What are the types of catalyst

The exhaust gas catalytic converter - petrol and diesel cat

see also CO | HC | NOx | Lambda control | Exhaust gas

What actually is a catalyst?

A catalyst contains chemical elements that cause and / or accelerate chemical reactions without taking part in them.

The exhaust gas catalytic converter in gasoline or diesel engines also ensures such a chemical reaction. More on this in the article.

Catalytic converter with lambda probe

Combustion products in the engine

When fuel is burned with the help of atmospheric oxygen, water and CO are produced in the engine2. Since the combustion is not completely complete, there are other combustion products that we call exhaust gas. The exhaust gas component is about 1%. Gasoline engines mainly produce: CO, HC and NO, NO2 (summarized to NOx). In the case of diesel engines, there are also soot particles and sulfur dioxide. Please also read our pages about exhaust gases (HC, CO, NOx, SO2, PM) and via the lambda control. The catalytic converter now helps to convert the pollutants into water and CO2 to transform. The catalyst is most effective when an exhaust gas composition is present.

Catalyst types

Single bed oxidation catalyst

It is also known as an unregulated catalyst (U-Kat) or oxidation catalyst. In the U-Kat, the pollutants HC and CO are converted into H with the residual oxygen or with additional air2O and CO2 transformed. There is no reduction in nitrogen oxides. It is actually only found in many older gasoline vehicles.

As a diesel catalytic converter, it is installed very close to the engine, as the exhaust gas temperatures are often very low here. Metal carrier cats are used, which have thin foils at the front for better jumping and thicker foils at the back for better storage capacity and thus ensure better conversion.

Double bed catalyst

2 catalytic converters are connected in series, the first is a reduction catalytic converter to split the nitrogen oxides into oxygen and nitrogen. A low-oxygen exhaust gas is required for this. The second is an oxidation catalyst for HC and CO2 as described above. Secondary air injection is often used here in order to obtain an oxygen-rich exhaust gas for post-combustion. The secondary air injection is particularly important in the warm-up phase of the engine to reduce the HC values. This catalytic converter is also no longer used today, as it only insufficiently reduces nitrogen oxides.

Three way catalytic converter

The regulated three-way catalytic converter, also known as the G-Kat, is now standard equipment in a gasoline car. The conversion rate of the exhaust gases (conversion rate) is over 90%. This type of catalyst is described below. Incidentally, the "regulated" refers to the engine management with its combustion and not to the catalytic converter itself.

Catalyst systems

Lean or Denox catalytic converter (NOx adsorber)

Denox-Kats can be found in diesel vehicles and in gasoline direct injection.

Diesel work with excess air, as does the gasoline DI in lean-burn mode. A conventional catalytic converter has difficulties here in reducing nitrogen oxides. A DeNOx-Kat is always a complex system. The requirement for the adsorber is low-sulfur diesel and low-ash engine oil.

Metal supports are used. They are either provided with small cross-sections or with structured channels. Desulfation by heating the catalyst from time to time is necessary. Further details can also be found on the Diesel exhaust page. Emitec has a pre-turbocharger metal carrier catalytic converter that effectively converts HC and CO.

V-catalyst

The V stands for pre-turbocharger but also for pre-oxidation catalyst. This increases the CO2Share of the NOx (Diesel) and improves the oxidation of the carbon particles and the reduction of NOx. This is followed by a urea catalytic converter, the SCR catalytic converter and an Oxi catalytic converter. See the page about diesel exhaust.

Potassium catalyst

Use with gasoline DI engines to cool the Denox catalytic converter and to expand the lean range. The metal carrier coating consists of potassium. A cooler is therefore not required.

Particle filter

see report FAP and diesel exhaust

Video:

Car catalytic converter - what does it do?

(from TheSimpleChemics)

Structure of the cat

The catalytic converter consists of four components:

So-called monoliths (AlMg-Silicate = ceramic) or metal supports are used as supports.

 

Ceramic catalyst

Ceramic carrier

As large a surface as possible is important in order to be able to process large amounts of exhaust gas.

The carrier is criss-crossed with several thousand channels through which the exhaust gas flows.

In order to keep the flow resistance as small as possible, the monolith is very thin-walled (approx. 0.3 mm).

 

Advantages of the ceramic carrier:

  • better recovery of precious metals,

  • cheaper,

  • more constant operating temperature

 

Metal catalyst / font>

Metal support

Since the risk of burning through or mechanical damage is lower with metal supports, the wall thickness can be designed to be much thinner (approx. 0.05 mm).

 

Advantages of the metal beam:

  • less sensitive to shock,
  • more heat resistant,
  • faster heating-up time,
  • lower exhaust back pressure.

 

The metal support Metalit consists of very thin steel foils (0.03 to 0.08 mm). It is suitable for different types of Kat.

 

  • the intermediate layer (wash coat or carrier layer)

To further enlarge the surface, the ceramic carrier is covered with a layer of aluminum oxide (Al2O3) Mistake. This increases the surface 7000 times. The oxygen storage capacity is also increased.

  • the catalytically active layer

The catalytically active layer consisting of the noble metals platinum and rhodium and also palladium is vapor-deposited onto the intermediate layer. The platinum favors the oxidation processes, the rhodium the reduction processes. However, the metals do not react themselves, they only provoke the reaction. The ratio of platinum to rhodium is about 5: 1. the total amount of the two precious metals per catalyst is between 4 and 9 grams.

Since the ceramic carrier is very brittle and also has a different thermal expansion than the housing, it is embedded in a damping layer, a wire mesh or a ceramic fiber mat. Metal supports do not need the damping layer.

   

schematic structure of the cat

 

Function of the cat

As already mentioned, the catalyst helps remove the pollutants in water and CO2 to transform.

  • Oxidation turns CO into CO2
  • Oxidation turns HC into H2O and CO2
  • Reduction turns NOx into N2.

That is why we also speak of a 3-way catalytic converter!

The reactions are actually a bit more extensive, since the unburned hydrocarbons are not only C2H6 but are also contained in many other compounds in fuel. However, the presentation so far is completely sufficient to derive the essential findings for the explanation of the 3-way cat:

  • Both oxidation and reduction processes take place in the catalytic converter.
  • Oxygen is required for oxidation and carbon monoxide for reduction.
  • The pollutants CO, HC, NOx and the reaction partners O2 and CO must be present in a certain ratio so that the highest possible conversion rate is achieved.
This produces the following results, which are also reflected in the diagram:
  • With a lean mixture composition, the catalytic converter has a high conversion rate for CO and HC because there is a lot of residual oxygen in the exhaust gas.
  • If the CO and HC amounts in the exhaust gas are too low, however, the conversion rates of NOx decrease
  • Conversely, the conversion rate of NOx is high in the case of a rich mixture, since there is enough CO for reduction in the exhaust gas.
  • In return, however, the conversion of CO and HC decreases because of the low oxygen content in the exhaust gas.

 

Conversion rate (fixed mixing ratio)

In order for the catalytic converter to even be able to achieve the high conversion rates, the engine management must ensure that the lambda value and thus the mixture are maintained within a narrow range around lambda = 1 (lambda window). The control unit is assisted by the lambda probe as a sensor, which measures the residual oxygen content of the exhaust gas after the catalytic converter. Read my lambda probe pages on this.

In order to trigger the chemical reactions, the catalytic converter also needs a minimum temperature, the so-called light-off temperature. In order to achieve a 50% conversion rate, the temperature of the cat must have already reached 250 to 280 ° C. The catalyst reaches this temperature approx. 30 - 90 s after a cold start. That is why the catalytic converter is installed as close as possible to the intake pipe. Aged cats only start at higher temperatures. The best working range is roughly between 400 and 800 ° C.

The following catalyst failures are typical

Surface reduction through sintering processes at temperatures above 800 ° C until melting

Chemical reactions with foreign substances (fuel, oil additives), the catalytic layer is destroyed

The active layer is covered by e.g. lead and sulfur from fuel and oil.

 

Additions:

  • Because diesel engines are operated with excess air, they are not suitable for the use of regulated three-way catalytic converters.

  • They are equipped with an oxidation catalyst.

  • In the oxidation catalytic converter, about 80% of the hydrocarbons (HC) that are not or only partially burned are converted into water vapor (H.2O) and carbon dioxide (CO2) transformed.

  • The poisonous carbon monoxide (CO) also turns into carbon dioxide (CO2).

  • Nitrogen oxides (NOX) cannot be reduced in the catalytic converter because of the excess air.

  • For this purpose, exhaust gas recirculation is required, which reduces the formation of stitches oxides by cooling the combustion chamber.

  • see also the pages on the diesel engine

Can a car with a cat be towed in?

In any case, cars with catalytic converters should not be towed in for too long. If the car does not start after a short towing phase, unburned fuel (HC) can get into the catalytic converter. Reinhold Bruners from TÜV Rheinland in Cologne warns against this. In such a case it is better to look for the cause why the car does not start immediately. Whether the cat was damaged during such an action can only be determined during the following emissions test. However, according to Bruners, even a short towing is not a problem for cars with a "Kat".

see also CO | HC | NOx | Lambda control | Diesel exhaust | Diesel exhaust technologies |

Also read how to recognize a defective catalytic converter and how kats are recycled

Sources for text and pictures: Examinations course emissions test of the TAK, Europe, Internet, dpa


Author: Johannes Wiesinger