Nanoscale chemical imaging of a working catalyst by scanning transmission X-ray microscopy

Abstract

The modern chemical industry uses heterogeneous catalysts in almost every production process. They commonly consist of nanometre-size active components (typically metals or metal oxides) dispersed on a high-surface-area solid support, with performance depending on the catalysts' nanometre-size features and on interactions involving the active components, the support and the reactant and product molecules. To gain insight into the mechanisms of heterogeneous catalysts, which could guide the design of improved or novel catalysts, it is thus necessary to have a detailed characterization of the physicochemical composition of heterogeneous catalysts in their working state at the nanometre scale. Scanning probe microscopy methods have been used to study inorganic catalyst phases at subnanometre resolution, but detailed chemical information of the materials in their working state is often difficult to obtain. By contrast, optical microspectroscopic approaches offer much flexibility for in situ chemical characterization; however, this comes at the expense of limited spatial resolution. A recent development promising high spatial resolution and chemical characterization capabilities is scanning transmission X-ray microscopy, which has been used in a proof-of-principle study to characterize a solid catalyst. Here we show that when adapting a nanoreactor specially designed for high-resolution electron microscopy, scanning transmission X-ray microscopy can be used at atmospheric pressure and up to 350 degrees C to monitor in situ phase changes in a complex iron-based Fisher-Tropsch catalyst and the nature and location of carbon species produced. We expect that our system, which is capable of operating up to 500 degrees C, will open new opportunities for nanometre-resolution imaging of a range of important chemical processes taking place on solids in gaseous or liquid environments.

Correlative electron and X-ray microscopy: probing chemistry and bonding with high spatial resolution

“…Equations 1 and 2 are similar in that both the electron-electron interaction operator q · r, and the photon-electron operator ε · r have a dipole form (within the dipole limit in the case of EELS). 55 Therefore, the selection rules that determine allowed final states are the same i.e. the change in angular momentum Δl = ±1.…”

“…23 As single-atom spectroscopy is limited by these sample instabilities, an increase in probe brightness or acquisition time alone may not be suitable for achieving better signal-to-noise ratios (SNR) or for investigating other elements with lower scattering cross-sections. Instead, improvements in detector 55 sensitivity are required for single-atom measurements to become more routinely achievable in EELS.…”

“…This is due in part to the shadowing of EDX detectors by liquid cell holders, however modified liquid cell holders have very 50 recently been developed, 60 and in a test sample EDX maps were acquired from Au nanoparticles in 150 nm of liquid with 30 nm spatial resolution. EELS measurements do not suffer the same geometrical constraints as EDX, but are once again limited by multiple scattering events which greatly reduce the signal to 55 background ratio. It has been suggested that while core-loss EELS measurements are only practical through < 3 inelastic mean free paths (which corresponds to a 300 nm cell thickness at 200 keV), valence-loss EELS signals at energies below the plasmon peak are less sensitive to thickness effects and may be 60 used to provide chemical information of samples up to six inelastic mean free paths (or 600-700 nm at 200 keV) in thickness.…”

“…Due to recent advances in diffractive optics for soft (100 eV to 2 keV 1 ) X-rays, as well as monochromators and aberration correctors for electron microscopes, the spatial and 50 spectral resolutions of STXM-XAS and STEM-EELS are converging. However, the fundamental diffraction limit for Xrays means STXM-XAS cannot match the spatial resolution of STEM-EELS, while it is unlikely that the combined high flux and high energy-resolution achievable in synchrotron based X-ray 55 microscopes will ever be matched in the electron microscope. Additionally, differences in primary damage mechanisms and detection efficiencies alter the suitability of each technique for different samples.…”

In Situ Industrial Bimetallic Catalyst Characterization using Scanning Transmission Electron Microscopy and X‐ray Absorption Spectroscopy at One Atmosphere and Elevated Temperature

“…Only transmission or scanning transmission electron microscopy (TEM, STEM) can provide nanoscale structural and elemental information for individual nanoparticles at the required length scale 16, 17, 18. When combined with electron energy loss spectroscopy (EELS) or energy dispersive X‐ray spectroscopy (EDS) this approach can be used to map the elemental distribution or even oxidation states for individual nanoparticles, with a full three dimensional characterisation at nanometre resolution achievable for model catalyst systems and ultra‐high vacuum (UHV) environments 19…”