Tracking dynamic structural changes in catalysis by rapid 2D-XANES microscopy.

Saba Alizadehfanaloo,Martin Seyrich,Andreas Schropp,Christian G Schroer,Vadim Murzin,Jan-Dierk Grunwaldt,Johannes Becher,Dmitry E Doronkin,Thomas L Sheppard,Jan Garrevoet

doi:10.1107/s1600577521007074

Abstract

Many processes and materials in heterogeneous catalysis undergo dynamic structural changes depending on their chemical environment. Monitoring such dynamic changes can be challenging using conventional spectroscopic characterization tools, due to the high time resolution required. Here, a high-resolution 2D X-ray camera operating at 50 Hz full-frame rate was synchronized with a QEXAFS monochromator, enabling rapid spectro-microscopic imaging with chemical contrast over individual pixels. This was used to monitor chemical gradients within a model Pt/Al2O3 catalyst during catalytic partial oxidation of methane to synthesis gas. The transition from methane combustion (partly oxidized Pt) to combustion-reforming and partial oxidation (fully reduced Pt) was observed by a characteristic reduction front, which progressed from the end of the catalyst bed towards its beginning on the second time scale. The full-field QEXAFS imaging method applied here allows acquisition of entire XANES spectra `on the fly' in a rapid and spatially resolved manner. The combination of high spatial and temporal resolution with spectroscopic data offers new opportunities for observing dynamic processes in catalysts and other functional materials at work. The methodology is flexible and can be applied at beamlines equipped with a QEXAFS or other fast-scanning monochromators and a suitable sample environment for gas phase analytics to allow for catalytic studies at the same time.

Highlights

Materials used in heterogeneous catalysis can show dynamic structural changes depending on their chemical environment, for example during ignition and extinction of reactions, activation and deactivation, or restructuring through oxidation and reduction processes (Kalz et al, 2017; Bergmann & Roldan Cuenya, 2019)
Considering time-resolved spectroscopy, QEXAFS has developed into a key technique, which combines the valuable structural characterization of X-ray absorption spectroscopy (XAS) with rapid data acquisition on millisecond timescales (Frahm, 1989), this is typically performed in single point measurements (Grunwaldt et al, 2009; Frenkel et al, 2013; Muller et al, 2016; Nachtegaal et al, 2016)
We have demonstrated that rapid 2D-XANES imaging can be implemented at beamline P64 at PETRA III

Summary

Introduction

Materials used in heterogeneous catalysis can show dynamic structural changes depending on their chemical environment, for example during ignition and extinction of reactions, activation and deactivation, or restructuring through oxidation and reduction processes (Kalz et al, 2017; Bergmann & Roldan Cuenya, 2019). Considering time-resolved spectroscopy, QEXAFS has developed into a key technique, which combines the valuable structural characterization of X-ray absorption spectroscopy (XAS) with rapid data acquisition on millisecond timescales (Frahm, 1989), this is typically performed in single point measurements (Grunwaldt et al, 2009; Frenkel et al, 2013; Muller et al, 2016; Nachtegaal et al, 2016) The combination of these two techniques offers the intriguing possibility to perform rapid spatially resolved spectroscopic imaging over millimetre-scale fields of view, though this has not been demonstrated in practice so far. This behaviour leads to transient conditions and catalyst structure, which is an ideal proof of principle for rapid spectroscopic imaging

Sample preparation

Results

Conclusions

Funding information