The catalyst is intended to simultaneously reduce nitrogen oxides (NOX) as a 3-way catalytic converter in the gasoline engine and to oxidize hydrocarbons (HC) and carbon monoxide (CO). As oxidation
catalyst for Diesel engines, it reduces the hydrocarbon content of the sooty particles by means of the oxygen in the diesel exhaust gas.
The 'catalyst' would have to be called a catalytic converter. But like so many not quite true expressions, he has established himself in the motor vehicle technology. The expression is actually for the coating, which
allowes or accelerates the reactions without being consumed. The channels visible in the pictures above reach a surface area of approx. 2 - 3 m². For the (not yet thoroughly investigated) processes in the
catalyst, however, the 7,000-fold area is necessary. This is done by the washcoat applied to the ceramic. Platinum, palladium or rhodium may be used as further coating materials in thousandths
millimeter in thickness. For example, a few grams of the surface of approx. 3 - 4 football fields are sufficient, which can be recovered by refining at around 98 percent.
|Price for a few grams of platinum in the normal catalyst: |
ca. 100 Euro (2010)
The wash coat layer is applied to the finest sheet metal comb in metal catalysts. Before being carefully pressed into the sheet metal housing of an exhaust system, it still receives an enveloping layer which protects it
even at extreme temperatures of more than 1000°C. Two wire mesh rings (Fig. 1) sometimes provide reinforcement and protection at the ends. Only asymmetrical ends are welded to the catalyst housing. In the case
of concentrically running out, the metal body is heated by induction and then tapered by automatic machines. By a test with compressed air is finished.
|The consumption increase starts at approx. 0.1 bar couter pressure.|
The catalyst is installed as close to the engine as possible, since it only starts working at about 450°C. The close to engine assignment is e.g. for V- and flat engines only by two catalysts possible. Two or more
parallel switched catalytic converters are also used in sports engines with high exhaust gas throughput in order to reduce congestion losses.
|In the case of the (possibly also still fluctuating prices for platinum, the recycling becomes more and more important.|
The surface area of a catalyst may depend on certain additives in the gasoline, e.g. tetraetyl lead, and impair the function. 09/10