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

Xinwei Ye; Ramon Oord; Matteo Monai; Joel E Schmidt; Tiehong Chen; Florian Meirer; Bert M Weckhuysen

doi:10.1039/d1cy02348a

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

Catal Sci Technol. 2022 Feb 28;12(8):2589-2603. doi: 10.1039/d1cy02348a. eCollection 2022 Apr 19.

Authors

Xinwei Ye^{1

2}, Ramon Oord², Matteo Monai², Joel E Schmidt², Tiehong Chen¹, Florian Meirer², Bert M Weckhuysen²

Affiliations

¹ Institute of New Catalytic Materials Science, School of Materials Science and Engineering, Key Laboratory of Advanced Energy Materials Chemistry (MOE), Nankai University Tianjin 300350 China.
² Inorganic Chemistry and Catalysis Group, Debye Institute for Nanomaterials Science, Utrecht University Universiteitsweg 99 3584 CG Utrecht The Netherlands b.m.weckhuysen@uu.nl.

Abstract

To control diesel vehicle NO _x 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, [Cu _x (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 HNO _x 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.