Nitrogen oxide (NOx) emissions are a major source of pollution, demanding ever-improving performance from catalytic after-treatment systems. However, catalyst development is often hindered by limited understanding of the catalyst at work, exacerbated by widespread use of model catalysts rather than technical catalysts, and by global rather than spatially resolved characterization tools. Here we combine operando X-ray absorption spectroscopy with microtomography to perform three-dimensional chemical imaging of the chemical state of copper species in a Cu-SSZ-13 washcoated monolith catalyst during NOx reduction. Gradients in copper oxidation state and coordination environment, resulting from an interplay of NOx reduction with adsorption–desorption of NH3 and mass transport phenomena, were revealed at micrometre spatial resolution while simultaneously determining catalytic performance. Crucially, direct three-dimensional visualization of complex reactions on non-model catalysts is feasible only by the use of operando X-ray spectrotomography, which can improve our understanding of structure–activity relationships, including the observation of mass and heat transport effects.
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Raw data were generated at the Swiss Light Source of the Paul Scherrer Institut (Switzerland). The collected and cleaned imaging data acquired before tomographic reconstruction that support the findings of this study are stored in KITopen, the central repository of the Karlsruhe Institute of Technology, and are freely available with the following DOIs: 200 °C dataset, https://doi.org/10.5445/IR/1000122874; 300 °C dataset, https://doi.org/10.5445/IR/1000122890; 350 °C dataset, https://doi.org/10.5445/IR/1000122892; 400 °C dataset, https://doi.org/10.5445/IR/1000122893; NH3 reference dataset, https://doi.org/10.5445/IR/1000122894; NO reference dataset, https://doi.org/10.5445/IR/1000122895. Additional data, including reconstructed and treated spectrotomography datasets, are available from the authors upon reasonable request. Source Data for Figs. 1–3 are provided with the paper.
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This work was supported by the German Federal Ministry of Education and Research (BMBF) projects MicTomoCat (no. 05K16VK1) and COSMIC (no. 05K19VK4), and the German Research Foundation project (no. GR 3987/5-1). Spectrotomography experiments were performed at beamline microXAS of the Swiss Light Source. We acknowledge the Paul Scherrer Institut, Villigen, Switzerland for provision of synchrotron radiation beamtime at beamline microXAS, and thank M. Birri, B. Meyer and D. Grolimund for technical assistance and scientific discussions. Additional energy-dispersive spectrotomography experiments were performed on beamline ID24 at the European Synchrotron Radiation Facility, Grenoble, France. We thank F. Perrin and S. Pasternak for technical assistance. Furthermore we thank T. Bergfeldt (IAM-AWP, Karlsruhe Institute of Technology) for elemental analysis of the ion-exchanged zeolite; we thank M. Casapu from the Karlsruhe Institute of Technology for discussions on NH3-SCR, P. Lott for assistance during beamtime and D. Yuda for assistance with design of the reaction set-up. D.Z. additionally thanks the Deutsche Bundesstiftung Umwelt for the scholarship provided. T.L.S. thanks the BMBF and project COSMIC for funding.
The authors declare no competing interests.
Peer review information Nature Catalysis thanks Jianjun Chen and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Percentage NO conversion versus time and temperature during ammonia SCR.
Example of Cu K XANES data derived from operando spectrotomography.
Cu XANES data from operando spectrotomography under SCR conditions from 200 to 400 °C and related linear combination fitting of XANES.
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Becher, J., Sanchez, D.F., Doronkin, D.E. et al. Chemical gradients in automotive Cu-SSZ-13 catalysts for NOx removal revealed by operando X-ray spectrotomography. Nat Catal 4, 46–53 (2021). https://doi.org/10.1038/s41929-020-00552-3