Hard X-ray Nanotomography of Catalytic Solids at Work

摘要

Advances in microscopy such as a spatial resolution in the nanometer range combined with higher chemical and morphological sensitivities have improved the imaging of functional materials. Optical and electron microscopy techniques are commonly employed; however, they may not always be applicable for nondestructive imaging if the sample is either optically opaque or too thick to be penetrated by electrons. On the other hand, X-rays offer superior imaging resolution compared to, for example, visible-light methods because of their short wavelength and their intrinsic penetration power, which allows studying thick materials. Scanning transmission X-ray microscopy (STXM) in the soft X-ray energy region (200-2000 eV) has been used extensively for biological imaging and has been recently also introduced in the field of materials chemistry, including heterogeneous catalysis. STXM combines a high spatial resolution of about 15 nm with chemical speciation by X-ray absorption spectroscopy using focused soft X-ray light as a probe. The sample thickness for soft X-rays must ideally be in the (sub-)micron range but the gap of imaging larger particles at higher pressures and temperatures remains an important drawback. Full-field transmission hard X-ray microscopy (TXM) appears to be an excellent method to counter the current limitations in view of its potential to image the nanoscale features of thick objects because of a high penetration power.Thanks to the advances in X-ray optics, synchrotron-radiation-based transmission hard X-ray microscopes have already achieved a resolution of 30-50 nm from an energy range of 4000-14000 eV. The advantage of TXM is the possibility of working at higher pressures and reaction temperatures and of implementing 3D imaging of larger catalyst particles (10-60 microns). In addition, collecting a stack of images at different energies across the absorption edge makes it possible to obtain X-ray absorption spectra, even under in situ conditions. X-ray absorption near-edge spectroscopy (XANES) provides unique information on the oxidation state and local environment of the absorber element in complex multi-component materials and thus provides crucial information on the nanoscale structure of heterogeneous catalysts. Here, we show that TXM combined with a specially designed in situ reactor makes it possible to investigate the dynamic changes in morphology, porosity, and chemical composition of a 20 μm catalyst particle at 10-30 bar and up to 600 °C in a reactant stream with a spatial resolution of 30 nm.