Chemical gradients in automotive Cu-SSZ-13 catalysts for NOx removal revealed by operando X-ray spectrotomography

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.

Automated summary

This study combined operando X-ray absorption spectroscopy with microtomography to perform three-dimensional chemical imaging of copper species in a Cu-SSZ-13 monolith catalyst during NOx reduction. The results showed gradients in copper oxidation state and coordination environment at micrometre spatial resolution, revealing the interplay of NOx reduction with adsorption-desorption of NH3 and mass transport phenomena, while simultaneously determining catalytic performance. The use of operando X-ray spectrotomography proved useful in direct three-dimensional visualization of complex reactions on non-model catalysts, improving understanding of structure-activity relationships and observing mass and heat transport effects.