New insights into the NH 3 -selective catalytic reduction of NO over Cu-ZSM-5 as revealed by operando spectroscopy.

To control diesel vehicle NO x emissions, Cu-exchanged zeolites have been applied in the selective catalytic reduction (SCR) of NO using NH 3 as reductant. However, the harsh hydrothermal environment of tailpipe conditions causes irreversible catalyst deactivation. The aggregation of isolated Cu 2+ brings about unselective ammonia oxidation along with the main NH 3 -SCR reaction. An unusual 'dip' shaped NO conversion curve was observed in the steamed zeolite Cu-ZSM-5, resulting from the undesired NH 3 oxidation that produced NO. Here we gain further insights into the NH 3 -SCR reaction and its deactivation by employing operando 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 2+ with associated NH 3 preferentially forms during low temperature NH 3 -SCR (<250 °C) in fresh Cu-ZSM-5. The high coordination number of Cu 2+ ensures the availability for high coverage of nitrate intermediates. Whilst in the steamed Cu-ZSM-5, [Cu x (OH) 2 x -1 ] + oligomers/clusters in pseudo-tetrahedral symmetry with coordinated NH 3 accumulated during the low-temperature NH 3 -SCR reaction. These clusters presented a strong adsorption of surface NH 3 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 3 with increased reaction temperature favors NH 3 oxidation that causes the drop of NO conversion at ∼275 °C. Moreover, competitive adsorption of NH 3 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 2 and HNO x points to the necessity of an acid adsorbent in practical applications. The structural similarity under the NH 3 -SCR reaction and unselective NH 3 oxidation confirmed the entanglement of these two reactions above 250 °C.