Three-Dimensional Nanoparticle Distribution and Local Curvature of Heterogeneous Catalysts Revealed by Electron Tomography

The size and shape of pores is a key factor in the successful deployment of mesoporous silicas as supports
for high-activity catalytic nanoparticles. Critical concerns are the accessibility of catalyst particles to reactant
species, the effect of the particle−support interaction on catalytic activity, and the stability of the system
with respect to degradation or sintering. In the present work, high-angle annular dark field scanning transmission
electron microscopy (HAADF−STEM) tomography has provided quantitative three-dimensional information
about the location of bimetallic nanoparticles supported on and within disordered mesoporous silica (Grace
Davison 634-type). The surface of the pore network was found to be fractal in nature (fractal dimension Ds
∼ 2.4) with implications for the selectivity of the catalyst−support system. By measuring the location of
catalyst particles as a function of the local curvature of the support, particle adsorption sites were classified.
The distribution of nanoparticles within the interior of the support showed preferential adsorption on anticlastic
(“saddle-shaped”) surfaces, whereas nanoparticles adsorbed on the exterior surfaces of the support structure
also demonstrated a strong preference for concave (“cup-like”) regions. These results highlight the critical
importance of three-dimensional characterization for quantitative evaluation of porous media and catalytic
supports.