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

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

doi:10.1039/d1cy02348a

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.

Highlights

Since the high NO decomposition activity of zeolite CuZSM-5 was discovered in 1980s, Cu-exchanged zeolites have been widely investigated for the NH3-selective catalytic reduction (SCR) reaction.[4]
We found that tetragonally distorted octahedral Cu2+ with associated NH3 preferentially forms during low temperature NH3-selective catalytic reduction (NH3-SCR) (
When the reaction temperature is higher than 250 °C, the contribution from unselective NH3 oxidation accelerated and produced NO, resulting in a ‘dip’ shape of the NO conversion curve in the NH3-SCR reaction in steamed Cu-ZSM-5 zeolites

Summary

Introduction

Such as MFI and BEA, are limited by their low hydrothermal stability, while the more robust small pore zeolite CHA (i.e., SSZ-13 and SAPO-34) has a higher cost. Various Cu species in the zeolites provide multiple possible sites for catalytic reactions at NH3-SCR reaction conditions. With multiple evolutionary Cu species in the Cu-exchanged zeolites, the unwanted side reactions such as NO oxidation (2NO + O2 = 2NO2) and unselective NH3 oxidation to NO (4NH3 + 5O2 = 4NO + 6H2O) can take place under standard NH3-SCR reaction conditions.[11–14]. Operando UV-vis diffuse reflectance spectroscopy (DRS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were conducted to gain mechanistic insight into the NH3-SCR reaction and its deactivation, and to gain a deeper understanding of the unusual catalytic behaviour of the steamed zeolite Cu-ZSM-5 material. The ‘dip’-shaped NO conversion curve (Fig. 1) could be explained by the side reaction of unselective NH3 oxidation, which is structurally ascribed to the possible formation of [Cux(OH)2x−1]+ oligomers/clusters with a pseudo-tetrahedral Cu2+ center, coordinated with NH3 in the steamed Cu-ZSM-5 material. The slow rate of surface reaction between adsorbed NH3 and surface nitrites/nitrates or nitrous/ nitric acid limits the low-temperature NH3-SCR

NH3-Selective catalytic reduction, NO oxidation and NH3 oxidation

Local damage of the framework structure

Loss of isolated Cu2+ sites

Cu2+ dynamics unravelled by operando UV-vis diffuse reflectance spectroscopy The

Replacement of ligands in the Cu complex

Adsorption competition revealed by operando diffuse reflectance infrared Fourier transform spectroscopy

Competitive adsorption of surface species

Nitrate mediated reaction network

Important mechanistic implications

Structure–intermediate–performance relationship

Practical implication of detected acidic products

Conclusions