Self-Supporting Monoliths from Hollow Core-Shell Ni@SiO2 Nanostructures

Metal nanoparticles are finding widespread application in various industries due to their unique properties compared to their bulk counterparts. However, beyond advantageous properties, metal NPs also show a greater tendency to form larger clusters, mitigating their "nano-effects". To counter this tendency, encapsulation of metal NPs in stable oxide shells, such as silica shells, has been shown previously to increase the thermal stability of the embedded metal NP due to reduced ability for Oswald Ripening and coalescence. While these core-shell configurations hence show great promise for robust application of NPs, these materials are still in nano-powder form and constitute significant health and safety concerns. Furthermore, nano-powders pose an engineering challenge due to excessive pressure drop when used in a catalytic bed, for example. While these issues could be overcome by depositing these core-shell structures onto conventional supports, a self-supporting monolithic structure made from such core-shell materials would be a uniquely efficient and potentially flexible configuration.

In the present work, we present results from an investigation into forming such self-supporting monolithic structures from hollow Ni@SiO2 core-shell catalysts. The aim of the study is the formation of monoliths from Ni@SiO2 without the aid of sacrificial chemicals (such as binders) or support structures, and, ultimately, to control the meso- and macroporosity of the resulting monolith while maintaining a minimum mechanical stability. Towards this goal, several different approaches were tested based on a cross-linking of the oxide shells with and without additional silica reinforcement. The resulting monoliths were characterized via SEM and TEM and tested for their rupture strength. Based on the results, we developed an optimized protocol to form stable, self-supporting monoliths from Ni@SiO2 core-shell materials. Since the procedure is based on cross-linking the silica matrix, it is expected to be transferrable to other hollow and non-hollow metal@SiO2 systems. We are currently evaluating these structures in catalytic experiments, comparing the monolith against a conventional catalytic fixed-bed.