JP2014113580A - Catalyst for exhaust gas purification and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas catalyst using an aluminum phosphate fired body and having more excellent performance, and a method for producing the same.SOLUTION: Provided is a catalyst for exhaust gas purification obtained by carrying at least one kind of platinum group metal selected from the group consisting of Pt, Rh and Pd onto a carrier of tridymite type aluminum phosphate fired body, in which the ratio x of a base content to an acid content in the carrier measured by a temperature rising desorption method satisfies 0.5%≤x≤1.0%, and also, the CO(probe gas) desorption peak temperature from the base point of the carrier is 600°C or higher.

Description

  The present invention relates to an exhaust gas purifying catalyst and a method for producing the same, and more particularly, a platinum group metal for exhaust gas purifying and a base metal-supported aluminum phosphate catalyst, for example, harmful components contained in exhaust gas discharged from an internal combustion engine such as an automobile. The present invention relates to a platinum group metal and base metal-supported aluminum phosphate catalyst to be purified, and a method for producing the same.

In recent years, exhaust gas regulations have been strengthened worldwide year by year from the viewpoint of protecting the global environment.
As a countermeasure, an exhaust gas purifying catalyst is used in an internal combustion engine. In this exhaust gas purification catalyst, platinum such as Pt, Pd, and Rh is used as a catalyst component in order to efficiently purify hydrocarbons (hereinafter sometimes abbreviated as HC), CO, and nitrogen oxides in the exhaust gas. Various catalysts including group elements are used.

Patent Document 1 includes a heat-resistant AlPO ₄ compound having a tridymite crystal structure and a BET specific surface area of 50 to 150 m ² / g, and a group consisting of Pt, Pd, and Rh supported on the AlPO ₄ compound. An exhaust gas purifying catalyst (Claim 1 of Patent Document 1) characterized by comprising at least one selected precious metal component is described.

International Publication No. 2009-142180A1 Pamphlet

  There is a need for an exhaust gas purification catalyst that reduces the amount of noble metal contained in the exhaust gas purification catalyst, and that does not easily deteriorate due to heat exhausted from the engine, sulfur components contained in fuel, and the like.

  However, the catalyst for exhaust gas purification shown in the prior art has been greatly reduced in catalytic activity due to sulfur poisoning, that is, poisoning of the catalyst by sulfur oxides in fuel and the like. Also, the activity is greatly reduced by sintering, that is, noble metal grain growth which is an active point of the catalyst, and there is room for improvement in order to increase the activity.

As a result of diligent efforts, the present inventors have determined the ratio of the amount of base to the amount of acid of the carrier measured by the temperature programmed desorption method in a catalyst supporting a platinum group metal using AlPO ₄ having a tridymite crystal structure as a carrier. By controlling the CO ₂ desorption peak temperature of the carrier base point to a certain temperature or more by controlling to a certain range, the above two problems can be solved at once, and very excellent results can be obtained. I found it.

Aspects of the present invention are as follows.
(1) An exhaust gas purifying catalyst comprising at least one platinum group metal selected from the group consisting of Pt, Rh, and Pd on a carrier of a tridymite type aluminum phosphate fired body, The ratio x of the base amount to the acid amount of the carrier measured by the desorption method is 0.5% ≦ x ≦ 1.0%, and the CO ₂ (probe gas) desorption peak temperature from the base point of the carrier is An exhaust gas purifying catalyst having a temperature of 600 ° C. or higher.
(2) The exhaust gas-purifying catalyst according to (1), wherein the platinum group metal is Pd.
(3) A step of baking an aluminum phosphate obtained from an aqueous solution adjusted to have a pH of 3.5 to 4.5 at a temperature of 1000 ° C. to 1200 ° C. for 2 hours or more to obtain an aluminum phosphate fired body,
Supporting at least one platinum group metal selected from the group consisting of Pt, Rh, and Pd on the carrier of the aluminum phosphate fired body;
The ratio x of the base amount to the acid amount of the carrier measured by the temperature-programmed desorption method is 0.5% ≦ x ≦ 1.0%, A method for producing an exhaust gas purifying catalyst, wherein a CO ₂ (probe gas) desorption peak temperature from a base point of the carrier is 600 ° C or higher.
(4) The method for producing an exhaust gas purification catalyst according to (3), wherein the platinum group metal is Pd.

  The exhaust gas purification catalyst according to the present invention suppresses a decrease in the activity of the catalyst by simultaneously solving the problem of grain growth of active sites due to the thermal durability of the exhaust gas purification catalyst and the problem of sulfur poisoning of the platinum group catalyst. Even after heat endurance treatment / sulfur poisoning treatment, the catalyst can exhibit not only excellent catalytic activity but also higher activity than conventional catalysts.

FIG. 1 is a graph showing the results of measuring the amount of acid and the amount of base with respect to temperature for the sample of Example 1. FIG. 2 is an image diagram showing the state of acid points and base points on each carrier. FIG. 3 is a graph showing measurement patterns of base amount / base strength and acid amount / acid strength. FIG. 4 is a graph showing a pattern of heat endurance / sulfur poisoning treatment of the catalyst. FIG. 5 is a graph showing a measurement pattern for evaluating the activity of the catalyst. FIG. 6 is a conceptual diagram of catalyst activity evaluation.

Note that in this specification, impurities or the like may be generated even if they are generated so as to have these compositions by the name of the inorganic compound or the notation using the ratio of the contained metal (as exemplified below). Including a composition that is actually generated. Therefore, according to the notation using the name of the inorganic compound or the ratio of the contained metal, for example, in the structure of the inorganic compound, for example, elements such as oxygen, hydrogen, and nitrogen are excessive in the chemical formula within ± 1 atom number or less. Or an inorganic compound with an under-existing composition, that is, for example, in the case of aluminum phosphate, AlPO ₄ also contains, for example, AlPO ₂ to AlPO ₅ when the number of oxygen is ± 1, and Al / P The ratio of 1 ± about 0.3 is also included, and further, those having hydrogen not represented in the compound as an impurity are included.

  The aluminum phosphate fired body according to the present invention has a tridymite crystal structure. However, in addition to the tridymite type crystal structure, a burrinite type crystal structure, a cristobalite type crystal structure, or an amorphous structure may be included.

  As the aluminum phosphate fired body according to the present invention, an aluminum phosphate fired body obtained by a known method can be used without particular limitation.

And the support | carrier of the purification catalyst which concerns on this invention has the ratio of the amount of bases with respect to the acid amount of a fixed range. The ratio x of the base amount to the acid amount of the carrier measured by the temperature programmed desorption method is about 0.40% or more, about 0.45% or more, about 0.50% or more, about 0.55% or more, about 0.60% or more, about 1.15% or less, about 1.10% or less, about 1.05% or less, about 1.00% or less, about 0.95% or less, about 0.90% or less, about 0 .85% or less.
As described below, x is preferably about 0.50 or more and about 1.0% or less from the viewpoint of suppressing poisoning by SOx and sintering of active sites due to heat.

Furthermore, the carrier of the purification catalyst according to the present invention is such that the CO ₂ (probe gas) desorption peak temperature from the base point of the carrier is a certain value or more. The CO ₂ (probe gas) desorption peak temperature from the base point of this carrier is about 550 ° C. or higher, about 580 ° C. or higher, about 590 ° C. or higher, about 600 ° C. or higher, about 605 ° C. or higher, about 610 ° C. or higher, about It can be 615 ° C. or higher, about 620 ° C. or higher, about 630 ° C. or higher, about 640 ° C. or higher, and about 1200 ° C. or lower.
As will be described below, the CO ₂ (probe gas) desorption peak temperature from the base point of this carrier is about 600 ° C. in order to prevent the catalyst activity from decreasing due to SOx poisoning that becomes noticeable around 600 ° C. The above is preferable.

The production method of the aluminum phosphate fired body is not particularly limited, and a known method such as a neutralization method can be employed. For example, in an aqueous solution of an Al-containing compound, a phosphoric acid aqueous solution is added so that the molar ratio of P to Al is approximately equivalent, and further, ammonia water is added to adjust the pH, and the resulting precipitate is separated and dried. Then, the method of baking at said baking temperature is mentioned. Examples of the Al-containing compound include metal salts such as hydroxide and nitrate, and specifically include Al (OH) ₃ , Al (NO ₃ ) _{3 and the} like.

  As the solvent used in the mixed solution containing the aluminum salt and phosphoric acid, any solvent that can dissolve the aluminum-containing compound and phosphoric acid, for example, an aqueous solvent such as water, an organic solvent, or the like can be used. it can.

And the aluminum phosphate sintered body according to the present invention has a tridymite type crystal structure, for example,
(A) a method of firing an aluminum phosphate obtained from an aqueous aluminum phosphate solution having a predetermined pH at a predetermined temperature for a predetermined time;
(B) a method of adding an excessive amount of phosphate ions to aluminum ions and producing unreacted phosphate ions in the product when producing an aluminum phosphate fired body,
(C) A foaming agent such as a thermoplastic resin such as an olefin resin having a diameter capable of generating a desired pore diameter, a thermoplastic resin such as a phenol resin, or the like is mixed in the raw material for producing the aluminum phosphate fired body, Those obtained by a wide range of methods such as a method of burning off the foaming agent in the firing step can be used.

  Here, the pH of the aqueous solution, the firing temperature, and the firing time during the formation of aluminum phosphate in the above (a) are not particularly limited as long as they do not adversely affect the tridymite crystal structure, and about 3.0 to about 10.0, respectively. Usually used conditions such as in the range of about 1000 ° C. to 1200 ° C., about 1 hour to about 10 hours can be used.

  A purification catalyst according to the present invention is obtained by supporting at least one platinum group metal selected from the group consisting of platinum (Pt), rhodium (Rh), and palladium (Pd) on the aluminum phosphate fired body. It is. There is no particular limitation on the form of loading, as long as the platinum group metal is supported substantially uniformly on the fired body.

  The supported platinum group metal nanoparticles have a particle size of about 0.40 nm or more, about 0.50 nm or more, about 0.60 nm or more, about 0.70 nm or more, about 0.80 nm or more, about 0.90 nm or more, about 1.0 nm or more, about 2.2 nm or less, about 2.1 nm or less, about 2.0 nm or less, about 1.9 nm or less, about 1.8 nm or less, about 1.7 nm or less, about 1.6 nm The thickness may be about 1.5 nm or less, about 1.4 nm or less, about 1.3 nm or less, about 1.2 nm or less, or about 1.1 nm or less.

The amount of platinum group metal with respect to the aluminum phosphate fired body is about 0.0001 wt% or more, about 0.001 wt% or more, about 0.01 wt% or more, about 0.1 wt% or more, about 0.20 wt% or more, about 0 30 wt% or less, about 2.0 wt% or less, about 1.9 wt% or less, about 1.8 wt% or less, about 1.7 wt% or less, about 1.6 wt% or less, about 1.5 wt% Below, about 1.4 wt% or less, about 1.3 wt% or less, about 1.2 wt% or less, about 1.1 wt% or less, about 1.0 wt% or less, about 0.90 wt% or less, about 0.80 wt% or less About 0.70 wt% or less, about 0.60 wt% or less, about 0.50 wt% or less, or about 0.40 wt% or less.
There is no particular limitation on the form of loading, as long as the platinum group metal is supported substantially uniformly on the carrier of the fired body.

  As a method for supporting the platinum group metal nanoparticles on the aluminum phosphate fired body, there is no particular limitation as long as it does not adversely affect the aluminum phosphate fired body. Can do.

  In order to equalize the particle size of the platinum group metal nanoparticles, a platinum group metal colloid that can provide platinum group metal particles having a desired particle size can be used. Other platinum group metal sources such as platinum acetate metal compounds, platinum nitrate metal compounds, platinum chloride metal compounds, and platinum group metal nanoparticles synthesized from these may also be used. However, since aluminum phosphate is easily soluble in a strong acid, it is preferable not to contain nitrate ions, chloride ions, and the like.

In the catalyst according to the present invention, as shown in Examples 1 to 3 below, the ratio x of the base amount to the acid amount in the aluminum phosphate is 0.5% ≦ x ≦ 1.0%, and the carrier When the CO ₂ desorption peak temperature from the base point of the catalyst is 600 ° C. or higher, an AlPO ₄ catalyst carrying a platinum group metal, particularly Pd particles by an impregnation method, is subjected to heat endurance treatment, sulfur poisoning by SO _{2 and the} like. Even after the treatment, the purification performance is 60% or more with respect to the exhaust gas such as NOx at 600 ° C., which is much better than the conventional catalysts (Comparative Examples 1 to 7) that show only a purification rate of 40% or less. It was found to have the ability.

Here, the ratio x of the amount of base to the amount of acid is, for example, that the amount of acid / base in Example 1 shows a maximum value in the vicinity of about 200 ° C. as shown by the solid line graph in FIG. 1, and the temperature is higher than that. Although both decrease as the value decreases, the ratio of the acid amount to the base amount is about 0.8 as calculated from the left and right axes. Further, as shown by the wide line having a peak on the right side of FIG. 1, the CO ₂ desorption peak temperature from the base point of the carrier is about 600 ° C. or higher.

Although not wishing to be bound by any theory, it is possible to suppress the adsorption of acidic SOx on the carrier when the amount of the base is small and the amount of acid is large on the aluminum phosphate carrier, which is the carrier, while the amount of the base is high and the acid is high. When the amount is small, the interaction between the active sites, that is, the supported platinum group metal nanoparticles and the carrier is strengthened, and it is considered that the growth of active sites due to thermal durability can be suppressed.
In addition, SOx adsorbed on the carrier moves to the active point when the exhaust gas temperature rises or becomes a rich atmosphere, and the sulfur-poisoned catalyst has a significant decrease in activity around 600 ° C. If SOx can be adsorbed and held up to about 600 ° C. at the base point, it is considered that the deactivation of the active point due to sulfur poisoning can be prevented.

The zeolite shown in Comparative Example 1 below has a sufficiently large number of acid points relative to the base point (FIG. 2B), so it is considered that the sulfur compound (SOx) in the fuel is difficult to adhere to the carrier. However, the strength of the base point is weak, that is, the SOx adsorption power is weak because the CO ₂ desorption temperature is low, and the SOx adsorbed on the carrier moves to the active point due to the rich change of the exhaust gas atmosphere, the temperature rise, etc. Therefore, it is considered that the activity is decreased by poisoning the active sites. Further, zeolite has low heat resistance of the carrier itself, and it is considered that active sites are aggregated due to shrinkage of the carrier due to heat, and the activity is greatly reduced.

Since SiO ₂ shown in Comparative Example 2 below has a sufficiently high acid point relative to the base point ((c) in FIG. 2), the sulfur compound (SOx) in the fuel is difficult to adhere to the carrier, and the strength of the base point Is also strong (CO ₂ desorption temperature is high), and it is considered that sulfur poisoning at the active site is also greatly suppressed. However, since the acidity of the carrier is too high, the interaction between the carrier and the active sites is reduced, and the active sites are considered to move and aggregate due to thermal durability, resulting in a decrease in activity.

Since Al ₂ O ₃ shown in Comparative Example 3 below has many basic points ((d) in FIG. 2), the interaction between the support and the active points is large, and the grain growth of the active points due to thermal endurance can be suppressed. On the other hand, since SOx adsorption is promoted, it is thought that it is easy to receive sulfur poisoning.

  With respect to these materials, the aluminum phosphate carrier shown in Examples 1 to 3 according to the present invention has 0.5% ≦ x ≦ 1.0%, in addition to a large amount of acid sites, a small amount of strong base sites. It is difficult to adhere to the support of the sulfur compound by having (FIG. 2 (a)) and the magnitude of the interaction between the support and the active site is balanced. Not only is it advantageous for suppressing grain growth, but also has excellent characteristics that it can suppress sulfur poisoning and decrease in activity due to SOx migration by adsorbing and holding SOx up to about 600 ° C. at the base point of the carrier (FIG. 1). It is what you have.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples.

<Synthesis Example 1: Synthesis of AlPO ₄ >
Step 1-1: 50 ml of ion-exchanged water was added to a beaker at room temperature, and a stir bar was added and stirred.
Step 1-2: 0.1 mol of aluminum nitrate nonahydrate (manufactured by Nacalai Tesque) was weighed, added to the ion-exchanged water, and dissolved while stirring.
Step 1-3: Weigh 0.1 mol of phosphoric acid (manufactured by Nacalai Tesque) in a separate beaker in terms of the amount of phosphoric acid, add it to the water in Step 1-2, and continue stirring at a stirring speed of 300 rpm. (The phosphoric acid remaining in the beaker was added to the mixed solution beaker using ion-exchanged water).
Step 1-4: 28 wt% aqueous ammonia (manufactured by Nacalai Tesque) was added dropwise to the beaker of the mixed solution little by little using a pipette, and the pH was adjusted to 4.3.
Step 1-5: The mixed solution beaker was covered and stirred at room temperature for 12 hours.
Step 1-6: The mixture was centrifuged (3000 rpm / min, 10 minutes), and separated into a precipitate and a supernatant.
Step 1-7: An appropriate amount of ion-exchanged water was added to the precipitate, and it was centrifuged again (washing step).
Step 1-8: The precipitate was dried with a dryer at 120 ° C. for 12 hours.
Step 1-9: The dried product was put in a mortar and crushed using a pestle to form a powder.
Step 1-10: The powder was fired at 1100 ° C. for 5 hours in an electric furnace to obtain about 12 g of powder.
<Production Example 1: Production of Pd carrier>
Step 2-1: At room temperature, 6 g of AlPO ₄ powder was weighed, placed in a beaker containing 30 ml of ion-exchanged water and a stirring bar, and stirred.
Step 2-2: A palladium nitrate aqueous solution having a Pd particle diameter of 0.7 nm was weighed so that the amount of Pd supported was 0.50 wt%, added to the beaker in Step 2-1, and stirred.
Step 2-3: The beaker of Step 2-2 was heated and stirred and evaporated to dryness until there was no water.
Step 2-4: The dried product of Step 2-3 was dried in a dryer at 120 ° C. for 12 hours.
Step 2-5: The dried product was put in a mortar and crushed using a pestle to form a powder.
Step 2-6: This supported powder was baked in an electric furnace at 500 ° C. for 3 hours.
Step 2-7: The powder after firing was molded into a pellet.

Example 1
A Pd-supported AlPO ₄ catalyst having a particle size of 2.0 nm was obtained by the procedures of <Synthesis Example 1> and <Production Example 1>.
(Examples 2 and 3)
The catalyst powder (Example 2, Example 3) was prepared in the same procedure as in (Example 1) except that the pH was adjusted to 4.0 and 10.0 in Steps 1-4, respectively. Got.
(Comparative Examples 1-3)
As carriers, ferrierite-type zeolite (manufactured by Tosoh, SiO ₂ : Al ₂ O ₃ = mass ratio, 18: 1, product number: HSZ-700 series 720KOA), SiO ₂ nanotech silica (manufactured by C-I Kasei), Al _{2, respectively.} Catalyst powders (Comparative Examples 1 to 3) were obtained in the same procedure as in (Example 1) except that O ₃ nanotech alumina (manufactured by C-I Kasei) was used.
(Comparative Examples 4-5)
In Step 1-3, the catalyst powders (Comparative Example 4 and Comparative Example 5) were prepared in the same manner as in (Example 1) except that the rotation speed of the stirrer was 100 rpm / 500 rpm. )
(Comparative Examples 6-7)
In Step 1-10, catalyst powders (Comparative Example 6 and Comparative Example 7) were obtained in the same procedure as in (Example 1) except that the firing temperature was 1000 ° C. and 1300 ° C., respectively.

<Measurement of photoelectron energy and crystal structure by XPS>
A sample horizontal strong X-ray diffraction apparatus RINT-TTRIII (manufacturer name: Rigaku Co., Ltd.) was used to irradiate X-rays, measure the energy of the generated photoelectrons, and observe the crystal structure.

<Measurement of particle size of platinum group metal nanoparticles>
The particle size of the platinum group metal nanoparticles was measured using a sample horizontal intense X-ray diffraction apparatus RINT-TTRIII (manufacturer name: Rigaku Corporation).
For calculation of the crystallite diameter, the vicinity of 2θ = 40.0 to 40.2 ° was used as the peak of Pd.

(Catalyst evaluation method)
The catalyst powder was compression-molded by applying a pressure of 2 t, then pulverized and compression-molded into pellets having a diameter of about 1.0 mm as an evaluation sample.
<Measurement of base amount and base strength (CO ₂ temperature programmed desorption method)>
According to the following steps 3-1 to 3-7 shown in FIG. 3, the measurement was performed using a catalyst adsorption / desorption gas analyzer (model number: RE-202, manufacturer: Nippon Vacuum Technology Co., Ltd.).
The sample amount was 1.0 g each, and the gas flow rate was 100 cc / min.
Step 3-1: He was allowed to flow while increasing the temperature of the sample from room temperature to 500 ° C. at a rate of temperature increase of 30 ° C./min over 15 minutes.
Step 3-2: He was allowed to flow while holding the sample at 500 ° C. for 10 minutes.
Step 3-3: He was allowed to flow while raising the temperature of the sample from 500 ° C. to 100 ° C. at a rate of temperature decrease of 20 ° C./min over 20 minutes.
Step 3-4: He was allowed to flow while holding the sample at 100 ° C. for 5 minutes.
Step 3-5: While maintaining the sample at 100 ° C. for 15 minutes, 90:10 He: CO ₂ was flowed at a sccm ratio, that is, a volume ratio.
Step 3-6: He was allowed to flow while holding the sample at 100 ° C. for 90 minutes.
Step 3-7: He was allowed to flow from 100 ° C. to 800 ° C. at a rate of temperature increase of 20 ° C./min for 35 minutes while increasing the temperature of the sample.

<Measurement of acid amount and acid strength (NH ₃ temperature programmed desorption method)>
In Step 3-5, instead of He: CO ₂ , the ratio of sccm, that is, volume ratio, except that 90:10 He: NH ₃ was flowed, <base amount / base strength measurement (CO ₂ temperature rising desorption) Separation procedure)>.
<Heat endurance treatment / sulfur poisoning treatment>
Step 4-1: 1.0 g of pellets carrying a white metal metal was placed in an alumina reaction tube. Next, as shown in FIG. 4, the catalyst was subjected to heat durability and sulfur poisoning by the following steps 4-2 to 4-6. In Steps 4-2 to 4-5, the gas flow rate is 5 liters / minute.
Step 4-2: While the temperature in the reaction tube was heated from room temperature to 1000 ° C. in an electric furnace at a rate of temperature increase of 5 ° C./min for 180 minutes, 100% by volume of N ₂ was allowed to flow into the reaction tube.
Step 4-3: While maintaining the temperature in the reaction tube at 1000 ° C., for 3 minutes, 3% by volume of CO (residual N ₂ ) gas and 5% by volume of O ₂ (residual N ₂ ) gas Were circulated alternately at intervals of 2 minutes.
Step 4-4: While the temperature in the reaction tube was lowered to 400 ° C. for 200 minutes at a rate of 3 ° C./min, 100% by volume of N ₂ was allowed to flow into the reaction tube.
Step 4-5: While maintaining the temperature in the reaction tube at 400 ° C., NO is 0.1%, CO is 0.65%, C ₃ H _{6 is} 0.1%, and CO ₂ by volume for 90 minutes. : 10%, O ₂ : 0.725%, H ₂ O: 3%, SO ₂ : 0.05%, N ₂ : The remaining gas was allowed to flow.
Process 4-6: It cooled to room temperature by natural cooling.

<Evaluation of catalyst activity>
By evaluating the activity of the catalyst after the heat endurance treatment / sulfur poisoning treatment by measuring the purification rate using a gas flow-type catalyst evaluation device according to the following steps 5-1 to 5-5 shown in FIG. went.
Each sample amount was 3.0 g.
Step 5-1: The temperature of the sample was raised from room temperature to 150 ° C. at a rate of temperature increase of 20 ° C./min.
Step 5-2: The sample was held at 150 ° C. for 5 minutes.
Step 5-3: The temperature of the sample was increased from 150 ° C. to 600 ° C. at a rate of temperature increase of 20 ° C./min.
Step 5-4: While maintaining the temperature of the sample at 600 ° C., by volume, as an evaluation gas, CO: 0.65%, C ₃ H ₆ : 3000 ppmC, NO: 1500 ppm, O ₂ : 0.7%, H ₂ O: 3%, CO ₂ : 10%, N ₂ : The remaining model gas was allowed to flow for 3 minutes at a gas flow rate of 15 L / min and SV of about 300,000 hours ⁻¹ to measure the purification rate.
As shown in FIG. 6, the purification rate is measured using an infrared spectrometer (manufacturer name: HORIBA, Ltd., model number: MEXA-6000FT) after passing the evaluation gas through the sample. The purification rate was calculated from the measured value by the following formula.
Purification rate (%) = (catalyst gas concentration (volume%) − catalyst gas concentration (volume%)) / catalyst gas concentration (volume%) × 100
Step 5-5: Cooled to room temperature by natural cooling.

(result)
The ratio of base amount to acid amount (%), CO ₂ desorption peak temperature from base point (° C.), heat endurance / sulfur ratio, obtained for each of Examples 1 to 3 and Comparative Examples 1 to 7 Table 1 shows the NOx purification rate (%) at 600 ° C. after poisoning.
The ratio of the amount of base to the amount of acid was calculated from both calibration curves and the peak area.

The ratio x of the amount of base to the amount of acid is 0.8% (Example 1), 0.5% (Example 2), 1.0% (Example 3), and CO ₂ desorption from the base point. In the catalyst according to the present invention having separation peak temperatures of 640 ° C., 620 ° C., and 650 ° C., surprisingly, the purification rate was 65% for NO at 600 ° C. even after heat endurance and sulfur poisoning treatment. Compared with conventional purification rates of 18% to 40 (Comparative Examples 1 to 7), 58% and 60%, markedly higher performance was exhibited.

Since the zeolite of Comparative Example 1 has a sufficiently high acid point relative to the base point, it is considered that the sulfur compound (SOx) in the fuel is difficult to adhere to the carrier, but the CO ₂ desorption peak temperature from the base point is 170. Since it was as low as 340 ° C., it was found that the strength of the base point seems to be weak.
Furthermore, the NOx purification rate at 600 ° C. after heat endurance and sulfur poisoning is as low as 18% because the heat resistance of the zeolite carrier itself is low, and the active sites are aggregated due to the shrinkage of the carrier due to heat, and the activity is increased. This is thought to be due to a large drop.

Since SiO _{2 of} Comparative Example 2 has a sufficiently large acid point relative to the base point, it is considered that the sulfur compound (SOx) in the fuel is difficult to adhere to the carrier, and the CO ₂ desorption peak temperature from the base point is also 680. Since it is ° C., it is considered that the strength of the base point is also strong.
However, since the acidity of the carrier is too large, the interaction between the carrier and the active point is reduced, and the active point moves and aggregates due to heat endurance, and the NOx purification rate at 600 ° C. after heat endurance and sulfur poisoning Is considered to have fallen to 35%.

Al ₂ O ₃ of Comparative Example 3, since it has many basic sites is believed that a large interaction between the carrier and the active sites. However, it is considered that the adsorption of acidic SOx was promoted by the large number of base points, and the NOx purification rate at 600 ° C. after heat endurance / sulfur poisoning was reduced to 30% due to sulfur poisoning.

  Further, even when aluminum phosphate having tridymite type crystals was used, the ratio x of the base amount to the acid amount exceeded 1.0%, 1.2% (Comparative Example 5), 2.0% (Comparative Example 7) ), It was found that as x increased, the NOx purification rate at 600 ° C. after heat endurance / sulfur poisoning decreased to 40% (Comparative Example 5) and 18% (Comparative Example 7).

  Further, when the baking condition of the aluminum phosphate carrier is 1000 ° C., amorphous aluminum phosphate is generated. It is considered that the amorphous aluminum phosphate having an amorphous structure breaks the structure due to the acid component of the supporting chemical solution when supporting the white metal metal, so that the white metal metal is not well supported on the support and is sintered after the heat endurance. Even at 0.8%, it has been found that the NOx purification rate at 600 ° C. after heat endurance and sulfur poisoning decreases to 24% (Comparative Example 6).

As described above, the performance difference between the catalyst according to the embodiment of the present invention, the comparative example, and the conventional catalyst is a tridymite-type aluminum phosphate fired body that does not contain an amorphous material, measured by a temperature programmed desorption method. The ratio x of the base amount to the acid amount of the carrier is 0.5% ≦ x ≦ 1.0%, and the CO ₂ (probe gas) desorption peak temperature from the base point of the carrier is 600 ° C. or higher. It has become clear that this has a significant effect.

  As described above, the exhaust gas purification catalyst according to the present invention has good performance as an exhaust gas purification catalyst even after the heat endurance treatment and sulfur poisoning treatment. For these reasons, the use of the oxidation catalyst according to the present invention is not limited to the exhaust gas purification catalyst, and can be used for various applications in a wide field.

Claims (4)

A catalyst for purifying exhaust gas, comprising at least one platinum group metal selected from the group consisting of Pt, Rh, and Pd on a carrier of a tridymite-type aluminum phosphate fired body, The ratio x of the base amount to the acid amount of the carrier measured by the above is 0.5% ≦ x ≦ 1.0%, and the CO ₂ (probe gas) desorption peak temperature from the base point of the carrier is 600 ° C. or higher A catalyst for exhaust gas purification.   The exhaust gas-purifying catalyst according to claim 1, wherein the platinum group metal is Pd. a step of calcining an aluminum phosphate obtained from an aqueous solution adjusted to have a pH of 3.5 to 4.5 at a temperature of 1000 ° C. to 1200 ° C. for 2 hours or more to obtain a calcined aluminum phosphate,
Supporting at least one platinum group metal selected from the group consisting of Pt, Rh, and Pd on the carrier of the aluminum phosphate fired body;
The ratio x of the base amount to the acid amount of the carrier measured by the temperature-programmed desorption method is 0.5% ≦ x ≦ 1.0%, A method for producing an exhaust gas purifying catalyst, wherein a CO ₂ (probe gas) desorption peak temperature from a base point of the carrier is 600 ° C or higher.   The method for producing an exhaust gas purification catalyst according to claim 3, wherein the platinum group metal is Pd.

Images