New insights into the NH-selective catalytic reduction of NO over Cu-ZSM-5 as revealed by spectroscopy.

To control diesel vehicle NO emissions, Cu-exchanged zeolites have been applied in the selective catalytic reduction (SCR) of NO using NH as reductant. However, the harsh hydrothermal environment of tailpipe conditions causes irreversible catalyst deactivation. The aggregation of isolated Cu brings about unselective ammonia oxidation along with the main NH-SCR reaction. An unusual 'dip' shaped NO conversion curve was observed in the steamed zeolite Cu-ZSM-5, resulting from the undesired NH oxidation that produced NO. Here we gain further insights into the NH-SCR reaction and its deactivation by employing UV-vis diffuse reflectance spectroscopy (DRS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) on fresh and steamed zeolite Cu-ZSM-5. We found that tetragonally distorted octahedral Cu with associated NH preferentially forms during low temperature NH-SCR (<250 °C) in fresh Cu-ZSM-5. The high coordination number of Cu ensures the availability for high coverage of nitrate intermediates. Whilst in the steamed Cu-ZSM-5, [Cu (OH)] oligomers/clusters in pseudo-tetrahedral symmetry with coordinated NH accumulated during the low-temperature NH-SCR reaction. These clusters presented a strong adsorption of surface NH and nitrates/nitric acid at low temperatures and therefore limited the reaction between surface species in the steamed Cu-ZSM-5. Further release of NH with increased reaction temperature favors NH oxidation that causes the drop of NO conversion at ∼275 °C. Moreover, competitive adsorption of NH and nitrates/nitric acid occurs on shared Lewis-acidic adsorption sites. Prompt removal of surface nitrates/nitric acid by NO avoids the surface blockage and tunes the selectivity by alternating nitrate-nitrite equilibrium. The formation of adsorbed NO and HNO points to the necessity of an acid adsorbent in practical applications. The structural similarity under the NH-SCR reaction and unselective NH oxidation confirmed the entanglement of these two reactions above 250 °C.