JPS6050490B2 - Method for manufacturing palladium catalyst - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a method for producing a supported palladium catalyst.

Carbon monoxide, hydrocarbons, and nitrogen oxides contained in the exhaust gas of internal combustion engines cause air pollution, so from the perspective of pollution prevention, reoxidation of carbon monoxide (Co) and hydrocarbons (HC) and These must be made non-toxic by reducing the oxides (NOx).

It is known that platinum group catalysts exhibit excellent activity as oxidation catalysts for exhaust gas purification. However, in gas conversion catalytic reactions, the reaction gases are captured on the surface of the catalyst, and there is a high probability that the conversion reaction will proceed there. Therefore, in order to make these expensive platinum catalysts work efficiently and with high performance, it is necessary to use a carrier. An important condition is to support catalytic active components such as platinum group metals as densely as possible on the surface of the catalyst.

The conventional method for supporting catalytic metals on a carrier is the impregnation method, that is, a method in which a granular carrier is impregnated with a platinum group metal-containing solution (impregnating solution), and the platinum group metal is deposited by adsorption and hydrolysis on the surface of the carrier. Most commonly used.

In this impregnation method, the impregnating solution is generally used as a hydrochloric acid solution or a nitric acid solution, but if the nitric acid solution is used, the nitric acid can be decomposed and removed only by calcination, compared to a hydrochloric acid solution that contains chlorine, which is difficult to remove. This is advantageous because it simplifies the catalyst manufacturing process and reduces manufacturing costs. More specifically, the present invention relates to an improvement in a method for manufacturing a palladium-supported catalyst using a so-called palladium nitrate solution impregnation method using palladium among platinum group metals using a nitric acid solution.

As mentioned above, in order to produce high-performance catalysts, it is important to support the catalytic metal at a high density on the surface of the carrier.Generally, when the carrier is impregnated by the impregnation method, the catalytic metal is deposited on the surface of the carrier. Whether the impregnating liquid is supported at a high density or by diffusion and penetration into the interior depends on factors such as the pH of the impregnating solution, the hydrolysis point, and the adsorption capacity of the support, among which pH is a major factor. Ru.

That is, when the pH is high, it is adsorbed and deposited on the surface of the carrier, but when the pH is low, it diffuses into the interior of the carrier. Therefore, in order to produce high-performance catalysts, a method of increasing the pH of the impregnating solution by adding alkali has been conventionally carried out, but the higher the pH, the more likely precipitation will occur, and the added alkali, especially sodium However, problems such as adverse effects on the activity of the catalyst have arisen. On the other hand, various methods have been tried to adjust the support layer by adding surfactants or organic acids instead of adding alkali.As a result of intensive study of these methods, the present inventors found that palladium nitrate In this case, surfactants have almost no effect, and among organic acids, only citric acid has been found to have an effect. This effect of citric acid is due to the fact that by adding citric acid to a palladium nitrate solution, a palladium-citrate complex is formed, and when the carrier is impregnated with the solution, the specificity of the formed complex allows it to enter the inside of the carrier. Diffusion and permeation are suppressed, and palladium is supported on the surface layer of the carrier. However, even when citric acid is used, if the palladium concentration is high and the free nitric acid concentration is high, that is, in a low pH range, this complex will not be formed and this effect will be lost.

In order to form a complex, the free nitric acid concentration of the citric acid-added palladium nitrate solution is lowered and the pH is increased to the limit where palladium does not precipitate, but the pH at which the complex is formed is still low.
It doesn't reach that point. This method of adding citric acid is effective only for catalysts with a low palladium concentration, and has the drawback of limiting its range of use. In order to obtain a large amount of catalyst, the only way is to repeatedly carry out the loading using an impregnating solution with a low concentration of panradium. The present inventors hereby propose the present invention as a result of intensive studies to eliminate the above-mentioned drawbacks.

The present invention is characterized by adding ammonium citrate to palladium nitrate instead of the conventional citric acid and immersing the carrier in this solution when supporting palladium on the carrier. According to the present invention, when ammonium citrate is added to palladium nitrate, it acts as an alkali, thereby increasing the pH of the solution and making it possible to form a complex even in the case of a palladium nitrate solution with a low pH.

Due to the action of this complex, even when the pH is low, palladium can be impregnated and supported on the surface layer of the carrier at a high density.
It is possible to produce catalysts with thin carrier layers. In the present invention, the amount of ammonium citrate added is such that a complex of palladium and citric acid is formed at a molar ratio of 1:1.
It is most effective when the amount is equivalent to or more than palladium, and when it is less than the equivalent amount, the supported density and the thickness of the supported layer can be adjusted depending on the amount added.

Further, the carrier used in the present invention is a refractory inorganic oxide such as alumina, silica, alumina-silica, alumina-magnesia, etc. Furthermore, the supporting method according to the present invention can be applied not only to impregnation methods but also to dipping methods, spray methods, etc., and has a wide range of applications.

The present invention will be specifically explained below with reference to Examples.

Example: To the palladium nitrate stock solution (palladium concentration 50 fI'),
Add an equimolar amount of ammonium citrate to palladium, dilute the solution with distilled water, and make a palladium concentration of 2.
．． Prepare an impregnating solution of 5y1e.

The activated alumina carrier is impregnated with this impregnating liquid in an amount corresponding to the water absorption rate, dried, and then fired at 600° C. for 1 hour. The thickness of the support layer obtained is measured using an optical microscope. Furthermore, palladium concentration 5
Prepare the impregnating solution of y1f by the above method and impregnate the support,
After firing, the thickness of the support layer is measured in the same manner. In addition, in order to investigate the performance of these catalysts, we investigated the exhaust gas from an engine for power generation using unleaded gasoline (composition: CO4.3%, HC36Op.
pm,. Using CO29.5%, 024.4%),
After a durability test of 800°C for 5 hours, the purification performance of carbon monoxide and hydrocarbons is measured at high and low temperatures.

The catalyst bed temperature was 500° C. in the high temperature test, and 500° C. in the low temperature test from room temperature after two millings. Further, the amount of catalyst was 10 m11, and the space velocity was 40000 Hr1. The activity test conditions in this example are shown in Table 1 below.
The above results are shown in Table 2.

Comparative Example After preparing a catalyst in the same manner as in the example using citric acid instead of the ammonium citrate added in the above example,
Examine the thickness of the support layer and catalyst performance.

Furthermore, the same procedure was carried out in the case of no additives.

*3 These results are shown in the table below. As is clear from the table above, the thickness of the support layer was the thinnest when ammonium citrate was added, and the activity was also the highest, indicating that these catalysts had excellent catalytic performance.

As described above, the manufacturing method of the present invention makes it possible to support palladium metal at high density on the surface layer of the carrier over a wide range of concentrations, and furthermore, by changing the amount added as necessary, the supporting density can be adjusted. Since the layer thickness can be adjusted, it has many advantages, such as allowing valuable palladium metal to be used efficiently and with high performance.

Claims (1)

[Claims] 1. A method for producing a palladium-supported catalyst, which comprises immersing a carrier in a solution of palladium nitrate and ammonium citrate.