Selective catalytic reduction of nitrogen oxides with ammonia is a technology that has been developed for exhaust-gas after-treatment for light- and heavy-duty vehicles. An efficient Selective Catalytic Reduction (SCR) system is a prerequisite for existing as well as future specifications for lean-burn motors, and particularly diesel engines. Improving the efficiency of diesel engines results in decreased exhaust-gas temperature, and therefore more strict stipulations for the thermal design of the exhaust-gas branch. The application of an active SCR-active coatings, i.e., a washcoat, on monolith diesel particulate filters is a current measure for thermal as well as packaging optimization.
The incorporation of diesel particulate filters with an ion-exchanged zeolite designed with catalytically active metals including iron and/or copper leads to attractive specifications for operation of the selective diesel particulate filter component in exhaust-gas aftertreatment. Unfortunately, high nitrogen oxide conversion as well as high soot-filtration efficiency cannot be achieved at the same time always. It has been demonstrated experimentally that soot-loaded diesel particulate filters coated with a SCR-active layer (SPDFs), while operated in the cake filtration arrangement, convert less nitrogen oxides as compared to soot-free selective diesel particulate filter.
Despite the various efforts made by researchers in trying to understand this phenomenon, the microscopic processes that dictate the interaction of soot with the catalytic components in selective particulate diesel filters is still not well understood. Therefore, researchers led by Professor Roger Gläser at Leipzig University in Germany, clarified the origin of the microscopic phenomena inside the filter wall of SCR-catalyst coated diesel particulate filters associated with interaction of soot and ammonia-selective catalyst reduction catalyst. The research work is now published in Chemical Engineering Science.
In order to realize their objective, the authors loaded SCR-catalyst coated diesel particulate filters with model-soot identical to that resulting from real diesel engine operation. Through model gas studies, the authors investigated the effect of soot on ammonia adsorption/desorption, and on the catalytic nitrogen oxide conversion in soot loaded diesel particulate filters coated with an active SCR catalyst. The authors distributed the soot over the entire porous filter wall of the selective diesel particulate filter. Also, various inhomogeneous soot distributions of depth-filtered soot within the porous wall were studied.
The research team observed a dramatic change in the ammonia slip behavior in the presence of soot within the SCR-catalyst coated diesel particulate filter. The soot was also observed to lead to a decreased nitrogen oxide conversion by approximately 20%. The researchers then developed 1-Dimensional resolved microscopic filter wall model with integration of the unit collector model as well as a kinetic selective catalytic reduction model. In the model, they adopted soot as a diffusion barrier limiting the transport of the reactants to the washcoat surface.
The governing process was found to be diffusion of reactants through the soot deposited in the porous filter wall on top of the SCR-catalyst washcoat. This phenomenon greatly influenced the entire ammonia adsorption and reduced the nitrogen oxide conversion, but it had no direct influence on the adsorption or reaction kinetics of the SCR-catalyst in the washcoat. The significant nitrogen oxide conversion drop from about 3% to 20% for SCR-catalyst coated diesel particulate filter loaded with soot could be explained through the variations in the soot distribution within the filter wall.
“The highlighted study supports the suitability of the layered SDPF design for prevention of soot penetrating into the washcoated wall of a diesel particulate filter. Reducing soot deposition on the filter wall will also result in improved fuel usage. The study also shows that in an ideal SDPF design, the catalytic washcoat is located close to the outlet channel in order avoid that soot covers SCR-active sites.“, said Professor Roger Gläser, the lead author.
Marcus Purfürst, Sergej Naumov, Kay-Jochen Langeheinecke, and Roger Gläser. Influence of soot on ammonia adsorption and catalytic DeNOx-properties of diesel particulate filters coated with SCR-catalysts. Chemical Engineering Science, volume 168 (2017), pages 423–436. (2017) 903-912.
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