New insights into the NH3-selective catalytic reduction of NO over Cu-ZSM-5 as revealed by operando spectroscopy

Abstract

To control diesel vehicle NOx emissions, Cu-exchanged zeolites have been applied in the selective catalytic reduction (SCR) of NO using NH3 as reductant. However, the harsh hydrothermal environment of tailpipe conditions causes irreversible catalyst deactivation. The aggregation of isolated Cu2+ brings about unselective ammonia oxidation along with the main NH3-SCR reaction. An unusual 'dip' shaped NO conversion curve was observed in the steamed zeolite Cu-ZSM-5, resulting from the undesired NH3 oxidation that produced NO. Here we gain further insights into the NH3-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 Cu2+ with associated NH3 preferentially forms during low temperature NH3-SCR (<250 °C) in fresh Cu-ZSM-5. The high coordination number of Cu2+ ensures the availability for high coverage of nitrate intermediates. Whilst in the steamed Cu-ZSM-5, [Cux(OH)2x-1]+ oligomers/clusters in pseudo-tetrahedral symmetry with coordinated NH3 accumulated during the low-temperature NH3-SCR reaction. These clusters presented a strong adsorption of surface NH3 and nitrates/nitric acid at low temperatures and therefore limited the reaction between surface species in the steamed Cu-ZSM-5. Further release of NH3 with increased reaction temperature favors NH3 oxidation that causes the drop of NO conversion at ∼275 °C. Moreover, competitive adsorption of NH3 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 NO2 and HNOx points to the necessity of an acid adsorbent in practical applications. The structural similarity under the NH3-SCR reaction and unselective NH3 oxidation confirmed the entanglement of these two reactions above 250 °C.