AdsorbersThere are specific situations where engine operating conditions may not be ideal for conventional catalysts to achieve their full potential. Three-way catalysts, for instance, are highly effective in gasoline engine exhaust, but there is too much oxygen present in diesel exhaust for their NOx-reduction function to operate properly. Also catalysts generally need to reach a certain minimum temperature for effective operation, and although modern systems can reach this 'light-off' temperature within a few seconds there may still be some emissions until that temperature is reached. Adsorbers offer ways to collect certain pollutants, specifically NOx or HC, during conditions which are not ideal, to store the pollutant and then to treat it when conditions are suitable. The two main current applications are NOx adsorbers, which can be used to treat NOx emissions from lean-burn gasoline engines or from diesel engines and hydrocarbon adsorbers which can be used with conventional three-way catalysts to 'trim' the HC emissions from fuel enrichment needed for cold start.
NOx adsorbers (NOx traps)
NOx adsorbers (NOx traps) adsorb and store NOx under lean conditions. A typical approach is to speed up the conversion of nitric oxide (NO) to nitrogen dioxide (NO2) using an oxidation catalyst so that NO2 can be rapidly stored as nitrate on alkaline earth oxides. A brief return to stoichiometric* or rich operation for one or two seconds is enough to desorb (remove) the stored NOx and provide the conditions for a conventional three-way catalyst mounted downstream to reduce (destroy) NOx.
*Stoichiometric conditions are those where the ratio of fuel and air is chemically correct for complete combustion. Under 'lean' conditions there is an excess of oxygen in the air:fuel ratio, under 'rich' conditions the opposite is true and there is more fuel than would be required for the chemically correct air:fuel ratio.
The NOx adsorber system
Unfortunately, NOx adsorbers also adsorb sulfur oxides resulting from the fuel sulfur content. For that reason fuels with a very low sulfur content (European 'zero' sulfur fuel contains less than 10ppm sulfur) are required. The sulfur compounds are more difficult to desorb, so periodically the system has to automatically run a short 'desulfation' cycle to remove them.