Cu-IM-5 as the Catalyst for Selective Catalytic Reduction of NO_x with NH₃: Role of Cu Species and Reaction Mechanism

The role of Cu species in Cu ion-exchanged IM-5 zeolite (Cu-IM-5) regarding the performance in selective catalytic reduction (SCR) of NOx with NH3 (NH3-SCR) and the reaction mechanism was studied. Based on H-2 temperature-programmed reduction (H-2-TPR) and electron paramagnetic resonance (EPR) results, Cu-O-Cu and isolated Cu species are suggested as main Cu species existing in Cu-IM-5 and are active for SCR reaction. Cu-O-Cu species show a good NH3-SCR activity at temperatures below 250 degrees C, whereas their NH3 oxidation activity at higher temperatures hinders the SCR performance. At low temperatures, NH4NO3 and NH4NO2 are key reaction intermediates. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) suggests a mixed Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanism over Cu-IM-5 at low temperatures.