Lean NOx reduction by alternative fuels - with focus on catalysts for dimethyl ether

Abstract: There is an increased demand for diesel engines running on non-fossil fuels such as dimethyl ether (DME), biodiesel and synthetic diesel. DME is a promising alternative fuel with potential for high energy efficiency and low CO2 emissions. However, during all high temperature combustion NOx is formed, which may require exhaust aftertreatment. Two technologies for NOx reduction under lean conditions utilizing the fuel as reducing agent are the lean NOx catalyst (LNC) and the lean NOx adsorber (LNA). Lean NOx reduction with alternative fuels was investigated in a flow reactor and showed that DME was an efficient reducing agent for NOx over a BaO-based LNA, but not over a Cu-ZSM-5-based LNC. Also methanol did not reduce NOx over Cu-ZSM-5, whereas higher ethers and alcohols did. Despite being more oxidised than DME, ethylene glycol reduced NOx. It was concluded that a C–C bond is required in hydrocarbon-based reducing agents for lean NOx reduction over Cu-ZSM-5. A comparison of biodiesel, synthetic diesel, conventional diesel and octane as reducing agents, showed that Cu-ZSM-5 reduced NOx at a lower temperature than Ag/Al2O3, which was more sensitive to the reductant used. RME gave the lowest, and octane the highest, NOx conversion over both catalysts. The oxygenated hydrocarbon triethylene glycol dimethyl ether (triglyme) efficiently reduced NOx over Ag/Al2O3, but not over Cu-ZSM-5. Adding a low amount of triglyme to propene as reducing agents drastically promoted the NOx conversion over Ag/Al2O3 at low temperature. A series of In2O3-, Ga2O3- or B2O3-promoted γ-Al2O3 catalysts were investigated for lean NOx reduction with DME. B2O3/Al2O3 and Ga2O3/Al2O3 catalysts gave a temperature window similar to Al2O3, but with higher NOx conversion. In2O3/Al2O3 showed the highest NOx conversion at low temperature, whereas pure In2O3 was inactive for NOx reduction with DME. It was concluded that a close interaction between In2O3 and Al2O3 is essential for the promoting effect of In2O3 at low temperature. In-situ DRIFT spectroscopy showed that the differences in activity of In2O3/Al2O3, pure Al2O3 and pure In2O3 correlated to differences in the amounts of surface species that are possible intermediates of the reaction.