HREM IMAGE SIMULATIONS FOR SMALL-PARTICLE CATALYSTS ON CRYSTALLINE SUPPORTS

The imaging conditions for electron microscope studies of supported ultrafine particle catalysts bane been investigated by multislice simulations. Images of Pt and ReO4 particles ranging from 0.4 to 2.3 nn in size were simulated in both plan view and profile view with a rutile (TiO2) support. It was shown that particle visibility varied greatly with the objective lens defocus. Optimum defocus was not favourable for supported particles in plan view since the ultrafine supported particles were least visible at this defocus. Underfocusing, especially at defoci corresponding to half-spacing fringes in the TiO2 support, led to improved visibility and resolution of the supported particles. Although the structure and shape of supported ultrafine particles should be resolved better with a 400-kV high-resolution electron microscope, their detectability is poorer than with a 200-kV instrument. An ReO4 cluster should be detectable at 200 kV on TiO2 supports up to 5 nm in thickness, whereas it is only likely to be detectable at 400 kV on supports up to 3 nm in thickness. The simulations confirmed that optimum defocus is most favourable for imaging supported particles in profile view. Atomic information for particles as small as a 13-atom Pt cuboctahedral cluster should be resolvable with a 400-kV instrument. The crystalline Ti monolayer observed on surfaces of Pt particles, which could explain the mechanism known as SMSI, was simulated as an example of profile imaging.