KR102332047B1 - Low-temperature de-NOx catalyst using ceria-alumina complex support and manufacturing method thereof - Google Patents

Abstract

The present invention discloses a low-temperature nitrogen oxide removal catalyst using a ceria-alumina composite support and a method for preparing the same. According to the present invention, a ceria-alumina composite support synthesized by treating a ceria precursor and an alumina precursor by a co-precipitation method at a preset mass ratio is impregnated with a noble metal and a metal oxide. Low-temperature nitrogen oxide removal using a ceria-alumina composite support A catalyst is provided.

Description

Low-temperature de-NOx catalyst using ceria-alumina complex support and manufacturing method thereof

The present invention relates to a low-temperature nitrogen oxide removal catalyst using a ceria-alumina composite support and a method for preparing the same, and more particularly, to a ceria-alumina composite support used for NOx removal at a low temperature of 200 °C or less. It relates to a synthesis method and additionally to an optimal synthesis method of noble metals and metal oxides.

As high fuel efficiency technology (lean burn engine) is applied to reduce fuel consumption of automobiles, it is difficult to remove NOx, one of the harmful substances in exhaust gas.

This is because the amount of air input during combustion increases and the amount of NOx produced itself increases, and the temperature of the exhaust gas is lowered, which is not suitable for the activation temperature of the existing catalyst.

In addition, in order to keep pace with the accelerating environmental regulations, it is important to secure the ability to remove NOx in the 'cold-start' section.

According to the survey, it can be confirmed that the amount of NOx emitted during the cold start section is larger than the amount of NOx emitted during normal driving. The cold start section refers to the initial stage of starting and refers to driving conditions in which the temperature of the exhaust gas purification device is not sufficient.

In order to improve these problems, the development of a catalyst capable of efficiently removing NOx at low temperatures, specifically below 200 °C, and adsorption and removal factors are being sought.

Existing lean NOx traps (hereinafter referred to as LNTs) include a support made of a porous material such as alumina, a metal oxide using an adsorption active material such as barium oxide, and a noble metal responsible for NOx oxidation or reduction catalyst such as platinum, This consists of 3 parts.

However, in order to enhance the NOx removal efficiency, it must be possible to remove NOx even under low temperature conditions, and lowering the desorption temperature as well as enhancing the adsorption capacity for NOx at low temperature is an important improvement direction.

If the adsorption capacity for NOx at low temperature is low, NOx slip phenomenon frequently occurs below the catalyst activation temperature, and the very high desorption temperature is another disadvantage of the existing LNT catalyst, which is disadvantageous for regeneration at low temperature operation. Therefore, performance is improved by introducing various types of elements, and among them, introducing ceria having excellent oxygen storage capacity (OSC) helps nitric oxide oxidation to increase NOx storage performance as well as Oxidation-reduction ability (redox property) is excellent, it is advantageous even when reducing the stored NOx.

According to the prior art, ceria was additionally supported on an inorganic carrier, or ceria was physically mixed with other inorganic oxides, such as alumina, silica or zirconia, with high thermal stability, and used together.

Ceria is an inorganic oxide with a high surface area and can serve as a support by itself, but is not suitable for use alone as a support for an automobile exhaust gas purification catalyst because of its low thermal stability.

According to the paper on the surface area according to the firing temperature of ceria, ^{it maintains a surface area of 130 m 2} /g when calcined at 600 °C and continues to decrease at higher temperatures, so that when calcined at 800 °C, the surface area is 46 m ² /g was recorded [M. Eberhardt et al., Topics in Catalysis 30-31 (2004) 135].

Conventionally, there have been cases in which ceria is additionally impregnated with noble metals, barium oxide, and the like into an inorganic support, but when the support is impregnated to a certain level or more, the surface area characteristics of the catalyst are deteriorated, and the adsorption performance of the catalyst is rather reduced.

Republic of Korea Patent Registration 10-1621110

In order to solve the problems of the prior art, the present invention provides a ceria-alumina complex capable of supplementing the thermal stability of the support and increasing the oxygen storage capacity and oxidation-reduction ability of ceria when an inorganic carrier and ceria are mixed and used. An object of the present invention is to propose a low-temperature nitrogen oxide removal catalyst using a support and a method for preparing the same.

In order to achieve the above object, according to an embodiment of the present invention, as a low-temperature nitrogen oxide removal catalyst, a ceria precursor and an alumina precursor are treated by a co-precipitation method at a preset mass ratio to a ceria-alumina composite support. A low-temperature nitrogen oxide removal catalyst using a ceria-alumina composite support prepared by impregnating a noble metal and a metal oxide is provided.

The ceria precursor may include one of cerium chloride (CeCl ₃ ), cerium sulfate (Ce(SO ₄ ) ₂ ), and cerium nitrate hydrate (Ce(NO ₃ ) ₃ 6H ₂ O).

The alumina precursor may include one of aluminum chloride (AlCl ₃ ), aluminum sulfate (Al ₂ (SO ₄ ) ₃ ), and aluminum nitrate hydrate (Al(NO ₃ )9H ₂ O).

A mass ratio of the ceria precursor to the alumina precursor may be 1:3 to 1:5.

The ceria-alumina composite carrier may be synthesized by dissolving the alumina precursor and then dissolving the alumina precursor for a preset time by introducing the ceria precursor after a preset time elapses.

The ceria-alumina composite carrier may be synthesized by stirring, washing, filtering, and calcining at a preset temperature to obtain a precipitate through pH adjustment after the dissolution.

The noble metal may include at least one of platinum, palladium, rhodium, and iridium, and the metal oxide may include an oxide containing at least one of magnesium, calcium, strontium, barium, sodium, potassium, rubidium, cesium, iron, and copper. can

After the precursor of the noble metal is first impregnated in the ceria-alumina composite carrier and dried, the precursor of the metal oxide may be impregnated and fired.

The precursor of the noble metal includes (NH ₄ ) ₂ PtCl ₄ , and the precursor of the metal oxide is copper nitrate hydrate (Cu(NO ₃ ) _{2 .3H2O) which is a copper oxide precursor, and barium acetate ((CH 3} ) which is a precursor of barium oxide. COO) ₂ Ba).

A mass ratio of the precursor of the copper oxide to the barium oxide may be 1:1.

According to another aspect of the present invention, as a method for preparing a low-temperature nitrogen oxide removal catalyst, after a preset time has elapsed after dissolving an alumina precursor in distilled water, the ceria precursor is introduced and dissolved again for a preset time, and pH is adjusted through obtaining a precipitate, and preparing a ceria-alumina composite carrier through drying, washing, filtering and calcining; impregnating the noble metal precursor into the ceria-alumina composite support; and impregnating one or more metals into the ceria-alumina composite support impregnated with the noble metal using one or more metal oxide precursors through co-impregnation. A method is provided.

According to the present invention, ceria can have a relatively high surface area even at a high temperature by co-precipitation with alumina, and there is an advantage in that the oxygen storage capacity and oxidation-reduction ability of ceria can be simultaneously increased.

In addition, according to the present invention, since ceria is present as a support, it is possible to avoid performance degradation of surface properties through impregnation.

Furthermore, the catalyst synthesized by synthesizing the catalyst by co-impregnating the ceria-alumina composite support formed through the co-precipitation method in addition to the noble metal and metal oxides. There is this.

1 is a view showing the synthesis process of the ceria-alumina composite carrier according to the present embodiment.
_{2 is a H 2} -TPR experiment results from 50 °C to 600 °C.
Figure 3 shows the change in the gas composition under conditions in which nitrogen monoxide and oxygen coexist from 50 °C to 600 °C in order to evaluate the nitrogen monoxide oxidation ability.
4 is a view showing a comparison of NOx storage efficiencies according to the temperature of the two catalysts through the above calculation method.

Since the present invention can have various changes and can have various embodiments, specific embodiments are illustrated in the drawings and described in detail.

However, this is not intended to limit the present invention to specific embodiments, and it should be understood to include all modifications, equivalents and substitutes included in the spirit and scope of the present invention.

According to a preferred embodiment of the present invention, the ceria-alumina composite carrier is synthesized through a co-precipitation method, rather than impregnating ceria into alumina.

The co-precipitation method is a treatment method in which the precipitate precursors containing metal ions are simultaneously and uniformly precipitated.

1 is a view showing the synthesis process of the ceria-alumina composite carrier according to the present embodiment.

Referring to FIG. 1 , a ceria precursor and an alumina precursor are dissolved in a solvent according to a predetermined order (step 100).

As a precursor of ceria, cerium chloride (CeCl ₃ ), cerium sulfate (Ce(SO ₄ ) ₂ ), cerium nitrate hydrate (Ce(NO ₃ ) ₃ 6H ₂ O), etc. may be used, and cerium nitrate hydrate is preferably used. can

As a precursor of alumina, aluminum chloride (AlCl ₃ ), aluminum sulfate (Al ₂ (SO ₄ ) ₃ ), aluminum nitrate hydrate (Al(NO ₃ )9H ₂ O), etc. may be used, and aluminum nitrate hydrate is preferably used. can

In step 100, the ceria precursor and the alumina precursor are dissolved in a preset mass ratio.

The mass ratio of the ceria precursor and the alumina precursor is a variable that can affect the surface area properties and the catalytic properties.

According to this embodiment, the mass ratio of the alumina precursor and the ceria precursor may be 1:1 to 9:1, preferably 4:1 to 5:1, more preferably 4:1.

When the co-precipitation method is performed, the dissolution order and dissolution time of the ceria precursor and the alumina precursor introduced into distilled water may be considered as factors that may affect the surface area characteristics of the composite carrier.

The alumina precursor may be dissolved after the ceria precursor is first dissolved, or the ceria precursor may be dissolved after the alumina precursor is first dissolved, and the precursor may be simultaneously introduced into distilled water.

According to this preferred embodiment, after a preset time elapses (for example, 3 hours later) after dissolving the alumina precursor, a method of introducing the ceria precursor and dissolving it again for a preset time is used.

Table 1 shows the BET surface area (m ² /g), pore volume, and pore diameter of _{Al 2} O ₃ and CeO ₂ according to this embodiment.

BET surface area
(m ² /g) Pore volume
(cm ³ /g) Pore diameter
(nm) Al ₂ O ₃ 294 0.26 5.6 Al1Ce1 204 0.29 4.6 Al2Ce1 212 0.27 5.5 Al4Ce1 299 0.20 5.8 Al9Ce1 271 0.18 5.4 Ce1Al2 239 0.16 3.6 Ce1Al4 204 0.22 4.2 Ce1Al2(co) 190 0.16 3.8 CeO ₂ 55 0.10 4.6

Referring to Table 1, in the case of dissolving the alumina precursor first and dissolving the ceria precursor, in an experiment in which the mass ratio of alumina and ceria precursor was changed to 1:1, 2:1, 4:1 and 9:1, 4: In the case of 1, it can be confirmed that the BET surface area is the largest. Also, looking at the case where the mass ratio of alumina and ceria precursor in the precipitation sequence is 4:1, the BET surface area is higher in the case where the alumina precursor is first dissolved and the ceria precursor is dissolved (Al4Ce1) compared to the opposite case (Ce1Al4). You can see the big one.

In order to obtain a precipitate from the alumina precursor and the ceria precursor, the pH is adjusted using NaOH and stirring is performed (step 102).

In step 102, the pH required to obtain a precipitate from each precursor is determined in the range of 10.0 to 11.0, and the stirring speed is maintained at 300 rpm.

After stirring is completed, a precipitate is obtained through washing and filtering (step 102).

In step 104, agitation is performed at 300 rpm in order to prevent the filtering speed from being slowed by the sediment as much as possible even when washing and filtering is performed.

After sufficient washing, a precipitate is obtained through filtering without stirring.

The precipitate is dried in an oven at 110 °C for 12 hours to remove moisture (step 106).

The dried precipitate is calcined to activate the ceria-alumina composite carrier (step 108).

The activation firing temperature of the composite carrier according to the present embodiment may be 600 °C, and proceeds for 4 hours.

The firing atmosphere is a factor that can change the surface properties. As the gas flowing during firing, nitrogen, argon, carbon dioxide, air, etc. may be selected, and the firing is preferably carried out in an air atmosphere.

Precious metals and metal oxide impregnation

Precious metals and metal oxides were impregnated using the ceria-alumina composite support synthesized by the above method.

The impregnation of noble metals and metal oxides uses continuous impregnation and co-impregnation methods.

As the noble metal, platinum, palladium, rhodium, iridium, etc. may be used, and platinum is preferably used, and the platinum precursor (NH ₄ ) ₂ PtCl ₄ was used.

Metal oxides were used to store NOx, and magnesium, calcium, strontium, barium, sodium, potassium, rubidium, cesium, iron, copper, etc. may be used, preferably barium and copper oxides.

_{Table 2 below shows the NO 2} adsorption capacity when the ceria-alumina composite carrier is synthesized, the precipitation sequence is performed at different mass ratios in the order of alumina and ceria, and then impregnated with platinum and barium.

Adsorption capacity
(mmol/g) Pt2-Ba20-Al 0.99 Pt2-Ba20-Al2Ce1 0.83 Pt2-Ba20-Al4Ce1 1.04 Pt2-Ba20-Al9Ce1 0.90

Table 2 shows the adsorption capacity for NOx of the catalyst synthesized by synthesizing the support with the same precipitation sequence and different ratios of Al and Ce, and impregnating the support with Pt and Ba corresponding to the LNT catalyst. Referring to Table 3, _{when comparing the adsorption capacity for the 10% NO 2} /He balance gas flow, when the mass ratio of alumina and ceria precursor was 4:1, it was found that the adsorption capacity was the highest compared to other mass ratios. can be checked

_{Barium acetate ((CH 3} COO) ₂ Ba) and copper nitrate hydrate (Cu(NO ₃ ) ₂ ·3H ₂ O) are used as precursors of barium and copper oxide, respectively.

The mass of each precursor is a variable that can affect the catalyst properties, and the noble metal can be adjusted to 0.5, 1, 2 wt%, etc. relative to the mass of the entire catalyst, preferably 2 wt%.

The metal oxide may be adjusted to 5, 10, 20 wt%, etc. of the total catalyst mass, preferably 20 wt%.

The mass ratio of the precursor of copper oxide to barium oxide can be adjusted to 2:1, 1:1, 1:2, etc., and preferably 1:1.

Copper oxide and barium oxide precursors were impregnated together or separately in 1.5 mL or 1.2 mL of distilled water dispersed in distilled water, this order may affect the catalytic properties, preferably the platinum precursor is impregnated first and then copper and barium sequentially The precursor was co-impregnated.

Specifically, after dispersing the platinum precursor in 1.5 mL of distilled water, and then impregnating 0.25 mL each in 1 g of ceria-alumina composite carrier, repeating drying at 110 °C 6 times, and drying at 110 °C for 12 hours, Next, copper and barium oxide precursors were dispersed in 1.2 mL of distilled water and then co-impregnated into the same ceria-alumina composite support by 0.2 mL each, followed by drying 6 times and then dried at 110 °C for 12 hours.

The impregnated catalyst was calcined for activation and was carried out at 600 °C for 2 hours. As the calcination atmosphere, nitrogen, argon, carbon dioxide, air, etc. can be selected as a variable that can affect the catalyst properties, and calcination was preferably carried out in an air atmosphere.

Catalyst Characterization

In order to compare the activities of the catalysts, a reduction experiment (H ₂ -TPR) according to the temperature increase using hydrogen was performed.

The name of the catalyst was divided into three parts: a noble metal catalyst, a metal oxide, and a support. When the metal oxide part and the support part consist of two or more, each element is denoted at the same time using a dash (-).

_{2 is a H 2} -TPR experiment results from 50 °C to 600 °C. The initial reduction of the Pt/Ba/Al catalyst corresponding to the existing LNT material recorded a peak at 218 °C, and the higher reduction recorded a peak at 514 °C, confirming that the reduction at high temperature was greater.

The initial reduction of the Pt/Ba/Al-Ce catalyst in which the support was replaced with a ceria-alumina composite support peaked at 176 °C, and it was observed that higher reduction occurred near 450 °C. Compared with Pt/Ba/Al, the amount of reduction was increased at low temperature and the reduction initiation temperature was lower.

_{Lastly, looking at the H 2} -TPR of the Pt/Cu-Ba/Al-Ce catalyst, the reduction temperature at low temperature was the lowest among the three types of catalysts and the size was the largest. Therefore, it can be seen that the catalyst with the highest activity at low temperature is Pt/Cu-Ba/Al-Ce.

In particular, the Pt/Cu-Ba/Al-Ce catalyst shows the highest reduction in the amount near 200 °C, confirming that it is highly useful for NOx removal at low temperatures below 200 °C.

In order to measure the specific surface area of the support and the catalyst, nitrogen adsorption analysis was performed, and the results are shown in Table 3.

Support and catalyst specific surface area
(m ² /g) pore volume
(cm ³ /g) Al 294 0.25 Al-Ce 289 0.23 Pt/Ba/Al 197 0.11 Pt/Cu-Ba/Al-Ce 163 0.16

Referring to Table 3, the specific surface area and pore volume of the alumina support and the ceria-alumina composite support did not differ significantly. However, it can be seen that the specific surface area and pore volume of both catalysts are greatly reduced after impregnation of the catalyst and the metal oxide. Through this, it can be seen that the impregnation occurred on the support and the surface area properties were reduced.

Figure 3 shows the change in the gas composition under conditions in which nitrogen monoxide and oxygen coexist from 50 °C to 600 °C in order to evaluate the nitrogen monoxide oxidation ability.

3a is Pt/Ba/Al, FIG. 3b is Pt/Ba/Al-Ce, and FIG. 3c shows gas composition change through temperature increase after nitrogen monoxide adsorption for Pt/Cu-Ba/Al-Ce.

Since barium oxide has a higher affinity for nitrogen dioxide than nitrogen monoxide, the oxidation ability for nitrogen monoxide at low temperatures is important to increase the adsorption capacity.

In the experiment, a gas of 200 ppm NO/10% O ₂ /N ₂ atmosphere was flowed from 50 °C to 200 ppm NO/10% O 2 /N 2 for 30 minutes in the same way for each catalyst, and then the temperature was raised to 600 °C while flowing the gas of the same composition.

Referring to FIG. 3, first, comparing the nitrogen monoxide adsorption section (10 to 40 minutes) at 50 °C, it can be seen that the concentration of NOx in Pt/Ba/Al is rapidly recovered, indicating that almost no adsorption is performed. .

Next, it can be seen that in Pt/Ba/Al-Ce, since the concentration of NOx was recovered more slowly than that of Pt/Ba/Al, the adsorption took place a little more. This is thought to be due to the high oxygen storage capacity and oxidation-reduction capacity of ceria present as a support. Lastly, in the Pt/Cu-Ba/Al-Ce catalyst, the NOx concentration was recovered the most slowly during the nitrogen monoxide adsorption section (10 to 40 minutes) compared to the previous two catalysts, indicating that the most adsorption occurred.

Comparing the results of analysis of the oxidation degree of nitrogen monoxide through temperature increase after adsorption for 30 minutes, Pt/Ba/Al and Pt/Ba/Al-Ce corresponding to the existing LNT catalyst showed almost the same trend, and the adsorbed amount was Pt/Ba/Al-Ce appeared to be more abundant, and the temperature range in which the adsorption capacity increased due to oxidation was similar. However, Pt/Cu-Ba/Al-Ce showed a different tendency from the previous two catalysts, and the nitrogen monoxide slip phenomenon from above 200 °C was a notable difference. This is a phenomenon in which nitrogen monoxide, which is weakly bound to copper oxide, falls off with increasing temperature. seemed to happen

The temperature at which desorption occurs was lowered by nearly 100 °C compared to Pt/Ba/Al or Pt/Ba/Al-Ce.

Adsorption performance evaluation

The adsorption capacity according to the temperature of the Pt/Ba/Al catalyst corresponding to the existing LNT catalyst and the Pt/Cu-Ba/Al-Ce catalyst was compared.

The adsorption capacity was compared through the NOx storage efficiency (hereinafter referred to as NSE, the difference between the inflow NOx and the exhaust NOx compared to the inflow NOx), and the formula to calculate the NOx storage efficiency is as follows.

4 is a view showing the comparison of NSE according to the temperature of the two catalysts through the above calculation method.

Referring to FIG. 4 , if the NSE according to the temperature of the Pt/Ba/Al catalyst is compared first, it can be confirmed that it is significantly lower at a low temperature and increases as the temperature increases. Next, comparing the NSE according to the temperature of the Pt/Cu-Ba/Al-Ce catalyst, it can be seen that the adsorption efficiency was significantly increased at low temperatures, especially in the initial section, and decreased with time.

At 250 °C, it can be seen that the NSE decreased in a small way due to the NOx slip phenomenon, and at 350 °C, the highest NSE was recorded, indicating that there is almost no amount of NOx emitted until about 15 minutes. Based on these results, it can be confirmed that the Pt/Cu-Ba/Al-Ce catalyst is advantageous for NOx adsorption at low temperature and the adsorption efficiency is the best in the high temperature region of 350 °C.

The above-described embodiments of the present invention are disclosed for purposes of illustration, and various modifications, changes, and additions will be possible within the spirit and scope of the present invention by those skilled in the art having ordinary knowledge of the present invention, and such modifications, changes and additions should be regarded as belonging to the following claims.

Claims (11)

In the low-temperature nitrogen oxide removal catalyst,
It is prepared by impregnating a noble metal and a metal oxide in a ceria-alumina composite carrier synthesized by treating a ceria precursor and an alumina precursor in a preset mass ratio by a co-precipitation method,
The mass ratio of the alumina precursor and the ceria precursor is 3:1 to 5:1,
The noble metal includes at least one of platinum, palladium, rhodium, and iridium,
The metal oxide includes an oxide containing at least one of magnesium, calcium, strontium, barium, sodium, potassium, rubidium, cesium, iron, and copper,
After the precursor of the noble metal is first impregnated in the ceria-alumina composite carrier and dried, the precursor of the metal oxide is impregnated and fired,
The noble metal includes 0.5 to 2 weight percent of the total catalyst mass, and the metal oxide includes 5 to 20 weight percent of the total catalyst mass. A low-temperature nitrogen oxide removal catalyst using a ceria-alumina composite support. According to claim 1,
The ceria precursor is ceria containing one of cerium chloride (CeCl ₃ ), cerium sulfate (Ce(SO ₄ ) ₂ ), and cerium nitrate hydrate (Ce(NO ₃ ) ₃ 6H ₂ O) using an alumina composite support. Low-temperature nitrogen oxide removal catalyst. According to claim 1,
As the alumina precursor, aluminum chloride (AlCl ₃ ), aluminum sulfate (Al ₂ (SO ₄ ) ₃ ), and aluminum nitrate hydrate (Al(NO ₃ )9H ₂ O) Using a ceria-alumina composite support. Low-temperature nitrogen oxide removal catalyst. delete According to claim 1,
The ceria-alumina composite carrier is synthesized by introducing the ceria precursor after a preset time has elapsed after dissolving the alumina precursor and dissolving it again for a preset time. Low-temperature nitrogen oxide removal catalyst using a ceria-alumina composite carrier . 6. The method of claim 5,
The ceria-alumina composite support is synthesized by stirring, washing, filtering and calcining at a preset temperature to obtain a precipitate through pH adjustment after the precipitation. A low-temperature nitrogen oxide removal catalyst using a ceria-alumina composite support. delete delete According to claim 1,
The precursor of the noble metal includes (NH ₄ ) ₂ PtCl ₄ , and the precursor of the metal oxide is copper nitrate hydrate (Cu(NO ₃ ) _{2 .3H2O) which is a copper oxide precursor, and barium acetate ((CH 3} ) which is a precursor of barium oxide. A low-temperature nitrogen oxide removal catalyst using a ceria-alumina composite support containing COO) _{2 Ba).} 10. The method of claim 9,
A low-temperature nitrogen oxide removal catalyst using a ceria-alumina composite support in which the mass ratio of the copper oxide and the precursor of barium oxide is 1:1. In the low-temperature nitrogen oxide removal catalyst manufacturing method,
After dissolving the alumina precursor in distilled water, after a preset time elapses, the ceria precursor is introduced and dissolved again for a preset time, a precipitate is obtained through pH adjustment, and a ceria-alumina complex through drying, washing, filtering and calcining preparing a carrier;
impregnating the noble metal precursor into the ceria-alumina composite support; and
impregnating the ceria-alumina composite support impregnated with the noble metal using one or more metal oxide precursors through co-impregnation with one or more metals,
The mass ratio of the alumina precursor and the ceria precursor is 3:1 to 5:1,
The noble metal includes at least one of platinum, palladium, rhodium, and iridium,
The metal oxide includes an oxide containing at least one of magnesium, calcium, strontium, barium, sodium, potassium, rubidium, cesium, iron, and copper,
After the precursor of the noble metal is first impregnated in the ceria-alumina composite carrier and dried, the precursor of the metal oxide is impregnated and fired,
The noble metal comprises 0.5 to 2 weight percent of the total catalyst mass, and the metal oxide comprises 5 to 20 weight percent of the total catalyst mass.

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