The influence of gas phase reactions on the design criteria for catalysts for lean NOx reduction with dimethyl ether

In the selective catalytic reduction (SCR) of NO by dimethyl ether (DME), the formation of unexpectedly high amounts of NO2 over 300 °C has previously been reported. In this study, we explain this phenomenon by radical reactions initiated by DME and O2, during which DME is partly oxidized and NO2 is formed in the presence of NO. For the design criteria of a DME-SCR catalyst, these gas phase reactions have mainly three consequences: (i) another type of reducing agent than that fed into the reactor reaches the catalyst, (ii) no activation of the reducing agent such as partial oxidation is required, and (iii) several of the proposed intermediate species for HC-SCR, e.g. NO2, HONO, CH3–NO2, and CH3–NO form already in the gas phase. An efficient DME-SCR catalyst should thus have high selectivity for reduction of NOx predominately by partially oxidized C1-compounds, and it should not have particularly strong oxidizing properties to avoid non-selective oxidation of these C1-compounds. These two requirements appear to be reasonable well met by the acidic zeolite H-ZSM-5.