Detection of key transient Cu intermediates in SSZ-13 during NH3-SCR deNO x by modulation excitation IR spectroscopy.

Alex G Greenaway,Oliver Kröcher,Adrian Marberger,C Richard A Catlow,Inés Lezcano-González,Adam Thetford,Andrew M Beale,Miren Agote-Arán,Davide Ferri,Maarten Nachtegaal

doi:10.1039/c9sc04905c

Abstract

The small pore zeolite Cu-SSZ-13 is an efficient material for the standard selective catalytic reduction of nitrogen oxides (NO x ) by ammonia (NH3). In this work, Cu-SSZ-13 has been studied at 250 °C under high conversion using a modulation excitation approach and analysed with phase sensitive detection (PSD). While the complementary X-ray absorption near edge structure (XANES) spectroscopy measurements showed that the experiments were performed under cyclic Cu+/Cu2+ redox, Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) experiments provide spectroscopic evidence for previously postulated intermediates Cu-N([double bond, length as m-dash]O)-NH2 and Cu-NO3 in the NH3-SCR deNO x mechanism and for the role of [Cu2+(OH-)]+. These results therefore help in building towards a more comprehensive understanding of the reaction mechanism which to date has only been postulated in silico.

Highlights

Ammonia selective catalytic reduction (NH3-SCR) is an efficient way to prevent nitrogen oxide (NO, NO2) emissions from heavy duty diesel powered vehicles.[1,2,3,4,5] Cu-exchanged chabazite, a small pore zeolite, is the subject of a considerable amount of research as it exhibits both a higher activity and selectivity and is less susceptible tothermal degradation than other zeolitebased systems (e.g. ZSM-5 and beta).[5,6,7] Mechanistically, the study of NH3-SCR is complicated and as of yet, not fully rationalised in terms of reaction kinetics, temperature regimes, and perhaps more relevant here, reaction intermediates
Cu-SSZ-13 has been studied by concentration modulation excitation (ME) using both DRIFTS and XANES
These results, are in line with predictions from theory, which helps benchmark the modulation excitation (ME) approach for obtaining potential mechanistic insight importantly obtained under more relevant conditions.[10]

Summary

Introduction

Ammonia selective catalytic reduction (NH3-SCR) is an efficient way to prevent nitrogen oxide (NO, NO2) emissions from heavy duty diesel powered vehicles.[1,2,3,4,5] Cu-exchanged chabazite (in its synthetic form SSZ-13), a small pore zeolite (maximum ring size 8 T-sites), is the subject of a considerable amount of research as it exhibits both a higher activity and selectivity and is less susceptible to (hydro)thermal degradation than other zeolitebased systems (e.g. ZSM-5 and beta).[5,6,7] Mechanistically, the study of NH3-SCR is complicated and as of yet, not fully rationalised in terms of reaction kinetics, temperature regimes, and perhaps more relevant here, reaction intermediates.

Results

Conclusion