Applications of Full-field Transmission X-ray Nanotomography and X-ray Nanospectroscopy at Stanford Synchrotron Radiation Lightsource

Abstract

There are many materials whose properties and performance are dependent on the local chemical and physical structure on a length scale in the tens of nanometer range. Such materials include catalysts, adsorbents and batteries. The development and application of nanoscale full-field X-ray tomography and X-ray absorption spectroscopy (XAS) has impacted our understanding of such materials. This talk will provide an introduction to full-field transmission X-ray nanospectroscopy and nanotomography, i.e. transmission X-ray microscopy (TXM), in 2D and 3D and will use specific examples from our research at Stanford Synchrotron Radiation Lightsource (SSRL) to illustrate the technique, the information content, and the new understanding that is gained from such studies. The TXM located at SSRL is designed to work over an energy range of ∼5-14 keV. A capillary condenser focuses the beam to a spot of a few tens of micrometers on to the sample. The nominal spatial resolution of this system is ≈30 nm. Typical exposure time for a single image is ~0.5 s. The system is fully automated for conducting complicated scans involving motions of the sample as well as all the optics in the system 1. The first case study is the investigation of the reaction kinetics in solid sorbent materials upon exposure to the reactive gas. For example, CuO is used to remove trace H2S in natural gas purification. The aim of the study was to investigate whether heterogeneity at the particle or sub-particle scale affects the performance of the solid phase. First, XAS tracked changes in the average chemical environment of Cu atoms during sulfidation, and allowed determination of CuO conversion with time, providing direct determination of the bulk sulfidation kinetics. These data were then complemented by in-situ TXM that probed individual mm sized CuO particles to quantify chemical and structural changes on a length scale of tens of nm. The data showed that sulfidation initiates on the outer portions of the particle and then propagates towards the center. Analysis of these data revealed similar kinetics of the sulfidation as those measured in a reactor containing many particles.