High-resolution X-ray imaging of catalytic reactions by using X-ray Absorption Spectroscopy

Heterogeneous catalysis plays an important role in many areas of the chemical industry such as oxidation reactions, and dehydrogenation reactions. In heterogeneouscatalytic processes, the materials experience changes in their structure depending on variations in their chemical environment and in the course of their application,which can lead to a reduced function of the catalyst. A precise understanding of the processes taking place and the structural changes occurring on various length scales - from the atomic scale of catalytically active sites to the macroscopic scale - is therefore of great importance for industrial applications. In order to characterize these structural and chemical changes different techniques can be employed. Conventional spectroscopic characterization tools such as Raman spectroscopy allow real-time measurements, quantitative analysis and identification of reaction mechanisms.X-ray microscopy can make an important contribution here, as it allows for the extraction of structural information from within samples in three dimensions, due to the relative weak interaction of hard X-rays with matter. Also, this technique can easily be combined with spectroscopy, allowing for the investigation of both the local density and the chemical state of a material. In this way, dynamic processes can be analyzed under operando conditions with high temporal and spatial resolution. In this work, methods were developed to combine X-ray full-field microscopy withX-ray absorption spectroscopy. The methodology was demonstrated on well-known chemical reactions as test systems at beamline P64 (PETRA III). This allowed for the investigation structural changes in the chemical oxidation state of a platinum catalyst during the catalytic partial oxidation of methane under operando conditions. This initial 2D demonstration of the method was further extended to 3D using X-ray tomography, and the oxidative dehydrogenation of ethane to ethylene was investigated using this method. This enabled us to investigate the local chemical oxidation state of Molybdenum as catalytic material during operation. Overall, the results demonstrate the great potential of the method for visualizing chemical catalysis processes with high spatial resolution in 3D. These methods can be easily implemented at beamlines equipped with high X-ray flux, fast-scanning X-ray monochromators, and a suitable gas-phase analysis environment.