SEARCH

SEARCH BY CITATION

Keywords:

  • electron microscopy;
  • heterogeneous catalysis;
  • nanoparticles;
  • nanoscale characterization;
  • structure determination

Abstract

From its initial appearance as a low-resolution commercial instrument in the early 1960s to its present ultra-high-resolution (aberration-corrected) state, the electron microscope has proved invaluable, often uniquely so, in the elucidation of structural aspects of solid catalysts. When one of us (JMT) used it for the study of surface reactivities of layered solids (graphite and molybdenite), topographical features at a resolution of 3.4 Å could be readily distinguished by using the so-called gold-decoration technique, which enabled the catalytic channelling of graphite surfaces (by nanoparticles of various metals) to be charted. This technique also enabled vacancy concentrations in individual (i.e., graphene) layers to be quantitatively determined with unequalled precision. Progressive improvements in electron, optical and other instrumental features became so significant that, by the early 1980s, high-resolution images of zeolites, intercalates and metal-carbide-tipped multiwall carbon nanotubes could be routinely retrieved. High-resolution studies of zeolites (natural and synthetic) proved particularly illuminating, as they uncovered the ubiquity of coherent (regular and irregular) intergrowths, especially amongst the important pentasil catalysts (ZSM-5 and ZSM-11, that is, MFI and MEL structural types).

By the mid-1980s, commercial conventional transmission electron microscopes operating at 200 keV yielded atomically-resolved structures of nanoparticles of supported noble-metal catalysts, such as Pt and Au, in which it was found that microtwinning and other unexpected structural irregularities were common. Parallel advances in scanning transmission electron microscopy, pioneered by Crewe and co-workers, and the use of high-angle annular dark-field scanning transmission electron microscopy introduced by Howie and Treacy, added further power to chemical electron microscopy, especially when spectroscopic information like electron energy-loss and X-ray emission signals could be routinely recorded.

Our own development (PAM and JMT) since the early 2000s of electron tomography (ET) provided deeper insights into the nature of the growing families of nanoporous solids, especially those designated as single-site heterogeneous catalysts (SSHCs). In these catalysts, the active site may be an integral part of the micro- or mesoporous framework (as in MFI and the so-called MAPOs), or, alternatively be a multinuclear-single site bimetallic nanocluster like Ru10Pt2 anchored at the pore walls of mesoporous solids. With the dramatic advances in instrumentation, ET now yields quantitatively, features such as fractal dimension in addition to surface areas, pore volumes and visual representation of tortuosity. With the added procedure of “compressed sensing”, far-reaching possibilities, particularly in ET, now exist for the future study of nanostructured catalysts that have, hitherto, been so vulnerable to electron beam damage as to render them inaccessible to electron microscopic investigations. Another major recent advance, that flows from the work of Vincent and Midgley in the 1990s, is the so-called precession electron diffraction (PED) procedure, where, by judicious recording (e.g. involving rocking the electron beam in a hollow cone), diffracted intensities can be used for structure determinations because of the elimination of multiple electron (i.e., dynamical) scattering. Solids of catalytic interest that have been structurally determined by PED include Ba6Mn5O15 and MCM-68. There are real prospects that the PED approach may be able to solve the structures of notoriously beam-sensitive metal–organic frameworks (MOFs), the catalytic potential of which is continually increasing.

Although two other major advances in chemical electron microscopy—the 4 D microscopy pioneered by Zewail, and the in situ electron microscopy of Gai and others—have not been experimentally explored by us, we also summarize their power and future potential in this Essay.