The technique, employed by Doug Blom and colleagues at the University of South Carolina and co-workers at the University of Delaware, relies on using an aberration-corrected electron microscope operating in STEM (scanning transmission electron microscopy) mode. The STEM operates under HAADF (high-angle, annular dark-field) conditions.
Model agreement
The researchers used this nano-imaging method to independently derive a structural model for an important new oxidation catalyst. "The model agrees very well with that obtained by the more laborious method of classical crystallographic refinement that took two years to complete," explained Thomas Vogt, Director of the NanoCenter at USC.
The investigated Mo-V-Nb-Te-O catalyst converts propane to acrylonitrile. This is an important process that will allow paraffin-based reactants to replace the more expensive currently employed olefin-based ones such as propene when making acrylate polymers. Acrylonitrile production has today reached the staggering scale of nearly 1 kg/year for every human being on Earth.
One major problem in electron microscopy is spherical aberration, which not only degrades the point resolution of samples by a factor of 1000 from the fundamental diffraction limit of electrons but also contributes to a lack of contrast in the final images. Although periodic structure can be resolved via high-resolution electron microscopy, interfaces, surfaces and defects at the atomic level are not readily accessible.
Higher point resolution
In contrast, the HAADF detector collects electrons that have interacted closely with the nucleus of atoms in the sample and therefore resemble the more well-known Z2 (where Z is the atomic number) Rutherford scattering. This is known as Z-contrast STEM and allows for a much higher point resolution and better contrast, with the contrast being proportional to Z2.
The team says it will use its technique to understand the composition and structure of other real industrial catalytic systems and relate these to the catalysts' performance. "Based on these investigations we will then be able to asses if the observed complexity is a sufficient and necessary condition for multifunctional heterogeneous catalysts or if it is possible to make structurally and compositionally simpler catalysts with the same selectivity and reactivity as the M1-phases we studied," said Vogt.
Vogt adds that the new study will allow for further important developments in elucidating the structure of other complex, technologically important solids. These include phosphors, ceramic membranes and heterogeneous catalysts that convert natural gas and liquified petroleum to consumer products such as acrylates and acrylonitriles. "Most of these solids are complex oxides consisting of four or more different cationic species and are extremely difficult to prepare as single crystals amenable to X-ray crystallography," explained Vogt. "Our approach will help to determine the atomic structures of these materials, in which heavy rare-earth elements with high Z are the major constituents."
The work was reported in Angewadte Chemie and The Journal of Physical Chemistry C