Studies of the Selective Catalytic Reduction of Nitrogen Oxides with Dimethyl Ether

Doctoral thesis, 2010

Dimethyl ether (DME) is one of the most energy-efficient and low CO2 emitting alternative fuels when produced from biomass. Similar to other vehicles with combustion engines, vehicles running on DME will most likely need after-treatment technologies for the reduction of NOx emissions to meet the most stringent upcoming legislations. One attractive technique would be selective catalytic reduction with DME as reducing agent (DME-SCR), which is in the focus of this thesis. The activity for NOx reduction with DME of several acidic catalysts was studied in a flow reactor and the accumulation and consumption of surface species was monitored in diffuse reflectance infrared Fourier transform (DRIFT) and transmission FTIR spectroscopy experiments over γ-Al2O3. It was shown that dimethyl ether is a special reducing agent since it induces radical reactions in the presence of O2 and NO above 300 °C before the catalyst, with NO2 and CO as the main products. Despite the changed feed gas composition, activity tests with DME as reducing agent in the flow reactor over a zeolite H-ZSM 5 and a γ-Al2O3 catalyst resulted in 28 and 47 % NOx reduction, respectively. During DRIFT and transmission FTIR spectroscopy experiments, methoxy, formate, nitrate, nitrite, NCO and likely formohydroxamic acid and formaldehyde-like species were observed on the γ-Al2O3 surface. A reaction mechanism which explains the involvement of these species in the selective catalytic reduction of NOx was proposed. For DME-SCR over γ-Al2O3 it was shown in experiments where the occurrence of the gas phase reactions could be controlled independently of the catalyst temperature, that the formation of NO2 in the gas phase reactions boosts the activity for NOx reduction at 250 °C, probably due to a more efficient reaction with NCO surface species. In contrast, at 350 °C, lower activity for NOx reduction was observed in the presence than in the absence of gas phase reactions. This negative effect was explained by partial oxidation of DME in the gas phase reactions partly consuming the limiting reducing agent at 350 °C.