We can separate catalytic converters in three major categories: diode catalysts, unregulated three-way catalysts and adjustable three-way catalysts. The latter type is widely used in Europe by all car manufacturers, although it is also possible to find diode catalysts in vehicles designed before 1988, or unregulated three-way cars from the US. We need to clarify the meaning of the word “Street” in a catalytic converter. Receiver means that the flue gases coming from two or three different directions, or there are two or three ceramic monoliths, but indicates the number of pollutants, which can convert the catalyst. Thus, a diode catalyst converts two emitters and a three-way catalyst converts three pollutants.Externally show no differences (apart from uncontrolled three-way catalyst having a short air intake tube diameter). Their differentiation is the type of intermediate alumina coating used, and the type of noble metal used as catalyst.

diode catalysts

They are also known as oxidizing catalysts as this kind of reaction out. They are an alternative to thermal converters which eliminate CO and HC.Achieve a large reduction of these pollutants, but the high temperatures at which these reactions are carried out, increase NOx emissions. Normally, these catalysts are used in machines operating with a lean mixture, since the HC and CO emissions are low, while the NOx treated with another process, for example. Recirculating exhaust. If used in machines operating with a rich mixture, to achieve a low NOx production initially, then be introduced additional air by means of a pump, so that there is adequate oxygen to the catalyst to effect the oxidation.

Unregulated three-way catalyst

Often called “three-way catalyst light adjustment system”. Usually not seen in Europe, it has been used exclusively in American-made vehicles. It consists of two ceramic monoliths mounted separately within a metal shell. Between the two monoliths is a steel pipe, which is connected to a pump tube that introduces air from the engine. The first catalyst causes reduction reactions, thereby converting NOx, while the second oxidize the CO and HC.

Three-way catalyst light adjustment system

To effectively carry out the reduction reactions in the first monolith should be a lack of oxygen in the exhaust gas, so the machine should be operated with a rich mixture, which is uneconomical. On the other hand, the second catalyst requires oxygen to function, so it must be supplied with additional air through an air pump.

Adjustable three-way catalysts

also called “three-way closed regulation system catalysts.” This name (as the corresponding “open three-way adjustment system” for unregulated three-way), refers to the existence or non-recipient lambda, respectively creates a closed or open system configuration. Unlike uncontrolled three-way, first perform the reducing and then oxidizing reactions adjustable three-way operating all three reactions simultaneously. The oxidation of HC and CO occurs simultaneously with the reduction of Nox.

closed adjustment system three-way catalyst

To become sufficiently reactions to the air / fuel mixture is very close to the perfect mix, therefore the use of an electronic fuel injection system and electronically controlled power system is necessary so that it can be used in a closed system of regulation. The efficiency of the three way catalyst is determined by the ratio lambda of the machine. Perfect catalytic reaction is possible only in the “lambda field”. When the ratio lambda is kept within these limits, the three chemical reactions (oxidation of CO, the oxidation of HC and reduction of NOx) are performed simultaneously and very efficiently. If the mixture is made leaner and the oxygen ratio is increased to an extent to overcome the limits of the “field lambda” the optimum operating range of the three-way catalyst, the amount of oxygen in the exhaust gas will prevent the realization of the reductive reaction and NOx emissions will increase rapidly. Similarly if the mixture enriched why oxygen is reduced, the oxygen shortage will worsen the oxidative reactions, increasing CO and HC emissions.

Chemical Reactions

Within the group of hydrocarbons several different reactions are carried out at various speeds. Those which react more slowly are saturated hydrocarbons (especially methane). Unsaturated hydrocarbons that are situated in oxygen-rich atmosphere) and polycyclic aromatic react at moderate speed, but as carbon monoxide (CO) and hydrogen (H2) react rapidly.
CnIm + (n + m / 4) O2 O nCO2 + m / 2H2 (1)
CO + 1 / 2O2 O CO2 (2)
H2 + 1 / 2O2 O H2 O (3)

These reactions are carried out in oxidizing catalysts and through these achieves drastic reduction of HC and CO. In three-way catalysts are also performed other reactions, either simultaneously (a closed regulation system catalysts) or sequentially (opened adjustment system catalysts).These are the reduction reactions that ultimately will remove the nitrogen oxides (NOx).
CO + NO O 1 / 2N2 + CO2 (4)
CnIm + 2 (n + m / 4) NO O (n + m / 4) N 2 + m / 2H2O + nCO2 (5)
H2 + NO O 1 / 2N2 + H2O (6)
the reactions (2) and (4) are the main reactions taking place in a three-way catalyst to remove the CO and HC. In an open adjustment system catalyst (TWC unconfigured) occur consecutively, while in a closed regulation system catalyst (three-way adjustable) occur simultaneously. In the second case, to carry out the reactions, the conditions must be adjusted perfectly. There are also some clear conditions that must be met so that the two reactions take place simultaneously. Reaction (2) will default if the mixture will be low and exhaust gas is rich in oxygen. If this occurs, the reaction (4) is slower, it will move to the left and the conversion of NO will be reduced. Conversely, if there is an obvious lack of oxygen, the concentration of this gas and NO will not be enough to obtain the necessary for the conversion of CO and HC levels. Given that the reaction (4) is slower than reaction (2), and to achieve more efficient conversion of CO and NO, you must move the reaction (4) to the right, so as to bring the same speed of the reaction (2). The ratio lambda of the locomotive must therefore be slightly smaller than 1. It follows that the three-way catalysts are useful only in the field of so-called lambda, close to where the mixture is perfect. If the catalyst is operating in this field, gave a very good reduction of toxic emissions. Research is continuing efforts to expand this lambda field as much as possible. Such an extension would be possible if we increase the rhodium ability to bind oxygen, ensuring several aluminum oxide (alumina intermediate layer) on the protective mat type expanding mat. When there is adequate oxygen in the exhaust gas, they can oxidize hydrocarbons and carbon monoxide for a short period of time.When there is a lack of oxygen will be retained by the exhaust gases, thereby slowing the process of oxidation. Another way to speed up the conversion of hydrocarbons and CO during the oxygen deficiency is the use of catalysts which accelerate the reactions (7) and (8). Thanks to the presence of water vapor, which are always in sufficient amounts, some contaminants may be oxidized and the absence of oxygen.
CO + H2 O CO2 + H2 (7)
CIm + 2H2 O CO2 + (2 + m / 2) H2 ( 8)
in fact the reactions carried out in a catalytic converter is very complicated, and beyond those we tested – all of which are desirable – parallel reactions can be carried out, which form other undesirable substances.
SO2 + 1 / 2O2 O of SO3 (9)
5 / 2H2 + NO O NH3 + h2o (10)
SO2 + 3H2 O H2S + 2H2O (11)
NH3 + CH4 O HCN + 3H2 (12)
This problem of “secondary pollution” has become the subject of extensive examination in USA, where catalytic converters are the norm, and the conclusion was that it is of little importance. By using regulated three-way catalysts which operate very close to the perfect mixture, it is possible to achieve favorable results where parallel reactions practically disappeared. Only when the car is new or the camera is not set correctly it is possible to detect secondary pollution. Particularly noteworthy is hydrogen sulfide (H2S) because of the permeant rotten egg odor.