In the field of chemistry, substances that are present at a chemical reaction and the speed, but not taking part in this known catalysts. When the reaction process is over, the catalyst is not changed and can be used again. In the car, the catalyst is a device installed in the exhaust system near the machine in order to maintain a high temperature. Its purpose is to neutralize the pollutants that are produced due to incomplete combustion and to convert them into harmless substances. In fact, the “catalyst” designation is not very accurate, because the catalyst is a group of noble metals, contained within the converter and view them to accelerate the chemical reactions that remove contaminants from the flue gas.

Catalytic conversion of exhaust gases

In the catalytic conversion is used an old well-known chemical process to accelerate the reaction which automatically occurs within the exhaust system. There are some specific substances, which, without taking part in the reaction, causing significant acceleration (eg noble metals such as platinum and rhodium). In the case of cars, the metals that are responsible for the catalytic conversion of exhaust gases are in a metal box that resembles the exhaust muffler and is disposed near the engine to maintain a high temperature. This accessory is called catalyst or catalytic converter.

The parts of the catalytic converter

To quickly and efficiently performed chemical reactions that reduce contaminants, to the exhaust gas in contact with the precious metals which accelerate the reactions for a period sufficient to complete the reaction. To achieve this, the exhaust gases are channeled through extremely thin tubes, so that practically each gas molecule is contacted with the precious metal. This flow separation is carried out from the interior material of the catalyst, which may be metallic or ceramic.

ceramic carrier

Commonly referred to as “ceramic monolith”. Manufactured by korditi, located in a pliable condition at celled with 400 channels per square inch and a wall thickness of 0.15mm. Given the thinness of the walls, the front of which is open to the flow of exhaust gases exceeds 70{61f99a9885186ea462a8cb31c9699d0c282007d5159d02395bdf5b202314d234} of the total area, which is why the backpressure is too small. This inner material is used in most catalytic converters.

metal body

Manufactured from stainless steel plates, 0.01mm thickness, forming a cellular matrix (as in ceramic) with 400 channels / square inch. The two main advantages of this type carrier is less backpressure caused because the open front surface exceeds 80{61f99a9885186ea462a8cb31c9699d0c282007d5159d02395bdf5b202314d234}, and greater sturdiness to high temperatures. Main disadvantage is its high cost. This type of carrier is normally used for small inverters or the procatalysts, who, being very close to the export, stressed at extremely high temperatures.

The inner material of the catalytic converter (either ceramic or metal) is not in itself sufficient area to achieve efficient contact of the exhaust gas to the noble metals. Furthermore, it needs to somehow affixed called noble metals on the inner material. That is the dual role of the intermediate alumina layer (between the inner material and the noble metals), which increases up to 100 times the active surface on which are reactions. The types of intermediate coating vary with the noble metals and the amounts thereof to be secured thereto, and the method of manufacture of the intermediate layer itself. All these factors influence the chemical composition, the efficiency of the catalyst, its resistance to high temperatures and durability of the final product. In the case of the ceramic catalyst is an intermediate component between the monolith and the metal shell, which connects the ceramic body and the cap, absorbing the expansion differences, since when the temperature increases, the steel shell expands with the ceramic monolith is not varied in its dimensions. The consequence of this is rising temperatures result in an increase of the space between the ceramic body and the metal cover. There are two solutions to this problem: the use of a protective mat or wire mesh type or type-expandable mat.

Protective mat type wire mesh

The manufacture of a catalyst with this intermediate component is very simple and relatively low cost. The main drawback of this method is that the thermal stress caused in the wire mesh after innumerable variations in temperature reduces its effectiveness. The problem is exacerbated when the sections of the monolith is not circular, because then the expansion becomes asymmetric. A further disadvantage is that it may adversely affect the efficiency of the catalyst: a possible failure of the wire grid to stop the exhaust gases will create an exhaust gas escape passage between the ceramic monolith and the outer shell without previously been processed.

Protective mat type expanding mat

This solution requires very precise planning dimensions. The ability to expand with temperature is much greater than the capacity of the metal wire so it covers the gap caused by the expansion of the metal shell very easily. It also functions as an insulating element, keeping the monolith at a high temperature and thus improves the performance of the catalyst. The main drawback is the ease with which decomposes when the density decreases, which means that it can easily be eroded if it comes into direct contact with the exhaust gases.

Outer metal shell

Manufactured from stainless steel, which improves the mechanical characteristics at high temperatures and shows strong resistance to corrosion.Used to strengthen the structure and to protect it from the constant use and exposure to high temperatures. Apart from these main components, the catalytic converter can be used and other minor components, such as protective rings for the protective matting type expanding mat, double shell, insulating ceramic fibers, protective grill (for any bumps, etc.) Etc. All these are part of the different designs of catalytic converters and their existence is a choice of the manufacturer.