Studies of hydrocarbon-assisted selective catalytic reduction of nitrogen oxides - DME as reductant and reaction mechanism over silver/alumina

Licentiate thesis, 2007

Increasing amounts of carbon dioxide are attaining worldwide growing attention as one probable cause for global warming. Together with the expected peak in oil production and an increasing oil demand, this results in a greater interest in more fuel effective lean burn engines (i.e. operation with excess oxygen). Nitrogen oxides are formed during combustion, and the excess oxygen obstructs NOx reduction in a conventional three-way catalyst. One concept for NOx reduction under lean conditions is hydrocarbon-assisted selective catalytic reduction (HC-SCR). In this thesis, two aspects of HC-SCR are studied, namely DME as reducing agent and the reaction mechanism of propene-SCR over a Ag/Al2O3 catalyst. Dimethyl ether is an energy efficient and low CO2 emitting alternative fuel, which can be used in diesel engines. For NOx reduction, however, many HC-SCR catalyst materials show only moderate activity with DME under dry conditions. This activity decreases further by the introduction of water. Comparably high conversions of 28 % NO and 37 % NO2 conversion were obtained in the presence of water over an H-ZSM-5 catalyst in this work. A comparison between H-ZSM 5 and the less acidic Ag/H-ZSM-5 and Ag/Na-ZSM 5 catalysts indicates that the acid sites in the catalyst are crucial since the activation of hydrocarbons and NOx is believed to occur on such sites. Although widely researched for HC-SCR, the reaction mechanism on Ag/Al2O3 catalysts is still not fully understood. With the aim of understanding further details of the role of nitrogen containing compounds, step-response experiments were carried out in a flow reactor and in a Diffuse Reflectance Infrared Fourier Transformed (DRIFT) spectroscopy cell. Hydrogen cyanide (HCN) and isocyanic acid (HNCO) were detected at the same time in the gas phase indicating a similar role in the reaction mechanism. Accumulation and consumption of the corresponding surface species R-CN and R-NCO, however, did not show any correlation to each other. Therefore two parallel reaction pathways for CN and NCO species are proposed, in contrast to literature.