JPS5950376B2 - Exhaust gas purification catalyst - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a catalyst for purifying exhaust gas for combustion.
In order to reduce the amount of flammable harmful gases (such as unburned hydrocarbon gas and carbon monoxide gas) emitted, especially in an oxygen-deficient atmosphere, a normal (full oxidation) combustion catalyst is used, which has low activity as a full oxidation catalyst. However, by adding substances that inhibit specific reactions, we are trying to create a new combustion catalyst that reduces multiple combustible harmful gas components in combustion exhaust gas in a well-balanced manner, resulting in an overall reduction in harmful components. It is something to do.

Conventionally, when removing flammable harmful gases (CO, HC, etc.), CO2 and H2O are removed by oxidizing and burning them.
The most reliable and easiest method is to convert the gas into harmless gases, such as catalytic combustion, which is the most reliable and simplest method compared to other methods in terms of conversion efficiency and equipment structure. Many combustion catalysts have been developed as excellent methods.

However, the numerous combustion catalysts that have been invented so far1 are, of course, intended for use in an atmosphere of oxygen sufficient for oxidation reactions, and for this reason, there is sufficient oxygen in the combustion exhaust gas for catalytic combustion. It was necessary to make the catalytic atmosphere rich in oxygen by introducing oxygen into the exhaust gas, or by introducing secondary air as an auxiliary combustion component into the exhaust gas for catalytic combustion.

This point is particularly important in the combustion of hydrocarbons.If catalytic combustion occurs in an oxygen-deficient atmosphere, the heat of reaction of the hydrocarbons will cause the catalyst bed to reach a high temperature, and this heat will advance the decomposition reaction of higher hydrocarbons, oxidizing them. and harmless CO2 and H2
Because there is not enough oxygen to convert to O, incomplete combustion occurs, resulting in the production of large amounts of harmful CO and carbon, which defeats the purpose of installing a catalyst in the exhaust system. .

In particular, if the exhaust gas contains unburned ω, the CO concentration after passing through the contactor may even be higher than the CO concentration in the exhaust gas, making it meaningless as a so-called pollution prevention device. Therefore, it can be said to be a harmful device.

The figure shows the CO and HC purification rate in the CO and HC mixed gas oxidation reaction of a platinum catalyst, which is a typical complete oxidation catalyst, in an oxygen-deficient atmosphere. A graph plotted against the ratio of the amount of oxygen supplied when the amount is 1.0 is shown as a conventional example.

Here, as the oxygen sufficiency rate approaches 1.0 at a constant gas temperature, the HC purification rate increases monotonically, but on the other hand, CO production increases depending on the incomplete combustion rate of HC, so CO It can be seen that the purification rate of is significantly reduced, and in extreme cases, even Ma-minus purification (generation) occurs.

The present invention provides a novel combustion catalyst that suppresses CO production due to incomplete combustion even when the reaction gas is in an oxygen-deficient atmosphere, and constantly converts combustible harmful components in the reaction gas into harmless components.

The present invention will be explained in detail below with reference to examples.

Example 1 Alumina grains were sufficiently immersed in a platinum and silver solution obtained by dissolving chloroplatinic acid and silver nitrate, which were prepared so that the metal weight ratio was 2 parts metal to 1 part silver, in ion-exchanged water, and then taken out. After drying at 120°C for several hours, it was dried at 35°C in a hydrogen stream.
A platinum and silver supported alumina particle catalyst was prepared by hydrogen reduction at 0°C for 3 hours.

Responsible. The amount of metal held was 1 wt% based on the weight of the carrier.

Example 2 Platinum and rhodium obtained by dissolving chloroplatinic acid, rhodium chloride, 1 silver perchlorate, and silver perchlorate in ion-exchanged water, prepared at a metal weight ratio of 8 parts rhodium to 1 part silver. A silica cloth whose surface was coated with alumina was fully immersed in a silver solution, dried at 120°C for 2 hours, and then hydrogen-reduced at 350°C for 2 hours in a hydrogen stream to obtain platinum,
Rhodium, silver-backed.

When a silica cloth catalyst was prepared.

The final amount of metal supported is 1w in weight ratio to the carrier weight.
It was t%.

Example 3 A honeycomb shaped carrier was sufficiently immersed in a palladium and silver solution obtained by dissolving palladium chloride and silver nitrate in ion-exchanged water, which were prepared so that the metal weight ratio was 3 parts palladium to 2 parts silver. After drying at 120℃ for 5 hours,
A honeycomb-shaped catalyst supporting palladium and silver was prepared by hydrogen reduction at 400° C. for 3 hours in a hydrogen stream.

The final supported metal amount is 0.95w relative to the carrier weight
It was t%.

Example 4 A mixture of platinum, palladium, and silver prepared by dissolving chloroplatinic acid, palladium chloride, and silver perchlorate in ion-exchanged water in a metal weight ratio of 1 part metal to 2 parts palladium to 1 part silver. A silica cloth whose surface was coated with alumina was thoroughly immersed in the solution, dried at 120°C for 2 hours, and then hydrogen-reduced at 350°C for 2 hours in a hydrogen stream to form platinum-, palladium-, and silver-supported silica cloth. A catalyst was prepared.

The amount of metal finally supported was 1 wt% based on the weight of the carrier.

In order to compare the catalyst characteristics of the above-mentioned Examples of the present invention, a platinum-supported alumina granular catalyst known as a complete oxidation catalyst was prepared by the following method.

Conventional example Alumina grains were sufficiently immersed in a platinum solution prepared by dissolving chloroplatinic acid in ion-exchanged water, dried at 120°C for 2 hours, and hydrogen-reduced at 400°C in a hydrogen stream for 3 hours to support platinum. An alumina granule catalyst was prepared.

The amount of metal finally supported was 1 wt% based on the weight of the carrier.

In order to see the effects of the present invention, the above-mentioned 4 Example catalysts and 1
CO when reacting the conventional catalyst under the following conditions
In addition, the HC conversion rate (purification rate) was investigated.

a) Reaction gaff: CO: 2vo1%, HC (as gasoline wet gas): 3vo1% (CH4 conversion)
, 02:2-9V01% (oxygen sufficiency rate fluctuation), remaining N2
Balanced with gas, b) Measured space velocity: 8 x 104 h-1 C) Periodic reaction gas inflow temperature on purification rate side: 250°C d) Reactor: Flow-through reactor Under the following measurement conditions, the catalysts of this example and the conventional example were used. The results of the reaction were as shown in the drawing and the table below.

In the drawings, the results of Example 1 and the conventional example are shown side by side in order to show the typical effects of the present invention in comparison with the conventional example.

Here, the HC purification rate is not much different between Example 1 and the conventional example, and as the oxygen sufficiency rate decreases, the HC purification rate also decreases monotonically, and although the present invention has no particular effect, the CO purification rate Their behavior with respect to the oxygen sufficiency rate shows an extreme difference.

In other words, in the conventional example, CO production due to incomplete combustion progresses despite the progress of HC purification, and the balance between the oxidation of CO flowing into the catalyst layer and the CO generated due to incomplete HC combustion causes the entire amount to flow out of the catalyst layer. The CO concentration as shown in FIG. The result is that the concentration exceeds the inlet concentration.

This concave curve has a correlation with the decomposition temperature of HC, and H
When C purification (oxidation or decomposition) is 70% or higher, the catalyst bed temperature due to the reaction heat reaches 700°C or higher, and the HC decomposition reaction is significantly accelerated, resulting in increased ω production, but when the oxygen sufficiency rate is 0. Below 5, there is little HC combustion (the purification rate is also small) and the calorific value is small, so HC decomposition is slow and HC decomposition is slow.
This shows that the purification of inflowing CO affects the total amount of CO since O production is also small.

In contrast, in Example 1 according to the present invention, the CO purification rate monotonically decreases with respect to the oxygen sufficiency rate, and does not show a concave curve as in the prior art example.

This is true even at low oxygen sufficiency rates.
This means that there is little or no production of oxygen, and the supplied oxygen is consumed only for the complete oxidation reaction, and it can be said that it is functioning to its fullest as a so-called combustion catalyst.

In addition, the above table summarizes the CO and HC purification rates of Examples 1 to 4 and the conventional example, especially when the oxygen sufficiency rate is 1.0.0.9.0.8.

The reason why we only compared oxygen sufficiency rates of 0.8 or higher is that if the oxygen sufficiency rate is 0.5 or less, it has little meaning as a combustion catalyst or catalytic purification device that utilizes so-called complete oxidation; This is because it is common to take measures to increase the amount of oxygen in the reaction gas by introducing secondary air or the like.

In the above table, all of the examples have both Co and HC of 8.
It shows a purification rate of 0% or more, and it can be seen that it has a high purification property that is incomparable to the conventional example, which is several percent.

As stated above, the main focus of the present invention is that the residual oxygen in the combustion exhaust gas is slightly insufficient to completely oxidize (combust) the combustible harmful gases in the exhaust gas, and with ordinary catalysts, the oxygen is The object of the present invention is to provide a combustion catalyst for use in an oxygen-deficient atmosphere, which can be processed only by a contact device without using such an additional device, where a combustion improver must be supplied by a method such as air.

For this reason, the oxygen concentration in the exhaust gas needs to be at least 0.6 or higher in terms of oxygen sufficiency, but under these conditions the exhaust gas from a combustion engine is surprisingly large, and the amount of high-hydrocarbon gas other than combustion engines is It is also suitable for purifying blower exhaust gas, etc.

In particular, two-stroke gasoline engines are one of the engines that have the most suitable exhaust gas composition for such situations.

This type of engine is used in many fields because of its light weight, low price, and high engine acceleration, but its exhaust gas components are characterized by high concentrations of CO and HC, and 02 concentration. Since the oxygen content is quite high, it is not necessarily necessary to introduce secondary air, which is normally thought of in four-cycle engines, and it is possible to achieve a considerable amount of oxygen sufficiency by changing the operating conditions of the engine.

However, it is impossible to ensure the required oxygen sufficiency under all operating conditions, and moreover, in practice, the heat of reaction due to HC combustion is very high, resulting in HC decomposition and subsequent CO production. In many cases, HC purification is possible, but CO purification is difficult.

When the catalyst of the present invention was used to purify the exhaust gas of a two-stroke engine, it was possible to purify both CO and HC by 60% or more.

The addition of Ag in this way does not impair the effect of the present invention, which suppresses the production of CO, which is a by-product of combustion reactions in an oxygen-deficient atmosphere, and reduces the combustible harmful components as a whole. Further, the amount of Ag added was preferably in the range of 1:1.5 to 1:12 in terms of weight ratio, as shown in this example, in order to exhibit the effects of the present invention.

Furthermore, the effect of the present invention will not be impaired even if the carrier or support to be supported is made of any material or shape other than those described in this example.

As described above, by using the catalyst of the present invention, catalytic combustion of combustible harmful components in an oxygen-deficient atmosphere, which was previously thought to be difficult, can be carried out without introducing secondary air or special operating restrictions on the engine. It has become possible to do this, and its utility value is great.

[Brief explanation of drawings]

The figure is a characteristic diagram of C0-HC purification rate with respect to oxygen sufficiency rate.

Claims (1)

[Claims] 1. A combination of silver and at least one of platinum, rhodium, and palladium for reducing combustible harmful gas components such as carbon monoxide and hydrocarbons in exhaust gas in an atmosphere below the stoichiometric oxygen amount. An exhaust gas purification catalyst characterized by: 2. The catalyst for exhaust gas purification according to claim 1, wherein the combination of silver and at least one of platinum, rhodium, and palladium is in a weight ratio of 1:1.5 to 1:12. .