogel an aqueous solution having a concentration of 0.1 to 10 moles of alkali metal silicate. 5. A catalyst support according to any preceding claim, wherein said support has a specific surface area of at least 20 m²/g after being heated at 1200°C in air. 6. A catalyst support according to any preceding claim, wherein said ageing step was carried out at a pH of 6.0 to 8.0. 7. A catalyst support according to any one of claims 1 to 5, wherein said composition is present as a layer formed on an inorganic oxide substrate. 8. A catalyst support comprising: a sintered ceramic and a composition as defined in any of claims 1 to 6 uniformly dispersed therein or formed as a layer thereon. 9 Combustion catalyst comprising a heat-resistant catalyst carrier molded body according to any preceding claim and a catalytically active metal component incorporated therein.