Coupling of soot oxidation and ammonia-mediated selective catalytic reduction of nitrogen oxides

Martinovity, Ferenc; Palkovits, Regina (Thesis advisor); Pirone, Raffaele (Thesis advisor)

Aachen : RWTH Aachen University (2021)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2021. - Dissertation, Politecnico di Torino, 2021

Abstract

Diesel engines present higher efficiency and fuel economy when compared to gasoline engines, however the emission of toxic pollutants, such as NOx, particulate matter and unburned hydrocarbons present a serious health hazard. The removal of the NOx is carried out through the selective catalytic reduction (SCR) with urea or hydrocarbons as reductant, while the particulate matter (soot) is removed by filtration followed by subsequent oxidation. One of the current state-of-the-art technologies for diesel pollution control in the automotive field is the integrated SCR and soot filtration on the so-called SCR on Filter (SCRoF) device. This thesis attempts to address some of the issues related to the coupling of the NOx SCR and soot oxidation reactions. Chapter 2 focuses on finding the proper soot oxidation catalyst that is highly active towards soot oxidation and does not interfere with the SCR reaction by oxidizing also the reductant NH3. It was found that K2CO3 loaded on CeO2-ZrO2 was highly active for soot oxidation and when mixed with Fe-ZSM-5 SCR catalyst it did not interfere with the NOx conversion. Chapter 3 investigates in much more detail the simultaneous interaction of the SCR and soot oxidation catalyst. It is shown that the NOx conversion over the SCR catalyst can be improved if the soot oxidation catalyst is also active for NO to NO2 oxidation. The CeO2-PrO2 catalyst impregnated with low amount potassium was used as soot oxidation catalyst that was physically mixed with Fe and Cu-ZSM5 and the w/f through the catalyst bed was maintained constant. In such mixtures the soot oxidation temperature was reduced by 150 °C and the SCR reaction improved In Chapter 4 the hydrocarbon poisoning of Cu-SSZ-13 was investigated and an effective solution proposed. A zeolite and mixed oxide “Composite” catalyst was made by ball-milling Cu-SSZ-13 and CeO2-SnO2 in 4:1 mass ratio. In the presence of 700 ppm C3H6 as model hydrocarbon the SCR reaction over Cu-SSZ-13 was markedly inhibited, while the Composite catalyst was resistant towards deactivation. Chapters 5 and 6 involve the development and investigation of LaCoO3 catalyst for NO oxidation and soot oxidation as potential replacement for Pt-based catalysts. The optimal perovskite, LaCo0.75Al0.25O3, obtained through a sol-gel method and calcination at 700 °C, could reach NO2/NOx ratio of 0.8 at 300 °C and was shown to be comparable to the Pt/Al2O3 catalyst. The SO2 poisoning took place in 2 stages, in the first stage the SO2 strongly adsorbed and blocked the active sites for NO oxidation and in the second stage lanthanum sulphates grew on the catalyst surface.