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 D-s similar to 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.