This work covers the development of a new high throughput technique for activity screening of catalysts for the abatement of nitrogen oxides from diesel exhaust gases and the application of the apparatus for the investigation of the chemical deactivation of Fe-zeolite catalysts for the selective catalytic reduction of nitrogen oxides with ammonia. Due to co-combustion of lube oil additives and fuel impurities, different kinds of inorganic particulates are put through the catalytic emission reduction system of a diesel car. The developed high throughput technology allowed for the fast screening of the deactivating potential of various inorganics to Fe-BEA and Fe-MFI NH3-SCR catalysts. Thereby, ceramic substrates coated with the catalytic active agent, so called “monoliths”, were used as catalyst libraries. These “monoliths” are also used as reactor system in the automobile, thus the developed high throughput technology is, from the view of reaction engineering, as close as possible to the industrial application. In high throughput investigations, a deactivating potential of alkaline metals, alkaline earth metals, phosphates and zinc towards Fe-BEA and Fe-MFI NH3-SCR catalysts was identified. The strongest catalyst poisons were magnesium and zinc. The uptake of 2 wt. % zinc or magnesium was followed by complete deactivation of the investigated Fe-BEA catalyst and by a reduction of the activity of the Fe-MFI NH3-SCR catalyst by approximately 70 %. Fe-zeolite catalysts loaded with 2 wt. % of alkaline metals (K, Na) or phosphates lost 40-60 % of their DeNOx-activity. Such important results from miniaturized high throughput investigations were verified in larger scale and in reactor geometrics close to industrial application (“scale up”). The results were further supplemented by characterization of selected samples by means of temperature programmed desorption of NH3 and NO2, temperature programmed reduction, X-Ray diffraction, nuclear magnetic resonance and physisorption of N2. Due to the uptake of alkaline metals, the catalysts sorption constant of the reducing agent ammonia is being reduced. Activity losses will be noted if the surface concentration of ammonia becomes low in comparison to the surface concentration of nitrogen oxides. Thus the deactivating potential of alkaline metals is dependant of the partial pressure of ammonia in the feed gas and will be worse in case of low concentration (as is the case if ammonia dosing is controlled by the concentration in the outlet stream in order to minimise ammonia slip). Phosphates react with the zeolite lattice in terms of an acid base reaction, but are also irreversibly chemisorbed at the iron center. Furthermore iron centers are partially blocked (“fouling”). Thus the catalysts sorption constant towards NO is being reduced and the rate determining step of the standard SCR reaction, the oxidation of NO to NO2 at the iron center, is slowed down. | English |
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