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

Abstract

NOx emissions are a major source of pollution, demanding ever improving performance from catalytic aftertreatment systems. However, catalyst development is often hindered by limited understanding of the catalyst at work, exacerbated by widespread use of model rather than technical catalysts, and global rather than spatially-resolved characterisation tools. Here we combine operando X-ray absorption spectroscopy with microtomography to perform 3D 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 with micrometre spatial resolution while simultaneously determining catalytic performance. Crucially, direct 3D visualisation of complex reactions on nonmodel catalysts is only feasible using operando X-ray spectrotomography, which can improve our understanding of structure-activity relationships including the observation of mass and heat transport effects.Due to health and environmental concerns, there is currently a strong societal demand for improved air quality, driving strict emission limits for CO, NOx, unburnt hydrocarbons and particulates. The primary means of mitigating such pollutants is through more efficient catalytic converters. Highly effective Cu-zeolite catalysts such as Cu-SSZ-13 (chabazite) are among the prominent recent examples of active materials for selective catalytic reduction of NOx with ammonia (NH3-SCR) 1,2 .

Sample Environment for Operando Hard X-ray Tomography—An Enabling Technology for Multimodal Characterization in Heterogeneous Catalysis

“…The first published demonstration of aRCTIC introduced full-field operando spectrotomography, whereby a model copper-zeolite exhaust gas catalyst for selective catalytic reduction of NO x was imaged under a series of reaction conditions. This resulted in 3D spatially resolved XAS data at the Cu K edge, allowing the observation of chemical gradients in copper oxidation state and local coordination environment within a complex technical catalyst at work [43]. This also demonstrates possible application in both focused beam (scanning) mode and full-field acquisition mode.…”

“…It should be noted that ED-XAS was performed here with primary slits fully open (i.e., full horizontal acceptance of the incident beam) and focused on a small 3 × 7 µm 2 spot, leading to relatively high flux on the sample compared to conventional XAS studies. It is possible in certain cases that beam-induced effects can likewise be more significant during ED-XAS than for conventional XAS, including desorption of adsorbed gas species [43] or unexpected variations in oxidation state induced by the beam. Nevertheless, the aRCTIC sample environment again demonstrated maintaining controlled gas and temperature environment during extended tomography studies, which is crucial for successful operando measurements.…”

“…The current implementation of aRCTIC is at the P06 microprobe beamline of PETRA III, which can perform XAS tomography, XRF tomography, and XRD tomography, among other methods. Permutations of the aRCTIC setup have also been tested at beamline ID24 of the ESRF, and beamline microXAS of the Swiss Light Source [43], and beamline P64 of PETRA III. This shows that the aRCTIC concept may be readily adapted for various beamlines, even XAS or scattering beamlines.…”

Spatio‐Chemical Heterogeneity of Defect‐Engineered Metal–Organic Framework Crystals Revealed by Full‐Field Tomographic X‐ray Absorption Spectroscopy

“…Forthe construction of this XANES tomogram, 600 equiangular projections over 1808 8 were acquired at each of 42 energies across the Cu K-edge (8.950 to 9.100 eV). The acquisition setup is illustrated in Figure 2a.W hile the preand post-edge regions were scanned in 5eVs teps,t he absorption edge itself was scanned in 1eVs teps.F ollowing, pre-processing including position-refinement to account for sample drift, [31] atomogram was reconstructed at each energy using as imultaneous iterative reconstruction algorithm (SIRT). [38,39] This allowed the assembly of as ingle spectraltomogram with av oxel size of about 650 nm and spatial resolution of about 1.8 mm( Figure S5).…”

“…Here,w eu tilized full-field X-ray absorption near-edge structure (XANES) spectro-microscopy [24] and cryo-full-field XANES computed tomography [25] to circumvent these limitations and complement existing approaches,probing the 3D chemical and structural heterogeneity found within aseries of entire HKUST-1 MOF crystals increasing in defective linker (3,5-pyridinedicarboxylate)concentration. XANES spectroscopy [26] and the corresponding tomography techniques [27][28][29][30][31][32] are capable of unveiling the chemical state,s tructure and coordination around as elected chemical element, here copper. Thel ocal coordination environment of copper can change around the defect sites compared to "perfect" defectfree MOF crystal sites, [12,13] as well as comparing different types of defect, which can attribute different functionalities to MOFs.…”