Special Issue: Advanced Electron Microscopy for Catalysis

Abstract

Catalysis plays a key role for the sustainable development of our society. The characterization of morphology, chemical composition, surface, and internal structure of catalysts is of great importance for the synthesis of materials of high selectivity, high conversion rate, with long cycle times favored for their reduced environmental impact. Modern electron microscopy with its arsenal of imaging and spectroscopic techniques gives access to the collective and individual properties of such materials. From the very beginning of electron microscopy, its potential to reveal the microscopic details of catalysts has been explored. In 1940, M. von Ardenne and D. Beischer published in Angewandte Chemie possibly the first paper to describe the usefulness of transmission electron microscopy (TEM) for extracting morphological and structural information on supported and unsupported catalysts.1 J. Turkevich reported in 1945 the use of the electron microscope for the study of several important catalysts.2 Catalysis, together with the materials science of semiconductors, have been the driving force for the continuous improvement of electron optics and microscopic design for transmission electron microscopes. Not to forget, is Crewe’s pioneering development of the annular dark field scanning transmission electron microscopy (STEM) technique revealing single or small clusters of heavy atoms as bright features against the low intensity level of their supports.3 Advanced electron microscopes are not only marked by their high resolution, they are also defined by their high brightness field emission gun with its highly coherent electron beam. Aberration correction significantly improves the quality of high-resolution images by eliminating the Frensel fringes and delocalization effects, so that the surface structure of industrial catalysts can be determined (see the example in Figure 1).4, 5 The implementation of monochromators, and the development of high resolution electron-energy loss spectrometery (EELS), mark a new generation of analytical electron miscroscopy. Modern, advanced, analytical electron microscopes with aberration correctors provide genuine atomic and energy resolution and high sensitivity to heavy or light atoms.6 EELS, by using a monochromatized beam, is able to measure the core-Level near-edge structure in loss spectra with an energy resolution that is comparable to that measured with a synchrotron.7 Element mapping, a technique that provides a 2D image of inhomogeously distributed multi-components catalyst, is a well-established technique, used not only by catalytic researchers, but also other materials scientists, an example of which is illustrated in Figure 2. A newly developed windowless X-ray energy-dispersive spectrometer (XEDS) based on silicon drift detector technology8 allows a high speed recording of energy-dispersive spectrum and element mapping, down from hours to minutes, even at nanoscales. The combination of element mapping with electron tomography, a technique that is widely used by biologists, can give a truly 3D distribution of the active sites/phase of supported or mixed catalysts. In situ or environmental transmission electron microscopy allows for the first time the visualization of chemical/physical processes at atomic resolution. Advanced electron microscopy comes out from the shadow of being a “passive” characterization tool to become an active research method in materials and in catalysis. a) An experimental high-resolution image of the surface of an Ag catalyst used for the hydrogenation of acrolein. b) The structural model obtained from DFT calculations showing the position of α- and γ-oxygen on Ag. c) A simulated high-resolution image using the structure model in b. For more details, see Ref. [4]. Element mapping of an inhomogeously distributed multi-component catalyst. Advanced electron microscopy generates more realistic views of catalysts, allowing optimization of their structure to improve their performance. Presentations at the 1st International Symposium on Advanced Electron Microscopy for Catalysis and Energy Storage Materials at the Fritz-Haber Institute in Berlin provided a fascinating snapshot of the latest developments in catalyst microscopy. Aberration-corrected microscopes, cryo-tomography, and specialty heating stages are shedding new light on the nanoworld of catalysts and are beginning to facilitate improvements in catalyst design.9 This Special Issue contains selected papers presented at this symposium. Since then, other new developments in electron microscopy have emerged. By using photons as input rather than as an output signal, particularly in the form of femtosecond pulses, Zewail and colleagues developed ultrafast microscopy.10–12 The production of vortex electron beam in an electron microscope is exciting.13 The applications of direct relevance in catalysis are still to be identified, the potential could be tremendous.14 1