Nanoscience is characterized by the ability to determine the structure and composition of nanoscopic objects ranging from mesoscopic dimensions, of about 100 nm, down to individual atoms and clusters. A rational science in this dimension has to rely on analytical capabilities to “see” and control the manipulation of atoms. Catalysis at the atomic or molecular level is a major player in nanoscience. The need to understand catalysts and other functional materials (including semiconductors) has driven the development of electron microscopy into dimensions that were hardly imaginable 10 years ago.
As shown in Figure 1, electron microscopy has experienced a revolutionary development through the past 80 years, both instrumentally and methodically.1 From the development of Bragg diffraction contrast and column approximation, which enable us to understand TEM images of crystals and their defects, to spherical aberration correction for sub-angstrom scale imaging, which is capable of revealing atomic structures with unprecedented precision, and all the powerful analytic modes and affiliated detectors, such as X-rays, backscattered electrons, and electron energy loss spectroscopy (EELS), electron microscopy has shown spiraling and steady advances. The well-established methods, such as diffraction contrast and mass thickness contrast, developed around the 1950’s are still vital imaging techniques used to obtain the basic morphological information of a catalyst. A revised form of mass thickness, called HAADF-STEM (Z contrast), established around the 1990’s, has proven to be a very powerful method to study supported catalysts of high atomic number. The left side of Figure 1 shows the tendency of spatial resolution achieved with TEM from 10 nm to sub-angstrom. More recently, aberration-corrected TEM has achieved resolution below 0.5 Å at a magnification of 50 million. Not summarized in Figure 1, but equally important for materials science and electron microscopy, are electron tomography and electron holography. These technologies in association with modern electron microscopy are becoming routine methods in many laboratories.
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 and high conversion rate with long cycle times favored for their reduced environmental impact. As shown in Figure 2, modern electron microscopy with its arsenal of diffraction, imaging, and spectroscopic techniques gives access to the collective and individual geometric, chemical, and electronic properties of such materials.2
Catalysts, however, are complex solids and this structural complexity is associated with their ability to undergo in every catalytic cycle structural changes of their active sites that are reverted into a metastable initial state of activity. In this sense, the ex situ obtained information from a catalyst, either by using electron microscopy or any other spectroscopic method, is often irrelevant to a working catalyst. The recently developed environmental TEM (in situ TEM) and 4D electron microscopy provide the possibility to study dynamic behavior of a catalyst under working conditions. This could be a true revolution in catalysis, if combined with other in situ spectroscopic methods, to give a full set of information for these catalysts and possibly access to key information for the reaction mechanism.
Apart from nanoscience and nanotechnology (but is the catalysis not a part of nanoscience?), catalysis is one of the most important areas where electron microscopy has found important application. It is said that the evolution of catalysis science is based on the ability to cope with the structural complexity of solid catalysts, which is largely made possible by the advancement of electron microscopy. Although, undeniably, the opposite is also true in that the analysis of catalysts is seen as a major driving force for the current revolution in electron microscopy.
The manuscripts in this Special Issue include selected contributions to ChemCatChem and a large number of invited articles based on the 2nd International Symposium on Advanced Electron Microscopy for Catalysis and Energy Storage Materials, EmCat2012, organized by us in Berlin in 2012. EmCat2012 covered the most recent developments, especially those after EmCat2010, in all areas of advanced electron microscopy applied to catalysis and energy storage materials.
I close by wholeheartedly thanking all the contributors to this Special Issue, the referees, and the editors of ChemCatChem. With best regards,
Dang Sheng Su